Stability of Superoxide Ion in Phosphonium-Based Ionic Liquids

Feb 3, 2015 - In this work the chemical generation of superoxide ion and determination of its stability in five phosphonium-based ionic liquids has be...
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Stability of Superoxide Ion in Phosphonium-Based Ionic Liquids Omar U. Ahmed,† Farouq S. Mjalli,*,† Talal Al-Wahaibi,† Yahya Al-Wahaibi,† and Inas M. AlNashef‡ †

Petroleum and Chemical Engineering Department, Sultan Qaboos University, Al Khoudh, Muscat 123, Sultanate of Oman Department of Chemical and Environmental Engineering, Masdar Institute of Science and Technology, Masdar City, Abu Dhabi, United Arab Emirates

‡

S Supporting Information *

ABSTRACT: In this work the chemical generation of superoxide ion and determination of its stability in five phosphoniumbased ionic liquids has been carried out. The stability of the generated superoxide ion depended on the anion. For the trihexyl(tetradecyl)phosphonium cation, the bis(2,4,4-trimethylpentyl)phosphinate anion (IL 104) has shown a relatively good stability with a rate constant of 3.34 × 10−5 s−1 for the reaction of the superoxide ion. Triisobutyl(methyl)phosphonium tosylate has also shown moderate stability (6.8 × 10−5 s−1). The order of stability, bis(2,4,4-trimethylpentyl)phosphinate > dicyanamide (6.97 × 10−5 s−1) > Br− (7.72 × 10−5 s−1) > Cl− (12.7 × 10−5 s−1), correlates well with the order of their respective ionic volumes. On application of the generated superoxide ion for the oxidation of two organic sulfur compounds, 15% conversion of thiophene was attained in 2 h while dibenzothiophene (DBT) was found to be unreactive to the ion in IL 104. This was attributed to higher electron density on the sulfur atom in DBT relative to thiophene and high nucleophilicity of the superoxide ion. Furthermore, the type of IL appears to slightly affect the conversion. The conversion of thiophene obtained was in the following order: IL 104 (15%) > [HMPyrr][TFSI] (8%) > [BMPyrr][TFSI] (7%) with the apparent differences in the magnitude of the alkyl chain length. highest reactivity.2 This allows the ODS to be well-placed downstream from an HDS unit. Furthermore, ODS can be conveniently integrated with extraction, adsorption, and distillation,3 thereby making ODS a flexible system. The type of oxidant, catalyst, and assistance (ultrasound, photo, or radiation) employed all add up to this flexibility as an additional degree of freedom. Hydrogen peroxide is the commonly used oxidant in ODS studies. It is a powerful oxidant with a redox potential of E° = 1.78 V vs SHE (standard hydrogen electrode) and generates water as the only byproduct of the reaction with sulfur compounds. However, H2O2 requires catalyst activation. It cannot be stored or shipped at a concentration greater than 52%, while all on-site methods of production, which include pyrolysis of water, electrochemically and biochemically, can only produce low concentrations (99%) and 1-hexyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([HMPyrr][TFSI], >99%) were supplied by io-li-tec (Germany). Dimethyl sulfoxide (AGR, 99.99%) and acetonitrile (high-performance liquid chromatography (HPLC) grade, 99.99%) were supplied by Fischer Scientific, while KO2 (96.5%) was supplied by Alfa Aesar. 2.2. Stability Experiments. To generate the superoxide ion, a total of 0.1 g of KO2 at a rate of 25 mg/min was added into 10 g of dimethyl sulfoxide (DMSO) in a screw-capped vial under vigorous mixing using a magnetic stirrer. After 30 min, the mixture was centrifuged at the rate of 1000 rpm for 10 min so as to separate any undissolved KO2. The upper clear phase was withdrawn for further analysis. The clear phase (solution) obtained above was diluted with DMSO at the mass ratio of 1:1.5−1:1.6 (solution/DMSO) depending on the initial absorbance of the solution. This is to reduce the concentration of the superoxide ion in DMSO. An appropriate amount of the IL under investigation was added to the solution to achieve a mass ratio of 0.01:1 (IL/solution), and it was tagged sample S. An appropriate amount of the same IL was added to 10 g of DMSO to achieve a mass ratio of 0.01:1 (IL/DMSO) to give sample R. The concentration of the generated superoxide ion can be evaluated as Abs/(ε260·l) ( DCA− > Br− > Cl−. This correlates well with the order of their respective ionic volumes, which are 488, 72, 36, and 24 Å3, respectively.25 Anions with smaller ionic volume are expected to be more electronegative, and because the K+ ion is generated alongside the superoxide ion, a strong interaction is more likely going to be formed with the more electronegative anion. Phosphonium ILs were reported to be relatively unsuitable for the generation of superoxide ion (Table 2). However, the ILs investigated in this work appear to provide a better stability for the superoxide ion compared to the previously investigated phosphoniumbased ILs. Relative to imidazolium- and pyrrolidinium-based ILs shown in Table 2, IL 104 can be categorized as moderately suitable for the generation of a stable superoxide ion depending on the anion. To have a better understanding of the reaction of the superoxide ion with the IL, GC-MS analysis was carried out for samples taken before and after the dissolution of KO2, and the results are presented in Figures 3 and 4, respectively, for IL 101. In Figures 3 and 4, peaks at retention times 5.5, 8.0, 8.8, 12.1, and 12.4 min in Figure 4 are silicon oxides and therefore are believed to be from the column material. Peaks at retention times 14.5, 14.7, 15.5, and 40.2 min in Figure 3 are absent in Figure 4. This may be attributed to possible reaction with the superoxide ion. However, none of these peaks represents the components of IL 101, suggesting that these are products of reaction with impurities. This is not strange because the purity of the IL as supplied is ≥95%.

Table 1. Stability of Superoxide Ion in ILs ILs IL IL IL IL IL

101 102 104 105 106

k′ (× 105 s−1)

R2

% O2· − conversiona

12.70 7.72 3.34 6.97 6.80

0.9949 0.9985 0.9921 0.9704 0.9993

62.00 40.99 21.90 42.34 38.26

Conversion after 120 min, k′ = observed rate constant, O2·‑ = superoxide ion.

a

Table 2. Values for the Rate Constant of the Reaction of Superoxide Ion with ILsa type

cation

anion

k′ (× 105 s−1)

imidazolium

[EMIm] [BMIm] [BDMIm]

phosphonium

[P14666]

pyrrolidinium

[BMPyrr]

[EtS] [HFP] [TPTP] [BF4] [TFSI] [TPTP] [TfO] [TFSI] [TFSI]

1.717 2.117 5.027 2.527 16.215 19.128 2.827 2.629 1.915

[HMPyrr] a

k′ = observed rate constant, [EtS] = ethylsulfate, [HFP] = hexafluorophosphate, [TPTP] = tris(pentafluoroethyl)trifluorophosphate, [BF4] = tetrafluoroborate, [TfO] = trifluoromethanesulfonate, and [TFSI] = bis(trifluoromethylsulfonyl)imide.

moderately stability to the superoxide ion. These values are comparable to those obtained for [BDMIm][TPTP] (butyldimethyl imidazolium tris(pentafluoroethyl)trifluorophosphate), which has a rate constant of 5.00 × 10−5 s−1. IL 104 appears to provide the highest degree of stability to the superoxide ion among the five tested ILs in this work. Its rate constant of 3.34 × 10−5 s−1, is comparable to some of the pyrrolidinium-based ILs shown in Table 2.

Figure 3. GC chromatogram for the Cyphos IL 101 sample before the addition of KO2. D

DOI: 10.1021/ie504893k Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. GC chromatogram for the Cyphos IL 101 sample after the addition of KO2.

refractory sulfur compound was low, reaching 15% after 2 h of reaction time. To ensure that this low rate of reaction is not as a result of competitive reaction with the impurities in IL 104, [BMPyrr][TFSI] and [HMPyrr][TFSI] were used as the medium for the oxidation of both thiophene and dibenzothiophene because these ILs have been reported to provide great stability for generated superoxide ion. As could be seen in Table 3, dibenzothiophene was found to be virtually unreactive to

The products of the reaction of the cation with the superoxide ion cannot be deduced directly from the GC-MS result. The product of this reaction is likely nonvolatile and thereby cannot be determined using the current method or GC-MS in general. 4.2. Superoxide Ion As an Oxidant for Oxidative Desulfurization. To evaluate efficiency of the generated superoxide ion to oxidize refractory sulfur compounds in ILs, eq 5 was used to calculate the conversion of the sulfuric compounds. Furthermore, IL 104, [BMPyrr][TFSI], and [HMPyrr][TFSI] were chosen as they provided higher stability of the superoxide ion. The conversion is defined as Conversion = (C0 − C)/C0 × 100

Table 3. Performance of Superoxide Ion on Oxidation of Refractory Sulfur Compounds Conversion (%)

(5)

dibenzothiophene thiophene

where C0 is the initial concentration (1000 ppm) and C is the concentration after time t. Figure 5 shows the % Conversion of thiophene with time in IL 104. It could be seen that, although the superoxide ion was generated and stable in IL 104, the rate of reaction with

IL 104

[HMPyrr][TFSI]

[BMPyrr][TFSI]

15.0

7.3

6.3

superoxide ion while thiophene was slightly reactive in the three ILs chosen. This can be explained through the difference in electron density of the two sulfur compounds. The sulfur atom attached to the aromatic ring in thiophenic compounds draws electrons toward itself and thereby possesses relatively high electron density. According to Otsuki et al., the electron densities on the sulfur atoms of thiophene and dibenzothiophene are 5.696 and 5.758, respectively.2 In other words, dibenzothiophene is relatively more nucleophilic than thiophene, and because superoxide ion is a strong nucleophile, thiophene is relatively more susceptible to attack by the anion. This places superoxide ion as a better oxidant for sulfur compounds with lower electron density on the sulfur atom and electrophilic compounds. Chlorobenzenic compounds, owing to the chlorine atom, have been reported to react readily with superoxide.15 Tetrachloromethane and dimethyl disulfide (RSSR), which has lower electron density than thiophene, also react with superoxide ion.26 There is also an indication of the slight influence of IL in the reaction with regards to thiophene in the order 1L104 >

Figure 5. Conversion of thiophene with time in Cyphos IL 104 using superoxide ion. E

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(2) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.; Ishihara, A.; Imai, T.; Kabe, T. Oxidative Desulfurization of Light Gas Oil and Vacuum Gas Oil by Oxidation and Solvent Extraction. Energy Fuels 2000, 14, 1232−1239. (3) Babich, I. V.; Moulijn, J. A. Science and Technology of Novel Processes for Deep Desulfurization of Oil Refinery Streams: A Review. Fuel 2003, 82 (6), 607−631. (4) Chan, N. Y.; Lin, T.-Y.; Yen, T. F. Superoxides: Alternative Oxidants for the Oxidative Desulfurization Process. Energy Fuels 2008, 22 (5), 3326−3328. (5) Sawyer, D. T.; J, L. R. Hydroxide ion: An effective one-electron reducing agent? Acc. Chem. Res. 1988, 21 (12), 469−476. (6) AlNashef, I. M.; Hashim, M. A.; Mjalli, F. S.; Hayyan, M. Benign Degradation of Chlorinated Benzene in Ionic Liquids. Int. J. Chem., Environ. Biol. Sci. 2013, 1 (1), 201−206. (7) Jeon, S.; Sawyer, D. T.; Tsang, P. K. S. Methods for Producing Superoxide Ion In Situ. U.S. Patent 5143710 A, 1992. (8) Sawyer, D. T.; Calderwood, T. S.; Yamaguchi, K.; Angelis, C. T. Synthesis and Characterization of Tetramethylammonium Superoxide. Inorg. Chem. 1983, 22 (18), 2578−2583. (9) Merritt, M. V.; Sawyer, D. T. Electrochemical studies of the Reactivity of Superoxide Ion with Several Alkyl Halides in Dimethyl Sulfoxide. J. Org. Chem. 1970, 35 (7), 2157−2159. (10) Moorcroft, M. J.; Hahn, C. E. W.; Compton, R. G. Electrochemical Studies of the Anaesthetic Agent Enflurane (2Chloro-1,1,2-Trifluoroethyl Difluoromethyl Ether) in the Presence of Oxygen: Reaction with Electrogenerated Superoxide. J. Electroanal. Chem. 2003, 541, 117−131. (11) Valentine, J. S.; Curtis, A. B. Convenient Preparation of Solutions of Superoxide Anion and the Reaction of Superoxide Anion with a Copper(II) Complex. J. Am. Chem. Soc. 1975, 97 (1), 224−226. (12) Johnson, R. A.; Nidy, E. G. Superoxide chemistry. Convenient synthesis of dialkyl peroxides. J. Org. Chem. 1975, 40 (11), 1680− 1681. (13) Peters, J. W.; Foote, C. S. Chemistry of superoxide ion. II. Reaction with hydroperoxides. J. Am. Chem. Soc. 1976, 98 (3), 873− 875. (14) Evans, R. G.; Klymenko, O. V.; Saddoughi, S. A.; Hardacre, C.; Compton, R. G. Electroreduction of Oxygen in a Series of Room Temperature Ionic Liquids Composed of Group 15-Centered Cations and Anions. J. Phys. Chem. B 2004, 108, 7878−7886. (15) Hayyan, M.; F S Mjalli, M. A. H.; AlNashef, I. M.; Al-Zahrani, S. M.; Chooi, K. M. Long Term Stability of Superoxide Ion in Piperidinium, Pyrrolidinium and Phosphonium Cations-Based Ionic Liquids and its Utilization in the Destruction. J. Electroanal. Chem. 2012, 664, 26−32. (16) Hayyan, M.; Mjalli, F. S.; Hashim, M. A.; AlNashef, I. M.; Tan, X. M.; Chooi, K. Generation of Superoxide Ion in Trihexyl (Tetradecyl) Phosphonium Bis(Trifluoromethylsulfonyl) Imide Room Temperature Ionic Liquid. J. Appl. Sci. 2010, 10 (12), 1176− 1180. (17) AlNashef, I. M.; Hashim, M. A.; Mjalli, F. S.; Ali, M. Q.; Hayyan, M. A Novel Method for the Synthesis of 2-Imidazolones. Tetrahedron Lett. 2010, 51, 1976−1978. (18) Li, F. T.; Liu, Y.; Sun, Z. M.; Chen, L. J.; Zhao, D. S.; Liu, R. H.; Kou, C. G. Deep Extractive Desulfurization of Gasoline with xEt3NHCl·FeCl3 Ionic Liquids. Energy Fuels 2010, 24, 4285−4289. (19) Li, F. T.; Liu, R. H.; Wen, J. H.; Zhao, D. S.; Sun, Z. M.; Liu, Y. Desulfurization of Dibenzothiophene by Chemical Oxidation and Solvent Extraction with Me3NCH2C6H5Cl·2ZnCl2 Ionic Liquid. Green Chem. 2009, 11, 883−888. (20) Stojanovic, A.; Morgenbesser, C.; Kogelnig, D.; Krachler, R.; Keppler, B. K., Quaternary Ammonium and Phosphonium Ionic Liquids in Chemical and Environmental Engineering. In Ionic Liquids: Theory, Properties, New Approaches; Kokorin, A., Ed. InTech: Rijeka, Croatia, 2011; pp 657−680. (21) Wolff, M. O.; Alexander, K. M.; Belder, G. Uses of Quaternary Phosphonium Compounds in Phase Transfer Catalysis. Chim. Oggi 2000, 18 (1/2), 29−32.

[HMPyrr][TFSI] > [BMPyrr][TFSI]. The apparent difference in these ILs is the alkyl chain length, which is also in the same order. The effect of solvent is not strange as superoxide ion readily accepts a proton to generate ions such as HOO−. In general, any protic IL can lead to generation of these ions, which are reactive, and thereby aid in the oxidation of thiophene.

5. CONCLUSION The stability of superoxide ion in five commercially available phosphonium-based IL was investigated. IL 101 (Cl− anion) provided the least stability for the generated superoxide ion with a rate constant of 12.70 × 10−5 s−1, while IL 104 (with anion bis(2,4,4-trimethylpentyl)phosphinate) has shown relatively good stability with a superoxide reaction rate of 3.34 × 10−5 s−1. The order of stability for the superoxide ion for the ILs with the trihexyl(tetradecyl)phosphonium cation is TMPP− > DCA− > Br− > Cl−. This correlates well with the order of their ionic volume. IL 104 can be considered suitable for the generation of superoxide ion and subsequent usage as an oxidant for oxidative desulfurization. KO2 was employed as an oxidant for the oxidation of dibenzothiophene and thiophene in Cyphos IL 106, [HMPyrr][TFSI], and [BMPyrr][TFSI]. Although dibenzothiophene appears to be unreactive to the generated superoxide ion, 15% conversion of thiophene was attained in 2 h. This was attributed to the relative difference in the electron densities on the sulfur atoms of thiophene and dibenzothiophene and nucleophilicity of superoxide ion. Furthermore, the type of solvent appears to have a slight impact on the conversion of thiophene. Thiophene conversion was to found decrease in the order IL 106 > [HMPyrr][TFSI] > [BMPyrr][TFSI], which corresponds to the magnitude of their alkyl chain length.

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ASSOCIATED CONTENT

S Supporting Information *

Plot of concentration of superoxide against time in the presence of ILs 02, 104, 105, and 106 and plots of data-fitted pseudofirst-order kinetic model for of ILs 02, 104, 105, and 106. Tables showing purity as well as freezing and glass transition temperature of used ILs. Table showing GC-MS and HPLC specifications. Table containing data used for fitting the pseudofirst-order kinetic model. Equation showing relationship between ln Abs and ln A. This material is available free of charge via the Internet at http://pubs.acs.org/

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support of The Research Council and Sultan Qaboos University, Muscat, Oman, under the project RC/ENG/PCED/12/02.

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REFERENCES

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DOI: 10.1021/ie504893k Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX