Aromatization of n-Hexane over metal modified H-ZSM-5 zeolite catalysts

摘要

The aromatization of n-hexane was studied over H-ZSM-5 (SiO2/Al2O3 = 70 and %XRDudcrystallinity = 66%), Ga/H-ZSM-5, Zn/H-ZSM-5 and Mo/H-ZSM-5 catalysts prepared by theudincipient impregnation method and calcined at 500oC. The aromatization reactions were carriedudout at 500oC. BET, NH3-TPD, H2-TPR and XRD techniques were used in characterization of theudcatalysts in a preliminary attempt to correlate structure and catalytic behaviour. The catalyticudactivity of H-ZSM-5 was improved when impregnated with gallium and zinc. High conversionsudwere obtained and the aromatic selectivity was above 50% when the gallium loading was 0.5udwt%. For Zn/H-ZSM-5 catalysts the activity increased with increase in zinc loading. Theud3%Zn/H-ZSM-5 was the most active catalyst attaining a conversion of 88% and aromaticudselectivity above 40%. The impregnation of H-ZSM-5 with molybdenum led to a decrease inudactivity and aromatic selectivity. As the molybdenum content was increased the deactivationudrate increased with time-on-stream. This may be attributed to less dispersion of molybdenumudspecies in the H-ZSM-5 channels leading to the blockage of the pore and active sites. The NH3-udTPD profiles suggested that an increase in the molybdenum content decreased the concentrationudof Brønsted acid sites hence decreased the activity of the H-SM-5 catalysts. The results obtainedudshowed that Ga/H-ZSM-5 and Zn/H-ZSM-5 catalysts are good catalysts for the aromatization ofudn-hexane due to their dehydrogenation activity. The results on the effect of percentage XRD crystallinity (from 5 to 86%) of H-ZSM-5 on theudactivity of H-ZSM-5 modified by loading 2 wt% of metal showed that conversion of n-hexaneudincreased with %XRD crystallinity. The Ga/H-ZSM-5 and Zn/H-ZSM-5 catalysts with %XRDudcrystallinity above 30% showed more aromatic selectivity than Mo/H-ZSM-5 catalysts. TheudMo/H-ZSM-5 catalysts were more selective to the cracked products due to the absence of theuddehydrogenation activity that is possessed by gallium and zinc metals. The effect of reaction temperature (between 500 and 600o) on the aromatization of n-hexane overudH-ZSM-5 containing 2 wt% metal content was investigated. The activity of catalysts increasedudwith temperature for the 1 hour on-stream studies and as the time-on-stream increased a decreaseudin activity was observed. But at 550oC Ga/H-ZSM-5 and Mo/H-ZSM-5 showed good stabilityudwith increase in time-on-stream. A rapid deactivation on Zn/H-SM-5 is associated with zincudleaving the catalyst bed.