Mechanistic modeling of enzymatic hydrolysis of lignocellulosic biomass with detailed structural and morphological considerations

Enzymatic hydrolysis of lignocellulosic biomass is a complicated process involving a multitude of physico-chemical transformations. Many of the mechanistic models that have been developed to date have emphasized the reaction kinetics of component species during enzyme hydrolysis and have neglected to include the effect that the structure and morphology of biomass particles have on the reaction. Recent microscopy and cellulase-tracking studies at the National Renewable Energy Laboratory (NREL) indicate that, after dilute acid pretreatment, the cell wall contributes significant resistance to cellulase transport. Thus, for modeling purposes, we propose that diffusion across the cell wall is the greatest obstacle to the accessibility of cellulose to cellulases, whereas the overall biomass particle size and shape and the internal macroporous structure provide negligible resistance to transport over the time-scale of the hydrolysis reactions. In light of these features, we propose a one-dimensional hindered diffusion model through the cell wall to account for the accessibility of cellulose to cellulases. Cell walls are modeled as one-dimensional sheets, of varying porosity, through which cellulases diffuse and catalyze solids degradation. We couple this one-dimensional diffusion model with detailed reaction kinetics models that include lignin and hemicellulose species. A few model simulations illustrate how the progression of enzymatic hydrolysis may be predicted based on chemical and structural measurements of pretreated lignocellulosic biomass.