Study On Preparation And Catalytic Performance Of Catalyst For MTA Reaction

HZSM-5 zeolite was the most suitable substrate material for the reaction of methanol to aromatics and its physicochemical properties were closely related to its activity.From the perspective of reaction,the key to obtain high selectivity target products lies in the reasonable regulation of acidity and pore structure of zeolite.The pore structure influences the number and distribution of active centers,the contact degree between active centers and raw materials,the diffusion rate of target products,shape selection efficiency and the carbon capacity of the catalyst,and is closely related to the life of the catalyst.Acid properties affect the conversion of methanol,the generation of carbon depositing precursor,the selectivity of aromatics especially light aromatics,and the service life of catalysts.In other words,the pore structure and acid property of the catalyst play their respective roles,but they are interrelated and inseparable.In the regulation of acid properties,the number of active centers in the pore should be ensured,and the target products should be timely dispersed out of the pore,so as to avoid the generation of carbon deposition caused by side reactions.This paper regulated the acidity,pore structure and crystal morphology of the substrate HZSM-5 zeolite to different degrees.The specific research contents are as follows:(1)HZSM-5 zeolite were treated with different concentrations of ammonium fluoride solution to remove amorphous silicon or aluminum from the zeolite to obtain pure zeolite with high crystallinity.The surface acidity of the obtained zeolite was regulated and the adverse reaction was inhibited by passivating the strong acidity of the outer surface.The physical and chemical properties of the catalysts were characterized by XRD,SEM,TEM,NH₃-TPD,Py-IR and N₂ adsorption/desorption.The effects of acidity and surface properties of the catalyst on the reaction performance were investigated.The effects of temperature,contact time and space velocity on the selectivity of aromatic hydrocarbons were also investigated.The results showed that under the optimal conditions of reaction temperature 400?,pressure 0.1 MPa,total flow rate of feed gas 20 m L/min and catalyst dosage 0.5 g,the selectivity of aromatics reached 45.3%with HZSM-5 0.09M as catalyst.After silanization,the surface acidity of HZSM-5-0.09M catalyst was passivated,which inhibited the occurrence of side reactions and obtained a higher xylene selectivity.Moreover,the stability of the silanization catalyst was obviously better than that of HZSM-5 catalyst after continuous operation for 12 h.(2)HZSM-5-0.09M samples were treated with different concentrations of nitric acid,citric acid and oxalic acid,respectively.The HZSM-5-0.09M-1N catalyst was successfully prepared.The surface acidity of HZSM-5-0.09M-1N catalyst was regulated by surface silanization and hydrothermal treatment.The physical and chemical properties of the catalysts were characterized by XRD,SEM,TEM,NH₃-TPD,Py-IR and N₂ adsorption/desorption.The effects of acidity and surface properties of the catalyst on the reaction performance were investigated.The catalytic performance of the treated catalyst in MTA reaction was investigated under the optimal conditions of reaction temperature 400?,total flow rate of feed gas 20 m L/min,pressure 0.1 MPa and catalyst dosage 0.5 g.The results showed that the total aromatics selectivity reached 57.9%,BTX selectivity reached 51.6%and heavy aromatics selectivity reached 6.3%on HZSM-5-0.09M-1N.(3)A series of Gd/HZSM-5-0.09M-1M catalysts were synthesized by incipient-wetness impregnation method and their surface properties and catalytic performances in methanol to hydrocarbons(MTH)reaction were studied in detail.The addition of gadolinium has little effect on the structure of zeolite and the specific surface area of catalyst.However,with the increase of Gd content,the strong acid density of catalysts shows a monotonously increasing trend and the Gd/HZSM-5-0.09M-1M catalysts exhibit high C₂?C₅ aliphatic hydrocarbon selectivity in MTH reaction which can reach 71.6%over the catalysts with the gadolinium content of 7%.In addition,the Gd/HZSM-5-0.09M-1M catalysts exhibit much lower carbon deposition rates than the parent HZSM-5-0.09M-1M catalyst in MTH reaction due to the low yield of C₉+aromatics.(4)Modification of HZSM-5 zeolite by adding metal components can effectively regulate the acidity of the zeolite and the distribution of MTA reaction products.Therefore,the core-shell 2%Gd/S-1@HZSM-5 catalyst was prepared by using Gd supported nanocrystalline Silicalite-1 as seed.The physical and chemical properties of 2%Gd/S-1@HZSM-5 catalyst were systematically analyzed by XRD,SEM,TEM,NH₃-TPD,Py-IR and N₂ physical adsorption and desorption.The catalytic performance of 2%Gd/S-1@HZSM-5 catalyst in MTA reaction was investigated under the optimal conditions.The results show that the selectivity of BTX over the core-shell catalyst is only 51.9%due to the position relationship of metal species Gd,Si and HZSM-5 zeolite,which does not achieve higher selectivity,but the hydrothermal stability of the catalyst was improved.(5)Limited by the diffusion of traditional microporous HZSM-5 zeolite.The hollow2%Gd/HZSM-5 catalyst with hierarchical pore structure was successfully prepared to readjust the position relationship of the three components of Gd,Si and HZSM-5 zeolite.XRD,SEM,TEM,NH₃-TPD,Py-IR and N₂ adsorption/desorption were used to systematically analyze the physicochemical properties of 2%Gd/HZSM-5-T catalysts.The results showed that the specific surface area,pore structure,morphology and acidity of the catalyst after TPAOH treatment had obvious influences,especially the concentration of strong Br?nsted acid increased significantly,the Br?nsted acid site was the main active site of aromatization,which was more conducive to the improvement of selectivity of BTX.The catalytic performance of 2%Gd/HZSM-5-T catalyst in the MTA reaction was also investigated under the conditions of the reaction temperature 400?,the total flow rate of raw gas 20 m L/min,and the catalyst consumption 0.5g.The results showed that the selectivity of aromatics on 2%Gd/HZSM-5-T catalyst increased from 38.8%to 68.9%,and that of BTX increased from 35.3%to 65.7%.The results of hydrothermal stability experiments show that the hydrothermal stability of 2%Gd/HZSM-5-T catalyst was significantly better than that of 2%Gd/HZSM-5 catalyst.