Mark Crocker

1 H NMR spectroscopy of titania. Chemical shift assignments for hydroxy groups in crystalline and amorphous forms of TiO2

1 H magnetic-angle spinning (MAS) NMR measurements have been performed on a number of crystalline titanias, and on amorphous silica-supported titania and titania–silica, with the aim of measuring the characteristic proton chemical shifts of hydroxy groups bound to titanias of different crystalline form. In the case of anatase, signals observed at δ= 2.3 and 6.7 ppm correspond to terminal and bridging hydroxy groups, the results of deuterium exchange experiments (using D2O) and IR data supporting these assignments. For rutile, signals observed at δ= 2.2 and 5.3 ppm are similarly assigned. Hydroxy groups bound to amorphous titania supported on silica (containing tetrahedrally coordinated TiIV) are found to possess a characteristic chemical shift of δ= 3.3 ppm. Deconvolution of 1H NMR spectra of titania–silica (containing 8 wt.% Ti) indicate the presence of a signal at δ≈ 3.3 ppm, which is similarly assigned to hydroxy groups bound to tetrahedrally coordinated TiIV, together with signals assigned to anatase and silanol groups. These observations are consistent with literature reports indicating the presence of two main titania phases in titania–silicas: an amorphous phase containing isolated Ti sites tetrahedrally coordinated by Si—O and OH groups, and segregated nanodomains of TiO2(anatase or rutile).

Preparation of acidic forms of montmorillonite clay via solid-state ion-exchange reactions

A solid-state procedure has been developed which enables A13+ and Fe3+ ion-exchanged montmorillonite to be prepared via co-grinding of the clay with the appropriate metal nitrate at room temperature. Pyridine adsorption/infrared spectroscopic studies indicate that montmorillonite treated in this manner contains Brønsted and Lewis acid sites in significant quantities, suggesting that these easily prepared materials may be of interest as acid catalysts.

Do titanyl groups exist in titanium silicates? An experimental study

Abstract Two synthetic routes have been investigated, aimed at the preparation of silica-supported titanyl (>TiO) and titanol (>Ti(OH) 2 ) groups, the latter corresponding to the hydrated form of the titanyl group. In the first synthetic route, the titanyl complex TiOCl 2 (NMe 3 ) 2 was reacted with an aerosil, and the resulting material thermally treated to remove residual Cl and NMe 3 ligands. In an alternative route, silica (aerosil and silica-gel) was reacted with Ti(CH 2 Ph) 4 to afford mainly anchored >Ti(CH 2 Ph) 2 moieties, which were subsequently hydrolysed. Characterization of the resulting materials using a combination of surface analytical techniques revealed that in all cases at least two titania phases were obtained, corresponding to isolated tetrahedral Ti sites, and an amorphous form of TiO 2 containing six-coordinate titanium. For the syntheses based on Ti(CH 2 Ph) 4 , UV-vis and XPS data indicated that the relative proportion of the two phases formed was dependent on the support employed, aerosil affording predominantly (≡SiO) 2 Ti(OH) 2 sites. No evidence was found for the presence of three-coordinate titanyl species, >TiO, even when the aerosil-supported >Ti(OH) 2 sites were calcined at 500°C. It is, therefore, concluded that titanyl groups are unlikely to be present in significant concentrations in titanium silicates. When tested in the epoxidation of 1-octene with tert -butyl hydroperoxide, the model systems were found to display epoxidation activity comparable with that of a wide-pore Ti–zeolite, Ti-MCM-41. The observed turnover frequency was found to increase with increasing dispersion of the titania, consistent with the notion that isolated, Lewis acidic Ti(IV) centres are the most active sites for epoxidation catalysis.

Sulfur dioxide as a chemical probe for titanyl groups in titanium silicalites

Abstract The reaction of titanyl (TiO) groups with SO 2 , which for molecular titanyl complexes is found to occur readily at room temperature, has been used to test for the presence of titanyl groups in titanium silicalite (TS-1) and Ti-MCM-41. Exposure of samples to SO 2 does not lead to any reduction in the intensity of the characteristic IR band observed at 960 cm −1 , implying that this absorption is not associated with the presence of titanyl groups.