A solid-acid-based process for the conversion of hemicellulose†

The conversion of biomass into chemicals, fuels and energy using the bio-refinery concept has attracted lot of attention. In particular, lignocellulosic biomass, consisting of cellulose (ca. 50%), hemicellulose (ca. 30%) and lignin (ca. 20%) is of special interest because it is non-edible and in plentiful supply. Hemicellulose is a heterogeneous polymer, made up of C5 and C6 sugars (such as xylose, arabinose, mannose, glucose, etc.) depending on the source. In the hydrolysis of hemicellulose, selective cleavage of the C–O bonds present between adjacent sugar molecules is very important to yield intact monomer sugar molecules. Conventionally, mineral acids and enzymes are used to hydrolyze these materials, but recently, it has been shown that heterogeneous catalysts can convert cellulose into sugars and its derivatives in a one-pot reaction. Conversion of hemicellulose into monomeric sugars in a two-step process over ion-exchange resins has also been reported. However, in these reports initial autohydrolysis of hemicellulose was carried out at around 160 ◦C to yield water-soluble oligomers. Ionexchange resins were then added to the solutions to yield sugar monomers. In this report, we describe for the first time a one-pot process for the conversion of solid hemicellulose (without any pretreatment) into xylose, arabinose and furfural using solid acid catalysts (zeolites, clays, and metal oxides) in aqueous media (Scheme 1).Also, we show that it is possible to selectively convert just the hemicellulose in lignocellulosicmaterials using solid acid catalysts. Moreover, we show that the catalysts can be re-used in 5 runs with almost the same activity. C5 and C6 sugars are important building blocks for the synthesis of chemicals and fuels, and the importance of xylose and furfural is reported in aUSDepartment of Energy report on the 12 top value-added chemicals. The hydrogenation of xylose using supportedmetal catalysts gives xylitol, which finds use as a sweetener in food and pharmaceutical industries. In addition, it is possible to convert xylose/xylitol into glycols, while under acidic conditions xylose undergoes dehydrocyclization to yield furfural. Furfural is used in the agricultural, plastics, and oil-refining industries, and is a raw material in the synthesis of furfuryl alcohols, furan and THF.