Process for production of acrylic acid

Abstract: Acrylic acid can be produced efficiently by subjecting acrolein to catalytic gas-phase oxidation in the presence of a molybdenum/vanadium-based oxide catalyst prepared by using particular substances as the raw materials of the individual metal elements constituting the catalyst. A preferable example of such a catalyst is prepared by using, as the raw materials of vanadium, ammonium metavanadate and at least one vanadium oxide in which the valency of vanadium is larger than 0 but smaller than 5; as the raw material of copper, copper nitrate; and, as at least part of the raw material(s) of antimony, at least one antimony oxide in which the valency of antimony is larger than 0 but smaller than 5.

Description: The present invention relates to a process for producing acrylic acid. More particularly, the present invention relates to a process for producing acrylic acid by subjecting acrolein or an acrolein-containing gas to catalytic gas-phase oxidation with molecular oxygen or a molecular oxygen-containing gas in the presence of a molybdenum/vanadium-based oxide catalyst prepared under particular conditions. Various improved catalysts were proposed for efficient production of acrylic acid by catalytic gas-phase oxidation of acrolein. Most of them are molybdenum/vanadium-based catalysts containing molybdenum and vanadium as main components. DE 1 924 496 discloses a process for the production of acrylic acid characterized in that acrolein is oxidized with molecular oxygen in the presence of a catalyst containing oxides or oxide-containing compositions of vanadium, molybdenum and at least one element X or Y. In this respect, X is selected from germanium, uranium, manganese, copper, gold or barium and Y represents selenium, cobalt, lead, silver, magnesium, boron, tungsten, strontium, calcium, zinc or cadmium. Some of the hitherto-proposed molybdenum/vanadium-based catalysts are at a fairly high level with respect to the yield of acrylic acid when viewed from an industrial production standpoint, and are actually used in industrial production of acrylic acid. Conventional molybdenum/vanadium-based catalysts, however, are not sufficiently satisfactory in view of the performance stability allowing for high yield of acrylic acid over a long period. Thus, it has been desired to develop a molybdenum/vanadium-based catalyst showing a long-term performance stability in production of acrylic acid by oxidation of acrolein. An approach to the development of such a molybdenum/vanadium-based catalyst includes a study on the improvement of catalyst preparation process. In Japanese Patent Publication No. 25914/1975, for example, it is disclosed that by using an organic acid (e.g. oxalic acid) in catalyst preparation process, the oxidation state of catalyst is controlled, or a molybdenumvanadium compound is formed. In this improved catalyst preparation process, however, a heat, which is generated by the decomposition of the organic acid in the heating and firing step of catalyst, deteriorates the performance of the catalyst obtained; moreover, when the resulting catalyst is used for oxidation of acrolein over a long period, the effect of the organic acid used in catalyst preparation process is difficult to maintain; therefore, the catalyst prepared by the above process is not satisfactory in acrylic acid yield and catalyst life when applied in industry. The object of the present invention is to (1) develop a molybdenum/vanadium-based catalyst for acrylic acid production, represented by the following general formula (I) Mo a V b W c Cu d X e Y f Z g O h (wherein the individual elements and their proportions are described later), which is excellent in activity, selectivity and life and which shows stable performance over a long period of time and (2) provide a process for producing acrylic acid at a high yield over a long period of time by subjecting acrolein or an acrolein-containing gas to catalytic gas-phase oxidation with molecular oxygen or a molecular oxygen-containing gas in the presence of the above-mentioned molybdenum/vanadium-based catalyst. It is reported in T. V. ANDRUSHKEVICH, CATAL. REV.-SCI. ENG., 35, p. 213 (1993) that in molybdenum/vanadium-based catalysts such as represented by the above general formula (I), the active compound is VMo 3 O 11 ; with the progress of oxidation of acrolein, this vanadium-molybdenum compound (hereinafter referred to as "vanadium-molybdenum active compound" in some cases) undergoes modification, resulting in the deterioration of catalyst; and when most of vanadium has been converted to a compound (e.g. V 2 O 5 ) in which the valency of vanadium is 5, the selectivity of acrylic acid from acrolein is reduced remarkably. The present inventors examined the property changes of molybdenum/vanadium-based catalysts, for example, the changes in surface area and pore volume and further, by using X-ray diffractometry, compared the differences in physical and chemical properties between used catalyst of reduced activity and unused catalyst. As a result, the present inventors found out that when a molybdenum-vanadium-based catalyst is measured for X-ray pattern, the peak intensity appearing at d = 4.00 Å has a relation to the performance of the catalyst and its change with time. According to the above-mentioned literature, the peak at d = 4.00 Å is assigned to the vanadium-molybdenum active compound (VMo 3 O 11 ). Specific description is made below on a molybdenum/vanadium-based catalyst having a catalyst composition (an atomic ratio when oxygen is excluded) of Mo 12 V 5 W 1 Cu 2.2 Sr 0.5 . This fresh catalyst and the used catalyst of reduced performance obtained when the fresh catalyst was subjected to catalytic gas-phase oxidation of acrolein continuously for 8,000 hours, were measured for surface area by BET method and pore volume. Their surface areas were 2.6 m 2 /g and 2.4 m 2 /g, respectively, and were not much different. Their pore volumes were 0.22 cc/g and 0.21 cc/g, respectively, and were not much different, either. In contrast, when the fresh and used catalysts were measured, by X-ray diffractometry, for peak at d = 4.00 Å (assigned to the vanadium-molybdenum active compound) and peak at d = 4.38 Å (assigned to V 2 O 5 , a five-valency vanadium compound), the intensity of the peak at d = 4.00 Å was very low at 65 in the used (8,000 hours) catalyst when that of the fresh catalyst was taken as 100. Thus, it was found that the catalytic activity of the catalyst examined has a strong relation to the intensity of the vanadium-molybdenum active compound having a peak at d = 4.00 Å and that one reason for reduction in catalytic activity during catalyst use is decrease of a crystal phase having a peak at d = 4.00 Å . Hence, the present inventors made an extensive study on the formation of the vanadium-molybdenum active compound having a peak at d = 4.00 Å . As a result, the present inventors found out that a catalyst obtained by using, as the raw materials of vanadium and copper, ammonium metavanadate and copper nitrate, respectively, and by using, in addition to the above raw materials, even a low-valency antimony oxide and/or a low-valency tin oxide, shows an increased peak-intensity at d = 4.00 Å assigned to the vanadi um-mol ybdenum active compound and a decreased peak intensity at d = 4.38 Å assigned to V 2 O 5 , and that the catalyst obtained as above has an improved catalytic activity and can show stable performance over a long period of time. The present invention has been completed based on the finding. According to the present invention there is provided a process for producing acrylic acid by subjecting acrolein or an acrolein-containing gas to gas-phase oxidation with molecular oxygen or a molecular oxygen-containing gas in the presence of a molybdenum/vanadium-based oxide catalyst represented by the following general formula (I): Mo a V b W c Cu d X e Y f Z g O h (wherein Mo is molybdenum; V is vanadium; W is tungsten; Cu is copper; X is at least one element selected from antimony and tin; Y is at least one element selected from magnesium calcium, strontium and barium; Z is at least one element selected from titanium, zirconium and cerium; O is oxygen; a, b, c, d, e, f, g and h are the atom numbers of Mo, V, W, Cu, X, Y and Z, respectively, with a proviso that when a is 12, 2 ≦ b ≦ 14, 0 ≦ c ≦ 12, 0 < d ≦ 6, 0 < e ≦ 5, 0 ≦ f ≦ 3 and 0 ≦ g ≦ 10; and h is a number determined by the oxidation states of the individual elements other than O), in which process the molybdenum/vanadium-based oxide catalyst is produced by using the following substances as the raw materials of vanadium, copper, antimony and tin: (A) the raw material of vanadium is ammonium metavanadate and part thereof is used in the form of a complex with an antimony compound in which the valency of antimony is larger than 0 but smaller than 5, and the raw material of copper is copper nitrate, or (B) the raw material of vanadium is ammonium metavanadate and part thereof is used in the form of a complex with a tin compound in which the valency of tin is larger than 0 but smaller than 4, and the raw material of copper is copper nitrate, or (C) the raw material of vanadium is ammonium metavanadate and part thereof is used in the form of a complex with (1) an antimony compound in which the valency of antimony is larger than 0 but smaller than 5 and (2) a tin compound in which the valency of tin is larger than 0 but smaller than 4, and the raw material of copper is copper nitrate. Preparation of a catalyst wherein 0 < e ≦ 5 Raw material(s) of molybdenum (Mo): Ammonium paramolybdate, molybdic acid, molybdenum oxide, etc. can be used singly or in admixture of two or more of them. Raw material(s) of tungsten (W): Ammonium paratunstate, tungstic acid, tungsten oxide, etc. can be used singly or in admixture of two or more of them. A nitrate, carbonate, ammonium salt, sulfate, etc. of magnesium, calcium, strontium or barium can be used singly or in admixture of two or more of them. Raw material(s) of Z component: A nitrate, carbonate, ammonium salt, sulfate, hydroxide, oxide, etc. of titanium, zirconium or cerium can be used singly or in admixture of two or more of them. Raw materials of vanadium: Ammonium metavanadate is used, and at least part of the ammonium metavanadate is used as a complex with a low-valency X compound(s) mentioned later, i.e. a V-X complex (specifically, a V-Sb complex or a V-Sn complex or a V-Sb-Sn complex). The amount of the ammonium metavanadate used for formation of the V-X complex may be 1-50% by weight, preferably 2-40% by weight of the total ammonium metavanadate amount used. Raw material of copper: Copper nitrate is used. Raw material(s) of antimony (X component): In the formation of the above-mentioned V-Sb complex, an antimony compound in which the valency of antimony is larger than 0 but smaller than 5, is used. Typical examples of this low-valency antimony compound are antimony trichloride, antimony oxychloride, antimony bromide, antimony trioxide, antimony tetraoxide, etc. These compounds can be used singly or in admixture of two or more of them. As the antimony compound used in the form other than the V-Sb complex, there can be used ammonium salt, sulfate, oxides, etc. of antimony, and they can be used singly or in admixture of two or more of them. No antimony halide, however, can be used because it reduces the performance of the resulting molybdenum/vanadium-based oxide catalyst. Raw material(s) of tin (X component): In the formation of the above-mentioned V-Sn complex, a tin compound in which the valency of tin is larger than 0 but smaller than 4, is used. Typical examples of this low-valency tin compound are stannous chloride, stannous sulfate, tin acetate, tin oxalate, stannous hydroxide, stannous oxide, etc. These compounds can be used singly or in admixture of two or more of them. As the tin compound used in the form other than the V-Sn complex, there can be used nitrate, ammonium salt, sulfate, oxides, etc. of tin, and they can be used singly or in admixture of two or more of them. No tin halide, however, can be used because it reduces the performance of the resulting molybdenum/vanadium-based oxide catalyst. The further feature of the catalyst used in the present invention is that when the catalyst is subjected to X-ray diffractometry, the catalyst gives a high peak intensity at d = 4.00 Å (the peak intensity at d = 4.00 Å is hereinafter referred to as d 4.00 ) assigned to the vanadium-molybdenum phase of the catalyst and a low peak intensity at d = 4.38 Å (the peak intensity at d = 4.38 Å is hereinafter referred to as d 4.38 ) assigned to the V 2 O 5 of the catalyst. When the ratio of the two peak intensities (d 4.38 /d 4.00 ) is less than 0.07, a favorable catalytic activity is obtained. When the ratio is 0.06 or less, a more favorable catalytic activity is obtained; and when the ratio is 0-0.05, a particularly favorable catalytic activity is obtained. When the ratio is 0.07 or more, the amount of the vanadium-molybdenum phase is small and the resulting catalytic activity is low. In the present invention, the catalyst desirably has a peak intensity retention of at least 80%, preferably at least 85% when the peak intensity retention is defined as percent of the peak intensity at d = 4.00 Å, of used catalyst after 4,000-hour operation to the peak intensity at d = 4.00 Å , of unused catalyst when the two catalysts are subjected to X-ray diffractometry. A catalyst having a peak intensity retention of lower than 80% contains a small amount of a active vanadium-molybdenum compound and consequently has low catalytic activity. The method for preparation of the catalyst of the present invention is essentially the same as generally used in preparation of similar type catalysts, except that the present catalyst is prepared under one of the above-mentioned conditions (A) to (C). The present catalyst can be prepared by any of the evaporation-to-dryness method, granulation method, extrusion method, etc. all known hitherto. The low-valency metal oxides may be added and dispersed at any step of catalyst preparation. In order to efficiently control the oxidation state of vanadium in catalyst preparation, the low-valency metal oxides are used preferably in small particles having an average particle diameter of 1-150 µm, preferably 5-100 µm. In preparation of the present catalyst, it is possible to add inorganic fibers (e.g. glass fiber) and various whiskers, which are generally known to be effective for the improvement of catalyst strength and abrasion loss. In order to obtain a catalyst having properties of well-controlled ranges, it is also possible to add additives generally known as a binder for powder, such as ammonium nitrate, cellulose, starch, polyvinyl alcohol, stearic acid and the like. The catalyst composition represented by general formula (I) may be used as it is, but is used preferably by being supported on an inert carrier such as alumina, silica-alumina, silicon carbide, titanium oxide, magnesium oxide, aluminum sponge, diatomaceous earth or the like. With respect to the conditions employed when acrolein or an acrolein-containing gas is subjected to a catalytic gas-phase oxidation reaction using the present catalyst prepared by the above-mentioned method, there is no particular restriction. The reaction can be carried out using the process and conditions which are well known in similar reactions. The reaction can be conducted, for example, by contacting, with the present catalyst, a mixed gas comprising 1-15 % by volume, preferably 4-12% by volume of acrolein, 0.5-25% by volume, preferably 2-20% by volume of oxygen, 0-30% by volume, preferably 3-25% by volume of steam and 20-80% by volume, preferably 50-70% by volume of an inert gas (e.g. nitrogen) at a temperature of 180-350°C, preferably 200-330°C at a pressure of normal pressure to 10 atm. (a reduced pressure may be used) at a space velocity (STP) of 500-20,000 hr -1 , preferably 1,000-10,000 hr -1 . As the raw material gas to be contacted with the present catalyst, there can be used not only a mixed gas comprising acrolein, oxygen and an inert gas but also an acrolein-containing gas obtained by direct oxidation of propylene. When the latter gas is used, the oxidation products as by-products (e.g. acrylic acid, acetaldehyde and acetic acid), carbon oxides, propane, unreacted propylene, etc. contained in the acrolein-containing gas give no harm to the catalyst of the present invention. The above catalytic gas-phase oxidation reaction can be carried out in a fixed bed or in a fluidized bed. The molybdenum/vanadium-based catalyst of the present invention, having a high catalytic activity, can produce acrylic acid at a high yield. The molybdenum/vanadium-based catalyst of the present invention, having a superior catalyst life, can maintain the excellent performance over a long period of time. As a result, even after the long-term use, the catalyst can produce acrylic acid at about the same high yield as at the reaction start, with no necessity of significant increase in reaction temperature. The molybdenum/vanadium-based catalyst of the present invention, showing excellent performance even under high-load conditions, can produce acrylic acid at a high yield under such conditions. The present invention is hereinafter described more specifically by way of Examples. However, the present invention is in no way restricted by these Examples. In the Examples, acrolein conversion, acrylic acid selectivity and acrylic acid per-pass yield were determined by the following formulas. Acrolein conversion (%) = (moles of acrolein reacted) ÷ (moles of acrolein fed) x 100 Acrylic acid selectivity (%) = (moles of acrylic acid formed) ÷ (moles of acrolein reacted) x 100 Acrylic acid per-pass yield (%) = (moles of acrylic acid formed) ÷ (moles of acrolein fed) x 100 Example 1 [Preparation of V-Sb complex] 9.7 g of ammonium metavanadate was dissolved in 500 ml of water being heated and stirred. To the solution was added 18.8 g of antimony trichloride. The mixture was placed in a porcelain-made evaporator set on a hot water bath, and concentrated to dryness. The resulting solid was dried at 350°C for 6 hours and then ground to obtain 20 g of a powder (A-1) of 150 µm or less. The powder (A-1) had the following metal composition (expressed in terms of atomic ratio when oxygen excluded, the same applies hereinafter). V 1 Sb 1 [Preparation of catalyst] In 2,500 ml of water being heated and stirred were dissolved 350 g of ammonium paramolybdate, 106 g of ammonium metavanadate and 44.6 g of ammonium paratungstate. Thereto was added 20 g of the powder (A-1). Separately, 87.8 g of copper nitrate was dissolved in 750 ml of water being heated and stirred. The resulting two fluids were mixed and placed in a porcelain-made evaporator set on a hot water bath. Thereto was added 1,000 ml of a spherical α-alumina carrier having diameters of 3-5 mm. The mixture was evaporated to dryness with stirring to adhere the above compounds onto the carrier, followed by firing at 400°C for 6 hours, to obtain a catalyst (1). The catalyst (1) had the following metal composition. Mo 12 V 6 W 1 Cu 2.2 Sb 0.5 [Oxidation reaction] 400 ml of the catalyst (1) was filled in a stainless steel-made U-tube having a diameter of 25 mm. A mixed gas comprising 4% by volume of acrolein, 4.5% by volume of oxygen, 25% by volume of steam and 66.5% by volume of nitrogen was introduced into the tube and subjected to an oxidation reaction at 250°C for 2 seconds (a contact time). The results are shown in Table 9. Separately, the catalyst (1) was subjected to X-ray diffractometry to measure its peak intensities at d = 4.00 Å and d = 4.38 Å, i.e. d 4.00 and d 4.38 ; and the latter peak intensity was reported in Table 1 as relative peak intensity when the former peak intensity was taken as 100. Comparative Example 1 A catalyst (2) having the same composition as that of the catalyst (1) was prepared in the same manner as in Example 1 except that ammonium meta-vanadate and antimony trichloride were used as they were, without being made into a V-Sb complex. Using the catalyst (2), an oxidation reaction was conducted under the same conditions as in Example 1. The results are shown in Table 1. Separately, the catalyst (2) was subjected to X-ray diffractometry in the same manner as in Example 1 to measure its peak intensities at d = 4.00 Å and d = 4.38 Å , i.e. d 4.00 and d 4.38 ; and they were reported in Table 1 as relative peak intensities when the peak intensity of the catalyst (1) at d = 4.00 Å was taken as 100. Comparative Example 2 A catalyst (3) was prepared in the same manner as in Example 1 except that antimony pentachloride was used in place of the antimony trichloride used in Example 1 in production of a V-Sb complex. Using the catalyst (3) an oxidation reaction was conducted under the same conditions as in Example 1. The results are shown in Table 1. Separately, the catalyst (3) was subjected to X-ray diffractometry in the same manner as in Example 1 to measure its peak intensities at d = 4.00 Å and d = 4.38 Å, i.e. d 4.00 and d 4.38 ; and they were reported in Table 1 as relative peak intensities when the peak intensity of the catalyst (1) at d = 4.00 Å was taken as 100. Example 2 An oxidation reaction was conducted for 4,000 hours using the catalyst (1) under the same conditions as in Example 1. Then, the product was collected and analyzed. The results are shown in Table 2. The used catalyst (1) after 4,000-hour reaction was subjected to X-ray diffractometry to measure its peak intensity at d = 4.00 Å, and it was reported in Table 2 as relative peak intensity at d = 4.00 Å when the peak intensity of the unused catalyst (1) at d = 4.00 Å was taken as 100. Comparative Example 3 An oxidation reaction was conducted in the same manner as in Example 2 except that the catalyst (2) was used in place of the catalyst (1). The results are shown in Table 2. The used catalyst (2) after 4,000-hour reaction was subjected to X-ray diffractometry to measure its peak intensity at d = 4.00 Å , and it was reported in Table 2 as relative peak intensity at d = 4.00 Å when the peak intensity of the unused catalyst (1) at d = 4.00 Å was taken as 100. Comparative Example 4 An oxidation reaction was conducted in the same manner as in Example 2 except that the catalyst (3) was used in place of the catalyst (1). The results are shown in Table 2. The used catalyst (3) after 4,000-hour reaction was subjected to X-ray diffractometry to measure its peak intensity at d = 4.00 Å, and it was reported in Table 2 as relative peak intensity at d = 4.00 Å when the peak intensity of the unused catalyst (1) at d = 4.00 Å was taken as 100. Example 3 An oxidation reaction was conducted in the same manner as in Example 1 using the catalyst (1) except that the contact time was changed to 1.5 seconds. The results are shown in Table 3. Comparative Example 5 An oxidation reaction was conducted in the same manner as in Example 3 except that the catalyst (2) was used in place of the catalyst (1). The results are shown in Table 3. Comparative Example 6 An oxidation reaction was conducted in the same manner as in Example 3 except that the catalyst (3) was used in place of the catalyst (1). The results are shown in Table 3. Example 4 An oxidation reaction was conducted in the same manner as in Example 1 using the catalyst (1) except that the proportions of acrolein and nitrogen in raw material gas were changed to 5% by volume and 65.5% by volume, respectively. The results are shown in Table 3. Comparative Example 7 An oxidation reaction was conducted in the same manner as in Example 4 except that the catalyst (2) was used in place of the catalyst (1). The results are shown in Table 3. Comparative Example 8 An oxidation reaction was conducted in the same manner as in Example 4 except that the catalyst (3) was used in place of the catalyst (1). The results are shown in Table 3. Example 5 [Preparation of V-Sb complex] 9.7 g of ammonium metavanadate was dissolved in 500 ml of water being heated and stirred. To the solution was added 25.4 g of antimony tetraoxide. The mixture was placed in a porcelain-made evaporator set on a hot water bath, and concentrated to dryness. The resulting solid was dried at 200°C for 6 hours and then ground to obtain 35 g of a powder (A-2) of 150 µm or less. The powder (A-2) had the following metal composition. V 1 Sb 2 [Preparation of catalyst] In 2,500 ml of water being heated and stirred were dissolved 350 g of ammonium paramolybdate, 106 g of ammonium metavanadate and 44.6 g of ammonium paratungstate. Thereto was added 35 g of the powder (A-2). Separately, 87.8 g of copper nitrate and 13 g of titanium oxide were added to 750 ml of water being heated and stirred. The resulting two fluids were mixed and placed in a porcelain-made evaporator set on a hot water bath. Thereto was added 1,000 ml of a spherical α-alumina carrier having diameters of 3-5 mm. The mixture was evaporated to dryness with stirring to adhere the above compounds onto the carrier, followed by firing at 400°C for 6 hours, to obtain a catalyst (4). The catalyst (4) had the following metal composition. Mo 12 V 6 W 1 Cu 2.2 Sb 1 Ti 1 [Oxidation reaction] Using the catalyst (4), an oxidation reaction was conducted under the same conditions as in Example 1. As a result, the acrolein conversion was 98.7%; the acrylic acid yield was 94.5%; and the acrylic acid selectivity was 95.7%. The ratio of peak intensities, i.e. d 4.38 /d 4.00 was 0.03. Example 6 [Preparation of V-Sb complex] 4.8 g of ammonium metavanadate was dissolved in 500 ml of water being heated and stirred. To the solution was added 6 g of antimony trioxide. The mixture was placed in a porcelain-made evaporator set on a hot water bath, and concentrated to dryness. The resulting solid was dried at 350°C for 6 hours and then ground to obtain a powder (A-3) of 150 µm or less. The powder (A-3) had the following metal composition. V 1 Sb 1 [Preparation of catalyst] In 2,500 ml of water being heated and stirred were dissolved 350 g of ammonium paramolybdate, 101.5 g of ammonium metavanadate and 44.6 g of ammonium paratungstate. Thereto was added 10 g of the powder (A~3). Separately, 87.8 g of copper nitrate was dissolved in 750 ml of water being heated and stirred. The resulting two fluids were mixed and placed in a porcelain-made evaporator set on a hot water bath. Thereto was added 1,000 ml of a spherical α-alumina carrier having diameters of 3-5 mm. The mixture was evaporated to dryness with stirring to adhere the above compounds onto the carrier, followed by firing at 400°C for 6 hours, to obtain a catalyst (5) . The catalyst (5) had the following metal composition. Mo 12 V 5.5 W 1 Cu 2.2 Sb 0.25 [Oxidation reaction] Using the catalyst (5) , an oxidation reaction was conducted under the same conditions as in Example 1. As a result, the acrolein conversion was 98.9%; the acrylic acid yield was 94.7%; and the acrylic acid selectivity was 95.8%. The ratio of peak intensities, i.e. d 4.38 /d 4.00 was 0.03. Example 7 [Preparation of V-Sb complex] 19.3 g of ammonium metavanadate was dissolved in 500 ml of water being heated and stirred. To the solution were added 12.7 g of antimony tetraoxide and 6 g of antimony trioxide. The mixture was placed in a porcelain-made evaporator set on a hot water bath, and concentrated to dryness. The resulting solid was dried at 300°C for 6 hours and then ground to obtain 33.5 g of a powder (A-4) of 150 µm or less. The powder (A-4) had the following metal composition. V 4 Sb 3 [Preparation of catalyst] In 2,500 ml of water being heated and stirred were dissolved 350 g of ammonium paramolybdate, 96.6 g of ammonium metavanadate and 44.6 g of ammonium paratungstate. Thereto was added 33.5 g of the powder (A-4). Separately, 87.8 g of copper nitrate and 10.2 g of zirconium oxide were added to 750 ml of water being heated and stirred. The resulting two fluids were mixed and placed in a porcelain-made evaporator set on a hot water bath. Thereto was added 1,000 ml of a spherical α-alumina carrier having diameters of 3-5 mm. The mixture was evaporated to dryness with stirring to adhere the above compounds onto the carrier, followed by firing at 400°C for 6 hours, to obtain a catalyst (6). The catalyst (6) had the following metal composition. Mo 12 V 6 W 1 Cu 2.2 Sb 0.75 Zr 0.5 [Oxidation reaction] Using the catalyst (6) , an oxidation reaction was conducted under the same conditions as in Example 1. As a result, the acrolein conversion was 99%; the acrylic acid yield was 94.6%; and the acrylic acid selectivity was 95.6%. The ratio of peak intensities, i.e. d 4.38 /d 4.00 was 0.04. Example 8 [Preparation of V-Sn complex] 38.7 g of ammonium metavanadate was dissolved in 500 ml of water being heated and stirred. To the solution was added 25 g of stannous hydroxide. The mixture was placed in a porcelain-made evaporator set on a hot water bath, and concentrated to dryness. The resulting solid was dried at 400°C for 6 hours and then ground to obtain 52 g of a powder (A-5) of 150 µm or less. The powder (A-5) had the following metal composition. V 2 Sn 1 [Preparation of catalyst] In 2,500 ml of water being heated and stirred were dissolved 350 g of ammonium paramolybdate, 87 g of ammonium metavanadate and 44.6 g of ammonium paratungstate. Thereto was added 52 g of the powder (A-5). Separately, 87.8 g of copper nitrate was dissolved in 750 ml of water being heated and stirred. The resulting two fluids were mixed and placed in a porcelain-made evaporator set on a hot water bath. Thereto was added 1,000 ml of a spherical α-alumina carrier having diameters of 3-5 mm. The mixture was evaporated to dryness with stirring to adhere the above compounds onto the carrier, followed by firing at 400°C for 6 hours, to obtain a catalyst (7). The catalyst (7) had the following metal composition. Mo 12 V 6.5 W 1 Cu 2.2 Sn 1 [Oxidation reaction] Using the catalyst (7), an oxidation reaction was conducted under the same conditions as in Example 1. As a result, the acrolein conversion was 99.0%; the acrylic acid yield was 94.0%; and the acrylic acid selectivity was 95.0%. The ratio of peak intensities, i.e. d 4.38 /d 4.00 was 0.05. Example 9 [Preparation of V-(Sb-Sn) complex] 19.3 g of ammonium metavanadate was dissolved in 500 ml of water being heated and stirred. To the solution were added 18.8 g of antimony trichloride and 3 g of stannous chloride. The mixture was placed in a porcelain-made evaporator set on a hot water bath, and concentrated to dryness. The resulting solid was dried at 500°C for 6 hours and then ground to obtain 27.5g of a powder (A-6) of 150 µm or less. The powder (A-6) had the following metal composition. V 10 Sb 5 Sn 1 [Preparation of catalyst] In 2,500 ml of water being heated and stirred were dissolved 350 g of ammonium paramolybdate, 96.6 g of ammonium metavanadate and 44.6 g of ammonium paratungstate. Thereto was added 27.5 g of the powder (A-6). Separately, 87.8 g of copper nitrate was dissolved in 750 ml of water being heated and stirred. The resulting two fluids were mixed and placed in a porcelain-made evaporator set on a hot water bath. Thereto was added 1,000 ml of a spherical α-alumina carrier having diameters of 3-5 mm. The mixture was evaporated to dryness with stirring to adhere the above compounds onto the carrier, followed by firing at 400°C for 6 hours, to obtain a catalyst (8). The catalyst (8) had the following metal composition. Mo 12 V 6 W 1 Cu 2.2 Sb 0.5 Sn 0.1 [Oxidation reaction] Using the catalyst (8), an oxidation reaction was conducted under the same conditions as in Example 1. As a result, the acrolein conversion was 99.2%; the acrylic acid yield was 94.5%; and the acrylic acid selectivity was 95.3%. The ratio of peak intensities, i.e. d 4.38 /d 4.00 was 0.04. Example 10 [Preparation of V-Sn complex] 9.7 g of ammonium metavanadate was dissolved in 500 ml of water being heated and stirred. To the solution was added 12.6 g of stannous hydroxide. The mixture was placed in a porcelain-made evaporator set on a hot water bath, and concentrated to dryness. The resulting solid was dried at 300°C for 6 hours and then ground to obtain 20 g of a powder (A-7) of 150 µm or less. The powder (A-7) had the following metal composition. V 1 Sn 1 [Preparation of catalyst] In 2,500 ml of water being heated and stirred were dissolved 350 g of ammonium paramolybdate, 106 g of ammonium metavanadate and 44.6 g of ammonium paratungstate. Thereto was added 20 g of the powder (A-7). Separately, 87.8 g of copper nitrate and 5.6 g of stannous oxide were added to 750 ml of water being heated and stirred. The resulting two fluids were mixed and placed in a porcelain-made evaporator set on a hot water bath. Thereto was added 1,000 ml of a spherical α-alumina carrier having diameters of 3-5 mm. The mixture was evaporated to dryness with stirring to adhere the above compounds onto the carrier, followed by firing at 400°C for 6 hours, to obtain a catalyst (9). The catalyst (9) had the following metal composition. Mo 12 V 6 W 1 Cu 2.2 Sn 0.75 [Oxidation reaction] Using the catalyst (9), an oxidation reaction was conducted under the same conditions as in Example 1. As a result, the acrolein conversion was 98.8%; the acrylic acid yield was 94.0%; and the acrylic acid selectivity was 95.1%. The ratio of peak intensities, i.e. d 4.38 /d 4.00 was 0.05. Example 11 Industrial propylene (purity: 94% or higher) was subjected to catalytic gas-phase oxidation in the presence of a molybdenum/bismuth-based catalyst to obtain a reaction mixture gas comprising 5% by volume of acrolein, 1.2% by volume of unreacted propylene and organic by-products, 4.5% by volume of oxygen, 20% by volume of steam and 69.3% by volume of a nitrogen-containing inert gas. Successively, the reaction mixture gas was introduced into a reaction tube filled with the catalyst (1) and subjected to an oxidation reaction under the conditions of temperature = 255°C and contact time = 2 seconds. An acrolein conversion of 98.9%, an acrylic acid selectivity of 95.6% and an acrylic acid per-pass yield of 94.5% were obtained based on an assumption that the propylene, propane, acrylic acid, acetic acid, etc. present in the reaction mixture gas introduced into the reaction -tube did not take part in the oxidation reaction. The above results confirmed that the catalyst produced by the present invention maintained a high activity and could produce acrylic acid from acrolein stably at a high yield.

Claims: A process for producing acrylic acid by subjecting acrolein or an acrolein-containing gas to gas-phase oxidation with molecular oxygen or a molecular oxygen-containing gas in the presence of a molybdenum/vanadium-based oxide catalyst represented by the following general formula (I): Mo a V b W c Cu d X e Y f Z g O h (wherein Mo is molybdenum: V is vanadium; W is tungsten; Cu is copper; X is at least one element selected from antimony and tin; Y is at least one element selected from magnesium calcium, strontium and barium; Z is at least one element selected from titanium, zirconium and cerium; O is oxygen ; a, b c, d, e, f, g and h are the atom numbers of Mo, V, W, Cu, X, Y and Z, respectively, with a proviso that when a is 12, 2 ≦ b ≦ 14, 0 ≦ c ≦ 12, 0 < d ≦ 6, 0 < e ≦ 5, 0 ≦ f ≦ 3 and 0 ≦ g ≦ 10; and h is a number determined by the oxidation states of the individual elements other than O), in which process the molybdenum/vanadium-based oxide catalyst is produced by using the following substances as the raw materials of vanadium, copper, antimony and tin: (A) the raw material of vanadium is ammonium metavanadate and part thereof is used in the form of a complex with an antimony compound in which the valency of antimony is larger than 0 but smaller than 5, and the raw material of copper is copper nitrate, or (B) the raw material of vanadium is ammonium metavanadate and part thereof is used in the form of a complex with a tin compound in which the valency of tin is larger than 0 but smaller than 4, and the raw material of copper is copper nitrate, or (C) the raw material of vanadium is ammonium metavanadate and part thereof is used in the form of a complex with (1) an antimony compound in which the valency of antimony is larger than 0 but smaller than 5 and (2) a tin compound in which the valency of tin is larger than 0 but smaller than 4, and the raw material of copper is copper nitrate. A process according to Claim 1, wherein the catalyst is supported on an inactive carrier. A process according to Claim 1 or 2, wherein the oxidation reaction is carried out by contacting, with the catalyst, a mixed gas comprising 1-15% by volume of acrolein, 0.5-25% by volume of oxygen, 0-30% by volume of steam and 20-80% by volume of an inert gas at a temperature of 180-350°C at a pressure of normal pressure to 10 atm. at a space velocity (STP) of 500-20,000 hr -1 . A process according to Claim 1 or 2, wherein the oxidation reaction is carried out by contacting, with the catalyst, a mixed gas comprising 4-12% by volume of acrolein, 2-20% by volume of oxygen, 3-25% by volume of steam and 50-70% by volume of an inert gas at a temperature of 200-330°C at a pressure of normal pressure to 10 atm. at a space velocity (STP) of 1,000-10,000 hr -1 . A process according to Claim 1, wherein the catalyst, when subjected to X-ray diffractometry, gives a ratio of peak intensity at d = 4.38 Å to peak intensity at d = 4.00 Å, of less than 0.07. A process according to Claim 1, wherein the catalyst has a peak intensity retention of at least 80% when the peak intensity retention is defined as percent of the peak intensity at d = 4.00 Å , of used catalyst after 4000-hour operation to the peak intensity at d = 4.00 Å , of unused catalyst when the two catalysts are subjected to X-ray diffractometry.

Inventors: KAWAJIRI TATSUYA, MIHARA ICHIRO, TANIMOTO MICHIO, KAWAJIRI, TATSUYA, MIHARA, ICHIRO, TANIMOTO, MICHIO
Applicants: NIPPON CATALYTIC CHEM IND, NIPPON SHOKUBAI CO., LTD.
Application year: 1995
Main classification: C07C51
IPC codes: B01J23, C07C57, C07C51