Cellulose is a renewable resource that can be used for the production of fuels and chemicals. Bifunctional solid acid catalyst have been proposed for hydrolysis of cellulose to glucose. However, there is little mechanistic understanding of the catalytic process, which limits rational development of such catalysts. Frequently overlooked control experiments include catalyst stability at hydrothermal reaction conditions. Catalyst degradation could result in the release of liquid phase acid that could have a significant effect on the hydrolysis rate of cellulose. Therefore, the contribution of liquid acid must be considered when interpreting the catalytic activity of a bifunctional solid acids.  In this study we investigated chloromethylpolystyrene based polymer solid acid catalyst. Chloromethyl moieties are hypothesized to function as binding groups, while sulfonic acids act as catalytic sites. Structural analysis revealed lack of chlorine and presence of hydroxyl groups on the external surface of the catalyst. The high catalytic activity towards cellulose hydrolysis pointed to the beneficial effect of the presence of these groups, but their contribution to the reaction mechanism was unclear. We tested the hydrothermal stability of the catalyst and found release of hydrochloric acid, evidenced by significant drop of reaction media pH and commensurate increase of chloride ion concentration. Time dependent studies revealed monotonic release of acid. Kinetic models for cellulose hydrolysis take into account acid concentration as a preexponential factor in the rate constant. We hypothesized that applying the acid concentration as a time dependent function in the hydrolysis model would predict glucose yields to an approximate degree. Good agreement between modeled and experimental glucose yield pointed to liquid acid acting as the primary catalyst, obscuring catalytic activity, if any, of the solid acids. The results in this investigation highlight the need for systematic study of proposed solid acids for cellulose hydrolysis and consideration of potential release of homogeneous acids in the reaction media.

Implications of homogeneous acids on interpretation of solid acid activity for celloluse hydrolysis

256th National Meeting (2018)

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