Ab-Initio Study of Glycerol Dehydration Mechanisms with Explicit Solvent Treatment

Glycerol produced as a waste product in biodiesel production has interesting potential as a low-cost biomass-derived feedstock for chemical processing. A process of current academic and commercial interest is the acid-catalyzed dehydration of glycerol to acrolein, a valuable platform chemical. Such a process would improve the economic viability of biodiesel production while also producing acrolein from renewable resources rather than from propylene – the current commercial process. Despite a breadth of study on catalysts for this chemistry, relatively little has been done to investigate the reaction mechanism at a fundamental level.

We investigated the acid-catalyzed dehydration mechanism of glycerol to acrolein using ab-initio methods. Solvent effects were accounted for using an implicit solvent model and a number of explicit water molecules. We show that the reaction proceeds first through a hydride transfer (dehydration) inhibited by the solvent (water molecules) and forms an aldol intermediate, which tautomerizes to the unsaturated diol via a solvent mediated proton transfer. Though the latter is easily reversible to the lower energy aldol, it can also form acrolein as a final product. We shall argue that the mechanism favors the formation of acrolein through the diol, in stark contrast to the mechanisms previously proposed in the literature, which form acrolein through the aldol. Gas phase calculations in the literature have suggested that production of acetaldehyde and formaldehyde are kinetically favored over production of acrolein. However, our study clearly demonstrates that systems with explicit water molecules correctly predict acrolein selectivity, in agreement with experiments. These insights provide leverage for the rational design of more effective catalysts to promote this chemistry.