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 chlori ne contents between 4 and 6% by weight, and differed in surface acidity. Th ey were tested as catalysts for the alkylation of isobutane with 2-butene u sing a fixed bed plug flow reactor. Alumina reacted with CCl4 was found ine fficient for this reaction. However, the solid chlorinated with HCl above 8 00 K was able to catalyze alkylation at a temperature as low as 273 K. More over, the CCl4-reacted solid could be activated upon further treatment with HCl at moderate temperatures (370 to 550 K). However, the catalytic activi ty decays after a few hours on stream. The composition of the alkylate vari ed somewhat with time on stream: large amounts of cracked products appeared during the initial period, after which the selectivity to trimethylpentane s (TMP) was comparable to that of other solid catalysts. The presence of hy drogen chloride bound to the catalyst surface was established by measuring the temperature-programmed desorption (TPD) of HCl from the various Al2O3-C l. 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 si tes of Al2O3-Cl, and creates the strong Bronsted sites required for catalyt ic alkylation. (C) 1999 Elsevier Science B.V. All rights reserved.