Kinetic coupling and hydrogen surface fugacities in heterogeneous catalysis: I. Alkane reactions on Te/NaX, H-ZSM5, and Ga/H-ZSM5

Iglesia, Enrique; Baumgartner, Joseph E.; Price, Geoffrey L.

doi:10.1016/0021-9517(92)90342-f

Cited by 158 publications

(126 citation statements)

References 25 publications

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“…This step precedes C-H bond activation and was found to be nearly irreversible. Reactions with 18 O 2 - 16 O 2 did not produce detectable amounts of 16 O-18 O; the reoxidation of lattice oxygen vacancies was found to be irreversible.…”

Section: Discussionmentioning

confidence: 91%

“…CH 3 16 OCH 3 , 5% 16 3 at 488 K to probe the involvement of lattice oxygen atoms in redox cycles and the reversibility of DME dissociation steps.…”

Section: Isotopic Exchange and Tracer Experimentsmentioning

confidence: 99%

“…Involvement by lattice oxygen atoms would lead to the initial formation of HCH 16 O until lattice 16 O atoms become ultimately depleted and replaced by 18 O from 18 O 2 co-reactants. Also, the involvement of lattice oxygen atoms would lead to significant 18 O introduction into unreacted DME, as lattice 16 O is replaced by 18 O from 18 O 2 , if DME dissociation steps are reversible and quasi-equilibrated.…”

Section: Isotopic Evidence For the Involvement Of Lattice Oxygen Atommentioning

confidence: 99%

“…Figure 7 shows total DME chemical conversion turnovers and methoxide recombination turnovers per Mo to form DME in the reverse of step 2. The latter were estimated as twice the number of CH 3 18 OCH 3 formation turnovers, because after initial stages of the reaction, lattice oxygens are mostly 18 O and methoxide groups contain equal numbers of 16 O and 18 O atoms. The ratio of the rates of DME chemical conversion of reversible DME dissociation by methoxide recombination, as calculated from the slope of the graphs, is ∼1.3, suggesting that DME dissociation may be reversible, but not quasi-equilibrated, during DME oxidation to HCHO on this catalyst.…”

Section: Isotopic Evidence For the Involvement Of Lattice Oxygen Atommentioning

confidence: 99%

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Kinetics and Mechanism of Dimethyl Ether Oxidation to Formaldehyde on Supported Molybdenum Oxide Domains

2004

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Kinetic isotope effect and isotopic tracer/exchange methods were combined with in situ infrared spectroscopy and kinetic data to determine the mechanism of dimethyl ether (DME, CH 3 OCH 3 ) oxidation to formaldehyde (HCHO) on MoO x /Al 2 O 3 . The reaction intermediates and elementary steps established a redox mechanism that led to kinetic rate equations that are consistent with observed dependencies of reactant pressures. Methoxide concentrations as detected by in situ infrared spectroscopy correlated directly with formation rates to establish their importance for the formation of HCHO and CH 3 OH. Reactant partial pressure studies showed that rates of HCHO and CH 3 OH formation are first-order in DME and zero-order in O 2 at low DME pressures. At high DME pressures, rates became independent of DME pressure and acquired positive-order O 2 dependencies. H-D kinetic isotope effects indicated that C-H bond activation is not involved in kinetically relevant steps and transient studies involving CH 3 16 OCH 3 -18 O 2 -Mo 16 O x /Al 2 O 3 confirmed the kinetic relevance of DME dissociative adsorption, the step that precedes C-H bond activation. These studies also indicated that mechanisms for HCHO formation do not discriminate between methoxide species formed from DME oxygen and those formed from lattice oxygen.

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Section: Discussionmentioning

confidence: 91%

“…CH 3 16 OCH 3 , 5% 16 3 at 488 K to probe the involvement of lattice oxygen atoms in redox cycles and the reversibility of DME dissociation steps.…”

Section: Isotopic Exchange and Tracer Experimentsmentioning

confidence: 99%

Section: Isotopic Evidence For the Involvement Of Lattice Oxygen Atommentioning

confidence: 99%

Section: Isotopic Evidence For the Involvement Of Lattice Oxygen Atommentioning

confidence: 99%

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Kinetics and Mechanism of Dimethyl Ether Oxidation to Formaldehyde on Supported Molybdenum Oxide Domains

2004

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“…Based on the reports of Iglesia, Boudart, and Temkin (39)(40)(41)(42) we propose the following model to explain this behavior. The initial dehydrogenation rate (on the additional site, not on the Brønsted acid sites) is high.…”

Section: A Second Dehydrogenation Pathwaymentioning

confidence: 96%

Dehydrogenation of Light Alkanes over Zeolites

Narbeshuber

Brait

Seshan

et al. 1997

Journal of Catalysis

114

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The conversion of light paraffins to olefins and the secondary reactions of the unsaturated compounds were investigated on H-ZSM5 and H-Y zeolites between 733 and 823 K. Steady stateand transient response-isotope tracing studies revealed that two mechanisms of dehydrogenation are operative. The main pathway is represented by monomolecular, protolytic dehydrogenation. This reaction contributes most to steady state olefin production. Additionally, at the initial stages of the reaction, extra framework aluminum moieties are speculated to participate in high dehydrogenation activity. This pathway is blocked at later stages of the reaction by product (hydrogen) inhibition. The intrinsic rates of protolytic dehydrogenation and olefin desorption range in the same order of magnitude. At high protolytic dehydrogenation rates, olefin desorption represents the rate determining step. Depending on the process conditions, olefins undergo secondary cracking, oligomerization, or isomerization. The latter proceeds via intramolecular rearrangement, possibly via a cyclopropylcarbenium ion at high temperatures and low conversions. At reaction conditions where bimolecular cracking prevails, isomerization is concluded to occur via secondary cracking of di-or oligomers.

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