Aromatization And Alkylation Of Methanol Over Nano-HZSM-5and HMCM-22Zeolites

The aromatization of methanol and alkylation of methanol with benzene are important reactions for producing high octane gasoline and light aromatics. In this dissertation, combined with the characterization of catalyst, thermodynamic analysis and theoretical calculation, the reaction performances of nano-HZSM-5and HMCM-22zeolites in the both reactions were investigated on fixed-bed reactor. The main results and conclusions are as followings:(1) For the aromatization of methanol, nano-HZSM-5(Si/Al=14.5) zeolite had higher resistance to coking deactivation than HMCM-22zeolite with similar Si/Al ratio. The anti-coking deactivation ability of the nano-HZSM-5was further enhanced by treating with NaOH solution, which turned the nano-HZSM-5into a hierarchical zeolite. At low temperature (ca.380℃), the single pass lifetime of methanol aromatization over the hierarchical zeolite reached620h, exceeding the parent nano-HZSM-5catalyst by77%. This low-temperature aromatization could give71%Cs+hydrocarbon yield (on CH2basis), and the liquid product had a RON value of94(calculated) and a aromatics content of no more than60%in the initial reaction period. Therefore, the low-temperature aromatization with the hierarchical zeolite is suitable for high octane gasoline production. Moreover, the hierarchical nano-HZSM-5zeolite may also be employed for aromatics production from methanol, since it could give57%C5+hydrocarbon yield with a high aromatics content of near70%in the initial reaction period at high temperature (ca.450℃).(2) The results of N2-physical adsorption, FTIR,27A1and29Si-NMR and theoretical calculation indicate that, the L acid site generated during the alkali-treatment of nano-HZSM-5with low Si/Al ratio should be ascribed to naked "=Alδ+". The desilication reaction of the low Si/Al ratio nano-HZSM-5proceeded more easily in the Si-rich region of zeolite framework, while the framework Si atoms that directly linked to framework Al were difficult to be removed. As a result, the desilication of the low Si/Al ratio nano-HZSM-5led to a hierarchical zeolite with narrow mesopore size distribution.(3) For the alkylation of benzene with methanol, HMCM-22zeolite has not only higher anti-coking deactivation ability, but also higher benzene conversion and aromatics methylation selectivity, than the nano-HZSM-5zeolite. The reaction temperature, pressure and benzene/methanol molar ratio were proved to be the main factors influencing the anti-coking deactivation ability of HMCM-22zeolite. Under optimum reaction conditions and with optimized HMCM-22zeolite catalyst, the alkylation could reach a single pass lifetime of more than1200h, a benzene conversion and aromatics methylation selectivity of30%and94%, respectively. In view of the low content of ethylbenzene in Cg aromatics and the low contents of propylbenzene, ethyltoluene in C9aromatics, the product of methanol with benzene over HMCM-22is very suitable for being used as the feedstocks of p-xylene production. In addition, the coked catalyst can be regenerated through coke-burning.(4) The theoretical calculation and experimental results indicate that, the reaction of methanol to olefins (MTO) was likely to be responsible for the formation of more ethylbenzene, propylbenzene and ethyltoluene by-products over nano-HZSM-5catalyst during the alkylation of benzene with methanol. The reason why the MTO reaction occurred more easily on nano-HZSM-5zeolite than on HMCM-22zeolite seems to relate to the different hydrocarbon pool species required for olefins formation over the two zeolites. HZSM-5zeolite needs xylene and trimethylbenzene, while HMCM-22zeolite needs hexamethylbenzene and heptamethyl-benzenium (heptaMB+). Under the alkylation reaction condition (high benzene/methanol ratio), xylene and trimethylbenzene were easily generated on nano-HZSM-5zeolite, but hexamethylbenzene and heptamethyl-benzenium (heptaMB+) were not on HMCM-22zeolite.