Hydrolysis of Poly(1-4-β-glucan) Strands Derived From Cellulose Using Mild Acidity and Temperature

Finding a selective synthetic catalyst for the hydrolysis of O-glycosidic bonds in cellulose is crucial for the transformation of biomass to fuels and chemicals. Whereas concentrated mineral acids used under harsh conditions (i.e. 230 ^oC; 500 psi) can rapidly accomplish this transformation with moderate selectivity, enzymes use a different mechanism to accomplish this transformation under mild conditions of pH and temperature while maintaining higher selectivity, albeit at a much slower rate. In this respect the bioinspired synthetic systems are exceptional because they demonstrate glycosidic bond hydrolysis under mild conditions of pH and temperature in aqueous solution. However, a major practical limitation is that they require the precise positioning of a carboxylic acid functional group adjacent to the glycosidic oxygen:  a molecule with carboxylic acid functionality in the ortho position has a 13,000-fold faster hydrolysis rate relative to a similar molecule where the carboxylic acid functionality is in the para position. To overcome such constraints, we have constructed a system that is capable of circumventing the rigid requirement of intramolecular acid catalyst positioning by essentially providing an array of acid groups in the vicinity of the O-glycosidic bond using a silica surface.  Our approach relies on the synthesis and characterization of a new class of materials comprising grafted, isolated poly(1-4-β-glucan) strands within an environment consisting of an ensemble of mildly acidic surface silanols in silica. We demonstrate that by careful control over the grafting density within a chain during synthesis, it is possible to control microenvironment as well as combustion temperature as measured using thermogravimetric analysis. We show that the close proximity of poly(1-4-β-glucan) to the weakly acidic silica surface is able to promote hydrolysis where H₃O⁺ activity is considered background. Finally the extent of hydrolysis of the grafted poly(1-4-β-glucan) strands is demonstrated under mild condition of pH and temperature.  This hydrolysis is shown to be highly dependent and controlled by the extent of surface interactions with adjacent silanols.