RU2429070C1 - Anion-exchange sorbent for simultaneous ion chromatographic determination of polarised and nonpolarised inorganic anions and its production method - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: disclosed is an anion-exchange sorbent for simultaneous ion chromatographic determination of polarised and nonpolarised inorganic anions which is based on a copolymer of styrene and divinyl benzene with a chemically grafted 2-hydroxypropyl-N,N-dimethylammonium group, which is separated from the benzene ring of the matrix by a tertiary carbon atom with a side metal group, having general formula

(1). The method of producing the sorbent involves successive acylation of the matrix in an organic solvent with acetic anhydride, reductive amination and alkylation with a reagent selected from oxiranes.

EFFECT: invention enables to obtain an efficient for ion chromatographic determination of inorganic anions.

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Description

The invention relates to a new polymer anion-exchange material, which can be used in ion chromatography as a sorbent for the simultaneous determination of polarizable and non-polarizable inorganic anions with increased efficiency, allowing selective separation of the mixture of more than seven anions, in particular for the analysis of water for the content of inorganic anions.

In recent years, there has been significant interest in the search for new sorbents for ion chromatography. As a matrix for such sorbents, it is most promising to use styrene-divinylbenzene copolymers with a high degree of crosslinking, which is determined by the high content of divinylbenzene in the copolymer, since they are characterized by greater mechanical stability, withstand high pressures in the system, making it possible to reduce particle size and increase separation efficiency.

However, when using ion-exchange resins based on styrene-divinylbenzene copolymers, the retention of anions can be determined not only by ion-exchange, but also by non-ion-exchange interactions. Adsorption types of interactions are observed, as a rule, with all polarizable inorganic and organic ions, which are characterized by low hydration energy, hydrophobicity, and strong ח-ח interactions (interactions of the ח electron shells of polarizable anions with the ח electron shells of aromatic rings that make up the matrix structure As a result of these interactions, the so-called ח-ח-complexes) are formed with the aromatic base of the ion exchanger. In some cases, even the separation of simple inorganic anions, such as nitrate and bromide, is largely determined by the adsorption properties of the sorbent. The negative influence of ח-ח interactions with the matrix is manifested in large retention times of polarizable anions and a decrease in the efficiency of their separation.

In this regard, the search for new approaches to the synthesis of anion exchangers based on matrices with a high degree of crosslinking, as well as ways to reduce the influence of the matrix on anion retention, is an urgent scientific problem.

Known (Bogolitsyna A.K., Pirogov A.V., Shpigun O.A. Synthesis of anion exchangers based on a copolymer of styrene and divinylbenzene // Izv. Universities of the North Caucasus. Natural Sciences. Appendix. 2006. No. 8. S.24-30) anion-exchange materials based on a copolymer of styrene and divinylbenzene with a degree of crosslinking of 50%, in which the chemically grafted functional groups are triethylammonium or dimethylethanolammonium groups removed from the benzene ring of the matrix by a methylene group.

However, the described sorbent does not allow to exclude the influence of the matrix on the retention of polarizable anions. Both sorbents exhibit an abnormally high retention of nitrate ions relative to sulfate and phosphate ions.

A method of obtaining such material includes sequentially carrying out the following operations:

1) chloromethylation of a copolymer of styrene and divinylbenzene with thionyl chloride and an aqueous solution of formaldehyde;

2) amination using triethylamine (to obtain a sorbent with trimethylammonium groups) or using 2-dimethylethanolamine (to obtain a sorbent with a dimethylethanammonium group).

However, the chloromethylation operation used in this method has several disadvantages. Firstly, the likelihood of side reactions, including those leading to additional crosslinking of polymer chains, is high. In addition, the reagents used for the synthesis are toxic.

Known (Xu H., Hu X. // Reactive and Functional Polymers. 1999. V.42. P.235-242) is the closest to the proposed anion-exchange material sorbent based on a macroporous copolymer of styrene and divinylbenzene (degree of crosslinking 8%, particle size 0.3-1.0 mm), in which the chemically grafted functional group is a trimethylammonium group remote from the benzene ring of the matrix with a tertiary carbon atom with a methyl side group.

However, the low degree of crosslinking of the polymer matrix and the large particle diameter make this sorbent unsuitable for highly efficient determination of inorganic anions.

A method of obtaining this sorbent is to carry out the following steps:

1) acylation of the matrix with acetic anhydride in the ratio of 1 g of reagent per 1 g of the matrix, the synthesis is carried out at a temperature of 50 ° C for 4 hours;

2) reductive amination with dimethylamine hydrochloride and sodium cyanoborohydride;

3) methyl iodide alkylation.

This synthesis method is simple to implement and does not require the use of highly toxic reagents. However, the trimethylammonium functional group with this synthesis method does not reduce the contribution of nonion exchange interactions of polarizable anions, such as nitrate and bromide, with the aromatic base of the sorbent. This affects the determination efficiency and relative retention of such anions.

The objective of the invention is the creation of an effective sorbent for the simultaneous determination of polarizable and non-polarizable inorganic anions with increased efficiency.

The technical effect in this case is to reduce the influence of the matrix on the retention of polarizable anions.

The problem is solved by creating an anion-exchange sorbent for simultaneous ion chromatographic determination of polarizable and non-polarizable inorganic anions based on a copolymer of styrene and divinylbenzene with a chemically grafted functional group remote from the benzene ring of the matrix with a tertiary carbon atom with a side methyl group, the novelty of which is that the functional group a 2-hydroxypropyl-N, N-dimethylammonium group of the general formula (1)

where R is selected from the series: H, OH, Hal (halogen), Alkyl (alkyl radical).

The most typical is a sorbent in which the chemically grafted functional group is a 3-chloro-2-hydroxypropyl-N, N-dimethylammonium group of the general formula (2)

Using a copolymer of styrene and divinylbenzene with a particle diameter of 3-10 microns allows you to achieve the most efficient separation of polarizable and non-polarizable inorganic anions. Moreover, a decrease in particle diameter of less than 3 μm leads to an unjustified increase in pressure in the system. Particles with a diameter of more than 10 microns are not used in ion chromatography.

The use of copolymer of styrene and divinylbenzene with a content of divinylbenzene, which is a crosslinking agent, of at least 20% as matrices for the synthesis of the anion-exchange sorbent allows increasing the mechanical stability of the sorbent. Such a sorbent is able to withstand high pressures in the system, thereby providing the ability to reduce particle size and increase separation efficiency. If you go beyond the specified lower limit of the content of divinylbenzene, the strength of the resulting sorbent will not satisfy the requirements of its industrial applicability. The maximum possible degree of crosslinking is determined by the content of divinylbenzene in the copolymer, which is 55%.

Another aspect of the invention is a method for producing an anion-exchange sorbent.

The method for producing an anion-exchange sorbent for simultaneous ion-chromatographic determination of polarizable and non-polarizable inorganic anions according to claim 1 includes sequentially carrying out the acylation of the matrix in an organic solvent with acetic anhydride, reductive amination and alkylation. The novelty of the method lies in the fact that the alkylation is carried out by a reagent from the group of oxiranes.

We found that it is the use of oxiranes at the alkylation stage that allows us to achieve the solution of the problem, i.e. reducing the influence of the matrix on the retention of polarizable anions and, as a result, a significant increase in the efficiency of these anions. Attempts to use various other classes of compounds at this stage did not lead to visible improvements.

As a solvent during all stages of the synthesis, any organic solvents inert to the matrix are used, leading to wetting of the matrix and its swelling, providing an environment for the effective occurrence of acylation reactions, reductive amination and alkylation of nitrogen atoms.

The most optimal results with simultaneous ion chromatographic determination of polarizable and non-polarizable inorganic anions appear when using a sorbent with a capacity of 0.02-0.08 mmol / g. A sorbent with such a capacity can be obtained by acylation for 4-6 hours, while 0.02-0.06 mol of acetic anhydride is taken per 1 g of styrene-divinylbenzene copolymer.

The optimal ratio of components to obtain a sorbent with the above capacity when carrying out the alkylation reaction is not less than 0.04 mol of oxirane per 1 g of amino copolymer.

Table 1 shows data illustrating the change in the ion exchange capacity of the sorbent depending on the amount of acylating agent.

Table 2 shows data illustrating the change in the ion exchange capacity of the sorbent depending on the amount of alkylating agent.

Table 3 presents the efficiency (tp / m) of the column with the sorbent according to the invention for polarizable and non-polarizable inorganic anions.

Table 4 presents the efficiency (tt / m) of the column with the sorbent obtained in accordance with the prototype method for polarizable and non-polarizable inorganic anions.

Table 5 shows data illustrating the change in efficiency for polarizable and non-polarizable inorganic anions depending on the particle diameter of the starting copolymer.

Table 6 shows data illustrating the change in the life of the column with the sorbent depending on the content of divinylbenzene in the copolymer.

Figure 1 presents the chromatogram of a mixture of anions determined using the proposed sorbent.

Figure 2 presents the chromatogram of a mixture of anions determined using a sorbent obtained in accordance with the prototype method.

The above examples confirm, but do not limit the proposed method.

Example 1. A method of obtaining an anion-exchange sorbent based on a copolymer of styrene and divinylbenzene with chemically grafted 3-chloro-2-hydroxypropyl-N, N-dimethylammonium functional group, remote from the benzene ring of the matrix with a tertiary carbon atom with a methyl side group.

1) Acylation

A suspension of 1 g of a styrene-divinylbenzene copolymer is prepared with a divinylbenzene content of 25% and a grain diameter of 3.3 ± 0.2 μm in 20 ml of carbon disulfide and allowed to swell for 2 hours. Then 3 g of anhydrous aluminum chloride powder was added and 0.04 mol of acetic anhydride was added dropwise with rapid stirring. The reaction mixture is stirred at a speed of 250 rpm for 6 hours at a temperature of 50 ° C. The resulting product is filtered on a glass filter and washed successively with 10% hydrochloric acid, 10% sodium hydroxide, water and ethanol, and then left on the filter until completely dried in a desiccator over CaCl ₂ .

2) reductive amination

1 g of the product obtained in the first stage is placed in a 100 ml flask, 5 g of dimethylamine hydrochloride, 0.35 g of sodium cyanoborohydride and 20 ml of methanol are added. The reaction mixture is stirred for 22 hours at a temperature of 67 ° C at a speed of 250 rpm, then acidified with concentrated hydrochloric acid to pH <2. The obtained dimethylamine-functional aminopolymer is filtered off, washed with 10% hydrochloric acid, 10% sodium hydroxide solution, water until neutral and ethanol, then dried on a filter in a desiccator over CaCl ₂ .

3) Alkylation with epichlorohydrin

A suspension of 1 g of the amino copolymer obtained in the second stage is prepared in alcohol, 0.16 mol of epichlorohydrin is added. The reaction mixture is stirred at 50 ° C for 2 hours (with increasing temperature, the synthesis time is reduced). After the specified time, the reaction product is filtered off, washed with water and ethanol.

The resulting sorbent is a copolymer of styrene and divinylbenzene with chemically grafted 3-chloro-2-hydroxypropyl-N, N-dimethylammonium functional group, remote from the benzene ring of the matrix with a tertiary carbon atom with a methyl side group. The anion exchange capacity is 0.065 ± 0.005 mmol / g (n = 3, P = 0.95).

Example 2. Determination of the optimal amount of acylating agent at the stage of acylation.

The same as in example 1, only at the acylation stage the amount of the acylating agent - acetic anhydride is varied, and dichloromethane is used as an inert organic solvent at this stage.

Table 1 shows data illustrating the change in the ion-exchange capacity of a sorbent based on a styrene-divinylbenzene copolymer with a divinylbenzene content of 25% and a particle diameter of 3.3 ± 0.2 μm depending on the amount of acylating agent in the first stage.

As can be seen from table 1, the capacity that is optimal for working in the variant of ion chromatography with suppression of the background conductivity (0.02-0.08 mmol / g) is achieved using 0.02-0.06 mol of acetic anhydride per 1 g of styrene copolymer and divinylbenzene.

Example 3. Determination of the optimal amount of reagent at the stage of alkylation.

The same as in example 1, only at the stage of alkylation vary the amount of alkylating agent - epichlorohydrin.

As can be seen from table 2, at least 0.04 mol of epichlorohydrin is required to provide a capacity in the range of 0.02-0.08 mmol / g, which is optimal for operation in the ion chromatography variant with suppression of the background electrical conductivity.

Example 4. Ion chromatographic determination of polarizable and non-polarizable inorganic anions using an anion-exchange sorbent based on a copolymer of styrene and divinylbenzene with chemically grafted 3-chloro-2-hydroxypropyl-N, N-dimethylammonium functional group, remote from the benzene ring of the matrix with a tertiary carbon atom with a lateral methyl atom a group

Obtained in example 1, an anion-exchange sorbent based on a copolymer of styrene and divinylbenzene with chemically grafted 3-chloro-2-hydroxypropyl-N, N-dimethylammonium functional group removed from the benzene ring of the matrix with a tertiary carbon atom with a methyl side group, is packed into a chromatographic column of size 4 × 50 mm under a pressure of 250 bar and carry out ion chromatographic separation of a mixture of polarizable and non-polarizable anions in the variant of ion chromatography with suppression of the background conductivity. As mobile phases, solutions of sodium carbonate and sodium bicarbonate are used.

The sorbent with 3-chloro-2-hydroxypropyl-N, N-dimethylammonium functional group, remote from the benzene ring of the matrix with a tertiary carbon atom with a side methyl group, shows good selectivity and allows the separation of a mixture of 7 polarizable and non-polarizable inorganic anions: fluoride, chloride, nitrite, nitrate, bromide, phosphate and sulfate. The chromatogram of the mixture is shown in Fig. 1 (eluent - 5 mM NaHCO ₃ ), and Table 3 shows the efficiencies (tp / m) of the sorbent column for polarizable and non-polarizable inorganic anions when using carbonate buffer solution of composition 1 as eluents, 8 mM Na ₂ CO ₃ + 1.7 mM NaHCO ₃ or 5 mM NaHCO ₃ . As can be seen from the table, the maximum efficiency is 50,000 tp / m (for phosphate ion). For polarizable nitrate and bromide ions, the maximum efficiencies are 8500 and 10000 tp / m, respectively.

In addition, when using hydrocarbonate eluents, the series of anion retention corresponds to the traditional one for ion chromatography, anomalous retention of nitrate and bromide ions and significant erosion of their peaks are not observed, which indicates a decrease in the influence of the matrix on the retention of polarizable anions.

Example 5

For comparison, an ion-chromatographic determination of polarizable and non-polarizable inorganic anions was carried out using an anion-exchange sorbent based on a copolymer of styrene and divinylbenzene with a chemically grafted trimethylammonium functional group, remote from the benzene ring of the matrix with a tertiary carbon atom with a methyl side group.

The sorbent was obtained similarly to the prototype, but the content of divinylbenzene in the copolymer was 25%, the particle diameter was 3.3 ± 0.2 μm, and 0.04 mol of acetic anhydride was used at the acylation stage. Table 4 shows the effectiveness of the sorbent column for polarizable and non-polarizable inorganic anions when using a carbonate buffer solution of 1.8 mM Na ₂ CO ₃ + 1.7 mM NaHCO ₃ or 8 mM NaHCO _{3 as} eluents.

As can be seen from table 4, the maximum efficiency is 30,000 tp / m (for phosphate ion). For polarizable nitrate and bromide ions, the maximum efficiencies are 2300 and 3000 tp / m, respectively, which is 6200 and 7000 tp / m, respectively, less than when using the sorbent that we offer.

On a sorbent with trimethylammonium functional group, a selective separation of a mixture of not more than five inorganic anions is possible (see figure 2). In this case, a significant smearing of the peaks of polarizable anions is observed, which makes their quantification impossible.

Example 6. Determination of the influence of the particle diameter of the styrene-divinylbenzene copolymer on the efficiency of the sorbent

The same as in example 4, only as the initial copolymer of styrene and divinylbenzene take copolymers, the particle diameter of which ranged from 3 to 10 microns.

Table 5 shows data illustrating the change in efficiency for polarizable and non-polarizable inorganic anions depending on the particle diameter of the initial styrene-divinylbenzene copolymer with a divinylbenzene content of 25%. Phosphate was taken as an example of a non-polarizable anion, since it is for it that the efficiency is usually the highest (there are no non-exchange interactions with the matrix). Nitrate is taken as an example of the most strongly polarizable ion. As an eluent, a carbonate buffer solution of 1.8 mM Na ₂ CO ₃ + 1.7 mM NaHCO _{3 was used} .

As can be seen from the table, a decrease in the particle diameter of styrene-divinylbenzene copolymer leads to a significant increase in efficiency for polarizable and non-polarizable anions.

Example 7. Determination of the effect of the content of divinylbenzene in the copolymer of styrene and divinylbenzene on the effectiveness of the sorbent

The same as in example 4, only copolymers of styrene and divinylbenzene with different divinylbenzene contents are taken.

To carry out high-performance ion chromatographic separation, it is preferable to use sorbents with a small particle diameter (3-10 μm), which is confirmed by Example 7. The use of particles of such a small diameter leads to a significant increase in pressure in the system. Therefore, preference is given to sorbents based on polymers with a high degree of crosslinking, determined by the content of divinylbenzene in the copolymer of ≥20%, which have greater mechanical stability and withstand high pressures in the system.

Table 6 shows data illustrating the change in the life of the column with the sorbent depending on the content of divinylbenzene in the copolymer.

As can be seen from the table, sorbents with a divinylbenzene content of less than 20% are not suitable for high-performance ion chromatographic separation, since under the action of high pressure in the system, the polymer structure of the sorbent is gradually destroyed, the sorbent sags and cracks and voids form in its dense packaging. Thus, the life of columns with such sorbents is significantly reduced.

The above examples confirm that the use of a sorbent based on a copolymer of styrene and divinylbenzene with a chemically grafted 2-hydroxypropyl-N, N-dimethylammonium group, of which the 3-chloro-2-hydroxypropyl-N, N-dimethylammonium group is a representative, reduces the influence of the matrix retention of polarizable anions. The result is a sharp increase in the efficiency of the sorbent for polarizable anions, the erosion of their peaks becomes insignificant, in the retention row they occupy the place traditional for these anions in ion chromatography. The proposed sorbent shows very good selectivity and provides the ability to separate a mixture of seven anions.

Table 1 The amount of acetic anhydride (per 1 g of copolymer), mol Capacity, mmol / g (n = 3, P = 0.95) 0.01 0.014 ± 0.002 0.02 0.029 ± 0.003 0,03 0.039 ± 0.003 0.04 0.065 ± 0.005 0.05 0.074 ± 0.006 0.06 0.083 ± 0.007 0,07 0.092 ± 0.007

table 2 The amount of epichlorohydrin (per 1 g of aminated copolymer), mol Capacity, mmol / g (n = 3, P = 0.95) 0.02 0.013 ± 0.002 0.04 0.022 ± 0.002 0.08 0.035 ± 0.003 0.012 0.049 ± 0.004 0.16 0.065 ± 0.005 0.20 0.067 ± 0.005

Table 3 Eluent Efficiency, t./m F ^- Cl ^- HPO ₄ ^2- SO ₄ ^2- NO ^2- Br ^- NO ^3- 1.8 mM Na ₂ COl ₃ +1.7 mM NaHCO ₃ 9500 15,000 50,000 30000 10,000 9000 8000 5 mM NaHCO ₃ 12000 17000 35,000 25,000 15,000 10,000 8500

Table 4 Eluent Efficiency, t./m F ^- Cl ^- HPO ₄ ^2- SO ₄ ^2- NO ^2- Br ^- NO ^3- 1.8 mM Na ₂ CO ₃ +1.7 mM NaHCO ₃ 7800 6600 28,000 16000 4000 2200 2000 8 mM NaHCO ₃ 8500 6000 29000 15,000 4200 3000 2300

Table 5 Particle diameter, microns Efficiency, t.t./m (for phosphate) Efficiency, t.t./m (for nitrate) 2 Pump operation not possible 3 50,000 8000 5 32000 6000 10 13000 3500 12 Not used in ion chromatography.

Table 6 The content of divinylbenzene in the copolymer of styrene and divinylbenzene,% Lifetime four 1 Week 8 2.5 weeks 12 1 month 25 3 months 35 5 months 55 > 6 months

Claims (7)

1. Anion-exchange sorbent for simultaneous ion chromatographic determination of polarizable and non-polarizable inorganic anions based on a copolymer of styrene and divinylbenzene with a chemically grafted functional group remote from the benzene ring of the matrix with a tertiary carbon atom with a methyl side group, characterized in that the functional group is 2-hydroxypropyl- N, N-dimethylammonium group, General formula 1:

where R is selected from the series: H, OH, HaI (halogen), Alkyl (alkyl radical). 2. Anion-exchange sorbent according to claim 1, characterized in that the chemically grafted functional group is a 3-chloro-2-hydroxypropyl-N, N-dimethylammonium group of the general formula (2):

3. The anion-exchange sorbent according to claim 1, characterized in that for the most efficient separation of polarizable and non-polarizable inorganic anions, a copolymer with a particle diameter of 3-10 μm is used as a matrix. 4. Anion-exchange sorbent according to claim 1, characterized in that in order to increase the mechanical stability of the sorbent, a copolymer with a divinylbenzene content of at least 20% is used as a matrix. 5. The method for producing an anion-exchange sorbent for simultaneous ion chromatographic determination of polarizable and non-polarizable inorganic anions according to claim 1, comprising sequentially performing in the organic solvent the acylation of the matrix from a styrene-divinylbenzene copolymer with acetic anhydride, reductive amination and alkylation, characterized in that the alkylation is carried out with a reagent groups of oxiranes. 6. The method according to claim 5, characterized in that the acylation is carried out for 4-6 hours, with 0.02-0.06 mol of acetic anhydride per 1 g of styrene-divinylbenzene copolymer. 7. The method according to claim 5, characterized in that at least 0.04 mol of oxirane is taken per 1 g of the amino copolymer obtained after reductive amination.

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