Active Solid Acid Research Articles

HPW/MCM-41 catalytic Simmons-Smith cyclopropanation of olefins for synthesis of high-energy–density fuel

Simmons-Smith cyclopropanation commonly promoted by CF3COOH is one important method for transforming olefin into high-energy–density cyclopropyl fuel, but it brings problems such as high cost and harm to environment. In this work, a series of highly active solid acid catalysts HPW/MCM-41 is prepared by impregnation method for the cyclopropanation of dicyclopentadiene. The composition, structural and acidity of catalyst were characterized by TEM, XRD, NH3-TPD, etc. The 5 % HPW/MCM-41 contains more Dawson-type P2W18 (H6P2W18O62) and defective-type P2W21 (H6P2W21O71) that can provide the most abundant acid sites. This catalyst shows the best catalytic activity, the conversion and bicyclic product selectivity for cyclopropanation of dicyclopentadiene are 93.4 % and 55.7 %, respectively, both higher than that of CF3COOH. Moreover, such catalyst shows good catalytic activity for several olefin substrates, and can be recycled in several runs. This work provides a green and efficient catalyst for cyclopropanation reactions for synthesis of high-energy–density fuel, pesticide and medicine.

Sulfonated ionic liquid immobilized SBA-16 as an active solid acid catalyst for the synthesis of biofuel precursor 5-hydroxymethylfurfural from fructose

The catalytic conversion of biomass-derived chemicals to 5-hydroxymethylfurfural (HMF) is of high current interest. In this work, an efficient solid acid catalyst was synthesized by anchoring sulfonic imidazolium-based ionic liquid (IL) over the surface of mercaptopropyl-modified SBA-16 via thiol-ene click reaction. Various characterization techniques exhibited that the mesostructure of SBA-16 was preserved after the immobilization of acidic IL on its surface. The catalytic activity of the synthesized catalyst was evaluated for the dehydration of fructose to HMF. The influence of different reaction parameters on the catalyst efficiency was examined and optimized. A maximum HMF yield of 98% was attained after 30 min of reaction at 120 °C in dimethyl sulfoxide (DMSO) with a catalyst load of 15 mg. Importantly, the catalyst was simply recoverable and can be reused for five successive runs.

Construction, formation mechanism and adsorption properties for ethyl mercaptan of H2Nb4O11 nanotubes

Designing an efficient adsorbent for removal mercaptans is of great importance to the purification of natural gas. Low-dimensional protonated nanomaterials with abundant surface hydroxyl and Brønsted acid (BA) sites have been the promising candidates for the adsorption desulfurization. Herein, a novel niobate nanotubes, H2Nb4O11 nanotubes (HNO-NTs), were constructed through a two-step intercalation strategy following freeze drying under vacuum as highly active solid acid adsorbents. The formation mechanism and textural features of the as-prepared HNO-NTs were investigated. The resulted HNO-NTs are more regular than H4Nb6O17 nanotubes, and the specific surface area of the former (ca. 222.6 m2 g−1) is higher than that of the latter (174.0 m2 g−1). The dynamic adsorption experiments shown that HNO-NTs have a high ethyl mercaptan (EM) adsorptive capacity of 52.27 mg∙g−1 with a gas hourly space velocity (GHSV) of 6000 h−1, which is greatly improved compare with that of H4Nb6O17 nanotubes (26.84 mg∙g−1). Moreover, the regenerated HNO-NTs still maintained an excellent performance after ten adsorption-desorption cycles. Finally, the possible mechanism of the adsorption of EM over HNO-NTs was proposed. This work is expected to provide a new strategy to synthesize polyniobate nanotubes and use as adsorption separation.

Functionalized Polyethylene Glycol as a Catalyst for Esterification of Salicylic Acid

In this work, two catalysts based on polyethylene glycol (PEG) containing sulfonic acid group and the −COOH group of citric acid (CA) were synthesized. Characterization of the −SO3H functionalized PEG and citric acid functionalized PEG has been carried out using FT–IR. The acidity of PEG–SO3H and PEG–CA has been explored to investigate their catalytic efficiency towards eco-friendly production of methyl salicylate via esterification of salicylic acid using methanol as a reactant as well as solvent. Methanol to acid molar ratios of 4:1, 6:1, 8:1, and 10:1 was applied. The sulfonated PEG is found to be a very active solid acid catalyst giving high yields (82%) under the optimized reaction conditions (10:1 M ratio of methanol to acid; reaction temperature, 65 °C; reaction time, 150 min with catalyst loading of 1.5%. In comparison with the catalytic activity of H2SO4, PEG–SO3H, and PEG-CA; the PEG–SO3H surpassed the catalytic activity of both H2SO4 and PEG–CA. The IEC of PEG–SO3H was 4 meq/g. And the maximum water uptake of PEG-SO3H was 8.25%.

Sulfonic acid-functionalized silica with controlled hydrophobicity as an effective catalyst for esterification of levulinic acid

As one of the essential additives in biodiesel and flavoring agents, solvents, and plasticizers in industry, ethyl levulinate (EL) has gained attention in the last two decades, and the global production of the compound is rapidly increased over the years. So far, esterification of biomass-derived levulinic acid (LA) with ethanol (EtOH) in the presence of highly corrosive mineral acid catalysts is the most common technique for the reaction. To avoid the harsh environment during the reaction and to ease the separation of the catalyst, the research has been recently focused on developing highly active solid acid catalysts. In this research, we have systematically investigated the catalytic activity of sulfonic acid-functionalized silica (SiO2-SO3H) with different hydrophobicity for esterification of LA with EtOH. The hydrophobicity of SiO2-SO3H was tuned by incorporating three different alkylsilanes, e.g., methyl (Me), octyl (Oct), and hexadecyl (HD) silanes on the surface of the material. We found that the longer the alkyl group, the lower the acidity and thus resulted in the lower the catalytic activity. On the other hand, the particle size and hydrophobicity increase with the length in the alkyl group. SiO2-SO3H modified with the methyl group (SiO2-SO3H/Me) showed the highest acidity (0.76 mmol g–1) and the highest catalytic performance (70.6% conversion). Moreover, while all alkyl-modified catalysts followed pseudo-first-order, the pseudo-first-order rate constant (k1p) for the reaction over SiO2-SO3H/Me was 5.4 × 10–3 min–1. The kinetic results of this study could be useful for reactor modeling and simulation in the future.

A Highly Effective Biomass-Derived Solid Acid Catalyst for Biodiesel Synthesis Through Esterification.

Efficient valorization of renewable liquid biomass for biodiesel production using the desirable biomass-based catalysts is being deemed to be an environmentally friendly process. Herein, a highly active biomass-based solid acid catalyst (SiO2@Cs-SO3H) with renewable chitosan as raw material through sulfonation procedure under the relatively mild condition was successfully manufactured. The SiO2@Cs-SO3H catalyst was systematically characterized, especially with a large specific surface area (21.82 m2/g) and acidity (3.47 mmol/g). The catalytic activity of SiO2@Cs-SO3H was evaluated by esterification of oleic acid (OA) and methanol for biodiesel production. The best biodiesel yield was acquired by Response Surface Methodology (RSM). The optimized reaction conditions were temperature of 92°C, time of 4.1 h, catalyst dosage of 6.8 wt%, and methanol to OA molar ratio of 31.4, respectively. In this case, the optimal experimental biodiesel yield was found to be 98.2%, which was close to that of the predicted value of 98.4%, indicating the good reliability of RSM employed in this study. Furthermore, SiO2@Cs-SO3H also exhibited good reusability in terms of five consecutive recycles with 87.0% biodiesel yield. As such, SiO2@Cs-SO3H can be considered and used as a bio-based sustainable catalyst of high-efficiency for biodiesel production.

Turkish perlite supported nickel oxide as the heterogeneous acid catalyst for a series of Claisen-Schmidt condensation reactions.

Potentially active and eco-friendly solid acid catalysts have been synthesized by loading different weight percentages (10, 15, and 50) of nickel oxide on thermally activated Turkish perlite through the deposition-precipitation method. Structural features of prepared catalysts were analyzed using BET surface area analysis, X-ray diffraction, scanning electron microscope (SEM), SEM-EDX, transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR), pyridine adsorbed FT-IR, UV-Vis diffuse reflectance spectroscopy (DRS), and thermogravimetric analysis (TGA) techniques. Pyridine adsorbed FT-IR analysis confirmed the presence of the optimum amount of Bronsted acidic sites in a catalyst having 15 wt. % loading of nickel oxide, which was tested for catalyzing a series of Claisen–Schmidt condensation of cyclohexanone and aromatic aldehydes to produce good isolated yield (90%–93%) of 2,6-bis(substituted benzylidene)cyclohexanones, significantly used in anti-tumor and cytotoxic activities. The high catalytic efficiency of the chosen catalyst remains almost intact up to six reaction cycles. On higher wt. % loading of nickel oxide, crystallite size increases along with agglomeration of larger nickel oxide particles on catalyst surface resulting in pore blockage and poor catalytic activity. Loading of NiO on the surface of thermally activated Turkish perlite was confirmed by SEM-EDX analysis, and TEM observations show that the particle size of the preferred catalyst was less than 50 nm. Based on results drawn from XRD, FT-IR, pyridine adsorbed FT-IR, UV-Vis DRS studies, model structures were proposed for Turkish perlite and all prepared catalysts. During this work, the catalytic potential of the preferred catalyst was compared with other previously reported catalysts, and it showed appreciable results. The formed products were further confirmed by their melting point and 1H-NMR analysis.

A highly active and stable organic-inorganic combined solid acid for the transesterification of glycerol under mild conditions

Solid acid catalyst plays a crucial role in the petroleum refinery industry and bio-refinery technology. In this work, p-phenolsulfonic acid (PSA) was successfully grafted onto the surface of KH560-modified zirconium phosphate (K-ZrP) in a facile routine. The structure and property of this organic-inorganic combined solid acid PSA/K-ZrP-x were characterized via XRD, FTIR, 13C solid-state NMR, TG, N2 adsorption-desorption, SEM, pyridine-adsorption FTIR and XPS technologies. The characterization results showed that KH560 can bond with ZrP and promote the grafting of PSA on the surface of K-ZrP via the condensation reaction between its epoxy ring and the phenolic hydroxyl group in PSA. Consequently, PSA/K-ZrP-2 exhibited excellent performance and stability in the transesterification between glycerol and methyl acetate among the tested H3PW12O40, Amberlyst-45, HBEA, HZSM-5, ZrP, AlCl3 and FeCl3 catalysts. The calculated conversion of glycerol reached 81.3% with a 97.9% selectivity for monoacetin (MAG) and diacetin (DAG) with a 2.2% dosage of [H+] at 100 °C for 4 h. The highest specific activity of PSA/K-ZrP-2 reached 24028.2 mg-glycerol/g-cat/h in a short reaction time (at 0.17 h), and it could be recycled five times without obvious deactivation.

Highly active solid oxide acid catalyst for the synthesis of benzaldehyde glycol acetal

A series of composite metal oxide catalysts, namely CeMnTiO, CeCoTiO, and CeFeTiO, were prepared by sol-gel method, and their physicochemical properties were characterized by various techniques, viz. SEM, XRD, XPS, and NH3-TPD. Their desirable acidity and surface area were suitable for the acid-catalyzed acetalization reaction of benzaldehyde (BzH) with ethylene glycol (EG). In particular, CeFeTiO catalyst exhibited superior activity and excellent durability, leading to an acetal yield of 96.8 %, which was in close agreement with the optimal yield (96.95 %) predicted by the response surface methodology (RSM) based on a Box-Behnken design (BBD). Moreover, a kinetic study under the optimal reaction conditions showed that the acetalization reaction was second-order with an active energy (Ea) of 46.65 kJ/mol. Results revealed that CeFeTiO catalyst had high efficiency to surpass conventional Amberlyst catalysts for acetalization reactions.

Solid acid catalyzed carboxymethylation of bio-derived alcohols: an efficient process for the synthesis of alkyl methyl carbonates

Acid catalyzed carboxymethylation of alcohols is an emerging organic transformation that has grabbed the attention of scientific community in recent years. In the present study, sulfonated mesoporous polymer (MP-SO3H) is presented as a highly active solid acid catalyst to convert a wide range of alcohols into alkyl methyl carbonates. The remarkable catalytic activity of MP-SO3H is comparable to that of reported homogeneous acid catalysts. A good correlation was established between the catalytic activity and textural properties of the material. An exceptional catalytic activity of MP-SO3H was observed for DMC mediated carboxymethylation of bio-derived alcohols which is unmatchable to conventional resins and zeolites. This superior activity of MP-SO3H is ascribed to its intrinsic mesoporosity, high acid strength and uniform coverage of surface area by active sites. The catalyst is recyclable, resistant towards leaching and can be used in successive runs without losing the original activity. To the best of our knowledge, MP-SO3H is the first solid acid catalyst to exemplify highest activity for the synthesis of different alkyl methyl carbonates using DMC. The protocol developed herein opens up new avenues to transform wide range of bio-alcohols into useful organic carbonates in the future.

Solid Acid Resin Amberlyst 45 as a Catalyst for the Transesterification of Vegetable Oil.

Commercial transesterification of vegetable oil to biodiesel using alkaline hydroxides requires expensive refined vegetable oil and anhydrous alcohols to avoid saponification. These issues are not present in the acid-catalyzed process; however, the challenge still lies in developing stable and active solid acid catalysts. Herein, Amberlyst 45, a resin for high-temperature application, was efficiently used for biodiesel production by the methanolysis or ethanolysis of vegetable oil. Yields of up to 80 and 84% were obtained for the fatty acid methyl ester and the fatty acid ethyl ester, respectively. Two processes are proposed and showed to be efficient: (i) incremental addition of alcohol along with the reaction for both methanolysis and ethanolysis; or (ii) one-pot reaction for ethanolysis using oil/ethanol molar ratio of 1/18. The catalytic system used also showed to be compatible with used oil (2.48 ± 0.03 mgNaOH ) and to the presence of water (10–20 wt. % based on the alcohol), allowing the use of waste oil and hydrated alcohol.

SO3H-functionalized porous organic polymer with amphiphilic and swelling properties: A highly efficient solid acid catalyst for organic transformations in water

Abstract SO3H-Functionalized porous organic polymers were successfully synthesized by a solvothermal, free radical copolymerization, and successive ion exchange method. Physicochemical characterizations suggested that the prepared polymers featured relatively large surface areas, large pore volumes, excellent surface amphiphilicity, and nice swelling properties. These characteristics enabled the polymers as efficient acid catalysts for organic transformations in water. We demonstrated the use of these solid catalysts for hydrolysis of cyclohexyl acetate and hydration of phenyl acetylene as two case reactions. The results of catalytic tests suggested that the prepared solid acid exhibited excellent catalytic activities in both cases, which outperformed the homogeneous sulfuric acid and phenylsulfonic acid, as well as commercial amberlyst-15 resin. Furthermore, the solid acid was stable in water, and could be used repetitively for at least four times. This study provides an active and water-compatible solid acid catalyst for organic transformations in water, as well as a facile copolymerization approach for the preparation of amphiphilic heterogeneous catalyst.

Synthesis of Solid Acid Catalyst from Fly Ash for Eugenol Esterification

A series of fly ash-based heterogeneous acid catalysts were prepared by chemical and thermal treatment. Fly ash was chemically activated using sulfuric acid and followed by thermal activation. Characterization methods of XRD, BET, SEM-EDX, and the performance in esterification of eugenyl acetate production was carried out to reveal the physical and chemical characteristics of prepared catalysts. Activated catalyst showed high silica content (96.5%) and high BET surface area of 70 m2.g-1. The catalyst was proven to be highly active solid acid catalyst for liquid phase esterification of eugenol with acetic acid yielding eugenyl acetate. A yield of 43-48% was obtained with activated fly ash catalysts for 90 minutes reaction. These catalysts may replace beneficially the conventional homogenous liquid acid to the eco-friendly heterogeneous one.

MIL-101(Fe) as an active heterogeneous solid acid catalyst for the regioselective ring opening of epoxides by indoles

Ring opening of an epoxide by a nucleophile is considered as one of the viable strategies to prepare biologically active molecules using homogeneous or heterogeneous Lewis acid catalysts. Very often, the ring opening of epoxide was performed with oxygen, sulfur and nitrogen based nucleophiles. Recently, metal organic frameworks (MOFs) were identified as one of the promising solid heterogeneous catalysts for ring opening of epoxide with oxygen and nitrogen based nucleophiles, but however, use of carbon nucleophile like indole is limited. Hence, the present work reports the catalytic performance of MIL-101(Fe) as a highly active regioselective heterogeneous solid Lewis acid catalyst in promoting the ring opening of styrene oxide by indole. Among the various catalysts employed for this reaction, MIL-101(Fe) exhibited the highest catalytic activity. A series of control experiments indicated the pivotal role played by Lewis acids within the frameworks of MIL-101(Fe) and further, the catalytic reaction was found to be heterogeneous in nature as evidenced by leaching test. Further, MIL-101(Fe) was reused four cycles with no decay in its activity. Also, MIL-101(Fe) exhibited wide substrate scope in providing series of heterocyclic compounds in moderate to higher yields. A suitable mechanism was also proposed considering the present results and the knowledge from the literature.

Effect of grafting solvent in the optimisation of Sba-15 acidity for levulinIc acid production

Valorisation of lignocellulosic biomass for chemical production is one of the main challenges of the 21st century. Levulinic acid (LA) has been chosen as the target product due to its potential as an intermediate to produce a wide variety of other chemicals. The attention has been focused on the formulation of an active solid acid catalyst for the hydrolysis of glucose to LA. Therefore, a deep modification of SBA-15 with sulfonic groups was performed using post-synthesis grafting method. In particular, this work focuses on the role of different grafting solvents. Since the traditional solvents such as toluene or hexane, are flammable and toxic, a safer and more environmentally friendly solvent, a mixture of water and NaCl has been investigated. It was found that the nature of the solvent highly affects the morphological and chemical features of the materials. Thus, the best catalytic results were obtained with the catalyst prepared in water and NaCl; indeed 30 % and 16 % of LA yield were obtained from fructose and glucose hydrolysis at 180 °C respectively. Water and NaCl mixture guarantees the best distribution of sulfonic groups over the surface, leading to the most balanced acid catalyst.

Ga/HZSM-5 Catalysed Acetic Acid Ketonisation for Upgrading of Biomass Pyrolysis Vapours

Pyrolysis bio-oils contain significant amounts of carboxylic acids which limit their utility as biofuels. Ketonisation of carboxylic acids within biomass pyrolysis vapours is a potential route to upgrade the energy content and stability of the resulting bio-oil condensate, but requires active, selective and coke-resistant solid acid catalysts. Here we explore the vapour phase ketonisation of acetic acid over Ga-doped HZSM-5. Weak Lewis acid sites were identified as the active species responsible for acetic acid ketonisation to acetone at 350 °C and 400 °C. Turnover frequencies were proportional to Ga loading, reaching ~6 min−1 at 400 °C for 10Ga/HZSM-5. Selectivity to the desired acetone product correlated with the weak:strong acid site ratio, being favoured over weak Lewis acid sites and reaching 30% for 10Ga/HZSM-5. Strong Brønsted acidity promoted competing unselective reactions and carbon laydown. 10Ga/HZSM-5 exhibited good stability for over 5 h on-stream acetic acid ketonisation.

Strongly Lewis Acidic Metal-Organic Frameworks for Continuous Flow Catalysis.

The synthesis of highly acidic metal-organic frameworks (MOFs) has attracted significant research interest in recent years. We report here the design of a strongly Lewis acidic MOF, ZrOTf-BTC, through two-step transformation of MOF-808 (Zr-BTC) secondary building units (SBUs). Zr-BTC was first treated with 1 M hydrochloric acid solution to afford ZrOH-BTC by replacing each bridging formate group with a pair of hydroxide and water groups. The resultant ZrOH-BTC was further treated with trimethylsilyl triflate (Me3SiOTf) to afford ZrOTf-BTC by taking advantage of the oxophilicity of the Me3Si group. Electron paramagnetic resonance spectra of Zr-bound superoxide and fluorescence spectra of Zr-bound N-methylacridone provided a quantitative measurement of Lewis acidity of ZrOTf-BTC with an energy splitting (ΔE) of 0.99 eV between the πx* and πy* orbitals, which is competitive to the homogeneous benchmark Sc(OTf)3. ZrOTf-BTC was shown to be a highly active solid Lewis acid catalyst for a broad range of important organic transformations under mild conditions, including Diels-Alder reaction, epoxide ring-opening reaction, Friedel-Crafts acylation, and alkene hydroalkoxylation reaction. The MOF catalyst outperformed Sc(OTf)3 in terms of both catalytic activity and catalyst lifetime. Moreover, we developed a ZrOTf-BTC@SiO2 composite as an efficient solid Lewis acid catalyst for continuous flow catalysis. The Zr centers in ZrOTf-BTC@SiO2 feature identical coordination environment to ZrOTf-BTC based on spectroscopic evidence. ZrOTf-BTC@SiO2 displayed exceptionally high turnover numbers (TONs) of 1700 for Diels-Alder reaction, 2700 for epoxide ring-opening reaction, and 326 for Friedel-Crafts acylation under flow conditions. We have thus created strongly Lewis acidic sites in MOFs via triflation and constructed the MOF@SiO2 composite for continuous flow catalysis of important organic transformations.

Mesoporous silica nanosphere with open-mouth stellate pore architecture as a promising carrier for highly active solid acid catalysts

The mesoporous silica nanosphere containing the open-mouth stellate pore architecture (SMSN) was firstly prepared through a facile and practical hydrothermal approach in which urea acts as a cheap depositing agent to replace poisonous and high-cost organic amines. Using SMSN as a catalyst support, the supported phosphotungstic acid (PTA) catalyst was obtained by adopting an impregnation method. The as-prepared solid acid catalyst demonstrates much superior catalytic activity for diverse reactions to the supported catalyst on a worm-like mesoporous silica nanosphere (WMSN) as a support, originating from the promoted transfer of reactants and products, unique acidic properties and also the higher surface area of the supported PTA catalyst although SMSN has a lower surface area than WMSN, which results from its unique opening-mouth pore structure of SMSN. This work also presents an efficient approach to prepare highly efficient solid acid catalysts for many catalytic reaction processes by modulating the pore architecture of supports, besides tuning the morphology of carrier. The as-formed mesoporous silica nanosphere with opening-mouth stellate pore architectures using as an excellent carrier could be extended to the other supported-type catalysts for diverse reactions. Moreover, besides heterogeneous catalysis, the developed mesoporous silica nanospheres with unique pore architectures by the scalable method have a great potential to use in cell imaging, disease diagnosis, drug delivery, separation, and the other applications.

Preparation of Ga₂O₃ Doped Sulfonated Tin Oxides as a Highly Active and Recyclable Heterogeneous Solid Acid Catalyst for Aldol Reactions.

Ga₂O₃ doped sulfonated tin oxide catalysts were prepared via co-condensation method in ethanol solvent, followed by sulfonation and calcination. The samples were characterized by isothermal nitrogen adsorption/desorption, powder X-ray diffraction (XRD), thermal gravimetric analysis (TG), Raman spectra and DRIFT spectra. The number of acid sites on the catalysts was measured with the potentiometric titration of butyl amine. The results showed that the addition of small amounts of Ga₂O₃ to sulfonated tin oxide resulted in an enhanced acid site density, which makes Ga₂O₃ doped sulfonated tin oxide catalysts highly active for aldol reactions. The catalyst containing 1.5% Ga₂O₃ exhibited much higher activity than those of SO₄²-/SnO₂, SO₄²-/ZrO₂ and H₃PO₄ in aldol condensation of prenal and prenol for citral precursor, which is a important in fragrance industry. Besides the high activity, the catalyst also exhibited good recyclability, making 1.5% GST an efficient and promising solid catalyst for aldol reactions.

Highly stable, active and recyclable solid acid catalyst based on polymer-coated magnetic composite particles

A facile approach has been adopted for coating cross-linked polystyrene (PS) shells on the surface of Fe3O4 magnetic clusters using reflux-precipitation polymerization (RPP). Treating the PS shell with chlorosulfonic acid yields magnetic composite particles with acid functionality. By adjusting the amount and proportion of monomers (styrene and divinylbenzene), the obtained magnetic composite particle solid acid (MPM-5S) exhibits a saturation magnetization value of 18 emu/g, a specific surface area of 243 m2/g and an acid density of 2.113 mmol/g. The MPM-5S magnetic solid acid catalyst was evaluated for esterification of oleic acid with methanol to prepare biodiesel. Under mild conditions, the conversion of oleic acid reached 91%, which was much higher than the catalytic activity of Amberlyst-15 and close to the catalytic activity of concentrated H2SO4. The solid acid catalyst can be recovered by magnetic separation and reused three times maintaining over 95% of its initial catalytic activity. Additionally, the solid acid can be used to catalyze the dehydration of fructose to 5-hydroxymethylfurfural.