Structural characteristics and reactivity properties of the tantalum modified mesoporous silicalite (MCM-41) catalysts

Abstract

The structural characteristics of the Ta–MCM-41 catalyst have been investigated by BET measurement, small angle X-ray diffraction, UV–vis–NIR diffuse reflectance spectroscopy (DRS) and Raman spectroscopy techniques. Three types of tantalum oxide species: an isolated TaO4 species in the MCM-41 framework, an isolated surface TaO4 species on the MCM-41 surface, and bulk Ta2O5, can be present individually or coexist on the Ta–MCM-41 catalysts, and its relatively intensity is dependent on the Ta concentration. The local structure of the Ta atoms in the Ta–MCM-41 catalyst forms an isolated active site with the bridging Ta–O–Si bonds on the surface and in the frame structure of MCM-41. The catalytic properties of the Ta–MCM-41 catalysts were chemically probed with propane oxidative dehydrogenation (ODH) and methanol oxidation reactions in order to distinguish the different surface active sites present on the catalyst. Consequently, the well-dispersed isolated active sites exhibit high redox catalytic properties to produce formaldehyde (HCHO) and methyl formate (MF), and bulk Ta2O5 only possesses acid sites to form dimethyl ether (DME) The presence of Ta–O–Si bonds in the catalyst is responsible for the high reactivity/selectivity of the oxidation reactions.