| Mo/HZSM-5 was found to be a good catalyst for the aromatization of CH4 in the atmosphere without oxidants, but the aromatization of CH4 over Mo/HZSM-5 has not been realized yet when oxidants are present. On the other hand, the molybdenum species of Mo/HZSM-5 catalyst is reduced and carburised during the aromatization of CH4 and this leads to the formation of different kinds of molybdenum species. The aromatization performance of Mo/HZSM-5 with different molybdenum species has not been well investigated. In this work, the aromatization of CH4 in the presence of oxidants (O2, N2O) and the aromatization of C4HIO on Mo/HZSM-5 with different molybdenum species are systematically studied, the main results are as follows:The reaction results show that the lifetime of Mo/HZSM-5 in the absence of oxidants is very short. But the deactivation of the catalyst becomes slow when suitable amount of oxidant (e.g, O2 or N2O) is introduced into the reactant. The higher the reaction temperature, the better (or longer) the aromatization performance and the lifetime of Mo/HZSM-5. It is found that there is a threshold concentration of oxidant that switches the reaction mechanism. When the concentration of oxidant is below the threshold value, the aromatization of methane is not only maintained but the performance and the durability of Mo/HZSM-5 is also greatly improved. When the concentration of oxidant is higher or even a little higher than the threshold value, the reaction completely changes from aromatization to the total oxidation of CH4. The threshold value varies with the reaction temperature, for instance, the value increases from ~6 x 10'3 (O2/CH4) at 700C to 5 x 10'2 (O2/CH4) at 800癈, and also depends on the different oxidants, the oxidant with weaker oxidation ability gives a higher value.The characterization of the coke deposits by UV-Raman spectroscopy during thearomatization of CH4 over Mo/HZSM-5 shows that the coke evolves from poly-olefinic and aromatic coke species on HZSM-5 into aromatic and poly-aromatic coke species on Mo/HZSM-5. When a trace amount of oxygen is introduced into the reactant, only poly-aromatic and graphite-like coke species are observed. But when the concentration of oxidant is higher than the threshold value, the amount of deposited coke is quite low and the molybdenum species exist as MoOx.Based on these results, it is believed that, when oxidant is introduced into the reactant during the aromatization of CH4 over Mo/HZSM-5, there is an equilibration between the carburisation of MoO3 by CH4 and the oxidation of Mo2C by the oxidant. When the amount of oxidant is below the threshold value, the oxidant can delete part of aromatic coke and maintain the catalyst phase as the active state for the aromatization of CH4, i.e, MoOxCy/HZSM-5. These two factors are responsible for the improvement of the aromatization performance and the durability of Mo/HZSM-5 when the aromatization of CH4 proceeds in the presence of oxidant.In order to distinguish the aromatization performance of Mo/HZSM-5 containing different molybdenum species, n-butane instead of CH4 was used. The reaction results and the analysis of molybdenum species of Mo/HZSM-5 using XRD disclosed the relationship between the catalytic performance of Mo/HZSM-5 and the structure of the molybdenum species in Mo/HZSM-5. MoO3 of MoO/HZSM-5 can be gradually transformed into MoO2/HZSM-5, (MoO2+Mo2C)/HZSM-5 and eventually into Mo2C/HZSM-5 by the reduction and carburisation with n-butane. It is found that all the Mo/HZSM-5 catalysts show the activity for the aromatization of n-butane. But for the catalyst containing oxygen, oxidation reaction and cracking reaction are dominant while the aromatizaton reaction is partially prohibited. The stability of Mo2C/HZSM-5is better than any other Mo/HZSM-5 catalyst and Mo2C/HZSM-5 has the best catalytic performance for the aromatization of n-butane.The phase change between the molybdenum species was investigated in detail. It is found that MoO3/HZSM-5 is carburised into a-Mo2C/HZSM-5 at 540~6...
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