Identifying Catalytically Active Mononuclear Peroxoniobate Anion of

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Identifying Catalytically Active Mononuclear Peroxoniobate Anion of Ionic Liquids in the Epoxidation of Olefins Wenbao Ma, Haiyang Yuan, Haifeng Wang, Qingqing Zhou, Kang Kong, Difan Li, Yefeng Yao, and Zhenshan Hou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04443 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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Identifying Catalytically Active Mononuclear Peroxoniobate Anion of Ionic Liquids in the Epoxidation of Olefins Wenbao Ma,†,§ Haiyang Yuan,†,§ Haifeng Wang,*, † Qingqing Zhou,† Kang Kong,† Difan Li†, Yefeng Yao,‡ and Zhenshan Hou*,† † Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, People’s Republic of China, E-mail: [email protected]; [email protected]. ‡Physics Department and Shanghai Key Laboratory of Magnetic Resonance, East China Normal University, Shanghai 200062, People’s Republic of China.

Abstract: The organic carboxylic acid-coordinated monomeric peroxoniobate-based ionic liquids (ILs) designated [TBA][NbO(OH)2(R)] (TBA= tetrabutylammonium; R=lactic acid (LA), glycolic acid (GLY) or malic acid (MA)) were prepared and fully characterized by elemental analysis, NMR, IR, Raman, TGA, 93Nb-NMR, and HRMS. These IL catalysts exhibited not only high catalytic activity for the epoxidation of olefins under very mild reaction conditions as the turnover frequency of [TBA][NbO(OH)2(LA)] reached up to 110 h-1 but also satisfactory recyclability in the epoxidation by using only one equivalent of hydrogen peroxide as an oxidant. Meanwhile, this work revealed that the ILs underwent structural transformation from [NbO(OH)2(R)]- to [Nb(O-O)2(R)]- (R=LA, GLY, MA) in the presence of H2O2 by the subsequent activity evaluation, characterization and the first-principles calculations. Moreover, the organic carboxylic acid-coordinated monomeric peroxoniobate-based ILs were investigated using density functional theory (DFT) calculations, which identified that [Nb(O-O)2LA]- was more advantageous than [Nb(O-O)2(OOH)2]- for the epoxidation of olefins. Due to the coordination between the α-hydroxy acids and the monomeric peroxoniobate anions, the functionalized ILs can efficiently catalyze the epoxidation of a wide range of olefins and allylic alcohols under very mild conditions. Additionally, the effect of solvents on the reaction is illustrated. It was found that methanol can lower the epoxidation barriers by forming a hydrogen bond with a peroxo ligand attached to the niobium center. Keywords: Peroxoniobate, Ionic liquid, Epoxidation, Olefins, DFT 1

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Introduction Oxidation reactions are essential in the synthesis of oxygen-containing compounds,1 and the epoxidation of olefins was one of the most important oxidation reactions in the fine chemical industry. Epoxides constitute important intermediates for the production of valuable chemicals, especially for the synthesis of polymers, surfactants, and a variety of natural products and chiral molecules.2 Expensive and environmentally harmful oxidants, such as peracids and hazardous organic solvents, are usually employed to produce epoxides.3 In contrast, hydrogen peroxide is an environmentally benign oxidant because water is the only product of H2O2 reduction, which obviates the need to sell a co-product alcohol when organic hydroperoxide is selected as an oxidant.4 The epoxidation of olefins with H2O2 usually requires the presence of a catalyst. Catalytic epoxidation reactions using non-noble metals and environmentally benign oxidants (e.g., H2O2) have been attracting much attention recently.5 As an important class of crystalline metallosilicates, microporous titanium silicate zeolite (e.g., TS-1) has a uniform and ordered network of micropores and is widely applied in industry for the epoxidation of C=C double bonds with hydrogen peroxide, which provides an environmentally friendly approach for producing epoxides.6 However, TS-1 has pores of 5.5 Å in diameter, and thus bulky substrates cannot easily access the active sites located inside the pores. Additionally, the leaching of active species and the stability of these catalysts towards H2O2 under the reaction conditions is still a considerable problem. Therefore, the epoxidation of bulky alkenes using microporous titanosilicate catalysts remains a significant challenge.7 In addition to titanosilicate catalysts, various transition metal complexes, especially the complexes of Mn, Fe and Mo-bearing ligands such as porphyrins, salen, pyridyl-amine and N-heterocyclic carbene have also been reported to be employed as catalysts in these processes.8-15 However, the common issues associated with homogeneous complex catalysts, such as their hydrolytic stability under oxidative conditions and the difficulty in recycling these catalysts, largely restricts their application. Moreover, expensive ligands and complicated preparation processes also lead to these catalysts not being cost-effective. Although tungsten-based catalysts 2


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display a high activity for the epoxidation of olefins, they usually exist in the form of polynuclear species, e.g., polyoxometalates (POMs) and peroxotungstate catalysts, which can degrade further into active species under reaction conditions. This leads to structure instability and the difficult characterization of catalysts under reaction conditions. Moreover, these catalysts typically have a limited turnover frequency.16-19 In Group V, the use of vanadium complexes as oxidative catalysts has been more often reported in previous research.20-24 The two heaviest elements, niobium and tantalum, have applications in advanced materials and catalysis. For example, the niobium compounds are normally involved in acid-catalyzing reactions and show high activity and stability.25-28 Additionally, niobium- and tantalum-based heterogeneous catalysts have been explored in the epoxidation of olefins.29-34 However, only a poor or moderate catalytic activity has been obtained. On the other hand, homogeneous catalysts have been widely studied owing to their high catalytic activity and remarkable structure designability, which allows for accurate reaction mechanism research at the molecular level. To our knowledge, only a limited number of niobium-based homogeneous catalysts have been used in the epoxidation of olefins and allylic alcohols.35 Peroxo complexes of niobium have attracted considerable attention in the last decade. They exhibit the potential ability to release active oxygen and can participate in oxidations of different kinds of substrates.36,37 These complexes correspond either to homoleptic tetraperoxoniobates of the general formula (M)3[Nb(O2)4] (M= K+, NH4+, Na+ and gu+) (gu=carbamidine) or to heteroleptic peroxoniobate compounds, designated (M)x[Nb(O2)y(L)]·zH2O, where L is tartrate, citrate, malate, ophenanthroline, oxalic acid, dipyridine, EDTA, DTPA, etc.38,39 Nevertheless, the catalytic performance of these complexes in the epoxidation reaction has rarely been explored. An IL is an organic salt, containing an organic cation and a suitable anion, in the liquid state.40 ILs are currently of great interest owing to their unique properties, such as a large liquid state range, high ionic conductivity, strong polarity, the contact ion pairs effect and their inherent nature, which has expanding ILs into many important 3


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areas.41 In particular, ILs can be used as novel green solvents or benign catalytic materials. Therefore, the functionalization of the IL cations and anions will surely provide an effective approach to impart particular capabilities to ILs and encourage more sustainable and rational development in this field.42-45 Recently, we have developed a series of highly catalytically active peroxoniobate (peroxotantalate)-based IL catalysts for the selective epoxidation of allylic alcohols.46,47 These IL catalysts showed a higher turnover frequency (TOF) compared to the previously reported Nb/Ta-based heterogeneous catalysts and dinuclear tungstate-based catalysts. However, these peroxoniobate (peroxotantalate)-based IL catalysts exhibited only a very limited catalytic activity for the epoxidation of olefins, which is an even more important reaction than the epoxidation of allylic alcohols in the chemical industry. On the basis of our previous work, in this work we have attempted to design several novel organic acid-coordinated peroxoniobate-based ILs. The new IL catalysts demonstrated excellent activity and reusability in the epoxidation of various kinds of olefins and allylic alcohols. To our knowledge, this is the first work that Nb-based catalysts have afforded a high catalytic activity for the epoxidation of various olefins. In particular, the structure of the monomeric peroxoniobate anion in the presence of H2O2 was identified in detail, and the reaction transition state was suggested through experimental measurements and DFT calculations. Furthermore, the role of methanol in the epoxidation has also been illustrated.

Results and Discussion Catalyst Preparation and Characterization A series of mononuclear peroxoniobate-based ionic liquids were synthesized by introducing L-(+)-lactic acid (LA), glycolic acid (GLY) and L-(-)-malic acid (MA) into the parent peroxoniobate anion [NbO(η-O2)(OH)2]-, which afforded the carboxylic acid-coordinated

peroxoniobate-based

[TBA][NbO(OH)2(GLY)]

and

ILs

[TBA][NbO(OH)2(MA)],

[TBA][NbO(OH)2(LA)], respectively.

These

compounds were dark red in color and showed the general characteristics of ILs, such 4


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as a viscous liquid at room temperature, the immiscibility with low polar solvents (alkane, toluene, diethyl ether, etc.) and non-volatility (Figure S1). The viscosity of the three

IL

catalysts,

[TBA][NbO(OH)2(LA)],


and

[TBA][NbO(OH)2(MA)], was determined (Table 1). In addition, the melting point of the ILs was measured by DSC, which is shown in Table 1. The melting points of [TBA][NbO(OH)2(GLY)] and [TBA][NbO(OH)2(MA) were found to be approximately -30.2 °C and -42.7 °C, respectively, while the melting point of [TBA][NbO(OH)2(LA) was below -75 °C. In the first step of their synthesis, TBAOH was obtained from TBACl and KOH by anionic exchange, where TBA represented a quaternary ammonium cation. Slowly mixing TBAOH with (NH4)3[Nb(O2)4] in dry ethanol resulted in the ionic liquid [TBA][NbO(η-O2)(OH)2]. Next, the organic acid-coordinated peroxoniobate-based ionic liquids were obtained accordingly via the reaction between [TBA][NbO(ηO2)(OH)2] and the different carboxylic acids. The structures of the synthesized catalysts were examined by 1H NMR, 13C NMR, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), elemental analysis, potential difference titration of Ce3+/Ce4+, and HRMS. The number of peroxide bonds were 0.1, 0.12, and 0.09 in [TBA][NbO(OH)2(LA)], [TBA][NbO(OH)2(GLY)] and [TBA][NbO(OH)2(MA)], respectively by potential difference titration of Ce3+/Ce4+, suggesting that the peroxo band in [NbO(η-O2)(OH)2] was substituted by carboxylato ligands in the course of catalyst preparation (Scheme 1). As a result, the reaction between [TBA][NbO(ηO2)(OH)2] and α-hydroxy acids in a 1:1 molar ratio led to new organic acid-coordinated ILs, where the α-hydroxy acids act as a bidentate ligand, coordinating with the Nb site by two oxygen atoms from the carboxylato and vicinal hydroxyl groups. Complexation with the metal atom therefore resulted in a five-membered chelate ring, which exhibited a similar structure as that of the peroxo-carboxylato Nb(V) complexes.48,49

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Scheme 1. Synthetic route of different IL catalysts.

These IL catalysts were first investigated by using FT-IR spectroscopy. As shown in Figure 1a (left), the band in [TBA][NbO(η-O2)(OH)2] at approximately 570 cmí1 was related to

as(Nb(O2)),

and the band near 850 cmí1 was characteristic for (O-O).50

In addition, the band at approximately 886 cmí1 was related to (Nb=O),51 and the band at 1640 cm-1 was assigned to the adsorption of water molecules on the catalyst.52 For the different organic acid-coordinated ILs (Figure 1b, 1c and 1d, left), the broadening of the

as(COO)

band and its significant shift from a value of approximately 1700 cm-1

in the free carboxylic acid to approximately 1665 cm-1 in the coordination complex indicated the coordination of the Nb site with a carboxylato group.39 In addition, the bands at 570 cmí1 and 850 cmí1 that arose from (O-O) species were not observed in the organic acid-coordinated ILs, revealing the absence of (O-O) species after coordination, which was well in agreement with that of the potential difference titration analysis. The band at approximately 1100 cmí1 of these ILs was related to the vibration of C-O;53 the band at approximately 882 cmí1 was attributed to v(Nb=O); and the band near 585 cmí1 and 545 cm-1 was characteristic for the (Nb-O) in the ILs.54 The absence of the stretching of the Nb-O-Nb bridge at 835 cm-1 indicated that the peroxoniobate anion existed as a mononuclear instead of a multinuclear form.55 Additionally, the band at approximately 1734 cmí1 in [TBA][NbO(OH)2(MA)] (Figure 1d, left) assigned to the vibration of the free carboxylic group of malic acid still existed, indicating that only one carboxylato group and the vicinal hydroxyl group in the malic acid molecule participated in the formation of complexes.56

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a

1640

865 580

886 850 570

Intensity (a.u.)

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

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Transmittance (a.u.)

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b c d

1734 1665

a b c d

1100 882 585 545

1676

1058 880 603

2400 2100 1800 1500 1200 900 600

2400 2100 1800 1500 1200 900 600

-1

-1

Raman Shift (cm )

Wavenumber (cm )

Figure 1. FT-IR spectra (left) and Raman spectra (right) of (a) [TBA][NbO(ηO2)(OH)2]; (b) [TBA][NbO(OH)2(LA)]; (c) [TBA][NbO(OH)2(GLY)]; and (d) [TBA][NbO(OH)2(MA)].

Raman spectroscopy gave similar results. As shown in Figure 1a (right), the band in [TBA][NbO(η-O2)(OH)2] at 580 cmí1 was related to s(Nb(O2)), and the band near 865 cmí1 was characteristic for (O-O).50 The band at approximately 880 cmí1 of the IL was related to v(Nb=O). After the IL [TBA][NbO(η-O2)(OH)2] was coordinated by carboxylic acid (Figure 1b, 1c and 1d, right), the bands at 580 cmí1 and 865 cmí1, attributed to the vibration of peroxo group, disappeared, as seen from the FT-IR spectra. The band at approximately 1676 cmí1 was related to the vibration of COO- coordinating with the niobium atom, while the band at approximately 1058 cmí1 was assigned to the vibration of C-O. The bands at approximately 880 cmí1 and 603 cm-1 of the ILs were related to (Nb=O) and (Nb-O), respectively.56 The TGA curves of the three ILs are sequentially shown in Figure S2. Obvious weight losses of approximately 72.5%, 71.7%, and 75.1% were observed between 40 °C and 700 °C, which could be assigned to the decomposition of the cation part and the carboxylato group in the anion moieties. Unlike [TBA][NbO(η-O2)(OH)2], which was mostly degraded at 200 °C,46 the organic acid-coordinated ILs decomposed completely at a higher temperature (approximately 300 °C), indicating a higher thermal stability. Table 1 summarizes the main physico-chemical data of the three ILs.

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Table 1. The physico-chemical data of the three IL catalysts. IL

[TBA][NbO(OH)2(LA)]

Color

Dark red

Tm

Td

°C

°C

< -75.0

334

IR

Raman

cm-1

cm-1

Viscosity [a]

Pa s

1.25

543, 589 ( 1660 (


Dark red

-30.2

330

1.37

Dark red

-42.7

329

1.46

as(COO))

604 ( 1667 (

as(Nb-O))

as(COO))

1123 (v(C-O))

1059 (v(C-O))

883 (v(Nb=O))

880 (v(Nb=O))

543, 579 ( 1669 (

[TBA][NbO(OH)2(MA)]

as(Nb-O))

as(Nb-O))

as(COO))

604 ( 1663 (

as(Nb-O))

as(COO))

1091 (v(C-O))

1059 (v(C-O))

883 (v(Nb=O))

878 (v(Nb=O))

542, 583 ( 1667 (

as(Nb-O))

as(COO))

591 ( 1680 (

as(Nb-O))

as(COO))

1100 (v(C-O))

1057 (v(C-O))

881 (v(Nb=O))

879 (v(Nb=O))

Tm and Td refer to the melting point and decomposition temperature, respectively. [a] viscosity data at 25 °C.

High-resolution electrospray ionization mass spectrometry (HR ESI-MS) analysis in the negative mode was also applied to determine the structure of these catalysts. The ESI-MS of [TBA][NbO(OH)2(LA)] a¡orded an ion centered at 230.9223 m/z, corresponding to [NbO(OH)2(LA)]-. Similarly, the ESI-MS of [TBA][NbO(OH)2(GLY)] showed an ion centered at 216.9066 m/z, corresponding to [NbO(OH)2(GLY)]- and an ion centered at 274.9121 m/z for [TBA][NbO(OH)2(MA)] was attributed to [NbO(OH)2(MA)]- (Figure S3-S5). Finally, [TBA][NbO(OH)2(LA)] was chosen as a model IL to examine the chemical environment changes of the Nb sites by using 93Nb NMR spectra (Figure 2). The [TBA][NbO(η-O2)(OH)2] IL displayed a resonance signal at í1000 ppm (Figure 2a). However, when coordinated by lactic acid, the signal was shifted to the low field and the chemical shift moved to approximately í890 ppm (Figure 2b). This was 8


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probably due to the decreasing electron density of the niobium atom when the peroxoniobate anions were coordinated by α-hydroxy acids, which acted as the electron withdrawing group.57 These results were highly consistent with those of the previous characterization, including FT-IR, Raman, titration analysis and HRMS, which all showed that the α-hydroxy acids functioned as a bidentate ligand. As a result, niobium exhibited coordination by five oxygen atoms belonging to one bidentate group, two hydroxyl groups and one doubly bound O atom in the three IL catalysts. -1000

a 0

-250 -500 -750 -1000 -1250 -1500 -1750 -2000 -890

b 0

-250 -500 -750 -1000 -1250 -1500 -1750 -2000 -1116

c 0

-250 -500 -750 -1000 -1250 -1500 -1750 -2000

Chemical Shift

Figure 2. 93Nb NMR spectra of different niobate anions in CD3OD: (a) [TBA][NbO(ηO2)(OH)2];

(b)

[TBA][NbO(OH)2(LA)];

and

(c)

oxidizing

species

of

[TBA][NbO(OH)2(LA)].

The Structural Identification of Peroxoniobate Anion in the Presence of H2O2 It was visually observed that the color of the ILs quickly changed from dark red to colorless when the IL catalysts were treated with H2O2, which revealed that the anion of the ILs could be oxidized by H2O2 to create a new species. To identify the new anion structure form, the IL [TBA][NbO(OH)2(LA)] (0.2 mmol) was added to methanol (1 ml), and then hydrogen peroxide (2 mmol) was added to the solution. After stirring for 10 min at 273 K, diethyl ether was rapidly poured into the solution to precipitate the oxidizing species, which was washed carefully with a small amount of diethyl ether, sequentially. The resultant material was then characterized. In the IR spectra (Figure 9


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3a), the vibration at approximately 882 cm-1, which was related to

(Nb=O),

disappeared, and a single band near 865 cmí1 emerged. This could be attributed to (OO).50 Only one vibration of (O-O) was observed, indicating that the oxidizing species only contained one type of (O-O) group. Additionally, a strong peak at 576 cm-1, referring to [Nb(O2)], appeared simultaneously. The Raman spectra gave similar information (Figure 3b). The oxidizing species displayed one strong (O-O) band at approximately 870 cmí1 and one broad v[Nb(O2)] band near 585 cmí1.50 In addition to IR and Raman spectroscopy, 93Nb NMR of the oxidizing species was also carried out. As shown in Figure 2c, when [TBA][NbO(OH)2(LA)] was treated with H2O2, the signal was shifted to high field (-1116 ppm), which was obviously different from that of both the IL [TBA][NbO(η-O2)(OH)2] and the coordinated IL [TBA][NbO(OH)2(LA)]. This indicated that the chemical environment around niobium changed, and a new peroxoniobate species was formed in the presence of H2O2.

a

865 870

576 585

b 2240 1960 1680 1400 1120 840 560 -1

Wavenumber (cm ) Figure 3. (a) IR spectrum and (b) Raman spectrum of oxidizing species of [TBA][NbO(OH)2(LA)].

Next, the peroxo number of the oxidizing species was determined by potential difference titration of Ce3+/Ce4+. The number of O-O was approximately 1.8, demonstrating that the oxidizing species could contain approximately two peroxo groups (O-O) in derived oxidizing species. Furthermore, HR ESI-MS was employed to determine the structure of the oxidizing intermediates. The ESI-MS of [TBA][NbO(OH)2(LA)] containing hydrogen peroxide a¡orded an ion centered at 10


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244.9015 m/z corresponding to [Nb(O-O)2(LA)]-, which can be recognized as the reaction intermediate (Figure S6). To gain more insight into the oxidizing species, five possible anions arising from [NbO(OH)2(LA)]- in the presence of H2O2, i.e., [Nb(O-O)(OH)2LA]-, [NbO(O-O)LA]-, [Nb(O-O)2(LA)]-, [Nb(O-O)3(LA)]3- and Nb[(O-O)(OOH)2(LA)]-, were explored by first principles calculations. Upon optimizing the structures of these five peroxoniobate anions, the relative stability was compared through computing the Gibbs free energy changes (ûG) for their formation reactions from [NbO(OH)2(LA)]- at 298 K (Table 2). Evidently, if ûG < 0 (> 0), the corresponding conversion is thermodynamically favored (hindered); the more negative ûG is, the more stable the generated Nb anion is. As indicated in Table 2, the formation of [Nb(O-O)2LA]- containing two peroxos is the most thermodynamically favored (ûG = -0.51 eV), while [Nb(O-O)3LA]3- is the least stable with the most positive formation energy (ûG =1.72 eV). Therefore, [Nb(OO)2LA]- can be theoretically expected to be the dominant form, which is in accordance with the above experimental identification.

Table 2. Gibbs free energy changes (ûG) of the conversion from [NbO(OH)2LA]- to [Nb(O-O)(OH)2LA]-, [NbO(O-O)LA]-, [Nb(O-O)2LA]-, Nb[(O-O)(OOH)2LA]- and [Nb(O-O)3LA]3-, in the presence of H2O2. Note that the solvation energy of the proton (H+) is set at the experimental value of approximately -11.53 eV. Reaction

ûG /eV

[NbO(OH)2LA]- + H2O2 : [Nb(O-O)(OH)2LA]- + H2O

-0.15

[NbO(OH)2LA]- + H2O2 : [NbO(O-O)LA]- + 2H2O

-0.24

[NbO(OH)2LA]- + 2H2O2 : [Nb(O-O)2LA]- + 3H2O

-0.51

[NbO(OH)2LA]- + 3H2O2 : Nb[(O-O)(OOH)2LA]- + 3H2O + 2H+

-0.18

[NbO(OH)2LA]- + 3H2O2 : [Nb(O-O)3LA]3- + 3H2O

1.72

In summary, according to the above experimental and theoretical results, the newly evolved Nb anion from [NbO(OH)2R]- oxidized by H2O2 in our system is proposed to be [Nb(O-O)2LA]- (R=LA, GLY, MA) (see Scheme 2). The optimized configuration of

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[Nb(O-O)2LA]- is illustrated in Figure 4.

OH O [N4,4,4,4] O Nb OH O

R' + 2H2O2 O

O O [N4,4,4,4] O Nb O O O

R' + 3H2O O

xxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxx R' = -CH3; [TBA][Nb(OO)2(LA)] xxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxx -H; [TBA][Nb(OO)2(GLY)] xxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxx -CH2COOH; [TBA][Nb(OO)2(MA)] xxxxxxxxxxxxxxxxxxxxxxxxx

Scheme 2. The structure evolution of the IL catalysts [TBA][NbO(OH)2(LA)] in the presence of hydrogen peroxide.

Figure 4. Ball-and-stick model of the optimized structure of [Nb(O-O)2LA]-. White, gray, red, blue balls indicate the H, C, O and Nb atoms, respectively.

Catalytic Performance Catalytic Epoxidation of Olefins The model substrate cyclooctene was epoxidized with H2O2 in the presence of organic acid-coordinated peroxoniobate-based ionic liquids in methanol at 30 °C. The results are shown in Table 3. The catalytic reaction could barely occur without catalysts (Table 3, entry 1), indicating that H2O2 could not oxidize the substrate without the catalysts. TBACl was also inactive for the epoxidation of olefins (Table 3, entry 2). It was clear that the organic acid itself could not catalyze the epoxidation reaction at all (Table 3, entries 3-5). In addition, (NH4)3[Nb(O2)4] afforded a very poor activity for the epoxidation of cyclooctene (Table 3, entry 6). The (NH4)3[Nb(O2)4] compound was nearly undissolved in the methanol, and the catalytic system displayed a liquid-solid biphase under the reaction conditions. In contrast, the IL catalyst [TBA][NbO(ηO2)(OH)2] was miscible with the methanol media and retained a homogeneous phase, and it exhibited a much higher catalytic activity than that of (NH4)3[Nb(O2)4] (Table 3, 12


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ACS Catalysis

entries 6 vs 7). The results suggest that the catalytic activity could arise from the moiety of peroxoniobate anions, while the substitution of the inorganic cation (NH4+) by a quaternary ammonium cation was much more favorable to the epoxidation reaction due to the better solubility of the catalyst in the organic solvent.58,59 Additionally, the different anion structure could have contributed to the improved activity. Interestingly, the epoxidation reaction proceeded very efficiently when [TBA][NbO(OH)2(LA)] was adopted as the catalyst (Table 3, entry 8). A similar reactivity could be obtained using [TBA][NbO(OH)2(GLY)] and [TBA][NbO(OH)2(MA)] (Table 3, entries 9 and 10). If lactic acid was added directly into the parent IL, a high conversion of approximately 85.3% was still achieved (Table 3, entry 11). However, it was observed that the introduction of acetic acid, succinic acid or 3-hydroxypropionic acid afforded the same catalytic activity as that of the parent IL (Table 3, entry 7 vs entries 12-14), implying that the addition of these acids cannot exert a positive effect on the epoxidation of olefins. These results clearly demonstrated that the α-hydroxy acids (LA, GLY and MA) as ligands played a crucial role in the enhancement of catalytic activity. In particular, these IL catalysts demonstrated their advantages in catalytic epoxidation compared with that of the reported tungsten-containing catalysts such as [TBA]2W2O11 or [TBA]3PW12O40 under similar reaction conditions (Table 3, entries 15 and 16).

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Table 3. Epoxidation of Cyclooctene with Different Catalysts[a] Sel. [%]

Entry

Catalysts

Con. /[%][b]

1

None

0.3

•99

Identifying Catalytically Active Mononuclear Peroxoniobate Anion of

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