The nonoxidative conversion of light alkanes over metal-loaded H-ZSM-5 zeolite catalysts

摘要

The study of the aromatisation of methane was conducted at 750oC over metalimpregnatedudH-ZSM-5 catalysts with a feed flow rate of 13 ml/min and theudcomposition of the feed was 90% methane balance argon. Typical products thatudwere detected from the outlet stream were ethene, ethane, benzene and toluene. Theudamount of coke produced was determined by using 10% argon as an internaludstandard. The effects of different parameters such as the type of the supportudmaterial, the molybdenum content, the %XRD crystallinity and SiO2/Al2O3 ratio ofudH-ZSM-5, the reaction temperature, the feed flow rate, the type of the molybdenumudprecursor, the catalysts preparation method, the addition of dopants, silanation andudthe regenerability of the catalysts were investigated.udThe results obtained showed that H-ZSM-5 was a better support for the preparationudof catalysts used for the aromatisation of methane. Mo/H-ZSM-5 catalysts wereudmore active when the molybdenum loading was between 2 and 4 wt% and loadingsudhigher than 4% led to lower activities. The lower activities observed at higherudmolybdenum loadings was related to the poor dispersion and decrease in the poreudvolumes and surface areas observed due to the formation of MoO3 crystallites.udFurthermore, the zeolite structure collapsed under the reaction conditions when theudmolybdenum loading was more than 4 wt%. The study showed that the conversionudof methane increased linearly with increasing reaction temperature and the apparentudactivation energy of the reaction was found to be 64.5 kJ/mol.udThe results of the effect of the %XRD crystallinity of H-ZSM-5 on the performanceudof H-ZSM-5 catalysts showed that 2%Mo/H-ZSM-5 catalysts were more activeudwhen the crystallinity of the zeolite was between 50 and 70%. The conversion ofudmethane decreased with an increase in the SiO2/Al2O3 ratio of H-ZSM-5. Higherudaromatisation activities were observed when the SiO2/Al2O3 ratio of H-ZSM-5 wasudiiiud60. The type of the molybdenum precursor used in the preparation of 2%Mo/HZSM-ud5 catalysts did not have a significant influence on the conversion of theudcatalysts, but higher selectivities for aromatics were observed when ammoniumudheptamolybdate was used as a source of molybdenum. The catalysts prepared byudphysical mixing of MoO3 and H-ZSM-5 catalysts were more active than thoseudprepared by impregnation with solutions of ammonium heptamolybdate.udThe presence of dopants such as boron, silver and alkali metal ions (Li+, Na+ and K+)udin 2%Mo/H-ZSM-5 catalysts was also investigated. Boron (0.05-0.2 wt%) did notudaffect the conversion level of the catalysts but changed their selectivity properties.udThe selectivity for C2 hydrocarbons increased with boron content, while theudselectivity for aromatics decreased. The addition of silver ions (0.5 wt%)udsignificantly improved the conversion of the catalysts. This was attributed to theudenhancement of the acvidity of the catalysts upon addition of silver ions which wasudobserved by temperature programmed desorption of ammonia and pyridineudadsorption studies of the infrared spectra of the catalysts. The addition of alkaliudmetal ions in the Mo:Metal ratio of 0.5 led to decreased catalytic activities, due to theudlowered acidities of the catalysts.udThe silanation of H-ZSM-5 improved the conversion of methane but lowered theudselectivity for aromatics. A comparative study of the W-based and Mo-basedudcatalyst at equivalent molar contents showed that molybdenum-based catalysts wereudmore active than tungsten based catalysts. The study also showed that the catalyticudperformance of 2%Mo/H-ZSM-5 catalysts could be regenerated to appreciable levelsudby treatment of the catalysts in air at 600oC.udThe possibility of using Mo/H-ZSM-5 catalysts for the aromatisation of propane wasudalso evaluated at 530oC, with consideration of three variables, namely, theudmolybdenum loading, the reaction temperature and %XRD crystallinity. The resultsudindicated that impregnation H-ZSM-5 catalysts with molybdenum led to lowerudivudpropane aromatisation activities. This lower activity was attributed to the lowerudBrønsted acid sites in the Mo/H-ZSM-5. The activities of the catalysts could beudimproved by operation at higher temperatures, but the rate of deactivation was alsoudimproved at higher temperatures. In line with the observations from the conversionudof methane, higher activities were observed when the %XRD crystallinity of theudcatalyst was 61%.