Tailoring the Characteristics of ZSM-5 by Alkaline Treatment for Preparing Superior MTP Catalysts

Author:Li Jun Jie

Supervisor:song chun shan guo xin wen


Degree Year:2019





Zeolites are microporous crystalline materials with tunable acidity,shape-selective pores,and high hydrothermal stability.They have been widely used in industrial applications including catalysis and separation.While zeolite of ZSM-5 type is important as catalytic material for the methanol to propylene(MTP)process,it deactivates quickly due to the intrinsically dominant micropores which can be blocked by the coke.Introducing additional mesopores or macropores into zeolites significantly increases the catalytic stability.In recent years,researchers have developed many ways for preparing hierarchical zeolites,including demetallation and template methods.This doctoral thesis studies the influences of alkaline treatment on ZSM-5 pore structure and acidity.It addresses the effects of pore structure and acidity on the catalytic performance.Superior MTP catalysts were obtained by tailoring pore structure and acid properties simultaneously.A series of materials and treatments were examined and the results are summarized as follows.TPAOH templated ZSM-5 showed an aluminum-rich exterior and a silicon-rich defective interior.NaOH treatment selectively removed the inner portion,while the external surface was less corroded,resulting in a hollow ZSM-5.This material was ineffective for suppressing coke deposition,and only limited improvement of stability was observed.The NH4F treatment can modify A1 coordination in ZSM-5,thus distinct hierarchical structures were introduced into ZSM-5 by a subsequent NaOH treatment.Interonnetcted and uniform mesopores were introduced into Al-zoned ZSM-5 by optimized treatment methods,which resulted in a longer lifetime that was three times longer than hollow ZSM-5,while the propylene selectivity increased by 12 percent.The acid property,including acid amount and distribution,can be tailored by alkaline treatment.TPAOH treatment selectively removed the silicon-rich interior of Al-zoned ZSM-5,meanwhile the removed Si species recrystallized on the zeolite surface by TPA+ template,and the acid sites distribution was varied by the redistribution of Si sites.Parent ZSM-5 showed a large amount of external Bronsted acid sites,while there were few external Bronsted acid sites on the hollow ZSM-5,which inhibited external coke deposition during the MTP reaction.Therefore the lifetime of the hollow ZSM-5 with more silica-rich shell was doubled when compared with the parent ZSM-5.TPAOH treatment of NH4F modified ZSM-5 removed both Si and A1 sites,meanwhile Si and Al sites redistributed through the recrystallization,obtaining a more uniform A1 distribution.Besides,mesoporous ZSM-5 was obtained by sequential NH4F and TPAOH treatments.Mesoporous ZSM-5 with a more uniform acid sites distribution showed four times longer lifetime of parent ZSM-5.NaOH treatment could modulate acid amount,and the variation of the acid amount was directly related with the A1 content in ZSM-5.The amount of strong acid decreased significantly after NaOH treatment of samples with a higher Al content,while it increased for samples with a lower A1 content.Alkaline treated ZSM-5 showed three times longer lifetime than the parent ZSM-5.But the propylene selectivity showed close relationship with the strong acid amount,the propylene selectivity decreased with the increasing amount of strong acid over microporous and mesoporous ZSM-5.A superior MTP catalyst may have a hierarchical structure and a suitable amount of acid sites.A suitable Si/Al ratio for alkaline treatment was between 25 and 50,the corresponding mesoporous ZSM-5 obtained by alkaline treatment contains too many acid sites,which showed a low propylene selectivity.Catalysts with suitable amounts of acid sites were observed by alkaline treatment of ZSM-5 with a higher Si/Al ratio,but it showed low solid yield below 40%,and some macropores were generated.Hierarchical alveolus-like ZSM-5,composed of numerous hollow crystals,was obtained by NaOH treatment of B-MFI nano-aggregates with an addition of Al(NO3)3 solution,it showed a higher pore volume of 1.28 cm3/g and a higher solid yield above 60%.The addition of boron facilitated the insertion of Al3+ in the framework structure,as framework tetrahedral Al sites.Alveolus-like ZSM-5 with a suitable amount of acid sites showed two times longer lifetime than ZSM-5 with a high Si/Al ratio and hierarchical ZSM-5 during MTP reaction,and the propylene selectivity increased by four percent.ZSM-5 with a much higher Si/Al ratio(>300)showed higher propylene selectivity(>50%)during the MTP reaction,but a low stability.Once the active sites were covered by coke,the methanol conversion and propylene selectivity decreased sharply.If some new active sites could be introduced into ZSM-5 with a high Si/Al ratio,during the reaction,higher propylene selectivity and stability could be obtained.A superior catalytic performance was obtained with otherwise inactive Silicalite-1 after AICl3 modification.By using NH3-TPD,27Al MAS NMR,and OH-IR characterization,it is determined that a portion of Al atoms were inserted into the framework structure,serving as four-coordinate framework Al.A series of superior catalysts were tailor-made by regulating in-situ Al migration.Large amounts of defects were generated after acid washing of borosilicate MFI,producing a superior catalyst with a longer lifetime of 360 h,stable propylene selectivity higher than 50%,and a high selectivity of light olefin above 80%before 200 h TOS.Al2O3 extruded Silicalite-1 showed a life-span of 437 h,a stable high propylene selectivity above 52%,and a high propylene/ethylene(P/E)ratio of 11 after acid washing.A superior catalyst,with longer lifetime of 968 h and higher propylene selectivity of 53%,was designed by introducing in-situ Al migration into hierarchical ZSM-5,which combined the modulation of acidity and pore structure.