Applied Catalysis A: General 272 (2004) 229–240 Hydrotalcites as precursors for Mg,Al-mixed oxides used as catalysts on the aldol condensation of citral with acetone C. Noda Pérez a,1 , C.A. Pérez a , C.A. Henriques b , J.L.F. Monteiro a,∗ a Núcleo de Catálise (NUCAT), COPPE/UFRJ, Caixa Postal 68502, CEP 21945-970, Rio de Janeiro, RJ, Brazil b Instituto de Qu´ ımica, UERJ, Rua São Francisco Xavier, 524, CEP 20559-900, Rio de Janeiro, RJ, Brazil Received 9 March 2004; received in revised form 25 May 2004; accepted 29 May 2004 Available online 24 July 2004 Abstract The use of Mg,Al-mixed oxides derived from hydrotalcites as basic catalysts for the citral/acetone aldol condensation was studied. Three series of hydrotalcite samples with different Al/(Al + Mg) molar ratios (0.2, 0.25, and 0.33), aged at different pH (10 or 13) and temperatures (333 and 473 K), were prepared as precursors. The corresponding Mg,Al-oxides were obtained by calcining the hydrotalcites at 723 K. They were characterized by N 2 physisorption, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Their relative basicity was evaluated by using diacetone alcohol (DAA) retroaldolization as a model reaction. The Mg,Al-mixed oxides were efficient catalysts for the aldol condensation of citral with acetone in heterogeneous media. The best catalytic activities and pseudoionone yields were obtained from the samples with Al/(Al + Mg) molar ratio equal to 0.2 derived from hydrotalcites aged at 333 K. On the other hand, neither the aluminum content nor the pH of the synthesis gel influenced significantly the reaction selectivity. For mixed oxides derived from hydrotalcites aged at pH = 10 citral conversion decreased with increasing aluminum content, whereas for those obtained from precursors aged at pH = 13 citral conversion passed through a minimum for the sample with Al/(Al + Mg) = 0.25. Both trends reflect the mesoporous characteristics of the samples and their relative basicity as measured by retroaldolization of DAA. Since the influence of aluminum content on basic properties could not be clearly established, the textural characteristics best explained the observed trends. Catalysts derived from hydrotalcites aged at 473K presented low catalytic activities, which was attributed to their lower mesoporosity. © 2004 Elsevier B.V. All rights reserved. Keywords: Citral; Pseudoionones; Basic catalysts; Hydrotalcite; Mg,Al-mixed oxides; Aldol condensation 1. Introduction Hydrotalcite-like compounds (HTLCs) or layered double hydroxides (LDHs) are anionic clays with general formula: (M 2+ 1-x M 3+ x (OH) 2 ) x+ (A x/m ) m- · nH 2 O where the divalent ion can be Mg 2+ , Ca 2+ , Zn 2+ , Cu 2+ , Co 2+ or Ni 2+ , the trivalent ion Al 3+ , Fe 3+ or Cr 3+ , and the compensation anion OH - , Cl - , NO 3 - , CO 3 2- or SO 4 2- . These materials have a structure similar to that of brucite (Mg(OH) 2 ), where each Mg 2+ ion is octahedrally sur- ∗ Corresponding author. Fax: +55 21 25626300. E-mail address: monteiro@peq.coppe.ufrj.br (J.L.F. Monteiro). 1 Present address: Universidade Estadual de Goi´ as, An´ apolis, GO, Brazil. rounded by six OH - ions and the different octahedra share edges to form infinite sheets. The sheets are stacked one on top of the other and are held together by weak interactions through hydrogen bonds. When M 2+ cations are replaced isomorphously by M 3+ cations with similar radius, the brucite-like layers become positively charged and the elec- trical neutrality is attained by compensating anions located in the interlayers along with water molecules [1–5]. The formation of a pure HTLC phase is favored by a Mg/Al molar ratio in the synthesis gel ranging between 4 and 2 (0.2 ≤ x ≤ 0.33, where x = Al/(Al + Mg)), al- though several authors claim that HTLCs with 0.1 ≤ x ≤ 0.5 can be prepared [1,2]. For higher x values, the increase in the number of neighboring Al-containing octahedra leads to the formation of Al(OH) 3 , whereas low values of x leads to the segregation of Mg(OH) 2 due to the high density of Mg-containing octahedra in the brucite-like sheets [6]. 0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.05.045