The role of extraframework aluminum species in USY catalysts during isobutane/2-butene alkylation

The liquid phase alkylation of isobutane with 2-butene has been carried out at 50°C reaction temperature and 2.5 MPa total pressure on three steam dealuminated USY catalysts with unit cell sizes (UCS) ranging from 24.55 to 24.31 A. These catalysts, which contained the extraframework aluminum (EFAL) generated during the steam stabilization, were subjected to an (NH4)2SiF6 (AHF) treatment to selectively remove the EFAL. The catalysts were characterized by XRD and IR spectroscopy with adsorbed pyridine. It has been shown that the AHF treatment is more effective in removing cationic EFAL species, which are the predominant species in mildly steamed USY samples, than for extracting condensed type EFAL present in highly dealuminated USY catalysts. The characteristics of the high UCS sample after the AHF treatment depended strongly on the conditions of calcination used after the treatment. Elimination of EFAL in mildly dealuminated USY samples caused a decrease of the initial cracking and alkylation activity, and increased the deactivation rate of the zeolite. However, both the alkylation activity and the catalyst life improved after elimination of EFAL from the highly dealuminated catalysts. These results have been interpreted in terms of changes in Bronsted acid site density and strength distribution produced after the AHF treatment on the different USY catalysts.