Chlorinated alumina as an alkylation catalyst: influence of superficial HCl

Chlorinated alumina catalysts were obtained by reacting gamma-alumina with gaseous CCl4 or hydrogen chloride under various conditions. They had chlorine contents between 4 and 6% by weight, and differed in surface acidity. They were tested as catalysts for the alkylation of isobutane with 2-butene using a fixed bed plug flow reactor. Alumina reacted with CCl4 was found inefficient for this reaction. However, the solid chlorinated with HCl above 800 K was able to catalyze alkylation at a temperature as low as 273 K. Moreover, the CCl4-reacted solid could be activated upon further treatment with HCl at moderate temperatures (370 to 550 K). However, the catalytic activity decays after a few hours on stream. The composition of the alkylate varied somewhat with time on stream: large amounts of cracked products appeared during the initial period, after which the selectivity to trimethylpentanes (TMP) was comparable to that of other solid catalysts. The presence of hydrogen chloride bound to the catalyst surface was established by measuring the temperature-programmed desorption (TPD) of HCl from the various Al2O3-Cl. For the active catalysts, desorption started at temperatures (350-400 K) well under those for the CCl4-treated sample, but all solids continuously released HCl above 650 K. Thus, HCl interacts with particular Lewis acid sites of Al2O3-Cl, and creates the strong Bronsted sites required for catalytic alkylation. (C) 1999 Elsevier Science B.V. All rights reserved.