WO2011145485A1

WO2011145485A1 - Method for producing propylene oxide

Info

Publication number: WO2011145485A1
Application number: PCT/JP2011/060793
Authority: WO
Inventors: Yoshihiko Ohishi; Avelino Corma
Original assignee: Sumitomo Chemical Company, Limited
Priority date: 2010-05-17
Filing date: 2011-04-28
Publication date: 2011-11-24
Also published as: JP2012001534A

Abstract

To provide a method for producing propylene oxide by reacting propylene with oxygen substantially without using hydrogen, the present invention provides a method for producing propylene oxide including an oxidation step of reacting propylene with oxygen in the presence of a catalyst containing silver, titanium, and an element of group II of the periodic table. The catalyst is preferably the one obtained by obtaining a first mixture from a compound containing an element of group II of the periodic table and a titanium compound and mixing the first mixture with at least one member selected from the group consisting of metallic silver and a silver compound.

Description

DESCRIPTION

Title of Invention

METHOD FOR PRODUCING PROPYLENE OXIDE

Technical Field

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

Background Art

[0002] A method including a step of oxidizing propylene is known as a method for producing propylene oxide. For example, Patent Literature 1 describes a method for producing propylene oxide, including supplying a mixed gas containing hydrogen, oxygen, and propylene to a fixed bed flow reactor filled with a catalyst prepared from silver nitrate and titanium dioxide to oxidize propylene in the fixed bed flow reactor to produce propylene 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 propylene oxide by reacting propylene with oxygen substantially without using hydrogen, and have resulted in the present invention.

Specifically, the present invention provides

<1> a method for producing propylene oxide, comprising: an oxidation step of reacting propylene with oxygen in the presence of a catalyst containing silver, titanium, and an element of group II of the periodic table.

Note that, in the following description, the catalyst is optionally referred to as "the present silver catalyst", and the reaction of propylene with oxygen in the oxidation step is optionally referred to as "the present reaction". Further, this method for producing propylene 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 element of group II of the periodic table is selected from the group consisting of calcium, strontium, and barium;

<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 and second step;

a first step: a step of obtaining a first mixture from a compound containing an element of group II of the periodic table and a titanium compound; and

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

<4> the method according to the <3>, wherein the first step includes the following step A and step B:

step A: a step of mixing the compound containing the element of group II of the periodic table with the titanium compound to obtain a mixture; and

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

<5> the method according to the <3> or the <4>, wherein the preparation method further includes the following third step:

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

<6> the method according to any one of the <3> to <5>, wherein the titanium compound is a titanium oxide;

<7> the method according to the <6>, wherein the titanium oxide forms a rutile structure;

<8> the method according to any one of the <3> to <7>, wherein the compound containing the element of group II of the periodic table is a salt;

<9> the method according to any one of the <3> to <8>, wherein the second 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 first mixture to obtain the second mixture;

<10> the method according to any one of the <1> to <9>, wherein the oxidation step is a step of reacting propylene with oxygen in the presence of an organic halogen compound in addition to the catalyst; and

<11> the method according to any one of the <1> to <10>, wherein the oxidation step is a step of reacting propylene with oxygen in the presence of water in addition to the catalyst.

Effects of Invention

[0007] According to the present invention, propylene oxide can be produced from propylene 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 an element of group II of the periodic table. 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. The present silver catalyst containing silver, higher-valent titanium and element of group II of the periodic table can be prepared with a suitable preparation method to be described below.

[0010] Specific examples of the element of group II of the periodic table include magnesium, calcium, strontium, and barium, and among these elements, calcium, strontium, and barium are preferred. When using the present silver catalyst containing an element of group II of the periodic table selected from the group consisting of calcium, strontium, and barium, a side reaction during the process of the present reaction tends to be suppressed. As a result, the catalyst is also effective to suppress formation of by-products (acetone, acrolein, carbon dioxide, and the like) to allow selective production of propylene oxide.

Further, the present silver catalyst preferably contains a titanium oxide such as titanium dioxide as a titanium compound. 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.

[0011] The present silver catalyst is preferably prepared by a preparation method comprising the following first step and second step: a first step: a step of obtaining a first mixture from a compound containing an element of group II of the periodic table and a titanium compound; and

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

Hereinafter, this suitable preparation method will be described.

This preparation method is optionally referred to as "the present preparation method"; and a compound containing an element of group II of the periodic table is referred to as "compound of group II element". [0012] The first step of the present preparation method is a step of obtaining a first mixture from a compound of group II element and a titanium compound.

Examples of the compound of group II element include a salt containing an element of group II of the periodic table and an oxide containing an element of group II of the periodic table. Among these compounds, a salt containing an element of group II of the periodic table is preferred. Examples of the salt include chlorides such as magnesium chloride, calcium chloride, strontium chloride, and barium chloride; bromides such as magnesium bromide, calcium bromide, strontium bromide, and barium bromide; iodides such as magnesium iodide, calcium iodide, strontium iodide, and barium iodide; nitrates such as magnesium nitrate, calcium nitrate, strontium nitrate, and barium nitrate; acetates such as magnesium acetate, calcium acetate, strontium acetate, and barium acetate; and carbonates such as magnesium carbonate, calcium carbonate, strontium carbonate, and barium carbonate, and among these salts, nitrates are preferred. With respect to these nitrates, acetates, and carbonates, a volatile component (for example, an anion component of a salt containing an element of group II of the periodic table, or the like) tends to be easily removed from the first mixture during heat treatment in step B of the first step to be described below.

[0013] As to the amount ratio of the titanium compound with the compound of group II element used in the first step, the amount of the compound of group II element 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.

[0014] 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.

[0015] The first step has a step of mixing a compound of group II element with the titanium compound to obtain a mixture (hereinafter referred to as "step A"). This mixing 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 compound of group II element and the titanium compound is easily dissolved therein.

[0016] As a preferred combination used in the first step, a salt (a salt containing an element of group II of the periodic table) is used as a compound of group II element, 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.

[0017] First, a salt containing an element of group II of the periodic table 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 (salts containing an element of group II of the periodic table) 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 an element of group II of the periodic table 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.

[0018] 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, more preferably in the 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.

[0019] 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.

[0020] One embodiment of the first step of the present preparation method has been described as above. When a compound of group II element to be used is insoluble or poorly soluble in a solvent such as water, the compound of group II element 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 compound of group II element.

[0021] The first mixture obtained may be subjected to the second step to be described below as it is or may be subjected to the second step after further being subjected to heat treatment. The first step preferably includes the step A and a step of subjecting the mixture to heat treatment (hereinafter optionally referred to as "step B"). The first step preferably includes the following step A and step B:

step A: a step of mixing a compound of group II element with a titanium compound to obtain a mixture; and

step B: a step of subjecting the mixture obtained in step A to heat treatment.

[0022] The heat treatment in step B is preferably a heat treatment in which the lower limit of treatment temperature is 200°C, and the lower limit of the treatment temperature is more preferably 250°C, further preferably 300°C. Further, although the upper limit of the treatment temperature can be controlled depending on the titanium compound used in step A and impurities which the titanium compound may contain and the amount thereof, the upper limit is preferably 1000°C, more preferably 800°C, further preferably 600°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 step B can be performed as follows. The following is an explanation of the heat treatment in the case where, in step A, a compound of group II element is mixed with a titanium compound in a solvent, followed by filtration operation or distillation operation to obtain a mixture in a solid form which contains the compound of group II element and the titanium compound. The mixture in a solid form 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 heat-treated first mixture can be obtained.

[0024] Next, the second mixture is obtained by mixing the first mixture obtained in the first step with at least one member selected from the group consisting of metallic silver and silver compounds (the second step of the present preparation method). In this second step, it is preferred to mix the first 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 step A of the first step.

[0025] Although metallic silver, silver compounds, or mixtures thereof can be used in the second 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 second step in the case of mixing the first 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 "solvent for a silver compound solution".

[0027] An acid, a nitrogen-containing compound, or a mixture thereof may be added to the solvent for a silver compound solution.

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, tetraethylammonium hydroxide, and tetrapropylammonium hydroxide; and quaternary ammonium halides such as tetramethylammonium chloride, tetramethylammonium bromide, tetraethylammonium chloride, and tetraethylammonium bromide. [0037] The silver compound may be mixed with the first mixture by mixing the silver compound solution with the first mixture. Preferably, the first mixture is also dispersed in a suitable solvent to obtain a first mixture dispersion, and then the first mixture dispersion is preferably mixed with the silver compound solution. It is preferred to select the solvent for preparing the first 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 first mixture dispersion be also water. Further, an acid or an alkali may be added to the solvent for preparing the first 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 first mixture dispersion and the like.

[0038] Although a method for mixing the silver compound solution with the first 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 first mixture dispersion.

[0039] The temperature during the mixing of the silver compound solution with the first mixture dispersion is in the range of 0 to 100°C. When the silver compound solution is dropwise added to the first 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 further stir the mixture for about 0.1 to 10 hours.

[0040] The amount ratio of the silver compound with the first mixture used in the second 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 first 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 second step, that is, a mixture containing the second mixture, takes a form in which the second 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 first mixture dispersion. Hereinafter, the mixture obtained in the second step is referred to as "second step mixture". Subsequently, the second step mixture is subjected to solid liquid separation operation such as filtration operation to separate the second 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 or the like to obtain the second mixture in a solid form. In order to separate the second mixture from the second step mixture, solid liquid separation operation and distillation operation can be performed in combination. The second mixture in a solid form obtained by separating the second mixture from the solvent may be optionally washed with a suitable solvent (for example, water washing). Further, the second mixture in a solid form may be dried using reduced pressure drying or the like. When an alkali metal component is contained in the second step mixture obtained in the second 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 second mixture by sufficiently washing the second mixture in a solid form removed from the second 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 second mixture in which the alkali metal component is not substantially mixed can be obtained by optionally performing washing or the like. And the second mixture in which an alkali metal component is not substantially mixed can be preferably used in the present silver catalyst. Herein, the second 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 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 second mixture.

[0042] The second mixture obtained in the present preparation method including the first step and the second 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 third step:

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

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

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

[0045] When the second 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 second 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 second 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 second step mixture, it can also be used as a reducing agent.

[0046] Further, when the second step mixture is used as it is 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 second step mixture may be employed, or the second step mixture may be sealed in a suitable pressure-resistant container, into which a reducing gas may be injected.

The treated material which has been subjected to the reduction can be subjected to, for example, filtration to remove a solvent and optionally subjected to washing and/or drying to obtain a third mixture.

[0047] Further, when the second mixture in a solid form is obtained by filtration or the like of the second step mixture, the reduction can be performed by bringing the second mixture in a solid form into contact with a reducing gas. The second 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 second mixture in a solid form and passing the reducing gas through the packed tube. In order to improve permeability of the reducing gas through the packed tube, the second mixture may be molded into a suitable shape, and then the packed tube may be filled with the molded second mixture. Examples of the reducing gas include hydrogen, carbon monoxide, methane, ethane, propane, butane, ethylene, propylene, butene, isobutene, 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 second mixture and passing the reducing gas through the packed tube, the optimum temperature can be selected from the range of 20 to 300°C according to the reducing gas, the second mixture (composition), and the dilution gas and the mixing ratio thereof. When the treatment temperature is within the range, the aggregation of the metallic silver particles in the third mixture 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 second 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 second mixture is thermally decomposed to be converted into metallic silver only by heat-treating the second mixture. Thus, when the silver oxide or silver carbonate in the second mixture is converted into metallic silver to obtain the third 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 second 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 second 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 selected from the range of 200 to 500°C, more preferably from 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 element contained in the second 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, the treated material is optionally cooled and then taken out from the packed tube to obtain the third mixture. The resulting third mixture can be used as the present silver catalyst as it is or by optionally molding or the like.

[0052] The silver content in the present silver catalyst 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 an element of group II of the periodic table) 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 a third 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 third mixture from the second mixture (the third step of the present preparation method); or after the third mixture is prepared, the prepared third mixture is sufficiently washed with a solvent. Since the third 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 propylene with oxygen in the presence of the present silver catalyst. Hereinafter, the gas containing propylene 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 propylene 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 propylene 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 propylene used in the present production method may also be diluted with an organic gas other than propylene as long as the gas is inert in the present reaction. Examples of such an organic gas include a lower alkane such as methane and ethane.

[0059] 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, propylene 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, and methyl chloride. 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 volume of the source gas.

[0060] 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.

[0061] 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 propylene 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.

[0062] Further, in the present production method, it is also possible to react propylene with oxygen in the presence of water in addition to the present silver catalyst.

Preferably, water is converted into steam, and the steam is then mixed with oxygen and propylene to prepare a source gas for use in the present production method. Further, the amount of the organic halogen compound used in the case of using water (steam) is preferably 1 to

1000 ppm by volume, more preferably 1 to 500 ppm by volume relative to the volume of the source gas excluding 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. Therefore, when steam is mixed with oxygen and propylene to prepare a source gas, the composition of the source gas is preferably controlled so that the amount of the water used with respect to propylene falls within the above range.

[0063] 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 propylene and oxygen is supplied into the reaction column from the gas supply port using a compressor or the like. As mentioned above, water and/or an organic halogen compound may be contained in the source gas. Propylene and oxygen are brought into contact with each other in the presence of the present silver catalyst when the source gas is brought into contact with the silver catalyst within the reaction column. The contact allows propylene and oxygen contained in the source gas to react with each other to produce propylene oxide, and the product gas containing the propylene 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 propylene 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.

[0064] In this way, produced propylene oxide, unreacted propylene and oxygen, and by-products such as carbon dioxide are contained in the product gas which has passed through the reaction column. Further, when propylene and oxygen are diluted and used, an inert gas used for dilution is mixed. After collecting the product gas, the target propylene oxide can be taken out by suitable separation means such as distillation.

[0065] 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. The 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, propylene oxide can be produced from propylene even if hydrogen is not contained in the source gas.

Examples

[0066] Hereinafter, embodiments of the present invention will be described by illustrating Examples.

[0067] Example 1

In 10 mL of water was dissolved 0.089 g of strontium nitrate to obtain an aqueous strontium nitrate solution, and thereto was then added 10 g of titanium dioxide (Ti0₂, 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 under reduced pressure at 60°C using an evaporator. A residue after distillation was further subjected to heat treatment at 400°C for 5 hours in a firing furnace (atmospheric gas: air) to thereby obtain a first mixture A.

The resulting first mixture A 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 second mixture A. A glass firing tube was filled with the resulting second mixture A, which was subjected to reduction by passing a mixed gas of carbon monoxide (CO)/nitrogen (N₂) (compositional ratio by volume: CO/N₂=1/10) therethrough at 55 mL/min. Through the CO/N₂ 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 second 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 third mixture A. The third 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:

[0068]

[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.

[0069] 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).

[0070] 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:

[0071]

[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))

[0072] The results of the propylene conversion and selectivity which were determined as described above are shown in Table 1.

[0073] [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 ran) 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 Ti0₂ with rutile structure at 27.4, 36.1, 39.2, 41.2, 44.1, 54.3 and o

56.6 of 20 were observed.

[0074] Example 2

The present silver catalyst (the present silver catalyst B) was obtained by the same method as in Example 1 except that the temperature condition (400°C) of the heat treatment for obtaining the first mixture was changed to 300°C. 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.

[0075] Example 3

The present silver catalyst (the present silver catalyst C) was obtained by the same method as in Example 1 except that the temperature condition (400°C) of the heat treatment for obtaining the first mixture was changed to 550°C. 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.

[0076] [Table 1]

[0077] Example 4

In 10 mL of water was dissolved 0.089 g of strontium nitrate to an aqueous strontium nitrate solution, and then 10 g of titanium dioxide (Ti0₂, manufactured by Aldrich, 99.99%, rutile structure) was added to the aqueous strontium nitrate solution, and the resulting mixture was stirred for 1 hour at 60°C. After stirring, water was removed by distillation at 60°C under a reduced pressure using an evaporator to thereby obtain a first mixture D.

The resulting first mixture D 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 to obtain a second mixture D. A glass firing tube was filled with the resulting second mixture D, which was subjected to reduction by passing a mixed gas of carbon monoxide (CO)/nitrogen (N₂) (composition ratio by volume: CO/N₂=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 second mixture D 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 third mixture D. The production of propylene oxide was performed under the same conditions as in Example 1 using the third mixture D obtained in this way as the present silver catalyst (the present silver catalyst D). The results of the propylene conversion and selectivity are shown in Table 2. [0078] Example 5

The present silver catalyst (the present silver catalyst E) was prepared by the same preparation method as in Example 1 and the production of propylene oxide was performed under the same conditions as in Example 1 except that 0.455 g of strontium nitrate was used instead of 0.089 g of strontium nitrate. The results of the propylene conversion and selectivity are shown in Table 2.

[0079] Example 6

The present silver catalyst (the present silver catalyst F) was prepared by the same preparation method as in Example 1 and the production of propylene oxide was performed under the same conditions as in Example 1 except that 0.0445 g of strontium nitrate was used instead of 0.089 g of strontium nitrate. The results of the propylene conversion and selectivity are shown in Table 2.

[0080] [Table 2]

[0081] Example 7

The present silver catalyst (the present silver catalyst G) was prepared by the same preparation method as in Example 1 and the production of propylene oxide was performed under the same conditions as in Example 1 except that 0.099 g of calcium nitrate was used instead of 0.089 g of strontium nitrate. The results of the propylene conversion and selectivity are shown in Table 3.

[0082] Example 8

The present silver catalyst (the present silver catalyst H) was prepared by the same preparation method as in Example 1 and the production of propylene oxide was performed under the same conditions as in Example 1 except that 0.11 g of barium nitrate was used instead of 0.089 g of strontium nitrate. The results of the propylene conversion and selectivity are shown in Table 3.

[0083] Reference Example 1

Titanium dioxide (Ti0₂, 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 (N₂) (compositional ratio by volume: CO/N₂=1/10) therethrough at 55 mL/min. Through the CO/N₂ 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

[0084] [Table 3]

Industrial Applicability

[0085] Propylene oxide is useful as a production intermediate of various engineering materials. According to the present invention, such propylene oxide can be produced without taking safety measures as described above, and thus, the industrial value of the present invention is high.

Claims

1. A method for producing propylene oxide comprising:

an oxidation step of reacting propylene with oxygen in the presence of a catalyst containing

silver,

titanium, and

an element of group II of the periodic table.

2. The method according to claim 1, wherein the element of group II of the periodic table is selected from the group consisting of calcium, strontium, and barium.

3. The method according to claim 1 or 2, wherein the catalyst is a catalyst obtained by a preparation method comprising the following first step and second step:

a second step: a step of mixing the first mixture with at least one member selected from the group consisting of metallic silver and a silver compound to obtain a second mixture.

4. The method according to claim 3, wherein the first step comprises the following step A and step B:

step B: a step of subjecting the mixture to heat treatment to obtain the first mixture.

5. The method according to claim 3 or 4, wherein the preparation method further comprises the following third step:

a third step: a step of subjecting the second mixture to reduction to obtain a third mixture.

6. The method according to any one of claims 3 to 5, wherein the titanium compound is a titanium oxide.

7. The method according to claim 6, wherein the titanium oxide forms a rutile structure.

8. The method according to any one of claims 3 to 7, wherein the compound containing the element of group II of the periodic table is a salt.

9. The method according to any one of claims 3 to 8, wherein the second step is a step of mixing the first mixture with at least one member selected from the group consisting of a silver salt and a silver oxide to obtain the second mixture.

10. The method according to any one of claims 1 to 9, wherein the oxidation step is a step of reacting propylene with oxygen in the presence of an organic halogen compound in addition to the catalyst.

11. The method according to any one of claims 1 to 10, wherein the oxidation step is a step of reacting propylene with oxygen in the presence of water in addition to the catalyst.