Influence of coke formation on the conversion of hydrocarbons II. i-buteneon HY-zeolites

The influence of coke formation on the conversion of i-butene was investiga ted on a series of MY-zeolites differing by the number of framework and ext ra framework Al (EFAL) atoms per unit cell. The reactions were carried out at 723 K, an inlet partial pressure of i-butene of 1 kPa and a total pressu re of 105 kPa, in a recycle electro-balance reactor, operating gradientless ly under high conversion, with on-line gas chromatographic analysis of the effluent. Coke formation not only leads to pore blockage but also to reactions involv ing coke molecules that cause changes in the activity and the selectivity o f the catalysts. Even in the presence of important pore blockage, the occur rence of these reactions can result in an increased activity of the catalys t. In the absence of EFAL, the conversion rate is found to decrease with coke content for H+/u.c.<29, while for H+/u.c.greater than or equal to 29, the c onversion rare of i-butene initially increases with increasing coke content until pore blockage becomes dominant. Up to 2 wt.% of coke, and in the abs ence of EFAL, the influence of coke formation on the formation rate of n-bu tenes is positive for catalysts with H+/u.c.greater than or equal to 25, wh ile it is negative for H+/u.c.<25. On all catalysts, the rate of the dimeri zation-beta-scission path decreases with coke content. Except for the zeoli te with H+/u.c.=6, the rate of hydride transfer and of coke formation are p ositively influenced by coke formation. The rate of hydride transfer parall els the rate of coke formation. To explain the influence of coke formation on the reaction rates, a reaction mechanism involving coke molecules is pro posed. (C) 2000 Elsevier Science B.V. All rights reserved.