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

Abstract

The aromatization of n-hexane was studied over H-ZSM-5 (SiO2/Al2O3 = 70 and %XRD crystallinity = 66%), Ga/H-ZSM-5, Zn/H-ZSM-5 and Mo/H-ZSM-5 catalysts prepared by the incipient impregnation method and calcined at 500oC. The aromatization reactions were carried out at 500oC. BET, NH3-TPD, H2-TPR and XRD techniques were used in characterization of the catalysts in a preliminary attempt to correlate structure and catalytic behaviour. The catalytic activity of H-ZSM-5 was improved when impregnated with gallium and zinc. High conversions were obtained and the aromatic selectivity was above 50% when the gallium loading was 0.5 wt%. For Zn/H-ZSM-5 catalysts the activity increased with increase in zinc loading. The 3%Zn/H-ZSM-5 was the most active catalyst attaining a conversion of 88% and aromatic selectivity above 40%. The impregnation of H-ZSM-5 with molybdenum led to a decrease in activity and aromatic selectivity. As the molybdenum content was increased the deactivation rate increased with time-on-stream. This may be attributed to less dispersion of molybdenum species in the H-ZSM-5 channels leading to the blockage of the pore and active sites. The NH3- TPD profiles suggested that an increase in the molybdenum content decreased the concentration of Brønsted acid sites hence decreased the activity of the H-SM-5 catalysts. The results obtained showed that Ga/H-ZSM-5 and Zn/H-ZSM-5 catalysts are good catalysts for the aromatization of n-hexane due to their dehydrogenation activity. The results on the effect of percentage XRD crystallinity (from 5 to 86%) of H-ZSM-5 on the activity of H-ZSM-5 modified by loading 2 wt% of metal showed that conversion of n-hexane increased with %XRD crystallinity. The Ga/H-ZSM-5 and Zn/H-ZSM-5 catalysts with %XRD crystallinity above 30% showed more aromatic selectivity than Mo/H-ZSM-5 catalysts. The Mo/H-ZSM-5 catalysts were more selective to the cracked products due to the absence of the dehydrogenation activity that is possessed by gallium and zinc metals. The effect of reaction temperature (between 500 and 600o) on the aromatization of n-hexane over H-ZSM-5 containing 2 wt% metal content was investigated. The activity of catalysts increased with temperature for the 1 hour on-stream studies and as the time-on-stream increased a decrease in activity was observed. But at 550oC Ga/H-ZSM-5 and Mo/H-ZSM-5 showed good stability with increase in time-on-stream. A rapid deactivation on Zn/H-SM-5 is associated with zinc leaving the catalyst bed.