Raman and X-Ray Absorption Studies of Mo Species in Mo/H-ZSM5 Catalysts for Non-Oxidative CH4 Reactions

Li, Wei; Meitzner, G.; Borry, Richard W.; Iglesia, Enrique

doi:10.1006/jcat.1999.2795

Cited by 188 publications

(166 citation statements)

References 40 publications

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“…21 Wafers were heated to 823 K at 0.17 K s -1 in dry air and held for 1 h to remove physisorbed H 2 O; they were then cooled to ambient temperature before measuring spectra using a rotating cell (∼7 Hz) that minimizes sample damage by laser heating. Spectra were recorded with a HoloLab 5000 research raman spectrometer (Kaiser Optical Systems, Inc.) using a 532 nm laser.…”

Section: Structural and Chemical Characterization Of V-zsm5mentioning

confidence: 99%

“…Crystalline oxides (e.g. V 2 O 5 ) typically give a second-shell M-M (M ) metal) feature (Figure 8a), but these are seldom observed in small oxide domains, because of weak scattering, nonidentical connectivity at nonuniform support anchoring sites, and possible destructive interference from multiple scattering paths, 21,55 as shown for Cu + cations exchanged onto H-ZSM5 (using sublimation of CuCl) with a Cu/Al f ratio of unity, in which only a Cu-O shell is detected. 56 X-ray absorption fine structure spectra are consistent with coexisting monomers and dimers for V/Al f,f ratios near unity.…”

Section: + Exchange Stoichiometry In H-zsm5mentioning

confidence: 99%

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Synthesis, Structure, and Catalytic Reactivity of Isolated V⁵⁺-Oxo Species Prepared by Sublimation of VOCl₃ onto H-ZSM5

Lacheen

Iglesia

2006

J. Phys. Chem. B

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Isolated and uniform V 5+ -oxo species were grafted onto H-ZSM5 at V/Al f ratios of 0.2-1 via sublimation of VOCl 3 precursors. These methods avoid the restricted diffusion of solvated oligomers in aqueous exchange, which leads to poorly dispersed V 2 O 5 at external zeolite surfaces. Sublimation methods led to stable and active V-ZSM5 catalysts for oxidative dehydrogenation (ODH) reactions; they led to an order of magnitude increase in primary C 2 H 6 ODH rates compared with impregnated ZSM5 catalysts at similar V/Al f ratios and showed similar activity to impregnated VO x /Al 2 O 3 . The structure of grafted V 5+ -oxo species was probed using spectroscopic and titration methods. Infrared spectra in the OH region and isotopic exchange of D 2 with residual OH groups showed that exposure to VOCl 3(g) at 473 K led to stoichiometric replacement of H + by each (VOCl 2 ) + species. Raman spectra supported by Density Functional Theory electronic structure and frequency calculations showed that, at V/Al f < 0.5, hydrolysis and subsequent dehydration led to the predominant formation of (VO 2 ) + species coordinated to one Al site with single-site catalytic behavior (0.7-0.9 × 10 -3 mol C 2 H 4 V -1 s -1 , 673 K). At higher V/Al f ratios, simulation of extended X-ray absorption fine structure spectra indicated that V 2 O 4 2+ dimers coexisted with VO 2 + monomers and led to an enhancement in ODH rates as a result of bridging V-O-V (1.3 × 10 -3 mol C 2 H 4 V -1 s -1 ). These V 5+ -oxo species form via initial reactions between VOCl 3(g) and OH groups to form HCl (g) , hydrolysis of grafted (VOCl 2 ) + to form HCl (g) and (VO(OH) 2 ) + , and intramolecular and intermolecular condensation to form monomers and dimers, respective with the concurrent evolution of H 2 O. Raman and X-ray spectroscopies did not detect crystalline V 2 O 5 at V/Al f ratios of 0.2-1, but V 2 O 5 crystals were apparent in samples prepared by impregnation or physical mixtures of V 2 O 5 /H-ZSM5. Framework Al atoms and zeolite crystal structures are maintained during VOCl 3 treatment and subsequent hydrolysis; 27 Al and 29 Si MAS NMR showed that these synthetic protocols removes <10% of the framework Al atoms (Al f ).

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Section: Structural and Chemical Characterization Of V-zsm5mentioning

confidence: 99%

Section: + Exchange Stoichiometry In H-zsm5mentioning

confidence: 99%

Synthesis, Structure, and Catalytic Reactivity of Isolated V⁵⁺-Oxo Species Prepared by Sublimation of VOCl₃ onto H-ZSM5

Lacheen

Iglesia

2006

J. Phys. Chem. B

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“…In their recent publications, Iglesia and co-workers studied Mo/HZSM-5 catalysts prepared by using mechanical mixtures of MoO 3 and HZSM-5 via solid exchange reaction [13][14][15][16]. The authors presented evidence that most of the Mo species migrated into the channels and were properly reduced by CH 4 into Mo 2 O x C y .…”

Section: Introductionmentioning

confidence: 99%

Study of the carbonaceous deposits formed on a Mo/HZSM-5 catalyst in methane dehydro-aromatization by using TG and temperature-programmed techniques

Tian

et al. 2001

Applied Catalysis A: General

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“…12 We question such a model due to the fact that a dimer, once introduced into the channels and exchanged with the Brønsted acid sites, requires the presence of Si(2Al) sites, which has been proved to be of little possibility to exist in HZSM-5 zeolites. Later, the model was revised and the authors claimed that two Mo atoms will bond to two Si(1Al) sites, 13 which seems to be more reasonable theoretically. Based on the experimental results above and our theoretical study by the density functional theory, 36 we suggest that a monomer structure of the Mo species (Scheme 2) may be more appropriate for representing the migrated Mo species.…”

Section: Ft-irmentioning

confidence: 99%

“…Nevertheless, more anchored Mo species results in less free Brønsted sites remaining inside the channels, and less consecutive channels. Therefore, for the best bi-functional catalyst, the amount of active Mo species, either in the form of Mo 2 C or MoO x C y , [13][14][15][16] and the amount of free Brønsted acid sites should be well balanced. One may put forward a question on what is the suitable balance between the free Brønsted acid sites and the anchored Mo species and thus to give the best catalytic performance.…”

Section: Catalytic Behavior and Bi-functional Feature Of Mo/mcm-22 Camentioning

confidence: 99%

The synergic effect between Mo species and acid sites in Mo/HMCM-22 catalysts for methane aromatization

Zhu

et al. 2005

Phys. Chem. Chem. Phys.

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The acid properties of Mo/HMCM-22 catalyst, which is the precursor form of the working catalyst for methane aromatization reaction, and the synergic effect between Mo species and acid sites were studied and characterized by various characterization techniques. It is concluded that Brønsted and Lewis acidities of HMCM-22 are modified due to the introduction of molybdenum. We suggest a monomer of Mo species is formed by the exchange of Mo species with the Brønsted acid sites. On the other hand, coordinate unsaturated sites (CUS) are suggested to be responsible for the formation of newly detected Lewis acid sites. Computer modelling is established and coupling with experimental results, it is then speculated that the effective activation of methane is properly accomplished on Mo species accommodated in the 12 MR supercages of MCM-22 zeolite whereas the Brønsted acid sites in the same channel system play a key role for the formation of benzene. A much more pronounced volcano-typed reactivity curve of the Mo/HMCM-22 catalysts, as compared with that of the Mo/HZSM-5, with respect to Mo loading is found and this can be well understood due to the unique channel structure of MCM-22 zeolite and synergic effect between Mo species and acid sites.

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Raman and X-Ray Absorption Studies of Mo Species in Mo/H-ZSM5 Catalysts for Non-Oxidative CH4 Reactions

Cited by 188 publications

References 40 publications

Synthesis, Structure, and Catalytic Reactivity of Isolated V⁵⁺-Oxo Species Prepared by Sublimation of VOCl₃ onto H-ZSM5

Synthesis, Structure, and Catalytic Reactivity of Isolated V⁵⁺-Oxo Species Prepared by Sublimation of VOCl₃ onto H-ZSM5

Study of the carbonaceous deposits formed on a Mo/HZSM-5 catalyst in methane dehydro-aromatization by using TG and temperature-programmed techniques

The synergic effect between Mo species and acid sites in Mo/HMCM-22 catalysts for methane aromatization

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Raman and X-Ray Absorption Studies of Mo Species in Mo/H-ZSM5 Catalysts for Non-Oxidative CH4 Reactions

Cited by 188 publications

References 40 publications

Synthesis, Structure, and Catalytic Reactivity of Isolated V5+-Oxo Species Prepared by Sublimation of VOCl3 onto H-ZSM5

Synthesis, Structure, and Catalytic Reactivity of Isolated V5+-Oxo Species Prepared by Sublimation of VOCl3 onto H-ZSM5

Study of the carbonaceous deposits formed on a Mo/HZSM-5 catalyst in methane dehydro-aromatization by using TG and temperature-programmed techniques

The synergic effect between Mo species and acid sites in Mo/HMCM-22 catalysts for methane aromatization

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Synthesis, Structure, and Catalytic Reactivity of Isolated V⁵⁺-Oxo Species Prepared by Sublimation of VOCl₃ onto H-ZSM5

Synthesis, Structure, and Catalytic Reactivity of Isolated V⁵⁺-Oxo Species Prepared by Sublimation of VOCl₃ onto H-ZSM5