Multiscale Dynamics of Hemicellulose Hydrolysis for Biofuel Production

Abstract

Hemicelluloses are the earth’s second most abundant structural polymers and are found in lignocellulosic biomass. This work uses a tightly coupled experimental and theoretical analysis to show that the enzymatic hydrolysis of hemicellulose is a two-phase multiscale reactive process, comprising the molecular scale, the pore scale, and the reactor scale. The pore scale is the smallest of these three scales, and the sequence of the length scales is given by reactor scale > molecular scale > pore scale. Enzyme adsorption to the solid xylan particles is the first important step in hemicellulose hydrolysis, followed by the cleaving of β-(1 → 4)-glycosidic bonds by an endoenzyme in solid and liquid phases. Our experiments show that enzyme adsorption and solid-phase hydrolysis primarily occur on the pore surface, with the former, though initially nonequilibrium, attaining equilibrium at 5 h. At the molecular scale, the products (xylose and xylobiose) inhibit the hydrolysis noncompetitively in both liquid- and solid-phase hydrolyses, and the nature of product inhibition remains unaltered by the mixing at the reactor scale. Model–experiment comparisons allow us to quantify the adsorption and desorption rate constants as well as the kinetic constants in the solid- and liquid-phase hydrolyses. The optimum solid loading is obtained as 5 mg/mL, above which substrate inhibition sets in. We develop an “optimal mixing” strategy comprising 55–70 min of initial reactor mixing followed by no mixing for the rest of the hydrolysis, which increases xylose and reducing sugar yields by 6.3–8% and 13–20%, respectively, over continuous mixing at 150 rpm for 1–5 mg/mL xylan, with an energy saving of 94–96% on reactor mixing over 24 h. We quantify the dominant phenomena and the determining timescales at each length scale, and we show that efficient depolymerization of solid carbohydrates results from coupled interscale interactions at various stages of the two-phase hydrolysis process.