Sulphonated polystyrene resin

When either of the reactants is sensitive to mineral acids, the esterification can often be successfully accomplished with the aid of a cation exchange resin (hydrogen form) in the presence of benzene. Zeo-Karb 225/H, a unifunctional sulphonated polystyrene resin in the hydrogen form, may be used. Thus good yields of isopropyl lactate may be obtained ... 

These will be represented by (Res.A )B , where Res. is the basic polymer of the resin, A is the anion attached to the polymeric framework, B+ is the active or mobile cation thus a sulphonated polystyrene resin in the hydrogen form would be written as (Res.SO J)H. A similar nomenclature will be employed for anion exchange resins, e.g. (Res. NMeJ )CI . 

Amino acids are separated in their native form on a sulphonated polystyrene resin using a system of sodium or lithium based buffers. Separation is effected by stepwise, rather than gradient elution, and the chromatography can be further optimized by carefully controlling the temperature of the analytical column. 

A C4 fraction and methyl alcohol (molar ratio methyl alcohol i-C4 = 1 1) is preheated to about 70°C before being sent to the reactor containing a fixed bed of sulphonated polystyrene resin. The reaction is exothermic (AH298=-37.7 kj/mol) and the heat produced is removed by means of cooling jackets so as to keep the temperature below 120°C. The reaction mixture from the top of the reactor is distilled and any unreacted butene is collected overhead with the azeotropic amount of methyl alcohol. The bottom contains pure MTBE. The distillate, together with additional methyl alcohol, is passed to a second reactor. The products from the second stage are extracted with water to remove residual methyl alcohol. The water-methyl alcohol solution is distilled to recover methyl alcohol, which is recycled. 

The catalysts used industrially in the MTBE process are sulphonated polystyrene resins of the macroreticular type. These strongly acidic materials are prepared by copolymerizing styrene and p-divinylbenzene in the presence of an organic compound that is a good solvent for the monomers but a poor swelling... 

Exchange resins are also employed for the concentration of ions present in very dilute solutions instances are the recovery of silver from photographic residues, chromate from the waste liquor of chromium plating and magnesium from sea water. They have also been used for the separation of rare earths (p. 426), and of uranium, plutonium and radio-active fission products (p. 437), and for plutonium and uranium-233 purification. A striking application was the historic separation of single atoms of mendelevium on a sulphonated polystyrene resin and their elution therefrom, at 87 , with a-hydroxyisobutyrate (Seaborg, 1955). 

Many schemes for fractionating nucleotides, nucleosides and bases on sulphonated polystyrene resins have been published. The main difficulty with these methods is variation between resin batches (e.g. Anderson et al. 1963). Nucleotide separations can be achieved on DEAE-cellulose (Whatman Data Sheet 13, 1967) and DEAE-Sephadex (Piers et al. 1965b) but these media do not seem to be widely used. Gel filtration columns will separate some nucleotide components. Ligand exchange chromatography and partition chromatography of nucleosides are useful for minor components. 

The essential feature of sulphonated polystyrene resins is that the sulphonic acid group offers a structure analogous to /7-toluene sulphonic acid (TSA), and hence in principle should be able to replace this acid in catalytic applications (Figure 6.23). In fact, typical acid resins are somewhat more acidic than TSA as shown by the values of the Hammett acidity function gathered together by Sharma [108] (Table 6.2) and correlates better with sulphuric acid ( 40%). The reason for this has been discussed before [99], and cooperative effects of adjacent sulphonic acid groups on the resin network probably play a role. Overall, however, the acidity of conventional resins is less than sulphuric acid (100%) and certainly lower than some of the solid inorganic acids. This automatically limits the useful application of these materials. However, acid resins do display remarkable selectivities and they have found some important niche applications (see below). Replacement of a mineral acid catalysed process by a polymer-... 

Apart from the ion-pairing aspect, the view that the activity of sulphonic acid groups does not markedly depend on local structure is perhaps supported by the observation by Sakurada et al. that a change in tacticity of the resin - which is expected to affect the geometrical distance between polymer groups - did not alter the catalytic behaviour of sulphonated polystyrene resins [26]. 

Strongly acidic cation exchangers (polystyrene sulphonic acid resins). These resins (Duolite C225, Amberlite 120, etc.) are usually marketed in the sodium form and to convert them into the hydrogen form (which, it may be noted, are also available commercially) the following procedure may be used. 

The separation takes place in a column of sulphonated cross-linked polystyrene resin, which is a strong cation exchanger. The matrix of the resin is strongly anionic in nature (S03 ) and at the low pH used initially, the amino acids will be positively charged and will be attracted to the negatively charged sulphonate groups. 

Approximate selectivity coefficients for the exchange of various cations for lithium ions on a sulphonated polystyrene, a typically strong acid resin, are given in Table 18.1. The values are relative to Li = 1.0. The selectivity coefficient between two ions is the ratio of their selectivities relative to lithium. Hence, for a sodium-hydrogen exchange ... 

Nafion-H was procurerd from Du Pont. It was repeatedly treated with 25% nitric acid at room temperature, washed free of acid, dried at 110°C for 16 hours and stored in airtight glass tube until use. Polystyrene sulphonic acid resin (Indian 130) was obtained from Ion Exchange (India) Ltd. Triflic acid was obtained from Fluka. All the chemicals used in the investigation were of Analytical Reagent1 grade. 

Strong base anion exchange resins are manufactured by chloromethylation of sulphonated polystyrene followed by reaction with a tertiary amine ... 

Jones et al. [280] used dithizone for post column derivativisation of cadmium, cobalt, copper, lead, nickel and zinc. The separation was achieved in aqueous media on a sulphonated 10% cross-linked polystyrene resin. Cadmium(II), cobalt(II), copper(II), lead(II), nickel(II) and zinc ions were detected down to nanogram levels. 

Possibly the simplest example of the first approach would be the substitution of a sulphonated polystyrene-divinylbenzene polymer ion-exchange resin (in free acid form) for p-toluene sulphonic acid as the catalyst in an esterification or other acid-catalysed chemical reaction (see section 11.7.2). The ion-exchange resin may be used in bead form and packed into a tubular reactor, or may be introduced as a powder and subsequently filtered off. Furthermore, the solid resin may prove less corrosive to the metal walls of an industrial reaction vessel. 

Sulphonated polystyrene-poly(divinylbenzene) resins Sulphonated PPS-PDVB -SO3H [H(H20) ]+ [H(ROH)] + Ether synthesis Olefin oUgomerization Liquid/ sohd 40-100... 

Typical polystyrene-divinylbenzene sulphonic acid (Rohm Haas). Fluorocarbon sulphonic acid resin (Du Pont). 

For the strongly acidic cation exchange resin such as cross linked polystyrene sulphonic acid resins, the ion-exchange capacity is virtually independent of the pH of the solutions. For weak acid cation exchangers, such as those containing carboxylate group, ionizations, occurs only in alkaline solution. Similarly weakly basic cation exchanger does not work above pH 9. 

Systematic studies of the acid-catalysed hydrolysis of esters in the presence of sulphonated polystyrene showed an increase in the value of q with decreasing solubility of the esters in water [17, 22]. Correspondingly, for soluble macromolecular acids the values of q have been found to decrease with increasing degree of sulphonation of the polymer. These effects are consistent with hydrophobic interaction between resin and substrate and its effect on the value of Ar. The importance of hydrophobic interaction can be reduced if, for example, acetone is added to the solvent. Such solvent changes have the opposite effect on hydrophilic substrates [26]. 

A pellicular chromatographic packing consists of a rigid core with a relatively thick outer shell. The outer shell is cross-linked by the inclusion of a difunctional monomer and for added stability the outer shell may be covalently attached to the core. An example of this type of packing is the material developed for carbohydrate analysis, CarboPac (Dionex), which is a pellicular anion exchange resin bead which has a 5 pm non-porous sulphonated polystyrene bead core with a 0.1 pm quaternary amine latex. These materials have gained widespread acceptance for analytical separations and small-scale fractionations. 

---