JPS6310083B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for photochemically producing hydrogen using a metal bipyridyl or metal porphyrin complex as a photocatalyst. In recent years, attempts have been made to directly utilize solar energy using molecular systems that artificially aggregate photosynthetic functions. Such a photochemical reaction system is roughly shown as follows. That is, when light reaches the photocatalyst PC, it becomes an excited state PC ^* due to the energy of the light. This PC ^*
transfers an electron to an electron acceptor and creates a cation radical
It is converted to PC〓, and then PC〓 takes electrons from the electron donor and is regenerated into the original state PC. on the other hand,
The electron acceptor that has received the electrons transfers the electrons to the oxidizing agent, producing a reduction product. In the above-mentioned system, generally manganese, zinc, magnesium, ruthenium, iron,
Metal complexes of bipyridyl such as copper or porphyrin and derivatives thereof, quinone, pyridine, viologen, or derivatives thereof as electron acceptors, and ethanolamines, EDTA, ascorbic acid, etc. as electron donors. , are used in appropriate combinations. Furthermore, it is also known that the reduced electron acceptor is reacted with hydrogen ions using a metal such as platinum as a catalyst to generate hydrogen and return the electron acceptor to its original oxidized state. However, a method for generating hydrogen, regenerating electron acceptors, and regenerating oxidized electron donors has not been established. That is, when hydrogen is generated using the photochemical reaction system as described above, the electron donor is consumed and hydrogen generation stops, and oxidation products of the electron donor accumulate in the system. Therefore, in order to continuously generate hydrogen, it is necessary to remove the oxidation product of the electron donor and add a new electron donor, and it is also necessary to reduce the oxidation product by some method. By the way, EDTA and triethanolamine, which are commonly used as electron donors, are irreversible, and once they donate electrons to a photocatalyst and are oxidized (become an oxidized product), they cannot be regenerated. Ascorbic acid, on the other hand, is oxidized to dehydroascorbic acid, which is a relatively strong reducing agent, e.g.
It can be easily regenerated into the original ascorbic acid by contacting it with hydrogen sulfide, dithionite, sulfite, or the like. However, performing regeneration using a reducing agent within a photochemical reaction system disturbs the hydrogen generation system, and there are also problems such as deactivation of the platinum group catalyst used for hydrogen generation. It is possible. Moreover, photochemical reaction systems are generally carried out in aqueous solutions, and most of the above compounds are water-soluble, and ascorbic acid, which is an electron donor, and dehydroascorbin, which is its oxidation product, are no exception. It has been impossible to selectively extract and regenerate it from within. As a result of intensive studies to solve the above problems, the present inventors found that a water-insoluble or poorly water-soluble ascorbic acid-containing polymer synthesized by reacting ascorbic acid with a polymer sufficiently acts as an electron donor. We obtained the knowledge that the oxidation products taken out of the system can be easily reduced and regenerated by a reducing agent. Furthermore, esters and ethers of ascorbic acid are water-soluble in their hydroforms, but
When it is oxidized and becomes a dehydro form, its solubility in water decreases significantly and it precipitates, so it is removed from the reaction system.
We have discovered that only this oxidation product can be selectively extracted and regenerated. The present invention has been made based on this knowledge, and an object of the present invention is to provide a method capable of continuously producing hydrogen using a photochemical reaction system. That is, the present invention provides a method for producing hydrogen by a photochemical reaction using a system containing a photocatalyst, an electron donor, an electron acceptor, and a metal catalyst of a metal bipyridyl complex or a metal porphyrin complex. The present invention relates to a photochemical method for producing hydrogen, characterized by using an ascorbic acid-containing polymer, or an ester or ether of ascorbic acid as an electron donor. The present invention will be explained in detail below. The photocatalyst of the present invention is a metal bipyridyl complex and a metal porphyrin complex, each represented by the following formula. Me in the above formula represents a metal, and in the present invention, metal complexes of ruthenium, zinc, manganese, magnesium, iron, and copper can be used. In addition, R ₁ , R ₂ , R ₃ , and R ₄ are particularly hydrogen, phenyl group,
4-hydroxysulfophenyl group, methylpyridyl group, etc. are preferred. These metal complexes are excited by light energy, transfer electrons to electron acceptors, and are oxidized to become oxidation products. This oxidized metal complex then comes into contact with an electron donor, accepts electrons, and returns to its original state. By repeating this process, light energy is introduced into the chemical reaction system. On the other hand, as the electron acceptor that receives electrons from the excited metal complex, the ones mentioned above are generally used, but when hydrogen is generated as a subsequent reaction as in the present invention, especially methyl viologen, benzyl Viologen and monoalkyl bipyridine having 1 to 8 carbon atoms or nicotinamide are preferred. These electron acceptors receive electrons from the metal complex and are reduced. This reduced product is
The catalytic action of the metal transfers electrons to hydrogen ions,
Generates hydrogen gas by reducing hydrogen ions. The metal used in this catalyst is preferably a platinum group metal,
In particular, it is preferable to use it in a colloidal form by supporting it on polyvinyl alcohol, polyethylene glycol, styrene-maleic acid copolymer, or the like. In the present invention, in particular, an ascorbic acid-containing polymer or an ester or ether of ascorbic acid is used as an electron donor for transferring the electrons to the oxidized metal complex. Water-insoluble or poorly water-soluble ascorbic acid-containing polymers include polymers such as polyacrylic acid,
It is obtained by reacting polyvinyl chloride, polyvinyl acetic acid, polymethacrylic acid, polyvinyl benzoic acid, chloromethylated polystyrene, etc. with ascorbic acid. In these products, the functional groups of the polymer react with the hydroxyl groups of ascorbic acid, resulting in ascorbic acid bonded to the polymer. In addition to the above ascorbic acid-containing polymer,
Esters or ethers of ascorbic acid can be used alone or in conjunction with the above polymers. When this ascorbic acid ester or ether becomes a dehydro form, its solubility in water is significantly reduced. Therefore, at the initial stage of the photochemical reaction, it is dissolved in the system, but as the reaction progresses, a dehydro compound is generated, which becomes a solid and precipitates. This precipitate may be subjected to solid-liquid separation. According to this method, light collection efficiency is better than when using an ascorbic acid-containing polymer. The ester of ascorbic acid is preferably a condensate obtained by reacting an organic acid having 1 to 11 carbon atoms with ascorbic acid by a known method in terms of solubility and the like. Particularly preferred are benzoic acid, formic acid, acetic acid, propionic acid, and the like. Further, as the ether, condensates with alcohols having 1 to 11 carbon atoms are preferred for the same reasons as above, and in particular, methyl alcohol, ethyl alcohol,
N-propyl alcohol and isopropyl alcohol are preferred. Photochemical reactions using compounds such as those described above are carried out in an aqueous phase system. Among the above compounds, bipyridyl complexes or porphyrin complexes are water-soluble,
The concentration is preferably at least 10 ^-6 molar concentration and at most the saturation solubility. The concentration of the electron acceptor is determined by the concentration of the photocatalyst, but is generally 10 to 10 ⁶ relative to the photocatalyst.
Appropriately selected within the range of 2 times. Furthermore, the amount of metal catalyst is 0.1 times or more that of the photocatalyst to fully exhibit its function. Next, the amount of ascorbic acid-containing polymer added is
The amount of ascorbic acid in the contained polymer is selected to be the same amount to 1000 times the amount of the photocatalyst. Further, ascorbic acid ether and ester are determined by the solubility of the dehydro form, and are added in an amount higher than this, but the hydro form does not necessarily need to be completely dissolved, and a portion may be suspended. By irradiating a reaction system in which the above compounds are dissolved or suspended in water, hydrogen is generated and ascorbic acid becomes a dehydro form. As this dehydro-form gradually accumulates, the rate of hydrogen generation decreases, but the dehydroascorbic acid
Generation of hydrogen can be maintained by separating and removing it by operations such as centrifugation or decantation and adding new ascorbic acid-containing polymer, ascorbic acid ester, or ether to the reaction system. The separated and removed dehydroascorbic acid is easily regenerated into a hydroform by contacting with hydrogen sulfide. for example,
Ascorbic acid-containing polymers are regenerated by suspending the dehydro form in water to a concentration of 1 to 80% and stirring by blowing in hydrogen sulfide gas. used. In addition, ascorbic acid esters or ethers are suspended or dissolved in water to a concentration of 1 to 80%, hydrogen sulfide is blown in and stirred to regenerate, and then this is diluted and extracted with a solvent such as water, methanol, or ether. By washing and removing solvent,
used again. The present invention can continuously generate hydrogen from sunlight through a series of operations as described above. Examples of the present invention will be described below. Examples 1 to 7, Reference Examples 1 to 2 5 ml of an aqueous solution containing the various compounds shown in Table 1
The sample was placed in a quartz cell type reactor with an internal volume of 10 ml, and the mixture was degassed and replaced with nitrogen in advance. Inject 1.00ml of argon gas as an internal standard gas into this, and cool it on ice.
It was irradiated with 500W tungsten lamp light (light with a wavelength of 430 nm or less was filtered out). Sample 50μ of the gas above the reactor at predetermined intervals and measure the hydrogen concentration using gas chromatography.
The amount of hydrogen generated was calculated. These results are shown in Table 1. In addition, the electron donor 6-benzoic acid ascorbic acid ester, 6-acetic acid ascorbic acid ester,
5,6-diacetic acid ascorbic acid ester, 6-O
- Each compound of methyl ascorbic acid ether and poly-6-acrylic acid ascorbic acid was prepared by the following method. 6-benzoic acid ascorbic acid ester is made by combining dehydroascorbic acid and equimolar benzoic acid with 6-benzoic acid.
Acetic acid ascorbic acid ester is obtained by reacting dehydroascorbic acid and equimolar acetic anhydride, and 5,6-diacetic acid ascorbic acid ester is obtained by reacting dehydroascorbic acid and excess acetic anhydride under reflux in a benzene solvent. The dehydroester was obtained by reducing it with sodium dithionite and recrystallizing it from an aqueous solution. 6-O-methylascorbic acid ether is produced by mixing dehydroascorbic acid and equimolar methyl chloride using sodium hydride as a catalyst.
The reaction was carried out in a DMF solvent at 50°C, sodium dithionite was further added, and the product was recrystallized from an aqueous solution. Poly6-acrylic acid ascorbic acid ester is prepared by mixing 1 part by weight of dehydroascorbic acid and 0.7 parts by weight of polyacrylic acid chloride in a dioxane solvent at 60% by weight.
℃, then cross-linked by adding 0.1 part by weight of ethylenediamine, filtered, immersed in an aqueous solution of sodium dithionite, and then washed with water.

[Table] As is clear from these results, compounds such as ascorbic acid-containing polymers, ascorbic acid esters, and ascorbic acid ethers act as electron donors in exactly the same way as ascorbic acid. Examples 8 to 14 In the above examples, after 6 hours of reaction, the precipitated or suspended electron donor was separated by filtration and washed with water. Next, the suspension was suspended in about 50 times its weight of water, and hydrogen sulfide gas was blown into it at room temperature for 2 hours. After standing still for two days, nitrogen gas was blown into the reactor to drive out excess hydrogen sulfide, and precipitated sulfur was removed for regeneration. The electron donor regenerated as above was photochemically reacted again under the same conditions as in Examples 1 to 7. These results are shown in Table 2.

[Table] From the above results, ascorbic acid ester,
It can be seen that the ether and the polymer contained therein are all regenerated from the dehydro form, and that the regenerated product acts well as an electron donor.

Claims (1)

[Claims] 1. A method for producing hydrogen by a photochemical reaction in a system containing a photocatalyst, an electron donor, an electron acceptor, and a metal catalyst of a metal bipyridyl complex or a metal porphyrin complex, in which the electron donor is water-insoluble or water-resistant. A method for photochemically producing hydrogen, characterized by using a soluble ascorbic acid-containing polymer, or an ester or ether of ascorbic acid.