Preparation Of Hierarchical Molecular Sieves Based On Coal Gangue And Their Ad/Desorption Performance Of VOCs

Coal gangue is one kind of black solid waste which generates in the process of coal mining and coal washing.The accumulation of coal gangue is large in China and spontaneous combustion easily occur which produce a great deal of greenhouse gas,resulting in a series of environmental problems.Volatile organic compounds（VOCs）are important precursors of aerosols and ozone.Most VOCs have pungent odor and carcinogenicity,which threaten human health.Adsorption is one kind of convenient,economical and applicable technology.Due to its high specific surface area and excellent hydrothermal stability,zeolites play an important role in VOCs treatment technology.The main components of coal gangue are Si O₂ and Al₂O₃,which can be used as raw materials to prepare molecular sieves.Therefore,it is of great significance to study the preparation of hierarchical ZSM-5 molecular sieve from coal gangue and its application in VOCs adsorption and desorption.In this paper,for the purpose of comprehensive utilization of coal gangue,the pretreatment method of coal gangue was explored by orthogonal experiment to determine the best combination of pretreatment conditions and reaction principle,and the physicochemical properties of intermediate product（CG-M）were also explored.Hierarchical ZSM-5 molecular sieves were synthesized by hydrothermal with cationic polymers----poly dimethyl diallyl ammonium chloride and organosilane----octyl trimethoxysilane,respectively.The effect of the amount of cationic polymer and organosilane on crystal state,pore structure,and morphology of zeolites were investigated by XRD,BET,SEM,TEM,FTIR and TG-DSC.The adsorption and desorption properties,water resistance and renewable properties of hierarchical ZSM-5 molecular sieve for toluene were investigated by static adsorption experiment and dynamic adsorption and desorption experiment.The quasi-first-order kinetics,quasi-second-order kinetics and intra-particle diffusion model were used to fit and analyze the adsorption data,the adsorption kinetic characteristics and adsorption process of hierarchical molecular sieve were investigated.The main conclusion are follows:The main crystalline phases of coal gangue are quartz and kaolinite,and the main components are silica and alumina by XRD,XRF and EA.the ratio of silicon to aluminum of coal gangue could be greatly changed by alkali melting and acid leaching,which made it meet the requirement for synthesis conditions of the ZSM-5 molecular sieves.The optimal treatment conditions were as follows:the roasting temperature of 800℃,the mixing ratio of coal gangue to sodium carbonate of 1:0.6,and the acid leaching conditions of 4 mol·L^-1 H₂SO₄.According to the characterization results,the reaction principle of pretreatment method was inferred,and it was found that the intermediate prepared under the optimum treatment conditions had amorphous mesoporous structure,and its main component was silica.The hierarchical ZSM-5 molecular sieve prepared with cationic polymers as the mesoporous template had both microporous and mesoporous structures.With the increase of the amount of cationic polymers,the crystallinity of the molecular sieve gradually decreased,and the specific surface and mesoporous pore volume increased first and then decreased.The spherical polymers on the surface of the molecular sieves increased and the surface gradually roughened.FTIR and TG-DSC results showed that cationic polymer could interact with intermediate（CG-M）to form hierarchical molecular sieves.The hierarchical zeolites had excellent adsorption properties for toluene(dynamic adsorption capacities of 47.02 mg·g^-1),water resistance（adsorption capacity reduction rates of 4.64%）and renewability（five times adsorption and desorption efficiencies of above 95%）for toluene.The hierarchical ZSM-5 molecular sieve prepared with organosilane as an additive showed coffin-like and coral-like crystals.With the increase of the amount of organosilane added,the crystallinity and specific surface of the sample gradually decreased,and the mesoporous pore capacity first increased and then decreased.The dynamic adsorption capacities of hierarchical zeolites for toluene was 42.08 mg·g^-1.In the water resistance experiments,the adsorption capacity reduction rates was 4.97%.In renewability experiments,the adsorption and desorption efficiencies of five times was above 85%.The adsorption process of toluene on hierarchical ZSM-5 zeolite prepared by the two methods was more in line with the quasi-first-order kinetic model.This adsorption process depends on the diffusion of toluene molecules in particles.This work provided an important reference for the comprehensive utilization of coal gangue,the control of mes-microporous structure of molecular sieve by template and treatment of VOCs with molecular sieves.