Near-Complete Recovery of Sugar Monomers from Cellulose and Lignocellulosic Biomass via a Two-Step Process Combining Mechanochemical Hydrolysis and Dilute Acid Hydrolysis

This study reports a two-step hydrolysis process for achieving near-complete recovery of sugar monomers from crystalline cellulose or lignocellulosic biomass. The first step is mechanochemical hydrolysis of the acid-impregnated sample in the solid state via ball milling at room temperature. It was found that mechanochemical hydrolysis not only effectively breaks the hydrogen-bonding network within the crystalline cellulose but also drives the acid-catalyzed hydrolysis reactions to form water-soluble products, mainly consisting of glucose and its oligomers, with a degree of polymerization up to 15. However, mechanochemical hydrolysis appears to be incapable of further hydrolyzing these oligomers into monomers and, hence, is not suitable for producing sugar monomers directly. Therefore, the second step is dilute acid hydrolysis of the mechanochemically hydrolyzed sample in the aqueous phase under low-severity conditions, i.e., at a low acid concentration of 0.25 wt % and a low temperature of 150 degrees C. The second dilute acid hydrolysis step can be completed rapidly (within 30 min) and achieves remarkable glucose recovery, up to similar to 91% from cellulose. A key innovation of the two-step hydrolysis process is that deep depolymerization in the first step (mechanochemical hydrolysis) is not required for completely converting crystalline cellulose into water-soluble products because all sugar oligomers can be effectively hydrolyzed into monomers in the second step (dilute acid hydrolysis). Our results also show that near-complete recovery of sugar monomers (similar to 94%) can be achieved from wood biomass via the two-step hydrolysis process, suggesting that this technology has the potential to replace the conventional enzymatic hydrolysis to recover sugar monomers from lignocellulosic biomass.