Studies On Sio₂-deposited Hzsm-5 Catalysts For Catalytic Cracking Of C₄ Olefins

By using fixed-bed micro-reactor, the reaction performance of the catalytic cracking of C4 olefins on HZSM-5 catalyst with different zeolite content and SiO2/Al2O3 ratio, and the basic composition was determined. Through modifying HZSM-5 zeolite catalysts which were untreated or treated with steam on certain temperature and pressure, with silica by chemical liquid deposition of organosiliconpolymer, the catalysts for catalytic cracking of C4 olefins were prepared. By means of XRD, nitrogen adsorption, IR, NH3-TPD, SEM and the probe reactions of 1,3,5-triisopropylbenzene and meta-xylene, the physicochemical properties and reaction performance were comparatively characterized. According to these studies, the mechanistic models of modification affected zeolite acidity and pore structure, the mechanistic model of object and secondary reactions have also been proposed.The experiment results showed that the chemical bonds of Si-O-Si can be obtained by the thermolysis of organosiliconpolymer which inactivate the external surface acidity of zeolite under the catalysis of hydroxides on the external surface of zeolite. When oxygen existed, the homogenizing deposition of Si-O-Si is induced by the thermal oxidation of organosiliconpolymer and that caused the narrowing of pore-opening size.The catalyst with suitable depositon amount of silica improved the yield and selectivity of propylene through suppressing the hydrogen transfer reactions and restraining the formation of aromatics. Depositing silica after steam treatment increased the stability and selectivity of catalysts.