The Performance Of Modified ZSM-5 Catalysts For Methanol To Propylene Reaction (MTP)

Currently, the conversion of methanol to olefins develops rapidly, while the demand of ethylene growing slowly. At the same time, the manufacturing process of ethylene tends to be light hydrocarbon-based raw materials. Therefore, as the subordination, the relation between supply and demand of propylene exists contradiction, and developing separate methanol conversion to propylene is imperative. Methanol to propylene process mainly uses ZSM-5 zeolite as the catalyst, thus for ZSM-5 it is necessary to obtain the suitable acid distribution. In view of the Si/Al ratio of B-Al-ZSM-5 zeolite is high relatively, therefore, we used B-Al-ZSM-5 zeolite as the catalyst on the fixed-bed reactor, investigated the best conditions for process amplification of MTP reaction.And we impregnated ZSM-5 zeolite with CeO2 to research the effects on stability for MTP reaction. Considering ZSM-11 has the similarity structure to ZSM-5 zeoliite, we analyzed and compared the difference between ZSM-11 and ZSM-5 for MTP reactions.The influences of WHSV (methanol), material composition for MTP reaction were researched based on B-Al-ZSM-5 catalyst, under the condition of H2O:MeOH=1:1 (mol) or WHSV=1 h-1 the catalyst showed relatively stable performance for MTP reaction, and was suitable for technology scale-up. Under the conditions of 1 atm,500 ?, WHSV=1 h-1, and H2O:MeOH=1:1 (mol) over B-Al-ZSM-5 catalyst for MTP, the catalyst got deactivative after 584 hours when the conversion of methonal dropped below 90%.We synthesized high silica B-Al-ZSM-5 catalyst (Si/Al=681) and impregnated with different amounts of CeO2. The propylene selectivity, light olefin selectivity and catalyst stability for MTP reaction over this series of catalysts was investigated under the reaction conditions ofl atm, H2O:MeOH=1:1 (mol), WHSV=3 h-1 and T= 500? on the fixed-bed reactor. The result showed that as the amount of CeO2 loading over B-Al-ZSM-5 increasing, the ratio of B acid sites and L acid sites decreased, and the catalytic lifetime declined. It is deduced that the high CeO2 loading B-Al-ZSM-5 catalyst attributes to higher concentration of weak acid sites, and then the reaction rate enhances, the more catalyst deactivation due to coke formation.It has been verified that Si/Al ratio is a radical factor in the MTP process. As the Si/Al ratio getting lower, the catalyst acidity will be stronger. We prepared ZSM-5 zeolite samples with different Si/Al ratios, studied the performance of Ce-modified ZSM-5 catalysts for MTP reaction. For non-modified ZSM-5 catalyst, when the silica to alumina ratio rised, the propylene selectivity increased firstly and then decreased, the lifetime of catalysts declined. Impregnated with CeO2 can cover the strong acid sites, change the distribution of acidity. Ce-modified leads to the growth rate of catalyst life increased compared with the unmodified catalysts when the Si/Al ratio became lower, which enhanced the effect for catalyst stability. When catalyst has sufficient amount of acid sites, the effect of CeO2 for catalyst stability is more obvious. Thus, the chosen optimum ZSM-5 zeolite is NK ZSM-5 in order to balance propylene selectivity and catalyst stability:the highest propylene selectivity,35.53%, and the longest lifetime,200 hours.We impregnated NK ZSM-5 catalyst (Si/Al=271) with different amounts of CeO2. Cerium modification led to partial elimination of the strong acid sites on the NK ZSM-5 catalyst, which resulted in enhancement of the catalytic performance. The catalyst with 2.5 wt% CeO2 loading is the optimum NK ZSM-5 zeolite, which increased the olefins selectivity and enhanced catalyst stability.ZSM-11 zeolite (MEL) and ZSM-5 zeolite (MFI) has similar structure, that two 10-ring channels intersect. Under the same conditions for MTP reation, B-Al-ZSM-11 has higher propylene selectivity than B-Al-ZSM-5 catalyst, while worse stability than B-Al-ZSM-5 catalyst. Since the molecular traffic control of ZSM-11 zeolite is weaker, the inverse diffusion of reactant and product molecules in catalyst is relatively more serious. Therefore, compared to the ZSM-5 zeolite, ZSM-11 zeolite showed worse stability.