Characterizations and Catalytic Properties of Chromium Silicalite-2 Prepared by Direct Hydrothermal Synthesis and Impregnation

A series of Cr-Si-2 molecular sieves have been prepared by direct hydrothermal synthesis using tetrabutylammonium hydroxide as a template. Cr/Si-2 was also prepared by conventional impregnation. The synthesized materials were characterized by X-ray diffraction (XRD), N-2 adsorption (-196 degrees C), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and diffuse reflectance (UV-vis) spectroscopic measurements. Cr-Si-2 sample with lower content of chromium (ca. 1.28wt%) exhibits excellent catalytic performance for the dehydrogenation of ethane with carbon dioxide, giving 45.5% ethylene yield with a selectivity of 87.9% at 650 degrees C. The catalytic performance of Cr/Si-2(1.31%) was slightly lower to that of Cr-Si-2(1.28%). Characterization indicates that the chromium species with high oxidation state on the Cr-Si-2(1.28%) and Cr/Si-2(1.31%) were partly reduced to aggregated Cr(III) during the dehydrogenation of ethane with carbon dioxide, which could account for their different catalytic behaviors in this reaction.