RU2626635C1 - Method for fulleren c60 purification from oxide impurities - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: crystalline fullerene C₆₀ is heat treated at 160-170°C in an inert gas stream to transfer the oxide impurity C₆₀O into the dioxide impurity C₁₂₀O. The treated crystalline product is then dissolved in a weakly polar aromatic solvent, for example, ortho-xylene. To convert C₁₂₀O to the precipitated phase, a strongly polar solvent is added, the molecule of which does not contain double bonds, and a nitrogen atom, for example isopropyl alcohol. The volume ratio of ortho-xylene:isopropyl alcohol is 1:(0.35-0.75), respectively. After this, C₁₂₀O is separated by filtration, and the desired product - fullerene C₆₀, remaining in the solvent mixture solution, is isolated by evaporation.

EFFECT: method is simplified, irreversible losses of expensive _C60 are reduced.

2 cl, 6 tbl

Description

The invention relates to a technology for the purification of fullerene C ₆₀ from oxide impurities.

The main technology for producing high-purity fullerene C ₆₀ is chromatographic purification on activated carbons of various grades.

In research practice, fullerene C _{60 of} different grades of purity and different manufacturers is used (Table 1).

From the table. 1 shows that, regardless of the degree of purity, fullerene C ₆₀ is an expensive product. It should be noted that manufacturers give only the level of impurities, and their composition is not specified. In this case, the content of oxide impurities is not indicated.

A feature of fullerene C ₆₀ is its tendency to oxidize its crystalline powders in air with the formation of monoxide, C ₆₀ O [1] (LC, 2013, v. 87. No. 10, pp. 1771-1778). The monoxide also appears during various effects on fullerene C ₆₀ , for example, when heated in air [2] (Synthetic Materials. 1996. V.77. P.299-302), under the influence of light [3] (J. Am. Chem Soc. 1992. V. 114. 1103-1105), and also as a result of mechanical action on fullerenes in an oxygen atmosphere and at ordinary temperature [4] (Tetrahedron Letters. 2007. V. 48. p.8132-8137).

It was found that monoxide, C ₆₀ O and dimer oxide, C ₁₂₀ O, are already formed when the crystalline fullerene C _{60 is} exposed to daylight for more than 40 hours, while the content of dimer oxide, C ₁₂₀ O, can reach 1% weight. [5] (Chem. Commun., 1998. p. 2497-2498). It is noted that oxide impurities were found in all crystalline samples of fullerene C ₆₀ , produced at different times and by different manufacturers, stored for a long time under ordinary conditions (for one year or more).

This means that already during storage of fullerene, C ₆₀ loses its purity and may become unsuitable for research purposes in the future. In this regard, the solution of the task of cleaning C ₆₀ from oxide impurities is necessary to restore the purity of fullerene.

A known method in which oxide impurities are removed from C ₆₀ fullerene by chromatographic purification on finely divided activated alumina and silica gel. [6] (Studies in Surface Science and Catalysis. Elsevier. 1996. v. 99. p. 902). Fullerene oxides are effectively adsorbed by these sorbents from a solution of fullerene in a solvent mixture of hexane / toluene.

However, such a mixture does not allow working with concentrated solutions of fullerenes, because the solubility of fullerenes in it is small and large volumes of solvents are required to clean even small amounts of C ₆₀ from oxide impurities. In addition, to ensure the movement of the eluent along the column of finely dispersed sorbent in this method requires the use of compressed gas. The method is applicable to obtain milligram quantities of pure fullerene C ₆₀ and therefore low productivity.

A method is known in which the dimer oxide, C ₁₂₀ O, is separated from C ₆₀ on a Nucleosil C ₁₈ preparative column (20 × 250 mm) with toluene-ethyl acetate eluent (20:80, vol.). About 2 mg of C ₆₀ / C ₁₂₀ O dissolved in 1,2-dichlorobenzene is periodically loaded into the column. After elution with C ₆₀ , the absorbed C ₁₂₀ O was eluted with 1,2-dichlorobenzene. [7] (Tetrahedron Letters. 1995. V.36. No. 28. p. 4971-4974). The method is inefficient and is not intended for the processing of gram amounts of fullerene C ₆₀ contaminated with an admixture of dimer oxide. The method is ineffective against impurities C ₆₀ O.

A known method for producing high-purity fullerene C ₆₀ (up to 99.99%), in which the impurity of C ₆₀ monoxide and all thermostable dimeric oxides are simultaneously removed by vacuum heat treatment at a temperature of 700-900 ° C in the sublimation mode. [8] (RF Patent No. 2456233). Carrying out vacuum sublimation heat treatment is characterized by harsh conditions (duration of heat treatment, the presence of special vacuum equipment, high vacuum and very high temperatures are required), therefore it is complicated and difficult to apply in ordinary laboratory conditions.

This method is advisable to apply in cases where fullerene C ₆₀ is required a very high degree of purification (up to 99.99%). In many cases, such a high degree of purification is not required and therefore there is a need for simpler methods for purifying fullerene from oxides.

As a prototype, a method for purifying fullerene C ₆₀ from oxide impurities described in the patent of the Russian Federation No. 2327635 is considered. [9] (RF Patent No. 2327635). The method consists in the fact that the extract of the fullerene mixture is dissolved in several stages in o-xylene at a temperature of 80-85 ° C until a C ₆₀ content of 96 ± 2% is reached, while alternating the dissolution and filtration of the precipitate at a temperature of 80-85 ° C . The obtained precipitate, enriched in fullerene C ₆₀ , is dissolved in o-xylene, the resulting solution is subjected to first chromatographic purification on a sorbent in the form of activated carbon, the product of the first purification is isolated from the solution by evaporation of the solvent, and the obtained crystals are kept in a vacuum of 10 ^-1 -10 ^-2 mm Hg for 2-4 hours at a temperature of 180-200 ° C, then the treated solid product is again dissolved in o-xylene, the resulting solution is subjected to second chromatographic purification on a sorbent in the form of activated carbon, the final product is isolated from the solution by evaporation in a rotary evaporator under vacuum 1 -10 ^-1 mm RT.article, washed with ethyl alcohol, and after washing with ethyl alcohol is treated with water vapor, followed by drying.

In this method, it was found that during the vacuum treatment, most of the C ₆₀ O is removed, but a significant amount of an oxide dimer, C ₁₂₀ O, is generated, which is removed by repeated chromatographic purification.

The disadvantages of the method. The method is characterized by increased irreversible loss of fullerene C ₆₀ due to several chromatographic procedures. The method is associated with carrying out a complicated technical implementation of vacuum heat treatment and chromatography, which requires the turnover of a large volume of organic solvent and a significant investment of time.

The objective of the invention is to simplify the method, reducing the irreversible loss of expensive C ₆₀ .

The technical result consists in the implementation of this method.

It is important that consumers, after a long storage of fullerene, using the inventive method, may be able to quite simply, with a minimum of losses, purify C ₆₀ from oxide impurities to an acceptable level.

The claimed technical effect is achieved by the fact that in the known method of purification of fullerene C ₆₀ from oxide impurities, including heat treatment of crystalline C ₆₀ with subsequent removal of oxide impurities from the solution, it is new that the heat treatment of crystalline fullerene C _{60 is} carried out at a temperature of 160-170 ° C inert gas flow (within 3-4 hours) to remove the oxide impurity C ₆₀ O, then dissolve the treated crystalline product in a weakly polar aromatic solvent, and to transfer C ₁₂₀ O to the precipitated ext They add a strongly polar solvent that does not contain double bonds and a nitrogen atom in its molecule, after which the precipitate containing C ₁₂₀ O is separated by filtration, and the target product containing pure C ₆₀ remaining in the solvent mixture is isolated by evaporation.

In the inventive method of purification of fullerene C ₆₀ from oxide impurities, ortho-xylene is used as a weakly polar solvent, and isopropyl alcohol in a volume ratio of ortho-xylene: isopropyl alcohol 1: (0.35-0.75), respectively, is used as a highly polar solvent.

Thus, conditions have been developed for the stepwise purification of crystalline fullerene C ₆₀ from C ₆₀ O and a solution of fullerene C ₆₀ from C ₁₂₀ O. These conditions are not associated with either high temperatures or the creation of vacuum and do not require complicated special equipment, applicable in ordinary laboratory conditions. The selection of bipolar solvents in a certain ratio eliminates the irreversible sorption loss of an expensive product (which is typical of the prototype and analogues).

The essence of the method is the targeted conversion of C ₆₀ monoxide to C ₁₂₀ O dimer oxide by means of a conventional heat treatment in a dynamic inert gas medium with further use of the ability of fullerenes to aggregate in a mixture of different polar solvents.

The combination of these features ensures the achievement of the task.

The inventive method is as follows.

The 1st stage of purification - purification of crystalline fullerene C ₆₀ from monoxide impurities. It was experimentally established that the heat treatment of crystalline fullerene powder in an inert gas atmosphere at a relatively low temperature leads to the almost complete removal of the thermally unstable impurity of C ₆₀ O monoxide.

Carrying out heat treatment of crystalline C ₆₀ in an inert gas environment is technically simple and does not require sophisticated equipment. To do this, a weak inert gas flow (for example, helium) was supplied through a sealed quartz ampoule with a loaded sample of crystalline fullerene C ₆₀ when heated. To prevent accidental ingress of air into the ampoule, a water shutter was installed at the outlet of the gas stream. After heat treatment, the fullerene sample was removed and analyzed by high performance liquid chromatography (HPLC) for the content of oxide impurities: residual monoxide and the formed C ₁₂₀ O. The heat treatment efficiency was estimated from the initial and final monoxide content. According to the difference in the weight of the sample before and after heat treatment, weight loss was estimated, which occurs mainly due to the partial removal of residual solvent. The weight of the residue insoluble in o-xylene was used to evaluate the degree of amorphization, which in all cases turned out to be zero, i.e. the crystalline powder under these conditions was completely dissolved in o-xylene.

The data confirming the claimed temperature range are presented in table. 2.

At temperatures above 170 °, the formation of an insoluble residue is observed, which indicates the beginning of the amorphization process (destruction of the fullerene structure). An important condition for preventing amorphization is the absence of air to the surface of the crystals, which is achieved by creating a dynamic atmosphere of inert gas.

The lower temperature does not allow to achieve high efficiency of the removal of monoxide even with prolonged heat treatment.

Purification from monoxide (C ₆₀ O) is carried out in the temperature range 160-170 ° C in an inert gas stream. The removal efficiency of the monoxide is very high and amounts to 98.5-100% of its initial content. As follows from the above results, the treated crystalline powder is enriched with an admixture of dimer oxide C ₁₂₀ O.

The 2nd stage of purification is the removal of impurities of the formed C ₆₀ O from the crystalline sample processed in the first stage. This operation to remove the C ₁₂₀ O impurity is associated with the ability of C _{60 to} form aggregates in solutions of various polar solvents that are stable over time. The formation of aggregates consisting of many fullerene molecules is determined by nonequilibrium states arising from the interaction of fullerene molecules with solvent molecules.

It is known that the aggregation of fullerene molecules in a solution occurs when a strongly polar solvent is added [10. Utilities. 2010.V. 84. No. 8. from. 1405-1416]. Fullerene C ₆₀ as a weakly polar component is practically insoluble in highly polar solvents such as water, acetone, acetonitrile, isopropyl alcohol, dioxane, dimethylformamide, etc. [eleven. J. Chem. Eng. Data 2010. v. 55 (1). p. 13-36]. When such solvents are added to a solution of C ₆₀ in a weakly polar solvent, aggregates form and grow, followed by their precipitation. When developing the method, it was found that the precipitate of aggregates of C ₆₀ fullerene aggregates is enriched with an admixture of C ₁₂₀ O dimer oxide, and a purer C ₆₀ fullerene remains in the solution.

When choosing a highly polar solvent, isopropyl alcohol (IPA) was proposed. Water was excluded because it does not mix with o-xylene, and solvents containing double bonds and a nitrogen atom in their molecule (such as acetone, acetonitrile, dioxane, dimethylformamide, etc.) were not used to exclude any chemical interaction of fullerenes with active groups of solvents, which can negatively affect the aggregation process or lead to the formation of by-products.

To ensure the most effective purification of C ₆₀ from C ₁₂₀ O, it is proposed to use o-xylene as a weakly polar solvent, which allows one to obtain highly concentrated solutions of fullerenes, which is important during aggregation. In slightly polar solvents such as benzene and toluene, the solubility of fullerene C ₆₀ is small (1.70 mg / ml and 2.8 mg / ml, respectively). And solvents such as o-dichlorobenzene or 1,2,4-trichlorobenzene having a high solubility of fullerene C ₆₀ (27 mg / ml and 8.5 mg / ml, respectively) are characterized by a higher boiling point than o-xylene, which makes it difficult to isolate of which crystalline C ₆₀ by evaporation. [12] (Phys. Uspekh. 1998. S. 1195-1220). It was also experimentally established that when isopropyl alcohol is added to a solution of C ₆₀ in o-dichlorobenzene or 1,2,4-trichlorobenzene, a mixture with a higher residual solubility of C _{60 is created} , and therefore the concentration effects on the components were noticeably lower than for solutions with xylene.

Therefore, they were experimentally selected: the volume fraction of the added IPA, the concentration of the initial solution of C ₆₀ in o-xylene, the exposure time required for the formation and growth of fullerene aggregates to ensure the best redistribution of C ₁₂₀ O between the deposited phase and the target phase and eliminate losses.

The choice of the optimal volume ratio of bipolar solvents was carried out on the basis of the results presented in table 3.

The results show that with an increase in the volume fraction of IPA, the yield of the precipitated phase increases, and the C ₁₂₀ O content in it decreases, but more pure C ₆₀ solutions are obtained. With a decrease in the volume fraction of IPA, an optimum high content of C ₁₂₀ O is achieved in the precipitated phase and, accordingly, a maximum decrease in this component in the liquid phase (in solution). The target product is a liquid phase containing more pure C ₆₀ , so it is necessary that the yield of the precipitated phase is optimally low.

The data show that the maximum decrease in the content of C ₁₂₀ O in the solution is achieved when the volume ratio of solvents is o-xylene: IPA = 1: (0.35 ÷ 0.75). With an increase in the proportion of IPA in the mixture, the yield of the precipitated phase increases, i.e. it turns out a solution with a lower concentration of C ₆₀ , which reduces the performance of the target phase (solution).

After heat treatment of fullerene C ₆₀ contaminated with an admixture of monoxide, a product with an increased impurity content of C ₁₂₀ O is obtained. Such a sample was dissolved in o-xylene, the resulting solution was filtered through a dense paper filter and steamed on a rotary evaporator to a concentration of ~ 10 mg / ml. To this solution, IPA was added dropwise in a predetermined volume ratio with vigorous stirring. Then the mixture was mixed for about 30-60 minutes to mature the precipitate (establish exchange equilibrium), the aggregated phase was separated by filtration, the precipitate was dried under vacuum and weighed.

IPA was removed from the filtrate under vacuum in a rotary evaporator at a temperature of 35-40 ° C. The composition of the precipitated (aggregated) phase and the o-xylene solution (target phase) was determined by HPLC. The results are presented in table. four.

The results presented in Table 4 show that, if a highly concentrated initial solution of fullerene C ₆₀ in o-xylene is used, a smaller volume fraction of IPA is required, and therefore it is possible to effectively remove the C ₁₂₀ O impurity even with a very significant initial content. In this case, a solution of fullerene C _{60 of a} high degree of purity is obtained in the form of the target phase and with a minimum content of oxide impurities. An increase in the volume fraction of IPA leads to a substantial increase in the yield of the precipitated phase, i.e. most of the material goes into precipitate, and accordingly, the yield of the target phase (C ₆₀ in solution) is significantly reduced. When the volume fraction of IPA is less than 0.35, aggregation of fullerene C ₆₀ and dimer oxide C ₁₂₀ O does not occur, which does not allow obtaining fullerene C _{60 of a} sufficiently high degree of purity (99.5% +) in the composition of the target phase. Therefore, the optimal volume ratio of o-xylene: IPA, allowing efficient removal of C ₁₂₀ O impurities from C ₆₀ fullerene with the highest yield of the target phase, according to the results of the experiments, is in the range 1: (0.35-0.75).

An example of a specific implementation.

Fullerene C ₆₀ , having an initial purity of 99.7%, stored for 3 years under normal conditions, has the composition:

C ₆₀ - 96.39%; C ₆₀ O - 0.20%; C ₇₀ - 0.02%; From ₁₂₀ O — 3.39% according to the results of HPLC analysis.

Stage 1: purification of crystalline fullerene C ₆₀ from monoxide impurities.

17.9 g of crystalline fullerene powder C _{60 was} heated in 3 g portions each in a quartz tube heated to a temperature of 160-170 ° C in a dynamic atmosphere of an inert gas (helium) for 3-4 hours. At the end of the procedure, the product was removed from the tube and weighed. The total weight of the product after heat treatment was 17.2 g, i.e. product loss was only 4.0% by weight. The product was dissolved in an excess of o-xylene and filtered. Insoluble residue was absent. The solution was analyzed by HPLC and steamed on a rotary evaporator to a concentration of ~ 10 mg / ml. The average content of product components after heat treatment was: C ₆₀ - 95.90%; C ₆₀ O 0 0.02%; C ₇₀ - 0.02%; C ₁₂₀ O — 4.06% by HPLC analysis.

Thus, after heat treatment in an inert gas stream, a material was obtained containing trace amounts of C ₆₀ monoxide along with a high content of C ₁₂₀ O dimer oxide, which was removed in the next stage.

Stage 2. Removal of the oxide of the dimer C ₁₂₀ O from a solution of fullerene C ₆₀ .

A crystalline sample of fullerene C ₆₀ , which passed the 1st stage, was dissolved in an excess of o-xylene, the solution was filtered from insoluble impurities and steamed on a rotary evaporator to a concentration of ~ 10 mg / ml. To 300 ml of the obtained fullerene C ₆₀ solution, IPA was added dropwise in a volume ratio of 1: (0.35-0.45) with vigorous stirring on a magnetic stirrer and at room temperature. The mixture was stirred for another 30-60 minutes and filtered. The precipitated phase was dried on a filter under vacuum, weighed and analyzed by HPLC. IPA was removed from the filtrate on a rotary evaporator under vacuum at a water bath temperature of 35-40 ° C. The concentration of the obtained o-xylene solution of purified C _{60 was} determined by spectrophotometric method, and the composition by HPLC. If necessary, this procedure was repeated to obtain fullerene C _{60 of the} maximum degree of purity in the target phase. In this case, the concentration of the o-xylene solution was adjusted if necessary (by removing excess solvent) to a concentration of ~ 10 mg / ml and IPA was again added to this solution.

The dependence of the content of C ₁₂₀ O in the o-xylene solution of C ₆₀ (target phase) on the volume ratio of solvents is presented in Table 5 (initial composition of the fullerene sample after stage 1,%: C ₆₀ - 95.90; C ₁₂₀ O - 4.06.

It should be noted that in the ongoing procedures the content of C ₆₀ monoxide and C ₇₀ fullerene in all phases remained at the trace level and therefore their content is not indicated.

The data presented in table. 5 show that by aggregating in one or two successive stages, it is possible to increase the content of fullerene C ₆₀ in solution from the initial 95.90% to the level of 99.5% +.

Moreover, aggregation can reduce the content of C ₁₂₀ O in pure fullerene C ₆₀ (99.5% +) to trace amounts. This possibility is additionally demonstrated by the results presented in table. 6.

To ~ 580 ml of a solution of C ₆₀ in o-xylene with a concentration of ~ 10 mg / ml (5.8 g of fullerene C ₆₀ ) IPA was added dropwise in a volume ratio of 1: 0.35 (~ 200 ml). The mixture was stirred for 1 hour and separated by filtration. The weight of the precipitated phase was 3.06 g, and the amount of material contained in the solution was 2.45 g according to the analysis. Moreover, the purity of the precipitated phase by C ₆₀ fullerene was kept at a high level.

The inventive method has the following advantages compared to the prototype: simple and affordable, provides a minimum loss of valuable fullerene C ₆₀ (excludes chromatography as the main source of losses in the prototype), provides removal of oxide impurities to the level of 0.02%.

The method is applicable for purification of fullerene C ₆₀ from oxide impurities, and also, if necessary, to restore the purity of fullerene C ₆₀ , contaminated during storage by oxide impurities, to a purity of 99.5% +.

Bibliography.

1. V.V. Aksenova, R.M. Nikonova, V.I. Ladyjanov, N.B. Tamm, E.V. Skokan, B.E. Pushkarev. Oxidation of fullerites C ₆₀ and C ₇₀ in air // ZhFKh. 2013.v. 87. No. 10. from. 1771-1778.

2. M. Wohlers, N. Werner, D. Herein, T. Schedel-Niedrig, A. Bauer, R. Schlogl. Reaction of C ₆₀ and C ₇₀ with molecular oxygen // Synthetic Materials. 1996. 77. p. 299-302.

3. K.M. Creegan, JL Robbins, WK Robbins, JM Millar, RD Sherwood, PJ Tindall, DM Cox, JM McCauley, DR Jones, R. Gallagher and AB Smith. Synthesis and characterization of C ₆₀ O, the first fullerene epoxide // J. Am. Chem. Soc. 1992. V. 114. p. 1103-1105.

4. H. Watanabe, E. Matsui, Y. Ishiyama, M. Senna. Solvent free mechanochemical oxygenation of fullerene under oxygen atmosphere // Tetrahedron Letters. 2007. V. 48. p. 8132-8137.

5. R. Taylor, MP Barrow and T. Drewello. C ₆₀ degrades to C ₁₂₀ O // Chem. Commun. 1998. p. 2497-2498.

6. The book Studies in Surface Science and Catalysis. In Delmon and J.T. Yates. Adsorption on new and modified inorganic sorbents. Editors: A. Dabrowski and V.A. Tertykh, Elsevier Publishing. 1996. v. 99. p. 902.

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8. The method of producing fullerene C ₆₀ . RF patent No. 2456233, 2010

9. A method of producing fullerene C ₆₀ . RF patent №2327635, 2008 - a prototype.

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Claims (2)

1. The method of purification of fullerene C ₆₀ from oxide impurities, including heat treatment of crystalline C ₆₀ with subsequent removal of oxide impurities from the solution, characterized in that the heat treatment of crystalline fullerene C _{60 is} carried out at a temperature of 160-170 ° C in an inert gas stream to transfer the oxide impurity C ₆₀ O into the C ₁₂₀ O dioxide impurity, then the treated crystalline product is dissolved in a weakly polar aromatic solvent, and to transfer the C ₁₂₀ O into the precipitated phase, a strongly polar solvent is added that does not contain the molecule has double bonds and a nitrogen atom, after which C ₁₂₀ O is separated by filtration, and the target product remaining in the solvent mixture solution is isolated by evaporation. 2. A method for purifying fullerene C ₆₀ from oxide impurities according to claim 1, characterized in that ortho-xylene is used as a weakly polar solvent, and isopropyl alcohol in a volume ratio of ortho-xylene: isopropyl alcohol 1: (0, 35-0.75), respectively.