| Volatile organic compounds(VOCs)are precursors of PM2.5 and O3,which are one of the vital atmospheric pollutants.The combined process of adsorption concentration and post-treatment can be adopted to control the VOCs exhaust gas from complex industrial sources.As aluminosilicates with homogeneous microporous or mesoporous structures and function of sieving molecules,zeolites have the advantages of high adsorption selectivity and nonflammability.However,the adsorption property of zeolites in the actual adsorption system of VOCs is not clear,the utilization efficiency of adsorption sites is low due to the limited mass transfer,and it is difficult to adsorb small molecular VOCs,such as mercaptan,thioether,etc.Therefore,it is of great significance to develop efficient modified zeolites of VOCs adsorption.In this paper,10 commercial zeolites were selected to systematically explore the key factors affecting the VOC adsorption properties of zeolites through the adsorption performance test and the characterization results.Then,the hierarchical zeolites were prepared by alkali treatment of high silica zeolites.The VOC adsorption of hierarchical zeolites were studied in detail and the effect of mesoporous structure on the adsorption and desorption properties was revealed.In addition,the ion-exchanged zeolites were prepared and their adsorption properties for sulfur-containing VOCs were investigated.Finally,the competitive adsorption of VOCs by each modified zeolite was studied.The composite modified zeolites which can effectively adsorb ordinary VOCs and sulfur-containing VOCs were obtained.The main innovative conclusions are as follows:(1)The adsorption properties of 5A,ZSM-5,Y(NaY and USY),NaX and MCM-41 mesoporous silica materials were studied under different water vapor contents.The relationship of structure-activity was analyzed based on the characterization results of adsorbents.It was found that the pore size and Si/Al ratio were the vital factors affecting the VOC adsorption.And the USY-3 zeolite(Si/Al atio=21.1)has the best adsorption property under humid condition.When the water content is 4.5%,its p-xylene(PX)adsorption capacity could reach approximately 100 mg·g-1.(2)The USY-3-0.2M hierarchical zeolite was prepared by alkali treatment,and its adsorption and desorption properties for VOCs(benzene,PX,chlorobenzene,butyl acetate)were studied.It resulted that the appropriate alkali treatment can not only generate mesoporous structure,but also can effectively increase the mass transfer rate of VOCs and improve the utilization efficiency of adsorption sites with maintaining the stability of zeolite structure,so as to improve the adsorption property of zeolite.In addition,the thermal desorption temperature of adsorbate is lower.(3)AgY,CuY,MgY,CeY,ZnY and CaY zeolites were prepared by ion exchange modification of NaY zeolte,and their adsorption and desorption properties for dimethyl sulfide(DMS)were systematically discussed.It proved that AgY zeolite can effectively adsorb DMS under 1.5%water content,while other ion-exchanged zeolites have almost no adsorption capacity,which is mainly due to strong adsorption capacity of Ag ions with DMS.The desorption of AgY zeolite is more difficult,but it can still maintain stable adsorption property after multi cycle regeneration.(4)The competitive adsorption properties of NaY,AgY and AgUSY-H zeolites for PX and DMS were investigated.It was found that NaY and AgY zeolites mainly adsorbed PX and DMS respectively,which indicated that PX had stronger competitive adsorption ability,but the faster diffusion of small molecular DMS in microporous AgY zeolite and the formation of Ag-S bond make DMS would be preferentially adsorbed.Due to the improved diffusion performance and the existence of chemical adsorption sites,AgUSY-H zeolite can effectively adsorb PX and DMS. |
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