Novel mesoporous (Cr)MCM-48 molecular sieves: Promising heterogeneous catalysts for selective oxidation reactions

A series of chromium-containing mesoporous MCM-48 molecular sieves, (Cr)MCM-48, with varying Si/Cr (25 - proportional to) ratios were synthesized hydrothermally, and characterized systematically by various analytical and spectroscopic techniques. The catalytic performance of the materials was evaluated for the oxidation reactions of certain industrially important organics such as ethylbenzene, cyclohexene and cyclohexane. For all the chosen reactions, the catalyst with a Si/Cr ratio of 50 showed very good conversion and excellent selectivity of the products. Furthermore, (Cr)MCM-48 exhibits much higher activity than the one-dimensional mesoporous (Cr)MCM-41 due to the incorporation of chromium ions in large concentrations as well as to the three-dimensional pore system of the former. Unlike many other chromium-based heterogeneous catalysts, the (Cr)MCM-48 showed only a slight decrease in catalytic activity during first recycling (or second run) experiments, and thereafter it remains nearly the same. Further, the quenching experiments as well as washing studies clearly support the above conclusion. In addition, it can also be deduced from several recycling, washing, filtrate and quenching studies that the leaching of the active chromium ions from the mesoporous matrix is minimal compared to almost all the chromium-based heterogeneous catalysts reported so far. The catalytic activity of (Cr)MCM-48 was also compared with various chromium-containing microporous silicalite-1 and beta, i.e., (Cr)S-1 and (Cr)beta, molecular sieves.