Synthesis and characterization of hydrotalcites by mechanical milling and conventional method Jose ´ Salmones a, * , Beatriz Zeifert b, * , Miguel Hesiquio Gardun ˜o c , Jose ´ Contreras-Larios d , Dwigh R. Acosta e , Antonio Romero Serrano b , L.A. Garcia d a Lab. Cata ´lisis y Materiales, ESIQIE-Instituto Polite ´cnico Nacional Zacatenco, Me ´xico 07738, D.F., Mexico b Instituto Polite ´cnico Nacional, ESIQIE-UPALM Zacatenco, Me ´xico 07738, D.F., Mexico c Instituto Polite ´cnico Nacional, ESFM-UPALM Zacatenco, Me ´xico 07738, D.F., Mexico d Universidad Auto ´noma Metropolitana-Azcapotzalco, Departamento de Energı ´a, Me ´xico, D.F., Mexico e UNAM, Instituto de Fı ´sica, A.P. 20-364, Me ´xico 01000, D.F., Mexico Available online 1 February 2008 Abstract Interest in the use of mixed oxides of Mg and Al has increased notably. These solids are used as catalysts in reactions of basic character and as catalysts supports, due to their structural properties. Mesoporous hydrotalcites, with the formula Mg 6 Al 2 (OH) 16 CO 3 4H 2 O, have been synthesized by two techniques: first, an innovative one, the mechanical milling (MM) and second, the conventional coprecipitation method (CM). The synthesis by CM was prepared from Mg(NO 3 ) 2 6H 2 O and Al(NO 3 ) 3 9H 2 O over a solution of NaOH and Na 2 CO 3 , with a Mg/Al molar ratio of 3. The MM technique was employed over the hydrotalcites in an aqueous media previously prepared by CM. In this case, the milling retained the pillared lamellar structure and resulted in the formation of hydrotalcites with refined thickness layers and higher specific areas, compared with those that were prepared by CM. The materials obtained by both methods were calcined at 500 8C and analyzed. # 2008 Elsevier B.V. All rights reserved. Keywords: Hydrotalcite-like compounds; Layered double hydroxides; Mechanical milling 1. Introduction Hydrotalcites (HT) constitutes a class of extremely useful materials in the field of catalysis [1–3]. They are synthetic or natural crystalline materials consisting of positively charged two-dimensional sheets with water and exchangeable charge- compensating anions in the interlayer region. The nature of both the layer cations and the interlayer anions can be changed and when it occurs, the compounds are known as hydrotalcite-like compounds (HTlcs). Specific metals can be incorporated either as a cation in the layer or as an anion by exchange. They have the general molecular formula ½M 1x 2þ M x 3þ ðOHÞ 2  xþ ðA x=n n Þ mH 2 O, where M 2+ and M 3+ are divalent and trivalent metal cations in the brucite-type layers respectively, A n is the interlayer charge-compensating n-valent anion, x is the molar ratio of M III /(M II +M III ) and can take values from 0.1 to 0.5, and m is the water of crystallization [4,5]. Because of their structure, HTlcs are also known as layered double hydroxides. The mineral hydrotalcite itself is a magnesium-aluminum hydroxycarbonate and the family of HTlcs comprises many isostructural and polytype forms. HTlcs are mainly applied because of their base properties, or as redox catalysts. HTlcs in the uncalcined form can be used as catalyst supports or after different pretreatments, and they can be successfully applied in a broad spectrum of organic reactions [6]. Its thermal decomposition leads to stable non-stoichio- metric mixed metal oxides. A particular advantage for base catalysis is that the number and strength of the basic sites can be tuned precisely to a specific reaction. HTlcs are excellent materials to design bifunctional redox-base catalysts or to control the acid–base properties around a heterogeneous metal complex. Whereas for the hydrated material, the active base sites are mainly structural hydroxyl anions, strong Lewis basic O 2 M n+ pairs are present in completely water-free calcined materials. www.elsevier.com/locate/cattod Available online at www.sciencedirect.com Catalysis Today 133–135 (2008) 886–890 * Corresponding authors. E-mail addresses: jose_salmones@yahoo.com.mx (J. Salmones), bzeifert@yahoo.com (B. Zeifert). 0920-5861/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2007.12.072