CATALYSTS

FIELD OF THE INVENTION

The invention relates to compositions and methods involving ionic liquids and dissolved metal compounds as homogeneous and heterogeneous catalysts useful for catalyzing various chemical reactions.

BACKGROUND

Alkanes comprise a significant fraction of the world's petroleum and natural gas resources and have the potential to be a useful source of carbon for large-scale synthesis. Methane, the smallest alkane and the principal component of natural gas, is an abundant and inexpensive natural resource. Despite these attributes, it is typically only used as a fuel for power generation. The. reason for methane's underutilization is that there are few .commercially viable methods for converting methane to a product that is chemically-useful due to the strength of its covalent carbon-hydrogen (C-H) bonds, which are among the strongest of all hydrocarbons. The search for catalysts that can facilitate C-H bond activation in methane and other low molecular- weight alkanes is an area of research with considerable industrial significance.

The most common chemical use of methane is to convert it (by an indirect oxidation process) to methanol, a commercially important alcohol that is one of the top 25 chemicals produced worldwide. The conversion is generally carried out at high temperatures and pressures in a two-stage steam reforming process to form synthesis gas (carbon monoxide and hydrogen), and is coupled with a methanol synthesis process that dates back to the 1920s. The process is expensive, energy intensive, and impractical for use in the remote locations where many of the methane reserves are found. As a result, the direct oxidative conversion of methane to an easily transportable liquid such as methanol has been extensively investigated for decades.

Although various parties have been seeking simpler, more efficient methods for converting methane to methanol, no such methods have been commercialized due to the difficulty in finding a sufficiently active and selective catalyst. Although low temperature selective methane oxidation by transition metal complexes in solution has been the focus of substantial effort since. the 1970s, the most promising catalysts so far described by Periana et al. 1993 and 1998 (Periana R.A., et al. (1993) Science 259:340, Periana R.A. et al, (1998) Science

280:560-564) have not yet been commercialized. The process described by Periana et a in 1998 utilizes a platinum bipyrimidine catalyst and requires concentrated sulfuric acid. This catalyst converts the methane to methyl bisulfate (CH ₃ OSO ₃ H), which can then be converted to methanol. Ionic liquids are salts consisting of ions that exist in the liquid state at ambient temperatures, or salts that have melting points below around 300 ⁰ C. Ionic liquids typically consist of organic nitrogen-containing heterocyclic cations and inorganic anions. Ionic liquids offer numerous advantages over conventional organic solvents for carrying out organic reactions, including very low vapor pressure, lack of flammability, and the capacity to be functionalized to suit particular reactions. Unlike conventional molten salts (for example, molten sodium chloride), ionic liquids often melt below 300 ⁰ C. Since the melting points are low, ionic liquids can act as solvents in which reactions can be performed, and because the liquid is made of ions rather than molecules, such reactions often provide distinct selectivities and reactivities as compared to conventional organic solvents. In addition, their non-volatility results in low impact on the environment and human health, and they are recognized as solvents for "green" chemistry.

Ionic liquids have been disclosed for use as solvents for a broad spectrum of chemical processes. These ionic liquids, which in some cases can serve as both catalyst and solvent, are attracting increasing interest from industry because they promise significant environmental benefits. Several patent applications, including international PCT publication Nos. WO 95/21871, WO 95/21872, and WO 95/21806 relate to ionic liquids and their use to catalyze hydrocarbon conversion reactions such as polymerization and alkylation reactions.

There is a significant need in the art for readily available hydrocarbon sources that require a less expensive plant, which is cheaper to run, uses less energy, and produces fewer pollutants than the current technology. Ionic liquids may provide a new approach for facilitating this difficult yet important chemical reaction.

SUMMARY OF THE INVENTION

The invention disclosed herein relates to composition and methods useful for producing organometallic catalysts comprising ionic liquids and metal compounds. Embodiments of the invention provide for methods of facilitating a homogeneous or heterogeneous catalytic reaction, comprising providing a quantity of an ionic liquid and a quantity of a metal compound; contacting the metal compound with the ionic liquid such that at least a portion of the metal

compound dissolves in the ionic liquid to produce an ionic liquid catalyst, and using the ionic liquid catalyst to facilitate a homogeneous or heterogeneous catalytic reaction.

Further embodiments relate to methods wherein the ionic liquid further comprises one or more cationic components and one or more anionic components. Other embodiments relate to methods wherein the cationic component is selected from the group consisting of imidazolium-based cations, pyridinium-based cations, ammonium-based cations, phosphonium-based cations, thiazolium-based cations, triazolium-based cations, oxazolium-based cations, pyrazinium-based cations, pyrazolium-based cations, and combinations thereof, and further embodiments relate to methods wherein the cationic component is selected from the group consisting of imidizolium, 1-methylimidizolium; 1,3- dimethylimidizolium, and combinations thereof.

Some embodiments of the invention relate to methods wherein the anionic component is selected from the group consisting of chloride, bromide, iodide, bisulfate, triflate, . trifluoroacetate, methanesulfate, and combinations thereof. Additional embodiments relate to methods wherein the ionic liquid is selected from the group consisting of 1-methylimidazolium chloride, 1-methylimidazolium bisulfate., 1- ^' methylimidazolium triflate, imidazolium chloride, imidazolium bisulfate, 1,3- dimethylimidazolium iodide, 1,3-dimethylimidazolium bisulfate, and combinations thereof. Further embodiments relate to methods wherein the ionic liquid has a melting point between about -100 °C and about 300 ⁰ C, and still further embodiments relate to methods wherein the ionic liquid has a melting point between about 30 ⁰ C and 300 ⁰ C.

Certain embodiments relate to methods wherein the metal compound comprises a metal selected from the group consisting of main group metals, transition metals, and combinations thereof, and further embodiments relate to methods wherein the metal compound comprises a metal selected from the group consisting of platinum, palladium, iridium, rhodium, ruthenium, rhenium, gold, silver, mercury, chromium, molybdenum, tungsten, titanium, zirconium, iron, manganese, technetium, osmium, copper, vanadium, niobium, tantalum, and cobalt.

Still further embodiments relate to methods wherein the metal compound is selected from the group consisting Of PtCl ₂ , PtCU, PtO ₂ , and combinations thereof. Other embodiments relate to methods wherein the molar ratio of the amount of ionic liquid to the amount of the metal compound is from about 1,000,000:1 to about 1:1.

Embodiments of the invention described herein provide for ionic liquid catalysts that comprise an ionic liquid and a metal compound, or a combination thereof. Further embodiments

comprise ionic liquid catalysts wherein the ionic liquid further comprises one or more cationic components and one or more anionic components.

Further embodiments relate to ionic liquid catalysts wherein the cationic component is selected from the group consisting of imidizolium-based cations, pyridinium-based cations, ammonium-based cations, phosphonium-based cations, thiazolium-based cations, triazolium- based cations, oxazolium-based cations, pyrazinium-based cations, pyrazolium-based cations, and combinations thereof.

Still further embodiments of the present invention relate to ionic liquid catalysts wherein the cationic component is selected from the group consisting of imidizolium, 1- methylimidizolium, 1,3-dimethylimidizolium, and combinations thereof.

Other embodiments of the present invention provide for ionic liquid catalysts wherein the anionic component is selected from the group consisting of chloride, bromide, iodide, bisulfate, triflate, trifluoroacetate, methanesulfate, and combinations thereof.

Additional embodiments of the present invention provide ionic liquid catalysts wherein the ionic liquid is selected from the group consisting of 1-methylimidazolium chloride, ^' 1- methylimidazolium bisulfate, 1-methylimidazolium triflate, imidazolium chloride, imidazolium bisulfate, 1,3-dimethylimidazolium iodide, 1,3-dimethylimidazolium bisulfate, and combinations thereof.

Further embodiments relate to ionic liquid catalysts wherein the ionic liquid has a melting point between about -100 ⁰ C and about 300 ⁰ C, and still further embodiments relate to methods wherein the ionic liquid has a melting point between about 30 ⁰ C and 300 ⁰ C.

Other embodiments of the invention include ionic liquid catalysts wherein the metal compound comprises metals selected, from the group consisting of main group metals; transition metals, and combinations thereof, including platinum, palladium, iridium, rhodium, ruthenium, rhenium, gold, silver, mercury, chromium, molybdenum, tungsten, titanium, zirconium, iron, manganese, technetium, osmium, copper, vanadium, niobium, tantalum, and cobalt. Further embodiments provide for ionic liquid catalysts wherein the metal compound is PtCl ₂ , PtCU, PtO ₂ , or combinations thereof.

Alternative embodiments of the invention provide for ionic liquid catalysts wherein the molar ratio of the amount of ionic liquid to the amount of the metal compound is from about 1,000,000: 1 to about 1:1.

Embodiments of the present invention also provide a method for converting methane into an oxidized product, comprising contacting methane gas with an ionic liquid catalyst in the

presence of sulfuric acid, and further provides methods wherein the ionic liquid catalyst comprises an ionic liquid and a metal compound.

Further embodiments of the invention provide methods wherein the ionic liquid is selected from the group consisting of 1-methylimidazolium chloride, 1-methylimidazolium bisulfate, 1-methylimidazolium triflate, imidazolium chloride, imidazolium bisulfate, 1,3- dimethylimidazolium iodide, 1,3-dimethylimidazolium bisulfate, and combinations thereof.

Still further embodiments of the invention provide for methods wherein the metal compound is selected from the group consisting OfPtCl ₂ , PtCl ₄ , PtO ₂ , and combinations thereof.

Other embodiments provide methods wherein the contacting of the methane to the ionic liquid catalyst is by bubbling methane through the ionic liquid catalyst, or by pressurizing a reaction system with methane.

Additional embodiments relate to methods wherein the molar ratio of the amount of ionic liquid to the amount of metal compound is from about 1,000,000:1 to 1:1, and wherein the oxidized product is methylbisulfate. Other embodiments relate to methods for producing methanol from methane gas comprising contacting methane gas with an ionic liquid catalyst in the presence of sulfuric acid.

Alternative embodiments provide for compositions comprising a quantity of methanol, produced by a process comprising providing an ionic liquid catalyst comprising an ionic liquid and a metal compound, contacting a quantity of methane with a quantity of the ionic liquid catalyst sufficient to convert at least a portion of the quantity methane to methylbisulfate, converting at least a portion of the methylbisulfate to methanol, and recovering at least a portion of the methanol.

• BRIEF DESCRJPTION OF THE DRAWINGS . Figure 1 shows examples of cationic components of ionic liquids in accordance with certain embodiments of the present invention. Figure Ia shows an ammonium ion. Figure Ib shows a sulfonium ion. Figure Ic shows a phosphonium ion. Figure Id shows a lithium ion. Figure Ie shows an imidazolium ion. Figure If shows a pyridinium ion. Figure Ig shows a picolinium ion. Figure Ih shows a thiazolium ion. Figure Ii shows a triazolium ion.. Figure Ij shows an oxazolium ion. Figure Ik shows a pyrazolium ion.

Figure 2 shows various structures for PtCl ₂ dissolved in [bmim][X] (X = Cl ^" or HSO ₄ ^" ) at 200 ⁰ C in air in accordance with an embodiment of the present invention. Figure 2a shows Pt(II) in [bmim][Cl]; Figure 2b shows Pt(IV) in [bmim][Cl] if oxidized; Figure 2c shows the product of Figure 2a in H ₂ SO ₄ ; Figure 2d shows the product of Figure 2b in H ₂ SO ₄ .

Figure 3 shows ¹ H NMR spectra of the liquid of H ₂ Sθ4/[bpym]PtCl ₂ /[bmim][Cl] ternary system after rate tests using the low pressure reactor, in accordance with an embodiment of the present invention. Figure 3a shows a spectrum at room temperature, as reference; Figure 3b shows a spectrum at 200 ⁰ C at 2 hr; and Figure 3c shows a spectrum at 200 ⁰ C at 27 hr. Figure 4 shows ¹ H NMR spectra of the liquid of EfeSCVCatalyst/CHL _f ternary system after methane oxidation tests using the high pressure reactor in accordance with an embodiment of the present invention. Figure 4a shows system #1, using [bpym]PtCl ₂ as the catalyst. Figure 4b shows system #2, using PtCl ₂ -IL003 as the catalyst. Acetic acid was used as the external standard. Figure 5 shows GC-MS spectra of (a) system #1; and (b) system #2 in accordance with an embodiment of the present invention. Following each run, about 0.2 mL liquid was hydrolyzed in 5 mL H ₂ O solution and then neutralized with NaOH. They differ in the total peak area integrated.

Figure 6 shows ¹ HNMR spectra of the liquid of H ₂ SO ₄ /Pt(IV) Catalyst/CIL, ternary system after methane oxidation tests using the high-pressure reactor in accordance with an embodiment of the present invention. Figure 6a shows a PtCl ₄ + IL-003 system; Figure 6b shows a PtO ₂ + IL006 system. Acetic acid was used as the internal standard.

Figure 7 shows ¹ HNMR spectra from the liquid of methane oxidation test using a non- chlorine-containing system (IL-004/ PtO ₂ / H ₂ SO ₄ ) in accordance with an embodiment of the present invention.

Figure 8 shows ¹ H NMR spectra from the liquid of Carbon- 13 methane oxidation test using PtCl ₂ and IL-004 in accordance with an embodiment of the present invention.

Figure 9 shows ¹ HNMR spectra from the liquid of methane oxidation test using pyridinium-based IL (IL-020) and PtCl ₂ in H ₂ SO ₄ in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention disclosed herein generally relates to the use of ionic liquids to dissolve metal compounds to produce organometallic catalyst solutions. Such organometallic catalyst solutions can be used to efficiently promote a number of chemical reactions, including reactions involving the activation of a carbon-hydrogen (C-H) bond, such as the oxidation of methane and the alkylation of organic compounds. The compositions and methods described herein improve upon the platinum-based catalyst system described by Periana et al. (Periana R.A. et al, (1998) Science 280:560-564).

DEFINITIONS

As used herein, the term "catalyst" is a substance that accelerates the rate of a chemical reaction at some temperature. A catalyst generally participates in the reaction but is neither a chemical reactant nor a chemical product.

The term "reactant" refers to a chemical substance that is present at the start of a chemical reaction.

The term "oxidation" refers to a chemical reaction that involves a loss of one or more electrons by one molecule. Oxidations often result the addition of oxygen to a compound that accompanies a loss of electrons.

A "homogeneous catalyst" is a substance that accelerates the rate of a chemical reaction in a system wherein the catalyst is in the same phase as the reaction medium. An example of a homogeneous catalytic system is one in which the catalyst is a liquid and the reaction medium is a liquid, or a system in which the catalyst is a solid that is dissolved in the reaction medium forming a homogeneous solution. It is also possible that the reaction medium itself be a component of the catalytic system. A homogeneous catalytic reaction is a reaction in which the catalyst, reactants, and reaction medium are in the same phase.

A "heterogeneous catalyst" is a substance that accelerates the rate of a chemical reaction wherein one or more of the catalysts are in a different phase than the reaction medium. For example, a solid metal catalyst may be used to in a liquid-phase reaction system. Another example of a heterogeneous catalytic solution is a slurry, wherein one or more of the catalysts exist in a solid phase. In situations involving a metal catalyst in a liquid reaction medium, the rate of transport of reactants and products to and from the solid catalytic surface may limit the overall rate of the reaction due to the inherent limitation of surface area. As a result, homogeneous catalytic systems are generally more efficient than heterogeneous catalytic systems. A heterogeneous catalytic reaction is a reaction in which the catalyst, reactants, and reaction medium exist in more than one phase.

An "ionic liquid" is defined herein as a salt that has a melting pointbetween around -100 ⁰ C and around 300 ⁰ C. Ionic liquids comprise one or more cations or cationic components, and one or more anions or anionic components. In some cases, the cations or anions may be related species in equilibrium. Many ionic liquids have been disclosed in the literature as well as in patents and patent applications. Published US Patent Application 2004/0035293A1 as well as Dongbin et al. (Dongbin, Z et al, (2002) Catalysis Today 74:157-189) disclose a number of ionic liquids. In addition, Table 1 shows a number of ionic liquid compounds that have been

synthesized by the inventors, but are by no means intended to limit the scope of the present invention.

Table 1. List of ionic liquids that may be used to produce ionic liquid catalysts.

IL Name Abbreviation

No.

IL-OOl l-neopentyl-3-methylimidazolium chloride [npmim][Cl]

IL-002 l-isopropyl-3-methylimidazolium bromide [ipmim][Br] .

IL-003 1-methylimidazolium chloride [l-mim][Cl]

IL-004 1-methylimidazolium bisulfate [l-mim][HS0 ₄ ]

IL-005 1-methylimidazolium triflate [l-mim][CF ₃ SO ₄ ]

IL-006 Imidazolium chloride [Un][Cl]

IL-007 Imidazolium bisulfate [im][HSO ₄ ]

IL-008 1,3-dimethylimidazolium iodide [mmim][I]

IL-009 1,3-dimethylimidazolium bisulfate [mmim] [HSO ₄ ]

IL-010 Teframethylammonium trifluoiOacetate ^■

IL-OI l 2-methylimidazolium bisulfate [2-mim] [HSO ₄ ]

IL-012 4-methylimidazoIium bisulfate [4-mim][HSO ₄ ]

IL-013 1 ,2-dimethylimidazolium bisulfate [l,2-dimim][HSO ₄ ]

IL-014 1,4-dimethylimidazolium bisulfate [l,4-dimim][HSO ₄ ]

IL-015 1 ,2,3 -trimethylimidazolium methanesulfate, [l,2,3-trimim][CH ₃ SO ₄ ]

IL-016 2,4,5- trimethyloxazolium bisulfate [2,4,5-trimox][HSO ₄ ]

IL-017 1 -trifluoroacetylimidazolium bisulfate

IL-018 1-methylbenzimidazolium bisulfate

IL-019 1,3-dimethylbenzimidazolium bisulfate

IL-020 Pyridinium bisulfate [pyr][HSO ₄ ]

IL-021 1,4-dimethylpyridinium bisulfate

IL-022 2,6-lutidinium bisulfate

IL-023 3,5-lutidinium bisulfate

IL-024 Pyrazinium bisulfate [pyz][HSO ₄ ]

IL-025 1 -methy lpyrazinium bisulfate

IL-026 2-methylpyrazinium bisulfate

IL-027 2,3-dimethylpyrazinium bisulfate

IL-028 2,3,5-trimethylpyrazinium bisulfate

IL-029 2,3,5,6-tetramethylρyrazinium bisulfate

IL-030 1,2,3,5,6-pentamethylpyrazinium bisulfate

IL-031 Quinoxalinium bisulfate

IL-032 Quinoxalinium chloride

IL-033 Pyrimidinium bisulfate

IL-034 1-methylpyrimidinium bisulfate

IL-035 4,6-dimethylpyrimidinium bisulfate

IL-036 Triaziniura bisulfate

IL-037 Bipyriinidinium bisulfate

IL-038 1-methylbipyrimidinium bisulfate

A "cation" is defined as a positively-charged atom, molecule or compound. Examples of cationic components of ionic liquids include but are not limited to l-neopentyl-3- methylimidazolium, l-isopropyl-3-methylimidazolium, 1-methylimidazolium, imidazolium, 1,3- dimethylimidazolium, tetramethylammonium, 2-methylimidazolium, 4-methylimidazolium, 1,2- dimethylimidazolium , 1,4-dimethylimidazolium , 1,2,3-trimethylimidazolium , 2,4,5- trimethyloxazolium, 1-trifluoroacetylimidazolium, 1-methylbenzimidazolium, 1,3- dimethylbenzimidazolium, pyridinium , 1,4-dimethylpyridinium, 2,6-lutidinium, 3,5-lutidinium, pyrazinium, l-methylpyrazinium, 2-methylpyrazinium, 2,3-dimethylpyrazinium, 2,3,5- trimethylpyrazinium, 2,3,5,6-tetramethylpyrazinium, 1,2,3, 5,6-pentamethylpyrazinium, quinoxalinium, pyrimidinium, 4,6-dimethylpyrimidinium, bipyrimidinium, and 1- methylbipyrimidinium. Figure 1 shows the chemical structures of some of the ionic liquid cations.

"Imidizolium-based cations" are cations wherein the cation contains an imidizolium group, which may be substituted. "Pyridinium-based cations" are cations wherein the cation contains a pyridinium group, which may be substituted. "Ammonium-based cations" are cations wherein the cation contains an ammonium group, which may be substituted: "Phosphonium- based cations" are cations wherein the cation contains a phosphonium group, which may be substituted. "Thiazolium-based cations" are cations wherein the cation contains a thiazolium group, which may be substituted. "Triazolium-based cations" are cations wherein the cation contains a triazolium group, which may be substituted. "Oxazolium-based cations" are cations wherein the cation contains an oxazolium group, which may be substituted. "Pyrazolium-based cations" are cations wherein the cation contains a pyrazolium group, which may be substituted. "Pyrazinium-based cations" are cations wherein the cation contains a pyrazinium group, which may be substituted.

The term "anion" refers to a negatively-charged atom, molecule, or compound. Examples of ionic liquid anions that may be used in the reaction system disclosed herein include, but are not limited to, chloride (Cl ^" ), bromide (Br ^' ), iodide (Y), bisulfate, (HSO ₄ ^" ), triflate (CF ₃ SO ₃ ^" ), tetrafluoroborate (BF ₄ ^" ), and methylsulfate (CH ₃ SO ₄ ^" ). The term "organometallic" refers to an organic compound containing metal atoms.

As used herein, the term "ionic liquid catalyst" refers to an ionic liquid that has" a metal compound dissolved hi it. Ionic liquid catalysts may be either homogeneous or heterogeneous solutions.

The term "metal compound" refers to compounds that comprise one or more metals. Metals in metal compounds may be either free metal ions, solvated metal ions in a coordinated complex form, which may have one or more replaceable ligands, alloys of metals, or any combination thereof. A metal compound may also comprise a nanocluster, nanoparticle, or other nanoform of pure metal, as well as metal compounds with other elements. Metal compounds may also comprise one or more metal cations and one or more anions, The metals in metal compounds may be in any possible oxidation state.

The term "metal" refers to elements classified on the Periodic Table of the Elements as "transition metals" or as "main group metals". Transition metals also include elements classified as lanthanides and actinides. Main group metals include the elements aluminum, gallium, indium, tin, thallium, lead, bismuth, and polonium. For reference, the inside cover of the text "Organic Chemistry" by Brown and Foote ((2002), Thompson Learning Inc.) denotes which elements belong to which groups in the Periodic Table of the Elements.

Examples of metal compounds include salts and oxides of metals. The term "metal salt" refers to a compound comprising a metal cation and an anion. Metal oxides are compounds comprising a metal cation combined with oxygen. The tenn "metal salt" also includes mixed oxides, acids, alkoxides, amides, azides, borates, borides, carbides, carboxylates, carbonates, clays, clusters, cyanides, halides, hydroxides, hydrates, hydrides, imides, isocyanides, nitrates, nitrides, phosphides, phosphates, sulfides, sulfates, silicates, suicides, superacids, thiocyanate, and thiolates of metals.

An extensive range of metal compounds may be dissolved in ionic liquids to form ionic liquid catalysts. Salts and oxides of transition metals, including platinum, palladium, iridium, rhodium, ruthenium, rhenium, gold, silver, mercury, chromium, molybdenum, tungsten, titanium, zirconium, iron, manganese, technetium, osmium, copper, vanadium, niobium, tantalum, and cobalt, as well as the lanthanide and actinide series of the Periodic Table of the elements, may be useful in the production of ionic liquid catalysts.

Specific salts and oxides of metals that may be dissolved in ionic liquids to produce ionic liquid catalysts include PtCl ₂ , PtCl ₄ , PtO ₂ , PdCl ₂ , RuCl ₃ , RhCl ₃ , IrCl ₄ , AuCl ₃ , Fe ₂ O ₃ , V ₂ O ₅ , MnO ₂ , CuCl ₂ , CuCl, MgSO ₄ , OsCl ₃ , AlCl ₃ , and FeCl ₃ .

A skilled artisan would recognize that there are many other metal compounds that may be used in the system that are readily identifiable without undue experimentation.

As used herein, the term "high-pressure reactor" refers to a reactor system that has greater than atmospheric pressure (around 14.7 psi). In the case of reactions involving.methane, high pressure reactors may have added methane gas that increases the pressure of the system. Additional gases may also be used to pressurize a reaction system. The reaction systems disclosed herein reference a reaction system described by Periana et al. (Periana R.A. et al, (1998) Science 280:560-564). One of the catalysts disclosed in this publication, dichloro(η-2-{2,2'-bipyrimidyl})platinum(II), (abbreviated as [bpym]PtCl ₂ ) is a produced by Catalytica Advanced Technologies Inc. of Mountain View CA. This compound may be referred to as the "Periana" catalyst or the "Catalytica" catalyst herein. Room-temperature ionic liquids are believed to be an excellent replacement for volatile organic solvents, concentrated acids, and concentrated basic solutions that are undesirable due to environmental concerns. In addition, they may. dissolve compounds that are not readily soluble in aqueous or organic solvent systems. Ionic liquids have been used as clean solvents and catalysts for green chemistry and as electrolytes for batteries, photochemistry and' electrosynthesis. They have no significant vapor pressure and thus create no volatile organic contaminants. They also allow for easy separation of organic molecules by direct distillation without loss of the ionic liquid. Their liquid range can be as large as 3,000 ⁰ C allowing for large reaction kinetic control, which, coupled with their good solvent properties, allows small reactor volumes to be used. By changing the anion or the alkyl chain on the cation of an ionic liquid, a number of properties, such as hydrophobicity, viscosity, density, and solvation may be varied. For example, ionic liquids may dissolve a wide range of organic molecules to an appreciable extent, the solubility being influenced by the nature of the counter anion.

Another beneficial feature of ionic liquids is their designability : miscibility with water or organic solvents can be tuned through sidechain lengths on the cation and choice of anion. Furthermore, they can be functionalized to act as acids, bases or ligands.

Due to the capacity for ionic liquids to serve as acids, bases, coordination ligands, and nucleophiles, it is possible that ionic liquids themselves catalyze chemical reactions as well as

solubilizing reaction components. It 'is known that several types of chemical reactions, such as Diels-Alder reactions and Friedel-Crafts reactions, occur in ionic liquids.

Metals have been shown to be involved in a large number of chemical reactions. In many cases, they are used as catalysts or cofactors to promote chemical reactions that would otherwise not occur under desirable reaction conditions. However, many of these catalytically- useful metals are insolμble in aqueous or organic reaction systems, leading to their use as heterogeneous catalysts. Heterogeneous catalysts are often rate-limited by the rate of diffusion of reactants and products to and from the metal surface. As a result, homogeneous catalytic systems are generally more efficient. Ionic liquids may provide a more efficient catalytic system due to their ability to dissolve many metal compounds that would not normally be soluble in other solvent systems. In some embodiments of the invention, ionic liquids that have a melting point between -100 ⁰ C and 300 ⁰ C may be used. Further embodiments provide for ionic liquids that have a melting point between 30 ⁰ C and 300 ⁰ C. Ionic liquids containing dissolved metals are referred to as "ionic liquid catalysts" and may catalyze a wide variety of different types of chemical reactions, including but not limited to oxidations, substitutions, isomerizations, additions, C-H bond activations, C-N bond activations, C-C bond activations, hydrogenations, dehydrogenations, alkylations, acylations, nucleophilic displacement reactions, and radical reactions.

Ionic liquids may be used as ligands for metal catalysts, whereby the catalytic metal center may be directly attached to the reaction media through coordination, rather than simply being dissolved in it. Because metals may be mobilized on ionic liquids, ionic liquids also provide a means to regenerate or recycle the metals. Additionally, catalytic systems comprising metals dissolved in ionic liquids containing larger organic side chains may mimic the catalytic behavior of macromolecules such as proteins or nucleic acids. Ionic liquid catalysts are generally produced by dissolving a metal compound in an ionic liquid. While many ionic liquid catalysts are generally homogeneous catalysts that enable chemical reactions to occur without rate-limiting steps associated with diffusion of reactants and products to and from solid surfaces, it is also possible for ionic liquid catalysts to comprise metal catalysts in a solid state, such as in a slurry. Dissolution of metal compounds in ionic liquids may occur at a variety of different temperatures, and some embodiments of the invention provide methods wherein dissolution generally occurs between about 15 ⁰ C and about 350 ⁰ C. The molar ratio of the amount of ionic liquid to the amount of metal compound in the ionic liquid catalyst may range from about 1,000,000:1 to about 1:1. Dissolution of ionic liquid catalysts, as well as reactions catalyzed out in the presence of ionic liquid catalysts may be

carried out under a range of different pressures. Certain embodiments of the invention-provide for reaction systems wherein the pressure is between around 1 and around 1,500 psi.

Likewise, catalytic reactions involving ionic liquid catalysts may occur at a variety of different temperatures. In certain embodiments of the invention, chemical reactions are carried out in systems between about 25 ⁰ C and about 350 °C.

A skilled artisan would recognize that there are a number of deletions, substitutions, and modifications of the ionic liquids in the ionic liquid catalysts that would achieve satisfactory dissolution of desired metal compounds that may be identified without undue experimentation. Reaction systems involving ionic liquid catalysts may be carried out under a variety of different pressures. In certain embodiments of the invention, chemical reactions may be carried out at ambient pressures, which range from about 14.7 psi to about 300 psi. In embodiments where a high-pressure system is used, such as one involving a gas-phase reactant, ^' system pressures may range from around 200 psi to around 5,000 psi.

Certain embodiments of the invention relate to methods wherein solutions comprising ionic salts and salts of platinum are contacted with methane in the presence of concentrated H ₂ SO ₄ , which may range from 1 to 99 percent by weight. Such reactions generally catalyze the oxidation of methane by the activation of a single C-H methane bond. Platinum is a powerful oxidant; depending on the ligands present, platinum may oxidize methane all the way to carbon dioxide (CO ₂ ). One of the benefits of the catalytic system disclosed herein is that the ionic liquid may provide a coordination environment for the platinum that allows for the oxidation of a single C-H bond to produce methanol rather than a complete oxidation of methane to CO ₂ .

Examples of platinum salts and oxides that are useful for these particular embodiments include PtCl ₂ , PtCl ₄ , and PtO ₂ . These metal compounds are relatively insoluble in concentrated H ₂ SO ₄ alone, but are readily dissolved in imidazolium and pyridinium-based ionic liquids with chloride or bisulfate as the anion. Heat may be used to promote the dissolution process. In some embodiments of the invention, the reaction occurs between about 150 ⁰ C and about 220 ⁰ C.

Methane may be introduced into the system by a variety of mechanisms, including pressurization of the reaction vessel with methane in a high-pressure reactor, or by bubbling methane through the reaction solution; The reaction vessel for this particular reaction as well as other reactions involving ionic liquid catalysts may be sealed or open to the atmosphere. The initial product between methane and ionic liquid catalysts in the presence of concentrated H ₂ SO ₄ is usually methyl bisulfate. Methyl bisulfate may be converted to methanol by a number of methods that are well-known in the art.

Some embodiments of the invention use ionic liquid catalysts wherein the cationic portions of the liquid comprise alkyl or heteroatom substituents. Certain embodiments provide for reaction systems wherein the cationic portion is unreactive towards the dissolved metal agents, for example, ionic liquids IL-003 through IL-009 shown in Table 1. The anionic component of an ionic liquid may comprise a number of different negatively-charged compounds. Often, the solubility and reactivity of ionic liquid catalysts may be mediated by its anionic component. Examples of ionic liquid anions that may be suitable for reactions involving the oxidation of methane include but are by no means limited to chloride, bromide, iodide, bisulfate, triflate, and methanesulfate. In some embodiments of the invention, imidazolium-based ionic liquid systems may demonstrate greater reactivity than the Catalytica catalyst system when tested under the same temperature and pressure conditions. The effect of cation structure (i.e., different alkyl group combinations on the imidazolium ring), anion (e.g., Cl ^" vs. HSO ₄ ^" ), and Pt species (i.e., PtCl ₂ vs. PtCl ₄ vs. PtO ₂ ) on the reactivity were systematically investigated and are shown in the Examples.

There are a number of other metals that can be dissolved in ionic liquids to yield catalytically-useful, homogeneous and heterogeneous organometallic solutions. Other ^' embodiments of the invention relate to the use of ionic liquid catalysts comprising an ionic liquid and a dissolved iridium (Ir) salt to catalyze the alkylation of benzene. Ionic liquids provide many possibilities to form catalytic solutions with metal - compounds. Therefore, additional embodiments provide for ionic liquid catalysts to be used for combinatorial and high-throughput screening applications.

For chemical reactions using an ionic liquid catalyst, additional substances may be added to the ionic liquid solution. Examples of other substances that may be added to ionic liquid catalyst solutions include but are not limited to water or aqueous solutions, organic solutions, organic or inorganic salts, metals, or other organic or inorganic compounds.

All patents, patent publications, provisional applications and publications referred to or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent that they are not inconsistent with the explicit teachings in this specification. It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.

EXAMPLES

Example 1

Compatibility of Oxidants and Reactants in Ionic Liquid Catalytic Systems

Tests were performed to determine the stability of oxidants and reactants for the conversion of methane to methanol, a process that requires H ₂ SO ₄ . Previous studies tested the compatibility for binary systems of ionic liquid - H ₂ SO ₄ and the Periana catalyst, [bpym]PtCl ₂ - H ₂ SO ₄ , at room temperature and at elevated temperatures (up to 220 ⁰ C). A binary system refers to a reaction system with two components. This example describes compatibility tests for two binary systems that include new varieties of ionic liquids as well as other types of Pt-based catalysts. Solubility tests of Pt-based catalysts in ionic liquids were also conducted. The solubility and stability of four types of ionic liquids (ammonium-based, phosphonium-based, pyridinium-based, and imidazolium-based) in H ₂ SO ₄ were analyzed as shown in Table 2. The cation and anion of each ionic liquid were separately evaluated for their stability in H ₂ SO ₄ . In some cases, the anions were released in the form of HX gas (X =■ Cl ^" or Br ^" ). It was also observed that the ionic liquid [bmim] [BF ₄ ] was stable at elevated temperatures based on ¹ H NMR observations, but the anion (BF ₄ ^" ) decomposed to HF and BF ₃ in H ₂ SO ₄ . As shown in Table 2, upon heating all cations stay stable but all anions except triflate (CF ₃ SO ₃ ^" ) decompose in H ₂ SO ₄ . It is possible that a more stable anion, bisulfate (HSO ₄ ^" ), may form after the decomposition.

Table 2. Compatibility between ionic liquids and concentrated sulfuric acid at room temperature (RT) and at elevated temperatures.

Ionic Liquid Solubility in Stability in H ₂ SO ₄ up to 200 ⁰ C

H ₂ SO ₄ at RT

Cation Anion

Ammonium-based

(CH ₃ ) ₃ N(C ₁₄ H ₂₉ )Br Soluble Stable HBr released

(CH ₃ ) ₄ NC1 Soluble Stable HCl released

Phosphon ium-based

(CH ₃ ) ₃ P(C ₁₆ H ₃₃ )Br Soluble Stable HBr released

Pyridin ium-based

[bpy][Cl] Soluble Stable HCl released

Imidazolium-based

[bmim][PF ₆ ] Slow and small

[nimim][CH ₃ SO ₄ ] Soluble Stable Decomposes

[bmim][Cl] Soluble Stable HCl released

[bmim][BF ₄ ] Soluble Stable HF/BF ₃ released

[emim] [CF ₃ SO ₃ ] • Soluble Stable Stable

IL-OOl (chloride) Soluble Stable HCl released

IL-002 (bromide) Soluble Stable HBr released

It has been shown previously that the Catalytica catalyst ([bpym]PtCl ₂ ) dissolves well and is stable in concentrated sulfuric acid at room temperature (Periana R.A. et al, (1998) Science 280:560-564). Several other Pt-based compounds including [NH ₃ ] ₂ PtCl ₂₎ K ₂ PtCl ₄ and PtCl ₂ were added to measure solubility and stability. For example, [NH ₃ J ₂ PtCl ₂ shows'better catalytic activity at 180 ⁰ C than [bpym]PtCl ₂ . PtCl ₂ is the product after the decomposition of [NH ₃ J ₂ PtCl ₂ and is insoluble in H ₂ SO ₄ , and K ₂ PtCl ₄ is the starting material for the synthesis of [bpym]PtCl ₂ (Periana R.A. et al, (1998) Science 280:560-564). The compatibility study of these four types of Pt-based compounds is presented in Table 3. The free ligand, bipyrimidine (bpym), was visible after [bpym]PtCl ₂ was heated at elevated temperatures, but no PtCl ₂ precipitated.

Table 3. Compatibility of Pt-based catalysts in concentrated sulfuric acid at room temperature and at elevated temperatures.

Pt-based Catalyst Solubility and Stability in H ₂ SO ₄

Room Temp. High Temp., up to 220

⁰ C [bpym]PtCl ₂ Yes Free ligand bpym is observed by NMR at > 110 ⁰ C. [NH ₃ ] ₂ PtCl ₂ ^' Yes _, Decomposes at > 18O ⁰ C and PtCl ₂ precipitates. '

K ₂ PtCl ₄ No, decomposes to PtCl ₂ No

PtCl ₂ No No

Ionic liquids themselves are often powerful solvents. The solubility of Pt-based catalysts in a variety of imidazolium-based ionic liquids was tested. The molar ratio of ionic liquid to Pt- compound was at least 4:1 to promote a complete dissolution if applicable. As shown in Table 4, the dissolution is greatly facilitated with heating and may depend on the anion. Chloride and bisulfate ionic liquids readily dissolve all Pt-based catalysts after heating at 200 °C. The cations, such as [bmim] and [emim], did not seem to significantly affect the solubility.

Since dialkylimidazolium choloroplatinates (both II and FV) have been synthesized under similar conditions (Hasan, M. et al., (2001) Inorg Chem 40:795-800), it has been proposed that the dissolution during heating may be a chelating process in which the anions of the ionic liquid coordinate to the Pt center to form a new anion. Using PtCl ₂ in [bmim] [Cl] as an example, a new structure is illustrated in Figure 2(a). The new compound (with excess ionic liquid) is soluble in concentrated H ₂ SO ₄ , presumably due to the replacement of Cl ^" by HSO ₄ ^' (Figure 2 (c)). It is possible that PtCl ₂ may be oxidized to Pt(FV) during heating, and two additional new structures associated with Pt(IV) are shown in Figure 2(b) and (d).

Table 4. Compatibility of Pt-based catalysts in ionic liquids, primarily dependent on the type of anion.

Ionic Liquid Pt-based Catalyst Solubility in IL, up to 22O ⁰ C

[bpym]PtCl ₂ [NH ₃ J ₂ PtCl ₂ K ₂ PtCl ₄ PtCl ₂

[bmim][BF ₄ ] No No No No

[emim] [CF ₃ SO ₃ ] No No No No

[bmim][Cl] Yes at 200 Yes upon Yes at 200 Yes upon

⁰ C; free bpym is melting ⁰ C melting seen

[bmim][HS0 ₄ ] Yes at 200 Yes at 200 Yes at 200 Yes at 200

⁰ C; free bpym is ⁰ C ⁰ C ⁰ C seen

After studies of binary systems, ternary systems at elevated temperatures were tested for stability. The catalyst [bpym]PtCl ₂ was used as the primary catalyst, but PtCl ₂ and K ₂ PtCl ₄ were also studied for comparison. The results are summarized in Table 5. Commercially available ionic liquids usually have dialkyl groups on the imidazolium ring. Table 5 shows that alkyl groups longer than methyl may not be stable in the presence of Pt-based catalysts in heated sulfuric acid.

It was found that K ₂ PtUl ₄ a ^' rtd PtCl ₂ had similar catalytic capability when dissolved in ionic liquids. Furthermore, seven new ionic liquids (IL-003 to IL-009; shown in Table 1) have been designed and synthesized in our laboratory in order to develop a stable ternary system for methane oxidation rather than oxidizing the ionic liquid itself.

Table 5. Stability of ternary systems of H ₂ S0 ₄ /Catalyst/Ionic Liquid at 200 ⁰ C

Ionic Liquid Pt-based Catalyst ⁸

[bpym]PtCl ₂ K ₂ PtCl ₄ system PtCl ₂ system system

[mmim] [CH ₃ SO ₄ ] Cation stable;

Anion decomposed (Insoluble) (Insoluble)

[emim] [CF ₃ SO ₃ ] Ethyl oxidized; (Insoluble) (Insoluble)

Anion stable

[bmim][BF ₄ ] Butyl oxidized;

BF ₄ decomposed (Insoluble) (Insoluble)

[bmim][Cl]/[HSO ₄ ] Butyl oxidized Butyl oxidized Butyl oxidized

IL-003 to IL-009 ^b Compatible Compatible Compatible ^a Pt-based catalysts were either dissolved in H ₂ SO ₄ first ^' or in ionic liquid first at elevated temperature. ^b Ionic liquids (IL-003 to IL-009) are home designed and synthesized and meet the compatibility requirement in the ternary system up to 220 °C.

Example 2 Conversion of Methyl Bisulfate to Methanol

The majority of the ionic liquids that have been tested have been shown to be miscible with methanol (CH ₃ OH) and are generally stable. Deuterated methanol (CD ₃ OD) may be a good NMR solvent for ionic liquids. A small amount of methanol in concentrated sulfuric acid may exist in the form of methyl bisulfate (CH ₃ HSO ₄ ), as evidenced by ¹ H NMR using-D ₂ SO ₄ as a solvent.

Extraction of methanol from concentrated sulfuric acid after the methane oxidation reaction may be one of the steps in the methane conversion process. The hydrolysis and neutralization of a standard methyl bisulfate solution in sulfuric acid was measured to model the process following oxidation of methane. About 55.7 μL CH ₃ OH was dissolved in 10 mL 96% H ₂ SO ₄ . From this solution, 0.2 mL was removed and hydrolyzed into 5 mL deionized water to

give a formal 100 ppm solution. Sodium hydroxide was added into the water solution to reach pH = 7, which is a desirable condition for GC-MS (gas chromatography mass spectrometry) measurement. Neutralization was conducted in an ice bath with magnetic stirring to prevent methanol loss. GC-MS produced two peaks that could be assigned to methanol; one representing the free CH ₃ OH and the other for the remaining CH ₃ HSO ₄ . This indicates that the hydrolyzation is incomplete in 5 mL of water solution. A direct 100 ppm methanol solution in water was prepared and subjected to GC-MS measurement under the same conditions. The methanol peak position matched one of the two peaks from the neutralized solution. However, the peak area (100 ppm) was larger than the two peaks combined in the latter case, implying hydrolysis and titration lead to some loss of methanol.

Example 3 Ambient Pressure Reactor Tests for Alkyl Oxidation

Studies using a low-pressure reactor were carried out to observe the kinetics of methane activation. Because the solubility of methane gas in sulfuric acid solution at low pressures is somewhat low, the hydrogen/deuterium exchange rate in D ₂ SO ₄ may be very small. An alternative method was used to model the effect of platinum-containing ionic liquid catalysts on a hydrocarbon moiety. Since l-methyl-3-butylimidazolium chloride ([bmim][Cl]) may dissolve all types of Pt-based catalysts (Table 4) and the butyl group may be oxidized in a ternary system (Table 5), it can be used instead of methane as a model compound for oxidation rate study. A systematic comparison between three catalytic systems, i.e., [bpym]PtCl ₂ , K ₂ PtCl ₄ , and PtCl ₂ , is presented in Table 6.

Table 6. Oxidation rate study of H ₂ SO ₄ /Catalyst/[bmim][Cl] ternary system at 200 ⁰ C. [bmim] was used as a model compound for oxidation. Rates were determined based on ¹ H NMR in D ₂ SO ₄ using acetic acid as the external standard.

Time at [bpym]PtC- ₂ system K ₂ PtCl ₄ system PtCl ₂ system

200 ⁰ C

Conversion ^a Yield ^a Conversion Yield Conversion Yield

R.T. (ref.) 0 0 0 0 0 0

70 min 37% 37% 47% 20% 24% 12%

2 hr 65% 55% -60% 24% -40% 21%

27 hr 100% 100% 80% . 72% 50% 50%

¹ See the definitions of conversion and yield in the text.

Typical experimental conditions were 0.05 mmol Pt-based catalyst and 0.73 mmol ionic liquid in 1 niL 96% H ₂ SO ₄ at atmospheric pressure using oil bath (200 ⁰ C) and magnetic stirring. The rates were all normalized to references that were prepared at room temperature. Only the butyl group on the imidazolium ring was observed to be oxidized and the methyl group was not. The conversion rate is arbitrarily defined from NMR as the intensity decrease of the - CH ₃ on the butyl group. When the butyl group changes, the electronic environment of the methyl group also changes, which leads to a new -CH ₃ peak adjacent to the old one. As expected, the total intensity remained nearly the same. The yield rate is thus defined as the amount of the newly-formed -CH ₃ peak intensity relative to the total.

As seen in Table 6, the oxidation rate was fastest in the [bpym]PtCl ₂ system and decreases in the order of [bpym]PtCl ₂ , K ₂ PtCl ₄ , and PtCl ₂ . The ¹ H NMR spectra for the [bpym]PtCl ₂ system reacted for different time were plotted in Figure 3.

^" Example 4

High Pressure Reactor Tests for Methane Oxidation to Methanol Using a Pt(IT)-Based

Ionic Liquid Catalyst

With compatible H ₂ SO ₄ /catalys1/ionic liquid systems developed (Table 5), high pressure reactor tests for direct methane conversion were conducted. About 0.05 mmol of a Pt-based catalyst was dissolved in 0.2 mmol ionic liquid. The resulting ionic liquid catalyst solution was added to 1 mL H ₂ SO ₄ in a glass tube. The glass tube was sealed in a 61 mL stainless steel reactor that also contains 500 psi high purity methane gas. The reactor was heated at 220 ⁰ C in an oil bath with constant magnetic stirring. After 2.5 hours, the reactor was cooled off and gas samples were collected in expansion tubes before the high pressure gas was released. Following the reaction, a sample of the liquid was removed for ¹ H NMR, and another sample was hydrolyzed and neutralized for GC-MS measurements. The sampled gas was subjected to GC analysis. Four systems were tested and the CH ₃ OH yields were reported in Table 7, in which NMR and GC-MS results for each run were consistent. As a comparison between system #1 and #2, their ¹ H NMR spectra are shown in Figure 4 highlighting the methyl bisulfate peak; and their GC-MS spectra are shown in Figure 5 highlighting both the free CH ₃ OH peak and the CH ₃ HSO ₄ peak.

Table 7. High pressure reactor tests in Pt-catalyst/ H ₂ SO ₄ /Ionic Liquids ternary systems. System CH ₃ OH Yield (relative to system #1)

By ¹ H NMR By GC-MS

System #1 1 1

EbPyTn]PtCl ₂ ZH ₂ SO ₄ ZCH ₄

System #2 7 5 ^■ -

PtCl ₂ -ILOOSZH ₂ SO ₄ ZCH ₄

System #3 3 3.5

PtCl ₂ -ILOOoZH ₂ SO ₄ ZCH ₄

System #4 1 1

[bpym]PtCl ₂ -IL006ZH ₂ SO ₄ ZCH ₄

System # 1 was designed to repeat the experiment conducted by Periana et al. , (Periana

R.A. et al., (1998) Science 280:560-564) which only differs in reactor design. The previously- published results were used as reference (Table 7). System #2 and #3 used PtCl ₂ -ionic liquid as the catalysts and gave appreciably higher methanol yield (3 - 5 times more). This demonstrates a great potential of using ionic liquids as a novel media to directly oxidize methane. ^■ System #4 tested the catalytic activity when some ionic liquid was added to system # 1 but did not improve any methanol yield. Figure 4 indicates that CH ₃ HSO ₄ comprises most of the liquid product from CH ₄ . The triplet peaks between 5 and 6 ppm have not been identified with a specific structure, but parallel experiment without CH ₄ showed that it originated from the interaction of imidazolium ring and Pt-based catalyst. Figure 5 indicates the hydrolysis is not complete and both peaks (free CH ₃ OH and CH ₃ HSO ₄ ) are counted as the product.

Example 5

High Pressure Reactor Tests for Methane Oxidation to Methanol Using a Pt(IV)-Based

Ionic Liquid Catalyst Selected H ₂ Sθ ₄ ZcatalystZionic liquid ternary systems were tested under several types of conditions. About 0.05 mmol Pt-based catalyst was first dissolved in 03 - 0.4 mmol ionic liquid and then the total solution was added into 1 niL H ₂ SO ₄ in a glass tube. The glass tube was sealed in a 69 mL stainless steel reactor that also contains 500 psi high purity methane gas. The reactor was heated at 220 ⁰ C in an oil bath with constant magnetic stirring. After 2.5 hours, the reactor

was cooled off and gas samples were collected via an expansion volume directly connected to the reactor and finally the remaining high pressure gas was released. Part of the liquid after reaction was taken for ¹ H NMR and part was hydrolyzed and neutralized for GC-MS measurements. The sampled gas was subjected to GC analysis. The two ¹ H NMR spectra in Figure 6 demonstrate that Pt(FV) species in ionic liquids have similar catalytic activity as Pt(II) species reported before. Methylbisulfate (CH ₃ HSO ₄ ) is the predominant product from CH ₄ . Both PtCl ₄ + EL-003 and PtO ₂ + IL-006 showed a higher methanol yield than the reference reaction, which used the Catalytica [bpym]Cl ₂ catalyst. While it has been proposed that methane C-H bond activation occurs through a Pt(II) center, the results shown here suggest that Pt(TV) may also activate methane and oxidize it to methanol in sulfuric acid environment.

It was found that the presence of chlorine in the catalytic system may play a role in catalysis. The methane oxidation test using IL-004 (bisulfate) and PtO ₂ in sulfuric acid showed no methanol generated (Figure 7). This contrasts with the results shown in Figure 2 (b) which also used PtO ₂ but with a chloride ionic liquid (IL-006). It is possible that the difference may be in the activation energy in these two systems, and that chlorine on Pt sites may help reduce the energy barrier.

In some cases, methylated imidazolium-based ionic liquids were used for solvent for platinum catalysts. It is possible that the product methylbisulfate results from the dissociation of the methyl group from the imidazole ring. To test this, an experiment was conducted to test the oxidation of methane using a pure ¹³ CH ₄ as opposed to ¹² CH ₄ with PtCl ₂ and IL-004. Figure 8 shows the majority of methylbisulfate was ¹³ CH ₃ HSO ₄ (> 95%). This suggests that the product is almost exclusively from the oxidation of the methane gas.

Methane oxidation studies were also conducted using other types of ionic liquids. One example was pyridinium-based ionic liquid (IL-020) and PtCl ₂ . Figure 9 shows the ¹ H NMR spectra from the reaction liquid.

An alternative reaction system for methane oxidation studies may be a 1 mL high- pressure gold tube. A gold tube may be loaded with 0.3 mL reaction liquid and 0.7 mL methane gas and then sealed. The sealed tube may be placed in a furnace and external hydraulic pressure up to 1000 psi may be applied. This mini-reactor may sustain uniform heating and pressure and provide a good mass balance. In addition, a commercial 25 mL Parr reactor may be used to provide accurate temperature, pressure, and mass balance control.