(159a) Cooperative Activation of Cellulose with Alkaline Earth Metals

Rapid thermal decomposition of lignocellulosic biomass yields a complex array of organic volatile compounds that form bio-oil when condensed and can be further upgraded to liquid fuels and chemicals. Lignocellulosic biomass contains inorganic metals such as alkaline earth metals that can catalyze and alter pyrolysis chemistry^[1]. However, the role these alkaline earth metals have on the fundamental chemical mechanisms involved in this chemistry remains unknown. This study utilizes experimental and computational techniques to detail the fundamental chemical mechanism on how alkaline earth metals assist in activation of cellulose. Time-resolved kinetic data is obtained via rapid thermal pulsing of α-cyclodextrin, a chemical surrogate of cellulose^[2], with the PHASR (Pulsed Heated Analysis of Solid Reactions) system^[3-5]. Thin-films of α-cyclodextrin were prepared with varying concentrations of CaNO₃ and MgNO₃ (up to 2 wt%) to model typical calcium and magnesium concentrations in lignocellulosic biomass. Cellulose conversion exhibits transitory kinetic behavior among all temperatures (370 - 430 °C) and alkaline earth metal concentrations. Catalytic kinetic rate constants were extracted and compared with values during non-catalyzed pyrolysis^[4]. Additionally, density functional theory calculations were performed on calcium/magnesium-catalyzed glycosidic bond cleavage and agree with experimentally obtained activation energies, allowing for the identification of an atomistic mechanism with good agreement between experiment and theory.

References:

1. Zhu, C.; Maduskar, S.; Paulsen A. D.; Dauenhauer, P. J., Alkaline-earth-metal-catalyzed thin film pyrolysis of cellulose. ChemCatChem, 2016, 8, 818-829.
2. Mettler, M. S.; Mushrif, S. H.; Paulsen, A. D.; Javadekar, A. D.; Vlachos, D. G.; Dauenhauer, P. J., Revealing pyrolysis chemistry for biofuels production: Conversion of cellulose to furans and small oxygenates. Energy & Environmental Science 2012,5, (1), 5414-5424.
3. Krumm, C.; Pfaendtner, J.; Dauenhauer, P.J., Millisecond pulsed films unify the mechanisms of cellulose fragmentation. Chemistry of Materials2016, 28, 3108-3114.
4. Zhu, C.; Krumm, C.; Facas, G. G.; Neurock, M.; Dauenhauer, P.J., Energetics of cellulose glycosidic bond cleavage. Reaction Chemistry and Engineering 2017, 2, 201-214.
5. Maduskar, S.; Facas, G. G.; Papageorgiou, C.; Williams, C.L.; Dauenhauer, P.J., Five rules for Measuring Biomass Pyrolysis Kinetics: PHASR Kinetics of Lignocellulosic Biomass. ACS Sustainable Chemistry & Engineering, 2018, 6, 1, 1387-1399.