Polym. Bull. DOI 10.1007/s00289-015-1502-5 ORIGINAL PAPER
Economical and environmentally friendly synthesis of strong cation-exchange resins from macroporous styrene–divinylbenzene copolymers Syed Wasim Ali1 • Muhammad Arif Malik2 Tariq Yasin3
•
Received: 1 February 2014 / Revised: 1 July 2015 / Accepted: 17 August 2015 Ó Springer-Verlag Berlin Heidelberg 2015
Abstract In the process of strong acid resin synthesis, the process step of acetone washing is meant to extract diluent and homopolymer/oligomers from styrene–divinylbenzene base copolymer. Acetone accounts for *80 % of the cost of the chemicals involved in the synthesis of the copolymer. Acetone also causes environmental pollution and poses health risks to workers. This study demonstrates that the acetone washing can be eliminated in the case of a broad range of solvents commonly employed as diluents in the synthesis of copolymers. Specifically, hydrocarbons such as petroleum ether, isooctane, n-heptane, xylene and toluene can be extracted based on steam distillation phenomena during the curing of the copolymer; esters such as diethylphthalate and butyl stearate are sulfonated and/or hydrolyzed and washed with water; alcohols such as 1-hexanol, tert-amylalcohol, benzoyl alcohol, and ketones such as cyclohexanone are partially soluble in water and can be washed with hot water. The residual homopolymers/oligomers are sulfonated and washed with water. Residual esters have no negative effect and traces of residual alcohols or ketones either have no negative effect or they significantly increase the sulfonation of the copolymer compared to that of acetone-washed copolymers. Keywords Styrene–divinylbenzene Macroporous copolymer Strong acid cation-exchange resin Diluent Suspension copolymerization
& Muhammad Arif Malik
[email protected];
[email protected] 1
Chemistry Division, Pakistan Institute of Nuclear Science and Technology, PO Nilore, Islamabad 44000, Pakistan
2
Frank Reidy Research Center for Bioelectrics, Old Dominion University, 4211 Monarch Way, Suite 300, Norfolk, VA 23508, USA
3
Department of Metallurgy and Material Engineering, Pakistan Institute for Engineering and Applied Sciences (PIEAS), PO Nilore, Islamabad, Pakistan
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Introduction The macroporous styrene–divinylbenzene copolymer beads form the basis of a broad range of commercially important products, including adsorbents, catalysts, support for catalysts and ion-exchange resins used as chromatographic media [1]. Synthesis of these materials was first reported in the 1960s [2, 3]. Since then, it has remained an active area of research and development [2–22]. The styrene– divinylbenzene base copolymers are produced in beaded form by an oil-in-water suspension polymerization technique [2, 3, 7–12]. Monomers and an initiator mixture are diluted with some inert organic liquid (called diluent or porogene) and suspended in water while being stirred. The suspended mixture is polymerized by heating. The copolymer is cured by raising the temperature close to the boiling point of water. Finally, the copolymer beads are filtered out and washed with hot water followed by acetone. Acetone is usually employed because: (i) it is soluble in water, (ii) it can extract most of the organic liquids employed as diluents from the water wet copolymers, (iii) it can extract homopolymers/oligomers, and (iv) being highly volatile, it is easily removed from the copolymer by heat treatment. Another solvent that meets these requirements is methanol, but it is not used as it is even more toxic. Major drawbacks of acetone washing are the following. The amount of acetone required for the washing is about four times the volume of the copolymer. Since the cost of acetone is comparable to the cost of monomers and diluents, it accounts for *80 % of the cost of chemicals involved in the synthesis of the copolymer. Further, being a toxic volatile organic solvent, acetone causes environmental pollution and poses a health risk to workers. One of the major products of the styrene–divinylbenzene copolymers is a strong acid cation-exchange resin derived by sulfonation using concentrated sulfuric acid [23–31]. Our study revealed that in the case of hydrocarbon diluents such as n-heptane, toluene and xylene, acetone washing of the copolymer is not necessary for the synthesis of the sulfonated resin [32, 33]. This is possible because the hydrocarbon solvents can be removed during the copolymer curing stage based on the steam distillation phenomenon. The homopolymers/oligomers in the pores of the copolymer become water-soluble upon sulfonation without any negative effect on the sulfonation of the copolymer. The sulfonated homopolymers/oligomers are extracted during water washing after the sulfonation reaction just like acetone extraction of them from the base copolymers. This study explores the possibility of eliminating acetone washing in the cases of all commonly employed diluents including hydrocarbons, esters, alcohols and ketones. Further, this study revealed that presence of residual cyclohexanone diluent in the pores of the copolymer enhanced sulfonation reaction and capacity of the resin. Socking the copolymer in cyclohexanone solvent before sulfonation further enhanced the capacity of the sulfonated resin so produced. The reasons for why the elimination of acetone washing is possible are explained for each type of diluent.
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Experimental The experimental procedures were the same as reported earlier [16, 29, 31, 32]. A summary of the procedures follows. Synthesis of the copolymers [16, 31] The polymerization mixture was prepared by mixing styrene (99 %), divinylbenzene (60 %, the remaining 40 % mainly comprising ethylvinylbenzene impurity) and diluent in a 1:1:2 ratio by volume and benzoyl peroxide (1 % of the polymerization mixture). One part (by volume) of the polymerization mixture was mixed with five parts of aqueous solution comprising 1 % each of gum arabic and gelatin in water and stirred with a two-blade turbine at about 200 rpm at room temperature for about half an hour. The temperature of the water circulating in the jacket of the polymerization vessel was raised to 80 °C in half an hour and maintained at 80 °C for 20 h followed by curing at 98 °C for 2 h. The copolymer beads were filtered through a Buckner funnel and washed with hot water. The copolymer beads were fractionated and the fraction having particle diameters in the range of 105–150 lm was processed further for the sulfonation reaction. The desired size fraction of the copolymer beads was divided into two portions. The first portion was further washed with acetone. A few drops of the acetone effluent formed a turbid solution upon mixing with about 10–20 ml of water. The turbidity occurs because the diluents/homopolymers extracted by the acetone become insoluble when the acetone is mixed with an excess of water. The acetone washing was continued until the effluent appeared clear when mixed with water. This portion of the copolymer is called ‘washed copolymer’ and the second portion that was not treated with acetone is called ‘unwashed copolymer’ in this report. Both the washed and the unwashed copolymers were air dried then oven dried at 110 °C for about 20 h. In some experiments, a portion of dried ‘acetone-washed’ copolymer was placed on a filter paper in a funnel and treated with the same diluent that was employed in the synthesis of the copolymer. The volume of the diluent in this step was about two times the volume of the copolymer. The copolymer was centrifuged to remove excess solvent before the sulfonation reaction. It is called the ‘spiked’ copolymer sample in this report. Characterization of the copolymers [34] The apparent density was calculated by dividing the weight by the volume of the dried washed copolymer [34]. The pore volume, surface area and pore size distribution of the dried washed copolymers were determined by the mercury penetration method using an Autopore II 29220 mercury porosimeter from Micromeritics, Norcross, GA. FTIR spectra of the dried copolymers and resin were recorded using a NICOLET 6700 FTIR spectrophotometer.
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Sulfonation of the copolymers to produce the resin [29, 31, 32] One part of the dried copolymer beads (by weight) was mixed with five parts (by volume) of 98 % sulfuric acid and the resulting slurry was stirred at 98 °C for 2 h. The slurry was then poured slowly along the inner sidewall of a beaker into about twenty times by volume of stirred, ice-cold, demineralized water. The resin beads were filtered through a Buckner funnel and washed with demineralized water till the effluent was free of acid. Characterization of the resin [29, 31, 32] The resin was packed in a column and washed with 3 resin bed volumes (RBVs) of 2 molar hydrochloric acid (2 M HCl) followed by washing with demineralized water till the effluent was free of acid. The moles of –SO3H groups per liter of resin bed volume (mol/L) and the moles of –COOH groups per liter of resin bed volume were calculated based on the moles of HCl produced and moles of sodium hydroxide (NaOH) consumed in the following reactions, respectively: P-u-COOH þ P-u-SO3 H þ NaClðaqÞ ! P-u-COOH þ P-u-SO3 Na þ HClðaqÞ P-u-COOH þ P-u-SO3 Na þ NaOHðaqÞ ! P-u-COONa þ P-u-SO3 Na þ H2 O,
ð1Þ
ð2Þ
where P represents the polymer backbone and u represents aromatic rings attached to the polymer backbone. The resin slurry in water was packed in a column and washed with water until a constant resin bed volume (RBV) was achieved. The resin was then treated with about 10 RBV of 2 M HCl followed by demineralized water until the effluent was neutral. About 4 RBV of 2 M NaCl followed by about 3 RBV of demineralized water was passed through the resin and 7 RBV of the effluent was titrated to determine moles of the HCl produced by reaction number 1. After that, 2 RBV of 0.20 M NaOH followed by about 3 RBV of demineralized water was passed through the resin and 5 RBV of the effluent was titrated to determine the moles of NaOH consumed in reaction number 2. The moles of –SO3H groups per liter of resin bed volume (mol/l) and the moles of –COOH groups per liter of resin bed volume were calculated based on the RBV, the moles of HCl produced, and the moles of NaOH consumed. The ion-exchange capacity of the resin is the moles of – SO3H ? –COOH groups per unit volume of the resin. The proportion of –COOH groups was 8–10 % the total of –SO3H ? –COOH groups, which is about the same as observed earlier [29]. A sample of the resin in the H-form obtained from the washed copolymer was air dried in an oven at 110 °C for about 20 h. Density (d) was calculated from dry weight (Wd) and dry volume (Vd) of the resin sample. The dried resin in H-form was allowed to swell in distilled water for about 24 h and the volume of the water-
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swollen resin after reaching equilibrium swelling (Vs) was recorded. The swelling coefficient in water (SCw) was calculated using the following equation: SCw ¼ 100 ðVs Vd Þ=Vd :
ð3Þ
Results and discussion Physical characteristics of the copolymers and sulfonated resins derived from them are listed in Table 1. Different diluents impart different porosities to the copolymers as shown in Table 1. In general, the larger the difference in the solubility parameter of the diluent and the copolymer, the higher the pore volume is, which is explained well in earlier literature [5, 9, 12, 35]. Figure 1 shows that the capacities of the sulfonated resins are dependent on the pore volume. The capacity reaches maximum value at pore volume in the range of 0.3–0.4 ml/g, which is in agreement with earlier literature [26]. The dried resins swell when in contact with water because the sulfonic acid groups in the resin are hydrophilic and ionize in water causing osmotic pressure that results in absorption of water. Therefore, the swelling coefficient increases as the capacity, i.e., concentration of sulfonic acid groups in the resin, increases as shown in Table 1. The pore size distribution curves (not shown) were related to pore volumes as reported earlier [16, 31]. The capacities of the resins derived from the washed and unwashed samples of the copolymers remained almost constant in ten consecutive cycles of resin loading/ regeneration. The effluent from the first cycle showed a slight brownish color in all the resins, which is also typically observed in commercially available equivalent products. The color gradually disappeared and was not observed after the third cycle of capacity measurement in every case. These observations are in agreement with Table 1 Characteristics of the styrene–divinylbenzene base copolymers synthesized using different diluents, i.e., density (dBP), pore volume (PV), surface area (SA) and characteristics of sulfonated resins derived from them, i.e., density (dr), swelling coefficient in water (SCw), capacity of resin from washed copolymer (CPw) and from unwashed copolymer (CPU) Diluent
Base copolymer
Sulfonated resin
dBP (g/ml)
PV (ml/g)
SA (m2/g)
dR (g/ml)
SCw (%)
CPW (mol/l)
CPU (mol/l)
Petroleum ethera
0.35
0.88
196
0.47
26
0.98 ± 0.12
1.0 ± 0.17
Isooctane
0.25
1.3
215
0.37
21
0.84 ± .05
0.86 ± 0.03
Diethyl phthalate
0.57
0.24
96
0.62
77
1.5 ± 0.03
Butyl stearate
0.31
1.1
201
0.41
33
1.0 ± 0.07
1.0 ± 0.12
1-Hexanol
0.26
1.3
109
0.35
27
0.87 ± 0.09
0.93 ± 0.05
tert-Amyl alcohol
0.51
0.28
137
0.62
39
1.7 ± 0.02
1.7 ± 0.02
Benzyl alcohol
0.56
0.21
108
0.67
38
1.2 ± 0.03
1.8 ± 0.01
Cyclohexanone
0.60
0.11
56
0.63
21
0.45 ± 0.02
1.3 ± 0.01
a
1.5 ± 0.06
Petroleum ether of boiling range 120–180 °C was employed
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Capacity (mol/L)
2
Washed
Unwashed
1.5
1
0.5
0 0
0.5
1
1.5
Pore volume (ml/g) Fig. 1 Average capacities along with error bars from ten consecutive cycles of loading/regeneration of resins obtained from ‘washed’ and ‘unwashed’ styrene–divinylbenzene copolymers synthesized using different diluents. The solid line is a curve of capacity versus pore volume of resins obtained from washed copolymers based on the data of an earlier publication [26]. The two cases where capacities from unwashed samples were significantly higher compared to those from washed samples are highlighted with gray shades
studies reported earlier [32, 33]. The earlier studies were limited to hydrocarbons, i.e., n-heptane, toluene and xylene. This study explores other classes of diluents also, i.e., esters, alcohols and ketones, in addition to hydrocarbons. The mechanism of removal of diluent from the copolymers in the case of unwashed copolymers was different for each class of diluent employed. The effect of the residual diluent on sulfonation of the copolymers was also dependent on the nature of diluent. These details are explained in the following discussion. In the first two experiments a hydrocarbon, i.e., petroleum ether or isooctane, was employed as the diluent. Capacities of the resins obtained from washed and unwashed copolymer were the same. In Fig. 2, FTIR spectra of the washed and unwashed copolymer obtained using isooctane as the diluent completely overlapped each other. These were almost the same as the FTIR spectra of the styrene– divinylbenzene copolymers reported earlier [32, 33]. FTIR spectra of the washed and unwashed copolymer obtained using petroleum ether as the diluent were also overlapped each other. These observations indicate that petroleum ether/isooctane was removed from the copolymer during the copolymer curing stage by steam distillation. Residual homopolymers/oligomers in the case of unwashed copolymer became water soluble after sulfonation and were removed during water washing of the resin just as homopolymers/oligomers were removed during acetone washing of the base copolymer. Sulfonation of the copolymer was not affected by the presence of the homopolymers/oligomers in the case of unwashed samples because the sulfuric acid reagent was employed in huge excess of the stoichiometric ratio. In the following two experiments, an ester, i.e., diethylphthalate or butyl stearate, was employed as the diluent. Although the capacities of the resins obtained from
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Fig. 2 FTIR spectra of acetone-washed and without acetone washing (unwashed) styrene– divinylbenzene copolymer samples obtained by employing different diluents
washed and the corresponding unwashed copolymer were the same, the FTIR spectra of the washed and unwashed copolymer did not overlap each other. Figure 2 shows that the FTIR spectrum of the washed base copolymer obtained using diethylphthalate completely overlapped with the FTIR spectrum of the washed base copolymer in isooctane diluent. These spectra were the same as that of the washed
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base copolymer of styrene–divinylbenzene reported earlier [32, 33]. Figure 2 shows that the FTIR spectrum of the unwashed copolymer in diethylphthalate diluent demonstrates stronger absorption in some peaks, while other peaks are due to the residual diluent in the unwashed copolymers. Both diethylphthalate and butyl stearate are insoluble in water, but are polar compounds having high boiling points close to or above 300 °C. Therefore, they are not removed during the curing or hot water washing steps. The esters are hydrolyzed by reacting with sulfuric acid and become water soluble during sulfonation [36, 37]. Additional reactions, such as sulfonation of the aromatic rings in the case of the phthalate ester and elimination of water molecules from alcohol produced by the hydrolysis, are also possible. Residual homopolymers/ oligomers are also sulfonated and become water soluble after the sulfonation reaction. The products of these by-reactions of the residual diluent/homopolymers/ oligomers are removed during water washing of the resin after the sulfonation reaction. Although the by-reactions consume a portion of the sulfonating reagent in the case of unwashed copolymer samples, they do not affect the sulfonation of the copolymer because the sulfuric acid reagent is employed in huge excess of the stoichiometric ratio. In the following three experiments, an alcohol, i.e., 1-hexanol, amyl alcohol or benzoyl alcohol, was employed as the diluent. FTIR spectra of the washed and the unwashed copolymers overlapped each other. The overlap of the FTIR spectrum in the unwashed copolymer with diluent benzoyl alcohol with the spectra of washed copolymers obtained using other diluents is shown in Fig. 2. This result suggests that the diluent was almost completely removed both in the washed and unwashed samples. All three alcohols are partially soluble in water which explains the fact that there is no difference in FTIR spectra of the washed and unwashed samples. The capacity of the resins obtained from washed and the respective unwashed copolymers was almost the same in the case of 1-hexanol and amyl alcohol diluent. However, in the case of benzoyl alcohol, the capacity of the resins obtained from unwashed copolymer was *1.5 times that of the resin obtained from the washed copolymer. This observation indicates that there was a trace level of residual diluent that was not visible through FTIR spectroscopy, but was sufficient to leave a noticeable positive effect on the sulfonation of the base copolymer. It should be mentioned here that pre-swelling of the styrene–divinylbenzene copolymer with some polar solvents such as methylene-chloride [38], or a mixture of methylenechloride and nitromethane [39] is known to increase the degree of sulfonation of the copolymer. It is explained on the basis of the fact that the solvents keep the copolymer swollen and, being polar, they facilitate the transfer of the polar sulfonating agent into the copolymer metrics. The hypothesis that residual benzoyl alcohol has a positive effect on sulfonation of the copolymer is supported by the results of the following experiments. A fraction of the washed and dried copolymer synthesized using benzoyl alcohol diluent was spiked with benzoyl alcohol. The copolymer re-swollen with the diluent was then sulfonated. Figure 3 compares the capacities of the resins obtained from ‘washed’, ‘unwashed’ and ‘washed and spiked’ copolymer samples. The capacity of the resin from the spiked sample was *1.3 times of the capacity of the resins
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obtained from washed sample. It supports the view that residual benzoyl alcohol facilitates the sulfonation reaction. In the following experiment, a ketone, i.e., cyclohexanone, was employed as the diluent. FTIR spectra of the washed and unwashed copolymers almost completely overlapped each other. Overlap of the FTIR spectrum of unwashed copolymer using cyclohexanone with the washed copolymers obtained using other diluents can be observed in Fig. 2. It shows that the residual diluent in the case of unwashed samples was below the detection limit of FTIR. It can be explained by the fact that cyclohexanone is partially soluble in water and has been removed during washing of the copolymers with hot water. The capacity of the resins obtained from unwashed copolymer was significantly higher compared to the corresponding resins obtained from the washed copolymers. It shows that the traces of residual diluent were sufficient to enhance the sulfonation of the base copolymer. When the washed and dried copolymer sample was spiked with the cyclohexanone diluent, the sulfonation of the copolymer was enhanced as shown in Fig. 4. The results were reproduced in a repeat experiment. In the case of cyclohexanone, the capacity of resins from the ‘washed and spiked’ sample was higher than those from ‘washed’ and from ‘unwashed’ samples. In the case of benzoyl alcohol, the capacity in the case of ‘washed and spiked’ sample is higher compared to ‘washed’ but lower compared to ‘unwashed’ case. These results show the positive effect of re-swelling or residual solvent dependent on the type and amount of diluent. These effects and their relationship with the characteristics of the copolymers need be explored further. Detailed studies on enhancing the sulfonation of macroporous styrene–divinylbenzene copolymers by re-swelling them with suitable solvents are in progress in our laboratory. The sulfonating agent (98 % sulfuric acid) was in huge excess of the stoichiometric amount. Further, the extent of sulfonation increases with temperature and/or duration of sulfonation reaction [1, 29, 40]. The conditions chosen in this study for sulfonation reactions were those that are usually used to achieve maximum sulfonation of the macroporous styrene–divinylbenzene copolymers. It should be 2
Capacity (mol/L)
Fig. 3 Capacities versus number of loading/regeneration cycle for the resins obtained from ‘washed’, ‘unwashed’ and ‘washed and spiked’ copolymers synthesized using benzoyl alcohol diluent
1.5
1
0.5 Washed
Unwashed
Spicked
0 0
1 2 3 4 5 6 7 8 9 10 Number of loading/regeneration cycle
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2
Capacity (mol/L)
Fig. 4 Capacities versus number loading/regeneration cycle for the resins obtained from ‘washed’, ‘unwashed’ and ‘washed and spiked’ copolymers synthesized using cyclohexanone diluent. The symbols with the thin lines in the case of the spiked experiments represent results of repeat experiments
1.5
1
0.5 Unwashed
Washed
Spicked
0 0
1
2
3
4
5
6
7
8
9
10
Number of loading/regeneration cycle
mentioned that the residual diluent in the case of unwashed copolymers may affect the sulfonation when lower temperature, shorter reaction durations, or lower amounts of the sulfonating agent may be employed to achieve partial sulfonation of the copolymers. Elimination of acetone washing proposed in this study is proven applicable specifically for the case of macroporous styrene–divinylbenzene copolymers synthesized using suspension polymerizing by employing common organic liquid diluents such as hydrocarbons, esters, alcohols or ketones. It may not be applicable when the porosity is generated by some other technique or using special diluents. For example, porous styrene–divinylbenzene foam may be synthesized by emulsion polymerization where significant amount of surfactant like Span 80 may be present in the unwashed copolymer [41]. Further studies are needed to figure out whether presence of compounds such as Span 80 in the unwashed copolymers affects the sulfonation or not. The sulfonic acid groups may be bound to aromatic rings of styrene or to aromatic rings of divinylbenzenes. All of these sulfonic acid groups allow ion exchange but some of their characteristics may be different. For example, they may affect thermal stability of the resins differently. Results of this study do not clarify whether the ratio of the two types of sulfonic acid groups was affected by the residual diluents in the case of unwashed copolymers or not. Therefore, further studies are needed to assess the relative effect on sulfonation of styrene and divinylbenzene units in the copolymers. This study leads to the following conclusions. 1.
The process step of ‘acetone washing’ of the base copolymer to remove commonly employed diluents can be eliminated in the case of synthesis of strong acid cation-exchange resins from macroporous styrene–divinylbenzene copolymers.
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2.
3.
The residual diluent in the case of the unwashed copolymers does not have an adverse effect, even improving the degree of sulfonation of the copolymers in some instances. Re-swelling the washed copolymer with solvents such as cyclohexanone or benzoyl alcohol before the sulfonation reaction can significantly improve the degree of sulfonation of the copolymer.
Acknowledgments Muhammad Arif Malik gratefully acknowledges financial support from the Frank Reidy Fellowship. Authors thank Mr. Amjad Ali of Applied Chemistry Laboratories, for performing Mercury Porosimetic analysis of the copolymers. Authors also thank Barbara C. Carroll of the Frank Reidy Research Center for Bioelectrics for improving the English of the manuscript.
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