Oligomerization of ethene and propene over composite zeolite catalysts

Abstract

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 SiO 2/Al 2O 3=35) zeolite in a silica-alumina matrix. The silica-alumina matrix contained 15 wt.-% Al 2O 3 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 (C 1–C 4), 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.