coordinence en catalyse hétèrogène appliquée coordination in applied heterogeneous catalysis
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ID: 258685
2006
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Abstract
L'exposé concerne l'activation des paraffines et des oléfines en présence d'hydrogène sur les métaux du groupe VIII. L'activation des paraffines fait intervenir les voies d'hydrogénolyse, d'isomérisation et de cyclisation. La profondeur d'hydrogénolyse permet de proposer un classement des métaux selon l'ordre croissant de la force de la liaison, paraffine chimisorbée-métal Pd-Pt-Ir-Rh-Ru-Os-Ni-Co-Fe L'activité spécifique d'hydrogénolyse, portée alors en fonction de la force de cette liaison, passe par un maximum autour de la position de Rh - Ru - Os. Les activités d'isomérisation et de cyclisation passent, par un maximum, pour la position de Pt. On retrouve ainsi la notion de force optimale de chimisorption pour catalyser une transformation précise. Sur la base des notions développées par Pearson, on peut alors considérer que la paraffine chimisorbée intervient plutôt en qualité de base «dure» de telle façon que la force de la liaison paraffine métal, augmente avec le caractère «dur» du métal. L'effet des ligands dans la sphère de coordination du site catalytique est ensuite analysé, en distinguant les ligands introduits lors de la préparation du catalyseur et ceux amenés par la phase réactionnelle. Les modifications ainsi apportées au caractère « dur ou mou » du site catalytique, conduisent à prévoir le sens de variation de l'activité d'hdrogénolyse ou de cyclisation. L'expérience confirme en gros les prévisions. L'étude de l'activation des oléfines est limitée au cas de l'éthylène, pour lequel on examine l'activité de l'hydrogénation et de l'échange éthylène-deutérium. La valeur de la multiplicité de cet échange permet de proposer un classement des métaux selon l'ordre croissant de la force de la liaison oléfine-métal Fe - Co-Ni-Ru-Rh-Os-Ir-Pt L'activité d'hydrogénation portée alors en fonction de la force de cette liaison, passe par un maximum autour (également) de la position de Rh. On retrouve donc aussi 'la notion de force optimale de chimisorption. On sait ici que les oléfines interviennent en qualité de base « douce», ce qui explique que la force de liaison de chimisorption augmente de Fe à Pt, et que le classement des métaux soit en gros l'inverse du précédent. L'effet des ligands dans la sphère de coordination du site catalytique est examiné comme précédemment, en cherchant à dégager certaines prévisions. Une comparaison générale peut alors être tentée entre les deux tranformations de type opposé : l'activation «exigeante » des paraffines et l'activation « non exigeante » des oléfines. On précise ainsi les différences essentielles entre ces deux modes d'activation. Les prévisions qui peuvent être dégagées fournissent des facteurs pratiques d'action, et doivent servir de pierre de touche pour délimiter le degré de validité de l'interprétation proposée.
This article deals with the activation of paraffins and olefins in the presence of hydrogen, on Group VIII metals. Paraffin activation involves processes of hydrogenolysis, isomerization and cyclization. The depth of hydrogenolysis can be used ta propose a classification of metals according ta an increasing order of chemisorbed paraffin-metai bond strength, Pd-Pt-Ir-Rh-Ru-Os-Ni-Co-Fe The specific activity of hydrogenolysis, determined as a function of the strength of this bond, goes via a maximum around the position of Rh - Ru - Os. The isomerization and cyclization activities go via a maximum for the position of Pt. We thus encuenter the concept of optimum chemisorption force for catalyzing a specific transformation. On the bases of the concepts developed by Pearson, we can thus consider than the chemisorbed paraffin tends rather to act as a « hard » base in such a way that the paraffin-metal bond strength increases with the « hard » nature of themetal. The influence of ligands in the coordination sphere of the catalytic site is then analyzed by distinguishing the ligands introduced during catalyst preparation from those brought in by the reaction phase. The changes thus made in the « hard » or « soft » nature of the catalytic site make it possible to predict the direction of variation of the hydrogenolysis or cyclization activity. Experience has more or less confirmed such predictions. Analysis of olefin activation is limited to the case of ethylene for which the hydrogenation activity and ethylene-deuterium exchange are examined. The multiplicity value of this exchange enables us to propose a classification of metals by increasing order of the olefin-metal bond strength : Fe- Co-Ni-Ru-Rh-Os-Ir-Pt The hydrogenation activity, thus determined as a function of the strength of this bond, goes via a maximum around (likewise) the position of Rh. Hence we also find the concept of optimum chemisorption force. We know that here the olefins intervene as a « mild » base, which explains why the chemisorption bond strength increases from Fe to Pt and why the metal classificaiion is roughly the reverse of the preceding one. The influence of ligands in the coordination sphere of the catalytic site is examined as before, by attempting ta determine various predictions. A general comparison can then be attempted between the two opposite-type transformations, i.e. the a demanding » activation of paraffins and the « undemanding » activation of olefins. In this way, the essential differences in these two activation modes are illustrated. The predictions that can be'determined provide practical factors for action and must be used as a touchstone for circumscribing the degre of validity of he proposéd interpretation.
This article deals with the activation of paraffins and olefins in the presence of hydrogen, on Group VIII metals. Paraffin activation involves processes of hydrogenolysis, isomerization and cyclization. The depth of hydrogenolysis can be used ta propose a classification of metals according ta an increasing order of chemisorbed paraffin-metai bond strength, Pd-Pt-Ir-Rh-Ru-Os-Ni-Co-Fe The specific activity of hydrogenolysis, determined as a function of the strength of this bond, goes via a maximum around the position of Rh - Ru - Os. The isomerization and cyclization activities go via a maximum for the position of Pt. We thus encuenter the concept of optimum chemisorption force for catalyzing a specific transformation. On the bases of the concepts developed by Pearson, we can thus consider than the chemisorbed paraffin tends rather to act as a « hard » base in such a way that the paraffin-metal bond strength increases with the « hard » nature of themetal. The influence of ligands in the coordination sphere of the catalytic site is then analyzed by distinguishing the ligands introduced during catalyst preparation from those brought in by the reaction phase. The changes thus made in the « hard » or « soft » nature of the catalytic site make it possible to predict the direction of variation of the hydrogenolysis or cyclization activity. Experience has more or less confirmed such predictions. Analysis of olefin activation is limited to the case of ethylene for which the hydrogenation activity and ethylene-deuterium exchange are examined. The multiplicity value of this exchange enables us to propose a classification of metals by increasing order of the olefin-metal bond strength : Fe- Co-Ni-Ru-Rh-Os-Ir-Pt The hydrogenation activity, thus determined as a function of the strength of this bond, goes via a maximum around (likewise) the position of Rh. Hence we also find the concept of optimum chemisorption force. We know that here the olefins intervene as a « mild » base, which explains why the chemisorption bond strength increases from Fe to Pt and why the metal classificaiion is roughly the reverse of the preceding one. The influence of ligands in the coordination sphere of the catalytic site is examined as before, by attempting ta determine various predictions. A general comparison can then be attempted between the two opposite-type transformations, i.e. the a demanding » activation of paraffins and the « undemanding » activation of olefins. In this way, the essential differences in these two activation modes are illustrated. The predictions that can be'determined provide practical factors for action and must be used as a touchstone for circumscribing the degre of validity of he proposéd interpretation.
| Reference Key |
r.2006oilcoordinence
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| Authors | |
| Journal | addiction (abingdon, england) |
| Year | 2006 |
| DOI |
10.2516/ogst:1977021
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