Isomerization and Alkylation of Biomass-Derived Compounds in

Review pubs.acs.org/IECR

Isomerization and Alkylation of Biomass-Derived Compounds in Aqueous Media over Hydrophobic Solid Acid Catalysts: A Mini Review Oki Muraza*,†,‡ and Ahmad Galadima† †

Center of Research Excellence in Nanotechnology, ‡Chemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia ABSTRACT: Isomerization and alkylation of biomass-derived compounds are key industrial processes for mitigating over dependence on fossil fuels and petrochemicals. Although designing the reactions in aqueous media is environmentally sustainable, cost-effective, and efficient, the most reliable catalysts are still being sought. We have therefore reviewed and analyzed classified literature on the prospects of hydrophobic zeolites, acidic oxides, and heteropoly acid catalysts. Emphasis was given to catalyst design, key active sites, modification, catalytic activity in related reactions, and stability properties.

1. INTRODUCTION Biomass, solid or liquid, is a carbon and oxygen rich mixture, which upon conversion via thermochemical processing can produce a range of compounds, with potentials as fuels or petrochemicals for the industry. Compounds such as diesel and gasoline range alkanes, alcohols, aromatics, and organic acids have been successfully derived from abundant biomass materials (Figure 1) that can be classified into primary,

therefore very attractive. The materials, if appropriately designed, are highly active, selective, and cost-effective. They can be prepared as bifunctional composites, providing an opportunity for having several active sites in a single catalyst system.6 This allows the chance to design one-pot system, which reduces number of unit operations and maximize process efficiency. Recently, Hara reported solid acid catalysts based on amorphous carbon as good candidates for esterification and transesterification of vegetable oils, due to associated Brønsted acidity.7 They were also tolerant to water in the reaction feed. Hara also shows similar systems as good catalysts for the saccharification of cellulose. Shimizu and Satsuma8 reported zeolites, mixed oxides, and silica-based solid acid catalysts as excellent materials for alkyl- and aromatic alcohols acetylation, a key organic process that allows for a cheaper and highly efficient option of OH groups protection during reactions such as peptide coupling and oxidation.8 Zhang et al. showed a SiO2/ TiO2−SO42− catalyst to be 20 times more effective for esterification of bio-oil than a basic K2CO3/Al2O3−NaOH catalyst.9 It was also good for simultaneous acetal formation from alcohols via carbonyl addition. We have reported herein, a critical but mini-review on the upgrading of biomass-derived compounds over water tolerance solid acid catalysts in the aqueous media. The paper emphasizes isomerization and alkylation as model upgrading processes. Numerous solid acid catalysts that can be classified into four groups, depending on their catalytic compositions and previous history (Figure 2), are available today for utilization at wide range of industrial scales, due to their excellent stability and acidity-structure properties. However, emphasis is given to the hydrophobic zeolites, heteropoly acids, and oxide systems with great potential for biomass-derived compound alkylation and/ or isomerization. These catalysts have been evaluated in other aqueous and refining reactions yielding positive results.

Figure 1. Abundant sources of global biomass feedstock.

secondary, and tertiary sources (i.e., biomass from agricultural and related activities, processing, and waste materials) and are also useful for direct energy services.1−3 For example, dimethylfurfural, an important chemical for liquid fuels, can be derived from carbohydrates to very high yield.4 Biomass conversion processes such as hydrolysis, isomerization, and dehydration can be achieved in aqueous media over watertolerant solid acid and/or base catalysts.5 Aqueous phase reactions are particularly of great interest due to abundant of water reserves, negligible environmental impact, and the chance to prepare high biomass concentrations. Among the catalysts systems for biomass conversion and upgrading, solid acids are by far given good considerations. One major challenge affecting the industry is the catalyst preparation and separation problems. Solid acids such as zeolites, oxides, and heteropoly acids are associated with ease of separation and © 2014 American Chemical Society

Received: Revised: Accepted: Published: 17869

August 19, 2014 September 25, 2014 October 2, 2014 October 2, 2014 dx.doi.org/10.1021/ie503310p | Ind. Eng. Chem. Res. 2014, 53, 17869−17877

Industrial & Engineering Chemistry Research

Review

hydrophobic heterogeneous catalysts is more likely to handle the situation. Catalyst topology and structure−acidity properties are other important factors that should be considered. Corma et al.11 showed channel dimensions to influence the activities of SSZ-33 and ITQ-22 zeolites during ethanol alkylation of benzene. Results of the gas phase reaction showed ITQ-22 and 10-membered ring ZSM-5 to yield similar selectivity to ethylbenzene, whereas SSZ-33 and the 12membered ring ZSM-5 showed similarity with Beta zeolite.11 Reactions over ITQ-22 were also unique due to densely populated Brønsted acid sites located at the 10 membered ring channels. Reports from other authors indicated similar observations for ethanol or ethylene as alkylation substrate.12−15 Xu et al.16 carried out similar studies with propylene and phenol as reactants. Both primary and secondary alkylation reactions occur at the external surfaces of the zeolites (Beta, mordenite, and chabazite). Ortho- and para-isopropylphenols were the main ones primarily formed over all catalysts.16 However, Beta zeolites produced highest selectivity to dialkylated phenols due to an established diffusion and steric balance. The results indicated that the catalysts could be employed for successful upgrading of biofuel derived from lignin feed. According to Zhu et al.,17 catalyst acidity could be more critical for xylene production from methanol and toluene alkylation. Acid sites of moderate strength are more favorable for reactions over zeolites such as ZSM-5 (MFI), ZSM-22 (TON) and MOR, whereas strong acid sites for SAPO-34 (CHA).17 The SAPO-34 catalyst was very selective to gaseous hydrocarbons because of its smaller channels. Here, reactions over 8-membered ring systems were unsuccessful, producing mainly longer chain hydrocarbons from methanol. This was similarly noticed with strongly acidic zeolites. Moderate acidity enhances catalyst stability and favors desired products selectivity.18 On the other hand, large zeolite pores are prone to coke deposition due to the formation of higher aromatics, with restricted diffusion difficulties.19 Biomass conversion is known for the production of higher aromatics such as naphthalene and long chains olefins under controlled conditions.20,21 These compounds could similarly be upgraded over solid acid catalysts via alkylation. Their derivatives are good candidates for used as surfactant in enhanced oil recovery. They are also applicable in dyeing and spinning industries. Guo et al.22 reported modification of zeolites H−Y and H-Beta with La3+ and cations of group IIA

Figure 2. Different categories of solid acid catalysts with potentials for industrial applications.

Therefore, their evaluation for the upgrading of biomass based materials is very relevant. The paper provides critical information on the key characteristics of these materials that will impact their choice for biomass-based alkylation and isomerization in the aqueous media. The reaction system covers both biomass-derived compounds and aqueous medium; therefore, solid acid catalysts, which mostly are hydrophilic, need to be modified to hydrophobic surface to some extent. The design of watertolerant solid acid catalysts is also dependent on the reaction of interest, substrate nature, reaction medium, reaction conditions, and other factors.

2. ALKYLATION IN AQUEOUS MEDIA Alkylation reaction is particularly important for designing new organic compounds, with potentials as fuels or petrochemicals. In the petroleum industry, the process involves reacting lower olefins, such as propene or n- and i-butenes, in the presence of catalyst, to generate alkylates with higher molecular weights and superior fuel properties. Isomeric paraffins with fundamentally high octane numbers are normally produced. The process can be extended for upgrading biomass-derived n- and iso-alkenes, aromatics, or even oxygenated compounds with wide range applications.10 The initial aqueous alkylation catalysts (i.e., hydrochloric and hydrofluoric acids) are known for serious corrosion, recycling, and disposal difficulties. Friedel−Craft Lewis acid catalysts such as AlCl3, FeCl3, and their analogues are associated with similar problems. Therefore, a shift to

Table 1. Behaviors of Some Solid Acid Catalysts in Alkylation Reactions catalyst H-ZSM-5 H-ZSM-5 H-Beta zeolites H-Beta zeolites SO42− modified TiO2, ZrO2, SnO2 MCM-22, ITQ-2 USY organosilane modified USY

alkylated products selectivity (%)

other products selectivity (%)

ref.

93.9 28.3−83.6

6.1 16.4−72.7

23 24

10.9−58.8

41.2−89.1

25

15−73

27−85

28

500 to 600 °C, 1 atm, 2 h−1

11−67

33−89

29

523 523 200 200

89.8 82 52 70

10.1 18 48 30

26 26 27 27

reactants toluene and ethanol ethyltoluene and ethanol isobutane and trans-2butene isobutane and trans-2butene isobutane and trans-2butene propene and bipheyl propene and bipheyl bio-oil in water bio-oil in water

conditions 673 K, 1 atm, toluene/ethanol = 4.0 250 to 450 °C, 1 atm, reactants ratio = 1−5.0 50 °C, 1 atm, reactant ratios = 1.0 323 K, 2.5 MPa, Reactants ratio = 15. 1, 1−8 h

K, 1 atm, reactant ratio = 4 mol/mol K, 1 atm, reactant ratio = 4 mol/mol °C, 0.5 g catalyst, 700 psig °C, 0.5 g catalyst, 700 psig

17870

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dx.doi.org/10.1021/ie503310p | Ind. Eng. Chem. Res. 2014, 53, 17869−17877


Review

acidic oxide catalysts also behave through the participation of the Lewis sites. For example, Nakajima et al.33 showed aqueous reactions over hydrated Nb2O5 to proceed consistently with Lewis acid sites density. Brønsted acid sites catalyzed dehydration of generated isomers into other compounds, consistent with Figure 3.33 Moliner et al.34 found that the large pore system of Beta zeolite was very beneficial during similar reaction. The reaction yields 40% of glucose isomers; they were not successful when carried over medium-pore systems.34 Similarly, a mesoporous MCM-41 catalyst produced very low concentration of fructose. Studies by Lew et al.35 reported glucose and xylose isomerization into fructose and xylulose over Sn modified Beta and MFI zeolites, to depend on pore structure and surface area. Modified Beta system formed the best catalyst, yielding 34 and 24% of fructose and xylulose, respectively, compared to 4 and 19% obtained over modified MFI.35 The respective surface areas of the two catalysts were 560 and 474 m2 g−1 and correlate linearly with their activities. The latter catalyst was more active for dihydroxyacetone isomerization to lactic acid, particularly due to lower substrate size. Recently, some authors showed the presence of primary alcohol, such as methanol, in the aqueous reaction media shifts the reaction mechanism over hydrophobic H−Y, H-USY, HZSM-5, and H-Beta zeolites at low temperatures.36 Formation of intermediate alkyl fructosides favors the isomerization process (with glucose for example, step 1 in Figure 4), with additional advantage of direct conversion into other useful industrial biochemicals. However, the zeolite catalyst must be dense in both Brønsted and Lewis acid sites. Under this conditions, the second role of water is the conversion of these intermediates into the isomerized sugar (step 2). Therefore, the degree of alkylation versus isomerization is also dependent on the water/alcohol ratio. In Table 2, a further summary of the activities of some solid acids during biomass-derived compounds isomerization in the aqueous phase was presented. Both the zeolites and oxide catalysts showed good activities for glucose, xylose and dihydroxyacetone isomerization at low temperatures between 70 and 160 °C.37−41 Mesoporous silica supported heteropoly acids were similarly active. The low catalyst loadings were stable for variable times ranging between 1 and 24 h. In addition to the isomerized products, the reactions also produced furfural, mannose, and organic acids of industrial significance.

elements in the periodic table to have positive outcomes for this reaction. Naphthalene alkylation with C11 and C12 olefins produced mainly monoalkylnaphthalenes at low temperatures.22 The catalysts were very stable, given up to 90% conversion and 100% selectivity. Table 1 reports an additional summary of the behavior of some solid acids. The various studies indicated solid acid catalysts to have good potentials for alkylation of compounds derived from biomass. They have no recorded corrosion difficulties and can be regenerated and reused, given optimal activity. Alkylation reactions over these system is reactant-, acidity-, channel-, and texture-dependent.23−29 Their water tolerance implies that high feed can be processed in aqueous media without difficulties. Oxide systems formed good catalysts at low water concentrations and became poisoned at high concentrations. An interesting issue is that, even though the catalysts are hydrophobic in nature, the acid sites can interact successfullly with the hydrophobic rectants without acidity, structure, or activity decay.

3. ISOMERIZATION IN AQUEOUS MEDIA Here, we view isomerization from two perspectives. One option involves the conversion of linear paraffins from biomass derived feeds into high grade gasoline and diesel range iso-paraffins. This process does not normally take place in the aqueous phase and is a special case in modern refineries. The aqueous phase reaction is one that mainly isomerized compounds such as sugars, alcohols, or organic acids derived from biomass into more useful biochemicals. Choudhary et al.30 used Sn modified Beta zeolite to isomerized a biomass-derived xylose into xylulose at 100 °C. They achieved 60% conversion and 27% yield. The reaction was also demonstrated to form lyxose as a secondary product to 11% yield, during the intermediate stages (Figure 3). Brønsted acid sites enhancement with HCl or

4. DESIGNING HYDROPHOBIC SOLID ACID CATALYSTS The choice and design of suitable water-tolerant solid acid catalysts for biomass-based conversions are much dependent on the reaction of interest (i.e., alkylation, isomerization, decarboxylation, and/or deoxygenation), substrate nature, reaction medium (aqueous or mixed-aqueous), reaction conditions, and the characteristics of the anticipated reaction products. For example, zeolites that are rich in Brønsted acidity are more appropriate for alkylation reactions whereas those with dense Lewis sites are good candidates for isomerization of biomass-derived sugars. Unlike hydrophilic solid acids, which are covered by water in aqueous media, thereby preventing them from interaction with organic materials, the hydrophobic materials are very active in water. Zeolite catalysts are designed to possess high silica/alumina (Si/Al) ratio. Decreasing the aluminum content reduces the hydrophilicity of the zeolitic catalyst and favors its water

Figure 3. Typical isomerization route for biomass derived sugars (xylose as an example) over a water tolerance zeolite, the mechanism also shows a chance to further upgrade to furfural. Figure reproduced with permission from ref 30. Copyright 2011, American Chemical Society.

Amberlyst-15 further converts the primary product into furfural, another important raw material for the chemical industry.30 Although the actual mechanism is still debatable, water shifts the reaction to proceed by intramolecular hydride shift, involving Lewis acid participation, a deviation from the proton transfer mechanism. Evidence of this mechanism was also recently reported by similar authors employing Brønsted acid modified CrCl3 at 145 °C.31 In a related development, glucose isomerization to fructose was achieved over Sn modified Beta zeolite catalyst via the same mechanism.32 Evidences from 1H and 13C NMR studies showed the −Sn− O−Si− sites to form the main reaction centers. Water-tolerant 17871



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Figure 4. Effect of alcohol presence in aqueous media during glucose isomerization to fructose. Figure reproduced with permission from ref 36. Copyright 2013, American Chemical Society.

Table 2. Behaviors of Some Solid Acids during Aqueous Biomass-Derived Compounds Upgrading biomass-derived compound

catalyst

glucose

Sn-Beta zeolite

dihydroxyacetone

Sn-Beta zeolite

xylose

Sn-Beta zeolite

xylose

SO42−/ZrO2

xylose

Al2O3 + SO42−/ZrO2

xylose

Al2O3 + SO42−/ZrO2 + MCM-41 mesoporous SiO2 supported H3PW12O40 H−Y zeolite

xylose glucose dihydroxyacetone, glyceraldehyde (trios sugars)

Sn-Beta

reaction conditions

conversions (%)

ref.

fructose (36%), mannose (9%), byproducts (55%) glyceraldehyde (95%), methyl lactate (5%) lyxose and xylulose (40%), byproducts (60%) xylulose to furfural (43%)

60

37

20

37

40

37

86

39

xylulose to furfural (39%)

95

39


50

39


68

38

fructose, formic acid, 4oxopentanoic acid, etc. lactic acid

100

40

95

41

reaction products

90 °C, 0.1 g cat., 10 wt % glucose, 120 min 70 °C, 0.08 g cat., 4.0 g methanol, 60 min 1000 °C, 0.178 g cat., 10 wt % xylose, 120 min 0.3/0.7 mL water/toluene, 0.03 g xylose, 0.02 g cat., 4 h 0.3/0.7 mL water/toluene, 0.03 g xylose, 0.02 g cat., 4 h 0.3/0.7 mL water/toluene, 0.03 g xylose, 0.02 g cat., 4 h 0.03 g xylose, 0.03 g cat., 4 h 24 h, 12 wt % glucose, 110 to 160 °C, 24 h 125 °C, 80 mg cat., 24 h, water

introduction of transition metal species such as Sn, Zr, or V into the zeolite frameworks, replacing some Si ions. In such circumstances, new water tolerance Lewis acid sites are generated, and therefore, the catalysts are good candidates for Lewis sites dependent reactions. Moliner44 has documented a substantial amount of literature on the introduction of such species. Sn and Ti-modified Beta and MFI zeolites were the earlier developed systems in alkaline media. The large pore systems and framework dimensions permitted the incorporation of metal species, thus enhancing hydrophobicity. The effect was more pronounced with Sn than Ti, particularly due to electronegativity effects.44 One important issue to note is that the metal particles must be well dispersed over the zeolite material to ensure optimal Lewis acidity/hydrophobicity balance. The hydrophobicity of most heteropoly acids is derived by partially replacing some of the protons with metals such as Cesium. This metal effectively increases the stability of these materials in aqueous media and modify their acidity properties. Kimura et al.45 showed a CS2.5H0.5PW12O40 derived from H3PW12O40 to have higher water tolerance than the parent material. It was particularly more active and stable, during aqueous hydrolysis of a range of esters, than other hydrophobic solid acids (Nb2O5, silica−alumina, SO42− modified zirconia, and some zeolites) evaluated under similar conditions.45 Introduction of Cs reduces acidity from 1.0 to 0.15 mmol/g

tolerance. One major problem with this is the possession of low to moderate number of acid sites due to the dealumination process. Namba et al.42 developed H-ZSM-5 and H−Y zeolites of various Si/Al ratios ranging between 3.1 and 47 using the regular conventional ion exchange method. They were evaluated for the aqueous hydrolysis of 5% ethyl acetate at 60 °C. It was observed that the H-ZSM-5 material with Si/Al ratio of 47 formed the best catalyst, yielding higher activity than all other catalysts with lower Si/Al ratios.42 Increasing the Si/Al ratio has a positive effect on hydrophobicity on the catalyst surface and, consequently, higher affinity for the ethyl acetate adsorption. Kono et al.43 synthesized various H-ZSM series (i.e., H-ZSM-5, H-ZSM-11, H-ZSM-12, and H-ZSM-35) and mordenite catalysts of Si/Al ratios between 10 and 45. All the catalysts were prepared via ion exchange and studied as cyclohexene hydration materials in the liquid phase. The results showed cyclohexane conversion and cyclohexanol selectivity to increase with increase in Si/Al ratio. The catalysts were also very stable for up to 72 h.43 However, the 10-membered ring H-ZSM-5 to H-ZSM-35 catalysts formed the best systems, given up to 100% selectivity to cyclohexanol. Hydrophobicity here favors the adsorption of both cyclohexanol and cyclohexene. The multidimensional mordenite zeolite produces 74% optimal cyclohexanol selectivity; therefore, the reaction was also shape dependent. Recently, authors have shown the activity of hydrophobic zeolites to be enhanced by the 17872



Review

Table 3. Some Hydrophobic Zeolites and Heteropoly Acids Applied for Aqueous Reactions catalyst (s)

reactant (s)

conditions

MFI, Beta, FAU, and MOR: Si/Al from 10 to 200

α-pinene

hydration; 3.3 mmol α-pinene in 24 mL water and 1,4dioxane, 343 K

Cs2.5H0.5PW12O40, amberlyst-15 H-Mordenite (MOR): Si/Al from 6.9 to 100. H−Y, H-Beta: Si/Al from 12.9 to 34.6 Ti-modified MFI: Si/Al of 8.0 and 6.2 H3PMo12O40, H3PW12O40

α-pinene furfuryl alcohol

hydration; 3.3 mmol α-pinene in 24 mL water and 1,4dioxane, 343 K 6 mL formalin, 0.025 g cat., 338 K, 30 min

anisole

150 mL anisole, 70 °C, 0.5 g cat.

styrene, 1-hexene amines, aldehydes and ketones acetic anhydride, anisole

epoxidation; 0.35 g cat., 30 wt % H2O2, 60 °C

H3PMo12O40, H3PW12O40, H4SiW12O40, CS2.5H0.5PW12O40

Mannich reaction; 5 mL water, 18 h, 0.01−0.02 g cat. acylation; 0.01 mol anisole, 90−110 °C

activity

ref.

up to 100% conversion and selectivity; activity generally increased with hydrophobicity 49−56% conversion and 23−57% selectivity

48 48

acidity decreases, hydroxymelation increases with Si/Al ratio; up to 220 ks−1 TOF best activity (45%) with H-Beta (Si/Al 34.6)

49

higher hydrophobicity with Ti-MFI (Si/Al 8.0) most active good stability and yield (63 to 94%); H3PMo12O40 most active

51

very active, up to 100% yield

76

50

52

Figure 5. (a) Effect of F− content on the specific surface area of an MFI zeolite and (b) effect of synthesis time on the specific surface area an MFI zeolite. The origin indicates zero concentration of F− ions (i.e., MFI only). Figure reproduced with permission from ref 55. Copyright 2008, American Chemical Society.

than all other catalysts.46 Okuhara et al.47 also evaluated H3PW12O40, CS2HPW12O40, and CS2.5H0.5PW12O40 as catalyst for aqueous alkylation of cyclohexene with di- and trimethylbenzenes. Catalyst stability and activity properties were dependent on Cs ratio. The two acidic salts were generally more effective than H3PW12O40. Similarly, CS2.5H0.5PW12O40

but produces 3-fold activity that was also constant for repeated cycles. Okuhara et al.46 prepared the same series of catalyst systems and evaluated their activity in dimethylbutene hydration and n-butanol esterification with acrylic acid at 80 °C. In a similar trend, the acidic Cs salt (CS2.5H0.5PW12O40) demonstrated superior activity and efficient aqueous stability 17873



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Figure 6. (a) Effect of F− content on the crystallinity of an MFI zeolite and (b) effect of synthesis time on crystallinity of an MFI zeolite. The origin indicates zero concentration of F− ions (i.e., MFI only). Figure reproduced with permission from ref 55. Copyright 2008, American Chemical Society.

zeolite pores thereby repelling the OH species from blocking the zeolite pores. There are many other advantages of the fluoride synthetic routes. Moliner44 reported the presence of fluoride ions in the frameworks to permits the incorporation of important species such as Sn, Ti, Ta, Zr, etc., with the view of enhancing the Lewis acid density of the zeolites that are beneficial for many aqueous reactions such as isomerization of sugars. Arichi and Louis55 prepared a number of MFI zeolites in the fluoride medium using ammonium fluoride as the fluoride source, by batch crystallization method. Their characterization data, showed parameters such as the extent of zeolite crystallinity and specific surface area to increase with an increase in the concentration of F− in the zeolite frameworks and synthesis time (Figures 5 and 6).55 The availability of F− also decreased the number of Brønsted acid sites and favored high activity and selectivity during benzoyl chloride conversion to 4-methylbenzophenone. Kim et al.56 showed the fluoride route combined with seeding to decreased the synthesis time and enhanced catalyst crystallinity during Beta zeolite preparation via microwave irradiation method.56 The fluoride medium was also reported to permit the preparation of Al-free Beta zeolites with subsequent Ti modification.57,58 There were also positive effects on the resulting zeolite crystallinity, hydrophobic properties, and thermal stability, with very high activity and selectivity to oxidation reactions. Similar properties were also observed with zeolites such as MFI, TON, and MTT systems.59−61 Recently, an aggregate of MCM-22 zeolite was synthesized in the fluoride medium, using hydrothermal preparation method.62 Hierarchical materials that are rich in both macro- and mesopores with inherent microporosity were obtained. The presence of F− ions increases the number of Brønsted acid sites. The resulting catalysts were very active to methane dehydroaromatization reactions, due to the formation of new coke inhibition active centers. Benzene selectivity and catalyst lifetime significantly increased with fluoridation. 4.2. Surface Modification Using Silanes. Another important alternative for improving zeolites hydrophobicity and reducing hydrophilicity is by modification with silanes, a process called silylation. Hydrophilicity in zeolites creates a

formed the best catalyst due to the larger number of acidic sites available at the surface.47 Table 3 presents the characteristics of some developed hydrophobic zeolites and heteropoly acids in reactions such as acylation, hydroxymelation, hydration, epoxidation, and Mannich reactions. An interesting fact is that increasing Si/Al for the zeolites favors hydrophobicity and catalytic activity increases with hydrophobicity.48−53 Yields as high as 100% could be achieved with some materials. These superior properties indicated their potentials as aqueous alkylation and isomerization catalysts. The Cs salts of heteropoly acids are good water-tolerant materials, with high activity and selectivity in biomass reactions, compared with some of the metal free systems (that can form homogeneous mixtures) (Table 3). 4.1. Zeolite Synthesized in Fluoride Routes. Synthesis of hydrophobic zeolites via the fluoride routes is a recently developed alternative for achieving optimal catalytic properties. The process involved the replacement of OH− anions in the zeolite frameworks by F− ions.54 Zeolites prepared in the fluoride medium possess significantly higher water tolerance than their counterparts synthesized in alkaline medium. This has also been beneficial to catalytic activity and products selectivity in the aqueous phase. Fluoride synthetic route proceeds with the incorporation of F− ions into the synthesis mixture, usually by the addition of fluoride source such as hydrofluoric acid solution. The pH of the solution must be maintained at nearly neutral (i.e., 8 to 9.5). The materials are then crystallized and calcined at appropriate temperatures. For example, Camblor et al.54 employed 48% hydrofluoric acid as the fluoride source during fluorinated zeolite Beta synthesis. The material was crystallized and calcined at 140 and 580 °C, respectively. Their FT-IR data showed vibration features at 500 to 600 cm−1, corresponding to the presence of F− ions in the frameworks.54 Fluoride ion is highly electronegative, and hence, it possesses very strong attraction between neighbor molecules. However, this attraction will become repulsion up to a certain level. Therefore, OH, which is polar and similarly electronegative, will be repelled from fluoride ion. In addition to incorporation into the frameworks, the fluoride ion can be incorporated into the 17874



Review

selective dehydration of the feed into mainly propan-1,2-diol and propan-1,3-diol to very high selectivity.73 Takagaki et al.70 showed layered and nanosheet catalysts based on HNbMoO6 and HTiNbO5 to behave as excellent materials for sucrose hydration into glucose. Both catalysts exhibited strong acidity and hydrophobicity properties at 353 K. Glucose yield as high as 82% was achieved in just 30 min. The activity was similarly higher than that obtained with catalysts such as amberlyst-15 and H-ZSM-5 under similar reaction conditions. Other modified oxide catalysts, containing ZrO2, evaluated for biomass-based conversion include those containing oxides of Rh, Re, and W or their composites with TiO2, CuO, ZnO, or SiO2. However, yields over these materials were demonstrated to be quite low.73−75 These materials have previously been prepared via impregnation or precipitation method with limited difficulties. Crystalline materials with good water tolerance and resistance to catalytic poisons that may be present in the reaction feed can easily be designed. Another important issue is that these oxides are cheaper than zeolites and can be obtained from naturally occurring deposits. Therefore, further studies with the view of deriving the optimum activities are necessary.

structural collapse during reactions in hot aqueous media. Hydrophobization process allows the zeolites to selectively and preferentially locate themselves at the water−oil interface.63,64 This enhances stability and favors transfer of reactants and generated products. Majority of the hydrophobization routes cause a significant decrease in the density and strength of Brønsted acid sites. This factor consequently lowers the zeolites activity in Brønsted acid sites dependent reactions such as alkylation and dehydration. However, the introduction of silane compounds to the external surface of zeolites has been successful without alteration to the acidity properties.27 Common silylation compounds include trimethylchlorosilane, hexamethyldisilazane, and triphenylchlorosilane, which are characterized by the possession of chloro-, bromo-, alkoxy-, or other hydrolyzable groups that enhance their silylation behaviors.65,66 Recently, Zapata et al.5 hydrophobized H−Y zeolites with octadecyltrichlorosilane in toluene at 100 °C. The functionalized materials were evaluated for aqueous phase dehydration and alkylation of m-cresol and propan-2-ol. The characterization data showed no any alteration to the zeolites acid densities. The catalysts were stable under hot aqueous reaction conditions without structural changes.5 However, unsilylated H−Y zeolite loses its structure at 200 °C in few minutes. Silylation prevents the contact of zeolites with bulk water and therefore enhances stability. The reaction results showed the silylated materials to be very active. Tiseanu et al.67 also functionalized terbium modified Beta zeolites with vinyl- and phenyl- silane derivatives via photosynthesis grafting method. All the materials showed significant water tolerance with no observable acidity decay or structural alterations.67 Vuong and Do68 reported silylation process to be suitable for the synthesis of nanosized zeolites both via water and organic phases. It is interesting to note that high postsynthesis calcination temperature does not alter the hydrophobic character or acidity properties of the prepared zeolites.68

6. CONCLUSION There is no doubt that water-tolerant (i.e., hydrophobic) solid acid catalysts are good candidates for alkylation and isomerization of biomass-derived compounds. This can be observed from their unique behaviors in these and similar aqueous reactions such as hydration, epoxidation, and acylation. They are highly stable and can easily be prepared by slight modification to the conventional methods. Zeolites with high Si/Al ratio modified with Sn, transition metals, or silanes or prepared via the fluoride routes are excellent catalysts. Heteropoly acids must be modified with metals such as Cs to derive substantial activity−stability properties. Reactions over oxides need to be further evaluated. Issues such as thermal stability and resistance to catalyst poisons that may be present in the biomass-derived compounds must be taken into full consideration, because the hydrophobic materials may encounter activity decay under uncontrolled conditions. For each catalyst category, more research is required with the view of identifying the best catalyst synthesis option(s), reaction conditions, and recyclability to ensure optimal performance. Actual reaction mechanisms over oxides and zeolites are not yet understood and should therefore be critically explored. It would be very important to identify the most suitable biomass feedstock to provide optimal activity with specific catalyst. Environmental sustainability and cost implications are other parameters that are of great interest to industries and must therefore be appropriately considered.

5. OTHER POTENTIAL OXIDES Among the known acidic oxides, pure or composite transitional metal oxides are attractive materials for reactions involving the participation of either Lewis or Brønsted acid sites.69,70 However, their adsorption behaviors to biomass-derived organic substrates in aqueous have not been fully established. There is some literature evidence that materials such as the nanosheets of molybdena and modified oxides of zirconia and tungsten, are been evaluated for aqueous phase reactions involving biomass compounds.70,71 Yoshikawa et al.71 reported the aqueous production of propene, alcohols, and organic acids from glycerol using iron oxide modified ZrO2 at 623 K. Catalyst synthesis via coprecipitation method was found very effective. Similarly, the water concentration should not exceed 50% to derive the optimal activity−stability properties. Modification of SO42−/ ZrO2 with SBA-15 was also found beneficial for glucose production from celloboise in aqueous media at 160 °C, due to the generation of new and strong Brønsted acid sites.72 The activity−stability properties could be correlated to the strength and availability of these sites during the reaction. High acid density favors glucose yield, with no noticeable effect of catalyst decay. Oh et al.73 reported the deposition of Pt particles to promote diols production from glycerol in the aqueous phase. Pt generates new Brønsted acid sites through hydrogen spill over. Their presence, in return, provides an opportunity for

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the funding provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit in Center of Research Excellence in Nanotechnology at King Fahd University of Petroleum & Minerals (KFUPM) for supporting this work 17875



Review

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through project No. 10-NAN1392-04 as part of the National Science, Technology, and Innovation Plan.

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