Applied Catalysis A: General, Vol.107, No.2, 147-164, 1994
Platinum Promoting Effects in Pt/Ga Zeolite Catalysts of Lower Alkane Aromatization .1. Ga and Pt Electronic States, Dispersion and Distribution in Zeolite Crystals in Dependence of Preparation Techniques - Dynamic Effects Caused by Reaction Mixture
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 Ga2O3 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 (30-50 angstrom) 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(II)-HZ 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).
Keywords:PROPANE AROMATIZATION;LIGHT ALKANES;ACTIVE-CENTERS;CONVERSION;HYDROCARBONS;PENTASILS;GALLIUM;GALLOSILICATES;ETHANE