The Study Of Catalysts Support On The Methane Non-oxidative Aromatization

With the increasing scarcity of petroleum, natural gas becomes particularly important. As methane is the primary component of natural gas, its activation and conversion is the key of natual gas chemical engineering. However, the utilization of methane is facing a major test, because, from the structure point of view, methane is a very stable symmetrical molecule,so it is very difficult to be activated. Therefore, methane conversion becomes one of the most valuable and challengeable research in both academic and industrial. In the past twenty years, many researchers have been focusing on methane non-oxidative aromatization, because methane non-oxidative aromatization is an important way to methane conversion. To be able to in-depth understand methane non-oxidative aromatization mechanism, many researchers has done a lot of researches to this reaction, such as choosing catalysts, reaction mechanism, the reasons resulting in deactivation and regeneration methods, However, the stability of catalysts is still to be a problem. Because the reaction was carried out under the condition of high temperature, and the carrier should possess stronger acid, which are very prone to the formation of coke. The formed coke is the chief reason for the rapid deactivation of the catalyst. In addition, it is important to a detailed study on the role of different pore strucuture of zeolite surports in the titled reaction. The purpose of the paper focuses on the two aspects to study. Firstly, improving catalyst stability with two methods, one is to modify the external surface of HZSM-5 through changing the acidic properties of zeolites with magnesium acetate, tetraethylorthosilicate and triethoxyphenylsilane, the other is to synthesize ZSM-5 with a few mesoporous system by introducing auxiliary materials into the synthesis system. Secondly, studying novel zeolite, we synthesize three novel 10MR channel materials - ITQ-13, TNU-9 and IM-5, and study the catalytic behavior of Mo based the three catalysts for the title reaction.1. Improving catalyst stabilityFirstly, we modify the external surface of HZSM-5 with Magnesium acetate, tetraethylorthosilicate and triethoxyphenylsilane. The catalytic performance of methane aromatization over modified Mo/ZSM-5 with different Mg contents was investigated in detail. the results show that when MgO contents is at 5%, modified Mo/ZSM-5 shows highest selectivity to benzene While it shows the lowest selectivity to naphthalene and coke. It was found that modified Mo/HZSM-5 reduced the densities of external acid sites, and promoted Mo species disperse in the channel of catalyst, which the modified Mo/HZSM-5 showed higher shape-selectivity to major product benzene, and lower selectivity to naphthalene and coke. However, when MgO content is higher than 5%, modified Mo/ZSM-5 shows lower activity. The possible reason is that overfull MgO builds up channel of catalysts. In addition, the catalytic performance over modified Mo/ZSM-5 with different modifiers in detail, and find that modified Mo/ZSM-5 with triethoxyphenylsilane shows highest catalytic activity and yield of aromatics. In a word, modification of the external surface reduces the densities of external acid site, and improves the activity and stability of the Mo-based ZSM-5 catalyst system. So we consider that the role of external surface acid sites is to promote the formation of larger hydrocarbons.Secondly, hierarchical ZSM-5 was applied on the title reaction. We synthesize hierarchical ZSM-5 zeolite with three different strategies, such as adding triethoxyphenylsilane (PTEOS) into the initial sol of the synthesis system, using SBA-15 and MCM-41 as silica sources and using mesoporous carbon as meso-template. And we apply the hierarchical ZSM-5 zeolites on the title reaction. Research results showed that the crystal size of the ZSM-5 zeolites could be adjusted in a certain range by introducing different contents of PTEOS. Besides, the resultant materials possess hierarchical porosity, in addition to those micropores generated by the MFI channels. Moreover, It was found that the Mo/ZSM-5 catalyst, bearing suitable crystal size and mesoporous characteristic showed high stability for the reaction of methane aromatization. In addition, Mo/ZSM-5 zeolite synthesized by using SBA-15 as silica source shows highest catalytic activity and yield of aromatics than that of synthesized by using MCM-41 as silica source. The probable reason is that ZSM-5 zeolite synthesized by using SBA-15 as silica sources possesses more mesoporous systems. So we consider that the presence of mesopore can let methane easier access to the active sites in micropores and the products can diffuse out of catalysts much easier.2. Studying novel zeoliteFirstly, Mo/ITQ-13 was applied to the title reaction. ITQ-13 is a tridirectional medium-pore zeolite containing 9-and 10-member ring pores. ITQ-13 zeolite was successfully synthesized using different silica sources (i.e., tetraethylorthosilicate (TEOS), colloidal silica and fumed silica). Research results showed that the ITQ-13 samples were synthesized using colloidal silica and fumed silica possess better crystallinity and larger crystals than those synthesized by the TEOS route. Besides, Mo/ITQ-13 was applied to methane non-oxidative aromatization. The catalytic performance of Mo/ITQ-13 is worse in comparison with Mo/ZSM-5 in the title reaction. So we consider that the presence of 9MR in ITQ-13 is disadvantageous for methane non-oxidative aromatization.Secondly, Mo/TNU-9 and Mo/IM-5 were applied to the title reaction. We study the two catalysts on the different synthesis conditions. Research results showed that the two catalysts begin to grow at the expense of MCM-22(P). Besides, Mo/IM-5 and Mo/TNU-9 were applied to methane non-oxidative aromatization. The catalytic performance of methane aromatization over the two catalysts with different Mo contents, SiO2/Al2O3 and reaction temperature was investigated in detail. Within the scope of our research, it was found that the two catalyts showed almost similar trend. Mo/IM-5 shows higher activity and yield of aromatics between 30 and 50 on the SiO2/Al2O3 ratio. When SiO2/Al2O3 ratio is at 50, TNU-9 shows higher activity and yield of aromatics. Mo content is between 6 and 8, Mo/TNU-9 and Mo/IM-5 shows higher catalytic activity and yield of aromatics. In addition, it is clear found that with reaction temperature increasing, catalytic activity evidently increases but stability clearly decreases.3. Effect of the structure of channel for the title reactionMo/TNU-9, Mo/IM-5, Mo/MCM-22 and Mo/ZSM-5 are applied on the title reaction for studying the role of supercage structure. MCM-22 contains the 12 MR supercages (7.1×7.1×18.2?), TNU-9 contains the 12 MR supercages(7.2?), IM-5 contains larger cavities than ZSM-5. Research results showed that the stability of catalysts follows the trend: Mo/MCM-22 > Mo/TNU-9 > Mo/IM-5 > Mo/ZSM-5. The result is accordance with size of supercage. Therefore, we conclude that this might be interrelated with surpercages. It is well-known that MCM-22 exhibits higher activity and stability due to its surpercages.In addition, it was found that MoOx disperses in the channel: Mo/MCM-22 > Mo/TNU-9 > Mo/IM-5 > Mo/ZSM-5.The result also is accordance with the stability of catalysts. So we conclude that supercage structure and Mo specis have a positive role to the title reaction.