Hydrotalcite-Type Anionic Clays: Preparation, Properties And
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coldysis T&y, I I ( I99 I ) 173-301 173 Elsevier Science Publishers B.V., Amsterdam HYDROTALCITE-TYPE ANlONlC CLAYS: PREPARATION, PROPERTIES AND APPLICATIONS. F. Cavani, F. Trifirb, A.Vaccari Dipartimento di Chimica Industriale e dei Materiali Wale de1 Risorgimento 4,40136 BOLOGNA (Italy). 1. HISTORICAL BACKGROUND Research into hydrotalcite-like compounds and catalysis followed separate parallel paths up to the year 1970, when the first patent appeared that referred specifically to a hydrotalcite-like strttctum as an optimal precursor for the preparation of hydrogenation catalysts (ref. 1). Hydrotalcite (a mineral that can be easily crushed into a white powder similar to talc, discovered in Sweden around 1842) is a hydroxycarbonate of magnesium and aluminium and occurs in nature in foliated and contorted plates and/or fibrous masses. At the same time that hydrotalcite was discovered, another mixed hydroxycarbonate of magnesium and iron was found, which was called pyroaurite (because of a likeness to gold when heated) and which w,as later recognized to be isostructural with hydrotalcite and other minerals containing different elements, all of which were recognized as having similar features. The fast exact formula for hydrotalcite, [Mg&l2(OH)16CO3.4H20], and of the other isomorphous minerals was presented by E. Manasse, professor of Mineralogy at the University of Florence (Italy), who was also the first to recognize that carbonate ions were essential for this type of structure (ref. 2). The opinion current at that time, which persisted for many years, was that such minerals were mixed hydroxides. On the basis of X-ray investigations, Aminoff and Broome (ref. 3) recognized the existence of two polytypes of hydrotalcite, the first one having rombohedral symmetry and the second having hexagonal symmetry, which was called manasseite in honour of Manasse. It was necessary to wait for Frondel’s paper published in 1941 (ref. 4), entitled ” Constitution and Polymorphism of the Pyroaurite and Sjisgrenite Groups” before the interrelations between the several minerals and their real constitutions were generally recognized. The confusion and the uncertainty were due to the lack of adequate crystallographic data, which, in turn, was a result of the complex and unusual composition of these minerals as well as of the fact that the papers by Manasse and Aminoff and Broome went unnoticed. In 1942 Feitknecht (mfs. 5,6) synthesized a large number of compounds with a hydrotalcite-like structure, to which he gave the name “doppelschichtstrukturen” (double sheet structures), assigning then the following structure: r 4 M&OH)2 Al(OH13 k._. The Feitknecht’s idea was that the compounds synthesized compounds were constituted by a 092~5861/91/$45.15 0 1991 Elsevier Science Publishers B.V. All rights reserved. 174 layer of hydroxide of one cation, intercalated with a layer of the second one. This hypothesis was definitively refuted by Alhnann (ref. 7) and Taylor (ref. 8) by means of the X-ray analysis of monocrystals. In fact, they concluded that the two cations are localized in the same layer and only the carbonate ions and the water am located in an interlayer. Thus, considerable time passed from the discovery of hydmtalcite to the publication of its structure, due to its non-stoichiometric nature and to the unavailability of sufficiently large crystals for X-ray analysis. In fact, the earlier works of Alhnann and Taylor dealt with the minerals sjbgmnite and pyroaurite (monocrystals of which were available), hydrotalcite being studied only later. In this review what we shall call a hydrotalcite-like compound corresponds to the hydroxycarbonate of the sjirgrenite and pyroaurite groups; these compounds are also referred to as Feitknecht’s compounds or mixed hydroxides in many papers. In the opinion of the authors of the present review, the reason why hydrotalcite is used as reference name in many applications of these compounds may be related to the fact that extensive physico-chemical characterization has been carried out on hydrotalcite by many authors, rather than on the other similar structures; the hydrotalcite is simple and relatively inexpensive to prepare in the laboratory (refs. 9-23). The parallel work on catalysis began with the work of Zelinski and Kommamwsky, published in 1924 (ref. 24), who recognized that coprecipitated Ni,Al catalysts presented good activity in hydrogenation reactions and, some years later, with the work of Molstad and Dodge (ref. 25) on the preparation of Zn,Cr mixed oxides for the synthesis of methanol. It has been recognized that coprecipitation is one of the most reliable and reproducible techniques for the preparation of non-noble metal-based catalysts. This technique allows homogeneous precursors to be used as starting materials, where two or more elements are intimately mixed together, and synergic effects am favoured. The papers on Ni,Al based catalysts by Milligan and Richardson (ref. 26). Langebeck (ref. 27), Dent et al. (ref. 28), Merlin et al. (ref. 29) and Rubinshtein et al. (ref. 30) am worth mention and it also is useful to mention a patent (ref. 31), which dealt with the same catalysts prepared by precipitation, in which a strong indication was given of the formation of a compound during the coprecipitation stage, having a composition later recognized as being an optimal one for the prepartion of hydmtalcite-like compounds. The thermal stability and the activity of the catalyst also had to be attributed to the nature of the precursor. The time was ripe for the recognition of the fact that the precipitate was a compound which was structurally similar to hydrotalcite: in fact, in 1970 the fast patent appeared in which it was claimed that the hydrotalcite-like compounds obtained by precipitation may be very good precursors for hydrogenation catalysts (ref. 1). The first papers in the open literature referring to hydmtalcite-like compounds appeared in 1971, written by Miyata et al., dealing with basic catalysts (ref. ll), in 1975 by B&her and Kaempfer (ref. 32), dealing with hydrogenation catalysts (even though it is worth noting that in this paper reference is made to a manasseite-like compound, the polytype form of hydrotalcite, which exists only in the natural form) and in 1977 by Miyata (ref. 33). This review begins at the time when the knowledge of hydrotalcite-like compounds was 175 introduced to the people working in catalysis. 2. INTRODUCTION Anionic clays, natural and synthetic layered mixed hydroxides containing exchangeable anions, are less well known and diffuse in nature than cationic clays. Hydrotalcite belongs to the large class of anionic clays, and will be taken as a reference name for many other isomorphous and polytype compounds. The anionic clays based on hydmtalcite-like compounds have found many practical applications (see Fig. 1). The hydrotalcites have been used as such or (mainly) after calcination. The most interesting properties of the oxides obtained by calcination are the following: 1) High surface area. 2) Basic properties. 3) Formation of homogeneous mixtures of oxides with very small crystal size, stable to thermal treatments, which by reduction form small and thermally stable metal crystallites. 4) “Memory effect”, which allows the reconstruction. under mild conditions, of the original hydrotalcite structure when contacting the product of the thermal treatment with water solutions containing various anions. Fig. 1. Schematic picture of the possible applications of hydrotalcite-like compounds. Properties 1, 2 and 3 have found application in the field of heterogeneous catalysis (hydrogenation, reforming, basic catalysts and as support). Properties 1, 2 and 4 are utilized in applications such as the scavenging of chlorine ions and the purification of water containing waste anions (organic and inorganic). The papers and patents dealing with hydrotalcite-like compounds are not only interesting for their industrial applications, but are also beautiful examples of the scientific preparation of catalysts. Ah the stages of the preparation of a catalyst based on a hydrotalcite-like precursor (i.e. choice of the optimal composition, nature and amount of promoters, precipitation conditions, type of reagents, aging, washing and, possibly, hydrothermal treatments, drying, calcination and activation) need 176 precise chemical foundations in order to avoid inhomogeneities and/or chemical segregations, which would be detrimental to the properties of the final compounds. The high quality of the work done by many scientists has made it a great pleasure as well as scientifically stimulating to work on this review. NOMENCLATURE The following nomenclature will be used in the remainder of this paper: HT= Hydrotalcite.= MgeAl2(OH)i6Co3.4H20 HTlc= Hydrotalcite-like compound= M(II)M(IlI)A-HT= [M(II)I-xM(III)x(OH~]x+(An-x/n).mH~O, where: A= anion. 3. STRUCTURAL PROPERTIES 3.1 The structure of hydrotalcite. The most detailed structural investigations (when monoctystals were available) on HTlcs were carried out by Allmann (refs. 7,34) and by Ingram and Taylor (ref. 35) on [email protected] and pyroaurite with the approximate composition Mg6F~(OH)t&03.H20, and later by Allmann (ref. 36) on hydrotalcite; the papers were reviewed by Allmann (ref. 37) and by Taylor (ref. 14). To understand the structure of these compounds it is necessary to start from the structure of brucite, Mg(OH)z, where octahedra of Mg2’ (6-fold coordinated to OH-) share edges to form infinite sheets. These. sheets are stacked on top of each other and are held together by hydrogen bonding (see Fig. 2a). When Mg2+ ions are substituted by a trivalent ion having not too different a radius (such as Fe3+ for pyroaurite and A13’ for hydmtalcite, respectively), a positive charge is generated in the hydroxyl sheet. This net positive charge is compensated for by (COS)~- anions, which lie in the interlayer region between the two brucite-like sheets (see Fig. 2b). In the free space of this interlayer the water of crystallization also finds a place (Fig.