Reactivity studies, structural characterization, and thermolysis of cubic titanosiloxanes: precursors to titanosilicate materials which catalyze olefin epoxidation

Abstract

The cubic titanosiloxane [RSiO₃Ti(OPrⁱ)]₄ (R = 2,6-Pr₂ⁱC₆H₃NSiMe₃) (1) is found to be relatively inert in its attempted reactions with alcohols and other acidic hydrogen containing compounds. The reaction of 1 with silanol (Bu^tO)₃SiOH however proceeds over a period of approximately 3 months to result in the hydrolysis of (Bu^tO)₃SiOH and yield the transesterification product [RSiO₃Ti(OBu^t)]₄ (2) rather than the expected [RSiO₃Ti(OSi(OBu^t)₃)]₄. Products 1 and 2 have been characterized by elemental analysis, thermal analysis, and spectroscopic techniques (IR, EI-MS, and NMR). The solid-state structures of both 1 and 2 have been determined by single-crystal X-ray diffraction studies. Compounds 1 and 2 are isomorphous and crystallize in a cubic space group with a central cubic Ti₄Si₄O₁₂ core. Solid state thermolysis of 1 was carried at 450, 600, 800, 900, 1000, and 1200 °C in air, and the resulting titanosilicate materials 1a-f were characterized by spectroscopic (IR and DR UV), powder XRD, and electron microscopic methods. While, the presence of Ti-O-Si linkages appears to be dominant in the samples prepared at lower temperatures (450-800 °C), phase separation of anatase and rutile forms of TiO₂ occurs at temperatures above 900 °C as revealed by IR spectral and PXRD studies. The presence of octahedral titanium centers was observed by DR UV spectroscopy for the samples heated at higher temperatures. The use of new titanosilicate materials as catalysts for olefin epoxidation has been investigated. The titanosilicate materials produced at temperatures below 800 °C with a large number of Ti-O-Si linkages (or tetrahedral titanium centers) were found to be more active catalysts compared to the materials produced above 900 °C. The observed conversion in the epoxidation reactions was found to be somewhat low although the selectivity of the epoxide formation over the other possible oxidized products was found to be very good.