Study On Co-production Of Low-carbon Olefins From Methanol To Para-xylene

Para-xylene is an important chemical basic raw material,and its production mainly depends on the petroleum production route;and the resource structure of China's"rich coal,low gas,and lack of oil"determines the need to develop coal-based para-xylene production technology.In this paper,low-cost coal-based methanol is used as a raw material,and under the action of modified ZSM-5 catalyst,p-xylene is produced with high selectivity through different process routes,which shortens the product separation process and greatly improves efficiency.The contents are shown as follows.In a fluidized bed reactor,the reaction performance of Zn,Si and P-modified ZSM-5 catalysts for methanol to p-xylene co-production of lower olefins was studied.XRD,BET,SEM,NH₃-TPD and other means were used for characterization analysis.The results show that the modification of Zn decreases the acid strength of the catalyst,increases the amount of medium and strong acids,and increases the selectivity of paraxylene and lower olefins.A certain amount of silicon deposition modification reduces the pore size of the catalyst while reducing the amount of acid on the outer surface of the catalyst.Immersion of an appropriate amount of phosphorus can modulate the strength of the acid center of the molecular sieve and the amount of acid.The results of the reaction of methanol to paraxylene in a fluidized bed reactor to produce low-carbon olefins showed that at the temperature of 425°C,normal pressure,a reaction time of 40 min,and a space velocity of 1 h^-1,the 3Zn-3Si-3P/ZSM-5 catalyst has the selectivity of 76%for para-xylene in xylene,24.4%for C₂-C₄ lower olefins.In particular,the total selectivity of aromatics and C₂-C₄ lower olefins is as high as 92.2%.A new process for the production of low-carbon olefins by coupling methanol-to-aromatic hydrocarbons with methanol-toluene alkylation was developed,and catalyst performance was studied for methanol aromatization and methanol-toluene alkylation reactions.Zn-Si-P-Mg composite modification of ZSM-5 molecular sieves with different silicon-aluminum ratios and different grain sizes was used to study the performance of methanol aromatization in fixed-bed reactors.The results show that the higher the silicon-aluminum ratio,the lower the acid amount and acid strength of the catalyst,the lower the aromatic selectivity,and the higher the selectivity of paraxylene in xylene and the selectivity of lower olefins.When the catalyst silicon-aluminum ratio is 130,the selectivity of C₂-C₄ low-carbon olefins is 55.9%,and the selectivity of para-xylene in xylene is increased to 84.0%.Studies have shown that the more seed crystals are added,the smaller the crystal grain size of the catalyst,the more micropore structure and shorter pore channels,which can improve the yield and stability of aromatics.When the catalyst seed amount is 1%,the aromatic yield is higher at 17.5%;when the seed amount is 0.01%,the selectivity of C₂-C₄ lower olefins is 49.9%,and the selectivity of para-xylene in xylene is78.6%.Study on the process conditions of 6Si-5P-3Mg modified ZSM-5 catalyst for the selective alkylation of methanol to toluene to p-xylene.The results showed that under optimized reaction conditions at a temperature of 460°C,a molar ratio of toluene to methanol of 2/1,WHSV=3h^-1,and a flow rate of N₂ of 77.9 ml/min,the conversion of toluene was 18.20%,the selectivity is as high as 98.94%,and the total selectivity of low-carbon olefins and para-xylene is 85.09%.After using different catalysts for coupling,there is a certain degree of difference in the improvement of reaction performance.Finally,under optimized process conditions,the use of different catalysts for coupling has a certain degree of improvement in reaction performance.The selectivity of aromatics is up to 36.5%,the selectivity of para-xylene in xylene can be increased to 86.0%,and the selectivity of C₂-C₄ lower olefins is about 56.3%.