| Propylene, as feedstock for the production of polypropylene and its derivatives, such as acrylonitrile, epoxypropane and oxoalcohols, is one of the most important and essential materials in petrochemical industry. Nowadays, the technologies to produce propylene mainly depend on the steam cracking and the fluidized catalytic cracking (FCC). With the increasing demand of propylene and the serious shortage situation of petroleum resource, development of new propylene production technology has drawn significant attention in the chemical industry. For the situation of poor petroleum but relatively ample coal resources in China, methanol can be successfully produced from coal, natural gas and biomass. Therefore, the development of methanol technology to propylene (MTP) has become a great strategy in our country.In the past few decades, many fundamental researches have been conducted to improve the high selectivity and stability of the catalysts in MTP reaction. ZSM-5 zeolite with a high Si/Al molar ratio has been selected as one of the prospective catalysts with high selectivity and stability for MTP reaction. However, Zeolite with particular acidity pore structure still tends to undergo rapid deactivation by coking, so more works focus on the structure design and surface acidity adjustment have been done. Our works mainly aim to prepare practical and efficient catalysts through different synthesis methods in order to control the morphology and acidity of zeolite and investigate it relation with catalytic activity.1. Synthesis of Al, Ga, B incorporated MFI catalysts and its application in MTP reaction.With incorporation of B、Al、Ga into the MFI zeolites framework, the acidity of catalysts is successfully adjusted. The catalysts performance showed that Aromatics has been successfully inhibited over Ga-MFI with lower acid amount and strength, that is beneficial to improvement of propylene selectivity. Ga-doping ZSM-5 exhibited longer catalytic lifetime about 320 h and the higher selectivity of propylene from 34.5% to 41.7% through improving the acidity density. B and Ga incorporated MFI catalyst is conductive to increasing the weak acid sites, inhibiting the aromatization reaction, reducing the coking and improving the stability of the catalyst.2. Hierarchical ZSM-5 Synthesized by ultrasonic-assisting method and its application in MTP reaction.The result of ultrasonic cavitation is advantageous to promote the fast forming of crystal nucleus. In this work, hierarchical zeolites are prepared by ultrasonic assistant method through pre-treatment of precursor solution. The nucleus grows fast in the crystallization process, ultrasonic assistant method is benefits to forming accumulated nanoparticles with multi-stage pore structure. The hierarchical ZSM-5 possesses a large external surface area and kept the same acid strength and acid density as commercial one. The selectivity of propylene can reach over 40% and the lifetime of the catalyst maintains 1100 h with above 95% conversion, intracrystalline pore is beneficial to enhancing mass transfer and exposing more active sites.3. Synthesis of boron-modified hierarchical nanocrystalline ZSM-5 zeolite and its application in MTP reaction.B-introducing hierarchically nanocrystalline ZSM-5 is successfully synthesized by a facile "salt-aided seed-induced" methods. The characterization results demonstrated that the zeolite has a shorter pore channel, a rough external surface and abundant intercrystal mesopores, which vastly improved diffusion ability of reactants and products in the pores. Moreover, incorporation of B into the catalyst framework increases the weak acid sites, but keeps the same high strength acid sites as before. The B-N-Z5 catalyst shows a very long lifetime of 720 h at WHSV=4.0 h-1 for the MTP reaction due to the cooperative effect of hierarchical nanocrystalline structure and boron-modified acidity. |
PDF Full Text Request