RESUMO
The precipitous decline in the rates of enzymatic hydrolysis of cellulose with conversion is one of the major limitations to the commercialization of second-generation biofuel. In this work, various rate-limiting factors (fractal kinetics, changes in crystallinity, accessibility, reactivity and hydrolysable fraction, enzyme clogging, and degree of polymerization) were investigated employing experimental as well as computational studies. Model-guided experiments showed cellulose accessibility and the hydrolysable fraction of accessible substrate (a previously undefined and unreported quantity) to decrease steadily until a conversion level of nearly 70%, while cellulose reactivity, defined in terms of hydrolytic activity per amount of actively adsorbed cellulase, remained constant. Substrate depletion, accessibility and hydrolysability decrease accounted for approximately 90% of rate retardation up to 70% conversion. Faster restart rates were observed on partially converted cellulose as compared to uninterrupted hydrolysis rates, supporting an enzyme clogging phenomenon that could possibly be responsible for the additional rate decrease.
