LOW-RINSE, HIGH-CAPACITY, WEAKLY BASIC ACRYLIC ION EXCHANGE RESINS PROCESS FOR PREPARING THEM, AND THEIR USE IN REMOVING ANIONS FROM A LIQUID

Abstract: The invention provides weakly basic anion exchange resins having both low rinse-volume requirements and high anion exchange capacity, the resins comprising copolymers formed from monomers comprising acrylonitrile and crosslinker having two or more unconjugated allylic groups, the copolymers being functionalized with polyamine. The invention also provides a process for preparing the weakly basic anion exchange resins, and a process for removing anions from a liquid using the resins.

Description: This invention is concerned with weakly basic, acrylic anion exchange resins having both high anion exchange capacity and low rinse requirements, a process for preparing them, and their use in removing anions from a liquid. Among anion exchange resins, those with acrylic polymer backbones are known for having higher ion exchange capacity and greater fouling resistance than styrenic resins. The weakly basic, gel acrylic resins prepared from conventional acrylic esters, however, are difficult to rinse after regeneration. One reason for this rinse difficulty is that trace amounts of carboxylic acid functionality remain in the functionalized resin, and retain cations from the regenerant, which cations are released only gradually into the rinse water. Prolonged rinsing eventually reduces the cations to a reasonable level, but this takes excessive time and amounts of rinse water. Macroreticular acrylic resins have been used to avoid this high-rinse-volume requirement, but these resins have significantly lower anion exchange capacity than the high-rinse-volume, gel acrylic resins. Corte et al., in U.S.-A-3,544,488, describes copolymers of acrylonitrile with aliphatic, including cycloaliphatic, crosslinkers containing two or more unconjugated allyl and vinyl groups; such copolymers resisting hydrolysis much better than acrylonitrile copolymers crosslinked with divinylbenzene or other, aromatic, crosslinkers. In the Corte reference these copolymers were used to prepare only cation exchange resins. Bufton et al., in U.S.-A-3,423,336, discloses the preparation of weakly basic anion exchange resins by functionalizing acrylonitrile copolymers with anhydrous polyamines. The copolymers are crosslinked with conventional crosslinkers such as esters and polyvinyl aromatic monomers, and suffer from the stability problems discussed by Corte et al. Abrams et al., in U.S.-A-3,389,096, disclose a similar reaction to that of the Bufton reference, but functionalization occurs in the presence of water. As the copolymer is the same as Bufton's, the disadvantages are the same. GB-A-972,897 discloses a process for the production of anion exchange resins which comprises reacting, with a polyamine, a crosslinked copolymer of an ethylenically unsaturated monomer containing nitrile groups and a polyethylenically unsaturated crosslinking agent. The crosslinking agent is preferably divinyl benzene or a substituted divinyl benzene. We have now unexpectedly found stable, weakly basic, acrylic anion exchange resins that combine the low rinse requirements of low-capacity, macroreticular anion exchange resins with the high capacity of high-rinse, gel anion exchange resins. The resins of the present invention are copolymers formed from monomers comprising acrylonitrile and aliphatic crosslinking monomers selected from monomers having two or more non-conjugated allyl groups, 1,4-pentadiene, 3,4-diethyl-1,5-hexadiene, and 1,2,4-trivinylcyclohexane, the copolymers being functionalized with a polyamine. The resins of the invention are conveniently prepared by a process comprising functionalizing a copolymer formed from monomers comprising acrylonitrile and aliphatic crosslinking monomer selected from monomers having two or more non-conjugated allyl groups, 1,4-pentadiene, 3,4-diethyl-1,5-hexadiene, and 1,2,4-trivinylcyclohexane, the functionalization of the copolymer being carried out by reacting it with a polyamine at a temperature of from 100 to 200°C and for from 5 to 20 hours. The functionalization may comprise hydrolyzing the copolymers and subsequently aminating them with polyamine or directly aminating the copolymers with a mixture of a polyamine and water. In use, the resins of the invention exhibit high capacity for removing anions from liquids, and low rinse requirements during regeneration. Copolymers useful in forming the resins of the present invention are prepared by copolymerizing monomers comprising acrylonitrile and aliphatic, crosslinking monomer selected from monomers containing two or more non-conjugated allyl groups, preferably 2 or 3 non-conjugated allyl groups, 1,4-pentadiene, 3,4-diethyl-1,5-hexadiene, and 1,2,4-trivinylcyclohexane. The aliphatic crosslinking monomer may be a cycloaliphatic monomer, and the aliphatic crosslinking monomer may be present at levels of from 0.05 to 20, preferably from 1 to 10, percent by weight of the total monomer weight. Polymerization can be initiated using any of the free-radical polymerization initiators, including azoperoxide and perester initiators, for example at levels from 0.1 to 15% by weight of the monomer mixture and at a temperature from 40° to 90°C, preferably from 50° to 70°C. Ultraviolet radiation may also be used to initiate polymerization. The monomer mixture is preferably dispersed, during polymerization, in from 1 to 5 times the monomer volume of water, containing from 0.1 to 2% by weight of the aqueous phase of a dispersing agent, and from 0.1 to 2% by weight of the aqueous phase of gelatin. The aliphatic crosslinking monomer conveniently is resistant to hydrolysis. Suitable aliphatic crosslinking monomers containing two or more non-conjugated allyl groups include: 1,5-hexadiene; 1,6-heptadiene; 1,7-octadiene; 1,8-nonadiene; n-1,4,9-decatriene; and other, related, aliphatic crosslinkers such as triallyl isocyanurate. Weakly basic anion exchange resins having high anion exchange capacity and low rinse requirements are prepared from the copolymer by functionalizing it with a polyamine, preferably a polyamine containing 2 to 4 amine groups. The amine groups of the polyamine are preferably separated by 2 to 4 methylene groups, more preferably by 3 methylene groups. Examples of polyamines that may be used to functionalize the resins of the present invention include diethylenetriamine, triethylenetetramine, tetramethyliminobispropylamine, N,N-dimethylaminoethylamine and N,N-dimethyl­ aminopropylamine. As the amines supply the functional groups to the resin, a high ratio of amine groups to total molecular weight of the resin is preferred for a high-capacity resin. Long alkyl chains, or large nonfunctional substituents between the amine groups, while operable, are not preferred because they reduce the number of anion exchange functional groups per gram of resin. The copolymer may be functionalized by hydrolyzing it in hot, aqueous caustic or acid, followed by treating it with from 3 to 8 times the copolymer weight of the polyamine for from 5 to 20 hours at from 100 to 200°C, or treating it directly with the polyamine in the presence of water, the water being present in an amount of up to 52% by weight of the polyamine. When the copolymer is functionalized directly with the polyamine and water, the water may optionally be distilled from the mixture during the functionalization. For aqueous functionalization (i.e. treatment of the copolymer directly with the polyamine and water) the copolymer is treated with a similar weight of the polyamine (i.e. similar to that used in the functionalization involving hydrolysis of the copolymer followed by treatment with the polyamine) containing from 5 to 50% water, initially at from 90° to 120°C for 0 to 6 hours, after which the temperature is raised to the range from 140° to 180°C and held there for from 12 to 20 hours. While the copolymer and functionalized resin according to the invention have a degree of macroreticular structure, resulting from the insolubility of the copolymer in the acrylonitrile monomer, the rinse requirements of these resins are significantly lower than would be expected based on their porosity. At the same time, their anion exchange capacity is significantly higher than any previously known resins having similar rinse requirements. This surprising combination of high capacity and low rinse requirement is one of the resin's most significant advantages. The present invention will now be further illustrated by way of the following examples which are for illustrative purposes only and are not to be construed as imposing any limitation on the scope of the invention as claimed. All percentages are by weight unless otherwise indicated, and all reagents are of good commercial quality, unless otherwise indicated. Example 1: This example illustrates the preparation of an acrylonitrile copolymer crosslinked with an non-conjugated polyvinyl aliphatic crosslinker, useful in the present invention. 11.6 g poly(diallyldimethylammonium chloride), 1.16 g boric acid, 1.67 g sodium nitrite, and 3.25 g gelatin were dissolved in 550 ml deionized water.The pH of the solution was adjusted to 10.5 with aqueous sodium hydroxide and the solution was stirred. To the stirred solution a mixture of 375 g acrylonitrile, 16 g trivinylcyclohexane and 4 g 2-t-butylazo-2-cyano-4-­ methoxy-4-methylpentane was added. The resulting suspension was heated to 55°C and maintained at that temperature for 6 hours, after which it was heated to 90°C and maintained at that temperature for 1 hour. The resulting opaque beads were washed with water and allowed to air dry. Example 2 (comparative) : This example illustrates the preparation of a comparative, aromatic-crosslinked copolymer. The procedure of Example 1 was used, except that the amount of acrylonitrile was 320 g, and the trivinylcyclohexane was replaced with 69 g divinylbenzene of 55% purity. Example 3: This example illustrates a more highly crosslinked acrylonitrile copolymer useful in the present invention. The procedure of Example 1 was used, except that the amount of acrylonitrile was 360 g and the amount of trivinylcyclohexane was 31 g. Example 4: This example illustrates the preparation of a resin of the present invention by directly aminating the copolymer of Example 3. A 40 g sample of the copolymer of Example 3 was added to a mixture of 85 ml diethylenetriamine and 10 ml deionized water, and the mixture was heated to 125°C and maintained at that temperature for 16 hours. The resulting resin solids content was 52.0%, and the yield of resin was 66.9% of theoretical yield. Example 5 (comparative): This example illustrates the preparation of an anion exchange resin from the comparative, aromatic-­ crosslinked copolymer of Example 2. The procedure of Example 4 was used, except that the copolymer was the copolymer of Example 2. The resulting resin solids content was 44.6%, and the yield of resin was 55.5% of theoretical yield. Example 6: This example illustrates an anion exchange resin of the present invention prepared by aminating the resin with dimethylaminopropylamine. A 40 g sample of the copolymer of Example 3 was added to a mixture of 340 g dimethylaminopropylamine and 45 g deionized water. The mixture was stirred and heated to 120°C, maintained at that temperature for 5 hours, heated to 160°C, and maintained at that temperature for 16 hours. The resulting resin solids content was 48.6%, and the yield of resin was 75.5% of theoretical yield. Example 7 (comparative): This example illustrates the comparative, aromatic-crosslinked copolymer of Example 2, aminated with dimethylaminopropylamine. The procedure of Example 6 was used, except that the copolymer of Example 2 was used. The resulting resin solids content was 33.0%, and the yield of resin was 33.9% of theoretical yield. Example 8: This example illustrates the two-step hydrolysis and amination of the copolymer of Example 1. 100 g of the copolymer of Example 1 were hydrolyzed by refluxing in 50% sulfuric acid for 6 hours. The hydrolyzed copolymer was rinsed 3 times with water and dried. 40 g of the dry, hydrolyzed copolymer were suspended in 240 g of dimethylaminopropylamine, the mixture was heated to 160°C, and maintained at that temperature for 20 hours. The properties of the resulting anion exchange resin are given in Table 1. Example 9: This example illustrates measurement of rinse-­ volume requirements for resins of the present invention and comparative resins. Approximately 250 ml of each resin were placed in a 1.27 cm ( 1 2 -inch)-diameter, glass column. The resin was exhausted with aqueous hydrochloric acid and subsequently regenerated with 4% aqueous sodium hydroxide solution; a 250% excess of regenerant, based on the anion exchange capacity of the resin, was used. The rinse requirement of the resin was then determined by passing deionized water through the freshly regenerated resin bed and measuring the electrical conductivity of the resin-bed effluent as a function of the number of bed volumes of rinse water passed through it. The resins tested according to this example were the following: 1. Anion and cation exchange capacity in milliequivalents per gram of dry resin. 2. Average size is the average diameter of the whole of the resin beads. 3. Anion exchange capacity of resin, expressed as kilograins CaCO₃ per cubic foot of resin. 4. Bed volumes of rinse water to reach effluent conductivity shown. Example 10: This example illustrates the amination of a polymer useful in the present invention, using a sulphur catalyst. A mixture of 40 g of resin prepared according to Example 1, 45 ml deionized water, 340 ml dimethylaminopropylamine and 3.40 g elemental sulphur were heated to 120°C and maintained at that temperature for 4 hours, then heated to 160°C and maintained at that temperature for 16 hours. The mixture was cooled, rinsed and drained. A total of 239 g of wet resin, having a solids content of 42.2%, was obtained, for a yield of 83%. Example 11: This example illustrates thioacetamide-­ catalyzed amination of a copolymer useful in the present invention. The materials and procedures of Example 10 were used except that the sulphur was replaced with 0.5 g thioacetamide. The yield of aminated resin was 86%.

Claims: 1. A weakly basic anion exchange resin having low rinse-volume requirements and high anion exchange capacity, characterized in that it comprises a copolymer formed from monomers comprising substantially acrylonitrile and aliphatic crosslinking monomer selected from monomers having two or more non-conjugated allyl groups, 1,4-pentadiene, 3,4-diethyl-1,5-hexadiene, and 1,2,4-trivinylcyclohexane, the copolymer being functionalized by reacting it with a polyamine. 2. A resin as claimed in claim 1, wherein the polyamine contains 2 to 4 amine groups. 3. A resin as claimed in claim 1 or claim 2, wherein the amine groups of the polyamine are separated by 2 to 4, e.g. 3, methylene groups. 4. A resin as claimed in claim 1, wherein the polyamine comprises diethylene triamine, triethylene tetramine, or dimethylaminopropylamine. 5. A resin as claimed in any of claims 1 to 4, wherein the aliphatic crosslinking monomer comprises cycloaliphatic monomer. 6. A resin as claimed in any of claims 1 to 5, wherein the aliphatic crosslinking monomer comprises 2 or 3 non-conjugated allyl groups. 7. A process for preparing a weakly basic anion exchange resin as claimed in any of claims 1 to 6, characterized in that it comprises functionalizing a copolymer formed from monomers comprising substantially acrylonitrile and aliphatic crosslinking monomer selected from monomers having two or more non-conjugated allyl groups, 1,4-pentadiene, 3,4-diethyl-1, 5-hexadiene, and 1,2,4-trivinylcyclohexane, the functionalization of the copolymer being carried out by reacting it with a polyamine at a temperature of from 100 to 200°C and for from 5 to 20 hours. 8. A process as claimed in claim 7, wherein the functionalization comprises hydrolyzing the copolymer in hot, aqueous caustic soda or acid, followed by treating it with from 3 to 8 times the copolymer weight of the polyamine. 9. A process as claimed in claim 7, wherein the functionalization occurs in the presence of up to 52% water, based on the weight of the polyamine. 10. A process for removing anions from a liquid, characterized in that it comprises contacting the liquid with a weakly basic anion exchange resin as claimed in any of claims 1 to 6.

Inventors: BIGWOOD, MICHAEL PETER, NAPLES, JOHN OTTO
Applicants: ROHM AND HAAS COMPANY
Application year: 1986
Main classification: C08F8
IPC codes: C08F8, B01J41