Hierarchical Structure ZSM-22 And ZSM-48 Zeolite And Performance For Hydroisomerization

As a consequence of petroleum feedstock composition degrading,social environmental protection and high quality gasoline,diesel fuel and lubricants demands,the petrochemical industry desperately looking for highly effective hydroisomerization catalysts.ZSM-22 and ZSM-48 are zeolite molecular sieves with linear channels which are developed by Mobil in the early 1980s.ZSM-22and ZSM-48 obtain excellent hydrothermal and thermal stability,which are ideal candidate catalysts for long chain n-alkanes hydroisomerization and cracking on account of suitable pore structure and appropriate acidity.However,due to micro porous structure nature of ZSM-22 and ZSM-48,many disadvantages such as large diffusion resistance,long diffusion path and low utilization rate of active center have limited their industrial application.Therefore,adjusting the pore structure of zeolites is quite essential.In dissertation,hierarchical pore structure ZSM-22 and ZSM-48 have been successfully synthesized by NaOH treatment,then the as-synthesized zeolites have been characterized and catalysis performance have been analyzed.According to recent studies,the aluminum-poor nature of ZSM-48 zeolite has dramatically hindered its catalytic performance due to weak acidity.Meanwhile,ZSM-48 with high Si/Al ratio has been attracted attentions for the stronger acidity recently.However,the synthesis of ZSM-48 with high Si/Al ratio requires synthesis procedure be severely controlled.Therefore,this dissertation aims of relatively low Si/Al ratio ZSM-48 zeolite synthesis.By using silica gel as silica source,sodium metaaluminate as alumina source,ZSM-48 which was hydrothermally synthesized.The effects of alkali concentration,crystallized temperature and duration have been researched by X-ray radiation diffraction?XRD?,Scanning Electron Microscope?SEM??Transmission Electron Microscope?TEM??MAS NMR?N₂ absorption-desorption,NH₃-TPD and Py-FTIR characterizations.Pure phase relatively low Si/Al ratio ZSM-48 can be synthesized at 180?during 17 h or 160?during 24h when alkali and silicon ratio is 0.09.Commercial ZSM-22?Si/Al=23?zeolites were used for further alkali treatment.Afterwards,the pore structure and acidity of ZSM-22 and ZSM-48 were analyzed after 0-0.5mol/L NaOH treated.The results confirmed the hierarchically pore structure for both ZSM-22 and ZSM-48.With the increase of NaOH concentration,the BET surface area,external surface area and mesoporous capacity of the two kinds of molecular sieves both increased.The acidity shows a reverse trend,the acidity of ZSM-22 decreased,however,the acidity of ZSM-48 increased after NaOH treatment.Pt impregnated bifunctional catalysts Pt/ZSM-22 and Pt/ZSM-48 have evaluated for hydroisomerization performance of n-hexane.The conversion of n-hexane and the yield of isomers were increased for both Pt supported ZSM-22 and ZSM-48 with different alkali treatments,meanwhile the selectivity of isomers was slightly decreased.Comprised with Pt/ZSM-22 and Pt/ZSM-48,the hydroisomerization performance of n-hexane over Pt/ZSM-22was superior to Pt/ZSM-48 at lower temperature?<300??,however at higher temperature?>300??,it occurred oppositely.The optimum reaction temperature for n-hexane hydroisomerization over Pt/ZSM-22 is 300?,which conversion reached 75%and selectivity and yield of product were 90%⁹5%and 70%,respectively.For Pt/ZSM-48,the optimum reaction temperature is 320?,the conversion was 70%and selectivity and yield of isomers were 95 and 75%,respectively.