Preparation Of ZSM-5 Molecular Sieve From Coal Fly Ash By Hydrothermal Activation Combined Interzeolite Conversion Method

Coal fly ash is the main solid waste generated during the combustion and transformation of coal.The main chemical components of coal fly ash are SiO₂and Al₂O₃,and their proportions can reach more than 60 wt.%,which are close to the chemical composition of zeolites.Thus,the way of synthesis of molecular sieve from coal fly ash was developed.ZSM-5 molecular sieve is one of the most important zeolites.Therefore,the synthesis of ZSM-5 molecular sieve from coal fly ash can improve the utilization value of coal fly ash resources and reduce the synthesis cost of ZSM-5 molecular sieve.Supercritical(Subcritical)hydrothermal activation method can greatly improve the activation efficiency of coal fly ash,promote the efficient utilization of silica and aluminum resources,and can quickly stimulate the conversion of silica and aluminum substances in coal fly ash into low Si/Al ratio zeolite.As a green and efficient method for the synthesis of high Si/Al ratio zeolite,interzeolite conversion method had attracted wide attention of scholars.In this paper,the primary zeolite–coal fly ash-based analcime was obtained from coal fly ash by supercritical(subcritical)hydrothermal activation method,and ZSM-5 molecular sieve was further prepared by interzeolite conversion method.The effects of crystallization conditions under different preparation methods on the surface morphology,relative crystallinity and pore structure parameters of the products were investigated.SEM,XRD,N₂adsorption-desorption,ICP-OES and other testing techniques were used to characterize the samples.The results showed that:(1)Using coal fly ash-based analcime as raw material,which was obtained from the hydrothermal activation of coal fly ash with 0.5 mol/L Na OH solution and 250?C,ZSM-5 molecular sieve was prepared by the combined interzeolite comversion method.The interzeolite comversion method can realize through the addition of template to realize.As TPAOH was used as the template agent,and the TPAOH:Al₂O₃ molar ratio was 10,and the ZSM-5 molecular sieve with high crystallinity and a specific surface area of about 370 m²/g can be prepared.In addition,compared with the synthesis of ZSM-5 molecular sieves with pure chemicals,the preparation of ZSM-5 molecular sieve by the interzeolite comversion of analcite was faster in crystallization speed and higher in crystallization degree.The interzeolite comversion method can also realize through the addition of seed.But the conversion of analcime was incomplete and the product ZSM-5 molecular sieve had low crystallinity and small specific surface area.(2)Using coal fly ash-based analcime as raw material,ZSM-5 molecular sieve was prepared by the method of interzeolite conversion with the addition templete and seed at the same time.When TPAOH was used as the template agent,the TPAOH:Al₂O₃ molar ratio was 2.5,and the commercial ZSM-5 seed was added with 5 wt.%,the ZSM-5 molecular sieve with high crystallinity and specific surface area of about 370 m²/g can be synthesized.In addition,by changing the ratios of Si O₂/Al₂O₃,ZSM-5 molecular sieve with different ratios of silicon to aluminum can be prepared.(3)Using the template-seed compound method and adding 5 wt.%of the synthesized nano-scale ZSM-5 molecular sieve as the seed,the nanoscale ZSM-5molecular sieve with high crystallinity specific surface area of more than 300 m²/g and grain size of about 400-500 nm can be prepared.Moreover,ZSM-5 molecular sieve grains could grow on the surface of analcite particles and gradually completely cover the surface of analcite particles.