The Alpha-Bet(a) of Glucose Pyrolysis: Computational and Experimental Investigations of 5-Hydroxymethylfurfural and Levoglucosan Formation Reveal Implications for Cellulose Pyrolysis

As biomass pyrolysis is a promising technology for producing renewable fuels, mechanistic descriptions of biomass thermal decomposition are of increasing interest. While previous studies have demonstrated that glucose is a key primary intermediate and have elucidated many important elementary mechanisms in its pyrolysis, key questions remain. For example, there are several proposed mechanisms for evolution of an important product and platform chemical, 5-hydroxymethylfurfural (5-HMF), but evaluation with different methodologies has hindered comparison. We evaluated a host of elementary mechanisms using a consistent quantum mechanics (QM) level of theory and reveal a mechanistic understanding of this important pyrolysis pathway. We also describe a novel route as a target for catalyst design, as it holds the promise of a more selective pathway to 5-HMF from glucose. We further demonstrate the effect of conformational and structural isomerization on dehydration reactivity. Additionally, we combined QM and experimental studies to address the question of whether only the reactions of beta-D-glucose, the cellulose monomer, are relevant to biomass pyrolysis, or if alpha-D-glucose needs to be considered in mechanistic models of glucose and cellulose pyrolysis. QM calculations show notable differences in elementary mechanisms between the anomers, especially in levoglucosan formation, which provide a means for evaluating experimental yields of alpha-D-glucose and,beta-D-glucose pyrolysis. The combined data indicate that both anomers are accessible under pyrolysis conditions. The kinetic and mechanistic discoveries in this work will aid catalyst design and mechanistic modeling to advance renewable fuels from nonfood biomass.