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Raman spectroscopy of sulfonated polystyrene resins
Vibrational Spectroscopy 24 Ž2000. 213–224 www.elsevier.comrlocatervibspec
Raman spectroscopy of sulfonated polystyrene resins H.G.M. Edwards a,) , D.R. Brown b, J.A. Dale c , S. Plant c b
a Department of Chemical and Forensic Sciences, UniÕersity of Bradford, Bradford, West Yorkshire BD7 1DP, UK Department of Chemical and Biological Sciences, UniÕersity of Huddersfield, Queensgate, Huddersfield HD1 3DH, UK c Roybinek Research and DeÕelopment Laboratory, Purolite International Ltd., Cowbridge Road, Pontyclun, Mid Glamorgan CF 72 8YL, UK
Received 26 January 2000; received in revised form 5 April 2000; accepted 17 April 2000
Abstract Raman spectroscopy has been applied to the determination of molecular structural features in sulfonated polystyrene resins. Comparison with model compounds has facilitated the identification of key molecular vibrations which are characteristic of sulfonates and their undissociated acids. Of particular importance in the current study is the ability to undertake analysis of these resins in the hydrated state, with various amounts of water being present in the resins. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Sulfonate; Raman spectroscopy; Polystyrene resin; Sulfonic acid; Industrial catalysts; Quantitative
1. Introduction Solid organic polymeric resin materials containing acidic functionalities have been used for several decades in effecting chemical processes as alternatives to inorganic acidic zeolites. Resins seem to have several advantages as catalysts in industrial processes because of their potentially high acidity levels, controllable surface areas and porosity. The catalytic performance of these acid resins is not that well understood and several factors are deemed to be of importance to their acidity, including possible
) Corresponding author. Tel.: q44-1274-233787; fax: q441274-235350. E-mail address: [email protected] ŽH.G.M. Edwards..
cooperative effects between acid groups, degree of hydration of the resin matrix, and the tendency towards loss of acidity at elevated temperatures w1x. The sulfonation of polyaromatic resins has been undertaken to extend the versatility of these materials in ion exchange applications and as a means of improving their hydrophilic properties. Sulfonated polyŽarylether sulfones. are finding applications as catalysts for industrial processes particularly as synthetic membranes and in biomedicine, where the hemocompatibility of these membranes is strongly dependent on the degree of sulfonation w2x. Acidcatalysed hydration of propene is used to produce 2 M tons of isopropyl alcohol Žipa. per annum worldwide; the acidity and sulfonic acid content of the resins are believed to be important factors w3x in this addition of water across the ŽC5C. double bond in propene.
0924-2031r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 0 0 . 0 0 0 7 0 - 9
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
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In the present work, we have examined a series of 15 different sulfonated resins as prototypes for industrial catalysis. From the detailed Raman spectroscopic analysis of the resins and model aromatic sulfonic acids, characteristic spectral markers have been identified for both qualitative and quantitative determination of structural information. Novel vibrational spectroscopic features, which are sensitive to sulfonation, have been identified in the resins. This offers the potential for off-line quantitative evaluation of the degrees of sulfonation in commercial resins. A particular advantage of Raman spectroscopy in this respect is the insensitivity to adsorbed water in the specimens. Furthermore, the removal of water results in major structural changes in the gel region of the resin.
2. Experimental 2.1. Samples The model compounds ŽAldrich Chemical. which were used for our vibrational spectroscopic characterisation were: methanesulfonic acid, trifluoromethanesulfonic acid, 4-toluene sulfonic acid, benzenesulfonic acid, phenol sulfonic acid, and sodium benzenesulfonate. Analysis of the Raman spectra of these compounds permitted the assignment of vibrational spectroscopic markers which were indicative of –SO 3 H and –SOy 3 groups as well as the location of these groups on the aromatic rings. Several sulfonated polystyrene resins, both commercial and experimental, were subjected to Raman spectroscopic analysis. Although a major advantage
of Raman spectroscopy for the analysis of hydrophilic materials is its insensitivity to varying water content, several of the resins were highly fluorescent even with near-infrared laser excitation. Details of the acidities and hydration of the commercial resins studied here are presented in Table 1. The range of resins studied encompass C-100H, in which sulfonation in every aromatic ring has been effected, to SP21r51, in which only one ring in four has been sulfonated. 2.2. Preparation of sulfonated resins The C-100H and D3178 resins were prepared by the common, heterogeneous method of sulfonating styrene–divinylbenzene copolymers using concentrated sulfuric acid at temperatures between 808C and 1008C or higher. The other resins studied in this paper were prepared by homogeneous sulfonation after U.S. Patent 3,133,030 Žissued May 12, 1964.. Relevant characteristics of the resin specimens are reported in Table 1; it should be noted that the C-100H sample, with an acidity value of 5.25 mEq gy1 dry weight of resin, represents at 97% sulfonation almost one sulfonic acid group per aromatic ring and this is termed a Ahighly sulfonated resinB. In contrast, SP21r51 with a degree of sulfonation of only 24% is a Avery lowly sulfonated resinB, having only one in four aromatic rings sulfonated. 2.3. Raman spectra The Raman spectra were obtained using a Bruker IFS 66 infrared spectrometer with FRA 106 Raman module attachment and Nd 3qrYAG laser excitation
Table 1 Characteristics of the sulfonated polystyrene resins Resin code
Copolymer morphology
X-linking
Sulfonation process
Dry weight capacity a
Moisture Ž%.
Sulfonation degree Ž%. b
C-100H D3178 SP22r53 SP21r165 SP22r166 SP21r15 SP21r51
gel macroporous A macroporous B macroporous B macroporous B macroporous B macroporous B
low high high high high high high
sulfuric acid sulfuric acid US Patent 3,133,030 US Patent 3,133,030 US Patent 3,133,030 US Patent 3,133,030 US Patent 3,133,030
5.25 4.90 4.83 4.18 3.28 2.97 1.82
58 42 56 56 56 54 52
97 89 87 70 49 43 24
a b
Equivalents per dry kilogram of resin, or mEq gy1 . Calculated from the measured dry-weight-capacities.
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
in the near infrared at 1064 nm. Sampling was effected in a macroscopic mode using compacted resin beads in an aluminium sample AcupB, and scattered radiation was collected from a 100-mm-diameter AfootprintB. Spectra from individual resin beads were also taken using a dedicated Raman microscope attachment, for which a sample footprint of about 8 mm is achievable using a 100 = microscope objective lens. Co-addition of sample spectra over 100 scans, at 4 cmy1 spectral resolution and 200–3500 cmy1 shift range, was used to increase spectral signal-to-noise ratios. Wavenumbers of spectral bands are correct to "1 cmy1 , and relative intensities are compared w2x with respect to the symmetric ring stretching mode of the aromatic ring near 1000 cmy1 .
3. Results and discussion The Raman spectra of the model compounds were of good quality and enabled the identification of some key vibrational bands characteristic of major molecular features to be made. The spectra of ben-
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zene sulfonic acid, 4-toluene sulfonic acid, phenol sulfonic acid and sodium benzene sulfonate are shown stackplotted over the wavenumber ranges 2700–3100 cmy1 and 100–1800 cmy1 in Figs. 1 and 2, respectively. Some of the important molecular vibrational assignments in these compounds are provided in Table 2. The assignments are made by comparison of the vibrational spectra of these compounds and those of species such as methanesulfonic and trifluromethanesulfonic acids already in the literature w4–8x. To summarise, some key molecular indicators are: n Žaromatic CH stretch. 3055 cmy1 n Žaromatic CCH 1598 cmy1 quadrant stretch. n Žaromatic CC stretch. 1001 cmy1 n ŽSO 2 . sulfone 1190 cmy1 1130 cmy1 d ŽSO 2 . sulfone and acid ; 635 cmy1 3y n ŽSO , sulfonate ion. ; 1070 cmy1 n ŽCS, sulfone. 797 cmy1 n ŽCS, sulfonate ion. 770 cmy1 n ŽCS, sulfonic acidr 760 cmy1 complex.
Fig. 1. FT-Raman spectra of Ža. 4-toluenesulfonic, Žb. benzenesulfonic acid, Žc. phenol sulfonic acid and Žd. sodium benzenesulfonate. Wavenumber range 2700–3100 cmy1 , excitation wavelength 1064 nm, spectral resolution 4 cmy1 , 2000 scans accumulated.
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
216
Fig. 2. FT-Raman spectra of Ža. 4-toluenesulfonic acid, Žb. benzenesulfonic acid, Žc. phenol sulfonic acid and Žd. sodium benzene sulfonate. Wavenumber range, 100–1800 cmy1 . Conditions as in Fig. 1.
Some useful general molecular markers for aromatic sulfonated species are as follows: 1012 cmy1 : characteristic of p-substituted aryl ring of sulfonated aromatic rings.
q 1025–35 cmy1 : in a sulfonate ion, –SOy 3 M , q q varies with the counterions Li to Cs in wavenumber position and intensity w2x; provides a measure of the degree of sulfonation in polymeric polysulfones.
Table 2 Raman vibrational wavenumbers and molecular assignments for key features in benzenesulfonic acid, 4-toluenesulfonic acid, phenolsulfonic acid and sodium benzenesulfonate
n˜rcmy1
Assignment of vibrational mode
Benezenesulfonic acid
4-Toluenesulfonic acid
Phenolsulfonic acid
Sodium benzenesulfonate
3071 ms 3061 m, sh – 1583 m –
3065 ms
3078 s 3058 mw
3065 ms
n ŽCH. aromatic
1598 mw –
– 1589 m –
n ŽCH. aliphatic n ŽCCH. ring quadrant d ŽCH 3 .
1170 m
–
n ŽSO.
1128 m 1040 m – 1002 mw 814 m 630 mw 814 mw – 140 s
– 1048 m – 997 s 740 ms 615 mw 325 m – 140 mw
n ŽSO. n ŽSO 3 . n ŽCC., p-substituted ring n ŽCC. aromatic ring n ŽCS. d ŽSO 3 . d ŽCCS. d ŽCH 3 . r ŽSO 3 .
1170 m 1158 m 1130 mw 1024 m – 997 s 726 ms 614 m 320 mw – 137 mw
2931 ms 1599 m 1452 mw 1378 m 1187 m 1125 s 1040 m 1013 mw – 800 s 635 m 317 mw 268 mw –
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
Previous literature w2x identifies this band with the q moiety, R-SOy 3 H 3 O . Increase in water content of resin produces increase in this band intensity; it is
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also dependent on the counterion, e.g. Liq produces a shift to 1027 cmy1 . Note that this band is specific for R-SOy 3 and that the presence of H 2 SO4 is not
Fig. 3. FT-Raman spectra of sulfonated polystyrene resins studied in the present work: ŽA. Ža. SP21r15, Žb. SP21r51, Žc. C100H 2800–3100 cmy1 . ŽB. Ža. 20r165, Žb. D3178. Wavenumber range and conditions as in Fig. 1.
218
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
interfering — unlike the methods such as titration, e.g. where the resins are treated in sulfuric acid solutions and in which residual H 2 SO4 might also be present.
The Raman spectra of the sulfonic acid model compounds and of the sodium benzenesulfonate have facilitated the identification of the –SO 3 H and –SOy 3 vibrational bands in these compounds. A stackplot of
Fig. 4. FT-Raman spectra of some sulfonated polystyrene resins studied in the present work. Wavenumber range 100–1800 cmy1 , conditions as in ŽA. Ža. SP21r15, Žb. SP21r51, Žc. C100H. ŽB. Ža. 20r165, Žb. D3178.
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
the Raman spectra of several of the polysulfonated resins studied in the present work is shown in Figs. 3 and 4 for the wavenumber ranges, 2700–3100 cmy1 and 100–1800 cmy1 , respectively. The six sulfonated resins, which yielded good quality Raman spectra ŽTable 1., had several common features related to the model sulfonic acid and sulfonate group modes which can be summarised as follows. Ži. Band groupings in the region of 800 cmy1 indicate the presence of p-substituted sulfonic acid groups in the polystyrene resins. Žii. There are two bands at 1127 and 1035 cmy1 in the resins, assigned to n ŽSO. modes of undissociated sulfonic acid and sulfonate groups, respectively w2,6x, which are present in all resin spectra studied here. This indicates that the resins contain both –SO 3 H and –SOy 3 groups. Žiii. The relative intensities of the 1127 and 1035 cmy1 bands, representing –SO 3 H and –SOy 3 groups, are significantly different in the six resin specimens. Table 3 gives the average values of Irel s I 1127rI 1035 from four band area measurements of representative samples from each resin studied. The Raman spectra of the two resins, SP21r15 and SP 21r51, which are two batch-processed resin specimens with degrees of sulfonation of 43% and 24%, respectively, relative to C-100H at 97% sulfonation, are reproduced over the region 950–1250cmy1 in
Table 3 Relative intensity measurements of the Raman 1127 and 1035 . cmy1 bands, Žrepresenting –SO 3 H and –SOy 3 groups Resin code
Acidity mEq gy1 Ždry. a
Relative band intensitiesb I 1127rI 1035 I 1127rI 1003 I 1035 rI 1003
SP 21r51 SP 21r15 SP 22r53 SP 21r165 SP 22r53 D 3178 C-100H
1.82 2.97 2.85 4.18 4.83 4.90 5.25
0.09 1.36 1.21 2.83 3.64 3.89 4.14
a
0.02 0.45 0.37 1.32 9.00 1.54 18.93
0.23 0.33 0.30 0.47 2.47 0.40 4.57
Measure of the total resin sulfonation; C-100H is essentially 100% sulfonated, representing one sulfonic acid group per phenyl ring. b Band intensities of 1127 and 1035 cmy1 features, relative to the aromatic ring stretching mode of the phenyl group at 1003 cmy1 .
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Fig. 5. Wavenumbers of observed bands for the SP21r15 and SP 21r51 specimens are given in Table 4. The Raman spectroscopic analysis indicates that SP21r51 has a ratio of –SOy 3 to –SO 3 H of approximately 10:1, compared with a 1:1.5 approximate ratio for the SP21r15 specimen. This situation seems to mirror that of sulfonic acids in aqueous solution w5,6x, where increased AsulfonationB, i.e. formal sulfonic acid concentration, is paralleled by an increase in –SO 3 H concentration as measured using Raman spectroscopy. Therefore, the role of water in the resin gel structure is clearly not a passive one w9,10x. Historically, it has been established that the water in the gel region of a resin is dependent on the degree of functionalisation and also on the crosslinking of the resin polymer matrix. Analysis of the spectroscopic band intensity data in Table 3 produces some interesting conclusions. Ži. The I 1127rI 1035 band ratio increases steadily from the resin specimen of lowest acid content Ž1.82 mEq gy1 . to the practically fully sulfonated specimen Ž5.25 mEq gy1 .. A plot of determined resin acidity against this band intensity ratio gives a straight line of slope 0.83 and an extrapolated intercept on the y-axis of 1.82 mEq gy1 . We conclude that a progressive increase in the sulfonation of the polystyrene resins results in an increased content of undissociated sulfonic acid groups at the expense of the ionised sulfonate. This is confirmed from the I 1127rI 1003 band ratios in Table 3, representing undissociated sulfonate groups relative to phenyl ring content in the resins. The effect of p-substitution on the sulfonic acid is a decrease in band intensity of the aromatic ring n ŽCC. stretching band near 1003 cmy1 and this is clearly seen in Fig. 2, where for the toluene sulfonic acid and phenol sulfonic acid species the phenyl ring mode has decreased significantly in intensity, compared with the monosubstituted analogues. However, Table 3 clearly indicates that for a nearly threefold increase in resin acidity, the I 1127rI 1003 band ratio increases nearly a hundredfold Žfrom specimens SP21r51 to D3178.. Over this same acidity range, there is only a twofold increase in the sulfonate ion band ratio, measured as I 1035rI 1003. The sulfonate ion band intensity ratio changes over this acidity range are therefore complicated by the intensity changes in the phenyl ring
220
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
Fig. 5. FT-Raman spectra of three sulfonated polystyrene resins: Ža. SP21r15, Žb. SP21r51, Žc. C100H; wavenumber range, 950–1250 cmy1 , conditions as in Fig. 4.
mode, with a mean value of 0.35 " 0.12, excluding the fully sulfonated C-100H specimen.
Žii. Fig. 6 indicates that at low acidity values near that represented by the SP21r51 specimen studied
Table 4 Raman band wavenumber and vibrational assignments for SP21r51 and SP21r15 resin samples Degree of sulfonation
SP21r51 24% nrcmy1
SP21r15 43% nrcmy1
Approximate assignment of vibrational mode
3057 3040 sh 2999 2972 2905 2852 sh 1600 1583 1450 1321 br 1200 sh 1182 1155 1127 w 1033 1001 775 754 638 620 221
3060 3040 sh 3000 2976 2908 2857 sh 1600 1583 1450 1326 br 1200 1183 1155 1127 1033 1001 772 756 635 620 220
aromatic CH stretch n ŽCH. aromatic n ŽCH. aliphatic n ŽCH. aliphatic n ŽCH 2 . n ŽCH 3 . aromatic ring quadrant stretching aromatic ring quadrant stretching d ŽCH 3 . n ŽSO 2 . asymmetric stretch n ŽSO 2 . symmetric Žfree acid. n ŽSO 2 . symmetric Žassociated. n ŽSO 2 . sulfone n ŽSO 2 . of –SO 2 OH . n ŽSOy 3 aromatic CC stretch breathing mode n ŽC–S. sulfonate ion n ŽC–S. of sulfonic acid d ŽSO 2 .
t ŽCH 3 .
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
221
Fig. 6. Graphical plot of I 1127rI 1003 band intensity ratios Ž'. against measured acidity values Žmilliequivalents per gram. for resin specimens studied in the present work.
here, the resins will have the smallest sulfonic acidrsulfonate ratio. This conclusion is supported by the observations of Zundel and Metzger w11x and Zundel w12x, who advocate the presence of aggregation of sulfonic acid groupings and stabilisation through internal hydrogen bonding at high acidities. Gates et al. w13x have also noted that sulfonated polymer resins were much more effective in promoting the dehydration of t-butyl alcohol than were the corresponding sulfonated soluble catalysts — this was ascribed to high local density of sulfonic acid groups acting in a concerted fashion. Žiii. To test of the use of the graphical plot in Fig. 6 as a predictor of resin acidity, we subjected a newly synthesised polystyrene sulfonated resin to Raman spectroscopic analysis. The spectrum of specimen D3895 is shown in Fig. 7 over the wavenumber range 100–1800 cmy1 ; the 1127, 1035 and 1003 cmy1 bands are clearly identifiable. From the relative intensity of the phenyl ring mode in the spectrum, we would conclude qualitatively that this resin specimen was in the Alow sulfonationB category. The I 1127rI 1035 band intensity ratio was measured at 1.12,
from which we deduce that it would have an acidity value of 2.60 " 0.08 mEq gy1 . This compares favourably with the value of A37% sulfonationB from a titration value of 2.63 " 0.05 mEq gy1 . Thus, Raman spectroscopy can be used to determine quantitatively and nondestructively the acidity value of sulfonated resins, and in their water-swollen state. These quantitative measurements, which we believe are the first of their kind to be made on such systems by Raman spectroscopic techniques, could be of key importance in devising new sulfonated resins with different activities for catalytic applications. It is maintained in the literature w3x that the total degree of sulfonation of a resin may be less important than the proportion of groups that are present as sulfonic acid, i.e. catalytic activity could depend not only on the total degree of sulfonation but strongly on the ratio of –SO 3 H to –SOy 3 moieties present w9x. In this respect, the degree of hydration of the resin should also be critically important since the ionisation of SO 3 H groups is strongly dependent on the formal solution concentration w5,6,10x. Buttersack w14x has assessed the accessibil-
222
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
Fig. 7. FT-Raman spectrum of an anonymous sulfonated resin specimen D3895; wavenumber range conditions as in Fig. 2.
ity and catalytic activity of resins in several solvents and has drawn attention to the influence of swelling in the gel phase affected by the association of –SO 3 H groups via hydrogen bonding. Zundel and Metzger w11x and Zundel w12x have deduced from infrared spectroscopic studies of cation exchange membranes that at least two molecules of water are necessary to effect ionisation of a sulfonic acid group. When the water to sulfonic acid molecular ratio was 1, no proton dissociation occurred, and the water molecule was strongly hydrogen-bonded to three –SO 3 H groups. The consequence of this model for the catalytic activity of hydrated sulfonated ion exchange resins has been explored by Kalynam and Sivaram w10x. Also, Delmas and Gaset w15x concluded that the maximum yield of reaction between styrene and paraformaldelyde catalysed by a sulfonated resin was obtained when the ratio of water molecules to sulfonic acid groups was 2. Examination of the Raman spectra of the simple model compounds, benzenesulfonic acid, phenolsulfonic acid and 4-toluenesulfonic acid, is also of interest from the point of view of discerning the presence of –SO 3 H and –SO 3 groups in these solid materials. All three model compounds have Raman bands at 1127 and 1035 cmy1 , indicating that they
q have both –SO 3 H and –SOy species in equilib3 H rium. However, the ratio of sulfonic acid to sulfonate anion band intensities is significantly different for these three acids, being approximately 1.5, 1.2 and 3.0 for benzenesulfonic, phenolsulfonic acid and 4toluenesulfonic acid, respectively. This indicates that in the solid state, 4-toluene sulfonic acid is significantly less ionised than either benzene sulfonic or phenol sulfonic acids. The sulfonated resins with higher acidity values in Table 3 are also deserving of comment. For example, C-100H, a 97%-sulfonated resin, has a significantly greater Raman band intensity at 1127 cmy1 relative to that at 1033 cmy1 ŽFig. 5.. The Irel value for C-100H at 4.14 means that there are significantly more SO 3 H groups than there are free –SOy 3 groups; this could indicate that undissociated SO 3 H groups on neighbouring aromatic rings are stabilised through a hydrogen-bonded structure, as represented in Fig. 8. Such hydrogen-bonded structures have already been identified in sulfonic acid chemistry and related to their acidities and metal complexation ability w16x. This is paralleled in the case of 4-toluenesulfonic acid where an Irel value of about 3.0 for the 1127 and 1033 cmy1 band intensities is also found in the present work. Similarly, higher alkyl-substituted sul-
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
Fig. 8. Diagrammatic representation of intramolecularly hydrogen-bonded sulfonic acid groups in a highly sulfonated resin such as C-100H, with one –SO 3 H group per aromatic ring.
fonic acids have lower dissociations observed by Raman spectroscopy than the parent methanesulfonic acid or fluoro-substituted analogues w5,6x. In more lowly sulfonated resins, such as SP21r15 or SP21r51, which are only 24–43% sulfonated, it is unlikely that many of the –SO 3 H groups can find another SO 3 H group for hydrogen bonding, on an nearby aromatic ring; hence, the competition with q the –SOy moiety in these sulfonated species 3 H 3O is much favoured towards the ionised structures in the presence of larger molar ratios of H 2 O:SO 3 H. Another interesting feature with C-100H are two modes at 635 and 620 cmy1 , which are clearly doublets of different intensities in the SP21r15 and
223
SP21r51 specimens ŽFig. 4.; whereas, a significantly stronger singlet at 635 cmy1 is observed in C-100H. Clearly, therefore, the 635 cmy1 band is a marker for the –SO 3 H group attached to the polystyrene backbone, and the large intensity of this mode swamps the feature at 620 cmy1 assigned to q the mode of the –SOy group. In SP21 and 3 H 3O SP21r51, however, the presence of significantly greater amounts of the ionised sulfonate groups results in the greater intensity of the 620 cmy1 band observed in these specimens; in Fig. 4, the significantly lower intensity of this 620 cmy1 band for C-100H reflects the much lower concentration of sulfonate ions and is too weak to be seen at this scale. Another significant difference between C-100H and SP21r15 or SP21r51 in the 100–1800 cmy1 region is the intensity of the n ŽCC. aromatic ring stretching mode at 1003 cmy1 . C-100H, a highly sulfonated resin, has a very weak band at 1003 cmy1 , similar to that of 4-toluenesulfonic acid Ža model A100% sulfonatedB, aromatic compound.. In contrast, SP21r15 and SP21r51 still have a strong Raman band at 1003 cmy1 , which has to reflect their unsubstituted aromatic ring content w17,18x.
Fig. 9. FT-Raman microscope spectra of sulfonated resin specimen single bead ŽSample SP21r15. 40 = microscope objective, 2000 scans, 1064 nm excitation.
224
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
The data presented in Table 3 and the graphical plot in Fig. 6 indicate that the 1127r1035 Raman band intensity ratio measurements are suitable for the determination of the acidity values of the sulfonated resins studied here. However, a comparison of resin samples SP22r53 and D3178, with nearly identical acidity values of 4.83 and 4.90 mEq gy1 , respectively, is of interest since these resins were prepared by two different sulfonation procedures. Although the 1127r1035 Raman band intensity ratios for the SP22r53 and D3178 are reassuringly similar at 3.64 and 3.89, respectively, clearly there is a significant difference between the 1127r1003 and 1035r1003 band ratios which are 9.00, 2.47 and 1.54, 0.40, respectively. We intend to investigate further this phenomenon with selected resins of similar acidity values which have been prepared by different sulfonation methods. Fourier transform ŽFT.-Raman microscopy was used to examine a single resin bead Žresin sample SP21r15. and a section of that bead. A typical spectrum is shown in Fig. 9, which is of significantly poorer quality than that of the resins obtained in a macroscopic sampling mode. However, the major features indicate that there are no significant differences between the Raman spectra of a single resin bead, a sectioned bead, or the bulk resin.
4. Conclusions Raman spectroscopy has demonstrated the ability to measure non-destructively the degree of sulfonation of hydrated polystyrene resins. Spectral markers have been identified which indicate the presence in the hydrated resins of both Ža. undissociated and Žb. ionised forms of the sulfonic acid group, i.e. –SO 3 H q and –SOy 3 H 3 O . The situation is further complicated, it is realised, by the influence of ion pairs between H 3 Oq and –SOy 3 and undissociated species in the Raman spectra of the sulfonated resins, and this will be the concern of future studies in conjunc-
tion with the degree of hydration. These studies will address a wider range of sulfonic acid resins and resins of catalytic importance, and relating to their activities and stabilities. Acknowledgements The authors are grateful to EPSRC and the DTI for financial support ŽACCP LINK grant GRr L87859. during which the work described was undertaken. References w1x P. Weil, D. Farcasiu, Eur. Chem. News 21 Ž1998. July issue. w2x S. Sollinger, M. Diamantoglou, J. Raman Spectrosc. 28 Ž1997. 811. w3x B.C. Gates, Catalytic Chemistry, Wiley, Chichester, 1992, p. 192. w4x D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Grasselli, Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, New York, 1991, p. 244. w5x H.G.M. Edwards, Spectrochim. Acta, Part A 45 Ž1989. 715. w6x H.G.M. Edwards, D.N. Smith, J. Mol. Struct. 238 Ž1990. 27. w7x H.G.M. Edwards, I.R. Lewis, J. Mol. Struct. 301 Ž1993. 37. w8x A.K. Covington, R. Thompson, J. Solution Chem. 3 Ž1974. 603. w9x R. Thornton, B.C. Gates, J. Catal. 34 Ž1974. 275. w10x N. Kalynam, S. Sivaram, in: D. Nader, M. Streat ŽEds.., Ion Exchange Technology, Ellis Horwood, Chichester, 1984, p. 458. w11x G. Zundel, H. Metzger, Z. Phys. Chem. 59 Ž1965. 225. w12x G. Zundel, Angew. Chem., Int. Ed. Engl. 8 Ž1966. 499. w13x B.C. Gates, J.S. Wisnouskas, H.W. Heath Jr., J. Catal. 24 Ž1972. 320. w14x C. Buttersack, React. Polym. 10 Ž1989. 143. w15x M. Delmas, A. Gaset, J. Mol. Catal. 17 Ž1982. 35. w16x N. Furukawa, H. Fujihara, in: S. Patai, Z. Rappoport ŽEds.., The Chemistry of Sulfonic Acids, Esters and Their Derivatives, Wiley, Chichester, 1991, p. 261, Chap. 7. w17x G. Ellis, A. Sanchez, P.J. Hendra, H.A. Willis, J.M. Chalmers, J.G. Eaves, W.G. Gaskin, K.-N. Kryer, J. Mol. Struct. 247 Ž1991. 385. w18x J.K. Agbenyega, G. Ellis, P.J. Hendra, W.F. Maddams, C. Passingham, J.M. Chalmers, H.A. Willis, Spectrochim. Acta, Part A 46 Ž1990. 197.
Raman spectroscopy of sulfonated polystyrene resins H.G.M. Edwards a,) , D.R. Brown b, J.A. Dale c , S. Plant c b
a Department of Chemical and Forensic Sciences, UniÕersity of Bradford, Bradford, West Yorkshire BD7 1DP, UK Department of Chemical and Biological Sciences, UniÕersity of Huddersfield, Queensgate, Huddersfield HD1 3DH, UK c Roybinek Research and DeÕelopment Laboratory, Purolite International Ltd., Cowbridge Road, Pontyclun, Mid Glamorgan CF 72 8YL, UK
Received 26 January 2000; received in revised form 5 April 2000; accepted 17 April 2000
Abstract Raman spectroscopy has been applied to the determination of molecular structural features in sulfonated polystyrene resins. Comparison with model compounds has facilitated the identification of key molecular vibrations which are characteristic of sulfonates and their undissociated acids. Of particular importance in the current study is the ability to undertake analysis of these resins in the hydrated state, with various amounts of water being present in the resins. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Sulfonate; Raman spectroscopy; Polystyrene resin; Sulfonic acid; Industrial catalysts; Quantitative
1. Introduction Solid organic polymeric resin materials containing acidic functionalities have been used for several decades in effecting chemical processes as alternatives to inorganic acidic zeolites. Resins seem to have several advantages as catalysts in industrial processes because of their potentially high acidity levels, controllable surface areas and porosity. The catalytic performance of these acid resins is not that well understood and several factors are deemed to be of importance to their acidity, including possible
) Corresponding author. Tel.: q44-1274-233787; fax: q441274-235350. E-mail address: [email protected] ŽH.G.M. Edwards..
cooperative effects between acid groups, degree of hydration of the resin matrix, and the tendency towards loss of acidity at elevated temperatures w1x. The sulfonation of polyaromatic resins has been undertaken to extend the versatility of these materials in ion exchange applications and as a means of improving their hydrophilic properties. Sulfonated polyŽarylether sulfones. are finding applications as catalysts for industrial processes particularly as synthetic membranes and in biomedicine, where the hemocompatibility of these membranes is strongly dependent on the degree of sulfonation w2x. Acidcatalysed hydration of propene is used to produce 2 M tons of isopropyl alcohol Žipa. per annum worldwide; the acidity and sulfonic acid content of the resins are believed to be important factors w3x in this addition of water across the ŽC5C. double bond in propene.
0924-2031r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 0 0 . 0 0 0 7 0 - 9
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
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In the present work, we have examined a series of 15 different sulfonated resins as prototypes for industrial catalysis. From the detailed Raman spectroscopic analysis of the resins and model aromatic sulfonic acids, characteristic spectral markers have been identified for both qualitative and quantitative determination of structural information. Novel vibrational spectroscopic features, which are sensitive to sulfonation, have been identified in the resins. This offers the potential for off-line quantitative evaluation of the degrees of sulfonation in commercial resins. A particular advantage of Raman spectroscopy in this respect is the insensitivity to adsorbed water in the specimens. Furthermore, the removal of water results in major structural changes in the gel region of the resin.
2. Experimental 2.1. Samples The model compounds ŽAldrich Chemical. which were used for our vibrational spectroscopic characterisation were: methanesulfonic acid, trifluoromethanesulfonic acid, 4-toluene sulfonic acid, benzenesulfonic acid, phenol sulfonic acid, and sodium benzenesulfonate. Analysis of the Raman spectra of these compounds permitted the assignment of vibrational spectroscopic markers which were indicative of –SO 3 H and –SOy 3 groups as well as the location of these groups on the aromatic rings. Several sulfonated polystyrene resins, both commercial and experimental, were subjected to Raman spectroscopic analysis. Although a major advantage
of Raman spectroscopy for the analysis of hydrophilic materials is its insensitivity to varying water content, several of the resins were highly fluorescent even with near-infrared laser excitation. Details of the acidities and hydration of the commercial resins studied here are presented in Table 1. The range of resins studied encompass C-100H, in which sulfonation in every aromatic ring has been effected, to SP21r51, in which only one ring in four has been sulfonated. 2.2. Preparation of sulfonated resins The C-100H and D3178 resins were prepared by the common, heterogeneous method of sulfonating styrene–divinylbenzene copolymers using concentrated sulfuric acid at temperatures between 808C and 1008C or higher. The other resins studied in this paper were prepared by homogeneous sulfonation after U.S. Patent 3,133,030 Žissued May 12, 1964.. Relevant characteristics of the resin specimens are reported in Table 1; it should be noted that the C-100H sample, with an acidity value of 5.25 mEq gy1 dry weight of resin, represents at 97% sulfonation almost one sulfonic acid group per aromatic ring and this is termed a Ahighly sulfonated resinB. In contrast, SP21r51 with a degree of sulfonation of only 24% is a Avery lowly sulfonated resinB, having only one in four aromatic rings sulfonated. 2.3. Raman spectra The Raman spectra were obtained using a Bruker IFS 66 infrared spectrometer with FRA 106 Raman module attachment and Nd 3qrYAG laser excitation
Table 1 Characteristics of the sulfonated polystyrene resins Resin code
Copolymer morphology
X-linking
Sulfonation process
Dry weight capacity a
Moisture Ž%.
Sulfonation degree Ž%. b
C-100H D3178 SP22r53 SP21r165 SP22r166 SP21r15 SP21r51
gel macroporous A macroporous B macroporous B macroporous B macroporous B macroporous B
low high high high high high high
sulfuric acid sulfuric acid US Patent 3,133,030 US Patent 3,133,030 US Patent 3,133,030 US Patent 3,133,030 US Patent 3,133,030
5.25 4.90 4.83 4.18 3.28 2.97 1.82
58 42 56 56 56 54 52
97 89 87 70 49 43 24
a b
Equivalents per dry kilogram of resin, or mEq gy1 . Calculated from the measured dry-weight-capacities.
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
in the near infrared at 1064 nm. Sampling was effected in a macroscopic mode using compacted resin beads in an aluminium sample AcupB, and scattered radiation was collected from a 100-mm-diameter AfootprintB. Spectra from individual resin beads were also taken using a dedicated Raman microscope attachment, for which a sample footprint of about 8 mm is achievable using a 100 = microscope objective lens. Co-addition of sample spectra over 100 scans, at 4 cmy1 spectral resolution and 200–3500 cmy1 shift range, was used to increase spectral signal-to-noise ratios. Wavenumbers of spectral bands are correct to "1 cmy1 , and relative intensities are compared w2x with respect to the symmetric ring stretching mode of the aromatic ring near 1000 cmy1 .
3. Results and discussion The Raman spectra of the model compounds were of good quality and enabled the identification of some key vibrational bands characteristic of major molecular features to be made. The spectra of ben-
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zene sulfonic acid, 4-toluene sulfonic acid, phenol sulfonic acid and sodium benzene sulfonate are shown stackplotted over the wavenumber ranges 2700–3100 cmy1 and 100–1800 cmy1 in Figs. 1 and 2, respectively. Some of the important molecular vibrational assignments in these compounds are provided in Table 2. The assignments are made by comparison of the vibrational spectra of these compounds and those of species such as methanesulfonic and trifluromethanesulfonic acids already in the literature w4–8x. To summarise, some key molecular indicators are: n Žaromatic CH stretch. 3055 cmy1 n Žaromatic CCH 1598 cmy1 quadrant stretch. n Žaromatic CC stretch. 1001 cmy1 n ŽSO 2 . sulfone 1190 cmy1 1130 cmy1 d ŽSO 2 . sulfone and acid ; 635 cmy1 3y n ŽSO , sulfonate ion. ; 1070 cmy1 n ŽCS, sulfone. 797 cmy1 n ŽCS, sulfonate ion. 770 cmy1 n ŽCS, sulfonic acidr 760 cmy1 complex.
Fig. 1. FT-Raman spectra of Ža. 4-toluenesulfonic, Žb. benzenesulfonic acid, Žc. phenol sulfonic acid and Žd. sodium benzenesulfonate. Wavenumber range 2700–3100 cmy1 , excitation wavelength 1064 nm, spectral resolution 4 cmy1 , 2000 scans accumulated.
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
216
Fig. 2. FT-Raman spectra of Ža. 4-toluenesulfonic acid, Žb. benzenesulfonic acid, Žc. phenol sulfonic acid and Žd. sodium benzene sulfonate. Wavenumber range, 100–1800 cmy1 . Conditions as in Fig. 1.
Some useful general molecular markers for aromatic sulfonated species are as follows: 1012 cmy1 : characteristic of p-substituted aryl ring of sulfonated aromatic rings.
q 1025–35 cmy1 : in a sulfonate ion, –SOy 3 M , q q varies with the counterions Li to Cs in wavenumber position and intensity w2x; provides a measure of the degree of sulfonation in polymeric polysulfones.
Table 2 Raman vibrational wavenumbers and molecular assignments for key features in benzenesulfonic acid, 4-toluenesulfonic acid, phenolsulfonic acid and sodium benzenesulfonate
n˜rcmy1
Assignment of vibrational mode
Benezenesulfonic acid
4-Toluenesulfonic acid
Phenolsulfonic acid
Sodium benzenesulfonate
3071 ms 3061 m, sh – 1583 m –
3065 ms
3078 s 3058 mw
3065 ms
n ŽCH. aromatic
1598 mw –
– 1589 m –
n ŽCH. aliphatic n ŽCCH. ring quadrant d ŽCH 3 .
1170 m
–
n ŽSO.
1128 m 1040 m – 1002 mw 814 m 630 mw 814 mw – 140 s
– 1048 m – 997 s 740 ms 615 mw 325 m – 140 mw
n ŽSO. n ŽSO 3 . n ŽCC., p-substituted ring n ŽCC. aromatic ring n ŽCS. d ŽSO 3 . d ŽCCS. d ŽCH 3 . r ŽSO 3 .
1170 m 1158 m 1130 mw 1024 m – 997 s 726 ms 614 m 320 mw – 137 mw
2931 ms 1599 m 1452 mw 1378 m 1187 m 1125 s 1040 m 1013 mw – 800 s 635 m 317 mw 268 mw –
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
Previous literature w2x identifies this band with the q moiety, R-SOy 3 H 3 O . Increase in water content of resin produces increase in this band intensity; it is
217
also dependent on the counterion, e.g. Liq produces a shift to 1027 cmy1 . Note that this band is specific for R-SOy 3 and that the presence of H 2 SO4 is not
Fig. 3. FT-Raman spectra of sulfonated polystyrene resins studied in the present work: ŽA. Ža. SP21r15, Žb. SP21r51, Žc. C100H 2800–3100 cmy1 . ŽB. Ža. 20r165, Žb. D3178. Wavenumber range and conditions as in Fig. 1.
218
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
interfering — unlike the methods such as titration, e.g. where the resins are treated in sulfuric acid solutions and in which residual H 2 SO4 might also be present.
The Raman spectra of the sulfonic acid model compounds and of the sodium benzenesulfonate have facilitated the identification of the –SO 3 H and –SOy 3 vibrational bands in these compounds. A stackplot of
Fig. 4. FT-Raman spectra of some sulfonated polystyrene resins studied in the present work. Wavenumber range 100–1800 cmy1 , conditions as in ŽA. Ža. SP21r15, Žb. SP21r51, Žc. C100H. ŽB. Ža. 20r165, Žb. D3178.
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
the Raman spectra of several of the polysulfonated resins studied in the present work is shown in Figs. 3 and 4 for the wavenumber ranges, 2700–3100 cmy1 and 100–1800 cmy1 , respectively. The six sulfonated resins, which yielded good quality Raman spectra ŽTable 1., had several common features related to the model sulfonic acid and sulfonate group modes which can be summarised as follows. Ži. Band groupings in the region of 800 cmy1 indicate the presence of p-substituted sulfonic acid groups in the polystyrene resins. Žii. There are two bands at 1127 and 1035 cmy1 in the resins, assigned to n ŽSO. modes of undissociated sulfonic acid and sulfonate groups, respectively w2,6x, which are present in all resin spectra studied here. This indicates that the resins contain both –SO 3 H and –SOy 3 groups. Žiii. The relative intensities of the 1127 and 1035 cmy1 bands, representing –SO 3 H and –SOy 3 groups, are significantly different in the six resin specimens. Table 3 gives the average values of Irel s I 1127rI 1035 from four band area measurements of representative samples from each resin studied. The Raman spectra of the two resins, SP21r15 and SP 21r51, which are two batch-processed resin specimens with degrees of sulfonation of 43% and 24%, respectively, relative to C-100H at 97% sulfonation, are reproduced over the region 950–1250cmy1 in
Table 3 Relative intensity measurements of the Raman 1127 and 1035 . cmy1 bands, Žrepresenting –SO 3 H and –SOy 3 groups Resin code
Acidity mEq gy1 Ždry. a
Relative band intensitiesb I 1127rI 1035 I 1127rI 1003 I 1035 rI 1003
SP 21r51 SP 21r15 SP 22r53 SP 21r165 SP 22r53 D 3178 C-100H
1.82 2.97 2.85 4.18 4.83 4.90 5.25
0.09 1.36 1.21 2.83 3.64 3.89 4.14
a
0.02 0.45 0.37 1.32 9.00 1.54 18.93
0.23 0.33 0.30 0.47 2.47 0.40 4.57
Measure of the total resin sulfonation; C-100H is essentially 100% sulfonated, representing one sulfonic acid group per phenyl ring. b Band intensities of 1127 and 1035 cmy1 features, relative to the aromatic ring stretching mode of the phenyl group at 1003 cmy1 .
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Fig. 5. Wavenumbers of observed bands for the SP21r15 and SP 21r51 specimens are given in Table 4. The Raman spectroscopic analysis indicates that SP21r51 has a ratio of –SOy 3 to –SO 3 H of approximately 10:1, compared with a 1:1.5 approximate ratio for the SP21r15 specimen. This situation seems to mirror that of sulfonic acids in aqueous solution w5,6x, where increased AsulfonationB, i.e. formal sulfonic acid concentration, is paralleled by an increase in –SO 3 H concentration as measured using Raman spectroscopy. Therefore, the role of water in the resin gel structure is clearly not a passive one w9,10x. Historically, it has been established that the water in the gel region of a resin is dependent on the degree of functionalisation and also on the crosslinking of the resin polymer matrix. Analysis of the spectroscopic band intensity data in Table 3 produces some interesting conclusions. Ži. The I 1127rI 1035 band ratio increases steadily from the resin specimen of lowest acid content Ž1.82 mEq gy1 . to the practically fully sulfonated specimen Ž5.25 mEq gy1 .. A plot of determined resin acidity against this band intensity ratio gives a straight line of slope 0.83 and an extrapolated intercept on the y-axis of 1.82 mEq gy1 . We conclude that a progressive increase in the sulfonation of the polystyrene resins results in an increased content of undissociated sulfonic acid groups at the expense of the ionised sulfonate. This is confirmed from the I 1127rI 1003 band ratios in Table 3, representing undissociated sulfonate groups relative to phenyl ring content in the resins. The effect of p-substitution on the sulfonic acid is a decrease in band intensity of the aromatic ring n ŽCC. stretching band near 1003 cmy1 and this is clearly seen in Fig. 2, where for the toluene sulfonic acid and phenol sulfonic acid species the phenyl ring mode has decreased significantly in intensity, compared with the monosubstituted analogues. However, Table 3 clearly indicates that for a nearly threefold increase in resin acidity, the I 1127rI 1003 band ratio increases nearly a hundredfold Žfrom specimens SP21r51 to D3178.. Over this same acidity range, there is only a twofold increase in the sulfonate ion band ratio, measured as I 1035rI 1003. The sulfonate ion band intensity ratio changes over this acidity range are therefore complicated by the intensity changes in the phenyl ring
220
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
Fig. 5. FT-Raman spectra of three sulfonated polystyrene resins: Ža. SP21r15, Žb. SP21r51, Žc. C100H; wavenumber range, 950–1250 cmy1 , conditions as in Fig. 4.
mode, with a mean value of 0.35 " 0.12, excluding the fully sulfonated C-100H specimen.
Žii. Fig. 6 indicates that at low acidity values near that represented by the SP21r51 specimen studied
Table 4 Raman band wavenumber and vibrational assignments for SP21r51 and SP21r15 resin samples Degree of sulfonation
SP21r51 24% nrcmy1
SP21r15 43% nrcmy1
Approximate assignment of vibrational mode
3057 3040 sh 2999 2972 2905 2852 sh 1600 1583 1450 1321 br 1200 sh 1182 1155 1127 w 1033 1001 775 754 638 620 221
3060 3040 sh 3000 2976 2908 2857 sh 1600 1583 1450 1326 br 1200 1183 1155 1127 1033 1001 772 756 635 620 220
aromatic CH stretch n ŽCH. aromatic n ŽCH. aliphatic n ŽCH. aliphatic n ŽCH 2 . n ŽCH 3 . aromatic ring quadrant stretching aromatic ring quadrant stretching d ŽCH 3 . n ŽSO 2 . asymmetric stretch n ŽSO 2 . symmetric Žfree acid. n ŽSO 2 . symmetric Žassociated. n ŽSO 2 . sulfone n ŽSO 2 . of –SO 2 OH . n ŽSOy 3 aromatic CC stretch breathing mode n ŽC–S. sulfonate ion n ŽC–S. of sulfonic acid d ŽSO 2 .
t ŽCH 3 .
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
221
Fig. 6. Graphical plot of I 1127rI 1003 band intensity ratios Ž'. against measured acidity values Žmilliequivalents per gram. for resin specimens studied in the present work.
here, the resins will have the smallest sulfonic acidrsulfonate ratio. This conclusion is supported by the observations of Zundel and Metzger w11x and Zundel w12x, who advocate the presence of aggregation of sulfonic acid groupings and stabilisation through internal hydrogen bonding at high acidities. Gates et al. w13x have also noted that sulfonated polymer resins were much more effective in promoting the dehydration of t-butyl alcohol than were the corresponding sulfonated soluble catalysts — this was ascribed to high local density of sulfonic acid groups acting in a concerted fashion. Žiii. To test of the use of the graphical plot in Fig. 6 as a predictor of resin acidity, we subjected a newly synthesised polystyrene sulfonated resin to Raman spectroscopic analysis. The spectrum of specimen D3895 is shown in Fig. 7 over the wavenumber range 100–1800 cmy1 ; the 1127, 1035 and 1003 cmy1 bands are clearly identifiable. From the relative intensity of the phenyl ring mode in the spectrum, we would conclude qualitatively that this resin specimen was in the Alow sulfonationB category. The I 1127rI 1035 band intensity ratio was measured at 1.12,
from which we deduce that it would have an acidity value of 2.60 " 0.08 mEq gy1 . This compares favourably with the value of A37% sulfonationB from a titration value of 2.63 " 0.05 mEq gy1 . Thus, Raman spectroscopy can be used to determine quantitatively and nondestructively the acidity value of sulfonated resins, and in their water-swollen state. These quantitative measurements, which we believe are the first of their kind to be made on such systems by Raman spectroscopic techniques, could be of key importance in devising new sulfonated resins with different activities for catalytic applications. It is maintained in the literature w3x that the total degree of sulfonation of a resin may be less important than the proportion of groups that are present as sulfonic acid, i.e. catalytic activity could depend not only on the total degree of sulfonation but strongly on the ratio of –SO 3 H to –SOy 3 moieties present w9x. In this respect, the degree of hydration of the resin should also be critically important since the ionisation of SO 3 H groups is strongly dependent on the formal solution concentration w5,6,10x. Buttersack w14x has assessed the accessibil-
222
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
Fig. 7. FT-Raman spectrum of an anonymous sulfonated resin specimen D3895; wavenumber range conditions as in Fig. 2.
ity and catalytic activity of resins in several solvents and has drawn attention to the influence of swelling in the gel phase affected by the association of –SO 3 H groups via hydrogen bonding. Zundel and Metzger w11x and Zundel w12x have deduced from infrared spectroscopic studies of cation exchange membranes that at least two molecules of water are necessary to effect ionisation of a sulfonic acid group. When the water to sulfonic acid molecular ratio was 1, no proton dissociation occurred, and the water molecule was strongly hydrogen-bonded to three –SO 3 H groups. The consequence of this model for the catalytic activity of hydrated sulfonated ion exchange resins has been explored by Kalynam and Sivaram w10x. Also, Delmas and Gaset w15x concluded that the maximum yield of reaction between styrene and paraformaldelyde catalysed by a sulfonated resin was obtained when the ratio of water molecules to sulfonic acid groups was 2. Examination of the Raman spectra of the simple model compounds, benzenesulfonic acid, phenolsulfonic acid and 4-toluenesulfonic acid, is also of interest from the point of view of discerning the presence of –SO 3 H and –SO 3 groups in these solid materials. All three model compounds have Raman bands at 1127 and 1035 cmy1 , indicating that they
q have both –SO 3 H and –SOy species in equilib3 H rium. However, the ratio of sulfonic acid to sulfonate anion band intensities is significantly different for these three acids, being approximately 1.5, 1.2 and 3.0 for benzenesulfonic, phenolsulfonic acid and 4toluenesulfonic acid, respectively. This indicates that in the solid state, 4-toluene sulfonic acid is significantly less ionised than either benzene sulfonic or phenol sulfonic acids. The sulfonated resins with higher acidity values in Table 3 are also deserving of comment. For example, C-100H, a 97%-sulfonated resin, has a significantly greater Raman band intensity at 1127 cmy1 relative to that at 1033 cmy1 ŽFig. 5.. The Irel value for C-100H at 4.14 means that there are significantly more SO 3 H groups than there are free –SOy 3 groups; this could indicate that undissociated SO 3 H groups on neighbouring aromatic rings are stabilised through a hydrogen-bonded structure, as represented in Fig. 8. Such hydrogen-bonded structures have already been identified in sulfonic acid chemistry and related to their acidities and metal complexation ability w16x. This is paralleled in the case of 4-toluenesulfonic acid where an Irel value of about 3.0 for the 1127 and 1033 cmy1 band intensities is also found in the present work. Similarly, higher alkyl-substituted sul-
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
Fig. 8. Diagrammatic representation of intramolecularly hydrogen-bonded sulfonic acid groups in a highly sulfonated resin such as C-100H, with one –SO 3 H group per aromatic ring.
fonic acids have lower dissociations observed by Raman spectroscopy than the parent methanesulfonic acid or fluoro-substituted analogues w5,6x. In more lowly sulfonated resins, such as SP21r15 or SP21r51, which are only 24–43% sulfonated, it is unlikely that many of the –SO 3 H groups can find another SO 3 H group for hydrogen bonding, on an nearby aromatic ring; hence, the competition with q the –SOy moiety in these sulfonated species 3 H 3O is much favoured towards the ionised structures in the presence of larger molar ratios of H 2 O:SO 3 H. Another interesting feature with C-100H are two modes at 635 and 620 cmy1 , which are clearly doublets of different intensities in the SP21r15 and
223
SP21r51 specimens ŽFig. 4.; whereas, a significantly stronger singlet at 635 cmy1 is observed in C-100H. Clearly, therefore, the 635 cmy1 band is a marker for the –SO 3 H group attached to the polystyrene backbone, and the large intensity of this mode swamps the feature at 620 cmy1 assigned to q the mode of the –SOy group. In SP21 and 3 H 3O SP21r51, however, the presence of significantly greater amounts of the ionised sulfonate groups results in the greater intensity of the 620 cmy1 band observed in these specimens; in Fig. 4, the significantly lower intensity of this 620 cmy1 band for C-100H reflects the much lower concentration of sulfonate ions and is too weak to be seen at this scale. Another significant difference between C-100H and SP21r15 or SP21r51 in the 100–1800 cmy1 region is the intensity of the n ŽCC. aromatic ring stretching mode at 1003 cmy1 . C-100H, a highly sulfonated resin, has a very weak band at 1003 cmy1 , similar to that of 4-toluenesulfonic acid Ža model A100% sulfonatedB, aromatic compound.. In contrast, SP21r15 and SP21r51 still have a strong Raman band at 1003 cmy1 , which has to reflect their unsubstituted aromatic ring content w17,18x.
Fig. 9. FT-Raman microscope spectra of sulfonated resin specimen single bead ŽSample SP21r15. 40 = microscope objective, 2000 scans, 1064 nm excitation.
224
H.G.M. Edwards et al.r Vibrational Spectroscopy 24 (2000) 213–224
The data presented in Table 3 and the graphical plot in Fig. 6 indicate that the 1127r1035 Raman band intensity ratio measurements are suitable for the determination of the acidity values of the sulfonated resins studied here. However, a comparison of resin samples SP22r53 and D3178, with nearly identical acidity values of 4.83 and 4.90 mEq gy1 , respectively, is of interest since these resins were prepared by two different sulfonation procedures. Although the 1127r1035 Raman band intensity ratios for the SP22r53 and D3178 are reassuringly similar at 3.64 and 3.89, respectively, clearly there is a significant difference between the 1127r1003 and 1035r1003 band ratios which are 9.00, 2.47 and 1.54, 0.40, respectively. We intend to investigate further this phenomenon with selected resins of similar acidity values which have been prepared by different sulfonation methods. Fourier transform ŽFT.-Raman microscopy was used to examine a single resin bead Žresin sample SP21r15. and a section of that bead. A typical spectrum is shown in Fig. 9, which is of significantly poorer quality than that of the resins obtained in a macroscopic sampling mode. However, the major features indicate that there are no significant differences between the Raman spectra of a single resin bead, a sectioned bead, or the bulk resin.
4. Conclusions Raman spectroscopy has demonstrated the ability to measure non-destructively the degree of sulfonation of hydrated polystyrene resins. Spectral markers have been identified which indicate the presence in the hydrated resins of both Ža. undissociated and Žb. ionised forms of the sulfonic acid group, i.e. –SO 3 H q and –SOy 3 H 3 O . The situation is further complicated, it is realised, by the influence of ion pairs between H 3 Oq and –SOy 3 and undissociated species in the Raman spectra of the sulfonated resins, and this will be the concern of future studies in conjunc-
tion with the degree of hydration. These studies will address a wider range of sulfonic acid resins and resins of catalytic importance, and relating to their activities and stabilities. Acknowledgements The authors are grateful to EPSRC and the DTI for financial support ŽACCP LINK grant GRr L87859. during which the work described was undertaken. References w1x P. Weil, D. Farcasiu, Eur. Chem. News 21 Ž1998. July issue. w2x S. Sollinger, M. Diamantoglou, J. Raman Spectrosc. 28 Ž1997. 811. w3x B.C. Gates, Catalytic Chemistry, Wiley, Chichester, 1992, p. 192. w4x D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Grasselli, Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, New York, 1991, p. 244. w5x H.G.M. Edwards, Spectrochim. Acta, Part A 45 Ž1989. 715. w6x H.G.M. Edwards, D.N. Smith, J. Mol. Struct. 238 Ž1990. 27. w7x H.G.M. Edwards, I.R. Lewis, J. Mol. Struct. 301 Ž1993. 37. w8x A.K. Covington, R. Thompson, J. Solution Chem. 3 Ž1974. 603. w9x R. Thornton, B.C. Gates, J. Catal. 34 Ž1974. 275. w10x N. Kalynam, S. Sivaram, in: D. Nader, M. Streat ŽEds.., Ion Exchange Technology, Ellis Horwood, Chichester, 1984, p. 458. w11x G. Zundel, H. Metzger, Z. Phys. Chem. 59 Ž1965. 225. w12x G. Zundel, Angew. Chem., Int. Ed. Engl. 8 Ž1966. 499. w13x B.C. Gates, J.S. Wisnouskas, H.W. Heath Jr., J. Catal. 24 Ž1972. 320. w14x C. Buttersack, React. Polym. 10 Ž1989. 143. w15x M. Delmas, A. Gaset, J. Mol. Catal. 17 Ž1982. 35. w16x N. Furukawa, H. Fujihara, in: S. Patai, Z. Rappoport ŽEds.., The Chemistry of Sulfonic Acids, Esters and Their Derivatives, Wiley, Chichester, 1991, p. 261, Chap. 7. w17x G. Ellis, A. Sanchez, P.J. Hendra, H.A. Willis, J.M. Chalmers, J.G. Eaves, W.G. Gaskin, K.-N. Kryer, J. Mol. Struct. 247 Ž1991. 385. w18x J.K. Agbenyega, G. Ellis, P.J. Hendra, W.F. Maddams, C. Passingham, J.M. Chalmers, H.A. Willis, Spectrochim. Acta, Part A 46 Ž1990. 197.