Synthesis Of Hierarchical ZSM-5 Zeolites And Their Catalytic Properties In MTP Reaction

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.In methanol to propene（MTP）reaction,the target product propene is a sort of middle reactant,and the reaction is strong exothermic.However,the typical ZSM-5 zeolite is rich in micropore structure,the long Zig-Zag chanel and small external surface area strictly restrain the release of middle reactant propene and the heat transfer during MTP reaction.Therefore,we need to shorten the molecular diffusion path and enhance the surface area of zeolite,that is,hierarchical zeolite is a good choice.In our work,the influence of crystallization conditions,such as the pH of reactants,crystallization temperature and time,has been investigated in details while low-cost silica sources and suitable template were used.Besides,those methods,including an accurate selection of NaX,addition of seed and modifiers,were successfully applied to fabricate hierarchical ZSM-5 molecular sieves with nano-particle aggregates in very short crystallization time.In addition,the performance of hierarchical ZSM-5 zeolite applied as a catalyst in methanol to propene process was detected.The synthesis of ZSM-5 zeolite containing cheap silica source as well as a spot of composite template and a higher alkalinity was studied first,XRD,N₂ adsorption-desorption,SEM,TEM and FTIR analysis confirms the purity and the high crystallinity of ZSM-5,while the SEM reveals that each individual particle gives a larger crystal size,and the TEM shows aggregate of nanoscale particle.In MTP reaction,sample AZ-0.07 has the most excellent catalytic property:the highest propene yield is 41.7 wt%and P/E ratio is 7.5 in 840 min.Recently,a few investigation of initial water content affect ZSM-5 zeolite synthesis,so the effect of initial water content on the synthesis of ZSM-5 zeolite and its reaction performance in methanol to propylene reaction was comprehensively investigated.The results show that the difference of H₂O adsorbed on the surface of ZSM-5 and gel concentration lead to many differences in zeolite synthesis,such as crystal size,pore structure.The more initial water content,the stronger adsorption effect,and the smaller concentration difference,the smaller crystal size,in addition,the pore structure of ZSM-5 change to hierarchical pore.In MTP reaction,the hierarchical zeolite has the better catalysis stability and a higher propene yield in 2h.NaX affect much on zeolite structure.The results showed that the addition of NaX improved the MTP catalysis property of ZSM-5 zeolite.The synthesized NaBr-ZSM-5 zeolite exhibited the best MTP catalysis property among NaX-assisted ZSM-5 zeolite;the nano-sized ZSM-5 aggregates can be obtained by adding NaF and NaBr,and the NaBr-ZSM-5 zeolite had the smaller nanocrystal and the bigger mesopore volume;the MTP catalysis property was enhanced with a wider external surface area and a bigger mesoporous volume.The high crystallinity ZSM-5 zeolite is synthesized in condition of 1.5h crystallization time and 180 ^oC crystallization temperature,with seed assisted.The shorter crystallization time and fewer template content did reduce raw material cost.During the synthesis of ZSM-5zeolite,increasing seed content was used to obtain smaller particle size,and when seed/SiO₂=0.03,the synthesized ZSM-5 zeolite catalyst has the most excellent MTP catalysis property.Comparied with non-seed zeolite samples,zeolite with seed assisted has smaller crystal size（about 1μm）,and has no aggregate,mesopore come from the crack on crystal surface.In MTP catalysis reaction,sample BZ-0.07 showed the most excellent catalytic properties,the highest propene yield was 31 wt%and P/E was 3.1 in 960min.