(383d) Mechanistic Insights into the Role of Alkaline Earth Metals in Primary and Secondary Reactions during Cellulose Pyrolysis

Thermal breakdown of lignocellulosic biomass via fast pyrolysis is a technology of interest for producing liquid fuels and oxygenates that can be catalytically upgraded to valuable platform chemicals. It is well known that these challenging feedstocks typically contain group I and II metal impurities in them which not only enhance the breakdown rates for the feedstock but also significantly alter the product distribution. However, despite experimental evidence of the significance of these natural metal impurities, there is little insight into the molecular role played by these metals in influencing elementary steps of pyrolytic reactions. This study utilizes first principle density functional theory(DFT) in conjunction with experimental techniques such as the Pulsed Heated Analysis of Solid Reactions(PHASR) to elucidate the role of group II metals in catalyzing cellulose activation. A detailed mechanism is provided for glycosidic bond activation in the presence of group II metals which shows that these metal ions play a primary role of stabilizing charged transition states as well as a secondary role of disrupting the local hydrogen bonding network thus leading to enhanced reactivity of the feedstock. Activation barriers and rate orders obtained from the DFT computations are in good agreement with the experimentally measured kinetics obtained using α-cyclodextrin, a cellulose kinetic surrogate, at temperature between 370-430°C. In addition, this study also elucidates the mechanism for the catalytic cracking of levoglucosan, a major product of cellulose pyrolysis, and shows that alkaline earth metals can significantly enhance levoglucosan degradation. This finding agrees well with previously reported experimental evidence of suppressed yields of levoglucosan in presence of metal impurities.