Controllable Design Of Hierarchical Porous ZSM-5 Zeolites And Its Application In Phenol Hydroxylation

Dihydroxybenzenes,including catechol and hydroquinone,are important chemical materials,which can be widely used in medicine,pesticide,dye and other industries.They are generally produced by phenol hydroxylation over metal-doped zeolite catalysts by using hydrogen peroxide as oxidant.Zeolites are a class of crystalline aluminosilicate materials,their microporous characteristic will lead to low catalytic activity and easy deactivation,which limits their practical application in phenol hydroxylation.In contrast,hierarchical zeolites have better diffusion properties and catalytic properties.Thus,the development of hierarchical zeolites is the key factor for green production of dihydroxybenzenes.This paper mainly studies the controllable design of hierarchical structure in ZSM-5 zeolites and the application in phenol hydroxylation.This research has important theoretical significance and application value.The main contents include:（1）Hierarchical ZSM-5 zeolites were prepared by using gemini quaternary ammonium salt as mesoporous structure-directing agent,and the regulations of their structure and property were realized by controlling the synthesis conditions.The results showed that the crystallization temperature is a key factor for the simultaneous construction of hollow morphology and hierarchical structure.The catalytic activity of the Fe-substituted hollow ZSM-5 with intracrystalline mesopores in phenol hydroxylation was evaluated.At the phenol/H₂O₂ molar ratio of 1:1 and 65℃,52.5%conversion rate of phenol can be obtained with 96.8%selectivity to dihydroxybenzenes.（2）Hierarchical ZSM-5 zeolite with short-range ordered mesoporosity was obtained via a hydrothermal synthesis by using organosilane as mesoporous srtucture-directing agent.The study showed that the increased concentration of organosilane can change the hierarchical porous structure from ordered mesoporous structure to worm-like mesopores,while the decrease of siloxane groups will decrease the order degree of mesopores.When the ZSM-5 zeolite with short-range ordered mesopores is employed as the catalyst,22.7%conversion rate of phenol can be obtained with almost 97.9%selectivity to dihydroxybenzenes at the phenol/H₂O₂ molar ratio of 3:1 and room temperature.（3）To explore an effective method for the directional design of hierarchical structure,hierarchical ZSM-5 zeolites were prepared through design and synthesis of bifunctional soft templates.First,the TPA⁺group was integrated with hydrophobic alkyl chain in one molecule,nanosheets of ZSM-5 zeolite can be obtained.Then,by increasing the number of hydrophobic alkyl chain,the control of the crystal growth direction of zeolite was realized,the growth of the crystal on the a-c plane was further restricted,resulting in the formation of single-unit-cell nanowires of ZSM-5 zeolite.With its excellent pore connectivity,26.1%conversion rate of phenol can be obtained with almost 98.9%selectivity to dihydroxybenzenes at the phenol/H₂O₂ molar ratio of3:1 and room temperature.（4）Hierarchical ZSM-5 zeolite and microporous-mesoporous ZSM-5/MCM-41composite was prepared by alkali treatment and recrystallization method,respectively.The prepared materials were iron doped by ion-exchange method and their catalytic properties in phenol hydroxylation were campared.The study showed that Fe-M-ZSM-5 catalyst with intracrystalline mesopores can achieve higher selectivity of dihydroxybenzenes,while Fe-ZSM-5/MCM-41 catalyst with two-dimensional hexagonal ordered mesopores has better catalytic stability.This paper found that the hierarchical ZSM-5 zeolites with shorter diffusion pathway and better pore connectivity can provide better catalytic properties for phenol hydroxylation.