Nanoporous Catalysts for Biomass Conversion

List of Contributors

Yong Cao Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, China

Sheng Dai Department of Chemistry, The University of Tennessee, USA

Chi-Linh Do-Thanh Department of Chemistry, The University of Tennessee, USA

Atsushi Fukuoka Institute for Catalysis, Hokkaido University, Japan

Emiel J.M. Hensen Laboratory of Inorganic Materials Chemistry, Schuit Institute of Catalysis, Eindhoven University of Technology, The Netherlands

Xiaoming Huang Laboratory of Inorganic Materials Chemistry, Schuit Institute of Catalysis, Eindhoven University of Technology, The Netherlands

Hirokazu Kobayashi Institute for Catalysis, Hokkaido University, Japan

Jiechen Kong Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, China

Tamás I. Korányi Laboratory of Inorganic Materials Chemistry, Schuit Institute of Catalysis, Eindhoven University of Technology, The Netherlands

Changzhi Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China

Guangyi Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China

Ning Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China

Shu-Shuang Li Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, China

Xin-Hao Li School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, China

Yao Lin Department of Chemistry and Institute of Materials Science, University of Connecticut, USA

Fei Liu State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China

Fujian Liu Department of Chemistry, Shaoxing University, China

Yong-Mei Liu Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, China

Zhicheng Luo Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, China

Xiangju Meng Key Laboratory of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, China

Jifeng Pang State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China

Abhijit Shrotri Institute for Catalysis, Hokkaido University, Japan

Hui Su School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, China

Lei Tao Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, China

Noritatsu Tsubaki Department of Applied Chemistry, School of Engineering, University of Toyama, Japan

Aiqin Wang State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China

Hong-Hui Wang School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, China

Liang Wang Key Laboratory of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, China

Yanqin Wang Shanghai Key Laboratory of Functional Materials Chemistry, Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, China

Liubi Wu Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, China

Qineng Xia Shanghai Key Laboratory of Functional Materials Chemistry, Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, China

Feng-Shou Xiao Key Laboratory of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, China

Chuang Xing School of Biological and Chemical Engineering, Zhejiang University of Science and Technology, China

Jinming Xu State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China

Shaodan Xu Key Laboratory of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, China

Guohui Yang Department of Applied Chemistry, School of Engineering, University of Toyama, Japan

Ruiqin Yang School of Biological and Chemical Engineering, Zhejiang University of Science and Technology, China

Tao Zhang State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China

Chen Zhao Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, China

Tian-Jian Zhao School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, China

Xiaochen Zhao State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China

Mingyuan Zheng State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China

Xiang Zhu Department of Chemistry, The University of Tennessee, USA

Series Preface

Renewable resources, their use and modification are involved in a multitude of important processes with a major influence on our everyday lives. Applications can be found in the energy sector, paints and coatings, and the chemical, pharmaceutical, and textile industry, to name but a few.

The area interconnects several scientific disciplines (agriculture, biochemistry, chemistry, technology, environmental sciences, forestry …), which makes it very difficult to have an expert view on the complicated interaction. Therefore, the idea to create a series of scientific books that will focus on specific topics concerning renewable resources, has been very opportune and can help to clarify some of the underlying connections in this area.

In a very fast changing world, trends are not only characteristic for fashion and political standpoints; science is also not free from hypes and buzzwords. The use of renewable resources is again more important nowadays; however, it is not part of a hype or a fashion. As the lively discussions among scientists continue about how many years we will still be able to use fossil fuels – opinions ranging from 50 to 500 years – they do agree that the reserve is limited and that it is essential not only to search for new energy carriers but also for new material sources.

In this respect, renewable resources are a crucial area in the search for alternatives for fossil-based raw materials and energy. In the field of energy supply, biomass and renewables-based resources will be part of the solution alongside other alternatives such as solar energy, wind energy, hydraulic power, hydrogen technology, and nuclear energy. In the field of material sciences, the impact of renewable resources will probably be even bigger. Integral utilization of crops and the use of waste streams in certain industries will grow in importance, leading to a more sustainable way of producing materials. Although our society was much more (almost exclusively) based on renewable resources centuries ago, this disappeared in the Western world in the nineteenth century. Now it is time to focus again on this field of research. However, it should not mean a retour à la nature, but it should be a multidisciplinary effort on a highly technological level to perform research towards new opportunities, to develop new crops and products from renewable resources. This will be essential to guarantee a level of comfort for a growing number of people living on our planet. It is the challenge for the coming generations of scientists to develop more sustainable ways to create prosperity and to fight poverty and hunger in the world. A global approach is certainly favoured.

This challenge can only be dealt with if scientists are attracted to this area and are recognized for their efforts in this interdisciplinary field. It is, therefore, also essential that consumers recognize the fate of renewable resources in a number of products.

Furthermore, scientists do need to communicate and discuss the relevance of their work. The use and modification of renewable resources may not follow the path of the genetic engineering concept in view of consumer acceptance in Europe. Related to this aspect, the series will certainly help to increase the visibility of the importance of renewable resources. Being convinced of the value of the renewables approach for the industrial world, as well as for developing countries, I was myself delighted to collaborate on this series of books focusing on different aspects of renewable resources. I hope that readers become aware of the complexity, the interaction and interconnections, and the challenges of this field and that they will help to communicate on the importance of renewable resources.

I certainly want to thank the people of Wiley's Chichester office, especially David Hughes, Jenny Cossham and Lyn Roberts, in seeing the need for such a series of books on renewable resources, for initiating and supporting it, and for helping to carry the project to the end.

Last, but not least, I want to thank my family, especially my wife Hilde and children Paulien and Pieter-Jan, for their patience and for giving me the time to work on the series when other activities seemed to be more inviting.

Christian V. Stevens,

Faculty of Bioscience Engineering

Ghent University, Belgium

Series Editor ‘Renewable Resources’

June 2005

Acknowledgements

We would like to thank the National Natural Science Foundation of China for the constant encouragement and financial support (NO. 91634201, 21333009, 21403192, 91645105, and U1462202) to our investigation in Nanoporous Catalyst Synthesis and Biomass Conversion.

We are also grateful to Shagun Chaudhary and Emma Strickland, from Wiley, whose great patience was much appreciated in ‘polishing’ the text of the book.

Chapter 1

Nanoporous Organic Frameworks for Biomass Conversion

Xiang Zhu, Chi-Linh Do-Thanh and Sheng Dai

Department of Chemistry, The University of Tennessee, USA

1.1 Introduction

Porosity, a profound concept that helps to understand Nature and create novel fascinating architectures, is inherent to natural processes, as seen in hollow bamboo, hexagonal honeycomb, and the alveoli in the lungs (Figure 1.1) [1, 2]. These advanced natural porous frameworks and their promising applications have widely inspired scientists with the idea of mimicking them in artificial structures down to the micro- and nanoscale range [1, 2]. The rational design and synthesis of advanced nanoporous materials, which play a crucial role in established processes such as catalysis and gas storage and separations and catalysis [3–12], have long been an important science subject and attracted tremendous attention. During the past two decades, the linking of molecular scaffolds by covalent bonds to create crystalline extended structures has afforded a broad family of novel nanoporous crystalline structures [13] such as like metal–organic frameworks (MOFs) [14] and covalent organic frameworks (COFs) [15]. The key advance in this regard has been the versatility of covalent chemistry and organic synthesis techniques, which give rise to a wide variety of target applications for these extended organic frameworks, for example, the use of MOFs and COFs in the context of biomass conversion. In addition to crystalline frameworks, nanoporous organic resins have long been extensively studied as heterogeneous catalysts for the conversion of biomass because of their commercial synthesis [16].

Figure 1.1 Illustration of porosity existing in Nature and synthesized frameworks with a decreasing pore size. (a) Bamboo; (b) honeycomb; (c) scanning electron microscopy (SEM) image of alveolar tissue in mouse lung; (d) SEM image of an ordered macroporous polymer; (e) SEM image of an ordered mesoporous polymer from self-assembly of block copolymers; (f) structural representation of the COF structure.

Upgrading biomass into fuel and fine chemicals has been considered a promising renewable and sustainable solution to replacing petroleum feedstocks, owing to the rich family of biomass raw materials, which mainly includes cellulose, hemicellulose, and lignin [17, 18]. For example, the carbohydrates, present in the cellulosic and hemicellulosic parts of biomass, can be converted into renewable platform chemicals such as 5-hydroxymethylfurfural (HMF), via acid-catalyzed dehydration for the production of a wide variety of fuels and chemical intermediates [19]. Despite great progress, including unprecedented yields and selectivities, having been made in biomass conversion using conventional homogeneous catalysts, the cycling abilities have long been the main drawbacks that inhibit their large-scale applications. As a result, heterogeneous nanoporous solid catalysts hold great promise in these diverse reactions [16]. High porosities of nanoporous catalysts may help to access reactants, mass transfer, and functionalization of task-specific active sites, such that the product selectivities can be easily controlled. To this end, nanoporous materials with high surface areas, tunable pore sizes and controllable surface functionalities have been extensively prepared and studied. Significantly, nanoporous crystalline organic frameworks, with well-defined spatial arrangements where their properties are influenced by the intricacies of the pores and ordered patterns onto which functional groups can be covalently attached to produce chemical complexity, exhibit distinct advantages over other porous catalysts. For instance, post-synthetic modification (PSM) techniques [20] provide a means of designing the intrinsic pore environment without losing their long-range order to improve the biomass conversion performance. The inherent ‘organic effect’ enables the architectures to function with task-specific moieties such as the acidic sulfonic acid (–SO3H) group. The desired microenvironment can also be generated by rationally modifying the organic building units or metal nodes. In addition, the attractive large porosity allows the frameworks to become robust solid supports to immobilize active units such as polyoxometalates and polymers [21]. In essence, nanoporous crystalline organic frameworks including MOFs and COFs have demonstrated strong potential as heterogeneous catalysts for biomass conversion [21]. The ability to reticulate task-specific functions into frameworks not only allows catalysis to be performed in a high-yield manner but also provides a means of facile control of product selectivity.

HMF, as a major scaffold for the preparation of furanic polyamides, polyesters, and polyurethane analogs, exhibits great promise in fuel and solvent applications [19, 22]. The efficient synthesis of HMF from biomass raw materials has recently attracted major research efforts [23–29]. Via a two-step acyclic mechanism, HMF can be prepared from the dehydration of C-6 sugars such as glucose and fructose. First, glucose undergoes an isomerization to form fructose in the presence of either base catalysts or Lewis acid catalysts by means of an intramolecular hydride shift [30]. Subsequently, Brønsted acid-catalyzed dehydration of the resultant fructose affords the successful formation of HMF with the loss of three molecules of H2O (Scheme 1.2) [31]. The development of novel nanoporous acidic catalysts for the catalytic dehydration of sugars to HMF is of great interest, and is highly desirable. Hence, design strategies for the construction of nanoporous crystalline organic frameworks that are capable of the efficient transformation of sugars to HMF are discussed in this chapter, and some nanoporous organic resins for the conversion of raw biomass materials are highlighted. By examining the common principles that govern catalysis for dehydration reactions, a systematic framework can be described that clarifies trends in developing nanoporous organic frameworks as new heterogeneous catalysts while highlighting any key gaps that need to be addressed.

Scheme 1.1 Possible valuable chemicals based on carbohydrate feedstock.

1.2 Nanoporous Crystalline Organic Frameworks

1.2.1 Metal–Organic Frameworks

The Brønsted acidity of nanoporous catalysts is very essential for the dehydration of carbohydrates towards the formation of HMF [32–34]. One significant advantage of metal–organic frameworks (MOFs) is their highly designable framework, which gives rise to a versatility of surface features within porous backbones. Whereas, a wide variety of functional groups has been incorporated into MOF frameworks, exploring the Brønsted acidity of MOFs [14], the introduction of sulfonic acid groups in the framework remains a challenge and less explored, mainly because of the weakened framework stability. In this regard, several different synthetic techniques have been developed and adopted to introduce sulfonic acid (–SO3H) groups for MOF-catalyzed dehydration processes: (i) de-novo synthesis using organic linkers with –SO3H moiety; (ii) pore wall engineering by the covalently postsynthetic modification [20] (PSM) route; and (iii) modification of the pore microenvironment through the introduction of additional active sites. These novel MOF materials featuring strong Brønsted acidity show great promise as solid nanoporous acid catalysts in biomass conversion.

1.2.1.1 De-Novo Synthesis

Inspired by the framework MIL-101 [35], which possesses strong stability in aqueous acidic solutions and is fabricated from a chromium oxide cluster and terephthalate ligands in hydrofluoric acid media, Kitagawa et al. for the first time reported the rational design and synthesis of a MIL-like MOF material for cellulose hydrolysis [36]. By, adopting the MIL-101 framework as a platform, these authors created a novel nanoporous acid catalyst with highly acidic –SO3H functions along the pore walls by the innovative use of 2-sulfoterephthalate instead of the unsubstituted terephthalate in MIL-101 (Figure 1.2). The resultant Cr-based MOF MIL-101-SO3H was shown to exhibit a clean catalytic activity for the cellulose hydrolysis reaction, thus opening a new window on the preparation of novel nanoporous catalysts for biomass conversion. On account of the unsatisfactory yields of mono- and disaccharides from cellulose hydrolysis being caused by the poor solubility of crystalline cellulose in water, the same group further studied isomerization reactions from glucose to fructose in aqueous media, where MIL-101-SO3H not only shows a high conversion of glucose but also selectively produces fructose [37]. A catalytic one-pot conversion of amylose to fructose was also achieved because of the high stability of the framework in an acidic solution, which suggests promising applications of compound in the biomass field.

Figure 1.2 Schematic representation of the structure of MIL-101-SO3H.

On account of the HMF formation mechanism, the Lewis acid featuring metal center – for example, chromium (II) – allows for a high-yield isomerization because of the coordinate effect between the Lewis acidic metal sites and glucose [24]. In addition to strong Brønsted acidity caused by the –SO3H moieties, MIL-101-SO3H also bears Cr(III) sites within the structure, which is similar to CrCl2 and may act as active sites for the isomerization of glucose to fructose, whereas the fructose dehydration can be initiated with the aid of –SO3H groups. Bao et al. carried out an integrated process using nanoporous MIL-101-SO3H as the catalyst and biomass-derived solvent (γ-valerolactone; GVL) for the conversion of glucose into HMF (Figure 1.3) [38]. The batch heterogeneous reaction was shown to give a HMF yield of 44.9% and a selectivity of 45.8%. The glucose isomerization in GVL with 10 wt% water was found to follow second-order kinetics, with an apparent activation energy of 100.9 kJ mol−1 according to the reaction kinetics study. Clearly, the bifunctional MIL-101-SO3H framework can serve as a potential platform for the dehydration reaction of biomass-derived carbohydrate to generate platform chemicals.

Figure 1.3 Bifunctional catalyst MIL-101(Cr)-SO3H used for glucose conversion to HMF.

Reproduced with permission from Ref. [38]. Copyright 2016, American Institute of Chemical Engineers.

Solvents play another crucial role in the green biomass conversion processes, and the development of water-based heterogeneous systems for dehydration is important for the industrial reaction of fructose conversion to HMF [19]. Janiak et al. adopted MIL-101Cr (MIL-SO3H) as the heterogeneous catalyst and achieved a 29% conversion of glucose to HMF in a THF : H2O (39 : 1, v : v) mixture [39]. Recently, Du et al. reported a 99.9% glucose conversion and an excellent HMF yield of 80.7% in water using a new bifunctional PCP(Cr)-SO3HCr(III) material [40]. These authors showed that the sulfonic acid group in the framework was the essential function center, and more Lewis acid sites resulted in a better catalyst activity.

Despite the aforementioned nanoporous Cr-based MOF materials displaying promising applications in dehydration processes, the stringent synthetic conditions and toxic inorganic reagents of Cr MOFs greatly limit their real use. Recently, the research group of Zhao reported a modulated hydrothermal (MHT) approach that can be used to synthesize a series of highly stable Brønsted acidic NUS-6(Zr) and NUS-6(Hf) MOFs in a green and scalable way, although the linker 2-sulfotherephthalate was previously reported to make unstable UiO-type frameworks. The hafnium (Hf)-based material NUS-6(Hf) exhibited a superior performance for the dehydration of fructose to HMF (Figure 1.4) [41], outperforming all other presently known MOFs or heterogeneous catalysts with a yield of 98% under the same reaction conditions. Nevertheless, although such a high transformation yield can be achieved in organic dimethyl sulfoxide (DMSO), the attempts at dehydration with the same MOF material in aqueous media resulted in only negligible amounts of HMF (ca. 5%). Therefore, it is very valuable and significant to rationally design and develop stable Brønsted acidic MOF materials for high-performance dehydrations to prepare HMF in green aqueous media, even though this is an enormous challenge.

Figure 1.4 NUS-6(Hf) used as a heterogeneous catalyst for fructose conversion to HMF.

Reproduced by permission of Ref. [41]. Copyright 2012 American Chemical Society.

1.2.1.2 Postsynthetic Modification

In addition to direct synthesis using building linkers to modify MOF materials in situ, the PSM route is becoming an important technique to introduce various functions inside the MOFs for diverse applications, such as heterogeneous catalysis and gas storage and separation [20]. Therefore, PSM provides another means of introducing Brønsted acid groups to MOF backbones for the dehydration of biomasses to HMF. The presence of organic scaffolds in MOFs allows for a convenient employment of a variety of organic transformations. Furthermore, the acid strength on the surface can be precisely controlled by the PSM through varying the grafting rate of the reaction. As shown in Figure 1.5, Chen et al. reported the synthesis of a family of MOF frameworks functionalized with the –SO3H group through the PSM of the organic linkers, using chlorosulfonic acid [42]. The resultant framework MIL-101(Cr) [MIL-101(Cr)-SO3H] exhibited a full fructose conversion with a HMF yield of 90% in DMSO. The –SO3H groups was found to have a significant effect on fructose-to-HMF transformation [42]. Both the conversions of fructose and selectivities towards HMF were increased with the sulfonic acid-site density of the MOF material. Kinetics studies further suggested that the dehydration of fructose to HMF using MIL-101(Cr)-SO3H followed pseudo-first-order kinetics with an activation energy of 55 kJ mol−1.

Figure 1.5 Synthetic routes to MOF-SO3H and the conversion of fructose into HMF.

1.2.1.3 Pore Microenvironment Modification

Pore microenvironment engineering inside MOF frameworks via either the physical impregnation of acidic compounds or in-situ polymerization, allows for the hybrid material to obtain promising catalytic activities towards the formation of HMF. MOF materials featuring large porosities not only give rise to easy access of voluminous reactant molecules diffusing into the pores, but also provide an appropriate channel for the product molecules to move out, and thereby improve the catalytic performance. Li and coworkers first introduced this concept to construct MOF materials for the dehydration of carbohydrates to HMF by introducing phosphotungstic acid, H3PW12O40 (PTA), to the network of MIL-101 [43]. PTA and other transition metal-substituted polyoxometalates have been widely encapsulated into MIL-101 for other catalysis processes, such as the oxidation of alkanes and alkenes and esterification of n-butanol with acetic acid in the liquid phase. Using ionic liquids as reaction media, PTA(3.0)/MIL-101 showed the best catalytic performance in fructose dehydration with a HMF yield of 79% [43]. The immobilization of PTA significantly changed the pore environment, which promoted the initiation of the proton-catalyzed dehydration. The exchange of protons between the PTA and the organic cations of the ionic liquid led to the presence of partial protons in the ionic liquid medium [43].

Inspired by this innovative finding, Chen et al. further reported the synthesis of PTA/MOF hybrid ruthenium catalysts, Ru-PTA/MIL-100(Cr), to selectively convert cellulose and cellobiose into sorbitol under aqueous hydrogenation conditions (Figure 1.6) [44]. It was shown that a 63.2% yield in hexitols, with a selectivity for sorbitol of 57.9% at complete conversion of cellulose, and 97.1% yield in hexitols with a selectivity for sorbitol of 95.1% at complete conversion of cellobiose, could be achieved with the creative use of a Ru-PTA/MIL-100(Cr) catalyst with loadings of 3.2 wt% for Ru and 16.7 wt% for PTA. This opened new avenues for the rational design of acid/metal bifunctional MOF catalysts for biomass conversion.

Figure 1.6 Ru-PTA/MIL-100(Cr) used as a heterogeneous catalyst for biomass conversion.

Reproduced with permission from Ref. [44]. Copyright 2013, Wiley.

In addition to the physical incorporation of metals and the phosphotungstic acid, Hatton and coworkers described a straightforward strategy for the construction of novel hybrid materials consisting of MOFs and polymer networks [45]. The porous MIL-101(Cr) framework was impregnated with organic monomers (maleimide) and crosslinkers for an in-situ free-radical (co)polymerization (P), and thereby the functionalization (F) of the pore environment with poly(N-bromomaleimide) (Figure 1.7). Although the porosity of the composite material was decreased because of the presence of polymers, the pore of the MOF material was still large enough to enable ready access of the reactants to the active sites inside the framework. As a result, the resultant MOF-brominated polymer hybrids exhibited potential applications for the industrially important reaction of fructose conversion to HMF, suggesting an innovative and effective method for controlling the pore environment for target applications and thus promoting the development of MOF materials for biomass conversion.

Figure 1.7 Scheme of fabrication of functional hybrids of MOFs and polymer networks.

Reprinted with permission from Ref. [45]. Copyright 2014, American Chemical Society.

Even though MOF compounds show significant advantages for biomass conversions over other heterogeneous catalysts such as zeolites and metal oxides, only a handful of studies have been reported dealing with the conversion of biomass using MOF catalysts. Herbst and Janiak recently highlighted the promising potential in the field of MOF-catalyzed biomass conversion [21]. These authors discussed the synthetic conditions of MOF materials, mechanisms of biomass reactions, product yields, and the corresponding selectivity in detail. Meanwhile, in-depth reviews by Jiang and Yaghi which focused on Brønsted acidic MOFs were recently published [14, 46]. Clearly, there exist great opportunities and potential to address investigations into MOF catalysis in the context of biomass-based fine chemical production. Large porosity, strong acidity, and stability should be considered simultaneously when designing a novel MOF catalyst.

1.2.2 Covalent Organic Frameworks

Covalent organic frameworks (COFs) possess molecular ordering and inherent porosity in addition to robust stability under a wide range of conditions, and have recently been identified as versatile new platforms for a wide range of applications such as gas storage or heterogeneous catalysis. Their promising performance is due in part to the extent to which they can be rationally tuned during the preparation of their organic constituents, as comparatively gentle synthesis conditions are required for the final COF material [47–52]. Although the scope for structural diversity and high-yielding synthesis suggests potential advantages [53–61], few COFs have been investigated for biomass conversion [62].

Recently, Zhao and coworkers reported a de-novo synthesis of a sulfonated two-dimensional crystalline COF termed TFP-DABA from the Schiff base condensation reaction between 1,3,5-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acid (Scheme 1.2) [63]. For the first time, remarkable fructose conversion yields (97% for HMF and 65% for 2,5-diformylfuran), good selectivity and recyclability were obtained using COF TFP-DABA as the heterogeneous catalyst. This innovative study paves a solid way towards the de-novo synthesis of novel catalytically active COFs and their applications in biobased chemical conversion.

Scheme 1.2 Scheme of fabrication of COF-SO3H for the conversion of fructose into HMF.

1.3 Nanoporous Organic Sulfonated Resins

Nanoporous organic sulfonated resins, such as Amberlyst® and Nafion®, have attracted tremendous attention and show significant promise for the conversion of biomass because of their facile synthesis and high catalytic activities [16]. The intrinsic inherent porosity allows for an easy installation of the Brønsted sulfonated group (–SO3H) on the pore surface, thereby affording an enhanced interaction between the catalyst framework and the reactants. A high-performance transformation of sugars to HMF is therefore achieved.

1.3.1 Amberlyst Resins

A facile polymerization of styrene and further sulfonation results in the formation of one famous nanoporous resin catalyst, Amberlyst® 15 (Figure 1.8), that enables a wide variety of acid-catalyzed liquid-phase biomass conversions, for example the dehydration of fructose to HMF. Shimizu et al. reported a 100% yield of HMF in a water-separation reactor, using Amberlyst® 15 powder in a size of 0.15–0.053 mm [64]. The removal of adsorbed water by the small-sized resin particles resulted in an excellent yield, which was supported by the near-infrared spectroscopic characterization. It should be noted that the intrinsic nanoporous structure of Amberlyst® 15 features a relatively low stability that needs to be enhanced in future investigations.

Figure 1.8 Chemical structure of Amberlyst® 15.

1.3.2 Nafion Resins

Compared with Amberlyst® 15, Nafion® perfluorosulfonic acid resins show better stability in the nanoporous structure and bear strong acid strength that is comparable to that of pure sulfuric acid. However, unswollen Nafion® resin suffers from a serious drawback due to its very low surface area that may impede application in the conversion of sugars to HMF (Figure 1.9) [16]. Commercialized Nafion® NR50, with a pellet diameter of 0.6–1.5 mm, was used as a catalyst for the preparation of HMF from fructose, and a moderate HMF yield (45–68%) was obtained in the presence of ionic liquid 1-butyl-3-methyl imidazolium chloride ([BMIM][Cl]) [65]. The ionic liquid and sulfonic ion-exchange resin could be recycled for seven successive trials.

Figure 1.9 Nafion® NR50 used for the conversion of fructose into HMF.

On account of these promising results and low surface areas of Nafion® resins, Shen and coworkers developed a novel Nafion®-modified mesocellular silica foam (MCF) hybrid material for the catalytic dehydration of D-fructose to HMF [66]. By using a physical impregnation approach, the Nafion® resin was highly dispersed in the ultra-large pores of the MCFs (Figure 1.10), affording a high conversion performance with an 89.3% HMF yield and 95.0% selectivity in DMSO. It was shown that the Nafion®(15)/MCF catalyst could be easily regenerated through an ion-exchange method, and a high yield was retained after five cycles.

Figure 1.10 Transmission electron microscopy (TEM) images of (a) MCF, (b) Nafion®(15)/MCF, (c) Nafion®(30)/MCF, and (d) Nafion®(45)/MCF.

As discussed above, the sulfonated resins clearly hold promise as heterogeneous acidic catalysts for biomass conversion, where the nanoporous structure plays crucial roles in the acid concentration and/or strength of the resins. However, the fact must be noted that the sulfonated resin catalysts still have a relatively low thermal stability due to collapse of the porous architectures, which greatly hinders their wide application in biomass conversion.

1.4 Conclusions and Perspective

This chapter has provided a brief overview of recent studies which have centered on the rational design, synthesis, and catalytic applications of nanoporous organic frameworks as acidic heterogeneous catalysts for the conversion of raw biomass materials to the renewable platform chemical HMF. Crystalline frameworks including MOFs and COFs and amorphous sulfonated resin-based networks were discussed in detail. Clearly, these novel catalysts display competitive activity towards the acid-catalyzed synthesis of HMF. The intrinsic nanoporous properties play a crucial role in the generation and dispersion of the acidic active sites, access of reactants, mass transfer, and control of product selectivity. Although in recent times extensive fundamental research and experiments have been carried out to optimize the conversion of biomass to HMF with the innovative use of these novel nanoporous organic frameworks, their commercial application for the production of HMF remains a challenge from an economic point of view. The following aspects should be considered simultaneously when designing new organic frameworks such as MOFs to improve the catalysis performance in the future: (i) better porosities consisting of hierarchical meso/macropores for faster mass transport of reactants and runoff of the products; (ii) better thermal and chemical stabilities that enable an efficient dehydration; and (iii) modulation of the degree of acidity and hydrophobicity on the surface via pore wall engineering, thereby providing a means for the controlled adsorption of substrates/intermediates. To summarize, the innovative utilization of novel nanoporous organic frameworks for the conversion of biomass to HMF as a bio-renewable energy source will lead to a sustainable and bright future.

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