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

The as-synthesized alkali-free [Ga,Al]-ZSM-5 zeolites were steamed at different steam pressures (68 kPa less than or equal to P less than or equal to 92 kPa), reaction times (0 h less than or equal to t less th an or equal to 9 h) and temperatures (530 degrees C less than or equal to T less than or equal to 800 degrees C). The obtained solids were c haracterized by XRD, FT-IR, N-2 adsorption, Al-27 and Ga-71 MAS-NMR. A cidity of the solids was characterized using n-heptane cracking (T=350 degrees C, P-N2/ P-C7=4, P=100 kPa) as a model reaction. Acidic or bi functional behavior of these catalysts was determined using propane ar omatization (530 degrees C, P-C3=100 kPa) as a model reaction. Steamin g of solids, even at low temperatures (530 degrees C), leads to a prof ound 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 me ans of direct calcination under dry air at higher temperatures (T>700 degrees C), or by conventional impregnation methods. Solids steamed un der more severe conditions (T greater than or equal to 600 degrees C o r t>6h) present a strong dealumination and total degalliation, which l eads to a strongly dehydrogenating catalyst, unable for aromatization, due to a drastic decrease of the number of acidic sites present on th e zeolite structure. (C) 1998 Elsevier Science B.V.