Alkylation Of Toluene Into Para-xylene Over Modified ZSM-5Zeolite

Para-xylene is an important chemicals for making various materials including those useful in human life. Toluene can be selectively converted to value-added para-xylene over modified ZSM-5zeolite through methylation. Shape-selective methylation is an important route, because of the advantages of little byproduct, high toluene utilization and easy separation of products. In this dissertation, effects of the modifiers (SiO2, P2O5and MgO) on the acidity and the pore structure of the nano-sized HZSM-5and their impacts on activity and selectivity of toluene methylation have been investigated in detail; the catalysts with high para-xylene selectivity were prepared. The HZSM-5catalyst coated with Si-P-Mg was further modified with NiO, which exhibited a high stability in the selective methylation of toluene with methanol. The performance of methylation with different methylating reagents was also discussed. The main results are as follows:(1) The effects of the modifiers (SiO2, P2O5and MgO) on the acidity and the pore structure of the nano-sized HZSM-5were comparatively studied. It was found that the passivation of Lewis acid sites occurs prior to that of the Bronsted acid sites over the external surface of ZSM-5modified by SiO2, while P2O5or MgO preferentially neutralizes the Bronsted acid sites of ZSM-5. The MgO deposition is more efficient in passivating the acid sites and narrowing the pore openings, compared to SiO2or P2O5with the same oxide loading (3-9wt%). The enhancing effect of the modifiers on para-xylene selectivity is in the following order:MgO> SiO2> P2O5.(2) After first modification with SiO2or P2O5, the further modification with MgO can not only effectively passivate the acidic sites near pore openings, but also narrow the pore openings of the zeolite. The multiple modifications, with a proper modification sequence, can demonstrate synergistic effects between the modifiers (SiO2, P2O5and MgO), which can properly passivate the acid sites on the external surface and near pore openings of the catalysts, and simultaneously further narrow the zeolite pore openings to a proper extent, and then markedly enhances the para-selectivity to a high value. When the ZSM-5was first modified by SiO2, then by P2O5, and finally by MgO, the prepared catalyst6Si-5P-3Mg/ZSM-5can give a higher para-selectivity than98%.(3) After further modification with NiO on the surface of the most selective6Si-5P-3Mg/ZSM-5catalyst, the stability can be enhanced under N2flow. Metal Ni can be detected on the spent catalysts, and the coke amount significantly decreased. It may be speculated that part of NiO species on the surface of the catalyst could be reduced to metal Ni during the reaction process, which can promote the reaction between the water and the coke, or inhibited the reaction of aromatic hydrocarbons into carbon precursor. When the loading of nickel is5wt%, the selectivity of para-xylene stabilized at～98%after100h time on stream, and the conversion of toluene maintained at～18%at the same reaction conditions over the prepared catalyst. The catalyst exhibits an excellent stability.(4) At the same reaction conditions, similar para-xylene selectivity (-98%) but higher toluene conversion can be reached by the alkylation of toluene with dimethyl carbonate over the catalyst6Si-5P-3Mg/ZSM-5, compared to the reaction using methanol as methylating agent.