Methane dehydroaromatization

MoZSM-5 Methane dehydroaromatization Higher activity and selectivity to aromatics Higher tolerance to coking [75]... 

Modulation of zeolite acidity by post-synthesis treatments in Mo/HZSM-5 catalysts for methane dehydroaromatization... 

Keywords methane dehydroaromatization, Mo/FIZSM-5, dealumination, acidity. 

Tessonnier, J.-P., Louis, B., Rigolet, S., Ledoux, M.J., and Pham-Huu, C. (2008) Methane dehydroaromatization on Mo/ ZSM-5 about the hidden role of Bronsted add sites. Appl. Catal. A,... 

Smdy of methane dehydroaromatization on impregnated Mo/ZSM-5 catalysts and characterization of nanostructured molybdenum phases and carbonaceous deposits. Ind. Eng. Chem. Res., 46, 4063 074. 

Methane dehydroaromatization on zeolites Mo/HZSM-5 was also investigated by solid-state MAS NMR spectroscopy 162. Both variation of the state of the transition metal component and products (such as ethane, benzene, and ethylene) adsorbed in zeolite were observed after reaction at high temperature (900-1000 K). Molybdenum carbide species, dispersed on the external surface or in the internal channels of the zeolite catalysts, had formed during the reaction 162. ... 

On the basis of the results obtained one may draw a conclusion that there exists a certain relationship between the concentration of acidic sites of different types of zeolite and Mo content. In this coimection, to produce a catalyst e diibiting a high activity in the process of methane dehydroaromatization, it is necessary to optimise the relationship between the acidic sites number of a zeolite and the number of active sites connected with different Mo forms. The highest methane conversion per one run and maximal yield of aromatic hydrocarbons are reached for the sample containing 4.0 mass% of Mo nanopowder. The development of the mesoporous zeolite structure is an important factor promoting the activity of Mo-ZSM-5 in the reactions of the formation of high-molecular aromatic compounds. 

Transition metal-incorporated zeolites have been shown to be effident catalysts for direct conversion of methane to benzene and toluene under nonoxidative conditions [45,46]. Bao and co-workers revealed that Mo/ H-MCM-22 catalysts are desirable bifiinctional catalysts for methane dehydroaromatization reaction [47]. In terms of catalytic performances of Mo/H-MCM-22 with varied metal loading, catalyst with a Mo loading of ca. 6 wt% was found to exhibit the optimal benzene selectivity, suppressed naphthalene yield, and prolonged catalyst hfe under a moderate methane conversion. Although both Bronsted and Lewis acid sites are capable of catalysing methane conversion reaction, active sites with higher acidic strengths are anticipated to play the dominant role. 

The Mo-containing MFl-type core-shell HZSM-5-Silicalite-l s (HZ5 S1) materials with various core-shell ratios prepared by the epitaxial growth of Silicalite-1 on HZSM-5 demonstrated the high shape selectivity to aromatics and stability in methane dehydroaromatization [66]. The silicalite-1 layer covering the HZSM-5 core can eliminate the external acid sites, and thus prevent the formation of active Mo species associated with Brpnsted acid sites on the external surface of catalysts. However, the overgrowth of the Silicalite-1 shell may lead to a severe inhibition of the Mo species migration into zeolite pores and consequent anchoring on the Brpnsted acid sites of the HZSM-5 core. Therefore, the catalytic performance is dependent on the core-shell ratio. 

Song Y Sun C, Shen W, Lin L. Hydrothermal post-synthesis of HZSM-5 zeolite to enhance the coke-resistance of Mo/HZSM-5 catalyst for methane dehydroaromatization reaction reconstruction of pore structure and modification of acidity. Appl Catal A 2007 317 266-74. 

The dehydroaromatization of light alkane feeds (methane to butanes) into aromatics has come into prominence as a method of converting the unreactive light paraffins into useful chemical precursors. In many of the world s markets, light alkanes are very undesired off-gasses which can not be used other than as fuel. To accomplish this difficult transformation, catalysts typically are bifunctional, containing a dehydrogenating component such as Pt, Ga, Zn or Mo with an acidic zeolite. 

Hu, B., Yang, Y., and Sayari, A. (2001) Non-oxidative dehydroaromatization of methane over Ga-promoted Mo/ HZSM-5-based catalysts. Appl. Catal. A, 214, 95-102. 

Metal oxides on zeolites have also found use as redox catalysts. High-temperature (700-750 °C) dehydroaromatization of methane under nonoxidizing conditions has been explored with a number of zeolitic catalysts modified with transition metal ions. Although coke formation at these high temperatures is a problem, calcined molybdate-impregnated ZSM-5 shows unparalleled activity of up to 8 % methane conversion with 100 % selectivity towards aromatics. Surface studies of these Mo HZSM-5 catalysts indicate that M0O3 crystals are on the external zeolite surface [123]. 

Dehydroaromatization of methane Hierarchical Mo/HZSM-5 Enhanced selectivity to aromatics because of a larger tolerance to coke [180]... 

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