The Alkylation Of Benzene With Methanol Over Modified Nanoscale HZSM-5

Para-xylene is the basic material of producing polyester fiber, synthetic resin and plastic. With the development of economy, the scale of polyester production becomes larger and the demand for para-xylene is on the increase day by day. Benzene is produced in quantity from petroleum chemical industry and coal chemical industry. Clean gasoline standard claimed that the content of benzene should be1.0volume percent (vol%) or less. Therefore, all of these will lead to a large surplus of benzene. Methanol supply is relative surplus for the rise of domestic coal chemical industry. Hence, methanol can be used as an alkylating agent which is easy to get and at a low price. The alkylation of benzene with methanol can obtain toluene and xylene of high concentration. This process can realize the optimization of the aromatic hydrocarbon resources and make up for the deficiency of the xylene resources. Meanwhile, the benzene can be utilized locally which will lay the foundation for the expansion of aromatic integrated unit.The performance for benzene alkylation with methanol over molecular sieve catalysts was researched. The thesis focused on the activity and stability of the reaction which was influenced by acid and pore structure of molecular sieves. The catalytic performance for benzene alkylation with methanol was investigated over HZSM-5, HZSM-11and HMCM-22in a continuous flow fixed-bed reactor. The results showed that HMCM-22had higher alkylation activity but the catalyst was deactivated easily for the existence of supercage in its structure. HZSM-11had good stability. However, it can only be synthesized by using TBA+or1,8-octanediamine as the template, which is not conducive to industrial production. HZSM-5had appropriate activity and stability.Si/P/Mg-modified nanoscale HZSM-5catalysts were prepared by impregnation method. The results for the alkylation of benzene with methanol showed that with the modification of Si, P, Mg, the selectivity of para-xylene increased obviously. The selectivity of para-xylene was70%over Si/P/Mg-modified HZSM-5. However, the stability of the reaction was not as well as over HZSM-5because the modification narrowed the channels of the molecular sieves. Feeding water, carrier gas and increasing the molar ratio of benzene to methanol can improve the stability of the reaction and extend the life of the catalyst. Nanoscale HZSM-5was treated with water vapour at different temperature. The molecular sieves were washed by dilute HNO3after hydrothermal treatment. Comparing with HZSM-5, the stability of the alkylation of benzene with methanol was improved after hydrothermal treatment. The alkylation can run310h with the activity keeping at31%.550℃was the appropriate hydrothermal treatment temperature. Hydrothermal treatment can remove the framework aluminum and decrease the acid amount and strength. So the formation of coke was restrained and the stability of the alkylation was improved. The acid-washing after hydrothermal treatment can remove the non-framework aluminum and the channels of the molecular sieves were dredged, which was beneficial to the diffusion of reactant and product. The presence of C8aromatics in mixed-benzene had impact on the stability of the alkylation. The stability of benzene alklyted with methanol.was better than mixed-benzene over the nanoscale HZSM-5treated at550℃and washed by dilute HNO3. When at atmospheric pressure, WHSV was4h-1and the molar ratio of aromatics to methanol was4:1, the alkylation of mixed-benzene can run300h with the conversion of benzene in mixed benzene keeping at17%.