Abstract

The as-synthesized alkali-free [Ga,Al]-ZSM-5 zeolites were steamed at different steam pressures (68 kPa≤ P≤92 kPa), reaction times (0 h≤ t≤9 h) and temperatures (530°C≤ T≤800°C). The obtained solids were characterized by XRD, FT-IR, N 2 adsorption, 27 Al and 71 Ga MAS-NMR. Acidity of the solids was characterized using n-heptane cracking ( T=350°C, P N2/ P C7=4, P=100 kPa) as a model reaction. Acidic or bifunctional behavior of these catalysts was determined using propane aromatization (530°C, P C3=100 kPa) as a model reaction. Steaming of solids, even at low temperatures (530°C), leads to a profound and progressive demetalliation (degalliation and/or dealumination). Catalysts obtained under mild steaming conditions show for propane aromatization, properties similar to those of catalysts prepared by means of direct calcination under dry air at higher temperatures ( T>700°C), or by conventional impregnation methods. Solids steamed under more severe conditions ( T≥600°C or t>6 h) present a strong dealumination and total degalliation, which leads to a strongly dehydrogenating catalyst, unable for aromatization, due to a drastic decrease of the number of acidic sites present on the zeolite structure.

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