Kinetic study of the selective hydrogenation of styrene over a Pd egg-shell composite catalyst

This is a study on the kinetics of the liquid-phase hydrogenation of styrene to ethylbenzene over a catalyst of palladium supported on an inorganic–organic composite. This support has a better mechanical resistance than other commercial supports, e.g. alumina, and yields catalysts with egg-shell structure and a very thin active Pd layer. Catalytic tests were carried out in a batch reactor by varying temperature, total pressure and styrene initial concentration between 353–393 K, 10–30 bar, and 0.26–0.60 mol L−1. Kinetic models were developed on the assumptions of dissociative hydrogen chemisorption and non-negligible adsorption of hydrogen and styrene. Final chemical reaction expressions useful for reactor design were obtained. The models that best fitted the experimental data were those ones that considered the surface reaction as the limiting step. In this sense, a two-step Horiuti–Polanyi working mechanism with half hydrogenation intermediates gave the best fit of the experimental data. The heats of adsorption of styrene and ethylbenzene were also estimated.The authors are gratefully indebted to CONICET, ANPCyT and Universidad Nacional del Litoral for financially sponsoring this research work

Préparation de catalyseurs au molybdène par greffage du pentachlorure de molybdène sur silice en milieu organique

La présente étude concerne l’amélioration de la technique de préparation de catalyseurs molybdène-silice, réalisée par greffage, c’est-à-dire par condensation de pentachlorure de molybdène sur les groupes silanols du support, dans divers solvants organiques.Des essais comparatifs effectués sur d’anciens échantillons (bleus), et sur ceux que nous avons préparés (marrons), permettent de tester la qualité du greffage et de la dispersion des ions molybdène sur la silice. Les techniques de RPE et d’absorption UV-visible montrent que ces deux types de catalyseurs contiennent, à l’issue de la préparation, des composés à valence mixte Mo5+ — Mo6+. Les produits marrons se distinguent des bleus par le fait qu’ils ne présentent pas en UV-visible de bande d’intervalence, ce qui tend à prouver que leurs ions molybdène sont mieux dispersés. Cette hypothèse est renforcée par des mesures plus quantitatives de la dispersion, basées sur le principe de l’adsorption d’oxygène à basse température. Des tests de lavage à l’eau, réalisés sur les deux séries d’échantillons montrent que le molybdène des produits marrons est mieux fixé que celui des bleus. Enfin, nous constatons que ces composés bleus présentent des propriétés analogues aux ’’bleus de molybdène"

Liquid-phase selective hydrogenation of hexa-1,5-diene and hexa-1,3-diene on palladium catalysts. Effect of tin and silver addition

The liquid-phase hydrogenation of hexa-1,5-diene and hexa-1,3-diene on alumina-supported palladium catalysts is investigated with special attention paid to the effects of tin or silver addition. All catalysts show a global selectivity near 100%; these high values persist at total conversion when the reactant is hexa-1,3-diene but decrease to about 70% in the case of hexa-1,5-diene. In the hexa-1,5-diene hydrogenation, monometallic palladium catalysts give mainly 1-hexene at conversions lower than 80% and E-hex-2-ene at higher conversions. The fractional selectivity to 1-hexene is significantly improved by tin or silver addition; however, significant yields are observed only on Pd-Sn catalysts with a low Sn/Pd atomic ratio (approximate to 0.1) and when the intermetallic compound Pd3Sn is present. On the other hand, in the hexa-1,3-diene hydrogenation, hex-1-ene is preferentially formed on monometallic catalysts with low dispersion; on bimetallic Pd-Sn or Pd-Ag catalysts, the selectivity to E-hex-3-ene is enhanced and this isomer is even predominant up to 100% conversion on the Pd-Sn catalysts containing the Pd3Sn intermetallic compound. These results are explained by the geometric effect of dilution of Pd atoms, which delays the palladium double-bond isomerization ability. The proposed mechanism suggests that strongly adsorbed alkadienes react with dissolved hydrogen, following a zero-order kinetics. (C) 2000 Academic Press.19519610

Self-influence of a femtosecond laser beam upon ablation of Ag in liquids

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Palladium, palladium-tin, and palladium-silver catalysts in the selective hydrogenation of hexadienes: TPR, Mossbauer, and infrared studies of adsorbed CO

Alumina-supported Pd-Sn catalysts prepared by successive impregnation of the precursors, calcination, and reduction contain SnO2 and SnAl2O5, Pd-Sn solid solutions, and intermetallic phases Pd2Sn and Pd3Sn. The formation of Pd3Sn is favored not only by lowering the Sn/Pd ratio but also by introducing tin through ethanolic impregnation. The dilution of surface Pd atoms is confirmed by infrared spectroscopy of adsorbed CO: when the Sn/Pd ratio increases, the linear/bridged intensity ratio increases and the dipole-dipole interactions decrease. TPR experiments show the influence of the support acidity and of the preparation procedure on the reducibility of the various Pd-Sn and Pd-Ag species; in the case of Pd-Ag catalysts, the presence of silver postpones the reduction of PdO in calcined samples. (C) 2000 Academic Press.1951889

Catalytic behaviour of cobalt exchanged hydroxyapatite in the oxidative dehydrogenation of ethane

Several samples of exchanged cobalt/calcium (Co$^{2+}$/Ca$^{2+})$ hydroxyapatite were synthesized and characterised by XRD, IR and UV-visible spectroscopy. The level of Co$^{2+}$/Ca$^{2+}$ exchanged was limited to 1.35 wt% of Co and the Co$^{2+}$ ions incorporated in the phosphate matrix were mainly hosted by octahedral sites. The catalytic activity of these materials was measured in the oxidative dehydrogenation of ethane into ethylene in the 450-550$^{\circ}$C range. The global conversion and the ethylene yield increase with temperature and depend upon the cobalt content. They reached 35% and 22% at 550$^{\circ}$C respectively for a Co wt% of about 1%

Characterization of active sites on W2−x_{2-x}Mox_x(PO4_{4})2_{2} (0≤x≤1.250 \le x \le 1.25) in butan-2-ol conversion

The molybdenum-tungsten phosphate (W$_{2-{\rm x}}$Mo$_{\rm x}$O$_{3}$(PO$_{4})_{2})$ was synthetised and characterised using several techniques. Its catalytic performances was studied in butan-2-ol conversion. The UV-visible investigations showed that the catalyst contains Mo(V) ions. This results was confirmed by the EPR spectra. Additional U.V. visible in-situ experimentss, in dynamic conditions revealed that butan-2-ol reduces Mo(VI) into Mo(V) ions. The active sites in the catalytic reaction appearto be built up arround the Mo(VI) ions

Hydrogenation of oleic acid over sol-gel ruthenium catalysts

The hydrogenation of oleic acid to oleyl alcohol over Ru-alumina and Ru-Sn-alumina sol-gel catalysts has been studied. It was found that, besides its hydrogenation activity, the Ru-Sn-alumina catalyst promotes the cis-trans isomerization of the unsaturated acid. This effect is used to explain some kinetic features of the system, like the high induction time observed in the formation of alcohols, and to reexamine the role of tin.66115516

Mössbauer Study Of 119sn-pd Catalysts For Selective Hydrogenation Of Hexa-1,5-diene

The addition of tin to palladium-alumina catalysts induces significant modifications of the catalytic properties of palladium in the hydrogenation of hexa-1,5-diene. The influence of the support, of the metal loading and of the preparation method on the chemical state of tin in Pd-Sn/Al2O3 catalysts are studied by Mössbauer spectroscopy. In reduced catalysts, about 60% of tin is present as Sn(0), which forms different Pd-Sn compounds, that are responsible for the changes in the catalytic performances.1121-41316Bacaud, R., Bussière, P., Figueras, F., (1981) J. Catal., 69, p. 399Fréty, R., Benaichouba, B., Bussière, P., Santos Cunha, D., Lam, Y.L., (1984) J. Mol. Catal., 25, p. 173Masai, M., Honda, K., Kubota, A., Ohnaka, S., Nishikawa, Y., Nakahara, K., Kishi, K., Ikeda, S., (1977) J. Catal., 50, p. 419Aduriz, H.R., Bodnariuk, P., Coq, B., Figueras, F., (1989) J. Catal., 119, p. 97Li, Y.-X., Klabunde, K.J., (1990) J. Catal., 126, p. 173Gray, P.R., Farha, F.E., (1976) Mössbauer Effect Methodology, 10, p. 47. , Ed. I. J. Gruverman and W. Seidel, Plenum Press, New YorkSnediker, D.K., (1966) Mössbauer Effect Methodology, 2, p. 161Cordey Hayes, M., Harris, I.R., (1967) Phys. Lett., 24 A, p. 80Elliott, R.P., (1986) Constitution of Binary Alloys, First Supplement, p. 732. , Mc Graw-Hill Book Company, New Yor