CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201402007

Efficient Solid Acid Catalyst Containing Lewis and Brønsted Acid Sites for the Production of Furfurals Michael G. Mazzotta,[a] Dinesh Gupta,[b] Basudeb Saha,*[a] Astam K. Patra,[c] Asim Bhaumik,[c] and Mahdi M. Abu-Omar*[a] Self-assembled nanoparticulates of porous sulfonated carbonaceous TiO2 material that contain Brønsted and Lewis acidic sites were prepared by a one-pot synthesis method. The material was characterized by XRD, FTIR spectroscopy, NH3 temperature-programmed desorption, pyridine FTIR spectroscopy, fieldemission scanning electron microscopy, high-resolution transmission electron microscopy, N2-sorption, atomic absorbance spectroscopy, and inductively coupled plasma optical emission spectroscopy. The carbonaceous heterogeneous catalyst (GluTsOH-Ti) with a Brønsted-to-Lewis acid density ratio of 1.2 and

more accessible acid sites was effective to produce 5-hydroxymethylfurfural and furfural from biomass-derived mono- and disaccharides and xylose in a biphasic solvent that comprised water and biorenewable methyltetrahydrofuran. The catalyst was recycled in four consecutive cycles with a total loss of only 3 % activity. Thus, Glu-TsOH-Ti, which contains isomerization and dehydration catalytic sites and is based on a cheap and biorenewable carbon support, is a sustainable catalyst for the production of furfurals, platform chemicals for biofuels and chemicals.

Introduction The irreversible consumption of carbon sources has resulted in the depletion of fossil fuel reserves and global warming by CO2 emission.[1] This issue has prompted researchers to explore nonconventional resources, which include nonfood biomass, for the large-scale production of chemicals and fuels, particularly after the US Department of Energy published a list of “ten bio-based chemicals” of top priority.[2] Among these priority chemicals, 5-hydroxymethylfurfural (HMF) and furfural (Ff) have received significant attention as platform chemicals for the production of a broad range of chemicals and liquid transportation fuels.[3] As a result of the versatile applications of furfurals, rapid progress in the development of efficient catalytic processes for the conversion of carbohydrates and biomass has been witnessed over the past few years, although sustainable and economically viable routes for their production in scalable quantities are yet to be developed. The direct transformation of carbohydrates into furfurals involves multiple steps,[3, 4] namely, hydrolysis, isomerization, and [a] M. G. Mazzotta, Dr. B. Saha, Prof. Dr. M. M. Abu-Omar Department of Chemistry and the Center for Catalytic Conversion of Biomass to Biofuels (C3Bio) Purdue University 560 Oval Drive, West Lafayette, Indiana 47907 (USA) E-mail: [email protected] [b] D. Gupta Department of Chemistry, University of Delhi Delhi 110007 (India) [c] Dr. A. K. Patra, Prof. Dr. A. Bhaumik Department of Material Science Indian Associate for the Cultivation of Science Kolkata 700032 (India) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402007.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

finally dehydration to Ff and HMF; therefore, the development of a single catalytic system is challenging. Several homogeneous catalytic systems, which include CrII/CrIII halide in imidazolium ionic liquids,[5] AlCl3 in aqueous and biphasic solvents,[6, 7] SnCl4 in 1-ethyl-3-imidazolium tetrafluoroborate ([EMIM]BF4),[8] GeCl4 in 1-butyl-3-imidazolium chloride ([BMIM]Cl),[9] maleic acid,[10] and Zr(O)Cl2/CrCl3 in DMA/LiCl (DMA = dimethylacetamide),[11] have been reported for the conversion of carbohydrates into furfurals. The conversion of cellulosic/hemicellulosic biomass that has a strong H-bonded network of sugar units has also been investigated using homogeneous Lewis acid catalysts, for example, CrCl2 for corn stover,[4] CuCl2/CrCl3 for cellulose,[12] Zr(OCl2)/CrCl3 for sugarcane bagasse,[11] and maleic acid catalyst for corn stover, switchgrass, and pinewood.[10] More recently, Brønsted acidic, N,N-dimethylacetamide methylsulfonate ([DMA][CH3SO3]) and N-methyl-2-pyrrolidinone methylsulfonate ([NMP][CH3SO3]) ionic liquid (IL) catalysts have been reported to be effective for both the hydrolysis of weed biomass and the subsequent dehydration of sugar units.[13] Although these homogeneous catalysts gave moderate to high yields of HMF and Ff, the separation of the catalysts and product purification can be a major challenge. To overcome these challenges, several heterogeneous Lewis acidic mesoporous materials have been tested as solid catalysts. Watanabe and co-workers used anatase TiO2 as catalyst for the conversion of fructose and glucose to HMF in 38 and 7.7 % yields, respectively.[14, 15] More recently, self-assembled mesoporous TiO2 nanospheres via templating pathways and hierarchically porous titanium phosphate (MTiP-1) that have different Lewis acidity and surface areas have been synthesized and tested for HMF production.[16] Although the Lewis acidic solid catalysts are promising in terms of recyclability and easy ChemSusChem 0000, 00, 1 – 10

&1&

These are not the final page numbers! ÞÞ

CHEMSUSCHEM FULL PAPERS separation, they suffer from poor yield and selectivity, particularly in aqueous media. In contrast to the poor activity of Lewis acidic heterogeneous catalysts, functionalized carbonaceous materials that have Brønsted acidic sulfonic acid groups are promising catalysts because of their high acidity and water tolerance. It is only recently that sulfonated graphene oxide has been demonstrated as an efficient carbocatalyst that enables an average 61 % yield of furfural from xylose at 200 8C.[17] Usually sulfonated carbonaceous materials are synthesized by two-step processes that involve the preparation of carbonaceous materials in the first step followed by the incorporation of SO3H groups under harsh oxidation conditions using fuming sulfuric acid.[18–20] To avoid such harsh reaction conditions, Wang et al. developed a one-pot method to prepare sulfonated carbonaceous materials by reacting glucose (Glu) and p-toluenesulfonic acid (TsOH) in a sealed autoclave at 180 8C and reported its catalytic activity for the esterification of succinic acid with ethanol.[21] In a subsequent communication, the authors used the Glu-TsOH catalyst for fructose dehydration and reported a maximum yield of 91 % HMF in DMSO at 130 8C in 1.5 h.[22] However, the effectiveness of the Glu-TsOH catalyst was neither tested in an aqueous medium nor with difficult carbohydrates such as glucose, xylose, and cellobiose. It has been demonstrated that catalytic systems that contain both Lewis and Brønsted acidity are more beneficial for HMF production than Lewis or Brønsted acidic catalysts alone.[23] Herein, we report the synthesis and characterization of a sulfonated carbonaceous material using a biorenewable carbon support that contains Brønsted acidic sulfonic acid groups and Lewis acidic TiO2 sites. This report also describes the catalytic effectiveness of the as-synthesized material for the conversions of fructose, glucose, xylose, and cellobiose to the corresponding furfurals in an environmentally benign aqueous medium that uses biorenewable methyltetrahydrofuran (MeTHF) as an organic phase in a biphasic system to extract furfurals into the organic phase. Some reactions were also performed in DMSO and DMA/LiCl (10 wt % LiCl) to compare the HMF and Ff yields.

Results and Discussion Material synthesis and characterization

www.chemsuschem.org

Figure 1. a) FTIR spectrum and b) wide-angle XRD profile of the Glu-TsOH-Ti catalyst.

XRD study The wide-angle XRD pattern of the Glu-TsOH-Ti catalyst (Figure 1 b) revealed that the material is crystalline, and the major crystalline peaks at 2 q = 25.3, 37.8, and 48.08 correspond to the anatase TiO2 (101), (004), and (200) crystal planes (JCPDS File Card No. 21-1272).[24] The crystalline planes that correspond to the peaks for anatase TiO2 are indexed in Figure 1 b. The presence of crystalline TiO2 particles in the carbonaceous material was further studied by high-resolution transmission electron microscopy (HRTEM; Figure 3). Elemental mapping by energy dispersive X-ray spectroscopy (EDS) in SEM confirmed the presence of both titanium and sulfur atoms in the material. Furthermore, atomic absorption spectroscopy (AAS) showed the presence of 3.2 wt % titanium in the sample. Based on the EDS-SEM data, the material contained 76 and 9.1 wt % C and O, respectively.

FTIR spectroscopy The FTIR spectrum of the Glu-TsOH-Ti catalyst is shown in Figure 1 a. It shows peaks at n˜ = 1010, 1035, and ~ 1115 cm 1 for SO3H groups, which indicate that Brønsted acidic sulfonic acid groups were deposited successfully on the carbon framework.[21] The O H stretching band at approximately n˜ = 3400 cm 1, the C H stretching band at n˜ = 2950–2875 cm 1, the C=O stretching band at approximately n˜ = 1680 cm 1, and the C=C stretching band at approximately n˜ = 1610 cm 1 were also observed.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Acidity measurement Temperature-programmed FTIR spectroscopy is one of the most important analytical tools to characterize the acidic properties of hybrid materials using pyridine as a Lewis base. The pyridine adsorbed on the Glu-TsOH-Ti samples has four characteristic adsorption bands at n˜ = 1587 (broad), 1539 (sharp), 1487 (sharp), and 1438 cm 1 (sharp; Figure 2). The band at n˜ = 1438 cm 1 could be attributed to the adsorbed pyridine at the Lewis acidic sites,[25] whereas that at n˜ = 1487 cm 1 could be asChemSusChem 0000, 00, 1 – 10

&2&

These are not the final page numbers! ÞÞ

CHEMSUSCHEM FULL PAPERS

www.chemsuschem.org

Figure 2. a) FTIR spectrum of Glu-TsOH-Ti and pyridine-desorption FTIR spectra at b) 25, c) 50, d) 150, and e) 250 8C.

signed to the overlap of the Brønsted and Lewis acid sites present in the sample. With an increase in temperature to 250 8C, these bands showed a very slow decrease in intensity caused by desorption of surface-bound pyridine from the GluTsOH-Ti surface. The adsorption bands at n˜ = 1587 (broad) and 1539 cm 1 (sharp) could be because of the presence of weakly chemisorbed pyridine (H bonded at the Brønsted acid sites of the SO3H groups).[26] In addition to the pyridine-IR results that show evidence for Brønsted and Lewis acid sites in the self-assembled Glu-TsOHTi nanomaterial, the quantification of the total acid density was evaluated by NH3 temperature-programmed desorption (NH3-TPD). The ammonia desorption experiment was performed at a linear heating rate of 10 8C min 1 up to 300 8C as thermogravimetric analysis (TGA) of the sample confirmed a mass loss at higher temperatures (Figure S2). The result shows desorption of ammonia at 194.7, 297.9, and 298.8 8C (Figure S3). Each of these desorption peaks was integrated to measure the corresponding acid densities as 0.284, 0.137, and 0.610 mmol g 1, respectively. Thus, the total acid density of the catalyst is 1.03 mmol g 1. As the identification of Lewis and Brønsted acidic sites from the NH3-TPD desorption plot could be erroneous, we measured the total sulfur content in the catalyst by inductively coupled plasma optical emission spectroscopy (ICP-OES) to determine the concentration of HSO3 groups. Notably, the HSO3 group is the only sulfur-containing functional group in the catalyst. This experiment shows the presence of 1.8 % sulfur in the catalyst, which corresponds to 0.56 mmol of HSO3 per gram of catalyst. As HSO3 is the only Brønsted acidic functional group in the catalyst, we subtracted this value from the total acidity to calculate the concentration of Lewis acidic sites as 0.47 mmol g 1. Thus, the ratio of Brønsted to Lewis acid density in the catalyst is 1.2. Nanostructural analysis SEM images of Glu-TsOH-Ti (Figure S4) show that the material is composed of spherical nanoparticles of approximately 20 nm. These spherical nanoparticles are almost uniform in size and form some big aggregated particles in some parts of the specimen. Tiny nanoparticles and their spherical morpho 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 3. a) HRTEM images of Glu-TsOH-Ti that show its nanostructure and b) a magnified view of this nanostructure that show the lattice fringes of TiO2

logical features could help with the diffusion of products from the active catalytic site during the reaction. High-resolution transmission electron microscopy (HRTEM) images of Glu-TsOH-Ti (Figure 3) at different magnifications show that the platelike particles of dimensions of 10–20 nm are self-assembled by the formation of self-aggregated (loosely assembled) nanostructures.[27] Close inspection of these images revealed that the interparticle porosity of this self-assembled nanomaterial is approximately 4.0–5.0 nm. The observed interparticle porosity was further evidenced from the results of N2 sorption studies as discussed below. N2 sorption study The porosity and BET surface area of the Glu-TsOH-Ti material were investigated by a N2 adsorption–desorption study at 77 K. The N2 sorption isotherm of hydrothermally synthesized Glu-TsOH-Ti is shown in Figure 4. This isotherm can be classified as a type IV isotherm with an H2 hysteresis loop that corresponds to mesoporous materials based on their adsorption isotherm at a low P/P0. The H2 hysteresis loop can be linked to narrow-necked pores with wide bodies and an ascending boundary curve of the isotherm that follows a trajectory similar ChemSusChem 0000, 00, 1 – 10

&3&

These are not the final page numbers! ÞÞ

CHEMSUSCHEM FULL PAPERS

Figure 4. a) N2 adsorption–desorption isotherm of Glu-TsOH-Ti at 77 K [(*) adsorption and (*) desorption]. b) Representative pore size distributions calculated by NLDFT.

to that obtained with medium-porosity adsorbents.[28] The BET surface area of Glu-TsOH-Ti was 42.5 m2 g 1 with a pore volume of 0.0543 cm3 g 1. In this isotherm, between P/P0 of 0.04–0.70 the N2 adsorption gradually increases. The pore size distribution of the sample, measured by the nonlocal density functional theory (NLDFT) method (using N2 adsorption on silica as a reference), suggested that the Glu-TsOH-Ti catalyst has an average pore size of approximately 4.5 nm. Catalysis for HMF production A preliminary reaction for the dehydration of 0.56 mmol of fructose with 50 mg of Glu-TsOH-Ti catalyst was performed in 4 mL of DMSO at 150 8C. The analysis of aliquots, collected at different times during the reaction, showed an increase of the HMF yield as a function of reaction time. A maximum 99 mol % HMF yield was achieved within 20 min (Figure 5 a) with the appearance of a pale yellow color in the solution. The pale yellow color in the product solution is an indication that blackcolored humin oligomers[29] did not form as a byproduct under these conditions. The clean 1H NMR spectrum of the product solution in [D6]acetone (Figure S5) further confirmed the purity of HMF as proton signals for the side products, levulinic and formic acids,[6] were not present in the NMR spectrum. Previous work on the dehydration of 2.8 mmol of fructose with 400 mg of Glu-TsOH catalyst in 6 mL of DMSO reported a 91 % yield of HMF in 1.5 h.[22] A rough comparison of the reported data for  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org

Figure 5. Time-dependent formation of HMF from a) fructose and b) glucose dehydration with and without Glu-TsOH-Ti in DMSO. Reaction conditions: a) 0.56 mmol fructose, 50 mg catalyst, 4 mL DMSO, T = 150 8C; b) 0.56 mmol glucose, 26 mg catalyst (filled symbols) or no catalyst (empty symbols), 2 mL DMSO, 140 (red lines) or 180 8C (black lines).

the Glu-TsOH catalyst that has a Brønsted acidity of 2.0 mmol g 1 and our result from using less Glu-TsOH-Ti catalyst suggests that the catalytic activity of our carbonaceous material, which contains both Brønsted and Lewis acid sites, is better. Although the total acidity of our titanium-containing carbonaceous material (1.03 mmol g 1) is lower than that of the reported titanium-free carbonaceous catalyst (2.0 mmol g 1),[22] the accessibility of acid sites as a result of porosity, the presence of Lewis as well as Brønsted acidic sites, and the higher surface area (42.5 m2 g 1) of our catalyst compared to the titanium-free material (< 1 m2 g 1) could be the reasons for the observed activity. A similar observation was also reported by Zhao et al.[30] in which the authors used heterogeneous catalysts of varying total acid density that contained only Brønsted acidity as well as combined Brønsted and Lewis acid sites. The results showed that the Amberlyst-15 catalyst with the highest acid-site density (but only Brønsted acid sites) exhibited the lowest HMF yield from fructose compared with a heteropolyacid catalyst, Cs2.5H0.5PW12O40, which has a significantly lower acid density. The latter catalyst, which contains both Brønsted and Lewis acid sites, is believed to have a better catalytic activity as a result of the minimization of secondary side reactions that involve HMF degradation. Additionally, the higher pore volume of the titanium-containing catalyst (4–5 nm) may also ChemSusChem 0000, 00, 1 – 10

&4&

These are not the final page numbers! ÞÞ

CHEMSUSCHEM FULL PAPERS

www.chemsuschem.org

favor the formation of a high HMF yield,[31] although the pore volume of the reported titanium-free carbonaceous catalyst was not given. The effectiveness of the Glu-TsOH-Ti catalyst was further tested for difficult substrates, such as glucose, the dehydration of which is known to occur by the isomerization of glucose to fructose. The dehydration of glucose with catalyst was performed at 140 and 180 8C in 2 mL of DMSO. The results plotted in Figure 5 b show an increase in HMF yield with the increase of the reaction time from 5 to 30 min at both temperatures. However, the rate of the formation of HMF at two different temperatures did not differ significantly, which could be attributable to the loss of HMF in the form of humin oligomers at higher temperatures. Although the yield of humin was not quantified, such humin formation was not observed in the fructose dehydration reaction at 150 8C in the same solvent (DMSO). This result suggests that the oligomerization of HMF could occur preferentially with glucose.[29] The blank experiments (without catalyst) for glucose dehydration under comparable conditions produced only 9 % HMF at 180 8C for 60 min, whereas at 140 8C the yield of HMF in the control experiment was low (Figure 5 b). As noted above, the yield of HMF from the fructose dehydration reaction in DMSO was impressive; the major challenge, however, is product extraction and purification from DMSO because of its miscibility with most organic solvents. Therefore, the effectiveness of the Glu-TOH-Ti catalyst was tested in water at 140 8C for 30 min. The yield of HMF was only 8 mol % (entry 1 in Table 1), which is because of the known effect of the slow dehydration of fructose as well as the rapid rehydration of HMF to the side products levulinic and formic acids in aqueous media.[6, 32] Thus, additional experiments were performed in MeTHF/H2O biphasic solvent in which HMF could accumulate in the MeTHF phase after its formation in the aqueous phase. MeTHF was chosen as an organic phase because it is a green, cost-effective, biorenewable alternative to oil-derived solvents, it is stable, and has a better extracting ability for furfurals. The beneficial effect of the MeTHF/H2O biphasic

solvent was realized immediately for the reaction between 0.27 mmol of fructose and 20 mg of Glu-TsOH-Ti catalyst performed in the biphasic solvent. As shown in entry 2 of Table 1, the reaction produced 26 % HMF yield in 40 min. Unless otherwise stated, the total HMF yield in the biphasic solvent system is the sum of HMF obtained from the organic and aqueous phases. If the reaction time was increased from 40 min to 1 h, the yield of HMF improved from 26 to 34 % (entry 3, Table 1). At a higher temperature (180 8C), fructose dehydration produced 52 % HMF in 10 min, followed by a slight increase in yield to 59 % if the reaction was continued to 60 min (entries 4–7, Table 1). Under comparable conditions, the dehydration of fructose catalyzed by anatase TiO2 and Glu-TsOH produced 14 and 44 wt % HMF, respectively, based on UV/Vis spectrophotometric analysis of the reaction products. These results further confirm that the Glu-TsOH-Ti catalyst, which contains Brønsted and Lewis acid sites, is more effective for HMF production than the individual Lewis (TiO2) and Brønsted (GluTsOH) acid sites. Notably, the conversions of fructose with the Glu-TsOH-Ti catalyst were approximately 30–40 % higher than the observed HMF yields, which suggests a significant loss of either HMF or fructose under these reaction conditions. To investigate the loss of the desired product HMF as its rehydration products, levulinic and formic acids, the 1H NMR spectrum of isolated HMF obtained from reaction 6 (entry 6, Table 1) was recorded in [D6]acetone (Figure S6). This NMR spectrum showed signals for formic acid and levulinic acid at d = 8.2 and 2.2 ppm, respectively. The integrated signal intensity for the formic acid proton was approximately 10 % with respect to the intensity of the aldehyde proton of HMF. Furthermore, the decomposition of HMF was studied separately in an aqueous solution of 0.2 mmol of pure HMF at 140 and 180 8C in the presence of 24 mg of catalyst. Although the degradation of HMF at 140 8C was not evidenced from a plot of HMF yield [%] versus time (Figure S7), approximately 10 % decomposition of HMF was noted at 180 8C in 20 min. A significant loss of HMF, which amounts to approximately 40 %, was also reported at 180 8C after 1.5 h in the case of the Glu-TsOH-catalyzed dehydration of fructose in DMSO.[22] Table 1. The catalytic effectiveness of Glu-TsOH-Ti for the dehydration of fructose, glucose, and cellobiose in The effect of catalyst loading MeTHF/H2O biphasic solvent. on the yield of HMF was also studied under the reaction conEntry Substrate Glu-TsOH-Ti Solvent T t Conversion HMF yield [mol %] ditions listed in entry 4 of [mmol] [mg] ([mL]) [8C] [min] [%] UV/Vis HPLC aq. organic aq. organic Table 1. The yield of HMF increased from 7 to 52 % if the cat1 fructose (0.27) 20 H2O (4) 140 30 18 8 – 6 – 20 7 19 4 19 2 fructose (0.27) 20 MeTHF (4)/H2O (2) 140 40 alyst loading was increased from – 9 25 – – 3 fructose (0.27) 20 MeTHF (4)/H2O (2) 140 60 5 to 22 mg (Figure 6). A further 81 6 46 11 35 4 fructose (0.2) 22 MeTHF (2)/H2O (1) 180 10 increase in the catalyst loading 87 5 51 4 47 5 fructose (0.2) 22 MeTHF (2)/H2O (1) 180 20 to 50 mg resulted in a small in93 6 50 3 48 6 fructose (0.2) 22 MeTHF (2)/H2O (1) 180 30 99 7 52 5 50 7 fructose (0.2) 22 MeTHF (2)/H2O (1) 180 60 crease in yield to 60 %. The ob– 8 10 – – 8 glucose (0.28) 23 MeTHF (4)/H2O (2) 180 10 served nonlinearity of the HMF 61 4 27 7 23 9 glucose (0.28) 23 MeTHF (4)/H2O (2) 180 60 yield as a function of catalyst 73 10 36 9 34 10 glucose (0.28) 23 MeTHF (4)/H2O (2) 180 120 loading could be because of 90 10 38 10 38 11 glucose (0.28) 22 MeTHF (2)/H2O (1) 220 120 94 6 20 4 18 12 cellobiose (0.2) 23 MeTHF (2)/H2O (1) 180 10 mass-transport limitations for 94 6 30 5 27 13 cellobiose (0.2) 23 MeTHF (2)/H2O (1) 180 20 the heterogeneous catalyst. The 14 cellobiose (0.2) 23 MeTHF (2)/H2O (1) 180 60 100 9 30 7 28 heterogeneity of the catalyst

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemSusChem 0000, 00, 1 – 10

&5&

These are not the final page numbers! ÞÞ

CHEMSUSCHEM FULL PAPERS

Figure 6. Yield of HMF as a function of Glu-TsOH-Ti catalyst dosage. Reaction conditions: 0.2 mmol fructose, T = 180 8C, t = 10 min, solvent: MeTHF (2 mL)/ H2O (1 mL).

was tested by a hot-filtration experiment. In this experiment, 0.2 mmol of fructose and 23 mg of catalyst were reacted in the MeTHF/H2O biphasic solvent at 140 8C for 2 min under microwave-assisted heating. The temperature of the microwave reactor needed to cool to 70 8C to allow the reactor chamber open. The reaction mixture, after the separation of the solid catalyst through a 0.22 mm cut-off syringe filter (25 mm diameter), was transferred immediately to a new microwave tube, and the reaction was resumed without catalyst at 140 8C for another 18 min. UV/Vis spectrophotometric analysis of aliquots, collected at 2 and 20 min, showed similar HMF yields of 4.1 and 4.5 mol % after 2 and 20 min, respectively. This result confirms that the catalyst is indeed heterogeneous. Although fructose is the preferred feedstock for HMF production, its occurrence in nature is limited. This drives our attention to utilize a more abundant carbohydrate, glucose, as the raw material for HMF synthesis. As shown above, the GluTsOH-Ti catalyst was effective for glucose dehydration in DMSO (vide supra); the major concern was the separation of HMF from the DMSO. Therefore, the dehydration of glucose with the Glu-TsOH-Ti catalyst was performed in MeTHF/H2O at 180 8C. The results shown in Table 1, entry 8 reveal the formation of 18 % HMF in 10 min, which increased to 46 % if the reaction time was increased to 2 h (entries 9 and 10, Table 1). To test the effect of temperature, a reaction between 0.28 mmol of glucose and 22 mg of catalyst was performed at 220 8C for 2 h, which produced 48 % HMF (entry 11, Table 1); a little improvement in the yield was observed at 220 8C. It is possible that the yield of HMF at a higher temperature (220 8C) is undermined by its decomposition. A previous report by Nikolla et al. described only 14 % HMF yield at 75 % glucose conversion using a Sn-beta catalyst at 160 8C in a water/1-butanol/NaCl biphasic solvent.[23] A significant improvement in yield (53 % HMF at 76 % glucose conversion at 180 8C) was recorded if 0.1 m HCl (Brønsted acid) was added with the Sn-beta catalyst. Similar to the observation of Dumesic et al.,[33] the higher yield in the latter reaction is because of the presence of both Lewis and Brønsted acidic com 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org ponents in which the Lewis acid facilitates the isomerization of glucose to fructose and the Brønsted acid sites facilitate the dehydration of fructose. Compared with the Sn-beta/HCl catalysis system, the activity of the Glu-TsOH-Ti catalyst to give an almost comparable HMF yield from glucose without using mineral acid (HCl) and NaCl in the reactive phase is significant. The presence of the Lewis acidic isomerization component and Brønsted acidic dehydration component in the Glu-TsOH-Ti catalyst is believed to make it an effective catalyst. The scope of the present investigation was further extended to the dehydration of cellobiose (a dimer of glucose units) with the Glu-TsOH-Ti catalyst. A reaction between 0.22 mmol of cellobiose and 23 mg of catalyst in the MeTHF/H2O biphasic solvent produced 26 % HMF (entry 12, Table 1) in 10 min at 180 8C based on the HPLC analysis of the reaction products. The yield of HMF increased from 26 to 36 % if the reaction was continued from 10 to 20 min followed by almost a plateau up to 60 min (entries 12–14, Table 1). Although the conversion of cellobiose was nearly 100 %, HPLC analysis of the aqueous phase of the reaction solution showed the presence of a large amount of unconverted glucose and a small amount of fructose (Figure S8). Previous reports on cellobiose dehydration with homogeneous GeCl4 and CrCl3 catalysts have shown the formation of 41 and 50 % HMF,[34, 35] respectively, in pure or mixed [BMIM]Cl ionic liquid. Compared to these reported HMF yields on using toxic and nonseparable Lewis acidic salts in ionic liquid, the present catalysis that uses a biorenewable, nontoxic, and recyclable Glu-TsOH-Ti catalyst in an aqueous medium to enable 39 % HMF is remarkable and advances green chemistry applications. Compared the activity of the Glu-TsOH-Ti catalyst for the conversion of different carbohydrate substrates under comparable reaction conditions (180 8C, 60 min reaction time, and a similar amount of catalyst), maximum HMF yields from fructose, glucose, and cellobiose were recorded as 59, 31, and 39 %, respectively (entries 7, 9, and 14 of Table 1). The involvement of an additional isomerization step in the glucose dehydration reaction is the reason for the lower HMF yield from glucose than that obtained from fructose. The conversion of cellobiose, which progresses through the hydrolysis of cellobiose to glucose, isomerization of glucose to fructose, and finally the dehydration of fructose, involved an additional hydrolysis step in the reaction sequence. Although cellobiose, as a dimer of two glucose units, theoretically contains double the amount of glucose, a slightly higher HMF yield from cellobiose than from glucose suggests that the overall HMF yield from cellobiose is perhaps limited by the glucose isomerization step as quantitative conversion of cellobiose was noted from HPLC analysis. Catalysis for Ff production Ff, another platform chemical with an annual production of more than 200 000 tons, is a feedstock for 2-methylfuran, 2MeTHF, furfural alcohol, ethyl levulinate, and biofuels.[3] Xylose dehydration usually proceeds through the isomerization of xylose to xylulose followed by the dehydration of xylulose to ChemSusChem 0000, 00, 1 – 10

&6&

These are not the final page numbers! ÞÞ

CHEMSUSCHEM FULL PAPERS

www.chemsuschem.org

Ff.[36] The conversion of xylose, xylane, and lignocellulosic biomass (corn stover, pinewood, switchgrass, and poplar) to Ff has been reported recently with homogeneous catalysts, such as maleic acid, AlCl3, and FeCl3.[10, 37, 38] As a result of the potential benefit of a heterogeneous catalyst over homogeneous analogues, we investigated the catalytic effectiveness of our sulfonated carbonaceous material for the conversion of xylose to Ff. Preliminary experiments for the conversion of xylose to Ff were performed in DMSO and DMA/LiCl (10 wt % LiCl). The reaction between xylose (0.7 mmol) and Glu-TsOH-Ti catalyst (50 mg) at 140 8C produced an Ff yield of 51 % in 60 min. In the case of the DMA/LiCl-mediated reaction between xylose (0.33 mmol) and the catalyst (20 mg) at 180 8C, a maximum yield of 37 % Ff was recorded in 5 min. The yield remained almost constant if the reaction time was further increased to 60 min (Figure S9). A previous report has shown the formation of 56 % Ff in 4 h in DMA/LiCl from the chromium(II)-catalyzed dehydration of xylose at 100 8C.[39] In comparison to the 56 % yield obtained by using a homogeneous catalyst, the observed 51 % Ff yield in the present reaction using a recyclable heterogeneous catalyst makes the process more sustainable. However, to avoid the complexity of Ff extraction and purification from these organic solvents, subsequent experiments for the conversion of xylose to Ff were performed in MeTHF/H2O biphasic solvent. The yields of Ff from a reaction between 0.33 mmol of xylose and 22 mg of Glu-TsOH-Ti catalyst at 180 8C in MeTHF/H2O were monitored as a function of time. The results are tabulated in Table 2 and reveal the formation of

Table 2. The yield of Ff from xylose with and without Glu-TsOH-Ti catalyst.[a] t [min]

with catalyst

without catalyst

5 10 20 30

25 40 46 51

4 8 14 17

adding fresh substrate and solvent. Fresh catalyst was not added to compensate for any loss of catalyst during recovery. The organic and aqueous phases of the reaction solution of each run were analyzed separately to quantify the total amount of HMF formed. The loss of activity of the catalyst, in terms of HMF yield, after four cycles was negligible (Figure 7).

Figure 7. Recyclability of the Glu-TsOH-Ti catalyst for the dehydration of fructose (0.2 mmol) to HMF using 22 mg of catalyst in MeTHF (2 mL)/H2O (1 mL) biphasic solvent at 180 8C for 60 min.

The FTIR spectrum of the used Glu-TsOH-Ti catalyst confirmed the presence of HSO3 groups after the fourth consecutive cycle (Figure S10). Field-emission scanning electron microscopy (FE-SEM) and TEM images of the used catalyst are shown in Figure S10. It is clear from these images that the particle morphology, spherical nanoparticle dimensions, and interparticle porosity of the used catalyst is quite similar to that of the fresh material.

Ff yield [mol %]

[a] Reaction conditions: 0.33 mmol xylose, 22 mg Glu-TsOH-Ti, 180 8C, 2 mL MeTHF, and 1 mL water.

a maximum 51 % yield of Ff over 30 min. The comparable Ff yield in the present reaction with that of a homogeneous catalytic system[39] again suggests that the use of Glu-TsOH-Ti catalyst for Ff production is beneficial and sustainable. In parallel, blank experiments were performed without catalyst (Table 2). Catalyst recyclability The reusability of the Glu-TsOH-Ti catalyst was examined for fructose dehydration in MeTHF/H2O by performing a reaction between 0.2 mmol of fructose and 22 mg of catalyst at 180 8C for 60 min. After completion of the reaction in 60 min, an aliquot was collected for analysis, and the solid catalyst left in the tube was collected and reused for three more cycles by  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Conclusions Self-assembled nanoparticulates of sulfonated carbonaceous material that contain Brønsted acidic sulfonic groups and Lewis acidic titania pores have been synthesized through the thermal treatment of biorenewable glucose, p-toluene sulfonic acid, and titanium isopropoxide. The presence of sulfonic acid ( HSO3) groups and Lewis acidic TiO2 in the material has been confirmed by FTIR and pyridine-desorption FTIR spectroscopy, NH3 temperature-programmed desorption, XRD, and elemental analysis. The total acid density and the ratio of Brønsted-toLewis acidity values are 1.03 mmol g 1 and 1.2, respectively. This material shows a good catalytic activity for the dehydration of biomass-derived fructose, glucose, and cellobiose to 5hydroxymethylfurfural (HMF), which enables maximum yields of 59, 48, and 39 %, respectively, in a methyltetrahydrofuran/ water biphasic solvent system. A higher yield of HMF (99 %) is observed in DMSO for the dehydration of fructose compared with a reported reaction catalyzed by a carbonaceous material that contained only Brønsted acid sites with a higher acid density. The catalyst is also effective for the conversion of xylose to give a maximum 51 % yield of furfural in the same biphasic solvent. Compared with glucose and xylose dehydration using ChemSusChem 0000, 00, 1 – 10

&7&

These are not the final page numbers! ÞÞ

CHEMSUSCHEM FULL PAPERS Sn-beta/HCl and homogeneous CrII catalytic systems, the present heterogeneous catalyst, which contains both isomerization and dehydration catalytic sites and a moderate pore size (4– 5 nm), gives comparable furfural yields in water as the reactive phase. The recyclability experiments show that the catalyst retained full activity after four consecutive cycles, and the loss in activity, in terms of HMF yield, was only 3 %.

Experimental Section Materials Fructose, glucose, xylose, sucrose, cellobiose, xylulose, MeTHF, DMSO, lithium chloride, DMA, titanium isopropoxide, pure HMF, and furfural were purchased from Sigma–Aldrich and used as received. Unless otherwise mentioned, Millipore water was used as the aqueous phase for all reactions.

Catalyst synthesis The biorenewable, solid, TiO2-containing carbonaceous acid catalyst, Glu-TsOH-Ti, was prepared by the thermal treatment of TsOH, glucose, and titanium(IV) isopropoxide at 180 8C. In a specific preparation, glucose (2 g), TsOH (2 g), and titanium isopropoxide (0.5 g) were mixed well, transferred to a 25 mL Teflon-sealed autoclave, and maintained at 180 8C for 24 h. The obtained black material was ground to powder by using a mortar and pestle, washed with water and ethanol, and oven-dried at 80 8C. Glu-TsOH catalyst was prepared by following a similar synthesis method but without using titanium isopropoxide.

www.chemsuschem.org The microwave-assisted conversions of all substrates were performed by using a CEM Corporation Discover TM Microwave reactor at the standard operating frequency and 100 W power. HMF and furfural yields were measured by both UV/Vis spectrophotometry by using a Shimadzu UV-2501PC spectrophotometer and HPLC by using a Waters HPLC instrument equipped with a Waters 2487 photodiode array (PDA) and 2414 refractive index detectors. 1 H NMR spectra of HMF were recorded by using a Bruker ARX 400 MHz instrument, and NMR data were processed with XWinNMR software.

Conversion of carbohydrates to HMF and Ff The dehydration reactions of carbohydrates were performed by charging a 10 mL microwave tube with the substrates, solvent, and catalyst. The loaded microwave tube was then inserted into the microwave reactor preset to the desired temperature and reaction time. Upon completion of the allotted reaction time, the reactor was opened. The reaction mass was cooled to RT, and the solution was filtered through a 0.22 mm cut-off syringe filter (25 mm diameter) for analysis. In the case of the biphasic-solvent-mediated reactions, both the organic and aqueous phase were analyzed separately for the quantification of furfurals yield and carbohydrate conversion. The conversion of the starting carbohydrate substrate was calculated from HPLC analysis by determining the unconverted substrate in the aqueous phase. For 1H NMR spectroscopic analysis, MeTHF was removed from the organic phase by rotary evaporation, and the oily product was dissolved in [D6]acetone. DMF was used as an internal standard.

Recyclability of Glu-TsOH-Ti Instrumentation Powder X-ray diffraction (PXRD) patterns were recorded by using a Bruker D-8 Advance diffractometer operated at 40 kV and 40 mA and calibrated with a standard silicon sample using nickel-filtered CuKa (l = 0.15406 nm) radiation. A JEOL JEM 6700F field-emission scanning electron microscope was used to determine the morphology of powder samples. The pore structure was investigated by using a JEOL JEM 2010 transmission electron microscope operated at an accelerating voltage of 200 kV. The FTIR spectrum of the material as well as pyridine desorption studies at variable temperatures were recorded by using a PerkinElmer Spectrum 100 spectrophotometer. For pyridine-IR studies, the sample was saturated with pyridine vapor in a closed vessel at 75 8C for 2 h, and desorption spectra were recorded at elevated temperatures. N2 adsorption–desorption isotherms were obtained by using a Beckman Coulter SA 3100 Surface Area Analyzer at 77 K. The sample was degassed at 135 8C for 4 h under high vacuum prior to the N2 sorption analysis. The NH3-TPD was conducted by using a Micrometrics AutoChem II 2920 V4.04 in the temperature range of 120–300 8C, which employed a thermal conductivity detector. This experiment was performed at the Micrometritics analytical facility, USA. For the NH3TPD measurement, the sample was activated at 300 8C inside the reactor of the TPD furnace under a He flow. After cooling to 120 8C, 10.02 % ammonia balanced with He was injected, and the system was allowed to equilibrate. The temperature was then raised at a linear heating rate of 10 8C min 1 up to 300 8C. AAS analysis of the sample was performed by using a Shimadzu AA-6300 atomic absorption spectrometer. The percentage of sulfur in the catalyst sample was measured by ICP-AES by using an ICP-OES optima instrument of model 3300DV at Galbraith Laboratory, USA.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

The recycling efficiency of the catalyst was determined for the dehydration of fructose as a representative reaction. In this study, a 10 mL microwave tube was charged with fructose (0.2 mmol), catalyst (22 mg), MeTHF (2 mL), and water (1 mL). The tube was placed in the microwave reactor, and the mixture was heated at 180 8C using 100 W microwave power for 60 min. After the reaction, the tube was cooled to RT, and an aliquot was collected for analysis. The solid catalyst left in the tube was collected and reused for three consecutive cycles by adding fresh substrate and solvent. Fresh catalyst was not added to compensate any loss of the catalyst during recovery. The yield of HMF was determined from each run.

Determination of HMF yield UV/Vis spectrophotometric method The UV/Vis spectra of pure HMF (Figure S1) and Ff have distinct peaks at lmax = 284 and 268 nm with corresponding extinction coefficient (e) values of 1.66  104 and 1.53  104 L mol 1 cm 1, respectively. The mole percentage of furfurals in each of the reaction product was calculated from the measured absorbance values at respective lmax values for HMF and Ff and the corresponding extinction coefficient values. First, standard HMF and Ff solutions of 98–99 % purity were analyzed separately to correlate the actual and calculated amounts of HMF and Ff. Once good correlations were established, HMF product samples were measured, and the percentage HMF and Ff yields were calculated. Repeated measurement of the same solution showed that the percentage of error associated with this measurement was in the range of  5 %. ChemSusChem 0000, 00, 1 – 10

&8&

These are not the final page numbers! ÞÞ

CHEMSUSCHEM FULL PAPERS HPLC method For MeTHF/H2O-mediated reactions, HPLC analyses of both the organic and aqueous phases were performed separately. The organic phase was analyzed by using a Waters HPLC instrument equipped with a Waters 152 pump, a XDB-C18 column (Agilent), and a Waters 2487 PDA detector. A solution of 80 % formic acid (0.1 %) and 20 % methanol was used as the mobile phase at a flow rate of 1 mL min 1. HPLC analysis of the aqueous phase was performed by using a Waters 2695 Separations Module equipped with an Aminex HPX-87H column (300  7.8 mm) set at 65 8C and Waters 2414 refractive index detector for the determination of unconverted carbohydrates as well as Ff. A solution of 5 % acetonitrile in sulfuric acid (0.005 m) was used as the mobile phase at a flow rate of 0.6 mL min 1. The characteristic peaks for Ff and unconverted carbohydrates in the product solutions were identified by their retention times in comparison with authentic samples. Each peak was integrated, and the actual concentrations of glucose, fructose, xylose, Ff, and HMF were calculated from their respective precalibrated plots of peak areas versus concentrations. The difference in Ff yields measured by the UV/Vis and HPLC methods are within  5 %.

Acknowledgements M.M.A.-O., B.S., and M.M. acknowledge support from the Center for direct Catalytic Conversion of Biomass to Biofuels (C3Bio), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-SC0000997, which supported research carried out at Purdue University (B.S. and M.M.). B.S. thanks CSIR, India for a research grant to partly support this work. D.G. and A.K.P. thank CSIR, India for their respective Senior Research Fellowships. B.S. and A.B thank DST, New Delhi for financial support through a DST-SERB project grant. Keywords: acidity · biomass · heterogeneous catalysis · nanoparticles · titanium [1] A. J. Ragauskas, C. K. Williams, B. H. Davidson, G. Britovsek, J. Cairney, W. J. Frederick, J. P. Hallett, D. J. Leak, C. L. Liotta, J. R. Mielenz, R. Murphy, R. Templet, T. Tschaplinski, Science 2006, 311, 484 – 489 and references therein. [2] T. Werpy, G. Petersen in Top Value Added Chemicals from Biomass, Volume I, Results of Screening for Potential Candidates from Sugar and Synthesis Gas, US Department of Energy DOE/GO-102004 – 1992, August 2004. www.eere.energy.gov/biomass/pdfs/35523.pdf. [3] S. Dutta, S. De, B. Saha, I. Alam, Catal. Sci. Technol. 2012, 2, 2025 – 2036 and references therein. [4] J. B. Binder, R. T. Raines, J. Am. Chem. Soc. 2009, 131, 1979-.1985. [5] H. Zhao, J. E. Holladay, H. Brown, Z. C. Zhang, Science 2007, 316, 1597 – 1600. [6] S. De, S. Dutta, B. Saha, Green Chem. 2011, 13, 2859 – 2868 and references therein. [7] Y. Yang, C. Hu, M. M. Abu-Omar, Green Chem. 2012, 14, 509 – 513. [8] S. Hu, Z. Zhang, J. Song, Y. Zhou, B. Han, Green Chem. 2009, 11, 1746 – 1749. [9] J. Y. G. Chan, Y. Zhang, ChemSusChem 2009, 2, 731 – 734.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org [10] E. S. Kim, S. Liu, M. M. Abu-Omar, N. S. Mosier, Energy Fuels 2012, 26, 1298 – 1304. [11] S. Dutta, S. De, I. Alam, M. M. Abu-Omar, J. Catal. 2012, 288, 8 – 15. [12] S. Zhao, M. Cheng, J. Li, J. Tian, X. Wang, Chem. Commun. 2011, 47, 2176 – 2178. [13] I. Alam, S. De, S. Dutta, B. Saha, RSC Adv. 2012, 2, 6890 – 6896. [14] X. Qi, M. Watanabe, T. M. Aida, R. L. Smith Jr., Catal. Commun. 2008, 9, 2244- 2249. [15] M. Watanabe, Y. Aizawa, T. Lida, R. Nishimura, H. Inomata, Appl. Catal. A 2005, 295, 150 – 156. [16] a) S. De, S. Dutta, A. K. Patra, A. Bhaumik, B. Saha, J. Mater. Chem. 2011, 21, 17505 – 17510; b) A. Dutta, A. K. Patra, S. Dutta, B. Saha, A. Bhaumik, J. Mater. Chem. 2012, 22, 14094 – 14100; c) S. Dutta, S. De, A. K. Patra, M. Sasidharan, A. Bhaumik, B. Saha, Appl. Catal. A 2011, 409 – 410, 133 – 139; d) S. De, S. Dutta, A. K. Patra, B. S. Rana, A. K. Sinha, B. Saha, A. Bhaumik, Appl. Catal. A 2012, 435 – 436, 197 – 203. [17] E. Lam, J. H. Chong, E. Majid, Y. Liu, S. Hrapovic, A. C. W. Leung, J. H. T. Luong, Carbon 2012, 50, 1033 – 1043. [18] M. Toda, A. Takagaki, M. Okamura, J. N. Kondo, S. Hayashi, K. Domen, M. Hara, Nature 2005, 438, 178. [19] R. Xing, Y. Liu, Y. Wang, L. Chen, H. Wu, Y. Jiang, M. He, P. Wu, Microporous Mesoporous Mater. 2007, 105, 41 – 48. [20] V. L. Budarin, J. H. Clark, R. Luque, D. J. Macquarrie, Chem. Commun. 2007, 634 – 636. [21] B. Zhang, J. Ren, Xiaohui Liu, Y. Guo, Y. Guo, G. Lu, Y. Wang, Catal. Commun. 2010, 11, 629 – 632. [22] J. Wang, W. Xu, J. Ren, X. Liu, G. Lu, Y. Wang, Green Chem. 2011, 13, 2678 – 2681. [23] E. Nikolla, Y. Roman-Leshkov, M. Moliner, M. E. Davis, ACS Catal. 2011, 1, 408 – 411. [24] a) A. K. Patra, S. K. Das, A. Bhaumik, J. Mater. Chem. 2011, 21, 3925 – 3930; b) S. Dutta, A. K. Patra, S. De, A. Bhaumik, B. Saha, ACS Appl. Mater. Interfaces 2012, 4, 1560 – 1564. [25] S. K. Das, M. K. Bhunia, A. K. Sinha, A. Bhaumik, ACS Catal. 2011, 1, 493 – 501. [26] R. W. Stevens, S. S. C. Chuang, B. H. Davis, Appl. Catal. A 2003, 252, 57 – 74. [27] S. K. Das, M. K. Bhunia, A. K. Sinha, A. Bhaumik, J. Phys. Chem. C 2009, 113, 8918 – 8923. [28] S. S. Lowell, J. E. Shields, M. A. Thomas, M. Thommes in Characterization of Porous Solids and Powders: Surface Area, Pore Size, and Density, Vol. 16 (Ed.: B. Scarlett), Springer Science, Amsterdam, 2004. [29] S. J. Dee, A. T. Bell, ChemSusChem 2011, 4, 1166 – 1173. [30] Q. Zhao, L. Wang, S. Zhao, X. Wang, S. Wang, Fuel 2011, 90, 2289 – 2293. [31] A. Dutta, D. Gupta, A. K. Patra, B. Saha, A. Bhaumik, ChemSusChem 2014, 7, 925 – 933. [32] X. Qi, M. Watanabe, T. M. Aida, R. L. Smith Jr., Green Chem. 2009, 11, 1327 – 1331. [33] Y. J. Pagn-Torres, T. Wang, J. M. R. Gallo, B. H. Shanks, J. A. Dumesic, ACS Catal. 2012, 2, 930 – 934. [34] Z. Zhang, Q. Wang, H. Xe, W. Liu, Z. K. Zhao, ChemSusChem 2011, 4, 131 – 138. [35] S. Lima, P. Neves, M. M. Antunes, M. Pillinger, N. Ignatyev, A. A. Valente, Appl. Catal. A 2009, 363, 93 – 99. [36] V. Choudhary, A. B. Pinat, S. I. Sandler, D. G. Vlachos, R. F. Lobo, ACS Catal. 2011, 1, 1724 – 1728. [37] Y. Yang, C.-W. Hu, M.-M. Abu-Omar, ChemSusChem 2012, 5, 405 – 410. [38] T. vom Stein, P. M. Grande, W. Leitner, P. Domnguez de Mara, ChemSusChem 2011, 4, 1592 – 1594. [39] J. B. Binder, J. J. Blank, A. V. Cefali, R. T. Raines, ChemSusChem 2010, 3, 1268 – 1272. Received: February 3, 2014 Revised: March 13, 2014 Published online on && &&, 0000

ChemSusChem 0000, 00, 1 – 10

&9&

These are not the final page numbers! ÞÞ

FULL PAPERS M. G. Mazzotta, D. Gupta, B. Saha,* A. K. Patra, A. Bhaumik, M. M. Abu-Omar* && – && Efficient Solid Acid Catalyst Containing Lewis and Brønsted Acid Sites for the Production of Furfurals

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

A tale of two acids: A Brønsted and Lewis acidic sulfonated carbonaceous catalyst is prepared by a one-pot synthesis method. The catalyst with a Lewis-to-Brønsted acid density ratio of 1.2 is effective for the production of 5hydroxymethylfurfural and furfural from carbohydrates in a biphasic solvent of water and methyltetrahydrofuran.

ChemSusChem 0000, 00, 1 – 10

&10&

These are not the final page numbers! ÞÞ