Synthesis Of Modified ZSM-5Catalysts And Their Application In The Methanol-to-Propylene Reaction

Propylene serves as an essential feedstock in petrochemical industry, which is widely used to produce polypropylene, acrylonitrile, propylene oxide and many downstream products with high added value. Nowadays, the technologies to produce propylene mainly depend on the petroleum resources. The oil crisis is getting aggravated while the demand of propylene is getting increased. China is oil-poor country butrelatively abundant charcoal resources, which makes developing methanol to propylene (MTP) technology, a non-petroleum way to produce propylene from charcoal and natural gas, of great strategic significance. Therefore, MTP technology has drawn more and more attraction. For the time being, there have been quite a few petrochemical giants which focus on the research of MTP process. However, the succeeful commercialmanufacture of MTP technology is still on the way.The key issues of MTP technology are to improve the stability and propylene selectivity of the catalysts. It makes the structure design and surface acdity control of the catalyst hot research interests in MTP technology. However, the process is still facing many problems such as heat transfer, mass transfer, rapid coking, etc, which means the stability and propylene selectivity has a potential improvement to be realized. Zeolite ZSM-5is a good catalyst with high propylene selectivity for MTP reaction and it has a brilliant prospect for industrilization. The catalytic performance of zeolite quite depends on its physical chemical properties. Therefore, it is of practical significance to control the morphology, size, acidity of zeolite through different synthesis and modification methods to improve its activity in MTP reaction.Our work mainly aims to improve the stability and propylene selectivity of the catalysts, and focuses on the synthesis of zeolites with novel pore systems and acidities, which includes the following parts:(1) Study of the synthesis of core-shell zeolites and their application in MTP reaction; comparison of the influences on the catalytic performance caused by microporous and hierarchical zeolite catalysts;(2) Synthesis of P species modified ZSM-5catalysts by impregnation method; study of the influences on the surface acidity, structure property and catalytic stability of the catalyst caused by P modification;(3) Incorporation of B into the zeolite framework through isomorphous substitution; the influences on the structure, acidity and catalytic performance of B-Al-ZSM-5zeolite catalysts; controlling of the acid strength and acid density of the catalyst by B incorporation have been invertigated, catalytic stability of MTP is greatly enhanced over B-Al-ZSM-5catalyst..1. Synthesis, characterization and catalytic activities of ZSM-5with core-shell structure.ZSM-5zeolites with core-shell structure are synthesized using an ultra-dilute liquid-phase coating strategy. The ZSM-5zeolites with a Si/Al=200are used as cores and two types of shells are coated onto the cores respectively, microporous silica Silicalite-1and mesoporous silica SBA-15. The products are denoted as Z-5@S1and Z-5@S15corresponding. The characterization results show that compared to the conventional ZSM-5, Z-5@S1posesses a external surface area decreased by37%, a similar micropore surface area of305m2/g and a total acidity decreased by27%while Z-5@S15ZSM-5, Z-5@S1posesses a increased external surface of228m2/g, a total acidity decreased by24%and a micropore surface area decreased by21%. The catalytic performances show that the introduction of the silica shells helps to suppress the ethylene formation, thus improving the Propylene Ethylene (P/E) ratio. However, since the core-shell zeolites contain only70wt%of ZSM-5zeolites, the catalytic lifetime is greatly decreased due to the relatively low total acid amount.2. Synthesis, characterization and catalytic activities of P/ZSM-5.ZSM-5(Si/Al=140) catalyst is modified by impregnation method using phosphoric acid. The characterizations show that the P modification leads to an unchanged structure, an unchanged micropore surface area and a decreased external surface area of the catalyst. After P modification, the weak acid amount of the catalyst is increased while the strong acid amount is decreased. For the0.25wt%P modified ZSM-5which maintains good catalytic stability, the C3=41.1%and C4=18.7%selectivities are improved. Overload of P (1wt%) leads to a rapid deactivation of the catalyst.3. Synthesis, characterization, catalytic activities of B incorporated B-AL-ZSM-5.We investigate the different effects on the catalyst acidity caused by incorporation of B and Al into the zeolite framework. In this way, the control of the acidity of the catalyst is realized. The catalytic tests show that the catalytic performance of the Bx-Al-ZSM-5series catalysts is significantly improved. Among the catalysts, B1-Al-ZSM-5posses a super long catalytic lifetime of700h with methanol conversion above95%and propylene selectity above40%. The characterizations show that this series of catalysts is featured by its unchanged strong acid amount and increasing weak acid amount. This indicates that the catalytic stability can be improved by increasing the weak acid sites of the zeolites. The scale-up in chemical engineering of this catalyst is also performed. The results show that the acid properties and catalytic performance can be well maintained after scale up of the synthesis.