Cobalt(II) acetylacetonate complex immobilized on aminosilane-modified SBA-15 as an efficient catalyst for epoxidation of trans-stilbene with molecular oxygen

From environmental and economic points of view, it is highly desirable to develop a clean and efficient catalytic process to produce epoxides. An attractive approach is to use a solid, recyclable catalyst and molecular oxygen as the oxidant without any sacrificial reductant or other additives. Nonetheless, the catalysts reported up to now still cannot balance catalytic activity with epoxide selectivity. It is of great importance to explore novel catalysts with both high activity and selectivity for the epoxidation of olefins. In this work, cobalt(II) acetylacetonate (Co(acac)₂) was covalently bonded to the silica surface of SBA-15 molecular sieve by multi-step grafting using 3-aminopropytrimethoxysilane (APTS) as coupling agent. Characterizations with nitrogen physisorption, X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis suggested that the metal complex was successfully immobilized on the aminosilane-modified SBA-15 surface and the channel structure remained intact. The synthesized Co(acac)₂APTS@SBA-15 catalyst was used in the epoxidation of trans-stilbene (TS) with molecular oxygen. Compared to the sample prepared by the impregnation method as well as Co(acac)₂ solutions under the same reaction conditions, the Co(acac)₂ immobilized catalyst exhibited remarkably higher TS conversion and trans-stilbene oxide (TSO) selectivity. An increase in TS conversion with Co content was observed when the Co loading was lower than 0.70% and the 0.70Co(acac)₂APTS@SBA-15 sample exhibited the best catalytic performance. Up to 50.1% of TS conversion could be achieved within 6 h, affording TSO selectivity as high as 96.7%. The superior catalytic performance of this particular catalyst is attributed to the high activity of the immobilized Co(acac)₂ species on SBA-15.