Synthesis, Characterization And Catalytic Performance Of The Y/Beta Composite Zeolites With Core-shell Structure

Zeolites are a family of crystalline microporous aluminosilicate materials andwidely used as catalysts in a variety of chemical reactions in the petrochemicalindustry, fine chemicals and some other fields, due to the presence of strong acid sitesand shape-selectivity induced by molecular-sized microporosity. However, in recentyears the single component zeolite could not meet the requirement of the developmentof petroleum and chemical industry. So, the composite zeolites, which contain two ormore kinds of zeolites with different topologies and mixed at the micro or nano-level,have attracted much attention, due to much more favorable synergistic effect ascompared with the single component zeolite. Especially, when the composite zeolitepresent core-shell structures, the diffusion resistance of the reactants and products isreduce effectively, accompanying with the enhanced synergistic effect. Besides that,the composite zeolite could take advantages of the different pore system andadjustable acidity between core and shell segment to design shape-selective catalysts.This thesis mainly focuses on the synthesis, structure characterization and catalyticperformance of the core-shell structured Y/Beta composite zeolite, the main resultsare as following:(1) The NaY zeolites were hydrothermally synthesized by additive directingagent method and direct way respectively. The results showed that the NaYzeolite couldbe synthesized rapidly within4h by directing agent method, with particle size between150and200nm, the microporesurfacearea of777.6m2/g and micropore volume of0.30cm3/g.However, the molar ratio of SiO2to Al2O3was just3.6, resulting in poor thermalstability during the NH+4exchange and calcination process. The synthesis of NaYzeolite bydirect method took muchlonger time (48h) viaadditive directing agent method.The resultant samples present theparticle size ofabout1.5μm, the micropore surfacearea of845.6m2/g and the micropore volume of0.33cm3/g. Unlike directing agent method, themolar ratio of SiO2to Al2O3was about5.4. The cracking activities were preliminaryevaluated with cumene as a probe molecule, using the zeolites synthesized by thedirect method. The best catalytic performance could achieve as high as88.3%at thebeginningofthereactionand maintain20.0%evenafterthereactiontimeof400min.(2) A bi-phase Y/Beta composite zeolite with obvious core-shell structure wassynthesized, using NaY zeolite crystallization solution (direct method) as precursor.Meantime, the relative amount of zeolite NaY or zeolite Beta in the composite zeolitecould be adjusted via controlling crystallization time. Furthermore, the possibleformation mechanism of Y/Beta composite zeolite was proposed as over growing a layer of zeolite Beta around the core of zeolite NaY. When the crystallization timewas less than68h, it was the induction period of the generation of Beta phase. Whilethe time was longer than68h, zeolite Beta started to growing slowly on the surface ofY untill70h, after that the zeolite Beta would experience a rapid growth processalong with the consumption of zeolite NaY and result in the successful construct ofcore-shell structure. But when the crystallization time was prolonged to84h, onlyzeolite Beta was appeared. A stepwise-induced method is therefore proposed to ownmany advantages such as easy handling, simple procedure and controllablecrystallization. Particularly, the tedious filtration and the surface treatment forpre-synthesized zeolite NaY were unnecessary during the whole synthetic pathway, ascompared with current preparations.(3) The Phi-type zeolite LZ-277was successfully synthesized from Na2O-Al2O3-SiO2-H2O system, with the uniform yarn-like spherical particle size of about4μm.Meanwhile, the particle size of the aluminosilicates materials could be controlledaround2-4μm, by adding the tween-20in the reactant mixture. The XRD pattern andSEM image was employed to characterize the extract structure of the Phi-type zeolite,which was proved to be pure phase. Besides that, the special yarn-like morphologywas favourable to adsorb the precursor of the other zeolites, because of the abundantfolds on the yarn-like particle surface. These investigates provide us a new method todesign and control the morphology of the core-shell composite zeolites.