PROCESS FOR PREPARATION OF AN OXIDATION CATALYST

Abstract: An oxidation catalyst formed from V₂0₅ and TiO₂ of rutile structure is obtained by a process which comprises the following stages: a) preparation of a solution of Ti(IV) by dissolution (partial hydrolysis) of TiCl₄ in an aqueous solution at a final pH below 1.0, b) preparation of a solution of V(IV) by heating, in order to dissolve it, solid V₂0₅ suspended in an aqueous oxalic acid solution, c) precipitation of metatitanic acid from the Ti(IV) solution at a pH not higher than 1.3, d) precipitation of vanadium oxide from the said V(IV) solution in the presence of the metatitanic acid obtained in (c), and e) separation of the precipitate, drying and calcination thereof at elevated temperature. This gives a catalyst formed from V₂0₅ and TiO₂ of rutile structure, which has a specific surface area of the order of 10-60 m²/g, is highly active, selective and stable under conditions of catalytic oxidation, especially under the conditions of the oxidation of o-xylene to phthalic anhydride.

Description: The present invention relates to a process for preparing an oxidation catalyst formed from V₂0₅ (vanadic anhydride or vanadium pentoxide) and TiO 2 (titanium dioxide) of rutile structure. Oxidation catalysts consisting of vanadium and titanium oxides, which may contain an oxidation promoter, especially an alkali metal halide, are known from the state of the art. These known catalysts can be obtained by impregnating a preformed TiO₂ support of rutile structure with an aqueous solution of a decomposable vanadium salt and decomposing the vanadium compound at an elevated temperature to obtain the corresponding oxide. These catalysts show in general a low activity in oxidation processes, presumably due to the low surface area of the support. In fact, the industrial preparation of rutile takes place by calcination at very high temperatures, of the order of 900-1,000°C, of anatase TiO₂ obtained by the sulfate or chloride process, and such a calcination at elevated temperatures leads to a diminution of the surface area of the rutile. In practice, the assumption that the rutile structure of the TiO₂ leads to poorer characteristics for the catalyst derives essentially from the experimental evidence that the transformation of the anatase structure into the rutile structure in the catalysts formed from vanadium and titanium oxides leads to the following undesirable phenomena: a sharp decrease in the values of the surface area, destruction of the porous structure (macropores) and formation of a solid solution of V⁴⁺ in the rutile lattice, with consequent destruction of the so-called "monolayer" of vanadium on the anatase. These phenomena contribute to the loss of activity of the catalyst in the oxidation of o-xylene to phthalic anhydride, and it has been reported that the formation of certain percentages of rutile in the catalyst leads to a significant reduction in the useful product yield from the reaction. It is therefore generally preferred to use catalysts formed from oxides of vanadium and titanium containing Ti0₂ of anatase structure, and these catalysts can be obtained either by impregnation of preformed anatase with an aqueous solution of a vanadium salt or by coprecipitation of the hydrated titanium and vanadium oxides from a solution of the corresponding soluble salts, followed by drying of the precipitate thus obtained and calcination of the dried precipitate at elevated temperature. In the technical and patent literature, the fact has been widely stressed that the catalysts for the oxidation of o-xylene to phthalic anhydride, based on V 2 O 5 and TiO 2 , require the anatase structure for the Ti0₂ namely in order to obtain active and selective catalysts, and reference is made in this respect to the descriptions by I.E. Wachs et al. in Applied Catalysis, 15 (1985) 339; J. Haber in Pure and Applied Chemistry, 56 (12) (1984) 1663; M.S. Wainwright et at. in Catal. Rev. -Sci. Eng., 19 (1977) 211; and also the descriptions in US Patents 4,228,038, 4,397,768 and 4,498,204, and German Patent 1,553,728. The prior art includes furtheron the US-Patent 4 238 370 describing a method for the manufacture of a total oxidation catalyst in which titanium dioxide is treated with vanadium pentoxide and then washed until the wash liquid is substantially free of vanadium pentoxide. This method produces a catalyst which will totally oxidize an oxidizable organic compound to carbon dioxide. Finally it is cited the DE-A 2 401 160 describing a process for preparing an oxide catalyst consisting of vanadium oxide and titanium oxide wherein more than 50% of the titanium oxide are present in cristalline rutile structure, characterized in that an intimate mixture of the vanadium compound and a titanium oxide hydrogel, resulting after the drying at 250°C in a titanium oxide having a crystallite size of not more than 70 Å calculated according to the Sherrer-Process on the basis of his x-ray diffraction peak at 20 = 25,4°, is calcined in an oxidizing atmosphere at a temperature of 400 to 600°C. Furtheron there is described an oxide catalyst being suitable for the application as fluid bed catalyst for producing acetic acicd by gas phase oxidation of butenes, essentially consisiting of vanadium oxide, titanium oxide and silicic acid having the following characteristic values: Various theories have been advanced to explain the need to have anatase TiO₂ in the catalysts under discussion, such as, for example, the presence of a crystallographic similarity between the most exposed surface planes in TiO₂ and the crystallographic planes of the V₂O₅ which contain the active and selective sites. Reference is made in this respect to the description by A. Vejux et al. in J- Solid State Chem., 23 (1978) 93. On the other hand, it must be remembered that the presence of vanadium in the catalyst favors the transformation of anatase TiO₂ to rutile and lovers the transformation temperature from 900-1,000°C to 500-600°C. Therefore, in spite of accurate control of the temperature profile and the hot spots in the catalyst bed, when the catalysts based on vanadium and titanium oxides are used, a partial transformation of the anatase into rutile, with a consequent reduction in the performance of the catalyst, takes place in the course of a certain period of time. It is the main object of the present invention to provide a process for preparing a V₂O₅ and TiO₂ catalyst which, while containing TiO₂ in the rutile structure, overcomes the disadvantages of the analogous catalysts of the state of the art, which is suitable for operation at temperatures lower than those normally used in industrial oxidation processes, in particular in the oxidation of o-xylene to phthalic anhydride, while maintaining the high productivity and conversions at the industrially desirable level and which has a longer service life. Yet another object of the present invention is to provide a process for preparing a V 2 O 5 and TiO 2 catalyst which allows the TiO₂ to be obtained directly in the rutile structure and leads therefore to a catalyst having the abovementioned advantages. according to the subject of the invention, the abovementioned objects are achieved by a process for preparing an oxidation catalyst which comprises the following stages: a) preparation of a solution of Ti(IV) by partial hydrolysis of TiCl₄ in an aqueous solution at a temperature below 50°C and a final pH below 1.0, b) preparation of a solution of V(IV) by dissolving, by means of heating, solid V₂O₅ suspended in an aqueous oxalic acid solution, c) precipitation of metatitanic acid from the said Ti(IV) solution, while maintaining the pH at no higher than 1.3, d) precipitation of vanadium oxide from the said V(IV) solution in the presence' of the metatitanic acid obtained in (c), and e) separation of the precipitate obtained in (d), drying and calcination thereof at temperatures above 300°C. According to an embodiment of the process, which is the subject of the invention, the procedure starts with the precipitation of metatitanic acid (stage (c)) from a mixture of the Ti(IV) and VI (IV) solutions prepared in (a) and (b) respectively. In this case, owing to the conditions used, which are explained in more detail below, a selective precipitation of metatitanic acid takes place in the absence or substantial absence of precipitation of the vanadium. According to an alternative embodiment of the process of the invention, the metatitanic acid is precipitated from the Ti(IV) solution in the absence of the vanadium compound, the V(IV) solution is then added to the suspension thus obtained and, finally, the vanadium oxide is precipitated onto the metatitanic acid. The process and the catalyst according to the invention are explained in more detail in the text which follows. Stage (a) According to the process of the present invention, a Ti(IV) solution is prepared by dissolution (partial-hydrolysis) of TiCl₄ in an aqueous hydrochloric acid solution. The final pH of the solution must be below 1.0 and is preferably about 0. The temperature during the dissolution must be maintained at values below about 50°C and preferably within the range from 20° to 40°C. If the temperature and pH conditions are not adhered to, total hydrolysis of TiCl₄ with precipitation of metatitanic acid can take place. The hydrolysis of TiCl₄ can also be carried out in water, under the temperature conditions indicated above, in the absence of added hydrochloric acid. In fact, liberation of hydrochloric acid takes place during the partial hydrolysis of TiCl₄, and this takes the acidity to values at which redissolution of the titanic acid, which may have precipitated, can occur. Nevertheless, in this case, the Ti(IV) solution obtained is less stable in time and leads in the course of a few days to the precipitation of TiO(OH)₂. Instead, when operating according to the preferred embodiment in the presence of HCl, a Ti(IV) solution which is stable for several months is obtained. Stage (b) According to the process of the present invention, a V(IV) solution is prepared by heating, in order to dissolve it, solid V₂O₅ suspended in an aqueous solution of oxalic acid. The initial molar ratio of oxalic acid and vanadium is advantageously maintained at a value in the range from 0.5/1 to 2/1, preferably about 1.5/1. In fact, an oxalic acid quantity lover than the abovementioned ratios leads to an incomplete reduction of the vanadium and, in such a case, a step for separating off the unreduced excess V2O5 is necessary. A greater quantity of oxalic acid can have an adverse effect in the subsequent precipitation of the metal oxides and lead to a final catalyst in which the TiO₂ contains a certain fraction of undesired anatase structure. In general, the V₂O₅ suspension in the aqueous oxalic acid solution is heated to a temperature between 40° and 70°C and, under these conditions, a complete or substantially complete reduction of V 2 O 5 to VI(IV) (blue solution) is achieved in a period of time from 20 minutes to about 2 hours, depending on the temperature applied. Stage (c) According to the process of the present invention, metatitanic acid TiO(OH)2 is precipitated from the solution obtained in stage (a). According to a possible embodiment of the process, the precipitation of metatitanic acid is carried out before mixing of the aqueous solutions obtained in stages (a) and (b) in proportions depending on the desired value of the Ti/V ratio in the resulting solution, which varies from 1 to 100, and hence in the catalyst obtained at the end. The quantity of vanadium in the catalyst, expressed as a percentage by weight of V 2 O 5 , can in general vary from I% to 50% and is preferably of the order of 10-20%. Conveniently, the mixing of the solutions is carried out at ambient temperature values (20-25°C) and the pH value of the resulting solution is below 1.0 and preferably is about 0. The solution is stirred at a temperature in the range from 20 to 90°C and at a temperature in the range from 55-95°C. The preferred alkalizing agent is ammonia, the use of which makes it possible to avoid the introduction of metallic cations into the solution. Moreover, the addition is carried out while maintaining the mixture under vigorous stirring, and preferably the alkalizing agent is added gradually in order to allow better pH control. In this treatment, an initial opalescence appears which increases progressively and finally leads to the complete or substantially complete precipitation of metatitanic acid. The precipitation temperature can vary from ambient values (20-25°C) to the boiling point of the solution. Nevertheless, ambient or slightly higher temperature values are preferred. The time required for achieving complete precipitation of metatitanic acid depends essentially on the preselected temperature of the precipitation and on the final pH value, and it is generally in the range from 1 to 24 hours. It is to be noted that the pH value during the phase of precipitating the metatitanic acid is critical. In fact, if the pH value of about 1.3 is exceeded, orthotitanic acid (Ti(OH)₄) is precipitated which, in the subsequent calcination treatment of the catalyst, leads to the formation of TiO₂ of anatase structure. According to an alternative form of stage (c), the metatitanic acid is precipitated by thermal hydrolysis of the solution resulting from the mixing of the Ti(IV) and V(IV) solutions, operating at pH values below 1.0 and preferably around zero. In this case, the mixture of the two solutions is heated to a temperature from about 55°C up to a temperature just below the boiling point of the mixture (about 100°c), values of the order of 80-85°c being preferred. Under these conditions, a substantially complete precipitation of metatitanic acid is achieved in a time of the order of 1 hour. Operating under the conditions described above leads to a selective precipitation of Ti in the absence of precipitation of vanadium or at least substantial precipitation thereof, since only small traces of vanadium can remain combined with the metatitanic acid precipitate. According to a further possible embodiment of stage (c), the metatitanic acid is precipitated from the Ti(IV) solution in the absence of the vanadium compound but following in other respects the procedure reported above. In this case, a suspension of the precipitate in the aqueous "mother" solution is obtained, to which the VI IV) solution is then added, and the mixture obtained is treated in the next stage (d) as explained below. Stage (d) In this stage, the vanadium oxide is precipitated in its hydrated form on the metatitanic acid either from the solution, from which the metatitanic acid was previously precipitated, or from the mixture of metatitanic acid precipitated beforehand with the V(IV) solution. For this purpose, an alkalizing agent, for example ammonia, is added to the suspension resulting from the preceding stage, in order to take the pH to a value of about 5 to cause the vanadium to precipitate in the form of hydrated oxide. Preferably, the alkalizing agent is added gradually to the suspension of metatitanic acid, with continued stirring, at temperatures from ambient (20-25°C) to about 90°C. Alternatively, the precipitation of the vanadium oxide is carried out by evaporating the solvent under atmospheric pressure or reduced pressure. Stage (e) The precipitate obtained in stage (d) is separated off, dried and calcined. The drying is conveniently carried out at temperatures below about 150°C and can be preceded by a washing of the precipitate with water. The calcination is carried out at temperatures above 300°C and preferably at temperatures from 380°c to 500°C, for a time longer than 1 hour and preferably of the order of 5 hours. This gives the catalyst according to the present invention, formed from V₂O₅ and TiO₂, with a percentage of V 2 O 5 of 1% to 50% by weight and preferably of the order of 10-20%, and containing Ti0₂ of rutile structure and having a surface area of the order of 10-60 m²/g. The catalyst according to the present invention can also contain an oxidation promoter in addition to the V₂0₅ and rutile Ti0₂. Such a promoter, which can be selected from those known from the state of the art, such as, for example, phosphorus, rubidium, cesium, potassium, boron, antimony, molybdenum, tungsten and their precursors or mixtures, is added in stage (a) or (b) or in that of mixing the Ti(IV) and V(IV) solutions, in such quantities that it is present in the finished catalyst in the percentages known from the state of the art, for example within the range from 0.1 to 10%. The catalyst of the present invention exhibits an exceptionally high stability under oxidation conditions, is suitable for operation at temperatures lower than those normally used on the industrial scale, thus leading to high productivity and conversions, and in addition has an extended service life in the production cycle. The said catalyst is particularly useful in the processes for the oxidation of o-xylene to phthalic anhydride. The experimental examples which follow are illustrative and do not restrict the scope of the invention. Example 1 10 g of V₂O₅ (vanadic anhydride) are suspended in a solution of H 2 C 2 O 4 .2H 2 O (20.8 g in 200 cm³ of distilled H₂O) in a three-necked glass flask; the suspension is heated, with stirring and under a nitrogen stream, to 70°C by means of a heating jacket, until the V 2 O 5 has been completely reduced to V(IV), that is to say until a clear blue solution is obtained. The time needed for complete reduction is about 2 hours. The solution is cooled and any unreacted residues of V 2 O 5 are separated off by filtration. 50 cm³ of TiCl₄ are slowly added dropwise to a second flask immersed in a water/ice bath and containing a solution of HCl in H 2 O (10 cm³ of 37% HCl in 300 cm³ of H 2 O); the solution is kept under vigorous stirring during the partial hydrolysis of the TiCl₄. The two solutions are then mixed while cold. The precipitation is carried out by adding dropwise, with stirring of the solution, a solution of NH 4 OH (30% of NH₃) until pH 1.0 is obtained in the solution. An initial opalescence forms, and the precipitation of metatitanic acid is complete after about 12 hours. By contrast, the vanadium remains in solution and is precipitated by concentrating, the solvent being evaporated in vacuo at 60°C. The solid obtained is dried at 80°C for 24 hours and calcined at 400°C for 3 hours. The surface area of the catalyst obtained in the end is 45 m²/g. The TiO₂ obtained is in the rutile crystal form, and the fraction of vanadium in the catalyst is 21.9%, expressed as % by weight of V₂O₅. Example 2 The V(IV) and Ti(IV) solutions are prepared in a manner analogous to that described in Example 1. By contrast, the precipitation is carried out by heating the mixture of the two solutions to 85°c, with stirring. The hydrolysis with precipitation of metatitanic acid is complete after about 3 hours. The vanadium which, however, regains in solution is precipitated in the same solution by concentrating, the solvent being evaporated In vacuo at 60°C. The precipitate is dried at 80°C for 24 hours and calcined at 400°C for 3 hours. Examples 3-5 The catalyst is prepared as in Example 2, but the solid product after drying is calcined at 450°C for 3 hours, or at 500° C for 3 hours or at 550°C for 3 hours. The surface areas obtained are 30 m 2 /g, 20 m 2 /g and 14 m 2 /g in these three cases respectively. Example 6 The catalyst is prepared analogously to Example 2, except that the initial quantity of V 2 O 5 used for preparing the V(IV) solution is 5.0 g. The final vanadium content after calcination at 400°C for 3 hours is 12.3% by weight, expressed as V 2 O 5. The surface area is 45 m 2 /g, and the TiO₂ is in the rutile crystal form. Example 7 The V(IV) and Ti(IV) solutions are prepared as indicated in Example 1; NH₄OH (30% of NH₃) is then added dropwise up to pH 1.0 to the Ti(IV) solution, with stirring. An initial opalescence forms, and the precipitation of metatitanic acid is complete after about 12 hours. The V(IV) solution is then added with stirring to the suspension; vanadium oxide then precipitates in its hydrated form when the solvent is evaporated in vacuo at 60°C. The treatment of the precipitate is analogous to that indicated in Example 1. After calcination, the catalyst shows the rutile structure of TiO₂, has a surface area of 45 m²/g and a vanadium content of 21.9%, expressed as % by weight of V₂O₅. Example 8 The V(IV) and Ti(IV) solutions are prepared in a manner analogous to that reported in Example 1; the Ti(IV) solution is heated to 85°C, with stirring; the hydrolysis with precipitation of metatitanic acid is complete after about 2 hours. The V(IV) solution is then added with stirring to the suspension; the vanadium oxide then precipitates in its hydrated form, when the solvent is evaporated in vacuo at 60°C. The treatment of the precipitate is analogous to that indicated in Example 1. After calcination, the catalyst shows the rutile structure of TiO₂, has a surface area of 45 m²/g and a vanadium content of 21.9%, expressed as % by weight of V₂O₅. Example 9 The preparation procedure is analogous to that reported in Example 7, but the metatitanic acid, once the precipitation is complete, is filtered off and the solid residue is washed with H₂O, dried at 80°C for 24 hours and then calcined at 400°C for 3 hours. This gives TiO₂ in the rutile crystal form with a surface area of 40 m²/g. The solid is then suspended in the V(IV) solution; the vanadium is precipitated in its hydrated form, when the solvent is evaporated in vacuo at 60°C. The precipitate is dried at 80°C for 24 hours and calcined at 400°C for 3 hours. Example 10 The preparation procedure is analogous to that reported in Example 8, but the metatitanic acid, once precipitation is complete, is filtered off and the solid residue is washed with H₂0, dried at 80°c for 24 hours and then calcined at 400°C for 3 hours. This gives TiO₂ in the rutile crystal form with a surface area of 40 m²/g. The solid is suspended in the V(IV) solution; the vanadium is precipitated in its hydrated form, when the solvent is evaporated in vacuo at 60°C. The precipitate is dried at 80°C for 24 hours and calcined at 400°C for 3 hours. Examples 11-14 The preparation procedure is analogous to that reported in Examples 7-10, except that the V(IV) solution is prepared by reduction of 5.0 g of V₂0₅. The catalysts finally obtained contain 12.3% by weight of V₂0₅. Examples 15-17 The catalysts prepared by the procedure reported in the preceding examples are used for the gas-phase oxidation with air of o-xylene to phthalic anhydride. The process is run at temperatures in the range from 290° to 330°C, that is to say lower than the temperatures 360-380°C which can be used with commercial catalysts, and at high hourly space velocities of the gas of the order of about 5,000 hours-¹, o-xylene conversions of about 99% and higher being obtained. These data precisely demonstrate the superiority of the catalysts which are the subject of the present invention and which show high activities at temperatures about 30-90°C lower than those used according to the state of the art, and at space velocities much higher than the usual ones, which leads to a superior productivity and longer life of the catalyst.

Claims: A process for preparing an oxidation catalyst which comprises vanadium pentoxide and titanium dioxide of rutile structure, the said catalyst containing 1-50% by weight of vanadium pentoxide and having a surface area in the range from 10 to 60 m 2 /g, which comprises the following stages: a) preparation of a solution of Ti(IV) by partial hydrolysis of TiCl₄ in an aqueous solution at a temperature below 50°C and a final pH below 1.0, b) preparation of a solution of V(IV) by dissolving, by means of heating, solid V₂O₅ suspended in an aqueous oxalic acid solution, c) precipitation of metatitanic acid from the said Ti(IV) solution, while maintaining the pH at no higher than 1.3, d) precipitation of vanadium oxide from the said V(IV) solution in the presence of the metatitanic acid obtained in (c), and e) separation of the precipitate obtained in (d), drying and calcination thereof at temperatures above 300°C. The process as claimed in claim 1, wherein the partial hydrolysis stage (a) is carried out in an aqueous hydrochloric acid solution to a final pH of 0. The process as claimed in claim 1, wherein the partial hydrolysis stage (a) is carried out in water in the absence of added acid. The process as claimed in any one of claims 1 to 3, wherein the molar oxalic acid/vanadium ratio varying from 0.5/1 to 2/1 is used in stage (b). The process as claimed in any one of claims 1 to 4, wherein stage (b) is carried out at a temperature from 40 to 70°. The process as claimed in any one of claims 1 to 5, wherein the metatitanic acid precipitation stage (c) is carried out by introducing an alkalizing agent into the Ti(IV) solution until a pH no higher than 1.3 is reached, the solution being stirred at a temperature in the range from 20 to 90°C. The process as claimed in claim 6 wherein the said alkalizing agent is ammonia. The process as claimed in any one of claims 1 to 5, wherein stage (c) is carried out in the absence of an alkalizing agent at a pH below 1 and at a temperature in the range from 55-95°C. The process according to any one of claims 1 to 8 , which comprises, before the said stage (c), a stage of mixing the said Ti(IV) and V(1V) solutions, prepared in stages (a) and (b) respectively, in such a ratio that a Ti/V atomic ratio from 1 to 100 is obtained in the resulting mixture, the said stage (c) carried out on this mixture leading to a selective precipitation of metatitanic acid. The process as claimed in any one of claims 1 to 9 , wherein the said stage (d) is carried out by introducing an alkalizing agent into a mixture of the said V(IV) solution and the metatitanic acid precipitated in (c), until a pH of about 5 is reached, the mixture being stirred at a temperature in the range from 20 to 90°C. The process as claimed in claim 10 wherein the said alkalizing agent is ammonia. The process as claimed in any one of claims 1 to 9 , therein the said stage (d) is carried out by subjecting a mixture of the said V(IV) solution and the metatitanic acid precipitated in (c) to evaporation under a pressure which varies from atmospheric pressure to a reduced pressure. The process as claimed in any one of claims 1 to 12, which also comprises the addition, in one of stages (a), (b) or the mixing of the Ti(IV) and V(IV) solutions before stage (c), of an oxidation promoter selected from the group comprising potassium, rubidium, cesium, antimony, boron, phosphorus, molybdenum, tungsten, their precursors and their mixtures.

Inventors: CAVANI, FABRIZIO, RIVA, ALFREDO
Applicants: ALUSUISSE ITALIA S.P.A.
Application year: 1987
Main classification: B01J23
IPC codes: C01G31, B01J27, B01J21, B01J35, C07C51, B01J37, C07D307, C01G23, B01J23