Modification Of ZSM-5 Zeolite And Its Performance In Catalytic Methanol To Aromatic Reaction

Aromatics are very important basic chemical raw materials,which are commonly used in the synthesis of rubber,resin,chemical fiber and other fields.Therefore,the demand of aromatic hydrocarbons is very gigantic.With the depletion of the world’s oil,the quantity of aromatic hydrocarbons produced by petroleum routes are no longer meet the market demand.It can be seen that the production of aromatic hydrocarbons by nonpetroleum routes is very important.In our country,the production of methanol aromatics from methanol as raw material is promising.ZSM-5 zeolite is a solid acid catalyst,which has a unique pore structure and a kinetic diameter comparable to xylene and other products,so it is selected as a catalyst for the reaction of methanol to aromatic hydrocarbons.At present,there are many problems in the reaction of methanol to aromatic hydrocarbons to achieve industrialization,such as low selectivity of aromatic hydrocarbons,short life of catalysts,and environmental pollution caused by the synthesis of catalysts.In view of the above situation,researchers have carried out a lot of research on ZSM-5 zeolite.The traditional synthesis of ZSM-5 zeolite is mainly based on industrial silicon aluminum source as raw material,using organic template agent under hydrothermal conditions.This method has high production cost,high pollution and low utilization rate,which do not conform to the concept of green chemistry.In addition,the microporous structure of industrial ZSM-5 zeolite will limit the product diffusion,promote the formation of carbon deposition,and reduce its catalytic life.Therefore,the main objective of this paper is to improve the diffusion properties of ZSM-5 zeolite.In order to improve the BTX selectivity and catalytic lifetime of ZSM-5 zeolite in the reaction of aromatics from methanol,a series of modification treatments were carried out.The specific research work was carried out as following:(1)In this experiment,the industrial ZSM-5(Si/Al = 38)was used as raw material and Na OH and HCl were used as reagents for desilication and desalification.The effects of acid and base concentration on pore structure,relative crystallinity,morphology and acidity of molecular sieve were discussed in detail.After the pore structure and other data analysis,the industrial molecular sieve and the parent molecular sieve treated with0.3 mol/L Na OH and 0.4 mol/L HCl were selected for the reaction of methanol to aromatic hydrocarbons.After that,the BTX selectivity and catalytic lifetime of the above samples were compared.The results showed that mesoporous could be introduced into the ZSM-5 by appropriate acid-base treatment.The specific surface area and pore volume of the molecular sieve were increased,and the diffusion property of the molecular sieve was improved.In the reaction of methanol to aromatic hydrocarbons,it was found that the lifetime of molecular sieve was extended from 8 h to 36 h,the BTX selectivity was increased from 38.97% to 51.39%,and both catalytic lifetime and BTX selectivity were greatly improved.(2)During the synthesis of industrial molecular sieve,a large amount of waste liquid will be produced and the environment will be polluted.Therefore,in subsequent experiments,ZSM-5 zeolite(38 Si/Al ratio)directly synthesized by illite template-free and solvent-free method was selected as the matrix for the above modification treatment and catalytic performance evaluation.The optimal treatment conditions were as follows:0.2 mol/L Na OH and 0.2 mol/L HCl.The results show that acid and alkali treatment makes the service life of molecular sieve from 13 h to 38 h,BTX selectively from 36.11%to 53.36%.It was verified that the catalytic life and selectivity of BTX could be effectively improved by appropriate alkali-acid treatment.(3)In addition to introducing mesoporous in molecular sieve,reducing the grain size of crystal can also improve the diffusion performance.In this paper,nanometer ZSM-5 zeolite(Si/Al=130)was synthesized from Illite by two-stage variable temperature method.After testing found that the specific surface area and pore volume of nano ZSM-5,far higher than the same silicon aluminum ratio of industrial molecular sieve.In order to further improve the catalytic performance of ZSM-5 zeolite catalyst,nano ZSM-5 was modified with zinc(2% zinc load).The catalytic results show that the longest catalytic life of the nano-ZSM-5 is 40 h,and the yield of BTX after zinc modification is the highest,reaching 50.47 %,but the catalytic life decreases to 30 h.The lifetime and BTX selectivity of nanometer zeolite are significantly higher than those of industrial zeolite,which indicates that reducing grain size can effectively improve the catalytic performance of molecular sieve.Although the life of the catalyst was reduced,the BTX selectivity increased significantly,indicating that the zinc modification treatment is an effective way to improve the BTX selectivity.