Catalysis Science & Technology PAPER Cite this: Catal. Sci. Technol., 2015, 5, 531 Received 12th August 2014, Accepted 16th September 2014 DOI: 10.1039/c4cy01047g www.rsc.org/catalysis Brönsted acidic ionic liquid-catalyzed conversion of hemicellulose into sugars† Babasaheb M. Matsagar and Paresh L. Dhepe* Development of a competent method for the conversion of hemicellulose, a lignocellulosic component, into sugars is essential. In a one-pot method, Brönsted acidic ionic liquid (BAIL)-catalyzed hydrolysis of hardwood hemicellulose at 160 °C in water media gave C5 sugars (xylose + arabinose) in 87% yield. The efficiency of the ILs and their acid strength both followed the similar trend, ijC 3 SO 3 HMIM]ijHSO 4 ] > [C 3 SO 3 HMIM][PTS] > [C 3 SO 3 HMIM][Cl] > [BMIM][Cl]. The ion–dipole-type interaction present between the BAIL and the substrate, which is proposed by the 1 H NMR study, is suggested to help in achieving better activity with BAIL than with mineral acid, H 2 SO 4 . 1. Introduction Efficient production of chemicals and fuels from alternative renewable resources, such as biomass, is a major challenge. 1–4 The second largest component (20–35%) of plant-derived lignocellulosic biomass, hemicellulose, is usually known as a complex heteropolysaccharide as it is made up of pentoses (xylose, arabinose) and hexoses (glucose, galactose, mannose) along with minor quantities of sugar acids. 5,6 Depending on the composition, hemicellulose can be named as xylan (xylose, arabinose), arabinogalactan (arabinose, galactose), glucomannan (glucose, mannose), etc. However, amongst all these hemicelluloses, xylans, which are usually present in hardwood, are available in large amounts. Generally, the degree of poly- merization (DP) for hardwood hemicellulose ranges from 70 to 250 and for softwood hemicellulose from 100 to 200. 7 Even though hemicellulose is available in an enormous quan- tity from paper and pulp industries, it is still considered as one of the most under-utilized substrates to produce chemicals. The possibility of hemicellulose undergoing hydrolysis reactions to produce sugar monomers is known from the literature. 1–14 These sugars in turn are capable of producing several other industrially important chemicals, such as sugar alcohols (low-calorie sweeteners), 8 furans, such as furfural and 5-hydroxymethyl furfural (HMF) as well as acids (gluconic, xylonic, levulinic, formic, etc.). 9 Additionally, their conversion into other value-added chemicals is well documented. 10–12 Thus, it is vital to synthesize sugars from hemicellulose with high efficiencies. Conventionally, mineral acid- and enzyme-catalyzed methods for the hydrolysis of hemicellulose into sugars are known. 13–15 But to overcome drawbacks associated with these methods, recently conversion of hemicellulose into sugars using solid acid catalysts, such as zeolites (HUSY, Hβ, HMOR) and silicoaluminophosphates (SAPOs) has been shown. 16–20 However, in these reactions it was difficult to attain higher yields of sugars in respectable time. 16,17 The selective conver- sion of hemicellulose present in bagasse has also been carried out in biphasic medium using the catalyst HUSY, obtaining 20% C5 sugars and 55% furfural. 20 Conversion of hemicellu- lose using the microwave irradiation technique is also claimed in the literature to yield 23–31% C5 sugars. 21 Since ionic liquids (ILs) show tunable properties 22,23 and are considered as green solvents, their uses in the biomass-related reac- tions have increased in the last couple of years. In one of the latest studies, hydrolysis of hemicellulose in the pres- ence of 1-ethyl-3-methylimidazolium chloride ionic liquid, [EMIM][Cl], and mineral acid, H 2 SO 4 (200 mM), has been reported to achieve xylose in 47% yield. However, in the same work, a substantial amount of the ionic liquid (500 μl) was used compared to that of the substrate (27 mg). 24 Application of the IL, 1-ethyl-3-methylimidazolium hydrogensulfate, [EMIM][HSO 4 ], in the synthesis of furfural (29%) has also been reported in the literature. 25 In yet another report, synthesis of furfural from xylose using dimethylacetamide (DMA)-LiCl, CrCl 2 and the IL [EMIM][Cl] has been described. 26 Ionic liquids are also widely used in the dehydration reactions of C6 sugars (glucose, fructose) to yield HMF. 27,28 In one of the reports, researchers have used 1-butyl-3-methyl imidazolium chloride, [BMIM][Cl], as a solvent and metal chlorides as catalysts to achieve HMF in 71% yield from glucose under Catal. Sci. Technol., 2015, 5, 531–539 | 531 This journal is © The Royal Society of Chemistry 2015 Catalysis and Inorganic Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India. E-mail: pl.dhepe@ncl.res.in; Fax: +912025902633; Tel: +912025902024 † Electronic supplementary information (ESI) available: Catalyst synthesis, 1 H and 13 C NMR data for ionic liquids, microanalysis data, TGA of ionic liquids and temperature-programmed desorption data for zeolites. See DOI: 10.1039/ c4cy01047g Published on 16 September 2014. Downloaded on 07/06/2017 09:46:51. View Article Online View Journal | View Issue