Analysis of Ion-Exchange Resins by Pyrolysis-Gas Chromatography J. R . Parrish Rhodes University, Grahamstown, South Africa
Ion-erchange resins can be characterized by pyrolysisgas chromatography. The principal products of pyrolysis of the most important resins have been identified, and the relative amount of xylene produced by polystyrene resins has been shown to depend on the crosslinking. The effects on the pyrogram of the pyrolysis conditions, the ionic form of the resin, and the nature of the column packing have been studied, and the reproducibility of the pyrograms has been estimated.
Ion-exchange resins have been characterized by their infrared absorption spectra ( 1 ) and by their nuclear magnetic resonance spectra (Z),but the only previous report of the pyrolysis-gas chromatography of ion-exchange resins is a note by Carpov and Hagen ( 3 ) .These authors showed that certain resins could be distinguished in this way, but the pyrograms they published represented only the most volatile pyrolysis products, such as the three methylamines and methanol. Pyrolysis-gas chromatography would be expected to yield more information if monomers such as styrene and divinylbenzene could be detected in the mixture of pyrolysis products. It might then be possible to determine the degree of crosslinking of the resin as well as the nature of the matrix and the functional groups. In this laboratory, ion-exchange resins were pyrolyzed under conditions that gave a good yield of styrene from polystyrene, and the products were chromatographed under conditions designed to separate compounds from benzene to derivatives such as divinylbenzene within a few minutes. EXPERIMENTAL Apparatus. Two types of flash pyrolyzer were used:-a Pye Curie Point Pyrolyzer (Phillips), and a, filament pyrolyzer (Figure l ) , made in this laboratory. The filament pyrolyzer was similar to one described by Stanford (4),but the filament was designed to hold a melting point tube, 1.5 mm in diameter, cut to a length of 4 mm. A new borosilicate tube was used for each sample of resin. Current was supplied from a variable transformer through a 6-V filament transformer, and the voltage was adjusted to produce a temperature of 800 "C in the tube within 10 sec. The temperature was estimated by observing the melting of sodium chloride crystals a t the bottom of the sample tube. The Curie Point Pyrolyzer was used a t Curie temperatures of 770 and 980 "C, and a single bead of resin was held in a hook formed a t the end of the wire. The pyrolysis time was 10 sec, as this gave the best compromise between the yield and the resolution of the pyrolysis products. For gas chromatography a Beckman GC 2A gas chromatograph was used with a flame ionization detector and nitrogen as the carrier gas. The flow rate for all columns was 60-65 ml/min. Three different columns were tried, with intermediate, nonpolar, and polar stationary phases: column 1-125 cm, 10% polyphenyl ether (5 rings) on Celite; column 2-138 cm, 5% SE-30 on Fluoropak 80; column 3-183 cm, 15% Carbowax 1000 on Gas-Chrom (1) D. Whittington and J. R. Millar. J. Appi. Chem., 18, 1 2 2 (1968). (2) J. P. de Villiers and J. R. Parrish, J. Polymer Sci., Part A , 2, 1331 (1964). (3) A. Carpov and E. Hagen, Piaste Kaut., 1 5 , 3 5 8 (1968). (4) F. G. Stanford,Analyst (London), 90,266 (1965).
P. Temperatures of 100-160 "C were used with columns 1 and 2, but column 3 was operated only a t 160 "C. The best resolution of the compounds of interest was obtained on column 1 a t 100 "C. Reagents. A wide variety of commercial ion-exchange resins was tested. The manufacturers were the Dow Chemical Co., the Rohm and Haas Co., and the Permutit Co., and the samples were purchased from laboratory suppliers. Procedure. The resins were washed with 1M HCl and then with water to convert cation exchangers into the hydrogen form and anion exchangers into the chloride form. For the most reproducible results, the resins were dried overnight a t 110 "C before pyrolysis. Samples were not weighed, but the weight taken could be controlled by sieving the resins and taking one or two beads of the required particle size, A single bead of resin of the usual particle size (16-60 mesh) weighs 40-400 fig, which is a satisfactory range. Many beads were used if the particle size was very small (100-400 mesh).
RESULTS AND DISCUSSION Distinctive pyrograms were obtained for each type of resin, and the chromatographic conditions were not critical for the purpose of comparing an unknown resin with a standard resin. Comparisons could be made rapidly with a single bead of resin. Thus polystyrene-based anion exchangers could be distinguished from polystyrene-based cation exchangers within 3 min, even on the most inefficient column that was tried (Figure 2). Under more favorable conditions, the main products of pyrolysis could be separated (Figure 3). These products have been identified by the comparison of their retention times with those of standard compounds on the three different columns. The identification of vinyltoluene and of styrene has been confirmed by the use of a n MS 30 mass spectrometer (Associated Electrical Industries) attached to a Pye Unicam Series 104 gas chromatograph. Once its pyrolysis products have been identified, a resin can be recognized without the use of standard resin samples, and without the difficulty of reproducing published pyrograms on different equipment. Polystyrene Resins. All ion-exchange resins derived from polystyrene produced some styrene on pyrolysis, but the styrene peak was not the largest peak, except in the pyrogram of Dowex 2. Acrylic and phenolic resins did not yield appreciable amounts of styrene and could thus be distinguished from polystyrene resins. Cation-Exchange Resins. Pyrolysis of arylsulfonic acids a t 700-800 "C has been reported to give a high yield of sulfur dioxide and a lower yield of the parent hydrocarbon ( 5 ) . Cation-exchange resins which consist of sulfonated copolymers of styrene and divinylbenzene, would be expected to pyrolyze to sulfur dioxide and the pyrolysis products of crosslinked polystyrene. The pyrolysis of linear polystyrene gives a high yield of monomer partly by "chain unzipping"-the reverse of the free-radical polymerization reaction, but at 825 "C ethylene, acetylene, benzene, toluene, and ethylbenzene are produced as well (6). These products would be expected from random scission of the polymeric chain, followed by hydrogen-transfer (5) S. Siggia and L. R. Whitlock, A m i . Chem., 42;1719 (1970). (6) F. A . Lehmann and G. M. Brauer, Anal. Chem., 33,673 (1961).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
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c
I IA
\
leads
c-Pt
Mi n u t e l
t
Figure 3. Pyrograrns of Z column 1 at 100 “C
I
2225
0
UI C 0
a UI
al
0:
L
I
0
1
1
1
2
3
0
4
1
2
3
4
Minutes
Figure 2. Pyrograms of Amberlite IRA 400 and Zeokarb column 2 at 100 “C
225
on
reactions. The ratio of these products to styrene increases with increasing temperature and is generally greater from crosslinked than from linear polystyrene (7). Cation-exchange resins were more difficult to pyrolyze than crosslinked polystyrene. The yield of volatile products was low when the Curie Point Pyrolyzer was used at 770 “C, but better results were obtained at 980 “C, or with the filament pyrolyzer a t temperatures above 800 “C. The flame ionization detector was not sensitive to sulfur diox(7) R . H. Wiley. G. De Venuto. and F. E. Martin, J. Macrornol. Chem., 1, 137 (1966).
1660
and De-Acidite H-IP on
( A ) Benzene, (5)toluene, (C) xylene and ethylbenzene. ( D ) styrene, (E)ethyltoluene, ( F ) vinyltoluene
Figure 1. The filament pyrolyzer
L
225, IRA 400,
ide, and the highest peak in the pyrogram was that of benzene (Figure 3). The initial peak represents very volatile compounds such as ethylene and acetylene (6),which were not separated. The fourth peak, hereafter called the xylene peak, represents the xylene isomers and ethylbenzene which usually cannot be separated from each other on general purpose columns. Toluene and styrene were always found among the pyrolysis products. Effects of Counterions. The sodium form of a sulfonic resin produced very little volatile material on pyrolysis, styrene being the only appreciable peak in the pyrogram. This is in agreement with the featureless pyrogram published by Carpov and Hagen ( 3 ) . The tetramethylammonium form of the resin pyrolyzed to give more volatiles than the hydrogen form, and the largest peak was that of styrene. However, the hydrogen form of cation exchangers was used in further work, because capacity and swelling measurements are based on this form, resins are usually supplied in this form, and its pyrogram is sufficiently characteristic for the identification of the resin. Effect of Crosslinking. When pure copolymers of styrene and divinylbenzene are pyrolyzed, divinylbenzene monomer i s seen in the pyrogram (7). Commercial hydrocarbon copolymers, as used for the manufacture of ionexchange resins, were pyrolyzed in this laboratory, and peaks corresponding to m- and p-divinylbenzene were found. The height of the m-divinylbenzene peak, relative to that of styrene, gave a linear plot against the nominal ratio of divinylbenzene to styrene from 0 to 8% divinylbenzene. In contrast, no divinylbenzene could be found in the pyrograms of cation-exchange resins. However, a linear correlation was found between the nominal percentage of divinylbenzene and the height of the xylene peak relative to the benzene peak (Table I). It seems probable that the divinylbenzene residues in sulfonated copolymers yield mainly xylene on pyrolysis. In the pyrolysis of pure poly(m-divinylbenzene), m-xylene constituted 12% of the volatile products (7). This correlation enables one to estimate the crosslinking of a cation-exchange resin by careful comparison with resins of known crosslinking. Effect of Capacity. The pyrograms of “Amberlyst 15” and “Amberlite 200” had markedly higher styrene peaks than those of “Bio-Rad AG 50” resins (Table,I). A possible explanation is that the two macroreticular resins had lower capacity than the gel-type resins, and that the
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, A U G U S T 1973
Table II. Peak Heightsa of Pyrolysis Products Relative to Vinyltoluene
Table I. Peak Heightsa of Pyrolysis Products Relative to Benzene Resin
CapacitF
Toluene
Xylene
AG50 X 2
5.16
AG50 X 4 AG50 X 8
5.09
0.47 0.48 0.59 0.61 0.75
0.37 0.41 0.57 0.61 0.69 0.83
AG50 X 12 Arnberlyst 15 Amberlite 200
5.03 5.02 4.86 4.80
CI Obtained on coiurnn 1 at 160 aients per gram.
0.75
’.
Styrene
Ethyltoluene
0.14
Resin
Xylene
Styrene
0.15
Bio-Rad AG1 X 8 Arnberlite IRA 400
0.43 0.51 0.59 0.58
0.49 0.42
0.13
0.13
0.23 0.24
0.18 0.25
De-Acidite FF-IP De-Acidite H-IP
0.64 0.67
Measured capacity in rnilliequiv-
Obtained on coiurnn 1 at 100 “C.
-$H-CHz-
unsulfonated benzene rings gave rise to an increased yield of styrene. This is supported by the measured ion-exchange capacities which appear in Table I. Anion-Exchange Resins. Similar pyrograms were obtained from the chloride and the hydroxide forms of an anion-exchange resin. Since only the relative peak heights differed, the more stable chloride forms were used for the comparison of different anion exchangers. The conditions used for gas chromatography did not allow the separation of methyl chloride and the methylamines from each other, but mass spectrometry showed that methyl chloride was present in the initial peak of the pyrogram of even the weak base resin, “Deacidite H-IP.” Previous work has indicated that all three methylamines are obtained from a Type 1 resin ( 3 ) .The largest peak in the pyrogram was that of vinyltoluene (Figure 3), which was not found a t all in pyrograms of cation-exchange resins. p-Vinyltoluene could be produced by deamination (homolysis of a C-N bond, and abstraction of hydrogen), followed by chain unzipping of the poly(vinyltoluene), as shown in Figure 4. The peak before vinyltoluene is probably p-ethyltoluene. Benzene, toluene, xylene, and styrene were produced, but the proportion of benzene was much smaller than that obtained from cation-exchange resins. In “Dowex 1” resins, the height of the xylene peak, relative to that of vinyltoluene, increased linearly with the crosslinking (X2 to X8). Thus the divinylbenzene residues in anion-exchange resins, like those in cation exchangers, are pyrolyzed mainly to xylene. In some pyrograms, a peak with retention time similar to that of m-divinylbenzene was found, but the yield of this monomer was very low. Isoporous Resins. In resins of this type (8) the crosslinks are methylene bridges instead of divinylbenzene residues. A proportion of this type of crosslinking may be present in commercial anion-exchange resins crosslinked with divinylbenzene, since additional crosslinking occurs as a side reaction during chloromethylation of the hydrocarbon copolymer (9). The methylene-bridged benzene rings would be expected to yield xylene on pyrolysis, but not in the same proportion as divinylbenzene crosslinks. In practice, Permutit “isoporous” resins H-IP and FF-IP gave a relatively higher xylene peak than normal resins of the same swelling (Table 11), and pyrograms of isoporous resins showed more ethyltoluene and less styrene than normal resins. The different pattern can be seen in Figure 3. Presumably, less chain unzipping and more random scission occurs when methylene crosslinks are present. The figures for xylene and styrene in Table I1 indicate that some Friedel-Crafts crosslinking (methylene-bridging) occurs in the manufacture of “Amberlite IRA 400.” (8) T. R. E. Kressman, Effluent Wafer Treat. J . , 6, 119 (1966). (9) K. W. Pepper, H. M. Paisley, and M. A. Young, J. Chern. SOC., 1953,4097.
0.15
0.13
-$H-CHz-
-
(Iy, CH,-N’-CH, I
CH,
CI-
QCH2-N-CH3 y, +cnp
-?H-CHz-
8
CH3
CH= CHz
YH3
cn3
+ HN-CH3
Figure 4. Production of vinyltoluene by pyrolysis of an anionexchange resin
Unlike isoporous resins, macroreticular resins gave very similar pyrograms to those of normal resins, although Amberlite IRA 900 produced relatively less styrene than IRA 400. Acrylic Resins. Weak acid cation-exchange resins are crosslinked polymers of acrylic or methacrylic acid. These resins appear to pyrolyze by random chain fission, producing a large number of compounds. A high initial peak in the pyrogram is followed by a series of very small peaks (Figure 5). The products have not been identified, but retention times of three of the peaks correspond to those of benzene, toluene, and xylene. These compounds may arise from the divinylbenzene used for crosslinking. In samples of “Zeokarb 226” with 2.5 and 4.5% nominal divinylbenzene content, the heights of the xylene peak relative to the initial peak were 0.022 and 0.044, respectively. This is approximately the correct ratio, but when a series of acrylic acid-divinylbenzene copolymers was made in this laboratory, a divinylbenzene content of 9% was necessary to produce a xylene peak of relative height 0.044 on pyrolysis. The discrepancy may result from different methods of synthesis. The laboratory resins were made by bulk polymerization in sealed tubes to avoid any loss of monomers, whereas the commercial resins are probably made by suspension polymerization in an aqueous medium. Polymethacrylic acid copolymers had slightly different pyrograms ( e . g . , the presence of a peak at 2.3 min in Figure 5 ) from those of polyacrylic acid copolymers. Anionexchange resins with an acrylic matrix, such as “Amberlites” IRA 68 and IRA 458, gave pyrograms which were similar to, but which could be distinguished from, those of weak acid resins. Miscellaneous Resins. Ion-exchange resins made by condensation of phenols or amines with formaldehyde are no longer in common use, but they can be distinguished easily by their pyrograms (Figure 6). Their pyrolysis products have not been identified. Pyrograms have been obtained from specialized resins such as “Biorex” 62 and 63, “Bio-Rad” AG llA8, and “Dowex” Chelating Resin A-1. It was always possible to determine which resins were based on polystyrene and nearly always possible to distinguish resins from each other, but Biorex 62 and 63 gave very similar pyrograms. Reproducibility of Pyrograms. Alteration of the pyrolysis conditions was not found to change the nature of the pyrolysis products, but variations in yield, resolution, and
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
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Minutes
I R A 458 I
0
1
2
3
4
5
Figure 7. Pyrograms of De-Acidite H-IP on column 3 at 160 with Curie Point Pyrolyzer (CPP) and filament pyrolyzer (FP)
“C
Minutes
Figure 5. Pyrograms of acrylic resins Zeokarb 226, crosslinked poly(acry1ic acid) [PAA]. crosslinked poly(methacrylic acid) [PMA], and Amberlite IRA 45@on column 1 at 100 “C
1
IR 100
Minutes
Figure 6. Pyrograms of Amberlite IR 100 H and Amberlite I R 4 8 OH on column 1 at 100 “C
relative proportions of the products occurred. Since ionexchange resins are insoluble and infusible, the sample cannot be coated as a thin film on the filament of the pyrolyzer.* Finely ground resin did not adhere to the wire of the Curie Point Pyrolyzer, and therefore single particles of unground resin were used. Similar results were obtained with the different pyrolyzers (Figure 7), but the 1662
filament pyrolyzer gave a higher yield of volatile products from the same size of resin particle. Apparently the pyrolysis was complete, whereas in the Curie Point Pyrolyzer the resin particle was pyrolyzed mainly a t the point of contact with the wire hook. The dimensions of the apparatus did not allow the particles to be completely surrounded by a coil of wire. In ten consecutive determinations of the height of the xylene peak relative to that of the benzene peak in pyrograms of “Zeokarb 225,” the relative standard deviation was 15% with the Curie Point Pyrolyzer and 12% in a similar series with the filament pyrolyzer. When peak areas were measured instead of heights, the relative standard deviations were 17 and 1170,respectively. The mean ratios of peak areas were different for the two pyrolyzers (1.51 and 0.91, respectively). These values indicate that only semiquantitative determinations of the crosslinking of sulfonic acid resins can be made, but it might be possible to refine the method. For instance, a constant weight of sample could be taken by sieving out a very narrow range of particle sizes. However, the interparticle homogeneity of a commercial sample is never certain, and commercial resins are crosslinked with a mixture of m- and p-divinylbenzene, so that accurate determinations of the divinylbenzene content would be difficult. The retention times of the main pyrolysis products were reproducible; their relative standard deviations were 1-2%. Therefore, it should be possible to determine the type and approximate crosslinking of any ion-exchange resin by pyrolysis-gas chromatography. Received for review November 6, 1972. Accepted January 19. 1973.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
