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Glycerol: Production, Structure and Applications : Production, Structure, and Applications [1 ed.]
 9781620811306, 9781620811207

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Copyright © 2012. Nova Science Publishers, Incorporated. All rights reserved. Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

Copyright © 2012. Nova Science Publishers, Incorporated. All rights reserved. Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

GLYCEROL

Copyright © 2012. Nova Science Publishers, Incorporated. All rights reserved.

PRODUCTION, STRUCTURE AND APPLICATIONS

No part of this digital document may be reproduced, stored in a retrieval system or transmitted in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services. Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

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Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

GLYCEROL PRODUCTION, STRUCTURE AND APPLICATIONS

MIGUEL DE SANTOS SILVA Copyright © 2012. Nova Science Publishers, Incorporated. All rights reserved.

AND

PAULO COSTA FERREIRA EDITORS

Nova Science Publishers, Inc. New York

Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

Copyright © 2012 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers‘ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works.

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Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

Library of Congress Cataloging-in-Publication Data

Glycerol : production, structure, and applications / editors, Miguel de Santos Silva and Paulo Costa Ferreira. p. cm. Includes bibliographical references and index. ISBN:  (eBook)

1. Glycerin. I. Silva, Miguel de Santos. II. Ferreira, Paulo Costa. TP973.G596 2011 668'.2--dc23 2012003611

Published by Nova Science Publishers, Inc. † New York Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

CONTENTS Preface Chapter 1

Thermochemical Conversion of Glycerol to Hydrogen Roger Molinder and Valerie Dupont

Chapter 2

Valorization of Glycerol in Propanediols Production by Catalytic Processes Francesco Mauriello, Maria Grazia Musolino and Rosario Pietropaolo

Chapter 3

Chapter 4

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Chapter 5

Chapter 6

Chapter 7

Chapter 8

Aqueous-Phase Reforming of Glycerol for Hydrogen Production Robinson L. Manfro and Mariana M. V. M. Souza The Pauper and the Prince: Glycerol in a View from Biofuels and Biorefineries Alejandro J. Beccaria, Alberto A. Iglesias and Raúl A. Comelli Glycerol as a Substrate for Bioprocesses in Different O2 Availability Conditions M. Julia Pettinari, Mariela P. Mezzina, Beatriz S. Méndez, Manuel S. Godoy and Pablo I. Nikel Hydrogen Production from Glycerol via Membrane Reactor Technology A. Iulianelli, S. Liguori and A. Basile Use of the Glycerol from Biodiesel to Production of Environmental Technologies Miguel Araujo Medeiros, Carla M. Macedo Leite and Rochel Montero Lago Diffusion in Glycerol-Water Binary and GlycerolWater-Sodium Chloride Ternary Solutions and the Viewpoint of Hydrogen Bonds Cong Chen, Wei Zhong Li, Yong Chen Song, Lin Dong Weng and Ning Zhang

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45

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vi Chapter 9

Contents Conversion of Glycerol into Bio-Fuel Additives over Heterogeneous Catalysts M. Caiado, N. F. Lopes, S. Carlota and J. E. Castanheiro

Chapter 10

Is Glycerol a Sustainable Reaction Medium? Adi Wolfson, Dorith Tavor and Giancarlo Cravotto

Chapter 11

Ruthenium-Catalyzed Nitrile Hydration Reactions Using Glycerol as Solvent Alba E. Díaz-Álvarez, Rocío García-Álvarez, Pascale Crochet and Victorio Cadierno

Chapter 12

Conversion of Glycerol into Products for Technological Applications Márcio Guimarães Coelho, Luiz Claudio de Melo Costa, Miguel Araujo Medeiros and Carla M. M. Leite

Chapter 13

Natural Activation of Commercial Glycerol Alexander I. Bol’shakov, Svetlana I. Kuzina and Dmitry P. Kiryukhin

Chapter 14

The Applications of Glycerol in Pharmaceutical Formulation Weien Yuan and Hui Li

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Index

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249

263

277

287 293

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PREFACE In this book, the authors present topical research in the study of the production, structure and applications of glycerol. Topics discussed include the use of glycerol from biodiesel to production of environmental technologies; thermochemical conversion of glycerol to hydrogen; the application of glycerol in pharmaceutical formulation; valorization of glycerol in propanediols production by catalytic processes; hydrogen production from glycerol via membrane reactor technology; ruthenium-catalyzed nitrile hydration reactions using glycerol as solvent; and natural activation of commercial glycerol. Chapter 1 - Hydrogen gas is at present mainly used for the production of nitrogen-rich inorganic fertilisers such as anhydrous ammonium nitrate and urea (via the ammonia intermediate). To a lesser extent, it is also a necessary reagent in common petroleum refinery operations such as hydrogenation, hydrocracking, hydrodesulphurisation, hydrodenitrogenation, hydrodemetallisation, and Fischer-Tropsch processes, in addition to being the coolant of choice in large electrical power generators. In more recent years, there has been a surge of interest in hydrogen production processes as hydrogen could be the low carbon energy vector of the future, with the aim of drastically curbing greenhouse gas emissions as well as air pollution from transport. Current production of hydrogen relies on reacting hydrocarbons with steam through steam reforming of mainly natural gas and petroleum naphtha, and also steam gasification of coal, and therefore carries a large carbon footprint. With the increase in hydrogen consumption expected from the growth in world population accompanied by food and fertiliser demand, concurrent with the development of a low carbon economy relying heavily on hydrogenation reactions at bio-refineries, and eventually the introduction of a hydrogen economy, sustainable methods of hydrogen production are required to replace the current reliance on fossil fuels. In this chapter, thermochemical processes of conversion of glycerol to hydrogen are reviewed from the literature. The heterogeneous catalytic processes of mainly steam reforming, but also aqueous reforming, supercritical water gasification, and thermal decomposition are considered through their process conditions and outputs. Chapter 2 - In this chapter a review on hydrogenolysis reactions, promoted by enzymatic, homogeneous and heterogeneous catalysts, with particular emphasis on the different reaction mechanisms, is proposed, having the aim to provide a bridge between catalysis and glycerol deriving applications. The selective hydrogenolysis of glycerol to propylene and ethylene glycols is one of the most attractive routes since it is a clean and economically competitive process that allows

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Miguel De Santos Silva and Paulo Costa Ferreira

formation of different valuable products, such as 1,2-propanediol (1,2-PDO) and 1,3propanediol (1,3-PDO). Glycerol can be converted into 1,3-propanediol in a two-step, enzyme-catalyzed reaction using bacterial strains attaining to the groups citrobacter, enterobacter, ilyobacter, klebsiella, lactobacillus, pelobacter and clostridium. In all cases, in the first step, an enzyme dehydratase catalyzes the conversion of glycerol into 3-hydroxypropanal (3-HPA) then reduced to 1,3PDO. The 1,3-PDO is not further metabolized and, as a result, accumulates in the medium. Generally, the hydrogenolysis of glycerol by homogeneous catalysts leads to a variety of by-products such as propanol or ethers and to a mixture of 1,2- and 1,3-PDO. Typical catalysts for this application are metals of the platinum group either in complexes having iodide and carbonyl ligands or compounds with phosphorous, arsenic or antimony containing ligands. Another possible way to obtain propanediols from glycerol is the direct hydrogenolysis of glycerol by classical heterogeneous hydrogenation catalysts. Different metal catalysts were, so far, widely employed: Cu, Ru, Rh, Pt and Pd supported on a wide range of qualitatively different carriers and sometimes in presence of Brønsted acids or bases as co-catalysts. Chapter 3 - The demand and supply of glycerin in the world market was in equilibrium by the end of the 1990s. With the production of biofuels, especially biodiesel, this equilibrium has been completely changed. Biodiesel is produced by the transesterification of vegetable oils and animal fats, and glycerol is a byproduct of this reaction. One ton of biodiesel yields about 110 kg of crude glycerol or 100 kg of pure glycerol. In Brazil, according to National Agency of Petroleum, Natural Gas and Biofuels (ANP), the production of biodiesel (B100) in 2010 was approximately 2.4 million m3, generating approximately 240,000 m3 of glycerin, creating a surplus of glycerin in the Brazilian market. One of the promising ways to utilize this crude glycerol is to produce hydrogen by aqueous phase reforming. The aqueous-phase reforming (APR) reaction is able to transform oxygenated hydrocarbons, as glycerol, in a gas phase composed mainly by H2 and CO2 with low concentration of CO and CH4. For this reaction, nickel catalysts supported on aluminum oxide, zirconium oxide and cerium oxide were used. They were synthesized by three different methodologies: wet impregnation, coprecipitation and combustion. The characterization of catalysts showed that the morphological and structural properties are strongly influenced by the preparation methodology. The analysis of N2 physisorption showed that catalysts synthesized by combustion presented lower BET areas, considering that all samples are mesoporosus (average diameter pore of 20 to 500 Å) with type IV isotherms and H3 hysteresis. Temperature programmed reduction analysis displayed that catalysts supported on aluminum oxide present a reduction level lower than 100%, indicating there is no complete reduction of nickel oxide to metallic nickel. However, the reduction degree of the other catalysts has values higher than 100%, indicating an additional reduction of the support, mainly in the case of the catalysts supported by cerium oxide. The APR of glycerol was performed in an autoclave reactor with 450 rpm of agitation, using 200 mg of catalyst, and 250 mL of aqueous solution of 1 or 10 wt.% glycerol at 250 and 270 °C. The gas phase is composed by H2, CO2, CO e CH4 gases, which were analyzed in a gas chromatograph. The results suggested that it is possible to achieve high concentrations of H2 at temperature around 250 ºC. The higher conversion obtained was 50%, using the Ni/Al2O3 catalyst prepared by impregnation. In the comparative tests with Pt catalysts, the Ni

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catalysts showed higher catalytic activity. X-ray diffraction analysis displayed that nickel catalysts present sintering, resulting in an increase of the Ni crystal size after reaction. Chapter 4 - Glycerol (1,2,3-propanetriol, abbrev. Gro), a sweet-tasting alcohol, was discovered in the 18th century (1779) by the Swedish chemist C.W. Scheele, who isolated the poly-alcohol after heating olive oil and litharge. The chemical is also known as glycerin or glycerine. It is abundant in nature after being a component of many lipids and a main compatible solute produced by cells for osmoregulation, to manage water activity variations in the medium. Also, many microorganisms are able to use glycerin as a source for carbon and energy. Main physicochemical properties of Gro can be summarized by being a colorless, odorless, highly hygroscopic and viscous liquid having a boiling point at 290 ºC, a specific gravity of 1.26 and a molecular weight of 92.09. It forms crystals at low temperatures that tend to melt at 17.9 ºC. All its characteristics make of Gro a compound of utility for inclusion in different industrial processes and/or for being a key constituent in several chemical preparations. Although the many faceted uses and applications for Gro, its market value is in downhill, principally because the rapid expansion of biodiesel production that generated a glut of the polyol (Johnson and Taconi, 2007). Biodiesel is produced from triacylglycerides contained in animal fats or vegetable oils through transesterification with low molecular weight alcohols (mainly methanol or ethanol). The process generates methyl-(or ethyl-) esters of fatty acids as the major biodiesel product and about 10% (v/v) of Gro as a by-product. This is not only converting the polyol in a low value chemical; but it is further creating an environmental problem because of the limited possibility for disposal, which is also quite expensive (da Silva et al., 2009). The situation is obligating to revisit current uses and the design of new processes for adding value to the propanetriol. For the latter, overall possibilities include the finding for new applications of the chemical as well as its conversion to other molecules serving in different industry systems, in a way improving competitiveness in the productive chain of the biodiesel business. Paradoxical to the relatively pauper state of affairs for Gro, it can be visualized as a key compound for developing processes in the emerging field of biorefineries. Limited reserves of petroleum and the associated dramatic increases in its price, together with critical environmental concerns and climate changes provoked by the use of fossil fuels are creating obligatory demands for generate renewable fuels as well as for reconvert industrial processes. The necessity is to make them more ecology-friendly and compatible with a sustainable environment. This leads to the concept of biorefinery, which embraces different technologies allowing convert biomass into materials, chemicals and energy. In this framework, the positioning of Gro as a by-product in a process to obtain biofuel that needs to be revalorized is relevant and it represents a fair challenge for the development of biorefinery tools. In the present work the authors analyze current and potential ways to valorize glycerol. Globally, the authors review chemical as well as biological approaches and the authors analyze possibilities for combine strategies to develop biorefinery-like solutions. Chapter 5 - In recent years, a significant increase in the production of biodiesel has caused a sharp fall in the cost of glycerol, the main by-product of biodiesel synthesis. As a result, glycerol has become a very attractive substrate for biotechnological processes. Many bacterial metabolites of industrial interest are products of aerobic metabolism, such as antibiotics, amino acids, and other compounds; but others, including molecules used as biofuels (e.g., ethanol, butanol, and hydrogen), and other relevant biochemicals, such as 1,3-

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propanediol and succinate, are products of fermentative metabolism. Since carbon atoms in glycerol are more reduced than in glucose or other substrates commonly used in bioprocesses, the catabolism of this substrate normally consumes high amounts of oxygen, and only a handful of bacteria, including some members of the genus Enterobacter, Clostridium, and Klebsiella, were thought to be able to ferment this substrate in anaerobiosis. Recent research has shown that the facultative aerobe Escherichia coli can ferment glycerol, producing a number of different biotechnologically relevant molecules, like succinate and ethanol, and the heterologous product poly(3-hydroxybutyrate), both in microaerobiosis and in anaerobiosis. Several strategies have been used to increase the number of applications for glycerol as a substrate for bacterial processes, mostly based on modified bacterial strains that can efficiently produce different chemicals from this substrate. Manipulations to enhance the synthesis of various metabolic products from glycerol include several approaches to increase its availability inside the cells, or to decrease the synthesis of other metabolites. Mutations in the glp genes, involved in glycerol metabolism, or in genes involved in competing pathways, like ldhA, that encodes a D-lactate dehydrogenase, have been tested to increase the synthesis of several products from glycerol in different bacterial species. On the other hand, mutations in global regulator genes, especially in the redox-control pair arcAB, have been introduced in E. coli resulting in enhanced biosynthesis of reduced products from glycerol, mainly under microaerobic growth conditions. All these manipulations increase the number of possible uses of glycerol as a substrate for the obtention of a wide diversity of different biotechnological products using bacteria. Chapter 6 - Bio-diesel fuel, when converted from vegetables oils, produces around 10 wt% of glycerol as a byproduct, which could be used for producing hydrogen through steam reforming reaction. The state of the art in the scientific literature on hydrogen production via reforming reaction of glycerol in aqueous or gas phase is mainly devoted to the utilization of conventional reactors. Thus, as main highlights present in the open literature on this process, both high reaction pressure and a relatively small catalyst deactivation are noticed when steam reforming of glycerol is carried out in aqueous phase, whereas the catalyst deactivation is the main disadvantage in gas phase. In this chapter, glycerol steam reforming reaction to produce hydrogen is the main topic, paying special attention to the application of membrane reactor technology to this process. As a further scope, the chapter also describes the utilization of perm-selective Pd-based membrane reactors, pointing out the ability of these systems to both extract a high purity hydrogen stream and enhance the performances of the reaction system in terms of both glycerol conversion and hydrogen yield. Furthermore, the benefits and drawbacks of the membrane reactor systems are descript as a mature technology compared to the conventional reactors. Chapter 7 - One important feature of the glycerol produced from biodiesel is the presence of large amounts of impurities, such as catalysts (usually different alkalis), oil, carboxylic acids, alcohols (usually methanol or ethanol), etc. and even after purification the glycerol from the biodiesel process usually contains relatively large amounts of H2O (5–10%) and NaCl (4–8%). In this chapter, several processes for the conversion of crude glycerol into products for environmental and technological applications are discussed. These processes are based on the controlled sequence of reactions involving the catalyst already present in the crude glycerol to produce oligomers, thermoplastic and biodegradable polymers, and carbonaceous material. Some of them obtained materials to be described as olygomers with

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application in mining industries, thermoplastic polymers with application in controlled and slow release fertilizers, or carbonaceous material for environmental applications. Chapter 8 - When glycerol is used as a cryoprotective agent, glycerol-water binary and glycerol-water-sodium chloride ternary solutions are of great importance in understanding the cryoprotective behavior of glycerol. In the present study, glycerol-water binary and glycerolwater-sodium chloride ternary solutions have been studied using molecular dynamics simulation study. Self-diffusion coefficients for water and glycerol molecules have been predicted using mean square displacement method. The concentration dependence of self-diffusion coefficients has been studied from the viewpoint of hydrogen bonds. It has been found that as glycerol concentration increases, water and glycerol self-diffusion coefficients both decrease. For water molecules, the decreasing of self-diffusion coefficients is caused by increasing glycerol-water hydrogen bonds and decreasing water-water hydrogen bonds, while for glycerol molecules, the decreasing of self-diffusion coefficients is caused by increasing glycerol-glycerol hydrogen bonds and decreasing glycerol-water hydrogen bonds. Chapter 9 - Biodiesel is produced from vegetable oils and animal fats, by transesterification of triglycerides and esterification of free fatty acids with methanol or ethanol, in presence of acid or basic catalysts. The glycerol is a by-product in the biodiesel production. For every 9 kg of biodiesel produced, about 1 kg of a crude glycerol is formed. Due to the increase of biodiesel production, an increase of glycerol production has been observed. Therefore, it is imperative to find new applications for the excess of glycerol produced from biodiesel. A possibility is to transform glycerol into fuel additives, by different reactions (etherification, esterification and acetalisation). Traditionally, strong homogeneous acid catalysts, e.g sulphuric acid, have been used in these acid reactions. Heterogeneous catalysts have some advantages over homogeneous ones. They can be separated from reaction mixture, are reusable, do less harm to the environment and have not corrosion or disposal of effluent problems. This present review focuses on solid acid catalysts as potential heterogeneous catalysts for glycerol conversion (by esterification, etherification and acetalisation) into biofuel additives, replacing homogenous ones. Chapter 10 - Glycerol, a by-product of oil transesterification form biodiesel production, has become a protagonist of sustainable chemical processes. This renewable feedstock is superior to most green solvents as a reaction medium because it is non-toxic, non-irritating and biodegradable. In addition to its environmentally-friendly character, such as its high boiling point, low vapour pressure, thermal stability and recyclability, glycerol has also been successfully employed as solvent in a wide variety of organic reactions and synthetic methodologies. Moreover glycerol can often enhance reaction rate and selectivity while facilitating the reaction work-up, the product separation and the catalyst recycling. Furthermore, its physicochemical properties enable a wide applicability as solvent in microwave- and ultrasound-assisted reactions. Chapter 11 - The rapid growth of the biodiesel industry has led to a large surplus of its major byproduct, i.e. glycerol, for which new applications need to be found. In line with the increasing interest by the chemical community in the use of ―green‖ solvents, an innovative way to revalorize glycerol has seen the light in recent years, i.e. its use as environmentally friendly reaction medium for synthetic organic chemistry. In this context, herein the authors would like to communicate that glycerol is an adequate solvent to perform nitrile hydration

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reactions. Thus, using the arene-ruthenium(II) complex [RuCl2(η6-C6Me6)(PTA-Bn)] (PTABn = 1-benzyl-3,5-diaza-1-azonia-7-phosphatricyclo [3.3.1.13,7]decane chloride) as catalyst (5 mol%), the authors have found that a large variety of aromatic, heteroaromatic, aliphatic and ,-unsaturated organonitriles can be selectively converted into the corresponding amides, in short times (0.5-3 h) and high yields (≥ 95% by GC), performing the catalytic reactions in a 1:1 v/v glycerol/water mixture at 160 ºC. Remarkably, the use of this inexpensive and green reaction medium also enables easy product separation by simple extraction with ethyl acetate, as well as the recycling of the catalytically active ruthenium species. Chapter 12 - The glycerol (1,2,3propanetriol or glycerine) was discovered by Scheele in 1779 during the saponification of olive oil. Glycerol is an oily liquid, colorless, viscous and sweet taste, soluble in water and alcohol in all proportions, no toxicity, color or odor. Because of these properties glycerol is a substance with a variety of applications such as: the manufacture of alkyd resins, dynamite, esters, pharmaceuticals, perfumes, plastics, cosmetics, foods, including sweets, rolls of tobacco, alcoholic beverages, paint thinner, polyurethane polyols, emulsifiers, rubber stamping, paints, cements and binders for mixing, special soaps, emollients and lubricants, penetrating, hydraulic fluids, humectants, nutrients and fermentation antifreeze mixture. Some physicochemical properties are presented in Table 1. Chapter 13 - The effect of activation of commercially pure glycerol (e.g. upon exposure to daylight) may be of interest for applied chemistry since this alcohol is being widely used in medicine, biology, food and cosmetic industry, in manufacturing explosives, etc. An important conclusion is that the production and storage of glycerol must be carried out in conditions that exclude the formation of peroxide compounds. Interaction of hydroperoxides with impurities present in commercially available glycerol may lead to generation of radicals and thus stimulate side reactions accompanied by accumulation of undesirable products. This finding seems to be of especial significance in relation to medicinal, perfume, and food products in view of important role of free-radical reactions in the development of malignant tumors and aging processes. Chapter 14 - Glycerol, because of its unique properties, is an important pharmaceutical excipient and has a wide variety of applications in the preparation of drug formulations including oral, parenteral, topical, ophthalmic and otic preparations. The aim of this review is to summarize the up-to-date applications of glycerol in pharmaceutical formulation with an emphasis on its function in different dosage forms. The safety issues and regulatory status of glycerol will also be considered and discussed.

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Chapter 1

THERMOCHEMICAL CONVERSION OF GLYCEROL TO HYDROGEN Roger Molinder and Valerie Dupont Energy Research Institute, School of Process, Environmental and Materials Engineering, The University of Leeds, Leeds, UK

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ABSTRACT Hydrogen gas is at present mainly used for the production of nitrogen-rich inorganic fertilisers such as anhydrous ammonium nitrate and urea (via the ammonia intermediate). To a lesser extent, it is also a necessary reagent in common petroleum refinery operations such as hydrogenation, hydrocracking, hydrodesulphurisation, hydrodenitrogenation, hydrodemetallisation, and Fischer-Tropsch processes, in addition to being the coolant of choice in large electrical power generators. In more recent years, there has been a surge of interest in hydrogen production processes as hydrogen could be the low carbon energy vector of the future, with the aim of drastically curbing greenhouse gas emissions as well as air pollution from transport. Current production of hydrogen relies on reacting hydrocarbons with steam through steam reforming of mainly natural gas and petroleum naphtha, and also steam gasification of coal, and therefore carries a large carbon footprint. With the increase in hydrogen consumption expected from the growth in world population accompanied by food and fertiliser demand, concurrent with the development of a low carbon economy relying heavily on hydrogenation reactions at bio-refineries, and eventually the introduction of a hydrogen economy, sustainable methods of hydrogen production are required to replace the current reliance on fossil fuels. In this chapter, thermochemical processes of conversion of glycerol to hydrogen are reviewed from the literature. The heterogeneous catalytic processes of mainly steam reforming, but also aqueous reforming, supercritical water gasification, and thermal decomposition are considered through their process conditions and outputs.

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INTRODUCTION

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World energy demand is expected to increase from 11.8 billion tons oil equivalent (toe) in 2010 and reach 16.4-16.8 billion toe in 2030. [1,2] Our current major sources of energy come from fossil fuels such as coal, oil and gas. The increase of CO2 in the atmosphere over the last 200 years has been attributed to the burning of fossil fuels and is expected to cause a change in climate just like previous changes in atmospheric CO2 have done in the past. [3-6] Negative effects such as reduced crop yields and increased frequency of storms are expected. [7-10] G8 leaders proclaimed a target of 50% reductions in CO2 emissions by 2050 from 2005 levels after their meeting in Lake Toya, Japan, in 2008. In 2009 it was recognized by the G8 leaders and the Major Economies Forum that the concentration of atmospheric greenhouse gases ought not to exceed 450 ppm CO2 equivalent. At the UN climate change conference (COP17/CMP7) in Durban, South Africa in 2011, a decision was made to write a legally binding agreement by 2015 which will come into force in 2020. Fossil fuel resources are spread unevenly over the planet and their use has made many nations dependent on energy imports. This represents a threat to national security in the case of shortages, high prices and high demand, but also in the case of conflict, both military and diplomatic. The use of fossil fuel for transportation is achieved mainly through the use of gasoline in internal combustion engine vehicles (ICEV). This results in the release of air pollutants, including particulates which have been linked to respiratory and cardiac disease. [11] These effects are worsened by increased population density and internal combustion engine vehicles which result in ever growing concentrations of particulates in populated areas. The efficiency of the process from energy source to end use is commonly quantified using the ‗well to wheel‘ (WtW) efficiency (WtW). The latter takes the efficiency of the manufacture, transportation and storage of the energy carrier into account as well as the efficiency of the end use. Efficiency at each step of the WtW calculations is defined according to Equation 1:



Energy output of process or product Energy input of process or product

Equation 1. Definition of efficiency used for WtW calculations. Crude oil as an energy source, gasoline as an energy carrier and ICEV as their end use has a WtW of 14-15% and results in WtW CO2 emissions of about 200 g km-1. [12,13] There are several alternatives to this route which have higher WtW efficiencies and lower WtW CO2 emissions such as refining the crude oil to diesel instead of gasoline, using ICE-hybrid vehicles, switching to biofuels like biodiesel and ethanol. However, two alternatives stand out as potential long term solutions to achieve near zero CO2 emissions. The first alternative is to directly use renewable electricity (e.g. wind, solar, hydro, tidal) in an electric vehicle, the second is to allow a renewable energy source (biomass, surplus wind/solar/hydro) to convert to H2 and use it to power a fuel cell vehicle. [12] Battery electric and fuel cell electric vehicles (BEV and FCEV) using electricity and H2 as energy carriers respectively are considered the best long term end uses with regard to the reduction of CO2 emissions.

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Thermochemical Conversion of Glycerol to Hydrogen

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H2 as energy carrier can solve the problems of intermittency of renewable electricity by acting as a grid buffer, and FCV (as end use) are quickly refilled for a similar range to that of current ICEV. This is unlike BEV which require frequent and lengthy recharging and are suited only to short journeys. Since the only endpoint emissions from a FCEV run on hydrogen is water, the health problems derived from particulate matter are also solved through this solution. Hydrogen can be produced from electricity and water using electrolysis; however it can also be produced from the combination of water and a number of different carbon-containing feedstocks. Therefore, this solution may also solve the problem of national security. The most economical production route of H2 is currently steam reforming of natural gas. Hekkert et al. [13] reported that producing H2 from natural gas using steam reforming, compressing the H2 for transportation and storage and using the H2 in a fuel cell vehicle was the scenario which had a higher WtW efficiency (21%) and the lower WtW CO2 emissions (115 g km-1) than directly fuelling a ICEV with compressed natural gas. The results were in agreement with Svensson et al. [12]. Adding carbon capture and storage (CCS) to the steam reforming process reduced WtW CO2 emissions further but also reduced WtW. [12] In summary, steam reforming of natural gas was considered a viable route to hydrogen production for the transport sector because the WtW efficiency was higher, and the WtW CO2 emissions lower than those of the alternative uses of natural gas for the transport sector. However, for a truly sustainable route of hydrogen production, renewable feedstock is needed. Biodiesel can be produced from vegetable oils and animal fats, either from fresh or waste streams. [14] An emerging energy source is algae. Biodiesel consists of methyl esters and is conventionally produced through direct transesterification of the triglycerides present in the energy source (Fig. 1). [15] Another route to biodiesel is through biomass gasification followed by Fischer-Tropsch synthesis.

Figure 1. Formation of biodiesel from oils. [15]

A major by-product formed during biodiesel production is crude glycerol, which is a mixture containing glycerol as well as alcohols, soaps, alkali hydroxides salts and ash as well as calcium, magnesium, phosphor and sulphur. [16-19] The composition of the crude glycerol varies depending on what energy source is used. [17] For every 100 kg of vegetable oil that is transesterified, about 10 kg of crude glycerol is formed and this makes it too large a byproduct stream to be economically disposed of. The increased production of biodiesel has caused a surplus of crude glycerol as evidenced by the reduced value of glycerol. Biodiesel production increased by 295% between 2000 and 2005 and the value of glycerol was reduced almost tenfold between 2004 and 2006. [20-22] Between 2005 and 2008 world biodiesel production was below capacity, which can be attributed in part by the reduced cost of glycerol. [23] This provides an economic incentive to find new ways of adding value to crude glycerol from biodiesel production. Biofuel production increased from 0.7 million barrels

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daily (mb/d) in 2007 to 1.8 mb/d in 2010 and has been predicted to increase further to 2.4 mb/d in 2015 and 6.5 mb/d in 2030. [1,2,24] Assuming a reference scenario in which the atmospheric level of greenhouse gas emissions were limited to 450 ppm CO2 equivalent, the use of biofuels would be 275 Mtoe in 2030. [1 ]It has also been reported that in order to meet a target of 50% reduction in CO2 emissions by 2050 from 2005 levels, biofuel production needs to equal 12 exajoules (EJ, where exa = ×1018) by 2030 and 32 EJ by 2050. [25] Therefore the incentives to add value to crude glycerol from biodiesel production will be stronger in the future, especially if the goals of CO2 emission reductions are to be reached. The glycerol and the alcohols in the crude glycerol can be thermochemically converted to hydrogen. The glycerol can also be purified prior to hydrogen production. This way two transport fuels (biodiesel and hydrogen) can be produced from the same renewable energy source. The yield of hydrogen from glycerol must be maximised as well as the purity of the hydrogen. The inefficiencies of the process must be minimised and any problems that arise such as catalyst deactivation must be addressed. In order to address all these issues, several thermochemical routes to hydrogen production from glycerol have been investigated. Thermodynamic equilibrium analysis has been performed with the aim of understanding the processes involved and to identify the optimum conditions for maximum hydrogen yield and purity. Experimental work has been carried out building on the experience from the thermodynamic equilibrium analysis. Catalysts have been manufactured, characterized and used for hydrogen production in order to understand the effects of catalyst characteristics on hydrogen production and to learn how to manufacture catalysts which optimise hydrogen yield and purity. The aim of this chapter is to review the work carried out on thermochemical conversion of glycerol to H2. First, the thermodynamics of the process is introduced followed by an introduction of the parameters used when studying glycerol conversion to H2 and how they are defined (e.g. glycerol conversion and hydrogen yield). The routes to conversion of glycerol to H2 are then reviewed. These are steam reforming (including sorption enhanced steam reforming), aqueous phase reforming, autothermal reforming, supercritical water gasification and thermal decomposition. Although the term ‗gasification‘ is more often applied to solid feedstock, it is also in some cases used instead of ‗reforming‘ to gaseous and liquid feedstock in the literature. Each route is first introduced and then the results from thermodynamic equilibrium analysis (if any) are reviewed followed by those from experimental work. The work regarding catalysts is then reviewed. The main focus of this chapter is on the effects of the process parameters of temperature, ratio of water to glycerol and catalyst characteristics on the H2 concentration and yield for each of the routes of thermochemical conversion of glycerol to H2.

THERMODYNAMICS OF CONVERSION OF GLYCEROL TO HYDROGEN Three main reactions are involved in the thermochemical conversion of glycerol to H2, namely glycerol decomposition (Reaction 1), methanation/methane steam reforming (Reaction 2) and the water gas shift reaction (WGS, Reaction 3). C3H8O3 → 3CO + 4H2 ΔH298K = +251.2 kJ mol-1

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Reaction 1, thermal decomposition. CO +3H2  CH4 + H2O ΔH298K = -206.1 kJ mol-1 Reaction 2, methanation of CO. H2O + CO  H2 + CO2 ΔH298K = -41.2 kJ mol-1 Reaction 3, water gas shift (WGS).

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The dry gas composition at temperatures between 27 and 1007 ˚C derived from thermodynamic equilibrium analysis of glycerol conversion in the presence of water using minimisation of Gibbs free energy is shown in Figure 2. In this example, the molar ratio of water to glycerol is 9:1 corresponding to a molar water or steam to carbon ratio (S:C) of 3:1 at 1 bar. With this method, glycerol conversion is complete at all temperatures in this range. At lower temperatures CH4 and CO2 are formed through the methanation (Reaction 2) and WGS (Reaction 3), respectively, from the CO and H2 produced by the glycerol decomposition reaction (Reaction 1). As the temperature is increased, the balance of Reactions 2 and 3 shifts further to the left resulting in decreasing concentrations of CH4 and CO2 in favour of increased concentrations of CO and H2. Note that the reverse of the methanation reaction is referred to as the steam methane reforming reaction (SMR). When SMR has gone to completion and the concentration of CH4 is negligible, the H2 concentration is maximised, and as the temperature is increased further the H2 concentration will decrease as the reverse WGS reaction is favoured. With increasing S:C, the H2 concentration is increased for all temperatures, and maximum H2 concentration is reached at an intermediate temperature (Figure 3). This is because an increase in water concentration will shift Reactions 2 and 3 towards more H2 production.

Figure 2. Concentrations of products from reaction of glycerol with water at molar steam to carbon ratio (S:C) of 3 at 1 bar. Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

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.

Figure 3. Hydrogen concentration with temperature from water-glycerol reaction for various S:C at 1 bar.

Combining Reactions 1 and 3 gives the ‗complete‘ steam reforming reaction of glycerol (Reaction 4).

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C3H8O3 + 3H2O → 7H2 + 3CO2 ΔH298K +127.67 kJ mol-1 Reaction 4. The theoretical maximum H2 production from reacting glycerol with water is 7 moles of H2 per mole of glycerol. H2 yield is a parameter used to measure the amount of H2 which is derived from glycerol and is commonly defined as either Equation 1 [26,27], Equation 3 [28,29] or Equation 4. [30-32] However, King et al. [33] refer to Equation 2 as H2 selectivity while Hu and Lu [34] and Slinn et al. [35] use Equation 3 for H2 selectivity. In the tables below their results will be presented as H2 yields according to equations 2 and 3.

H 2 yield  100 

moles of H 2 produced 7  (moles of glycerol converted)

Equation 2

H 2 yield  100 

moles of H 2 produced 7  (moles of glycerol supplied)

Equation 3

H 2 yield 

moles of H 2 produced moles of glycerol supplied

Equation 4

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Thermochemical Conversion of Glycerol to Hydrogen

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The difference between Equation 2 and Equations 3 and 4 is the amount of moles of glycerol which are taken into account. Equation 2 only takes the amount of converted glycerol into account hence disregarding unconverted glycerol, while Equations 3 and 4 take all the glycerol which is supplied to the reactor into account. When considering thermodynamic equilibrium analysis using minimization of Gibbs free energy Equation 2 and 3 will provide the same result since thermodynamic equilibrium results in complete conversion of glycerol. However, all work on thermodynamic equilibrium analysis reviewed here used Equation 4 to define the H2 yield. In the case of Equations 2 and 3, the maximum H2 yield is 100% while in the case of Equation 4 the maximum H2 yield is 7. Another useful parameter is the glycerol conversion. This parameter is used for experimental work and is a measure of how much of the glycerol supplied to the experimental setup was converted. Combined with the H2 yield, this parameter can provide useful information on the ability of the experimental setup to convert the glycerol to H2. Glycerol conversion is defined in different ways by different authors, and the definition used is often dictated by the methodology employed to quantify the conversion products. If for example the amount of glycerol in the product condensate (volatile products condensed by cooling) is quantified by measurement, Eq. 5 can be applied. If only gaseous products are quantified, then Eq. 6 and 7 can be applied. Eq. 6 and 7 only differ in the amount of carbonaceous gaseous products that are quantified. However, they are defined here with separate definitions since CO, CO2 and CH4 are by far the most common gaseous carbonaceous species to be quantified, but additional species such as C2H6 or C2H4 are sometimes also measured. Due to the variety of additional species that are reported in the literature, Eq. 7 is used to describe the definition of glycerol conversion when species other than just CO, CO2 and CH4 are quantified. Eq. 8 is used when both liquid and gaseous products are taken into account to calculate glycerol conversion.

glycerol conversion (%)  100 

moles of glycerol ((supplied) - (in the products)) moles of glycerol supplied

Equation 5

glycerol conversion (%)  100 

moles of C in CO, CO2 and CH 4 produced 3  (moles of glycerol supplied)

Equation 6

glycerol conversion (%)  100 

moles of C in all gas products 3  (moles of glycerol supplied)

Equation 7

glycerol conversion (%)  100 

moles of C in gas and liquid products 3  (moles of glycerol supplied)

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The final parameter used when describing glycerol conversion to hydrogen which will be considered here is the selectivity to products. Conventional definition for selectivity to a product containing a specific element is the ratio of molar production of this species to the sum of the molar productions of all the products containing the same element. For example, for selectivity to hydrogen gas from the products containing the hydrogen element, all the hydrogen containing products are considered in the ratio (H2, CH4, NH3, hydrocarbons), for selectivity to the carbon containing products CO or CO2, all the carbon containing products are considered (CH4, CO, CO2, carbonaceous deposits, carbonates, hydrocarbons, volatile organic compounds). But other definitions have appeared in the literature such as Equations 9 to 11. Among the non-conventional definitions of selectivity, for H2 selectivity Equation 9 is by far the most common. It compares the ratio of H2 in the gas to gaseous carbon containing species to the ratio of H2 to CO2 under thermodynamic equilibrium. Note that only H2 and CO2 are formed (in the ratio 7/3) under thermodynamic equilibrium according to Reaction 4. In the Tables below, only H2 selectivity is listed for clarity.

H 2 selectivi ty(%)  100 

moles of H 2 produced 1  moles of C produced in gas phase RR

Equation 9. Where RR is the reforming ratio (7/3), defined as the ratio of moles of H2 to CO2 formed at thermodynamic equilibrium.

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H 2 selectivi ty (%)  100 

moles of H 2 produced  moles of H 2 , CO,CH 4 and CO2 produced

Equation 10. Where α is H2, CO2, CH4 or CO.

Selectivity to carbon species

 (%)  100 

moles of species  produced  moles of C produced in the gas phase

Equation 11 Where α is CO, CO2, CH4 etc. Other characteristics which are less common (maybe even specific to one particular reference) are listed in the text. When the characteristics listed above are referred to in the text or in a Table the definition used is given in brackets. Temperatures are given in degrees Celsius (˚C) and pressures are given in bar.

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Thermochemical Conversion of Glycerol to Hydrogen

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GLYCEROL CONVERSION REACTION PATHWAY The reaction pathway for the conversion of glycerol to hydrogen is complex and many reaction intermediates are possible on the route from glycerol through 2- and 3-carbon compounds to CO, CO2 and CH4. Conversion of glycerol is commonly carried out using a catalyst containing a metal on a support material. The metal has the ability to bind to C and O atoms in the glycerol molecule and then break C-C or C-O bonds. [36] By binding to two C atoms in the glycerol molecule, the catalyst can break a C-C bond resulting in the formation of ethylene glycol (ethane-1,2-diol), CO and H2 and thus promoting Reaction 1. By binding to one C and one O atoms in the glycerol molecule, the catalyst can break a C-O bond resulting in the formation of an 1,2 or 1,3-propanediol and H2O. This is referred to as dehydrogenation since water is removed from the compound. Further C-C and C-O breakage of the ethylene glycol, 1,2 and 1,3-propanediol by the metal catalyst then follows. Liu and Greeley [37] constructed free energy diagrams for a series of glycerol conversion intermediates on Pt(111) using density functional theory (DFT) calculations correlation schemes for binding energies and Brønsted–Evans-Polanyi (BEP) relationships for transition state energies (Fig. 4). They reported that for the first 6 dehydrogenation decomposition intermediates the energy of the dehydrogenation transition state was lower than the C-C bond cleavage transition state. The results showed that decomposition of glycerol is more likely to first proceed through a series of dehydrogenation reactions and then through C-C bond cleavage reactions. A metal catalyst can also induce the WGS reaction by adsorption of CO and H2O molecules, followed by dissociation of the H2O molecules into OH and then O. [38] The CO molecules can then react with adsorbed OH and H to form CO2 and H2 with COOH as an intermediate or alternatively the CO molecules can react with O to form CO2. In the latter case, adsorbed H form H2 without interfering with the reaction between CO and O. Hence a metal catalyst can induce the WGS reaction by dissociating O-H bonds in the H2O and OH molecules and promote C-O bonds in the CO2 molecules.

Figure 4. Free energy diagram of glycerol decomposition intermediates at 210 ˚C and 1 bar on Pt(111) for all levels of dehydrogenation states of glycerol. Black squares =the adsorption thermochemistry of the most stable dehydration intermediate for a given dehydrogenation state, diamonds = the most stable dehydrogenation transition state for a given dehydrogenation state, triangles = the corresponding energetics for the C-C cleavage transition state for a given dehydrogenation state.37

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A part from gaseous and liquids compounds, it is also possible that elemental carbon (or coke) is formed during glycerol conversion. A common route to coke formation is Reaction 5. Coke formation can cause catalyst deactivation and measures are therefore taken to minimize it.

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CO + H2  H2O + C(s) ΔH298K = -131.3 kJ mol-1 Reaction 5. By analysing the gaseous products and the liquid products formed during glycerol conversion as well as analyzing the coke, the understanding of the reaction pathways can be increased. Dupont et al. [39] for example studied thermal decomposition of glycerol using thermogravimetric analysis (TGA) by heating it with a temperature ramp rate of 5 ˚C min-1 in N2. Mass loss began at 150 ˚C and ended at 230 ˚C. The crude glycerol decomposed into 94 wt% volatiles and 6 wt% residues. The results showed that glycerol will readily decompose at temperatures below 230 ˚C in N2. Thermal decomposition of glycerol at temperatures between 500 and 700 ˚C produced H2, CO, CO2 and CH4 as well as ethane, ethene, propane, propene, n-butane, butane, and significant amounts of coke. [40] In this study, up to 50% of the carbon in the glycerol converted to coke during catalytic cracking at 500 ˚C with a S:C of 1.7 using a commercial FCC catalyst containing Y-zeolite in a Si/Al matrix. Valliyappan et al. [41] carried out thermal decomposition of glycerol at 650 ˚C with N2 as a carrier gas and analysed the off gas using gas chromatography. The off gas consisted mainly of CO (54.0%) while the second most common species was H2 (17%). Other species detected in the off gas were CO2 (0.2 %), CH4 (14.2 %), C2H4 (10.1%), C2H6 (2.2%) and C3H6 (2.4%). Chiodo et al. [26] investigated glycerol‘s thermal decomposition at 800 ˚C in the presence of steam (S:C of 3) and analysed the reactor off gas and condensate using GC and GS-MS respectively. With regard to the gas phase, CO had the highest carbon selectivity (Eq. 11) (50%) followed by C2H2 and C3H6 (25% in total), CH4 (13%), H2 (10%) and CO2 (1%). The condensate contained acetone, acetaldehyde, ethanol, propanol, acetid acid and 2,3dyhydroxylpropanal. Stein et al. [42] performed thermal decomposition of glycerol in the presence of steam at temperatures between 650 and 700 ˚C and analysed both the condensate and the off gas using gas chromatography. At 650 ˚C only acetaldehyde and acrolein were formed (the moles of acetaldehyde and acrolein formed per mole of converted glycerol were 0.48 and 0.52 respectively). The absence of any other species could be explained by the short residence time used in the experiment (0.1 s). As the temperature increased, the moles of acetaldehyde and acrolein per mole of converted glycerol decreased while the moles of CO, CO2, CH4 and H2 per mole of converted glycerol increased. The results indicated that glycerol first decomposed to acetaldehyde and acrolein which then reacted further to form CO, CO2 CH4 and H2. Stein et al. [42] also carried out thermal decomposition in the presence of steam at 700 ˚C and (S:C 168.7, 1 bar) with and without the presence of NO2 gas (which has the ability to inhibit radical initiated reactions). The presence of NO2 gas had no effect on the yield of acetaldehyde but reduced the yield of acrolein by 71%. They concluded that acetaldehyde was formed by a bond cleavage reaction and acrolein was formed by a radical initiated reaction.

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STEAM REFORMING (SR) Steam reforming is the most extensively investigated production route to hydrogen from glycerol. It involves adding glycerol and water to a reactor where the glycerol is gasified and the water is turned into steam. The water can also be turned into steam before it reaches the reactor using a preheating system. A Ni based catalyst is commonly placed inside the reactor. A CO2 sorbent can also be incorporatd to the reactor, which has the ability to capture the CO2 formed from the WGS reaction and thus shifting favourably its equilibrium and as a knock-on effect also that of the decomposition of glycerol. The SR process with in situ CO2 capture is referred to as sorption enhanced steam reforming (SESR).

Thermodynamic Equilibrium Analysis

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Thermodynamic equilibrium analysis with reactor temperatures between 77 and 1127 ˚C, S:C between ⅓ and 4⅔ has been undertaken by a number of research groups. [43-52] The H2 concentration increased as the temperature increased and reached a maximum level. [43-45,48,49,51,52] The maximum H2 concentration was dependent on the S:C so that the higher the S:C, the lower the temperature of maximum H2 concentration. [43-45,48] A higher S:C also resulted in higher H2 concentrations over a range of temperatures. [43-45,4850] Thermodynamic equilibrium analysis has shown that the optimum temperature for steam reforming of glycerol with regard to maximum H2 yield was 540-677 ˚C. [31,46,49-52] The maximum H2 yield (Eq. 4) was 6-6.9 (Table 1). S:C affected the H2 yield such that a higher S:C resulted in a higher H2 yield, [46,51,52] as illustrated by figure 3. Table 1. Summary of thermodynamic equilibrium analysis of SR. Temperatures are given in ˚C Reference

Temperatures

S:C

Best results

52 50

350-700 427-827

3 3

49 46

277-727 227-1177

0-4 0.5 - 4⅔

31 45 43 44

300-800 327-927 327-727 327-727

6.8 - 15⅓ 0-7 0.3-3 0.3-3

47 48

327-927 327-727

0.3-3.3 0.3-3

Yield (Eq. 4) 6.2 Concentration 67% Yield (Eq. 4) 6 Yield (Eq. 4) 6.2 Concentration 70% Yield (Eq. 4) 6.3 Yield (Eq. 4) 6.9 Concentration 65%* Yield (Eq. 4) 6 Concentration 65% Yield (Eq. 4) 6 Yield (Eq, 4) 6 Selectivity (Eq. 9) 78% Yield (Eq 4) 5.8

Optimum conditions Temperature S:C 580 3 652 3 652 3 652 4 727 2 667 4⅔ 540 15⅓ 577 4 627 3 627 3 627 3 627 3.3 627 3 652 3

* while allowing a maximum CO concentration of 5%.

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Experimental Work

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Experimental work with reactor temperatures between 200 and 803 ˚C and S:C between 0.5 and 15.3 have been carried out (Table 3). Both electrical heating and microwave heating were used. [53] Hydrogen hydrogen yields of 46-67% (Eq. 2), 56.5-95% (Eq. 3) and 2.5-6.5 (Eq. 4) were achieved (Table 3). High temperatures and high S:C resulted in improved performance. Buffoni et al. [54] for example carried out steam reforming between 450 and 600 ˚C (S:C 6) and reported that the H2 concentration increased from 59 to 71% as the temperature was raised from 450 to 600 ˚C while the concentrations of CO, CO2 and CH4 were reduced. This is in agreement with Zhang et al. [55], Adhikari et al. [44] and Pompeo et al. [56] who all reported increased H2 concentrations with rising temperature in the 350-650 ˚C range. The effect of S:C was studied by Slinn et al. [35] who performed steam reforming of crude glycerol at 850 ˚C and reported an increase in H2 concentration when the S:C grew from 0.5 to 2.5. The effects on the steam reforming process of the WGS reaction were demonstrated by Doutte et al. [57] who carried out steam reforming with a separate WGS reactor. The steam reforming reactor was at 1 bar, 804 ˚C and S:C of 2.2 while the WGS reactor was between 320 and 420 ˚C. The H2 yield (as defined by Equation 3) increased from 4.3 to 5.9 for WGS temperature from 320 to 380˚C (Figure 5). As the temperature increased further to 420˚C the H2 yield (Eq. 4) reduced to 5.3. The results demonstrated the ability of the WGS reaction to aid in H2 production at lower temperatures but also to have the opposite effect when it is reversed as temperature increases, as explained by the thermodynamics of the steam reforming process.

Figure 5. H2 yield as a function of WGS reactor temperature following steam reforming of glycerol with S:C 2.2 at 804 ˚C. [57]

The means of heating the steam reforming reactor can also affect the steam reforming process as demonstrated by Fernandez et al. [53], who steam reformed glycerol using an Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

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Thermochemical Conversion of Glycerol to Hydrogen

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activated carbon (AC) catalyst at 1 bar, 800 ˚C and S:C between 1 and 9 using both electrical heating and microwave heating. With electrical heating, H2 in the reactor off gas increased with S:C from 30.9 to 38.2% while the amount of CO reduced from 47.2 to 44.1%. As a result the H2:CO ratio augmented from 0.7 to 0.9. With microwave heating, H2 increased from 40.9 to 49.1% as S:C increased while CO reduced from 44.0 to 42.7%, causing the H2:CO ratio to increase from 0.9 to 1.2. However, the conversion to gaseous products (Eq. 7) was lowered from 67.8 to 52.3% under electrical heating and from 70.4 to 62.3% under microwave heating. The results showed that increased S:C augmented the H2:CO ratio and that microwave heating produced a higher H2:CO ratio for a given S:C compared to electrical heating. However, there was a trade off between conversion to gaseous products and H2:CO ratio. The improved performance of microwave heating compared to electrical heating was discussed by Fernandez et al. [58] (reviewed in the section on thermal decomposition). During analysis of the gaseous products from glycerol steam reforming, only the concentrations of H2, CH4, CO and CO2 are normally measured since the formation of other compounds with higher numbers of carbon atoms that can form is negligible. [50-52,59] The reason for this is that the enthalpy of formation for carbon containing compounds is increased with the number of carbon atoms (Table 2). [59] This subsequently increases the Gibbs free energy of formation with the number of carbon atoms, making it less likely for compounds containing more carbon atoms to form compared to compounds containing less carbon atoms. In fact, thermodynamic analyses using minimisation of Gibbs free energy have shown that the compounds methanal, methanol, formaldehyde, ethanal, ethanol, ethane, ethene, ethylene, propanal, propane, propene, propanone, propionaldehyde, acetone, acetic acid, acrolein, allyl alcohol and acetaldehyde either exist in negligible concentrations (molar fractions 99% H2 purity with CO, CO2 and CH4 levels 80%) using a CuO/ZnO or CuO/ZnO/Al2O3 catalyst. Conversion of glycerol is substantially complete under H2 pressure between 10-15 MPa and temperatures in the range of 513-543 K. Montassier et al. [40, 41, 69] describe the hydrogenolysis of aqueous glycerol at 533 K and H2 pressure between 3-6 MPa in the presence of Ru, Rh, Ir, Co, Cu supported on SiO2 and C, Raney nickel and Raney copper

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Valorization of Glycerol in Propanediols Production by Catalytic Processes

53

catalysts. 1,2-PDO was found as the main reaction product only when copper systems were used. On other systems formation of ethylene glycol and gaseous hydrocarbons, mainly methane, was detected. Furthermore, experimental results, obtained with a sulphided Ru supported on carbon, show a significant improvement of the selectivity to 1,2-PDO, but the activity decreases, due to poisoining, by sulphur, of the Ru surface [41-43, 45]. Chaminand et al. [60] reported a low conversion of glycerol in the reaction carried out over CuO/ZnO, Pd/C, Rh/C at 8 MPa H2 pressure and 453 K; however a high selectivity to 1,2-PDO (100%) in the presence of CuO/ZnO was detected. Suppes et al. [44, 47] compared the catalytic performance, in glycerol hydrogenolysis, of different commercial catalysts, including Ru, Pt, Pd, Ni supported on activated C and Raney copper, Raney nickel and copper-chromite, at low pressure and 473 K. On supported noble metals lower conversion and selectivity, than on Raney systems, were observed. Copper–chromite was identified as the most effective catalyst with 54.8% glycerol conversion and 85% selectivity to 1,2-PDO. Furthermore, since 1-hydroxyacetone was detected, as intermediate, authors proposed a two-step reaction mechanism, involving the dehydration of glycerol to acetol and its further hydrogenation to 1,2-PDO. The hydrogenolysis process with copper-chromite results more attractive than traditional processes mainly due to the mild reaction conditions used (low pressure and temperature). Liang et al. [74] synthesized, via the template preparation method, high surface area Cu–Cr catalysts showing a high selectivity to 1,2-PDO (97% at 51% conversion) at 4.15 MPa H2 and 483 K, using a 40 wt% aqueous glycerol solution. The high activity and selectivity were ascribed both to the good dispersion of copper on Cr2O3 and the strong interaction between copper and Cr2O3, leading to CuCr2O4. Also binary Cu/Cr catalysts, containing different molar ratios of copper to chromium, were synthesized by co-precipitation, and the system containing Cu and Cr with a 1:2 ratio shows the best catalytic performance at 8 MPa H2 and 493 K (83.9% selectivity to 1,2-PDO at 80.3% conversion) [77]. However, chromium is not an environmentally friendly material, due to its toxicity, and copper-chromite based catalysts are undesirable. After a screening of various supported metal catalysts (Rh, Ru, Pt, Pd on C, SiO2 and Al2O3), Furikado et al. [51] found that the Rh/SiO2 catalyst is more active and selective, towards hydrogenolysis products, such as propanediols and propanols, than Ru/C. All other supported samples tested show very low activity. Furthermore, the selectivity to CC cleavage products (degradation products: ethylene glycol + ethanol + methanol + methane) on Rh/SiO2 (total 7.1%) was much lower than that on Ru/C (42%). Many works reported in the literature deal with use of supported ruthenium catalysts, since ruthenium is considered an effective catalytic metal in glycerol hydrogenolysis. However, Ru promotes C-C cleavage, leading to excessive degradation of glycerol to form lower alcohols. Feng et al. [54] report that the intrinsic properties of the support used (SiO2, NaY, -Al2O3, C, TiO2) can affect the particles size of metallic Ru on the catalyst surface and the reaction pathway. In fact, the hydrogenolysis of glycerol is more active on smaller metal particles, as those on Ru/TiO2, whereas Ru/SiO2 is the most selective for 1,2propanediol production. Ru-based samples, prepared using the chloride salt precursor, show higher activity compared to the corresponding one obtained from Ru(NO)(NO3)3, but the selectivity to 1,2-PDO is lower due to the promotion of excessive hydrogenolysis to propanols [55]. The results were explained with the retention of Cl- ions on the support surface (-Al2O3, SiO2, ZrO2) that makes more acidic the catalyst. Increased acidity

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improves activity, but results in poor selectivity values due to over-hydrogenolysis to propanols and C-C bond cleavage reactions. Recently, Pietropaolo et al. [62] demonstrated that palladium catalysts supported on appropriate carriers, prepared by co–precipitation, were more active and selective to 1,2-PDO than Ru based systems. A 71.2% selectivity to 1,2PDO, at complete conversion of glycerol, was found in presence of Pd/Fe2O3 under mild reaction conditions, using a 4 wt% isopropanol solution of glycerol. The best performance of the catalyst, in terms of activity and selectivity, was ascribed to a good palladium-support interaction. Copper is well known for its lower ability to cleave C-C bonds and to be highly efficient for alcohols dehydration-hydrogenation reactions. Then, copper-based catalysts show high catalytic performance, mainly in terms of selectivity to 1,2-PDO, in glycerol hydrogenolysis under mild reaction conditions. Furthermore addition of Al2O3 to Cu-ZnO was found to increase the catalyst stability. Co-precipitated Cu-ZnO catalysts were tested and selectivity > 80% to 1,2-PDO, at low glycerol conversion, together with formation of small amounts of ethylene glycol, as a by-product, were reported [70, 71]. The glycerol conversion and selectivity are affected by Cu and ZnO particles size: smaller particles lead to higher conversion and selectivity to 1,2-PDO and sintering of particles is avoided. Pre-reduction of fresh Cu-ZnO catalysts by H2 stream is essential to avoid aggregation of Cu particles. Huang et al. [72] synthesized an efficient and stable CuO/SiO2 catalyst with highly dispersed nanoparticles, by using the precipitation-gel method, achieving much higher activity and selectivity to 1,2-PDO (> 94%) than the catalyst prepared by the conventional impregnation method. The results reported from Vasiliadou et co-workers [79] indicate that glycerol hydrogenolysis effectively proceeds in the presence of Cu monometallic supported on silicates (silica and high surface area hexagonal mesoporous silica). High activity and selectivity were obtained with 20% Cu supported on hexagonal mesoporous silica (HSM) and the different reactivity was correlated with the catalyst surface area. Similarly, Bienholz et al. [81] explain the essential role of copper surface area in the catalyst activity of both the dehydration and hydrogenation steps in 1,2-PDO formation. Other studies [58] report the performance of Cu-based catalysts (supports used: -Al2O3, HY zeolite, 13X zeolite, HZSM5 zeolite and H-zeolite); it was found that a Cu/Al2O3 sample, with the optimized amount of Cu, is fairly active and very selective to propanediols (about 97%) under relatively low pressure (1.5 MPa initial H2 pressure). On this line the hydrogenolysis of glycerol over CuO/ZnO/MnO2, achieving a high selectivity to 1,2-PDO (97.5%) at 8 MPa H2 and 473 K, using neat glycerol, was patented [76]. Very high 1,2-PDO selectivity can be also obtained, in gas-phase hydrogenolysis, over copper-based catalysts. The vapour phase process presents several advantages with respect to the traditional process using a batch reactor: milder reaction conditions, continuous operation and a solvent free system. Huang and co-workers [53] used the Cu/ZnO/Al2O3 catalyst, chosen after a preliminary screening performed using an autoclave, as container, of a series of silica supported metal catalysts, such as Co, Cu, Ni, Pd, Ru, in the vapour-phase hydrogenolysis in a fixed–bed flow reactor. A selectivity of 92.2% to 1,2-PDO at 96.2% conversion was achieved at 463 K and 0.64 MPa H2. By using the same catalyst, the liquidphase hydrogenolysis requires more severe conditions (473 K and 5 MPa H2) and shows lower selectivity to 1,2-PDO (80.1% at 20.4% conversion). Several results over Cu/ZnO/Al2O3 catalysts, prepared by three different methods: incipient wetness

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impregnation, co-precipitation and sol-gel, were obtained by Panyad et al. [80]. In particular, it was found that the catalyst, prepared by incipient wetness impregnation, shows the highest stability as compared to the other samples prepared in a different way. Vapour phase hydrogenolysis of glycerol over copper metal catalysts at ambient hydrogen pressure and gradient temperatures was also reported [61, 63]. It was found that glycerol is dehydrated to form 1-hydroxyacetone, followed by the hydrogenation to 1,2-PDO. Therefore, glycerol can be converted into 1,2-PDO with high yield (> 96%) over Cu/Al2O3 at gradient temperatures (393-473 K), while the yield of 1,2-PDO is almost 80% at a constant temperature of 463 K, since the higher temperature favours the dehydration forming acetol whereas the lower temperature favours the exothermic hydrogenation of acetol to 1,2-PDO. Raney Cu was used as catalyst in a liquid phase fixed bed hydrogenolysis process, that appears to have higher potential applications for high space–time yield than a vapor phase process [78]. A yield of 94% to 1,2-PDO was obtained at 478 K and 1.4 MPa H2. Nickel supported on different carriers, such as SiO2, -Al2O3, TiO2, C, NaY, ZrO2, and Raney nickel catalysts were also tested in the hydrogenolysis of glycerol. Perosa and Tundo [82] showed that the Raney Ni catalyst is active at relatively low pressure (1 MPa H2), with a 91% glycerol conversion and 48% selectivity to 1,2-propanediol. Ethanol and CO2 are the only by-products. The reaction was carried out without solvent and the product can be easily separated, by distillation, from the reaction mixture. Also a Ni catalyst supported on NaX zeolite was found to be effective in the glycerol hydrogenolysis [66]. The better performance of this catalyst was ascribed to the acidity of the zeolite support. Under optimized conditions, 86.6% conversion with 80.4% selectivity to 1,2-PDO was obtained at 473 K and 6 MPa H2 after 10 h reaction.

4.1.2. Metal Catalysts Modified by Brønsted Acids or Bases Use of acids or bases, as co-catalysts, as well as supports in glycerol hydrogenolysis improves both propanediols production rate and selectivity and allows milder reaction conditions (Table 3). In particular, 1,2-PDO formation is favoured both by dehydration of glycerol to acetol, promoted by acid sites, followed by hydrogenation on the surface metal sites [46, 48, 49, 51, 60, 66, 83-86] or dehydrogenation of glycerol to glyceraldehyde, in alkaline solution, followed by dehydration to 2-hydroxyacrolein, rapidly isomerized to pyruvaldehyde that undergoes hydrogenation [40, 41, 50, 87, 88]. The U.S. Pat. issued by Schuster et al. [89] describes the catalytic hydrogenation of glycerol at elevated pressure and temperature to give 1,2-PDO in high yield, in presence of a mixed catalyst of cobalt, copper, manganese, molybdenum and an inorganic polyacid, using neat glycerol or a solution having a water content up to 20 wt%. A yield to 1,2-PDO of about 95%, with associated formation of npropanol and lower alcohols, was obtained at complete conversion of glycerol. Preferred process conditions include a pressure ranging from 20 to 32.5 MPa and a temperature from 473 K to 523 K. Addition of a solid acid, such as H2WO4, to Rh/C increases the glycerol conversion (2.5% and 10% without and with H2WO4, respectively) and the selectivity to propanediols [60]. Furthermore, use of sulfolane as solvent allows to obtain a major conversion and selectivity to 1,3-propanediol (two times higher than to 1,2-PDO). Combination of active carbon supported Ru with the ion-exchange resin Amberlyst 15 is effective for the glycerol hydrogenolysis under mild reaction conditions, compared with other Brønsted acids, such as homogeneous HCl, homogeneous H2SO4, various zeolites, sulphated zirconia and H2WO4 [46, 48, 49, 83]. The presence of HCl significantly decreases the

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activity, due to the presence of Cl- ions that can be adsorbed on the Ru surface, acting as catalyst poison. Amberlyst results the most effective additive for a higher acid strength than other solid acids used. Addition of Amberlyst to Rh/C [48] or Rh/SiO2 [51] as well as to Ru/C increases the activity but a major formation of over-hydrogenolysis products (1- and 2propanol) is observed. Table 3. Selected hydrogenolysis results of aqueous glycerol to 1,2-PDO over acid or base modified catalysts

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Catalyst

a

PH2 T (K) Time Conv Selectivity to Ref. (Mpa) (h) (%) 1,2-PDO (%)

CoO/CuO/MnO2/MoO3/H3PO4

Glycerol Concentration (wt%) 100a

25

523

6

100

95.8

89

5% Ru/C + Amberlyst 15

20

8

453

10

15.0

53.6

48

5% Ru/C + Amberlyst 70

20

8

453

10

48.8

70.2

83

5% Ru/C + Nb2O5

20

6

453

8

62.8

66.5

85

4.9% Ru-CaZnMg/Al

20

2.5

453

18

58.5

85.5

58

Raney Ni + (C6H13)3C14H29PCl

100a

1

463

20

47.0

81.0

82

1% Pt/SiO2-Al2O3

20

4.5

493

24

19.8

31.9

86

3% Pt/C + CaO

1

4

473

5

40.0

71.0

50

5% Ru/C + CaO

1

4

473

5

50.0

46.0

50

5% Ru/TiO2 + LiOH

20

3

443

12

89.6

86.8

87

Co/MgO

10

2

473

9

44.8

42.2

88

Co-Zn-Al oxide (40.1 wt% Co)

10

2

473

12

70.6

57.8

90

2% Pt/hydrotalcite

20

3

493

20

92.1

93.0

91

Cu0.4Mg5.6Al2O8.6

75

3

453

20

80.0

98.2

92

Neat glycerol.

The promoting effect of Amberlyst is not significant for Pt/C and Pd/C. It was also reported that a Ru/C catalyst, prepared by using Ru(NO)(NO3)3, as precursor, and an active carbon with a low surface area (~ 250 m2/g), treated under Ar flowing at the optimal temperature (573 K), gives higher performance than the commercial Ru/C sample. A glycerol conversion of 79% and a selectivity of 70% to 1,2-PDO were obtained at 393 K in presence of home-made Ru/C catalyst and Amberlyst 15 as co-catalyst and by using a 2 wt% aqueous glycerol solution. However, use of ion-exchange resins, as co-catalysts, limits the range of reaction temperature, since they are thermally unstable. On increasing the reaction temperature, in fact, a decrease of glycerol conversion is observed, due to the poisoning of the catalyst promoted by sulphur containing compounds, formed by thermal decomposition of the resin starting from 393 K. The combination of Ru/C and an ion-exchange resin with higher heat resistance (Amberlyst 70) than the Amberlyst 15 allows to carry out the glycerol

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hydrogenolysis reaction at 453 K, achieving higher conversion and selectivity [83]. The influence of the catalysts acidity, on glycerol conversion and selectivity, was investigated by Lingaiah et co-workers, using the combination of a commercial Ru/C catalyst with different thermally stable solid acids, such as niobia, ZrO2 supported tungstophosphoric acid (TPA), cesium salt of TPA, cesium salt of TPA supported on ZrO2 [85]. A linear correlation between the acidity of the additives and the glycerol conversion was reported. The niobia and ZrO2 supported tungstophosphoric acid co-catalysts, that possess moderate acid sites, show better catalytic performance. Alhanash et al. [84] synthesized bifunctional catalysts by loading Ru or Rh on the heterpolyacid salt, Cs2.5H0.5 [PW12O40]. At 423 K a high selectivity to 1,2-PDO (~ 96%) was obtained at 21% glycerol conversion with ruthenium supported on Cs2.5H0.5 [PW12O40]. The analogous rhodium catalyst results considerably less active (6.3% glycerol conversion) than the Ru one, but more selective to 1,3-PDO (7.1%) whereas 1,2-PDO is the main product (65%). Ru-based hydrotalcite like catalysts, doped with Ca and Zn and prepared by solid phase crystallization and impregnation methods, are effective in glycerol hydrogenolysis [58]. A selectivity to 1,2-PDO of 85.5% at 58.5% conversion is achieved under mild reaction conditions. The performance of the catalysts is ascribed to the enhanced acidity due to addition of Ca and Zn ions acting as Lewis acid sites over hydrotalcite (layered double hydroxide and known as weak base). Addition of a liquid phosphonium salt improves, to a small extent, the reactivity of Raney Ni [82]. A high selectivity to 1,2-PDO (81% at 47% conversion) is obtained without solvent. The role of acid and metal sites on amorphous silicaalumina supported Pt catalysts in the glycerol hydrogenolysis was investigated by Gandarias and co-workers [86]. A low selectivity to 1,2-PDO (31.9% at 19% conversion) is obtained at 493 K and 4.5 MPa H2 pressure, due to over-hydrogenolysis reactions, leading to 1- and 2propanol (53. 8% selectivity to 1-propanol + 2-propanol). Addition of a base, such as NaOH and CaO, to commercial Pt/C and Ru/C catalysts enhances the reactivity of Pt to a greater extent than that of Ru/C; however, the base catalyzes the C-C bond cleavage leading to ethylene glycol. Furthermore formation of lactate is noticeable at higher pH [50]. The effect of the nature and amount of the base additive, such as hydroxides and carbonates of lithium, sodium and potassium, over Ru/TiO2 catalysts was reported by Chen et al. [87]. The best catalytic performance was obtained by using a lithium or sodium base and an influence of alkali metal cations on the activity in the hydrogenolysis reaction was observed. The activity trend Li+ > Na+ > K+ detected was associated with the radius of the cation. The increase of the LiOH amount increases the activity and selectivity to 1,2-PDO up to reach a maximum, after which the conversion begins to decrease. On increasing the amount of LiOH from 0 to 1 mmol, the selectivity to 1,2-PDO increases from 47.7 to 86.8% and, conversely, the selectivity to ethylene glycol decreases. An efficient Co/MgO bifunctional catalyst, where the solid MgO acts as basic component and support of cobalt nanoparticles, for hydrogenolysis of glycerol was reported [88]. The higher reactivity and stability of the catalyst is due to the interaction between Co3O4 and MgO (stronger at higher calcination temperature) that prevents aggregation of Co particles under the reaction conditions. Cobalt supported on a hydrotalcite material results active and selective to 1,2PDO production [90]. A selectivity of 57.8% to 1,2-PDO and a glycerol conversion of 70.6% were achieved at 473 K and 2 MPa H2 pressure due to the homogeneous dispersion of cobalt particles on the solid base matrix. Yuan et al. [91] evaluated the catalytic performance of platinum catalysts supported on different solid bases, such as H-, H-ZSM-5, MgO, hydrotalcite precursor. It was found that the activity of the tested catalysts depends mainly on

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their alkalinity and the particles size of Pt. A hydrotalcite material supported Pt catalyst with strong alkalinity and highly dispersed Pt particles exhibits the best performance (with a 93% selectivity to 1,2-PDO at 92.1% conversion) at lower pressure in aqueous glycerol solution. Also a Cu0.4Mg5.6Al2O8.6 catalyst, prepared by thermal decomposition of the Cu0.4Mg5.6Al2(OH)16CO3 layered double hydroxides, was found extremely effective for hydrogenolysis of glycerol [92]. The activity of this catalyst is due to highly and homogeneously dispersed copper species on the matrix of the hydrotalcite-like solid base. Furthermore, this kind of solid base is a material full of mesopores that enhances the accessibility of reactants. A little amount of added NaOH further increases the activity without cleavage of C-C bonds. A conversion of 80% and a selectivity of 98.2% to 1,2-PDO were obtained at 453 K and 3 MPa H2.

4.1.3. Bimetallic Catalysts Bimetallic catalysts are widely used in hydrogenolysis reactions of glycerol and often afford higher activity and selectivity to 1,2-PDO, than monometallic systems (Table 4). Carbon supported Pt-Ru and Au-Ru species, tested in glycerol hydrogenolysis at 473 K and 4 MPa H2, show a performance similar to that observed over monometallic Ru catalysts [93]. Both bimetallic systems favour formation of ethylene glycol over 1,2-PDO. However, whereas the PtRu catalyst appears to be stable under the aqueous phase reaction conditions, the AuRu is modified, since gold migrates off Ru and agglomerates on the carbon support during the hydrogenolysis reaction. Table 4. Selected hydrogenolysis results of aqueous glycerol to 1,2-PDO over bimetallic catalysts. a ethanol as solvent; b 2-propanol as solvent

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Catalyst

T (K) Time Conv (h) (%)

Selectivity to 1,2-PDO (%)

Ref.

1.9% Pt-4.7% Ru/C

Glycerol PH2 Concentration (Mpa) (wt%) 1 4

473

5

42.0

24.0

93

5% Ru/Al2O3 + Re2(CO)10 3.64 % Ru-4.12%Re/ZrO2

40 40

8 8

433 433

8 8

53.4 56.9

50.1 47.2

52 56

3.23% Ru-3.57%Re/SiO2

40

8

433

8

51.7

44.8

56

3% Ru –Cu/ TMG -bentonite Pd0.04Cu0.4Mg5..56Al2O8.56

30 75a

8 2

503 453

18 10

100 88.0

85.4 99.6

95 96

3.7% Pd/CoO

4b

0.5

453

24

100

10.2

62

b

5% Pd/NiO

4

0.5

453

24

90.1

84.5

62

5% Ni -0.5%Pt/SiO2

5

8

453

48

79

14.0

99

7% Ni -1.4%Re/C + NaOH

25

4.14

503

4

70

54.0

97

Ni -Ce/C

25

5

473

6

90.4

65.7

98

A remarkable promoting effect of rhenium on the catalytic performance of supported Ru catalysts both on glycerol conversion and selectivity to propanediols was reported [52, 56, 94]. The conversion of glycerol increases from 1.5 to 2.2 times when, using heterogeneous Re2(CO)10 as precursor, rhenium is added to ruthenium. Also the selectivity to propanediols increases, whereas the selectivity to degradation products, such as ethylene glycol,

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significantly decreases. Therefore, addition of rhenium inhibits degradation of glycerol to undesired products. A glycerol conversion of 53.4% and a selectivity of 50.1% to 1,2-PDO were obtained over Ru/Al2O3 containing Re2(CO)10 at 433 K and 8 MPa H2. A less pronounced increase of the conversion is observed over Ru/ZrO2 + Re2(CO)10, although a higher selectivity to 1,3-PDO is obtained (12.6%). Supported Ru-Re bimetallic catalysts, prepared by impregnation of different supports, such as SiO2, ZrO2, TiO2, H-, H-ZSM5, using aqueous solutions of RuCl3 · 4 H2O and HReO4, show a much higher activity than supported ruthenium containing Re2(CO)10. However, on the last catalysts, the increase of the selectivity to propanediols was more pronounced. The results were explained by considering a synergic effect between Ru and Re species on bimetallic catalysts where the presence of Re promotes dispersion of Ru on the surface of the support and prevents aggregation of Ru metal particles during the reaction. The influence of the pre-treatment on the catalytic performance of Ru-Re/SiO2 catalysts in glycerol hydrogenolysis was also investigated [94]. High prereduction temperature (723 K) under H2 flow decreases the reactivity of the catalyst, since it accelerates aggregation of particles. Characterization data indicate that Ru is present as Ru°, while Re is mostly as rhenium oxide on the spent Ru-Re/SiO2. Low reduction temperature (< 573 K) or ―in situ reduction‖ prevents the growth of Ru° particles and favours the interaction of rhenium oxide with ruthenium metallic species. Ru-Cu bimetallic catalysts supported on bentonite, modified with the functional ionic liquid 1,1,3,3-tetramethylguanidinium lactate (TMGL), were prepared and tested in the glycerol hydrogenolysis [95]. The bimetallic catalysts were very efficient and a 100% conversion and 85% yield of 1,2-propanediol were obtained at 503 K and 8 MPa H2. TMGL plays a crucial role in the preparation of the catalysts, stabilizing the metal particles with a combination of coordination and electrostatic forces. Both the coordination between metal particles and TMG and the electrostatic forces between TMG and the clay inhibit aggregation of particles. The activity and selectivity to 1,2PDO of bimetallic Pd-Cu/solid-base, prepared by thermal decomposition of PdxCu0.4Mg5.6xAl2(OH)16CO3 layered double hydroxides precursors, were investigated by Hou et al. [96]. On Pd0.04Cu0.4Mg5.56Al2O8.56 the glycerol conversion and the selectivity reach values of 88% and 99.6%, respectively, at 2 MPa H2 and 453 K. The high performance of Pd-Cu/solid-base catalysts was ascribed to a H2 spillover from Pd to the Cu surface. Supported bimetallic palladium catalysts (Pd-Co, Pd-Ni, Pd-Zn), prepared by co-precipitation, were found effective in the hydrogenolysis of glycerol under mild reaction conditions [62]. The Pd/CoO catalyst results the most active: a complete conversion of glycerol was obtained at 0.5 MPa H2 and 453 K although the selectivity to 1,2-PDO was low (10.2%), due to an over-hydrogenolysis reaction to 1-propanol (selectivity 80.9%). The main peculiarity of these catalysts is formation of bimetallic particles, evidenced by characterization data, that allows modification of the electronic density of palladium and promotion of the hydrogenolysis reaction. The nature of the co-metal (particulary cobalt) may be synergically involved in the mechanism and constitutes an additional factor that contributes to further increasing the reactivity. Addition of different promoters, such as cerium, copper, zinc, aluminum, cobalt, rhenium, platinum, to supported nickel catalysts was reported to favour glycerol hydrogenolysis [9799]. Modification of a Ni/SiO2 catalyst with a small amount of platinum favours conversion of glycerol to ethylene glycol. Addition of cerium to activated carbon supported nickel shows a remarkable promoting effect compared with other metals studied. At 473 K and 5 MPa H2 a 90.4% conversion was obtained in presence of Ni-Ce/C. The best performance of this system was attributed to a metal-metal interaction, as evidenced by the decrease of the reduction

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temperature of nickel, and formation of smaller particles with respect to the unpromoted catalyst.

4.1.4. Hydrogenolysis in Absence of Hydrogen The combination of the ―in situ‖ hydrogen production and glycerol hydrogenolysis reaction is a new and efficient catalytic route for obtaining 1,2-PDO and ensuring several advantages over previous cited reactions using externally added hydrogen (Table 5). Hydrogen is currently derived from non-renewable natural gas and petroleum. The new route provides a feasible approach since hydrogen is directly generated in the reaction medium and the ensuing hydrogenolysis process is certainly facilitated, since the reaction can be carried out at mild pressure using an inert gas. Two different routes were, so far, considered to generate ―in situ‖ hydrogen, allowing transformation of glycerol into 1,2propanediol: aqueous phase reforming (APR) and catalytic transfer hydrogenation (CTH). Aqueous phase reforming of glycerol, carried out over metallic catalysts at temperatures near 500 K, is a well known process and converts glycerol into H2 and CO2 [100]. Jacobs et al. [101] reported the direct conversion of glycerol to 1,2-PDO over a Pt/NaY catalyst carried out at 503 K, using the hydrogen generated from the glycerol APR process to accomplish the subsequent hydrogenolysis of the remaining glycerol to 1,2-PDO. A selectivity of 64% to 1,2-PDO was obtained at 85% conversion of glycerol. An admixture of Pt/Al2O3 and Ru/Al2O3 as catalyst for reforming and hydrogenolysis reactions was also reported for production of 1,2-PDO without external hydrogen addition [102]. The Ru-Pt admixture catalyst in the ratio (w/w) 1:1 showed a better performance at 493 K (50% conversion and 47% selectivity to 1,2-PDO) compared to that of the monometallic catalysts, suggesting a beneficial synergic effect between Pt and Ru in the mixture.

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Table 5. Selected hydrogenolysis results of aqueous glycerol to 1,2-PDO in absence of hydrogen Catalyst

2.7% Pt/NaY 5% Ru/Al2O3 + 5% Pt/Al2O3 (Ru/Pt = 1:1) Raney Ni 10% Pd/Fe2O3 7.7% Ni-28%Cu/ Al2O3 a

Glycerol Concetration (wt%) 20

PN2 (MPa)

T (K)

Time (h)

Conv (%)

Selectivity to Ref. 1,2-PDO (%)

0.1

503

15

85.4

64.0

101

10

1.4

493

6

50.2

47.2

102

10

0.1

453

1

100

43.0

103

a

0.5

453

24

100

94.0

104

4.5

493

24

60.4

64.6

105

4

b

4

2-propanol as solvent and hydrogen donor; propanol = 1:1.5.

b

2-propanol added as hydrogen donor – glycerol/2-

Yin et al [103] developed an one-pot aqueous phase process for production of 1,2-PDO and ethylene glycol, using a commercial Raney Ni as catalyst under 0.1 MPa nitrogen pressure. A complete conversion of glycerol was obtained at 453 K, although the yield to 1,2PDO (43%) is lower than that to EG (55%).

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Catalytic transfer hydrogenation, involving transfer of hydrogen from a hydrogen donor molecule, as an alcohol, to glycerol, is an interesting process, since it allows using of the same solvent for the entire biodiesel process involving also the chemical valorization of glycerol. The Pd/Fe2O3 catalyst, prepared by co-precipitation, was found very active and selective in glycerol hydrogenolysis under inert atmosphere. The hydrogen derives from the dehydrogenation of the solvent (2-propanol or ethanol), promoted by supported palladium, itself reduced ―in situ‖ by alcohols [104]. A high selectivity to 1,2-PDO (94%) at total conversion of glycerol was obtained at 453 K. The feasibility of this process, in absence of hydrogen, is due to three factors: 1) the easy reducibility of palladium cations; 2) the dehydrogenation efficiency of palladium metal and 3) the use of an appropriate support. The transfer hydrogenation process of glycerol with 2-propanol was also investigated on Al2O3 supported Ni and/or Cu systems, prepared by the sol-gel method [105]. The best performance was obtained on the Ni-Cu/Al2O3 catalyst with a selectivity of 64.6% to 1,2-PDO and a conversion of 60.4% at 493 K.

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4.2. Glycerol to 1,3-Propanediol The development of an efficient hydrogenolysis process of glycerol to 1,3-PDO is particularly interesting because it makes the biodiesel process highly profitable. However, selective hydrogenolysis of glycerol to 1,3-PDO over heterogeneous catalysts is more difficult since generally 1,2-PDO is predominantly formed. In order that 1,3-PDO is formed the dehydration of glycerol to 3-hydroxypropanal has to come before the hydrogenation of the ensuing aldehyde to 1,3-PDO. Therefore the selective conversion of glycerol to 1,3-PDO requires a bi-functional catalyst, able to promote the coupled dehydration/hydrogenation reaction and to minimize the side products. Significant amounts of 1,3-PDO were obtained in presence of solid acids, such as tungstic acid, or transition metal oxides as co-catalysts and polar aprotic solvents, such as sulfolane or 1,3-dimethyl-2-imidazolidinone (DMI) (Table 6). In 2004 Chaminand et al. reported that addition of H2WO4 to Rh/C improves the selectivity to 1,3-PDO in the glycerol hydrogenolysis carried out in sulfolane [60]. The 1,3-PDO/1,2-PDO ratio of 2 was obtained at 453 K and 8 MPa H2 pressure with 12% selectivity to 1,3-PDO at 32% glycerol conversion. Several metal catalysts supported on WO3/ZrO2 were tested in glycerol hydrogenolysis, using DMI as solvent, considered more stable than sulfolane in the reaction conditions used [106]. A selectivity of 28.2% to 1,3-PDO at ~ 86% conversion was obtained at 443 K and 8 MPa H2 over Pt/WO3/ZrO2. The hydrogenolysis of glycerol, in aqueous phase, over the same catalyst was also investigated in a fixed-bed continuous reactor at low temperatures (383-413 K) and hydrogen pressure of 2-5 MPa [107]. At 403 K, 4 MPa H2 and 70% conversion a 45.6% selectivity to 1,3-PDO, with a 1,3-PDO/1,2-PDO ratio equal to 17.7, was achieved over a 3% Pt/WO3/ZrO2 catalyst.

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Francesco Mauriello, Maria Grazia Musolino and Rosario Pietropaolo Table 6. S Selected hydrogenolysis results of aqueous glycerol to 1,3-PDO

Catalyst

2%Pt/WO3/ZrO2

Glycerol Concetration (wt%)

56a

PH2 (MPa)

8

T (K)

Time (h)

Conv (%) Selectivity to 1,2-PDO (%)

Ref.

443

18

85.8

28.2

106

b

3%Pt/WO3/ZrO2

60

4

403

24

70.2

45.6

107

2%Pt/WO3/TiO2/ SiO2 10%CuH4SiW12O40/SiO2 4%Rh-ReOx/SiO2 (Re/Rh = 0.5) 4%Ir-ReOx/SiO2 (Re/Ir = 1)d 5.7%Pt 4.6%Re/C Pt – sulphated/ZrO2

10

5.5

453

12

15.3

50.5

108

100c

0.54

483

0.1 h-1

83.4

32.1

109

20

8

393

5

79.0

13.8

110

80

8

393

36

81.0

46.3

111

1

4

443

2

20.0

34.0

112

n.r.a

7.3

443

24

66.5

55.6

113

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n.r. = not reported; a DMI solvent; b continuous reactor; c neat glycerol; d sulphuric acid added.

An ionic mechanism, involving a proton transfer step, was proposed in order to explain the high selectivity to 1,3-PDO taking into account that the protonation of the secondary alcohol is preferred over the primary. SiO2 supported Pt/WO3/TiO2 systems were found more active and selective than Pt/WO3/TiO2 in glycerol hydrogenolysis to 1,3-PDO in water [108]. A selectivity of 50.5% (43.7% over Pt/WO3/TiO2) to 1,3-PDO at 15.3% conversion (7.5% over Pt/WO3/TiO2) was obtained at 453 K and 5.5 MPa H2. The better performance of Pt/WO3/TiO2/SiO2 was ascribed not only to the high surface area of SiO2, but also to the synergic effect between Pt/WO3/TiO2 and silica. Furthermore, it was found that the presence of TiO2 in the catalyst favours the dispersion of platinum whereas Brønsted acid sites, formed by addition of WO3 to the catalyst, are essential for 1,3-PDO formation. Huang et al. [109] reported conversion of glycerol into 1,3-PDO over silica supported copper and H4SiW12O40 in vapour phase without any added solvent. A good selectivity to 1,3-PDO (32.1% at 83.4% conversion) was obtained at 483 K and 0.54 MPa H2. Tomishige et al. [110] compared the catalytic performance of silica supported rhodium catalysts modified with various metals, such as Re, W, Mo, V, Zr, in glycerol hydrogenolysis at 393 K using water as solvent. Addition of Mo, W and Re to Rh/SiO2 remarkably improves conversion of glycerol with respect to the monometallic system. In particular, modification with Re oxides gives the highest conversion and selectivity to 1,3-PDO, although 1,2-PDO is produced in greater amount. Characterization data of Rh-ReOx/SiO2 indicate that the Re species is present as Re7+on the calcined catalyst and, after reduction under hydrogen, the average oxidation state is Re2.5+. In particular, EXAFS analysis suggests that the structure includes ReOx clusters attached to surfacial Rh metal particles. The synergic effect between ReOx and Rh promotes glycerol hydrogenolysis. A higher selectivity to 1,3-PDO (46% at 81% conversion) was obtained in presence of the Ir-ReOx/SiO2, indicating a much more noticeable additive effect of rhenium over iridium than over the Rh catalyst [111]. However, sulphuric acid is

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necessary, in the hydrogenolysis reaction, in order to increase the low catalytic activity of iridium. Bimetallic Pt-Re nanoparticles, supported on activated carbon, were found more active and selective towards production of 1,3-PDO than the analogous monometallic systems [112]. On Pt-Re/C bimetallic and highly dispersed particles on the carbon support were revealed by microstructural characterization. Treatment, under hydrogen, of the catalyst at higher temperatures results in a better atomic mixing of Pt and Re components, without decreasing the dispersion of the metals, and affords a better catalytic performance. At 443 K and 4 MPa H2 pressure a selectivity of 34% to 1,3-PDO was found. An effective catalyst for the selective conversion of glycerol to 1,3-propanediol was obtained by depositing the platinum on sulphated zirconia, containing super acidic sites [113]. Platinun and sulphate ions are stabilized into the more active tetragonal zirconia phase. Deposition of Pt favours adsorption of hydrogen and hydrogen spillover on the sulphated zirconia surface, providing more Brønsted acid sites. Large amount of Brønsted acid sites promotes removal of the hydroxyl group from the secondary carbon of glycerol, driving preferentially the reaction to 1,3-PDO. A selectivity of 83.6% to 1,3-PDO and a conversion of 66.5% were achieved using DMI as solvent. However, use of metals different than Pt, such as Ru, Ni, Cu, or substitution of sulphate ions with heteropolyacids, such as silicotungstic acid or posphotungstic acid, lower the yield to 1,3-PDO.

5. INSIGHT THE REACTION MECHANISM OF GLYCEROL HYDROGENOLYSIS

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5.1. Critical Approach to the Hydrogenolysis Mechanism of Glycerol There is an imporant difference between hydrocarbons and polyols in valuable chemicals production. The lasts are highly functionalized and, therefore, only a short list of compounds may be obtained. In particular, the hydrogenolysis of glycerol gives, in principle, three main different compounds, 1,2-propanediol, 1,3-propanediol and ethylene glycol (Scheme 1), besides some intermediates such as 1-hydroxyacetone and 3-hydroxypropanal or degradation products (ethyl alcohol, 1- and 2-propanol, methane and CO). The challenge in the industrialization of a given process is to make the desired reaction as more selective as possible. Therefore, achieving the necessary improvement in glycols selectivity, in a hydrogenolysis process, with a reasonable reaction rate, requires a deep understanding of the reaction mechanism, being convinced that such knowledge will bring about a more rational approach to the design of a suitable catalyst. In order to do this we need to realize that hydrogenolysis, in principle, is a simple reaction in wich the hydrogen promoted C-OH bond breaking is accompained by C-H bond formation. However the way to accomplish this result may be different, depending on the desired product, and delicate factors, both energetic and mechanistic, have to be taken into account in order to tailor, as thoroughly as possible, an appropriate catalyst. Therefore, for a careful assessment of the argument, we consider fundamental to recall: Basic and surface chemistry concepts relative to alcohols and polyols reactions. Thermodynamic variations involved in glycerol hydrogenolysis reactions. Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

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A. Basic and Surface Chemistry Concepts Relative to Alcohols and Polyols Reactions Three main general reactions of alcohols have to be accounted in order to understand the complex mechanism involved in hydrogenolysis of glycerol: Dehydration Dehydrogenation Hydrogenolysis of alcoholic groups.

i. The dehydration of alcohols is an acid catalyzed reaction, that occurs through the E1 mechanism (Scheme 3), previously involving hydroxyl protonation followed by water elimination.

Scheme 3. E1 mechanism relative to the dehydration of alcohols.

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The ensuing carbocation looses a proton affording an alkene: since a carbocation is formed, as intermediate, the E1 water elimination is favoured by polar solvents. The feasibility of the reaction, reported in Scheme 3, follows the order: primary alcohol < secondary alcohol < tertiary alcohol.

Figure1. Relative stability of carboradicals and carbocations.

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Figure 1 reports the potential energy of differently substituted organic radicals and carbocations that follows the order:

As expected, the reactivity of alcohols parallels the stability order of carbocations: tertiary > secondary > primary > CH3+. Interestingly, the carbocations stability differences are much higher than those of carboradicals. The stability trend observed stems from the general physical principle that as more the charge is dispersed as more stable the system is. Dehydration of diols or polyols is an easy reaction and first affords an enolic species that fast rearranges giving a carbonyl molecule (Scheme 4). Interestingly, if an organic molecule contains both primary and secondary alcoholic groups, the secondary is preferentially protonated in accord with the higher stability of the relative carbocation.

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Scheme 4. Dehydration of vicinal diols.

ii. Dehydrogenation reactions of alcohols are typically endothermic, need a metal catalyst to be performed and conversions are equilibrium controlled. In addition low H2 pressures may enhance deactivation of the catalyst whereas the selectivity towards carbonyls may suffer the negative effect of the competitive dehydration reaction. Cu-chromite, Cu/ZnO and Al2O3 are well known active catalysts [114-116] and Pt, Ni and Ru have also shown high activity for alcohols dehydrogenation in vapor phase [117-119]. The same reaction also occurs at lower temperatures (363 K) in liquid phase, in presence of styrene as hydrogen acceptor, following the order of reactivity: 2-octanol > cyclohexanol > benzyl alcohol > 1-octanol [120] in good agreement with other results [121, 122]. Again, secondary alcohols are largely more active than primary alcohols. The heat of adsorption value of 2-propanol on a clean Pt(III) is relatively low (H≅ -41.8 kJmol-1) and is close to that of acetone [123]. Similar low values are expected for other metals. However there is actually a significant evidence, in the surface science literature, that an alkoxide species is formed during the decomposition of various alcohols on single crystals. In other words, the O-H bonds of alcohols breaks, upon adsorption, already at low temperature, on metal surfaces producing an adsorbed alkoxide RO- and surface hydrogen [124]. The C-O bond of alcohols is generally retained on the mid to late transition metals such as Cu [125, 126], Pt [127, 128], Pd [129, 130] Rh [124, 131] and Ru [132, 133]. These metals are also particularly suitable for hydrogenolysis reactions of glycerol. The metal surface alkoxide formation may have important mechanistic implications driving the selectivity of the ensuing reactions. In particular, the C-H bond adjacent to the

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oxygen atom, the so called -C-H bond, is more readily broken than others in surfacial alkoxides. This is a direct consequence of the withdrawal of electron density from the carbon bonded to oxygen such that the C-H bond is weaker than others, so affording a carbonyl compound. Interestingly, the OH bond is several kJ stronger than the C-H one [123] and the easier OH breaking is probably due to the orientation of the hydroxyl group on the surface. By comparing these results with reactivity data [120-122] it is possible to infer that surfacial secondary alkoxides are better formed than the primary analogous. iii. C-OH hydrogenolysis of aromatic alcohols is a well known metal catalyzed reaction and occurs both by reacting H2 with alcohols [134-136] or as transfer hydrogenolysis [137]. The OH substitution occurs through one of two possible steps: a) a SN2 type mechanism in which the surface hydrogen displaces the hydroxyl group from the carbon atom or b) a more stepwise mechanism starting with the dissociation of the C-O bond and temporary bonding of the OH group and hydrocarbon residue on the metal surface followed by fast transfer of the activated hydrogen from the surfacial metal-H to the unsaturated carbon (SN1 type mechanism). No similar reaction was so far reported for aliphatic alcohols and this may be attributed to two main factors: a) the larger C-OH dissociation energy of aliphatic than aromatic analogous (326 kJmol-1 for benzyl alcohol compared to 380 kJmol-1 for 1- and 2-propanol; b) the energetically much higher LUMO orbital in aliphatic than in aromatic alcohols.

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B. Thermodynamic Variations Involved in Glycerol Hydrogenolysis Reaction It is well ascertained that the reaction from glycerol to diols is quite complex and implies several steps. Therefore the knowledge of thermodynamic variations involved is fundamental to understand the sequences of reactions forming the catalytic process. The relative H° variations at 473 K are reported in Scheme 5.

Scheme 5. Thermodynamic data, at 473K, for sequential reactions involved in hydrogenolysis of glycerol to diols. Values were obtained from elaboration of data reported in ref. [138] and [139].

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Several features are to be considered: 1. Direct hydrogenolysis of glycerol to glycols follows the order: H°(1,2-PDO) < H°(1,3PDO) < H°(EG) in accord with the relative stability of the reaction products involved [138]. 2. Dehydration of glycerol to acetol is thermodynamically more favoured than that to 3hydroxypropanal being acetol a more stable intermediate. On the other hand, since dehydration products (carbonyl intermediate and water) are more strongly adsorbed than glycerol, dehydration processes become more exothermic once they are carried out on a metal catalyst surface [139]. 3. H° values relatives to hydrogenation of acetol and 3-hydroxypropanal, as expected, are very close to those of acetone to 2-propanol (H°= -57 kJmol-1) and propanal to 1-propanol (H°= -69 kJmol-1) [63].

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5.2. The Hydrogenolysis Mechanism of Glycerol: General Consideration The rate of a given reaction mainly depends on several variables: temperature, pressure, nature of metal or support, surface area and particles size. There is, actually, a large amount of data that clearly demonstrate that 1,2-PDO is the main hydrogenolysis product formed, with high selectivity, when supported monometallic or bimetallic catalysts are used, sometimes added with thermally stable solid acids, possessing moderate acid sites. A low selectivity to 1,3-PDO is also generally detected in these cases. On the other hand, conversion of glycerol to 1,3-PDO is difficult: only in few cases, a selectivity to 1,3-PDO higher than that to 1,2-PDO was reported and, in this case, the system contains a metal (mainly platinum) supported on WO3/ZrO2 or a superacid system such as platinum supported on sulfonated zirconia [113]. Therefore different mechanisms leading to 1,2- or 1,3-PDO are operating. It follows that it is appropriate to discuss separately the steps involved in the one or the other reaction taking into account all previous considerations, involving both basic chemistry concepts and thermodynamic aspects of every single step.

5.2.1. The Mechanism to 1,2-Propanediol Several mechanisms relative to 1,2-PDO formation have, so far, been suggested and a sinthetic rappresentation of them was also reported [53]. However, a convincing route leading to 1,2-PDO needs to take into account several experimental findings: a) The reaction occurs in presence of monometallic or bimetallic supported catalysts often in absence of Brønsted or Lewis acids or bases. b) The selectivity to 1,2-PDO is generally high, whereas 1,3-PDO is obtained in low (or very low) amount. c) Acetol was often detected as intermediate. d) No 1,3-dihydroxyacetone was ever detected. Furthermore, the most stable conformers of glycerol, in gas phase, are intramolecularly hydrogen-bonded [139]. Therefore, adsorption of the polyol molecule on a metal needs to break H bonds and to form new bonds with the surface. Experimental evidences suggest that

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the secondary alcoholic group is the one indicated to preferentially link a surfacial metal particle. This is not unexpected and confirms previously reported observations [120-122]. The subsequent O-H breaking affords metal-alkoxide and metal-H bonds:

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The surfacial secondary alkoxide (C) becomes the key to interpret further steps leading to 1,2-PDO [62]. The metal-alkoxide bond formation, in fact, can make it easier to break the adjacent C-OH bond through the so-called neighboring group partecipation, involving a Pd-H species. The phenomenon is well known in organic and coordination chemistry [140] and can be transferred to catalytic surfaces. In the specific case, such an event may occur throug one of two possible ways: i. Interaction between a metal bonded hydrogen and a primary alcohol group leading to a vinylic alkoxide, which rapidly converts to a vinylic alcohol and then rearranges to acetol through a cheto-enol equilibrium:

ii. Substitution of the carbon-bonded OH group by an incoming hydrogen to afford directly 1,2-PDO. However, since the LUMO orbital, located on the primary carbon, should be energetically high, a SN2 type mechanism seems disfavoured. Conversely, a SN1 type mechanism involving previous breaking of the C-OH bond followed by fast incoming of hydrogen appears more likely:

Accordingly, the vapour phase reaction of glycerol over copper catalysts, in absence of hydrogen, produces acetol through a previous breaking of the primary C-OH group [61]. No 1,3-dihydroxyacetone was detected in this case, suggesting that the dehydration occurs prefentially than the dehydrogenation. Further reaction of 1-hydroxyacetone with H2 affords, through an easy carbonyl reduction, 1,2-PDO:

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The proposed mechanism well explains: (i) why the selectivity to 1,3-PDO is generally low: formation of a primary alkoxide is less favoured, on a metal surface, than the secondary analogous and, furthermore, 3-hydroxypropanal is less stable than 1-hydroxyacetone; (ii) why a combination of Ru/C and thermally stable solid acids, Amberlyst, Nb2O5 and phosphotungstic acid supported on ZrO2, possessing moderate acid sites, increase both conversion and selectivity to 1,2-PDO [49, 83, 85]: in this case dehydration to hydroxyacetone is facilitated and probably occurs with an ionic mechanism [61].

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5.2.2. The Mechanism to 1,3-Propanediol There is a common feature that gets together catalytic systems able to give a selectivity to 1,3-PDO higher than that to 1,2-PDO with a moderate to good conversion: all contain strong Brønsted acids. This is, in fact, the case of Pt-sulphated zirconia [113] (conv. = 66.5%; yield to 1,3-PDO = 55.6%; 1,3-PDO/1,2PDO = 19.17) or Pt/WO3/ZrO2 [107] (conv. = 70.2%; yield to 1,3-PDO = 32.0%; 1,3-PDO/1,2PDO = 17.7). The behaviour of Pt/WO3/ZrO2 is, on this regard, paradigmatic [107]. At low temperature (403 K) aqueous glycerol affords 1,3-PDO with large conversion and selectivity since activated hydrogen on platinum can spillover onto the WO3/ZrO2 surface. Here, through a dismutation reaction, an activated hydrogen atom, on one hand, transfers an electron to a support Lewis acidic site forming a proton and, on the other hand, accepts an electron to form a hydride stabilized by acid Lewis sites:

The mechanism to 1,3-PDO becomes, in this case, quite straightforward:

and reflects the easier proton attack on the secondary alcohol since the ensuing secondary carbocation is, by far, more stabilized than the primary.

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Francesco Mauriello, Maria Grazia Musolino and Rosario Pietropaolo The route to 1,3-PDO shows however, some drawbacks: 1. the necessity of both Brønsted acids and active metals may favour also the route to 1,2-PDO (promoted by transition metals) so limiting the selectivity to 1,3-PDO. 2. The presence of Brønsted acids, into the reaction system, may favour further transformation of the intermediate 3-hydroxypropanal to acrolein.

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CONCLUSION The major glycerol hydrogenolysis products are 1,2- and 1,3-Propanediol. With respect to enzymatic catalysis, it was shown above that among all the possible bacterial species to carry out the glycerol conversion the most appropriate are Klebsiella Pneumoniae and Clostridium Butyricum. The biotechnological method seems to be an attractive alternative to the traditional production of 1,3-PDO; however, their pathogenic properties strongly limitates their industrial applicability. In the course of the years, a limited number of processes for the catalytic hydrogenolysis of glycerol, in the presence of a homogeneous catalyst, were presented. In general, the glycerol conversion route consists of an acid-catalyzed dehydration, followed by a metal complex-catalyzed hydrogenation. However, homogeneous production of 1,3-PDO is difficult since glycerol is easily dehydrated directly to acrolein. Different supported metals, mainly Cu but also Ru, Rh, Pt and Pd (suitably prepared) afford, with higher conversion and selectivity, 1,2-PDO and a lot of papers have, so far, appeared on the argument. Supported Ru catalysts, often afford, through carbon-carbon hydrogenolysis, appreciable amounts of ethylene glycol that, in the context, has to be considered a waste product. The many results obtained make possible the industrialization of the process from glycerol to 1,2-PDO. Neverthless, the true challenge is the preparation, with high conversion and yield, of 1,3-PDO (the most important industrial product) for its interesting applications. Papers reporting yields to 1,3-PDO higher than 1,2-PDO are very limited and much more research is welcome. The ideal catalyst should have several features: 1. it must include surely a strong Brønsted acid; 2. the catalytic bed should be made up so that further reaction to acrolein should be limited (continuous flow); 3. partecipation of the metal should occur, as much as possible, in the second stage (hydrogenation of 3-hydroxypropanal).

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In: Glycerol: Production, Structure and Applications Editors: M. De Santos Silva and P. Costa Ferreira

ISBN: 978-1-62081-120-7 © 2012 Nova Science Publishers, Inc.

Chapter 3

AQUEOUS-PHASE REFORMING OF GLYCEROL FOR HYDROGEN PRODUCTION Robinson L. Manfro and Mariana M. V. M. Souza* Laboratory of Hydrogen Technology, School of Chemistry, Federal University of Rio de Janeiro (UFRJ), Centro de Tecnologia, Cidade Universitária, Rio de Janeiro/RJ, Brazil

ABSTRACT

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The demand and supply of glycerin in the world market was in equilibrium by the end of the 1990s. With the production of biofuels, especially biodiesel, this equilibrium has been completely changed. Biodiesel is produced by the transesterification of vegetable oils and animal fats, and glycerol is a byproduct of this reaction. One ton of biodiesel yields about 110 kg of crude glycerol or 100 kg of pure glycerol. In Brazil, according to National Agency of Petroleum, Natural Gas and Biofuels (ANP), the production of biodiesel (B100) in 2010 was approximately 2.4 million m3, generating approximately 240,000 m3 of glycerin, creating a surplus of glycerin in the Brazilian market. One of the promising ways to utilize this crude glycerol is to produce hydrogen by aqueous phase reforming. The aqueous-phase reforming (APR) reaction is able to transform oxygenated hydrocarbons, as glycerol, in a gas phase composed mainly by H2 and CO2 with low concentration of CO and CH4. For this reaction, nickel catalysts supported on aluminum oxide, zirconium oxide and cerium oxide were used. They were synthesized by three different methodologies: wet impregnation, coprecipitation and combustion. The characterization of catalysts showed that the morphological and structural properties are strongly influenced by the preparation methodology. The analysis of N2 physisorption showed that catalysts synthesized by combustion presented lower BET areas, considering that all samples are mesoporosus (average diameter pore of 20 to 500 Å) with type IV isotherms and H3 hysteresis. Temperature programmed reduction analysis displayed that catalysts supported on aluminum oxide present a reduction level lower than 100%, indicating there is no complete reduction of nickel oxide to metallic nickel. However, the reduction degree of the other catalysts has values higher than 100%, indicating an *

Corresponding author (Fax: 55-21-25627598; E-mail: [email protected]).

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Robinson L. Manfro and Mariana M.V. M. Souza additional reduction of the support, mainly in the case of the catalysts supported by cerium oxide. The APR of glycerol was performed in an autoclave reactor with 450 rpm of agitation, using 200 mg of catalyst, and 250 mL of aqueous solution of 1 or 10 wt.% glycerol at 250 and 270 °C. The gas phase is composed by H2, CO2, CO e CH4 gases, which were analyzed in a gas chromatograph. The results suggested that it is possible to achieve high concentrations of H2 at temperature around 250 ºC. The higher conversion obtained was 50%, using the Ni/Al2O3 catalyst prepared by impregnation. In the comparative tests with Pt catalysts, the Ni catalysts showed higher catalytic activity. Xray diffraction analysis displayed that nickel catalysts present sintering, resulting in an increase of the Ni crystal size after reaction.

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1. INTRODUCTION Fossil fuels are the main energy source used by humanity and by the use of this nonrenewable resource it has been possible to develop the industrial, transport, and agriculture sectors, among many other basic human needs. The search for alternative energy sources is increasing in the world motivated by the predictions that point to a progressive decrease in the production of fossil fuels. Besides the shortage, another problem associated with the use of fossil fuels is the continued increasing in emissions of pollutants, especially those related to global warming. These pollutant gases affect human health and also cause imbalances in the fauna and flora, such as acid rain. Therefore, there is a great need for alternative fuels that do not affect the environment [1]. Motivated by concerns about air pollution, safe energy production and climate change, the notion of "hydrogen economy" is moved beyond the area of the scientists and engineers, it is a matter of policy issues and discussed by business leaders. The interest in hydrogen, the simplest element and most abundant in the universe, is increasing due to technological advances in fuel cells, a potential successor of portable batteries in electronics, power stations and internal combustion engines [2]. Several alternative fuels such as hydrogen, ethanol and biodiesel are currently being exploited with the purpose to develop sustainable energy. For hydrogen to become a truly sustainable energy source, it should be promoted its production from renewable resources. However, over 95% of hydrogen produced today results from nonrenewable resources, mainly fossil fuels [3]. According to Kirtay [4], currently 48 % of hydrogen is produced from natural gas, 30 % from heavy oils and naphtha, 18 % coal and only 4% from water electrolysis, Figure 1 (A). The H2 can be used as fuel both in internal combustion engines and in fuel cells [5]. However, the main users of H2 are the fertilizer and petroleum industries, which consume 49 % and 37 %, respectively, of the H2 produced, Figure 1 (B) [6]. Recent studies have shown the possibility of production of hydrogen from aqueous-phase reforming (APR) of oxygenated organics derived from biomass, such as ethylene glycol, sorbitol and glycerol [7]. The hydrogen generation by APR from oxygenated organics has advantages of eliminating steps of vaporization of water and raw material to be reformed, significantly reducing energy demand in the process. In addition, hydrogen and carbon dioxide are the main products of this process, providing a reformed gas with low levels of carbon monoxide [8], with great potential for application in fuel cells.

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The biodiesel, an alternative fuel for diesel engines, is normally produced by reaction between a vegetable oil or animal fat with alcohol, such as ethanol or methanol, in the presence of catalysts. The transesterification reaction between the triglycerides of vegetable or animal origin with alcohol produces fatty acid ester (biodiesel) and glycerol (propane1,2,3-triol) [9]. One ton of biodiesel yields about 110 kg of crude glycerol or 100 kg of pure glycerol [3].

(A) 18% Coal 4% Electrolysis

48%

(B)

49% Ammonia

Natural Gas 8% Methanol

30% Heavy Oils and Naphtha

37% Petroleum Refinery

6% Other

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Figure 1. (A) Worldwide production of hydrogen by source, (B) Worldwide use of hydrogen per sector.

The use of glycerol in the APR has been aroused great interest in Brazil and worldwide, since glycerol is a byproduct of biodiesel production. Thus, the glycerin is a product that has a large potential market and with increasing biodiesel production, its supply has risen substantially and the price of crude glycerin has fallen to about U$ 0.11 kg-1 [10]. The hydrogen production by APR of sugars and alcohols occurs at relatively low temperatures (close to 230 °C) with heterogeneous catalysts [11]. Supported catalysts of Pt, Ru, Pd and Ni are the most used in the reforming of oxygenated organics, both APR and steam reforming, showing good catalytic activity and selectivity for hydrogen production. Undesired products can be formed due to the occurrence of parallel reactions by breaking of the C-O bonds, forming alcohols or organic acids. Thus, a good catalyst to produce hydrogen by reforming reactions has to be more active for cleavage of the C-C, O-H and C-H bonds rather than the C-O bonds and promote the removal of adsorbed carbon monoxide by the water gas shift reaction [7]. In this work, we have investigated APR of glycerol to produce hydrogen using nickel catalysts supported on Al2O3, ZrO2 and CeO2 prepared by three different methods: wet impregnation, combustion and coprecipitation. The effect of the preparation methodology on the catalytic activity for reforming reaction and hydrogen selectivity was investigated, correlating with structural and morphological properties of the catalysts.

2. AQUEOUS-PHASE REFORMING The reaction of aqueous-phase reforming (APR) of oxygenated organics has become an attractive alternative for hydrogen production due to several advantages over traditional methods. The main advantages are associated with energy savings because there is no need for vaporization of the reactants, in addition the reaction can be conducted at low temperatures, around 250 °C. The operation at low temperatures prevents parallel reactions of

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decomposition of oxygenated compounds and still favors the shift reaction, making it possible to generate hydrogen with low concentration of carbon monoxide in a single stage. Additionally, the oxygenated compounds used in the reaction are non flammable and non toxic, allowing them to be stored and handled safely. When purification of hydrogen produced is needed, this process is also favored because the reaction is conducted under pressure, typically in the range of 15 and 50 atm, and at this pressure the hydrogen can be purified by adsorption processes or by using membrane technology and carbon dioxide can be easily separated by CO2 sequestration [12]. Cortright et al. [11] presented the pioneering work where it was demonstrated that it is possible to produce hydrogen from sugars and alcohols by APR reaction at low temperatures and in a single reactor. The authors evaluated the APR of methanol, ethylene glycol, glycerol, sorbitol and glucose. The platinum catalyst supported on alumina showed high activity and good selectivity for hydrogen production from these compounds, but improvements are needed to make the process more profitable. New researches are needed to develop new catalysts with lower cost and higher activity in reactions at low temperatures, to minimize the undesirable effects of homogeneous decomposition reactions.

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2.1. Catalysts for Aqueous-Phase Reforming The generation of H2 and CO2 by APR at low temperatures is accompanied by challenges in terms of selectivity, because the reaction of H2 with CO2 or CO to form alkanes is highly favored at low temperatures. Thus, a good catalyst to produce H2 by APR has to be more active for the cleavage of the C-C, O-H and C-H bonds rather than the C-O bond and promote the removal of adsorbed carbon monoxide by the water-gas shift reaction [13]. Davda et al. [14] studied different metals of group VIII supported on silica, on APR of ethylene glycol at 210 °C. The Pt and Pd catalysts were the most active and selective for H2 production. The H2 selectivity decreased in the following order: Pd > Pt > Ni > Ru > Rh. An opposite trend was found for selectivity for alkane production, which increases in the following order: Pd < Pt < Ni < Ru ≈ Rh. It was observed that the rate of reforming of ethylene glycol, measured from the rate of CO2 production, was relatively high for Ni, Pt and Ru. Metals such as Rh and Ru showed a low selectivity for H2 and a high selectivity for alkanes. Although Pt, Ni and Ru catalysts have presented high activity in reforming of ethylene glycol, only Pt, Pd and Ni catalysts were selective for H2. Thus, the overall catalytic activity for aqueous-phase reforming of ethylene glycol of these metals supported on silica decreased in the following order: Pt ~ Ni > Ru > Rh ~ Pd > Ir [14]. Shabaker et al. [15] analyzed the influence of the support for Pt catalysts in the APR of ethylene glycol. The supports used were SiO2, ZrO2, carbon, TiO2, SiO2-Al2O3, CeO2, Al2O3 and ZnO. Amongst the evaluated catalysts, Pt supported in TiO2 presented higher activity for H2 production and also good activity was found for Pt supported in carbon and Al 2O3 and non-supported platinum. A moderate catalytic activity was obtained with the Pt/SiO2-Al2O3 and Pt/ZrO2 catalysts. Catalysts that showed low activity for production of hydrogen, such as Pt/CeO2, Pt/ZnO and Pt/SiO2, lead to the formation of larger amounts of two-carbon aldehydes and acids, particularly glycolaldehyde and glycolic acid. The appearance of these compounds may be related to initial dehydrogenation of ethylene glycol before breaking of

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the C–C bond. The Pt/ZnO catalyst showed higher selectivity toward the formation of glycolic and acetic acids than all other catalysts. Huber et al. [7] and Shabaker et al. [13] studied the effect of the addition of promoters on nickel catalysts for APR of sorbitol, glycerol and ethylene glycol, and the authors also analyzed the effect of addition of Sn in Ni-Raney catalyst. It was found that the Raney Ni-Sn catalyst showed good activity and selectivity for hydrogen production in the APR of oxygenated organics. Using an atomic ratio of Ni:Sn ratio of 14:1 the Raney Ni-Sn catalyst showed catalytic properties similar to the catalyst with 3 % of Pt supported on Al2O3. The addition of Sn significantly reduces the rate of methanation, increasing the selectivity to hydrogen. With intention to produce hydrogen from the APR using renewable resource as raw material, the kinetic of the reaction must be studied, optimizing the selectivity in hydrogen. The main products of the reaction are the carbon dioxide and hydrogen, and these are thermodynamically unstable in low temperatures [16-17]. Alkanes, especially methane, can be formed by the reaction of Fischer-Tropsch or methanation of H2 with CO or CO2. The catalytic activity of different metals for the C-C bond breaking during the reaction of ethane hydrogenolysis was studied by Sinflet and Yates [18]. The catalytic activity for the water-gas shift reaction of different metals supported in alumina was studied by Grenoble et al. [19]. Vannice et al. [20] studied the reaction of methanation catalyzed by different metals supported in silica. In these studies, it was observed that the metals Ru, Ir, Ni and Rh present high catalytic activity to break C-C bonds. However, as cited previously, an effective catalyst for reforming of oxygenated organics does not only have to be active in breaking C-C bonds, but must also be active for the water-gas shift reaction to remove the CO adsorbed in the surface of the metal [14]. Taking into account the activity of the metals for shift reaction, Cu exhibits the highest activity among all the metals, but this metal is not active to break C-C bonds and thus it is not appropriate for APR reaction. Pt, Ru and Ni show considerable activity for the shift reaction. Finally, to obtain high selectivity for hydrogen, the catalyst should inhibit the undesirable reactions, such as CO methanation and synthesis of FischerTropsch. Ru, Rh and Ni show the highest rate of methanation, whereas Pt, Ir and Pd present lower catalytic activity. Comparing the catalytic activity of all metals studied, it can be concluded that Pt and Pd would be the most appropriate metals, having good catalytic activity and selectivity for hydrogen from the reforming of oxygenated organics. Ni presents high rate of methanation, but, together with the Ru, has the highest catalytic activity to break C-C bonds and good activity for shift reaction. Thus, Ni catalysts become attractive for the APR, with the advantage of being cheaper than noble metals [11]. Iriondo et al. [21] examined the effect of addition of promoters such as Mg, Zr, Ce and La on Ni catalysts supported on Al2O3 in the reactions of steam reforming and APR of glycerol. The catalytic tests were carried out with a glycerol solution of 1 wt%. APR reactions were performed at a pressure of 30 atm, 225 °C and weight hourly space velocity (WHSV) of 1.25 h-1. The Ni/Al2O3 catalysts doped with Mg, Zr, Ce and La were called Ni/A-M, Ni/A-Z, Ni/A-C and Ni/A-L, respectively. All catalysts showed a pronounced deactivation in the initial hours of APR of glycerol. It was observed that the conversion of glycerol decreased in the following order: Ni/A–L > Ni/A–C ≈ Ni/A–Z > Ni/A ≈ Ni/A–M. At the end of 30 h of reaction, all catalysts showed a conversion lower than 3%. When the initial composition of gaseous products is compared, catalysts promoted with Ce and La exhibited relatively higher rates of H2 production. The catalyst promoted with Mg presented the highest molar

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composition of hydrogen, though with very low conversion of glycerol. In all catalysts the gaseous products are saturated with water and do not contain significant amounts of CO. Analysis of the liquid effluent from APR of glycerol showed that propylene glycol and ethylene glycol are the main by-products formed in reaction for all catalysts.

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2.2. Reaction Mechanism Cortright et al. [11] proposed a sequence of chemical reactions involved in the formation of H2, CO2 and alkanes, from APR of oxygenated organics using Pt-based catalysts. In their proposal the reagent first undergoes dehydrogenation steps on the metal surface, producing adsorbed intermediates, before the cleavage of C-C or C-O bonds. The activation energies required to break the bonds of O-H and C-H are similar [22], however, the Pt-C bond is more stable than Pt-O bonds, and thus intermediate species are preferentially adsorbed to metal by Pt-C bond. Subsequent cleavage of C-C bonds leads to formation of CO and H2. The CO produced reacts with water to form CO2 and H2 by the shift reaction. The reactions of CO and/or CO2 with H2 produce alkanes and water by methanation and Fischer-Tropsch reaction. Alkanes can also be formed by the cleavage of C-O bonds followed by hydrogenation. These H2 consuming reactions represent a challenge to improve the selectivity of H2. The cleavage of C-O bonds, through the dehydration reaction catalyzed by acid sites of the catalytic support or catalyzed by protons from aqueous solution, followed by hydrogenation reaction, are also challenges for selectivity. Organic acids may be formed by dehydrogenation followed by a rearrangement in solution, and these organic acids give rise to alkanes [11]. Shabaker et al. [13] presented a reaction mechanism for APR of ethylene glycol. Initially, ethylene glycol undergoes dehydrogenation to form intermediate species, and then a rapid cleavage of the C-C bonds occurs to form adsorbed CO on the active site. This prevents the unwanted parallel reactions that are conducted by the cleavage of C-O bonds followed by dehydration, leading to the formation of ethanol, which can be transformed into methane and ethane. The adsorbed CO on active site forms CO2 and H2 by shift reaction, or can form methane by the methanation reaction. Soares et al. [23] proposed a reaction scheme for the APR of oxygenated organic compounds to obtain a gaseous mixture concentrated of hydrogen. The scheme consists of two reaction steps; in the first step the oxygenated organic component generates a mixture of CO and H2, according to equation 1: CnHmOk + (n-k)H2O → nCO + (n + m/2 – k)H2

(1)

The second step is the water-gas shift reaction, where CO formed in the equation 1 reacts with H2O to form CO2 and H2, equation 2: nCO + nH2O ↔ nCO2 + nH2

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3. EXPERIMENTAL 3.1. Synthesis of the Catalysts

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Three different supports were used to prepare the catalysts: aluminum oxide (Al2O3), zirconium oxide (ZrO2) and cerium oxide (CeO2). The catalysts were prepared by three different synthesis method for each support: combustion, coprecipitation and wet impregnation. It resulted in a total of nine catalysts, and all catalysts were prepared with a nominal content of 20 wt.% of nickel oxide (NiO). Two catalysts with 2 wt.% of Pt, supported on Al2O3 and ZrO2, were prepared by dry impregnation method to compare with Ni catalysts. Table 1 shows the nomenclature of the catalysts and the preparation methods used.

3.1.1. Combustion Method The combustion method is based on a redox reaction between a metal nitrate, acting as an oxidant, and an organic fuel such as urea, playing the role of reductant. For preparing the catalysts NiCe-Com, NiAl-Com e NiZr-Com it was used cerium nitrate (Ce(NO3)3.6H2O), aluminum nitrate (Al(NO3)3.9H2O), zirconium nitrate (ZrO(NO3) 2.6H2O) and nickel nitrate (Ni(NO3)2.6H2O), in specific amounts to obtain a catalyst with NiO content of 20 wt.%. The calculation of the amount of urea to be used in the preparation of these catalysts is based on the theory of propellant, according to the following expression:  ni  vi  0 , where ni is the number of moles of nitrates and urea, and vi is the oxidation number of the respective reagents, which is calculated according to the valence of their elements. Thus, it was possible to calculate the stoichiometric amount of urea to be used in the reactions [2426]. Table 2 shows the oxidation numbers for the reagents used. The nitrates were dissolved in deionized water, heated to 150 °C and in sequence urea (CO(NH2)2) was added in stoichiometric amount. The solution remained on a hot plate for a partial evaporation of water, forming a viscous gel-like solution. Then, it was introduced in a furnace heated at 600 °C for combustion. The combustion reaction is highly exothermic, it is possible to observe the formation of flames on the solution (Figure 2). Table 1. Nomenclature and preparation methods used Catalyst Ni/Al2O3 Ni/CeO2 Ni/ZrO2 Ni/Al2O3 Ni/CeO2 Ni/ZrO2 Ni/Al2O3 Ni/CeO2 Ni/ZrO2 Pt/Al2O3 Pt/ZrO2

Nomenclature NiAl-Com NiCe-Com NiZr-Com NiAl-CP NiCe-CP NiZr-CP NiAl-Imp NiCe-Imp NiZr-Imp PtAl PtZr

Method of Preparation Combustion

Coprecipitation

Wet impregnation Dry impregnation

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Robinson L. Manfro and Mariana M.V. M. Souza Table 2. Oxidation number of the reagents used Reagents Ni(NO3)2 · 6H2O Al(NO3)3 · 9H2O ZrO(NO3)2 · 6H2O Ce(NO3)3 · 6H2O CO(NH2)2

Oxidation number -10 -15 -10 -15 +6

Figure 2. Synthesis of NiAl-Com sample: combustion of the reactants (in the left), and the product obtained after the complete combustion of the reactants (in the right). Nickel nitrate

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Aluminum nitrate

Cerium nitrate

Solubilization Urea

Zirconium nitrate

Evaporation 150 °C

Furnace 600 °C

Calcination 3 hs, 700 °C

NiAl-Com

NiCe-Com

NiZr-Com

Figure 3. Schematic representation of the combustion synthesis of the catalysts.

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The product formed was ground to obtain a powder, which was calcined at 700 °C for 3 h with a heating rate of 10 °C.min-1 under air flow (60 mL.min-1). Figure 3 shows a schematic diagram representing the synthesis of combustion.

3.1.2. Coprecipitation Method The methodology used was adapted from Zhang et al [27] and Li et al. [28] to synthesize NiAl-CP and NiZr-CP catalysts, using as precipitating agent ammonium carbonate ((NH4)2CO3). NiCe-CP catalyst was synthesized using sodium carbonate (Na2CO3) as precipitating agent, with methodology adapted from Li et al. [29]. Each solution containing the corresponding nitrates was heated up to 60 °C on a hot plate, followed by dropwise addition of 2M precipitant solution under strong agitation. With addition of ammonium carbonate the pH increased to 9 and then the temperature was elevated up to 90 °C for 1 h. In the preparation of NiCe-CP catalyst the addition of sodium carbonate to the solution of nitrates was done at 80 °C, remaining for 1 h after reaching pH 9. It was followed by vacuum filtration of the solids obtained and washing using deionized water at room temperature for NiAl-CP and CP-NiZr and hot water for NiCe-CP. The filtered solids were dried overnight at 110 °C, in sequence they were ground and calcined at 500 °C for 3 h at a heating rate of 10 °C.min-1 under air flow (60 mL.min-1). Figure 4 shows schematically the synthesis of the catalysts prepared by coprecipitation method.

Nickel nitrate

Precipitating agent

Nitrate Solution

Coprecipitation pH = 9

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Aluminum nitrate

Cerium nitrate

Zirconium nitrate

Washing and filtration

1 h – 90 °C

Drying at 110 °C

Calcination 3 hs, 500 °C

NiAl-CP

NiCe-CP

NiZr-CP

Figure 4. Schematic representation of the coprecipitation synthesis of the catalysts. Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

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3.1.3. Wet Impregnation Method The NiAl-Imp and NiZr-Imp catalysts were prepared using γ-Al2O3 and ZrO2 supports (Saint-Gobain NorPro), with specific surface area (BET method) of 260 m2.g-1 and 70 m2.g-1, respectively. These supports were first calcined for 16 h at 500 °C to remove water and possible undesirable organic materials, since they were received in the form of pellets. The CeO2 support was prepared from the cerium nitrate by heating in furnace at 250 °C for 1 h to decompose part of the nitrate and eliminate water, followed by calcination at 500 °C for 3 h with a heating rate of 10 °C.min-1 under air flow (60 mL.min-1). An aqueous solution of nickel nitrate was added to the support to be impregnated in a rotary evaporator and kept in rotation for 1 h for homogenization of the mixture. Then the mixture was heated to 80 °C for removal of the water under vacuum. After completed evaporation, the solid obtained was dried overnight at 110 °C, ground and calcined at 500 °C for 3 h with a heating rate of 10 °C.min-1 under air flow (60 mL.min-1). Figure 5 represents the scheme of the synthesis process by wet impregnation method.

Cerium nitrate

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Furnace 1 h, 250 °C

Nickel nitrate

Calcination 16 hs, 500 °C

Homogenization 1h

Aluminum nitrate

Evaporation 80 °C

Zirconium nitrate

Drying at 110 °C

Calcination 3 hs, 500 °C

NiAl-Imp

NiCe-Imp

NiZr-Imp

Figure 5. Schematic representation of the wet impregnation synthesis of the catalysts.

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3.1.4. Dry Impregnation Method The PtAl and PtZr catalysts were prepared by dry impregnation using γ-Al2O3 and ZrO2 supports (Saint-Gobain NorPro). It was used a solution of chloroplatinic acid (H2PtCl6) (Umicore) to prepare catalysts with 2 wt.% of Pt. This solution was dripped slowly in the support and continuously homogenized. The maximum volume to be dripped was always inferior to the pore volume of the support, which was determined by fisisorption of nitrogen. After this step the samples were dried overnight at 110 °C and calcined at 500 °C for 3 h with a heating rate of 10 °C.min-1 under air flow (60 mL.min-1). Figure 6 represents the scheme of the synthesis process by dry impregnation method. Calcination 16 hs, 500 °C

Determination of pore volume. Solution of platinum

γ-Al2O3 Impregnated material ZrO2 Drying at 110 °C

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Calcination 3 hs, 500 °C

PtAl

PtZr

Figure 6. Schematic representation of the dry impregnation synthesis of the Pt catalysts.

3.2. Characterization of Catalysts The chemical composition of the catalysts was determined by X-ray fluorescence (XRF) using a Rigaku (RIX 3100) spectrometer. X-ray powder diffraction (XRD) patterns were recorded in a Rigaku Miniflex II X-ray diffractometer equipped with a graphite monochromator using CuKα radiation (30 kV and 15 mA). The measurements were carried out with steps of 0.05° using a counting time of 1 second per step and over the 2θ range of 10° and 90º. The textural properties of the catalysts were determined by N2 adsorption-desorption at 196 °C in a Micromeritcs ASAP 2000. The specific area was obtained using the BET method and pore volume and diameter were obtained by BJH method. Prior to the analysis the samples were outgassed for 24 h at 200 °C.

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The reducibility of the catalysts was analyzed by temperature programmed reduction (TPR), carried out in a microflow reactor operating at atmospheric pressure. The samples were firstly dehydrated at 150 °C under flowing Ar before the reduction. A mixture of 1.59 % H2/Ar flowed at 30 mL.min-1 through the sample, raising the temperature at a heating rate of 10 °C.min-1 up to 1000 °C. The outflowing gases were detected by thermal conductivity detector (TCD). Analysis of diffuse reflectance UV-Vis spectroscopy (DRS-UV-Vis) was performed using a Varian Cary 5000 spectrophotometer with diffuse reflectance accessory (Harrick), in the range of 200 - 800 nm. The spectra were obtained at room temperature with the calcined sample without any type of pre-treatment. The contribution of the support was subtracted from the analysis. The function of the Schuster-Kubelka-Munk was used to express the results F(R∞), where R∞ corresponds to the reflectance of the samples without the contribution of support.

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3.3. Catalytic Tests The reactions of APR of glycerol were carried out in an autoclave batch reactor of 600 mL, with 250 mL of aqueous solution of 1 or 10 vol.% glycerol and 450 rpm of agitation. Before the reaction, the reactor was purged with He to remove air inside. The catalytic tests were performed at 250 °C and 270 °C, resulting in autogeneous pressures of 37 and 52 atm, respectively. The catalysts were reduced ex situ under 75 mL.min-1 of 20 % H2/N2 using a heating rate of 10 °C.min-1. Table 3 presents the final temperatures of reduction of the different catalysts. These temperatures were chosen based on TPR results. Gas products were collected every hour and analyzed online by gas chromatography (GC-1000), equipped with a Hayesep D column and TCD. The products detected in the gas phase were H2, CH4, CO2 and CO. The molar fractions of these products were calculated without considering water. In the liquid phase only glycerol was quantified by a colorimetric method, using an enzymatic kit for triglycerides and a spectrophotometer. Conversion was calculated based on the moles of glycerol in the feed. Table 3. Final temperature of reduction used for the different catalysts Catalyst NiAl-Com NiCe-Com NiZr-Com NiAl-CP NiCe-CP NiZr-CP NiAl-Imp NiCe-Imp NiZr-Imp PtAl PtZr

Temperature (°C) 1000 700 900 1000 700 900 900 700 650 500 500

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4. RESULTS AND DISCUSSIONS The synthesis and physicochemical characterization of the catalysts will be presented, with the exception of platinum catalysts, whose characterizations were presented by Souza [30]. The results of APR of glycerol will be presented for all catalysts, where platinum catalysts were used as reference.

4.1. Chemical Composition The chemical composition of the synthesized catalysts was determined by X-ray fluorescence (XRF) and is presented in Table 4. The nominal composition of the catalysts was 20 wt% NiO. As expected, the chemical composition of the catalysts was very close to the nominal loadings and small differences could be related to losses during preparation and/or due to impurities in the metallic precursors. Small quantity of impurities such as silica (SiO2) for Al2O3 catalysts, hafnium (HfO2) and sulfur trioxide (SO3) for ZrO2 catalysts were observed. For CeO2 catalysts it was not observed the presence of impurities. The agreement between the nominal and real values shows the good performance of the synthesis techniques used. The technique of coprecipitation showed the highest possibility of loading error among all the synthesis methods used due to the critical stage of the precipitation of metallic precursors. The correct choice of precipitating agent and pH of synthesis is essential for a good performance of this method.

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Table 4. Results of chemical composition obtained by XRF analysis

Catalyst NiAl-Com NiZr-Com NiCe-Com NiAl-CP NiZr-CP NiCe-CP NiAl-Imp NiZr-Imp NiCe-Imp

NiO 21.7 20.5 18.6 22.3 16.5 20.6 22.1 20.2 23.7

Mass content of oxides (%) MO Impurity 78.3 0.0 77.3 2.2 81.4 0.0 77.5 0.2 81.1 2.4 79.4 0.0 77.7 0.2 77.7 2.1 76.3 0.0

MO → CeO2 ou ZrO2 ou Al2O3.

4.2. Textural Analysis The textural analysis of the catalysts was performed by physisorption of N2. The specific surface area was calculated according to Brunauer-Emmett-Teller (BET) method. The volume and average pore diameters were determined by Barrett-Joyner-Halenda (BJH) method, from

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the N2 adsorption data, because it represents the better approximation of the equilibrium conditions [31]. The results obtained for different catalysts are shown in Table 5. Table 5. Textural analysis of the synthesized catalysts

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Catalyst NiAl-Com NiZr-Com NiCe-Com NiAl-CP NiZr-CP NiCe-CP NiAl-Imp NiZr-Imp NiCe-Imp

SBET (m2/gcat) 14 coprecipitation > combustion, except the NiZr-CP catalyst, which was more active than NiZr-Imp catalyst. Considering the same methodology of synthesis the catalysts with different supports can be placed in the following order: aluminum oxide > zirconia oxide > cerium oxide, except for the combustion synthesis where the NiCe-Com catalyst was more active than the NiZr-Com catalyst. The liquid phase was analyzed using an enzymatic kit where only glycerol can be detected, but it is known that small amounts of other components are formed during the APR of glycerol as propylene glycol, ethylene glycol, acetol, ethanol and aldehydes [8,21]. The performance of the catalysts was also measured as a function of the composition of the gas phase. The gaseous products identified from the APR of glycerol were H2, CO, CH4 and CO2. The gas phase was analyzed whenever possible during the 12 h of reaction, but in some cases the gas phase did not have sufficient volume for the large number of samples, mainly with catalyst that showed low conversions. The gaseous composition of the catalysts supported on alumina is shown in Figure 20. The composition of H2 and CO2 remained practically unchanged in the reaction with NiAlImp and NiAl-Com catalysts, with molar fraction around 77 and 80 % for H2 and 20 and 17 % for CO2, respectively. However, molar fraction of CO2 in the reaction with the NiAl-CP catalyst increased from 20 to 30 % and the H2 reduced from 71 to 60 % in the tenth hour of reaction. The formation of CO and CH4 was small for all catalysts. For the NiAl-CP catalyst the CO molar fraction was approximately 2 % and for NiAl-Imp and NiAl-Com less than 1

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%. The CH4 molar fraction was approximately 1, 6 and 10 % in reactions with NiAl-Com, NiAl-Imp and NiAl-CP catalysts, respectively. These compositions were practically constant during the reactions, except with the NiAl-CP catalyst, which showed an increase from 5 % to 10 % in the sixth hour. Figure 21 shows the results obtained using NiZr-Imp and NiZr-CP catalysts. The catalyst prepared by the combustion method (NiZr-Com) did not show sufficient activity to produce gas phase. The molar fraction of H2 stayed between 75 and 80 % in the reaction with NiZrImp catalyst until the eighth hour of reaction, however in the tenth hour there was a small reduction. The molar fraction of CO2 remained in the range between 10 and 15 % up to the eighth hour, and then increased to 21.5 % on the tenth hour of reaction. The reaction with NiZr-CP catalyst produced a molar fraction of H2 in the range 69 to 75 % and CO2 molar fraction in the range of 17 to 25.5 %. The production of CH4 and CO was very similar for the reactions with NiZr-Imp and NiZr-CP catalysts, showing a molar fraction in the range of 2 to 4 % for CH4 and in the range of 2 to 5 % for CO, except with NiZr-Imp catalyst in the first hours of reaction, where the molar fraction of CO reached 8.5 %, but subsequently reduced to 5 %.

100 90 80 70 60 50 40 30 20 10 0

Molar Fraction (%)

(a)

(b)

Time (h) 100 90 80 70 60 50 40 30 20 10 0

Time (h) (c)

Molar Fraction (%)

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1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10

H2 CO CH4 CO2

1 2 3 4 5 6 7 8

Time (h)

Figure 20. H2, CH4, CO2 and CO molar fractions as a function of time for aqueous-phase reforming of 1 vol.% glycerol solution at 250 ºC with 200 mg of catalyst NiAl-Imp (a), NiAl-CP (b) and NiAl-Com (c).

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Molar Fraction (%)

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100 90 80 70 60 50 40 30 20 10 0

103

(a)

(b)

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10 11 12

H2 CO CH4 CO2

Time (h)

Time (h)

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Figure 21. H2, CH4, CO2 and CO molar fractions as a function of time on stream for aqueous-phase reforming of 1 vol.% glycerol solution at 250 ºC with 200 mg of catalyst NiAl-Imp (a) and NiAl-CP (b).

Figure 22 shows the gas phase composition obtained using NiCe-Imp, NiCe-CP and NiCe-Com catalysts. It can be clearly noticed the higher H2 selectivity of these catalysts compared with those supported on alumina and zirconia. Reactions with NiCe-Imp and NiCeCP catalysts presented an initial H2 molar fraction higher than 90 %, but during the reaction decreased to 77 %, while the catalyst NiCe-Com presented the molar fraction of H2 constant at around 85 %. The molar fraction of CO did not show large variations during the reaction with these catalysts, in the range of 2 to 8 %. The molar fraction of CH4 is insignificant for all catalysts, less than 1 % during all reaction time. The molar fraction of CO2 in reactions with NiCe-Imp and NiCe-CP catalysts presented initially low values, around 1 %, but stabilized approximately at 20 and 15 %, respectively, while the reaction with NiCe-Com catalyst showed small variation and stabilization around 10 %. The APR of glycerol was also tested in the same conditions using platinum catalysts supported on alumina (PtAl) and zirconia (PtZr) as reference to the nickel catalysts. Figure 23 shows the composition of the gas phase obtained using platinum catalysts. The behavior of these catalysts is very similar, with H2 molar fraction around 68 %. The molar fraction of CH4 was slightly higher in the test with PtAl catalyst and the molar fraction of CO2 was slightly higher with the PtZr catalyst. The molar fraction of CO in the reactions was insignificant, less than 1 %. Comparing the results of the nickel with the platinum catalysts, Pt catalysts exhibited higher activity in methanation reaction, because the average molar fraction of CH4 in tests with nickel catalysts was lower than 3 %, while platinum catalysts produced an average molar fraction of 9 %. Considering the shift reaction, which converts CO to CO2 producing H2, the platinum catalysts also showed a higher activity compared to the nickel catalysts, because the average molar fraction of CO on platinum catalysts was only 0.7 %, while nickel catalysts showed an average molar fraction of 3.5 %.

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100 90 80 70 60 50 40 30 20 10 0

(b)

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8 9 101112

Molar Fraction (%)

(a)

Time (h)

Time (h)

100 90 80 70 60 50 40 30 20 10 0

Molar Fraction (%)

(c) H2 CO CH4 CO2

Figure 22. H2, CH4, CO2 and CO molar fractions as a function of time on stream for aqueous-phase reforming of 1 vol.% glycerol solution at 250 ºC with 200 mg of catalyst NiCe-Imp (a), NiCe-CP (b) and NiCe-Com (c).

100 90 80 70 60 50 40 30 20 10 0

Molar Fraction (%)

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1 2 3 4 5 6 7 8 Time (h)

(a)

(b)

H2 CO CH4 CO2

1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12

Time (h)

Time (h)

Figure 23. H2, CH4, CO2 and CO molar fractions as a function of time on stream for aqueous-phase reforming of 1 vol.% glycerol solution at 250 ºC with 200 mg of catalyst PtAl (a) and PtZr (b).

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Conversion (%)

60

105

PtAl PtZr

50 40 30 20 10 0

1 2 3 4 5 6 7 8 9 10 11 12

Time (h)

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Figure 24. Conversion of glycerol for Pt catalysts. Reaction conditions: 200 mg of catalyst, 250 °C, 1 vol.% glycerol solution and agitation speed of 450 rpm.

The conversion of glycerol using platinum catalysts is shown in Figure 24. The conversion did not reach stability, even after 12 h, with a continuous increase. The PtZr catalyst presented a slightly higher activity compared to PtAl, reaching at the end of 12 h of reaction 37 % and 32 % of conversion, respectively. In all catalytic tests performed with platinum-based catalysts, the catalyst mass used was the same of the tests performed with nickel-based catalysts and no study of the influence of catalyst loading was performed with the former. Considering that the nominal content of platinum was 2 wt.% and nickel was 20 wt.%, this may justify the lower conversion achieved in the catalytic test at 250 °C with 1 vol.% glycerol solution using platinum catalysts as well the lower molar fraction of H2 in the gas phase. Since PtAl and PtZr catalysts presented no significant differences between them and the catalysts supported on alumina are the most traditional, the PtAl catalyst was chosen to be tested in APR of glycerol at 270 °C. The NiAl-Imp catalyst was also tested at 270 °C, because it showed the best catalytic performance among the Ni catalysts at 250 °C. All reaction conditions were the same, with exception of the temperature and consequently the pressure increased to 55 atm. Figure 25 shows the gas phase composition of the reaction performed at 270 °C with PtAl and NiAl-Imp catalysts. Under these conditions both catalysts presented a similar behavior; the molar fraction of H2 is initially close to 85 %, but then there is a successive reduction stabilizing around to 63 % for NiAl-Imp catalyst and 50 % for PtAl catalyst. The concentration of CO2 follows an opposite behavior, initially the reaction presents a low molar fraction, around 8 %, but in the course of the reaction it increases and stabilizes around 25 % for NiAl-Imp catalyst and 38 % for PtAl catalyst. The molar fraction of CH4 and CO did not show large fluctuations during the reaction for both catalysts, the CH4 remained with molar fraction around 9 % and the CO with molar fraction lower than 1 %. The most notable difference when comparing the gas phase of the reactions performed at 250 °C with 270 °C using NiAl-Imp catalyst was the increase in methanation reaction, where the average CH4 molar fraction increased from 5.4 % to 9.5 % approximately; however in the

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case of platinum catalyst, with increasing the reaction temperature the molar fraction of CH4 did not change significantly, from 10.2 % to 9.3 %.

Molar Fraction (%)

100 90 80 70 60 50 40 30 20 10 0

(a)

(b)

H2 CO CH4 CO2

1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12

Time (h)

Time (h)

Figure 25. H2, CH4, CO2 and CO molar fractions as a function of time on stream for aqueous-phase reforming of 1 vol.% glycerol solution at 270 ºC with 200 mg of catalyst NiAl-Imp (a) and PtAl (b).

100 90 80 70 60 50 40 30 20 10 0

Conversion (%)

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The conversion of glycerol in the reaction performed at 270 °C reached high values, as shown in Figure 26: 77 % with NiAl-Imp catalyst and 88 % with PtAl catalyst. The NiAl-Imp catalyst presented a stabilization of conversion near to 77 % from the sixth hour of reaction, but the PtAl catalyst presented an increasing conversion until the end of the reaction, indicating the possibility to achieve a complete conversion in a long time.

NiAl-Imp PtAl

1 2 3 4 5 6 7 8 9 10 11 12

Time (h)

Figure 26. Conversion of 1 vol.% glycerol solution at 270 °C, using 200 mg of PtAl and NiAl-Imp catalysts.

4.6.4. Evaluation of the Catalysts with 10% Glycerol Solution For the catalytic tests with 10 vol.% glycerol solution it was evaluated only the catalysts that showed the best performance for each support in the tests with 1% glycerol solution. In this context the following catalysts were selected: NiAl-Imp, NiZr-CP and NiCe-Imp. Figure 27 shows the molar fractions of the gas products. The reactions performed with NiAl-Imp and

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100 90 80 70 60 50 40 30 20 10 0

(a)

(b)

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10 11 12

Molar Fraction (%)

Time (h)

Time (h)

100 90 80 70 60 50 40 30 20 10 0

(c)

Molar Fraction (%)

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NiZr-CP catalysts showed a reduction in H2 production after 6 h of reaction, reaching a molar fraction of 42 % with NiAl-Imp catalyst after 10 h and 35 % with NiZr-CP catalyst after 12 h. This decreased molar fraction of H2 may be related to secondary reactions in the liquid phase. Hydrogen produced by APR can be consumed in the hydrogenation of products from dehydration of glycerol, as acetol, 3-hydroxypropanal, pyruvaldehyde and acetaldehyde [47,48]. The NiCe-Imp catalyst showed a smaller reduction in the molar fraction of H2, from 83 to 67 %. The molar fractions of CO and CH4 were practically constant during the reaction with this catalyst, while CO2 increased significantly from 2 to 12 %. This increase of CO2 molar fraction was also observed in the other two catalysts, reaching 42 % with NiAl-Imp and 36 % with NiZr-CP. The molar fractions of CH4 and CO were lower than 10 % with NiAl-Imp catalyst. The NiZr-CP catalyst presented a molar fraction of CH4 below 3 % and produced a large amount of CO, reaching 28 %. Comparing the results of these tests with those performed with 1 vol.% glycerol solution, it was observed that increasing the glycerol concentration, keeping the other variables equal, results on the reduction of the catalytic activity for shift reaction or the amount of catalyst is very small in this glycerol concentration. This effect is more pronounced for NiZr-CP and NiCe-Imp catalysts, where the average molar fraction of CO increased from 5.5 % to 20.6 % for NiZr-CP catalyst and 5.3 % to 15.8 % for NiCe-Imp catalyst.

H2 CO CH4 CO2

1 2 3 4 5 6 7 8 9 10

Time (h) Figure 27. H2, CH4, CO2 and CO molar fractions as a function of time on stream for aqueous-phase reforming of 10 vol.% glycerol solution at 250 ºC with 200 mg of catalyst NiAl-Imp (a) NiZr-CP (b) and NiCe-Imp (c ).

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The glycerol conversion in these reactions is shown in Figure 28. The increase of the glycerol concentration from 1 to 10 vol.% resulted in a considerable reduction of the conversion. The NiAl-Imp catalyst exhibited a reduction in conversion of glycerol from 50 to 14 %, NiZr-CP from 40 to 6.5 % and NiCe-Imp from 20 to 8 %. Thus, the NiZr-CP catalyst showed a larger reduction in catalytic activity, around 84 %. This reduction was expected since the amount of catalyst used was the same.

Conversion (%)

25

NiAl-Imp NiZr-CP NiCe-Imp

20 15 10 5 0

1 2 3 4 5 6 7 8 9 10 11 12 Time (h)

100 90 80 70 60 50 40 30 20 10 0

30

H2

Molar Fraction (%)

CO CH4

Conversion (%)

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Figure 28. Conversion of 10 vol.% glycerol solution at 250 °C for 12 hours, using 200 mg of catalyst.

CO2

1

2

3

4

5

Time (h)

6

7

8

25 20 15 10 5 0

1 2 3 4 5 6 7 8 9 10

Time (h)

Figure 29. Catalytic test at 250 °C with 10 vol.% glycerol solution using PtAl catalyst: (a) molar fraction of H2, CO, CH4 and CO2, and (b) glycerol conversion.

The APR reaction using 10 vol.% glycerol solution was also evaluated with PtAl catalyst and the result is presented in Figure 29. The molar fraction of H2 reduced gradually from 80 % to 40 % in 8 h of reaction, as occurred with the NiAl-Imp and NiZr-CP catalyst. The molar fraction of CO2 increased successively from 16 % to 56 %. The molar fraction of CH4 produced under these conditions was very close to the results obtained with the Ni catalysts (Figure 27), with values below 5 %. However, comparing with the catalytic test that was carried out under the same conditions, except for the concentration of glycerol that was

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1 vol.% (Figure 23), it is observed that the increase in concentration of glycerol decreased the formation of CH4 in the reaction, which reduced from 10 % to 5 %, approximately. The reactions performed with platinum catalysts showed low formation of CO, presenting a molar fraction below 1 %. The reactions with nickel catalysts showed relatively low formation of CO when performed with 1 vol.% glycerol solution, but the catalytic tests with 10 vol.% glycerol solution showed higher formation of CO, reaching a molar fraction of 27 % with the NiZr-CP catalyst. In relation to glycerol conversion, the PtAl catalyst practically did not have variations in conversion within 10 h of reaction at 250 °C due to increased concentration of glycerol in the reaction: the reactions with 1 and 10 vol.% glycerol solution presented 28 and 25 % of conversion, respectively. On the other hand, the conversion of glycerol over Ni catalysts was strongly influenced by the concentration, when the glycerol concentration increased from 1 vol.% to 10 vol.% the conversion reduced from 50 % to 14 % with the NiAl-Imp catalyst.

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CONCLUSIONS In the present work nickel catalysts supported on alumina, zirconia and ceria were prepared using three different methods: wet impregnation, coprecipitation and combustion, all with 20 wt.% of NiO. Analysis of X-ray fluorescence (XRF) showed that the synthesis methods are efficient to produce catalysts with the desired composition. The analysis of N2 physisorption revealed that the preparation method has a strong influence on the textural properties of catalysts. The catalysts prepared by combustion method presented BET area lower than 10 m2/gcat, which is the result of the high temperatures reached during synthesis, leading to sintering of the particles. The catalysts prepared by wet impregnation showed BET areas smaller than their supports, due to partial blockage of the pores of the support by the nickel particles. The coprecipitation method resulted in higher BET areas compared to other methods of synthesis, especially for NiAl-CP catalyst, which achieved a BET area of 353 m2/gcat. The analysis of X-ray diffraction (XRD) permitted to identify the crystalline phases presented in the catalysts, as well as to calculate the average crystallite size. The catalysts supported on CeO2 exhibited two crystalline phases, NiO and CeO2. For the catalysts supported on Al2O3 it was observed three different phases, NiO, Al2O3 and NiAl2O4, while for the combustion method NiAl2O4 was the only phase formed. The catalysts supported on ZrO2 showed two crystalline phases, as observed in the catalysts supported on CeO2. The catalyst prepared by combustion method presented the formation of the cubic phase of ZrO2 and NiO. Wet impregnation method resulted in the formation of monoclinic phase of ZrO2 and cubic phase of NiO. Finally, synthesis by coprecipitation method resulted in the formation of amorphous structures. The average size of crystalline particles was calculated by the Scherrer equation. For the catalysts with cerium, the largest average crystallite size of NiO was found in the NiCe-Com catalyst, due to the sintering during the preparation of the catalyst, which results in large particles. However, NiZr-Com catalyst presented one of the smallest NiO crystallite sizes of all catalysts, suggesting that the temperature and/or time of combustion during the synthesis for the zirconia precursors are smaller.

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In the analysis of temperature-programmed reduction (TPR) the reduction degree of the catalysts was calculated considering only the reduction of NiO; however, there was also a partial reduction of CeO2 and ZrO2 supports, especially for CeO2. The XRD analysis showed that the catalysts supported on Al2O3 present a spinel NiAl2O4 phase, but the NiAl-Imp and NiAl-CP catalysts showed, in addition to the phase of NiAl2O4, the formation of Al2O3 and NiO phases. As the reduction of the NiAl2O4 only occurs at high temperatures, it results in reduction peak at high temperatures for the catalysts supported on Al2O3, around 730 °C, 860 °C and 1000 °C, for NiAl-Imp, NiAl-CP and NiAl-Com, respectively. Diffuse reflectance spectroscopy (DRS) was used to study the coordination of nickel species in the catalysts. It was observed that the catalysts presented the tetrahedral and/or octahedral coordination forms of Ni2+ in NiO and NiAl2O4 phases. In some catalysts only one form of coordination was found; the NiAl-Com, NiCe-Imp, NiCe-CP and NiZr-Com catalysts presented both forms of Ni2+coordination. The catalytic tests showed that the nickel catalysts have great potential in the reaction of aqueous-phase reforming of glycerol for hydrogen production. In the catalytic tests at 250 °C with 1 vol.% glycerol solution, the catalyst with the highest activity in terms of glycerol conversion was NiAl-Imp, reaching 50 % of conversion, and NiZr-Com had the lowest activity, with 7 % of conversion. Analyzing the performance in terms of H2 production, the nickel catalysts supported on CeO2 presented the best results. Under the same operating conditions all the nickel catalysts showed good stability in the composition of the gas phase, without many fluctuations during the reaction. In general, the formation of CO and CH4 in these tests was low and H2 and CO2 were the major products. With the increase of temperature reaction from 250 °C to 270 °C in tests with 1 vol.% glycerol solution, the conversion of glycerol increased from 50 to 77 % using the NiAl-Imp catalyst, however the molar fraction of H2 decreased from 75 to 65 %. Thus, increasing the reaction temperature reduces the molar fraction of H2 in the gas phase, and this may be related to the occurrence of reactions in liquid phase at higher temperatures and/or may be due to increased decomposition of glycerol. The catalytic tests performed with platinum catalysts at 250 °C with 1 vol.% glycerol solution showed the higher activity in the methanation reaction. The molar fractions of H2 and CH4 in the tests with platinum catalysts were lower and higher, respectively, compared to the results obtained with the nickel catalysts. It was also tested 10 vol.% glycerol solution at 250 °C and the results obtained in the gas phase showed a reduction in production of H2 with a significant increase in molar fraction of CO, reaching values in the range of 20 to 30 %. Thus it is clear that the shift reaction is inhibited with the increase of glycerol concentration in the solution. In these tests there was a significant reduction in the conversion of glycerol. In the catalytic test with NiAl-Imp catalyst the glycerol conversion decreased from 50 to 14 % compared to the test with 1 vol.% glycerol solution at 250 °C. Catalytic tests with 10 vol.% glycerol solution at 250 °C were also performed with platinum catalysts, and the result obtained in the gas phase was similar to that obtained in the tests with NiAl-Imp catalyst, in the same condition. The conversion of glycerol in the reactions with platinum catalysts was independent of glycerol concentration. Depending on the reaction conditions, it was observed that the activity of nickel catalysts can be higher than the activity of platinum catalysts, as in the tests with 1 vol.% glycerol solution at 250 °C, where the conversion obtained with nickel catalysts was around 50 % and platinum catalysts

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around 30 %. However, when using 10 vol.% glycerol solution, the platinum catalysts keep their activity in terms of conversion, while nickel catalysts suffer a drastic reduction.

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REFERENCES Shahid, E.M.; Jamal, Y. Renew. Sust. Energy Rev. 2008, 12, 2484-2494. Dunn, S. Int. J. Hydrogen Energy. 2002, 27, 235-264. Adhikari, S.; Fernando, S.D.; Haryanto, A. Renew. Energy. 2008, 33, 1097-1100. Kirtay, E. Energy Conv. Manag. 2011, 52, 1778-1789. Kotay, S.M.; Das, D. Int. J. Hydrogen Energy. 2008, 33, 258-263. Konieczny, A.; Mondal, K.; Wiltowski, T.; Dydo, P. Int. J. Hydrogen Energy. 2008, 33, 264272. Huber, G.W.; Shabaker, J.W.; Dumesic, J.A. Science. 2003, 300, 2075-2077. Menezes, A.O.; Rodrigues, M.T.; Borges, L.E.P.; Fraga, M.A. 14° Congresso Brasileiro de Catálise 2007, ISSN 1980-9263. Gerpen V. J. Fuel Process. Technol. 2005, 86, 1097-1107. Johnson, D.T.; Taconi, K.A. Environ. Prog. 2007, 26, 338-348. Cortright, R.D.; Davda, R.R.; Dumesic, J.A. Nature. 2002, 418, 964-967. Davda, R.R.; Shabaker, J.W.; Huber, G.W.; Cortright, R.D.; Dumesic, J.A. Appl. Catal. BEnviron. 2005, 56, 171-186. Shabaker, J.W.; Huber, G.W.; Dumesic, J.A. J. Catal. 2004, 222, 180-191. Davda, R.R.; Shabaker, J.W.; Huber, G.W.; Cortright, R.D.; Dumesic, J.A. Appl. Catal. BEnviron. 2003, 43,13-26. Shabaker, J.W.; Huber, G.W.; Davda, R.R.; Cortright, R.D.; Dumesic, J.A. Catal. Letters. 2003, 88,1-8. Iglesia, E.; Soled, S.L.; Fiato, R.A. J. Catal. 1992, 137, 212-224. Kellner, C.S.; Bell, A.T. J. Catal. 1981, 70, 418-432. Sinflet, J.H.; Yates, D.J.C., J. Catal. 1967, 8, 82-90. Grenoble, D.C.; Estadt, M.M.; Ollis, D.F., J. Catal. 1981, 67, 90-102. Vannice, M.A. J. Catal. 1977, 50, 228-236. Iriondo, A.; Barrio, V.L.; Cambra, J.F.; Arias, P.L.; Guemez, M.B.; Navarro, R.M.; SanchezSanchez, M.C.; Fierro, J.L.G. Top. Catal. 2008, 49, 46-58. Greeley, J.; Mavrikakis, M. J. Amer. Chem. Soc. 2002, 124, 7193-7201. Soares, R.R.; Simonetti, D.A.; Kunkes, E.L.; Dumesic, J.A. 14° Brazilian congress of Catalysis. 2007. Li, C.P.; Chen, Y.W. Thermochim Acta. 1995, 256, 457-465. Avgouropoulos, G.; Ioannides, T. Appl. Catal. A-Gen. 2003, 244, 155-167. Tahmasebi, K.; Paydar, M.H. Mater. Chem. Phys. 2008, 109, 156-163. Zhang, J.; Xu, H.Y.; Jin, X.L.; Ge, Q.J.; Li, W.Z. Appl. Catal. A-Gen. 2005, 290, 87-96. Li, G.H.; Hu, L.J.; Hill, J.M. Appl. Catal. A-Gen. 2006, 301, 16-24. Li, Y.; Zhang, B.C.; Tang, X.L.; Xu, Y.D.; Shen, W.J. Catal. Comm. 2006, 7, 380-386. Souza, M.M.V.M., Ph.D. Thesis, COPPE/UFRJ, Rio de Janeiro, Brazil, 2001. Arean, C.O.; Mentruit, M.P.; Lopez, A.J.L.; Parra, J.B. Colloid Surface A 2001, 180, 253258.

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Conceição, L.; Ribeiro, N.F.P.; Furtado, J.G.M.; Souza, M.M.V.M. Ceram. Int. 2009, 35, 1683-1687. Sahli, N.; Petit, C.; Roger, A.C.; Kiennemann, A.; Libs, S.; Bettahar, M.M. Catal. Today. 2006, 113, 187-193. Chary, K.V.R.; Rao, P.V.R.; Rao, V.V. Catal. Comm. 2008, 9, 886-893. Jun, K.W.; Roh, H.S.; Chary, K.V.R. Catal. Surv. Asia 2007, 11, 97-113. Xu, S.; Wang, X.L. Fuel. 2005, 84, 563-567. Chary, K.V.R.; Rao, P.V.R.; Vishwanathan, V. Catal. Comm. 2006, 7, 974-978. Almeida, R.M. De; Fajardo, H.V.; Mezalira, D.Z.; Nuernberg, G.B.; Noda, L.K.; Probst, L.F.D.; Carreno, N.L.V. J. Mol. Catal. A-Chem. 2006, 259, 328-335. Becerra, A.M.; Castro-Luna, A.E. J. Chil. Chem. Soc. 2005, 50, 465-469. Heracleous, E.; Lee, A.F.; Wilson, K.; Lemonidou, A.A. J. Catal. 2005, 231, 159-171. Kis, E.; Marinkovicneducin, R.; Lomic, G.; Boskovic, G.; Obadovic, D.Z.; Kiurski, J.; Putanov, P. Polyhedron. 1998, 17, 27-34. Wang, J.; Dong, L.; Hu, Y.H.; Zheng, G.S.; Hu, Z.; Chen, Y. J. Solid State Chem. 2001, 157, 274-282. Boukha, Z.; Kacimi, M.; Pereira, M.F.R.; Faria, J.L.; Figueiredo, J.L.; Ziyad, M. Appl. Catal. A-Gen. 2007, 317, 299-309. Ribeiro, N.F.P.; Moya, S.F.; Neto, R.C.R.; Souza, M.M.V.M.; Schmal, M. Int. J. Hydrogen Energy. 2010, 35, 11725-11732. Kim, P.; Kim, Y.; Kim, H.; Song, I.K.; Yi, J. Appl. Catal. A-Gen. 2004, 272, 157-166. Dong, W.S.; Roh, H.S.; Jun, K.W.; Park, S.E.; Oh, Y.S. Appl. Catal. A-Gen. 2002, 226, 6372. King, D.L.; Zhang, L.A.; Xia, G.; Karim, A.M.; Heldebrant, D.J.; Wang, X.Q.; Peterson, T.; Wang, Y. Appl. Catal. B-Environ. 2010, 99, 206-213. Wawrzetz, A.; Peng, B.; Hrabar, A.; Jentys, A.; Lemonidou, A.A.; Lercher, J.A. J. Catal. 2010, 269, 411-420.

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Chapter 4

THE PAUPER AND THE PRINCE: GLYCEROL IN A VIEW FROM BIOFUELS AND BIOREFINERIES Alejandro J. Beccaria1, Alberto A. Iglesias1 and Raúl A. Comelli*,2 Instituto de Agrobiotecnología del Litoral – IAL (UNL-CONICET). FBCB, Santa Fe, Argentina 2 Instituto de Investigaciones en Catálisis y Petroquímica – INCAPE (FIQ-UNL, CONICET), Santa Fe, Argentina

1

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1. INTRODUCTION Glycerol (1,2,3-propanetriol, abbrev. Gro), a sweet-tasting alcohol, was discovered in the 18th century (1779) by the Swedish chemist C.W. Scheele, who isolated the poly-alcohol after heating olive oil and litharge. The chemical is also known as glycerin or glycerine. It is abundant in nature after being a component of many lipids and a main compatible solute produced by cells for osmoregulation, to manage water activity variations in the medium. Also, many microorganisms are able to use glycerin as a source for carbon and energy. Main physicochemical properties of Gro can be summarized by being a colorless, odorless, highly hygroscopic and viscous liquid having a boiling point at 290 ºC, a specific gravity of 1.26 and a molecular weight of 92.09 (Pagliaro and Rossi, 2008). It forms crystals at low temperatures that tend to melt at 17.9 ºC. All its characteristics make of Gro a compound of utility for inclusion in different industrial processes and/or for being a key constituent in several chemical preparations. Without trying to make a detailed and complete listing of its applications, chief uses of the polyol include: 

*

Gro is extensively used in the pharmaceutical and cosmetic industries taken advantage of its emollient, humectant and demulcent properties (Pagliaro and Rossi,

Correspondence to: Prof. Raúl A. Comelli, INCAPE, Instituto de Investigaciones en Catálisis y Petroquímica, (FIQ-UNL, CONICET), Santiago del Estero 2654, S3000AOJ – Santa Fe, Argentina, Telephone 54 0342 4571164 (int. 2739), Fax +54-342 4531068, Email: [email protected].

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2008). Also, its capability as a solvent (alone or as stabilizer in combination with water and alcohols) for preparation of different formulations and elixirs carrying active ingredients for variable purposes going from tinctures to key agents benign for specific treatments (i.e. theophyllin and xanthine derivatives with clinical applications for respiratory diseases). Food industry has Gro as a key compound of many utilities: preservative moistening agent for baked goods, preservative for juices, anti-crystallization mediator in candies, and solvent of chemicals serving as carriers of color and flavors (Pagliaro and Rossi, 2008). Gro is used as a plasticizer of different polysaccharides, such as starches (Qiao et al., 2011) or chitosan (Epure et al., 2011), this application being decidedly relevant for the production of biodegradable polymers with improved properties as plastic-like compounds. Gro conversion into nitroglycerin serves in contrasting applications (Pagliaro and Rossi, 2008). Thus, the nitro-derivative is a main component in production of dynamites and explosives; although it also has a laudable use as an active component in medication for heart disease, since nitroglycerin has the capacity of generate nitric oxide (Agvald et al., 2002).

Although the many faceted uses and applications for Gro, its market value is in downhill, principally because the rapid expansion of biodiesel production that generated a glut of the polyol (Johnson and Taconi, 2007). Biodiesel is produced from triacylglycerides contained in animal fats or vegetable oils through transesterification with low molecular weight alcohols (mainly methanol or ethanol). The process generates methyl-(or ethyl-) esters of fatty acids as the major biodiesel product and about 10% (v/v) of Gro as a by-product. This is not only converting the polyol in a low value chemical; but it is further creating an environmental problem because of the limited possibility for disposal, which is also quite expensive (da Silva et al., 2009). The situation is obligating to revisit current uses and the design of new processes for adding value to the propanetriol. For the latter, overall possibilities include the finding for new applications of the chemical as well as its conversion to other molecules serving in different industry systems, in a way improving competitiveness in the productive chain of the biodiesel business. Paradoxical to the relatively pauper state of affairs for Gro, it can be visualized as a key compound for developing processes in the emerging field of biorefineries. Limited reserves of petroleum and the associated dramatic increases in its price, together with critical environmental concerns and climate changes provoked by the use of fossil fuels are creating obligatory demands for generate renewable fuels as well as for reconvert industrial processes. The necessity is to make them more ecology-friendly and compatible with a sustainable environment. This leads to the concept of biorefinery, which embraces different technologies allowing convert biomass into materials, chemicals and energy (Ohara, 2003; Kamm and Kamm, 2004, 2007; Schell et al., 2008; Amidon and Liu, 2009). In this framework, the positioning of Gro as a by-product in a process to obtain biofuel that needs to be revalorized is relevant and it represents a fair challenge for the development of biorefinery tools. In the present work we analyze current and potential ways to valorize glycerol. Globally, we review

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chemical as well as biological approaches and we analyze possibilities for combine strategies to develop biorefinery-like solutions.

2. CHEMICAL APPROACHES TO VALORIZE GLYCEROL

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Different alternatives for valorize Gro have been proposed from the chemical field. Figure 1 illustrates on the different compounds that can be obtained from the polyol by utilizing chemical strategies. It is important to consider that they are not the only ones. Then, Gro can be a versatile raw material and plays an important role as a key compound in the environment of the future biorefinery.

Figure 1. Glycerol valorization by catalytic ways.

2.1. Dehydration and Oxyhydration Reactions Dehydration of Gro can directly produce important chemicals such as 3-hydroxypropionaldehyde (3-HPA), monohydroxyacetone (acetol), and acrolein. 3-HPA is a precursor

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for 1,3-propanediol (1,3-PDO), acrolein, and acrylic acid, and used for polymer production; it also exhibits antimicrobial activity towards a wide range of pathogens and food spoilage organisms, and it finds use both as a food preservative and as a therapeutic auxiliary in the pharmaceutical industry. Currently 3-HPA is produced by synthesis from petrochemicals; Degussa and Shell chemical processes produce it as an intermediate in the formation of 1,3PDO. The Degussa catalytic process transforms propylene into acrolein, which is hydrated to 3-HPA. The Shell process transforms ethylene into ethylene oxide, which is converted to 3HPA by a hydroformylation reaction with syngas at 150 bar pressure; ethylene is inexpensive and intermediate products are not toxic, but 3-HPA has to be recovered from the organic phase. More recently, dehydration of Gro to 3-HPA which is latter hydrogenated to 1,3-PDO was suggested by Miyazawa et al. (2006). Dasari et al. (2005) proposed that Gro is dehydrated to acetol which is then hydrogenated to 1,2-propanediol (1,2-PDO). Metal supported catalysts, especially Ru/C, combined with a strong solid acid as an Amberlyst ion-exchange resin, were active in the hydrogenolysis of Gro to 1,2-PDO (Kusunoki et al., 2005); the ion-exchange resin was responsible to produce acetol, which is an intermediate product (Miyazawa et al., 2006). Acrolein is an important intermediate for the chemical industry; it can be polymerized to acrylic resins and forms the basis of superabsorbent polymers, widely employed in the baby hygiene market. Acrolein is currently produced by the oxidation of propylene, however, the increase of propylene price making commercially attractive its production from Gro. A hybrid process coupling the oxidation of propylene with the dehydration of Gro is conduced by Arkema. By considering a one-step oxydehydration of Gro in the presence of molecular oxygen and at atmospheric pressure, the commercially important acrylic acid can be produced; the dehydration reaction to acrolein is followed by the direct oxidation to acrylic acid. Thermal balance of the overall process is improved coupling both exothermic oxidation and endothermic dehydration reactions. The reactor contains two catalytic beds, a first one of tungstated zirconia, and a second one of a mixed W–Sr–V–Cu–Mo oxidation catalyst with acetic acid as binder. At 280 °C and feeding a 20wt% Gro aqueous solution, conversion was complete while acrylic acid reached 74% yield.

2.2. Selective Reduction Reactions The selective reduction of Gro through the hydrogenolysis reaction allows to obtain compounds with added value, such as 1,2-PDO, ethylene glycol (EG), and 1,3-PDO. 1,2PDO, also known as propyleneglycol, is a major commodity chemical traditionally derived from propylene oxide, with applications in medicines, cosmetics, food, liquid detergents, tobacco humectants, flavors and fragrances, personal care products, paints, and animal feedstuffs, being also used as functional fluids (antifreeze, deicing fluid, and heat transfer liquids), and a solvent for the production of unsaturated polyester resins (Corma Canos et al., 2007). The selective production of 1,2-PDO by hydrogenolysis in liquid phase has been reported using catalysts such as Raney Ni, Ru/C, Pt/C, Ni/C, and copper chromite, which reached the best performance. That hydrogenolysis reaction was evaluated on catalysts containing Ru, Cu, Pt, Ni, Co, Rh, and Re, following catalytic performance the order Cu ≈ Ni

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≈ Ru > Pt > Pd (Roy et al., 2010). Pt-Ru/C is more stable, being Au-Ru/C altered by the harsh operating conditions (Maris et al., 2007). Using Ru-containing catalysts, the type of support (TiO2, SiO2, NaY, γ-Al2O3, and activated carbon) can influence the metal particle size and the reaction pathways (Feng et al., 2008). Ma et al. (2008) studied the promoting effect of Re on Ru impregnated on SiO2, ZrO2, Al2O3, C, and ZSM-5. Vasiliadou et al. (2009) reported the effect of both support and Ru precursor, obtaining a linear relationship between the total acidity of catalyst and the activity in hydrogenolysis. Lahr and Shanks (2005) increasing sulphur loading on Ru/C during the hydrogenolysis at high pH, increased the selectivity to 1,2-PDO, without modifying the selectivity to EG. Balaraju et al. (2009, 2010) reported Nb2O5 and TPA/ZrO2 (TPA: 12-phosphotungstic acid) having moderate acid sites as the most active co-catalysts combined with Ru/C, influencing the amount of acidity in the acid solid its catalytic activity; the effect of preparation conditions of Ru/TiO2 on the behavior during the hydrogenolysis of Gro was also studied (Balaraju et al., 2010). Roy et al. (2010) used a mixture of Ru/Al2O3 and Pt/Al2O3 to produce 1,2-PDO from Gro without added external hydrogen. Unfortunately, Ru catalysts promote an excessive breakage of C-C bonds decreasing the selectivity to 1,2-PDO. Copper-containing catalysts have a poor activity in the rupture of C-C bonds and a high efficiency for the hydrogenating/dehydrogenating of C-O bonds, then they were proposed as an alternative material (Huang et al., 2009). Commercial copper chromite and samples prepared by impregnation and co-precipitation, have been reported as efficient catalysts to produce 1,2-PDO. Cu/ZnO (Wang and Liu, 2007), Cu/ZnO/Al2O3 (Meher et al., 2009), Cu/Al2O3 (Akiyama et al., 2009; Guo et al., 2009), Cu/SiO2 (Huang et al., 2009), and Cu/MgO (Yuan et al., 2010) were suitable materials for hydrogenolysis, while Cu supported on zeolites such as HY, 13X, H-ZSM5, and Hβ did not produce 1,2-PDO from Gro (Guo et al., 2009). Catalytic behavior of copper chromite without and with barium and Cu/H-FER (being FER: ferrierite zeolite), Cu/K-FER, and Ni/13X, was measured in the hydrogenolysis of Gro in gas phase at 200ºC and atmospheric pressure (Comelli, 2011). Copper chromite stabilized with barium was the most active catalyst, reaching 83.0% conversion while both copper chromite samples were the most efficient in selectivity to acetol and 1,2-PDO; sample without barium was the most selective to acetol (67.8%), while the one with barium improved the selectivity to 1,2-PDO up to 29.7%. Cu/HFER was more active than Cu/K-FER, which was more selective to acetol and 1,2-PDO. Ni/13X was very active, 79.9% conversion, but few selective to acetol and 1,2-PDO; methane was the only product detected in the gas stream while EG was the major one in the condensed fraction. 1,3-PDO is used as a solvent for the production of adhesives, laminates, and paints, and in coolant formulations, although it can have the same applications as EG, 1,2-PDO, 1,3butanediol, and 1,4-butanediol. Nevertheless, its use as monomer in polycondensation reactions concentrates attention to produce polyesters, polyethers, and polyurethanes; 1,3PDO copolymerized with acid produces polytrimethylene-terephthalate (PTT) polymers, recognized for their excellent elastic properties, and marketed with the commercial name Corterra™ and Sorona®, by Shell Chemical Company and DuPont, respectively. Both companies produce the largest commercial amount of 1,3-PDO, DuPont hydrating acrolein to 3-HPA followed by hydrogenation to 1,3-PDO, while Shell produces it by hydroformylation of ethylene oxide followed by hydrogenation. The high pressure applied in both hydroformylation and hydrogenation steps together with the high temperature, the use of expensive catalysts, and the release of toxic intermediate compounds are problems in those

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processes (Saxena et al., 2009). By considering 1,3-PDO, a selective hydroxylation involving three steps, acetalization, tosylation, and detosylation was reported as an alternative synthesis way (Wang et al., 2003). Kurosaka et al. (2008) studied the selective hydrogenolysis of Gro to 1,3-PDO on Pt/WO3/ZrO2, using 1,3-dimethyl-2-imidazolidinone (DMI) as a solvent, while Gong et al. (2009) studied the effect of both protic and aprotic solvents such as sulfolane, DMI, ethanol, and water, using the same catalyst. Other materials employed to produce propanediols by the hydrogenolysis of Gro are Pt/WO3/TiO2/SiO2 (Gong et al., 2010), Pt/amorphous silica-alumina (Gandarias et al., 2010), and Pt on MgO, HLT (hydrotalcite), and Al2O3 (Yuan et al., 2009). Pt/WO3/ZrO2 and Pt/WO3/Zr(OH)4 allow forming 1,3-PDO at ratios higher or similar to 1,2-PDO; different ratios allow to consider the effect of catalyst preparation on the catalytic behavior which can be associated to the interaction of Pt and tungsten, the modifying of active sites, and/or the formation of new ones. It has been previously reported for this catalytic system (Vaudagna et al., 1997). The production of 1,3-PDO by hydrogenolysis of Gro on Pt/WO3-ZrO2 was previously reported at higher pressures, 5.5 and 8.0 MPa, higher weigh ratio of catalyst/Gro, and in the presence of organic solvents (Kurosaka et al., 2008). In 2007, Davy Process Technology Ltd., a Johnson Matthey company in a joint venture with Ashland and Cargill announced the production of 1,2-PDO, starting from renewable resources in the context of sustainable chemical technology search; Gro is reacted with hydrogen on a heterogeneous copper catalyst under relatively moderate conditions (20 bar, 200 ºC), being conversion around 99%, adequate selectivity to the desired product, and byproducts removed by distillation. Senergy Chemical, a consortium of propylene glycol consumers and marketers has also licensed a process for the first commercial facility. Renewable Alternatives was the first to focus on creating an antifreeze based on Gro mixed with propylene glycol. Products containing propylene glycol are slightly more expensive, but the new process will bring the price down and make it the predominant product. There is also a significant positive environmental advantage, since the ethylene glycol currently in almost universal use is toxic, whereas propylene glycol is not.

2.3. Selective Oxidation Reactions Primary products of oxidation of Gro are glyceraldehyde, glyceric acid, tartronic acid, and dihydroxyacetone (DHA); tartronic acid can be oxidized to glycolic, glyoxylic, oxalic, and mesoxalic acids, while the latter one can be obtained by oxidation of DHA, being hydroxypyruvic acid an intermediate product. The main products have not been yet developed due to low selectivities and yields reached with the existing processes, which operate with low concentration solutions of Gro. Glyceric acid, is mostly produced by a fermentation process (Kenji et al., 1989; Teruyuki and Yoshinori, 1989). Hydroxypyruvic acid is obtained by oxidation of Gro or sugars using mineral acids and it is the precursor of serine amino acid. Tartronic acid is widely used as a precursor of major products such as oxalic acid (Fordham et al., 1995). In aqueous solution, DHA is found as monomer, which can gradually tautomerize to glyceraldehyde. The equilibrium between both compounds depends largely on pH; DHA is favored in an acid medium (greater stability at pH 3) while glyceraldehyde in an alkaline one (Yaylayan et al., 1999). DHA is a chemical used in the cosmetic industry to make artificial

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tans, varying its concentration between 2 and 5%; people with sensitive skin should limit exposure to sun due to the consequences that may ensue. Due to its potential capacity of taining, DHA is also used for the treatment of vitiligo, an autoimmune disease that affects the melanocytes. DHA also finds an important application in the chemical industry as reagent of great versatility for the production of compounds like lactic acid, hydroxypyruvic acid, or 1,2PDO (Hekmat et al., 2003; Bicker et al., 2005). Considering the oxidation of Gro in liquid phase using heterogeneous catalysts, both nature of metal and pH of medium affect the reaction selectivity. Catalytic oxidation of Gro using Pt supported on carbon (Pt/C) was more efficient than the conventional fermentation process, decreasing drastically the DHA selectivity in a basic medium (pH 8) and being less than 10% in an acid one (2-3 pH), increasing that selectivity up to 80% by incorporating bismuth on Pt/C (Kimura et al., 1993). In the same way, Abbadi and van Bekkum (1996) studying the oxidation of both Gro and DHA on Bi-Pt/C at 65°C, reached 95% conversion of Gro and 93% selectivity to glyceric acid at pH 5-6, being DHA, hydroxypyruvic acid, and oxalic acid the main products obtained under acidic conditions. The oxidation of primary or secondary hydroxyl group can be favored controlling reaction conditions (Garcia et al., 1995); Gro conversion was 90% with 70% glyceric acid and 8% DHA using Pd/C, while the DHA yield was 37% with 75% conversion on Bi-Pt/C at pH 2. Using gold catalysts supported on carbon, the selectivity to glyceric acid decreased markedly when gold nanoparticles with average diameter of 6 nm are well dispersed on the surface, while the selectivity increased to 92-95% with nanoparticles larger than 20 nm, indicating the importance of preparation method on performance in the oxidation reaction of Gro (Porta and Prati, 2004). Monometallic catalysts such as Pt/C, Pd/C, and Au/C are able to produce glyceric acid, while bimetallic ones as Au-Pd/C and Au-Pt/C produce tartronic acid and glyceraldehyde, respectively (Bianchi et al., 2005). Supported catalysts were prepared by impregnation following the incipient-wetness techniques, being identified as Pt/C, Pt-Bi/C, Pt/K-FER and Pt-Bi/K-FER (Antuña and Comelli, 2010). Pt/C showed activity and oxidized Gro (34.9% conversion) but without producing DHA, agreeing with previous results indicating this catalyst as selective to glyceric acid; the addition of a second metal as Bi on Pt/C promoted the selective oxidation of Gro to DHA, being it associated to the changing on the environment of the active site (Kimura et al., 1993). Pt/K-FER having only Pt displayed activity and selectivity to DHA, allowing that zeolitic support the oxidation of the secondary hydroxyl group of Gro; Pt-Bi/K-FER reached the best performance in the selective oxidation to DHA with 48.3% Gro conversion and 90.5% selectivity to DHA.

2.4. Reforming Reactions In the last few years, there has been a growing interest in environmentally clean renewable sources for hydrogen production. In this context, new technologies have been developed for Gro reforming. A complete review related to the processes capable to convert Gro into hydrogen was carried out by Adhikari et al. (2009). The processes include steam reforming, partial oxidation, autothermal reforming, aqueous-phase reforming, and supercritical water reforming; their main characteristics are:

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Steam reforming process: the substrate is reacted with steam in the presence of a catalyst to produce hydrogen, carbon dioxide, and carbon monoxide, being a highly endothermic process. Thermodynamically, it is favored by high temperatures, low pressures, and an excess of steam. This process is the most exhaustively studied to produce hydrogen from Gro. Partial oxidation process: the substrate is reacted with oxygen at sub-stoichiometric ratios. The oxidation reaction results in heat generation and high temperature. Reforming in the presence of air allows to balance the energy required for the process by oxidizing some of the substrate. Autothermal process: it combines the effect of partial oxidation and steam reforming by feeding together fuel, air, and water. This process is carried out in the presence of a catalyst. The steam reforming process absorbs the heat generated by the partial oxidation one; then, the main benefit is that, ideally, it should not require any energy for reaction to occur whereas the steam reforming is a highly endothermic process. Although the autothermal steam reforming process has advantages over the conventional steam reforming one, the amount of hydrogen produced from that process would be less on a thermodynamic basis. Aqueous-phase reforming process: it is a relatively new process and has opened a new pathway for hydrogen production from alcohols and sugars. It operates at relatively higher pressures and lower temperatures than the steam reforming process. The main advantage is that it is a liquid phase process and most biomass based liquids are difficult to vaporize; another advantage is the process also produces less amount of CO. Supercritical water reforming: it is performed under the critical temperature (374ºC) and pressure (22.1 MPa) of water. Hydrogen has been produced from Gro by this process using a Ru/Al2O3 catalyst.

By considering the steam-reforming of Gro, Adhikari et al. (2007a) performed a thermodynamic equilibrium study, resulting temperature higher than 900 K, atmospheric pressure, and 9:1 water:Gro molar ratio as the best conditions for producing hydrogen and minimizing the methane production. Wang et al. (2008) determined optima conditions for hydrogen production between 925 and 975 K and water:Gro ratios of 9-12 at atmospheric pressure, whereas higher temperatures and lower reactant ratios at 20-50 atm favored the production of synthesis gas. Hirai et al. (2005) reported the production of hydrogen using ruthenium catalyst, reaching Ru/Y2O3 the best performance; at 500°C, the hydrogen yield increased by increasing the ruthenium loading up to 3 wt.%, while a further increment to 5 wt.% did not affect the behavior. Zhang et al. (2007) studied the hydrogen production by reforming of ethanol and Gro over ceria-supported Ir, Co, and Ni catalysts, showing Ir/CeO2 a quite promising catalytic performance with 100% Gro conversion and hydrogen selectivity higher than 85% at 400ºC. Adhikari et al. (2008) measured the hydrogen production using Ni/CeO2, Ni/MgO, and Ni/TiO2 catalysts; Ni/CeO2 having the highest surface area and metal dispersion, reached the best performance in the steam-reforming process gaving the maximum hydrogen selectivity (74.7%) at 600ºC, 12:1 water:Gro molar ratio, and 0.5 mL/min feed flow rate, while the maximum hydrogen yield was obtained at 650ºC using MgO. Cui et al. (2009) evaluated the steam reforming of Gro on non-substituted and partially

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Ce substituted La1_xCexNiO3 mixed oxides, being La0.3Ce0.7NiO3 highly active with conversions nearly to the equilibrium at 500 and 700ºC, and forming the smallest amount of carbonaceous deposits. The viability of steam reforming by combining Gro and water byproduct streams of a biodiesel plant was studied on a platinum alumina catalyst; a high gas yield (almost 100%) with 70% selectivity (dry basis) can be reached at high temperatures (Slinn et al., 2008). Adhikari et al. (2007b) reported the behavior of fourteen catalysts prepared on ceramic foam monoliths (92% Al2O3 and 8% SiO2) following the incipient wetness impregnating technique; Ni/Al2O3 and Rh/CeO2/Al2O3 reached the best conversion and selectivity, respectively. Valliyappan et al. (2008) evaluated the steam gasification in the presence of a commercial Ni/Al2O3 catalyst in the range of steam to Gro weight ratio of 0:100–50:50 to produce either hydrogen or syngas. Pure Gro was completely converted to gas containing 92 mol% syngas at 50:50 weight ratio of steam to Gro. Monometallic (Ni or Pt) and bimetallic (Pt–Ni) catalysts and the effect of lanthana modified alumina support was evaluated during the Gro steam reforming; the lanthana addition on alumina improved catalytic activity of Ni catalysts, while the best selectivity to hydrogen was reached with the catalyst modified with the intermediate content of lanthana (Iriondo et al., 2009). Profeti et al. (2009) investigated the activity of Ni/CeO2–Al2O3 catalysts modified with noble metals (Pt, Ir, Pd, and Ru); the formation of inactive nickel aluminate was prevented by the presence of CeO2 dispersed on alumina while the highest catalytic performance for the Gro steam reforming was obtained with the NiPt catalyst, producing an effluent gaseous mixture with the highest H2 yield and low amounts of CO. Sánchez et al. (2010) using a Ni-alumina catalyst reached 96.8% conversion at 600ºC increasing it to 99.4% at 700ºC. Dou et al. (2009) evaluated a commercial Ni-based catalyst and a dolomite sorbent for the steam reforming reaction and in situ CO2 removal, respectively. Hydrogen productivity was greatly increased with increasing temperature and the formation of methane by-product became negligible above 500ºC. Chiodo et al. (2010) investigated features of Rh and Ni supported catalysts in the steam reforming of Gro to produce syngas; at temperature higher than 720 K, Gro drastically decomposes before to reach the catalyst surface, and Rh/Al2O3 resulted more active and stable than Ni supported catalysts. The hydrogen production from Gro by the aqueous phase reforming process using supported-Pt catalysts was studied (Luo et al., 2008). The reaction pathways showed that hydrogen generation is accompanied by side reactions to form alkane and liquid products. Lehnert and Claus (2008) studied the influence of Pt particle size and support type on the aqueous-phase reforming of Gro; independent of the metal precursors employed, similar Gro conversion (45%), and identical selectivity to hydrogen (85%) were obtained. The use of crude Gro as starting material was successful, although the rate of hydrogen production was lower than the one obtained feeding pure Gro, and decreased dramatically after 4 h-onstream; it was related to impurities present in the crude Gro. Iriondo et al. (2008) compared the hydrogen production from Gro reforming in both liquid (aqueous phase reforming) and vapor (steam reforming) phase over alumina-supported nickel catalysts modified with Ce, Mg, Zr, and La; catalytic activity indicated different catalyst functionalities necessary to carry out aqueous-phase and vapor-phase reforming of Gro. For the aqueous phase reforming, the addition of Ce, La, and Zr to Ni/Al2O3 improved the Gro conversion respect to the unpromoted Ni/Al2O3; moreover, all samples showed important deactivation which was associated to oxidation of the active metallic Ni during reaction. In the Gro reforming in vapor phase, Ce, La, Mg, and Zr on Ni based catalysts promoted the hydrogen selectivity.

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The operating conditions affect stability of Ni-supported catalysts, causing deactivation. Carbon deposition on the catalyst surface will result several undesirable reactions and products affecting the purity of the reforming products. Carbon occurrence may arise due to the decomposition of CO or CH4 or the reaction of CO2 or CO with H2 (Adhikari et al., 2007b); carbon formation is thermodynamically inhibited at temperature higher than 900 K, atmospheric pressure, and 9:1 water:Gro molar ratio (Adhikari et al., 2007c). At water:Gro molar feed ratios lower than 3:1, the insufficient steam supply produced the methane decomposition forming solid carbon, which is an undesirable substance that decreases the hydrogen production and also causing catalyst deactivation (Rossi et al., 2009). The use of a CO2 adsorbent can suppress the carbon-formation reaction and substantially reduce the lower limit of the water:Gro feed ratio (Chen et al., 2009). A minimal degradation of a platinumalumina catalyst took place after several days of continuous operating under the optimum conditions for Gro reforming, only 0.4% of feed was deposited (Slinn et al., 2008). Hydrogen production on a supported-Pt catalyst is accompanied by side reactions which form carbonaceous entities on the surface causing catalyst activity drop (Luo et al., 2008). Nickel catalysts supported on commercial Al2O3 and Al2O3 modified by addition of ZrO2 and CeO2 were evaluated; Ni/CeO2/Al2O3 was the most stable system; it was associated to the Ce effect in inhibition of secondary dehydration reactions forming unsaturated hydrocarbons that are coke precursors generating fast catalyst deactivation (Buffoni et al., 2009). Using Ni/CeO2– Al2O3 catalysts modified with Pt, Ir, Pd, and Ru, the presence of noble metals stabilized the Ni sites in the reduced state along the reforming reaction, increasing conversion and decreasing coke formation (Profeti et al., 2009). Chiodo et al. (2010) investigated features of Rh and Ni supported catalysts in the steam reforming of Gro, independently of both impregnated metal and temperature, reaction is affected by coke formation mainly promoted by the presence of olefins formed by Gro thermal decomposition. Recently, characterization of deactivation processes of Ni impregnated on alumina during the hydrogen production from Gro and evaluation of regenerating conditions was made, indicating that selecting adequate conditions, activity can be restored (Sánchez and Comelli, 2012). Finally, reforming of Gro can also produce synthesis gas which containing mainly carbon monoxide and hydrogen; syngas can be converted into methanol using conventional technology, and methanol used for methyl esterification of vegetable oils, then, 100% biomass-based bio-diesel fuel could be obtained. In the same way, BioMethanol Chemie Nederland BV uses this technology to produce biomethanol. Another commercial potential of the aqueous phase reforming technology is reinforced by coupling with the Fisher–Tropsch process, providing the characteristic product streams.

2.5. Miscelaneous Reactions: Chlorination, Etherification, Esterification, Nitration, and Polymerization Direct hydrochlorination of Gro is an attractive way to produce 1,3-dichloro-2-propanol, an intermediate to obtain epichlorohydrin, which interests to produce ether polymers, including the commercially valuable epoxy resins. From 2007, Solvay started the production of epichlorohydrin from Gro obtained as by-product of biodiesel. Previously, Solvay produced both epichlorohydrin and Gro from propene, but the increment of propene prices

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and the decrease of the Gro one, induced that company stop the production of synthetic Gro from epichlorohydrin, reversing the process, and converting the plant to produce epichlorohydrin from Gro. This process based on organic acid catalysts displays advantages over the propene route. Reaction to produce dichloropropanol takes place using anhydrous hydrochloric with 30% caprylic acid as catalyst at above 120 ºC, in order to minimized corrosion; then, epichlorohydrin is obtained by dechlorination with NaOH, recovering NaCl which is used to produce chlorine by electrolysis and recycling the fraction rich in water to the hydrodechlorination step. Reacting Gro with isobutene or tert-butanol produces glycerol ethers, which can be added to fuels, also including polyglycerols and glycosyl glycerol. Both high polarity and hygroscopic character of Gro make it unsuitable as a fuel additive in unmodified form; other problems are the Gro polymerization at high temperatures and the partially oxidised into toxic acrolein. Then, glycerol ether formulations can be commercialized as oxygenate fuel additives for use in gasoline engines. The CPS Biofuels company react Gro coming from both biodiesel and ethanol facilities with low molecular weight alkenes supplied by petrochemical producers, for producing glycerol tert-butyl ether (GTBE), a green, renewable, sustainable, non-toxic fuel additive. Esterification of Gro with carboxylic acids results in monoacylglycerols (MAGs) and diacylglycerol (DAG), which have well established industrial applications, particularly in food and oleochemical industries. MAGs are amphilic molecules and useful as nonionic surfactants and emulsifiers, while DAGs have the advantage of stability to decomposition at cooking temperatures and their human nutritional characteristics compared to triacylglycerol (TAG) clearly indicates the important suppressive effect by DAG on body fat accumulation. Reacting Gro with nitrating agents, the nitroglycerine which is an explosive and also used as an antianginal drug, can be produced; it is currently produced in modular plants. Another reaction product can also be dinitroglycerol, which treated with a cyclizing agent is converted into glycidyl nitrate, that can be polymerized to poly(glycidyl nitrate) (PGN). Polyglycerol is a clear viscous liquid, highly soluble in water and in polar organic solvents such as methanol, being nonvolatile at room temperature. Its viscosity increases with the molecular weight. Combined with Amberlyst 35 ion-exchange resin is an excellent catalyst for glycerol alkylation with isobutene. Polyglycerols are commercially available for different applications including from cosmetics to controlled drug release; biocompatibility is an attractive feature of aliphatic polyether structures containing hydroxyl end-groups.

3. BIOLOGICAL APPROACHES TO VALORIZE GLYCEROL The abundance of Gro in nature established that many organisms, mainly microorganisms can metabolize the polyol, utilizing it as a source of carbon and energy. Different metabolic pathways followed by glycerine are found depending on the microorganism. These differences are relevant in considering biological tools to add value to the polyol, because many intermediate metabolites or final products are of utility for specific industries and for development of biorefinery strategies. We will analyze possibilities for using entire microorganisms or enzymes involved in key metabolic steps as biological and molecular tools. Biological strategies can allow utilize Gro for different purposes, mainly: (i) as a raw

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substrate for growing microorganisms of strategic value by themselves (i.e. as single cell additive to cattle feeding); (ii) as a raw substrate to grow transgenic cells producing recombinant proteins and enzymes, medicinal drugs, pigments, biosurfactants, or metabolites of high value; or (iii) to convert Gro in key metabolites of value as chemicals by using specific set of enzymes or genetically altered metabolic pathways. Figure 2 schematize about variants in the use of Gro for industrial microbiology.

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Figure 2. Glycerol substrate in industrial microbiology. References I and II: glycerol conversion to 1,3PDO and DHA, respectively, by specific enzymes (see biochemical reactions in text).

3.1. The Meeting History between Glycerol and Microorganisms Relationships between Gro and microorganisms were first documented by Pasteur, who reported, in 1858, the polyol presence in yeast culture broth, thus evidencing the capacity of microorganisms to metabolize Gro by fermentation. This scientific finding brought forth the microbial industrial production of Gro, especially during World War I (Wang et al., 2001); creating a successful biotechnological enterprise that short after was gloomed by the development of processes for its chemical synthesis, especially before the World War II. Great relevance that existed for obtain Gro in relation with army conflicts is clear considering the use of nitroglycerin for production of dynamites and explosives. The surplus of Gro originated lately by the rapid expansion of biodiesel production is forcing to manufacture companies to shut down plants for its chemical synthesis (McCoy, 2006). On the other hand, the glut is refreshing the old association between the polyol and microorganisms; but now in the substrate sense. Thus, Gro is visualized as a carbon-energy source for microbial growth, paving the way to produce biomass (Juszczyk and Rymowicz, 2009) recombinant proteins (Giordano et al., 2010), bioplastics (Mothes and Otto, 2009), single cell oils (Papanikolaou et al., 2008) or valuable metabolites (Papanikolaou et al., 2008; Mantzouridou, 2009; Musial and Rymowicz, 2009; Rywinska and Rymowicz, 2009). Thus, the overall scenario for

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biological processing of Gro is clearly related with the use of the polyol as a key compound for applications in biorefinery-like procedures.

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3.2. Glycerol as Carbon and Energy Microbial Source. The First Step to Production Gro qualifies as a suitable source for carbon-energy satisfying demands for cell proliferation of many chemo-organo-heterotrophic microorganisms (Peters, 2007). Between them, worth to highlight is the enterobacterium Escherichia coli and the yeast Saccharomyces cereviseae, because they are model organisms profusely employed in bioprocesses producing recombinant proteins or other transgenic or non-transgenic technologies. Other microorganisms able to metabolize Gro aerobically or anaerobically include species form the genus Klebsiella, Clostridium, Enterobacter, Citrobacter, and Lactobacillus (da Silva et al., 2009). The first condition for bioprocessing of Gro, is its uptake by the cell. Although Gro can cross the plasmatic membrane through passive diffusion, its more effective transport is mediated by specific permeases (see Figs. 3 and 4). E. coli permeases belong to the group of aquaporines, which function as a passive carrier (Sanno et al., 1968; Richey and Lin, 1972; Sweet et al., 1990). The permease is also known as the Gro facilitator (Fu et al., 2000), and it acts as a channel with stereo- and enantio-selectivity for many non-ionic compounds including Gro, polyalcohols, and urea derivatives. In S. cereviseae, in addition to a channel protein for facilitated diffusion, it was also identified an active mechanism for transport of the polyol (da Silva et al., 2009). Once inside the cell, Gro can be metabolize by different pathways, depending of the specific microorganism (da Silva et al., 2009). The polyol oxidation for obtaining cellular energy can occur by respirative or fermentative routes, depending on which is the final electron acceptor of the process. Oxidation can also take place via primary conversion to glycerol-3-phosphate (Gro3P) or by the route producing DHA (see Figs. 3 and 4, respectively, for details). Under aerobic respiration (Fig. 3), such as that able to occur in E. coli, oxygen is the molecule accepting electrons and full oxidation of Gro to carbon dioxide involves the tricarboxylic acid cycle (TCA). But in anaerobiosis other compounds (such as nitrate or formate) are the chemical species reduced, and this process for energy obtaining is referred as anaerobic respiration. The latter oxidation can also be performed by E. coli strains in the absence of oxygen. Oxidative metabolisms for glycerine are fully active in yeasts. The intermediate metabolite Gro3P, produced in the pathway involving Gro kinase, can also serve as a precursor for lipid biosynthesis. Utilization of Gro as a source of carbon and energy for growing a microorganism employed in a productive process is a relevant strategy in adding value to the polyol, and the associate use for make recombinant proteins is a key option (Giordano et al., 2010). Normally, aerobic conditions are the preferred for developing a bioprocess for recombinant protein production in E. coli. In them, higher biomass yield can be obtained, and proteins synthesis is proportional to formed biomass. Cultures under aerobiosis perform maximal substrate oxidation and produce higher energy (ATP) levels. However, and despite the lesser energy obtained by a fermentative process, the latter is the more selected when the aim is the synthesis of different metabolites (Fig. 4). It was in 1936 when probably the first report (but not concluding) about Gro fermentation by E. coli was published (Dozois et al., 1936). Then,

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employing classical microbial assays, they were detected acid and gas production in fermentative culture conditions of this bacterium. Contemporary studies call in doubt the Gro fermentation by E. coli [see comment and references in (Gonzalez et al., 2008)]. However, recent reports vindicate that capability by applying molecular methodologies and fermentation technologies (Dharmadi et al., 2006; Gonzalez et al., 2008; Murarka et al., 2008), as well as modelling (Cintolesi et al., 2012). As it is detailed in Figure 4, in wild type E. coli relevant fermentative products are ethanol, hydrogen and carbon dioxide. From the latter, they are relevant ethanol and hydrogen as key biofuels that can be produced by processing Gro by biotechnological processes (Fig. 2).

Figure 3. Main intermediaries and pathways during glycerol oxidation via respiration in E. coli. References: 1': Glycerol kinase (EC 2.7.1.30); 2': Glycerol-3-phosphate dehydrogenase (EC 1.1.5.3); 3: Triosephosphate isomerase (EC 5.3.1.1); 4: Glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12); 5: Phosphoglycerate kinase (EC 2.7.2.3); 6: Phosphoglycerate mutase (EC 5.4.2.1); 7: Enolase (EC 4.2.1.11); 8': Pyruvate kinase (EC 2.7.1.40); 9': Pyruvate dehydrogenase (EC 1.2.1.51). Gro-3-P: glycerol-3-phosphate; DHAP: dihydroxyacetone phosphate; G-3-P: Glyceraldehyde 3-phosphate; 1,3DPG: 1,3-diphosphoglycerate; 3-PG: 3-phosphoglycerate; 2-PG: 2-phosphoglycerate; PEP: phosphoenolpyruvate; PYR: pyruvate; AcCoA: Acetyl coenzyme A; TCA: tricarboxylic acid cycle.

In Klebsiella, Clostridium, Enterobacter and Citrobacter the polyol can be metabolized by oxidation or by a reductive route involving two enzymatic steps (da Silva et al., 2009). Firstly, Gro is converted to 3-hydroxy-propionaldehyde by Gro dehydratase [an enzyme using cobalamin (vitamin B12) as a coenzyme] and related diol dehydratases (Seifert et al., 2001).

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The second step is mediated by 1,3-PDO dehydrogenase, catalyzing the NADH-dependent reduction of 3-hydroxypropionaldehyde to 1,3-PDO. This reductive pathway for Gro is unique for obtaining 1,3-PDO by biological fermentation.

Figure 4. Fermentation pathway of glycerol in E. coli. References: Glycerol dehydrogenase (EC 1.1.1.6); 2: Dihydroxyacetone kinase (EC 2.7.1.29); 3: Triosephosphate isomerase (EC 5.3.1.1); 4: Glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12); 5: Phosphoglycerate kinase (EC 2.7.2.3); 6: Phosphoglycerate mutase (EC 5.4.2.1); 7: Enolase (EC 4.2.1.11); 8: Pyruvate formate-lyase (EC 2.3.1.54); 9: Formate hydrogen lyase complex (EC 1.2.1.2 plus EC 1.12.1.2; or EC 1.1.99.33); 10: Alcohol / acetaldehyde dehydrogenase (EC 1.1.1.1; EC 1.2.1.10); Gro: glycerol; DHA: dihydroxyacetone; DHAP: dihydroxyacetone phosphate; G-3-P: Glyceraldehyde 3-phosphate; 1,3DPG: 1,3-diphosphoglycerate; 3-PG: 3-phosphoglycerate; 2-PG: 2-phosphoglycerate; PEP: phosphoenolpyruvate; PYR: pyruvate; FA: formate; AcCoA: Acetyl-coenzyme A; EtOH: ethanol; P: phosphate; H: NADH+ + H+ or NADPH+ + H+.

3.3. Relevant Products from Glycerol Bioconversion As described above, biological processing of Gro may pursue the use of the polyol as the raw material for growing cells. Such cells may be of value by themselves (i.e. for feeding usage; namely SCP for Single Cell Proteins), or because they are genetically modified microorganisms generating recombinant proteins or enzymes of highly added commercial value. Other relevant approach is the biological use of glycerin to obtain key metabolites that are valuable compounds. The latter can be reached by using entire wild-type or transgenic organisms, or enzyme(s) catalyzing key steps in the metabolism of Gro in one specific cell.

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3.3.1. Bioconversion of Glycerol to Dihydroxyacetone DHA is one compound that theoretically could be straightforwardly obtained by bioconversion of Gro. The process requires of a unique step catalyzed by Gro dehydrogenase (GroDHase):

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Gro + NAD+

DHA + NADH + H+

Depending on the microorganism, two type of GroDHases can be found; the soluble enzyme (EC 1.1.1.6), typically occurring in E. coli (Asnis and Brodie, 1953; Truniger and Boos, 1994), or the membrane bound enzyme (EC 1.1.99.22) as that present in Gluconobacter oxydans (Gatgens et al., 2007). Globally, GroDHases have been identified in different organisms, including bacteria, yeasts and mammals (Ruzheinikov et al., 2001). Given the high commercial relevance of DHA because it is used in cosmetics and is a key compound for organic synthesis of valuable chemicals, different processes were developed for its production (da Silva et al., 2009). Biological procedures for synthesize DHA are economically more convenient than those of chemical production. G. oxydans is one microorganism frequently utilized to convert Gro into DHA with some degree of efficiency (Gatgens et al., 2007). Although different variants were implemented to optimize procedures, the microbial process has as a main trouble the fact that both, the substrate and the product negatively affect because they inhibit bacterial growth (da Silva et al., 2009). A possibility that has not yet completely explored is the development of methodology to use isolated enzymes for transforming the polyol into the hydroxyketone. For the latter it could be convenient to consider the soluble rather than the membrane-bound GroDHase and also some characteristics of the E. coli enzyme, as in example that it exhibits thermostability (Asnis and Brodie, 1953). A main obstacle in the use of a system of isolated enzyme is the necessity of regenerating the oxidized form of the coenzyme (NAD+) from NADH, and some strategies have been proposed to solve the problem (Nemeth and Sevella, 2008). However, this type of biotechnological designs are quite promising for improve the production of a valuable chemical as DHA.

3.3.2. Bioproduction of 1,3-Propanediol From Glycerol 1,3-PDO is the major product of microbial reductive fermentation of Gro (da Silva et al., 2009). The biological pathway involves two steps. It courses with Gro dehydration catalyzed by vitamin B12-dependent Gro dehydratase (EC 4.2.1.30) and associated diol dehydratases (EC 4.2.1.28), followed by the reductive stage mediated by NADH-dependent 1,3-PDO dehydrogenase (EC 1.1.1.202); as respectively detailed by next equations: Gro

3-hydroxy-propionaldehyde + H2O

3-hydroxypropionaldehyde + NADH + H+

1,3-PDO + NAD+

The relevance of 1,3-PDO as a commodity chemical, mainly utilized in the industrial production of plastics (such as polyester fibers and polyurethanes) as well as cyclic derivatives, has been highlighted above (see section 2.1.2). Although the high value of the compound, its chemical synthesis is relatively expensive, and the current situation is demanding designs for more convenient biotechnological procedures. Many developments for

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the production of 1,3-PDO have been established, with the uses of wild-type or genetically modified microorganisms. For obtaining molecular tools related with 1,3-PDO metabolism, they are important species of the genus: Clostridium, Citrobacter, Klebsiella, Lactobacillus, and Bacillus, between others (da Silva et al., 2009). In these organisms, the enzymes of the metabolic pathway are coded by genes grouped in the dha regulon. The kinetic characteristics of the enzymes involved critically limit the biological production, since they undergo a suicide-inactivation exerted by the substrate (Daniel et al., 1998; Knietsch et al., 2003). Gro dehydratase (strictly glycerol hydro-lyase) and diol dehydratase (specifically D,L-1,2propanediol hydro-lyase) utilize cobalamin as an essential cofactor, in a reaction proceeding via a radical mechanism that leads to the irreversible cleavage of the Co—C bound present in vitamin B12. These enzymes are multisubunit (of the type α2β2γ2) and reactivation of the Groinactivated forms involves a heterodimeric protein complex and requires ATP (Daniel et al., 1998).

3.3.3. Bioconversion of Glycerol to Commercially Valuable Organic Acids Organic acids of use in different industries that can be obtained form microbial conversion of Gro include succinic acid, propionic acid, and citric acid (da Silva et al., 2009). As detailed in Figures 3 and 4, the metabolism of the polyol produces phospho-enol-pyruvate (PEP), which is the metabolite from which initiates the pathway leading to succinic acid. PEP can be converted to oxaloacetic acid (OAA) by reactions catalyzed by PEP carboxykinase (4.1.1.32) or PEP carboxylase (EC 4.1.1.31), respectively: PEP + CO2 + ATP OAA + ADP ‾ PEP + HCO3 OAA + inorganic orthophosphate (Pi)

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OAA is then reduced by NADH-dependent malate dehydrogenase (EC 1.1.1.37): OAA + NADH + H+

malic acid + NAD+

and malic acid is reversibly dehydrated to fumaric acid (also of value for some industries) by fumarase (or fumarate hydratase, EC 4.2.1.2): malic acid

fumaric acid + H2O

Finally, two distinct membrane-bound enzyme complexes (EC 1.3.99.1) can catalyze the interconversion of fumaric acid and succinic acid: fumaric acid + XH2

succinic acid + X

Under anaerobic growth, the enzyme is fumarate reductase, whereas succinate dehydrogenase is involved in aerobic growth. The XH2 electron donor is a component of the membrane. Both complexes are formed by a membrane-extrinsic component containing a FAD-binding flavoprotein and an iron-sulfur protein, plus a hydrophobic membrane anchor protein and/or cytochrome-b (Blaut et al., 1989). In a further step, succinic acid can be converted into propionic acid in a reaction mediated by enzymes of the type propionyl-

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CoA:succinate CoA transferase (EC 2.8.3.-) related to the crotonase superfamily (Haller et al., 2000): propionyl-CoA + succinate

propionate + succinyl-CoA

On the other hand, citric acid is an intermediate of the Krebs cycle, being synthesized by the reaction of citrate synthase (EC 2.3.3.1): Acetyl-CoA + H2O + OAA

citrate + CoA + H+

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Succinic acid is a chemical of important apply in food and pharmaceutical industries, also being of utility to chemically produce 1,4-butanediol as well as to obtain surfactants, detergents and biodegradable plastics. Microbial making of succinic acid from Gro fermentation currently employs Anaerobiospirillum succiniproducens. For propionic acid production, microorganisms useful are Propionibacterium spp. and Clostridium propionicum; being this acid taken up by industries of biodegradable plastics, thermoplastics, herbicides, arthritis drugs and perfumes. Whereas with respect to citric acid, its application is principally to give fruity flavour to foods and beverages; also serving as additive in detergents, and pharmaceuticals. Biotechnologies to produce citric acid are based in Aspergillus niger and the yeast Yarrowia lipolytica, with this latter organisms growing conveniently on Gro (da Silva et al., 2009).

3.3.4. Other Valuable Bioconversions of Glycerol As detailed in Figures 2-4, bioconversion of Gro can also produce metabolites (intermediate of final products) of importance as biofuels. Main examples are the production of ethanol and, interestingly, the possibility to obtain hydrogen (considered an ideal clean fuel after that its oxidation gives water: H2 + ½O2  H2O). By fermentation it is also possible to generate methane and similar biogases. Of interest are the studies from Dharmadi et al. (2006) showing that fermentation of Gro by E. coli is pH-dependent and that conditions can be settled to improve the process to generate ethanol and hydrogen. On the other hand, glycerine can also be utilized as a substrate for growing of microorganisms producing industrially key derivatives, as in examples: (i) the plastic-like polymer polyhydroxyalcanoate (polyesters accumulated by some bacteria intracellularly as a reserve of carbon and energy); (ii) biosurfactants, of the ionic and non-ionic types, as rhamnolipids produced by Pseudomonas aeruginosa; and (iii) pigments, like in the production of the red/orange compound astaxanthin (utilized to feed salmon, trout and crustaceans) or prodigiosin, a red pigment produced by Serratia marcescens that is an active immunosupressor and inductor of apoptosis, after which is of value in anti-cancer drugs (da Silva et al., 2009). A main product of biological oxidation is CO2, a gas useful in several industries.

CONCLUDING REMARKS Gro is a chemical of wide application in diverse industries that, paradoxically, is experiencing a striking loss in its value in the last decade. The main reason for the latter is the

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fact that Gro is a main by-product in the generation of biodiesel, after which the former is experiencing a huge over-offer. The situation created a commitment to find new uses and applications for glycerine and to develop (bio)technological strategies to rise-up its commercial value. The scenario is interesting when it is associated with development of processes of the biorefinery type. In such a way, Gro is a compound with key properties that are suitable for application in procedures for the generation of different goods of value as biofuels, bioplastics, food-additives, and active ingredients in pharmaceutical, cosmetic, and chemical industries. Many strategies to add-value to Gro are currently underway, with basis either in chemical or biological approaches. Besides the relevance that each approach has by itself, it is worth to have an integrated management, thus combining the toughness of the tools each one affords. The latter is a specific biorefinery-like activity, where expertises in the areas of chemistry and biology come together to solve complex technological problems in a framework compatible with a sustainable environment.

ACKNOWLEDGMENTS Work in our laboratories are supported by grants from Universidad Nacional del Litoral (CAI+D Orientado y Redes), ANPCyT (PICT‘08 1754), and CONICET (PIP 2519). AAI and RAC are members of the Investigator Career from CONICET.

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Rossi CCRS, Alonso CG, Antunes OAC, Guirardello R, Cardozo-Filho L (2009) Thermodynamic analysis of steam reforming of ethanol and glycerine for hydrogen production. International Journal of Hydrogen Energy 34: 323-332. Roy D, Subramaniam B, Chaudhari RV (2010) Aqueous phase hydrogenolysis of glycerol to 1,2-propanediol without external hydrogen addition. Catalysis Today 156: 31-37. Ruzheinikov SN, Burke J, Sedelnikova S, Baker PJ, Taylor R, Bullough PA, Muir NM, Gore MG, Rice DW (2001) Glycerol dehydrogenase. structure, specificity, and mechanism of a family III polyol dehydrogenase. Structure 9: 789-802. Rywinska K, Rymowicz W (2009) Citric acid production from raw glycerol by Yarrowia lipolytica Wratislavia 1.31. In G Aggelis, ed, Microbial conversions in raw glycerol. Nova Science Publishers, Inc., New York, pp 19-30. Sánchez EA, Comelli, R. A. (2012) Hydrogen by glycerol steam reforming on a nickelealumina catalyst: Deactivation processes and regeneration. International Journal of Hydrogen Energy. doi:10.1016/j.ijhydene.2011.12.088. Sánchez EA, D'Angelo MA, Comelli RA (2010) Hydrogen production from glycerol on Ni/Al2O3 catalyst. International Journal of Hydrogen Energy 35: 5902-5907. Sanno Y, Wilson TH, Lin EC (1968) Control of permeation to glycerol in cells of Escherichia coli. Biochem Biophys Res Commun 32: 344-349. Saxena RK, Anand P, Saran S, Isar J (2009) Microbial production of 1,3-propanediol: Recent developments and emerging opportunities. Biotechnology Advances 27: 895-913. Schell C, Riley C, Petersen GR (2008) Pathways for development of a biorenewables industry. Bioresour Technol 99: 5160-5164. Seifert C, Bowien S, Gottschalk G, Daniel R (2001) Identification and expression of the genes and purification and characterization of the gene products involved in reactivation of coenzyme B12-dependent glycerol dehydratase of Citrobacter freundii. Eur J Biochem 268: 2369-2378. Slinn M, Kendall K, Mallon C, Andrews J (2008) Steam reforming of biodiesel by-product to make renewable hydrogen. Bioresource Technology 99: 5851-5858. Sweet G, Gandor C, Voegele R, Wittekindt N, Beuerle J, Truniger V, Lin EC, Boos W (1990) Glycerol facilitator of Escherichia coli: cloning of glpF and identification of the glpF product. J Bacteriol 172: 424-430. Teruyuki N, Yoshinori, K. (1989). In. 1168292, JP. Truniger V, Boos W (1994) Mapping and cloning of gldA, the structural gene of the Escherichia coli glycerol dehydrogenase. J Bacteriol 176: 1796-1800. Valliyappan T, Ferdous D, Bakhshi NN, Dalai AK (2008) Production of hydrogen and syngas via steam gasification of glycerol in a fixed-bed reactor. Topics in Catalysis 49: 59-67. Vasiliadou ES, Heracleous E, Vasalos IA, Lemonidou AA (2009) Ru-based catalysts for glycerol hydrogenolysis-Effect of support and metal precursor. Applied Catalysis B: Environmental 92: 90-99. Vaudagna SR, Comelli RA , Fígoli NS (1997) Influence of the tungsten oxide precursor on WOx-ZrO2 and Pt/WOx-ZrO2 properties. Applied Catalysis A: General 164: 265-280. Wang X, Li S, Wang H, Liu B, Ma X (2008) Thermodynamic analysis of glycerin steam reforming. Energy and Fuels 22: 4285-4291. Wang K, Hawley MC, DeAthos SJ (2003) Conversion of glycerol to 1,3-propanediol via selective dehydroxylation. Industrial and Engineering Chemistry Research 42: 29132923.

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Wang ZX, Zhuge J, Fang H, Prior BA (2001) Glycerol production by microbial fermentation: a review. Biotechnol Adv 19: 201-223. Wang S, Liu H (2007) Selective hydrogenolysis of glycerol to propylene glycol on Cu-ZnO catalysts. Catalysis Letters 117: 62-67. Yaylayan AV, Harty-Majors S, A. Ismail A (1999) Investigation of dl-glyceraldehyde– dihydroxyacetone interconversion by FTIR spectroscopy. Carbohydrate Research 318: 20-25. Yuan Z, Wang J, Wang L, Xie W, Chen P, Hou Z, Zheng X (2010) Biodiesel derived glycerol hydrogenolysis to 1,2-propanediol on Cu/MgO catalysts. Bioresource Technology 101: 7088-7092. Yuan Z, Wu P, Gao J, Lu X, Hou Z, Zheng X (2009) Pt/solid-base: A predominant catalyst for glycerol hydrogenolysis in a base-free aqueous solution. Catalysis Letters 130: 261265. Zhang B, Tang X, Li Y, Xu Y, Shen W (2007) Hydrogen production from steam reforming of ethanol and glycerol over ceria-supported metal catalysts. International Journal of Hydrogen Energy 32: 2367-2373.

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In: Glycerol: Production, Structure and Applications Editors: M. De Santos Silva and P. Costa Ferreira

ISBN: 978-1-62081-120-7 © 2012 Nova Science Publishers, Inc.

Chapter 5

GLYCEROL AS A SUBSTRATE FOR BIOPROCESSES IN DIFFERENT O2 AVAILABILITY CONDITIONS M. Julia Pettinari1, Mariela P. Mezzina1, Beatriz S. Méndez1, Manuel S. Godoy1 and Pablo I. Nikel1,2 1

Department of Biological Chemistry, Faculty of Sciences, University of Buenos Aires and National Council for Research (CONICET) 2 Institute for Research in Biotechnology, University of San Martín, Argentina

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ABSTRACT In recent years, a significant increase in the production of biodiesel has caused a sharp fall in the cost of glycerol, the main by-product of biodiesel synthesis. As a result, glycerol has become a very attractive substrate for biotechnological processes. Many bacterial metabolites of industrial interest are products of aerobic metabolism, such as antibiotics, amino acids, and other compounds; but others, including molecules used as biofuels (e.g., ethanol, butanol, and hydrogen), and other relevant biochemicals, such as 1,3-propanediol and succinate, are products of fermentative metabolism. Since carbon atoms in glycerol are more reduced than in glucose or other substrates commonly used in bioprocesses, the catabolism of this substrate normally consumes high amounts of oxygen, and only a handful of bacteria, including some members of the genus Enterobacter, Clostridium, and Klebsiella, were thought to be able to ferment this substrate in anaerobiosis. Recent research has shown that the facultative aerobe Escherichia coli can ferment glycerol, producing a number of different biotechnologically relevant molecules, like succinate and ethanol, and the heterologous product poly(3-hydroxybutyrate), both in microaerobiosis and in anaerobiosis. Several strategies have been used to increase the number of applications for glycerol as a substrate for bacterial processes, mostly based on modified bacterial strains that can efficiently produce different chemicals from this substrate. Manipulations to enhance the synthesis of various metabolic products from glycerol include several approaches to increase its availability inside the cells, or to decrease the synthesis of other metabolites. Mutations in the glp genes, involved in glycerol metabolism, or in genes involved in

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M. Julia Pettinari, Mariela P. Mezzina, Beatriz S. Méndez et al. competing pathways, like ldhA, that encodes a D-lactate dehydrogenase, have been tested to increase the synthesis of several products from glycerol in different bacterial species. On the other hand, mutations in global regulator genes, especially in the redox-control pair arcAB, have been introduced in E. coli resulting in enhanced biosynthesis of reduced products from glycerol, mainly under microaerobic growth conditions. All these manipulations increase the number of possible uses of glycerol as a substrate for the obtention of a wide diversity of different biotechnological products using bacteria.

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INTRODUCTION Global biodiesel production has seen a remarkable growth in the last ten years, and this growth has been especially significant in South America. Germany is the leader of global biodiesel production, followed by Brazil and Argentina, with rapidly expanding biodiesel industries. These two countries have increased their participation in the biodiesel market, and, with a production of 2.3 and 2.1 billion liters in 2010, have become the second and third largest world producers (Shrank and Farahmand, 2011). As a result, the availability of glycerol, the main by-product of biodiesel synthesis, has increased greatly in the last years, especially in South America, and several articles dealing with the different industrial uses of either pure or crude glycerol have been published in the last years (Dobson et al.; Mothes et al., 2007). In particular, glycerol has become a very attractive substrate for bacterial fermentations (da Silva et al., 2009) especially for reduced products (Solaiman et al., 2006). Cells growing on glycerol are in a more reduced intracellular state than when glucose is used in similar conditions of oxygen availability. This has a significant effect on the intracellular NADH/NAD+ ratio, that causes the cells to direct carbon flow towards the synthesis of more reduced products when using glycerol compared to glucose in order to achieve redox balance (San et al., 2002). Because of this, glycerol is an ideal substrate for the biotechnological production of reduced molecules, including many fermentation products. Several bacterial species have been known to ferment glycerol, but these microorganisms are not easily cultured or manipulated. Until the last decade it was thought that Escherichia coli, the best known bacterial species, amenable to genetic and metabolic manipulation, was unable to use glycerol as a substrate in the absence of external electron acceptors. In the last decade, several manuscripts describing the fermentation of glycerol by different wild type or mutant E. coli strains have opened the way for new possibilities for the biotechnological use of this cheap and readily available substrate.

1. AEROBIC VERSUS ANAEROBIC METABOLISM OF GLYCEROL A great number of bacteria are able to grow using glycerol as sole carbon source. The highly reduced nature of carbon atoms in this substrate makes it difficult for microorganisms to use it in the absence of external electron acceptors. In his extensive review on glycerol catabolism in bacteria, E. C. C. Lin described glycerol assimilation in several bacterial species, with special focus on E. coli and other Enterobacteriaceae (Lin, 1976).

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Glycerol as a Substrate for Bioprocesses …

GlpF

ATP

Glycerol GlpK

GldA

Fig. 1

141

NADH Biomass ATP NADH

Glycerol-3-phosphate Dihydroxyacetone GlpABC

QH2

GlpD

DhaKLM

QH2 Dihydroxyacetone phosphate

Phosphoenolpyruvate Pyruvate

Glycolytic intermediates

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Figure. 1. Glycerol metabolism in Escherichia coli under aerobic and anaerobic growth conditions. These alternative catabolic pathways ultimately lead to the generation of dihydroxyacetone phosphate, which is later transformed into the glycolytic intermediates phosphoenolpyruvate and pyruvate (shown in the diagram as a gray arrow). These intermediates can be further carboxylated to metabolic precursors of succinate, processed to lactate, ethanol, or acetate, or completely oxidized to CO2. Enzymes involved in glycerol metabolism are shown in gray boxes; GlpF, glycerol facilitator (aquaglyceroporin); GldA, glycerol dehydrogenase; GlpK, glycerol kinase; GlpABC, anaerobic glycerol-3-phosphate dehydrogenase; GlpD, aerobic glycerol-3-phosphate dehydrogenase; and DhaKLM, dihydroxyacetone kinase. QH2 denotes where a flavin-containing enzyme interacts with a quinone (e.g., ubiquinone/menaquinone) to provide or acquire reducing equivalents.

In this seminal work, he described glycerol uptake and conversion, that in all cases involved phosphorylation and dehydrogenation steps to convert glycerol into dihydroxyacetone phosphate (DHAP), both aerobically or anaerobically (Figure 1). Almost all Enterobacteriaceae that he studied needed an external electron acceptor, such as nitrate or fumarate, to use glycerol in anaerobiosis, and only Klebsiella pneumoniae was found to be able to ferment glycerol in the absence of an external electron acceptor. A later report (Bouvet et al., 1995) described other species of Enterobacteria capable of fermenting glycerol, belonging to the genera Citrobacter, Enterobacter, and Klebsiella. In these bacteria there is a reductive pathway of glycerol utilization, in which glycerol is dehydrated by a vitamin B12dependent enzyme to form 3-hydroxypropionaldehyde that is then reduced to 1,3-propanediol (1,3-PD) by an NADH-linked oxidoreductase (1,3-PD dehydrogenase; i.e., 1,3-PD-DH) thereby regenerating NAD+. The fermentation of glycerol with the production of 1,3-PD was first described in Lactobacillus and Clostridium species (Biebl et al., 1999). Until the last decade it was thought that E. coli was unable to use glycerol as a substrate in the absence of external electron acceptors. The genes that encode the enzymes involved in the conversion of glycerol to 1,3-PD in bacteria are part of the dihydroxyacetone (dha) regulon, which encodes genes needed for the anaerobic metabolism of glycerol (Sun et al., 2003), and is absent in bacteria such as E. coli.

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Furthermore, expressing the dha regulon of K. pneumoniae in E. coli was shown to enable the recombinants to grow anaerobically on glycerol in the absence of external electron acceptors (Sprenger et al., 1989). In the last decade, interest in using glycerol as a substrate increased, and new efforts regarding the use of this substrate were published, such as a report that appeared in 2006, describing the use of glycerol by E. coli in the absence of external electron acceptors, with ethanol and succinate as the main fermentation products (Dharmadi et al., 2006). This opened new possibilities for the investigation of biotechnological applications for glycerol, as the best known bacterial species, amenable to genetic and metabolic manipulation, makes metabolic engineering studies a lot more accessible.

2. SYNTHESIS OF BIOTECHNOLOGICAL PRODUCTS FROM GLYCEROL a. Products of Aerobic Metabolism Glycerol can be efficiently used for growth and for the production of numerous Fig. 2 biotechnological products (Figure 2) by many bacterial species in aerobic conditions.

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Glycerol

Aerobic metabolism

Micro and anaerobic metabolism

Biomass (inoculants, single cell protein) Polyhydroxyalkanoates Single cell oil Amino acids Rhamnolipids Citrate Pigments

Biofuels (ethanol, butanol, 2,3-butanediol, H2, CH4) 1,2-Propanediol 1,3-Propanediol Polyhydroxyalkanoates Succinate Propionate Specialty chemicals

Figure 2. Biotechnological uses of glycerol. Some added-value bioproducts are listed under the relevant metabolic regime (regarding oxygen availability) under which they are formed. These examples correspond to bioproducts synthesized by both bacteria and fungi.

i. Biomass, Enzymes and Recombinant Proteins Oxidative glycerol catabolism favors the formation of biomass over other metabolic products, so it can be used as a substrate to grow different microorganisms that are themselves biotechnological products. Some examples include yeasts, such as Saccharomyces cerevisiae, that are then sold for different applications, mostly in the food industry. Different species of bacteria that are used as inoculants for crops, such as Rhizobium, Mesorhizobium, and Bradyrhizobium can also use glycerol as carbon source, and in fact it is the substrate of choice for the growth of some slow-growing Rhizobia (Arias and Martinez-Drets, 1976). The strictly aerobic yeast Yarrowia lipolytica is a ―non-conventional‖ yeast considered a single cell oil and lipase producer. This microorganism is able to accumulate oils

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intracellularly (Makri et al.) and produce extracellular and cell-bound lipases of great industrial interest (Galvagno et al., 2011), using crude glycerol as a substrate. Glycerol is also a very good substrate for recombinant protein production in yeast such as Pichia pastoris, as it favors biomass accumulation, diminishing the production of undesired products (Cereghino et al., 2002).

ii. Amino Acids Since the establishment of an efficient process for the biotechnological production of glutamate using Corynebacterium glutamicum more than 50 years ago, biotechnological production processes have been increasingly used for the industrial production of amino acids, mainly for use as human food and animal feed additives. Current processes employ high performance bacterial strains, such as C. glutamicum, E. coli, Brevibacterium flavum, and Methylobacterium sp., using molasses, sucrose, or glucose as carbon sources (Leuchtenberger et al., 2005). Glycerol has been studied as a substrate for the microbial production of amino acids, both using strains that can naturally use this substrate or modifying bacterial species that have been traditionally used in aminoacid production, such as C. glutamicum, so that they can produce amino acids from glycerol (Rittmann et al., 2008).

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iii. Antibiotics Glycerol has also been analyzed as a substrate for antibiotic production using both fungi and bacteria. For instance, the production of cephalosporin C by Acremonium chrysogenum M35 was shown to be increased by this substrate (Shin et al., 2011). Antibiotic production in bacteria using glycerol has also been analyzed. Streptomyces clavuligerus has been modified to enhance production of specific antibiotics, such as clavulanic acid (Baños et al., 2009), and antibiotic production from glycerol has been described in natural Streptomyces isolates (Kayali et al., 2011). iv. Polyhydroxyalkanoates Polyhydroxyalkanoates, PHAs, are natural bioplastics produced by many bacteria from different substrates. These polymers are totally biodegradable, as all organisms that produce PHAs can degrade them, and they are also degraded by many other organisms, both bacteria and fungi, in aerobic or anaerobic conditions. Two classes of PHAs are distinguished according to their monomer composition: shortchain length (SCL) PHAs, with three to five carbon monomers, such as poly(3hydroxybutyrate) (PHB), the most common PHA; and medium-chain length (MCL) PHAs, composed of monomers with six to 16 carbons (Steinbüchel and Valentin, 1995). All of them are optically active R-() compounds. PHAs can be synthesized from many different carbon sources, including sugars, typically glucose, and fatty acids. The use of glycerol for microbial PHA synthesis has been analyzed in natural PHA producers, such as Methylobacterium rhodesianum, Ralstonia eutropha (Borman and Roth, 1999), several Pseudomonas strains (Solaiman et al., 2006), the recently described Zobellella denitrificans (Ibrahim and Steinbüchel, 2009; Ibrahim and Steinbüchel, 2010), and Bacillus sp. (Reddy et al., 2009), among others. Glycerol has also been investigated as a substrate for PHB synthesis in recombinant E. coli carrying the PHB synthesis genes from Streptomyces aureofaciens (Mahishi et al., 2003) and Azotobacter sp. strain FA8 (Nikel et al., 2008a).

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PHAs obtained from glycerol have been reported to have a significantly lower molecular weight than polymer synthesized from other substrates, typically less than 1 MDa. In Methylobacterium extorquens and R. eutropha, PHB obtained from glycerol, ethanol, or methanol, had a lower molecular mass than that obtained from other substrates (such as succinate, glucose, and fructose), and the molecular mass of the polymer was shown to decrease with increasing glycerol concentrations (Taidi et al., 1994). This effect was further analyzed and attributed to premature chain termination caused by glycerol (Madden et al., 1999). A recent study performed using R. eutropha described PHB obtained from commercial glycerol and from waste glycerol with a molecular mass of 957 and 786 kDa, respectively, less than half of that of PHB obtained from glucose (Cavalheiro et al., 2009). A low molecular mass is undesirable for industrial processing of the polymer, so the results available in the literature pointed to a drawback in the use of glycerol as a substrate for the microbial production of PHAs. However, recent results obtained with recombinant E. coli showed that it is possible to obtain PHB from glycerol with molecular weights similar to those of the polymer obtained from glucose or lactose (de Almeida et al., 2010).

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v. Other Products of Aerobic Metabolism Many other products, such as several biosurfactants, fatty acids, lipids, and a variety of pigments, can be produced either by fungal or bacterial species using glycerol as a substrate (da Silva et al., 2009; Yazdani et al., 2010; Kośmider et al., 2011; Wendisch et al., 2011). Several aromatic compounds like 2-phenylethanol, p-hydroxycinnamic acid, phydroxystyrene, p-hydroxybenzoate, anthranilate, and cyclohexadiene-transdiols can also be synthesized using engineered E. coli and Pseudomonas putida strains and glycerol as a substrate (Gosset, 2009).

b. Products of Anaerobic Metabolism i. 1,3-Propanediol 1,3-PD is used in many synthetic reactions, particularly as a monomer for polycondensations to produce polyesters, polyethers, and polyurethanes, among other uses (Zeng and Biebl, 2002). This compound is primarily produced through chemical synthesis from petroleum derivatives, with high production costs, but it can also be obtained by microbial fermentation, a more environmentally favorable process that also has lower costs. 1,3-PD was identified in a glycerol fermentation mixed culture containing Clostridium pasteurianum in 1881 by August Freund (Freund, 1881), and was later found to be produced by the fermentation of glycerol by many bacteria including Citrobacter, Clostridium, Enterobacter, Klebsiella, and Lactobacillus species (Nakamura and Whited, 2003). Anaerobic growth on glycerol generates an excess of reducing equivalents, causing a redox imbalance. In order to achieve redox balance, cells synthesize products that serve as electron sinks, such as 1,3-PD. This molecule is synthesized from glycerol in two steps: dehydrogenation to 3-hydroxypropionaldehyde (3-HPA) followed by an NADH-dependent reduction to 1,3-PD, that is accumulated in the medium (Nakamura and Whited, 2003; Saxena et al., 2009).

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A number of by-products are formed during glycerol fermentation, i.e., ethanol, lactate, succinate, and 2,3-butanediol by the enterobacteria K. pneumoniae, Citrobacter freundii, and Enterobacter agglomerans; butyrate by Clostridium butyricum; and butanol by Clostridium pasteurianum (Zeng and Biebl, 2002). Several recombinant E. coli strains have been constructed to optimize 1,3-PD synthesis from glycerol, normally containing genes from K. pneumoniae (Altaras and Cameron, 1999), or C. butyricum (Tang et al., 2009).

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ii. Succinate Succinate is currently used as a specialty chemical in the agricultural, food, and pharmaceutical industries, and as a building block chemical because it can be converted into a wide variety of products, including green solvents, pharmaceutical products, and biodegradable plastics (McKinlay et al., 2007; Zeikus and Elankovan, 1999). Like 1,3-PD, succinate is primarily produced from petroleum-derived compounds but can also be synthesized from renewable substrates by microbial fermentation. Synthesis of succinate from glycerol can be achieved using either natural succinate-producing bacteria, such as Anaerobiospirillum succiniciproducens (Lee et al., 2001), Actinobacillus succinogenes (Vlysidis, 2009), or by metabolically engineered E. coli strains (McKinlay et al., 2007; Zhang et al., 2009). Recombinant E. coli strains that can efficiently produce succinate from glycerol have been recently described (Zhang et al., 2010). Several metabolic manipulations that enhance succinate synthesis from glycerol in E. coli are further discussed in a special section of this chapter. iii. Ethanol Ethanol is currently produced by fermentation using the yeast S. cerevisiae from several agroindustrial by-products as carbon source. This microorganism is highly efficient for the fermentation of several sugars to ethanol, but it is supposedly unable to grow anaerobically on glycerol. Moreover, it produces glycerol as a by-product of ethanol synthesis when it ferments sugars under ethanologenic (i.e., anaerobic) conditions in order to achieve redox and carbon balances (Medina et al., 2009). However, it has recently been reported that a recombinant strain of S. cerevisiae containing several genetic modifications can produce ethanol when growing on crude glycerol in aerobiosis (Yu et al., 2010). In contrast, ethanol synthesis from glycerol in recombinant E. coli strains has been extensively analyzed, as ethanol is the main product of anaerobic or microaerobic glycerol catabolism of this facultative anaerobe (Durnin et al., 2009; Gonzalez et al., 2008). Several metabolic manipulations that enhance ethanol synthesis from glycerol in E. coli are further discussed in a special section of this chapter. iv. Hydrogen This fuel gas can be produced from glycerol together with ethanol, both by natural glycerol fermenters, such as Enterobacter aerogenes, or by recombinant E. coli. E. aerogenes is able to produce both ethanol and H2 from pure glycerol or from glycerol-containing wastes discharged from a biodiesel fuel production plant (Ito et al., 2005; Sakai and Yagishita, 2007). Several metabolic manipulations, described in a later section of this chapter, enabled recombinant E. coli to produce both ethanol and H2 from unrefined glycerol with remarkable yields (Yazdani and Gonzalez, 2008).

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3. EFFECTS OF AERATION IN PRODUCT DISTRIBUTION E. coli is the best known bacterial species. This facultative aerobe adjusts its metabolism to optimize cell growth in each environmental condition by using different combinations of metabolic pathways. As a result, the metabolic product distribution varies according to growth conditions, such as carbon source and terminal electron acceptor availability. There is an intimate association between carbon and electron flow, as carbon will be directed towards the synthesis of more reduced or more oxidized products according to intracellular redox conditions. Even small variations in oxygen availability can lead to significant changes in the metabolite distribution of E. coli cultures. These changes vary when using carbon sources with different oxidation states, for instance glucose and glycerol, reflecting the metabolic adjustments that take place in order to optimize cell growth. Most metabolic studies are performed using glucose, but many of the effects observed in cultures using this substrate cannot be extrapolated to other carbon sources. Carbon atoms in glycerol have a lower oxidation state (–2) than in glucose (0). As a consequence, glycerol catabolism produces more reducing equivalents. This has a significant effect on the intracellular NADH/NAD+ ratio, that causes the cells to direct carbon flow towards the synthesis of more reduced products when using glycerol compared to glucose in order to achieve redox balance. On the other hand, different culture agitation speeds results in differences in oxygen availability, which also affects the oxidation state of the cells, producing variations in the product pattern (de Almeida et al., 2010). The ratio ethanol/acetate can be used as an indicator of the redox state of the cells (Nikel et al., 2008b; San et al., 2002). Cultures growing on glycerol produce much more ethanol than those using glucose, and significant amounts of ethanol are accumulated in the glycerol cultures even at high aeration. Both the carbon sources and the aeration conditions were observed to affect carbon distribution among different metabolic products (de Almeida et al., 2010), for instance, lactate and formate concentrations increase as aeration decreases (Shalel-Levanon et al., 2005a), and are higher with glucose than with glycerol. In a PHB producing recombinant strain, the effects of using two different agitation speeds were totally different when using glucose or glycerol. Reduced aeration favored the formation of PHB and ethanol in the glycerol cultures, probably as a result of redirecting carbon metabolism to increase the consumption of reducing power (de Almeida et al., 2010). In glucose cultures, carbon flow is mostly directed towards lactate and formate when oxygen availability is lower (San et al., 2002), resulting in a decrease in PHB synthesis (de Almeida et al., 2010). The internal redox state of the cells is one of the main signals driving the metabolic changes that result in the differences in metabolic product distribution. Cells growing on glycerol are in a more reduced intracellular state than when glucose is used in similar conditions of oxygen availability. As a result, cultures using these substrates will respond to the same aeration conditions with different metabolic profiles in order to tune their metabolism, adjusting the consumption of reducing equivalents. This occurs because cells growing on glycerol will switch to high reducing power consuming metabolism in conditions in which glucose cultures are still able to sustain an oxidative metabolism.

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4. METABOLIC MANIPULATIONS TO ENHANCE SYNTHESIS OF BIOTECHNOLOGICAL PRODUCTS FROM GLYCEROL a. Mutations in Competing Pathways

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E. coli is known to adjust its metabolism to optimize cell growth in each environmental condition by using different combinations of metabolic pathways. As there is an intimate association between carbon and electron flow, carbon skeletons will be directed towards the synthesis of more reduced or more oxidized products according to intracellular redox conditions. In this context, manipulations to enhance the synthesis of a metabolic product include several approaches, either to increase the availability of substrates (and often also cofactors) needed for its formation, or to inhibit competing pathways.

i. Mutations in Competing Pathways for the Synthesis of Ethanol from Glycerol in E. coli Soon after the discovery of the ability of E. coli to use glycerol under anaerobic conditions (Dharmadi et al., 2006), an in-depth study about the involvement of metabolic pathways in this process was undertaken (Murarka et al., 2008). In that work, elimination of candidate genes with products involved in respiratory (cydA and cyoB) and metabolic (glpA, glpD, and glpK) processes, as well as fermentative pathways (frdA and pta), allowed to sketch a catabolic roadmap from glycerol to final fermentation products. A striking conclusion therein derived is the absolute dependence of the glycerol fermentation process on ethanol synthesis, as the elimination of adhE (encoding an alcohol-acetaldehyde dehydrogenase) rendered E. coli altogether unable to grow on glycerol under anoxic conditions. The quasi-homoethanologenic nature of anaerobic glycerol catabolism is an attractive feature of the process that can be further manipulated in order to increase the yield of ethanol on substrate. Fermentation of glycerol by E. coli results in a mixture of metabolic products comprising predominantly ethanol but also acetate, succinate, and minor amounts of formate. Unlike acetate and succinate, which are competing pathways, formate (and also H2, one of its decomposition products) represents a potentially interesting co-product that could enhance the value of the fermentation process itself. Genes encoding fumarate reductase (frdA) and phosphotransacetylase (pta), two key enzymes in the synthesis of succinate and acetate, respectively, were disrupted (Yazdani and Gonzalez, 2008) in an attempt to re-route carbon precursors into ethanol biosynthesis. The corresponding double-mutant strain produced negligible amounts of succinate and acetate, and, when cultivated at acidic conditions (pH = 6.3), was able to convert all the produced formate to H2 and CO2. In consequence, equimolar amounts of ethanol and hydrogen produced during glycerol fermentation attained yields that approached the maximum theoretical yield (1.01 and 1.02 mol product · mol glycerol-1, respectively, with a carbon recovery of 106%). In the same line of reasoning, the authors evaluated whether lower catabolic fluxes in the upper steps of glycerol catabolism could impose a restriction in the availability of metabolic precursors for ethanol synthesis. A metabolic engineering approach implemented to increase the rate of glycerol utilization involved the coordinated overexpression of gldA and dhaK (encoding glycerol dehydrogenase and DHAP kinase, respectively), the outcome of which would be an accelerated conversion of glycerol to DHAP. As common glycolytic enzymes are known to be normally expressed at

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high levels, this strategy supported high carbon fluxes from dihydroxyacetone phosphate to pyruvate. Indeed, overexpression of both gldA and dhaK in a strain bearing deletions in frdA, pta, and fdhF (a key component of formate-hydrogen lyase) resulted in a biocatalyst able to efficiently convert glycerol into ethanol and formate (i.e., no degradation of formate to H2 and CO2). Product yields consistently reached values above 90% of the maximum theoretical, ca. 1.04 and 0.92 mol ethanol and formate, respectively, for each mol of glycerol fermented, with a carbon recovery of 102%. Elimination of ldhA (encoding D-lactate dehydrogenase) has been shown to have an important effect on the metabolic product distribution in a recombinant E. coli using glycerol as carbon source, promoting ethanol synthesis (Nikel et al., 2010a). The recombinant used in that work, an arcA and creC(Con) mutant, overexpressed a bifunctional alcohol-acetaldehyde dehydrogenase from Leuconostoc mesenteroides. A two-stage bioreactor culture was implemented to increase the volumetric productivity of ethanol, using an evolved derivative of the original strain capable of withstanding up to 35 g · liter-1 ethanol. The rationale behind this strategy was that acetyl-CoA would be actively accumulated during a first aerobic stage and, after oxygen deprivation, excess acetyl-CoA not consumed in oxidative metabolism could be transformed into ethanol. After 48 h, ethanol reached 15 g · liter-1 with a yield on substrate of 0.39 g · g-1 (ca. 81% of the maximum theoretical). Interestingly, under these culture conditions it was unnecessary to supplement antibiotics for plasmid maintenance. The intracellular NADH/NAD+ content suggested that regeneration of oxidized co-factors by the plasmid-encoded heterologous bioreaction played a relevant role in plasmid maintenance. Trinh and Srienc (2009) used a combined in silico and wet-lab strategy to construct an E. coli strain able to convert glycerol into ethanol under microaerobic growth conditions. First, the authors investigated the minimal set of elementary metabolic modes needed to support coupled growth and efficient ethanol synthesis. It was postulated that by using this approach, metabolic evolution of such a minimal cell would improve ethanol productivity because faster-growing cells would also produce ethanol at a higher rate. The genetic background that purportedly enables biomass generation and efficient ethanol synthesis was a strain carrying deletions in zwf (glucose-6-phosphate 1dehydrogenase), ndh (NADH dehydrogenase), sfcA (NAD-dependent malic enzyme), maeB (NADP-dependent malic enzyme), ldhA, frdA, poxB (pyruvate oxidase), pta, and mdh (malate dehydrogenase). Plasmid pLOI297 (carrying pyruvate decarboxylase and alcohol dehydrogenase B from the obligate anaerobe Zymomonas mobilis) was introduced in that metabolic background, and the resulting strain was subjected to directed evolution through serial sub-culture for at least 50 enrichment transfers. At a kLa = 0.15 min-1, the evolved strain produced ethanol with a yield on substrate of 0.49 g · g-1 (i.e., 97% of the maximum theoretical) and a volumetric productivity of 0.46 g · liter-1 · h-1.

ii. Mutations in Competing Pathways for the Synthesis of Succinate From Glycerol in E. coli Another interesting approach was applied to rationally manipulate the fermentative production of succinate from glycerol (Zhang et al., 2010). The authors reasoned that no net ATP would be produced during the anaerobic metabolism of glycerol to succinate using the native phosphoenolpyruvate carboxylase reaction for carboxylation to oxaloacetate. In stark contrast, the high energy of phosphoenolpyruvate can be easily conserved by replacing this

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enzyme with the pck-encoded phosphoenolpyruvate carboxykinase to produce 1 mol ATP · mol succinate-1. A chromosomal promoter mutation was used to upregulate pck expression by ca. 8-fold, which significantly enhanced the succinate yield on glycerol (from 0.25 to 0.44 mol · mol-1 in the parental strain and its pck-overexpressing derivative, respectively). This sharp increase in the succinate synthesis was predictably accompanied by a lower production of both formate and ethanol. Further improvements in succinate formation were achieved by eliminating pstI (encoding an enzyme involved in the phosphorelay system for dihydroxyacetone phosphorylation that uses phosphoenolpyruvate as phosphate donor) and pflB (encoding pyruvate-formate lyase) in the pck-overexpressing background. In such a strain, the succinate concentration after 6 days of fermentation was 102 ± 25 mM, with a succinate yield on glycerol of 0.80 mol · mol-1. Blankschien et al. (2010) exploited the microaerobic conversion of glycerol to succinate by introducing a pyruvate carboxylase from Lactococcus lactis in an E. coli strain devoid of adhE, pta, poxB, ldhA, and ppc (phosphoenolpyruvate carboxylase). As mentioned above, glycerol is mainly converted into ethanol under culture conditions with restricted oxygen supply, therefore the AdhE activity was an obvious target for manipulation in order to divert carbon precursors into succinate. When cultured for 72 h in bioreactor batch cultures, this recombinant consumed 20 g · liter-1 of glycerol and produced 14 g · liter-1 (ca. 120 mM) of succinate, with a succinate yield on substrate of 0.54 mol · mol-1. Under these conditions, the maximum volumetric and specific productivities of succinate reached 0.26 g · liter-1 · h-1 and 0.4 g· g cell dry weight-1 h-1, respectively. Even when these results proved promising, a significant fraction of the carbon source was still converted into CO2, thus decreasing the overall yield of succinate.

iii. Mutations in Competing Pathways for the Synthesis of PHB from Glycerol in E. coli The effect of eliminating competing pathways on PHB production from glucose in recombinant E. coli strains has been investigated in extenso through the inactivation of different genes, such as those encoding enzymes participating in the synthesis of acetate (ackA, pta, poxB), or lactate (ldhA). Relevant examples are a pta mutant, which produces very little acetate (Chang et al., 1999), and an frdA ldhA double mutant (Wlaschin et al., 2006), which showed increased PHB accumulation, presumably because acetyl-CoA was directed towards polymer biosynthesis instead of these unwanted by-products. Similarly, an ackA, pta, poxB, ldhA, and adhE mutant of E. coli (Jian et al., 2010), unable to produce acetate, lactate, and ethanol, showed increased PHB synthesis under microaerobic conditions. Though into a lesser extent, PHB synthesis from glycerol has also been evaluated in genetic contexts in which the synthesis of by-products is impaired. For instance, in an ldhA mutant (Nikel et al., 2010b), the increased availability of carbon and reducing power affected the distribution of cofactors as well as the carbon partitioning among the different metabolic products, resulting in a significant increase of PHB and ethanol synthesis. In 60-h fed-batch cultures, the ldhA mutant carrying the pha genes from Azotobacter sp. strain FA8 reached 41.9 g · liter-1 biomass and accumulated PHB up to 63% (wt/wt), with a PHB yield on glycerol of 0.41 g · g-1, the highest so far reported using this substrate. Kang et al. (2010) used a multiple-knockout strain, comprising deletions in fadR (a multifunctional dual regulator of the fatty acid degradative regulon), fadA (3-ketoacyl-CoA thiolase), atoC (the response regulator of the AtoSC two-component signal transduction system), ptsG (a

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component of the sugar phosphotransferase system), sdhA (one of the catalytic subunits of succinate dehydrogenase), and pta for PHA and succinate co-production from a mixture of glycerol and fatty acids. Introduction of the phaC1 gene from Pseudomonas aeruginosa enabled this strain to produce 21.07 g · liter-1 succinate and 0.54 g · liter-1 PHA (consisting of 3-hydroxyoctanoate and 3-hydroxydecanoate at a molar ratio of 58.7 and 41.3 mol · mol-1, respectively) after 90-h cultivation.

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b. Mutations in Redox Regulators Most of the current processes for the bacterial synthesis of reduced products of commercial interest, such as ethanol, 1,3-PD, and PHAs, among others, require fully aerobic conditions, which means that they are high energy-consuming processes. The energy used is needed to fulfill aeration and agitation inside the culture vessel of the bioreactor. Therefore, manipulations that improve the respiratory capacity of the host strains under micro-aerobic growth conditions can help reduce aeration, thus reducing energy requirements. E. coli, the workhorse of biotechnological developments, is a facultative microorganism that can adapt its metabolism to oxygen availability. The two-component signal transduction system ArcAB (for aerobic respiration control) modulates, at the transcriptional level, the expression of many operons according to the redox state of the cell (Liu and De Wulf, 2004; Lynch and Lin, 1996). ArcB is a transmembrane sensor kinase which under anaerobic or microaerobic conditions undergoes stable phosphorylation and then transphosphorylates the response regulator ArcA (Kwon et al., 2000). The main targets for repression by the phosphorylated regulator are the genes that code for enzymes involved in aerobic respiration. On the other hand, the cytochrome d oxidase, with high affinity for oxygen, and fermentation enzymes such as pflB are activated in anaerobic conditions (Iuchi and Lin, 1988; Sawers and Suppmann, 1992). E. coli arc mutants are unregulated for aerobic respiration in microaerobic conditions. As a consequence, the tricarboxylic acid cycle enzymes are not repressed and the pool of reducing equivalents, such as NADH or NADPH, is elevated (Alexeeva et al., 2000; ShalelLevanon et al., 2005b). Great amounts of reducing equivalents are thus available to be funneled into reduced compounds making the mutants extremely suitable for the synthesis of reduced bioproducts. This approach enabled the increase of PHA content up to 35% (wt/wt) in an arcA mutant strain grown in a semi-synthetic medium with gentle (75 rpm) agitation, conditions in which no PHA was accumulated by the wild-type strain (Nikel et al., 2006). Another global regulatory system manipulated to increase the synthesis of reduced products is CreBC, a two-component signal transduction pair, where CreB is the regulator and CreC the sensor kinase. The cre regulon includes different genes, and some of them are involved in carbon metabolism (Avison et al., 2001). As E. coli can use glycerol as a carbon source under anaerobic conditions (Dharmadi et al., 2006), experiments were performed using mutants affected in global regulators in media supplemented with glycerol under low oxygen availability. The metabolic profile of an arcA and an arcA and creC(Con) double mutant, grown in these conditions were compared with the arcA+ parental strain. Both mutants achieved lower concentrations of fermentation acids and higher ethanol values than the wild-type strain. Accordingly, the NADH/NAD+ ratios for the three strains were 0.63, 0.97, and 0.18 mol · mol-1, respectively. The ratio obtained for the

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double mutant strain (ca. 1 mol · mol-1), made it adequate as a candidate host for the synthesis of reduced bioproducts (Nikel et al., 2008b). One of the few reports in the scientific literature on fed-batch cultivation in micro-aerobiosis describes a process for the synthesis of PHA using glycerol as substrate with the arcA creC(Con) double mutant. Micro-aerobic fed-batch cultures allowed a 2.57-fold increase in volumetric productivity when compared with batch cultures, attaining a PHA content of 51% (wt/wt). A concomitant synthesis of a valuable by product, bio-ethanol, was also observed (Nikel et al., 2008a). Other approach based on the control of the bacterial redox state was developed by Wei et al. (2009). Nine anaerobic promoters were cloned upstream of PHB synthesis genes from R. eutropha for the micro- or anaerobic production of PHB in recombinant E. coli. Among the promoters, the one for alcohol dehydrogenase (PadhE) was found to be the most effective. Recombinant E. coli harboring pha genes under the control of PadhE achieved a 48% (wt/wt) PHB accumulation after 48 h of static culture compared with only 30% (wt/wt) using the native promoter.

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CONCLUSION Glycerol is a versatile compound, that can be used in a variety of chemical and biological conversions. It is a good substrate for microbial growth, as it can be used by many different microorganisms for the synthesis of a wide range of bioproducts. Due to the highly reduced nature of the carbon atoms in glycerol, this substrate is mostly used in oxidative metabolism, in highly aerobic processes. There are, however, several bacteria that can ferment this substrate, such as some Clostridia and a few Enterobacteria. Genetic and metabolic manipulations, mainly in E. coli strains, have allowed the use of glycerol to synthesize a variety of biotechnologically relevant products with different oxygen availability conditions, increasing the efficiency and sustainability of fermentation processes using glycerol as a substrate.

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Makri, A., Fakas, S., and Aggelis, G. (2010) Metabolic activities of biotechnological interest in Yarrowia lipolytica grown on glycerol in repeated batch cultures. Biores. Technol. 101: 2351-2358. McKinlay, J. B., Vieille, C., and Zeikus, J. G. (2007) Prospects for a bio-based succinate industry. Appl. Microbiol. Biotechnol. 76: 727-740. Medina, V. G., Almering, M. J., van Maris, A. J., and Pronk, J. T. (2009) Elimination of glycerol production in anaerobic cultures of a Saccharomyces cerevisiae strain engineered to use acetic acid as an electron acceptor. Appl. Environ. Microbiol. 76: 190195. Mothes, G., Schnorpfeil, C., and Ackermann, J. U. (2007) Production of PHB from crude glycerol. Eng. Life Sci. 7: 475-479. Murarka, A., Dharmadi, Y., Yazdani, S. S., and Gonzalez, R. (2008) Fermentative utilization of glycerol by Escherichia coli and its implications for the production of fuels and chemicals. Appl. Environ. Microbiol. 74: 1124-1135. Nakamura, C. E., and Whited, G. M. (2003) Metabolic engineering for the microbial production of 1,3-propanediol. Curr. Opin. Biotechnol. 14: 454-459. Nikel, P. I., Pettinari, M. J., Galvagno, M. A., and Méndez, B. S. (2006) Poly(3hydroxybutyrate) synthesis by recombinant Escherichia coli arcA mutants in microaerobiosis. Appl. Environ. Microbiol. 72: 2614-2620. Nikel, P. I., Pettinari, M. J., Galvagno, M. A., and Méndez, B. S. (2008a) Poly(3hydroxybutyrate) synthesis from glycerol by a recombinant Escherichia coli arcA mutant in fed-batch microaerobic cultures. Appl. Microbiol. Biotechnol. 77: 1337-1343. Nikel, P. I., Pettinari, M. J., Ramirez, M. C., Galvagno, M. A., and Méndez, B. S. (2008b) Escherichia coli arcA mutants: metabolic profile characterization of microaerobic cultures using glycerol as a carbon source. J. Mol. Microbiol. Biotechnol. 15: 48-54. Nikel, P. I., Ramirez, M. C., Pettinari, M. J., Méndez, B. S., and Galvagno, M. A. (2010a) Ethanol synthesis from glycerol by Escherichia coli redox mutants expressing adhE from Leuconostoc mesenteroides. J. Appl. Microbiol. 109: 492-504. Nikel, P. I., Giordano, A. M., de Almeida, A., Godoy, M. S., and Pettinari, M. J. (2010b) Elimination of D-lactate synthesis increases poly(3-hydroxybutyrate) and ethanol synthesis from glycerol and affects cofactor distribution in recombinant Escherichia coli. Appl. Environ. Microbiol. 76: 7400-7406. Reddy, S. V., Thirumala, M., and Mahmood, S. K. (2009) A novel Bacillus sp. accumulating poly(3-hydroxybutyrate-co-3-hydroxyvalerate) from a single carbon substrate. J. Ind. Microbiol. Biotechnol. 36: 837-843. Rittmann, D., Lindner, S. N., and Wendisch, V. F. (2008) Engineering of a glycerol utilization pathway for amino acid production by Corynebacterium glutamicum. Appl. Environ. Microbiol. 74: 6216-6222. Sakai, S., and Yagishita, T. (2007) Microbial production of hydrogen and ethanol from glycerol-containing wastes discharged from a biodiesel fuel production plant in a bioelectrochemical reactor with thionine. Biotechnol. Bioeng. 98: 340-348. San, K. Y., Bennett, G. N., Berríos-Rivera, S. J., Vadali, R. V., Yang, Y. T., Horton, R. E., Rudolph, F. B., Sariyar, B., and Blackwood, K. (2002) Metabolic engineering through cofactor manipulation and its effect on metabolic flux redistribution in Escherichia coli. Metab. Eng. 4: 182-192.

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Sawers, G., and Suppmann, B. (1992) Anaerobic induction of pyruvate formate-lyase gene expression is mediated by the ArcA and FNR proteins. J. Bacteriol. 174: 3474-3478. Saxena, R. K., Anand, P., Saran, S., and Isar, J. (2009) Microbial production of 1,3propanediol: recent developments and emerging opportunities. Biotechnol. Adv. 27: 895913. Shalel-Levanon, S., San, K. Y., and Bennett, G. N. (2005a) Effect of oxygen on the Escherichia coli ArcA and FNR regulation systems and metabolic responses. Biotechnol. Bioeng. 89: 556-564. Shalel-Levanon, S., San, K. Y., and Bennett, G. N. (2005b) Effect of oxygen, and ArcA and FNR regulators on the expression of genes related to the electron transfer chain and the TCA cycle in Escherichia coli. Metab. Eng. 7: 364-374. Shin, H. Y., Lee, J. Y., Park, C., and Kim, S. W. (2011) Utilization of glycerol as cysteine and carbon sources for cephalosporin C production by Acremonium chrysogenum M35 in methionine-unsupplemented culture. J. Biotechnol. 151: 363-368. Shrank, S., and Farahmand, F. (2011) Biofuels regain momentum. Available on line at www.vitalsigns.worldwatch.org. Solaiman, D. K. Y., Ashby, R. D., Foglia, T., and Marmer, W. N. (2006) Conversion of agricultural feedstock and coproducts into poly(hydroxyalkanoates). Appl. Microbiol. Biotechnol. 71: 783-789. Sprenger, G. A., Hammer, B. A., Johnson, E. A., and Lin, E. C. C. (1989) Anaerobic growth of Escherichia coli on glycerol by importing genes of the dha regulon from Klebsiella pneumoniae. J. Gen. Microbiol. 135: 1255-1262. Steinbüchel, A., and Valentin, H. E. (1995) Diversity of bacterial polyhydroxyalkanoic acids. FEMS Microbiol. Lett. 128: 219-228. Sun, J., van den Heuvel, J., Soucaille, P., Qu, Y., and Zeng, A. P. (2003) Comparative genomic analysis of dha regulon and related genes for anaerobic glycerol metabolism in bacteria. Biotechnol. Prog. 19: 263-272. Taidi, B., Anderson, A. J., Dawes, E. A., and Byrom, D. (1994) Effect of carbon source and concentration on the molecular mass of poly(3-hydroxybutyrate) produced by Methylobacterium extorquens and Alcaligenes eutrophus. Appl. Microbiol. Biotechnol. 40: 786-790. Tang, X., Tan, Y., Zhu, H., Zhao, K., and Shen, W. (2009) Microbial conversion of glycerol to 1,3-propanediol by an engineered strain of Escherichia coli. Appl. Environ. Microbiol. 75: 1628-1634. Trinh, C. T., and Srienc, F. (2009) Metabolic engineering of Escherichia coli for efficient conversion of glycerol to ethanol. Appl. Environ. Microbiol. 75: 6696-6705. Vlysidis, A., Binns, M., Webb, C., and Theodoropoulos, C. (2009) Utilisation of glycerol to platfortm chemicals within the biorefinery concept: a case for succinate production. Chem. Eng. Trans. 18: 537-542. Wei, X. X., Shi, Z. Y., Yuan, M. Q., and Chen, G. Q. (2009) Effect of anaerobic promoters on the microaerobic production of polyhydroxybutyrate (PHB) in recombinant Escherichia coli. Appl. Microbiol. Biotechnol. 82: 703-712. Wendisch, V. F., Lindner, S. N., and Meiswinkel, T. M. (2011) Use of glycerol in biotechnological applications. In: Biodiesel – Quality, emissions and by-products. Montero, G., and Stoytchevam, M. (eds.). Rijeka, Croatia: InTech.

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Wlaschin, A. P., Trinh, C. T., Carlson, R., and Srienc, F. (2006) The fractional contributions of elementary nodes to the metabolism of Escherichia coli and their estimation from reaction entropies. Metab. Eng. 8: 338-352. Yazdani, S. S., and Gonzalez, R. (2008) Engineering Escherichia coli for the efficient conversion of glycerol to ethanol and co-products. Metab. Eng. 10: 340-351. Yazdani, S. S., Mattam, A.J., and Gonzalez, R. (2010) Fuel and chemical production from glycerol, a biodiesel waste product. In: Biofuels from agricultural wastes and byproducts. Blaschek, H.P., Ezeji, T.C., and Scheffran, J. (eds.): Wiley Blackwell. Yu, K. O., Kim, S. W., and Han, S. O. (2010) Engineering of glycerol utilization pathway for ethanol production by Saccharomyces cerevisiae. Biores. Technol. 101: 4157-4161. Zeikus, J. G., Jain, M. K., and Elankovan, P. (1999) Biotechnology of succinic acid production and markets for derived industrial products. Appl. Microbiol. Biotechnol. 51: 545-552. Zeng, A. P., and Biebl, H. (2002) Bulk chemicals from biotechnology: the case of 1,3propanediol production and the new trends. Adv. Biochem. Eng. Biotechnol. 74: 239-259. Zhang, X., Shanmugam, K. T., and Ingram, L. O. (2010) Fermentation of glycerol to succinate by metabolically engineered strains of Escherichia coli. Appl. Environ. Microbiol. 76: 2397-2401. Zhang, X., Jantama, K., Moore, J. C., Jarboe, L. R., Shanmugam, K. T., and Ingram, L. O. (2009) Metabolic evolution of energy-conserving pathways for succinate production in Escherichia coli. Proc. Natl. Acad. Sci. USA 106: 20180-20185.

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In: Glycerol: Production, Structure and Applications Editors: M. De Santos Silva and P. Costa Ferreira

ISBN: 978-1-62081-120-7 © 2012 Nova Science Publishers, Inc.

Chapter 6

HYDROGEN PRODUCTION FROM GLYCEROL VIA MEMBRANE REACTOR TECHNOLOGY A. Iulianelli, S. Liguori and A. Basile Institute on Membrane Technology of National Research Council (ITM-CNR), Rende, Italy

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ABSTRACT Bio-diesel fuel, when converted from vegetables oils, produces around 10 wt% of glycerol as a byproduct, which could be used for producing hydrogen through steam reforming reaction. The state of the art in the scientific literature on hydrogen production via reforming reaction of glycerol in aqueous or gas phase is mainly devoted to the utilization of conventional reactors. Thus, as main highlights present in the open literature on this process, both high reaction pressure and a relatively small catalyst deactivation are noticed when steam reforming of glycerol is carried out in aqueous phase, whereas the catalyst deactivation is the main disadvantage in gas phase. In this chapter, glycerol steam reforming reaction to produce hydrogen is the main topic, paying special attention to the application of membrane reactor technology to this process. As a further scope, the chapter also describes the utilization of perm-selective Pd-based membrane reactors, pointing out the ability of these systems to both extract a high purity hydrogen stream and enhance the performances of the reaction system in terms of both glycerol conversion and hydrogen yield. Furthermore, the benefits and drawbacks of the membrane reactor systems are descript as a mature technology compared to the conventional reactors.

Keywords: Glycerol steam reforming, membrane reactor, hydrogen production



Corresponding author: [email protected].

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ABBREVIATIONS FBR MR HT LT WGS PROX PSA GSR PEM GC H PSS pore JH2 Pe δ pH2-retentate pH2-permeate n

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Pe0 Ea R T

Fixed bed reactor Membrane reactor High temperature Low temperature Water gas shift Preferential oxidation Pressure Swing Adsorption Glycerol steam reforming Polymer electrolyte membrane Gas-chromatograph Permeation rate to reaction rate ratio Porous stainless steel Pore‘s diameter Hydrogen flux permeating through the membrane Hydrogen permeability Membrane thickness Hydrogen partial pressures in the retentate side Hydrogen partial pressures in the permeate side Dependence factor of the hydrogen flux on the hydrogen partial pressure Pre-exponential factor Apparent activation energy Universal gas constant Absolute temperature

1. INTRODUCTION TO MEMBRANE REACTOR TECHNOLOGY In the last years, the interest on membrane reactor (MR) technology is growing up for the applications in separation processes owing to their continuous operation, modularity and reduced costs with respect to the conventional systems [1-4]. As a special field of interest, metal membranes are particularly applied in the hydrogen separation area because hydrogen can selectively permeate through a dense metal wall [1,5,6]. Among various metals, Pd and its alloys have been extensively studied. Unfortunately, high selectivity is associated to low permeability (and vice versa), while the cost of the membranes is strictly correlated to the thickness of the precious metal (Pd alloy) layer. Composite membranes consisting of thin metal films coated over porous supports have been also studied [7-11]: this kind of membranes exhibits hydrogen permeability and selectivity values depending on the Pd-alloy layer covering the support‘s pores. By using inorganic membrane reactor technology, a catalyzed reaction and a selective addition of reactants or removal of products are simultaneously performed in the same apparatus and, so, conversion values beyond the thermodynamic limit could be attained. Therefore, in a membrane reactor, the effect promoting the enhancement of the reaction

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conversion over the correspondent conventional fixed bed reactors (FBRs) is called "shift effect" [1,3,5]. In this contest, membrane reformers have been studied for producing hydrogen via reforming of hydrocarbons and alcohols. The use of dense and thin wall Pdbased membranes having full hydrogen perm-selectivity allows to give both high-purity hydrogen stream and high hydrogen recovery [12,13]. Furthermore, considering that most of the reforming reactions are endothermic processes thermodynamically favoured at high temperature, the utilization of membranes selective to hydrogen permeation could allow to attain higher conversion than a FBR operating at same temperature, or, otherwise, the same conversion of a FBR, but operating at lower temperature with a consequent benefit due to the energy saving [5]. For these reasons, in the last years Pd-based membranes and MRs have been mainly applied to the production of hydrogen, which is seen as a clean energy carrier for the future [14,15]. Nowadays, hydrogen is largely produced from fossil sources such as methane and coke, while the production from renewable feedstocks (bio-ethanol, glycerol, as a by-product of bio-diesel production, and so on) could rise for the next future [16-19]. The latter could be useful to produce hydrogen via reforming reactions and performed in Pd-based MRs. This choice could give several benefits in terms of reaction conversion and process efficiency as well as the production of a high-purity hydrogen stream and lower costs related to the absence of further hydrogen separation/purification devices, necessary in conventional systems. As a special case of application, this chapter describes the recent findings concerning hydrogen production via glycerol steam reforming reaction coupled to Pd-based MRs with the aim of manufacturing high-purity hydrogen.

2. PD-BASED MEMBRANES

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2.1. Membrane Classification Classification by nature subdivides membranes into both biological and synthetic. Biological membranes are easily manufactured, although they present relevant drawbacks related to the limited operating temperature (below 100 °C), pH range, etc. Synthetic membranes can be further classified into organic (polymeric) and inorganic (ceramic, metallic), which differ for functionality owing to their temperature limit. Indeed, polymeric membranes generally operate up to 200 °C, preferentially under 100-150 °C, making them unusable for reforming process in MRs. Otherwise, inorganic membranes operate above 250 °C and are stable between 300 - 800 ºC. In some cases, they can operate at elevated temperatures (ceramic membranes) over 1000 ºC [20]. However, they may be further subdivided into porous and dense. Porous membranes are classified according to their pore diameter in microporous (pore < 2nm), mesoporous (2nm < pore < 50nm) and macroporous (pore > 50nm) [21]. Based on the dimension of the membrane pores, Table 1 summarizes the mechanism taking place in porous and Pd-based dense membranes.

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A. Iulianelli, S. Liguori and A. Basile Table 1. Transport mechanisms in both porous and dense membranes. Membrane Dense (Pd-based) Microporous Mesoporous Macroporous

pore (nm) 50

Transport mechanism Fick Activated process Knudsen Poiseuille (Viscous flow)

Inorganic membranes are also categorized depending on the structural material:

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   

metal membranes, ceramic membranes, carbon membranes, zeolite membranes.

In this chapter, metal membranes are reported with particular reference to those based on palladium and its alloys, which are the dominant materials for preparing this kind of membranes owing to their aforementioned high solubility and selectivity to hydrogen permeation. Nevertheless, also other dense membranes are selectively permeable only to hydrogen such as those based on tantalum, vanadium, nickel and titanium, which are less expensive than palladium and its alloys. Unfortunately, they have also the important drawback of surface poisoning, which is more significant when they are very thin. Ceramic membranes are generally made from aluminium, titanium or silica oxides, which are chemically inert and stable at high temperatures and, therefore, particularly useful for food, biotechnology and pharmaceutical applications as well as for gas separation and in MR applications. Carbon membranes are constituted of porous solids containing constricted apertures approaching the molecular dimensions of diffusing gas molecules, which are separated through molecular sieving. Zeolites are microporous crystalline alumina-silicate with a uniform pore size and are mainly used as catalysts or adsorbents in a form of micron or submicron-sized crystallites embedded in millimeter-sized granules.

2.2. Short Overview on Supported and Self-Supported Pd-Membranes One of the most relevant drawback for the large industrial applications of Pd-based membranes and, then, membrane reactors is the high cost of the precious materials (Pd and Pd alloys). As a consequence, the reduction of Pd content in a such membrane has been approached by the scientists under different techniques. However, most of the approaches for containing the costs of palladium-based membranes is oriented to coat Pd-based layers onto both ceramic and metallic porous supports using several procedures in order to both produce composite membranes and minimize the amount of precious metal inside and, then, the membrane cost.

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Generally, composite Pd-ceramic membranes consist of three layers: a Pd or Pd alloy film, a ceramic porous support and an intermediate ceramic layer of reduced porosity, which improve the adhesion between metal and support. An example of composite membranes production is given by Apte et al. [22], who performed the electroless plating of Pd applied to a bi-layer ceramic porous support. In this case, as a main drawback, the presence of the intermediate layer in the electroless plated composite membranes reduce significantly the hydrogen permeance. In this view, Hou et al. [23] developed a modified electroless procedure for producing Pd-ceramic membranes characterized by high hydrogen permeation fluxes. The procedure disclaimed in this invention consists of filling the ceramic porous support with Al(OH)3, electroless plating Pd or Pd alloy layer and, then, removing (also partial) by the filler material. For instance, the Al(OH)3 is decomposed to porous Al2O3 in order to maintain open the pores of the substrate and form free passages for the hydrogen permeation [23]. In order to overcome the mismatching between the Pd alloy films and the stainless steel support, Dardas et al. [24] used an intermediate ceramic layer of appropriate thermal expansion coefficient. In detail, Pd-Ag 23%wt. and Pd-Cu 40% wt. films (thickness less than 10 μm), having a thermal expansion coefficient of 11 μm m-1 K-1, are coated over an intermediate oxide yittria-zirconia layer (average porosity 0,1 μm, thickness less than 3 μm and thermal expansion coefficient of 13,9 μm m-1 K-1). A more general concept of composite Pd-based membrane has been disclaimed by Friedberger et al. [25]: a dense Pd or Pd alloy thin layer is supported by a silicon carrier with round or rectangular perforations. Pinacci et al. [26] proposed the electroless plating method for manufacturing a Pd-based supported onto porous stainless steel (PSS) membrane. PSS support has been chosen owing to its thermal expansion coefficient (14.7 μm/m °C at 20°C [27]) close to that of palladium (11.8 μm/m °C at 20°C [27]). This Pd-based membrane on metallic support offers good stability up to 400 °C with a hydrogen/nitrogen ideal selectivity equal to around 300/1 and hydrogen/carbon dioxide equal to 550/1 at T = 400 °C and total pressure difference between retentate and permeate of 1.0 bar. The electroless plating procedure was applied for manufacturing the membrane, consisting of the pre-treatment of the PSS support, oxidation and activation of support and, at the end, the Pd plating. In particular, the pre-treatment of the PSS support included complete removal of foreign contaminants (grease, oil, dirt, corrosion products), while activation of oxidized support consisted of dipping into a stannous chloride solution and a palladium chloride solution. The Pd plating is obtained by circulating a solution, constituted by palladium chloride, ammonia, EDTANa2 and hydrazine, in a reactor where the membrane is immersed and kept in rotation. Nevertheless, in the last years also other applications involving dense and self supported Pd-based membranes have been studied for producing Pd-membranes with full hydrogen perm-selectivity. For example, dense self-supported Pd-Ag permeator tubes have been produced via cold-rolling and diffusion welding of metal foils [28]. The resulting membrane tubes have a wall thickness of about 50 μm: the metal membrane is dense and defect-free, thus exhibiting the full hydrogen selectivity beside significant hydrogen permeation fluxes [29]. Unfortunately, due to the high thickness, the cost of such membrane is too high. The diffusion bonding has been also applied to the production of Pd-Cu membranes [30]. The composition of Pd-Cu-40% wt alloy exhibits the maximum hydrogen permeability. In this patent, Pd-Cu foils of a thickness of 25-63 μm are welded to a Cu frame by pressing the parts to be joined in hydrogen atmosphere furnace at about 300 °C for several hours. The

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procedure can be applied also to connect the Pd-Cu dense membranes to the membrane module through Cu metal frames.

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3. MEMBRANE REACTORS At industrial level, several heterogeneous gas–solid catalytic processes (conventionally performed by means of fixed, fluidised or trickle bed reactors) are developed by combining operations at high temperatures and in a chemically harsh environment. Therefore, inorganic membranes are more adequate than polymeric ones as useful materials for MRs. Generally, the separation capability of an inorganic membrane housed in a MR is utilized as a benefit for improving the performances of a catalytic system. Today, it is possible to distinguish two main generic approaches concerning a MR as a selective product separation (Extractor) and selective reactant addition (Distributor) [31]. The first MR approach makes possible the in– situ removal of one of the reaction products. As an example, when a steam reforming reaction is performed in a conventional reactor, hydrogen yield and product selectivities are limited by Thermodynamics. Using a MR (with an Extractor approach), the selective removal of hydrogen from the reaction side allows the thermodynamic equilibrium restrictions to be overcome. This potentiality is the well-known and aforementioned ―shift effect‖, through which both higher hydrogen yields and reaction conversion than those of a conventional reactor can be reached. Moreover, this effect allows operation at milder reaction conditions in terms of temperature and pressure than the FBRs [32]. The second MR approach involves the use of the membrane to control the contact within reactants. Both full perm-selective and non-full perm-selective membranes can be useful to distribute the supplying of one of the reactants in the reaction zone. For example, in partial oxidation reactions performed in FBRs, on the one hand oxygen rich-feed results in low product selectivity and high reactant conversions. On the other hand, low oxygen content means high product selectivity but lower conversions. Using a membrane for distributive oxygen supply in the reaction zone, both high conversions and product selectivities are reached [33,34]. Furthermore, since the reactants and oxygen are not premixed, mixtures are avoided and the possibility of flame back firing into the feed are considerably lowered. However, the inorganic membranes discussed in this chapter are devoted to favour the selective permeation of hydrogen as a product of glycerol steam reforming reaction (Extractor approach), allowing as much as possible a high purity of hydrogen collection in the permeate stream.

3.1. Palladium-Based Membrane Reactors Owing to the ―shift effect‖ that allows the thermodynamic equilibrium restrictions of a FBR to be overcome, the main behaviour and benefit of MRs used for reforming reactions are related to the improvement of the reaction conversion, hydrogen yield, etc. Concerning the inorganic membranes housed in MRs, in the open literature it is reported that the MR performances in terms of, for example, conversion are a function of the parameter ―H‖, defined as the permeation rate to reaction rate ratio [35]. When H = 0, no permeation takes

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place (FBR). Otherwise, H > 0 corresponds to different type of MRs. In the case of low H values, microporous, dense and mesoporous MRs show the same behaviour. At higher H values, the differences among the different kind of MRs is evident. In particular, MRs with a finite separation factor (representing the ratio between the permeability of a certain gas to a gas assumed as a blank, generally an inert such as He or N2) achieve an optimum in terms of permeability/reaction rate. Above the optimum, the reactant loss caused by permeation induces a detrimental effect on the conversion. On the contrary, higher separation factors always correspond to higher conversions. MRs showing infinite separation factors, for example for hydrogen, do not give any drawbacks in terms of conversion because loss of reactants does not occur. In summary, in this work [35] it has been shown that by using MRs it is possible to realize better performances in terms, for example, of conversion than FBRs. As previously introduced, dense palladium membranes are most considered owing to their full hydrogen perm-selectivity, even if, between 0 – 700 °C other metals such as niobium, vanadium and tantalum offer higher hydrogen permeability than palladium. Nevertheless, these metals show greater surface resistance to hydrogen transport due to their easier oxidation. Therefore, much scientific attention has been paid on dense palladium membranes despite of their low hydrogen permeability and high cost. The hydrogen molecular transport in palladium membranes takes place through a solution/diffusion mechanism and, as indicated by Koros and Fleming [36], it involves six different activated steps: dissociation of molecular hydrogen at the gas/metal interface, adsorption of the atomic hydrogen on membrane surface, dissolution of atomic hydrogen into the palladium matrix, diffusion of atomic hydrogen through the membrane, re-combination of atomic hydrogen to form hydrogen molecules at the gas/metal interface and desorption of hydrogen molecules. Basile [37] identified that each of these steps may control hydrogen permeation through the dense film as a dependence on temperature, pressure, gas mixture composition and membrane thickness. As a result, the hydrogen flux permeating through the membrane can be expressed [38]: J

H2

= Pe (pn H2

H2,retentate

 pnH

)/δ

(1)

2,permeate

where JH2 is the hydrogen flux permeating through the membrane, Pe the hydrogen permeability, δ the membrane thickness, pH2-retentate and pH2-permeate the hydrogen partial pressures in the retentate (reaction side) and permeate (side in which hydrogen permeating through the membrane is collected) zones, respectively, n (variable in the range 0.5 - 1) the dependence factor of the hydrogen flux on the hydrogen partial pressure. The membrane thickness plays an important role. Indeed, as thin the membrane as high the hydrogen permeance, even affecting the membrane mechanical resistance. Therefore, for real applications, to ensure the mechanical resistance and strength of the membrane, thicker membranes are necessary. As a consequence, lower hydrogen permeances are reached. However, when the pressure is relatively low, the rate-limiting step of the hydrogen permeation through the membrane is assumed to be the diffusion [39]. When the membrane thickness is greater than 5 m, equation (1) becomes the Sieverts-Fick‘s law (2): JH

2

,Sieverts-Fick

= PeH · (p0.5H 2

2

,retentate 

p0.5H

2

,permeate )/δ

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(2)

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Otherwise, when the hydrogen-hydrogen interactions in the palladium bulk are not negligible owing to the high pressures, n becomes equal to 1: JH = PeH · (pH 2

2

2

,retentate 

pH

2

,permeate )/δ

(3)

The hydrogen permeability dependence on the temperature can be expressed by an Arrhenius-like equation: PeH = Pe0H exp (-Ea/RT) 2

(4)

2

where Pe0 is the pre-exponential factor, Ea the apparent activation energy, R the universal gas constant and T the absolute temperature. Therefore, when the Sieverts-Fick‘s law is valid, the hydrogen flux is expressed as indicated by the Richardson‘s equation (5): JH = Pe0H [exp (-Ea /RT)] · (p0.5H 2

2

2

,retentate 

p0.5H

2

,permeate )/δ

(5)

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In various studies [37,40-42] a method, as economically more advantageous for developing a single system combining the reaction for hydrogen production and its purification as a pure stream is proposed. According to the scheme shown in Figure 1, where the conventional system for pure hydrogen production, followed by WGS reactors, CO PROX and PSA, could be replaced by a dense Pd-based MR, useful to perform the reaction as well as to collect a high purity hydrogen stream in only one system, representing a costeffective solution owing to a lower number of devices.

Figure 1. Conventional system with a FBR, a two stages WGS reaction and hydrogen purification system (a); dense Pd-based MR for pure H2 production from reforming reaction (b).

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3.2. The Critical Issues of Palladium-Based MRs MRs housing dense Pd-based membranes may present critical issues as summarized in the following. Associated to pure palladium membranes, there is the ―hydrogen embrittlement‖ phenomenon, taking place below 300 °C and 2.0 MPa. It can be avoided by alloying palladium with other metals, commonly with silver, which displays its electron donating behaviour, being largely similar to that of the hydrogen atom in palladium, making possible competition for the filling of electron holes within the silver and hydrogen atoms [1]. Another critical drawback for the Pd-based MRs is that the palladium surface can be contaminated by the presence of hydrogen sulfide, SO2, Hg vapour, thiophene, arsenic, unsaturated hydrocarbons, or chlorine carbon from organic materials, etc. More in detail, the poisoning through exposure to hydrogen sulphide negatively affects Pd-coated membranes, which can be destroyed rapidly and the poisoning effects would be irreversible [43,44]. Typically, in some reforming reactions such as, for example, the steam reforming of glycerol some coke-precursors can take place as byproducts besides H2, CO, CO2 and CH4. Indeed, coke affects negatively both the hydrogen permeation (covering the membrane surface and lowering the hydrogen permeating flux) and the catalyst (lowering its activity), reducing the overall performances of the reaction. Furthermore, when a MR allocates thin palladium membranes in contact with coke at high temperature, the hydrogen permeation characteristics are lowered. This poisoning is caused by the carbon atoms, which penetrate into the palladium lattice, provoking membrane failure owing to the expansion of the palladium lattice [45]. A further drawback when using Pd-based MR is constituted by CO effect, which could cause the decrease of the hydrogen permeation performances of the membrane, because the adsorbed CO displaces the adsorbed hydrogen, blocking the hydrogen adsorption sites. This reduction is more significant at low temperature (below 150 °C) or at high CO feed concentration [46]. The poisoning effect of water vapour on hydrogen permeability can be more consistent than the CO effect. It affects the water vapour dissociation/recombinative desorption, which contaminates the palladium surface with adsorbed oxygen [47].

4. A SHORT OVERVIEW ON HYDROGEN PRODUCTION THROUGH PALLADIUM-BASED MRS Since 1866, the first application of palladium membrane was done by Graham, who was the first to apply palladium membranes for separating hydrogen from gases mixtures [48]. In 1915, Snelling patented a system constituted of palladium or platinum tubes for the hydrogen removal from a reactor, packed with a granular catalyst for dehydrogenation reactions [49]. In 1964, Gryaznov studied the application of a tubular and full perm-selective palladium-based membrane reactor for hydrogen consumption, where the palladium acts also as a catalyst [50]. In the same year, the first commercial application of a dense Pd-Ag membrane was realized by Johnson Matthey for purifying a hydrogen rich-stream [51]. Johnson Matthey also developed a Pd-based MR for producing hydrogen from a methanol/water mixture. This system was used on a small scale by the British Antarctic Survey in 1975 [52,53]. In the last

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decades, Tokyo Gas Company Ltd. realized the first pilot-scale Pd-based MR for direct ultrapure hydrogen production [54]. Nowadays, palladium-based MR technology is particularly studied and developed at lab scale and it is proposed as an alternative solution to conventional systems for high-purity hydrogen production. In detail, Table 2 summarizes some of the chemical reactions performed by using Pd-based MRs technology with the purpose of producing pure hydrogen. Table 2. Representative list of chemical reactions for producing pure hydrogen by using Pd-based MR technology

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Reaction type Decomposition of ammonia Dehydrogenation of cyclohexane to benzene Dehydrogenation of ethylbenzene to styrene Dehydrogenation of ethane to ethylene Dehydrogenation of isopropyl alcohol to acetone Dehydrogenation of water-gas shift reaction Dehydrogenation of n-heptane to toluene + benzene Dehydrogenation of butane to butadiene Dehydrogenation of 1,2-cyclohexanediol Dry reforming of methane Methane conversion into hydrogen rich-gas Octane reforming Partial oxidation of methane Steam reforming of ethanol Steam reforming of methane Steam reforming of glycerol Steam reforming of acetic acid Steam reforming of methanol Water gas shift

Membrane dense dense porous dense dense dense porous dense dense dense dense porous dense porous dense dense dense dense dense Dense-porous

Material Pd Pd/Ag Pd Pd/Ag Pd Pd, Pd/Ag Pd Pd/Rh Pd Pd/Cu Pd-alloy Pd-al1oy Pd and Pd-al1oy Pd-alloy Pd and Pd-al1oy Pd-alloy Pd-alloy Pd-alloy Pd/Pd-al1oy P/Pd-alloy– Silica/etc.

At the moment, different processes are based on hydrogen production via reforming reaction of biofuels, such as methanol, glycerol, ethanol, biogas, etc. and there is a consistent number of publications to be found in the open literature concerning the application of Pdbased MR technology.

5. GLYCEROL AS A BIO-FEEDSTOCK Nowadays, air pollution represents one of the most hard environmental problem caused by several solid, liquid or gaseous substances, able to alter the natural environmental conditions, with harmful effects on humans. One of the most relevant contributor to the environmental pollution is represented by the automotive industry and, in particular, by the combustion engines fuelled by derived fossil fuels. A viable solution for depleting the derived

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fossil fuels utilization could be constituted by alternative and clean energy sources. As an example, biodiesel is a renewable fuel targeted for compression ignition engines. In detail, it is renewable, obtained by cultivation of oil plants, biodegradable, capable of ensuring an energy efficiency equal to that of fossil fuels with an excellent reliability in performance of vehicles. Today, owing to the high cost of vegetable oils [55], the biodiesel is more expensive than the traditional diesel, but by performing the process of bio-diesel production, the exploitation of the byproducts could match an interesting and economical benefit. For instance, when biodiesel is produced through a process of transesterification of vegetable oils, glycerol is produced as a byproduct, constituting another bio-feedstock potentially usable for further applications.

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5.1. Glycerol Steam Reforming Reaction to Produce Hydrogen An alternative and interesting way to the exploitation of derived of fossil fuels for producing hydrogen or synthesis gas could be represented by the steam reforming process of glycerol. From the open literature, glycerol steam reforming (GSR) reaction has been only studied in conventional reactors. In particular, GSR can be performed both in aqueous or gas phase. When operated in aqueous phase, it shows, as an advantage, low catalyst deactivation, but as a drawback it needs to be operated at high pressures [56]. Otherwise, in gas phase it can be carried out at atmospheric pressure, showing a great catalyst deactivation as a main issue [56]. In this field of scientific research, a reaction kinetics study on aqueous-phase GSR reaction indicates that Pt and Pd catalysts are selective for producing hydrogen, with Pt showing high catalytic activity [57]. Metals such as Ni and Ru exhibit good catalytic activity but lead to alkanes formation. On the contrary, Ir, Co, Cu, Ag, Au and Fe show low catalytic activity. Furthermore, Huber et al. [57] used a heterogeneous catalyst based on Ni, Sn and Al, active and selective for hydrogen production by aqueous-phase GSR reaction. Zhang et al. [58] studied both steam reforming reaction of ethanol and glycerol to produce hydrogen over Ir, Co and Ni-based catalysts, determining that Ir-based catalyst is significantly more active and selective toward hydrogen production. Iriondo et al. [59] studied GSR reaction in both aqueous and gas phase over aluminasupported Ni catalysts, modified with Ce, Mg, Zr and La. As a result of this investigation, in aqueous phase reforming, the addition of Ce, La and Zr to Ni-Al2O3 catalyst improves glycerol conversion with respect to the use of only a Ni-Al2O3 catalyst. The authors pointed out that the differences in catalytic activity are related to geometric effects caused by the Ni and La or the close interaction between Ni and Zr. Furthermore, this study also highlighted that the catalyst deactivation becomes relevant after a few hours under operation, owing to the oxidation of the active catalyst metallic phase. In gas phase, using Ce, La, Mg and Zr as promoters of Ni-based catalysts, the authors noticed the enhancement of the catalytic activity compared to the reference catalyst supported on alumina. Hirai et al. [56] proposed the following catalytic activity scale for the gas phase GSR reaction: Ru ≈ Rh > Ni > Ir > Co > Pt > Pd > Fe. Although a noble metal such as Rh is greatly effective to steam reforming of hydrocarbons and less susceptible to carbon formation, Rhbased catalysts are not common in industrial applications owing to their high cost.

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However, GSR reaction performed in FBRs involves a complex reaction system that produces undesirable byproducts besides hydrogen. In this contest, Valliyappan et al. [55] concluded that the overall system of reactions, taking place during GSR reaction at T < 700 °C, may be represented by the reactions as reported below: Steam reforming of pure glycerol:  Steam reforming of pure glycerol: H2O

C3H8O3

3CO + 4H2

(6)

 Water gas shift reaction: CO + H2O

CO2 + H2

(7)

 Overall glycerol steam reforming reaction: C3H8O3 + 3H2O

3CO2 + 7H2

(8)

 Steam reforming of methane: CH4 + H2O

CO + 3H2

(9)

 Steam reforming of ethanol: C2H5OH + H2O

CO2 + CH4 + 2H2

(10)

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 Steam reforming of aldehyde: CH3CHO + H2O

CO2 + CH4 + 2H2

(11)

With the aim of exploiting glycerol as a bio-source via steam reforming reaction to produce high purity hydrogen, the reformed hydrogen rich-gas stream coming out from a FBR needs to further separation/purification steps. A single stage process to reach high purity hydrogen stream is attained by means of palladium-based membrane reactor technology, in which both GSR reaction and the selective removal of hydrogen take place in the same device. Unfortunately, to the best of our knowledge, only a few scientific studies deal with GSR reaction performed in MRs. In the following it is reported what obtained about GSR reaction performed in Pd-Ag MR for extracting high-purity hydrogen, showing some advantages with respect to the FBRs. Firstly, Figure 2 shows a short summary of the most significative scientific results in terms of glycerol conversion via FBRs at different temperatures, compared to the results obtained by using a Pd-Ag MR. Clearly, what illustrated in the figure does not represent a quantitative comparison because the results are obtained using different operating conditions and catalysts. Therefore, these results should be seen only from a qualitative point of view. However, the main indication given by the graph is that the MR operates at lower temperature than the FBRs. This result is greatly important because, lower operating temperatures means

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higher energy saving and, as a consequence, the MR could result a cheaper solution for performing the reaction. In particular, Iulianelli et al. [71] performed GSR in both FBR and a full hydrogen permselective Pd-Ag MR and, at the same operating conditions, the MR gives better result in terms of higher conversion than FBR (Figure 2). 100

Glycerol conversion Glycerol conversion [%] [%]

100

MR

80 MR 80

MR

60 MR 60

40 40

20 20

0 0

400 400

600 600

800 800

Temperature [°C]

1000 1000

Temperature [°C]

FBR - Adhikari et al - Catal Tod 129 (2007) 355–364 (Ni/Al O ) [60]

3 FBR - Adhikari et al - Catal Tod 129 (2007) 355–364 (Ni/Al2O3)2[60]

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FBR -- Adhikari Adhikarietetalal- Catal - Catal Tod 129 (2007) 355–364 - (Rh/CeO 2/Al2O3) [60] FBR Tod 129 (2007) 355–364 - (Rh/CeO 2/Al2O3) [60] FBR -- Adhikari Adhikarietetalal- Energy - Energy & Fuels 2008, 1220–1226 - Ni/MgO FBR & Fuels 2008, 22,22, 1220–1226 - Ni/MgO [61] [61] FBR- Adhikari Adhikarietetalal- Energy - Energy Fuels 2008, 1220–1226 - (Ni/TiO FBR&& Fuels 2008, 22,22, 1220–1226 - (Ni/TiO 2) [61] 2) [61]

FBR Comm 1010 (2009) 1656–1660 - Ni/CeO FBR -- Buffoni Buffonietetalal- Catal - Catal Comm (2009) 1656–1660 - Ni/CeO 2/Al2O 3 [62] 2/Al 2O3 [62] ppl Cat B:B: Env 9090 (2009) 29–37 - Pt/CeZrO FBR - Cui et al - Appl Cat Env (2009) 29–37 - Pt/CeZrO 2/Y2O 3 [63] 2/Y 2O3 [63]

FBR Comm 1212 (2010) 292–298 - Co/Al 2O3 [64] FBR -- Cheng Chengetetalal- Catal - Catal Comm (2010) 292–298 - Co/Al 2O3 [64] FBR - Chiodo et al - Appl Cat A: Gen 381 (2010) 1–7 - Rh/Al2O3 [65]

FBR - Chiodo et al - Appl Cat A: Gen 381 (2010) 1–7 - Rh/Al2O3 [65]

FBR - Pompeo et al - Int J Hydrogen En 35 (2010) 8912–8920 - Pt/Al2O3 [66]

FBR - Pompeo et al - Int J Hydrogen En 35 (2010) 8912–8920 - Pt/Al2O3 [66]

FBR - Chen et al - Ren En 36 (2011) 779–788 - Ni/CrO2 [67]

FBR - Chen et al - Ren En 36 (2011) 779–788 - Ni/CrO [67]

FBR - Chen et al - doi:10.1016/j.cattod.2011.07.011 - Ni/Al22O3 [68]

FBR - Chen et al - doi:10.1016/j.cattod.2011.07.011 - Ni/Al O [68]

2 3 FBR - Iriondo et al - Topics in Catal 49 (2008) 46–58 - Ni/ZrO2 [59]

FBR -- Dauenhauer Iriondo et alet- Topics in Catal 49 (2008) 46–58 - Ni/ZrO[69] 2 [59] FBR al - J Catal 244 (2006) 238–247 - RhCeWc FBR et al - Fuel 2956–2960 - Ru/Al O [70] FBR -- Byrd Dauenhauer et87 al (2008) - J Catal 244 (2006) 238–247 RhCeWc [69] 2 3 MR Asia87 Pac J Chem Eng 5 (2010) 138–145 - Co/Al2O3 [19] FBR- Iulianelli - Byrd etetalal- -Fuel (2008) 2956–2960 - Ru/Al 2O3 [70] FBR Iulianelli et 5 (2010) 138–145 - Co/Al MR -- Iulianelli et al al -- Asia AsiaPac PacJ JChem ChemEng Eng 5 (2010) 138–145 - Co/Al O [19] 2O3 [19] 2

3

MR et et al al -Int- JAsia Hydrogen 36 (2011) - Ru/Al-2O 3 [71] O [19] FBR- Iulianelli - Iulianelli Pac J En Chem Eng 5 3827–3834 (2010) 138–145 Co/Al 2 3 FBR - Iulianelli et al -Int J Hydrogen En 36 (2011) 3827–3834 - Ru/Al2O3 [71]

MR - Iulianelli et al -Int J Hydrogen En 36 (2011) 3827–3834 - Ru/Al2O3 [71]

FBR - Iulianelli et al -Int J Hydrogen En 36 (2011) 3827–3834 - Ru/Al2O3 [71]

Figure 2. Short summary of the main scientific results on GSR reaction in both fixed bed and membrane reactors.

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In detail, both the reactors are packed with a Ru-Al2O3 catalyst and the reaction is carried at 400 °C, water/glycerol feed molar ratio = 6/1 and reaction pressure of 5 bar. The choice of the Ru-Al2O3 catalyst was done on the basis of literature information about efficiency of several catalysts for GSR reaction: ruthenium-based catalysts is confirmed to be one of the most efficient catalyst for this reaction. In this case, around 60% glycerol conversion was attained using the MR with respect to around 40% for the FBR at the same operating conditions. In order to reach a conversion as much as possible high with a limited byproducts production, it is necessary a selective catalyst able to both convert byproducts (ethanol, etc.) and well catalyze GSR reaction. Using Co-Al2O3 catalyst in a Pd-Ag MR (Figure 3), at 400 °C and 5 bar Iulianelli et al. [19] achieved a glycerol conversion of more than 90% with respect to 40% of FBR. The high conversion obtained is also emphasized by the high purity hydrogen collected in the shell side of the MR.

Figure 3. Scheme of a Pd-Ag MR used for GSR reaction.

More in detail, the catalyst is packed in the membrane lumen where the feed is supplied. A sweep-gas is used in the shell side to lower the partial pressure of hydrogen permeated through the membrane and, then, to favour the hydrogen permeation driving force. Other gases cannot permeate owing to the full hydrogen perm-selectivity of the dense self-supported Pd-Ag membrane. The stream not permeated through the membrane is called ―retentate‖, while the ―permeate‖ is the high purity hydrogen stream coming out from the shell side. The Pd-Ag MR gives better results in terms of conversion with the further benefit of collecting a high purity hydrogen stream, useful for further applications such as, for instance, PEM fuel cells. Table 3 resumes other significative results obtained by using Pd-Ag MR and FBR in GSR reaction.

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The MR gives better yield of hydrogen using different catalysts. Nevertheless, when Rubased catalyst is used, a lower content (0.5%) of the metal dispersed on the catalytic support with respect to the Co-based one (18%) is present, provoking lower performance in terms of both hydrogen yield and hydrogen recovery. However, hydrogen yield around 40% and hydrogen recovery higher than 60% represent the best result of the Pd-Ag MR. Table 3. Literature data on GSR reaction: comparison between MR and FBR at the same experimental conditions and catalysts

Reactor type

Pd-Ag MR FBR Pd-Ag MR FBR

Catalyst

Co/Al2O3 Co/Al2O3 Ru/Al2O3 Ru/Al2O3

T [°C]

400 400 400 400

p [bar]

1.0 1.0 5.0 5.0

Feed molar ratio (steam/glycerol)

Sweep Factor

H2 Recovery

H2 Yield

(a)

(b)

(c)

6/1 6/1 6/1 6/1

22.8 11.9 -

63 56 -

39 31 28 18

Ref.

[19] [19] [71] [71]

(a) Sweep Factor = Sweep gas to glycerol molar ratio. (b) H2 Recovery = Molar ratio between the COx-free hydrogen permeated stream and the total hydrogen really produced. (c) H2 Yield = Molar ratio between the hydrogen stream in the permeate side and the total hydrogen theoretically producible from the stoichiometry of reaction.

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CONCLUSION Hydrogen will play a strategic role as an energy carrier for the future energy systems such as PEMFCs. Therefore, industrial companies and the scientific community are devoted to study the production hydrogen in a more technically, environmentally and economically attractive way, based on the idea of sustainable development. From a techno-economic point of view, steam reforming process and the exploitation of renewable sources are currently the most favourable and alternative route for hydrogen production. Inorganic membranes are excellent candidates for hydrogen purification, especially when incorporated into MRs, combining the reaction and separation processes in a single unit. Meanwhile, glycerol as a bio-derived source is becoming an attracting candidate for producing hydrogen via reforming reaction and, combined to inorganic MR technology, could constitute an important goal in this area. In this chapter, glycerol reforming process performed in both conventional and inorganic membrane reactors has been presented, paying particular attention to the effect of the inorganic membranes on the reaction performances in terms of hydrogen yield, hydrogen recovery and glycerol conversion. Pd-based MRs seem to be the dominant applications in this field, particularly because of the hydrogen perm-selectivity characteristics of these membranes. In summary, the future perspectives on performing the glycerol reforming in inorganic MRs are listed below:

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Development of low or non-palladium-based membranes useful for glycerol reforming in MRs. This task could represent a novel step in the viewpoint of MR cost reduction. Scale-up of glycerol reforming MRs is one of the most important issues. Developing low-cost, defect-free, effective membranes could be a chance for realistic application of MRs on an industrial scale. Future researches should be aimed at the improvement of membrane mechanical resistance during the glycerol reaction processes, at both relatively high reaction temperatures and pressures. Great attention should be paid to evaluate the effective balance between benefits and drawbacks of applying MR technology to glycerol reforming process to produce hydrogen. Specifically, research should be devoted to studying the increase of operating or capital costs related to the use of relatively high reaction pressure and temperature in order to improve the hydrogen permeation driving force in hydrogen fully perm-selective MRs. More experimental studies on the lifetime of MRs utilized for carrying out glycerol reforming processes should be undertaken in order to validate them as a possible alternative to the conventional systems at larger scales.

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[30] Juda W, Krueger CW, Todd Bombard R, ―Diffusion-bonded palladium-copper alloy framed membrane for pure hydrogen generators and the like and method of preparing the same‖, patent: US5904754 (1999). [31] Julbe A, Farrusseng D, Guizard C, ―Porous ceramic membranes for catalytic reactorsoverview and new ideas‖, J. Membr. Sci., 181 (2001) 3–20. [32] Zaman J, Chakma A, ―Inorganic membrane reactors‖, J. Membr. Sci., 92 (1994) 1–28. [33] Saracco G, Neomagus HWJP, Versteeg GF, van Swaaij WPM, ―High-temperature membrane reactors: potential and problems‖, Chem. Eng. Sci., 54 (1999) 1997–2017. [34] Coronas J, Santamaria J, ―Catalytic reactors based on porous ceramic membranes‖, Catal. Tod., 51 (1999) 377–389. [35] Keizer K, Zaspalis VT, De Lange RSA, Harold MP, Burggraaf AJ, Membrane reactors for partial oxidation and dehydrogenation reactions‖, Membr. Proc. Sep. Pur., JG Crespo and Boddeker, eds., Netherlands, 415–429 (1994). [36] Koros WJ, Fleming GK, ―Membrane-based gas separation‖, J. Membr. Sci., 83 (1993) 1–80. [37] Basile A, ―Hydrogen production using Pd-based membrane reactors for fuel cells‖, Top. Catal., 51 (2008) 107–122. [38] Gallucci F, Tosti S, Basile A, ―Synthesis, Characterization and applications of palladium membranes‖, Elsevier, chapter 8, 255–323, Membrane science and technology, Eds. R. Malada, M. Menendez, (2008). [39] Dolan MD, Dave NC, Ilyushechkin AY, Morpeth LD, McLennan KG, ―Composition and operation of hydrogen-selective amorphous alloy membranes‖, J. Membr. Sci., 285 (2006) 30–55. [40] Tosti S, Bettinali L, Violante V, ―Rolled thin Pd and Pd–Ag membranes for hydrogen separation and production‖, Int. J. Hydrogen. En., 25 (2000) 319–325. [41] Wieland S, Melin T, Lamm A, ―Membrane reactors for hydrogen production‖, Chem. Eng. Sci., 57 (2002) 1571–1576. [42] Damle AS, ―Hydrogen production by reforming of liquid hydrocarbons in a membrane reactor for portable power generation – Experimental studies‖, J. Power. Sou., 186 (2009) 167–177. [43] Edlund DJ, Pledger WA, ―Thermolysis of hydrogen sulfide in a metal-membrane reactor‖, J. Membr. Sci., 77 (1993) 255–264. [44] Edlund DJ, Pledger WA, ―Catalytic platinum-based membrane reactor for removal of H2S from natural gas streams‖, J. Membr. Sci., 94 (1994) 111–119. [45] McCool BA, Lin YS, ―Nanostructured thin palladium-silver membranes: Effects of grain size on gas permeation properties‖, J. Mater. Sci. 36 (2001) 3221–3227. [46] Li A, Liang W, Hughes R, ―The effect of carbon monoxide and steam on the hydrogen permeability of a Pd/stainless steel membrane‖, J. Membr. Sci., 165 (2000) 135–141. [47] Amandusson H, Ekedahl LG, Dannetun H, ―The effect of CO and O2 on hydrogen permeation through a palladium membrane‖, Appl. Surf. Sci., 153 (2000) 259–267. [48] Graham T, ―On the occlusion of hydrogen gas by meteoric iron‖, Proc. Roy. Soc. (London), 15 (1866) pp 502–503. [49] Mallada R, Menéndez M, ―Inorganic membranes: synthesis, characterization and applications‖, Technology and Engineering, Elsevier ISBN: 978-0-444-53070-7, (2008). [50] Gryaznov VM, Patent, USSR Author‘s Certificate, 274092 (1964).

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[51] Booth JCS, Doyle ML, Gee SM, Miller J, Scholtz LA, Walker PA, ―Advanced hydrogen separation via thin supported Pd membranes‖, Proc. of the 11th World Hydrogen Energy Conf., Stuttgart, Germany, June 23-28 (1996) pp. 867–878. [52] Cole MJ, ―The generator of pure hydrogen for industrial applications‖, Plat. Met. Rev., 25 (1981) 12–13. [53] Philpott J, ―Hydrogen diffusion technology. Commercial applications of palladium membrane‖, Plat. Met. Rev., 29 (1985) 12–16. [54] Shirasaki Y, Tsuneki T, Ota Y, Yasuda I, Tachibana S, Nakajima H, Kobayashi K, ―Development of membrane reformer system for highly efficient hydrogen production from natural gas‖, Int. J. Hydrogen. En., 34 (2009) 4482–4487. [55] Valliyappan T, Ferdous D, Bakhshi NN, Dalai AK, ―Production of hydrogen and syngas via steam gasification of glycerol in a fixed-bed reactor‖, Top. Catal., 49 (2008) 59–67. [56] Hirai T, Ikenaga N, Miyake T, Suzuki T, ―Production of hydrogen by steam reforming of glycerin on ruthenium catalyst‖, Energy and Fuel, 19 (2005) 1761–1762. [57] Huber GW, Shabaker JW, Dumesic JA, ―Raney Ni-Sn catalyst for H2 production from biomass-derived hydrocarbons‖, Science, 30 (2003) 2075–2077. [58] Zhang B, Tang X, Li Y, Xu Y, Shen W, ―Hydrogen production from steam reforming of ethanol and glycerol over ceria-supported metal catalysts‖, Int. J. Hydrogen. En., 32 (2007) 2367–2373. [59] Iriondo A, Barrio VL, Cambra JF, Arias PL, Guëmez MB, Navarro RM, SánchezSánchez MC, Fierro JLG, ―Hydrogen production from glycerol over nickel catalysts supported on Al2O3 modified by Mg, Zr, Ce or La‖, Top. Catal., 49 (2008) 46–58. [60] Adhikari S, Fernando S, Haryanto A, ―Production of hydrogen by SR of glycerin over alumina-supported metal catalyst‖, Catal. Tod., 129 (2007) 355–364. [61] Adhikari S, Fernando S, To SDF, Bricka RM, Steele PH, Haryanto A, ―Conversion of glycerol to hydrogen via a steam reforming process over nickel catalysts, Energy and Fuels ,2008, 22, 1220–1226. [62] Buffoni IN, Pompeo F, Santori GF, Nichio NN, ―Nickel catalysts applied in steam reforming of glycerol for hydrogen production‖, Catal. Comm., 10 (2009) 1656–1660. [63] Cui Y, Galvita V, Rihko-Struckmann L, Lorenz H, Sundmacher K, ―Steam reforming of glycerol: the experimental activity of La1-xCexNiO3 catalyst in comparison to the thermodynamic reaction equilibrium‖, Appl. Catal. B: Env, 90 (2009) 29–37. [64] Cheng CK, Foo SY, Adesina AA, ―H2-rich synthesis gas production over Co/Al2O3 catalyst via glycerol steam reforming‖, Catal. Comm., 12 (2010) 292–298 [65] Chiodo V, Freni S, Galvagno A, Mondello N, Frusteri F, ―Catalytic features of Rh and Ni supported catalysts in the steam reforming of glycerol to produce hydrogen‖, Appl. Catal. A: Gen., 381 (2010) 1–7 [66] Pompeo F, Santori G, Nichio NN, ―Hydrogen and/or syngas from steam reforming of glycerol. Study of platinum catalysts‖, Int. J. Hydrogen. En., 35 (2010) 8912–8920. [67] Chen H, Ding Y, Cong NT, Dou B, Dupont V, Ghadiri M, Williams PT ―A comparative study on hydrogen production from steam-glycerol reforming: thermodynamics and experimental‖, Ren. En., 36 (2011) 779–788. [68] Cheng CK, Foo SY, Adesina AA, ―Steam reforming of glycerol over Ni/Al2O3 catalyst‖, Cat Today, doi:10.1016/j.cattod.2011.07.011.

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[69] Dauenhauer PJ, Salge JR, Schmidt LD, ―Renewable hydrogen by autothermal steam reforming of volatile carbohydrates‖, J. Catal., 244 (2006) 238–247. [70] Byrd AJ, Pant KK, Gupta RB, ―Hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst‖, Fuel, 87 (2008) 2956–2960. [71] Iulianelli A, Seelam PK, Liguori S, Longo T, Keiski R, Calabrò V, Basile A, ―Hydrogen production for PEM fuel cell by gas phase reforming of glycerol as byproduct of bio-diesel. The use of a Pd–Ag membrane reactor at middle reaction temperature‖, Int. J. Hydrogen. En., 36 (2011) 3827–3834.

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ISBN: 978-1-62081-120-7 © 2012 Nova Science Publishers, Inc.

Chapter 7

USE OF THE GLYCEROL FROM BIODIESEL TO PRODUCTION OF ENVIRONMENTAL TECHNOLOGIES Miguel Araujo Medeiros1, Carla M. Macedo Leite2 and Rochel Montero Lago2 1

2

Universidade Federal do Tocantins, Brazil Universidade Federal de Minas Gerais, Brazil

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ABSTRACT One important feature of the glycerol produced from biodiesel is the presence of large amounts of impurities, such as catalysts (usually different alkalis), oil, carboxylic acids, alcohols (usually methanol or ethanol), etc. and even after purification the glycerol from the biodiesel process usually contains relatively large amounts of H2O (5–10%) and NaCl (4–8%). In this chapter, several processes for the conversion of crude glycerol into products for environmental and technological applications are discussed. These processes are based on the controlled sequence of reactions involving the catalyst already present in the crude glycerol to produce oligomers, thermoplastic and biodegradable polymers, and carbonaceous material. Some of them obtained materials to be described as olygomers with application in mining industries, thermoplastic polymers with application in controlled and slow release fertilizers, or carbonaceous material for environmental applications.

1. INTRODUCTION Currently, fossil fuels represent over 80% of energy consumption in the world. However, due to environmental and geopolitical issues the development of new energy sources is mandatory. For example, only the Middle East holds 63% of global reserves, which directly influences in the final price of fuel. In developed nations there is a growing trend towards employing modern technologies and efficient bioenergy conversion using a range of biofuels, which are becoming cost

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competitive with fossil fuels (Puhan et al., 2005). In Brazil, this work is focused on the production of bioethanol and biodiesel. There are discussions around the world on the feasibility of using renewable fuels, which may cause a much smaller impact to global warming, because the balance of CO2 emissions decreases when using these fuels. (Demirbas, 2008). In 1997 at a meeting in Kyoto, Japan, many of the developed nations agreed to limit their greenhouse gas emissions, relative to the levels emitted in 1990. In this occasion Brazil established social and environmental policies to collaborate with those global goals (Puhan et al., 2005). An example is the biodiesel program which in 2008 implemented the use of B2(2% biodiesel into conventional diesel) and the B5 in 2010.

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1.1. Biodiesel Production Biodiesel, a renewable biofuel produced from biomass, is biodegradable and does not cause significant contamination with emissions containing sulfur or aromatics. Biodiesel, is an viable alternative for compression-ignition engines (Puhan et al., 2005), in total or partial substitution of fossil diesel (Chiang, 2007). Biodiesel is fuel produced mainly by transesterification of vegetable oils, but can also be obtained by the reaction of animal fat (Pinto et al. 2005; Puhan et al., 2005; Chiang, 2007) soybean (Costa Neto and Rossi, 2000), Cotton (Pinto et al., 2005; Puhan et al., 2005), castor bean (Pinto et al., 2005), canola (Pinto et al., 2005; Catharino et al., 2007; Kocak et al., 2007; Puhan et al., 2005), palm (Pinto et al., 2005; Catharino et al., 2007; Puhan et al., 2005), sunflower (Pinto et al., 2005; Catharino et al., 2007; Puhan et al., 2005; Costa Neto and Rossi, 2000), peanut and babassu. Synthesis of biodiesel can be accomplished by using acid, basic (Costa Neto and Rossi, 2000; Puhan et al., 2005; Chiang, 2007; Pinto et al., 2005) or enzymes (Talukder et al., 2007; Schuchardt, 1990) catalysts or even in supercritical methanol (Puhan et al., 2005). Transesterification (Figure 1) is the reaction of triglycerides with an alcohol to form esters and glycerol (Chiang et al., 2007; Georgogianni et al., 2007; Krishna et al., 2007; Wu et al., 2007; Talukder et al., 2007; Aparício et al., 2007; Zuhair, 2005; Vicente et al., 2005; Medeiros et al., 2008; Stern and Hillion, 1990; Freedman et al., 1984; Encinar et al, 2002; Vicente et al., 2006; Bunyakiat et al., 2006; Karinen and Krause, 2006). This process decreases the viscosity of the oil and transforms the large, branched molecular structure of bio-oils into smaller molecules, of type required in regular diesel engines.

Figure 1. Synthesis of biodiesel by transesterification of triglyceride.

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Figure 2. (a) crude glycerol, (b) pre-purified glycerol, (c) glycerol purified.

In the transesterification for biodiesel production, a large amount of glycerol as a byproduct (about 10% compared to the mass of ester produced) (Puhan et al., 2005; Medeiros et al., 2010) is produced. The separation step of glycerol can be accomplished by decanting, in which the lower phase has the glycerol, the catalyst of the process (usually homogeneous and and high polar character), alcohol and oil residue without reacting (crude glycerol, Figure 2, a). The biodiesel separates from the upper stage, almost pure. The transesterification using methanol is the most used process around the world (Chiang, 2007) offering several advantages, such as: (i) small volume of alcohol recovery, (ii) lower cost of alcohol compared to ethanol (not in Brazil) and (iii) shorter reaction times (Pinto et al., 2005). The use of ethanol proves more advantageous, when considering its lower toxicity.

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1.2. New Uses for Glycerol The investigation of new uses for glycerol is critical for the success of the biodiesel program, especially in relation to the crude glycerol, which has few direct uses and market value marginalized. Currently, the demand for purified glycerol for the pharmaceuticals, food additives, personal care (Puhan et al., 2005) and industry is supplied by the petrochemical industry. The biodiesel production will produce a large increase in the amount of glycerol in the market, causing a decrease in the prices significantly, in the world. In the European Union, for example, the price of glycerol, in 1995 was € 1500 t-1 and reduced to 330 € t-1 in 2006 (Puhan et al., 2005). In Brazil, in 2005 the price of glycerol reached € 1270 t-1, but already in 2007 the price dropped to 720 € t-1. And in regions close to the price of biodiesel plants did not exceed € 300 t-1, in 2010. Different routes have been investigated to transform this glycerol to new products and new applications. Some of these processes are listed in Table 1. Oxidation products of glycerol, for example, can be used in cosmetics and pharmaceuticals intermediates (Davis et al., 2000; Pachauri and He, 2006; Krishna et al., 2007) and even suntan lotion (Kimura, 1993). The products of oligomerization of glycerol can be used as additives for cosmetics and foods, the raw material for resins and foams (Shenoy, 2006; Lemke, 2003; Werpy, 2004; Pagliaro and Rossi, 2008), lubricants (Pagliaro and Rossi, 2008), cement additives (retains

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moisture) and are synthetic intermediates and possible substitutes of polyols, e.g. polyvinyl alcohol, in some applications (Werpy, 2004; Pagliaro and Rossi, 2008; Medeiros et al., 2008). Table1. Conversion of glycerol to different products Process Polymerization

Pyrolise steam reforming Esterification

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Oxidation

Conditions T = 210-230°C; reduced pressure (~0,3atm); 0,51,5% NaOH. T = 650°C. T = 200-250°C; 1% cat. Níquel-Raney (Ni-Sn). T = 200-240°C; 0,1-0,3% NaOH; t =0,5 h; 100% methanol. T = 50°C; Pd/C (5-8% of Pd), t = 8h; pH = 5-11.

Etherization

T = 90°C; 1-7,5% amberlist 15; t = 2-3h.

Oligomerization

T = 260°C; 2% Mg25Al20 (cat.); t = 8h.

Products Cyclic polymers.

Ref. (Blytas and Frank, 1993)

CO; acetaldehyde; acrolein. 50-70% H2; 30-40% CO; 2-11% of alkanes. Carbohydrates and esters.

(Chiang, 2007)

Dihydroxyacetone. 70% of 3-tert-butoxi-1,2propanodiol (mono ether). 87% of mono ether. 65% of diglycerol; 20% of triglycerol and 15% of tetraglycerol.

(Stein et al., 1983) (Noureddini and Medikonduru, 1997) (Garcia et al., 1995) (Klepácová et al., 2003, 2006)

(Barrault et al., 2004)

It is noteworthy that many of the applications mentioned for the glycerol require high degree of purity, which for glycerol derived from biodiesel requires several stages of treatment, increasing its cost. The main impurities in the glycerol from biodiesel is methanol or ethanol, water, inorganic salts and catalyst residues, free fatty acids, unreacted mono, di and triglycerides and various other matter organic non-glycerol (MONG) (Pagliaro and Rossi, 2008). Thus, it is necessary to develop new routes for the consumption of glycerol from biodiesel. In this chapter, the transformation of glycerol, from biodiesel, based on the production of ethers from condensation of two (or more) molecules of glycerol will be showed. One of the 

mechanisms favorable for the formation of ethers is by alcohol protonation (ROH 2 ), followed by condensation of other alcohol and water loss.

Scheme 1.

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The condensation reaction of glycerol (Scheme 1), is usually catalyzed by acids or bases producing small polymers, called oligomers, and water. Along the text will be described the oligomers, polymers and carbons obtained from polyglycerol and its applications.

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2. POLYGLYCEROL APPLIED IN THE MINING INDUSTRY Many environmental and economic problems in the mining industry are related to the significant release of iron ore dust to the atmosphere during railroad transport and storage. Because of its small particle size (in average 0.15 mm), this iron ore is carried by the wind, causing heavy loss during the production process. These losses begin in the ore piles stored in the yards of mining. However, the most significant loss occurs during railroad transport of ore. At this stage, most ore in the railroad car is carried by the wind, causing an enormous environmental impact and generating a considerable amount of loss for mining companies. To minimize this problem, mining companies use aqueous solutions of polymers sprayed on the surface of the ore in wagons. Over time, water of solution evaporates and the material accumulated on the surface of the ore forms a thin protective layer against the wind (Figure 1). This polymeric material not have to be removed from the ore, before it was submitted for metallurgical methods for obtaining iron or steel (the polymer undergo pyrolysis at high temperatures), because the amount used is very small in relation to iron ore present in each wagons (not a significant contamination). Most commercially employed dust suppressants are derived from polyvinyl alcohol and acrylic acid polymers (Baeck et al, 2003; Talomani, 2008). The development of dust suppressants from glycerol derived from the manufacture of biodiesel fuel was motivated by: (i) similarities between the chemical structure of polyvinyl alcohol and oligomers and / or polymers of glycerol (Behr et al., 2008; Barrault et al., 2005; Lemke, 2003; Jeromin et al., 1998; Barrault et al., 2001; Roberts et al., 1995; Jakobson et al., 1988; Clacens et al., 2002; Barrault et al., 2004; Medeiros et al., 2009; Eshuis et al., 1994; Seiden and Martin, 1976); (ii) physical and chemical properties of glycerol oligomers similar to those of commercial products; (iii) allow the consumption of significant amounts of glycerol, a byproduct of biodiesel production, increasing the value of this byproduct. For a better understanding the transformation of glycerol in dust suppressant, two different variables were investigated: (i) the nature and concentration of the catalyst and (ii) the nature of the substrate (ultrapure glycerol and crude glycerol – the polar fraction obtained from biodiesel synthesis without treatment).

Effect of Catalyst and its Concentration The condensation of glycerol (Scheme 1) is usually catalyzed by acids or bases producing oligomers and water (Medeiros et al., 2009; Zhou et al., 2008; Klepacova et al., 2006).

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Figure 1. Application of dust suppressant solution on the wagon train with iron ore.

In order to assess the action of the catalyst in the process of oligomerization of glycerol a temperature of 140°C was selected for the assessment of the influence of the catalyst using a molar ratio of 3% H2SO4, H3PO4 and NaOH, in a reaction lasting 4 hours, as shown in Figure 3. Figure 3 shows that the NaOH and H3PO4 catalysts have little oligomerization activity of glycerol, even after 4 hours of reaction. However, oligomerization promoted by H2SO4, was very active since the first hours of reaction, forming 30% of oligomers. Three hours later, the product of reaction was a rigid polymer, determined by scanning electronic microscopy (Figure 4). In Figure 4 (a) and (b), the SEM images show the main forms of structuring the polymer material at microscopic level. These images show, in longitudinal section, spherical structures derived from the extensive release of water during oligomerization/polymerization. Undulations are seen on the surfaces of these bubbles and the material (Figure 4 (b) and (c)) that typically occur when the volume of the polymer is reduced, after the release of water vapor, when the polymer was till malleable (Medeiros et al., 2010).

Figure 3. Polycondensation of glycerol using different catalysts (H2SO4, H3PO4 and NaOH). Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

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Figure 4. SEM images of: (a), (b) and (c) polymer obtained with 3 mol% H2SO4, 140°C/3 h; (d) carbonaceous material obtained by polymer carbonization (800°C/3h).

Figure 5. ESI (+)-MS of the acid-catalyzed (3 mol% H2SO4) oligomerization of glycerol conducted at 140°C/ 1h. The unnamed ions refer to dehydration products.

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Figure 4 (d) shows the preservation of the basic structure of the polymer even after carbonization at 800°C/3h, suggesting the resulting polymer is thermosetting. The oligomerization/polymerization of glycerol is based on the production of ethers from: (i) condensation of two (up to five) molecules of the substrate, as shown in the ESI-MS spectrum of the sample with molar ratio of 3% H2SO4 and 1h (Figure 5); (ii) condensation of various substrate molecules, forming a cross-linked material (thermosetting polymer), with its structure growing in three dimensions (Medeiros et al., 2010). One of the mechanisms most favorable to the formation of these ethers is the protonation of the alcohol group (ROH) of glycerol, followed by condensation of other glycerol and water loss (smith and March, 2001). These results indicate that H2SO4 is more likely to donate protons to alcohol groups of glycerol than H3PO4, favoring condensation reactions. The mechanism promoted by NaOH, in turn, is not favored by the conditions of these reactions.

Variation in Concentration of H2SO4

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The increase in the amount of catalyst led to a higher rate of condensation of glycerol molecules, which has made the formation of a solid polymer possible. Thus, the variation of the molar ratio of H2SO4 (0.5, 1, 2 and 3%), 140°C/4h was studied. Note that for a system with molar ratio equal to 3%, the maximum amount of time taken to obtain oligomers is 1 hour. In order to measure the variation of the relative viscosity of the solution, the ratio of the viscosity of the solution and the viscosity of glycerol, in a reaction period of 60 min, were obtained (Figure 6).

Figure 6. Variation in the relative viscosity of the solution for the oligomerization of glycerol, with molar ratio of 0.5, 1, 2 and 3% H2SO4 (viscosity values related to glycerol).

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The curves in Figure 6 indicate a considerable increase in the viscosity of the solution, with a variation in the molar percentage of the catalyst of 0.5-3%. It can be seen that the solution containing only 0.5% H2SO4 has almost no viscosity change after 1h, suggesting that the process of condensation in this system favored the formation of short and simple chain oligomers. Regarding the solutions with higher molar ratios of catalyst (1, 2 and 3%), there is a considerable value added in viscosity, as the reaction proceeds. This greater viscosity observed can be attributed to the increased size and complexity of the structures of oligomers, because of the higher number of simultaneous condensation processes and lower selectivity of the catalyst for the OH groups of glycerol, forming a structure with branches and/or links between chains of oligomers and products arises from dehydration.

Variation in the Concentration of H3PO4

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Sulfuric acid is a very effective catalyst for the reaction of oligomerization/ polymerization of glycerol, even at molar ratios of 3%, as shown in TGA results (Figure 3). Phosphoric acid, however, has low ionization in the reaction medium, slightly favoring the condensation of molecules of glycerol. In order to verify whether or not the higher concentrations of phosphoric acid had a positive influence on the polycondensation of glycerol, experiments were conducted using different molar ratios (3, 5, 10 and 15%). The yields of oligomers/polymers and the variation of viscosity of the solution are shown in Figure 7.

Figure 7. Yields for the oligomers/polymers and viscosity of the solution at various concentrations of H3PO4. Reaction conditions: T = 140°C and t = 8 h. Data obtained by TGA.

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Analysis of the graphic in Figure 7 shows that the increase in catalyst concentration favors the oligomerization / polymerization of glycerol and the increase in relative viscosity of the solution. However, even with a molar ratio of 5% H3PO4, only a 21% yield for oligomers was found. Molar ratios of 10 and 15% favored the formation of a thermosetting polymer (12 and 18% of the sample, respectively). However, the amount of oligomers falls to 9 and 12% in these systems, respectively. With respect to the viscosity of the solution obtained, increasing the molar ratio from 5 to 15% resulted in a slight increase in this parameter (9 to 12). These results indicate that the increase in the concentration of H3PO4 favors polycondensation of glycerol. However, the condensation process required to form oligomers is smaller and less complex than that obtained with H2SO4. The highest relative viscosity value obtained with H3PO4 (15%), 8 h, is much lower than the viscosity values obtained for the systems with lower molar ratios of H2SO4 in a 1 h reaction period.

Tests of Dust Suppression

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Viscosity is a physicochemical property that measures the degree of movement of layers of molecules moving over each other. The more restricted the molecular movement, the greater the viscosity of the solution. High viscosity is a desired characteristic of a dust suppression product, because the higher the viscosity, the lesser the trepidation during transport and the winds will be able to break through the layers of product on the particulate material. Thus, tests were conducted in a wind tunnel to simulate the conditions of a railroad car loaded with ore transporting the ore from the mine to the port (exposure to wind speed of 70 km/h and constant trepidation). Table 2. Efficiency of different products in dust suppression, using iron ore (dparticle 99% > 99%

a

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Reactions performed under N2 atmosphere at 130 ºC using 1 mmol of benzonitrile (0.33 M solution). Substrate/Ru ratio: 100/5. b Yields determined by GC (uncorrected GC areas). c Technical grade glycerol was employed.

As expected, performing the catalytic reaction in the absence of water, that is, using exclusively pharmaceutical grade glycerol (99.5% purity), only a very minor amount (7%; determined by Gas Chromatography (GC)) of benzamide (14a) was formed after heating the reaction mixture for 24 hours (entry 1). Formation of 14a was obviously due to the traces of water present in the solvent. The same reaction performed in technical grade glycerol (87% purity), which is assumed to contain about 12-13% of water, allowed to increase the yield of 14a to 45% (entry 2), thus confirming the need for an appreciable amount of water for the hydration process to proceed. In accord with this, further experiments carried out using pharmaceutical grade glycerol/water mixtures, with v/v ratios ranging from 90:10 to 50:50 (entries 3-7), indicated that an improvement of the effectiveness of the process takes place with increasing the proportion of water present in the reaction medium. In particular, the highest conversions toward the desired benzamide (14a) were reached employing 60:40 and 50:50 v/v glycerol/water mixtures (entries 6-7). Under these conditions, complete disappearance of the starting material was observed after 24 h with quantitative and selective formation of 14a (benzoic acid was not detected by GC in the crude reaction mixtures). Based on these results, a 50:50 v/v glycerol/water mixture was chosen as the solvent for the rest of our studies.

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Alba E. Díaz-Álvarez, Rocío García-Álvarez, Pascale Crochet et al. Table 2. Hydration of Benzonitrile (13a) into Benzamide (14a) Catalyzed by Complex [RuCl2(η6-C6Me6)(PTA-Bn)] (7) in Glycerol/Water: Influence of the Temperaturea

Entry 1 2 3 4 5 6 7

Temperature 100 ºC 110 ºC 120 ºC 130 ºC 140 ºC 150 ºC 160 ºC

Time 24 h 24 h 24 h 24 h 5h 5h 2h

Yieldb 41% 81% 90% > 99% > 99% > 99% > 99%

a

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Reactions performed under N2 atmosphere using 1 mmol of benzonitrile (0.33 M solution). Substrate/Ru ratio: 100/5. b Yields determined by GC (uncorrected GC areas).

Using this optimal mixture of solvents, the catalytic hydration of benzonitrile (13a) by complex 7 (5 mol%; 0.33 M solution of the substrate) was studied at different temperatures, ranging from 100 to 160 ºC. As shown in Table 2, increasing the temperature from 130 ºC to 140-160 ºC resulted in a remarkable acceleration of the process (entry 4 vs entries 5-7). In particular, performing the catalytic reaction at 160 ºC, quantitative formation of benzamide (14a) was observed by GC after only 2 h (entry 7). Conversely, although complex [RuCl2(η6C6Me6)(PTA-Bn)] (7) remained active, remarkably poorer results were obtained below 130 ºC (entries 1-3). Table 3. Hydration of Benzonitrile (13a) into Benzamide (14a) Catalyzed by Complex [RuCl2(η6-C6Me6)(PTA-Bn)] (7) in Glycerol/Water: Influence of the Concentrationa

Entry 1 2 3 4 5

Concentration 0.20 M 0.25 M 0.33 M 0.50 M 1.00 M

Time 1 h (2 h) 1 h (2 h) 1 h (2 h) 1 h (2 h) 1h

Yieldb 89% (99%) 91% (> 99%) 91% (> 99%) 91% (> 99%) > 99%

a

Reactions performed under N2 atmosphere at 160 ºC using 1 mmol of benzonitrile (0.2-1 M solutions). Substrate/Ru ratio: 100/5. b Yields determined by GC (uncorrected GC areas). Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

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In order to improve the efficiency of the process, some experiments were also performed at different concentrations of the substrate (from 0.20 M to 1.00 M solutions), using 5 mol% of complex 7 and a constant temperature regime of 160 ºC (see Table 3). Thus, while no marked differences in activity were observed working in the concentration range 0.20-0.50 M (entries 1-3), the rate of the reaction slightly increased when a more concentrated 1 M solution of benzonitrile (13a) was employed. Under these new conditions, quantitative and selective formation of the desired benzamide (14a) was achieved in only 1 hour, leading to a turnover frequency (TOF) of 20 h-1 (entry 5). Such a TOF value could be further increased to 120 h-1 employing microwave irradiation, a non-classical low-energy-consuming heating source of prime interest within the Green Chemistry context [23]. Thus, using an MWirradiation power of 300 W, formation of 14a was quantitative after only 10 minutes at 160 ºC. However, we must note that partial degradation of the catalyst upon irradiation takes place. This fact is appreciable with the naked eye as the solution, initially orange, darkens and a black precipitate appears in suspension. Such a phenomenon was not observed employing conventional thermal heating in oil-bath. Remarkably, the TOF values reached in glycerol/water are comparable to those previously described for complex [RuCl2(η6-C6Me6)(PTA-Bn)] (7) in the hydration of benzonitrile in pure aqueous medium (TOF = 9.9 and 126.7 h-1 under classical thermal or MW heating, respectively) [14a]. Consequently, we can conclude that the use of a 1 M solution of the substrate, in a 50:50 v/v glycerol/water mixture of solvents, and a reaction temperature of 160 ºC (oil-bath) are also optimal conditions for the maximum efficiency of this ruthenium catalyst. We must also note that the use of these unprecedented reaction conditions allows the reduction of the catalyst loading. In this sense, we have observed that, even with only 2 mol% of complex [RuCl2(η6-C6Me6)(PTA-Bn)] (7), the hydration of benzonitrile proceeded to completion. However, as shown in Table 4, this reduction of the catalyst loading is associated with an increase in the reaction time (from 1 to 7 h) and a decrease of the turnover frequency (from 20 to 7.1 h-1). Table 4. Hydration of Benzonitrile (13a) into Benzamide (14a) Catalyzed by Complex [RuCl2(η6-C6Me6)(PTA-Bn)] (7) in Glycerol/Water: Influence of the Ruthenium Loadinga

Entry 1 2 3 4

mol% of Ru 5 mol% 4 mol% 3 mol% 2 mol%

Time 1h 2h 3h 7h

Yieldb > 99% > 99% > 99% > 99%

TOFc 20.0 h-1 12.5 h-1 11.1 h-1 7.1 h-1

a

Reactions performed under N2 atmosphere at 160 ºC using 1 mmol of benzonitrile (1 M solution). Substrate/Ru ratio: from 100/5 to 100/2. b Yields determined by GC (uncorrected GC areas). c Turnover frequencies ((mol product/mol Ru)/time) were calculated at the time indicated in each case.

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Alba E. Díaz-Álvarez, Rocío García-Álvarez, Pascale Crochet et al. Table 5. Hydration of Nitriles into Amides Catalyzed by Complex [RuCl2(η6-C6Me6)(PTA-Bn)] (7) in Glycerol/Water: Scope of the Reactiona

Entry 1 2 3 4 5 6 7 8 9 10 11 12 13

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a

Substrate R = Ph (13a) R = 2-C6H4F (13b) R = 4-C6H4Cl (13c) R = 3-C6H4Br (13d) R = C6F5 (13e) R = 4-C6H4Me (13f) R = 3-C6H4OMe (13g) R = 3-C5H4N (13h) R = 2-Thienyl (13i) R = n-C5H11 (13j) R = (CH2)3Ph (13k) R = CH2-4-C6H4Cl (13l) R = (E)-CH=CHPh (13m)

Time 1h 1h 30 min 30 min 30 min 1h 1h 30 min 1h 1h 3h 1h 1h

Yieldb 14a; > 99% (82%) 14b; > 99% (86%) 14c; > 99% (88%) 14d; > 99% (85%) 14e; > 99% (87%) 14f; > 99% (90%) 14g; > 99% (84%) 14h; > 99% (89%) 14i; 95% (80%) 14j; > 99% (87%) 14k; 98% (83%) 14l; > 99% (89%) 14m; > 99% (91%)

TOFc 20.0 h-1 20.0 h-1 40.0 h-1 40.0 h-1 40.0 h-1 20.0 h-1 20.0 h-1 40.0 h-1 19.0 h-1 20.0 h-1 6.5 h-1 20.0 h-1 20.0 h-1

Reactions performed under N2 atmosphere at 160 ºC using 1 mmol of of the corresponding nitrile 13am (1 M solution). Substrate/Ru ratio: 100/5. b Yields of 14a-m determined by GC (uncorrected GC areas). Isolated yields after appropriate work-up are given in brackets. c Turnover frequencies ((mol product/mol Ru)/time) were calculated at the time indicated in each case.

In order to assess the scope of this new protocol, the hydration of other nitriles was explored employing the best reaction conditions found, i.e. using a ruthenium loading of 5 mol% in a 50:50 v/v glycerol/water mixture at 160 ºC (oil-bath). The results obtained are collected in Table 5. Gratifyingly, we have found that, as observed for benzonitrile (entry 1), other aromatic (13b-g; entries 2-7) and heteroaromatic (13h-i; entries 8-9) substrates could also be selectively converted into the corresponding amides 14b-i (≥ 95% GC yields) in short times (30 min or 1 h). Influence of the electronic properties of the aryl rings on the reaction rates was observed. Thus, aromatic nitriles with electron-withdrawing groups showed in general a higher reactivity as compared to those with electron-donating substituents (entries 2-5 vs 6-7). In the case of 2-fluorobenzonitrile (13b), the steric effects associated with its ortho-substitution may be behind its anomalous behaviour (entry 2). As shown in entries 10-13, the effectiveness of the process is not restricted to aromatic organonitriles, the hydration of substrates 13j-m containing alkyl- or alkenyl-CN bonds being also conveniently achieved under the standard reaction conditions. Remarkably, upon completion of the reaction, the final amides could be in all cases easily separated from the water-glycerol phase by selective extraction with ethyl acetate (3 x 1 mL), thus avoiding the use of ―destructive‖ chromatographic methods. Further purification by recrystallization from hot water (or hot methanol) afforded analytically pure samples of the amides 14a-m in 8091% isolated yields. Finally, the possible recycling of complex [RuCl2(η6-C6Me6)(PTA-Bn)]

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(7) was also investigated. In this sense, we have found that, after extraction of 4chlorobenzamide (14c) and pentafluorobenzamide (14e) with EtOAc, the respective waterglycerol phases containing 7 remained active in the hydration process when a new charge of 4-chlorobenzonitrile (13c) and pentafluorobenzonitrile (13e), respectively, was added. Almost complete (≥ 95% GC yield) and selective conversions into 14c,e were reached in 1-3 h in these second runs.

CONCLUSIONS In summary, we have demonstrated that selective hydration of nitriles into amides by means of the arene-ruthenium(II) complex [RuCl2(η6-C6Me6)(PTA-Bn)] (7) can be conveniently performed in glycerol/water. Remarkably, the use of this inexpensive and green reaction medium enables easy product separation and catalyst recycling, thus providing a simpler method for the recycling of this type of catalysts as compared with the immobilization on the surface of silica-coated magnetic Fe3O4-nanoparticles previously described by us [17]. Overall, the results reported herein represents a new example of the utility of glycerol as solvent for synthetic organic chemistry [19-22], an emerging research field that has as main objective the revalorization of a waste generated by the biodiesel industry [18]. Although there is already a body of work in this area, we are still at the beginning to learn what the real potential of this green solvent in organic synthesis is. Obviously, the area remains open with many opportunities for new discoveries and further advances in this field can be expected in the near future.

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EXPERIMENTAL SECTION General Comments All catalytic reactions were performed under an atmosphere of dry nitrogen using vacuum-line and standard sealed-tube techniques. All reagents were obtained from commercial suppliers and used without further purification, with the exception of compound [RuCl2(η6-C6Me6)(PTA-Bn)] (7) which was prepared by following the method reported in the literature [14a]. Pharmaceutical (99.5%) and technical (87%) grade glycerol were purchased from VWR International and used as received. GC measurements were made on a HewlettPackard HP6890 equipment using a Supelco Beta-DexTM 120 column (30 m length; 250 µm diameter). NMR spectra were recorded on Bruker DPX300 or AV400 instruments. Chemical shifts are given in ppm, relative to internal tetramethylsilane (1H and 13C) or CFCl3 (19F) standards.

General Procedure for the Catalytic Hydration Reactions Under nitrogen atmosphere, the corresponding nitrile 13a-m (1 mmol), pharmaceutical grade glycerol (0.5 mL), water (0.5 mL), and the ruthenium catalyst [RuCl2(η6-C6Me6)(PTA-

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Alba E. Díaz-Álvarez, Rocío García-Álvarez, Pascale Crochet et al.

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Bn)] (7) (31 mg; 5 mol% of Ru) were introduced into a Teflon-capped sealed tube, and the resulting solution stirred at 160 °C (oil-bath) for the indicated time (see Table 5). The course of the reaction was monitored by taking regularly samples of ca. 20 µL which after extraction with ethyl acetate (3 mL) were analyzed by GC. After that, the reaction mixture was washed with ethyl acetate (3 x 1 mL), the upper organic phase separated from the aqueous glycerol phase, and evaporated under reduced pressure. The resulting solid residue was then recrystallized from hot water (or hot methanol) to yield amides 14a-m in pure form. The identity of 14a-m was assessed by comparison of their 1H and 13C{1H} NMR spectroscopic data with those reported in the literature: Benzamide (14a) [24]: White solid. 0.099 g (82%). 1H NMR (CDCl3): δ = 6.31 (br, 2H), 7.41-7.55 (m, 3H), 7.82 (m, 2H) ppm. 13C{1H} NMR (CDCl3): δ = 127.3, 128.5, 131.9, 133.3, 169.7 ppm. 2-Fluorobenzamide (14b) [24]: White solid. 0.119 g (86%). 1H NMR (CDCl3): δ = 6.73 (br, 2H), 7.01-7.50 (m, 3H), 8.07 (m, 1H) ppm. 13C{1H} NMR (CDCl3): δ = 116.0 (d, 2 JFC = 24.4 Hz), 120.2 (d, 2JFC = 11.6 Hz), 124.7 (d, 4JFC = 2.9 Hz), 132.1, 133.7 (d, 3 JFC = 9.3 Hz), 160.8 (d, 1JFC = 248.6 Hz), 165.3 ppm. 19F{1H} NMR (CDCl3): δ = 112.8 (s) ppm. 4-Chlorobenzamide (14c) [24]: White solid. 0.137 g (88%). 1H NMR (CDCl3): δ = 5.97 (br, 2H), 7.42-7.75 (m, 4H) ppm. 13C{1H} NMR (CDCl3): δ = 128.7, 128.8, 131.6, 168.0 ppm. 3-Bromobenzamide (14d) [25]: White solid. 0.170 g (85%). 1H NMR (CD3OD): δ = 7.387.83 (m, 3H), 8.03 (br, 1H) ppm; NH2 protons not observed. 13C{1H} NMR (CD3OD): δ = 125.4, 129.3, 133.3, 133.7, 137.7, 139.1, 172.5 ppm. Pentafluorobenzamide (14e) [26]: White solid. 0.183 g (87%). 13C{1H} NMR (CD3OD): δ = 113.5 (m), 135.6-146.6 (m), 160.3 (br) ppm. 19F{1H} NMR (CD3OD): δ = -164.0 (m), -143.8 (m), -155.7 (m) ppm. 4-Methylbenzamide (14f) [25]: White solid. 0.105 g (90%). 1H NMR (CDCl3): δ = 2.39 (s, 3H), 6.06 (br, 2H), 7.24 and 7.70 (d, 2H each, 3JHH = 6.9 Hz) ppm. 13C{1H} NMR (CDCl3): δ = 21.4, 127.3, 129.2, 130.4, 142.5, 169.4 ppm. 3-Methoxybenzamide (14g) [24]: White solid. 0.127 g (84%). 1H NMR (CDCl3): δ = 3.85 (s, 3H), 6.08 (br, 2H), 7.07 (m, 1H), 7.31-7.40 (m, 3H) ppm. 13C{1H} NMR (CDCl3): δ = 55.4, 112.5, 118.3, 119.1, 129.5, 134.4, 159.8, 169.3 ppm. Nicotinamide (14h) [24]: White solid. 0.109 g (89%). 1H NMR (CD3OD): δ = 7.54 (dd, 1H, 3JHH = 7.9 and 5.0 Hz), 8.28 (dt, 1H, 3JHH = 7.9 Hz, 4JHH = 2.5 Hz), 8.67 (dd, 1H, 3 JHH = 5.0 Hz, 4JHH = 2.5 Hz), 9.02 (s, 1H) ppm; NH2 protons not observed. 13C{1H} NMR (CD3OD): δ = 123.1, 129.4, 135.4, 147.4, 150.8, 167.8 ppm. 2-Thienylamide (14i) [27]: White solid. 0.102 g (80%). 1H NMR (CD3OD): δ = 7.11 (dd, 1H, 3JHH = 5.0 and 3.8 Hz), 7.65 (dd, 1H, 3JHH = 5.0 Hz, 4JHH = 1.0 Hz), 7.70 (dd, 1H, 3JHH = 3.8 Hz, 4JHH = 1.0 Hz) ppm; NH2 protons not observed. 13C{1H} NMR (CD3OD): δ = 127.3, 129.0, 130.7, 148.3, 169.4 ppm. Hexanamide (14j) [24]: White solid. 0.100 g (87%). 1H NMR (CD3OD): δ = 0.91 (t, 3H, 3 JHH = 6.9 Hz), 1.32 (m, 4H), 1.60 (m, 2H), 2.18 (m, 2H) ppm; NH2 protons not observed. 13C{1H} NMR (CD3OD): δ = 11.8, 21.0, 24.2, 30.1, 34.1, 176.9 ppm.

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4-Phenylbutyramide (14k) [24]: White solid. 0.135 g (83%). 1H NMR (CD3OD): δ = 1.94 (m, 2H), 2.25 (m, 2H), 2.65 (m, 2H), 7.19-7.31 (m, 5H) ppm; NH2 protons not observed. 13C{1H} NMR (CD3OD): δ = 25.7, 32.9, 33.3, 123.9, 126.4, 126.5, 140.0, 175.9 ppm. (4-Chlorophenyl)acetamide (14l) [28]: White solid. 0.151 g (89%). 1H NMR (CD3OD): δ = 3.50 (s, 2H), 7.30 (br, 4H) ppm; NH2 protons not observed. 13C{1H} NMR (CD3OD): δ = 40.6, 127.7, 129.9, 131.9, 133.9, 174.5 ppm. (E)-3-Phenylacrylamide (14m) [29]: White solid. 0.134 g (91%). 1H NMR (CD3OD): δ = 6.63 and 7.54 (d, 1H each, 3JHH = 15.8 Hz), 7.37 (m, 3H), 7.59 (m, 2H) ppm; NH2 protons not observed. 13C{1H} NMR (CD3OD): δ = 119.5, 127.0, 128.1, 129.1, 134.3, 140.8, 169.1 ppm.

Catalyst Recycling After completion of the reaction the mixture was allowed to reach the room temperature. Ethyl acetate (1 mL) was then added and two phases appeared. The upper organic phase containing the desired amide was separated with the aid of a Pasteur pipette, and the extraction process was repeated twice more. After that, to the aqueous glycerol phase a new load of the nitrile was added, and the mixture heated at 160 ºC in an oil-bath for the indicated time (see the discussion section).

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ACKNOWLEDGMENTS Financial support by the Ministerio de Ciencia e Innovación (MICINN) of Spain (Projects CTQ2010-14796/BQU and CSD2007-00006) and the Gobierno del Principado de Asturias (FICYT Project FC-11-COF11-13) is gratefully acknowledged.

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[2] [3]

See, for example: (a) The Chemistry of Amides; Zabicky, J., Ed.; Wiley: New York, 1970; (b) The Amide Linkage: Structural Significance in Chemistry, Biochemistry and Materials Science; Greenberg, A.; Breneman, C. M.; Liebman, J. F., Eds.; Wiley: New York, 2000; (c) Polyesters and Polyamides; Deopura, B. L.; Gupta, B.; Joshi, M.; Alagirusami, R., Eds.; CRC Press: Boca Raton, 2008; (d) Johansson, I. In Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 2004; Vol. 2, pp 442-463. For a recent review, see: Allen, C. L.; Williams, J. M. J. Chem. Soc. Rev. 2011, 40, 3405-3415. See, for example: (a) Bailey, P. D.; Mills, T. J.; Pettecrew, R.; Price, R. A. In Comprehensive Organic Functional Group Transformations II; Katritzky, A. R.; Taylor, R. J. K., Eds.; Elsevier: Oxford, 2005; Vol. 5, pp 201-294; (b) Methoden Org. Chem. (Houben Weyl); Dopp, D.; Dopp, H., Eds.; Thieme Verlag: Stuttgart, 1985; Vol. E5(2), pp 1024-1031.

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Alba E. Díaz-Álvarez, Rocío García-Álvarez, Pascale Crochet et al. For reviews covering this field, see: (a) Parkins, A. W. Platinum Metals Rev. 1996, 40, 169-174; (b) Kukushkin, V. Y.; Pombeiro, A. J. L. Chem. Rev. 2002, 102, 1771-1902; (c) Kukushkin, V. Y.; Pombeiro, A. J. L. Inorg. Chim. Acta 2005, 358, 1-21; (d) Ahmed, T. J.; Knapp, S. M. M.; Tyler, D. R. Coord. Chem. Rev. 2011, 255, 949-974. (a) Murahashi, S.-I.; Sasao, S.; Saito, E.; Naota, T. J. Org. Chem. 1992, 57, 2521-2523; (b) Murahashi, S.-I.; Sasao, S.; Saito, E.; Naota, T. Tetrahedron 1993, 49, 8805-8826; (c) Murahashi, S.-I.; Takaya, H. Acc. Chem. Res. 2000, 33, 225-233. (a) Ghaffar, T.; Parkins, A. W. Tetrahedron Lett. 1995, 36, 8657-8660; (b) Akisanya, J.; Parkins, A. W.; Steed, J. W. Org. Proc. Res. Dev. 1998, 2, 274-276; (c) Ghaffar, T.; Parkins, A. W. J. Mol. Catal. A: Chem. 2000, 160, 249-261; (d) Jiang, X.-B.; Minnaard, A. J.; Feringa, B. L.; de Vries, J. G. J. Org. Chem. 2004, 69, 2327-2331. Oshiki, T.; Yamashita, H.; Sawada, K.; Utsunomiya, M.; Takahashi, K.; Takai, K. Organometallics 2005, 24, 6287-6290. Goto, A.; Endo, K.; Saito, S. Angew. Chem. Int. Ed. 2008, 47, 3607-3609. Gieling, R. G.; Groen, M. B. PCT Int. Appl. WO2006/097342, 2006 (Synthon B. V.). Harrington, P. M.; Jung, M. E. US Patent US5354868, 1994 (American Cyanamid Co.). See, for example: (a) Yamada, H.; Kobayashi, M. Biosci. Biotech. Biochem. 1996, 60, 1391-1400; (b) van Pelt, S.; van Rantwijk, F.; Sheldon, R. A. In Focus in Catalysis Applications (supplement to Chimica Oggi/Chemistry Today); Teknoscienze Srl: Milano, Italy, 2008; Vol. 26, pp 2-4; (c) Sanchez, S.; Demain, A. L. Org. Process Res. Dev. 2011, 15, 224-230; (d) Li, B.; Su, J.; Tao, J. Org. Process Res. Dev. 2011, 15, 291293. (a) Anastas, P. T.; Warner, J. C. In Green Chemistry: Theory and Practice; Oxford University Press: Oxford, 1998; (b) Matlack, A. S. In Introduction to Green Chemistry; Marcel Dekker: New York, 2001; (c) Lancaster, M. In Handbook of Green Chemistry and Technology; Clark, J. H., Macquarrie, D. J., Eds.; Blackwell Publishing: Abingdon, 2002; (d) Lancaster, M. In Green Chemistry: An Introductory Text; RSC Publishing: Cambridge, 2010. (a) Nelson, W. M. In Green Solvents for Chemistry: Perspectives and Practice; Oxford University Press: New York, 2003; (b) Clark, J. H.; Taverner, S. J. Org. Process Res. Dev. 2007, 11, 149-155; (c) Kerton, F. M. In Alternative Solvents for Green Chemistry; RSC Publishing: Cambridge, 2009. (a) Cadierno, V.; Francos, J.; Gimeno, J. Chem. Eur. J. 2008, 14, 6601-6605; (b) Cadierno, V.; Díez, J.; Francos, J.; Gimeno, J. Chem. Eur. J. 2010, 16, 9808-9817; (c) García-Álvarez, R.; Díez, J.; Crochet, P.; Cadierno, V. Organometallics 2010, 29, 3955-3965; (d) García-Álvarez, R.; Francos, J.; Crochet, P.; Cadierno V. Tetrahedron Lett. 2011, 52, 4218-4220; (e) García-Álvarez, R.; Díez, J.; Crochet, P.; Cadierno V. Organometallics 2011, 30, 5442-5451. See, for example: (a) Sheldon, R. A.; Arends, I.; Hanefeld, U. In Green Chemistry and Catalysis; Wiley-VCH: Weinheim, 2007; (b) Recoverable and Recyclable Catalysts; Benaglia, M., Ed.; John Wiley & Sons: Chichester, 2009. See, for example: (a) Aqueous Organometallic Chemistry and Catalysis; Horváth, I. T.; Joó, F., Eds.; Kluwer: Dodrecht, 1995; (b) Aqueous-Phase Organometallic Catalysis: Concepts and Applications; Cornils, B.; Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, 1998; (c) Aqueous Organometallic Catalysis; Horváth, I. T.; Joó, F., Eds.; Kluwer: Dodrecht, 2001.

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[17] García-Garrido, S. E.; Francos, J.; Cadierno, V.; Basset, J. M.; Polshettiwar, V. ChemSusChem 2011, 4, 104-111. [18] Pagliaro, M.; Rossi, M. In The Future of Glycerol: New Usages for a Versatile Raw Material; RSC Publishing: Cambridge, 2008. [19] For reviews on this topic, see: (a) Gu, Y.; Jérôme, F. Green Chem. 2010, 12, 11271138; (b) Díaz-Álvarez, A. E.; Francos, J.; Lastra-Barreira, B.; Crochet, P.; Cadierno, V. Chem. Commun. 2011, 47, 6208-6227; (c) Wolfson, A.; Dlugy, C.; Tavor, D. In Homogeneous Catalysis: Types, Reactions and Applications; Poehler, A. C., Ed.; Nova Science Publishers: New York, 2011, pp 185-203. [20] Francos, J.; Cadierno, V. Green Chem. 2010, 12, 1552-1555. [21] Lastra-Barreira, B.; Francos, J.; Crochet, P.; Cadierno, V. Green Chem. 2011, 13, 307313. [22] Catalyst recycling was also possible in the ruthenium-catalyzed reduction of allylic alcohols, using glycerol as solvent and hydrogen donor, recently developed by us: DíazÁlvarez, A. E.; Crochet, P.; Cadierno, V. Catal. Commun. 2011, 13, 91-96. [23] For recent reviews and a book on microwave-assisted chemistry in aqueous media, see: (a) Dallinger, D.; Kappe, O. C. Chem. Rev. 2007, 107, 2563-2591; (b) Polshettiwar, V.; Varma, R. S. Chem. Soc. Rev. 2008, 37, 1546-1557; (c) Polshettiwar, V.; Varma, R. S. Acc. Chem. Res. 2008, 41, 629-639; (d) Aqueous Microwave-Assisted Chemistry; Polshettiwar, V.; Varma, R. S., Eds.; RSC Publishing: Cambridge, 2010. [24] AIST: RIO-DB Spectral Database for Organic Compounds; National Institute of Advanced Industrial Science and Technology (AIST), Japan. [25] Li, X.-Q.; Wang, W.-K.; Han, Y.-X.; Zhang, C. Adv. Synth. Catal. 2010, 352, 25882598. [26] Zibarev, A. V.; Furin, G. G.; Yakobson, G. G. Izv. Sibir. Otd. Akad Nauk, Ser. Khim. 1980, 107-112. [27] Ali, M. A.; Punniyamurthy, T. Adv. Synth. Catal. 2010, 352, 288-292. [28] Van Baelen, G.; Maes, B. U. W. Tetrahedron 2008, 64, 5604-5619. [29] Yi, C. S.; Zeczycki, T. N.; Lindeman, S. V. Organometallics 2008, 27, 2030-2035.

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In: Glycerol: Production, Structure and Applications Editors: M. De Santos Silva and P. Costa Ferreira

ISBN: 978-1-62081-120-7 © 2012 Nova Science Publishers, Inc.

Chapter 12

CONVERSION OF GLYCEROL INTO PRODUCTS FOR TECHNOLOGICAL APPLICATIONS Márcio Guimarães Coelho1, Luiz Claudio de Melo Costa2, Miguel Araujo Medeiros3 and Carla M.M. Leite4 1

Centro Universitário Newton Paiva, Brazil 2 Verti Ecotecnologias S. A., Brazil 3 Universidade Federal do Tocantins, Brazil 4 Universidade Federal de Minas Gerais, Minas Gerais, Brazil

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INTRODUCTION The glycerol (1,2,3propanetriol or glycerine) was discovered by Scheele in 1779 during the saponification of olive oil (Arruda et al, 2006). It is a polyol with the structural formula shown in Figure 1.

Figure 1. Structure of the glycerol.

Glycerol is an oily liquid, colorless, viscous and sweet taste, soluble in water and alcohol in all proportions, no toxicity, color or odor. Because of these properties glycerol is a substance with a variety of applications such as: the manufacture of alkyd resins, dynamite, esters, pharmaceuticals, perfumes, plastics, cosmetics, foods, including sweets, rolls of tobacco, alcoholic beverages, paint thinner, polyurethane polyols, emulsifiers, rubber stamping, paints, cements and binders for mixing, special soaps, emollients and lubricants, penetrating, hydraulic fluids, humectants, nutrients and fermentation antifreeze mixture. Some physicochemical properties are presented in Table 1(Morrison, 1994).

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Márcio Guimarães Coelho, Luiz Claudio de Melo Costa et al. Table 1. Some physicochemical properties of glycerol Molar mass Density (25 °C) Viscosity Boiling point Melting point Flash point SurfaceTension (20 °C) Specificheat (99.94% glycerol, 26 °C) Heatofdissolution Heatofformation 55 ° C heatvaporization ThermalConductivity

92.09 g/mol 1.262 kg / m3 939 cps 290 °C 18 °C 177 °C 63.4 mN / m 2.435 J / g 5.8 kJ / mol 667.8 kJ / mol 88.12 J / mol 0.28 W / mK

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GLYCEROL PRODUCTION The main routes of glycerol are obtained by microbial fermentation, chemical synthesis from petrochemical raw materials, can be recovered as a byproduct of the manufacture of soap from the fat or as a byproduct of biodiesel production. The microbial production of glycerol is known about 150 years ago when Pasteur (1858) investigated the production of glycerol by fermentation. Glycerol is a byproduct of ethanol fermentation of sugar made from Saccharomyces cerevisiae, was produced on a large scale during the First World War. The fermentation also produces ethanol and carbon dioxide, secondary compounds such as glycerol. The route of microbial synthesis was not able to compete with the synthesis from petroleum, due to a low yield of the reaction and the difficulty of extraction and purification of glycerol mixture obtained. The synthesis of glycerol from petroleum is performed from the late 40s. Glycerol is produced from epichlorohydrin obtained from propylene (Scheme 1) which in turn is derived from oil (Beatriz et al, 2011; Wang et al, 2001).

Scheme 1. Glycerol Productionfrom propylene.

With the high cost of propylene and due to concerns about the environment, since 1970 a decrease of its availability, thereby, the production of glycerol by fermentation has become an interesting alternative route (Wang et al, 2001). More recently, production plants and glycerol are closing plants that use glycerol as a raw material are being opened as a result of the large surplus of glycerol obtained as a byproduct of biodiesel production (Beatriz et al, 2011). During the production process of biodiesel by

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acidic or basic trasesterificação from vegetable oils or animal fats using methanol or ethanol as part of the reagents (Scheme 2), is the production of glycerol as a byproduct of the reaction. Approximately 10% compared to the mass of the ester of glycerin is produced (Araújo et al, 2010; Alptekin et al, 2011; Mendow et al, 2011). The glycerin byproduct of the manufacture of biodiesel (crude glycerin or blonde) has about 20% impurities. The main impurities in the glycerine from biodiesel are catalyst, alcohol, fatty acids, salts and water. These impurities depend on the type of oilseed and type of catalyst used in biodiesel production (Beatriz et al, 2011). According to estimates, the production of glycerol in the world would exceed 1.2 million tons per year from 2010 due to increased of biodiesel production (Figure 2) (Zhou et al, 2008). The investigation of new uses for glycerol produced from biodiesel synthesis is critical to the success of the biodiesel program.

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Scheme 2. Generationof glycerolas a byproductof the synthesis ofbiodiesel.

Figure 2. Production of glycerol in the world. Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

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Glycerol can also be obtained from the basic hydrolysis of triglycerides in the production of soaps (Scheme 3) (Barbosa et al, 1995).

Scheme 3. Generation ofglycerolas a byproduct ofgetting thesoap.

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MAIN PRODUCTS OBTAINED FROM THE GLYCEROL Although glycerol can generate energy when burned, it can also be turned into valuable chemicals. By having highly functionalized molecule, glycerol can be easily oxidized, reduced, halogenated, etherified and esterified generating new chemicals (Zheng et al, 2008, Zhou et al, 2008). The glycerol from the biodiesel production process, is usually contaminated with water, monoglycerides, diglycerides, salt, soap, catalyst residue and esters (biodiesel). After going through a process of purification (distillation), glycerin can be used in numerous products such as biogas production, pharmaceuticals, cosmetics industry (emollient), chemicals (glyceraldehyde), solvent for paints and varnishes, lubricants Composites (biodegradable plastics) and substrate for biotechnological processes. Since residual crude glycerin, a byproduct of biodiesel production, is a raw material with great potential for production of hydrogen (H2) and syngasby pyrolysis, gasification or catalytic reforming (Valliyappan et al, 2004). Following are some routes for the chemical transformation of glycerol into products.

OXIDATION OF GLYCEROL A series of products can be obtained from the oxidation of glycerol, such as glyceraldehyde, dihydroxyacetone, glyceric acid, glycolic acid, hydroxipiruvico, mesoxalicmesoxalico acid, oxalic acid and tartronico among others (Figure 4). Dihydroxyacetone is used as a tanning agent by the cosmetics industry, hidroxipiruvico acid can be used as raw material for synthesis of D, L-serine, acidmesoxalico has potential for use as a complexing agent and as a precursor in organic synthesis.

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Scheme 4. Some products derivedfrom the oxidation ofglycerol.

Scheme 5. Products obtainedfrom thereduction ofglycerol.

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REDUCTION OF GLYCEROL The main products resulting from the reduction of glycerol are 1,3-propanediol and 1,2 propanediol. The 1,3-diol is copolymerized with terephthalic acid to produce polyester fiber used in the manufacture of carpets and textiles (Figure 5).

SELECTIVE DEHYDROXYLATION OF GLYCEROL The 1,3-propanediol can be obtained from the selective dehydroxylation of glycerol. The process consists of three steps: (i) acetylation, aims to protect the first and third hydroxyl groups, (ii) tosilação, this group is easier to come out and be replaced by a hydride ion compared to the hydroxyl group and (iii) detosilação, which removes the tosyl group, followed by hydrolysis to remove the protection first and third hydroxyl groups (Scheme 6).

Sheme 6. Process of obtaining 1,3-propanediol.

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DEHYDRATION OF GLYCEROL The dehydration of glycerol can generate acrolein, a substance used as raw material for the production of acrylic acid, pharmaceuticals and others. The dehydration of glycerol can still generate the 3-hidroxipropanaldeído, a substance that can be used as a precursor in the production of acrylic acid (Scheme 7).

Scheme7. Dehydration reactionof glycerol.

HALOGENATION OF GLYCEROL A major product obtained from the halogenation of glycerol is 1,3-dicloropropanol which is an intermediate in the synthesis of epicloridina. This substance is an important raw material for the production of materials such as resins, synthetic elastomers, among others. The process for the synthesis of 1,3-dicloropropanal from glycerol is described in Scheme 8.

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Scheme 8. Halogenationof glycerol.

ETHERIFICATION OF GLYCEROL

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Glycerol ethers can be used as fuel additives. Mixtures of mono, di, and glycerol trialquil are suitable to be added to diesel. The addition of these esters have a positive effect on the final quality of diesel oil. The polyglycerol ethers can be used in surfactants, lubricants and cosmetics. You can control the polyglycerol chain length, degree of etherification. The etherification of glycerol to form polyglycerol is demonstrated in scheme 9.

Scheme 9. Etherificationof glycerol.

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ESTERIFICATION OF GLYCEROL The reactions of esterification of glycerol has been the subject of much research. These can be divided into three types: esterification with carboxylic acids, carboxylation, and nitration. The esterification of glycerol with carboxylic acid or diacylglycerolmonoglicerol way. The monoacilgliceroisamphiphilic molecules are useful as surfactants and emulsifiers. The diglycerides are present edible fats and oils. Recent studies suggest that diacylglycerols play a role in reducing serum triacylglycerol and, consequently, decreases body weight. Glycerol carbonate is a stable colorless liquid, is useful as a solvent for plastics and resins, reacts readily with phenols, alcohols, carboxylic acids and when heated to form the ethers of glycerol. From the nitration of glycerol can potentially produce polymers suitable for use empropelentes, explosives, gas generators, and pyrotechnics.

TECHNOLOGIES DEVELOPED IN BRAZIL

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Brazil is among the largest producers and consumers of biodiesel in the world with an annual production in 2010 of 2.4 billion liters and an installed capacity in the same year, to about 5.8 billion liters (ANP, 2010 ). With the implementation of the program B5 in 2010 by the federal government, which requires the addition of 5% biodiesel in the diesel market, the production of biodiesel in Brazil has increased and therefore the generation of glycerol which is a byproduct of the reaction. The Brazilian government is encouraged to conduct research aimed at developing technologies that consume the excess of glycerol produced. In the following sections it will be discuss some technologies researched in Brazil.

OBTAININ GHYDROGEN FROM GLYCEROL Hydrogen is considered a clean fuel because its combustion results in the generation of water only, no emissions of air pollutants, greenhouse gases or particles. However, about 95% hydrogen used today is derived from fossil fuels, while the remaining 5% comes from the electrolysis water (Melo et al, 2011, Manfro et al, 2011). Because this process involves the use of nonrenewable resources and requires high energy consumption, this form of hydrogen production is not sustainable or economically viable. Several research groups have searched the world to obtain hydrogen from glycerol using different methods such as steam reforming (Hashaikeh et al, 2006) gasification (Authayanun et al, 2010), autothermal reforming (Wena et al, 2008) , reform in the aqueous phase (Marshall et al, 2008) Electrochemical reform (Sabourin et al, 2009) and fotofermentação (Byrd et al, 2008). Obtaining hydrogen from glycerol is an interesting technology because it uses renewable resources to generate clean fuel, consuming part of the glycerol produced in the world due to the incentive to produce biodiesel. Most studies on the production of hydrogen from glycerol have focused on thermochemical routes. However, hydrogen can be produced from biomass, under ambient conditions from the photocatalytic route, researched technology in Brazil. The photocatalysis

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has been widely investigated in recent years as an efficient technology for degradation of contaminants in aqueous solution. The photocatalytic degradation of organic compounds can occur with the simultaneous production of hydrogen since it is performed in the absence of oxygen. The photocatalytic oxidation of organic compounds occurs in the presence of oxygen with the participation of photogenerated holes in the semiconductor, which act directly or indirectly by generating hydroxyl radicals (OH •), leading to production of CO2 and H2O. The photogenerated electrons are consumed by the semiconductor chemisorbed oxygen to form superoxide radicals (O2-•). For the occurrence of hydrogen gas generation process should happen in the absence of oxygen, this results in the oxidation of organic substrate through holes generating CO2, which is accompanied by the production of gaseous hydrogen from the reduction of protons from the water by photogenerated electrons, thus the formation of superoxide radicals is suppressed. Wide range of biomass-such as monosaccharides including pentoses (ribose, arabinose), hexoses (glucose, galactose, fructose, and mannose), alcohols (ethanol, methanol, propanol and butanol), organic acids (acetic acid, formic acid) and has been studied for hydrogen generation glycerol (Melo et al, 2011). In all these cases, the amounts of H2 and CO2 are produced in accordance with the following chemical equation:

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CxHyOz + (2x-z)H2O → xCO2 + (2x - z + y/2)H2 Few studies have focused on obtaining hydrogen from glycerol through the photocatalytic route. The main research carried out using TiO2 as catalyst. The big disadvantage of TiO2 is its inability to use visible light to be activated due to its bangap of 3.2 eV, which corresponds to a wavelength in the ultraviolet (UV). Sunlight is known po contain only about 3% of UV light. Research has been conducted in order to obtain photocatalysts that can be activated by visible light in order to use the sunlight. Brazilian researchers have produced the material Pt-CdS-TiO2 which can be activated by visible light, to be effec generation of hydrogen gas from glycerol (Melo et al, 2011). Another way to produce hydrogen from glycerol investigated in Brazil's reform in the aqueous phase. This process can be performed at low temperatures, reducing the cost of the process, moreover, it is possible to generate H2 and CO2 in a single step with low levels of CO, which is important for applications in fuel cells. Some metals such as supported Pt, Ru, Rh, Pd, Ir and Ni catalysts were tested in the process and have shown good activity and selectivity for hydrogen production. The process involves the cleavage of C-C bonds and the C-H bonds to form adsorbed species (especially CO) on the catalyst surface, then CO is converted to CO2 and H2 by reaction of water-gas shift. C3H8O3→ 3CO + 4H2 CO + H2O → CO2 + H2 A good catalyst of the process must be active in the cleavage of CC bonds, but should inhibit the cleavage of CO bonds. Brazilian researchers have shown in their study that the catalyst Ni / CeO 2 has great potential to be used in the aqueous phase reforming of glycerol, with good activity and high production of hydrogen (Manfro et al, 2011).

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GASIFICATION OF GLYCEROL One of the cases cited in the previous item is the gasification of glycerine. However, in this case, the main aim of gasification is the production not only H2 but also for CO to form the so-called synthesis gas. Several studies have been conducted in this field in Brazil today (Peres, 2010 (thesis), Peres et al, 2010; Lima, 2008) all focused on the study of residual glycerin from biodiesel. The results show that the production of syngasusing glycerol is both technically feasible as economically. That is a gasification process converts organic or fossil based carbonaceous materials into carbon monoxide, hydrogen, carbon dioxide and methane, in this case a mixture of hydrogen and carbon monoxide called syngas. This process occurs by reacting the material at high temperatures (> 700 ° C), without combustion, with a controlled amount of oxygen and / or steam.

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CnHmOk + (nk) NCO H2O + [(n + m/2-k)] H2(Czernik et al., 2002) The syngas is a mixture of H2 and CO, whose stoichiometric ratio varies with process and the raw material used (Gerosa and Mata, 2006). Thesyngas can be produced by any source of carbon, like coal or biomass. It can be used in the Fischer-Tropsch process to produce some fuels like methane, methanol, gasoline or diesel andothers organic compounds in catalytic processes. The synthesis of hydrocarbons from syngas was discovered by Sabatier and Sanderens around 1902, when methane obtained by passing hydrogen (H2) and carbon monoxide (CO), for catalysts based on nickel, iron and cobalt . During World Wars I and II, many techniques for obtaining syngas were developed in Germany, because of scarcity of natural resources. On this occasion, the syngas was obtained from the gasification of coal (Gerosa, 2006). From the 1980s, there was the development of the natural gas industry in Europe, this fact has made the raw material for this process becomes the methane contained in natural gas (Marschner et al, 2002). The syngas is produced on an industrial scale by catalytic reforming of natural gas in the presence of CO2 and H2O, since the 1980s.

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PRODUCTION OF POLYMERS FROM GLYCEROL The basis of this technology in oligomerization is controlled directly from the waste glycerin from biodiesel. In the oligomerization reaction, the glycerol units are attached through the hydroxyl groups to form ether type connector. A simplified representation of this process is shown in Figure 11.

Scheme 10.Schematic representation of theoligomerizationprocess.

The oligomerization process can be quite complex and different types of large molecules and other types of connections can also be formed. The product of oligomerization has interesting properties that can result in various applications, which will be described later. The process can be led to the formation of the polymer polyglycerol that due to form a crosslinked thermoset material with interesting properties for replacement of resins, eg, phenolic. And finally from the polyglycerol-based resin can be produced special carbons with controlled porosity.

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DUST SUPRESSOR Many environmental and economic problems in the miningindustry are related to the significant release of iron ore dust tothe atmosphere during railroad transport and storage. Because ofits small particle size (in average 0.15 mm), this iron ore is carried bythe wind, causing heavy loss during the production process. Theselosses begin in the ore piles stored in the yards of mining. However,the most significant loss occurs during railroad transport of ore. Atthis stage, most ore in the railroad car is carried by the wind, causingan enormous environmental impact and generating a considerableamount of loss for mining companies (Lago et al, 2011) To minimize this problem, mining companies use aqueous solutions of polymers sprayed on the surface of the ore in wagons

Adapted of Medeiros et al, 2011.

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The process consists in the production of long chair polymer based on glycerol by a thermal process. The product is dissolved in water to form a solution that, when sprayed on a surface such as iron, forms a protective layer which prevents the dispersion of solid particles. Studies have shown that formulations made with glycerol-based polymers have been more effective dust suppressionthan commercial products.

HIDROFILIC ADSORBENT Brazil is the third largest producer of vermiculite in the clay world. The main use of vermiculite is now in construction for insulation, and agriculture to absorb, hold and release water, interesting for use in dry locations. A major problem in the marketing of fresh vermiculite is its low value. The clay can be expanded vermiculite forming a low density material that floats in water and shows great potential for the removal of oily waste spill accidents (Figure 14). However, due to its hydrophilic nature, the vermiculite has a low potential for oil absorption. The hidrofobization its surface, by using glycerol as a carbon source, can substantially increase the potential for absorption of oils.

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Adapted from Medeiros et al, 2009.

The process consists in the impregnation followed by polymerization and carbonization of glycerol on the surface of expanded vermiculite. The reaction formed a highly resistant thermofix composed of a crosslinked resin Polyether:

Medeiros et al, 2009.

Studies show that after hidrofobization, the adsorption capacity of vermiculite increases by almost 500% for oily materials such as soybean oil and diesel oil (Lake et al, 2009). Glycerol: Production, Structure and Applications : Production, Structure, and Applications, Nova Science Publishers, Incorporated, 2012. ProQuest

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REFERENCES Alptekin, E., Canakci, M., Fuel, 90, 2630-2638, 2011. Arruda, P. V. de, Rodrigues, R. de C. L. B., Felipe, M. das G. de A., Revista Analytica, 26,5662, 2006. Authayanun, S., Arpornwichanop, A., Paengjuntuek, W., Assabumrungrat, S., Barbosa, A. B., Silva, R. R., Química Nova Na Escola, 2, 3-6, 1995. Beatriz, A.,Araújo , Y. J. K., de Lima, D. P., Química Nova, 34, 306-319, 2011. Byrd, A.J., Pant, K.K., Gupta, R.B., Fuel 87, 2956-2960, 2008. Hashaikeh, R., Butler, I.S.,Kozinski, J.A., Energy Fuels 20, 2743-2747, 2006. Int. J. Hydrogen Energy 35, 6617-6623, 2010. Lima, D. R., Utilização de Glicerina Residual para Produção de Gás de Síntese, Master Dissertation, Chemical Engineering Departament-UNICAMP, 2008. Manfro,R. L., Costa, A. F., Ribeiro, N. F.P., Souza ,M.M.V.M.,Fuel Processing Technology 92, 330–335, 2011. Marshall ,A.T., Haverkamp, R.G., Int. J. Hydrogen Energy 33, 4649-4654, 2008. Medeiros, M. de A. Leite, C. M. M., Lago, R. M., Chemical Engineering Journal (article in press), 2011. Medeiros, M. de A., Araújo, M. H., Sansiviero, M. T. C., Lago, R. M., Applied Clay Science 45, 213-219, 2009. Medeiros, M. de A., Lago, R. M.,Araújo, M. H., Novas rotas para a conversão do glicerol (subproduto do biodiesel) em materiais para aplicação tecnológica,Tese de Doutorado, DQ, ICEx-UFMG,2010. Melo, M. de O.,Silva, L.A., Journal of Photochemistry and Photobiology A: Chemistry 226, 36-41, 2011 Mendow, G., Veizaga, N. S., Querini, C. A., Bioresource Technology, 102, 6385-6391, 2011. Morrison, L.R. Encyclopedia of Chemical Technology. New York: Wiley, pp. 921-932, 1994. Peres, A. P. G., Produção De Gás De Síntese A Partir Da Glicerina, Doctoral Thesis, Chemical Engineering Departament-UNICAMP, 2010. Peres, A. P. G., Silva Lima, N., Wolf, M. R., Chemical Engineering Transactions 20, 333338, 2010. Provost, G. S., Hallenbeck, P.C., Bioresour.Technol. 100, 3513-3517, 2009. Wang, Z. X., Zhuge, J., Fang, H., Prior, B. A., Biotechnology Advances, 19, 201-223, 2001. Wena,G.,Xu Y., Ma, H.,Xu, Z., Tian, Z., Int. J. Hydrogen Energy 33, 6657-6666, 2008. Zheng,Y., Chen, X., Shen, Y., Chemical Reviews, 108, 5253–5277, 2008. Zhou, C. H., Beltramini, J. N., Fan, Y. X., Lu, G. Q.,Chemical. Society Reviews, 37, 527-549, 2008.

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In: Glycerol: Production, Structure and Applications Editors: M. De Santos Silva and P. Costa Ferreira

ISBN: 978-1-62081-120-7 © 2012 Nova Science Publishers, Inc.

Chapter 13

NATURAL ACTIVATION OF COMMERCIAL GLYCEROL Alexander I. Bol’shakov, Svetlana I. Kuzina and Dmitry P. Kiryukhin Institute of Problems of Chemical Physics, Russian Academy of Sciences, Moscow, Russia

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ABSTRACT The effect of activation of commercially pure glycerol (e.g. upon exposure to daylight) may be of interest for applied chemistry since this alcohol is being widely used in medicine, biology, food and cosmetic industry, in manufacturing explosives, etc. An important conclusion is that the production and storage of glycerol must be carried out in conditions that exclude the formation of peroxide compounds. Interaction of hydroperoxides with impurities present in commercially available glycerol may lead to generation of radicals and thus stimulate side reactions accompanied by accumulation of undesirable products. This finding seems to be of especial significance in relation to medicinal, perfume, and food products in view of important role of free-radical reactions in the development of malignant tumors and aging processes.

Keywords: Photoactivated glycerol, acrylamide, polymerization in solution



E-mail: [email protected].

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1. INTRODUCTION Activation of commercial glycerol was discovered via the spontaneous polymerization taking place during dissolution of acrylamide in glycerol. As is known, the reactive centers initiating polymerization are normally generated by decomposition of specially added agents, by activation with actinic light or ionizing radiation, and upon addition of some redox systems. The spontaneous polymerization taking place in some systems is usually attributed to random generation of reactive centers (radicals, ion-radicals, ions) that initiate the growth of polymer chains. So, the presence of impurity hydroperoxide in monomers is known [1, 2] to result in spontaneous formation of polysulfones. Uncontrollable spontaneous polymerization is extremely undesirable in the production, storage, and use of monomers. In order to suppress spontaneous polymerization, special inhibitors and stabilizers are being added to monomers. Spontaneous polymerization is a radical process that is believed [3] to proceed more readily when the monomers are capable of generating stabilized radicals (styrene, methyl metacrylate). Recently, we have noticed (by an increase in viscosity) the specific spontaneous polymerization of acrylamide in glycerol. In this communication, we report on the phenomenological features of this process with special emphasis on the nature of the reactive species initiating polymerization in the binary system under consideration.

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2.1. Photoactivation of Glycerol and Formation of Hydroperoxide Storage of acrylamide (AA) solutions in glycerol (Gl) at room temperature (r.t.) was found accompanied by an increase in their viscosity growing with time. This was associated with the polymerization of AA. The polymerization occurred spontaneously upon dissolution of a finely dispersed suspension of solid AA in the absence of any initiating agents. In some batches of Gl, no AA polymerization was detected. This observation gave grounds to assume that commercially available glycerol might contain some reactive species capable of initiating the polymerization of dissolved AA. Tentatively, these could be the complexes of glycerol with impurities or with other compounds formed during fabrication/storage of glycerol under the action of external influences (illumination, irradiation, heating, etc.). We have found that the activation of glycerol occurs upon exposure to common daylight or to -radiation even to low doses. Thus, the limiting yield of the polymer (~70%) was achieved upon glycerol -irradiation to a dose of ~0.5 kGy. Upon photolysis ( > 360 nm) or exposure to daylight, the limiting yield was achieved in 30 min or several days, respectively (Figure 1). These data clearly show that preliminary photolysis (or radiolysis) of glycerol indeed leads to generation and stabilization of active intermediate species (AIS) capable of initiating the polymerization of added AA.

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Figure 1. The polymer yield as a function of (1) photolysis (λ ≥ 360 nm) duration t and (2) dose of glycerol γ-irradiation.

The excitation spectrum for photoactivation of Gl was determined in the following experiments. Several samples of starting Gl were illuminated at different excitation wavelength ex (cut out with appropriate glass filters) for 30 min and, after dissolution of AA in the above samples, the yields of polymer were detected. The results are presented in Figure

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2 (curve 1). As is seen, commercially pure glycerol is activated in the range of ex = 236–450 nm. Strictly speaking, pure glycerol does not absorb in this spectral range since it contains only the C–C, C–H, C–O, and O–H bonds exhibiting absorption in vacuum UV spectral range.

Figure 2. The polymer yield as a function of excitation wavelength λ for glycerol activation (1) and absorption spectra of starting (2) and photoactivated (3) glycerol (l = 1 cm).

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Therefore, the activation of glycerol is evidently a photosensitized process caused by the presence of micro impurities such as sulfates/chlorides of iron, lead, and silver and esters of fatty acids that are present in commercially glycerol. Therefore, a band peaked at 275 nm (Figure 2, curve 2) can be assigned to the absorption of the impurities. Upon prolonged photolysis, the intensity of this band decreased (Figure 2, curve 3) thus indicating photodecomposition of impurities. The process of Gl activation was studied by inhibition method using stable nitroxyl radical, NR, as an inhibitor. NRs were added to starting Gl at 293 K and their concentration was monitored by ESR method. Photolysis of inhibitor-containing Gl led to disappearance of radicals in 30 min of activation (Figure 3). The addition of AA to thus processed Gl did not result in polymerization of added AA. Hence in the presence of NRs, AIS are not stabilized and the decay of NRs can be regarded as an indication for generating some radical species—presumably such as R•, HO•, and H•— arising upon sensitized photolysis of glycerol molecules. The dissociation of the СН bond in Gl was confirmed by the mass spectra of gaseous products (mass 2.0 was detected). Reactions of H, HO, and R radicals with glycerol molecules were found (in inhibition experiments) to result in formation of AIS at a rate of about 2  1012 сm3 s1 (for   236 nm) and 4.2  1011 сm3 s1 (for   360 nm). As estimated from the initial slope of the plot in Figure 3, the accumulation of Gl radicals R (for   360 nm) proceeded at a rate of around 4.2  1016 сm3 s1. Therefore, the fraction of AIS formed upon Gl photolysis relative to the total amount of formed Gl radicals, can be expected to have a value of about 105. The AIS are unstable at normal conditions and may decompose during dissolution of solid AA in Gl. We attempted to estimate the initial rates for AIS decomposition at different temperatures. Figure 4 presents the polymer yield as a function of termostating time t at different temperatures T (for   360 nm and   236 nm). Since the polymerization rate and polymer yield are proportional to initial AIS concentration [AIS]0, a decrease in the yield with increasing t (for   360 nm) can be related to decomposition of AIS.

Figure 3. Relative concentration of NR ([R]/[R]0) vs. glycerol photoactivation time t (λ ≥ 360 nm, [R]0 = 3.8 × 1018 cm−3).

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Figure 4. The polymer yield (%) vs. termostating time t [at T = 298 (1−5) and 318 K (6)]; Gl was photoactivated with λ ≥ 236 nm (1, 2) and λ ≥ 360 nm (3−6) for 1 (2−6) a and 5 h (1).

Temperature rise and addition of solvent (e.g. Н2О) were found to accelerate the decomposition of AIS. Given that [AIS]0 is about 1.5  1015 сm3, the rate of their autodecomposition can be estimated as 6.2  108 at 273 K, 4.3  109 at 293 K, 1.7  1012 at 318 K, and 7.0  109 сm3 s1 at 293 K in the presence of water. Based on the above data, the activation energy Ea for autodecomposition of AIS was estimated as 24 kcal/mol, which confirms the instability of AIS at r.t. In case of Gl photoactivated at   236 nm, the polymer yield remained unchanged for t  240 h and [AIS] > 1015 сm3. Apparently, a major part of AIS is not involved in the formation of polymer. For this reason, repeated use of recovered Gl resulted in effective polymerization of AA in yields up to 80%. The properties of AIS were studied by spectral and kinetic methods. The ESR spectra (sensitivity threshold 1014 spin/сm3) of activated Gl ([AIS]  1.4  1017 сm3) did not show the presence of any paramagnetic species. The absorption spectra of photoactivated Gl did not show the presence of new bands in the range 200500 nm, which could be associated with formation of complexes. Therefore, the AIS can be identified as diamagnetic molecular compounds which, upon decomposition into radicals, are capable of initiating AA polymerization. A radical nature of AIS was confirmed by experiments with NR as an inhibitor added to activated Gl. Fast decay of NRs and subsequent inhibition of AA polymerization evidenced the decomposition of AIS by a radical mechanism. The above observations allowed us to conclude that, due to low activation energy (Ea  24 kcal/mol), diamagnetic AIS species slowly decompose at r.t. into radicals. The process gets intensified during dissolution of solid AA in activated Gl. Our prophesy is that such a behavior is typical of peroxide compounds. In order to check this assumption, we carried out the photolysis (  236 nm, 20 h) of thoroughly degassed Gl and determined the formation of hydroperoxide compounds by iodometric titration. Their amount (~1.2  1017 сm3) was found to be practically the same as the amount of AIS detected in inhibition experiments (~1.4  1017 сm3). Thus we have concluded that the activation of Gl actually represents the accumulation of hydroperoxides ROOH (AIS) and that their generation is not related to the presence of atmospheric oxygen. The reaction scheme for formation of ROOH can be represented as shown below:

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282

Alexander I. Bol‘shakov, Svetlana I. Kuzina and Dmitry P. Kiryukhin hv М   M*  М* + Gl  М + Gl*  R + Н (НО)

(1)

СН2ОН–СНОН–СН2ОН + Н  СН2ОН–СНО–СН2ОН + Н2

(2)

СН2ОН–СНО–СН2ОН + НО  СН2ОН–СНООН–СН2ОН (RООН)

(3)

RООН  RO + HO

(4)

RO (НО) + AA  R 0AA (initiation)

(5)

R 0AA + АА  RР (chain growth)

(6)

Photoexcitation of impurity molecule М is followed by transfer of excitation energy from М* to a Gl molecule and decomposition of exited glycerol molecules Gl* into radicals R, Н, and НО (reaction 1). Radicals rapidly disappear in the liquid but some minor part of them (~0.001%) have enough time to react with glycerol to give hydroperoxide ROOH (reactions 2, 3). Upon addition of solid monomer, activated Gl (containing ROOH) undergoes decomposition into radicals RO and HO (reaction 4). Addition of these radicals to the double bond of AA leads to formation of the primary acrylamide radical, R 0AA , which

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initiates the growth of polymer chains. Reactions 5 and 6 are responsible for the initiation and propagation of polymer chains. Low activation energy for rupture of the RO–OH bond (~24 kcal/mol) in reaction 4 can be attributed to the influence of hydrogen bonding between ОН groups in a glycerol molecule.

2.2. Polymer Structure and Molecular Weight A 70-% polymer yield required that active species (ROOH) be present in the system in amounts of 1014–1015 cm–3. During polymerization (for 60 min), about 1013 cm–3 of ROOH species are decomposed, which is insufficient for obtaining such an amount of the polymer, even if active species are fully consumed for the formation of R 0AA . For this reason, spontaneous polymerization cannot be associated only with activation-less decomposition of ROOH. Our previous experiments have shown that the rate of polymerization and yield of the polymer increase with increasing amount of undissolved AA. In homogeneous solutions (fully dissolved AA), spontaneous polymerization proceeds in a low yield (~10%). Consequently, the main fraction of R 0AA (1014–1015 cm–3) initiating the propagation of polymer chains is formed via decomposition of ROOH during dissolution of AA in Gl. Intermolecular stresses arising (due to concentration gradients) at the interface between the solid surface of AA particles and solvent can accelerate the decomposition of ROOH into radicals. The R 0AA radicals formed on the surface of undissolved AA particles give rise to development of polymer shells on the surface of monomer particles, which results in a globular structure of resultant polymers.

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Figure 5. Still frames of polymer globules in Gl (293 K): reaction time t = 5 (a), 45 (b), 120 (c), and 200 min (d). Starting globule size was around 100 μm.

Figure 5 shows the still frames of AA particles in Gl taken at 293 K with an optical microscope. The globular structure is seen to be retained until almost complete consumption of solid AA. Upon formation of soluble polymer, the globules become transparent and invisible. The molecular weight of the polymer (Мw  105) was determined viscosimetrically. It has been found that Mw is independent of sample preparation and reaction conditions. This implies that Мw is restricted by the effective reaction of chain transfer. Our data [4] on radiation-induced post-polymerization of AA in glassy glycerol suggest that the length of kinetic chains exceeded the mean length of material chain by a factor of 104, that is, one active center initiates 104 acts of chain transfer. Apparently, the high efficiency of spontaneous polymerization of AA in Gl can be associated with a high chain transfer coefficient. In a viscous medium of Gl solution, the reaction proceeds without chain break, without induction period, and the effective chain transfer process (~104) at low concentrations of [AIS] (