Oxidation-resistant cation-exchange resins

Abstract: Cation-exchange resins are provided which have improved resistance to copolymer degradation caused by oxidizing agents like molecular oxygen. The resins are prepared by functionalizing a plurality of certain cross-linked copolymer beads with cation-exchange groups. The cross-linked copolymer beads are prepared from a para-substituted monovinyl aromatic monomer comprising an aromatic ring substituted with an oxidation-stabilizing moiety in a position para to a polymerizable vinyl moiety. The oxidation-stabilizing moiety is substantially inert during substitution of the copolymer beads with the cation-exchange functional groups, which avoids substitution at the para position. Exemplary of such para-substituted monovinyl aromatic monomers are para-methylstyrene, para-ethylstyrene, and para-chlorostyrene. The cation-exchange resins are capable of being used in a separation process for longer periods of time without detrimental increases in bed pressure drops, loss of operating capacity, and leaching of organic contaminants.

Description: This invention concerns ion-exchange resins generally, and in particular, cation-exchange resins which are resistant to oxidative degradation during use in a separation process, such as an ion-exchange or chromatographic process. Cation-exchange resins are widely employed by industry to remove impurities from liquid mixtures, or to separate various components contained in such mixtures. Conventional ion-exchange resins are prepared by functionalizing a copolymer matrix with groups having anions or cations which are capable of being exchanged for, or associated with, ions or molecules having the same charge when the resin is in contact with a liquid mixture. Such resins have generally been used in water treatment and purification, food preparation, pharmaceutical manufacturing, chemical processing, and metal extraction, as discussed by R. M. Wheaton et al. in, "Ion Exchange", 11 Kirk-Othmer Ency. Chem. Tech. pp. 871-899 (2nd Ed. 1966). One problem associated with conventional cation-exchange resins is oxidative degradation of the copolymer matrix over an extended period of time. Liquid mixtures which are typically in contact with the resin inherently contain quantities of oxidizing species such as molecular oxygen, or may have an elevated temperature, both of which may promote more rapid degradation of the copolymer matrix. It is believed that during oxidative degradation, carbon-carbon bonds rupture with respect to cross-links between individual polymer chains and/or links between individual styrene moieties. The loss of such bonds results in an increase in water retention capacity and, ultimately, in the release of organic contaminants, such as segments of functionalized linear polystyrene. As used hereinafter, the term "oxidative degradation" refers to such degradation of the copolymer matrix. Degradation of the copolymer matrix is undesirable for commercial operation of an ion-exchange or chromatographic process. For example, resins which decross-link become relatively soft and swell to a greater extent. Such changes ultimately result in an increased bed pressure drop, a reduced flow rate for the liquid mixture being treated, and reduced operating capacity for removing chemical species when such resins are employed in a typical ion-exchange or chromatographic column. Additionally, the release of organic contaminants into column effluent can be unacceptable in some applications, such as those used in preparing food products. Organic contaminants may also create a source of potential corrosion to process equipment. Organic contaminants may also foul anion-exchange resins associated with a process, such as in a mixed-bed application where both cation- and anion-exchange resins are mixed together, or when an anion-exchange resin is employed in a subsequent process step. Previously, industry attempted to remedy oxidative degradation and its associated problems by increasing the amount of cross-linking monomer used in preparing the copolymer matrix. However, an increase in the number of cross-links renders the resulting resin bead less compatible with liquid mixtures, thereby resulting in reduced diffusion into the bead and poor operating capacity. A highly cross-linked resin also generally exhibits poor regeneration efficiencies and may be impermeable with respect to large molecules, such as glucose, fructose, and other sugars. Furthermore, increasing the cross-link density does not address problems associated with release of organic contaminants, since degradation still occurs. U.S. Patent 3,342,755 issued to Calmon et al. discloses a postulated mechanism for degradation of the copolymer matrix by identifying a so-called "weak link" at tertiary carbons adjacent to the benzene ring of a styrene moiety. This weak link is said to exist, due to the tendency of hydrogen attached to the tertiary carbons to form hydroperoxides with oxidizing agents like molecular oxygen or chlorine. The hydroperoxides are said to eventually lead to splitting of carbon chains associated with the copolymer. Calmon et al. attempt to solve the problem by substituting a halogen for the hydrogen at the tertiary carbon, and further disclose that nuclear halogenation, i.e., employing a monomer such as ortho-chlorostyrene, does not contribute to resin stability. As can be seen, it is desirable to develop cation-exchange resins which have improved resistance to oxidative degradation. Use of such resins in an ion-exchange exchange or chromatographic separation process would promote a more efficient and reliable process. Thermally stable cation-exchange resins are disclosed in GB-1393594. The thermal stability is conferred by the inclusion into the resin of at least one sulphonated aromatic nucleus which is substituted with a halogen substituent. The halogen substituent, preferably chlorine, can be substituted at any position on the ring relative to the sulphonic group, and stabilises the ring by virtue of its ring deactivating properties. Monohalogen substituted styrene/divinylbenzene copolymers for use as ion-exchangers are also disclosed in DE-A-2218126. Chlorine is the preferred halogen substituent. In accordance with the invention there is provided a cation-exchange resin which is resistant to oxidative degradation. The resin comprises a plurality of cross-linked copolymer beads substituted with cation-exchanged functional groups. The copolymer beads are prepared from a cross-linking monomer and a para-substituted monovinyl aromatic monomer. The para-substituted monovinyl aromatic monomer comprises an aromatic ring which is substituted with an oxidation-stabilizing moiety in a position para to a polymerizable vinyl moiety. The oxidation-stabilizing moiety is phenyl or a C 1 to C 8 alphatic moiety, and is substantially inert during substitution of the copolymer beads with the cation-exchange functional groups, thereby substantially preventing functionalization of the aromatic ring at the para position. The novel cation-exchange resins of the present invention are prepared by functionalizing a plurality of copolymer beads. The copolymer beads may be prepared by suspension polymerization methods wherein a suspended monomer phase is polymerized within a continuous phase that is substantially immiscible with the monomer phase. According to the present invention, the monomer phase is characterized by having a major amount of para-substituted, monovinyl aromatic monomer for example methylstyrene, along with a minor amount of a cross-linking monomer for example divinylbenzene, and an effective amount of a free-radical polymerization initiator. After polymerization, the resulting copolymer beads are converted to cation-exchange resins by functionalization with, for example, sulfonic, carboxylic, or phosphonic acid ion-exchange moieties according to known methods. The resulting resins are characterized by high resistance to oxidative degradation as discussed hereinafter. It has now been discovered that use of certain para-substituted, monovinyl aromatic monomers in preparing the copolymer beads results in resins which are highly resistant to oxidative degradation. As used hereinafter, the term "para-substituted monovinyl aromatic monomer" refers to monomers comprising an aromatic ring having, as substituents to the ring, an addition-polymerizable vinyl moiety in a para position to an oxidation-stabilizing moiety. By "aromatic ring", it is meant a six-membered carbon ring, i.e., a benzene ring. The term "oxidation-stabilizing moiety" refers to phenyl or a C 1 -C 8 aliphatic moiety which is substantially inert during functionalization of the copolymer beads, thereby substantially preventing substitution of ion-exchange functional groups at the para position. The term "addition-polymerizable vinyl moiety" refers to any ethylenically unsaturated substituent which is capable of rendering the para-substituted monovinyl aromatic monomer susceptible to addition-type polymerization. The term "conventional cation-exchange resin" as used herein refers to resins prepared substantially from conventional monovinyl aromatic monomers which are not para substituted with an oxidation stabilizing moiety. Para-substituted monovinyl aromatic monomers in accordance with the invention correspond to Formula I: wherein R is phenyl or an aliphatic moiety having up to 8 carbon atoms. Where R is aliphatic, it preferably has less than 6 carbons, more preferably less than 3 carbons, and most preferably one carbon. Examples of preferred aliphatic moieties are alkyls like methyl, ethyl, propyl, isopropyl, or butyl. Examples of preferred para-substituted monovinyl aromatic monomers are para-methylstyrene and para-ethylstyrene. A variety of cross-linking monomers may be employed in preparing the copolymer beads. Suitable cross-linking monomers are addition polymerizable polyvinyl compounds. Such monomers are well-known in the art and reference is made to Polymer Processes , edited by Calvin E. Schildknecht, published in 1956 by Interscience Publishers, Inc., New York, Chapter III. Of particular interest are water-insoluble polyvinyl aromatics such as divinylbenzene, divinyltoluene, divinylxylene, divinylnaphthalene, trivinylbenzene, divinyldiphenyl ether, diisopropenylbenzene, triisopropenylbensene and divinyldiphenylsulfone. Also suitable for use as a cross-linking monomer are polyvinyl α,β-ethylenically unsaturated carboxylic acids and esters thereof. Preferred cross-linking monomers are the polyvinyl aromatic compounds such as divinylbenzene, trivinylbenzene, diisopropenylbenzene and triisopropenylbenzene. The copolymer beads may be prepared by suspension polymerization methods generally known in the art. Suitable polymerization techniques include, for example, a single-stage polymerization process described by F. Helfferich, Ion Exchange , (McGraw-Hill 1962) at pages 35-36, wherein a water-immiscible monomer mixture is suspension polymerized in a continuous aqueous phase to produce copolymer beads. Generally, the cross-linking monomer is employed in an amount of from 0.1 to 20 weight percent, and preferably from 2 to 12 weight percent based on total weight of the monomers employed. The balance of the monomer mixture is essentially the para-substituted monovinyl aromatic monomer. The weight ratio of the monomer phase to the continuous aqueous phase is generally from 0.5 to 1.5. Also suitable for preparing the copolymer bead matrix is a so-called multi-staged polymerization process. A multi-stage polymerization, also known as a "seeded" polymerization, adds monomers in two or more increments, each increment comprising at least 5 percent, and preferably at least 10 percent of the weight of the monomers employed. Each increment is followed by complete or substantial polymerization of the monomers therein before adding a subsequent increment. These so-called seeded polymerizations, as well as continuous or semi-continuous staged polymerizations, are described in U.S. Patents 4,419,245 and 4,564,644. A multi-stage polymerization is advantageously and preferably conducted as a suspension polymerization process, wherein the monomers are dispersed as a plurality of droplets in a continuous aqueous phase and polymerized therein. In this process, staged polymerization is readily accomplished by forming a suspension of an initial increment of monomers, wholly or partially polymerizing the same, and then adding the remaining monomers in one or more increments. Each increment may be added at once, or continuously. Due to the insolubility of the monomers in the aqueous phase and their solubility in the copolymer particles, the monomers migrate to the copolymer particles, are imbibed thereby and polymerize therein. The multi-stage polymerization techniques can vary in terms of the type and amount of monomers used for each stage, the proportion of cross-linking monomer used in each stage, and the conditions under which the monomers are added in each stage. In a preferred multi-staged process, the last polymerization stage employs a monomer mixture containing substantially no free-radical initiator. This mixture, which preferably comprises 10 to 80, preferably 25 to 75 weight percent of the final copolymer particles, is added to the suspension under polymerization conditions. The copolymer particles formed in earlier stages contain a source of free radicals so that, upon migration to the copolymer particles, the monomer mixture polymerizes therein. Such a free-radical source may be previously imbibed initiator, or the residue of a partially polymerized monomer mixture from a preceding polymerization stage. Such a process is described in U.S Patent 4,564,644. In a preferred multi-staged process, the copolymer beads are prepared by swelling seed copolymer particles in a first step with a first monomer mixture having, in terms of weight percent of the monomer mixture, desirably from 1 to 20, preferably 3 to 14, and more preferably from 5 to 10 weight percent of a cross-linking monomer, the balance being essentially the para-substituted monovinyl aromatic monomer and an amount of a free-radical initiator sufficient to initiate polymerization. The swollen seed copolymer particles are then partially polymerized with the first monomer mixture. The seed particles advantageously comprise 10 to 70, preferably 25 to 60 weight percent of the final product copolymer bead and have preferably from 0.1 to 8 and more preferably from 0.3 to 5 weight percent of cross-linking monomer, with the balance being essentially the para-substituted monovinyl aromatic monomer. The first monomer mixture is advantageously polymerized in this stage to 20 to 90, and preferably 50 to 75 percent conversion. Thereafter, a second monomer mixture is added continuously to the suspended seed copolymer particles under polymerization conditions such that the monomers are imbibed by the particles and polymerize therewith. The second monomer mixture contains, in terms of weight percentage of the mixture, preferably from 0 to 12, more preferably 2 to 10, and most preferably 3 to 9 weight percent of a cross-linking monomer, the balance of the second monomer mixture being essentially the para-substituted monovinyl aromatic monomer and substantially no free radical initiator. Copolymer beads formed by a multi-staged process exhibit a core/shell morphology which is more fully described in U.S. Patent 4,564,644. Generally, the term "core/shell morphology" means that the polymeric structure of the copolymer bead varies with the distance from the center of the bead. For example, the changes in polymeric structure may be gradual and, thus, give rise to a bead having a gradient of polymeric structure along any radius thereof. Alternatively, the change in polymeric structure may be relatively abrupt at some point along the radius, thereby yielding a bead having a substantially distinct inner core of one polymeric structure and a substantially distinct outer shell having another polymeric structure. In either case, the terms "core" and "shell" generally refer to the polymeric structure of the inner and outer portions of the bead, respectively, and do not necessarily imply a sharp interface between the core or shell. It is possible to prepare oxidation-resistant resins from monomer mixtures having minor amounts of conventional monovinyl monomers, i.e., monomers which do not correspond to the description for the para-substituted monovinyl aromatic monomers, provided that oxidation resistance is not significantly compromised thereby. Such conventional monomers are known in the art and reference is made to Polymer Processes , edited by Calvin E. Schildknecht, published in 1956 by Interscience Publishers, Inc., New York, Chapter III, the relevant teachings of which are incorporated by reference. Of interest are water-insoluble monomers including monovinyl aromatics such as styrene, vinyl naphthalene, alkyl-substituted styrenes (particularly monoalkyl-substituted styrenes such as vinyltoluene and ethyl vinylbenzene) and halo-substituted styrenes like bromo- or chlorostyrene; esters of α,β-ethylenically unsaturated carboxylic acids, such as acrylic or methacrylic acids; and mixtures of one or more of said monomers. However, it is preferred that the amount of such monovinyl monomers be less than 50 percent, more preferably less than 20 percent, and most preferably less than 10 percent, based on the weight of all monomers employed. With respect to resins prepared from copolymer beads made in a multi-stage polymerization process, it is possible to employ conventional monovinyl monomers in one or more of the polymerization stages and still obtain a resin having a beneficial amount of oxidation resistance. For example, the seed copolymer particles may be made of the para-substituted monovinyl aromatic monomer, with a monovinyl monomer, as previously described, used in subsequent polymerization stages. Similarly, the seed copolymer particle may be made of a monovinyl monomer and, in subsequent polymerization stages, the para-substituted monovinyl aromatic monomer is used. In these instances, the para-substituted monovinyl aromatic monomer is preferably employed in polymerization stages using the least amount of cross-linking monomer. Also, it is preferred that the para-substituted monovinyl aromatic monomer be employed in those stages using less than 4 weight percent of a cross-linking monomer, based on the weight of monomers employed in such stage. The free-radical initiator employed may be any of a diverse number of conventional initiators well-known in the art for polymerization of the monomers as previously described. Representative of such initiators are UV radiation and chemical initiators including azo compounds such as azobisisobutyronitrile; peroxygen compounds such as benzoyl peroxide, t-butyl peroctoate, t-butyl perbenzoate and isopropyl percarbonate. Other suitable initiators are disclosed in U.S. Patents 4,192,921; 4,246,386; and 4,283,499. The free-radical initiator is employed in an effective amount sufficient to cause polymerization of the monomeric components of a monomer mixture. An effective amount will generally vary depending on a variety of factors including the type of initiator, temperature employed, and the type and proportion of monomers being polymerized. Generally, the initiator is employed in amounts from 0.02 to 1 weight percent, and preferably from 0.05 to 0.5 weight percent, based on the total weight of the monomers employed. A liquid diluent which is substantially inert under polymerization conditions may also be incorporated into the monomer phase to obtain macroporous copolymer beads. The term "macroporous" (also referred to as macroreticular) is well known in the art and, in general, refers to resins prepared from copolymer beads which have regions of densely packed polymer chains exhibiting molecular-sized porosity which are separated by copolymer-free voids, often referred to as mesopores (50-200 Å) and macropores (>200 Å). In contrast, gel-type, or microporous, resins have pores generally of molecular-size (generally less than about 50Å). Macroporous and microporous resins are further described in U.S. Patents 4,224,415 and 4,382,124. Resins of the present invention may be prepared from either macroporous or microporous copolymer beads, but the benefits of the invention are particularly applicable to resins based upon microporous copolymer beads. Resins prepared from a microporous copolymer typically have a relatively smaller proportion of cross-linking monomer therein to promote resin permeability with respect to the liquid being treated. Such lightly cross-linked copolymer beads are more sensitive to oxidative degradation. The resulting copolymer beads may be converted to sulfonated cation-exchange resin using known techniques. For example, sulfonated cation-exchange resins may be prepared from copolymer beads using methods described, for example, in U.S. Patents 3,266,007; 2,500,149; 2,631,127; 2,664,801; and 2,764,564 and the book by F. Helfferich previously mentioned herein. Such sulfonated resins are the most common type of cation-exchange resin presently used by industry. In general, sulfonated cation-exchange resins are prepared by reacting the copolymer beads with a sulfonation agent, such as concentrated sulfuric acid (acid which has greater than 95 weight percent sulfuric acid based upon total weight), oleum, chlorosulfonic acid, or sulfur trioxide, at an elevated temperature and for a time sufficient to achieve a desired degree of sulfonation. A preferred sulfonation agent is sulfuric acid. The amount of concentrated sulfuric acid should be sufficient to provide adequate mixing during reaction, with a weight ratio of acid to beads of from 5:1 to 10:1 being generally sufficient. Typically, the acid and copolymer beads are maintained at a temperature of from 50°C to 150°C for a time sufficient to obtain resin having a dry weight capacity of at least 2.5 milliequivalents per gram (meq/g). Other sulfonation conditions are known in the art. Prior to sulfonation, the copolymer beads are typically allowed to swell for at least 30 minutes with both a swelling agent and the sulfonation agent to allow for more uniform penetration of the sulfonation agent into the copolymer beads. Suitable swelling agents are organic solvents which do not react with the copolymer beads or the sulfonation agent employed. Examples of suitable swelling agents are chlorinated hydrocarbons like chlorobenzene, methylene chloride, or tetrachloroethylene. After sulfonation, the resin is hydrolyzed and for some applications, it may be advantageous to convert it to a metal salt. The resin is advantageously hydrolyzed by washing it with a series of sulfuric acid solutions, each of which is successively more dilute in comparison to the sulfuric acid solution used in the preceding acid wash, and finally with water. Other methods to hydrolyze the resin are also suitable. After hydrolysis, the washed resin may be converted to a desired metal salt form by contact with a dilute aqueous solution of a water-soluble metal salt or base having a desired metal counterion. For example, the resin can be converted to its calcium form by contact with a 5-10 percent by weight calcium chloride solution or a saturated calcium hydroxide solution. The resin may be converted to other forms in like manner using water-soluble salts of the desired metal counterion. Preferred metal counterions are calcium, magnesium, sodium, and potassium, with calcium being most preferred. After sulfonation, the resin suitably has a dry weight capacity of at least 2.5 milliequivalents per gram (meq/g), preferably at least 4.0 meq/g, more preferably at least 5.0 meq/g. Dry weight capacity may be measured by analytical techniques well known in the art. The present invention is applicable to other types of cation-exchange resins, provided the copolymer matrix employed is one which is subject to oxidative degradation as previously mentioned. For example, F. Helfferich in Ion Exchange at pps. 29-43, describes preparation of cation-exchange resins having phosphonic, phosphinic, arsenic and carboxylic acid groups substituted onto a copolymer matrix. The cation-exchange resin suitably exhibits a water retention capacity of less than 70 percent. Water retention capacity is determined by swelling a weighed amount of resin with water, removing excess water, and then weighing the fully swollen resin. The swollen resin is then dried until a constant weight is obtained. Water retention capacity is the ratio of water imbibed to the total combined weight of the resin plus imbibed water. Water retention capacity, on a fully functionalized basis, is advantageously from 40 percent to 65 percent, with from 50 to 60 percent being preferred. As used herein, water retention capacity is measured with the resin in its calcium form. The size of the resin particles is not critical to obtain the benefits of the invention. For most commercial applications, the resin desirably has a volume average particle diameter from 200 to 1000, preferably from 250 to 800, and more preferably from 300 to 400 micrometers (µm). In addition, the resin preferably has a relatively narrow particle diameter distribution, with at least 50 volume percent and more preferably at least 90 volume percent of the particles having a diameter which is between 0.9 and 1.1 times the average diameter. Volume average particle diameter and particle size distribution may be determined by any one of a number of commercially available instruments designed to make such measurements, such as a Criterion Model PC-320 Particle Size Analyzer available from The Pacific Scientific Company. The oxidation-resistant resins disclosed herein may be employed in a separation process, such as a chromatographic separation process or an ion-exchange separation process. In such a process, the resins exhibit substantial resistance to oxidative degradation which is characterized, for example, by comparing changes in water retention capacity over time for the present resins with the same type of data for otherwise similar conventional resins exposed to substantially similar conditions. For example, Fig. 1 contains a graphical presentation of data associated with Examples 1-2 and Comparative Examples A-B herein. Figure 1 plots the natural logarithm (log e ) of resin water retention capacity vs. days of resin exposure to a simulated oxidizing environment. The resins of the invention (Examples 1-2) display water retention capacity values which increase at a rate which is substantially less than the rate associated with otherwise similar conventional resins (Comparative Examples A-B). Such reduced rates allow a separation process to operate efficiently for longer periods of time relative to use of conventional resins, without undesirable increases in bed pressure drop, decreases in flow rate of the liquid mixture being treated, or release of undesirable organic contaminants. The resins disclosed herein are also useful in chemical processes where oxidation resistance is desirable. For example, cation-exchange resins are commonly employed in a fixed bed as catalysts for numerous organic reactions, such as the alkylation of aromatic rings, esterification of alcohols, and in particular, preparation of methyl tert-butyl ether. Oxidation resistance allows for use of the resins in these applications for longer periods of time without undesirable increases in bed pressure drop, decreases in flow rate of reactants involved, or release of undesirable organic impurities. The following examples illustrate the present invention and should not be construed, by implication or otherwise, as limiting the scope thereof. All parts and percentages are by weight and all temperatures are in degrees Celsius (°C) unless otherwise indicated. EXAMPLE 1 A. Resin Preparation Copolymer beads were prepared by suspension polymerization of a monomer mixture in a continuous aqueous phase. A 3 liter, stainless steel reactor was loaded, under agitation, with 1000 grams of an aqueous solution consisting of demineralized water, 0.14 weight percent of carboxymethyl methylcellulose, and 0.13 weight percent of sodium dichromate. Thereafter, a monomer mixture was added to the reactor, with agitation, consisting of 123 grams of a commercially available divinylbenzene mixture, 1077 grams of para-methylstyrene, and, as chemical initiators, 0.43 grams of a 50 percent t-butylperoctoate solution and 0.60 grams of t-butylperbenzoate. The divinylbenzene mixture was obtained from The Dow Chemical Company and it consists of 55.6 percent divinylbenzene, the balance being essentially ethyl vinyl benzene. After loading the above-described reactants, the reactor was sealed, purged with nitrogen, and the contents heated to a temperature of 75°C which was maintained for 10 hours. To complete polymerization, the reactor contents were heated to a temperature of 95°C for an additional 1.5 hours and, thereafter, to a temperature of 110°C for another 1.5 hours. The copolymer beads were recovered from the reactor, washed, and dried. The copolymer beads were then screened to remove any particles above 30 mesh (595 µm) or below 60 mesh (250 µm) in size. The resulting copolymer beads were sulfonated according to well-known methods. A 3-necked, round-bottom, glass reactor fitted with a mechanical stirrer and a drain valve was initially charged with 260 milliliters (ml) of a 99 weight percent sulfuric acid solution. A 50 gram portion of the copolymer beads, as prepared above, was added to the reactor and agitation was thereafter initiated. The reactor was then charged with 15 ml of methylene chloride, a swelling agent, and the reactor contents were agitated for a 30-minute period. The reactor was thereafter heated to a temperature of 115°C over a 60-minute period, and the temperature was maintained for an additional 2 hours to substantially sulfonate of the copolymer beads. The resulting sulfonated beads were hydrolyzed by successive rinses with dilute sulfuric acid solutions, wherein each acid rinse was less dilute in comparison to the immediately preceding acid rinse. The sulfonated beads were initially rinsed with 397 ml of a 43 percent aqueous sulfuric acid solution which is added dropwise into the reactor at a rate of about 10 ml/min. This was followed by rinsing with 467 ml of a 19 percent aqueous sulfuric acid solution and 512 ml of a 5 percent aqueous sulfuric acid solution in the same manner. After each rinse, a volume of acid solution approximately equal to the amount added, was removed before proceeding with the next rinse. The resins were finally rinsed with deionized water to remove residual acid. The resin was converted to its calcium form by gradually adding calcium hydroxide powder until reaching a pH of 8 for the reactor contents. The resin was washed a final time with water until the reactor contents reached a neutral pH. Water retention capacity for the resin was determined by the procedure previously described herein. The sulfonated resin exhibited a water retention capacity of 50.0 percent in the calcium form. B. Determination of Resin Oxidation Stability Oxidation stability was determined by placing the sulfonated resin in a simulated oxidizing environment and, thereafter, sampling the resin at selected intervals to determine resin water retention capacity at the time each sample is collected. A 100 gram portion of the sulfonated resin, in its calcium form, and 500 ml of deionized water were placed into a 1 liter, 4-necked, round-bottom flask. The flask was equipped with an overhead mechanical stirrer, a water inlet port, a water outlet port, and a glass sparge tube having a fritted end. A continuous flow of demineralized water through the flask via the inlet and outlet ports is provided by use of a pump. The flow of water was such that the volume of water in the flask was maintained at a constant level. The water was also maintained at a temperature of 80°C. Oxygen gas was sparged into the flask contemporaneously with the deionized water flow. The flow of oxygen gas was maintained at 50 ml/min. At selected intervals, generally once every few days, a resin sample of approximately 10 grams was recovered from the flask. Water retention capacity ("WRC") was determined for each respective sample according to the procedure previously described herein. The time of sampling, in terms of days after being exposed to the simulated oxidizing environment, and water retention capacity data are given in Table I and presented in graphical form by Fig. 1. The data illustrates that resin water retention capacity for Example 1 remained essentially constant over a period of 16 days, thereby indicating no significant degradation of the copolymer matrix. COMPARATIVE EXAMPLE A The procedure of Example 1 was repeated using styrene instead of para-methylstyrene. The results of the oxidation stability test are also given in Table I and Fig. 1 for comparison with the results of Example 1. The data for Comparative Example A show a substantial rise in water retention capacity over a period of ten days, thereby indicating that the copolymer bead matrix was substantially degrading. EXAMPLE 2 Example 2 concerns oxidation-resistant cation-exchange resin made from copolymer beads prepared in a multi-staged polymerization process. Copolymer beads employed were prepared by following the seeded polymerization process described in Example 1 of U.S. Patent 4,564,644 as well as column 7, line 3 to column 9, line 53 of the patent. A 3 liter stainless steel reactor was initially loaded, under agitation, with 400 grams of seed copolymer particles and 900 grams of water. The seed copolymer particles were 2.5 percent by weight cross-linked para-methylstyrene/divinylbenzene copolymer particles having a volume average particle diameter of 220 µm. Thereafter, the reactor was charged with a first monomer mixture containing 164 grams of para-methylstyrene, 16.2 grams of a commercially available divinylbenzene mixture, and 0.72 grams of a 50 percent t-butyl peroctoate solution with 0.36 grams of t-butyl perbenzoate as polymerization initiators. The divinylbenzene mixture was obtained from The Dow Chemical Company and consisted of 55.6 percent divinylbenzene, the balance of the divinylbenzene mixture being essentially ethyl vinyl benzene. The first monomer mixture therefore had a divinylbenzene content of 5 percent. The seed copolymer particles were allowed to imbibe the monomers and initiators for a 30-minute period. Thereafter, an additional 275 grams of water, 24 grams of an aqueous one percent sodium lauryl sulfate solution, 1.8 grams each of gelatin A and B, and 4 grams of a 58 percent aqueous sodium dichromate solution were added to the reactor. The reactor was then sealed, purged with nitrogen, and the reactor contents heated to a temperature of 80°C which was maintained for about 70 minutes to obtain partial conversion of the monomers within the seed copolymer particles. At the conclusion of the 70-minute period, a second monomer mixture was continuously fed to the reactor at a rate of 2.9 grams per minute over a 225-minute period. The second monomer mixture contained 613 grams of para-methylstyrene and 35 grams of the divinylbenzene mixture previously described, which yields a divinylbenzene content of 3 percent based upon the total weight of the second monomer mixture. The reactor contents were maintained at 80°C during addition of the para-methylstyrene-divinylbenzene feed and, thereafter, for another 6 hours. The polymerization was concluded by heating the reactor contents to 95°C which is maintained for 1.5 hours and, subsequently, to 110°C which was maintained for an additional 1.5 hours. The resulting copolymer beads were recovered from the reactor. The beads exhibit a volume average particle diameter of 350 µm. The copolymer beads were then sulfonated as in Example 1 and water retention capacity was measured with the resin in its calcium form. The procedure of Example 1 was substantially repeated to determine oxidation stability. The results are given in Table I and are graphed in Fig. 1. COMPARATIVE EXAMPLE B The procedure of Example 2 was repeated using styrene, instead of para-methylstyrene, in the seed copolymer particles and the first and second monomer mixtures. The results for oxidation stability testing are given in Table I and Fig. 1 for comparison with Example 2. The results show that oxidative degradation, as measured by an increase in water retention capacity over time, for Example 2 is considerably reduced in comparison with Comparative Example B. EXAMPLE 3 The procedure of Example 2 was substantially repeated, except that para-methylstyrene was used only in making the seed copolymer particles. Also, larger amounts of a cross-linking monomer were employed in subsequent polymerization stages. The seed copolymer particles employed were 220 grams of 2.5 percent by weight cross-linked para-methylstyrene/divinylbenzene copolymer particles having a volume average particle diameter of 280 µm. The first monomer mixture contained 291 grams of styrene, 49 grams of the divinylbenzene mixture, and 1.22 grams of the 50 percent t-butyl peroctoate solution with 0.38 grams of t-butyl perbenzoate. The first monomer mixture therefore had a divinylbenzene content of 8 percent. The second monomer mixture was continuously fed to the reactor at a rate of 2.25 grams per minute over a 250-minute period. The second monomer mixture contained 482 grams of styrene and 81 grams of the divinylbenzene mixture, which yields a divinylbenzene content of 8 percent based upon the total weight of the second monomer mixture. All other procedures were substantially the same. The oxidation stability data appears in Table I. EXAMPLE 4 The procedure of Example 2 was substantially repeated, except that para-methylstyrene was used only in the second monomer mixture. The seed copolymer particles were 2.5 percent by weight cross-linked styrene/divinylbenzene copolymer particles having a volume average particle diameter of 210 µm. The first monomer mixture contained 164 grams of styrene, 16.2 grams of the divinylbenzene mixture, and 0.65 grams of the 50 percent t-butyl peroctoate solution with 0.34 grams of t-butyl perbenzoate. The first monomer mixture therefore had a divinylbenzene content of 5 percent. The second monomer mixture was continuously fed to the reactor at a rate of 2.6 grams per minute over a 240-minute period. The second monomer mixture contained 57.9 grams of para-methylstyrene and 33 grams of the divinylbenzene mixture, which yields a divinylbenzene content of 3 percent based upon the total weight of the second monomer mixture. All remaining procedures were the same. The oxidation stability data appears in Table I. EXAMPLE 5 The procedure of Example 2 was substantially repeated, except that para-methylstyrene was not used in the seed copolymer particles. The seed copolymer particles were 2.5 percent by weight cross-linked styrene/divinylbenzene copolymer particles having a volume average particle diameter of 210 µm. The first monomer mixture contained 164 grams of para-methylstyrene, 16.2 grams of the divinylbenzene mixture, and 0.65 grams of the 50 percent t-butyl peroctoate solution with 0.34 grams of t-butyl perbenzoate. The first monomer mixture therefore had a divinylbenzene content of 5 percent. The second monomer mixture was continuously fed to the reactor at a rate of 2.6 grams per minute over a 240-minute period. The second monomer mixture contained 583 grams of para-methylstyrene and 33.2 grams of the divinylbenzene mixture, which yields a divinylbenzene content of 3 percent based upon the total weight of the second monomer mixture. All remaining procedures were the same. The oxidation stability data appears in Table I. EXAMPLE 6 Chromatographic Separation of Fructose and Glucose The resin prepared in Example 2 was evaluated for chromatographic performance in separating fructose and glucose. The resin had an average particle diameter of 440 µm. A 800 ml portion of the resin was placed in a 25 mm x 1280 mm jacketed glass chromatography column with an attached expansion column. The resin was backflushed with degassed, deionized water for 60 minutes which was followed by 2 hours of backflushing with an aqueous calcium chloride solution (7 grams CaCl 2 /100 ml solution). The resin was then equilibrated with the calcium chloride solution for at least 6 hours, tapped down to provide a uniform packing, and sealed. The column was then flushed with degassed, deionized water and heated to a temperature 60°C. A total of 78 ml of a hot (60°C) commercial high fructose corn syrup of 50 percent dissolved solids content was fed to the column at a rate of 11-13 ml/min. The composition of the solids was fructose: 41-43 percent; glucose: 50-52 percent; and oligosaccharides: 6-7 percent. Immediately after the syrup was loaded on the column, it was eluted with 60°C degassed, deionized water, at a rate of about 11-13 ml/min. Samples of effluent from the column were collected at half-minute intervals and quantitatively analyzed for the individual components eluting from the column. The experiment was continued until the product stream contained less than 1 percent dissolved solids. Glucose was eluted faster than fructose, so earlier samples are rich in glucose and later fractions are rich in fructose. The results indicated that the percent yield of fructose was 31 percent. Percent yield was determined from the consecutive samples collected at the end of the experiment which cumulatively have a fructose purity of 80 percent, i.e., the weight ratio of fructose to total carbohydrates contained in the samples, was 80 percent. The percent yield was 100 times the total amount of fructose contained in those samples divided by the total amount of fructose recovered before the experiment was terminated. The average production rate, in pounds of fructose per cubic foot of resin per hour at 80 percent purity, was 0.56. The average concentration of carbohydrate in the product cut was 4.8 grams/100 ml. No oxidative degradation of the resin was apparent.

Claims: A styrenic cation-exchange resin in the form of copolymer beads substituted with cation-exchange functional groups, wherein the copolymer contains units of a cross-linking monomer, and units of a para-substituted styrenic monomer of the formula wherein R is phenyl or an aliphatic moiety having up to eight carbon atoms. A cation-exchange resin as claimed in Claim 1, wherein R is methyl, ethyl, propyl, isopropyl, or butyl. A cation-exchange resin as claimed in Claim 2, wherein R is methyl. A cation-exchange resin as claimed in any one of the preceding Claims, wherein the cross-linking monomer is divinylbenzene, trivinylbenzene, diisopropenylbenzene, or triisopropenylbenzene. A cation-exchange resin as claimed in any one of the preceding claims, wherein the copolymer beads are prepared by a seeded polymerisation process. A cation-exchange resin as claimed in any one of the preceding claims, wherein the copolymer beads are prepared using from 2 to 12 weight percent of the cross-linking monomer and from 88 to 98 weight percent of the para-substituted monovinyl aromatic monomer based upon the weight of the monomers. A cation-exchange resin as claimed in any one of the preceding claims, wherein the copolymer beads are microporous. A cation-exchange resin as claimed in any one of the preceding Claims 1 to 6, wherein the copolymer beads are macroporous. A cation-exchange resin as claimed in any one of the preceding claims, wherein the cation-exchange functional groups are sulfonic acid, phosphonic acid, phosphinic acid, arsenic acid, or carboxylic acid groups. A separation process employing a cation-exchange resin as claimed in any one of the preceding claims. The use in the preparation of styrenic cation-exchange resin beads resistant to oxidative degradation of a para-substituted styrenic monomer of the formula wherein R is phenyl or an aliphatic moiety having up to eight carbon atoms.

Inventors: DHINGRA YOG R, VAN TOL PAUL R, DHINGRA, YOG R., VAN TOL, PAUL R.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1992
Main classification: B01J39
IPC codes: C08F212, C08F12, B01J39