Understanding the Role of Defect Sites in Glucan Hydrolysis on Surfaces

Abstract

Though unfunctionalized mesoporous carbon consisting of weakly Brønsted acidic OH-defect sites depolymerizes cellulose under mild conditions, the nature of the active site and how this affects hydrolysis kinetics--the rate-limiting step of this process--has remained a puzzle. Here, in this manuscript, we quantify the effect of surface OH-defect site density during hydrolysis catalysis on the rate of reaction. Our comparative approach relies on synthesis and characterization of grafted poly(1→4-β-glucan) (β-glu) strands on alumina. Grafted β-glu strands on alumina have a 9-fold higher hydrolysis rate per glucan relative to the highest rate measured for β-glu strands on silica. This amounts to a hydrolysis rate per grafted center on alumina that is 2.7-fold more active than on silica. These data are supported by the lower measured activation energy for hydrolysis of grafted β-glu strands on alumina being 70 kJ/mol relative to 87 kJ/mol on silica. The observed linear increase of hydrolysis rate with increasing OH-defect site density during catalysis suggests that the formation of hydrogen bonds between weakly Brønsted acidic OH-defect sites and constrained glycosidic oxygens (i.e., those juxtaposed adjacent to the surface) activates the latter for hydrolysis catalysis. Altogether, these data elucidate crucial structural requirements for glucan hydrolysis on surfaces and, when coupled with our recent demonstration of long-chain glucan binding to mesoporous carbon, present a unified picture, for the first time, of adsorbed glucan hydrolysis on OH-defect site-containing surfaces, such as unfunctionalized mesoporous carbon.

Mechanism and Kinetic Analysis of the Hydrogenolysis of Cellulose to Polyols

“…Carbons are hydrothermally stable under the reaction conditions typically at temperatures of 423-573 K. Moreover, the surface of carbon materials can be furnished with plenty of OH and carboxyl groups. Such weak acidic sites have been found to contribute to the activation of the glycosidic bonds in cellulose and consequently promote the hydrolysis of cellulose [27,35,[47][48][49][50][51]. Conversely, strong acid sites on carbon materials are unequivocally effective for cellulose hydrolysis [52][53][54].…”

“…A whole expression for the reaction kinetics of cellulose hydrogenolysis can be obtained according to the kinetics of individual reactions and their correlation in a model. For the first reaction step, i.e., cellulose hydrolysis, there have been a plenty of studies conducted because this is a really important topic for biomass utilizations [35,51,59,. Several recent reviews presented very comprehensive summary on the reaction kinetics for cellulose hydrolysis [59,75,76,82,85,94].…”

“…The weak acid sites adsorbed soluble oligosaccharides onto the surface of carbon material and promoted the overall reaction rate [104]. Katz et al further manifested the promoting function of weak acidic OH-defect sites on the surface of supports for cellulose hydrolysis by comparatively studying the performance of modified silica and alumina in β-glucan hydrolysis [51]. The authors concluded that (1) OH-defect sites on the surface favored adsorption of the β-glu strand in highly constrained environment and (2) OH-defect sites activated glycosidic bonding for hydrolysis via hydrogen bonding.…”

Depolymerization of Cellulosic Biomass Catalyzed by Activated Carbons

“…7) in trace hydrochloric acid at pH 4 [ 24 , 25 ]. The catalytic activity correlated with the density of surface OH groups [ 26 ]. In contrast, free cellulose cannot be depolymerized by the catalysts under the same conditions.…”

Grafting of Different Monomers onto Nano Aluminum Surface and Studying Their Biological Activities

“…Alumina has attracted a particular attention as a filler to augment the properties of polymer materials due to its high refractive index, its high thermal conductivity, and the absence of light absorption in the visible range [8−11]. There are only few studies on introducing polymer brushes onto the surface of α-alumina [12,13]. Van der Waals forces and the electrostatic repulsive energy between colloidal particles are significantly favorites domination of ceramic particles dispersion characteristics.…”