HIGH YIELD, HIGH RESISTANCE CATALYST, PARTICULARLY SUITABLE FOR THE OXIDATION OF METHYL ALCOHOL TO FORMALDEHYDE

Abstract: Catalyst suitable for the oxidation of CH₃OH to CH₂O, comprising a mixture of a well known catalytic system, herein indicated as mixed oxides, based on Fe₂(MoO₄)₃ and molybdenum trioxide (MoO₃) with a quantity of Fe₂O₃ from 5 to 90% by weight of the sum of Fe₂O₃ plus mixed oxides, the superficial area of the Fe₂O₃ being from 1 to 10 m²/g and the superficial area of the whole catalyst being from 1 to 6 m²/g.

Description: BACKGROUND The oxidation of CH₃OH to CH₂O, with O₂ or other gas con­ taining oxygen, is usually carried out in the presence of catalysts based on Ag or ferro-molybdate; sometimes a com­ bination of these two catalysts is used in two stages of a same plant. The highly exothermic oxidation with Fe-Mo catalysts is carried out in static bed and tube nest reactors in which heat is removed indirectly by a thermic fluid. In the most common catalytic beds of the Mo/Fe type, air and CH₃OH are fed, as well as streams of recycled gas, the con­ tent of CH₃OH being between 6 and 9,5% by volume and the maximum temperature of the hot spots being between 300 and 430°C while the average temperature should be from 240 to 400°C and better still 270-370°C. It is a good rule, moreover, to add an inert seal at the beginning and the end of the catalytic layer as suggested in British Patent No. 1,057,080. The Mo/Fe catalysts used up to now, hereinafter referred to as "mixed oxides", are prepared by coprecipitation from solutions of ferric and molybdic salts and are made up to mixtures of ferro-molybdate and Mo oxide, uncombined. These mixed oxides in which molybdenum prevails over iron, do not, however, give completely satisfactory yields, especially when a double catalytic layer is used (see British Patent No. 1,463,174) to better control the temperature and increase the selectivity of oxidation into CH₂O. US-Patent No. 2,812,308 owned by Reichold Company teaches the improved use of the combination of mixed oxides with ferric oxide or silicon carbide. With regard to the ferric oxide, the patent specifies that a sintered oxide must be used, sintering temperatures usually being from 1090 to 1370°C (see Encyclopedia of Chemical Technology; Wiley-Inter­ science Publication Editor; Third Edition (1983) volume 21, page 94). At these temperatures, however, the porosity of the oxides disappears and some chemical transformations take place. For instance, if ferric oxide is heated above the 800-900°C range in which there is an equilibrium between Fe₂O₃ and magnetite, ferric oxide loses oxygen and is gradually con­ verted into Fe₃O₄. Mixtures of "mixed oxides" with porous rhombohedral pulverized ferric oxide of the type usually used as a pigment having a superficial area (B.E.T.) generally higher than 20 m²/g do not give satisfacory results. We have unexpectedly found catalysts which are much more effective based on mixtures of "mixed oxides" plus ferric oxides which possess high mechanical resistance and thermal stability as well as a very long life. The mechanical re­ sistance, as known, eliminates the pressure losses due to catalyst crumbling (see USP 3,978,136). Another result we obtain is that of a much higher selectivity CH₂O in such a way that it is possible to use higher concentrations of CH₃OH feed, thus obtaining higher final yields in CH₂O. DESCRIPTION OF THE INVENTION In its most general form, the present invention concerns a high yield, high resistance catalyst suitable for the oxidation of CH₃OH to CH₂O comprising the mixture of a particular known catalytic system, herein indicated as "mixed oxides" based on ferro-molybdate and Mo oxide in which system, obtained through coprecipitation, the Mo/Fe molar ratio is from 1.8 to 5 and preferably from 2 to 4 with a quantity of ferric oxide, Fe₂O₃, not combined with Mo having a superficial area of from 1 to 10, preferably from 2 to 6 m²/g, from 5 to 90% by weight, preferably from 10 to 70% and more, preferably from 20 to 50% of the sum of Fe₂O₃ plus mixed oxides, the superficial area of the entire catalyst being from 1 to 6 m²/g. It was surprisingly found that the introduction of a ferric oxide having the above mentioned specific sur­ face, not only is not the reason of substantial changes in the phase composition of the "mixed oxides", the oxide being little reactive in regard to the uncombined molybdenum trioxide, but also gives to the catalysts valued properties of mechanical resistance and yield, even for a very long time of use. The new catalysts, in fact, possess exceptional mechanical resistance, up to 18 kg per pellet and more with reference to the bored pellet prepared according to Example 3 hereof, and very high yield, for very long periods, up to approximately 1 kg/h of CH₂O per dm³ of catalyst (apparent volume), said yield being independent of the activity and selectivity of the catalyst as well as of the oxidation temperature. The Mo quantities necessary for the preparation of the catalyst are very reduced, and this is a great advantage since molybdenum is a very expensive raw material. The selectivity of the new catalyst both with regard to formaldehyde and to the un­ desired by-products (CO and CO₂) is practically independent of the temperature and that corresponds to high reactor sta­ bility; the importance of a constant selectivity is put well in evidence in the above-mentioned US-Patent. The greatest part of the excellent characteristics described above dis­ appear or are reduced if the superficial area of the ferric oxide is too high or if it is practically reduced to zero. These new catalysts can be prepared by different methods, the method described hereinbelow being only an example and in no way limiting. A solution of a molybdic salt, for instance, ammonium dimolybdate (NH₄)₂Mo₂O₇ or ammonium paramolybdate having formula (NH₄)₆Mo₇O₂₄.4H₂O, and a solution of ferric salt, for instance, hexahydrated ferric chloride or non-hydrated ferric nitrate, are mixed in such quantities as to obtain a molar ratio Mo/Fe from 1.8 to 5 in the final product. The addition of an acid, for instance HCl, suitably adjusting the pH, can accelerate the coprecipitation which brings about the formation of the mixed oxides, as described, for instance, in US-Patents Nos. 3,408,309; 3,198,753 and 3,152,997. The water is then removed, for instance by siphoning, centrifugation or evaporation. Pulverized Fe₂O₃ together with stearic acid, graphite or other conventional lubricant is added to the mixed oxides which may be wet, dried, calcined or, possibly, ground. Excellent results are obtained, particularly with respect to extruded products, by also adding a cellulosic binder such as hydroxypropyl cellulose with the Fe₂O₃. Preferably, the amount of the binder added with the Fe₂O₃ is from 0,5 to 5% by weight of the sum of Fe₂O₃ plus the mixed xodes. The powder or paste is then formed, for instance as cylinders, rings, balls, granules, saddles, extrusions, etc. The cata­ lyst obtained is gradually heated in an oxidizing atmosphere, for a good activation, from room temperature up to 300-600°C, preferably 400-550°C, in a time of approximately 12-36 hours, the final temperature being maintained for at least a further 4 hours. Before activation it is better to dry the extruded catalysts at 100-120°C. The average diameter of the granules is generally from 3 to 5 mm. The invention is illustrated in a non-limiting way in the accompanying drawing in which Fig. 1 shows the results obtained with catalysts formed only by mixed oxides as in Example 2, Fig. 2 shows the results obtained with catalysts con­ taining 30% by weight of Fe₂O₃ as in Example 4. In obtaining the results shown in Fig. 1 and 2, the operating conditions were maintained constant except for the tempera­ ture which is considered as an independent variable. The following examples are provided for illustrating the in­ vention in more detail and are not intended to be limiting of the spirit and scope thereof. EXAMPLE 1 - PREPARATION OF THE "MIXED OXIDES" To a solution at pH = 5.6 of 5 kg of ammonium dimolybdate at 56% by weight of Mo in 90 dm³ of deionized H₂O heated to 60°C, the following were added gradually over a period of 60 minutes: 3,15 kg of FeCl₃6H₂O at 20% by weight of Fe, dissolved in 50 dm³ of deionized H₂O at room temperature (pH of the chloride solution = 1.4). The dispersion was then diluted up to 180 dm³ (pH of the diluted solution = 1.7) and was left to decant until it was possible to siphon off 120 dm³ of liquid; the Mo/Fe molar ratio was 2.58 in the reagents. It was washed 5 times by decantation,using approximately 120 dm³ of deionized H₂O each time. The product wherein the Mo/Fe molar ratio was 2,53 was filtered and the precipitate was dried, first at 120°C for 8 hours and then at 190°C for about 6 hours. The scales obtained were ground reducing their dimension to under 1.5 mm, and the powder was added with stearic acid and at last tableted in the form of bored cylinders (dimensions: 5 x 5 mm; bore diameter = 2.5 mm).Activation was carried out in air by gradually heating from room temperature to 500°C, in 18 hours, and maintaining the final temperature for 4 hours. The oven was then turned off, and the product was ex­ tracted at below 100°C. EXAMPLE 2 - (COMPARATIVE) USE OF THE MIXED OXIDES 221 cm³ (apparent volume) of the bored cylinders prepared according to Example 1 were loaded in a reactor having an internal diameter of 20.4 mm. The reactor was placed in a bath of molten salts and was then fed with a gaseous mixture containing 9% of CH₃OH and 10.5% of O₂ (by volume), the rest being N₂ at a space velocity of 6,000 h⁻¹ (6,000 normal l/h of gas per liter of apparent catalytic volume) with a tempera­ ture of the salts of 280°C. The conversion of methyl alcohol (see Table 1) was 97.3% and the CH₂O yield was 90.5%. EXAMPLE 3 - PREPARATION OF Fe₂O₃ FOR MIXTURES 25 kg of pulverized ferric oxide having a specific surface of 2.7 m²/g were placed in a mixer and added with: 1.36 kg of stearic acid; 0,54 kg of hydroxypropyl cellulose; 0,27 kg of graphite. After dry mixing, 6.6 dm³ of deionized water were added to the mass, and the mixing continued for 2 hours after which the paste was dried for approximately 8 hours at 110-115°C, cooled down, ground and riddled. The granulometric fraction between 0,2 and 1 mm (Fe₂O₃ content = 92% by weight) was used for the preparation of the catalyst of Example 4. In all the other examples the ferric oxide was used as such(in powder). EXAMPLE 4 70 kg of mixed oxides, dried and ground as in Example 1 were intimately mixed with 32.6 kg of Fe₂O₃, prepared according to Example 3. The mixture was added with the lubricant, tableted to obtain cylinders as in Example 1 and activated as in Example 1. Afterwards 221 cm³ of catalyst were loaded in the reactor of Example 2 and tested in the same operating conditions. The CH₂O yield (see Table 1) was 92.5%. EXAMPLE 5 The catalyst of Example 4 was tested in slightly different conditions; data and results are shown in Table 1. EXAMPLE 6 50 kg of mixed oxides, prepared, dried and ground according to Example 1 were intimately mixed with 50 kg of Fe₂O₃ in powder, the lubricant was added, and the mass tableted to obtain bored cylinders (dimensions: 4 x 4 mm; bore = 2mm), and activated according to Example 1. The resulting cylinders (220 cm³) were loaded in the reactor of Example 2. Data and results are shown in Table 1. EXAMPLE 7 Example 6 was repeated, slightly varying Fe₂O₃/mixed oxides ratio. Data and results (totally satisfying) are shown in Table 1. EXAMPLE 8 (COMPARATIVE) A catalyst in the form of 4 x 4 mm bored cylinders having a bore of 2 mm, prepared and activated according to Example 1, i.e. omitting the addition of Fe₂O₃, was loaded in the reactor of Example 2. Data and results are shown in Table 1. EXAMPLE 9 Example 6 was repeated, modifying the Fe₂O₃/mixed oxides ratio as well as the operating conditions. Data and results are shown in Table 1. EXAMPLE 10 50 kg of mixed oxides prepared according to Example 1, dried to 120°C and ground below 0,5 mm were intimately mixed with 50 kg of Fe₂O₃ in powder, added with 1 kg of hydroxypropyl cellulose as binder, and mixed with water and extruded in solid cylinders (approximate dimensions: 4 x 4 mm). The cy­ linders were dried at 100-110°C for a night and then acti­ vated according to Example 1. Data and results are shown in Table 1. EXAMPLE 11 30 kg of mixed oxides prepared according to Example 1,dried to 120°C and powdered below 0,5 mm were intimately mixed with 70 kg of Fe₂O₃ in powder, additioned with 1 kg of hydroxy­ propyl cellulose and then mixed with water and extruded in solid cylinders (approximate dimensions: 4 x 4 mm). The cy­ linders were dried at 100-110°C overnight and activated according to Example 1. Data and results are shown in Table 1.

Claims: 1. A high yield, high mechanical resistance catalyst, particularly suitable for the oxidation of methyl alcohol to formaldehyde, comprising a mixture of a specific ca­ talytic system referred to as "mixed oxides" based on ferro-molybdate and molybdenum trioxide and in which system, obtained through coprecipitation, the Mo/Fe molar ratio is from 1.8 to 5, with a quantity of ferric oxide from 5 to 90% by weight of the sum of Fe₂O₃ plus mixed oxides, the superficial area of said Fe₂O₃ being from 1 to 10 m²/g and the superficial area of the catalyst as a whole being from 1 to 6 m²/g. 2. A catalyst according to claim 1 characterized in that the Mo/Fe molar ratio in the mixed oxides is from 1.8 to 4 and the quantity of Fe₂O₃ is from 10 to 70% by weight with respect to the sum of Fe₂O₃ plus the mixed oxides. 3. A method for the preparation of the catalyst according to claim 1, in which the mixed oxides and the ferric oxide are mechanically mixed, in the presence of an adjuvant selected from the group consisting of lubricants and binders. 4. The method according to claim 3, in which said binder is hydroxypropyl cellulose. 5. A catalyst according to claim 2, in which the Mo/Fe molar ratio in the mixed oxides is from 2 to 3 and the quantity of Fe₂O₃ is from 20 to 50% with respect to the sum of Fe₂O₃ plus the mixed oxides. 6. A method according to claim 4, in which the amount of hydroxypropyl cellulose is from 0.5 to 5% by weight with respect to the sum of Fe₂O₃ plus the mixed oxides. 7. A method according to claim 3, in which the ferric oxide has a specific surface not higher than 5. 8. The method according to claim 3, in which the ferric oxide has a specific surface not higher than 3 m²/g. 9. A method according to claim 3, in which the ferric oxide is in the form of particles having an average size not higher than 0.2 mm. 10. The method according to claim 9, in which the ferric oxide particles have a size of from 0.2 to 1.0 mm. 11. Use of the catalyst according to claim 1 to oxidize CH₃OH to CH₂O in a stream of feed gas with O₂, air or other gas containing oxygen, through cooling with a thermic fluid the temperature of which is from 24° to 400°C, the content of methyl alcohol being from 6 to 9.5% by volume, the space velocity being from 5,000 to 21,000 normal l/h of gas per liter of apparent catalytic volume, the pressure being substantially atmospheric and higher than the pressure losses. 12. Use according to claim 11, in which the stream of feed gas is mixed with a stream of recycled gas poor in oxygen. 13. Use of catalyst according to claim 1 in the second stage of a plant for the oxidation of methyl alcohol in two stages, the first stage being carried out in the presence of silver-based catalysts. 14. Use according to claim 11, in which the space velocity is from 6,000 to 9,000 h⁻¹.

Inventors: CAVALLI, LUIGI, CUCCHETTO, MARIO, PETRINI, GUIDO, VIOLA, AUGUSTO
Applicants: AUSIMONT S.R.L.
Application year: 1986
Main classification: B01J23
IPC codes: C07C67, B01J37, C07C47, B01J23, C07C45, C07B61