Influence of coke formation on the conversion of hydrocarbons: II. i-Butene on HY-zeolites Original Research Article

The influence of coke formation on the conversion of i-butene was investigated on a series of HY-zeolites differing by the number of framework and extra 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 pressure of 105 kPa, in a recycle electro-balance reactor, operating gradientlessly under high conversion, with on-line gas chromatographic analysis of the effluent. Coke formation not only leads to pore blockage but also to reactions involving coke molecules that cause changes in the activity and the selectivity of the catalysts. Even in the presence of important pore blockage, the occurrence of these reactions can result in an increased activity of the catalyst. 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.≥29, the conversion rate of i-butene initially increases with increasing coke content until pore blockage becomes dominant. Up to 2 wt.% of coke, and in the absence of EFAL, the influence of coke formation on the formation rate of n-butenes is positive for catalysts with H+/u.c.≥25, while it is negative for H+/u.c.<25. On all catalysts, the rate of the dimerization-β-scission path decreases with coke content. Except for the zeolite with H+/u.c.=6, the rate of hydride transfer and of coke formation are positively influenced by coke formation. The rate of hydride transfer parallels the rate of coke formation. To explain the influence of coke formation on the reaction rates, a reaction mechanism involving coke molecules is proposed.