(51b) Synthesis of Sulfonated 3DOm Carbon Catalysts for Hydrolysis of β(1-4) Glucan
Selective hydrolysis of cellulose into glucose is the key process for the utilization of biomass. It is the first step in the reaction network for converting biomass into fuels and high-value chemicals1-3. Conventional processes for the hydrolysis of cellulose using mineral acids, cellulase enzymes, and supercritical water have several drawbacks, such as the high cost for separation of products and catalysts, corrosion hazards, severe controls of enzymes, low selectivity and waste fluids. The increasing need for environmentally sustainable chemical processes has stimulated the use of recyclable heterogeneous acid catalysts as replacements for the unrecyclable homogeneous acid catalysts and enzyme-based techniques. In particular, sulfonated carbon catalysts (C-SO3H) have shown promising activity for the selective hydrolysis of cellulose in water under moderate conditions. However, due to the limited understanding of the interaction between the solid catalyst and water-insoluble cellulose, the catalytic property of carbon catalysts for this reaction is still elusive.
The aim of this study is to design a bifunctional heterogeneous catalyst with both acid and adsorption sites for the selective hydrolysis of β(1-4) glucan to glucose. Three dimensionally ordered mesoporous (3DOm) carbon with a cage size of 35 nm was successfully functionalized with sulfonic acid through covalent attachment of sulfonic acid-containing aryl radicals produced by homogeneous reduction of diazonium salt with hypophosphorous acid. It is found that the adsorption of saccharides on this carbon catalyst could be dominated by the hydrophobic interaction between the π-domains on the carbon surface and the CH groups on the saccharides.
The carbon catalyst was used to hydrolyze acidulated ball-milled cellulose (ABMC) to produce glucose. ABMC was prepared by incipient wetness of microcrystalline cellulose with dilute sulfuric acid and ball-milled, producing short β(1-4) chains (DP < 10) with α(1-6) branching. This ABMC has excellent solubility (10% in water) compared to ordinary β(1-4) glucan. The turn over frequency (TOF, (mole glucose)(mole H+)-1h-1) for ABMC reacted with the carbon catalyst and HCl was 1.5 and 0.18, respectively. A max glucose yield of 65% was obtained by reacting ABMC with the carbon catalyst. NMR studies revealed that β(1-4) glycosidic bonds break down rapidly while the α(1-6) bonds remain mostly unreacted using the carbon catalyst. The high reaction rate of the β(1-4) glycosidic bonds in ABMC when using the carbon catalyst is likely due to the interaction between α(1-6) branched β(1-4) glucan chains and adsorption on the carbon surface.
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