A new way to obtain acid or bifunctional catalysts. II. Straightforward calcination of as-synthesized [Ga,Al]-ZSM-5 zeolites obtained from alkali-free media

摘要

The acid or bifunctional behavior of catalysts obtained by the straightforward calcination (530-800℃) of as-synthesized [Ga,Al]-ZSM-5 zeolite, obtained from alkali-free media, was determined using propane, cyclohexane and cyclohexene transformations (T= 530℃, p = 1 atm) as reaction models. Solids were characterized by XRD, FTIR, N_2 adsorption, ~(27)Al and ~(71)Ga MAS NMR and chemical analysis. The catalyst acidities were evaluated using n-heptane cracking and m-xylene isomerization as reaction models. Results show that calcination temperatures (T_c) between 530 and 550℃, causes only the removal of organic molecules used for the synthesis of the zeolite, leading then to pure acidic catalysts. At 700 ≤ T_c≤ 800℃, simultaneously with the removal of those organic molecules, a significant chemical change of the zeolite framework composition take place (degalliation and dealumination). The presence of extraframework gallium species (EFGS), led to acid gallium promoted bifunctional catalysts with catalytic properties for propane and naphthenes aromatization almost identical to those for gallium promoted zeolites obtained by conventional methods (ion exchange and/or impregnation).