A new way to obtain acid or bifunctional catalysts: V. Considerations on bifunctionality of the propane aromatization reaction over [Ga,Al]-ZSM-5 catalysts

Abstract

The as-synthesized alkali-free TPA, MeNH 3-[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 solids obtained 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 N 2 / P C 7 =4, P=100 kPa) as a model reaction. Acidic or bifunctional behavior of these catalysts was determined using propane aromatization (530°C, P C 3 =100 kPa) as a model reaction. Steaming of these solids leads, even at low temperatures (530°C), to a profound and progressive demetalization (degalliation and/or dealumination). Catalysts obtained under mild steaming conditions show 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. A clear synergistic effect is observed between acidic sites in the structure and dehydrogenating species composed by extraframework gallium species (EFGS). Linear dependence of initial activity in propane aromatization per protonic acidic site ( A 0/H +), as a function of EFGS/H + relation, shows that this aromatization reaction should be kinetically limited by the initial dehydrogenation step produced on EFGS, which demonstrates bifunctionality of this reaction. Finally, inactivation of acidic sites of the structure of the solid (H +) is produced by EFGS, and the numeric ratio between them shows that EFGS present on galloaluminosilicates, under reaction conditions, should be species with a positive charge equal to 1.