苯-乙烯超临界烷基化反应体系在ZSM-5中吸附行为的分子模拟

Adsorption of Benzene-Ethylene Alkylation System in ZSM-5 under Supercritical Condition by Molecular Simulation

运用巨正则蒙特卡罗的方法研究了超临界苯 -乙烯混合体系在ZSM 5分子筛中的吸附行为 .首先模拟了苯 -乙烯混合物在ZSM 5分子筛中的吸附等温线 .苯的吸附等温线呈现出先增大 ,而后渐趋平衡 ,最后又有减小的趋势 .而苯在分子筛中的吸附位集中在弯曲通道和直通道的交叉处 ,且不随压力的变化而变化 .从苯在两通道的交叉处产生了“类凝结”(commensuratefreezing)现象的角度解释了这种现象 .乙烯的情况有所不同 ,其吸附等温线始终呈增大的趋势 ,其吸附位随着压力的变化呈现明显的变化 .探讨了不同主体相苯 /乙烯比 (进料比 )下超临界的苯 -乙烯的混合吸附行为 .表明苯的平衡吸附量曲线基本上不随主体相苯 /乙烯比的变化而变化 ,而乙烯随主体相苯 /乙烯比的增大而减小 ,它们在主体相苯 /乙烯比为 3∶1时相交 .因此 ,由苯和乙烯生成乙苯的反应机理出发 ,从理论上得到了超临界条件下最优的进料比为 3∶1.此外 ,在超临界条件下 ,观察到苯的吸附位集中在两通道交叉处 ,乙烯的吸附位集中在除两通道交叉处之外的直通道和弯曲通道中 ,它们不随主体相苯

Binary adsorption of benzene-ethylene in ZSM-5 under supercritical conditions has been studied by grand canonical Monte Carlo simulation. The adsorption isotherm of benzene in ZSM-5 increases at low pressures, then achieves equilibrium gradually, and decreases slightly at high pressures. Localization of benzene molecules in ZSM-5 is mainly in the interconnections of sinusoidal and straight channels covering different pressures. This observation is in agreement with the 'commensurate freezing' phenomenon taking place in the interconnections of the two types of channels. In contrast, adsorption and localization of ethylene molecules are distinctly different from that of benzene. The adsorption isotherm of ethylene rises with pressure, and localization of ethylene molecules varies dramatically with pressure. Furthermore, the effect of the feed ratio of benzene/ethylene under supercritical conditions has been investigated. Simulation results indicate that the mean loading of benzene is almost unchanged, while the mean loading of ethylene decreases with the increase of the feed ratio of benzene/ethylene. The two curves of the mean loadings intersect at the feed ratio of benzene/ethylene about 3:1, which corresponds to equimolecular adsorption of benzene and ethylene in the pores of ZSM-5, and is in favor of the mechanism of alkylation of benzene with ethylene. Moreover, it is found that localization of benzene molecules is in the interconnections of two types of channels. On the other hand, localization of ethylene molecules is in the sinusoidal and straight channels, rather than the interconnections, under supercritical conditions. In addition, it is found that different feed ratios of benzene/ethylene have little effect on localizations of benzene and ethylene molecules in the pores of ZSM-5.