| As an important chemical product,isoolefins play an important role in daily life and chemical production.In the process of isomerization of n-pentene skeleton,HZSM-35 molecular sieve is widely used due to its high activity and good shape selection.The industrial HZSM-35(silica-alumina ratio 20)molecular sieve used in this paper has a large amount of acid and the surface of the catalyst is amorphous and the small particle species are more,which affects the diffusion of the product,resulting in the catalyst being easily deactivated by carbon deposition.In this paper,HZSM-35 molecular sieve was treated by acid to obtain a catalyst with high isomerization activity and good stability by investigating the difference of treatment between different acids,different treatment conditions of the same acid and secondary modification of the molding catalyst.The isomerization performance of HZSM-35 molecular sieve before and after treated with nitric acid,hydrochloric acid,citric acid and oxalic acid under the same conditions was compared.Studies showed that the untreated Z-35 catalyst has a large amount of strong acid,resulting in serious carbon deposition in the catalyst,and the catalyst is deactivated faster.The catalytic activity of the catalysts treated with the four kinds of acid increased to different extents.However,the strong acidity of the catalysts treated with hydrochloric acid and oxalic acid is not much different from that of Z-35 catalyst,which results in the surface area of the catalyst having a serious carbon deposition.The catalysts treated with nitric acid and citric acid have a suitable amount of weak and strong acid,so the stability of the catalyst is obviously improved compared with the Z-35 catalyst.The mesoporous pore volume and external surface area of the catalyst treated with nitric acid increased,while the mesoporous pore volume of the catalysts after the treatments of the other three acids decreased.When the concentration is 1M,the treatment temperature is 80?,and the treatment time is 6 h,the treatment effect of nitric acid and citric acid is better.The effects of treatment conditions of nitric acid and oxalic acid on the isomerization performance were further studied.It was found that when the concentration of oxalic acid is 1M,the treatment temperature is 80?,and the treatment time is 12 h,the conversion rate of the catalyst is 25 percentage points higher than Z-35 catalyst.when the concentration of nitric acid is 3.7M,the treatment temperature is 120?,and the treatment time is 12 h,the conversion rate of the catalyst is 40 percentage points higher than Z-35 catalyst.In order to meet the requirements of industrial applications,the XZ-35 catalyst with the best treatment effect of nitric acid was extruded.Secondary modification of different elements was applied to the shaped catalyst J to improve the stability.It was found that after the four elements of M1,M2,M3 and M4 are modified,the M1 element reacts with the binder reducing the amount of strong acid in the catalyst,forming a strong acid center with a peak temperature of 640?,decreasing the carbon deposition rate of the catalyst which makes the stability of the catalyst JM1 better,and the conversion rate only drops by 3 percentage points after 10 hours of reaction.When the loading content of M1 is 1.25% and the calcination temperature is 600?,the catalyst M1-600 has a suitable amount of medium and strong acid,so that the catalyst has high activity and is not deactivated by carbon deposition.When the reaction temperature is 325?,the amount of diluent is 30%,the reaction pressure is 0.4 MPa,and the carrier gas flow rate is 20 m L/min,the skeletal isomerization performance of the catalyst is the best.Under the optimal reaction conditions,the reaction performance of the shaped catalyst J and M1-600 was compared.After 24 h of reaction,the conversion rate of the catalyst J decreased by 12 percentage points,while the catalyst M1-600 decreased by only 2 percentage points and the conversion rate is still 60%. |
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