Catalyst for producing unsaturated aldehyde, and method for producing unsaturated aldehyde using same

Abstract

[Problem] To provide a method for improving the selectivity of an unsaturated aldehyde. [Solution] A method for producing an unsaturated aldehyde, which uses a catalyst including a complex oxide containing molybdenum, bismuth, iron, and cobalt, and subjects a compound selected from propylene, isobutylene, and a tertiary alcohol to catalytic gas-phase oxidation in the presence of a molecular oxygen-containing gas in a fixed-bed multi-tubular reactor, wherein the catalyst is obtained by reduction treatment followed by oxidation treatment.

Description

Catalyst for producing unsaturated aldehyde and method for producing unsaturated aldehyde using the same

[0001] The present invention relates to a method for producing an unsaturated aldehyde. In particular, the present invention relates to a catalyst suitable for producing an unsaturated aldehyde by catalytic vapor phase oxidation of propylene in the presence of a molecular oxygen-containing gas, and a method for producing an unsaturated aldehyde using the catalyst.

[0002] Acrolein is widely used worldwide as a raw material for industrial products such as polyester resins and polyurethanes, as well as for methionine, which is used in feed additives and pharmaceuticals. It is also used as a raw material for glutaraldehyde, a disinfectant, and pyridine, a raw material for antibacterial agents and pharmaceuticals, as well as for textile treatments. The global acrolein market has been growing at an average annual rate of more than 3%, and demand is expected to continue to grow. Given the recent rise in raw material prices and utility costs, improving acrolein selectivity by even 0.1 mol% in industrial-scale acrolein production has enormous economic significance.

[0003] The most commonly used method for producing acrolein is catalytic vapor-phase oxidation of propylene in the presence of a molecular oxygen-containing gas. In this type of acrolein production by catalytic vapor-phase oxidation of propylene, selectivity is of utmost importance from the perspective of productivity. However, the sequential oxidation of acrolein produces by-products such as acrylic acid, acetaldehyde, and furfural. Increasing the reaction bath temperature to increase propylene conversion in an attempt to obtain more acrolein increases the sequential oxidation, resulting in increased production of by-products such as acrylic acid and a decrease in acrolein selectivity. Lowering the reaction bath temperature to suppress sequential oxidation results in abrupt fluctuations in propylene conversion or acrolein selectivity relative to the reaction bath temperature, resulting in insufficient acrolein production. In other words, in acrolein production by catalytic vapor-phase oxidation of propylene, reactor and process designs that prevent sequential oxidation are required.

[0004] For example, Patent Document 1 discloses a method for producing acrolein and acrylic acid by gas-phase catalytic oxidation of propylene with molecular oxygen in the presence of a solid oxidation catalyst in a fixed-bed tubular reactor, in which the temperature is raised to the range of 250 to 400°C while flowing a gas containing oxygen, nitrogen, and water vapor and containing 0 to 0.5% by volume of propylene through the catalyst layer, and then a gas containing 1 to 3.8% by volume of propylene, 7 to 16% by volume of oxygen, and 5 to 50% by volume of water vapor is flowed at 250 to 400°C for one hour or more, thereby sufficiently suppressing temperature rise in hot spots and enabling acrolein to be produced with high selectivity.

[0005] Japanese Patent Application Laid-Open No. 2003-012589

[0006] However, even with the above-mentioned measures, it is difficult to say that the selectivity of the target product, acrolein, is sufficiently improved by suppressing the by-production of acrylic acid and other products due to successive oxidation. This results in fluctuations in the amount of propylene used in production, significantly affecting production costs. Furthermore, continued operation at low selectivity results in the production of large amounts of by-products, which places a burden on the purification process and increases operating costs. Depending on the type of by-product, it may accumulate on the catalyst surface or in the gas flow path near the catalyst, deactivating the catalyst's active sites, making it necessary to increase the reaction bath temperature. As a result, the catalyst is subjected to thermal stress, shortening its lifespan and reducing selectivity.

[0007] Therefore, an object of the present invention is to provide a method for producing an unsaturated aldehyde such as acrolein by catalytic gas-phase oxidation of a compound selected from propylene, isobutylene, and a tertiary alcohol using a fixed-bed multi-tubular reactor, while improving the conversion of the raw material compound such as propylene, while suppressing the production of by-products such as acrylic acid due to the successive oxidation of acrolein. Another object of the present invention is to provide a catalyst that improves the selectivity of an unsaturated aldehyde such as acrolein while improving the conversion of the raw material compound such as propylene.

[0008] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that the above-mentioned problems can be solved by using a catalyst that has been subjected to a reduction treatment and then an oxidation treatment in a method for producing an unsaturated aldehyde by catalytic vapor-phase oxidation of a compound selected from propylene, isobutylene, and a tertiary alcohol in the presence of a molecular oxygen-containing gas in a fixed-bed multi-tubular reactor using a catalyst containing a composite oxide containing molybdenum, bismuth, iron, and cobalt. Furthermore, they have found that the above-mentioned problems can be solved by providing a catalyst obtained by performing a reduction treatment with a reducing gas and then an oxidation treatment, and the catalyst has an oxidation-reduction volume ratio of 1.10 or more and 1.92 or less, for the production of an unsaturated aldehyde, and they have completed the present invention.

[0009] In this specification, a "catalyst for producing an unsaturated aldehyde" is also referred to simply as a "catalyst for producing an unsaturated aldehyde" or a "catalyst according to the present invention." An "oxidation treatment of a catalyst using an oxidation gas" is also referred to simply as an "oxidation treatment." A "reduction treatment of a catalyst using a reducing gas" is also referred to simply as a "reduction treatment." A "compound selected from propylene, isobutylene, and a tertiary alcohol" is also referred to simply as a "raw material compound such as propylene" or simply as a "raw material compound." A "fixed-bed multi-tubular reactor" is also referred to simply as a "reaction tube." Unless otherwise specified, operations and measurements of physical properties are performed under conditions of room temperature (20 to 35°C) and a relative humidity of 40 to 90% RH. When a certain element is described in terms of content, ratio, proportion, concentration, etc., and two or more types of that element are included, the total amount can be used for calculations.

[0010] As used herein, the term "X to Y" indicating a range includes X and Y and means "X or more and Y or less." As used herein, "A and / or B" means at least one of A and B, and includes both A and B, or either A or B. As used herein, the expression "at least one selected from ..." encompasses both embodiments using only one of the listed elements and embodiments using two or more of them in combination. Furthermore, when the listed elements include elements specified by a range, such as "a hydrocarbon having 1 to 6 carbon atoms," the expression also encompasses embodiments using only one of each individual component within this range, embodiments using two or more of the individual components in combination, and embodiments using elements other than those specified by this range.

[0011] One aspect of the present invention is a method for producing an unsaturated aldehyde by catalytic gas-phase oxidation of a compound selected from propylene, isobutylene, and a tertiary alcohol using a catalyst. The catalyst contains a composite oxide containing molybdenum, bismuth, iron, and cobalt, and is subjected to a reduction treatment followed by an oxidation treatment. This configuration provides a method for improving the conversion of a raw material compound such as propylene while also improving the selectivity of an unsaturated aldehyde such as acrolein. Another aspect of the present invention is a catalyst for producing an unsaturated aldehyde, which contains a composite oxide containing molybdenum, bismuth, iron, and cobalt, has an oxidation-reduction volume ratio of 1.10 or more and 1.92 or less, and is obtained by performing a reduction treatment with a reducing gas followed by an oxidation treatment. This configuration provides a catalyst that improves the conversion of a raw material compound such as propylene while also improving the selectivity of an unsaturated aldehyde such as acrolein. Preferred embodiments of the present invention are described below. The present invention is not limited to the following embodiments and can be modified in various ways within the scope of the claims. Furthermore, the embodiments described in this specification can be combined in any manner to form other embodiments.

[0012] [Catalyst for producing unsaturated aldehyde] The catalyst for producing unsaturated aldehyde used in the present invention is not particularly limited as long as it is a catalyst that can produce unsaturated aldehyde by catalytic gas phase oxidation of a compound selected from propylene, isobutylene, and tertiary alcohol (also referred to as a compound such as propylene), and a conventionally known oxide catalyst can be used. By subjecting this to a specific treatment, it becomes the catalyst according to the present invention (catalyst for producing unsaturated aldehyde). Specifically, an oxide catalyst having a catalytic component represented by the following general formula 1 can be suitably used as the catalyst for producing unsaturated aldehyde. Formula 1: Mo ａ Bi ｂ Fe ｃ Co ｄ X1 ｅ X2 ｆ X3 ｇ O ｘ (wherein Mo is molybdenum, Bi is bismuth, Fe is iron, Co is cobalt, X1 is at least one element selected from nickel, copper, silver, phosphorus, tellurium, antimony, tin, cerium, lead, niobium, manganese, arsenic and zinc, X2 is at least one element selected from alkali metals, alkaline earth metals, boron and thallium, X3 is at least one element selected from tungsten, silicon, aluminum, zirconium and titanium. and O represents oxygen; and a, b, c, d, e, f, g, and x represent the atomic ratios of Mo, Bi, Fe, Co, X1, X2, X3, and O, respectively, where when a = 12, b = 0.1 to 20, c = 0.1 to 20, d = 0.1 to 20, e = 0 to 20, f = 0.001 to 10, and g = 0 to 20; and x is a number determined by the oxidation state of each element, but is 0 to 60.

[0013] The catalyst used in the present invention may be a supported catalyst in which the above-mentioned catalyst components are supported on a carrier (also referred to as a supported catalyst), or a non-supported catalyst in which the catalyst components containing the above-mentioned catalyst components are not supported on a carrier. Preferably, the catalyst according to the present invention is a supported catalyst in which the catalyst components are supported on a carrier.

[0014] When the catalyst used in the present invention is a supported catalyst, the support that can be used is not particularly limited, and known supports can be used, but it is preferable that they are inactive against acrolein and acrylic acid. Specific examples include inorganic oxides such as silica, alumina, titania, zirconia, magnesia, steatite, and cordierite, and composite oxides thereof such as alumina-silica, silica-magnesia, and silica-magnesia-alumina; crystalline metallosilicates such as zeolite; metals and alloys such as stainless steel and aluminum; and inorganic compounds such as activated carbon, silicon carbide, and silicon nitride. Of these, it is preferable to use silica, alumina, titania, zirconia, or composite oxides thereof as the support.

[0015] When the catalyst used in the present invention is a supported catalyst, the shape of the support is not particularly limited, and examples thereof include spherical, cylindrical, columnar, and irregular shapes. The size of the support is also not particularly limited, and a support of a size that can be used when packed in a reaction tube may be selected. Specifically, the maximum outer diameter of the support is preferably 3 to 20 mm, and more preferably 4 to 8 mm.

[0016] The catalytic component loading rate can be appropriately selected depending on the reduction treatment time and oxidation treatment time. Specifically, the catalytic component loading rate is preferably 50 to 200 wt %, more preferably 100 to 150 wt %, relative to 100 wt % of the carrier. In this specification, the catalytic component loading rate in a supported catalyst is measured by the following method.

[0017] [Measurement of Support Ratio of Supported Catalyst] The support ratio of catalyst components in a supported catalyst is the percentage of the total weight of catalyst components relative to the total weight of the support, and can be calculated using the following formula 2. Here, the total weight of catalyst components is the value obtained by subtracting the total weight of the support (B) from the total weight of the catalyst (A). Note that separation of catalyst components from a supported catalyst can be carried out by the following procedure.

[0018] Equation 2: Loading rate (wt %) = (total weight of catalyst [g] (A) - total weight of support [g] (B)) / (total weight of support [g] (B)) × 100

[0019] The catalyst packed into the reaction tube does not necessarily have to be a single catalyst; it is also possible to pack multiple types of known catalysts so that each catalyst forms a layer (catalyst layer). For example, multiple types of catalysts with different activity can be packed as different catalyst layers depending on the purpose. Specifically, a method of packing catalysts with different loading rates so that the loading rate increases from the raw gas inlet side to the outlet side, a method of diluting a portion of the catalyst with an inert carrier, or a combination of these methods may be employed. Furthermore, a portion of the catalyst layer may be diluted with an inert carrier such as inorganic oxides such as silica, alumina, titania, zirconia, magnesia, steatite, cordierite, or composite oxides thereof such as alumina-silica, silica-magnesia, or silica-magnesia-alumina; crystalline metallosilicates such as zeolite; metals or alloys such as stainless steel and aluminum; activated carbon, silicon carbide, or silicon nitride.

[0020] An inert substance layer may be provided between the end of the reaction tube on the raw material gas inlet side and the catalyst layer, and / or between the end of the reaction tube on the raw material gas outlet side and the end of the catalyst layer, for the purpose of supporting the catalyst, preheating the raw material gas, etc. The inert substance layer consists solely of an inert substance, and the inert substance may be inorganic oxides such as silica, alumina, titania, zirconia, magnesia, steatite, cordierite, etc., composite oxides thereof such as alumina-silica, silica-magnesia, and silica-magnesia-alumina, etc.; crystalline metallosilicates such as zeolite; metals and alloys such as stainless steel and aluminum; activated carbon, silicon carbide, silicon nitride, etc., as long as it does not contribute to the target reaction. However, if a dilution operation with an inert carrier is performed when forming the catalyst layer, it is preferable to use the same inert substance as the inert carrier, as this simplifies subsequent catalyst replacement and sieving operations.

[0021] The reaction tube filled with the catalyst used in the present invention is preferably a fixed-bed, fluidized-bed, or plate-type reactor. A fluidized-bed reactor is not preferred because the pressure loss during the reduction and oxidation treatments of the present invention increases, which may lead to excessive progress of the reduction and oxidation treatments and a significant decrease in catalytic performance. On the other hand, a fixed-bed multi-tubular reactor is preferred because the pressure loss during the reduction and oxidation treatments of the present invention decreases, allowing the reduction and oxidation treatments to be carried out under appropriate pressure, eliminating the risk of a decrease in catalytic performance. The reaction tube may be a single-tube type, but a multi-tube type is more preferred, and a fixed-bed multi-tubular reactor is particularly preferred. The inner diameter of the reaction tube in a fixed-bed multi-tubular reactor is usually 15 to 50 mm, more preferably 20 to 40 mm, and even more preferably 22 to 38 mm.

[0022] The amount of catalyst (weight of catalyst packed in one reaction tube) is preferably 100 to 5000 g, more preferably 200 to 4500 g, and most preferably 350 to 4000 g.

[0023] The method for producing an unsaturated aldehyde according to the present invention essentially involves the use of a catalyst (the catalyst according to the present invention) that has undergone a reduction treatment followed by an oxidation treatment. The reduction treatment causes grain growth of a composite oxide containing molybdenum and bismuth (hereinafter referred to as BiMo composite oxide) on a composite oxide containing molybdenum and cobalt (hereinafter referred to as CoMo composite oxide), and iron is further dissolved and dispersed in the CoMo composite oxide. Next, oxidation treatment refines the BiMo composite oxide grains grown on the CoMo composite oxide, and simultaneously oxidizes the iron dissolved and dispersed in the CoMo composite oxide, thereby creating a state in which oxygen is smoothly supplied to the fine BiMo composite oxide on the CoMo composite oxide. It is presumed that this improves the conversion of compounds such as propylene and the selectivity for the unsaturated aldehyde. However, the technical scope of the present invention is not limited by this presumption. <Reduction Treatment>

[0024] The method for producing an unsaturated aldehyde according to the present invention uses a catalyst that has been subjected to a reduction treatment and then an oxidation treatment after a catalyst packed in a reaction tube has been subjected to the reduction treatment. The reduction treatment includes a (Rd1) reduction preparation step, a (Rd2) reduction step, and a (Rd3) reduction termination step, and refers to an operation in which a reducing gas is passed through a catalyst layer at a reduction treatment temperature Tr at a predetermined space velocity for a predetermined time, and then the flow of the reducing gas is terminated at a predetermined timing.

[0025] In the reduction preparation step (Rd1), the temperature of the catalyst layer formed by the catalyst packed in the reaction tube is adjusted to be equal to the reduction treatment temperature Tr. The temperature of the catalyst layer is also referred to as the catalyst layer temperature.

[0026] The reduction treatment temperature Tr means the temperature of the heat transfer medium (temperature of the reaction bath) flowing around the reaction tube inside the jacket (also referred to as shell) that covers the reaction tube in the reactor. Specifically, a heat transfer medium temperature measurement tube separate from the reaction tube is installed inside the jacket, and a thermocouple is installed at a position 500 to 1000 mm in the depth direction from the inlet end face of the heat transfer medium temperature measurement tube, thereby measuring the reduction treatment temperature Tr. The number of thermocouples may be one or more. When one thermocouple is used, the temperature measured by the thermocouple is taken as the reduction treatment temperature Tr. When multiple thermocouples are used, the arithmetic mean value of the temperatures measured by the thermocouples is taken as the reduction treatment temperature Tr.

[0027] The reduction treatment temperature Tr can be changed depending on the target unsaturated aldehyde. When the target unsaturated aldehyde is acrolein, the reduction treatment temperature Tr is preferably 310°C or higher, more preferably 320°C or higher, and even more preferably 330°C or higher. Also, the reduction treatment temperature Tr is preferably 345°C or lower, more preferably 340°C or lower, and even more preferably 335°C or lower. A reduction treatment temperature Tr lower than 310°C is undesirable because it reduces the effect of the reduction treatment on the catalyst, and there is a risk that the conversion rate of propylene and the like and the unsaturated aldehyde selectivity of the catalyst according to the present invention after undergoing the oxidation treatment described below may not be efficiently improved. Furthermore, a reduction treatment temperature Tr higher than 345°C is undesirable because excessive reduction of the catalyst may proceed, and there is a risk that the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after undergoing the oxidation treatment described below may decrease. The reduction treatment temperature Tr is preferably, for example, 310°C or higher and 345°C or lower.

[0028] In the reduction step (Rd2), the catalyst layer temperature is adjusted to a reduction treatment temperature Tr in the reduction preparation step (Rd1), and then a reducing gas is passed through the catalyst layer at a predetermined space velocity for a predetermined time. The start of the reduction treatment is determined as the point at which the reducing gas starts to pass through the catalyst layer in the reduction step (Rd2).

[0029] The reducing gas is preferably a mixed gas containing 5.0 to 10.0 vol% of a reducing gas and 65 to 95 vol% of a reducing diluent gas. The reducing gas is preferably present in an amount of 5.0 vol% or more, more preferably 7.0 vol% or more, and even more preferably 8.0 vol% or more, relative to the reducing gas. The reducing gas is preferably present in an amount of 10.0 vol% or less, more preferably 9.0 vol% or less, and even more preferably 8.5 vol% or less, relative to the reducing gas. The reducing gas is preferably present in an amount of 5.0 to 9.0 vol% or more, more preferably 7.0 to 9.0 vol%, even more preferably 7.0 to 8.5 vol%, and most preferably 8.0 to 8.5 vol%. A reducing gas content of less than 5.0 vol% is undesirable because it reduces the effect of the reduction treatment on the catalyst, potentially preventing efficient improvement in the conversion of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after the oxidation treatment described below. Furthermore, if the amount of the reducing gas exceeds 10.0%, the reduction of the catalyst will proceed excessively, which is undesirable because it may result in a decrease in the conversion of raw material compounds such as propylene and the selectivity for unsaturated aldehyde of the catalyst according to the present invention after undergoing the oxidation treatment described below.

[0030] The reducing gas in the reducing gas is at least one selected from hydrogen, ammonia, carbon monoxide (CO), hydrocarbons having 1 to 6 carbon atoms, alcohols having 1 to 6 carbon atoms, aldehydes having 1 to 6 carbon atoms, and amines having 1 to 6 carbon atoms. Here, since the reducing gas in the reducing gas is desirably one that enhances the effects of the reduction treatment, the reducing gas is preferably at least one selected from CO, hydrocarbons having 1 to 6 carbon atoms, alcohols having 1 to 6 carbon atoms, aldehydes having 1 to 6 carbon atoms, and amines having 1 to 6 carbon atoms, more preferably at least one selected from hydrocarbons having 2 to 5 carbon atoms, alcohols having 2 to 5 carbon atoms, aldehydes having 2 to 5 carbon atoms, and amines having 2 to 5 carbon atoms, and even more preferably at least one selected from propane, butane, propylene, isobutene, acrolein, methacrolein, propionaldehyde, butylaldehyde, 1-propanol, 2-propanol, 1-butanol, 2-butanol, tertiary butyl alcohol, ethylene glycol, glycerin, propylamine, and butylamine, with propylene being particularly preferred. Furthermore, a mixture of multiple types of reducing gases may be used, but a mixed gas of propylene and propane is preferred.

[0031] The reducing diluent gas is preferably present in an amount of 65% by volume or more, more preferably 70% by volume or more, and even more preferably 75% by volume or more, relative to the reducing gas. The reducing diluent gas is preferably present in an amount of 95% by volume or less, more preferably 85% by volume or less, and even more preferably 80% by volume or less, relative to the reducing gas. The reducing diluent gas is preferably present in an amount of 65 to 95% by volume, more preferably 65 to 85% by volume, even more preferably 70 to 85% by volume, particularly preferably 70 to 80% by volume, and most preferably 75 to 80% by volume. If the reducing diluent gas is present in an amount of less than 65% by volume, the reducing gas concentration will be too high, causing excessive catalyst reduction, which may result in a decrease in the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after the oxidation treatment described below. If the reducing diluent gas is present in an amount of more than 95% by volume, the reducing gas concentration will be too low, reducing the effectiveness of the reduction treatment on the catalyst. As a result, there is a risk that the conversion rate of raw material compounds such as propylene and the selectivity for unsaturated aldehyde of the catalyst according to the present invention after undergoing the oxidation treatment described below may not be efficiently improved, which is undesirable.

[0032] The reducing diluent gas in the reducing gas is at least one selected from nitrogen, water vapor, argon, helium, and carbon dioxide. Here, from the viewpoint of availability, at least one selected from nitrogen, water vapor, and carbon dioxide is preferable. Furthermore, when the unsaturated aldehyde is acrolein, from the viewpoint of availability and overall cost, at least one of nitrogen and water vapor is more preferable. The reducing diluent gas in the reducing gas may be a mixture of multiple gases, and is preferably a mixed gas of nitrogen and water vapor.

[0033] When the reducing diluent gas contains nitrogen, it is preferably 45.0 vol% or more, more preferably 60.0 vol% or more, and even more preferably 70.0 vol% or more relative to the reducing gas. Furthermore, it is preferable that the nitrogen be 90.0 vol% or less, more preferably 80.0 vol% or less, and even more preferably 75.0 vol% or less relative to the reducing gas. When the reducing diluent gas contains nitrogen, it is preferably 45.0 vol% or more and 90.0 vol% or less relative to the reducing gas. If the nitrogen content is less than 45.0 vol%, the reducing gas concentration will be too high, causing excessive reduction of the catalyst, which may result in a decrease in the conversion of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after undergoing the oxidation treatment described below, which is undesirable. Furthermore, if the nitrogen content exceeds 90.0% by volume, the reducing gas concentration will be too low, reducing the effect of the reduction treatment on the catalyst, and there is a risk that the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after undergoing the oxidation treatment described below may not be efficiently improved, which is undesirable.

[0034] When the reducing dilution gas contains water vapor, it is preferably 1.0 vol% or more, more preferably 3.0 vol% or more, and even more preferably 5.0 vol% or more relative to the reducing gas. The water vapor content is preferably 35.0 vol% or less, more preferably 30.0 vol% or less, and even more preferably 27.0 vol% or less relative to the reducing gas. If the water vapor content is less than 1.0 vol%, the heat capacity of the reducing gas decreases, making it impossible to completely suppress heat generation during reduction, resulting in an increase in the catalyst layer temperature. This can lead to excessive reduction of the catalyst, which may result in a decrease in the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after undergoing the oxidation treatment described below. If the water vapor content exceeds 35.0 vol%, the reducing gas concentration is too low, reducing the effectiveness of the reduction treatment on the catalyst. As a result, it is not preferable to efficiently improve the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after undergoing the oxidation treatment described below. When the reducing diluent gas contains water vapor, the water vapor content is preferably 1.0% by volume or more and 35.0% by volume or less relative to the reducing gas.

[0035] When the reducing gas contains an oxidizing gas such as oxygen, the oxidizing gas is preferably contained in an amount of 7.0 vol% or more, more preferably 10.0 vol% or more, and even more preferably 12.0 vol% or more relative to the reducing gas. The oxidizing gas is preferably contained in an amount of 21.0 vol% or less, more preferably 19.0 vol% or less, and even more preferably 16.0 vol% or less relative to the reducing gas. When the reducing gas contains an oxidizing gas such as oxygen, the oxidizing gas is preferably contained in an amount of 7.0 vol% or more and 21.0 vol% or less relative to the reducing gas. When the oxidizing gas is contained, the reduced catalyst is partially oxidized and then reduced again by the reducing gas. This suppresses excessive reduction of the catalyst, and prevents a decrease in the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after undergoing the oxidation treatment described below. If the proportion of the oxidizing gas is less than 7.0% by volume, the concentration of the reducing gas will be relatively high, which may cause excessive reduction of the catalyst, resulting in a decrease in the conversion rate of raw material compounds such as propylene and the selectivity for unsaturated aldehydes of the catalyst according to the present invention after undergoing the oxidation treatment described below, which is undesirable. Also, if the proportion of the oxidizing gas is more than 21.0% by volume, the oxidizing gas concentration will be too high to reduce the catalyst, which may prevent an improvement in the conversion rate of raw material compounds such as propylene and the selectivity for unsaturated aldehydes of the catalyst according to the present invention after undergoing the oxidation treatment described below, which is undesirable.

[0036] When the reducing gas contains an oxidizing gas such as oxygen, the ratio by volume of the oxidizing gas to the volume of the reducing gas (also referred to as the redox volume ratio) represented by Equation 3 is preferably 1.10 or more, more preferably 1.30 or more, and even more preferably 1.38 or more. It is also preferably 1.92 or less, more preferably 1.91 or less, even more preferably 1.90 or less, and particularly preferably 1.88 or less. The redox volume is preferably 1.10 to 1.92, more preferably 1.10 to 1.91, even more preferably 1.30 to 1.91, particularly preferably 1.30 to 1.90, and most preferably 1.38 to 1.88. By achieving this ratio, the effect of the reduction treatment on the catalyst is enhanced, and the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after undergoing the oxidation treatment described below can be efficiently improved.

[0037] Equation 3: Redox volume ratio = (volume % of oxidizing gas) / (volume % of reducing gas)

[0038] The space velocity in the reduction step (Rd2) was 900 h －１ It is preferable that the time is 1100 hours or more. －１ More preferably, 1350 h or more. －１ It is more preferable that the temperature is 2200 h or more. －１ It is preferable that the time is 2100 hours or less. －１ More preferably, it is 1750 h or less. －１ The space velocity in the reduction step (Rd2) is preferably 900 to 2200 h －１ It is preferable that the time is 900 to 2100 hours. －１ More preferably, it is 900 to 1750 h －１ More preferably, 1100 to 1750 h －１ It is particularly preferable that the time is 1350 to 1750 h. －１ It is most preferable that the space velocity in the reduction step (Rd2) is 900 h －１If the space velocity in the reduction treatment is less than 2200 h , the effect of the reduction treatment on the catalyst is reduced, and there is a risk that the conversion of raw material compounds such as propylene and the selectivity for unsaturated aldehyde of the catalyst according to the present invention after the oxidation treatment described below cannot be efficiently improved. －１ If the temperature exceeds this range, the reduction of the catalyst may proceed excessively, which may result in a decrease in the conversion of raw material compounds such as propylene and the selectivity for unsaturated aldehyde of the catalyst according to the present invention after the oxidation treatment described below, and this is not preferable.

[0039] In the reduction step (Rd2), the catalyst generates heat, causing an increase in the catalyst layer temperature. If the catalyst layer temperature becomes too high, the reduction may proceed excessively, resulting in a decrease in catalyst performance. Therefore, it is preferable to perform the reduction step (Rd2) so that the catalyst layer temperature is higher than the reduction treatment temperature Tr and 420°C or lower. The catalyst layer temperature can be measured by inserting a thermocouple at any position in the depth direction from the end face on the inlet side or the end face on the outlet side of the reaction tube filled with the catalyst. However, it is preferable to measure the catalyst layer temperature by installing a thermocouple at a hot spot where the catalyst layer generates the most heat.

[0040] In the reduction termination step (Rd3), the flow of the reducing gas is terminated. Terminating the flow of the reducing gas reduces the catalyst layer temperature. In the reduction termination step (Rd3), the point at which the catalyst layer temperature reaches the reduction treatment temperature Tr is defined as the end point of the reduction treatment. The difference between the start point and end point of the reduction treatment is defined as the reduction treatment time. However, if at least one optional step different from the reduction step (Rd2), such as the reduction termination step (Rd4), the oxidation step (Ox2'), and the treatment step using an inert gas (In1), which will be described later, is intervened, the time for that step (total time) is subtracted.

[0041] The treatment time (reduction treatment time) in the reduction step (Rd2) or (Rd2') is preferably 1 hour or more, more preferably 10 hours or more, and even more preferably 45 hours or more. It is also preferably 1,800 hours or less, more preferably 1,000 hours or less, and even more preferably 500 hours or less. The reduction treatment time is preferably 1 to 1,800 hours, more preferably 1 to 1,000 hours, even more preferably 10 to 1,000 hours, particularly preferably 45 to 1,000 hours, and most preferably 45 to 500 hours. A reduction treatment time of less than 1 hour is undesirable because the effect of the reduction treatment on the catalyst is reduced, and the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after undergoing the oxidation treatment described below cannot be efficiently improved. A reduction treatment time of more than 1,800 hours is also undesirable because the reduction of the catalyst proceeds excessively, resulting in a decrease in the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after undergoing the oxidation treatment described below. Here, the effect of the reduction treatment differs depending on the catalyst amount and the loading rate, so the reduction treatment time can be determined depending on the catalyst amount and the loading rate. For example, when the loading rate is 100 to 150 wt%, the reduction treatment time is preferably 45 to 1,000 hours, more preferably 45 to 500 hours.

[0042] Here, the reduction treatment may be performed continuously for the predetermined time, or may be performed intermittently. An example of an "intermittent" reduction treatment is a treatment performed in the following order: (Rd1) reduction preparation step, (Rd2) reduction step, (Rd4) reduction stop step, (Rd2) reduction step, and (Rd3) reduction termination step. Another example of an "intermittent" reduction treatment is a treatment performed in the following order: (Rd1) reduction preparation step, (Rd2) reduction step, (Rd4) reduction stop step, (Ox2') oxidation step, (Rd2) reduction step, and (Rd3) reduction termination step. Another example of an "intermittent" reduction treatment is a treatment performed in the following order: (Rd1) reduction preparation step, (Rd2) reduction step, (Rd4) reduction stop step, (In1) inert gas treatment step, (Rd2) reduction step, and (Rd3) reduction termination step.

[0043] The reduction termination step (Rd4) refers to a step of stopping the flow of the reducing gas. The oxidation step (Ox2') refers to a step of flowing an oxidation gas containing an oxidizing gas and an oxidizing dilution gas (which does not contain a reducing gas, or if it does contain a reducing gas, its concentration is 0.1 volume % or less). The inert gas treatment step (In1) refers to a step of flowing an inert gas such as nitrogen, argon, or helium. All of steps (Rd4), (Ox2'), and (In1) are performed under conditions such that the temperature of the catalyst layer does not fall below the reduction treatment temperature Tr. Here, (Rd4) and (Rd2), which are performed between (Rd1) and (Rd3), may be repeated, or (Ox2') and (Rd2) may be repeated, or a combination of (Rd4), (Ox2'), and (Rd2) may be repeated. However, it is preferable that the time for steps other than reduction performed between the reduction step (Rd2) be shorter than the reduction treatment time. Furthermore, in any reduction treatment, it is preferable that the final step of the reduction treatment is the reduction termination step (Rd3), the step immediately before the final step is the reduction step (Rd2), and the total time of the reduction treatment is the above-mentioned predetermined time.

[0044] In the oxidation step (Ox2'), after the reduction termination step (Rd4), an oxidation gas containing an oxidizing gas and an oxidizing dilution gas is passed through at a predetermined space velocity for a predetermined time. <Oxidation Treatment>

[0045] The method for producing an unsaturated aldehyde according to the present invention uses a catalyst that has been subjected to a reduction treatment followed by an oxidation treatment. The oxidation treatment includes an oxidation preparation step (Ox1), an oxidation step (Ox2), and an oxidation termination step (Ox3), and refers to an operation in which an oxidation gas is passed through a catalyst layer at a predetermined space velocity for a predetermined time at a temperature To such that To-Tr, which is the difference between the oxidation treatment temperature To and the reduction treatment temperature Tr, falls within a desired range, and the flow of the oxidation gas is then terminated at a predetermined timing.

[0046] In the oxidation preparation step (Ox1), the temperature of the catalyst layer (catalyst layer temperature) formed from the catalyst packed in the reaction tube is adjusted to be equal to the oxidation treatment temperature To. At this time, the temperature To is adjusted so that the difference To-Tr (ΔTor) between the oxidation treatment temperature To and the reduction treatment temperature Tr is within a desired range.

[0047] The oxidation treatment temperature To refers to the temperature of the heat transfer medium (reaction bath temperature) flowing around the reaction tube inside the jacket (also referred to as shell) that covers the reaction tube in the reactor. A separate heat transfer medium temperature measurement tube is installed inside this jacket, and the oxidation treatment temperature To can be measured by installing a thermocouple at a depth of 500 to 1000 mm from the inlet end of the heat transfer medium temperature measurement tube. The number of thermocouples may be one or more. When one thermocouple is used, the temperature measured by that thermocouple is taken as the oxidation treatment temperature To. When multiple thermocouples are used, the arithmetic mean value of the temperatures measured by those thermocouples is taken as the oxidation treatment temperature To.

[0048] The oxidation treatment temperature To can be changed depending on the target unsaturated aldehyde. ΔTor, which is the difference between the oxidation treatment temperature To in the oxidation treatment and the reduction treatment temperature Tr in the reduction treatment, is expressed by Equation 4 and is preferably 3°C or higher, more preferably 5°C or higher, and even more preferably 15°C or higher. Furthermore, the oxidation treatment is preferably performed at an oxidation treatment temperature To of 45°C or lower, more preferably 40°C or lower, and even more preferably 30°C or lower. ΔTor is preferably 3 to 45°C, more preferably 3 to 40°C, even more preferably 3 to 35°C, particularly preferably 5 to 35°C, and most preferably 15 to 30°C.

[0049] The ΔTor indicates the standard for the oxidation treatment temperature To sufficient to remove or reduce the reducing gas remaining in the catalyst after reduction treatment. By increasing the oxidation treatment temperature above the reduction treatment temperature (To > Tr), the reducing gas remaining in the catalyst after reduction treatment can be efficiently removed or reduced, thereby improving the conversion of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst of the present invention after oxidation treatment. However, if the ΔTor is less than 3°C, the removal of the reducing gas remaining in the catalyst after reduction treatment may not proceed smoothly, resulting in the remaining reducing gas poisoning the catalyst, which may reduce the conversion of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst of the present invention after oxidation treatment. Furthermore, if the ΔTor exceeds 45°C, the catalyst may be excessively oxidized, which may reduce the conversion of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst of the present invention after oxidation treatment.

[0050] Equation 4: ΔTor=To−Tr

[0051] When the target unsaturated aldehyde is acrolein, the oxidation treatment temperature To is preferably 313°C or higher, more preferably 315°C or higher, and even more preferably 320°C or higher. Also, it is preferably 420°C or lower, more preferably 400°C or lower, and even more preferably 380°C or lower. An oxidation treatment temperature To below 313°C is undesirable because the oxidation effect on the catalyst is reduced, and the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after oxidation treatment may not be efficiently improved. Furthermore, an oxidation treatment temperature above 420°C is undesirable because excessive oxidation of the catalyst may proceed, resulting in a reduction in the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after oxidation treatment. The oxidation treatment temperature To is preferably 313°C or higher and 420°C or lower.

[0052] In the oxidation step (Ox2), after the catalyst layer temperature is adjusted to the oxidation treatment temperature To in the oxidation preparation step (Ox1), an oxidation gas containing an oxidizing gas and an oxidation dilution gas is passed through at a predetermined space velocity for a predetermined time. The point at which the flow of the oxidation gas begins in the oxidation step (Ox2) is set as the starting point for calculating the oxidation treatment time.

[0053] The oxidizing gas is a mixed gas containing an oxidizing gas and an oxidizing diluent gas. The oxidizing gas does not contain a reducing gas, or if it does contain a reducing gas, the amount is 0.1 vol% or less. Here, the oxidizing gas is preferably 5.0 vol% or more, more preferably 10.0 vol% or more, more preferably 15.0 vol% or more, and even more preferably 20.0 vol% or more, relative to the oxidizing gas. The oxidizing gas is preferably 30.0 vol% or less, more preferably 25.0 vol% or less, and even more preferably 22.0 vol% or less, relative to the oxidizing gas. If the oxidizing gas concentration is less than 1.0 vol%, the oxidizing gas concentration is too low, which reduces the effect of the oxidation treatment on the catalyst and may prevent efficient improvement of the conversion rate of raw material compounds such as propylene and the selectivity for unsaturated aldehyde after the oxidation treatment, which is undesirable. On the other hand, if the oxidizing gas concentration exceeds 30.0% by volume, the oxidizing gas concentration will be too high, causing excessive oxidation of the catalyst, which may result in a decrease in the conversion of raw material compounds such as propylene and the selectivity for unsaturated aldehyde of the catalyst according to the present invention after oxidation treatment. The oxidizing gas concentration is preferably 5.0% by volume or more and 30.0% by volume or less relative to the oxidizing gas.

[0054] The oxidizing gas in the oxidizing gas is preferably a gas having oxidizing properties such as oxygen, more preferably oxygen. The oxidizing gas may be oxygen alone or a mixed gas containing oxygen such as air. When air is used, the oxygen in the air corresponds to the oxidizing gas, and the nitrogen and carbon dioxide in the air correspond to the oxidizing diluent gas.

[0055] The oxidizing dilution gas in the oxidizing gas may be mixed in a proportion that can increase the heat capacity of the oxidizing gas and thereby exert the effect of the oxidation treatment, preferably 70 vol% or more, more preferably 75 vol% or more, and even more preferably 78 vol% or more relative to the oxidizing gas. Also, 95 vol% or less is preferred, more preferably 90 vol% or less, and even more preferably 80 vol% or less. The inclusion of 70 vol% or more but 95 vol% or less of the oxidizing dilution gas in the oxidizing gas increases the heat capacity of the oxidizing gas, improving the effect of the oxidation treatment on the catalyst, while avoiding a decrease in the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity due to excessive oxidation of the catalyst according to the present invention after reduction treatment, which is preferable.

[0056] The oxidizing dilution gas in the oxidizing gas is at least one selected from water vapor, nitrogen, carbon dioxide, helium, and argon. From the viewpoint of availability, at least one of water vapor and nitrogen is preferred, and nitrogen is more preferred. The oxidizing dilution gas in the oxidizing gas may be a mixture of multiple gases, and is preferably a mixed gas of nitrogen and water vapor. The use of these oxidizing dilution gases increases the heat capacity of the oxidizing gas, thereby improving the efficiency of the oxidation treatment on the catalyst.

[0057] When the oxidation dilution gas contains nitrogen, the nitrogen content is preferably 30.0 vol% or more, more preferably 40.0 vol% or more, and even more preferably 60.0 vol% or more relative to the oxidation gas. Furthermore, the nitrogen content is preferably 90.0 vol% or less, more preferably 85.0 vol% or less, and even more preferably 80.0 vol% or less relative to the reducing gas. When the oxidation dilution gas contains nitrogen, the nitrogen content is preferably 30.0 vol% or more and 90.0 vol% or less relative to the oxidation gas. If the nitrogen content is less than 30.0 vol%, the oxidation gas concentration becomes too high, causing excessive oxidation of the catalyst, which may result in a decrease in the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after oxidation treatment. Furthermore, if the nitrogen content exceeds 85.0% by volume, the concentration of the oxidizing gas will be too low, reducing the effect of the oxidation treatment on the catalyst, and there is a risk that the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after the oxidation treatment may not be efficiently improved, which is undesirable.

[0058] Water vapor, which is one type of oxidation dilution gas, can increase the heat capacity during oxidation treatment and improve the efficiency of the oxidation treatment on the catalyst. However, if the content is too high, it may promote sublimation of the molybdenum contained in the catalyst, shortening its lifespan. Therefore, it is most preferable that the oxidation dilution gas does not contain water vapor.

[0059] When the oxidation dilution gas contains water vapor to improve the oxidation treatment efficiency, the water vapor content is preferably 60.0 vol% or less, more preferably 30.0 vol% or less, and even more preferably 10.0 vol% or less, relative to the oxidation gas. By setting the water vapor content to 60.0 vol% or less, the oxidation treatment of the catalyst proceeds while suppressing sublimation of molybdenum contained in the catalyst, and the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after the oxidation treatment can be efficiently improved. When the oxidation dilution gas contains water vapor, the water vapor content is, for example, 20 vol% or more and 60.0 vol% or less, or 40 vol% or more and 55.0 vol% or less.

[0060] In one embodiment of the present invention, the oxidation gas is substantially free of water vapor. The term "substantially free of water vapor" means that the oxidation gas does not contain water vapor, or, even if it contains water vapor, the water vapor content in the oxidation gas is 5% by volume or less, 1% by volume or less, or 0.1% by volume or less. The oxidation gas being substantially free of water vapor more significantly enhances the technical effect of improving the selectivity of unsaturated aldehydes such as acrolein while improving the conversion rate of raw material compounds such as propylene.

[0061] The space velocity of the oxidation gas in the oxidation step (Ox2) or (Ox2') is 100 h －１ It is preferable that the time is 300 hours or more. －１ More preferably, 500 h or more －１ It is more preferable that the temperature is 1500 h or more. －１ It is preferable that the time is 1300 h or less. －１ More preferably, it is 1000 h or less. －１ The space velocity of the oxidation gas in the oxidation step (Ox2) or (Ox2') is preferably 100 h －１ More than 1500h －１ It is preferable that the space velocity of the oxidation gas in the oxidation step is 100 h －１ If the oxidation gas is used in the oxidation step, the effect of the oxidation treatment on the catalyst is reduced, and the conversion of raw material compounds such as propylene and the selectivity for unsaturated aldehyde of the catalyst according to the present invention after the oxidation treatment may not be efficiently improved. －１ If the temperature exceeds this range, oxidation of the catalyst will proceed excessively, which is undesirable because it may result in a decrease in the conversion of raw material compounds such as propylene and the selectivity for unsaturated aldehyde of the catalyst according to the present invention after the oxidation treatment.

[0062] In the oxidation step (Ox2) or (Ox2'), it is preferable to flow the oxidation gas so that the difference between the catalyst layer temperature immediately before the oxidation gas is flowed and the catalyst layer temperature while the oxidation gas is flowing is 20°C or less. The catalyst layer temperature immediately before the oxidation gas is flowed is To, but when the flow of the oxidation gas starts, the catalyst layer temperature rises as the reducing gas adsorbed on the catalyst and remaining therein burns during the oxidation treatment. It is not preferable for the difference between the catalyst layer temperature immediately before the oxidation gas is flowed and the catalyst layer temperature while the oxidation gas is flowing to exceed 20°C, as this may result in a decrease in catalytic performance. The catalyst layer temperature can be measured by inserting a thermocouple at any position in the depth direction from the end face on the inlet side or the end face on the outlet side of the reaction tube filled with the catalyst. However, it is preferable to measure the temperature by placing a thermocouple at a hot spot where the catalyst layer generates the most heat.

[0063] In the oxidation termination step (Ox3), the flow of the oxidation gas is terminated, and the time when the difference between the catalyst layer temperature (To) before the oxidation treatment and the catalyst layer temperature during the oxidation treatment reaches 0°C is defined as the end point of the oxidation treatment, and the time difference between the start point and the end point is defined as the oxidation treatment time. However, if at least one optional step different from the oxidation step (Ox2), such as the oxidation termination step (Ox4), the reduction step (Rd2'), and the treatment step using an inert gas (In1), which will be described later, is intervened, the time (total time) of that step is subtracted. The flow of the oxidation gas is preferably terminated when it is confirmed that the concentration of carbon monoxide (CO) contained in the outlet gas discharged from the reaction tube during the oxidation treatment has reached a certain value or less. As described above, the oxidation step (Ox2) is terminated when the difference between the catalyst layer temperature (To) before the oxidation treatment and the catalyst layer temperature during the oxidation treatment reaches 0°C. Here, in the production of unsaturated aldehydes, the reducing gas adsorbed on the catalyst is oxidized by the oxidation treatment to generate CO.

[0064] There is a risk that the reducing gas contained in the reducing gas used in the reduction treatment will remain adsorbed on the catalyst even after the reduction treatment, resulting in a decrease in catalytic performance. Here, when the reducing gas in the reducing gas used in the reduction treatment contains carbon, that is, when the reducing gas is at least one of CO, a hydrocarbon having 1 to 6 carbon atoms, an alcohol having 1 to 6 carbon atoms, an aldehyde having 1 to 6 carbon atoms, and an amine having 1 to 6 carbon atoms, it is preferable to remove the reducing gas remaining on the catalyst by oxidation.

[0065] The reducing gas is oxidized to carbon dioxide (CO ２ The oxidation gas is converted into carbon monoxide (CO) or carbon monoxide (CO) and discharged from the outlet. It is preferable to terminate the flow of the oxidation gas after confirming that the concentration of carbon monoxide (CO) contained in the outlet gas discharged during the oxidation treatment has reached a certain value or less. It is particularly preferable to terminate the oxidation treatment after confirming that the CO concentration during the oxidation treatment has reached 200 ppm by volume or less, more preferably after confirming that the CO concentration has reached 100 ppm by volume or less, and even more preferably after confirming that the CO concentration has reached 50 ppm by volume or less. Terminating the oxidation treatment when the CO concentration exceeds 200 ppm by volume is undesirable because the reducing gas may not be completely burned and may remain adsorbed on the catalyst, resulting in a decrease in the conversion of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst according to the present invention after the oxidation treatment.

[0066] The timing for ending the oxidation step (Ox2) is the timing when the difference between the catalyst layer temperature (To) before the oxidation treatment and the catalyst layer temperature during the oxidation treatment becomes 0°C. Although the catalyst layer temperature before the oxidation treatment is To, when the oxidation treatment is started, the reducing gas remaining adsorbed on the catalyst is burned during the oxidation treatment, causing the catalyst layer temperature to rise. Therefore, if the oxidation step (Ox2) is ended when there is a difference between the catalyst layer temperature before the oxidation treatment and the catalyst layer temperature during the oxidation treatment, reducing gas will remain in the catalyst, which may result in a decrease in catalytic performance, and this is not preferable.

[0067] The oxidation treatment time is preferably 1 hour or more, more preferably 20 hours or more, and even more preferably 40 hours or more. It is also preferably 70 hours or less, more preferably 60 hours or less, and even more preferably 50 hours or less. The oxidation treatment time is also preferably 1 to 70 hours, more preferably 1 to 60 hours, even more preferably 1 to 50 hours, particularly preferably 20 to 70 hours, and most preferably 40 to 50 hours. An oxidation treatment time of less than 1 hour is undesirable because it reduces the effect of the oxidation treatment on the catalyst, and the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst of the present invention after oxidation treatment may not be efficiently improved. An oxidation treatment time of more than 70 hours is also undesirable because it may cause excessive oxidation of the catalyst, resulting in a decrease in the conversion rate of raw material compounds such as propylene and the unsaturated aldehyde selectivity of the catalyst of the present invention after oxidation treatment. The effect of the oxidation treatment varies depending on the catalyst amount and support rate, so the oxidation treatment time can be determined depending on the catalyst amount and support rate. For example, when the loading rate is 100 to 150% by weight, the oxidation treatment time is preferably 20 to 60 hours, more preferably 40 to 50 hours.

[0068] Here, the oxidation treatment may be performed continuously for the predetermined time, or may be performed intermittently. An example of an "intermittent" oxidation treatment is a treatment performed in the following order: (Ox1) oxidation preparation step, (Ox2) oxidation step, (Ox4) oxidation stopping step, (Ox2) oxidation step, and (Ox3) oxidation termination step. Another example of an "intermittent" oxidation treatment is a treatment performed in the following order: (Ox1) oxidation preparation step, (Ox2) oxidation step, (Ox4) oxidation stopping step, (Rd2') reduction step, (Ox2) oxidation step, and (Ox3) oxidation termination step. Another example of an "intermittent" oxidation treatment is a treatment performed in the following order: (Ox1) oxidation preparation step, (Ox2) oxidation step, (Ox4) oxidation stopping step, (In1) inert gas treatment step, (Ox2) oxidation step, and (Ox3) oxidation termination step.

[0069] In the reduction step (Rd2'), a reducing gas is circulated at a predetermined space velocity for a predetermined time after the oxidation stopping step (Ox4). In the inert gas treatment step (In1), an inert gas is circulated after the oxidation stopping step (Ox4). Here, (Ox4) and (Rd2'), which are performed between (Ox1) and (Ox3), may be repeated, or (Ox2) and (Rd2') may be repeated, or a combination of (Ox4), (Ox2), and (Rd2') may be repeated. However, it is preferable that the time for steps other than the oxidation treatment performed between the oxidation step (Ox2) is shorter than the oxidation treatment time. Furthermore, in any oxidation treatment, it is preferable that the final step of the oxidation treatment is the oxidation termination step (Ox3), and the step immediately before the final step is the oxidation step (Ox2), and it is preferable that the total time for the reduction treatment is the above-mentioned predetermined time.

[0070] [Production of Acrolein] The method for producing an unsaturated aldehyde according to the present invention essentially involves using a catalyst that has undergone a reduction treatment followed by an oxidation treatment. The production method according to the present invention can be suitably used in processes for producing unsaturated aldehydes such as acrolein and methacrolein. Examples of raw material compounds used in the production of the unsaturated aldehyde include compounds selected from propylene, isobutylene, tertiary alcohols, and isopropanol. Examples of tertiary alcohol raw material compounds include glycerin and 2-methylpropan-2-ol. Among these, propylene is preferred as the raw material compound. When producing acrolein by catalytic vapor-phase oxidation of propylene, using a catalyst that has undergone a reduction treatment followed by an oxidation treatment can improve the conversion rate of propylene while suitably suppressing the production of acrylic acid. As a result, acrolein can be produced with high selectivity. Therefore, the production method according to the present invention can be suitably used as a production method for producing acrolein.

[0071] Hereinafter, one embodiment of the method for producing acrolein using the method for producing an unsaturated aldehyde according to the present invention will be described.

[0072] In an embodiment of the present invention, the catalyst after the oxidation treatment is used for producing an unsaturated aldehyde. A raw material gas containing propylene and molecular oxygen is introduced into a reaction tube, and a catalytic gas-phase oxidation reaction is carried out using the catalyst that has undergone the oxidation treatment, thereby producing acrolein.

[0073] The raw material gas flowing through the reaction tube is a mixed gas consisting of preferably 1 to 15 vol%, more preferably 4 to 12 vol% propylene; preferably 0.5 to 25 vol%, more preferably 2 to 20 vol% molecular oxygen; preferably 0 to 35 vol%, more preferably 0 to 30 vol% water vapor; and the remainder an inert gas such as nitrogen. The mixture is heated at a temperature of 250 to 450°C under a pressure of 0.1 to 1.0 MPa for 300 to 5,000 hours. －１ The catalyst may be brought into contact with the fuel at a space velocity (standard condition).

[0074] The grade of propylene used as the raw material gas is not particularly limited, and polymer-grade or chemical-grade propylene can be used. A propylene-containing mixed gas obtained by the oxidative dehydrogenation reaction of propane can also be used, and air or oxygen can be added to this mixed gas as needed. One aspect of the present invention is a catalyst for producing unsaturated aldehydes, which contains a composite oxide containing molybdenum, bismuth, iron, and cobalt, and has a redox volume ratio of 1.10 or more and 1.92 or less, and is obtained by performing a reduction treatment with a reducing gas followed by an oxidation treatment. The above descriptions of the composite oxide containing molybdenum, bismuth, iron, and cobalt, the redox volume ratio, the reducing gas, the reduction treatment, and the oxidation treatment are equally applicable. The present invention encompasses the following aspects and configurations. 1. 1. A method for producing an unsaturated aldehyde by catalytic vapor phase oxidation of a compound selected from propylene, isobutylene, and a tertiary alcohol using a catalyst, wherein the catalyst contains a composite oxide containing molybdenum, bismuth, iron, and cobalt, and the oxidation treatment is carried out after a reduction treatment. 2. A method for producing an unsaturated aldehyde according to 1., wherein the reducing gas used in the reduction treatment contains 5.0 volume % or more and 10.0 volume % or less of a reducing gas and 65 volume % or more and 95 volume % or less of a reducing diluent gas. 3. A method for producing an unsaturated aldehyde according to 2., wherein the reducing gas is at least one selected from the following: hydrogen, ammonia, carbon monoxide, at least one hydrocarbon having 1 to 6 carbon atoms, at least one alcohol having 1 to 6 carbon atoms, at least one aldehyde having 1 to 6 carbon atoms, and at least one amine having 1 to 6 carbon atoms. 4. The method for producing an unsaturated aldehyde according to 2. or 3., wherein the reduction dilution gas is at least one selected from nitrogen, water vapor, argon, helium, and carbon dioxide. 5. The method for producing an unsaturated aldehyde according to any one of 1. to 4., wherein the reducing gas used in the reduction treatment is allowed to flow through the catalyst for 1 hour or more and 1,800 hours or less. 6. The method for producing an unsaturated aldehyde according to any one of 1. to 4., wherein the space velocity in the reduction treatment is 900 h －１ More than 2200h －１The method for producing an unsaturated aldehyde according to any one of 1. to 5. below: 7. The method for producing an unsaturated aldehyde according to any one of 1. to 6. above, wherein the oxidation gas used in the oxidation treatment is a mixed gas containing an oxidizing gas and an oxidizing diluent gas, the oxidizing gas in the oxidation gas is a gas having oxidizing properties, and the oxidizing diluent gas in the oxidation gas is at least one selected from water vapor, nitrogen, carbon dioxide, helium, and argon. 8. 8. The method for producing an unsaturated aldehyde according to any one of 1. to 7., wherein the reducing gas used in the reduction treatment is a mixed gas containing a reducing gas and a reducing diluent gas, and the reducing gas is at least one selected from the following: hydrogen, ammonia, carbon monoxide, at least one hydrocarbon having 1 to 6 carbon atoms, at least one alcohol having 1 to 6 carbon atoms, at least one aldehyde having 1 to 6 carbon atoms, and at least one amine having 1 to 6 carbon atoms, and the reducing diluent gas contains water vapor and is at least one selected from nitrogen, argon, helium, and carbon dioxide, and the oxidizing gas used in the oxidation treatment is a mixed gas containing an oxidizing gas and an oxidizing diluent gas, and the oxidizing gas in the oxidizing gas is a gas having oxidizing properties, and the oxidizing diluent gas in the oxidizing gas is at least one selected from nitrogen, carbon dioxide, helium, and argon, and does not substantially contain water vapor. 9. The method for producing an unsaturated aldehyde according to any one of 1. to 7., wherein the redox volume ratio in the reduction treatment is 1.10 or more and 1.92 or less. 10. A method for producing an unsaturated aldehyde according to any one of 1. to 9., wherein the oxidation treatment temperature To is such that ΔTor, which is the difference between the oxidation treatment temperature To in the oxidation treatment and the reduction treatment temperature Tr in the reduction treatment, is 3°C or more and 45°C or less, and the oxidation treatment time is 1 hour or more and 70 hours or less. 11. A method for producing an unsaturated aldehyde according to any one of 1. to 10., wherein the oxidation gas used in the oxidation treatment contains 5.0 volume % or more and 30.0 volume % or less of an oxidizing gas and 70 volume % or more and 95 volume % or less of an oxidizing diluent gas.12. The method for producing an unsaturated aldehyde according to any one of 1. to 11., wherein the flow of the oxidation gas is terminated when the concentration of the reducing gas contained in the outlet gas discharged in the oxidation treatment becomes 200 ppm by volume or less. 13. A catalyst for producing an unsaturated aldehyde, which contains a complex oxide containing molybdenum, bismuth, iron, and cobalt, and has an oxidation-reduction volume ratio of 1.10 or more and 1.92 or less, and is obtained by reduction treatment with a reducing gas and then oxidation treatment. 14. A method for producing an unsaturated aldehyde, comprising: obtaining a catalyst by reduction treatment and then oxidation treatment; and catalytically oxidizing a compound selected from propylene, isobutylene, and a tertiary alcohol using the catalyst, wherein the catalyst contains a complex oxide containing molybdenum, bismuth, iron, and cobalt. 15. A method for producing an unsaturated aldehyde by catalytically oxidizing a compound selected from propylene, isobutylene, and a tertiary alcohol in a gas phase using a catalyst, wherein the catalyst contains a composite oxide containing molybdenum, bismuth, iron, and cobalt, and is subjected to a reduction treatment followed by an oxidation treatment, or the method comprises obtaining a catalyst by performing a reduction treatment followed by an oxidation treatment, and catalytically oxidizing a compound selected from propylene, isobutylene, and a tertiary alcohol in the gas phase using the catalyst, wherein the catalyst contains a composite oxide containing molybdenum, bismuth, iron, and cobalt, the reduction treatment comprises a reduction preparation step (Rd1), a reduction step (Rd2), and a reduction completion step (Rd3), the oxidation treatment comprises an oxidation preparation step (Ox1), an oxidation step (Ox2), and an oxidation completion step (Ox3), The (Rd1) reduction preparation step is a step of adjusting the temperature of a catalyst layer in a reaction tube to a reduction treatment temperature Tr, wherein the reduction treatment temperature Tr is adjusted to be within a range of 310°C or higher and 345°C or lower, and the (Rd2) reduction step is performed at a space velocity of 900 to 2200 h with respect to the catalyst layer adjusted to the reduction treatment temperature Tr. －１the (Rd3) reduction termination step is a step of terminating the flow of the reducing gas to make the temperature of the catalyst layer equal to or lower than the reduction treatment temperature Tr, the (Ox1) oxidation preparation step is a step of adjusting the temperature of the catalyst layer that has been subjected to the reduction treatment to an oxidation treatment temperature To, the (Ox2) oxidation step is a step of adjusting the temperature of the catalyst layer that has been subjected to the reduction treatment to an oxidation treatment temperature To, the (Ox3) oxidation step is a step of adjusting the temperature of the catalyst layer that has been adjusted to the oxidation treatment temperature To at a space velocity of 100 h －１ More than 1500h －１16. A method for producing an unsaturated aldehyde, wherein the oxidation gas is a mixed gas containing 5.0 to 10.0 volume % of a reducing gas and 65 to 95 volume % of a reducing diluent gas, and the temperature of the catalyst layer is adjusted to be within a range of not less than the oxidation treatment temperature To but not more than the oxidation treatment temperature To + 20°C, and the (Ox3) oxidation termination step is a step of terminating the flow of the oxidation gas to bring the temperature of the catalyst layer to not more than the oxidation treatment temperature To. 16. A method for producing an unsaturated aldehyde according to 15., wherein the reducing gas is a mixed gas containing 5.0 to 10.0 volume % of a reducing gas and 65 to 95 volume % of a reducing diluent gas. 17. A method for producing an unsaturated aldehyde according to 15. or 16., wherein, when the reducing diluent gas contains nitrogen, the nitrogen content is 45.0 volume % to 90.0 volume % relative to the reducing gas. 18. A method for producing an unsaturated aldehyde according to any of 15. to 17., wherein, when the reducing diluent gas contains water vapor, the water vapor content is 1.0 volume % to 35.0 volume % relative to the reducing gas. 19. A method for producing an unsaturated aldehyde according to any one of 15. to 18., wherein, when the reducing gas contains an oxidizing gas, the oxidizing gas accounts for 7.0 volume % or more and 21.0 volume % or less relative to the reducing gas. 20. A method for producing an unsaturated aldehyde according to 19., wherein the volume % of the oxidizing gas relative to the volume % of the reducing gas (oxidation-reduction volume ratio) is 1.10 to 1.92. 21. A method for producing an unsaturated aldehyde according to any one of 15. to 20., wherein the reduction treatment time in the reduction treatment is 1 to 1,800 hours. 22. A method for producing an unsaturated aldehyde according to any one of 15. to 21., wherein the oxidation gas is a mixed gas containing an oxidizing gas and an oxidizing diluent gas, and does not contain a reducing gas, or if it does contain a reducing gas, the amount is 0.1 volume % or less. 23. The method for producing an unsaturated aldehyde according to any one of 15. to 22., wherein the oxidation gas used in the oxidation treatment contains 5.0 volume % to 30.0 volume % of an oxidizing gas and 70 volume % to 95 volume % of an oxidizing diluent gas. 24. The method for producing an unsaturated aldehyde according to any one of 15. to 23., wherein, when the oxidizing diluent gas contains nitrogen, the nitrogen content is 30.0 volume % to 90.0 volume % of the oxidizing gas. 25. The method for producing an unsaturated aldehyde according to any one of 15. to 24., wherein the oxidizing gas is substantially free of water vapor.26. The method for producing an unsaturated aldehyde according to any one of 15. to 25., wherein the flow of the oxidation gas is terminated after it is confirmed that the concentration of the reducing gas contained in the outlet gas discharged in the oxidation treatment has reached 200 ppm by volume or less.

[0075] Although embodiments of the present invention have been described in detail, they are illustrative or exemplary and not restrictive.

[0076] The effects of the present invention will be explained using the following examples and comparative examples. However, the technical scope of the present invention is not limited to the following examples, and examples obtained by appropriately combining the technical means disclosed in each example are also included in the scope of the present invention. In the following examples, unless otherwise specified, operations were performed at 25°C. Furthermore, unless otherwise specified, "parts" means "parts by mass."

[0077] Example 1 [Catalyst Preparation] 1500 parts of ion-exchanged water was heated and stirred, and ammonium paramolybdate (VI) tetrahydrate ((NH ４ ) ６ {Mo (Mo ６ O ２４ )・4H ２ Separately, 800 parts of cobalt (II) nitrate hexahydrate (Co(NO)) was dissolved in 500 parts of ion-exchanged water (liquid A). ３ ) ２ ・6H ２ 330 parts of nickel (II) nitrate hexahydrate (Ni(NO ３ ) ２ ・6H ２ Separately, 253 parts of iron (III) nitrate nonahydrate (Fe(NO)) was dissolved in 350 parts of ion-exchanged water (liquid B), and 24 parts of concentrated nitric acid (65 wt%) was added to the acidic solution. ３ ) ３ ・9H ２ 305 parts of bismuth (III) nitrate pentahydrate (Bi(NO ３ ) ３ ・5H ２147 parts of ammonium nitrate (O) were dissolved in the mixture (Liquid C). While maintaining Liquid A at 60°C, Liquids B and C were added dropwise. Subsequently, a liquid (Liquid D) prepared by dissolving 3.0 parts of potassium nitrate in 50 parts of distilled water was added. The resulting mixture was evaporated to dryness at 150°C for 3 hours to obtain a cake-like solid. The resulting solid was dried in an air atmosphere at 210°C for 5 hours using a box dryer and then pulverized to 500 μm or less to obtain a catalyst powder. 500 g of α-alumina spherical carriers with an average particle size of 4.0 mm were placed in a centrifugal fluidized bed coating apparatus, and then 800 g of catalyst powder was loaded together with a 30 wt% aqueous ammonium nitrate solution as a binder to support the carrier. The catalyst was then dried in a tunnel dryer in an air atmosphere at 250°C for 7 hours. The resulting dried product was calcined in an air atmosphere at 490°C for 5 hours using a tunnel calcination furnace to obtain Catalyst 1. The loading rate of Catalyst 1 was calculated using the above-mentioned Equation 2 to be 120 wt%. The composition of the metal elements other than oxygen in the active components (other than the support) of this catalyst was as follows: Mo １２ Bi ０．８ Fe ２．０ Co ６．２ Ni ２．３ K ０．０８

[0078] [Reduction Treatment] Catalyst 1 obtained above was subjected to reduction treatment according to the following method.

[0079] A reactor was prepared vertically, consisting of a stainless steel reaction tube with a total length of 4000 mm and an inner diameter of 25 mm, and a shell surrounding the reaction tube for passing a heat transfer medium. A separate reaction tube for measuring the heat transfer medium temperature inside the shell was prepared inside the shell (hereinafter referred to as the heat transfer medium temperature measurement tube). A thermocouple was inserted from the lower inlet end of the heat transfer medium temperature measurement tube in the depth direction and fixed at a position of 600 mm. Catalyst 1 was dropped from the top of the reaction tube and filled to a layer length of 3500 mm. The amount of catalyst filled was 2300 g.

[0080] Next, the heat transfer medium temperature (reduction treatment temperature) Tr of the reactor was adjusted to 343°C (reduction preparation step). Next, a reducing gas having the gas composition and redox volume ratio shown in Table 1 was introduced from the bottom of the reaction tube filled with the catalyst at a space velocity of 1,370 hr －１(standard state) (reduction step). Next, the flow of the reducing gas was stopped (reduction termination step). The time from the start of the flow of the reducing gas until the catalyst layer temperature decreased and became equal to the reduction treatment temperature Tr after the stop of the flow of the reducing gas (reduction treatment time) was 45 hours.

[0081] [Oxidation Treatment] With the catalyst after reduction treatment still packed in the reaction tube, the heat transfer medium temperature (oxidation treatment temperature) To was changed to 350°C (oxidation preparation step). At this time, ΔTor (=To-Tr) was 7°C. An oxidation gas having the gas composition shown in Table 2 was introduced from the bottom of the reaction tube packed with the catalyst at a space velocity of 1,400 hr. －１ (standard conditions) (oxidation step). The flow of the oxidation gas was terminated after it was confirmed that the concentration of carbon monoxide (CO) contained in the outlet gas discharged during the flow of the oxidation gas had reached 64 volume ppm (oxidation termination step). The time from the start of the flow of the oxidation gas until the catalyst layer temperature decreased and became equal to the oxidation treatment temperature To after the flow of the oxidation gas was terminated (oxidation treatment time) was 25 hours. After the oxidation treatment was completed, the catalyst was removed from the reaction tube to obtain catalyst A.

[0082] Measurement of CO concentration in outlet gas during oxidation treatment: The outlet gas discharged during oxidation gas flow was collected in a gas sampler and measured by gas chromatography. Analytical equipment: GC-8A manufactured by Shimadzu Corporation, Column: Molecular Sieve 5A, Shincarbon-ST 50 / 80, Detector: TCD.

[0083] Examples 2 to 7 Catalysts B to F and J were obtained by carrying out reduction treatment and oxidation treatment on catalyst 1 in the same manner as in Example 1, except that the conditions for the reduction treatment and oxidation treatment in Example 1 were changed to the conditions for Examples 2 to 7 shown in Tables 1 and 2.

[0084] Comparative Example 1 Catalyst G was obtained by carrying out reduction treatment and oxidation treatment on catalyst 1 in the same manner as in Example 1, except that the conditions for the reduction treatment and oxidation treatment in Example 1 were changed to the conditions for Comparative Example 1 shown in Tables 1 and 2.

[0085] Comparative Example 2 Catalyst H was obtained by treating catalyst 1 in the same manner as in Example 1, except that the reduction treatment in Example 1 was not performed and the conditions for the oxidation treatment were changed to the conditions in Comparative Example 2 shown in Table 2.

[0086] Comparative Example 3 Catalyst 1 was treated in the same manner as in Example 1, except that the conditions for the reduction treatment in Example 1 were changed to the conditions for Comparative Example 3 shown in Table 1 and that the oxidation treatment was not performed, thereby obtaining catalyst I.

[0087]

[0088]

[0089] [Performance Evaluation] For the catalysts A to J obtained above, propylene was subjected to catalytic vapor phase oxidation in the presence of molecular oxygen or a molecular oxygen-containing gas according to the following method, and the conversion of propylene and the selectivities to acrolein and acrylic acid were measured.

[0090] A reactor was prepared vertically, consisting of a stainless steel reaction tube with a total length of 3000 mm and an inner diameter of 25 mm, and a shell surrounding the reaction tube for passing a heat transfer medium. Each catalyst was dropped from the top of the reaction tube to fill the reactor so that the layer length was 2100 mm.

[0091] Next, for catalysts A to E, catalyst J, and catalysts G to I, the heat transfer medium temperature (reaction temperature) was kept at 320°C, and a mixed gas consisting of 8.0 vol% propylene, 14.0 vol% oxygen, 10.0 vol% water vapor, and the remainder nitrogen gas was introduced from the bottom of the reaction tube packed with the catalyst at a space velocity of 1600 hr. －１ (standard conditions). The amount of propylene remaining after the reaction, the amount of acrolein produced, and the amount of acrylic acid produced were measured by gas chromatography. The number of moles of propylene consumed in the reaction was calculated from the amount of propylene supplied and the amount of propylene remaining after the reaction.

[0092] Next, for catalyst F, a mixed gas consisting of 5.0 vol% propylene, 14.0 vol% oxygen, 10.0 vol% steam, and the remainder nitrogen gas was passed through the catalyst at a space velocity of 1600 hr. －１The number of moles of propylene consumed in the reaction was calculated from the amount of propylene supplied and the amount of propylene remaining after the reaction, in the same manner as above, except that propylene was introduced at 1000 kJ / min (standard condition).

[0093] Based on these values, the propylene conversion, the acrylic acid selectivity, and the acrolein selectivity were calculated according to the following formulas 5, 6, and 7. The results are shown in Table 3.

[0094] (Conditions for measuring the amounts of propylene, acrolein, and acrylic acid) Analytical equipment: GC-8A and GC-2010 manufactured by Shimadzu Corporation Columns: Molecular Sieve 5A, Shincarbon-ST 50 / 80, and ULBON HR-20M 0.53 mm ID × 30 m 3.0 μm Detectors: TCD and FID

[0095] Equation 5: Propylene conversion rate [mol %] = (number of moles of reacted propylene) / (number of moles of supplied propylene) × 100

[0096] Equation 6: Selectivity of acrylic acid [mol %] = {(number of moles of acrylic acid produced) / (number of moles of propylene reacted)} × 100

[0097] Equation 7 Acrolein selectivity [mol %] = {(number of moles of acrolein produced) / (number of moles of propylene reacted)} × 100

[0098]

[0099] From Table 3, it can be seen that catalysts A, B, C, D, E and J have higher propylene conversion and higher acrolein selectivity than catalysts G, H and I. On the other hand, it can be seen that catalysts A, B, C, D, E and J have lower acrylic acid selectivity than catalysts G, H and I. It can also be seen that catalyst F, which has a low reducing gas concentration and a low space velocity, has high acrolein selectivity and low acrylic acid selectivity.

[0100] This application is based on Japanese Patent Application No. 2024-154571, filed on September 9, 2024, the disclosure of which is incorporated herein by reference in its entirety.

Claims

1. A method for producing an unsaturated aldehyde, which comprises catalytically oxidizing a compound selected from propylene, isobutylene, and a tertiary alcohol in a gas phase using a catalyst, wherein the catalyst contains a composite oxide containing molybdenum, bismuth, iron, and cobalt, and the method comprises carrying out a reduction treatment followed by an oxidation treatment.

2. The method for producing an unsaturated aldehyde according to claim 1, wherein the reducing gas used in the reduction treatment contains 5.0% by volume or more and 10.0% by volume or less of a reducing gas and 65% by volume or more and 95% by volume or less of a reducing diluent gas.

3. The method for producing an unsaturated aldehyde according to claim 2, wherein the reducing gas is at least one selected from the following: hydrogen, ammonia, carbon monoxide, at least one hydrocarbon having 1 to 6 carbon atoms, at least one alcohol having 1 to 6 carbon atoms, at least one aldehyde having 1 to 6 carbon atoms, and at least one amine having 1 to 6 carbon atoms.

4. The method for producing an unsaturated aldehyde according to claim 2 or 3, wherein the reducing diluent gas is at least one selected from the group consisting of nitrogen, water vapor, argon, helium, and carbon dioxide.

5. The method for producing an unsaturated aldehyde according to any one of claims 1 to 4, wherein the reducing gas used in the reduction treatment is allowed to flow through the catalyst for 1 hour or more and 1,800 hours or less.

6. The space velocity in the reduction treatment is 900 h －１ More than 2200h －１ The method for producing an unsaturated aldehyde according to any one of claims 1 to 5, wherein the following is true:

7. The method for producing an unsaturated aldehyde according to any one of claims 1 to 6, wherein the oxidation gas used in the oxidation treatment is a mixed gas containing an oxidizing gas and an oxidizing diluent gas, the oxidizing gas in the oxidation gas is a gas having oxidizing properties, and the oxidizing diluent gas in the oxidation gas is at least one selected from water vapor, nitrogen, carbon dioxide, helium, and argon.

8. The method for producing an unsaturated aldehyde according to any one of claims 1 to 7, wherein the reducing gas used in the reduction treatment is a mixed gas containing a reducing gas and a reducing diluent gas, and the reducing gas is at least one selected from the following: hydrogen, ammonia, carbon monoxide, at least one hydrocarbon having 1 to 6 carbon atoms, at least one alcohol having 1 to 6 carbon atoms, at least one aldehyde having 1 to 6 carbon atoms, and at least one amine having 1 to 6 carbon atoms; the reducing diluent gas contains water vapor and is at least one selected from nitrogen, argon, helium, and carbon dioxide; the oxidizing gas used in the oxidation treatment is a mixed gas containing an oxidizing gas and an oxidizing diluent gas, and the oxidizing gas in the oxidizing gas is a gas having oxidizing properties; and the oxidizing diluent gas in the oxidizing gas is at least one selected from nitrogen, carbon dioxide, helium, and argon, and does not substantially contain water vapor.

9. The method for producing an unsaturated aldehyde according to any one of claims 1 to 8, wherein the oxidation-reduction volume ratio in the reduction treatment is 1.10 or more and 1.92 or less.

10. A method for producing an unsaturated aldehyde according to any one of claims 1 to 9, wherein the oxidation treatment temperature To is such that ΔTor, which is the difference between the oxidation treatment temperature To in the oxidation treatment and the reduction treatment temperature Tr in the reduction treatment, is 3°C or more and 45°C or less, and the oxidation treatment time is 1 hour or more and 70 hours or less.

11. A method for producing an unsaturated aldehyde according to any one of claims 1 to 10, wherein the oxidation gas used in the oxidation treatment contains 5.0 vol% or more and 30.0 vol% or less of an oxidizing gas and 70 vol% or more and 95 vol% or less of an oxidizing dilution gas.

12. A method for producing an unsaturated aldehyde according to any one of claims 1 to 11, wherein the flow of the oxidation gas is terminated after it is confirmed that the concentration of the reducing gas contained in the outlet gas discharged in the oxidation treatment has reached 200 ppm by volume or less.

13. A catalyst for producing unsaturated aldehydes, which contains a composite oxide containing molybdenum, bismuth, iron, and cobalt, has an oxidation-reduction volume ratio of 1.10 or more and 1.92 or less, and is obtained by reduction treatment with a reducing gas followed by oxidation treatment.

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