Tetrachlorocobaltate-Catalyzed Methane Oxidation to Methyl Trifluoroacetate

Abstract

In ongoing attempts to efficiently utilize abundant natural gas, there has been steady scientific and industrial interest in using an environmentally benign and inexpensive oxidant (dioxygen O₂) for the direct catalytic oxidation of methane to oxygenate products under mild conditions. Here, we report the homogeneous bis(tetramethylammonium) tetrachlorocobaltate ([Me₄N]₂CoCl₄)-catalyzed methane oxidation to methyl trifluoroacetate (MeTFA) with dioxygen O₂ in trifluoroacetic acid (HTFA) media. [Me₄N]₂CoCl₄ had the highest catalytic activity among previously reported homogeneous cobalt-based catalyst systems; the turnover of methane to MeTFA reached 8.26 mol_ester mol_metal⁻¹h⁻¹ at 180 °C. Results suggest that the ionic form of the catalyst makes the Co species more soluble in the HTFA media; consequently, an active catalyst form, [CoTFA_xCl_y]²⁻, can form very rapidly. Furthermore, chloride anions dissociated from CoCl₄²⁻ appear to suppress oxidation of the solvent HTFA, thereby driving the reaction toward methane oxidation. The effects of reaction time, catalyst concentration, O₂ and methane pressure, and reaction temperature on MeTFA production were also investigated.

1. Introduction

Methane is the primary constituent of natural gas, a vital part of the world’s energy supply. Methanol demand and production have also been steadily increasing over the last two decades for use as a platform chemical and fuel [1,2]. Developing new methods to convert an affordable starting material (methane) into a strategic product (methanol) is an important process in the chemical industry. While several commercial technologies are currently used to convert methane to syngas and for the subsequent transformation of syngas to value-added chemicals such as methanol [3,4,5], this is an indirect and energy-intensive pathway. Finding a way to directly convert methane to oxygenates under benign conditions is a challenge that has captivated both scientific and industrial communities. However, high-value product formation and selectivity via direct oxidation are limited because of the stability of the C-H bond [6], and this can result in the over-oxidation of methanol to carbon dioxide [7]. To avoid this problem, in recent decades, various methods have been proposed and studied to protect the methanol in the form of an ester, such as methyl bisulfate [8,9,10,11,12] or methyl trifluoroacetate (MeTFA) [13,14,15,16,17,18,19,20,21,22]. Trifluoroacetic acid (HTFA)-based MeTFA synthesis from methane has received attention because it occurs in non-super acidic media under milder conditions; and because of its low boiling temperature (43 °C), methyl trifluoroacetate can be easily separated from the mixture by simple distillation after oxidation.

KIO₃, KIO₄, K₂S₂O₈, and O₂ are frequently used as oxidants to directly synthesize MeTFA from methane in HTFA media. Among them, the most promising oxidant candidate is dioxygen O₂ since it is inexpensive, plentiful, non-harmful, and simple to eliminate after the reaction. Accordingly, in recent decades, many efforts have been dedicated to utilizing dioxygen O₂ for methane oxidation in HTFA [16,23,24,25,26,27,28,29,30].

For example, the one-pot aerobic oxidation of methane to MeTFA using a catalyst system consisting of three redox couples, Pd²⁺/Pd⁰, Q/H₂Q (p-benzoquinone/p-hydroquinone), and NO₂/NO, was reported by Z. An et al. [25]. The turnover frequency with this catalyst system was only 0.7 per hour due to the limitation of the initial activation step. To improve its catalytic performance, J. Yuan et al. suggested adding perfluorooctane (C₈F₁₈) into the Pd(OAc)_2/benzoquinone/H₅PMo₁₀V₂O₄₀ system [26]. The resulting increase in the oxygen solubility of the HTFA/C₈F₁₈ media reduced the tension of the liquid/gas interface and resulted in a dramatic increase in MeTFA formation; the turnover number (TON) reached over 100 at 80 °C after 8 h.

Compared to the Pd-based multi-redox couple oxidation system, Co-based catalysts are known to act as a direct oxidation catalyst for hydrocarbon in HTFA. R. Tang et al. reported that Co(III) reacted instantaneously as an electron-transfer oxidant for the activation of the C-H bond in cyclohexane, and O₂ was an essential contributor to the regeneration cycle between the Co(II) and Co(III) species [31]. M.N. Vargaftik et al. tested various metals, including Pd, Mn, Fe, Co, Cu, and Pb, on aerobic methane oxidation and showed Co(TFA)₂ gave the highest activity for MeTFA formation with a TON of 4 at 180 °C after 4 h [32]. Nevertheless, the precipitation of Co species into inactive CoF₂ during the reaction was reported to decrease the rate of MeTFA production.

T. Strassner and co-workers investigated the activities of various cobalt salts, such as Co(OAc)₂, CoCl₂, Co(NO₃)₂, Co₂O₃, CoBr₂, Co(acac)₃, and CoSO₄, on aerobic methane oxidation and found the catalytic activities were highly dependent on the anions of cobalt salts [24]. Among the tested catalysts, Co(NO₃)₂ and Co(OAc)₂ were the most potent for this reaction; TONs of 8.4 and 7.3 were obtained at 160 °C after 24 h, respectively. Furthermore, catalyst deactivation to CoF₂ could be prevented by the addition of trifluoroacetic anhydride (TFAA). They also noted that CoBr₂, CoF₂, and CoSO₄ did not show any activity in the reaction due to their poor solubilities in HTFA.

Many inorganic metal compounds have been disregarded as potential homogeneous catalysts because they are insoluble in typical organic solvents, aqueous solutions, or concentrated acids. One way of improving their solubilities is by transforming inorganic metal compounds into organometallic or ionic complexes. For example, in our previous research, we found that transforming neutral PdCl₂ to anionic PdCl₄²⁻ led to greatly enhanced catalytic performance (from 7.3% to 33%) as methane oxidation proceeded in a K₂S₂O₈ oxidant and HTFA solvent system [21]. CoCl₂ is the most basic form of Co catalyst, and its performance on aerobic methane oxidation was reported to be TON 2–4.7 after a reaction time of 3–72 h at 180 °C, which corresponds to 0.6 to 0.06 mol_ester mol_metal⁻¹h⁻¹ [24].

Here, we introduce an anionic Co complex, bis(tetramethylammonium) tetrachlorocobaltate ([Me₄N]₂CoCl₄), as a methane oxidation catalyst in an O₂ oxidant system. Like PdCl₄²⁻ mentioned above, by changing CoCl₂ to an anionic form, CoCl₄²⁻, the catalytic activity can be greatly increased to 8.26 mol_ester mol_metal⁻¹h⁻¹, which corresponds to the highest value among the homogeneous cobalt catalysts reported. To understand the structure of the real catalytic species, ICP, XPS, and FT-IR analyses were conducted, and the results showed that chloride in CoCl₄²⁻ reversibly exchanged with trifluoroacetate to form CoCl_xTFA_y²⁻ in the HTFA solvent. The effect of reaction time, catalyst concentration, and dioxygen amount were also investigated.

2. Results and Discussion

[Me₄N]₂CoCl₄ can be made from CoCl₂ and Me₄NCl by dissolving them in ethanol, as described in the Experimental section. Elemental Analysis of the CHN contents and ICP analysis of the Co content revealed the [Me₄N]₂CoCl₄ was successfully synthesized. Figure 1 provides an X-ray diffractogram of the prepared catalyst which matches the reference [Me₄N]₂CoCl₄ well [33].

Using the synthesized metal catalyst and other conventional Co catalysts, a methane oxidation reaction was conducted in the presence of O₂ as an oxidant, trifluoroacetic acid (HTFA) as a solvent, and trifluoroacetic anhydride (TFAA) as a dehydrating agent at 180 °C for 1 h (Scheme 1).

It can be expected that Co(II) is oxidized to Co(III) by dioxygen with the liberation of water, which will be removed quickly by TFAA. After that, the oxidation of methane by Co(III) will take place via radical mechanism in the HTFA media to produce MeTFA, as previously reported [24,32,34].

Table 1. Aerobic methane oxidation using various Co catalysts ^a.

Entry	Catalyst System	MeTFA (mmol, molester molmetal−1h−1)	CO2 (mmol)
1	[Me4N]2CoCl4	8.26	5.00
2 b	[Me4N]2CoCl4	0	10.59
3 c	[Me4N]2CoCl4	9.73	9.62
4	CoCl2	0.33	1.50
5	CoCl2 + 2Me4NCl	5.52	3.82
6	Co(NO3)2.6H2O	4.38	9.18
7	Co(TFA)2	4.74	9.86
8	CoBr2	0.12	2.49
9	CoSO4.7H2O	0.10	1.93
10	CoTFA4(Me4N)2	5.40	7.56
11	CoTFA4(Me4N)2 + 2Me4NCl	5.75	3.76
12	-	-	0.25

^a Reaction conditions: 1 mmol of catalyst, 5 bar of O₂, 20 bar of methane, 5 g of TFAA, 25 g of HTFA, 180 °C, 1 h. ^b The reaction was carried out in the absence of methane. ^c 10 mmol of MeTFA was reacted instead of methane.

shows that, among the various cobalt catalysts tested, the synthesized [Me₄N]₂CoCl₄ gave the highest activity; 1.00 mmol of [Me₄N]₂CoCl₄ produced 8.26 mmol of MeTFA (Table 1, Entry 1) during a 1 h reaction at 180 °C. Table S1 in Supplementary Materials reveals that, among the O₂-based oxidant system, a productivity (mol_ester mol_metal⁻¹h⁻¹) of 8.26 for MeTFA formation was the highest value among the homogeneous aerobic catalysts reported (Table S1 in Supplementary Materials). The same reaction conducted in the absence of methane (Entry 2) gave no MeTFA, suggesting the tetramethylammonium cation in [Me₄N]₂CoCl₄ does not lead to the formation of MeTFA. Similarly, using MeTFA as a starting material (Entry 3) showed that MeTFA is quite stable under this reaction condition. CoCl₂, the precursor of [Me₄N]₂CoCl₄, only produced a negligible amount of MeTFA (0.33 mmol). Interestingly, the in situ addition of Me₄NCl to CoCl₂ prior to the oxidation reaction appeared to be effective, increasing the MeTFA formation to some extent; the addition of 2 equiv. of Me₄NCl resulted in an increase in MeTFA formation to 5.52 mmol (Entry 5).

On the other hand, Co(NO₃)₂ and Co(TFA)₂ resulted in MeTFA formations of 4.38 mmol and 4.74 mmol, respectively. The other cobalt salts, such as CoBr₂ and CoSO₄, exhibited very low catalytic performance, with only 0.12 and 0.10 mmol of MeTFA, respectively.

2.1. [Me₄N]₂CoCl₄-Catalyzed Methane Oxidation

As T. Strassner et al. mentioned, the solubility of inorganic cobalt compounds such as CoCl₂, CoBr₂, and CoSO₄ are very poor in HTFA [24]. However, as expected, [Me₄N]₂CoCl₄ was instantly dissolved in HTFA, even at room temperature, forming a pink-colored solution (Figure 2b). Furthermore, this pink-colored solution was also observed during the reaction (Figure 2c); however, only a sky-blue heterogeneous solid remained after the reaction (Figure 2d). When this pink solution and sky-blue solid were used as a catalyst for this methane oxidation, respectively, the quantitation pink solution showed some MeTFA formation, but no product was observed after the sky-blue solid-catalyzed reaction, indicating active catalysts remained in the pink solution phase.

To understand the catalyst form in HTFA in the [Me₄N]₂CoCl₄-catalyzed reaction, the HTFA solvent was removed by vacuum from the pink solution after the reaction, and a pink powder was obtained. Characterization of the obtained pink solid using XPS, IR, and elemental analysis revealed the isolated solid was [Me₄N]₂CoTFA₄, suggesting CoCl₄²⁻ was turned into CoTFA₄²⁻ in HTFA.

The XPS data shown in Figure 3 reveals that Co, Cl, C, and N atoms exist in the fresh [Me₄N]₂CoCl₄, as expected, while the Cl peak disappeared with the appearance of an F peak in the solid isolated from the pink solution produced by the [Me₄N]₂CoCl₄-catalyzed reaction.

The detailed C1s and Co2p XPS analyses exhibited in Figure 4 suggest the transformation of CoCl₄²⁻ to CoTFA₄²⁻ in HTFA during the reaction. According to the C1s XPS analysis, apart from a peak at 286.5 eV, which was ascribed to a methyl group in the Me₄N⁺ cation, the pink powder obtained from the [Me₄N]₂CoCl₄-catalyzed reaction showed two new peaks at 288.8 eV and 292.5 eV, which correspond to C=O and CF₃, respectively. These peaks were also observed in the [Me₄N]₂CoTFA₄ synthesized from [Me₄N]₂CoCl₄ and AgTFA, according to the literature [35,36]. Moreover, in the Co2p XPS analysis data, similar peak positions at 781.7 and 781.6 eV were observed both in the isolated solid after the reaction and in the synthesized [Me₄N]₂CoTFA₄, a clear difference from the 780.8 eV of [Me₄N]₂CoCl₄.

The FT-IR pattern of the pink solid was also very similar to that of synthesized [Me₄N]₂CoTFA₄ (Figure 5b,c). In addition to two peaks at 1489 and 949 cm⁻¹, which were assigned to the CH₃ groups in [Me₄N]⁺, a strong peak at 1685 cm⁻¹ was observed in both the isolated pink solid and synthesized [Me₄N]₂CoTFA₄, which can be interpreted to be the carbonyl group (C=O) in TFA (CF₃CO₂⁻) binding to a Co metal center. Furthermore, two strong peaks at 1185 cm⁻¹ and 1130 cm⁻¹, assigned to the C-F stretching vibration, and two medium intensity peaks at 840 cm⁻¹ and 794 cm⁻¹, assigned to the C-C stretching and CF₃ symmetric stretching vibrations, respectively, were also observed in the pink solid and synthesized [Me₄N]₂CoTFA₄ [37,38].

Interestingly, the ligand exchange of CoCl₄²⁻ to CoTFA₄²⁻ seems to happen very quickly, even at room temperature. As Figure 5d shows, the IR spectra of the solid obtained from just-dissolved [Me₄N]₂CoCl₄ in HTFA and synthesized [Me₄N]₂CoTFA₄ are almost similar.

However, although [Me₄N]₂CoCl₄ turns to [Me₄N]₂CoTFA₄ in HTFA, the catalytic activity of [Me₄N]₂CoTFA₄ for the methane oxidation reaction was lower than that of [Me₄N]₂CoCl₄; 5.40 mmol of MeTFA was produced, which is lower than the 8.26 mmol for [Me₄N]₂CoCl₄ (Entry 10 vs Entry 1 in Table 1). Because the chloride anion is dissociated from [Me₄N]₂CoCl₄ in the course of changing to CoTFA₄²⁻, the effect of chloride anions was investigated by adding 2 equiv. of Me₄NCl into the [Me₄N]₂CoTFA₄-catalyzed reaction. Entry 11 in Table 1 reveals that the presence of the free chloride anion did not have much effect on the amount of MeTFA; the amount was changed from 5.40 mmol to 5.75 mmol, while CO₂ formation noticeably decreased from 7.56 mmol to 3.76 mmol.

M. N. Vargaftik et al. reported that, besides methane oxidation, solvent HTFA oxidation occurs simultaneously in this aerobic Co-catalyzed methane oxidation reaction [32]. We also observed CO₂ in the gas phase after methane oxidation in all the reactions shown in Table 1 and found that used Co catalysts also catalyzed HTFA oxidation to generate CO₂ and F compounds like HF and CF₃Cl. Meanwhile, MeTFA oxidation in the presence of [Me₄N]₂CoCl₄ was negligible (Entry 3 in Table 1).

Table 1 also shows that the catalysts with very low activities for methane oxidation, CoCl₂, CoBr₂, and CoSO₄, also showed marginal CO₂ production between 1.50 and 2.49 mmol. The catalysts showing higher activity for MeTFA production also showed increased CO₂ formation. However, the catalytic activity on the solvent oxidation was not always proportional to the catalytic activity on the methane oxidation. The catalyst showing the highest activity on MeTFA production, [Me₄N]₂CoCl₄, gave 5.00 mmol of CO₂ formation, while Co(NO₃)₂ and Co(TFA)₂, which showed a lower MeTFA amount than [Me₄N]₂CoCl₄, gave increased CO₂ formation of 9.18 and 9.86 mmol, respectively.

It was previously reported that the Co(III) complex oxidizes organic aliphatic acid to produce alkyl radicals and CO₂ [39,40]. Similarly, the solvent HTFA in the Co catalyst system could be decomposed to CF₃ radicals and CO₂ as in Equation (1).

$$\mathrm{Co}\left(\mathrm{III}\right) + {\mathrm{CF}}_3{\mathrm{CO}}_2\mathrm{H}\to \mathrm{Co}\left(\mathrm{II}\right) + {{\mathrm{CF}}_3}^{•} + {\mathrm{CO}}_2 + {\mathrm{H}}^{+}$$

(1)

In fact, a large amount of CF₃Cl was detected in the [Me₄N]₂CoCl₄-catalyzed reaction in the gas phase of the reaction product when conducted in the absence of methane, suggesting the CF₃ radical from HTFA was captured by the chloride in the reaction media (Figure S5 in Supplementary Materials), and when the same reaction proceeded in the presence of methane, CHF₃ and CF₃Cl were observed together (Figure S6 in Supplementary Materials). The detection of CHF₃ could suggest the formation of MeTFA and CF₃ radicals linked by the methyl radical formation [41]. However, considering the formed amounts of CHF₃ (0.42 mmol) and MeTFA (8.26 mmol), it can be concluded that the major amount of MeTFA was formed by the direct reaction between Co(III) and methane.

On the other hand, in the [Me₄N]₂CoTFA₄-catalyzed reaction, no CHF₃ and CF₃Cl were observed (Figure S7 in Supplementary Materials), suggesting the CF₃ radicals from the HTFA oxidation further degraded to HF and CO₂ in the reaction media [42]. The formation of HF could increase the acid concentration on the reaction media, which is known to accelerate the oxidation of organic acid in a Co catalyst system [39,40]. In other words, the chloride in the reaction media could capture CF₃ radicals in the form of CHF₃ or CF₃Cl, thereby suppressing excessive oxidation of HTFA, compared to a chloride-free system. Entry 10 and Entry 11 in Table 1 show that the CO₂ formation decreased from 7.56 mmol to 3.76 mmol with the addition of 2 equiv. of Me₄NCl to CoTFA₄(Me₄N)₂.

Overall, the higher catalytic activity of [Me₄N]₂CoCl₄ can be attributed to the high solubility of the catalytically active Co species on HTFA and the presence of chloride anions, which appears to suppress solvent oxidation. Besides this, we suspect that the real catalytic species formed from [Me₄N]₂CoCl₄ during the reaction may not be CoTFA₄²⁻, which is an isolated species from the [Me₄N]₂CoCl₄-catalyzed reaction. Instead, the partially substituted CoCl_xTFA_y²⁻ species may be the real form in this methane oxidation reaction.

Figure 2 shows that when the reaction was stopped at 30 min, besides the homogeneous pink solution, a sky-blue solid was observed in the bottom of the reactor liner. After 1 h reaction, a transparent solution was attained with a sky-blue solid. This precipitated sky-blue solid appeared to be CoCl₂, based on XRD and XPS analyses (Figure 6).

This result indicates that chloride and TFA reversibly interacted with the Co species in the HTFA media to form CoCl_xTFA_y²⁻ but finally precipitated in the form of poorly soluble CoCl₂ at the reaction temperature of 180 °C. The reason [Me₄N]₂CoTFA₄ is isolated from the dissolved [Me₄N]₂CoCl₄ in HTFA can possibly be explained as follows: during the HTFA removal process, the chloride could be removed in the form of HCl, which is more volatile than HTFA. On the other hand, no CoF₂ was observed in the XRD analysis of the precipitated sky-blue solid, which was assumed to be a deactivated catalyst form [32].

2.2. Effects of Reaction Conditions

The effect of [Me₄N]₂CoCl₄ concentration on the MeTFA formation was investigated. Figure 7a confirms that by increasing [Me₄N]₂CoCl₄ catalyst concentration from 0.2 mmol to 1.0 mmol, the amount of produced MeTFA linearly increased from 1.85 mmol to 8.26 mmol. Further increasing the [Me₄N]₂CoCl₄ concentration to 1.5 mmol resulted in a slight decrease in MeTFA formation to 8.01 mmol, suggesting the probability of MeTFA oxidation at a higher catalyst concentration. The increased amount of dioxygen, from 5.0 bar to 7.5 bar and 10 bar, resulted in a slight increase in MeTFA formation from 8.26 mmol to 9.03 and 9.60 mmol, respectively (Figure 7b), suggesting the effect of O₂ amount on this methane oxidation was insignificant. In contrast, the effect of methane pressure was more pronounced than that of dioxygen (Figure 7c). At the lower pressures of 5 and 10 bar, MeTFA formation was 1.52 mmol and 3.28 mmol, respectively. At the methane pressure of 20 bar, it increased to 8.26 mmol; however, a further increase to 30 bar did not result in a proportional increase in MeTFA formation.

The influence of reaction time on the formation of MeTFA was also explored (Figure 7d). By increasing the reaction time from 30 min to 3 h, the formation of MeTFA continuously increased from 5.23 mmol to 9.69 mmol. At 3 h reaction, although 9.69 mmol of MeTFA was produced, the reactor was severely corroded, forming a dark contaminated solution which originated from the SUS reactor (Supplementary Materials, Figure S8). The effect of reaction temperature was also explored at temperatures between 150–220 °C. Figure S9 in Supplementary Materials revealed the formation of MeTFA was the highest at 180 °C. At the temperature of 150 °C, the produced MeTFA was 2.85 mmol, while at the reaction temperatures of 200 and 220 °C the MeTFA amounts were 7.52 and 5.37 mmol, which were lower than the 8.26 mmol produced at 180 °C. This result indicates MeTFA decomposed to a significant degree at higher reaction temperatures above 200 °C.

3. Experimental Section

3.1. General Information

All chemicals were of analytical reagent grade and used without further purification. Cobalt salts (CoCl₂, Co(NO₃)₂.6H₂O, CoBr₂, CoSO₄.7H₂O), tetramethyl ammonium chloride, and other chemicals for the synthesis of catalysts were purchased from Sigma Aldrich Chemical Co.(St. Louis, MO, USA), trifluoroacetic acid and trifluoroacetic anhydride were obtained from Alfa Aesar Co.(Haverhill, MA, USA), and high-purity methane containing 1% of Ar and dioxygen O₂ gas (99.995 %) were supplied from Shinyang Gas Co.(Gyeonggi-do, Korea).

3.2. Catalyst Synthesis

[Me₄N]₂CoCl₄ catalyst was synthesized by a simple ligand exchange method presented in the previous report [43]. In a typical sample preparation, cobalt dichloride CoCl₂ (0.50 g, 3.85 mmol) and tetramethylammonium chloride (CH₃)₄NCl (0.84 g, 7.70 mmol) were separately dissolved in ethanol. A sky-blue solid was precipitated by mixing the two solutions at room temperature. After being filtered, washed, and dried under vacuum, bis(tetramethylammonium) tetrachlorocobaltate (II) was obtained. Elemental Analysis (Thermo Scientific Flash 2000 CHNS/O, Thermo Fisher Scientific, Cambridge, UK) Cal. C (27.53 wt%), H (6.93 wt%), N (8.03 wt%). Anal. C (27.31 wt%), H (6.84 wt%), N (7.80 wt%), ICP analysis Cal. Co (16.88 wt%), Anal. Co (16.52 wt%).

Co(TFA)₂ catalyst was prepared by the metathesis reaction between CoCl₂ (0.46 g, 3.51 mmol) and AgTFA (1.55 g, 7.02 mmol) in 30 g of CH₃NO₂, as described in the literature [37]. The round flask containing CoCl₂ and AgTFA was fully covered using aluminum foil and stirred at rt overnight. After the precipitated AgCl was filtered out, Co(TFA)₂ was obtained by removing nitromethane under reduced pressure. Elemental Analysis for Co(TFA)₂ Cal. C (16.86 wt%), Anal. C (17.05 wt%), ICP analysis Cal. Co (20.68 wt%), Anal. Co (21.30 wt%).

[Me₄N]₂CoTFA₄ was synthesized by the reaction of [Me₄N]₂CoCl₄ (0.670 g, 2.0 mmol) and AgTFA (1.77g, 8.0 mmol) in nitromethane at room temperature overnight [35,43]. After AgCl was filtered out, a violet-colored product could be obtained by evaporation of the solvent under vacuum. Elemental Analysis for [Me₄N]₂CoTFA₄ Cal. C (29.15 wt%), H (3.67 wt%), N (4.25 wt%), Anal. C (29.31 wt%), H (3.58 wt%), N (4.50 wt%), ICP analysis Cal. Co (8.94 wt%), Anal. Co (8.82 wt%).

3.3. Methane Oxidation Reaction

The partial oxidation of methane was conducted using a bomb reactor (SS304, 100 mL) equipped with a glass liner, thermocouple, pressure gauge, and heating jacket. In a typical reaction, a glass liner containing 1.00 mmol of catalyst, 25 g of trifluoroacetic acid, and 5 g of trifluoroacetic anhydride was introduced into the reactor and pressurized with 5 bar of O₂ (17.2 mmol) and 20 bar of CH₄ (68.6 mmol) at room temperature. The reactor was subsequently heated to 180 °C and stirred (600 rpm) for 1 h.

3.4. Analysis

After the reaction, a liquid product containing MeTFA was analyzed using GC (7890A, Agilent Technologies, Santa Clara, CA, USA). CHCl₃ was used as an external reference for the quantitation of MeTFA. For the quantitation of CO₂ formed during the oxidation, the gas collected from the reactor was analyzed using GC-MS (HP 6890 GC/5973 MSD, Agilent Technologies, Santa Clara, CA, USA) equipped with a capillary column (Poraplot Q 30 m × 25 um). Ar gas which was included in methane (1%) was used as an internal reference. The cobalt concentration in the liquid phase was determined by ICP-OES (iCAP 700 series-ICP spectrometer, Thermo Fisher Scientific, Waltham, MA, USA) and the X-ray diffraction patterns (XRD) were recorded on a Shimadzu X-ray diffractometer (XRD-6000, Shimadzu, Kyoto, Japan) using nickel-filtered CuKα radiation with 2θ angle from 10 to 80°. FT-IR spectra were obtained by Thermo Nicolet iS10 FTIR (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a Smart Diamond ATR accessory. CNH analysis was conducted using Thermal scientific flash 2000 CHNS/O (Thermo Fisher Scientific, Cambridge, UK). XPS analysis was taken by Nexsa (Thermo Fischer Scientific, Waltham, MA, USA) with a microfocus monochromatic X-ray source (Al-Kα = 1486.6 eV).

4. Conclusions

An [Me₄N]₂CoCl₄-catalyzed aerobic methane oxidation was conducted in HTFA solvent at 180 °C. Because of its ionic structure, it dissolved well in HTFA, even at room temperature, and exhibited the highest catalytic activity among the tested Co catalysts, including CoCl₂, CoBr₂, CoSO₄, Co(NO₃)₂, Co(TFA)₂, and [Me₄N]₂CoTFA₄. Although [Me₄N]₂CoTFA₄ was isolated from the reaction media, the real catalytic species were assumed to be [CoCl_xTFA_y]²⁻, and it finally turned into CoCl₂. It was found that HTFA oxidation to CO₂ was also catalyzed by the methane oxidation catalyst and that the presence of chloride in the reaction media appeared to suppress HTFA oxidation to some extent. This could be another reason for the high catalytic activity of [Me₄N]₂CoCl₄ compared to the other chloride-free Co catalysts, such as Co(TFA)₂ and CoTFA₄(Me₄N)₂. The production of MeTFA was heavily affected by the reaction condition, including reaction time, catalyst amount, and methane pressure, but the effect of O₂ pressure was insignificant. MeTFA was quite stable at 180 °C, but it was further oxidized above 200 °C.