JP2022053181A - Oxygen carrier and method for producing gas - Google Patents

Abstract

To provide an oxygen carrier that uses copper (Cu), which is an inexpensive metal element, as an activity species, but selects an appropriate second element for high conversion efficiency of carbon dioxide to carbon monoxide, and is economically advantageous, and a method for producing a gas using the oxygen carrier.SOLUTION: The inventive oxygen carrier is brought into contact with a raw material gas containing carbon dioxide to reduce the carbon dioxide, generating a product gas containing carbon monoxide, wherein the oxygen carrier contains a metal oxide containing copper (Cu) and a second element different from copper, and wherein the metal oxide has a crystal phase of copper oxide and a crystal phase of oxides of the second element.SELECTED DRAWING: None

Description

The present invention relates to an oxygen carrier and a method for producing a gas, and more particularly to, for example, an oxygen carrier that can be used in a chemical looping method, and a method for producing a gas using such an oxygen carrier.

In recent years, the concentration of carbon dioxide, which is a kind of greenhouse gas, has been increasing in the atmosphere. Increased concentrations of carbon dioxide in the atmosphere contribute to global warming. Therefore, it is important to recover the carbon dioxide released into the atmosphere, and if the recovered carbon dioxide can be converted into valuable substances and reused, a carbon cycle society can be realized.
Conventionally, as a method for producing carbon monoxide from carbon dioxide, a method using a reverse water-gas shift reaction is known. However, in this conventional water-gas shift reaction, carbon monoxide, which is a product, and water coexist in the system, so that the conversion efficiency of carbon dioxide to carbon monoxide is low due to the restriction of chemical equilibrium. There was a problem.

Therefore, in order to solve the above problem, conversion (synthesis) of carbon monoxide from carbon dioxide is performed using a chemical looping method. The chemical looping method referred to here divides the above-mentioned reverse water-gas shift reaction into two reactions, a reduction reaction with hydrogen and a reaction for producing carbon monoxide from carbon dioxide, and these reactions are divided into metal oxides (MO). It is a method of bridging by _x ) (see the formula below).
H ₂ + MO _x → H ₂ O + MO _x-1
CO ₂ + MO _x-1 → CO + MO _x
In the above formula, MO _x-1 indicates a state in which a part or all of the metal oxide is reduced.

In the chemical looping method, since water and carbon monoxide, which are substrates for the reverse reaction, do not coexist during each reaction, it is possible to obtain a conversion efficiency of carbon dioxide to carbon monoxide, which is higher than the chemical equilibrium of the reverse water-gas shift reaction. There is sex.
In this chemical looping method, cerium oxide or the like, which easily causes oxygen deficiency, is widely used as the metal oxide that bridges the reaction.
For example, Patent Document 1 describes that the use of cerium oxide in which zirconium is dissolved is more likely to cause oxygen deficiency than cerium oxide alone.

Patent No. 5858926

If the metal oxide described in Patent Document 1 is used, carbon monoxide can certainly be produced from carbon dioxide. However, cerium, which is an active species, is a rare earth metal and has a problem in that it is expensive, which is economically disadvantageous.
The present invention has been made in view of such circumstances, and an object thereof is to select an appropriate second element while using copper (Cu), which is an inexpensive metal element, as an active species. It is an object of the present invention to provide an oxygen carrier having high conversion efficiency of carbon dioxide to carbon monoxide and economically advantageous, and to provide a method for producing a gas using such an oxygen carrier.

Such an object is achieved by the following invention.
(1) The oxygen carrier of the present invention is an oxygen carrier used for producing a produced gas containing carbon monoxide by reducing the carbon dioxide by contacting it with a raw material gas containing carbon dioxide. ,
It contains a metal oxide containing copper (Cu) and a second element different from copper, and the metal oxide has a crystal phase of copper oxide and a crystal phase of the oxide of the second element. It is characterized by that.

(2) In the oxygen carrier of the present invention, when the metal oxide is measured by an X-ray diffraction method, at least two diffraction peaks are observed in the range of 2θ of 60 to 65 ° in the X-ray diffraction profile. Is preferable.
(3) In the oxygen carrier of the present invention, the diffraction peak having the highest diffraction intensity and the diffraction peak having the second highest diffraction intensity are selected from the at least two diffraction peaks, and the above two diffraction peaks are described. In the X-ray diffraction profile, when the diffraction intensity of the diffraction peak having the smaller value of 2θ is A and the diffraction intensity of the diffraction peak having the larger value is B, the A / B is 0.05 to 2. Is preferable.

(4) In the oxygen carrier of the present invention, the molar ratio of the second element to the total of the copper and the second element in the metal oxide is preferably 40 to 90 mol%.
(5) In the oxygen carrier of the present invention, the second element is preferably zinc (Zn).
(6) The oxygen carrier of the present invention preferably further contains a binder that binds the metal oxide.

(7) The oxygen carrier of the present invention preferably further contains a binder that binds the metal oxide.
(8) In the oxygen carrier of the present invention, the amount of the binder is preferably 70 parts by mass or less with respect to 100 parts by mass of the oxygen carrier.
(9) The method for producing a gas of the present invention is to reduce the carbon dioxide by contacting the oxygen carrier of the present invention with a raw material gas containing carbon dioxide to produce a produced gas containing carbon monoxide. It is characterized by.

According to the present invention, a produced gas containing carbon monoxide can be efficiently produced from a raw material gas containing carbon dioxide. Further, the oxygen carrier of the present invention is inexpensive and can be used, for example, in a chemical looping method.

Hereinafter, the method for producing an oxygen carrier and a gas of the present invention will be described in detail based on a preferred embodiment.
[Oxygen carrier]
The oxygen carrier of the present invention is used in the production of a produced gas containing carbon monoxide by reducing carbon dioxide by contacting it with a raw material gas containing carbon dioxide. Further, the oxygen carrier can be reduced (regenerated) by bringing the reducing gas into contact with the oxidized oxygen carrier.
At this time, preferably, carbon dioxide is converted to carbon monoxide by the oxygen carrier by alternately passing the raw material gas and the reducing gas into the reaction tube (reaction vessel) filled with the oxygen carrier of the present invention. The oxygen carrier in the oxidized state is regenerated by the reducing gas.

The metal oxide in the oxygen carrier of the present invention is a compound capable of causing a reversible oxygen deficiency, and the oxygen element is deficient by reduction from itself, but in a state where the oxygen element is deficient (reduced state), it is combined with carbon dioxide. A compound that, when in contact with it, has the effect of depriving carbon dioxide of oxygen elements and reducing them.
The oxygen carrier of the present invention contains a metal oxide containing copper (Cu) and a second element M different from copper (hereinafter, also simply referred to as "element M"). Such a metal oxide may be in a crystalline state by dissolving the element M in copper oxide, but the crystal phase of copper oxide (crystal grain boundary) and the crystal phase of the oxide of element M (crystal grain boundary). It is preferable that these crystal phases (crystal grain boundaries) are bonded to each other via an amorphous phase (atypical phase) containing an oxygen element. Is more preferable.

In this case, the bond of Cu—OM can be highly activated by selecting an appropriate element M. As a result, the metal oxide is more efficiently desorbed from the oxygen element by contact with the reducing gas to induce an oxygen deficiency, and a lattice defect is generated, and the strain of the lattice defect deprives the carbon dioxide of the oxygen element. It is thought that it will be easier.
In addition, it is difficult for copper oxide alone to adsorb to the raw material gas (carbon dioxide) and the reducing gas (hydrogen). On the other hand, the metal oxide is likely to be adsorbed with the raw material gas or the reducing gas, and the contact time with carbon dioxide or hydrogen can be maintained for a long time. As a result, the efficiency of conversion of carbon dioxide to carbon monoxide by oxygen carriers can be increased.

Such a metal oxide is an oxide that can be represented by Cu _{1-x (} M) _x Oy.
Here, x is a positive real number, preferably 0.3 to 0.9, and more preferably 0.5 to 0.8. In this case, lattice defects due to oxygen deficiency are more likely to occur in the metal oxide, and the degree of distortion of the lattice defects is likely to be appropriate.
Further, y is an integer of 1 to 4, preferably an integer of 1 to 3, and more preferably an integer of 1 to 2. In this case, the stability of the metal oxide tends to be improved.

As the element M, it is preferable to select an element having an electronegativity close to the electronegativity of Cu (1.90) and an element having an electronegativity lower than that of Cu. This makes it easy to increase the degree of activation of the Cu—OM bond.
The specific value of the electronegativity of the element M is preferably 1.10 to 2.33, more preferably 1.30 to 1.93, and 1.55 to 1.83. Is even more preferable.

The element M is preferably at least one of the elements belonging to the third to sixth cycles, and more preferably at least one of the elements belonging to the third to fifth cycles. In this case, since the ionic radius of the element M and the ionic radius of Cu can be brought close to each other, it is easy to synthesize a highly stable metal oxide.
Further, the element M is preferably at least one of the elements belonging to the 2nd to 14th groups, and more preferably at least one of the elements belonging to the 4th to 14th groups. , 8th group, 12th group and 13th group are more preferably at least one of the elements. The present inventors have found that the efficiency of conversion of carbon dioxide to carbon monoxide by a metal oxide (oxygen carrier) can be particularly enhanced by selecting these elements M.

From the above, the element M includes, for example, aluminum (Al), silicon (Si), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), and cobalt. (Co), nickel (Ni), gallium (Ga), germanium (Ge), samarium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), indium (In), tin (Sn), ruthenium (Hf), tantalum (Ta), lanthanum (La), cerium (Ce), samarium (Sm), zinc (Zn) , Gadrinium (Gd), Scandium (Sc) and the like, and one or more of these can be selected.
Among these, the element M is at least one selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Zr, Nb, Ag, Hf, In and Ta. It is preferably a species, and more preferably at least one selected from the group consisting of Al, Fe and Zn.

The oxygen carrier of the present invention may be composed only of a metal oxide, or may contain a binder (carrier) for binding the metal oxide. By including such a binder, the shape of the oxygen carrier can be stably maintained when it is formed into a molded product.
The binder may be any as long as it is difficult to denature depending on the raw material gas, reaction conditions and the like, and is not particularly limited. Specific examples of the binder include carbon materials (graphite, graphene, etc.), zeolite, montmorillonite, SiO ₂ , ZrO ₂ , TiO ₂ , V ₂ O ₅ , MgO, Al ₂ O ₃ , or a composite oxide containing these. And so on. Among these, the binder is preferably at least one selected from the group consisting of zeolite, Al ₂ O ₃ , TIO ₂ , SiO ₂ , ZrO ₂ and MgO. This is because these binders have excellent binding ability of metal oxides.

Even when the oxygen carrier of the present invention contains a binder, the amount thereof is preferably 70 parts by mass or less, more preferably 50 parts by mass or less, based on 100 parts by mass of the oxygen carrier. It is more preferably 30 parts by mass or less, and particularly preferably 10 parts by mass or less. The amount of the binder is preferably 2 parts by mass or more, and more preferably 5 parts by mass or more with respect to 100 parts by mass of the oxygen carrier.
By setting the amount of the binder contained in the oxygen carrier within the above range, it is possible to promote the conversion of carbon dioxide into carbon monoxide by the oxygen carrier, that is, the conversion efficiency can be sufficiently further improved. Further, it is possible to further improve the shape stability (mechanical strength) when the oxygen carrier is used as a molded product.

The packing density of the oxygen carrier is preferably 1.1 g / mL or less, more preferably 0.4 to 1 g / mL, and even more preferably 0.5 to 0.9 g / mL. If the filling density is too low, the gas passing speed becomes too high, and the time for contact between the oxygen carrier and the raw material gas and the reducing gas is reduced. As a result, the conversion efficiency of carbon dioxide into carbon monoxide by the oxygen carrier and the regeneration efficiency of the oxygen carrier in the oxidized state by the reducing gas tend to decrease. On the other hand, if the filling density is too high, the passing speed of the gas becomes too slow, the reaction becomes difficult to proceed, and it takes a long time to produce the produced gas.

The pore volume of the oxygen carrier is preferably 0.4 cm ³ / g or more, more preferably 1 to 30 cm ³ / g, and even more preferably 5 to 20 cm ³ / g. If the pore volume is too small, it becomes difficult for the raw material gas and the reducing gas to enter the inside of the oxygen carrier. As a result, the contact area between the oxygen carrier and the raw material gas and the reducing gas is reduced, and the conversion efficiency of carbon dioxide into carbon monoxide by the oxygen carrier and the regeneration efficiency of the oxygen carrier in the oxidized state by the reducing gas are likely to decrease. On the other hand, even if the pore volume is increased beyond the upper limit, no further increase in the effect can be expected, and the mechanical strength tends to decrease depending on the type of oxygen carrier.

The shape of the oxygen carrier is not particularly limited, but for example, granules are preferable. If it is granular, it is easy to adjust the packing density of the oxygen carrier within the above range.
Here, the term "granular" is a concept including powder, particle, lump, pellet, etc., and the form thereof may be spherical, plate-like, polygonal, crushed, columnar, needle-like, scale-like, or the like. ..
The average particle size of the oxygen carrier is preferably 0.1 μm to 5 mm, more preferably 0.5 μm to 1 mm, and even more preferably 1 μm to 0.5 mm. If the oxygen carrier has such an average particle size, the packing density tends to be in the above range.

In the present specification, the average particle size means the average value of the particle size of any 200 oxygen carriers in one field of view observed with an electron microscope. At this time, the "particle size" means the maximum length of the distance between two points on the contour line of the oxygen carrier. When the oxygen carrier is columnar, the maximum length of the distance between two points on the contour line of the end face is defined as "particle size". The average particle size is, for example, lumpy, and when the primary particles are agglomerated, it means the average particle size of the secondary particles.
The BET specific surface area of the oxygen carrier is preferably 1 to 500 m ² / g, more preferably 3 to 450 m ² / g, and even more preferably 5 to 400 m ² / g. When the BET specific surface area is within the above range, it becomes easy to improve the conversion efficiency of carbon dioxide into carbon monoxide by oxygen carriers.

Further, in the present invention, since the strain of the lattice defect of the metal oxide can be sufficiently increased, the oxygen capacity of the oxygen carrier is high in a wide range from low temperature (about 400 ° C.) to high temperature (about 650 ° C.). Can be maintained at. That is, the oxygen carrier can efficiently convert carbon dioxide into carbon monoxide over a wide temperature range.
The oxygen capacity of the oxygen carrier at 400 ° C. is preferably 1 to 40% by mass, more preferably 2 to 30% by mass. If the oxygen capacity of the oxygen carrier at a low temperature is within the above range, it means that the oxygen capacity is sufficiently high even at the actual operating temperature (about 650 ° C.), and the conversion efficiency of carbon dioxide to carbon monoxide is high. It can be said that it is an extremely high oxygen carrier.

In the oxygen carrier of the present invention, it is preferable that at least two diffraction peaks are observed in the range of 2θ of 60 to 65 ° in the X-ray diffraction profile when the metal oxide is measured by the X-ray diffraction method. Here, in the present specification, a diffraction peak is defined as a peak whose half width at half maximum is 1 ° or less among the peaks in the X-ray diffraction profile. The number of diffraction peaks may increase or decrease depending on the number of elements M.
When at least two diffraction peaks are observed in the range of 2θ of 60 to 65 °, these diffraction peaks include a diffraction peak derived from copper oxide and a diffraction beak derived from an oxide of the element M, and each oxidation It can be determined that the crystal state of the crystal phase of the object is good. It can also be considered that the crystal phases of each oxide are uniformly dispersed and exist in the metal oxide. In this case, the Cu—OM bond is activated to a higher degree, and the conversion efficiency from carbon dioxide to carbon monoxide is further enhanced.

Further, the diffraction peak having the highest diffraction intensity and the diffraction peak having the second highest diffraction intensity are selected from at least two diffraction peaks, and the value of 2θ is smaller in the X-ray diffraction profile among the two diffraction peaks. When the diffraction intensity of the one diffraction peak is A and the diffraction intensity of the larger diffraction peak is B, the A / B is preferably 0.05 to 2, and more preferably 0.1 to 1. It is preferably 0.1 to 0.6, and more preferably 0.1 to 0.6. When A / B satisfies the above range, copper and element M are present in an appropriate ratio in the metal oxide, so that the above effect can be further improved.
The specific value of the diffraction intensity A is preferably 300 to 4000 cps, more preferably 400 to 3500 cps, and the specific value of the diffraction intensity B is preferably 100 to 3000 cps, more preferably 500 to 2500 cps.

In the metal oxide, the molar ratio of the element M to the total of the copper and the element M is preferably 40 to 90 mol%, more preferably 50 to 80 mol%. With such a metal oxide containing a sufficient amount of element M, the degree of activation of the Cu—OM bond can be further increased.
Under the above conditions, zinc (Zn) is preferable as the element M from the viewpoint of particularly increasing the conversion efficiency of carbon dioxide to carbon monoxide.
According to the present invention, while copper (Cu), which is an inexpensive metal element, is used as an active species, by selecting an appropriate element M, the conversion efficiency of carbon dioxide to carbon monoxide is high and the economy is economical. An advantageous oxygen carrier can be obtained.

[Manufacturing method of oxygen carrier]
Next, a method for manufacturing an oxygen carrier will be described.
The metal oxide can be synthesized by, for example, a coprecipitation method, a sol-gel method, a solid phase method, a hydrothermal synthesis method, or the like.
As an example, the metal oxide can be synthesized as follows, for example. First, an aqueous solution of a raw material is prepared by dissolving salts of a plurality of elements constituting a metal oxide (hereinafter, also referred to as “constituent elements”) in water. Then, the raw material aqueous solution and the alkaline aqueous solution are added dropwise to water to obtain a mixed solution. Then, after recovering the precipitate formed in this mixed solution, it is dried and fired. That is, the metal oxide in the present invention can be easily and surely synthesized by the so-called coprecipitation method.

Examples of the salt of the constituent element include nitrate, sulfate, chloride, hydroxide, carbonate or a composite thereof, and among these, nitrate is preferable. Further, as the salt of the constituent element, a hydrate may be used if necessary.
Examples of the alkali include sodium carbonate, potassium carbonate, rubidium carbonate, cesium carbonate, sodium hydroxide, potassium hydroxide, ammonia, urea and the like.
The dropping speed of the raw material aqueous solution into water is not particularly limited, but is preferably 0.1 to 50 mL / min, and more preferably 1 to 25 mL / min. When the raw material aqueous solution is dropped at such a dropping rate, a precipitate containing each constituent element is individually generated, and it is possible to prevent aggregation and generate a sufficient amount of a precipitate containing copper and the element M. ..

The concentration of the raw material aqueous solution and the concentration of the alkaline aqueous solution are not particularly limited, but are preferably 0.1 to 5 mol / L, and more preferably 0.25 to 2.5 mol / L. By using an aqueous solution having such a concentration, a precipitate containing each constituent element can be individually generated and prevented from agglomerating, and a sufficient amount of a precipitate containing copper and the element M can be produced. ..
The temperature of the mixed solution is also not particularly limited, but is preferably 20 to 90 ° C, more preferably 40 to 70 ° C. By adjusting to such a temperature, a precipitate containing copper and element M can be smoothly formed.
It is preferable to stir the mixed solution for a predetermined time in order to promote the formation (ripening) of the precipitate containing copper and the element M. in this case. The stirring speed of the mixed solution is preferably 50 rpm or more, more preferably 75 to 100 rpm. The stirring time is preferably 0.5 to 5 hours, more preferably 1 to 3 hours.

The precipitate may be dried at a temperature of preferably 20 to 200 ° C., more preferably 50 to 150 ° C., preferably 0.5 to 20 hours, more preferably 1 to 15 hours.
The calcining of the precipitate is preferably carried out at a temperature of preferably 300 to 1200 ° C., more preferably 350 to 800 ° C., preferably 1 to 24 hours, and more preferably 1.5 to 20 hours.
Until the firing temperature is reached, the temperature may be raised at a heating rate of 1 to 20 ° C./min, preferably 2 to 10 ° C./min. As a result, it is possible to promote the growth of the crystal phase of copper oxide and avoid cracking of the metal oxide.
The metal oxide obtained at this stage may be used as it is, or may be pulverized (classified if necessary) to obtain the oxygen carrier of the present invention, or the molded product obtained through the following steps may be used as the oxygen carrier. ..

Next, the obtained metal oxide (particles or lumps) is pulverized, if necessary, and then mixed with the particles of the binder to obtain a mixture. Then, the mixture is fired to obtain a sintered body, and the sintered body is crushed and then classified as necessary. As a result, an oxygen carrier formed by binding the metal oxide with a binder can be obtained.
The mixture is fired at a temperature of preferably 300 to 1200 ° C, more preferably 350 to 800 ° C, still more preferably 350 to 500 ° C, preferably 1 to 24 hours, more preferably 1.5 to. It is recommended to do it in 20 hours.
The sintered body can be pulverized by using, for example, a jet mill, a ball mill, a bead mill, a rod mill, or the like.

[How to use oxygen carrier]
As described above, the oxygen carrier of the present invention can be used, for example, in a chemical looping method. Further, as described above, the oxygen carrier of the present invention can be used for the purpose of reducing carbon dioxide.
More specifically, it is preferable to carry out a carbon dioxide reduction reaction and an oxygen carrier reduction reaction, and the oxygen carrier should be used so as to circulate between the carbon dioxide reduction reaction and the oxygen carrier reduction reaction. Is preferable. In the reduction reaction of the oxygen carrier, another reducing agent (reducing gas) is used.

Further, the oxygen carrier of the present invention is preferably used for a so-called reverse water-gas shift reaction. The reverse water-gas shift reaction is a reaction that produces carbon monoxide and water from carbon dioxide and hydrogen. When the chemical looping method is applied, the reverse water-gas shift reaction is divided into an oxygen carrier reduction reaction (first process) and a carbon dioxide reduction reaction (second process), and the oxygen carrier reduction reaction is as follows. The reaction is represented by the formula (A), and the carbon dioxide reduction reaction is the reaction represented by the following formula (B).

H ₂ (gas) + Cu _1-x (M) _{x Oy} (solid)
→ H ₂ O (gas) + Cu _1-x (M) _{x Oyn} (solid) (A)
CO ₂ (gas) + Cu _1-x (M) _{x Oyn} (solid)
→ CO (gas) + Cu _1-x (M) _{x Oy} (B)
In the formulas (A) and (B), n is usually a value smaller than 2, preferably 0.05 to 1.7, and more preferably 0.1 to 1.5. It is more preferably 0.15 to 1.3.
That is, in the reduction reaction of oxygen carriers, hydrogen, which is a kind of reducing gas, is oxidized to generate water. Further, in the carbon dioxide reduction reaction, carbon dioxide is reduced to produce carbon monoxide.

The reaction temperature in the reduction reaction of the oxygen carrier may be any temperature at which the reduction reaction can proceed, but is preferably 300 ° C. or higher, more preferably 400 ° C. or higher, still more preferably 500 ° C. or higher. It is particularly preferable that the temperature is 550 ° C. or higher. Within such a temperature range, an efficient reduction reaction of oxygen carriers can proceed.
The upper limit of the reaction temperature is preferably 850 ° C. or lower, more preferably 750 ° C. or lower, and even more preferably 700 ° C. or lower. By setting the upper limit of the reaction temperature within the above range, economic efficiency can be improved.

The reaction temperature in the carbon dioxide reduction reaction is preferably 300 ° C. or higher, more preferably 350 ° C. or higher, and even more preferably 400 ° C. or higher. Within such a temperature range, an efficient carbon dioxide reduction reaction can proceed.
The upper limit of the reaction temperature is preferably 1000 ° C. or lower, more preferably 850 ° C. or lower, further preferably 700 ° C. or lower, and particularly preferably 650 ° C. or lower. Since the oxygen carrier can carry out the reduction reaction of carbon dioxide to carbon monoxide with high efficiency even at a low temperature, the reduction reaction of carbon dioxide can be set to a relatively low temperature. Further, by setting the upper limit of the reaction temperature in the above range, not only the waste heat can be easily utilized, but also the economic efficiency can be further improved.

In the present invention, the reduced product obtained by the reduction reaction of carbon dioxide may be a substance other than carbon monoxide, and specific examples thereof include methane. It is preferable that the reduced product such as carbon monoxide obtained by the reduction reaction of carbon dioxide is further converted into an organic substance or the like by microbial fermentation or the like. Examples of microbial fermentation include anaerobic fermentation. Examples of the obtained organic substance include methanol, ethanol, acetic acid, butanol, derivatives thereof, mixtures thereof, compounds of C5 or higher such as isoprene, and the like.
Further, the reduced product such as carbon monoxide may be converted into a compound from C1 to C20 containing a hydrocarbon and an alcohol conventionally synthesized by petrochemicals by a metal oxide or the like. Specific examples of the obtained compound include methane, ethane, propylene, methanol, ethanol, propanol, acetaldehyde, diethyl ether, acetic acid, butyric acid, diethyl carbonate, butadiene and the like.

[Characteristics of oxygen carrier]
The oxygen carrier of the present invention preferably has the following characteristics.
That is, when a stainless steel reaction tube having an inner diameter of 8 mm in which a pressure gauge is arranged in the flow path is filled with an oxygen carrier at a height of 40 cm and nitrogen gas having a concentration of 100% by volume is passed through at 30 mL / min. The pressure increase in 10 minutes is preferably 0.03 MPaG or less, and more preferably 0.01 MPaG or less.
It can be determined that the oxygen carrier exhibiting such characteristics has the filling density and the pore volume satisfying the above ranges, and the conversion efficiency of carbon dioxide into carbon monoxide can be sufficiently increased.

Although the method for producing the oxygen carrier and the gas of the present invention has been described above, the present invention is not limited thereto.
For example, the method for producing an oxygen carrier and a gas of the present invention may have any other additional configuration with respect to the above embodiment, and is replaced with any configuration exhibiting the same function. Often, some configurations may be omitted.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.
1. 1. Production of Oxygen Carrier (Example 1)
First, as raw materials, copper (II) nitrate trihydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) and zinc nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) , Purity: 99.0%).
Next, this raw material was dissolved in distilled water to prepare a 1 mol / L raw material aqueous solution (Liquid A). The molar ratio of Cu and Zn in the liquid A was set to 0.1: 0.9.
Further, sodium carbonate was dissolved in distilled water to prepare a 1 mol / L sodium carbonate aqueous solution (liquid B).

Next, solutions A and B were simultaneously added dropwise to a container containing 20 mL of distilled water at 70 ° C. to prepare a mixed solution. The dropping rate of the liquid A was 10 mL / min, and the mixed liquid was stirred so that the generated precipitate did not aggregate.
In addition, the temperature of the mixed solution was maintained at 70 ° C., and the dropping rate of the solution B was also adjusted so that the pH was 7.0.
Then, while maintaining the mixed solution at 70 ° C., stirring was continued at a speed of 50 rpm or more, and aging was carried out for 3 hours. After the aging was completed, the precipitate was collected by filtration and washed thoroughly with water.
Then, the recovered precipitate was dried at 120 ° C. for 12 hours by a dryer and then calcined at 400 ° C. for 3 hours to obtain a metal oxide.
Next, the metal oxide was pulverized by a ball mill to obtain granular oxygen carriers (average particle size: 0.5 μm) consisting only of the metal oxide. It was confirmed by ICP measurement of the oxygen carrier (metal oxide) that the molar ratio of Cu and Zn was 0.1: 0.9.

(Example 2)
Granular oxygen carriers (average particle size: 0.5 μm) were produced in the same manner as in Example 1 except that the molar ratio of Cu and Zn in the liquid A was 0.2: 0.8. did. It was confirmed by ICP measurement of the oxygen carrier (metal oxide) that the molar ratio of Cu and Zn was 0.2: 0.8.

(Example 3)
Granular oxygen carriers (average particle size: 0.5 μm) were produced in the same manner as in Example 1 except that the molar ratio of Cu and Zn in the liquid A was 0.3: 0.7. did. It was confirmed by ICP measurement of the oxygen carrier (metal oxide) that the molar ratio of Cu and Zn was 0.3: 0.7.

(Example 4)
Granular oxygen carriers (average particle size: 0.5 μm) were produced in the same manner as in Example 1 except that the molar ratio of Cu and Zn in the liquid A was 0.4: 0.6. did. It was confirmed by ICP measurement of the oxygen carrier (metal oxide) that the molar ratio of Cu and Zn was 0.4: 0.6.

(Example 5)
Granular oxygen carriers (average particle size: 0.5 μm) were produced in the same manner as in Example 1 except that the molar ratio of Cu and Zn in the liquid A was 0.5: 0.5. did. It was confirmed by ICP measurement of the oxygen carrier (metal oxide) that the molar ratio of Cu and Zn was 0.5: 0.5.

(Example 6)
Granular oxygen carriers (average particle size: 0.5 μm) were produced in the same manner as in Example 1 except that the molar ratio of Cu and Zn in the liquid A was 0.6: 0.4. did. It was confirmed by ICP measurement of the oxygen carrier (metal oxide) that the molar ratio of Cu and Zn was 0.6: 0.4.

(Example 7)
Granular oxygen carriers (average particle size: 0.5 μm) were produced in the same manner as in Example 1 except that the molar ratio of Cu and Zn in the liquid A was 0.7: 0.3. did. It was confirmed by ICP measurement of the oxygen carrier (metal oxide) that the molar ratio of Cu and Zn was 0.7: 0.3.

(Example 8)
First, as raw materials, copper (II) nitrate trihydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity: 99.9%) and zinc nitrate hexahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) , Purity: 99.0%).
Next, this raw material was dissolved in ethanol to prepare a 1 mol / L raw material aqueous solution (Liquid A). The molar ratio of Cu and Zn in the liquid A was set to 0.5: 0.5.
Next, zeolite particles (manufactured by Tosoh Zeolite Co., Ltd., "330HUA") were added to this liquid A. The amount of the zeolite particles added was set to 50 parts by mass with respect to 100 parts by mass of the total (oxygen carrier) of the generated metal oxide and the zeolite particles.

Then, ethanol was distilled off from the liquid A containing the zeolite particles using an evaporator, and the residue was recovered.
Next, the recovered residue was dried at 120 ° C. for 12 hours in a dryer and then calcined at 700 ° C. for 3 hours to carry out granular oxygen carriers (average particle size) composed of zeolite particles carrying (bonding) a metal oxide. : 0.5 μm) was obtained. It was confirmed by ICP measurement of the oxygen carrier (metal oxide) that the molar ratio of Cu and Zn was 0.5: 0.5.

(Comparative Example 1)
Copper (II) oxide particles (manufactured by Wako Pure Chemical Industries, Ltd., purity: 99.9%, average particle size: 0.5 μm) were used as oxygen carriers.

2. 2. X-ray diffraction measurement Before performing the X-ray diffraction measurement, a sample of oxygen carrier was prepared.
First, about 100 mg of oxygen carrier was measured in a mortar and pestle and ground using a pestle. Then, the oxygen carrier was uniformly filled in the hole of the sample filling portion of the sample plate, and the surface of the sample plate was adjusted so that the surface of the sample plate and the surface of the oxygen carrier were flush with each other.
A sample horizontal multipurpose X-ray diffractometer (“Ultima III” manufactured by Rigaku Co., Ltd.) was used for the X-ray diffraction measurement.
A copper tube made of pure copper was used as the counter-cathode, and the characteristic X-ray of CuKα (wavelength (λ) = 1.5418 Å (0.15418 nm)) was used for diffraction.
In the diffractometer, the divergence slit was set to 1/2 °, the divergence vertical limiting slit was set to 10 mm, the scattering slit was set to 2 °, and the light receiving slit was set to 0.15 mm.

Then, the prepared oxygen carrier sample was irradiated with X-rays under the conditions of a tube voltage of 40 kV and a tube current of 20 mA.
The scanning angle of the goniometer was set in the range of 3 to 60 °, and the measurement was performed at a scanning speed of 2 ° / min.
After the measurement was completed, the obtained data was analyzed. The data was displayed in a graph (X-ray diffraction profile) as the horizontal axis angle and vertical axis intensity (cps) using Excel or the like, and the presence or absence of a diffraction peak was confirmed. In the graph, the value with the highest intensity of the peak was taken as the apex, and the difference between the angle of the apex and the angle of the part showing the intensity value of half of the intensity value of the apex was calculated, and this was defined as the half width at half maximum. Then, a peak having a half width of 1 ° or less was defined as a diffraction peak.

When the half width was different on the left and right of the peak, the larger one was adopted as the half width. When there were multiple peaks, the half width of each was calculated.
If the peaks overlap and the full width at half maximum cannot be calculated, peak fitting using the least squares method or the like is performed to separate the peaks, and the full width at half maximum is calculated.
Further, the diffraction intensity of each specified diffraction peak is calculated, the diffraction peak having the highest diffraction intensity and the diffraction peak having the second highest diffraction intensity are selected, and among the two diffraction peaks, in the X-ray diffraction profile, the diffraction peak is selected. The diffraction intensity of the diffraction peak having the smaller value of 2θ was defined as A, the diffraction intensity of the diffraction peak having the larger value was defined as B, and the ratio (A / B) thereof was determined.
The X-ray diffraction measurement was performed three times for each sample, and the average value was used as the measured value.

3. 3. Oxygen carrier characterization (conversion efficiency)
Using a rapid catalyst evaluation system ("Single μ-Reactor Rx-3050SR" manufactured by Frontier Lab Co., Ltd.) equipped with a microreactor and a gas chromatograph mass spectrometer (GC / MS) directly connected to the microreactor, the following The characteristics of the oxygen carrier were evaluated by the procedure.
First, in evaluating the characteristics of oxygen carriers in the microreactor, the following process was performed in order to activate the oxygen carriers. A reaction tube made of quartz having an inner diameter of 3 mm and a length of 78 mm was filled with 0.2 g of oxygen carriers. Then, while flowing helium gas at a flow rate of 20 mL / min, the temperature was raised at a heating rate of 40 ° C./min and heated for 20 minutes.

Next, hydrogen gas (reducing gas) was passed through the reaction tube of the microreactor at a flow rate of 20 mL / min for 5 minutes to carry out a reduction reaction (first process) of oxygen carriers to reduce oxygen carriers. At this time, the gas discharged from the discharge port of the microreactor contained water vapor.
Then, for gas exchange, helium gas was flowed at a flow rate of 20 mL / min for 5 minutes, and then carbon dioxide gas was flowed at a flow rate of 20 mL / min for 5 minutes to carry out a carbon dioxide reduction reaction (second process). , Carbon dioxide gas (raw material gas) was reduced. At this time, the produced gas discharged from the discharge port of the microreactor contained carbon monoxide.
Then, for gas exchange, helium gas was flowed at a flow rate of 20 mL / min for 5 minutes.

Next, the following process was performed for this test. Hydrogen gas (reducing gas) was passed through the reaction tube of the microreactor at a flow rate of 30 mL / min for 20 minutes to carry out a reduction reaction of oxygen carriers (first process) to reduce oxygen carriers. At this time, the gas discharged from the discharge port of the microreactor contained water vapor.
Then, for gas exchange, helium gas was flowed at a flow rate of 20 mL / min for 5 minutes, and then carbon dioxide gas was flowed at a flow rate of 3 mL / min for 20 minutes to carry out a carbon dioxide reduction reaction (second process). , Carbon dioxide gas (raw material gas) was reduced. At this time, the produced gas discharged from the discharge port of the microreactor contained carbon monoxide.
The above process was carried out under atmospheric pressure conditions while maintaining the temperature of the oxygen carrier at 550 ° C. when any of the gases was flown.

The conversion efficiency of carbon dioxide to carbon monoxide by oxygen carriers was calculated by the following formula.
The conversion efficiency is the average conversion efficiency for 1 minute after the distribution of carbon dioxide gas into the reaction tube is started.
X _CO (%) = n _{CO, out} / (n _{CO2, in} ) × 100
In the above formula, n is a mole fraction of carbon dioxide or carbon monoxide in the raw material gas or the produced gas.
Moreover, since the selectivity of the reaction is 100%, the total n _CO2 = n _{CO2, in} + n _{CO, out} .

The measurement conditions in the gas chromatograph mass spectrometer are as follows.
Column temperature: 200 ° C
Injection temperature: 200 ° C
Detector temperature: 250 ° C
Column: EGA tube (L: 2.5 m, φ (inner diameter): 0.15 mm, t: 0 mm)
Column flow rate: 1.00 mL / min Split ratio: 250
Purge flow rate: 3.0 mL / min

These results are shown in Table 1 below.

The oxygen carriers of each example had high conversion efficiency of carbon dioxide to carbon monoxide. Further, by changing the type and amount of the element M (doped metal element) constituting the oxygen carrier and the amount of the binder, the conversion efficiency of carbon dioxide into carbon monoxide could be adjusted.
On the other hand, the oxygen carriers of each comparative example had a low efficiency of converting carbon dioxide to carbon monoxide.

Claims (9)

An oxygen carrier used to reduce carbon dioxide by contacting it with a raw material gas containing carbon dioxide to produce a produced gas containing carbon monoxide.
It contains a metal oxide containing copper (Cu) and a second element different from copper, and the metal oxide has a crystal phase of copper oxide and a crystal phase of the oxide of the second element. An oxygen carrier characterized by that. The oxygen carrier according to claim 1, wherein when the metal oxide is measured by an X-ray diffraction method, at least two diffraction peaks are observed in the range of 2θ of 60 to 65 ° in the X-ray diffraction profile. Of the at least two diffraction peaks, the diffraction peak having the highest diffraction intensity and the diffraction peak having the second highest diffraction intensity are selected, and among the two diffraction peaks, the value of 2θ in the X-ray diffraction profile. The oxygen carrier according to claim 2, wherein when the diffraction intensity of the diffraction peak having a smaller value is A and the diffraction intensity of the diffraction peak having a larger value is B, the A / B is 0.05 to 2. .. The oxygen according to any one of claims 1 to 3, wherein in the metal oxide, the molar ratio of the second element to the total of the copper and the second element is 40 to 90 mol%. Career. The oxygen carrier according to any one of claims 1 to 4, wherein the second element is zinc (Zn). The oxygen carrier according to any one of claims 1 to 5, further comprising a binder that binds the metal oxide. The oxygen carrier according to claim 6, wherein the amount of the binder is 70 parts by mass or less with respect to 100 parts by mass of the oxygen carrier. The oxygen carrier according to claim 6 or 7, wherein the binder is at least one selected from the group consisting of zeolite, Al ₂ O ₃ , TiO ₂ , SiO ₂ , ZrO ₂ and MgO. The oxygen carrier according to any one of claims 1 to 8 is brought into contact with a raw material gas containing carbon dioxide to reduce the carbon dioxide to produce a produced gas containing carbon monoxide. The method of manufacturing gas.