Thiophene as Internal Promoter of Selectivity for the Liquid Phase Hydrogenation of Citral Over Ru/KL Catalysts
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Thiophene as Internal Promoter of Selectivity for the Liquid Phase Hydrogenation of Citral Over Ru/KL Catalysts ´ lvarez-Rodrı´guez Æ A. Guerrero-Ruiz Æ J. A A. Arcoya Æ I. Rodrı´guez-Ramos
Received: 2 January 2009 / Accepted: 2 January 2009 / Published online: 21 January 2009 Ó Springer Science+Business Media, LLC 2009
Abstract The effect of thiophene (Th) addition, in the concentration range 0–6 ppm, on the performance of Ru/KL catalysts is analyzed for the liquid phase hydrogenation of citral at 323 K and 4 MPa. The results show a promoter effect of thiophene on the overall hydrogenation activity of the catalysts, with a maximum at 3 ppm concentration of Th. Particularly, it is evidenced an increase in the selectivity towards the hydrogenation of the carbonyl group of citral. Keywords Ru/KL catalysts Citral hydrogenation Thiophene as selectivity promoter
1 Introduction Selective hydrogenation of citral is an important reaction in the sector of fine chemicals, unsaturated alcohols being the more valuable reaction products. In this line many research works have been published because, in addition to the importance of their hydrogenation products in the perfumes and pharmaceutical industries, this reaction is frequently ´ lvarez-Rodrı´guez A. Guerrero-Ruiz J. A Departamento de Quı´mica Inorga´nica y Quı´mica Te´cnica, Facultad de Ciencias, UNED, C/Senda del Rey no. 9, 28040 Madrid, Spain ´ lvarez-Rodrı´guez A. Guerrero-Ruiz A. Arcoya J. A I. Rodrı´guez-Ramos Grupo de Disen˜o y Aplicacio´n de Catalizadores Heteroge´neos, Unidad Asociada UNED-ICP (CSIC), Barcelona, Spain A. Arcoya I. Rodrı´guez-Ramos (&) Instituto de Cata´lisis y Petroleoquı´mica, CSIC, C/Marie Curie no. 2, 28049 Madrid, Spain e-mail: [email protected]
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used as a ‘‘reaction-model’’ to analyze new formulations for selective catalysts. The reaction pathway generally accepted for the hydrogenation of citral [1], including competitive parallel and consecutive reactions, is depicted in Fig. 1. Hydrogenation of C=O in Z and E isomers yields the unsaturated alcohols nerol and geraniol, respectively, whereas, hydrogenation of the conjugated C=C double bond produces citronellal. In addition, consecutive hydrogenation, isomerization and/or cyclization reactions of these primary products can occur, depending on the surface properties of the catalysts used. In general, ruthenium catalysts direct the reaction mainly to citronellal, and for this reason formulations of metal catalysts, highly selective toward unsaturated alcohols, are required. The influence of the catalyst components and of the catalyst preparation method on the reaction selectivity is very well documented in the literature and reviewed by Kluson and Cerveny [2, 3]. In this sense several authors proposed addition of promoters such as Ge, Sn, Fe [4–6] and Cu [7], which can positively modify the selectivity toward unsaturated alcohols of the supported transition metals. On the other hand Hutchings [8] considers that organic sulfur compounds, added in very low concentration l
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