Oligomerization of ethene and propene over composite zeolite catalysts

The conversion of ethene and propene over a series of composite zeolite catalysts has been studied in a fixed-bed reactor. The composite catalysts were synthesized by incorporating 1 to 10 wt.-% of HAlZSM-5 (molar ratio SiO₂/Al₂O₃=35) zeolite in a silica-alumina matrix. The silica-alumina matrix contained 15 wt.-% Al₂O₃ and was prepared by the cogelation method. Pure ethene and propene were used as the feed gases. Experiments were conducted over a temperature range of 200 to 600°C at a space velocity of 3.4±0.1 g_alkene/g_catalyst/h and atmospheric pressure. In addition, a few experiments were carried out at the space velocities of 34 and 68 h^-1. For all the catalysts, ethene and propene conversion increases with increasing reaction temperature. In the case of propene, the conversion increases rapidly over a narrow temperature range, for a ZSM-5 zeolite content of 10 wt.-%. However, a moderate increase with temperature was observed when ethene was the reactant and the composite catalyst had a zeolite loading of less than 10 wt.-%. With both reactants, silica-alumina catalysts containing 5 and 10 wt.-% ZSM-5 exhibited activities much greater than the pure silica-alumina. The selectivity of the reaction remained comparable to that of silica-alumina however, not of HAIZSM-5. The concomitant formation of aromatic hydrocarbons and lower alkanes (C₁-C₄), indicates that hydrogen transfer reactions occur on all catalysts. This transfer reaction is sensitive to the zeolite content of the composite. Both the amorphous silica-alumina and the pure HAIZSM-5 zeolite exhibit high aromatization activity, because of hydrogen transfer, whereas, at intermediate zeolite loading (5 wt.-%), the hydrogen transfer activity is lower, resulting in higher alkenes. For both the 5% and 10% zeolite composites, the coke content expressed as a percentage of the converted propene was less than 1.5 wt.-%, a value close to that for the pure HAIZSM-5 zeolite. In contrast, the amorphous silica-alumina as well as the 1.5 wt.-% of zeolite composite resulted in a larger coke fraction. Furthermore, the temperature-programmed oxidation profiles indicate that the coke could be oxidized more easily in the case of 5% zeolite composite than the 10% zeolite composite or the pure zeolite.