Preparation And Modification Of Zeolite ZSM-5for Catalyzing Methanol-to-Gasoline Reaction

Methanol to gasoline ?MTG? is an important non-petrochemical route for producinghigh octane number gasoline. MTG can not only make good use of the over-producedmethanol but also reduce the dependence on oil for gasoline production. Increasingmethanol conversion, improving gasoline selectivity and prolonging life-span of thecatalyst zeolite ZSM-5are the key for commercialization. Therefore, modification ofzeolite ZSM-5to improve its catalytic performances is of great importance.Four ZSM-5zeolites with different Si/Al ratios ?25,38,50and80? were used tocatalyze the MTG reaction, respectively. The gasoline yield reached only15.8%whenthe Si/Al ratio was80, indicating that this zeolite is not suitable for catalyzing theMTG reaction. The other three zeolites gave almost the same gasoline yields?46.2-48.7%?. Based on the aromatics and durene contents in the gasoline products,the optimal Si/Al ratio of the catalysts was found to be50. In terms of the anti-cokedeactivation capability, the zeolite with a lower Si/Al ratio has a stronger acidity anddeactivates faster.Four transition metal oxides ?Fe₃O₄, CoO, CuO and ZnO? were loaded ?3%? on thezeolite ZSM-5with a Si/Al ratio of25to modify the catalyst. The MTG reactionscatalyzed by the modified zeolites showed that metal oxides formed an L acid site onthe surface of ZSM-5. After modification with CuO, the L and B acid sitessynergistically acted increasing the gasoline yield by ca10%. The aromatics anddurene contents were also increased by ca40%and60%, respectively. The zeolitemodified by ZnO showed a high surface dispersion and strong capability ofanti-carbon deactivation. After24h's MTG reaction, the methanol conversion reducedto from90%to65%?25%reduction?, compared to98%to54%?44%reduction?when the unmodified catalyst was used.Mesoporous multistage zeolite was prepared by desilication of ZSM-5with a Si/Alratio of50using0.2M NaOH. The specific surface area, pore size and number of acidsites of the modified zeolite were all increased. The gasoline yield was increased by37%, the aromatics and durene contents were increased by12%and34%, respectively,giving a lower quality of gasoline product. The anti-coke deactivation capability wasincreased. After24h's reaction, the methanol conversion reduced from97%to91%?6%reduction?, compared to91%to78%?13%reduction? before the modification of the catalyst.Small crystal size of ZSM-5with a Si/Al ratio of25was prepared by the seedcrystal inducing method. This process has the advantages of no template agent, lowpollution and low cost. After further modification of the catalyst with amorphousaluminosilicate, large surface area and moderate acidity were achieved. The optimalreaction conditions for the MTG reaction catalyzed by the modified catalyst were:380-420?,1MPa, WHSV of methanol1-5h-1. After6h's MTG reaction catalyzed bythe modified small crystal size of ZSM-5, the gasoline yield was increased by28%,the aromatics and durene contents were increased by30%and11%, respectively,compared to those catalyzed by the unmodified catalyst, giving a higher quality ofgasoline product. In terms of the anti-coke deactivation capability, after24h'sreaction, the methanol conversion reduced from99%to70%?29%reduction?, whichis quite similar to that of the ZnO-loaded zeolite.