Methane dehydrogenation and aromatization in the absence of oxygen on MoHZSM-5: A study on the interaction between Mo species and HZSM-5 by using 27Al and 29Si MAS NMR

Liu, W.; Xu, Yide; Wong, Samson S. Y.; Wang, Linsheng; Qiu, Jian; Yang, Nan

doi:10.1016/s1381-1169(96)00427-x

Cited by 103 publications

(26 citation statements)

References 14 publications

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“…Thus, it appears that the unusual shape of the H 2 O desorption curve for 6.3 wt % Mo/H-ZSM5 reflects the extraction of Al from the framework to form Al 2 (MoO 4 ) 3 and the concomitant collapse of the zeolite pore structure. H 2 O desorption rates from mixtures initially containing 4 and 8 wt % Mo were identical up to about 850 K (Figure ), indicating that MoO x -induced extraction of framework Al did not occur below 850 K. This agrees with 27 Al NMR data for 15 wt % Mo/H-ZSM5, which showed Al 2 (MoO 4 ) 3 in samples treated in air at 873 or 973 K, but not in those treated at 773 K . The ratio of OH groups removed as H 2 O to Mo atoms in the 6.3 wt % Mo sample is only 0.64; this value is very similar to that expected for Al 2 (MoO 4 ) 3 (0.67), but it is smaller than in Mo/H-ZSM5 samples with lower Mo content.…”

Section: Resultssupporting

confidence: 76%

“…27 Al NMR lines corresponding to crystalline Al 2 (MoO 4 ) 3 appeared in the 6.3 wt % Mo/H-ZSM5 sample . The Al 2 (MoO 4 ) 3 crystallites are too small to be detected by X-ray diffraction in this sample, but they become detectable in Mo/H-ZSM5 samples with higher Mo content (15 wt %) 7 …”

Section: Resultsmentioning

confidence: 84%

“…27 Al NMR spectroscopy can probe the local structure of Al cations in Mo/H-ZSM5 samples. Tetrahedral Al centers in the zeolite framework bonded to terminal O−H groups have a chemical shift of 56 ppm; extraframework octahedral Al centers (in ZSM5 and in Al 2 O 3 ) show no shift (δ = 0 ppm), while crystalline Al 2 (MoO 4 ) 3 (octahedral Al, tetrahedral Mo) shows a line at −13 ppm ) showed that the density of unperturbed framework Al centers (corresponding to Bronsted acid sites) decreased with increasing Mo content.…”

Section: Resultsmentioning

confidence: 99%

“…Similar condensation reactions of OH groups lead to the extraction of Al ions from the framework, with the formation of water and the disappearance of two Bronsted acid sites. In Mo/H-ZSM5, extraframework Al atoms form small domains of Al 2 O 3 or Al 2 (MoO 4 ) 3 ; the latter species were detected by 27 Al NMR in Mo/H-ZSM5 samples with high Mo content (>10 wt %) …”

Section: Introductionmentioning

confidence: 99%

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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: Resultssupporting

confidence: 76%

Section: Resultsmentioning

confidence: 84%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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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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“…The concentration of the impregnating solutions is varied in a wide range. The Mo content in the dried catalyst samples is within a range of 1-20 wt.% [68][69][70][71][72]. The conditions for the preparation of Мо-ZSM-5 catalysts (conditions of impregnation, duration and temperature of thermal treatment, pretreatment with different gases, etc.)…”

Section: Dehydroaromatization Of Methanementioning

confidence: 99%

Promoters for Improvement of the Catalyst Performance in Methane Valorization Processes

Исмагилов

Восмериков

Коробицына

et al. 2021

Eurasian Chem. Tech. J.

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In this work, the introduction of modifying additives in the composition of catalysts is considered as an effective mode of improving functional characteristics of materials for two processes of methane conversion into valuable products – methane dehydroaromatization (DHA of CH4) into benzene and hydrogen and autothermal reforming of methane (ATR of CH4) into synthesis gas. The effect of type and content of promoters on the structural and electronic state of the active component as well as catalyst activity and stability against deactivation is discussed. For DHA of CH4 the operation mode of additives M = Ag, Ni, Fe in the composition of Mo-M/ZSM-5 catalysts was elucidated and correlated with the product yield and coke content. It was shown that when Ag serves as a promoter, the duration of the catalyst stable operation is enhanced due to a decrease in the rate of the coke formation. In the case of Ni and Fe additives, the Ni-Мо and Fe-Mo alloys are formed that retain the catalytic activity for a long time in spite of the carbon accumulation. For ATR of CH4, the influence of M = Pd, Pt, Re, Mo, Sn in the composition of Ni-M catalysts supported on La2O3 or Ce0.5Zr0.5O2/Al2O3 was elucidated. It was demonstrated that for Ni-M/La2O3 catalysts, Pd is a more efficient promoter that improves the reducibility of Ni cations and increases the content of active Nio centers. In the case of Ni-M/Ce0.5Zr0.5O2/Al2O3 samples, Re is considered the best promoter due to the formation of an alloy with anti-coking and anti-sintering properties. The use of catalysts with optimal promoter type and its content provides high efficiency of methane valorization processes.

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The Use of²⁷Al NMRto Study Aluminum Compounds: A Survey of the Last 25 Years

Martineau

Taulèlle

Haouas

2016

Patai's Chemistry of Functional Groups

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Use of 27 Al nuclear magnetic resonance (NMR) spectroscopy for the study of the chemistry of aluminum in a variety of compounds during the last 25 years is reported. A compilation of NMR data, including chemical shifts and heteronuclear scalar couplings, is provided for a wide range of families of aluminum compounds in both solution and solid state. Recent methodological and technological advances in the field of quadrupolar NMR are detailed and their applications to 27 Al nucleus are illustrated. The potential of this technique is exemplified with some representative studies of relevance to mineral, biological, and organic aluminum‐based systems.

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Methane dehydrogenation and aromatization in the absence of oxygen on MoHZSM-5: A study on the interaction between Mo species and HZSM-5 by using 27Al and 29Si MAS NMR

Cited by 103 publications

References 14 publications

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

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

Promoters for Improvement of the Catalyst Performance in Methane Valorization Processes

The Use of²⁷Al NMRto Study Aluminum Compounds: A Survey of the Last 25 Years

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Methane dehydrogenation and aromatization in the absence of oxygen on MoHZSM-5: A study on the interaction between Mo species and HZSM-5 by using 27Al and 29Si MAS NMR

Cited by 103 publications

References 14 publications

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

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

Promoters for Improvement of the Catalyst Performance in Methane Valorization Processes

The Use of27Al NMRto Study Aluminum Compounds: A Survey of the Last 25 Years

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Product

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The Use of²⁷Al NMRto Study Aluminum Compounds: A Survey of the Last 25 Years