Ionic Liquids in Selective Oxidation: Catalysts and ... - ACS Publications

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Ionic Liquids in Selective Oxidation: Catalysts and Solvents Chengna Dai, Jie Zhang, Chongpin Huang,* and Zhigang Lei* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 266, Beijing 100029, China S Supporting Information *

ABSTRACT: Selective oxidation has an important role in environmental and green chemistry (e.g., oxidative desulfurization of fuels and oxidative removal of mercury) as well as chemicals and intermediates chemistry to obtain high-value-added special products (e.g., organic sulfoxides and sulfones, aldehydes, ketones, carboxylic acids, epoxides, esters, and lactones). Due to their unique physical properties such as the nonvolatility, thermal stability, nonexplosion, high polarity, and temperature-dependent miscibility with water, ionic liquids (ILs) have attracted considerable attention as reaction solvents and media for selective oxidations and are considered as green alternatives to volatile organic solvents. Moreover, for easy separation and recyclable utilization, IL catalysts have attracted unprecedented attention as “biphasic catalyst” or “immobilized catalyst” by immobilizing metal- or nonmetal-containing ILs onto mineral or polymer supports to combine the unique properties of ILs (chemical and thermal stability, capacity for extraction of polar substrates and reaction products) with the extended surface of the supports. This review highlights the most recent outcomes on ILs in several important typical oxidation reactions. The contents are arranged in the series of oxidation of sulfides, oxidation of alcohols, epoxidation of alkenes, Baeyer− Villiger oxidation reaction, oxidation of alkanes, and oxidation of other compounds step by step involving ILs as solvents, catalysts, reagents, or their combinations.

CONTENTS 1. Introduction 2. Oxidation of Sulfides 2.1. Oxidative Desulfurization 2.1.1. ILs as Solvents or Extractants for Extraction and Oxidative Desulfurization 2.1.2. IL-Based Catalysts for Oxidative Desulfurization 2.1.3. ILs as Both Extractants and Catalysts for Extraction and Oxidative Desulfurization 2.2. Preparation of Sulfoxides and Sulfones 2.2.1. ILs as Solvents 2.2.2. ILs or IL-Based Catalysts 2.2.3. ILs as Reagents 3. Oxidation of Alcohols 3.1. ILs as Solvents 3.2. ILs or IL-Based Catalysts 4. Epoxidation of Alkenes 5. Baeyer−Villiger Oxidation 6. Oxidation of Alkanes 7. Oxidations of Other Compounds 7.1. Oxidation of Oximes 7.2. Oxidation of Benzene and Its Derivatives 7.3. Oxidation of Halides 7.4. Oxidative Removal of Mercury 8. Conclusions and Perspective Associated Content Supporting Information Author Information © 2017 American Chemical Society

Corresponding Authors ORCID Notes Biographies Acknowledgments References

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1. INTRODUCTION Ionic liquids (ILs) have attracted considerable attention due to their unique properties (such as nonvolatility, strong dissolution, low flash point, high thermal stability, nonexplosion, high polarity, relatively high electroconductivity, good catalytic activity, and easy operation at liquid state). Thus, ILs have become good alternatives to conventional volatile liquid solvents in chemical reaction and separation, which served as environmental recyclable reaction solvents and media to exhibit a rate acceleration effect on catalytic reactions.1−11 In many circumstances, for easy separation and recyclable utilization, IL catalysts have attracted unprecedented attention as “biphasic catalyst” or “immobilized catalyst” for the heterogeneous reactions and do not require an additional organometallic complex as catalyst. Various immobilized metal- or nonmetalcontaining ILs were considered as highly efficient catalysts, which combine the unique properties of ILs (i.e., chemical and

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Special Issue: Ionic Liquids Received: January 13, 2017 Published: May 1, 2017 6929

DOI: 10.1021/acs.chemrev.7b00030 Chem. Rev. 2017, 117, 6929−6983

Chemical Reviews

Review

2. OXIDATION OF SULFIDES

thermal stability, capacity for extraction of polar substrates and reaction products) with the extended surfaces of minerals or polymer supports.12−21 The oxidation reaction, an essential transformation from the standpoint of organic synthesis and industrial manufacturing, plays a key role in both environmental and synthetic processes. Oxidation reactions are the most important transformations in synthesis chemistry and offer an important methodology for the introduction and modification of functional groups. During the last two decades there has been a significant development in this field, and a large number of novel and useful oxidation reactions have been discovered. Different metals have been reported as catalysts for oxidation reactions, but in some cases, a stoichiometric amount of the metal or overstoichiometric oxidant is still necessary for higher efficiency of the reactions. Several works have been focused on developing greener catalytic oxidation processes: (1) the oxidant gives no waste as byproducts; (2) the used catalysts are in a small amount, inexpensive, having long-term stability, and easy to prepare, handle, regenerate, and recycle; (3) mild reaction conditions (near room temperature and atmospheric pressure) are needed; (4) no explosive or toxic volatile solvents are involved; and (5) the product separation is efficient and easy. Consequently, the potential applications of ILs for oxidation reactions as solvents and/or catalysts have been paid more attention by chemists and chemical engineers. Extensive studies on ILs have been carried out in selective oxidation during the past decade.22−26 Thus, this urges us to write this review in the special issue for ILs. This review highlights the most recent outcomes on ILs in selective oxidation, including the oxidation of sulfides, alcohols, alkanes, oximes, benzene and its derivatives, halides, and mercury, epoxidation of alkenes, and Baeyer−Villiger oxidation reactions. The ILs concerned in this review can be used as catalysts, reaction solvents, extractants, reaction reagents, or their combinations in oxidation processes. The meanings of abbreviations, names, and structures for ILs and other components throughout this review are given in Table S1 in the Supporting Information.

2.1. Oxidative Desulfurization

Sulfur-containing toxic compounds in fuel oil can be converted into sulfur oxides during the combustion of car engines, leading to serious environmental problems.27−32 To deal with increasingly critical atmospheric pollution, governments all over the world are imposing more stringent regulations to limit the sulfur concentration in transportation fuels. Since 2009, the sulfur content of gasoline in Europe must be less than 10 ppm.33−35 Thus, the deep desulfurization of fuel oil has become an environmentally urgent subject and attracted wide interest.36−40 At present, the most used process for the removal of sulfur compounds in industry is the hydrodesulfurization (HDS), which requires a high temperature and pressure. Furthermore, the HDS method is not effective to remove aromatic heterocyclic sulfur compounds, such as thiophene, dibenzothiophene (DBT), benzothiophene (BT), and their derivatives. Thus, several nonhydrodesulfurization strategies for deep desulfurization have been developed, such as extraction, adsorption, biodesulfurization, and oxidation. Among others, oxidative desulfurization as a promising method has attracted more attention. In 2001, ILs have been reported for the first time for the removal of sulfur compounds from diesel as extractants.41 Since then more and more attention has been focused on this application of ILs in oxidative desulfurization. This section aims at the removal of sulfur compounds in fuels using ILs as solvents (or extractants), catalysts, or their dual function. 2.1.1. ILs as Solvents or Extractants for Extraction and Oxidative Desulfurization. The technology of extraction and oxidative desulfurization (EODS) from fuels in ILs is developed to achieve a deep desulfurization level in which H2O2 is usually chosen as oxidant and ILs are used as both extractants and reaction media. In the process, sulfur compounds are first extracted from oil into IL phase and then oxidized by H2O2 in the presence of catalyst. Scheme 1 shows a typical procedure for the removal of DBT using H2O2 as oxidant. With the increase of oxidation time, the concentration of sulfurcontaining compounds (S compounds) in IL decreases. Then more S compounds are extracted from oil into IL phase, resulting in a continuous decrease of sulfur content in oil and achieving a deep desulfurization. 2.1.1.1. Molybdenum-Based Catalysts. The recent studies on extraction and oxidative desulfurization (EODS) using molybdenum-based (Mo) catalysts and ILs as solvents or extractants with H2O2 oxidant in model oil, real diesel, and gasoline are summarized in Table 1. For more details, please see Table S2 in the Supporting Information. Li et al.42 proposed using the peroxotungsten complex [WO(O2)2·Phenanthroline·H2O] and peroxomolybdenum complex [MoO(O2)2·Phenanthroline·H2O] as catalysts in three common ILs ([BMIM][BF4], [OMIM][BF4], and [OMIM][PF6]) for extraction and catalytic oxidation of DBT in model oil (n-octane). The sulfur removal efficiency over Mobased catalysts in IL solvent is mainly dependent on the hydrophilicity of ILs, water-miscible ILs (e.g., [BMIM][BF4]) exhibiting better performance. Moreover, as the alkyl side chain on the cation increases, the removal efficiency decreases. The sulfur removal can reach 98.6% in the case of WO(O2)2· Phenanthroline·H2O as catalyst, H2O2 as oxidant, and [BMIM][BF4] as extractant. However, a large amount of H2O2 (n(O):n(S) = 10:1) was required. In addition, W element

Scheme 1. Typical Extraction and Oxidative Desulfurization (EODS) in ILs for the Removal of Dibenzothiophene (DBT) Using H2O2 as Oxidant

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Table 1. Summary of EODS Using Mo-Based Catalysts and ILs as Extractants with H2O2 Oxidant entry

S compounds

oil

catalysts

ILs

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61

DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT BT BT BT BT BT BT 4,6-DMDBT 4,6-DMDBT 4,6-DMDBT 4,6-DMDBT 4,6-DMDBT 4,6-DMDBT 4,6-DMDBT

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

[MoO(O2)2·Phen·H2O] [MoO(O2)2· Phen·H2O] [MoO(O2)2·Phen·H2O] [MoO(O2)2·Phen·H2O] Na2Mo4·2H2O Na2Mo4·2H2O Na2Mo4·2H2O Na2Mo4·2H2O Na2Mo4·2H2O Na2Mo4·2H2O H2Mo4 (NH4)6Mo7O24·4H2O H3PMo12O40·13H2O (NH4)3PMo12O40·7H2O Na3PMo12O40·7H2O Na2Mo4·2H2O H2Mo4 [(C4H9)4N]3{PO4[MoO(O2)2]4} [C14H29N(CH3)3]3{PO4[MoO(O2)2]4} [C16H33NC5H5]3{PO4[MoO(O2)2]4} MoO(O2)2·C2H5NO2 MoO(O2)2·C2H5NO2 MoO(O2)2C3H7NO2·H2O MoO(O2)2C3H7NO2·H2O MoO(O2)2C5H9NO4·H2O MoO(O2)2C5H9NO4·H2O MoO(O2)2·C2H5NO2 MoO(O2)2·C2H5NO2 H3PMo12O40·26H2O [(C4H9)4N]6[Mo7O24] [(C4H9)4N]6[Mo7O24] [(C4H9)4N]6[Mo7O24] [(C4H9)4N]6[Mo7O24] [C4H9N]4NiMo6‑xWxO24H6 [C4H9N]4NiMo6O24H6 [C4H9N]4NiMo6O24H6 [(C4H9)4N]4[Mo8O26] [(C12H25)N(CH3)3]4[Mo8O26] [(C14H29)N(CH3)3]4[Mo8O26] [(C16H33)N(CH3)3]4[Mo8O26] [(C12H25)N(CH3)3]4[Mo8O26] [(C12H25)N(CH3)3]4[Mo8O26] [(C12H25)N(CH3)3]4[Mo8O26] [(C12H25)N(CH3)3]4[Mo8O26] [(C12H25)N(CH3)3]4[Mo8O26] [(C12H25)N(CH3)3]4[Mo8O26] [(C12H25)N(CH3)3]4[Mo8O26] [(C12H25)N(CH3)3]4[Mo8O26] Na2Mo4·2H2O MoO(O2)2·C2H5NO2 [(C4H9)4N]3{PO4[MoO(O2)2]4} H3PMo12O40·26H2O [(C4H9)4N]6[Mo7O24] [C4H9N]4NiMo6O24H6 Na2Mo4·2H2O MoO(O2)2·C2H5NO2 (NH4)6Mo7O24·4H2O (NH4)6Mo7O24·4H2O (NH4)6Mo7O24·4H2O (NH4)6Mo7O24·4H2O [(C4H9)4N]3{PO4[MoO(O2)2]4}

[BMIM][BF4] [OMIM][BF4] [BMIM][PF6] [OMIM][PF6] [BMIM][BF4] [OMIM][BF4] [BMIM][PF6] [OMIM][PF6] [BMIM][TA] [OMIM][TA] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][PF6] [BMIM][BF4] [BMIM][PF6] [BMIM][BF4] [BMIM][PF6] [OMIM][PF6] [OMIM][BF4] [BMIM][BF4] [OMIM][PF6] [BMIM][PF6] [BMIM][BF4] [BMIM][PF6] [BMIM][PF6] [BMIM][BF4] [OMIM][PF6] [(CH2)3SO3HMIM][BF4] [(CH2)3SO3HMIM][BF4] [(CH2)3SO3HMIM][BF4] [(CH2)3SO3HMIM][BF4] [(CH2)3SO3HPY][H2PO4] [(CH2)3SO3HPY][BF4] [(CH2)3SO3HPY][HSO4] [(CH2)3SO3HTEA][HSO4] [(CH2)3SO3HTEA][BF4] [(CH2)3SO3HTEA][H2PO4] [(CH2)3SO3HMIM][HSO4] [(CH2)3SO3HMIM][H2PO4] [BMIM][BF4] [BMIM][PF6] [BMIM][BF4] [BMIM][BF4] [BMIM][PF6] [BMIM][PF6] [BMIM][BF4] [BMIM][PF6] [MMIM][DMP] [EMIM][DEP] [BMIM][DBP] [MMIM][DMP] [BMIM][BF4]

42 42 42 42 43 43 43 43 43 43 43 43 43 43 43 43 43 44 44 44 45 45 45 45 45 45 45 45 46 48 48 48 48 49 49 49 51 51 51 51 51 51 51 51 51 51 51 51 43 45 44 46 48 49 43 45 50 50 50 50 44

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Table 1. continued entry

S compounds

oil

62 63 64 65 66 67 68 69 70 71 72 73 74 75

4,6-DMDBT 4,6-DMDBT 4,6-DMDBT 4-MDBT 4-MDBT thiophene thiophene thiophene thiophene thiophene thiophene thiophene 5-MBT actual diesel

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

catalysts H3PMo12O40·26H2O [(C4H9)4N]6[Mo7O24] [C4H9N]4NiMo6O24H6 [(C4H9)4N]6[Mo7O24] [C4H9N]4NiMo6O24H6 (NH4)6Mo7O24·4H2O H2MoO4 Na2MoO4·2H2O MoO3 H3PMo12O40 H3PMo12O40 H3PMo10V2O40 [C4H9N]4NiMo6O24H6 [C4H9N]4NiMo6O24H6

ILs [BMIM][BF4] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [HNMP][BF4] [HNMP][BF4] [HNMP][BF4] [HNMP][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][PF6] [BMIM][PF6]

ref 46 48 49 48 49 52 52 52 52 53 53 53 49 49

highest catalytic activity, with the result that 98% of DBT in model diesel (n-octane) can be removed at 30 °C after 3 h. However, DBT was hardly oxidized in the absence of ILs. The reactivity of different S compounds in model diesel was evaluated, and the sulfur removal efficiency followed the order of DBT > 4-MDBT > 4,6-DMDBT > BT > 5-MBT, indicating that the desulfurization performance is very sensitive to the electron density on sulfur atoms and the steric hindrance of substituted groups of S compounds. Moreover, the sulfur content in an actual commercial diesel can be reduced from 700 to about 30 ppm, and the sulfur removal ratio can achieve 96%. The desulfurization system for the actual commercial diesel can be recycled 10 times with an unnoticeable decrease in activity. If [(C4H9)4N]6Mo7O24 was used as catalyst, the sulfur removal of DBT could reach 99.0%. Shao et al. investigated the removal of 4,6-dimethyldienzothiophene (4,6-DMDBT) from a model oil (n-octane) using hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24· 4H2O) as catalyst and H2O2 as oxidant. A series of imidazolium-based phosphoric ILs ([MMIM][DMP], [EMIM][DEP], and [BMIM][DBP]) was used as extractants and reaction media.50 The removal of 4,6-DMDBT can reach 89.2% at 50 °C after 3 h with the initial S content of 100 ppm for the (NH4)6Mo7O24·4H2O/H2O2/[BMIM][DBP] system. It is worth noting that the removal ratio is only 9.29% using mere solvent extraction with IL and 5.34% using mere catalytic oxidation. Ge et al. evaluated four surfactant-type octamolybdates catalysts in acidic ILs for deep removal of organic sulfur in fuels.51 DBT could be 100% removed with the H2O2/S molar ratio of 8:1 using [(CH2)3SO3HMIM][BF4] as extractant and [(C12H25)N(CH3)3]4[Mo8O26] as catalyst. 2.1.1.2. Tungsten-Based Catalysts. Simple tungsten-containing compounds, tungstate-based materials with carbon chains as cations, PW(phosphotungstic) species, and other tungsten-based materials were widely used as catalysts for the oxidation of S compounds. The recent studies on extraction and oxidative desulfurization (EODS) using the tungsten-based (W) catalysts, ILs as solvents or extractants, and H2O2 as oxidant are summarized in Table 2. The detailed outcomes are listed in Table S3 in the Supporting Information for the interested reader to refer to. Liu et al.54 investigated the influence of halogen-free taskspecific ILs with different combinations of cations and anions on the S-compounds removal in the extraction process. The IL [(CH2)4SO3HMIM][Tos] shows the best extraction capacity,

would contaminate the oil phase and make a second pollution due to the amphiphilic feature of the catalyst. For this reason, they employed several Mo-based catalysts including commercially available molybdic compounds (e.g., Na2MoO4·2H2O, H 2 MoO 4 , (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, H 3 PMo 12 O 40 ·13H 2 O, (NH4)3PMo12O40·7H2O, and Na3PMo12O40·7H2O) and peroxophosphomolybdates (e.g., Q3{PO4[MoO(O2)2]4}, Q = [(C4H9)4N], [C14H29N(CH3)3], and [C16H33NC5H5]) for the removal of dibenzothiophene (DBT), benzothiophene (BT), and 4,6-dimethyldibenzothiphene (4,6-DMDBT) in a model oil (n-octane) by means of IL extraction combined with oxidation desulfurization, where H2O2 was used as oxidant.43,44 The removal of DBT can achieve 99.0% using Na2MoO4·2H2O and the water-miscible IL ([BMIM][BF4]) with a small molar ratio of H 2 O 2 to DBT (4:1). In the case of [C 14 H 29 N(CH3)3]3{PO4[MoO(O2)2]4} as catalyst, the removal of DBT can reach 97.3% with a stoichiometric amount of H2O2. However, the commercially available catalyst Na2MoO4·2H2O exhibits low sulfur removal in water-immiscible ILs, e.g., only 69.8% removal of DBT in [BMIM][PF6]. In 2011, the same group synthesized a series of peroxo−molybdenum amino acid complexes (PMAACs), and their catalytic performance for extraction and oxidative desulfurization using water-immiscible ILs as extractant was evaluated.45 The results showed that when MoO(O2)2·C2H5NO2 was used as catalyst, 99.2% of DBT could be removed in water-immiscible IL [BMIM][PF6] and the removal of DBT in water-miscible IL [BMIM][BF4] could reach 99.0%, indicating that the PMAACs catalysts are wideranging effective in different ILs. Moreover, the removal of BT and 4,6-DMDBT could also achieve 93.2% and 99.6%, respectively, under optimal operating conditions. Zhu et al. reported the use of phosphomolybdic acid (H3PMo12O40· 26H2O, PMo12) as catalyst and [BMIM][BF4] as extractant for the removal of sulfur compounds.46 The removal of DBT could achieve 99.4% in 1 h, which seems higher than that performed by Li et al. A series of Anderson-type polyoxometalate Q4NiMo6−xWxO24H6 (x = 0, 2, 4, 6; Q stands for the group C4H9N, C12H25(CH3)3N, C16H33(CH3)3N, or NH4) catalysts containing nickel (Ni), molybdenum (Mo), and tungsten (W) was employed together with ILs for the oxidation desulfurization using H2O2 as oxidant to remove benzothiophene (BT), dibenzothiophene (DBT), and their derivates in model and actual commercial diesels.47−49 The results showed that the catalyst [C4H9N]4NiMo6O24H6 in [BMIM][PF6] exhibits the 6932


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Table 2. Summary of EODS Using W-Based Catalysts and ILs as Extractants with H2O2 Oxidant entry

S compounds

oil

catalysts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61

DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT BT 4,6-DMDBT thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene BT 2,3,5-3MT 2,5-2MT 3-MT 2-MT DBT DBT DBT DBT BT 4,6-DMDBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

Na2WO4·2H2O Na2WO4·2H2O Na2WO4·2H2O Na2WO4·2H2O Na2WO4·2H2O Na2WO4·2H2O H2WO4 WO3 (NH4)3PWO12O40·3H2O H4SiWO12O40 Na4SiWO12O40 Na2W4·2H2O [WO(O2)2·Phenanthroline·H2O] [WO(O2)2·Phenanthroline·H2O] [WO(O2)2·Phenanthroline·H2O] [WO(O2)2·Phenanthroline·H2O] [(C4H9)4N]4W10O32 [(CH3)4N]4W10O32 [(C2H5)4NC7H7]4W10O32 Na4W10O32 [(C8H17)3NCH3]2W6O19 [(C8H17)3NCH3]2W6O19 [(C8H17)3NCH3]2W6O19 [(C8H17)3NCH3]2W6O19 [(C8H17)3NCH3]2W6O19 [(C8H17)3NCH3]2W6O19 (NH4)10H2(W2O7)6·H2O (NH4)10H2(W2O7)6·H2O (NH4)10H2(W2O7)6·H2O (NH4)10H2(W2O7)6·H2O (NH4)10H2(W2O7)6·H2O (NH4)10H2(W2O7)6·H2O H2WO4 Na2WO4·H2O WO3 H3PW12O40·xH2O (NH4)10H2(W2O7)6·H2O (NH4)10H2(W2O7)6·H2O (NH4)10H2(W2O7)6·H2O (NH4)10H2(W2O7)6·H2O (NH4)10H2(W2O7)6·H2O SiO2/NH3+/LaW10 SiO2/NH3+/LaW10 SiO2/NH3+/LaW10 SiO2/NH3+/LaW10 SiO2/NH3+/LaW10 SiO2/NH3+/LaW10 Na2WO4·2H2O + DDTMAB Na2WO4·2H2O + DDTMAB Na2WO4·2H2O + DDTMAB Na2WO4·2H2O + DDTMAB Na2WO4·2H2O + DDTMAB Na2WO4·2H2O + DDTMAB Na2WO4·2H2O + DDTMAB Na2WO4·2H2O + DDTMAB Na2WO4·2H2O + DDTMAB Na2WO4·2H2O Na2WO4·2H2O + TBAB Na2WO4·2H2O + DDTMAB Na2WO4·2H2O + TDTMAB Na2WO4·2H2O + CTMAB 6933

ILs

ref

[BMIM][BF4] [OMIM][BF4] [BMIM][PF6] [OMIM][PF6] [BMIM][TA] [OMIM][TA] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [(CH2)4SO3HMIM][Tos] [BMIM][BF4] [OMIM][BF4] [BMIM][PF6] [OMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][BF4] [OMIM][BF4] [BMIM][PF6] [OMIM][PF6] [OMIM][PF6] [OMIM][PF6] [BMIM][BF4] [HMIM][HSO4] [Hnmp][SO3CH3] [HMIM][BF4] [Hnmp][HSO4] [Hnmp][BF4] [Hnmp][BF4] [Hnmp][BF4] [Hnmp][BF4] [Hnmp][BF4] [Hnmp][BF4] [Hnmp][BF4] [Hnmp][BF4] [Hnmp][BF4] [Hnmp][BF4] [OMIM][BF4] [OMIM][PF6] [BMIM][BF4] [BMIM][PF6] [BMIM][BF4] [BMIM][BF4] [(CH2)3SO3HMIM][BF4] [(CH2)3SO3HMIM][HSO4] [(CH2)3SO3HMIM][H2PO4] [(CH2)3SO3HTEA][BF4] [(CH2)3SO3HTEA][HSO4] [(CH2)3SO3HTEA][H2PO4] [(CH2)3SO3HPy][BF4] [(CH2)3SO3HPy][HSO4] [(CH2)3SO3HPy][H2PO4] [(CH2)3SO3HMIM][BF4] [(CH2)3SO3HMIM][BF4] [(CH2)3SO3HMIM][BF4] [(CH2)3SO3HMIM][BF4] [(CH2)3SO3HMIM][BF4]

57 57 57 57 57 57 57 57 57 57 57 55 42 42 42 42 60 60 60 60 61 61 61 61 61 61 52 52 52 52 52 52 52 52 52 52 52 52 52 52 52 66 66 66 66 66 66 58 58 58 58 58 58 58 58 58 58 58 58 58 58


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S compounds

oil

62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115

DBT DBT DBT DBT BT 4,6-DMDBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT BT 4,6-DMDBT DBT DBT DBT BT 4,6-DMDBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT BT BT BT 4,6-DMDBT 4,6-DMDBT 4,6-DMDBT real diesel real gasoline light gas oil light gas oil light gas oil

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

catalysts WC WC WC WC WC WC 30%HPW-CeO2(100) 30%HPW-CeO2(200) 30%HPW-CeO2(300) 30%HPW-CeO2(400) 30%HPW-CeO2(500) 20%HPW-CeO2(400) 10%HPW-CeO2(400) 30%HPW-CeO2(400) 30%HPW-CeO2(400) 30%HPW-CeO2(400) 30%HPW-CeO2(400) 30%HPW-CeO2(400) Na7H2LaW10O36 LaW10 EuW10 LaW10 LaW10 EuW10 EuW10 EuW10 LaW10 LaW10 LaW10 YW10 CeW10 NdW10 SmW10 GdW10 TbW10 ErW10 YdW10 (DODA)9LaW10 (DODA)9LaW10 (DODA)9LaW10 (DODA)9LaW10 (DDA)9LaW10 (TSA)9LaW10 (DODA)9LaW10 (DDA)9LaW10 (TSA)9LaW10 (DODA)9LaW10 (DDA)9LaW10 (TSA)9LaW10 Na2W4·2H2O [(C8H17)3NCH3]2W6O19 TPA TPA TPA

ILs

ref

[BMIM][BF4] [OMIM][BF4] [BMIM][PF6] [OMIM][PF6] [OMIM][PF6] [OMIM][PF6] [OMIM][BF4] [OMIM][BF4] [OMIM][BF4] [OMIM][BF4] [OMIM][BF4] [OMIM][BF4] [OMIM][BF4] [BMIM][BF4] [OMIM][PF6] [BMIM][PF6] [OMIM][BF4] [OMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [OMIM][BF4] [OMIM][PF6] [BMIM][PF6] [OMIM][BF4] [OMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][BF4] [OMIM][PF6] [OMIM][BF4] [OMIM][PF6] [OMIM][PF6] [OMIM][PF6] [OMIM][PF6] [OMIM][PF6] [OMIM][PF6] [OMIM][PF6] [OMIM][PF6] [(CH2)4SO3HMIM][Tos]/NMP [OMIM][PF6] [BMIM][BF4] [BMIM][PF6] [OMIM][PF6]

59 59 59 59 59 59 64 64 64 64 64 64 64 64 64 64 64 64 53 53 53 53 53 53 53 53 53 53 53 53 53 53 53 53 53 53 53 65 65 65 65 65 65 65 65 65 65 65 65 56 61 63 63 63

dissolved in [(CH2)4SO3HMIM][Tos].55 The sulfur content could be reduced from the initial 500 to 3 μg·g−1 at 323 K, and the removal ratio can achieve 99.4%. A possible mechanism that the new peroxytungstate−IL complex may be generated to make the catalytic system homogeneous was proposed. In 2013,

and the sulfur content in real hydrotreated diesel can be reduced from 438 to 45 μg·g−1. For a further removal of S content, they proposed using oxidative desulfurization to remove dibenzothiophene (DBT) from model diesel (ntetradecane) by oxidation with H2O2 using Na2WO4·2H2O 6934


Chemical Reviews

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Scheme 2. Mechanism for the Oxidation of DBT Using HPW-CeO2 Catalyst and IL Extractanta

a

(A) Before oxidation (original oil); (B) during oxidation; (C) after oxidation (clean oil); and (D) catalytic oxidation process of DBT. Adapted from from ref 64 with permission. Copyright 2013 Elsevier.

the same group studied the catalytic performance of Na2WO4 catalyst in the IL ([(CH2)4SO3HMIM][Tos]) for deep desulfurization of real diesel.56 For the oxidative desulfurization of real diesel, not all sulfur compounds are in the IL phase after reaction, and partial sulfur compounds still exist in the diesel phase. The main sulfur compounds in diesel phase after oxidation are sulfones, i.e., the oxidation products of alkyl derivatives of DBT. Thus, an additional extraction procedure was required to extract the remaining sulfones from diesel with NMP (N-methyl-2-pyrrolidinone). It was found that the sulfur content in the real diesel could decrease from 200 to 23.2 ppm, and the removal ratio could reach 88.4%. On the other hand, the catalytic performance of Na2WO4· 2H2O catalyst using conventional imidazolium-based ILs as extractant for extraction and oxidative desulfurization was explored.57 The removal of DBT could reach 99.7% at 60 °C for 3 h. Zhang et al. investigated several commercial tungstate catalysts in Brønsted acidic ILs for the removal of thiophene, BT, and their derivatives from model oil using H2O2 as oxidant.52 Sulfur content in model oil containing BT could be reduced from 700 to less than 1 ppm using ammonium tungstate catalyst in [Hnmp][BF4], and the removal of thiophene could also be achieved up to 99%. Ge et al. studied

Scheme 3. Proposed Extraction and Oxidative Desulfurization Process Using SiO2/NH3+/LaW10 Catalyst in ILa

a

Reprinted with permission from ref 66. Copyright 2013 Elsevier.

6935


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Table 3. Summary of EODS Using Other Catalysts and ILs as Extractants entry

S compounds

oil

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61

DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT BT 4,6-DMDBT DBT DBT DBT DBT BT 4,6-DMDBT DBT DBT DBT DBT DBT DBT DBT thiophene thiophene thiophene thiophene thiophene gasoline DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT 4,6-DMDBT 4,6-DMDBT 4,6-DMDBT 4,6-DMDBT 4,6-DMDBT 4,6-DMDBT

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

catalysts FeCl3 FeCl3 FeCl3 FeCl3 CoCl2·6H2O CuCl2·2H2O NiCl2·6H2O MnCl2·4H2O CrCl3·6H2O FeCl2·4H2O Fe(NO3)3·9H2O Fe2(SO4)3 FeCl3 FeCl3 FeCl3 [(CH3)4N]FeCl4 [C14H29N(CH3)3FeCl4 [C16H37N(CH3)3FeCl4 [(CH3)4N]FeCl4 [(CH3)4N]FeCl4 [(CH3)4N]FeCl4 [(CH3)4N]FeCl4 [(CH3)4N]FeCl4 [BMPIP]FeCl4 [BMPIP]FeCl4 [BMPIP]FeCl4 [BMPIP]FeCl4 V2O5 MTO MTO MTO MTO MTO [EMIM]3Fe(CN)6 VO(acac)2 VO(acac)2 VO(acac)2 VO(Cl-acac)2 VO(Cl-acac)2 VO(Cl-acac)2 VO(Cl-acac)2 VO(Me-acac)2 VO(Me-acac)2 VO(Me-acac)2 VO(Me-acac)2 VO(Me-acac)2 VO(Me-acac)2 VO(Et-acac)2 VO(Et-acac)2 VO(Et-acac)2 VO(Et-acac)2 VO(Et-acac)2 VO(Et-acac)2 VO(Me-acac)2 VO(Me-acac)2 VO(acac)2 VO(acac)2 VO(acac)2 VO(Me-acac)2 VO(Me-acac)2 VO(Me-acac)2 6936

ILs [BMIM][BF4] [OMIM][BF4] [BMIM][PF6] [OMIM][PF6] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][PF6] [OMIM][BF4] [OMIM][PF6] [BMIM][BF4] [BMIM][PF6] [OMIM][BF4] [OMIM][PF6] [Hnmp][BF4] [BMIM][BF4] [OMIM][BF4] [BMIM][PF6] [OMIM][PF6] [BMIM][BF4] [BMIM][BF4] [BMIM][Tf2N] [BM2IM][Tf2N] [BMPY][Tf2N] [BMIM][BF4] [BMIM][TfO] [BMIM][BF4] [BMIM][TfO] [BMIM][BF4] [BMIM][TfO] [BMIM][Tf2N] [BMIM][PF6] [BBIM][Tf2N] [BMPy][Tf2N] [BMIM][BF4] [BMIM][TfO] [BMIM][Tf2N] [BMIM][PF6] [BBIM][Tf2N] [BMPy][Tf2N] [BMIM][Tf2N] [BMIM][PF6] [BMIM][Tf2N] [BMIM][PF6] [BBIM][Tf2N] [BMIM][Tf2N] [BMIM][BF4] [BMIM][TfO]

oxidants H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

ref 77 77 77 77 77 77 77 77 77 77 77 77 77 77 78 78 78 78 78 78 78 78 78 79 79 79 79 52 69 69 69 69 69 80 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82


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S compounds

oil

catalysts

ILs

oxidants

ref

62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102

4,6-DMDBT 4,6-DMDBT 4,6-DMDBT 4,6-DMDBT BT BT BT BT BT BT BT BT BT DBT diesel DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT BT 4,6-DMDBT TS DBT thiophene MT BT 4,6-DMDBT real gasoline DBT DBT BT 4,6-DMDBT

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

VO(Et-acac)2 VO(Et-acac)2 VO(Et-acac)2 VO(acac)2 VO(acac)2 VO(acac)2 VO(acac)2 VO(Me-acac)2 VO(Me-acac)2 VO(Me-acac)2 VO(Et-acac)2 VO(Et-acac)2 VO(Et-acac)2 CH3COOH CH3COOH [PSPy]3PW [PSPy]3PW [PSPy]3PW [PSPy]3PW [C3MIM]FeCl4-MCM-41 [C3MIM]FeCl4-MCM-41 [C3MIM]FeCl4-MCM-41 [C3MIM]FeCl4-MCM-41 [C3MIM]FeCl4−SBA-15 [C3MIM]FeCl4−SBA-15 [C3MIM]FeCl4−SBA-15 [C3MIM]FeCl4−SBA-15 MnO2 MnO2 MnO2 MnO2 CoPc(Cl)n CoPc(Cl)16 CoPc(Cl)16 CoPc(Cl)16 CoPc(Cl)16 CoPc(Cl)16 CoPc(Cl)16 NHPI NHPI NHPI

[BMIM][Tf2N] [BMIM][BF4] [BMIM][TfO] [BMIM][Tf2N] [BMIM][Tf2N] [BMIM][PF6] [BBIM][Tf2N] [BMIM][Tf2N] [BMIM][BF4] [BMIM][TfO] [BMIM][Tf2N] [BMIM][BF4] [BMIM][TfO] [BzMIM][Tf2N] [BzMIM][Tf2N] [BMIM][BF4] [BMIM][PF6] [OMIM][BF4] [OMIM][PF6] [BMIM][BF4] [OMIM][BF4] [BMIM][PF6] [OMIM][PF6] [OMIM][BF4] [BMIM][BF4] [BMIM][PF6] [OMIM][PF6] [BMIM][Ac] [BMIM][Ac] [BMIM][Ac] [BMIM][Ac] [PBy][BF4] [PBy][BF4] [PBy][BF4] [PBy][BF4] [PBy][BF4] [PBy][BF4] [PBy][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4]

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 DBD plasma DBD plasma DBD plasma DBD plasma O2 O2 O2 O2 O2 O2 O2 O2 O2 O2

82 82 82 82 82 82 82 82 82 82 82 82 82 84 84 86 86 86 86 87 87 87 87 88 88 88 88 89 89 89 89 90 90 90 90 90 90 90 91 91 91

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

the desulfurization efficiency using the Na2WO4·2H2O catalyst in acidic IL ([(CH2)3SO3HMIM][BF4]) with the addition of amphiphilic quaternary ammonium salts as phase transfer catalyst (PTC).58 The sulfur compound may be virtually completely removed from model oil. Xu et al.59 studied the extraction with [OMIM][PF6] in conjunction with adsorption and catalytic oxidation over tungsten carbide catalyst. The ultradeep removal of DBT could be achieved in model oil. During the process, H2O2 first reacted with WC to form [perW] center, which dispersed well in IL phase. DBT was extracted into IL phase and adsorbed onto per-W center, and then DBT was oxidized to sulfones, which easily escaped into IL phase due to their strong polarity. Li et al.60 synthesized three decatungstates catalysts for oxidative desulfurization with [BMIM][PF6] as extractant and H2O2 as oxidant. When using [(C4H9)4N]4W10O32 as catalyst, deep desulfurization could be achieved with the sulfur content of 8 ppm in model oil under the optimal operating conditions.

After reaction, the tungsten content in oil phase was 7.4 ppm for the [(C4H9)4N]4W10O32 catalyst, that is, about 0.7% of the catalyst was leaching. In 2011, a water-in-IL emulsion system was proposed for the catalytic oxidative desulfurization in a three-phase reaction system, where the hexatungstates acted as catalyst, hydrophobic IL [OMIM][PF6] as extractant, and H2O2 as oxidant.61 The authors claimed that after recycling 15 times sulfur removal could still reach a high level (sulfur removal > 98.0%, and sulfur content < 10 ppm) without any decrease. Lo et al.62 reported a one-pot desulfurization of light oils by a combination of both chemical oxidation and solvent extraction using ILs as extractants and H2O2−acetic acid as oxidant. The water-immiscible IL ([BMIM][PF6]) was found to be more effective than the water-soluble [BMIM][BF4], providing a suitable solvent environment for the oxidation of dibenzothiophene (DBT). Without oxidation, the DBT contents in model light oil (tetradecane) only using [BMIM][PF6] and [BMIM]6937


Chemical Reviews

Review

Figure 1. Schematic diagram for desulfurization process via DBD plasma oxidation combined with IL extraction. Reprinted with permission from ref 89. Copyright 2013 Elsevier.

lanthanide-containing polyoxometalate Na 7 H 2 LaW 10 O 36 · 32H2O (LaW10) onto the amino-modified mesoporous silica to act as a heterogeneous catalyst (SiO2/NH3+/LaW10).66 In this case, deep desulfurization of DBT, BT, and 4,6-DMDBT could be achieved using [BMIM][BF4] as extractant and H2O2 as oxidant within 35 min. Even when the initial content of DBT in model oil was as low as 100 ppm, the removal efficiency could reach up to 99% within 5 min. When the capacity of model oil was scaled up to 1000 mL with the initial content of 1000 ppm, 99% sulfur removal could be achieved in 45 min, indicating a great potential for practical application. The proposed extraction and oxidative desulfurization process using SiO2/NH3+/LaW10 catalyst in IL is shown in Scheme 3. DBT was first extracted from oil to IL phases and then oxidized by the W−peroxo species to DBTO2. The active center LaW10 can be anchored tightly onto SiO2 support due to the strong electronic interaction between LaW10 and the protonated NH3 group. The results showed no presence of La and W in water phase after reaction. 2.1.1.3. Other Catalysts. Fenton-like reagents, vanadiumbased, rhenium-based, and several nonmetal compounds were proved efficient for deep desulfurization using ILs as extractants or reaction media.67−76 The recent works on extraction and oxidative desulfurization are summarized in Table 3. For more details, please see Table S4 in the Supporting Information. Li et al.77 investigated the catalytic performance of several Fenton-like reagents as catalysts in ILs for deep oxidative desulfurization. The system composed of low-cost metal salt (FeCl3), H2O2, and [BMIM][BF4] exhibits a high catalytic activity, and DBT removal in model oil could achieve 96.1% with the H2O2/DBT molar ratio of 6:1. When H2O2 was added into the oxidative system in four equal batches, deep desulfurization could be achieved with the result that the sulfur content can be reduced down to 5 ppm. Magnetic Fenton-like hybrid materials based on a quaternary ammonium cation (Q+, Q = (CH3)4N, C14H29N(CH3)3, and C18H37N(CH3)3) and [FeCl4] anion were used as catalysts in oxidative desulfurization combined with IL extraction.78 The removal of DBT in model oil using [(CH3)4N]FeCl4 catalyst in [BMIM][BF4] could reach 97.0% at 30 °C for 1 h, while the catalytic system

[BF4] as extractants decreased from 758 to 400 ppm and to 466 ppm, respectively. For comparison, after 6 h of oxidation in [BMIM][PF6] the DBT content could be reduced to 7.8 ppm. When an actual light oil containing a sulfur content of 8040 ppm was tested, after several hours of oxidation in [BMIM][PF6] and [BMIM][BF4] the sulfur content could decrease to 1300 and 3640 ppm, respectively. Later, the same group63 investigated the sulfur removal efficiency from diesel light oil in ILs using tungstophosphoric acid (TPA) as catalyst. DBT and 4,6-dimethyldibenzothiophene (4,6-DMDBT) dissolved in hexadecane were taken on as model oil system, and the water-immiscible ILs offered higher removal efficiency. Given the optimal IL [OMIM][PF6], the sulfur content in diesel light oil can be reduced from 897 to 42 ppm (corresponding to 95% desulfurization efficiency), and the alkyl benzothiophene (C4BT) appears to be the main residual sulfur compound in oil product. In 2013, Li et al. proposed to use the polyoxometalates (POM)-based hybrid materials (phosphotungstic acid supported ceria, HPW-CeO2) as catalyst in ILs, where CeO2 acted as the support.64 When using [OMIM][BF4] as extractant, the removal ratio of DBT could reach 99.4%. The proposed mechanism is illustrated in Scheme 2. DBT was first extracted from oil to IL phase, then absorbed onto the surface of CeO2 support, and finally reacted with the active peroxo species (formed by HPW in the presence of H2O2) to produce sulfone. Song et al.53 proposed using the lanthanide-containing POMs LnW10 (Na7H2LnW10O36·32H2O, Ln = Eu, La) as catalyst and [BMIM][BF4] as extractant. In the case of LaW10 catalyst, DBT can be completely removed from model oil with the initial content of 1000 ppm in 25 min. For LaW10/ [BMIM][BF4], the removal efficiencies of BT and 4,6-DMDBT under mild conditions with the O/S molar ratio of 5:1 could reach 80% and 92%, respectively. To overcome the phase transfer limitation, the Na7H2LnW10O36·32H2O encapsulated by the surfactants such as dodecyltrimethylammonium bromide and trimethylstearylammonium bromide were used as catalysts in [OMIM][PF6] for desulfurization. The three-liquid IL emulsion system exhibits not only high deep desulfurization efficiency under mild conditions but also easy separation and recyclability of catalysts. Furthermore, they anchored the 6938


Chemical Reviews

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Table 4. Summary of Oxidative Desulfurization Using IL-Based Catalysts reaction conditions

entry

S compounds

oil

initial content (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

thiophene BT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT

n-dodecane n-dodecane n-dodecane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

511 524 530 1000 1000 1000 1000 1000 1000 1000 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500

IL-based catalysts [BMIM][MeSO4] [BMIM][MeSO4] [BMIM][MeSO4] no no [C18H37N(CH3)3]7[PW11O39] [C18H37N(CH3)3]7[PW11O39] [C18H37N(CH3)3]7[PW11O39] [C18H37N(CH3)3]7[PW11O39] [C18H37N(CH3)3]7[PW11O39] [(C18H37)3N(CH3)]2Mo2O11 [(C18H37)3N(CH3)]2W2O11 [(C18H37)3N(CH3)]2Cl K2W2O11 Na2WO4·2H2O Na2MoO4·2H2O [(C18H37)3N(CH3)]2Mo2O11 [(C18H37)3N(CH3)]2Mo2O11 [(C18H37)3N(CH3)]2Mo2O11 [(C18H37)3N(CH3)]2Mo2O11 [(C18H37)3N(CH3)]2Mo2O11 [(C18H37)3N(CH3)]2Mo2O11 [(C18H37)3N(CH3)]2Mo2O11 [(C18H37)3N(CH3)]2W2O11 [(C18H37)3N(CH3)]2W2O11 [(C18H37)3N(CH3)]2W2O11 [(C18H37)3N(CH3)]2W2O11 [(C18H37)3N(CH3)]2W2O11 [(C18H37)3N(CH3)]2W2O11 [(C18H37)3N(CH3)]2W2O11 [MIMPS]3PW12O40·2H2O [MIMPS]3PW12O40·2H2O [MIMPS]3PW12O40·2H2O [MIMPS]3PW12O40·2H2O [MIMPS]3PW12O40·2H2O [BMIM]3PW12O40 [BMIM]3PW12O40 [BMIM]3PW12O40 [BMIM]3PW12O40 [BMIM]3PW12O40 [BMIM]3PMo12O40 [BMIM]3PMo12O40 [BMIM]3PMo12O40 [BMIM]3PMo12O40 [BMIM]3PMo12O40 [BMIM]4SiW12O40 [BMIM]4SiW12O40 [BMIM]4SiW12O40 [BMIM]4SiW12O40 [BMIM]4SiW12O40 [(C8H17)3N(CH3)]Cl [(C12H25)3N(CH3)]Cl H3PMo12O4026H2O(PMo12) H3PMo12O4014H2O(PW12) [(C8H17)3NCH3]3{PO4[MoO(O2)2]4} [(C12H25)3NCH3]3{PO4[MoO(O2)2]4} [(C8H17)3NCH3]3{PO4[WO(O2)2]4} [(C12H25)3NCH3]3{PO4[WO(O2)2]4} 6939

n(cat)/n(S) n(O)/n(S) 1:1 (wt) 1:1 (wt) 1:1 (wt) no no 1:150 1:150 1:150 1:150 1:150 1:40 (wt) 1:40 (wt) 1:40 (wt) 1:40 (wt) 1:40 (wt) 1:40 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:120 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt) 1:50 (wt)

1:1 (wt) 1:1 (wt) 1:1 (wt) no 4:1 4:1 4:1 4:1 3:1 2:1 8:1 8:1 8:1 8:1 8:1 8:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1

t

T (°C)

sulfur removal (%)

170 min 170 min 170 min 1h 1h 1h 1h 3h 3h 3h 2h 2h 2h 2h 2h 2h 2h 2h 2h 2h 2h 2h 2h 2h 2h 2h 2h 2h 2h 2h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 2h 2h 2h 2h 2h 2h 2h 2h

50 50 50 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 60 60 60 60 60 60 60 60

97.6 99.4 98.9 14.8 30.7 97.6 99.1 100.0 99.1 94.6 96.2 86.7 7.4 2.3 1.1 1.5 83.1 56.3 80.6 96.8 97.8 94.9 99.7 68.3 17.8 54.8 83.1 85.9 45.7 98.1 90.6 30.2 82.7 100 3.3 19.1 28.3 41.9 97.9 3 19.2 59.9 52.1 95.1 1.8 21.2 21.8 34.9 36.7 2.6 5.7 5.2 25.4 23.2 100 100 100 100

ref 92 92 92 93 93 93 93 93 93 93 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 101 101 101 101 101 101 101 101


Chemical Reviews

Review

Table 4. continued reaction conditions

oil


59

BT

n-heptane

2000

PIL

60

BT

n-heptane

2000

monomer

61

BT

n-heptane

2000

PSMIM

62

BT

n-heptane

2000

PSMIMHSO4

63

BT

n-heptane

2000

SO42−/ZrO2

64 65 66 67 68 69 70 71 72

DBT DBT diesel diesel diesel diesel diesel diesel DBT

toluene toluene

n-octane

2000 2000 530 530 530 530 530 530 500

[BMIM]3PMo12O40/SiO2 [BMIM]3PMo12O40/SiO2 [BMIM]3PMo12O40/SiO2 [BMIM]3PMo12O40/SiO2 [BMIM]3PMo12O40/SiO2 [BMIM]3PMo12O40/SiO2 [BMIM]3PMo12O40/SiO2 [BMIM]3PMo12O40/SiO2 [BMIM]PW/HMS

73

BT

n-octane

500

[BMIM]PW/HMS

74

DMDBT

n-octane

500

[BMIM]PW/HMS

75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101

DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT BT BT BT BT BT BT BT BT BT BT BT BT DBT

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

500 500 500 500 500 500 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 500

W-SiO2-350 W-SiO2-450 W-SiO2-550 W-SiO2-650 W-SiO2-750 W-SiO2-850 LaW10 LaW10 LaW10 LaW10 LaW10 LaW10 LaW10 LaW10 SiO2-BisILs[(PW12O40)3−] SiO2-BisILs[(PW12O40)3−] SiO2-BisILs[(PW12O40)3−] SiO2-BisILs[(PW12O40)3−] SiO2-BisILs[(PW12O40)3−] SiO2-BisILs[(PW12O40)3−] SiO2-BisILs[(PW12O40)3−] SiO2-BisILs[(PW12O40)3−] SiO2-BisILs[(PW12O40)3−] SiO2-BisILs[(PW12O40)3−] SiO2-BisILs[(PW12O40)3−] SiO2-BisILs[(PW12O40)3−] MCM-41

102

DBT

n-octane

500

0.2W-MCM-41

103

DBT

n-octane

500

0.1W-MCM-41

104

DBT

n-octane

500

0.05W-MCM-41

105

DBT

n-octane

500

0.025W-MCM-41

106

BT

n-heptane

2000

UIO-66

entry

S compounds

fuel fuel fuel fuel fuel fuel

IL-based catalysts

6940

t

T (°C)

sulfur removal (%)

5:1

20 min

50

95.5

102

5:1

20 min

50

84.8

102

5:1

20 min

50

77.6

102

5:1

20 min

50

83.7

102

5:1

20 min

50

81.6

102

3:1 3:1 5:1 5:1 5:1 5:1 3:1 10:1 3:1

2 1 3 3 3 3 3 3 1

h h h h h h h h h

60 50 50 60 70 80 50 50 60

100 64.8 84.1 91.3 95.1 97.9 80.2 86.5 98.3

103 103 103 103 103 103 103 103 104

3:1

1h

60

79.4

104

3:1

1h

60

87.8

104

32 μL 32 μL 32 μL 32 μL 32 μL 32 μL 5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 2.5:1

1h 1h 1h 1h 1h 1h 80 min 80 min 80 min 40 min 25 min 35 min 25 min 10 min 10 h 10 h 10 h 10 h 10 h 10 h 2h 4h 6h 8h 12 h 14 h 20 min

60 60 60 60 60 60 30 40 50 60 70 70 70 70 50 50 50 50 50 50 50 50 50 50 50 50 60

75.6 99.6 92.6 89.2 79 63 32 79 83 >99 >99 >99 >99 >99 43.2 62.3 84.6 85.9 98.5 97.9 28.9 66.9 76.7 88.3 98.7 98.9 7.5

105 105 105 105 105 105 106 106 106 106 106 106 106 106 107 107 107 107 107 107 107 107 107 107 107 107 108

2.5:1

20 min

60

99.8

108

2.5:1

20 min

60

99.6

108

2.5:1

20 min

60

93

108

2.5:1

20 min

60

77.4

108

7:1

20 min

30

62.3

110

n(cat)/n(S) n(O)/n(S) 80 mg:5 mL oil 80 mg:5 mL oil 80 mg:5 mL oil 80 mg:5 mL oil 80 mg:5 mL oil 0.021:1 0.021:1 0.021:1 0.021:1 0.021:1 0.021:1 0.021:1 0.021:1 0.06 g:10 mL oil 0.06 g:10 mL oil 0.06 g:10 mL oil 5 mg 5 mg 5 mg 5 mg 5 mg 5 mg 1:30 1:30 1:30 1:30 1:30 1:60 1:30 1:15 0.005:1 0.010:1 0.015:1 0.020:1 0.025:1 0.030:1 0.025:1 0.025:1 0.025:1 0.025:1 0.025:1 0.025:1 0.01 g:5 mL oil 0.01 g:5 mL oil 0.01 g:5 mL oil 0.01 g:5 mL oil 0.01 g:5 mL oil 400 mg:10 mL oil

ref


Chemical Reviews

Review


oil


107

BT

n-heptane

2000

10% PSMIMHSO4/UIO-66

108

BT

n-heptane

2000


109

BT

n-heptane

2000


110

BT

n-heptane

2000


111

BT

n-heptane

2000


112

DBT

n-octane

500

G-h-BN

113

DBT

n-octane

500

M-h-BN

114

DBT

n-octane

500

commercial bulk BN

115

DBT

n-octane

500

IL/G-h-BN

116

DBT

n-octane

500

IL/G-h-BN

117

DBT

n-octane

500

IL

118

DBT

n-octane

500

IL

119

DBT

n-octane

500

IL+G-h-BN

120

DBT

n-octane

500

IL/M-h-BN

121

DBT

n-octane

500

IL/commercial bulk BN

entry

S compounds

IL-based catalysts

T (°C)

7:1

20 min

30

69.4

110

7:1

20 min

30

77.8

110

7:1

20 min

30

88.3

110

7:1

20 min

30

94.6

110

7:1

20 min

30

92.9

110

No

80 min

30

39.4

111

No

80 min

30

32.5

111

No

80 min

30

2

111

No

80 min

30

26.3

111

4:1

80 min

30

99.3

111

4:1

80 min

30

10.6

111

4:1

80 min

30

93.5

111

4:1

80 min

30

47.4

111

4:1

80 min

30

53.5

111

4:1

80 min

30

7.7

111

n(cat)/n(S) n(O)/n(S) 400 mg:10 mL oil 400 mg:10 mL oil 400 mg:10 mL oil 400 mg:10 mL oil 400 mg:10 mL oil 0.05 g:5 mL oil 0.05 g:5 mL oil 0.05 g:5 mL oil 0.05 g:5 mL oil 0.05 g:5 mL oil 0.005 g:5 mL oil 0.4 g:5 mL oil 0.05 g:5 mL oil 0.05 g:5 mL oil 0.05 g:5 mL oil

ref

consisting of IL and catalyst could be easily separated by applying an external magnetic field and regenerated by washing with water. Furthermore, Li et al.79,80 proposed using dialkylpiperidinium tetrachloroferrates and organic hexacyanoferrates as catalysts in ILs for catalytic oxidation of S compounds. The removal of DBT could reach 97.1% with [BMPIP]FeCl4 as catalyst in [OMIM][BF4] at 30 °C in 60 min in the case of a H2O2/sulfur molar ratio of 3.5:1. When the organic hexacyanoferrate catalyst [BMIM]3Fe(CN)6 was used in [BMIM][BF4], the high removal ratios for DBT (97.9%) and BT (91.1%) could be reached using the low-concentration H2O2 (7.5 wt %) as oxidant with the O/S molar ratio of 4:1. Recently, two hexacyanoferrate-based ILs ([BPy]3Fe(CN)6 and [C16Py]3Fe(CN)6), which are not sensitive to water, have been chosen as Fenton-like catalysts in oxidative desulfurization of model oil using ILs, water, or organic solvents as extractants.81 The results showed that the removal of DBT could reach 97.1% using [BPy]3Fe(CN)6 as catalyst and [OMIM][PF6] as extractant under optimal condition. Vanadium and rhenium compounds have also been examined as catalysts in ILs for oxidative desulfurization.52,82,83 Mota et al.82 employed the acetylacetonate complexes of oxovanadium(IV) VO(X-acac)2 (X = Cl, CH3, or CH3CH2) as catalysts in ILs for oxidative desulfurization with H2O2. It was observed that over 95% conversion of BT and DBT to sulfones was achieved in [BMIM][Tf2N]; however, in [BMIM][TfO] and [BMIM][BF4] the conversion was moderate, which may be due to the higher coordinating ability of these two anions than [Tf2N]. Zhu et al.83 also investigated the catalytic activity of VO(acac)2 in [BMIM][BF4], achieving 99.6% DBT removal in 2 h.

Scheme 4. Proposed Oxidative Desulfurization with H2O2 over SPIL [(n-C8H17)3NCH3]3{PO4[MoO(O2)2]4} Catalysta

a

t

sulfur removal (%)

Adapted from ref 101 with permission. Copyright 2013 Elsevier.

6941


Chemical Reviews

Review

Scheme 5. Proposed Oxidative Reaction Mechanism of DBT on IL/G-h-BNa

a

Reprinted with permission from ref 111. Copyright 2015 American Chemical Society.

Scheme 6. Synthesis Procedure of the Catalyst (a), and the Proposed Desulfurization Procedure (b)a

a


Ren et al.84 used four benzyl-based ILs as extractants for deep desulfurization of model oil and real diesel by an oxidative− extractive two-step method, where sulfides was first oxidized by H2O2 with CH3COOH catalyst, and then the unreacted sulfides and uncrystallized bibenzothiophene sulfoxide in model oil were extracted by ILs. In the end, after one-stage extraction with [BzMIM][Tf2N] the removal efficiencies could reach 98.4% and 96.4% for DBT and 4,6-DMDBT, respectively. Ribeiro et al.85 encapsulated the tetrabutylammonium salt of phosphotungstic acid in the chromium terephthalate metal− organic framework MIL-101, and the obtained PW12@MIL101 was employed as the catalyst in one-pot extraction and oxidative desulfurization with H2O2 in [BMIM][PF6]. The catalytic activity of the catalyst in IL medium was much higher than that in the conventional solvent methanol. The sulfur removal from model oil with the initial sulfur content of 500 ppm using [BMIM][PF6] could reach 100% for DBT within 1

h, 100% for 1-BT within 3 h, and 99% for 4,6-DMDBT within 4 h. The heteropolyanion-based IL, [(3-sulfonic acid) propylpyridine]3PW12O40·2H2O (abbreviated as [PSPy]3PW), was proved to be an effective catalyst for desulfurization of fuels in [OMIM][PF6], the removal of DBT being 99.4%.86 For 4,6DMDBT and BT, the removal ratios could reach 98.8% and 69.9%, respectively. For real FCC gasoline, the sulfur level could be reduced from 360 to 70 ppm. The MCM-41supported Fenton-like IL ([PMIM)FeCl4-MCM-41) as catalyst with H2O2 as oxidant and [OMIM][BF4] as extractant showed a high catalytic activity at 30 °C for oxidative desulfurization.87 After reaction, the upper layer oil was decanted to reuse the catalyst and extractant. On the other hand, SBA-supported Fenton-like IL ([PMIM)FeCl4-SBA-15) was prepared and used as catalyst in extractive catalytic oxidative desulfurization using [OMIM][BF4] as extractant and reaction media.88 Under the 6942


Chemical Reviews

Review

Table 5. Summary of Oxidative Desulfurization Using ILs as Both Extractants and Catalysts reaction conditions

oil


entry

S compounds

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 1000 1000 1000 500

41

DBT

n-octane

500

42

DBT

n-octane

500

43

DBT

n-octane

500

44

DBT

n-octane

500

45

DBT

n-octane

500

46

DBT

n-octane

500

47 48

DBT DBT

n-octane n-octane

500 500

49

DBT

n-octane

500

50 51 52 53

DBT DBT DBT DBT

n-octane n-octane n-octane n-octane

500 500 500 500

ILs (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO (C4H9)4NBr·2C6H11NO [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [Hnmp][H2PO4] [BMIM]Cl/FeCl3 [OMIM]Cl/FeCl3 [Et3NH]Cl/FeCl3 [(C8H17)3CH3N]Cl/ FeCl3 [(C8H17)3CH3N]Cl/ CuCl2 [(C8H17)3CH3N]Cl/ SnCl2 [(C8H17)3CH3N]Cl/ ZnCl2 [(C4H9)3CH3N]Cl/ FeCl3 [(C10H21)3CH3N]Cl/ FeCl3 [(C10H21)2(CH3)2N]Cl/ FeCl3 [(C8H17)3CH3N]Cl [(C8H17)3CH3N]Cl/ 0.5FeCl3 [(C8H17)3CH3N]Cl/ 1.5FeCl3 [Et3NH]Cl/0.5FeCl3 [Et3NH]Cl/FeCl3 [Et3NH]Cl/1.5FeCl3 [BPY]Cl

t

T (°C)

sulfur removal (%)

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

1:3 1:2 2:1 3:1 1:1 2:1 3:1 4:1 3:1 4:1 1:1 2:1 4:1 3:1 2:1 1:1 2:1 1:1 4:1 3:1 1:1 1:2 1:3 1:4 1:2 1:1 1:4 1:3 1:3 1:4 1:1 1:2 1:4 1:3 1:2 1:1 3:1a 3:1a 3:1a 5 mL:0.702 mmol

16:1 16:1 16:1 16:1 4:1 8:1 12:1 16:1 8:1 4:1 16:1 12:1 12:1 16:1 4:1 8:1 16:1 12 8:1 4:1 4:1 8:1 12:1 16:1 16:1 12:1 8:1 4:1 8:1 4:1 16:1 12:1 12:1 16:1 4:1 8:1 3:1 3:1 3:1 14:1

0.5 h 0.5 h 0.5 h 0.5 h 0.25 h 0.25 h 0.25 h 0.25 h 0.5 h 0.5 h 0.5 h 0.5 h 0.75 h 0.75 h 0.75 h 0.75 h 1h 1h 1h 1h 3h 4h 5h 6h 3h 4h 5h 6h 3h 4h 5h 6h 3h 4h 5h 6h 10 min 10 min 10 min 1h

50 50 50 50 30 40 50 60 30 40 50 60 30 40 50 60 30 40 50 60 40 40 40 40 50 50 50 50 60 60 60 60 70 70 70 70 10 10 10 25

97.5 98.2 93.2 90.1 86.8 97.9 89.1 89.1 91.9 82.4 98.6 92.8 84.5 92 94.1 98 93.8 93.7 87.4 89.5 37.7 8.1 4.8 76.1 48.8 71.1 52 82.6 83.9 34.3 99.8 99.3 91.4 98.5 81.9 99 99.2 87.0 37.2 97.9

116 116 116 116 116 116 116 116 116 116 116 116 116 116 116 116 116 116 116 116 117 117 117 117 117 117 117 117 117 117 117 117 117 117 117 117 120 120 120 121

H2O2

5 mL:0.702 mmol

14:1

1h

25

26.2

121

H2O2

5 mL: 0.702 mmol

14:1

1h

25

25.8

121

H2O2

5 mL:0.702 mmol

14:1

1h

25

19.8

121

H2O2

5 mL:0.702 mmol

14:1

1h

25

95.8

121

H2O2

5 mL:0.702 mmol

14:1

1h

25

93.4

121

H2O2

5 mL:0.702 mmol

14:1

1h

25

98.7

121

H2O2 H2O2

5 mL:0.702 mmol 5 mL:0.702 mmol

14:1 14:1

1h 1h

25 25

20.1 98.3

121 121

H2O2

5 mL:0.702 mmol

14:1

1h

25

97.9

121

H2O2 H2O2 H2O2 H2O2

5:2 5:2 5:2 5 mL:0.78 mmol

6:1 6:1 6:1 8:1

5h 5h 5h 10 min

30 30 30 40

71.6 97.1 64.6 1.4

122 122 122 123

oxidants

6943

V(oil)/V(IL)

n(O)/n(S)

ref


Chemical Reviews

Review


oil


entry

S compounds

54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

500 500 500 500 500 500 500 500 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 500 500 500 500 1000 1000 1000 1000 1000 1000 1000 1000 1000 500

87

DBT

n-octane

1000

88

DBT

n-octane

1000

89

DBT

n-octane

1000

90

DBT

n-octane

1000

91

DBT

n-octane

1000

92

DBT

n-octane

1000

93

DBT

n-octane

1000

94

DBT

n-octane

1000

95 96 97 98

DBT DBT DBT DBT

n-octane n-octane n-octane n-octane

500 500 500 1000

99

DBT

n-octane

1000

100

DBT

n-octane

1000

101 102

DBT DBT

n-octane n-octane

1000 1000

103

DBT

n-octane

1000

ILs [BPY][FeCl4] [BPY][FeCl4] [BPY][FeCl4]/0.5FeCl3 [BPY][FeCl4] [BPY][FeCl4] [BPY][FeCl4] [BPY][FeCl4] [BPY][FeCl4] [HMPY]Cl/0.5FeCl3 [HMPY]Cl/FeCl3 [HMPY]Cl/2FeCl3 [HMPY]Cl/3FeCl3 [HMIM]Cl/FeCl3 [HMPY]Cl/FeCl3 [HMPY]Cl/FeCl3 [HMPY]Cl/FeCl3 [BMPY]/[FeCl4] [BMPY]/[FeCl4] [BMPY]/[FeCl4] [OMPY]/[FeCl4] [OMPY]/[FeCl4] [OMPY]/[FeCl4] [OMPY]/[FeCl4] [OMPY]/[FeCl4] [OMPY]/[FeCl4] [BMPY]/[FeCl4] [BMPY]Cl/3ZnCl2 [BMPY]Cl/3ZnCl2 [BMPY]Cl/3ZnCl2 [BMPY]Cl/3ZnCl2 [BMPY]Cl/3ZnCl2 [BMPY]Cl/3ZnCl2 [C3H6COOHMIM]Cl/ 2FeCl3 [(CH2)2COOHMIM] Cl/FeCl3 [(CH2)2COOHMIM] Cl/FeCl3 [(CH2)2COOHMIM] Cl/FeCl3 [(CH2)2COOHMIM] Cl/FeCl3 [(CH2)2COOHMIM] Cl/ZnCl2 [(CH2)2COOHMIM] Cl/ZnCl2 [(CH2)2COOHMIM] Cl/ZnCl2 [(CH2)2COOHMIM] Cl/ZnCl2 [AMIM]Cl/FeCl3 [AMIM]Cl/FeCl3 [AMIM]Cl/FeCl3 2[(CH2)2COOHMIM] Cl/ZnCl2 [(CH2)2COOHMIM] Cl/2ZnCl2 [(CH2)2COOHMIM] Cl/3ZnCl2 [BMIM]Cl/ZnCl2 [(CH2)2COOHMIM] Cl/2ZnCl2 [BMIM]Cl/FeCl3

t

T (°C)

sulfur removal (%)

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

5 mL:0.78 mmol 5 mL:0.78 mmol 5 mL:0.78 mmol 5 mL:0.156 mmol 5 mL:0.936 mmol 5 mL:0.78 mmol 5 mL:0.78 mmol 5 mL:0.78 mmol 3:1a 3:1a 3:1a 3:1a 3:1a 3:1a 3:1a 3:1a 3:1a 3:1a 3:1a 3:1a 3:1a 3:1a 3:1a 5:1a 5:1a 5:1a 3:1a 3:1a 3:1a 3:1a 3:1a 3:1a 5 mL:1 g

8:1 8:1 8:1 8:1 8:1 2:1 8:1 8:1 4:1 4:1 4:1 4:1 4:1 8:1 4:1 4:1 6:1 6:1 6:1 18:1 6:1 24:1 6:1 6:1 6:1 6:1 8:1 8:1 8:1 2:1 4:1 6:1 6:1

10 min 10 min 10 min 10 min 10 min 10 min 10 min 10 min 20 min 20 min 20 min 20 min 20 min 20 min 20 min 20 min 20 min 20 min 20 min 20 min 20 min 20 min 20 min 20 min 5 min 5 min 4h 4h 4h 4h 4h 4h 10 min

40 30 40 40 40 40 40 60 25 25 25 25 25 25 35 50 25 35 50 50 50 25 25 25 25 25 25 50 60 50 50 50 30

95.3 51.2 68.9 32.8 95.3 64.0 95.3 52.3 81.0 100 59.5 50.9 80.0 82.8 100 75.8 100 99.6 85.6 99.5 99.9 99.7 100 100 89.6 70.9 64.2 100 99.9 67.3 95.7 99.6 100

123 123 123 123 123 123 123 123 124 124 124 124 124 124 124 124 125 125 125 125 125 125 125 125 125 125 126 126 126 126 126 126 127

H2O2

3:1

2:1

20 min

25

79.9

128

H2O2

3:1

4:1

20 min

25

95.7

128

H2O2

3:1

6:1

20 min

25

90.6

128

H2O2

3:1

8:1

20 min

25

72.2

128

H2O2

3:1

2:1

4h

60

62.0

128

H2O2

3:1

4:1

4h

60

81.9

128

H2O2

3:1

6:1

4h

60

92.7

128

H2O2

3:1

8:1

4h

60

99.6

128

H2O2 H2O2 H2O2 H2O2

3:1 3:1 3:1 3:1

4:1 6:1 8:1 6:1

20 min 20 min 20 min 4h

25 25 25 60

81.9 92.7 97.0 11.6

128 128 128 128

H2O2

3:1

6:1

4h

60

23.2

128

H2O2

3:1

6:1

4h

60

12.8

128

H2O2 H2O2

2:1 3:1

8:1 8:1

6h 4h

45 40

48.6 87.1

128 128

H2O2

3:1

3:1

10 min

30

99.2

128

oxidants

6944

V(oil)/V(IL)

n(O)/n(S)

ref


Chemical Reviews

Review


oil


entry

S compounds

104 105 106

DBT DBT DBT

n-octane n-octane n-octane

1000 1000 1000

107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160

DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT DBT

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-tetradecane

505 505 505 505 505 505 505 505 505 505 505 505 505 505 505 505 505 505 505 505 505 224.6 224.6 224.6 224.6 224.6 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1600 1600 1600 1600 500

161

thiophene

n-heptane

1500

ILs [OMIM]Cl/FeCl3 [AMIM]Cl/FeCl3 [(CH2)2COOHMIM] Cl/FeCl3 [BMIM]Cl/ZnCl2 [BMIM]Cl/FeCl2 [BMIM]Cl/CoCl2 [BMIM]Cl/MgCl2 [BMIM]Cl/CuCl2 [BMIM]Cl/SnCl2 [BMIM]Cl/(1/2)ZnCl2 [BMIM]Cl/(1/3)ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/3ZnCl2 [BMIM]Cl/3ZnCl2 [BMIM]Cl/3ZnCl2 [BMIM]Cl/3ZnCl2 [BMIM]Cl/3ZnCl2 [BMIM]Cl/3ZnCl2 [BMIM]Cl/3ZnCl2 [BMIM]Cl/3ZnCl2 [BMIM]Cl/3ZnCl2 [BMIM]Cl/3ZnCl2 [BMIM]Cl/3ZnCl2 [MPY]Cl/ZnCl2 [MPY]Cl/ZnCl2 [MPY]Cl/ZnCl2 [MPY]Cl/ZnCl2 [MPY]Cl/ZnCl2 [BMIM][HSO4] [BMIM][HSO4] [BMIM][HSO4] [BMIM][HSO4] [BMIM][HSO4] [BMIM][HSO4] [BPY][HSO4] [BMIM][HSO4] [BMIM][HSO4] [BMIM][HSO4] [BMIM][HSO4] [BMIM][HSO4] [BMIM][HSO4] [BMIM][HSO4] [CH2COOHPY][HSO4] [HDMF][BF4] [HDMF][TFA] [HDMAC][TFA] [HNMP][TFA] [HCPL][TFA] [HCPL][NO3] [HCPL][HSO4] [HNMC][TFA] [EIMC4SO3H][Tf2N] [EIMC4SO3H][Tf2N] [EIMC4SO3H][Tf2N] [EIMC4SO3H][Tf2N] [(CH2)2COOHMIM] [HSO4] [C5BIM][CH3COO]

t

T (°C)

sulfur removal (%)

H2O2 H2O2 H2O2

3:1 3:1 3:1

3:1 4:1 4:1

10 min 10 min 10 min

30 25 25

87.0 90.4 95.6

128 128 128

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 NaClO NaClO H2O2

2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 2:1a 5:1a 1:1a 3:1a 3:1a 3:1a 3:1a 5:1a 2:1 2:1 2:1 2:1 2:1 2:1 2:1 5 mL:3 g 5 mL:3 g 5 mL:3 g 5 mL:2 g 5 mL:4 g 5 mL:3 g 5 mL:3 g 20:1.2 1:1a 1:1a 1:1a 1:1a 1:1a 1:1a 1:1a 1:1a 5:1a 1:1a 5:1a 1:1a 2.5 mmol:10 mL

8:1 8:1 8:1 8:1 8:1 8:1 8:1 8:1 8:1 8:1 8:1 8:1 8:1 8:1 8:1 8:1 8:1 2:1 9:1 8:1 8:1 2:1 3:1 4:1 4:1 4:1 5:1 5:1 2:1 3:1 4:1 6:1 5:1 2:1 3:1 4:1 3:1 3:1 4:1 4:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 2:5a 2:5a 2:5a 2:5a 1:10b

6h 6h 6h 6h 6h 6h 7h 7h 2h 3h 2h 3h 3h 3h 7h 7h 7h 3h 3h 3h 3h 20 min 20 min 20 min 20 min 20 min 1.5 h 1.5 h 1.5 h 1.5 h 1.5 h 1.5 h 1.5 h 0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 1h 2h 2h 2h 2h 2h 2h 2h 2h 3h 3h 3h 3h 7h

45 45 45 45 45 45 45 45 45 45 45 45 30 60 45 30 60 45 45 45 45 75 75 75 75 75 25 60 25 25 25 25 25 70 70 70 70 70 60 80 30 30 30 30 30 30 30 30 30 60 60 60 60 25

48.6 35.5 27.0 19.0 17.0 14.0 12.8 22.1 88.9 96.9 99.4 99.9 47.4 62.1 99.9 65.7 89.0 18.6 49.3 49.1 87.8 93.5 99.3 99.9 99.9 94.4 99.6 64.6 84.6 92.0 93.1 99.7 32.8 86.0 97.0 99.0 72.0 90.0 100 96.0 99.9 81.1 99.2 99.3 99.4 100 99.6 73.3 99.4 55.0 99.1 89.5 98.8 96.7

130 130 130 130 130 130 130 130 130 130 130 130 130 130 130 130 130 130 130 130 130 132 132 132 132 132 133 133 133 133 133 133 133 134 134 134 134 134 134 134 135 137 137 137 137 137 137 137 137 138 138 138 138 139

H2O2

10:9.5

10.5:10b

3h

70

68.3

119

oxidants

6945

V(oil)/V(IL)

n(O)/n(S)

ref


Chemical Reviews

Review


oil


entry

S compounds

162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182

thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene

n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane n-octane n-octane

1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1000 1000

183

thiophene

n-octane

1000

184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199

thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene thiophene BT BT BT BT BT

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

500 1000 202.544 202.544 202.544 202.544 202.544 202.544 202.544 202.544 202.544 1000 500 1000 250 250

200

BT

n-octane

250

201

BT

n-octane

250

202

BT

n-octane

250

203

BT

n-octane

1000

204

BT

n-octane

1000

205 206 207 208 209 210 211 212 213 214 215

BT BT BT BT BT BT BT BT BT BT BT

n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane n-octane

500 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000

ILs [HBIM][CH3COO] [C7BIM][CH3COO] [OBIM][CH3COO] [C5BIM][CH3COO] [HBIM][CH3COO] [C7BIM][CH3COO] [OBIM][CH3COO] [C7BIM][CH3COO] [OBIM][CH3COO] [C5BIM][CH3COO] [HBIM][CH3COO] [OBIM][CH3COO] [C7BIM][CH3COO] [HBIM][CH3COO] [C5BIM][CH3COO] [HBIM][CH3COO] [C5BIM][CH3COO] [OBIM][CH3COO] [C7BIM][CH3COO] [BMPY]Cl/3ZnCl2 [(CH2)2COOHMIM] Cl/FeCl3 [(CH2)2COOHMIM] Cl/ZnCl2 [AMIM]Cl/FeCl3 [BMIM][HSO4] [BMIM][TFA] [BMIM][TFA] [BMIM][TFA] [BMIM][TFA] [BMIM][TFA] [BMIM][TFA] [BMIM][TFA] [BMIM][TFA] [BMIM][TFA] [BMIM]Cl/FeCl3 [BPY][FeCl4] [BMPY]Cl/3ZnCl2 [C3H6COOHMIM]Cl [C3H6COOHmim]Cl/ 0.5FeCl3 [C3H6COOHmim]Cl/ FeCl3 [C3H6COOHmim]Cl/ 1.5FeCl3 [C3H6COOHmim]Cl/ 2FeCl3 [(CH2)2COOHMIM] Cl/FeCl3 [(CH2)2COOHMIM] Cl/ZnCl2 [AMIM]Cl/FeCl3 [BMIM][HSO4] [CH2COOHPY][HSO4] [HDMF][BF4] [HDMF][TFA] [HDMAC][TFA] [HNMP][TFA] [HCPL][TFA] [HCPL][NO3] [HCPL][HSO4] [HNMC][TFA]

n(O)/n(S)

t

T (°C)

sulfur removal (%)

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

10:9.5 10:9.5 10:9.5 5:9.5 10:10 15:9.5 20:9.1 10:9.1 5:9.5 20:10 15:9.5 15:10 20:9.5 5:9.1 10:9.5 20:9.5 15:9.1 10:9.5 5:9.5 3:1a 3:1

10.5:10b 10.5:10b 10.5:10b 5:10.5b 10:10b 15:10.5b 20:10.9b 10:10.9b 5:10.5b 20:10b 15:10.5b 15:10b 20:10.5b 5:10.9b 10:10.5b 20:10.5b 15:10.9b 10:10.5b 5:10.5b 6:1 4:1

3h 3h 3h 0.50 h 1.33 h 2.17 h 3.00 h 0.50 h 1.33 h 2.17 h 3.00 h 0.50 h 1.33 h 2.17 h 3.00 h 0.50 h 1.33 h 2.17 h 3.00 h 4h 20 min

70 70 70 40 40 40 40 50 50 50 50 60 60 60 60 70 70 70 70 50 25

77.7 83.2 87.5 30.7 66.2 45.7 69.1 73.9 86.6 48.8 59.4 86.9 54.6 66.3 85.6 83.9 76.2 84.8 87.1 35.2 33.0

119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 126 128

H2O2

3:1

8:1

4h

60

94.6

128

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

3:1 2:1 25:1 25:1 25:1 30:1 30:1 30:1 35:1 35:1 35:1 3:1a 5 mL:0.78 mmol 3:1a 5 mL:1 g 5 mL:1 g

8:1 5:1 1:25b 2:25b 4:25b 1:25b 2:25b 4:25b 1:25b 2:25b 4:25b 3:1 8:1 6:1 6:1 6:1

20 min 1.5 h 0.3 h 1h 2h 2h 0.5 h 1h 1h 2h 0.5 h 10 min 10 min 4h 10 min 10 min

25 25 60 70 80 70 80 60 80 60 70 10 40 50 30 30

36.9 86.6 78 92 100 83 90 88 86 82 96 75.9 75.0 60.1 19.1 21.0

128 133 136 136 136 136 136 136 136 136 136 120 123 126 127 127

H2O2

5 mL:1 g

6:1

10 min

30

13.2

127

H2O2

5 mL:1 g

6:1

10 min

30

10.2

127

H2O2

5 mL:1 g

6:1

10 min

30

100

127

H2O2

3:1

4:1

20 min

25

42.2

128

H2O2

3:1

8:1

4h

60

96.5

128

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

3:1 2:1 16.67 2:1a 2:1a 2:1a 1:1a 1:1a 1:1a 1:1a 1:1a

8:1 5:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1

20 min 1.5 h 1h 2h 2h 2h 2h 2h 2h 2h 2h

25 25 30 30 30 30 30 30 30 30 30

92.0 94.2 82.5 69.8 80.1 86.5 92.2 100 94.5 56.5 93.1

128 133 135 137 137 137 137 137 137 137 137

oxidants

6946

V(oil)/V(IL)

ref


Chemical Reviews

Review

Table 5. continued reaction conditions initial content (ppm)

entry

S compounds

oil

216 217 218

4,6-DMDBT 4,6-DMDBT 4,6-DMDBT

n-octane n-octane n-octane

500 500 250

219 220 221

4,6-DMDBT 4,6-DMDBT 4,6-DMDBT

n-octane n-octane n-tetradecane

500 1000 500

222 223

actual diesel FCC gasoline

224 225 226 227 228 229 230 231 232 233 234 235 236 237

actual gasoline commercial diesel commercial diesel commercial diesel commercial diesel commercial diesel commercial diesel commercial diesel coke diesel fuel coke diesel fuel coke diesel fuel coke diesel fuel coke diesel fuel diesel fuel

238

diesel fuel

225

239

diesel fuel

225

240

diesel fuel

225

241

diesel fuel

225

242

diesel fuel

225

243

diesel fuel

225

244 245 246 247 248

hydrogenated diesel hydrogenated diesel straight-run diesel straight-run diesel real diesel

659.70 77.2 11034 983.41 200

a

1078 360

fuel fuel fuel fuel fuel fuel fuel

468 64 64 64 64 64 64 64 5380 5380 5380 5380 5380 225

ILs [BMIM]Cl/FeCl3 [BPY][FeCl4] [C3H6COOHmim]Cl/ 2FeCl3 [BMIM][HSO4] [CH2COOHPY][HSO4] [(CH2)2COOHMIM] [HSO4] [BMIM]Cl/FeCl3 [(C8H17)3CH3N]Cl/ FeCl3 [OMPY]/[FeCl4] [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [BMIM]Cl/2ZnCl2 [(CH2)4SO3HMIM] [ZnCl3] [(CH2)4SO3HMIM] [Tos] [(CH2)4SO3HMIM] [Tos] [(CH2)4SO3HMIM] [HSO4] [(CH2)4SO3HMIM] [H2PO4] [(CH2)4SO3HMIM] [ZnCl3] [(CH2)4SO3HMIM] [FeCl4] [HCPL][TFA] [HCPL][TFA] [HCPL][TFA] [HCPL][TFA] [(CH2)2COOHMIM] [HSO4]

t

T (°C)

sulfur removal (%)

H2O2 H2O2 H2O2

3:1a 5 mL:0.78 mmol 5 mL:1 g

3:1 8:1 6:1

10 min 10 min 10 min

10 40 30

90.3 54.8 93.7

120 123 127

H2O2 H2O2 H2O2

2:1 16.67 2.5 mmol:10 mL

5:1 6:1 1:10b

1.5 h 1h 7h

25 30 25

85.2 89.1 95.1

133 135 139

H2O2 H2O2

0.5 h 1h

30 25

71.3 69.4

120 121

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

1.5:1a 20:1 1.404 mmol of IL, 112 μL of H2 O2 3:1a 6:1 2:1a 13:1a 2:1a 29:1a a 2:1 57:1a 2:1a 141:1a a 2:1 214:1a a 2:1 284:1a 2:1a 354:1a a 2:1 14:1a a 2:1 28:1a 2:1a 57:1a a 2:1 142:1a 2:1a 213:1a 2:1a 30:1

0.5 h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 3h

25 90 90 90 90 90 90 90 90 90 90 90 90 60

44.0 12.5 23.1 35.6 59.8 70.0 77.5 87.7 49.5 56.2 39.7 28.7 29.9 40.7

125 129 129 129 129 129 129 129 129 129 129 129 129 131

H2O2

2:1a

40:1

3h

75

43.7

131

H2O2

2:1a

8:1

2h

60

32.6

131

H2O2

2:1a

8:1

2h

60

21.1

131

H2O2

2:1a

8:1

2h

60

22.9

131

H2O2

2:1a

8:1

2h

60

35.4

131

H2O2

2:1a

8:1

2h

60

16.6

131

H2O2 H2O2 H2O2 H2O2 H2O2

1:1a 6:1 1:1a 6:1 1:1a 6:1 1:1a 6:1 3 mL of diesel, 6 mmol of IL, 0.5 mL of H2O2

1h 1h 1h 1h 2.5 h

40 40 40 40 25

88.3 88.8 91.1 90.9 89.8

137 137 137 137 140

oxidants

V(oil)/V(IL)

n(O)/n(S)

ref

The values are based on the mass ratio. bThe values are based on the volume ratio.

using H2O2 as oxidant, which may be due to the reaction occurring in a gas−solid−oil triphasic system. Zhang et al.90 reported that cobalt phthalocyanines (CoPc(Cl)n) exhibited effective activity for the oxidative desulfurization of DBT using air as oxidant in IL at room temperature, and the activity of cobalt phthalocyanines increases in the order of CoPc(Cl)4 < CoPc(Cl)8 < CoPc(Cl)12 < CoPc(Cl)16. The removal ratio could reach 90% using CoPc(Cl) 16 catalyst. For the desulfurization of real gasoline, the sulfur content can be reduced from 1000 to 30 ppm. Wang et al. reported the oxidative and extractive desulfurization of BT and DBTs in fuel oil over N-hydroxyphthalimide (NHPI) catalyst in [BMIM][BF4] with molecular oxygen as oxidant.91 The removal of DBT could reach 100% with O2 at the initial pressure of 0.3 MPa and reaction temperature of 120 °C.

optimal conditions, the removal of DBT from model oil could reach 94.3%. To avoid the oil−aqueous biphasic problems, several researchers proposed using gas as the oxidant in oxidative desulfurization process.89−91 The dielectric barrier discharge (DBD) air plasma atmosphere oxidation using MnO2 catalyst combined with IL extraction was used for the removal of sulfur compounds including thiophene (TS), benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in model oil (n-octane).89 The schematic diagram of experimental apparatus is shown in Figure 1. The removal ratios of TS, BT, DBT, and 4,6-DMDBT in model oil with the initial S content of 107 ppm can achieve 99.9%, 99.9%, 97.9%, and 90.7%, respectively. The oxidation activities of TS and BT were higher than that of DBT, being different from those reported with solid−oil−water three-phase oxidation 6947


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Figure 2. Concept process for deep desulfurization in a refinery combining hydrodesulfurization (HDS) and extraction and oxidative desulfurization (EODS) using IL. Adapted from ref 129 with permission. Copyright 2011 American Chemical Society.

Zhu et al.99−101 prepared several surfactant-type metal-based a nd PO M- b ase d ILs ( SPI L s ), e. g. , [( C H 3 ) N(nC8H17)3]2Mo2O11, [(CH3)N(n-C8H17)3]2W2O11, [Q3NCH3]3{PO4[MoO(O2)2]4}, and [Q3NCH3]3{PO4[WO(O2)2]4} (Q = n-C8H17 and n-C12H25), and investigated their oxidative desulfurization performance. The use of SPILs can solve the major drawbacks of conventional peroxo polyoxometalate catalysts in organic solvents such as catalyst deactivation, volatilization, and toxicity. The supposed mechanism as shown in Scheme 4 and kinetics studies on the [(nC8H17)3NCH3]3{PO4[MoO(O2)2]4} catalyst revealed that the oxidative desulfurization of organosulfur compounds presents a pseudo-first-order kinetics. Almost 100% DBT could be removed using SPILs catalysts, whereas the S content could be reduced from 150 to 15 ppm for the desulfurization of actual prehydrotreated fuel. Wu et al.102 investigated the oxidative activities of solid acidic IL polymer (PIL) prepared by the copolymerization of acidic IL oligomers and divinylbenzene (DVB). The results showed that the new solid catalyst led to 95.5% removal of BT at 50 °C in 20 min. Under the same reaction conditions, the catalytic activity of PIL is much higher than those of sulfonic acid groupfunctionalized IL monomer, PSMIM (1-methyl-imidazolium-3propylsulfonate), PSMIMHSO4 (1-methyl-imidazolium-3-propylsulfonate hydrosulfate), and traditional SO42−/ZrO2. Wang et al. 1 0 3 found that the SiO 2 -supported [BMIM]3PMo12O40 exhibited a high catalytic activity in the oxidation of DBT, and 100% DBT conversion was achieved at 60 °C with the loading of 20 wt % [BMIM]3PMo12O40 on SiO2. The high catalytic performance was attributed to its amphiphilicity, which could enhance the adsorption of both H2O2 and S compounds. For the sulfur removal from a real diesel, the efficiency could reach 97.9% and the S content was reduced from 530 to 12 ppm after oxidation and separation. The [BMIM]PW/HMS catalyst was synthesized through impregnating the hexagonal mesoporous silica (HMS) support

Scheme 7. Proposed Mechanism of Fast Oxidation of Heterocyclic S Compounds with [HCPL][TFA]a

a


2.1.2. IL-Based Catalysts for Oxidative Desulfurization. Recently, a number of quaternary ammonium-coordinated ILs, metal-based surfactant-type ILs, IL polymers (ILPs), immobilized ILs based on different supports (e.g., MCM-41, SBA-15, NaY, MOFs, and porous carbons), and others were proposed as homogeneous or heterogeneous catalysts for ultradeep desulfurization of fuel oil.92−98 The recent outcomes on oxidative desulfurization using IL-based catalysts are summarized in Table 4. 6948


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Scheme 8. Synthesis Routes of Oxidative−Thermoregulated Phosphotungstate Bifunctional ILsa

a

Reprinted with permission from refs 141 (a) and 142 (b). Copyright 2014 Elsevier.

Scheme 9. Oxidation of Sulfides to Sulfoxides (a) and Sulfones (b)

molar ratio of H2O2, S, and catalyst is 10:1:0.025. In the case of DBT, it can achieve 100% removal in 4 h at 50 °C with the same ratio. The authors claimed that the sulfur removal process has no apparent scale-up effect when the treatment capacity of model oil increases from 50 to 1500 mL. The active tungsten species were successfully introduced into the mesoporous MCM-41 by using [C16MIM]Br as template and [C16MIM]3PW12O40 as tungsten source. The prepared catalyst exhibited an excellent catalytic activity on the removal of DBT without additional solvent and extractant.108 The sulfur can be completely removed at the initial content of 500 ppm in n-octane and a low H2O2/DBT molar ratio (2.5) at 60 °C in 30 min. Recently, the same group109 introduced the POM-based IL ([C16MIM]3PW12O40) directly into mesoporous silica in which the imidazole cation and polyoxometalate anion acted as template and metal source, respectively. The obtained hybrid materials (W-SiO2) exhibited excellent activity for the oxidative desulfurization from a model oil within 40 min. The process did not need the addition of organic solvents as extractant. After seven cycles, the catalyst still showed high desulfurization efficiency (DBT removal from 90.3% to 100%). Wu et al.110 used IL-supported Zr metal−organic framework as catalyst for deep desulfurization, where 1-methylimidazolium-3-propylsulfonate hydrosulfate [PSMIM][HSO4] was supported onto Zr metal−organic framework (MOF) UIO66. The BT removal can achieve 94.6% at 30 °C in 20 min from

by phosphotungstic acid (HPW) and the IL [BMIM][HSO4] and was evaluated by the oxidative desulfurization process to remove BT, DBT, and 4,6-DMDBT from model oil with H2O2 as oxidant.104 Under the optimal reaction conditions, the removal of BT, DBT, and 4,6-DMDBT could reach 79%, 98%, and 88%, respectively. The tungsten-containing mesoporous silica was synthesized using POM-based IL [(nC8H17)3NCH3]2W2O11 as template and W precursor.105 Under the optimal conditions, the removal of DBT from model oil could reach 99.6%. Noteworthily, the sulfur removal ratios of 4,6-DMDBT and BT, which are often regarded as a difficult task, can achieve up to 100% after 50 min and 95.7% after 120 min, respectively. After eight recycles, the catalyst still exhibited a high sulfur removal efficiency (99.2%) with only slight decrease. The polyoxometalate (POM) cluster (Na7H2LaW10O36·32H2O) was immobilized onto dihydroimidazolium-based IL-modified mesoporous silica to form a new catalyst (LaW10/IL-SiO2), and the prepared catalyst could achieve deep desulfurization for DBT, BT, and 4,6-DMDBT.106 Shi et al.107 used peroxophosphotungstate held in IL brush as heterogeneous catalyst and methanol as solvent for the removal of BT and DBT from model oil using 30 wt % H2O2 as oxidant. The IL brush catalyst SiO2-BisILs[(PW12O40)3− is based on SiO2 support and shows a high activity for the oxidation of S compounds. The highest removal of BT in n-octane with the content of 1000 ppm can reach 100% in 7 h at 70 °C when the 6949


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Table 6. Summary of the Oxidation of Sulfides to Sulfoxides in ILs

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Table 6. continued

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Table 6. continued

a

The yield was obtained by the conversion based on sulfide substrate multiplied by the selectivity toward sulfoxide.

the initial content of 2000 ppm in n-heptane with the O/S molar ratio of 7:1. After reaction, the solution was separated by centrifugation. Recently, the graphene analogue hexagonal boron nitride (Gh-BN) was used as support to coat with W-based IL as heterogeneous catalyst (IL/G-h-BN) for oxidative desulfurization.111 The catalytic activity of the few-layer materials supported with IL was more excellent for the oxidation of DBT and superior to the homogeneous catalysts of ILs themselves. The sulfur removal of DBT from model oil could reach 99.3% at 30 °C. The authors stated that the oxidative desulfurization initiated with the adsorption of H2O2 and S compound over the surface of IL/G-h-BN simultaneously or subsequently, and then H2O2 reacted with the active center W6O192− to form peroxo species, which further reacted with adsorbed DBT. After the products desorbed from the IL/G-hBN surface, the catalyst could be reused. The proposed reaction mechanism is shown in Scheme 5. The POM-based IL [C16MIM]3PMo12O40 was employed for the synthesis of Mo-containing ordered mesoporous silica (MoOMS), where the long-chain imidazole cation [C16MIM] acts

as the template of ordered mesoporous and the POM anion acts as the source of active metal sites.112 The synthesis procedure of the catalyst as well as the proposed desulfurization procedure is shown in Scheme 6. The catalyst was very effective for the S compounds removal with a low usage of oxidant (H2O2), and after 30 min the sulfur removal of 4,6-DMDBT, BT, and DBT was 100%, 87%, and 89%, respectively. 2.1.3. ILs as Both Extractants and Catalysts for Extraction and Oxidative Desulfurization. The ILs based on metal halide anions, Brønsted acidic ILs, and several other ILs can not only serve as extractants and reaction media but also act as catalysts in the extraction and oxidative desulfurization processes.113−119 The additional catalysts are not needed, thus avoiding the problem of separation and regeneration of catalysts. The recent outcomes on oxidative desulfurization using ILs as dual extractants and catalysts are summarized in Table 5. Li et al.120 investigated the performance of three redox ILs based on FeCl3 in the extraction and oxidative desulfurization process for the removal of benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,66952


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Table 7. Summary of the Oxidation of Sulfides to Sulfones in ILs

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Table 7. continued

a

The yield was obtained by the conversion based on sulfide substrate multiplied by the selectivity toward sulfone.

[C10H21(CH3)3N]Cl/FeCl3, and [(C10H21)2(CH3)2N]Cl/ FeCl3. The oxidation reactivity for different S compounds was in the order of DBT > BT > 4,6-DMDBT, and the removal of DBT, BT, and 4,6-DMDBT from model oil could reach 97.9%, 83.5%, and 58.5%, respectively. In contrast, the desulfurization efficiency of [(C4H9)3CH3N]Cl/MCl2 (M = Cu, Sn, and Zn) ILs is not so good. The sulfur content in FCC gasoline can be reduced from 360 to 110 ppm using [(C4H9)3CH3N]Cl/FeCl3. Moreover, [Et3NHCl]FeCl3 exhibits the best sulfur removal efficiency among the ILs [Et3NHCl]-

DMDBT) in model oil (n-octane) using H2O2 as oxidant. The results showed that the sulfur removal ratio can achieve 99.2% for [BMIM]Cl/FeCl3/H2O2 under mild reaction conditions. However, for the mere extraction with the same IL, only 68.3% of sulfur can be removed. The same group also investigated the performance of several Fenton-like ILs for the removal of S compounds.121 The results showed that the iron-containing ILs reveal high oxidative desulfurization ability, and the sulfur removal of DBT-containing model oil is above 90% using three FeCl3-based ILs, i.e., [(C4H9)3CH3N]Cl/FeCl3, 6954


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could reach 99.7% after seven runs. Nie et al.125 went a further step to investigate a series of Lewis ILs containing Fe halide anions (e.g., [BMPy]FeCl4, [HMPy]FeCl4, and [OMPy]FeCl4) for the desulfurization of DBT from model oil. It was concluded that DBT could be completely removed in 10 min at 298 K. When [OMPy]FeCl4 acted as both extractant and catalyst, the S content in actual gasoline could be reduced from 468 to 261 ppm. Recently, a series of zinc chloride Lewis acid ILs [BMPy]Cl/nZnCl2 (n = 1, 1.5, 2, 3) wasproposed as both extractant and catalyst in the presence of H2 O 2 for desulfurization of the fuels.126 The results showed that [BMPy]Cl/3ZnCl2 exhibits almost 100% DBT removal from model oil under optimal conditions. After six cycles of regeneration, the sulfur removal efficiency only had a slight decrease. Li et al.127 prepared a series of magnetic ILs [C3H6COOHMIM]Cl/xFeCl3 (x = 0.5, 1, 1.5, 2) for the removal of refractory aromatic sulfur compounds by the combination of extraction and oxidation. The IL [C3H6COOHMIM]Cl/2FeCl3 was proved to be highly active, and the removal of BT and DBT could be achieved 100% in 10 min. After reaction, the magnetic IL could be easily separated by applying an external magnetic field due to its paramagnetic property. Nie et al.128 investigated the performance of three functionalized ILs ([(CH 2 ) 2 COOHMIM]Cl/nFeCl 3 , [(CH2)2COOHMIM]Cl/nZnCl2, and [AMIM]Cl/nFeCl3) for extraction and catalytic oxidative desulfurization of liquid fuels. It was proved that the nature of the functional groups (−COOH, −CH2−CHCH2) in cations and the acid strength of anions play an important role for S removal. It is interesting to find that the FeCl3-based ILs present high sulfur removal in a short reaction time at 25 °C, while the ZnCl2-based ILs do it in a long reaction time at 60 °C. At the same time, the nitrogencontaining compounds (e.g., pyridine, pyrrole, and quinolone) could be removed simultaneously. Chen et al.129 studied the deep oxidative desulfurization using acidic ILs like Lewis acidic ILs ([BMIM]Cl/2ZnCl2 and [BMIM]Cl/ZnCl2) and Brønsted acidic ILs ([CH2COOHMIM]HSO4, [SO3H-C4MIM]HSO4, [BMIM]HSO4, and [HMIM]HSO4). It was found that for acidic ILs both anion and cation play an important role in the extractive and catalytic ability. The authors claimed that 100% DBT can be removed from model diesel fuel using [BMIM]Cl/2ZnCl2 and [SO3H-C4MIM]HSO4. For commercial diesel fuel, the

Scheme 10. Suggested Mechanism of Oxidation of Sulfide in [BMIM][BF4]

MCln (M = Fe, Cu, Zn, Co, Sn, and Cr). The deep desulfurization, i.e., sulfur content in model oil less than 10 mg·L−1, can be achieved in [Et3NHCl]FeCl3 in only 5 min.122 Furthermore, the sulfur removal from prehydrotreated gasoline for the [Et3NHCl]FeCl3/H2O2 system at 30 °C under the optimal experimental conditions was tested, the S content being reduced from 150 to 15 mg·L−1 after two rounds of reaction. The Raman spectra indicate no transformation of the [FeCl4]− anion during the reaction, and the possible existence of nitrogen compounds (Lewis base) in real oil could not deactivate the catalyst. The same authors reported the use of a temperature-responsive magnetic pyridinium-based IL [BPy][FeCl4] as both extractant and catalyst for deep desulfurization.123 At room temperature (30 °C) [BPy][FeCl4] is at solid state, while it starts to melt at 38.4 °C and becomes a liquid at 40 °C. Thus, at low temperatures the solid [BPy][FeCl4] is only as catalyst, and only 51.2% sulfur can be removed at 30 °C. When [BPy][FeCl4] turns to a liquid, the IL acts as both catalyst and extractant. At 40 °C the removal ratio of sulfur can reach 95.3%. Furthermore, [BPy][FeCl4] has a strong response to a magnet, indicating that it can be easily recovered from model oil only by applying an additional magnetic field. The sulfur removal of DBT, BT, and 4,6-DMDBT from model oil in [BPy][FeCl4] could achieve 95.3%, 75.0%, and 54.8% at 10 min, respectively. Dong et al.124 reported that nearly 100% DBT could be removed from model oil using [HMPy]Cl/ FeCl3 in 20 min at 298 K, while the S removal of real gasoline

Scheme 11. Possible Mechanism for the Oxidation of Methyl(phenyl)sulfane for H2O2−V2O5 in [C12MIM][HSO4]a

a

Adapted from ref 152 with permission. Copyright 2014 Royal Society of Chemistry. 6955


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initial sulfur content (5380 ppm) the extraction and oxidative desulfurization performance is not so predominant. A proposed process combining hydrodesulfurization with oxidative extraction to obtain the sulfur-free or ultralow sulfur fuels is illustrated in Figure 2. For a series of Lewis acidic ILs [BMIM]Cl/MCl2 (M = Zn, Fe, Cu, Mg, Sn, or Co), the Sremoval efficiencies decrease in the order of Zn > Fe > Co > Mg > Cu > Sn.130 The S content in real diesel fuel could be reduced from 460 to 43 ppm using [BMIM]Cl/3ZnCl2 after five runs. Two acidic ILs ([(CH2)4SO3HMIM][Tos] and [(CH2)4SO3HMIM][ZnCl3]) were tested for the desulfurization of real diesel fuel in a coupled oxidative−extractive way.131 However, only 43.7% S-removal efficiency was obtained using [(CH2)4SO3HMIM][Tos] as catalyst and H2O2 as oxidant after one-step oxidation. Even after eight-step oxidation, the remaining S content was still 48 ppm, which cannot meet the requirement of stringent legislation. Thus, the authors proposed a coupled extractive−oxidative desulfurization process, where the acidic ILs as catalyst were used for oxidative desulfurization followed by extraction of the residual sulfones in oil phase with conventional ILs. The S content in real diesel fuel could be reduced from 225 to 4.6 ppm. Later, they synthesized a series of N-methylpyrrolidonium zinc chloride ILs [HnMP]Clx/(ZnCl2)y (x:y ranging from 2:1 to 1:2) as both extractants and catalysts for the desulfurization from model oil and real FCC feedstock.132 For the desulfurization of real FCC diesel fuel the S-removal efficiency (less than 38.2%) was much lower than that for model diesel oil (99.9%). Thus, the authors proposed the strategy of multistage oxidative desulfurization to achieve a satisfied desulfurization result. After five-stage oxidative desulfurization with one more extractive desulfurization using furfural as extractant, the total S removal could reach 97.6% for FCC diesel fuel with the final S content of 5.3 ppm. The Brønsted acidic ILs [BMIM][HSO4] and [BPy][HSO4] were suggested as both extractant and catalyst for the desulfurization of diesel fuel.133 The results showed that [BMIM][HSO4] was better than [BPy][HSO4] during the desulfurization process, and the removal of DBT in model oil could achieve 99.6% in 90 min for [BMIM][HSO4]. For diesel fuel, the S content could be reduced from 97 to 13.8 ppm at room temperature. Zhang et al.134 also tested the activity of [BMIM][HSO4] for the desulfurization of DBT and found that under optimal conditions DBT could be 100% removed in model oil. The Brønsted acidic ILs [CH2COOHPy][HSO4] and [(CH2)2COOHPy][HSO4] were used as extractant and catalyst, respectively, for the desulfurization of model oil (noctane).135 The results showed that the IL [CH2COOHPy][HSO4] has the stronger acidity and better desulfurization performance. Under appropriate reaction conditions, the sulfur removal of DBT in model oil with the initial S content of 1000 ppm could reach 99.9% with [CH2COOHPy][HSO4], and the IL can be recycled 9 times without a significant loss in sulfur removal. The removal ratios of BT and 4,6-DMDBT with [CH2COOHPy][HSO4] can reach 82.5% and 89.1%, respectively. To identify the structure−property relation, the extraction−oxidative desulfurization of thiophene from model oil (n-octane) was exemplified.136 The desulfurization capability for various ILs with the same imidazolium cation and different anions follows TFA− > HSO4− > COO− > AlO4− > AcO−, indicating that the catalytic and extractive capability of ILs with stronger acidity is better. The thiophene content can be decreased from 202.544 to 7.320 μg·mL−1 with the volume ratio of model oil to IL ([C4MIM][TFA]) 35:1 at 70 °C after

Scheme 12. Peroxotungstates Immobilized on IL-Modified Silica SiO2-W2-Im (a), SiO2-W2-Py (b), and Peroxotungstates Held in Bislayer Ionic Liquid Brushes (c)

sulfur content can be reduced from 64 to 7.9 ppm using [BMIM]Cl/2ZnCl2, whereas for coke diesel fuel with the high 6956


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Scheme 13. Preparation of the POM Catalyst [Cn+2mim]3PM

Scheme 14. Proposed Mechanism for the Oxidation of Sulfide with [bMImB]·(Br3)2a

a

Adapted from ref 176 with permission. Copyright 2015 Royal Society of Chemistry.

30 min. Recently, Jiang et al.137 synthesized a series of Brønsted acidic ILs with amide- or lactam-based cations as extractants and catalysts for the desulfurization of both model oil and diesel fuel. Among the ILs investigated, [HCPL][TFA] exhibited the highest S-removal efficiency, and BT and DBT in model oil with the initial content of 1000 ppm could be completely removed within 2 h. For the hydrogenated diesel with the S content of 659.7 ppm through a two-step desulfurization process using [HCPL][TFA], the S content could be reduced to 8.62 ppm. In the case of straight-run diesel, the S content could be reduced from 11034 to 89.36 ppm. The proposed mechanism with [HCPL][TFA] is shown in Scheme 7. The special structure of [HCPL] cation allows it to share two active hydrogen atoms simultaneously with one [TFA] anion to form peroxide for catalytic oxidation. Chi et al.138 used the functional acidic IL [EIMC4SO3H][Tf2N] as both extractant and catalyst for the removal of DBT in n-octane using H2O2 or NaClO as oxidant. It was found that for five different types of ILs the extractive capability is in the order of [EIMC4SO3H][Tf2N] > [EIMC4SO3H][HSO4] > [BPy][Tf2N] > [BMIM][Tf2N] > [BMIM][PF6], and the DBT content decreases from 1600 to less than 20 ppm in the case of [EIMC4SO3H][Tf2N]. Moreover, the extraction and oxidation process with different oxidants obeys different mechanisms. In the case of NaClO as oxidant, DBT is oxidized continuously at the oil/IL interface, and the oxidized product DBTO2 dissolves rapidly into the IL phase; in the case of H2O2 as oxidant, DBT is first extracted from oil to IL phases and then oxidized by hydroxyl radicals provided by the decomposition of H2O2. A halogen-free task-specific IL (TSIL) [(CH2)2COOHMIM][HSO4] was also suggested as catalyst and reaction media for the deep oxidative desulfurization of both model diesel and real diesel.139,140 During the desulfurization process, the TSILs first extract the S compounds from the diesel, and then the carboxylic acid group reacts with H2O2 to generate a peroxycarboxylic acid group in situ. Finally, the S compounds are oxidized to the corresponding sulfones. The removal of DBT and 4,6-DMDBT from model diesel could reach 96.7% and 95.1%, respectively. The sulfur level in real diesel could be decreased from 200 to 20.5 ppm at room temperature.

However, an additional extractant NMP is needed to remove the residual oxidized sulfur compounds. Yu et al.141 developed several ammonium oxidative− thermoregulated bifunctional ILs for oxidative desulfurization of fuels by introducing polyoxyethylene chain as the thermoregulated structure unit and polyoxyethylene as catalytic oxidation group, and the synthesis route is shown in Scheme 8a. The synthesized ILs have their own critical solution temperature in the toluene/n-octane or n-dodecane mixed solvents, enabling the system to exhibit the so-called thermoregulated feature of “homogeneous at high temperature and heterogeneous at low temperature” and to achieve an integration of homogeneous catalysis and heterogeneous separation. The removal of thiophene from n-octane and ndodecane model oils using IL (n = 96, R = C4H9) as catalyst and H2O2 as oxidant could be achieved 92.3% and 93.2% under optimal conditions, respectively. They also synthesized the thermoregulated imidazolium ILs based on heteropolyanion, as shown in Scheme 8b.142 The desulfurization rate of thiophene from model oil could reach 85.3% using IL (n = 19) as catalyst and O2 as oxidant. After reaction, the upper oil phase was withdrawn and then extracted with DMF. 2.2. Preparation of Sulfoxides and Sulfones

Organic sulfoxides and sulfones are important synthetic intermediates for the construction of various fine chemicals and biologically active molecules. The selective oxidation of sulfides is an attractive and important method to obtain sulfoxides and sulfones. Since hydrogen peroxide is widely regarded as a “green” and effective oxidant in organic oxidations, the catalytic oxidation of sulfides with aqueous H2O2 as oxidant has attracted great interest as a direct route for selective production of sulfoxides or sulfones. Undoubtedly, a suitable and efficient catalyst for the oxidation of sulfides must be highly selective for either sulfones or sulfoxides, inexpensive, easy to prepare and handle, operative under mild conditions, and stable long term. The catalysts based on tungsten (W), molybdenum (Mo), vanadium(V), titanium (Ti), and other metals, as well as several nonmetallic catalysts, have been proved to have high activities in many oxidation procedures using hydrogen peroxide as oxidant.143−149 However, there is 6957


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2.2.2. ILs or IL-Based Catalysts. Hajipour et al.154 reported the oxidation of sulfides to sulfoxides using metal nitrates or several non-nitrates oxidants in the presence of 1hexyl-3-methylimidazolium bisulfate ([HMIM][HSO4]) as Brønsted acidic IL under solvent-free conditions. Ceric ammonium nitrate (CAN) was found to be the best oxidant. Besides, no byproducts such as overoxidation to sulfones or nitration of aromatic rings were produced. The catalytic properties of imidazolium perrhenate ILs (IPILs) for the oxidation of sulfides to sulfones using aqueous H2O2 and hydrogen peroxide urea adduct (UHP) as oxidants were tested by Zhang et al.155 It was found that the oxidation of thioanisole with H2O2 was strongly dependent on the selection of solvents, and [BMIM][BF4] turned out to be the best solvent, providing the highest yield and selectivity. The approach is a significant improvement when compared to that using homogeneous Re(V) catalyst and UHP for the oxidation of sulfides in acetonitrile,156 in which the reaction time is very long and the catalyst cannot be reused. In 2005, Karimi et al.157 reported a type of silicafunctionalized ammonium tungstate heterogeneous catalyst for selective oxidation of sulfide to sulfoxides using 30% H2O2. As a result, a good yield (up to 91%) of sulfoxides could be obtained, but excessive H2O2 was required (3−8 equiv, based on the amount of sulfide). Shi and Wei158,159 investigated the catalytic activities of two kinds of catalysts of peroxotungstates immobilized on IL-modified silica, i.e., SiO2-W2-Im and SiO2W2-Py, as shown in Scheme 12a and 12b. The selectivity either to the formation of sulfoxides or to the formation of sulfones can be controlled by changing the ratio of catalyst, H2O2, and sulfide. A yield of sulfoxides from 80.4% to 94.8% can be obtained in the case of the molar ratio of catalyst, H2O2, and sulfide being 1.5:110:100 in the methanol and CH2Cl2 mixture (molar ratio of 1:1) as solvent and reacting 2.5−4 h at room temperature. When the molar ratio of catalyst, H2O2, and sulfide was changed to 2:250:100, the same amount of sulfone can be produced with the reaction time ranging from 4 to 7 h at room temperature. Meanwhile, the yield of methyl phenyl sulfoxide was still satisfying after 6 cycles of the catalyst. For conventional immobilized IL catalyst (e.g., SiO2-W2-Im and SiO2-W2-Py, as shown in Scheme 12a and 12b) only one imidazolium is presented on each of the side chains, and thus, the amount of catalytic active centers is limited. For this reason, Shi et al.160−162 proposed using the supported multilayered ILs (referred to as “ionic liquid brush”, as shown in Scheme 12c), which have more imidazoliums on each of the side chains. It seems that the multilayer catalysts outperform the monolayer catalysts except that the catalytic activity of the trilayer analogue is lower than that of the dilayer analogues. Furthermore, the catalyst exhibits high chemoselectivity toward sulfur groups with unsaturated double bonds, and there is no apparent loss of catalytic efficiency until the eighth cycle. The peroxotungstate catalyst held in ionic liquid brush was proved to be also efficient in water as solvent for the selective oxidation of sulfides using 30% H2O2 as oxidant, while no organic cosolvents or other additives were required. The IL-based polyoxometalates (POM) salts were proposed as catalysts due to the controllable redox and acidic properties with high activity and convenient recovery and reuse. Wang et al.163−165 prepared a series of IL-based POM catalysts with different carbon chain in alkyls and various Keggin-type POMs. The synthesis procedure is shown in Scheme 13. It was confirmed that the imidazolium POM salts are very active and

still a demand to develop alternative systems to avoid the drawbacks such as long reaction times, difficulty in catalyst isolation and recycling, needing excessive catalyst or oxidant, environmental hazards, and poor recovery of expensive metal catalysts. Noticeably, ILs are a fascinating class of solvents and catalysts in the application of oxidation of sulfides to selective preparation of sulfoxides or sulfones under homogeneous, liquid−liquid, and liquid−solid heterogeneous phases conditions. Recently, the immobilized metal-containing ILs as catalysts combining with the unique properties of ILs, i.e., chemical and thermal stability, capacity for extraction of polar substrates and reaction products, and the extended surface of mineral or polymer supports, have been proposed for the heterogeneous selective oxidation of sulfides. The ILs concerned in this review may act as solvents, catalysts, reagents, or their combinations. Scheme 9 shows the reaction mechanism of the oxidation of sulfides to sulfoxides and sulfones. The recent outcomes on the oxidation of sulfides to prepare sulfoxides and sulfones in relation with ILs are summarized in Tables 6 and 7, including the substrates, solvents, oxidants, catalysts, reaction conditions, yields, and selectivities. 2.2.1. ILs as Solvents. ILs as a class of potentially greener solvents has been widely proposed for the synthesis of sulfoxides and sulfones. Zhang et al.150 presented an efficient means to selectively oxidize sulfides to sulfoxides in ILs without catalyst using aqueous H2O2 (35%) as oxidant. It showed that a high yield of sulfoxides can be obtained only if the oxidant is dissolved in the solvent to form a homogeneous reaction system. Although the solvent methanol exhibits very good conversion and yield, it needs a long reaction time (18 h) and functional group decomposition occurs as well. However, the reaction becomes much faster using ILs as solvents than using conventional organic solvents. Within 4 h, the conversion of thioanisole and the selectivity of sulfoxide using [BMIM][BF4] as solvent can achieve 98% and 95%, respectively. The reaction mechanism was explored by the techniques of IR, Raman, and NMR spectroscopies. As shown in Scheme 10, the crucial step is the formation of a hydrogen bond between IL and H2O2. Hu et al.151 studied the aerobic oxidation of sulfides to sulfoxides catalyzed by Mn(OAc)2/[C12MIM][NO3] using molecular oxygen as oxidant. The targeted sulfoxide products with high yields were obtained, and no overoxidation was observed under mild reaction conditions. During the reaction, the Mn(OAc)2 catalyst provides a source of Mn(II), which reacts with the oxidant O2 in IL to form OMn(IV)O. Then, OMn(IV)O reacts with sulfide to form the corresponding sulfoxide. A simple, efficient, and ecofriendly method for the oxidation of sulfides to sulfones using H2O2 in IL solvent catalyzed by V2O5 was presented.152 It was observed that the IL [C12MIM][HSO4] gives the best results. The difference of catalytic performance among various ILs may be due to the abilities and capacities to stabilize and dissolve the oxidant and catalyst. The possible mechanism for the oxidation of methyl(phenyl)sulfane for H2O2−V2O5 in [C12MIM][HSO4] is shown in Scheme 11. In addition, Hu et al.153 also reported the oxidation of sulfides using molecular oxygen as oxidant catalyzed by manganese acetate (Mn(OAc)2) in IL. It was found that among the ILs investigated, [C12MIM][NO3] exhibits the best oxidation performance for methyl(phenyl)sulfide, mainly due to the high solubility of O2. No overoxidized product was detected by 1H NMR analysis. The yield of sulfoxide can achieve 97% within 2 h. 6958


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H2O2 under solvent-free condition. Compared with other catalysts for catalytic oxidation of methyl phenyl sulfide by H2O2 under solvent condition, the PILW catalyst shows equal or even higher yields under solvent-free condition. 2.2.3. ILs as Reagents. With the ultimate goal of hazardfree and waste-free efficient synthesis of sulfoxides, tribromide IL was proposed as reagent for selective oxidation of sulfides.176−184 The reactions for a number of substrates using the IL 1,4-bis(3-methylimidazolium-1-yl) butane ditribromide [bMImB]·(Br3)2 as reagent and the mixed acetonitrile−water (volume ratio of 2:1) solvent were performed within 5−20 min with high yields, and no overoxidation byproducts were detected. A proposed mechanism for the oxidation of sulfide is shown in Scheme 14. The molecular bromine generated in situ is treated as the active species and acts as an electrophile. An electrophilic attack of Br2 on sulfur leads to the formation of intermediate (A), which is then hydrolyzed by H2O to obtain the corresponding sulfoxide. The product sulfoxide was extracted with ethyl acetate. The byproduct [bMImB]·(Br)2 in aqueous layer was recovered by evaporation, dried at 40 °C for 12 h, and then treated with

selective heterogeneous catalysts for the oxidations of sulfides. The oxidation process almost exclusively produces sulfoxide at room temperature in the solvent methanol with the substrate and H2O2 molar ratio of 1:1.1 within 0.5 h. However, over the same catalyst sulfone can be selectively produced at a higher temperature of 45 °C and with excess H2O2 (20 equiv of substrate). The POM salt catalysts with the [BMIM] cation and the Keggin-structured 12-phosphomolybdic (PM), 12-phosphotungstic (PW), and 12-silicotungstic (SiW) anions resulted in similar heterogeneous catalysis. When the alkyl side chain was prolonged to [HMIM], the POM [HMIM]3PM was still not soluble in the reaction, whereas [C12MIM]3PM would lead to a homogeneous reaction due to the enhancement of lipophilicity. Spectroscopic analysis reveals that the heterogeneous nature of the oxidation process associates with the good crystallinities of butyl- and hexylimidazolium POM salts and the hydrogen-bonding networks among cations and anions. The excellent performance of heterogeneous POM salts in selective oxidation of sulfides is mainly due to the improved redox property of the POM anion by the intermolecular charge transfer from the cation of IL. The formation of ionic linkage between the cation and the POM anion was verified. Moreover, phase transfer catalysis may also play a great role in the high activity of such POM salts. Carrasco et al.166 investigated the supported IL phase (SILP) catalyst SBA-15 + ImCl + MoO5, which was synthesized by immobilization of an oxodiperoxomolybdenum complex onto the IL thin film of SILP materials (SBA-15 + ImCl, prepared by reaction of 1-methyl-3-(3-(triethoxysilyl)propyl)-1H-imidazol3-ium chloride with a mesoporous SBA-15 silica) for oxidation of sulfides to sulfoxide with aqueous H2O2 in the solvent methanol at room temperature. It was found that the conversion of sulfide (95%) and the selectivity toward sulfoxide (98%) for SBA-15 + ImCl + MoO5 are much higher than those for SBA-15 + ImCl or SBA-15 + MoO5 under similar conditions. Moreover, the conversion and selectivity are also higher than those obtained in the process catalyzed by a molecular molybdenum complex in the [BMIM][PF6] solvent (conversion 80% and selectivity 96% for [Mo(O)(O2)2(Mepz)2]).153 The supported Brønsted solid acidic catalyst IL-HSO4@SBA-15 was synthesized by incorporating SBA-15 with the metal and halogen-free hydrogensulfate IL, which was used for oxidation of aromatic and aliphatic sulfides to sulfoxides using H2O2 as oxidant.167 The oxidation process showed high selectivity toward sulfoxides. The polymer-immobilized IL catalysts (IL immobilized in the form of a cation-decorated copolymer) integrate the favorable properties of ILs with the advantages of polymer support.168−174 [PO4{WO(O2)2}4]@PIILP by immobilizing peroxophosphotungstate [PO4{WO(O2)2}4]3− on a ROMPderived pyrrolidinium-decorated polymer is an efficient catalyst for sulfide oxidation by H2O2. A high selectivity to sulfoxide can be obtained in batch, segmented, and even continuous flow processes under mild conditions and in short reaction times at room temperature in methanol (1−3 equiv of substrate, 20 °C). Meanwhile, the sulfone can be obtained in acetonitrile at higher temperature (45 °C) with excess H2O2 (5 equiv of substrate). The catalyst is robust and ideally suited for scale up because it still remains active and stable for 8 h under continuous flow operation. Recently, Pourjavadi et al.175 proposed a new heterogeneous catalyst by immobilization of tungstate ions on a cross-linked poly(ionic liquid) nanogel (PILW), which exhibits high yields for selective oxidation of sulfides to sulfoxides by

Scheme 15. 3-(3-(1,2-Dicarboxyethylamino)-3-oxopropyl)1-methyl-1H-imidazol-3-ium Bromide (L-AAIL)

molecular bromine in an ice bath for 2 h to regenerate [bMImB]·(Br3)2. The IL can be recovered several times and reused without any significant loss of the activity. Ahammed et al.185 reported an oxidation process of organosulfides to sulfones using the bifunctional IL [pmim]IO4 without any other oxidants, metals, and organic solvents at ambient temperature. It was assumed that the oxidation was mediated by the IO4 ion.

3. OXIDATION OF ALCOHOLS Oxidation of alcohols to carbonyl compounds (e.g., aldehyde, ketone, and carboxylic acid), which are essential constituents of chemical platform, pharmaceuticals, and other important chemicals, is one of the most important fundamental transformations in organic chemistry. In most instances, oxidation of alcohols is performed using transition metal catalysts in organic solvents, which are neither environmentally friendly nor economical and usually have serious toxicity issues. In recent years, much attention was focused on the utilization of ILs as both solvents and catalysts aiming to improve the selectivity and yield. In this section, the recent developments on the oxidation of alcohols using ILs as solvents, catalysts, or both will be discussed and highlighted. In addition, the detailed reaction conditions are listed in Table S5 in the Supporting Information. 3.1. ILs as Solvents

In the past decade, various transition metals, e.g., palladium, ruthenium, tungsten, rhenium, iron, copper, manganese, and vanadium, have been used as catalysts for alcohol oxidation with ILs as green solvents.186−196 A series of square planar nickel(II) complexes containing N, O-donor Schiff ligand was used as catalysts for oxidation of 6959


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Scheme 16. Structures of S4SiIL and S3PILa

a


alcohols in IL media using NaClO as oxidant. 197 All experiments were carried out at air atmosphere, and nickel(II) complexes are air stable. Rong et al.198 reported a selective oxidation process of alcohols to carbonyl compounds in the IL [BMIM][BF4] using sodium hypochlorite NaClO as oxygen source. The oxidation process proceeded more slowly in [PF6]or [CF3COO]-type IL, possibly due to the free oxygen atoms easily derived from oxidant in [BF4]-type IL. The authors claimed that [BMIM][BF4] acts as both catalyst and solvent. Moreover, other oxidants (e.g., t-BuOOH, NaClO2, and H2O2) exhibited very poor oxidative performance in conversion and selectivity. Chen et al.199 pointed out that IL as solvent could effectively intensify cyclohexanol oxidation when compared to three conventional solvents using WO3 as catalyst and H2O2 as oxidant. The use of [OMIM]Cl as solvent leads to 100% conversion of cyclohexanol and 100% selectivity to cyclohexanone at 70 °C. The authors claimed that hydrophobic ILs are more suitable for the reaction investigated. The COSMORS (conductor-like screening model for real solvent) theory was applied for predicting the polarities of the solvents to better understand the solvent effect, and the polarities of the solvents show the same trend as catalytic performance. Oda et al.200 reported that the [BMIM][PF6]/PhCF3 biphasic solvent system could uniquely accelerate the transition-metal-free aerobic oxidation of benzylic alcohols to obtain the corresponding ketones in good yields with molecular oxygen as the sole oxidant. Rong et al.201 studied the selective oxidation of alcohols to carbonyl compounds using the cheap novel Mn-based catalyst (salicylaldehyd amino acid Schiff base manganese ligand) in ILs. The results showed that the oxidation of α-phenylethanol proceeded rapidly in [BF4]-type ILs, much slower in [PF6]- and [CF3COO]-type ILs, while in conventional organic solvents (acetone and methanol) no oxidative reaction occurred. In [BMIM][BF4] all primary alcohols and secondary alcohols had been selectively oxidized to carboxylic acids and ketones, respectively. Moreover, the reaction of aromatic alcohols was fast and efficient, whereas aliphatic alcohols reacted more slowly.

of ILs as the catalyst for aerobic oxidation of primary and secondary alcohols to aldehydes and ketones, in which acetonitrile was used as solvent and α,α-azobis(isobutyronitrile) (AIBN) or tert-butylhydroperoxide (TBHP) was used as a radical initiator. High conversions of primary and secondary alcohols and high yields of aldehydes and ketones were obtained, and no overoxidation to carboxylic acids was observed. Moreover, the obtained product yields could be comparable to those of the homogeneous analogue under the same reaction conditions. The catalyst by loading phosphotungstic acid (H3PW12O40) onto amino-functionalized ILmodified mesoporous silica SBA-15 was proved to be versatile for the solvent-free oxidation of alcohols with H2O2 oxidant, resulting in high efficiency, convenient recovery, and steady reuse.205 Recently, the immobilized iron chloride IL on SBA-15 has been used as catalyst for the oxidation of benzyl alcohol to benzaldehyde under solvent-free condition.206 The results showed that IL-FeCl3 groups distribute mainly on the surface of the immobilized composite, and the coordination linkage of imidazole ring to FeCl3 was strong enough to keep it from leaching from the surface. Under solvent-free condition, the benzyl alcohol could be oxidized to benzaldehyde with a high TON (conversion moles per mole of FeCl3, 125.8 mol/molFe). However, the yield of benzaldehyde was not so high. It is known that nitroxyl radicals like 2,2,6,6-tetramethyl-1piperidinyloxyl free radical (TEMPO) have emerged as metalfree catalysts for oxidation of various alcohols to the corresponding carbonyl compounds effectively under mild reaction conditions.207−209 However, separation of TEMPO from the products could be a great problem. Besides, TEMPO is a rather expensive chemical agent. Several immobilized TEMPO variants including TEMPO-derived ILs have been synthesized and used for oxidation of alcohols to aldehydes and ketones.210−214 The results showed that the TEMPO-IL catalyst exhibits similar catalytic activity to TEMPO in the oxidation of a wide range of alcohols into the corresponding carbonyl compounds with excellent yields, and the carbonyl compounds can be easily separated from the TEMPO-IL catalyst. Moreover, to avoid the use of a toxic volatile organic compound (VOC) such as dichloromethane as reaction solvent, the common ILs such as [HMIM][BF4], [BMIM][PF6], and others were suggested. A homogeneous mixture composed of vanadate, acid, and TEMPO- functionalized ILs was proved to be an efficient catalytic system for oxidation of a

3.2. ILs or IL-Based Catalysts

Bordoloi et al.202−204 immobilized the POM-based compounds H5[PMo10V2O40]·32.5H2O onto the IL-modified mesoporous support SBA-15 (V2ILSBA) in the manner of supported films 6960


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epoxidation reactions. The recent outcomes on alkene epoxidation are summaried in Table S6 in the Supporting Information. Wong et al.225,226 demonstrated the catalytic oxidation of alkenes by MnII/Me4NHCO3 in the IL [BMIM][BF4] with H2O2 as the terminal oxidant and found that the lipophilic alkenes could be effectively epoxidated in this case. However, the MnII/Me4NHCO3 and [BMIM][BF4] system was inactive for the epoxidation of aliphatic terminal alkenes. Later, they227 reported a robust IL system ([PMPyr][C12H25SO4] + 1methyl-1-propylpyrrolidinium dodcyl sulfate) for rapid epoxidation of various aliphatic terminal alkenes to epoxides using MnII acetate (Mn(OAc)2) as catalyst and peracetic acid as oxidant. Tangestaninejad et al.228 studied the efficient epoxidation of alkenes with sodium periodate NaIO4 catalyzed by manganese porphyrins in IL, and a high yield of epoxides was obtained under mild reaction conditions. Introducing electron-withdrawing substituents such as bromine could increase the catalyst robustness. Commercially available molybdenum(VI) compounds were proved to be good catalysts for the epoxidation of olefins (cyclooctene) with the UHP or aqueous H2O2 as oxidant in the IL [BMIM][PF6].229−231 The epoxidation reaction proceeded more quickly in the IL media than in the conventional solvent chloroform. However, the system was somewhat less active toward terminal aliphatic olefin. The peroxy IL, 1-butyl-3-methylimidazolium peroxymonosulfate [BMIM][HSO5], was used as both oxidant and solvent for the synthesis of epoxides.232 The reaction system contains 1,1,1-trifluoroacetone (TFA) as an oxirane precursor, as illustrated in Scheme 17. Under optimum conditions the epoxidation of 4-bromocinnamic acid led to the epoxide formation in 30 min with high yields of final epoxides. When compared to the strong oxidant m-chloroperbenzoic acid (mCPBA), the epoxidation of olefins in the presence of TFA and peroxy IL has the advantages of high yields of epoxides and short reaction time. Gharnati et al.233 studied the epoxidation of cyclooctene over the Venturello catalyst [C8H17]3N(CH3)3]3[PO4{WO(O2)2}4] in guanidinium-based ILs (GILs). The yields of epoxide lie in a broad range from 13% to 79% depending on the types of anions and substituents on the guanidinium moiety. More specifically, the new guanidinium phosphotungstates with the PW12O403− anion were used as catalyst in the solvents GILs and acetonitrile. Li et al.234 used the peroxopolyoxometalate-based roomtemperature IL (POM-RTIL) as catalyst for efficient epoxidation of various olefins. The RTIL catalyst is well dissolved in the solvent ethyl acetate and can self-separate from the reaction media after the reaction is completed. The PMA@ POM-ILs were prepared by the immobilization of 12phosphomolybdic acid (PMA) on POM-IL support, which were catalytically active with nearly 100% selectivity to cyclooctene epoxide using tert-butyl hydroperoxide as oxidant.235 Recently, Leng et al.236,237 synthesized a series of polyoxometalate (POM)-based stable polymeric hybrids based on polyhedral oligomeric vinylsilsesquioxanes (POSS) and ILs bearing hydrophobic alkyl chains as the building blocks followed by ion exchange with Keggin-type phosphotungstic acid (PW), as shown in Scheme 18. The obtained hybrids POSS-ILx-PW exhibit extraordinary catalytic activities, catalytic rates, and quite stable reusability as a heterogeneous catalyst for the epoxidation of cyclooctene. The excellent catalytic

wide range of alcohols to achieve high conversion and selectivity.215 N-n-Dodecylpyridinium vanadate was used as catalyst, N-(propyl-1-sulfonic acid) pyridinium tetrafluoroborate as cocatalyst, 4-(propanoate-2,2,6,6,-tetramethylpiperidine1-oxyl) pyridinium tetrafluoroborate as co-oxidant, and the common hydrophilic IL [BPy][BF4] as reaction solvent. The toxic vanadate could be successfully locked in the IL phase, and thus, the content in organic phase was below 0.1 ppm. A clean and highly efficient oxidation method for selective conversion of alcohols to carbonyl compounds using L-aspartic acid-coupled imidazolium-based IL (L-AAIL, as illustrated in Scheme 15) as catalyst and H2O2 as oxidant was proposed under solvent-free condition.216 A variety of alcohols including aromatic, aliphatic, cyclic, and heterocyclic alcohols could be completely oxidized with high conversion and selectivity. Meanwhile, the IL-supported selenium reagents were found to be an excellent catalyst and solvent in the oxidation of alcohols to aldehydes or ketones using H2O2 as oxidant.217 The organoselenium compounds are nonvolatile by anchoring the selenium function to the IL moiety. Li et al. used two novel long chain multi-SO3H-functionalized heteropolyanion-based ILs (as shown in Scheme 16) as homogeneous catalyst for selective oxidation of alcohols under solvent-free conditions and without any phase transfer catalyst.218 The substituted Scheme 17. Epoxidation of 4-Bromocinnamic Acid with [BMIM][HSO5]a

a

Adapted from ref 232 with permission. Copyright 2015 Royal Society of Chemisty.

benzyl alcohols could be oxidized smoothly to their corresponding benzaldehydes or benzoic acids with high yields, and cyclohexanol, cyclopentanol, and 2-octanol were oxidized to the corresponding ketones under optimal conditions. The high catalytic activity of the two ILs was ascribed to two factors: one is the high Brønsted acidity and acid content, and the other is the synergetic catalytic effect of the oxidation and acid catalytic sites.

4. EPOXIDATION OF ALKENES In view of the industrial importance of epoxides, which are valuable and versatile intermediates in organic synthesis and useful starting materials for the preparation of industrial chemicals (such as epoxy resins, surfactants, paints, adhesives, and surface-coating agents), the environmentally friendly methods for epoxidation of alkenes have become a subject of interest.219−224 Much attention has been focused on the use of ILs as environmentally benign solvents or catalysts for 6961


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Scheme 18. Schematic Preparation of the POSS-ILx-PW Catalysts and the Possible Mechanism for Epoxidation of Alkenesa

a


Scheme 19. Proposed Mechanism for Expoxidation of Allylic Alcohols with H2O2 Catalyzed by the Monomeric Peroxoniobate ILa

Scheme 20. Expoxidation of Styrene to 1,2Epoxyethylbenzene with H2O2 Catalyzed by PMo/ILMCM41 under Solvent-Free Conditiona

a


Chen et al.238 reported that the monomeric peroxoniobatebased functionalized ILs [Q][NbO(O−O) (OH)2] (Q = tetrapropylammonium, tetrabutylammonium, or tetrahexylammonium) exhibit excellent catalytic activity and recyclability for the epoxidation of various alkylic alcohols with H2O2 as oxidant under solvent-free and ice bath conditions. The reaction mechanism was explored at the atomic level by DFT (density functional theory) calculations, and the calculated results together with the activity tests and catalyst characterization indicated that the parent [NbO(O−O) (OH)2] (A) anion is oxidized into [NbO(O−O)2(OOH)2] (B) anion, which constitutes the real catalytically active species during reaction (see Scheme 19). Thus, the reaction occurred through a hydrogen-bond mechanism, in which the peroxo group of [NbO(O−O)2(OOH)2] (B) serves as the adsorption site to

a

Adapted from ref 238 with permission. Copyright 2016 American Chemical Society.

performance is attributed to its amphiphilic surface and mesoporous structure, which allow for a rapid diffusion of the reactants into the reactive PW centers. The possible mechanism for epoxidation of alkenes is illustrated in Scheme 18, confirming the amphipathicity of the POSS-ILx-PW catalysts. 6962


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anchor the substrate OH group by forming a hydrogen bond, while OOH as the active oxygen species attracks the CC bond in substrates to produce the corresponding epoxide (C). With the desorption of epoxy alcohol, B could be converted into [Nb(O−O)2(OOH)−(OH)] (D), which would be regenerated into the oxidizing species (B) with H2O2, thus completing the whole catalytic cycle. The quaternary ammonium cation of IL plays an important role in not only balancing the negative charge of anion but also offering the appropriate hydrophobicity to facilitate the accessibility of organic substrates to active sites. Recently, Vafaeezadeh et al.239 synthesized glutaric acid (GA) from the oxidation of cyclopentene with 30% H2O2 in the IL bis(1-buty-3-methylimidazolium) tungstate [BMIM]2[WO4] as phase transfer catalyst (PTC) to provide a suitable reaction media. It is worth noting that a high yield of GA could be obtained without using special methodologies or precautions. When compared to the current GA synthetic procedures, the process is less hazardous and significantly improved from the environmental viewpoint. An IL-immobilized copper complex by immobilizing a copper complex onto an IL support ([EMIM][PF6]) was used as catalyst for olefin and terpene epoxidation with H2O2 oxidant.240 Both the oxidant and the catalyst were completely soluble in the IL, giving rise to a homogeneous oxidation solution, and after reaction the products were separated by extraction with an immiscible solvent. Various POM supported on IL-modified MCM-based or other mesoporous materials as heterogeneous catalysts were used for the selective epoxidation of alkylenes.241−244 The heterogeneous catalyst PMo/ILMCM-41 (as shown in Scheme 20) showed good reusability in the selective oxidation of styrene to 1,2-epoxyethylbenzene using H2O2 as oxidant. The catalyst was easily separated by centrifugation and reused without deactivation after six runs.245 The PMo/ILMCM-41 catalyst possesses both hydrophilic and hydrophobic properties and thus can adsorb H2O2 and styrene from aqueous and organic phases simultaneously to avoid the disadvantage of mass interface transfer limitation. The conversion of styrene and the selectivity to 1,2-epoxyethylbenzene could achieve 95.4% and 90.2%, respectively, at 50 °C with a H2O2/styrene molar ratio of 1.2 within 3 h under solvent-free condition. A series of Fe(III) Schiff base complexes immobilized on MCM-41 was used for epoxidation of cyclohexene and other alkenes with 30% H2O2 in the presence and absence of the IL [EMIM][Cl].246 It showed that the presence of IL solvent could efficiently improve the catalytic performances. Moreover, the catalytic performance of immobilized complexes was proved better than their homogeneous analogue, which is closely related to their different structures of the used Schiff base ligands. Hua et al.247 immobilized the IL consisting of a PEGfunctionalized ammonium cation and a lacunary-type phos-

Scheme 22. Structures of [Pt(dppd)(μ-OH)]2(BF4)2

Scheme 23. Proposed Mechanism of the BV Oxidation of Ketones with BTSP Using [BMIM][TfO] as Solvent and Catalysta

a

Adapted from ref 255 with permission. Copyright 2009 Royal Society of Chemistry.

Scheme 24. Immobilization Procedure of IL onto the Silica Surface To Obtain the [pMIM]HSO4SiO2 Catalysta

a


photungstate anion onto environmentally benign polymer− carboxymethyl cellulose, which was used as catalyst for epoxidation of olefin with H2O2. Evidently, the immobilized IL catalyst (PEG−PW11) shows better catalytic activities and stability than the homogeneous analogue in consecutive runs. Moreover, the catalytic performance is greatly dependent on the interactions between the heteropolyanions and the polymer supports.

Scheme 21. Baeyer−Villiger Oxidation of Cyclic (a) and Linear (b) Ketones in IL

5. BAEYER−VILLIGER OXIDATION The Baeyer−Villiger (BV) oxidation is commonly encountered in the synthesis of esters and lactones by the oxidation of linear 6963


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ACHN as free radical initiator and oxone as oxidant in IL solvents for the synthesis of lactones. High conversions and yields of ε-caprolactone were observed for the BV reactions carried out in either hydrophobic ILs based on Tf2N anion or other hydrophilic ILs. The [Tf2N]-based ILs give the best results, because they create an anhydrous reaction environment so that the hydrolysis of lactone to the corresponding hydroxyacid is less competitive. In 2013, they attempted to use the silyl peroxides (including bis(silyl) and alkyl silyl peroxides) as oxidants and the chloroaluminate(III) ILs as Lewis acidic catalysts and reaction media for the BV oxidation of cyclic ketones.258 The presented reaction system was the most efficient for the synthesis of substituted γ-butyrolactones with high yields (80−97%). However, in the case of other lactones such as ε-caprolactones, only moderate yields were obtained. The generation of peroxy anions, R3SiOO− or R3COO−, during the BV oxidation was confirmed due to the participance of chloroaluminate(III) ILs. For the chemoenzymatic BV oxidation of cyclic ketones to lactones, free Candida antarctica lipase B or Novozyme-436 was used as catalyst and 30% aqueous H2O2 as oxidant in various ILs.259 It was found that the catalytic activity of lipases is highly dependent on the characteristics of ILs, and the hydrophobic IL [BMIM][Tf2N] are more desirable. The heterogeneous solid acid catalyst ([pMIM]HSO4SiO2), where IL was anchored onto solid silica as support via cation− anion stays unbounded to the surface, was used for the synthesis of lactones through BV reaction. The immobilization process is illustrated in Scheme 24.260 The cyclic ketones were readily oxidized with 68% H2O2 in dichloromethane solvent, and high yields (60−91%) of lactones were obtained at 50 °C within a short time (5−20 h). The most important advantages are the relatively inexpensive catalyst and direct isolation of the products from reagents by simple filtration. On the other hand, the crystalline zirconium phosphates intercalated with chloridebased ILs were used as catalyst for the BV oxidation of omethoxybenzaldehyde, p-methoxybenzaldehyde, and 2,4,6trimethylbenzaldehyde by H2O2 under glacial acetic acid or solvent-free condition.261 The highest selectivities of omethoxybenzaldehyde, p-methoxybenzaldehyde, and 2,4,6trimethylbenzaldehyde without solvent could achieve 95%, 100%, and 55%, respectively. Recently, Chrobok et al.262 confirmed that the extremely high activity of gallium(III) chlorides (GaCl3) was irrespective of solvent in promoting BV

and cyclic ketones in many applications, including the synthesis of antibiotics, steroids, pheromones, and monomers for polymerization. The recent outcomes on BV oxidation in relation with ILs are summaried in Table S7 in the Supporting Information. In 2003, Bernini et al.248 first reported the BV oxidation of cyclic ketones in the IL [BMIM][BF 4 ]. Methyltroxorhenium (CH3ReO3) was immobilized into IL, performing an efficient BV oxidation of cyclic ketones with H2O2. Product separation was accomplished by simple extraction with diethyl ether. Later, Panchgalle et al.249 reported the BV oxidation of aryl ketones to esters with 30% aqueous H2O2 catalyzed by Sn-β molecular sieve in IL at room temperature. The esters were obtained in good yields after 10 h. Yadav et al.250 studied the BV oxidation of a variety of cyclic and linear ketones with m-chloroperbenzoic acid (m-CPBA) in [BMIM][BF4] to obtain esters and lactones, as shown in Scheme 21. The BV reaction proceeded smoothly in IL without the need of any additional acid or base catalyst. The BV oxidation of cyclohexanone with H2O2 in a series of imidazolium-based ILs catalyzed by [Pt(dppd)(μ-OH)]2(BF4)2 (as shown in Scheme 22) was investigated by Conte et al.251 The results showed that the oxidation of cyclohexanone was significantly improved in ILs, and the reaction was faster and more efficient than in conventional solvents. The yield of lactone in [BMIM][Tf2N] was 47% at a temperature of 40 °C within 2 h, but the catalytic activity decreased after recycling, the yield of lactone being 31% in the second run. Kotlewska et al.252 investigated the BV oxidation of ketones in the hydrogenbond-donating (HBD) ILs using a lipase as catalyst and H2O2 as the terminal oxidant, but the BV reaction was much faster than in organic solvents such as ethyl acetate and acetonitrile. In this case, 99% yield of lactone produced from the oxidation of cyclopentanone in 1-(3-hydroxypropyl)-3-methyl imidazolium nitrate could be obtained within 5 h. The authors thought that the rearrangement of the so-called Criegee intermediate in BV reactions was generally rate determining and would be facilitated by proton-donating solvents. Hu et al.253 reported the use of molybdovanadophosphoric cobalt heteropoly acid salt (Co4HP2Mo15V62) as catalyst for the oxidation of aldehydes and ketones to carboxylic acids and esters with H2O2 as oxidant in the IL [TEBSA][BF4]. The high isolated yields of the corresponding carboxylic acids and esters (85−97%) were obtained under mild reaction conditions. Chrobok et al.254 used acidic ILs for the BV oxidation of cyclic ketones, and these ILs act as both solvents and catalysts. For example, the reagents [BMIM][HSO4] and [BMIM][CF3COO], which are easily purchased from chemical markets, were very successful in the oxidation process. High yields (85− 92%) of δ-valerolactone were obtained at 50 °C at an amount of H2O2 3.5 times higher than the stoichiometric ratio within a reasonable reaction time (5 h). Subsequently, the same research group proposed a new method for the synthesis of lactone with bis(rimethylsilyl) peroxide (BTSP) as oxidant and BF3·OEt2 as catalyst in the IL solvent [BMIM][Tf2N], resulting in the increased product yields compared with those in the classical solvent CH2Cl2.255 Moreover, they went a further step to replace the original catalyst by the IL [BMIM][TfO] acting as both solvent and catalyst. High yields (72−91%) of lactones were obtained from the oxidation of cyclic ketones under mild conditions within a short time. It was assumed that the anion [TfO] plays a key role during the reaction. The reaction mechanism is illustrated in Scheme 23. In 2010, Chrobok et al.256,257 proposed using O2/benzaldehyde as oxidant with

Scheme 25. Oxidation of Cyclohexane

oxidation of cyclic ketones with H2O2 as oxidant, and GaCl3 provided the fastest known homogeneous activation with lactone yields of 89−94%. Like GaCl3, the Lewis acidic chlorogallate(III) IL is also extremely active and allows for elimination of volatile molecular solvents. In terms of Raman and 71Ga NMR spectroscopies, the active species are the hydroxychlorogallate(III) anions, which are intermediate products of the hydrolysis of GaCl3 and chlorogallate(III) ILs. In this regard, water is crucial for efficient activation. 6964


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Table 8. Peroxidative Oxidation of Cyclohexane in ILs yield (%) entry

catalysts

solvents

oxidants

time

conversion (%)

CyOH

CyO

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

TS-1 TS TS TS TS TS TS HZSM-5 HZSM-5 FeZSM-5 HZSM-5 NiZSM-5 CoZSM-5 MnZSM-5 FeZSM-5 CuZSM-5 Z1 Z2 Z3 Cu(II) complex Cu(II) complex Cu(II) complex Cu(II) complex Cu(II) complex Cu(II) complex Cu(II) complex Cu(II) complex Cu(II) complex Cu(II) complex Cu(II) complex Cu(II) complex CuL CuL CuL CuL CuL CuL

acetone no solvent [EMIM][BF4] [EMIM][BF4] [EMIM][BF4] [EMIM][BF4] [EMIM][BF4] no solvent acetone acetone [EMIM][BF4] [EMIM][BF4] [EMIM][BF4] [EMIM][BF4] [EMIM][BF4] [EMIM][BF4] [EMIM][BF4] [EMIM][BF4] [EMIM][BF4] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6] [BMIM][BF4] [BMIM][BF4] [BMIM][BF4] [BMIM][PF6] [BMIM][PF6] [BMIM][PF6]

TBHP TBHP H2O2 TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

24 h 24 h 24 h 6h 12 h 18 h 24 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 15 min 1h 2h 4h 15 min 1h 2h 4h 15 min 1h 2h 4h 1h 2h 3h 1h 2h 3h

3.62 1.25 0.18 6.77 10.5 13.2 13.0 0.98 3.29 3.81 15.8 15.9 14.2 15.5 20.9 9.5 15.8 12.8 10.8 4.8 12.4 23.1 24.0 7.0 15.3 34.9 35.0 9.0 19.0 36.1 36.3

1.7 0.9 0.1 5.4 8.4 10.4 10.7 0.3 0 0.6 6.98 8.2 9.82 10.3 12.1 6.15 7.0 5.9 4.8 1.1 2.4 4.0 5.1 1.8 3.9 8.9 9.0 1.5 3.2 7.1 9.7 3.7 7.7 10.3 1.4 5.1 6.4

1.1 0.4 0.0 1.3 1.9 2.4 1.9 0.68 1.29 1.35 2.70 2.84 3.53 3.06 3.27 3.31 2.7 1.9 1.6 3.2 9.8 19.0 15.2 5.0 10.2 25.3 22.0 7.3 14.8 28.9 23.6 1.6 2.6 3.1 1 3.2 4

272a 272a 272a 272a 272a 272a 272a 273b 273b 273b 273b 273b 273b 273b 273b 273b 274c 274c 274c 275d 275d 275d 275d 275e 275e 275e 275e 275f 275f 275f 275f 275g 275g 275g 275g 275g 275g

Reaction conditions: 0.15 g of catalyst, 27.8 mmol of cyclohexane, 55.6 mmol of TBHP (80% in H2O), 5 mL (6.25 g) of solvent, 90 °C. bReaction conditions: 0.15 g of catalyst, 27.8 mmol of cyclohexane, 55.6 mmol of TBHP (85% in H2O), 5 mL (6.25 g) of IL, 90 °C. Z1, Z2, and Z3 represent H-ZSM-5 catalysts with Si/Al ratios of 25, 38, and 50, respectively. cReaction conditions: 0.15 g of catalyst, 27.8 mmol of cyclohexane, 55.6 mmol of TBHP (85% in H2O), 5 mL (6.25 g) of IL, 90 °C. Z1, Z2, and Z3 represent H-ZSM-5 catalysts with Si/Al ratios of 25, 38, and 50, respectively. d Reaction conditions: [CyH]0 = 0.46 mol·L−1, [total H2O2] = 2.2 mol·L−1 (50% aqueous), 5 mL total volume, 50 °C, catalyst amount 2 × 10−4 mol· L−1. eReaction conditions: [CyH]0 = 0.46 mol·L−1, [total H2O2] = 2.2 mol·L−1 (50% aqueous), 5 mL total volume, 50 °C, catalyst amount 2 × 10−3 mol·L−1. fReaction conditions: [CyH]0 = 0.46 mol·L−1, [total H2O2] = 2.2 mol·L−1 (50% aqueous), 5 mL total volume, 50 °C, catalyst amount 2 × 10−2 mol·L−1. gReaction conditions: [CyH]0 = 0.46 mol·L−1, [total H2O2] = 2.2 mol·L−1 (50% aqueous), [CuL] = 2 × 10−5 M, 5 mL total volume, 40 °C. a

Scheme 26. Structure of the Monocopper(II) Complex CuLa

a

6. OXIDATION OF ALKANES Selective oxidation of chemically inert alkanes to value-added products such as alcohols, carboxylic acids, and alkenes is an important and challenging field in industry and commodity chemistry.263−267 Most of the current processes use toxic and often stoichiometric oxidants and molecular solvents, eventually producing wastes. However, in recent years the need for designing clean and green industrial processes has attracted much attention on the use of nonvolatile ILs for oxidation. In 2005, Bianchini et al.268 investigated the C−H insertion reaction with H2O2 catalyzed by homogeneous methyltrioxorhenium, heterogeneous poly(4-vinylpyridine)/methyltrioxo-


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The generated carbonyl compounds as the oxidation products could be easily isolated from the reaction media. The oxidation results for various oximes in ILs are listed in Table 9. Safaei-Ghomi and Hajipour281 investigated the oxidative cleavage of some oximes with KMnO4 as oxidant in the presence of [BMIM][Br] under solvent-free conditions. Yields above 81% could be obtained with an oxime:IL:KMnO4 ratio of 1:0.7:1.4 at room temperature without any further oxidation of aldehydes to acids. Shaabani and Farhangi282 investigated the aerobic regenerations of aldehydes and ketones from aldoximes and ketoximes catalyzed by various metallophthalocyanine in imidazolium-based ILs. The best results arose from cobalt(II) phthalocyanine (Co-Pc) in [BMIM][Br], and the catalyst could be reused after the extraction of product with n-hexane with only a slight drop in activity. Lumb et al.283 studied the oxidation of oximes to carbonyl compounds with sodium bromate in the presence of [BMIM][HSO4]. The optimum yields of the products (68−98%) appeared in a molar ratio of oxime:oxidant (NaBrO3) = 1:1 in the presence of [BMIM][HSO4]:H2O (3:1, v/v) at 60 °C. All reactions were complete in 10−25 min.

rhenium (PVP/MTO), and microencapsulated polystyrene/ methyltrioxorhenium (PS/MTO) systems in ILs. In some cases, a higher activity was observed in ILs than those in molecular solvents under the same reaction conditions. Since then a lot of work on the oxidation of alkanes using ILs has been done. In particular, the partial oxidation of cyclohexane to cyclohexanol (CyOH) and cyclohexanone (CyO) (as shown in Scheme 25) has attracted more interest.269−271 The recent outcomes on the oxidation of cyclohexane involving ILs are summarized in Table 8. Wang et al.272−274 studied the oxidation of cyclohexane with metal-containing molecular sieves (TS-1, MZSM-5, and HZSM-5 catalysts with different Si/Al ratios) as catalysts and tert-butyl-hydroperoxide (TBHP) as oxidant in ILs. Higher yields and selectivities were obtained in ILs than those in molecular solvents under mild conditions. FeZSM-5 exhibited the highest catalytic activity among all MZSM-5 catalysts investigated, and the conversion of cyclohexane and the selectivity of desired products could achieve 20.9% and 98.2%, respectively. The catalyst/IL system could be successfully recycled by a simple decantation unit without significant loss of activity. Ribeiro et al.275 were concerned with the catalytic peroxidative oxidation of cyclohexane with H2O2 in an IL using tetracopper(II) complex as catalyst. Significant improvement in the catalytic performances, i.e., product yield up to 36% and TON up to 529 in a short reaction time (2 h), was observed in [BMIM][PF6]. Moreover, the copper(II) compound CuL belonging to the Schiff base type (as shown in Scheme 26) exhibited high catalytic activity for alkane peroxidative oxidation in IL media.276 The use of IL as reaction media promotes the catalytic activity and allows catalyst recycling when compared to a typical organic solvent.

7.2. Oxidation of Benzene and Its Derivatives

Direct oxidation of benzene and its derivatives is still a hot topic.284,285 In 2003, Deng et al.286 investigated the direct oxidation of benzene to phenol with an equal molar of H2O2 as oxidant and metal dodecanesulfonate salts as catalyst in a green aqueous-IL biphasic medium without any addition of volatile organic solvent. In this case, excellent selectivity and enhanced conversion were obtained. However, when the system was used for oxidation of toluene, only 1% conversion of toluene was obtained. It is interesting to find that the conversion of toluene would be greatly enhanced to 3% in the presence of a small amount of benzene, indicating that benzene could promote the oxidation of toluene. Meng et al.287 studied the effect and mechanism of the liquid-phase oxidation of toluene with O2 over cobalt naphthenate catalyst in hydrophilic ILs. The results showed that toluene conversion increased with the increase of the hydrophilicity of ILs, and the conversion and selectivity toward benzaldehyde could reach 19.6% and 19.5%, respectively, in [EMIM][BF4]. The low solubility of toluene in IL indicates a heterogeneous oxidation mechanism. This indicates that the improvement of conversion and selectivity is attributed to the coupling effect of catalytic reaction and extraction separation. Kumari et al.288 proposed an efficient oxidative system for oxidation of polycyclic aromatic hydrocarbons with H2O2 catalyzed by 5,10,15-triarylcorrolatoiron(IV) chloride in the imidazolium-based IL and organic solvent mixed reaction media ([BMIM][BF4]/CH2Cl2 ratio of 1:1, v/v). The presence of electron-withdrawing groups at the aromatic ring of the catalyst (as shown in Scheme 28) and the use of noncoordinating IL and coordinating organic solvent improve the yields. Lu et al.289,290 proposed using the PEG-1000-based dicationic acidic ILs (PEG1000-DAIL) as reaction solvent for aerobic oxidation of alkylaromatics over the N-hydroxyphthalimide (NHPI)-cobalt acetate (Co(OAc)2) and N′,N″,N‴trihydroxyisocyanuric acid (THICA)/dimethylglyoxime (DMG) catalysts. The PEG1000-DAIL could successfully replace the conventional solvent acetic acid, leading to the enhancement of catalytic activity, short reaction time, operational simplicity, and environmentally benign.

Scheme 27. Oxidation of Oximes with H2O2 Catalyzed by Cl8TAPS4Fe(III) or H3PW12O40 in [BMIM][BF4]

Recently, Mncube and Bala277 presented the use of simple metal salts in triazolium IL as recyclable catalysts for the oxidation of alkanes using H2O2 as oxidant to obtain oxygenated products in an aqueous biphasic system. The results showed that the IL is required for catalyst stability and enhancement of its activity, and the chemical constitution of IL by virtue of the substituents around the triazole ring has an influence on the oxidation performance. For example, the IL containing hydrophobic groups was observed to be more active.

7. OXIDATIONS OF OTHER COMPOUNDS 7.1. Oxidation of Oximes

Compounds such as aldoximes and ketoximes are derivatives of carbonyl compounds, which are derived by the oxidation of oximes.278,279 Jain et al.280 investigated the oxidation of several oximes with H2O2 to carbonyl compounds using the watersoluble Fe(III) porphyrins (Cl8TAPS4Fe(III)) and phosphotungstic acid (H3PW12O40) as catalyst in the IL [BMIM][BF4] under mild reaction conditions (as shown in Scheme 27). Among others, H3PW12O40 was found to be the most efficient. 6966


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Table 9. Oxidation of Oximes in ILs

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Table 9. continued

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Table 9. continued

a

Reaction conditions: oxime 1.0 mmol, oxidant (H2O2) 1.0 mmol, catalyst (H3PW12O40) 0.01 mmol, solvent ([BMIM][BF4]) 3.0 mL, room temperature. bReaction conditions: oxime 1.0 mmol, oxidant (H2O2) 1.0 mmol, catalyst (Cl8TAPS4Fe(III)) 0.01 mmol, solvent ([BMIM][BF4]) 3.0 mL, room temperature. cReaction conditions: oxime 1.0 mmol, (KMnO4) 0.4 mmol, catalyst ([BMIM][Br]) 0.7 mmol, without solvent, room temperature. dReaction conditions: oxime 1.0 mmol, catalyst (Co-Pc) 0.01 g, solvent ([BMIM][Br]) 0.3 g, oxidant (O2) 5 mL/min, 70 °C. e Reaction conditions: 1:1 of oxime:oxidant (NaBrO3) (molar ratio), 3:1 of IL([BMIM][HSO4]):H2O (v/v), 60 °C. 6969


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ILs from gram to ton scales. They used the chlorocuprate(II)based ILs for direct oxidation of elemental mercury. Mercury was treated by reactive capture from gas streams by means of a solid-supported IL phase (SILP). This new technology has been commercialized in a petroleum gas production plant in Malaysia. The transformation from lab-scale preparation of chlorocuprate(II) SILPs through to pilot scale with 100 cm3 of SILP on production plant gas feeds to full-scale mercury removal units with 20 m3 of SILP at a gas processing plant site is illustrated in Figure 3.

Scheme 28. Structure of the 5,10,15Triarylcorrolatoiron(IV) Chloride Catalystsa

8. CONCLUSIONS AND PERSPECTIVE This review highlights the most recent outcomes on the application of ILs in selective oxidation. The contents on oxidation of sulfides, oxidation of alcohols, epoxidation of alkenes, Baeyer−Villiger oxidation reaction, oxidation of alkanes, and oxidation of other compounds in relation with ILs as solvents, catalysts, reagents, or their combinations under different conditions are comprehensively summarized and provided for the readers to refer conveniently. Clearly, due to their unique properties, ILs have attracted more and more attention in selective oxidations as catalysts (having high catalytic activity, phase transfer catalysis, and active redox property) and solvents (having nonvolatility, reaction rate acceleration effect, and high thermal stability). However, to the best of our knowledge, there is rare industrial application for selective oxidation with ILs except for the oxidative removal of mercury. The main unsolved problems are the recyclability of both ILs and catalysts, economic and ecological concerns due to the high cost and viscosity, unknown toxicity and stability of ILs, and lack of dynamic and thermodynamic models, which should be addressed in the future work. With respect to the oxidation using ILs, chemists and chemical engineers will encounter with a lot of challenges in the near future, including at least the following aspects: (1) For the oxidative desulfurization of diesel, the combination of ILs as efficient solvent or catalyst with air as oxidant is expected to be more explored in the near future; (2) more works should be carried out on the study of the oxidation reaction mechanism at the molecular scale using DFT technique; (3) more attention should be paid to the thermodynamic model and basic data relevant to selective oxidation, which can be realized through experiments or the powerful predictive COSMO-RS model; (4) it is desirable to understand how the physicochemical properties of ILs (e.g., charge, bulkiness, ionic mobility, viscosity, and density) affect the catalytic routes; (5) although ILs exhibit several advantages in selective oxidation, their cost, viscosity, toxicity, and biodegradability should be taken into account at the same time; and (6) studies on reactor hydrodynamics (e.g., mixing performance, mass transfer coefficients, pressure drop, and CFD modeling) of selective oxidation involving ILs, which are fundamental and essential for real reactor design and operation, are still scarce and should be focused on in future work.

a

R = H or F, X = Cl or Br. Adapted from ref 288 with permission. Copyright 2012 Elsevier.

Scheme 29. Oxidation of Organic Halides to Ketones and Aldehydes in ILsa

a

R1 = aryls or alkyls, R2 = H, aryls, or alkyls, X = Cl, Br, or I.

Various dialkyl imidazolium−metal chloride ILs were suggested as catalysts in selective oxidation of toluene with H2O2 as oxidant, and the IL [BMIM]Br-FeCl3 showed higher catalytic activity than other metal-based ILs, with 10.2% conversion and 99% selectivity of benzaldehyde and benzyl alcohol.291 The high catalytic activity was likely ascribed to the formation of the IL via the coordination of imidazolium cation with transition metal chloride as well as the nucleophilicity of the matching bromide anions. The high selectivity was related to the nature of biphasic catalysis generated by organic phase (containing toluene, benzaldehyde, and benzyl alcohol) and H2O2 aqueous phase. 7.3. Oxidation of Halides

Oxidation of organic halides to ketones and aldehydes with H5IO6 in ILs is illustrated in Scheme 29.292−294 Various types of benzylic, allylic, and aliphatic halides can be successfully oxidized to the corresponding ketones and aldehydes in high yields (88−98%) in [C12MIM][FeCl4] at 30 °C. However, oxidation of less reactive aliphatic halides needs longer reaction times than other compounds. Further investigations were made using the V2O5 catalyst in [BMPY][PF6], as listed in Table 10. The conversion of organic halides into carbonyl derivatives took place via the microwave-assisted oxidation with Nmethylmorpholine N-oxide (NMO) in IL within a short reaction time.295 It was found that the reaction in [EMIM][Cl] gives rise to a 2-fold product yield compared to dimethyl sulfoxide, which was regarded as the best organic solvent for nucleophilic substitution reactions previously.296 The reaction conversion was further improved in the presence of additives, and the highest yield was obtained with KI. The authors claimed that the IL could be recovered and reused in several reaction cycles, and no significant loss in activity was observed.

ASSOCIATED CONTENT

7.4. Oxidative Removal of Mercury

S Supporting Information *

It is known that mercury is extremely corrosive, causing destructive damage to process equipment through liquid metal embrittlement. The mercury control and treatment are increasingly recognized as important for protection of both equipment and human beings.297−301 Abai et al.302 systematically investigated the mercury removal from natural gas using

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemrev.7b00030. Meaning of abbreviations, names and structures for ILs used throughout this review, oxidative desulfurization 6970


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Table 10. Oxidation of Organic Halides in ILs

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Table 10. continued

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Table 10. continued

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Table 10. continued

Reaction conditions: Organic halide (10 mmol), H5IO6 (11 mmol), [C12MIM][FeCl4] (0.4 mmol), 30 °C. bReaction conditions: Organic halide (10 mmol), V2O5 (0.3 mmol), H5IO6 (11 mmol), [BMIM][PF6] (5 mL), 50 °C. cReaction conditions: Organic halide (0.4 mmol), NMO (0.8 mmol), KI (10 mol %) in [EMIM][Cl] (0.4 g) with a MW (150 W), 100 °C.

a

Figure 3. Transformation of the IL process from (a) lab-scale preparation of chlorocuprate(II) SILPs through to (b) pilot scale using 100 cm3 of SILP on production plant gas feeds to (c) full-scale mercury removal units with 20 m3 of SILP at a gas processing plant site. Reprinted with permission from ref 302. Copyright 2015 Royal Society of Chemistry.

results using ILs as solvent or extractant, oxidation data

epoxidation results of alkenes, and Baeyer−Villiger

of alcohols to the corresponding carbonyl compounds,

oxidation data using ILs (XLS) 6974


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AUTHOR INFORMATION

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China under Grant Nos. 21476009, 21406007, and U1462104.

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

REFERENCES

Zhigang Lei: 0000-0001-7838-7207

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Notes

The authors declare no competing financial interest. Biographies Chengna Dai was born in Hebei Province (China) in 1984 and received her Ph.D. degree in 2013 from Beijing University of Chemical Technology (BUCT) under the supervision of Prof. Zhigang Lei. Thereafter she worked in BUCT, and she is now an associate professor. Her current research interest is reaction and separation science with ionic liquids as well as the corresponding predictive molecular thermodynamics. She has contributed to more than 30 papers in international journals. Jie Zhang was born in Tianjin (China) in 1978. He received his B.S. degree in 2001 and Ph.D. degree in 2008 at College of Chemical Engineering, Beijing University of Chemical Technology (BUCT). Then he became a postdoctoral researcher at the College of Materials Science and Technology of BUCT. He began to work as Lecturer in 2011 and was promoted to Associate Professor in 2014 at the State Key Laboratory of Chemical Resource Engineering (BUCT, China). From September 2014 to March 2015, he worked as an academic visitor at Imperial College London hosted by Prof. David Chadwick under the scholarship of the China Scholarship Council. His current research focuses on the fundamental research on ionic liquids and zeolites in chemical processes. He has contributed to more than 40 papers in international journals. Chongpin Huang was born in Hubei Province (China) in 1974 and received his Ph.D. degree in 2003 from the China University of Petroleum (Beijing). Then he became a postdoctoral researcher at the Beijing University of Chemical Technology (BUCT). In 2005, He was promoted to Associate Professor at the State Key Laboratory of Chemical Resource Engineering (BUCT, China). His main research focuses on the science and technology of ionic liquids in environmental and green chemistry, and he has contributed to more than 50 papers in international journals. Zhigang Lei was born in Hubei Province (China) in 1973. He received his B.S. degree in 1995 from Wuhan Institute of Technology and Ph.D. degree in 2000 from Tsinghua University. Then he became a postdoctoral researcher in Beijing University of Chemical Technology working with Professor Chengyue Li. In 2003−2005, he worked as a researcher in the Research Center of Supercritical Fluid Technology (Tohoku University, Sendai, Japan). In 2005−2006, he got the worldfamous Humboldt Fellowship and carried out his research in Chair of Separation Science and Technology (Universität Erlange-Nürnberg, Erlangen, Germany). In 2006, he came back to China. He is now Professor in the State Key Laboratory of Chemical Resource Engineering (BUCT, China). His current research interests include chemical process intensification and predictive molecular thermodynamics. He has contributed to about 110 papers in international journals and one book entitled Special Distillation Processes published by Elsevier B.V. (2005). 6975


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