Green Synthesis Of Heteroatom ZSM-5 Zeolite From Natural Attapulgite And Its Catalytic Application

As an important industrial catalyst,heteroatom ZSM-5 zeolite has been widely concerned and deeply studied by academia and industry.At present,the processes of heteroatom ZSM-5zeolite using chemical reagents are high-energy,high-cost and non-green.Attapulgite is a natural mineral with the advantages of rich resources and low price.In this work,attapulgite was purified by acid treatment and activated by low-temperature alkali fusion to obtain silicon,aluminium,iron and titanium sources of hetero-atom substituted ZSM-5 zeolite.Then,the high value-added heteroatom ZSM-5 zeolite was synthesized by hydrothermal method and dry gel method in amine-free system,respectively.Finally,the denitrification performance of heteroatom ZSM-5 zeolite was evaluated.The results show that:(1)after acid pretreatment,the crystal structure of natural attapulgite destroyed and inorganic and organic impurities removed from the minerals.The optimum conditions for alkali fusion of acidified attapulgite by 4% HCl solution are as follows: the temperature of alkali fusion is 250 ℃,the time of alkali fusion is 1 h,and the mass ratio of attapulgite to sodium hydroxide is 1 : 2.Under this condition,the alkali fusion rate of acidified attapulgite can reach over 95%,which indicates the full activation of attapulgite.(2)Pure heteroatom ZSM-5 zeolite with high crystallinity can be directly synthesized by hydrothermal method and dry gel method under suitable conditions.Alkali fusion activated solution of attapulgite was used as silicon-aluminium source and heteroatom source(such as iron,titanium,magnesium,etc.),without chemical reagents,organic templates and crystal seeds.By comparing the crystal morphologies and crystallization conditions of zeolite samples synthesized by the two methods,it can be found that dry gel method can greatly shorten the crystallization time and reduce the crystal particle size of zeolite.(3)In heteroatom ZSM-5zeolite,the existing forms of heteroatom iron include oligomeric,high-polymer and isolated coordination states,in which the skeleton structure of isolated coordination is dominant.The results of denitrification performance evaluation show that the heteroatom ZSM-5 catalyst has better denitrification activity at medium and high temperatures(350-500 ℃)than the commercial iron-containing zeolite(Fe/C-Z)catalyst obtained by impregnation method,and the conversion rate remains above 99%.It can meet the requirements of industrial coal-burning process for the catalysts.