Probing Reaction Details Critical to Converting Biomass to Fuel Using Molecular Simulation

Background: Developing alternative energy sources, specifically transportation fuels, to slow climate change has been a major research priority across all fields of science and engineering for decades. Biofuels can be implemented in the current transportation infrastructure, unlike stationary electricity sources (i.e. solar and wind power); however, production of a truly sustainable fuel is still too expensive to compete with fossil fuels. Corn and other starch-based biofuels are cheap to produce, but are shown to release more net greenhouse gases (GHGs) than fossil fuels. Biofuels derived from algae or cellulosic plants release less GHGs than fossil fuels, but require enzymes from bacteria and fungi that are expensive to produce on a large scale. Efficiency of enzymatic biomass conversion to fuel can be improved using solvents; recently a class of solvents called ionic liquids (ILs) have shown promise for this application, however little is understood about how the enzymatic reaction mechanism is influenced at the atomic level. Using molecular simulation we are able to understand the interactions of solvents and enzymes at the atomic level to engineer these essential reactions to be more efficient by optimizing novel solvents and catalysts. Methods: Using density functional theory (DFT) the detailed reaction mechanism of the hydrolysis of the β-1,4 glycosidic bond that links cellulose was examined. Atomistic details of solvent interactions with solute atoms involved in the reaction were determined using ab initio molecular dynamics (AIMD) to model relevant reaction steps based on results from DFT calculations. Similarly, the esterification reaction that converts bio-oils, such as those from algae, to biodiesel (alkyl ester) was modeled with DFT and AIMD to determine the influence of solvents on the reaction mechanism. Energy states corresponding to transition state geometries were compared for catalyzed and uncatalyzed reactions and varying solvents. Results: DFT simulations elucidated the order of events during hydrolysis and esterification reactions. The previously unknown order that bonds break and form in the hydrolysis of cellulose were uncovered along with corresponding transition state energies. Surrounding solvent atoms exhibit a distinct, previously unidentified, effect on the transition state energy and geometric features of reacting atoms for both the esterification and hydrolysis reactions. Conclusions: Molecular simulations of two reactions important for biofuel production uncovered critical details for interpreting reaction mechanisms relevant to fuel production from cellulosic biomass and bio-oils. Atomistic details will aid in the development of efficient catalysts and solvents to provide cost-effective, sustainable transportation fuels to help the world combat global climate change.