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

This study presents experimental and theoretical evidence for distorted Ti–OH groups supported on amorphous SiO₂ 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.