JPH07112945A - How to convert carbon dioxide - Google Patents

Abstract

(57) [Summary] [Purpose] A method of highly selectively converting carbon dioxide into methanol or formic acid by using a catalyst that responds to visible light having a high energy distribution to sunlight. [Constitution] Cuprous oxide is used as a catalyst. This catalyst may be supported on a carrier, or a noble metal catalyst (ruthenium, palladium) may be used in combination. When photoreduction is carried out in water, methanol is selectively produced, and when it is carried out in acetonitrile, formic acid is selectively produced.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for converting carbon dioxide. More specifically, it relates to a method for converting carbon dioxide into useful compounds such as methanol and formic acid by a photoreduction reaction in the presence of a catalyst.

[0002]

2. Description of the Related Art It is well known that global warming due to an increase in carbon dioxide concentration in the atmosphere is currently a major environmental problem. Therefore, consuming carbon dioxide and effectively utilizing it for chemical synthesis is not only an economical aspect that raw materials are cheap and a resource protection aspect that does not consume resources, but it is also very useful as a measure to prevent global warming. It is desirable.

The method of converting carbon dioxide can be classified into a catalytic hydrogenation reaction using thermal energy, an electrochemical reaction using electric energy, and a photochemical reaction using light energy.

The conversion of carbon dioxide by heat energy or electric energy is not preferable because energy of fossil fuel or the like is required to obtain the reaction energy and, at that time, secondary emission of carbon dioxide is involved. On the other hand, the conversion of carbon dioxide by light energy is extremely important from the standpoint of both economic efficiency and environmental protection, since inexhaustible and clean solar energy is directly used for conversion of chemical substances.

As the conversion reaction from carbon dioxide to a useful substance, for example, the following various reduction reactions are possible, but from the one-electron reduction process represented by the formula (1),
The multi-electron reduction process represented by equation (6) is thermodynamically advantageous.

[0006] The redox potential _{(vs. NHE) CO 2 + e} - → CO 2 - E 0 = -2.0 V ‥‥ (1) CO 2 + 2H + + 2e - → HCOOH E 0 = -0.61V ‥‥ ( ^{_{2) CO 2 + 2H + +}} 2e - → CO + H 2 OE 0 = -0.52V ‥‥ (3) CO 2 + 4H + + 2e - → HCHO + H 2 OE 0 = -0.48V ‥‥ (4) ^{_{CO 2 + 6H + + 2e -}} → CH 3 OH + H 2 OE 0 = -0.38V ‥‥ (5) CO 2 + 8H + + 2e - → CH 4 + 2H 2 OE 0 = -0.24V ‥‥ (6 In order to carry out the multi-electron reduction process by light, it is known that it is advantageous to use a semiconductor catalyst to photochemically generate a redox potential and perform multi-electron injection into carbon dioxide.

Photoreduction of carbon dioxide using a semiconductor catalyst has been carried out using a catalyst such as titanium oxide, strontium titanate, and silicon carbide [K. Honda et al., Nature, Vo.
l. 277, 637, (1979), and JP-A-4-122453. However, any of the catalysts requires ultraviolet light for photoexcitation, and is not satisfactory for effectively utilizing sunlight having a large energy distribution in the visible light region. Further, in the photoreduction reaction, there is a problem that the reaction selectivity of the product is low.

In the reduction reaction of carbon dioxide by visible light using a semiconductor catalyst, a method using cadmium sulfide as a catalyst has been tried [BR Eggins et al., J. Chem. Soc., Chem.
Commun., 1123 (1988)]. In this case, the reaction product will be a mixture of glycolic acid, acetic acid, formic acid, formalin, etc., so not only the reaction product needs to be separated, but also the cadmium used in the catalyst may elute, which may increase safety. Also had a problem.

Visible light can also be used in the photoreduction reaction of carbon dioxide using a metal complex, and Ru (bipy) ₃ ²⁺ (b
Many systems using ipy = bipyridyl) as photosensitizers have been reported [eg, JL Grant et al., J. Chem. Soc., Dalto
n Trans., 2105 (1987), and H. Ishida et al., Chem.
Lett., 1035 (1987)]. However, in the metal complex system, the presence of an electron relay and a co-catalyst is required, and the reaction product is a low reduction product such as carbon monoxide or formic acid, and a high reduction product such as methanol is given. Is extremely small. In addition, reduction in reaction activity due to decomposition of the metal complex itself has been pointed out.

[0010]

[Problems to be Solved by the Invention] The conversion of carbon dioxide is
It is important as one of the measures to prevent global warming. Therefore,
Exhausting the energy of the conversion from inexhaustible and clean sunlight and reducing carbon dioxide to useful methanol, formic acid, etc. does not involve new energy consumption during conversion, and the product is an intermediate raw material for chemicals. In particular, since methanol can be used as a fuel for automobiles, it is considered to be an extremely effective means for reducing carbon dioxide.

In consideration of the above points, an object of the present invention is to provide a method for highly selectively converting carbon dioxide into methanol or formic acid by using a catalyst which responds to visible light having a high energy distribution in sunlight. It is to be.

[0012]

In order to solve the above-mentioned problems, the present inventors have used a variety of oxide and sulfide semiconductors having absorption in the visible light region to provide a photocatalyst for the reduction of carbon dioxide. As a result of trial and error about the action, it was found that cuprous oxide photoreduces carbon dioxide in water and acetonitrile solutions to give methanol and formic acid, respectively, and the present invention was completed.

Here, the gist of the present invention is a method for converting carbon dioxide by photoreduction in the presence of a catalyst, which comprises using cuprous oxide as a catalyst.

[0014]

According to the present invention, by photochemically reducing carbon dioxide in the presence of a cuprous oxide catalyst, methanol,
Manufacture useful chemicals such as formic acid.

The cuprous oxide used in this reaction is copper acetate (I
I), copper (II) hydroxide, copper (II) sulphate, copper (II) chloride, etc. An aqueous solution of a divalent copper compound is reacted with a reducing agent such as hydrazine, carbon monoxide or hydrogen to cause reductive precipitation, or Or copper (I) chloride
, Copper iodide, copper (I) thiocyanate, and the like are obtained by subjecting an aqueous solution of a monovalent copper compound to a hydrolysis reaction to cause precipitation. In particular, cuprous oxide obtained by reduction of copper (II) acetate with hydrazine is preferable as a catalyst.

The cuprous oxide catalyst used is preferably in the form of fine powder having a particle size of 10 μm or less in order to exhibit good activity.
In order to prepare such fine powdery cuprous oxide, the concentration of the precursor copper compound aqueous solution may be made as dilute as possible.

Cuprous oxide is preferably used by supporting it on a suitable carrier in order to improve reaction efficiency and reaction selectivity. As the carrier, conventional carrier materials such as alumina, silica, titania, zeolite and carbon are used. The photocatalyst can be supported on the carrier by a kneading method, an impregnation method, a precipitation method, an ion exchange method, or the like. The amount of the catalyst loaded on the carrier is preferably 10 wt% or less, which is considered to have good dispersibility in the carrier system. More preferably, it is 5 wt% or less. Also when used as a supported catalyst, the carrier itself is preferably in the form of fine powder as described above.

When a noble metal catalyst is carried on the catalyst carrier in addition to cuprous oxide, the catalytic activity is improved. As the noble metal catalyst, a noble metal such as palladium or ruthenium can be used. As the supporting method, a kneading method or a photoreduction method of a noble metal salt is used. The noble metal catalyst and the cuprous oxide catalyst may be supported on the same carrier or different carriers, but it is preferable to support them on the same carrier in terms of reaction efficiency. Further, a noble metal catalyst can be supported on the cuprous oxide of the catalyst.

The amount of the noble metal supported is preferably 5 wt% or less.
When it exceeds 5 wt%, the dispersibility of the supported metal is poor and the catalyst itself tends to aggregate, so that the photoreaction itself becomes difficult to proceed.

To carry out the photoreduction reaction according to the present invention, a catalyst solution prepared by dispersing the prepared cuprous oxide catalyst in a suitable liquid is charged into a reaction vessel. As the liquid used as the dispersion medium of this catalyst, water, acetonitrile, THF, acetone, butyronitrile or the like can be used because of its redox potential, but water or acetonitrile is particularly preferable. As described above, when the cuprous oxide catalyst is dispersed in water, methanol is produced by photoreduction of carbon dioxide, and when dispersed in acetonitrile, formic acid is produced.

As a reaction method, first, carbon dioxide is bubbled through this catalyst liquid to absorb CO ₂ in the liquid. Even if the catalyst is present in the liquid, simply bubbling carbon dioxide into the catalyst dispersion does not reduce carbon dioxide. The catalyst dispersion liquid that has absorbed carbon dioxide is then irradiated with light while the catalyst is suspended in the liquid by stirring. The catalyst absorbs the irradiated light and is excited, and reduction occurs as shown in the above reaction formula, and carbon dioxide is converted into useful chemical substances such as methanol and formic acid.

Regarding light irradiation, in principle, the photoreduction reaction also proceeds by ultraviolet light, but it is desirable to irradiate sunlight with visible light (> 400 nm) having a large energy distribution.

If stirring is stopped during this irradiation, the catalyst settles down and the amount of light absorbed by the system itself decreases, which is disadvantageous in terms of reaction efficiency. The reaction temperature is preferably room temperature (about 25 ° C) or lower. It should be noted that the absorption of carbon dioxide and the conversion of carbon dioxide by light irradiation can be carried out simultaneously in parallel instead of sequentially in one reactor as described above. In addition, two reactors are prepared, carbon dioxide is absorbed by the catalyst liquid in the first reactor, and the catalyst liquid that has absorbed the carbon dioxide is transferred to the second reactor, where it is irradiated with light to oxidize carbon dioxide. It is also possible to carry out photoreduction of carbon. The catalyst liquid in which carbon dioxide has been consumed may be circulated from the second reactor to the first reactor for use. In this way, the carbon dioxide conversion method of the present invention can be carried out by either a batch system or a continuous system. Reaction products such as methanol and formic acid are dissolved in the catalyst liquid. The recovery can be performed, for example, by separating the catalyst from the catalyst liquid and then separating the reaction product by an appropriate means such as distillation.

[0024]

【Example】

(Example 1) 2.0 g of copper acetate monohydrate (manufactured by Aldrich)
It is dissolved in 50 cc of distilled water, 2 ml of a 10% aqueous solution of hydrazine monohydrate (Tokyo Kasei) is added dropwise, and the mixture is stirred for several hours in an ice bath. The obtained yellow suspension was washed 5 times by decantation using a centrifuge to obtain 10 ml of an aqueous catalyst solution containing cuprous oxide in a suspended state. The average particle size of the cuprous oxide particles was about 2 μm.

Next, 0.03 ml (catalyst amount of about 2.0 mg) taken out from the aqueous catalyst solution containing cuprous oxide was added to 2 ml of an aqueous solution containing triethanolamine 1.0M, and carbon dioxide was added at 0 ° C. for 30 minutes. I bubbled. Then, while stirring at 25 ° C, a wavelength of 400 nm was passed from a 300 W halogen lamp through a saturated sodium nitrite solution filter.
The above light was applied. The reaction product was analyzed by gas chromatography (column: PEG1000).

As a result, methanol was produced at 0.021 g / h per 1.0 g of cuprous oxide used as a catalyst, and a trace amount of hydrogen was produced. Other reduction products (carbon monoxide,
Formic acid and methane) did not form.

Example 2 From the cuprous oxide suspension prepared in the same manner as in Example 1, water was removed by decantation. After adding acetonitrile to this residue, acetonitrile was removed by decantation using a centrifugal separator again. This operation was repeated 3 times to obtain 10 ml of a catalyst liquid containing cuprous oxide in a suspended state.

For the photoreduction of carbon dioxide, the reaction solution was changed to acetonitrile and the analysis was carried out by ion chromatography.
The procedure was performed in the same manner as in Example 1 except that [Column: Toso TSC gel-SCX (H +)] was changed. As a result, 0.036 g / h of formic acid was produced per 1.0 g of cuprous oxide used as a catalyst. Other reduction products (carbon monoxide, methanol, methane) were not produced.

Example 3 Copper acetate monohydrate 0.5 g 5.3 g
25c containing silica (made by Aldrich, average particle size 10μm)
It was dissolved in distilled water of c, 2 ml of a 10% aqueous solution of hydrazine monohydrate was added dropwise, and the mixture was stirred in an ice bath for several hours. The resulting precipitate was filtered, washed thoroughly with distilled water and ethanol, and vacuum dried in a desiccator. As a result, 5.25 g of cuprous oxide / silica having silica as a carrier was obtained.

Photocatalytic reduction of carbon dioxide was carried out in the same manner as in Example 1 by using 83 mg of the silica-supported catalyst thus prepared (about 2 mg of cuprous oxide equivalent). As a result, the catalyst cuprous oxide
0.034 g / h of methanol was produced per 1.0 g.

Example 4 Carbon dioxide was photoreduced in the same manner as in Example 3 except that the carrier was changed to alumina (made by Aldrich). 0.041 g / h of methanol was produced per 1.0 g of cuprous oxide used as a catalyst.

Example 5 The cuprous oxide suspension obtained by the method described in Example 1 was filtered and dried, and then the obtained cuprous oxide (0.65 g) and palladium black (3 wt%) were added. It was put in a mortar and kneaded well for 20 minutes.

Photocatalytic reduction of carbon dioxide was carried out in the same manner as in Example 1 using 2 mg of the catalyst thus prepared, which had palladium supported on cuprous oxide. As a result, cuprous oxide used as a catalyst
0.033 g / h of methanol was produced per 1.0 g.

Comparative Example 1 Photoreduction of carbon dioxide was carried out in the same manner as in Example 1 except that cadmium sulfide (manufactured by Aldrich, purity 99.999%) was used as the catalyst. as a result,
Formic acid, carbon monoxide and hydrogen were produced at 0.028 g / h, 0.043 g / h and 0.009 g / h, respectively, per 1.0 g of the catalyst.
That is, with this catalyst, the selectivity of the reaction was low, and various reaction products were produced.

Comparative Example 2 A photoreaction was performed in the same manner as in Example 1 except that gallium phosphide (made by Strem) was used as a catalyst and triethanolamine was replaced with isopropyl alcohol. As a result, methanol is 0.00 per 0.00 g of catalyst.
6 g / h produced.

[0036]

As is clear from the results of the examples, according to the present invention, by using cuprous oxide as a photocatalyst and irradiating sunlight with visible light having a large energy distribution, carbon dioxide is removed at room temperature. Can be converted. Therefore, the present invention provides a method for treating carbon dioxide that prevents global warming without using new energy consumption and secondary carbon dioxide emission by using inexhaustible solar energy. it makes sense.

Moreover, in the photoconversion of carbon dioxide by the method of the present invention, formic acid and further a highly reduced product, methanol, are selectively produced without producing other products as by-products. These reaction products are used as intermediate raw materials for various chemically synthesized products, and since methanol is particularly drawing attention as a next-generation automobile fuel, the present invention provides a simple and inexpensive method for producing methanol and formic acid. It is also effective as a method.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C07C 53/02 // C07B 61/00 300

Claims (3)

[Claims] 1. A method for converting carbon dioxide by photoreduction in the presence of a catalyst, which comprises using cuprous oxide as a catalyst. 2. The method of claim 1, wherein the catalyst comprises cuprous oxide supported on a carrier. 3. The method according to claim 2, wherein a noble metal catalyst supported on a carrier is also used.