Surface Silicon-rich Catalysts For Aromatization Of Methanol To P-xylene

P-xylene?PX?is an important chemical raw material and the leader of polyester product chain.The concentration of p-xylene produced by the traditional route is about 24%.Further adsorption separation is needed to separate PX from the mixture of isomers with similar boiling point to obtain p-xylene with high purity.This process requires a lot of energy consumption.In recent years,with the continuous development of the polyester industry,the demand for PX has increased year by year,which has led to the development of technologies related to the production of p-xylene.PX produced by the traditional route has been unable to meet the market demand,and since October 2018,the international oil price has been rising,which leads to an increase in the cost of producing p-xylene from the oil route.The energy structure of"more coal,less oil and less gas"in China also determines that it is necessary to develop the production technology of p-xylene from coal-based methanol.Use cheap and readily available methanol to produce high concentration of p-xylene.In this paper,starting from the screening of Si/Al ratio of HZSM-5 molecular sieve for methanol aromatization,surface silicon-rich ZSM-5 shape-selective catalysts were prepared by surface chemical modification.The effects of various modification methods on the catalytic performance of methanol aromatization to p-xylene were investigated in a fixed-bed reactor.The pore structure and acid properties of the catalysts were analyzed by XRD,BET,TEM,NH₃-TPD and Py-IR.The results show that:?1?Commercial HZSM-5 catalysts with different Si/Al ratios?Si/Al=18,23,30,35?were selected for methanol aromatization,and the effects of different Si/Al ratios on the performance of methanol aromatization were studied.The results show that the acid properties of HZSM-5 catalysts with different Si/Al ratios are different,which further affects the distribution of aromatization products.With the increase of Si/Al ratio of the catalyst,the aromatics selectivity showed an inverted U-shaped trend,and the catalyst had higher aromatics selectivity at Si/Al=23.?2?HZSM-5 was modified by Zn by impregnation method and the effect of metal ion Zn on the catalytic performance was studied.It was found that the introduction of metal Zn could transform a part of the strong acid in the catalyst into medium strong acid and increase the Zn-L acid center with dehydrogenation,which could effectively improve the aromatization activity of the catalyst.In the range of load investigated,when the catalyst was loaded with 3%Zn,The highest selectivity of aromatics is 69.18%;?3?On the basis of Zn/ZSM-5 catalyst,the surface silicon-rich catalyst was prepared by dry gel conversion method and polysiloxane modification,and the effects of the amount of Si O₂,crystallization time and the concentration of template n-butylamine on the catalytic performance were investigated by orthogonal experiment.The results show that the influence order of the three factors on the selectivity of p-xylene is as follows:the amount of Si O₂>the concentration of n-butylamine>the crystallization time.Within the range of the factors investigated,the best reaction conditions are 30%of Si O₂,100%of n-butylamine and 3 d of crystallization time,while further increasing the amount of Si O₂will reduce the activity of the catalyst,and the more suitable amount of Si O₂in the dry glue conversion is 50%.The modification of polysiloxane to passivate the acid site on the outer surface of the catalyst and effectively modify the pore structure of the catalyst to enhance the shape selectivity of the catalyst can obviously improve the para-selectivity.When the loading amount of polysiloxane is constant,the modification of a small amount of silicon for many times is more effective to improve the selectivity of PX,and the selectivity of C⁼_2-3in the product is 45.04%.The characterization of BET,NH₃-TPD and Py-IR showed that multiple silicon deposition from different silicon sources enabled Si O₂to deposit more evenly on the outer surface and adjust the acid properties and pore structure of the catalyst.The evaluation results showed that the selectivity of p-xylene was greatly improved.After silicon modification,Mg O modification of the catalyst can not only adjust the shape selectivity of the pore,but also increase the number of L acid sites,which is beneficial to the formation of PX and further improve the selectivity of p-xylene.Finally,the PX selectivity in xylene can reach 87.1%,the selectivity of PX+C⁼_2-3reaches 61%,the selectivity of ethylene in C₂hydrocarbons is 97.8%,and the selectivity of propylene in C₃hydrocarbons is 90.6%.