Synthesis And Characterization Of Silicalite-1 Zeolite

High-silica three-dimensional zeolite ZSM-5 has been widely applied in the production of propene from the catalytic pyrolysis or methanol conversion process owing to its special pore structure,pore size and high hydrothermal stability.Silicalite-1 is the stablealuminum-free form of this zeolite.Silicalite-1 zeolite has a better hydrophobic lipophilic property,adsorption ability,thermal stability.Thus,it has a huge application in the process of recycling important chemical products,selective adsorption,and the gas phase catalytic exchanges,etc.Because of its specific properties and applications in various catalyticprocesses,the synthesis of these materials has attracted attention of numerous investigators.However,the synthsis methods of Silicalite-1 zeolite that have been reported have a high cost and a long crystallization time.In our work,the influence of crystallization conditions,such as the pH of reactants,crystallization tempreture and time,has been investigated indetails while low-cost silica sources and suitable template were used.Besides,those methods,including an accurate selection of reactants,addition of seeds and modifiers,weresuccessfully applied to fabricate Silicalite-1 molecular sieves with perfect morphology,ideal size distribution and different crystal sizes in a very short crystallization time.In addition,the performance of Silicalite-1 zeolite applied as a catalyst in methanol to propene process was detected.The synthesis of Silicalite-1 containing cheap silia sources as well as a spot of template?TPABr?as a single template was studied first.XRD,FTIR and NMR analysis confirms the purity and the high crystallinity of Silicalite-1,while the SEM reveals that each individual particle gives a large crystal size.TPAOH has been recognized as the best structure-directing agent to synthesize ZSM-5.But its high price limits its wide application.Most templates that used in the crstallization of ZSM-5 is TPAOH,so the influence of different templates on Silicalite-1 zeolite was studied.It was investigated that there is little balance charge in non-Al system and with the increase of Si/Al ratio,it's more difficult for silicon species enter into zeolite framework,so n-butylamine is unfavorable in the synthesis of Silicalite-1 zeolite.While,TEA,another template,can also direct formation of MFI structure.Unlike Al-ZSM-5,its very hard to obtain Silicalite-1zeolite by crystal seeds or mother liquid without templates.During the syntheis of Silicalite-1,increasing nucleation center and inhibit smaller particles aggregated to form larger particles were used to obtain smaller crystallites.It was found that the existence of seed particles did a great contribution to the fast crystallization rate of systems.The particle size of products could be feasibly tuned from 11?m to 1-2?m by two-stages crystallization and adding seed crystal.The morpholgies,crystal size and pore structure data were obtained by SEM and N₂ adsorption analysis.It was deduced that seeds play an important role in increasing nucleus and obtaining small crystallites with uiform size distribution:give active sites for materials grow and give active species.Using salts instead strong alkili as pH regulator is a widely use method to increase crystal size.The use of a weakly alkaline medium reduces silica solubility and nucleation rate,so that large crystals can be prepared.The effect of various silica sources on the zeolite synthesis was also investigated.Different silica material has different solubility in alkaline system,which affect the polymerization of silicate oligomers,zeolite structure and surface acid content.The silonal groups population of its surfsce has catalytic activity in MTP reaction.Silicalite-1 catalyst exhibits a high selectivity to desired products because the acid strength and acid density decrease deeply and inhibit low carbon olefin hydrogen transfer and aromatization reaction.