Design And Synthesis Of The Catalyst For Methanol-to-olefins

Light olefins such as ethylene and propylene, one of the most importantintermediates for the production of plastics, synthetic fibers, synthetic rubber, andother petrochemical and fine chemicals plays a very important role in the modernchemical industry. They are mainly derived from petroleum cracking, reforming andnaphtha steam-cracking in traditional petrochemical process. But with the growingmarket denmand, the soaring of oil price, and the overuse of crude oil, it has inspiredpeople to find a new sustainable route to substitute petroleum for production of lightolefins. In addition, the combination of the actual distribution of Chinese fossilresources that the reserves of petroleum and natural gas is relative less than that ofcoal, change and improvement of coal chemiacal technoledy is benefit to nationalsecurity and energy strategy. Because methanol could be easily produced from syngas,a mixture of CO and H2, which in turn, can be made from biomass, natural gas, orcoal, so chemical transformation less value-added C1raw material methanol into highvalue-added petrochemicals has attracted significant attention. Comparation to themajor ethylene product in tradotional petroleum route, propylene is one of byproducts.Therefore, high yield of propylene from methanol on an industrial scale is verydesirable.It is well known that zeolite used as acid catalyst or carrier is one of most importantheterogeneous catalysts. Zeolite with unique, regular crystals, unique pore size andstructure, large surface area, high thermal and hydrothermal stability, strong acid siteson the surface, Coulomb filed and polarity effect in crystal is regarded as a excellentsolid acid catalyst having the unique “shape selectivity” catalytic functions and strongadsorption performance. MTO using H-type zeolite as the catalyst is a typical “shape selectivity” catalytic reaction. The catalyst is the focus of research on methanolconversion, therefore, the design, modification, and synthesis of the catalyst forconversion methanol to light olefins is of great importance.Methanol conversion is divided into MTO, MTP, MTG (methanol-to-gasoline), andMTA (methanol-to-aromatics) depending on the difference of the major productresulting from the types and different modifications of zeolite catalysts. Among thesecatalysts, the most studied and high applicable catalysts are HZSM-5and SAPO-34zeolite. HZSM-5zeolite with medium pore sizes usually results in the formation of alarge amount of relatively bulky hydrocarbons, while SAPO-34zeolite, although highselectivity towards light olefins，especially ethylene, is more sensitive to deactivationas compared to HZSM-5. Independent of the catalytic activities, both catalystsdeactivate with time-on-stream due to the formation of coke deposits, thereby limitingtheir potential application on an industrial scale which is an important aspect ofefficient use of methanol.Based on the above background, effort was made to design, modification, andsynthesis of the catalyst for methanol conversion.A fast, organotemplate-free, and seed-directed synthesis of ZSM-34zeolite havingvery high selectivity towards propylene was demonstrated here. When the temperatureis at140-180°C, ZSM-34zeolite could be crystallized for2-6hours in the presenceof ZSM-34crystals as seeds. For industry, fast and organotemplate-free synthesis,typically green route, means significant savings in energy and costs. After thehydrothermal treatment, partial skeleton Al was broken off, which is important foradjustment of Si/Al ratios in ZSM-34framework. Catalytic tests in MTO reactionshow that as-synthesized HZSM-34has very high selectivity for propylene (55.2%),which is even higher than that (41.6%) of SAPO-34zeolite under the same conditions.Moreover, after the hydrothermal treatment HZSM-34significantly improves catalystlife in MTO reaction. In addition, as-synthesized ZSM-34exhibits higher propyleneselectivity and longer catalyst life than that from other routes due to the small crystalsize. The combination of “green” synthesis and good catalytic performance ofZSM-34would be potentially important for highly effective conversion of methanolwhich can be easily obtained from coal, natural gas, or biomass at a large scale.Single-crystalline, high-silica HZSM-5zeolites with mesoporosity were preparedvia dual templates method, where cheap cationic polymer as the soft template, played an important role in mesoporosity development. Various characterizations show thathierarchical ZSM-5zeolite remain relative high degree of crystallinity，and have largeexternal surface area, small crystal size, and low acid density. Catalytic tests inconversion of methanol to propylene exhibit when the mesoporosity was introduced tomicropore system, hierarchical HZSM-5zeolite significantly improved the propyleneselectivity and P/E ratio (the ratio of propylene and ethylene) up to47.5%and12.1,respectively. Interestingly, when the crystallographic symmetry of ZSM-5zeolitecontaining mesoporosity chang from the typical orthorhomibic system to monoclinicsystem, the propylene selectivity (50.5%), light olefins (ethylene and propylene,60%),and catalyst life are improved. Meanwhile, compared with that of HZSM-5zeolitewith the similar Si/Al ratio synthesized in the obscene of soft template, longer catalystlife was also obtained. These results could be ascribed to the enhanced accessibility toreactants and diffuse out from pore system resulting from the high Si/Al ratio, smallcrystal size, and mesoporosity developed in crystals.ZSM-5/RUB-41composite zeolite materials were obtained by calcinations ofZSM-5/RUB-39zeolites which were prepared by introducing Al3+species into thestarting gel of the layered silicate RUB-39. All-silica RUB-41zeolite with RROtopology derived from the topotactic layer condensation of the layered materialRUB-39, where a two-dimensional pore system including intersecting8-and10-membered ring pores (5.8×4.1,5.9×4.1) is created in between thelayers. The amount of ZSM-5zeolite can be adjusted by the amount of Al3+added.Active ZSM-5components are partially confined by the surface of layered poresilicate RUB-41and thereby modified to enhance its the shape selectivity towardspropylene. HZSM-5/RUB-41zeolite with high Si/Al ratio can improve thepropylene selectivity and P/E ratio to52.5%and7.9, respectively. Moreover, theselectivity to bulky molecular also decrease, however, ZSM-5synthesized inpresence of DMDPAOH, Al-RUB-41, and mechanical mixture of ZSM-5andRUB-41exhibit low selectivity to light olefins. These results can be attributed to themodification of surface of HZSM-5by layered pore silicate RUB-41.Core-shell structure ZSM-5/AlPO-18zeolite was synthesized by addition of thetreated ZSM-5zeolite seeds into the starting gel of nanoscale AlPO-18zeolite. ForZSM-5zeolite, especially for solide phase synthesized large ZSM-5crystal, theuse of non-active AlPO-18partially covered the surface and pore, thereby enhancing the selectivity to light olefins. The catalytic test is good agreement withthe dedication.