Systematic Study on the General Preparation of Ionic Liquids with

Mar 18, 2014 - High-purity ionic liquids were prepared from halide precursors by an optimized synthesis route. This preparation route consisted of thr...
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Systematic Study on the General Preparation of Ionic Liquids with High Purity via Hydroxide Intermediates Da-Niu Cai, Kuan Huang, Yong-Le Chen, Xing-Bang Hu, and You-Ting Wu* Separation Engineering Research Center, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *

ABSTRACT: High-purity ionic liquids were prepared from halide precursors by an optimized synthesis route. This preparation route consisted of three steps, i.e. methathesis, ion exchange, and acid/base neutralization. A significant amount of halide impurity remained in the ionic liquids. Metal and halide impurities were effectively removed by utilizing an ionic exchange column. Multiple parameters that affected the amounts of impurities in the final product, such as molar ratio of halide precursor to potassium hydroxide, amount of ethanol solvent, type of resin, and concentration of hydroxide, were investigated in detail. Two high-purity hydroxide intermediates ([N2221]OH and [C6mim]OH) were prepared by metathesis and ion exchange, and they were then neutralized with four different acids to produce a series of ionic liquids. The yields of cations and anions in the ionic liquids as well as the concentrations of impurities in each step were determined to verify the feasibility of the method. The new preparation route is of great potential in preparing various high-purity ionic liquids.

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INTRODUCTION Ionic liquids (ILs), as a special group of salts that are liquid at ambient temperature, have several unique advantages, including negligible vapor pressure, large dissolving capacity, high chemical stability and conductivity, and easy chemical modification.1−3 Hence, ILs have attracted considerable interest from both academic and industrial communities over past decades, and have been widely investigated in many fields such as catalysis, electrochemistry, gas separation, and organic synthesis.4−7 To accelerate these applications, it is of great importance to prepare high-purity ILs in a green and industry-scalable way. Nowadays, two-step synthesis is one of the most common methods for preparing desired ILs. It starts with the quaternization reaction that yields cations, followed by the metathesis reaction that introduces anions (Scheme 1). Silver salts, Brønsted acids, and alkali metal salts are commonly used in the second reaction step. However, the first anion source is limited by the tremendously high cost of silver salts, whereas the second is hampered by the byproduct HX that is extremely corrosive and causes the decomposition of ILs over time. As for the third one, although it has low cost and toxicity, incomplete reaction produces residual halide and inorganic metal ions in ILs. The halide residuals are detrimental to most transition metal catalyzed reactions,8−11 while the inorganic impurities can severely affect the physical properties of ILs.12 To remove these impurities, conventional purification routes have been practically applicable (Scheme S1, Supporting Information). The routes are based on the high solubility of impurities in water, but they have different procedures for hydrophobic and hydrophilic ILs. Obviously, the route is not green for the preparation of hydrophilic ILs, since there is a significant loss of the product in the water phase and the use of CH2Cl2 is harmful to both humans and the environment.13 Traditional synthetic and purification methods have been systematically reviewed.14−17 © 2014 American Chemical Society

To synthesize ILs with high purity, a novel strategy is to develop intermediates of desired cations that are low in halogen or even halogen free. Two ways are applied to obtain these intermediates: (1) Synthesize from materials that are intrinsically free of halogen. 1,3-Dimethylimidazolium-2-carboxylate,18 1,3-dialkylimidazolium alkanesulfonate salts,19 and 1,3-dialkylimidazolium alkanesulfate salts20,21 are prepared by this method. (2) Eliminate the halogen of halide salts (i.e., [RR′im]+X− or [R3R′N]+X− obtained from the quaternization reaction) to the greatest extent by converting them to N-heterocyclic carbenes22 or hydroxides of desired cations ([RR′im]+OH− or [R3R′N]+OH−). Among the methods mentioned above, the hydroxides of desired cations are promising intermediates due to the following advantages: (1) Their raw materials (halide salts) are commercially available from gram to multiton scale.23 (2) They can be easily prepared using various methods, such as ion exchange of halide salts with anion exchange resin,24−26 metathesis with potassium hydroxide,27,28 reaction of water with N-heterocyclic carbene,29 and electrodialysis through bipolar membrane (EDBM).30 (3) Acid/base neutralization of hydroxide intermediates with acids of desired anions to prepare ILs is widely applicable for almost all kinds of cations and anions.26,31 In this study, a combinational method was proposed to prepare hydroxides of desired cations with very low contents of halogen and alkali metal. The method consisted of the metathesis of halide salts with potassium hydroxide in ethanol to remove most KCl, followed by ion exchanges in anion exchange and cation exchange resin columns to purify the hydroxide intermediates. The resulting hydroxide intermediates Received: Revised: Accepted: Published: 6871

January 13, 2014 March 12, 2014 March 18, 2014 March 18, 2014 dx.doi.org/10.1021/ie500086r | Ind. Eng. Chem. Res. 2014, 53, 6871−6880

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Scheme 1. Typical Ionic Liquid Synthetic Routes

Figure 1. Three-step strategy for preparing ILs with the desired anion.

effect can be neglected.32 Columns were packed with 90 mL of wet resins, and a bed height of 45 cm was obtained. To reduce the experimental cost, two smaller columns were used during synthesis instead of the ones used in the column studies. However, the key parameters are consistent with the ones in the column studies. Aqueous solution containing known concentrations of tetramethylammonium cation ([N1111]+), OH−, Cl−, and K+ was prepared by dissolving [N1111]OH·5H2O and KCl in deionized water, and was used as the working fluid for the column study. The working fluid was fed to the top of the columns at desired flow rates with peristaltic pumps until the breakthrough curves were completed. Effluent samples were taken at preset time intervals. Cl− and K+ concentrations were measured using chlorine and potassium ion selective electrodes. Cationic and anionic selective electrodes were calibrated in the linear range of 5 × 10−5 to 10−1 mol/L with R2 > 0.999 and in the linear range of 1 × 10−5 to 10−1 mol/L with R2 > 0.999, respectively. Synthesis and Characterization of ILs. A typical threestep strategy of preparing ILs with the desired anion was adopted (Figure 1). The method consisted of metathesis, ion exchange, and neutralization steps. For example, the synthesis of triethylmethylammonium alanine ([N2221][L-Ala]) was described in detail, and other ILs were prepared following the same procedure unless otherwise specified. In the metathesis step, triethylmethylammonium chloride ([N2221][Cl], 15.2 g, 0.10 mol) was dissolved in ethanol (20.0 g), and a solution of KOH (6.4 g, 0.10 mol) in ethanol (35.0 g) was added dropwise under stirring. White precipitate could be immediately observed, and the addition of KOH was finished within 0.5 h. After being stirred for 24 h at 10 °C in a thermostatic bath equipped with a cooling machine, the

were then neutralized as the last step with free acids of desired anions to prepare a series of ILs with high purity. Each step of this route was investigated and the influencing factors in each step were explored in detail to verify the feasibility of the new method.

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EXPERIMENTAL SECTION Chemicals. All halide salts of desired cations (Shanghai Cheng Jie Chemical Co., Ltd.) were dried before use. The purities of these salts after drying were higher than 99.0% as measured by precipitation titration with standard AgNO3 solution. All ion exchange resins were commercially available from Anhui Sanxing Resin Technology Co., Ltd., China (Table S1, Supporting Information). Prior to use, the resins were washed with 1 M NaOH and 1 M HCl solutions to remove possible impurities. Anion exchange resins (201×7 and 202×7) were converted to the OH− form by flushing the resin column with 4 M NaOH until the concentration of Cl− was lower than 10−4 mol/L as detected using a Cl− selective electrode. Cation exchange resins (001×7, 001×10, and 001×12) were converted to the H+ form with 1 M HCl. Deionized water was used to wash off free HCl or NaOH resident in resins. Other chemicals were analytical reagents and were used without further purification. Ion Exchange Column Study. In order to optimize the operating conditions, a column study for anion and cation exchange processes was carried out. The experiments employed two glass columns with an inner diameter (i.d.) of 1.6 cm and a length of 50.0 cm, with one for anion exchange and the other for cation exchange. Since the diameter of the columns was over 20 times larger than that of the resin particles, the wall 6872

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reaction mixture was centrifuged at 4000 rev/min for 10 min, and the obtained supernatant was [N2221]OH solution with small amounts of chlorine and potassium residues. The precipitate was washed with 5.0 g of ethanol, and the effluent was combined with [N2221]OH solution. To lower the OH− concentration to 0.5 M, the obtained ethanol solution of [N2221]OH was diluted to 0.2 L by deionized water. Then, this solution was fed to the top of an ion exchange setup at the desired flow rate by a peristaltic pump, and the remaining [N2221]OH was washed out with deionized water. All effluents were collected at the bottom of the ion exchange setup. The [N2221]OH content in the effluent was determined using acid titration. The ion exchange setup referred to above consisted of two columns, both with an inner diameter of 1.0 cm. The two columns were loaded with anion exchange resins (201×7, OH−, wet, 25 mL) and cation exchange resins (001×12, H+, wet, 10 mL), respectively (Figure S1, Supporting Information). In the last reaction step, L-alanine was dissolved in water and was added dropwise with an equimolar quantity of purified [N2221]OH under stirring. After reaction for 3 h, the solvents were removed by evaporation, and the remaining solution was further dried in a vacuum for 2 days at 80 °C to generate the desired IL. IL samples were analyzed by 1H NMR spectroscopy (Varian XL-300) and elemental analysis (Elementar Vario EL) to determine their structure and purity, and the results are presented in the Supporting Information. The Cl− content was determined with AgNO3 potentiometric titration, and the K+ content was analyzed using atomic absorption spectroscopy (AAS; Varian AA240FS).

X (wt %) =

Q=

MNOH Q2 ·100 − 1 MX

nMOH nNX

msol SsolMX mNOH

(5)

(6)

where msol and mNOH are the masses (g) of solvent and onium hydroxide, respectively, Ssol is the solubility (mol/g) of MX in the reaction solvent, and MM, MX, and MNOH are the molecular weights (g/mol) of M, X, and NOH, respectively. As shown in eqs 2−6, the residual mass concentrations of M and X in this process are influenced by three factors: (1) solubility of alkali metal halide in the reaction solvent (Ssol); (2) molar ratio of MOH to NX (nMOH/nNX); (3) mass ratio of solvent to onium hydroxide (msol/mNOH). The effect of each factor is discussed below. Effect of Ssol. Ssol is equal to the solubility of alkali metal halide in organic solvent if the reaction system is assumed to be completely anhydrous. However, water is difficult to avoid in this process because all the reactants used are extremely hygroscopic. The hygroscopicity of reactants was measured by an experiment in which the amount of water absorbed from the air at ambient temperature was recorded as a function of time as shown in Figure 2. An uptake of 9−15 wt % after 3 h of

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RESULTS AND DISCUSSION Synthesis of Onium Hydroxide by Metathesis. The synthesis process of onium hydroxide (NOH, “N” represents the onium cation such as [RR′im]+ and [R3R′N]+) from alkali metal hydroxide (MOH) is essentially a metathesis reaction driven by the difference of solubility between the starting material (NX) and alkali metal halide (MX) in a reaction solvent, as shown in eq 1. organic solvent

N+X− + M+OH− ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ N+OH− + MX↓

Figure 2. Mass percentage of water absorbed into reactants from air at ambient temperature as a function of time.

(1)

The reaction is fast as observed in the experiments, and is nearly complete due to the insolubility of alkali metal halide. However, a small amount of alkali metal halide that remains in the system should be further removed. The residual mass percentages of M+ and X−, which are experimentally determined using chlorine and potassium ion selective electrodes, can also be calculated from eqs 2−6 according to the precipitation− dissolution equilibrium theory. If the molar ratio of MOH to NX (nMOH/nNX) is equal to 1 msol SsolMM ·100 M (wt %) = mNOH (2) X (wt %) =

msol SsolMX ·100 mNOH

exposure is considered quite high. The hygroscopicity should be paid attention to during the experiments because even a small amount of water dramatically increases the solubility of alkali metal halide.33 In the experiments, the water content in the reaction system is controlled to within 2 wt % as detected by coulometric Karl Fischer titration. In addition, we use a rough but easy-tounderstand estimation to improve the calculation accuracy of Ssol as shown in eq 7. It should be noted that this approximation is applicable to a very small range of water mass fraction (1−2%). A more accurate model for the calculation of Ssol at high water concentrations has been reported by Li et al.34 Ssol = (1 − waq)Sorg + waqSaq

(3)

where Saq is the solubility of MX in water (mol/g), Sorg is the solubility of MX in organic solvent (mol/g), and waq is the mass percentage of water in the reaction system (wt %). Sorg and Saq in eq 7 are dependent on the types of organic solvent and alkali metal halide. In the selection of an organic

If nMOH/nNX is greater than 1 ⎞ M ⎛n M (wt %) = ⎜ MOH − 1⎟ M ·100 ⎠ MNOH ⎝ nNX

(7)

(4) 6873

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Effect of Mass Ratio of Onium Hydroxide to Ethanol. The amount of solvent is another important factor that influences the impurity concentration in the supernatant, and this was validated by the experiments in which the Cl− and K+ contents were measured as functions of the mass ratio of onium hydroxide to ethanol. As shown in Figure 4, the contents of

solvent, MOH and onium halide salt should be sufficiently soluble. Therefore, some common organic solvents such as ethers and hydrocarbons are excluded. Ketones and esters are also unsuitable due to their chemical instability in the presence of a strong base. Alcohols meet the requirement of large solubility (Table S2, Supporting Information), of which ethanol is the most suitable because of low cost, suitable boiling point, and nontoxicity.35 In this study, ethanol is selected as the reaction solvent. Table S2 in the Supporting Information shows that the solubilities of KCl in ethanol and water are both the lowest compared to those of other alkali metal halides. Meanwhile, KOH is more soluble in ethanol than NaOH. Therefore, KOH and onium chloride are selected as reactants since the reaction can be easily driven to be complete by the most insoluble KCl.36 Notably, onium bromide is also competitive because it is obtained more easily due to the higher reactivity of bromoalkanes than that of chloroalkanes in nucleophilic substitution reactions.16 However, onium iodide salt is not an option in any case due to the extra-high solubility of alkali metal iodide in alcohols and in water.37 To prepare high-purity ILs, KOH and onium chloride are selected as reactants in most of our experiments, and two examples of using bromide salts as starting materials are also performed. Effect of Molar Ratio of MOH to NX. Figure 3 shows that the molar ratio of KOH to [N2221]Cl simultaneously influences

Figure 4. Mass percentage of K+ and Cl− in the supernatant as a function of the mass ratio of [N2221]OH to ethanol.

Cl− and K+ decrease from 1.89 and 1.79% to 0.82 and 0.87% respectively as m[N2221]OH/methanol rises from 0.10 to 0.25. This can be explained from two aspects: reduction of ethanol (msol is small) reduces the capacity for the dissolution of KCl; increase of ionic concentration (mNOH is large) enhances the salting-out effect to minimize the solubility of KCl. However, when the ratio varies from 0.25 to 0.30, the mass concentrations of impurities increase slightly from 0.82 to 0.84% for Cl− and from 0.87 to 0.89% for K+, while the calculated curves using eqs 2 and 3 suggest reductions. Probably the elevated viscosity was adverse to the crystallization and subsequent precipitation of KCl crystals in the reaction system. Hence, the optimized mass ratio of onium hydroxide to ethanol is about 0.25. Removal of Inorganic Impurities by Ion Exchange. The onium hydroxide solution obtained from the metathesis step still contains small amounts of Cl− and K+ residues that should be further removed. In this study, the ion exchange process was applied due to high efficiency, low cost, and ease of operation. As shown in eqs 8 and 9, the impurity removal is driven by the difference of the affinity between Cl− and OH− for the anion exchange resin and between K+ and H+ for the cation exchange resin.

Figure 3. Mass percentage of K+ and Cl− in the supernatant as a function of the molar ratio of KOH to [N2221]Cl.

the residual concentrations of K+ and Cl− in the supernatant solution of onium hydroxide. With increasing molar ratio from 1 to 1.15, the mass percentage of Cl− in the supernatant decreases from 0.82 to 0.19% because excess K+ drives the reaction equilibrium to the right. However, the mass percentage of K+ in the supernatant significantly rises from 0.87 to 4.18%. If the minimization of halide content is focused on, KOH should be introduced as much as possible. Nevertheless, although the presence of alkali metal in ILs may work in most cases, the physicochemical properties of ILs may be affected.16,38 In addition, excess KOH in the supernatant leads to undesirable loss of the desired cation in the following ion exchange process or an extra consumption of the acid of the desired anion in the last neutralization process. The two effects raise the raw material cost, which is unfavorable to the IL production.16,39,40 In order to balance the impurity control with the cost of reactant consumption, the parameter nKOH/n[N2221]Cl should be optimized and was kept at 1 in the subsequent experiments.

The fixed bed column operation for ion exchange allows efficient utilization of the exchange capacity, and the performance is usually described through a breakthrough curve which shows the loading behavior of ions to be removed from the solution onto the fixed bed. A column study using the working 6874

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Figure 8. Breakthrough curves of K+ in different cation exchange resin beds (COH−= 0.5 M, flow rate = 10 BV/h, bed i.d. = 16 mm, bed volume = 90 mL).

Figure 5. Breakthrough curves of Cl− in different anion exchange resin beds (COH− = 0.5 M, flow rate = 10 BV/h, bed i.d. = 16 mm, bed volume = 90 mL).

Figure 9. Breakthrough curves of K+ in the cation exchange resin bed at different flow rates (resin type 001×12, COH− = 0.5 M, bed i.d. = 16 mm, bed volume = 90 mL).

−

Figure 6. Breakthrough curves of Cl in the anion exchange resin bed at different OH− concentrations of the feeding solution (resin type 201×7, flow rate = 10 BV/h, bed i.d. = 16 mm, bed volume = 90 mL).

assumes symmetrical nature of the breakthrough curve and neglects the effect of axial dispersion, which agrees well with the experimental conditions in this study. The Yoon and Nelson model for a single component can be expressed as follows:42,43 Ce 1 = C0 1 + exp[k(δ − v)]

(10)

where C0 and Ce are the inlet and outlet adsorbate concentrations respectively, δ is the solution volume required for 50% adsorbate breakthrough (BV, bed volume), v is the solution volume passed through when sampling (BV), and k is the proportionality constant related to the shape of the breakthrough curves (BV−1). The Yoon and Nelson model can be used to represent the experimental breakthrough curves with R2 values ranging from 0.985 to 0.998. The fitted model parameters are summarized in Tables S3 and S4 (Supporting Information), and the calculation results are shown in Figures 5−9. Anion Exchange for the Removal of Cl−. The functional group of anion resin is crucial to the removal of Cl−. Two of the most common strong base anion exchange resins (SBAER, 201×7 and 202×7) were tested to remove residual Cl− ions from the solution while other operating parameters were kept constant. Significant performance differences are observed (Figure 5 and Table S3 in the Supporting Information). For 201×7 resin, the breakthrough of Cl− is greatly delayed, and the

Figure 7. Breakthrough curves of Cl− in the anion exchange resin bed at different flow rates (resin type 201×7, COH−= 0.5 M, bed i.d. = 16 mm, bed volume = 90 mL).

fluid was carried out to investigate the effects of operating conditions on the breakthrough curves (Figures 5−9). Meanwhile, the Yoon and Nelson model was selected to fit these breakthrough curves, because it can be written in a simple form and requires no detailed data concerning the characteristics of the adsorbate(s), the type of adsorbent, or the physical properties of the adsorption bed.41 In addition, the model 6875

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value of δ (34.44) is nearly 4 times that for 202×7 (7.29). This comes from the functional group of −N+(CH3)2C2H4OH in 202×7 resin that has a lower Cl− selectivity than that of −N+(CH3)3 in 201×7 resin, because hydroxyethyl can weaken the electron-withdrawing ability of the nitrogen atom. In addition, 202×7 resin (OH− form) is easily oxidized and turns dark brown hours after exposure to air. On the contrary, 201×7 resin (OH− form) changes little in appearance due to high chemical stability. All these make 201×7 resin more suitable for the removal of Cl−. Onium hydroxide concentration in the supernatant is another important factor that influences the performance of Cl− removal. The supernatant from metathesis should be diluted to decrease the OH− concentration in the resin phase before ion exchange. Three OH− concentrations in the supernatant, 0.5, 0.75, and 1.0 M, are selected to investigate the breakthrough of Cl−. As shown in Figure 6 and Table S3 in the Supporting Information, the breakthrough of Cl− is accelerated with increasing OH− concentration, and δ decreases from 34.44 to 9.48 as the OH− concentration increases from 0.5 to 1 M. This favors a low concentration of onium hydroxide in the supernatant to get a good performance in the ion exchange step. The mechanism can be easily explained from eq 8 that excess OH− moves the reaction equilibrium to the left and makes the bonding of Cl− more difficult. The removal of Cl− is also dependent on the flow rate of the feeding solution. As shown in Figure 7 and Table S3 in the Supporting Information, δ increases from 25.66 to 34.44 and then decreases to 27.57 as the flow rate rises from 3.3 to 10 and then to 16.7 BV/h. There exists an optimized flow rate for the process probably because, as the flow rate increases, the liquid membrane on the resin is thinned to reduce the mass transfer resistance and then to improve the ion exchange efficiency, so that the breakthrough point is delayed. However, further increase in the flow rate shortens the contact time for Cl− with resin, which leads to insufficient ion exchange at elevated flow rates. Therefore, the optimum flow rate is around 10 BV/h. Cation Exchange for the Removal of K+. Strong acid cation exchange resin with −SO3H group (SACER, 001 series) is the most commonly used since it can remove nearly all kinds of cations. Therefore, besides K+ residue, onium cation in the solution also reacts with SACER because the initial 3−6 bed volumes of the collected solution are neutral. This implies the occurrence of the following reaction:

can react only with the functional groups exposed on the surface of the resin, whereas K+ can exchange with all available sites due to its small size. Figure 8 exhibits that SACER with DVB of 12% is suitable for effective removal of K+. Besides the micropore size of the resin or the cross-linking degree, the size of the target cation is another determinant factor that influences the size exclusion in the cation exchange as discussed above. To this end, the complete exchange capacity (ECN+) of 001×12 resin for [Cnmim]+ and [R4N]+ is measured as a function of alkyl chain length in batch experiments according to ASTM D2187-94(R2004),47 and the results are available in Table 1. With rising length of the alkyl chain, the Table 1. Complete Exchange Capacity of 001×12 for K+, [R4N]+, and [Cnmim]+ Cations cation +

K [N1111]+ [N2222]+ [N4444]+

ECN+ (mmol/g) 4.76 4.36 4.15 2.38

cation

ECN+ (mmol/g) +

[C2mim] [C4mim]+ [C6mim]+ [C8mim]+

4.31 4.26 3.77 3.03

complete exchange capacity reduces for both types of cations, and the ratio of EC to ECK+ (the exchange capacity for K+) decreases finally to 50% for [N4444]+ and to 64% for [C8mim]+. Such incomplete exchange at a high cross-linking degree for organic cations was reported as early as 50 s.48 However, this was the first time to determine the exchange capacity for the imidazolium cations to the best of our knowledge. The incomplete exchange can reduce the loss of the desired cation in resins and correspondingly enhance the exchange capacity for residual K+, especially when the desired cation has a large size. The flow rate of feeding solution is also an important parameter that influences the removal efficiency of K+. The effect of flow rate on K+ removal was examined under the same conditions as those for Cl− removal, and the results are shown in Figure 9 and Table S4 in the Supporting Information. As the flow rate increases, the breakthrough curve becomes flatter, and the breakthrough appears earlier, which disfavors effective removal of K+. A flow rate as low as 3.3 BV/h is appropriate since the breakthrough curve is steep enough, which is in contradiction to the case of Cl− removal where an intermediate value of the flow rate is preferred. The results may be attributed to the resin of high cross-linking degree that affects the mass transfer of K+ in the interior of the resin at high flow rates. Synthesis of ILs by Neutralization. The onium hydroxide solution purified in the ion exchange process can react directly with the acid of the desired anion, and the target IL is generated in the neutralization reaction with H2O as the only byproduct (eq 12). After the removal of water and ethanol by evaporation, the target ILs with high purity can be obtained.

It is obvious that the reaction in eq 11 leads to not only the loss of desired cations but also the reduced exchange capacity for K+. To overcome this problem, SACER with a high content of divinylbenzene (DVB) cross-linking agent should be introduced. Figure 8 and Table S4 in the Supporting Information show the effect of DVB content on the performance of K+ removal. The breakthrough of K+ is remarkably delayed at DVB contents higher than 10%, and δ increases significantly from 36.96 to 84.04 as the DVB content increases from 7 to 12%. The reason is that the micropore size of the resin decreases with increasing cross-linking degree, so the cation with large size for the construction of ILs is excluded from cation exchange in the interior of the resin at high DVB concentrations.44−46 The target cation

N+OH− + H+Y − → N+Y − + H 2O

(12)

The simple neutralization process has been widely applicable in industry. Theoretically, any cation that exists in the form of hydroxide can be combined with any anion that exists in the form of free acid during neutralization to obtain the corresponding IL. Actually, most cations commonly used for the formation of ILs (imidazolium, quaternary ammonium, quaternary phosphonium, pyrrolidinium, and pyridinium cations) were successfully introduced by this method,24,25,31,35,49 and so were the cations with modified structure when designing functionalized 6876

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Table 2. Concentrations of Cl− and K+ in Each Step and Yields of Cations and Anions of ILs metathesis step

ion exchange step

final yield

neutralization step

product

Cl−/ppm

K+/ppm

YM/%

Cl−/ppm

K+/ppm

YI/%

Cl−/ppm

K+/ppm

YN/%

Ycation/%

Yanion/%

[N2221][HSO4] [N2221][BF4] [N2221][OAc] [N2221][L-Ala] [C6mim][HSO4] [C6mim][BF4] [C6mim][OAc] [C6mim][L-Ala]

10679

9656

98

115

89

87

8414

12495

98

96

100

92

33 27 90 88 67 70 78 69

56 60 68 58 70 72 81 72

95 95 94 96 95 96 94 95

81 81 80 82 85 86 84 85

95 95 94 96 95 96 94 95

ionic liquids.26 It should be pointed out that some cations are prone to degradation in aqueous solutions with high OH− concentration and/or at high temperature, primarily due to the attack of hydroxide counterion through an SN2 pathway which leads to Hofmann elimination.50,51 Therefore, appropriate conditions, such as a OH− concentration of lower than 1.2 mol/L and a reaction/storage temperature of lower than 30 °C, should be ensured during neutralization.35 As for anions, different acids ranging from simple inorganic acids to complex organic acids of functional groups are reactive with the hydroxides, even if they are not fully soluble in water.26,35 In general, it is the neutralization process that unifies the preparation of various ILs into a generic methodology. Under optimal conditions obtained above, eight salts ([N2221][HSO4], [N2221][BF4], [N2221][OAc], [N2221][L-Ala], [C 6 mim][HSO 4 ], [C 6 mim][BF 4 ], [C 6 mim][OAc], and [C6mim][L-Ala]) were synthesized from the precursors of [N2221]OH or [C6mim]OH and the corresponding acids with the desired anions by acid−base neutralization. All the eight salts are submitted for 1H NMR and elemental analysis (EA), and the results indicate that the samples are target products without additional peaks in the NMR spectra and with element concentration deviations of less than 0.5%. The NMR and EA data are available in the Supporting Information. In the eight salts, [N2221][HSO4] and [N2221][BF4] do not belong to ILs due to high melting points. However, since quaternary ammonium salts were well studied as the most favorite solutes for nonaqueous electrolyte systems, these two salts verify the possibility of using the method in this paper for the preparation of nonaqueous electrolytes in electrochemical capacitors.52,53 The other six samples are consistent with the definition of ILs, and have been investigated in several different research fields; e.g., [N2221][L-Ala] was studied as a task-specific ionic liquid for fast and reversible CO2 absorption49 and [C6mim][HSO4] was used to catalyze esterification as a recyclable reaction medium due to immiscibility with the produced esters.54 Concentrations of Halogen and Alkali Metal in ILs. The Cl− and K+ contents in each preparation step of the eight salts are measured and shown in Table 2. Most Cl− and K+ in the reactants are removed in the metathesis step in the form of KCl precipitate, and 8000−13 000 ppm (mg/kg, the same as below) Cl− or K+ is left in the obtained solutions. Residual Cl− or K+ in the supernatants of the metathesis step can be further removed in the ion exchange step to have a concentration of about 100 ppm, with a removal efficiency of around 99%, despite the target cation ([N2221]+ or [C6mim]+) used for the construction of ILs. In fact, all Cl− and K+ ions can be stripped from the solutions if the ion exchange is finished at an early cutoff, but not at the breakthrough point of 5% feeding

concentration. The effluent from the exchange column before the breakthrough point is completely collected and mixed in the experiments to make full use of the ionic exchange resin. As a result, the purified solutions after ion exchange contain about 100 ppm Cl− or K+ ions. Nevertheless, after purification by the metathesis and ion exchange steps, the Cl− and K+ contents in the final products are both controlled within 100 ppm. For comparison, [C6mim]Br was also used as the starting cation source, and two ILs, [C6mim][BF4] and [C6mim][L-Ala], were prepared following the same procedure. The bromide contents (337 and 184 ppm) in these two ILs are about 3−5 times those of the chloride values in the ILs obtained from the [C6mim]Cl precursor. This is a consequence of the higher solubility of KBr than that of KCl in the reaction system. The halogen contents in the ILs obtained from the traditional two-step method were usually 10 000−20 000 ppm (1−2%) for Cl−35,55,56 and 30 000−60 000 ppm (3−6%) for Br−,35,56 and the values could reach up to 32.7%35 if the solubility difference between MY (“Y” is the anion of ILs) and MX was inadequate. Thus, the residual halide came from unreacted stating materials, which implies the uncompleted metathesis reaction between the reactants.55 The concentration of alkali metal residual resulting from the dissolved MX in ILs was lower but still reached 1000−1500 ppm (0.1−0.15%).55 Although halogen and alkali metal impurities can be removed by being washed with deionized water, it is simple and effective for hydrophobic ILs. For hydrophilic ILs, the removal of these impurities is more complex, and the ILs should be either chilled to form a biphasic system (e.g., [C4mim][BF4]) or dissolved in a water immiscible solvent (e.g., CH2Cl2). However, these cumbersome techniques usually lead to significant loss of ILs (around 30% for both cation and anion)57 and incomplete removal of inorganic impurities (around 600 ppm halogen and 500 ppm alkali metal).58 Therefore, halogen and alkali metal impurities in the ILs prepared with the method in this study are far less than those obtained from the traditional two-step method. Yields of Cation and Anion. When preparing ILs on a large scale, the raw material cost predominantly controls the price of ILs especially in industrial production. Besides the price of the cation and anion sources, the raw material cost is also decided by the yields of cation and anion. The yield herein is defined as follows: yield =

mol of cation or anion in production ·100% mol of cation or anion in material

It should be mentioned that the yield was always simply referred to as that of the cation (Ycation) in previous literature.58−61 The yield of the anion (Yanion), although not clearly pointed out, was less than or equal to Ycation since excess or an 6877

dx.doi.org/10.1021/ie500086r | Ind. Eng. Chem. Res. 2014, 53, 6871−6880

Industrial & Engineering Chemistry Research

Article

shown in eq 8, equilibrium can be represented by the follow equation.62

equivalent anion source was used in most synthesis processes.59,60 This is acceptable on a laboratory scale, but more detailed information about both Ycation and Yanion should be given in commercial production.16 The yields of metathesis (YM), ion exchange (YI), and neutralization (YN) steps are also shown in Table 2. According to stoichiometry, for a multistep process, the overall yield of the process can be calculated by multiplying the yield of each step. Therefore, Ycation and Yanion can be calculated as follows:

Ycation = YMYIYN

(13)

Yanion = YN

(14)

−

X − = K OH

[R ‐ X][OH−] [R ‐ OH][X−]

(15)

−

where KXOH− is the selectivity coefficient for a particular ion exchange reaction, [R-X] is the concentration of halide ion (X−) in the resin (mol/L), [R-OH] is the concentration of OH− in the resin (mol/L), [OH−] is the concentration of OH− in the solution (mol/L), and [X−] is the concentration of X− in the solution (mol/L). − KXOH− values are quite different for different halide ions but

Table 2 presents that Ycation is around 80% for [N2221]+ and 85% for [C6mim]+. The cations are mainly lost in the cation exchange step. The yield difference between [N2221]+ and [C6mim]+ verifies once again that the loss of cation decreases with increasing size of the cation. Yanion is as high as around 95%, which may be ascribed to the high purity of onium hydroxide (without excess of KOH) and the facile neutralization. In the traditional two-step preparation of ILs, the yield of anion is strongly dependent on the type of anion source. The yield can exceed 85%58,61 if silver salts are used as anion sources. However, this route is unfortunately limited by the exorbitant price of silver salts and the high-polluting byproduct. Alternatively, alkali metal salts are low in price and pollution, but the yields of anion generally range from 70 to 80%60 and decrease to 50−70% if the desired ILs are highly soluble in water, not to mention the fact that this route always leaves considerable contaminations in the ILs as discussed in the Introduction. Hence, the method proposed in this paper is of potential in minimizing the consumption of both cation and anion sources and in elevating the purity of ILs. Comparison of the Present Route with Two Simplified Ones. As stated in the Introduction, hydroxide intermediates have been synthesized by metathesis or ion exchange. The routes, metathesis + neutralization and ion exchange + neutralization, can be regarded as two simplified ones compared with the route studied in this study. However, these two simplified routes suffered from a few disadvantages in large-scale production of ILs. When hydroxide intermediates are synthesized only by metathesis, excessive KOH is needed to lower the halide content according to eq 1. However, this leads to unwanted consumption of anion source in the neutralization step, not to mention that the halide content is still too high. For example, onium chlorides reacted with excess KOH, and the resulting onium hydroxide−KOH solution was directly neutralized with the acid of the desired anion (KOH:NCl:HY = 1.2:1:1.2). The potassium salt (KY) byproduct was crystallized out from the IL solution after the evaporation of ethanol, removed by filtration, and finally discarded. Three amino acid ILs ([N1114][Gly], [N2221][Gly], and [Bmim][Gly]) were synthesized following this procedure, and the Cl− contents were 699, 700, and 738 ppm, respectively. Gao et al.35 prepared a series of ILs from [Cnmim]Br precursors by setting the molar ratio of KOH to [Cnmim]Br at 1.5. The Br− contents in their final products were in the range 1700−6900 ppm. When hydroxide intermediates are synthesized only by ion exchange, a tremendous amount of anion exchange resin is required. This can be roughly explained according to the ion exchange equilibrium theory. For the ion exchange reaction

−

−

−

Br I are in the range 20−200. For example, KCl OH−, KOH−, and KOH− 63 are 22, 50, and 175, respectively for Diaion A-101D resin. If X− needs to be controlled within a low concentration, such as 100 ppm (i.e., [X−]/[OH−] = 10−4), [R-X] can be calculated from [R-OH] according to eq 15, since [R-OH] is approximately the exchange capacity of the resin (1.4 mol/L for 201×7). Since most halide ions are exchanged onto the resin, the required volume of the resin can be calculated as follows.

V≈

n [R ‐ X]

(16)

where V is the required volume of resin (L) and n is the moles of onium halide (mol). According to the information above, to prepare 0.1 mol of hydroxide intermediate with the halide ion concentration lower than 100 ppm only by ion exchange, the required volume, V, is estimated to be in the range 3.57−35.7 L (depending on the type of halide). This agrees well with the experimental observation in our previous work49 during the preparation of tetraalkylammonium hydroxide only by ion exchange. However, the required resin volume in the present route is lowered to less than 100 mL, being only 1/35 to 1/350 of the amount in the simplified route (only by ion exchange). Since the regeneration of ion exchange resin is reagent- and time-consuming, the method proposed in this paper is much more efficient and economical.

■

CONCLUSIONS A general preparation route consisting of metathesis, ion exchange, and acid/base neutralization steps was described to prepare ILs with high purity via hydroxide intermediates. The reactions in the route were investigated in detail in this paper. Multiple parameters can be flexibly adjusted to reach the purity and cost requirements, and the route is generally applicable for the preparation of various ILs. A series of ILs were then prepared under the optimum conditions. The yields of both cations and anions (>80 and >95%, respectively) were much higher than those in the traditional two-step routes, while the concentrations of halogen and alkali metal residues are far lower (