Factors limiting selectivity in C3 and C4 amm(oxidation) reactions

Costine, Allan; Hodnett, B.K.

doi:10.1016/j.apcata.2005.05.032

Cited by 23 publications

(6 citation statements)

References 68 publications

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“…The catalyst has to perform dual functions of dehydrogenating the starting hydrocarbon while at the same time minimizing or eliminating desorption of the unsaturated intermediate alkene. 43 The trend in activity: Mo-V-O < Mo-V-Te-O < Mo-V-Te-Nb-O, was consistent with the oxidative behavior found here.…”

Section: Mo-v-te-o Electrocatalystsupporting

confidence: 89%

Mo-V-O Based Electrocatalysts for Low Temperature Alcohol Oxidation

Pacquette

Gewirth

2015

J. Phys. Chem. C

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There is a growing interest in alcohol oxidation electrochemistry due to its role in renewable energy technologies. The goal of this work was to develop active non-precious metal electrocatalysts based on the Mo-V-(M)-O (M is Nb, Te) lattice. Selective gaseous alkane oxidation had been previously observed on these catalysts at elevated temperatures above 300°C. In this study, the activity of the catalysts at lower temperatures, 25−60°C, was investigated. Hydrothermal conditions were used to synthesize the Mo-V-(M)-O mixed oxides. Physical characterization of the catalysts were obtained by powder X-ray diffraction (XRD), scanning electron micrography (SEM) equipped with energy dispersive X-ray (EDX), transmission electron micrography (TEM), and X-ray photoelectron spectroscopy (XPS). The catalytic activity for the oxidation of cyclohexanol was studied electrochemically. Chronoamperometric studies were used to evaluate the long-term performance of the catalysts. The onset of alcohol oxidative current was observed between 0.2 and 0.6 V versus Ag/AgCl. Gas chromatography−mass spectrometry analysis was used to determine the nature of the oxidative products. The mild oxidation products, cyclohexanone and cyclohexene, were observed after oxidation at 60°C. The catalytic activity increased in the order Mo-V-O < Mo-V-Te-O < Mo-V-Te-Nb-O. Mo-V-(Te,Nb)-O based electrocatalysts efficiently catalyzed the oxidation of alcohols at low temperatures.

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Section: Mo-v-te-o Electrocatalystsupporting

confidence: 89%

Mo-V-O Based Electrocatalysts for Low Temperature Alcohol Oxidation

Pacquette

Gewirth

2015

J. Phys. Chem. C

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“…Previous work , has suggested that the rate of C−H activation for a given alkane is governed by the rate of activation of the weakest C−H bond. This implies that an Arrhenius plot of ( k 1 + k 2 )/ n w , where n w is the number of weakest C−H bonds, would give a systematic trend between the apparent activation energy and the energy of the weakest C−H bond for various alkanes.…”

Section: Resultsmentioning

confidence: 99%

“…Detailed kinetic and isotopic methods have confirmed these conclusions for VO x and MoO x catalysts. [3][4][5][6][7][8] Hodnett et al 9,10 proposed empirical relations between selectivity and the strength of C-H bonds in reactant and products for reactions involving the activation of these bonds. These relations predict that achievable selectivities at a given reactant conversion depend on the differences in dissociation energies between the weakest C-H bonds in reactants and in products for a broad range of catalytic oxidation reactions.…”

Section: Introductionmentioning

confidence: 99%

Role of C−H Bond Strength in the Rate and Selectivity of Oxidative Dehydrogenation of Alkanes

2009

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The oxidative dehydrogenation of alkanes (C2H6, C3H8, i-C4H10, and n-C4H10) was investigated on VO x supported on Al2O3. Rate constants for alkane dehydrogenation (k 1), alkane combustion (k 2), and alkene combustion (k 3) were measured, and a model was developed to describe the effects of alkane composition on these rate constants. The proposed model accounts for the effects of the number of C−H bonds available for activation and the relative strengths of these bonds in both the reactant and the product molecules. The Brønsted−Evans−Polanyi (BEP) relationship is used to relate activation energies of secondary and tertiary C−H bonds to that of primary C−H bonds. The model gives a reasonable approximation of the relative order of alkane reactivity, expressed by k 1 + k 2, and the relative ranking of alkanes with respct to combustion versus oxidative dehydrogenation, expressed by k 2/k 1. The ratio of k 2/k 1 is described by the product of two components: one that depends on the nature, number, and relative strength of C−H bonds of surface alkoxides, and a second one that is independent of the alkoxide composition and structure but depends on the difference in the entropy of activation for CO x precursor versus alkene formation. The model also explains the observed variation of k 3 with alkene composition by considering two precursor states for alkenes. One is strongly bound through π-orbital interactions with Lewis acid centers, and the second weakly binds via H bonding and van der Waals interactions, similar to the binding of alkanes. As a result, the rate of alkene combustion depends strongly on the large heats of adsorption of alkenes and only slightly on the presence of weak allylic C−H bonds. The high rate of C2H4 combustion is thus a consequence of its high heat of adsorption.

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“…In their study, the selectivity of n-butane to butenes could only reach 60%, when the conversion was 30%. 14 The effects of the supports, e.g. TiO 2 , CeO 2 , ZrO 2 , Al 2 O 3 , and MgO, have been widely investigated for V-based catalysts.…”

Section: Introductionmentioning

confidence: 99%