Intrazeolitic Transition-metal Ion Complexes

BY R. KELLERMAN AND K. KLIER

1 Introduction

Zeolitic science is sufficiently well established to have formed the subject for three tri-annual international conferences and a comprehensive book, and sufficiently topical to give rise currently to several hundred publications each year. Against this large background the present review will consider only a single important property of zeolites: their ability to stabilize transition-metal ions in unusual chemical environments.

Transition-metal ions introduced by ion exchange into zeolites may, and frequently do, exhibit physical and chemical properties not found in their homogeneous analogues, even if such analogues exist. These properties are of both scientific interest and technological importance, and consequently continue to be extensively investigated using most of the techniques of modern experimental science. A brief discussion of these techniques as they apply to zeolite research and of the results and understanding obtained through their use will be the aim of this review, and although data from various probes will be discussed, emphasis will be placed upon the collection and interpretation of optical spectroscopic data. Properties of the transition-metal intrazeolitic complexes per se will be emphasized. The periodic table serves as a framework for organizational purposes and accordingly, following a consideration of zeolite frameworks and ion exchange, and the collection and interpretation of data on intrazeolitic transition-metal ion complexes, the 3d block of elements is discussed sequentially.

Zeolite Frameworks. — Although many different zeolite frameworks have been characterized (Meier and Olson give stereo-pairs of 27 well-established structures) attention in this review will be restricted to the synthetic zeolites X, Y, and A. The X and Y types are, because of their large pore size, of great catalytic importance, and the A type, as will be emphasized, is in its simplicity a very attractive host or matrix for the study of well-defined surface complexes. Each of the three is available in high purity, each has a well-understood structure, and each has been thoroughly studied.

The structures of all three are based on the sodalite unit, a truncated octahedral structure consisting of 24 silicon- or aluminium-centred oxygen tetrahedra which are linked through shared oxygen atoms (Figure la). By joining sodalite units through their square (square with respect to oxygen atoms) faces, an f.c.c. type cubic lattice is built up in which truncated cubo-octahedral voids of ca. 1.1 nm diameter are linked via ca. 0.42 nm channels, as shown in Figure I b where a stereoscopic pair of the A-type framework is given.

The sodalite units, joined octahedrally through their square faces to yield the A-type structure, can be joined tetrahedrally through their hexagonal faces to give the isotypes X or Y, as is shown in stereo pair form in Figure 2. Again, a three-dimensional network of large (1.3 nm) voids linked by somewhat smaller (0.9 nm) channels is the dominant characteristic of the structure.

The structures of zeolites A, X, and Y and the effects of water of hydration on those structures have been discussed by several authors in considerable detail.

Ion Exchange. — The partial substitution of aluminium for silicon in the tetrahedral oxygen units which make up zeolite frameworks results in a net negative charge on the framework, which, for zeolites as they are normally prepared and available, is balanced by sodium ions. It is these sodium ions which may be exchanged, under favourable circumstances, for an equivalent number of multiply-charged ions. Ions so exchanged are an integral part of the zeolitic 'macromolecule' and as such differ in their properties from ions which are merely adsorbed, along with their counter-ions, on other high surface area solids. Though ion exchange can take place from non-aqueous media, including liquid ammonia, fused salts, and ethanol, many of the transition-metal ions can be introduced into zeolites A, X, and Y from aqueous solutions. Unfortunately, no systematic study of the ion-exchange equilibria and kinetics for either A, X, or Y types with respect to transition-metal ions has been reported, though Sherry has reviewed developments occurring between the first and second International Conferences, and Breck devotes an entire chapter to the subject of cation exchange.

In the absence of published information for ion-exchange behaviour of all of the transition-metal ions, it is important to observe several precautions to ensure that 'true' ion exchange occurs. These include thorough washing of the zeolite prior to use to remove any occluded sodium hydroxide, careful control of pH to prevent either hydrolysis of the transition-metal ion and possible formation of intracavity hydroxy-oxides or loss of zeolite crystallinity due to dissolution of aluminium, and extensive post-exchange washing to remove any occluded salt molecules. Careful chemical analysis can ensure that the ion exchange was successful, and either X-ray diffraction or low-temperature adsorption of argon or nitrogen can be used to confirm that the structural integrity of the zeolite was maintained.

Sites for Transition-metal Ions in Anhydrous Zeolites. — Though the location of transition-metal ions exchanged into A, X, and Y zeolites is to some extent the subject of this review it is useful to anticipate somewhat and to make use of structural data available for sodium, alkaline earth and other non-transition-metal ion exchanged materials in their anhydrous state.

The structure of sodium A (NaA1has been the subject of several investigations and has recently been re-examined by single-crystal X-ray crystallographic techniques. 6 The structure of Tl(I)11A has also been established by single-crystal techniques. 20 An important outcome of these studies is the accurate location of the charge-balancing cations in the anhydrous zeolite. Progressive dehydration of the porous zeolite eventually leaves a negatively charged framework and 'bare' sodium ions which must be accommodated by this framework. In A-type zeolite there are three non-equivalent sites at which Na+ ions are localized: the eight oxygen-six rings which open into the eight sodalite units, one eighth of each of which make up the A-type unit cell (see Figure 1b), the six oxygen-eight rings which make up the six faces of the cubic unit cell, and the twelve oxygen-four rings which link the sodalite units together and which open into the large cavity. Eight of...