Isobutane/2-butene alkylation on zeolite beta: Influence of post-synthesis treatments

Abstract

The influence of the framework and extraframework composition of zeolite beta on its activity, selectivity, and deactivation behavior during the alkylation of isobutane with 2-butene has been studied by submitting a H-beta sample to different post-synthesis treatments. The parent Hβ sample was prepared by calcination, conventional NH 4 + exchange, and further calcination of a commercial TEA-beta. These treatments were seen to produce some framework dealumination with formation of dispersed cationic extraframework aluminum (EFAL) species. The Hβ was then dealuminated by two chemical treatments (hydrochloric acid and ammonium hexafluorosilicate) and by steam calcination, the former procedures leading to almost EFAL-free catalysts. Furthermore, Hβ was also treated with the required amount of ammonium hexafluorosilicate (HFS) in order to remove EFAL species generated during the activation of the parent TEA-beta sample. The catalysts were characterized by XRD, N 2 adsorption-desorption isotherms, 27Al and 29Si MAS-NMR, XPS, and IR spectroscopy with adsorption-desorption of pyridine. In general, dealumination of Hβ leads to less active, selective, and stable catalysts for isobutane/butene alkylation. The chemically dealuminated EFAL-free catalysts were much more active than the steamed sample, which was attributed to the presence of both cationic EFAL compensating framework charge and condensed type EFAL blocking acid sites in the latter catalyst. Furthermore, elimination of EFAL in Hβ by HFS treatment also decreased the alkylation activity of the zeolite, suggesting a synergistic effect of dispersed cationic EFAL species with framework hydroxyls of Hβ to form Brönsted acid sites of enhanced acid strength.