Effect of synthesis parameters on the chromium content and catalytic activities of mesoporous Cr-MSU-x prepared under acidic conditions

A series of chromium-incorporated MSU-x mesoporous molecular sieves were synthesized under different templates, initial Si/Cr molar ratios, aging times, and temperatures in acid solution. The synthesis was performed by using sodium silicate, chromium nitrate, and nonionic poly(ethylene oxide) surfactant as the source of silicone, metal, and the template, respectively. The Cr-MSU-x products were analyzed by inductively couple plasma-optical emission spectrometry to determine the actual Cr content and were characterized by X-ray diffraction, N-2 adsorption-desorption, scanning electron microscopy, high-resolution transmission electron microscopy, diffuse reflectance UV-visible, X-ray adsorption near-edge spectroscopy, and temperature-programmed reduction techniques. The Cr species were mostly formed as Cr(VI) in tetrahedral coordination. Two kinds of Cr(VI) species with different reduction abilities were distinguished. The catalytic activities of Cr-MSU-x in the dehydrogenation of ethane to ethylene with CO2 were investigated at the same time. The synthesis parameters explored strongly influence the chromium content in Cr-MSU-x and, subsequently, the catalytic activities. The Cr-MSU-x synthesized with Si/Cr = 20, aging at 25 degrees C for 22 h, and templating by fatty alcohol polyoxyethylene ether gave the best activities, resulting in 58.0% ethane conversion and 92.1% ethylene selectivity. The Cr species in Cr-MSU-x are more efficient in activating and converting ethane molecules than are conventional catalysts.