- Mechanistic Insights into Catalytic Ethanol Steam Reforming Using Isotope-Labeled Reactants
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The low-temperature ethanol steam reforming (ESR) reaction mechanism over a supported Rh/Pt catalyst has been investigated using isotope-labeled EtOH and H2O. Through strategic isotope labeling, all nonhydrogen atoms were distinct from one another, and allowed an unprecedented level of understanding of the dominant reaction pathways. All combinations of isotope- and non-isotope-labeled atoms were detected in the products, thus there are multiple pathways involved in H2, CO, CO2, CH4, C2H4, and C2H6product formation. Both the recombination of C species on the surface of the catalyst and preservation of the C?C bond within ethanol are responsible for C2product formation. Ethylene is not detected until conversion drops below 100 % at t=1.25 h. Also, quantitatively, 57 % of the observed ethylene is formed directly through ethanol dehydration. Finally there is clear evidence to show that oxygen in the SiO2-ZrO2support constitutes 10 % of the CO formed during the reaction.
- Crowley, Stephen,Castaldi, Marco J.
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supporting information
p. 10650 - 10655
(2016/09/03)
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- The mechanism of aromatic dealkylation in methanol-to-hydrocarbons conversion on H-ZSM-5: What are the aromatic precursors to light olefins?
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Co-reactions of 7.5-9.3 kPa of DME with 4 kPa of toluene, p-xylene, and 4-ethyltoluene on H-ZSM-5 at 523-723 K at low conversions (13C/12C show that carbons originating from the aromatic ring are incorporated into ethene and propene. A comparison of the predicted 13C-contents of ethene and propene postulated on the basis of the paring, side-chain, and ring-expansion aromatic dealkylation mechanisms based on the experimentally observed isotopologue distribution of 1,2,4-trimethylbenzene, 1,2,4,5-tetramethylbenzene, and 4-ethyltoluene reveal that the predicted 13C-content of ethene and propene from 1,2,4,5-tetramethylbenzene via the paring mechanism most closely match the experimentally observed 13C-contents of ethene and propene (a 200 K range in temperature.
- Ilias, Samia,Bhan, Aditya
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- The influence of catalyst acid strength on the methanol to hydrocarbons (MTH) reaction
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The methanol to hydrocarbons (MTH) reaction was studied over two isostructural zeotype catalysts of different acid strength, H-SAPO-5 and H-SSZ-24. Conversion of methanol alone was performed at 350-450 C and WHSV = 0.31-2.48 h-1. The product selectivities of the two catalysts were compared at similar conversion. The strongly acidic H-SSZ-24 was found to be more selective towards aromatic products and C2-C3 hydrocarbons as compared to the moderately acidic H-SAPO-5, which produced more non-aromatic C4+ hydrocarbons. Co-reactions of 13CH 3OH and benzene at 250-300 C with low conversion of both reactants revealed that both catalysts produced ethene and propene from polymethylbenzenes via a paring mechanism. However, this reaction proceeded more readily in H-SSZ-24 than in H-SAPO-5. Furthermore, isobutene formation was found to be mainly associated with aromatic intermediates in H-SSZ-24, whereas isobutene produced over H-SAPO-5 was mainly formed via alkene intermediates. Overall, the results obtained in this study suggest that a lower acid strength promotes an alkene-mediated MTH reaction mechanism.
- Westg?rd Erichsen, Marius,Svelle, Stian,Olsbye, Unni
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p. 216 - 223
(2013/09/02)
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- Tracing the interstitial carbide of the nitrogenase cofactor during substrate turnover
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The fate of the interstitial atom of the nitrogenase cofactor during substrate turnover has remained a topic of interest since the discovery of this atom more than a decade ago. In this study, we labeled the interstitial carbide atom with 14C and 13C isotopes and traced the fate of the isotope under turnover conditions. Our results show that the interstitial carbide cannot be exchanged upon turnover, nor can it be used as a substrate and incorporated into the products. These observations point to a role of the interstitial carbide in stabilizing the cofactor structure, although a function of this atom in indirectly modulating the reactivity of the cofactor or directly interacting with the substrate cannot be excluded.
- Wiig, Jared A.,Lee, Chi Chung,Hu, Yilin,Ribbe, Markus W.
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supporting information
p. 4982 - 4983
(2013/06/04)
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- Cationic zirconium hydrides supported by an nnnn-type macrocyclic ligand: Synthesis, structure, and reactivity
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An air- and light-sensitive, but thermally stable tris[(trimethylsilyl) methyl]zirconium complex containing an NNNN-type macrocyclic ligand [Zr(Me 3TACD)(CH2SiMe3)3] (1; Me 3TACD = Me3[12]aneN4: 1,4,7-trimethyl-1,4,7,10- tetraazacyclododecane) was prepared by reacting [Zr(CH2SiMe 3)4] with (Me3TACD)H. Reaction of the zirconium tris(alkyl) 1 with a Lewis or Bronsted acid gave a dialkyl cation with a weakly coordinating anion [Zr(Me3TACD)(CH2SiMe 3)2][A] [A = Al{OC(CF3)3} 4 (2a), B{3,5-C6H3(CF3) 2}4 (2b), B(3,5-C6H3Cl 2)4 (2c), and BPh4) (2d)]. Hydrogenolysis of 2a-2c resulted in the formation of the dinuclear tetrahydride dication [{Zr(Me3TACD)(μ-H)2}2][A]2 (3a-3c). Compounds 1-3 were characterized by multinuclear NMR spectroscopy, and the solid-state structures of 1, 2c, and 3b were established by single-crystal X-ray diffraction studies. The dinuclear hydride complex 3b exhibits a quadruply bridged {Zr2(μ-H)4} core in solution and in the solid state with a relatively short Zr...Zr distance of 2.8752(11) . Density functional theory computations at the B3PW91 level reproduced this structure (Zr...Zr distance of 2.900 ). The cationic hydride complex 3b reacted with excess carbon monoxide in tetrahydrofuran at room temperature to give ethylene in 25% yield based on 3b. Upon analysis of 13C NMR spectra of the reaction mixture using 13CO, oxymethylene and enolate complexes were detected as intermediates among other complexes.
- Kulinna, Heiko,Spaniol, Thomas P.,Maron, Laurent,Okuda, Jun
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p. 12462 - 12472
(2013/01/15)
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- ATP-independent formation of hydrocarbons catalyzed by isolated nitrogenase cofactors
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Reduce to produce: Molybdenum- and vanadium-nitrogenase cofactors have been isolated and shown to reduce carbon monoxide and cyanide ions to a mixture of alkanes and alkenes in the presence of a strong reductant, europium(II) diethylenetriaminepentaacetate (see scheme). Various hydrocarbons of up to seven carbon atoms in length are detected as products in these ATP-free reactions.
- Lee, Chi Chung,Hu, Yilin,Ribbe, Markus W.
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experimental part
p. 1947 - 1949
(2012/04/04)
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- Tetraethylborate as an ethyl transfer reagent
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Reaction of NaBEt4 with RhCl(C2H4)(triphos) (I), where triphos is MeC(CH2PPh2)3, gives RhH-(C2H4)(triphos). Coordinated 13C2H4 in labeled I is liberated during the reaction, proving that NaBEt4 is the source of the ethylene coordinated in the product. Compound I reacts at 1 atm and 25°C with 13C2H4 or H2 to give RhCl(13C2H4)(triphos) or Rh(H)2Cl(triphos), respectively. It is proposed that all three reactions proceed by a mechanism in which one arm of the triphos ligand in I dissociates to yield a planar RhCl-(C2H4)(η2-triphos) transient. Both five-coordinate species RhX(C2H4)(triphos) (X = H, Cl) are unusual in showing relatively high barriers for both phosphorus site exchange and olefin rotation. In contrast to the reaction of I with NaBEt4, the reaction with NaBPh4 stops at Rh(η2-triphos) (η6-PhBPh3) and shows no further conversion to RhPh(C2H4)(triphos).
- Thaler, Eric G.,Caulton, Kenneth G.
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p. 1871 - 1876
(2008/10/08)
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