Biomass-derived carbohydrates are important platform molecules for the production of renewable fuels and chemicals. Their production requires the depolymerization of cellulose and hemicellulose, the main polysaccharides in lignocellulosic biomass, to glucose and xylose, respectively. However, in inexpensive systems such as pure water or aqueous dilute acid mixtures, carbohydrate monomer degradation rates exceed hemicellulose and especially cellulose depolymerization rates at most easily accessible temperature, leading to low sugar yields. Because of this kinetic problem, cellulose is often depolymerized in the presence of expensive enzymes, concentrated mineral acids or even ionic liquids. All of these systems are used because they feature higher rates of polysaccharide depolymerization compared to sugar degradation, enabling its recovery at higher yields. We have demonstrated an alternative strategy that involves the reversible stabilization of xylose and glucose by acetal formation notably with formaldehyde. In solvent-water systems at low water concentrations (&lt;20%) and dilute acid environments, the diols present on glucose and xylose readily react with formaldehyde to form acetals preventing their dehydration to furans. Once the resulting protected sugars are treated in high water environments (&gt;50% H2O), the acetals hydrolyze quantitatively releasing formaldehyde and simple sugars. When performing an extensive pretreatment of beech in the presence of formaldehyde, over 90% of xylan could be converted to diformylxylose. In comparison, the same pretreatment without formaldehyde lead to almost full xylose degradation into furfural with only 16% xylose recovery. Similarly, depolymerization of cellulose led to over 70% yields of glucose and a final concentration of ~5 wt%, whereas the same reaction in the absence of formaldehyde led to yields below 30%. Therefore, this stabilization strategy pushes back the longstanding kinetic limitations in bio-derived polysaccharide depolymerization and enables a new method for recovering biomass-derived carbohydrates at high yields and concentrations. Furthermore, acetal protected sugars can be produced almost quantitatively from glucan and xylan and, depending on the aldehyde used for functionalization, can have widely differing properties. Therefore, we are also beginning to explore the unique properties and reactivity of these protected sugars to develop new upgrading or separation routes for biomass-derived sugars.

Carbohydrate stabilization by acetal formation prevents degradation and leads to new reactivity during biomass depolymerization

2019 Fall National Meeting

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