Preparation and Study of Decavanadate-Pillared Hydrotalcite-like Anionic Clays Containing Cobalt and Chromium M. del Arco, † M. V. G. Galiano, † V. Rives,* ,† R. Trujillano, † and P. Malet ‡ Departamento de Quı ´mica Inorga ´nica, Facultad de Farmacia, Universidad de Salamanca, Salamanca, Spain, and Departamento de Quı ´mica Inorga ´nica, Facultad de Quı ´mica, Instituto de Ciencia de Materiales, Centro Mixto CSIC, Universidad de Sevilla, Sevilla, Spain ReceiVed February 9, 1996 X Hydrotalcite-like compounds containing Co(II) and Cr(III) in the brucite-like layers have been prepared. The interlayer anion was carbonate or decavanadate. The chemical formulas of the samples are [Co 0.65 Cr 0.35 (OH) 2 ] (CO 3 ) 0.175 ‚1.22H 2 O (sample CoCrC) and [Co 0.61 Cr 0.39 (OH) 2 ](V 10 O 28 ) 0.065 ‚1.35H 2 O (sample CoCrV). The compounds have been characterized by X-ray diffraction, XAS, vis-UV, FT-IR, and Raman spectroscopies, while the surface textures were assessed by nitrogen adsorption. Reducibility has been studied by temperature- programmed reduction. A similar characterization study has been carried out on samples obtained after calcination of the parent samples in air at increasing temperatures. Results indicate an ordered structure, with Co(II) and Cr(III) ions in octahedral holes of the brucite-like layers and, in CoCrV, decavanadate species with its main C 2 axis parallel to the layers. Thermal decomposition at increasing temperatures takes place, for CoCrC, through intermediate formation of Cr(VI) species, which are again reduced to Cr(III) at higher temperatures; simultaneously, Co(II) is oxidized to Co(III) (even at 673 K), thus leading to formation of Co II Co III Cr III O 4 . However, decomposition of CoCrV takes place through partial depolymerization of decavanadate species and formation of Co II Cr 2 O 4 and Co II 2 V 2 O 7 , without intermediate formation of Cr(VI) species. Introduction Layered double hydroxides (LDH) are materials with an increasing interest because of the large number of fields where they find applications, such as pharmaceuticals, adsorbents, etc., as well as the use of their decomposition products as catalysts and catalyst supports, mainly in hydrogenation, aldol condensa- tion, re-forming, and also selective oxidation. 1 Probably, the unique advantage of hydrotalcites occurs because the different cations are well apart from each other, thus allowing formation of well-dispersed mixed oxides upon decomposition. Although they are commonly known as “hydrotalcite-like” compounds, they should be more properly named “anionic clays”, as hydroxyl is not (most usually) the only anion existing in the structure. Their formula can be written as [M 1-x M′ x (OH) 2 ] x+ (X m- ) x/m ‚nH 2 O, where M ) divalent cation, M′ ) trivalent cation, X ) anion, usually Cl - , OH - , CO 3 2- , and carboxylate, etc. The name comes from the mineral hydrotalcite, Mg 6 Al 2 (OH) 16 (CO 3 )‚4H 2 O, or [Mg 0.75 Al 0.25 (OH) 2 ] (CO 3 ) 0.125 ‚0.5H 2 O, according to the formulation method above given. Their structure consists of brucite-like layers positively charged after partial M(II)/M′(III) substitution, the excess in positive charge being balanced by interlayer anions; water molecules also are located in the interlayers. One of the most useful properties of hydrotalcites is the easy exchange of the interlayer anions, leading to a large number of different compounds. Exchange of carbonate, chloride, nitrate, etc., with polyoxometallates may lead to formation of pillared materials. One of the methods reported in the literature for exchanging the interlayer anion is based on the ability of LDH to recover their layered structure after a mild calcination (ca. 700 K, depending on the nature of the ions in the layers) when immersed in aqueous solutions containing the entering anion. 2 Bish 3 has described another method consisting of reaction of the LDH with dilute HCl; this reaction gives rise to decomposi- tion of carbonate (because of the lower pH) and introduction of chloride in the interlayers; chloride is more easily exchanged than carbonate, and then a further exchange may lead to the desired compound. This reaction can be summarized as follows: This method has been successfully used by Kwon et al. 4 and Woltermann 5 to introduce decavanadate, V 10 O 28 6- , in hydro- talcites containing Zn-Al, Zn-Cr, or Ni-Al in the brucite- like layers. Drezdzon 6 has reported that the procedure is even easier if the interlayer is previously expanded by introduction of organic molecules such as terephtalate. Ulibarri et al. 7 have recently reported a comparative study on the use of different methods to intercalate decavanadate into hydrotalcite (i.e., direct exchange, intermediate use of terphtalate, and reconstruction from the product calcined at 823 K). Preparation of mixed oxides containing two or three different cations is a very interesting field, as these sorts of materials find application in reactions involving catalytic partial and selective oxidation processes. The use of hydrotalcites seems rather interesting in this field, because, with use of different atomic ratios, the chemical composition of the layers can be changed; moreover, the introduction of interlayer polyoxomet- allates is then a method for introducing a third cation in the mixed oxide obtained after calcination of the resulting ex- changed hydrotalcite. * Author to whom correspondence should be addressed. † Universidad de Salamanca. ‡ Universidad de Sevilla. X Abstract published in AdVance ACS Abstracts, September 15, 1996. (1) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 1. (2) Reichle, W. T. Solid State Ionics 1986, 22, 135. (3) Bish, D. L. Bull. Mineral. 1980, 52, 1036. (4) Kwon, T.; Tsigdinos, A.; Pinnavaia, T. J. J. Am. Chem. Soc. 1988, 110, 3653. (5) Woltermann, G. M. U. S. Patent 4, 454, 244, 1988. (6) Drezdzon, M. A. Inorg. Chem. 1988, 27, 4628. (7) Ulibarri, M. A.; Labajos, F. M.; Rives, V.; Trujillano, R.; Kagunya,W.; Jones, W. Inorg. Chem. 1994, 33, 2592. [LDH]CO 3 + 2HCl f [LDH]Cl 2 + CO 2 + H 2 O 6362 Inorg. Chem. 1996, 35, 6362-6372 S0020-1669(96)00155-3 CCC: $12.00 © 1996 American Chemical Society