(407e) Metal Catalysts for Cooperative Activation of Cellulose

Heterolytic cleavage of cellulose glycosidic bonds enables the depolymerization and deconstruction of long-chain glucan biopolymers to small molecules for fuels and chemicals. The homogeneity of cellulose ether linkages (β, 1-4) allows for a common activating mechanism leading to rapid chain scission. As originally proposed by Shafizadeh and co-workers^[1], the activating mechanism of cellulose dominates the overall chemistry at 500 °C. In this work, we reveal the existence of two kinetic regimes of intra-chain cellulose activation transitioning at 467 °C^[2]. The high temperature activation mechanism associated with energy technologies (e.g. gasification, pyrolysis) exhibits high activation energy (~53 kcal/mol) and pre-exponential (10¹⁶ s^-1) consistent with the mechanism of concerted transglycosylation^[3]. The insertion of alkaline earth metals including calcium substantially enhance the rate of high temperature activation, by lowering the transition temperature at which this mechanism dominates^[4]. Measured apparent activation energy, pre-exponential, and rate expressions are reported for calcium catalysis consistent with computations of cooperating metal atoms.

[1] Bradbury, A. G. W.; Sakai, Y.; Shafizadeh, F.; A kinetic model for pyrolysis of cellulose. J. Appl. Polym. Sci. 1979, 23, 3271−3280.

[2] Krumm, C.; Pfaendtner, J.; Dauenhauer, P.J.; Millisecond Pulsed Films Unify the Mechanisms of Cellulose Fragmentation. Chem. Materials 2016, 28(9), 3108-3114.

[3] Zhu, C.; Krumm, C.; Facas, G.; Neurock, M.; Dauenhauer, P.J.; Energetics of Cellulose and Cyclodextrin Glycosidic Bond Cleavage. React. Chem. Eng. 2017, 2, 201-214.

[4] Zhu, C.; Maduskar, S.; Paulsen, A.D.; Dauenhauer, P.J.; Alkaline earth metal catalyzed thin-film pyrolysis of cellulose. ChemCatChem 2016, 8(4), 818-829.