Characterizations and catalytic properties of Cr-MCM-41 prepared by direct hydrothermal synthesis and template-ion exchange

Cr-MCM-41 prepared by direct hydrothermal synthesis (DHT) and template-ion exchange (TIE) has been characterized by X-ray diffraction (XRD), N-2 adsorption (77 K), diffuse reflectance UV-vis, X-ray absorption (XANES and EXAFS), and UV-Raman spectroscopic measurements. It is suggested that monochromate species mainly exist on the Cr-MCM-41 by the DHT method while several types of chromate species including both monochromates and polychromates coexist on that by the TIE method. The two kinds of samples exhibit similar catalytic property in the dehydrogenation of propane with carbon dioxide. The selectivity to propylene is higher than 90% and the presence of carbon dioxide enhances propane conversion. The chromate species on both types of samples are reduced to aggregated Cr(III) with octahedral coordination during the dehydrogenation reactions. On the other hand, the selectivity to formaldehyde in the partial oxidation of methane with oxygen is remarkably higher over the sample by the DHT method than that by the TIE method. The structure of the chromate species is kept during the oxidation of methane, and the high dispersion of monochromate species probably accounts for the higher selectivity over the sample by the DHT method. (C) 2003 Elsevier Inc. All rights reserved.