Study of pre-coking modified ZSM-5 molecular sieve and its benzene and syngas alkylation properties

Toluene, xylene and other alkylbenzenes are important basic chemical raw materials, which mainly come from the petrochemical industry. With the increasing foreign dependence of petroleum resources in China, the development of coal-to-aromatics technology can not only alleviate the problem of shortage of petroleum resources in China, but also promote the transformation and upgrading of consumption in the traditional coal chemical industry and promote the clean and efficient use of coal. In this paper, methanol-modified ZSM-5 molecular sieves were used and mechanically milled with ZnCrO_x in a mass ratio of 3∶1 as a composite catalyst, so as to investigate the catalytic effect of pre-preg carbon modified ZSM-5 molecular sieves. Under the reaction conditions of 450 ℃ and 4.0 MPa, the catalytic performance of the catalyst modified with pre-built carbon for 24 h was the best, in which the conversion of benzene reached 20.18% and that of CO reached 46.55%. It was found that the catalytic performance of the pre-built carbon modified ZSM-5 sieve was mainly affected by the B acid/L acid ratio, and the pre-built carbon covered some B acid sites on the ZSM-5 sieve, which reduced the B acid/L acid ratio and led to an increase in the benzene conversion, while the crystalline and pore structures of the pre-built carbon modified ZSM-5 sieve did not undergo significant changes. changed significantly. In order to better understand the deactivation mechanism of the carbon-modified ZSM-5 catalyst, TG-MS, Raman and GC-MS were used to further analyze the carbon species, and it was found that the carbon species of the carbon-modified ZSM-5 molecular sieves were mainly low-temperature carbon, which was mainly due to the fact that the strong acid was partially covered by the carbon-modified composite catalyst with the reduced amount of acid, and the carbon species were alkanes, olefins, aromatics and other oligomers mainly. Soluble carbon, high-temperature carbon and graphitic carbon are less, so as to avoid the reduction of catalyst activity caused by excessive carbonisation, while the carbon species on the composite catalyst after the reaction are mainly high-temperature carbon, which is mainly because the carbon species after the reaction of the composite catalyst are mainly large-molecule aromatic hydrocarbons and olefins, and even insoluble carbon such as graphitic carbon. In addition, the deactivation mechanism of ZSM-5 molecular sieve in the alkylation reaction of benzene and syngas was speculated based on the hydrocarbon pool mechanism: in the alkylation reaction of benzene and syngas, syngas firstly forms methanol in the active site of metal oxides, and then the methanol migrates in the acidic site of the molecular sieve to generate alkylbenzene with benzene, but as the reaction proceeds, the alkylbenzene will be alkylated deeply in the molecular sieve to form heavy polycyclic aromatic hydrocarbons or graphitic. However, as the reaction proceeds, alkylbenzene will be deeply alkylated on the molecular sieve, forming heavy polycyclic aromatic hydrocarbons or graphite carbon, etc. Such substances are not easy to diffuse in the pores and will continuously cover the acid centre and pores of the molecular sieve, which will ultimately lead to catalyst deactivation.