Solid-State NMR in Zeolite Catalysis [1st ed.] 978-981-13-6965-0;978-981-13-6967-4

Table of contents :
Front Matter ....Pages i-xi
Solid-State NMR Principles and Techniques (Jun Xu, Qiang Wang, Shenhui Li, Feng Deng)....Pages 1-55
Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis (Jun Xu, Qiang Wang, Shenhui Li, Feng Deng)....Pages 57-91
Solid-State NMR Characterization of Framework Structure of Zeolites and Zeotype Materials (Jun Xu, Qiang Wang, Shenhui Li, Feng Deng)....Pages 93-132
Solid-State NMR Characterization of Host-Guest Interactions (Jun Xu, Qiang Wang, Shenhui Li, Feng Deng)....Pages 133-157
Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid Catalysts (Jun Xu, Qiang Wang, Shenhui Li, Feng Deng)....Pages 159-197
In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites (Jun Xu, Qiang Wang, Shenhui Li, Feng Deng)....Pages 199-254
Back Matter ....Pages 255-260

Citation preview

Lecture Notes in Chemistry 103

Jun Xu Qiang Wang Shenhui Li Feng Deng

Solid-State NMR in Zeolite Catalysis

Lecture Notes in Chemistry Volume 103

Series Editors Barry Carpenter, Cardiff, UK Paola Ceroni, Bologna, Italy Barbara Kirchner, Bonn, Germany Katharina Landfester, Mainz, Germany Jerzy Leszczynski, Jackson, USA Tien-Yau Luh, Taipei, Taiwan Eva Perlt, Bonn, Germany Nicolas C. Polfer, Gainesville, USA Reiner Salzer, Dresden, Germany

The Lecture Notes in Chemistry The series Lecture Notes in Chemistry (LNC), reports new developments in chemistry and molecular science - quickly and informally, but with a high quality and the explicit aim to summarize and communicate current knowledge for teaching and training purposes. Books published in this series are conceived as bridging material between advanced graduate textbooks and the forefront of research. They will serve the following purposes: • provide an accessible introduction to the field to postgraduate students and nonspecialist researchers from related areas, • provide a source of advanced teaching material for specialized seminars, courses and schools, and • be readily accessible in print and online. The series covers all established fields of chemistry such as analytical chemistry, organic chemistry, inorganic chemistry, physical chemistry including electrochemistry, theoretical and computational chemistry, industrial chemistry, and catalysis. It is also a particularly suitable forum for volumes addressing the interfaces of chemistry with other disciplines, such as biology, medicine, physics, engineering, materials science including polymer and nanoscience, or earth and environmental science. Both authored and edited volumes will be considered for publication. Edited volumes should however consist of a very limited number of contributions only. Proceedings will not be considered for LNC. The year 2010 marks the relaunch of LNC.

More information about this series at http://www.springer.com/series/632

Jun Xu Qiang Wang Shenhui Li Feng Deng •

•

•

Solid-State NMR in Zeolite Catalysis

123

Jun Xu Wuhan Institute of Physics and Mathematics Chinese Academy of Sciences Wuhan, Hubei, China

Qiang Wang Wuhan Institute of Physics and Mathematics Chinese Academy of Sciences Wuhan, Hubei, China

Shenhui Li Wuhan Institute of Physics and Mathematics Chinese Academy of Sciences Wuhan, Hubei, China

Feng Deng Wuhan Institute of Physics and Mathematics Chinese Academy of Sciences Wuhan, Hubei, China

ISSN 0342-4901 ISSN 2192-6603 (electronic) Lecture Notes in Chemistry ISBN 978-981-13-6965-0 ISBN 978-981-13-6967-4 (eBook) https://doi.org/10.1007/978-981-13-6967-4 Library of Congress Control Number: 2019934515 © Springer Nature Singapore Pte Ltd. 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Since the first observation of nuclear magnetic resonance (NMR) phenomenon by Bloch and Purcell in 1946, the strong impact of NMR has found in a wide range of fields from fundamental and applied science. Nowadays, it is inarguably that the NMR spectroscopy is an indispensable and powerful analytical method for scientists from chemistry to biology as NMR is endowed with the capability to describe the structure and dynamic behavior of inorganic or bimolecular system at atomic level. Solid-state NMR is sensitive to short-range ordering of solids in structure and geometry determination, which, however, did not initially get popularity before the 1980s due to the lower resolution as compared to conventional liquid-state NMR. The establishment of solid-state NMR as a powerful, often unique, technique in most of the major topics in the area of solid-state material research considerably benefits from the novel theories and techniques enabling the achievement of high resolution and high sensitivity. Recent advances in solid-state NMR from one-pulse and one-dimensional experiments to multi-pulse and multi-dimensional experiments make it routinely to yield invaluable information about the electronic structure, the spatial arrangement, and the connectivity of given nuclei in solids that often cannot be achieved by other techniques. Heterogeneous catalysis plays an essential role in the modern industrial process for the conversion of petroleum and natural gas into fuels and bulk chemicals, fine chemicals, and intermediates. Synthesizing an efficient catalyst that produces the desired products at high rates and selectivity with long lifetime requires a deep understanding of the function of the catalyst as well as the reaction catalyzed at a specific condition and thus of the establishment of the structure–activity relationship. Heterogeneous catalysts include zeolites, zeotypes, oxides, heteropolyacids, etc. Zeolites are one kind of the most important heterogeneous catalysts, and they are crystalline nanoporous inorganic materials formed by TO4 tetrahedra (T = Si, Al, P, etc.). Not only in petrochemistry, zeolites as catalysts are applied in manufacturing organic intermediates and fine chemicals and also show

v

vi

Preface

promising results in the conversion of biomass. The success of zeolites in catalysis benefits from their rich and tunable properties and structures. Up to now, there are more than 200 different structural types of zeolites. A substantial progress has been made in the zeolite catalysis in terms of synthesis, characterization, and testing. Solid-state NMR provides valuable information on the structure of catalysts and the dynamics of the catalytic reactions. The successful application of solid-state NMR in zeolites catalysis has resulted in a better understanding of the property and structure of zeolites and the related catalytic reactions and thus advances in the design of new catalysts and discovery of new catalytic processes. Chapter 1 of the book deals with the basics of solid-state NMR, which covers the general principles of NMR and NMR techniques. It introduces the theory of NMR and illustrates how the nuclear spins are modulated by radio-frequency (RF) pulse sequences to enhance the NMR sensitivity and resolution. The spectral characteristics in solid-state NMR, mainly various interactions like dipolar-dipolar and quadrupolar couplings, are also described. In Chapter 2, the application of solid-state NMR in the synthesis of zeolite and zeotype materials is demonstrated. This chapter introduces the strategy to characterize the evolution of the structure of the materials from normally amorphous state to crystallite by collecting the NMR data on the constituent elements such as 31P, 27 Al, 29Si. Ex situ and in situ NMR methods are described for the monitoring of the synthesis of the catalysts. Chapter 3 describes the intensive use of solid-state NMR for revealing the structure features of solid catalysts. It focuses on the zeolites and zeotype materials of direct relevance to the heterogeneous catalysis. Besides the information on chemical environment of the coordinated atoms (29Si, 27Al, 17O, 11B, 71Ga, etc.) on the framework, using 129Xe as an NMR probe for investigating the microstructure of porous catalysts and the state of metal species confined in zeolites is described. Characterization of host-guest interaction in zeolites by solid-state NMR is introduced in Chapter 4. This chapter describes the advanced solid-state NMR experiments including heteronuclear double-resonance, 2D homonuclear, and heteronuclear correlation NMR for the establishment of the correlation between zeolite framework and adsorbed molecules. The detection of the host-guest interaction for the understanding of molecular sieve synthesis, active center, and catalytic reaction mechanism is demonstrated. Chapter 5 discusses the solid-state NMR experiments on nuclei including 1H and 27 Al serving as the active sites in solid acid catalysts. This chapter demonstrates the strategies for the analysis of nature (type and strength), location, and spatial proximity of the acid sites. Probe molecule technique is presented for the detection of the surface acid sites on catalysts. Chapter 6 describes the solid-state NMR applied in heterogeneous reactions. In situ solid-state NMR techniques that allow monitoring the catalytic reaction on zeolites are demonstrated. This chapter also introduces the joint application of in situ NMR and other techniques for the elucidation of kinetics and mechanism of catalytic reactions.

Preface

vii

The book is mainly oriented to audiences composed of graduate students, faculty members, researchers of both academia and industry, and R&D managers directly or indirectly involved in materials and heterogeneous catalysis. Wuhan, China

Jun Xu Qiang Wang Shenhui Li Feng Deng

Contents

1 Solid-State NMR Principles and Techniques . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Nuclear Spin Interactions in Solids . . . . . . . . . . . . . . . . . 1.2.1 Chemical Shift Interaction . . . . . . . . . . . . . . . . . . 1.2.2 Dipole-Dipole Interaction . . . . . . . . . . . . . . . . . . . 1.2.3 Quadrupolar Interaction . . . . . . . . . . . . . . . . . . . . 1.2.4 Spin-Spin Interaction . . . . . . . . . . . . . . . . . . . . . . 1.3 Manipulations of Spin Interactions in Solids . . . . . . . . . . 1.3.1 Magic-Angle Spinning (MAS) . . . . . . . . . . . . . . . 1.3.2 Cross-Polarization . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Dipolar Decoupling Methods in Rotating Solid . . . 1.3.4 Dipolar Recoupling Methods in Rotating Solid . . . 1.3.5 High-Resolution Techniques for Half-Integer-Spin Quadrupolar Nuclei . . . . . . . . . . . . . . . . . . . . . . . 1.3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . .

1 1 2 5 6 7 10 11 11 13 15 18

...... ...... ......

34 45 45

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Zeolite Synthesis Route and Procedures . . . . . . . . . . . . . . 2.3 Zeolite Synthesis Process and Crystallization Mechanism . 2.4 NMR Strategy in Characterization of Zeolite Synthesis . . . 2.4.1 Microporous Aluminosilicates . . . . . . . . . . . . . . . 2.4.2 Microporous Aluminophosphates . . . . . . . . . . . . . 2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .

57 57 59 59 62 63 73 88 88

. . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . .

. . . . . . . . .

ix

x

3 Solid-State NMR Characterization of Framework Structure of Zeolites and Zeotype Materials . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Solid-State NMR Characterization of Zeolite Framework Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 27Al MAS NMR . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 29Si MAS NMR . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 31P MAS NMR . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4 17O MAS NMR . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.5 129Xe NMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.6 Other Framework Elements . . . . . . . . . . . . . . . . . 3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Contents

...... ...... . . . . . . . . .

. . . . . . . . .

4 Solid-State NMR Characterization of Host-Guest Interactions . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Solid-State NMR Characterization of Host-Guest Interactions 4.2.1 Host-Guest Interaction Between Adsorbed Molecule and Zeolite Framework . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Host-Guest Interaction in Molecular Sieve Synthesis . 4.2.3 Host-Guest Interaction in Zeolite Catalysis . . . . . . . . 4.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . .

. . . . . . . . .

94 94 99 107 110 116 122 125 126

. . . . 133 . . . . 133 . . . . 134 . . . . .

. . . . .

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Solid-State NMR Characterization of Acidic Property . . . . . . . . 5.2.1 Acid Sites Containing Hydroxyl Groups . . . . . . . . . . . . 5.2.2 Acidic Nature and Strength . . . . . . . . . . . . . . . . . . . . . 5.2.3 Location and Distribution of Acid Sites . . . . . . . . . . . . 5.2.4 Spatial Proximities and Synergy Effects of Different Acid Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 In Situ Solid-State NMR Approaches . . . . . . . . . . . . . . . . . 6.2.1 Batch Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Flow Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Mechanistic Study of the Catalytic Reactions by In Situ NMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .

93 93

. . . . .

. . . . .

. . . . .

. . . . .

134 138 146 153 154

. . . . . .

. . . . . .

159 159 160 160 168 175

. . 179 . . 191 . . 192 . . . . .

. . . . .

199 199 200 201 203

. . . . . 209

Contents

6.3.1 Activation and Conversion of Light Alkanes . 6.3.2 Methanol-to-Olefins (MTO) Conversion . . . . 6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xi

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

209 226 242 243

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

Chapter 1

Solid-State NMR Principles and Techniques

Abstract This chapter gives a brief introduction of nuclear spin interactions in solids. Then techniques to manipulate these internal interactions via space and spin aspects are presented, including magic-angle spinning (MAS), cross-polarization (CP), dipolar decoupling as well as dipolar recoupling methods. One- (1D) and twodimensional (2D) homo- and heteronuclear correlation MAS NMR experiments are also discussed here. Particularly, we provide a snapshot of the current state of methodology development on quadrupolar nuclei (I > 1/2) in solids, e.g., the high-resolution 2D multiple-quantum MAS (MQMAS) experiment and strategies, to enhance NMR signals of the central transition (CT) of half-integer quadrupolar nuclei. Other issues concerning hyperpolarization approaches such as dynamic nuclear polarization (DNP) and parahydrogen-induced polarization (PHIP) that improve the NMR sensitivity are also described in this chapter. Keywords Solid-state NMR · Spin interactions · Dipolar decoupling · Dipolar recoupling · Magic-angle spinning · Quadrupolar nuclei · Hyperpolarization

1.1 Introduction The explorations in the field of nuclear magnetic resonance (NMR) spectroscopy have never been slowed down since the NMR phenomenon in bulk condensed phase was observed for the first time in 1946 by Bloch et al. and Purcell et al. independently. After the development of the pulse Fourier transform NMR spectroscopy by Ernst and Anderson [1] and the proposal of and demonstration of multi-dimensional NMR by Jeener and by Ernst [2], respectively, the study of NMR is widespread in physics, chemistry, biology, and even medicine for investigating the structural and dynamic properties of materials at atomic level. As an important branch of NMR spectroscopy, solid-state (SS) NMR is a powerful tool widely used to characterize heterogeneous catalysts, polymer materials, and biological molecules. However, the existence of different internal spin interactions in solids broadens the breadth of the NMR absorption line compared with that in liquids, thus leading to significant loss of resolution and sensitivity of NMR © Springer Nature Singapore Pte Ltd. 2019 J. Xu et al., Solid-State NMR in Zeolite Catalysis, Lecture Notes in Chemistry 103, https://doi.org/10.1007/978-981-13-6967-4_1

1

2

1 Solid-State NMR Principles and Techniques

signals. Over the past few decades, scientists have dedicated to developing new methods or techniques to improve the resolution and sensitivity of NMR spectroscopy, which prompted great progress of SSNMR spectroscopy and in-depth researches with SSNMR characterizations. In fact, there are many valuable books [3–6] describing the NMR phenomenon in terms of basic quantum-mechanical concepts, which also provide full quantum-mechanical treatment of the nuclear magnetic interactions in the solid state. Concerning the readership among chemists who are of interest for the state-of-the-art solid-state NMR characterizations, we will not cover a detailed analysis of NMR theory, and there is no rigorous physical deduction/derivation described in this book. However, an overview of the spin interactions in solids and the methods or techniques developed to manipulate the interactions in pulsed SSNMR will be presented in this chapter. We first consider a brief description of the interactions of nuclear spins, such as chemical shift, dipolar or quadrupolar interactions, originated by their ambient structure at atomic level. Then various strategies to manipulate these internal interactions will be introduced: Sample rotation together with dipolar decoupling methods can be used to improve the resolution/sensitivity of NMR signal in solids, while dipolar- or J-mediated methods under sample rotation could be used to investigate the spatial proximity or bond connectivity between spins. In particular, SSNMR methods involving quadrupolar nuclei (spin number I > 1/2) are also discussed. The quadrupolar nuclei represent about two-thirds of stable NMR-active nuclei and are present in materials ranging from catalysts, glasses, ceramics, and superconductors to biological systems. In addition, recently burgeoning hyperpolarization methods are also concerned in this chapter.

1.2 Nuclear Spin Interactions in Solids In NMR spectroscopy, when a nucleus with a nonzero spin quantum number (I = 0) is placed in a magnetic field, the non-degenerate energy levels of spin states are separated, which is known as the Zeeman effect. Additionally, the energy levels are also perturbed by internal spin interactions, mainly including chemical shift, J-coupling, dipole-dipole interaction (also dipolar coupling), and quadrupolar interactions, which are associated with the chemical and structural environments around the nucleus. Thus, the appearance of NMR spectra is determined by the various anisotropic interactions of the nuclear spins with each other and the external magnetic field. These interactions affect not only the apparent resonance frequencies, but also the lineshapes and relaxation times of the NMR signals. In solutions, the rapid Brownian motion of molecules typically leads to averaging the anisotropic interactions, which broaden the resonance lines, into isotropic interactions, such as the isotropic chemical shift and J-coupling terms. Hence, high-resolution spectra can be easily obtained in solution NMR. For solid samples, the slow molecular motions cannot average out the anisotropic interactions. Thus, NMR signals of powder samples normally exhibit broad resonance lines, which result in the low-resolution characteristics and the difficulties to distinguish and assign all the non-equivalent nuclei.

1.2 Nuclear Spin Interactions in Solids

3

Therefore, effectively designing and applying various SSNMR techniques to suppress or manipulate the anisotropic terms/interactions is an important issue to achieve high-resolution SSNMR spectra, which is essential for extracting chemical and structural information of materials at the atomic/molecular level. Ignoring the relaxation part, the Hamiltonian of spin interactions has the form: H = HExt + HInt

(1.1)

where H Ext is the Zeeman interaction between spin angular moment and applied external magnetic field (i.e., the static field B0 and the radio-frequency field B1 ) and H Int corresponds to internal interactions. In this chapter, we focus on the second term. Usually, H int includes chemical shift interaction H CS , dipolar interaction H DD , nuclear quadrupolar interaction H Q (if spin I > 1/2), and spin-spin interaction (Jcoupling) H J . HInt = HCS + HDD + HQ + HJ

(1.2)

These interactions can be represented in a uniform way: 3 

Hλ = C λ

λ Imλ Rmn Sλn

(1.3)

m,n=1

Here λ = CS, DD, Q, or J; C λ is a constant which relates to the characteristics of the nucleus; Im represents the nuclear spin angular momentum vector; Rmn is the mn element in the second-rank Cartesian tensor; Sn could be the external field (λ = CS), the same spin angular momentum vector (λ = Q) or another spin angular momentum vector (λ = J, DD). Thus, these interactions can be written as follows, respectively. HCS =



·γ i Ii · σi · B0

(1.4a)

i

HJ =



Ii · Jij · Ij

(1.4b)

i50 kHz) MAS conditions. Moreover, it has been demonstrated that the combination of Lee–Goldburg decoupling and cross-polarization can significantly suppress the 1 H homonuclear dipolar interactions during the period of polarization transfer from 1 H to 13 C. Thus, 1 H–13 C distances r CH on uniformly 13 C-enriched tyrosine·HCl was precisely extracted from the time-oscillating CP build-up curves with LG-CP under fast MAS [51].

1.3.4 Dipolar Recoupling Methods in Rotating Solid Many applications of solid-state NMR to molecular structure determination are based on dipole-dipole interactions between nuclei since they encode important structural information on the spatial proximity/interaction of atoms through their dependence on the inverse cube of the internuclear distance. A wide variety of dipolar recoupling methods have been developed for measuring the dipole-dipole interactions that are averaged out by magic-angle spinning (MAS). Heteronuclear dipolar recoupling Since dipolar interaction contains the structural information about the distance of a coupled spin pair in addition to the orientation of the related internuclear vector, a particularly important class of recoupling techniques has been developed to restore the dipole-dipole interactions between spins of the different isotopic types. The most commonly established and useful technique for heteronuclear dipolar recoupling is cross-polarization (CP). Besides, many other recoupling schemes have been developed under fast MAS. Rotational-Echo DOuble Resonance (REDOR) [52] is one of the most prevalent heteronuclear recoupling methods under magic-angle

(a)

xy

yx

τ τ 2τ τ τ

(b)

xy y x x y yx

τ τ 2τ τ 2τ τ 2τ τ τ

(c) xy y x x y

yx y x x y y x yx x y y x x y x y

τ τ 2τ τ 2τ τ 2τ τ 2τ τ 2τ τ 2τ τ 2τ τ 2τ τ 2τ τ 2τ τ 2τ τ τ Fig. 1.13 Schematic representation of the homonuclear dipolar decoupling RF pulse sequences employed under low MAS conditions: WHH4 (a), MREV8 (b), BR24 (c). The vertical rectangles in (a–c) represent 90° pulses of finite length. The interpulse delays τ and 2τ can be well-optimized to compensate for the finite lengths of the pulses. The x, y labels denote the phase of each pulse. The phases x¯ and y¯ correspond to a 180° phase shift of phase x and y

1.3 Manipulations of Spin Interactions in Solids

(a)

x

x

Vω

−Vω

19

(b)

6

5 4

1 2 3 45 6 7 8 9 9 8 7 65 4 3 2 1

2 9 1 8 7 3 4 5

(c)

Phase (degree)

3

7 8 1 9 2 6

360 300 240 180 120 60 0 -60 0

10

20

30

40

50

60

(d)

Fig. 1.14 Schematic representation of the homonuclear dipolar decoupling RF pulse sequences employed under moderate to ultra-fast MAS. (a) The FSLG pulse scheme (shown on the left) consists of two consecutive 2π pulses with opposite RF phase, x and x¯ , and opposite frequency √ offsets ω and −ω. The frequency offset must obey the relation  ω = ω1 / 2 in order to tilt the RF effective field by the magic angle with respect to the Z-axis. (b) For PMLG, the frequency offset is replaced by a phase list made of a limited number of phase steps. The case of PMLG9 using 9 × 2 phase steps is displayed. (c) DUMBO-1 employs constant irradiation divided into 64 fragments. The horizontal and vertical coordinates represent the ordinal number of the pulse and the phase of corresponding pulse, respectively. (d) SAMn (here n = 3), 3 cycles of cosine-shape(ω1max cos (3ωR t)) pulses irradiate for each rotor period in series

20

1 Solid-State NMR Principles and Techniques

π/2

τre/2

π

τre/2

I π S N

N Rotor Period 0

1

2

3

4

Fig. 1.15 The typical REDOR pulse sequence, for the measurement of heteronuclear dipolar couplings under magic-angle spinning. Two separate experiments have to be performed with I as the observed spin. In the first experiment, there is no irradiation on the S spin (no pulse on S channel), and the heteronuclear dipolar coupling between the I–S spin pair is refocused at the end of echo time (τ re ) under magic-angle spinning. The intensity of reference I spin signal is denoted S 0 (τ re ). In the second experiment, as shown in the figure, the refocusing of the dipolar coupling is prevented by a series of rotor-synchronized π pulses applied to S spins. Thus, the intensity of I spin signal is modulated by the I–S heteronuclear dipolar coupling. Here, the signal is denoted as S  (τ re ). Both signals are collected with the increasing of the dephasing time, τ re , stepped by a multiple of 2τ R , where τ R is the rotor period. The dipolar coupling constant can be deduced by simulating or fitting  the dephase curve 1 − [S (τ re )/S 0 (τ re )]

spinning. Consider the case of a spin pair of spin-1/2, labeled here I and S. The typical pulse sequence used in the REDOR experiment is shown in Fig. 1.15. There are two π pulses applied to the I or S spins in each rotational period to prevent the dipolar coupling of I–S from refocusing at the time of rotational echoes. The difference spectrum (S(τ re )) of I spin signal is obtained by subtracting the I spin signal of the spin-echo with irradiation (π pulses) on S spin (S (τ re )) from that without irradiation on S spin (S0 (τ re )), which depends quantitatively on the dipolar coupling strength, where τ re = N c T r , N c = 2N is the number of rotor periods in the dipolar evolution periods, and T r is the sample rotation period. Therefore, the normalized REDOR dipolar-dephasing signal [S (τ re )/S0 (τ re )] of I spin is obtained by gradually increasing the number of rotor cycles and observing signals both with and without the dipolar dephasing. For this reason, REDOR method can be used to measure the dipolar coupling constant that is associated with the internuclear distance within an isolated (or dilute) spin-pair I–S. Figure 1.16 shows two universal curves for S (τ re )/S0 (τ re ) and for S(τ re )/S0 (τ re ), where S(τ re ) = S0 (τ re ) − S (τ re ) [53], which could also be reported as S = (S 0 − S)/S 0 as a function of recoupling time (τ re ) and can be subsequently analyzed in terms of the analytical REDOR transform techniques [54] or numerical simulations [55]. Figure 1.17 displays the S 0 (bottom) and S (top) 13 C REDOR spectra of [1-13 C, 15 N]acetyl-l-carnitine hydrochloride with a fixed recoupling/dephasing time (N c = 34) [53].

1.3 Manipulations of Spin Interactions in Solids

21

Fig. 1.16 Dependence of the REDOR ratios S r /S 0 and S/S0 on the dimensionless parameter λ. The dipolar evolution time is the product of the number of rotor cycles, N c , in the dipolar evolution period and the sample rotation period, T r . The dipolar coupling, D, is expressed in Hz for the graphs. S r represents the same parameters as the S  denoted in this chapter. Reproduced from Ref. [53] by permission of Wiley

However, in the case where the I spin is coupled to more than one S spins, the data can often be fitted by many different sets of dipolar coupling constants and so are more difficult to be quantitatively analyzed. For REDOR experiment, it commonly demands two short π pulses separated by a certain interval in each rotor period to recover the heteronuclear dipolar interactions averaged out by fast MAS, while the rotary resonance recoupling (R3 ) [56, 57] is proposed for continuous-wave irradiation that requires a nutation frequency (RF field) matching the rotary resonance condition ω1I = nωR , n = 1 or 2 under MAS. In case of n = 1, it will reintroduce both heteronuclear and homonuclear dipolar interactions; thus, the n = 2 condition is usually selected to only recover the heteronuclear dipolar interactions. For heteronuclear dipolar coupling and chemical shift anisotropy (CSA), the rotary resonance occurs at n = 1 or 2. And there is another rotary resonance condition with n = 1/2 that affects only the homonuclear dipolar interaction, which will be discussed in the next section.

22

1 Solid-State NMR Principles and Techniques

Fig. 1.17 13 C REDOR spectra for a powder sample of [1-13 C, 15 N]acetyl-l-carnitine hydrochloride diluted 1 in 20 parts natural-abundant acetyl-l-carnitine. These spectra were obtained with N c = 34 and a spinning speed of 3205 Hz. 13 C and 15 N RF field strengths were 38 kHz. The proton decoupling field strength was 110 kHz. Reproduced from Ref. [53] by permission of Wiley

In fact, we can also employ phase or amplitude modulation with respect to the MAS frequency ωR to reintroduce the anisotropic dipolar interactions. The simultaneous frequency and amplitude modulation (SFAM) [58] is designed to follow this concept where the carrier frequency of the RF field is modulated cosinusoidally while its amplitude is modulated sinusoidally: ωI (t) = ω cos(N ωR t) ω1I (t) = ω1I sin(N ωR t)

(1.21)

where N = 1 or 2, denoted SFAMN , the frequency modulation can be achieved with the fast modulation of RF phase φ I due to ωI (t) = dφI (t)/dt, and thus, we have φI (t) = ω sin(N ωR t)/N ωR

(1.22)

It has also been shown that, with respect to heteronuclear dipolar interaction, continuous SFAM1 behaves exactly the same as REDOR with ideal π pulses. However, SFAM1 also recouples the homonuclear dipolar interactions and thus should not be used when these are not negligible. In this case, in a similar way as with R3 to eliminate homonuclear dipolar interactions, the modulation frequency should be twice the spinning speed, leading to the SFAM2 method. Nevertheless, SFAM1 and SFAM2 are both efficient, especially at very high spinning speed [59]. As for dipolar coupling measurement of heteronuclear systems involving halfinteger quadrupolar spins, useful techniques such as transfer of populations in double resonance (TRAPDOR) [60–63], rotational-echo adiabatic-passage double resonance (REAPDOR) [52, 64–66], rotary resonance-echo saturation-pulse double

1.3 Manipulations of Spin Interactions in Solids

23

resonance (R-RESPDOR) [67], and symmetry-based resonance-echo saturationpulse double resonance (S-RESPDOR) [68, 69] are commonly used. TRAPDOR and REAPDOR use an adiabatic-passage pulse to change the spin states of the quadrupolar nuclei to reintroduce the heteronuclear dipolar coupling. Therefore, the RF field experienced by the quadrupolar nuclei must fulfill the condition: α = ν 21I /ν R ν Q , where α the adiabaticity parameter, ν 1I the RF-field amplitude on the quadrupolar nuclei channel, ν R = 1/T R the MAS frequency, and ν Q = 3C Q /2I(2I − 1) with C Q = e2 qQ/h [60, 70–72]. This condition requires high-power RF field, especially in the case of large quadrupole interactions and high spinning frequencies. Therefore, the use of TRAPDOR and REAPDOR may be prevented under fast MAS owing to the limited RF field. Note that TRAPDOR is more demanding than REAPDOR since the adiabatic-passage irradiation lasts for one-half of the dipolar evolution period in the former instead of a fraction of the rotor period in the latter. Moreover, REAPDOR and REDOR suffer from the same drawbacks, especially under fast MAS: (i) They recouple the homonuclear dipolar interaction between the spin-1/2 nuclei [73], and (ii) their dephasing curves depend on the ratio between the π -pulse length and T R [74]. An alternative was proposed recently [67], in which adiabatic-passage pulses were replaced by saturation pulses. These are advantageous since it only needs moderate RF fields, even at high MAS speed. Saturation pulse was first incorporated in R-RESPDOR experiment, which uses R3 sequence as a heteronuclear dipolar recoupling method. The R3 technique has the advantage to be γ -encoded and compatible with high MAS frequency [75]. However, R-RESPDOR method exhibits several limitations: (i) It is sensitive to RF-field inhomogeneity [76], (ii) it is influenced by chemical shift anisotropy (CSA) which does not commute with heteronuclear dipolar terms [69], (iii) it is sensitive to homonuclear dipolar interactions of the observed nuclei, (iv) it suffers from dipolar truncation, which prevents the measurement of medium- and long-range distances. However, these can be circumvented by replacing the R3 recoupling by SR42 1 sequence [77] (see the next section for details). This symmetry-based resonance-echo saturation-pulse double-resonance (S-RESPDOR) method was first applied to determine 13 C–14 N distance in uniformly 13 C-labeled amino acid [69]. And it also can be used to measure distances between spin-1/2 and half-integer quadrupolar nuclei, and a general analytical formula describes its dephasing curve for all spin values as shown in Eq. 1.23.  √    2I kπ λ kπ λ f π 2  S J−1/ 4 = 2I − [4I − 2(k − 1)]J1/ 4 S0 2I + 1 4(2I + 1) 4 4 k=1

(1.23) where I is the spin quantum number of the quadrupolar nucleus and J ±1/4 denotes ±1/4-order Bessel function of the first kind. The τ is the dephasing time. This formula depends on the dimensionless parameter λ = τ * |bIS | and the pre-factor parameter f. Note that the parameter f represents the fraction of sample in which the spin states of quadrupolar nucleus are saturated by the saturation pulse, and is equal

24

1 Solid-State NMR Principles and Techniques

Fig. 1.18 S-REDOR and S-RESPDOR signal fractions as function of the dimensionless parameter k. The (blue) points represent the simulated S/S 0 values calculated with PULSAR [78] at ν R = 20 kHz and B0 = 9.4 T for an S = 13 C–I spin pair. An infinitely short central π -pulse is used on the 13 C-observed channel. For S-RESPDOR, the saturation pulse lasts 75 μs = 1.5T . The anisotropic R chemical deshielding constant, δ aniso (13 C), is equal to 8 kHz and ηCSA (13 C) = 0. The continuous lines represent the fits of simulated S/S 0 values using Eq. (1.1). For each curve, the simulation and fit parameters are given below: a I = 1/2 (15 N). PULSAR: bIS = 937 Hz, ideal central π -pulse on non-observed 15 N channel. Fit: bIS = 924 Hz, f = 1.0. b I = 1 (14 N). PULSAR: bIS = 674 Hz, C Q = 1.14 MHz, ηQ = 0.24, ν 14N = 50 kHz. Fit: bIS = 659 Hz, f = 0.96. c I = 3/2 (11 B). PULSAR: bIS = 800 Hz, C Q = 2.5 MHz, ηQ = 0.8, ν 11B = 60 kHz. Fit: bIS = 793 Hz, f = 1.02. d I = 5/2 (17 O). PULSAR: bIS = 300 Hz, C Q = 8.52 MHz, ηQ = 0.74, ν 17O = 65 kHz. Fit: bIS = 290 Hz, f = 0.99. (e) I = 7/2 (51 V). PULSAR: bIS = 270 Hz, C Q = 1.73 MHz, ηQ = 0.37, ν 51V = 50 kHz. Fit: bIS = 267 Hz, f = 1.0. (f) I = 9/2 (73 Ge). PULSAR: bIS = 320 Hz, C Q = 5 MHz, ηQ = 1, ν 73Ge = 50 kHz. Fit: bIS = 319 Hz, f = 0.98. There are two parameters introduced into the fit: f and bIS . These two parameters can be fully determined as long as the recoupling time is long enough to reach the plateau of the signal fraction. When it is not the case, these two parameters are strongly correlated, and then the pre-factor f must be fixed. Reproduced from Ref. [68] by permission of Elsevier

to 1 only for 100%-abundant I nuclei experiencing a complete saturation. Equation 1.23 also describes results observed with the symmetry-based resonance-echo double-resonance (S-REDOR) method [69], which is the analogue of S-RESPDOR for a pair of spin-1/2 nuclei. Figure 1.18 displays the S-RESPDOR fraction in the case of I = 1, 3/2, 5/2, 7/2, and 9/2, as well as the S-REDOR (I = ½) fraction. Nevertheless, all these methods were designed to make the heteronuclear dipolar interaction participate in the NMR signal evolution in a certain period under MAS, and thus, the time-dependent dipolar dephasing could be quantitatively or qualitatively collected. More recently, the C and R symmetry-based pulse sequences [79, 80] have been introduced by Levitt et al. for the purpose of selecting symmetry-allowed space components and spin components of spin interactions under magic-angle spinning. Table 1.3 displays the space and spin rank of different spin interactions in the interac-

1.3 Manipulations of Spin Interactions in Solids

25

Table 1.3 Components of spin interactions in the interaction frame of an applied RF field, in the case of exact magic-angle spinning Interaction

Space rank (l)

Space component (m)

Spin rank (p)

Spin component (μ)

Isotropy chemical shift

0

0

1

1, 0, −1

Chemical shift anisotropy

2

2, 1, −1, −2

1

1, 0, −1

Homonuclear Dipole–dipole

2

2, 1, −1, −2

2

2, 1, 0, −1, −2

Heteronuclear Dipole–dipole

2

2, 1, −1, −2

1 (spin I) 1 (spin S)

1, 0, −1 (spin I) 1, 0, −1 (spin S)

J-coupling

0

0

0

0

tion frame. In the presence of the RF field on one spin species, the interaction frame Hamiltonian may be described as a superposition of many rotational components: Hλ =

p l  

λ Hlmpμ

(1.24)

m=−l μ=−p

Based on their definition, there are a number of symmetry-based sequences classified into CNnv and RNnv (Fig. 1.19) by two types of selection rules, respectively: For CNnv sequences, we have (1)

H lmpμ = 0, , if mn − vμ = NZ

(1.25)

where Z is any integer such as 0, ±1, ±2, ±3. For RNnv sequences, the following selection rule applies (1)

H lmpμ = 0, if mn − vμ =

N Zp 2

(1.26)

where Z p is an integer with the same parity as the spin rank p. Thus, we can recover any spin interaction averaged out under MAS by designing such pulse sequence according to the above selection rules. At present, the symmetrybased R sequences [81–83] such as SR4 (R42 1 R4−2 1 ), R1225 3 , and R209 5 , have been widely used for the efficient heteronuclear dipolar recoupling in SSNMR. Here we only give the selection rules of first-order average Hamiltonian. If the first-order average Hamiltonian is not a sufficiently good approximation of the effective Hamiltonian, the higher-order selection rule should be also considered [80].

26

1 Solid-State NMR Principles and Techniques

(a)

(b)

Fig. 1.19 The schematic diagrams of symmetry-based pulse sequences. a Construction of CNnv sequences: N equivalent 360° pulses (or composite 360° pulses) occupy n rotor periods with RF phase increment of +2π ν/N per step in this pulse train. b Construction of RNnv sequences: N/2 equivalent 180° pulse pairs (or composite 180° pulse pairs) occupy n rotor periods, and in each pulse pair, two pulses have opposite RF phases +π ν/N and −π ν/N

Through-bond and through-space polarization transfer The polarization/coherence transfer between heteronuclei is usually achieved through-bond via the J-coupling or through-space via the dipolar interaction (e.g., CP). As mentioned above, the isotopic J-coupling is not affected by the fast MAS, and recent results on spin-1/2 nuclei in solid materials have shown that the measurement and utilization of J-couplings are nevertheless possible in the case that the line broadening caused by other anisotropic interactions could be sufficiently suppressed. Thus, it is possible to transpose liquid-state NMR-pulse sequences based on scalar coupling like J-RINEPT and J-HMQC experiments (see Fig. 1.20a, b) into solid-state NMR experiments. These methods are referred as through-bond experiments since they can provide bond-connectivity information about molecular structure. However, in most solids, the relative weak J-couplings prevent efficient polarization transfer between different spins. Thus, it is usually preferable to employ dipolar coupling as the medium to achieve the internuclear polarization transfer. Besides CP method, D-HMQC experiment (Fig. 1.20c) is one of the successful designs in solid-state MAS NMR to establish heteronuclear through-space correlations. Comparing with J-HMQC experiment, the delay τ in J-HMQC (Fig. 1.20b) is replaced by dipolar recoupling segments in D-HMQC experiment. In fact, many heteronuclear dipolar recoupling methods such as R3 , SFAM, and R symmetry-based sequences introduced in the context can be also used as the basic dipolar recoupling element in the

1.3 Manipulations of Spin Interactions in Solids

π

π/2 (a)

π

π

π/2

π

τ'/2

τ'/2

τ/2

τ/2

t1/2

I

27

π/2

π t2

S

π/2

(b)

π τ/2

τ/2

I π

t2

π t1/2

S

t1/2

N*τR π/2

(c)

π

τ/2

I

τ/2

Dipolar recoupling

Dipolar recoupling

π

π

S

t2

t1/2

t1/2

Fig. 1.20 Two-dimensional (2D) J-RINEPT (a), J-HMQC (b), and D-HMQC (c) pulse sequences under magic-angle spinning

D-HMQC experiment. Since dipole-dipole interaction is proportional to the inverse cube of the distance between interacting nuclei, these through-space experiments can provide spatial proximities information. A major limitation of above J-mediated HETeronuclear CORrelation (J-HETCOR) experiments involving quadrupolar nuclei is their low sensitivity, since the heteronuclear J-couplings are often smaller than the rate, 1/T2  , of transverse losses. Meanwhile, fast transverse losses might also be an obstacle for D-HETCOR experiments for the observation of long-range distances or the study of paramagnetic materials. In particular, in the presence of quadrupolar nuclei, the complex interference between the RF field and the large quadrupolar interactions and the broadening of the resonances caused by second-order quadrupolar dephasings lead to additional

28

1 Solid-State NMR Principles and Techniques

Fig. 1.21 Pulse scheme for I-{S > 1 PT-J-HMQC experiment. The magnetization of I spins is generated by cross-polarization (CP) from proton. During the defocusing and refocusing delays, τ , the polarization transfer is accelerated by applying N loops of WURST shape pulses with a frequency sweep equal to the MAS frequency, ν R , at an offset of hundreds of kHz from the CT resonance frequency. Reproduced from Ref. [9] by permission of Royal Society of Chemistry

difficulties for these heteronuclear experiments [84]. Many approaches have been developed to overcome the poor sensitivity of J- and D-HETCOR experiments involving half-integer spin quadrupolar nucleus. So far, most of the successful experiments to probe the proximities and connectivities between spin-1/2 (1 H, 13 C, 31 P, 15 N, etc.) and half-integer quadrupolar (11 B, 23 Na, 17 O, 27 Al, etc.) nuclei have been based on the HMQC indirect detection [66, 85, 86]. These methods use RF pulses with low power on the quadrupolar nucleus to only selectively irradiate its central transition (CT: ½ ↔ −½). Due to the second-order quadrupolar broadenings, the sensitivity of HMQC experiments then remains low. This type of method can be advantageous since (i) it benefits from higher sensitivity for correlation between low-γ quadrupolar isotopes (e.g., 17 O, 43 Ca) and high-γ nuclei (e.g., 1 H or 19 F); (ii) it requires a small number of t 1 increments owing to second-order quadrupolar broadening in the indirect dimension and hence shorter acquisition time in the sampling-limited data collection regime; (iii) it provides a strategy to identify the connectivity between two different quadrupolar isotopes (e.g., 27 Al–17 O). Recently, the so-called population transfer (PT)-J-HMQC experiment (Fig. 1.21) was introduced to enhance the sensitivity of J-HMQC with indirect observation of half-integer quadrupolar nuclei in solids [9, 87]. It has been shown that the manipulation of the population of satellite transitions efficiently accelerates the coherence transfer between two nuclei and hence improves the sensitivity when the J-HMQC transfer efficiency is reduced by T2  losses. This PT-J-HMQC experiment has been successfully tested for 31 P–27 Al J-HMQC experiments on a layered aluminophosphate Mu-4. Signal to noise ratio (S/N) gains in the range of 3–5 have been achieved (Fig. 1.22). When the quadrupolar nucleus is directly observed, its CT signal can be further enhanced by adiabatically manipulating the populations of the satellite transitions (STs) at the beginning of the sequence prior to the excitation of CT transverse magnetization. Another way to increase the CT signal is by recycling the signal at

1.3 Manipulations of Spin Interactions in Solids

29

Fig. 1.22 2D 31 P-{27 Al} J-HMQC spectra of Mu-4 sample recorded at B0 = 9.4 T and ν R = 12.5 kHz, with (a) conventional J-HMQC and 2τ = 5.12 ms, and (b) PT-J-HMQC and 2τ = 3.84 ms, and skyline projections along F1 and F2. (c) 1D 27 Al MAS spectrum of Mu-4 sample (black solid line) and simulated four different 27 Al sites (red dash lines) derived from the 2D 3QMAS spectrum. Reproduced from Ref. [9] by permission of Royal Society of Chemistry

the end with a quadrupolar Carr–Purcell–Meiboom–Gill (QCPMG) accumulation [88], when the T2  time of the detected nucleus is longer than its apparent transverse dephasing time constant, T2 ∗ . Homonuclear dipolar recoupling In the same way as heteronuclear dipolar couplings, the investigation of homonuclear dipolar couplings can provide information about spatial connectivity and local geometry between homonuclear spins. However, in solid-state MAS NMR experiments, the orientation-dependent dipolar interactions are attenuated under fast MAS. Therefore, in order to obtain structure information from MAS experiments, the homonuclear couplings/interactions must be also restored first. The first homonuclear dipolar recoupling pulse sequence is the dipolar recovery at the magic angle (DRAMA) [89] introduced by Tycko et al. This sequence is shown in Fig. 1.23a. There are two 90° pulses symmetrically placed in each rotor period. Subsequently, a number of different pulse sequences have been developed to reintroduce homonuclear dipolar interactions, such as BAck to BAck (BaBa) [90], finite pulse radio field dipolar-driven recoupling (fp-RFDR) [91, 92], homonuclear rotary resonance (HORROR) [75], Post-C7 [93], and others. Under fast MAS conditions, because of different RF pulse requirements, these sequences are not equally well suited for dipolar recoupling. For example, the PostC7 is one of the best efficient recoupling pulse sequences and insensitive to the resonance offset under moderate MAS rate ( 1/2) ones, and they suffer from spectral resolution and sensitivity problems caused by either the anisotropic quadrupolar interactions or small gyromagnetic ratio in the solid-state NMR. In particular, for half-integer quadrupolar nuclei, many of them (e.g., 11 B, 23 Na, 17 O, 27 Al, etc.) play important roles in NMR spectroscopy because of their abundant existence in materials ranging from catalysts, glasses, ceramics, and superconductors to biological systems. Therefore, great efforts have been made to design experiments that can improve the resolution and sensitivity of SSNMR spectra of these nuclei. Here we will discuss the effect of the second-order quadrupolar interaction under MAS condition. According to Eq. (1.14), the second-order quadrupolar frequency correction between energy level m and −m (anti-symmetry transition) may be written as a sum of Legendre polynomials with ranks 0, 2, and 4, taking the following general form ωQ(2) (m ↔ −m) =

ωQ2

Q

A0 (I , m, −m)B0 (ηQ )P0 (cos θ ) ω0 ωQ2 Q + A2 (I , m, −m)B2 (ηQ , αQ , βQ )P2 (cos θ ) ω0 ωQ2 Q A4 (I , m, −m)B4 (ηQ , αQ , βQ )P4 (cos θ ) + ω0

(1.27)

where An (I, m, −m)BQ n (ηQ , α Q , β Q ) coefficients are functions specific of quadrupolar interaction that depend (except for n = 0) on the orientation of the crystallites in the rotor frame; In particular, An (I, m, −m) are zero-, second- and fourth-rank spin coefficients depending on the spin I and magnetic quantum number m : A0 (I , m, −m) = 2m[I (I + 1) − 3m2 ] A2 (I , m, −m) = 2m[8I (I + 1) − 12m2 − 3] A4 (I , m, −m) = 2m[18I (I + 1) − 34m2 − 5]

(1.28)

and Pn ( cos θ ) are the rank n Legendre polynomials: P0 (cos θ ) = 1 1 P2 (cos θ ) = (3 cos2 θ − 1) 2 1 P4 (cos θ ) = (35 cos4 θ − 30 cos2 θ + 3) 8

(1.29)

1.3 Manipulations of Spin Interactions in Solids

35

Fig. 1.28 Schematic diagram of the DOR experiment. In Ref. [102], the outer rotor, inclined by 54.74° with respect to the external field, rotated at about 400 Hz, and had a diameter of 20 mm. The sample in the inner rotor, 5 mm diameter, rotated at about 2 kHz, and the angle between the two axes of rotation was 30.56°

The MAS technique √ consists of rotating a sample around an axis inclined at an angle θm = cos−1 (1/ 3), in such case, P2 ( cos θ m ) = 0 and P4 ( cos θ m ) = 0, thus complete removal of the anisotropic quadrupolar effect only by MAS is impossible, and the second-order quadrupolar broadening remains a major obstacle for high resolution under MAS. Because it is impossible to find a value of θ satisfying the condition P2 ( cos θ ) = P4 ( cos θ ) = 0, other two experimental approaches which can completely remove the anisotropic part of the second-order quadrupolar interaction were introduced: DOR [107], the double-rotation technique, was accomplished by Samoson, Lippmaa, and Pines with a homemade double-rotation probe designed in 1988. The outer rotor is inclined by magic angle θ 1 = 54.74° (the same as MAS) with respect to the external field, whereas the inner rotor is rotated such that the angle between both axes of rotations remains θ 2 = 30.56° and P4 ( cos θ 2 ) = 0 (Fig. 1.28). The sample is spun about two axes simultaneously, and it can average both first- and second-order quadrupolar interactions and obtain high-resolution NMR spectra of quadrupolar nuclei. DAS [108–110], the dynamic-angle spinning technique, designed by Pines et al., is a two-dimensional NMR experiment to achieve isotropic signal in indirect dimension. It is based on the sample spinning about an axis inclined at a time-dependent angle θ (t) with respect to the external field. In the most simple case, θ (t) toggles between the two different fixed angles (e.g., 37.38° and 79.19°) for two equal periods of time. DOR and DAS methods both require elaborate design of the NMR probe, and hence, their wide applications are still limited with technical problems. In 1995, a robust two-dimensional NMR technique known as multiple-quantum magic-angle spinning (MQMAS) was proposed by Frydman and Harword [111]

36

1 Solid-State NMR Principles and Techniques

φ1

P 3 2 1 0 -1 -2 -3

3 3 R( I , − , )t1 2 2

t1

φ2

3 3 R( I , , − )t1 2 2

t2

Fig. 1.29 Pulse sequence (top) and coherence-level diagram for two-pulse 3QMAS NMR. Isotropic echoes form at time t 2 = R(I, m, −m)t 1 , coherence-level P = (−m)−m; the solid line echo pathway corresponds to the correlation of P = −3 and P = −1, and the dashed line corresponds to the correlation of P = 3 and P = −1. Φ 1 and Φ 2 are the phases of the first and second pulses. Specific phase cycling is used to choose the expected coherence

for acquiring high-resolution spectra of half-integer quadrupolar nuclei while the rotational axis of the sample is only inclined by magic angle. Figure 1.29 illustrates the simple schematic diagram of the two-pulse 3QMAS NMR experiment with its coherence pathway. The first RF pulse is applied to generate multiple-quantum coherences (Fig. 1.29; m = 3/2; coherence order is −2m = −3), these coherences evolve for a time t 1 , and then the second RF pulse is used to convert the multiple-quantum coherences to observable the single quantum coherence which evolves for a time t 2 . Fortunately, these anti-symmetry transitions (m ↔ −m) are affected not by the firstorder quadrupolar interaction but only by the second-order quadrupolar interaction (see Eq. 1.27). The first term of Eq. (1.27) is independent of crystallite orientation. Therefore, it does not contribute to the phase dispersion and only induces an isotropic second-order quadrupolar shift, and the second term of Eq. (1.27) vanishes under MAS condition because of the P2 ( cos θ R ) = 0. Thus, only the third term of Eq. (1.27) remains. In MQMAS, the global evolution phase of an anti-symmetry coherence reduces to φ(t) =

ωQ2 ω0

Q

B4 (ηQ , αQ , βQ )P4 (cos θ )[A4 (I , m, −m)t1 + A4 (I , 1/2, −1/2)t2 ] (1.30)

In the two-dimensional MQMAS experiment, in order to obtain an isotropic spectrum, Eq. (1.30) should be equal to zero, which entails: t2 = −

A4 (I , m, −m) t1 = R(I , m, −m)t1 A4 (I , 1/2, −1/2)

(1.31)

1.3 Manipulations of Spin Interactions in Solids

37

Table 1.4 The R(I, m, −m) values for the MQMAS experiment for different spin quantum numbers (I) and different multiple-quantum transitions (m → −m) m = ±3/2

m = ±5/2

m = ±7/2

I = 3/2

±7/9

I = 5/2

∓19/12

±25/12

I = 7/2

∓101/45

∓11/9

±161/45

I = 9/2

∓91/36

∓95/36

∓7/19

m = ±9/2

± 31/6

Fig. 1.30 (a) 2D 27 Al sheared 3QMAS spectrum of Mu-4 observed with B0 = 9.4 T, ν R = 12.5 kHz. (b) Fittings of the slices of 3QMAS spectrum. Reproduced from Ref. [9] by permission of Royal Society of Chemistry

When t 2 fulfills the above condition, the nuclear magnetizations of all crystallites are refocused at the same time (Table 1.4) giving rise to an isotropic echo. After two-dimensional Fourier transform, the projection of the spectrum onto the axis perpendicular to the anisotropic axis with a slope of R(I, m, −m) yields an isotropic, highly resolved spectrum. For example, Fig. 1.30 shows 27 Al (I = 5/2)-sheared 3QMAS spectrum of a layered aluminophosphate, Mu-4 [9]. Four different 27 Al sites can be clearly observed in the 27 Al 2D 3QMAS spectrum, and the projection of the indirect (isotopic) dimension exhibits much better resolution. Compared with DOR and DAS, this method is more convenient to implement. Based on the similar concept, another two-dimensional method denoted STMAS [112–115] was introduced by Gan in 2000, and the main difference between MQMAS and STMAS is that the latter selects satellite transition (ST) instead of symmetrical multiple-quantum transition, evolving during an evolution time t 1 . In STMAS, to remove the first-order quadrupolar broadening of satellite transition, it is necessary to set the magic angle as accurate as possible, while in MQMAS the effect of first-order

38

1 Solid-State NMR Principles and Techniques

quadrupolar coupling was vanished due to the selection of anti-symmetry transitions (m ↔ −m) of half-integer quadrupolar nuclei. On the other hand, much effort has also been devoted to the development of pulse sequences aimed at improving S/N of quadrupolar nuclei in solid-state NMR. For half-integer quadrupolar nuclei, the population difference between the 1/2 and −1/2 energy levels can be increased by inversing or saturating the spin population from the satellite transitions (STs), which can theoretically lead to an enhancement of the central transition (CT) signal by a factor of 2I or I + 1/2. Hence, several pulse sequences were successfully developed to adiabatically manipulate the populations of the satellite transitions (STs) at the beginning of the sequence prior to the excitation of CT transverse magnetization, including “rotor-assisted population transfer” (RAPT) [116], “fast amplitude modulation” (FAM) [117], “double-frequency sweeps” (DFS) [118, 119], “hyperbolic secant pulse” (HS) [120, 121], “wideband, uniform rate, and smooth truncation” (WURST) [122], and their variants [123–126]. The other way to increase sensitivity is by recycling at the end of the signal with a quadrupolar Carr–Purcell–Meiboom–Gill (QCPMG) accumulation [88, 127]. A long train of spin-echoes can be acquired with the QCPMG sequence. After being Fouriertransformed, a spectrum consisting of a series of equally spaced sharp spikelets can be obtained. With a judicious optimization of the experimental parameters, even greater sensitivity enhancement (greater than an order of magnitude) can be achieved over the conventional spin-echo method (Figs. 1.31 and 1.32) [128]. Hyperpolarization Methods Although useful structural and dynamic information can be obtained by elaborately manipulating nuclear spins at atomic level, NMR is still an intrinsically insensitive technique compared with many other spectroscopic methods, such as infrared (IR), Raman, and ultraviolet (UV), since the population difference between “up” and “down” states is only ~1/105 order of magnitude. Thus, breakthrough ideas to create the non-Boltzmann spin population via “hyperpolarization” methods have been developed [128–136] to significantly boost the sensitivity of NMR. Among these methods, dynamic nuclear polarization (DNP) and parahydrogen-induced polarization (PHIP), exhibit renewed vitality in the field of heterogenous catalysis in recent years [137, 138], although the phenomena and their underlying physical or chemical principles have been already found/explored several decades ago. DNP technique [131, 139] was demonstrated to be one of the most revolutionary and promising approaches that could enhance the NMR sensitivity by 2–3 orders of magnitude via polarization transfer from unpaired electrons of a radical polarizing agent to nuclear spins. Theoretically, such hyperpolarization of nuclear spins can potentially provoke NMR signal enhancements by a factor of ε = γ e /γ 1H ≈ 660 or γ e /γ 13C ≈ 2640. The modern DNP instrument, using high-field gyrotron microwave (MW) sources [140, 141], combined with MAS, has opened avenues to many advanced studies of solids under high magnetic fields (up to 21 T). These developments have triggered a fantastic interest of the solid-state NMR community in MAS–DNP experiments, for both biological systems [142–144] and materials

1.3 Manipulations of Spin Interactions in Solids

39

Fig. 1.31 (a) QCPMG, (b) DFS-QCPMG, and (c) RAPT-QCPMG pulse sequences. Reproduced from Ref. [128] by permission of Elsevier

[145–148]. In particular, the applications of DNP-surface-enhanced NMR spectroscopy (DNP-SENS) in solids enable rapid and atom-molecule-level characterizations of the surface of various materials [123, 144, 149–159]. Taking 17 O NMR in solids for example, by using 17 O DNP-NMR, Griffin et al. demonstrated a dramatically enhanced S/N ratio in water-glycerol glass (Fig. 1.33), which makes the heteronuclear correlation experiments possible as more than 6000-fold experiment time could be saved [160, 161]. 17 O DNP-NMR investigations of Mg(OH)2 and Ca(OH)2 compounds without isotopic labeling opened an avenue for 17 O NMR characterization of natural-abundant (0.037%) solid materials [154, 162, 163]. For the surface oxygen species, Pruski et al. investigated the surface hydroxyl groups in mesoporous silica nanoparticles by 1 H → 17 O cross-polarization (CP) DNP-NMR, where hydrogen-bonded and isolated surface silanols could be distinguished [164]. Most recently, the surface-selective direct/indirect 17 O DNP-NMR was demonstrated on isotope-labeled CeO2 [165] and γ-Al2 O3 [166] nanoparticles. The direct 17 O DNPNMR (hyperpolarization from electrons to 17 O nuclei) spectrum could be achieved on CeO2 nanoparticles (Fig. 1.34), where the first three layers can be distinguished

40

1 Solid-State NMR Principles and Techniques

Fig. 1.32 (a) Static 87 Rb Hahn-echo and DFS-echo NMR spectra of RbClO4 along with corresponding QCPMG spectra. The vertical scale of both echo spectra is augmented by a factor of 4. (b) 87 Rb MAS Hahn-echo. RAPT-echo, DFS-echo, QCPMG, RAPT-QCPMG, and DFS-QCPMG NMR spectra of RbClO4 . All spectra were acquired with 128 scans. Integrated intensities with respect to the static and MAS 87 Rb Hahn-echo experiments are indicated on the right of each spectrum. Reproduced from Ref. [128] by permission of Elsevier

1.3 Manipulations of Spin Interactions in Solids

41

Fig. 1.33 Direct polarization of 17 O in 60/30/10 (v/v) d 8 -glycerol/D2 O/H17 2 O using 35% labeled 17 O water and 40 mM electrons, using 9 W of microwave power. Radicals are arranged from highest enhancement to lowest, with trityl (16 scans, ∼2 min), mixture (64 scans, ∼8 min), TOTAPOL (608 scans, ∼60 min), and SA-BDPA (1664 scans and ∼180 min), and the microwaves off ×15 (6646 scans, ∼12 h) spectrum from the trityl sample. Reproduced from Ref. [160] by permission of American Chemical Society

with high selectivity due to the slow spin diffusion of 17 O polarization into the bulk. The complex (sub-)surface oxygen species on surface-selectively labeled γ-Al2 O3 were investigated by 17 O DNP-SENS. Direct 17 O MAS and indirect 1 H–17 O crosspolarization (CP)/MAS DNP experiments allowed to probe the (sub-)surface bare oxygen species and hydroxyl groups (Fig. 1.35). In particular, a two-dimensional (2D) 17 O 3QMAS DNP spectrum was achieved for γ-Al2 O3 (Fig. 1.36), in which two surface O(Al)4 and one O(Al)3 bare oxygen species were identified. Different from DNP achieved via a physical process, PHIP is realized to perturb nuclear spin-state distributions in situ during an ongoing chemical process by using para-H2 as the source of polarization. It was first reported by Bowers and Weitekamp [167, 168] and is now an important hyperpolarization technique [134, 169–171]. The spin polarization of hydrogenated molecules can theoretically lead to signal enhancements by a factor of 104 –105 . Significant NMR signal enhancement provided by PHIP allows one to study reaction kinetics of hydrogenation processes [172, 173] and reaction mechanisms [171, 174], as it provides a unique way to monitor the pairwise addition route and polarization pattern of the intermediates and products during the reaction. Increasing the potential application of the PHIP technique, such as in magnetic resonance imaging (MRI) [175, 176], is substantially driven by the

42

1 Solid-State NMR Principles and Techniques

Fig. 1.34 17 O NMR (14.1 T) spectra of 17 O enriched CeO2 nanoparticles mixed with the TEKPol radical in TCE, with and without microwave irradiation, using a presaturated Hahn-echo experiment. The spectra were recorded at 95 K. The OFF spectrum was recorded at 12.5 kHz MAS, whereas the ON spectrum was recorded at 10 kHz in order to separate the spinning sidebands from the signal arising from the first layer. Spinning sidebands are labeled according to the layer of the signal from which they arise. Reproduced from Ref. [165] by permission of Royal Society of Chemistry

Fig. 1.35 17 O MAS DNP spectra (a, b, and d) and 1 H–17 O CP DNP spectra (c, e) of γ-Al2 O3 (O) labeled by 17 O2 gas at 773 K and γ-Al2 O3 (W ) labeled by H17 2 O at 723 K. MW-on and MW-off denote microwave irradiation on and off, respectively. A total of 600 scans (a), 120 scans (b and d), and 320 scans (c and e) were accumulated. *Denotes spinning sidebands. Reproduced from Ref. [166] by permission of Royal Society of Chemistry

1.3 Manipulations of Spin Interactions in Solids

43

Fig. 1.36 2D 17 O 3QMAS DNP spectrum of γ-Al2 O3 (O) (a) and 17 O MAS DNP spectrum and simulated spectra of different sites (sites 1, 2, and 3) based on the parameters extracted from the 3QMAS spectrum (b). The acquisition time for the 3QMAS DNP spectrum is 1.6 h. The dotted lines denote the simulated spectra. Reproduced from Ref. [166] by permission of Royal Society of Chemistry

introduction of PHIP in heterogeneous reactions (HET-PHIP) using bulk metal or metal oxide [177] and supported metal (like Pt [174], Ir [171], Rh [178], Pd [169, 179], Au-Pd [180], Pt-Sn [181]) nanoparticle catalysts. The HET-PHIP benefits from developing well-defined and robust heterogeneous catalysts for hydrogenation. Koptyug et al. first demonstrated [182] that supported metal catalysts such as Pt/Al2 O3 and Pd/Al2 O3 do exhibit PHIP effects by using either PASADENA (parahydrogen and synthesis allow dramatic enhancement of nuclear alignment) [134, 167, 168] or ALTADENA (adiabatic longitudinal transport after dissociation engenders net alignment) [183] schemes. Significant enhanced 1 H NMR signals of the heterogeneous hydrogenation product (propane) could be observed when hydrogen enriched in the para-H2 spin isomer is used in the reaction, which indicates that the hydrogen addition occurs pairwise to a measurable extent. Very recently, Bowers introduced the SWAMP (surface waters are magnetized by parahydrogen) effect [184] in liquid water as well as methanol and ethanol, where the spin polarization of the solvent protons is enhanced by the bubbling of parahydrogen through a suspension of Pt3 Sn intermetallic nanoparticles (iNPs) encapsulated within a protective mesoporous silica shell (Pt3 Sn@mSiO2 ). Symmetry-breaking interactions on the surface of the iNPs induced the conversion of singlet spin order into magnetization. It enables observations of hyperpolarized NMR signals of the exchangeable hydroxy protons as well as non-exchangeable methyl or methylene protons (Fig. 1.37).

44

1 Solid-State NMR Principles and Techniques

Fig. 1.37 Proton NMR spectra of hyperpolarized and thermally polarized water, methanol, and ethanol. The 9.4 T (400 MHz) liquid-state 1 H NMR spectra acquired about 10 s after 7 bar pH2 (upper spectra) or n-H2 (lower spectra) were bubbled for 20 s at a flow rate of 350 mL/min through a suspension containing 50 mg Pt3 Sn@mSiO2 and (a) water-d2 (D2 O) at 120 °C, (b) methanol-d4 (CD3 OD) at 105 °C, and (c) ethanol-d6 (CD3 CD2 OD) at 105 °C. Proton signals of the non-exchangeable methyl and methylene groups arose only from proton isotopic impurities in the perdeuterated neat liquids. In (c), the asterisk (*) indicates a methanol impurity. Reproduced from Ref. [184] by permission of Elsevier

Simon et al. showed that a metal complex could facilitate the reversible interaction of parahydrogen with a suitable organic substrate, and it can boost 1 H, 13 C, and 15 N NMR signals up to 800-fold on the substrate without its hydrogenation [176, 185]. This means the transfer of polarization could occur without the direct hydrogenation of materials, which is accomplished by temporarily associating a substrate with para-H2 via a transition metal center in low magnetic field. Thus, NMR signal amplification by reversible exchange (NMR-SABRE) is achieved without any chemical modification of the hyperpolarized material. These results suggest a potential route for the use of naturally occurring molecules as probe agents in catalytic processes. The increasing number of applications in the past decade has driven further developments of the methodology of hyperpolarization with promising examples. More detailed descriptions and general concepts of DNP and PHIP methods can be found in numbers of specific monographs or reviews [137–139, 156, 173, 185–189].

1.3 Manipulations of Spin Interactions in Solids

45

1.3.6 Summary After over 70 years of rapid development, nuclear magnetic resonance still glows with vitality. Considerable progress of methodology and instruments exhibits a broad prospect of solid-state NMR spectroscopy. The increasing applications in the past decades have prompted further developments of the methodology in promising fields of scientific research. This chapter summarizes the current state-of-the-art solid-state NMR methods, concentrating on the design of pulse sequences to explore specific nuclear spins at the atomic scale, which have proven themselves important techniques for materials science investigations. It must be emphasized that the combination of the available hardware and improved NMR methods can extend considerably the depth of our research in many fields; however, we still require reasonable knowledge and ingenious experimentation of NMR. Continuous developments aiming to improve the resolution and sensitivity will further facilitate the challenging application to study catalyst and related catalytic reactions at working condition by using in situ NMR techniques.

References 1. Ernst RR, Anderson WA (1966) Application of Fourier transform spectroscopy to magnetic resonance. Rev Sci Instrum 37(1):93–102. https://doi.org/10.1063/1.1719961 2. Muller L, Kumar A, Ernst RR (1975) 2-Dimensional C-13 NMR-spectroscopy/copy. J Chem Phys 63(12):5490–5491 3. Abragam A (1983) The principle of nuclear magnetism. Clarendon Press, Oxford 4. Andrew ER (1956) Nuclear magnetic resonance. Cambridge University Press, Cambridge 5. Duer MJ (2002) Solid state NMR spectroscopy. Blackwell Science, Oxford 6. Mehring M (1980) Principle of high resolution NMR in solid, 2nd edn. Springer, Berlin 7. Spiess HW (1978) Rotation of molecules and nuclear spin relaxation. In: NMR-basic principles and progress. Springer, Berlin, pp 55–214 8. Ulrich AS (2005) Solid state 19F NMR methods for studying biomembranes. Prog Nucl Magn Reson Spectrosc 46(1):1–21. https://doi.org/10.1016/j.pnmrs.2004.11.001 9. Wang Q, Trebosc J, Li Y, Xu J, Hu B, Feng N, Chen Q, Lafon O, Amoureux J-P, Deng F (2013) Signal enhancement of J-HMQC experiments in solid-state NMR involving half-integer quadrupolar nuclei. Chem Commun 49(59):6653–6655. https://doi.org/10.1039/c3cc42961j 10. Gan Z, Gor’kov P, Cross TA, Samoson A, Massiot D (2002) Seeking higher resolution and sensitivity for NMR of quadrupolar nuclei at ultrahigh magnetic fields. J Am Chem Soc 124(20):5634–5635. https://doi.org/10.1021/ja025849p 11. Brown SP, Pérez-Torralba M, Sanz D, Claramunt RM, Emsley L (2002) Determining hydrogen-bond strengths in the solid state by NMR: the quantitative measurement of homonuclear J couplings. Chem Commun 17:1852–1853. https://doi.org/10.1039/b205324a 12. Andrew ER, Bradbury A, Eades RG (1959) Removal of dipolar broadening of nuclear magnetic resonance spectra of solids by specimen rotation. Nature 183(4678):1802–1803. https:// doi.org/10.1038/1831802a0 13. Lowe IJ (1959) Free induction decays of rotating solids. Phys Rev Lett 2(7):285–287. https:// doi.org/10.1103/physrevlett.2.285 14. Wickramasinghe NP, Shaibat M, Ishii Y (2005) Enhanced sensitivity and resolution in 1H solid-state NMR spectroscopy of paramagnetic complexes under very fast magic angle spinning. J Am Chem Soc 127(16):5796–5797. https://doi.org/10.1021/ja042188i

46

1 Solid-State NMR Principles and Techniques

15. Deschamps M (2014) Ultrafast magic angle spinning nuclear magnetic resonance. In: Webb GA (ed) Annual reports on nmr spectroscopy, vol 81. Annual reports on NMR spectroscopy, pp 109–144. https://doi.org/10.1016/b978-0-12-800185-1.00003-6 16. Pines A, Gibby MG, Waugh JS (1973) Proton-enhanced nmr of dilute spins in solids. J Chem Phys 59(2):569–590. https://doi.org/10.1063/1.1680061 17. Muller L, Eckman R, Pines A (1980) Proton-enhanced deuterium nmr in rotating solids. Chem Phys Lett 76(1):149–154. https://doi.org/10.1016/0009-2614(80)80625-7 18. Schaefer J, Stejskal EO (1976) C-13 Nuclear magnetic-resonance of polymers spinning at magic angle. J Am Chem Soc 98(4):1031–1032. https://doi.org/10.1021/ja00420a036 19. Levitt MH, Suter D, Ernst RR (1986) Spin dynamics and thermodynamics in solid-state nmr cross polarization. J Chem Phys 84(8):4243–4255. https://doi.org/10.1063/1.450046 20. Wu XL, Zilm KW (1993) Cross polarization with high-speed magic-angle spinning. J Magn Reson Ser A 104(2):154–165. https://doi.org/10.1006/jmra.1993.1203 21. Peersen OB, Wu XL, Kustanovich I, Smith SO (1993) Variable-amplitude cross-polarization MAS NMR. J Magn Reson Ser A 104(3):334–339. https://doi.org/10.1006/jmra.1993.1231 22. Hediger S, Meier BH, Ernst RR (1993) Cross-polarization under fast magic-angle sample-spinning using amplitude-modulated spin-lock sequences. Chem Phys Lett 213(5–6):627–635. https://doi.org/10.1016/0009-2614(93)89172-e 23. Metz G, Wu XL, Smith SO (1994) Ramped-amplitude cross-polarization in magic-anglespinning nmr. J Magn Reson Ser A 110(2):219–227. https://doi.org/10.1006/jmra.1994.1208 24. Amoureux JP, Trebosc J, Wiench J, Pruski M (2007) HMQC and refocused-INEPT experiments involving half-integer quadrupolar nuclei in solids. J Magn Reson 184(1):1–14. https:// doi.org/10.1016/j.jmr.2006.09.009 25. Kao HM, Grey GP (1998) INEPT experiments involving quadrupolar nuclei in solids. J Magn Reson 133(2):313–323. https://doi.org/10.1006/jmre.1998.1455 26. Wickramasinghe NP, Ishii Y (2006) Sensitivity enhancement, assignment, and distance measurement in C-13 solid-state NMR spectroscopy for paramagnetic systems under fast magic angle spinning. J Magn Reson 181(2):233–243. https://doi.org/10.1016/j.jmr.2006.05.008 27. Lesage A, Steuernagel S, Emsley L (1998) Carbon-13 spectral editing in solid-state NMR using heteronuclear scalar couplings. J Am Chem Soc 120(28):7095–7100. https://doi.org/ 10.1021/ja981019t 28. Bennett AE, Rienstra CM, Auger M, Lakshmi KV, Griffin RG (1995) Heteronuclear decoupling in rotating solids. J Chem Phys 103(16):6951–6958. https://doi.org/10.1063/1.470372 29. Fung BM, Khitrin AK, Ermolaev K (2000) An improved broadband decoupling sequence for liquid crystals and solids. J Magn Reson 142(1):97–101. https://doi.org/10.1006/jmre.1999. 1896 30. Detken A, Hardy EH, Ernst M, Meier BH (2002) Simple and efficient decoupling in magicangle spinning solid-state NMR: the XiX scheme. Chem Phys Lett 356(3–4):298–304. https:// doi.org/10.1016/s0009-2614(02)00335-4 31. Weingarth M, Tekely P, Bodenhausen G (2008) Efficient heteronuclear decoupling by quenching rotary resonance in solid-state NMR. Chem Phys Lett 466(4–6):247–251. https://doi.org/ 10.1016/j.cplett.2008.10.041 32. Weingarth M, Bodenhausen G, Tekely P (2009) Low-power decoupling at high spinning frequencies in high static fields. J Magn Reson 199(2):238–241. https://doi.org/10.1016/j. jmr.2009.04.015 33. Orr RM, Duer MJ (2006) Decoupling residual dipolar coupling between C-13 and N-14 spin pairs in CPMAS NMR. Solid State Nucl Magn Reson 30(3–4):130–134. https://doi.org/10. 1016/j.ssnmr.2006.06.002 34. Delevoye L, Trebosc J, Gan Z, Montagne L, Amoureux JP (2007) Resolution enhancement using a new multiple-pulse decoupling sequence for quadrupolar nuclei. J Magn Reson 186(1):94–99. https://doi.org/10.1016/j.jmr.2007.01.018 35. Goldburg WI, Lee M (1963) Nuclear magnetic resonance line narrowing by a rotating rf field. Phys Rev Lett 11(6):255–000. https://doi.org/10.1103/physrevlett.11.255

References

47

36. Lee M, Goldburg WI (1965) Nuclear-magnetic-resonance line narrowing by a rotating rf field. Phys Rev 140(4A):1261–000. https://doi.org/10.1103/physrev.140.a1261 37. Waugh JS, Huber LM, Haeberlen U (1968) Approach to high-resolution nmr in solids. Phys Rev Lett 20 (5):180. https://doi.org/10.1103/physrevlett.20.180 38. Mansfield P (1971) Symmetrized pulse sequences in high resolution nmr in solids. J Phys Part C Solid State Phys 4(11):1444. https://doi.org/10.1088/0022-3719/4/11/020 39. Rhim WK, Elleman DD, Vaughan RW (1973) Analysis of multiple pulse nmr in solids. J Chem Phys 59(7):3740–3749. https://doi.org/10.1063/1.1680545 40. Burum DP, Rhim WK (1979) Analysis of multiple pulse nmr in solids. J Chem Phys 71(2):944–956. https://doi.org/10.1063/1.438385 41. vanRossum BJ, Forster H, deGroot HJM (1997) High-field and high-speed CP-MAS C-13 NMR heteronuclear dipolar-correlation spectroscopy of solids with frequency-switched LeeGoldburg homonuclear decoupling. J Magn Reson 124(2):516–519. https://doi.org/10.1006/ jmre.1996.1089 42. Lesage A, Sakellariou D, Steuernagel S, Emsley L (1998) Carbon-proton chemical shift correlation in solid-state NMR by through-bond multiple-quantum spectroscopy. J Am Chem Soc 120(50):13194–13201. https://doi.org/10.1021/ja983048+ 43. Lesage A, Duma L, Sakellariou D, Emsley L (2001) Improved resolution in proton NMR spectroscopy of powdered solids. J Am Chem Soc 123(24):5747–5752. https://doi.org/10. 1021/ja0039740 44. Vinogradov E, Madhu PK, Vega S (1999) High-resolution proton solid-state NMR spectroscopy by phase-modulated Lee-Goldburg experiment. Chem Phys Lett 314(5–6):443–450. https://doi.org/10.1016/s0009-2614(99)01174-4 45. Vinogradov E, Madhu PK, Vega S (2001) Phase modulated Lee-Goldburg magic angle spinning proton nuclear magnetic resonance experiments in the solid state: a bimodal Floquet theoretical treatment. J Chem Phys 115(19):8983–9000. https://doi.org/10.1063/1.1408287 46. Sakellariou D, Lesage A, Hodgkinson P, Emsley L (2000) Homonuclear dipolar decoupling in solid-state NMR using continuous phase modulation. Chem Phys Lett 319(3–4):253–260. https://doi.org/10.1016/s0009-2614(00)00127-5 47. Lesage A, Sakellariou D, Hediger S, Elena B, Charmont P, Steuernagel S, Emsley L (2003) Experimental aspects of proton NMR spectroscopy in solids using phase-modulated homonuclear dipolar decoupling. J Magn Reson 163(1):105–113. https://doi.org/10.1016/s10907807(03)00104-6 48. Leskes M, Madhu PK, Vega S (2009) Why does PMLG proton decoupling work at 65 kHz MAS? J Magn Reson 199(2):208–213. https://doi.org/10.1016/j.jmr.2009.05.003 49. Salager E, Stein RS, Steuernagel S, Lesage A, Elena B, Emsley L (2009) Enhanced sensitivity in high-resolution H-1 solid-state NMR spectroscopy with DUMBO dipolar decoupling under ultra-fast MAS. Chem Phys Lett 469(4–6):336–341. https://doi.org/10.1016/j.cplett.2008.12. 073 50. Amoureux JP, Hu B, Trebosc J (2008) Enhanced resolution in proton solid-state NMR with very-fast MAS experiments. J Magn Reson 193(2):305–307. https://doi.org/10.1016/j.jmr. 2008.05.002 51. van Rossum BJ, de Groot CP, Ladizhansky V, Vega S, de Groot HJM (2000) A method for measuring heteronuclear (H-1-C-13) distances in high speed MAS NMR. J Am Chem Soc 122(14):3465–3472 52. Gullion T, Schaefer J (1989) Rotational-echo double-resonance NMR. J Magn Reson 81(1):196–200. https://doi.org/10.1016/0022-2364(89)90280-1 53. Terry G (1998) Introduction to rotational-echo, double-resonance NMR. Concepts Magn Reson 10(5):277–289. https://doi.org/10.1002/(sici)1099-0534(1998)10:5%3c277:aidcmr1%3e3.0.co;2-u 54. Mueller KT (1995) Analytic solutions for the time evolution of dipolar-dephasing NMR signals. J Magn Reson Ser A 113(1):81–93. https://doi.org/10.1006/jmra.1995.1059 55. Bak M, Rasmussen JT, Nielsen NC (2000) SIMPSON: a general simulation program for solid-state NMR spectroscopy. J Magn Reson 147(2):296–330. https://doi.org/10.1006/jmre. 2000.2179

48

1 Solid-State NMR Principles and Techniques

56. Levitt MH, Oas TG, Griffin RG (1988) Rotary resonance recoupling in heteronuclear spin pair systems. Isr J Chem 28(4):271–282 57. Oas TG, Griffin RG, Levitt MH (1988) Rotary resonance recoupling of dipolar interactions in solid-state nuclear magnetic-resonance spectroscopy. J Chem Phys 89(2):692–695. https:// doi.org/10.1063/1.455191 58. Fu RQ, Smith SA, Bodenhausen G (1997) Recoupling of heteronuclear dipolar interactions in solid state magic-angle spinning NMR by simultaneous frequency and amplitude modulation. Chem Phys Lett 272(5–6):361–369. https://doi.org/10.1016/s0009-2614(97)00537-x 59. Lafon O, Wang Q, Hu B, Vasconcelos F, Trébosc J, Cristol S, Deng F, Amoureux J-P (2009) Indirect detection via spin-1/2 nuclei in solid state NMR spectroscopy: application to the observation of proximities between protons and quadrupolar nuclei. J Phys Chem A 113(46):12864–12878. https://doi.org/10.1021/jp906099k 60. Vaneck ERH, Janssen R, Maas W, Veeman WS (1990) A novel application of nuclear spin-echo double-resonance to aluminophosphates and aluminosilicates. Chem Phys Lett 174(5):428–432. https://doi.org/10.1016/s0009-2614(90)87174-p 61. Grey CP, Veeman WS (1992) The detection of weak heteronuclear coupling between spin-1 and spin-1/2 nuclei in MAS NMR - N-14/C-13/H-1 triple resonance experiments. Chem Phys Lett 192(4):379–385. https://doi.org/10.1016/0009-2614(92)85486-t 62. Grey CP, Vega AJ (1995) Determination of the quadrupole coupling-constant of the invisible aluminum spins in zeolite HY with H-1/AL-27 TRAPDOR NMR. J Am Chem Soc 117(31):8232–8242. https://doi.org/10.1021/ja00136a022 63. Deng F, Yue Y, Ye C (1998) 1H/27Al TRAPDOR NMR studies on aluminum species in dealuminated zeolites. Solid State Nucl Magn Reson 10(3):151–160. https://doi.org/10.1016/ S0926-2040(97)00028-3 64. Gullion T, Vega AJ (2005) Measuring heteronuclear dipolar couplings for I = 1/2, S > 1/2 spin pairs by REDOR and REAPDOR NMR. Prog Nucl Magn Reson Spectrosc 47(3–4):123–136. https://doi.org/10.1016/j.pnmrs.2005.08.004 65. Kalwei M, Koller H (2002) Quantitative comparison of REAPDOR and TRAPDOR experiments by numerical simulations and determination of H-Al distances in zeolites. Solid State Nucl Magn Reson 21(3–4):145–157. https://doi.org/10.1006/snmr.2002.0055 66. Hung I, Uldry A-C, Becker-Baldus J, Webber AL, Wong A, Smith ME, Joyce SA, Yates JR, Pickard CJ, Dupree R, Brown SP (2009) Probing heteronuclear 15 N–17O and 13C–17O connectivities and proximities by solid-state NMR spectroscopy. J Am Chem Soc 131(5):1820–1834. https://doi.org/10.1021/ja805898d 67. Gan Z (2006) Measuring multiple carbon-nitrogen distances in natural abundant solids using R-RESPDOR NMR. Chem Commun 45:4712–4714. https://doi.org/10.1039/b611447d 68. Chen L, Lu X, Wang Q, Lafon O, Trebosc J, Deng F, Amoureux J-P (2010) Distance measurement between a spin-1/2 and a half-integer quadrupolar nuclei by solid-state NMR using exact analytical expressions. J Magn Reson 206(2):269–273. https://doi.org/10.1016/j.jmr. 2010.07.009 69. Chen L, Wang Q, Hu B, Lafon O, Trebosc J, Deng F, Amoureux J-P (2010) Measurement of hetero-nuclear distances using a symmetry-based pulse sequence in solid-state NMR. Phys Chem Chem Phys 12(32):9395–9405. https://doi.org/10.1039/b926546e 70. Grey CP, Vega AJ (1995) Determination of the quadrupole coupling constant of the invisible aluminum spins in zeolite HY with 1H/27Al TRAPDOR NMR. J Am Chem Soc 117(31):8232–8242. https://doi.org/10.1021/ja00136a022 71. Vega AJ (1992) MAS NMR spin locking of half-integer quadrupolar nuclei. Journal of Magnetic Resonance (1969) 96(1):50–68. doi:https://doi.org/10.1016/0022-2364(92)90287-H 72. Gullion T, Vega AJ (2005) Measuring heteronuclear dipolar couplings for I = 1/2, S > 1/2 spin pairs by REDOR and REAPDOR NMR. Prog Nucl Magn Reson Spectrosc 47(3):123–136. https://doi.org/10.1016/j.pnmrs.2005.08.004 73. Goetz JM, Schaefer J (1997) REDOR dephasing by multiple spins in the presence of molecular motion. J Magn Reson 127(2):147–154. https://doi.org/10.1006/jmre.1997.1198

References

49

74. Jaroniec CP, Tounge BA, Rienstra CM, Herzfeld J, Griffin RG (2000) Recoupling of heteronuclear dipolar interactions with rotational-echo double-resonance at high magic-angle spinning frequencies. J Magn Reson 146(1):132–139. https://doi.org/10.1006/jmre.2000.2128 75. Nielsen NC, Bildsoe H, Jakobsen HJ, Levitt MH (1994) Double-quantum homonuclear rotary resonance—efficient dipolar recovery in magic-angle-spinning nuclear-magnetic-resonance. J Chem Phys 101(3):1805–1812. https://doi.org/10.1063/1.467759 76. Hu B, Trébosc J, Amoureux JP (2008) Comparison of several hetero-nuclear dipolar recoupling NMR methods to be used in MAS HMQC/HSQC. J Magn Reson 192(1):112–122. https://doi.org/10.1016/j.jmr.2008.02.004 77. Brinkmann A, Kentgens APM (2006) Proton-selective 17O–H distance measurements in fast magic-angle-spinning solid-state NMR spectroscopy for the determination of hydrogen bond lengths. J Am Chem Soc 128(46):14758–14759. https://doi.org/10.1021/ja065415k 78. Nimerovsky E, Goldbourt A (2010) Efficient rotational echo double resonance recoupling of a spin-1/2 and a quadrupolar spin at high spinning rates and weak irradiation fields. J Magn Reson 206(1):52–58. https://doi.org/10.1016/j.jmr.2010.05.019 79. Levitt MH (2008) Symmetry in the design of NMR multiple-pulse sequences. J Chem Phys 128(5). https://doi.org/10.1063/1.2831927 80. Levitt MH (2002) Symmetry-based pulse sequences in magic-angle spinning solid-state NMR. In: Grant DM, Harris RK (ed) Encyclopedia of nuclear magnetic resonance. Wiley, Chichester 81. Brinkmann A, Levitt MH (2001) Symmetry principles in the nuclear magnetic resonance of spinning solids: Heteronuclear recoupling by generalized Hartmann-Hahn sequences. J Chem Phys 115(1):357–384. https://doi.org/10.1063/1.1377031 82. Brinkmann A, Kentgens APM (2006) Sensitivity enhancement and heteronuclear distance measurements in biological O-17 solid-state NMR. J Phys Chem B 110(32):16089–16101. https://doi.org/10.1021/jp062809p 83. Zhao X, Eden M, Levitt MH (2001) Recoupling of heteronuclear dipolar interactions in solid-state NMR using symmetry-based pulse sequences. Chem Phys Lett 342(3–4):353–361. https://doi.org/10.1016/s0009-2614(01)00593-0 84. Ashbrook SE, Wimperis S (2009) Spin-locking of half-integer quadrupolar nuclei in nuclear magnetic resonance of solids: Second-order quadrupolar and resonance offset effects. J Chem Phys 131(19):194509. https://doi.org/10.1063/1.3263904 85. Etienne M, Julien T, Anne B, Olivier B, Jérémie P, Jean-Marie B, J. VM, P. NC, M. GR, Mostafa T, Laurent D (2010) Heteronuclear NMR correlations to probe the local structure of catalytically active surface aluminum hydride species on γ-alumina. Angewandte Chemie International Edition 49(51):9854–9858. doi:https://doi.org/10.1002/anie.201004310 86. Merle N, Trébosc J, Baudouin A, Rosal ID, Maron L, Szeto K, Genelot M, Mortreux A, Taoufik M, Delevoye L, Gauvin RM (2012) 17O NMR gives unprecedented insights into the structure of supported catalysts and their interaction with the silica carrier. J Am Chem Soc 134(22):9263–9275. https://doi.org/10.1021/ja301085m 87. Wang Q, Li Y, Trébosc J, Lafon O, Xu J, Hu B, Feng N, Chen Q, Amoureux J-P, Deng F (2015) Population transfer HMQC for half-integer quadrupolar nuclei. J Chem Phys 142(9):094201. https://doi.org/10.1063/1.4913683 88. Larsen FH, Jakobsen HJ, Ellis PD, Nielsen NC (1997) Sensitivity-enhanced quadrupolar-echo NMR of half-integer quadrupolar nuclei. Magnitudes and relative orientation of chemical shielding and quadrupolar coupling tensors. J Phys Chem A 101(46):8597–8606. https://doi. org/10.1021/jp971547b 89. Tycko R, Dabbagh G (1991) Double-quantum filtering in magic-angle-spinning nmrspectroscopy—an approach to spectral simplification and molecular-structure determination. J Am Chem Soc 113(25):9444–9448. https://doi.org/10.1021/ja00025a003 90. Feike M, Demco DE, Graf R, Gottwald J, Hafner S, Spiess HW (1996) Broadband multiplequantum NMR spectroscopy. J Magn Reson Ser A 122(2):214–221. https://doi.org/10.1006/ jmra.1996.0197 91. Bayro MJ, Ramachandran R, Caporini MA, Eddy MT, Griffin RG (2008) Radio frequencydriven recoupling at high magic-angle spinning frequencies: Homonuclear recoupling sans heteronuclear decoupling. J Chem Phys 128(5). https://doi.org/10.1063/1.2834736

50

1 Solid-State NMR Principles and Techniques

92. Bennett AE, Rienstra CM, Griffiths JM, Zhen WG, Lansbury PT, Griffin RG (1998) Homonuclear radio frequency-driven recoupling in rotating solids. J Chem Phys 108(22):9463–9479. https://doi.org/10.1063/1.476420 93. Hohwy M, Rienstra CM, Jaroniec CP, Griffin RG (1999) Fivefold symmetric homonuclear dipolar recoupling in rotating solids: Application to double quantum spectroscopy. J Chem Phys 110(16):7983–7992. https://doi.org/10.1063/1.478702 94. Gullion T, Baker DB, Conradi MS (1990) New, compensated Carr-Purcell sequences. J Magn Reson 89(3):479–484. https://doi.org/10.1016/0022-2364(90)90331-3 95. Brinkmann A, Eden M, Levitt MH (2000) Synchronous helical pulse sequences in magicangle spinning nuclear magnetic resonance: Double quantum recoupling of multiple-spin systems. J Chem Phys 112(19):8539–8554. https://doi.org/10.1063/1.481458 96. Rienstra CM, Hatcher ME, Mueller LJ, Sun BQ, Fesik SW, Griffin RG (1998) Efficient multispin homonuclear double-quantum recoupling for magic-angle spinning NMR: C-13-C-13 correlation spectroscopy of U-C-13-erythromycin A. J Am Chem Soc 120(41):10602–10612. https://doi.org/10.1021/ja9810181 97. Geen H, Titman JJ, Gottwald J, Spiess HW (1994) Solid-state proton multiple-quantum nmrspectroscopy with fast magic-angle-spinning. Chem Phys Lett 227(1–2):79–86. https://doi. org/10.1016/0009-2614(94)00789-6 98. Carravetta M, Edén M, Johannessen OG, Luthman H, Verdegem PJE, Lugtenburg J, Sebald A, Levitt MH (2001) Estimation of carbon–carbon bond lengths and medium-range internuclear distances by solid-state nuclear magnetic resonance. J Am Chem Soc 123(43):10628–10638. https://doi.org/10.1021/ja016027f 99. Vega S (1978) Fictitious spin 1/2 operator formalism for multiple quantum NMR. J Chem Phys 68(12):5518–5527. https://doi.org/10.1063/1.435679 100. Baldus M, Rovnyak D, Griffin RG (2000) Radio-frequency-mediated dipolar recoupling among half-integer quadrupolar spins. J Chem Phys 112(13):5902–5909. https://doi.org/10. 1063/1.481187 101. Wi S, Logan JW, Sakellariou D, Walls JD, Pines A (2002) Rotary resonance recoupling for half-integer quadrupolar nuclei in solid-state nuclear magnetic resonance spectroscopy. J Chem Phys 117(15):7024–7033. https://doi.org/10.1063/1.1506907 102. Painter AJ, Duer MJ (2002) Double-quantum-filtered nuclear magnetic resonance spectroscopy applied to quadrupolar nuclei in solids. J Chem Phys 116(2):710–722. https://doi. org/10.1063/1.1425831 103. Mali G, Fink G, Taulelle F (2004) Double-quantum homonuclear correlation magic angle sample spinning nuclear magnetic resonance spectroscopy of dipolar-coupled quadrupolar nuclei. J Chem Phys 120(6):2835–2845. https://doi.org/10.1063/1.1638741 104. Edén M, Annersten H, Zazzi Å (2005) Pulse-assisted homonuclear dipolar recoupling of half-integer quadrupolar spins in magic-angle spinning NMR. Chem Phys Lett 410(1):24–30. https://doi.org/10.1016/j.cplett.2005.04.030 105. Edén M, Zhou D, Yu J (2006) Improved double-quantum NMR correlation spectroscopy of dipolar-coupled quadrupolar spins. Chem Phys Lett 431(4):397–403. https://doi.org/10.1016/ j.cplett.2006.09.081 106. Wang Q, Hu B, Lafon O, Trébosc J, Deng F, Amoureux JP (2009) Double-quantum homonuclear NMR correlation spectroscopy of quadrupolar nuclei subjected to magic-angle spinning and high magnetic field. J Magn Reson 200(2):251–260. https://doi.org/10.1016/j.jmr.2009. 07.009 107. Samoson A, Lippmaa E, Pines A (1988) High-resolution solid-state nmr averaging of 2ndorder effects by means of a double-rotor. Mol Phys 65(4):1013–1018. https://doi.org/10.1080/ 00268978800101571 108. Mueller KT, Sun BQ, Chingas GC, Zwanziger JW, Terao T, Pines A (1990) Dynamic-angle spinning of quadrupolar nuclei. J Magn Reson 86(3):470–487. https://doi.org/10.1016/00222364(90)90025-5 109. Grandinetti PJ, Lee YK, Baltisberger JH, Sun BQ, Pines A (1993) Side-band patterns in dynamic-angle-spinning nmr. J Magn Reson Ser A 102(2):195–204. https://doi.org/10.1006/ jmra.1993.1091

References

51

110. Llor A, Virlet J (1988) Towards high-resolution nmr of more nuclei in solids—sample spinning with time-dependent spinner axis angle. Chem Phys Lett 152(2–3):248–253. https://doi.org/ 10.1016/0009-2614(88)87362-7 111. Frydman L, Harwood JS (1995) Isotropic spectra of half-integer quadrupolar spins from bidimensional magic-angle-spinning nmr. J Am Chem Soc 117(19):5367–5368. https://doi. org/10.1021/ja00124a023 112. Ashbrook SE, Wimperis S (2002) High-resolution NMR spectroscopy of quadrupolar nuclei in solids: Satellite-transition MAS with self-compensation for magic-angle misset. J Am Chem Soc 124(39):11602–11603. https://doi.org/10.1021/ja0203869 113. Ashbrook SE, Wimperis S (2004) High-resolution NMR of quadrupolar nuclei in solids: the satellite-transition magic angle spinning (STMAS) experiment. Prog Nucl Magn Reson Spectrosc 45(1–2):53–108. https://doi.org/10.1016/j.pnmrs.2004.04.002 114. Gan ZH (2000) Isotropic NMR spectra of half-integer quadrupolar nuclei using satellite transitions and magic-angle spinning. J Am Chem Soc 122(13):3242–3243. https://doi.org/ 10.1021/ja9939791 115. Gan ZH (2001) Satellite transition magic-angle spinning nuclear magnetic resonance spectroscopy of half-integer quadrupolar nuclei. J Chem Phys 114(24):10845–10853. https://doi. org/10.1063/1.1374958 116. Yao Z, Kwak HT, Sakellariou D, Emsley L, Grandinetti PJ (2000) Sensitivity enhancement of the central transition NMR signal of quadrupolar nuclei under magic-angle spinning. Chem Phys Lett 327(1–2):85–90. https://doi.org/10.1016/s0009-2614(00)00805-8 117. Madhu PK, Goldbourt A, Frydman L, Vega S (2000) Fast radio-frequency amplitude modulation in multiple-quantum magic-angle-spinning nuclear magnetic resonance: Theory and experiments. J Chem Phys 112(5):2377–2391. https://doi.org/10.1063/1.480804 118. Iuga D, Schafer H, Verhagen R, Kentgens APM (2000) Population and coherence transfer induced by double frequency sweeps in half-integer quadrupolar spin systems. J Magn Reson 147(2):192–209. https://doi.org/10.1006/jmre.2000.2192 119. Kentgens APM, Verhagen R (1999) Advantages of double frequency sweeps in static, MAS and MQMAS NMR of spin I = 3/2 nuclei. Chem Phys Lett 300(3–4):435–443. https://doi. org/10.1016/s0009-2614(98)01402-x 120. Siegel R, Nakashima TT, Wasylishen RE (2007) Sensitivity enhancement of NMR spectra of half-integer spin quadrupolar nuclei in solids using hyperbolic secant pulses. J Magn Reson 184(1):85–100. https://doi.org/10.1016/j.jmr.2006.09.007 121. Siegel R, Nakashima TT, Wasylishen RE (2004) Signal enhancement of NMR spectra of half-integer quadrupolar nuclei in solids using hyperbolic secant pulses. Chem Phys Lett 388(4–6):441–445. https://doi.org/10.1016/j.cplett.2004.03.047 122. Dey KK, Prasad S, Ash JT, Deschamps M, Grandinetti PJ (2007) Spectral editing in, solidstate MAS NMR of quadrupolar nuclei using selective satellite inversion. J Magn Reson 185(2):326–330. https://doi.org/10.1016/j.jmr.2006.12.013 123. Vitzthum V, Miéville P, Carnevale D, Caporini MA, Gajan D, Copéret C, Lelli M, Zagdoun A, Rossini AJ, Lesage A (2012) Dynamic nuclear polarization of quadrupolar nuclei using cross polarization from protons: surface-enhanced aluminium-27 NMR. Chem Commun 48(14):1988–1990 124. Brauniger T (2012) Enhancing the central-transition NMR signal of quadrupolar nuclei by spin population transfer using SW-FAM pulse trains with a tangent-shaped sweep profile. Solid State Nucl Magn Reson 45–46:16–22. https://doi.org/10.1016/j.ssnmr.2012.04.002 125. Brauniger T, Hempel G, Madhu PK (2006) Fast amplitude-modulated pulse trains with frequency sweep (SW-FAM) in static NMR of half-integer spin quadrupolar nuclei. J Magn Reson 181(1):68–78. https://doi.org/10.1016/j.jmr.2006.03.016 126. Wang Q, Trébosc J, Li Y, Lafon O, Xin S, Xu J, Hu B, Feng N, Amoureux J-P, Deng F (2018) Uniform signal enhancement in MAS NMR of half-integer quadrupolar nuclei using quadruple-frequency sweeps. J Magn Reson 293:92–103. https://doi.org/10.1016/j.jmr.2018. 06.005

52

1 Solid-State NMR Principles and Techniques

127. Larsen FH, Nielsen NC (1999) Effects of finite rf pulses and sample spinning speed in multiplequantum magic-angle spinning (MQ-MAS) and multiple-quantum quadrupolar Carr-PurcellMeiboom-Gill magic-angle spinning (MQ-QCPMG-MAS) nuclear magnetic resonance of half-integer quadrupolar nuclei. J Phys Chem A 103(50):10825–10832. https://doi.org/10. 1021/jp992798i 128. Schurko RW, Hung I, Widdifield CM (2003) Signal enhancement in NMR spectra of halfinteger quadrupolar nuclei via DFS–QCPMG and RAPT–QCPMG pulse sequences. Chem Phys Lett 379(1):1–10. https://doi.org/10.1016/S0009-2614(03)01345-9 129. Ward HR, Lawler RG (1967) Nuclear magnetic resonance emission and enhanced absorption in rapid organometallic reactions. J Am Chem Soc 89(21):5518–5519. https://doi.org/10. 1021/ja00997a078 130. Closs GL, Closs LE (1969) Induced dynamic nuclear spin polarization in reactions of photochemically and thermally generated triplet diphenylmethylene. J Am Chem Soc 91(16):4549–4550. https://doi.org/10.1021/ja01044a041 131. Overhauser AW (1953) Polarization of nuclei in metals. Phys Rev 92(2):411–415. https://doi. org/10.1103/physrev.92.411 132. Bouchiat MA, Carver TR, Varnum CM (1960) Nuclear polarization in He3 gas induced by optical pumping and dipolar exchange. Phys Rev Lett 5(8):373–375. https://doi.org/10.1103/ physrevlett.5.373 133. Lampel G (1968) Nuclear dynamic polarization by optical electronic saturation and optical pumping in semiconductors. Phys Rev Lett 20(10):491–493. https://doi.org/10.1103/ PhysRevLett.20.491 134. Eisenschmid TC, Kirss RU, Deutsch PP, Hommeltoft SI, Eisenberg R, Bargon J, Lawler RG, Balch AL (1987) Para hydrogen induced polarization in hydrogenation reactions. J Am Chem Soc 109(26):8089–8091. https://doi.org/10.1021/ja00260a026 135. Bifone A, Song Y-Q, Seydoux R, Taylor RE, Goodson BM, Pietrass T, Budinger TF, Navon G, Pines A (1996) NMR of laser-polarized xenon in human blood. Proc Natl Acad Sci 93(23):12932–12936. https://doi.org/10.1073/pnas.93.23.12932 136. Daviso E, Janssen GJ, Alia A, Jeschke G, Matysik J, Tessari M (2011) A 10 000-fold Nuclear Hyperpolarization of a Membrane Protein in the Liquid Phase via a Solid-State Mechanism. J Am Chem Soc 133(42):16754–16757. https://doi.org/10.1021/ja206689t 137. Akbey Ü, Franks WT, Linden A, Orwick-Rydmark M, Lange S, Oschkinat H (2013) Dynamic nuclear polarization enhanced NMR in the solid-state. In: Kuhn LT (ed) Hyperpolarization methods in NMR spectroscopy. Springer, Berlin pp 181–228. https://doi.org/10.1007/128_ 2013_436 138. Kovtunov KV, Zhivonitko VV, Skovpin IV, Barskiy DA, Koptyug IV (2013) Parahydrogeninduced polarization in heterogeneous catalytic processes. In: Kuhn LT (ed) Hyperpolarization methods in NMR spectroscopy. Springer, Berlin, pp 123–180. https://doi.org/10.1007/128_ 2012_371 139. Abragam A, Goldman M (1978) Principles of dynamic nuclear polarisation. Rep Prog Phys 41(3):395 140. Rosay M, Tometich L, Pawsey S, Bader R, Schauwecker R, Blank M, Borchard PM, Cauffman SR, Felch KL, Weber RT, Temkin RJ, Griffin RG, Maas WE (2010) Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results. Phys Chem Chem Phys 12(22):5850–5860. https://doi.org/10.1039/C003685B 141. Mathies G, Caporini MA, Michaelis VK, Liu Y, Hu KN, Mance D, Zweier JL, Rosay M, Baldus M, Griffin RG (2015) Efficient dynamic nuclear polarization at 800 MHz/527 GHz with Trityl-Nitroxide Biradicals. Angewandte Chemie International Edition 54(40):11770–11774. https://doi.org/10.1002/anie.201504292 142. Hu K-N, Song C, Yu H-H, Swager TM, Griffin RG (2008) High-frequency dynamic nuclear polarization using biradicals: A multifrequency EPR lineshape analysis. J Chem Phys 128(5). https://doi.org/10.1063/1.2816783

References

53

143. Bayro MJ, Debelouchina GT, Eddy MT, Birkett NR, MacPhee CE, Rosay M, Maas WE, Dobson CM, Griffin RG (2011) Intermolecular structure determination of amyloid fibrils with magic-angle spinning and dynamic nuclear polarization NMR. J Am Chem Soc 133(35):13967–13974. https://doi.org/10.1021/ja203756x 144. Debelouchina GT, Bayro MJ, Fitzpatrick AW, Ladizhansky V, Colvin MT, Caporini MA, Jaroniec CP, Bajaj VS, Rosay M, MacPhee CE, Vendruscolo M, Maas WE, Dobson CM, Griffin RG (2013) Higher order amyloid fibril structure by MAS NMR and DNP spectroscopy. J Am Chem Soc 135(51):19237–19247. https://doi.org/10.1021/ja409050a 145. Lee D, Takahashi H, Thankamony ASL, Dacquin J-P, Bardet M, Lafon O, Paëpe GD (2012) Enhanced solid-state NMR correlation spectroscopy of quadrupolar nuclei using dynamic nuclear polarization. J Am Chem Soc 134(45):18491–18494. https://doi.org/10. 1021/ja307755t 146. Lafon O, Thankamony ASL, Kobayashi T, Carnevale D, Vitzthum V, Slowing II, Kandel K, Vezin H, Amoureux J-P, Bodenhausen G, Pruski M (2013) Mesoporous silica nanoparticles loaded with surfactant: low temperature magic angle spinning 13C and 29Si NMR enhanced by dynamic nuclear polarization. J Phys Chem C 117(3):1375–1382. https://doi.org/10.1021/ jp310109s 147. Rossini AJ, Zagdoun A, Lelli M, Lesage A, Copéret C, Emsley L (2013) Dynamic nuclear polarization surface enhanced NMR spectroscopy. Acc Chem Res 46(9):1942–1951. https:// doi.org/10.1021/ar300322x 148. Blanc F, Sperrin L, Lee D, Dervisoglu R, Yamazaki Y, Haile SM, De Paepe G, Grey CP (2014) Dynamic nuclear polarization NMR of low-gamma nuclei: structural insights into hydrated yttrium-doped BaZrO3 . J Phys Chem Lett 5(14):2431–2436. https://doi.org/10.1021/ jz5007669 149. Lesage A, Lelli M, Gajan D, Caporini MA, Vitzthum V, Miéville P, Alauzun J, Roussey A, Thieuleux C, Mehdi A (2010) Surface enhanced NMR spectroscopy by dynamic nuclear polarization. J Am Chem Soc 132(44):15459–15461 150. Lafon O, Rosay M, Aussenac F, Lu X, Trébosc J, Cristini O, Kinowski C, Touati N, Vezin H, Amoureux JP (2011) Beyond the silica surface by direct silicon-29 dynamic nuclear polarization. Angew Chem Int Ed 50(36):8367–8370 151. Zagdoun A, Rossini AJ, Gajan D, Bourdolle A, Ouari O, Rosay M, Maas WE, Tordo P, Lelli M, Emsley L (2012) Non-aqueous solvents for DNP surface enhanced NMR spectroscopy. Chem Commun 48(5):654–656 152. Piveteau L, Ong T-C, Rossini AJ, Emsley L, Copéret C, Kovalenko MV (2015) Structure of colloidal quantum dots from dynamic nuclear polarization surface enhanced nmr spectroscopy. J Am Chem Soc 137(43):13964–13971 153. Rossini AJ, Zagdoun A, Lelli M, Gajan D, Rascón F, Rosay M, Maas WE, Copéret C, Lesage A, Emsley L (2012) One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition. Chem Sci 3(1):108–115 154. Perras FA, Kobayashi T, Pruski M (2015) Natural abundance 17O DNP two-dimensional and surface-enhanced NMR spectroscopy. J Am Chem Soc 137(26):8336–8339 155. Kobayashi T, Perras FA, Slowing II, Sadow AD, Pruski M (2015) Dynamic nuclear polarization solid-state NMR in heterogeneous catalysis research. ACS Catal 5(12):7055–7062. https://doi.org/10.1021/acscatal.5b02039 156. Rossini A, Zagdoun A, Lelli M, Lesage A, Copéret C, Emsley L (2013) Dynamic nuclear polarization surface enhanced NMR spectroscopy. Acc Chem Res 46(9):1942–1951 157. Lee D, Takahashi H, Thankamony AS, Dacquin JP, Bardet M, Lafon O, Paepe GD (2012) Enhanced solid-state NMR correlation spectroscopy of quadrupolar nuclei using dynamic nuclear polarization. J Am Chem Soc 134(45):18491–18494. https://doi.org/10. 1021/ja307755t 158. Rankin AGM, Webb PB, Dawson DM, Viger-Gravel J, Walder BJ, Emsley L, Ashbrook SE (2017) Determining the surface structure of silicated alumina catalysts via isotopic enrichment and dynamic nuclear polarization surface-enhanced NMR spectroscopy. J Phys Chem C 121:22977–22984

54

1 Solid-State NMR Principles and Techniques

159. Perras FA, Padmos JD, Johnson RL, Wang LL, Schwartz TJ, Kobayashi T, Horton JH, Dumesic JA, Shanks BH, Johnson DD, Pruski M (2017) Characterizing substrate-surface interactions on alumina-supported metal catalysts by dynamic nuclear polarization-enhanced double-resonance NMR spectroscopy. J Am Chem Soc 139(7):2702–2709. https://doi.org/ 10.1021/jacs.6b11408 160. Michaelis VK, Br Corzilius, Smith AA, Griffin RG (2013) Dynamic nuclear polarization of 17O: direct polarization. J Phys Chem B 117(48):14894–14906 161. Kobayashi T, Perras FA, Chaudhary U, Slowing II, Huang W, Sadow AD, Pruski M (2017) Improved strategies for DNP-enhanced 2D 1 H-X heteronuclear correlation spectroscopy of surfaces. Solid State Nucl Magn Reson 87:38–44. https://doi.org/10.1016/j.ssnmr.2017.08. 002 162. Blanc F, Sperrin L, Jefferson DA, Pawsey S, Rosay M, Grey CP (2013) Dynamic nuclear polarization enhanced natural abundance 17O spectroscopy. J Am Chem Soc 135(8):2975–2978 163. Perras FA, Wang Z, Naik P, Slowing II, Pruski M (2017) Natural abundance 17O DNP NMR provides precise O–H distances and insights into the Brønsted acidity of heterogeneous catalysts. Angew Chem 129(31):9293–9297. https://doi.org/10.1002/ange.201704032 164. FdrA Perras, Chaudhary U, Slowing II, Pruski M (2016) Probing surface hydrogen bonding and dynamics by natural abundance, multidimensional, 17O DNP-NMR spectroscopy. J Phys Chem C 120(21):11535–11544 165. Hope MA, Halat DM, Magusin PC, Paul S, Peng L, Grey CP (2017) Surface-selective direct 17O DNP NMR of CeO2 nanoparticles. Chem Commun 53(13):2142–2145 166. Li W, Wang Q, Xu J, Aussenac F, Qi G, Zhao X, Gao P, Wang C, Deng F (2018) Probing the surface of [gamma]-Al2O3 by oxygen-17 dynamic nuclear polarization enhanced solid-state NMR spectroscopy. Phys Chem Chem Phys 20(25):17218–17225. https://doi.org/10.1039/ C8CP03132K 167. Bowers CR, Weitekamp DP (1986) Transformation of symmetrization order to nuclearspin magnetization by chemical-reaction and nuclear-magnetic-resonance. Phys Rev Lett 57(21):2645–2648. https://doi.org/10.1103/PhysRevLett.57.2645 168. Bowers CR, Weitekamp DP (1987) Para-hydrogen and synthesis allow dramatically enhanced nuclear alignment. J Am Chem Soc 109(18):5541–5542. https://doi.org/10.1021/ja00252a049 169. Gloeggler S, Grunfeld AM, Ertas YN, McCormick J, Wagner S, Bouchard LS (2016) Surface ligand-directed pair-wise hydrogenation for heterogeneous phase hyperpolarization. Chem Commun 52(3):605–608. https://doi.org/10.1039/c5cc08648e 170. Barskiy DA, Coffey AM, Nikolaou P, Mikhaylov DM, Goodson BM, Branca RT, Lu GJ, Shapiro MG, Telkki V-V, Zhivonitko VV, Koptyug IV, Salnikov OG, Kovtunov KV, Bukhtiyarov VI, Rosen MS, Barlow MJ, Safavi S, Hall IP, Schroeder L, Chekmenev EY (2017) NMR hyperpolarization techniques of gases. Chem-A Eur J 23(4). https://doi.org/10.1002/ chem.201603884 171. Zhao EW, Zheng H, Ludden K, Xin Y, Hagelin-Weaver HE, Bowers CR (2016) Strong metalsupport interactions enhance the pairwise selectivity of parahydrogen addition over Ir/TiO2 . Acs Catal 6(2):974–978. https://doi.org/10.1021/acscatal.5b02632 172. Balu AM, Duckett SB, Luque R (2009) Para-hydrogen induced polarisation effects in liquid phase hydrogenations catalysed by supported metal nanoparticles. Dalton Trans 26:5074–5076. https://doi.org/10.1039/b906449d 173. Green RA, Adams RW, Duckett SB, Mewis RE, Williamson DC, Green GGR (2012) The theory and practice of hyperpolarization in magnetic resonance using parahydrogen. Prog Nucl Magn Reson Spectrosc 67:1–48. https://doi.org/10.1016/j.pnmrs.2012.03.001 174. Salnikov OG, Burueva DB, Barskiy DA, Bukhtiyarova GA, Kovtunov KV, Koptyug IV (2015) A mechanistic study of thiophene hydrodesulfurization by the parahydrogen-induced polarization technique. Chemcatchem 7(21):3508–3512. https://doi.org/10.1002/cctc.201500691 175. Bouchard L-S, Burt SR, Anwar MS, Kovtunov KV, Koptyug IV, Pines A (2008) NMR imaging of catalytic hydrogenation in microreactors with the use of para-hydrogen. Science 319(5862):442–445. https://doi.org/10.1126/science.1151787

References

55

176. Adams RW, Aguilar JA, Atkinson KD, Cowley MJ, Elliott PIP, Duckett SB, Green GGR, Khazal IG, Lopez-Serrano J, Williamson DC (2009) Reversible interactions with parahydrogen enhance NMR sensitivity by polarization transfer. Science 323(5922):1708–1711. https://doi.org/10.1126/science.1168877 177. Kovtunov KV, Barskiy DA, Salnikov OG, Khudorozhkov AK, Bukhtiyarov VI, Prosvirin IP, Koptyug IV (2014) Parahydrogen-induced polarization (PHIP) in heterogeneous hydrogenation over bulk metals and metal oxides. Chem Commun 50(7):875–878. https://doi.org/10. 1039/c3cc44939d 178. Salnikov OG, Kovtunov KV, Koptyug IV (2015) Production of catalyst-free hyperpolarised ethanol aqueous solution via heterogeneous hydrogenation with parahydrogen. Sci Rep 5. https://doi.org/10.1038/srep13930 179. Wang W, Xu J, Zhao Y, Qi G, Wang Q, Wang C, Li J, Deng F (2017) Facet dependent pairwise addition of hydrogen over Pd nanocrystal catalysts revealed via NMR using parahydrogen-induced polarization. Phys Chem Chem Phys 19(14):9349–9353. https://doi.org/ 10.1039/C7CP00352H 180. Wang W, Hu H, Xu J, Wang Q, Qi G, Wang C, Zhao X, Zhou X, Deng F (2018) Tuning Pd–Au bimetallic catalysts for heterogeneous parahydrogen-induced polarization. J Phys Chem C 122(2):1248–1257. https://doi.org/10.1021/acs.jpcc.7b11801 181. Zhao EW, Maligal-Ganesh R, Xiao C, Goh T-W, Qi Z, Pei Y, Hagelin-Weaver HE, Huang W, Bowers CR (2017) Silica-encapsulated Pt-Sn intermetallic nanoparticles: a robust catalytic platform for parahydrogen-induced polarization of gases and liquids. Angew Chem Int Ed 56(14):3925–3929. https://doi.org/10.1002/anie.201701314 182. Kovtunov KV, Beck IE, Bukhtiyarov VI, Koptyug IV (2008) Observation of parahydrogeninduced polarization in heterogeneous hydrogenation on supported metal catalysts. Angewandte Chemie-International Edition 47(8):1492–1495. https://doi.org/10.1002/anie. 200704881 183. Pravica MG, Weitekamp DP (1988) Net NMR alignment by adiabatic transport of parahydrogen addition products to high magnetic field. Chem Phys Lett 145(4):255–258. https://doi. org/10.1016/0009-2614(88)80002-2 184. Zhao EW, Maligal-Ganesh R, Du Y, Zhao TY, Collins J, Ma T, Zhou L, Goh T-W, Huang W, Bowers CR (2018) Surface-mediated hyperpolarization of liquid water from parahydrogen. Chem 4(6):1387–1403. https://doi.org/10.1016/j.chempr.2018.03.004 185. Duckett SB, Mewis RE (2012) Application of parahydrogen induced polarization techniques in nmr spectroscopy and imaging. Acc Chem Res 45(8):1247–1257. https://doi.org/10.1021/ ar2003094 186. Bowers CR (2007) Sensitivity enhancement utilizing parahydrogen. In: Wasylishen RKHaRL (ed) eMagRes. Wiley. https://doi.org/10.1002/9780470034590.emrstm0489 187. Hovav Y, Feintuch A, Vega S (2010) Theoretical aspects of dynamic nuclear polarization in the solid state—the solid effect. J Magn Reson 207(2):176–189. https://doi.org/10.1016/j.jmr. 2010.10.016 188. Hovav Y, Feintuch A, Vega S (2012) Theoretical aspects of dynamic nuclear polarization in the solid state—the cross effect. J Magn Reson 214:29–41. https://doi.org/10.1016/j.jmr. 2011.09.047 189. Hovav Y, Feintuch A, Vega S (2013) Theoretical aspects of dynamic nuclear polarization in the solid state—spin temperature and thermal mixing. Phys Chem Chem Phys 15(1):188–203. https://doi.org/10.1039/C2CP42897K

Chapter 2

Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

Abstract Understanding of synthesis mechanism is a prerequisite for the rational design of materials with desired structure and property. NMR is a powerful and unique tool that can probe the local or atomic environments of solid or liquid phase during the synthesis. The study of the crystallization of zeolites and zeotype materials with NMR spectroscopy is dealt with in this chapter. Synthesis routes and procedures as well as self-assembling mechanisms for zeolites are briefly introduced. 1D NMR spectroscopy allows to perform the structural analysis of the species leading to the construction of the zeolite framework, while the 2D NMR reveals the connectivity and correlations between the same or different species during the synthesis. Ex situ and in situ NMR approaches are presented, which allow to obtain detailed information on the chemical reactions and kinetics involved in synthesis process. The crystallization of aluminosilicates and aluminophosphates are discussed to show how and to what extent the molecular-level insights can be obtained by using multi-nuclear NMR and different NMR protocols. Keywords Zeolites · Synthesis · Crystallization · Mechanism · Ex situ NMR · In situ NMR

2.1 Introduction Zeolites are a well-known family of microporous materials, which are usually referred to as molecular sieves, possessing regular pores or voids in size range of 5–20 Å, high surface area of several hundred m2 /g, and excellent hydrothermal stability. After the successful synthesis of the first artificial zeolite in the 1950s [1, 2], hundreds of zeolites with various topological structures have been synthesized [3]. Owing to their special architecture and properties, zeolites have been widely used in industrial applications as heterogeneous catalysts, adsorbents, ion-exchange agents, and materials for molecular recognition operations. In the early 1990s, a new family of porous material appeared, and the most famous members were M41S series (MCM-41) [4] and SBA series (SBA-15) [5]. They are often referred to as “mesoporous zeolites” and are featured by regular mesoporosity with much larger size © Springer Nature Singapore Pte Ltd. 2019 J. Xu et al., Solid-State NMR in Zeolite Catalysis, Lecture Notes in Chemistry 103, https://doi.org/10.1007/978-981-13-6967-4_2

57

58

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

(20–500 Å) and significantly higher surface area (above 700 m2 /g), which render them a broad range of promising identities as heterogeneous catalysts, semiconducting materials, optic devices, etc. The discovery of the mesoporous materials remarkably boosts the development of the porous materials in synthesis and application. In addition to conventional zeolites which exhibit excellent catalytic properties, mesoporous materials have been intensively studied as catalysts since its discovery, thanks to their ability to accommodate the relatively large molecules of particularly petrochemistry interest for catalytic reaction. Because of the significant importance of porous materials in the catalysis community, this chapter is dedicated to the synthesis mechanism of representative zeolites and zeotype materials, with focus on the crystalline aluminosilicates and aluminophosphates. The studies on zeolite synthesis have been making a great contribution to the design and applications of zeolites. This is demonstrated by the discovery of porous materials with new framework structures with advanced synthetic routes and techniques. The optimization of catalytic process and elucidation of the reaction mechanism both benefit from the understanding of the zeolite synthesis as well. Trialand-error strategy is typically used with the aim to obtain new phase and structure. Although new synthetic zeolite materials are regularly discovered, the rational “priori design” of molecular sieves now is not possible because of a lack of full understanding of their synthesis mechanism. Actually, the mechanism study of the synthesis is accompanied by the discovery of the porous materials. However, the extreme complexity of the molecular process and the great diversity of different reaction systems more or less hamper the understanding of synthesis process, in which non-stoichiometric reactions, bond formation and cleavage, and various non-bond interactions between the dissolved and non-dissolved species are all involved. The synthesis mechanism of the porous materials remains one of the most challenging topics in chemistry. Many advanced techniques like X-ray diffraction [6, 7], NMR [8–13], IR/Raman [14], SAXS/WAXS (small- and wide-angle X-ray scattering) [15], and neutron diffraction [16] have been employed to study the crystallization mechanism of zeolites. Among them, NMR is a powerful and unique tool that can probe the local or atomic environments of solid or liquid phase during the synthesis. Sensitive to the short-range ordering of porous materials, NMR enables the analysis of the structure of aggregated species in different configurations. Multi-nuclear one- (1D) and two-dimensional (2D) NMR techniques can be used to establish the correlation and connectivity between different nuclei, providing information about the host-guest interactions within the aggregates. The self-assembling process, particularly in the sol–gel phase, can be monitored by in situ NMR technique, from which the obtained information on the species formation and evolution offers direct experimental evidence for uncovering the synthesis mechanism. In the past 20 years, the development and application of advanced NMR techniques benefit the knowledge accumulation on the synthesis of porous materials. The self-assembling process in the synthesis is becoming clear though not well understood, which is helpful for the rational design and synthesis of zeolites with new structure and property.

2.2 Zeolite Synthesis Route and Procedures

59

2.2 Zeolite Synthesis Route and Procedures The synthesis of zeolite can be dated back to the mid of nineteenth century [17]. The first attempt is to use special conditions by high temperature (about 200 °C) and pressure (about 10 MPa) that mimic the environment for the formation of natural zeolites. The successful zeolite synthesis was commenced by the first synthetic zeolite ZK-5 by Barrer in the late 1940s [1]. Milton from UCC (Union Carbide Corporation) developed new synthesis route under milder conditions, leading to the discovery of zeolites A [18] and X [18], which do not exist as natural minerals. The introduction of quaternary ammonium cations into the synthesis system in the 1960s has a great impact on zeolite synthesis, enabling the production of high-silica and even pure silica zeolite. The first high-silica zeolite beta (5 < Si/Al < 100) was reported in 1967 by Wadlinger et al. [19]. The archetypal high-silica zeolite ZSM-5 was synthesized by using tetrapropylammonium cation in 1972 [20]. Since then a large number of zeolites and zeolite-like materials has been discovered by the “templated” synthesis approach with the organics acting as structure-directing agents (SDAs). The further step to the zeotype material synthesis is the discovery of microporous aluminophosphates by Wilson and Flanigen [21]. Since many metals can be introduced in the framework, such materials display a great compositional diversity. Hydrothermal synthesis is the best approach for zeolites synthesis. For an aluminosilicate, a typical synthesis route can be summarized as follows: Silica and alumina reactants are mixed in a strong basic medium and stirred to form gel; after aging, the mixture is heated in a sealed autoclave at a medium temperature of ca. 200 °C for a period (from several hours to days); zeolite crystals are generated in the autoclave and after washing, drying, and calcination, zeolite crystals are transformed into the final products.

2.3 Zeolite Synthesis Process and Crystallization Mechanism The diversity of zeolite structure and synthesis conditions exerts great complex on the synthesis system and thus the difficulty in understanding the synthesis process and mechanism. Although the molecular mechanism responsible for the formation of zeolite is still poorly understood [22], two processes have been recognized to describe the zeolite synthesis, solution-mediated reaction transformation [23, 24], and solid hydrogel transformation [23, 25] as shown in Fig. 2.1. In the solution-mediated reaction transformation process, precursor materials are dissolved and the zeolite crystals are generated from the resulting solution. Zhdanov et al. [27] gave a detailed description of this process which was supported by experiments. It claims that the nucleus is formed in solution or on the interface of the gel; the growth of the nucleus consumes the silicates and aluminates in solution which provides the soluble structure units for the growth of zeolite crystals; the consump-

60

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

Fig. 2.1 Zeolite crystallization mechanisms: a solution-mediated process (type A); b hydrogel reconstruction (type B). Reprinted from Ref. [26] by permission of Elsevier

tion of the soluble species in solution leads to further dissolution of the solid gel till the formation of zeolite product. This offers a reasonable explanation for the experimental observations of zeolite nucleation and crystal growth. For the formation of microporous crystals, nucleation and growth do not likely take place through atom deposition. There exists a considerable amount of smallest structural units like secondary building units (SBUs) [28] or oligomeric species in solution phase. The thermally stable nuclei are formed by the polymerization of structural units. Thus, the orderly assembling of the SBUs and oligomeric species in solution lays the basis for the nucleation and growth. The difference between the solid hydrogel transformation process and the solution-mediated reaction transformation process lies in the role of the soluble components in the crystallization of zeolite. The work of Breck and Flanigen in the

2.3 Zeolite Synthesis Process and Crystallization Mechanism

61

Fig. 2.2 High-resolution electron micrograph of quasi-crystalline zeolite A. Reprinted from Ref. [26] by permission of Elsevier

late 1960s found that the formation and transformation of amorphous aluminosilicates prevail in the crystallization process [17], and the composition of the formed solid gels resembles the final zeolite products. This leads to the conclusion that the dissolution of solid gels does not occur in the crystallization and the soluble components are not directly involved in the zeolite nucleation and crystal growth. The solid gel is formed under hydrothermal conditions from which the rearrangement of aluminosilicate framework generates the crystallized products. The postulation of “solid-state transformation” was confirmed by McNicol et al. in the synthesis of zeolite A [29]. It was found that secondary building units do not exist in liquid phase while the solid gel grows into zeolite framework. Besides the experimental observations on the chemical phenomena, the work of Thomas et al. gave direct evidence on the solid hydrogel transformation process in zeolite crystallization [30]. They used high-resolution electron micrograph to analyze the amorphous gel in the formation of zeolite A. As shown in Fig. 2.2, the quasi-crystallines of zeolite A in 5–10 nm were identified in the solid gel which demonstrated that the zeolite nucleus was originated from solid phase.

62

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

2.4 NMR Strategy in Characterization of Zeolite Synthesis NMR spectroscopy can provide detailed information about the local environments of framework building elements in the solid and liquid phases, which is complementary to X-ray diffraction and electron microscopy methods that are often used to gain structural information in long-range ordering [31]. In principle, all the framework atoms of aluminosilicates and aluminophosphates can be studied by NMR. However, low natural abundance of isotope and low gyromagnetic ratio of nucleus like 17 O (0.037%) make its NMR observation difficult due to the low detection sensitivity. The most often analyzed nuclei are 1 H, 29 Si, 27 Al, 31 P of the framework as well as substituted heteroatoms like 71 Ga and 11 B. For 17 O NMR observation, isotope enrichment is usually required. The coordination geometry and local chemical environment of the T atoms in solid framework as well as speciation and concentration of the soluble component can be determined by 1D NMR spectroscopy, while advanced 2D NMR methods concerning homonuclear or heteronuclear correlation enable the establishment of the connectivity of solid framework and that of soluble species. Two approaches have been developed for the application of NMR spectroscopy in zeolite crystallization, in situ and ex situ methods. In zeolite synthesis, the nucleation and growth are governed by kinetic factors. Thus, some meta-stable rather than thermodynamically stable solids are often produced initially, and the crystallization often involves different transient solids before the zeolite crystal is formed. In order to optimize and control such processes, it is essential to understand the sequence of events that occur in the evolution of the solid phase, rather than simply characterizing the final product formed at the end of the process. The in situ NMR experiment is applied during crystallization to monitor the chemical reactions in the synthesis process. The advantage of the in situ approach is that the crystallization process is not or less perturbed. Thus, the analysis of short-lived intermediates formed in the crystallization and the kinetics can be achieved. Importantly, the effect on the crystallization by reducing temperature and pressure for room temperature analysis could be largely minimized by this approach, since in a hydrothermal reaction the chemical equilibria may be shifted by perturbation of the reaction conditions. From the experimental point of view, the data points obtained during the synthesis are considerably more than under ex situ conditions, which allows to gain detailed information about the kinetic of the crystallization. There is a high technical demanding on the application of in situ NMR study on conventional NMR instrument. The difficulty is the design of a hydrothermal reaction cell that can withstand high temperature and pressure, and it can be properly fitted in the NMR probe in magnet for signal acquisition. The commercial NMR rotors that usually hold the sample often do not meet the demanding. Thus, delicate modification is often required to use the NMR rotor as the reaction cell. Another issue that should be considered is that crystallization time of the investigated zeolite must be not shorter than the NMR time scale. Therefore, in spite of the numerous merits, in situ NMR method is not widely applied due to the technical limitation. The ex situ NMR method alternatively does not have that technical demanding on the NMR instrument and thus is widely applied. The ex situ

2.4 NMR Strategy in Characterization of Zeolite Synthesis

63

method is realized by quenching the crystallization process at preset temperature and time. Soluble species and solids are separated from the reaction at room temperature. Liquid-state NMR is used to analyze the soluble species, while the solids are examined by solid-state NMR. Since no further reaction occurs in both soluble species and solids after quenching the crystallization, the obtained information from the NMR analysis can be related to the formation of intermediate products in the crystallization. Thus, a series of such experiments during a whole reaction allows to some extent the establishment of crystallization process in terms of species evolution.

2.4.1 Microporous Aluminosilicates Zeolites are crystalline aluminosilicates composed of silicon, aluminum, and oxygen atoms that form a framework with cavities and channels where cations, water, and/or small molecules may reside. Zeolite is crystallized from a liquid phase which contains silicate, aluminate, and solid aluminosilicate gel. Because of the difficulty in analyzing the liquid and solid phase simultaneously, they are often investigated separately. Engelhardt et al. [32] studied the structural properties of the solid gel phase in zeolite A synthesis and its transformation into the crystalline phase by solid-state 29 Si and 27 Al NMR. There is a direct influence of four-coordinated Al substituting for Si on the second-coordination sphere of Si. In the aluminosilicate gels, non-equivalent Q4 units of different degrees of Al substitution reflected by different 29 Si chemical shifts indicate different extents of substitution of Si by Al atom in the units. It was revealed that the initial gel has an amorphous structure and is composed of tetrahedral Si(OAl)4 and Al(OSi)4 building units. The short-range ordering of Si and A1 in the gel framework is confirmed by the highly symmetrical 27 A1 NMR signals. With increasing heating time, the amorphous gel phase transforms into crystalline zeolite as evidenced by the upper-field shift of the 29 Si signal of the Si(OAl)4 units from the amorphous gel (−85 ppm) to the crystalline zeolite A (−89 ppm). The observed aluminosilicate gel with Si/A1 = 1 and an alternating Si, Al ordering is different from previous studies, in which silica-rich initial gel phases have been observed. Thus, it was concluded that the synthesis of zeolite A is determined by the specific preparation method, i.e., the nature of the starting materials, the procedure of mixing the silicate and aluminate solutions, the pH value of the solutions, concentration, aging, etc. This effect was further investigated by the same group [33]. Two reactant mixtures of equal overall composition for zeolite A synthesis were prepared with different sodium silicate solution. The 29 Si NMR spectra of solid aluminosilicate gel showed that a silica-rich amorphous gel phase with a molar Si/A1 ratio of 2.4 and an aluminum-rich initial gel with Si/Al = 1 were obtained, respectively. The difference in the formation of initial gel can be accounted for by the chemical composition of the starting silicate and aluminate solutions because the overall composition of the reaction mixture is the same in both reactant mixtures. The thermodynamic variables including the temperature, pressure, and overall chemical composition of the reactant mixtures affect the zeolite synthesis and the products

64

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

obtained in the hydrothermal reactions. While the non-thermodynamic factor like aging that is the treatment of reactants prior to crystallization may influence the kinetic stage of nucleation and thus the crystalline product. Aging is often used prior to the crystallization of zeolites which play an important role in suppressing the formation of phase other than the target zeolite. The synthesis of zeolite faujasite (FAU), in particular, require room temperature aging of the hydrogel that is formed by mixing the silicate and aluminate prior to heating for the crystallization [34]. It is supposed that structural rearrangements may occur during aging, which gives rise to the formation of zeolite FAU nuclei and the amorphous aluminosilicate gels serving as the precursors for the crystallization of FAU. The structural changes in the aluminosilicate gels of FAU were confirmed by use of 29 Si MAS NMR. Okubo et al. [35] found the changes in Si/Al ratios and Q units during the aging and the crystallizing processes. As shown in Fig. 2.3 for the products with 2 and 7 days of aging, respectively, different modes of building to FAU in the course of crystallization were revealed by the 29 Si MAS NMR spectra. The broad peak centered at −84 ppm due to solid aluminosilicates (Q4 (4Al) unit) grew into some sharp peaks which gradually developed after FAU phase was formed from the mixture with 2 days of aging. The broad signal centered at −110 ppm was due to the Q4 (0Al) unit from unsolved silica source. The relative intensities of the peaks in the products with 7 days of aging, in contrast, did not show obvious change after the formation of FAU. The Si/Al ratios and the Q units can be further analyzed on the basis of the NMR spectroscopy. The aluminosilicate species obtained after 7 days of aging is dominated by Q4 (4Al) unit, leading to crystalline faujasite with higher crystallinity, sharper particle size distribution, and a lower Si/Al ratio, while shorter aging time produces unreacted silicate species derived from the starting silica source. In combination with XRD, the formation of aluminosilicate precursors during the aging process and the formation mechanism of FAU and other zeolitic phases were proposed as shown in Fig. 2.4. In the aging stage, the dissolution of colloidal silica is involved in the formation of Q4 (4Al) unit. Gismondine (GIS), analcime (ANA), and hydrated sodalite (SOD) could be formed from the clear sodium aluminosilicate solution without the intervention of hydrogel. During the crystallization period, six-membered aluminosilicate species are generated in the solid amorphous phase, which play a structure-directing role in the formation of FAU and a less amount of GIS. The NMR observation of gel transformation by using 27 Al and/or 29 Si NMR also includes the crystallization of mordenite [36] and ZSM-5 [37, 38]. The organic molecules such as amines and alkylammonium ions (structure-directing agents) are often added to zeolite synthesis gels, which lead to the formation of a particular structure with different framework chemical compositions. Understanding the exact role of the structure-directing agent and the mechanism by which it affects the formation of specific structure is critical to reveal the route that the crystalline zeolites are produced. A pioneer work of Davis et al. reported the structure-directing mechanism of tetrapropylammonium (TPA) ions in the synthesis of pure silica ZSM-5 (Si-ZSM-5) by using solid-state NMR [39]. In particular, 1 H–29 Si CP NMR was utilized to probe

2.4 NMR Strategy in Characterization of Zeolite Synthesis

65

Fig. 2.3 29 Si MAS NMR spectra of the products obtained with 2 days (left) and 7 days (right) of aging prior to crystallization. Reprinted from Ref. [35] by permission of American Chemical Society

Fig. 2.4 Illustration of the proposed formation mechanism and the promotional effect of aging on the zeolites crystallization. Reprinted from Ref. [35] by permission of American Chemical Society

66

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

Fig. 2.5 29 Si MAS NMR and 1 H–29 Si CP MAS NMR (contact times as indicated) spectra of the freeze-dried samples collected during the TPA-mediated synthesis of Si-ZSM-5: a unheated gel, b heated 1 day, c heated 2 days, d heated 3 days, e heated 10 days. The spectra in a–c are shown on the same intensity scale, and the spectra in (d) and e are reduced by a factor of approximately 3; i.e., all of the spectra obtained without CP have approximately the same maximum intensity. Reprinted from Ref. [39] by permission of American Chemical Society

the interactions between the TPA cation and the silicate species that ultimately form the zeolite framework. 1 H–13 C CP NMR was used to gain information on the conformation of the TPA cation in the gel. The polarization transfer efficiency from 1 H to 13 C or 29 Si is determined by the internuclear dipole interactions. The strength of the dipole interactions is proportional to the inverse internuclear distance (r−6 ) and related to the molecular motion. Valuable information about the spatial proximity of concerned nuclei and molecular dynamics can be obtained from the CP NMR studies. In the work of Davis, the Si-ZSM-5 gels were prepared in a reactant mixture in which the only source of protons was the TPA cation and all other reagents were used in their deuterated forms. The samples were collected from the unheated gel and after 1, 2, 3, and 10 days of heating, and then freeze-dry was applied by immediately quenching of the hot gels in order to preserve the silicate structures that were present at synthesis conditions. Figure 2.5 shows the 1 H–29 Si CP MAS NMR spectra with varying contact times to demonstrate the degree of molecular motion within different samples. Increasing contact time notably enhances the signals on the relatively immobilized systems because polarization can be transferred to more distant nuclei as well as to nuclei within the van der Waals contact distance. According to Mentzen’s work [40], the distance of a Si-to-H van der Waals contact is approximately 3.3 Å. The signals at −102 and −112 ppm were observable on the samples, assigned to Q3 (Si(OSi)3 (OH)) and Q4 (Si(OSi)4 ) species, respectively. Interestingly, Q3 species are polarized more rapidly than Q4 species at 2 ms of contact time, implying the shorter distance from the Q3 species to the organic protons due to a weak coulombic attraction. Thus, the 1 H–29 Si CP MAS NMR spectra show the relative positions of the TPA and the evolution of framework silicon atoms during the synthesis. Figure 2.6 shows 1 H–13 C CP MAS NMR spectra of solid TPABr and TPA occluded into Si-ZSM-5 and the freeze-dried samples collected at various time inter-

2.4 NMR Strategy in Characterization of Zeolite Synthesis Fig. 2.6 1 H–13 C CP MAS NMR spectra of the freeze-dried samples collected during the TPA-mediated synthesis of Si-ZSM-5: a unheated gel (same as TPABr), b heated 1 day, c heated 2 days, d heated 3 days, e heated 10 days, f heated 10 days (D2 O-washed) (TPA occluded in Si-ZSM-5). Reprinted from Ref. [39] by permission of American Chemical Society

67

68

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

vals during the synthesis. The TPABr in the unheated gel produces 13 C signals at 60, 16, and 12.6 ppm, due to the methylene carbon bonded to the nitrogen, methylene carbon in the propyl chain, and methyl carbon, respectively (Fig. 2.6a). When the TPA is occluded into ZSM-5 as shown in Fig. 2.6f, the signals of the methylene carbon bonded to the nitrogen are down-field shifted to 60.6 ppm and broadened, and a splitting of the methyl carbon signals at 10.1 and 11.3 ppm is observable. After 1 day of heating, it is difficult to discern whether any TPA is occluded into the sample. However, after 2 days of heating the TPA molecules are present in ZSM-5 channels as reflected by the signal at 10.1 ppm. This is supported by the XRD results that some crystalline Si-ZSM-5 is already present at this time. The growth of the signals intensity at 10.4, 11.3, and 62.6 ppm indicates the increase of the amount of occluded TPA in ZSM-5 channels. This suggests that the TPA molecules within these structures adopt a conformation different from that of solid TPABr or of a TPA ion in aqueous solution [41]. Regarding the synthesis mechanism, the CP NMR results confirmed the model proposed by Gies and Marler by indicating that weak, non-covalent intermolecular interactions such as van der Waals or hydrophobic interactions between TPA ions and silicate species are involved in an organic-mediated zeolite synthesis [42] (Fig. 2.7). The interactions between the TPA cations and the silicate species were identified by 1 H–29 Si CP MAS NMR during heating of the synthesis gel before the long-range ordered crystalline was formed. It is noted that all these studies were undertaken on separated solids which were analyzed ex situ. Using zeolite A as a model system, Shi et al. performed the first in situ solid-state NMR study on the crystallization which allows to follow the speciation as a function of time [43]. The synthesis gel was prepared by sodium silicate, sodium aluminate, and NaOH. The gel was transferred into a 7-mm Chemagnetics NMR rotor sealed with Teflon end caps at room temperature and heated at 65 or 70 °C with nitrogen gas flow as shown in Fig. 2.8. The rotor was spun at a speed of 2100 Hz. Both liquid and solid species could be in situ monitored by 29 Si and 27 Al MAS NMR (Fig. 2.9). The in situ 29 Si MAS NMR spectra of the synthesis of zeolite A at 65 °C shows that after heating for 40 min the broad 29 Si signals at from −75 to −95 ppm due to Si(0Al)~Si(4Al) units do not have apparent change. This period of crystallization can be considered as induction and nucleation stage, in which either the rearrangements in the gel are too small to be observed by NMR or the changes are taking place in the longer-range order of the gel. Thus, little information can be offered by the in situ NMR experiment at this period. After 56-min heating, the signals at −85 ppm and −89 ppm increase, indicating the enrichment of the Si(3Al) and Si(4Al) units in the gel. The appearance of the −89 ppm signal evidences the formation of crystalline zeolite A. The enhancement of the signal intensity at −89 ppm and the decreasing of its half-height width from 100 to 150 min indicate an ordering of the local environment of the Si(4Al) units because of a rapid increase in crystallinity. After 150 min, the crystallization goes into a period of crystal growth reflected by the constant half-height width at −89 ppm (Fig. 2.10). In the corresponding 27 Al NMR spectra, two different chemical environments exist in the initial gel, producing the

2.4 NMR Strategy in Characterization of Zeolite Synthesis

69

Fig. 2.7 Schematic of the proposed mechanism of structure direction and crystal growth involving inorganic–organic composite species in the TPA-mediated synthesis of Si-ZSM-5. The interactions between TPA and silicate within the composite species are detailed in the upper portion of the diagram. Reprinted from Ref. [39] by permission of American Chemical Society

signals at ca. 60 and ca. 77 ppm. The former is ascribed to aluminosilicate units, that is, tetrahedrally coordinated aluminum linked by oxygen bridges to silicon, while the latter is due to the soluble [Al(OH)4 ]− . A variety of silicon coordination results in a broad chemical shift distribution on 27 Al nuclei. The 27 Al MAS NMR result is in agreement with the observation from the 29 Si NMR. After 70 min, the signal of [Al(OH)4 ]− at 77 ppm decreases, indicative of the incorporation of free aluminum species into the zeolite. Similarly, the reduction in half-width of the 60 ppm signal as the crystallization proceeds reflects the formation of crystalline zeolite A. The

70

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

Fig. 2.8 Diagram illustrating the method used to seal the zeolite A gels in NMR rotors. Reprinted from Ref. [43] by permission of American Chemical Society

initiation of crystallization is associated with the slow reduction of free [Al(OH)4 ]− ions at the initial period while a rapid consumption of the free aluminate is associated with the rapid crystallization. The combination of 27 Si and 27 Al MAS NMR and XRD analysis provides a comprehensive picture of the crystallization of zeolite A. However, the NMR data show that NMR is not sensitive enough to probe the early changes in the gel chemistry. The formation of secondary building units in zeolite synthesis has been intensively discussed in the literatures [28, 44]. The in situ 29 Si and 27 Al MAS NMR did not show any observable complex soluble aluminosilicate or silicate species as the secondary building units. As shown in the NMR spectra (Fig. 2.9), only (OSi)4 units, Q0 (Si) and [Al(OH)4 ]− species are definitely identified. With respect to the mechanism, the results support the solution-mediated route, in which Q0 (Si) and [Al(OH)4 ]− ions are formed by dissolution of the gel. The rapid deposition of the formed species on the growing zeolite surface promotes the crystallization. The transformations are supposed to take place in the vicinity of the solid gel phase. Focusing on the kinetics, Miladinovic et al. used an in situ 27 Al NMR method to monitor zeolite A crystallization [45]. Unlike the previous in situ NMR investigation that needs an NMR rotor, all experiments were performed in self-made 10-mm glass NMR tubes under static conditions. 27 Al NMR spectra were acquired which show similar observation to the work of Shi et al. Signals at 77 and 60 ppm were assigned to Al(OH)− 4 species and tetrahedral Al(OSi)4 building blocks, respectively (Fig. 2.11). Importantly, a subtle departure (humps) from a smooth S-curve of the signal intensity was observed during the crystal-forming phase (Fig. 2.12), similar to the observations by other techniques such as in situ energy-dispersive X-ray diffraction (EDXRD) [46]. The “autocatalytic nucleation” model of zeolite crystallization was responsible for the phenomena. This has been noticed by Walton in the in situ EDXRD study of

2.4 NMR Strategy in Characterization of Zeolite Synthesis

71

Fig. 2.9 In situ 29 Si NMR and 27 Al MAS NMR spectra on zeolite A synthesis from gel at 65 °C. Reprinted from Ref. [43] by permission of American Chemical Society Fig. 2.10 Plot of the half-height width of the peak at −89 ppm versus heating time. Reprinted from Ref. [43] by permission of American Chemical Society

72

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

Fig. 2.11 27 Al NMR spectra at different time intervals during the course of zeolite A synthesis. Reprinted from Ref. [45] by permission of Elsevier

Fig. 2.12 Changes of relative intensity of 27 Al NMR lines at 79 ppm (open circles) and 59 ppm (closed circles) during the zeolite synthesis. Open triangles denote changes of width at half-height of 27 Al NMR line at 59 ppm in absolute units. The arrows indicate lags at intensity curves. Reprinted from Ref. [45] by permission of Elsevier

zeolite A synthesis that the nucleation occurs not only in solution, but also within the amorphous aluminosilicate gel [46]. The dissolution of the amorphous gel occurs for the dormant nucleus to be released from the gel and the crystal growth to take place.

2.4 NMR Strategy in Characterization of Zeolite Synthesis

73

2.4.2 Microporous Aluminophosphates Up to now, over 200 types of microporous aluminophosphates have been identified [3]. Aluminophosphates shows a large variety in framework as compared to aluminosilicates [47], while the similarity to zeolites is that aluminophosphates are often synthesized at hydrothermal conditions in which amorphous solid gels appear in the initial crystallization stage and grow into crystalline product. In the framework, Al atoms of aluminophosphates have different coordination, from 4 to 6 with P atoms, with 27 Al NMR chemical shift being at −10 to 50 ppm; P atoms have a 31 P NMR chemical shift of −30 to 0 ppm. In the synthesis mechanism studies, NMR techniques are widely used and can provide valuable information for understanding the complex chemistry involved in the hydrothermal synthesis process. Deng et al. monitored the hydrothermal synthesis of AlPO4 -5 (AFI topology) in the fluoride medium by ex situ NMR [48], and the attention was given to the intermediate gels. A starting mixture with a composition of Al(OPri)3 (triisopropylate aluminum), H3 PO4 , TEA (triethylamine), HF, and H2 O was prepared. The visible gel formed in the synthesis and liquid phase was separated by centrifugation. The gel was washed with distilled water and dried at room temperature before NMR analysis. Figure 2.13 shows 27 Al MAS spectra of the selected gels with different lengths of heating time. For the initial gel, a weak signal at 41.9 ppm and a strong signal at − 6.6 ppm with a shoulder at 6.7 ppm are observable. The weak signal arises from the tetrahedral Al environments (Al(OP)4 ), while the strong signal is probably due to octahedral Al atoms in aluminophosphates. Heating the gel for 120 min results in almost equal intensity of the tetrahedral and the octahedral Al signals, suggesting an increase of the ordered phase at the expense of the disordered moieties. After 150 min of heating, the tetrahedral signal (shifting to 37.6 ppm) dominates the 27 Al spectrum. At the same time, a new weak signal at 10.2 ppm appears, and its intensity increases with the heating time, which is due to the pentacoordinated Al associated with F–Al complex. In combination with the PXRD result, the 27 Al spectrum of the gel heated for 180 min corresponds to the crystalline AlPO4 -5. The only one broad 31 P signal observed at −11.4 ppm in the initial gel is assigned to the amorphous aluminophosphates characterized by P–O–Al units or to some free phosphate species [49]. When the heating time is increased to 120 min, a narrow signal appears at ca. −29 ppm. By spectral deconvolution, two other new signals at ca. −8 and −22 ppm can be found. The two signals at ca. −22 and −29 ppm can be ascribed to the tetrahedral P in the crystalline framework evidenced by the appearance of a weak diffraction peak characteristic of AlPO4 -5 in PXRD [50]. Further lengthening the heating time leads to the consumption of the amorphous or disordered moieties. Heteronuclear correlation (HETCOR) spectroscopy is a 2D technique based on cross-polarization to detect the connectivity between two different nuclei in proximity of less than 1 nm. 31 P{27 Al} HETCOR spectrum of the initial gel (Fig. 2.14) shows that the broad 31 P signal at −12 ppm exhibits strong correlations with both

74

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

Fig. 2.13 27 Al MAS (a) and 31 P MAS (b) spectra of the isolated gels throughout the hydrothermal treatment period. Reprinted from Ref. [48] by permission of American Chemical Society

tetrahedral and octahedral 27 Al signals (−7.8 and 42 ppm). In addition, a sharp 31 P signal at −16 ppm can be observed, which is also correlated with the two 27 Al signals. The two signals at −12 and −16 ppm can be assigned to the partially condensed P with less than four Al atoms in its coordination sphere. It is concluded that prior to hydrothermal treatment the initial gel forms amorphous aluminophosphate species immediately after mixing the reactants, which is characterized by the different partially condensed P and different Al environments. For the 120-min heated gel, it is interesting to note that three 31 P signals at −11.6, −22.2, and −29.3 ppm with almost equal intensity appear in the 31 P projection, showing distinctly different patterns compared with the corresponding 31 P MAS spectrum. The two high-field 31 P signals at ca. −22 and −29 ppm are correlated with both the tetrahedral Al (38.6 ppm) and the pentacoordinated Al (9.7 ppm) sites but not with the octahedral Al sites (−14.8 ppm). The 31 P signal at −11.6 ppm exhibits correlations with both tetrahedral and octahedral Al sites but not with the pentacoordinated Al site. There-

2.4 NMR Strategy in Characterization of Zeolite Synthesis

75

Fig. 2.14 31 P{27 Al} HETCOR spectra of initial gel (a) and 120-min heated gel (b). Reprinted from Ref. [48] by permission of American Chemical Society

fore, it can be concluded that the number of Al atoms in the coordination sphere of the two P sites corresponding to the two high-field 31 P signals is more than that of the P site corresponding to the low-field 31 P signal, indicating that the former two P sites have a fully condensed coordination sphere, whereas the latter can be considered as a partially condensed P site. In combination with 31 P{27 Al} TRAPDOR, 1 H → 31 P CP/MAS NMR, and 19 F → 27 Al CP/MAS experimental results, a clear crystallization process of AlPO4 -5 in the presence of HF can be pictured (Fig. 2.15). In the first stage, prior to hydrothermal treatment, amorphous fluoraluminophosphate phase is formed which is characterized by two main components with one being composed of aluminophosphate species (Altet –O–Ppar and F–Aloct –O–Ppar , par indicates partially condensed) units and the other being composed of some free species, such as protonated phosphates, unreacted phosphoric acid, and unreacted Al source. The amorphous nature of the phase does not change until the heating time reaches 120 min, when the formation of a semiordered network occurs. Two microdomains can be clearly identified in the 120-min gel. The periodic crystalline structure characterized by Altet –O–Pful and F–Alpen –O–Pful (ful denotes fully condensed) units appears to be the new domain II, and amorphous fluoroaluminophosphate component (F–Aloct –O–Ppar ) and aluminophosphate component (Altet –O–Ppar ) found in the former stage are present as the domain I. In the final stage, the amorphous component (domain I) and the free species are almost completely consumed in favor of the formation of the highly condensed Al(OPful )4 and Al(OPful )4 F units that exist in the crystalline AlPO4 -5. Isomorphic substitution of framework Al3+ and P5+ ions by metal cations (V, Co, Mg, Ga, Fe, Zn, etc.) or silicon gives rise to MeAPO and SAPO family materials, respectively. These materials not only exhibit characteristics of zeolites but also show novel physicochemical properties that are linked to their unique composition [21]. The crystallization process of magnesium-containing aluminophosphate molecular sieve MgAPO-36 (ATS type) is characterized by the evolution of intermediate gels

Fig. 2.15 Schematic of the proposed mechanism of AlPO-5 synthesis in the fluoride medium. Reprinted from Ref. [48] by permission of American Chemical Society

76 2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

2.4 NMR Strategy in Characterization of Zeolite Synthesis

77

Fig. 2.16 27 Al MAS (a) and 31 P MAS (b) spectra of the gels throughout the hydrothermal treatment period. Reprinted from Ref. [51] by permission of Elsevier

with ex situ NMR [51]. Figure 2.16 shows 27 Al and 31 P MAS NMR spectra of the selected gels. The Al species in the amorphous produces the signal at 7 ppm with a broad shoulder at ca. −9.5 ppm, which gradually grew into tetrahedral Al atoms (at 37 ppm) of the crystalline MgAPO-36 framework. The formation of amorphous aluminophosphate phase in the initial stage was evidenced by the asymmetrical broad 31 P signal at −7.5 ppm. The up-field 31 P signal at ca. −29 ppm appeared upon heating the get at 423 K is ascribed to the tetrahedral P atoms of crystalline MgAPO-36 framework, in consistent with the appearance of a weak diffraction peak characteristic of ATS-type structure in PXRD. For the gel heated for 18 h, the corresponding 31 P spectrum shows a main peak at −29 ppm along with a shoulder peak at −23 ppm, due to P(4Al) and P(Mg, 3Al) units of crystalline MgAPO-36, respectively. The difference in the 31 P chemical shift was ascribed to the P atoms bound to different number of Mg atoms via a bridging oxygen [52, 53], namely an increase in the number of adjacently substituted Mg atoms corresponds to an increase in the 31 P chemical shift. On the basis of the proposed binomial theorem [36], the two P signals agree well with the observed intensity patterns, indicating a random ordering distribution of the magnesium in framework. The 31 P{27 Al} HETCOR experiments demonstrated that two types of microstructure regions, characterized by the partially and fully condensed P species, are found to coexist in the 1.5-h heated (423 K) gel. The detailed local environment around the P species associated with various kinds

78

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

Fig. 2.17 Fitting curves of 31 P signals at −8 ppm (a), −14 ppm (b), −20 ppm (c), −23 ppm (d), and −29 ppm (e) in 1 H–31 P CP MAS experiment on 1.5-h heated gel, and of 31 P signals at −23 and −29 ppm for the final product (f). Reprinted from Ref. [51] by permission of Elsevier

of proton sources (water molecules, template molecules, surface P–OH and Al–OH groups) was probed by 1 H–31 P CP NMR experiments, and the nature of these P species was explored by analyzing the cross-polarization dynamics for each P site. The build-up curves of the five 31 P signals produced through cross-polarization as a function of contact time is shown in Fig. 2.17. The polarization dynamics can be described by the simplified formula [54]. By fitting the CP data, the Tcp values are 0.23, 0.25, 0.48, 0.52, and 0.64 ms for the signals at ca. −8, −14, −20, −23, and − 29 ppm, respectively.

2.4 NMR Strategy in Characterization of Zeolite Synthesis

79

Combined with the PXRD and HETCOR results, different regions (domains) in the 1.5-h heated (423 K) gel was differentiated from the Tcp values for the five P sites, namely the fully condensed domain characterized by the signals at −23 and −29 ppm, belonging to the crystalline region; the partially condensed domain characterized by the signals at −8, −14, and −20 ppm, belonging to the amorphous region. The authors also showed that the detailed information about the Al and P coordination sphere in the amorphous phase like 2-h heated gel can be determined by using 31 P{27 Al}TRAPDOR experiments [55] which otherwise is hard to be obtained by other techniques. In addition to the hydrothermal method, dry-gel conversion (DGC) has received much interest in the synthesis of zeolite and AlPOs since it is particularly effective in producing zeolite crystals with uniform particle size [56, 57] and high metal content [58, 59]. SAPO-34 is an important silicon-substituted aluminophosphate catalyst, showing distinct catalytic property in the methanol-to-olefins conversion. The synthesis of SAPO-34 under dry-gel conversion (DGC) conditions was investigated by Huang et al. [60]. The changes in the structure of solid gel phases were monitored by solid-state NMR techniques in combination with powder XRD and SEM. 31 27 P, Al, 29 Si, and 13 C multi-nuclear NMR analysis was performed on the selected gel during the reaction. The results indicated that semicrystalline precursor with a layered structure assembled by weak non-bonding interactions leads to the crystallization of SAPO-34. Si incorporation into the framework is induced by amorphous aluminosilicate species. It also shows that the steam-assisted conversion and vaporphase transport method have no apparent difference in the reaction pathway. Using a similar strategy, Huang et al. investigated the synthesis of other AlPO [61] and the substituted materials [62–64]. To reveal the crystallization processes under the DGC condition, it is important to understand the role of water vapor in the transformation from initial dry-gel powder to zeolitic framework. The involvement of 17 O-enriched water vapor in crystallization of AlPO4 -11 was monitored by Huang et al. using 17 O solid-state NMR spectroscopy [65]. Figure 2.18 shows the 17 O MAS NMR spectra of selected samples as a function of crystallization time. No 17 O signal is observed in the gel after the first 30 min of heating, indicating that water molecules bound to the solid have not yet exchanged with the 17 O-enriched water in vapor. Heating the initial dry-gel powder for 80 min results in the formation of 17 O(–Al)4 (at 72 ppm) and 17 O(–Al)3 (34 ppm) units in unreacted alumina, while the restricted water molecule bound to the solid produces a signal at −40 ppm. The assignments are supported by a series of 17 O{27 Al}, 17 O{31 P}REDOR as well as 1 H–17 O cross-polarization experiments. After 160-min heating, the formation of P–17 O–H and Al–17 O–P units in the layered phase as an intermediate to AlPO-11 is observed under the broad envelope with two maxima at 33 and -31 ppm, which, however, can only be identified by simulating the overlapping 17 O MAS signals in the 17 O{31 P} REDOR difference spectrum (Fig. 2.19). In combination with 31 P NMR experiments on the selected samples, it is concluded that 17 O atoms are preferably bonded only to the P(2) and P(3) sites in AlPO4 -11 in the initial crystallization (Fig. 2.20). The H-bonding of protonated Di-n-propylamine (structure-directing agent) with the oxygen atoms in the first coordination sphere

80

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

Fig. 2.18 17 O MAS NMR spectra of selected dry-gel samples. Asterisks indicate spinning sidebands. Reprinted from Ref. [65] by permission of American Chemical Society

of P1 site hinders the 17 O exchange on this site, which is explained by a kinetic selectivity. Like in the synthesis study of aluminosilicates, in situ NMR methods are developed to observe the crystallization in the synthesis of aluminophosphates. Taullelle et al. monitored the crystallization process of SAPO-34 under hydrothermal condition [8, 66]. They designed an NMR tube in 10 mm diameter as an autoclave as shown in Fig. 2.21. It withstands temperatures up to 200 °C and pressures up to 50 bar, which guarantees the safety of the hydrothermal reaction in the NMR probe [9, 67, 68]. The synthesis of SAPO-34 was conducted under hydrothermal conditions (170 °C) in the presence of HF, using morpholine as the structure-directing agent. The dissolved species in the liquid phase were examined by 13 C, 19 F, 27 Al, and 31 P solutionstate NMR. The solid phase was isolated from the mixture and analyzed by ex situ XRD. To ensure the synthesis conditions in the NMR tube were comparable to the conventional autoclave synthesis, reference synthesis were performed in Teflon-lined steel autoclaves at 170–190 °C. As shown in Fig. 2.22, four reaction routes have been examined: (a) from a SAPO gel with HF (triclinic SAPO-34), (b) from a SAPO gel without HF (trigonal SAPO-34), (c) from an AlPO4 gel with HF (triclinic AlPO4 -34), and (d) from a slurry containing the layered AlPO4 F prephase (triclinic AlPO4 -34). In this work, 29 Si NMR has not been acquired due to the low sensitivity of 29 Si,

2.4 NMR Strategy in Characterization of Zeolite Synthesis

81

Fig. 2.19 17 O{31 P} REDOR spectra of 160-min dry-gel sample (10 rotor periods were applied at 6.5 kHz (2 Hz)). A total of 65,908 scans were acquired with a recycle time of 0.5 s. The total experimental time was 19 h. a 17 O spin-echo (S 0 ); b REDOR (S); and c REDOR difference (S) spectra. Asterisks indicate spinning sidebands. d Simulated REDOR difference spectrum using two components in a 2.3:1 ratio: (1) Cq = 5.7 MHz, η = 0, δ iso = 65 ppm, 2500 Hz line broadening. (2) Cq = 5.1 MHz, η = 0.55, δ iso = 95 ppm, 2400 Hz line broadening. Reprinted from Ref. [65] by permission of American Chemical Society

which requires long acquisition time and is not applicable at short synthesis time in the NMR tube. According to the NMR results from the dissolved species in situ formed in the synthesis, the authors proposed the crystallization pathways (Fig. 2.23). The dissolution of the amorphous gel produces AlPO4 F species containing 4R type I units. Upon increasing temperature, layered AlPO4 F prephase is formed by alternating stacking of these 4R type I units. The AlPO4 F prophase is subsequently dissolved at temperature higher than 120 °C, producing 4R type II units. 4R type III units are formed by defluorination in which the aluminum coordination is reduced from six to five.

82

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

Fig. 2.20 a Projection of AlPO4 -11 structure along [001] direction with three crystallographically non-equivalent P sites and b illustration of the interaction between protonated DPA and the oxygen atoms bonded to the P at the junction of four- and six-membered ring. Reprinted from Ref. [65] by permission of American Chemical Society

Further defluorination produces 4R type V units, and the condensation of 4R types III and V results in the SAPO-34 with CHA topology. It is supposed that the silicon could be incorporated into framework by substitution of aluminum or phosphorus in the 4R type I units. Similar in situ NMR studies were also performed on the synthesis of AlPO4 -CJ2 by following the evolution of the signal of each atom in the dissolved species containing 19 F, 27 Al, and 31 P nucleus [9]. A particular interest is that organic amine 1,4-diazabicycol-2,2,2-octane is not only used as the structure-directing agent, but also as an in situ pH probe by monitoring the 14 N NMR signals in the synthesis. In order to get more information on the crystallization process, it is highly desirable to monitor the evolution of the species in both solution and solid parts. In situ MAS NMR setup that endures high-temperature and pressure hydrothermal conditions is needed to get reasonable spectral resolution on the solids. Recently, Hu et al.

2.4 NMR Strategy in Characterization of Zeolite Synthesis

83

Fig. 2.21 (Left) Photograph of the NMR tube and its components, and (right) schematic showing the design of the hydrothermal NMR tube. Figure legend: a Teflon stopper, b titanium, c polyimide (Vespel or Torlon), d volume restrictor (Teflon), e Teflon sleeve, and f sample volume. Reprinted from Ref. [67] by permission of EDP Sciences

developed a high-temperature (250 °C) and high-pressure (up to 10 MPa) HTHP MAS rotor, enabling the in situ MAS NMR study of material synthesis under typical hydrothermal conditions [69]. Figure 2.24 shows the schematic diagram of the developed 7.5-mm in situ MAS rotor. The sample cell with a volume up to 300 μL is tightly sealed by O-ring. The spinning rate is up to 4 kHz. Different from previous design [70, 71], no plastic inserts are used within the sample cell space, which show advantages in suppressing the 13 C and 1 H background signals. The NMR analysis of the crystallization of AlPO4 -5 molecular sieves was successfully performed in this HTHP MAS NMR rotor. AlPO4 -5 crystallizes from a mixture of aluminum isopropoxide, H3 PO4 , triethylamine, and water. A small amount of water was used to reduce the influence of the strong signal of excessive H2 O solvent and signal exchanges. The obtained homogenous gel was transferred into (typically 250–300 mg) the HTHP rotor for in situ MAS NMR experiment at 150 °C for 12 h. The XRD indicated that the synthesis of AlPO4 -5 in the NMR rotor could be well reproduced as in the autoclave. Figure 2.25 shows the timeon-stream in situ MAS NMR spectra. At temperature of 150 °C, four-coordinated aluminum appeared at 37–34 ppm ((Fig. 2.25a), while the signal at 46 ppm was due to five-coordinated aluminum complex bonded to HPO4 2− ions in solution. Octahedral coordinated Al species was also observed, as reflected by the broad peak near 12 ppm. In 31 P NMR spectra, the broad peak at −8 to −6 ppm can be ascribed to terminal phosphate (PO3 (OAl)) units. The oscillating of the chemical shift indicated the repeated hydrolysis and condensation reaction during the whole crystallization process, which is accompanied by producing and consuming terminal phosphate. During heating, several short-range ordered structures were formed which produced the broad peaks at −14 to −30 ppm. It is not easy to make the assignment of each signal in 1 H spectra as the proton-exchange at high temperature. The signal at 5.0 ppm

Fig. 2.22 The NMR spectra obtained from the four synthesis routes. Both 3D and stack plots 27 Al spectra are displayed, the former showing the intensity evolution and latter illustrating the chemical shift variations. Reprinted from Ref. [8] by permission of American Chemical Society

84 2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

Fig. 2.23 Proposed synthesis mechanism of SAPO-34, including the equilibriums and synthesis parameters. Reprinted from Ref. [8] by permission of American Chemical Society

2.4 NMR Strategy in Characterization of Zeolite Synthesis 85

86

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

Fig. 2.24 Schematic diagram of 7.5-mm HTHP MAS rotor: (1) ZrO2 rotor sleeve with thread; (2) sample cell; (3) ZrO2 screw; (4) O-ring; (5) Kel-F drive cap. Reprinted from Ref. [69] by permission of American Chemical Society

can be tentatively attributed to hydrogen-bonded proton between phosphate and TEA (structure-directing agent) experiencing proton-exchange with water. The chemical shift of the 1 H NMR signal can be accounted for by the change of pH value. The release of excess phosphate in amorphous gel has happened and then was consumed during the growth of crystalline product, which is confirmed by the 31 P spectra (Fig. 2.25b). This process led to the change of the pH value of the solution. 13 C MAS NMR experiments were also performed to monitor the structure-directing agent TEA in the synthesis. It was shown that the protonated TEA has a strong interaction with the inorganic species in the synthesis and the interactions become stronger as the cross-linking of Al and P proceeds. The formation of the channel-like structure of AlPO4 -5 should be directed by these interactions. Interestingly, the role of water was reflected by the 1 H NMR spectra particularly between 30 and 70 min of nucleation phase; the sharp signal at 4.3 ppm at 30 min grew into well-distinguishable signals at 4–5 ppm at 50–70 min. For example, the appearance of 5.0 ppm 1 H signal indicated the formation of hydrogen-bond interaction between TEA and phosphate, which directed the channel-like structure of AlPO4 -5. Based on the in situ multi-nuclear NMR analysis, the AlPO4 -5 crystallization mechanism was proposed. It is noted that the normalized signal intensities of 31 P and 1 H MAS NMR (Fig. 2.25d) demonstrate an overall decreasing trend with crystallization. The change of Q factor of the probe is supposed to result in the signal loss. In the synthesis, the pH, conductivity, dielectric constant, etc., would induce effects on the Q factor of the probe [72]. Regarding the in situ MAS NMR technique, there

Fig. 2.25 Time-on-stream in situ MAS NMR spectra and their relative signal intensities of synthesis gel crystallized at 150°C: a 27 Al, b 31 P, c 1 H MAS NMR, and d normalized signal intensities (each curve was normalized individually). Asterisks denote spinning sidebands. Reprinted from Ref. [69] by permission of American Chemical Society

2.4 NMR Strategy in Characterization of Zeolite Synthesis 87

88

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

is a trade-off between time resolution on the acquisition and signal-to-noise ratio. The relaxation time for the P species changes remarkably from condensed P(OAl)4 to partially condensed ones, often longer than 100 s for the former and shorter than 5 s for the later. Thus, to ensure the time resolution on each time point would sacrifice the signal-to-noise ratio for the species with long relaxation time, and the real concentration of the species would not be reflected truly by the signal intensity.

2.5 Summary Understanding the synthesis mechanism of zeolites and molecule sieves is critical for developing a route by which designing porous materials with target structure and property becomes possible. Hydrothermal synthesis that is often employed for zeolites is composed of heterogeneous mixtures containing gel, supersaturated solutions, and crystals. The characterization of the synthesis process still remains as a great challenging task. Multi-nuclear NMR spectroscopy is demonstrated to be a powerful technique for identifying the active species and detecting their transformations in the zeolite synthesis. The combination of NMR that provides the atomic-level insights with XRD and SEM/TEM that provide longer-range structural and morphological information has deepened our understanding of the formation mechanisms of zeolites and zeotype materials from precursor solutions to crystals. The in situ NMR techniques allow for monitoring the crystallization process, providing more details about the solution- and solid-state species especially at the early stages of zeolite crystal nucleation and growth. The advances in the in situ NMR with high spatial and time resolution would enable the deep atomic-level insights into the zeolite chemistry to be obtained.

References 1. Barrer RM (1948) 33. Synthesis of a zeolitic mineral with chabazite-like sorptive properties. J Chem Soc (Resumed) (0):127–132. https://doi.org/10.1039/jr9480000127 2. Kerr GT (1963) Zeolite ZK-5: a new molecular sieve. Science 140(3574):1412–1412. https:// doi.org/10.1126/science.140.3574.1412 3. Baerlocher C, McCusker LB, Olson DH (eds) (2007) Atlas of zeolite structure types, 6th edn. Elsevier, Amsterdam 4. Beck JS, Vartuli JC, Roth WJ, Leonowicz ME, Kresge CT, Schmitt KD, Chu CTW, Olson DH, Sheppard EW, McCullen SB, Higgins JB, Schlenker JL (1992) A new family of mesoporous molecular sieves prepared with liquid crystal templates. J Am Chem Soc 114(27):10834–10843. https://doi.org/10.1021/ja00053a020 5. Zhao D, Feng J, Huo Q, Melosh N, Fredrickson GH, Chmelka BF, Stucky GD (1998) Triblock copolymer synthesis of mesoporous silica with periodic 50 to 300 angstrom pores. Science 279(5350):548–552. https://doi.org/10.1126/science.279.5350.548 6. Walton RI, O’Hare D (2000) Watching solids crystallise using powder diffraction. Chem Commun 23:2283–2291. https://doi.org/10.1039/B007795J

References

89

7. Francis RJ, O’Hare D (1998) The kinetics and mechanisms of the crystallisation of microporous materials. J Chem Soc, Dalton Trans 19:3133–3148. https://doi.org/10.1039/A802330A 8. Vistad ØB, Akporiaye DE, Taulelle F, Lillerud KP (2003) In situ nmr of sapo-34 crystallization. Chem Mater 15(8):1639–1649. https://doi.org/10.1021/cm021317w 9. Taulelle F, Haouas M, Gerardin C, Estournes C, Loiseau T, Ferey G (1999) NMR of microporous compounds: from in situ reactions to solid paving. Colloids Surf A 158(1–2):299–311. https:// doi.org/10.1016/S0927-7757(99)00200-9 10. Serre C, Lorentz C, Taulelle F, Férey G (2003) Hydrothermal synthesis of nanoporous metalofluorophosphates. 2. In situ and ex situ 19F and 31P NMR of nano- and mesostructured titanium phosphates crystallogenesis. Chem Mater 15 (12):2328–2337. https://doi.org/10.1021/ cm021347z 11. Huang Y, Machado D, Kirby CW (2004) A study of the formation of molecular sieve SAPO-44. J Phys Chem B 108(6):1855–1865. https://doi.org/10.1021/jp0308256 12. Huang Y, Richer R, Kirby CW (2003) Characterization of the gel phases of AlPO4 -11 molecular sieve synthesis by solid-state NMR. J Phys Chem B 107(6):1326–1337. https://doi.org/10.1021/ jp021878a 13. Huang Y, Demko BA, Kirby CW (2003) Investigation of the evolution of intermediate phases of AlPO4 -18 molecular sieve synthesis. Chem Mater 15(12):2437–2444. https://doi.org/10. 1021/cm021728c 14. Twu J, Dutta PK, Kresge CT (1991) Raman spectroscopic studies of the synthesis of faujasitic zeolites: comparison of two silica sources. Zeolites 11(7):672–679. https://doi.org/10.1016/ S0144-2449(05)80170-8 15. Grandjean D, Beale AM, Petukhov AV, Weckhuysen BM (2005) Unraveling the crystallization mechanism of CoAPO-5 molecular sieves under hydrothermal conditions. J Am Chem Soc 127(41):14454–14465. https://doi.org/10.1021/ja054014m 16. Walton RI, Smith RI, O’Hare D (2001) Following the hydrothermal crystallisation of zeolites using time-resolved in situ powder neutron diffraction. Micropor Mesopor Mat 48(1–3):79–88. https://doi.org/10.1016/S1387-1811(01)00333-X 17. Cundy CS, Cox PA (2003) The hydrothermal synthesis of zeolites: history and development from the earliest days to the present time. Chem Rev 103(3):663–702. https://doi.org/10.1021/ cr020060i 18. Milton RM (1959). US Patent 2,882,244 19. Wadlinger RL Kerr GT, Rosinski EJ (1967). US Patent 3,308,069 20. Argauer RJ Landolt GR (1972). US Patent 3,702,886 21. Flanigen EM, Patton RL, Wilson ST (1988) Studies in surface science and catalysis 37:13 22. Grand J, Awala H, Mintova S (2016) Mechanism of zeolites crystal growth: new findings and open questions. CrystEngComm 18(5):650–664. https://doi.org/10.1039/C5CE02286J 23. Kerr GT (1966) Chemistry of crystalline aluminosilicates. I. Factors affecting the formation of zeolite A. J Phys Chem 70(4):1047–1050. https://doi.org/10.1021/j100876a015 24. Ciric J (1968) Kinetics of zeolite A crystallization. J Colloid Interf Sci 28(2):315–324. https:// doi.org/10.1016/0021-9797(68)90135-5 25. Breck DW (1964) Crystalline molecular sieves. J Chem Educ 41(12):678. https://doi.org/10. 1021/ed041p678 26. Derouane EG, Determmerie S, Gabelica Z, Blom N (1981) Synthesis and characterization of ZSM-5 type zeolites I. Physico-chemical properties of precursors and intermediates. Appl Catal 1(3):201–224. http://dx.doi.org/10.1016/0166-9834(81)80007-3 27. Zhdanov SP (1974). In: Flanigen E M SLB, eds. (ed) Molecular sieve zeolites-I. American Chemical Society, Washington D C, pp 20–43 28. Smith JV (1988) Topochemistry of zeolites and related materials. 1. Topology and geometry. Chem Rev 88(1):149–182. https://doi.org/10.1021/cr00083a008 29. McNicol BD, Pott GT, Loos KR, Mulder N (1973) Spectroscopic studies of zeolite synthesis: evidence for a solid-state mechanism. In: Molecular sieves, vol 121. Advances in Chemistry, vol 121. American Chemical Society, pp 152–161. doi:https://doi.org/10.1021/ba-1973-0121. ch012

90

2 Solid-State NMR Studies of Zeolites and Zeotype Materials Synthesis

30. Thomas JM, Bursill LA (1980) Amorphous Zeolites. Angew Chem Int Ed Engl 19(9):745–746. https://doi.org/10.1002/anie.198007451 31. Epping JD, Chmelka BF (2006) Nucleation and growth of zeolites and inorganic mesoporous solids: molecular insights from magnetic resonance spectroscopy. Curr Opin Colloid In 11(2–3):81–117. https://doi.org/10.1016/j.cocis.2005.12.002 32. Engelhardt G, Fahlke B, Mägi M, Lippmaa E (1983) High-resolution solid-state 29Si and 27Al n.m.r. of aluminosilicate intermediates in zeolite A synthesis. Zeolites 3(4):292–294. http://dx. doi.org/10.1016/0144-2449(83)90170-7 33. Engelhardt G, Fahlke B, Mägi M, Lippmaa E (1985) High-resolution solid-state 29Si and 27Al n.m.r. of aluminosilicate intermediates in the synthesis of zeolite A. Part II. Zeolites 5 (1):49–52. http://dx.doi.org/10.1016/0144-2449(85)90012-0 34. Barrer RM (1982) The hydrothermal chemistry of zeolites. Academic Press, London 35. Ogura M, Kawazu Y, Takahashi H, Okubo T (2003) Aluminosilicate species in the hydrogel phase formed during the aging process for the crystallization of FAU zeolite. Chem Mater 15(13):2661–2667. https://doi.org/10.1021/cm0218209 36. Engelhardt G, Michel D (1987) High resolution solid-state NMR of silicates and zeolites. Wiley, Chichester 37. Ginter DM, Radke CJ, A.T. Bell (1989). Stud Surf Sci Catal 49A:161 38. Chang CD, Bell AT Studies on the mechanism of ZSM-5 formation. Catal Lett 8(5):305–316. https://doi.org/10.1007/bf00764192 39. Burkett SL, Davis ME (1994) Mechanism of structure direction in the synthesis of Si-ZSM-5: an investigation by intermolecular 1H-29Si CP MAS NMR. J Phys Chem 98(17):4647–4653. https://doi.org/10.1021/j100068a027 40. Lefebvre F, Sacerdote-Peronnet M, Mentzen BF (1993) C R Acad Sci Paris Ser 2 316:1549 41. Chao K-J, Lin J-C, Wang Y, Lee GH (1986) Single crystal structure refinement of TPA ZSM-5 zeolite. Zeolites 6(1):35–38. https://doi.org/10.1016/0144-2449(86)90009-6 42. Gies H, Marker B (1992) The structure-controlling role of organic templates for the synthesis of porosils in the systems SiO2/template/H2O. Zeolites 12(1):42–49. https://doi.org/10.1016/ 0144-2449(92)90008-D 43. Shi J, Anderson MW, Carr SW (1996) Direct observation of zeolite A synthesis by in situ solid-state NMR. Chem Mater 8(2):369–375. https://doi.org/10.1021/cm950028n 44. Knight CTG (1990) Are zeolite secondary building units really red herrings? Zeolites 10(2):140–144. https://doi.org/10.1016/0144-2449(90)90036-Q 45. Miladinovi´c Z, Zakrzewska J, Kovaˇcevi´c B, Baˇci´c G (2007) Monitoring of crystallization processes during synthesis of zeolite A by in situ 27Al NMR spectroscopy. Mater Chem Phys 104(2–3):384–389. https://doi.org/10.1016/j.matchemphys.2007.03.029 46. Walton RI, Millange F, O’Hare D, Davies AT, Sankar G, Catlow CRA (2001) An in situ energydispersive x-ray diffraction study of the hydrothermal crystallization of zeolite A. 1. Influence of reaction conditions and transformation into sodalite. J Phys Chem B 105(1):83–90. https:// doi.org/10.1021/jp002711p 47. Ruren X (2007) Chemistry of zeolites and related porous materials: synthesis and structure. Wiley, Singapore (Asia) ©2007 48. Xu J, Chen L, Zeng D, Yang J, Zhang M, Ye C, Deng F (2007) Crystallization of AlPO4 5 aluminophosphate molecular sieve prepared in fluoride medium: a multinuclear solid-state NMR study. J Phys Chem B 111(25):7105–7113. https://doi.org/10.1021/jp0710133 49. Hartmann P, Vogel J, Schnabel B (1994) The influence of short-range geometry on the 31P chemical-shift tensor in protonated. Phosphates. J Magn Reson, Ser A 111 (1):110–114. doi:http://dx.doi.org/10.1006/jmra.1994.1234 50. Gougeon RD, Brouwer EB, Bodart PR, Delmotte L, Marichal C, Chézeau J-M, Harris RK (2001) Solid-State NMR Studies of the As-Synthesized AlPO4 -5/TPAF Microporous Aluminophosphate. J Phys Chem B 105(49):12249–12256. https://doi.org/10.1021/jp0111214 51. Xu J, Zhou D, Song X, Chen L, Yu J, Ye C, Deng F (2008) Crystallization of magnesium substituted aluminophosphate of type-36 as studied by solid-state NMR spectroscopy. Micropor Mesopor Mat 115(3):576–584. https://doi.org/10.1016/j.micromeso.2008.02.037

References

91

52. Barrie PJ, Klinowski J (1989) Ordering in the framework of a magnesium aluminophosphate molecular sieve. J Phys Chem 93(16):5972–5974. https://doi.org/10.1021/j100353a007 53. Deng F, Yue Y, Xiao T, Du Y, Ye C, An L, Wang H (1995) Substitution of aluminum in aluminophosphate molecular sieve by magnesium: a combined NMR and XRD study. J Phys Chem 99(16):6029–6035. https://doi.org/10.1021/j100016a045 54. Kolodziejski W, Klinowski J (2002) Kinetics of cross-polarization in solid-state NMR: a guide for chemists. Chem Rev 102(3):613–628. https://doi.org/10.1021/cr000060n 55. Zhou D, Xu J, Yu J, Chen L, Deng F, Xu R (2006) Solid-state NMR spectroscopy of anionic framework aluminophosphates: a new method to determine the Al/P ratio. J Phys Chem B 110(5):2131–2137. https://doi.org/10.1021/jp056335q 56. Hari Prasad Rao PR, Ueyama K, Matsukata M (1998) Crystallization of high silica BEA by dry gel conversion. Appl Catal A 166(1):97–103. https://doi.org/10.1016/S0926-860X(98)80005-7 57. Rao PRHP, Matsukata M (1996) Dry-gel conversion technique for synthesis of zeolite BEA. Chem Commun 12:1441–1442. https://doi.org/10.1039/cc9960001441 58. Arnold A, Hunger M, Weitkamp J (2004) Dry-gel synthesis of zeolites [Al]EU-1 and [Ga]EU-1. Micropor Mesopor Mat 67(2–3):205–213. https://doi.org/10.1016/j.micromeso.2003.10.010 59. Zhang LR, Gavalas G (1999) Vapor-phase transport synthesis of ZnAPO-34 molecular sieve. Chem Commun 1:97–98. https://doi.org/10.1039/a809399g 60. Zhang L, Bates J, Chen D, Nie H-Y, Huang Y (2011) Investigations of formation of molecular sieve SAPO-34. J Phys Chem C 115(45):22309–22319. https://doi.org/10.1021/jp208560t 61. Chen B, Huang Y (2011) Formation of microporous material AlPO4-18 under dry-gel conversion conditions. Micropor Mesopor Mat 143(1):14–21. https://doi.org/10.1016/j.micromeso. 2011.02.002 62. Yan Z, Chen B, Huang Y (2009) A solid-state NMR study of the formation of molecular sieve SAPO-34. Solid State Nucl Mag 35(2):49–60. https://doi.org/10.1016/j.ssnmr.2008.12.006 63. Chen B, Huang Y (2009) Dry gel conversion synthesis of SAPO- and CoAPO-based molecular sieves by using structurally related preformed AlPO precursors as the starting materials. Micropor Mesopor Mat 123(1–3):71–77. https://doi.org/10.1016/j.micromeso.2009.03.025 64. MacIntosh AR, Huang Y (2013) Formation of and silicon incorporation in SAPO-5 synthesized via dry-gel conversion. Micropor Mesopor Mat 182:40–49. https://doi.org/10.1016/j. micromeso.2013.08.016 65. Chen B, Huang Y (2006) 17O solid-state NMR spectroscopic studies of the involvement of water vapor in molecular sieve formation by dry-gel conversion. J Am Chem Soc 128(19):6437–6446. https://doi.org/10.1021/ja060286t 66. Vistad ØB, Akporiaye DE, Taulelle F, Lillerud KP (2003) Morpholine, an in situ 13C NMR pH meter for hydrothermal crystallogenesis of SAPO-34. Chem Mater 15(8):1650–1654. https://doi.org/10.1021/cm021318o 67. Haouas M, Gerardin C, Taulelle F, Estournes C, Loiseau T, Férey GJ (1998) J Chim Phys Chim Biol 95:302 68. Taulelle F (2001) Crystallogenesis of microporous metallophosphates. Curr Opin Solid State Mater Sci 5(5):397–405. https://doi.org/10.1016/S1359-0286(01)00037-7 69. Zhao Z, Xu S, Hu MY, Bao X, Hu JZ (2016) In Situ high temperature high pressure MAS NMR study on the crystallization of AlPO4 -5. J Phys Chem C 120(3):1701–1708. https://doi. org/10.1021/acs.jpcc.5b11294 70. Yonker CR, Linehan JC (2005) The use of supercritical fluids as solvents for NMR spectroscopy. Prog Nucl Mag Res Sp 47(1–2):95–109. https://doi.org/10.1016/j.pnmrs.2005.08.002 71. Turcu RVF, Hoyt DW, Rosso KM, Sears JA, Loring JS, Felmy AR, Hu JZ (2013) Rotor design for high pressure magic angle spinning nuclear magnetic resonance. J Magn Reson 226:64–69. https://doi.org/10.1016/j.jmr.2012.08.009 72. Gerardin C, Haouas M, Lorentz C, Taulelle F (2000) NMR quantification in hydrothermal in situ synthesis. Magn Reson Chem 38(6):429–435. https://doi.org/10.1002/1097458x(200006)38:6%3c429:aid-mrc671%3e3.0.co;2-s

Chapter 3

Solid-State NMR Characterization of Framework Structure of Zeolites and Zeotype Materials

Abstract This chapter introduces the application of solid-state NMR to characterize the framework structure of zeolite and zeotype materials. Zeolites are inorganic crystallites containing pores and cavities of molecular dimensions with well-defined structures. The framework of zeolite is composed of tetrahedra (TO4 , T = Si, Al, B, P, etc.), which can be comprehensively characterized by the well-established and robust solid-state NMR techniques. Chemical environment of the metal or nonmetal elements in zeolites and zeotype materials could be studied by the multinuclear MAS NMR spectroscopy including 29 Si, 27 Al, 31 P, 17 O NMR. Additionally, the detailed information about coordination, connectivity, and framework ordering can be obtained from multi-nuclear and two-dimensional NMR spectroscopy as well as distance constraints’ measurement. Moreover, the structure features and communication of cages and channels in porous materials can be extracted by 129 Xe NMR spectroscopy. Keywords Zeolite · Solid-state NMR · Structural characterization · Framework topology · Chemical shift assignments · Spatial proximity · Distance measurement

3.1 Introduction Zeolites are inorganic crystallites containing micropores and cavities of molecular dimensions with well-defined structures. The framework of zeolite is composed of tetrahedra (TO4 , T = Si, Al, B, P, etc.). These TO4 serving as the primary building units of zeolite structure can then link together by their corners to form different membered rings which are the so-called secondary building units (SBUs). The SBU can be linked to form cages or channels within the structure. Although all zeolites are constructed from TO4 tetrahedra, the different ways in which they can be connected lead to a rich variety of zeolite structures [1]. Incorporation of heteroatoms, such as Al into siliceous zeolites, will introduce negative charge in the framework which is counterbalanced by protons, thus generating Brønsted acid sites. However, the Lewis acid sites can be formed by either the dealumination of H-form zeolites or introduction of metal species into zeolites and zeotype materials [2]. Due to their unique pore © Springer Nature Singapore Pte Ltd. 2019 J. Xu et al., Solid-State NMR in Zeolite Catalysis, Lecture Notes in Chemistry 103, https://doi.org/10.1007/978-981-13-6967-4_3

93

94

3 Solid-State NMR Characterization of Framework Structure …

structures and acid–base property, zeolites are widely used in petrochemical industry and fine chemical industry as catalysts, adsorbents, and ion-exchangers [3]. Solid-state NMR is a well-established tool to explore the chemical environments of the observed nuclei, such as 1 H, 17 O, 27 Al, and 29 Si in zeolites [4]. In this chapter, the utilization of multi-nuclear and multi-dimensional solid-state NMR spectroscopy to characterize zeolite structure is briefly introduced.

3.2 Solid-State NMR Characterization of Zeolite Framework Structure 3.2.1

27 Al

MAS NMR

Solid-state 27 Al and 29 Si MAS NMR have been widely used to characterize the local environments in zeolites such as HY, HZSM-5, and Beta in hydrated or dehydrated state [5]. The coordination environments of aluminum species in zeolite could be clearly distinguished from 27 Al MAS NMR spectroscopy [6]. As shown in Fig. 3.1, four-, five-, and six-coordinate Al resonances at ca. 51–65 ppm, ca. 30–40 ppm, and ca. −10 to 15 ppm, respectively. Highly distorted Al sites, especially in the dehydrated zeolites, are often subject to very large second-order quadrupole interactions and may appear as unusually broad signals, which is beyond NMR detection. These Al sites are often considered as “invisible” aluminum [6]. Figure 3.2 shows the 27 Al MAS NMR spectra of hydrated H–Y zeolites with calcination treatments at various designated temperatures. In the 27 Al MAS NMR spectrum of parent hydrated H–Y zeolite as shown in Fig. 3.2a, only a single peak

Fig. 3.1 Ranges of 27 Al NMR chemical shifts for four-, five-, and six-coordinate Al in zeolites. Reproduced from Ref. [6] by permission of American Chemical Society

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

95

Fig. 3.2 27 Al MAS NMR spectra of parent HY (a) and dealuminated HY zeolites upon calcination treatment at 400 (b), 500 (c), 600 (d), and 700 °C (e). Reproduced from Ref. [7] by permission of American Chemical Society

at 60 ppm arising from four-coordinate framework Al species can be observed. However, in calcined H–Y zeolite as shown in Fig. 3.2b–e, additional resonances appear at 30 and −2 ppm which can be assigned to extra-framework five- and sixcoordinate Al species, respectively [7]. They gradually grow up at the expense of the signal of four-coordinate framework Al with the increasing calcination temperature, indicative of a progressive increase of dealumination [7]. 27 Al MAS NMR can also be employed to study the aluminum states in mesoporous materials such as Al-MCM-41 and Al-SBA-15. Figure 3.3 shows the 27 Al MAS NMR spectra of calcined and hydrated Al-SBA-15 zeolite [8]. The intense resonance at 54 ppm can be assigned to four-coordinate framework Al formed in the mesoporous wall of Al-SBA-15. The concentrations of the four-coordinate framework Al atoms in the mesoporous wall were determined from the corresponding 27 Al MAS NMR spectra. It is obvious that the intensity of the 54 ppm signal gradually increases with the decrease of silicon-to-aluminum ratios, indicating that the amount of Al atoms that are incorporated into the mesoporous wall of SBA-15 increases as well. The other signal at 0 ppm is due to six-coordinate Al caused by the sample calcination. Subsequent adsorption of ammonia on these calcined and hydrated samples can convert the six-coordinate Al back to four-coordinate framework Al. It is interesting that no signal at ca. 30 ppm, due to five-coordinate aluminum, can be observed in the 27 Al MAS NMR spectra of the Al-SBA-15 materials [8]. In most of the NMR studies of framework and extra-framework Al species, the experiments are often performed on hydrated samples. Since the extra-framework aluminum species in dealuminated zeolites can be cationic compounds, such as Al3+ , AlO+ , Al(OH)2+ , and Al(OH)+2 , or neutral and polymerized compounds, such as AlO(OH), Al(OH)3 , and Al2 O3 [9], the presence of water in the hydrated zeolites

96

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.3 27 Al MAS spectra of Al-SBA-15 materials with various silicon-to-aluminum ratios: a 10, b 20, and c 40. The spectra were recorded on the calcined and hydrated samples (left) and after subsequent adsorption of ammonia onto the calcined and hydrated samples (right). Reproduced from Ref. [8] by permission of Elsevier

have significant influence on the nature of the extra-framework Al. It is desirable to investigate the extra-framework Al in non-hydrated zeolites. The simple NMR method for studying aluminum atoms in non-hydrated zeolites is the static 27 Al spin-echo NMR experiment, from which both the quadrupole coupling constants (QCCs) and the relative amounts of the corresponding species can be determined [10]. A series of HY zeolite dealuminated by steaming with different water vapor pressure (7.4, 31.3, and 81.5 kPa) in non-hydrous state were investigated by 27 Al spin-echo NMR [11]. As shown in Fig. 3.4, signal 1 has a QCC value of 14.4–15.0 MHz, characteristic for four-coordinate framework Al of the bridging hydroxyl (SiOHAl) groups. However, signal 2 with a QCC value of 6.4–9.1 MHz is due to the fourcoordinate framework Al which is charge-balanced by residual sodium cation. It is revealed that the relative signal intensity of the framework Al species in the bridging OH groups decreases dramatically with the increase of dealumination by steaming with higher water vapor pressure. Meantime, the signal 2 grows and is characterized by a QCC of ca. 9 MHz in the strongly dealuminated samples, indicating the formation of extra-framework aluminum species. Since Al is a spin 5/2 nucleus, the 27 Al MAS NMR spectrum is often significantly affected by the second-order quadrupolar interaction. The using of high magnetic field is an effective approach to reduce the quadrupolar interaction because it is inversely proportional to the strength of external magnetic field [12]. Figure 3.5 shows the 27 Al MAS NMR spectra of two HBEA zeolites (HBEA150a, Si/Al = 71, and HBEA150b, Si/Al = 75) acquired at different (7.05–19.97 T) magnetic fields [13]. The signals around 50–60 ppm can be assigned to the four-coordinate framework Al, and the signal at ~0 ppm is due to the six-coordinate extra-framework Al. The signal from the extra-framework Al does not show much difference at different magnetic fields. For the tetrahedral framework Al atoms at ~50–60 ppm, the overlapped signals

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

97

Fig. 3.4 27 Al spin-echo NMR spectra of anhydrated HY zeolites: a parent sample, b deH–Y/7.4, c deH–Y/31.1, and d deH–Y/81.5. Experimental spectra (top) are compared with simulated spectra (bottom). Reproduced from Ref. [10] by permission of American Chemical Society

are observed at low field of 7.05 T, which, however, become two resolved signals at higher field of 11.75 T and 19.97 T for both HBEA150 samples. This demonstrates the broadening effect in 27 Al spectra caused by second-order quadrupolar interactions at low magnetic field. Although the 27 Al NMR resolution can be increased by working at higher magnetic fields, very high-field NMR spectrometers are not available for most of the researchers. An alternative way to reduce the second-order effects is the utilization of double-rotation (DOR) NMR [14]. The ultra-large pore (18-membered rings) aluminophosphate molecular sieve VPI-5 (VFI) consists of alternating AlO4 and PO4 tetrahedral units [15] as shown in Fig. 3.6. High-resolution synchrotron powder diffraction data show three different crystallographic sites [15]. The 27 A1 MAS NMR spectrum (Fig. 3.7) of VPI-5 shows the presence of six-coordinate Al at about 19 ppm and one four-coordinate Al atom at about 41 ppm, which is contradictory to the X-ray findings. It is most likely that the observed four-coordinate Al consists of two heavily overlapped signals due to relatively small differences in the corresponding crystallographic sites or to the second-order quadrupolar broadening [15].

98

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.5 27 Al MAS NMR spectra of HBEA zeolites at magnetic field strengths from 7.05, 11.75, to 19.97 T: a HBEA150a and b HBEA150b. Asterisks denote background signal from the empty rotor. Reproduced from Ref. [13] by permission of American Chemical Society

In Fig. 3.7, the DOR spectrum of hydrated VPI-5 shows two partially resolved signals at 41.2 (a) and 40.4 ppm (b), as well as a single signal at −18.4 ppm (c). The intensities of the signals, taken into account the respective sidebands, are in a 1:1:1 ratio. The quadrupolar broadened signal is effectively averaged out into narrow and completely symmetrical lines at DOR conditions, which correspond to two different four-coordinate Al sites. Multiple-quantum magic-angle spinning (MQ-MAS) is another powerful solidstate NMR method for obtaining high-resolution spectra of half-integer spin quadrupolar nuclei [16]. It is performed as a two-dimensional NMR experiment, establishing the correlations between isotropic (high resolution) dimension and anisotropic central transition MAS dimension. Two-dimensional (2D) 27 Al MQ MAS NMR provides a more resolved pattern compared with one-dimensional (1D) 27 Al MAS NMR after averaging out the second-order quadrupole interaction. Figure 3.8 shows 27 Al 3Q MAS spectra of hydrated H–Y zeolites calcined at different temperatures [17]. Only four-coordinate framework Al species are present in the parent HY. Calcination at increasing temperature leads to a progressive formation of extraframework Al species: six-coordinate extra-framework aluminum (EFAL) at 0 ppm, five-coordinate EFAL at 30 ppm, and four-coordinate EFAL at 56 ppm. The isotropic chemical shift δiso as well as the second-order quadrupolar interaction parameter PQ of each Al species can be calculated according to the position of signals in the 2D MQ MAS spectra by the following Formulas (3.1) and (3.2) [18].

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

99

Fig. 3.6 The VPI-5 framework structure. Top, [001] projection, where the black lines represent oxygen atoms bonded to aluminum and phosphorus atoms at the vertices. Centre, the octahedral coordination around the Al I site. Bottom, the hexagonal unit cell. Reproduced from Ref. [15] by permission of American Chemical Society

δiso = (17δF1 + 10δF2 )/27  PQ = CQCC 1 +

η2 = 3



17 ν 2 (δF1 − δF2 ) 162000 L

(3.1) 1/2 (3.2)

Combined with 27 Al MAS NMR, 27 Al MQ MAS NMR has been extensively used to investigate the dealumination process, water hydration process, and the aluminum states in various zeolites [19–21].

3.2.2

29 Si

MAS NMR

There are five different possible environments for a silicon atom in zeolite framework: Si(OAl)4 , Si(OAl)3 (OSi), Si(OAl)2 (OSi)2 , Si(OAl)(OSi)3 , and Si(OSi)4 , which can

100

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.7 27 A1NMR spectra of VPI-5 acquired at 9.4 T: the MAS (spinning rate = 15 kHz) and the DOR (outer rotor spinning rate = 1 kHz) spectra. Peaks not indicated by arrows in the DOR spectrum are spinning sidebands. Reproduced from Ref. [15] by permission of American Chemical Society

be denoted as Si(nAl) with n ≤ 4. 29 Si NMR chemical shifts normally vary for different chemical environments in the zeolite framework. Thus, the local framework structure characterized by different chemical environments in the zeolite can be reflected from the 29 Si NMR chemical shifts [22, 23]. Figure 3.9 shows the range of 29 Si chemical shift for each type Si(nAl) unit [5]. Because of the absence of Al–O–Al linkages and the presence of 0.25 Al atoms in each Si–O–Al linkage in a Si(nAl) unit, the Si/Al ratio of a zeolite can be calculated from the 29 Si MAS NMR spectra on the basis of Formula (3.3) [24, 25]. Si/Al =

4  n=0

ISi(nAl)

 4 

  0.25n ISi(nAl)

(3.3)

n=0

where ISi(nAl) is the integrated intensity of the NMR signal corresponding to the Si(nAl) unit. By comparing the values of (Si/Al) from NMR with the results of chemical analysis, the amount of extra-framework aluminum in zeolites can be determined. It should be noted that the 29 Si NMR signals from Si[3Si, 1Al] and Si[3Si, 1OH] structural units are difficult to be discriminated from one another, since they share the similar chemical shift at around −105 ppm [26], rending the uncertainty for the calculation of Si/Al ratio. Figure 3.10 shows the 29 Si MAS NMR spectrum of H–Y zeolite upon calcination treatments at different temperatures [7]. Four different peaks at −89, −95, −101, and −106 ppm arising from Si[3Al, 1Si], Si[2Al, 2Si], Si[1A1,3Si] and Si[0Al,4Si], respectively, could be clearly resolved. According to Formula (3.3), the framework

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

101

Fig. 3.8 27 Al 3Q MAS NMR spectra of parent HY zeolite (a) and dealuminated HY zeolites upon calcination treatment at 500 (b), 600 (c), and 700 °C (d). Spectra were recorded on hydrated samples. Reproduced from Ref. [17] by permission of Wiley

Si/Al ratio could be quantitatively determined after careful deconvolution of these four different peaks. The corresponding framework Si/Al ratios were determined to be 2.8, 3.0, 3.5, 4.6, and 5.3, for parent HY and HY zeolites calcined at 400, 500, 600, and 700 °C [7]. In addition, 29 Si MAS NMR is also suitable for observing the chemically distinct and the crystallographic non-equivalent silicon sites in a topologically ordered framework of zeolite and the mapping of various sites to specific tetrahedral locations in the unit cell [27, 28]. 29 Si isotropic NMR chemical shifts may vary for different chemical environments in the zeolite framework. Thus, the topology framework structure can be reflected from the 29 Si NMR chemical shifts [29]. Hammond et al. [30] demonstrated that the

102

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.9 29 Si chemical shift ranges for Si(nAl) building blocks in zeolites. Reproduced from Ref. [5] by permission of American Chemical Society

framework structure of N-doped zeolites could be elucidated from 29 Si MAS NMR spectra in conjunction with quantum chemical calculations. Figure 3.11 shows the 29 Si NMR spectra of nitrogen-substituted HY zeolites. By using 29 Si MAS NMR spectra in conjunction with quantum chemical calculation, the 29 Si resonances at −89, −80, and −68 ppm have been ascribed to SiNH2 Al, Al–OH–Si–NH2 –Al, and Si(NH2 Al)2 , respectively. The framework structure of newly designed N-doped zeolites could be elucidated accordingly. For the pure siliceous zeolite, the 29 Si NMR chemical shift strongly depends on chemical environment of different silicon sites, which could be further verified from DFT theoretical calculations. Other related work in which the zeolite structures were resolved from 29 Si NMR chemical shifts and scalar couplings in conjunction with quantum chemical calculations was reported as well [31, 32]. Cadars et al. [32] proposed that 29 Si J-coupling interaction could be used as a sensitive probe for the local structure of zeolite frameworks and offer new opportunities for refining and solving complicated zeolite structures. As shown in Fig. 3.12, the one-bond 29 Si–29 Si J-coupling constant for each pair could be extracted

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

103

Fig. 3.10 29 Si MAS NMR spectra of parent HY (a) and dealuminated HY zeolites upon calcination treatment at 400 (b), 500 (c), 600 (d), and 700 °C (e). Reproduced from Ref. [7] by permission of American Chemical Society

from 2D INADEQUATE 29 Si{29 Si} NMR measurements. It was also shown that the 2 J(Si–O–Si) couplings are strongly related to the local framework topology [32]. 29 Si MAS NMR can also be employed to extract structural information on mesoporous materials such as Al-MCM-41 and Al-SBA-15 [8, 33]. Figure 3.13 shows the 29 Si MAS NMR spectra of Al-SBA-15 samples with various Si/Al ratios compared with pure siliceous SBA-15 material, and a low-field shift of 1 ppm can be observed after the introduction of aluminum into the framework of SBA-15. In the corresponding 1 H/29 Si CP/MAS NMR spectra, the three resonances at ca. −90, −100, and −110 ppm can be assigned to Q2 , Q3 , and Q4 sites, respectively, where n in Qn denotes the number of the neighboring silicon atoms that are connected with an oxygen bridge. The Q3 site probably results from two structure units including Si(OSi)3 (OH) and Si(OSi)3 (OAl), and the Q2 site stems from either Si(OSi)2 (OH)2 or Si(OSi)2 (OAl)(OH) [8]. The intramolecular distance constraints are of great importance for solving the 3D topology structure of zeolites. To establish the 29 Si–29 Si correlations and probe the long-range Si–Si distances in the zeolite frameworks, Kristiansen et al. introduced a new 29 Si solid-state MAS NMR experiment by incorporating the 29 Si DQ MAS homonuclear dipolar recoupling sequence SR2611 4 [34]. In the 2D NMR spectrum of a zeolite Sigma-2 with natural abundance as shown in Fig. 3.14a, the spatial proximity among different 29 Si sites has been demonstrated. In addition, long-range 29 Si–29 Si distances of different Si pairs can be elucidated from the 29 Si DQ bulid-up curves as a function of homonuclear recoupling time. Moreover, as

104

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.11 29 Si NMR spectra of HY zeolite with 0% (top), 20% (below) of oxygen substituted for NH groups. Reproduced from Ref. [29] by permission of American Chemical Society

Fig. 3.12 Probing local structures of siliceous zeolite frameworks by solid-state NMR and firstprinciples calculations of 29 Si–O–29 Si scalar couplings. Reproduced from Ref. [32] by permission of Royal Society of Chemistry

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

105

Fig. 3.13 29 Si MAS (left) and CP/MAS (right) spectra of Al-SBA-15 materials with various siliconto-aluminum ratios: (a) 10, (b) 20, and (c) 40. Reproduced from Ref. [8] by permission of Elsevier

Fig. 3.14 29 Si DQ correlation spectra of Sigma-2 obtained with (a) the SR2611 4 dipolar recoupling sequence and (b) the J-coupling-based refocused INADEQUATE experiment. Reproduced from Ref. [28] by permission of American Chemical Society

manifested in Fig. 3.14b, the two-bond 29 Si–29 Si connectivity in Sigma-2 could be obtained through 2D J-coupling-based refocused INADEQUATE experiment. Both the spatial proximity and bond connection information can be achieved separately from the 29 Si–29 Si correlations through dipolar and J-coupling interactions [28].

106

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.15 Structure determination strategy for zeolite crystal structures by a combination of 29 Si DQ MAS NMR spectroscopy and powder X-ray diffraction. Reproduced from Ref. [27] by permission of American Chemical Society

Determination of zeolite topology structure usually relies on powder X-ray diffraction (XRD) approach. However, it is difficult for powder XRD to solve the topological structure of zeolites which cannot form single crystals. As is well known, the distance constraints are very important for 3D de novo structure determination of proteins through solid-state NMR [35–40]. Similarly, short- and long-range 29 Si–29 Si distance constraints are of great importance to determine the 3D structure of zeolites. 29 Si–29 Si distances of different Si pairs can be elucidated from the 29 Si DQ bulidup curves as a function of homonuclear recoupling time. By incorporation with the unit cell parameters and space group from the powder XRD, the 3D topology structure of zeolites can be accurately determined based on the long-range and shortrange 29 Si–29 Si distance constraints which were obtained from the 2D 29 Si DQ MAS NMR experiments (Figure 3.15). This method opens up a new perspective for highresolution structure determination of zeolites by solid-state NMR spectroscopy [27]. The anisotropies of the 29 Si shielding interactions, arising from Si atoms in different tetrahedral geometries, strongly depend on the topology structure of zeolites. Therefore, the topology structure of zeolite can also be extracted by incorporation of 29 Si chemical shift anisotropy (CSA) [41–43]. Figure 3.16 shows the CSA pattern of 7 distinct Si sites of ZSM-12 zeolite obtained by using a robust 2D CSA recoupling pulse sequence [43]. After carefully line shapes fitting for the CSA pattern, the principal components of 29 Si CSA tensors of the 7 distinct Si sites can be determined. The best-fit values of isotropic chemical shift (δiso ) and asymmetry parameter (η) and their uncertainties for each distinct Si sites can be calculated accordingly. By using Hartree-Fock ab initio calculations on the clusters derived from crystal structures, the computational 29 Si magnetic shielding tensors are in excellent agreement with the experimental measurements. The 29 Si chemical shift anisotropy is very sensitive to the local structure around each Si atom. Thus, the measurement and calculation of 29 Si shielding tensors can be incorporated into the NMR crystallography of zeolites to refine the topology structure of zeolite. After incorporation of the 29 Si CSA to zeolite structure constraints, the zeolite structure can be determined more

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

107

Fig. 3.16 a 1D 29 Si MAS NMR spectrum of ZSM-12 zeolite. b Experimental (solid lines) and best-fit simulated (dashed lines) quasi-static CSA recoupled line shapes for the indicated Si sites extracted from the 2D spectrum for ZSM-12. Reproduced from Ref. [43] by permission of American Chemical Society

accurately [43]. The strategy from 29 Si CSA might be helpful in structural determination of zeolites in which diffraction experiments provide very limited information due to the structural complexity of zeolites.

3.2.3 31

31 P

MAS NMR

P MAS NMR has been extensively applied to characterize the PO4 tetrahedra in aluminophosphate (AlPO), silicoaluminophosphate (SAPO), and metal-substituted aluminophosphate (MAPO) molecular sieves [44–49]. Typically, the PO4 units in aluminophosphates produce a chemical shift range between −20 and −30 ppm. The incorporation of a divalent or trivalent metal atom into AlPOs affects the 31 P chemical shift of PO4 tetrahedra. Barrie and Klinowski [50] studied MgAPO-20 (sodalite structure) using 31 P MAS NMR and observed four signals at −14.0, −21.1, −28.0, and −34.9 ppm in the spectrum (Fig. 3.17). They ascribed these resonance lines to P (lAl, 3 Mg), P(2Al, 2 Mg), P (3Al, 1 Mg), and P (4Al) structural units, respectively, and were able to calculate the framework composition from the 31 P MAS spectrum.

108

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.17 31 P MAS NMR spectrum of MgAPO-20 recorded on a Bruker MSL-400 spectrometer at 4.5 kHz spinning speed: ssb denotes spinning sidebands. Reproduced from Ref. [8] by permission of American Chemical Society

The population ratio of the framework elements (i.e., P/Al) can be calculated from the 31 P MAS spectra in a similar manner to that used for calculating the Si/Al ratios from the 29 Si MAS spectra of zeolites. In the absence of Al–O–Al linkages (this follows from the Loewenstein rule, which is usually obeyed in aluminophosphates) and Al–O–Mg linkages (Mg substitutes only for Al rather than for P), the populations of P, Al, and Mg can be obtained directly from the 31 P MAS NMR spectra. All the Al is in an A1(4P) environment, and all the Mg is in an Mg(4P) environment. Each P–O–Al linkage in a P(nAl) unit therefore incorporates 1/4Al atom and the whole unit (n/4) Al atoms. The P/Al ratio in the MgAPO aluminophosphate framework is thus can be expressed by the following Eq. (3.4) P/Al =

4  n=0

IP(nAl)

 4 

  0.25n IP(nAl)

(3.4)

n=0

where I p(nAl) is the intensity of the NMR signal corresponding to the P(nAl) unit. Since 50% of the tetrahedral sites of MgAPOs are occupied by P and the other 50% by both Al and Mg, the Mg/Al ratio and the fraction of T sites occupied by Mg in MgAPOs can be expressed by Eqs. (3.5) and (3.6), respectively [51]. It was demonstrated that magnesium in the framework of MgAPO-20 was strictly ordered, while in the study of MgAPO-5 and MgAPO-34 by 31 P MAS NMR, Deng’s work unambiguously confirmed a restricted random distribution of Mg in these MgAPO molecular sieves [51].

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

109

Fig. 3.18 2D 31 P{27 Al} MQ-D-HETCOR spectra of AlPO4 -14 obtained using cross-polarization at 18.8 T. Reproduced from Ref. [52] by permission of Elsevier



Mg Al

 = nmr

4 

IP(nAl)

n=0

4    Mg = (4 − n)IP(nAl) n=0

 4 

 8

n=0 4 

  0.25n IP(nAl) − 1

  n IP(nAl) (supposing [P]=0.5)

(3.5)

(3.6)

n=0

The framework of AlPOs often consists of different Al and P sites. In order to assign these sites, 27 Al → 31 P HETCOR experiments are often employed to establish the connectivity between P sites and neighboring Al sites. AlPO4 -14 is a well-studied aluminophosphate, containing four different aluminum sites and four different phosphorus sites [52, 53]. Figure 3.18 shows the dipolar CP-based MQD-HETCOR spectrum of AlPO4 -14, which correlated 27 Al and 31 P nuclei under the isotropic resolution offered by multiple-quantum magic-angle spinning (MQMAS) NMR [54]. The complex Al–P connectivities are revealed by the MQ-J-HETCOR spectrum: Al1{1P1, 1P2, 1P3, 1P4}, Al2{2P1, 1P2, 1P4}, Al3{1P1, 2P3, 1P4}, and Al4 {2P2, 1P3,1P4}. The connectivities can be drawn in Fig. 3.19, which shows that not all Al and P sites are connected by Al–O–P links in the correlation matrix. Anionic framework aluminophosphates are featured by structural and compositional diversities. The Al polyhedra may include AlO4 , AlO5 , and AlO6 , and the P tetrahedral may include PO4b (Q4 ), PO3b Ot (Q3 ), PO2b O2t (Q2 ), and POb O3t (Q1 ) (b represents bridging oxygens and t terminal oxygens) [55]. The double-resonance NMR techniques such as REDOR and TRAPDOR can be used to characterize the chemical environments of P and Al in the ALPO -based materials, and the average numbers of Al atoms and P atoms in the second-coordination spheres for each P and Al site can be estimated [56, 57]. Figure 3.20 shows the 31 P{27 Al} TRAPDOR

110

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.19 Expected Al/P connectivities for As-synthesized AlPO4 -14. As “X” stands for one neighbor of this kind. Note that there are some Al and P sites which are not directly connected via oxygen atoms. Reproduced from Ref. [52] by permission of Elsevier

spectra of Mu-4 and AlPO-HDA. The observation of the 31 P signals in the TRAPDOR difference spectrum clearly indicates that both P sites are connected to the Al via P–O–Al linkages. In the TRAPDOR experiments, the initial parts of the S/S 0 versus evolution time curves only depend on the strength of I-S dipolar interaction (i.e., the number of S spins and internuclear I-S distances) and are independent of the exact geometry involved [58]. For aluminophosphate-based materials, the Al–O–P distance does not change much in a different polyhedral unit. The difference in the initial slopes of 31 P{27 Al} TRAPDOR curves reflects average number of Al atoms coordinated to P. Figure 3.21 shows the plots of the 31 P{27 Al} TRAPDOR fraction (S/S 0 ) as function of dephasing time (denoted TRAPDOR curve) for four kinds of P coordination forms [57]. It is obvious that the slopes of the TRAPDOR curves are well separated for different P forms with an order of PO4b > PO3b Ot > PO2b O2t > POb O3t .

3.2.4 17

17 O

MAS NMR

O NMR can provide valuable information about the local environments of oxygen atoms in inorganic materials including zeolites, oxides, and other oxygen-containing materials [59–64]. One challenge for the application of 17 O NMR is that the extremely low natural abundance of 17 O nuclei (0.037%) usually requires isotopic enrichment for NMR investigations. Another difficulty is that the quadrupolar interaction of 17 O (I = 5/2) and the distribution of its NMR parameters due to disorder in the local structure usually hampers the analysis of oxygen species. Nevertheless, the 17 O MAS NMR has been used to observe the zeolite framework oxygen species since the 1980s [65–69]. The framework oxygen sites (Si–O–Si and Al–O–Si) could be directly detected from 17 O MAS spectroscopy. Figure 3.22 shows the projection of 17 O DAS NMR spectrum of the original enriched stilbite and back-reacted stilbite. The chemical shift of the Si–O–Si site is in the range of −20 to 20 ppm, whereas the chemical shift of the Si–O–Al site is in the range from 10 to 40 ppm [70]. However,

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

111

Fig. 3.20 31 P MAS NMR spectra and 31 P{27 Al} TRAPDOR experiments of Mu-4 (left) and AlPOHDA (right). 31 P spin-echo (S0 ), TRAPDOR (S) and TRAPDOR difference spectra (S). Reproduced from Ref. [57] by permission of American Chemical Society

the Brønsted acid site (Al–OH–Si) cannot be clearly distinguished by 17 O NMR due to its relatively low concentration on zeolite surface. To detect the surface 17 O sites in HY, H-ZSM-5, and H-mordenite zeolites, several double-resonance solid-state NMR techniques including 1 H–17 O TRAPDOR, 17 O–1 H REDOR, 1 H–17 O REDOR, and 1 H–17 O HETCOR experiments were utilized by Grey et al. [71–73]. Figure 3.23 shows the 17 O{1 H} REDOR NMR spectra of 17 O-enriched HY zeolite. The shoulder signal at −24 ppm decreases significantly in the double-resonance spectrum, indicating the presence of protons in close proximity to the oxygen atom. While the broad peak at 21 ppm remains almost unchanged in the doubleresonance spectrum and unobservable in the difference spectrum [72]. According to this REDOR experiment, the broad peak could be assigned to the framework oxygen

112

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.21 31 P{27 Al} TRAPDOR fraction (S/S0) as a function of evolution time NTr for different P coordination forms. Reproduced from Ref. [57] by permission of American Chemical Society

Fig. 3.22 The projection of 17 O DAS NMR spectrum of the original enriched stilbite (a) and back-reacted stilbite zeolite (b). Reproduced from Ref. [70] by permission of American Chemical Society

atoms in the Si–O–Al and Si–O–Si linkages, whereas the shoulder signal could be associated to the oxygen atom in the Brønsted acid site (Si–OH–Al) [72]. 1 H–17 O CP-REDOR NMR experiment was further applied to determine the O–H distance in zeolite HY. Numerical simulations with typical O–H distances are compared with the experimental REDOR fractions of zeolite HY in Fig. 3.24a. All these simulations were performed by calculating the 17 O spectrum obtained following a REDOR dephasing sequence. The center band and the corresponding spinning sidebands were then integrated allowing comparison with the REDOR fractions extracted from the experimental data. The calculated data qualitatively fit the experimental data, indicating an O–H distance of 1.04 Å [73]. To distinguish the 17 O-NMR signal of Brønsted acid site in supercage from that in sodalite cage in HY zeolite, two-dimensional 1 H–17 O HETCOR NMR experiment was carried out [74]. As shown in Fig. 3.25a, in the 1 H–17 O HETCOR spectrum of HY zeolite, the correlations between 17 O sites and Brønsted acidic protons in the supercage and sodalite cage can be established [74]. The 17 O isotropic chemical shifts

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

113

Fig. 3.23 17 O{1 H} REDOR NMR spectra of 17 O-enriched HY zeolite. The difference spectrum is obtained by subtracting the double-resonance spectrum from the control spectrum. Reproduced from Ref. [72] by permission of Nature Publishing Group

Fig. 3.24 a Comparison of experimental 1 H–17 O REDOR fractions with those obtained by using the SIMPSON package. Dipolar coupling constants of 17308, 15811, 14482, and 13298 Hz were used in simulations, corresponding to O–H distances of 0.98, 1.01, 1.04, and 1.07 Å, respectively. b The orientation of the principal axis of the electric-field gradient (EFG). Reproduced from Ref. [73] by permission of American Chemical Society

114

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.25 (a) 2D 1 H–17 O HETCOR NMR spectrum of zeolite HY. Slices and simulations of the 17 O dimension corresponding to the Brønsted acid site in the supercage (b and d) and sodalite cage (c and e) are shown. Reproduced from Ref. [74] by permission of American Chemical Society

(δCS ) and quadrupolar coupling parameters (quadrupolar coupling constant, QCC, and asymmetry parameter, η) for the two oxygen sites could be separately extracted from Fig. 3.25b–e. The Brønsted acid sites in other zeolites such as H-ZSM-5 and H-mordenite [73] were also studied by 17 O MAS NMR spectroscopy, which provides insights into the active center in the acidic zeolite catalysts. All of the oxygen sites in HY zeolite can be distinguished from the 2D 17 O MQMAS NMR experiment [75]. As shown in Fig. 3.26, the 17 O NMR signals due to all the different oxygen environments have been observed, including the readily resolved signals arising from the two types of oxygen atoms of the zeolite framework (Si–O–Al and Si–O–Si), and oxygen atoms directly bound to Brønsted acid sites

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

115

Fig. 3.26 17 O MQMAS NMR spectra of HY at 14.1 T. a 2D MQMAS spectrum. The projections of F2 and F1 dimensions are shown on the above and right side of the 2D spectrum, respectively. b–e Slices of anisotropic dimension at 35.1, 40.0, 57.2, and 63.0 ppm in F1 dimension (top), and the simulations of the spectra (bottom). Reproduced from Ref. [75] by permission of Elsevier

116

3 Solid-State NMR Characterization of Framework Structure …

(Si–O(H)–Al) [75]. The 17 O NMR parameters such as the isotropic chemical shift (δCS ) and quadrupolar coupling parameters (quadrupolar coupling constant, QCC, and asymmetry parameter, η) for these oxygen species could be extracted and found to be consistent with the results from previous NMR measurements. The NMR parameters of the framework oxygen sites reveal a greater distribution of T–O–T bond angles in hydrated zeolites.

3.2.5

129 Xe

NMR

In addition to zeolite framework structure, the structure of cages and channels plays key roles in the adsorption and desorption behavior, significantly affecting catalytic activity of zeolites. 129 Xe is an inert, non-polar, spherical atom with a large electron cloud sensitive to its environments and interactions, which results in a wide 129 Xe NMR chemical shift range. The chemical shifts of 129 Xe are very sensitive to the pore geometry and the chemical surroundings. Thus, 129 Xe is an ideal and promising NMR probe for investigating the microstructure of porous materials [76–80]. For xenon adsorbed in a porous material, the observed 129 Xe NMR chemical shift is the weighted average of different types of interactions on the NMR time scale. In particular, for xenon adsorbed on zeolites at room temperature, it was found that the chemical shift of adsorbed xenon is the sum of several terms corresponding to the various perturbations. δ = δ0 + δXe − Xe ρXe + δ E + δ M + δSAS where δ0 is the reference chemical shift, δS is the shift due to interaction of Xe with the pore surface and is related to the pore size [81], δXe–Xe arises from Xe–Xe collisions and can be also used to determine the sizes of channels or micropores [82], δE is the shift due to the electric field caused by cations, δM is an extra term accounting for the presence of paramagnetic species, and δSAS is the shift due to strong adsorption sites. 129 Xe NMR has been used to characterize the pore structure and adsorption properties in various zeolites such as MCM-22, MAS-7, and ZSM-5 [83]. By using 129 Xe NMR, Chen et al. [83] found that Xe atoms were preferentially adsorbed in the supercages of zeolite MCM-22 at low Xe pressure, while Xe atoms could penetrate into the two-dimensional sinusoidal channels at high Xe pressure. No direct exchange between the Xe in the supercages and the sinusoidal channels was observed in variable-temperature (VT) 129 Xe NMR experiments (Fig. 3.27). This is in line with the crystal structure of the zeolite that there is no communication between the two micropore systems. However, when the Xe adsorption pressure is larger than 6 atm, two different kinds of xenon exchange were observed by VT experiments (Fig. 3.27). As revealed by the 2D 129 Xe exchange NMR spectra (Fig. 3.28), exchange of xenon at the different adsorption sites in the same supercage, i.e., xenon atoms in the two

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

117

Fig. 3.27 Temperaturedependent 129 Xe NMR spectra of Xe adsorbed in the H-MCM-22 zeolite with a Xe pressure of 6.0 atm. Reproduced from Ref. [83] by permission of American Chemical Society

pockets and those in the central part of the supercage, occurs at lower temperatures (170–122 K). Exchange between xenon in the supercage and the sinusoidal channel takes place through gaseous xenon in the interparticle space at higher temperatures (280–350 K). In addition, Chen et al. [84] demonstrated that two different independent micropore systems having good communication with mesopores were present in the walls of an ordered mesoporous aluminosilicate material MAS-7. Since the chemical shift of the xenon is very sensitive to the types of atoms with which it collides, 129 Xe NMR is especially successful in investigating valence, location, and state of cations in the metal-exchanged zeolites [85, 86]. 129 Xe NMR spectroscopy was employed to characterize the state of metal species in zeolites. Figure 3.29 shows the 129 Xe chemical shift variation as a function of the xenon concentration in different ion-exchanged zeolites [87]. The high polarizability of xenon and the distortion of the xenon electron cloud by the strong electric fields created by the 2+ cations resulted in the parabolic behavior of the curves in the case

118

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.28 Two-dimensional 129 Xe exchange NMR spectra of Xe adsorbed in the H-MCM-22 zeolite (with a Xe pressure of 8.2 atm) at different mixing times, which were recorded at 145 K. Reproduced from Ref. [83] by permission of American Chemical Society

of CoNaY, CdX, and MgY zeolites. For analogous zeolites containing monovalent cations such as HY, NaY, and CuX, it was shown that the chemical shifts vary linearly with xenon loading. Two examples regarding the state of metal species in zeolite used for catalytic reactions have been briefly introduced as follows. Figure 3.30 shows the 129 Xe chemical shifts of xenon adsorbed in H-ZSM-5, fresh and deactivated Ag-ZSM-5 as a function of Xe loading [86]. The H-ZSM-5 and fresh air-calcined Ag-ZSM-5 catalysts exhibit a linear correlation between the chemical shift and the xenon concentration, indicating that the collisions among xenon atoms are isotropically distributed. It is interesting that the deactivated air-calcined Ag-ZSM-5 zeolite shows a parabolic correlation between 129 Xe chemical shift and xenon loading. The curve of the deactivated air-calcined Ag-ZSM-5 catalysts has the classical form of zeolite-supported metals: The chemical shift is large at low Xe loading because of the strong metalxenon interaction. Silver oxide could be dissolved in zeolite, yielding Ag+ ions. It is

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

119

Fig. 3.29 Chemical shift variation versus xenon concentration from up to down for CoNaY (15% Na exchanged by Co), CdX (100% exchanged), MgY (100% Na exchanged by Mg), HY and NaY, CuX (100% exchanged), AgX (100% exchanged). Reproduced from Ref. [87] by permission of Wiley

very likely that a small amount of Ag+ was formed from the residual silver oxides. The low-frequency shift of the deactivated air-calcined Ag-ZSM-5 at moderate Xe loading is probably due to the specific interaction of Xe with Ag+ cations. Compared with the silver metal, the amount of the silver ions is much lower, so the curve of the deactivated air-calcined Ag-ZSM-5 catalysts still has the classical form of zeolite-supported metals. On the basis of experimental observations, it is concluded that silver oxide is predominant on the Ag-ZSM-5 catalyst though a small amount of silver oxide is probably reduced into silver metal [86]. The techniques have been successfully applied to investigate the presence of metal active center in zeolites catalysts including Zn/ZSM-5 [88], platinum complexes in zeolite LTL [89], and Ni-Y zeolite [90]. Metal Zn species on zeolite surface are normally considered as the Lewis acid centers, which play key roles in the activation of alkanes over Zn-modified H-ZSM-5 zeolite. 129 Xe NMR spectra of xenon adsorbed in Zn/ZSM-5 catalyst as a function of xenon loading are shown in Fig. 3.31a. The 129 Xe chemical shifts of the two observed signals at 260 and 158 ppm as a function of Xe adsorption density are plotted in

120

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.30 129 Xe NMR chemical shifts versus xenon loading for different zeolites: (black square), HZSM-5; (black circle), fresh air-calcined Ag-ZSM-5; (black up-pointing triangle), deactivated air-calcined Ag-ZSM-5; (black down-pointing triangle), fresh vacuum-calcined Ag-ZSM-5; (black diamond), deactivated vacuum-calcined Ag-ZSM-5. Reproduced from Ref. [86] by permission of Elsevier

Fig. 3.31 a 129 Xe NMR spectra of xenon adsorbed on Zn/ZSM-5 as a function of xenon density. b 129 Xe chemical shift as a function of xenon density. Reproduced from Ref. [88] by permission of Royal Society of Chemistry

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

121

Fig. 3.32 ITQ-6 hyperpolarized Xe 2D-exchange spectrum obtained for a mixing time of 50 ms. The sample was rotated at a speed of 3 kHz. Reproduced from Ref. [92] by permission of Royal Society of Chemistry

Fig. 3.31b. The variation of 129 Xe NMR chemical shifts suggests the presence of two types of Zn2+ ions in the channel of Zn/ZSM-5 catalyst, which is characterized by two typical parabolic curves that are usually observed for xenon adsorbed on divalent metal ions. The bottom curve is similar to the adsorption behavior of xenon in the Zn2+ /ZSM-5 catalyst in which only isolated Zn2+ ions are present. The large chemical shift (260 ppm) at low xenon density could be possibly resulted from another type of Zn2+ cation species. In combination with other characterizations and DFT calculations, it is demonstrated that Zn–O–Zn clusters in open shell are associated with these new Zn2+ ions, which serve as active centers for the activation of methane at room temperature [88]. This species enables a facile hemolytic cleavage of C–H bond of methane with the formation of methyl radical. Adsorption of atomic xenon facilitates the extraction of the information about the porosity of solid materials. A remarkable increase in the sensitivity can be obtained using an optical-pumping device that allows hyperpolarization of the 129 Xe nuclear spins [91]. Application of hyperpolarized (HP) 129 Xe NMR is very promising as it is possible to obtain porosity information faster than normal way. The HP 129 Xe NMR spectroscopy of porous materials under MAS and continuous-flow conditions was introduced to study the adsorption of hyperpolarized xenon on ITQ-6 zeolite. The high stability of the HP xenon flow allowed to perform two-dimensional exchange experiments under MAS conditions, in a short time and with a very good resolution [92]. Figure 3.32 shows the HP 129 Xe 2D-exchange spectrum of ITQ-6 obtained for a mixing time of 50 ms. As shown in Fig. 3.30, the cross-peak a indicated the exchange between Xe adsorbed in channels and cavities in ITQ-6 zeolite. However, the appearance of cross-peak b corresponds to the exchange between Xe adsorbed

122

3 Solid-State NMR Characterization of Framework Structure …

in cavities and in interlamellar space. Additionally, the cross-peak c suggests the presence of exchange between Xe adsorbed in interlamellar space and Xe gas in the free space of the rotor. This method provided 2D-exchange 129 Xe MAS NMR experiments under continuous-flow conditions [92].

3.2.6 Other Framework Elements The isomorphous substitution of framework Si of zeolites by other heteroatoms such as B, Ti, Ga, Sn, Fe, Ce, and Fe gives rise to specific catalytic properties related to the coordination state of the heteroatoms [93]. The structural information of the isomorphously framework-substituted zeolites can be obtained from the multinuclear NMR analysis. Tetrahedral BO4 or trigonal BO3 units may be formed in boron-substituted zeolites. In hydrated boron-containing zeolites, tetrahedrally coordinated B atoms (I = 3/2) experience a symmetrical environment with a negligible electric-field gradient, while the B atoms in three-coordination have large quadrupolar interaction, leading to a broad second-order quadrupolar pattern [94]. A superposition of the tetrahedral and trigonal boron atoms signals are often observed on 11 B NMR spectra. Figure 3.33 shows the 11 B MAS NMR spectra of mordenite zeolites with different boron contents (2, 6, and 10%). By spectral simulation, it was found that the spectra consisted of a typical second-order quadrupolar pattern with a δiso of 16 ppm, a Qcc of 2.8 MHz, arising from trigonal BO3 , and two sharp lines at 3.1 and 0.3 ppm, probably due to two different types of tetrahedral BO4 units. Figure 3.34 shows the 11 B MQMAS NMR spectra of the mordenite zeolites [95]. A strong signal (C) due to the tri-coordinated extra-framework boron was observed in the 11 B MQMAS NMR spectra. Additionally, the existence of two different tetrahedral BO4 units in the samples could be verified by 11 B MQMAS NMR experiments. The association of the new tetrahe-

Fig. 3.33 11 B MAS NMR spectra of mordenite containing various boron contents. (a) 2 wt%, (b) 6 wt%, (c) 10 wt%. Reproduced from Ref. [95] by permission of Elsevier

3.2 Solid-State NMR Characterization of Zeolite Framework Structure Fig. 3.34 11 B MQ MAS NMR spectra of mordenite containing various boron loadings. (a) 2 wt%, (b) 6 wt%, (c) 10 wt%. Reproduced from Ref. [95] by permission of Elsevier

123

124

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.35 In the 71 Ga MAS NMR spectra of the gallium source Ga2 O3 (a) and of as-synthesized materials obtained after DGC times of 0 (b), 3.5 (c), 16 (d), and 65 h (e). Reproduced from Ref. [96] by permission of Elsevier

dral BO4 unit with the framework of mordenite was confirmed using 11 B{29 Si} rotational-echo double-resonance (REDOR) experiment, and its detailed structure is determined by ab initio calculation, which suggests that the new tetrahedral BO4 is connected to two hydroxyl groups [95]. Dry-gel conversion (DGC) method was usually used in synthesis of zeolite [Ga]Beta (structure-type BEA). Incorporation of gallium atoms (I = 3/2) into the zeolite framework during synthesis was investigated by 71 Ga MAS NMR spectroscopy [96]. As shown in Fig. 3.35, the signals at −6 to 24 ppm were due to octahedrally coordinated Ga species, while the broad signal centered at 174 ppm can be assigned to tetrahedrally coordinated Ga. The decrease of the signal for octahedrally coordinated gallium species on the fresh dry gel indicated the dissolution of the gallium source. Increasing the DGC times leads to an increase of the tetrahedrally coordinated framework gallium species, reflected by the increasing signal intensity at 150 ppm. Only one signal at 150 ppm can be observed at DGC time of 65 h, indicating a complete transformation of the gallium atoms to tetrahedrally coordinated species. The 71 Ga MAS NMR spectra provide detailed information on the incorporation of gallium atoms into the zeolite framework during the whole DGC process. It is not difficult to obtain Ga-containing zeolites, but the ion-exchange of Na ions with NH4 ions followed by calcination often leads to the collapse of the framework structure. The stability of Ga-containing zeolites can be examined by 71 Ga MAS NMR spectroscopy. The octahedrally coordinated gallium species is a clear sign of formation of extra-framework Ga(VI) species caused by framework collapse [97].

3.2 Solid-State NMR Characterization of Zeolite Framework Structure

125

Dynamic nuclear polarization (DNP) NMR makes it possible to transfer the large Boltzman polarization of electron spin reservoir to nuclear spin reservoir to provide a boost in NMR signal intensities by several orders of magnitude, thus dramatically increasing the signal intensity and data acquisition rate in the NMR experiments [98]. The development of DNP techniques is very promising to probe surface functionality in functional materials. In dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS) [99], a porous or particulate sample usually is wetted with a radical solution. The large polarization of the radicals’ unpaired electrons is then transferred to surrounding nuclear spins, with a typical signal enhancement of between 10 and 100. Recently, the DNP-SENS technique has been used to characterize natural-abundant 119 Sn-Beta zeolite, in which a 119 Sn NMR signal enhancement (ε) of 75 can be gained using biradicals [100]. Figure 3.36 shows 119 Sn DNP-SENS CP magic-angle turning (CPMAT) spectra of 5 wt% Sn-β zeolite [101]. The isotropic NMR spectrum can be observed in the indirect dimension (F1) and the normal anisotropic CP spectrum can be observed in the direct dimension (F2). The chemical shift tensor parameters (δiso , , and k) are extracted by fitting the sideband manifolds. The signal S3 at δ = −614 ppm is similar to that observed for bulk SnO2 , which is consistent with the octahedral environment of Sn in SnO2 . In addition, the other two signals at δ = −659 (S2) and δ = −685 ppm (S1) are associated with slightly different CSA parameter. The chemical shifts and the skews (k) close to 0 indicate a slightly distorted octahedral Sn environment. The 2D CP/MAT experiment on 1 wt% Sn-Beta shows the two peaks at δ = −659 ppm and δ = −685 ppm with similar CSA parameters as observed in the 5 wt% Sn-Beta sample, consistent with the presence of similar species, suggesting that increased loading does not change the nature of Sn species, but only their ratio [101]. To relate these observations directly to the local structure of the Sn environment, DFT calculations on cluster models were carried out to assign the observed NMR signatures as shown in Fig. 3.36c. Combining DNP-SENS spectroscopy and DFT calculations, it is shown that the active sites of Sn-Beta zeolite correspond to an octahedrally coordinated SnIV , involving the tetrahedral Sn sites [101].

3.3 Summary Solid-state NMR spectroscopy has been demonstrated to be a powerful tool for the characterization of framework atoms including metal or non-metal elements in zeolites and zeotype materials. The detailed information about coordination, connectivity, and framework ordering can be obtained from multi-nuclear and multi-dimensional NMR spectroscopy. Additionally, the structure features and communication of cages and channels in porous materials can be extracted by 129 Xe NMR spectroscopy.

126

3 Solid-State NMR Characterization of Framework Structure …

Fig. 3.36 a 105 K 119 Sn DNP-SENS CP magic-angle turning (CPMAT) spectra of 5 wt% Sn-Beta zeolite. b The extracted CS tensor parameters for the three different isotropic shifts are indicated. c Computed 119 Sn isotropic chemical shifts in ppm for each model. Reproduced from Ref. [101] by permission of Wiley

References 1. Tao YS, Kanoh H, Abrams L, Kaneko K (2006) Mesopore-modified zeolites: preparation, characterization, and applications. Chem Rev 106(3):896–910. https://doi.org/10.1021/ cr040204o 2. Corma A (2003) State of the art and future challenges of zeolites as catalysts. J Catal 216(1–2):298–312. https://doi.org/10.1016/s0021-9517(02)00132-x 3. Barthomeuf D (1996) Basic zeolites: characterization and uses in adsorption and catalysis. Catal Rev Sci Eng 38(4):521–612. https://doi.org/10.1080/01614949608006465 4. Li S, Deng F (2013) Chapter one—Recent advances of solid-state NMR studies on zeolites. In: Graham AW (ed) Annual reports on NMR spectroscopy, vol 78. Academic Press, pp 1–54. https://doi.org/10.1016/B978-0-12-404716-7.00001-8

References

127

5. Klinowski J (1991) Solid-state NMR-studies of molecular-sieve catalysts. Chem Rev 91(7):1459–1479. https://doi.org/10.1021/cr00007a010 6. Haouas M, Taulelle F, Martineau C (2016) Recent advances in application of 27Al NMR spectroscopy to materials science. Prog Nucl Magn Reson Spectrosc 94–95:11–36. https:// doi.org/10.1016/j.pnmrs.2016.01.003 7. Li SH, Huang SJ, Shen WL, Zhang HL, Fang HJ, Zheng AM, Liu SB, Deng F (2008) Probing the spatial proximities among acid sites in dealuminated H–Y zeolite by solid-state NMR spectroscopy. J Phys Chem C 112(37):14486–14494. https://doi.org/10.1021/jp803494n 8. Hu W, Luo Q, Su YC, Chen L, Yue Y, Ye CH, Deng F (2006) Acid sites in mesoporous Al-SBA-15 material as revealed by solid-state NMR spectroscopy. Microporous Mesoporous Mater 92(1–3):22–30. https://doi.org/10.1016/j.micromeso.2005.12.013 9. Shannon RD, Gardner KH, Staley RH, Bergeret G, Gallezot P, Auroux A (1985) The nature of the nonframework aluminum species formed during the dehydroxylation of H–Y. J Phys Chem 89(22):4778–4788. https://doi.org/10.1021/j100268a025 10. Jiao J, Altwasser S, Wang W, Weitkamp J, Hunger M (2004) state of aluminum in dealuminated, nonhydrated zeolites Y investigated by multinuclear solid-state NMR spectroscopy. J Phys Chem B 108(38):14305–14310. https://doi.org/10.1021/jp040081b 11. Comotti A, Bracco S, Valsesia P, Ferretti L, Sozzani P (2007) 2D multinuclear NMR, hyperpolarized xenon and gas storage in organosilica nanochannels with crystalline order in the walls. J Am Chem Soc 129(27):8566–8576. https://doi.org/10.1021/ja071348y 12. Gan ZH (2000) Isotropic NMR spectra of half-integer quadrupolar nuclei using satellite transitions and magic-angle spinning. J Am Chem Soc 122(13):3242–3243. https://doi.org/ 10.1021/ja9939791 13. Christiansen SC, Zhao DY, Janicke MT, Landry CC, Stucky GD, Chmelka BF (2001) Molecularly ordered inorganic frameworks in layered silicate surfactant mesophases. J Am Chem Soc 123(19):4519–4529. https://doi.org/10.1021/ja004310t 14. Samoson A, Lippmaa E, Pines A (1988) High-resolution solid-state NMR averaging of 2ndorder effects by means of a double-rotor. Mol Phys 65(4):1013–1018. https://doi.org/10.1080/ 00268978800101571 15. Xu L, Ji X, Li S, Zhou Z, Du X, Sun J, Deng F, Che S, Wu P (2016) Self-assembly of cetyltrimethylammonium bromide and lamellar zeolite precursor for the preparation of hierarchical MWW zeolite. Chem Mater 28(12):4512–4521. https://doi.org/10.1021/acs. chemmater.6b02155 16. Medek A, Harwood JS, Frydman L (1995) Multiple-quantum magic-angle spinning NMR: A new method for the study of quadrupolar nuclei in solids. J Am Chem Soc 117(51):12779–12787. https://doi.org/10.1021/ja00156a015 17. Yu ZW, Zheng AM, Wang QA, Chen L, Xu J, Amoureux JP, Deng F (2010) Insights into the dealumination of zeolite HY revealed by sensitivity-enhanced Al-27 DQ-MAS NMR spectroscopy at high field. Angew Chem Int Ed 49(46):8657–8661. https://doi.org/10.1002/ anie.201004007 18. Amoureux JP, Fernandez C, Steuernagel S (1996) Z filtering in MQMAS NMR. J Magn Reson Ser A 123(1):116–118. https://doi.org/10.1006/jmra.1996.0221 19. van Bokhoven JA, Koningsberger DC, Kunkeler P, van Bekkum H, Kentgens APM (2000) Stepwise dealumination of zeolite beta at specific T-sites observed with Al-27 MAS and Al-27 MQ MAS NMR. J Am Chem Soc 122(51):12842–12847. https://doi.org/10.1021/ja002689d 20. van Bokhoven JA, Roest AL, Koningsberger DC, Miller JT, Nachtegaal GH, Kentgens APM (2000) Changes in structural and electronic properties of the zeolite framework induced by extraframework Al and La in H-USY and La(x)NaY: A Si-29 and Al-27 MAS NMR and Al-27 MQ MAS NMR study. J Phys Chem B 104(29):6743–6754. https://doi.org/10.1021/ jp000147c 21. Li SH, Zheng AM, Su YC, Fang HJ, Shen WL, Yu ZW, Chen L, Deng F (2010) Extra-framework aluminium species in hydrated faujasite zeolite as investigated by twodimensional solid-state NMR spectroscopy and theoretical calculations. Phys Chem Chem Phys 12(15):3895–3903. https://doi.org/10.1039/b915401a

128

3 Solid-State NMR Characterization of Framework Structure …

22. Burkett SL, Davis ME (1994) MECHANISM Of Structure Direction In The Synthesis Of SI-ZSM-5—an investigation by intermolecular H-1-SI-29 CP MAS NMR. J Phys Chem 98(17):4647–4653. https://doi.org/10.1021/j100068a027 23. Muller M, Harvey G, Prins R (2000) Comparison of the dealumination of zeolites beta, mordenite, ZSM-5 and ferrierite by thermal treatment, leaching with oxalic acid and treatment with SiCl4 by H-1, Si-29 and Al-27 MAS NMR. Microporous Mesoporous Mater 34(2):135–147. https://doi.org/10.1016/s1387-1811(99)00167-5 24. Barrie PJ, Klinowski J (1989) Ordering in the framework of a magnesium aluminophosphate molecular-sieve. J Phys Chem 93(16):5972–5974. https://doi.org/10.1021/j100353a007 25. Klinowski J, Ramdas S, Thomas JM, Fyfe CA, Hartman JS (1982) A reexamination of Si, Al ordering in zeolites NaX and NaY. J Chem Soc Faraday Trans 2 78:1025–1050. https://doi. org/10.1039/f29827801025 26. Luo Q, Yang J, Hu W, Zhang MJ, Yue Y, Ye CH, Deng F (2005) Unambiguously distinguishing Si 3Si,1Al and Si 3Si,1OH stuctural units in zeolite by H-1/Si-29/Al-27 triple resonance solid state NMR spectroscopy. Solid State Nucl Magn Reson 28(1):9–12. https://doi.org/10.1016/ j.ssnmr.2005.02.004 27. Brouwer DH, Darton RJ, Morris RE, Levitt MH (2005) A solid-state NMR method for solution of zeolite crystal structures. J Am Chem Soc 127(29):10365–10370. https://doi.org/10.1021/ ja052306h 28. Brouwer DH, Kristiansen PE, Fyfe CA, Levitt MH (2005) Symmetry-based Si-29 dipolar recoupling magic angle spinning NMR spectroscopy: a new method for investigating threedimensional structures of zeolite frameworks. J Am Chem Soc 127(2):542–543. https://doi. org/10.1021/ja043228l 29. Hammond KD, Dogan F, Tompsett GA, Agarwal V, Conner WC Jr, Grey CP, Auerbach SM (2008) Spectroscopic signatures of nitrogen-substituted zeolites. J Am Chem Soc 130(45):14912–14913. https://doi.org/10.1021/ja8044844 30. Freeman D, Wells RPK, Hutchings GJ (2001) Methanol to hydrocarbons: enhanced aromatic formation using a composite GaO-H-ZSM-5 catalyst. Chem Commun 18:1754–1755. https:// doi.org/10.1039/B104844A 31. Dogan F, Hammond KD, Tompsett GA, Huo H, Conner WC Jr, Auerbach SM, Grey CP (2009) Searching for microporous, strongly basic catalysts: experimental and calculated 29Si NMR spectra of heavily nitrogen-doped Y zeolites. J Am Chem Soc 131(31):11062–11079. https://doi.org/10.1021/ja9031133 32. Cadars S, Brouwer DH, Chmelka BF (2009) Probing local structures of siliceous zeolite frameworks by solid-state NMR and first-principles calculations of 29 Si–O–29 Si scalar couplings. Phys Chem Chem Phys 11(11):1825–1837. https://doi.org/10.1039/b815361b 33. Luo Q, Deng F, Yuan ZY, Yang J, Zhang MJ, Yue Y, Ye CH (2003) Using trimethylphosphine as a probe molecule to study the acid states in Al-MCM-41 materials by solid-state NMR spectroscopy. J Phys Chem B 107(11):2435–2442. https://doi.org/10.1021/jp0213093 34. Kristiansen PE, Carravetta M, Lai WC, Levitt MH (2004) A robust pulse sequence for the determination of small homonuclear dipolar couplings in magic-angle spinning NMR. Chem Phys Lett 390(1–3):1–7. https://doi.org/10.1016/j.cplett.2004.03.075 35. Loquet A, Bardiaux B, Gardiennet C, Blanchet C, Baldus M, Nilges M, Malliavin T, Boeckmann A (2008) 3D structure determination of the Crh protein from highly ambiguous solidstate NMR restraints. J Am Chem Soc 130(11):3579–3589. https://doi.org/10.1021/ja078014t 36. Castellani F, van Rossum B, Diehl A, Schubert M, Rehbein K, Oschkinat H (2002) Structure of a protein determined by solid-state magic-angle-spinning NMR spectroscopy. Nature 420(6911):98–102. https://doi.org/10.1038/nature01070 37. Franks WT, Wylie BJ, Schmidt HLF, Nieuwkoop AJ, Mayrhofer R-M, Shah GJ, Graesser DT, Rienstra CM (2008) Dipole tensor-based atomic-resolution structure determination of a nanocrystalline protein by solid-state NMR. Proc Natl Acad Sci USA 105(12):4621–4626. https://doi.org/10.1073/pnas.0712393105 38. Li S, Zhang Y, Hong M (2010) 3D 13C-13C-13C correlation NMR for de novo distance determination of solid proteins and application to a human alpha-defensin. J Magn Reson 202 (2):203–210. https://doi.org/10.1016/j.jmr.2009.11.011

References

129

39. Li S, Su Y, Hong M (2012) Intramolecular 1H-13C distance measurement in uniformly 13C, 15 N labeled peptides by solid-state NMR. Solid State Nucl Magn Reson 45–46:51–58 40. Franks WT, Zhou DH, Wylie BJ, Money BG, Graesser DT, Frericks HL, Sahota G, Rienstra CM (2005) Magic-angle spinning solid-state NMR spectroscopy of the beta 1 immunoglobulin binding domain of protein G (GB1): N-15 and C-13 chemical shift assignments and conformational analysis. J Am Chem Soc 127(35):12291–12305. https://doi.org/10.1021/ ja044497e 41. Brouwer DH (2008) NMR crystallography of zeolites: refinement of an NMR-solved crystal structure using ab initio calculations of 29 Si chemical shift tensors. J Am Chem Soc 130(20):6306–6307. https://doi.org/10.1021/ja800227f 42. Brouwer DH (2008) A structure refinement strategy for NMR crystallography: an improved crystal structure of silica-ZSM-12 zeolite from 29 Si chemical shift tensors. J Magn Reson 194(1):136–146. https://doi.org/10.1016/j.jmr.2008.06.020 43. Brouwer DH, Enright GD (2008) Probing local structure in zeolite frameworks: ultrahighfield NMR measurements and accurate first-principles calculations of zeolite 29 Si magnetic shielding tensors. J Am Chem Soc 130(10):3095–3105. https://doi.org/10.1021/ja077430a 44. Zibrowius B, Loffler E, Hunger M (1992) Multinuclear MAS NMR and IR spectroscopic study of silicon incorporation into SAPO-5, SAPO-31, and SAPO-34 molecular-sieves. Zeolites 12(2):167–174. https://doi.org/10.1016/0144-2449(92)90079-5 45. Ojo AF, Dwyer J, Dewing J, Karim K (1991) Synthesis and properties of SAPO-37. J Chem Soc Faraday Trans 87(16):2679–2684. https://doi.org/10.1039/ft9918702679 46. Lutz W, Kurzhals R, Sauerbeck S, Toufar H, Buhl JC, Gesing T, Altenburg W, Jaeger C (2010) Hydrothermal stability of zeolite SAPO-11. Microporous Mesoporous Mater 132(1–2):31–36. https://doi.org/10.1016/j.micromeso.2009.08.003 47. Xu J, Chen L, Zeng D, Yang J, Zhang M, Ye C, Deng F (2007) Crystallization of AlPO4-5 aluminophosphate molecular sieve prepared in fluoride medium: a multinuclear solid-state NMR study. J Phys Chem B 111(25):7105–7113. https://doi.org/10.1021/jp0710133 48. Longstaffe JG, Chen B, Huang Y (2007) Characterization of the amorphous phases formed during the synthesis of microporous material AlPO4-5. Microporous Mesoporous Mater 98(1–3):21–28. https://doi.org/10.1016/j.micromeso.2006.08.009 49. Liu ZQ, Xu WG, Yang GD, Xu RR (1998) New insights into the crystallization mechanism of microporous AlPO4-21(1). Microporous Mesoporous Mater 22(1–3):33–41 50. Barrie PJ, Klinowski J (1989) Ordering in the framework of a magnesium aluminophosphate molecular sieve. J Phys Chem 93(16):5972–5974. https://doi.org/10.1021/j100353a007 51. Deng F, Yue Y, Xiao T, Du Y, Ye C, An L, Wang H (1995) Substitution of aluminum in aluminophosphate molecular sieve by magnesium: a combined NMR and XRD study. J Phys Chem 99(16):6029–6035. https://doi.org/10.1021/j100016a045 52. Fyfe CA, zu Altenschildesche HM, Wong-Moon KC, Grondey H, Chezeau JM (1997) 1D and 2D solid state NMR investigations of the framework structure of As-synthesized AlPO4-14. Solid State Nucl Magn Reson 9(2):97–106. https://doi.org/10.1016/S0926-2040(97)00049-0 53. Fernandez C, Amoureux JP, Chezeau JM, Delmotte L, Kessler H (1996) 27Al MAS NMR characterization of AlPO4-14 enhanced resolution and information by MQMAS Dr. Hellmut G. Karge on the occasion of his 65th birthday. Microporous Mater 6 (5):331–340. https://doi. org/10.1016/0927-6513(96)00040-5 54. Amoureux JP, Trebosc J, Wiench J, Pruski M (2007) HMQC and refocused-INEPT experiments involving half-integer quadrupolar nuclei in solids. J Magn Reson 184(1):1–14. https:// doi.org/10.1016/j.jmr.2006.09.009 55. Yu J, Xu R (2003) Rich structure chemistry in the aluminophosphate family. Acc Chem Res 36(7):481–490. https://doi.org/10.1021/ar0201557 56. Huang Y, Yan Z (2005) Probing the local environments of phosphorus in aluminophosphatebased mesostructured lamellar materials by solid-state NMR spectroscopy. J Am Chem Soc 127(8):2731–2740. https://doi.org/10.1021/ja040167i 57. Zhou D, Xu J, Yu J, Chen L, Deng F, Xu R (2006) Solid-state NMR spectroscopy of anionic framework aluminophosphates: A new method to determine the Al/P ratio. J Phys Chem B 110(5):2131–2137. https://doi.org/10.1021/jp056335q

130

3 Solid-State NMR Characterization of Framework Structure …

58. Gougeon RD, Bodart PR, Harris RK, Kolonia DM, Petrakis DE, Pomonis PJ (2000) Solidstate NMR study of mesoporous phosphoro-vanado-aluminas. Phys Chem Chem Phys 2(22):5286–5292. https://doi.org/10.1039/B005598K 59. Gerothanassis IP (2010) Oxygen-17 NMR spectroscopy: basic principles and applications (part I). Prog Nucl Magn Res Sp 56(2):95–197 60. Gerothanassis IP (2010) Oxygen-17 NMR spectroscopy: basic principles and applications (part II). Prog Nucl Magn Res Sp 57(1):1–110 61. Ashbrook SE, Smith ME (2006) Solid state 17O NMR—an introduction to the background principles and applications to inorganic materials. Chem Soc Rev 35(8):718–735 62. Wang M, Wu X-P, Zheng S, Zhao L, Li L, Shen L, Gao Y, Xue N, Guo X, Huang W, Gan Z, Blanc F, Yu Z, Ke X, Ding W, Gong X-Q, Grey CP, Peng L (2015) Identification of different oxygen species in oxide nanostructures with 17 O solid-state NMR spectroscopy. Sci Adv 1(1). https://doi.org/10.1126/sciadv.1400133 63. Peng L, Liu Y, Kim N, Readman JE, Grey CP (2005) Detection of Brønsted acid sites in zeolite HY with high-field 17O-MAS-NMR techniques. Nat Mater 4(3):216–219 64. Peng L, Huo H, Liu Y, Grey CP (2007) 17O magic angle spinning NMR studies of Brønsted acid sites in zeolites HY and HZSM-5. J Am Chem Soc 129(2):335–346 65. Janes N, Oldfield E (1986) Oxygen-17 NMR study of bonding in silicates: the d-orbital controversy. J Am Chem Soc 108(19):5743–5753. https://doi.org/10.1021/ja00279a014 66. Amoureux JP, Bauer F, Ernst H, Fernandez C, Freude D, Michel D, Pingel UT (1998) 17O multiple-quantum and 1H MAS NMR studies of zeolite ZSM-5. Chem Phys Lett 285(1–2):10–14. https://doi.org/10.1016/s0009-2614(97)01462-0 67. Bull LM, Bussemer B, Anupold T, Reinhold A, Samoson A, Sauer J, Cheetham AK, Dupree R (2000) A high-resolution 17O and 29Si NMR study of zeolite siliceous ferrierite and ab initio calculations of NMR parameters. J Am Chem Soc 122(20):4948–4958. https://doi.org/10. 1021/ja993339y 68. Bull LM, Cheetham AK, Anupold T, Reinhold A, Samoson A, Sauer J, Bussemer B, Lee Y, Gann S, Shore J, Pines A, Dupree R (1998) A high-resolution 17O NMR study of siliceous zeolite faujasite. J Am Chem Soc 120(14):3510–3511. https://doi.org/10.1021/ja9743001 69. Timken HKC, Turner GL, Gilson JP, Welsh LB, Oldfield E (1986) Solid-state oxygen-17 nuclear magnetic resonance spectroscopic studies of zeolites and related systems. 1. J Am Chem Soc 108(23):7231–7235. https://doi.org/10.1021/ja00283a017 70. Xu Z, Stebbins JF (1998) Oxygen sites in the zeolite stilbite: a comparison of static, MAS, VAS, DAS and triple quantum MAS NMR techniques. Solid State Nucl Magn Reson 11(3–4):243–251. https://doi.org/10.1016/s0926-2040(97)00019-2 71. Huo H, Peng LM, Gan ZH, Grey CP (2012) Solid-state MAS NMR studies of Bronsted acid sites in zeolite H-mordenite. J Am Chem Soc 134(23):9708–9720. https://doi.org/10.1021/ ja301963e 72. Peng LM, Liu Y, Kim NJ, Readman JE, Grey CP (2005) Detection of Bronsted acid sites in zeolite HY with high-field O-17-MAS-NMR techniques. Nat Mater 4(3):216–219. https:// doi.org/10.1038/nmat1332 73. Peng L, Huo H, Liu Y, Grey CP (2007) O-17 magic angle spinning NMR studies of Bronsted acid sites in zeolites HY and HZSM-5. J Am Chem Soc 129(2):335–346. https://doi.org/10. 1021/ja064922z 74. Peng L, Huo H, Liu Y, Grey CP (2007) 17O magic angle spinning NMR studies of Bronsted acid sites in zeolites HY and HZSM-5. J Am Chem Soc 129(2):335–346. https://doi.org/10. 1021/ja064922z 75. Peng L, Huo H, Gan Z, Grey CP (2008) 17O MQMAS NMR studies of zeolite HY. Microporous Mesoporous Mater 109(1–3):156–162. https://doi.org/10.1016/j.micromeso.2007.04. 039 76. Demarquay J, Fraissard J (1987) Xe-129 NMR of xenon adsorbed on zeolites—relationship between the chemical-shift and the void space. Chem Phys Lett 136(3–4):314–318. https:// doi.org/10.1016/0009-2614(87)80258-0

References

131

77. Fraissard J, Ito T (1988) Xe-129 NMR-study of adsorbed xenon—a new method for studying zeolites and metal-zeolites. Zeolites 8(5):350–361. https://doi.org/10.1016/s01442449(88)80171-4 78. Ito T, Fraissard J (1982) Xe-129 NMR-study of xenon adsorbed on Y zeolites. J Chem Phys 76(11):5225–5229. https://doi.org/10.1063/1.442917 79. Raftery D (2006) Xenon NMR spectroscopy. Annu Rep NMR Spectrosc 57:205–270. https:// doi.org/10.1016/s0066-4103(05)57005-4 80. Barrie PJ, Klinowski J (1992) 129Xe NMR as a probe for the study of microporous solids: a critical review. Prog Nucl Magn Reson Spectrosc 24:91–108. https://doi.org/10.1016/00796565(92)80006-2 81. Bonardet JL, Fraissard J, Gedeon A, Springuel-Huet MA (1999) Nuclear magnetic resonance of physisorbed Xe-129 used as a probe to investigate porous solids. Catal Rev Sci Eng 41(2):115–225. https://doi.org/10.1080/01614949909353779 82. Chen F, Chen C-L, Ding S, Yue Y, Ye C, Deng F (2004) A new approach to determination of micropore size by 129Xe NMR spectroscopy. Chem Phys Lett 383(3–4):309–313 83. Chen F, Deng F, Cheng MJ, Yue Y, Ye CH, Bao XH (2001) Preferential occupation of xenon in zeolite MCM-22 as revealed by Xe-129 NMR spectroscopy. J Phys Chem B 105(39):9426–9432. https://doi.org/10.1021/jp011710+ 84. Chen F, Zhang MJ, Han Y, Xiao FS, Yue Y, Ye CH, Deng F (2004) Characterization of microporosity in ordered mesoporous material MAS-7 by 129Xe NMR spectroscopy. J Phys Chem B 108(12):3728–3734. https://doi.org/10.1021/jp031187u 85. Liu SB, Fung BM, Yang TC, Hong EC, Chang CT, Shih PC, Tong FH, Chen TL (1994) Effect of cation substitution on the adsorption of xenon on zeolite nay and on the Xe-129 chemical-shifts. J Phys Chem 98(16):4393–4401. https://doi.org/10.1021/j100067a030 86. Zeng DF, Yang J, Wang JQ, Xu J, Yang YX, Ye CH, Deng F (2007) Solid-state NMR studies of methanol-to-aromatics reaction over silver exchanged HZSM-5 zeolite. Microporous Mesoporous Mater 98(1–3):214–219. https://doi.org/10.1016/j.micromeso.2006.09.012 87. Springuel-Huet M-A, Bonardet J-L, Gédéon A, Fraissard J (1999) 129Xe NMR overview of xenon physisorbed in porous solids. Magn Reson Chem 37(13):S1–S13. https://doi.org/10. 1002/(SICI)1097-458X(199912)37:13%3cS1:AID-MRC578%3e3.0.CO;2-X 88. Xu J, Zheng AM, Wang XM, Qi GD, Su JH, Du JF, Gan ZH, Wu JF, Wang W, Deng F (2012) Room temperature activation of methane over Zn modified H-ZSM-5 zeolites: insight from solid-state NMR and theoretical calculations. Chem Sci 3(10):2932–2940. https://doi.org/10. 1039/c2sc20434g 89. Enderle BA, Labouriau A, Ott KC, Gates BC (2002) Nanoclusters in nanocages: platinum clusters and platinum complexes in zeolite LTL probed by Xe-129 NMR spectroscopy. Nano Lett 2(11):1269–1271. https://doi.org/10.1021/nl025697w 90. Gedeon A, Bonardet JL, Ito T, Fraissard J (1989) Application of xenon-129 NMR to the study of Ni2+ Y zeolites. J Phys Chem 93(6):2563–2569. https://doi.org/10.1021/j100343a064 91. Barskiy DA, Coffey AM, Nikolaou P, Mikhaylov DM, Goodson BM, Branca RT, Lu GJ, Shapiro MG, Telkki V-V, Zhivonitko VV, Koptyug IV, Salnikov OG, Kovtunov KV, Bukhtiyarov VI, Rosen MS, Barlow MJ, Safavi S, Hall IP, Schroeder L, Chekmenev EY (2017) NMR hyperpolarization techniques of gases. Chem Eur J 23(4). https://doi.org/10.1002/chem. 201603884 92. Nossov A, Guenneau F, Springuel-Huet M-A, Haddad E, Montouillout V, Knott B, Engelke F, Fernandez C, Gédéon A (2003) Continuous flow hyperpolarized 129Xe-MAS NMR studies of microporous materials. Phys Chem Chem Phys 5(20):4479–4483. https://doi.org/10.1039/ B305793N 93. Nagy JB, Aiello R, Giordano G, Katovic A, Testa F, Kónya Z, Kiricsi I (2007) Isomorphous substitution in zeolites. In: Karge HG, Weitkamp J (eds) Characterization II. Springer Berlin Heidelberg, Berlin, Heidelberg, pp 365–478. https://doi.org/10.1007/3829_006 94. Kessler H, Chezeau JM, Guth JL, Strub H, Coudurier G (1987) N.m.r. and i.r. study of B and B Al substitution in zeolites of the MFI-structure type obtained in non-alkaline fluoride medium. Zeolites 7(4):360–366. https://doi.org/10.1016/0144-2449(87)90040-6

132

3 Solid-State NMR Characterization of Framework Structure …

95. Chen L, Zhang MJ, Yue Y, Ye CH, Deng F (2004) NMR and theoretical studies of boronmodified mordenite. Microporous Mesoporous Mater 76(1–3):151–156. https://doi.org/10. 1016/j.micromeso.2004.08.007 96. Arnold A, Steuernagel S, Hunger M, Weitkamp J (2003) Insight into the dry-gel synthesis of gallium-rich zeolite [Ga]Beta. Microporous Mesoporous Mater 62(1):97–106. https://doi. org/10.1016/S1387-1811(03)00397-4 97. Occelli ML, Schwering G, Fild C, Eckert H, Auroux A, Iyer PS (2000) Galliosilicate molecular sieves with the faujasite structure. Microporous Mesoporous Mater 34(1):15–22. https://doi. org/10.1016/S1387-1811(99)00151-1 98. Ni QZ, Daviso E, Can TV, Markhasin E, Jawla SK, Swager TM, Temkin RJ, Herzfeld J, Griffin RG (2013) High frequency dynamic nuclear polarization. Acc Chem Res 46(9):1933–1941. https://doi.org/10.1021/ar300348n 99. Rossini AJ, Zagdoun A, Lelli M, Lesage A, Copéret C, Emsley L (2013) Dynamic nuclear polarization surface enhanced NMR spectroscopy. Acc Chem Res 46(9):1942–1951. https:// doi.org/10.1021/ar300322x 100. Gunther WR, Michaelis VK, Caporini MA, Griffin RG, Román-Leshkov Y (2014) Dynamic nuclear polarization NMR enables the analysis of Sn-Beta zeolite prepared with natural abundance 119 Sn precursors. J Am Chem Soc 136(17):6219–6222. https://doi.org/10.1021/ ja502113d 101. Wolf P, Valla M, Rossini AJ, Comas-Vives A, Núñez-Zarur F, Malaman B, Lesage A, Emsley L, Copéret C, Hermans I (2014) NMR signatures of the active sites in Sn-β zeolite. Angew Chem Int Ed 53(38):10179–10183. https://doi.org/10.1002/anie.201403905

Chapter 4

Solid-State NMR Characterization of Host-Guest Interactions

Abstract This chapter introduces the application of solid-state NMR to elucidate the host-guest interaction in zelite and zeotype materials. Host-guest interactions between adsorbed molecules and active sites strongly influence the performances (activity and selectivity) of solid catalysts because they play essential roles in adsorption, desorption, and catalytic reaction process. Solid-state NMR provides a versatile approach for exploring the host-guest interactions between organic molecules and inorganic moieties of zeolites. The location of the guest molecule confined inside zeolite channels can be deduced by cross-polarization build-up curves. Additionally, the interaction between organic surfactant agent and the framework during the synthesis of porous materials can be monitored by 2D heteronuclear correlation (HETCOR) MAS NMR spectroscopy. The advanced 13 C–27 Al double-resonance MAS NMR spectroscopy for detecting the host-guest interaction during heterogeneous catalytic processes is also discussed in this chapter. Keywords Zeolite · Double-resonance NMR · Host-guest interaction · 2D homoand heteronuclear correlation NMR · Heterogeneous catalysis

4.1 Introduction Host-guest interactions between adsorbed molecules and active sites strongly influence the performances (activity and selectivity) of solid catalysts because they play essential roles in (i) adsorption, (ii) desorption, and (iii) catalytic reaction process [1, 2]. The interactions between the guest molecules and the zeolite framework dictate the size and shape selectivity of products formed in catalytic reactions. Thus, it is essential to understand the reliable structures of zeolite-sorbate host-guest complexes to establish the structure–property relationship [3, 4]. Numerous efforts have been made to investigate the location of guest molecules inside of the zeolite cages by combining various spectroscopies (magnetic resonance, infrared, Raman, X-ray absorption, etc.) and density functional theory (DFT) calculations [5, 6]. Solid-state NMR is a robust spectroscopic tool to gain the detailed information about local structures, dynamic behaviors, as well as host-guest interactions in complicated systems. © Springer Nature Singapore Pte Ltd. 2019 J. Xu et al., Solid-State NMR in Zeolite Catalysis, Lecture Notes in Chemistry 103, https://doi.org/10.1007/978-981-13-6967-4_4

133

134

4 Solid-State NMR Characterization of Host-Guest Interactions

To reveal the zeolite-sorbate host-guest interaction, it is desirable to establish the correlation between zeolite framework and guest molecules, which can be achieved by polarization transfer from guest molecules to zeolite framework via either J-coupling (through-bond) or dipolar interaction (through-space) [7]. The spatial proximity and the distance information between guest molecule and the host zeolite could be extracted from a series of advanced solid-state NMR experiments including 2D homonuclear and heteronuclear correlation NMR and REDOR [8, 9]. Herein, the application of solid-state NMR to some representative host-guest interaction systems including gaseous adsorption, zeolite synthesis process, and heterogeneous catalysis process are briefly introduced.

4.2 Solid-State NMR Characterization of Host-Guest Interactions 4.2.1 Host-Guest Interaction Between Adsorbed Molecule and Zeolite Framework Although single-crystal X-ray diffraction experiments can be used to determine the structure of zeolite-sorbate complexes, it is limited to a very few zeolites with suitable size and quality of single crystal. Fyfe et al. [10] developed NMR strategies to determine the structures of these complexes, which is independent of crystal size of zeolite. The general strategy consists of three steps: (1) using two-dimensional (2D) correlation experiments to assign Si sites of the zeolite framework, (2) determining the internuclear interactions and distance between the nuclei of the guest species and the nuclei of the zeolite framework, and (3) determining the location of the guest species in zeolite channel. By this strategy, the guest molecules (pDCB, p-dichlorobenzene) in very highly siliceous zeolite ZSM-5 were studied [10]. A 2D 29 Si INADEQUATE experiment [11] was used to assign the Si sites in the zeolite framework. As shown in Fig. 4.1, 12 unique Si sites were clearly observed and their connectivities were established. The 1 H/29 Si CP MAS NMR experiments were further performed for the determination of the location of the guest molecules in the zeolite framework. Since the rate of magnetization transfer between 1 H and 29 Si nuclei depends on the strength of the dipolar couplings, the 1 H/29 Si CP MAS NMR provides information about 1 H–29 Si internuclear distances. Figure 4.2 shows the 29 Si MAS and 1 H/29 Si CP MAS spectra. The relatively increased peaks in the CP spectrum show that the 1 H nuclei of the pDCB molecule are close to Si8 and Si9 sites, while far away from Si4 and Si10 sites, and at a medium distance from the other Si sites. Considering the locations of these Si sites, the molecules are likely located in the straight channels or channel intersections and not in the zigzag channels. The exact location of the guest molecules in zeolite framework can be determined from the 1 H/29 Si CP rate constants, which are measured by performing a variable contact time 1 H/29 Si CP MAS experiment and plotting the 29 Si peak areas as functions of the contact time. The relative CP rate constants (k IS ) can be obtained from the

4.2 Solid-State NMR Characterization of Host-Guest Interactions

135

Fig. 4.1 Two-dimensional 29 Si MAS INADEQUATE spectrum of the low-loaded pDCB/ZSM-5 complex at 275 K. The indicated peak assignments were determined from the observed correlations using a peak assignment algorithm. Reproduced from Ref. [12] by permission of Elsevier Fig. 4.2 a Quantitative 29 Si MAS and b 1 H/29 Si CP MAS spectra of the low-loaded pDCB/ZSM-5 complex at 280 K. The quantitative spectrum was acquired with a 30-s recycle delay. Reproduced from Ref. [12] by permission of Elsevier

136

4 Solid-State NMR Characterization of Host-Guest Interactions

CP experiments, and absolute k IS for the CP rate constants can be obtained from CP experiments [13]. The k IS is proportional to the heteronuclear second moments drain M2IS which is a constant as well. For the spin-1/2 nuclei, the heteronuclear second moment for powder can be calculated as a pairwise summation of the inverse sixth powers of the I-S internuclear distances (in units of Hz2 ) as described in Eq. 4.1 [14]:  M2 =

1 2π

2   4 1 μ0 2  1 γ I γS  6 5 2 4π rIS

(4.1)

Assuming the structure is rigid, in that case, for the determination of the guest molecule locations in host zeolite frameworks, the proportionality between the 1 H/29 Si CP rate constants and the heteronuclear second moments obtained H–Si distances should be satisfied (Eq. 4.2). kIS ∝ M2 ∝



r −6

(4.2)

The location of adsorbed molecule was found by systematically searching for the calculated sorbate-zeolite complexes which were in good consistence with the experimental data. The location of adsorbed molecule was determined from a large number of solutions in combination with the 29 Si CP MAS NMR data as illustrated in Fig. 4.2. This method provides insights into the host-guest interaction model from cross-polarization from adsorbate to zeolite framework. Understanding the interactions between guest species and host zeolite frameworks, which is essential to the synthesis and applications of zeolites, requires detailed structural information for these complexes [4, 15]. The location and distribution of molecular oxygen confined inside the zeolite could be determined from the paramagnetic effect of molecular oxygen. Fyfe and Brouwer [16] found that removal of the oxygen by purging the sample with nitrogen gas reduces the paramagnetic effect and leads to a dramatic increase in the resolution of the 29 Si MAS NMR spectrum of p-dibromobenzene/ZSM-5 complex as shown in Fig. 4.3. The true nature of the line broadening in 29 Si MAS NMR spectra is ascribed to the variation of 29 Si relaxation times. The individual 29 Si nuclei have quite different relaxation times, suggesting that the oxygen molecules are localized relatively specifically in the zeolite channel system. As shown in Fig. 4.4, different Si atoms of the ZSM-5 framework are coded according to their T1 relaxation times. The shortest T1 values belong to Si4 , Si5 , Si6 , and Si9 which are part of the “zigzag” channel, while the longest belong to Si8 and Si11 which are in the channel intersection. These different relaxation times suggest that the oxygen molecules are localized mainly in the zigzag channels. This in turn suggests that the pDBB molecules are located in the channel intersection, blocking Si8 and Si11 from the O2 molecules [16]. This work provides a simple method for roughly determining the location of the sorbate molecules. Inorganic materials such as zeolites and layered materials have potential uses as new intelligent materials since the porous framework and the interlayer space

4.2 Solid-State NMR Characterization of Host-Guest Interactions

137

Fig. 4.3 Variable-temperature 29 Si MAS NMR spectra of the pDBB/ZSM-5 complex with a loading of 4.0 molecules/u.c. a Sample packed in air and spun with air as the drive gas. b Sample purged with N2 gas for 24 h and spun with N2 as the drive gas. Reproduced from Ref. [16] by permission of American Chemical Society

can accommodate various guest molecules. Characterization of the nanocomposites is one of the most important subjects from the viewpoint of efficient designs of functional materials [17, 18]. The ZSM-5/p-nitroaniline (pNA) system is a typical example of the nanocomposite. The dynamics of p-nitroaniline (pNA) molecules in the micropores of siliceous ZSM-5 zeolite have also been investigated by means of solid-state 2 H NMR [19]. Figure 4.5 shows the results of the simulation at selected temperatures together with the corresponding experimental spectra. The most appro-

138

4 Solid-State NMR Characterization of Host-Guest Interactions

Fig. 4.4 ZSM-5 framework with Si sites coded according to 29 Si T1 relaxation times for the pDBB/ZSM-5 complex with 3.5 molecules/u.c. at 300 K. Reproduced from Ref. [16] by permission of American Chemical Society

priate rates of the 180° flip-flop motion of aromatic ring are 20, 50, 300, 1, and 3 MHz at 286, 297, 323, 342, and 361 K, respectively [19]. Figure 4.6 shows a plot of the rate versus inverse temperature. The relation between the rate of the motion and the temperature obeys the Arrhenius law, and the apparent activation energy is estimated to be 57 kJ mol−1 [19]. These results indicate that the motion of pNA in the micropores of ZSM-5 depended on the adsorbed amount of pNA and that the interaction between the guest molecule and zeolite framework influences the dynamics of the guest molecules.

4.2.2 Host-Guest Interaction in Molecular Sieve Synthesis Solid-state NMR spectroscopy has also been employed to investigate the hostguest interactions in various ordered mesoporous materials. Periodic mesoporous organosilicas (PMOs) attract a great deal of interest because of their duality in the framework that reflects the hybrid nature of the channel walls [20, 21]. Twodimensional 1 H–13 C and 1 H–29 Si solid-state HETCOR NMR techniques were used to study the structure–property relationship of periodic mesoporous organosilicas (PMOs) hybrid p-phenylenesilica with crystalline order in the walls [22]. As shown in 1 H–29 Si 2D Lee–Goldburg HETCOR NMR spectrum (Fig. 4.7a), the silanol

4.2 Solid-State NMR Characterization of Host-Guest Interactions

139

Fig. 4.5 2 H NMR spectra of ZSM-5/p-nitroaniline-d with the medium loading level (left column) and their simulated results (right column). The sample temperatures and the rates of the motion are attached to the spectra. Reproduced from Ref. [19] by permission of Royal Society of Chemistry

groups are correlated only with T2 silicons within two bonds, whereas the aromatic hydrogens are correlated with both silicon species T2 and T3 sites. According to the schematic structure, each T2 and T3 silicon site locates at the same distance of ca. 3 Å from the aromatic hydrogens. Interestingly, in the 1 H–13 C HETCOR spectrum (Fig. 4.7b), the silanols correlate with both the aromatic carbons, suggesting that the dangling Si–OH bond adopts conformations folded toward the aromatic ring [22]. Moreover, in the meosoporous material with surfactants, the interactions between the polar head groups and aromatic layers of the matrix walls could be revealed by using 1 H–13 C and 1 H–29 Si Lee–Goldburg [23] HETCOR NMR spectroscopy, which indicates that the aromatic wall faces to the nanochannel as shown in Fig. 4.8. These experimental results reveal the structure of interfaces between the matrix walls and the guest molecules confined in the nanochannels [22].

140

4 Solid-State NMR Characterization of Host-Guest Interactions

Fig. 4.6 A plot of the rate of the motion, k (Hz), versus inverse temperature for ZSM-5/pnitroaniline-d with medium loading level. Reproduced from Ref. [19] by permission of Royal Society of Chemistry

Fig. 4.7 a 2D 1 H–29 Si and b 2D 1 H–13 C Lee–Goldburg HETCOR NMR spectra of mesoporous p-phenylenedilica acquired at a spinning speed of 15 kHz and a contact time of 2 ms. Reproduced from Ref. [22] by permission of American Chemical Society

4.2 Solid-State NMR Characterization of Host-Guest Interactions

141

Fig. 4.8 Structure of the nanocomposite interface showing the hybrid building blocks of the nanochannel walls interacting with the polar heads of the surfactant molecules as highlighted by 1 H–13 C and 1 H–29 Si 2D MAS NMR. Reproduced from Ref. [22] by permission of American Chemical Society

The incorporation of aluminum into the silica framework is important for the generation of active sites for catalytic reactions. Tetrahedrally coordinated Al is generally considered to be in framework, while octahedrally coordinated Al may be formed as the undesirable framework species. 2D 1 H–27 Al and 1 H–29 Si HETCOR techniques have been used to identify interfacial species and establish framework locations of aluminum atoms incorporated in aluminosilicate MCM-41 mesophases [24]. In the work of Chmelka et al., modification of the 2D 1 H–27 Al HETCOR with incorporated proton spin diffusion allows to observe 1 H–27 Al correlations including weakly coupled species. Figure 4.9 shows 2D spin-diffusion 1 H–27 Al HETCOR spectra of as-synthesized hexagonal aluminosilica MCM-41 prepared under hydrothermal conditions acquired using mixing times of 15 and 25 ms, during which 1 H magnetization is mixed among nearby proton species. At short mixing time of 15 ms (Fig. 4.9a), all of the different proton species from water and cetyltrimethylammonium bromide (CTAB) surfactant species show strong correlations with the four-coordinated aluminum. This confirms the strong electrostatic interactions between the cationic ammonium methyl head groups and anionic tetrahedral framework Al sites. Strong correlation between six-coordinated aluminum and water was observed as well. As shown in Fig. 4.9b, a longer mixing times (t mix = 25 ms) makes the relatively weakly coupled 1 H and 27 Al species more visible. The correlation of protons from alkyl CTA+ chains with the six-coordinated Al can be observed. Therefore, it can be concluded that the six-coordinated Al sites are formed in the framework and not present as phase-separated Al2 O3 . In addition, it was also recognized that the transformation of disordered frameworks into molecularly ordered silicate sheets depended strongly on the charge-density of the surfactant head groups [25].

142

4 Solid-State NMR Characterization of Host-Guest Interactions

Fig. 4.9 1 H–27 Al HETCOR spectra of the hydrothermally synthesized aluminosilicate MCM-41 with a mixing time of 15 ms (a) 25 ms (b). Reproduced from Ref. [24] by permission of American Chemical Society

4.2 Solid-State NMR Characterization of Host-Guest Interactions

143

The host-guest interaction between CTAB surfactant and zeolite framework plays an essential role in the crystallization and self-assembly during zeolite synthesis. The spatial proximity between zeolite framework and CTAB surfactant could be established with 1 H–29 Si HETCOR experiment. 1 H MAS NMR provided direct information about the surfactant species in zeolites. The 1 H NMR chemical shifts were unambiguously assigned according to the 1 H–13 C HETCOR spectra (Fig. 4.10a), in which only short-range correlation could be discerned with a short contact time of 0.3 ms. As shown in Fig. 4.10a, there were two main peaks at ca. 1.4 and 3.2 ppm in the 1 H dimension, which were attributed to the segment methylene (CH2 ) group and methyl (CH3 ) species directly connected to the ammonium groups of CTAB, respectively. The 2D 1 H–29 Si HETCOR NMR spectrum (Fig. 4.10b) revealed the essential role of the CTAB surfactant in supporting the swollen structure of multi-lamellar MWW zeolites [26]. The 29 Si signals in the chemical shift range of −110 to −118 ppm and at −95 ppm were assigned to Q4 and Q3 sites in Si-ECNU7P zeolite, respectively. The framework Q4 sites exhibited the strongest correlation peaks with −N+ (CH3 )3 segment in CTAB, implying the existence of strong intermolecular interactions between the quaternary ammonium head groups of CTAB molecule and the zeolite framework. A skeletal drawing of 3D MWW structure showing only Si atoms as T sites is shown in Fig. 4.10c. The adjacent MWW layers were joined by Si–O–Si linkages, forming a two-dimensional channel between layers. There were 12-membered-ring (MR) hemicavities on the layer surface, and a large supercage formed between layers in the 3D MWW zeolite (Fig. 4.10d). Thus, it could be deduced that only small parts of CTA+ surfactants may interact with the free silanols (T6 site). Besides, as the synthesis of ECNU-7P was performed under basic conditions, the head group of the remaining CTA+ (OH– ) may be embedded into the hemicavities of MWW layers through the intermolecular hydrogen bonding with the bridge oxygen atoms attached to the framework Q4 sites (Fig. 4.10e, f). This result confirmed the evolution process of ECNU-7P crystallization at the early stage, during which the parent ITQ-1 was dissolved to silica segments initially and then self-assembled into weakly ordered mesophase in the presence of CTAB surfactant [26]. The NMR analysis of these two samples provided more evidence about the development and evolution process of transient framework structures during the hydrothermal synthesis. The multi-lamellar Si-ECNU-7P was constructed through the dissolution–recrystallization process. The crystallization and the self-assembly were highly coupled, as the simple surfactant CTAB acted as the structure-directing agent for mesophase or pillaring agent for MWW monolayers at different crystallization stages [26]. Aluminophosphate molecular sieves (AlPO4 -n) are usually prepared by hydrothermal synthesis, and the crystalline molecular sieves are produced from the gel phases. In order to better understand the mechanisms of molecular sieve crystallization, it is desirable to obtain the detailed structural information on the status of the surfactant upon the evolution of intermediate phases [27]. 2D 1 H–1 H DQ MAS NMR was utilized to probe proton−proton proximities among various components of surfactant CTA+ in order to investigate the arrangement of the organic surfactant in the mesostructured aluminophosphate [28]. Figure 4.11 shows 2D 1 H–1 H

144

4 Solid-State NMR Characterization of Host-Guest Interactions

Fig. 4.10 a 1 H–13 C and b 1 H–29 Si HETCOR NMR spectra of Si-ECNU-7P. 1D 13 C CP/MAS, 29 Si CP/MAS, and 1 H MAS spectra are shown along the axes. The CP contact time was set to be 0.3 and 4 ms, respectively, to acquire 1 H–13 C and 1 H–29 Si HETCOR NMR spectra. Skeletal drawings of c the framework of 3D MWW structure, d the MWW supercage with labeled T atoms, and the structure of ECNU-7P viewed along e a axis and f c axis. Reproduced from Ref. [26] by permission of American Chemical Society

DQ MAS NMR spectra of aluminophosphate solids with different hydrothermal treatment times. The peaks A, B, and C represent alkyl chain CH3 protons, alkyl chain CH2 protons, and polar head N−CH3 /N−CH2 −protons, respectively. In the 2D 1 H–1 H DQ MAS NMR spectrum of the 1-h heated sample (Fig. 4.10a), three autocorrelation peaks (AA, BB, and CC) and two pairs of off-diagonal peaks (AB and BC) can be clearly observable, which could be resulted from hexagonal array of cylindrical micelles of CTA+ cations. As the solid is heated for 3 h (Fig. 4.11b), a new off-diagonal peak AC appears, indicative of the intermolecularly spatial proximity between the non-polar tail CH3 protons and the polar head N−CH3 /N−CH2 −or N(CH3 )+4 protons in a planar structure, in which the surfactant is aggregated in the interdigitated bilayer [28]. It is revealed that there might be a tilt angle for the long axes of the cetyl chains with respect to the aluminophosphate layers. In the spectrum of the 50-h heated sample (Fig. 4.11c), the off-diagonal peak AC is still observable, suggesting that the CTA+ bilayer also exists in a tail-to-head form. By utilizing the 2D 1 H–1 H DQ MAS NMR spectroscopy, the status of the surfactant was found to

4.2 Solid-State NMR Characterization of Host-Guest Interactions

145

Fig. 4.11 2D 1 H–1 H DQ MAS NMR spectra of the aluminophosphate solids with hydrothermal treatment times of 1 (a), 3 (b), and 50 h (c). The arrangement of the organic surfactant in the mesostructured aluminophosphate during crystallization process (d). Reproduced from Ref. [28] by permission of American Chemical Society

be arrayed interdigitated in a bilayer with a tilt angle in L1 phase whereas perpendicular in L2 phase, which facilitates the phase transition during molecular sieve crystallization process [28]. Solid-state NMR spectroscopy is also robust for characterizing inorganic-organic hybrid mesostructured materials, which offers the possibility to directly investigate both the bulk (silica and/or alumina) and surface functionalities such as adsorbates, grafted molecules, and organic fragments. However, the relatively lower sensitivity of conventional solid-state NMR methods prevents the precise exploration of hostguest interaction in the functionalized mesostructured materials. Recently, Lesage et al. utilized DNP-enhanced solid-state NMR to investigate the host-guest interaction during the functionalization of siloxanes [29]. Figure 4.12 displays the 1 H–13 C CP/MAS DNP spectra of organic-inorganic mesostructured material in which the surface is functionalized with phenol. The spectra were obtained with and without microwave irradiation. It was found that the signal enhancement factor was estimated to be greater than 56, which enabled the investigation of the surface substrate at natural isotopic abundance. Additionally, 2D surface-enhanced 1 H–29 Si CP DNPNMR was also employed to identify and compare the bonding topology of functional groups in materials obtained via a sol−gel process or by post-grafting reactions [30].

146

4 Solid-State NMR Characterization of Host-Guest Interactions

Fig. 4.12 1 H–13 C CP/MAS DNP spectra of organic-inorganic mesostructured materials (in which the surface is functionalized with phenol) acquired with and without MW irradiation at 263 GHz. Reproduced from Ref. [29] by permission of American Chemical Society

4.2.3 Host-Guest Interaction in Zeolite Catalysis The study of host-guest interactions in zeolites is a straightforward and practical way for better understanding the detailed mechanism of catalytic reaction [31]. 13 C MAS NMR of adsorbed 2–13 C-acetone molecule is frequently used as a probe for characterizing the acidic properties of Brønsted and Lewis acid sites, which are associated in zeolites with framework (FAL) and extra-framework (EFAL) Al species, respectively [32, 33]. In such systems, knowing carbon−aluminum proximities is essential to probe the interactions between zeolite and acetone (and its reaction products). This kind of information can be achieved by 13 C–{27 Al} symmetry-based rotational-echo saturation-pulse double-resonance (S-RESPDOR) experiments [34]. The doubleresonance 13 C and 27 Al NMR experiments are challenging due to their very close Larmor frequencies (about 3.6 MHz difference at 9.4 T). Simultaneous observation of 13 C and 27 Al nuclei is precluded with conventional NMR probes. However, with the help of a frequency splitter and advanced new NMR methods, carbon−aluminum dipolar couplings, and hence distances, can be evaluated [35–37]. Figure 4.13 shows 13 C–{27 Al} S-RESPDOR spectra of 2–13 C-acetone adsorbed on dealuminated HY zeolite acquired with and without 13 C–27 Al dipolar dephasing [37]. The interaction strength between the acetone molecules and the acid sites was related to the dipole dephasing ratio between S and S0 . It could be clearly observed that all of the 13 C signals in the chemical shift range of 228–240 ppm are subject to a strong 13 C–27 Al dipolar dephasing, which could be ascribed to either the hydrogenbond interaction between the carbonyl oxygen of acetone and the Brønsted acid proton or to acetone directly bound to the Al atom of the Lewis acid site. The 13 C signals at 228 and 234 ppm could be resulted from acetone adsorbed on the Brønsted acid site having interaction with Lewis acid site, whereas the signal at 240 ppm could be due to acetone directly adsorbed on the Lewis acid site [38]. The relatively

4.2 Solid-State NMR Characterization of Host-Guest Interactions

147

Fig. 4.13 13 C–{27 Al} S-RESPDOR NMR spectra for 2–13 C-acetone loaded on dealuminated HY zeolite. The blue and red lines represent the spectra observed with (S) and without (S0 ) 13 C–{27 Al} S-RESPDOR dipolar dephasing. Reproduced from Ref. [37] by permission of American Chemical Society

smaller dephasing for the carbonyl group (214 ppm) [39] can probably be due to steric hindrance effects due to a relatively greater size of diacetone alcohol/mesityl oxide. The vinyl groups of mesityl oxide associated with signals within the 188–199 ppm range shows smaller 13 C–27 Al dipolar dephasing. The quaternary carbon at 75 ppm was subject to an intense 13 C–27 Al dipolar dephasing, which is possibly due to the strong hydrogen-bond interaction between the neighboring OH group in diacetone alcohol and the Brønsted acid site. Therefore, the 13 C–27 Al S-RESPDOR solid-state NMR provided experimental evidence of the interaction models between acetone and Brønsted and Lewis acid sites in dealuminated HY zeolite. The distances between the adsorbed acetone and reaction active centers can be quantitatively determined. As shown in Fig. 4.14, the 13 C−27 Al dipolar coupling constants between acetone and the Brønsted acid with the Lewis acid site being in close proximity (234 ppm) and that between acetone and Lewis acid (240 ppm) sites were determined to be 205 and 315 Hz, respectively. Accordingly, the distances between the carbonyl carbon of acetone and the nearby FAL and EFAL species were estimated to be 3.4 and 2.9 Å, respectively. In addition, the spatial interaction between the adsorbed reactants with different active centers (Brønsted and Lewis acids) can be also clearly revealed by 2D 27 Al–{13 C} D-HMQC experiment [40]. Figure 4.15 shows 2D 27 Al MQMAS and 2D 27 Al–{13 C} D-HMQC spectra of 2–13 Cacetone adsorbed on dealuminated HY. Four 27 Al signals were observable in the 27 Al MQMAS spectrum acquired at 18.8 T (Fig. 4.15a), which can be unambiguously assigned to four-coordinated FAL (A), four-coordinate EFAL (B), five-coordinate EFAL (C), and six-coordinate EFAL (D), respectively. Meanwhile, the interaction between acetone and FAL/EFAL (or Brønsted/Lewis acid, respectively) species is observable in the 27 Al-{13 C} D-HMQC spectrum (Fig. 4.15b). The appearance of correlation peak pairs at (228, 53) and (229, 22) ppm suggested that the acetone molecule is in spatial proximity to both FAL and EFAL species, which agreed with the previous experimental and computational results [37], confirming the existence of Brønsted/Lewis acid synergy and the close spatial proximity of FAL–EFAL in the

148

4 Solid-State NMR Characterization of Host-Guest Interactions

Fig. 4.14 S/S 0 experimental 13 C–{27 Al} S-RESPDOR fractions for the signals at a 234 and b 240 ppm. The best-fit 13 C–27 Al dipolar coupling constants are 205 and 315 Hz, respectively. Reproduced from Ref. [37] by permission of American Chemical Society

dealuminated HY zeolite [38]. Similar results can be found on the basis of the appearance of the correlations at (234, 48) and (234, 28) ppm. In addition, the correlation peak at (240, 29) ppm justified the proposed model of acetone bounded with EFAL species (Lewis acid site). Moreover, the strong correlations between 13 C signals at 183 and 214 ppm and 27 Al signals at 26–60 ppm are clearly observable, suggesting the spatial proximity of acetic acid and carbonyl sites of aldol reaction products to the FAL and/or five-coordinated EFAL [41]. Even though the host-guest interactions between reactants/intermediates and the acid sites could be easily demonstrated from the DFT theoretical calculations, the experimental evidence to describe this kind of interaction is still lacking so far. The combination of in situ 13 C MAS NMR and 13 C–{27 Al} S-RESPDOR experiment may pave a new avenue for monitoring different reaction processes and elucidating the reaction mechanism in heterogeneous catalysis. The requirement of organic compounds on active catalysts to form reaction centers is not common in catalysis but distinctive in methanol-to-olefins (MTO) chemistry on acidic zeolites [42–45]. Haw et al. proposed supramolecular entity to describe the reaction center constituted by zeolite channel or cage and trapped organic component (hydrocarbon pool, HP species) [46]. Large organic molecules like methylbenzenes (MBs) are readily formed in the MTO reaction and are trapped in zeolite channels. Additionally, various C5 and C6 cyclic carbocations have been identified as trapped compounds on different zeolites and zeotype catalysts [47–51]. Both organic and inorganic com-

4.2 Solid-State NMR Characterization of Host-Guest Interactions

149

Fig. 4.15 a 27 Al 3QMAS and b 27 Al–{13 C} D-HMQC spectra of 2–13 C-acetone loaded on dealuminated HY zeolite acquired at 18.8 T under 20 kHz MAS. The asterisk denotes spinning sidebands. Reproduced from Ref. [37] by permission of American Chemical Society

ponents are indispensable and work together to generate olefins. The formation of supramolecular reaction center can be identified by 13 C–{27 Al} S-RESPDOR experiment [52]. Figure 4.16 shows 13 C spectra of trapped products obtained from reactions of methanol over H-ZSM-5 at 300 and 350 °C for 15 min. At 300 °C, as shown in Fig. 4.16a, all the signals in the up-field from 0 to 60 ppm are subject to different degrees of 13 C–27 Al dipolar dephasing, demonstrating the spatial interaction/proximity between different 13 C species and framework 27 Al sites. The signals

150

4 Solid-State NMR Characterization of Host-Guest Interactions

Fig. 4.16 13 C MAS NMR spectra of trapped products obtained from reactions of methanol over HZSM-5 at 300 and 350 °C for 15 min. The red spectrum is detected with 13 C–{27 Al} S-RESPDOR dipolar dephasing and the black spectrum without 13 C–{27 Al} S-RESPDOR dipolar dephasing. Reproduced from Ref. [52] by permission of Wiley

at 50 and 60 ppm could be assigned to methanol and dimethyl ether (DME) reactants, respectively, while the signals at 59 and 48 ppm were due to methoxy species typically formed in the MTO reaction and saturated carbon atoms in the rings of cyclopentenyl cations, respectively. With the assistance of 2D 1 H–13 C HETCOR NMR experiment, the signals at 17 and 18 ppm were assigned to the methyl groups of aromatics (mainly pentamethylbenzene and 1, 2, 3, 5-tetramethylbenzene) and the signal at 25 ppm to the methyl groups of cyclopentenyl cations and pentamethylbenzenium ion. The 13 C–{27 Al} S-RESPDOR experimental observations suggest the existence of spatial interaction/proximity between trapped HP species and zeolite framework (Brønsted acid/base site), indicative of the formation of supramolecular reaction centers (SMCs). Further analysis of the 13 C–27 Al dipole interaction shows that MBs interact with Brønsted acid site (SiOHAl) by forming a π-complex, while cyclic carbocations interact with Brønsted base site (SiO− Al) by forming an ion-pair complex. Increasing the reaction temperature to 350 °C leads to a considerable consumption of DME (60 ppm) and methanol (50 ppm) reactants (Fig. 4.15b). A slight reduction of S/S 0 occurs on the signals at 48 and 25 ppm, whereas the S/S 0 values for the signals at 18 and 17 ppm are dramatically decreased, implying that the C5 and C6 cyclic carbocations are still in close proximity to but the MBs become more distant from the framework Al sites. This is likely due to the facile transformation and diffusion of MBs at higher temperature, and much broader distribution of MBs results in a large average 13 C–27 Al distance to the framework Al sites.

4.2 Solid-State NMR Characterization of Host-Guest Interactions

151

Table 4.1 S/S 0 (%) experimental 13 C–{27 Al} S-RESPDOR fractions for the signals of trapped organic compounds obtained from the reactions of methanol over H-ZSM-5 at 300 and 350 °C for 15 min Chemical shifts (ppm)

17

18

25

48

59

S/S 0 (300 °C)a

16.9

39.3

40.7

52.1

72.6

S/S 0 (350 °C)b

2.2

1.3

35.5

46.3

77.8

a The b The

S/S 0 value for the reaction at 300 °C S/S 0 value for the reaction at 350 °C

In combination with 12 C/13 C isotope exchange experiments, it was found that the internuclear spatial interaction/proximity between the 13 C nuclei (associated with HP species) and the 27 Al nuclei (associated with Brønsted acid/base sites) determined the reactivity of the HP species. The closer the HP species to zeolite framework Al, the higher reactivity they possess in the MTO reaction [52] (Table 4.1). The topology structure of zeolites has a significant influence on the MTO reaction. The conversion, products selectivity, catalyst deactivation, as well as the reaction routes can be related to the variation of the channel system (shape and dimension) in zeolites [53–57]. The confinement effect imposed by zeolite channels often leads to host-guest interactions between the zeolite framework host and the HP species guest. This implied that the product shape selectivity of zeolite might affect the host-guest interaction, leading to variable reactivity of HP species. The host-guest interactions during the MTO reaction were explored by the 13 C–27 Al double-resonance NMR on H-ZSM-5 (MFI, sinusoidal channel: 5.5 × 5.1 Å2 , straight channel: 5.3 × 5.6 Å2 ), H-SSZ-13 (CHA cage: 6.7 × 10.9 Å2 , 8-MR window opening: 3.8 × 3.8 Å2 ) and HMOR (MOR, 12-MR channel: 6.7 × 7.0 Å2 ) zeolites which are featured by different shape selectivity [58]. It was demonstrated that both carbenzenium ions (mainly cyclopentenyl cations) and aromatics (mainly MBs) can form the SMCs over HZSM-5, H-SSZ-13, and H-MOR zeolites. The cyclic cations strongly interact with the Brønsted acid/base sites on zeolites. The obtained S/S 0 from the double-resonance NMR experiments reflects the reactivity of the SMCs and their behavior over the three zeolites. For comparison, the SMCs composed by the retained MBs display different behavior over the three zeolites. At higher reaction temperature, the MBs gradually move away from the Brønsted acid sites over H-ZSM-5, indicative of the possibly decreasing catalytic function of this HP species in the MTO reaction. But this is not the case for H-SSZ-13 and H-MOR where the retained MBs are keeping close to the framework active sites throughout the reaction. This means the reactivity of the MBs would not markedly change and play an important role in the whole reaction over H-SSZ-13 and H-MOR. These observations evidence the influence of the shape selectivity of zeolite on the host-guest interactions and the formation of SMCs. The 13 C–27 Al double-resonance experiments can be used to detect the spatial proximity between coking species and active sites. Figure 4.17 shows the 13 C–27 Al double-resonance NMR spectra of deactivated H-MOR and H-SSZ-13 zeolites.

152

4 Solid-State NMR Characterization of Host-Guest Interactions

Fig. 4.17 13 C MAS NMR spectra of deactivated H-SSZ-13 (a) and H-MOR (b) at 400 °C for 250 and 100 min, respectively. The black and red lines represent the spectrum observed with (S) and without (S 0 ) 13 C–{27 Al} S-RESPDOR dipolar dephasing, respectively. The S/S 0 is indicated in bracket. Reproduced from Ref. [58] by permission of American Chemical Society

Compared with the non-deactivated catalysts, the methyl groups of polyaromatics (coking species) show different spatial proximities to the active sites on the deactivated samples. On deactivated H-SSZ-13, the methyl groups at 16.3 ppm and 19.4 ppm (S/S 0 is 8.5% and 20.9%, respectively) are further away from the active sites compared to those on the non-deactivated one (S/S 0 is 26.8% and 27.5%, respectively), while the methyl groups at the 24.0 and 21.5 ppm signals exhibit increased interactions with the active sites reflected by the higher S/S 0 which are 37.4 and 30.3% compared to 31.0 and 21.6%, respectively on the non-deactivated H-SSZ-13. The increased interactions between the two methyl groups (24.0 and 21.5 ppm) and active sites evidence the formation and growth of the carbonaceous species in the CHA cage of H-SSZ-13. The weaker interactions reflected on the methyl groups (19.4 and 16.3 ppm) suggest that the orientation of carbonaceous species in the cage makes some of the methyl groups farther from the active sites. Another possible explanation is the formation of external coke species. In fact, the formation of larger carbonaceous deposits on the outer surface of SSZ-13 zeolites

4.2 Solid-State NMR Characterization of Host-Guest Interactions

153

Fig. 4.18 Schematic of the coke species (polyaromatics) depositing on the active site (Brønsted acid site) and blocking the zeolite pore in CHA cage of H-SSZ-13 and 12-MR channel of H-MOR zeolite. Reproduced from Ref. [58] by permission of American Chemical Society

has been observed [59]. Similar to H-SSZ-13, the different interactions of the carbonaceous species with active sites are also observed on deactivated H-MOR zeolite. Since the growth of the extended coke compounds on the external zeolite surface has been suggested by Svelle et al. [60], the accumulation of these species would lead to blockage of the pores, resulting in the complete deactivation. The schematic illustration of the distribution of the coking species in the CHA cage of H-SSZ-13 and 12-MR channel of H-MOR is shown in Fig. 4.18. The active sites are covered and the channels are blocked in the deactivated zeolites.

4.3 Summary The host-guest chemistry is widely involved in material synthesis and catalytic reactions. Solid-state NMR especially 2D or double-resonance technique provides a versatile approach for the spatial proximity/investigation between organic molecules and inorganic moieties of solid catalysts. Firstly, the location of the guest molecule confined inside zeolite channels can be explored by cross-polarization build-up curves or paramagnetic relaxation effect. Additionally, the interaction between organic surfactant agent and inorganic framework during the synthesis of porous materials can be probed by multi-nuclear and multi-dimensional solid-state NMR spectroscopy. Moreover, advanced 13 C–27 Al double-resonance MAS NMR spectroscopy enables observation of the host-guest interaction during heterogeneous catalytic reaction processes on zeolites. The detailed insights into the host-guest chemistry obtained from the NMR characterization are essential for the optimizing synthesis conditions to prepare desired zeolites and for the elucidation of reaction mechanism toward the establishment of structure–activity relationship.

154

4 Solid-State NMR Characterization of Host-Guest Interactions

References 1. Wu D, Hwang S-J, Zones SI, Navrotsky A (2014) Guest-host interactions of a rigid organic molecule in porous silica frameworks. Proc Natl Acad Sci 111(5):1720–1725. https://doi.org/ 10.1073/pnas.1323989111 2. Corma A (2003) State of the art and future challenges of zeolites as catalysts. J Catal 216(1):298–312. https://doi.org/10.1016/S0021-9517(02)00132-X 3. Shimizu LS, Salpage SR, Korous AA (2014) Functional materials from self-assembled bis-urea macrocycles. Acc Chem Res 47(7):2116–2127. https://doi.org/10.1021/ar500106f 4. Van Speybroeck V, Hemelsoet K, Joos L, Waroquier M, Bell RG, Catlow CRA (2015) Advances in theory and their application within the field of zeolite chemistry. Chem Soc Rev 44(20):7044–7111. https://doi.org/10.1039/c5cs00029g 5. Brogaard RY, Weckhuysen BM, Nørskov JK (2013) Guest-host interactions of arenes in H-ZSM-5 and their impact on methanol-to-hydrocarbons deactivation processes. J Catal 300:235–241. https://doi.org/10.1016/j.jcat.2013.01.009 6. Li S, Deng F (2013) Chapter one—recent advances of solid-state NMR studies on zeolites. In: Webb GA (ed) Annual reports on NMR spectroscopy, vol 78. Academic Press, pp 1–54. https://doi.org/10.1016/B978-0-12-404716-7.00001-8 7. Lesage A, Emsley L (2001) Through-bond heteronuclear single-quantum correlation spectroscopy in solid-state NMR, and comparison to other through-bond and through-space experiments. J Magn Reson 148(2):449–454. https://doi.org/10.1006/jmre.2000.2249 8. Ramamoorthy A, Wu CH, Opella SJ (1999) Experimental aspects of multidimensional solidstate NMR correlation spectroscopy. J Magn Reson 140(1):131–140. https://doi.org/10.1006/ jmre.1999.1827 9. Hing AW, Vega S, Schaefer J (1992) Transferred-echo double-resonance NMR. J Magn Reson 96(1):205–209. https://doi.org/10.1016/0022-2364(92)90305-q 10. Fyfe CA, Brouwer DH (2006) Optimization, standardization, and testing of a new NMR method for the determination of zeolite host-organic guest crystal structures. J Am Chem Soc 128(36):11860–11871. https://doi.org/10.1021/ja060744y 11. Lesage A, Bardet M, Emsley L (1999) Through-bond carbon-carbon connectivities in disordered solids by NMR. J Am Chem Soc 121(47):10987–10993. https://doi.org/10.1021/ ja992272b 12. Brouwer DH (2003) An efficient peak assignment algorithm for two-dimensional NMR correlation spectra of framework structures. J Magn Reson 164(1):10–18. https://doi.org/10.1016/ S1090-7807(03)00190-3 13. Schaefer J, McKay RA, Stejskal EO (1979) Double-cross-polarization NMR of solids. J Magn Reson 34(2):443–447. https://doi.org/10.1016/0022-2364(79)90022-2 14. Van Vleck JH (1948) The dipolar broadening of magnetic resonance lines in crystals. Phys Rev 74(9):1168–1183. https://doi.org/10.1103/PhysRev.74.1168 15. Kim HJ, Heo J, Jeon WS, Lee E, Kim J, Sakamoto S, Yamaguchi K, Kim K (2001) Selective inclusion of a hetero-guest pair in a molecular host: formation of stable charge-transfer complexes in cucurbit 8 uril. Angew Chem-Int Edit 40(8):1526–1529. https://doi.org/10.1002/ 1521-3773(20010417)40:8%3c1526::aid-anie1526%3e3.0.co;2-t 16. Fyfe CA, Brouwer DH (2004) Effect of molecular oxygen on the variable-temperature 29 Si MAS NMR spectra of Zeolite−sorbate complexes. J Am Chem Soc 126(5):1306–1307. https:// doi.org/10.1021/ja0367905 17. Giannelis EP (1996) Polymer layered silicate nanocomposites. Adv Mater 8(1):29–35. https:// doi.org/10.1002/adma.19960080104 18. Ray SS, Okamoto M (2003) Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 28(11):1539–1641. https://doi.org/10.1016/j.progpolymsci. 2003.08.002 19. Komori Y, Hayashi S (2003) Dynamics of p-nitroaniline molecules in siliceous ZSM-5 studied by solid-state NMR. Phys Chem Chem Phys 5(17):3777–3783. https://doi.org/10.1039/ b305837a

References

155

20. Asefa T, MacLachlan MJ, Coombs N, Ozin GA (1999) Periodic mesoporous organosilicas with organic groups inside the channel walls. Nature 402(6764):867–871 21. Yoshina-Ishii C, Asefa T, Coombs N, MacLachlan MJ, Ozin GA (1999) Periodic mesoporous organosilicas, PMOs: fusion of organic and inorganic chemistry ‘inside’ the channel walls of hexagonal mesoporous silica. Chem Commun 24:2539–2540. https://doi.org/10.1039/ a908252b 22. Comotti A, Bracco S, Valsesia P, Ferretti L, Sozzani P (2007) 2D multinuclear NMR, hyperpolarized xenon and gas storage in organosilica nanochannels with crystalline order in the walls. J Am Chem Soc 129(27):8566–8576. https://doi.org/10.1021/ja071348y 23. Lee M, Goldburg WI (1965) Nuclear-magnetic-resonance line narrowing by a rotating RF field. Phys Rev 140(4A):A1261–A1271 24. Janicke MT, Landry CC, Christiansen SC, Kumar D, Stucky GD, Chmelka BF (1998) Aluminum incorporation and interfacial structures in MCM-41 mesoporous molecular sieves. J Am Chem Soc 120(28):6940–6951. https://doi.org/10.1021/ja972633s 25. Christiansen SC, Zhao DY, Janicke MT, Landry CC, Stucky GD, Chmelka BF (2001) Molecularly ordered inorganic frameworks in layered silicate surfactant mesophases. J Am Chem Soc 123(19):4519–4529. https://doi.org/10.1021/ja004310t 26. Xu L, Ji X, Li S, Zhou Z, Du X, Sun J, Deng F, Che S, Wu P (2016) Self-assembly of cetyltrimethylammonium bromide and lamellar zeolite precursor for the preparation of hierarchical MWW zeolite. Chem Mater 28(12):4512–4521. https://doi.org/10.1021/acs.chemmater. 6b02155 27. Huang Y, Demko BA, Kirby CW (2003) Investigation of the evolution of intermediate phases of AlPO4-18 molecular sieve synthesis. Chem Mater 15(12):2437–2444. https://doi.org/10. 1021/cm021728c 28. Shen W, Li S, Xu J, Zhang H, Hu W, Zhou D, Zhang J, Yu J, Xu W, Xu Y, Deng F (2010) A novel phase transformation phenomenon in mesostructured aluminophosphate. J Phys Chem C 114(15):7076–7084. https://doi.org/10.1021/jp911959u 29. Lesage A, Lelli M, Gajan D, Caporini MA, Vitzthum V, Miéville P, Alauzun J, Roussey A, Thieuleux C, Mehdi A, Bodenhausen G, Coperet C, Emsley L (2010) Surface enhanced NMR spectroscopy by dynamic nuclear polarization. J Am Chem Soc 132(44):15459–15461. https:// doi.org/10.1021/ja104771z 30. Lelli M, Gajan D, Lesage A, Caporini MA, Vitzthum V, Miéville P, Héroguel F, Rascón F, Roussey A, Thieuleux C, Boualleg M, Veyre L, Bodenhausen G, Coperet C, Emsley L (2011) Fast characterization of functionalized silica materials by silicon-29 surface-enhanced NMR spectroscopy using dynamic nuclear polarization. J Am Chem Soc 133(7):2104–2107. https:// doi.org/10.1021/ja110791d 31. Li S, Deng F (2013) Chapter one—recent advances of solid-state NMR studies on zeolites. In: Graham AW (ed) Annual reports on NMR spectroscopy, vol 78. Academic Press, pp 1–54. http://dx.doi.org/10.1016/B978-0-12-404716-7.00001-8 32. Jiao J, Kanellopoulos J, Wang W, Ray SS, Foerster H, Freude D, Hunger M (2005) Characterization of framework and extra-framework aluminum species in non-hydrated zeolites Y by 27 Al spin-echo, high-speed MAS, and MQMAS NMR spectroscopy at B = 9.4 to 17.6 T. 0 Phys Chem Chem Phys 7(17):3221–3226. https://doi.org/10.1039/b508358c 33. Jiao J, Wang W, Sulikowski B, Weitkamp J, Hunger M (2006) 29 Si and 27 Al MAS NMR characterization of non-hydrated zeolites Y upon adsorption of ammonia. Microporous Mesoporous Mater 90(1):246–250. https://doi.org/10.1016/j.micromeso.2005.08.006 34. Gan Z (2006) Measuring multiple carbon-nitrogen distances in natural abundant solids using R-RESPDOR NMR. Chem Commun 45:4712–4714. https://doi.org/10.1039/b611447d 35. Pourpoint F, Trébosc J, Gauvin RM, Wang Q, Lafon O, Deng F, Amoureux JP (2012) Measurement of aluminum–carbon distances using S-RESPDOR NMR experiments. Chem Phys Chem 13(16):3605–3615. https://doi.org/10.1002/cphc.201200490 36. Pourpoint F, Morin Y, Gauvin RM, Trébosc J, Capet F, Lafon O, Amoureux J-P (2013) Advances in structural studies on alkylaluminum species in the solid state via challenging 27 Al–13 C NMR spectroscopy and X-ray diffraction. J Phys Chem C 117(35):18091–18099. https://doi.org/10. 1021/jp4055044

156

4 Solid-State NMR Characterization of Host-Guest Interactions

37. Li S, Pourpoint F, Trébosc J, Zhou L, Lafon O, Shen M, Zheng A, Wang Q, Amoureux J-P, Deng F (2014) Host-guest interactions in dealuminated HY zeolite probed by 13 C–27 Al solid-state NMR spectroscopy. J Phys Chem Lett 5(17):3068–3072. https://doi.org/10.1021/jz501389z 38. Li S, Zheng A, Su Y, Zhang H, Chen L, Yang J, Ye C, Deng F (2007) Brønsted/Lewis acid synergy in dealuminated HY zeolite: a combined solid-state NMR and theoretical calculation study. J Am Chem Soc 129(36):11161–11171. https://doi.org/10.1021/ja072767y 39. Xu T, Munson EJ, Haw JF (1994) Toward a systematic chemistry of organic reactions in zeolites: in situ nmr studies of ketones. J Am Chem Soc 116(5):1962–1972. https://doi.org/10. 1021/ja00084a041 40. Tricot G, Lafon O, Trebosc J, Delevoye L, Mear F, Montagne L, Amoureux JP (2011) Structural characterisation of phosphate materials: new insights into the spatial proximities between phosphorus and quadrupolar nuclei using the D-HMQC MAS NMR technique. Phys Chem Chem Phys 13(37):16786–16794. https://doi.org/10.1039/c1cp20993k 41. Li S, Huang S-J, Shen W, Zhang H, Fang H, Zheng A, Liu S-B, Deng F (2008) Probing the spatial proximities among acid sites in dealuminated H–Y zeolite by solid-state NMR spectroscopy. J Phys Chem C 112(37):14486–14494. https://doi.org/10.1021/jp803494n 42. Haw JF, Song WG, Marcus DM, Nicholas JB (2003) The mechanism of methanol to hydrocarbon catalysis. Acc Chem Res 36(5):317–326. https://doi.org/10.1021/ar020006o 43. Stocker M (1999) Methanol-to-hydrocarbons: catalytic materials and their behavior. Microporous Mesoporous Mater 29(1–2):3–48. https://doi.org/10.1016/s1387-1811(98)00319-9 44. Tian P, Wei Y, Ye M, Liu Z (2015) Methanol to olefins (MTO): from fundamentals to commercialization. Acs Catalysis 5(3):1922–1938. https://doi.org/10.1021/acscatal.5b00007 45. Wilson S, Barger P (1999) The characteristics of SAPO-34 which influence the conversion of methanol to light olefins. Microporous Mesoporous Mater 29(1–2):117–126. https://doi.org/ 10.1016/s1387-1811(98)00325-4 46. Haw J, Marcus D (2005) Well-defined (supra)molecular structures in zeolite methanol-to-olefin catalysis. Top Catal 34(1–4):41–48. https://doi.org/10.1007/s11244-005-3798-0 47. Xu T, Barich DH, Goguen PW, Song WG, Wang ZK, Nicholas JB, Haw JF (1998) Synthesis of a benzenium ion in a zeolite with use of a catalytic flow reactor. J Am Chem Soc 120(16):4025–4026. https://doi.org/10.1021/ja973791m 48. Song W, Nicholas J, Sassi A, Haw J (2002) Synthesis of the heptamethylbenzenium cation in zeolite-β: in situ NMR and theory. Catal Lett 81(1–2):49–53. https://doi.org/10.1023/a: 1016003905167 49. Wang C, Chu Y, Zheng A, Xu J, Wang Q, Gao P, Qi G, Gong Y, Deng F (2014) New insight into the hydrocarbon-pool chemistry of the methanol-to-olefins conversion over zeolite HZSM-5 from GC-MS, solid-state NMR spectroscopy, and DFT calculations. Chem Eur J 20(39):12432–12443. https://doi.org/10.1002/chem.201403972 50. Wang C, Yi X, Xu J, Qi G, Gao P, Wang W, Chu Y, Wang Q, Feng N, Liu X, Zheng A, Deng F (2015) experimental evidence on the formation of ethene through carbocations in methanol conversion over H-ZSM-5 zeolite. Chem Eur J 21(34):12061–12068. https://doi.org/10.1002/ chem.201501355 51. Wang C, Xu J, Qi G, Gong Y, Wang W, Gao P, Wang Q, Feng N, Liu X, Deng F (2015) Methylbenzene hydrocarbon pool in methanol-to-olefins conversion over zeolite H-ZSM-5. J Catal 332:127–137. https://doi.org/10.1016/j.jcat.2015.10.001 52. Wang C, Wang Q, Xu J, Qi GD, Gao P, Wang WY, Zou YY, Feng ND, Liu XL, Deng F (2016) Direct detection of supramolecular reaction centers in the methanolto- olefins conversion over zeolite H-ZSM-5 by C–13–Al–27 solid-state NMR spectroscopy. Angew Chem-Int Edit 55(7):2507–2511. https://doi.org/10.1002/anie.201510920 53. Bhawe Y, Moliner-Marin M, Lunn JD, Liu Y, Malek A, Davis M (2012) Effect of cage size on the selective conversion of methanol to light olefins. ACS Catal 2(12):2490–2495. https://doi. org/10.1021/cs300558x 54. Teketel S, Skistad W, Benard S, Olsbye U, Lillerud KP, Beato P, Svelle S (2012) Shape selectivity in the conversion of methanol to hydrocarbons: the catalytic performance of one-dimensional 10-ring zeolites: ZSM-22, ZSM-23, ZSM-48, and EU-1. ACS Catal 2(1):26–37. https://doi. org/10.1021/cs200517u

References

157

55. Wang Q, Cui Z-M, Cao C-Y, Song W-G (2011) 0.3 Å makes the difference: dramatic changes in methanol-to-olefin activities between H-ZSM-12 and H-ZSM-22 zeolites. J Phys Chem C 115(50):24987–24992. https://doi.org/10.1021/jp209182u 56. Lesthaeghe D, De Sterck B, Van Speybroeck V, Marin GB, Waroquier M (2007) Zeolite shape-selectivity in the gem-methylation of aromatic hydrocarbons. Angew Chem Int Ed 46(8):1311–1314. https://doi.org/10.1002/anie.200604309 57. Cui Z-M, Liu Q, Song W-G, Wan L-J (2006) Insights into the mechanism of methanol-to-olefin conversion at zeolites with systematically selected framework structures. Angew Chem Int Ed 45(39):6512–6515. https://doi.org/10.1002/anie.200602488 58. Wang C, Xu J, Wang Q, Zhou X, Qi G, Feng N, Liu X, Meng X, Xiao F, Deng F (2017) Hostguest interactions and their catalytic consequences in methanol to olefins conversion on zeolites studied by 13 C–27 Al double-resonance solid-state NMR spectroscopy. ACS Catal:6094–6103. https://doi.org/10.1021/acscatal.7b01738 59. Goetze J, Meirer F, Yarulina I, Gascon J, Kapteijn F, Ruiz-Martínez J, Weckhuysen BM (2017) Insights into the activity and deactivation of the methanol-to-olefins process over different small-pore zeolites as studied with Operando UV–vis spectroscopy. ACS Catal:4033–4046. https://doi.org/10.1021/acscatal.6b03677 60. Rojo-Gama D, Signorile M, Bonino F, Bordiga S, Olsbye U, Lillerud KP, Beato P, Svelle S (2017) Structure—deactivation relationships in zeolites during the methanol-to-hydrocarbons reaction: complementary assessments of the coke content. J Catal 351:33–48. https://doi.org/ 10.1016/j.jcat.2017.04.015

Chapter 5

Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid Catalysts

Abstract This chapter introduces the application of solid-state NMR to investigate the acid features of solid acid catalysts, which have been widely used in advanced petrochemical processes because of their environmental friendliness, high product selectivity, and easy product separation. The acidic features of solid acids including acid type, acid concentration, acid distribution, acid strength, and the spatial interaction of acid sites are essential factors in dictating their reactivity and selectivity. Solid-state NMR is presented to be a powerful tool for the characterization of the surface acidic properties of solid acid catalysts including zeolites, oxides and heteropolyacids etc. The acid strength of solid acids can be quantitatively measured from the chemical shifts of adsorbed probe molecules such as pyridine, acetone, trialkylphosphine oxides, and trimethylphosphine. The spatial proximity and synergetic effect of various acid sites on solid acid catalysts can be ascertained by either double-resonance or two-dimensional (2D) double-quantum magic-angle spinning (DQ MAS) NMR spectroscopy. Keywords Solid-state NMR · Zeolite · Solid acid catalyst · Probe molecule · Acidic property · Brønsted/Lewis acid synergy

5.1 Introduction Solid acids such as zeolites, metal oxides, and heteropolyacids are by far the most important catalysts employed by industries including the fields of oil refining, petrochemicals, and chemicals [1]. Solid acids are characterized by Brønsted/Lewis acidity with varied strength and number of acid sites. A Brønsted acid site is usually associated with a bridging hydroxyl group (e.g., SiOHAl in zeolites) which can act as a proton donor and protonate the adsorbed molecule, while a Lewis acid site is an electron acceptor which is often associated with metal species. In zeolites, Lewis acid sites can be generated by dealumination treatments, during which the Al atom is released from zeolite framework, forming the extra-framework Al as Lewis acid. On the other hand, the introduction of metal (Zn, Ga, Ag, Mo, etc.) species into the channels or the framework of zeolite can also produce Lewis acid sites. The acidity © Springer Nature Singapore Pte Ltd. 2019 J. Xu et al., Solid-State NMR in Zeolite Catalysis, Lecture Notes in Chemistry 103, https://doi.org/10.1007/978-981-13-6967-4_5

159

160

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

of solid acids dictates their catalytic performance during various heterogeneous catalytic reactions [2, 3]. Therefore, a comprehensive study of the acidic properties is crucial for the design of robust catalysts and improvement of their catalytic performance. The most important points for the characterization of solid acid catalyst lie in the precise determination of the acid type such as Brønsted and Lewis, acid concentration, acid strength, acid distribution (or location), and spatial proximity/interaction of acid sites [4, 5]. Solid-state NMR is one of the mostly used techniques for the study of the acidic property of solid acid catalysts [6, 7]. 1D 1 H and 27 Al MAS NMR can provide structural information about the acid sites, whereas it cannot reliably enable the quantitative determination of acid strength. Solid-state NMR in combination with basic probe molecules is a well-established tool to quantitatively assess the acid strength [3]. Meanwhile, the internal or external distribution of the acid sites in solid acids with porous structure can be explored with probe molecules having different molecular diameters. Additionally, the interactions between different acid sites can be revealed by detecting the spatial proximity between these sites, which can be achieved with 2D 1 H–1 H or 27 Al–27 Al DQ MAS and 1D 1 H{M}(M denotes metal) double-resonance solid-state NMR spectroscopy [8–10]. Herein, characterization of the acidity property of solid acid catalysts with various solid-state NMR techniques will be briefly introduced.

5.2 Solid-State NMR Characterization of Acidic Property 5.2.1 Acid Sites Containing Hydroxyl Groups 1

H MAS NMR can provide direct information about various hydroxyl groups in solid acid catalysts such as zeolites, oxides, heteropolyacids [6, 11, 12]. The successful utilization of zeolite in petrochemistry is largely based on its strong Brønsted acidity. Derouane et al. [13] summarized a variety of ways to introduce the Brønsted acidity on zeolites: (1) direct proton-exchange of the charge-compensating metal cations; (2) ammonium-exchange of the same compensating metal cations followed by calcination to decompose the ammonium cation leaving a proton on the surface; (3) exchange with polyvalent cations that can generate protons via partial hydrolysis of water molecules; and (4) exchange by metal cations that can be reduced by hydrogen to a lower valence state, again generating protons on the surface. The chemical shift ranges for the Brønsted acidic proton, non-acidic SiOH, and AlOH groups in various zeolites are listed in Table 5.1 [14]. In addition, their concentrations can be directly obtained by measuring the integrated area of 1 H signals, which provides quantitative information on the content of hydroxyl groups in zeolites. As shown in Fig. 5.1, in the 1 H MAS NMR spectrum of parent HY zeolite, the two signals at 5.0 and 4.3 ppm are assigned to bridging SiOHAl groups (Brønsted acid sites) in the sodalite and the supercage of HY zeolite, respectively [8]. The peak at 2.2 ppm is due to non-acidic

5.2 Solid-State NMR Characterization of Acidic Property Table 5.1 Assignments of 1 H NMR chemical shift for various hydroxyl groups in dehydrated zeolites

161

δ 1H /ppm

Hydroxyl group

Assignment

1.2–2.2

SiOH

Silanol groups on the external surfaces or lattice defect sites

0.6–3.6

AlOH

Non-framework Al hydroxyl groups in the pore channels

3.6–4.3

SiOHAl

Bridging hydroxyl groups in large channels

4.6–5.2

SiOHAl

Bridging hydroxyl group in small channels

Fig. 5.1 1 H MAS NMR spectra of HY (a), dealuminated HY without 27 Al irradiation (b) and with 27 Al irradiation (c), and difference spectrum (d) of (b) and (c). Reproduced from Ref. [8] by permission of American Chemical Society

SiOH groups. For dealuminated HY (Fig. 5.1), the resonances at 2.8 and 1.0 ppm are associated with two different types of AlOH hydroxyl groups. In the transfer of populations in double-resonance (TRAPDOR) NMR experiment [15], the 1 H signals of hydroxyl groups in close proximity to Al are reduced under 27 Al irradiation, while those of hydroxyl groups far away from Al remain unchanged. Thus, the hydroxyl groups such as AlOH and Brønsted acidic proton in spatial proximity to aluminum can thus be distinguished from silanol groups. As shown in Fig. 5.1, apart from the signals at 5.0 and 4.3 ppm, the signals at 2.8 and 1.0 ppm are largely reduced as well (Fig. 5.1c–d), indicating that these hydroxyl groups are close to aluminum nuclei and are due to extra-framework AlOH groups. Note that the signal of silanol groups remains almost unchanged under 27 Al irradiation.

162

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.2 1 H MAS NMR spectra acquired at room temperature for dehydrated H-ZSM-5 using a simple spin-echo sequence (top) without and (bottom) with an 27 Al pulse applied during the echo evolution periods. Reproduced from Ref. [16] by permission of American Chemical Society

Recently, White et al. showed the direct spectroscopic identification of at least three types of protons exhibiting Brönsted acid character in zeolite HZSM-5 by using one- and two-dimensional 1 H solid-state NMR methods [16]. In the 1 H{27 Al} TRAPDOR spectra as shown in Fig. 5.2, Si–OH peak intensity is the same in both experiments, due to the fact that those hydroxyl groups are not close to Al atoms. The 12–15 ppm peak was completely attenuated by the double-resonance echo, suggesting that like the AlOH protons and the Brønsted acid sites (BAS) protons, the associated protons are close to Al atoms. This species could contain a hydroxyl group proximate to either a framework acid site Al or an extra-framework Al species. 1 H 2D-exchange MAS NMR was used to detect the proton dynamics. As shown in Fig. 5.3, all the signals observed in Fig. 5.3 appear along the diagonal of the contour plot, and obvious off-diagonal signals are also observed. The signals appeared on the spectrum extracted along the row at 13 ppm, as shown to the right of the contour plot, show that the same three protons that are attenuated in Fig. 5.3 are undergoing chemical exchange with one another. This indicates the protonic character of the protons represented by the 12–15 ppm signals. In addition, by comparing the relaxation time constant for all the protons on the catalyst with and without water molecules, the authors concluded that 12–15 ppm peak is not originated from surface-stabilized H3 O+ ions as suggested in the previous studies [17–19]. Since the 12–15 ppm signal is obviously a broad signal, different species will contribute to it, as shown in Scheme 5.1. Either hydroxyl groups from extra-lattice aluminum oxides or the BAS proton interacting with extra-lattice aluminum oxides could lead to the 12–15 ppm signal. ZrO2 has been found to be excellent catalytic support due to its high thermal stability, more active when interacts with dopant phase [20]. Thus considerable interests have been invested in the study of MoOx /ZrO2 and WOx /ZrO2 catalysts [21–23],

5.2 Solid-State NMR Characterization of Acidic Property

163

Fig. 5.3 1 H 2D-exchange MAS NMR spectra acquired at room temperature for dehydrated HZSM-5 (Si:Al = 15) catalyst prepared under inert gas flow and heating. The row extracted from the contour plot at 13 ppm is shown on the right. Reproduced from Ref. [16] by permission of American Chemical Society

Scheme 5.1 Proposed chemical structures for the hydroxyl groups leading to the broad 12–15 ppm signal (indicated in red) on H-ZSM-5. Reproduced from Ref. [16] by permission of American Chemical Society

including their physical and chemical properties and catalytic performance. 1 H MAS NMR was used to investigate the hydroxyl groups in mesoporous MoOx /ZrO2 and WOx /ZrO2 [24]. 1 H MAS NMR spectrum of the dehydrated zirconia shown in Fig. 5.4 consists of an intense signal at 4.4 ppm and two weak shoulder peaks at 1.8 and 0.4 ppm. The 4.4 ppm signal is ascribed to acidic hydroxyl groups, and the other two peaks to weak or non-acidic hydroxyl groups present on ZrO2 . After the introduction of Mo species, the peak at 4.4 ppm completely disappears and two new signals at 7.2 and 5.6 ppm are present, indicating that interaction between the acidic OH groups of ZrO2 and Mo species occurs and two types of new acidic OH groups are likely formed. Generally, the chemical shift of hydroxyl groups increases with increasing acidity if hydrogen bonding is absent. The 7.5 and 5.6 ppm signals are assigned to the strong acidic OH groups, while those at 1.7 and 0.6 ppm are assigned to weak or non-acidic OH groups. The concentration of the non-acidic OH groups considerably increases after the addition of Mo species. Deuterated pyridine is one of the probe molecules widely used for the determination of the acid strength of surface

164

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.4 1 H MAS NMR spectra of mesoporous materials: a zirconia, b zirconia loaded with pyridine-d 5, c MoOx /ZrO2 , d MoOx /ZrO2 loaded with pyridine-d 5, e WOx /ZrO2 , and f WOx /ZrO2 loaded with pyridine-d 5. Asterisks denote spinning sidebands. Reproduced from Ref. [24] by permission of American Chemical Society

OH groups [24]. In case of acidic OH groups (Brønsted acid sites), the adsorption of pyridine-d 5 results in 1 H NMR signals at chemical shifts in the range of 12–19 ppm. The down-field signals result from a proton transfer to the probe molecule, forming pyridine ions. The adsorption of pyridine-d 5 onto ZrO2 results in three new signals at 8.7, 7.5, and 7.2 ppm at the expense of the signal at 4.4 ppm (Fig. 5.4b), implying that only hydrogen-bonded pyridines are formed, and thus, the acid strength of the acidic OH groups present on ZrO2 is not strong enough to protonate the adsorbed pyridine molecules. In contrast, after the adsorption of pyridine-d 5 onto the MoOx /ZrO2 mate-

5.2 Solid-State NMR Characterization of Acidic Property

165

Scheme 5.2 Proposed scheme for the generation of Brønsted acid sites on mesoporous MoOx /ZrO2 and WOx /ZrO2 . Reproduced from Ref. [24] by permission of American Chemical Society

rial (Fig. 5.4d), two signals appear at 19.3 and 16.6 ppm, resulting from the formation of pyridine ions between the adsorbed pyridine molecules and the two distinct kinds of acidic OH groups (at 7.2 and 5.6 ppm). The signals at 8.7 and 7.5 ppm are probably due to the H/D exchange between deuterons bound to the ring of pyridine and the acidic proton. The 1 H MAS NMR results indicate that the introduction of Mo or W species leads to the formation of two different types of acidic OH groups (Brønsted acid sites), with acid strength much stronger than that of ZrO2 . A possible scheme for the formation of Brønsted acid sites is illustrated in Scheme 5.2. The coordination of Mo–OH or W–OH to the unsaturated Zr4+ sites leads to the appearance of bridging Mo–OH–Zr (or W–OH–Zr) hydroxyl groups that act as Brönsted acid sites on the mesoporous MoOx /ZrO2 and WOx /ZrO2 materials, and a remarkable decrease in the concentration of Lewis acid sites present on the surface of ZrO2 . This is likely created by electron-deficient regions resulted from the anionic dopants (MoO2− 4 or ) −δ +δ that attract electrons and increase the polarity of the acidic O –H , as in the WO2− 4 case of the bridging Al–OH–Si groups in zeolites. Two types of Brønsted acid sites with different acid strength may exist on the surface of the mesoporous MoOx /ZrO2 and WOx /ZrO2 materials, namely monomer or oligomer Mo (or W) species, which were confirmed by DFT calculations as well. BF3 /γ -Al2 O3 is one of the most promising alkylation catalysts for HF replacement [25]. Strong Brønsted acid sites are proposed to be formed by reaction between BF3 and the hydroxyl groups present on the surface of γ -Al2 O3 . The acid sites formed on the surface of the BF3 /γ -Al2 O3 catalyst were revealed by 1 H MAS NMR [26]. Figure 5.5a shows 1 H MAS NMR spectrum of parent γ -Al2 O3 . Only three types of hydroxyl groups could be resolved by 1 H MAS NMR: a terminal OH group attached

166

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.5 1 H spin-echo spectra of a γ -Al2 O3 (without Al irradiation), b BF3 /γ -Al2 O3 (without Al or B irradiation), c BF3 /γ -Al2 O3 (with B irradiation), d difference spectrum of parts b and c, e BF3 /γ -Al2 O3 (with Al irradiation), (f) difference spectrum of parts (b–e). Reproduced from Ref. [26] by permission of American Chemical Society

to a single Al (0.3 ppm), a bridging OH coordinated to two Al (2.1 ppm), and a bridging OH attached to three octahedral Al (4.3 ppm). Adsorption of BF3 onto the surface of γ -Al2 O3 results in substantial changes in the 1 H spectrum (Fig. 5.5b). The signal at 0.3 ppm disappears completely, indicating that BF3 reacts with the most basic OH groups. Another change is that a new signal at 3.7 ppm with a shoulder peak at 5.0 ppm appears. To reveal the nature of these signals, 1 H{11 B} and 1 H{27 Al} TRAPDOR experiments were performed. Under on-resonance 11 B irradiation, the two signals at 3.7 and 2.3 ppm are partially reduced (Fig. 5.5c–d), suggesting that corresponding hydroxyl groups are all in close proximity to the boron atom. Similarly, the 1 H{27 Al} TRAPDOR experiment (Figs. 5.5e–f) indicates the hydroxyl groups corresponding to the 3.7 and 2.3 ppm signals are also close to Al atoms. According to the chemical shifts of these signals and the TRAPDOR experimental results, the 3.7 ppm signal was ascribed to bridging Al–OH–B hydroxyl groups that most likely act as the Brønsted acid site, while the 2.3 ppm signal was due to an AlOH group associated with BF3 species. The 5.0 ppm signal was assigned to water adsorbed on the surface of γ -Al2 O3 . A model for the Brønsted acid site was proposed on BF3 /γ Al2 O3 catalyst (Scheme 5.3). The interaction of BF3 with one AlOH group and one neighboring Al defect site leads to the formation of Brønsted acid sites. Sn-β zeolite is an atom-efficient solid Lewis acid catalyst for green and sustainable production of chemicals and fuels because of its unparalleled catalytic performance in transformation of biomass and biomass-derived feedstocks [27, 28]. The so-called

5.2 Solid-State NMR Characterization of Acidic Property

167

Scheme 5.3 Proposed model for the Brønsted acid sites formed on BF3 /γ -Al2 O3 . Reproduced from Ref. [26] by permission of American Chemical Society

open (e.g., (SiO)3 Sn–OH) and closed (e.g., (SiO)4 Sn) Sn sites are proposed on Sn-β [29, 30]. With respect to the activity of the Sn sites, the hydroxyl group associated with the open one was proved to be the active site in the reactions such as Baeyer-Villiger oxidation of cyclohexanone [29] and glucose isomerization [30–32]. Proton-detected one-dimensional (1D) and two-dimensional (2D) 1 H{119 Sn} D-HMQC NMR experiments were used to characterize the open Sn site in Sn-β zeolite [33]. Figure 5.6 shows 2D 1 H {119 Sn} D-HMQC spectra of Sn-β zeolite. Both hydrated 119 Sn-β and the corresponding sample dehydrated at 298 K exhibit 1 H–119 Sn correlation peak at (5.4, −686) ppm (Fig. 5.6a, b), revealing the presence of water molecule-bound 6-coordinated Sn sites. For 119 Sn-β dehydrated at 393 K, four 1 H–119 Sn correlation peaks are observable in the HMQC spectrum (Fig. 5.6c). The two 4-coordinated Sn sites at −443 ppm and −429 ppm are correlated with 1 H species at 0.1–0.4 ppm: The two 1 H signals at 0.37 and 0.20 ppm have correlations with the Sn signal at −443 ppm, while the two 1 H signals at 0.34 and 0.17 ppm exhibit correlations with the Sn signal at −429 ppm (Fig. 5.6c). When 119 Sn decoupling was applied during 1 H acquisition in the D-HMQC experiments, the four correlation peaks merge into two ones centered at (0.28, −443) and (0.26, −429) ppm (Fig. 5.6d). Therefore, the four resolved correlation peaks in Fig. 5.6c should be due to the doublet splitting of two types of 1 H signals caused by J-coupling between spin pairs of 1 H and 119 Sn. The measured the J-coupling constant from the peak distance between the doublet fine structure is ~136 Hz, in consistent with the 2 J (119 Sn–1 H) constant of 130 Hz for Sn–OH species in monoalkyl-SnCl3−x (OH)x solution [34, 35]. These results show that there are two types of Sn–OH groups; one corresponds to the Sn atom at −443 ppm bound to hydroxyl group at 0.28 ppm, and the other is the Sn atom at −429 ppm bound to hydroxyl group at 0.26 ppm. They can be attributed to two types of open Sn sites ((SiO)3 Sn–OH) located at different T sites on the framework of β-zeolite. This is in agreement with the recent DFT study, which indicated that there are two T sites to stabilize the open Sn sites on β zeolite [36]. The direct observation of the Sn–OH groups indicates that either defect- or hydrolyzed-open sites are present, although they are formed differently on the sample [37]. In combination with 1D 1 H MAS NMR and 2D 1 H {29 Si} D-HMQC MAS NMR experimental results, a reversible transformation between the open and closed Sn sites is pictured as shown in Scheme 5.4.

168

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.6 Identification of open Sn sites by proton-detected 1 H/119 Sn correlation NMR. Twodimensional 1 H {119 Sn} HMQC MAS NMR spectra of 119 Sn-β a without dehydration, b dehydrated at 298 K, c dehydrated at 393 K without 119 Sn decoupling, and d dehydrated at 393 K with 119 Sn decoupling. Projections of 1 H and 119 Sn dimensions are shown in black. Representative slices along −429 ppm (red) and −443 ppm (blue) in the F1 dimension are also displayed in (c) and (d). Reproduced from Ref. [33] by permission of Nature Publishing Group

5.2.2 Acidic Nature and Strength The utilization of probe molecules was widely used to investigate the acidity of zeolites. Pioneered by Lunsford et al. [38],trimethylphosphine (TMP) was adopted as a probe molecule to characterize the acidity of H-form faujasite-type zeolites based on the observed 31 P NMR chemical shifts. Figure 5.7 shows the 31 P MAS NMR spectra of TMP adsorbed on H-Y zeolites calcined at specified temperatures. As a TMP molecule adsorbed onto a Brønsted acid site, the formation of a TMPH+ ionic complex would give rise to a 31 P chemical shifts ranging from ca. −1 to −6 ppm, whereas TMP molecules bound to Lewis acid sites normally result in 31 P chemical shifts in the range of −30 to −70 ppm [38] (see Fig. 5.7). Therefore, Brønsted and Lewis acid sites in zeolites can be clearly distinguished and their concentrations can be quantitatively determined. The technique has been widely used for acidity

5.2 Solid-State NMR Characterization of Acidic Property

169

Scheme 5.4 Proposed model for interconversion between open and closed Sn sites in Sn-β zeolite. a 6-coordinated open Sn site, b 4-coordinated open Sn site, and c 4-coordinated closed Sn site. Reproduced from Ref. [33] by permission of Nature Publishing Group

characterization of other solid acid catalysts such as sulfated zirconia catalysts [39], H-ZSM-5 [40], Al-MCM-41 [41], and MCM-22 [42]. In order to accurately determine the Brønsted acid strength of zeolites, several spectroscopic and analytical methods such as IR spectroscopy, temperatureprogrammed desorption (TPD), and microcalorimetry are usually employed. However, in the conventional NH3 -TPD and pyridine IR experiments, the probe molecules are normally too basic to distinguish the differences of Brønsted acid strengths. It is very promising to characterize the acidic strength of zeolites by utilizing solid-state MAS NMR spectroscopy of various probe molecules. To this end, 13 C MAS NMR of adsorbed 2-13 C-acetone [43, 44], 31 P MAS NMR of adsorbed trimethylphosphine oxide (TMPO) [45, 46], 1 H MAS NMR of adsorbed pyridine-d5 [6, 47] and acetonitrile-d3 [48] are usually utilized. It is well accepted that the increased acidity of dealuminated zeolites is due to the reduction of the number of framework Al as well as the presence of extra-framework aluminum species [49, 50]. 2-13 C-acetone is well-established NMR probe molecule for measuring the relative Brønsted acid strengths of solid acids. The hydrogen-bond interaction between the acidic proton and the carbonyl oxygen of adsorbed 2-13Cacetone normally causes a down-field shift of the 13 C NMR resonance of the carbonyl to higher 13 C chemical shifts [43]. In general, the stronger the Brønsted acidity, the higher is 13 C chemical shift for adsorbed 2-13 C-acetone. This approach was utilized to determine the Brønsted acid strength of dealuminated HY zeolite [8]. In the 13 C NMR spectrum of 2-13 C-acetone adsorbed on HY (Fig. 5.8a), only one sharp resonance at ca. 220 ppm due to unreacted acetone adsorbed on the Brønsted acid site of HY was observed. For the dealuminated HY zeolite as shown in Fig. 5.8b, with the assistance of DFT calculations, the signals at 228 and 234 ppm were attributed to acetone adsorbed on two Brønsted acid sites in close proximity to Lewis acid sites

170

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.7 Effect of calcination temperature on the 31 P MAS NMR spectra of TMP adsorbed on H-Y zeolite calcined at a 400 °C, b 500 °C, c 600 °C, d 700 °C. Reproduced from Ref. [38] by permission of American Chemical Society

with enhanced acid strength, whereas the weak shouldering peak at ca. 242 ppm was attributed to acetone adsorbed on Lewis acid sites. The results from 13 C NMR of adsorbed acetone as well as DFT calculation demonstrated that the Brønsted/Lewis acid synergy considerably enhanced the Brønsted acid strength of dealuminated HY zeolite [8]. The threshold acid strength of a solid superacid, which corresponds to a 100% H2 SO4 solution, gives rise to an isotropic 13 C NMR chemical shift of 245 ppm of adsorbed 2-13 C-acetone [44]. Heteropolyoxometalates have attracted great attention for many years due to their strong acidity and redox properties. They have led to broad application in industry as homogeneous catalysts and extensive investigations as promising heterogeneous catalysts [51, 52]. Figure 5.9 shows the 13 C MAS NMR spectra of 2-13 C-acetone (ca. 0.3 molecules/KU) adsorbed on 12-H3 PW12 O40 (HPW) samples dehydrated at different temperatures [53]. Two signals at 219 and 235 ppm were observed in the 13 C MAS spectrum. After dehydration at 373 K for 2 h, the 219 ppm signal in the 13 C MAS spectrum decreased in intensity slightly while the 235 ppm signal increased in intensity. In the corresponding 1 H–13 C CP/MAS spectrum as shown in Fig. 5.9, the 235 ppm signal was enhanced while the 219 ppm signal decreased significantly [53]. After dehydrating at 493 K for 2 h, the 219 ppm

5.2 Solid-State NMR Characterization of Acidic Property

171

Fig. 5.8 13 C CP/MAS NMR spectra of 2-13 C-acetone adsorbed on HY and dealuminated HY zeolites with different loadings: a HY, 2.4 acetone/u.c. (unit cell); b dealuminated HY, 1.2 acetone/u.c.; c dealuminated HY, 2.4 acetone/u.c.; d dealuminated HY, 4.8 acetone/u.c.; e dealuminated HY, 7.2 acetone/u.c. Reproduced from Ref. [8] by permission of American Chemical Society

signal in the 13 C MAS NMR spectrum disappeared completely, and a new signal at 246 ppm appeared. The signals of 235 and 246 ppm were still present in the HPW samples dehydrated at 573 K. Therefore, the signal at 219 ppm could be assigned to hydrated acidic proton with weak acid strength, while the signal at 235 was due to isolated acidic proton with strong acid strength. Moreover, the signal at 246 ppm confirms the presence of another isolated acidic proton with superacidity. Moreover, solid-state NMR spectroscopy of adsorbed TMPO was employed to probe the acid strength of various solid acid catalysts such as phosphomolybdic acid (H3 PMo12 O40 , HPMo) [54] and tungstophosphoric acid (H3 PW12 O40 , HPW) [55]. Figure 5.10 shows the 31 P MAS NMR spectra of TMPO adsorbed on the phosphomolybdic acid with different loadings. With the assistance of DFT calculation, resonance peaks with chemical shifts greater than 84 ppm are assigned to the adsorption of one TMPO per Keggin unit (KU) whereas those with chemical shifts in the range of 80–84 ppm are attributed to (TMPOH+ )2 /KU. It was conclusive that the acid strengths of Brønsted acidic protons in HPMo were much stronger than those in typical zeolites, representing solid acid catalysts with superacidic characteristics [55]. The acid strength and their relative concentration play an essential role in the heterogeneously catalyzed reaction process. Figure 5.11 shows the 31 P MAS NMR spectra of TMPO adsorbed in H-Beta zeolites [56]. Significant changes can be found in the lineshape after dealumination. All spectra can be deconvoluted by the isotropic peaks at ca. 81–85, 67–71, 65–66, 55–64, 49–50, and 42 ppm, respectively. The 31 P NMR chemical shifts ranging from 49 to 85 ppm are ascribed to chemically adsorbed TMPO complexes (TMPO molecules interact with Brønsted acid sites or Lewis acid sites). The signal at 42 ppm is attributed to the physisorbed TMPO [56]. The peaks

172

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.9 13 C MAS (left) and CP/MAS (right) NMR spectra of 2-13 C-acetone (0.3 molecule/KU) adsorbed on 12-H3 PW12 O40 after dehydration at a room temperature for 4 h, b 373 K for 2 h, c 493 K for 2 h, and d 573 K for 2 h. Reproduced from Ref. [53] by permission of American Chemical Society

at 65–66 and 49–50 ppm are attributed to TMPO adsorbed on Lewis acid sites, while those at 81–85, 67–71, and 55–64 ppm are attributed to TMPO adsorbed on three types of Brønsted acid sites [56]. Using this method, the acidity property of series H-Beta zeolites derived from dealumination of Al-rich H-Beta zeolite was quantitatively identified [56]. Recently, pyridine-d5 acting as probe molecule was employed to characterize the acidity of metal-modified H-ZSM-5 zeolite [57]. 1 H MAS NMR experiments were performed without and with adsorbed pyridine-d 5 to distinguish different hydroxyl groups on Zn/ZSM-5 zeolites with zinc species being loaded with different methods. As shown in Fig. 5.12b, the low-field signals at 15.5 and 19.3 ppm can be assigned to pyridine-d 5 adsorbed on the acidic protons of ZSM-5 zeolite with different acidic strength; the signals at 8.3 and 6.7 ppm come from the formation of hydrogen bonds between pyridine-d 5 and non-acidic SiOH groups [6]. Actually, different types of SiOH groups with a chemical shift distribution could contribute to the observed broad signal at 2.3 ppm on H-ZSM-5 [58] (Fig. 5.12a). The newly resolved signal at 1.9 ppm is due to the SiOH group inaccessible to pyridine. It is notable that a weak shoulder peak at 13.4 ppm appears in the 1 H MAS spectra of pyridine-d5 adsorbed on ZSM-5(I2) (zinc loading is 2 wt%) and ZSM-5(I6) (zinc loading is 6 wt%), but it is absent on ZSM-5(G2) (zinc oxide loading is 2 wt%) and H-ZSM-5. For pyridined 5 adsorbed on Brønsted acidic protons of zeolites, pyridinium ion complexes are formed with 1 H chemical shifts of 12–20 ppm [6, 47], and a smaller chemical shift

5.2 Solid-State NMR Characterization of Acidic Property

173

Fig. 5.10 31 P MAS NMR spectra of bare HPMo and TMPO/HPMo samples with varied TMPO loading ranging from 0.5 to 3.3 TMPO/KU. Reproduced from Ref. [54] by permission of American Chemical Society

corresponds to a stronger acid strength [24, 59]. Thus, the 13.4 ppm signal indicates that the acid strength of a small fraction of Brønsted acid sites is largely increased on ZSM-5(I2) and ZSM-5(I6), due to the interaction of acidic protons and the introduced zinc ion as evidenced by 1 H–67 Zn S-RESPDOR NMR experiments. Besides the experimental observations, the correlations between the 1 H, 13 C, and 31 P NMR chemical shifts of probe molecules and the acid strength of solid acid catalysts have been established by Deng et al. with the help of theoretical DFT calculations [3]. Proton affinity (PA) or deprotonation energy (DPE) can serve as an indicator for the intrinsic acid strengths of Brønsted acid sites. Namely, a smaller PA or DPE corresponds to a stronger Brønsted acid. In the DFT calculations, Brønsted acid sites with different strengths are represented by a series of 8T zeolite models with varying terminal Si–H bond lengths. As such, proton affinities of Brønsted acid sites varying from 246.7 to 310.8 kcal/mol can be generated from these selected models, covering from weak, medium strong, strong, to superacid. In addition, the 1 H, 13 C, and 31 P NMR chemical shifts can be derived from optimized complexes of the basic probe molecules adsorbed on the Brønsted acid sites. Thus, the correlation between the Brønsted acid strength and the NMR chemical shift is established.

174

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.11 Deconvoluted 31 P MAS NMR spectra of TMPO adsorbed in H-Beta zeolites with varied Si/Al ratios: a H-Beta-7, b H-Beta-22, and c H-Beta-36. Reproduced from Ref. [56] by permission of American Chemical Society

As shown in Fig. 5.13a, for the pyridine-d5 probe molecule, it was found that the 1 H chemical shift of pyridinium ions decreases linearly with the decrease of PA or the increase of Brønsted acid strength, indicating that the 1 H chemical shift of adsorbed pyridine-d5 can be used as a scale for quantitatively measuring the Brønsted acid strength [59]. Furthermore, since a threshold DPE value of ca. 250 kcal/mol was predicted for superacidity (with acid strength stronger than 100% H2 SO4 ), a 1 H chemical shift of ca. 12.3 ppm is achieved as the threshold for pyridine-d5 adsorbed on superacid sites. For the 2-13 C-acetone probe molecule [60], it was found that three adsorption conformations (hydrogen-bonded, proton-shared, and ion-pair) exist, corresponding to different extents of proton transfer from the Brønsted acid site to the adsorbed acetone. A correlation of three broken lines was obtained for the 13 C chemical shift of acetone versus the DPE value [60]. The correlation can be used as a scale for quantitatively measuring the Brønsted acid strength of zeolites (Fig. 5.13b). In this case, the threshold 13 C chemical shift for superacidity was found to be ca. 245 ppm. DFT calculation results indicate that the 31 P NMR chemical shifts of adsorbed TMP molecules can be used as a scale to quantitatively measure the Lewis acid strength; i.e., the Lewis acid strength linearly increases with the increase of the 31 P NMR chemical shift (Fig. 5.13c). Deng et al. [61, 62] also demonstrated that the 31 P chemical shifts of the trialkylphosphine oxide (R3 PO) probe molecules including trimethylphosphine oxide (TMPO), triethylphosphine (TEPO), tributylphosphine oxide (TBPO), and trioctylphosphine oxide (TOPO) could also be utilized as a scale for quantitatively measuring the Brønsted acid strength (Fig. 5.13d). It is interesting to note that the linear curves observed for the TEPO, TBPO, and TOPO probe molecules

5.2 Solid-State NMR Characterization of Acidic Property

175

Fig. 5.12 1 H MAS NMR spectra of H-ZSM-5, ZSM-5(G2), ZSM-5(I2), and ZSM-5(I6) (a) and pyridine-d5 adsorbed on these samples (b). ZSM-5(I2) and ZSM-5(I6) were prepared by incipient wetness impregnation, while ZSM-5(G2) was prepared by mechanically mixing zinc oxide with H-ZSM-5. Reproduced from Ref. [57] by permission of Wiley

coincide with one another, but deviate from TMPO by a consistent 31 P chemical shift offset of ca. 8 ± 2 ppm. The threshold 31 P chemical shift for superacidity was found to be 86 ppm for TMPO and 92–94 ppm for TEPO, TBPO, and TOPO, respectively. By choosing the R3 PO molecules with appropriate sizes, acid sites residing on the external and internal surfaces of porous catalysts may readily be differentiated and analyzed [63].

5.2.3 Location and Distribution of Acid Sites The characterization of acid site distributions is of great importance to understand the detailed mechanism of catalytic reactions occurring on different sites. In order to distinguish the acid sites located in the internal voids and on the external surfaces of zeolites, a solid-state 31 P NMR technique was proposed by using trialkylphosphine oxides probe molecules with different sizes, such as trimethylphosphine oxide (TMPO) and tributylphosphine oxide (TBPO) [63]. The size of TMPO (kinetic diam-

176

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.13 Correlations of (a) 1 H (b) 13 C and (d) 31 P chemical shifts of adsorbed pyridine-d5, 213 C-acetone, and trialkylphosphine oxide against the deprotonation energy (DPE) of the Brønsted acid. c Correlations of calculated 31 P chemical shifts against the binding energy of TMP adsorbed on acid sites with B-, Al-, and Ti-Lewis centers. Reproduced from Ref. [3] by permission of American Chemical Society

eter ca. 0.55 nm) is smaller than the typical pore aperture of the 10-membered ring (ca. 0.60 nm) of zeolite ZSM-5. The small size of TMPO enables it to diffuse into the intracrystalline channels and pores of the zeolite. Thus, both the internal and external acid sites are accessible to TMPO. However, the size of TBPO (ca. 0.82 nm) is too large to penetrate into the channels and can only detect acid sites located on the external surface of the zeolite. Therefore, the concentration of the internal acid sites can be obtained from the difference between those determined from TMPO and TMBO as shown in Fig. 5.14 [63]. By using DFT calculations and solid-state NMR of adsorbed TMPO [64], it could be found that the accessible Brønsted acidic protons reside in both the supercages (at the Al8–OH–Si8 and Al1–OH–Si2 sites) and the external surface pockets (at the Al8–OH–Si8 site) rather than in the sinusoidal channels (at the Al5–OH–Si7 site) of zeolite H-MCM-22, with the Al1–OH–Si2 site having the strongest acid strength. This finding may partially explain the special selectivity of acid-catalyzed reactions occurring inside the channels of the zeolite. 1 H MAS NMR spectra of perfluorotributylamine adsorbed on zeolites were found to be a promising probe molecule for distinguishing the internal and external acidic

5.2 Solid-State NMR Characterization of Acidic Property

177

Fig. 5.14 31 P MAS NMR spectra of (a) TMPO and (b) TBPO adsorbed on a mesoporous Al-MCM-41 sample (Si/Al = 70; averaged pore size 2.54 nm). The lower spectrum in (c) was obtained from the TMPO/MCM-41 sample exposed to humidity for 1.5 h. Reproduced from Ref. [63] by permission of American Chemical Society

sites in zeolites [65]. Figure 5.15 shows the 1 H MAS NMR spectra of H-ZSM-5 zeolite and 6 wt% Mo-HZSM-5 with and without adsorption of perfluorotributylamine. The intensity of the peak at 3.9 decreases, while that of the peak at about 5.8 ppm increases after adsorption of perfluorotributylamine. The signal at 5.8 ppm shifted to 6.0 ppm upon adsorption of perfluorotributylamine, suggesting that the bridging hydroxy groups located on the external surface of the zeolite interacted with the perfluorotributylamine. While those at the internal surface are not accessible to perfluorotributylamine, so their chemical shift at 3.9 ppm is unchanged [65]. This also

178

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.15 1 H MAS NMR spectra, with deconvolution, of a HZSM-5 zeolite and b 6 wt% MoHZSM-5, (top) before and (bottom) after adsorption of perfluorotributylamine. Reproduced from Ref. [65] by permission of Royal Society of Chemistry

provides a feasible method for distinguishing the internal and external acidic sites in zeolites [65]. Nanometal oxides are widely used in semiconductors, optics, solar cells, catalysts, paints, cosmetics, sun-cream lotions [66–68]. However, the relationship of surface features (exposed planes, defects, and chemical functionalities) with physiochemical properties is not well understood primarily due to the lack of experimental characterization. Solid-state 31 P MAS NMR was used to map surfaces on various ZnO samples with the assistance of trimethylphosphine (TMP) as a chemical probe [1]. As shown in Fig. 5.16a, XRD analysis confirms the crystal structure of three ZnO samples. The intensity of the (002) peak in the plates is higher than that in the other morphologies, with this predominantly polar (002) facet covering the structure, whereas both rod and powder samples show relatively higher proportions of the non-polar (100) and (101) planes. Figure 5.16b shows the 31 P solid-state NMR spectra of TMP adsorbed on ZnO plate, rod, and powder. In general, a stronger interaction of the surface acidic site with the basic TMP molecule gives a more down-field shift in the NMR spectrum. The intensity of the peak at −55 ppm is shown to decrease pronouncedly compared to the peak at −48 ppm (plate) and −43 ppm (rod), suggesting a weaker interaction with the TMP molecule. It is interesting to note from Fig. 5.16b that each sample reveals two large distinct 31 P resonances (−48 and −55 ppm for plate, −43 and −55 ppm for rod, −48 and −61 ppm for powder) in the range from −30 to −70 ppm [1]. Generally, TMP molecules bound to Lewis acid sites result in a δ31P

5.2 Solid-State NMR Characterization of Acidic Property

179

in the range from ca. −30 to −61 ppm and a larger δ31P corresponds to a stronger Lewis acid strength [54]. The peaks at −43, −48, −55, and −61 ppm can be initially assigned as the interactions between TMP and surface Lewis acid sites. The spectrum of each ZnO sample is thus deconvoluted into four components, namely Site I (−43 ppm), Site II (−48 ppm), Site III (−55 ppm), and Site IV (−61 ppm), and the concentration of adsorbed TMP on each site is calculated according to the corresponding peak area (Fig. 5.16c). The resonance at around −43 ppm (Site I) can be attributed to TMP on (100)Zn3C site. The −48 ppm signal (Site II) is due to the interaction between TMP and Zn-(002)Zn3C. The signal at −55 ppm of Site III is assigned to the adsorption of TMP on Zn-(002)Zn–OH and O-(002)Zn–OH, which shows comparable adsorption energies. The signal at −61 ppm of Site IV can be assigned to weak adsorption of TMP on non-polar (100)Zn–OH or related surfaces [1]. It is demonstrated that this new surface-fingerprint technique not only provides qualitative (chemical shift) but also quantitative (peak intensity) information on the concentration and distribution of cations and anions, oxygen vacancies, and hydroxyl groups on various facets from a single deconvoluted 31 P NMR spectrum [1].

5.2.4 Spatial Proximities and Synergy Effects of Different Acid Sites In order to better understand the possible synergy effect between Brønsted and Lewis acids, advanced 2D solid-state NMR methods, such as 1 H–1 H [8], 27 Al–27 Al [10], and 31 P–31 P [69] double-quantum (DQ) magic-angle spinning (MAS) NMR spectroscopy methods have been developed and applied to investigate the spatial proximity among different acid sites in zeolites. 2D 1 H DQ MAS NMR experiment is a robust technique for probing proton–proton proximities in functional materials [70]. It was first employed by Deng et al. to explore the spatial proximities among various acid sites in dealuminated HY zeolites [8]. As shown in Fig. 5.17, several types of correlation peaks can be clearly discerned in the 1 H DQ MAS spectrum of dealuminated HY. The autocorrelation peaks at (4.3, 8.6) ppm and (5.0, 10.0) ppm suggest the spatial proximity of Brønsted acid sites in the supercage and sodalite cage, respectively. The autocorrelation peak at (2.2, 4.4) ppm results from the formation of silanol groups during the dealumination process. Another autocorrelation peak at (2.8, 5.6) ppm is due to EFAL species containing more than one hydroxyl group such as Al(OH)3 and Al(OH)+2 . Additionally, the off-diagonal peak pair at (1.0, 6.0) and (5.0, 6.0) ppm corresponds to the correlation between the extra-framework AlOH group and the bridging hydroxyl group in the sodalite cage, suggesting the spatial proximity between Brønsted and Lewis acid sites. The appearance of another off-diagonal peak pair at (2.8, 7.1) and (4.3, 7.1) ppm confirms the spatial proximity between the Lewis and Brønsted acid sites in the supercage [8]. By using 13 C NMR of adsorbed 2-13 C-acetone probe, it was demonstrated the spatial proximity results in a synergy effect between Brønsted and Lewis acid sites, which eventually enhances the Brønsted acid strength of dealuminated HY zeolite.

180

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.16 a XRD and b solid-state 31 P MAS NMR spectra (TMP adsorbed) of ZnO plate, rod and powder. c Spectral deconvolution of (b) and the corresponding data for each peak. The total coverage of OH on the surface was calculated from Site III and Site IV. Reproduced from Ref. [1] by permission of American Chemical Society

Meanwhile, 1 H–1 H DQ MAS NMR experiments were performed with varying DQ recoupling time to determine the average 1 H–1 H distance between various hydroxyl groups [9]. Accordingly, the 1 H–1 H distance between a Brønsted acidic proton and an extra-framework AlOH species in both the supercage and the sodalite cage was determined to be 4.3 Å, whereas the average distance between two nearby Brønsted acidic protons in the supercages was measured to be 5.0 Å (Fig. 5.18) [9]. By using the similar techniques, a comprehensive study was carried out to probe the spatial proximities between different acid sites in H-Y zeolites modified with different dealumination treatments (including calcination, steaming, and acid-leaching) [9]. It was found that the Brønsted/Lewis acid synergy effect was always present in the samples prepared by thermal and hydrothermal treatments, but absent in the samples prepared by acid-leaching treatment. The 1 H–1 H DQ MAS NMR spectroscopy was also employed to investigate the spatial proximity of acid sites in highly siliceous zeolites, such as H-ZSM-5, H-mordenite, and H-MCM-22 [71, 72]. All these find-

5.2 Solid-State NMR Characterization of Acidic Property

181

Fig. 5.17 1 H DQ MAS NMR spectra of dealuminated HY. Reproduced from Ref. [8] by permission of American Chemical Society

ings have provided insights into the roles of Lewis acid and its synergy with the Brønsted acid in zeolite-mediated hydrocarbon reactions. Apart from 1 H–1 H DQ MAS NMR, 27 Al–27 Al DQ MAS NMR can also be employed to investigate the Brønsted/Lewis acid synergy in dealuminated zeolites as well [10]. By utilizing a sensitivity-enhanced 2D 27 Al DQ MAS NMR technique [73], the spatial proximities among various Al species in dealuminated HY zeolites were demonstrated [3]. Figure 5.19 shows 2D 27 Al DQ MAS NMR spectra of parent HY and HY zeolites calcined at 500, 600, 650, 700 °C. For the parent HY as shown in Fig. 5.19a, the autocorrelation peak at (61, 122) ppm in the 27 Al–27 Al DQ MAS NMR spectrum indicates that four-coordinated framework Al (FAL) species are in close proximity to one another. For the HY-500 zeolite, the cross-peak pair at (61, 61) and (0, 61) ppm results from the spatial proximity between the four-coordinate FAL and the six-coordinate EFAL (Fig. 5.19b), implying the existence of Brønsted/Lewis acid synergy in the dealuminated HY zeolite [10]. For the HY-600 zeolite, it is revealed that three kinds of aluminum species including four-coordinate FAL, five-coordinate EFAL, and six-coordinate EFAL are in close proximity to one another (Fig. 5.19c). In the 2D 27 Al DQ MAS NMR spectrum of HY-700 (Fig. 5.19d), the correlation peak pair at (55, 87) and (32, 87) ppm is ascribed to the spatial proximity between four-coordinate EFAL species and five-coordinate EFAL species. On the basis of the experimental observations, the detailed spatial correlations among various aluminum species in hydrated HY zeolites after dealumination treatment are clearly identified [10]. With the assistance of DFT theoretical calculation, a new dealumination mechanism was proposed and three types of EFAL species, Al(OH)3 , Al(OH)+2 and Al(OH)2+ , in close proximity to framework aluminum were identified in dealu-

182

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.18 1 H–1 H DQ intensity as a function of DQ recoupling time for various hydroxyl groups in the dealuminated HY sample. 1 H–1 H distances were obtained by fitting the curves. Reproduced from Ref. [9] by permission of American Chemical Society

5.2 Solid-State NMR Characterization of Acidic Property

183

Fig. 5.19 27 Al MAS and DQMAS NMR spectra of a parent HY, b HY-500, c HY-600, and d HY-700 zeolites. Reproduced from Ref. [10] by permission of Wiley

minated HY zeolites [10]. The spatial proximities of Brønsted and Lewis acid sites in the highly siliceous zeolites, such as H-MOR, H-ZSM-5, and MCM-22 zeolites, were also investigated by 2D 27 Al DQ MAS experiments [71, 72]. It was found that the Brønsted/Lewis acid synergy was present in these highly siliceous zeolites as well. Based on the experimental observation, the spatial proximity/interaction between various acid sites can be manifested as shown in Fig. 5.20. The spatial proximity between various acid sites can also be indirectly detected by the 2D 31 P–31 P DQ MAS NMR spectra of probe molecules. To measure the Brønsted acid densities in zeolites, Grey et al. [69] proposed a new method by using diphenyldiphosphines, Ph2 P(CH2 )nPPh2 molecules having two basic sites. 2D 31 P–31 P DQ MAS NMR experiments were employed to probe 31 P–31 P internuclear distances and distinguish the non-protonated, single-protonated, or double-

184

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.20 Experimentally observed spatial proximities of FAL and EFAL species and Brønsted/Lewis acid synergy in dealuminated H-Y zeolites. Reproduced from Ref. [3] by permission of American Chemical Society

protonated diphenyldiphosphines binding sites in the zeolite. Figure 5.21 shows the 2D 31 P–31 P DQ MAS NMR spectra of Ph2 P(CH2 )nPPh2 adsorbed on the HY zeolite. Two diagonal peaks are observed at (8, 16) and (−1, −2), but there is no peak at (−28, −56). These results indicate that the resonance at −28 ppm is due to the non-protonated end of the singly protonated diphosphine. The protonated end of this diphosphine gives rise to a broad resonance with a maximum at 14 ppm, while the doubly protonated phosphines resonate at 8 and −1 ppm [69]. With the assistance of the chemical shifts assignment, the Brønsted acid density in zeolite HY can be quantitatively determined from the 1D 31 P MAS NMR spectra as shown in Fig. 5.22 [69]. More than 90% of the Ph2 P(CH2 )6 PPh2 molecules are doubly protonated on zeolite HY at a loading level of 12 molecules per unit cell, indicating that there are at least 12 pairs of Brønsted acid sites about 9 Å apart. It is revealed that there are only six pairs of Brønsted acid sites separated by a distance of 6 Å. The density and distribution of Brønsted acid sites in HY and H-ZSM-5 zeolites with different Si/Al ratios were investigated by using these NMR techniques as well [74]. Synergetic catalysis causes catalytic enhancements in terms of both activity and selectivity in a broad range of catalytic reactions [75]. The introduction of metal species like Zn and Ga onto acidic zeolite supports as Lewis acid sites often leads to bifunctionality of modified zeolites [76, 77] on which the synergistic effect was also found [78, 79]. The synergistic effect is supposed to be originated from the interfacial interactions between the Brønsted and Lewis acid sites, but the structure of synergistic active sites has remained elusive [80]. Moreover, the quantification of synergistic active sites that is prerequisite for comparison of intrinsic activity between

5.2 Solid-State NMR Characterization of Acidic Property

185

Fig. 5.21 2D 31 P–31 P DQ NMR spectra of Ph2 PCH2 PPh2 adsorbed on HY zeolite (loading level = 8 molecules/unit cell). Reproduced from Ref. [69] by permission of American Chemical Society

Fig. 5.22 31 P MAS NMR spectra of Ph2 P(CH2 )n PPh2 adsorbed on HY (loading level = 12 molecules/unit cell). Reproduced from Ref. [69] by permission of American Chemical Society

186

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

different catalysts in terms of turnover frequencies still remains challenging. The direct detection of surface Zn species and their spatial interaction with the Brønsted acid sites (BAS) of zeolite on Zn-modified ZSM-5 zeolite was realized by using 67 Zn and 1 H–67 Zn double-resonance solid-state NMR spectroscopy [57]. In the work of Deng et al. [57], 67 Zn-enriched Zn/ZSM-5 (Si/Al = 21) samples containing 2 wt% (ZSM-5(I2)) and 6 wt% Zn (ZSM-5(I6)) were prepared by incipient wetness impregnation. Solid-state 67 Zn NMR on Zn-modified zeolites is challenging due to the unfavorable NMR characteristics of Zn (I = 5/2) nucleus, very low gyromagnetic ratio (γ = 1.678 × 107 rad T−1 s−1 ), and low natural abundance of the NMR-active isotope (4.1%, 67 Zn). Moreover, the difficulty in observing 67 Zn NMR signal is exacerbated by the low concentration of zinc species present on zeolite support. A sensitivity-enhanced HS-QCPMG NMR technique [81] was employed at high magnetic field (18.8 T) to characterize the Zn-modified ZSM-5 samples. To further improve the NMR sensitivity, 67 Zn (67 Zn, 89.6%) enriched precursors were used in the preparation of the Zn-modified zeolite samples. As shown in Fig. 5.23, 67 Zn HS-QCPMG NMR spectrum of ZSM-5(G2) (mechanical mixture of zinc oxide with H-ZSM-5) exhibits a typical second-order quadrupolar line shape with an isotropic chemical shift of 238 ppm, evidencing the formation of ZnO particles on ZSM-5. Two 67 Zn signals with isotropic chemical shifts of 238 ppm and 224 ppm, respectively, are observable for ZSM-5(I2) and ZSM-5(I6) (Fig. 5.23b and c). The lack of quadrupolar lineshape of the 238 ppm signal indicates that the formation of ZnO particles is highly dispersed on the ZSM-5 support. The 224 ppm signal is assigned to frameworkbound Zn ions on the cation-exchange sites of ZSM-5 zeolite. The local structural asymmetry induced by the zeolite framework may lead to an increase of quadrupolar interaction, which results in the line broadening. Figure 5.24 shows the 1 H–{67 Zn} S-RESPDOR NMR spectra of ZSM-5(I2) and ZSM-5(I6) recorded at a magnetic field of 18.8 T, which can provide direct experimental information on the spatial proximity/interaction between various protons and zinc species. Under 67 Zn irradiation, the 1 H signal of protons that were in close proximity to 67 Zn atoms would be modulated by 1 H–67 Zn dipolar interaction. In the difference spectra (Fig. 5.23a), the 1 H signals from SiOHAl (at 4.3 ppm), SiOH (at 2.3 ppm), and ZnOH (at 1.2 ppm) were observable, due to direct dipolar interactions between the protons and Zn species. Figure 5.24b displays that the 1 H–{67 Zn} S-RESPDOR signals build up curves (S/S0 ) as a function of recoupling time. The recoupling time for acidic proton and ZnOH to reach the maximum dipolar dephasing was approximately 12 and 7.2 ms, respectively, while the S/S0 of SiOH group keeps increasing. This indicated that the three types of hydroxy groups experience different degrees of dipolar interactions with Zn atoms. The longer maximum recoupling time for acidic proton as compared to ZnOH suggests that the 1 H–67 Zn internuclear dipolar interaction between acidic proton and zinc species is slightly weaker than that in ZnOH. The close spatial proximity between Zn2+ ion and acidic proton probably lead to the observed strong interaction. As the S/S0 of silanol group increased even after 14 ms recoupling time, it may be located farther away from zinc species. The approximate 1 H–67 Zn nuclear distances between the three types of protons and zinc atoms were extracted from both analytical formula and numerical simulations on the

5.2 Solid-State NMR Characterization of Acidic Property

187

Fig. 5.23 67 Zn HS-QCPMG NMR spectra recorded at 18.8 T of a ZSM-5(G2), b ZSM-5(I2) and c ZSM-5(I6). The isotropic chemical shifts are indicated on the top of the peaks. Reproduced from Ref. [57] by permission of Wiley

1

H–{67 Zn} S-RESPDOR dephasing data, which provides structural information on these moieties. The internuclear 1 H–67 Zn distance is 2.70–3.34 Å for acidic proton, 2.43–2.86 Å for ZnOH. The spatial proximity/interaction between the Zn2+ ion and BAS on Zn-modified ZSM-5 zeolites leads to an enhanced Brønsted acid strength as evidenced by 1 H MAS NMR of adsorbed pyridine-d 5 . A quantitative determination of the synergistic active sites can be made by the 1 H–{67 Zn} S-RESPDOR experiment [57]. Combining the maximum S/S0 value of the build-up curve, the residual acidic protons on zeolite, and the scaling factor of the 1 H dephasing fraction, the concentration of the synergistic active sites can be calculated. The observed promotion of C–H bond activation of methane is rationalized by the enhanced Brønsted acidity generated by synergic effects arising from the spatial proximity/interaction between Zn2+ ions and Brønsted acidic protons. Ga-modified ZSM-5 zeolites have attracted intensive studies for the methanol-toaromatics (MTA) reaction, because of their distinct performance toward the selective formation of aromatics [82, 83]. Introduction of metals on zeolites leads to the formation of strong Lewis acid sites, which promotes the dehydrogen-aromatization process. The Ga species on Ga/ZSM-5 prepared with impregnation method were probed by 71 Ga solid-state NMR spectroscopy [84, 85]. Ga2 O3 /ZSM-5 (physical mixture of Ga2 O3 with H-ZSM-5) exhibits a signal with a typical second-order

188

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.24 1 H–{67 Zn} S-RESPDOR NMR spectra recorded at 18.8 T of ZSM-5(I2) and ZSM-5(I6) with a recoupling time of 9.6 ms (a) and S/S0 signal fraction versus the total recoupling time (b). Reproduced from Ref. [57] by permission of Wiley

5.2 Solid-State NMR Characterization of Acidic Property

189

Fig. 5.25 71 Ga QCPMG MAS NMR spectra of Ga2 O3 /ZSM-5, Ga/ZSM-5(IM), and Ga/ZSM-5(redox). The asterisks denote spinning sidebands. Reproduced from Ref. [85] by permission of American Chemical Society

quadrupolar lineshape and an isotropic chemical shift at 58 ppm, due to octahedrally coordinated Ga atom from Ga2 O3 particles [86] (Fig. 5.25). Both Ga/ZSM-5(IM) (prepared by wetness impregnation) and Ga/ZSM-5(redox) (prepared by reduction of Ga/ZSM-5(IM)) produce two 71 Ga signals with isotropic chemical shifts at 58 and 190 ppm. The lack of quadrupolar lineshape of the 58 ppm signal as well as the linewidth broadening is an indication of the formation of highly dispersed amorphous Ga2 O3 particles on the two samples. The low-field signal (190 ppm) suggests the formation of Ga species with lower coordination. The quadrupolar coupling constant of this signal is 7.6 MHz determined by the simulation of the 71 Ga QCPMG NMR spectrum, notably larger than that (2–3 MHz) of Ga sites in zeolite framework which are in a symmetrical tetrahedral coordination [87]. This indicates a much lower coordination symmetry around this type of Ga species on Ga/ZSM-5, which can be ascribed to extra-framework cationic Ga species in the form of GaO+ or its hydrated Ca(OH)2+ [88], which substitutes the acidic proton of BAS and resides on its conjugate-base site (Si–O− –Al). The importance of the close spatial proximity between Lewis acid sites (metal species) and Brønsted acid site of zeolite has been widely noted in MTA reactions over metal-modified zeolites [82, 83, 89, 90]. The synergy of the Ga species and the Brønsted acid site in H-ZSM-5 was revealed by using 1 H–71 Ga double-resonance solid-state NMR [85]. Figure 5.26 shows the 1 H–71 Ga S-RESPDOR NMR spectra

190

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.26 1 H–71 Ga S-RESPDOR NMR spectra (a, b, c) of Ga2 O3 /ZSM-5, Ga/ZSM-5(IM) and Ga/ZSM-5(redox) with a recoupling time of 12 ms, and 1 H–71 Ga S-RESPDOR build-up curves (d, e, f) of the BAS, AlOH, and GaOH obtained from Ga/ZSM-5(redox) fitted by analytical formula. DIS and rH-Ga represent the dipolar interaction constant and internuclear distance of 1 H–71 Ga spin pair, respectively. Reproduced from Ref. [85] by permission of American Chemical Society

and build-up curves of Ga2 O3 /ZSM-5, Ga/ZSM-5(IM), and Ga/ZSM-5(redox). For Ga2 O3 /ZSM-5 (Fig. 5.26a), no proton is in close proximity to Ga species as none of the three 1 H signals exhibit any observable 1 H–71 Ga dipolar dephasing. This is probably because the Ga2 O3 particles are too large to enter the zeolite channel where the BAS is mainly located. On the contrary, an extremely weak 1 H–71 Ga dipolar dephasing effect is observable for Ga/ZSM-5(IM) as reflected by the weak signals in the difference spectrum (S) (Fig. 5.24b). Interestingly, for Ga/HZSM-5(redox), a strong 1 H–71 Ga dipolar dephasing effect is evident for the BAS (Fig. 5.26c), indicating that a moderate amount of neighboring BAS-Ga pairs is formed. The cationic Ga species in ZSM-5 channels should be involved in the formation of neighboring BAS-Ga pairs. The spatial interactions between protons and Ga species can be further quantitatively analyzed from the 1 H–71 Ga S-RESPDOR signal build-up curves. The internuclear distance between the GaO+ ion and the acidic proton of BAS is determined to be 5.05 Å. 2D 1 H–1 H DQ MAS NMR experiments provide more information on the formation of neighboring BAS-Ga pairs on Ga-modified samples (Fig. 5.27). A strong autocorrelation peak (4.3, 8.6) ppm characterizing the neighboring BAS pair was observed on Ga2 O3 /ZSM-5 which is similar to that of parent H-ZSM-5 (Fig. 5.27a and b), suggesting that the BAS pair remains almost unchanged in Ga2 O3 /HZSM-5. The characteristic autocorrelation peak disappears completely in Ga/ZSM-5(redox) (Fig. 5.27d), implying that most of the remaining BAS are no longer in close proximity or present in pairs. The isolation of the BAS should be resulted from the

5.2 Solid-State NMR Characterization of Acidic Property

191

Fig. 5.27 1 H–1 H DQ MAS NMR spectra of a H-ZSM-5, b Ga2 O3 /ZSM-5, c Ga/ZSM-5(IM), and d Ga/ZSM-5(redox). Reproduced from Ref. [85] by permission of American Chemical Society

substitution of acidic protons by Ga species. Taking the 1 H–1 H DQ and 1 H–71 Ga S-RESPDOR NMR results together, the distribution of Brønsted acid sites and Ga species on the samples can be illustrated in Fig. 5.28. As revealed by 1 H MAS NMR of adsorbed pyridine-d 5 , the Brønsted acidity of the Ga-modified zeolite is considerably enhanced due to the synergic effect. Similarly, the synergic active sites can be quantified by 1 H–71 Ga double-resonance solid-state NMR, which exhibits a correlation with the aromatics selectivity in the MTA reaction [85].

5.3 Summary Characterization of the surface acidic properties of solid acid catalysts is a key issue to understand their catalytic performance in heterogeneous catalysis. The acidic features

192

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

Fig. 5.28 Schematic distributions of Brønsted acid sites (acidic protons) and Ga species in the zeolite channels of a H-ZSM-5, b Ga2 O3 /ZSM-5, c Ga/ZSM-5(IM), and d Ga/ZSM-5(redox). Reproduced from Ref. [85] by permission of American Chemical Society

of solid acids including acid type, acid concentration, acid distribution, acid strength, and the spatial proximity/interaction of acid sites are essential factors in dictating their reactivity and selectivity. Solid-state NMR techniques in conjunction with probe molecules such as pyridine, acetone, trimethylphosphine, trialkylphosphine oxide, and acetonitrile provide a versatile approach for a systematic investigation of acidic property of zeolites and other solid acid catalysts. In addition, the spatial proximity and synergetic effect of different acid sites can be ascertained by 2D DQ MAS NMR and 1D 1 H{Metal} double-resonance solid-state NMR spectroscopy.

References 1. Peng Y-K, Ye L, Qu J, Zhang L, Fu Y, Teixeira IF, McPherson IJ, He H, Tsang SCE (2016) Trimethylphosphine-assisted surface fingerprinting of metal oxide nanoparticle by 31P solidstate NMR: a zinc oxide case study. J Am Chem Soc 138(7):2225–2234. https://doi.org/10. 1021/jacs.5b12080 2. Hunger M (1997) Brnsted acid sites in zeolites characterized by multinuclear solid-state nmr spectroscopy. Catal Rev 39(4):345–393. https://doi.org/10.1080/01614949708007100 3. Zheng AM, Li SH, Liu SB, Deng F (2016) Acidic properties and structure-activity correlations of solid acid catalysts revealed by solid-state NMR spectroscopy. Acc Chem Res 49(4):655–663. https://doi.org/10.1021/acs.accounts.6b00007 4. Hadjiivanov KI, Vayssilov GN (2002) Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule. In: Gates BC, Knozinger H (eds) Advances in catalysis 47:307–511. https://doi.org/10.1016/s0360-0564(02)47008-3 5. Song XM, Sayari A (1996) Sulfated zirconia-based strong solid-acid catalysts: recent progress. Catal Rev Sci Eng 38(3):329–412. https://doi.org/10.1080/01614949608006462 6. Hunger M (1996) Multinuclear solid-state NMR studies of acidic and non-acidic hydroxyl protons in zeolites. Solid State Nucl Magn Reson 6(1):1–29. https://doi.org/10.1016/09262040(95)01201-x 7. Hunger M (1997) Bronsted acid sites in zeolites characterized by multinuclear solid-state NMR spectroscopy. Catal Rev Sci Eng 39(4):345–393. https://doi.org/10.1080/01614949708007100 8. Li S, Zheng A, Su Y, Zhang H, Chen L, Yang J, Ye C, Deng F (2007) Brønsted/lewis acid synergy in dealuminated HY zeolite: a combined solid-state NMR and theoretical calculation study. J Am Chem Soc 129(36):11161–11171. https://doi.org/10.1021/ja072767y

References

193

9. Li SH, Huang SJ, Shen WL, Zhang HL, Fang HJ, Zheng AM, Liu SB, Deng F (2008) Probing the spatial proximities among acid sites in dealuminated H-Y zeolite by solid-state NMR spectroscopy. J Phys Chem C 112(37):14486–14494. https://doi.org/10.1021/jp803494n 10. Yu ZW, Zheng AM, Wang QA, Chen L, Xu J, Amoureux JP, Deng F (2010) Insights into the dealumination of zeolite HY revealed by sensitivity-enhanced Al-27 DQ-MAS NMR spectroscopy at high field. Angew Chem Int Ed 49(46):8657–8661. https://doi.org/10.1002/anie. 201004007 11. Freude D, Hunger M, Pfeifer H, Schwieger W (1986) H-1 MAS NMR-Studies On The Acidity Of Zeolites. Chem Phys Lett 128(1):62–66. https://doi.org/10.1016/0009-2614(86)80146-4 12. Lohse U, Loffler E, Hunger M, Stockner J, Patzelova V (1987) Hydroxyl-groups of the nonframework aluminum species in dealuminated Y-zeolites. Zeolites 7(1):11–13. https://doi.org/ 10.1016/0144-2449(87)90111-4 13. Derouane EG, Védrine JC, Pinto RR, Borges PM, Costa L, Lemos MANDA, Lemos F, Ribeiro FR (2013) The acidity of zeolites: concepts, measurements and relation to catalysis: a review on experimental and theoretical methods for the study of zeolite acidity. Catal Rev 55(4):454–515. https://doi.org/10.1080/01614940.2013.822266 14. Jiang Y, Huang J, Dai W, Hunger M (2011) Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts. Solid State Nucl Magn Reson 39(3–4):116–141. https://doi.org/10.1016/j.ssnmr.2011.03.007 15. Grey CP, Vega AJ (1995) Determination of the quadrupole coupling-constant of the invisible aluminum spins in zeolite HY with H-1/AL-27 TRAPDOR NMR. J Am Chem Soc 117(31):8232–8242. https://doi.org/10.1021/ja00136a022 16. Chen K, Abdolrhamani M, Sheets E, Freeman J, Ward G, White JL (2017) Direct detection of multiple acidic proton sites in zeolite HZSM-5. J Am Chem Soc 139(51):18698–18704. https://doi.org/10.1021/jacs.7b10940 17. Sauer J (1989) Acidic sites in heterogeneous catalysis: structure, properties and activity. J Mol Catal 54(3):312–323. https://doi.org/10.1016/0304-5102(89)80149-X 18. Ratcliffe CI, Ripmeester JA, Tse JS (1985) NMR chemical shifts of dilute 1H in inorganic solids. Chem Phys Lett 120(4):427–432. https://doi.org/10.1016/0009-2614(85)85634-7 19. Batamack P, Doremieux-Morin C, Fraissard J, Freude D (1991) Broad-line and highresolution NMR studies concerning the hydroxonium ion in HZSM-5 zeolites. J Phys Chem 95(9):3790–3796. https://doi.org/10.1021/j100162a064 20. Tanabe K (1985) Surface and catalytic properties of ZrO2 . Mater Chem Phys 13(3–4):347–364 21. Bartona DG, Soledb SL, Meitznerc GD, Fuentesd GA, Iglesia E (1999) Structural and catalytic characterization of solid acids based on zirconia modified by Tungsten Oxide. J Catal 181(1):57–72 22. Iglesia E, Barton DG, Soled SL, Miseo S, Baumgartner JE, Gates WE, Fuentes GA, Meitzner GD (1996) Selective isomerization of alkanes on supported tungsten oxide acids. Stud Surf Sci Catal 101:533–542 23. Khodakov A, Olthof B, Bell AT, Iglesia E (1999) Structure and catalytic properties of supported vanadium oxides: support effects on oxidative dehydrogenation reactions. J Catal 181:205–216 24. Xu J, Zheng AM, Yang J, Su YC, Wang JQ, Zeng DL, Zhang MJ, Ye CH, Deng F (2006) Acidity of mesoporous MoOx/ZrO2 and WOx/ZrO2 materials: a combined solid-state NMR and theoretical calculation study. J Phys Chem B 110(22):10662–10671. https://doi.org/10. 1021/jp0614087 25. Tanabe K, Hölderich WF (1999) Industrial application of solid acid–base catalysts. Appl Catal A 181(2):399–434. https://doi.org/10.1016/S0926-860X(98)00397-4 26. Yang J, Zheng AM, Zhang MJ, Luo Q, Yue Y, Ye CH, Lu X, Deng F (2005) Bronsted and lewis acidity of the BF3/gamma-Al2 O3 alkylation catalyst as revealed by solid-state NMR spectroscopy and DFT quantum chemical calculations. J Phys Chem B 109(27):13124–13131. https://doi.org/10.1021/jp051268l 27. Taarning E, Osmundsen CM, Yang X, Voss B, Andersen SI, Christensen CH (2011) Zeolitecatalyzed biomass conversion to fuels and chemicals. Energy Environ Sci 4(3):793–804. https:// doi.org/10.1039/C004518G

194

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

28. Roman-Leskkov Y, Moliner M, Labinger JA, Davis DM (2010) Mechanism of glucose isomerization using a solid lewis acid catalyst in water. Angewandte Chem Int Ed 49(47):8954–8957. https://doi.org/10.1002/anie.201004689 29. Boronat M, Concepcion P, Corma A, Renz M, Valencia S (2005) Determination of the catalytically active oxidation Lewis acid sites in Sn-beta zeolites, and their optimisation by the combination of theoretical and experimental studies. J Catal 234(1):111–118. https://doi.org/ 10.1016/j.jcat.2005.05.023 30. Harris JW, Cordon MJ, Di Iorio JR, Vega-Vila JC, Ribeiro FH, Gounder R (2016) Titration and quantification of open and closed Lewis acid sites in Sn-Beta zeolites that catalyze glucose isomerization. J Catal 335(Supplement C):141–154. https://doi.org/10.1016/j.jcat.2015.12.024 31. Bermejo-Deval R, Orazov M, Gounder R, Hwang SJ, Davis ME (2014) Active sites in Sn-Beta for glucose isomerization to fructose and epimerization to mannose. ACS Catal 4(7):2288–2297. https://doi.org/10.1021/cs500466j 32. Hwang S-J, Gounder R, Bhawe Y, Orazov M, Bermejo-Deval R, Davis ME (2015) Solid state NMR characterization of Sn-Beta zeolites that catalyze glucose isomerization and epimerization. Top Catal 58(7):435–440. https://doi.org/10.1007/s11244-015-0388-7 33. Qi G, Wang Q, Xu J, Wu Q, Wang C, Zhao X, Meng X, Xiao F, Deng F (2018) Direct observation of tin sites and their reversible interconversion in zeolites by solid-state NMR spectroscopy. Commun Chem 1(1):22. https://doi.org/10.1038/s42004-018-0023-1 34. Blunden SJ, Hill R (1990) An investigation of the base hydrolysis of methyl- and butyltin trichloride in aqueous solution by 1 H and 119 Sn NMR spectroscopy. Inorg Chim Acta 177(2):219–223. https://doi.org/10.1016/S0020-1693(00)85979-4 35. Blunden SJ, Smith PJ, Gillies DG (1982) An investigation of the hydrolysis products of monoalkyltin trichlorides by 119m Sn Mössbauer, and 1 H and 119 Sn NMR spectroscopy. Inorg Chim Acta 60:105–109. https://doi.org/10.1016/S0020-1693(00)91158-7 36. Josephson TR, Jenness GR, Vlachos DG, Caratzoulas S (2017) Distribution of open sites in Sn-Beta zeolite. Micropor Mesopor Mat 245:45–50. https://doi.org/10.1016/j.micromeso. 2017.02.065 37. Wolf P, Valla M, Núñez-Zarur F, Comas-Vives A, Rossini AJ, Firth C, Kallas H, Lesage A, Emsley L, Copéret C, Hermans I (2016) Correlating synthetic methods, morphology, atomiclevel structure, and catalytic activity of Sn-β catalysts. ACS Catal 6(7):4047–4063. https://doi. org/10.1021/acscatal.6b00114 38. Lunsford JH, Rothwell WP, Shen W (1985) Acid sites in zeolite Y: a solid-state NMR and infrared study using trimethylphosphine as a probe molecule. J Am Chem Soc 107(6):1540–1547. https://doi.org/10.1021/ja00292a015 39. Haw JF, Zhang JH, Shimizu K, Venkatraman TN, Luigi DP, Song WG, Barich DH, Nicholas JB (2000) NMR and theoretical study of acidity probes on sulfated zirconia catalysts. J Am Chem Soc 122(50):12561–12570. https://doi.org/10.1021/ja0027721 40. Zhang WP, Han XW, Liu XM, Bao XH (2003) Characterization of the acid sites in dealuminated nanosized HZSM-5 zeolite with the probe molecule trimethylphosphine. J Mol Catal A Chem 194(1–2):107–113. https://doi.org/10.1016/S1381-1169(02)00466-1/S1381-1169(02)00466-1 41. Luo Q, Deng F, Yuan ZY, Yang J, Zhang MJ, Yue Y, Ye CH (2003) Using trimethylphosphine as a probe molecule to study the acid states in Al-MCM-41 materials by solid-state NMR spectroscopy. J Phys Chem B 107(11):2435–2442. https://doi.org/10.1021/jp0213093 42. Ma D, Han XW, Xie SJ, Bao XH, Hu HB, Au-Yeung SCF (2002) An investigation of the roles of surface aluminum and acid sites in the zeolite MCM-22. Chem A Eur J 8(1):162–170. https://doi.org/10.1002/1521-3765(20020104)8:1%3c162:aid-chem162%3e3.0.co;2-4 43. Biaglow AI, Gorte RJ, Kokotailo GT, White D (1994) A probe of bronsted site acidity in zeolites—C-13 chemical-shift of acetone. J Catal 148(2):779–786. https://doi.org/10.1006/ jcat.1994.1264 44. Xu T, Munson EJ, Haw JF (1994) Toward a systematic chemistry of organic-reactions in zeolites—in-situ nmr-studies of ketones. J Am Chem Soc 116(5):1962–1972. https://doi.org/ 10.1021/ja00084a041

References

195

45. Kao HM, Yu CY, Yeh MC (2002) Detection of the inhomogeneity of Bronsted acidity in H-mordenite and H-P zeolites: a comparative NMR study using trimethylphosphine and trimethylphosphine oxide as P-31 NMR probes. Microporous Mesoporous Mater 53(1–3):1–12. https://doi.org/10.1016/S1387-1811(02)00279-2 46. Rakiewicz EF, Peters AW, Wormsbecher F, Sutovich KJ, Mueller KT (1998) Characterization of acid sites in zeolitic and other inorganic systems using solid-state P-31 NMR of the probe molecule trimethylphosphine oxide. J Phys Chem B 102(16):2890–2896. https://doi.org/10. 1021/jp980808u 47. Freude D, Hunger M, Pfeifer H (1982) Study of Brønsted acidity of zeolites using high-resolution proton magnetic resonance with magic-angle spinning. Chem Phys Lett 91(4):307–310. https://doi.org/10.1016/0009-2614(82)80162-0 48. Simperler A, Bell RG, Foster MD, Gray AE, Lewis DW, Anderson MW (2004) Probing the acid strength of Bronsted acidic zeolites with acetonitrile: an atomistic and quantum chemical study. J Phys Chem B 108(22):7152–7161. https://doi.org/10.1021/jp035673t 49. DeCanio SJ, Sohn JR, Fritz PO, Lunsford JH (1986) Acid catalysis by dealuminated zeolite-Y: I. Methanol dehydration and cumene dealkylation. J Catal 101(1):132–141. https://doi.org/10. 1016/0021-9517(86)90236-8 50. Sohn JR, DeCanio SJ, Fritz PO, Lunsford JH (1986) Acid catalysis by dealuminated zeolite Y. 2. The roles of aluminum. J Phys Chem 90(20):4847–4851. https://doi.org/10.1021/j100411a026 51. Misono M (2001) Unique acid catalysis of heteropoly compounds (heteropolyoxometalates) in the solid state. Chem Commun 13:1141–1152. https://doi.org/10.1039/b102573m 52. Howell RC, Perez FG, Jain S, Horrocks WD, Rheingold AL, Francesconi LC (2001) A new type of heteropolyoxometalates formed from lacunary polyoxotungstate ions and europium or yttrium cations. Angew Chem Int Ed 40 (21):4031. https://doi.org/10.1002/15213773(20011105)40:21%3c4031::aid-anie4031%3e3.0.co;2-8 53. Yang J, Janik MJ, Ma D, Zheng A, Zhang M, Neurock M, Davis RJ, Ye C, Deng F (2005) Location, acid strength, and mobility of the acidic protons in Keggin 12–H3PW12O40: a combined solid-state NMR spectroscopy and DFT quantum chemical calculation study. J Am Chem Soc 127(51):18274–18280. https://doi.org/10.1021/ja055925z 54. Chu YY, Yu ZW, Zheng AM, Fang HJ, Zhang HL, Huang SJ, Liu SB, Deng F (2011) Acidic strengths of Bronsted and lewis acid sites in solid acids scaled by P-31 NMR chemical shifts of adsorbed trimethylphosphine. J Phys Chem C 115(15):7660–7667. https://doi.org/10.1021/ jp200811b 55. Feng ND, Zheng AM, Huang SJ, Zhang HL, Yu NY, Yang CY, Liu SB, Deng F (2010) Combined solid-state NMR and theoretical calculation studies of Bronsted acid properties in anhydrous 12-molybdophosphoric acid. J Phys Chem C 114(36):15464–15472. https://doi.org/10.1021/ jp105683y 56. Zhao RR, Zhao ZC, Li SK, Zhang WP (2017) Insights into the correlation of aluminum distribution and Bronsted acidity in H-Beta zeolites from solid-state NMR spectroscopy and DFT calculations. J Phys Chem Lett 8(10):2323–2327. https://doi.org/10.1021/acs.jpclett.7b00711 57. Qi G, Wang Q, Xu J, Trébosc J, Lafon O, Wang C, Amoureux J-P, Deng F (2016) Synergic effect of active sites in zinc-modified ZSM-5 zeolites as revealed by high-field solid-state NMR spectroscopy. Angew Chem Int Ed 55(51):15826–15830. https://doi.org/10.1002/anie.201608322 58. Hunger M, Freude D, Fröhlich T, Pfeifer H, Schwieger W (1987) 1H-MAS n.m.r. studies of ZSM-5 type zeolites. Zeolites 7 (2):108–110. http://dx.doi.org/10.1016/0144-2449(87)90068-6 59. Zheng A, Zhang H, Chen L, Yue Y, Ye C, Deng F (2007) Relationship between 1H chemical shifts of deuterated pyridinium ions and Brønsted acid strength of solid acids. J Phys Chem B 111(12):3085–3089. https://doi.org/10.1021/jp067340c 60. Fang H, Zheng A, Chu Y, Deng F (2010) 13C chemical shift of adsorbed acetone for measuring the acid strength of solid acids: a theoretical calculation study. J Phys Chem C 114(29):12711–12718. https://doi.org/10.1021/jp1044749 61. Zheng A, Huang S-J, Chen W-H, Wu P-H, Zhang H, Lee H-K, de Menorval L-C, Deng F, Liu S-B (2008) 31P chemical shift of adsorbed trialkylphosphine oxides for acidity characterization of solid acids catalysts. J Phys Chem A 112(32):7349–7356. https://doi.org/10.1021/jp8027319

196

5 Solid-State NMR Characterization of Acid Properties of Zeolites and Solid Acid …

62. Zheng A, Huang S-J, Liu S-B, Deng F (2011) Acid properties of solid acid catalysts characterized by solid-state 31P NMR of adsorbed phosphorous probe molecules. Phys Chem Chem Phys 13(33):14889–14901. https://doi.org/10.1039/c1cp20417c 63. Zhao Q, Chen WH, Huang SJ, Wu YC, Lee HK, Liu SB (2002) Discernment and quantification of internal and external acid sites on zeolites. J Phys Chem B 106(17):4462–4469. https://doi. org/10.1021/jp015574k 64. Zheng AM, Chen L, Yang J, Zhang MJ, Su YC, Yue Y, Ye CH, Deng F (2005) Combined DFT theoretical calculation and solid-state NMR studies of Al substitution and acid sites in zeolite MCM-22. J Phys Chem B 109(51):24273–24279. https://doi.org/10.1021/jp0527249 65. Zhang WP, Ma D, Liu XC, Liu XM, Bao XH (1999) Perfluorotributylamine as a probe molecule for distinguishing internal and external acidic sites in zeolites by high-resolution H-1 MAS NMR spectroscopy. Chem Commun 12:1091–1092. https://doi.org/10.1039/a900396g 66. Comini E (2006) Metal oxide nano-crystals for gas sensing. Anal Chim Acta 568(1–2):28–40. https://doi.org/10.1016/j.aca.2005.10.069 67. Jang M, Shin EW, Park JK, Choi SI (2003) Mechanisms of arsenate adsorption by highlyordered nano-structured silicate media impregnated with metal oxides. Environ Sci Technol 37(21):5062–5070. https://doi.org/10.1021/es0343712 68. Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon JM (2000) Nano-sized transition-metaloxides as negative-electrode materials for lithium-ion batteries. Nature 407(6803):496–499 69. Peng LM, Chupas PJ, Grey CP (2004) Measuring Bronsted acid densites in zeolite HY with diphosphine molecules and solid state NMR spectroscopy. J Am Chem Soc 126(39):12254–12255. https://doi.org/10.1021/ja0467519 70. Brown SP, Spiess HW (2001) Advanced solid-state NMR methods for the elucidation of structure and dynamics of molecular, macromolecular, and supramolecular systems. Chem Rev 101(12):4125–4155. https://doi.org/10.1021/cr990132e 71. Yu Z, Li S, Wang Q, Zheng A, Jun X, Chen L, Deng F (2011) Brønsted/lewis acid synergy in H-ZSM-5 and H–MOR zeolites studied by 1H and 27Al DQ-MAS solid-state NMR spectroscopy. J Phys Chem C 115(45):22320–22327. https://doi.org/10.1021/jp203923z 72. Yu ZW, Wang Q, Chen L, Deng F (2012) Bronsted/lewis acid sites synergy in H-MCM-22 zeolite studied by H-1 and Al-27 DQ-MAS NMR spectroscopy. Chin J Catal 33(1):129–139. https://doi.org/10.1016/s1872-2067(10)60287-2 73. Wang Q, Hu B, Lafon O, Trebosc J, Deng F, Amoureux JP (2009) Double-quantum homonuclear NMR correlation spectroscopy of quadrupolar nuclei subjected to magic-angle spinning and high magnetic field. J Magn Reson 200(2):251–260. https://doi.org/10.1016/j.jmr.2009. 07.009 74. Peng L, Grey CP (2008) Diphosphine probe molecules and solid-state NMR investigations of proximity between acidic sites in zeolite HY. Microporous Mesoporous Mater 116(1–3):277–283. https://doi.org/10.1016/j.micromeso.2008.04.014 75. Shi JL (2013) On the synergetic catalytic effect in heterogeneous nanocomposite catalysts. Chem Rev 113(3):2139–2181. https://doi.org/10.1021/cr3002752 76. Iglesia E, Barton DG, Biscardi JA, Gines MJL, Soled SL (1997) Bifunctional pathways in catalysis by solid acids and bases. Catal Today 38(3):339–360. https://doi.org/10.1016/s09205861(97)81503-7 77. Meriaudeau P, Naccache C (1997) Dehydrocyclization of alkanes over zeolite-supported metal catalysts: monofunctional or bifunctional route. Catal Rev 39(1–2):5–48. https://doi.org/10. 1080/01614949708006467 78. Meriaudeau P, Naccache C (1990) The role of Ga2 O3 and proton acidity on the dehydrogenating activity of GA2 O3 -hzsm-5 catalysts—evidence of a bifunctional mechanism. J Mol Catal 59(3):L31–L36. https://doi.org/10.1016/0304-5102(90)85100-v 79. Stepanov AG, Arzumanov SS, Gabrienko AA, Parmon VN, Ivanova II, Freude D (2008) Significant Influence of Zn on activation of the C–H bonds of small alkanes by Brønsted acid sites of zeolite. ChemPhysChem 9(17):2559–2563. https://doi.org/10.1002/cphc.200800569

References

197

80. Ono Y (1992) Transformation of lower alkanes into aromatic-hydrocarbons over ZSM-5 zeolites. Catal Rev 34(3):179–226. https://doi.org/10.1080/01614949208020306 81. Siegel R, Nakashima TT, Wasylishen RE (2004) Signal enhancement of NMR spectra of half-integer quadrupolar nuclei in solids using hyperbolic secant pulses. Chem Phys Lett 388(4–6):441–445. https://doi.org/10.1016/j.cplett.2004.03.047 82. Freeman D, Wells RPK, Hutchings GJ (2001) Methanol to hydrocarbons: enhanced aromatic formation using a composite GaO-H-ZSM-5 catalyst. Chem Commun 18:1754–1755. https:// doi.org/10.1039/B104844A 83. Freeman D, Wells RPK, Hutchings GJ (2002) Conversion of methanol to hydrocarbons over Ga2 O3 /H-ZSM-5 and Ga2 O3 /WO3 catalysts. J Catal 205(2):358–365. https://doi.org/10.1006/ jcat.2001.3446 84. Gao P, Xu J, Qi G, Wang C, Wang Q, Zhao Y, Zhang Y, Feng N, Zhao X, Li J, Deng F (2018) A mechanistic study of methanol-to-aromatics reaction over Ga-modified ZSM-5 zeolites: understanding the dehydrogenation process. ACS Catal 8(10):9809–9820. https://doi.org/10. 1021/acscatal.8b03076 85. Gao P, Wang Q, Xu J, Qi GD, Wang C, Zhou X, Zhou XL, Feng ND, Liu XL, Deng F (2018) Bronsted/lewis acid synergy in methanol-to-aromatics conversion on Ga-modified ZSM-5 zeolites, as studied by solid-state NMR spectroscopy. ACS Catal 8(1):69–74. https://doi.org/ 10.1021/acscatal.7b03211 86. Massiot D, Farnan I, Gautier N, Trumeau D, Trokiner A, Coutures JP (1995) 71 Ga and 69 Ga nuclear magnetic resonance study of β-Ga2 O3 : resolution of four- and six-fold coordinated Ga sites in static conditions. Solid State Nucl Magn Reson 4(4):241–248. https://doi.org/10. 1016/0926-2040(95)00002-8 87. Occelli ML, Schwering G, Fild C, Eckert H, Auroux A, Iyer PS (2000) Galliosilicate molecular sieves with the faujasite structure. Microporous Mesoporous Mater 34(1):15–22. https://doi. org/10.1016/S1387-1811(99)00151-1 88. Garc´ıa-Sánchez M, Magusin PCMM, Hensen EJM, Thüne PC, Rozanska X, van Santen RA (2003) Characterization of Ga/HZSM-5 and Ga/HMOR synthesized by chemical vapor deposition of trimethylgallium. J Catal 219(2):352–361. https://doi.org/10.1016/S00219517(03)00231-8 89. Lopez-Sanchez JA, Conte M, Landon P, Zhou W, Bartley JK, Taylor SH, Carley AF, Kiely CJ, Khalid K, Hutchings GJ (2012) Reactivity of Ga2 O3 clusters on zeolite ZSM-5 for the conversion of methanol to aromatics. Catal Lett 142(9):1049–1056. https://doi.org/10.1007/ s10562-012-0869-2 90. Lai P-C, Chen C-H, Hsu H-Y, Lee C-H, Lin Y-C (2016) Methanol aromatization over Ga-doped desilicated HZSM-5. Rsc Adv 6(71):67361–67371. https://doi.org/10.1039/C6RA16052B

Chapter 6

In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

Abstract The design of an efficient catalyst for a specific reaction depends on the understanding of the structure–activity relationship, which requires the fundamental knowledge about the reaction mechanisms. This chapter deals with the development and application of in situ solid-state NMR in the investigation of reactions catalyzed by zeolites. The state of the art of the in situ MAS NMR approaches that are performed at either bath or flow conditions were described. A comprehensive introduction was given to the mechanistic study of the catalytic reactions by in situ NMR with focus on the activation of light alkanes (C1-C3) on metal-modified zeolites as well as methanol-to-hydrocarbons conversion on acidic zeolites. The identification of active intermediates and analysis of the kinetics by in situ NMR enable detailed information about the reaction mechanisms to be obtained. An outlook is discussed on the development of the in situ MAS NMR technique that will contribute to the further advancement in understanding zeolite catalysis. Keywords Heterogeneous catalysts · Zeolites · Mechanism · In situ NMR · Intermediates · Kinetics

6.1 Introduction Heterogeneous catalysts are intensively used in chemical, petrochemical, environmental, food, and pharmaceutical industries. Heterogeneous catalysis is estimated to be responsible for the production of more than 85% of all bulk chemicals, intermediates, and fine chemicals [1]. In order to enhance the catalytic efficiency in terms of activity, selectivity, and longevity, persistent efforts have been made on modification of the structure and property of the present catalysts as well as the design of new catalysts, which requires the atomic-level insight into the catalytic mechanism. The information related to the reaction mechanism can be partially obtained by analysis of the structure change of catalysts, the adsorbed species, and reaction products after the reactions. However, it is often difficult to determine whether the adsorbed species are reaction intermediates or arespectator species. Thus, the reactor where the reaction taking place is often treated as a “black box.” The interpretations and © Springer Nature Singapore Pte Ltd. 2019 J. Xu et al., Solid-State NMR in Zeolite Catalysis, Lecture Notes in Chemistry 103, https://doi.org/10.1007/978-981-13-6967-4_6

199

200

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

deductions based on the ex situ analysis inevitably result in misunderstanding and to some extent conflict on reaction mechanism. This would hinder the rational design of better catalysts. Nowadays, researchers largely benefit from the advances in the characterization technologies which provide detailed structural insights into the catalysts and chemical reactions in controlled environments [2]. The in situ study of working catalysts at real reaction conditions, e.g., gas flow and high temperatures, is the ideal and direct way to reveal catalytic reaction mechanism. This means monitoring the dynamic chemical reactions taking place on catalysts in the reactor, including the observation of reactant adsorption, reaction intermediates, diffusion of products, as well as the discrimination between spectator species and active species. In situ IR [3, 4], Raman [5, 6], UV–Vis [7, 8], XAS [9, 10], and EPR [11, 12] spectroscopies among others are the widely used techniques. High-resolution magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is unique in probing molecular structure and dynamics of heterogeneous catalysts. This makes MAS NMR a powerful and versatile tool for in situ investigations of heterogeneous catalytic reactions [13–18]. It is very often that the catalytic reactions involve one or more carbon-containing reactants. The in situ 13 C or 1 H NMR spectroscopy enables both qualitative and quantitative determination of organic compounds adsorbed on solid catalysts. Thus, the transformation of reactants and the reaction kinetics can be followed. Besides, the structure of the catalyst under reaction conditions can be monitored by NMR spectroscopy of framework nuclei like 27 Al and 29 Si. It is not surprised that the in situ NMR technique has been increasingly used in the study of reaction mechanisms in heterogeneous catalysis since the beginning of the 1970s [19] and shows growing impact in fundamental understanding of the structure–property relationship of heterogeneous catalysts. Zeolite is one of the most widely used catalysts in heterogeneous catalysis and attracts intensive NMR studies in the characterization of its structure and related catalytic process. This chapter focuses on the application of in situ MAS NMR in the investigation of reactions catalyzed by zeolites and the advances in the identification of intermediates and elucidation of the kinetics in revealing the catalytic reaction mechanisms.

6.2 In Situ Solid-State NMR Approaches Derouane and Nagy et al. were the pioneers to use MAS NMR spectroscopy to study the hydrocarbon conversion on zeolites [19, 20], in which the products obtained from adsorption of alcohol and alkanes were characterized by 13 C CP/MAS NMR at room temperature. The research in this field was significantly facilitated by the development of various in situ NMR techniques. Experiments at high MAS spinning rate and at elevated temperatures is now achieved on the commercial NMR probe, which provides more and accurate information about the catalytic reactions on the “working catalyst.” According to the conditions at which the catalytic reaction was

6.2 In Situ Solid-State NMR Approaches

201

performed, two types of in situ NMR techniques were developed for the reactions under batch [21] or flow [22] conditions.

6.2.1 Batch Reaction For the batch reactions, surface adsorbates, intermediates, and products formed in or after the reaction were analyzed by NMR in a batch mode. There are two approaches to carry out the catalysis and NMR experiments at the batch conditions. The first makes use of glass ampoules as reactors containing the catalyst. The gaseous or liquid-state reactants are introduced onto the dehydrated catalyst in a glass ampoule which can be attached to a vacuum line. As shown in Fig. 6.1, the glass ampoule is sealed by flame after controlled adsorption of reactants. The sealed ampoule containing catalyst is usually heated in an oven at preset temperature, allowing the reaction to take place. After a certain time of heating, the reaction is quenched by liquid nitrogen and the glass ampoule is transferred into NMR rotor at room temperature for measurements. One of the technical demands for this method is that the glass ampoule used as an insert should tightly fit the size of NMR rotor. The most frequently used rotors are in diameter of 7 or 4 mm for NMR probe. Another point is that this approach requires highly symmetrical sealing of the glass ampoule to achieve high MAS spinning rates for increasing the spectral resolution. The glass ampoule can be homemade from suitable glass tube or commercially available as the MAS rotor insert [24]. The advantage of this method is that the reaction process can be controlled to some extent by quenching or restarting the reaction at desired temperature and heating time between NMR experiments. Since the glass ampoule is used as a “closed reactor,” the reactants and all the products will not come out of the “reactor.” This allows observation of both adsorbed and gaseous species during the reaction. Importantly, the transformation of the observed species can be quantified, which is important for the mechanism anal-

Fig. 6.1 Diagram of magic-angle spinning and glass ampoule for in situ NMR experiments at batch condition. Reprinted from Ref. [23] by permission of Elsevier

202

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

Fig. 6.2 Diagram of CAVERN apparatus used for catalyst activation and reactant adsorption onto catalyst for in situ NMR study. A shallow bed of catalyst is spread over the trapdoor, and the device is connected to the vacuum line. Reprinted from Ref. [27] by permission of Springer

ysis. But the reactants can only be introduced onto the catalyst before the reaction and re-introduction of reactants requires the preparation of a new glass ampoule. The second strategy to perform the experiments at batch conditions is the utilization of Cryogenic Adsorption Vessel Enabling Rotor Nestling (CAVERN) apparatus designed by Haw et al. [25–27]. A commercial 7-mm pencil-style MAS rotor is usually used as the reactor. As shown in Fig. 6.2, the rotor and catalyst are put into the glass apparatus connected to vacuum line. The activation of catalyst is conducted in the apparatus, and then a certain amount of reactants is condensed and frozen onto the pre-activated sample in the CAVERN device at liquid N2 temperature. The catalyst containing reactants is transferred into the rotor and sealed with airtight cap. To avoid the occurrence of reaction at room temperature, the rotor can be kept at liquid N2 temperature and transferred into the pre-cooled probe. The reactions are performed in a variable-temperature (VT) MAS NMR probe by heating the rotor from low temperature (often blow room temperature) up to 300 °C. Several or tens of min is often needed to reach the target temperature. The use of high-power infrared

6.2 In Situ Solid-State NMR Approaches

203

laser can significantly speed up the heating [28, 29]. A short period of intense IR heating on the rotor could produce a temperature up to several hundred degrees; i.e., 600 K can be achieved within 15 s. An alternative way to rapidly heat the sample has been developed by coating the MAS rotor with Pt metal [30]. The high-power radiofrequency irradiation on Pt metal inductively heats the sample. It was demonstrated that a 30-s irradiation produces inductive heating of 350 °C on methanol conversion on zeolite H-ZSM-5 [30].

6.2.2 Flow Reaction For heterogeneous catalysis, most industrial processes are working under flow conditions. To simulate the conditions in a flow catalytic reactor, various groups were devoted to the development of in situ NMR techniques to probe the catalytic reactions under flow conditions [13–18, 31]. Reimer and Bell developed one of the first in situ static NMR probes [32], which provides the opportunity to gain insight into the chemistry involved in the flow reactor. The NMR probe allows the reagent flow in the microreactors, but no sample rotation largely decreases spectroscopic resolution due to signal linewidth broadening; i.e., chemical shift anisotropy for 13 C often produces linewidths of 200 ppm or more. The low spectroscopic resolution limits the application of this in situ static NMR probe. Among others, a distinct technique is from Hunger and Horvath who designed the first in situ MAS NMR probe, featured by coupled sample spinning with continuous reagent flow [22, 32]. Figure 6.3 shows the diagram of the in situ MAS NMR system. This technique is based on a 7-mm Bruker MAS probe. A glass tube is inserted into the MAS NMR rotor packed with catalyst through an axially placed hole in the rotor cap. The reactant flows into the inner space of the catalyst from the top of the MAS

Fig. 6.3 Diagram of design of the in situ MAS NMR rotor under continuous-flow conditions (left) and the design of the reactant supply system (right). Reprinted from Ref. [35] by permission of Springer

204

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

NMR rotor and diffuses out through the gap in the rotor cap. The species adsorbed on the catalyst can be in situ-detected by NMR while the effluent products can be analyzed by online gas chromatography [33]. The in situ MAS NMR device can also be combined with UV–Vis spectroscopy by adding a quartz glass window and a glass fiber at the bottom of the NMR rotor so as to probe the catalytic system simultaneously by UV–Vis spectrometer [34]. The heating of samples up to 423 K and a spinning rate of 2 kHz can be usually achieved on this probe. Modification of the reaction introduction system on the probe allows a higher heating temperature of 837 K and a spinning rate of up to 3.5 kHz [35]. Note that this kind of in situ NMR experiments suffers from sensitivity loss at high temperatures. Additionally, it is hard to conduct in situ NMR experiments at high pressure due to the gap in the rotor cap. Although this flow MAS NMR probe does not completely reproduce the reactions in a real flow catalytic reactor where high temperature and pressure are often required, more close insight into the working catalyst can be achieved than that by in situ NMR under batch conditions. This has been notably demonstrated in the mechanistic study of methanol-to-olefins conversion on zeolites using the in situ NMR spectroscopy under continuous-flow conditions [15]. To observe and identify the reaction intermediate is critical for the elucidation of reaction mechanism. In the NMR observation, the in situ-formed surface species on catalyst are not necessarily the reaction intermediates. They could instead be spectators and do not contribute to the final product formation. Discriminating intermediates from spectators represents an important issue in the reaction mechanism study. In this respect, Hunger et al. proposed stopped-flow (SF) strategy to determine real intermediates of heterogeneously catalyzed reactions [36–38]. Figure 6.4 shows the protocol of the in situ SF MAS NMR study of reactions. The method is working on a continuous flow to observe the further transformation of surface species at reaction temperatures after stopping the reactant flow. It has three steps: (i) conducting the continuous-flow in situ MAS NMR experiments at steady-state conditions; (ii) stopping the reactant flow and purging the catalyst with inner gas, recording the

Fig. 6.4 Protocol of the in situ SF MAS NMR experiment consisting of periods i–iii specified in the text, allowing the study of steady-state reactions and of the progressive transformation of surface species previously formed during period i. Reprinted from Ref. [36] by permission of American Chemical Society

6.2 In Situ Solid-State NMR Approaches

205

Fig. 6.5 Diagram of flow MAS spinning system. Reprinted from Ref. [31] by permission of Elsevier

MAS NMR spectra at room temperature; (iii) heating the sample without reactant and recording the NMR spectra. The intermediate can be picked out, and the nature of the intermediate is revealed by analysis of the NMR spectra obtained before and after stopping the reactant flow. For example, in the methylation of aniline with methanol on a basic CsOH/Cs, Na–Y zeolite, N-methyleneaniline was identified as an intermediate in the formation of N-methylaniline [38]. The proposed reaction pathway is in consistent with that obtained from in situ MAS NMR study on HY zeolite at batch conditions where the role of surface methoxy species was emphasized [39]. Importantly, the in situ SF MAS NMR method provides the first direct evidence on the intermediary role of N-methyleneaniline in the methylation reaction. Haw [31] and Munson [40] also developed a flow MAS NMR probe using pencilstyle rotor, enabling simultaneous study of the reactions in the probe with in situ analysis of the product stream leaving out the probe by an external analytical instrument like GC. Figure 6.5 shows Haw’s design of the flow MAS module, which differs from the flow MAS probe reported by Hunger and Horvath [22]. One important feature of the MAS module is the central chamber in which the sample region of the MAS rotor is heated or cooled by nitrogen gas to achieve temperature regulation between ca. 85 and 523 K. The MAS rotor was a 7.5 mm (o.d.) zirconia tube with a Kel-F drive tip and spacers being drilled out to provide a pathway for regent flow. Reagent gas is introduced into the MAS rotor through a Torlon needle, which is positioned inside the upper endcap along the spinning axis. The pressure differential between the top of the rotor and the stator area in the MAS spinning system creates a suction force, which facilitates the reagent gas passing through the rotor and over

206

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

Fig. 6.6 Diagram of GRASSHopper II probe. Reprinted from Ref. [41] by permission of Elsevier

the sample bed. The spinning speed for this MAS module can achieve 2.5 kHz. The ability of the flow MAS NMR in the in situ study of the heterogeneous catalysis reactions is demonstrated in the in situ activation of H-ZSM-5 zeolite. The obtained 1 H spectra concerning the silanols and Brønsted acid sites are similar to those obtained in standard MAS probes for zeolites activated off-line. Additionally, the dynamics of the organic compound on catalyst can be monitored using the flow MAS system. The 13 C spectra obtained in the flow MAS probe for the adsorption and desorption of benzene-13 C on H-ZSM-5 zeolite were also similar to those obtained for the change of benzene loaded on H-ZSM-5 in a standard MAS probe. Considering the difficulty in achieving MAS line-narrowing in a flow reactor, Maciel et al. [41] developed an approach alternative to conventional in situ NMR reactor/cell called gas reactor and solid sample hopper (GRASSHopper) based on the “magic-angle hopping” (MAH) technique [42, 43]. As shown in Fig. 6.6, the sample hops by 120° around the axis of the magic angle. During three jumps (each having duration t1 /3), the sample is stationary. The idea of the technique is that for a crystallite or amorphous sample, the isotropic chemical shift δiso obtained by the normal MAS experiment is the average of three chemical shifts (1/3)(δ11 + δ22 + δ33 ) corresponding to three orientations at 120° about the MAS axis. The hopping is implemented by a suitably controlled step motor, driving a shaft that is coupled via

6.2 In Situ Solid-State NMR Approaches

207

a gear system to the sample cell. It is demonstrated that resolution and sensitivity of MAH are sufficiently good for a variety of chemical applications. The advantage of this technique is that hopping the sample periodically makes the rapid spinning of sample avoidable which is basic for the normal MAS experiment. The additional advantage is that after a complete MAH cycle of three orientations, the MAH device can be re-initialized by rotating backward to the initial orientation. The application of this technique shows that the 120° hop time (thop ) is 30 ms and the requirement is that thop be smaller than the spin–lattice relaxation time T1 of the resonating nuclei. Importantly, the hopping must be accomplished in such a manner that it does not induce leak of the particles in the sample. For all of the above designs of flow MAS NMR, 13 C isotope-enriched reactants are required for sensitivity enhancement. Note that the isotope-enriched compounds are often expensive which prohibits the constant flow of the reactants under reaction conditions. Moreover, some reactants are even commercially unavailable. Thus, it is desirable to use the reactants with carbon atoms in natural abundance, which, however, is a challenge for the signal sensitivity. Since the signal-to-noise ratio (S/N) is proportional to the nuclear spins, large sample volume would effectively increase the S/N as an alternative to the use of 13 C-labeled reactants. To this end, Hu et al. designed a 9.5-mm outside diameter large-sample-volume constant flow MAS probe [44]. Figure 6.7 shows the diagram of the MAS probe. It is much larger than the 7- or 7.5-mm probes used in the previous designs, allowing the sensitivity enhancement using reactants in natural abundance. The flow of reactants in a carrier gas into the catalyst bed in MAS rotor is facilitated by the pressure difference generated by an attached vacuum pump at the end of the rotor. The large sample volume in the rotor allows to trap high concentration of reactants and products in natural abundance, enhancing the S/N in a reasonable acquisition time. In the study of natural abundance 2-butanol dehydration reaction over a mesoporous silicalite-supported heteropolyacid catalyst (HPA/meso-silicalite-1) at 73 °C on the flow probe, it was shown that reactants, products (trans- and cis-2-butene) and possible intermediates could all be clearly observed and determined by in situ 13 C NMR. The 2-butanol dehydration was also probed by using 1 H MAS NMR. Benefited from the large-sample-volume coupled with the high 1 H NMR sensitivity, the in situ 1 H MAS NMR spectrum with high S/N ratios was achieved by on scan, generating a time resolution as short as one second. This is very attractive to the study of catalytic reaction dynamics using in situ NMR, particularly for the fast reactions. Regarding the technical features, the sample can be spun at 3.5 kHz. Similar to the design of Hunger, only atmospheric pressure reactions could be conducted on the probe as the rotor is not sealed leading to an open system. For a heterogeneous catalytic reaction on zeolites, the contact time is typically from several tenths of a second to several tens of seconds. The above-mentioned in situ NMR experiments are not able to probe the transient reactions in such a short time after the introduction of reactants to the catalyst bed. The mechanism study would benefit from the high-time-resolution analysis of the reactions. At the end of the 1990s, Haw et al. designed an in situ NMR-pulse-quench reactor [45, 46].

208

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

Fig. 6.7 Schematic of the large-sample-volume CF-MAS probe. a The assembled MAS rotor. b Cross-sectional view of the various internal components of the MAS rotor. The labeled parts in (a) and (b) are the zirconium rotor sleeve “1,” spin tip “2,” sample cell space “3,” glass wool spacers “4” and “5,” and end plugs “6” and “7.” c The module for sample spinning inside the CFMAS probe with the MAS rotor inserted in place. Labeled parts in (c) are the static injection tube “8” mounted in support/block “9,” vacuum tube “10” mounted in support/block “11,” pencil-type driving mechanism for sample spinning “12,” NMR RF coil “13,” channel for variable-temperature gas input “14,” and magic-angle adjustment “15.” Reprinted from Ref. [44] by permission of Royal Society of Chemistry

The design of the system is shown in Fig. 6.8. Both continuous-flow and pulsed reactions can be conducted on this device. The volatile products that exit the reactor are analyzed online by GC or GC-MS. Specifically, the reaction can be rapidly quenched by introducing cryogenically cooled nitrogen over the catalyst bed so as to rapidly reduce the catalyst temperature. By using computer-controlled high-speed valves, the temperature of the catalyst can be reduced by 150 °C within 170 ms. This allows to probe the fast reaction within 200 ms. The possible reaction intermediates formed at high temperature can be “fossilized” on the catalysts and analyzed by solidstate NMR, which allows the detailed reaction mechanism to be identified. After the reaction being quenched, the catalyst containing trapped species is transferred into the rotor for NMR measurement at room temperature. For the air sensitive species, the sample transfer can be conducted in a glove box to avoid exposure to oxygen or moisture. Another significant feature of the pulse-quench reactor is that the same or different reagents can be quantitatively introduced onto catalyst bed in a number of pulses. A certain amount of reactant is first pulsed onto the catalyst, and the reaction is allowed to proceed for a period under inert carrier gas flow. The same or different reactant is subsequently pulsed onto the catalyst, and then the reaction is rapidly

6.2 In Situ Solid-State NMR Approaches

209

Fig. 6.8 Schematic of the pulse-quench catalytic reactor (a) and timing diagram of pulse quench (b). Reprinted from Ref. [46] by permission of American Chemical Society

quenched. Figure 6.8 shows the timing diagram for single pulse and double pulse of the experiments. This experimental protocol can be employed to study the transient isotope exchange. The pulse-quench catalytic reactor has been found an intensive application in the investigation of the methanol conversion on zeolites, which is featured by an induction period with complex reaction network.

6.3 Mechanistic Study of the Catalytic Reactions by In Situ NMR 6.3.1 Activation and Conversion of Light Alkanes Light alkanes (C1–C4), or saturated hydrocarbons, are main constituents of natural gas. However, using light alkanes for heating and transportation greatly surpasses applications as chemical feedstock. This is due to their chemical inertness resulting in the difficulty in chemical transformation. Methane, for example, is the least reactive hydrocarbon molecule because of its strong C–H bond strength (104 kcal/mol). High temperatures are often used for alkanes conversion, but the reactions usually proceed to form thermodynamically stable and economically unattractive products, like carbon dioxide, resulting in low selectivity. Thus, milder and better-controlled conversions of alkanes into high-valued chemicals will provide an attractive alternative to the processes that are currently based on alkenes and aromatics and thus generate enormous economic benefits. From the academic point of view, the understanding of the C–H and C–C bond activation of alkanes is of great importance for the design of robust catalysts capable of catalyzing selective conversion of light alkanes under mild conditions. Solid-state NMR plays a critical role in the elucidation of the

210

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

conversion mechanism of light alkanes. Acidic zeolites are the mostly used catalysts for in situ solid-state NMR studies under either batch or flow conditions.

6.3.1.1

Activation of Methane Over Zeolite Catalyst

In spite of the many efforts that have been made since the past decades, the commercial utilization of methane is still largely dependent on the multi-step syn-gas (CO + H2 ) strategy, which incurs energy costs and lacks selectivity [47]. Although the homogenous catalysis system (such as Pd/H2 SO4 ) is the most efficient system to date for the direct conversion of methane under mild conditions [48–51], the environmental concerns and the utilization of precious metals reduce its commercial potential. On the other hand, pioneered by Choudhary et al. [52, 53], the environmentally benign heterogeneous catalysts in which transition metal (Ga, Zn, Mo, etc.)-modified zeolites could catalyze the co-conversion of methane with alkanes and alcohol to form hydrocarbons (C2 –C12 ) at relatively low temperatures ( cyclopentenyl cations > aromatics. Accordingly, the conversion is first accelerated by the transformation of formed alkenes. The consequent involvement of cyclopentenyl cations and aromatics facilitates further conversion. The co-catalysis of hydrocarbon-pool species at different contact times shows that the retained methylbenzenes and their reactivity in catalyst bed decrease from top to bottom, which leads to the secondary reactions of alkenes prevailing to produce C3 –C8 alkanes and aromatics from benzene to tetramethylbenzene. The significant reduction of bulkier methylbenzenes such as pentamethylbenzene compared with light methylbenzene results in a decrease of the propene-to-ethene ratio in the cat-

240

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

Fig. 6.40 a Solid-state 13 C MAS NMR and b liquid-state 13 C NMR spectra (acquired at 18.8 T) of the trapped products obtained from the reaction of 13 C-labeled methanol over H-ZSM-5 at 350 °C for 30 min. The 13 C chemical shifts of both the solid-state and the liquid-state NMR spectra are indicated for the observed carbocations (those from the liquid-state NMR experiment are given in parentheses). Asterisks denote spinning sidebands. Reprinted from Ref. [162] by permission of Wiley

6.3 Mechanistic Study of the Catalytic Reactions by In Situ NMR

241

Scheme 6.3 Proposed catalytic cycle for the formation of ethene from lower methylbenzenes over H-ZSM-5. Reprinted from Ref. [162] by permission of Wiley

alyst bed from top to bottom, while the cyclopentenyl cations and alkenes maintain high reactivity in the formation of olefins throughout the catalytic bed. H-SAPO-34 is a CHA topologic zeolite containing a 3D cage (ca. 6.7 × 12 Å) structure with an 8-ring opening (3.8 × 3.8 Å). Theoretical calculations indicate that side-chain mechanism accounts for the olefins formation due to the larger reaction cage [196, 197]. To probe the MTO reactions taking place on SAPO-34, the isotopic labeling experiments were performed in such a way that at a given time on stream the feed was switched from ordinary methanol to 13 C-labeled methanol. By monitoring the rate of incorporation of 13 C-atoms, Kolboe et al. [198] found that the carbon atoms from methanol were incorporated into the aromatic rings trapped in the CHA cages, indicating the presence of paring mechanism in the reaction. The direct way to elucidate the reaction route is to observe and identify the key intermediates involved in the MTO reaction. SSZ-13 zeolite has the same topologic structure to SAPO-34 but stronger acidity. Liu et al. [199] studied and compared MTO reactions on SAPO-34 and SSZ-13 zeolites by in situ 13 C MAS NMR. Figure 6.41 shows the 13 C MAS NMR spectra obtained from the methanol conversion over HSAPO-34 and H-SSZ-13 catalysts at 275 °C. In addition to the dominant hydrocarbons (10–50 ppm) and methylbenzenes (120–140 ppm), polymethylcyclopentenyl cations were formed over the two catalysts. However, for H-SSZ-13, heptamethylbenzenium (heptaMB+ ) was observed as well, characterized by the signals at 203, 189, and 144 ppm. This six-ring benzenium ion has been confirmed over H-Beta [154] and DNL-6 [167] catalysts, which was considered as an important intermediate in the MTO reaction. The invisibility of heptamethylbenzenium ion over H-SAPO-34 was attributed to its weaker acidity than SSZ-13, resulting in low stability and short lifetime, but it was believed to be involved in the MTO reaction over H-SAPO-34. On the basis of experimental observations, both the side-chain and paring mechanisms were proposed over the CHA zeolites, while the former could be preferred from the energy point of view.

242

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

Fig. 6.41 13 C MAS NMR spectra of a retained organic species in H-SAPO-34 after continuous-flow 13 CH3 OH reaction at 300 °C for 15 min and b retained organic species in H-SSZ-13 after continuous-flow 13 CH3 OH reaction at 275 °C for 25 min. The asterisk denotes spinning sidebands. Reprinted from Ref. [199] by permission of Wiley

6.4 Summary In situ MAS NMR has been demonstrated to be a powerful technique for the investigation of the mechanisms of heterogeneously catalyzed reactions. Our understanding of the heterogeneous reactions of industrial interest will be definitely advanced by the development of these in situ NMR techniques. Till now, the reactants are mostly concerned in the in situ NMR investigation by tracing the 1 H or 13 C nucleus, although NMR is versatile also in the characterization of nuclei like 27 Al and 29 Si in the catalyst. The progress of in situ NMR is expected to enable obtaining simultaneous information on the stucture of catalysts, the transformation of reactants, and the interaction between them during the reaction. This requires the development of new NMR experimental approaches to enhance signal sensitivity and selectivity in the monitoring of the reactions. Hyperpolarization techniques like dynamic nuclear polarization (DNP) [200] and parahydrogen-induced polarization (PHIP) [201] can enhance the NMR spectroscopy signals by several orders of magnitude. They may provide a promising way to get multi-dimensional information in the heterogeneous catalysis which is now unapproachable with conventional MAS NMR because of the low sensitivity under working conditions. Note that the PHIP is emerging as a technique for investigating hydrogenation reactions [202]. Thus, the experimental techniques of in situ MAS NMR spectroscopy could be significantly improved by integration of DNP and PHIP into the heterogeneous reaction systems.

References

243

References 1. Ertl G, Knozinger H, Schüth F, Weitkamp J (2008) Handbook of heterogeneous catalysis, 2nd edn. Wiley, Weinheim 2. Topsøe H (2003) Developments in operando studies and in situ characterization of heterogeneous catalysts. J Catal 216(1–2):155–164. https://doi.org/10.1016/S0021-9517(02)00133-1 3. Fisher IA, Bell AT (1998) In situ infrared study of methanol synthesis from H2 /CO over Cu/SiO2 and Cu/ZrO2 /SiO2 . J Catal 178(1):153–173. https://doi.org/10.1006/jcat.1998.2134 4. Lercher JA, Veefkind V, Fajerwerg K (1999) In situ IR spectroscopy for developing catalysts and catalytic processes. Vib Spectrosc 19(1):107–121. https://doi.org/10.1016/S09242031(98)00094-0 5. Knözinger H, Mestl G (1999) Laser Raman spectroscopy—a powerful tool for in situ studies of catalytic materials. Top Catal 8(1):45–55. https://doi.org/10.1023/a:1019184321666 6. Wachs IE (1999) In situ raman spectroscopy studies of catalysts. Top Catal 8(1):57–63. https://doi.org/10.1023/a:1019100925300 7. Weckhuysen BM, Schoonheydt RA (1999) Recent progress in diffuse reflectance spectroscopy of supported metal oxide catalysts. Catal Today 49(4):441–451. https://doi.org/10. 1016/S0920-5861(98)00458-1 8. Bañares MA, Mart´ınez-Huerta MV, Gao X, Fierro JLG, Wachs IE (2000) Dynamic behavior of supported vanadia catalysts in the selective oxidation of ethane: in situ Raman, UV–Vis DRS and reactivity studies. Catal Today 61(1–4):295–301. http://dx.doi.org/10.1016/S09205861(00)00388-6 9. Singh J, Lamberti C, van Bokhoven JA (2010) Advanced X-ray absorption and emission spectroscopy: in situ catalytic studies. Chem Soc Rev 39(12):4754–4766. https://doi.org/10. 1039/C0CS00054J 10. Sankar G, Thomas JM (1999) In situ combined X-ray absorption spectroscopic and X-ray diffractometric studies of solid catalysts. Top Catal 8(1):1–21. https://doi.org/10.1023/a: 1019188422574 11. Bruckner A (2010) In situ electron paramagnetic resonance: a unique tool for analyzing structure-reactivity relationships in heterogeneous catalysis. Chem Soc Rev 39(12):4673–4684. https://doi.org/10.1039/B919541F 12. Brückner A, Kubias B, Lücke B (1996) In situ-electron spin resonance: a useful tool for the investigation of vanadium phosphate catalysts (VPO) under working conditions. Catal Today 32(1–4):215–222. https://doi.org/10.1016/S0920-5861(96)00077-6 13. Haw JF (1994) In situ NMR. In: Bell A, Pines A (eds) NMR techniques in catalysis. Dekker, New York, p 139 14. Hunger M (2008) In situ flow MAS NMR spectroscopy: state of the art and applications in heterogeneous catalysis. Prog Nucl Mag Res Sp 53(3):105–127. https://doi.org/10.1016/j. pnmrs.2007.08.001 15. Derouane EG, He H, Derouane-Abd Hamid SB, Ivanova II (1999) In situ MAS NMR investigations of molecular sieves and zeolite-catalyzed reactions. Catal Lett 58(1):1–19. https://doi.org/10.1023/a:1019044926400 16. Zhang W, Xu S, Han X, Bao X (2012) In situ solid-state NMR for heterogeneous catalysis: a joint experimental and theoretical approach. Chem Soc Rev 41(1):192–210. https://doi.org/ 10.1039/c1cs15009j 17. Zhang L, Ren Y, Yue B, He H (2012) Recent development in in situ NMR study on heterogeneous catalysis: mechanisms of light alkane functionalisation. Chem Commun 48(18):2370–2384. https://doi.org/10.1039/c2cc16882k 18. Ivanova II, Kolyagin YG (2010) Impact of in situ MAS NMR techniques to the understanding of the mechanisms of zeolite catalyzed reactions. Chem Soc Rev 39(12):5018–5050. https:// doi.org/10.1039/c0cs00011f 19. Derouane EG, Fraissard J, Fripiat JJ, Stone WEE (1972) NMR studies in adsorption and heterogeneous catalysis. Catal Rev 7(2):121–212. https://doi.org/10.1080/01614947208062257

244

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

20. Nagy JB, Engelhardt G, Michel D (1985) High resolution NMR on adsorbate-adsorbent systems. Adv Colloid Interfac 23:67–128. https://doi.org/10.1016/0001-8686(85)80017-8 21. Taylor RE, Ryan LM, Tindall P, Gerstein BC (1980) Protonic species in H1.7MoO3. The Journal of Chemical Physics 73 (11):5500–5507. http://dx.doi.org/10.1063/1.440069 22. Hunger M, Horvath T (1995) A new MAS NMR probe for in situ investigations of hydrocarbon conversion on solid catalysts under continuous-flow conditions. J Chem Soc, Chem Commun 14:1423–1424. https://doi.org/10.1039/C39950001423 23. Adrian Carpenter T, Klinowski J, Tilak D, Tennakoon B, Smith CJ, Edwards DC (1986) Sealed capsules for convenient acquisition of variable-temperature controlled-atmosphere magic-angle-spinning NMR spectra of solids. J Magn Reson (1969) 68(3):561–563. https:// doi.org/10.1016/0022-2364(86)90347-1 24. Giammatteo PJ, Hellmuth WW, Gordon Ticehurst F, Cope PW (1987) An easy sealing MAS rotor insert for liquids, gels, and air-sensitive solid samples. J Magn Reson (1969) 71(1):147–150. https://doi.org/10.1016/0022-2364(87)90136-3 25. Xu T, Haw JF (1997) The development and applications of CAVERN methods for in situ NMR studies of reactions on solid acids. Top Catal 4(1–2):109–118. https://doi.org/10.1023/ A:1019163500342 26. Munson EJ, Ferguson DB, Kheir AA, Haw JF (1992) Applications of a new CAVERN design to the study of reactions on catalysts using in situ solid-state NMR. J Catal 136(2):504–509 27. Munson EJ, Murray DK, Haw JF (1993) Shallow-bed CAVERN design for in situ solid-state NMR STUDIES OF CATALYTIC REACTIONS. J Catal 141(2):733–736. https://doi.org/ 10.1006/jcat.1993.1179 28. Ernst H, Freude D, Mildner T (1994) Temperature-switched MAS NMR: a new method for time-resolved in situ studies of reaction steps in heterogeneous catalysis. Chem Phys Lett 229(3):291–296. https://doi.org/10.1016/0009-2614(94)01045-5 29. Ernst H, Freude D, Mildner T, Wolf I (1996) Laser-supported high-temperature MAS NMR for time-resolved in situ studies of reaction steps in heterogeneous catalysis. Solid State Nucl Mag 6(2):147–156. https://doi.org/10.1016/0926-2040(95)01214-1 30. Ferguson DB, Haw JF (1995) Transient methods for in situ NMR of reactions on solid catalysts using temperature jumps. Anal Chem 67(18):3342–3348. https://doi.org/10.1021/ ac00114a034 31. Goguen P, Haw JF (1996) Anin SituNMR probe with reagent flow and magic angle spinning. J Catal 161(2):870–872. https://doi.org/10.1006/jcat.1996.0250 32. Haddix GW, Reimer JA, Bell AT (1987) A nuclear magnetic resonance probe for in situ studies of adsorbed species on catalysts. J Catal 106(1):111–115. https://doi.org/10.1016/ 0021-9517(87)90216-8 33. Hunger M, Seiler M, Horvath T (1999) A technique for simultaneous in situ MAS NMR and on-line gas chromatographic studies of hydrocarbon conversions on solid catalysts under flow conditions. Catal Lett 57(4):199–204. https://doi.org/10.1023/a:1019064003201 34. Seiler M, Schenk U, Hunger M (1999) Conversion of methanol to hydrocarbons on zeolite HZSM-5 investigated by in situ MAS NMR spectroscopy under flow conditions and on-line gas chromatography. Catal Lett 62(2):139–145. https://doi.org/10.1023/a:1019086603511 35. Hunger M, Wang W (2004) Formation of cyclic compounds and carbenium ions by conversion of methanol on weakly dealuminated zeolite H-ZSM-5 investigated via a novel in situ CF MAS NMR/UV-Vis technique. Chem Commun 5:584–585 36. Wang W, Seiler M, Ivanova II, Weitkamp J, Hunger M (2001) stopped-flow (SF) MAS NMR spectroscopy: a novel NMR technique applied for the study of aniline methylation on a solid base catalyst. Chem Commun 15:1362–1363. https://doi.org/10.1039/b104115k 37. Wang W, Seiler M, Ivanova II, Sternberg U, Weitkamp J, Hunger M (2002) Formation and decomposition of N, N, N-Trimethylanilinium cations on zeolite H−Y investigated by in situ stopped-Flow MAS NMR spectroscopy. J Am Chem Soc 124(25):7548–7554. https://doi. org/10.1021/ja012675n 38. Wang W, Seiler M, Hunger M (2001) Role of surface methoxy species in the conversion of methanol to dimethyl ether on acidic zeolites investigated by in situ stopped-flow MAS NMR spectroscopy. J Phys Chem B 105(50):12553–12558. https://doi.org/10.1021/jp0129784

References

245

39. Ivanova II, Pomakhina EB, Rebrov AI, Hunger M, Kolyagin YG, Weitkamp J (2001) Surface species formed during aniline methylation on zeolite H–Y investigated by in situ MAS NMR spectroscopy. J Catal 203(2):375–381. https://doi.org/10.1006/jcat.2001.3300 40. Isbester PK, Zalusky A, Lewis DH, Douskey MC, Pomije MJ, Mann KR, Munson EJ (1999) NMR probe for heterogeneous catalysis with isolated reagent flow and magic-angle spinning. Catal Today 49(4):363–375. https://doi.org/10.1016/S0920-5861(98)00450-7 41. Keeler C, Xiong J, Lock H, Dec S, Tao T, Maciel GE (1999) A new in situ chemical reactor for studying heterogeneous catalysis by NMR: the GRASSHopper. Catal Today 49(4):377–383. https://doi.org/10.1016/S0920-5861(98)00451-9 42. Bax A, Szeverenyi NM, Maciel GE (1983) Correlation of isotropic shifts and chemical shift anisotropies by two-dimensional fourier-transform magic-angle hopping nmr spectroscopy. J Magn Reson (1969) 52(1):147–152. https://doi.org/10.1016/0022-2364(83)90267-6 43. Szeverenyi NM, Bax A, Maciel GE (1985) Magic-angle hopping as an alternative to magic-angle spinning for solid state NMR. J Magn Reson (1969) 61(3):440–447. https://doi. org/10.1016/0022-2364(85)90184-2 44. Hu JZ, Sears JA, Mehta HS, Ford JJ, Kwak JH, Zhu K, Wang Y, Liu J, Hoyt DW, Peden CHF (2012) A large sample volume magic angle spinning nuclear magnetic resonance probe for in situ investigations with constant flow of reactants. Phys Chem Chem Phys 14(7):2137–2143. https://doi.org/10.1039/c1cp22692d 45. Haw JF, Goguen PW, Xu T, Skloss TW, Song W, Wang Z (1998) In situ NMR investigations of heterogeneous catalysis with samples prepared under standard reaction conditions. Angew Chem Int Ed 37(7):948–949. https://doi.org/10.1002/(SICI)1521-3773(19980420)37:7% 3c948:AID-ANIE948%3e3.0.CO;2-L 46. Goguen PW, Xu T, Barich DH, Skloss TW, Song W, Wang Z, Nicholas JB, Haw JF (1998) Pulse-quench catalytic reactor studies reveal a carbon-pool mechanism in methanol-togasoline chemistry on zeolite HZSM-5. J Am Chem Soc 120(11):2650–2651. https://doi. org/10.1021/ja973920z 47. Crabtree RH (1995) Aspects of methane chemistry. Chem Rev 95(4):987–1007 48. Periana RA, Taube DJ, Evitt ER, Loffler DG, Wentrcek PR, Voss G, Masuda T (1993) A mercury-catalyzed, high-yield system for the oxidation of methane to methanol. Science 259(5093):340–343 49. Kao LC, Hutson AC, Sen A (1991) Low-temperature, palladium(Ii)-catalyzed, solution-phase oxidation of methane to a methanol derivative. J Am Chem Soc 113(2):700–701 50. Periana RA, Taube DJ, Gamble S, Taube H, Satoh T, Fujii H (1998) Platinum catalysts for the high-yield oxidation of methane to a methanol derivative. Science 280(5363):560–564 51. Periana RA, Mironov O, Taube D, Bhalla G, Jones CJ (2003) Catalytic, oxidative condensation of CH4 to CH3 COOH in one step via CH activation. Science 301(5634):814–818 52. Choudhary VR, Mondal KC, Mulla SAR (2005) Simultaneous conversion of methane and methanol into gasoline over bifunctional Ga-, Zn-, In-, and/or Mo-modified ZSM-5 zeolites. Angew Chem Int Edit 44(28):4381–4385 53. Choudhary VR, Kinage AK, Choudhary TV (1997) Low-temperature nonoxidative activation of methane over H-galloaluminosilicate (MFI) zeolite. Science 275(5304):1286–1288 54. Kolyagin YG, Ivanova II, Ordomsky VV, Gedeon A, Pirogov YA (2008) methane activation over Zn-modified MFI zeolite: NMR evidence for Zn-methyl surface species formation. J Phys Chem C 112(50):20065–20069 55. Kazansky VB, Borovkov VY, Serikh AI, van Santen RA, Anderson BG (2000) Nature of the sites of dissociative adsorption of dihydrogen and light paraffins in ZnHZSM-5 zeolite prepared by incipient wetness impregnation. Catal Lett 66(1):39–47. https://doi.org/10.1023/ a:1019031119325 56. Pidko EA, Xu J, Mojet BL, Lefferts L, Subbotina IR, Kazansky VB, van Santen RA (2006) Interplay of bonding and geometry of the adsorption complexes of light alkanes within cationic faujasites. Combined spectroscopic and computational study. J Phys Chem B 110 (45):22618–22627. https://doi.org/10.1021/jp0634757

246

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

57. Kazansky VB, Serykh AI, Pidko EA (2004) DRIFT study of molecular and dissociative adsorption of light paraffins by HZSM-5 zeolite modified with zinc ions: methane adsorption. J Catal 225(2):369–373. https://doi.org/10.1016/j.jcat.2004.04.029 58. Biscardi JA, Meitzner GD, Iglesia E (1998) Structure and density of active Zn species in Zn/H-ZSM5 propane aromatization catalysts. J Catal 179(1):192–202. https://doi.org/10. 1006/jcat.1998.2177 59. Kazansky VB, Subbotina IR, Rane N, van Santen RA, Hensen EJM (2005) On two alternative mechanisms of ethane activation over ZSM-5 zeolite modified by Zn2+ and Ga1+ cations. Phys Chem Chem Phys 7(16):3088–3092. https://doi.org/10.1039/b506782k 60. Kolyagin YG, Ordomsky VV, Khimyak YZ, Rebrov AI, Fajula F, Ivanova II (2006) Initial stages of propane activation over Zn/MFI catalyst studied by in situ NMR and IR spectroscopic techniques. J Catal 238(1):122–133. https://doi.org/10.1016/j.jcat.2005.11.037 61. Gabrienko AA, Arzumanov SS, Freude D, Stepanov AG (2010) Propane aromatization on Zn-modified zeolite BEA studied by solid-state NMR in Situ. J Phy Chem C 114(29):12681–12688. https://doi.org/10.1021/jp103580f 62. Kolyagin YG, Ivanova II, Pirogov YA (2009) H-1 and C-13 MAS NMR studies of light alkanes activation over MFI zeolite modified by Zn vapour. Solid State Nucl Mag 35(2):104–112. https://doi.org/10.1016/j.ssnmr.2009.01.005 63. Barbosa L, Zhidomirov GM, van Santen RA (2000) Theoretical study of methane adsorption on Zn(II) zeolites. Phys Chem Chem Phys 2(17):3909–3918 64. Pidko EA, van Santen RA (2007) Activation of light alkanes over zinc species stabilized in ZSM-5 zeolite: a comprehensive DFT study. J Phy Chem C 111(6):2643–2655. https://doi. org/10.1021/jp065911v 65. Benco L, Bucko T, Hafner J, Toulhoat H (2005) Periodic DFT calculations of the stability of Al/Si substitutions and extraframework Zn2+ cations in mordenite and reaction pathway for the dissociation of H-2 and CH4 . J Phys Chem B 109(43):20361–20369. https://doi.org/10. 1021/Jp0530597 66. Frash MV, van Santen RA (2000) Activation of ethane in Zn-exchanged zeolites: a theoretical study. Phys Chem Chem Phys 2(5):1085–1089 67. Xu J, Zheng A, Wang X, Qi G, Su J, Du J, Gan Z, Wu J, Wang W, Deng F (2012) Room temperature activation of methane over Zn modified H-ZSM-5 zeolites: insight from solid-state NMR and theoretical calculations. Chem Sci 3(10):2932–2940. https://doi.org/10.1039/c2sc20434g 68. Wang W, Hunger M (2008) Reactivity of surface alkoxy species on acidic zeolite catalysts. Acc Chem Res 41(8):895–904. https://doi.org/10.1021/ar700210f 69. van Santen RA, Kramer GJ (1995) Reactivity theory of zeolitic broensted acidic sites. Chem Rev 95(3):637–660. https://doi.org/10.1021/cr00035a008 70. Corma A (1995) Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions. Chem Rev 95(3):559–614. https://doi.org/10.1021/cr00035a006 71. Inoue S, Yokoo Y (1972) Reactions of organozinc coordination compounds: IV. Reactions with carbon dioxide in relation to the action of carbonic anhydrase. J Organomet Chem 39(1):11–16. https://doi.org/10.1016/S0022-328X(00)88899-3 72. Wu JF, Wang WD, Xu J, Deng F, Wang W (2010) Reactivity of C1 surface species formed in Methane activation on Zn-modified H-ZSM-5 zeolite. Chem—A Eur J 16 (47):14016–14025. https://doi.org/10.1002/chem.201002258 73. Anderson MW, Klinowski J (1989) Direct observation of shape selectivity in zeolite ZSM-5 by magic-angle-spinning NMR. Nature 339(6221):200–203 74. Munson EJ, Kheir AA, Lazo ND, Haw JF (1992) In situ solid-state NMR study of methanolto-gasoline chemistry in zeolite HZSM-5. J Phys Chem 96(19):7740–7746. https://doi.org/ 10.1021/j100198a046 75. Wang X, Qi G, Xu J, Li B, Wang C, Deng F (2012) NMR-spectroscopic evidence of intermediate-dependent pathways for acetic acid formation from methane and carbon monoxide over a ZnZSM-5 zeolite catalyst. Angew Chem Int Edit 51(16):3850–3853. https:// doi.org/10.1002/anie.201108634

References

247

76. Luzgin MV, Rogov VA, Arzumanov SS, Toktarev AV, Stepanov AG, Parmon VN (2008) Understanding methane aromatization on a Zn-modified high-silica zeolite. Angew Chem Int Ed 47(24):4559–4562. https://doi.org/10.1002/anie.200800317 77. Luzgin MV, Rogov VA, Kotsarenko NS, Shmachkova VP, Stepanov AG (2007) Methane carbonylation with CO on sulfated zirconia: evidence from solid-state NMR for the selective formation of acetic acid. J Phys Chem C 111(28):10624–10629. https://doi.org/10.1021/ jp0728757 78. Zerella M, Mukhopadhyay S, Bell AT (2003) Synthesis of mixed acid anhydrides from methane and carbon dioxide in acid solvents. Org Lett 5(18):3193–3196. https://doi.org/10. 1021/Ol0348856 79. Luzgin MV, Rogov VA, Arzumanov SS, Toktarev AV, Stepanov AG, Parmon VN (2008) Understanding methane aromatization on a Zn-modified high-silica zeolite. Angew Chem 120(24):4635–4638. https://doi.org/10.1002/ange.200800317 80. Stepanov AG, Arzumanov SS, Gabrienko AA, Toktarev AV, Parmon VN, Freude D (2008) Zn-promoted hydrogen exchange for methane and ethane on Zn/H-BEA zeolite: in situ 1H MAS NMR kinetic study. J Catal 253(1):11–21. https://doi.org/10.1016/j.jcat.2007.11.002 81. Kramer GJ, van Santen RA, Emeis CA, Nowak AK (1993) Understanding the acid behaviour of zeolites from theory and experiment. Nature 363(6429):529–531 82. Schoofs B, Martens JA, Jacobs PA, Schoonheydt RA (1999) Kinetics of hydrogen-deuterium exchange reactions of methane and deuterated acid FAU- and MFI-type zeolites. J Catal 183(2):355–367. https://doi.org/10.1006/jcat.1999.2401 83. Blaszkowski SR, Jansen APJ, Nascimento MAC, van Santen RA (1994) Density functional theory calculations of the transition states for hydrogen exchange and dehydrogenation of methane by a Broensted zeolitic proton. J Phys Chem 98(49):12938–12944. https://doi.org/ 10.1021/j100100a021 84. Vollmer JM, Truong TN (2000) Mechanisms of hydrogen exchange of methane with H-zeolite Y: an ab Initio embedded cluster study. J Phys Chem B 104(26):6308–6312. https://doi.org/ 10.1021/jp0008445 85. Zheng X, Blowers P (2006) A computational study of methane catalytic reactions on zeolites. J Mol Catal A Chem 246(1–2):1–10. https://doi.org/10.1016/j.molcata.2005.10.009 86. Zheng X, Blowers P (2005) A computational study of alkane hydrogen-exchange reactions on zeolites. J Mol Catal A: Chem 242(1–2):18–25. https://doi.org/10.1016/j.molcata.2005. 07.029 87. Blaszkowski SR, Nascimento MAC, van Santen RA (1996) Activation of C−H and C−C bonds by an acidic zeolite: a density functional study. J Phys Chem 100(9):3463–3472. https://doi.org/10.1021/jp9523231 88. Gabrienko AA, Arzumanov SS, Moroz IB, Toktarev AV, Wang W, Stepanov AG (2013) Methane activation and transformation on Ag/H-ZSM-5 zeolite studied with solid-state NMR. J Phys Chem C 117(15):7690–7702. https://doi.org/10.1021/jp4006795 89. Luzgin MV, Gabrienko AA, Rogov VA, Toktarev AV, Parmon VN, Stepanov AG (2010) The “Alkyl” and “Carbenium” pathways of methane activation on Ga-modified zeolite BEA: 13C solid-state NMR and GC-MS study of methane aromatization in the presence of higher alkane. J Phys Chem C 114(49):21555–21561. https://doi.org/10.1021/jp1078899 90. Arzumanov SS, Moroz IB, Freude D, Haase J, Stepanov AG (2014) Methane activation on in-modified ZSM-5 zeolite. H/D hydrogen exchange of the alkane with Brønsted acid sites. J Phys Chem C 118(26):14427–14432. https://doi.org/10.1021/jp5037316 91. Hu JZ, Kwak JH, Wang Y, Peden CHF, Zheng H, Ma D, Bao X (2009) Studies of the active sites for methane dehydroaromatization using ultrahigh-field solid-state 95Mo NMR spectroscopy. J Phys Chem C 113(7):2936–2942. https://doi.org/10.1021/jp8107914 92. Ma D, Shu Y, Zhang W, Han, Xiuwen, Xu Y, Bao X (2000) In situ 1H MAS NMR spectroscopic observation of proton species on a Mo-modified HZSM-5 zeolite catalyst for the dehydroaromatization of methane. Angew Chem Int Ed 39(16):2928–2931. https://doi.org/ 10.1002/1521-3773(20000818)39:163.0.Co;2-t

248

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

93. Zheng H, Ma D, Bao X, Hu JZ, Kwak JH, Wang Y, Peden CHF (2008) Direct observation of the active center for methane dehydroaromatization using an ultrahigh field 95Mo NMR spectroscopy. J Am Chem Soc 130(12):3722–3723. https://doi.org/10.1021/ja7110916 94. Kazansky VB, Senchenya IN (1989) Quantum chemical study of the electronic structure and geometry of surface alkoxy groups as probable active intermediates of heterogeneous acidic catalysts: What are the adsorbed carbenium ions? J Catal 119(1):108–120. https://doi.org/ 10.1016/0021-9517(89)90139-5 95. Rigby AM, Kramer GJ, van Santen RA (1997) Mechanisms of Hydrocarbon Conversion in Zeolites: A Quantum Mechanical Study. J Catal 170(1):1–10. https://doi.org/10.1006/jcat. 1997.1574 96. Olah GA (1972) Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center bound penta- of tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions. J Am Chem Soc 94 (3):808–820. https://doi.org/10.1021/ja00758a020 97. Olah GA, Schilling P, Staral JS, Halpern Y, Olah JA (1975) Electrophilic reactions at single bonds. XIV. Anhydrous fluoroantimonic acid catalyzed alkylation of benzene with alkanes and alkane-alkene and alkane-alkylbenzene mixtures. J Am Chem Soc 97 (23):6807–6810. https://doi.org/10.1021/ja00856a034 98. Olah GA, White AM (1969) Stable carbonium ions. XCI. Carbon-13 nuclear magnetic resonance spectroscopic study of carbonium ions. J Am Chem Soc 91 (21):5801–5810. https://doi.org/10.1021/ja01049a017 99. Corma A, González-Alfaro V, Orchillés AV (1999) The role of pore topology on the behaviour of FCC zeolite additives. Appl Catal A 187(2):245–254. https://doi.org/10.1016/S0926860X(99)00226-4 100. Mirodatos C, Barthomeuf D (1988) Cracking of n-decane on zeolite catalysts: enhancement of light hydrocarbon formation by the zeolite field gradient. J Catal 114(1):121–135. https:// doi.org/10.1016/0021-9517(88)90014-0 101. Ivanova II, Pomakhina EB, Rebrov AI, Derouane EG (1998) 13C MAS NMR mechanistic study of the initial stages of propane activation over H-ZSM-5 zeolite. Top Catal 6(1):49–59. https://doi.org/10.1023/a:1019139111671 102. Ivanova II, Rebrov AI, Pomakhina EB, Derouane EG (1999) 13C MAS NMR mechanistic study of propane conversion into butanes over H-MFI catalyst. J Mol Catal A Chem 141(1–3):107–116. https://doi.org/10.1016/S1381-1169(98)00254-4 103. Ono Y (1992) Transformation of Lower Alkanes into Aromatic Hydrocarbons over ZSM-5 Zeolites. Catalysis Reviews 34(3):179–226. https://doi.org/10.1080/01614949208020306 104. Derouane EG, Abdul Hamid SB, Ivanova II, Blom N, Højlund-Nielsen P-E (1994) Thermodynamic and mechanistic studies of initial stages in propane aromatisation over Ga-modified H-ZSM-5 catalysts. J Mol Catal 86(1–3):371–400. https://doi.org/10.1016/ 0304-5102(93)E0176-H 105. Derouane EG, He H, Derouane-Abd Hamid SB, Lambert D, Ivanova I (2000) In situ MAS NMR spectroscopy study of catalytic reaction mechanisms. J Mol Catal A: Chem 158(1):5–17. https://doi.org/10.1016/S1381-1169(00)00039-X 106. Anunziata OA, Pierella LB (1993) Nature of the active sites in H-ZSM-11 zeolite modified with Zn(2+) and Ga(3+). Catal Lett 19(2):143–151. https://doi.org/10.1007/bf00771749 107. Mériaudeau P, Naccache C (1996) Gallium based MFI zeolites for the aromatization of propane. Catal Today 31(3–4):265–273. https://doi.org/10.1016/S0920-5861(96)00017-X 108. Buckles G, Hutchings GJ, Williams CD (1991) Propane conversion over zeolite catalysts: comments on the role of Ga. Catal Lett 8(2):115–123. https://doi.org/10.1007/bf00764107 109. Buckles G, Hutchings GJ, Williams CD (1991) Aromatization of propane over Ga/H-ZSM-5: an explanation of the synergy observed between Ga3+ and H+ . Catal Lett 11(1):89–93. https://doi.org/10.1007/bf00866905 110. Le van Mao R, Yao J, Dufresne LA, Carli R (1996) Gallium-loaded zeolites and related systems hybrid catalysts containing zeolite ZSM-5 and supported gallium oxide in the aromatization of n-butane. Catal Today 31(3):247–255. https://doi.org/10.1016/S0920-5861(96)00020-X

References

249

111. Biscardi JA, Iglesia E (1999) Reaction pathways and rate-determining steps in reactions of alkanes on H-ZSM5 and Zn/H-ZSM5 catalysts. J Catal 182(1):117–128. https://doi.org/10. 1006/jcat.1998.2312 112. Le Van Mao R, Dufresne L (1989) Enhancement of the aromatizing activity of ZSM-5 zeolite induced by hydrogen back-spillover: aromatizing the outstream gases of a propane steam-cracker. Appl Catal 52(1):1–18. https://doi.org/10.1016/S0166-9834(00)83368-0 113. Biscardi JA, Iglesia E (1996) Gallium-loaded zeolites and related systemsstructure and function of metal cations in light alkane reactions catalyzed by modified H-ZSM5. Catal Today 31(3):207–231. https://doi.org/10.1016/S0920-5861(96)00028-4 114. Yu SY, Biscardi JA, Iglesia E (2002) Kinetic relevance of hydrogen desorption steps and virtual pressures on catalytic surfaces during reactions of light alkanes. J Phys Chem B 106(37):9642–9648. https://doi.org/10.1021/jp020780t 115. Biscardi JA, Iglesia E (1999) Non-oxidative reactions of propane on Zn/Na-ZSM5. Phys Chem Chem Phys 1 (24):5753-5759. https://doi.org/10.1039/a906550d 116. Le Van Mao R, Dufresne L, Yao J (1990) Long distance hydrogen back-spillover (LD-HBS) phenomena in the aromatization of light alkanes. Appl Catal 65(1):143–157. https://doi.org/ 10.1016/S0166-9834(00)81594-8 117. Shubin AA, Zhidomirov GM, Kazansky VB, van Santen RA (2003) DFT cluster modeling of molecular and dissociative hydrogen adsorption on Zn2+ Ions with distant placing of aluminum in the framework of high-silica zeolites. Catal Lett 90(3):137–142. https://doi.org/ 10.1023/B:CATL.0000004107.24576.1c 118. Barbosa LAMM, van Santen RA (2003) Influence of zeolite framework geometry structure on the stability of the [ZnOZn]2+ cluster by periodical density functional theory. J Phys Chem B 107(19):4532–4536. https://doi.org/10.1021/jp022384g 119. Dent AL, Kokes RJ (1970) Intermediates in ethylene hydrogenation over zinc oxide. J Phys Chem 74(20):3653–3662. https://doi.org/10.1021/j100714a018 120. Luzgin MV, Stepanov AG, Arzumanov SS, Rogov VA, Parmon VN, Wang W, Hunger M, Freude D (2006) Mechanism studies of the conversion of 13C-labeled n-Butane on zeolite H-ZSM-5 by using 13C magic angle spinning NMR spectroscopy and GC–MS analysis. Chem A Eur J 12(2):457–465. https://doi.org/10.1002/chem.200500382 121. Luzgin MV, Arzumanov SS, Shmachkova VP, Kotsarenko NS, Rogov VA, Stepanov AG (2003) n-Butane conversion on sulfated zirconia: the mechanism of isomerization and 13C-label scrambling as studied by in situ 13C MAS NMR and ex situ GC-MS. J Catal 220(1):233–239. https://doi.org/10.1016/S0021-9517(03)00243-4 122. Stepanov AG, Luzgin MV, Arzumanov SS, Wang W, Hunger M, Freude D (2005) n-Butane conversion on sulfated zirconia: in situ 13C MAS NMR monitoring of the kinetics of the 13C-label scrambling and isomerization. Catal Lett 101(3):181–185. https://doi.org/10.1007/ s10562-005-4887-1 123. Chang CD, Silvestri AJ (1977) The conversion of methanol and other O-compounds to hydrocarbons over zeolite catalysts. J Catal 47(2):249–259. https://doi.org/10.1016/00219517(77)90172-5 124. Koempel H, Liebner W (2007) Lurgi’s methanol to propylene (MTP® ) report on a successful commercialisation. In: Fábio Bellot Noronha MS, Eduardo Falabella S-A (eds) Studies in surface science and catalysis, vol 167, pp 261–267. http://dx.doi.org/10.1016/S01672991(07)80142-X 125. Vora BV, Marker TL, Barger PT, Nilsen HR, Kvisle S, Fuglerud T (1997) Economic route for natural gas conversion to ethylene and propylene. In: M. de Pontes RLECPNJHS, Scurrell MS (eds) Studies in surface science and catalysis, vol Volume 107. Elsevier, pp 87–98. http:// dx.doi.org/10.1016/S0167-2991(97)80321-7 126. Stöcker M (1999) Methanol-to-hydrocarbons: catalytic materials and their behavior. Micropor Mesopor Mat 29(1–2):3–48. https://doi.org/10.1016/S1387-1811(98)00319-9 127. Haw JF, Song W, Marcus DM, Nicholas JB (2003) The mechanism of methanol to hydrocarbon catalysis. Acc Chem Res 36(5):317–326. https://doi.org/10.1021/ar020006o

250

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

128. Olsbye U, Svelle S, Bjørgen M, Beato P, Janssens TVW, Joensen F, Bordiga S, Lillerud KP (2012) Conversion of methanol to hydrocarbons: how zeolite cavity and pore size controls product selectivity. Angew Chem Int Ed 51(24):5810–5831. https://doi.org/10.1002/anie. 201103657 129. Hemelsoet K, Van der Mynsbrugge J, De Wispelaere K, Waroquier M, Van Speybroeck V (2013) Unraveling the reaction mechanisms governing methanol-to-olefins catalysis by theory and experiment. ChemPhysChem 14(8):1526–1545. https://doi.org/10.1002/cphc.201201023 130. Ilias S, Bhan A (2012) Mechanism of the catalytic conversion of methanol to hydrocarbons. ACS Catal 3(1):18–31. https://doi.org/10.1021/cs3006583 131. Wang W, Jiang Y, Hunger M (2006) Mechanistic investigations of the methanol-to-olefin (MTO) process on acidic zeolite catalysts by in situ solid-state NMR spectroscopy. Catal Today 113(1–2):102–114. https://doi.org/10.1016/j.cattod.2005.11.015 132. Jiang Y, Wang W, Reddy Marthala VR, Huang J, Sulikowski B, Hunger M (2006) Effect of organic impurities on the hydrocarbon formation via the decomposition of surface methoxy groups on acidic zeolite catalysts. J Catal 238(1):21–27. https://doi.org/10.1016/j.jcat.2005. 11.029 133. Wang W, Buchholz A, Seiler M, Hunger M (2003) Evidence for an initiation of the methanolto-olefin process by reactive surface methoxy groups on acidic zeolite catalysts. J Am Chem Soc 125(49):15260–15267. https://doi.org/10.1021/ja0304244 134. Li J, Wei Z, Chen Y, Jing B, He Y, Dong M, Jiao H, Li X, Qin Z, Wang J (2014) A route to form initial hydrocarbon pool species in methanol conversion to olefins over zeolites. J Catal 317(317):277–283 135. Lesthaeghe D, Van Speybroeck V, Marin GB, Waroquier M (2006) Understanding the failure of direct C–C coupling in the zeolite-catalyzed methanol-to-olefin process. Angew Chem Int Ed 45(11):1714–1719. https://doi.org/10.1002/anie.200503824 136. Marcus DM, McLachlan KA, Wildman MA, Ehresmann JO, Kletnieks PW, Haw JF (2006) Experimental evidence from H/D exchange studies for the failure of direct C–C coupling mechanisms in the methanol-to-olefin process catalyzed by HSAPO-34. Angew Chem Int Ed 45(19):3133–3136. https://doi.org/10.1002/anie.200504372 137. Song W, Marcus DM, Fu H, Ehresmann JO, Haw JF (2002) An oft-studied reaction that may never have been: direct catalytic conversion of methanol or dimethyl ether to hydrocarbons on the solid acids HZSM-5 or HSAPO-34. J Am Chem Soc 124(15):3844–3845. https://doi. org/10.1021/ja016499u 138. Dessau RM, LaPierre RB (1982) On the mechanism of methanol conversion to hydrocarbons over HZSM-5. J Catal 78(1):136–141. https://doi.org/10.1016/0021-9517(82)90292-5 139. Dessau RM (1986) On the H-ZSM-5 catalyzed formation of ethylene from methanol or higher olefins. J Catal 99(1):111–116. https://doi.org/10.1016/0021-9517(86)90204-6 140. Mole T, Bett G, Seddon D (1983) Conversion of methanol to hydrocarbons over ZSM-5 zeolite: an examination of the role of aromatic hydrocarbons using 13carbon- and deuteriumlabeled feeds. J Catal 84(2):435–445. https://doi.org/10.1016/0021-9517(83)90014-3 141. Mole T, Whiteside JA, Seddon D (1983) Aromatic co-catalysis of methanol conversion over zeolite catalysts. J Catal 82(2):261–266. https://doi.org/10.1016/0021-9517(83)90192-6 142. Langner BE (1982) Reactions of methanol on zeolites with different pore structures. Appl Catal 2(4–5):289–302. https://doi.org/10.1016/0166-9834(82)80075-4 143. Dahl I, Kolboe S (1993) On the reaction mechanism for propene formation in the MTO reaction over SAPO-34. Catal Lett 20(3–4):329–336. https://doi.org/10.1007/bf00769305 144. Dahl IM, Kolboe S (1994) On the reaction mechanism for hydrocarbon formation from methanol over SAPO-34: I. Isotopic labeling studies of the co-reaction of ethene and methanol. J Catal 149 (2):458–464. http://dx.doi.org/10.1006/jcat.1994.1312 145. Dahl IM, Kolboe S (1996) On the reaction mechanism for hydrocarbon formation from methanol over SAPO-34: 2. Isotopic labeling studies of the co-reaction of propene and methanol. J Catal 161(1):304–309. http://dx.doi.org/10.1006/jcat.1996.0188 146. Svelle S, Joensen F, Nerlov J, Olsbye U, Lillerud K-P, Kolboe S, Bjørgen M (2006) Conversion of methanol into hydrocarbons over zeolite H-ZSM-5: ethene formation is mechanistically

References

147.

148.

149.

150.

151.

152. 153. 154.

155.

156.

157.

158.

159.

160.

161.

162.

163.

251

separated from the formation of higher alkenes. J Am Chem Soc 128(46):14770–14771. https://doi.org/10.1021/ja065810a Bjorgen M, Svelle S, Joensen F, Nerlov J, Kolboe S, Bonino F, Palumbo L, Bordiga S, Olsbye U (2007) Conversion of methanol to hydrocarbons over zeolite H-ZSM-5: on the origin of the olefinic species. J Catal 249(2):195–207. https://doi.org/10.1016/j.jcat.2007.04.006 Teketel S, Olsbye U, Lillerud K-P, Beato P, Svelle S (2010) Selectivity control through fundamental mechanistic insight in the conversion of methanol to hydrocarbons over zeolites. Micropor Mesopor Mat 136(1–3):33–41. https://doi.org/10.1016/j.micromeso.2010.07.013 Westgård Erichsen M, Svelle S, Olsbye U (2013) H-SAPO-5 as methanol-to-olefins (MTO) model catalyst: towards elucidating the effects of acid strength. J Catal 298:94. https://doi. org/10.1016/j.jcat.2012.11.004 Westgård Erichsen M, Svelle S, Olsbye U (2013) The influence of catalyst acid strength on the methanol to hydrocarbons (MTH) reaction. Catal Today 215:216–223. https://doi.org/10. 1016/j.cattod.2013.03.017 Hunger M, Weitkamp J (2001) In situ IR, NMR, EPR, and UV/Vis spectroscopy: tools for New insight into the mechanisms of heterogeneous catalysis. Angew Chem Int Ed 40(16):2954–2971. https://doi.org/10.1002/1521-3773(20010817)40:16%3c2954:AIDANIE2954%3e3.0.CO;2-%23 Haw JF (2004) In-situ spectroscopy in heterogeneous catalysis Hunger M (2005) Applications of in situ spectroscopy in zeolite catalysis. Micropor Mesopor Mat 82(3):241–255. https://doi.org/10.1016/j.micromeso.2005.01.037 Song W, Nicholas J, Sassi A, Haw J (2002) Synthesis of the heptamethylbenzenium cation in zeolite-β: in situ NMR and theory. Catal Lett 81(1–2):49–53. https://doi.org/10.1023/A: 1016003905167 Munson EJ, Haw JF (1991) NMR observation of trimethyloxonium formation from dimethyl ether on zeolite HZSM-5. J Am Chem Soc 113(16):6303–6305. https://doi.org/10.1021/ ja00016a075 Wu X, Xu S, Zhang W, Huang J, Li J, Yu B, Wei Y, Liu Z (2017) Direct mechanism of the first carbon-carbon bond formation in the methanol-to-hydrocarbons process. Angew Chem Int Ed 56(31):9039–9043. https://doi.org/10.1002/anie.201703902 Wang C, Chu Y, Xu J, Wang Q, Qi G, Gao P, Zhou X, Deng F (2018) Extra-framework aluminum-assisted initial C−C bond formation in methanol-to-olefins conversion on zeolite H-ZSM-5. Angew Chem Int Ed 57(32):10197–10201. https://doi.org/10.1002/anie. 201805609 Anderson MW, Klinowski J (1990) Solid-state NMR studies of the shape-selective catalytic conversion of methanol into gasoline on zeolite ZSM-5. J Am Chem Soc 112(1):10–16. https://doi.org/10.1021/ja00157a004 Haw JF, Nicholas JB, Song W, Deng F, Wang Z, Xu T, Heneghan CS (2000) Roles for cyclopentenyl cations in the synthesis of hydrocarbons from methanol on zeolite catalyst HZSM-5. J Am Chem Soc 122(19):4763–4775. https://doi.org/10.1021/ja994103x Song W, Nicholas JB, Haw JF (2001) A persistent carbenium Ion on the methanol-to-olefin catalyst HSAPO-34: acetone shows the way. J Phys Chem B 105(19):4317–4323. https:// doi.org/10.1021/jp0041407 Wang C, Chu Y, Zheng A, Xu J, Wang Q, Gao P, Qi G, Gong Y, Deng F (2014) New Insight into the hydrocarbon-pool chemistry of the methanol-to-olefins conversion over zeolite H-ZSM-5 from GC-MS, solid-state NMR spectroscopy, and DFT calculations. Chemistry A Eur J 20 (39):12432–12443. https://doi.org/10.1002/chem.201403972 Wang C, Yi X, Xu J, Qi G, Gao P, Wang W, Chu Y, Wang Q, Feng N, Liu X, Zheng A , Deng F (2015) Mechanistic study of the formation of ethene via carbocations in methanol conversion over H-ZSM-5 zeolite. Chem Euro J 21(34):12061–12068. https://doi.org/10. 1002/chem.201501355 Sun X, Mueller S, Shi H, Haller GL, Sanchez-Sanchez M, van Veen AC, Lercher JA (2014) On the impact of co-feeding aromatics and olefins for the methanol-to-olefins reaction on HZSM-5. J Catal 314:21–31. https://doi.org/10.1016/j.jcat.2014.03.013

252

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

164. Wang J, Wei Y, Li J, Xu S, Zhang W, He Y, Chen J, Zhang M, Zheng A, Deng F, Guo X, Liu Z (2016) Direct observation of methylcyclopentenyl cations (MCP+) and olefin generation in methanol conversion over TON zeolite. Catal Sci Technol 6(1):89–97. https://doi.org/10. 1039/C5CY01420D 165. Zhang M, Xu S, Li J, Wei Y, Gong Y, Chu Y, Zheng A, Wang J, Zhang W, Wu X, Deng F, Liu Z (2016) Methanol to hydrocarbons reaction over Hβ zeolites studied by high resolution solid-state NMR spectroscopy: carbenium ions formation and reaction mechanism. J Catal 335:47–57. https://doi.org/10.1016/j.jcat.2015.12.007 166. Xu T, Barich DH, Goguen PW, Song W, Wang Z, Nicholas JB, Haw JF (1998) Synthesis of a benzenium ion in a zeolite with use of a catalytic flow reactor. J Am Chem Soc 120(16):4025–4026. https://doi.org/10.1021/ja973791m 167. Li J, Wei Y, Chen J, Tian P, Su X, Xu S, Qi Y, Wang Q, Zhou Y, He Y, Liu Z (2012) Observation of heptamethylbenzenium cation over SAPO-type molecular sieve DNL-6 under real MTO conversion conditions. J Am Chem Soc 134(2):836–839. https://doi.org/10.1021/ja209950x 168. Ono Y, Mori T (1981) Mechanism of methanol conversion into hydrocarbons over ZSM-5 zeolite. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 77 (9):2209–2221. https://doi.org/10.1039/f19817702209 169. Thursfield A, Anderson MW (1996) H, 2H, and 13C solid-state NMR studies of methanol adsorbed on a series of acidic microporous zeotype materials. J Phys Chem 100(16):6698–6707. https://doi.org/10.1021/jp9530919 170. Anderson MW, Occelli ML, Klinowski J (1992) Carbon-13 and proton magic-angle-spinning NMR studies of the conversion of methanol over offretite/erionite intergrowths. J Phys Chem 96(1):388–392. https://doi.org/10.1021/j100180a072 171. Salehirad F, Anderson MW (1998) Solid-state NMR study of methanol conversion over ZSM-23, SAPO-11 and SAPO-5 molecular sieves. Part 2. Journal of the Chemical Society, Faraday Transactions 94 (18):2857–2866. https://doi.org/10.1039/a803754j 172. Olah GA, Molnár A (2003) Hydrocarbon chemistry, 2nd edn. Wiley, New York 173. Luzgin MV, Romannikov VN, Stepanov AG, Zamaraev KI (1996) Interaction of olefins with carbon monoxide on zeolite H-ZSM-5. NMR observation of the friedel–crafts acylation of alkenes at ambient Temperature. J Am Chem Soc 118(44):10890–10891. https://doi.org/10. 1021/ja9615381 174. Li T, Tsumori N, Souma Y, Xu Q (2003) Highly active and stable performance of catalytic vapor phase Koch-type carbonylation of tert-butyl alcohol over H-zeolites. Chem Commun 16:2070–2071. https://doi.org/10.1039/b304353c 175. Jiang Y, Hunger M, Wang W (2006) On the reactivity of surface methoxy species in acidic zeolites. J Am Chem Soc 128(35):11679–11692. https://doi.org/10.1021/ja061018y 176. Li B, Xu J, Han B, Wang X, Qi G, Zhang Z, Wang C, Deng F (2013) Insight into dimethyl ether carbonylation reaction over mordenite zeolite from in-situ solid-state NMR spectroscopy. J Phys Chem C 117(11):5840–5847. https://doi.org/10.1021/jp400331m 177. Liu Y, Müller S, Berger D, Jelic J, Reuter K, Tonigold M, Sanchez-Sanchez M, Lercher JA (2016) Formation mechanism of the first carbon–carbon bond and the first olefin in the methanol conversion into hydrocarbons. Angew Chem Int Ed 178. Tajima N, Tsuneda T, Toyama F, Hirao K (1998) A new mechanism for the first carbon−carbon bond formation in the MTG process: a theoretical study. J Am Chem Soc 120(32):8222–8229. https://doi.org/10.1021/ja9741483 179. Govind N, Andzelm J, Reindel K, Fitzgerald G (2002) Zeolite-catalyzed hydrocarbon formation from methanol: density functional simulations. Int J Mol Sci 3(4):423 180. Blaszkowski SR, van Santen RA (1997) Theoretical study of C−C bond formation in the methanol-to-gasoline process. J Am Chem Soc 119(21):5020–5027. https://doi.org/10.1021/ ja963530x 181. Chowdhury AD, Houben K, Whiting GT, Mokhtar M, Asiri AM, Al-Thabaiti SA, Basahel SN, Baldus M, Weckhuysen BM (2016) Initial carbon–carbon bond formation during the early stages of the methanol-to-olefin process proven by zeolite-trapped acetate and methyl acetate. Angewandte Chem Int Ed 55(51):15840–15845. https://doi.org/10.1002/anie.201608643

References

253

182. Comas-Vives A, Valla M, Copéret C, Sautet P (2015) Cooperativity between Al sites promotes hydrogen transfer and carbon-carbon bond formation upon dimethyl ether activation on alumina. ACS Cent Sci 1(6):313–319. https://doi.org/10.1021/acscentsci.5b00226 183. Muller S, Liu Y, Kirchberger FM, Tonigold M, Sanchezsanchez M, Lercher JA (2016) Hydrogen transfer pathways during zeolite catalyzed methanol conversion to hydrocarbons. J Am Chem Soc 138(49):15994–16003 184. Wang C, Wang Q, Xu J, Qi G, Gao P, Wang W, Zou Y, Feng N, Liu X, Deng F (2016) Direct detection of supramolecular reaction centers in the methanol-to-olefins conversion over zeolite H-ZSM-5 by 13C–27Al solid-state NMR spectroscopy. Angew Chem Int Edit 55(7):2507–2511. https://doi.org/10.1002/anie.201510920 185. Pourpoint F, Trébosc J, Gauvin RM, Wang Q, Lafon O, Deng F, Amoureux JP (2012) Measurement of aluminum-carbon distances using S-RESPDOR NMR experiments. ChemPhysChem 13(16):3605–3615. https://doi.org/10.1002/cphc.201200490 186. Sullivan RF, Egan CJ, Langlois GE, Sieg RP (1961) A new reaction that occurs in the hydrocracking of certain aromatic hydrocarbons. J Am Chem Soc 83(5):1156–1160. https:// doi.org/10.1021/ja01466a036 187. McCann DM, Lesthaeghe D, Kletnieks PW, Guenther DR, Hayman MJ, Van Speybroeck V, Waroquier M, Haw JF (2008) A complete catalytic cycle for supramolecular methanol-toolefins conversion by linking theory with experiment. Angew Chem Int Ed 47(28):5179–5182. https://doi.org/10.1002/anie.200705453 188. Bjørgen M, Olsbye U, Petersen D, Kolboe S (2004) The methanol-to-hydrocarbons reaction: insight into the reaction mechanism from [12C] benzene and [13C] methanol coreactions over zeolite H-beta. J Catal 221(1):1–10. https://doi.org/10.1016/S0021-9517(03)00284-7 189. Ilias S, Bhan A (2014) The mechanism of aromatic dealkylation in methanol-to-hydrocarbons conversion on H-ZSM-5: what are the aromatic precursors to light olefins? J Catal 311:6–16. https://doi.org/10.1016/j.jcat.2013.11.003 190. Arstad B, Kolboe S, Swang O (2004) Theoretical study of protonated xylenes: ethene elimination and H C-scrambling reactions. J Phys Org Chem 17(11):1023–1032. https://doi. org/10.1002/poc.830 191. Kiricsi I, Förster H, Tasi G, Nagy JB (1999) Generation, characterization, and transformations of unsaturated carbenium ions in zeolites. Chem Rev 99(8):2085–2114. https://doi.org/10. 1021/cr9600767 192. Wang C, Xu J, Qi G, Gong Y, Wang W, Gao P, Wang Q, Feng N, Liu X, Deng F (2015) Methylbenzene hydrocarbon pool in methanol-to-olefins conversion over zeolite H-ZSM-5. J Catal 332:127–137. https://doi.org/10.1016/j.jcat.2015.10.001 193. Miyake K, Hirota Y, Ono K, Uchida Y, Tanaka S, Nishiyama N (2016) Direct and selective conversion of methanol to para-xylene over Zn ion doped ZSM-5/silicalite-1 core-shell zeolite catalyst. J Catal 342:63–66. https://doi.org/10.1016/j.jcat.2016.07.008 194. Schulz H (2010) “Coking” of zeolites during methanol conversion: basic reactions of the MTO-MTP- and MTG processes. Catal Today 154(3):183–194. https://doi.org/10.1016/j. cattod.2010.05.012 195. Wang C, Sun X, Xu J, Qi G, Wang W, Zhao X, Li W, Wang Q, Deng F (2017) Impact of temporal and spatial distribution of hydrocarbon pool on methanol conversion over H-ZSM-5. J Catal 354:138–151. https://doi.org/10.1016/j.jcat.2017.08.003 196. Arstad B, Nicholas JB, Haw JF (2004) Theoretical study of the methylbenzene sidechain hydrocarbon pool mechanism in methanol to olefin catalysis. J Am Chem Soc 126(9):2991–3001. https://doi.org/10.1021/ja035923j 197. De Wispelaere K, Hemelsoet K, Waroquier M, Van Speybroeck V (2013) Complete lowbarrier side-chain route for olefin formation during methanol conversion in H-SAPO-34. J Catal 305:76–80. https://doi.org/10.1016/j.jcat.2013.04.015 198. Arstad B, Kolboe S (2001) The reactivity of molecules trapped within the SAPO-34 cavities in the methanol-to-hydrocarbons reaction. J Am Chem Soc 123(33):8137–8138. https://doi. org/10.1021/ja010668t

254

6 In Situ Solid-State NMR Investigation of Catalytic Reactions on Zeolites

199. Xu S, Zheng A, Wei Y, Chen J, Li J, Chu Y, Zhang M, Wang Q, Zhou Y, Wang J, Deng F, Liu Z (2013) Direct observation of cyclic carbenium ions and their role in the catalytic cycle of the methanol-to-olefin reaction over chabazite zeolites. Angew Chem Int Ed 52(44):11564–11568. https://doi.org/10.1002/anie.201303586 200. Maly T, Debelouchina GT, Bajaj VS, Hu K-N, Joo C-G, Mak–Jurkauskas ML, Sirigiri JR, van der Wel PCA, Herzfeld J, Temkin RJ, Griffin RG (2008) Dynamic nuclear polarization at high magnetic fields. J Chem Phys 128(5):052211. http://dx.doi.org/10.1063/1.2833582 201. Bowers CR, Weitekamp DP (1987) Parahydrogen and synthesis allow dramatically enhanced nuclear alignment. J Am Chem Soc 109(18):5541–5542. https://doi.org/10.1021/ja00252a049 202. Kovtunov KV, Zhivonitko VV, Skovpin IV, Barskiy DA, Koptyug IV (2013) Parahydrogeninduced polarization in heterogeneous catalytic processes. In: Kuhn TL (ed) Hyperpolarization methods in NMR spectroscopy. Springer, Berlin, Heidelberg, pp 123–180. https://doi.org/10. 1007/128-2012-371

Index

A Acid–base property, 94 Acid concentration, 160 Acid distribution, 160 Acidic zeolites, 199 Acidity property, 160 Acid site distributions, 175 Acid strength, 160 Acid type, 160 Activation energy, 216 Activation of alkanes, 119, 209 Active intermediates, 199 Active sites, 133 Active species, 200 Alkanes conversion, 209 Alkene-based cycle, 226 Alkoxy species, 211 AlPO4-5, 83 AlPO4-34, 80 AlPO4-CJ2, 82 Aluminophosphate (AlPO), 79, 107 Aluminophosphate molecular sieves, 143 Aluminophosphates, 58 Aluminosilicates, 58 Aluminum states, 95 Aromatics-based cycle, 226 Aromatics selectivity, 191 Aromatization of propane, 220 Asymmetry parameter, 106, 114 Atomic-level, 88 Autoclave, 80 B BaBa, 29 BAck to BAck, 29

Batch reactions, 201 BEA, 216 Bifunctional, 220 Bifunctionality of modified zeolites, 184 Bimolecular mechanism, 225 Black box, 199 Boltzman polarization, 125 Bridging hydroxyl group, 159 Brønsted acid densities, 183 Brønsted acid site, 93, 111, 159, 206 Brønsted acid strength, 169 Brønsted/Lewis acid synergy, 181 Build up curves, 186 C 2-13C-acetone, 169 Carbenium ion mechanism, 220 Carbenium ions, 211, 220 Carbocations, 235 Carbocations intermediates, 232 Carbonium ions, 220 Carbonylation, 213 Catalytic efficiency, 199 Catalytic mechanism, 199 C2–C4 alkanes, 219 Central transition, 8 CHA, 241 C–H bond activation, 223 Chemically distinct, 101 Chemical shift, 3, 77 chemical shift anisotropy, 4 CSA, 6 See also chemical shift anisotropy isotropic chemical shift, 5 Chemical shift anisotropy, 106 Classical carbenium ion mechanism, 220

© Springer Nature Singapore Pte Ltd. 2019 J. Xu et al., Solid-State NMR in Zeolite Catalysis, Lecture Notes in Chemistry 103, https://doi.org/10.1007/978-981-13-6967-4

255

256 Cleavage of C–H bond, 121 Co-conversion, 210 Confinement effect, 151 Continuous-flow, 204 Continuous-flow condition, 122 Contraction/expansion process, 232 Coordination environment, 94 CP/MAT, 125 CRAMPS, 17 combined rotation and multiple-pulse spectroscopy, 17 Cross-polarization, 13 Cryogenic Adsorption Vessel Enabling Rotor Nestling (CAVERN), 202 Crystal growth, 61 Crystallization, 57 Crystallographic non-equivalent, 101 CSA parameters, 125 CSA pattern, 106 Cyclic intermediate, 220 Cyclopentadienes, 239 Cyclopentenyl cations, 150 Cycloproponium ion, 220 D DAS, 35 Dealuminated zeolites, 230 Dealumination by steaming, 96 Dealumination treatments, 180 Deprotonation energy (DPE), 173 2D-exchange MAS NMR, 162 2D-exchange spectrum, 121 DFS, 38 double-frequency sweeps, 38 DFT calculation, 121, 213 D-HMQC, 147, 167 2D homonuclear and heteronuclear correlation NMR, 134 1,3-dimethylcyclopentenyl cation, 234 Dimethyl ether, 226 2D INADEQUATE, 103 Diphenyldiphosphines, Ph2P(CH2)nPPh2 molecules, 183 Dipolar decoupling, 15 homonuclear dipolar decoupling, 16 Dipolar interaction, 3 heteronuclear dipolar interaction, 24 Dipolar interaction (through-space), 134 Dipolar recoupling, 18 heteronuclear dipolar recoupling, 18 Dipole-dipole interaction, 6 See also dipolar interaction Direct route, 226 Distance constraints, 103

Index DME, 226 DNP, 38, 125, 242 dynamic nuclear polarization, 38 DNP-SENS, 39 DNP-surface-enhanced NMR spectroscopy, 39 Double-resonance NMR, 109 Double-rotation (DOR), 35, 97 DQ, 30, 179 double-quantum, 30 DQ bulid-up curves, 106 DQ MAS NMR, 143 DQ recoupling time, 180 DRAMA, 29 dipolar recovery at the magic angle, 29 Dry-gel conversion (DGC), 79, 124 Dual-cycle, 226 DUMBO, 17 decoupling using mind-boggling optimization, 17 Dynamics, 200 E EDXRD, 70 Electric-field gradient (EFG), 4, 113, 122 Electric quadrupole moment, 7 Ethylcyclopentenyl cations, 236 Ex situ, 62, 77 Ex situ analysis, 200 Extra-framework, 222 Extra-framework aluminum (EFAL), 98, 159, 230 F FAM, 38 fast amplitude modulation, 38 FAU, 64 First C–C bond formation, 226, 230 Five-coordinate aluminum, 95 Five-coordinated, 83 Flow catalytic cracking, 219 Flow conditions, 203 Four-coordinated, 83 Four-coordinate framework, 95 Framework and extra-framework Al, 95 Framework structures, 58 FSLG, 17 frequency-switched Lee–Goldburg, 17 G Ga/H-ZSM-5, 220 Gallium alkoxide, 223 Gallium hydride, 223 Ga-modified ZSM-5, 187

Index GC-MS, 208 GIS, 64 Glass ampoules, 201 GRASSHopper, 206 Gyromagnetic ratio, 7, 62 H Half-integer spin quadrupolar nuclei, 98 Hartmann-Hahn matching condition, 13 H/D exchange, 216 Heptamethylbenzenium ion, 241 HETCOR, 27, 73, 79, 109, 143 HETeronuclear CORrelation, 27 Heterogeneous catalysis, 199 Heterogeneous catalysts, 199 Heterolytic dissociation, 212 Heteronuclear, 62 Heteronuclear correlation, 73 Heteropolyacid catalyst, 207 Heteropolyacids, 159 Heteropolyoxometalates, 170 High-resolution, 200 High-silica zeolite, 216 H-MOR, 153 HMQC, 26 Homolytic cleavage, 213 Homonuclear, 62 HORROR, 29 homonuclear rotary resonance, 29 Host-guest complexes, 133 Host-guest interactions, 58, 133 HS, 38 hyperbolic secant pulse, 38 HS-QCPMG NMR, 186 HTHP MAS rotor, 83 Hydrocarbon-pool, 226 Hydrocarbon pool, HP species, 148 Hydrogenation process, 216 Hydrogen transfer, 230 Hydrothermal, 57 Hydrothermal treatment, 144 Hyperpolarized (HP) 129Xe NMR, 121 H-ZSM-5, 203 I INADEQUATE, 134 Indirect route, 226 In situ, 62, 68 In situ MAS NMR, 200 In situ NMR technique, 200 In situ SF MAS NMR, 204 In situ solid-state NMR, 199 In situ static NMR, 203 Intermediate gels, 73

257 Internal and external acid sites, 176 Intrinsic acid strengths, 173 “Invisible” aluminum, 94 Ion-exchange, 57 IR spectroscopy, 169 Isomorphic substitution, 75 Isomorphously framework-substituted zeolites, 122 Isotope-enriched reactants, 207 Isotope enrichment, 62 Isotropic chemical shift, 106 ITQ-1, 143 ITQ-6, 121 J J-coupling (through-bond), 134 K Kinetic study, 216 Koch carbonylation, 229 L Larmor frequency, 5 Lewis acid catalyst, 166 Lewis acid site, 93, 159 Lewis acid strength, 174 Lewis sites, 220 Light alkanes, 199 Line-narrowing, 206 Linewidth broadening, 203 Loewenstein rule, 108 M Magic-angle hopping (MAH), 206 Magic-angle spinning (MAS) NMR, 179 Magic-Angle Spinning. See magic-angle spinning MAS-7, 116 MAS rotor insert, 201 MCM-22, 116 MeAPO, 75 Mechanistic study, 199 12-membered-ring (MR), 143 Membered rings, 93 Mesoporous materials, 58 Metal-exchanged zeolites, 117 Metal-modified zeolites, 199 Metal oxides, 159 Metal-substituted aluminophosphate (MAPO), 107 Methane, 209 Methane activation, 210 Methanol conversion, 211 Methanol-to-aromatics (MTA), 187

258 Methanol-to-gasoline (MTG), 212 Methanol-to-hydrocarbons, 199 Methanol-to-olefins (MTO), 148, 204, 226 Methoxy species, 210 Methylbenzenes (MBs), 148 MgAPO-36, 75 Microwave irradiation, 145 Molecular recognition, 57 Molecular sieves, 57 Molecular structure, 200 Monomolecular mechanism, 225 MQMAS, 35, 109 multiple-quantum magic-angle spinning, 35, 98 Multi-nuclear, 86 N Natural abundance, 207 N-doped zeolite, 102 Neutron diffraction, 58 NMR probe, 200 Nuclear magnetic resonance, 200 O Octahedral, 73 Overlapping, 79 P Paramagnetic effect, 136 Paring pathway, 232 PASADENA, 43 parahydrogen and synthesis allow dramatic enhancement of nuclear alignment, 43 Pencil-style MAS rotor, 202 Pentacoordinated, 73 Pentamethylbenzenium ions, 233 Perfluorotributylamine, 176 PHIP, 38, 242 parahydrogen-induced polarization, 38, 242 Phosphomolybdic acid, 171 PISSARRO, 15 phase-inverted supercycled sequence for attenuation of rotary resonance, 15 PMLG, 17 phase-modulated Lee–Goldburg, 17 Polarization transfer, 134 Post-C7, 29 PPCP, 222 Probe molecules, 160 Propene-to-ethene ratio (P/E), 239 Proton affinity (PA), 173 Proton–proton proximities, 179 Pseudo-cyclopropane, 222 Pulse-quench reactor, 207

Index Pyridine IR, 169 Pyridinium ions, 174 Q QCPMG, 29 quadrupolar Carr–Purcell–Meiboom–Gill, 29 QCPMG NMR, 189 Quadrupolar coupling constant, 114 Quadrupolar coupling parameters, 114 Quadrupolar interaction, 3, 110 Quadrupole coupling constant, 8 Quantitative determination of acid strength, 160 Quantum chemical calculations, 102 Quenching, 201 Q units, 64 R R3, 32 rotary resonance recoupling. See rotary resonance condition RA-MP, 16 rotor-synchronized multiple-pulse, 16 RAPT, 38 rotor-assisted population transfer, 38 Reactant adsorption, 200 Reaction intermediates, 199 Reaction mechanism, 58, 199 REAPDOR, 22 rotational-echo adiabatic-passage double resonance, 22 REDOR, 18, 79, 109, 124, 134 Rotational-Echo DOuble Resonance, 18 Reverse spillover, 223 RFDR, 29 radio field dipolar-driven recoupling, 29 RINEPT, 26 Rotary resonance condition, 15 R-RESPDOR, 23 rotary resonance-echo saturation-pulse double resonance, 23 RS-MP, 16 S SABRE, 44 signal amplification by reversible exchange, 44 SAM, 18 smooth amplitude modulation, 18 SAPO-34, 80, 226 SAPO-5, 226 Saturated hydrocarbons, 209 SAXS/WAXS, 58

Index Scalar couplings, 102 Scrambling, 220 Secondary building units (SBUs), 60, 93 Second-order quadrupolar interaction, 96 Secular parts, 4 Self-assembling, 58 Sensitivity enhancement, 207 SFAM, 22 simultaneous frequency and amplitude modulation, 22 Shielding interactions, 106 Shielding tensors, 106 Short-range ordering, 58 Si/Al ratio, 100 Signal-to-noise ratio (S/N), 207 Silanols, 206 Silicoaluminophosphate (SAPO), 75, 107 SIMPSON, 113 Si(nAl) unit, 100 Single-crystal X-ray diffraction, 134 Six-coordinate Al, 95 SOD, 64 Sodalite cage, 112 Sol–gel phase, 58 Solid acid, 159 Solid acid catalysts, 160 Solid hydrogel transformation, 59 Solid superacid, 170 Solution-mediated reaction transformation, 59 Spatial and time resolution, 88 Spatial interaction/proximity, 149 Spatial proximity/interaction, 103, 160 Spectator species, 199 Spectroscopic resolution, 203 SPINAL, 15 small-phase incremental alternation decoupling, 15 Spin–lattice relaxation time, 207 Spinning rate, 204 Spinning sidebands, 12 Spin-spin interaction, 3 S-RESPDOR, 23, 146, 186, 231 symmetry-based resonance-echo saturation-pulse double resonance, 23 SSZ-13, 152 ST, 38 STMAS, 37 Stopped-flow (SF), 204 Structure–activity relationship, 153, 199 Structure-directing agents, 59, 64 Structure–property relationship, 133 Sulfated zirconia, 225

259 Superacid chemistry, 220 Superacidity, 174 Supercage, 112 Surface adsorbates, 201 Surface area, 57 Surface-bound species, 211 Surface-enhanced, 145 SWAMP, 43 surface waters are magnetized by parahydrogen, 43 Symmetry-based sequences, 25 Synergetic effect, 223 Syn-gas, 210 T Temperature-programmed desorption (TPD), 169 Tetrahedra, 93 Tetrahedral, 73 Tetrahedral geometries, 106 Theoretical calculations, 210 Threshold acid strength, 170 Through-bond, 26 Through-space, 26 Time-on-stream, 83 TPA, 64 TPPM, 15 two-pulse phase modulation decoupling, 15 Transition metal, 210 TRAPDOR, 22, 79, 109, 161 transfer of populations in double resonance, 22 Trapped species, 236 Tributylphosphine oxide (TBPO), 174 Triethylphosphine (TEPO), 174 Trimethylphosphine oxide (TMPO), 169, 174 Trimethylphosphine (TMP), 168 Trioctylphosphine oxide (TOPO), 174 Tungstophosphoric acid, 171 U Unit cell, 101 Unit cell parameters, 106 UV–Vis spectroscopy, 204 V Vacuum line, 201 Variable-temperature (VT), 116, 137, 202 W Wetness impregnation, 189 Working catalyst, 200

260 WURST, 38 wideband, uniform rate, and smooth truncation, 38 X XiX, 15 X-ray diffraction, 58

Index Z Zeolite, 159 Zeolite A, 68 Zeolite catalysis, 199 Zeolite structure, 94 Zinc methyl species, 210 Zn-modified ZSM-5, 210 ZSM-5, 64, 116 ZSM-22, 226