A new way to obtain acid or bifunctional catalysts. IV. Effect of calcination conditions on the catalytic properties of TPA, MeNH3-[Ga, Al]-ZSM-5 zeolites synthesized in the absence of inorganic cations Original Research Article

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, N2 adsorption, 27Al and 71Ga MAS-NMR. Acidity of the solids was characterized using n-heptane cracking (T=350°C, PN2/PC7=4, P=100 kPa) as a model reaction. Acidic or bifunctional behavior of these catalysts was determined using propane aromatization (530°C, PC3=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.