(563g) High-Temperature Molecular Dynamics Simulations of Carbohydrates

Biofuels are still the only source of short-term renewable liquid fuels. For this reason, advancements in biomass conversion technologies continue to be of utmost priority. In addition to providing renewable fuels and chemical feedstock, developments in the biofuels industry will aid in the mitigation of global climate change and may provide opportunities to revitalize global economies.

Pyrolysis is a thermochemical conversion route where biomass is heated at high temperatures in an inert environment to produce permanent gases, bio-oil, and char. Pyrolysis products can further be upgraded to renewable fuels and chemicals. To this end it is important to understand the high-temperature behavior of the starting material. Cellulose, hemicellulose, and lignin comprise the bulk of plant matter. Extensive experimental and computational research is needed on the individual biomass components and raw feedstock to determine reaction mechanisms, product distributions, and ultimately kinetic models for the development of reliable conversion equipment. More recently, molecular dynamics (MD) modeling has been used to probe the decomposition of α-cyclodextrin during pyrolysis¹. The results indicate that the product distribution of α-cyclodextrin closely resembles that of cellulose, and thus might be a suitable small, surrogate compound for cellulose decomposition. This prior work further suggests that α-cyclodextrin (and cellulose) decomposes through various simultaneous reactions involving homolytic cleavage of glycosidic and pyran ring bonds.

In the present study, semi-empirical quantum mechanical methods were used in an attempt to validate the aforementioned hypotheses. High-temperature MD simulations of α-cyclodextrin show susceptibility to cleave bonds near the α-glycosidic linkage joining the individual glucose monomers. To establish a point of reference for using the semi-empirical method, MD simulations of α- and β-linked disaccharides were performed. When compared, the β-coupled disaccharide showed bond lengthening at the C-O bonds of the glucose monomer, whereas bond cleavage is more likely to occur at the glycosidic link of the α-coupled disaccharide, as observed in MD simulations of α-cyclodextrin.  Model compounds, such as α-cyclodextrin, may offer a fast and accurate alternative to modeling large molecules when using such advanced computational methods, which will at last allow for a deeper understanding of the fundamental chemistry governing pyrolytic decomposition of biomass.

¹ 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. (2011) Energy Environ. Sci. DOI:10.1039/c1ee02743.