Mechanism of Hydrodeoxygenation of Acrolein on a Cluster Model of MoO3

We have explored the potential energy surface for reactions of acrolein on a Mo3O9 cluster model of the MoO3 surface to investigate the thermodynamics and kinetics of hydrogenation and selective hydrodeoxygenation. In the presence of hydrogen, conversions of acrolein to allyl alcohol, 1-propanol, and propene are all thermodynamically favorable, but the selectivity is controlled kinetically to form the least favorable product, allyl alcohol. We propose a mechanism in which coordinatively unsaturated Mo sites (i.e., oxygen vacancies) selectively chemisorb acrolein. On the basis of experimental and theoretical evidence, the active phase of the catalyst is a reduced hydrogen bronze, H x MoO3−y , and surface hydroxyl sites are occupied when x is in the range 1.1−1.2. Surface hydroxyls are important for both oxygen vacancy formation and as Brønsted acids. The reaction rate is essentially controlled by protonation of the C-1 carbon of chemisorbed acrolein. Additional reaction barriers for proton donation to the C-2 or C-3 sites are similar in magnitude (104−134 kJ/mol), limiting the rates of formation of propene and 1-propanol, respectively. In contrast, the selectivity toward allyl alcohol is determined by a smaller O−H bond formation barrier (33 kJ/mol). The estimated reaction rate is comparable to the rate of oxygen vacancy creation, so that operation in a continuous flow process appears to be feasible. The calculated reaction barrier for C−O scission is 104 kJ/mol; we discuss the advantages of other oxides, particularly WO3, that have stronger metal oxygen bonds and stronger Brønsted acidity of surface hydroxyls.