Molybdenum ZSM-5 zeolite catalysts for the conversion of methane to benzene

Zhang, Jun-Zhong; Long, Mervyn A.; Howe, Russell F.

doi:10.1016/s0920-5861(98)00202-8

Cited by 106 publications

(72 citation statements)

References 25 publications

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“…Low pH conditions may alter the density and nature of external OH groups on zeolite crystals, 31 which can lead to MoO x binding and may account for the high external Mo concentrations detected by X-ray photoelectron spectroscopy on samples prepared by aqueous exchange. 26,31 Methane conversion rates and selectivities are similar on samples prepared by the two methods, suggesting that active site densities and aromatization pathways are similar in MoO x /H-ZSM5 samples prepared from physical mixtures (this study) and in those prepared using aqueous exchange. 1,3,6,28,32,33 Migration of MoO x Species during Treatment in Air.…”

Section: Discussionmentioning

confidence: 99%

Structure and Density of Mo and Acid Sites in Mo-Exchanged H-ZSM5 Catalysts for Nonoxidative Methane Conversion

et al. 1999

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Mo/H-ZSM5 (1.0-6.3 wt % Mo; Mo/Al ) 0.11-0.68) catalysts for CH 4 aromatization were prepared from physical mixtures of MoO 3 and H-ZSM5 (Si/Al ) 14.3). X-ray diffraction and elemental analysis of physical mixtures treated in air indicate that MoO x species migrate onto the external ZSM5 surface at about 623 K. Between 773 and 973 K, MoO x species migrate inside zeolite channels via surface and gas phase transport, exchange at acid sites, and react to form H 2 O. The amount of H 2 O evolved during exchange and the amount of residual OH groups detected by isotopic equilibration with D 2 showed that each Mo atom replaces one H + during exchange. This stoichiometry and the requirement for charge compensation suggest that exchanged species consist of (Mo 2 O 5 ) 2+ ditetrahedral structures interacting with two cation exchange sites. The proposed mechanism may provide a general framework to describe the exchange of multivalent cations onto Al sites in zeolites. As the Mo concentration exceeds that required to form a MoO x monolayer on the external zeolite surface (∼4 wt % Mo for the H-ZSM5 used), Mo species sublime as (MoO 3 ) n oligomers or extract Al from the zeolite framework to form inactive Al 2 (MoO 4 ) 3 domains detectable by 27 Al NMR. These (Mo 2 O 5 ) 2+ species reduce to form the active MoC x species during the initial stages of CH 4 conversion reactions. Optimum CH 4 aromatization rates were obtained on catalysts with intermediate Mo contents (∼0.4 Mo/Al), because both MoC x and acid sites are required to activate CH 4 and to convert the initial C 2 H 4 products into C 6+ aromatics favored by thermodynamics.

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Section: Discussionmentioning

confidence: 99%

Structure and Density of Mo and Acid Sites in Mo-Exchanged H-ZSM5 Catalysts for Nonoxidative Methane Conversion

et al. 1999

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“…3) 5), 10) , XAFS (X-ray absorption fine structure) 11) 13) , FT-IR (Fourier transform infrared spectroscopy) 5), 12) , NH3-TPD (temperature programmed desorption) 14) , ISS (ion scattering spectroscopy) 5) and XRD (X-ray diffraction) 10) carried out to reveal the chemical state of the active phases and to study the reaction mechanism have demonstrated the bifunctional nature of catalysis over Mo carbide particles and acidic sites on H-ZSM-5 10), 13) . Recently, the activity and stability of Mo/H-ZSM-5 were found to be significantly increased by the addition of small amounts of CO or CO2 in the CH4 feed 15) .…”

Section: Introductionmentioning

confidence: 99%

XAFS Characterization of Mo/ZSM-5 Catalysts for Methane Conversion to Benzene: Effect of Additives

岳志

伸之

康行

et al. 2006

J. Jpn. Petrol. Inst.

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ZSM-5 supported Mo catalysts for the dehydroaromatization reaction of methane were characterized by Mo K-edge EXAFS, NH3-TPD, and XPS techniques. Mo/ZSM-5 catalysts showed the maximum activity for benzene formation at 5 wt% Mo. The activity of the catalysts was related to the particle size of Mo2C supported on ZSM-5 estimated by Mo K-edge EXAFS. The addition of a second metal did not increase the activity for benzene formation. These results are ascribed to the weakening of the interaction between the acid sites of ZSM-5 and Mo oxide precursors due to competitive consumption of zeolite protons by added metal atoms, resulting in the formation of less dispersed Mo carbide particles and the decreased number of acid sites.

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“…It is significant that methane can be converted into benzene under nonoxidative conditions over Mo/HZSM-5 (5), and this process can be regarded as an analogy to propane or ethane aromatization over Ga/Zn-modified HZSM-5 catalysts (6). Recently, the reaction mechanism (7)(8)(9), the location of loaded TMI (10)(11)(12), the coupled reaction (13), etc., have been extensively investigated by various authors. It is verified that a smooth running of the reaction requires proper cooperation between the Brønsted acid sites and the molybdenum species (12).…”

Section: Introductionmentioning

confidence: 99%