Heterogeneous Catalysis for Today's Challenges: Synthesis, Characterization and Applications
138
Heterogeneous Catalysis for Today's Challenges: Synthesis, Characterization and Applications
138Hardcover
-
SHIP THIS ITEMNot Eligible for Free ShippingPICK UP IN STORECheck Availability at Nearby Stores
Available within 2 business hours
Related collections and offers
Overview
About the Author
Read an Excerpt
Heterogeneous Catalysis for Today's Challenges
Synthesis, Characterization and Applications
By Brian Trewyn
The Royal Society of Chemistry
Copyright © 2015 The Royal Society of ChemistryAll rights reserved.
ISBN: 978-1-84973-627-5
CHAPTER 1
Synthesis of Multi-functionalized Mesoporous Silica Nanoparticles for Cellulosic Biomass Conversion
KEVIN C.-W. WU
Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
Email: kevinwu@ntu.edu.tw
1.1 Introduction
1.1.1 Background of Cellulosic Biomass Conversion
The usage of fossil fuels causes serious problems like the energy crisis and global warming. In order to solve these problems, so far much attention has been paid to the development of renewable energies such as solar or wind energy. Biofuel produced from biomass is one of the potential alternatives. First-generation biofuels [i.e. biodiesel) produced from corn and soybean oil have proved that biomass-to-biofuel conversion is possible; however, the use of edible agriculture as a source will cause other problems such as food deficiency. Therefore, second-generation biofuels generated from non-edible lignocellulosic biomass have attracted more attention recently.
Lignocellulosic (or so-called 'wood-based') biomass consists of three major components: cellulose (41%), hemicellulose (28%), and lignin (27%). Generally, cellulose and hemicellulose can be used to produce bioethanol, and lignin offers a broad spectrum of conversion (thermal cracking, fast pyrolysis, and complete gasification) to achieve valuable chemicals and transportation fuels. So far, a great deal of effort has been put toward the degradation of cellulose with enzymes, mineral acids, bases, and supercritical water. The enzymatic hydrolysis of cellulose is effective, but the system is sensitive to contaminants originating from other biomass components. Furthermore, pre-treatment of cellulose (e.g., ammonia or steam treatments in a high-pressure process or mechanical milling) is typically required to increase the accessible area of cellulose for a reasonable rate of enzymatic hydrolysis. Mineral acids have been extensively investigated to catalyze hydrolysis at a variety of acid concentrations and temperatures. A rather high temperature (180–230 °C) has been used in order to obtain an acceptable rate of cellulose hydrolysis. Furthermore, degradation of the resulting glucose becomes an issue at such high temperatures.
1.1.2 Cellulosic Conversion in Ionic Liquid Systems
Recently, ionic liquids (ILs) have attracted a lot of attention and have been utilized as solvents for the degradation of the lignocellulosic biomass. The importance of ionic liquids in cellulose dissolution has been emphasized in several reviews. ILs are a kind of novel green solvent. They are organic salts with relatively low melting points. In other words, ILs usually appear as crystals under normal conditions; however, they can be melted and dissociated into two ionic parts at relatively high temperatures (usually less than 100 °C). In contrast to other crystalline salts (e.g. NaCl), the attractive characteristic of ILs is that they can transform into a liquid phase.
The utilization of ILs for the dissolution of lignocellulose started in early 2000. Numerous papers have been published on controlling the viscosity and polarity of ionic liquids by varying their ionic structures. The main focus of these papers was the solubility of the synthesized ILs toward different carbohydrates such as glucose, sucrose, amylose, cellulose, and so on. In 2002, Rogers et al. reported that cellulose could be dissolved in ILs at 100 °C. The solubility of cellulose in ionic liquids results from its anions. It can disrupt the hydrogen bonds between polysaccharide chains of cellulose and then dissolve it. This discovery started a new pathway to deal with cellulose at low temperatures and ambient pressure.
In 2007, Zhang and his co-workers discovered that CrCl2 in [EMIM]Cl (1-ethyl-3-methylimidazolium chloride, an imidazolium-type ionic liquid) can efficiently catalyze the glucose-to-HMF conversion. HMF is a promising platform chemical because it can further transform to a widely used biofuel called 2,5-dimethylfuran (DMF) and other useful materials. Since then, many have worked on the production of HMF from cellulose or glucose in ionic liquid systems. Binder and Raines combined HCl, CrCl2 or CrCl3, DMA/LiCl and [EMIM]Cl to convert cellulose to HMF; Zhang and his co-workers used CrCl2/CuCl2 as catalysts in [EMIM]Cl; Han and his co-workers also discovered SnCl4 in [EMIM]BF4 can convert glucose to HMF with a high yield; Riisager and his co-workers discussed HMF produced from lanthanide-containing ionic liquid systems; Bell and Chidambaram discovered that 12-molybdophosphoric acid in [EMIMjCl/acetonitrile or [BMIM]Cl/acetonitrile can selectively convert glucose to HMF. Although there has been much research focused on the addition of various kinds of catalysts in ionic liquid systems, very few papers discussed the effects of reaction conditions (such as dissolution temperatures and times of ILs, reaction temperatures and times, and the amounts of water) on the conversion efficiency in ionic liquids without additional catalysts. In fact, in the above-mentioned papers, HMF could still be produced when using ILs only (no other additives), although the yields were very low. This indicates that ILs in these systems serve not only as solvents but also as catalysts. We suggest that the low HMF yield was because the reaction conditions for HMF production in these cases were not optimized. For example, Zhao et al. has shown that the yield of HMF converted from fructose was greatly affected by the reaction temperature in an [EMIM]Cl only system. Very recently, Binder and Raines discussed the sequence and timing of the addition of water into the cellulosic conversion and showed that an optimal sequence and timing strongly affected the conversion efficiency.
1.1.3 Enzyme-assisted Cellulose Conversion
In recent decades, cellulase was broadly studied for the hydrolysis of cellulose. Cellulase is a mixture of enzymes containing three main components: (1) endo-1,4-beta -D-glucanase (EG) which randomly cleaves the cellulose chain to lower the crystallinity; (2) cellobiohydrolase (CBH) which degrades cellulose by releasing cellobiose units; (3) beta-glucosidase which hydrolyzes cellobiose and other oligomers to get glucose units. To date, the reaction conditions and the hydrolytic processes of hydrolyzing cellulose by using free cellulase have been optimized with a glucose yield as high as 70%. However, one critical problem when using cellulase as a catalyst is the easy deactivation of cellulase by environmental factors [e.g., temperature), which greatly hinder its practical use in industry. In order to overcome such difficulties, the immobilization of cellulase onto solid materials is a feasible way to enhance its stability.
Many research papers have demonstrated that immobilizing cellulase onto organic and inorganic materials could improve the stability and reusability of cellulase without reducing its catalytic ability. Among the host materials, mesoporous silica materials have gained much attention because of their large specific surface area, high mechanical strength, and tunable surface functionality. Recently, Sakaguchi et al. studied the encapsulation of cellulase by using mesoporous silica SBA-15 with various pore sizes as hosts. They found that the enzymatic activity of cellulase strongly depended on the pore size of SBA-15. The best performance of cellulase could be obtained when using SBA-15 with pore diameter around 8.9 nm. However, the structure of SBA-15 is 2D hexagonal with length of several µm, which would inhibit the adsorption of cellulase into the inner surface of the SBA-15 and result in a low adsorption amount. Lu et al. studied the effect of surface functionalities of a mesoporous silica FDU-12 (pore size is around 25.4 nm) on the immobilization of cellulase. They functionalized FDU-12 with phenyl, thiol, amino and vinyl groups. The results showed that the electrostatic and hydrophobic interactions between cellulase and functionalized FDU-12 play significant roles on the activity and stability of immobilized cellulase. Amine-functionalized FDU-12 adsorbed the largest amount of cellulase but exhibited the lowest activity. They explained this was due to the interaction between amine groups of FDU-12 and the carboxyl groups of catalytic site of cellulase which thereby inhibited the activity of cellulase. In contrast, vinyl-functionalized FDU-12 not only maintained the activity of cellulase up to 80% but also temporal enzyme stability owing to the existence of hydrophobic groups. Despite these pioneering studies, none of them has studied different immobilization methods [i.e., physical adsorption and chemical binding) on the efficiency of cellulase, and cellulosic hydrolysis by immobilized cellulase has never been reported yet.
1.1.4 Production of 5-Hydroxymethylfurfural from Cellulosic Conversion
5-Hydroxymethylfurfural (HMF), converted from lignocellulosic biomass, is considered one of the "top value-added chemicals"; this results from its utilization as a building-block platform between biomass and promising chemical intermediates, such as 2,5-furandicarboxylic acid (FDCA), 2,5-dimethylfuran (DMF), 5-ethoxymethylfurfural (EMF), and ethyl levulinate (EL), which have been studied extensively in recent years and demonstrate the significance of HMF.
HMF has been successfully generated from fructose, glucose, and cellulose using various kinds of reaction systems with homogeneous or heterogeneous catalysts. The mechanism of cellulose-to-HMF conversion is still unclear, but the conversion can be divided into several reactions. First, cellulose is usually pre-treated by alkaline, acid, or certain ionic solutions to destroy its rigid framework. The pre-treated cellulose then goes through the depolymerization process in an acidic system in order to break the 1,4-βglycosidic bonds of cellulose and produce glucose. Subsequently, glucose converts to fructose via isomerization, which is a so-called Lobry de Bruyn-Alberda van Ekenstein transformation. Finally, the dehydration of fructose generates HMF. The mechanism of fructose-to-HMF conversion has been discussed in numerous studies. The micro-kinetic model for this three-stage water-removed process has been constructed to determine an apparent activation energy. In addition, according to computational results, both the estimated equilibrium constant and activation energy can be greatly influenced by reaction conditions, including temperature, solvent, and catalysts.
Different solvents have been used in the fructose-to-HMF conversion because of the contrast between water-soluble reactants (e.g., fructose and glucose) and organic-solvent-soluble products (e.g., HMF). The careful selection of solvents can promote the preferential reaction and enhance product yield. The Dumesic group studied the effects of solvents on the dehydration of fructose in biphasic systems, and demonstrated the catalytic ability of dimethylsulfoxide (DMSO), which is able to suppress the undesired side reactions effectively. Recently, ILs have been widely used as both catalysts and solvents for producing HMF from lignocelluloses because of their comparatively higher catalytic activity and adjustable composition. However, despite the excellent activity and recyclability of ILs, their potential is restricted to laboratory-scale experiments due to high costs. Therefore, a low-price solvent with the desired properties (e.g., high boiling point and low viscosity) such as DMSO can have more potential in industrial applications.
In recent years, several groups have reported the production of HMF from fructose in DMSO-based reaction systems via homogeneous and heterogeneous catalysts, including acids, salts, and metal ions. The Dumesic group has investigated the catalytic capabilities of various homogeneous mineral acids. Recently, Wang et al. used carbon-based p-toluenesulfonic acid (TsOH) at 130 °C for 1.5 h resulting in a 91.2% yield of HMF. Although these pioneering studies showed high yields of HMF, harsher reaction conditions are always needed in such homogeneous catalytic systems. From economic and sustainable viewpoints, scientists have turned to heterogeneous solid catalysts and mild reaction conditions. For example, the Sidhpuria group immobilized ILs onto silica particles as an efficient heterogeneous catalyst for fructose-to-HMF conversion with a yield of 63% in a DMSO system.
1.1.5 Mesoporous Catalysts from Cellulosic Conversion
Mesoporous silica nanoparticles (MSNs) have attracted a great deal of attention in the field of catalysis because of their high surface area and controllable pore size. In addition, abundant SiOH groups on the surface of MSNs provide the possibility of further functionalization with other organic groups. For example, the Lin group has used a co-condensation method to functionalize MSNs with a general acid (i.e., a ureidopropyl (UDP) group) and a base (i.e., a 3-[2-(2-aminoethylamino)ethylamino]-propyl (AEP) group) as a cooperative acid-base catalyst for aldol, Henry, and cyanosilylation reactions. We also have used a grafting method to functionalize MSNs with several metal-histidine complexes for H2O2-assisted tooth bleaching. However, the conventional MSNs synthesized from the cationic surfactant cetyl-trimethylammonium bromide (CTAB) exhibit a pore size of around 2 nm. For several catalytic reactions involving large molecules (e.g., proteins or cellulose), this pore size is too small to allow the reactants to diffuse into the mesopores, thus losing the advantage of high surface area inside the MSNs. Therefore, the synthesis of MSNs with pore sizes large than 10 nm is highly desirable.
The MSNs with large pore sizes can be synthesized through two approaches: (1) using high-molecular-weight surfactants as templates; and (2) adding hydrophobic additives as swelling agents. For example, the Zhao group succeeded in synthesising mesoporous silica with an ultra-large pore size of approximately 37.0 nm by using a high-molecular-weight surfactant (polyethylene oxide)-b-poly(methyl methacrylate); PEO-b-PMMA). The same group has also reported the addition of 1,3, 5-trimethylbenzene (TMB) as a swelling agent to synthesize mesoporous silica with a large pore of 25.4 nm (as denoted as FDU-12). The Lu group has functionalized FDU-12 materials with phenyl, thiol, amino, and vinyl groups and studied the effect of these functional groups on the immobilization efficacy of an enzyme (i.e., cellulase). Despite these pioneering studies, researchers have not yet utilized large-pored MSNs with various functional groups for cellulosic conversion in ionic liquid systems. In general, cellulosic conversion involves three main reactions: (1) cellobiose-to-glucose depolymerization, (2) glucose-to-fructose isomerization, (3) fructose-to-HMF dehydration. These reactions need acid, base, and acid catalysts, respectively, as illustrated in Scheme 1.1. Consequently, to synthesize large-pored MSNs with both acid and base functionalities as a new cooperative solid catalyst would be helpful for one-pot cellulose-to-HMF conversion.
1.2 Cellulase-immobilized Mesoporous Silica Nanocatalysts for Efficient Cellulose-to-glucose Conversion
1.2.1 Optimization of Reaction Conditions
Optimal reaction conditions with respect to temperature, the amount of catalyst and the reaction time are crucial to maximising the final yield of cellulosic hydrolysis when using cellulase as a catalyst. Therefore, we first optimized the reaction temperatures, the amount of free cellulase and the reaction time. As shown in Figure 1.1a, 15 mg of cellulose was hydrolyzed using free cellulase (50 Unit, 1 Unit indicates the amount of enzyme that can catalyze 1 × 10-6 mole substrate in one minute) as the catalyst at different temperatures for 24 h; the maximum yield of glucose was obtained at 50 °C. By repeating the experiment, we found that 50 °C is also the most stable operating condition for cellulose. Thus, we chose 50 °C as the suitable temperature for cellulase-assisted cellulose hydrolysis. From the economic point of view, an optimal amount of cellulase means the minimum amount of cellulase while keeping the maximum yield of glucose. Various amounts of cellulase, ranging from 1 mg to 23 mg, were used for the hydrolysis of cellulose (15 mg) at 50 °C for 24 h. Based on the results in Figure 1.1b, we found that the optimal amount of celluase was 25 Unit (i.e., 4.5 mg). More cellulase than 25 Unit did not increase the yield of glucose at the current operation coniditions. After obtaining the optimal reaction temperature and the amount of cellulase, the optimal reaction time was also examined. 15 mg of cellulose was hydrolyzed using free cellulase (25 Unit) at 50 °C for various reaction times (i.e., ranging from 3 to 48 hours). According to the results shown in Figure 1.1c, in order to reach 90% glucose yield, cellulose has to be hydrolyzed for at least 24 h although 80% glucose yield could be obtained in 12 h. For consistency, here we chose 24 hours as the optimal reaction time.
(Continues...)
Excerpted from Heterogeneous Catalysis for Today's Challenges by Brian Trewyn. Copyright © 2015 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.