Title of Invention

METHOD FOR PRODUCING OLEFIN OXIDE

Technical Field

[0001] The present invention relates to a method for producing olefin oxide.

Background Art

[0002] A method including a step of oxidizing olefin is known as a method for producing olefin oxide. For example, Patent Literature 1 describes a method for producing olefin oxide (propylene oxide), including preparing a fixed bed flow reactor filled with a catalyst prepared from silver nitrate and titanium dioxide and supplying a mixed gas containing hydrogen, oxygen, and olefin (propylene) to the fixed bed flow reactor to oxidize olefin in the fixed bed flow reactor to produce olefin oxide.

Citation List

Patent Literature

[0003]

[Patent Literature 1 ] WO 99/00188 (Example)

Summary of Invention

Technical Problem

[0004] In the production method according to Patent Literature 1, a mixed gas containing oxygen and hydrogen in almost the same volume is used. In order to perform this production method, it is therefore necessary to take safety measures or the like for preventing the combustion reaction which may be caused by oxygen and hydrogen. Solution to Problem

[0005] The present inventors have intensively studied on a method for producing olefin oxide by reacting olefin with oxygen substantially without using hydrogen, and have resulted in the present invention.

Specifically, the present invention provides

<1> a method for producing olefin oxide, comprising: an oxidation step of reacting olefin with oxygen in the presence of a catalyst containing silver, titanium, and a lanthanoid.

Note that, in the following description, the catalyst is optionally referred to as "the present silver catalyst", and the reaction of olefin with oxygen in the oxidation step is optionally referred to as "the present reaction". Further, this method for producing olefin oxide is optionally referred to as "the present production method".

[0006] Further, the present invention provides, as the specific embodiments of the <1>:

<2> the method according to the <1>, wherein the lanthanoid is selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, samarium, holmium, and ytterbium;

<3> the method according to the <1> or the <2>, wherein the catalyst is a catalyst obtained by a preparation method comprising the following first step, second step and third step:

a first step: a step of mixing a compound containing a lanthanoid with a titanium compound to obtain a first mixture;

a second step: a step of subjecting the first mixture to heat treatment to obtain a second mixture; and

a third step: a step of mixing the second mixture with at least one member selected from the group consisting of metallic silver and silver compounds to obtain a third mixture;

<4> the method according to the <3>, wherein the preparation method further includes the following fourth step:

a fourth step: a step of subjecting the third mixture to reduction to obtain a fourth mixture;

<5> the method according to the <3> or the <4>, wherein the titanium compound is a titanium oxide;

<6> the method according to the <5>, wherein the titanium oxide forms a rutile structure;

<7> the method according to any one of the <3> to <6>, wherein the compound containing the lanthanoid is a salt;

<8> the method according to any one of the <3> to <7>, wherein the third step is a step of mixing at least one member selected from the group consisting of a silver salt and a silver oxide with the second mixture obtained in the second step to obtain the third mixture;

<9> the method according to any one of the <1> to <8>, wherein the oxidation step is a step of reacting olefin with oxygen in the presence of an organic halogen compound in addition to the catalyst;

<10> the method according to any one of the <1> to <9>, wherein the oxidation step is a step of reacting olefin with oxygen in the presence of water in addition to the catalyst; and

<11> the method according to any one of the <1> to <10>, wherein the olefin is propylene.

Effects of Invention

[0007] According to the present invention, olefin oxide can be produced from olefin and oxygen substantially without using hydrogen. Brief Description of Drawings

[0008] Fig. 1 is a powder X-ray diffraction pattern of the catalyst obtained in Example 1. The abscissa represents a diffraction angle (2Θ), and the ordinate represents peak intensity.

Description of Embodiments

[0009] <The present silver catalyst>

The present silver catalyst contains silver, titanium, and a lanthanoid. The silver contained in the present silver catalyst may be zero-valent silver, may be mono-valent silver (I), or may be mixed-valent silver (0, I), preferably zero-valent silver. On the other hand, the titanium in the present silver catalyst is preferably higher-valent titanium. Further, lanthanoid in the present silver catalyst is preferably a higher-valent lanthanoid. The present silver catalyst containing silver, titanium and lanthanoid can be prepared with a suitable preparation method to be described below.

[0010] The lanthanoids include lanthanum (atomic number: 57), cerium (atomic number: 58), praseodymium (atomic number: 59), neodymium (atomic number: 60), promethium (atomic number: 61), samarium (atomic number: 62), europium (atomic number: 63), gadolinium

(atomic number: 64), terbium (atomic number: 65), dysprosium (atomic number: 66), holmium (atomic number: 67), erbium (atomic number: 68), thulium (atomic number: 69), ytterbium (atomic number: 70), and lutetium (atomic number: 71), and among these elements, lanthanoids selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, samarium, holmium, and ytterbium are preferred. The present silver catalyst containing a lanthanoid selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, samarium, holmium, and ytterbium has a tendency to suppress the side reaction in the process of the present reaction. This can also result suppression of production of a by-product to selectively produce an olefin oxide. A lanthanoid selected from the group consisting of lanthanum, praseodymium, neodymium, holmium, and ytterbium is more preferred in that an olefin oxide can be produced more selectively.

[0011] The present silver catalyst contains silver and titanium in addition to the above-mentioned lanthanoid. Further, the present silver catalyst preferably contains a titanium oxide as a titanium compound. At least part of the titanium in the present silver catalyst is preferably contained in the form of a titanium oxide such as titanium dioxide. Further, the present silver catalyst preferably contains zero-valent silver dispersed on the surface of the titanium oxide. Zero-valent silver dispersed on the surface of the titanium oxide in the present silver catalyst can be confirmed by observing the catalyst with a scanning electron microscope or the like.

[0012] The present silver catalyst is preferably prepared by a preparation method comprising the following first step, second step and third step:

a first step: a step of mixing a compound containing a lanthanoid with a titanium compound to obtain a first mixture;

a second step: a step of subjecting the first mixture to heat treatment to obtain a second mixture; and a third step: a step of mixing the second mixture with at least one member selected f om the group consisting of metallic silver and silver compounds to obtain a third mixture.

Hereinafter, a suitable one as a preparation method of the present silver catalyst will be described in detail. This preparation method is optionally referred to as "the present preparation method"; and a compound containing the lanthanoid is referred to as "lanthanoid compound".

[0013] In a first step of the present preparation method, a lanthanoid compound is mixed with a titanium compound to obtain a first mixture.

Examples of the lanthanoid compounds include a salt containing a lanthanoid and an oxide containing a lanthanoid. Among these compounds, a salt containing a lanthanoid is preferred. Examples of such salts include chlorides such as lanthanum chloride, cerium chloride, praseodymium chloride, neodymium chloride, samarium chloride, holmium chloride, and ytterbium chloride; bromides such as lanthanum bromide, cerium bromide, praseodymium bromide, neodymium bromide, samarium bromide, holmium bromide, and ytterbium bromide; iodides such as lanthanum iodide, cerium iodide, neodymium iodide, praseodymium iodide, samarium iodide, holmium iodide, and ytterbium iodide; nitrates such as lanthanum nitrate, cerium nitrate, praseodymium nitrate, neodymium nitrate, samarium nitrate, holmium nitrate, and ytterbium nitrate; acetates such as lanthanum acetate, cerium acetate, praseodymium acetate, neodymium acetate, holmium acetate, and ytterbium acetate; carbonates such as lanthanum carbonate, cerium carbonate, praseodymium carbonate, neodymium carbonate, samarium carbonate, and ytterbium carbonate; and the like. Among these compounds, the nitrates containing a lanthanoid, acetates containing a lanthanoid, and carbonates containing a lanthanoid are preferred because it becomes easy to remove a volatile component (for example, an anion component of a salt containing a lanthanoid, or the like) from the first mixture at the time of heat treatment in the second step of the present preparation method to be described below. With respect to the suitable salts as illustrated here, removal of an anion component from the first mixture is made easy by volatilizing the anion component in the form of nitrogen, nitrogen oxide gas, carbon dioxide gas, water vapor, or the like in the second step of the present preparation method to be described below.

[0014] As to the amount ratio of the titanium compound with the lanthanoid compound used in the first step, the amount of the lanthanoid compound is preferably in the range of 0.001 to 5 parts by weight, more preferably in the range of 0.01 to 3 parts by weight relative to 100 parts by weight of the titanium compound.

[0015] Examples of the titanium compound include oxides such as titanium monoxide and titanium dioxide; halides such as titanium chloride, titanium bromide, and titanium iodide; and titanium alkoxides such as titanium methoxide, titanium ethoxide, titanium isobutoxide, and titanium tetraisopropoxide, and as mentioned above, among these compounds, oxides are preferred, and titanium dioxide is particularly preferred. Further, the crystal form of the oxides is not particularly limited, and a titanium oxide (particularly, titanium dioxide) used for the present catalyst may be a titanium oxide forming an anatase structure, a titanium oxide forming a rutile structure, or a mixture of these two structures. For the present catalyst, a titanium oxide forming a rutile structure is preferably used. It is particularly preferred that substantially all of the titanium oxide used in the first step forms a rutile structure.

[0016] In the first step, the mixing of the lanthanoid compound with the titanium compound is preferably carried out in the presence of a solvent. Examples of such a solvent include water, an organic solvent or a mixed solvent of water and an organic solvent (a water/organic solvent mixed solvent). Specific examples of the organic solvents as described herein include alcohols such as methanol, ethanol, and propanol; ethers such as tetrahydrofuran; and hydrocarbons such as toluene and hexane. Water or water/organic solvent mixed solvent is preferred, and particularly water is preferred, in that at least one of the lanthanoid compound and the titanium compound is easily dissolved therein.

[0017] As a preferred combination used in the first step, a salt containing a lanthanoid is used as a lanthanoid compound, titanium dioxide is used as a titanium compound, and water is used as a solvent. Hereinafter, one embodiment of the first step of obtaining the first mixture will be described in detail by taking such a preferred combination as an example.

[0018] First, a salt containing a lanthanoid is dissolved in water to prepare an aqueous salt solution. Subsequently, the aqueous salt solution is mixed with titanium dioxide. The concentration of the salt in the aqueous salt solution can be controlled in a suitable range depending on the salt to be used, but is preferably in the range of 0.01 to 50% by weight. Note that two or more salts can also be used for the preparation of the aqueous salt solution, and in this case, the total weight concentration of the two or more salts used may be in the above range. Further, in preparing the aqueous salt solution, the salt containing a lanthanoid is mixed with water, and then the resulting mixture may be optionally heated or cooled, and the temperature at this time can be controlled in the range of 0 to 100°C. Further, filtration or the like may be performed in order to remove an undissolved portion slightly remaining after dissolution.

[0019] The temperature at the time of mixing the aqueous salt solution with the titanium dioxide is preferably in the range of 0 to 150°C, preferably in range of 10 to 80°C. The mixing time can be controlled in the range of 0.1 to 10 hours depending on the temperature during the mixing.

[0020] A mixture obtained in this way takes a form in which the first mixture is dispersed or precipitated in water which is a solvent. Subsequently, the first mixture is separated from water by solid liquid separation operation such as filtration operation, or volatile components such as water are removed by distillation operation such as distillation under reduced pressure, thereby separating the water used as a solvent from the mixture to obtain the first mixture. In order to separate the first mixture from the mixture, solid liquid separation operation and distillation operation can be performed in combination. When the first mixture is separated from water by filtration operation, the first mixture in a solid form obtained by the filtration may be optionally washed with a suitable solvent (for example, washed with water). Further, the first mixture obtained by filtration operation may be dried by performing reduced pressure drying or the like.

[0021] One embodiment of the first step of the present preparation method has been described as above. When a lanthanoid compound to be used is insoluble or poorly soluble in a solvent such as water, the lanthanoid compound may be mixed with a solvent to prepare a dispersion. And if the aqueous salt solution is replaced by the dispersion and the first step is performed as described above, the first mixture can be obtained also in the case of using the insoluble or poorly soluble lanthanoid compound.

[0022] Next, the second mixture is obtained by subjecting the first mixture obtained in the first step to heat treatment (the second step of the present preparation method). The heat treatment is preferably a heat treatment in which the lower limit of treatment temperature is 250°C, and the lower limit of the treatment temperature is more preferably 300°C. Further, although the upper limit of the treatment temperature can be controlled depending on the titanium compound used in the first step and impurities which may be contained in the titanium compound and the amount thereof, it is preferably 1000°C, more preferably 800°C. When the titanium compound is titanium oxide or the like, for example, the upper limit of the treatment temperature may be controlled depending on the specific surface area of the titanium oxide or the like.

[0023] The heat treatment in the second step can be performed, for example, as follows. The first mixture is set in a suitable heat-resistant container and put in a firing furnace together with the heat-resistant container, and the temperature of the firing furnace is increased to a predetermined treatment temperature. The heat-resistant container in which the mixture has been set may be put in a firing furnace previously maintained at a predetermined treatment temperature. The treatment time of heat treatment (heat treatment time) is controlled in the range of

0.1 to 20 hours depending on treatment temperature or the like. The heat treatment may be performed in the presence of any atmospheric gas such as oxygen, nitrogen, carbon dioxide, helium, and argon, or may be performed in the presence of an atmospheric gas in which two or more selected from these gases are mixed (for example, air or the like).

Among these gases, the atmospheric gas is preferably air or oxygen, more preferably air. The heat-treated first mixture is optionally cooled after a lapse of a predetermined heat treatment time. Thus, the second mixture can be obtained.

[0024] Next, the third mixture is obtained by mixing the second mixture obtained in the second step with at least one member selected from the group consisting of metallic silver and silver compounds (the third step of the present preparation method). In this third step, it is preferred to mix the second mixture with the at least one member selected from the group consisting of metallic silver and silver compounds in the presence of a solvent. This solvent may be the same solvents as used for the first step.

[0025] Although metallic silver, silver compounds, or mixtures thereof can be used in the third step, it is preferred to use a silver compound among them. Examples of the silver compounds include oxides such as silver oxide; silver salts such as silver carbonate, silver nitrate, silver sulfate, silver cyanide, silver chloride, silver bromide, silver iodide, silver acetate, silver benzoate, and silver lactate; and silver complexes such as silver acetylacetonate, and among these silver compounds, oxides and/or silver salts are preferred; silver nitrate, silver carbonate, silver oxide, or a mixture of two or more selected therefrom are more preferred; and silver nitrate is particularly preferred.

[0026] Hereinafter, the third step in the case of mixing the second mixture with a silver compound using a solvent will be described in detail. First, a silver compound solution is prepared from a solvent and a silver compound. Hereinafter, the solvent for preparing a silver compound solution is sometimes referred to as a "solvent for a silver compound solution". An acid, a nitrogen-containing compound, or a mixture thereof may be added to the solvent for a silver compound solution.

[0027] The acid may be any of inorganic acids and organic acids.

Examples of the inorganic acids include hydrochloric acid, nitric acid, nitrous acid, sulfuric acid and perchloric acid. Examples of the organic acids include aliphatic carboxylic acids such as acetic acid, oxalic acid, propionic acid, butyric acid, citric acid, maleic acid, fumaric acid, and tartaric acid; and aromatic carboxylic acids such as benzoic acid, dicarboxy benzene, tricarboxy benzene, dicarboxy naphthalene, and dicarboxy anthracene. Among these acids, organic acids are preferred; aliphatic carboxylic acids are more preferred; and oxalic acid and citric acid are most preferred. When an acid is added to a solvent for a silver compound solution, the amount of the acid used is preferably in the range of 0.1 to 10 mol relative to 1 mol of silver contained in the silver compound to be used. When a plurality of silver compounds are used, the amount of the acid used may be in the range of 0.1 to 10 mol relative to 1 mol of the total of the silver contained in these silver compounds.

[0028] Examples of the nitrogen-containing compounds include nitrogen-containing organic compounds such as amine compounds, imine compounds, amide compounds, hydrazine compounds having an organic group, nitryl compounds, nitro compounds, and nitroso compounds; nitrogen-containing inorganic compounds such as ammonia, hydroxylamine, hydrazine, and hydroxyamines; and quaternary ammonium salts. Amine compounds are preferred as the nitrogen-containing compounds. The amine compounds may be acid addition salts such as amine hydrochlorides and amine acetates. When adding the nitrogen-containing compound to the solvent for a silver compound solution, the amount of the nitrogen-containing compound used is preferably in the range of 0.1 to 20 mol relative to 1 mol of silver contained in the silver compound to be used. When a plurality of silver compounds are used, the amount of the nitrogen-containing compound used may be in the range of 0.1 to 20 mol relative to 1 mol of the total of the silver contained in these silver compounds.

[0029] Examples of the amine compounds include aliphatic amines having 1 to 20 carbons, nitrogen heterocyclic compounds having 3 to 20 carbons, or aromatic amines having 6 to 20 carbons such as methylamine, ethylamine, propylamine, butylamine, amylamine, hexylamine, heptylamine, octylamine, decylamine, dodecylamine, stearylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine, trimethylamine, triethylamine, ethanolamine, dimethylethanolamine, diethanolamine, triethanolamine, ethylenediamine, tetramethylenediamine, pentamethylenediamine, diethylenetriamine, pyrrolidine, piperidine, piperazine, aniline, benzylamine, and phenylenediamine; amino acids such as glycine; and the like.

[0030] Examples of the imine compounds include ethyleneimine, and the like.

[0031] Examples of the amide compounds include acetamide and benzamide.

[0032] Examples of the hydrazine compounds having an organic group include methylhydrazine and phenylhydrazine.

[0033] Examples of the nitryl compounds include benzonitrile and butyronitrile.

[0034] Examples of the nitro compounds include nitrobenzene and nitropyridine.

[0035] Examples of the nitroso compounds include nitrosodimethylaniline and nitrosonaphthol.

[0036] Examples of the quaternary ammonium salts include quaternary ammonium hydroxides such as tetramethylammonium hydroxide, tetraethylammomum hydroxide, and tetrapropylammonium hydroxide; quaternary ammonium halides such as tetramethylammonium chloride, tetramethylammonium bromide, tetraethylammonium chloride, and tetraethylammonium bromide.

[0037] The silver compound may be mixed with the second mixture by mixing the silver compound solution with the second mixture obtained through the second step. Preferably, the second mixture is also dispersed in a suitable solvent to obtain a second mixture dispersion, and then the second mixture dispersion is preferably mixed with the silver compound solution. It is preferred to select the solvent for preparing the second mixture dispersion and the solvent for a silver compound solution so that these solvents are miscible with each other. When water is used as the solvent for a silver compound solution, it is preferred that the solvent for preparing the second mixture dispersion be also water. Further, an acid or an alkali may be added to the solvent for preparing the second mixture dispersion. As the acid, it is possible to use the same one as mentioned as an acid which can be arbitrarily added to the solvent for a silver compound solution. As the alkali, it is possible to use a nitrogen-containing compounds having alkalinity and capable of being added to the solvent for a silver compound solution, specifically, amine compounds, imine compounds, hydrazine or hydrazine compounds, ammonia, hydroxylamine, hydroxyamines and ammonium hydroxide. As the alkali, alkali metal hydroxides such as sodium hydroxide can also be used other than the nitrogen-containing compounds. Such acid and alkali can be suitably selected according to the solvent used for preparing the second mixture dispersion and the like.

[0038] Although a method for mixing the silver compound solution with the second mixture dispersion is not particularly limited, it is preferred to mix them while adding one of the both in small amounts to the other, and it is more preferred to mix them while dropwise adding the silver compound solution to the second mixture dispersion. [0039] The temperature during the mixing of the silver compound solution with the second mixture dispersion is in the range of 0 to 100°C. When the silver compound solution is dropwise added to the second mixture dispersion, the dropwise addition rate may be controlled while maintaining the above temperature range. After the completion of the dropwise addition, it is preferred to stir the mixture for about 0.1 to 10 hours.

[0040] The amount ratio of the silver compound with the second mixture used in the third step is determined so that the content of silver contained in the present silver catalyst (silver content) may be within an optimum range to be described below. Preferably, the second mixture is in the range of 0.1 to 200 parts by weight per part by weight of silver contained in the silver compound.

[0041] A mixture obtained in the third step, that is, a mixture containing the third mixture, takes a form in which the third mixture is dispersed or precipitated in the solvent. The solvent is a mixture of the solvents each used for preparing the silver compound solution and the second mixture dispersion. Hereinafter, the mixture obtained in the third step is referred to as "third step mixture". Subsequently, the third step mixture is subjected to solid liquid separation operation such as filtration operation to separate the third mixture from the solvent or to distillation operation such as distillation under reduced pressure to remove volatile components such as a solvent, thereby capable of separating the solvent to obtain the third mixture in a solid form. In order to separate the third mixture from the third step mixture, solid liquid separation operation and distillation operation can be performed in combination. The third mixture in a solid form obtained by separating the third mixture from the solvent may be optionally washed with a suitable solvent (for example, water washing). Further, the third mixture in a solid form may be dried using reduced pressure drying or the like. When an alkali metal component is contained in the third step mixture obtained in the third step, it is preferred to reduce the mixed amount of the alkali metal component (such as lithium, sodium, potassium, rubidium, and cesium) contained in the third mixture by sufficiently washing the third mixture in a solid form removed from the third step mixture using a solvent or the like. The reason is that, when the alkali metal component is mixed, the catalytic activity of the present silver catalyst tends to be reduced. Thus, a third mixture in which the alkali metal component is not substantially mixed can be obtained by optionally performing washing or the like. And the third mixture in which an alkali metal component is not substantially mixed can be preferably used in the present silver catalyst. Herein, the third mixture in which an alkali metal component is not substantially mixed as described herein means the one in which the content of the alkali metal component is below the minimum limit of detection of an ICP spectrometry or XRF analytical method as will be described below. The content of the alkali metal component is more preferably 1500 ppm by weight or less relative to the total weight of the third mixture.

[0042] The third mixture obtained in the present preparation method including the first step, the second step and the third step, which has been described above, can be used as the present silver catalyst as it is or after being optionally subjected to molding or the like. Moreover, in order to further increase the catalytic activity in the present reaction to be described below, it is preferred to further perform the following fourth step:

a fourth step: a step of subjecting the third mixture obtained in the third step to reduction to obtain a fourth mixture.

[0043] As described above, the solvent used for preparing the silver compound solution or the like is contained in the third step mixture obtained in the third step. The third step mixture may be subjected to reduction in the fourth step as it is, or the solvent may be separated to remove the third mixture in a solid form and then the third mixture in a solid form may be subjected to reduction. Further, as described above, it is preferred to reduce the mixed amount of the alkali metal component by washing or the like of the third mixture in a solid form removed from the third step mixture. In the case of such washing, if the same solvent as the solvent used for preparing the silver compound solution is used as a washing solvent, a very small amount of silver compound adhering to the third mixture can also be sufficiently removed.

[0044] The reduction in the fourth step means a treatment that converts any or all of silver ions (monovalent silver ions) contained in the third mixture into zero-valent silver. In the reduction, it is preferred that substantially all the silver ions contained in the third mixture have been converted into zero-valent silver.

[0045] When the third step mixture is used as it is as a material to be treated in the reduction, the reduction can be performed by adding, to the third step mixture, a reducing agent such as alcohols such as methanol, ethanol, propanol, butanol, ethylene glycol, propylene glycol, glycerin, aminoethanol, and dimethylamino ethanol; saccharides such as glucose, fructose, and galactose; aldehydes such as formaldehyde, acetaldehyde, propylaldehyde, butyraldehyde, and phenylaldehyde; hydrazines such as hydrazine, methylhydrazine, ethylhydrazine, propylhydrazine, butylhydrazine, and phenylhydrazine; metal hydrides such as lithium hydride, sodium hydride, potassium hydride, calcium hydride, and magnesium hydride; boron compounds such as boron hydride, sodium borohydride, potassium borohydride, and dimethylamine borane; and phosphites such as sodium hydrogenphosphite and potassium hydrogenphosphite. The amount of the reducing agent used can be controlled on the basis of the amount of the silver compound used in the third step, but the amount is preferably 1 mol or more relative to 1 mol of silver contained in the silver compound. The treatment conditions of the reduction can be suitably controlled according to the silver compound used, the reducing agent used, and the like. Further, when an alcohol, hydrazine, or a hydrazine compound among the above-mentioned reducing agents has already been contained in the third step mixture, it can also be used as a reducing agent.

[0046] Further, when the third step mixture is used as a material to be treated in the reduction, the reduction can also be performed using a reducing gas to be described below. In this case, a method of bubbling a reducing gas into the third step mixture may be employed, or the third step mixture may be sealed in a suitable pressure-resistant container, into which a reducing gas may be injected.

A mixture obtained through the fourth step can be subjected to, for example, filtration to remove a solvent and then optionally subjected to washing and/or drying to obtain a fourth mixture.

[0047] Further, when a third mixture is obtained by filtration or the like of the third step mixture, the third mixture will be obtained in a solid form. The third mixture in a solid form can be subjected to reduction by using a reducing gas. The third mixture in a solid form before being subjected to the reduction may be what is wet with a washing solvent or the like after having been subjected to filtration and washing, or what has been dried by drying treatment including heating, pressure reduction, or a combination thereof.

[0048] When a reducing gas is used, the reduction can be performed by a simple operation including filling a suitable packed tube with the third mixture and passing the reducing gas through the packed tube. In order to improve permeability of the reducing gas through the packed tube, the third mixture may be molded into a suitable shape, and then the packed tube may be filled with the molded third mixture. Examples of the reducing gas include hydrogen, carbon monoxide, methane, ethane, propane, butane, ethylene, propylene, butene, and butadiene, and a mixed gas in which two or more selected from these gases are mixed. Especially, carbon monoxide, hydrogen, and propylene are preferred. Further, the reducing gas may be diluted with, for example, nitrogen, helium, argon, water vapor (steam), or the like, or a dilution gas in which two or more selected from these gases are mixed, wherein the mixing ratio is arbitrary. A suitable example includes a reduction of using hydrogen as the reducing gas and using water vapor (steam) as the dilution gas, and in this case, the steam may be entrained when passing the reducing gas (hydrogen) through the packed tube, at this time, the mixing ratio of the steam in the gas flow passed through the packed tube is preferably 5 to 70% by volume.

[0049] With respect to the treatment temperature of the reduction including filling the suitable packed tube with the third mixture in a solid form and passing the reducing gas through the packed tube, the optimum temperature can be in the range of 20 to 300°C according to the reducing gas, of the third mixture (composition), the dilution gas and the mixing ratio thereof. When the treatment temperature is within the range, the aggregation of the metallic silver particles hardly occurs due to the reduction so as to avoid reducing, the effective surface area of silver in the present silver catalyst. Therefore, the upper limit of the treatment temperature is more preferably 250°C, most preferably 220°C.

[0050] When the silver contained in the third mixture, which is a material to be treated in the reduction, is contained in the state of silver oxide or silver carbonate, the silver oxide or silver carbonate contained in the third mixture is thermally decomposed to be converted into metallic silver only by heat-treating the third mixture. Thus, when the silver oxide or silver carbonate in the third mixture is converted into metallic silver to obtain the fourth mixture by heat treatment, the reducing gas is not required, but nitrogen, rare gas such as helium and argon can be used as the atmospheric gas, or oxygen or air can also be used. In the case of performing the reduction by such heat treatment, if a suitable packed tube is filled with the third mixture in a solid form in the same manner as that described in the reduction using a reducing gas, and the packed tube is heated while passing an atmospheric gas or without passing an atmospheric gas, the third mixture with which the packed tube is filled can be heat-treated. With respect to the treatment temperature in this case, a sufficient temperature to cause the silver oxide or silver carbonate to thermally decompose is required, and the temperature is preferably in the range of 200 to 500°C, more preferably in the range of 250 to 450°C. When the temperature is within the range, the aggregation of metallic silver particles hardly occurs as mentioned above. Therefore, even when the silver atom contained in the third mixture is contained in the state of silver oxide or silver carbonate, it is preferred to use the reduction by thermal decomposition and the reduction using a reducing gas in combination, and the reduction using the reducing gas is more preferred in that the reduction can be performed at a lower temperature.

[0051] After the reduction is performed by using a reducing gas or by heat treatment in this way, the treated material is optionally cooled and then taken out from the packed tube to obtain the fourth mixture. The resulting fourth mixture can be used as the present silver catalyst as it is or by optionally molding or the like.

[0052] The content of silver in the present silver catalyst (silver content) is preferably 0.1% by weight or more, more preferably 0.5% by weight or more relative to the total weight of the present silver catalyst. It is preferred to determine the amount of each raw material used for producing the present silver catalyst so that the silver content falls within the above range. The silver content can be determined by using ICP spectrometry or an XRF analytical method. [0053] Further, the present silver catalyst may contain other elements (elements other than silver, titanium, and a lanthanoid) if the amount is very small, but as already described above, the contamination of an alkali metal component is preferably reduced as much as possible in order not to significantly impair the catalytic activity of the present silver catalyst. In the case of using the fourth mixture as the present silver catalyst, the following operation can be performed: a raw material containing an alkali metal component is not used in the process for preparing the fourth mixture from the third mixture (the fourth step of the present preparation method); or after the fourth mixture is prepared, the prepared fourth mixture is sufficiently washed with a solvent. Since the fourth mixture obtained in this way does not substantially contain an alkali metal component, it is particularly preferred as the present silver catalyst. The present silver catalyst which does not substantially contain an alkali metal component means the one in which when determining the silver content in the present silver catalyst using ICP spectrometry or an XRF analytical method, the content of the alkali metal component is below the minimum limit of detection of these analytical methods; and the content of the alkali metal component is more preferably 1500 ppm by weight or less relative to the total weight of the present silver catalyst.

[0054] <The present production method>

Next, the present production method using the present silver catalyst will be described. The present production method comprises an oxidation step of reacting olefin with oxygen in the presence of the present silver catalyst. Hereinafter, the gas containing olefin and oxygen may be referred to as "source gas".

[0055] The present production method may be performed in any one of a batch reactor and a continuous reactor, but it is preferred to perform the present production method in a continuous reactor from the viewpoint of performing the present production method as commercial production.

[0056] In the present production method, the amount of the present silver catalyst used for 1 mol of olefin to be used is such an amount that the silver contained in the present silver catalyst amounts to preferably 0.00005 mol or more, more preferably 0.0001 mol or more. The upper limit is not particularly limited and a larger amount of olefin oxide can be produced if the amount of the present silver catalyst used is increased, but the upper limit of the amount of the present silver catalyst used is controlled in consideration of economical efficiency such as cost of the present silver catalyst.

[0057] The oxygen used in the present production method may be oxygen alone, that is, high purity oxygen, or an oxygen mixed with an inert gas in the present reaction (nitrogen, carbon dioxide, and the like) i.e. air or the like. Although the amount of the oxygen used can be suitably controlled according to a reaction form (continuous or batch), the present silver catalyst, and the like, the amount of the oxygen is preferably in the range of 0.01 to 100 mol, more preferably in the range of 0.03 to 30 mol relative to 1 mol of propylene.

[0058] The olefin used in the present production method may be a compound having a carbon-carbon double bond in the molecule and typically includes a compound represented by the following formula (1):

wherein R independently represents a hydrogen atom or an alky group, and the total number of carbon atoms contained in the four R is in the range of 0 to 10.

[0059] The olefin is also preferably present as a gas under the temperature and pressure conditions in the reaction system of the present reaction; and specific examples include ethylene, propylene, butylene, hexene, octene and decene, and among the above, propylene is preferred. Propylene oxide derived from propylene is particularly useful as an engineering material, and since propylene oxide can be produced with good selectivity according to the present production method using propylene as an olefin, the benefit of a further effect of the present invention can be obtained.

[0060] The olefin used in the present production method may also be diluted with an organic gas other than propylene as long as the gas is inert to the present reaction. Examples of such an organic gas include a lower alkane such as methane and ethane.

[0061] It is preferred to allow an organic halogen compound, particularly a halogenated hydrocarbon, to be contained in the source gas used in the present production method. If the present reaction is performed in the presence of an organic halogen compound, olefin oxide can be effectively produced with higher yield. The organic halogen compound is preferably an organic chlorine compound, and examples of the organic chlorine compound include ethyl chloride, 1,2-ethylene dichloride, methyl chloride, and the like. The organic halogen compound is preferably a compound which is present as a gas on the temperature and the pressure conditions in the reaction system of the present reaction. When the organic halogen compound is used, the amount used is preferably 1 to 1000 ppm by volume, more preferably 1 to 500 ppm by volume relative to the source gas.

[0062] The reaction temperature of the present reaction is preferably in the range of 100 to 400°C, more preferably in the range of 120 to 300°C.

[0063] The reaction pressure of the present reaction is not particularly limited and can be selected from a wide range of from a reduced pressure condition to a pressurization condition. The pressurization condition is preferred in that oxygen and olefin can be sufficiently brought into contact with the silver catalyst. The reaction pressure is preferably selected from the range of 0.01 to 3 MPa, more preferably selected from the range of 0.02 to 2 MPa, as represented by absolute pressure. The reaction pressure is also determined by taking the pressure resistance ability of the reactor used in the present production method into account. The reduced pressure condition means that the reaction pressure is reduced to a pressure lower than atmospheric pressure, and the pressurization condition means that the reaction pressure is pressurized to a pressure higher than atmospheric pressure.

[0064] Further, in the present production method, it is also possible to react olefin with oxygen in the presence of water in addition to the present silver catalyst and an organic halogen compound. Water can be used in the present production method by converting the water into steam and then mixing the steam with oxygen and propylene to prepare a mixed gas, or can be used in the present production method by wetting the present silver catalyst with water. Among the above, it is preferred to use water as steam.

When water is used, the amount of the water used is preferably selected from the range of 0.1 to 20 mol, more preferably selected from the range of 0.2 to 10 mol, further preferably selected from the range of 0.3 to 8 mol relative to 1 mol of propylene.

[0065] Hereinafter, one embodiment of the present production method in the case of using a continuous reactor which is a suitable reaction form will be described.

First, a reaction column (reactor) provided with a gas supply port and a gas discharge port is filled with a predetermined amount of the present silver catalyst. The reaction column may be provided with suitable heating means, and the temperature inside the reaction column is increased to a predetermined reaction temperature by the heating means. Subsequently, a source gas containing olefin (preferably, propylene), oxygen or a mixed gas containing oxygen, and an organic halogen compound are supplied into the reaction column from the gas supply port using a compressor or the like. As mentioned above, water may be contained in the source gas. Olefin and oxygen are brought into contact with each other in the presence of the present silver catalyst and the organic halogen compound when the source gas is brought into contact with the silver catalyst within the reaction column. The contact allows olefin and oxygen contained in the source gas to react with each other to produce olefin oxide, and the product gas containing the olefin oxide produced is discharged from the discharge port. The linear velocity of the source gas to be passed through the reaction column is determined so that the residence time in which the source gas and the present silver catalyst can sufficiently produce olefin oxide may be obtained. Although the case where the reaction column is provided with heating means has been described in the above embodiment, an embodiment may be employed in which the reaction column is maintained around room temperature, and the source gas is supplied to the reaction column after being heated to a predetermined reaction temperature. Further, another embodiment may be employed in which the reaction column is provided with suitable stirring means, and the source gas is supplied while stirring the present silver catalyst in the reaction column.

[0066] In this way, produced olefin oxide, unreacted olefin and oxygen, and by-products such as carbon dioxide are contained in the product gas which has passed through the reaction column. Further, when olefin and oxygen are diluted and used, an inert gas used for dilution is mixed. After collecting the product gas, the target olefin oxide can be taken out by suitable separation means such as distillation.

[0067] According to the present production method, it is unnecessary to use hydrogen like the invention described in the Patent Literature 1. Therefore, it is not necessary to take safety measures against the combustion reaction which may be caused from hydrogen and oxygen. That is, when a source gas which does not substantially contain hydrogen is used as the source gas, it is unnecessary to take the safety measures. A source gas which does not substantially contain hydrogen as described herein refers to the one in which a very small amount of hydrogen may be contained in the source gas if the amount is such a degree that does not cause combustion reaction from oxygen and hydrogen in the source gas. The degree that does not cause combustion reaction from oxygen and hydrogen means that the source gas may contain hydrogen if the content is below the combustible range of oxygen and hydrogen. The limit of the content of hydrogen in the source gas can be determined by determining the combustible range under the reaction pressure in consideration of the reaction pressure of the present reaction. Thus, hydrogen may be contained in the source gas as long as the content is in the range that can sufficiently prevent the combustion reaction which may be caused from hydrogen and oxygen, but in the present production method, olefin oxide can be produced from olefin even if hydrogen is not contained in the source gas.

Examples

[0068] Hereinafter, embodiments of the present invention will be described by illustrating Examples.

[0069] Example 1

In 10 mL of water was dissolved 0.18 g of lanthanum nitrate to obtain an aqueous calcium nitrate solution, and thereto was then added 10 g of titanium dioxide (Ti0 ₂ , manufactured by Aldrich, 99.99%, rutile structure), and the resulting mixture was stirred for 1 hour at 60°C. After stirring, water was removed by distillation at 60°C under reduced pressure using an evaporator to obtain a first mixture A. The first mixture A was further subjected to firing at 400°C for 5 hours in a firing furnace under air to thereby obtain a first mixture A.

The resulting second mixture in an amount of 5 g was used and dispersed in 50 g of water, and thereto was then added 0.93 g of sodium hydroxide. The resulting slurry was ice-cooled, and then thereto was dropwise added an aqueous silver nitrate solution (an aqueous solution in which 2.64 g of silver nitrate is dissolved in 10 mL of water). After stirring the resulting mixture for 3 hours with cooling, a precipitate was collected by filtration and then washed four times with 200 mL of ion-exchanged water to obtain a third mixture A. A glass firing tube was filled with the resulting third mixture A, which was subjected to reduction by passing a mixed gas of carbon monoxide (CO)/nitrogen (N ₂ ) (compositional ratio by volume: CO/N ₂ =1/10) therethrough at 55 mL/min. Through the CO/N ₂ mixed gas was then passed 1 mL/hour of water with a syringe pump, and the temperature of the glass firing tube was increased to 110°C and held at the same temperature for 1 hour. Subsequently, the third mixture A was subjected to reduction by increasing the temperature of the glass firing tube to 210°C over 5 hours to thereby convert it to a fourth mixture A. The fourth mixture A obtained in this way was used as the present silver catalyst (the present silver catalyst A) in the following method for producing propylene oxide.

A stainless steel reaction tube having a diameter of 1/2 inch was filled with 1 mL of the present silver catalyst A as described above, and the temperature of the reaction tube was increased to 200°C. The production of propylene oxide was performed by supplying a source gas containing propylene, air, nitrogen, water, and ethyl chloride to the stainless steel reaction tube filled with the present silver catalyst under a pressurization condition (equivalent to 0.3 MPa in gauge pressure). The feed rate of each gas contained in the source gas was 450 mL/hour for propylene, 900 mL/hour for air, 990 mL/hour for nitrogen, and 1.2 mL/hour for water, and ethyl chloride was controlled so that it is contained in an amount of 50 ppm by volume in the source gas. The source gas was supplied to the reaction tube, and a product gas which has passed through the reaction tube was injected into methanol for 1 hour to thereby allow produced propylene oxide and by-products (acrolein, acetone, and the like) to be absorbed in methanol to obtain a methanol solution containing these components. The amount of the produced propylene oxide and the amount of the by-products (the amount of the produced acrolein and the amount of the produced acetone) were determined by subjecting the methanol solution to gas chromatography (Detector: FID) analysis.

Further, the product gas, which had passed through the packed tube, at the time of the completion of the injection of the product gas into methanol was on-line introduced into gas chromatography (Detector: TCD) to thereby analyze unreacted propylene and a by-product (carbon dioxide) to determine the amount of the unreacted propylene and the amount of the produced carbon dioxide. From the amount of propylene supplied during the reaction (the amount of supplied propylene) and the amount of unreacted propylene determined by the gas chromatography analysis, propylene conversion (%) was determined according to the following formula:

[0070] [Propylene conversion] (%)

= ([the amount of reacted propylene] (mol) ÷ ([the amount of unreacted propylene] (mol) + [the amount of produced propylene oxide] (mol) + [the amount of produced carbon dioxide] ÷ 3 (mol) + [the amount of produced acrolein] (mol) + [the amount of produced acetone] (mol)) x

100.

[0071] The amount of the reacted propylene was determined by the following formula:

[The amount of reacted propylene] (mol)

= [the amount of produced propylene oxide] (mol) + [the amount of produced carbon dioxide] ÷ 3 (mol) + [the amount of produced acrolein]

(mol) + [the amount of produced acetone] (mol).

[0072] Further, from the amount of propylene oxide produced which was determined by the gas chromatography analysis, selectivity (propylene oxide selectivity) was determined according to the following formula:

[0073]

[Selectivity] (%)

= [the amount of produced propylene oxide] (mol) ÷ ([the amount of produced propylene oxide] (mol) + [the amount of produced carbon dioxide] ÷ 3 (mol) + [the amount of produced acrolein] (mol) + [the amount of produced acetone] (mol))

[0074] The results of the propylene conversion and selectivity which were determined as described above are shown in Table 1.

[0075] [Record of powder X-ray diffraction pattern]

The powder X-ray diffraction of the catalyst obtained in Example 1 was recorded on a Rigaku powder diffraction unit, RINT-2500V, with mono-chromatized Cu Ka radiation (λ= 0.154 nm) at 40 kVand 300 mA. The diffraction pattern was identified by comparing with those included in the JCPDS database (Joint Committee of Powder Diffraction Standards).

Fig. 1 shows XRD patterns of the catalyst. The peaks assigned to Ti0 ₂ with rutile structure at 27.4, 36.1, 39.2, 41.2, 44.1, 54.3 and o

56.6 of 2Θ were observed.

[0076] Example 2

The present silver catalyst (the present silver catalyst B) was prepared by the same preparation method as in Example 1 except that 0.18 g of cerium nitrate was used instead of 0.18 g of lanthanum nitrate.

The production of propylene oxide was performed under the same conditions as in Example 1 using the present silver catalyst B. The results of the propylene conversion and selectivity are shown in

Table 1.

[0077] Example 3

The present silver catalyst (the present silver catalyst C) was prepared by the same preparation method as in Example 1 except that 0.14 g of praseodymium nitrate was used instead of 0.18 g of lanthanum nitrate.

The production of propylene oxide was performed under the same conditions as in Example 1 using the present silver catalyst C. The results of the propylene conversion and selectivity are shown in Table 1. [0078] [Table 1]

[0079] Example 4

The present silver catalyst (the present silver catalyst D) was prepared by the same preparation method as in Example 1 except that 0.14 g of neodymium nitrate was used instead of 0.18 g of lanthanum nitrate.

The production of propylene oxide was performed under the same conditions as in Example 1 using the present silver catalyst D. The results of the propylene conversion and selectivity are shown in Table 2.

[0080] Example 5

The present silver catalyst (the present silver catalyst E) was prepared by the same preparation method as in Example 1 except that 0.19 g of samarium nitrate was used instead of 0.18 g of lanthanum nitrate.

The production of propylene oxide was performed under the same conditions as in Example 1 using the present silver catalyst E. The results of the propylene conversion and selectivity are shown in Table 2.

[0081] Example 6 The present silver catalyst (the present silver catalyst F) was prepared by the same preparation method as in Example 1 except that 0.15 g of holmium nitrate was used instead of 0.18 g of lanthanum nitrate.

The production of propylene oxide was performed under the same conditions as in Example 1 using the present silver catalyst F. The results of the propylene conversion and selectivity are shown in

Table 2.

[0082] [Table 2]

[0083] Example 7

The present silver catalyst (the present silver catalyst G) was prepared by the same preparation method as in Example 1 except that 0.15 g of ytterbium nitrate was used instead of 0.18 g of lanthanum nitrate.

The production of propylene oxide was performed under the same conditions as in Example 1 using the present silver catalyst G. The results of the propylene conversion and selectivity are shown in Table 3.

[0084] Reference Example 1

Titanium dioxide (Ti0 ₂ , manufactured by Aldrich, 99.99%, rutile structure) in an amount of 5 g was used and dispersed in 50 g of water, and thereto was then added 0.93 g of sodium hydroxide. The resulting slurry was cooled, and then thereto was dropwise added an aqueous silver nitrate solution (an aqueous solution in which 2.64 g of silver nitrate is dissolved in 10 mL of water). After stirring the resulting mixture for 3 hours with cooling, a precipitate was collected by filtration and then washed four times with 200 mL of ion-exchanged water. A glass firing tube was filled with the resulting washed product, which was subjected to reduction by passing a mixed gas of carbon monoxide (CO)/nitrogen (N ₂ ) (compositional ratio by volume: CO/N ₂ =1/10) therethrough at 55 mL/min. Through the CO/ ₂ mixed gas was then passed 1 mL/hour of water with a syringe pump, and the temperature of the glass firing tube was increased to 110°C and held at the same temperature for 1 hour. Subsequently, the temperature of the glass firing tube was increased to 210°C over 5 hours to obtain a comparison silver catalyst A. The production of propylene oxide was performed under the same conditions as in Example 1 using 1 mL of the comparison silver catalyst A. The results of the propylene conversion and selectivity are shown in Table 3.

[0085] [Table 3]

Industrial Applicability

[0086] Olefin oxide, particularly, propylene oxide is useful as a production intermediate of various engineering materials. According to the present invention, such olefin oxide can be produced without taking safety measures as described above, and thus, the industrial value of the present invention is high.