Platinum promoting effects in Pt/Ga zeolite Catalysts of lower alkane aromatization. I. Ga and Pt electronic states, dispersion and distribution in zeolite crystals in dependence of preparation techniques. Dynamic effects caused by reaction mixture

Abstract

X-ray photoelectron spectroscopy (XPS), infrared (IR), electron microscopy (EM) and energy dispersive X-ray (EDAX) studies of in various ways prepared Ga/H-ZSM-5, Pt-Ga/H-ZSM-5 as well as Ga-silicate and Pt/Ga-silicate have been performed. The external surface of the zeolite crystal was originally strongly enriched with a Ga 2O 3 phase the dispersion of which depends on the preparation as well. Ga reduction, evidenced by XPS Ga 3d line shifts, occurred during reduction and/or the reaction mixture treatment and it was accompanied by migration of Ga I cations into the zeolite channels. The migration of unreduced Ga III species via solid-state reactions is less pronounced. The gallium reduction and migration is dramatically promoted by platinum introduced into both Ga/H-ZSM-5 and Ga-silicate; the reduction degree in the last sample is limited by the concentration of the extra-framework Ga III ions. Platinum was shown to be finely dispersed in highly acidic Pt-Ga/H-ZSM-5 while most of the platinum formed aggregates (30–50A˚) on the external surface of Ga-silicate. IR of adsorbed hydrogen and pyridine confirmed that reduction results in the formation of new strong Lewis acidic sites. Along with Ga I-HZ and Ga IIHZ structures mixed Pt-Ga particles located within the zeolite structure are most likely to be responsible for the dramatic enhancement of the aromatic selectivity found with Pt-Ga/zeolite catalysts (see Part II).