Direct methane conversion to methanol by ionic liquid-dissolved platinum catalysts

Abstract

Ternary systems of inorganic Pt salts and oxides, ionic liquids and concentrated sulfuric acid are effective at catalyzing the direct, selective oxidation of methane to methanol and appear to be more water tolerant than the Catalytica reaction.

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Homogeneous catalysis using transition metal complexes for the direct conversion of natural gas to liquid products under mild conditions (such as T < 300 °C) has attracted considerable interest since the 1970s due to their scientific and industrial significance.^1–3 Shilov chemistry in aqueous solution was the earliest reported catalytic system, in which Pt(II) (added to the reaction as PtCl₄²⁻) was the catalyst and Pt(IV) (in the form of PtCl₆²⁻) was a stoichiometric oxidant to oxidize methane to methanol and chloromethane.² More recently, the Catalytica catalyst ((bpym)PtCl₂) in oleum has been found to be highly efficient and highly selective, with up to 72% yields of methanol (in the form of methylbisulfate that can be hydrolyzed later).³ Though the Catalytica system appears to be the one most suited to practical applications in terms of conversion and selectivity, the catalyst is unfortunately deactivated by the water and methanol that are produced in situ during the reaction.⁴

We demonstrate here a versatile method to prepare highly active ternary systems that involve ionic liquids (ILs), inorganic Pt compounds and sulfuric acid (98% and below). Ionic liquids are promising “green” media for organic synthesis due to their low volatility and good thermal stability;⁵ however, their potential in other applications has not been fully recognized. Many inorganic platinum compounds, such as PtCl₂ or PtO₂, are rarely used in homogeneous catalysis because they are insoluble in typical organic or aqueous solutions, and even in concentrated acids, although some of them have been used as heterogeneous catalysts.⁶ Another platinum salt, PtCl₄, and the reagents of Shilov chemistry, K₂PtCl₄ and H₂PtCl₆, are soluble in water but are not compatible with concentrated sulfuric acid. We found that all five of these Pt compounds could be readily dissolved in a variety of ILs upon heating, and were subsequently soluble in concentrated sulfuric acid, forming a homogeneous solution.⁷ Suitable ILs found so far include imidazolium, pyridinium, pyrazolium and triazolium-based examples, with chloride (Cl⁻) or bisulfate (HSO₄⁻) as the anion (Table 1). The initial dissolution of inorganic Pt compounds into ILs was presumably through Cl⁻ or HSO₄⁻ coordination.⁸ Studies of the detailed dissolution mechanism for binary systems of Pt/IL and ternary systems of Pt/IL/H₂SO₄ are ongoing. A prepared ternary solution with [Pt] = 50 mM and methane gas (∼3.4 MPa) were introduced into a high pressure reactor and reactions carried out at 180–220 °C for 2.5 h.‡

Table 1 Catalytic oxidation of methane to methanol in 96% H₂SO₄^a

Entry	Catalyst	Ionic liquid^b	T/°C	CH3OH/mM^c	TON
a Reaction conditions: 0.05 mmol Pt species + 0.3 mmol IL + 1 mL 96% H2SO4 + 3.4 MPa CH4 in a 69 mL reactor at the chosen temperature for 2.5 h. b IL: im = imidazolium; 1-mim = 1-methylimidazolium; mmim = 1,3-dimethylimidazolium; pyrid = pyridinium; pyraz = pyrazolium; 1-mpyraz = 1-methylpyrazolium; mmpyraz = 1,2-dimethylpyrazolium; triaz = 1,2,4-triazolium. c The methanol concentration was determined by ^1H NMR from the sum of the free or protonated methanol and the ester (CH3OSO3H).
1	(bpym)PtCl2		220	31	0.6
2	(bpym)PtCl2	[im][Cl]	220	26	0.5
3	PtCl2	[1-mim][Cl]	220	173	3.5
4	PtCl2	[im][Cl]	220	100	2.0
5	PtCl2	[mmim][HSO4]	220	73	1.5
6	K2PtCl4	[1-mim][Cl]	220	105	2.1
7	PtCl4	[1-mim][Cl]	220	89	1.8
8	PtO2	[im][Cl]	220	52	1.0
9	PtO2	[1-mim][HSO4]	220	0	0
10	H2PtCl6	[1-mim][Cl]	220	157	3.1
11	PtCl2	[pyrid][HSO4]	220	1	∼0
12	PtCl2	[pyraz][HSO4]	200	42	0.8
13	PtCl2	[1-mpyraz][HSO4]	200	79	1.6
14	PtCl2	[mmpyraz][HSO4]	200	94	1.9
15	PtCl2	[triaz][HSO4]	200	0	0

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Analyses by ¹H NMR and GC-MS on the crude reaction solutions showed that the only liquid products derived from methane, if any, were methanol and methylbisulfate (CH₃OSO₃H), which is similar to the Catalytica system.³ The data in Table 1 show that all Pt species (PtCl₂, PtCl₄, PtO₂, K₂PtCl₄ and H₂PtCl₆) could exhibit significant catalytic activity depending on the nature of the IL used. In the case of PtCl₂ + [1-mim][Cl] in 96% H₂SO₄ (Table 1, entry 3) a methanol concentration of 0.17 M was demonstrated, which was about 5 times higher than that for (bpym)PtCl₂ (Table 1, entry 1). As a sharp contrast to the imidazolium- and pyrazolium-based IL systems, the pyridinium-based (Table 1, entry 11) and triazolium-based (Table 1, entry 15) IL systems exhibited either negligible or no measurable activity. The reason for this striking difference is not as yet well understood. Furthermore, comparison between Table 1, entries 8 and 9 suggests that having at least one chlorine coordinated to Pt might be essential in the catalysis. This is consistent with a previous theoretical study of the Catalytica system,^4b which revealed that chlorine participated in the most active catalysis state.

In many other applications, the most widely used ILs are those with long alkyl chains, e.g., 1-butyl-3-methylimidazolium ([bmim]).⁵ However, in methane oxidation, it has been found that alkyl chains longer than –CH₃ would be oxidized quickly by Pt catalysis in concentrated H₂SO₄. This is understandable as methane is more inert than any longer alkyl group. We have actually observed intermediate oxygenated products due to partial oxidation of an ethyl group in an IL, 1-ethyl-3-methylimidazolium chloride ([emim][Cl]). Thus, only no-methyl, 1-methyl or dimethyl, imidazolium or pyrazolium ILs were used in the methane oxidation tests (Table 1). To eliminate the suspicion that the products might be coming from the methyl substituent on the IL ring, control experiments using ¹³C-enriched methane for a system of PtCl₂ + [1-mim][HSO₄] were carried out under similar conditions. Subsequent analyses by GC-MS and ¹H NMR confirmed that the produced methylbisulfate was indeed ¹³CH₃OSO₃H.

The stability of an IL's ring structure could itself be a challenge under the harsh catalytic conditions, since they are also hydrocarbons. Slight decomposition of the imidazolium ring was observed after the reaction had been run for a few hours, as monitored by the products due to deep oxidation, CO₂ and NH₄⁺. The formation of extra CO₂ complicated the calculation of the selectivity of methane-to-methanol conversion. We subsequently developed the pyrazolium-based ILs (Table 1), in which the ring structure proved to be stable against oxidation in the presence of Pt catalysts in concentrated sulfuric acid within the duration of our tests. Selectivity in these ternary systems was then found to be comparable to the Catalytica reaction under similar conditions.

Because of the desired use of a dilute sulfuric acid solution in practice, the effect of water concentration on the reactivity of Pt/IL/H₂SO₄ ternary systems was studied. Methane oxidation to methanol in concentrated H₂SO₄ by the Catalytica catalyst can be written as: CH₄ + 2H₂SO₄ → CH₃OSO₃H + 2H₂O + SO₂ in which water is generated in situ during the reaction. The reactivity of (bpym)PtCl₂ was found to be extremely sensitive to even small amounts of water. Quantum chemistry computations suggest that the water complex [(Hbpym)PtCl(H₂O)]²⁺ is about 30 kJ mol⁻¹ more thermodynamically stable than the starting active Pt complex [(Hbpym)PtCl(HSO₄)]²⁺, which is the so-called ground state effect for C–H activation.⁴ Experimentally demonstrated in Fig. 1, the catalytic activity dropped sharply when the sulfuric acid was diluted from the oleum to below 100%. This leads to uneconomical catalysis rates and high separation costs for the methanol.^4a As a comparison, the Pt/IL/H₂SO₄ systems at lower H₂SO₄ concentration (90 to 100%) exhibited higher methanol yields than the Catalytica system and hence were more water-tolerant.

Fig. 1 Methanol yield as a function of H₂SO₄ concentration (reaction conditions: 200 °C, 2.5 h). The methanol concentration from the Catalytica reaction in 2% oleum was 600 mM and is not indicated on the plot.

This trend was further examined through methane C–H bond activation tests via H/D exchange between the regular CH₄ and deuterated reaction media at 150 °C. The data are summarized in Table 2. At this temperature, the Catalytica system catalyzed extensive H/D exchange while producing no measurable amount of methanol.³Table 2, entries 1–4 indicate that ternary systems of Pt/IL/H₂SO₄ are more active than the Catalytica reaction, both in the C–H activation and oxidation steps, prompting a different mechanistic interpretation.

Table 2 Effects of water concentration in different catalyst systems on methane C–H activation and oxidation to methanol

Entry	System	Activation at 150 °C^a		Oxidation at 200 °C^b
		[CHxD4−x]/mmol	TON	[CH3OH]/mmol	TON
a H/D exchange occurred between CH4 and deuterated liquid media D2SO4, D2O and [1-mim-d4][DSO4]. TONs were determined from the gas analysis of methane isotopomers by GC-MS. Reaction conditions: 0.05 mmol Pt(II) + 0.3 mmol IL + 1 mL D2SO4 + 3.4 MPa CH4 in a 15 mL reactor; 150 °C, 2.0 h. b Tests were performed using non-deuterated liquid media. The reaction conditions were the same as those in Table 1.
1	(bpym)PtCl2 + 2% water	0.367	7.35	0.047	0.94
2	(bpym)PtCl2 + 6% water	0.276	5.51	0.016	0.31
3	PtCl2 + [1-mim][HSO4] + 2% water	0.480	9.60	0.065	1.31
4	PtCl2 + [1-mim][HSO4] + 6% water	0.420	8.39	0.037	0.73

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In summary, the ILs in the homogeneous catalysis of methane to methanol conversion not only acted as a dissolution media for those otherwise insoluble Pt salts/oxide, but also played a key role in promoting Pt reactivity, possibly through coordination and/or intermolecular interactions. The versatile method described here could also be used in other chemical reactions.

Notes and references

1. (a) R. H. Crabtree, J. Organomet. Chem., 2004, 689, 4083; (b) S. S. Stahl, J. A. Labinger and J. E. Bercaw, Angew. Chem., Int. Ed., 1998, 37, 2180; (c) J. A. Labinger and J. E. Bercaw, Nature, 2002, 417, 507.
2. (a) A. E. Shilov and G. B. Shul'pin, Chem. Rev., 1997, 97, 2879; (b) A. E. Shilov and G. B. Shul'pin, Activation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes, Kluwer Academic Publishers, Dordrecht, 2000.
3. R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh and H. Fujii, Science, 1998, 280, 560.
4. (a) R. A. Periana, G. Bhalla, W. J. Tenn, III, K. J. H. Young, X. Y. Liu, O. Mironov, C. J. Jones and V. R. Ziatdinov, J. Mol. Catal. A: Chem., 2004, 220, 7; (b) J. Kua, X. Xu, R. A. Periana and W. A. Goddard, III, Organometallics, 2002, 21, 511.
5. (a) M. J. Earle and K. R. Seddon, Pure Appl. Chem., 2000, 72, 1391; (b) Ionic Liquids in Synthesis, ed. P. Wasserscheid and T. Welton, Wiley-VCH, Weinheim, 2003.
6. (a) N. Chatani, N. Furukawa, H. Sakurai and S. Murai, Organometallics, 1996, 15, 901; (b) V. Voorhees and R. Adams, J. Am. Chem. Soc., 1922, 44, 1397.
7. The coordination capability of ILs in transition metal-involved homogeneous catalysis has been reported in a recent US patent application by the authors: Z. Li, Y. Tang and J. Cheng, US Patent, 2005, 60/673,705.
8. M. Hasan, I. V. Kozhevnikov, M. R. H. Siddiqui, C. Femoni, A. Sterner and N. Winterton, Inorg. Chem., 2001, 40, 795.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/b610328f

‡ Methane oxidation in 1 mL of these ternary solutions was conducted in a 69 mL cylindrical stainless steel reactor (outside diameter = 1 inch) with a glass liner inside. Mild stirring was provided via a magnetic stirring bar.

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