[1st Edition] 9780128110058, 9780128110041

Table of contents :
Content:
Series PagePage ii
CopyrightPage iv
ContributorsPage vii
PrefacePages ix-xChunshan Song
Chapter One - Catalysis Science of NOx Selective Catalytic Reduction With Ammonia Over Cu-SSZ-13 and Cu-SAPO-34Pages 1-107C. Paolucci, J.R. Di Iorio, F.H. Ribeiro, R. Gounder, W.F. Schneider
Chapter Two - Multiscale Aspects in Hydrocracking: From Reaction Mechanism Over Catalysts to Kinetics and Industrial ApplicationPages 109-238J.W. Thybaut, G.B. Marin
IndexPages 239-243

Citation preview

EDITOR IN CHIEF C.S. SONG University Park, Pennsylvania, USA

ADVISORY BOARD M. CHE Paris, France

A. CORMA CANÓS Valencia, Spain

D.D. ELEY Nottingham, England

G. ERTL Berlin/Dahlem, Germany

B.C. GATES Davis, California, USA

G. HUTCHINGS Cardiff, UK

E. IGLESIA Berkeley, California, USA

P.W.N.M. VAN LEEUWEN Tarragona, Spain

J. ROSTRUP-NIELSEN Lyngby, Denmark

R.A. VAN SANTEN Eindhoven, The Netherlands

€ F. SCHUTH M€ ulheim, Germany

J.M. THOMAS London/Cambridge, England

H. TOPSØE Lyngby, Denmark

Academic Press is an imprint of Elsevier 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States 525 B Street, Suite 1800, San Diego, CA 92101-4495, United States The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 125 London Wall, London, EC2Y 5AS, United Kingdom First edition 2016 Copyright © 2016 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-12-811004-1 ISSN: 0360-0564 For information on all Academic Press publications visit our website at https://www.elsevier.com/

Publisher: Zoe Kruze Acquisition Editor: Poppy Garraway Editorial Project Manager: Shellie Bryant Production Project Manager: Surya Narayanan Jayachandran Cover Designer: Victoria Pearson Typeset by SPi Global, India

CONTRIBUTORS J.R. Di Iorio School of Chemical Engineering, Purdue University, West Lafayette, IN, United States R. Gounder School of Chemical Engineering, Purdue University, West Lafayette, IN, United States G.B. Marin Laboratory for Chemical Technology, Ghent University, Ghent, Belgium C. Paolucci University of Notre Dame, Notre Dame, IN, United States F.H. Ribeiro School of Chemical Engineering, Purdue University, West Lafayette, IN, United States W.F. Schneider University of Notre Dame, Notre Dame, IN, United States J.W. Thybaut Laboratory for Chemical Technology, Ghent University, Ghent, Belgium

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PREFACE I am honored and humbled to have been appointed as the new Editor in Chief for Advances in Catalysis starting April 2016. Advances in Catalysis has been a flagship publication in heterogeneous catalysis and influenced generations of researchers in catalysis. Personally I have benefited greatly from reading the reviews published in this book series since my days of graduate study over 30 years ago. Two of the previous editors, the late Dr. Paul B. Weisz and the late Dr. Werner O. Haag, had also been very helpful to me through their visits and discussions on my research on shape-selective zeolite catalysis for converting polycyclic hydrocarbons in the early stage of my career at Penn State. I would like to thank the advisory board members, particularly Dr. Michel Che, and Elsevier publishing team, particularly Poppy Garraway, for their support and advice, and the previous editors, particularly Dr. Bruce C. Gates and Dr. Friederike C. Jentoft, for their helpful suggestions. I look forward to working with the catalysis community worldwide to further advance the catalytic science and technology through publishing in Advances in Catalysis. This volume of Advances in Catalysis contains two chapters focusing on catalysis in important conversion processes that have broad industrial and environmental applications. The first chapter by Paolucci, Di Iorio, Ribeiro, Gounder, and Schneider provides a comprehensive review on selective catalytic reduction (SCR) of NOx compounds with ammonia over chabazitetype zeolite-based catalysts for diesel automotive exhaust aftertreatment, with a focus on Cu-chabazite (Cu-CHA) catalysts which were commercialized in 2009. Selective reduction of NOx takes place through reactions with ammonia (often formed from decomposition of urea injected) inside the catalytic converter to form inert nitrogen (N2) and water, thus removing the NOx pollutants. The authors review the fundamental scientific advances that have been made in the molecular-level understanding of the active sites and mechanisms responsible for NOx SCR with NH3 on Cu-CHA catalysts. Many experimental studies have been reported in the literature on the “single site” of Cu species, but the authors of this review make new and unifying connections among the seemingly disparate findings of experimental investigations of Cu-CHA catalysts that differ in origin and treatment history, using ex situ and in situ characterizations, and operando characterization during catalysis. From the theory-based studies, the authors highlight how molecular-level descriptions of the active sites and mechanisms can provide insight into the chemical factors that influence practical SCR performance and behavior, including the ix

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onset of low-temperature NOx conversion, and the critical role of NH3 solvation of Cu active sites for low-temperature activity. The authors highlight the interactions between experimental observation and molecular models and also the scientific knowledge available to improve current materials and the knowledge gaps that remain to be addressed. The second chapter by Thybaut and Marin is a review on catalytic hydrocracking including hydroisomerization, which is widely used in the refining industry worldwide. The authors discuss multiscale aspects in hydrocracking from reaction mechanism over bifunctional acid catalysts to reaction kinetics and industrial application. Increasing demands for cleaner liquid transportation fuels and declining quality of crude oils have made hydrocracking processing more important in the refining industry. Hydroisomerization is not only a part of the reactions occurring during hydrocracking for fuels production but also an important step in producing other important industrial products. The authors review the catalytic reaction chemistry and bifunctional reaction mechanism as an essential feature where the synergy between metal and acid sites stems from the dehydrogenation of saturated carbon–carbon bonds on the metal sites prior to the isomerization or cracking on the acid sites. It is a comprehensive overview of the catalytic hydrocracking process starting from its commercialization in the 1960s up to the present, most advanced state of the art in process optimization. In this context, it is worth noting that this review can also be viewed as a follow-up to the pioneering work on polyfunctional heterogeneous catalysis in Advances in Catalysis by the late Paul Weisz (1962, Vol. 13, pp 137–190), a long-time editor for Advances in Catalysis during 1956–1993. The bifunctional mechanism comprises acid-catalyzed rearrangement and cracking in addition to metal-catalyzed (de)hydrogenation and is effective at relatively mild operating conditions, for which numerous combinations of metal functions (ranging from transition metals to noble metals) and acid functions (ranging from crystalline, microporous to wider pore, amorphous materials) and the catalytic reaction mechanisms have been reviewed. It should be noted that this review focusing on hydroisomerization and hydrocracking of hydrocarbon processing is also relevant for hydroprocessing of alternate feedstocks such as biomass- and waste plastics-based feeds. It is my hope that this volume will help researchers and graduate students in catalysis-related research areas worldwide and contribute to advancing both the science and technology of SCR and hydrocracking in years to come. CHUNSHAN SONG Pennsylvania State University October 23, 2016

CHAPTER ONE

Catalysis Science of NOx Selective Catalytic Reduction With Ammonia Over Cu-SSZ-13 and Cu-SAPO-34 C. Paolucci*, J.R. Di Iorio†, F.H. Ribeiro†, R. Gounder†, W.F. Schneider*,1 *University of Notre Dame, Notre Dame, IN, United States † School of Chemical Engineering, Purdue University, West Lafayette, IN, United States 1 Corresponding author: e-mail address: [email protected]

Contents 1. Introduction 1.1 Selective Catalytic Reduction 1.2 Metal-Exchanged Zeolite Catalysts 1.3 Scope and Structure of This Review 2. Synthesis of Zeolites 2.1 Synthetic Control of Al Distribution in Zeolites 2.2 Synthesis of SSZ-13 Zeolites 2.3 Synthesis of SAPO-34 Molecular Sieves 2.4 Copper Exchange in Zeolite and SAPO Frameworks 3. Ex Situ Characterization of Cu-SSZ-13 and Cu-SAPO-34 3.1 DFT-Based Analysis of Cu Speciation 3.2 Ambient Conditions 3.3 High-Temperature Oxidative Conditions 3.4 Vacuum and Inert Pretreatments 3.5 Hydrogen Temperature-Programmed Reduction 3.6 Characterization Following NO Dosing 4. In Situ and Operando Characterization 4.1 DFT Models of NH3 Adsorption in Cu-SSZ-13 4.2 Selective NH3 Titration of H+ Sites in H-Form and Cu-Exchanged Zeolites 4.3 Vibrational Spectroscopy 4.4 Magnetic Spectroscopy With NH3 4.5 X-Ray Spectroscopy With NH3 4.6 Operando X-Ray Spectroscopy 5. Catalytic Activity and Mechanism 5.1 Differential Standard SCR Kinetics 5.2 Standard SCR Mechanism Near 200°C

Advances in Catalysis, Volume 59 ISSN 0360-0564 http://dx.doi.org/10.1016/bs.acat.2016.10.002

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5.3 Standard SCR Mechanism Near 350°C 5.4 Parallel and Competing Reactions 6. Conclusions and Perspective Acknowledgments References

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Abstract Copper-exchanged, small-pore chabazite (CHA) zeolites were commercialized in 2009 for the selective catalytic reduction (SCR) of NOx compounds with ammonia, as an emissions control strategy in diesel automotive exhaust aftertreatment. Here, we review the fundamental scientific advances that have since been made in the molecular-level understanding of the active sites and mechanisms responsible for NOx SCR with NH3 on Cu-CHA catalysts. A large body of experimental and theoretical characterization has identified that these “single-site” catalysts contain Cu sites of different local coordinations and structures, influenced by synthetic and environmental factors. The speciation of isolated Cu ions is inextricably linked to the support composition and the conditions of exposure. We make new and unifying connections among the seemingly disparate findings of experimental investigations of Cu-CHA catalysts that differ in origin and treatment history, using ex situ and in situ characterizations, and operando characterization during catalysis. We discuss theory-based studies, in conjunction with multiple experimental spectroscopic methods performed on model Cu-CHA catalysts, that provide precise molecular assignments across a wide range of catalysts and conditions. We highlight how molecular-level descriptions of the active sites and mechanisms can provide insight into the chemical factors that influence practical SCR performance and behavior, including the onset of low-temperature NOx conversion, and the critical role of NH3 solvation of Cu active sites for low-temperature activity. Finally, we describe how the fundamental heterogeneous catalysis science approaches used to interrogate Cu-CHA catalysts used for NOx SCR with NH3 can be used to elucidate the molecular-level details of chemistry that occurs on other single-site catalysts.

ABBREVIATIONS 1 Al isolated Al atom 2 Al paired Al atoms 2NN next nearest neighbor 3NN next–next nearest neighbor AIMD Ab initio molecular dynamics B3LYP Becke, three-parameter, Lee–Yang–Parr exchange and correlation functional BEA structure code for beta framework BEEF Bayesian error estimation functional BEEFvdw BEEF functional with van der Waals (vdW) dispersive corrections BLYP Becke, Lee, Yang, and Parr exchange and correlation functional BTMA + benzyltrimethylammonium cations CHA structure code for chabazite framework CN coordination number

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Cu/Al molar ratio of copper to aluminum Cu-BEA BEA zeolite ion exchanged with copper Cu-CHA CHA molecular sieve (either SSZ-13 or SAPO-34) ion exchanged with copper Cu-MOR MOR zeolite ion exchanged with copper Cu-SAPO-34 SAPO-34 molecular sieve ion exchanged with copper Cu-SSZ-13 SSZ-13 zeolite ion exchanged with copper Cu-TEPA copper tetraethylenepentamine Cu-Y Y-zeolite ion exchanged with copper Cu-ZSM-5 ZSM-5 zeolite ion exchanged with copper DEF diesel emission fluid DFT density functional theory DFT-D2 density functional theory with dispersion corrections DFT-D3 density functional theory with coordination number dependent dispersion corrections DOC diesel oxidation catalyst DPF diesel particulate filter Ea activation energy EDDs electron-density distributions EPR electron paramagnetic resonance eV electron volts EXAFS X-ray absorption fine structure F2 fluoride anions FAU structure code for faujasite framework FEFF automated program for ab initio multiple scattering calculations FER structure code for ferrierite framework GGA generalized gradient approximation H+/Al molar ratio of protons to aluminum of the zeolite H-SAPO-34 proton form SAPO-34 molecular sieve HSE06 Heyd–Scuseria–Ernzerhof exchange and correlation functional HSE06-TSvdW HSE06 functional with TSvdW dispersion corrections H-SSZ-13 proton form SSZ-13 zeolite HTA hydrothermally aged H-ZSM-5 proton form ZSM-5 zeolite ICP-OES inductively coupled plasma optical emission spectroscopy IR Fourier transform infrared spectroscopy keV kilo electron volts M2+ divalent cation MAS NMR magic angle spinning nuclear magnetic resonance MFI structure code for mordenite framework inverted framework MOR structure code for mordenite framework MR membered ring Na+/TMAda+ molar ratio of sodium to TMAda+ cations in the synthesis solution NH3-SCR ammonia selective catalytic reduction NMR nuclear magnetic resonance NOx nitrogen oxides (NO and NO2) Of framework oxygen atom OH2 hydroxide anions

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PBE Perdew–Burke–Ernzerhof exchange-correlation functional PBE0 PBE functional augmented with Hartree–Fock exchange PJTE pseudo Jahn–Teller effect PMO partial methane oxidation PW91 Perdew–Wang exchange and correlation functional RDS rate-determining step ReaxFF reactive force field RR Rietveld refinement SAPO silicoaluminophosphate molecular sieve SAPO-34 silicoaluminophosphate molecular sieve with the chabazite framework SCR selective catalytic reduction SDA structure-directing agent Si/Al molar ratio of silicon to aluminum SSIE solid-state ion exchange SSZ-13 aluminosilicate zeolite of the chabazite framework T-atom tetrahedrally coordinated atom TEAOH tetraethylammonium hydroxide TEPA tetraethylenepentamine TMAda+ N,N,N-trimethyl-1-admantylammonium cations TOF turnover frequency TOR turnover rate T–O–T zeolite framework T-atom–oxygen–T-atom bonds TPA+ tetrapropylammonium cations TPD temperature-programmed desorption TPR temperature-programmed reduction T-site tetrahedral framework site TSvdW Tkatchenko–Scheffler functional with van der Waals dispersive forces UV–vis ultraviolet–visible spectroscopy vdW van der Waals dispersive forces XANES X-ray absorption near-edge spectroscopy XAS X-ray absorption spectroscopy XES X-ray emission spectroscopy XRD X-ray diffraction Y-zeolite aluminosilicate zeolite of the faujasite framework (Si/Al > 2) Z2Cu divalent Cu2+ exchanged at a paired Al site Z2M divalent cation exchanged at a paired Al site ZCu monovalent Cu+ exchanged at an Al site ZCuOH [CuOH]+ exchanged at an isolated Al site ZMOH [MOH]+ cation exchanged at an isolated Al ZSM-5 zeolite Socony Mobil Five, an aluminosilicate zeolite of the MFI framework

1. INTRODUCTION Molecular sieves have been explored as heterogeneous catalysts for over 50 years (1,2). These nanoporous materials are comprised of a

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crystalline, inorganic, and anionic host framework and chargecompensating extraframework guest cations that can be as simple as protons or as complex as large organic cations. Transition metal ions, present as isolated sites or in small clusters, are of particular interest as chargecompensating species because they may enable catalytic transformations analogous to those of homogeneous inorganic complexes. The cations coordinate with the zeolite framework in well-defined ways depending on their size and valence (3), making them well suited to investigations of their fundamental catalysis science (4) and to molecular design and optimization for practical application. Metal-exchanged zeolites have drawn interest as catalysts for a variety of transformations (5), including the partial oxidation of methane (6). An application that has received significant attention in the past 40 years is the removal of nitrogen oxides from lean (excess air) combustion gases, in particular for diesel and lean-burn exhaust aftertreatment (7). The emergence of hydrothermally stable metal-exchanged molecular sieves for NOx abatement catalysis has coincided with the development of analytical techniques capable of interrogating the materials in molecular detail across a range of experimental conditions and of powerful firstprinciples computational methods to model these systems at the molecular scale (8). As a result, details about the nature of the active sites and the catalytic mechanism have advanced well beyond those gleaned from early investigations in this field and are approaching a level of fidelity matched by few heterogeneous catalytic systems. In this contribution, we review the state of knowledge for the two most widely studied small-pore, Cu-exchanged molecular sieves, the zeolite SSZ-13 and the silicoaluminophosphate SAPO-34, both of the chabazite (CHA) topology. While the literature contains conflicting conclusions about the nature of the active sites and mechanisms responsible for NOx abatement catalysis, we demonstrate that these conflicts can be largely resolved after considering the influence of catalyst composition, synthesis conditions, and sample history on catalyst structure and state. We begin the review with a critical discussion of catalyst synthesis procedures and their implications for the molecular-scale structures that define catalytic function. We then describe how differences in structure manifest themselves in ex situ and in situ characterizations, and in differences in catalytic performance. We consider the state of understanding of the selective catalytic reduction (SCR) mechanism on these materials. Along the way, we uncover unifying insights, reconcile seemingly disparate literature observations, and identify needs and opportunities for further research.

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1.1 Selective Catalytic Reduction The nitrogen oxides NO and NO2, collectively known as NOx, are ubiquitous by-products of combustion reactions in air. Because of their adverse effects on human health and the environment, NOx emissions from stationary and mobile sources are strictly regulated in the United States, Europe, and elsewhere. In SCR, NOx is reduced to N2 through reaction with a reductant, most commonly NH3. NH3-SCR occurs primarily via three parallel routes (8,9), known as the “fast” SCR reaction: 4NH3 + 2NO + 2NO2 ! 4N2 + 6H2 O

(1)

the “standard” SCR reaction: 4NH3 + 4NO + O2 ! 4N2 + 6H2 O

(2)

and the “slow” SCR reaction: 8NH3 + 6NO2 ! 7N2 + 12H2 O

(3)

As its name suggests, the fast SCR reaction typically proceeds at higher rates than the standard SCR reaction (10,11), but the latter is the more prevalent reaction under typical conditions of application. Standard SCR competes with the undesirable direct oxidation of NH3 (8,9): 4NH3 + 5O2 ! 4NO + 6H2 O 4NH3 + 3O2 ! 2N2 + 6H2 O

(4) (5)

NH3 is a three-electron reductant and NO only a two-electron oxidant; with H2O as the only oxygen containing product, chemical balance demands an additional oxidant. In fast and standard SCR, the additional oxidizing capacity is provided by the four-electron oxidants NO2 and O2, respectively. Therefore, standard SCR catalysts must satisfy the seemingly contradictory requirements of selectivity toward reactions of NH3 with NO and against reactions of NH3 with O2, in a gas stream in which O2 is present in excess by several orders of magnitude over NO. NH3 SCR as a technology was first developed to treat emissions from nitric acid production, and the earliest catalysts were precious-metal-based (12). Base metal oxide catalysts, primarily vanadia on titania, were introduced in the 1970s. As early as 1977, Seiyama et al. (13) described the performance of Y-zeolites exchanged with a variety of metal cations for NH3 SCR. They noted the particularly favorable low-temperature activity

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of Cu2+-exchanged Y and speculated on a mechanism involving NO and NH3 coadsorbed on a Cu site.

1.2 Metal-Exchanged Zeolite Catalysts Interest in SCR for on-road applications emerged in the 1990s to address the conflicting demands of higher fuel efficiency and lower NOx emissions (14). The three-way catalysts employed to remove NOx from stoichiometric combustion exhaust, as from spark-ignited gasoline engines, are ineffective for NOx control at the higher air-to-fuel ratios characteristic of more efficient diesel and lean-burn engines. On-board lean-NOx control is ideally accomplished by direct NO decomposition or SCR using a fuel-derived hydrocarbon reductant. In the 1980s, Iwamoto reported that a Cu-exchanged ZSM-5 zeolite promoted the catalytic decomposition of NO to the elements (15–17) and soon after that the same material could selectively reduce NO to N2 with a hydrocarbon reductant (18). Metalexchanged zeolites, in particular Cu-exchanged ZSM-5, were widely explored for both of these reactions in the 1990s, but performance was found to be insufficient to meet practical requirements (14,19). NH3 SCR on Cu-exchanged zeolites was similarly limited by insufficient low-temperature activity, susceptibility to hydrocarbon fouling and catalyst degradation at high temperatures, and the practical challenges of storing and delivering NH3 reliably on-board a vehicle. While more hydrothermally stable than vanadia-based catalysts (20), as illustrated in Fig. 1 (21), the metal-exchanged zeolites of the 1980s and 1990s, in particular Cu- and Fe-exchanged ZSM-5 and BEA, still degraded substantially over time at 500°C (22). Metalexchanged zeolites also store substantially more NH3 than do vanadia catalysts, also shown in Fig. 1 (21). Excess ammonia storage contributes to NH3 slip and in some cases undesirable production of nitrous oxide (N2O) over N2. As a result, for a time lean-NOx traps were preferred over NOx SCR for on-board lean-NOx aftertreatment (23). Parallel to these developments in NOx SCR, a number of research groups were discovering methods to synthesize small-pore zeolites. As discussed further later, access to the inner cages of the molecular sieves is regulated by pores whose size is defined by the number of oxygen ions contained in the largest windows (24). Small-pore zeolites, with eight or fewer oxygen atoms in a window, exclude larger hydrocarbon molecules from accessing the inner cages; these hydrocarbons are otherwise able to access the micropores within medium-pore (10 oxygen) ZSM-5 or within

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Fig. 1 (A) Steady-state NOx conversion efficiency maps for state-of-the-art SCR catalysts, with 200 ppm NO and 200 ppm NH3. The vanadia-based catalyst was aged at a lower temperature due to its poor hydrothermal stability, compared to the zeolite-based catalysts. (B) Total NH3 storage on the state-of-the-art SCR catalysts, measured with 200 ppm NH3, 10% O2, 8% CO2, 7% H2O. Reprinted from Kamasamudram et al., Catal. Today 2010, 151, 212–222, Copyright (2010), with permission from Elsevier.

large-pore (12 oxygen) zeolites like BEA or FAU. Zones reported the synthesis of the small-pore zeolite SSZ-13 in 1985 (25) and Lok the isomorphous silicoaluminophosphate SAPO-34 in 1984 (26). Although the acid form of these materials was originally explored as catalysts for methanol to olefins (27), in 2009 Bull et al. (28) reported that Cu-exchanged

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SSZ-13 had comparable NOx SCR conversion and greater hydrothermal stability and resistance to hydrocarbon poisoning than medium-pore zeolites. As shown in Fig. 2, Kwak et al. found that the SCR performance of Cu-SSZ-13 hydrothermally aged (HTA) in 10% H2O at 800°C for 10 h was superior to that of other similarly aged medium- and small-pore zeolites A

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(22,29). Shortly thereafter, Fickel et al. (30) reported that Cu-SAPO-34 had catalytic performance and hydrothermal aging resistance similar to Cu-SSZ-13. These features enabled the first on-road commercial applications of SCR to appear in the US in 2010, and today it is not uncommon to find diesel emission fluid (DEF), an aqueous solution of urea, advertised on highway billboards. The pace of this commercialization has been remarkable. A commercially viable diesel aftertreatment systems based on SCR must meet stringent particulate matter and NOx emissions targets (21). As illustrated in Fig. 3, allowable NOx and particulate matter (PM) emissions have steadily decreased in Europe and the United States over the last 10 years. As a result, in modern diesel applications the SCR catalyst is part of a multicomponent aftertreatment system. A typical configuration, from engineout inlet at far left to outlet as far right, is depicted in Fig. 4. An exhaust gas mixture containing a few hundred ppm of particulate matter, CO, uncombusted hydrocarbons, NOx (90% of which is NO), along with

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Fig. 4 Layout of a Ford DOC–SCR–DPF system. Reproduced from Beale et al., Chem. Soc. Rev. 2015, 44, 7371–7405 with permission of The Royal Society of Chemistry.

roughly 10% O2, 3% H2O, 12% CO2, and balance N2 first passes through a platinum-based diesel oxidation catalyst (DOC) that catalyzes oxidation of residual CO, hydrocarbons and a fraction of particulate matter to CO2 and H2O as well as oxidation of some NO to NO2 (31). Urea, which thermally decomposes into NH3 (9), is injected and mixed into the exhaust stream after the DOC and before the SCR catalyst. A subsequent “NH3 slip” catalyst (not pictured), typically platinum-based, catalyzes excess NH3 oxidation. Finally, the remaining particulate matter is captured and oxidized in the diesel particulate filter (DPF) (32). Thus, the SCR catalyst must achieve high NOx conversion with as little NH3 slip and oxidation as possible over the wide range of temperatures and gas compositions characteristic of engine and emissions system operation while tolerating the occasional hightemperature excursions necessary to accommodate regeneration of the DPF. Excess NH3 storage remains (21) particularly troublesome for mobile applications because the size of the NH3 slip catalyst is limited by space constraints (10). Despite this drawback, the resistance of Cu-SSZ-13 and Cu-SAPO-34 to deactivation after hydrothermal aging proved to be sufficient to receive considerable commercial and scientific attention.

1.3 Scope and Structure of This Review The Cu-SSZ-13 and Cu-SAPO-34 literature grew rapidly from 2010 to 2016. For a review of SCR technology up until this point, the reader is directed to Brandenberger et al. (9). More recent and comprehensive reviews of the molecular sieve catalysts, their characterization and performance, and their application to catalytic NOx transformations include Gao et al. (33), Chen (7), Beale et al. (34), and Zhang et al. (8). A high-level overview of publications from the last 6 years reveals the evolution in both the objectives and the approaches to Cu-SSZ-13 and

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Cu-SAPO-34 research. The earliest work focused on comparing the performance of small-pore materials against other metal-exchanged zeolites and vanadia/titania catalysts. Subsequent work attempted to locate the exchanged Cu ions within the molecular sieve cages, typically in materials that have been treated at high temperatures under oxidative conditions (i.e., calcined) to anneal the Cu locations but that have not been exposed to SCR conditions. A large variety of chemical (e.g., CO chemisorption or H2 temperatureprogrammed reduction), spectroscopic (vibrational, X-ray), and computational (density functional theory, DFT) characterizations of Cu-CHA materials have subsequently been used. The most recent investigations interrogate catalyst state at reaction conditions (“in situ”), sometimes while simultaneously measuring catalyst rates (“operando”). While this work has demonstrated that isolated, Cu-exchanged ions are largely responsible for catalytic function and revealed numerous aspects of the materials and their mechanisms of function, several uncertainties remain. A complete picture that integrates all aspects of the structure–function relationship across the range of application conditions and material lifetime remains to be achieved. In this contribution, we review the current Cu-SSZ-13 and Cu-SAPO-34 literature from the perspective of the catalytic science of NOx SCR, highlighting the current state of knowledge regarding the relationship between microscopic structure and composition and catalytic function. As we will highlight later, the microscopic structure is a function not only of the gross compositional variables (e.g., Si/Al and Cu/Al ratios) but also of synthetic procedures, pretreatments, and the conditions of observation. As evidence, Fig. 5 (33) shows the many different locations accessible to exchanged Cu in SSZ-13 depending on material and circumstances of observation. Thus, in this review, we emphasize the preparation, characterization, performance, modeling, and analysis of well-defined catalytic materials under well-defined reaction conditions. We begin by comparing the primary synthesis routes to Cu-SSZ-13 and contrast them with the less well defined synthesis routes to Cu-SAPO-34, emphasizing in particular the relationship between synthesis, framework composition, and Cu exchange sites. We consider the results of ex situ characterizations and their relationship to catalyst preparation and microscopic structure. We next relate these results to in situ spectroscopic observations and observed reaction kinetics. Finally, we consider the current state of understanding of the catalytic mechanism. Throughout the review, we highlight the interactions between experimental observation and molecular models; in doing so, we identify scientific

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Fig. 5 Possible cation positions in the CHA structure. Note that position I is inside the double-6-membered rings and positions II–IV are within the large CHA cavities. Adapted from Almeida, R. K.; Gómez-Hortig€ uela, L.; Pinar, A. B.; Perez-Pariente, J. Microporous Mesoporous Mater. 2016, 232, 218–226. Reproduced with permission from Gao, F.; Kwak, J. H.; Szanyi, J.; Peden, C. H. F. Top. Catal. 2013, 56 (15), 1441–1459.

knowledge available to improve current materials and the knowledge gaps that remain to be addressed.

2. SYNTHESIS OF ZEOLITES 2.1 Synthetic Control of Al Distribution in Zeolites Zeolites are crystalline oxides composed of corner-sharing SiO4 tetrahedra that can be arranged to create molecular sieve frameworks that vary in crystalline topology and micropore size and interconnectivity (35). They are strictly aluminosilicates, a class of molecular sieves that contain a fraction of lattice silicon (Si4+) atoms substituted with aluminum atoms (Al3+), which generate anionic charges that are compensated by extralattice cations (e.g., H+, Na+). Fig. 5 shows the topology of the chabazite (CHA) framework. Nodes represent the locations of the lattice Si4+ or Al3+ ions, lines indicate bridging framework oxygen, and spheres indicate various potential extralattice cation locations. Synthetic zeolites are typically crystallized in the presence of either hydroxide (OH) or fluoride (F), which act as mineralizing agents that

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facilitate the disassembly and reassembly of silica during crystallization. The specific zeolite topology formed in the laboratory depends, in part, on the inorganic and organic compounds that are present during crystallization, referred to as structure-directing agents (SDAs), that guide the organization of embryonic building units into a certain crystal structure (36–38). SDAs can also contain cationic charge, such as those present in inorganic alkali or at quaternized ammonium centers in organic molecules. The total number of Al atoms incorporated into the zeolite framework (Si/Al ratio) depends on the number of cationic SDAs that are incorporated within the crystal product, because they direct the siting of anionic [AlO4] tetrahedra within the crystallizing lattice (38,39). Cationic charges in SDAs of different size and geometry, however, can reside within void spaces of different size and shape present within a given microporous framework. This enables directing the siting of Al atoms at specific T-sites, as demonstrated clearly in the case of ferrierite (FER; Si/Al ¼ 10–20) zeolites wherein Al atoms can be selectively incorporated at T-sites located within 8-membered ring (8-MR) or 10-MR structures by using organic SDAs that differ in size (40–43). In the case of ZSM-5 zeolites (MFI framework), tetrapropylammonium (TPA+) has been used as an SDA to preferentially incorporate Al at T-sites at 10-MR channel intersections, while the mixtures of TPA+ and Na+ have been used to position Al atoms more uniformly between 10-MR channels and their intersections (44). As a result, the specific synthesis method used to prepare a given zeolite will not only influence its framework elemental composition (Si/Al ratio) but also the arrangement and distribution of Al atoms throughout the lattice and, in turn, the number and types of extralattice cationic species that can be exchanged onto the zeolite. L€ owenstein’s rule states that two neighboring T-site positions cannot be occupied by Al atoms, and this rule is followed under typical laboratory synthetic conditions (45). Here, we use functional definitions of the local Al arrangement, in which 2 Al atoms separated by 1 or 2 Si atoms (Al–O(–Si–O)x–Al, x ¼ 1, 2) form a configuration that can exchange a divalent complex (“paired Al”), and Al atoms separated by 3 or more Si atoms (Al–O(–Si–O)x–Al, x  3) can only exchange monovalent complexes (“isolated Al”) (46). Paired Al sites can be quantified by saturation-exchange capacities of a divalent cation, such as Co2+ (46,47), as long as characterization data (e.g., Co2+ d–d transition: 19,000 cm1 in UV–visible (UV–vis) spectroscopy) and the expected 1:2 M2+:H+ exchange stoichiometry (selective NH3 titration of residual H+ sites) verifies sole exchange of isolated divalent cations (48). The number of paired Al sites in ZSM-5 zeolites as quantified by Co2+ titration has been shown to depend

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Isolated Al

Paired Al Si Al

Al

Si

Si Si Si

Al

Si

Si

Al

Si Si

Al

Si

Si Si

Si

Fig. 6 Different arrangements of framework Al atoms between paired and isolated configurations in a six-membered ring (6-MR). Adapted from Di Iorio, J. R.; Gounder, R. Chem Mater 2016, 28, 2236–2247.

on the specific synthesis conditions and SDAs (TPA+, Na+) used to crystallize them (49–51), but do not appear to depend systematically on a single parameter (46). This complexity has precluded the development of correlations that predict how synthesis procedures directly influence Al arrangement. In the CHA topology, isolated and paired Al sites are defined by 6-MR building units containing 1 or 2 Al atoms, respectively (Fig. 6). When extralattice ions are exchanged onto a given CHA zeolite, monovalent complexes will exchange onto isolated Al configurations and divalent complexes will exchange onto paired Al configurations in order to preserve framework charge-balancing requirements. Among the earlier literature, studies of the nature of Cu active sites for SCR catalysis considered primarily the consequences of Cu-CHA bulk elemental composition (Si/Al and Cu/Al ratio) on Cu cation speciation and resulted in inconsistent and contradictory findings. As we discuss next, accounting for the consequences of the different synthesis methods used on the resulting Al distribution can reconcile seemingly contradictory Cu characterization and speciation data in earlier studies of Cu-CHA of similar elemental composition.

2.2 Synthesis of SSZ-13 Zeolites The CHA framework (Fig. 7) is composed of a single, crystallographically unique T-site and its unit cell contains 36 tetrahedral centers (T-atoms) connected by 4-, 6-, and 8-MR units. SSZ-13 specifically refers to molecular sieve materials of the CHA topology that contain an aluminosilicate

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Fig. 7 Left: Side view of the chabazite cage. Right: HSE06-optimized structures of (A and B) dehydrated oxidized and reduced Cu sites and (C) hydrated oxidized sites. Label indicates the location of Cu ion within the chabazite cage. Reproduced with permission from Paolucci, C.; Parekh, A. A.; Khurana, I.; Di Iorio, J. R.; Li, H.; Albarracin Caballero, J. D.; Shih, A. J.; Anggara, T.; Delgass, W. N.; Miller, J. T. J. Am. Chem. Soc. 2016, 138 (18), 6028–6048.

composition. A three-dimensional pore system formed by stacking of a hexagonal array of planar 6-MR connected in an AABBCC-type stacking scheme that form hexagonal prisms (double 6-MR). The stacking of these double 6-MR units forms large chabazite cages (diameter: 0.73 nm), where access into the cage is limited by symmetric 8-MR windows (diameter: 0.38 nm). As a result of containing a single T-site, only the arrangement of framework Al atoms between isolated and paired sites (Fig. 6) will influence the exchange of extraframework cations between monovalent and divalent cations. Low-silica SSZ-13 zeolites (Si/Al < 8) are prepared using low-silica FAU zeolites (Si/Al < 5) as the Al source, which share the double 6-MR as a common structural building unit and can be converted into the CHA topology when recrystallization occurs in the presence of N,N,Ntrimethyl-1-admantylammonium (TMAda+) cations (52,53), as each of these organic cations can occupy the void space within a single CHA cage (52). Low-silica SSZ-13 can also be prepared using alternative organic SDAs (e.g., tetraethylammonium hydroxide) using high-silica FAU zeolites (Si/Al ¼ 10) as the Al source (54), and in the absence of organic SDAs using FAU (55) and amorphous aluminosilicate precursors (56) in the presence of alkali cations. Low-silica SSZ-13 zeolites synthesized from FAU precursors, using TMAda+, contain a large percentage of Al pairs (up to 45% at Si/Al ¼ 4.5) that exchange divalent cations (e.g., Cu2+, Co2+) (48,57,58), as would be expected from a high density of Al atoms present in the lattice. Low-silica SSZ-13 samples prepared by FAU to CHA conversion routes

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tend to contain extraframework Al (15% (57,59)) detectable in 27Al MAS NMR spectra (lines at 0 ppm) and, in turn, fewer protons than the number of Al atoms present (48,57–59) and that vary widely (H +/Al ¼ 0.45–0.85) even among replicate syntheses (58). High-silica SSZ-13 zeolites (Si/Al > 10) are typically synthesized via direct hydrothermal methods using molecular Al (e.g., Al(OH)3, Al2O3, NaAlO2) and Si (e.g., fumed silica, colloidal silica, tetraethyl orthosilicate) precursors in the presence of TMAda+ and Na+ in hydroxide media (Si/Al ¼ 10–65) (60), in the presence of only TMAda+ in hydroxide media (Si/Al ¼ 15–30) (48), or in the presence of only TMAda+ in fluoride media (Si/Al ¼ 10–∞) (61). The synthesis of high-silica SSZ-13 has also been reported using dealuminated FAU zeolites in the presence of benzyltrimethylammonium cations (BTMA+) (62,63). Hydroxide-mediated synthesis of high-silica SSZ-13 (with and without Na+) produces zeolites with high percentages of framework Al atoms (>90%) detected by 27Al MAS NMR (lines at 60 ppm) and corresponding H+/Al counts that are near unity across a wide range of Si/Al ratios (Si/Al ¼ 10–65) (48,59). Attempts to crystallize SSZ-13 at Si/Al < 10 in hydroxide media using only TMAda+ cations result in amorphous phases, presumably due to high concentrations of AlO4  tetrahedra in precursor solutions that generate anionic charge densities greater than what TMAda+ cations alone can stabilize (48). Similarly, synthesis solutions with Si/Al > 100 result in amorphous phases apparently because crystallization becomes hindered by the formation of anionic framework defects, which are required to balance excess cationic charges of occluded cationic SDAs that do not balance framework Al (48,64). Similar to hydroxide-mediated crystallization of high-silica SSZ-13, fluoride-mediated crystallization also creates SSZ-13 with predominantly framework Al atoms (61) and does not crystallize SSZ-13 zeolites at Si/Al < 10 for similar reasons, but can crystallize SSZ-13 at very high Si/Al ratios (>60) because fluoride anions compensate excess cationic charges introduced by the SDA and obviate the formation of framework defects (65). Synthesis of high-silica SSZ-13 using equimolar amounts of Na+ and TMAda+ in hydroxide media (Na+/TMAda+ ¼ 1), across a range of Si/Al ratios (Si/Al ¼ 10–55), results in a density of paired Al sites that is consistent with that predicted for a random Al distribution subject to L€ owenstein’s rule (66), as characterized by Co2+ (48) and Cu2+ (48,67) titration of paired Al sites (Fig. 8). SSZ-13 zeolites (Si/Al ¼ 15–30) crystallized in hydroxide media containing only TMAda+ are unable to exchange divalent Co2+ cations, but can exchange monovalent cations (e.g., Na+, NH4 + ) in an amount

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0.25

Experimental M2+/Al

0.20

0.15

0.10

0.05

0.00 0.00

0.05

0.10

0.15

Theoretical

0.20

0.25

M2+/Al

Fig. 8 Co2+ (squares) and Cu2+ (circles) saturation values on H-SSZ-13 samples of Si/ Al ¼ 5, 15, and 25 synthesized in hydroxide media (equimolar Na+ and TMAda+) plotted against the predicted maximum M2+/Al assuming a random Al distribution. Open circles are Cu2+ saturation values on H-SSZ-13 samples synthesized by the same route with Si/ Al ¼ 4–45 reported from Deimund et al. (67) The dashed line represents a parity line. The data in this figure are originally reported in Di Iorio, J. R.; Gounder, R. Chem Mater 2016, 28, 2236–2247.

equimolar to the total Al content, indicating that these zeolites contain exclusively isolated framework Al atoms (48). Addition of increasing amounts of Na+ cations to the synthesis solution in hydroxide media (at constant Si/Al ratio, OH/Si, and (Na++TMAda+)/Si) resulted in SSZ-13 zeolites with similar bulk elemental composition (Si/Al ¼ 15), but with fractions of paired Al sites that increased systematically with increasing Na+/TMAda+ ratio in the synthesis solution (0–18% paired Al; Na+/TMAda+ ¼ 0–1) (48). Additionally, the number of paired Al in SSZ-13 zeolites (Si/Al ¼ 15) increased linearly with the Na+/TMAda+ ratio of the crystalline zeolite product, suggesting that the number of Al pairs (i.e., framework anionic charge density) correlates with the Na+/TMAda+ ratio present within the solid zeolite (i.e., extraframework cationic charge density), which is determined by the Na+/TMAda+ ratio in the synthesis solution. These results indicate that the fraction of paired Al sites, which determines the anionic charge density of the zeolite framework, in high-silica SSZ-13 samples

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synthesized in hydroxide media is controlled by the relative ratio of Na+ and TMAda+ in the synthesis solution, a proxy for the cationic charge density in the precursor solution. In fluoride media, the Gounder group has synthesized SSZ-13 zeolites at various Si/Al ratios (Si/Al ¼ 15–60) to investigate the effect of the counteranion used together with TMAda+ as the organic SDA cation, on the Al distribution in SSZ-13. Similar to TMAda+ in hydroxide media, TMAda+ in fluoride media crystallized SSZ-13 with only isolated Al atoms, as measured by Co2+ titration (unpublished results). These findings are consistent with the ability of TMAda+ cations, which contain a single, isolated quaternary ammonium cation, to isolate single Al atoms in the CHA framework (48). Additionally, the presence of fluoride anions enables crystallization of SSZ-13 in the presence of only TMAda+ at higher Si/Al ratios (Si/ Al > 20; H+/Al > 0.9) than are accessible to hydroxide media because incorporation of F anions avoids formation of framework defects in the crystallized product. These results show that subtle changes in synthetic parameters can confer dramatic but predictable changes in the spatial distribution of framework Al atoms, even for the same zeolite topology and bulk composition.

2.3 Synthesis of SAPO-34 Molecular Sieves The silicoaluminophosphate (SAPO) of the CHA topology, SAPO-34, has also been widely studied for NOx SCR. The basic material is an aluminophosphate (AlPO) framework that contains alternating Al3+ and P5+ atoms in tetrahedral locations, and SAPO materials are formed from Si4+ substitution for P5+ in the framework to generate the anionic framework charge, analogous to Al3+ substitution for Si4+ in zeolites. Pure-silica CHA molecular sieves contain hydrophobic micropores that result from the presence of only nonpolar siloxane (Si–O–Si) linkages (38). This causes significantly lower water adsorption (1 mol kg1 at P/P0 ¼ 0.2) (68) in puresilica CHA than in SAPO-34 at similar relative pressures (16 mol kg1 at P/P0 ¼ 0.2) (68), which is comprised of a polar framework that imparts hydrophilic character (26). With increasing Al content, SSZ-13 zeolites show increased water adsorption uptakes at similar relative pressures (1–6 mmol g1 at P/P0 ¼ 0.2 for Si/Al ¼ 10–∞) (69), and approach values characteristic of SAPO-34 in the limit of very aluminum-rich CHA zeolites (Si/Al  2) (70). SAPO-34 can be synthesized using a variety of organic SDAs (e.g., tetraethylammonium hydroxide (71,72), morpholine (71–75),

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trimethylamine (71,72,75–77), diethylamine (71,72,78,79), dipropylamine (71), piperidine (80)) and even through “one-pot” syntheses using Cu-tetraethylenepentamine (81) in acidic media due to the use of concentrated phosphoric acid as the phosphorus source. The distribution of Si atoms throughout the SAPO frameworks also influences their ion-exchange properties, because isolated Si atoms generate framework anionic charges, but proximal Si atoms that aggregate in the framework form charge-neutral silica islands. Si isolation appears best controlled by choosing a neutral SDA (e.g., morpholine (71–75), trimethylamine (71,72,75–77), diethylamine (71,72,78,79), and dipropylamine (71)), which produce SAPO-34 materials with sharp NMR lines typical of isolated Si atoms with 4 Al neighbors, Si(4 Al), characterized by 29 Si MAS NMR (δ ¼ 95 ppm) and minor contributions from NMR lines reflecting Si–O–Si linkages (e.g., 29Si MAS NMR lines at δ ¼ 100 ppm, Si(3 Al); 105 ppm, Si(2 Al); 110 ppm, Si(1 Al); 115 ppm, Si(0 Al)). In contrast, charged SDAs such as TEAOH (71,72) and Cu-TEPA (81) lead to 29Si MAS NMR lines for Si–O–Si islanding (e.g., δ ¼ 100 ppm, Si(3 Al); 105 ppm, Si(2 Al); 110 ppm, Si(1 Al); 115 ppm, Si(0 Al)) that are more pronounced than those for isolated Si atoms (Si(4 Al), δ ¼ 95 ppm) (72). Differences in framework Si arrangements for SAPO-34 synthesized using the same organic SDA (TEA) and similar synthesis procedures have been reported, with either predominately isolated Si sites (71,76,77) or Si islands (72,75). Moreover, framework Si undergoes rearrangement to form Si islands in the presence of liquid water (75), which inhibits methods for aqueous phase ion-exchange of hydrated cations, such as Cu2+ (81). The influence of different organic SDAs on the formation of paired and isolated Si sites in SAPO-34 is not well understood, and quantification of such sites is challenging because direct titrations through aqueous ion exchange will be convoluted by redistribution of framework Si atoms.

2.4 Copper Exchange in Zeolite and SAPO Frameworks The distribution of Al atoms between isolated and paired Al sites in SSZ-13 will also influence the speciation of extraframework copper cations because divalent Cu2+ exchanges solely at paired Al sites (indicated here as Z2Cu), while [CuOH]+ exchanges at isolated Al sites (indicated here as ZCuOH). SSZ-13 synthesized using only TMAda+ contain no paired Al sites but still exchanges [CuOH]+ species (48) that catalyze NOx SCR (58,82). These

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materials contain both isolated Cu2+ and [CuOH]+ at high Cu loadings, which can convolute interpretations of site-specific characteristics inferred from bulk characterization techniques. Additionally, assuming a random distribution of Al on the framework lattice, the fraction of paired Al sites decreases as the Si/Al ratio increases, resulting in materials at high Si/Al ratios that contain very low quantities of paired Al (48,58). As a result, both Cu2+ (58,59,83,84) cations that exchange at paired Al sites and [CuOH]+ (58,59,85,86) complexes that exchange at isolated Al sites have been implicated as the active site for NOx SCR in high-silica Cu-SSZ-13 zeolites. Aqueous phase ion-exchange of cations into zeolites has been widely used to remove pollutants from water (87) and was used to prepare the first copper-exchanged zeolites for NOx SCR (15,16,88,89). The total copper content in a zeolite catalyst can be controlled during ion exchange by changing the exchange solution molarity, independent of Cu salt used (e.g., Cu(NO3)2, Cu(CH3CO2)2, CuSO4, CuCl2), and results in isolated Cu species at low copper loadings in SSZ-13 and ZSM-5 (Cu wt% < 3%) (15,57–59,88). The speciation of copper cations from aqueous ion exchanges depends on both the temperature and pH of the ion-exchange solution (90) and results in the formation of Cu oxides at high Cu molarities that have been adjusted to higher pH values (>5) in the presence of amines (66,88,91). Additionally, Cu oxides have also been observed in low-silica SSZ-13 (Si/Al ¼ 4.5) at loadings above 4 wt% Cu (Cu/Al ¼ 0.21) after ion-exchange in pH 5 solution through addition of NH4OH (57,66,91). As mentioned previously, aqueous phase ion exchange of Cu into SAPO-34 presents many challenges due to the hydrophilicity of the framework and apparent restructuring of isolated Si atoms under aqueous conditions (75,81). Ion exchange using aqueous copper solutions provides a simple and effective method to exchange controlled amounts of copper into zeolites, but care needs to be taken when adapting procedures because different zeolite frameworks preferentially stabilize different copper complexes. Alternatively, Cu-exchanged zeolites can be prepared by means of solidstate ion exchange (SSIE) where the zeolite, in its H-form, is physically mixed with a bulk Cu salt (e.g., CuO, CuCl2, CuF2, Cu3(PO4)2) or Cu metal powder and then heated (>900K) to allow for migration of copper species into the zeolite pores (92,93). The migration of Cu species into the pores is proposed to occur by oxidation of the Cu precursor to CuO followed by adsorption into the pores and acid-catalyzed dehydration at a H+ site to produce an isolated Cu2+ site (92–96). A major benefit of SSIE is that it allows for copper ion exchange into SAPO-34 frameworks without

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disruption of the framework Si structure (94) and it can enable faster preparation of Cu-SSZ-13 catalysts with similar properties to those prepared using aqueous ion exchange (95–97). SSIE provides a convenient means for introducing isolated Cu2+ species into zeolites and SAPOs and removes the need for successive washing and recovery steps, but due to the intense temperatures (>900K) required to facilitate SSIE, this technique requires thermally stable frameworks. More recently, ammonia-assisted exchange of Cu, from CuO precursors, into MFI, BEA, and SSZ-13 zeolites was reported at temperatures 10) by including TMAda+ during synthesis (103). As-synthesized Cu-TEPA SSZ-13 contains up to 10 wt% Cu, some of which must be removed through postsynthetic washing steps with either NH4NO3 (99,100) or HNO3 (102) to prevent the formation of bulk CuO after combustion of the organic complex. (101) Highly dispersed copper species are reported to exist in as-synthesized materials (99,100,102), but isolated Cu species are obtained only after removal of excess Cu followed by combustion of the organic TEPA complex. The use of Cu-TEPA as an SDA, while less expensive than TMAda+ (99), restricts the accessible composition range (Si/Al and Cu/Al ratio) and, as a result, limits the control over the Al distribution and Cu density when compared to first hydrothermally crystallizing SSZ-13 using

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Table 1 Summary of Relationships Between Zeolite Synthesis Route, Si/Al Ratios, and Al Distribution Synthesis Route Accessible Si/Al 6-MR Paired Al Extra-Framework Al

FAU conversion Hydroxide mediated Fluoride mediated

d

b

10) can be synthesized in hydroxide media using mixtures of Na+ and TMAda+ that allow for the fraction of paired Al to be controlled between complete Al isolation (0% paired Al) and an apparent random Al distribution (18% paired Al). Fluoride media can also be used to synthesize high-silica SSZ-13 with only TMAda+ over a wide Si/Al range (Si/Al ¼ 10–∞) and yields exclusively isolated Al sites, similar to SSZ-13 crystallized using only TMAda+ in hydroxide media. Isolated Cu2+ cations can be exchanged through either aqueous phase or solid-state ion-exchange methods, whose speciation depends on the distribution of framework Al atoms. As a result, care must be taken to carefully characterize the Al arrangement in SSZ-13 zeolites, because Al distribution depends on the specific synthesis and crystallization conditions and determines the speciation of extraframework Cu2+ cations. Table 1 summarizes the consequences of synthesis route on final amount and location of Al ions in the SSZ-13 zeolites.

3. EX SITU CHARACTERIZATION OF CU-SSZ-13 AND CU-SAPO-34 Solid catalysts are often interrogated under noncatalytic, or ex situ, conditions to make inferences about reactivity. In this section, we review the literature of ex situ Cu-CHA characterization. Following Section 2, we emphasize the relationship between ex situ observations and microscopic

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structure as determined by the synthesis conditions and catalyst composition. Over the last several years, an extensive literature has developed around the ex situ characterization of Cu-SSZ-13 and SAPO-34 materials under five types of conditions: (1) ambient temperature and exposed to atmosphere, (2) high-temperature oxidative conditions, (3) high-temperature inert or vacuum conditions, (4) high-temperature reductive (H2) conditions, and (5) exposure to NO, a probe molecule, and SCR reactant. Ammonia, which plays many different roles in SCR and in Cu coordination chemistry, is discussed separately in Section 4.

3.1 DFT-Based Analysis of Cu Speciation First-principles quantum mechanical molecular models have been applied to Cu ions exchanged within zeolites for more than 20 years (106–114) and in principle are capable of predicting the preferred locations, coordination environments, and spectroscopies of the exchanged species. Historically, the most common modeling approach was to excise some small set of T-sites from a zeolite lattice (historically ZSM-5) as a discrete cluster and terminate either by hydrogen atoms (107,108,110–116) or by embedding into a larger classical model of the zeolite lattice (117–119). Cluster models have the advantages that they are amenable to the most powerful and accurate first-principles methods (often termed “ab initio,” although conventional usage varies from community to community), that they are well suited to studying the details of local chemical bonding, and that computational cost is not tied to the unit cell size. Over the last 10 years, plane wave, supercell calculations began to appear for Cu-SSZ-13. These models explicitly treat the fully periodic zeolite framework, providing the ability to vary Si/Al and Cu/Al ratios, to compare Al sites, and to capture the full three-dimensional influence of the framework on Cu siting and mobility (120–122). The fully periodic models are typically restricted to a DFT description, which is more efficient but not as uniformly accurate as the ab initio methods. The small size of the chabazite unit cell allows a suite of DFT models to be employed and compared, from the conventional generalized gradient approximation (GGA) in its various incarnations (PW91, PBE, BEEF), to hybrid methods that incorporate fractions of exact electron exchange (B3LYP, HSE06, PBE0), to methods that additionally incorporate corrections for long-range dispersion that are particularly important for weak interactions and large molecules.

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As noted earlier, every T-site in the CHA framework is connected to the same symmetry-equivalent set of rings. A Cu+ ion charge compensating a single Al, which we refer to as ZCu, is computed to prefer to locate within the 6-MR, a site that is 50 kJ mol1 lower in energy than the adjacent 8-MR site (122). The same energy preference is computed for a Cu+ ion near a Si center in Cu-SAPO-34 (123). The Cu+ ion coordinates to two framework oxygen atoms (58,82,122,124), a coordination preference that persists in other environments. Greater structural diversity is possible with 2 Al located in the same ring, even within the constraints of L€ owenstein’s rule that is observed among laboratory-synthesized materials. L€ owenstein’s rule states that Al–Al pairs are separated by at least 1 Si T-site or at secondnearest-neighbor (2NN) separation. Bates et al. used a 24-T-site SSZ-13 supercell containing 5 Al to compare the energies of Cu2+ exchanged with various combinations of 2 Al. While not comprehensive, the reported results show that a ligand-free Cu2+ located adjacent to 2 Al within the 6-MR is 100 kJ mol1 lower in energy than comparable sites within a 4-MR or a 8-MR. (105) G€ oltl et al. compared the GGA energies of Cu2+ with Al pairs in a variety of configurations (Fig. 9) and found that the 6-MR site with either 2NN or 3NN Al is at least 50 kJ mol1 lower in energy than other combinations (125). Cu2+ in these sites is coordinated to four framework oxygen atoms in a distorted square-planar arrangement. We refer to a Cu2+ ion exchanged near 2 Al in a 6-MR as Z2Cu throughout this review. These site models have been widely used to study the structures and energetics of adsorption and reaction at Cu centers. For instance, a single H2O molecule is computed to adsorb to a Z2Cu site by displacing one of the four framework oxygen atoms, preserving the fourfold coordination of the Cu center (58). The H2O binding energy is computed as the difference in total energies of products and reactants: Z2 Cu + H2 O ! Z2 CuH2 O

(6)

DFT ΔE ¼ EZDFT  EZDFT  EH 2 CuH2 O 2 Cu 2O

(7)

Reported DFT-computed H2O adsorption energies to Cu sites in SSZ-13 are 70 to 90 kJ mol1 among commonly used functionals and on models of both exchanged Cu+ and Cu2+ (58,86,122,126). Values in this same range are reported for Cu exchanged into SAPO-34 (123). The structures and binding energies of successive adsorption of up to six H2O have been reported for models of Z2Cu and the corresponding oxidized form of Cu at a 1 Al site, ZCuOH (58). To seek low-energy structures, preliminary

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Fig. 9 Eight different cation positions for Cu(II) in SSZ-13. Each Cu cation, together with a proton, has to be associated with 2 Al atoms. For sites (A–E), both Al atoms are found in the same unit cell, while sites (F–H) correspond to the limit of large Al separation. In the atomistic pictures, Si is displayed in yellow, O in red, Al in silver, and cations in 1 blue. In the lower right-hand corner, the relative energetic stability (ΔECu(II) exc (kJ mol )) of these sites is displayed. Reprinted with permission from Go€lt et al., J. Phys. Chem. Lett. 2013, 4 (14), 2244–2249. Copyright 2013 American Chemical Society.

structures were annealed using ab initio molecular dynamics (AIMD) before performing geometry relaxations. Results show both Z2Cu and ZCuOH ions to prefer fourfold coordination, for H2O to progressively displace framework oxygen (Of) from the first-coordination sphere, for the fully hydrated species to be liberated from the framework, and for the successive

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Fig. 10 Cu positions (gray balls) visited during 90 ps of NVT AIMD at 298K. Fixed zeolite framework shown for ease of visualization; framework was unconstrained during dynamics. Inset illustrates discretization used to compute relative Cu mobilities. Reprinted with permission from Paolucci et al., J. Am. Chem. Soc. 2016, 138 (18), 6028–6048. Copyright 2016 American Chemical Society.

binding energies to be in a similar 70 to 90 kJ moll range across the series (58). By observing the volume of space visited by the Cu ions in the course of the AIMD simulation (Fig. 10), hydration was estimated to increase the mobility of Cu ions exchanged at 2 Al sites (Z2Cu) and 1 Al sites (ZCuOH) by up to 10-fold. Psofogiannakis et al. (127) used a semiempirical reactive force field (ReaxFF) to model the dynamics of Cu hydration in the SSZ-13 cage. Each 500 ps simulation started with 30 water molecules and a combination of exchanged Cu2+ and CuOH+ ions and was performed at temperatures ranging from 100K to 1100K. Cu ions were observed to detach from the zeolite framework and form hydrated Cu ions at long simulation time and/or elevated temperature. Partially hydrated Cu ions formed transient dimeric Cu complexes during the simulations. At 900K and 1100K, hydrated Cu atoms were observed to traverse through an 8-MR window into a new cage during the 500 ps simulations, indicating that this event is likely common at macroscopic timescales. Taken together, both semiempirical and AIMD indicate that H2O-solvated Cu ions are highly mobile within and between cages. Under wet and oxidizing conditions, exchanged Cu ions may exist in the 1 + or 2 + oxidation state and may have any number of associated H2O

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ligands, as well as OH, O2, or other species in the first-coordination sphere. The relative free energies of these combinations can be modeled using a firstprinciples free energy analysis. In this approach, the formation of a given ligand set, generally ZCuOxHy, is written relative to some reference species, in this case H2O and O2 (122): ZCu +

x ð2y  xÞ H2 O + O2 ! ZCuHx Oy 2 4

(8)

The structures and energies of a wide range of ZCuOxHy and Z2CuOxHy structures have been computed based on a combination of AIMD anneal and geometry optimization with an HSE06 hybrid-exchange functional (58). The free energy of formation of ZCuOxHy is well approximated as
form ST ΔGxform , y T ,ΔμO2 ,ΔμH2 O ¼ ΔEx , y  T ΔSx, y ðT Þ  x 1 y  ΔμH2 O  ΔμO2  ΔμO2 (9) 2 2  2 x 1 y ΔExform EH2 O  EO2  EO2 , y ¼ EZ* CuHx Oy  EZ* Cu  2 2 2 ST where ΔEform x,y is the reaction energy, ΔSx,y(T) is the difference in entropy between a bare and adsorbate-covered site at temperature T, and the △μ are changes in gas-phase chemical potentials relative to 0K values. In general, the finite-temperature ΔEform x,y is well approximated by the raw DFT energies. The entropy change requires a more careful treatment. Paolucci et al. compared the standard harmonic oscillator model for adsorbate entropy to a more accurate, AIMD-based computation for H2O or NH3 bound to a Z2Cu site (126). Consistent with the mobility seen in Fig. 10, the Cu-adsorbate combination is found to have a significantly higher entropy than the harmonic oscillator model would predict. Fig. 11 shows the results of a thermodynamic analysis based on an entropy correlation derived from the AIMD work. Plotted are formation free energies computed at 25 and 400°C, using △μ appropriate to 20% O2 and 2% H2O, and comparing 2 Al and 1 Al Cu sites. The lowest free energy species at 25°C are Cu2+ and CuOH+ ions fully hydrated by H2O and liberated from the framework. At the higher temperature, preference shifts to the framework-bound, H2Ofree Cu2+ and CuOH+. The first-principles thermodynamics models thus predicted Cu to be present in the 2 + oxidation state at wet and oxidizing conditions, for the degree of hydration to vary with temperature across the range of experimental interest, and for these Cu ions to retain fourfold oxygen coordination across these conditions.

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Fig. 11 Formation free energies (ΔGform) CuHxOy species at (left) 298K, 2% H2O, 20% O2, and at (right) 673K; 2% H2O, 20% O2 on the 2 Al (Z2Cu) and 1 Al (ZCu) sites. Common energy reference set through Eq. (6). Reprinted with permission from Paolucci et al., J. Am. Chem. Soc. 2016, 138 (18), 6028–6048. Copyright 2016 American Chemical Society.

Fig. 12 plots the same model results in the form of a phase diagram in temperature and O2 pressure at fixed 2% H2O. Points labeled 1 and 2 correspond to the two conditions shown in Fig. 11. Point 3 corresponds to 400°C and 1 ppm O2, a condition representative of a high-temperature purge in inert. The state of the Z2Cu site is predicted to be insensitive to the O2 pressure. In contrast, the lowest free energy state of the 1 Al site changes from framework-bound CuOH+ to framework-bound Cu+ with decrease in O2 pressure. Thus, the first-principles thermodynamics predicts that Cu exchanged at 1 Al but not 2 Al sites can autoreduce by loss of Cu-bound OH.

3.2 Ambient Conditions We next review spectroscopic characterizations of Cu-exchanged SSZ-13 and SAPO-34 materials exposed to ambient atmosphere. A suite of complementary characterization techniques indicate that hydrated Cu2+ is the

1273

1273

1173

1173

1 Al

2 Al

l

[ZH]/ [ZCu ]

1073

1073

973

973 l

[ZCu ]

873 Temperature (K)

Temperature (K)

873 773 3

673

[ZCulO2]

2 [ZCullOH]

573 473

−6

3

[Z2CullH2O]

−4

Z[Cu (OH)(H2O)3](H2O)2

1 −2

0

2

log10 (PO /P°) (atm.)

4

6

8

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273 −10

1 −8

−6

−4

2

[ZCullOH]

−2

0

2

4

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8

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log10 (PO /P°) (atm.) 2

1

Z[Cull(OH)(H2O)3](H2O)3

Z2[Cull(H2O)4](H2O)

Z2[Cull(H2O)4](H2O)2

373

ll

2

1

2

573

ll [ZCu (OH)(H2O)]

Z[Cull(OH)(H2O)3](H2O)3 −8

[Z2Cull]O2 673

473

l

[ZCu H2O]

373 273 −10

lI [Z2Cu ]

773

3

[ZCul]

Atmosphere

2

O2

3

He

1

Z2[Cull(H2O)4](H2O)2

2

3

[Z2CulI]

Fig. 12 Ex situ Cu speciation phase diagrams based on HSE06-TsvdW calculations on 1 Al (left) and 2 Al (right) Cu exchange sites. Regions indicate site composition that minimizes free energy at 2% H2O and given T and PO2 . Labeled on the phase diagram and illustrated below the phase diagram are minimum free energy species at (1) ambient (298K, 20% O2), (2) oxidizing (673K, 20% O2), and (3) inert (673 K, 106 atm. O2). Reprinted with permission from Paolucci et al., J. Am. Chem. Soc. 2016, 138 (18), 6028–6048. Copyright 2016 American Chemical Society.

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primary state of Cu at these conditions, regardless of the synthesis method used and the specific elemental composition. Thus, spectroscopic characterization indicates that both Cu2+ that compensate 2 Al and CuOH+ that compensate 1 Al are fully hydrated under ambient conditions. Additional features attributed to Cu2+ oxides appear at high Cu/Al and low Si/Al ratios. 3.2.1 X-Ray Absorption Spectroscopy X-ray absorption spectra (XAS) provide information about the Cu oxidation state and coordination. X-ray near-edge spectra (XANES) collected under ambient conditions of Cu-SSZ-13 synthesized via FAU conversion (Fig. 13) (66,128,129), synthesized using TMAda+ in fluoride medium (82,130), and hydroxide-mediated routes (58,84) at a variety of Si/Al and Cu/Al ratios contain a preedge feature near 8978 eV and edge energy near 8980 eV. These XANES features are consistent with those for an aqueous Cu2+ ion (105,129,131) and are the only features present in samples that contain only exchanged Cu cations (58,84,128,129,66). Samples at higher 1.8 Cu/Al = 0.16 1.6

[Cu(H2O)6]

Cu/Al = 0.31 Norm. absorption (a.u.)

1.4 Cu/Al = 1.6 1.2 1.0 Cu(II)O 0.8 0.6 0.4

Cu/Al = 0.09

0.2 0.0 8.98

Cu/Al = 0.04 8.98

8.99 8.99 9.00 Photon energy (keV)

9.00

Fig. 13 Ex situ XANES spectra of various hydrated Cu-SSZ-13 catalysts with Cu/Altot ranging from 0.04 to 1.6, measured under ambient conditions. XANES of bulk Cu(II)O and [Cu(H2O)6]2+ measured under ambient conditions have been added as references to do the linear combination XANES fitting. Reproduced with permission from Verma, A. A.; Bates, S. A.; Anggara, T.; Paolucci, C.; Parekh, A. A.; Kamasamudram, K.; Yezerets, A.; Miller, J. T.; Delgass, W. N.; Schneider, W. F. J. Catal. 2014, 312, 179–190.

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Cu loading (e.g., Si/Al ¼ 5, Cu:Al ¼ 0.31 and 1.6) show an additional shoulder in the XANES spectra near 8985 eV (Fig. 13) assigned to Cu oxides (66). Additional information about the first (and sometimes second) neighbor Cu-heavy atom distances and the Cu spatial location within the framework can be extracted from extended X-ray absorption fine structure (EXAFS). Under ambient conditions, EXAFS spectra of Cu-SSZ-13 samples synthesized via all three routes report Cu that are fourfold coordinated in the first shell and that lack persistent order in the second coordination shell (58,82). With the exception of samples containing bulk Cu oxides, XANES and EXAFS spectra of Cu-SSZ-13 are virtually identical to those of aqueous [Cu(H2O)6]2+ complexes (105) and of Cu-BEA and Cu-ZSM-5 at ambient conditions (130). While XAS spectra of Cu-SAPO-34 at ambient conditions have not been reported, first-principles DFT models predict similar XAS spectra for Cu of similar coordination environment in SSZ-13 and SAPO-34 (123).

3.2.2 UV–Vis Spectroscopy UV–vis spectroscopy probes electronic transitions between filled and empty valence states, and the d states of metal ions produce feature-rich spectra that can be diagnostic of specific chemical environments. The spectra collected between Cu/Al ¼ 0.02 and 0.2 shown in Fig. 14 are representative of the UV–vis spectra collected at ambient conditions of Cu-SSZ-13 synthesized via FAU conversion (66) and hydroxide- (83) and fluoride-mediated routes (130). All spectra exhibit a broad Cu2+ d–d transition from 6000 to 17,000 cm1 and a shoulder at 35,000 cm1 and main feature at 45,000 cm1 arising from ligand-to-metal charge transfer from Of to Cu d states (66,130,132). The position of the Cu2+ d–d transition is consistent with that of aqueous [Cu(H2O)6]2+ (122). The width of these absorption features likely reflects the dynamic Cu coordination environment at ambient temperature. The intensity of the Cu d–d transitions in a Si/Al ¼ 5 sample increase linearly with Cu/Al ratio up to 0.35, beyond which Cu oxide features appear in the spectrum (105). Those Cu oxide features appear as shoulders at 20,000 and 32,000 cm1 in Fig. 14. The UV–vis spectrum of Cu-SAPO-34 prepared via a direct synthetic route and exposed to ambient conditions (84) is similar to that of Cu-SSZ-13 (83). The d–d transition is red shifted by 2000 cm1 relative to the Cu-SSZ-13 spectra in Fig. 14 but is qualitatively similar to that of aqueous [Cu(H2O)6]2+. The Cu charge transfer region is qualitatively

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0.6

0.5

Kubelka Munk units

Cu/Al = 0.2

0.4

Cu/Al = 0.16 Cu/Al = 0.09 Cu/Al = 0.04

0.3

Cu/Al = 0.02 Cu/Al = 0

0.2

Cu/Al = 0.35

0.1

0 7000

17,000

27,000

37,000

47,000

Wavenumber (cm−1)

Fig. 14 Ex situ UV–vis–NIR spectra of series of hydrated Cu-SSZ-13 catalysts with Cu/ Altot atomic ratio ranging from 0 to 0.35. The peak centered at 12,500 cm1 is the contribution from the d–d transition of hydrated isolated Cu2+ ions. The peaks between 30,000 and 50,000 cm1 have been assigned to a combination of oxygen to Cu2+ charge transfer and bare zeolite absorption edge. Reproduced with permission from Verma, A. A.; Bates, S. A.; Anggara, T.; Paolucci, C.; Parekh, A. A.; Kamasamudram, K.; Yezerets, A.; Miller, J. T.; Delgass, W. N.; Schneider, W. F. J. Catal. 2014, 312, 179–190.

different from the corresponding SSZ-13 region, reflecting the different electronic properties of lattice oxide ions in the two materials. 3.2.3 Electron Paramagnetic Resonance Electron paramagnetic resonance (EPR) detects the magnetic environment experienced by unpaired electrons and thus is sensitive to the presence of Cu2+, its local coordination environment, and coupling to other magnetic species. EPR spectra of Cu-SSZ-13 zeolites with low Cu content or measured at subambient temperature exhibit a single feature (133,134) consistent with that of aqueous [Cu(H2O)6]2+ collected under similar conditions (92,135,136). At higher Cu content and/or higher temperature, additional features (Figs. 9–10) appear in the EPR spectrum that have been interpreted as evidence of dipolar interactions between the solvated and mobile Cu2+ ions (Fig. 15) (133,134). Similar spectra are measured under ambient conditions for Cu-SSZ-13 materials synthesized via FAU conversion (Si/Al ¼ 6, Cu/Al ¼ 0.12–0.45) or the fluoride-mediated route

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Fig. 15 EPR spectra of hydrated Cu-SSZ-13 samples measured at 294K. Samples with different ion-exchange levels are displayed with different colors. Reprinted from Kamasamudram et al., J. Catal. 2013, 300, 20–29, Copyright (2013), with permission from Elsevier.

(Si/Al ¼ 14, Cu/Al ¼ 0.44) (130,132). The total solvated Cu2+ content quantified using EPR is comparable to Cu loadings determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) (132). Some, but not all, of this Cu2+ loses intensity or becomes silent as the samples are dehydrated, evidence that there is more than one location and coordination environment of Cu2+ in the absence of water. Differences in EPR spectra among hydrated Cu-SSZ-13, Cu-BEA, and Cu-ZSM-5 under ambient conditions (130) have been interpreted as evidence of differences in mobility of the hydrated Cu species. The EPR spectra of Cu-SAPO-34 synthesized using either tetraethylammonium or morpholine as the SDA and subsequently exchanged with aqueous Cu2+ present features similar to Cu-SSZ-13 (75). The Cu-SAPO-34 samples exhibit an additional weak feature assigned to products of framework hydrolysis, consistent with structural damage to the SAPO-34 framework during wet ion exchange (137). In exploring Cu SSIE as a route that avoids SAPO framework hydrolysis, Gao et al. (94) prepared a series of Cu-SAPO-34 materials of Cu content from 0.30 to 2.31 wt%. After calcination, ambient exposure, and cooling to 120K, EPR spectra of all but the 2.31 wt% sample are similar to that of Cu-SSZ-13 synthesized via FAU

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conversion, indicating the SSIE method avoided framework damage and generated isolated Cu exchange sites. On the 2.3 wt% Cu-SAPO-34 material, an additional feature appears that has been assigned to persistent Cu oxides (66). This observation may reflect the incomplete conversion of CuO to isolated Cu ion sites during the SSIE procedure. Complete exchange may require a longer time or higher temperature thermal treatment (94). 3.2.4 Vibrational Spectroscopy Infrared spectroscopy probes the vibrational signatures of chemical functional groups. The zeolite framework has characteristic T–O–T vibrations near 800 and 1030 cm1. These modes are observed to intensify upon aqueous Cu exchange in ZSM-5 (138,139). In addition, aqueous Cu exchange to Cu/Al ¼ 0.4 of an SSZ-13 sample (Si/Al ¼ 6) prepared by FAU conversion generates a new band at 950 cm1, absent in H-SSZ-13 and attributed to a T–O–T mode perturbed by hydrated Cu2+ (129). The intensity of this band increases with dehydration of the sample, reflecting stronger interactions between Cu and the zeolite framework. The appearance of a single feature for Cu-perturbed T–O–T vibrations is consistent with a nearly homogeneous distribution of Cu. Features observed in the vibrational spectrum around 1630 cm1 on samples exposed to ambient conditions and that disappear upon heating to 400°C have been assigned to the H–O–H bending modes of Cu-bound H2O. Broad bands near 2200–3800 cm1 that persist up to 220°C have been assigned to various O–H stretches from H2O physically adsorbed to the zeolite (129), Brønsted acid sites (140,141), and silanol groups (142). 3.2.5 Summary of Ambient Conditions With the exception of persistent Cu oxides that form at high Cu densities, both experimental characterization and DFT models indicate that Cu exchanges as isolated Cu2+ and CuOH+ ions that are solvated by H2O when observed at ambient conditions. This solvation occurs regardless of the zeolite synthesis route or the Cu exchange method, and even of the zeolite framework topology (Table 2). EPR and AIMD show that these hydrated Cu ions are mobile, an intuitive finding considering that aqueous exchange is often used to introduce Cu2+ ions into the zeolite. At ambient, solvated Cu2+ and CuOH+ ions are spectroscopically indistinguishable from aqueous [Cu(H2O)6]2+and interact only weakly with the zeolite framework. As will be discussed in Sections 4 and 5, NH3 plays a similarly significant solvating role during SCR.

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Table 2 Summary of Cu-CHA Characterizations at Ambient Conditions Synthesis Route

Low Cu Density

High Cu Density

2+

2+

Hydrated Cu /Cu oxides

References

XAS (105,128,129,66) UV–vis (105,66) EPR (133,134) IR (129)

FAU conversion

Hydrated Cu

Hydroxide mediated

Hydrated Cu2+ Hydrated Cu2+

XAS (58,84) UV–vis (83,84)

Fluoride mediated

Hydrated Cu2+ Hydrated Cu2+

XAS (82,143) EPR (130,132) UV–vis (130,143) IR (82)

Cu-SAPO-34

Hydrated Cu2+ Hydrated Cu2+ CuAl2O4

UV–vis (84) EPR (75,94)

3.3 High-Temperature Oxidative Conditions Although H2O solvation at ambient conditions masks differences among Cu2+ ions exchanged at different locations within CHA, these differences become evident upon treatment under oxidative conditions at elevated temperature. In early work, XRD and XAS were used to characterize Cu sites in Cu-SSZ-13 materials synthesized via FAU conversion (Si/Al ¼ 4–9, Cu/Al ¼ 0.02–0.35) or via hydroxide-mediated routes (Si/Al ¼ 18, Cu/Al ¼ 0.5) and subsequently treated at 400°C in 20% O2 (53,57,83,105,128). XAS shows that Cu ions are present in a 2 + oxidation state and XRD that Cu ions are sited in or near the 6-MR. None of this work suggests the presence of more than one type of exchanged Cu. In 2012, Kwak et al. (144) reported the synthesis of a series of Cu-SSZ-13 samples (Si/Al ¼ 6, Cu/Al ¼ 0.2–0.5) prepared via FAU conversion and characterized with H2 TPR. The TPR profiles contain a single H2 consumption peak at low Cu loading, but two H2 consumption features at higher Cu content, which was interpreted as evidence for two distinct Cu2+ exchange sites of different reducibility. Subsequently, Giordanino et al. (143) used XAS to show that the majority of Cu2+ ions in a Cu-SSZ-13 sample prepared to Si/Al ¼ 14 and Cu/Al ¼ 0.44 via TMAda+ in fluoride media are present in the 8-MR rather than the 6-MR. Subsequent experimental and computational studies described in this section have clarified that the primary difference among these materials reflects the relative amount of Cu exchanged near single (ZCuOH) and paired Al (Z2Cu) sites.

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3.3.1 DFT Models The thermodynamic phase diagram model described earlier (Fig. 12) indicates that at 25°C and oxidative conditions, Cu exchanged near 1 Al and 2 Al sites will be present as H2O-free ions bound to framework oxygen, the former as threefold coordinated ZCuOH, with Cu oriented toward the 8-MR and the latter as fourfold coordinated Z2Cu in the 6-MR. Supercell DFT calculations have been used to predict the energetic preference for Cu2+ to exchange near 1 vs 2 Al. As noted earlier, Cu2+ exchanges near 2NN and 3NN Al in the 6-MR with nearly equal energy, while exchange at all other 2 Al site configurations is much higher in energy. To contrast exchange near 1 vs 2 Al, the energy of the hypothetical exchange reaction Z2 Cu + ZH + H2 O ! ZCuOH + 2ZH

(10)

was computed using a large supercell and HSE06-TSvdW functional to be +66 kJ mol1 (58). This energy difference appears as the offset in the free energies shown in Fig. 11. These results indicate an energetic preference for exchanged Cu to first occupy available 2 Al 6-MR sites as Cu2+ before populating remaining 1 Al sites as CuOH+. The number density and relative proportion of these two types of sites as a function of Si/Al ratio will also depend on the specific synthesis conditions used to crystallize the zeolite because Al incorporation in zeolites is not thermodynamically controlled. A useful limiting model assumes that Al are randomly distributed subject to L€ owenstein’s rule (145) and that exchanged Cu first populates all 6-MR 2 Al before populating 1 Al sites. The corresponding composition phase diagram as determined by numerical simulation is shown in Fig. 16 (105). The white line demarcates the Cu/Al ratio at which the 2 Al 6-MR sites saturate with Cu2+ and coloring indicates the fraction of total Cu present as CuOH+ sites. Over a large range of composition space, this model predicts both types of exchanged Cu to be present. As described in Section 2, SSZ13 zeolites synthesized by FAU conversion and by the hydroxide-mediated route (Na+/SDA¼ 1) are consistent with this random Al distribution model, while zeolites synthesized with only TMADa+ as the SDA are not. 3.3.2 X-Ray Absorption Spectroscopy XANES spectra collected under high-temperature oxidizing conditions (20–50% O2) on Cu-SSZ-13 zeolites containing a majority of exchanged either Z2Cu (58) or ZCuOH (58,82–85,143) have a shoulder near 8.985 keV that is absent in XANES spectra collected at ambient conditions (Fig. 17). This shoulder is located at slightly different positions in the Z2Cu and ZCuOH samples (Fig. 18) and has been attributed (131) to Cu in planar

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1.0

0.45

0.9

0.40

0.8 0.7

Cu:Al

0.35 0.30 0.25

0.6 0.5

0.20

0.4

0.15

0.3

0.10 0.05

0.2

0.00

CuOH:Cu total

0.50

0.1 5

Synthesized

10

15

20

25 Si:Al

30

35

40

45

0.0

Fig. 16 Predicted Cu site compositional phase diagram vs Si:Al and Cu:Al ratios. Color scale indicates predicted fraction of CuOH. White line demarcates transition from [Z2CuII]-only region to mixed [Z2CuII]/[ZCuIIOH] region. White circles indicated compositions of synthesized Cu-SSZ-13 samples. Reprinted with permission from Paolucci et al., J. Am. Chem. Soc. 2016, 138 (18), 6028–6048. Copyright 2016 American Chemical Society.

Fig. 17 Cu K-edge XANES spectra of Cu-SSZ-13 in its hydrated state (blue curve) collected at room temperature and after the activation process at 400°C, performed in both in vacuo conditions (green curve) and O2/He flux (black curve). The principal XANES features are labeled with the A–E letters, and the inset reports a magnification of the background-subtracted XANES spectra in the region where the weak Cu2+ 1s ! 3d peak (A) is observed. Reprinted with permission from Giordanino et al., J. Phys. Chem. Lett. 2014, 5 (9), 1552–1559. Copyright 2014 American Chemical Society.

Fig. 18 Left: XANES spectra collected on the 1 Al (top) and 2 Al (bottom) samples after treatment in 20% O2 at 673K (solid blue lines), He at 673K (dashed teal lines), and 3% H2 at 523 K (dot–dash red lines). Middle: Corresponding EXAFS spectra. Right: AIMD Cu–Si/O/Al RDFs for ZCuOH and ZCu (top) and Z2Cu (bottom). Insets show integrated RDFs. Reprinted with permission from Paolucci et al., J. Am. Chem. Soc. 2016, 138 (18), 6028–6048. Copyright 2016 American Chemical Society.

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or distorted planar conformations. XES spectra collected under the same conditions on a Cu-SSZ-13 sample synthesized by the hydroxide route (Si/Al ¼ 16, Cu/Al ¼ 0.2) have emission features at locations and intensities consistent with Cu bonded to O (85). XANES spectra of a Cu-SAPO-34 material (84) show the same shoulder at a position close to that ascribed to the ZCuOH species in Cu-SSZ-13 zeolites. The absence of any significant 8.983 keV edge in any of these materials is consistent with the majority presence of Cu2+ in materials treated at 400°C in >20% O2. Z2Cu are more readily distinguished from ZCuOH in the EXAFS (Fig. 18). EXAFS spectra following high-temperature oxidation of CuSSZ-13 synthesized by the hydroxide route ((Si/Al ¼ 18 and Cu/Al¼ 0.5) (83) and (Si/Al ¼ 15, Cu/Al ¼ 0.44) (58)) or using TMAda+ in fluoride medium (Si/Al ¼ 14, Cu/Al¼ 0.44) (82) are best fit to threefold coordinated Cu, as expected from the DFT models for ZCuOH in the 8-MR. The EXAFS spectra of samples containing a majority of ZCuOH or Z2Cu and collected at 400°C and 20% O2 are distinctly different (Fig. 18) (58). The EXAFS fits show a Cu coordination number of three for samples containing primarily ZCuOH and a Cu coordination of four for samples containing primarily Z2Cu. Further, the second shell scatter from neighbor Si or Al is of higher intensity in the more highly coordinated Z2Cu containing sample. This spectrum is consistent with the DFT-predicted structure of Z2Cu in a 6-MR. (58,125,126) EXAFS spectra of Cu-SSZ-13 synthesized by the FAU conversion method (Si/Al ¼ 9, Cu/Al ¼ 0.18) (128) or (Si/Al ¼ 18, Cu/Al¼ 0.11) (122) are similar to that of samples contain a majority Z2Cu sites, consistent with the predictions of Cu speciation in Fig. 16. Thus, EXAFS spectra collected during or after high-temperature oxidation are distinctly different for materials containing primarily ZCuOH or Z2Cu sites. 3.3.3 X-Ray Diffraction X-ray diffraction (XRD) averages over the ensemble of all ion locations within a crystal and thus is not well suited to distinguishing Z2Cu from ZCuOH sites. As a result, inferences about Cu speciation based on XRD may conflict with determinations from other methods. For example, the Cu ions in a Cu-SSZ-13 samples synthesized by the hydroxide route (Si/Al ¼ 18, Cu/Al ¼ 0.5) and treated at 500°C in 10% O2 have Cu-O bond lengths, determined by EXAFS, consistent with ZCuOH and thus in the 8-MR, while a Rietveld refinement of the XRD places the Cu in the 6-MR. (83) In contrast, Cu ions are determined to be threefold coordinated and outside of the 6-MR based on XRD of Cu-SSZ-13 (Si/Al ¼ 6, Cu/Al ¼ 0.35) zeolite synthesized by FAU conversion and treated at

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Fig. 19 Two-dimensional MEM EDDs of the 6-MR (top) and 8-MR (bottom), going from 0 e Å2 (blue) to 1 e Å2 (red), of (A) H-CHA, (B) Cu-CHA using only the framework as an MEM prior, and (C) Cu-CHA using the improved structure factors obtained from Rietveld analysis including the A0 and B sites in the structure model. (D) Ball-and-stick drawings of the 6R (top) and 8R (bottom) of Cu-CHA, showing the Cu2+ sites A0 (orange) and B (green), and the T-sites (Si/Al, white) oxygen (red). Reproduced with permission from Andersen, C. W.; Bremholm, M.; Vennestrøm, P. N. R.; Blichfeld, A. B.; Lundegaard, L. F.; Iversen, B. B. IUCrJ 2014, 1, 382–386.

435°C in air (53), while other methods would infer a large fraction of Z2Cu in this material. Cu ions have similarly been assigned to a location above the plane of the 6-MR based on XRD analysis of Cu-SAPO-34 (84). Fig. 19 (146) reports electron-density distributions (EDDs) obtained from the Rietveld refinement of (A) H-SSZ-13, (B) the framework ions of Cu-SSZ-13, and (C) the Cu and framework of a Si/Al ¼ 16, Cu/Al ¼ 0.45 SSZ-13 zeolite synthesized by TMAda+ in fluoride medium and dehydrated in air. By comparison of the EDDs to DFT-computed structures, 80% of exchanged Cu is attributed to 8-MR ZCuOH and 20% to Z2Cu in the 6-MR. In contrast, the unpublished Co2+ titration data described in Section 2 find no potential Z2Cu sites in a similarly prepared sample, indicating that further work is necessary to reconcile XRD results with other characterizations. 3.3.4 UV-Visible Spectroscopy The UV–vis spectral signatures of dry Z2Cu and ZCuOH remain controversial. The UV–vis spectrum of a Cu-SSZ-13 sample (Si/Al ¼ 9, Cu/Al ¼ 0.18) synthesized by the FAU conversion method and subsequently dehydrated in air at 500°C contains one broad feature between 10,000 and 20,000 cm1 and a ligand-to-metal transition between 25,000 and 50,000 cm1 (128). XRD and XAS characterizations of the same material are consistent with a majority of Z2Cu species (128). In contrast, the UV–vis spectrum of a Cu-SSZ-13 (Si/Al ¼ 15, Cu/Al ¼ 0.44) sample synthesized with TMAda+ in fluoride medium and subsequently oxidized at

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Fig. 20 Comparison between UV–vis–NIR spectra of hydrated (black line) and O2 activated (blue line) Cu-Zeolites. Photographs of the O2-activated powders are also reported in order to show the visual differences: Cu-SSZ-13, dark blue; Cu-ZSM-5 pale green. The color of O2-activated Cu-β (not reported) is as that of Cu-ZSM-5. Spectra are not normalized. Markers are also reported as an attempt of assignment to the observed bands: 2+ 2+ 2 2 2+ . ¼ isolated Cu ions, • ¼ [Cu–O–Cu] dimers, * ¼ [Cu2(μ-η :η -O2)] dimers. Reproduced from Giordanino et al., Dalton Trans. 2013, 42, 12741–12761, with permission of the Royal Society of Chemistry.

400°C in 50% O2 has four distinct features in the d–d region (Fig. 20) (130). These four spectral features have been assigned to the d–d transitions of Cu2+ exchanged at 2NN and 3NN Al sites in 6-MR sites on the basis of DFT– GGA cluster calculations (132), while other characterizations would suggest the presence of only ZCuOH sites. These authors suggest that ZCuOH sites may be UV–vis silent as a result of degeneracies arising from the trigonal coordination environment. While complete computed UV–vis spectra have not been reported, G€ oltl et al. used a supercell DFT model to show that d–d orbital splittings of Cu2+ computed using the GGA significantly underestimate those computed with the more reliable PBE0 and HSE03 hybrid functionals (124), calling GGA-based assignments into question. Dehydration and oxidation of the Si/Al ¼ 5, Cu/Al > 0.31 Cu-SSZ-13 sample (Fig. 20) also result in the appearance of higher energy transitions near 32,000 cm1 that are assigned to oxygen-bridged Cu dimers (130). These features are absent from the spectrum collected after exposure to ambient, suggesting that the oxides form during the dehydration process. 3.3.5 Electron Paramagnetic Resonance The EPR spectra shown in Fig. 21 are representative of those collected on Cu-SSZ-13 samples containing predominantly Z2Cu sites (Si/Al ¼ 6, Cu/Al ¼ 0.03, synthesized by FAU conversion) (134) or ZCuOH sites

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Fig. 21 Left: Experimental EPR spectra of Cu-CHA recorded at room temperature. (a) Hydrated (blue), (b) dehydrated ex situ at room temperature under helium flow (green), and (c) dehydrated at 400°C ex situ in an O2/He flow (black). Spectrum (c) was simulated (red) using the spin Hamiltonian given in the main text and the parameters in Table 1. It is shifted on the y-axis to allow for comparison. Right: The low-field part of the experimental and simulated spectrum of Cu-CHA dehydrated at 400°C is shown in greater detail. The simulation is the sum of the three different species A1 (blue), A2 (dark red), and B (dark green). The positions of the set of four parallel hyperfine peaks of the simulated spectrum of each species are marked. Reprinted with permission from Godiksen et al., J. Phys. Chem. C 2014, 118 (40), 23126–23138. Copyright 2014 American Chemical Society.

(Si/Al ¼ 14, Cu/Al ¼ 0.44, synthesized using only TMAda+ by fluoridemediated routes) (132,147). The intensities of the EPR signals on both samples are attenuated after dehydration in flowing O2 of N2 (134). Similar attenuation has been noted in the EPR of Cu-Y (148) and Cu-ZSM-5 (149–152). Several explanations have been proposed for the attenuation, including reduction of paramagnetic Cu2+ to diamagnetic Cu+, antiferromagnetic or ferromagnetic coupling in oxo- or hydroxo-bridged Cu dimers, or a pseudo-Jahn–Teller effect (PJTE) that broadens the Cu2+ signal as a consequence of near degeneracies in the ground electronic state (153,154). The percentage of the total EPR signal intensity (i.e., double integral) that remains during a dehydration treatment in 50% O2 of a sample that contains predominantly ZCuOH sites is shown as a function of temperature Fig. 22 (132). By 400°C, the EPR signal intensity is only 67% of its intensity at ambient. XAS spectra collected on samples of similar composition and similarly dehydrated at 400°C in flowing 50% O2 do not evidence any loss in Cu2+ signal (82,143). Godiksen et al. ascribe the intensity loss to signal broadening accompanying dehydration of ZCuOH sites (132). The 33% of the EPR signal intensity that remains is assigned to Z2Cu species

44 EPR signal intensity relative to hydrated Cu zeolite (%)

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100 Cu-CHA Cu-CHA @ RT Cu-*BEA Cu-MFI

75 50 25 0 0

100

200

300

400

Dehydration temperature (°C)

Fig. 22 Total intensity of the EPR signal, calculated as a double integral, of the samples after ex situ dehydration at the given temperature Cu-CHA (black stars), Cu-*BEA (red triangles), and Cu-MFI (blue triangles). The black triangle shows the 14 h roomtemperature dehydration of CHA shown in green in Fig. 21 (left). The EPR intensity is given in % relative to the value for the hydrated spectrum for each sample. Each point corresponds to one dehydration experiment. Multiple experiments performed on the same batch of material are plotted at some temperatures. Reprinted with permission from Godiksen et al., J. Phys. Chem. C 2014, 118 (40), 23126–23138. Copyright 2014 American Chemical Society.

(21%) and Cu exchanged at defect locations (12%) (132). This fraction of Z2Cu is larger than that expected based on the synthesis conditions. An alternative explanation, then, is that dehydration attenuates but does not silence ZCuOH sites. A similar analysis of signal attenuation in a sample containing predominantly Z2Cu sites has not been reported. 3.3.6 IR and Raman The 3550–3750 cm1 region in the IR corresponds to X–H stretching modes that can be diagnostic of various species in dehydrated SSZ-13 samples. An IR spectrum typical of SSZ-13 zeolites synthesized using TMAda+ in fluoride medium to contain only 1 Al sites is shown in Fig. 23 (155) for the H-form (a: Si/Al ¼ 15) and for two Cu-exchanged forms (b: Si/Al ¼ 15, Cu/Al ¼ 0.33; c: Si/Al ¼ 22, Cu/Al ¼ 0.5). IR bands at 3585 and 3605 cm1 correspond to Brønsted acid sites (Si–OH–Al) (140,141) and at 3735 cm1 to isolated silanols (SiOH) (142). ZCuOH has been shown to produce a characteristic IR feature at 3655 cm1 that can be used to track the appearance of this species as a function of Cu exchange level (58,82,130,143). The intensity of the 3655 cm1 feature has been followed as a function of Cu exchange level on a series of SSZ-13 materials (Si/Al ¼ 15) synthesized by the hydroxide route and subsequently dehydrated (58). The feature is absent up to an exchange level of

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Fig. 23 FTIR spectra of H-SSZ-13 (a), Cu-SSZ-13(67) (b), Cu-SSZ-13(100) (c), H-CHA (d), and Cu-CHA (e) zeolites, activated with 10% O2 in He, at 250°C for 12 h. Reproduced from Lezcano-Gonzalez et al., Phys. Chem. Chem. Phys. 2014, 16, 1639–1650, with permission of the Royal Society of Chemistry.

Cu/Al ¼ 0.10, corresponding to complete saturation of Z2Cu sites, and then increases linearly in integrated intensity from Cu/Al ¼ 0.21 to 0.44. The ZCuO-H stretch in the IR is thus both diagnostic and quantifiable. The 800–1000 cm1 region in the IR corresponds to framework T–O–T vibrations (Si–O–Si or Al–O–Si in zeolites) that are perturbed by the presence of exchanged extraframework Cu ions. IR spectra collected on dehydrated Cu-SSZ-13 zeolites (Si/Al ¼ 6, synthesized by FAU conversion) with increasing Cu/Al ratio (0.1–0.5) are shown in Fig. 24 (144). The Z2Cu sites in these samples saturate at Cu/Al 0.2, and up to this Cu loading a single perturbed T–O–T band is present at 890 cm1. At Cu loadings beyond this point, a new band appears at 940 cm1 and increases in intensity with Cu loading. This feature likely corresponds to T–O–T vibrations perturbed by ZCuOH. The 890 cm1 band is attenuated in Cu-SSZ-13 samples that are concurrently exchanged with other divalent ions (e.g., Mg2+ or Ca2+) that can compete with Cu2+ for 2 Al sites (156). Luo et al. (157) observe both the 890 and 940 cm1 bands on a commercial Cu-SSZ-13 sample,

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Fig. 24 DRIFT spectra of Cu-SSZ-13 samples after (A) oxidation at 400°C, and (B) reduction in H2 at 200°C (reduction time increases from the top spectrum to the bottom one). Reproduced from Kwak et al., Chem. Commun. 2012, 48, 4758–4760, with permission of the Royal Society of Chemistry.

indicating that the synthesis route and Si/Al composition used led to a mixture of both 2 Al and 1 Al exchange sites in detectable populations. Because of the nature of the Raman scattering experiment and the differences in selection rules, Raman is sensitive to low-frequency modes difficult or impossible to observe in the IR. The Raman spectra of Cu-SSZ-13 samples prepared by aqueous Cu exchange and by one-pot synthesis have been reported by Guo et al. (100). A Raman band near 475 cm1 is a characteristic of framework vibrations (100,158) and is the only band present on Cu/Al ¼ 0.04 and Cu/Al ¼ 0.09 materials. At higher Cu content additional features appear that have been assigned to oxobridged Cu dimers (66). These features are at frequencies similar to those computed for [CuOCu]2+ dimers in ZSM-5. The Cu–O–Cu angles in these dimers and thus their vibrational spectra are sensitive to the location of charge-compensating Al (6). The sample prepared by aqueous ion exchange (a: Si/Al ¼ 4.3, Cu/Al 0.36) has an additional feature at 610 cm1 in the Raman that is consistent with spectroscopic observations and DFTcomputed vibrations of a [CuO2Cu]2+ dimer (6,100,159). Raman spectra have not been reported for samples synthesized via TMAda+ in fluoride medium reported to contain Cu oxides (130,132). 3.3.7 Summary of High-Temperature Oxidative Conditions High-temperature oxidative treatments lead to dehydration of Cu ions and coordination to specific binding sites on the zeolite framework. As a result,

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Table 3 Summary of Cu-CHA Characterizations at High-Temperature Oxidative Conditions Low Cu Synthesis Route Density High Cu Density References

FAU conversion

Z2Cu

Hydroxide mediated Z2Cu

Fluoride mediated

Z2Cu/ZCuOH/Cu XAS (58,82,122,128,129) XRD (53) oxides UV–vis (128) EPR (134) IR (129,144,156) Z2Cu/ZCuOH

ZCuOH ZCuOH

Cu-SAPO-34

XAS (58,83–85) XRD (83,84) IR (58) XAS (82,143) XRD (146) EPR (130,132,147) UV–vis (130,143) IR (82,130,143,155) XAS (84) XRD (84,94)

treatment of Cu-SSZ-13 samples under these conditions reveals the microscopic differences that originate from the synthesis methods used, as summarized in Table 3. Low Cu density Cu-SSZ-13 materials synthesized by FAU conversion or by hydroxide media (in the presence of Na+ and TMAda+ cations) show XAS and IR spectra reflecting the presence of only Z2Cu, in alignment with DFT models and titration data that predict Z2Cu sites to saturate before ZCuOH sites become populated. Higher Cu densities form Cu-SSZ-13 materials with spectral features attributable to ZCuOH sites, and even higher Cu densities with spectral features characteristic of Cu oxides. In contrast, SSZ-13 materials synthesized with TMAda+ only, in hydroxide or fluoride media, appear to have solely isolated single Al sites that preferentially exchange ZCuOH sites at all Cu densities. These materials do not follow the random Al distribution model. Presently, molecularlevel descriptions of Cu site types under these conditions are less clear for Cu-SAPO-34.

3.4 Vacuum and Inert Pretreatments Vacuum ( Cu1 + + OH Δ

½Cu2 + OH  + OH> Cu2 + O + H2 O +

(11)

+ Δ

2½Cu2 + OH  > Cu1 + + Cu2 + O + H2 O or through condensation of two ZCuOH (17,161,167): + Δ

2½Cu2 + OH  > ½CuOCu2 + + H2 O Δ 1 ½CuOCu2 + > 2Cu1 + + O2 2

(12)

The first mechanism requires breaking of the ZCu–OH bond and invokes a ZCuO intermediate that is expected to be high in energy (Fig. 11). The second mechanism involves an O-bridged Cu dimer, a species with some precedent in ZSM-5. Exact reaction energies are dependent on the location of the charge-compensating Al relative to the [CuOCu]2+ intermediate (66). The autoreduction mechanism in SSZ-13 and in Cu-SAPO-34 requires further investigation. 3.4.1 X-Ray Absorption Spectroscopy XANES and EXAFS of He or vacuum-treated samples of various compositions and synthesis routes show that Z2Cu and ZCuOH sites are not susceptible to autoreduction to the same extent. This difference was noted earlier in the first-principles phase diagram in Fig. 12. The XANES spectra of samples expected to contain a significant fraction of ZCuOH, including a Cu-SSZ-13 zeolite (Si/Al ¼ 13.1, Cu/Al ¼ 0.44) synthesized via the fluoride route (143) and vacuum treated at 400°C and a Cu-SSZ-13 zeolite (Si/Al ¼ 12, Cu/Al ¼ 0.44) synthesized via the fluoride route (82) and exposed to He at 400°C both exhibit a 8.989 keV feature in the XANES that is absent in ambient or dehydrated samples. The 8.983 keV XANES edge is assigned to the Cu+ 1s ! 4p transition and thus is diagnostic of

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Cu+. Further, the Cu2+ feature is observed to decrease concomitantly with the increase in Cu+. The first-shell coordination number inferred from EXAFS fitting decreases from three under dry oxidizing conditions to two following autoreduction. Further, the ZCuO-H band at 3665 cm1 present in the IR prior to autoreduction decreases in intensity following the He treatment. All these results are consistent with ZCuOH that lose OH through some mechanism to form reduced ZCu. Paolucci et al. compared the XANES of a sample prepared to contain 80% ZCuOH (hydroxide synthesis, Si/Al ¼ 15, Cu/Al ¼ 0.44) and one prepared to contain 100% Z2Cu (FAU conversion, Si/Al ¼ 5, Cu/Al ¼ 0.08) (Fig. 18). At 400°C in flowing He, the XANES of the ZCuOH sample is nearly identical to that of similar samples (82,130,143). Quantification of the XAS indicates reduction of 50% exchanged Cu sites. In contrast, an 8.893 keV feature is undetectable in the Z2Cu sample. 3.4.2 Electron Paramagnetic Resonance Because Cu+ is EPR silent, EPR is an indirect reporter of Cu autoreduction. Loss of the Cu2+ EPR signal under autoreduction conditions has been reported in Cu-Y (148) and Cu-ZSM-5 (149,168,169) zeolites. Godiksen et al. collected EPR on a fluoride-synthesized SSZ-13 sample (Si/Al ¼ 14, Cu/Al ¼ 0.44) (132) following dehydration at 400°C in O2 and subsequent purge in He at 400°C. Based on synthesis conditions, Cu2+ is expected to be present primarily as ZCuOH. Consistent with the conversion of ZCuOH to ZCu observed in XAS, the Cu2+ signal in the EPR diminishes 66% relative to the hydrated sample following He treatment. An analogous experiment at 400°C has not been reported for Z2Cu-dominated samples. However, EPR results have been reported for a hydrated Cu-SSZ-13 sample (Si/Al ¼ 6, Cu/Al ¼ 0.032) synthesized by FAU conversion, thus containing Z2Cu sites (134), treated in N2 at 250°C. The Cu2+ EPR signal decreases from the hydrated to the 250°C treatment, but the origins of this decrease are unclear. 3.4.3 Summary of Vacuum and Inert Pretreatments Autoreduction of Cu2+ to Cu+ during high-temperature inert and vacuum conditions has been noted in the Cu-zeolite literature for over 30 years. Materials containing only Z2Cu are more resistant to Cu autoreduction than those containing other Cu ion types. Specifically, those containing a mixture of Z2Cu/ZCuOH or solely ZCuOH are observed to form Cu+ species, nominally ZCu, upon high-temperature vacuum or inert pretreatments. Complete reduction of all Cu sites to Cu+ is not observed on any materials,

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Table 4 Summary of Cu-CHA Characterizations After Vacuum and Inert Pretreatments Synthesis Route Low Cu Density High Cu Density References

FAU conversion

Z2Cu

Z2Cu/ZCuOH/ZCu XAS (58,129) EPR (134) IR (144)

Hydroxide mediated Z2Cu

Z2Cu/ZCuOH/ZCu XAS (58,85)

Fluoride mediated

ZCuOH/ZCu

Cu-SAPO-34

ZCuOH/ZCu

XAS (82,143) EPR (130,132) IR (82) XAS (84) XRD (84,94)

suggesting that the autoreduction mechanism is multistep and is potentially sensitive to the microscopic material structure and distribution of ZCuOH sites. The effect of vacuum or inert pretreatments on a Cu-CHA material must be considered before subsequent characterization experiments are performed (Table 4).

3.5 Hydrogen Temperature-Programmed Reduction Hydrogen temperature-programmed reduction has historically been used as a probe to infer the number and types of Cu sites in Cu-zeolites. Prior to the H2 TPR experiment, catalysts are commonly pretreated under dry conditions to remove H2O. As just noted, this pretreatment can autoreduce some Cu before the H2-TPR experiment is performed. During a typical H2 TPR experiment, the catalyst is exposed to a gas feed of 2% H2 and balance inert and the temperature is subsequently increased, while monitoring the H2 consumed as Cu reduces from Cu2+ to Cu+ to Cu0. Chen et al. (170) reported a set of H2 TPR collected from 100°C to 900°C on Cu-SSZ-13 materials synthesized via a hydroxide route and on commercial Cu-BEA and Cu-ZSM-5 zeolites. H2 consumption begins around 200°C on all three materials and, on the Cu-SSZ-13 sample, continues through several convoluted peaks up to 500°C. In contrast, H2 TPR reported by Kwak et al. (22) on Cu-SSZ-13, Cu-BEA, and Cu-ZSM-5 samples differ in both number and location of H2 consumption peaks. These differences can be traced to differences in synthesis protocols, the number and type of Cu sites, and pretreatments used prior to TPR experiments. These factors can only be decoupled by analysis of H2 TPR from samples from specified synthetic routes and with known Si/Al and Cu/Al ratios.

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Fig. 25 H2 consumption profiles during H2-TPR on 500°C-calcined Cu-SSZ-13 (wtcat. ¼ 50 mg; heating rate ¼ 10°C min1; total flow rate ¼ 60 mL min1 of (A) 2% H2/Ar and (B) 2% H2/Ar + 1% H2O). Cu ion-exchange level: 20% (red), 40% (green), 60% (blue), 80% (purple), and 100% (black)—bottom to top. Reproduced from Kwak et al., Chem. Commun. 2012, 48, 4758–4760, with permission of the Royal Society of Chemistry.

Fig. 25 presents H2 TPR results (offset on the y-axis) from a Cu-SSZ-13 zeolite synthesized by FAU conversion (Si/Al ¼ 6, Cu/Al ¼ 0.1–0.5) and subsequently dehydrated at 500°C in O2. The Cu-SSZ-13 sample containing a Cu/Al ¼ 0.1, which contains primarily Z2Cu sites based on the composition phase diagram (Fig. 16), shows one H2 consumption peak centered at 340°C. Samples with progressively higher Cu/Al ratio retain this 340°C feature and develop a new shoulder centered at 250°C that grows in intensity with increasing Cu/Al ratio. This behavior is consistent with the preferential exchange of Z2Cu sites to saturation prior to exchange of ZCuOH sites, a transition expected to occur near Cu/Al ¼ 0.18 on the Si/Al ¼ 6 sample. In another sample characterized to contain a majority of ZCuOH sites, H2 consumption is also observed at 250°C (58), indicating that ZCuOH sites reduce at lower temperature in H2 than Z2Cu sites. At the highest exchange level shown in Fig. 25, H2 consumption begins to occur at 230°C, likely evidence for formation of more readily reducible Cu oxide particles (66,100,144,171). The top panels in Fig. 26 compare H2-TPR profiles for SSZ-13 samples synthesized via FAU conversion at Si/Al ¼ 6, and the hydroxide route in the presence of Na+ and TMAda+ cations at Si/Al ¼ 12 and 35 (59). The samples were initially dehydrated at 550°C in 5% O2 and balance He. The low loaded Si/Al ¼ 12, Cu/Al ¼ 0.06 sample is expected to

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Fig. 26 Temperature-programmed reduction (TPR) data for Cu/SSZ-13 samples with various Cu loadings for (A) Si/Al ¼ 6, (B) Si/Al ¼ 12, and (C) Si/Al ¼ 35. The upper panels show TPR for dehydrated samples, while the lower panels show TPR for fully hydrated samples. Reprinted from Gao et al., J. Catal. 2015, 331, 25–38, Copyright (2015), with permission from Elsevier.

contain exclusively Z2Cu sites and, consistent with this expectation, begins to consume H2 at 400°C. The total H2 consumption is in 1:2 stoichiometric ratio with Cu2+: 1 Z2 Cu + H2 ! ZH=ZCu 2

(13)

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The Si/Al ¼ 12, Cu/Al ¼ 0.13 and 0.23 samples exhibit an additional H2 consumption peak near 230°C, again likely associated with ZCuOH reduction. H2 consumption is less than 1:2, potentially because some of the ZCuOH autoreduce to ZCu during the high-temperature dehydration pretreatment. Ma et al. report H2 TPR (172) profiles on commercial Cu-SSZ-13 and Cu-SAPO-34 materials aged in 14% O2, 5% CO2, and 5% H2O at varying temperatures and times. The Cu-SSZ-13 material exhibits low- and hightemperature consumption features consistent with ZCuOH and Z2Cu sites. H2 TPR of the SAPO-34 sample also contains two consumption peaks at similar temperatures, potentially evidencing the analogs of Z2Cu and ZCuOH exchange sites. As shown across the lower panels in Fig. 26, H2-TPR profiles collected in 1% H2O and H2 exhibit a single, broad consumption peak centered near 210°C (59). H2O is expected to hydrate and mobilize both Cu2+ and CuOH+ at these conditions, and some combination of these effects evidently facilitates Cu reduction.

3.6 Characterization Following NO Dosing NO adsorption has been widely used as a reporter of Cu site chemistry, because adsorbed NO has clear spectroscopic signatures in the IR region and because NO is the pollutant targeted in SCR chemistry (130,169,173–179). Precise assignments of spectroscopic features are complicated by sensitivity to catalyst pretreatments, to Cu speciation, and to the presence of defects and impurities. DFT calculations have been used extensively to assign these features. 3.6.1 DFT Models of NO Adsorption DFT calculations have a long history of application to NO adsorption to Cu-zeolites (108,112–114,116,180–184). Because NO is a neutral radical, it can act as an oxidizing (NO) and reducing (NO+) ligand, in addition to a charge-neutral adsorbate. Early DFT calculations showed that NO binds to a Cu+ ion in bent configuration as a neutral radical (114). Fig. 27 shows the results of recent supercell calculations on NO at a ZCu site (86); Bader charge analysis confirms that the Cu oxidation state is unchanged by NO adsorption, and harmonic frequency calculations show that the N–O stretch mode is blue shifted relative to free NO, to about 1800 cm1 (86,124,126,185,186). Binding energies within the GGA and HSE06 models are 120 and 90 kJ mol1, respectively, consistent with adsorption on Cu+ sites in NO dosing experiments.

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Fig. 27 DFT-computed structures for NO adsorbed to (A) ZCu, (B) Z2Cu, (C), and (D) two configurations of ZCuOH. Adapted from Zhang, R.; McEwen, J.-S.; Kollár, M.; Gao, F.; Wang, Y.; Szanyi, J.; Peden, C. H. F. ACS Catal. 2014, 4, 4093–4105.

NO interactions with Cu2+ centers are more subtle than with Cu+, as they involve a balance between neutral (Cu2+–NO) and charge transfer (Cu+–NO+) states, the latter of which is expected to be repulsive (114). DFT models have a difficult time with this type of electron (de)localization problem, and as a consequence, results are even more sensitive to the functional used. Fig. 27 shows the GGA-optimized geometry of an NO at a Z2Cu site (125). The N–O bond is shortened relative to ZCuN–O, consistent with charge transfer from NO to the Cu2+ ion; Bader charge analysis, however, reveals that the Cu center remains in the 2 + oxidation state (58,86,126). While the GGA-computed NO binding energy is 100 kJ mol1 (124,186), this value increases to near zero within the HSE06 model (58,126,124,186). The NO harmonic stretch frequency is computed to be blue shifted relative to ZCuNO by more than 100 cm1, to the 1925–1945 cm1 range. This mode has been proposed to split into a multipeak spectrum based on AIMD simulation results (185); further work is necessary to determine the extent to which the NO overbinding within the GGA model affects these results. Fig. 27 also shows two competing local minima identified within the GGA for NO adsorption at a ZCuOH site, one in which the Cu ion is in approximately square-planar coordination and the other in pseudotetrahedral coordination. N–O separations are slightly longer than NO associated with Z2Cu, and corresponding computed harmonic N–O stretch frequencies are 1782 and 1874 cm1 for structures (c) and (d), respectively. While the connectivity in the former is reminiscent of an HONO adsorbate, ˚ longer than that expected the computed HO–NO separation is about 0.5 A

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for an intact HONO species. GGA computed binding energies of the (c) and (d) isomers are 130 and 100 kJ mol1 (86). HSE06 results have only been reported for the (c) isomer; the optimized structure is similar but binding energy decreases to 70 kJ mol1. Bader analysis again reports the Cu center to have a 2 + oxidation state. 3.6.2 XAS and IR XANES spectra collected from a Cu-SSZ-13 zeolite expected to contain primarily ZCuOH sites (Si/Al ¼ 15, Cu/Al ¼ 0.48, synthesized with only TMAda+), after calcination and subsequent exposure to 1000 ppm NO and balance inert at 200°C, are shown in Fig. 28, left. The low-intensity 8.983 keV edge indicates that 5–10% of Cu is present as Cu+ under these conditions, the same as expected from the calcination treatment alone. XANES spectra collected from Cu-SSZ-13 samples containing varying proportions of Z2Cu and ZCuOH sites held in NO and He at 200°C (58,85) or at 25°C (187) are indistinguishable from that shown in Fig. 28. XES of Cu-SSZ-13 with exposure to NO in He or O2 at 200°C lacks the 8.958 eV feature characteristic of a Cu-bound N atom (85). Taken together, the XAS and XES indicate that NO neither reduces nor adsorbs to ZCuOH

Fig. 28 In situ XANES at the Cu K-edge (left panel) and EPR (right panel) showing the reducing capability at 200°C of 1200 ppm of NH3 (b: solid blue curve), 1000 ppm of NO (c: solid orange curve), and a mixture of 1200 ppm of NH3 and 1000 ppm of NO (d: solid red curve) on the Cu(II) state obtained after initial oxidation in a mixture of 1000 ppm of NO and 10% O2 (a: dashed black curve). Inset left panel: Development of the intensity at 8983 eV with time with NH3 only (a ! b) and with a mixture of NH3 + NO (a ! d), visualizing the different reduction behavior with time in these cases. In EPR, a stable state is obtained after 11 min in NH3 + NO (red), whereas the EPR spectra are still developing after 11 min in NH3 alone (blue) or 30 min in NO (orange). Reprinted with permission from Janssens et al., ACS Catal. 2015, 5 (5), 2832–2845. Copyright 2015 American Chemical Society.

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Cu2+–NO3–

2.04 2.01

FTIR

0.5 Å–3

1

2.83 8980

8990

2 3 R(Å)

9000

Incident energy (eV)

After 1 h of NO2 exposure

Absorption units

2.02

NO2

0.2

IFTI (Å–3)

2.44

Normalized absorption

2.03

NO + O2

XAS 3.65

0.2

After 2 h of NO + O2 exposure

4

9010

1650

1600

1550

1500

1450

–1 Wavenumber (cm )

Fig. 29 Structure of the bidentate Cu–NO3 species (left panel). In situ XAS and k2-weighted, phase uncorrected Fourier transformed EXAFS (middle panel) and FTIR (right panel) spectra after exposure of dehydrated Cu-CHA to 1000 ppm of NO2 (solid green curves), and to a mixture of 1000 ppm of NO and 10% O2 (dashed black curves) at 200°C. The left panel indicates the distances from the central Cu atom to the neighboring atoms in angstroms. Color code atoms: Cu, green; O, red; N, blue; Al, yellow. Reprinted with permission from Janssens et al., ACS Catal. 2015, 5 (5), 2832–2845. Copyright 2015 American Chemical Society.

or Z2Cu sites in Cu-SSZ-13 at 200°C, consistent with the hybrid-exchange DFT results described earlier. Janssens et al. (98) reported XAS and IR spectra collected from a Cu-SSZ-13 zeolite expected to contain primarily ZCuOH sites after exposure to NO and O2 at 200°C for 1–2 h (Fig. 29). XANES and EXAFS indicate that 100% of the Cu is present as four-coordinate Cu2+, and IR spectra contain 1550–1630 cm1 bands assigned to Cu-bound nitrate (188–190). XES spectra of Cu-SSZ-13 exposed to similar conditions indicates Cu coordination to O atoms only (85), consistent with the structure of an O-bound Cu bidentate nitrate shown in Fig. 29 (98,186). The XAS spectrum after NO2 dosing (Fig. 29 solid line) is similar to that with NO and O2 exposure, while the nitrate bands in the IR after NO2 dosing are higher intensity than in NO and O2 (98). NO2 dosing apparently leads to higher nitrate coverages than does NO and O2 dosing. Similarly, observations have been made on a commercial Cu-CHA material (191). Fig. 30 shows IR spectra collected upon cryogenic NO dosing (173°C) to a Cu-SSZ-13 zeolite containing primarily ZCuOH sites (Si/Al ¼ 14, Cu/Al¼ 0.44, synthesized with only TMAda+) (130). Samples were first pretreated either at 400°C in vacuum (top) or at 400°C in O2 followed by inert at 150°C (bottom). The vacuum-treated sample is expected based on results described earlier to contain some fraction of autoreduced Cu+, and the appearance of an N–O stretch band at 1815 cm1 widely associated with Cu+-bound NO supports this expectation. A similar N–O stretch band is present on many other Cu-zeolites containing some fraction of Cu+

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Fig. 30 Low-temperature (100K) IR spectra of NO dosed at increasing equilibrium pressure (from 7  102 to 7  101 Torr) on Cu-zeolites. Red curves show lowest NO coverage, black curves refer to highest NO coverage, while fading curves refer to intermediate adsorption steps. Upper parts (A–C) refer to vacuum-activated zeolites, showing the Cu+(NO) ! Cu+(NO)2 evolution; insets report low coverage spectra corresponding to Cu+(NO) complexes. Lower parts (C–E) refer to O2-activated zeolites showing Cu2+(NO) and residual Cu+(NO)2 adducts. Inset in (D) reports the corresponding bands in the v(OH) region, testifying the erosion of the 3650 cm1 band upon formation of NO adducts with dehydrated [Cu–OH]+ species. Reproduced from Giordanino et al., Dalton Trans. 2013, 42, 12741–12761, with permission of the Royal Society of Chemistry.

(160,173–177,190,192). If ZCuOH sites remain in these samples, they are not evidenced by a separate NO feature in the IR. At higher NO exposure, the 1815 cm1 band shifts to 1825 cm1 and is joined by a new band near 1730 cm1, features assigned to the symmetric and asymmetric stretches of Cu+-bound dinitrosyl complexes (150,176,190,193). In the O2-treated sample, the features in the region expected for Cu+-bound NO decrease in intensity and an envelope of features centered on 1900 cm1 appear at the same frequency as gas-phase NO. These features may represent NO condensed in the pores, possibly associated with ZCuOH (130). Additional features appear in similarly treated samples containing a mixture of ZCuOH and Z2Cu sites. Fig. 31 reports IR spectra collected from a Cu-SSZ-13 zeolite (Si/Al ¼ 6, Cu/Al ¼ 0.45, FAU conversion) following a high-temperature vacuum treatment (500°C) and subsequent NO exposure at 25°C. In addition to the 1815 cm1 band, additional features are present near 1950 and 1900 cm1 (192), the latter two presumably corresponding to NO perturbed by interactions with Z2Cu and/or ZCuOH. The 1815 cm1 band is diminished in intensity and 1900 cm1 enhanced and broadened

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Fig. 31 Series of selected IR spectra obtained after exposure of annealed Cu-SSZ-13 samples to 14NO (A) and 15NO (B) at 300K. The samples were annealed in vacuum at 773K for 2 h prior to IR measurements. Reproduced from Szanyi et al., Phys. Chem. Chem. Phys. 2013, 15, 2368–2380, with permission of the Royal Society of Chemistry.

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when the sample is first pretreated in O2 at 400°C (192). The 1950 cm1 band persists in both the vacuum- and O2-activated samples and appears to be associated with Z2Cu that survives both of these pretreatments. A band near 2165 cm1 appears on all these samples following either high-temperature vacuum or O2 pretreatments and ambient or subambient NO dosing (Fig. 31). This band is strongly blue shifted relative to gasphase NO, and as a result has been assigned to NO+ associated with either Cu+ or Brønsted sites. IR spectra collected on H-SZZ-13 after calcination and exposure to NO at 300°C reveal an NO+ band that increases in intensity concomitantly with a decrease in the 3610 cm1 Brønsted O–H stretch (194).

4. IN SITU AND OPERANDO CHARACTERIZATION Among all of the SCR reactants, NH3 is singularly significant because it adsorbs on different site types and in several configurations under typical reaction conditions, and because some NH3 species primarily coordinate to active sites to form intermediate complexes in the mechanism, while other NH3 species behave as co-reductants. NH3 is a uniquely suited chemical probe of Cu-exchanged zeolites that mediate SCR redox cycles because it can physically access all active sites that potentially catalyze SCR and also interrogate the state of Cu-zeolite surfaces after treatments that mimic oxidation and reduction half-cycles. In this section, we first discuss NH3 interactions with Cu-zeolites, and then its utility as a probe to characterize the sites present within small-pore Cu-zeolites. We examine in situ spectroscopic characterization of Cu-SSZ-13 and Cu-SAPO-34 in the presence of NH3 only and then discuss operando characterizations that include the complete suite of standard SCR reactants during catalytic turnover.

4.1 DFT Models of NH3 Adsorption in Cu-SSZ-13 As preface to this section, it is helpful to describe the results of DFT calculations for NH3 adsorbed on the sites expected to be present in Cu-SSZ-13. Fig. 32A shows the structure of NH3 bound to a Brønsted acid site, as computed in a 12 T-site CHA supercell at HSE06-TSvdW level of theory (58). NH3 extracts the H+ from the framework oxygen to form a symmetric ammonium cation that sits within an 8-MR. The structure of the ammonium cation is insensitive to the computational model used; NH3 binding

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Fig. 32 DFT-computed structures for (A) [ZNH4 + ], (B) [Z2CuNH3], (C) Z2[Cu(NH3)4], (D) Z [Cu(NH3)2], and (E) Z[Cu(NH3)2](NH3).

energies to the Brønsted site are 118 kJ mol1 based on a BEEFvdW (195) model and 151 kJ mol1 based on an HSE06-TSvdW model (58). The NH3 adsorption energy on H-SSZ-13 as determined by microcalorimetry is 155 kJ mol1 (196). A number of groups have reported DFT results for NH3 adsorbed on various Cu ion models (58,98,126,147,195). Table 5 (58) summarizes the results of DFT-computed equilibrium structures and sequential adsorption energies of NH3 to supercell models of exchanged Cu2+, including Z2Cu and ZCuOH, and Cu+, including ZCu and ZCu/ZNH4. Results are representative of the available literature and reveal behavior salient to interpretation of the experiments described later. In all cases, adsorbed NH3 are computed to displace framework oxygen (Of) from Cu firstcoordination spheres, so that sequential NH3 adsorption results in a progression through a series of mixed coordination structures. Fig. 32B shows one such representative structure in which a Cu2+ ion is coordinated by three Of and one NH3. Each series terminates in a Cu ion that is fully liberated from framework oxygen and fully coordinated to NH3 ligands. AIMD simulations show these NH3-solvated ions to be mobile within the CHA cage (58). In the case of Z2Cu, the NH3 adsorption sequence terminates at a square-planar [Cu(NH3)4]2+ ion, shown in Fig. 32C. Sequential adsorption energies across this series, as computed with an HSE06-TSvdW model, are in a narrow range of 120 to 135 kJ mol1, which are at least 20 kJ mol1 less exothermic than adsorption energies on a Brønsted site computed within the same DFT model. NH3 binding on

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Table 5 HSE06-TSvdW-Computed Sequential NH3 Adsorption Structures and Energiesa +xNH3 1 2 3 4

II

[Z2Cu ]

ΔEads (kJ mol1)

132

136

123

132

Cage location

A

A

B

C

Of/total CN

3/4

2/4

1/4

0/4

ΔEads (kJ mol )

134

150

72

73

Cage location

B

C

C

C

Of/total CN

1/2

0/2

0/2

0/2

ΔEads (kJ mol )

117

119

116

47

Cage location

B

C, Bb

C

C

0/4

0/4

1

I

[ZNH4]/[ZCu ]

1

[ZCuIIOH]

Of/total CN

2/4

0/3, 1/4

ΔEads (kJ mol )

137

151

75

41

Cage location

B

C

C

C

Of/total CN

1/2

0/2

0/2

0/2

1

[ZCuI]

b

a Cage location indicates optimized ion location referenced to Fig. 1. CN and Of indicate total Cu coordination number and number of close framework O contracts, respectively. b Trigonal planar, square-planar values. Source: Paolucci, C.; Parekh, A. A.; Khurana, I.; Di Iorio, J. R.; Li, H.; Albarracin Caballero, J. D.; Shih, A. J.; Anggara, T.; Delgass, W. N.; Miller, J. T. J. Am. Chem. Soc. 2016, 138 (18), 6028–6048.

ZCuOH is 10 kJ mol1 less exothermic than that on Z2Cu and saturates at a [CuOH(NH3)3]2+ ion, which preserves the same fourfold square-planar coordination but with a mixture of oxygen- and nitrogen-based ligands. Only two NH3 are necessary to fully liberate a Cu+ ion from a ZCu site, forming the twofold-coordinated and linear [Cu(NH3)2]+ species shown in Fig. 32D. Sequential NH3 binding on ZCu is computed to be 10 kJ mol1 more exothermic than that on Z2Cu. Thus, NH3 adsorbs to Lewis acidic Cu2+ and Cu+ ions with similar energies, saturating at different numbers of adsorbed NH3, and always displacing Of from the Cu firstcoordination sphere. Both the NH4 + ion and the Lewis-bound NH3 provide additional acidic sites for NH3 physisorption. Fig. 32E shows a typical result in which an NH3 is hydrogen bonded to an NH3 that is ligated to a Cu+ ion. Computed NH3 physisorption energies are in the range of 40 to 70 kJ mol1 depending on the specific Cu or ammonium ion to which it is bound (Table 5). For comparison, these physisorption energies have been estimated to be 50 to 70 kJ mol1 from analysis of TPD data (197).

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4.2 Selective NH3 Titration of H+ Sites in H-Form and Cu-Exchanged Zeolites NH3 titrations are used routinely to quantify H+ sites in H-form zeolites (198,199). NH3 is a particularly appropriate titrant for small-pore zeolites, because larger bases (e.g., n-propylamine, pyridine) are unable to traverse 8-MR or smaller apertures (57). This size-exclusion phenomenon has been well documented in FER and MOR zeolites, where large titrants such as n-hexane and pyridine cannot access H+ sites located within 8-MR pores in these frameworks (200–203). The kinetic diameter of NH3 (0.26 nm) (204) is small enough to allow access to protons within 8-MR voids. Thus, the Brønsted O–H stretching bands (3600 cm1) in IR spectra of H-SSZ-13 are observed to disappear completely after saturation with NH3 (57). When both Lewis and Brønsted acid sites are present in a sample, as is generally the case in metal-exchanged zeolites, selective NH3 titration of the H+ sites can be used to quantify the extraframework metal ion-exchange stoichiometry (57). For example, a divalent cation will displace two protons upon charge compensation of two framework anionic sites: 2ZH + M2 + ! Z2 M + 2H +

(14)

or a single proton upon charge compensation of a single anionic Z site: ZH + MOH + ! ZMOH + H +

(15)

Further, because it participates in the SCR redox catalysis, NH3 dosing or pretreatment can be used to prepare a material in a specific oxidation state and to probe this influence of redox on the generation or consumption of surface H+ sites (126). Because NH3 adsorbs to both Brønsted and Lewis acid sites (205), to extraframework Al species, and hydrogen bonds to chemisorbed NH3 and NH4 + , selective quantification of H+ sites becomes more complicated. The TPD profile of an H-SSZ-13 zeolite following saturation with gaseous NH3 has two partially overlapping desorption features (155,206), as shown in Fig. 34. The feature centered near 450K arises from the evolution of physisorbed NH3 located within microporous voids, located at preadsorbed acidic surface NH3 and NH4 + species (e.g., Fig. 32E), and also at extraframework Lewis acid sites (e.g., partial-extraframework or extraframework Al). The feature centered near 633K arises from NH3 desorption from NH4 + sites (Fig. 32A), leaving

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behind H+ sites. The convolution of the two features in the intermediatetemperature range interferes with quantification of the H + sites. Physisorbed and Lewis acid-bound NH3 can be removed from NH3saturated H-SSZ-13 zeolites by treatment in wet (up to 3% H2O) helium at 433K (57,206). A subsequent TPD experiment then contains only a single high-temperature desorption feature that corresponds to NH3 desorbing from NH4 + species (Fig. 33, blue line). Such methods are useful to quantify H+ sites only, especially when insufficient resolution between low- and high-temperature desorption features results from the presence of weaker Brønsted acid sites, or from differences in sample properties (e.g., crystallite size) or the experimental conditions chosen (e.g., temperature ramp rates) (207,208). The same NH3 saturation and wet helium pretreatment followed by TPD has been used to selectively titrate H+ sites in H-ZSM-5 (205), H-SSZ-13 (59,156), and H-SAPO-34 (94). Exchange of Cu into the SSZ-13 zeolite introduces additional features in the NH3 TPD. The TPD profile of Cu-exchanged SSZ-13 following NH3 dosing, without subsequent treatment or purge step, contains a new intermediate-temperature desorption feature (155) (Fig. 34, “IT”) that overlaps with the low- and high-temperature desorption features observed on H-SSZ-13 (Fig. 34, dashed line). Cu-SSZ-13 materials prepared via 0.30 No purge NH3 desorption rate (10−5 mol NH3 g−1 s−1)

0.25 0.20 0.15

Dry purge

0.10 0.05 0.00 323

Wet purge 423

523

623

723

823

Temperature (K)

Fig. 33 NH3 temperature-programmed desorption profiles on H-SSZ-13 (Si/Al¼ 4.5) measured after gaseous NH3 saturation at 323K followed by no helium purge (black trace), or gaseous NH3 saturation at 433K followed by a dry helium purge (red trace) or a wet (3% H2O) helium purge (blue trace); titration and TPD protocols reported in Di Iorio et al. (206). From Di Iorio, Topics Catal. 2015, 58 (7), 424–434, with permission of Springer.

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H-SSZ-13 Cu-SSZ-13 (67) Cu-SSZ-13 (100)

LT 8 × 10−10

MS signal (a.u)

HT

6 × 10−10

IT

4 × 10−10

2 × 10−10 100

200

300

400

500

600

T (°C)

Fig. 34 NH3-TPD profiles of H-SSZ-13, Cu-SSZ-13(67) and Cu-SSZ-13(100) zeolites; the number in parentheses denotes the Cu exchange-level percentage (100*2*Cu/AI). HT, high-temperature peak; IT, intermediate-temperature peak; LT: low-temperaturepeak. Reproduced with permission from Lezcano-Gonzalez, I.; Deka, U.; Arstad, B.; Van Yperen-De Deyne, A.; Hemelsoet, K.; Waroquier, M.; Van Speybroeck, V.; Weckhuysen, B. M.; Beale, A. M. Phys. Chem. Chem. Phys. 2014, 16, 1639–1650.

FAU conversion (57,59), hydroxide- (59,97) and fluoride-mediated crystallization (143,155), and SSIE of Cu (96) all exhibit these low-, intermediate-, and high-temperature NH3 desorption features in the TPD profile following NH3 dosing, as do commercial Cu-SSZ-13 (172,209) and Cu-SAPO-34 (172,210) samples. Olsson and coworkers (211) were able to fit the NH3 TPD data collected from a high Al content SSZ-13 zeolite synthesized by FAU conversion (Si/Al ¼ 3.56, Cu/Al ¼ 0.16) to a three-site model using binding energies of 18.6, 137.8, and 149.0 kJ mol1. The least exothermic binding energy is consistent with that expected for desorption of nonspecifically adsorbed NH3 (212). The intermediate value is in the range (120 to 140 kJ mol1) computed for NH3 bound to oxidized (Z2Cu, ZCuOH) or reduced (ZCu) sites using supercell DFT and a van der Waals corrected hybrid functional (Table 5) (58). The same DFT model predicts the NH3 desorption energy from a Brønsted site to be 151 kJ mol1 (58). Thus, the DFT results support the assignment of the intermediate and strong binding site to Lewis and Brønsted sites, respectively. The scheme shown in Fig. 35 has been shown to be a robust method to selectively titrate H+ sites on Cu-exchanged zeolites without disrupting the coordination of exchanged Cu cations (206). As with the method described

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H+

Cu2+ O

O Si

Al

O

O Si

Al

A

O Si

Al

NH3 adsorption (433K)

NH3 NH4+

NH3

Cu2+ NH3 O Si

Al

O

O

O

O Si

Al

B

He purge (433K) NH4+

Cu2+ O Al

O Si

O

O Si

Al

C

O Al

Al

H+ O

O Si

Si

TPD in He (to 873K)

Cu2+ O

Si

Al

Al

O Si

O Al

Si

Fig. 35 (a) NH3 saturation at 433K leads to adsorption on Brønsted and Lewis acid sites, and physisorption near pore walls, but (b) purging in wet helium at 433K (8 h) desorbs NH3 at nonprotonic sites and enables quantification of Brønsted acid sites via (c) TPD to 873K. Reproduced with permission from Di Iorio, J. R.; Bates, S. A.; Verma, A. A.; Delgass, W. N.; Ribeiro, F. H.; Miller, J. T.; Gounder, R. Top. Catal. 2015, 58, 424–434.

earlier for isolating the NH4 + feature in H-form zeolites, the method involves gas-phase saturation of the Cu-zeolite sample with NH3 followed by purge in wet helium at 433K. The wet purge removes the low- and intermediatetemperature features from the NH3 TPD profile. Fig. 36 (57) compares the NH3 TPD profiles measured on SSZ-13 zeolites (Si/Al ¼ 5) of increasing Cu/Al ratio (0.00, 0.09, 0.20) after oxidation treatments. From a comparison of Figs. 34 and 36, it is evident that all NH3 species present on the Cu-zeolite

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Fig. 36 The NH3-TPD profiles for selected H-SSZ-13 and Cu-SSZ-13 samples (Si/Al ¼ 4.5) after NH3 saturation at 433K and subsequent purge in flowing wet helium. Reproduced with permission from Bates, S. A.; Delgass, W. N.; Ribeiro, F. H.; Miller, J. T.; Gounder, R. J. Catal. 2014, 312, 26–36.

surface other than those adsorbed at H+ sites are removed by the wet purge treatment. The desorption peak areas in Fig. 36 decrease with increasing Cu loading (or increasing Si/Al) (59) as more H+ sites are substituted by Cu2+ cations in the sample. The decrease in the NH4 + feature with increasing Cu exchange can be used to quantify the Cu exchange stoichiometry.

4.3 Vibrational Spectroscopy 4.3.1 NH3 IR and Assignments Fig. 37 reports difference IR spectra collected before and after NH3 saturation of H-form (Si/Al ¼ 15) and Cu-exchanged (Si/Al ¼ 15, Cu/Al ¼ 0.33 and 0.50) SSZ-13 samples at 250°C that were pretreated in 10% O2 at 250°C for 12 h to prevent autoreduction of ZCuOH centers (155). The H-form spectrum has features at 1448 and 1393 cm1 that correspond to NH4 + bends, in locations the same as those attributed to NH4 + ion bends in other zeolites (213–217). A supercell DFT model of bound NH4 + recovers the same two vibrational frequencies; harmonic BLYP frequencies computed in a supercell model are 1432 and 1370 cm1 (155). Two new features appear

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Fig. 37 FTIR difference spectra of NH3 adsorbed on H-SSZ-13 (a), Cu-SSZ-13(67) (b), and Cu-SSZ-13(100) (c) zeolites at 250°C. Note the presence of at least two different NH3 species: NH4 + ions (1448 and 1393 cm1) and Cu[(NH3)4]2+ complexes (1619 and 1278 cm1). Reproduced with permission from Lezcano-Gonzalez, I.; Deka, U.; Arstad, B.; Van Yperen-De Deyne, A.; Hemelsoet, K.; Waroquier, M.; Van Speybroeck, V.; Weckhuysen, B. M.; Beale, A. M. Phys. Chem. Chem. Phys. 2014, 16, 1639–1650.

in the Cu-exchanged samples (Fig. 37) that are attributed to bending modes of Cu2+-bound NH3, based in part on comparison of DFT-computed vibrational spectrum of a [Cu(NH3)4]2+ complex contained within the SSZ-13 cage. These features grow in intensity at the expense of the NH4 + features with increasing Cu exchange. The same spectral features are observed on Cu-SSZ-13 samples of various Si/Al and Cu/Al ratios that are synthesized using TMAda+ in fluoride medium (143). The intensities but not the peak locations are sensitive to sample composition. The vibrational spectra of NH3-saturated commercial Cu-SSZ-13 and Cu-SAPO-34 (218) are consistent with those of fluoride-synthesized materials (143,155). 4.3.2 NH3 Transient IR Experiments Giordanino et al. (143) collected transient IR spectra (Fig. 38) during NH3 adsorption and desorption from a Cu-SSZ-13 sample (Si/Al ¼ 13.1,

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3800

63 14 56

14

24

12

0.2

15

90

13

14 1

0

31

86

32

37 36 56 36 11 35 84

37

Absorbance

21

0.5

16

33

32

33

80

79

B 41

A

3600

3400 3200 Wavenumber (cm–1)

3000

1600 1500 1400 Wavenumber (cm–1)

Fig. 38 FTIR spectra of Cu-SSZ-13 at increasing contact time with 1800 ppm of NH3/He mixture (100°C). Spectra are reported in both v(NH) (A) and δ(NH) (B, spectra reported after the subtraction of the spectrum of the dehydrated zeolite sample) regions. Orange curve refers to activated sample, red curve refers to highest contact time, that is, highest ammonia loading, and gray curves relate to intermediate ammonia loadings. Green curve refers to the total consumption of Brønsted sites. Dotted black curve in (B) refers to the spectrum of Cu-SSZ-13 upon NH3 adsorption at room temperature. Reproduced with permission from Giordanino, F.; Borfecchia, E.; Lomachenko, K. A.; Lazzarini, A.; Agostini, G.; Gallo, E.; Soldatov, A. V.; Beato, P.; Bordiga, S.; Lamberti, C. J. Phys. Chem. Lett. 2014, 5, 1552–1559.

Cu/Al ¼ 0.44) synthesized using TMAda+ in fluoride medium, which contains primarily Brønsted sites and ZCuOH sites. Initial NH3 dosing results in the simultaneous appearance of features attributed to NH4 + and to Cu2+bound NH3, consistent with similar binding energies of NH3 on both types of sites. Further, the ZCuO-H stretch mode (3656 cm1) is unchanged upon NH3 adsorption, which reflects the NH3 preference for binding to the Cu center over binding to the ZCuOH proton. The ZCuO-H stretch mode disappears upon further NH3 dosing, potentially due to reduction of ZCuOH to ZCu by NH3. The ammonium bending features become blue shifted at the highest NH3 dosing levels, most likely due to NH3 physisorption onto NH4 + (58,142,219,220). Giordanino et al. further monitored changes in these vibrational features while increasing the temperature from 100°C to 500°C (Fig. 39) (143). From 100°C to 200°C, IR features for ammonium bending modes red-shift to the locations observed for bare NH4 + ions. The NH4 + modes are at the same location as that reported by Leczano-Gonzalez et al. at 250°C (155).

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Fig. 39 Helium NH3-temperature-programmed desorption (TPD) over Cu-SSZ-13 followed by FTIR in the 100–500°C temperature range. Spectra are reported in both v(NH) (A) and δ(NH) (B, spectra reported after the subtraction of the spectrum of the dehydrated zeolite sample) regions. Gray curves refer to spectra recorded at intermediate temperature. In (A), the spectrum of dehydrated sample is also reported (orange curve). (C) The deconvoluted δ(NH) spectra of NH4 + ions at different desorption temperatures. Filled area refers to NH4 + .nNH3 associations band observed at 1460 cm1. The two other components at 1450 and 1400 cm1 refer to the antisymmetric and symmetric bending vibrations of NH4 + . Reproduced with permission from Giordanino, F.; Borfecchia, E.; Lomachenko, K. A.; Lazzarini, A.; Agostini, G.; Gallo, E.; Soldatov, A. V.; Beato, P.; Bordiga, S.; Lamberti, C. J. Phys. Chem. Lett. 2014, 5, 1552–1559.

Also between 100°C and 200°C, the bending features associated with Cu-bound-NH3 (1620 cm1) decrease in intensity as NH3 desorbs from the Cu sites (206). By 400°C (purple line), the 1620 cm1 bending mode feature assigned to Cu2+-bound NH3 has disappeared, and by 500°C the NH4 + features diminish in intensity. The ZCuO-H stretch mode near 3656 cm1 does not reappear during the temperature ramp in flowing He. The autoreduction process that leads to the loss of this feature upon heating is thus not reversible in the absence of an added oxidant. The Cu-bound-NH3 features disappear at lower temperatures in the temperature ramp experiments on SAPO-34 (218) than that on SSZ-13, suggesting that the support affects the NH3 binding energy on Cu ions.

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4.4 Magnetic Spectroscopy With NH3 Moreno-Gonza´lez et al. (147) studied the effects of NH3 on Cu2+ species with EPR, employing a Cu-SSZ-13 sample synthesized via a one-pot method (103) to achieve a high density of isolated Cu ions. A signal attributed to a single isolated Cu2+ ion is present in the calcined material, and it evolves into a signal indicative of NH3-solvated Cu2+ upon ambient temperature NH3 dosing. The number of NH3 in the first-coordination sphere decreases with increasing temperature, ultimately evolving into the signal for unsolvated Cu2+ at 350°C. Upon heating to 350°C, a fraction of Cu2+ is reduced to Cu+, evidenced by the decrease in Cu2+ signal intensity. Temperature-dependent nuclear magnetic resonance (NMR) spectra of 15 N-enriched samples indicate that Cu2+ cations are fully coordinated by NH3 at ambient temperature but evolve to Cu2+ ions with two NH3 and two framework oxygen atoms in the first-coordination sphere at 150°C (147). Thermodynamics models based on DFT-computed energies similarly predict a substitution of NH3 ligands by framework oxygen at >200°C temperature range, as discussed further in Section 5 (58).

4.5 X-Ray Spectroscopy With NH3 The NH3-induced reduction of Cu2+ observed in EPR is more directly probed with XAS and XES, which observe changes in Cu oxidation state and coordination environment as a function of gas composition. X-ray experiments have explored catalysts exposed to NH3 alone, NH3 with O2 or NO also present, and at standard SCR conditions. 4.5.1 Ammonia and Oxygen XANES spectra of Cu-SSZ-13 zeolites, whether prepared by FAU conversion, by the hydroxide route, or by using TMAda+ in fluoride medium, all indicate reduction of approximately 20% of Cu2+ ions to Cu+ after exposure to NH3 and O2 at 200°C (58,98,126). The EXAFS spectra can be fit to a fourfold coordinated Cu2+ ion that lacks higher shell structure (58,98,187). The XANES spectra of Cu-SSZ-13 samples synthesized via the hydroxide route and treated in NH3 and O2 at 200°C indicate that 50% of Cu2+ ions are reduced to Cu+, higher than that reported on the materials discussed earlier (85). XANES and XES are further consistent with coordination of both oxidized and reduced Cu ions by NH3.

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4.5.2 Ammonia and Helium NH3 is a more effective reductant of Cu2+ centers in the absence of O2, at least on samples containing primarily Cu2+ only in the isolated ZCuOH form. Giordanino et al. (143) exposed a Cu-SSZ-13 catalyst prepared by the fluoride route, which contains nearly 100% ZCuOH, to 1300 ppm NH3 in He at 120°C. Resultant XANES spectra resembled that of [Cu(NH3)2]+ (Fig. 40) (221). The intensity and location of Cu emission in the XES are indicative of a Cu ion linearly coordinated to nitrogen ligands (222). This loss of Cu2+ observed in XAS spectra is consistent with the irreversible loss of CuOH+ vibrational signatures after NH3 dosing observed by IR spectroscopy (143). Gunter et al. report that 100% of Cu2+ is reduced to Cu+ in NH3 and He at 200°C on a Cu-SSZ-13 zeolite synthesized by the hydroxide route (Si/Al ¼ 16, Cu/Al ¼ 0.2) that contains both ZCuOH and Z2Cu sites (85). Despite the fact that these samples were prepared using the hydroxide-mediated synthesis route, they appear to have a systematically increased susceptibility to Cu2+ reduction relative to samples in other reports (143). Similar experiments on Cu-SSZ-13 materials containing predominantly Z2Cu have not been reported. The mechanisms by which NH3 reduces CuOH+ or Cu2+, and their relevance to SCR catalysis or NH3 oxidation, are not currently well understood. DFT calculations have been used to examine the dissociation of NH3 across the Cu(II)–O framework bond of a Z2Cu site (126,147): Z2 Cu + NH3 ! ZCuNH2 =ZH

(16)

Paolucci et al. compute the NH3 dissociation barrier to be 120 kJ mol1 using an HSE06-DFT model and, from analysis of the computed Cu charge density, find that Cu is not fully reduced to Cu+ by adsorbed NH2 (126). Moreno-Gonza´lez et al. report that the dissociation barrier is decreased by additional physisorbed NH3 in the supercell (147), but this result is unlikely to alter the oxidation state of the product. Similar calculations for the direct dissociation of NH3 on a ZCuOH model site have not been reported. Thus, the available DFT results do not fully explain experimentally observed NH3 reduction of Cu2+, and there exists a gap in our understanding of the relationship between NH3 adsorption and Cu reduction.

Fig. 40 Left: XANES spectra of the 1 Al (top) and 2 Al (bottom) Cu-SSZ-13 samples under treatment in 2% H2O, 10% O2, 300 ppm of NH3 at 473K (O2 + NH3, blue traces), 2% H2O and 300 ppm of NO/NH3 at 473K (NO + NH3, red lines), and in 2% H2O, 10% O2, 300 ppm of NO/NH3 at 473K (black traces). Middle: EXAFS spectra collected under same conditions. Right: AIMD Cu–Si/O/Al RDFs for the most stable Cu1 (red lines) and CuII (blue traces) species on the 1 and 2 Al sites in the presence of NH3. Insets, integrated RDFs. Reprinted with permission from Paolucci et al., J. Am. Chem. Soc. 2016, 138 (18), 6028–6048. Copyright 2016 American Chemical Society.

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4.5.3 Ammonia and Nitric Oxide While the details of redox chemistry of Z2Cu and ZCuOH sites with NH3 are ambiguous, the behavior of both of these sites with NH3 and NO together is more clearly established. XANES have been reported for Cu-SSZ-13 zeolites prepared using TMAda+ only in fluoride medium (98), hydroxidemediated (58), and FAU conversion (58,126) routes, which is a suite of samples that contain both Z2Cu and ZCuOH sites. XANES spectra collected after saturation with stoichiometric mixtures of NO and NH3 reveal complete conversion of Cu2+ to Cu+, as evidenced by the appearance and intensity of the edge feature at 8.983 keV (58,98,126) (Fig. 40) that is identical to that observed on aqueous [Cu(NH3)2]+. EXAFS spectra are similar among all samples and reveal twofold-coordinated Cu without significantly intense higher coordination shells, again consistent with EXAFS of an aqueous [Cu(NH3)2]+ reference (98,221,223,224). The reduction occurs over a period of approximately 2 min in transient XANES experiments (200°C) (126) and thus occurs more rapidly than Cu2+ reduction by NH3 or by NH3 and O2 (126,143). The consistent in situ reduction behavior among Cu-SSZ-13 samples made by various synthetic methods stands in sharp contrast with the differences observed in ex situ experiments. DFT calculations have been more successful in explaining Cu reduction by NO and NH3 than reduction by NH3 alone. Supercell DFT calculations using a hybrid-exchange functional (HSE06) predict NH3 to outbind NO at a Z2Cu site by 100 kJ mol1 and at a ZCuOH site by 70 kJ mol1 (58,126). While direct NH3 dissociation is computed to have an activation energy of 120 kJ mol1 and does not fully reduce Cu, an NO-assisted NH3 dissociation process (Fig. 41) is computed to have an energy barrier of only 70 kJ mol1 on NH3-solvated Z2Cu or ZCuOH sites (58): Z2 CuNH3 + NO ! ZCuðH2 NNOÞ=ZH ! ZCu=ZH + N2 + H2 O

(17)

ZCuOHðNH3 Þ + NO ! ZCuðH2 NNOÞ + H2 O ! ZCu + N2 + 2H2 O

(18)

The reaction involves attack of NO on NH3 with simultaneous lengthening and dissociation of an N–H bond. Both reactions are predicted to produce a nitrosamine (H2NNO) intermediate that will decompose rapidly to N2 and H2O, leaving behind a fully reduced Cu+ site and either a new Brønsted acid site or H2O. These two reduction routes of Z2Cu and ZCuOH are distinguishable by the formation of one or zero additional Brønsted acid sites, respectively. Brønsted acid site titrations performed

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Fig. 41 HSE06 CI-NEB calculated activation (Ea) and reaction energies for NO-assisted redaction of NH3 solvated CuII 1 Al (black) and 2 Al (green) sites. Transition state structures are shown boxed. For ease of visualization, most of the zeolite framework is hidden. Reprinted with permission from Paolucci et al., J. Am. Chem. Soc. 2016, 138 (18), 6028–6048. Copyright 2016 American Chemical Society.

before and after reduction experiments confirm that the number of new Brønsted sites formed via reduction of Cu-SSZ-13 samples is directly proportional to Z2Cu sites and independent of ZCuOH sites (58,126).

4.6 Operando X-Ray Spectroscopy Many groups have reported XAS spectra collected under standard SCR conditions near 150–200°C and gas streams containing 10% O2 and 300–1000 ppm NO and NH3. The results are similar on Cu-SSZ-13 zeolites synthesized by FAU conversion (126), hydroxide-assisted synthesis (85), and fluoride-assisted synthesis using TMAda+ only (225), and on Cu-SAPO-34 (226). Deconvolution of the XANES suggests an approximately 50:50 mixture of Cu+ and Cu2+ to be present at these reaction conditions. EXAFS analysis yields an average Cu coordination number of three without any higher shell structure, as would be expected from a 50:50 mixture of twofold-coordinated [Cu(NH3)2]+ and fourfold-coordinated [Cu(NH3)4]2+ (Fig. 40). The exact kinetic and mechanistic origins of the 50:50 mixture remain to be resolved, although these results suggest that

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75

the reduction and oxidation half-cycles are both rate controlling (58,126). Under fast SCR conditions with NO2 as oxidant, XAS analysis shows 100% Cu2+ during steady-state catalysis, suggesting that NO2 is able to more effectively reoxidize Cu+ than O2 alone at these conditions (122). Under standard SCR conditions at 200°C, the location and intensity of features in the Cu XES are also consistent with Cu ions of two oxidation states, both associated with NH3 (222). EPR (147), IR (143), and XAS (98) spectra collected under these conditions all support this picture. Lomachenko et al. (225) reported XES spectra collected under operando SCR conditions at 150°C. The calcined catalyst has an O-bound Cu spectral feature that evolves to an N-bound Cu feature in the presence of an SCR gas mixture. The observed intensities are not as pronounced as the aqueous ammoniated Cu references, which may reflect the presence of framework oxygen or H2O in competition with NH3 for the Cu first-coordination sphere. XAS spectra collected under SCR conditions have been reported at temperatures up to 400°C on Cu-SSZ-13 zeolites prepared by the hydroxide route (83) and by the fluoride route using only TMAda+ cations (225). Above 200°C, the Cu+ feature diminishes at the expense of Cu2+, and the Cu coordination number increases to 4. Deka et al. noted the similarities between the XANES and EXAFS spectra collected at higher temperatures under SCR conditions and those of calcined samples, concluding that the Cu2+ is bound to framework oxygen at both conditions, likely in the 6-MR of CHA (83). Lomachenko et al. collected EXAFS spectra in the 250–300°C range and observed fourfold coordination of Cu and a second shell scatter attributed to framework Al, noting the difficulty in making unambiguous assignments because of the presence of multiple Cu species (227). They assign the primary Cu2+ state, based on comparison to DFTcomputed structures, to a Cu2+ ion twofold coordinated to the framework oxygen near an Al and twofold coordinated to a nitrate, a structure that can be represented as [ZCuNO3] (227). As the temperature is increased to 400°C, the Cu coordination number decreases to 3 and the second shell scatter attributed to Al of a ZCuOH site appears (227), consistent with the majority CuOH+ form of Cu species expected from the zeolite synthesis approach used in this study. XES experiments are also consistent with the loss of NH3 solvation and appearance of framework oxygen coordination in the 300–400°C temperature range (225). Although analogous experiments on samples containing only Z2Cu sites have not been reported, these results show that the majority Cu oxidation state and coordination number depend strongly on the SCR temperature.

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Operando XAS have been reported from measurements made at different locations along an SCR catalyst bed containing Cu-SAPO-34 held at 255°C (228). The entrance of the reactor has the highest concentrations of NO and NH3, and the XANES and EXAFS spectra collected near the entrance indicate that Cu ions are NH3-solvated Cu and present in a 55:45 ratio between the 2 + and 1 + oxidation states. The prevalent Cu oxidation states are thus similar to that observed in operando XAS at 150°C (225) and 200°C (58,126) over Cu-SSZ-13 zeolites. The fraction of Cu1+ decreases along the length of the catalyst bed, as expected from the concomitant decrease in NO and NH3 concentrations. The ratio of Cu oxidation state is thus sensitive to gas composition as well as temperature (225). A complete SCR mechanism will account for these dependencies.

5. CATALYTIC ACTIVITY AND MECHANISM Catalytic performance in the NOx SCR literature is most commonly reported in the form of a “light-off” curve, which shows the dependence of NOx conversion on temperature from experiments performed at fixed space velocity, as illustrated in Figs. 1 and 2. When measured at space velocities relevant for practical aftertreatment systems, these data provide a convenient way to estimate the performance of a catalyst against that required to meet emissions standards. Such data integrate information about reaction and transport rates at varying gas composition across the catalyst bed, which may be a powder or a monolith. Therefore, such data provide only indirect information about the underlying kinetic behavior and its relationship to catalyst structure and composition. Different explanations can be used to rationalize light-off curve behavior that differs on two catalysts, such as differences in the number of active sites or in their intrinsic activity, or even to interpret the light-off behavior of a given catalyst. For example, the minimum in the NOx conversion observed on many SCR catalysts near 350°C (Fig. 42), referred to as the “seagull” phenomenon, has been attributed to changes in the active site structure, in the reaction mechanism, in the prevalence of competing side reactions, or some combination of these factors (134,229). Differential rate measurements collected in the absence of mass and heat transfer resistances provide direct information about the kinetic behavior of catalytic systems and, in turn, how this behavior changes with catalyst structure and composition (230). In a differential reactor, the entire catalyst is maintained in contact with uniform gas composition and temperature. Parameters determined from variations in differential rates with temperature (apparent activation energy) and gas compositions (apparent reaction rate

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Catalysis Science of NOx SCR

100 90

NOx conversion (%)

80 70 60 50 40

Commercial Cu-Chabazite Catalyst

30

NO = NH3 = 200 ppm,

20

10% O2, 8% CO2, 7% H2O, balance N2,

10

GHSV = 40,000 h–1

0 150 200 250 300 350 400 450 500 550 600 650 Temperature (°C)

Fig. 42 Steady-state NOx conversion on a degreened commercial Cu/Chabazite catalyst. Reproduced with permission from Gao, F.; Walter, E. D.; Kollar, M.; Wang, Y.; Szanyi, J.; Peden, C. H. F. J. Catal. 2014, 319, 1–14.

orders) can be related directly to microscopic mechanistic models. Differential rates are expressed quantitatively as turnover rates, which are normalized to the number of putative active sites (230). The discrete nature of exchanged Cu sites in Cu-SSZ-13 and Cu-SAPO-34 renders them convenient to use in normalization of rate data, in order to quantitatively compare the kinetic behavior of samples of different structures and compositions, and to ultimately establish a microscopically-detailed SCR mechanism. In this section, we discuss the literature of SCR rate measurements and their mechanistic interpretations, first those relevant to low-temperature ( 150–200°C) standard SCR operation and then their extensions to higher temperature regimes of operation (300–400°C). We then consider findings reported on NO and NH3 oxidation, which provide complementary information about the mechanism and reaction selectivity in SCR. Finally, we use these fundamental kinetic insights to rationalize observations reported regarding nondifferential light-off behavior and observations on commercial catalysts. As noted in Section 4, the structure of active Cu sites in SCR catalysts depends on the temperature and gas compositions, and thus a complete mechanistic understanding must incorporate this information. We discuss how experiment and theory have converged on several aspects of SCR mechanism and its dependence on catalyst composition and structure, and discuss certain aspects of the mechanism that still are not precisely understood.

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5.1 Differential Standard SCR Kinetics A principal challenge in practical SCR application is to increase the NOx conversion over a wider temperature range to realize further emissions reductions, specifically at the low temperatures experienced during coldstart and low-load conditions that account for the majority of NOx emissions. For this reason, a significant amount of fundamental research has focused on SCR measurements and mechanistic studies in the 150–200°C temperature range. Laboratory measurements of standard SCR rates (where “standard” refers to the use of O2 as the oxidant) are typically made in the presence of gas streams comprising 10–15% O2, a stoichiometric mixture of 300–500 ppm of NO and NH3, 2–5% H2O, 2–5% CO2, and balance N2. The catalyst compositional variables introduced in Section 2, including Si/Al and Cu/Al ratios, have been studied widely because they influence the SCR rate and mechanism. Fig. 43 (105) reports standard SCR rates (per g, 200°C) collected on an H-SSZ-13 material synthesized by FAU conversion (Si/Al ¼ 5) and exchanged with Cu to achieve compositions ranging from 0.31 to

Std. SCR rate (10–6 mol NO g–1 s–1)

4 3.5 3 2.5 2 1.5 1 0.5 0 0

0.1

0.2

0.3

0.4

Cu:Altot

Fig. 43 The standard SCR rates per gram of Cu-SSZ-13 catalysts with Cu:Altot ranging from 0 to 0.35. The rates are reported at 473K. The standard SCR conditions used were 320 ppm NO, 320 ppm NH3, 10% O2, 8% CO2, 6% H2O, and balance Helium at 473K. The 90% confidence interval for rate per gram was 0.2 mol NO g1 s1. Reprinted from Bates et al., J. Catal. 2014, 312, 87–97, Copyright (2014), with permission from Elsevier.

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6.39 wt% Cu, corresponding to Cu/Al 0.02–0.35, respectively. Cu-SSZ-13 catalysts in the Cu/Al 0.01–0.20 composition range were characterized to contain only Z2Cu sites by UV–vis and XAS spectroscopies and quantification of residual H+ sites by NH3 titration, consistent with the saturation of Z2Cu sites computed to occur at Cu/Al 0.23 for an SSZ-13 zeolite with Si/Al ¼ 5 and a random framework Al distribution subject to L€ owenstein’s rule (57). The SCR turnover rate (per g, 200°C) increases linearly with Cu content until a Cu/Al composition of 0.2, suggesting that all Cu ions are catalytically equivalent in this regime and that the kinetic data are uncorrupted by internal transport artifacts by the Koros–Nowak criterion (231). Beyond 4 wt% Cu, the SCR turnover rate (per g) decreases, because these catalysts contain some Cu present as Cu oxide clusters (66) that do not contribute equivalently to the SCR rate. On a Cu-SSZ-13 (Si/Al ¼ 6) with compositions ranging from 0.5 to 5 wt% Cu, SCR rates also increased linearly with Cu content up to 3 wt% and then depended weakly on Cu content with further increases in Cu loading (134). The same dependence on rate as a function of Cu wt% was observed at 140, 155, 170, and 185°C (134). These data all indicate that isolated Cu cations, but not Cu oxides, are predominantly responsible for the observed SCR activity on Cu-SSZ-13; in turn, these isolated Cu ions are a logical choice to normalize low-temperature SCR rates. Differential standard SCR turnover rates at 200°C on Cu-SSZ-13 materials synthesized via FAU conversion (58,105,122,126,133), hydroxidemediated (58,59), and fluoride-mediated routes (98,232) are approximately 8  103 (mol NO mol Cu1 s1), which corresponds to roughly one turnover per Cu site every 2 min. Apparent SCR activation energies on Cu-SSZ-13 materials containing >0.5 wt% Cu are in the range of 70  10 kJ mol1 (58,59,98,105,126,133,232,233). Fig. 44A (105) reports representative Arrhenius plots collected on Cu-SSZ-13 materials synthesized by FAU conversion in compositions ranging from 0.31 to 6.39 wt% Cu. Apparent activation energies are constant within error on all materials except that with the lowest Cu content (0.31 Cu wt%). Apparent rate orders measured on materials with >0.5 wt% Cu that are prepared by FAU conversion (58,105) and hydroxide synthesis (58) are similar and approximately 0.3, 0.8, and 0 for O2, NO, and NH3, respectively (Fig. 44B) and are 0 for both H2O and CO2 (105). Cu-SAPO-34 materials synthesized either in one-pot (75) (0.63–1.01 wt% Cu) or by aqueous Cu ion exchange (232) (1.2 wt% Cu) exhibit apparent activation energies of 40–50 kJ mol1. Thermal annealing at temperatures >750°C that disperse Cu more uniformly in

80

B

–4

1.5 1.3

–4.5 –5 Cu:Altot = 0.2

–5.5 –6

Cu:Altot = 0.09 Cu:Altot = 0.04

–6.5 –7

Cu:Altot = 0.02

–7.5

Cu:Altot = 0.16

Apparent order of reaction

Ln (TOF/mol NO mol Cu(II)–1 S–1)

A

C. Paolucci et al.

NO

1.1 0.9 0.7

O2

0.5 0.3 0.1 –0.1

NH3

–0.3

Cu:Altot = 0.35

–8 0.002

–0.5 0.0021

0.0022

0.0023

T –1 (K–1)

0.0024

0

0.1

0.2

0.3

0.4

Cu:Altot

Fig. 44 (A and B) Arrhenius plots and reaction orders of the 6 Cu-SSZ-13 catalysts active for standard SCR. The temperature range used for the Arrhenius plots was 433–473K. The standard SCR conditions used are 320 ppm NO, 320 ppm NH3, 10% O2, 6% H2O, and 8% CO2. Reaction orders for NO, NH3, and O2 are shown as a function of the Cu:Altot ratio. Individual gas concentrations were changed, while all other gases were held constant. No orders were taken with NO concentrations ranging from 75 to 600 ppm, NH3 orders were taken from 250 to 600 ppm, and O2 orders were taken from 2.5% to 20% of the feed. The 90% confidence interval for activation energies and reaction orders was 5 kJ mol1 and 0.1, respectively. Reprinted from Bates et al., J. Catal. 2014, 312, 87–97, Copyright (2014), with permission from Elsevier.

SAPO-34, as inferred by X-ray photoelectron spectroscopy, exhibits higher apparent activation energies of 70 kJ mol1. Similar TOR and apparent activation energies are thus reported on Cu-SSZ-13 samples that contain only Z2Cu (58,126), ZCuOH (98,232), or a mixture of the two sites (58,59,171) as determined by ex situ characterizations, and on Cu-Beta and Cu-ZSM-5 zeolites of similar composition range (1.5–3.5 wt% Cu) (58), suggesting that neither the Cu speciation nor the zeolite topology strongly influences the rate or the mechanism in this kinetic regime. Differential standard SCR rates (per g, 185°C) reported by Gao et al. (134) over a wide range of Cu loadings on an H-SSZ-13 material of Si/Al ¼ 6 prepared by FAU conversion (Fig. 45) appear to deviate from the linear SCR rate dependence on Cu content for samples in which the Cu loading is type B1 cracking  type B2 cracking > type C cracking > type D cracking The high rate for type A β-scission compared to PCP branching limits the maximum observed branching degree within the product slate to three as rapid consecutive alkyl shifts convert tribranched species, once formed, to highly reactive isomers in the α,γ,γ-configuration, i.e., one branch in α and two branches in γ position with respect to the charge. In contrast to the monobranched and dibranched isomer lump in which concentrations close to thermodynamic equilibrium are usually measured (35,42), thermodynamic equilibrium within the tribranched isomer lump is not reached due to fast type A cracking (41,43). Isomerization toward tribranched species requires a sufficiently long hydrocarbon chain length and is only possible from heptane molecules on. However, any carbenium ion derived from 2,2,3-trimethylbutane as the only heptane isomer with three branches is unable to undergo type A β-scission, or any other cracking type. The hydrocracking product slate of reactants up to heptane is therefore primarily composed of feed isomers, while the contribution of monobranched cracking products increases significantly from octane feeds on. This results in a typically symmetric cracking product distribution, as shown for n-octane hydrocracking in Fig. 2 if the contributions were added per carbon number (41). iso-Butane is predominantly formed via the only type A β-scission 70

Contribution (%)

60 50 40 30 20 10 0

Propane

Butane Cracking product

Pentane

Fig. 2 Percentual contribution of linear (shaded) and branched (white) cracking products within the cracking product distribution of n-octane hydrocracking on a Pt/USY catalyst (41). Reprinted from Martens, J. A.; Jacobs, P. A.; Weitkamp, J. Appl. Catal. 1986, 20, 283–303, Copyright (1986), with permission from Elsevier.

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reaction in the n-octane reaction network, vide Scheme 3, which, remarkably, is primarily responsible for the drastic reduction in maximum attainable isomer yield compared to the heptane hydrocracking product slate (8,35). The typical consecutive nature of the classical reaction network yields a product distribution shown in Fig. 3 observed for n-decane hydrocracking over a USY catalyst loaded with 0.5 wt% Pt (42). Monobranched decane isomers constitute the primary products formed via PCP branching of n-decane. Further, isomerization leads to dibranched species, while, at the same moment, only a minor amount of cracking products are produced via type B1, B2, and C β-scission. Tribranched species, once formed, contribute only slightly to the final product distribution owing to their rapid cracking toward lighter products. It is not until the formation of these isomers that cracking products become dominantly present in the product slate. Extension toward cycloalkane molecules comprises the incorporation of additional acid-catalyzed elementary steps in the reaction network. Martens et al. (44) considered isomerization via intraring alkyl shifts which either leads to a change in ring size, or a change in branch position on the ring structure, and via cyclic PCP branching. In addition, cracking could also occur through endocyclic and exocyclic β-scission, vide Scheme 4. The rates of these elementary steps also depend strongly on the reactant and product ion type and are generally of the same magnitude of the corresponding 40

CR Prodcuct yield (%)

30

Mono 20

10

Di

Tri 0

0

20

40

60

80

100

Conversion (%)

Fig. 3 Yields of mono-, di-, and tribranched isomers and cracked products as a function of the total conversion of n-decane over Pt/USY (42). Reprinted from Martens, J. A.; Tielen, M.; Jacobs, P. A. Catal. Today 1987, 1, 435–453. Copyright (1987), with permission from Elsevier.

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A

B +

+

+ +

Intraring alkyl shift :(A) ring contraction or expansion without altering the branching degree (B) altering the relative positions of the substituents on the ring

+ + +

+

Cyclic PCP branching: ring contraction or expansion altering the branching degree

+

+

+

+

Endocyclic β-scission

+

Exocyclic β-scission

Scheme 4 Additional acid-catalyzed elementary steps involved in the hydrocracking of cycloalkanes (44). Reprinted with permission from Martens, G. G.; Thybaut, J. W.; Marin, G. B. Ind. Eng. Chem. Res. 2001, 40, 1832–1844. Copyright (2001) American Chemical Society.

isomerization and cracking reactions involved with acyclic carbenium ions, except for endocyclic β-scission which is considerably slower (8,44). Hydrocracking of mono- and polyaromatics implies either partial or complete hydrogenation to (poly)naphthenes, essentially leading to the same reaction network as obtained for cycloalkanes (45,46), but could also involve cyclization of alkyl substituents to polycyclic structures. As an illustration, Sullivan et al. (47) observed significant tetralin formation, i.e., a selectivity of about 39%, during n-decylbenzene hydrocracking over a sulfided Ni/SiO2– Al2O3 catalyst. The presence of unsaturated bonds within aromatic structures leads to strong competition with paraffinic and naphthenic species for the (de)hydrogenation reactions on the metal sites (48), but also with the olefinic intermediates for chemisorption on the acid sites (49–51). The classical bifunctional reaction mechanism described earlier is followed solely when the activities of the metal and the acid function are well balanced. A too low acidity relative to the metal function would lead

Multiscale Aspects in Hydrocracking

125

to primarily hydrogenolysis, i.e., cracking on the metal sites followed by hydrogenation, while too strong acid sites lead to several acid-catalyzed reaction steps in a single catalytic cycle (8). The latter mechanism strongly resembles the FCC mechanism and is often referred to as Haag–Dessau hydrocracking. Reaction temperatures exceeding 623K could additionally induce thermal hydrocracking via free radicals which are hydrogenated by active atomic hydrogen atoms formed on the metal phase (52,53). In Section 2.1.4, conditions required for the establishment of so-called ideal hydrocracking, which gives rise to the typical consecutive reaction mechanism depicted in Scheme 1, are discussed in more detail. 2.1.1 Nature of the Protonated Intermediate The reaction network described in Section 2.1 considers carbenium ions as the reactive species associated with the catalyst acid sites. However, at that time, a publication by Kazansky and Senchenya (36) questioned the ionic nature of this reaction intermediate based on ab initio simulation results. It was shown that proton donation to physisorbed alkenes onto various silica–oxygen or alumina–oxygen clusters with dangling bonds saturated by hydrogen atoms rather resulted in an alkoxy species covalently bound to the surface. This was evident from the calculated distance between the carbon atom and the basic oxygen bridge atom bound to the surface Al. In addition, the bond angles in the hydrocarbon fragments suggested an approximately tetrahedral configuration which were incompatible with the presence of an electron-deficient carbon atom. Methoxyl, ethoxyl, and isopropoxyl substituents were used as model protonated species and showed a decreasing trend in partial positive charge on the carbon atom bound to the surface oxygen. A large alkyl group chemisorbed on a Brønsted acid site should therefore resemble more to the alkyl fragment in the corresponding alcohol than to a free carbenium ion (36). The formation of a surface alkoxy intermediate rather than a carbenium ion upon protonation was confirmed by quantum mechanical calculations reported in other publications which predict covalently bound alkoxy species derived from propane (54), iso-hexane (55,56), and C3-to-C6 alkanes (57). Scheme 5 shows the proposed protonation mechanism for ethylene on an acid aluminum hydroxyl group. Excitation and, hence, prolongation of the C–O bond increase the carbenium ion character of the surface alkoxy group and, more importantly, require a relatively low activation barrier to be overcome (36). Therefore, desorption from the acid site is likely to occur over a transition state with an

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H

H C

H

H

C H

H

O

H

H

C

C H

CH2

H

O

O

O

O

O

O

O

O

O Al

Al

Al O

CH3

O

Scheme 5 Formation of a covalently bound ethoxy group via protonation (36). Reprinted from Kazansky, V. B.; Senchenya, I. N. J. Catal. 1989, 119, 108–120. Copyright (1989), with permission from Elsevier.

ionic character in which the C–O bond is elongated, hence, following the reverse reaction path depicted in Scheme 5. As a consequence, the transition state strongly resembles a nearly free carbenium ion still experiencing electrostatic interactions with the catalyst surface (55,58). Where fundamental principles propose surface alkoxy species as the reactive intermediates within the hydrocracking network, experimental research tends to be in favor of the carbenium ions. In response to the earlier discussion, Denayer et al. (59) performed C6-to-C9 hydrocracking experiments over three Pt-loaded faujasites which differed in acidity. A distinct maximum was observed in activity for monobranching, multibranching, and cracking rates at intermediate Si/Al ratio, vide Table 2 for monobranching through a PCP, owing to the inverse relation between the number of acid sites and the individual acid site strength. However, the intrinsic rate coefficients for n-heptane, n-octane, and n-nonane isomerization relative to the coefficient obtained for n-hexane and scaled to the number of possible PCP structures were found practically independent of the catalyst acidity. According to the surface alkoxy mechanism, the transition state formation should in theory depend on the basicity of the bridging oxygen atom which in turn is related to the acid strength of the hydroxyl group related to that site (16). This was obviously not confirmed from experimental practice. Following the work of Denayer et al., other arguments were given which tilt the earlier discussion toward the free carbenium ion mechanism. Ab initio studies found steric constraints in surface alkoxy formation from species bulkier than isobutane due to tail rotation restrictions, and metastable tertiary carbenium ion states were rather formed instead (55). Fundamental microkinetic modeling results of a n-octane hydrocracking data set showed a pronounced difference in standard alkene protonation enthalpy amounting to 30 kJ mol
1 between secondary and tertiary ion formation (60), while

Table 2 Absolute and Relative Intrinsic Rate Coefficients for Monomethyl Branching Through a PCP Branching Reaction as Obtained on Three Pt-Loaded Faujasites with Different Si/Al Ratios (59) Pt/HY (Si/Al 2.7) Pt/USY (Si/Al 13) Pt/USY (Si/Al 30) Number of Component Reactions Absolute Rate (s21) Relative Rate Absolute rate (s21) Relative Rate Absolute Rate (s21) Relative Rate

n-Hexane

4

100

1

2600

1

530

1

n-Heptane

6

110

0.8

4100

1.1

650

0.8

n-Octane

8

220

1.1

5800

1.1

1100

1.1

n-Nonane

10

340

1.3

8100

1.3

1500

1.2

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J.W. Thybaut and G.B. Marin

surface alkoxy species have rather similar energies regardless of their primary, secondary, or tertiary nature (55,56). In response, Surla et al. (61) stated that both approaches become equivalent if, concordantly, a stabilization energy term is introduced which accounts for the energy difference between gasphase carbenium ions and their corresponding surface-bonded alkoxy counterpart. 2.1.2 Isomerization Alkyl chain isomerization could be categorized in the relatively fast type A isomerization which comprises either a hydrogen or an alkyl branch rearrangement, or type B isomerization which involves an increase or a reduction in branching degree. In hydride and alkyl shifts, a threemembered C–H–C or C–C–C ring structure is initially formed, vide Fig. 4, after which the C–H or C–C bond originally present in the reactant ion is broken. Apart from (de)protonation, hydride shifts are the fastest reactions within the acid-catalyzed part of the hydrocracking reaction network, while alkyl shifts occur at only a slightly lower rate (56). The formation of a branch was proposed to occur via a one-step methyl shift mechanism or via the formation of a PCP structure in the transition state (56). The former reaction mechanism was identified during butane isomerization, but is unlikely for longer feed alkanes. A one-step methyl shift inevitably involves the formation of an unstable primary ion and an energetically less demanding mechanism is available for such longer alkanes, i.e., the PCP mechanism. It comprises the transformation of the carbenium ion reactant to a PCP carbonium ion complex after which the ring structure is again

Fig. 4 Optimized transition states for 1,2-hydrideshift from 2-methylpent-2-yl ion to 4-methylpent-4-yl ion (left) and 1,2-alkylshift from 2-methylpent-3-yl ion to 3-methylpent-2-yl ion (right); elongated C–H bonds are indicated by black hydrogen molecules (56). Reprinted from Natal-Santiago, M. A.; Alcala, R.; Dumesic, J. A. J. Catal. 1999, 181, 124–144. Copyright (1999), with permission from Elsevier.

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opened at a different C–C bond subsequent to a proton transfer within the PCP (57). The mechanism of these proton jumps has still not been completely elucidated, but is believed to occur over edge- or cornerprotonated cyclopropane structures (33,54,56). The edge-protonated variant of the transition state is shown in Fig. 5 in the formation of 2,2dimethylbut-2-yl ion from 2-methylpent-2-yl ion. Scheme 6 illustrates the formation of 2-methylpent-3-yl and 3-methylpent-2-yl ion from a hex-2-yl ion via a corner-to-corner proton jump in the PCP complex (37). Protonated cycloalkane complexes larger than cyclopropane, virtually up to n-2 carbon atoms with n the carbon number of the feed alkane, were proposed to contribute to the overall isomerization mechanism even though their formation energy is significantly higher than that of a PCP, e.g., 130 kJ mol
1 higher in case of a protonated cyclobutane complex (37). Accounting for isomerization through such complexes improved the agreement between modeled and experimental monomethyl isomer yields and, additionally, offered an explanation for the early formation of ethyl side

Fig. 5 Optimized transition state for the skeletal isomerization of 2-methylpent-2-yl ion to 2,2-dimethylbut-2-yl ion via an edge-protonated cyclopropane; elongated C–H bonds are indicated by black hydrogen molecules (56). Reprinted from Natal-Santiago, M. A.; Alcala, R.; Dumesic, J. A. J. Catal. 1999, 181, 124–144. Copyright (1999), with permission from Elsevier.

H H +

H

CH3 H + H C2H5

H

CH3 H

H H

+ H

C2H5

H

CH3 H

H H

+

H

+

+

C2H5

Scheme 6 Formation of 2-methylpent-3-yl and 3-methylpent-2-yl ion from hex-2-yl ion through PCP branching (37). Reprinted from Martens, J. A.; Jacobs, P. A. J. Catal. 1990, 124, 357–366. Copyright (1990), with permission from Elsevier.

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branches during hydrocracking (37,62). Other researches assumed fast 1,2alkyl shifts within methyl branched isomers in the formation of ethylbranched species (15,34,63). The formation of longer side chains up to butyl was evident from n-hexadecane hydrocracking data over an amorphous Pt/SiO2–Al2O3 (39). Isomers with propyl and butyl branches were not found as primary products, hence confirming the marginal contribution of isomerization over protonated cyclopentanes and larger ring structures (62). In general, the yield of individual isomer species decreased significantly with the length of the side chains (33). Therefore, fundamental kinetic models described in Section 4 often only consider side-chain formation of methyl and ethyl groups only. 2.1.3 Cracking Cracking reactions within the classical reaction network were classified into four distinct types of β-scission, while neglecting primary carbenium ion formation. β-Scission involves hybridization of the bond between the charge-bearing carbon atom and an α neighbor to sp2, followed by the breakage of the C–C bond in β position and charge transfer, vide Scheme 3 for the cracking of various iso-octyl ions. Sie (64) proposed a different cracking mechanism based on a misinterpretation of the type A to C β-scission mechanism established in the 1980s and postulated that dibranched isomers needed to be formed prior to any cracking in order to avoid primary ion formation during reaction. Scission toward a branched fragment directly from a linear alkyl ion was proposed to occur through the formation of a PCP structure similarly as in type B isomerization, followed by double-bond creation in an alkyl substituent of the PCP ring via two consecutive hydride shifts, vide Scheme 7. This mechanism could reasonably predict the chain length dependence of the alkane reactivity in catalytic cracking (65) and in hydrocracking (66), but excludes physisorption effects which were identified as the primary cause for reactivity differences between alkanes of different sizes, vide Section 2.2 (67). Moreover, fundamental microkinetic models based on the classical reaction mechanism, which are described in Section 4.4, could nearly perfectly reproduce the typical symmetric hydrocracking product distribution as shown earlier in Fig. 2 (43), a feature which was not believed possible in Sie’s publication (66). In a more recent research by Berner et al. (68), cracking of free carbenium ions at elevated temperatures was simulated using ab initio calculated values for the first time. Only β-scission was found to

H C +

H C H2

n

H2 C

H2 C C H2

C H2

C H2

H C

H

H

C H2

m

H

H2 C

n

+ C CH3

n

CH3

H2 C

H C H+

C H2

+

H2C

H C H2

C H

C H2

H2 C

H C

H C H2

m

m

H

+ H C H H

n

H C

H C H2

n

H C

H C H

+ H C H H

H2 C

H C

H

H

C H2

m

H2 C

H C H

C H2

m

Scheme 7 Cracking mechanism via protonated cyclopropane formation and consecutive hydride shifts according to Sie (64). Reprinted with permission from Sie, S. T. Ind. Eng. Chem. Res. 1992, 31, 1881–1889. Copyright (1992) American Chemical Society.

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occur in contrast to cracking over PCP transition states. In addition, the authors discarded the mechanism proposed by Sie as it involves two carbon atoms bonded fivefold, hence violating the octet rule (Scheme 7, fourth structure), and because it describes γ-scission rather than β-scission which is not in agreement with experimental data. Blomsma et al. (69) discovered a different cracking mechanism from n-heptane hydrocracking data obtained over a Pd/H-Beta zeolite. The considerable formation of pentane and hexane molecules did not occur via type D β-scission as the methane and ethane production remained absent. Dimerization toward C14 carbenium ions immediately followed by β-scission could explain the observed deviations from the symmetric hydrocracking product distributions that were typically obtained. Cracking of these dimers could also form two C7 compounds which differ in branching degree with the starting “monomers.” (70) The contribution of dimerization cracking was attenuated by increasing the amount of Pd loaded on the zeolite, or by weakening the acid function of the zeolite, but could not be completely ruled out as shown in Fig. 6 (69). The dimerization-cracking mechanism is dominant for light alkanes such as butane for which conventional cracking modes are restricted or even absent. It becomes less important, however, with increasing chain length of the reactant molecule (16). In case the activity of the metal function significantly exceeds that of the acid function, hydrogenolysis may become important. Hydrogenolysis essentially constitutes partial or full dehydrogenation on the metal sites, followed by consecutive cracking and hydrogenation (71). The activation barrier for hydrogenolysis exceeds that for conventional β-scission typically

Cracking

HC

DC Pd content

Fig. 6 Contribution of classic hydrocracking (HC) and dimerization-cracking (DC) to the overall cracking yield as a function of the metal content in Pd/H-Beta (69). Reprinted from Blomsma, E.; Martens, J. A.; Jacobs, P. A. J. Catal. 1995, 155, 141–147. Copyright (1995), with permission from Elsevier.

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Multiscale Aspects in Hydrocracking

by about 45–55 kJ mol
1 according to Lugstein et al. (72) and, hence, becomes increasingly important at higher metal loadings and elevated reaction temperatures. Alkane hydrogenolysis leads to significant methane and ethane formation and its contribution greatly depends on the metal type. The latter was evident from n-heptane-hydrocracking experiments over a Ni/H-ZSM5 and a Co/H-ZSM5 where Co was clearly the better hydrogenolysis site (72). Other metals such as Pt and Mo are renowned for their hydrogenolysis capabilities, but often exhibit a different hydrogenolysis selectivity based on their preferential coordination with carbon atoms of specific electron densities (73,74). For instance, Ni mainly catalyzes demethylation, while Pt unselectively cracks any bond in the sorbate (74). The question remains whether small alkanes such as methane are formed directly out of the parent molecule or via secondary hydrogenolysis reactions, a discussion point which has still not been completely resolved (75). A different cracking mechanism over sulfided transition metals supported on nonacidic alumina was proposed by Roussel et al. (76,77) involving consecutive alkene chemisorption and proton abstraction by a S2
anion, vide Scheme 8. The mechanism could explain the high yields of n-octane and ethane and the absence of methane formation during n-decane hydrocracking over such catalysts, but was not further elaborated in any other succeeding research. 2.1.4 Ideal Hydrocracking The activity and product selectivity of a bifunctional hydrocracking catalyst strongly depend on the balance between the acid and the metal function. During the short lifetime of olefinic intermediates, several acid sites can be encountered during the transport between two metal sites and, H

H

H HH W

H S S

W

S

W

S S

W

W

S

S

W

W

S

W

S S

H

W

H

H

H H H

S W

H +

H

S

H

H

H

H

H

S

W

S S

W

W

S S

H

H W

S

W

S S

W

H

H

Scheme 8 Cracking of n-dec-4-ene over tungsten sulfides as proposed by Roussel et al. (76). Reprinted from Roussel, M.; Lemberton, J. L.; Guisnet, M.; Cseri, T.; Benazzi, E. J. Catal. 2003, 218, 427–437. Copyright (2003), with permission from Elsevier.

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consequently, could result into more than a single isomerization or cracking reaction in between consecutive (de)hydrogenation steps. The latter phenomenon is often referred to as secondary cracking and gives rise to a higher cracking selectivity than would normally be expected (34,78,79). As a result, multibranched isomers and cracking products appear as primary products, while, in reality, they are still formed from monobranched isomer intermediates which preferably chemisorb on a second acid site than hydrogenate to an alkane (80). An “ideal” balance in number and strength of the metal and acid sites results in quasi-equilibration of the (de)hydrogenation reactions, and under these conditions, secondary reactions are eliminated as much as possible inducing a maximum obtainable isomerization yield. Moreover, at ideal hydrocracking conditions, a maximum insight can be acquired in the acid-catalyzed part of the hydrocracking reaction network. The term “ideal hydrocracking” was introduced as an indication for this quasi-equilibrium between alkanes and alkenes (34). Given specific operating conditions, it may be achieved by providing sufficient, well-dispersed metal particles on the acidic zeolite framework (79). As shown in Scheme 9, a too low metal content would lead to large diffusion times for the alkenes to reach a metal site allowing them to interact significantly more with multiple acid sites (34,62,81). This additionally leads to a more pronounced deactivation due to coking owing to coke precursor formation via excessive cracking (16,78). On the other hand, too much metal might catalyze hydrogenolysis (82–84) or cause pore plugging (63,85,86). The dimerization and secondary cracking rates are also affected by the catalyst Coke

A

A

A

A

Monobranched nC8=

nC8

Multibranched

Me

Mono= A Me

C=

Multi= A

A Me

C= A

Me

Me

Cracked nC8 Monobranched Multibranched Cracked

Cracked

Scheme 9 Reaction network in case of nonideal n-octane hydrocracking involving multiple elementary steps occurring on the acid (A) sites in between two (de)hydrogenation steps on the metal (Me) sites (88). Reprinted from de Lucas, A.; Sanchez, P.; Dorado, F.; Ramos, M. J.; Valverde, J. L. Appl. Catal. A Gen. 2005, 294, 215–225. Copyright (2005), with permission from Elsevier.

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Multiscale Aspects in Hydrocracking

acidity as the latter determines the frequency and the ease at which the olefinic intermediates are protonated to a reactive carbenium ion (80,87). The optimum amount of deposited metal on the catalyst varies strongly with the metal and the support type. For instance, de Lucas et al. (88) found that impregnating 1 wt% Pt or Pd is sufficient to prepare an ideal H-Beta hydrocracking catalyst (Si/Al ¼ 12.5), while depositing 1.5 wt% Pt by ion exchange with [Pt(NH3)4] is not enough to attain ideal behavior of an H-Y support (Si/Al ¼ 3) (62). Ion exchange aiming at 2 wt% Pt was required to obtain an ideal Pt/H-Mordenite catalyst (89), while only 0.5 wt% Pd or Pt by incipient wetness impregnation was sufficient for a SAPO11 (90), for a SAPO41, (91) and for another H-Beta (Si/Al ¼ 25) (92). A striking 10 wt% Ni by incipient wetness impregnation was needed to accomplish a maximum activity for different zeolite frameworks (86). The required metal loading for a particular support could be different from research to research and is likely due to differences in metal dispersion (93,94). As such, Guisnet et al. (63) found that for a H-ZSM5 zeolite (Si/Al ¼ 45) with a Pt loading of 1 wt%, the catalyst activity increased up to a metal dispersion of 35%. Degnan and Kennedy (79) constructed a basic kinetic model which provided a mechanistic interpretation of the impact of the catalyst acid–metal balance on the n-heptane hydrocracking product distribution. They considered the alkane dehydrogenation reaction as rate determining and performed a sensitivity analysis on the so-called acid–metal balance parameter σ which is essentially the ratio of the free acid site number and the free metal site number: σ¼

t Cacid θacid t Cmetal θmetal

(2)

with Ct the total concentration of active sites and θ* the unoccupied fraction. They found that with increasing σ, thus with increasing acidity, the apparent pathway for multibranched isomer formation shifted to direct transformation of n-heptane, while monobranched species appeared much more reactive toward cracking. The model was later extended and validated for larger n-alkanes and naphtha streams (80,95). Rather than focusing on the catalyst properties, Thybaut et al. (96) investigated the effect of the reaction conditions on the establishment of ideal hydrocracking using fundamental microkinetic model simulations. It was shown that for ideal hydrocracking, i.e., when the (de)hydrogenation

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reactions are potentially much faster than the isomerization and cracking steps on the acid sites, the total pressure pt as well as the molar hydrogento-hydrocarbon ratio γ exhibit no or a negative effect on the initial net rate of formation of the n-alkane reactant, vide Eq. (3). Herein, Csat represents the alkane physisorption saturation concentration, KL the Langmuir physisorption coefficient, yi the molar fraction of component i, Kdeh the dehydrogenation equilibrium coefficient, and kdeh * and k*iso a composite dehydrogenation and isomerization coefficient, respectively. When the acid-catalyzed reactions are much faster, hence, under nonideal hydrocracking conditions, Eq. (4) is obtained instead in which the total pressure effect on the net production rate is now either nonexisting or positive. Distinction of ideal from nonideal hydrocracking can, hence, be performed by investigating the total pressure effect on the overall n-alkane conversion (96): Rn
P ¼


L 1+γ Csat Kn
P X Kdeh kiso yn
P γ 1+γ+ KiL yi pt

(3)

L Csat Kn
P X k pt yn
P L y pt deh 1+γ+ K i i i

(4)

i

Rn
P ¼


In the same research (96), it was experimentally proven and theoretically deduced that decreasing total pressures, increasing temperatures, increasing inlet hydrogen-to-hydrocarbon molar ratios and increasing reactant carbon numbers promote nonideal hydrocracking. Another peculiarity about ideal hydrocracking is that the isomer yield becomes a unique function of the total n-alkane conversion, vide Fig. 7, a salient feature which was already observed experimentally (32,62,63,97,98).

2.2 Physisorption In one of the earlier studies on hydrocracking kinetics, Flinn et al. (29) found a pronounced difference in reactivity between n-hexadecane and n-octane over a Ni/SiO2–Al2O3. Identical operating conditions resulted in only 53% n-octane conversion in contrast to 95% n-hexadecane conversion. The significantly higher reactivity for the larger reactant alkanes in gas-phase hydrocracking was also observed over other catalyst frameworks such as zeolites and aluminophosphates (67,99,100) and could not be explained through an increase in the number of elementary reactions with the alkane carbon number only. Additionally, experimental evidence as well as fundamental microkinetic modeling results showed that the degree of stabilization

137

Multiscale Aspects in Hydrocracking

Fig. 7 Simulated isomerization conversion of n-alkane on Pt/USY as a function of the total n-alkane conversion under ideal hydrocracking conditions: at 480K (squares), 500K (triangles), and 520K (diamonds) and at 0.1 MPa (white), 1 MPa (gray), and 10 MPa (black) (96). Reprinted with permission from Thybaut, J. W.; Narasimhan, C. S. L.; Denayer, J. F.; Baron, G. V.; Jacobs, P. A.; Martens, J. A.; Marin, G. B. Ind. Eng. Chem. Res. 2005, 44, 5159–5169. Copyright (2005) American Chemical Society.

during alkene protonation rather depends on the acid strength of the catalyst and the ion type formed than on the size of the chemisorbing alkene (37,43). As shown in Scheme 1, physical Van Der Waals interactions between the alkane sorbate and sorbent precede any reaction and become stronger with the sorbate size. Early kinetic analysis showed that the explicit incorporation of alkane physisorption was compulsory to attain an adequate model (101). Discrimination of kinetic models applying different physisorption isotherms, including Freundlich and Drachsel, showed that a simple Langmuir equation was sufficient to determine the physisorbed alkane concentration, vide Eq. (5)(102). At low physisorbed alkane concentrations, the second term in the denominator can be neglected and an apparent first-order rate dependence on the feed partial pressure is obtained. In contrast, strong physisorption implies a near zero-order rate expression with respect to the feed concentration/partial pressure, and a negative total pressure effect (97): Ci ¼

Csat, i KiL pi X 1+ K Lp j j j

(5)

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J.W. Thybaut and G.B. Marin

In gas-phase hydrocracking, the Langmuir physisorption coefficient KL is related to the Henry coefficient H through the physisorbed alkane saturation concentration and can be expressed as a function of the standard physisorption enthalpy, ΔH0phys, and entropy, ΔS0phys: H 1 ΔSphys
ΔHphys K ¼ ¼ 0 e R e RT Csat 2p 0

0

L

(6)

with p0 representing atmospheric pressure. Denayer and coworkers performed chromatographic studies to determine the Henry physisorption coefficient of linear alkanes up to n-dodecane on various faujasites (103), and other zeolites such as Beta, Mordenite, and ZSM22 (104). They found that the absolute values of the standard physisorption enthalpy and entropy increase linearly with the sorbate carbon number in which the enthalpy effect prevails over the loss in entropy. This results in an exponentially increasing trend of the Henry coefficient with the carbon number, vide Fig. 8 for various zeolites, and provides an explanation for the pronounced differences in apparent reactivity between alkanes of different chain lengths. Next to the sorbate size, narrower pores induce stronger Van der Waals interactions between the catalyst framework and the sorbate and are also often referred to as a confinement effect (106). A higher heat of physisorption is, however, compensated by stronger entropy losses. Therefore, 1E–01

H (mol kg–1 Pa–1)

1E–02 1E–03 1E–04 1E–05 1E–06 1E–07

4

6

8 10 Carbon number

12

Fig. 8 Henry coefficients of n-alkanes on zeolites Mordenite (Si/Al ¼ 2.7) (black triangles), Y (2.7) (white triangles), USY (30) (black squares), Beta (12.5) (white squares), and ZSM22 (30) (black circles) at 598K (105). Reprinted with permission from Denayer, J. F.; Baron, G. V.; Martens, J. A.; Jacobs, P. A. J. Phys. Chem. B. 1998, 102, 3077–3081. Copyright (1998) American Chemical Society.

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Multiscale Aspects in Hydrocracking

physisorption in ZSM22 which contains the smallest pores of all zeolites depicted in Fig. 8 is least favored because of excessive losses in degrees of freedom relatively to the other framework types (105). An inverse in hydrocracking selectivity according to the carbon number was observed for eight-membered pore structures owing to the prohibited access for the longer sorbate alkanes. This phenomenon is denoted as the “window effect.” (107) Similarly, Toulhoat et al. (106) found from molecular simulations that the standard physisorption enthalpy suddenly dropped in value when the sorbate size approached or exceeded the pore diameter. Differences in hydrocracking activity of different zeolites could, hence, not be entirely attributed to differences in acid–metal balance only, but also by differences in physisorption stabilization (89). Transition from the Henry regime toward higher sorbate concentrations implies increasing physisorption competition between the different reacting species. Intermolecular interactions arise at near-saturation conditions and induce a disappearance and/or even an inversion in physisorption selectivity (108). Denayer et al. (67,109) introduced the empirical interaction parameter w in the Langmuir expression, vide Eq. (7), in order to account for such interactions. Herein, θ represents the total molecular loading of the catalyst with physisorbed species: Ci ¼

Csat, i KiL pi ewi θ X 1+ K L p ewj θ j j j

(7)

The change in physisorption behavior at near-saturation loadings is based on additional entropy losses which manifest themselves when a sorbate molecule needs to coil up in a specific position to fill up the remaining space at the physisorption sites. This “packing efficiency” effect was denoted as “size entropy” and “configurational entropy” by Krishna et al. (110) and assesses increasing entropy losses with increasing sorbate chain length and branching degree, respectively. Schenk et al. (111) extensively investigated both entropy effects on the physisorption selectivities of 18 different molecular sieves via configurational bias Monte Carlo calculations. They reported an extreme case of physisorption competition during hydrocracking in which n-hexane and n-hexadecane physisorb in equal amounts at pore saturation. Similarly, Vandegehuchte et al. (112) attributed the occurrence of secondary cracking during gas-phase hydrocracking of n-hexadecane on Pt/ H-Beta (Si/Al ¼ 13) to an inversion in physisorption selectivity from carbon number 7 on, vide Fig. 9 simulated with a single-event microkinetic model

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Separation factor

1.0E+06

1.0E+04

1.0E+02

1.0E+00

1.0E-02 4

6

8

10 12 Carbon number

14

16

Fig. 9 Simulated separation factors relative to n-heptane over Pt/H-Beta, determined via Eq. (8), as a function of the alkane carbon number at a temperature of 513K; dashed line, Henry regime, no saturation; full line, saturation regime, including size entropy (112). Reprinted from Vandegehuchte, B. D.; Thybaut, J. W.; Martinez, A.; Arribas, M. A.; Marin, G. B. Appl. Catal. A Gen. 2012, 441, 10–20. Copyright (2012), with permission from Elsevier.

(Section 4.4). The separation factor αi, j between two components i and j quantifies the ratio of both physisorption coefficients and saturation concentrations and equals the ratio of the Henry coefficients at low sorbate loadings: αi, j ¼

KiL Csat, i KjL Csat, j

(8)

Differences in hydrocracking reactivity between alkanes of different chain lengths are perhaps even less pronounced under liquid phase conditions as was observed from various experimental researches (113–115). The higher reactivity of the larger component usually remained, but was rather ascribed to a larger number of possible reaction pathways and, more importantly, to a higher concentration in the liquid phase at vapor–liquid equilibrium than to physisorption effects (4,116,117). Full saturation of the catalyst pores during liquid phase hydrocracking similarly induces destabilization effects during physisorption in the catalyst pores. Narasimhan et al. (118) could satisfactorily reproduce the alkane separation factors from Eq. (8) which equaled one relatively to n-heptane regardless of the sorbate carbon number, by introducing the liquid fugacity coefficient ϕliq and an excess physisorption coefficient KE in the Langmuir expression:

141

Multiscale Aspects in Hydrocracking

L, vap

Ci ¼

liq

liq

Csat, i Ki K E ϕ pt V C X L, vap i i liq m, i i liq 1+ K KjE ϕj pt Vm, j Cj j j

(9)

with pt the total pressure and Vm the sorbate molar volume. The fugacity coefficients account for bulk phase nonideality, while the physisorption excess quantifies the degree of destabilization owing to compression and solvation on a saturated surface. The excess physisorption parameter depends therefore on the density of the bulk fluid and the sorbent force field acting on the sorbate molecules and could be expressed as a function of the liquid fugacity coefficients and an excess parameter, cE: cE

K E ¼ ϕliq RT

(10)

The excess parameter depends on the catalyst type and decreases with the sorbate size, as such it can be considered as a compensation factor for the strong Van Der Waals interactions the larger sorbate molecules experience with the catalyst framework (119).

2.3 Hydrogen Spillover In the mid-1990s, Roessner and Roland (120) urged on an extension of the classical reaction mechanism based on an experimentally observed synergetic effect between a layered Pt/Al2O3 and H-Erionite system in n-hexane hydrocracking. They excluded gas-phase diffusion of alkane intermediates and attributed any hydrocracking activity to the dissociation of molecular hydrogen on Pt to hydrogen species which are transferred as H+ onto the acid support in a subsequent step, illustrated in Scheme 10 for n-pentane hydroisomerization on a Pt/SiO2 and H-ZSM5 powder mixture (121). This phenomenon, first suggested by Fujimoto and coworkers (122,123), was defined as “spillover” which points to the transport of active species, H2 H Pt H SiO2

n-C5 H

i-C5 H

H-ZSM-S

H2 H Pt H SiO2

Scheme 10 Hydride transfer to pentane in the hydrogen spillover mechanism (121). Reprinted from Fujimoto, K. Hydrotreatment and Hydrocracking of Oil Fractions. Vol. 127, 1999; pp 37–49. Copyright (1999), with permission from Elsevier.

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+H+ −H2

+

+H−

Scheme 11 Direct alkane activation with spilt-over hydrogen (121). Reprinted from Fujimoto, K. Hydrotreatment and Hydrocracking of Oil Fractions. Vol. 127, 1999; pp 37–49. Copyright (1999), with permission from Elsevier.

chemisorbed or formed on a first phase, toward another phase which, under the same conditions, cannot sorb or form such species (124). Direct observation of hydrogen spillover was possible via isotopic exchange experiments using deuterium, followed by FT-IR. Following this mechanism, the formed protons increase the Brønsted acidity of the support by donating an electron to a Lewis site and enable direct alkane activation through H+ transfer and H2 abstraction, vide Scheme 11. The formation of hydride species enables desorption of the ionic species directly to alkanes according to Fujimoto’s mechanism (121). As such, the lifetime of the ionic intermediates is reduced and secondary cracking or coking occur to a much lower extent. In later research, Najafabadi et al. (125) exploited the hydrogen spillover phenomenon in diphenylmethane hydrocracking in a membrane reactor where the hydrocarbons and a sulfided NiMo/Al2O3 catalyst were physically separated by means of an alumina membrane. The latter could transfer active hydrogen formed in the inner tube of the reactor to the hydrocarbon in the reactor shell for further reaction and, hence, coking or any other form of catalyst deactivation could be avoided. The role of spilt-over hydrogen hence prevails in cases where alkene diffusion is hampered, but is probably less pronounced under ideal hydrocracking conditions. This mechanism is mainly associated with highly acidic supports where, due to their high activity, alkene formation becomes unlikely owing to the relatively low reaction temperature applied. Sulfated and tungstated zirconia are classified as such catalysts and are described in Section 3.4.

2.4 Conclusions Although various alternative reaction pathways were suggested in the literature, which, for sure, are relevant on specific catalyst types, see Section 3, a general consensus is established concerning the hydrocracking mechanism over conventional catalysts which comprises consecutive formation of unsaturated bonds on a metal function, and protonation to alkylcarbenium ions on an acid function. Branching primarily occurs through PCP

Multiscale Aspects in Hydrocracking

143

intermediates prior to any cracking via β-scission. An ideal balance in activity between the metal and the acid function leads to an optimal catalyst performance with respect to the global isomer yield and resilience against coking. Different cracking and alkane activation mechanisms were identified in case an ideal acid–metal balance was not obtained. These include hydrogenolysis, dimerization cracking, and hydrogen spillover. At extreme hydrocracking temperature, thermal hydrocracking over alkyl radicals could occur as well.

3. CATALYSTS This section gives a concise overview of the different metal functions and acidic supports that have been explored for hydrocracking. The supports are classified as microporous or mesoporous and mainly comprise silica-alumina-based zeolites and amorphous substrates but also aluminophosphates and clays. Strongly acidic supports may give rise to direct activation of physisorbed alkanes via hydride transfer. In that case, the metal function merely serves as a hydride transfer promoting agent. The latter class of catalysts comprises mostly sulfated and tungstated zirconium oxides and is discussed in Section 3.4. Finally, molybdenum- and tungsten -based oxides and oxycarbides for which the hydrocracking mechanism is not fully elucidated yet are described separately in Section 3.5.

3.1 Metal Function The metal function is generally deposited on the support via impregnation or ion exchange with the metal itself typically selected from the VIII or the VIb group. In particular, Ni, Pt, Pd, Co, W, and Mo are most commonly used (3). Noble metals such as Pt and Pd are renowned for their hydrogenation activities but are more expensive than any of the other metals mentioned earlier (19,126). Various comparative studies on monometallic (de)hydrogenation functions have been carried out in the pursuit of suitable hydrocracking catalysts (see Table 3). Although the noble metals Pt and Pd exhibit superior performance with respect to isomer yield, which is indicative of so-called ideal hydrocracking (see also Section 2.1.4), in practice sulfided NiW, NiMo, and CoMo catalysts are applied in many industrial hydrocracking processes. Despite their lower stability with respect to coking compared to Pt- or Pd-loaded catalysts, they form a less expensive alternative to the noble metal-based catalysts. Moreover, the latter are much more sensitive to sulfur poisons making them less suited for, e.g., residue oil processing (138).

Table 3 Overview of Comparative Studies on the Metal Function in Bifunctional Hydrocracking Catalysts Catalyst Feed Summary

0.5 wt% Pt, Pd, or Ni on H-Beta (25)a

n-Hexane

References

Both the isomer selectivity of Pt- and Pd-loaded Modhera et al. zeolites exceeded 90%, while the Ni-loaded catalyst (92) exhibited a 22% selectivity toward cracking

0.14–2 wt% Pt or 0.7–1.8 wt% Pd over SAPO11 n-Heptane

Pd induced substantially more cracking products Chaar and Butt and aromatics at elevated reaction temperatures up (127) to 773K

0.25–1 wt% Pt or Pd on H-Beta (12.5)

n-Octane

The higher activity and isomer selectivity of the Pt-loaded zeolites were ascribed to a higher hydrogenation activity

De Lucas et al. (88)

1 wt% Pt or Pd on H-Beta (12.5)

nC6–nC8

A higher activity was obtained on the Pt-based zeolite because of a higher metal dispersion, i.e., 73% compared to 25% for Pd

Sanchez et al. (100)

1 wt% Pt or Pd on H-Mordenite (10.4), H-Beta C7–C8 (13), and H-ZSM5 (15.6) naphtha fraction 1 wt% Pt or Pd on SiO2–Al2O3 (40)

Ramos et al. Higher alkanes and aromatics conversion were (128,129) observed over Pt-loaded zeolites. Differences in activity were attributed to a poor dispersion of Pd, up to 57% less than for Pt

C21–C36 wax A slightly higher activity and an isomer selectivity Kim et al. (130) were obtained over the Pt-loaded zeolite which could be attributed to a difference in metal dispersion of 9%

1 wt% Pt, Ni, Fe, or Zn, or 1.7 wt% Co on USY n-Heptane (5.4) and H-X (3.4)

Pt outperformed all other metals based on activity Pope et al. (131) and isomer selectivity. Ni loading led to a substantially higher cracking activity

0.3–0.5 wt% Pt, 1–3.5 wt% Ni or 3–9 wt% Mo C6–C7 FCC The Pt-based catalyst showed the highest activity. Gonzalez et al. on H-ZSM5 (80) mixed with Al2O3 gasoline The performances of Ni- and Mo-loaded H-ZSM5 (132) were comparable although the latter catalyzed much more dimerization 1 wt% Pt or 0.34 wt% Ni on H-Beta (13)

C6–C8 naphtha stream

An equal molar amount of metal was deposited on Funez et al. each zeolite. A higher activity and a monobranched (133) isomer selectivity were observed over the Pt-loaded zeolite

0.9 wt% Pt or 0.8 wt% Ni on dealuminated H-Y n-Octane (5.9)

Ni/H-Y was more active ascribed to a lower Ni Santos et al. (134) particle size and, consequently, lower acid site blockage compared to Pt/H-Y. The latter catalyst was much more stable and selective toward isomerization

0.01–0.2 wt% Pt or 3–9 wt% Mo on Al2O3 + 20 wt% H-ZSM5 (80)

C7 naphtha stream

Sulfided Mo promoted dimerization and cracking Ocaranza et al. irrespective of the metal loading, in contrast to Pt (95) where the isomer yield increased with the loading

0.35 wt% Pt, Ir, Rh, Re, or U on Al2O3

C5–C6 naphtha stream

For n-hexane conversion, Rh-based catalysts exhibited the highest activity characterized by significant hydrogenolysis. Re and U loaded on Al2O3 were rather inactive

1.51 wt% Ni or 0.27 wt% Ru on H-ZSM5 (45) n-Heptane

a

Si/Al ratio.

Ali et al. (135)

Akhmedov Ru/H-ZSM5 was much more active than the et al. (136,137) Ni-based catalyst and catalyzed primarily hydrogenolysis toward methane. Ni also induced a high cracking selectivity

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Next to the metal type and the loading, the (de)hydrogenation activity depends strongly on the metal dispersion. A higher dispersion implies a better accessibility of the metal for the olefinic intermediates and, hence, the easier hydrocracking conditions can be reached. Some attempts have been made to improve metal dispersion (84,139,140) using incipient wetness impregnation and the so-called metal vapor method, the latter based on metal evaporation and co-condensation with butadiene; however the simplest strategy to increase the metal dispersion may be simply to select smaller support particles which contain sufficient mesoporosity (141). Addition of a second promoter metal could change the electronic state of the first metal and increase its activity and/or dispersion. Numerous metal combinations have been investigated to improve the hydrogenation function of the catalyst. Combining both Pt and Pd on an acidic framework led to a pronounced increase in hydrocracking performance compared to the corresponding monometallic catalysts (92,142) due to higher dispersion attributed to the fixation of the more active Pt clusters by the smaller Pd particles. Ni is more often used to increase the performance of nonideal Pt- or Pd-loaded catalysts. The formation of bimetallic Ni–noble metal clusters increases the ideality of the catalyst as evidenced by higher activities and stabilities of faujasites (131,143,144), zeolites Beta (145–147), and Mordenite (145,148), and of aluminophosphates (149). In order to achieve such an improvement, it was imperative to perform a simultaneous ion exchange technique instead of depositing each metal sequentially on the support (147). Le Van Mao and coworkers (150–152) studied the beneficial effect of Zn, Al, or Cd cationic species simultaneously deposited with Pt on an H-Y zeolite (Si/Al ¼ 5.2), on heptane isomerization. They attributed an increase in catalyst performance to subtle changes in electrostatic interactions between the chemisorbed carbenium ions and the acid sites, induced by the cation in proximity with the acid and the Pt sites. This led to a presumably better desorption step of the carbenium ion from the acid sites, hence avoiding several acid-catalyzed reaction steps in a single catalytic cycle (152). The authors therefore denoted the catalyst as “trifunctional” with the desorption site being the third function. The location of the actual active site for (de)hydrogenation on a sulfided metal surface is still under debate. Threefold coordinatively unsaturated metal ions, located at the edges of the metal phase and formed after a reductive treatment under hydrogen, were first considered as the active sites for alkene hydrogenation (153). Conversely, recent research suggested that saturated metal sulfides were primarily responsible for alkene chemisorption

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prior to hydrogenation (154). Herein, hydrogen physisorbed on the metal surface acted as a coreactant. Increasing the metal sulfidation degree induced a higher hydrotreating activity (155) and could also lead to the formation of additional Lewis or Brønsted acid sites (132,140,156–158), or to a direct cracking mechanism over sulfide anions as proposed by Roussel et al. (76,77), vide Section 2.1.3 and Scheme 8. A considerable fraction of the promoter metal is dispersed in a sulfided form at the step edges of MoS2 or WS2 slabs and probably increases the sulfidation rate of the base metal, hence improving the overall catalyst performance (155). More hydrogenolysis products are usually formed, especially with Co as promoter, as well as small amounts of alkenes (159,160). An optimal Ni or Co loading can be determined based on the catalyst activity and isomer selectivity. For both criteria, a Ni/(Ni + Mo) atomic ratio between 0.4 and 0.5 was found as depicted in Fig. 10 (159). Egia et al. (140) found a value of 0.35 for a NiMo/H-Y catalyst. A similar range of 0.4–0.5 for the optimal Ni/(Ni + W) atomic ratio was found for sulfided NiW catalysts (161,162), while a much higher ratio of about 0.8 was determined for the nonsulfided variant (163). Advanced metal deposition techniques, reviewed by Hensen and Van Veen (164), are tested to achieve high Ni or Co metal dispersions in the confined spaces of microporous supports, in order to attain the synergetic effect between the catalytically active and the promoter metal (165). Other metal combinations in hydrocracking catalyst formulations, including Cr–Pt (166), Cr–Pd (139), Re–Pt (135), Ir–Pt (135), Rh–Pt (135), Sn–Pt (167), La–Pt (166), Ce–Pt (166), U–Pt (135), Fe–Co (131), Ni–Re (136), Ni–Ru (137), and Ru–Re (137), were investigated to explore similar synergetic effects as described earlier.

Initial selectivity

Hydrogenolysis

Cracking

Isomerization

0.1

0.5

1.0

0.05

0.25

0.5

0.2

0.4

0.6 1.0

0.2 0.4 0.6 Ni / Ni + Mo (atomic ratio)

0.2

0.4

0.6

Fig. 10 Initial selectivity for hydrogenolysis, cracking, and isomerization reactions as a function of the Ni/(Ni + Mo) atomic ratio of the NiMo/USY catalyst in n-heptane hydrocracking (159). Reprinted with permission from Vazquez, M. I.; Escardino, A.; Corma, A. Ind. Eng. Chem. Res. 1987, 26, 1495–1500. Copyright (1987) American Chemical Society.

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3.2 Microporous Supports 3.2.1 Zeolites Zeolites are arguably the most commonly investigated materials for acidic supports in bifunctional catalysis. They combine a relatively high activity with a high resistance against coking and deactivation by heteroatomcontaining compounds. Moreover, they exhibit a good regenerability and allow product selectivity manipulation via molecular shape selectivity (3,25). Additionally, an enormous library of zeolite framework structures is currently at hand (168). A recent comprehensive review covering the use of zeolites and zeotypes for oil and gas conversion by Vogt et al. (169) includes details of their use for hydrocracking. Zeolites are rigorously classified according to the number of Si or Al atoms in the pore ring. Table 4 gives an overview of the zeolite frameworks which were explored for their hydrocracking capabilities. Large 12-membered pores usually exhibit no or minor shape selectivity in alkane hydrocracking and, additionally, allow a more efficient mass transport of bulky reactant molecules than medium 10-membered pore frameworks. Faujasites Y and USY contain a three-dimensional structure of circular 0.74 nm pores and, additionally, supercages of about 1.2 nm3 (168), and give rise to a maximum alkane isomer yield of approximately 50% as shown in Fig. 7 for n-octane (96). From all zeolite frameworks, faujasites are the most Table 4 Overview of Commonly Applied Zeolite Catalysts in Hydrocracking Studies Along With Their Pore Dimensions (126,168) Framework Catalysts Pore Dimensions (nm)

FAU

Y, USY

0.74  0.74

BEA

Beta

0.66  0.67 and 0.54  0.54

MTW

ZSM12

0.56  0.60

MOR

Mordenite

0.65  0.70 and 0.26  0.57

MAZ

Mazzite

0.74  0.74 and 0.31  0.31

MFI

ZSM5

0.51  0.55 and 0.53  0.56

MWW

MCM21

0.40  0.55 and 0.41  0.51

TON

ZSM22

0.46  0.57

MTT

ZSM23

0.45  0.52

MRE

ZSM48

0.53  0.56

Multiscale Aspects in Hydrocracking

149

applied in hydrocracking of heavy feeds (3) and are therefore the subject of many research projects on structural optimization via a posteriori treatment. The creation of mesopores through steaming or acid leaching induced a further improvement in product diffusion and catalyst stability (134). Other modification techniques involve twinning across the [111] crystallographic plane to create hypocages with a void volume of only 0.5 nm3 (170). As a result, the formation of α,α,γ-branched ions was restricted leading to a lower cracking selectivity. Chemical vapor deposition of silicon oxide reduced the catalyst pore size and covered a fraction of the acid sites in order to shift the acid–metal balance more to the metal side (134). Treatment with an aqueous Ti(SO4) solution removed extra-framework Al species, increased the global mesoporosity, and enhanced the MoS2 hydrogenation function in residue oil conversion (171). Atomic layer deposition of alumina onto USY substrates increased the n-decane hydrocracking performance because of the creation of new and stronger acid sites (172). Other large-pore zeolites such as Beta and ZSM12 showed a similar hydrocracking product distribution as obtained over faujasites, but were generally found more active owing to a stronger acidity (156,173–175), and/or a higher physisorption stabilization by its narrower pore structure (112,176). Beta zeolites also exhibited a lower selectivity toward ethyland propyl-branched feed isomers which could be attributed to a slightly pronounced transition state shape selectivity induced by its smaller supercages (112,177). Minor diffusion limitations were observed with the unidirectional ZSM12 framework leading to slight deviations from the product distributions usually obtained over faujasites (178–180). Zeolites such as Mordenite and the slightly more active Mazzite contain large straight pores interconnected by eight-membered pore channels through which alkane transport is restricted (181). It is believed that only one-third of the acid sites in the catalyst are accessible for reaction as a large fraction resides in the eight-membered side pockets (129). Nevertheless, Carvill et al. (182) observed an unexpected increase in n-heptane hydrocracking conversion at lower Pt metal dispersion which was related to framework destruction by the growing metal particles, and which leads to a fully accessible three-dimensional structure. Dealumination procedures essentially led to a similar observation (183). The unidirectional character of the Mordenite framework induces intracrystalline mass transport limitations and, consequently, a higher cracking selectivity compared to zeolites Y and Beta, vide Fig. 11 for n-octane hydrocracking (128,129,176). As a result, the zeolite is also more prone to deactivation by coking (63).

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100

Selectivity (%)

80

60

40

20

0 1

2

3 4 5 6 Carbon number

7

8

Fig. 11 Product selectivities in n-octane hydrocracking over 10.9 wt% Ni-loaded ZSM5 (squares) and 8.3 wt% Ni-loaded Beta (crosses) and Mordenite (circles) at 533K and 2 MPa (185). Reprinted from Lugstein, A.; Jentys, A.; Vinek, H. Appl. Catal. A Gen. 176, 119–128. Copyright (1999), with permission from Elsevier.

A unidirectional catalyst with similar pore size but larger side pockets, denoted as ITQ-4, was synthesized by Chica et al. (184) to attenuate such diffusion effects and, hence, to increase the isomerization yield in C5–C7 hydrocracking. Pentasyl-based ZSM5 catalysts are renowned in industry for their high cracking affinity induced by their peculiar 10-membered pore structure containing intersecting straight and sinusoidal channels. Strong diffusion limitations restrict the diffusion of bulky dibranched species out of the zeolite crystallites, once formed, and are compelled to react further to either monobranched or cracking products (63,72,114,126). A more uniform cracking product distribution with paraffinic feeds is obtained, indicating that any C–C bond in the reactant ion is nearly equally susceptible to β-scission, vide Fig. 11 (86). Type A β-scission therefore only occurs to a negligible extent owing to the fully restricted formation of bulkier tribranched species (86,128,129,185). A higher selectivity to the terminally monomethyl branched isomers was observed than expected from thermodynamic equilibrium which could be attributed to either product shape

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Multiscale Aspects in Hydrocracking

selectivity, and/or transition state shape selectivity in the formation of the PCP complex (86,186–188). Ethyl branching was generally not observed below the conversion level of 95% (188,189). MWW frameworks such as MCM22 which are built from straight intersecting medium pores with large 12-membered ring side pockets exhibit a similar hydrocracking behavior as MFI zeolites; i.e., multibranching occurs in the pockets, while diffusion through the channels is limited (114,190,191). Corma et al. (192) synthesized an ITQ-2 catalyst consisting of both 12- and 10-membered pore channels from delamination of such MWW zeolite structures. After loading with Ni and Mo, this catalyst showed a VGO hydrocracking activity and selectivity to light and middle distillates superior to conventional USY zeolites. ZSM22 and other medium-pore catalysts with straight unidirectional pores exhibit extreme shape selectivity leading to very high isomerization selectivities which, consequently, establishes them as superior octane boosting catalysts. In the pioneering work of Martens et al. (189), a total isomerization yield of 75% in n-decane hydrocracking could be obtained, vide Fig. 12. Zooming in on the isomer product distributions, Claude and 100

Isomer yield (%)

90

80

70

60

50 60

80

100

40

20

0 0

20

40

60

80

100

n-Decane conversion (%)

Fig. 12 Experimental (symbol) and modeled (lines) isomer yields obtained from n-decane hydrocracking over a Pt/NaH-Y (Si/Al¼ 2.56) (♦), a Pt/H-ZSM22 (Si/Al ¼ 45) (■), a 80:20 (ж), 60:40 (+), 40:60 (•), and 20:80 (▲) wt% Pt/NaH-Y:wt% Pt/H-ZSM22 mixture (203). Reprinted from Choudhury, I. R.; Thybaut, J. W.; Balasubramanian, P.; Denayer, J. F. M.; Martens, J. A.; Marin, G. B. Chem. Eng. Sci. 2010, 65, 174–178, Copyright (2010), with permission from Elsevier.

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Pore mouth mode

Key-lock mode

Fig. 13 Pore mouth and key lock physisorption modes for C21 alkanes on ZSM22 (200). Reprinted from Narasimhan, C. S. L.; Thybaut, J. W.; Marin, G. B.; Martens, J. A.; Denayer, J. F.; Baron, G.V. J. Catal. 2003, 218, 135–147. Copyright (2003), with permission from Elsevier.

Martens (193) found that monobranching near the reactant chain end was preferred resulting in higher yields of 3-methyl and especially 2-methylalkanes. Another salient feature was observed for long alkane reactants where simultaneous physisorption of both reactant chain ends in two adjacent pore mouths induced a new maximum in methyl isomer yields with the branch located more to the middle of the hydrocarbon skeleton. Both observations were explained via respectively pore mouth and key lock catalysis (194), illustrated in Fig. 13, and was confirmed in many subsequent publications for various paraffinic and naphthenic feeds (89,195–199). ZSM22 micropores allow unconstrained access for linear species only and are too narrow for diffusion of branched isomers. As a result, the latter species are formed and react at acid sites located near the pore mouths. They penetrate the micropore with one of the straight ends of their hydrocarbon skeleton, which leads to the necessary physisorption stabilization. The alkyl chain dangling outside the pore potentially aligns along the zeolite surface leading to a stronger interaction mode and, if sufficiently long, may penetrate a second pore provided that the latter is unoccupied. The degree of stabilization is directly related to the number of carbon atoms in the pores (194,200) and could explain the local maxima in isomer yields observed by Claude et al. (193,201) In addition, specific reaction rules depending on the distance between the different pore mouth sites and those at the pore bridges restrict the formation of multibranched species significantly and, consequently, of undesired cracking products (202). Cracking products are primarily formed from the thermodynamically restricted type D β-scission of linear carbenium ions inside the micropores. The number of acid sites inside the micropores considerably exceeds that at the pore mouths and compensates for the slower kinetics of the type D β-scission, such

Multiscale Aspects in Hydrocracking

153

that it leads to a significant, observable amount of, primarily, linear cracked products especially in the lower conversion range (202). Strategies to reduce the contribution of undesired cracking mainly comprised lowering the acidity at the external surface and within the micropores. Omission of the calcination step during ZSM-22 synthesis by applying successive ion exchange steps with a NH4Cl solution to remove the template led to distinctly less type D β-scission in the micropores and, hence, an even higher maximum isomerization yield as found from fundamental modeling simulations (204). Other frameworks such as ZSM23 and ZSM48 which have a slightly larger pore size than ZSM22 exhibited a similar hydrocracking pattern, suggesting that these catalysts were also susceptible to pore mouth and key lock effects (126,180,205–208). Combination of ZSM22 with ZSM48 results in an even further increase in n-octadecane hydroisomerization selectivity which was ascribed to an increase in key lock modes for multibranching reactions (209). Similarly, synergetic effects were found between a ZSM22 and a large-pore Y in n-decane hydrocracking, vide Fig. 12, where additional dibranching could be achieved on the Y zeolite of which the activity was adequately tuned toward that of the ZSM22 zeolite (203,210). Numerous zeolite types have been synthesized aiming at a maximum achievable selectivity toward the desired products. Herein, mass transport of reaction products and intermediates appeared to be a vital issue for improving the hydrocracking performance of the catalyst. Nanocrystalline Beta (49,92,156,196) and ZSM5 nanoslabs and zeogrid materials composed of stapled nanoslabs (211) were synthesized to decrease the intracrystalline pathway toward the active sites and to increase the contribution of the external surface acidity. Another strategy is to increase the catalyst mesoporosity especially for unidirectional catalyst frameworks such as Mordenite (212,213). Besides conventional dealumination and acid leaching techniques, other methods to introduce meso- and macropores include the synthesis of intergrowths of different zeolites, e.g., Beta and Y (214), mixing carbon black particles with conventional synthesis sol-gels (215), and introducing a trimodal porosity via base leaching (216). The microporosity of the catalyst, on the other hand, determines the intrinsic product selectivity of the catalyst. As shown especially for mediumpore zeolites, shape selectivity could shift the hydrocracking product slate more toward the cracking or the isomerization side depending on the pore size and connectivity. Many authors devised a criterion to compare the extent of shape selectivity between different frameworks based on the ratio

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of two typical reaction products. Frillette et al. (217) were the first to introduce the so-called constraint index which compares the catalytic cracking conversion of n-hexane to that of 3-methylpentane in an equimolar mixture. The higher the ratio in favor of n-hexane, the higher the transport limitations for 3-methylpentane and, hence, the more shape selectivity the catalyst exhibits. Weitkamp and Ernst (218) reviewed the different approaches to probe shape selectivity with model reactions. Particularly for n-alkane hydrocracking, Martens et al. (219) introduced the modified constraint index focusing on the ratio of the 2-methylnonane yield and 5-methylnonane yield at a total n-decane isomerization yield of 5%. Weitkamp et al. (220) focused rather on the iso-butane/n-butane yield ratio from butylcyclohexane hydrocracking to eliminate mass transport effects as much as possible and denoted it as the spaciousness index. More recently, Chen et al. (221) devised a more robust criterion based on the yields of monobranched and dibranched species at the maximum n-hexane hydroisomerization yield. To conclude, the role of the mesopores is mainly to provide unconstrained access of the larger molecules to the catalyst microporosity. The latter is primarily responsible for the isomerization and cracking reactions owing to the acid strength of the sites in the micropores and the confinement experienced by the molecules interacting with these sites (222). An unbalance between meso- and microporosity could induce a detrimental effect on either the catalyst activity or the stability. A proper trade-off between both structural contributions is pursued during catalyst synthesis and may differ from feed to feed. The numerous suitable framework types together with the wide variety of metals that can be used in bifunctional catalysis make that the design of optimal zeolite-based materials remains a challenging task (223). 3.2.2 Aluminophosphates Aluminophosphate (AlPO)-based supports constitute an alternative to zeolite materials and essentially are built from similar, ordered framework structures but with weaker acid sites. The latter implies that AlPOs are less likely to lead to nonideal hydrocracking and, hence, are often claimed as suitable materials for selective alkane isomerization. An AlPO framework is a neutral structure constructed from an alternating sequence of aluminum and phosphor atoms interconnected by bridging oxygen atoms. Isomorphous substitution of Si4+ for P5+, or of a divalent metal ion for Al3+, creates a negative charge which can then be compensated

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Multiscale Aspects in Hydrocracking

Si Al Si Si Si

Al

P

Al P

Al

Si Si Si Al Si

Si

Al

P Al

P Al

Si Si Al Si Si

Al

P

Al Si

Al Si Al

Si Si Si Si Si

Si

Al

P Al

P Al

Si Al Si Si Si

Al

P

Al P

Al

Si Si Si Si Si

Si

Al

P Al Si

SA

P Al P

P

P Al Al

P

SAPO

Fig. 14 Schematic representation of a silica-alumina and a SAPO surface. Encircled elements introduce Brønsted acid sites (225). Reprinted from Martens, J. A.; Grobet, P. J.; Jacobs, P. A. J. Catal. 1990, 126, 299–305. Copyright (1990), with permission from Elsevier. Table 5 Explored Monodirectional Silicoaluminophosphate Frameworks in n-Alkane Hydrocracking Along With Their Pore Dimensions (168) Framework Catalyst Pore Dimensions (nm)

AFI

AlPO-5

0.73  0.73

AEL

AlPO-11

0.40  0.65

ATO

AlPO-31

0.54  0.54

AFO

AlPO-41

0.43  0.70

by a proton, as illustrated in Fig. 14. After silicon incorporation, i.e., transformation to a silicoaluminophosphate (SAPO) structure, bridged Si–Al– OH groups represent the main active sites for isomerization and cracking reactions. Simultaneous substitution of a Al–P pair by two Si atoms leaves, however, the net global charge of the structure unaffected and is generally avoided (224). In addition, fewer but stronger Brønsted sites are created at the interfaces between Si and SAPO domains (224–227). Advanced preparation techniques were developed to exclusively target phosphor atoms in the framework, such as the use of specific surfactants during synthesis (224,226), or nonaqueous media (99). A large number of AlPO frameworks were synthesized in the past decades, but only a few were intensively investigated in hydrocracking (see Table 5). SAPO-5 is built from a 12-membered pore framework which does not impose shape-selective constraints to the reaction mechanism. As a

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result, a n-alkane hydrocracking behavior comparable to that of faujasites was observed on such catalysts (149,180,228,229). SAPO-37, which is isostructural to faujasite zeolites, was tested in n-decane hydrocracking and even approached the activity of a Pt/H-Y with a Si/Al ratio of 2.5 (225). SAPO-11 contains straight nonintersecting medium-sized channels of which the pore apertures are slightly larger than those of conventional ZSM22 zeolites. Although often attributed to mass transport limitations or transition state shape selectivity within the micropores (90), similar product distributions were observed over such materials, indicating that pore mouth effects might also govern the reaction mechanism as shown from experimentation with n-hexane (149), n-heptane (228–232), n-octane (180,226,233), n-decane (234), n-dodecane (167,224,235), n-tetradecane (82), and n-hexadecane (236,237), and a linear C13–C20 alkane fraction as feed (238). SAPO-41 also consists of a unidirectional 10-membered pore structure with slightly larger pore dimensions. As a result, its performance in the selective hydroisomerization of n-alkanes is comparable to that of SAPO-11 (91,227,233,234). Remarkably, SAPO-31 structures which are built from narrow 12-membered pore channels exhibited a similar or even higher selectivity toward isomers in n-heptane (232), n-octane (180,207), n-decane (234), and n-hexadecane hydrocracking (239). Instead of silicon, divalent metal ions could be incorporated into the catalyst framework to generate metal–P–OH Brønsted sites. These sites were found to be stronger than the Si–Al–OH groups that are inherent to SAPO frameworks, from n-heptane hydrocracking tests with CoAPO-5 and CoAPO-11 (229). Hartmann and Elangovan (240) performed n-decane hydrocracking experiments on MgAPO5, MgAPO11, and MgAPO41 and observed a similar decreasing trend of the maximum isomer yield with the catalyst pore diameter. As with zeolites, aluminophosphates can be physically mixed with metal-loaded mesoporous materials, such as MgAPO-5 and MgAPO-11 with Pt/MCM41 (241), to exploit both the microporosity of the former material and the facilitated diffusion induced by the latter.

3.3 Mesoporous Supports 3.3.1 Alumina and Silica-Alumina-Based Materials As elaborated in Section 3.2.1, increasing mesoporosity could tackle intracrystalline mass transport issues during heavy feed hydrocracking processes. Metal-loaded mesoporous alumina and silica-alumina are particularly interesting for this purpose, but they usually have a weaker acid function

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than microporous zeolite materials. Alumina-based materials normally become active at temperatures above 570K, but exhibit a reasonably high selectivity toward products in the middle distillate range once loaded with Pd (242,243). However, sulfided molybdenum and tungsten metals are more commonly deposited on mesoporous alumina in high severity hydrocracking of heavy feedstocks. Advanced surfactant-based preparation techniques, such as sol gel synthesis with stearic and lauric acids (244), were developed to tailor the pore size, distribution, and acidity of the support. The (de)hydrogenation function of such catalysts may be enhanced by partial fluoridation of the alumina prior to metal deposition and sulfidation (245). Similarly, the addition of 3.4 wt% P2O5 to an alumina support prior to Ni and Mo impregnation led to a higher metal dispersion, an increase in acid site strength, a higher thermal and coking stability, and, concordantly, a higher activity in crude oil hydrocracking (246). Another way to increase the activity of the alumina support is via chlorination by a chlorine-containing organic compound. In one of the earliest investigations, Giannetti et al. (247) used S2Cl2 and CCl4 as chlorinating agents for Pt-loaded alumina in the hydrocracking of a naphtha fraction to LPG and already observed a significant catalyst activity from 473K on. The role of chlorine in promoting the catalytic activity is still not completely understood, but it could increase the dispersion of the metal component, the Lewis acidity of the support, or facilitate carbenium ion desorption via hydrogen spillover (135). In addition, further doping with chlorinecontaining compounds could, after conversion into HCl, potentially transform Lewis acid sites into Brønsted acid sites, via the step proposed in Scheme 12 (248). Such Brønsted acid sites are, however, sensitive to water poisoning as evident from the continuous HCl release during reaction and require continuous spiking of the feed with chlorinating agents (247,248). Ali et al. (135) found an optimum chlorine content of 3.0 wt% with respect to the activity and alkane hydroisomerization yield.

Cl

Cl Al O

+ HCl

H+ − Cl Cl Cl Al O

Scheme 12 Dissociative chemisorption of HCl on a chlorinated Al2O3 surface (248). Reprinted from Ducourty, B.; Szabo, G.; Dath, J. P.; Gilson, J. P.; Goupil, J. M.; Cornet, D. Appl. Catal. A Gen. 2004, 269, 203–214. Copyright (2004), with permission from Elsevier.

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Amorphous silica-alumina contains stronger acid sites compared to an alumina support and could be considered as an alumina polymer grafted on a silica backbone (12). Its weak acidity compared to zeolite supports and its mesoporosity are advantageous for the selective isomerization of bulky alkanes as shown from various studies on the Fischer–Tropsch wax conversion (4,39,113,249). The activity of SiO2–Al2O3-based supports greatly depends on its composition and usually increases with the acid site concentration (12,130,249). In turn, the acidity of the support showed a volcano-shaped trend with respect to the SiO2 content (249,250). In the search for optimal SiO2–Al2O3 supports, Corma et al. (251) managed to synthesize a mesoporous silica–alumina material with a uniform pore diameter. More recently, Lee et al. (252) explored amorphous titania–silica-based materials for the hydrocracking of paraffin waxes. Since the discovery of crystalline mesoporous MCM41 aluminosilicates in the early 1990s, their potential in VGO hydrocracking was explored not long after (253). After impregnation with Ni and Mo, the catalyst showed a superior gas oil hydrocracking activity compared to conventional SiO2–Al2O3 supports, and a higher selectivity toward middle distillates compared to USY likely owing to a better accessibility of the active sites. Other investigations explored MCM41 catalysts in long n-alkane hydrocracking and found similar product distributions as obtained on large-pore zeolites (142,175,254). The activity of MCM41 usually decreases with the Si/Al ratio because of a reduction in acid site number (83,255). A superior MCM41 support was synthesized from a hexadecylamine organic template (256). Alsobaai et al. (257) and Campelo et al. (84) rather focused on MCM48 which, in contrast to MCM41, is composed of a three-dimensional structure, but requires a more advanced synthesis route. Other mesoporous sieves explored for long n-alkane hydrocracking involve SBA15, SBA16, and UTL (258–261). In summary, the acidity of mesoporous nonzeolitic supports is generally lower than observed over zeolites, but the former are much less prone to diffusion limitations. Combination of both types of acidic supports leads to hybrid materials which potentially outperform the individual catalysts. To this purpose, Kinger et al. (191) physically mixed an acid MCM22 and Beta with a Pt-loaded and rather inactive MCM41 support. Growth of zeolite particles onto mesoporous substrates could induce a similar improvement in hydrocracking performance. In this respect, advanced synthesis methods were developed for ZSM5/MCM41 (262), Beta/MCM41 (263), Mordenite/MCM41 (264), USY/MCM48 (265), and Y/SBA15 composite materials for n-alkane and VGO hydrocracking purposes (266).

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3.3.2 Heteropolyacids Heteropolyacids represent highly acidic solid acid catalysts with the Keggin structure containing both micro- and mesopores. They were first reported in 1984 for the hydrocracking of n-hexane over Pd-loaded H3PW12O40, of which the structure is depicted in Fig. 15. The catalyst exhibited a considerable activity from 443K on (267). The additional role of the noble metal is to dissociate H2 into hydrogen atoms which, in turn, react with the heteropoly anion to form a Brønsted site (16). Yi and coworkers (268–270), however, stated from H2 TPD measurements that direct activation of alkanes via hydride transfers could also be occurring owing to hydrogen spillover. Partial replacement of the protons with Cs gave highly acidic materials with large surface areas and with similar n-heptane isomerization selectivities as observed over Beta zeolites (272). Even more, the activity of Beta zeolites could be matched with a Pd-H4SiW12O40 supported on silica which was ascribed to a higher acid strength than obtained with the phosphorous variants (269,273). The beneficial effect of Cs on the catalyst surface area was confirmed by Liu et al. (274) in the hydrocracking of Fischer–Tropsch waxes. An optimal Cs/H molar ratio of 0.5 within the heteropolyacid structure was found from n-decane hydrocracking experimentation in the presence of thiophene and pyridine (270).

Fig. 15 Structure of a PW12O40 Keggin unit (271). Reproduced from Newman, A. D.; Brown, D.R.; Siril, P.; Lee, A. F.; Wilson, K. Phys. Chem. Chem. Phys. 2006, 8, 2893–2902 with permission from the PCCP Owner Societies.

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An important issue with heteropoly compounds is their lack of thermal stability. Hence, they are commonly supported by solids with high surface areas such as ZrO2, SiO2, silica-alumina, and γ-alumina (274–276). In addition, the metal dispersion and the accessibility of the active sites within the Keggin structures could be increased this way. A volcano-shaped relation exists between the catalyst performance and the heteropolyacid loading on the support which was probably related to the attainment of an optimal acid–metal balance (273,277). Gagea et al. (277) synthesized a welldispersed Pt-H3PW12O40 phase on an SBA15 support which outperformed conventional USY catalysts in n-decane isomerization selectivity. 3.3.3 Pillared Clays Natural and synthetic clays such as montmorillonite and beidellite contain Brønsted sites originating from AlO4 tetrahedra and from Mg2+ substitution for octahedral Al3+ at Si8Al4O20(OH)4 sites (278). The clay layers have a positive charge deficiency which could be compensated by large hydrated inorganic metallic polyoxocations, illustrated in Scheme 13, which form pillars to support the silicate layers after calcination (278). The interlayer regions contain significant mesoporosity which leads to an efficient mass transport of bulky products such as cycloalkanes and aromatics, and to a persistent activity even at high levels of coking (278–280). Molina et al. (281) assessed the n-decane hydrocracking performance of pillared montmorillonite, beidellite, and two transition metal-rich clays loaded with 1 wt% Pt. The synthetic beidellite clay pillared with a hydroxy-aluminum solution showed superior activity and isomer selectivity compared to the other catalysts and approached the hydrocracking performance of commercially available and highly active Pt/USY zeolites. The

: O2− : OH− : H2O

9.3 Å Na+

Na+

Na+

d001 = 19.2 Å

9.9 Å

12.4 Å + [AlO4Al12(OH)24(H2O)12]7+

Na–Mont

Al13–Mont

Scheme 13 Transformation of a natural Na-exchanged montmorillonite to an Al-pillared structure via ion exchange with a [AlO4Al12(OH)24(H2O)12]7+ solution (278). Reprinted from Liu, Y.; Murata, K.; Okabe, K.; Inaba, M.; Takahara, I.; Hanaoka, T.; Sakanishi, K. Top. Catal. 2009, 52, 597–608, with permission of Springer.

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capacity of pillared beidellite in alkane hydrocracking was confirmed in a later research (282). Modification of hydrothermally treated vermiculites with Al–Me solutions (Me ¼ Hf, Zr, or Ce) also led to a promising catalyst for n-decane isomerization (283). Owing to its rather weak acidity, Al-pillared montmorillonite showed a higher selectivity toward middle distillate products in nC36 hydrocracking than conventional amorphous silica-alumina catalysts (278). The high stability and activity of pillared clays in heavy feed hydrocracking was illustrated by Bodman and coworkers (284,285) with Cr-pillared montmorillonite and Sn-pillared laponite in the processing of a coal liquefaction extract. Al-Saleh et al. (286) explored the VGO hydrocracking activity of a Co-exchanged saponite, loaded with Pt and Rh to increase the catalyst’s cycle length. 3.3.4 Activated Carbon Recently, the potential of activated carbon as the acidic support in hydrocracking was investigated. The flexible coordination of carbon atoms allows the synthesis of any three-dimensional structure with high stability, internal surface area, and a versatile surface chemistry (287). Fukuyama and Terai (288) prepared activated carbon from Australian Yallourn brown coal char, which showed a high resilience against coking during residue oil hydrocracking at 523K and 10 MPa hydrogen pressure. Fernandes et al. (287) loaded 1 wt% Pt on a carbon-based carrier prepared from a silica SBA15 template. The catalyst showed a moderate activity for n-decane hydrocracking at reaction temperatures exceeding 573K which could point to a rather weak acidity compared to zeolites. Nevertheless, the advantages of activated carbon-based materials over conventional catalysts are mostly economic in nature as they can be synthesized from waste materials, and full metal recovery is possible simply by burning off the support.

3.4 Sulfated and Tungstated Zirconia Highly acidic catalysts may alleviate the need for alkene formation over the metal through direct alkane activation. In this respect, Friedel–Crafts catalysts such as AlCl3 with additives such as SbCl3 and HCl were previously used industrially, but were later discarded because of corrosion and regenerability issues (16). Therefore, sulfated and tungstated solid zirconia oxides were found more applicable for hydrocracking purposes. Sulfated ZrO2 was initially, and perhaps erroneously, assigned the status of super acid as it could induce a color change of Hammett indicators and catalyze the isomerization of butane at very low temperatures (16). In later investigations, its acid

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δ−

O

O

δ−

δ−

O

S O

H2O

H+ O

O

O

δ−

S

H+ O

O +

+

Zr

Zr

H

Zr

Zr

O H

O

O

Lewis acid sites

Bronsted acid sites

Scheme 14 Formation of Lewis and Brønsted acid sites in sulfated zirconia (15). Reprinted from Akhmedov, V. M.; Al-Khowaiter, S. H. Catal. Rev. 2007, 49, 33–139, with permission from Taylor & Francis.

strength was found to be of the same order of magnitude to that of zeolite materials and, hence, the term super acid was no longer applied. The acidity of sulfated zirconia originates from electron withdrawing anion groups creating electron-deficient Zr centers that behave as strong Lewis acids (see Scheme 14) (289). Reaction with any water molecules present in the catalyst converts the latter to strong Brønsted sites, implying that the catalytic activity increases substantially when the catalyst is exposed to moisture (290). The sulfate anion also stabilizes the tetragonal configuration of the Zr atoms. Other proposed structures for the active sites in solid zirconia catalysts were reviewed by Davis et al. (21). Pt is often deposited on the zirconia substrate to further increase the overall activity, isomer selectivity, and, more importantly, its resistance against coking. The role of Pt and hydrogen in the reaction mechanism is still a matter of discussion, but considering the relatively low reaction temperatures required for these catalysts, alkene formation through dehydrogenation over the noble metal is generally discarded. Ebitani et al. (291) also found from propene hydrogenation experiments that the presence of sulfate ions on the substrate greatly suppressed the activity of the Pt phase. Many researchers assumed direct carbenium ion formation from alkanes at the acid sites, while Pt heterolytically dissociates hydrogen to a proton and a hydride, the latter being transferred to the reactive carbenium ions (289,292–295), assisting their desorption from the acid sites. In this manner, consecutive cracking and dimerization reactions, which could lead to coking, are avoided. The mechanism is bifunctional in nature, but does not involve the formation of alkene intermediates and, hence, differs with the conventional scheme in the carbenium ion formation step. The presence of Pt, however, could still imply that the latter mechanism takes place, e.g., to initiate or facilitate the hydride transfer chain reactions (289). With respect to Pd and Ni, Pt

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outperformed other metals as hydride transfer promoting agent (296,297). Other compounds such as adamantane could facilitate hydride transfer because of highly reactive hydrogen atoms within the structure (289). Inspired by the potential of sulfated zirconia, the deposition of tungsten oxides onto ZrO2 substrates induced a similarly high catalytic activity. As with the sulfate anions, the tungsten phase could stabilize the zirconium atoms in their tetragonal coordination. Barton et al. (298) suggested that Brønsted sites were generated from slight reduction of and negative charge delocalization in the WOx domains, vide Scheme 15, which could explain the induction period usually observed during experimentation (299). A maximum activity was obtained at tungsten loadings in the range of 6.5–8 wt% which corresponded to a monolayer of tungsten oxide (299,300). A negative order with respect to the hydrogen pressure is found owing to the competitive chemisorption between alkanes and protons, in contrast to the sulfated zirconia where hydrogen does not take part in acid site creation. In that case, hydride transfer and, consequently, carbenium ion desorption were facilitated at higher H2 pressures resulting in a positive order. Compared with the sulfated form, tungstated zirconia exhibits a lower activity but a much higher isomer selectivity in alkane hydrocracking. Whereas Pt/SO42
–ZrO2 catalysts induced a relatively low maximum isomer yield of 20–25% in n-hexadecane hydrocracking regardless of the Pt content (296,301), this value exceeded 70% with Pt/WO3– ZrO2(296,300). Table 6 clearly illustrates the pronounced differences in

Scheme 15 Brønsted site generation on tungstated ZrO2 substrates as proposed by Barton et al. (298). Reprinted Barton, D.G.; Soled, S. L.; Meitzner, G. D.; Fuentes, G. A.; Iglesia, E. J. Catal. 1999, 181, 57–72. Copyright (1999), with permission from Elsevier. Table 6 n-C24 Hydrocracking Results Using Sulfated and Tungstated Zirconia Loaded With 0.5 wt% Pt, at 473K, and a H2 Pressure of 3.5 MPa (302) Selectivity (wt%) Catalyst/n-C24 Reaction 21 Ratio (kg kg ) Time (min) C5–C9 C10–C20 iso-C24 Catalyst

Pt/SO4 2
–ZrO2

0.08

15

53

30

17

Pt/WO3–ZrO2

0.25

25

2

2

96

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product selectivities between sulfated and tungstated zirconia catalysts obtained from n-C24 hydrocracking batch experimentation (302). A few investigations elaborated on physical mixtures of sulfated and tungstated zirconia in order to obtain intermediate cracking and isomerization yields (302,303). Other metal oxides such as HfO2 were found less active than the zirconia-based counterpart, but usually exhibited a higher affinity toward isomerization. Combination of both HfO2 and ZrO2 could even increase the overall catalytic activity compared to the monometal oxide catalysts (290,304). Sulfated and tungstated zirconia-based catalysts generally exhibit a higher activity than obtained over conventional large-pore zeolites. However, their sulfur resistance, thermal stability, and surface area are substantially worse making them less applicable for industrial feed processing (305,306). The latter two issues were tackled by supporting the modified zirconia onto mesoporous substrates such as MCM41 (306). The surface area and acidity were also increased by carefully selecting the calcination temperature during synthesis (294,307), or by applying other, more advanced preparative routes, e.g., via coprecipitation of tungsten and zirconium precursors (308).

3.5 Mo- and W-Based Oxides and Oxycarbides Transition metals of group VIb are almost exclusively found in an oxygenated state and exhibit interesting catalytic properties with respect to both metal and acid catalysis. In a series of pioneering works, Iglesia and Ribeiro (309,310) explored the performance of tungsten carbides and oxycarbides in the hydrocracking of light n-alkanes. After full carburization of WO3 to the corresponding carbide, the WC and β-W2C phases showed an extremely high hydrogenolysis activity at 430K. Recently, it was found that the carburization of a MoO3/ZrO2 catalyst could be facilitated by doping with rhodium which, however, further increased the hydrogenolysis activity (311). Oxygen chemisorption leading to local WOx phases resulted in isomer formation which became more pronounced as the oxygen treatment was prolonged. These observations suggested that isolated, tetrahedrally coordinated WOx species incorporated acidity into the parent material transforming the latter into a selective isomerization function, while the metal carbides provided the (de)hydrogenation function. In a series of subsequent studies, Ledoux and coworkers (312–316) investigated a partially carburized MoO3 catalyst in the hydrocracking of n-heptane and n-octane. The surface area could be increased by hydrogen

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H M H

H M H

H M H

H M H

Scheme 16 Isomerization of n-heptane via metallocyclobutane intermediates (16). Reprinted from Ono, Y. Catal. Today 2003, 81, 3–16. Copyright (2003), with permission from Elsevier.

reduction and carbon atom insertion leading to partial collapse of the structure to MoO2.4C0.2H0.8 and presumably inactive MoO2 species. Compared to a conventional Pt/H-Beta, the molybdenum oxycarbide showed a much higher isomer selectivity in the higher conversion range which was attributed to a different isomerization mechanism over metallocyclobutane transition states, vide Scheme 16 for n-heptane isomerization. This mechanism occurs on one type of active sites and does not involve the formation of intermediates with an ionic character. The activity of the Mo oxycarbide phase would originate from the high amounts of metal vacancies and from the carbon atoms which stabilize the metastable MoO2.4C0.2H0.8 phase (315). The addition of Pt to the catalyst promotes the formation of such oxycarbide phases owing to their prominent role in hydrogen dissociation and hydrocarbon activation (316). The catalytic active phase of tungsten and molybdenum oxides in n-alkane hydrocracking was further investigated by Katrib and coworkers (317–323). Partial reduction of TiO2-supported MeO3 (Me ¼ W or Mo) led to a MeO2 surface layer with Me2O5 interfaces. Chemically bonded hydrogen species to electronically active oxygen atoms in the MeO2 phases were assumed to act as active dual-functional sites and, hence, carburization as stated previously was excluded. A similar conclusion was drawn from publications by Matsuda et al. (324–326) in which only subtle differences in n-heptane hydrocracking product selectivities were observed between reduced molybdenum oxides and Pt-loaded USY zeolites. In slight contrast with the previous approach, they attributed the Brønsted acidity of the catalyst rather to MoOxHy phases obtained from H2 reduction. They did see, however, a beneficial effect of an additionally loaded noble metal on the formation of a hydrogen molybdenum bronze during the metal reduction step and on the global isomerization selectivity (327,328). Wang et al. (329) reported similar observations using MoO3 doped with NiO. High isomer yields were reported in each publication as illustrated in Fig. 16 for n-octane as feed. A consensus on the hydrocracking mechanism and the active sites associated with tungsten- and molybdenum oxides has clearly not been achieved yet. The theory of Ledoux et al. (312) was not confirmed by any other

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100 Conversion 80

60 %

Mono-iC8 C3–nC7

40 Multi-iC8

iC4–iC7

20 Aromatics

0 570

590

610 630 Temperature (K)

650

670

Fig. 16 Experimental n-octane hydrocracking product distribution obtained on a partially reduced MoO3/TiO2 catalyst as a function of the reaction temperature at 0.5 MPa (322). Reprinted from Al-Kandari, H.; Al-Kharafi, F.; Katrib, A. Appl. Catal. A Gen. 2010, 383, 141–148. Copyright (2010), with permission from Elsevier.

research group in the following decade, and the observed methane and ethane formation during their experimentation could be related to hydrogenolysis as observed earlier on tungsten carbides. The classical bifunctional mechanism is generally assumed although it could not unambiguously describe the entire product distribution (330). Even though the mechanism has not been entirely elucidated yet, it remains that WO3 and MoO3-based solids exhibit desirable isomerization capacities. Comprehensive studies using heavier alkanes as feed, however, are still missing in order to explore their potential in, for instance, the Fischer–Tropsch wax conversion.

3.6 Conclusions A highly active metal function can be established by using noble metals. However, to limit the expenditures with respect to the hydrocracking catalyst and to increase the catalyst resistance against sulfur poisoning, sulfided transition metals such as Ni and Mo are commercially used instead. Many acidic supports were explored which differ in number of acid sites, their corresponding strength, mesoporosity, and stability. Amorphous mesoporous alumina and silica-alumina are applied most often in industrial

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environments. In addition, zeolite frameworks were established as appropriate alternatives as they exhibit a generally higher acid strength, thermostability, and coking resistance. They also allow product distribution manipulation via shape selectivity induced by their microporosity. Considerable research was attributed to the incorporation of mesoporosity in zeolite frameworks to improve the diffusion of bulkier compounds such as polyaromatics, to increase their applicability for commercial upgrading processes. Other frameworks comprise aluminophosphates, anion exchanged zirconia, molybdenum and tungsten oxides and oxycarbides, tungsten-based heteropolyacids, pillared clays, and activated carbon.

4. KINETIC MODEL DEVELOPMENT Kinetic models are well established and indispensable tools in process optimization and reaction path analysis. In this section, a concise overview is given of the different kinetic modeling approaches that have been followed. The traditional, discrete lumped models are the simplest in nature and are generally used in process optimization via interpolation within the range of reaction conditions in which the model was validated. More advanced lumped models have also been developed that account for increasing detail in the reaction kinetics to extend the area of applicability. Models based on continuous kinetics constitute a more sophisticated approach in which the composition of the reacting mixture is considering as a continuously changing variable. Finally, fundamental models, e.g., based on single-event microkinetics, offer the most comprehensive insight and extrapolative capability for the hydrocracking reaction network and corresponding kinetics while retaining the number of model parameters to an acceptable number.

4.1 Discrete Lumped Models Models based on discrete lumped kinetics focus on the reaction pathways between different component lumps which are generally defined on the distillation temperature or the molecular structure. These models offer basic insight into the general reaction mechanism and allow process optimization for a narrow range of operating conditions in which the model parameters were estimated. 4.1.1 Lumped Models Based on Distillation Ranges Lumping based on a distillation temperature range is probably the earliest lumping technique developed in describing the hydrocracking kinetics of

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industrial feeds. One of the first models was developed by Qader and Hill (331) for the hydrocracking of heavy gas oil at 673–773K over a sulfided NiW/SiO2–Al2O3 catalyst. They only considered the conversion of the feed mixture and found an apparent first-order rate expression with respect to the feed concentration, with an activation energy of 88.3 kJ mol
1. The latter value depends strongly on the feed characteristics and can, hence, not be extended toward the hydrocracking of other feeds. However, because of the simplicity of this two-lump model, the extremely low number of kinetic parameters and the minor insight it already provides in the reaction mechanism, this lumping technique received considerable interest over the years. Ancheyta et al. (22) critically reviewed different discrete lumping approaches for conventional hydrocracking feeds and classified them according the number of defined lumps. A still simple three-lump model was developed by Callejas and Martinez (332) containing only two rate coefficients for the hydrocracking of Maya residue, vide Scheme 17, in which products were classified in either light oils or gases. A reevaluation of the parameter values by Ancheyta et al. (22) showed that, even with such a general approach, a satisfactory agreement between model and experiment was obtained at different reaction temperatures, vide Fig. 17. The downside of this model, however, is that it provides little information on the product composition and the actual reaction pathways. A higher level of detail is achieved by increasing the number of lumps and possible reaction paths in between the different lumps. In response to the work by Callejas and Martinez, Sanchez et al. (333) designed a five-lump model for the hydrocracking of Maya residue in which the products were classified in either VGO, distillates, naphtha, or gases depending on the boiling point, vide Scheme 18. Ten kinetic parameters were introduced, of which seven were significantly different from zero and indicated that gases were exclusively formed from residue cracking. This model was extended a few years later to incorporate catalyst deactivation (334). Maya residue

k1 Light oils

k2 Gases

Scheme 17 Proposed three-lump scheme for the hydrocracking of Maya residue by Callejas and Martinez (332). Reprinted with permission from Callejas, M. A.; Martinez, M. T. Ind. Eng. Chem. Res. 1999, 36, 3285–3289. Copyright (1999) American Chemical Society.

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Calculated product yield (wt%)

80 70 60 50 40 30 20 10 0 0

10

20 30 40 50 60 Experimental product yield (wt%)

70

80

Fig. 17 Parity diagram for the experimental and simulated Maya residue hydrocracking conversion (circles), light oils yield (squares), and gases yield (triangles) using the threelump model shown in Scheme 17 and the kinetic parameters reported by Callejas and Martinez (full symbols) (332), and by Ancheyta (open symbols) (22). Reprinted from Ancheyta, J.; Sanchez, S.; Rodriguez, M. A. Catal. Today 2010, 109, 76–92. Copyright (2010), with permission from Elsevier.

Residue k1 VGO k5 k2

k6

Distillates

k7

k8 k3

k9

Naphtha k10 k4

Gases

Scheme 18 Proposed lumped reaction scheme for the hydrocracking of Maya residue by Sanchez et al. (333). Reprinted with permission from Sanchez, S.; Rodriguez, M. A.; Ancheyta, J. Ind. Eng. Chem. Res. 2005, 44, 9409–9413. Copyright (2005) American Chemical Society.

Sadighi et al. (335,336) first constructed a four-lump model followed by a refined six-lump one for the hydrocracking of VGO. In the latter model, the distillate lump is split into kerosene and diesel, and naphtha into a heavy and a light fraction. They also accounted for catalyst deactivation in each rate equation via a deactivation factor α, vide Eq. (11) for the VGO lump, and

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also explicitly incorporated the hydrogen consumption. In Eq. (11), k0VGO represents the initial VGO cracking rate coefficient and AF the accumulated feed, i.e., a dimensionless measure for the accumulated amount of feed which has been in contact with the catalyst after a specific time on stream (336): kVGO ¼ exp ð
αVGO AFÞk0VGO exp

 
Eact, VGO RT

(11)

Other lumping strategies were designed depending on the processed feedstock. Botchwey et al. (337) considered six lumps in the hydrotreating and mild hydrocracking of Athabasca bitumen-derived gas oil. A four-lump model was constructed for both thermal and catalytic hydrocracking of an asphaltenic coal residue in which coke was considered as a distinct lump (53,338). The high-temperature hydrocracking of Marlim vacuum residue was modeled via a five-lump approach in which each reaction between lumps was categorized in either a thermally or a catalytically activated pathway (339). Mosby et al. (340) developed an expanded seven-lump model for the hydrocracking of vacuum residue in which the feed lump was split into “hard” and “soft” residue related to their cracking activity, and the gas oil fraction into a reactant and a product lump. The model was later validated using an Athabasca bitumen feedstock and came out overdetermined; i.e., a conventional five-lump model was sufficient for accurate data reproduction (341). A summary of the simulation results from different discrete lumped models showed indeed that the estimated parameters largely depended on the process conditions (22). In general, the reported activation energies for the cracking from one lump to another increased as the reactant lump became heavier. Therefore, an accurate initial guess of the real parameter values is difficult to extrapolate from one feedstock to another. Sophisticated parameter estimation algorithms for traditional lumped models were designed to avoid convergence to local optima, such as the protein inspired RNA genetic algorithm designed for the lumped reaction network in Scheme 18 (342). In order to tackle the high number of parameters, a different approach by Lababidi et al. (343) considered a simple relationship between the boiling point from the distillation curve and the cumulative weight fraction. The methodology failed, however, in accurately predicting product yields at low to moderate VGO hydrocracking conversions. Nevertheless, discrete models based on distillation ranges allow basic catalyst

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screening and process optimization studies. Such models lack detail and are less useful in predicting product yields outside the process design specifications in which they have been validated. 4.1.2 Lumped Models Based on Pseudo-Components Instead of introducing lumps based on their distillation range, pseudocomponents are often defined which lump components within the reaction network according to their structure and/or carbon number. An early model approach was elaborated by Krishna and Saxena (344) to describe the hydrocracking of VGO. Herein, they defined distinct lumps for the aromatics, cycloalkanes, acyclic alkanes, and sulfur-containing compounds, with the former three lumps split into a heavy and a light fraction. A different lumped network, vide Scheme 19, was proposed for the hydrocracking of pyrolysis gasoline, containing about 70 wt% aromatics and 14% alkenes, on a Pt–Pd/ H-ZSM5 catalyst and at 623–673K and 2–5 MPa (345). Thermal hydrocracking to methane owing to the relatively high reaction temperature applied was incorporated as a separate pathway with kinetic constant k5. Alkene dehydrogenation was found to be potentially much faster than any other reaction as evidenced by the negligible alkene concentration in the product mixture. Recently, Anand and Sinha (346) performed a reaction path analysis for triglyceride hydrocracking based on eight distinct five-lump models. A more detailed lumping approach was developed for pure alkane feeds. The reaction products were classified in feed isomer and cracking product lumps. As a result, additional information on the reaction mechanism could be obtained from the relative reaction rates between the different lumps. In a series of publications, Calemma and coworkers (39,113) introduced a lumped reaction scheme for the hydrocracking on a Pt/SiO2–Al2O3 catalyst of long n-alkanes with carbon numbers 16, 28, 36, and 44 as representative Hydrogenation

Primary cracking (RO)

Secondary cracking

Ternary cracking

k5 k1

k2 Cycloalkanes

Aromatics

Isoalkanes

k6

k4

C2+ n-Alkanes

Methane

k7

k9

k8 Alkenes

k3

Scheme 19 Proposed lumped hydrocracking scheme for pyrolysis gas oil by Gutierrez et al. (345). Reprinted from Gutierrez, A.; Castano, P.; Azkoiti, M. J.; Bilbao, J.; Arandes, J. M. Chem. Eng. J. 2011, 176, 141–148. Copyright (2011), with permission from Elsevier.

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molecules for a Fischer–Tropsch wax and found that, for any reactant, direct cracking of the linear alkane occurs at a much slower rate than isomerization and cracking of the isomer products. A decrease in alkane conversion with the chain length was also evidenced from the estimated activation energies for any reaction depicted in Scheme 20 and could be attributed to saturation phenomena occurring during reaction from the liquid phase for which is referred to Section 2.2. In Scheme 20, a distinction is made between the feed isomers which have a sufficiently low pour point (iso-Cn lube), and those which have not and, hence, are not suitable as a base oil (iso-Cn nolube). More detail was implemented by separating the physisorption step from the lumped rate coefficients (97). In the first instance, a Langmuir expression was introduced to describe the physisorption step, vide Eq. (5), and was later adjusted to account for possible interactions between the different sorbate molecules according to Eq. (7)(67). Assuming ideal hydrocracking, the rate coefficients for monobranching, multibranching, and cracking were estimated for C6–C9 n-alkanes over various faujasites following Scheme 21, and of which the values for monobranching are reported in Table 2. Even though this lumped reaction scheme is quite simple in nature, important information on the reaction mechanism was already extracted from the parameter estimates as elaborated in Section 2.1.1 (59). A more sophisticated approach was elaborated by Pellegrini et al. (347) for Fischer–Tropsch wax hydrocracking. They defined carbon number and branching degree-based lumps, vide Scheme 22, wherein the reactivity and

k1

iso-Cn lube (B)

n-Paraffin (A)

k5 Cracking products

k4 k2

iso-Cn nolube (C)

k6

k3

Scheme 20 Proposed lumped reaction scheme for the hydrocracking of long-chain n-alkanes by Calemma et al. (113). Reprinted with permission from Calemma, V.; Peratello, S.; Stroppa, F.; Giardino, R.; Perego, C. Ind. Eng. Chem. Res. 2004, 43, 934–940. Copyright (2004) American Chemical Society.

n-Alkane

kMB

MonoB

kMTB

MultiB

kCR

CrackP

Scheme 21 Proposed lumped reaction scheme for the hydrocracking of n-alkanes by Froment (97). Reprinted from Froment, G.F. Catal. Today 1987, 1, 455–473. Copyright (1989), with permission from Elsevier.

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n-C22+

1

iso-C22+

m 5*

m5

5 2 n-C15–22

iso-C15–22 6

n-C10–14

3

m6

iso-C10–14 7

n-C5–9

4

iso-C5–9 8 C1–4

Scheme 22 Proposed lumped reaction scheme for the hydrocracking of Fischer– Tropsch waxes by Pellegrini et al. (347). Reprinted from Pellegrini, L.; Locatelli, S.; Rasella, S.; Bonomi, S.; Calemma, V. Chem. Eng. Sci. 59, 4781–4787. Copyright (2011), with permission from Elsevier.

physisorption of each component were assumed identical. Only centralchain cracking was considered with the stoichiometric coefficients μ5, μ*, 5 and μ6, indicating the fraction of the cracking products which do not belong to the reactant lump. The values for the above coefficients were calculated as, respectively, 0.33, 0.14, and 0.56 assuming a uniform distribution of the components inside each lump. A strong increase of the physisorption coefficient with the lump carbon number range was estimated, while its dependence on the branching degree was relatively small. In later researches, Fernandes and Teles (348) and Moller et al. (349) removed the linear alkane lumps from the model for the simulation of a C4–C30 Fischer–Tropsch wax and a pure C80 feed, respectively. The more detail accounted for by these models increases the applicability range of the corresponding parameters with respect to the investigated reaction conditions and, additionally, provides more insight in the reaction mechanism. Model improvement was achieved by defining a n- and iso-lump per carbon number and by implementing an exponential increase for the Langmuir physisorption coefficients with the carbon number (350). Hosukoglu et al. (351) adopted this model to investigate the performance of microchannel reactors designed to cope with the exothermicity of the alkene

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hydrogenation and alcohol hydrodeoxygenation reactions during Fischer– Tropsch wax hydrocracking. The extension toward a three-phase hydrocracking model involved the substitution of the partial pressures in the rate equations by fugacity coefficients calculated from vapor–liquid equilibrium data (352). In the next step of the model development (353), a breakage probability function based on the work by Martens et al. (35) was introduced which allowed the scission of any C–C bond within the reactant molecule. Finally, Gambaro et al. (354) published their definitive version of the lumped model in which the rate coefficients as well as the physisorption, dehydrogenation, and protonation equilibrium coefficients are defined as a function of the reactant carbon number. A hyperbolic function for the physisorption coefficient with respect to the sorbate carbon number could account for saturation phenomena related to the liquid phase present in the catalyst pores, vide Section 2.2. Introducing a Langmuir–Hinshelwood approach for the rate coefficients led to a superior agreement between model and experiment, vide Fig. 18. The introduction of pseudo-components as lumps clearly provided more insight into the reaction mechanism, but could still not eliminate the feedstock dependence of the kinetic parameters. The latter issue was tackled by use of fundamental microkinetic models which are described in Section 4.4.

4.2 Lumping Based on Continuous Kinetics Some of the disadvantages inherently related to discrete lumped models were first addressed by Stangeland (355) via assuming a continuously changing hydrocracking product distribution with respect to the true boiling point (TBP). The cracking rate coefficient was expressed as an empirical function iso-Paraffins distribution Product concentration (wt%)

Product concentration (wt%)

n-Paraffins distribution 4.0 experimental Model 1 Model 2

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

10

20

30 40 Nc [–]

50

60

70

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

experimental Model 1 Model 2

0

10

20

30 40 Nc [–]

50

60

70

Fig. 18 Simulated and experimental n- and iso-alkane concentrations in the product distribution from C5–C70 n-alkane hydrocracking over a Pt/SiO2–Al2O3 catalyst at 632K and 4.125 MPa and at a C22+ conversion of 60.9% (354). From Gambaro, C.; Calemma, V.; Molinari, D.; Denayer, J. AIChE J. 2011, 57, 711–723. Copyright 2011 by John Wiley Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

Multiscale Aspects in Hydrocracking

175

of the TBP/1000 (TBP*) and an adjustable parameter A situated between 0 and 1:   kðTBP Þ ¼ TBP + A TBP 3
TBP (12) A discontinuity in the product yields was observed for the light gases with a TBP below 283K and was accounted for by an empirical function of the feed TBP and the model parameter C: Y 0 if

ΔHrs0

mesopores > macropores > external surface (394,395). Millan and coworkers (279,396) identified coking as a dynamic process during which a continuous exchange of material between the liquid phase and the catalyst surface takes place, eventually resulting in a gradually increasing carbonaceous layer. In a recent publication, Castano et al. (394) classified coke according to the condensation level. The most developed coke was likely related to highly polyaromatic species or a distorted graphite-type structure and could be mainly found on the external surface (395). The contribution of coking on the external layer increased with the reaction temperature and, eventually, became impermeable. Table 10 gives an overview of the main hydrocarbon and heteroatomic compounds found in the less developed coke extracted using dichloromethane (394). Besides active site coverage or pore blockage due to coking, organic heterocompounds could compete for chemisorption on the active sites leading to inhibition. The most abundant poisons in hydroprocessing are sulfurand nitrogen-containing compounds which chemisorb on, respectively, the metal and the acid sites (7,397). The nitrogenous poisons generally comprise five- and six-membered heteroatom rings and anilines, of which the latter two have the most strongly inhibiting character. Pyridine and aniline are therefore often used as model components to investigate the corresponding poisoning effect (7,76,398). Galperin (234) on the other hand used tributylamine which rapidly decomposed to ammonia that covers the acid sites of the catalyst. Dufresne et al. (398) found that ammonia sorption is quickly reversible except at zero ammonia pressure. The latter was ascribed to slow ammonia desorption from the strongest acid sites of the catalyst. For 770 ppm of nitrogen in the feed, a temperature increase of 100K was required to reach a similar total n-decane hydrocracking conversion as obtained over unpoisoned Pt-loaded zeolites, vide Fig. 28 (234). As part of the same work and also depicted in the figure, the addition of 1000 ppm H2S clearly inhibited the Pt function as evidenced by a pronounced decrease in the isomer yield apart from the reduction in catalyst

Table 10 Main Hydrocarbon and Heteroatomic Compounds in Coke, Extracted With Dichloromethane From a Deactivated Catalyst From Light Cycle Oil Hydrocracking (394) Name Structure Name Structure

1,3-Dimethylnaphthalene

3-Methyl-benzo[j]aceanthrylene

9-Methylfenantrene

4-Methyl-benzo[g,h,i]perylene

9-Methylanthracene

Indene[1,2,3-cd]fluoranthene

o-Terphenyl

2-[2,4-Dimethylphenyl]indole N H

9-Methylene 9H-fluorene

p,p0 -Ditolylamine

Pyrene

5,8-Dimethylquinoline

NH

N

11H-benzo[a]fluorene

4-Phenyl-2-[4-tolylamino]-thiazole

N

NH S

2-Methylfluoranthracene

1-Phenylthio-napthalene s

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B 100

iso-C10 select, (%)

n-C10 conversion (%)

A 80 60 40 20 0 300

350

400

450

Temperature (°C)

500

100 90 80 70 60 50 40 0

20 40 60 80 n-C10 conversion (%)

100

Fig. 28 Hydrocracking conversion and isomer selectivity on Pt/MAPSO-31 of a pure n-decane feed (▲), of n-decane with 1000 ppm H2S (♦), with 770 ppm tributylamine (■), and with 1000 ppm H2S and 770 ppm tributylamine (ж) (234). Reprinted from Galperin, L. B. Appl. Catal. A Gen. 2001, 209, 257–268. Copyright (2001), with permission from Elsevier.

activity. The former effect is a direct consequence of irreversible metal phase poisoning and leads to nonideal hydrocracking behavior. Noble metals are much more sensitive to sulfur poisons in such a way that their apparent activity becomes inferior to those of early transition metals at high sulfur contents (399). Thomazeau et al. (400) also found that the coking rate increased with the H2S pressure resulting in polysulfide molecules. Not only H2S but also thiophene, 2-methylthiophene, and dimethyldisulfide are commonly added to mimic the poisoning effect of a petroleum-based feed on the catalyst performance (49,148,179,305,397). In general, the presence of both nitrogen and sulfur in the reaction medium causes a deterioration in catalyst activity and pushes the acid–metal balance toward the metal or the acid side, respectively. An “ideal” combination of both poisons could again result into an ideally operating catalyst, as shown in Fig. 28 for a feed containing 1000 ppm sulfur and 770 ppm nitrogen (234). It should, however, be noted that an optimum presence of sulfur in the reaction medium is required for rendering the (de)hydrogenation function of metals such as Mo and W active, vide Section 3.1. Water is mostly present in the reactor because of dissolved water molecules in the feed, the scrubbed recycle gas or because it is produced by the conversion of oxygenates. The latter are mainly alcohols, aldehydes, ketones, and carboxylic acids and are abundantly present in bio-oils (126,401). In the early work, Yan (402) investigated the effect of isopentanol on the hydrocracking of n-hexadecane over faujasites. After decomposition, water molecules hydrated the catalytic Brønsted sites but, conversely, could also activate Lewis sites for alkene chemisorption.

Multiscale Aspects in Hydrocracking

205

Additionally, inhibition of the strongest Brønsted sites brings the catalyst closer to one exhibiting “ideal” hydrocracking and, hence, may lead to a more attractive product distribution, such as a higher middle distillate yield from Fischer–Tropsch waxes (403). However, the corresponding oxygenates content should remain low as higher concentrations may result in such an amount of water that it could induce structural changes to the catalyst (7). E.g., de Klerk (401) reported metal leaching by carboxylic acids and dealumination. The effect of carboxylic acids on the catalyst performance was also different than found from alcohols as the acid nature of the former tends to interact with the catalyst metal function rather than with the acid function (403). Vanadium and nickel are the predominant metals present in petroleum, heavy oils, and shale oil, while iron and titanium mostly appear in coalderived liquids (7). Deactivation by metals always occurs simultaneously with coking and is irreversible. V and Ni are abundantly found as porphyrin structures in crude oil, shown in Scheme 26, which are hydrogenated to chlorine intermediates and subsequently decomposed through C–N bond hydrogenolysis. Metal precipitation is facilitated with H2S via a mechanism which is not fully untangled yet, leading to a metal sulfide which is deposited

Scheme 26 Proposed hydrodemetallation mechanism of a metal etioporphyrin (M ¼ Ni or VO) (7). Reprinted from Furimsky, E.; Massoth, F.E. Catal. Today 1999, 52, 381–495. Copyright (1999), with permission from Elsevier.

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on the catalyst reducing its surface area and porosity. The metal deposition mechanism of nonporphyrin structures has not been investigated as intensively because of their much less defined molecular structure. Deactivation owing to Fe and Ti is less documented, but it is generally believed to occur through respectively sulfided and oxidized structures (7). Alkali metals such as Mg and Ca could also be present in the feed and mainly inhibit the acid function of the catalyst. For hydrocracking in specific, a decreasing order of the deactivation rate with respect to the different metal poisons was determined (7): For hydrogenation: V > Fe > Na > Mg > Ni For hydrocracking: Na ≫ Mg > Ni > Fe Other deactivating species which are commonly found in hydrocracking feedstocks comprise silicates and aluminosilicates in tar sand-derived oils, and boron which often resides in coal liquefaction feeds (7). Specifically for Fischer–Tropsch syncrudes, metal carboxylates such as sodium, potassium, and aluminum acetate could only be thermally decomposed and induced catalyst bed plugging (401). In commercial practice, catalyst deactivation is coped with by increasing the reaction temperature up to the point that comparable product yields are obtained as initially observed over a new catalyst batch. Nevertheless, the catalyst lifetime remains a crucial aspect in achieving an economically viable process (404). Besides the catalyst type, the selection of the reactor system is critical in improving the overall process tolerance against deposits and contaminants. Furimsky and Massoth (7) reviewed the potential of various reactor types, such as ebullated reactors and multiple bed systems, specifically for hydrotreating over mainly NiMo- and CoMo-loaded alumina supports. More information on the different commercial hydrocracker configurations is given in Section 5.2.2.

5.2 Processing of Industrial Feedstocks This section describes the versatility and flexibility of the hydrocracking process in much detail. Its versatility is reflected in the various feedstocks which are converted toward high-value distillate products. Most of the industrial plants are constructed particularly for VGO hydrocracking, while recent technologies are aiming at heavy residue and Fischer–Tropsch wax conversion. The wide variety of reactor types and configurations is exemplary for the versatility of the hydrocracking process.

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5.2.1 Isomerization of Naphtha Streams Exploitation of light and mid-cut naphtha streams directly formed from crude oil distillation led to numerous hydrocracking technologies producing high octane number and environmentally benign gasoline fuels. Early commercial light naphtha isomerization technologies to high-value C5–C8 compounds were classified according to the applied catalyst type, i.e., Pt-loaded chlorinated alumina or zeolites. Licensers involved UOP, IFP, ABB Lummus Global, Exxon, Mobil, Gulf, and Chevron (15,25). As observed from UOP’s Penex process, chlorinated alumina exhibited a high catalytic activity, but urged on more advanced process design specifications related to its sensitivity to water and sulfur in the feed and its corrosive nature (405). Mordenite zeolites were more convenient, but showed a distinctly lower activity than the chlorinated alumina catalysts. Shell’s Hysomer process is based on Pt/Mordenite catalysts and comprises once-through gas-phase operation at intermediate pressures and temperatures of 2–5 MPa and 513–553K, respectively (25). The process operates under thermodynamic equilibrium conditions with respect to the feed components and their isomers and could apply a feed recycle to achieve higher conversion levels similarly as in Fig. 29 for the UOP LLC Par-Isom process (405). Separation of C5–C6 alkanes was achieved with molecular sieves composed of eight-membered pores. Another drawback with Mordenite-based catalysts is related to product shape-selective effects, implying a higher cracking yield at the expense of Makeup H2 Stabilizer Off gas

Separator Reactor

Feed oil Isomerate

Fig. 29 Flow sheet of the Par-Isom process licensed by UOP LLC (405). Reprinted from Kimura, T. Catal. Today 2003, 81, 57–63. Copyright (2003), with permission from Elsevier.

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multibranched isomers (17). Gulf researchers and Mobil’s M-forming process rather used ZSM5 and ferrierite zeolite catalysts, respectively, for gasoline upgrading (25). In collaboration with Repsol-YPF, Ramos et al. (128,129,406) explored the potential of a highly stable Pt/H-Beta agglomerated on bentonite in the hydroisomerization of a C6–C8 naphtha cut. A higher activity for alkane isomerization and aromatics hydrogenation was generally observed than obtained over a Mordenite zeolite, and a more pronounced improvement in research octane number (RON) was achieved. Therefore, new catalyst formulations were explored for the isomerization of € C5–C6 cuts such as sulfated zirconia. SUD-CHEMIE developed the HYSOPAR-SA process based on such catalysts which exhibit an activity intermediate to that of zeolites and chlorinated alumina, combined with a substantially higher water and sulfur resistance (407). Cosmo Oil Co. and Mitsubishi Heavy Industries developed a similar Pt/SO4 2
–ZrO2 catalyst which was later licensed by UOP LLC (405). The loading of a small amount of Pt successfully increased the stability of the catalyst against coke formation. The corresponding Par-Isom process, depicted earlier in Fig. 29, was commercially applied in 1996 for the first time. Other light and aromatics-rich feedstocks such as FCC gasoline and pyrolysis gasoline were subjected to hydrocracking for fuel upgrading (132). Mo-, Ni-, or noble metal-loaded H-ZSM5 zeolites were commonly used for this purpose. In case of FCC gasoline, RON improvement from 84.8 to 88.3 without pronounced losses in liquid products could be achieved (132). Transformation of monoaromatic compounds to high-value isoalkanes and environmentally friendlier cycloalkanes, or to C2+n-alkanes which contribute as feed to stream cracking units, is generally pursued with pyrolysis gasoline feeds (345). 5.2.2 Fuel Production From Gas Oils, FCC Cycle Oils, and Shale Oils Hydrocracking was first applied in the early 1960s to convert low-value distillate feeds such as cycle oils and heavy gas oils to high-value gasoline products (3). In the 1970s, a broader range of hydrocracking products were targeted in order to address the shifting market demand toward turbine (jet) fuel and diesel fuels. Nowadays, modern hydrocracking technologies are still mainly focused on the production of diesel, kerosene, and gasoline from heavy vacuum gas oils (8). Owing to their high flexibility, a large variety of reaction products can be obtained aiming at either molecular weight reduction primarily to naphtha or LPG, or to middle distillates via selective dewaxing. Choudhary and Saraf (12) and Mohanty et al. (18) reviewed the

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Multiscale Aspects in Hydrocracking

commercial VGO upgrading technologies developed up to 1990, which included BASF-IFP one-stage and two-stage hydrocracking, the ISOMAX process patented by UOP and Chevron, and the Unicracking-JHC process for catalytic cycle oils and naphtha licensed by Union Oil Company and Esso Research and Engineering. These technologies primarily use trickle-bed reactors and silica-alumina-based catalysts, and a few were successfully modified for heavy residue conversion. Bezman (408) reviewed Chevron’s twostage Isocracking process for VGO feeds and stated that, in the year of 1993, the process provided 29% of the company’s catalytic fuel production capacity. Commercial gas oil feeds contain a significant amount of sulfur, nitrogen, and aromatic contaminants, vide Table 11 for various gas oil mixtures, and, hence, are often hydrotreated prior to or simultaneously with hydrocracking to attain the targeted fuel quality specifications. The catalyst (de)hydrogenation function is usually provided by sulfided early transition metals such as Ni, Co, Mo, and W. Although exclusively used at first, amorphous oxide supports are nowadays more and more replaced by more active zeolitic materials which, thanks to advances in catalyst synthesis, yield similar middle distillate products (3). In addition, zeolites are hydrothermally more stable and are more resistant against coking. Commercial Y and ZSM5 supports Table 11 Properties of Commercial Gas Oil and FCC Cycle Oil Mixtures Qader and Hill (331) Bezman (408) Castano et al. (394)

Gravity (API)

31.8

32.4

Distillation T (K)

ASTM E13358

ASTM D1160 ASTM D2887

IBP

573

530

375

50%

615

640

550

FBP

703

732

739

Hydrocarbon content vol%

vol%

wt%

Saturates

65

83

11

Alkenes

7

0

2

Aromatics

28

17

Sulfur (wt%)

0.94

0.50

Nitrogen (wt%)

0.81

0.30

a

By addition of di-(tert-nonyl)-polysulfide.

19.3

87 a

0.6 0.2

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are most commonly used for cracking and dewaxing purposes, respectively. In this respect, Shell developed a selective Y-based zeolite catalyst with small unit cell and a considerable mesoporosity for a second-stage hydrocracker (409). From the academic point of view, the following composite catalysts were experimentally tested in VGO hydrocracking aiming at a high activity combined with a high selectivity to middle distillates: sulfided catalysts such as mesoporous NiMo/MCM-41 (253), NiMo/ITQ-2 (192), Pt–Rhloaded Co-saponite (286), CoMo and NiW impregnated on USY/Al2O3 composites (141,160), NiMo-loaded H-Y/H-Beta composites (214), NiW-loaded Y/SBA-15 composites (266), and NiMo/H-Y with trimodal porosity (216). Cycle oils from FCC units are of the same boiling point range and also constitute a potential fuel source, but contain substantially more aromatics as illustrated in Table 11 (right column) (394). Therefore, a stronger (de) hydrogenation function of the catalyst is required to prevent excessive coking. Gutierrez and coworkers (410) recently explored fuel production from light cycle oil hydrocracking over Pt- and Pd-loaded faujasites and Beta zeolites. Both supports exhibited a high activity and naphtha and middle distillate selectivities at 633K and 5 MPa. It was subsequently found that high reaction temperatures, high pressures, and intermediate space times are beneficial for naphtha production at the expense of middle distillates (411–413). In a series of publications, Landau and coworkers (414–416) investigated the mild hydrocracking of Israeli shale oil on various catalysts in a singlestage and a two-stage trickle-bed reactor system with, in the former case, two catalyst beds loaded in series. Owing to the high level of contamination of shale oils with oxygen, nitrogen, sulfur, and metals such as arsenic, iron, and nickel, a first hydrotreating stage is often desired and, in this case, employed a sulfided NiMo catalyst with bimodal pore distribution for sulfur and nitrogen removal. The second hydrotreating/hydrocracking step at 623K and 14 MPa applied a Cr2O3 promoted H-Y and/or H-ZSM5 zeolite mixed with the NiMo/Al2O3 to increase the global conversion. The process versatility is reflected in the numerous process configurations developed in accordance with the feed characteristics and the targeted product yields. Ward (3,417) distinguished four major types of configurations: single-stage, two-stage, once-through, and with separate hydrotreating. Single-stage hydrocracking is the most common design configuration and could convert typically 40–70% of the feed in a single pass. Fig. 30 illustrates a single-stage hydrocracking process scheme with feed and gas recycle. In case of highly contaminated feedstocks, hydrotreating on

211

Multiscale Aspects in Hydrocracking

R·1 REACTOR

Recycle gas compressor

H.P. seperator

Sour water

Fresh feed Hydrogen makeup Wash water

Makeup compressor

L.P. separator

To gas plant

To fractionation

Recycle oil (fractionator bottoms)

Fig. 30 Simplified process flow scheme of a single-stage hydrocracker (417). Reprinted from Ward, J. W. Stud. Surf. Sci. Catal. 1990, 53, 193–215, Copyright (1990), with permission from Elsevier.

early transition metal sulfides is usually employed prior to hydrocracking, to selectively convert organic sulfur and nitrogen compounds to less detrimental and easily removable H2S and NH3. Once-through hydrocracking is a variation of the single-stage configuration without recycle. Partial conversion of the feeds could lead to high middle distillate yields and is typically applied for the coproduction of distillates and catalytic cracker feeds, or for the production of lube blending stocks from mixtures which are more difficult to recycle (3). Two-stage hydrocracking is more flexible as it allows further conversion of the unconverted oil from the first stage at more sever process conditions. They are particularly useful to convert feedstocks containing a substantial amount of nitrogen poisons (398). Two-stage processes involve higher capital investments and operating costs which are partially, or completely, compensated by the higher distillate yields usually obtained. Recycling the unconverted feed to the second-stage reactor aiming at a maximum conversion is often referred to as “recycle to extinction” hydrocracking (138,418). Minderhoud and Van Veen (26) also distinguished series-flow from two-stage operation in which a separate recycle for each stage is used. The first hydrocracker in the series-flow diagram is primarily applied for the removal of organic nitrogen compounds while already providing a moderate hydrocracking conversion prior to the second stage. A composite of a microporous zeolite and a mesoporous amorphous silica-alumina is often

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used for the second stage in order to achieve sufficient conversion of bulky polycyclic compounds which, in case a pure zeolite is applied, would build up in the recycle stream (418). The latter would lead to equipment fouling and coking acceleration and demands for extremer reaction temperatures implying undesired gas formation and, unfortunately, more polycyclics formation. Removal of polycyclics is a different and presumably more efficient strategy and is applied in the HC-Unibon unit of the Abu Dhabi National Oil Company to increase the lifetime of the catalyst (419). The flexibility of the hydrocracking process is established by the broad product range that can be obtained in a single hydrocracker by varying the process conditions, illustrated in Fig. 31. A striking example is the singlestage Akzo-Fina CFI process, of which two units were started up in Europe each aiming at a different product slate with low sulfur content (420). Unit 1 operates under mild hydrocracking conditions aiming at a cloud point reduction of 31–40K depending on the feed, while Unit 2 is rather used for heavy gas oil dewaxing to diesel yields of 85–90% and a cloud point reduction of 22K. Mild hydrocracking conditions were initially applied in Pressure (bar)

150

Hydrocracking 100 Mild hydrocracking Gasoline upgrading Catalytic dewaxing

50

Isomerization

0 200

300

400

500

Temperature (°c)

Fig. 31 Different modes of operation in petroleum feed hydrocracking (25). Reprinted from Maxwell, I. E. Catal. Today 1987, 1, 385–413, Copyright (1987), with permission from Elsevier.

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the 1980s in response to the growing demand of middle distillates from mainly vacuum gas oils and aim at the lower conversion range to mildly dewaxed and desulfurized gas oils (421). A much lower hydrogen pressure is applied, i.e., 3–7 MPa instead of 10–14 MPa in conventional hydrocracking, leading to only moderate hydrogenation of aromatics and nitrogen compounds. The process resembles strongly to once-through hydrotreating and, consequently, hydrotreaters were converted into mild hydrocrackers by increasing the reaction temperature or, to avoid more coke deposition, by loading or combining more suitable catalysts for this purpose (421–423). 5.2.3 Processing of Distillate Bottoms and Coal Extracts Following the recent trends in fuel demands, heavy petroleum fractions such as residue oil and bitumen are currently rather upgraded than blended-off toward hydrocarbon streams of lower quality (2). Vacuum residue is the largest stranded stream in current refinery technologies which, unfortunately, do not possess the capability of further processing of such streams. The current applications of vacuum residue involve the production of high sulfur fuel oils, bunker fuels, and road asphalt. Their application to catalytic upgrading processes has been limited due to high asphaltenes and metals content, vide Table 12 for various residue streams of different origins (424), which potentially lead to rapid catalyst deactivation and reactor fouling (425). Research work over the last two decades primarily focused on the Table 12 Composition Analysis of Various Residue Streams of Different Origins (424) Feed Athabasca Ural Duri Arabian Light

Density (kg m
3)a

1.044

1.003

0.964

1.022

Saturates

6.8

11.7

21.0

11.6

Aromatics

31.2

46.1

28.6

48.6

Resins

42.1

36.1

36.8

31.3

Asphaltenes

14.1

4.6

5.7

7.7

Other

5.8

1.5

7.9

0.8

Sulfur (wt%)

5.74

2.72

0.58

4.22

Nitrogen (ppm)

6275

5800

5934

4011

Ni + V (ppm)

431

220

76

125

Composition (wt%)

a

At 288K.

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adjustment of existing technologies for gas oil hydrocracking in order to cope with the issues described earlier. In this respect, two-stage hydrocracking combining an aromatics hydrogenation step prior to hydrocracking at temperatures exceeding 670K was explored to avoid sludge formation from insoluble aromatics particles (426). Other technologies involve the use of ebullated-bed reactor technologies with sulfided NiMo/Al2O3 catalysts mixed and suspended in the feed (395,427,428), and the addition of a supercritical hydrocarbon solvent such as toluene or n-decane, which likely acts as a hydrogen transfer agent (429–431). Strausz et al. (432) applied a gaseous HFBF3 super acid which operates at much lower temperatures (473K) and which can be easily recovered, but which also induces reactor corrosion and significant volatiles production. Chang et al. (433) facilitated bitumen conversion via oxidation forming peroxide radicals during thermal hydrocracking on a sulfided Ni/Al2O3. In any case, vacuum residue and any other heavy organics mixture constitute an important source of feeds and, especially over the last years, a significant amount of research was dedicated to the transformation of asphaltenic and highly polluted feeds toward high-value distillate streams. Intensive research led to the upgrading of existing plants, or the commercialization of new hydrocrackers such as the UOP Uniflex process, depicted in Fig. 32, which essentially is a combination of a slurry phase CANMET Makeup H2

Recycle H2

Hot separator

Flash gas Cold separator

Uniflex reactor Recycle gas heater

C4−

Naphtha

Cold flash drum

Diesel

Hot flash drum

Vacuum fractionator Stripper/ product fractionator

Feed Feed heater Catalyst HVGO recycle

LVGO HVGO

Fractionator heater Pitch

Fig. 32 Uniflex process flow scheme (5).

Multiscale Aspects in Hydrocracking

215

hydrocracker and UOP’s Unicracking and Unionfining process technology for gas oil upgrading (5). Any of the residue hydrocracking technologies mentioned earlier have met the same commercial success as the dispersion or dissolution of catalysts in asphaltenic “slurries.” Slurry phase hydrocracking resembles strongly to thermal hydrocracking over radical intermediates in which the catalyst merely forms H radicals via the spillover mechanism. The latter remove heteroatom and metallic impurities and avoid coking on the reactor wall. Operating conditions easily exceed 670K and typically apply high hydrogen pressures of 10 MPa or more. Zhang et al. (27) reviewed the development of commercial slurry phase hydrocracking technologies and stated that, at the year of 2007, 10 pilot plants were operational worldwide for vacuum residue conversion. Those involve the VEBA-combi-cracking process developed in Germany, Exxon Mobil’s M-Coke technology, HDH technology by INTEVEP, SOC technology developed by Asahi Kasei Industrial Company, UOP’s Aurabon process, EST technology developed by Eni, (HC)3 technology by Alberta Research Company, and, as already mentioned, the CANMET hydrocracker used in UOP’s Uniflex process. Two types of catalysts were developed for slurry phase hydrocracking, i.e., heterogeneous solid powder catalysts and homogeneous dispersed catalysts, of which the latter are in turn classified in water-soluble and oil-soluble catalysts (27). Sulfided iron oxides originating from natural ore powders are usually less expensive, but exhibit a lower activity and tend to pile up in the bottom oil. Nguyen-Huy et al. (434) recently reported on the application of a red mud which is a by-product from the manufacturing of alumina in the Bayer process, and which is composed of mainly iron, titanium, and aluminum oxides. Disposal and regeneration issues are less common with oil-soluble catalysts which, in addition, do not restrain large asphaltenic molecules from hydrogenation (27). They consist of an organic oxide, sulfide, or salt of a metal from groups IV to VIII such as vanadium, chromium, and tungsten and are inherently the most active catalysts in slurry phase hydrocracking. Fig. 33 shows the performance of sulfided NiMo naphthenates in the conversion of Athabasca bitumen (435). However, as they are also costly and, hence, their industrial use limited; less expensive, water-soluble catalysts are often recurred to (27). Examples of water-soluble catalysts are ammonium heptamolybdate, ammonium phosphomolybdate, and ammonium tetrathiomolybdate. Liu et al. (436,437) more recently tested sulfided Mo and Ni salts or oxides doped with phosphoric acid (0.1–3 wt% P). Galarraga

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Maltene (wt%)

90 80 70

Coke (wt%)

3.0 2.0 1.0

Gases and volatiles (wt%)

0 30 20 10

Asphaltene conversion (%)

0 80 70 60 50

Hydrogen consumption (cu ft bbl−1)

40 1000 900 800

0

50

200 100 150 Ni + Mo (ppm)

250

300

Fig. 33 Asphaltene conversion, hydrogen consumption, gas yields, coke, and resin production during Athabasca bitumen hydrocracking at 698K and 14 MPa as a function of the dissolved NiMo naphthenate amount (435). Reprinted from Chen, H. H.; Montgomery, D. S.; Strausz, O. P.; George, Z. M. Stud. Surf. Sci. Catal. 1989, 53, 439–450. Copyright (1989), with permission from Elsevier.

and Pereira-Almao (438) synthesized a water-soluble trimetallic catalyst from aqueous solutions of ammonium metatungstate, ammonium heptamolybdate, and nickel acetate. Apart from their lower manufacturing cost, water-soluble catalysts also induce less coking, but the presulfidation procedure is usually more complex. Owing to the presence of an aqueous phase, an additional separation from the product oils is subsequently required. The presence of water also decreases the hydrogen partial pressure and could be detrimental to the catalyst activity (438). Therefore, recent research mainly focuses on the development of cheap oil-soluble catalysts, such as organic Ni

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Multiscale Aspects in Hydrocracking

Ni

X

A

Rn

Fig. 34 Representation of an oil-soluble Ni-based catalyst precursor. A is an aromatic ring composed of six carbon atoms and interacts with the asphaltene molecules, Rn is a long alkyl group, and X is an independent linker group (439).

precursors, vide Fig. 34, which thermally decompose to active species (439), and sulfided ammonium cobalt molybdates with chemisorbed lipophilic oleic acid species (440). The current trend in fuel oil demand also stimulates the exploitation of other organic sources such as coal. Coal tars and extracts are composed of high amounts of asphaltenic substances exceeding 60 wt% and additionally contain a significant amount of oxygenated compounds ranging from 5 to 7 wt% oxygen (53,338,441). In addition to desulfurization, the transformation of oxygenates and nitrogen compounds to hydrocarbons and gaseous byproducts is of primary importance to upgrade the quality of coal liquefaction residue to more conventional oils. Sulfided Ni- and/or Mo-loaded supports such as alumina, silica, titania, and zirconia were explored in academic or industrial researches for this purpose (338,396,442,443). Similar strategies for crude residue conversion are essentially applicable here as well including intensive mixing and dispersion of the catalyst within the feed (444,445). Zhang et al. (446) explored the use of highly dispersed and oil-soluble catalysts including molybdenum naphthenate, octoate, and carbon-based fullerites and found that their initial activity approximates that of conventional NiMo catalysts. The hydrogen transfer capability of a fullerite consisting of 90% C60 and 10% C70 was confirmed in a subsequent research, but the catalyst unfortunately tended to lose its integrity owing to hydrogen addition (447). Bodman et al. (284) and Millan et al. (279) focused on pillared montmorillonite and laponite clays which combined a high hydrocracking and heteroatom removal activity with a high resistance against coking. 5.2.4 Fischer–Tropsch Wax Hydrocracking The importance of synthesis gas (syngas), i.e., H2 and CO, in fuel production has grown rapidly in the past decades. The transformation of coal, biomass, or natural gas to syngas via gasification or steam reforming enables the petrochemical industry to cope with the declining supplies of conventional crude oils. Methanol synthesis from syngas indirectly leads to the production of oxyfuels such as dimethyl ether, which constitute excellent gasoline and diesel fuel blends (448). A different route almost exclusively leading to

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J.W. Thybaut and G.B. Marin

hydrocarbon fuels involves Fischer–Tropsch (FT) synthesis mainly over cobalt or iron catalysts, converting syngas into long linear alkanes and α-alkenes via a chain growth mechanism. The carbon number distribution of the products follows the Anderson–Flory–Schulz polymerization model as depicted in Fig. 35(126). Heavy paraffinic waxes obtained in case of sufficiently high chain growth probabilities embody potential high-quality, zero-sulfur middle distillate sources and are concordantly hydrocracked at typical temperatures and pressures of 597–645K and 3.5–7 MPa (126). SASOL commercialized an FT synthesis reactor in the 1950s for the exploitation of the large coal reserves discovered in South Africa and expanded its research toward the hydrocracking of FT waxes since the 1970s (403,448). Other chemical companies such as Exxon and a joint venture between SASOL and Chevron also commercialized a Fischer–Tropsch and hydrocracker plant focusing on the transformation of natural gas to 100 90 80

70

C35–C120

Selectivity

60

C1 C5–C11

50 40

30

C12–C18

C2

C24–C35

20 C3 10

0.1

0.2

0.4 0.5 0.6 0.7 0.3 Probability of chain growth

0.8

0.9

1.0

Fig. 35 Product selectivities as a function of the chain growth probability assuming an ideal Anderson–Flory–Schulz distribution (126).

219

Multiscale Aspects in Hydrocracking

liquid fuels (14). Similarly, Shell commercialized its Middle Distillate Synthesis (SMDS) process built in Malaysia in the early 1990s, which integrates syngas production from methane, Fischer–Tropsch synthesis, and mild wax hydrocracking into a single plant (449,450). The SMDS technology was recently incorporated in the Shell’s Pearl GTL process in Qatar which is around 10 times the capacity of the plan built in Malaysia (451). A distinction is made between high-temperature Fischer–Tropsch (HTFT) synthesis yielding a significant amount of alkenes, oxygenates, and aromatics, and the often more desired low-temperature Fischer– Tropsch synthesis which exhibits a higher selectivity toward paraffinic waxes. Isomerization of the n-alkane mixture plays a crucial role in cold flow property improvement and is, for this reason, pursued rather than molecular weight reduction. Only moderate cracking is pursued in most cases of FT wax conversion to maximize the diesel yield as shown in Fig. 36 for the hydrocracking of an FT wax over a Pd/Al2O3 catalyst. Table 13 shows a typical composition of a highly isomerized diesel fraction obtained from hydrocracking of a C22+ FT wax on a Pt-loaded silica-alumina (4). The fraction shows also an extremely low sulfur and aromatics content and exhibits a cetane number of about 80 which makes it an excellent diesels blend mixture. 6

Selectivity (wt%)

5

4

3

2

1

0 10

15

20 25 Carbon number

30

Fig. 36 Carbon number distribution of a hydrocracked FT wax over Pd/Al2O3 at 603K and 1.2 MPa (full). The wax was obtained from FT synthesis over a Co/TiO2 at 493K and 1.2 MPa (shaded) (242). Reprinted from Nam, I.; Cho, K. M.; Seo, J. G.; Hwang, S.; Jun, K. W.; Song, I. K. Catal. Lett. 2009, 130, 192–197 with permission of Springer.

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J.W. Thybaut and G.B. Marin

Table 13 Detailed GC Analysis of a C11–C21 Diesel Fraction Obtained From a C22+ FT Wax Hydrocracking Over a Pt/SiO2–Al2O3 Catalyst (4) Isomer iso-Alkane n-Alkane iso-Alkane Mono/Multi Carbon Number (wt%) (wt%) Fractiona (wt%)

11

0.1

0.9

13.9

3.0

12

3.2

6.3

33.3

2.1

13

7.5

6.5

53.6

2.3

14

8.1

5.4

60.4

1.7

15

8.5

4.4

65.6

2.1

16

8.1

3.5

69.8

1.9

17

8.0

2.6

75.5

1.4

18

7.7

2.0

79.5

1.3

19

7.0

1.3

83.8

1.2

20

5.9

1.0

85.5

0.9

21

2.2

0.0

99.0

0.8

66.3

33.9

Total a

Within carbon number lump. Before the second stage, the bio-oil was distilled to remove volatiles (