Acid-catalysed conversion of cellulosic biomass into value added small molecules

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Acid-catalysed conversion of carbohydrates into organic building block molecules is a promising way to create renewable replacements for fossil fuel-based products. Despite this promise, it is presently not known how to usefully and economically convert native non-food-competitive cellulosic materials into sustainable carbon zero fuels and chemicals in high yields and with low losses. With the aim to remove the blockage towards the biorefinery, this project systematically studies the acid-catalysed transformation of cellulosic (poly)carbohydrates and provides innovative methods to efficiently convert raw, unrefined biomass into value added derivative products. To explore catalytic reactions of cellulosic polysaccharides, this work starts with model transformations of monomer glucose under the action of Lewis acidic metal trifluoromethanesulfonates (metal triflates), Brønsted acids or combined Lewis/Brønsted acid catalysts in water and methanol. The work underscores the notion that metal triflates are highly tunable catalysts, which under optimised conditions can selectively convert glucose into disaccharides and oligosaccharides, fructose, methyl glycosides, or methyl levulinate. The tunable acidic catalyst systems are further employed in the high-yielding transformation of microcrystalline cellulose into ethyl levulinate in ethanol. The pretreatment of raw and unrefined cellulosic biomass with a biobased deep eutectic solvent affords similarly efficient transformation thereof into ethyl levulinate. In parallel, the project interrogates the valorisation of cellulosic biomass in ionic liquids. Firstly, it researches zinc chloride hydrates with a molecular formula ZnCl₂·𝘯H₂O (𝘯 = 2.5–4.5) as solvent-catalyst media for the production of low molecular weight saccharides and furan type molecules. It defines the preferred reaction conditions to select furyl hydroxymethyl ketone and furfural, or low molecular weight saccharides and 5-(hydroxymethyl)furfural, from the processing of cellulosic materials. In addition, the work employs a co-solvent system, comprising 1-butyl-3-methylimidazolium chloride and the deep eutectic solvent choline chloride/oxalic acid for the selective depolymerisation of cellulosic biomass into derivative monomer sugars and water-soluble oligoglucans. It separately probes the reactivity of native polysaccharides in the neat deep eutectic solvent and identifies preferred conditions for the direct transformation of structurally branched polysaccharides into monosaccharides and furans, simultaneously providing fine cellulosic powder. The unreacted cellulose may be further beneficiated into additional useful chemicals, optimising towards total use of the biomass. Finally, the work targets a deeper understanding of the dissolution, recovery, and characterisation of cellulose in various classes of ionic solvents. It combines the obtained knowledge of the processing of cellulose in the abovementioned ionic systems, providing practical recommendations for their use in cellulose refinery.
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