Experimental and Theoretical Evidence for Distorted Tetrahedral ≡Ti–OH Sites Supported on Amorphous Silica and Their Effect on the Adsorption of Polar Molecules

Abstract

This study presents experimental and theoretical evidence for distorted �Ti−OH groups supported on amorphous SiO 2 and examines the influence of distortion on the strength of adsorption of polar molecules. For the theoretical part of this effort, we developed a model for isolated �Si−OH or �Ti−OH groups on the surface of amorphous silica. The �M−OH group is represented by a T8−11 cluster surrounded by a T747 cluster representing the surrounding amorphous silica. The properties of the small cluster are described by high-level density functional theory (DFT) (i.e., the quantum mechanical (QM) region), whereas the large surroundings are represented by molecular mechanics (MM). The QM/MM model was validated by demonstrating that the predicted enthalpy of adsorption for seven polar molecules on �Si−OH groups agrees satisfactorily with experimentally measured values determined by microcalorimetry. We also found that enthalpies of adsorption on isolated �Si−OH groups determined from isotherms obtained by infrared (IR) spectroscopy agree very well with the microcalorimetric values. IR spectroscopy was then used to measure isotherms for pyridine adsorption to Lewis acidic Ti isolated �Ti−OH groups grafted to amorphous silica. The isosteric enthalpy of adsorption decreased in magnitude with increasing pyridine coverages up to a coverage of 15% and then remained relatively constant for higher coverages. QM/MM calculations made with our model revealed that the enthalpy of adsorption is proportional to the area of the triangular O−Ti−O facets of the �Ti−OH group to which pyridine is bound. �Ti−OH sites exhibiting facet areas consistent with those deduced from X-ray absorption spectroscopy (XAS/EXAFS) measurements bind pyridine with enthalpies of adsorption consistent with the observed plateau for pyridine coverage above 15%. Larger tetrahedral facet areas are required to explain the coverage-dependent enthalpies of adsorption below 15% coverage, evidencing distorted �Ti−OH structures. Distorted �Ti−OH sites are also qualitatively consistent with reduced-intensity pre-edge X-ray absorption near-edge structure (XANES) measurements reported in this study. Energy decomposition analysis (EDA) calculations were carried out to understand the underlying physical phenomenon governing the change in the enthalpies of adsorption pyridine as a function of tetrahedral facet area.

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

“…We have recently reported that a small fraction (<15%) of silica-supported Ti–OH sites are heavily distorted. In comparison, the remaining 85% have structures consistent with those derived from analysis of EXAFS data . These conclusions were drawn from a combination of experimental and theoretical studies of pyridine adsorption on silica-supported Ti–OH sites.…”

“…The straddled geometry is more favorable for facet areas less than 3.925 Å 2 , whereas the in-plane geometry is more favorable above this value. Since most Ti–OH sites in Ti/SiO 2 exhibit facet areas smaller than 3.925 Å 2 , the straddled configuration is likely the more common one . However, the minority of in-plane transition states for facets larger than 3.925 Å 2 are relatively stable.…”

“…The QM/MM approach used for this study is identical to that used in our previous work and is discussed there in detail . Briefly, the surface is divided into a central QM region, composed of approximately 8–11 tetrahedral (T) atoms, surrounded by a model for the amorphous silica, described by MM, that extends 29 Å in radius from the central Ti atom.…”

“…Here, we report the results of a computational investigation of the effects of Ti–OH site distortion on the mechanism and kinetics of cyclohexene (C 6 H 10 ) epoxidation with H 2 O 2 to form cyclohexene oxide (C 6 H 10 O). We aim to establish the connection between these effects and experiments by leveraging our recently acquired knowledge of the Ti–OH facet area distribution . We chose this particular reaction because its gas-phase kinetics have been reported recently by Notestein and co-workers, and, hence, we can use the kinetic parameters reported by these authors for comparison .…”

“…We begin our analysis of Mechanism 1 by computing the Gibbs free energies of each step using our experimentally validated Quantum-Mechanics/Molecular Mechanics (QM/MM) approach . These free energies are then used to predict the rate coefficients for each elementary step in Mechanism 1.…”