Fast pyrolysis is a promising approach for the production of renewable fuels and value-added chemicals from non-edible, second generation lignocellulosic biomass. The product distribution of pyrolysis, particularly the quality and yield of bio-oil, is significantly altered by the operating conditions and the nature of the feedstock. Pyrolysis chemistry literature also suggests that the presence of naturally occurring alkali and alkaline-earth-metals considerably modifies the product distribution. However, the possible catalytic role of these metals in altering the product distribution has not yet been investigated in detail. In this work, we first present experimental results for thin-film pyrolysis (transport-free effects) and then perform density functional theory (DFT) calculations to understand the stereoelectronic basis of Na+, K+, Ca2+ and Mg2+ ions (majorly found in biomass) on the product distribution. β-D-Glucose (monomer of cellulose) and cellobiose are used as model biomass compounds for the study. The metal-free thin-film experimental cellobiose pyrolysis exhibits preferential formation of levoglucosan (LGA) and furans. Cellobiose/alkali ions show ~40% decrease in bio-oil with an increase in char yield while Ca2+ and Mg2+ strongly favour furans. Further, the sequential breakdown of intermediates formed post glycosidic-bond-breaking is explored by glucose thin-film experiments. Glucose in absence of metals forms pyrans predominantly. The glucose/Na+ complex forms anhydroglucofuranose (AGF), furfural in glucose/Mg2+ while levoglucoseneone (LGO) in glucose/Ca2+. Succeeding the experiments, the atomistic insights on the reaction pathways of sugar/metal are modelled using DFT. The Ea for glycosidic-bond-breaking in metal-free cellobiose are 54 and 66 kcal.mol-1 for LGA and furans, respectively. The opposite trend in Ea (~50 and 58 kcal.mol-1 for LGA and furans, respectively) and yields of bio-oil for cellobiose/alkali ions can be attributed to higher preference (low Ea) for repolymerisation by Na+ and K+ ions favouring char. Further, the Ea for metal-free glucose exhibits 60 kcal.mol-1 for pyrans, 49 kcal.mol-1 for AGF in glucose/Na+ and 76 kcal.mol-1 for LGO by glucose/Ca2+. Thus, the experimental product distribution and DFT calculated barriers suggests that presence of naturally occurring metals in biomass significantly perturbs the reaction kinetics depending upon the magnitude of metal ion interaction with charge-separated transition states.

Catalytic influence of naturally occurring salts on biomass pyrolysis chemistry: A combined experimental and DFT study

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.