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Reproducible preparation of low-capacity anion-exchange resins
Reactive Polymers, 1 (1983) 215-226 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
215
REPRODUCIBLE PREPARATION OF LOW-CAPACITY ANION-EXCHANGE RESINS ROBERT E. BARRON and JAMES S. FRITZ
Ames Laboratory * and Department of Chemistry, Iowa State University, Ames, Iowa 50011 (U.S.A.) (Received December 3, 1982; accepted in revised form April 11, 1983)
A procedure is described for the reproducible preparation of very low-capacity strong-base anion-exchange resins. The resins are intended for use in single column ion chromatography and range in capacity from less than 0.005 m e q / g to 0.16 m e q / g when XAD-1 is used as a substrate. Capacities can typically be reproduced to within a few microequivalents per gram on successive batches. It is believed that the ion-exchange groups are introduced on the surface of the resin.
INTRODUCTION Since its debut in 1975, ion chromatography has become an analytical method of great importance, particularly for the separation and determination of inorganic anions [1]. The dual-column method [2] uses a separation column containing a proprietary anion-exchange resin of low exchange capacity. The singlecolumn method [3,4] employs a separation column containing an anion-exchange resin of even lower exchange capacity, typically between 0.005 and 0.100 m e q / g . This resin is made by chloromethylation and subsequent amination of a macroporous, poly(styrene-divinylbenzene) resin, Rohm and Haas XAD-1. In the method published by Gjerde et al. [5], chloromethylation was accomplished using
* Operated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-ENG-83. 0167-6989/83/$03.00
chloromethyl methyl ether (CMME) with a zinc chloride catalyst. The exchange capacity of the resin was controlled by the chloromethylation step, rather than by the step in which the resin was aminated with trimethylamine. The procedure used had several drawbacks. The first is that chloromethyl methyl ether contains small amounts of bis-chloromethyl ether which is a mutagenic and carcinogenic agent, and thus CMME is subject to governmental restrictions on its use. In addition, the capacities of the resins were hard to control because of the erratic dissolution behavior of the zinc chloride in the solvent system that was used. Now. a procedure is proposed for introducing a low concentration of chloromethyl groups into a macroporous resin using paraformaldehyde and hydrochloric acid. The various experimental parameters are carefully examined so that, after subsequent amination, anion-exchange resins can be prepared very
~ 1983 Elsevier Science Publishers B.V.
216
reproducibly with any desired capacity in the range appropriate for ion chromatography.
BACKGROUND INFORMATION One of the earliest uses of chloromethyl methyl ether for chloromethylation was reported in 1937 [6]. Prior to this date the reaction was carried out with formaldehyde and hydrogen chloride gas, usually with a catalyst of some sort. The catalyst was generally a Lewis acid catalyst, such as aluminum chloride or zinc chloride, or a protic acid such as sulfuric acid or acetic acid. Lewis acid catalysis was used when the reaction was first reported in 1898 by Grassi and Maselli [7]. Since 1937 CMME and other haloethers have been used for chloromethylating styrene-divinylbenzene copolymers because of the high yields of the reaction. The haloethers are excellent swelling agents, especially for gel-type resins. But the toxic nature of haloethers and the difficulty in controlling chloromethylation of polymers where a very low concentration of chloromethyl groups is desired has now led to the search for a reasonable alternative. There are examples of the use of paraformaldehyde and hydrochloric acid for chloromethylation of styrene polymers. Trostyanskaya et al. [8] used paraformaldehyde and HC1 in dichloroethane with a zinc chloride catalyst to effect the reaction with gel copolymers. McBurney also noted that paraformaldehyde and HC1 may be used in the reaction [9]. However, both of these papers dealt with the preparation of high-capacity exchangers. Nelles and von Braun [10] used formalin in aqueous HC1 without a catalyst in order to effect chloromethylation of non-polymeric moieties. Fuson and McKeever [11] note that these conditions are especially suitable for highly alkylated benzenes because the ring is more activated toward electrophilic substitution.
Macroporous, poly(styrene-divinylbenzene) resins are highly crosslinked with commercial divinylbenzene, which contains about 45% ethyl styrenes and other alkylated styrenes in addition to the divinylbenzene. Thus, the typical macroporous resin contains a large proportion of multiply alkylated benzene rings, making it an ideal substrate for chloromethylation with paraformaldehyde and hydrochloric acid. The absence of a catalyst also simplifies control of the reaction and should eliminate the secondary crosslinking commonly found in the presence of Lewis acid catalysts. Another advantage of this approach is that the aqueous solution used does not allow the polymer to be swollen. This should confine the introduction of the functional groups to the easily accessible surfaces of the resin. This means that the final ion-exchange resin should have a fast exchange rate, a property that is highly desirable for ion chromatography. Finally, the direct handling of CMME is avoided. However, it is known that the formation of bis-chloromethyl ether can occur upon reaction of paraformaldehyde and concentrated HC1 [12]. But, because we run the reaction at room temperature, with short reaction times, and with limited amounts of reagents, the quantity of haloether produced should be small. This estimate notwithstanding, proper precautions should be used; i.e., a good fume hood and a water trap for removing any vapors which issue from the reaction vessel. Water hydrolyzes the bis-chloromethyl ether to formaldehyde. The mechanism of chloromethylation with paraformaldehyde and aqueous hydrochloric acid has been investigated by a number of workers. Ogata and Okano [13] showed by a kinetic analysis that HOCH~- is probably the reactive intermediate in hydroxylic solvents. Once the HOCH~- is substituted onto the aromatic nucleus, the - O H is replaced by -C1, with the equilibrium lying far to the right in this replacement step. Nazarov and
217
Semenovsky [12] confirmed this hypothesis. They also showed that chloromethylation and bromomethylation gave the same isomer distribution on a number of substituted aromatics [14]. This suggests a common attacking species, otherwise the steric bulk of the bromine should have had an effect on the isomer distribution. They also showed that under chloromethylation conditions, the hydroxyl group of benzyl alcohol is completely replaced by chlorine [12], thus strengthening their interpretation of the mechanism. The goal of the present research is the reproducible synthesis of resins having an exchange capacity suitable for use in ion chromatography. Rohm and Haas XAD-1 was selected for most of the studies because it has large pores, good mechanical strength, and the anion-exchange resins have favorable properties for chromatography.
EXPERIMENTAL Materials and equipment The XAD-I, XAD-2, and XAD-4 resins were obtained from Rohm and Haas Co. (Philadelphia, Pennsylvania). The ES-863 resin was obtained from Diamond Shamrock Corp. (Cleveland, Ohio) and the A-128 resin was obtained from the Benson Co. (Reno, Nevada). Concentrated HC1 was of reagent grade. Paraformaldehyde was obtained from a number of sources and was 96-98% pure. All solvents used in resin preparation were of reagent grade except for ethylene glycol, which was of histological purity. The HCI gas was obtained from Matheson (Joliet, Illinois) and was 99.0% pure. A 25% solution of trimethylamine in methanol from Eastman (Rochester, New York) was used for amination. Ion chromatography was carried out with a liquid chromatograph consisting of a Milton Roy Mini-pump manufactured by Laboratory Data Control (Riviera Beach, Florida), a
Model 7010 sample injector made by Rheodyne (Berkeley, California), and a Model 213A conductivity detector manufactured by Wescan Instruments (Santa Clara, California). The columns were 500 x 2.0 mm glass columns manufactured in-house. Procedures All resins were prepared in the following manner. Each resin was Soxhlet extracted for 24 hours each with methanol, acetonitrile, and diethyl ether in that order. After extraction with ether, the resins were dried overnight at 70°C before further treatment. The XAD resins were then ground in either a ball mill or a shear mill. The fractured beads were sieved through U.S. Standard Mesh sieves to obtain various size fractions. The fines were removed by slurrying with methanol and allowing the bulk of the resin to settle before decanting the fines. The XAD-1 that was to be used in chromatography was prepared from the resin fraction smaller than 38 ~m. This material was slurried in methanol and the resin was allowed to settle. All material that did not settle in 10 minutes was poured off. This process was repeated several times until the supernatant liquid was almost devoid of particles after the ten minute settling period. The resulting material was fairly homogeneous in size as determined by microscopic examination, but no attempt was made to measure the size distribution. The 500 mm long columns of this material normally had a back pressure of less than 2100 kPa ( - 300 psi). Chloromethylations were carried out in a round bottom flask. The desired initial concentration of HC1 in the mixture dictated the exact procedure. If a concentration of HC1 of less than 12 M was to be used, the following procedure could be employed. The HCI and part of the wetting agent (glacial acetic acid) were added to the flask and the appropriate quantity of paraformaldehyde was dissolved in the liquid. The dissolution was aided by
218
bubbling HCI gas through the mixture. When the solid was dissolved, the resin was wetted with the remaining wetting agent and added to the flask. When 12 M HC1 was to be used, a slightly different procedure was necessary. The paraformaldehyde was dissolved only in concentrated (12 M) HC1. The resin was wetted with glacial acetic acid which was then filtered off in a sintered glass filter crucible. The wetted resin was then rinsed once or twice with concentrated HC1 to displace the acetic acid. Finally, the damp resin was added to the mixture of reagents in the flask. This procedure allowed the resin to be wetted, but caused no dilution of the HC1. Agitation of the resin suspension was accomplished only by the bubbling HC1 gas. Macroporous resins tend to be somewhat friable under mechanical agitation, especially at higher temperatures, so the above method of agitation was used as a precaution. The addition of HC1 gas throughout the reaction also kept the mixture saturated. While only a small amount of HC1 should be consumed in the reaction, it was our experience that capacities would vary unacceptably unless HCI gas was bubbled through the mixture. Mechanical agitation equivalent to that achieved with the bubbling gas was wholly inadequate as far as reproducing capacities and achieving higher capacities were concerned. When the reaction period was completed, the reaction was quenched with water. The product was filtered and washed successively with water, isopropyl alcohol, water, and methanol. The chloromethylated product was immediately suspended in an excess of 25% trimethylamine in methanol. Equivalent results were obtained when liquefied trimethylamine or 25% trimethylamine in water were used, but the methanolic solution was used in almost all experiments. The resin was allowed to react for 24 hours in the unstirred solution. When this reaction was over, the resin was washed with water, 2 M HC1, water, isopropyl al-
cohol, water and methanol. The resin was then air dried for 48 hours. Strong-base anion-exchange capacities were determined by converting the chloride form of the resin to the nitrate form with the resin packed in a gravity-flow column. The bed was then thoroughly rinsed and the bound nitrate was eluted into a volumetric flask using sulfate ion. The nitrate ion was then determined by ion chromatography or UV spectrophotometry. The capacities of most of these resins were too low for titrimetric capacity determinations. Nitrogen analyses were performed with a Perkin-Elmer Model 240 Elemental Analyzer. All eluents were prepared in distilled, deionized water and filtered through a 0.45 ~m membrane filter. Columns were packed using an upward packing, stirred slurry technique with 40% ethylene glycol/water used as the packing fluid. The columns were usually conditioned overnight before use. Controlled chloromethylation - - Effect of experimental parameters
The main effort of this work was directed at the chloromethylation reaction, because this reaction determines the final capacity of the resin when the amination reaction is allowed to go to completion. The resin could also be heavily chloromethylated and then lightly aminated to the desired capacity. This route was rejected because the effect of all of the hydrolyzable chloromethyl groups remaining on the resin was unknown. Since chloromethylation was to be used to control capacity, the time required for the amination reaction to go to completion was determined. These studies were performed on XAD-4, which has the smallest average pore size of the XAD resins used, and appeared to react most slowly in the amination step. Several 0.5 g samples of chloromethylated XAD-4 were each suspended in 5 ml of 25% trimethylamine in methanol. After various
219
time intervals, the reaction was quenched with 2 M hydrochloric acid and the anion-exchange capacity was determined. The reaction proceeded very rapidly at first but the capacity then leveled out. A reaction period of 24 hours was chosen because there was virtually no increase in exchange capacity after that time.
Concentration of HCl and formaldehyde The concentrations of hydrochloric acid and paraformaldehyde were studied concurrently in various combinations. The paraformaldehyde concentration was varied from 0.22 M (as formaldehyde) to 2.8 M, which approached the amount that would easily go into solution. Hydrochloric acid concentrations ranged from 6 M to 12 M. Below 6M HCI the resin capacities were very low. The concentrations of hydrochloric acid reported are the initial concentrations, no corrections being made for volume changes on mixing or for any additional HC1 that might have been absorbed while the gas was bubbled through the reaction mixture. Although it seems unlikely that these effects could be ignored, literally hundreds of batches of resin were prepared and the final capacity could always be related to the initial concentration of HCI. Because the polymer matrix is so hydrophobic, a wetting agent must be used to facilitate the suspension of the resin in the aqueous medium. Glacial acetic acid was used
TABLE
in these initial investigations. A total volume of 30 ml of liquid was used and one gram of resin was chloromethylated in each reaction. The reaction was repeated at least twice at each set of conditions and the capacity of each batch of resin was determined using duplicate samples. The reactions were carried out at room temperature (23°C) for one hour so that small absolute timing errors would not be large relative to the reaction time. The results are summarized in Table 1. The first fact of note is that, above 1.7 M, the formaldehyde concentration seems to have no influence on the final capacity of the resin. A reasonable explanation for this behavior is that the diffusion of the electrophile, +CH2OH, from the bulk solution to the polymer surface is the limiting step. Once the concentration of attacking species crosses a threshold value, the rate of diffusion then controls the degree of substitution. When the formaldehyde concentration is at least 1.7 M, it seems likely that the amount of hydroxymethyl groups introduced into the polymer is the same for all acid concentrations examined. This assumes that all reactions are run for an equal time period. Theoretically, only a small, catalytic amount of hydrogen ion is necessary for the reaction, so there is actually a large excess in each case. The fact that a plateau is evident at all HC1 concentrations examined supports this explanation. The reason that the capacities are so much higher
1
Effect of r e a g e n t c o n c e n t r a t i o n s o n i o n - e x c h a n g e c a p a c i t y Formaldehyde concentration, M 0.22 0.56 1.1 1.7 2.2 2.8
R e s i n c a p a c i t y , m e q / g , at t h e H C I c o n c e n t r a t i o n listed 6M
8M
10M
12M
0.0056 0.0069 0.0052 0.0080
0.0082 0.013 0.014 0.014
0.010 0.019 0.028 0.033 0.030 0.032
0.046 0.067 0.086 0.088 0.080 0.084
220
at 12 M HC1 than at 6 M HC1 is that the degree of conversion of the hydroxymethyl substituent to the chloromethyl group is highly dependent on the concentration of HC1. It therefore seems likely that the resins made with less than 12 M HC1 have a certain amount of residual hydroxymethyl groups remaining on their surfaces, with the lower capacity resins having the largest amount of unconverted groups.
Time, min
Capacity b, m e q / g
20 40 60 90 120 180 480
0.018 0.028 0.033 0.040 0.044 0.052 0.079
Particle size
a 10 M HC1, 2.2 M formaldehyde, room temperature. b Average of two batches.
The effect of resin particle size on final resin capacity was examined using the following size fractions of resin: 38-45 ttm, 45-53 /xm, 53-75 ttm, 75-104 ttm, 104-150 ttm, 150-180 ttm, and whole beads whose size range was 250-425 ttm. The HC1 concentration used for all batches was 10 M and the formaldehyde concentration was 2.2 M. The reactions were carried out at room temperature for 1.0 hour. Each particle size range was represented by three batches of resin and the capacity of each of these batches was determined in duplicate. There was no detectable change of capacity with a change in particle size. This is a reasonable result because the inner surface area of a macroporous resin is much greater than the increase in surface area that would be expected from grinding the beads into smaller particles.
Time of chloromethylation The time of chloromethylation was studied while holding all other parameters constant. An HC1 concentration of 10 M was once again used. Likewise, the formaldehyde concentration was 2.2 M for all reactions. One gram of XAD-1 resin was used for each batch and two batches were made at each reaction time. The reactions were all run at room temperature. Capacity determinations were done in duplicate on each batch of resin. The results are shown in Table 2. Typically, the shorter the reaction time chosen for chloromethylation, the more difficult it is to predict
TABLE 2 Dependence of capacity on chloromethylation time a
the final exchange capacity. However, chloromethylation times of 5 minutes have been used successfully to produce batches of resin whose capacities agree quite closely.
Reaction temperature The effect of reaction temperature on the extent of chloromethylation was also studied. It is sometimes desirable to have a resin with an exchange capacity greater than 0.086 meq/g, which is about the maximum attainable at room temperature using an XAD-1 substrate. Accordingly, reactions were run at temperatures of 10°C, 30°C, 50°C, and 70°C in a constant temperature bath with one gram each of XAD-1 resin for 1.0 hour. The reactions were run using a 2.2 M formaldehyde concentration and 10 M and 12 M HC1 concentrations. The results of this study are summarized in Table 3. The results obtained at TABLE 3 Dependence of capacity on chloromethylation temperature a Temp, °C
10 30 50 70
Capacity b, m e q / g 10 M HCI
12 M HC1
0.019 0.031 0.050 0.076
0.086 0.116 0.159
a 2.2 M formaldehyde, one hour reactions. b Average of two batches.
221
the higher temperatures were slightly less reproducible than those at room temperature, but were still quite good. Also, the results at 30°C appear to match those reported earlier for room temperature. This is because the reactions run at 30°C were carried out in a thermostatted bath, as compared to a nonthermostatted flask used for the reactions run at room temperature. This allowed the temperature to increase somewhat during the reaction, causing results to be equivalent to those obtained at 30°C in the constant temperature bath.
Wetting agent Several solvents were compared as the wetting agent for the chloromethylation reaction. The reaction mixture contained 25 ml HC1, 5 ml of the wetting agent being tested, 2.2 M formaldehyde and 1.0 g of XAD-1; reaction time was 1.0 hour. The results are shown in Table 4. Clearly, glacial acetic acid and dimethyl sulfoxide are the best wetting agents. Glacial acetic acid was selected as being more agreeable to use than dimethyl sulfoxide.
Miscellaneous considerations The ion-exchange capacities of the resins seem to be independent of the amount of TABLE 4 Effect of wetting agent identity a Wetting agent
Resin capacity, meq/g
Glacial acetic acid Dimethyl sulfoxide Ethylene glycol Propionic acid Propylene glycol Ethanol (95%) Isopropyl alcohol Tetrahydrofuran
0.032 0.030 0.014 0.013 0.011 0.0068 0.0067 0.0062
a All reaction conditions were identical except for the identity of the wetting agent. Conditions were l0 M HC1, 2.2 M formaldehyde and one hour reactions at room temperature.
resin present in a given volume of reaction mixture. Most reactions were done using 1.0 g of resin in 30 ml of reagent solution. In addition, several batches of resin were made with 1.0 g of resin and volumes of up to 90 ml of reagents. The capacities of the resins obtained through these reactions were the same as those with 30 ml of reagents. Working with reagent volumes below 30 ml is less convenient. The preparation of these resins was normally done in small batches of about one gram in order to conserve carefully sized copolymer, but batches of up to five grams have been made without any difficulty. The only precaution necessary is to make sure that the method of gas dispersion used for theHC1 is efficient enough to saturate the increased volumes associated with the larger batches. When the resins are dried prior to capacity determination, it is imperative that they be air dried rather than oven dried. The resins in this study were originally oven dried and this practice resulted in erratic, occasionally large losses of capacity. Some batches were not affected, whereas some lost up to 50% of their expected capacity. Resins with a quaternary ammonium group are known to deteriorate at high temperatures when in the hydroxide form; however, all of these resins were in the chloride form. The mechanism responsible for the loss of capacity in these resins is unknown. An air drying period of 48 hours was used thereafter and no unexplained losses of capacity were noted in subsequent batches of resins.
Comparison of resin substrates Although a large majority of the work was performed on XAD-1 resin, several anion exchangers were made starting with other resins. These experiments showed that the chloromethylation reaction works in the same general way as with XAD-1. The other copolymers examined were Rohm and Haas XAD-
222 TABLE 5 Comparative behavior of copolymers under chloromethylation conditions" Copolymer
Approx. surface area b m2/g
Capacity with 6 M HC1, 1.1 M formaldehyde, meq/g
Capacity with 6 M HCI, 2.8 M formaldehyde, meq/g
Capacity with 12 M HCI, 1.1 M formaldehyde, meq/g
Capacity with 12 M HCI, 2.8 M formaldehyde, meq/g
XAD-l XAD-2 A-128 ES-863
ll0 300 450 500
0.0056 0.057 0.169 0.281
0.0080 0.065 0.186 0.302
0.086 0.197 0.237 0.596
0.084 0.194 0.232 0.538
Reactions were run for one hour at room temperature. h Determined by adsorption of p-nitrophenol.
2, Benson A-128, and Diamond Shamrock ES-863. All three are macroporous, styrenedivinylbenzene copolymers, although the macroporosity of each is generated in a different manner. In particular, ES-863 is different from the others as it will increase in volume by - 80% when soaked in methanol, whereas the other resins are not noticeably affected by methanol. Table 5 summarizes the behavior of all resins examined under two sets of conditions which bracket the concentrations found to be useful with XAD-1. The capacities of all four copolymers show relatively little sensitivity to the amount of formaldehyde present in the range which was examined. Most of the time, the ion-exchange capacity of the resin was easily varied by changing the concentration of HC1 used during chloromethylation. The capacities of repetitive batches agree fairly well, though not quite as well as those made from XAD- 1. Preparing resins of a desired capacity The reaction conditions needed to attain a given ion-exchange capacity are easily estimated by plotting ion-exchange capacity versus molarity of HC1. At high HC1 concentrations where the capacity increases rapidly with a small change in concentration, it is easier to adjust the time of chloromethylation rather than try to control the acid con-
centration. The capacities obtained via this reaction are extremely easy to reproduce. The reproducibility was demonstrated by preparing eighteen different batches of resin under varying conditions that should give the same capacity. The HC1 concentration was 8 M in all reactions, while five batches were made with 1.7 M formaldehyde, eight batches were made with 2.2 M formaldehyde, and five batches were made with 2.8 M formaldehyde. These reactions were spaced over the period of about a year in order to involve different lots of reagents. The average capacity of these eighteen batches of resin was 0.013 m e q / g with a standard deviation of 0.002 m e q / g . This small variation does not significantly affect the chromatographic behavior of the resin. Similar precision may be achieved at other HC1 concentrations. The probability of some residual hydroxymethyl groups existing on the polymer surface was noted earlier. There are also unreacted chloromethyl groups in the final ion-exchange resin. This may be shown by comparing the capacity based on chlorine analysis of the chloromethylated resin to the final ion-exchange capacity. Each copolymer exhibits a different degree of conversion of chloromethyl groups to quaternary ammonium ions. This is undoubtedly because of the different combinations of pore size and shape and surface area. The conversion for XAD-1 is
223 TABLE 6 Comparison of ion-exchange capacity and chlorine capacity Resin
Chlorine Ioncap., exchange meq/g cap., meq/g
AN328 (XAD-1) 0.38 AN331 (XAD-I) 0.22
0.15 0.082
Ratio (ion-exchange/ chlorine) × 100 39.7 37.3
about 38% for resins made by the reactions described in this paper. The pertinent data for two batches of resin are shown in Table 6. The influence of these residual groups on chromatographic behavior has not been investigated. Some of the nitrogen introduced into the resin is not present as quaternary ammonium ions. This became apparent when the nitrogen analyses of several copolymers indicated that more nitrogen was present than could be accounted for by the strong-base ion-exchange capacity. The percentage variation between nitrogen analysis and ion-exchange capacity was fairly constant despite changes in capacity. Table 7 shows the results of the nitrogen analyses and the ion-exchange capacities for a few batches of resin. Because XAD- 1 was the main copolymer studied, a batch with a capacity of 0.15 m e q / g was prepared so that a direct acidimetric titration of the resin could be performed. The strong-base capacity was determined in the manner described in
the experimental section. Another portion of the resin was converted to the hydroxide form, washed thoroughly, and then tit~ated potentiometrically with strong acid. Two inflection points were found, indicating the presence of both strong-base and weak-base ion-exchange sites. It is possible that some of the weak-base nitrogen is actually trimethylamine trapped in the pores of the resin. When the resins were stored for a while in a tightly sealed bottle, a slight odor of trimethylamine was noticeable when the bottles were reopened. However, the resins retain their separating ability for long periods of time with no significant change in characteristics, so it appears that this phenomenon is not detrimental to performance. Some columns have been operated as long as six months during which time they exhibited no loss of efficiency. It is believed that the functional groups are introduced on the fixed, inner surface of the macroporous copolymers without any penetration of the polymer matrix. This is suggested by three observations: (1) gel-type polymers (which have no easily accessible inner surface) show no evidence of ion-exchange capacity under conditions which would produce fairly high capacities in XAD-1 (recall that this copolymer has the lowest surface area of all of the macroporous resins examined); (2) the higher the surface area of the copolymer, the more ion-exchange groups are introduced in a given reaction time; and (3) each polymeric substrate reaches a limit of ion-exchange capacity when reacted according
TABLE 7 Comparison of strong-base ion-exchange capacity calculated from nitrogen analysis and actual strong-base ion-exchange capacity Resin
Nitrogen, %
Calculated ionexchange cap., meq/g
Found ion-exchange cap., meq/g
(Found/Calc.) × 100
AN237 (ES-863) AN241 (ES-863) AN244 (ES-863) AN328 (XAD-I)
0.54 0.56 0.59 0.28
0.38 0.40 0.42 0.20
0.284 0.294 0.306 0.151
75 74 73 76
224
to the procedures in this paper. This limit is proportional to surface area. The limiting capacities for XAD-1, XAD-2 and XAD-4 are about 0.16, 0.46 and 0.57 meq/g, respectively. These capacities are not exceeded even at high concentrations of HC1, high temperatures and long reaction times. These capacities are lower than one would expect if every aromatic nucleus was to contain a functional group. They are also lower than the capacity that can be achieved when a polymer-swelling reagent such as chloromethyl methyl ether is used to effect the chloromethylation.
Chromatographic separations The resins made via the procedure described in this paper exhibit good chromatographic
performance within their limitations. It appears that the limiting conditions which now exist are the large size and irregular shape of the particles. A typical performance of the - 3 0 - 3 7 /~m XAD-1 would be about 2800 plates/meter based on the chloride peak. Performance is easily reproduced. As long as the capacity of the different resin batches is the same, the performance will be equivalent if the columns are well-packed. Figures 1 and 2 show chromatograms which are routinely obtained with these resins. Chromatographic conditions are noted in the figure captions. Table 8 and Figs. 3 and 4 compare resins made via this procedure and conventional chloromethylation. The "conventional" chloromethylation was carried out by generating CMME in situ via the reaction of methylal and acetyl chloride in ethylene dichloride solvent. The catalyst used was BiCI 3 because it is less moisture sensitive than ZnC12
CI~o~
L/L_ I 0
!
I
I
110
I
~
I
210
b
g
I
3
minutes
Z CI-
L i
Fig. 1. Trimethylamine resin, 0.027 m e q / g in a 500 x 2.0 mm column. Eluent: 0.001 M benzoic acid at 0.93 m l / m i n . Peak identities and concentrations are as follows in order of elution: acetate (20 ppm), azide ( - 10 ppm), glycolate (14 ppm), formate (6.2 ppm), fluoride (4.0 ppm), dihydrogen phosphate (8.1 ppm), chloride (8.3 ppm), nitrate (8.3 ppm), chlorate (10.2 ppm), 1-propane sulfonate (16.7 ppm), and iodide (15.3 ppm).
i
/
~
.
A
L i
o
~o~
$
minutes
Fig. 2. Trimethylamine resin, 0.027 m e q / g in a 500 × 2.0 mm column. Eluent: 4 × l 0 - 4 M K H P at pH 5.6 with a flow rate of 0.93 m l / m i n .
225 TABLE 8 Comparison of relative retention times for various ions on resins prepared via two chloromethylation routes (same conditions as Figs. 3 and 4) Ion
CI FBrI formate acetate nicotinate glycolate lactate C10 3 BrO 3 IO~ CH 3SO3CH3CH2SO~ CH3CH2CH2SO3NO 2 NO 3 HzPO 4
N3 BF4
Resin 1, a HCl/formaldehyde chloromethylation, lr ion/tr CI1,00 c 0.66 1.19 2.37 0.51 0.22 0.30 0.44 0.46 1.51 1.04 0.82 1.00 1.15 2.00 0.90 1.26 0.83 0.34 2.57
:CI-
Resin 2, b CMME/BiC1 s chloromethylation, trion/trc I formate
1.00 0.67 1.23 2.72 0.56 0.26 0.34 0.46 0.47 1.53 1.04 0.83 1.00 1.17 2.01 0.82 1.32 0.83 0.46 3.39
of resin 1 = 0.027 meq/g. b Capacity of resin 2 = 0.022 meq/g. ~trc I =8.9rain. dt~c ~ =7.6 min.
gl ycolate _w ~ia
azide
L___ I 0
I
I
I
I
I 10
minutes
~' Capacity
a n d is o n l y slightly g r e a t e r in c a t a l y s t strength. T h e s a m e c o l u m n was used for each resin a n d e q u i v a l e n t elution c o n d i t i o n s were used. T h e b e h a v i o r of the two resins is similar, b u t n o t identical. T h e figures show that the c o n v e n tional resin c a n n o t resolve glycolate a n d azide w h e r e a s the H C 1 / f o r m a l d e h y d e resin c a n d o so. T a b l e 8 i n d i c a t e s that the polarizable, late-eluting ions such as I a n d BF 4 h a v e longer relative r e t e n t i o n times on the c o n v e n tional resin. Also, the p e a k s o n the c o n v e n tional resin are slightly b r o a d e r . T h e s e last t w o o b s e r v a t i o n s suggest t h a t p e r h a p s the
Fig. 3. Trimethylamine resin, 0.027 meq/g in a 500 × 2.0 mm column. Eluent: 0.001 M benzoic acid at 0.93 ml/min. Resin prepared via formaldehyde/HCl chloromethylation.
C M M E c h l o r o m e t h y l a t i o n allows s o m e ione x c h a n g e g r o u p s to b e located d e e p e r in the polymer m a t r i x t h a n d o e s the H C 1 / f o r m a l d e h y d e c h l o r o m e t h y l a t i o n . T h i s is p r o b a b l y b e c a u s e of the swelling ability of the solvents used in the c o n v e n t i o n a l reaction. Future work
W o r k is c u r r e n t l y u n d e r w a y which is a i m e d at v a r y i n g the t y p e of p o l y m e r and f u n c t i o n a l
226
Benson of the Benson Co., for providing the unfunctionalized copolymers used in this and other investigations. Thanks are also extended to Dr. David Burge and Dr. Thomas Jupille of Wescan Instruments for providing the conductivity detectors used in our work. This work was supported by the Director of Energy Research, Office of Basic Energy Sciences.
Clformate
azide \
t
glycolate/'~l~
0 0.7 C~ n~
REFERENCES
<.~
I
II
0 ]
I
I
I
I
10
I
I
I
minutes
Fig. 4. Trimethylamine resin, 0.022 meq/g in a 500 × 2.0 mm column. Eluent: 0.001 M benzoic acid at 0.93 ml/min. Resin prepared via in situ generation of CMME with BiCI 3 catalysis.
group attached to the polymer in order to change selectivity of the resins. Polymer physical properties are also being examined in order to identify the most critical parameters in the choice of a resin substrate.
ACKNOWLEDGEMENTS The authors would like to thank Dr. R.L. Albright of Rohm and Haas and Dr. Jim
1 H. Small, T.S. Stevens and W.C. Bauman, Anal. Chem., 47 (1975) 1801. ~ 2 T.S. Stevens and M.A. Langhorst, Anal. Chem., 54 (1982) 950. 3 D.T. Gjerde, J.S. Fritz and G. Schmuckler, J. Chromatogr., 186 (1979) 509. 4 D.T. Gjerde, G. Schmuckler and J.S. Fritz, J. Chromatogr., 198 (1980) 35. .~ 5 D.T. Gjerde and J.S. Fritz, J. Chromatogr., 176 (1979) 199. 6 G. Vavon and J. Bolle, Compt. Rend., 204 (1937) 1826. 7 G. Grassi and C. Maselli, Gazz. Chim. Ital., 28 (II) (1898) 477. 8 E.B. Trostyanskaya, I.P. Losev and Ly. Syanzhao, Zhur. Vsesoyuz. Khim. Obshchestva im. D.I. Mendeleeva, 5 (1960) 116. 9 C.H. McBurney, U.S. Patent 2,591,573, 1952. 10 J. von Braun and J. Nelles, Chem. Ber., 67 (1934) 1094. 11 R.C. Fuson and C.H. McKeever, in: R. Adams, W.E. Bachmann, L.F. Feiser, J.R. Johnson and H.R. Snyder (Eds.), Organic Reactions, Vol. I, Wiley, New York, 1942, Chap. 3. 12 I.N. Nazarov and A.V. Semenovsky, Izv. Akad. Nauk S.S.S.R. Otd. Khim. Nauk, (1957) 972. Bull. Acad. Sci., U.S.S.R., Div. Chem. Sci., (English Trans.), (1957) 997. 13 Y. Ogata and M. Okano, J. Amer. Chem. Soc., 78 (1956) 5423. 14 I.N. Nazarov and A.V. Semenovsky, Izv. Akad. Nauk S.S.S.R. Otd. Khim. Nauk, (1957) 212. Bull. Acad. Sci. U.S.S.R., Div. Chem. Sci., (English Trans.), (1957) 225.
215
REPRODUCIBLE PREPARATION OF LOW-CAPACITY ANION-EXCHANGE RESINS ROBERT E. BARRON and JAMES S. FRITZ
Ames Laboratory * and Department of Chemistry, Iowa State University, Ames, Iowa 50011 (U.S.A.) (Received December 3, 1982; accepted in revised form April 11, 1983)
A procedure is described for the reproducible preparation of very low-capacity strong-base anion-exchange resins. The resins are intended for use in single column ion chromatography and range in capacity from less than 0.005 m e q / g to 0.16 m e q / g when XAD-1 is used as a substrate. Capacities can typically be reproduced to within a few microequivalents per gram on successive batches. It is believed that the ion-exchange groups are introduced on the surface of the resin.
INTRODUCTION Since its debut in 1975, ion chromatography has become an analytical method of great importance, particularly for the separation and determination of inorganic anions [1]. The dual-column method [2] uses a separation column containing a proprietary anion-exchange resin of low exchange capacity. The singlecolumn method [3,4] employs a separation column containing an anion-exchange resin of even lower exchange capacity, typically between 0.005 and 0.100 m e q / g . This resin is made by chloromethylation and subsequent amination of a macroporous, poly(styrene-divinylbenzene) resin, Rohm and Haas XAD-1. In the method published by Gjerde et al. [5], chloromethylation was accomplished using
* Operated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-ENG-83. 0167-6989/83/$03.00
chloromethyl methyl ether (CMME) with a zinc chloride catalyst. The exchange capacity of the resin was controlled by the chloromethylation step, rather than by the step in which the resin was aminated with trimethylamine. The procedure used had several drawbacks. The first is that chloromethyl methyl ether contains small amounts of bis-chloromethyl ether which is a mutagenic and carcinogenic agent, and thus CMME is subject to governmental restrictions on its use. In addition, the capacities of the resins were hard to control because of the erratic dissolution behavior of the zinc chloride in the solvent system that was used. Now. a procedure is proposed for introducing a low concentration of chloromethyl groups into a macroporous resin using paraformaldehyde and hydrochloric acid. The various experimental parameters are carefully examined so that, after subsequent amination, anion-exchange resins can be prepared very
~ 1983 Elsevier Science Publishers B.V.
216
reproducibly with any desired capacity in the range appropriate for ion chromatography.
BACKGROUND INFORMATION One of the earliest uses of chloromethyl methyl ether for chloromethylation was reported in 1937 [6]. Prior to this date the reaction was carried out with formaldehyde and hydrogen chloride gas, usually with a catalyst of some sort. The catalyst was generally a Lewis acid catalyst, such as aluminum chloride or zinc chloride, or a protic acid such as sulfuric acid or acetic acid. Lewis acid catalysis was used when the reaction was first reported in 1898 by Grassi and Maselli [7]. Since 1937 CMME and other haloethers have been used for chloromethylating styrene-divinylbenzene copolymers because of the high yields of the reaction. The haloethers are excellent swelling agents, especially for gel-type resins. But the toxic nature of haloethers and the difficulty in controlling chloromethylation of polymers where a very low concentration of chloromethyl groups is desired has now led to the search for a reasonable alternative. There are examples of the use of paraformaldehyde and hydrochloric acid for chloromethylation of styrene polymers. Trostyanskaya et al. [8] used paraformaldehyde and HC1 in dichloroethane with a zinc chloride catalyst to effect the reaction with gel copolymers. McBurney also noted that paraformaldehyde and HC1 may be used in the reaction [9]. However, both of these papers dealt with the preparation of high-capacity exchangers. Nelles and von Braun [10] used formalin in aqueous HC1 without a catalyst in order to effect chloromethylation of non-polymeric moieties. Fuson and McKeever [11] note that these conditions are especially suitable for highly alkylated benzenes because the ring is more activated toward electrophilic substitution.
Macroporous, poly(styrene-divinylbenzene) resins are highly crosslinked with commercial divinylbenzene, which contains about 45% ethyl styrenes and other alkylated styrenes in addition to the divinylbenzene. Thus, the typical macroporous resin contains a large proportion of multiply alkylated benzene rings, making it an ideal substrate for chloromethylation with paraformaldehyde and hydrochloric acid. The absence of a catalyst also simplifies control of the reaction and should eliminate the secondary crosslinking commonly found in the presence of Lewis acid catalysts. Another advantage of this approach is that the aqueous solution used does not allow the polymer to be swollen. This should confine the introduction of the functional groups to the easily accessible surfaces of the resin. This means that the final ion-exchange resin should have a fast exchange rate, a property that is highly desirable for ion chromatography. Finally, the direct handling of CMME is avoided. However, it is known that the formation of bis-chloromethyl ether can occur upon reaction of paraformaldehyde and concentrated HC1 [12]. But, because we run the reaction at room temperature, with short reaction times, and with limited amounts of reagents, the quantity of haloether produced should be small. This estimate notwithstanding, proper precautions should be used; i.e., a good fume hood and a water trap for removing any vapors which issue from the reaction vessel. Water hydrolyzes the bis-chloromethyl ether to formaldehyde. The mechanism of chloromethylation with paraformaldehyde and aqueous hydrochloric acid has been investigated by a number of workers. Ogata and Okano [13] showed by a kinetic analysis that HOCH~- is probably the reactive intermediate in hydroxylic solvents. Once the HOCH~- is substituted onto the aromatic nucleus, the - O H is replaced by -C1, with the equilibrium lying far to the right in this replacement step. Nazarov and
217
Semenovsky [12] confirmed this hypothesis. They also showed that chloromethylation and bromomethylation gave the same isomer distribution on a number of substituted aromatics [14]. This suggests a common attacking species, otherwise the steric bulk of the bromine should have had an effect on the isomer distribution. They also showed that under chloromethylation conditions, the hydroxyl group of benzyl alcohol is completely replaced by chlorine [12], thus strengthening their interpretation of the mechanism. The goal of the present research is the reproducible synthesis of resins having an exchange capacity suitable for use in ion chromatography. Rohm and Haas XAD-1 was selected for most of the studies because it has large pores, good mechanical strength, and the anion-exchange resins have favorable properties for chromatography.
EXPERIMENTAL Materials and equipment The XAD-I, XAD-2, and XAD-4 resins were obtained from Rohm and Haas Co. (Philadelphia, Pennsylvania). The ES-863 resin was obtained from Diamond Shamrock Corp. (Cleveland, Ohio) and the A-128 resin was obtained from the Benson Co. (Reno, Nevada). Concentrated HC1 was of reagent grade. Paraformaldehyde was obtained from a number of sources and was 96-98% pure. All solvents used in resin preparation were of reagent grade except for ethylene glycol, which was of histological purity. The HCI gas was obtained from Matheson (Joliet, Illinois) and was 99.0% pure. A 25% solution of trimethylamine in methanol from Eastman (Rochester, New York) was used for amination. Ion chromatography was carried out with a liquid chromatograph consisting of a Milton Roy Mini-pump manufactured by Laboratory Data Control (Riviera Beach, Florida), a
Model 7010 sample injector made by Rheodyne (Berkeley, California), and a Model 213A conductivity detector manufactured by Wescan Instruments (Santa Clara, California). The columns were 500 x 2.0 mm glass columns manufactured in-house. Procedures All resins were prepared in the following manner. Each resin was Soxhlet extracted for 24 hours each with methanol, acetonitrile, and diethyl ether in that order. After extraction with ether, the resins were dried overnight at 70°C before further treatment. The XAD resins were then ground in either a ball mill or a shear mill. The fractured beads were sieved through U.S. Standard Mesh sieves to obtain various size fractions. The fines were removed by slurrying with methanol and allowing the bulk of the resin to settle before decanting the fines. The XAD-1 that was to be used in chromatography was prepared from the resin fraction smaller than 38 ~m. This material was slurried in methanol and the resin was allowed to settle. All material that did not settle in 10 minutes was poured off. This process was repeated several times until the supernatant liquid was almost devoid of particles after the ten minute settling period. The resulting material was fairly homogeneous in size as determined by microscopic examination, but no attempt was made to measure the size distribution. The 500 mm long columns of this material normally had a back pressure of less than 2100 kPa ( - 300 psi). Chloromethylations were carried out in a round bottom flask. The desired initial concentration of HC1 in the mixture dictated the exact procedure. If a concentration of HC1 of less than 12 M was to be used, the following procedure could be employed. The HCI and part of the wetting agent (glacial acetic acid) were added to the flask and the appropriate quantity of paraformaldehyde was dissolved in the liquid. The dissolution was aided by
218
bubbling HCI gas through the mixture. When the solid was dissolved, the resin was wetted with the remaining wetting agent and added to the flask. When 12 M HC1 was to be used, a slightly different procedure was necessary. The paraformaldehyde was dissolved only in concentrated (12 M) HC1. The resin was wetted with glacial acetic acid which was then filtered off in a sintered glass filter crucible. The wetted resin was then rinsed once or twice with concentrated HC1 to displace the acetic acid. Finally, the damp resin was added to the mixture of reagents in the flask. This procedure allowed the resin to be wetted, but caused no dilution of the HC1. Agitation of the resin suspension was accomplished only by the bubbling HC1 gas. Macroporous resins tend to be somewhat friable under mechanical agitation, especially at higher temperatures, so the above method of agitation was used as a precaution. The addition of HC1 gas throughout the reaction also kept the mixture saturated. While only a small amount of HC1 should be consumed in the reaction, it was our experience that capacities would vary unacceptably unless HCI gas was bubbled through the mixture. Mechanical agitation equivalent to that achieved with the bubbling gas was wholly inadequate as far as reproducing capacities and achieving higher capacities were concerned. When the reaction period was completed, the reaction was quenched with water. The product was filtered and washed successively with water, isopropyl alcohol, water, and methanol. The chloromethylated product was immediately suspended in an excess of 25% trimethylamine in methanol. Equivalent results were obtained when liquefied trimethylamine or 25% trimethylamine in water were used, but the methanolic solution was used in almost all experiments. The resin was allowed to react for 24 hours in the unstirred solution. When this reaction was over, the resin was washed with water, 2 M HC1, water, isopropyl al-
cohol, water and methanol. The resin was then air dried for 48 hours. Strong-base anion-exchange capacities were determined by converting the chloride form of the resin to the nitrate form with the resin packed in a gravity-flow column. The bed was then thoroughly rinsed and the bound nitrate was eluted into a volumetric flask using sulfate ion. The nitrate ion was then determined by ion chromatography or UV spectrophotometry. The capacities of most of these resins were too low for titrimetric capacity determinations. Nitrogen analyses were performed with a Perkin-Elmer Model 240 Elemental Analyzer. All eluents were prepared in distilled, deionized water and filtered through a 0.45 ~m membrane filter. Columns were packed using an upward packing, stirred slurry technique with 40% ethylene glycol/water used as the packing fluid. The columns were usually conditioned overnight before use. Controlled chloromethylation - - Effect of experimental parameters
The main effort of this work was directed at the chloromethylation reaction, because this reaction determines the final capacity of the resin when the amination reaction is allowed to go to completion. The resin could also be heavily chloromethylated and then lightly aminated to the desired capacity. This route was rejected because the effect of all of the hydrolyzable chloromethyl groups remaining on the resin was unknown. Since chloromethylation was to be used to control capacity, the time required for the amination reaction to go to completion was determined. These studies were performed on XAD-4, which has the smallest average pore size of the XAD resins used, and appeared to react most slowly in the amination step. Several 0.5 g samples of chloromethylated XAD-4 were each suspended in 5 ml of 25% trimethylamine in methanol. After various
219
time intervals, the reaction was quenched with 2 M hydrochloric acid and the anion-exchange capacity was determined. The reaction proceeded very rapidly at first but the capacity then leveled out. A reaction period of 24 hours was chosen because there was virtually no increase in exchange capacity after that time.
Concentration of HCl and formaldehyde The concentrations of hydrochloric acid and paraformaldehyde were studied concurrently in various combinations. The paraformaldehyde concentration was varied from 0.22 M (as formaldehyde) to 2.8 M, which approached the amount that would easily go into solution. Hydrochloric acid concentrations ranged from 6 M to 12 M. Below 6M HCI the resin capacities were very low. The concentrations of hydrochloric acid reported are the initial concentrations, no corrections being made for volume changes on mixing or for any additional HC1 that might have been absorbed while the gas was bubbled through the reaction mixture. Although it seems unlikely that these effects could be ignored, literally hundreds of batches of resin were prepared and the final capacity could always be related to the initial concentration of HCI. Because the polymer matrix is so hydrophobic, a wetting agent must be used to facilitate the suspension of the resin in the aqueous medium. Glacial acetic acid was used
TABLE
in these initial investigations. A total volume of 30 ml of liquid was used and one gram of resin was chloromethylated in each reaction. The reaction was repeated at least twice at each set of conditions and the capacity of each batch of resin was determined using duplicate samples. The reactions were carried out at room temperature (23°C) for one hour so that small absolute timing errors would not be large relative to the reaction time. The results are summarized in Table 1. The first fact of note is that, above 1.7 M, the formaldehyde concentration seems to have no influence on the final capacity of the resin. A reasonable explanation for this behavior is that the diffusion of the electrophile, +CH2OH, from the bulk solution to the polymer surface is the limiting step. Once the concentration of attacking species crosses a threshold value, the rate of diffusion then controls the degree of substitution. When the formaldehyde concentration is at least 1.7 M, it seems likely that the amount of hydroxymethyl groups introduced into the polymer is the same for all acid concentrations examined. This assumes that all reactions are run for an equal time period. Theoretically, only a small, catalytic amount of hydrogen ion is necessary for the reaction, so there is actually a large excess in each case. The fact that a plateau is evident at all HC1 concentrations examined supports this explanation. The reason that the capacities are so much higher
1
Effect of r e a g e n t c o n c e n t r a t i o n s o n i o n - e x c h a n g e c a p a c i t y Formaldehyde concentration, M 0.22 0.56 1.1 1.7 2.2 2.8
R e s i n c a p a c i t y , m e q / g , at t h e H C I c o n c e n t r a t i o n listed 6M
8M
10M
12M
0.0056 0.0069 0.0052 0.0080
0.0082 0.013 0.014 0.014
0.010 0.019 0.028 0.033 0.030 0.032
0.046 0.067 0.086 0.088 0.080 0.084
220
at 12 M HC1 than at 6 M HC1 is that the degree of conversion of the hydroxymethyl substituent to the chloromethyl group is highly dependent on the concentration of HC1. It therefore seems likely that the resins made with less than 12 M HC1 have a certain amount of residual hydroxymethyl groups remaining on their surfaces, with the lower capacity resins having the largest amount of unconverted groups.
Time, min
Capacity b, m e q / g
20 40 60 90 120 180 480
0.018 0.028 0.033 0.040 0.044 0.052 0.079
Particle size
a 10 M HC1, 2.2 M formaldehyde, room temperature. b Average of two batches.
The effect of resin particle size on final resin capacity was examined using the following size fractions of resin: 38-45 ttm, 45-53 /xm, 53-75 ttm, 75-104 ttm, 104-150 ttm, 150-180 ttm, and whole beads whose size range was 250-425 ttm. The HC1 concentration used for all batches was 10 M and the formaldehyde concentration was 2.2 M. The reactions were carried out at room temperature for 1.0 hour. Each particle size range was represented by three batches of resin and the capacity of each of these batches was determined in duplicate. There was no detectable change of capacity with a change in particle size. This is a reasonable result because the inner surface area of a macroporous resin is much greater than the increase in surface area that would be expected from grinding the beads into smaller particles.
Time of chloromethylation The time of chloromethylation was studied while holding all other parameters constant. An HC1 concentration of 10 M was once again used. Likewise, the formaldehyde concentration was 2.2 M for all reactions. One gram of XAD-1 resin was used for each batch and two batches were made at each reaction time. The reactions were all run at room temperature. Capacity determinations were done in duplicate on each batch of resin. The results are shown in Table 2. Typically, the shorter the reaction time chosen for chloromethylation, the more difficult it is to predict
TABLE 2 Dependence of capacity on chloromethylation time a
the final exchange capacity. However, chloromethylation times of 5 minutes have been used successfully to produce batches of resin whose capacities agree quite closely.
Reaction temperature The effect of reaction temperature on the extent of chloromethylation was also studied. It is sometimes desirable to have a resin with an exchange capacity greater than 0.086 meq/g, which is about the maximum attainable at room temperature using an XAD-1 substrate. Accordingly, reactions were run at temperatures of 10°C, 30°C, 50°C, and 70°C in a constant temperature bath with one gram each of XAD-1 resin for 1.0 hour. The reactions were run using a 2.2 M formaldehyde concentration and 10 M and 12 M HC1 concentrations. The results of this study are summarized in Table 3. The results obtained at TABLE 3 Dependence of capacity on chloromethylation temperature a Temp, °C
10 30 50 70
Capacity b, m e q / g 10 M HCI
12 M HC1
0.019 0.031 0.050 0.076
0.086 0.116 0.159
a 2.2 M formaldehyde, one hour reactions. b Average of two batches.
221
the higher temperatures were slightly less reproducible than those at room temperature, but were still quite good. Also, the results at 30°C appear to match those reported earlier for room temperature. This is because the reactions run at 30°C were carried out in a thermostatted bath, as compared to a nonthermostatted flask used for the reactions run at room temperature. This allowed the temperature to increase somewhat during the reaction, causing results to be equivalent to those obtained at 30°C in the constant temperature bath.
Wetting agent Several solvents were compared as the wetting agent for the chloromethylation reaction. The reaction mixture contained 25 ml HC1, 5 ml of the wetting agent being tested, 2.2 M formaldehyde and 1.0 g of XAD-1; reaction time was 1.0 hour. The results are shown in Table 4. Clearly, glacial acetic acid and dimethyl sulfoxide are the best wetting agents. Glacial acetic acid was selected as being more agreeable to use than dimethyl sulfoxide.
Miscellaneous considerations The ion-exchange capacities of the resins seem to be independent of the amount of TABLE 4 Effect of wetting agent identity a Wetting agent
Resin capacity, meq/g
Glacial acetic acid Dimethyl sulfoxide Ethylene glycol Propionic acid Propylene glycol Ethanol (95%) Isopropyl alcohol Tetrahydrofuran
0.032 0.030 0.014 0.013 0.011 0.0068 0.0067 0.0062
a All reaction conditions were identical except for the identity of the wetting agent. Conditions were l0 M HC1, 2.2 M formaldehyde and one hour reactions at room temperature.
resin present in a given volume of reaction mixture. Most reactions were done using 1.0 g of resin in 30 ml of reagent solution. In addition, several batches of resin were made with 1.0 g of resin and volumes of up to 90 ml of reagents. The capacities of the resins obtained through these reactions were the same as those with 30 ml of reagents. Working with reagent volumes below 30 ml is less convenient. The preparation of these resins was normally done in small batches of about one gram in order to conserve carefully sized copolymer, but batches of up to five grams have been made without any difficulty. The only precaution necessary is to make sure that the method of gas dispersion used for theHC1 is efficient enough to saturate the increased volumes associated with the larger batches. When the resins are dried prior to capacity determination, it is imperative that they be air dried rather than oven dried. The resins in this study were originally oven dried and this practice resulted in erratic, occasionally large losses of capacity. Some batches were not affected, whereas some lost up to 50% of their expected capacity. Resins with a quaternary ammonium group are known to deteriorate at high temperatures when in the hydroxide form; however, all of these resins were in the chloride form. The mechanism responsible for the loss of capacity in these resins is unknown. An air drying period of 48 hours was used thereafter and no unexplained losses of capacity were noted in subsequent batches of resins.
Comparison of resin substrates Although a large majority of the work was performed on XAD-1 resin, several anion exchangers were made starting with other resins. These experiments showed that the chloromethylation reaction works in the same general way as with XAD-1. The other copolymers examined were Rohm and Haas XAD-
222 TABLE 5 Comparative behavior of copolymers under chloromethylation conditions" Copolymer
Approx. surface area b m2/g
Capacity with 6 M HC1, 1.1 M formaldehyde, meq/g
Capacity with 6 M HCI, 2.8 M formaldehyde, meq/g
Capacity with 12 M HCI, 1.1 M formaldehyde, meq/g
Capacity with 12 M HCI, 2.8 M formaldehyde, meq/g
XAD-l XAD-2 A-128 ES-863
ll0 300 450 500
0.0056 0.057 0.169 0.281
0.0080 0.065 0.186 0.302
0.086 0.197 0.237 0.596
0.084 0.194 0.232 0.538
Reactions were run for one hour at room temperature. h Determined by adsorption of p-nitrophenol.
2, Benson A-128, and Diamond Shamrock ES-863. All three are macroporous, styrenedivinylbenzene copolymers, although the macroporosity of each is generated in a different manner. In particular, ES-863 is different from the others as it will increase in volume by - 80% when soaked in methanol, whereas the other resins are not noticeably affected by methanol. Table 5 summarizes the behavior of all resins examined under two sets of conditions which bracket the concentrations found to be useful with XAD-1. The capacities of all four copolymers show relatively little sensitivity to the amount of formaldehyde present in the range which was examined. Most of the time, the ion-exchange capacity of the resin was easily varied by changing the concentration of HC1 used during chloromethylation. The capacities of repetitive batches agree fairly well, though not quite as well as those made from XAD- 1. Preparing resins of a desired capacity The reaction conditions needed to attain a given ion-exchange capacity are easily estimated by plotting ion-exchange capacity versus molarity of HC1. At high HC1 concentrations where the capacity increases rapidly with a small change in concentration, it is easier to adjust the time of chloromethylation rather than try to control the acid con-
centration. The capacities obtained via this reaction are extremely easy to reproduce. The reproducibility was demonstrated by preparing eighteen different batches of resin under varying conditions that should give the same capacity. The HC1 concentration was 8 M in all reactions, while five batches were made with 1.7 M formaldehyde, eight batches were made with 2.2 M formaldehyde, and five batches were made with 2.8 M formaldehyde. These reactions were spaced over the period of about a year in order to involve different lots of reagents. The average capacity of these eighteen batches of resin was 0.013 m e q / g with a standard deviation of 0.002 m e q / g . This small variation does not significantly affect the chromatographic behavior of the resin. Similar precision may be achieved at other HC1 concentrations. The probability of some residual hydroxymethyl groups existing on the polymer surface was noted earlier. There are also unreacted chloromethyl groups in the final ion-exchange resin. This may be shown by comparing the capacity based on chlorine analysis of the chloromethylated resin to the final ion-exchange capacity. Each copolymer exhibits a different degree of conversion of chloromethyl groups to quaternary ammonium ions. This is undoubtedly because of the different combinations of pore size and shape and surface area. The conversion for XAD-1 is
223 TABLE 6 Comparison of ion-exchange capacity and chlorine capacity Resin
Chlorine Ioncap., exchange meq/g cap., meq/g
AN328 (XAD-1) 0.38 AN331 (XAD-I) 0.22
0.15 0.082
Ratio (ion-exchange/ chlorine) × 100 39.7 37.3
about 38% for resins made by the reactions described in this paper. The pertinent data for two batches of resin are shown in Table 6. The influence of these residual groups on chromatographic behavior has not been investigated. Some of the nitrogen introduced into the resin is not present as quaternary ammonium ions. This became apparent when the nitrogen analyses of several copolymers indicated that more nitrogen was present than could be accounted for by the strong-base ion-exchange capacity. The percentage variation between nitrogen analysis and ion-exchange capacity was fairly constant despite changes in capacity. Table 7 shows the results of the nitrogen analyses and the ion-exchange capacities for a few batches of resin. Because XAD- 1 was the main copolymer studied, a batch with a capacity of 0.15 m e q / g was prepared so that a direct acidimetric titration of the resin could be performed. The strong-base capacity was determined in the manner described in
the experimental section. Another portion of the resin was converted to the hydroxide form, washed thoroughly, and then tit~ated potentiometrically with strong acid. Two inflection points were found, indicating the presence of both strong-base and weak-base ion-exchange sites. It is possible that some of the weak-base nitrogen is actually trimethylamine trapped in the pores of the resin. When the resins were stored for a while in a tightly sealed bottle, a slight odor of trimethylamine was noticeable when the bottles were reopened. However, the resins retain their separating ability for long periods of time with no significant change in characteristics, so it appears that this phenomenon is not detrimental to performance. Some columns have been operated as long as six months during which time they exhibited no loss of efficiency. It is believed that the functional groups are introduced on the fixed, inner surface of the macroporous copolymers without any penetration of the polymer matrix. This is suggested by three observations: (1) gel-type polymers (which have no easily accessible inner surface) show no evidence of ion-exchange capacity under conditions which would produce fairly high capacities in XAD-1 (recall that this copolymer has the lowest surface area of all of the macroporous resins examined); (2) the higher the surface area of the copolymer, the more ion-exchange groups are introduced in a given reaction time; and (3) each polymeric substrate reaches a limit of ion-exchange capacity when reacted according
TABLE 7 Comparison of strong-base ion-exchange capacity calculated from nitrogen analysis and actual strong-base ion-exchange capacity Resin
Nitrogen, %
Calculated ionexchange cap., meq/g
Found ion-exchange cap., meq/g
(Found/Calc.) × 100
AN237 (ES-863) AN241 (ES-863) AN244 (ES-863) AN328 (XAD-I)
0.54 0.56 0.59 0.28
0.38 0.40 0.42 0.20
0.284 0.294 0.306 0.151
75 74 73 76
224
to the procedures in this paper. This limit is proportional to surface area. The limiting capacities for XAD-1, XAD-2 and XAD-4 are about 0.16, 0.46 and 0.57 meq/g, respectively. These capacities are not exceeded even at high concentrations of HC1, high temperatures and long reaction times. These capacities are lower than one would expect if every aromatic nucleus was to contain a functional group. They are also lower than the capacity that can be achieved when a polymer-swelling reagent such as chloromethyl methyl ether is used to effect the chloromethylation.
Chromatographic separations The resins made via the procedure described in this paper exhibit good chromatographic
performance within their limitations. It appears that the limiting conditions which now exist are the large size and irregular shape of the particles. A typical performance of the - 3 0 - 3 7 /~m XAD-1 would be about 2800 plates/meter based on the chloride peak. Performance is easily reproduced. As long as the capacity of the different resin batches is the same, the performance will be equivalent if the columns are well-packed. Figures 1 and 2 show chromatograms which are routinely obtained with these resins. Chromatographic conditions are noted in the figure captions. Table 8 and Figs. 3 and 4 compare resins made via this procedure and conventional chloromethylation. The "conventional" chloromethylation was carried out by generating CMME in situ via the reaction of methylal and acetyl chloride in ethylene dichloride solvent. The catalyst used was BiCI 3 because it is less moisture sensitive than ZnC12
CI~o~
L/L_ I 0
!
I
I
110
I
~
I
210
b
g
I
3
minutes
Z CI-
L i
Fig. 1. Trimethylamine resin, 0.027 m e q / g in a 500 x 2.0 mm column. Eluent: 0.001 M benzoic acid at 0.93 m l / m i n . Peak identities and concentrations are as follows in order of elution: acetate (20 ppm), azide ( - 10 ppm), glycolate (14 ppm), formate (6.2 ppm), fluoride (4.0 ppm), dihydrogen phosphate (8.1 ppm), chloride (8.3 ppm), nitrate (8.3 ppm), chlorate (10.2 ppm), 1-propane sulfonate (16.7 ppm), and iodide (15.3 ppm).
i
/
~
.
A
L i
o
~o~
$
minutes
Fig. 2. Trimethylamine resin, 0.027 m e q / g in a 500 × 2.0 mm column. Eluent: 4 × l 0 - 4 M K H P at pH 5.6 with a flow rate of 0.93 m l / m i n .
225 TABLE 8 Comparison of relative retention times for various ions on resins prepared via two chloromethylation routes (same conditions as Figs. 3 and 4) Ion
CI FBrI formate acetate nicotinate glycolate lactate C10 3 BrO 3 IO~ CH 3SO3CH3CH2SO~ CH3CH2CH2SO3NO 2 NO 3 HzPO 4
N3 BF4
Resin 1, a HCl/formaldehyde chloromethylation, lr ion/tr CI1,00 c 0.66 1.19 2.37 0.51 0.22 0.30 0.44 0.46 1.51 1.04 0.82 1.00 1.15 2.00 0.90 1.26 0.83 0.34 2.57
:CI-
Resin 2, b CMME/BiC1 s chloromethylation, trion/trc I formate
1.00 0.67 1.23 2.72 0.56 0.26 0.34 0.46 0.47 1.53 1.04 0.83 1.00 1.17 2.01 0.82 1.32 0.83 0.46 3.39
of resin 1 = 0.027 meq/g. b Capacity of resin 2 = 0.022 meq/g. ~trc I =8.9rain. dt~c ~ =7.6 min.
gl ycolate _w ~ia
azide
L___ I 0
I
I
I
I
I 10
minutes
~' Capacity
a n d is o n l y slightly g r e a t e r in c a t a l y s t strength. T h e s a m e c o l u m n was used for each resin a n d e q u i v a l e n t elution c o n d i t i o n s were used. T h e b e h a v i o r of the two resins is similar, b u t n o t identical. T h e figures show that the c o n v e n tional resin c a n n o t resolve glycolate a n d azide w h e r e a s the H C 1 / f o r m a l d e h y d e resin c a n d o so. T a b l e 8 i n d i c a t e s that the polarizable, late-eluting ions such as I a n d BF 4 h a v e longer relative r e t e n t i o n times on the c o n v e n tional resin. Also, the p e a k s o n the c o n v e n tional resin are slightly b r o a d e r . T h e s e last t w o o b s e r v a t i o n s suggest t h a t p e r h a p s the
Fig. 3. Trimethylamine resin, 0.027 meq/g in a 500 × 2.0 mm column. Eluent: 0.001 M benzoic acid at 0.93 ml/min. Resin prepared via formaldehyde/HCl chloromethylation.
C M M E c h l o r o m e t h y l a t i o n allows s o m e ione x c h a n g e g r o u p s to b e located d e e p e r in the polymer m a t r i x t h a n d o e s the H C 1 / f o r m a l d e h y d e c h l o r o m e t h y l a t i o n . T h i s is p r o b a b l y b e c a u s e of the swelling ability of the solvents used in the c o n v e n t i o n a l reaction. Future work
W o r k is c u r r e n t l y u n d e r w a y which is a i m e d at v a r y i n g the t y p e of p o l y m e r and f u n c t i o n a l
226
Benson of the Benson Co., for providing the unfunctionalized copolymers used in this and other investigations. Thanks are also extended to Dr. David Burge and Dr. Thomas Jupille of Wescan Instruments for providing the conductivity detectors used in our work. This work was supported by the Director of Energy Research, Office of Basic Energy Sciences.
Clformate
azide \
t
glycolate/'~l~
0 0.7 C~ n~
REFERENCES
<.~
I
II
0 ]
I
I
I
I
10
I
I
I
minutes
Fig. 4. Trimethylamine resin, 0.022 meq/g in a 500 × 2.0 mm column. Eluent: 0.001 M benzoic acid at 0.93 ml/min. Resin prepared via in situ generation of CMME with BiCI 3 catalysis.
group attached to the polymer in order to change selectivity of the resins. Polymer physical properties are also being examined in order to identify the most critical parameters in the choice of a resin substrate.
ACKNOWLEDGEMENTS The authors would like to thank Dr. R.L. Albright of Rohm and Haas and Dr. Jim
1 H. Small, T.S. Stevens and W.C. Bauman, Anal. Chem., 47 (1975) 1801. ~ 2 T.S. Stevens and M.A. Langhorst, Anal. Chem., 54 (1982) 950. 3 D.T. Gjerde, J.S. Fritz and G. Schmuckler, J. Chromatogr., 186 (1979) 509. 4 D.T. Gjerde, G. Schmuckler and J.S. Fritz, J. Chromatogr., 198 (1980) 35. .~ 5 D.T. Gjerde and J.S. Fritz, J. Chromatogr., 176 (1979) 199. 6 G. Vavon and J. Bolle, Compt. Rend., 204 (1937) 1826. 7 G. Grassi and C. Maselli, Gazz. Chim. Ital., 28 (II) (1898) 477. 8 E.B. Trostyanskaya, I.P. Losev and Ly. Syanzhao, Zhur. Vsesoyuz. Khim. Obshchestva im. D.I. Mendeleeva, 5 (1960) 116. 9 C.H. McBurney, U.S. Patent 2,591,573, 1952. 10 J. von Braun and J. Nelles, Chem. Ber., 67 (1934) 1094. 11 R.C. Fuson and C.H. McKeever, in: R. Adams, W.E. Bachmann, L.F. Feiser, J.R. Johnson and H.R. Snyder (Eds.), Organic Reactions, Vol. I, Wiley, New York, 1942, Chap. 3. 12 I.N. Nazarov and A.V. Semenovsky, Izv. Akad. Nauk S.S.S.R. Otd. Khim. Nauk, (1957) 972. Bull. Acad. Sci., U.S.S.R., Div. Chem. Sci., (English Trans.), (1957) 997. 13 Y. Ogata and M. Okano, J. Amer. Chem. Soc., 78 (1956) 5423. 14 I.N. Nazarov and A.V. Semenovsky, Izv. Akad. Nauk S.S.S.R. Otd. Khim. Nauk, (1957) 212. Bull. Acad. Sci. U.S.S.R., Div. Chem. Sci., (English Trans.), (1957) 225.