The Effects of ≡Ti–OH Site Distortion and Product Adsorption on the Mechanism and Kinetics of Cyclohexene Epoxidation over Ti/SiO₂

This work presents the effects of ≡Ti–OH site distortion in Ti/SiO2 on the kinetics and mechanism of gas-phase cyclohexene (C6H10) epoxidation to form cyclohexene oxide (C6H10O). We utilize an experimentally validated computational method to calculate enthalpies of adsorption and transition states along a well-established mechanism for the catalytic cycle. We discover that adsorption enthalpies correlate with the facet area of the tetrahedral O–Ti–O facets of the ≡Ti–OH group through which the adsorbate binds to Ti. In contrast, enthalpies of H2O2 activation and O atom transfer are relatively insensitive. We then develop a steady-state microkinetic model (MKM) to investigate the effects of distortion on predicted kinetic observables (apparent activation energy (Ea) and reaction orders in the partial pressures of C6H10 and H2O2) and to establish whether the mechanism is consistent with observed kinetics. Product inhibition increases with increasing facet area, significantly impacting the predicted activity. Building on our recent findings that ≡Ti–OH sites in the absence of reaction exhibit facet areas equal to, or greater than, that derived from X-ray absorption spectroscopy (XAS) measurements (≥3.76 Å2), we discover that the predicted kinetics for such sites are inconsistent with experiments. Much smaller facets are required for good agreement (<3.54 Å2) since C6H10O does not inhibit these facets. We show that the adsorption of C6H10O to one facet significantly reduces the facet area of the vacant facets on the opposite side of the same ≡Ti–OH site. C6H10O adsorption also considerably narrows the area distribution of these vacant facets for the set of ≡Ti–OH sites that have more than one fluid-accessible facet. We show that these reduced-area facets can catalyze the epoxidation of cyclohexene, while C6H10O remains co-adsorbed to the other facet (known as Pathway B in this paper). Using our MKM with an expanded mechanism for epoxidation that includes Pathway B, we find that Pathway B dominates the net rate of production of C6H10O at nearly all partial pressures of product expected along the length of a packed bed reactor. We also find that the predicted activity remains essentially constant with the level of ≡Ti–OH site distortion. This mechanism shows quantitative agreement between experiments and our predictions for the Ea, reaction orders in the partial pressures of reactants, and the Gibbs free energy barrier. It also illustrates the key role of adsorbates in influencing the degree of distortion of ≡Ti–OH sites, in addition to the amorphous support itself.