WO2017093782A1

WO2017093782A1 - Method for producing methanol from carbon dioxide and hydrogen gas in homogeneously catalyzed reactions and in an aqueous medium

Info

Publication number: WO2017093782A1
Application number: PCT/IB2015/059242
Authority: WO
Inventors: Katerina Stefania SORDAKIS; Gabor Laurenczy; Yuichiro Himeda; Hajime Kawanami; Akihiro TSURUSAKI; Masayuki Iguchi
Original assignee: Ecole Polytechnique Federale De Lausanne (Epfl); National Institute Of Advanced Industrial Science And Technology
Priority date: 2015-12-01
Filing date: 2015-12-01
Publication date: 2017-06-08
Also published as: JP2018537461A; JP6579561B2

Abstract

The invention relates to a method for producing methanol from hydrogen and carbon dioxide gas in homogeneously catalyzed reactions, composed of the carbon dioxide hydrogenation reaction to formic acid and the formic acid disproportionation reaction into methanol, both being conducted in aqueous media at mild conditions (temperature in the range from 20 to 100 °C, total hydrogen and carbon dioxide gas pressure up to 100 bar).

Description

Method for producing methanol from carbon dioxide and hydrogen gas in homogeneously catalyzed reactions and in an aqueous medium

Technical Field

The present invention relates to a method for producing methanol and/or for producing methanol and formic acid from carbon dioxide (CO₂) and hydrogen gas (H₂) in homogeneously catalyzed reactions and in aqueous medium at mild temperatures and gas pressures.

Prior Art and the Problem Underlying the Invention

Anthropogenic carbon dioxide may be regarded as a virtually unlimited source of fuels if chemically and energetically efficient processes are developed, that directly utilize it as chemical feedstock. The catalytic reduction of atmospheric carbon dioxide (CO₂) in the presence of hydrogen gas (H₂) constitutes a promising approach for the resolution of some of the major challenges faced by modern societies. Primarily, it can provide a pathway for the utilization of a troublesome, yet inexpensive and widely available carbon source for the production of value-added chemicals. In addition, it constitutes a means for storing H₂ in convenient and easy-to-handle liquid chemical carriers, such as formic acid (FA) and methanol (MeOH).

Hydrogen has been recognized as an attractive energy vector for a sustainable future energy system in view of rapidly declining fossil fuel resources and soaring environmental concerns. Both FA and MeOH have been proposed as liquid hydrogen storage media due to their attractive inherent properties. Methanol is superior in terms of gravimetric hydrogen storage capacity (12.5 wt% vs. 4.4 wt% for FA), whereas formic acid stands out due to reduced toxicity and inflammability. Furthermore FA and MeOH provide a number of important advantages as high-energy transportation fuels such as compatibility with current infrastructure. In addition, contrary to hydrogen, methanol and formic acid do not require any energy intensive procedures for pressurization or liquefaction and can therefore act as ideal hydrogen carriers for fuel cell vehicles.

Currently methanol is produced industrially from syngas at high temperatures (250-300 °C) and high pressures (50-100 bar) using a copper and zinc-based catalyst. Formic acid is derived from methanol under a pressurized carbon monoxide atmosphere, in a two-step synthesis process. Avoiding the utilization of toxic carbon monoxide as well as fossil fuel - ? - feedstock in the industrial synthesis of both methanol and formic acid is clearly desirable and represents a non-negligible advantage.

The production of FA by direct C(¾ hydrogenation in aqueous media in absence of organic solvents and additives has been demonstrated (Moret et al., Direct synthesis of formic acid from carbon dioxide by hydrogenation in acidic media, Nat. Commun. 5, 2014). Several methods for FA production in the presence of homogeneous catalysts have been developed, with reasonable formic acid yields. However, methanol production through the direct or indirect CO₂ hydrogenation with reasonable MeOH yields is rarely reported. The development of innovative production processes with improved methanol yields under mild reaction conditions is highly desirable.

Milstein and co-workers firstly reported indirect CO₂ hydrogenation to methanol via utilization of urea derivatives as well as organic carbonates, carbamates and formates as substrates in the presence of homogeneous ruthenium-based pincer catalysts at 110 °C. In most of the cases secondary by-products were formed (Balaraman et al.. Catalytic Hydrogenation of Urea Derivatives to Amines and Methanol, 201 1 , Angew. Chem. Int. Ed.

50, pp. 11702-11705, Balaraman et al, Efficient hydrogenation of organic carbonates, carbamates and formates indicates alternative routes to methanol based on CO₂ and CO, 201 1 , Nat. Chem. 3, pp. 609-614). At the same time, Sanford et al. presented a combination of three catalytic systems in order to progressively obtain methanol from CO₂ in a multistep- synthesis approach at 135 °C (Huff & Sanford, Cascade Catalysis for the Homogeneous Hydrogenation of CO₂ to Methanol, J. Am. Chem. Soc. 133, 201 1 , pp. 18122-18125). The same group reported on a ruthenium catalyst in THF able to reduce CO₂ to MeOH under basic conditions at temperatures up to 155 °C (Rezayee, N. M., Huff, C. A. & Sanford, M. S., Tandem Amine and Ruthenium-Catalyzed Hydrogenation of C02 to Methanol. J. Am. Chem. Soc. 137, 2015, pp. 1028-1031). Ding and coworkers followed a similar method to Milstein^'s, however, with ethylene carbonate in THF (at 145 °C) instead of methylene carbonate as the substrate, since the latter is rather difficult to access from CO₂ (Han, Z. et al., Catalytic Hydrogenation of Cyclic Carbonates: A Practical Approach from C02 and Epoxides to Methanol and Diols. Angew. Chem. Int. Ed. 51 , 2012, pp. 13041-13045). Direct CO₂ hydrogenation with a homogeneous ruthenium-phosphine catalytic system in THF, in the presence of an alcohol and an acid additive at 140 °C, has been developed by Klankermayer and Leitner (Wesselbaum et al., Hydrogenation of Carbon Dioxide to Methanol by Using a Homogeneous Ruthenium-Phosphine Catalyst, Angew. Chem. Int. Ed.

51 , 2012, pp. 7499-7502). They later reported on a similar system able to catalyse CO₂ hydrogenation to methanol under identical conditions but in the absence of alcohol additive (Wesselbaum et al. Hydrogenation of carbon dioxide to methanol using a homogeneous ruthenium-Triphos catalyst: from mechanistic investigations to multiphase catalysis. Chem. Sci. 6, 2014, pp. 693-704). Common features of these works were the harsh reaction temperatures and/or low selectivities. Another indirect approach for methanol production involves stepwise reduction of CO₂ to formic acid, followed by subsequent disproportionation of formic acid to provide methanol. The disproportionation reaction of aqueous formic acid solutions in the presence of a [Cp*Ir(bpy)-H₂O)](OTf)₂ (Cp*=pentamethylcyclopentadienyl, bpy=2,2'-bipyridine) catalyst was firstly reported by Miller and Goldberg, however with low methanol selectivities of 12% at low formic acid conversions (Miller et al., Catalytic Disproportionation of Formic Acid to Generate Methanol, Angew. Chem. Int. Ed. 52, 2013, pp. 3981-3984). Methanol yields up to 50% were obtained with a ruthenium catalytic precursor and phosphine ligand in THF, in the presence of an acidic additive, albeit at a very high temperature of 150 °C (Savourey et al. Efficient Disproportionation of Formic Acid to Methanol Using Molecular Ruthenium Catalysts. Angew. Chem. Int. Ed. 53, 2014, pp. 10466610470). Recently, Neary and Parkin reported a CpMo(CO)₃H (Cp=cyclopentadienyl) system in benzene, that catalysed FA disproportionation with a relatively low selectivity of 21 % at 100 °C (Neary and Parkin, Dehydrogenation, disproportionation and transfer hydrogenation reactions of formic acid catalyzed by molybdenum hydride compounds. Chem. Sci. 6, 2015, pp. 1859-1865).

The development of different catalysts for the aforementioned reactions in order to obtain a viable methanol production process with reasonable yields and high selectivities clearly represents a challenge. An iridium catalyst, [(Cp*)Ir(4dhbp)(OH₂)]SO4 (4dhbp=4,4'- dihydroxy-2,2'-bipyridine, Cp*=pentamethylcyclopentadienide), has been developed for the formic acid dehydrogenation into CO₂ without any detectable carbon monoxide being produced (Himeda, Highly efficient hydrogen evolution by decomposition of formic acid using an iridium catalyst with 4,4'-dihydroxy-2,2'-bipyridine, Green Chem. 1 1 , 2009, pp. 2018-2022). The present invention addresses the disadvantages of methanol production from a fossil fuel feedstock (syngas), the use of drastic reaction conditions (high temperatures and pressures) and the use of non-aqueous media.

The present invention also addresses the disadvantages of a multistep-synthesis reaction for indirect CO₂ hydrogenation to produce methanol and the use of multiple catalysts, e.g. a different catalyst for each different reactions, or substrates different from CO₂ and H₂. The present invention also addresses the disadvantages of the utilization of high temperatures and/or pressures for methanol production through direct CO₂ hydrogenation albeit with low methanol selectivities. The present invention addresses the problems depicted above, which are part of the invention.

Summary of Invention The inventors of the present invention provide a method for producing methanol from carbon dioxide gas (CO₂) and hydrogen gas (H₂) in a homogeneously catalyzed total reaction, which meets the objectives discussed above and which solves the problems of the prior art. The present invention relates to a method for producing methanol from hydrogen gas and carbon dioxide gas in a homogeneously catalyzed total reaction comprising a carbon dioxide hydrogenation reaction to formic acid and a formic acid disproportionation reaction to methanol, said homogeneously catalyzed total reaction being conducted:

in aqueous media;

at a temperature range of 20 to 100 °C;

at a total hydrogen gas and carbon dioxide gas pressure up to 100 bar;

in the presence of a catalyst comprising a complex of the general formula (I):

wherein,

M₁ is a metal selected from Ir, Ru, Rh or Co;

Ri is pentamethylcyclopentadienide group or hexamethylbenzene group;

R₂ is H₂O group or C1 group;

m is selected from 1 to 4 according to the oxidation state of metal M₁ ;

x is 1 or 2;

B is selected from F-, C1-, Br , Γ, OH-, H-, or S^2~, C0₃ ^2-, SO₄ ^2-, or PO₄ ^3~;

L₁ is a ligand selected from a conjugated system or a system of fused aromatic rings comprising at least one heteroatom N, wherein the aromatic rings may be further substituted by substituents being independently selected from -H, -OH group, -COOH group, -CF₃ group, -NH₂ group, C1 -C12 alkyl, C1 -C12 alkoxy group, C4-C12 aryl, C4-C12 aryloxy group, or a metal selected from Ir, Ru, Rh or Co, which is further substituted by one substituent R₃ being selected from pentamethylcyclopentadienyl group, unsubstituted or substituted benzene group, and one substituent R4 being selected from H₂O group, C1 group, halide group and hydride group.

Further aspects and preferred embodiments of the invention are defined herein below and in the appended claims.

The inventors have surprisingly found that a catalyst of the invention is active in the CO₂ hydrogenation to formic acid as well as in the formic acid disproportionation reaction to methanol in aqueous media and at ambient or near to ambient temperatures. Thus, such a catalytic system lead to overcome the four known major limitations in the CO₂-to-methanol process, namely high reaction temperatures, low MeOH selectivities in the FA disproportionation reaction, utilization of organic solvents as well as formation of cumbersome by-products that require additional purification steps.

The method comprises a homogeneously catalyzed total reaction, which includes the homogeneously catalyzed methanol production from CO₂ gas, being composed of the direct CO₂ (gas) hydrogenation reaction to formic acid and the formic acid disproportionation reaction to methanol, conducted in aqueous media at mild temperature and pressure conditions, namely from room temperature (20 °C) to 100 °C, and at a total H₂ gas and CO₂ gas pressure up to 100 bar, with a single homogeneous catalyst for both reactions. The method of the invention is believed to be highly advantageous because it can be performed in aqueous media being environmentally friendly, cheap and abundant, and via direct CO₂ hydrogenation with a homogeneous catalyst in non-drastic conditions, i.e. relatively low temperatures and total gas pressures. The absence of organic solvents and additives is highly beneficial not only from an environmental and financial point of view, but also because complicated separation and potential by-products are avoided. Furthermore the method has high methanol selectivity at formic acid conversions up to 99%. The methanol selectivity of the formic acid disproportionation reaction may be increased by the addition of a strong acid, which may be used to "tune" the direction of the overall reaction. Conveniently, methanol- water solutions do not form any azeo trope (in contrast to methanol-organic solvent mixtures), rendering their separation via simple distillation straightforward.

Taking into account all the described features and advantages, renders the method of the present invention an extremely valuable tool for producing formic acid and methanol for various purposes. Formic acid has a wide area of applications, from food preservation to leather processing, while methanol serves as an important raw material in the production of other high-value chemicals. Both formic acid and methanol can be utilized as high energy fuels. Further features and advantages of the invention will also become apparent to the skilled person from the description of the preferred embodiments given below.

Brief Description of the Figures Figure 1 shows the series of reactions related to CO₂ transformation to value-added chemicals and hydrogen storage realized in the presence of complex (1) of formula (13).

Figure 2 shows the direct, aqueous CO₂ hydrogenation to FA and MeOH realized by complex (I) of formula (13). 100 MHz ¹³C NMR spectra, recorded every 3 h, showing A: the increase of the FA doublet at 25 °C. B: the increase of the FA doublet and MeOH quartet in 2.5 M H₂SO₄ at 70 °C. C: FA concentrations obtained at 25 °C (triangles), at 60 °C (crosses) and in 2.5 M H₂SO₄ at 70 °C (squares). Significant MeOH (squares) amounts were only formed with H₂SO₄ under these conditions. P_totai(¹³CO₂ + 3H₂) = 80 bar, = 15.9 μmol, VH2O ⁼ 2.0 ml.

Figure 3 shows the time course of a FA disproportionation/dehydrogenation reaction. A: ¹³C NMR spectra, recorded every 1.5 h, showing the decrease of the FA doublet and the increase of the MeOH quartet (enlarged view) in the presence of complex (I) of formula (13) and 50 mol% H₂SO₄ under isochoric conditions. B: Concentrations of decomposed FA and formed MeOH derived from (a), n_FA = 10 mmol, n_cat - 15.9 μmol, V_H20 = 2.0 ml, T = 50 °C.

Figure 4 shows the FA dehydrogenation/disproportionation reaction pressure as a function of time. A: FA dehydrogenation activity of the complex (I) of formula (13) under isochoric conditions, n_FA ⁼ 10 mmol, n_cat = 15.9 μmol, V_H2o ⁼ 2.0 ml, T = 50 °C, first bottom curve: pressure, second bottom curve: conversion. B: Effect of H₂SO₄ concentration on FA dehydrogenation/disproportionation in the presence of the complex (I) of formula (13), n_FA ⁼ 10 mmol, n_cat= 15.9 μmol, V_H2o = 2.0 ml, T = 50 °C, no H₂SO₄ added (first top curve), 3.5 mol% H₂SO₄ (second top curve), 7.5 mol% H₂SO₄ (third top curve), 15 mol% H₂SO₄ (fourth top curve), 25 mol% H₂SO₄ (fifth top curve), 35 mol% H₂SO₄ (sixth top curve), 50 mol% H₂SO₄ (seventh top curve). Formic acid disproportionation has also contributed to the pressure increase. Figure 5 shows the ambient temperature FA dehydrogenation/disproportionation reaction under isochoric conditions in the absence of

2.0 ml, T = 20 °C, circles: FA concentration; triangles: MeOH concentration. Figure 6 shows the initial hydrogen pressure effect on the concentration of formed MeOH.

pressure: none (crosses), 10 bar (circles), 20 bar (triangles) and 40 bar (squares).

Detailed Description

The present invention provides a method for producing methanol from hydrogengas and carbon dioxide gas in a homogeneously catalyzed total reaction composed of carbon dioxide hydrogenation reaction to formic acid and formic acid disproportionation reaction to methanol, said homogeneously catalyzed total reaction being conducted in aqueous media. In particular, the method of the invention has a high methanol selectivity in the formic acid disproportionation reaction in an acidic medium.

The present invention provides a method for producing methanol from hydrogen gas and carbon dioxide gas in a homogeneously catalyzed total reaction composed of carbon dioxide hydrogenation reaction to formic acid and formic acid disproportionation reaction to methanol, said homogeneously catalyzed total reaction being conducted in aqueous media, at a temperature range from 20 to 100 °C, preferably at 50 °C, and at a total H₂ and CO₂ gas pressure up to 100 bar, partial hydrogen gas pressure being up to 100 bar. Said homogeneously catalysed total reaction is preferably conducted at 50°C to promote the FA disproportionation reaction.

The present invention relates to a method for producing methanol from hydrogen gas and carbon dioxide gas in a homogeneously catalyzed total reaction, comprising a carbon dioxide hydrogenation reaction to formic acid and a formic acid disproportionation reaction to methanol, said reaction being conducted:

in aqueous media;

at a temperature range of 20 to 100 °C;

at a total hydrogen gas and carbon dioxide gas pressure up to 100 bar;

in the presence of a catalyst comprising a complex of the general formula (I):

wherein,

M₁ is a metal selected from Ir, Ru, Rh or Co;

R] is pentamethylcyclopentadienide group or hexamethylbenzene group;

R₂ is H₂O group or C1 group;

m is selected from 1 to 4 according to the oxidation state of metal Mj ;

x is 1 or 2;

B is selected from F-, C1-, Br-, Γ, OH-, H-, or S^2- CO₃ ^2-, SO₄ ^2-, or PO₄ ^3- (I); L₁ is a ligand selected from a conjugated system or a system of fused aromatic rings comprising at least one heteroatom N, wherein the aromatic rings may be further substituted by substituents being independently selected from -H, -OH group, -COOH group, -CF₃ group, -NH₂ group, C1 - C12 alkyl, C1 -C12 alkoxy group, C4-C12 aryl, C4-C12 aryloxy group, or a metal selected from Ir, Ru, Rh or Co, which is further substituted by one substituent R₃ being selected from pentamethylcyclopentadienyl group, unsubstituted or substituted benzene group, and one substituent R4 being selected from H?0 group, C1 group, halide group and hydride group.

The catalyst, preferably used in the method of the present invention, is highly stable at the reaction temperatures and in the acidic environment of the reaction.

The catalyst used in the reaction of the present invention is soluble in a polar solvent at a concentration of at least 5 g/L at 25 °C. The polar solvent selected for the reactions is water. The catalyst is dissolved in the aqueous medium of the reaction at a concentration in the range from 0.5 to 8 mM, from 0.1 mM to 8.0 mM, from 0.6 mM to 7.0 mM, or from 1.0 mM to 6.0 mM. The catalyst in the homogenous catalysed reaction of the method of the invention is at a concentration from 0.00625 to 0.20 mol%. The catalyst in the homogeneously catalyzed formic acid disproportionation reaction of the method of the invention is at a concentration preferably from 0.00625 to 0.1 6 mol%.

Furthermore, the catalyst is stable at temperatures≤ 70 °C, more preferably≤ 60 °C. Stable, for the purpose of the present invention, means that the catalyst catalyses at least 10, preferably 30 or more reaction cycles, with small decrease of activity. The catalyst is stable at the pH of the reaction, as defined below, i.e. pH from 0.1 to 2, or at a concentration of H₂SO₄ in the range from 1 to 7.5 M.

The catalyst of the method comprises a com lex of formula (1) or is a catalyst of formula (1),

as defined above.

In an embodiment the ligand L₁ is selected as a moiety according to any one of formulae (1) to (12).

wherein wherein

A is the N atom;

M₂ is a metal selected from Ir, Ru, Rh or Co,

R₃ is selected from pentamethylcyclopentadienyl group, unsubstituted or substituted benzene group, preferably pentamethylcyclopentadienide group or

hexamethylbenzene group;

R4 is selected from H₂O group, C1 group, halide group and hydride group, preferably H₂O group or C1 group;

R₅, R₆, R₇ and R₈ are independently selected from -H, -OH group, -COOH group, - CF₃ group, -NH₂ group, C1 - C12 alkyl, C1 -C12 alkoxy group, C4-C12 aryl, C4-C12 aryloxy group.

The dotted lines represent the bonds between the ligand L₁ and metal M₁. Preferably, L₁ is a moiety according to formula (1), as defined above. M₁ and/or M₂ are metals independently selected from Ir, Ru, Rh or Co, preferably Ir and Co, more preferably Ir. M₂ may be the same metal as M₁ or a different metal. R₅, R₆, R₇ and R₈ of substituents of ligand L₁ , if present, are independently selected from -H, -OH group, -COOH group, -CF₃ group, -NH₂ group, C1 -C12 alkyl, C1 -C12 alkoxy group, C4-C12 aryl, C4-C12 aryloxy group. Preferably R₅, R₆, R₇ and Rg are independently selected from -H, -OH group, -COOH group, -CF₃ group, -NH₂ group, C1-C8 alkyl, C1-C8 alkoxy group, C4-C8 aryl, C4-C8 aryloxy group. Or R₅, R₆, R₇ and R₈ are independently selected from -H, -OH group, -COOH group, -CF₃ group, -NH₂ group, methyl group, hexyl group, methoxy group, hexyloxy group, phenyl group, hexamethyl benzene group, phenoxy group.

In an embodiment, the complex of formula (I) or the catalyst of formula (I) is under the form of a salt.

B represents the counterion (anion) of the complex of the invention and is selected from F-,

The complex of formula (I) or the catalyst of formula (I) may be selected from a compounds according to any one of formulae (13) to (37):

All complexes or catalysts are represented in the form of a salt with a formal charge of 4+, 2+ or 1+.

In a preferred embodiment, the complex or the catalyst of formula (I) is selected from a compound of formula (13) or a compound of formula (14).

In an embodiment, the method of the invention comprises two steps, wherein the step comprising the carbon dioxide hydrogenation reaction precedes the step comprising the formic acid disproportionation reaction. The homogeneously catalyzed total reaction, composed of carbon dioxide hydrogenation reaction to formic acid and formic acid disproportionation reaction to methanol, may take place in the same container, without spatial and/or temporar separation of the C(¾ hydrogenation reaction from the formic acid disproportionation reaction. Thus, the method of the invention can be characterized as a "one-pot" reaction. The homogeneously catalyzed total reaction of the method may be performed in one or two steps, in at least one or more containers, wherein the CO₂ hydrogenation reaction step is conducted before the formic acid disproportionation reaction step, and/or which can be performed successively if desirable.

When the homogeneously catalyzed total reaction is performed in one or two steps, the catalyst comprising a complex of formula (I) is preferably the same for the two reactions. When the catalyst of the invention comprises a complex of formula (I), it may comprise one or more complexes or a mixture of complexes selected from a complex of formula (I).

In an embodiment, the temperature is in the range from 20 to 100 °C, preferably at 50°C for promoting the formic acid disproportionation reaction.

The presence of an acidic compound or acid may act as catalyst for methanol formation and for the esterification reaction of formic acid and methanol to produce methyl formate, the latter reaction being highly reversible. The addition of a strong acid results in the increase of the selectivity of the homogeneously catalyzed total reaction for methanol formation, and in particular of the formic acid disproportionation reaction.

In a further embodiment of the method of the invention, an acid or acidic compound being selected from H₂SO₄, CF₃SO₃H, MeSO₃H, HN0₃, HC1O₄, HBF₄, or H₃PO₄, preferably H₂SO₄ is added into the aqueous medium. The acid may be added at the beginning of the homogeneously catalyzed total reaction directly into the aqueous medium, or it can be added into the aqueous medium in the formic acid disproportionation reaction. The acid is added at a concentration in the range from 2.0 mol% to 75.0 mol%, preferably at 50 mol%. This corresponds to a concentration in the range from 0.1 to 3.8 M, 0.5 to 2.5M or 1 to 2 M. The pH of the reaction is from 0.1 to 2.

The reaction selectivity may be in the range from 60% to 80%, of 55% to 90% and of 70% to 100%. The method of the invention has reaction methanol selectivities from 50% to 99%, from 50% to 97%, in particular for the conversion of formic acid into methanol, from 60% to 80% or from 55% to 98%. The selectivity for methanol formation of the method of the invention may be controlled by the addition of the acidic compound to the aqueous reaction medium, the variation of the catalyst and the temperature. The selectivity for methanol formation is determined by quantitative ¹³C NMR spectroscopy and is calculated as being 3 times the moles of methanol produced divided by the amount of consumed formic acid.

In a further embodiment, the partial pressure of hydrogen gas in the homogeneously catalyzed total reaction of the method of the invention is up to 100 bar. In particular, the partial pressure of hydrogen gas in the formic acid disproportionation reaction is in the range from 50 to 100 bar, 60 to 90 bar or 70 to 80 bar. Preferably, the formic acid disproportionation reaction is conducted at a partial hydrogen pressure of 50 bar. The homogeneously catalyzed total reaction of the method of the invention, or in particular, the formic acid disproportionation total reaction may be conducted at a partial hydrogen gas pressure from 50 to 100 bar, and at a temperature from 20 to 50 °C with or without the addition of an acidic compound at a concentration up to 50 mol%. Under such conditions, the selectivity for methanol formation may increase up to 97 or 96%. Preferably, the homogeneously catalyzed total reaction of the method of the invention, or in particular, the formic acid disproportionation reaction may be conducted at a partial hydrogen gas pressure of 50 bar and at a temperature of 50 °C, without or, preferably, with the addition of an acidic compound at a concentration up to 50 mol%. In an embodiment, the homogeneously catalyzed total reaction composed of carbon dioxide hydrogenation reaction to formic acid and formic acid disproportionation reaction into methanol is conducted at a partial carbon dioxide gas pressure up to 20 bar. The catalytic solutions of the reaction of the method are pressurized by adding carbon dioxide gas and subsequently hydrogen gas up to the total desired gas pressure.

In another embodiment, formic acid may be added into the aqueous medium of the homogeneously catalyzed total reaction. In particular, this addition is performed during the step of the method comprising the formic acid disproportionation reaction. In a preferred embodiment, successive amounts of formic acid are added into the aqueous medium of the homogeneously catalyzed total reaction. Formic acid may be added into the aqueous medium, preferably acidified by the presence of a strong acid, as defined above in the reaction medium. The addition of formic acid in the homogeneously catalyzed total reaction or in the step comprising the formic acid disproportionation may be continuous.

The present invention is described more concretely with reference to the following examples, which, however, are not intended to restrict the scope of the invention. Examples

Example 1 : synthesis of the complexes

The complex of the formula (13) was synthesized according to previously published procedures (Himeda et al., pH-Dependent Catalytic Activity and Chemoselectivity in Transfer Hydrogenation Catalyzed by Iridium Complex with 4,4'-Dihydroxy-2,2'-bipyridine, Chem. - Eur. J., 14, 2008, pp. 1 1076-1 1081). All other chemicals were commercial products and were used without any further purification. Example 2: direct CO? hydrogenation reaction

The CO₂ hydrogenation experiments were performed either in sapphire NMR tubes (for P<100bar) or in Parr autoclaves. In a typical reaction, 15.9 μmol of iridium catalyst (10 mg) were dissolved in H₂O and subsequently pressurized with 20 bar ¹³CO₂ and completed to 80 bar H₂. The system was heated to the required temperature and shaken/stirred. In the sapphire NMR tubes the evolution of formic acid and methanol concentrations was followed by quantitative ¹³C NMR spectroscopy. In case of autoclaves, the yields of formic acid/methanol were determined by quantitative ¹³C NMR measurements in the final solutions, with an added external standard. When the catalytic solutions were analysed in the absence of pressure, the tubes were cooled to 10 °C inside the NMR spectrometer to avoid decomposition of the formed formic acid that occurs even at room temperature. All experiments were prepared without exclusion of air.

The homogeneously catalyzed reaction, composed of the direct CO₂ hydrogenation to formic acid and the formic acid disproportionation to methanol, was performed with the iridium complex of formula (13) at 50 °C, in an aqueous medium acidified with 2.5 M H₂SO₄, the whole reaction being pressurized with CO₂ and H₂ gas. Both formic acid and methanol were detected in solution. Increasing the sulfuric acid concentration resulted in a better methanol to formic ratio, albeit at the expense of the reaction rates. This result is the first example of direct CO₂ hydro genation to methanol in a "one-pot" reaction under such mild temperatures (Figure 2). Pressurization of an aqueous solution of the complex of formula (13) with CO₂ gas and H₂ gas, notably without any organic solvents and additives, instantly afforded FA at room temperature, with a 93 mM FA solution obtained after 60 h at 25 °C. An increase of the reaction temperature to 60 °C, resulted in an enhanced FA formation rate (completion after 1.5 h), albeit at the expense of the FA yield, as expected for an exothermic reaction. Acidification of the reaction mixture (1 M H₂SO₄) decreased the rate of FA production but led to concomitant formation of methanol in solution after 5h at 70 °C. Monitoring the reaction by ¹³C NMR spectroscopy revealed that FA formation preceded that of MeOH, indicative of the latter resulting from formic acid disproportionation rather than direct CO₂ hydrogenation. The presence of H₂SO₄ was not indispensable for methanol production, however it did significantly increase the respective reaction rate and selectivity. Traces of methanol were also detected in purely aqueous solution of (13) after 20 h of heating at 50 °C or 6 d at 25 °C. When a reaction solution was left to equilibrate at a truly ambient temperature (20 °C) over an extended period of time (40 days), a very high FA concentration of 0.16 M along with 13 mM MeOH were obtained. This value is well comparable to the best results reported for direct CO₂ hydrogenation to FA under similar conditions (a 0.2 M aqueous FA solution was obtained at 60 °C under 200 bar H₂/CO₂ pressure (3:1 pressure ratio), without any organic solvents and additives (Moret et al., Direct synthesis of formic acid from carbon dioxide by hydrogenation in acidic media, Nat. Commun. 5, 2014)). This result constitutes the first example of direct CO₂ hydrogenation to methanol in a "one-pot" reaction, mediated by the formed FA disproportionation, in aqueous solution without any additives and at ambient temperature.

Example 3: formic acid dehydrogenation/disproportionation reactions In a typical formic acid dehydrogenation/disproportionation reaction, the catalyst, in particular the iridium catalyst of formula (13), was dissolved in 2.0 mL of H₂O to yield a pale yellow solution. Addition of formic acid led to the formation of a clear, bright yellow reaction mixture from which immediate gas evolution occurred, even at room temperature. In certain experiments, sulfuric acid was added dropwise to the reaction solution, which was occasionally cooled in order to avoid formic acid evaporation due to the highly exothermic H₂SO₄ hydration reaction. Sulfuric acid addition resulted in the change of the solution color back to pale yellow, while gas evolution stopped. Subsequently, the tube was sealed and, in the case of sapphire NMR tubes, thermostated either with an external heating jacket, pre-set to the desired temperature, or directly in the NMR spectrometer, the temperature of which was determined before and after the measurement using an external temperature probe. The reaction was followed by monitoring the pressure increase due to gas evolution as a function of time through a pressure transducer connected to the tube via a high pressure capillary, with an in-house LabView 8.2 program with a NI USB 6008 interface, and/or in situ by 1H and/or ¹³C NMR spectroscopy. In the latter case, spectra were taken at regular time intervals. At the end of the reaction the sapphire NMR tube was cooled and carefully depressurized to avoid loss of organic volatiles along with the gases. An internal standard was added (CH₃CN) and the methanol yield as well as formic acid conversion were determined from quantitative ¹³C NMR spectroscopy. For the recycling experiments, at the end of the catalytic reaction the tube was cooled, carefully depressurized and subsequently 0.38 ml H¹³COOH added to initiate a new cycle. When the desired number of cycles was completed, an internal standard was added and the solution analysed as described above. When the reaction was performed under isobaric conditions, the Schlenk tube containing the reaction mixture was connected to a condenser (in order to avoid loss of methanol) and heated under a nitrogen flow with an external oil bath, pre-set to the desired temperature. After certain time intervals, the solution was cooled and transferred to an NMR tube together with an internal standard for analysis.

When the catalytic reactions were preceded by a pressurization step (and the total pressures were over 100 bar), Parr autoclaves (25 or 75 ml) were used instead of sapphire NMR tubes. In these cases, reaction mixtures were prepared in an identical manner as described above and subsequently transferred to an autoclave for pressurization. The autoclaves were heated in an oil bath that was pre-set to the desired temperature. The temperature gradient between the oil bath and the solution inside the autoclave was measured to be 2.5±0.5 °C {i.e. when the oil bath temperature was 52-53 °C, the solution temperature was 49-50 °C). After certain time intervals, the autoclave was cooled, carefully depressurized and the aqueous solution was transferred to a standard 10 mm NMR tube for analysis.

Heating a 4.2 M aqueous formic solution at 50 °C in the presence of catalyst comprising a complex of formula (13), resulted in >98% dehydrogenation with a TOF of 324 h^-1 (2500 h^-1 using 0.2 mM of catalyst). Even though H₂ and CO₂ were the primary reaction products, 20 mM of methanol were also detected in solution corresponding to a selectivity of 1.2%. Methanol is the product of the disproportionation reaction of formic acid according to equation (1):

3HCOOH(aq) CH₃OH(aq) + H₂O(aq) + 2CO₂(g), (1)

Out of a series of catalytic complexes tested, complex with formula (13) gave the best results. The utilization of H COOH as a substrate resulted in the formation of CH₃OH and ^{, 3}CO₂, while no traces of ¹³CO were detected by ¹³C NMR spectroscopy. Increasing the reaction temperature favored FA dehydrogenation and lowered the obtained methanol selectivity from 1.4% at 20 °C to 0.3% at 90 °C.

When the reaction was performed at near ambient temperature under isobaric conditions (1 bar), formic acid dehydrogenation was complete while the methanol selectivity dropped to 0.8%) (Table 1 , Entries 1 and 2). This influence of pressure can be rationalized by Le Chateliers principle; elevated pressures will favor the disproportionation over the dehydrogenation of FA due to the respective reaction stoichiometries (2 mol versus 2/3 mol of gases produced by dehydrogenation and disproportionation respectively, per mol FA consumed).

In order to take advantage of the beneficial pressure effect, we repeated the reaction in the presence of 100 bar H₂ which resulted in a two-fold increase in the MeOH selectivity to 3.2% at 20 °C (Table 1 , Entries 1 and 3). However, when pressure influence was investigated at 50 °C, the obtained increase in MeOH selectivity was less pronounced, indicating that the negative effect of temperature was predominant over the favoring of pressure under the specific reaction conditions (Table 1, Entries 4 and 5).

Initially, the formation of MeOH proceeded almost linearly without any activation period even at 20 °C. A deceleration of the rate was clearly observed after 15 h when FA conversion reached 50% . Subsequently MeOH fonnation under constantly acidic conditions, by addition of sulfuric acid to the initial reaction solution, was examined.

Sulfuric acid acted as a catalyst for both methanol formation as well as in the esterification reaction of FA and methanol to produce methyl formate, with the latter, however, being highly reversible. When 3.5 mol% sulfuric acid were utilized, a 73 mM MeOH (4.4% yield) solution was obtained, which constitutes an over three-fold increase compared to our initial experiment. Sulfuric acid resulted in the best MeOH selectivities among a series of acids tested. Optimization of the H₂SO₄ concentration resulted in a MeOH selectivity of almost 60% in presence of 50 mol% H₂SO₄ for 99% FA decomposition at 50 °C (Table 1 , Entry 8), which is the highest value ever reported at such mild temperatures and in aqueous solution. Under these reaction conditions the catalytic system was active for five cycles, which resulted in an overall MeOH concentration of 3.1 M for 99% FA conversion. Doubling the initial FA concentration resulted in an identical MeOH selectivity of 60 % and a very high MeOH concentration of 2 M (TON = 250) after only one catalytic cycle. Sulfuric acid concentrations above 50 mol% did not further promote FA disproportionation. Monitoring pressure evolution in the presence of H₂SO₄ showed a significant decrease in the amount of produced gases with increasing solution acidity, in agreement with a much lower FA dehydrogenation yield and enhanced disproportionation. The occurrence of the FA disproportionation reaction was unequivocally confirmed by the increased CO₂ concentration in the produced gas mixture, which rose from 50% to 64% in the absence of H₂SO₄ (originating only from FA dehydrogenation). In order to exclude gas formation from FA dehydration catalyzed by sulfuric acid, a blank test was performed. Heating a 3.6 M aqueous FA/sulfuric acid mixture (1 :0.75) at 70 °C for 48h in absence of catalyst, resulted in no pressure increase. However, the highly acidic environment itself was not sufficient for promoting FA disproportionation. Performing the reaction in the presence of 35 mol% sulfuric acid under isobaric conditions (1 arm), reduced the methanol selectivity from 29 to 3%) (Table 1 , Entries 6 and 7). Consequently, both acidic conditions and elevated pressure were beneficial to favor MeOH production and obtained optimized selectivities. In view of the significantly reduced methanol selectivity under isobaric conditions, we examined the possibility of reaction (Equation 2) taking place and acting as the main source of methanol.

HCOOH(aq) + 2 H₂(aq) CH₃OH(aq) + H₂O(aq) (2) "Self-reduction" of FA to methanol by hydrogen gas originating from FA dehydrogenation would require a significant build-up of hydrogen pressure in order to ensure its adequate solubility in the aqueous solution. However, this did not occur at the beginning of the reaction, especially in the presence of sulfuric acid that significantly limited the rate of FA dehydrogenation. Therefore, methanol formation without an activation period was unlikely to happen via/result from reaction (2). The decreasing MeOH selectivity under isobaric conditions may be the result of an equilibrium in the catalytic cycle involving H₂ gas- via a hydrido-dihydrogen species. A similar phenomena, i.e. decrease of the formic acid dehydrogenation rate under H₂ pressures, has already been observed (Boddien et al., Efficient Dehydrogenation of Formic Acid Using an Iron Catalyst. Science 333, (201 1 ), pp. 1733-1736). Nevertheless we could not exclude some minor contribution of reaction (2) under elevated pressures. When the reaction was performed at ambient temperature (20 °C) in the presence of 50 mol% H₂SO₄, a methanol selectivity of 27.5% for 30% FA conversion was obtained (Table 1, Entry 10). In this case the decreased MeOH selectivity was attributed to the lower final pressure, whose beneficial effect has been rationalized above. Therefore an identical reaction with a preceding H₂ pressurization step was performed. In this case, a very high MeOH selectivity of 97% was achieved alongside 60% FA conversion (Table 1 , Entry 1 1). Likewise at a higher reaction temperature of 50 °C, under 50 bar of initial H₂ pressure, practically complete FA conversion was achieved with 96% selectivity for methanol (Table 1, Entry 9). This opens a new way for homogeneous catalytic methanol production under mild experimental conditions in aqueous solution.

The catalyst comprising a complex of formula (14) with the -OH groups in 6,6' positions was found to be less active in the formic acid disproportionation reaction. Heating a 4.2 M aqueous FA solution to 50 °C resulted in the production of 0.3% methanol, significantly lower compared to the respective selectivity in presence of (13) (Table 1 , Entries 4 and 12). The addition of 50 mol% sulfuric acid only resulted in a moderate improvement of the methanol selectivity to 10% (Table 1 , Entry 13). When the reaction was performed at 20 °C the methanol selectivity dropped to 6.5% (Table 1 , Entry 14) for 42% FA conversion, which we attributed to the decreased overall pressure in our system. Since methanol formation is enhanced in an acidic environment while FA dehydrogenation becomes more sluggish, we expected that short reaction times will afford higher methanol selectivities. Indeed when the overall FA conversion was limited to 8% and the reaction solution pre-pressurized with 100 bar H₂ to compensate for the lack of evolved pressure, a 95.5% methanol selectivity was obtained (Table 1, Entry 15). In summary this work demonstrated that the stable iridium complex of formula (13) possesses multiple functionalities that can be "tuned" in various directions, depending on the process of interest. First of all, it efficiently hydrogenates CO₂ to formic acid in aqueous media, with reasonable rates at ambient temperatures and under aerobic conditions. Formic acid itself is a high-value industrial chemical with a broad range of applications. Additionally, the formed FA can subsequently be disproportionated into methanol with selectivities up to 96% (with 98% FA conversion) under mild conditions, with the same iridium-based catalytic system. Methanol is considered a high energy fuel and a convenient liquid hydrogen carrier. The production of both fonnic acid (0.16 M) and methanol (13 mM) was demonstrated in a single reaction performed at 20 °C, where an aqueous solution of the complex of formula (13), without any additives, was simply pressurized with CO₂ and H₂ and left to equilibrate. It should be noted that the methanol selectivity can be significantly improved by adjustment of the reaction conditions. In particular, it was found that sulfuric acid acts as a co-catalyst in the formic acid disproportionation reaction and greatly limits the extent of formic acid dehydrogenation. The catalytic system was recycled several times affording a very concentrated 3.1 M methanol solution. When an aqueous 8.4 M FA solution was disproportionated, 2 M methanol were obtained after a single catalytic run. Conveniently, methanol-water solutions do not form any azeotrope (in contrast to methanol- organic solvent mixtures), rendering their separation via simple distillation straightforward. It was demonstrated that the iridium complex of formula (13) is catalyzing a series of reactions, which are likely to find applications in major fields of interest, such as CO? sequestration and hydrogen storage.

Example 4: type of acid effect on catalytic formic acid disproportionation reactions to methanol

A series of different acids was evaluated for the formic acid disproportionation reaction (see Table 3).

Table 3. Results for catalytic formic disproportionation/dehydrogenation reactions in presence of various acids with complex of formula (13).

a The general experimental procedure, as described in the Examples, was followed. 10 mmol H₁₃COOH, VH₂O = 2.0 ml, T = 50 °C, isochoric conditions, yields and conversion calculated by quantitative ¹³C NMR spectroscopy using acetonitrile as an internal standard.

For the experiments in Table 3 (Entries 2-8), complex (13) (3.15 mg, 5 mmol) was added to a stainless steel autoclave, and air was replaced to argon. A 4 M aqueous formic acid solution (4 mL, 16 mmol) including 10% acid (1.6 mmol), which was degassed by three cycles of freeze-pump-thaw, was charged into an autoclave. The autoclave was pressurized by 30 bar of H₂/CO₂ (1 : 1) and was stirred at 1500 rpm at 50 °C for 20 h. The pressure was kept at 30 bar during the reaction by equipped with back-pressure regulating valve. After finishing the reaction, the autoclave was cooled to 0 °C for 30 min, and then slowly depressurized to atmospheric pressure. The concentrations of generated MeOH and HCO₂Me were analyzed by NMR (Bruker Avance 500 NMR spectrometer). The remaining formic acid concentration was analyzed by HPLC (SHIMADZU SIL-20A) with an anion- exclusion column an [Tosoh TSKgel SCX(H⁺)] using aqueous phosphate solution (20 mM) as eluent and a UV detector (1 = 210 nm).

Claims

1. A method for producing methanol from hydrogen gas and carbon dioxide gas in a homogeneously catalyzed total reaction, comprising a carbon dioxide hydrogenation reaction to formic acid and a formic acid disproportionation reaction to methanol, said reaction being conducted:

in aqueous media;

at a temperature range of 20 to 100 °C;

at a total hydrogen gas and carbon dioxide gas pressure up to 100 bar;

in the presence of a catalyst comprising a complex of the general formula (I):

M₁ is a metal selected from Ir, Ru, Rh or Co;

R₁ is pentamethylcyclopentadienide group or hexamethylbenzene group;

R₂ is H₂O group or C1 group;

m is selected from 1 to 4 according to the oxidation state of metal M₁;

x is 1 or 2;

B is selected from F\ C1-, Br-, Γ, OH-, H-, or S^2-, CO₃ ^2-, SO₄ ^2- or PO₄ ^3-;

L₁ is a ligand selected from a conjugated system or a system of fused aromatic rings comprising at least one heteroatom N, wherein the aromatic rings may be further substituted by moieties being independently selected from -H, -OH group, -COOH group, -CF₃ group, -NH₂ group, C1-C12 alkyl, C1 -C12 alkoxy group, C4-C12 aryl, C4-C12 aryloxy group, or a metal selected from Ir, Ru, Rh or Co, which is further substituted by one substituent R₃ being selected from pentamethylcyclopentadienyl group, unsubstituted or substituted benzene group, and one substituent R4 being selected from H₂O group, C1 group, halide group and hydride group.

2. The method of claim 1 , wherein L₁ is selected from a moiety according to any one of formulae (1) to (12).

wherein

A is N atom;

M₂ is a metal selected from Ir, Ru, Rh or Co,

R₃ is selected from pentamethylcyclopentadienyl group, unsubstituted or substituted benzene;

R4 is selected from H₂O group, C1 group, halide group and hydride group;

R₅, R₆, R₇ and R« are independently selected from -H, -OH group, -COOH group, -CF₃ group, -NH₂ group, C1 -C12 alkyl, C1 -C12 alkoxy group, C4-C12 aryl, C4-C12 aryloxy group.

3. The method of any one of the preceding claims, wherein the homogeneously catalyzed reactions are conducted further at a partial hydrogen gas pressure up to 100 bar.

4. The method of any one of the preceding claims comprising two steps, wherein the step comprising the carbon dioxide hydrogenation reaction precedes the step comprising the formic acid disproportionation reaction.

5. The method of any one of the preceding claims, wherein an acidic compound being selected from H₂SO₄, CF₃SO₃H, MeSO₃H, HN0₃, HC1O₄, HBF₄, and H₃PO₄ is added into the aqueous media.

6. The method of claim 5, wherein the acidic compound is added into the aqueous medium in the formic acid disproportionation reaction.

7. The method of any one of claims 5-6, wherein the acidic compound is added into the aqueous medium at a concentration up to 50 mol%.

8. The method of any one of the preceding claims, wherein the temperature is in the range from 20 to 50°C.

9. The method of any one of the preceding claims, wherein the homogeneously catalyzed reactions are conducted at a partial carbon dioxide gas pressure up to 20 bar.

10. The method of any one of the preceding claims, wherein formic acid is added into the aqueous medium of the homogeneously catalyzed reactions.

11. The method of claim 12, wherein successive amounts of formic acid are added into the aqueous medium of the homogeneously catalyzed reactions.