Lewis Acid Site and Hydrogen-Bond-Mediated Polarization Synergy in the Catalysis of Diels-Alder Cycloaddition by Band-Gap Transition-Metal Oxides

We present evidence from kinetic studies and electronic structure calculations that monoclinic ZrO2 and HfO2 can catalyze Diels-Alder [4 + 2] cycloaddition between furan and methyl acrylate. The two oxides present the same apparent activation energies, in the range of 11.5 kcal/mol, but HfO2 seems intrinsically more active. We use density functional theory (DFT) calculations to investigate catalytic pathways and the influence of surface hydration on activity. Partially hydroxylated surfaces are more reactive than the dehydrated or completely hydroxylated surfaces on account of a synergy between Lewis acidic metal centers and surface hydroxyl groups that lowers the LUMO of surface-bound methyl acrylate. We argue that surface hydroxyl groups present a polar environment which, via hydrogen bonding, are solely responsible for the rate acceleration on completely hydroxylated surfaces without necessarily proton transfer. metal centers of completely dehydrated surfaces are marginally effective active sites. Upon comparison with gamma-Al2O3, which has been reported to catalyze the Diels-Alder of cyclopentadiene with methyl acrylate, we argue that without compromising catalytic efficiency, ZrO2 and HfO2 present the advantage of selective catalysis without need for surface activation or a controlled environment.