Studies On Hierarchical Catalysts And Membrane Catalytic Reactor In Methane Dehydroaromatization

Methane dehydroaromatization (MDA), which has been considered as an important branch of C1 chemistry and a promising route for direct conversion of methane into high value-added chemicals like benzene, toluene and naphthalene, is of great scientific significance and industrial interest in the effective utilization of natural gas. Up to date, many researchers have paid attention to this process and focused their interests on the development of catalysts and the study of catalytic mechanism, and many important features and findings have been obtained. However, the major problems of MDA reaction are the low methane conversion and the pronounced deactivation of the catalyst due to the heavy coke formation on the catalysts. Besides, the problem of decrease in catalytic activity with time on stream is very intractable, which restricts the industrial application of the MDA reaction.A novel facile, effective route was proposed in this doctor thesis to dramatically improve catalytic performance in MDA reaction, especially for the catalyst stability. The demonstrated feasible route, preparing hierarchical pore structure and hollow spherical catalysts to enhance catalytic performance, was different from other strategies such as adding the second various transition metal promoters, adding gas dopants to methane feed and the post-treatment catalyst. Base on the most recently development and obtained insights on self-assembly, zeolite synthesis chemistry and hierarchical pore structure zeolites, hierarchical pore structure zeolites (ZSM-5, MCM-22) catalysts were obtained by one pot hydrothermal synthesis through carefully manipulating the synthesis parameters without using any secondary template. Furthermore, a novel method of carbon templating one pot rotating hydrothermal synthesis was first developed for hollow MCM-22 which showed an exceptional catalytic performance. The relevance between the structure and properties were studied in details in this doctor thesis. Besides, an exploratory study, the non-metal porous membrane reactors running the membrane catalytic reaction for MDA, was presented in this paper. Improvement on the methane conversion and benzene yield were expected by continuous removal of co-produced hydrogen so as to shift the equilibrium-limited MDA reaction toward the formation of aromatics. The findings are as follow:A novel hierarchical ZSM-5 microsphere was synthesized through one pot hydrothermal synthesis by using SBA-15 as the silica source. Hierarchical ZSM-5 nano-aggregates (ZSM-5-HA) were synthesized through facile one-pot hydrothermal crystallization by means of carefully controlling the synthesis parameters (mol ratio of synthesis mixture, ageing treatment, and temperature etc.) without using any second template. The hierarchical Mo-based catalysts exhibited good catalytic performance in MDA reaction. A growth model via self-assembly combined with hydrothermal crystallization process was proposed based on the time trial observations.MCM-22 hierarchical aggregates (MCM-22-HA) were also synthesized through facile one-pot hydrothermal crystallization by means of carefully controlling the synthesis parameters without using any second template. The Mo/HMCM-22-HA catalysts exhibited remarkably enhanced catalytic performance in MDA reaction. After investigating deeply the chemical properties (Mo content, Si/Al ratio, acid center etc.) of the catalyst, the conclusion was proposed. The results showed that faster transfer and higher dispersion of Mo species on the hierarchical pore structure supports because of the introduction of mesopores were beneficial to the reaction between the Mo species and zeolites. In this way, high quality Mo/zeolite bifunctional catalysts were obtained easily, which would brought higher catalytic activity and aimed product benzene yield. The Mo/HMCM-22-HA catalyst exhibits remarkably enhanced yield of benzene product of above 8% even after reaction run for 60 h. Furthermore, the coke-tolerance of catalyst was significantly improved due to the good diffusibility of the hierarchical aggregate catalysts, which resulted in exceptionally enhanced catalysts stability. Mo/HMCM-22-HA catalyst was subjected to MDA reaction for 72 h, which was about three times of that for the conventional Mo/HMCM-22 catalyst under the same reaction conditions.A novel route, preparing an unusual MCM-22 zeolite hollow sphere (MCM-22-HS) by one pot rotating hydrothermal synthesis using M700 carbon black microsphere as a hard template via self-assembly combined hydrothermal crystallization techniques, was proposed. The hierarchical shell of MCM-22 hollow sphere was hierarchically constructed by many small flaky MCM-22 crystals. These special structures were undoubtedly beneficial to the formation of bifunctional catalysts. Under the same conditions, the Mo species would react more effectively with the Bronsted acid sites to form the active centres for activation of methane and aromatization. Therefore, the hollow sphere catalyst exhibited significantly improved catalytic performance in MDA reaction. During the 50 h reaction, the MCM-22-HS catalyst exhibited remarkably high CH4 conversion increased by 25-30% with high benzene yield of about 9%. The developed easy and feasible route may be extended to synthesize other types of zeolites hollow spheres.Pd/zeolite composite membrane reactor was adopted to run MDA membrane reaction. Compared with the fixed bed, the methane conversion and aromatics yields were improved effectively about one time in the Pd/SAPO-34 composite membrane reactor. MDA membrane catalytic reaction was firstly run in porous SAPO-34 zeolite membrane reactor and carbon/SAPO-34 composite membrane reactor under high temperature (973 K). The results showed that C/SAPO-34 composite membrane reactor possessing much higher H2/CH4 separation factor exhibited good catalytic performance. Higher aimed product benzene yield was obtained in the C/SAPO-34 composite membrane reactor, which was about 1.5 times of that in fixed bed reaction under the same reaction conditions. This result reasonably proves the feasibility of using porous inorganic membrane reactor in MDA reaction.