Journal of Catalysis 219 (2003) 167–175 www.elsevier.com/locate/jcat The aldol condensation of acetaldehyde and heptanal on hydrotalcite-type catalysts Didier Tichit, a,∗ Doina Lutic, a,1 Bernard Coq, a Robert Durand, a and Rémy Teissier b a Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS-ENSCM, 8, Rue Ecole Normale, 34296 Montpellier cedex 5, France b ATOFina, CRRA, Rue Henri Moissan BP 63, 69310 Pierre-Bénite cedex, France Received 13 December 2002; revised 18 March 2003; accepted 8 April 2003 Abstract The aldol condensation of acetaldehyde and heptanal has been carried out in the liquid phase between 353 and 413 K using different types of solid base catalysts: MgO with strong Lewis basic sites, Mg(Al)O mixed oxides with acid–base pairs of the Lewis type obtained from hydrotalcite precursor, and rehydrated Mg(Al)O mixed oxides with Brønsted basic sites. The influence of several reaction parameters, temperature, acetaldehyde to heptanal molar ratio, nature of solvent (hexane, toluene, ethanol), has been investigated. A comparative study of the catalysts has been performed in the such defined optimal reaction conditions, i.e., 373 K, acetaldehyde/heptanal molar ratio, 2/1; and ethanol/reactants molar ratio, 5/1. Mg(Al)O mixed oxides calcined below 673 K are the most selective catalyst to 2-nonenal, the cross- condensation product formed when in the first step proton abstraction occurs from acetaldehyde. Acid–base pairs of moderate basic strength are suitable when this cross-condensation is the desired reaction. Stronger Lewis basic sites of MgO or Brønsted-type basic sites of the rehydrated mixed oxide tend to favor the formation of carbanion from heptanal. This latter leads to the formation of 2-pentyl-2-butenal and 2-pentyl-2-nonenal by cross-condensation with acetaldehyde and self-condensation, respectively.  2003 Elsevier Inc. All rights reserved. Keywords: Acetaldehyde; Heptanal; 2-Nonenal; Hydrotalcite; Aldol condensation 1. Introduction Reactions involving C–C bond formation are of utmost importance for obtaining many fine chemicals of commer- cial interest. The base-catalyzed aldol condensation, which belong to this type of reaction is indeed very useful for the preparation of higher molecular weight aldehydes and/or ke- tones from lower easily available homologs. The exciting possibility of carrying out these reactions through a het- erogeneous catalytic process can now be contemplated by taking advantage of the increasing number of available ma- terials with finely tunable basicities. This is particularly the case of the hydrotalcite-type catalysts which have recently attracted much attention for various base-catalyzed reactions in fine chemistry [1]. Hydrotalcite-like compounds of gen- eral formula [M II 1-x M III x (OH) 2 ] x + [A n- x/n ]· mH 2 O, also de- * Corresponding author. E-mail address: tichit@cit.enscm.fr (D. Tichit). 1 On leave from Al.I. Cuza University of Iasi, Faculty of Chemistry, 11 Bd. Carol I, Iasi, Romania. nominated layered double hydroxides or anionic clays, con- stitute a class of compounds with positively charged layers and exchangeable anions in the interlayer space [2–4]. The structure is similar to that of brucite Mg(OH) 2 , where Mg 2+ - centered octahedra are linked by the edges to form infinite sheets. In the natural mineral hydrotalcite, whose name has been extended to this family of materials, some Mg 2+ are isomorphously substituted for Al 3+ and the formal positive charge thus appearing in the hydroxyl layers is usually com- pensated by carbonates linked by hydrogen bonds to water molecules. Dehydroxylation takes place and volatile anions are decomposed upon calcination, leading to mixed oxides. These materials obtained from hydrotalcite are the catalysts most largely used until now, though interest in activated hydrotalcites obtained by rehydration of the mixed oxides is growing. Using hydrotalcite-like catalysts, attractive re- sults have been thus reported in: (i) the self-condensations of acetone [5–7], acetaldehyde [8], or butyraldehyde [9]; (ii) the condensation between aromatic aldehydes like ben- zaldehyde or substituted benzaldehydes and acetone [10], 0021-9517/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9517(03)00192-1