WO2013175311A2

WO2013175311A2 - Conversion of carbon dioxide to methanol using visible light

Info

Publication number: WO2013175311A2
Application number: PCT/IB2013/001587
Authority: WO
Inventors: Bengt Norden
Original assignee: Bengt Norden
Priority date: 2012-05-23
Filing date: 2013-05-23
Publication date: 2013-11-28
Also published as: WO2013175311A3

Abstract

The present invention is directed to methods of preparing methanol from carbon dioxide. In preferred embodiments, the method comprises combining a photoactive multi- electron collector and an electron donor in the presence of light to form a two electron reduction product; and combining the two electron reduction product and carbon dioxide in the presence of one or more catalysts, thereby converting carbon dioxide to methanol.

Description

CONVERSION OF CARBON DIOXIDE TO METHANOL USING VISIBLE

LIGHT

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/650,787 filed on May 23, 2012, the entire disclosure of which is hereby incorporated in its entirety.

BACKGROUND

Methanol is a convenient, safe liquid that is easily obtained from existing coal or natural gas sources via methods developed and practiced since the 1920's. However, these methods using conversion (reforming) of coal and subsequently natural gas to syn-gas (a mixture of H₂ and CO) are highly energy consuming and produce large amounts of C0₂ as a by-product. This is notably an economic disadvantage, but also represents a serious environmental problem by increasing one of the main greenhouse gas causing global warming.

Methanol represents a convenient and safe way to store and transport energy, and is a desirable choice as a transportation fuel due to its efficient combustion, ease of distribution and wide availability. Methanol is used in transportation in 3 main ways - directly as fuel or blended with gasoline, converted in dimethyl ether (DME) to be used as a diesel replacement, or as a part of the biodiesel production process. DME is easily obtained from methanol by dehydration or from methane (natural gas) with C0₂ via a bi-reforming process. It is a particularly effective fuel for diesel engines because of its high cetane number and favorable combustion properties. Methanol and DME blend well with gasoline or diesel oil to be used as fuels in internal combustion engines or electricity generators. One of the most efficient use of methanol is in fuel cells, particularly in direct methanol fuel cells (DMFC), in which methanol is directly oxidized with air to carbon dioxide and water while producing electricity.

In addition to use as fuels, methanol, dimethyl ether and derived chemicals have significant applications in the chemical industry. Today, methanol is one of the most important feedstock in the chemical industry. The majority of the some 35 million tons of the annually produced methanol are used to manufacture a large variety of chemical products and materials, including basic chemicals such as formaldehyde, acetic acid, MTBE, as well as various polymers, paints, adhesives, construction materials, and others. Methanol is also a feedstock for chloromethanes, methylamines, methyl methacrylate, and dimethyl terephthalate, among others. These chemical intermediates are then processed to manufacture products such as paints, resins, adhesives, antifreeze, and plastics.

Capture and subsequent hydrogenative reduction of C0₂ could provide C0₂-neutral alternatives to fossil fuels. [1,2] Methanol, a potential product from proton coupled multiple- electron reduction of C0₂,[3] is an excellent fuel in its own right (ΔΗ = -726.55 kJ/mol) and more convenient for storage and transportation than hydrogen (ΔΗ = -285.8 kJ/mol).[4] Direct light-driven reduction of C0₂ to methanol could potentially represent a revolutionary solution to solar energy conversion into a convenient fuel, and become a bridge towards a renewable-energy future. Despite numerous attempts to achieve photochemical C0₂ reduction and conversion to hydrocarbon fuel (primarily methanol), methanol has been found difficult to obtain as main product. [5- 10] In combination with enzyme catalysts or homogeneous catalysts, selective C0₂ reduction into methanol could be achieved if ZnS or p- type GaP is used as photosensitizers.[ 11,12] However, generally, rather high energy (UV) photons are needed due to the large band gap of the semiconductors, and the efficiency drops dramatically when visible light is used as energy source. [7, 12] Therefore, in order to use the most intense part of the solar spectrum, there remains a need in the art for C0₂ reduction that is achievable with visible light.

SUMMARY OF THE INVENTION

The present invention provides the first report of effective homogeneous catalysis of C0₂ reduction to methanol driven by visible light. Converting C0₂ at low-pressure from the atmosphere, water and solar light into C0₂ neutral hydrocarbon fuel has been a desirable goal for a long time.

Accordingly, in a first aspect, the invention features a method of preparing methanol from carbon dioxide comprising combining a photoactive multi-electron collector and an electron donor in the presence of light to form a two electron reduction product, and combining the two electron reduction product and carbon dioxide in the presence of one or more catalysts, thereby converting carbon dioxide to methanol.

In one embodiment, the photoactive multi-electron collector is a ruthenium complex. In another embodiment, the ruthenium complex further comprises a quinone moiety. In another further embodiment, the ruthenium complex is the complex set forth as Formula I:

In still another embodiment, the two electron reduction product is the complex set forth as Formula II:

In another embodiment, the electron donor is ascorbic acid. In another further embodiment, the electron donor is water. In yet another embodiment, the one or more catalysts are selected from the group consisting of: an alcohol and a cobalt catalyst. In still another further embodiment, the catalysts are a primary alcohol and [Co(NDS)2]^2".

In another embodiment of the present invention, the method further comprises the addition of a co-catalyst. In a related embodiment, the co-catalyst is selected from the group consisting of: methanol and ethanol. In still another further embodiment, the preparation of methanol from carbon dioxide in the presence of light is conducted at a temperature of 0-150 ° C.

In a further embodiment, the light is visible light. In related embodiments, the visible light is a wavelength 300-900 nm.

In another embodiment, the method further comprises dehydrating the methanol thus produced to form dimethyl ether.

In another aspect, the present invention is directed to methanol that is produced from carbon dioxide by the process comprising combining a photoactive multi-electron collector and an electron donor in the presence of light to form a two electron reduction product, and combining the two electron reduction product and carbon dioxide in the presence of one or more catalysts, thereby producing methanol from carbon dioxide.

In one embodiment, the photoactive multi-electron collector is a ruthenium complex. In another embodiment, the ruthenium complex further comprises a quinone moiety. In another further embodiment, the ruthenium complex is the complex set forth as Formula I.

In another further embodiment, the two electron reduction product is the complex set forth as Formula II. In still another embodiment, the electron donor is ascorbic acid. In another further embodiment, the electron donor is water.

In another further embodiment, the one or more catalysts are selected from the group consisting of: an alcohol and a cobalt catalyst. In a further related embodiment, the catalysts are a primary alcohol and [Co(NDS)₂]^2~.

In another embodiment, methanol produced by the process described herein comprises the addition of a co-catalyst. In a related embodiment, the co-catalyst is selected from the group consisting of: methanol and ethanol.

In another embodiment, the preparation of methanol from carbon dioxide in the presence of light is conducted at a temperature of 0-150 ° C.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention, which should be considered in all its aspects, will become apparent from the following description, which is given by way of example only, with reference to the accompanying figures.

Figure 1 is a graph that shows absorption spectral response to dithionite reduction of complex 1 in aqueous solution. Figure 2 is a graph that shows absorption spectral response upon irradiation of complex 1 (0.03 mM) in aqueous solution buffered at pH = 4 with ascorbic acid/sodium ascorbate, [H₂A] = [NaHA] = 0.5 M. Figure 3 shows the C NMR spectra of the solution before (A) and after (B) photoreaction. Both the spectra were obtained with same scans of 64 repetitions. The peak at δ 62 ppm in the spectra was from ascorbic acid. Figure 4 shows cyclic voltammograms of complex 1 (0.5 mM) in unbuffered aqueous solution at pH = 5 under N₂ (a) and C0₂ (b), scan rate 100 mV/s. The reference electrode was based on the Ag/AgCl couples.

Figure 5 shows NMR integration of CH₃OH at δ 3.37 before (top) and after (bottom) photoreaction compared with internal standard DMSO-H₆ at δ 2.73 (0.54 mg). Sample was from a mixture of reaction solution/D₂0, (1:1, v/v).

Figure 6 shows ¹³C NMR calibration based on the ascorbic acid (0.1 M) in aqueous solution with addition of 50 mM (top) and 100 mM (bottom) of normal MeOH, respectively. Both the spectra were obtained with same scans of 64 repetitions.

Figure 7 shows gas chromatograph traces of a sample from reaction solution compared with a standard sample containing methanol, ethanol and n-propanol (top). GC traces of standard samples containing 10 or 100 ppm of methanol (1.8 min), ethanol (2.1 min) and n-propanol (2.8 min), respectively, the initial peak at 1.5 min was from the systematic error during injection (bottom).

Figure 8 shows cyclic voltammograms of [Co(NDS)₂]^2~ in unbuffered aqueous solution at pH = 5 under N₂ (solid) and C0₂ (dash), scan rate 100 mV/s.

Figure 9 shows cyclic voltammograms of complex 1 in buffered aqueous solution at pH = 5 under N₂ (solid) and C0₂ (dash), scan rate 100 mV/s.

Figure 10 shows the NMR spectrum from the reaction solution after photoreaction with ethyl formate as substrate. Inset: 1H NMR signal of CH₃OH at δ 3.37 obtained with scans of 128 repetitions.

Figure 11 shows the spectrum of the LED lamp. DETAILED DESCRIPTION

The present invention relates to methods of preparing methanol from carbon dioxide using light.

The invention features a method of preparing methanol from carbon dioxide comprising combining a photoactive multi-electron collector and an electron donor in the presence of light to form a two electron reduction product, and combining the two electron reduction product and carbon dioxide in the presence of one or more catalysts, thereby converting carbon dioxide to methanol.

Any suitable source of carbon dioxide obtained from any available source can be used, such as carbon dioxide obtained from emissions of power plants burning fossil fuels, fermentation processes, calcination of limestone, other industrial sources, or even the atmosphere is utilized via its chemical recycling providing renewable carbon fuels into mitigating the environmentally harmful effect of excess C0₂. A carbon dioxide source obtained from an exhaust stream from fossil fuel burning power or industrial plant, or a source accompanying natural gas can be used. According to the process of the invention, carbon dioxide is recycled instead of it being sequestered, which provides a way of disposal to the carbon dioxide produced by coal and other fossil fuel burning power plants and industries producing large amounts of carbon dioxide.

Methanol and dimethyl ether formed by the processes described herein can find utility in numerous applications, either alone, or upon subsequent conversion to other products. Without being limiting, methanol, dimethyl ether and their derived products can be used as synthetic internal combustion engine (ICE) fuels, effective diesel fuels (including mixing varied amounts of dimethyl ether (DME) with conventional diesel fuel), gasoline-methanol mixed fuels (prepared by adding methanol to gasoline with the fuel having a minimum gasoline content of at least 15% by volume). Without being limited as to other uses, methanol and/or dimethyl ether are convenient energy storage and transportation materials in order to minimize or eliminate the disadvantages or dangers inherent in the use and transportation of LNG or LPG. Dimethyl ether is also a convenient household gas to replace natural gas. They are also convenient raw materials for producing olefins (ethylene, propylene etc.) synthetic hydrocarbons, their products and materials, even for preparing single cell proteins for human or animal consumption.

In one embodiment, the photoactive multi-electron collector is a ruthenium complex. Ruthenium polypyridine complexes, owing to their absorption spectra reaching far into the visible region due to strong metal-to-ligand charge transfer (MLCT) excitations, have been successfully used as light harvesting agents in many light-driven processes including C0₂ reduction. [5,6] However, most ruthenium complexes only undergo one-electron processes upon excitation, which make them less efficient for multiple-electron reduction of CO₂ to give a sufficiently reduced, practically useful fuel product such as methanol. Efforts have been made to attach more than one photoactive unit to the catalyst center, but still hardly any highly reduced product from C0₂ reduction was observed. [13] Therefore, attention should instead be focused on photosensitizers as electron mediator that could collect more than one electron by the action of light, and be likely to efficiently support CO₂ activation as well as subsequent transfer of several reductive equivalents to the CO₂ reduction cycle. Indeed, the binuclear ruthenium complex [(phen)₂Ru-tatpq-Ru(phen)₂]⁴⁺ (phen = 1,10- phenathroline; tatpq = 9,l l,20,22-tetraazatetrapyrido[3,2-a:2',3'-c:3",2"-l:2'",3"'- n]pentacene-10,21-quinone), is known as a photoactive four-electron collector, storing the first two electrons in the quinone moiety of the central tatpq ligand,[14] but has never been utilized to drive any catalytic reaction. Interestingly, the reduced, dianionic form of benzoquinone derivatives display very high efficiency for CO₂ capture in organic

solvents. [15, 16] As a next step, the benzoquinone/hydroquinone redox couple has been envisioned as an electron mediator in the electrochemical reduction of CO₂ into methanol, for example, using aquapentacyanoferrate(II) ([Fe(CN)s(H₂0)]₃ ^") as a homogeneous catalyst in aqueous solution. [17].

Accordingly, the present inventors have shown that water soluble mononuclear ruthenium complex 1, carrying a quinone structure on one of its ligands, is able to collect two electrons, presumably in the quinone part, upon action of light in the presence of ascorbate. Together with [Co(NDS)₂]^2" (NDS = l-nitroso-2-naphthol-3,6-disulphonate) and a primary alcohol as catalysts, [18] methanol was produced from the photoreaction system under normal low pressure of C02 (Scheme 1, shown below). This is the first report of homogenous catalysis of C02 reduction to methanol driven by visible-light.

Accordingly, in another embodiment, the ruthenium complex further comprises a quinone moiety. In certain exemplary embodiments, the ruthenium complex is the complex set forth as Formula I:

An electron donor is a chemical entity that donates electrons to another compound. Accordingly, an electron donor known to one of skill in the art is envisioned to be used in the methods of the invention described herein. Ascorbic acid is an essential reductant in biology, and in according to exemplary embodiments of the present invention, the electron donor is ascorbic acid. In other embodiments of the invention, water is used as an electron donor.

In yet another embodiment, the one or more catalysts are selected from the group consisting of: an alcohol and a cobalt catalyst. Certain cobalt-based catalysts exhibit high levels of activity at room temperature and operate under neutral pH conditions. Accordingly, in certain exemplary embodiments, the catalysts are a primary alcohol and [Co(NDS)₂]^2~.

In another embodiment of the present invention, the method further comprises the addition of a co-catalyst, for example, but not limited to methanol and ethanol.

In still another further embodiment, the preparation of methanol from carbon dioxide in the presence of light is conducted at a temperature of 0-150 ° C.

The invention also encompasses subsequent methanol dehydration in the presence of a different catalyst (for example, but not limited to, silica-alumina) resulting in the production of dimethyl ether (DME). This is a two-step (indirect synthesis) process that starts with methanol synthesis, as described herein, and ends with DME synthesis (methanol dehydration).

The present invention also features methanol that is produced from carbon dioxide by the process comprising combining a photoactive multi-electron collector and an electron donor in the presence of light to form a two electron reduction product, and combining the two electron reduction product and carbon dioxide in the presence of one or more catalysts, thereby producing methanol from carbon dioxide.

The following examples illustrate preferred embodiments of the invention without limiting it.

EXAMPLES

The following Examples were carried out with, but not limited to, the Materials and Methods described below.

Materials and Instrumentation

2,3-Diaminonaphthalene, tetrabutylammonium chloride (Bu₄NCl), disodium 1- nitroso-2-naphthol-3,6-disulfonate, ascorbic acid, sodium ascorbate, ethyl formate, NH₄PF₆, NaS₂0₄, Na₂S₂0₈, CoS0₄-7H20 and ¹³C02 gas (99 atom % 13C, 2 atm) were used as received from Sigma- Aldrich. NMR spectra were obtained on a Varian spectrometer at 400 MHz for 1H and 100 MHz for 13C detection. UV/vis absorption spectra were recorded on a Varian Cary 50-Bio UV-visible spectrophotometer. The pH value of solution was determined by Jenway 3510 pH meter.

Synthesis

The ligand l,10-phenanthroline-5,6-dione and ds-Ru(bpy)₂Cl₂- 2H₂0 were synthesized according to the literature procedures. [28,29] The ligand benzodipyrido[3,2- a:2',3'-c]phenazine (P) was prepared by refluxing 1:1 molar ratio of l,10-phenanthroline-5,6- dione and 2,3-diaminonaphthalene in ethanol for 6 h in the presence of catalytic amount of acetic acid.

[ Ru(bpy )2(dipyrido[ 3, 2-a:2 3 '-c ]benzo[ 3, 4 ]phenazine-l 1,16-quinone ) ]( PF^)₂ ([1( PF^)₂] )

The ligand P (166 mg, 0.5 mmol) and cis-Ru(bpy)₂Cl₂- 2H20 (208 mg, 0.4 mmol) were suspended in 95% ethanol solution (40 ml). The reaction mixture was refluxed for 12 h and then cooled down to room temperature. The unreacted ligand was removed by filtration to get a clear solution. Solvent (mainly ethanol) was evaporated under reduced pressure before water (30 ml) and sodium persulfate (476 mg, 2 mmol) were added. And then the reaction solution was heated at reflux for 6 h. NH₄PF₆ was added to the solution to precipitate the crude product which was collected by filtration and washed with water. The crude product was then dissolved in a little amount of acetone to which ethyl ether was carefully added to get the very pure complex [1(PF₆ ^~)₂] by precipitation. The total yield based on ruthenium after two steps was 68% (290 mg).

^!H NMR (CD₃CN): δ 9.70 (d, 2H, J_HH = 8.4 Hz, H_n), 8.58 (d, 2H, J_HH = 8.4 Hz, H₅), 8.54 (d, 2H, J_HH =8.0 Hz, H₄), 8.51 and 8.50 (2 x d, 2 x 1 H, J_HH = 4 Hz, H₁₂), 8.30 (d, 2H, J_HH = 5.6 Hz, H₉), 8.15 (t, 2H, J_HH = 8.4 Hz, J_HH = 8.0 Hz, H₆), 8.09 and 8.08 (2 x d, 2 x 1 H, J_HH = 4 Hz, H₁₃), 8.05 (t, 2H, J_HH = 8.0 Hz, J_HH = 7.6 Hz, H₃), 7.99 (dd, 2H, J_HH = 8.4 Hz, J_HH = 5.6 Hz, Hio), 7.77 (d, 2H, J_HH = 5.0 Hz, H₈), 7.72 (d, 2H, J_HH = 5.6 Hz, Hj), 7.50 (t, 2H, J_HH = 5.0 Hz, J_HH = 8.0 Hz, H₇), 7.28 (t, 2H, J_HH = 7.6 Hz, J_HH = 5.6 Hz, H₂).

In a typical experiment for photoctatalysis, disodium l-nitroso-2-naphthol-3,6- disulfonate (15.2 mg, 40 μιηοΐ), CoS0₄-7H₂0 (5.6 mg, 20 μιηοΐ) and H₂A (176 mg, 1 mmol) were added to a Pyrex-glass test tube (vol. 40mL, dl4 cm). 10 ml of water was added to the test tube before the mixture was stirred for 0.5 h. Complex 1 (2.5 mg, 3 μιηοΐ) was added to the above solution. The test tube was sealed with a rubber septum and put into ice-water bath. The solution was bubbled with C0₂ at 2 atm for 1 h. Before the ice-water bath and the needle with C0₂ were removed, 8 μL· (0.2 mmol) of methanol was carefully injected into the solution through the rubber septum under protection of C0₂. The rubber septum was further sealed by parafilm immediately when the needle with C0₂ was removed. The reaction solution was irradiated at room temperature using a LED lamp (0.8 W) as visible light source (see Fig. 11 for light spectrum). The liquid phase of the reaction system was analyzed on a HP 5890 GC instrument with a FID detector, which housed a DB-Wax column (Agilent Technologies.), and with N2 as carrying gas. The amount of methanol produced was determined with iso-propanol as the internal standard. The methanol produced after 15 h reaction was a little more than that after 10 h but with similar amount to the reaction for 20 h.

When ¹³C0₂ was used as carbon source, the experimental procedure was the same as above description but with 12 μΐ_^ (0.3 mmol) of methanol as co-catalyst. The received ¹³C0₂ was 0.46 liter of gas at 2 atm, the pressure dropped to nearly normal pressure after 30 min of bubbling. Therefore, only less than 10 μιηοΐ of ¹³CH₃0H was produced from this

photoreaction. A sample for ¹³ C NMR detection was prepared from a mixture of reaction solution (0.3mL) and D₂0 (0.3mL).

Photolysis of complex 1

Complex 1 (30 μΜ) was dissolved in an ascorbic acid/sodium ascorbate buffer solution at pH = 4. The solution was sealed in a quartz cuvette with a rubber septum and degassed for 30 min with nitrogen prior to irradiation. The cuvette was irradiated by a 1000 W Xe/Hg lamp running at 450 W with a 380-nm-cutoff UV filter. The progress of the photochemical reaction was monitored by periodically recording the absorption spectra (Cary 50 Bio UV-visible spectrophotometer).

Electrochemical Measurements

Cyclic voltammograms of 0.5 mM parent complexes in aqueous solution containing

0.1 M KC1 as supporting electrolyte were recorded using a CH Instruments potentiostat at a scan rate of 100 mV/s. The electrolyte solution was adjusted to pH = 5 by 0.02 M HC1 or buffered at pH = 3.6. Electrochemical experiments were carried out in a three-electrode cell under N₂ or C0₂ atmosphere. The working electrode was a glassy carbon disk (diameter 3 mm) polished with 0.3-μιη A1₂C>3 powder prior to use, and the auxiliary electrode was a platinum wire. The reference electrode was based on the Ag/AgCl couples.

Example

As discussed herein, the present inventors have shown that water soluble mononuclear ruthenium complex 1, carrying a quinone structure on one of its ligands, is able to collect two electrons, presumably in the quinone part, upon action of light in the presence of ascorbate. Together with [Co(NDS)₂]^2" (NDS = l-nitroso-2-naphthol-3,6-disulphonate) and a primary alcohol as catalysts, [18] methanol was produced from the photoreaction system under normal low pressure of C02 (Scheme 1).

Synthesis of complex 1 followed the reported procedure for the analogous binuclear ruthenium complex. [19] Treatment of [(bpy)2Ru(P)]²⁺ (P = benzodipyrido[3,2-a:2^',3 ^'- c]phenazine) with 5 equiv. of sodium persulfate in refluxing water solution for 6 hours gave the desired complex 1 with counter anion PF₆ ^" in 68% yield. The structure was confirmed by NMR which spectrum was completely in accordance with that reported. [20] The PF₆ ^" counter anion was converted into CI^" for making it water soluble. In order to investigate the electron collecting properties of complex 1 upon action of light, the absorption spectrum for its reduced state was first calibrated by dithionite reduction according to the procedure for two-electron reduction of a similar ruthenium complex carrying a quinone-containing ligand.[21] Complex 1 displayed a characteristic MLCT band at 447 nm (ε = 1.7 x 10⁴ M^" 'cm^'1) in the visible region in water solution. Reduction of complex 1 by 4 equivalents of sodium dithionite under N₂ atmosphere resulted in a reduced species showing two absorption bands at 428 and 452 nm. Presumably, the new species is two-electron reduced complex 1 that combined two protons from water to give RuQH₂ as reported in the literature (Figure 1).[21] Figure 1 shows the absorption spectral response to dithionite reduction of complex 1 in aqueous solution. The difference in the absorption spectra between the oxidation and reduction state for complex 1 makes it feasible to monitor photochemical properties of complex 1 in the presence of a weaker electron donor than dithionite. To meet with the pH requirements for C02 reduction into methanol, [22] ascorbic acid was chosen as electron donor since it could provide both electrons and protons. [11] Irradiation of complex 1 in aqueous solution buffered at pH = 4 with ascorbic acid/sodium ascorbate under N2 atmosphere indeed resulted in a change in the absorption spectra confirming the same redox reaction as with the strong reductant dithionite (Figure 2). Figure 2 shows the absorption spectral response upon irradiation of complex 1 (0.03 mM) in aqueous solution buffered at pH = 4 with ascorbic acid/sodium ascorbate, [H₂A] = [NaHA] = 0.5 M.

The confirmation of a two-electron collection by complex 1 upon action of light next led to investigating the feasibility of C02 reduction into methanol initiated by the RuQ complex in the presence of [Co(NDS)₂]^2" and methanol, the most efficient catalyst combination found in earlier electrochemical studies. [18] The multi-component system of complex 1, H₂A and [Co(NDS)₂]^2" was first saturated with C0₂ by bubbling at pressure of 2 atm. Then 2 x 10^"4 mol of MeOH was added before the reaction solution was continuously irradiated for 15 h with visible light from a LED lamp. The amount of methanol before and after the reaction was determined to be 2.00 x 10^"4 mol and 2.48 x 10^"4 mol, respectively, based on the integration of CH30H peak in 1H NMR spectrum with DMSO as internal standard (see Figure 5). Thus, 4.8 x 10^"5 mol of methanol was produced, corresponding to 48 turnovers based on complex 1 and 2.4 turnovers based on [Co(NDS)₂]^2" during the photoreduction. The amounts of methanol before and after photoreaction were further confirmed by GC analysis that gave a result of methanol increment by 5.1 x 10^"5 mol.

In order to confirm that the increased methanol was indeed originating from the reduction of C0₂ isotope labeling was used: ¹³C0₂ was used as carbon source instead of normal ¹²C0₂. There was no detectable signal assig ned to ³CH₃OH in the ¹³C NMR spectrum of the reaction solution before photoirradiation (Figure 3A), in which the concentration of ³Ο¾0Η was only 0.3 mM calculated based on the 1% abundance of ¹³C in the normal MeOH (30 mM). With the same scans as above, the signal at δ 49.02 ppm, attributed to ³Ο¾0Η could be easily observed in the ¹³C NMR spectrum of the reaction solution after photoreaction (Figure 3B), indicating that the content of ³Ο¾0Η had increased remarkably in the resulting solution. A further calibration was carried out with addition of normal MeOH to the aqueous solution of ascorbic acid (0.1 M) that was the same condition as reaction solution. ¹³C NMR detection gave a similar signal when the

concentration of normal MeOH was increased to 100 mM which means the concentration of 3C¾OH in the solution is about ImM (see Figure 6). Since other components with ¹²C- carbon source in the reaction solution could not lead to any obvious difference in the sensitivity during ¹³C NMR detection, the ³C¾OH increment could thus be ascribed to the conversion from ¹³C0₂. As a control, the reaction was also carried out under N₂ atmosphere (absence of C0₂): then, the amount of methanol was not increased in support of our conclusion. The selectivity of C02 reduction into methanol was also investigated by determination of the potential products such as formic acid and formaldehyde according to the established colorimetric methods. [12] No obvious yield of formic acid or formaldehyde was found within the detection limits of the method.

Several other control experiments were carried out to ascertain that C0₂ reduction into methanol was indeed promoted by the above multi-component catalyst system: Methanol was not produced when the reaction was carried out in the dark. Moreover, omission of any of the components of the reaction system inhibited formation of methanol, suggesting that all of the four components were required for the methanol producing process. In a parallel study, methanol was not produced if the photoelectron collector complex 1 was replaced by intermolecular single electron transfer system, Ru(bpy)3²⁺ and 4 or 10 equiv. of

benzoquinone.[23] The results of the control experiments indicated that photocatalytic reduction of C02 into methanol was indeed initiated by the two-electron collector RuQ and catalyzed by the [Co(NDS)₂]^2".

It is noteworthy that methanol was not produced without initial addition of MeOH itself as co-catalyst. Increasing the amount of co-catalyst resulted in higher yield of methanol production (Table 1, shown below), indicating that an alcohol was very important for C0₂ fixation during the methanol producing process as reported by electrochemical studies in the literature. [24] Fafcfe f« Effect <sC afcstooi as co-catalyst on p^ioctenca! S isthansi; eo-caial¾¾§

{aicofcoS, id) based-en

MeQH, 20 51 ^o? 51 2,5

&feOH, 30 mM 73 350§ 73 3.6

and H {Q.1 ) tr*& a ssous sc_ tioi¾ w¾ different stesho? as co- catsSyss, 1:5 M ¾» moj&ciite of fcfeOH pradii ian i^e-eds ihrae

In order to study the function of the primary alcohol during the catalysis process, ethanol and iso-propanol were also employed as co-catalyst to investigate the effect of primary alcohol on methanol production in the present system. GC analysis of methanol formation was shown in Table 1. When ethanol was used as co-catalyst, only one peak attributed to ethanol was observed during GC analysis before reaction. Besides ethanol, a new peak was appeared during GC analysis after 15 h of photoreaction (see Figure 7).

Compared with the standard sample, the new peak observed was ascribed to methanol.

However, the yield of methanol production decreased dramatically to 3.2 μιηοΐ. A similar difference in catalytic efficiency between methanol and ethanol as co-catalysts was also found during the electrochemical reduction of C0₂ to methanol when a Fe-based complex was used as homogeneous catalyst, and the steric hindrance during C0₂ inserting into metal- alcohol bond was argued. [24] In further study of our system, only trace amount of methanol was found during GC analysis when iso-propanol was used as co-catalyst instead of ethanol, suggesting that the steric hindrance indeed exists in C0₂ insertion or the intermediates during C0₂ reduction.

The initial step in the present C0₂ reduction system is that C0₂ was activated and brought to the reduction cycle. C0₂ captured by dianionic form of the benzoquinone derivatives has been confirmed by electrochemical studies in organic solvent, but it is more significant to explore their interaction in aqueous solution because of the advantages of water as solvent. [16] Therefore, the possibility of C0₂ captured by two-electron reduced complex 1 in aqueous solution was studied by cyclic voltammetry (CV) experiments in which RuQ^2~ and RuQH₂ could selectively generate near the electrode. [25] In unbuffered aqueous solution at pH = 5 under N2 atmosphere, complex 1 (0.5 mM) displayed a reversible reduction peak at -0.49 V (£^"1_/2) in the cyclic voltammogram (see curve a in Figure 4). It has been demonstrated that a quinone species underwent non-proton coupled 2e^" reduction to produce quinone dianion near the electrode during CV scan when the concentration of proton was lower than that of quinone in unbuffered aqueous solution. [25] Therefore, the reduction wave at -0.56 V (E_pc) in Figure 4 (curve a) was attributed to non-proton coupled 2e^~ reduction of complex 1, RuQ to RuQ^2". When the solution of complex 1 was saturated with CO2 by bubbling, the RuQ to RuQ^2" reduction wave negatively moved by ca. 100 mV probably due to the pH change in the solution. However, the reversibility lost dramatically during the reverse scan, indicating the amount of RuQ^2"species was less than that generated during the reduction event, which caused a decrease in the current intensity of the corresponding oxidation wave (see curve b in Figure 4). This was mainly because of the interaction of RuQ^2" with CO2. Since benzoquinone derivatives showed good affinity for CO2 after non-proton coupled 2e^" reduction in organic solvent to form adduct BQ²-[CC>2], it is deduced that C02 could be also captured by RuQ^2" in aqueous solution. Being different from the case in organic solvent, the inner-sphere interaction between RuQ^2" and CO2 in aqueous solution would probably result in the solvated hydroxyformyl radical form, RuQ^"[ COOH], inspired by the CO2 reduction by the pyridinium radical. [26] Efforts were made to further investigate the current intensity increasing with addition of [Co(NDS)2]2^" catalyst and primary alcohol, but the observation of the current intensity change at the reduction event of RuQ to RuQ^2" was thwarted by the reduction event of [Co(NDS)2]^2" ahead at -0.4 V (see Figure 8), which was extensively studied under CO2 elsewhere.[18] The electrochemical property of complex 1 in buffered aqueous solution was also studied, in which 2e^", 2H+ reduction of complex 1 into RuQI¾ should be occurred near the electrode as reported in the literature. [25] A reversible reduction peak at -0.36 V (£^"1_/2) could be also observed in the cyclic voltammogram (see Figure 9). The reversibility lost little when the solution was saturated with CO2 by bubbling, suggesting no interaction between RuQI¾ and CO2. The electrochemical studies of complex 1 confirm that two-electron reduction product, RuQ^2", is an important species that can combine with CO2 in aqueous solution, providing insight into initial CO2 activation in the present photocatalyst system.

Although the reduced complex 1 could react with CO2, no detectable methanol was produced without cobalt catalyst in the control experiments. Therefore, it could be expected that CO2 was captured by the photosensitizer and brought to the reduction center,

[Co(NDS)2]^2"' in our photocatalyst system. To gain insight into the pathway for methanol production from the present CO2 reduction system, preliminary exploration was conducted with a potential intermediate: ethyl formate was used as substrate instead of C0₂ gas (reaction protected by N₂). There was no signal attributed to methanol in the 1H NMR spectrum of the reaction solution before photoirradiation. Surprisingly, besides the peaks of ethanol produced from the hydrolysis of ethyl formate in acidic aqueous solution, sporadic amount of methanol was observed in the 1H NMR spectrum of the reaction solution after photoirradiation (see Figure 10). It could be imaged that the intermediate during ethyl formate hydrolysis coordinated to the cobalt complex in the reaction solution and was further reduced to methanol. In combination of the aforementioned results from photocatalytic C0₂ reduction with a primary alcohol as co-catalyst, the methanol generation in our system probably went through a key intermediate like reduced form of formate ester. Unfortunately, subsequent experiment with formaldehyde as substrate was thwarted by methanol generation by disproportionation from formaldehyde itself. [26]

With combination of the results from our experiments and the electrochemical studies on C0₂ reduction from other groups, [17, 18,22,26] a plausible process for C02 reduction to methanol in our photocatalyst system is illustrated in Scheme 2, shown below.

Upon irradiation with visible light, the photosensitizer RuQ collected two electrons from ascorbate anions to generate RuQ^2" which captured C02 in aqueous solution to form an adduct RuQ^"[ COOH]. One would expect an adduct with two molecules of C0₂, but the kinetic rate of addition of the second C0₂ molecule to the reduced quinone derivatives was really slow. [15] The hydroxyformyl radical then dissociated from the adduct RuQ^" [COOH], and inserted between the cobalt center and the primary alcohol to form a reduced hydroxyformate ester intermediate, [Co-OCH(OH)OR]. The reduced hydroxyformate ester intermediate could be subsequently converted into reduced formaldehyde and finally methanol with H20 as byproduct. Although addition of primary alcohol (MeOH or EtOH) to the aqueous solution of [Co(NDS)2]^2" did not result in any change in absorption spectra, the coordination between the cobalt center and an primary alcohol could not be excluded since the primary alcohol was shown as an important co-catalyst in the present photoctalyst system. Moreover, we re-considered the difference in the catalytic efficency between MeOH and EtOH as co-catalysts, which might also be caused by the different kinetic rate for the reduced hydroxyformate ester formation since esterification with MeOH is faster than that with EtOH due to the shorter chain of MeOH and its higher polarity. [27] The mechanism proposed here is highly speculated. There might be the electron transfer from dianionic RuQ^2~to the cobalt center since [Co(NDS)2]^2" is easy to be reduced in electrochemical studies and hydrocarbon substrates were then reduced by cobalt species. Further exploring the mechanism is in progress aiming at improving the efficiency of C0₂ conversion into methanol.

In summary, the foregoing experiments have shown that complex 1 can collect two electrons upon action of light, presumably to be located on the quinone moiety where they can combine with C0₂ and activate the C0₂ reduction cycle in aqueous solution. Visible light-driven C0₂ selective reduction to methanol initiated by complex 1 could be observed for the multi-component system of complex 1, [Co(NDS) ₂]^2~, a primary alcohol and H₂A in an aqueous solution under low pressure of C0₂ atmosphere. High total turnover numbers were observed: 73 observed based on complex 1 and nearly 4 based on [Co(NDS) ₂]^2~ indicating that the photochemical C0₂ reduction to MeOH is catalytic both for complex 1 and in the cobalt catalyst. Primary alcohols as co-catalysts had effect on methanol production with MeOH itself breaking the record with a catalytic activity much better than EtOH.

Although the efficiency is not very high at this stage, the results illustrate the working principle, i. e. , photoactive multi-electron collectors as electron mediator are superior to single electron transfer system for C0₂ reduction to hydrocarbon fuels such as methanol, potentially providing a solution to solar energy conversion into fuel, with mitigation the green house effect in the meanwhile. EQUIVALENTS

The invention described and claimed herein is not to be limited in scope by the specific embodiments herein disclosed, as these embodiments are intended as illustrative of several aspects of the invention. Any equivalent embodiments are intended to be within the scope of this invention, as they will become apparent to those skilled in the art from the present description. Such embodiments also intended to fall within the scope of the appended claims.

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Claims

What is claimed is:

1. A method of preparing methanol from carbon dioxide comprising:

combining a photoactive multi-electron collector and an electron donor in the presence of light to form a two electron reduction product; and

combining the two electron reduction product and carbon dioxide in the presence of one or more catalysts,

thereby converting carbon dioxide to methanol.

2. The method of claim 1, wherein the photoactive multi-electron collector is a ruthenium complex.

3. The method of claim 1, wherein the ruthenium complex further comprises a quinone moiety.

4. The method of claim 3, wherein the ruthenium complex is the complex set forth as Formula I:

5. The method of claim 1, wherein the two electron reduction product is the complex set forth as Formula II:

6. The method of claim 1, wherein the electron donor is selected from the group consisting of: ascorbic acid and water.

7. The method of claim 1, wherein the one or more catalysts are selected from the group consisting of: an alcohol and a cobalt catalyst.

8. The method of claim 1, wherein the catalysts are a primary alcohol and [Co(NDS)2]^2".

9. The method of claim 1, further comprising the addition of a co-catalyst.

10. The method of claim 1, wherein the co-catalyst is selected from the group consisting of: methanol and ethanol.

11. The method of claim 1, wherein the preparation of methanol from carbon dioxide in the presence of light is conducted at a temperature of 0-150 ° C.

12. The method of claim 1, wherein the light is visible light:

13. The method of claim 1, further comprising dehydrating the methanol thus produced to form dimethyl ether.

14. Methanol that is produced from carbon dioxide by the process comprising:

thereby producing methanol from carbon dioxide.

15. Methanol produced by the process of claim 14, wherein the photoactive multi-electron collector is a ruthenium complex.

16. Methanol produced by the process of claim 14, wherein the ruthenium complex further comprises a quinone moiety.

17. Methanol produced by the process of claim 16, wherein the ruthenium complex is the complex set forth as Formula I.

18. Methanol produced by the process of claim 14, wherein the two electron reduction product is the complex set forth as Formula II:

19. Methanol produced by the process of claim 14, wherein the electron donor is selected from the group consisting of: ascorbic acid and water.

20. Methanol produced by the process of claim 14, wherein the one or more catalysts are selected from the group consisting of: an alcohol and a cobalt catalyst.

21. Methanol produced by the process of claim 14, wherein the catalysts are a primary alcohol and [Co(NDS)2]2-.

22. Methanol produced by the process of claim 14, further comprising the addition of a co-catalyst.

23. Methanol produced by the process of claim 14, wherein the co-catalyst is selected from the group consisting of: methanol and ethanol.

24. Methanol produced by the process of claim 14, wherein the preparation of methanol from carbon dioxide in the presence of light is conducted at a temperature of 0-150 ° C.

25. Methanol produced by the process of claim 14, wherein the light is visible light.

26. Methanol produced by the process of claim 14, further comprising dehydrating the methanol thus produced to form dimethyl ether.