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 the twelve Na+ ions per unit cell can be accommodated in the six-rings, three in the eight-rings and the remaining ion outside the four-ring, which is much too small to allow entry of an Na+ ion. The location of the Na+ ions in these sites is shown in the stereo pair of Figure 3.

Of the three non-equivalent sites only the six-ring is able to stabilize bi- and tervalent ions, and it is the uniqueness of this site which makes A-type frameworks such attractive hosts for model studies of surface complexes.

The structure of X and Y types is also well understood and five non-equivalent cation sites have been identified. The nomenclature suggested by Smith will be used in identifying these sites, which are shown in Figure 4a and b. There are 16 hexagonal prisms (SI) per unit cell of X or Y, 32 six-rings linking the hexagonal prisms sodalite units (SI'), and 16 six-rings opening into the large cavity (SII, SII', and SII*) so that there will always be more six-ring sites than there are multivalent ions to fill them and frequently more SI sites also. The actual occupancy of the identifiable sites is still a matter of some controversy. Again, the location of transition-metal ions and their complexes with guest molecules within the X and Y framework will be discussed on a case-by-case basis. It may be noted, however, that the attractive feature of a large pore size in zeolites X and Y (the windows linking the large cavities are 0.9 nm compared with the 0.42 nm of A type) is to a considerable extent mitigated by the existence of several sites which might stabilize transition-metal ions in the anydrous material.

Surface Complexes. — A sample of zeolite A containing a single exchanged transition-metal ion per large cavity (i.e. per formula weight) contains, on a volume basis, more or less the same concentration of that ion as would a one molar solution, and indeed cobalt(II) ion-exchanged zeolites, for instance, are visibly coloured. The high concentration of ions of interest greatly facilitates their study and allows the use of standard chemical techniques such as optical spectroscopy. On the other hand, the mean separation of ions at a level of one per unit cell (of A or X–Y types) is of the order of 1 nm so that electronic interaction is negligible. These two features of zeolitic transition-metal ions are attractive, but even more importance attaches to the fact that the negatively charged, non-reactive framework substitutes for the solvents and/or ligands which must normally stabilize such ions. This feature of zeolitic transition-metal ion chemistry (the complete absence of solvent effects and interactions) assumes further importance when it is remembered that the porous framework of type A can admit molecules as large as imidazole while types X and Y can accommodate molecules with at least one dimension having a maximum length of 0.8 nm. Such molecules, upon entry into an anhydrous zeolite containing a transition-metal ion, are free to complex or otherwise interact with that ion and do so without competition from solvent molecules. For this reason complexes having, by normal standards, very low enthalpies of formation (ca. 60 kJ mol-1 are stable and can be characterized intrazeolitically. It is exactly such complexes which must be involved in both heterogeneous and homogeneous catalysis where a prerequisite to catalytic action is that reactants be bound strongly enough to perturb their electronic structure sufficiently to promote reaction, and not so strongly that the excited- or ground-state product molecule cannot desorb. An additional and important feature of intrazeolitic transition-metal ion complexes is their 'co-ordinative unsaturation'. Co-ordinative unsaturation, the absence of one or more ligands from the coordination sphere of an ion, is another prerequisite to catalytic activity. The oxygen-six ring sites (in either zeolite A or zeolites X and Y) which bind bi- or ter-valent ions in the anhydrous materials produce, to a first approximation, trigonal-planar complexes which are highly co-ordinatively unsaturated.

In the discussion of intrazeolitic complexes below three distinct cases will be considered: 'bare' ions, complexed to or stabilized by only zeolitic framework oxyn atoms, 'wall' complexes in which an ion is bound in part to the framework and in part to a 'guest' molecule, and 'floating' complexes in which a sufficient concentration of a sufficiently good ligand exists within the zeolite to complex the exchanged ion and completely remove it from the wall.

2 Experimental Techniques

Techniques which have been used to study transition-metal ions and complexes within zeolites include optical, electron paramagnetic resonance, Mossbauer effect, nuclear magnetic resonance, and infrared spectroscopies, X-ray crystallography, magnetic susceptibility measurements, and adsorption studies. As is the case in non-zeolitic transition-metal chemistry, each of these methods contributes to our understanding of chemical behaviour and each has limitations.

Optical Spectroscopy. — Because zeolites consist of particles in the 1 —10 µm size range and, accordingly, strongly scatter visible light, their optical spectra are conveniently recorded by diffuse reflectance spectroscopy (d.r.s.). The theory and practice of d.r.s. have been discussed in detail by Kortum, and its application to the study of solid surfaces has been reviewed by one of the present authors, who also described a suitable sample container for d.r.s. measurements (Figure 8 of ref. 23). Sample cells of the same geometry with an 'lnfrasil 1' window fused directly to a quartz body can be made without difficulty, allowing wide variations in sample temperature.

In a d.r.s. experiment, what is actually measured is the ratio of the reflectivity of the sample to that of a 'white' standard, the choice of which has been discussed elsewhere. For the energy range accessible to Cary 14 or 17 series spectrophotometers (4800 — 40 000 cm-1), an anhydrous sodium zeolite is a convenient standard. At energies < 33 000 cm-1 anhydrous MgO may also be used, but at > 33 000 cm-1 the hydrated form must be used in order to avoid absorption due to defects produced during the dehydration of MgO. It may be noted that the lower energy region of the Cary 14 — 17 range will include i.r. overtone and combination band information in the event that a hydrogen-containing ligand is present in the sample.

The reflection ratio, R∞ = sample reflectance/standard reflectance, can, through a simple transformation, be linked to the true absorption coefficient k by:

[MATHEMATICAL EXPRESSION OMITTED] (1)

F(R∞) is the Schuster–Kubeika–Munk function and represents, but for a multiplicative constant σ(the scattering coefficient), the true absorption coefficient (k) of the sample. It is convenient to display spectra as the logarithm of F(R∞) to allow immediate comparison with the logarithm of absorbance obtained by transmission spectroscopy from which log F(R∞) differs by only an additive constant.

The colours and associated absorption maxima characteristic of the local environments and oxidation states of transition-metal ions have been the subject of intensive theoretical study which, for complexes having high symmetry (Oh or Td), has given rise to a powerful general theory able to identify absorption maxima with specific electronic transitions and, through parameterization to measured spectral energies, to give quantum mechanical wavefunctions for ground and excited electronic states. It is this latter quality which makes ligand field theory of particular utility in investigating transition-metal ion complexes, for once a set of ground- and excited-state wavefunctions has been obtained, it can be used to predict and explain other observables of the system, either in its ground or excited states.

Partially because application of ligand field theory to ions of low symmetry is more tedious than its application to highly symmetric Oh or Td complexes, and partially because the vast majority of transition-metal complexes belong to the Oh or Td point groups or slight distortions thereof, relatively little effort has been made to offer theoretical interpretation of the optical spectra of low-symmetry complexes. As is already apparent from the discussion of zeolite frameworks above, however, highly symmetric environments for transition-metal ions are not to be anticipated in anhydrous molecular sieves where, with the exception of the hexagonal prisms in types X and Y, all available sites have at best D3h symmetry and, in the event either that the ion does not lie in the plane of its ligands, or that different charge distributions associated with the large and small cavities are taken into account, more probably C3v symmetry. Accordingly, a general model for zeolitic transition-metal ion complexes and an outline of the ligand field theoretical calculations based on this model are now given.

As discussed above, the dominant site available for cation stabilization in anhydrous A, X, or Y zeolites is an oxygen-six ring. This ring, though six-membered, more effectively stabilizes even univalent ions by distorting so as to produce, with respect to the cation, three proximal and three distal oxygen atoms (see Figure 3). The same distortion is anticipated for multiply-charged ions and accordingly the model shown in Figure 5 includes only three basal or equatorial ligands L1, L2, L3, which represent the three proximal oxygens of the six-ring. Allowance is made for the possibility of non-planarity of the complex through the parameter β and for the presence of an axial ligand which might for instance be a zeolitic guest molecule, by L4. Two special cases arise for β = 90°, L1 = L2 = L3, L4 = 0 (D3h) and β = 109.48° L1 = L2 = L3 = L4 (Td). All other values of β and L4 give rise to the point group C3v, provided of course that L1 = L2 = L3.

The ligand field potential in a C3v complex is given, following Griffith, by

[MATHEMATICAL EXPRESSION OMITTED] (2)

where

[MATHEMATICAL EXPRESSION OMITTED]

and

[MATHEMATICAL EXPRESSION OMITTED]

The superscripts A, B in the γ's refer to axial or basal ligands, respectively. Vc3v is the potential felt by an electron at r [equivalent] (r,θ,φ) and ρ(R) is the charge density of the ligands at R [equivalent] (R,Θ,Φ). The Yim and Yim are the spherical harmonics and their complex conjugates.