Synthesis Of Hierarchical ZSM-5 Zeolite And Its Catalytic Properties In Methanol To Gasoline Reaction

With increasing global demand on the transportation fuel and the shortage of crude oil,searching for new routes to produce gasoline from crude oil substitutes becomes one of the most important issues in the field of chemical industry.Methanol to gasoline（MTG）reaction is considered to be a promising process to produce synthetic transportation fuel.The resources are independent of the traditional petroleum,which makes the MTG reaction attract widespread attentions of scholars.ZSM-5 zeolite is proven to be the effective catalyst in the MTG reaction due to its moderate acidic properties and uniform microporosity.However,the microporous structure of the ZSM-5 zeolite restricts the diffusion of macromolecules,leading to easy deactivation of the catalyst due to the formation of coke and low gasoline yield in the MTG reaction.This paper focuses on the synthesis of hierarchical ZSM-5zeolite to solve the diffusion limitation problems of the microporous ZSM-5 zeolite and improve the stability and gasoline yield of the catalyst in the MTG reaction.Firstly,a series of hierarchical ZSM-5 aggregates with a relatively low Si O₂/Al₂O₃ ratio（⁵0）were synthesized using alkali-treated commercial ZSM-5zeolite as seeds and cetyltrimethylammonium bromide（CTAB）as mesoporous template.The optimum crystallization condition for the synthesis of ZSM-5aggregates with the assistance of CTAB was investigated.The optimum crystallization condition was that the initial gel was aged at 120°C to accelerate the nucleation,and then crystallized at 160°C to complete the crystal growth.The two-step crystallization method was beneficial to the synthesis of the hierarchical ZSM-5 aggregates with small particle size.Secondly,under the optimal crystallization conditions,the influence of different amounts of CTAB on the physicochemical properties and catalytic performance of the synthesized ZSM-5 catalysts was investigated in detail.The results indicated that the mesoporosity of the hierarchical ZSM-5 aggregates was greatly enhanced with the addition of CTAB.When the molar ratio of CTAB/Si O₂ was 0.04,the ZSM-5aggregates showed the smallest primary crystal size,the largest external surface areas and mesoporous volume,while the acid amounts slightly decreased.Compared with the conventional ZSM-5 zeolite prepared in the absence of CTAB,the catalytic lifetime of the synthesized hierarchical ZSM-5 aggregates was remarkably improved and the gasoline yield increased by 10%in the MTG reaction.Thirdly,at the molar ratio of CTAB/Si O₂ of 0.04,the crystallization mechanism of the hierarchical ZSM-5 aggregates was investigated.The results showed the possible crystallization mechanism as follows:the alkali-treated commercial ZSM-5formed dispersed nuclei and the initial gel aged at 120°C for 24 h firstly in the synthesis facilitated the formation of more nuclei,inducing the formation of the ZSM-5 zeolite with primary nanocrystals.The drilling effect of CTAB promoted the formation of abundant intercrystalline mesopores,while the protective effect of CTAB inhibited the growth of the crystals,which was conducive to the formation of nanoscale primary particles.Thus,the hierarchical ZSM-5 aggregates with plenty of intercrystalline mesopores and primary nanocrystals were formed.Finally,binder-free ZSM-5 zeolite was directly synthesized by a vapor phase transport method.Mesopores were introduced into the synthesized zeolite by Na OH alkali treatment.The external surface areas and mesoporous volume of the treated zeolite significantly increased,and the acid amounts decreased.Thus,the stability of the zeolite in the MTG reaction was greatly improved.And the optimum alkali treatment time was 0.5h.