1

SEMINAR

ON

STYRENE BASED ION EXCHANGER

PREPARED BY:

PATEL YASH

GUIDED BY: CO-GUIDED BY:

Prof. RANJAN SENGUPTA Mr. HIMANSHU KOHLI

2014-15

DEPARMENT OFCHEMICAL ENGINEERING

FACULTY OF TECHNOLOGY AND ENGINEERING

THE MAHARAJA SAYAJIRAO UNIVERSITY

OF

BARODA

2

DEPARMENT OFCHEMICAL ENGINEERING

FACULTY OF TECHNOLOGY AND ENGINEERING

THE MAHARAJA SAYAJIRAO UNIVERSITY

OF

BARODA

CERTIFICA TE

This is to certify that Mr. Yash Patel, a student of B.E.-IV Chemical Engineering, Roll

number-834 Styrene based Ion

exchanger in bmit his report in partial

fulfillment of the degree of B.E. (Chemical) for the year 2014-15

Guide Head of Department

Mr. Himanshu Kohli Dr. Bina Sengupta

3

ACKNOWLEDGEMENT

I express my deepest sense of gratitude to my respected guide Mr.Himanshu Kohli, for

his valuable guidance, constructive criticism and constant encouragement during the entire

course of studies, till the completion of this seminar.

I would like to express my sincere thanks to Head of the Department of Chemical

Engineering Dr. Bina Sengupta for granting the permission to do work on my seminar.

Presented By:

Yash B. Patel

4

Index

CHAPTER

NO.

CONTENT PAGE

NO.

1 INTRODUCTION 6

2 HISTORICAL ASPECTS 7

3 ION EXCHANGE MATERIAL 10

3.1 NATURALLY OCCURING

ION EXCHANGERS

10

3.2 SYNTHETIC ION EXCHANGERS 11

3.3 SPECIFIC ION EXCHANGER 14

3.4 ION EXCHANGE MEMBRANES 15

4 STYRENIC ION EXCHANGER 16

4.1 DEVELOPMENT 16

4.2 STYRENIC CATION EXCHANGE RESINS 19

4.3 STYRENIC ANION EXCHANGE RESINS 21

5 STRUCTURE OF RESIN 22

6 PROPERTIES OF

STYRENIC ION EXCHANGERS

25

7 APPLICATIONS OF

STYRENIC ION EXCHANGERS

32

8 GUIDELINES TO SELECT RESIN

STRUCTURE ACCORDING TO

OPERATIONAL REQUIREMENT

33

9 BIBLOGRAPHY 34

5

LIST OF FIGURES

NUMBER FIGURE PAGE

NO.

1 STRUCTURE OF A CATION

EXCHANGER

6

2 CROSSLINKED POLYSTYRENE 13

3 SYNTHESIS OF A STYRENE SULFONIC

ACID CATION

EXCHANGE RESIN

19

4 SYNTHESIS OFSTYRENIC ANION

EXCHANGE RESINS

21

5 RESIN STRUCTURE 23

6 RESIN GEL-ELECTROLYTE PHASE 24

LIST OF TABLES

TABLE 1 COMMON SPECIFIC ION

EXCHANGERS

15

TABLE 2 PROPERTIES OF SOME TYPICAL

CATION EXCHANGE RESINS

26

TABLE 3 PROPERTIES OF SOME TYPICAL

ANION EXCHANGE RESINS

27

TABLE 4 CCAPACITY VALUES FOR NEW AND

USED STRONGLY BASIC ANION

EXCHANGE RESINS

30

TABLE 5 DENSITY VALUES FOR SOME TYPICAL

ION EXCHANGE RESINS

31

TABLE 6 IDEALIZED RESIN SELECTION CRITERIA 32

6

CHAPTER 1- INTRODUCTION

In ion exchange, ions of a given charge (either cations or anions) in a solution are adsorbed on

a solid material (the ion exchanger) and are replaced by equivalent quantities of other ions of

the same charge released by the solid.[1]

The ion exchanger may be a salt, acid, or base in solid form that is insoluble in water but

hydrated. Exchange reactions take place in the water, retained by the ion exchanger; this is

generally termed swelling water or gel water. The water content of the apparently dry material

may constitute more than 50 % of its total mass.

Figure 1. shows the partial structure of a cation exchanger; each positive or negative ion is

surrounded by water molecules.

Figure 1. Structure of a cation exchanger that exchanges H

+

for Na+

ions swelling water is represented in the

inset.

Ion exchange forms the basis of a large number of chemical processes which can be divided

into three main categories: substitution, separation, and removal of ions.

7

1. Substitution. A valuable ion (e.g., copper) can be recovered from solution and replaced by a

worthless one. Similarly, a toxic ion (e.g., cyanide) can be removed from solution and replaced

by a nontoxic ion.

2. Separation. A solution containing a number of different ions passes through a column

containing beads of an ion-exchange resin. The ions are separated and emerge in order of their

increasing affinity for the resin.

3. Removal. By using a combination of a cation resin (in the H+ form) and an anion resin (in the

OH form), all ions are removed and replaced by water (H+OH ). The solution is thus

demineralized.

8

CHAPTER 2- HISTORICAL ASPECTS

The discovery of ion exchange dates from the middle of the nineteenth century when

THOMSON and WAY noticed that ammonium sulfate was transformed into calcium sulfate after

percolation through a tube filled with soil.[1]

In 1905, GANS softened water for the first time by passing it through a column of sodium

aluminosilicate that could be regenerated with sodium chloride solution. In 1935, LIEBKNECHT

and SMIT discovered that certain types of coal could be sulfonated to give a chemically and

mechanically stable cation exchanger.

In addition, ADAMS and HOLMES produced the first synthetic cation and anion exchangers by

polycondensation of phenol with formaldehyde and a polyamine, respectively.

Demineralization then became possible. At present, aluminosilicates and phenol

formaldehyde resins are reserved for special applications and sulfonated coal has been replaced

by sulfonated polystyrene.

Another interesting event in the early history of macroporous resin development was a

directive from the U.S. government around 1941-42 to build a pilot plant for the production of

dried DuoliteTM A2, an aminated porous phenolic resin, following tests which showed that

such products were capable of adsorbing relatively large quantities of acids from no aqueous or

gaseous media. In particular, these resins proved effective in the removal of toxic agents (such

as mustard gas, etc.) from the atmosphere, and DuoliteTM A-2 in admixture with activated

carbon was contemplated for use in gas mask canisters during World War II.[2]

Industrial firms manufacturing ion exchange resins need to protect their inventions. Thus,

between 1957 and 1960, Bayer A.G., Dow Chemical Co., The Permutit Co. Ltd., Rohm and

Haas Co., Resindion (Italy) and Varion (Hungary) had all applied for patents in their own

countries and subsequently (in most cases) in other countries. Since not all countries have the

same definition for invention or criteria for novelty and since all of these processes for making

macroporous resins are so similar, it was not surprising that patent interferences were declared

and that litigation resulted. In 1957 and 1958, four patent applications, all based on the same

fundamental principle, were filed in the U.S. Patent Office. The history of this particular area

of ion-

9

applications concerned. In the words of a Rohm and Haas publication

perspective on the complexities of our patent system as it involves highly complex, technical

[3]

Some final dates of issue were as follows: 1970 for Dow, 1972 for Bayer, and 1980 for Rohm

and Haas. The latter was granted only after prolonged U.S. Patent Office examination,

involv

by the Court of Customs and Patent Appeals; at one point, an appeal was made to the U.S.

Supreme Court on constitutional grounds. It might be added that at least one of the patent

examiners involved did not survive the twenty-two years of litigation, and most of the

personnel involved are now either retired or dead[4]

.

The eventual master patent, originally applied for by Rohm and Haas in 1958 was reissued in

1980 and is still valid as of this writing. While this is not of any great significance to

manufacturers outside of the United States, concern in both the Senate and House of

Representatives for the Protection of U. may well result in legal

difficulties in the exportation of such materials from Europe or Japan to the United States.

Even today, after nearly 50 years of macroporous resins, the story is still incomplete.

Polyacrylic Anion Exchangers. Between 1970 and 1972, a new type of anion-exchange resin

with a polyacrylic matrix appeared on the market. This possesses exceptional resistance to

organic fouling and a very high mechanical stability due to the elasticity of the polymer.

10

CHAPTER 3- ION EXCHANGE MATERIAL

A wide range of materials is available for the ion exchange treatment. These materials are

available in a variety of forms, have widely differing chemical and physical properties and can

be naturally occurring or synthetic. This section focuses on materials that are commercially

available and that can be readily obtained.

3.1. NATURALLY OCCURRING ION EXCHANGERS

3.1.1. NATURAL INORGANIC ION EXCHANGERS

Many natural mineral compounds, such as clays (e.g. bentonite, kaolinite andillite), vermiculite

and zeolites (e.g. analcite, chabazite, sodalite and clinoptilolite), exhibit ion exchange

properties. Natural zeolites were the first materials to be used in ion exchange processes. Clay

materials are often employed as backfill or buffer materials for radioactive waste disposal sites

because of their ion exchange properties, low permeability and easy workability. Clays can

also be used in batch ion exchange processes but are not generally suited to column operation

because their physical properties restrict the flow through the bed.

In 1985 British Nuclear Fuels plc (BNFL) successfully commissioned the Site Ion Exchange

Effluent Plant (SIXEP), which uses naturally occurring clinoptilolite to remove caesium and

strontium from fuel cooling pond water [5]

. Other natural aluminosilicate materials, such as

green sand, are also used in some waste treatment applications, generally in column or large

deep bed designs. In this capacity they can be used as a combination of an ion exchanger and a

particulate filter.

3.1.2. NATURAL ORGANIC ION EXCHANGERS

A large number of organic materials exhibit ion exchange properties; these include

polysaccharides (such as cellulos1e, algic acid, straw and peat), proteins (such as casein,

keratin and collagen) and carbonaceous materials (such as charcoals, lignites and coals). Of

these, only charcoals, coal, lignite and peat are used commercially. Although they exhibit a

very low ion exchange capacity compared with synthetics, they are widely available at a very

11

low cost. They are normally used as general sorbents, with their ion exchange properties being

a secondary consideration. Commercially available materials are often treated or stabilized

with other additives to improve their uniformity or stability. Some materials, such as charcoals,

can be doped with chemicals to improve their capacity or selectivity.

3.2 SYNTHETIC ION EXCHANGERS

Synthetic ion exchangers are produced by creating chemical compounds with the desired

physical and chemical properties. They can be inorganic (mineral) or organic (generally

polymer) based.

3.2.1. SYNTHETIC INORGANIC ION EXCHANGERS

Some of the important synthetic inorganic ion exchangers are described below.

3.2.1.1. Zeolites

Zeolites were the first inorganic materials to be used for the large scale removal of

radionuclides from nuclear waste effluents. Zeolites are crystalline aluminosilicate based

materials and can be prepared as microcrystalline powders, pellets or beads.

The main advantages of synthetic zeolites when compared with naturally occurring zeolites are

that they can be engineered with a wide variety of chemical properties and pore sizes, and that

they are stable at higher temperatures.

3.3.1.2. Titanates and silico-titanates

For many years the oxide and hydroxide of titanium have been known to be effective in

removing metal ions from solution. In 1955 studies in the UK and later in Germany and Japan

identified a hydrous titanium oxide as the preferred exchange material for the large scale

extraction of uranium from sea water. Subsequent studies found that this material also had a

strong affinity for actinide metal ions and for ions with a charge of 2+ or more. Titanates and

hydrous titanium oxide (known both as HTiO or HTO) are known to be highly selective

exchangers for strontium . These materials have been prepared on a large scale and used for in-

tank precipitation at the Savannah River Site in the United States of America [6]

.

3.2.2. SYNTHETIC ORGANIC ION EXCHANGERS

12

The largest group of ion exchangers available today are synthetic organic resins in a powdered

(5 2 mm diameter) form. The framework, or matrix, of the resins is a

flexible random network of hydrocarbon chains. This matrix carries fixed ionic charges at

various locations. The resins are made insoluble by cross-linking the various hydrocarbon

chains. The degree of cross-linking determines the mesh width of the matrix, swelling ability,

movement of mobile ions, hardness and mechanical durability. Highly cross-linked resins are

harder, more resistant to mechanical degradation, less porous and swell less in solvents.[7]

When an organic ion exchanger is placed in a solvent or solution it will expand or swell. The

degree of swelling depends both on the characteristics of the solution/solvent and the

exchanger itself and is influenced by a number of conditions, such as:

The degree of cross-linking,

The exchange capacity,

A strong or weak solvation tendency of the fixed ion groups,

The size and extent of the solvation of counter ions,

The concentration of the external solution,

The extent of the ionic dissociation of functional groups.

The main advantages of synthetic organic ion exchange resins are their high capacity, wide

applicability, wide versatility and low cost relative to some synthetic inorganic media. The

main limitations are their limited radiation and thermal stabilities. At a total absorbed radiation

dose of 109 to 1010 rads most organic resins will exhibit a severe reduction in their ion

exchange capacity (a 10 to 100% capacity loss),owing to physical degradation at both the

molecular and macroscopic level. Cation exchange resins are generally limited to operational

temperatures below about 150C, while anion exchange resins are usually limited to less than

70oC. This requires that some streams, such as reactor coolant water, be precooled substantially

before their introduction to the ion exchange media. The main groups of synthetic organic ion

exchange resins are described below.

3.2.2.1 POLYSTYRENE MATRIX

The polymerization of styrene (vinyl benzene) under the influence of a catalyst (usually an

organic peroxide) yields linear polystyrene. Linear polystyrene is a clear mouldable plastic fi

which is soluble in certain solvents (e.g., styrene or toluene) and has a well-defined softening

point. If a proportion of divinylbenzene is mixed with styrene, the resultant polymer becomes

13

cross-linked and is then completely insoluble. In the manufacture of ion-exchange resins,

polymerization generally occurs in suspension. Monomer droplets are formed in water and,

upon completion of the polymerization process, become hard spherical beads of the polymer.[1]

Figure 2.Crosslinked polystyrene

3.2.2.2. PHENOLIC

Phenol-formaldehyde condensation products, with the phenolic OH groups as the fixed ionic

groups, are very weak acid exchangers. Sulphonation of the phenol prior to polymerization can

be used to increase the acid strength. Phenolsulphonic acid resins are bifunctional with both

strong acid SO3H and weak acid OH groups included. The degree of cross-linking is

controlled by the amount of formaldehyde. A resorcinol-formaldehyde polycondensate resin

was recently developed, characterized and tested extensively in India for the efficient removal

of radio caesium from alkaline reprocessing waste containing a large concentration of

competing sodium ions [8]

. Under alkaline conditions the phenolic OH groups ionize and

serve as cation exchange sites with a high selectivity for caesium ions. Incorporation of

iminodiacetic acid functional groups in the phenolic polymer gives it the additional property of

strontium uptake by chelation; such a resin is presently being used in an industrial scale plant

at Tarapur in India for the treatment of alkaline intermediate level reprocessing waste [9]

. The

development of a similar resin for the removal or recovery of radiocaesium from neutralized

high level waste has also been reported in the USA.

Other phenolic type resins are produced using resorcinol-formaldehyde, which incorporates

phosphoric acid or arsonic acid functional groups.

3.2.2.3. ACRYLIC

A weak acid ion exchange resin with weakly ionized carboxylic acid groups is prepared by the

suspension copolymerization of acrylic or methacrylic acid with divinylbenzene. The COOH

14

functional groups have very little salt splitting capacity, but under alkaline conditions exhibit a

strong affinity for Ca2+

and similar ions (such as strontium). The acid strength can be increased

by using various phosphoric acid derivative groups, such as PO32

, PO33

and HPO2 .

3.3 SPECIFIC ION EXCHANGERS

If an ion forms strong complexes with, or is precipitated by, a certain class of chemical reagent,

ion exchange resins incorporating such a class of compound as its functional group usually

exhibit a high affinity for such ions. Such an exchanger which strongly takes up one (or at least

not more than a few) counter-ion species relative to all others has great potential usefulness in

analytical chemistry, and as a possible basis of commercial recovery or purification processes.

The first specific cation exchanger was patented by A. Skogseid in 1947 containing functional

groups similar to -hexanitrodiphenylamine and showed a specific affinity for

potassium ions.

The chemistry of the common heavy metals is characterized by their readily forming co-

ordination complexes or chelates with electron pair donating ligands. Therefore it is not

surprising that a styrenic exchanger containing iminodiacetate functional groups should show

particularly strong affinity towards many polyvalent and transition metal cations.. Numerous

specific ion exchangers have been reported, but those more commonly encountered are listed

in Table 1. It is important to realize that specificity does not necessarily mean an affinity by the

exchanger for one ion only, and in many ways the

or chelating ion exchange is a better description since commonly the relative affinities of the

resin for several ions, not just one ion, are enhanced compared with conventional exchangers.

For example the aminophosphonic chelating resins are highly selective towards divalent

alkaline earth cations over monovalent ions; and certain types of quaternary

benzyltrialkylammonium strong base anion exchangers are more selective for the nitrate ion

over the sulfate ion which is a reversal of the normal sequence.

It is equally important to appreciate that a high selectivity for a particular ion does not

necessarily mean that the resin concerned is bound to have immediate commercial application.

The reason for this is that the property of high affinity is always associated with a reduction in

the degree of reversibility in cyclic operations thereby rendering regeneration difficult.

Tailored copolymer resins are not the only exchangers to exhibit specific affinities towards

selected ions. Many types of inorganic materials such as clays, zeolites, amphoteric oxides,

heteropolyacid salts, and phosphates exhibit useful specificity towards selected monovalent

15

and polyvalent ions. In the laboratory such media are often the basis of chromatographic

separations, whilst industrially many such materials offer benefits in radioactive waste effluent

treatment for removing nucleides such as caesium ( 137

Cs) and strontium ( 90

Sr).[10]

Table 1. Matrix, functional group, and ion affinities of some common speczfic ion exchangers

3.4 ION EXCHANGE MEMBRANES

There are two principal types of ion exchange membranes: heterogeneous and homogeneous.

Heterogeneous membranes can be prepared using almost any ion exchanger [11]

. They are

prepared by dispersing colloidal or finely ground ion exchange materials throughout an inert

thermoplastic binder such as polyethylene, polystyrene or synthetic rubber, followed by

rolling, compressing or extruding them into discs, films or ribbons. Ion exchange particles

must be in contact with one another in the binder, but not to the complete exclusion of the

binder otherwise the membrane will have a poor mechanical strength. Typically, the ion

exchange media comprises 50 to 75% by volume of the membrane. Homogeneous membranes

are condensation products of sulphonated phenol and formaldehyde or of nitrogen-containing

compounds and formaldehyde. These strong acid or strong base condensates are laid out in thin

sheets on mercury or acidresistant plates. They can also be prepared by heating a

precondensed, viscous reaction mixture between plates. If additional strength is required,

membranes can be prepared on a mesh or fibrous backing.

16

CHAPTER-4- STYRENIC ION EXCHANGER

The most common form of ion exchange resin is based on a copolymer of styrene and

divinylbenzene. The degree of cross-linking is adjusted by varying the divinylbenzene content

and is expressed as the percentage of divinylbenzene in the matrix; for example, 5% cross-

linking means 5 mol % divinylbenzene in the matrix. Low divinylbenzene content resins are

soft and gelatinous and swell strongly in solvents.

Fixed ionic groups are introduced into resin matrices, for example by sulphonation, to create an

ion exchanger. In sulphonation eight to ten SO3H groups are added for every ten benzene

rings in the structure. The H+ of the SO3H group then becomes the mobile or counter ion. It

can be replaced by a treatment with a solution containing another cation; for example, a

solution of NaOH will produce the SO3Na group, with Na+ as the mobile ion.[12]

4.1 DEVELOPMENT IN STYRENIC ION EXCHANGER

The first polystyrene-based resin was invented by D'ALELIO in 1944. Two years later,

MCBURNEY produced polystyrene anion-exchange resins by chloromethylation and amination

of the matrix.

The anion exchangers known until then were weakly basic and took up only strong mineral

acids. The new resins produced by the McBurney process were stronger bases and could

adsorb weak acids such as carbon dioxide or silica, allowing complete demineralization of

water with a purity previously obtainable only by multiple distillations in platinum. Even

today, ion exchange is still the only process capable of producing the water quality needed for

high-pressure boilers. Reverse osmosis and electrodialysis can demineralize solutions with 50

90 % efficiency. Only ion exchange can "polish" the predemineralized solution with a

demineralization efficiency of 99 99.99 % [1]

.

By 1950, the four principal classes of ion exchange resins were commercially available, the

major strong-acid, strong-base, and weak-base resins being derived from styrene-DVB

copolymers, while the weak-acid resins were cross linked acrylics. Except for the latter group,

all were single-phase (gel-type) resins. Among the gel-type strong-acid cation (SAC)

exchangers, the nominal degree of cross linking (%DVB based on total volume or weight of

17

monomers) ranged from about 5% to 12%. Most of those used for water treatment lay in the

range of 7-l0%, although where oxidation by chlorine was a factor, the resins of higher cross

linking had a longer working life and were therefore preferred. In water deionization, where

lower initial cost and lower regeneration costs were significant factors, the lower cross linked

resins were preferred for many applications, notably for sugar and wine treatment. Not only

were they more efficiently regenerated, which meant lower operating costs, but their removal

of proteinaceous materials and colour bodies was much more effective than the higher cross

linked resins. As the number of applications grew, users began to recognize the deficiencies of

the gel-type cation-exchange resins. Low cross linked products (4-6% DVB) suffered from

poor resistance to oxidation, leading to excessive irreversible swelling and increased moisture

retention, leading to physical degradation and thus the need for frequent resin replacement. On

the other hand, the resins of higher cross linking (12-16% DVB) were more costly, both to

make and to operate. Their cation selectivity coefficients were higher, entailing higher acid

costs for regeneration; moisture retention capacities were lower, resulting in slower diffusion

or even partial exclusion of larger cations. Suffocation of copolymers with DVB contents of

16% or more was extremely slow; in fact, the practical limit for commercial SAC exchangers

was around 12% DVB. Similar considerations applied to the normal gel-type strong-base anion

(SBA) exchangers, particularly those aminated with trimethylamine, the so-called Type-l

quaternary resins Those with higher crosslinking were tough, but suffered from low

regeneration efficiency, whereas the so- - with high

water-holding capacity, made by the functionalization of low-DVB copolymers, while more

efficient, frequently suffered from severe attrition losses. The need for improvements in both

cation and anion exchangers was recognized by resin manufacturers in many parts of the

world. For example, an attempt was made to produce SBA

and large surface - (i.e., proliferous) polymers [3].

Although the idea was well directed, it was also unfortunately impracticable.

In 1952, John T. Clarke of lonics Inc., makers of permselective membranes, applied for the

first of a series of patents for the synthesis of their styrene-DVB polymer matrices which could

subsequently be functionalized to give anion- and cation-exchange membranes in unfractured

form.

By the end of the fifties, it was becoming clear that most of the ion-exchange manufacturers

had decided to offer macroporous resins, both for water treatment and for other special

applications. Such materials initially were of necessity more costly than their conventional gel

18

counterparts, as the most economical large-scale production techniques were still being

perfected.

In the then Communist or Socialist countries of Eastern Europe and the U.S.S.R., a great deal

of interesting research was also being carried out along the same or similar lines. This; led to a

series of important publications (in Czechoslovakia) from Josef Seidl and co-workers at the

Research Institute of Synthetic Resins and Lacquers at Pardubice and from the Institute oif

Macromolecular Chemistry in Prague. An excellent review by Seidl et al., entitled

Styrene-Divinylbenzene Copolymers, Their Use in Chromatography and in the

Preparation of Ion s ~ and relevant work worldwide - up

to 1966. Work done in the Soviet Union and in China was reviewed a couple of years later by

Ergozhin.

During 1970s and 1990s chelating ion exchangers has been developed which are used for

separation and determination of rare earths. [13]

Solid acid zirconium sulfonated oligo-polystyrenylphosphonate-phosphate supported on ZrO2

was prepared by Yan Sui and Xiangkai Fu. [14]

Several non-ionic styrenic polymer resins like amberlite, purolite etc. has been developed in

last ten years. [15]

4.2 STYRENIC CATION EXCHANGE RESINS

The miscible monomers, ethenylbenzene (styrene) and diethenylbenzene (divinylbenzene,

DVB), undergo a free radical induced copolymerization reaction initiated by a benzoyl

peroxide catalyst. The exothermic reaction is carried out in an aqueous suspension whereby the

mixed monomers are immiscibly dispersed as spherical droplets throughout the reacting

medium resulting in discreet beads of copolymer being formed. Correct reaction conditions and

the use of suspension stabilizers enable the particle size distribution of the copolymer to be

closely controlled. The extent to which the copolymer is cross linked depends upon the

proportion of cross linking agent (divinylbenzene) employed in the synthesis and has a

pronounced impact upon both the mechanical and chemical behaviour of the derived ion

exchange resin. [16]

Activation of the copolymer is carried out by sulfonation of the matrix with hot sulfuric acid

thereby introducing the sulfonic acid functional group giving a strongly acidic cation exchange

resin. The reaction in Figure 3. shows sulfonic acid substitution occurring within the 'styrene'

19

nucleus only, but whether or not all aromatic nuclei become sulfonated is a subject of some

debate. Subsequent treatment of the sulfonic acid resin (RSO3H) with brine or sodium

Figure 3. Addition pobmerization synthesis of a styrene sulphuric acid cation exchange resin

20

hydroxide solution gives, via ion exchange, the sodium sulfonate salt form (RSO3Na). Finally rinsing

and grading produces the now so familiar bead form product characteristic of addition polymerized

resins.

4.3 STYRENIC ANION EXCHANGE RESINS

The suspension polymerized ethenylbenzene-diethenylbenzene copolymer is also the host

matrix for most anion exchange resins. The preformed copolymer is subject to two further

synthesis steps, first developed by McBurriey in 1947, as described below and by Figure 4.

1. Chloromethylation. A Friedel- Crafts reaction between the copolymer and

chloromethoxymethane with aluminium chloride as the catalyst introduces chloromethyl

groups (-CH2C1) into the ethenylbenzene nuclei. What appears to be a simple step is in fact a

critical stage in the synthesis, demanding strict techniques to firstly minimize undesirable side

reactions such as the formation of 1,2-dichloromethoxymethane (bis-chloromethyl ether), and

secondly to control the degree of secondary cross linking through 'methylene group' bridging

as illustrated by Figure 4.

2. Amination. The final stage after purification of the chloromethylated copolymer is the

substitution of the functional group by reaction with various alkyl substituted aliphatic amines

as shown in Figure 4. Trimethylamine, (CH3)3N gives the quaternary

benzyltrimethylammonium chloride functional group, RCH2N(CH3)3+C

- which is characteristic

of most Type I strongly basic anion exchange resins. The equivalent reaction using

dimethylethanolamine, (CH3)2(C2H4OH)N, gives the Type II class of strong base anion

exchange resins, RCH2N(CH3)2(C2H4OH)+Cl

-. If instead of using tertiary trimethylamine,

methylamine or dimethylamine is employed, the resulting resins are weakly basic with

secondary, RCH2NH(CH3), or tertiary, RCH2N(CH3)2 , functionality. The range of amine

functional group configurations is quite large because of the many suitable copolymers amine

derivatives available. However, in the main, most commercially available styrene based

anion exchange resins are based on weakly basic secondary and tertiary amine functional

groups or the strongly basic quaternary ammonium grouping.

21

Figure 4. Addition polymerization synthesis of styrenic anion exchange resins

Figure 5. Secondary cross linking through 'methylene group ' bridging