A diagnostic test of the kinetic regime in a packed bed reactor

The areal rate of liquid phase hydrogenation of cyclohexene on supported platinum catalysts does not depend on the nature of the support. Nor does it depend on particle size of the metal. The rate constant is independent of the nature of the solvent when the concentration of hydrogen in it is expressed as its measured or calculated solubility. All observations are compatible with the idea that the measured rate is that of chemisorption of dihydrogen on a metal surface covered with reactive hydrocarbon intermediates. SCOPEThe effect of the dimensions of metallic particles on the catalytic activity of supported metals is of theoretical and practical importance. The present work was undertaken to examine the effect of platinum particle size on the liquid phase hydrogenation of cyclohexene in a rocking slurry reactor. The reaction was studied in a static system at constant hydrogen pressure, normally atmospheric pressure, between 275O and 316OK. As diffusion coefficients in the liquid phase are often two or more orders of magnitude lower than in the vapor phase, diffusional influence can play a substantial role in slurry reactors. It is thus imperative that reactions studied in slurry reactors be examined for diffusional effects and proof be given that observed rates have not been disguised by either intraparticle or interphase mass transport. We set out to obtain true kinetic data devoid of either the influence of transport phenomena or catalyst deactivation during an experimental run.The present study also focused on two important aspects of liquid phase reactions, When heterogeneous catalytic reactions are carried out in the gas phase, ideal gas behavior is ordinarily assumed. However, for kinetics in the liquid phase, as in a slurry or trickle bed reactor, it is necessary first of all to ascertain whether concentrations or activities of dissolved reactants should be used in rate expressions. Second, it is also important to check whether different solvents affect the reaction rate or the rate constant. The literature of catalytic reaction engineering offers few experimental or theoretical guidelines bearing on these two important questions. CONCLUSIONS AND SIGNIFICANCEKinetic data have been obtained for the liquid phase hydrogenation of cyclohexene on supported platinum catalysts in a rocking slurry reactor. First of all, values of the turnover frequency N , defined as molecules reacted per site per second, were obtained on catalysts containing different amounts of platinum of the same dispersion, defined as the ratio of surface metal atoms to total metal atoms. Values of N were found to be identical. As this re- Page 904September, 1978 sult was obtained at two different temperatures, it offers sufficient proof that rates were not masked by artifacts such as the influence of interphase and intraparticle heat and mass transfer, or deactivation and poor contacting of the catalyst by reactants. The main result of this work is that values of N were identical on nine different supported platinum catalysts where the me...

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“…The criterion used was that proposed by Koros and Nowak (1967) and discussed more fully elsewhere (Madon, 1974).…”

Section: Resultsmentioning

confidence: 99%

Catalytic hydrogenation of cyclohexene: Part II. Liquid phase reaction on supported platinum in a gradientless slurry reactor

1978

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“…We investigate next whether the subsequent O 2 * dissociation steps require two vacancies (according to step 2 of Scheme 1) by considering the interaction of 18 O 2 (g) with surfaces nearly saturated by 16 O 2 ). The involvement of fast mobile oxygen species on O*-covered surfaces, detected here from isotopic exchange rates and selectivities, has been previously proposed to account for the effects of O 2 pressure on the rates of NO decomposition.…”

Section: Experimental Methods 21 Catalyst Synthesis and Characterizmentioning

confidence: 99%

NO Oxidation Catalysis on Pt Clusters: Elementary Steps, Structural Requirements, and Synergistic Effects of NO₂Adsorption Sites

2009

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Kinetic and isotopic methods show that NO oxidation on supported Pt clusters involves kinetically relevant reaction of O 2 with vacancy sites on surfaces nearly saturated with oxygen adatoms (O*). The oxygen chemical potential at Pt surfaces that determines the O* coverage is rigorously described by an O 2 virtual pressure and determined by the thermodynamics of NO 2 -NO interconversion reactions. NO oxidation and oxygen isotopic exchange processes are described by the same rate constant, consistent with similar kinetically relevant O 2 dissociation steps for both reactions. NO oxidation, NO 2 decomposition, andO 2 exchange rates increased markedly with increasing Pt cluster size (1-8 nm); these clusters remain metallic at all O 2 virtual pressures prevalent during NO oxidation. These effects of cluster size reflect the higher vacancy concentrations and more facile oxygen desorption on larger Pt clusters, which bind oxygen adatoms weaker than more coordinatively unsaturated surface Pt atoms on smaller clusters. These trends are similar to those found for methane and dimethyl ether combustion on Pt and Pd catalysts, which also require vacancy sites on O*-saturated cluster surfaces in their respective kinetically relevant steps. Inhibition of NO oxidation by NO 2 persists to undetectable NO 2 concentrations; thus, NO oxidation turnover rates increase significantly when NO 2 adsorption sites present on BaCO 3 /Al 2 O 3 are placed within diffusion distances of Pt clusters. NO oxidation rates on intrapellet catalyst-adsorbent mixtures are described accurately by a simple reaction-adsorption model in which NO 2 adsorbs via displacement of CO 2 on BaCO 3 surfaces.

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“…A Koros-Nowak test [55,56] was employed to check the effect of heat and mass transfer limitations on the catalytic rate. This test consists of running the reaction on three catalysts with different dosage over KCa/MMT-30.…”

Section: Kinetic Studymentioning

confidence: 99%

Montmorillonite-supported KF/CaO: a new solid base catalyst for biodiesel production

Jia

Chen

et al. 2015

Res Chem Intermed

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Biodiesel is an alternative to petroleum-derived diesel fuel; development of a high-efficiency base catalyst to be used in heterogeneous biodiesel production is still a challenge. In this paper, a novel solid base catalyst, KF-and CaOsupported montmorillonite (KCa/MMT) was successfully synthesized by a facile impregnation method, and used for producing biodiesel in transesterification of commercial soybean oil with methanol. The catalysts were characterized by X-ray diffraction, carbon dioxide temperature-programmed desorption and scanning electron microscopy. Effects of the parameters, such as the loading amount of KF, the amount of KCa/MMT, and the methanol to oil molar ratios, on the yield of biodiesel were investigated. A maximum biodiesel yield of 98 % was obtained under the optimal reaction conditions. The separated catalyst can be directly used in the next round of reactions and gave a satisfactory yield. Furthermore, analysis of the catalyst's tolerance to oil-containing water or free fatty acids, and a kinetic study were also carried out. Koros-Nowak tests were designed and conducted, and it was proven that the heat and mass transfer were not limited by the reaction rate.

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A diagnostic test of the kinetic regime in a packed bed reactor

Cited by 266 publications

References 5 publications

Catalytic hydrogenation of cyclohexene: Part II. Liquid phase reaction on supported platinum in a gradientless slurry reactor

Catalytic hydrogenation of cyclohexene: Part II. Liquid phase reaction on supported platinum in a gradientless slurry reactor

NO Oxidation Catalysis on Pt Clusters: Elementary Steps, Structural Requirements, and Synergistic Effects of NO₂Adsorption Sites

Montmorillonite-supported KF/CaO: a new solid base catalyst for biodiesel production

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A diagnostic test of the kinetic regime in a packed bed reactor

Cited by 266 publications

References 5 publications

Catalytic hydrogenation of cyclohexene: Part II. Liquid phase reaction on supported platinum in a gradientless slurry reactor

Catalytic hydrogenation of cyclohexene: Part II. Liquid phase reaction on supported platinum in a gradientless slurry reactor

NO Oxidation Catalysis on Pt Clusters: Elementary Steps, Structural Requirements, and Synergistic Effects of NO2Adsorption Sites

Montmorillonite-supported KF/CaO: a new solid base catalyst for biodiesel production

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NO Oxidation Catalysis on Pt Clusters: Elementary Steps, Structural Requirements, and Synergistic Effects of NO₂Adsorption Sites