Study Of The Catalytic Performance Of Zeolite Catalysts In Benzene Alkylation With Methanol And The Effect Of Pore Structure And Surface Acidity

Benzene alkylation with methanol to produce toluene and xylene is receiving increasing attentions, because it can effectively make use of the market surplus of benzene and methanol and cope with the high market demand of toluene and xylene. Moreover, as compared to the traditional aromatics production process which is strongly dependent on the petroleum resources, the raw materials used in the alkylation process can be produced from coal. As for our country, the available energy resource is rich in coal and lack in oil and gas, so producing aromatics via alkylation process is more in line with our national conditions. But until now, the industrial application of this production technology was not yet implemented. Through the literatures, we found that some problems, such as the high selectivity of by-products ethylbenzene, the low effective utilization of methanol, the low selectivity of xylene, the rapid deactivation of catalyst, and so on, have not been solved effectively. As is known to all that the catalyst is the core of alkylation technology and the physicochemical properties of itself determine the conversion activity and product selectivity. The existing literatures showed that the catalytic performance of zeolite in benzene alkylation with methanol was influenced by its pore structure and acid property. Therefore, in this work, we systematically investigated the effect of pore structure and surface acid on the performance of catalyst in benzene alkylation with methanol by pore structure adjustment and surface acid modification. The main research content of this paper and the related conclusion are as follows:(1) The noble metal Pt was introduced into the ZSM-5 zeolites, and the effect of hydrogenation reaction on benzene alkylation with methanol was investigated. The results indicated that the addition of Pt could effectively suppress the formation of ethylbenzene and this could be attributed to the rapid hydrogenation of ethylene into ethane which avoided the alkylation of ethylene and benzene. Moreover, the decrease of olefins also reduced the rate of carbon deposition and this in turn improved the stability of catalyst.(2) Considering the presenting diffusion limitation of ZSM-5 zeolites, the hierarchical porous ZSM-5 was synthesized and the catalytic performance of the catalyst with different pore structure was compared in detail. The results showed that the introducing of mesoporous not only could offer easier transport and access to the active sites, but also promoted the diffusion of bulk aromatic molecules and carbon precursor. Thus, the hierarchical porous ZSM-5 zeolites exhibited the higher benzene conversion, xylene selectivity and carbon deposition resistance.(3) A large amount of ethylbenzene was observed even over hierarchical porous ZSM-5 zeolites. It was found that Pt modified hierarchical porous ZSM-5 could successfully combine the catalytic advantages of hierarchical porous ZSM-5 and the high suppression to ethylbenzene formation of Pt. In addition, as compared to the Pt modified catalyst prepared by calcination method, employing direct reduction method could improve the utilization of Pt by avoiding Pt particles sintering.(4) Modification the hierarchical porous ZSM-5 by Zn loading, P loading or nitridation could adjust the surface acidity. The addition of Zn could convert Br?nsted acid into Lewis acid, the introduction of P could convert strong Br?nsted acid into weak Br?nsted acid, and the nitridation could reduce the amount of Br?nsted acid as well as Lewis acid. The results showed that the catalysts prepared by these modification methods could suppress the reaction of methanol to olefins and this in turn inhibited the formation of ethylbenzene. Therefore, it was reasonable to conclude that reducing the amount of strong Br?nsted acid played an important role in suppressing the reaction of methanol to olefins. As compared to suppressing the generation of ethylbenzene by ethylene hydrogenation, inhibiting methanol turning into olefins would help to improve the utilization of methanol and promote the alkylation reaction.(5) On the hierarchical porous ZSM-5 modified by Zn, P or nitridation, the selectivity of ethylbenzene could only be decreased to 0.3%. In order to further reduce the selectivity of ethylbenzene, the dual modified hierarchical porous ZSM-5(MgO and Pd) was prepared. The results indicated that the modification of MgO could effectively suppress the reaction of methanol to olefins by the reduction of Br?nsted acidity, and the introduction of Pd could further reduce the amount of ethylene by hydrogenation. With the promoting effect of MgO and Pd modification, the high benzene conversion(56%) and xylene selectivity(39.1%) was maintained and the lowest ethylbenzene selectivity(0.13%) and coke content(0.4 wt%, 10h) was obtained.(6) In order to investigate whether the adjusting of surface acid could completely suppress the formation of ethylbenzene, the hierarchical porous ZSM-5 with different Si/Al ratio was synthesized by reducing the amount of Al in the synthesis process. It was found that with the adjusting of Si/Al ratio, the change of structural framework, crystal morphology and porous structure was slightly, while the change of Br?nsted acidity was obviously. The hierarchical porous ZSM-5 with Si/Al ratio of 1800 was with appropriate Br?nsted acidity which highly suppressed the reaction of methanol to olefins and in turn reduced the selectivity of ethylbenzene to 0.1% below without the addition of noble metal.