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Cooperative Activation of Cellulose with Natural Calcium
preprintsubmitted on 02.10.2020, 02:04 and posted on 05.10.2020, 07:18 by Gregory G. Facas, Vineet Maliekkal, Cheng Zhu, Matthew Neurock, Paul Dauenhauer
Naturally occurring metals such as calcium catalytically activate the inter-monomer β-glycosidic bonds in long chains of cellulose initiating reactions to volatile oxygenates for renewable applications. In this work, the millisecond kinetics of calcium catalyzed reactions were measured via the method of pulse-heated analysis of solid/surface reactions (PHASR) at high temperature (370-430 °C) to reveal accelerated glycosidic ether scission with a second order rate dependence on Ca2+ ions. First principles density functional theory (DFT) calculations were used to identify stable binding configurations for two Ca2+ ions that demonstrated accelerated transglycosylation kinetics with an apparent activation barrier of 50 kcal mol-1 for a cooperative calcium catalyzed cycle. The agreement of mechanism with calcium cooperativity to the experimental barrier (48.7 ± 2.8 kcal mol-1) suggests that calcium enhances reactivity through a dual role of disrupting native H-bonding and stabilizing charged transition states.