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.
The Lewis metal centers of completely dehydrated surfaces are marginally
effective active sites. Upon comparison with γ-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.