Inorganic Ion Exchangers.pdf

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II / ION EXCHANGE / Inorganic Ion Exchangers

Further Reading

Helfferich F (1962) Ion Exchange , 2nd edn. New

York: McGraw-Hill.

Hwang S-T and Kammermeyer K (1975) Membranes in

Separations . New York: Wiley.

Marinsky JA and Marcus Y (eds) (1973) Ion Exchange

and

Alberti G, Casciola M, Costantino U and Vivani R (1996)

Layered and pillared metal(IV) phosphates and phos-

phonates. Advanced Materials 8(4): 291.

Amphlett CB (1964) Inorganic Ion Exchangers . Amster-

dam: Elsevier.

Clear

Solvent

Extraction . New York: Marcel

Dekker.

Osborn GH (1961) Synthetic Ion-Exchangers : Recent De-

velopment in Theory and Application . London: Chap-

man

eld A (ed.) (1982) Inorganic Ion Exchange Mater-

ials . Boca Raton, FL: CRC Press.

Fritz JS, Gjerde DT and Pohlandt C (1982) Ion Chromato-

graphy . Heidelberg: Hu

Hall.

Weiss J (1994) Ion Chromatography , 2nd edn. Weinheim:

Wiley.

thig.

Greig JA (ed.) (1996) Ion Exchange Developments and

Applications . Cambridge: Royal Society of Chemistry.

Inorganic Ion Exchangers

2. charge-compensating groups, electrostatically as-

sociated with, and not covalently bonded to,

a charged moiety

Type 1 sites, illustrated in Figure 1A , are respon-

sible for the ion exchange properties of materials

such as hydrous oxides and single-layer clays.

All oxidic materials have these sites to some degree,

at the surfaces of particles or crystals or at defect

sites within the structure. Ion exchange reactions

involving these types of sites may be regarded as

chemical reactions, which may display amphoteric

nature.

Type 2 sites, illustrated in Figure 1B, are respon-

sible for most of the ion exchange capacity of zeolites,

double-layer clays and zirconium phosphates. These

sites arise in structures possessing, for instance,

charged layers or charged porous frameworks. The

exchangeable ions are present to retain overall elec-

troneutrality. When materials such as zeolites are

concerned, a mixture of Type 1 and Type 2 sites is

available, although Type 2 sites will usually greatly

outnumber Type 1 sites, and the latter are often

ignored. Exchange interactions involving Type 2 sites

are physical in nature, as chemical bonds are neither

made nor broken.

E. N. Coker , BP Amoco Chemicals,

Sunbury-on-Thames, Middlesex, UK

2000 Academic Press

Summary

In the

rst part of this chapter, the origins of

ion exchange in inorganic materials are discussed

in relation to the structure of the exchanger.

Thereafter, the various types of inorganic ion

exchangers are introduced and categorized according

to their ion exchange properties. Descriptions of

particular materials follow, with special emphasis

on some structure-speci

ion exchange properties. The materials which are

discussed include zeolites and zeolite-like materials,

clays and other layered materials, zirconium

phosphates, heteropolyoxometalates and hydrous

oxides.

c and composition-speci

Types of Ion Exchange Sites in

Inorganic Materials and their

Origin

For the purposes of

this chapter,

ion exchange

interactions will be de

ned as those involving the

interchange of positively or negatively charged

species (atomic or molecular) at an ion exchange

site.

There are two types of chemical species which

constitute the vast majority of ion exchange sites in

inorganic materials:

Types of Inorganic Ion Exchange

Material

An important distinction between ion exchange ma-

terials is whether they exhibit capacity for cations,

anions, or both. Cation exchangers, and in particular

zeolites, clays and zirconium phosphates, are the

most common and best understood of the ion ex-

changers. Anion exchangers are also important but

1. structure-terminating, covalently bonded groups

such as

}

II / ION EXCHANGE / Inorganic Ion Exchangers

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Most crystalline inorganic ion exchangers are por-

ous. This porosity may arise through the presence of

void space between the layers in clay materials and

layered double hydroxides, or through the intrinsic

microporosity present in zeolitic materials. Many of

the layered materials have the versatility to (revers-

ibly) change their interlayer spacing and hence the

size of the voids, which allows the ion exchange

properties to be adjusted. The more rigid zeolite

structures give rise to exchange reactions which may

show extremely high selectivity to certain cations, or

perform ion sieving.

Zeolites

Zeolites are microporous crystalline aluminosilicate

minerals which occur naturally and may be syn-

thesized easily in the laboratory. An introduction

to the structures and properties of zeolites is given

in the article by Dyer. Zeolites are used on a large

scale as ion exchangers in many

elds; most notable

are their use as ‘builders’ or water softeners for laun-

dry detergents, and their use in the decontamination

of various types of waste streams. Typical applica-

tions of zeolites as ion exchangers are given in

Table 1 . Additionally, the ion exchange capability of

zeolites can be used as a tool to modify their catalytic

and sorptive properties. Some attention will be paid

to structural parameters which in

Figure 1 The two major types of ion exchange site. (A) Type 1,

structure-terminating and defect groups; (B) Type 2, charge-com-

pensating groups. M is an oxide-forming metal with oxidation

state 4; T is an oxide-forming metal with oxidation state 3. The

regions enclosed in dotted lines are those giving rise to ion

exchange where Z # (or Z

uence the ion ex-

change properties of zeolites in the following para-

graphs.

Besides the conditions under which an ion ex-

change reaction is performed, a number of factors

may in

) is exchangeable. Shaded areas

represent a continuation of the oxidic network.

}

uence the ion exchange properties of zeolites,

including:

the exchange of anions is often not fully reversible,

thus the exchangers cannot be easily regenerated and

the reactions are more dif

the structure of the zeolite, particularly the dia-

meters of the windows allowing access to the pores

and cavities

cult to treat thermo-

dynamically. Multiply charged anions, in particular,

may be held tenaciously by the exchanger. Examples

of anion exchangers are certain clays such as hydroxy

double salts (e.g. [CuNi(OH) 3 ]Cl) and layered

double hydroxides (e.g. hydrotalcite, Mg 6 Al 2 (OH) 16

(CO 3 )

the location of the ion exchange sites; different

cation environments lead to different ion ex-

change properties. The number of charge-balanc-

ing cations required for an electroneutral material

is often less than the number of available ion ex-

change sites, thus partial occupancy of sites is com-

mon. Some of the possible cation positions in

zeolites A and X (two of the most widely used

synthetic zeolite ion exchangers) are indicated in

Figure 2

4H 2 O). Amphoteric ion exchangers possess

predominantly Type 1 exchange sites, e.g. hydrous

oxides.

While ion exchange properties may be exhibited by

both amorphous and crystalline solids, studies of the

ion exchange properties of amorphous solids are of-

ten hampered by dif

)

culties in preparing mater-

ials reproducibly and the dif

the composition of the zeolite framework;

varying the Si : Al ratio or changing the frame-

work substituent elements may change, for

example, the density of exchange sites, the electric

culties in character-

izing them fully. With crystalline materials, however,

reproducible preparations can be easily veri

ed and

well-de

ned structural data aids in the interpretation

of the results of ion exchange experiments.

eld strength or the hydrophobicity of the sample

as a whole

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Table 1 Principal applications of zeolites as ion exchangers

Application

Typeof zeolitefrequentlyused

Ionexchangeprocess

Detergent building

A (synthetic)

Removal of Ca 2# and Mg 2# from solution

MAP (synthetic)

X (synthetic)

Wastewater treatment

Clinoptilolite (natural)

Uptake of NH 4

and heavy metals from waste

Chabazite (natural)

streams

Mordenite (natural)

Phillipsite (natural)

Nuclear waste treatment

Clinoptilolite (natural)

Uptake of 137 Cs # , 90 Sr 2# and other radionuclides

Chabazite (natural)

Phillipsite (natural)

Mordenite (natural)

Mordenite (synthetic)

Ionsiv IE-96 (synthetic)

Ionsiv A-51 (synthetic)

Animal food supplement

Various (natural)

Regulation of NH 4

and NH 3 levels in stomach

Animal food supplement

Various (natural)

Scavenging of radionuclides following contamina-

tion of livestock

Fertilizer

Various NH 4 forms (natural), often those

used to remove NH 4

Slow release of NH 4

(and other cations)

from wastewater

nM n # cations

are present to counterbalance the x units of

negative charge on the framework due to the presence

of x AlO 2 groups. In many cases, ion exchange

reactions in zeolites may reach completion, that is,

all of the charge-balancing cations ( M ) initially

present are capable of being replaced by the ingoing

cation.

The

empirical

structural

formula

for

occupy the interstitial space. The x

aluminosilicate zeolite may be given as

M (n)

n [(AlO 2 ) x (SiO 2 ) y ]

w H 2 O

)

where the framework is constructed from the

entities within the square brackets and the water

molecules

and

charge-balancing

cations

( M )

Figure 2 A representation of some of the possible positions of exchangeable cations in the structures of zeolites A (A) and X (B).

Note: the two structures are not shown on the same scale. Reproduced with permission from Stucky GD and Dwyer FG (eds) (1983)

IntrazeoliteChemistry. ACS Symposium Series, vol. 218, p. 288. Washington, DC: American Chemical Society.

II / ION EXCHANGE / Inorganic Ion Exchangers

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Incomplete ion exchange reactions In some cases,

some of the cations are constrained within the struc-

ture and are nonexchangeable. Such cations are intro-

duced into small cavities in the structure during

growth of the zeolite crystal. This situation is

common with feldspars and feldspathoids, which are

similar in composition to zeolites, but possess more

limited porosity. Even in instances when all charge-

balancing cations in the zeolite are physically ex-

changeable, the total theoretical exchange capacity

might not be obtained practically.

There are several reasons for incomplete ion ex-

change; the three most important of these are given

below and illustrated schematically in Figure 3 .

1. The most obvious cause of partial or nonexistent

exchange is ion-sieving, where the cation to be

exchanged into the zeolite is too large, or has

a hydration sphere which is too large and robust

for it to have unrestricted access to the pores of the

zeolite. Univalent cations will typically reach

100% exchange, except in limiting cases such as

large cations combined with small-pore zeolites.

Ion-sieving is more commonly observed with

multiply charged cations, which tend to have lar-

ger hydration spheres on account of their higher

charge densities. Zeolites which possess more than

one ion exchange site (see Figure 2) may display

ion-sieving properties depending on the thermo-

dynamics of the exchange reactions occurring at

the various sites. The sites which offer the

greatest thermodynamic advantage are exchanged

rst, while the less favourable sites may not ex-

change at all.

2. Volumetric exclusion may occur if bulky (organic)

cations are exchanged into zeolites of high charge

density. Here, the volume occupied by the cations

may reach that available in the pores of the crystal

before complete exchange has occurred.

3. A third reason for limited exchange to be observed

is when multivalent cations are exchanged into

zeolites of low charge density. As the density of

ion exchange sites decreases, the mean separation

between adjacent sites increases, until a point is

reached where multivalent cations are unable to

satisfy two or more cation exchange sites because

of the distance between them. Table 2 illustrates

this point by listing the maximum exchange limits

observed for several multivalent cations in samples

of zeolites ZSM-5 and EU-1 possessing a range of

Figure 3 The principal reasons for limitations to ion exchange

reactions found in zeolites. (A) Ion-sieving; (B) volume exclusion;

shaded regions represent an extract of the zeolite framework. For

clarity, only ingoingcations are shown.

exchange is kinetically limited but still capable of

reaching 100% of the theoretical capacity is the sof-

tening of water.

Zeolites are used in vast quantities in the detergent

industry as a water-softening additive for laundry

detergents

up to 30% by weight of most modern

washing powders is zeolite. The zeolite is added prin-

cipally to remove calcium and magnesium and thus

prevent their precipitation with surfactant molecules.

Zeolite A is most commonly used, due to its high ion

exchange capacity, which is a consequence of the

framework possessing the maximum possible number

of aluminium atoms (Si : Al

}

Al ratios.

It is easy to visualize the limiting factors of ion

exchange under equilibrium conditions; however,

practical ion exchange may have also kinetic limita-

tions. A particular example of when the desired ion

1 : 1). Recently, zeolite

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II / ION EXCHANGE / Inorganic Ion Exchangers

Table 2 Ion exchange limits (mole fraction) for various multivalent cations and temperatures in samples of zeolites ZSM-5 and EU-1

with varying numbers of aluminium atoms in the framework. In all cases, the ingoing cation replaces sodium

Zeolitetype Alper

u.c. a

Ca 2#

(253C)

Sr 2#

(253C)

Ba 2#

(253C)

La 3#

(253C)

Ca 2#

(653C)

Sr 2#

(653C)

Ba 2#

(653C)

La 3#

(653C)

ZSM-5

1.1

0.28

0.31

0.36

0.50

0.51

0.52

ZSM-5

2.0

0.31

0.36

0.56

0.54

0.64

0.76

ZSM-5

2.4

0.36

0.48

0.67

0.39

0.50

0.67

0.77

0.48

ZSM-5

4.2

0.37

0.42

0.90

0.62

0.85

0.93

EU-1

1.2

0.54

0.56

EU-1

2.1

0.62

0.67

0.85

0.89

EU-1

3.8

0.86

0.93

0.96

0.97

a Number of aluminium atoms in framework per unit cell.

order to provide acceptable kinetics of Ca 2 #

exchange.

MAP

(Maximum Aluminium P),

also with

Si : Al

1 : 1, has been introduced into some deter-

gents. Although the Mg 2 # ion (radius 0.07 nm) is

considerably smaller than the Ca 2 # ion (radius

0.1 nm), its exchange into the zeolite is far less

facile than that of Ca 2 # , due to its large, tight

hydration sphere (the radii of the hydrated Ca 2 #

and Mg 2 # cations are estimated to be 0.42 and

0.44 nm, respectively). Figure 4 shows the kinetics of

exchange of Ca 2 # and Mg 2 # into Na-A zeolite.

The major restriction to the hydrated Mg 2 # cation

is the 0.42 nm window in zeolite A through which

it must pass to gain access to the exchange sites

within the structure. In order for the ion exchanger

to be effective as a water softener for detergents,

it must reduce water hardness within a fewminutes of

beginning the wash cycle. While zeolites A and MAP

perform well at removing calcium from hard water

quickly, their performance towards magnesium is

generally poor. Despite the kinetic limitations, Ca 2 #

and Mg 2 # are fully exchangeable into zeolite A, al-

though selectivity is greater for Ca 2 # ( Figure 5 ). De-

tergent-grade zeolites possess small crystallite sizes in

Materials closely related to zeolites

Semicrystalline zeolites Some interest has been

shown in the ion exchange properties of zeolite pre-

cursors, which are obtained by quenching a zeolite

synthesis mixture before it has fully crystallized. In

these semicrystalline materials, some larger windows

and pores are present than in the crystalline counter-

part because the structure has not fully formed. This

leads to ion exchange selectivities which are dif-

ferent from the crystalline material. Also, their ion

exchange capacities are lower than the corresponding

crystalline zeolites. The materials typically show

weak zeolite X-ray diffraction patterns, and are

Figure 4 Kinetics of exchange of Ca 2# and Mg 2# for 2Na # in

zeolite A. Circles, Ca 2# exchange; triangles, Mg 2# exchange.

Data were determined at 25

Figure 5 Isotherms for Ca 2# /

2Na # exchange

in zeolite A. Circles, Ca 2# exchange; triangles, Mg 2# exchange.

Data were determined at 25

2Na # and Mg 2# /

C, pH 10 and at a solution concentra-

tion of 0.05 mol equiv. L

1 .

tion of 0.05 mol equiv. L

1 .

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