Amination of vinylbenzyl chloride-divinylbenzene (VBC-DVB) copolymers is an effective method for preparation of ion-exchange resins. Conventionally, the starting polymer is produced by chloromethylation of a styrene-divinylbenzene copolymer that utilizes chloromethyl methyl ether, a known carcinogen. An alterative approach is to copolymerize vinylbenzyl chloride with divinylbenzene to generate the necessary VBC-DVB. This method provides precise control over the density of the ion-exchange groups. The regiochemistry of the vinylbenzyl chloride methods was realized using solvent-ion exchange groups. In this investigation, an improved solvent system was found for the preparation of anion exchange resins by the vinylbenzyl chloride route. The effectiveness of amination of the intermediate VBC-DVB polymers with a variety of trimethylamine reagents was investigated, and ethanolic trimethylamine produced the highest degree of amination. These resulting ion-exchange polymers were characterized by a variety of techniques such as analytical titrations, nitrogen analysis, Fourier transform infrared spectroscopy and thermal gravimetric analysis. Testing of these copolymers for breakthrough was performed. The results indicate that these anion exchangers have a meaningful increase in thermal stability over commercial anionic exchange beads.
Ion exchange membranes (IEMs) can be used for numerous applications including desalination of seawater, softening of hard water, recovery of metal ions, and purification of products and water [
The use of chloromethylating reagents can be avoided by synthesizing the desired vinylbenzyl chloride/divinylbenzene copolymer utilizing vinylbenzyl chloride as a starting material [
In the subsequent amination step, the chloromethylated copolymer is conventionally swelled in a suitable solvent such as dioxane, butanone, tetrahydrofuran, or benzene to allow for a uniform reaction and to help reduce the effects of osmotic shock [
In addition to the synthesis and characterization of the ion exchange systems, an area of specific interest for this study was the removal of excess nitrate, phosphate, sulfate, and chloride ions from brackish and moderately saline waters. These anions, along with cations such as sodium, calcium, and magnesium, contribute to the salinity of water. It is worth mentioning that little work has been done using anion exchange resins for multianion removal from water. Among these are the removal of arsenic [
The starting monomer used in this work was vinylbenzyl chloride (VBC) 90% (Aldrich) that is a mixture of para and meta isomers. Divinylbenzene (DVB), 80% (Sigma-Aldrich), was used as a cross-linker, and benzoyl peroxide (Luperox A98) reagent grade, ≥98% (Sigma-Aldrich), was used as a radical initiator. The following chemicals or solutions were used as functionalizing agents: trimethylamine solution purum, 31–35 wt.% in ethanol (~4.2 M) (Sigma-Aldrich), trimethylamine solution purum, ~45 wt.% in H2O (Sigma-Aldrich), pure trimethylamine (Sigma-Aldrich), and triethylamine (Sigma-Aldrich). All of the aforementioned chemicals were used without further purification. Other solvents were reagent grade and used as received.
A mixture of vinylbenzyl chloride (VBC) and divinylbenzene in a ratio of 6.2 to 1 by weight was dissolved in a solvent mixture, and then 3% benzoyl peroxide was added as a radical initiator. Tetrahydronaphthalene (THN), Xylene, and a 1 : 1 v : v mixture of tetrahydronaphthalene and xylene were used as solvents in order to determine the influence of these on the resulting polymer. In all cases, the amounts were 62% VBC, 10% DVB, 3% initiator, and 25% solvent by weight. Radical polymerization was carried out by heating the solutions for 10 h at 80
Polymerization and functionalization of anion exchange polymer.
To determine the influence of the degree of cross-linking (Figure
Scheme showing the preparing procedures while different factors have been changed.
The copolymers were functionalized by reaction with trimethylamine gas and trimethylamine solutions in ethanol and water at 60°C for four days. An excess of trimethylamine was used in comparison to the chloromethyl content of the copolymer. Typically, 3 g of polymer was reacted with 30 mL of solution. In the case of the gas-phase reaction, a volume of 250 mL of trimethylamine was used. The resulting resins were washed with water/methanol, and water and dried at 55°C in a vacuum oven until they reached a constant weight (typically 12 hours). The yields were 3.035, 3.32, and 3.34 g for the solventless, aqueous, and ethanolic reactions, respectively. A reaction was also performed between the copolymer and neat triethylamine using the same procedure.
Uptake of water was determined by the difference in weight between the water-swelled and the dried polymer. Swelling is represented as milligrams of water per gram of dry resin. The water uptake (WU) was calculated by the following equation:
Copolymers were dried at
The aminated polymers were characterized by combustion analysis to determine the nitrogen content. The degree of amination and the amine group density were subsequently calculated. The upper limit of the expected nitrogen content should be 6.26, 6.18, 6.08, and 5.96% for the 6, 8, 10, and 12% DVB resins, respectively, assuming the functional groups in VBC completely reacted with trimethylamine.
A strong base anion-exchange resin generally has a greater affinity for more highly charged anions over lower-charged ones [
Breakthrough experiments were conducted using a VWR Mini-Peristaltic Pump-Variable Flow pump. Dynamic mode tests were conducted using both the novel anion exchanger and commercial beads. About 3 g of the novel polymer was packed vertically in a 0.4 inch ID column for testing using an initial solution containing 50 ppm of both sulfate and chloride ions with a flow rate of 5 mL/min. Using a 20 mL/min flow rate, solutions containing varying influent concentrations were pumped through both the commercial and novel resins beads. Twenty ml of effluent was collected repeatedly for analysis. All samples were analyzed by ion chromatography.
In this investigation, anion exchange polymers were prepared for potential application in the removal of anions that contribute to the salinity of brackish waters. The synthetic procedure involved copolymerization of vinylbenzyl chloride with divinylbenzene to generate a cross-linked polymer. The chloromethyl groups were subsequently converted to benzyltrimethylammonium groups by reaction with trimethylamine. For comparison purposes, the preparation of the triethylammonium analog was performed using triethylamine as the amination reagent. Several factors in the synthetic procedure were varied to identify a method that maximizes the number of functional groups in the final polymer. These factors included the solvent used for the initial polymer synthesis, the percentage of the cross-linker (divinylbenzene), and the solvent and temperature for the amination reaction.
Figure
Effect of amine type on the nitrogen content (after immersing in 6% DVB polymer in TMA at room temperature for 4 days).
The degree of amination can be derived from the nitrogen content of the resin. However, since the presence of variable amounts of water can complicate the comparison of one resin to another. Therefore, combustion analyses were performed for both carbon and nitrogen so that their ratio could be used to remove any influence of variable water content on the analyses. The results, presented in Table
Carbon and nitrogen analysis for the various experimental runs.
Effect of Functionalization agent on the amount of Nitrogen in polymer (after immersing polymer in TEA/EtOH, anhydrous TMA, aqueous TMA and ethanolic TMA at room temp. for 4 days). (VBC 66 wt%, DVB 6 wt%, BPO 3 wt% and 25 wt% Solvent (Xylene).
Type of amine | TN% | TC% |
---|---|---|
TEA/EtOH | 1.86 | 67.5 |
Anhydrous TMA | 0.42 | 75.1 |
TMA/H2O | 3.98 | 69.6 |
TMA/EtOH | 4.06 | 65.4 |
Effect of cross-linker on the Nitrogen content (after immersing polymer in TMA/EtOH at room temp. for 4 days) (VBC 66, 64, 62, and 60 wt%, DVB 6,8,10,12, respectively, BPO 3 wt% and 25 wt% Solvent (Xylene).
Divenylbenzene ratio | TN% | TC% |
---|---|---|
6 wt% DVB | 3.09 | 69.50 |
8 wt% DVB | 2.02 | 72.00 |
10 wt% DVB | 1.97 | 75.50 |
12 wt% DVB | 0.97 | 76.90 |
Effect of Solvent on the Nitrogen content (after immersing polymer in TMA/EtOH at
Solvent | TN% | TC% |
---|---|---|
THF (Tetrahydrofuran) | 2.12 | 76.10 |
Xyl. | 4.77 | 66.1 |
Xyl., THN (1:1) | 5.2 | 67.7 |
Effect of temperature on the Nitrogen content (after immersing polymer in TMA/EtOH for 4 days). (VBC (64 wt%, DVB 8 wt%, BPO 3 wt% and 25 wt% Solvent (THN, Xyl 1 : 1)
Temp | TN% | TC% |
---|---|---|
Room temp. | 4.06 | 65.4 |
40°C | 4.34 | 61.2 |
60°C | 5.2 | 67.7 |
TN% and TC% for different DVB ratios at
DVB ratio | TN% | TC% |
---|---|---|
6% | 5.25 | 67.7 |
8% | 5.2 | 67.7 |
10% | 5.05 | 67.5 |
12% | 4.09 | 72.3 |
The effect of cross-linking was investigated by preparing polymers with a range of divinylbenzene contents using xylene as a solvent and then aminating with an ethanolic trimethylamine solution. Figure
Effect of cross-linker (%DVB) on the nitrogen content (after immersing in ethanolic TMA at room temperature for 4 days).
It is possible that the solvent was used for the initial polymerization reaction to influence the microstructure of the cross-linked co-polymer and thereby affect the amination reaction. Therefore, besides the conventional xylene solvent, tetrahydrofuran (THF) and tetrahydronaphthalene (THN) were employed in this study (Figure
Effect of solvent on the nitrogen content after immersing polymer 8% DVB polymer in amination reagent at
Effect of temperature on the nitrogen content after immersing 8% DVB polymer in TMA for 4 days).
The percentage water uptake by an ion exchanger correlates with its hydrophilicity, a parameter that positively influences the rate of ion exchange [
Water percent, nitrogen percent, and exchange capacity for polymer prepared with different DVB ratios prepared by polymerization in xylene and THN then aminated using ethanolic trimethylamine.
DVB % | Water | Nitrogen | Exchange capacity meq/g | wt% gain after amination | wt% loss after extraction | wt% loss after treatment with 0.1 N NaOH |
---|---|---|---|---|---|---|
6% | 186 | 5.25 | 2.06 | 27.98 | 27.67 | 3.19 |
8% | 167 | 5.20 | 1.87 | 24.12 | 16.87 | 1.95 |
10% | 146 | 5.05 | 1.78 | 23.41 | 11.73 | 1.30 |
12% | 91 | 4.09 | 0.99 | 16.97 | 9.40 | 1.10 |
Although the cross-linking degree plays an important role on the degree of amination as well as the ion exchange capacity, the resin containing 10% DVB was utilized in the subsequent investigation since the yield of polymer was significantly enhanced over polymers prepared with lower amounts of DVB. It was found that there were markedly higher weight losses during the Soxhlet extraction with methanol for the 6 and 8% polymers, reflecting the generation of higher amounts of soluble, poorly cross-linked polymer in these syntheses. It should be noted also that the observed ion exchange capacities of both 8% DVB and 10% DVB are very close to each other. Moreover, the thermal properties of 10% DVB type are superior to those of the polymers with lower DVB content. Regeneration studies were conducted by taking the samples, with different DVB ratios, soaking them in 0.1 M NaOH solutions for 48 hours at room temperature, washing them with deionized water, and then vacuum drying and weighing them before they were reused. The regeneration of the resins led to minimal weight losses, especially for the 10% and 12% DVB types (Table
The FT-IR spectra of VBC-N(CH3)
FT-IR spectra of the anion exchange polymer.
The thermal stabilities of the base polymer, P-N(CH3)
Thermogravimetric curves (
The new anion exchange resin was tested in its hydroxide form in a breakthrough experiment when it was challenged with a solution containing 50 ppm of both sulfate and chloride ions (Figure
Breakthrough results using 50 ppm SO4 and Cl.
The polymer is more efficient at removing sulfate than chloride because of the selectivity preference for sulfate over chloride. As a result, sulfate replaces both hydroxide and chloride, while chloride replaces only hydroxide sites at these concentrations. Functionally, this is due to the strongly basic functional groups (benzyltrimethylammonium) which are highly selective toward divalent ions [
The performance of the novel anion exchange resin was further tested with a solution containing 50 ppm of each of several anions (
Breakthrough results for the novel ion exchanger using 50 ppm Cl, SO4, NO3, and PO4.
Breakthrough results for DOWEX ion exchanger using 50 ppm Cl, SO4, NO3, and PO4.
Breakthrough results for prepared ion exchanger using 50 ppm Cl and 10 ppm SO4.
In Figure
Breakthrough results for prepared ion exchanger using groundwater sample.
Several methods were used to prepare anion exchange polymers with higher cross-linking and reasonable ion exchange capacity. The type of solvent influenced the degree of amination when the cross-linker ratio was increased. The anion exchanger prepared in this investigation had improved thermal stability as compared to commercial resins. The results from the ion-exchange experiments suggest that this kind of anion exchanger is more selective for divalent anions rather than monovalent anions especially chloride. However, it works well with chloride when the concentrations of other divalent anions (e.g., sulfate) were significantly lower than that of the chloride.
The authors wish to thank Dr. Eliot Atekwana, Associate Professor, Boone Pickens School of Geology, Oklahoma State University for allowing them use lab to run many samples on the ion chromatograph, and they really value his cooperation.