Modeling the Structural Heterogeneity of Vicinal Silanols and Its Effects on TiCl4 Grafting onto Amorphous Silica

Molecular metal complexes such as MClxreactreadily with hydroxyl-terminated surfaces to produce some of the"single-atom"catalysts used in important large-scale commercialreactions, including olefin metathesis, polymerization, and epox-idation. While the local oxide environment can vary at each metalsite, the manner and degree to which these differences impact thecatalytic activities of individual sites are poorly understood. In thiswork, we develop a computational framework to model the graftingof metal complexes onto amorphous supports and apply theframework to examine TiCl4grafting onto amorphous silica. We usedensity functional theory (DFT) to calculate free energies for TiCl4reactions at vicinal silanol sites. The kinetics and thermodynamicsdepend on the dihedral angle between the silanols. Vicinal silanolpairs with small dihedral angles are predicted to yield bipodal [(???SiO)2TiCl2] sites initially, but they react further with TiCl4vaporto give vicinal pairs of monopodal sites, [???SiOTiCl3]. In contrast, silanol pairs with large dihedral angles yield vicinal pairs ofmonopodal Ti sites directly. The DFT results were used to construct a population balance model to describe the kinetics of TiCl4grafting onto nonuniform sites of an atomistic model for amorphous silica. The solution of the population balance model predictsthat the majority of the vicinal pairs graft as bipodal [(???SiO)2TiCl2] sitesfirst and then slowly convert to monopodal [???SiOTiCl3]sites. The predictions provide a plausible explanation for the variability in the populations of monopodal and bipodal sites previouslyreported for TiCl4grafting