Solvent-free Synthesis Of ZSM-5 Zeolite From Coal Fly Ash And Its Performance In Toluene Adsorption And Desorption

Thermal power plays an important role in China’s power system,but a controllable amount of coal fly ash is produced in the process of coal combustion.Due to its single utilization mode and low utilization rate,a large amount of untreated coal fly ash is accumulated for a long time,occupying land,polluting soil and atmosphere and even endangering human health.It is of far-reaching significance to find an efficient resource utilization way of coal fly ash.In this paper,the idea of synthesizing ZSM-5 zeolites through solvent-free synthesis method is creatively introduced into the synthesis of fly ash based ZSM-5 zeolites.A unique new path of synthesizing ZSM-5 zeolites from fly ash is explored and various parameters as well as conditions in the synthesis process are optimized.Finally,excellent adsorption and desorption performance is shown in the toluene desorption removal experiment and it is compared with commercial ZSM-5 zeolite.The specific conclusions are as follows:（1）Adding sodium carbonate to coal fly ash and roasting at high temperature can destroy its stable structure,and then adding hydrochloric acid can obtain silica gel and aluminum containing solution to realize the extraction and separation of silicon and aluminum elements in fly ash.The quartz and mullite phases can be completely transformed by roasting at 800℃for 2 hours.When 4 mol/L hydrochloric acid and fly ash:hydrochloric acid solid-liquid ratio is 1:10（g/ml）,the leaching rate of Si O₂ can reach 95%.The purity of silica gel obtained by extraction is as high as 96%,and it also contains 1.65%alumina.（2）ZSM-5 zeolite was prepared by using the aqueous silica gel extracted from（1）as raw material,adding an appropriate amount of sodium carbonate and a small amount of template,grinding,mixing and high-temperature crystallizing.The characteristic peak of ZSM-5 zeolite appeared at 3 h of high-temperature synthesis.When the crystallization time was extended to 6h,the zeolite crystallized completely,with complete morphology and uniform size.The crystallization condition was the best when the crystallization time was 12 h.Compared with the preparation of zeolites from silica powder with water,the preparation of ZSM-5 zeolite from aqueous silica gel has the advantages of short synthesis time,uniform sample and good morphology.In addition,when the ratio of sodium carbonate to silica gel（ZSM-5）is greater than that of silica gel（ZSM-5）,the yield of hetero carbonate increases.In the synthesis of molecular sieves,the optimum moisture content range of silica gel is 30%～60%.When the moisture content is less than 30%,the crystallinity is low,and when the moisture content is more than 60%,quartz hetero phase appears.（3）The ZSM-5 zeolites prepared in this study present a variety of morphologies.The increase of moisture content of raw silica gel induces the evolution of zeolite morphology from the long strip crystal morphology of S-30 to the uniform sphere of S-60 with the acidity enhancing.ZSM-5 zeolite synthesized by solvent-free method has a certain amount of intergranular mesoporous.The mesoporous content of the prepared zeolites increase significantly with the increasing of the moisture content of raw silica gel,from 85.3 m²·g^-1 of S-30 to 183.1 m²·g^-1 of S-60.The moisture content of silica gel has a significant linear relationship with the specific surface area and pore volume of mesoporous pores of zeolites.Therefore,it can be realized by adjusting the moisture content of raw silica gel to control the product morphology and mesoporous content of multistage porous ZSM-5 zeolites.（4）The results of toluene adsorption and desorption experiment show that the ZSM-5zeolites prepared by the solvent-free synthesis method in this experiment show excellent toluene adsorption capacity（71.4 mg/g-75.1 mg/g）,which is close to the toluene adsorption capacity of commercial ZSM-5 zeolite（78.2 mg/g）,but the toluene adsorption penetration time is slightly lower.The increase of moisture content of raw silica gel will lead to the increase of mesoporous content,which makes the desorption activation energy of toluene on the prepared ZSM-5 zeolite（39.97 k J/mol-50.08 k J/mol）significantly lower than that of commercial ZSM-5 zeolite（53.03 k J/mol）,which has a negative linear correlation with the mesoporous content of ZSM-5 zeolites.So it has great application potential in industrial fields such as VOCs adsorption removal.