| Being the feed stock for aromatic diacid monomer synthesis, xylenes are the most important aromatic compounds for polymer industry. Nowdays, there are two major processes for xylene production, the disproportionation of toluene and the transalkylation between toluene and multi methyl substituted benzene. However, in both of the processes, the toluene conversion is very low and a large mount of benzene is obtained as by-product. In recent years, in order to produce xylenes in lower cost, toluene methylation by methanol is widely investigated in many companies. Although the methylation of toluene with methanol has very high methanol conversion and no benzene by-product, the stability of catalyst is always very poor. Futher more, because of the formation of olefins from methanol, the xylene selectivity is low based on calculations of methanol. Hence, although the conversion of methanol is high in the reaction, the utilization ratio of methanol is low. The drawbacks of the process result in the difficult of the commercialization of it. The purpose of this investigation is to design a new process for xylene synthesis in a more efficient way. In the new process, bromomethane was employed as methyl resource for the methylation of toluene.All reactions in this paper were conducted in a fixed reactor. The investigation started with testing different zeolites as catalyst supporters to select the best support for our catalyst. In the subsequent work, the influences of different active compounds concentration of active compound, the Si to Al molar ratio of zeolites, the mol ratio of reactant, the space velocity of feedings, and reaction temperature on reaction were investigated. After carrying out the previous investigations,2wt%ZnO/ZSM-5 (Si: Al=40:1) was found to be the best catalyst for toluene methylation reaction by running reactions in the following conditions:CH3Br flow 5.3 ml/min, N2 flow 10.0 ml/min, toluene to CH3Br mol ratio 2:1. Toluene and CH3Br conversions of 41.0% and 100% were reached, respectively. During all of the reactions, the gas phase was analyzed on a GC/MS and only a trace of ethene and propene had been detected.The life-time of catalyst was tested. The life-time testing reaction was carried out at 300℃on the best catalyst (2wt%ZnO/ZSM-5 (Si:Al=40:1)). The CH3Br conversion of almost 100% was maintained within 204 h. After the catalyst partially deactivated, it was regenerated in air at 450℃. The regenerated catalyst was subjected to farther reaction. Over the regenerated catalyst,99.9% of CH3Br conversion was maintained within 228 h. The results indicate that the catalyst is regeneratable. Finally, the catalyst deactivation mechanism was investigated. The XRD characterization of the fresh catalyst and the regenerated catalyst show that the phase structure of the catalyst did not change in the catalytic reaction. After long time online reaction, the specific surface aera of catalyst decreased because of coking formation, which could be the reason that resulting in the deactivation of the catalyst. After analyzing the residue of the deactivated catalyst, multi-methyl substituted aromatic compounds were found to be the substances that clogged in the channels of zeolites catalyst. |
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