Catalytic Performance Of Hierarchical ZSM-5 Zeolite In Methanol To Aromatics

ZSM-5 zeolite has been widely used in petrochemicals,fine chemicals and environmental catalysis because of its special three-dimensional pore structure,acid resistance and thermal/hydrothermal stability.However,ZSM-5 zeolite itself is a microporous zeolite with a cubic lattice,and the microporous structure limits the access of reactants to the active sites on the inner surface of the zeolite,inhibiting the full utilization of the zeolite catalyst.Diffusion limitation is the main cause of zeolite catalyst deactivation and low utilization,which is an important problem limiting the application of zeolite catalysts.In this paper,we try to prepare hierarchical ZSM-5 zeolites by using hard and soft templates starting from the introduction of mesopores in order to solve the diffusion problem of ZSM-5 zeolites in catalytic reactions and improve their catalytic reaction activities.1.Ordered mesoporous Si O₂ colloidal crystals of 20,30 and 40 nm were attempted as templates,and ordered mesoporous carbon templates with controllable particle size were synthesized by the idea of reverse structure replication,and finally ordered mesoporous ZSM-5 zeolites with different particle sizes with a silica-alumina ratio of 50 were constrained within the ordered mesoporous carbon.The structure,morphology and surface acidity of the synthesized samples were characterized by XRD,SEM,TEM,N₂isothermal adsorption/desorption and Py-IR,and their catalytic activities in the methanol to aromatics process were tested on a fixed-bed reactor.The experimental results showed that the catalytic performance of ordered mesoporous ZSM-5 zeolites with different particle sizes was different for the methanol to aromatics reaction.Among them,the ordered mesoporous ZSM-5 zeolite of 20 nm showed excellent catalytic performance with a selectivity of up to 60%for light aromatics,and the methanol conversion started to decrease after 51 h of continuous operation.2.Hierarchical ZSM-5 zeolites with different Si/Al were successfully synthesized by the steam-assisted crystallization soft-templating method.The steam-assisted crystallization synthesis process resulted in the absence of liquid aqueous phase in the dry gel system,which successfully avoided the phase separation between the templating agent and the solid silica.The properties such as structure,morphology and surface acidity of the synthesized samples were characterized by XRD,SEM,TEM,N₂ isothermal adsorption/desorption,NH₃-TPD,Py-IR and ²⁷Al MAS NMR,and their catalytic activity and anti-deactivation performance in the methanol to aromatics reaction were tested on a fixed-bed reactor.The results showed that the hierarchical structure led to the reduction of diffusion resistance,increase of external surface area and reduction of carbon accumulation of ZSM-5 zeolite.The hierarchical ZSM-5 zeolite with ethanol as solvent,F₁₂₇ as soft templating agent,and Si/Al of 60 has large specific surface area and suitable acid strength,and exhibits excellent catalytic performance in the methanol-to-aromatics reaction,and the selectivity of BTX reaches 65.8%,the methanol conversion started to decrease after 107 h of continuous operation.Finally,the reaction mechanism of the methanol-to-aromatics process was explored by using the transition state search of density functional theory.The lepton states and energy barriers of methanol to aromatics were obtained by the minimum energy search of LST/QST lepton states.