Acid Strength of Pt-Zeolite Catalysts: Evidence of Metal-Support Interaction from Linear Free-Energy Relationship
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Acid Strength of Pt-Zeolite Catalysts: Evidence of Metal-Support Interaction from Linear Free-Energy Relationship Roberta C. Tourinho • Igor F. de Oliveira Pedro A. Arroyo • Claudio J. A. Mota
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Received: 2 June 2011 / Accepted: 4 August 2011 / Published online: 16 August 2011 Ó Springer Science+Business Media, LLC 2011
Abstract The acid strength of Pt-HBeta and Pt-HMordenite zeolite catalysts were estimated by H/D exchange of substituted benzenes with the use of linear free-energy relationship, through the use of the Hammet–Brown equation. The results indicated that compared with the parent acidic zeolite, the incorporation of Pt metal leads to a significant decrease in the acid strength, estimated by the slope (q) of the logarithm of the relative rate (kX/kH) with the r? constant of the substituent, indicating a lower degree of proton transfer to the ring in the transition state. The results support the model of metal-support interaction in Pt-zeolite catalyst, with formation of a metal-proton adduct. Keywords Zeolites Acid strength Catalysis Linear free-energy relationship
1 Introduction Bifunctional metal-zeolite catalysts are widely used in hydroisomerization and hydrocracking of hydrocarbons
R. C. Tourinho I. F. de Oliveira C. J. A. Mota Instituto de Quı´mica, Universidade Federal do Rio de Janeiro, Av Athos da Silveira Ramos 149, CT Bloco A, Rio de Janeiro 21941-909, Brazil P. A. Arroyo Departamento Engenharia Quı´mica, Universidade Estadual de Maringa´, Av. Colombo 5790, Maringa´, PR 87020-900, Brazil C. J. A. Mota (&) INCT Energia e Ambiente, UFRJ, Rio de Janeiro, RJ 21941-909, Brazil e-mail: [email protected]
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[1]. The metal function acts as dehydrogenating/hydrogenating site, whereas the acid function catalyzes branching and b-scission reactions, which normally involves carbocation intermediates. Another feature of bifunctional catalysts is the catalytic stability, with virtually no formation of coke that leads to deactivation. Although this simple picture may explain the catalytic behavior of bifunctional catalysts, the real situation is far more complex. Sachtler studied [2, 3] by XPS the electronic state of Pd clusters in protonic Y zeolite, observing an electron deficiency on the Pd atoms. He proposed the existence of metal-proton adducts to account for these results. However, this model does not explain the increasing electron density on metal cluster in alkaline zeolites [4]. Thus, another hypothesis was proposed by Koningsberger and co-workers, who studied Pd and Pt on L and Y zeolites [5, 6]. This model relies on the interaction of the metal particles with the oxygen atoms of the zeolite structure, implying the modification of the Madelung potential of the support [7]. Recently [8], we developed a technique for estimating the acid strength of solid acid catalysts, through the H/D exchange of substituted benzene derivatives (Scheme 1) and the use of linear free-energy relationship (LFER). The method is based on the Hammett–Brown equation, which plots the logarithm of the relative
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