Computational Investigation of the Role of Active Site Heterogeneity for a Supported Organovanadium(III) Hydrogenation Catalyst

A crucial consideration
for supported heterogeneous catalysts is
the nonuniformity of the active sites, particularly for supported
organometallic catalysts. Standard spectroscopic techniques, such
as X-ray absorption spectroscopy, reflect the nature of the most populated
sites, which are often intrinsically structurally distinct from the
most active catalytic sites. With computational models, often, only
a few representative structures are used to depict catalytic active
sites on a surface, even though there are numerous observable factors
of surface heterogeneity that contribute to the kinetically favorable
active species. A previously reported study on the mechanism of a
surface organovanadium­(III) catalyst [(SiO2)­VIII(Mes)­(THF)] for styrene hydrogenation yielded two possible mechanisms:
heterolytic cleavage and redox cycling. These two mechanistic scenarios
are challenging to differentiate experimentally since the kinetic
readouts of the catalyst are identical. To showcase the importance
of modeling surface heterogeneity and its effect on catalytic activity,
density functional theory (DFT) computational models of a series of
potential active sites of [(SiO2)­VIII(Mes)­(THF)]
for the reaction pathways are applied in combination with kinetic
Monte Carlo (kMC) simulations. Computed results were then compared
to the previously reported experimental kinetic study: (1) DFT free-energy
reaction pathways indicated the likely active site and pathway for
styrene hydrogenation, a heterolytic cleavage pathway requiring a
bare tripodal vanadium site. (2) From the kMC simulations, a mixture
of different bond lengths from the support oxygen to the metal center
was required to qualitatively describe the experimentally observed
kinetic aspects of a supported organovanadium­(III) catalyst for olefin
hydrogenation. This work underscores the importance of modeling surface
heterogeneity in computational catalysis.