Inorganic Ion Exchange Materials

Classification There are countless inorganic substances for which ion exchange properties have been claimed. Unfortunately a large number of these reports lack essential details of a reproducible synthesis, proper characterization and checks for reversibility. It is clear that many of the materials are amorphous and are often obtainable only as fine particles unsuited for column use. These pitfalls notwithstanding, there are many instances when inorganic exchangers are highly crystalline, well-characterized compounds, as well as instances when they can be made in a form appropriate for column use (even when amorphous). It also needs to be said that even a poorly defined ion exchanger may still be invaluable to scavenge toxic moieties from aqueous environments. This circumstance is valid in the treatment of aqueous nuclear waste and often drives the less rigorous studies mentioned earlier.

The traditional classification of inorganic ion exchange materials is:

• hydrous oxides

• acidic salts of polyvalent metals

• salts of heteropolyacids

• insoluble ferrocyanides

• aluminosilicates.

A more modern overview tends to blur some of these classes, but they still serve their purpose here with an addendum for the more recent materials of interest.

Hydrous oxides The compounds described in this section are 'oxides' precipitated from water. They retain OH groups on their surfaces and usually have loosely bound water molecules held in their structures. They can function either as anion exchangers, via replaceable OH~ groups, or as cation exchangers, when the OH groups ionize to release H + (H3O + ) ions. The tendency to ionize depends on the basicity

Large aggregates (nm) (20-50 nm)

Figure 7 Schematic representation of the pores present in a macroporous resin. (Reproduced from Dyer et a/., 1997, with permission.)

Large aggregates (nm) (20-50 nm)

Figure 7 Schematic representation of the pores present in a macroporous resin. (Reproduced from Dyer et a/., 1997, with permission.)

of the metal atom attached to the OH group, and the strength of the metal-oxide bond relative to the O-H bond. Some materials are able to function as both anion and cation exchangers, depending upon solution pH, i.e. they are amphoteric. Capacities lie in the range 0.3-4.0 meq g-1.

Hydrous oxides of the divalent metals Be, Mg, Zn have exchange properties, usually anionic, often in combination with similar materials derived from trivalent metals.

The most well-known trivalent hydrous oxides are those of iron and aluminium. Both produce more than one hydrous oxide. Examples of the iron oxides are the amorphous substances a-FeOOH (goethite), ft-FeOOH, and y-FeOOH (lepidicrocite). The similar compounds which can be prepared from aluminium are complex, and have been thoroughly researched because of their use as catalyst support materials and chromatographic substrates. Those that exhibit exchange are a-Al2O3, a-Al OOH and a-Al(OH)3. Certain of the Fe and Al oxides are amphoteric; Figure 8 demonstrates this via a pH titration. This is a common method of study for inorganic exchangers of this type, as well as those in the other classes which contain exchangeable protons. Other trivalent oxides with exchange properties are known for gallium, indium, manganese, chromium, bismuth, antimony and lanthanum.

Amphoteric exchange is known in the hydrous oxides of the tervalent ions of manganese, silica, tin, titanium, thorium and zirconium. Silica gel is particularly well studied because of its use as a chromato-graphic medium. It has weak cation exchange capacity (1.5 meq g_1 K + at pH 10.2) and can function as a weak anion exchanger at pH&3. Zirconia and titania phases also have been the subject of much interest, particularly for nuclear waste treatment, and manganese dioxide is unique in its high capacity for strontium isotopes.

Hydrous oxides of elements of higher valency are known but only one has merited much study namely, antimony oxide (also called antimonic acid and hydrated antimony pentoxide, or HAP). This exists in crystalline, amorphous and glassy forms and is an example of a material that is amenable to a reproducible synthesis. It can also be well characterized by, for example, X-ray diffraction and infrared spectros-copy. Many proposed applications have been suggested, especially based on the separations of metals that can be carried out on crystalline and other forms. An example of this is the ability of the crystalline phase selectively to take up the alkaline metals from nitric acid solution where the selectivity sequence is Na > Rb > Cs > K»Li. HAP has the sequences Na > Rb = K > Cs in nitric acid, Na > Rb >

Figure 8 Titration curve for the titration of a commercial alumina with 0.02 mol L"1: O, LiOH; •, KOH; A, HCl; ▲, HNO3. (Reproduced from Clearfield, 1982, with permission.)

Cs > K in hydrochloric acid. This unique ability to selectively take up sodium finds wide use in neutron activation analysis where the presence of sodium isotopes is a constant hindrance to the y-spectroscopy vital to the sensitivity of the technique. This is particularly important in environmental and clinical assays.

Acidic salts of polyvalent metals

Amorphous compounds The recognition that phosphates and arsenates of such metals as zirconium and titanium have ion exchange capabilities can be traced back to the 1950s. Around that time studies into the possible benefits of inorganic materials as scavengers of radioisotopes from aqueous nuclear waste were being initiated and amorphous zirconium phosphate gels were developed for that purpose, and used on a plant scale.

Later similar products of thorium, cerium, and uranium were studied, and also the analogous tungstates, molybdates, antimonates, vanadates and silicates. These compounds turn out to be of limited interest, and value because of the inherent difficulties in their sound characterization. In addition they often have a liability to hydrolyse, and these difficulties prompted the search for more crystalline phases of related compounds.

Polyvalent metal salts with enhanced crystal-linity The most success in producing crystalline, reproducible and characterizable compounds has been in the layered phosphates exemplified by those of zirconium and, to a lesser extent, titanium.

Zirconium phosphates Extensive refluxing of zirconium phosphate gel in phosphoric acid, or direct precipitation from HF, yields a layered material

Figure 9 Schematic diagram of adjacent layers of a-ZrP; inter-layer water and protons not included. (Reproduced from Williams and Hudson, 1987, with permission.)

A more hydrated phase of zirconium phosphate has been prepared with the formula Zr(HPO4)2 • 2H2O, designated y-ZrP. It has a different arrangement of layers, with the phosphate groups being sited above each other rather than being staggered as in the a-phase (Figure 9). y-ZrP also exchanges protons in two stages, creating half-exchanged materials; the replacement of the first proton takes place at about pH 2-3, and the second above pH 7.

Alkali metal ion selectivities are in the following order at low pH: K > Rb > Na > Cs > Li, but at high pH this becomes Li > Na > K > Rb. Recently a novel y-phase containing mixed zirconium phosphate/phosphite layers has been synthesized. This has one replaceable proton, and hence a lower capacity for cation exchange (3.3meqg_1) than the other layered materials described in this section.

with a diagnostic X-ray diffraction pattern. Its stoichiometry corresponds to Zr(HPO4)2H2O-zirco-nium bis(monohydrogen orthophosphate) monohydrate, with an exchange capacity of 6.64 meqg-1. It has been designated as an a-phase and given the shorthand notation 'a-ZrP'. Its idealized structure is shown in Figure 9; the layers form a series of cavities in which reside protons and water molecules.

Potentiometric titration demonstrates that the structural unit shown above has two replaceable protons. Detailed studies of its cation exchange properties have been done, including determinations of the thermodynamic quantities for the exchange of the protons for most common metals-especially those of Groups 1 and 2. An interesting feature of these studies is the finding that the layer structure expands to accommodate monovalent ions, with the interlayer spacing being a function of ionic radius and water content. Intermediate 'half-full' phases are stable and well characterized. When the divalent ions of the alkaline earths are the ingoing ions a size restriction operates that is a complex function of the hydrated ion size and instability to shed water of hydration.

For these reasons calcium and strontium exchanges proceed, but magnesium and barium exchanges are very limited (if at all). If, however, a-ZrP is first converted to a half-exchanged sodium form, then Mg and Ba phases can be obtained.

Selectivity series have been constructed, with the half-exchanged Na phase (a-NaH ZrP • 5H2O) as the initial exchanger, for the alkali metals and the divalent cations of first row transitional elements. They are: K > Cs > Na > Li, and Cu > Zn, Mn > Fe > Co > Ni. Exchange into ZrP (amorphous and crystalline) from fused salts gives a selectivity series LiiNa > K.

Other layer compounds Layered phosphate materials of the a type have been prepared for titanium, tin and hafnium. Arsenates of tin, titanium and zirconium with similar structures also are known. Only titanium and zirconium phosphates have y-phases. Table 2 illustrates the interlayer spacings of a- and y-phases of tetravalent acid salts.

Intercalates A considerable body of work exists in which workers have shown that layered substances derived from salts of tetravalent acids can readily expand their layers to accommodate organic molecules capable of protonation. They include amines, alcohols, amino acids and metallocene derivative and they have large interlayer distances. Figure 10 shows the formation of intercalation compounds of a-ZrP containing n-alkyl-monoamines.

Other salts of polyvalent acids Cerium phosphate readily precipitates as a fibrous material that can be formed into sheets. This material has attracted interest,

Table 2 Tetravalent acid salts with their interlayer distances

Compound Interlayer distance (nm) 7-Salts

Titanium phosphate 0.756

Zirconium phosphate 0.756

Hafnium phosphate 0.756

Germanium phosphate 0.760

Tin phosphate 0.776

Lead phosphate 0.780

Titanium arsenate 0.777

Zirconium arsenate 0.778

Tin arsenate 0.780

y-Salts

Zirconium phosphate 1.22

Titanium phosphate 1.16

both as an inorganic ion exchange paper and as a thin-layer material for the separation of inorganic ions. Cerium phosphate can also be prepared in robust granules for column use. Cerium phosphate is semicrystalline, as are the fibrous forms of thorium phosphate, titanium phosphate and titanium arsenate, which have also been prepared.

Salts of heteropolyacids These are the well-known salts of the parent 12-heteropolyacids, having the general formula HmXY12O40 • nH2O, where m = 3-5, X = P, As, Si, Ge or B, and Y = Mo, W, V (and others). Their structures have been known from the early days of X-ray single crystal analysis, and are examples of three-dimensional assemblages of linked [XO4]4~ tetrahedra and [YO6]6~ octahedra. The resulting frameworks contains voids large enough to contain replaceable cations.

The two most studied salts are the molybdophos-phates and the tungstophosphates. Until recently it was thought that exchange was facilitated by the presence of water in the framework voids, but it now seems that ammonium molybdophosphate (AMP) is anhydrous and that any water noted as present 'as-synthesized' is contained in the solid by capillary condensation. This means that entering cations must be stripped of hydration water. AMP was one of the first inorganic materials to be used to scavenge radiocaesium from aqueous nuclear waste on a plant scale. Ammonium tungstophosphate (ATP) has a similar selectivity. A large number of organic salts of these acids have been synthesized, e.g. di-, tri- and tetramethylammonium 12-molybdophosphate and pyridinium 12-tungstophosphate.

Insoluble ferrocyanides A variety of compounds have been reported with metal cations held in a framework of linked [FeCN6]4~ octahedra. They include those with mixed metal hexacyano anions, and when cobalt is included in the composition a useful exchanger is obtained.

The product can be written as K^-^CoJCoFe (CN)6], • yH2O. When x = 0.6-0.7, a stable granular material results that has a high selectivity for caesium. It is used to scavenge caesium radioisotopes from waste emanating from the Lovissa Nuclear Power Plant, Finland.

Several other compounds of this type are being investigated for nuclear waste management, and include the potassium and sodium forms of nickel and copper hexacyanoferrates. In these hexacyanoferrates the exchange is restricted to the surface of the exchanger, but even this restriction still gives an adequate caesium capacity (0.35 mmol g_1) and acceptable kinetics.

Aluminosilicates

Clay minerals These are another group of compounds whose structures have been studied since the earliest days of X-ray crystallography. Their structures are composed of layers of linked polyhedra, and a convenient subdivision of their structural types is into those with:

1. single layers;

2. nonexpandable double layers;

3. expandable double layers.

The most common single-layered clays are the kaolins. These have layers of [SiO4]4~ tetrahedra linked by three corners to create sheets. Between these layers are aluminium ions held to the fourth corners of the [SiO4]4~ tetrahedra that provide, on average, three hydroxyls from one layer and one hydroxyl plus two oxygens from the next layer. This results in another layer of hexagonally coordinated aluminium ions resembling those of gibbsite, a natural aluminium hydroxide mineral. In these clays ion exchange can take place at structural defects (broken bonds), or at exposed (edge) hydroxyls. The possibility also exists that a small amount of Al3+, or Fe3+, can isomor-phously replace silicon from some tetrahedral

Figure 10 Arrangement of n-alkyl monoamines in a-ZrP, illustrating the change in interlayer spacing with increased loading. (Reproduced from Williams and Hudson, 1987, with permission.)

environments. The presence of [FeO4]5~ entities confers a negative charge on the silica layers, which can then be compensated by exchangeable cations sited between the layers.

Whatever the mechanisms whereby cations are accommodated into single-layer clays, their exchange capacities are low. The minerals can also exhibit a low anion capacity via labile hydroxyl groups.

In double-layered clays the element of structure is that of two sheets of tetrahedra separated by cations. The unexpandable double layer aluminosilicates, like the micas, have isomorphous substitution of aluminium for silicon in the double layers. This creates strong ionic bonding between the negatively charged layers and the interlayer cations. The cations are in an environment virtually water free so ion exchange is difficult, and confirmed mainly to defect and edge effects. The micas, and similar minerals, are examples of exchangers where the potential cation ion exchange capacity, expected from their stoichiometry, is not experimentally achieved. The need for the concept of loading is thereby illustrated.

In the expandable double-layered silicates hydrated cations are held in interlayer positions by weak electrostatic forces between their hydration shells and the silica sheets. Some isomorphous substitution in the tetrahedral layers is present but does not have a major effect on ion exchange behaviour. The loosely held cations are readily exchanged, and the interlayer distance changes as a function of hydrated cation size. Further ingress of solvent is easy. More water, or even organic molecules such as glycols, can penetrate the layers to further swell the structure. Montmorillo-nites are typical expanding double-layered clays. Examples of cation capacities for the clay minerals are listed in Table 3.

The ability of clays to act as exchangers is, of course, a major property of soils related to their ability to sustain plant nutrients. Incorporation of metals, such as copper and nickel, aids their use as catalysts. Their use as ion exchangers seems to be limited to wastewater treatment by glauconite (green sand), sometimes in manganese form, and often erroneously described as a zeolite.

Zeolites In these aluminosilicate minerals the constituent [SiO4]4~ and [AlO4]5~ share all corners to create a three-dimensional framework structure carrying a negative charge. This framework charge is balanced by the presence of cations contained in channels and cavities within the framework. Many zeolites are able to contain a large amount of water in these cavities and channels, which can have void volumes as high as 50% of their total volume. They are known both as natural species and as synthetic

Table 3 Examples of the cation exchange capacities of some clay minerals

Mineral

Capacity (meqg) ~

Single layer

Kaolinite 0.03-0.15

Halloysite 0.05-0.10

Double layer (nonexpanding)

Muscovite (mica) 0.10

Illite 0.10-0.40

Glauconite 0.11-0.20

Pyrophyllite 0.40

Talc 0.01

Double layer (expanding)

Montmorillonite 0.70-1.00

Vermiculite 1.00-1.50

Nontronite 0.57-0.64

Saponite 0.69-0.81

minerals capable of being manufactured on the tonnes scale.

Nearly 100 different frameworks have been crystallographically defined for zeolites, and related structures, each one having a unique molecular architecture. The internal dimensions of their channels and cavities are close to molecular dimensions and this has led to their employment as 'molecular sieves' and catalysts. Usually synthetic zeolites perform these functions and thereby make an incalculable contribution to the world economy, particularly in the oil industry. Examples of zeolite structures are provided in Figures 11 and 12.

Most zeolites readily exchange the cations from their voids. This facile process is vital to their utility as both molecular sieves and catalysts, and has been responsible for most of the literature describing their cation exchange properties. Because detailed crystal-lographic data is available for some zeolites (even including the positions of water molecules within their frameworks), they have been used to model theories of ion exchange kinetics and equilibria.

These minerals can exhibit very high selectivities, with high capacities, and have been extensively studied for use as such, while being restricted by their instability in acid environments. Examples where use can be made of cation exchange properties will be considered in a later section.

Zeolite cation capacities are a function of the extent of aluminium substitution into framework interstices; examples are listed in Table 4. As with the clays, the loading may not correspond to the cation content. The reasons why full exchange cannot be attained in some zeolites is a complex subject - sometimes framework charges are too low to strip hydration spheres, and the ingoing ions (bare, hydrated or even partially hydrated) may be too large to readily

Figure 11 Linkage of Si, AlO4 tetrahedra to form a chain structure that, when joined in three dimensions, forms the framework of the zeolite natrolite.

pass through the restricting dimensions in the channels. These effects contribute to the 'ion-sieve' effects noted in some zeolites.

Other framework structures Earlier the point was made that adherence to the traditional classifications of inorganic ion exchangers was not entirely appropriate. This has arisen for a number of reasons, a major one being the endless search for novel catalysts. The synthetic routes chosen frequently mimic those of zeolite synthesis with the aim to produce robust framework structures based on the assemblage of coordination polyhedra. Implicit in this is the likelihood that a microporous medium possessing ion exchange properties will result. To date very little attention has been given to the numerous substances appearing from this source in the context of ion exchange, so only a brief survey will be given.

Prominent in the novel frameworks produced by assemblage of tetrahedral units has been the incorporation of [TO4] units into zeolite-like structures. Numerous varieties with T = P, B, Co, Cu, Zn, Mn, Mg, Ga, Ge, Be have been identified, and some have also been found in nature. The possibility has already been claimed that a new family of anion exchangers can be made with a framework in which an excess of [PO4]5~ over [Si, AlO4]n~ tetrahedra prevails, rendering the framework positively charged.

Discoveries like these have prompted a reassessment of the opportunities to make inorganic 'molecu-

larly ordered solids' (MOS) based on well-understood coordination polyhedra (tetrahedral and octahedral) formed by metals such as Ti, As, Zr, Hf, Nb, Mo, W, V, etc. They will prove a fruitful area in which to pursue the search for selective exchangers.

Other layer structures

Pillared materials Mention has already been made of the exchange of organic molecules into clays and layered phosphates/phosphites. Similar expansions can arise when a large inorganic ion is introduced between the layers. An early example of this was the exchange of a 'Keegin' ion, such as [Al13O4(OH)24]7+, into expandable clays. Subsequent calcination leaves

Figure 12 Structure of (A) synthetic zeolite A and (B) synthetic faujasite (zeolites X and Y) showing the internal cavities of molecular dimension. (Each line represents an oxygen and each junction a silicon or aluminium.)
Table 4 The cation exchange capacities of some zeolite minerals

Zeolite

Capacity (meqg )

Natural

Analcime

4.95

Chabazite

4.95

Phillipsite

4.67

Clinoptilolite

2.64

Mordenite

2.62

Synthetic

A

4.95

X

6.34

Y

4.10

an alumina pillar propping open the layers to 1-2 nm. Again, the intention of this work was to develop wide-pored cracking catalysts. When ion exchange capacities were measured they proved to be orders of magnitude greater than the equivalent amount of alumina. No significant residual capacity arising from the clay was detected. Many similar pillared substances have now been made with a variety of different layered compounds (natural and synthetic) and inorganic pillars. Little is known of their ion exchange properties.

Hydrotalcites The natural mineral, hydrotalcite, is a layered compound of composition Mg6Al2(Co3) (OH)16 • 4H2O. Synthetic analogues can be obtained with other metals replacing magnesium and aluminium (Co, Ni, Fe for example). They are commercially available as anion exchangers.

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