2011 Chinese Journal of Catalysis Vol. 32 No. 7 Article ID: 0253-9837(2011)07-1185-06 DOI: 10.1016/S1872-2067(10)60240-9 Article: 1185–1190 Rh/ZnO-Al 2 O 3 Catalysts for Selective Hydrogenation of Crotonaldehyde Mouna AOUN 1,2,* , Aicha BENAMAR 2 , Mohamed CHATER 2 1 Centre de Recherche Scientifique et Technique en Analyses Physico-Chimiques (C.R.A.P.C), BP 248 Alger RP, 16004, Algeria 2 Laboratoire d’Etude Physico-Chimique des Matériaux et Application à l’Environnement, Faculté de Chimie, Université des Sciences et de la Technologie, Houari Boumediene, BP 32 Bab Ezzouar, 16111, Alger, Algeria Abstract: Gas phase hydrogenation of crotonaldehyde was performed over 1 wt% Rh/ZnO-Al 2 O 3 catalysts with various Zn/Rh atomic ratios. Monometallic Rh/Al 2 O 3 was also prepared for comparison. The samples were prepared by the successive impregnation of Al 2 O 3 with chlo- ride precursors of zinc and rhodium. The solids have been characterized by H 2 chemisorption, temperature-programmed reduction, scanning electron microscopy, and cyclohexane dehydrogenation. Their catalytic behaviour in the gas phase crotonaldehyde hydrogenation reaction after reduction treatment in flowing hydrogen at 723 K was investigated. The relationship between catalytic activity, selectivity for crotyl alcohol, and physicochemical properties of the catalysts was examined. Results obtained showed that the presence of Zn clearly promotes the hydrogenation of the carbonyl bond. The catalyst with Zn/Rh atomic ratio of 5 displayed good catalytic stability and the highest selectivity for crotyl alcohol (70%) along with alloy formation. Key words: rhodium-based catalyst; crotonaldehyde; hydrogenation; zinc oxide; selectivity CLC number: O643 Document code: A Received 24 December 2010. Accepted 18 April 2011. *Corresponding author. Tel: +213-555800547; Fax: +213-21247311; E-mail: aounea@yahoo.fr This work was supported by the Centre de Recherche Scientifique et Technique en Analyses Physico-Chimiques (C.R.A.P.C) and Université des Sciences et de la Technologie, Houari Boumediene (USTHB). English edition available online at Elsevier ScienceDirect (http://www.sciencedirect.com/science/journal/18722067). The unsaturated alcohols obtained by selective hydro- genation of the C=O group of α,β-unsaturated aldehydes are a very important intermediate for the production of per- fumes, flavours, and pharmaceuticals [1–4]. The selective hydrogenation of the carbonyl bond in the presence of an olefinic bond [5] is very difficult because the C=C group hydrogenation is thermodynamically and kinetically favored [6–8]. Thus, monometallic catalysts supported on Al 2 O 3 and SiO 2 led mostly to the formation of saturated aldehyde [9]. Therefore it is necessary to promote the metal to increase selectivity of the carbonyl bond hydrogenation. This can be achieved by using bimetallic catalysts [10–15] as well as supports such as TiO 2 [16,17], ZnO [18], and CeO 2 [19–21] that can interact with the metal after a reduction treatment. The use of zinc as a support for platinum in different re- actions is currently being the subject of an intense research work. In the field of selective hydrogenations, the perform- ance of Pt/ZnO catalysts in gas phase crotonaldehyde hy- drogenation was investigated by Consoni et al. [22]. The authors [22] observed a correlation between reduction tem- perature and both activity and selectivity. An increase in the reduction temperature leads to a decrease in catalytic activ- ity and an increase in crotyl alcohol selectivity along with the formation of Pt-Zn alloy. Recently, Ammari et al. [23] reported a high crotyl alcohol selectivity of 80% during the reaction at 400 °C over Pt/ZnCl 2 /SiO 2 . This fact has been explained by a synergetic effect between Zn and chlorine. A high amount of chlorine on the catalyst surface leads to an increase of the support Lewis acidity, along with the forma- tion of Pt-Zn alloy. Zn, used as promoter, improves unsaturated alcohol for- mation over Pt catalysts in the crotonaldehyde hydrogena- tion reaction [24,25]. A recent study [26] showed that addi- tion of zinc to a silica-supported Co catalyst produced an increase in unsaturated alcohol selectivity. This promotion was favored by increasing reduction temperature. A similar favorable effect of Zn has been recently obtained over Cu/ZnO/SiO 2 catalysts [27]. The selectivity enhancement for hydrogenation of the C=O bond was explained by con- sidering that the high temperature hydrogen treatment forms mobile ZnO x reduced species that strongly interact with Cu 0 crystallites. The resulting Cu 0 -ZnO x species preferentially catalyze the crotyl alcohol formation from crotonaldehyde. The hydrogenation of unsaturated aldehydes can proceed via different reaction pathways as shown in Scheme 1. The 1,2-addition of hydrogen gives the unsaturated alcohol, the