Hydrophobic Solid Acids and Their Catalytic Applications in Green

Review Cite This: ACS Catal. 2018, 8, 372−391

pubs.acs.org/acscatalysis

Hydrophobic Solid Acids and Their Catalytic Applications in Green and Sustainable Chemistry Fujian Liu,† Kuan Huang,# Anmin Zheng,*,‡ Feng-Shou Xiao,*,§ and Sheng Dai*,¶ †

National Engineering Research Center of Chemical Fertilizer Catalyst (NERC−CFC), School of Chemical Engineering, Fuzhou University, Gongye Road No. 523, Fuzhou 350002, P. R. China ‡ National Center for Magnetic Resonance in Wuhan, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China # Key Laboratory of Poyang Lake Environment and Resource Utilization of Ministry of Education, School of Resources Environmental and Chemical Engineering, Nanchang University, Nanchang 330031, China § Department of Chemistry, Zhejiang University, Hangzhou, 310028, China ¶ Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ABSTRACT: Acid catalysts are widely used in petrochemical reactions, the synthesis of fine chemicals, and biomass conversions in industry. To comply with the principles of green and sustainable chemistry, much attention is being paid to the replacement of traditional liquid acids with solid acid catalysts. Normally, solid acids exhibit hydrophilicity because of the unique hydrophilic nature of the acidic sites on their surfaces. Water, as a typical solvent, byproduct, or negative component in a variety of acid-catalyzed reactions, may be adsorbed on solid acids and then cause the deactivation of catalytic sites or hydrolysis of the frameworks. The development of solid acids with suitable hydrophobicity largely overcomes these issues and enhances their catalytic activities and reusability. This Review discusses some recent advances in the preparation of novel solid acids with controllable wettability and suitable hydrophobicity and highlights their application in catalyzing various reactions such as esterification, transesterification, acylation, condensation, hydration, and depolymerization of crystalline cellulose. In addition, this Review discusses how the hydrophobicity of solid acids is affected by their structures, surface characteristics, and acid centers, and determines the principles for designing solid acids with high catalytic activity and good reusability. It is instructive for researchers who are interested in designing new kinds of solid acids with improved efficiency and reusability for applications in green and sustainable chemistry. KEYWORDS: solid acids, nanoporous materials, hydrophobicity, acid catalysis, green and sustainable chemistry

1. INTRODUCTION Acid catalysts are widely used in industry to catalyze a variety of reactions in the oil refining process, fine-chemical synthesis, and biomass conversions.1−12 In these areas, traditional mineral acids such as HCl, H2SO4, HF, and AlCl3 exhibit very high catalytic activity. However, their toxicity, corrosivity, and cost of regeneration significantly limit their use in industry, where green, sustainable, and low-cost catalysts are much in demand.1−11,13−31 Replacing traditional mineral acids with solid acids is a good solution, given that solid acids have low corrosivity and higher selectivity and can easily be separated from a reaction system for subsequent use.13−38 Among the various types of solid acids, heteropolyacids, metal oxides,6 sulfonated metal oxides, phosphates, and strongly acidic resins39 have shown good catalytic activity and improved reusability in a wide range of acid-catalyzed reactions. However, because of their low surface areas, the number of active sites that can be accessed by substrates is very limited, and the diffusion barrier is very high.40 Zeolitic solid acids, which have abundant micropores embedded in their frameworks, have been © XXXX American Chemical Society

shown to have increased surface areas and improved catalytic performance. They offer excellent stability for long-term use, selectivity for target products, and tunability for acidic strength.41 However, the relatively small sizes of the micropores in zeolites limit their applications in catalyzing the conversion of bulky molecules such as biomass and heavy oils.42−50 To overcome this limitation, ordered mesoporous silicas such as MCM-41 and SBA-15 with enlarged pore widths have been developed.51,52 To incorporate acidic sites, sulfonic groups, acidic ionic liquid segments, and Lewis acids such as aluminum (Al) and tin (Sn) have been immobilized onto the walls of mesoporous silicas.21,24,53−64 The resultant mesostructured solid acids have shown excellent catalytic activity and good reusability for converting bulky substrates into desired products.21,24,53−64 Received: October 2, 2017 Revised: November 13, 2017 Published: November 27, 2017 372

DOI: 10.1021/acscatal.7b03369 ACS Catal. 2018, 8, 372−391

Review

ACS Catalysis

Figure 1. Upper: illustration of the application of hydrophobic HY zeolites in the alkylation of m-cresol with 2-propanol. Lower: alkylation of mcresol with 2-propanol in the biphasic (emulsion) system at 200 °C and 700 psi of He over octadecyltrichlorosilane (OTS) functionalized HY zeolite (Si/Al = 30) as a function of reaction time. Left axis: product distribution. Right axis: overall m-cresol conversion. The feed had a total concentration of 2 M with a 2-propanol/m-cresol molar ratio of 3:1. Reproduced with permission from ref 75. Copyright 2012 American Chemical Society.

adjustable structures, different frameworks, versatile acid sites, and controllable surface characteristics have been successfully developed. In this regard, the challenge is to develop new routes for the rational design and synthesis of porous solid acids with controllable surface wettability and suitable hydrophobicity, and to correlate their catalytic performance with their structural properties so as to understand the underlying principles for designing superior solid acid catalysts. This Review discusses recent developments in the design and synthesis of hydrophobic solid acids, including zeolites, organic−inorganic hybrid mesostructured solid acids, carbonaceous solid acids, metal oxide-based solid acids, polymer resinsilica composites, sulfonated metal organic frameworks (MOFs), and nanostructured polymeric solid acids. We mainly focus on the relationship between the structural characteristics of solid acids and their catalytic performance. In particular, mesoporous polymeric solid acids are described in detail to demonstrate how the integration of superhydrophobicity and highly dense acidic sites can facilitate the superior performance of solid acid catalysts. Finally, the current challenges and the outlook for the synthesis and catalytic applications of hydrophobic solid acids are introduced.

To date, most porous solid acids are hydrophilic, owing to the hydrophilic nature of their frameworks (e.g., inorganic supports) and the acidic sites on their surfaces. Although some organic frameworks, such as carbonaceous materials, are themselves hydrophobic, they become much less so if modified with acidic groups such as sulfonic acids.25−28,65−67 Water, a typical byproduct and negative component of acid-catalyzed reactions, will coadsorb on hydrophilic solid acids, causing the partial deactivation of acidic sites and, in some cases, the hydrolysis of frameworks.4,35,40,68,69 In addition, many metalbased Lewis acids are unstable in water-containing environments and tend to evolve into metal hydroxides.70 Therefore, it is hypothesized that solid acids with suitable hydrophobicity may have further enhanced the adsorption of oleophilic reactants and desorption of hydrophilic reactants, which strongly enhance stability and reusability and catalytic performance in many reactions involving organic substrates.4,35,40,68,69 The significance of hydrophobic solid acids in the area of acid catalysis has been fully acknowledged, and several early classical reviews describe the development of hydrophobic solid acids, which reveal the essential relationship between catalytic performance and hydrophobicity. It is noteworthy that these reviews were mainly focused on zeolites, metal oxides, and hybrid solid acids with poor porosity.4,6,39 Within the last 10 years, many original and novel hydrophobic solid acids with 373


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2. HYDROPHOBIC SOLID ACIDS AND THEIR CATALYTIC APPLICATIONS 2.1. Hydrophobic Zeolites. Zeolites have long been widely used as solid acid catalysts in industry. Normally, the density of Brønsted acids and Lewis acid sites in zeolites increases with an increase in the Al/Si ratio; however, zeolites with high Al/Si ratios usually have low hydrothermal stability because of their decreased hydrophobicity.71 Enhancing the hydrophobicity of zeolites can effectively increase their hydrothermal stability, but at the same time, it may decrease the number of acidic sites. Yashima and co-workers investigated the catalytic application of high-silica zeolite in hydrolysis of ethyl acetate in aqueous solution. The high-silica zeolite showed better affinity for organic materials than water because of its unique hydrophobic properties, which results in its good activity and stability in the water contained reaction.72 The surface modification with hydrophobic groups such as alkyl groups was another effective approach for improving the hydrophobic properties of zeolites,73,74 which has been extensively reported and used in various acid-catalyzed reactions. For instance, Resasco and co-workers report the surface modification of HY zeolites with organic silicas, which resulted in suitable hydrophobicity and oleophilicity (Figure 1) without sacrificing their acid sites. As a result, the octadecyltrichlorosilane (OTS) functionalized HY zeolites could be homogeneously dispersed in the organic phase of a water/decalin biphasic system. In contrast, pristine HY zeolites tended to disperse in the water phase of the same biphasic system. Because the surface modification facilitated the diffusion of organic reactants into the nanopores of HY zeolites, the hydrophobic zeolites showed much improved activity (Figure 1) and good reusability in the alkylation of mcresol with 2-propanol, in both single-phase and biphasic (emulsion) systems, because of their enhanced compatibility in the reaction system.75 Moreover, the characterization of zeolites after reaction was also investigated. The untreated and OTS modified zeolites show a great difference in the resistance to deactivation of crystallinity and acid centers, which typically occurs during reaction in the liquid phase at high temperature. From the XRD profiles, the structure of hydrophobic zeolite was very stable after exposure to the aqueous environment at 200 °C. By contrast, untreated hydrophilic zeolite exhibited a dramatic drop in diffraction intensity, which indicates a significant loss of its crystallinity (Figure 2). Such obviously difference in the structures before and after catalytic reaction clearly confirmed that the severely deactivating effect of hot liquid water can be minimized by hydrophobic functionalization because of its enhanced property for decreasing the adsorption of water in the sample. Therefore, hydrophobic environment largely contributes the enhancements of acidity, hydrothermal stability, and reusability of the synthesized solid acids. Sn-Beta, a Lewis acid-containing zeolite with hydrophobic architecture, has been extensively used in the conversion of biomass and Baeyer−Villiger oxidation of ketones and aldehydes in aqueous solutions.76,77 The origin of Sn-Beta’s unique catalytic performance has aroused much interest across the academic community.78 Solid-state 119Sn nuclear magnetic resonance spectroscopy (NMR) was used to probe the detailed structure of Sn in the framework of a Sn-Beta zeolite. In hydrated Sn-Beta, the 119Sn NMR signals were in the range from −685 to −736 ppm, which is assigned to octahedral Sn species. After dehydration, the 119Sn NMR signals shifted to

Figure 2. X-ray diffraction of the HY zeolites (Si/Al = 30). Untreated zeolite, before (a) and after reaction (c); OTS-functionalized zeolite, before (b) and after reaction (d). Reproduced with permission from ref 75. Copyright 2012 American Chemical Society.

downfield at around −425 and −445 ppm. These peaks were attributed to tetrahedral open and closed Sn sites, respectively. To provide more precise structures of Sn in the framework of Sn-Beta zeolite, Hermans et al. used the sensitivity-enhanced dynamic nuclear polarization (DNP) surface-enhanced NMR approach to determine the local structural information for active sites in Sn-Beta zeolite,79 thus providing more precise structures of Sn species. Combining NMR experiment and theoretical calculation, the local structures of open and closed active Sn sites in an octahedral environment were unambiguously determined.79 The detailed hydrated pathways of Sn-Beta has been shown in Figure 3, it is observed that different

Figure 3. Detailed hydrated pathways of Sn-Beta zeolite in the presence of adsorbed water. Reproduced with permission from ref 79. Copyright 2014 John Wiley & Sons, Inc.

activation sites (i.e., two water molecules coordinated Sn (closed site) and Sn−O−Si bridges (open site) with one adsorbed water molecule) were readily formed in the aqueous environment, which explains well the resistance of Sn-Beta zeolite to water in variously catalytic reactions.79 In contrast to the traditional solid acidic catalysts, the Lewis acidity of Sn-Beta zeolite was maintained and will show excellent catalytic activity in the presence of water. Furthermore, Lewis acid zeolite active sites of Sn-Beta were quantitatively characterized by 31P solidstate NMR spectra with by trimethylphosphine oxide (TMPO) probe molecule. It is demonstrated that the Lewis acidic property derived from 31P spectra was correlated to catalytic activity.76 Except for Sn-Beta zeolite, other types of Lewis acid centers such as Nb and Ta have also been successfully incorporated into the frameworks of zeolites, which have been 374


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ACS Catalysis extensively used in the intramolecular carbonyl-ene reaction of citronellal to isopulegol.80 Mesoporous acidic zeolites such as ZSM-5 and Beta act as the state-of-the-art materials, show the advantages of both zeolites and mesoporous materials, which have strong acidity and hierarchical nanochannels for the transfer of bulky substrates.81−84 Up to now, mesoporous zeolites have been widely used in a variety of acid-catalyzed reactions such as bulky substrates condensation. Except for Al/Si ratios, the unique nanostructure of hierarchically porous zeolites may result in their improved hydrophobicity as well, which has positive effect on their compatibility with oleophilic reactants, further resulting in their enhanced catalytic activities.81−84 The above characteristics have a certain effect on their catalytic performances and products selectivity in various acid-catalyzed reactions.81−84 It should be also noted that the acid strength, acid density, and hydrophobicity of zeolitic solid acids still need to be improved for their wide applications in various acidcatalyzed reactions. 2.2. Hydrophobic Metal Halide-Based Solid Acids. Metal halide-based Lewis acids (e.g., AlCl3, BF3, and TiCl4) play important roles in the catalytic formation of carbon− carbon bonds, such as Friedel−Crafts alkylation and acylation of aromatic compounds. However, most metal halide-based Lewis acids easily decompose into their corresponding metal hydroxides in the presence of water, resulting in the loss of catalytic activity. Therefore, Lewis acids with water tolerance are very attractive for applications in catalytic reactions. Hara et al. investigated the Lewis acidity of various trifluoromethanesulfonates (OTf) and chlorides containing different metals (Sc, Y, La, Lu, In, and Zn) in water by 31P trimethylphosphine oxide (TMPO) NMR spectroscopy.85 The change in the 31P NMR chemical shift is an effective indicator to identify the formation of a TMPO-Lewis acid complex in water and determine watertolerant Lewis acid catalysts. Classical inorganic Brønsted acids (H2SO4 and H3PO4) were used as references; they showed a very slight difference in the chemical shifts of the 31P signal when the concentration ratio of the acidic proton and TMPO was set at 0.25, as shown in Figure 4. The chemical shifts of the 31 P signal in H2SO4 and H3PO4 were very close to that of hydrous TMPO (ca. δ = 53.5 ppm). Similarly, Y(OTf)3, La(OTf)3, Lu(OTf)3, Zn(OTf)2, YCl3 and LaCl3 had sharp TMPO signals at 53−55 ppm, and the slight difference in the chemical shifts of the 31P signal suggested that the TMPO molecules coordinated with these metal trifluoromethanesulfonates and chlorides were readily replaced by H2O molecules. In contrast, Sc(OTf)3, ScCl3, and In(OTf)3 had TMPO signals at 64.5, 63.4, and 61.9 ppm after TMPO adsorption, respectively, indicating that these metal trifluoromethanesulfonates and chlorides were capable of forming stable TMPO-acid complexes in the presence of water (Figure 4). Therefore, Sc(OTf)3, ScCl3, and In(OTf)3 are water-tolerant Lewis acids and thus exhibit high catalytic activity for lactic acid formation from 1,3-dihydroxyacetone (1,3-DHA) and pyruvic aldehyde in water: a strong interaction with carbonyl groups in a reactant on a Lewis acid is a dominant factor in lactic acid formation.85 2.3. Hydrophobic Ordered Mesoporous Silica-Based Solid Acids. Sulfonic group−functionalized ordered mesoporous silicas (OMSs) can overcome the limitations of zeolites for applications in the conversion of bulky molecules. They exhibit some unique characteristics, such as strong acidity and large surface areas, as well as abundant and adjustable ordered mesopores.53−64 These characteristics endow them with very

Figure 4. 31P NMR spectra of TMPO in TMPO/D2O solutions with different Lewis acid catalysts (the concentration ratio of Lewis acid (cacid) and TMPO (cTMPO) was set at 0.25: Lewis acid/TMPO = 0.25; a: TMPO without Lewis acid catalyst, b: Sc(OTf)3, c: Y(OTf)3, d: La(OTf)3, e: Lu(OTf)3, f: In(OTf)3, g: Zn(OTf)2, h: ScCl3, i: YCl3, j: LaCl3, k: H2SO4, and l: H3PO4). Reproduced with permission from ref 85. Copyright 2014 Elsevier.

good activities for catalyzing the conversion of bulky molecules. However, OMSs usually show low hydrothermal stability because of their hydrophilic frameworks and amorphous morphology. As a result, their acidic sites are easily deactivated, and their frameworks are vulnerable to hydrolysis in many acidcatalyzed reactions because of the easy adsorption of water on their surfaces. This characteristic strongly constrains their catalytic activity and reusability.86−88 The introduction of organic building blocks can increase the hydrophobicity of OMS-based solid acids owing to the unique hydrophobic nature of organic groups. For example, Melero and co-workers successfully grafted organic acid moieties, such as arenesulfonic acid groups, onto the networks of OMSs (Ph-PMO−SO3H, Figure 5). The resulting samples exhibited relatively low

Figure 5. TEM images (left) and CP MAS 13C CP NMR spectra (right) of extracted pristine SBA-15 (A) and arenesulfonic-modified SBA-15 (B). Reproduced with permission from ref 53. Copyright 2002 Royal Society of Chemistry. 375


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Figure 6. (A) 1H MAS NMR spectra of dehydrated Ph-PMO−SO3H (a: without propylsulfonic acid group, b: without sulfonic acid group, c−l: with different concentrations of propylsulfonic acid groups); (B) 1H MAS NMR spectra of hydrated Ph-PMO−SO3H (a: without propylsulfonic acid group, b: without sulfonic acid group, c−f: with different concentrations of propylsulfonic acid groups; asterisks depict ethanol impurities); (C) evolution of the chemical shift of 1H signal in a sulfonic acid group as a function of acid concentration; (D) 2D 1H−1H spin-exchange NMR spectra of dehydrated Ph-PMO−SO3H containing 0.21 mmol/g of sulfonic acid groups (the red filled circles emphasize the off-diagonal cross peaks for simplicity). Reproduced with permission from ref 89. Copyright 2012 Royal Society of Chemistry.

sulfonic group loadings but good stability.53,61 Notably, they showed superior activity in the catalytic condensation of indole to benzaldehyde in water because of their increased hydrophobicity and oleophilicity, which contributed to their good water tolerance and compatibility with various reactants. The structures of hydrophobic OMS-based solid acids were also investigated by solid NMR spectra (Figure 6). The chemical shifts of 1H signals in sulfonic groups were in the range of 3− 8.4 ppm and moved downfield as the concentrations of propylsulfonic acid in dehydrated samples increased. Because the extent of the changes in the chemical shifts of 1H signals corresponded to the strength of the Brønsted acidity, the shift of the 1H signal from 4.6 to 8.4 ppm indicated that the acidity of sulfonic acid groups was enhanced as the concentrations of propylsulfonic groups increased from 0.21 to 1.3 mmol/g (Figure 6A). As shown in Figure 6B, the strength of the acidity was significantly reduced in hydrated samples, which is indicated by the smaller chemical shift of the 1H signals (Figure 6B). Surprisingly, the effect of water on the strength of

the acidity was almost negligible when the loading of sulfonic groups was 0.21 mmol/g (Figure 6C); the reason was likely the hydrophobic environment constructed by the phenyl rings on the walls of the inner pores. The distinctively hydrophobic environment further protected the sulfonic acid groups from water solvation. In addition, 2D 1H−1H spin-exchange NMR spectra were used to determine the distance between hydrophobic phenyl rings and anchored sulfonic acid groups. As shown in Figure 6D, the cross-peaks between the protons in −SO3H (4.6 ppm) and those in phenyl (6.9 ppm) (red filled circles) start to appear when the mixing time is above 10 ms in the 2D 1H−1H spin-exchange NMR spectra, indicating that the propylsulfonic acid groups are close to the hydrophobic phenyl rings. Such structural characteristics may play key roles in protecting acidic sites against water, and they provide a reasonable explanation for the unexpectedly high turnover frequency values for the samples with the lowest loadings of sulfonic groups (0.21 mmol/g) in aqueous solutions.89 376


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Figure 7. Left: illustration showing the preparation and structure of catalysts: octyl and 3-aminopropyl groups were grafted onto the channel walls of SBA-15, followed by the immobilization of H3PW12O40 into the nanopores; right: water adsorption isotherms of SBA-15 (a), C8-AP-SBA-15 (b), and PW/C8-AP-SBA-15 (c). Reproduced with permission from ref 90. Copyright 2007, John Wiley & Sons, Inc.

Figure 8. Left: density of NH3 adsorbed on the surface of MoO3−ZrO2 as a function of the calcination temperature (○: 9 at. % MoO3−ZrO2, ●: 23 at. % MoO3−ZrO2, □: ZrO2); right: adsorption isotherms of water on 23 at. % MoO3−ZrO2 calcined at different temperatures (○: 573 L, ●: 673 L, △: 773 K, ▲: 873 K, ■: 973 K, □: 1073 K, ◊: 1173 K). Reproduced with permission from ref 94. Copyright 1999 Royal Society of Chemistry.

Yamanaka and co-workers reported novel water-tolerant heteropolyacid-based solid acids synthesized by impregnating H3PW12O40 into the nanopores of hydrophobic OMSs (e.g., SBA- 15). The hydrophobicity of an SBA-15 support can be controlled by grafting different Cn-aminopropyl (Cn-AP) groups onto the mesopore walls (Figure 7). Notably, unmodified SBA-15 exhibited large water uptake at P/P0 = 0.7 owing to the capillary condensation of water, indicating that the nanopores in SBA-15 were filled with water. In contrast, the organo-modified counterpart (C8-AP-SBA-15) showed negligible water condensation in the nanopores, indicating its excellent hydrophobic nature. After the impregnation of H3PW12O40 into the mesopores of C8-AP-SBA-15, the resultant PW/C8-AP-SBA-15 showed remarkable water condensation at P/P0 = 0.8−0.9, which was even more significant than the results obtained from pristine SBA-15. The amount of water adsorbed coincided with the pore volume of PW/C8-AP-SBA determined by N2 adsorption (0.15 cm3/g). This result demonstrates that reactants in aqueous solutions can penetrate into the nanospace of PW/C8-AP-SBA-15, although PW/C8AP-SBA-15 is still more hydrophobic than pristine SBA-15. This quality allows reactants to readily access the acidic sites in the nanospace of hydrophobic acid catalysts (Figure 7). As a consequence, the acidic sites in the hydrophobic environment of organo-modified SBA-15 showed extremely high catalytic activities for the hydrolysis of esters in water. Therefore, suitable hydrophobicity in solid acids can enhance their compatibility with organic reactants and protect the acidic

centers from deactivation by water, which is very favorable for the improvement of their catalytic activity and reusability.90 In fact, the high cost, limited acid density, and relatively low hydrothermal stability of mesoporous silica-based solid acids still need to be resolved for their practical applications. 2.4. Hydrophobic Metal Oxide-Based Solid Acids. Oxide-based solid acidssuch as clay minerals, sulfonated metal oxides, and heteropolyacids exhibit characteristics such as relatively low cost, strong and controllable acidity, and good catalytic activities in various reactions, including esterification, transesterification, hydration, isomerization, biomass transformation, and condensation.39,91 Therefore, they have great potential for industrial application. To further improve their catalytic activity and reusability, the design and synthesis of water-tolerant oxide-based solid acids have received considerable attention in recent years, which is very important for their practical industrial applications.91−95 Okuhara and co-workers successfully synthesized Mo-Zr mixed oxides with strong acidity at elevated calcination temperatures.94 The high calcination temperatures greatly increased the surface hydrophobicity because the number of acidic sites was reduced. For ZrO2, the optimal calcination temperature was centered at 673 K, which exhibits the highest acid density. However, the resultant ZrO2 showed bad hydrophobicity. Although the high calcination temperatures largely improved its hydrophobicity, the limited acid density strongly constrains its catalytic application (Figure 8).94 After modification of ZrO2 with MoO3 to give Mo-Zr mixed oxides, 377


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Figure 9. Left: structure of Nb2O5·nH2O hydrophobic solid acid; right: the catalytic activity of reused Nb2O5·nH2O for HMF production from Dglucose at 393 K in water. Reproduced with permission from ref 95. Copyright 2011 American Chemistry Society).

the optimal calcination temperature was increased to 773 K. A further increase in the calcination temperature up to 1173 K results in samples with much improved hydrophilicity. Meanwhile, the Mo-Zr mixed oxides still show high acid density in comparison with ZrO2. The mentioned characteristics result in the Mo-Zr mixed oxides with good activities in hydrolysis of ethyl acetate and esterification of acetic acid with ethanol, which act as new types of “water tolerant” and reusable catalysts in various water-contained reactions. Inspired by the pioneering works, Hara and co-workers successfully developed Nb2O5·nH2O-based solid acids, which acts as novel heterogeneous Lewis acids with good water-tolerant property (Figure 9).95 More interestingly, the synthesized Nb2O5·nH2O showed good activities and stabilities in allylation of benzaldehyde with tetraallyl tin and the conversion of glucose into HMF in water systems (Figure 9).95 Unfortunately, the oxide-based solid acids still showed disadvantages such as limited density of acidic sites and low BET surface areas, which still need to be improved for their wide industrial applications. To overcome the low BET surface areas and poor porosities of conventional oxide-based solid acids, Domen and co-workers successfully synthesized a layered transition-metal oxide solid acid (HNbMoO6). NH3-TPD and 31P NMR spectra with a TMPO probe confirmed the very strong acidity of HNbMoO6, which is an efficient and water-tolerant catalyst in Friedel− Crafts alkylation, esterification, and hydration (Table 1). Therefore, hydrophobicity of metal oxides solid acids was determined by their compositions and unique nanostructures, which effects the mass transfer of various guest molecules and exposure degree of acid sites in these samples.92 Other porous metal oxide-based solid acids, such as sulfonated MOFs and MOF-supported heteropolyacids, have also been synthesized in recent years. The controllability of nanostructures and building blocks in MOF-based supports effectively enhances their hydrophobicity and compatibility with various organic molecules in catalytic reactions. For example, Liu and co-workers report that heteropolyacids were facilely confined in the nanopores of MOFs by physical adsorption. The corresponding heteropolyacid-confined MOFs acted as efficient catalysts for the production of biodiesel. Their high activity should be attributed to the higher number of exposable Lewis acidic sites on {100} facets than on {111} facets; the result was fairly considerable adsorption of substrates on the solid acids materials.96 On the other hand, sulfonic groups can also be grafted onto MOF networks. For example, Chen and coworkers synthesized a series of sulfonic group−functionalized

Table 1. Friedel-Crafts Alkylation over Some Solid Acid Catalystsa

yield of alkylate (%)b catalysts

acid amount (mmol g−1)

R OCH3c

RCH3

RH

HNbMoO6d HNbMoO6g Nb2O5·nH2O Nafion NR50 Amberlyst-15 H-ZSM-5h H-Betai

1.9e 1.9e 0.3 0.9 4.8 0.2 1.0

99.0f (89) 94.0f (86) 1.2 (2) 42.3 (23) 42.1 (5) 8.6 (22) 30.6 (16)

74.1 (4.9) 22.5 (1.5) N.D. 19.8 (2.8) 14.0 (0.4) 0.9 (0.6) 0.6 (0.1)

21.9 (1.4) 7.7 (0.5) N.D. 9.7 (1.4) 6.7 (0.2) N.D. N.D.

a

Reaction conditions: 1 (100 mmol), benzyl alcohol (10 mmol), catalyst (0.2 g), 353 K, 4 h. bValues in parentheses are turnover rate (/h−1). N.D., not determined. cReaction temperature, 373 K; reaction time, 1 h. dProtonated with H3PO4. eDetermined by 31P NMR using trimethylphosphine oxide. fReaction time, 30 min. gProtonated with HNO3. hSiO2/Al2O3 = 90, JRC-Z-5−90H. iSiO2/Al2O3 = 25, JRC-ZHB25. Reproduced with permission from ref 92. Copyright 2008 American Chemical Society.

MOFs by using HSO3Cl for the sulfonation process. Because of the highly porous structures, suitable hydrophobic frameworks, and abundant acidic sites of the resultant solid acids, they proved to be efficient and reusable catalysts for the conversion of fructose into 5-hydroxymethylfurfural.97 However, in many cases, the relatively weak chemical stability of MOFs support cannot withstand the harsh functionalization conditions. 2.5. Polymer−Silica Nanocomposites. The introduction of polymer additives into the matrices of inorganic solid acids such as porous silicas, producing polymer−silica nanocomposites, is an alternative way of changing the surface wettability and thereby increasing the hydrophobicity of solid acids.98−102 Such hybrid materials normally exhibit good compatibility with organic substrates and water tolerance, which are favorable for improving their performance for applications in various acid-catalyzed reactions. Recently, various polymer−silica nanocomposites with excellent catalytic activities have been prepared.102,103 For example, Jones and coworkers synthesized a class of brush-type polymer−silica nanocomposites, and the polymeric brushes were further 378


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sulfonic group−functionalized ordered mesoporous phenolic resin-silica nanocomposites via the self-assembly of block copolymers with mixed precursors containing resol and 3mercaptopropyltrimethoxysilane.105 The synthesized nanocomposites exhibited a controllable density of acidic sites and large BET surface areas. Compared with pristine mesoporous silicas (e.g., SBA-15 and MCM-41), the polymer−silica nanocomposited solid acids had enhanced hydrophobicity. Figure 11 shows the adsorption isotherms of water vapor on various samples, including pristine SBA-15 and MCM-41 and polymer−silica nanocomposites. Pristine SBA-15 and MCM-41 showed a small amount of water uptake in the low relative pressure region, suggesting weak interactions between water molecules and pristine mesoporous silicas. The steep increase in the middle relative pressure region was attributed to the capillary condensation of water molecules in the mesopores. The Henry’s constants for water adsorption on pristine SBA-15 and MCM-41 calculated from the isotherm data at low relative pressures are quite similar (0.86 and 0.65 mol/g−1 Pa−1); the similarity is attributed to the similar hydrophilicity of the two types of mesoporous silicas. On the contrary, the polymerbased MP possesses an almost linear adsorption isotherm in the full relative pressure region. Considering the high surface area, large pore volume, and large pore width of mesoporous polymer−silica nanocomposites, this observation implies that the capillary condensation of water vapor may not occur under the experimental conditions. The calculated Henry’s constant is only 0.09 mol g−1 Pa−1 for the adsorption of water on polymerbased MP, much lower than those of pristine SBA-15 and MCM-41. This comparison further validates the much more hydrophobic nature of polymer-based MP. As a result, polymerbased MP shows enhanced activities for catalyzing the production of bisphenol A and condensation of 1,4-butanediol with ketones or aldehydes relative to hydrophilic SBA-15-SO3H and Amberlyst 15. These results confirm that the incorporation of polymer additives into the matrices of mesoporous silicas can greatly increase the hydrophobicity of the resultant polymer− silica nanocomposites. This finding opens a new avenue for the synthesis of hydrophobic solid acids. Meanwhile, other types of water-repellent organic−inorganic hybrid solid acids, such as Fe3O4@SiO2 core-Me&Et-PhSO3H shell nanostructures, have

modified by sulfonation to yield polymer−silica compositebased solid acids (see Figure 10). The hybrid solid acids

Figure 10. Structure of brush-type silica-supported poly(styrene sulfonic acid). Reproduced with permission from ref 104. Copyright 2011 American Chemistry Society.

showed comparable performance to homogeneous catalysts (e.g., p-toluenesulfonic acid), with excellent activity and good reusability in the hydrolysis of esters. Hybrid solid acids with the unique brush-type structure usually have hydrophobic polymeric brushes, abundant acidic sites, suitable hydrophobicity, and an enhanced area of exposure of the acidic sites,104 which are beneficial for the improvement of their catalytic activity in water-contained reactions. Similar results have been reported previously in other work.68 Ordered mesopores can also be incorporated into the frameworks of polymer−silica nanocomposites. The introduction of ordered mesopores significantly improves the mass transfer of reactants, as well as the homogeneous dispersion of acidic sites. In addition, the surface wettability of polymer− silica nanocomposites is changed by introducing controllable and ordered mesopores. Wan and co-workers synthesized

Figure 11. Left: adsorption isotherms of water vapor on pristine SBA-15, MCM-41 and polymer-based MP at 25 °C; right: TEM images of MPSO3H-22.7 (a and b) and MP-SO3H-27.4 (c and d) with −SO3H weight percentages of 22.7 and 27.4%, respectively (a and c: viewed along the pore channels, b and d: viewed perpendicularly to the pore channels, inset picture in c: EDX pattern). Reproduced with permission from ref 105. Copyright 2012 Royal Society of Chemistry). 379


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ultrahigh density of acidic sites (up to 4.9 mmol/g), which contributed to their excellent catalytic activity in esterification and hydrolysis reactions.107 However, their poor porosity constrains their practical application in industry. To overcome this problem, there is extensive research interest in the rational introduction of nanopores into the matrices of carbonaceous solid acids. In this area, sulfonic group−functionalized mesoporous carbons, carbon nanotubes, and graphene are the most important representatives. Feng and co-workers successfully synthesized sulfonic group−functionalized ordered mesoporous carbons (OMCs) with large BET surface areas, abundant mesopores and controllable hydrophobicity.25 The OMCs could be acted as efficient solid acid catalysts in various reactions such as catalyze synthesis of bisphenol A (see Figure 14).25 Sulfonic group− functionalized OMCs can also be synthesized by using other methods, the synthesized OMCs-based solid acids have been extensively used in a variety of acid-catalyzed reactions.26 To endow other functionality and enhance the efficiency of regeneration of OMCs solid acids, Liu et al. successfully synthesized magnetically active and sulfonic group−functionalized OMCs by direct sulfonation of magnetic OMCs.108 The magnetic OMCs supports were synthesized from carbonization of magnetic active ordered mesoporous polymers. The resultant magnetic solid acids showed good catalytic activity in esterification and condensation and were easily separated and recycled by applying a constant magnetic field (Figure 15), in which water acts as typical byproduct. Their good catalytic activity should be resulted from their hydrophobic and oleophilic carbon network. Furthermore, such magnetically active properties of carbonaceous solid acids can significantly simplify the separation and recycling of catalysts in various reaction systems. Therefore, magnetically active and sulfonic group−functionalized OMCs have great potential for acid catalysis in the industry. Compared with amorphous carbon materials, carbon nanotubes and graphene exhibit enhanced hydrophobicity, much higher thermal and mechanical stability because of their graphitized structures.109−117 Therefore, using carbon nanotubes or graphene as supports for functionalization with sulfonic groups is a way to obtain carbonaceous solid acids with high thermal and chemical stabilities. For example, Peng and co-workers successfully synthesized sulfonated carbon nanotubes, which can act as strong protonic acids and reusable catalysts for the esterification because of their good stability and controllable surface wettability (Figure 16)109−111 Graphenebased solid acids were also investigated because of their excellent stability, large surface areas, controllable surface wettability, and unique 2D layered structures, which endow them with decreased diffusion limitations and enhanced dispersion in various catalytic reactions.112−116 Fan and coworkers report the synthesis of sulfonated graphene with unique water tolerance, leading to a very good distribution of sulfonic groups and excellent activity in catalyzing the hydrolysis of ethyl acetate.117 Figure 17 shows the process for the synthesis of sulfonated graphene, which was realized by treating pristine graphene with 4-benzenediazoniumsulfonate. The resultant graphene-based solid acids showed good activity in catalyzing the hydrolysis of ethyl acetate. The activity of sulfonated graphene was much higher than that of Nafion resin, which is one of the most widely used commercial resins. The excellent activity of sulfonated graphene is attributable to their good water tolerance and unique nanosheet structures, which

also been synthesized. They show enhanced water-repellent properties owing to the presence of abundant organic groups, and they have been extensively used for catalyzing threecomponent Strecker reactions (see Figure 12).106 Their good

Figure 12. Structure of water-repellent Fe3O4@SiO2 core-Me&EtPhSO3H shell solid acids and its application in catalyzing threecomponent Strecker reactions. Reproduced with permission from ref 106. Copyright 2014 American Chemical Society.

catalytic performance should be resulted from the structural characteristics of superior water-repellent properties, which strongly enhance the antihydrolysis of acid sites by water and compatibility with the organic substrates. However, it remains challenge to scalable preparation of the solid acids because of their complicated synthetic procedures, which play important roles for their wide applications. 2.6. Hydrophobic Carbon-Based Solid Acids. Carbonaceous materials such as fullerenes, carbon nanotubes, and graphene have received considerable attention because of their unique properties, including high stability, large BET surface areas, and good resistance to corrosion by strong acids. They have wide application in the areas of adsorption, separation, energy storage, electrochemistry, and heterogeneous catalysis. Compared with other inorganic materials, carbonaceous materials are ideal candidates for functionalization with acidic sites because of their suitable hydrophobicity and controllable wettability for various organic substrates, which can greatly improve their activity and reusability in a variety of acidcatalyzed reactions. Hara and co-workers successfully synthesized carbonaceous solid acids by heating aromatic compounds (e.g., naphthalene) and sugars in the presence of sulfuric acid at 473−573 K, which resulted in the incomplete carbonization of the sulfopolycyclic aromatic compound CH0.30O0.33S0.16 (Figure 13). The resulting low-cost carbonaceous solid acids showed an 380


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Figure 13. Powder XRD pattern (a), 13C MAS NMR spectra (spinning sidebands (SSBs) appear at 50 and 210 ppm) (b), and proposed structure of the carbon-based solid acid CH0.30O0.33S0.16 (c) (CH0.30O0.33S0.16 was evacuated at 180 °C for 1 h prior to 13C-MAS NMR measurement; spinning sidebands (SSBs) appear at d = 50 and 210 ppm; CH0.30O0.33S0.16 is amorphous and composed of small sulfopolycyclic aromatic carbon rings oriented in a random fashion; −SO3H groups are attached to 11% of the aromatic carbon atoms). Reproduced with permission from ref 107. Copyright 2004 John Wiley & Sons, Inc.

greatly enhanced the mass transfer of reactants and resistance to the deactivation of acidic sites. Liu et al. successfully synthesized sulfonated graphene by treating pristine graphene with fuming sulfuric acid. The resultant graphene-based solid acids showed hydrophobic network, very good dispersion, and an enhanced degree of exposure of acidic sites in various reaction systems, which contributed to their excellent catalytic activity in esterification and condensation.113 Although a variety of porous carbon solid acids have been successfully prepared to date, the number of acidic sites on them is very limited because of their inert networks, which constrain their activity in various reactions. Also, it should be noted here that the hydrophobicity of carbonaceous solid acids needs to be further promoted to enhance their catalytic activity in reaction systems containing water.68,69 2.7. Hydrophobic Porous Organic Polymeric Solid Acids. The synthesis of ordered mesoporous polymers occurred relatively later than the synthesis of porous silicas, metal oxides, and carbons. Compared with inorganic and

Figure 14. Synthesis of sulfonic group−functionalized ordered mesoporous carbons. Reproduced with permission from ref 25. Copyright 2007 American Chemistry Society.

Figure 15. Synthesis of magnetically active sulfonic group-functionalized ordered mesoporous carbons. Reproduced with permission from ref 108. Copyright 2012 Elsevier. 381


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process is very important for the formation of controlled nanopores in the products. Zhao and co-workers successfully synthesized a family of ordered mesoporous phenol-formaldehyde resins by the selfassembly of resol with block copolymers via a soft-template route. The synthesized ordered mesoporous polymers exhibited cubic and hexagonal mesoporous structures.119,120 The successful preparation of ordered mesoporous polymers offers a great opportunity to use them as highly effective supports for functionalization with acidic groups. To this end, Wu and coworkers reported the synthesis of sulfonic group-functionalized ordered mesoporous polymers by direct sulfonation of FDU-14 and FDU-15 using fuming sulfuric acid (see Figure 18).23 The resultant polymeric solid acids showed large BET surface areas, a high density of acidic sites, and suitable hydrophobicity, providing them with good catalytic activity in alcohol-aldehyde condensations. To further improve their acidity, Liu and coworkers synthesized strong acidic ionic liquid−functionalized ordered mesoporous polymers by treating N-containing ordered mesoporous phenol-formaldehyde resins with 1,3propane sultone and then exchanging with sulfuric acid or HSO3CF3 (Figure 19).128 The enhanced acidity of the resultant polymeric solid acids arose from the strong acidic ionic liquid moiety. The enhanced acidity, hydrophobic networks, and controllable surface wettability of these polymeric compounds endowed them with excellent activity in catalyzing the conversion of triglyceride into biodiesel, relative to many other solid acids.127,128 However, the use of large amounts of templates and networks containing abundant oxygen resulted in high costs, unsatisfactory hydrophobicity, and low thermal stability for ordered mesoporous phenol-formaldehyde resins, which constrains their use as efficient, stable solid acid catalysts in industry. To expand the database of porous organic polymers, Xiao and co-workers reported superhydrophobic and swollen mesoporous polydivinylbenzene (PDVB) that can selectively adsorb organic compounds from aqueous solutions. Mesoporous PDVB was synthesized from the copolymerization of

Figure 16. SEM image (a), EDX spectrum (b), and catalytic performance in esterification (c) of sulfonated carbon nanotubes. Reproduced with permission from ref 110. Copyright 2005 Elsevier.

carbonaceous materials, porous polymers have a number of advantages, such as extremely flexible networks, excellent stability under acidic or basic conditions, much greater hydrophobicity, and controllable wettability for various organic substrates.118−128 In addition, the polymeric frameworks can be easily functionalized with various groups. All these features are favorable for the application of porous polymers in the catalytic conversion of various organic substrates into useful chemicals.118−130 However, there are two problems associated with the synthesis of porous organic polymers: (1) how to select suitable thermosetting polymeric networks to control the polymerization process and degree of cross-linking and (2) how to select suitable templates to control the self-assembly process. Matching the polymerization process with the self-assembly

Figure 17. Left: synthetic process, SEM image, elemental maps; right: catalytic performance and reusability of sulfonated graphene. Reproduced with permission from ref 117. Copyright 2007 Royal Society of Chemistry. 382


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Figure 18. Scheme for the preparation of sulfonic group-functionalized ordered mesoporous polymers with different mesostructures. Reproduced with permission from ref 23. Copyright 2007 John Wiley & Sons, Inc.

Figure 19. Synthetic scheme of OMR-[C4HMTA][SO4H]. Reproduced with permission from ref 127. Copyright 2014 Elsevier.

divinylbenzene (DVB) under solvothermal conditions without the use of any template. The unique synthetic technology resulted in products with abundant nanopores, large BET surface areas, and a high degree of cross-linking. The superhydrophobicity of mesoporous PDVB makes it compatible with various organic compounds, and as a result, it has extraordinary adsorption capacity and surprising swollen property for various organic compounds.118 Moreover, it has significantly higher thermal stability because of the oxygen-free and highly cross-linked networks. Therefore, mesoporous PDVB is an ideal support for functionalization with acidic groups to produce efficient and reusable porous solid acids with excellent hydrophobicity, controllable wettability, and good stability. Inspired by the work mentioned above, Liu et al. successfully synthesized hydrophobic solid acids by direct sulfonation of mesoporous PDVB with HSO3Cl in CH2Cl2 (PDVB-SO3Hs).68

Because of the good swelling, superhydrophobicity, and superoleophilicity of mesoporous PDVB supports, the polymeric networks were easily exposed to HSO3Cl and CH2Cl2, which resulted in PDVB-SO3Hs with a very high density of acidic sites and suitable hydrophobicity. Interestingly, because of their high density of acidic sites, suitable hydrophobicity, and surface wettability, PDVB-SO3Hs showed much better activity in catalyzing the esterification of cyclohexanol with acetic acid and in catalyzing acylation than did commercial Amberlyst 15 and many other inorganic solid acids (Figure 20). The superhydrophobicity and superoleophilicity of the PDVB support result in their high density of acidic sites, good catalytic activity, and reusability. Moreover, PDVB-SO3Hs can be used as highly efficient solid acids to catalyze transesterification to produce biodiesel from raw feedstocks because of their high density of acidic sites, abundant nanopores, and superoleophilic network. PDVB-SO3Hs showed much better 383


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has negative effect on their long-term usage in the watercontained reactions. To further improve the hydrophobicity of mesoporous PDVB-based solid acids, Liu et al. synthesized a class of superhydrophobic mesoporous polymer-based solid acids by direct copolymerization of DVB with sodium p-styrenesulfonate under solvothermal conditions for the first time.35 The N2 adsorption measurements and contact angle tests showed that the synthesized solid acids had large surface areas, a controllable density of acidic sites, a high degree of cross-linking, and superhydrophobic/superoleophilic networks. The superhydrophobic and superoleophilic properties should be resulted from their unique nanostructures and polymeric networks. These properties were very beneficial for the enhancement of their antideactivation of catalytic sites or hydrolysis of the frameworks, fast diffusion of various reactants and products in the deep channels, and they significantly increased the degree of exposure of acidic sites on the solid acids (Figure 22). These

Figure 20. Photographs of PDVB-0.1 before (A) and after (B) swelling in CH2Cl2; sulfonated PDVB-0.1 (C); sulfonation procedures of PDVB-x samples with HSO3Cl in CH2Cl2 (D). Reproduced with permission from ref 68. Copyright 2010 Elsevier.

Figure 22. Synthesis of superhydrophobic mesoporous polymer-based solid acids. Reproduced with permission from ref 35. Copyright 2012 American Chemical Society.

characteristics contributed to the remarkable improvement in its catalytic activity, such as the good reusability and ultrahigh turnover frequency values (TOF: 172−4431 h −1 ) in esterification, condensation, and transesterification to biodiesels.12,35,40 It is noteworthy that their high efficiencies were much higher than variously reported solid acids with bad hydrophobicity35 in these reactions. Superhydrophobicity of PDVB-based solid acids results in their good water tolerant property, which strongly contributes their acidity strength, stability, good activities, and very high TOF values in the watercontained reactions. In fact, the surface wettability and acidity of solid acids play key roles in determining their catalytic activity in various reactions. To further improve the acid strength of mesoporous PDVB-based solid acids, strong electron-withdrawing groups (e.g., −SO3CF3) were effectively grafted onto the PDVBSO3Hs networks. As illustrated in Figure 23, the relatively larger 31P chemical shift of adsorbed trimethylphosphine oxide (TMPO) was apparently demonstrated the acidic strength of PDVB-SO3H−SO2CF3 has been considerably improved.12 Furthermore, it is found that the introduction of −SO3CF3 significantly enhanced the catalytic activity toward transesterification of oil to biodiesel and depolymerization of crystalline cellulose to sugars (Figure 23).12 To further improve the acidity of mesoporous PDVB-based solid acids, Liu et al. grafted various strong acidic ionic liquid moieties onto the frameworks of mesoporous PDVB-vinylimidazole copolymers.

performances than commercial Amberlyst 15, heteropolyacids, sulfonated SBA-15, and sulfonated zirconia (see Figure 21).129,130 However, the high density of acidic sites on PDVB-SO3Hs partially decreased its hydrophobicity, which

Figure 21. Kinetics curves of the transesterification of tripalmitin with methanol over (a) PDVB-SO3H-24 (b) H3PO40W12 (c) SBA-15-SO3H (d) Amberlyst 15, and (e) S-ZrO2. Reproduced with permission from ref 129. Copyright 2012 Elsevier. 384


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Figure 23. Upper left: solid-state 31P MAS NMR spectra of adsorbed TMPO on (a) PDVB-SO3H and (b) PDVB-SO3H−SO2CF3; upper right: TEM image of PDVB-SO3H−SO2CF3; lower: kinetic curves of the depolymerization of crystalline cellulose monitored by DNS assay (A) and HPLC (B) over Amberlyst 15 (a), PDVB-SO3H (b), and PDVB-SO3H−SO2CF3 (c). Reproduced with permission from ref 12. Copyright 2013 Elsevier.

The resultant solid acids had abundant nanopores, strong acidity, good hydrophobicity, and controllable wettability for various reactants, which greatly increased the degree of exposure of acidic sites and decreased the diffusion resistance of various reactants in catalytic reactions (Figure 24). These characteristics resulted in solid acid catalysts with even higher activities than those of their homogeneous counterparts in the

transesterification of oil with methanol into biodiesels (Table 2). Moreover, sulfonic groups and acidic ionic liquid moieties can be simultaneously grafted onto the networks of mesoporous PDVB. This was done by treating mesoporous PDVB-vinylimidazole copolymers with 1,3-propanesulfonate and then ion exchanging with a strong acid (e.g., HSO3CF3). The resultant functionalized mesoporous polymers can be used as highly efficient solid acid catalysts for biodiesel production and depolymerization of crystalline cellulose into glucose, cellobiose, and HMF. Their activities were even higher than those of mineral acids such as HCl and H2SO4. The excellent activities of mesoporous PDVB-based solid acids can be attributed to their unique structural characteristics, including abundant strong acidic centers, controllable surface wettability, large BET surface areas, and large pore widths. All these features resulted in their enhanced compatibility with various reactants, surface polarity for destroying crystallinity of cellulose and significantly improved the accessibility of their acidic sites (Figures 24 and 25 and Tables 2 and 3).19,40 In addition, PDVB-based solid acids also show good reusability in biomass conversions. The obvious decreasing of activity in PDVBSO3H-[C3vim][SO3CF3] could not be observed after five times recycling in the presence of abundant water (Table 3), which indicates its excellent water tolerant property. Good water-

Figure 24. Synthesis of PDVB-[C1vim][SO3CF3] from PDVB-vim (A); Contact angles of water droplet on the surface of PDVB-vim (B) and PDVB-[C1vim][SO3CF3] (C); Contact angles of droplet of methanol (D) and tripalmitin (E) on the surface of PDVB[C1vim][SO3CF3]. Reproduced with permission from ref 40. Copyright 2012 American Chemical Society. 385


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run

catalyst

yield of methyl palmitate (%)

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

PDVB-[C1vim][SO3CF3] PDVB-[C3vim][SO3CF3] PDVB-[C4vim][SO3CF3] PDVB-[C3vpy][ SO3CF3] PDVB-[C4vpy][SO3CF3] PDVB-[C3vpr][SO3CF3] [C1vim][SO3CF3] SBA-15-[C1vim][SO3CF3] Amberlyst 15 PDVB-[C1vim][SO3CF3] PDVB-[C3vim][SO3CF3] Amberlyst 15

96.9 >99.9 97.4 99.5 92.9 99.3 89.1 24.1 28.6 93.5 98.1 20.2

Table 3. Yield of Sugars and Dehydration Products in the Depolymerization of Avicel Catalyzed by Various Solid Acids and Mineral Acids catalysts

glucose yield (%)a

cellobiose yield (%)a

TRS (%)b

Amberlyst 15 HCl [C3vim][SO3CF3]c H2SO4 PDVB-SO3H PDVB-SO3H-[C3vim][SO3CF3] PDVB-SO3H-[C3vim][SO3CF3]d PDVB-[C3vim][SO3CF3] PDVB-SO3H-[C3vim][SO4H] PDVB-SO3H-[C3vim][Cl]

25.1 63.4 66.2 59.6 56.8 77.0 72.7 75.9 76.8 74.1

14.8 13.4 12.2 9.8 10.2 8.2 5.9 6.8 5.6 6.4

56.2 94.1 93.8 86.8 82.6 99.6 94.3 98.1 98.5 96.3

a

Measured by high-performance liquid chromatography (HPLC) method; the reaction time was 5 h. bTotal reducing sugar (TRS) was measured by DNS method. cThe same number of acidic sites as in PDVB-SO3H-[C3vim][SO3CF3]. dThe catalyst has been recycled for five times.

tolerant property results from its unique hydrophobic PDVB networks. On the basis of the synthesis of solid acid catalysts with suitable hydrophobicity, the including suitable hydrophobic solid acids and bases to catalyze biomass transformed reactions have also been investigated recently. For example, Wang et al. reported the synergic actions from the solid acid and base catalysts for transformation of crystalline cellulose into valuable

a

0.05 g of catalyst, 1.04 mmol of tripalmitin, 92.7 mmol of methanol, reaction temperature: 65 °C, reaction time: 16 h. bThe same number of active sites as on PDVB-[C1vim][SO3CF3]. cRecycled for 5 times. Reproduced with permission from ref 40. Copyright 2012 American Chemical Society.

Figure 25. Upper (A,B): scanning electron microscope images of PDVB-SO3H-[C3vim][SO3CF3]; lower: their catalytic applications in the depolymerization of crystalline cellulose. Reproduced with permission from ref 19. Copyright 2013 Royal Chemical Society. 386


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ACS Catalysis fine chemicals such as 5-hydroxymehylfurural (HMF).34 In this process, they demonstrated a one-pot process to directly transform glucose into HMF by an integrated catalytic system that included both PDVB-based solid acids and solid bases (Figure 26). The excellent activities and selectivity found in this

hydrophobic solid acids with enhanced acidity and controllable nanoscale morphology. To further develop new kinds of polymer-based solid acids with controllable surface wettability, Yang and co-workers synthesized novel polystyrene sulfonic acid resins with controllable core−shell structures via macromolecular self-assembly within a confined nanospace.20,133 The resultant solid acids show enhanced acidity because of the formation of enhanced H-bond interactions between adjacent acidic sites, which induced a cooperative effect to draw −SO3H groups closer (Figures 27 and 28). The enhanced acidity, unique nanostructure and suitable hydrophobic networks largely decreased the activation energy and increased the degree of exposure of the acidic sites in various acid-catalyzed reactions. These factors determined their much improved catalytic activity and good reusability in esterification, Friedel− Crafts alkylation and Cumene hydroperoxide (CHP) cleavage relative to many traditional solid acid catalysts. Notably, the activities of the synthesized PS-SO3H@mesosilicas DSNs were much higher than commercial acidic resins of Amberlyst 15 and Nafion NR50, which were even higher than that of H2SO4 in esterification. Meanwhile, PS-SO3H@mesosilicas DSNs showed very high TOF values up to 27.9−611.5 h−1, much higher than those of Amberlyst 15 (2.4−189.4 h−1), Nafion NR50 (14.7 h−1) and H2SO4 (31.9 h−1). The high TOF values of PSSO3H@mesosilicas DSNs were resulted from their unique nanostructures and superior hydrophobic networks, which largely contribute the exposure degree and stability of acidic sites. The mentioned characteristics result in their very high efficiency of acid sites in these reactions.

Figure 26. Water contact-angle and catalytic performance of various catalysts. (A) Photographs and contact angles of water droplets on the surface of PSO3H-154 and Amberlyst-15; (B) Hydration of HMF over various catalysts with different water contact angles in DMSO (□) and DMSO-water solvents (△). Reaction conditions: 1 mmol of HMF, 5 g of DMSO (or DMSO−water with weight ratio at 9:1), 50 mg of solid catalyst, 1008C for 5 h; (C) Time dependence in catalytic conversion of fructose over P-SO3H-154 catalyst (yield of HMF: □, yield of LA: □) and over Amberlyst-15 catalyst (yield of HMF: △,yield of LA: △). Reaction conditions: 100 mg of fructose, 50 mg of solid catalyst, 5 g of THF−DMSO solvent (weight ratio at 1.5), 100 °C. Reproduced with permission from ref 34. Copyright 2014 John Wiley & Sons, Inc.

3. CONCLUSIONS AND OUTLOOK Solid acids are suitable candidates to replace mineral acids in catalyzing the production of various useful chemicals to advance green and sustainable chemistry. They have potential for important industrial applications. Hydrophobicity plays a crucial role in the catalytic activity of solid acid catalysts because water usually is a byproduct of reactions such as esterification and condensation and has negative effects on the equilibrium in these reactions. Moreover, the presence of water usually results in the deactivation, poisoning, and leaching of acidic sites in solid acids, further limiting their reusability in various acidcatalyzed liquid reactions. Therefore, the development of highly hydrophobic solid acids is essential to enable the adsorption of oleophilic reactants and desorption of hydrophilic reactants, which make them to be used as efficient, long-lived catalysts in various acid-catalyzed reactions. In principle, solid acids usually show unique hydrophilicity because of the hydrophilic characteristics of the various acidic sites in nature. This Review presents recent work focused on the design and synthesis of solid acids with controllable acidity and suitable hydrophobicity and research in their application in various acid-catalyzed liquid reactions. Solid acids with suitable hydrophobicity include zeolites, sulfonated metal oxides, acid group−functionalized ordered mesoporous silicas, nanoporous resins, polymer−silica composites, amorphous carbon, graphene, carbon nanotubes, mesoporous PDVB, and polystyrene-based solid acids with unique yolk−shell nanostructures. Different frameworks and nanostructures result in samples with various degrees of hydrophobicity. For these solid acids, the frameworks and structures play key roles in their hydrophobicity,20,35,40 and each type of hydrophobic solid acid has different characteristics. Inorganic solid acids show good thermal stability and controllable and

system was attributed to the cooperative effects of suitable hydrophobic solid acids and hydrophilic solid bases. In the isomerization of glucose, PDVB solid bases showed good wettability for glucose, and the product of fructose could also be quickly transformed into HMF catalyzed by PDVB solid acids. Moreover the HMF product in this reaction could be quickly dispersed into organic phase, which could not be transformed into other products by water in organic phase. More importantly, the solid acids and bases could be easily separated from each other based on their quite different water wettability after the catalytic reactions. This work may open up a cost-effective way to prepare solid acid catalysts with controllable wettability and suitable hydrophobicity, which show high activities and selectivity in various acid-catalyzed reactions. 2.8. Yolk−Shell Polymer-Based Solid Acids. Nafion resins (e.g., Nafion NR50 and Nafion NR117) are a well-known class of commercially available acidic resins. They have large numbers of sulfonic groups and fluorine atoms on their networks. These structural characteristics make their networks with suitable hydrophobicity and enhanced acidity, endowing Nafion resins with excellent activity in various acid-catalyzed reactions.131,132 However, it remains a challenge to synthesize 387


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Figure 27. Synthesis and characterization of PS-SO3H@mesosilicas DSNs: general procedure for the synthesis of PS-SO3H@mesosilicas DSNs. (a) Schematic illustration showing the general procedure for the synthesis of PS-SO3H@mesosilicas DSNs. (b) Transmission electron microscope (TEM) image of PS template. Scale bar, 100 nm. (c) TEM image of PS@mesosilicas YSNs. Scale bar, 200 nm. (d) TEM image of PS-SO3H@ mesosilicas DSNs. Scale bar, 200 nm. Reproduced with permission from ref 20. Copyright 2014 Nature Publishing Group.

Figure 28. Left: sulfur contents (a), acidic exchange capacities (b) and molar percentage of sulfur to acidic exchange capacity (c) of the samples prepared at different sulphonation time; Right: 31P MAS NMR spectra of PS-SO3H@mesosilicas DSNs of adsorbed TEPO probe before (a) and after (b) DMF treatment, and Amberlyst-15 (c). Reproduced with permission from ref 20. Copyright 2014 Nature Publishing Group.

intricate nanostructures; however, their low concentrations of acidic sites and low hydrophobicity constrain their catalytic activity and reusability. Carbonaceous solid acids show suitable hydrophobicity and good hydrothermal stability, which enhance the compatibility between reactants and carbonaceous solid acids and improve their reusability. However, their concentrations of acidic sites and surface hydrophobicity need improvement. The synthesis of solid acids with high concentrations of acidic sites, enhanced hydrophobicity, and controllable wettability for various reactants remains a challenge. Much effort has been focused on the development of porous organic polymer solid acids, such as polystyrenebased networks, because of their unique hydrophobicity. The hydrophobicity of the resultant solid acids can also be controlled by engineering their networks and structures. Moreover, acidic sites and acid strength can easily be controlled in polymeric solid acids. However, the research points to the

need for research to improve the thermal stability and adjust the surface wettability and number of acidic sites of nanoporous polymeric solid acids. These characteristics strongly influence catalytic performance and reusability in various acid-catalyzed reactions. We emphasize that the hydrophobicity of solid acids is fundamental to their catalytic application in acid-catalyzed liquid reactions. This Review briefly summarizes recent studies of the synthesis of solid acids with suitable hydrophobicity and excellent catalytic activity, which might be favorable for the rational design of highly active and reusable hydrophobic solid acids in the future. For the wide application of hydrophobic solid acids on an industrial scale, the following issues need to be addressed by scientists: (1) control of the wettability of solid acids for both reactants and products in acid-catalyzed reactions to enhance the fast adsorption of reactants and desorption of products and 388


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promote good catalytic activity and reusability in various reactions; (2) increasing the hydrophobicity of solid acids, which usually is done by using hydrophobic organo-groups or designing unique nanostructures in the samples; (3) improving the reusability of hydrophobic solid acids by improving their thermal and hydrothermal stabilities, which may be achieved by increasing the degree of cross-linking in their networks or introducing stable units; (4) increasing the acid concentrations of hydrophobic solid acids without sacrificing their hydrophobicity, which could greatly increase their catalytic activity; (5) developing porous hydrophobic solid acids with various Lewis acidic centers and heterogeneous Lewis acids. Apparently, the practical application of hydrophobic porous solid acids in heterogeneous catalysis on an industrial scale is a great challenge that requires considerable further research.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for A.Z.: [email protected]. *E-mail for F-S.X.: [email protected]. *E-mail for S.D.: [email protected]. ORCID

Fujian Liu: 0000-0002-6694-582X Anmin Zheng: 0000-0001-7115-6510 Feng-Shou Xiao: 0000-0001-9744-3067 Sheng Dai: 0000-0002-8046-3931 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21573150, 21203122, 21522310, 91645112, 21403192), Natural Science Foundation of Zhejiang Province (LY15B030002),and Key Research Program of Frontier Sciences, CAS (Grant QYZDB-SSW-SLH026). S.D. was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, & Biosciences Division.

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Hydrophobic Solid Acids and Their Catalytic Applications in Green

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