Study Of The Catalyst Stability And Regeneration In Methane Dehydro-aromatization

Recently, direct conversion of methane into aromatics over Mo-based zeolite catalysts, i.e., methane dehydro-aromatization (MDA) is of importance in effective utilization of natural gas. Many catalysts have been studied in conjunction with their reactivity, reaction mechanism and catalyst characterization. One of the most serious problems of this reaction is the catalyst deactivated in a few hours due to serious coke formation.In this thesis, in order to improve the stability of Mo-based zeolite catalysts the effect of optimization of Lewis to Bronsted acid sites on the catalyst stability, the variation hi the shape selectivity of Mo/MCM-22 in 100h dehydro-aromatization of methane with 2(vol)%CO2, as well as the regeneration of Mo/MCM-22 at reaction temperature were investigated using XRD, Benzene/NH3-TPD, TG, TPO and FT-IR techniques.First, the 6Mo/HZSM-5 catalyst was modified by MgO oxide. The study of coke deposited on the 6Mo-Mg/HZSM-5 after MDA revealed that, when the ratio of Lewis to Bronsted acid sites on the 6Mo/HZSM-5 catalysts was adjusted in the range from 0.2~0.3:1 to 0.6:1, the stability of the catalyst could be enhanced. This could be attained by introducing 0.75(wt)%Mg on/in the 6Mo/HZSM-5 catalysts. The result suggested that the Lewis acid sites hi MDA catalyst play a role to reduce the density of Bronsted acid sites, so lead to suppression on the formation of coke that had not been reported.A detailed study of the effect of coke on the pore blockage and the acid sites coverage make it clear that, there were two kinds of coke deposited on the Mo/MCM-22 catalyst after running the reaction for several hours, one is active carbonaceous deposits containing hydrogen, and the other is inert one almost without hydrogen. It was the inert coke, which blocked the pore system of the MCM-22 zeolite, is responsible for the deactivation of the 6Mo/MCM-22 catalyst.On the basis of above-mentioned results, the variation of carbonaceous deposits and the change of shape selectivity over Mo/MCM-22 within 100 h on stream havebeen studied. It was found that the shape selectivity of the catalyst is dynamical. That is, because of coke formation and accumulation on the catalyst surface, the shape selectivity to benzene of the catalyst Varied with the reaction time on stream. There is a critical value for coke deposition, i.e., 17wt%, beyond which the shape selectivity of the Mo/MCM-22 was lost at all. Only was the coke deposition smaller than 17wt%, the Mo/MCM-22 could exhibit a good MDA reactivity.In the regeneration experiment of 6Mo/MCM-22 using a gas mixture of l(vol)%O2 and 99(vol)%N2 at reaction temperature, the catalyst activity could be regenerated, and the products distribution on the regenerated catalyst was similar to that on the original one. This indicates that the reaction and regeneration can be carried out in a methane-dilute oxygen switched mode.In summary, several factors that affect the stability of the Mo-based zeolite catalysts have been investigated in mis thesis, and the results are helpful for further consideration of the industrial application of the MDA process.