CATALYST AND PROCESS FOR THE CATALYTIC HETEROGENEOUS GASPHASE OXIDATION OF ALKENES AND CYCLOALKENES BY THE WACKER ROUTE

Abstract: The invention relates to a catalyst as well as a process for the catalytic heterogeneous gas-phase, oxidation of an alkene or cycloalkene to the corresponding ketone or aldehyde in the presence of H₂O and O₂, the catalyst containing a vanadium and a palladium component mounted on a carrier, the improvement being that the surface of the carrier is covered with hydroxyl groups and the vanadium is present in the form of a surface vanadate.

Description: The invention relates to a catalyst as well as a process for the catalytic heterogeneous gas-phase oxidation of an alkene or cycloalkene to the corresponding ketone or aldehyde in the presence of H₂O and O₂, the catalyst containing a vanadium and a palladium com­ ponent mounted on a carrier. Such a process is also referred to as 'heterogeneous Wacker oxidation'. Ethylene, for instance, can be oxi­ dized to acetaldehyde, butylene-1 to methyl ethyl ketone, and cyclo­ hexene to cyclohexanone. The reaction takes place with palladium-2+ as catalyst. For the Wacker oxidation of ethylene, the reactions taking place can be indicated as follows: Such a process is known from DE-B-2,065,585, which describes the H₂PdCl₄/V₂O₄/V₂O₅ system on a carrier material as catalyst for the heterogeneous Wacker oxidation of olefins to aldehydes and ketones; this was later also described in J. of Catalysis 30 , 109-17 (1973); as redox system, use is made of vanadium 5⁺ / vanadiu7m 4⁺. The disadvantage of such a catalyst, however, is its unstable behaviour. This phenomenon is reported in an article by L. Forni et al. (Ind. Engng. Chem.; Proc. Des. Dev. 16 288-93 (1077): in a period of about 200 hours, the activity decreases by 50 %, necessitating catalyst regeneration at about 350 ◊C. The carrier that in the majority of cases is used for said catalyst is α-alumina, which is a carrier with a small specific surface (up to 20 m²/g), the outer layer consisting of O²⁻ ions. It has been found (L. Forni, loc. cit.) that this requires the use of cata­ lysts having a small specific surface (0.5-1.5 m²/g), since at larger specific surfaces the small pores are filled up or clogged with the divanadium pentoxide. The above-mentioned and other disadvantages of the known heterogeneous Waker oxidation are obviated by the use according to the subject invention of a catalyst, the carrier surface of which is covered with hydroxyl groups, while the vanadium is present in the form of a surface vanadate. Here and hereinafter the term 'surface vanadate' is used in the same sense as in the article by F. Roozenboom c.s. (Z. Anorg. Allg. Chem. 449 , 25-40 (1979)). For the catalyst according to the invention, a chemical bond of the vanadium to the carrier is obtained in the form of a molecularly disperse layer of a vanadium oxide on the carrier. Besides the components already mentioned, the palladium component, for instance dihydropalladium tetrachloride (H₂PdCl₄), is present, deposited according to known techniques. In itself, the preparation of a vanadium oxide - monolayer catalyst on a carrier with hydroxyl groups is known from the publica­ tion by F. Roozenboom c.s. (loc. cit.). Such catalysts have been described to be active for the oxidation of carbon monoxide and metha­ nol. Surprisingly, it has now been found that such a surface vanadate catalyst, provided with an amount of H₂PdCl₄, is a suitable and stable catalyst for the heterogeneous Wacker oxidation. As carrier material use may be made of, for instance, oxides such as Cr₂O₃, TiO₂, CeO₂, ZrO₂ and γ-alumina, all of which contain hydroxyl groups on their surface. By preference, use is made of γ-alumina, which, by the chamical bond to the vanadium, yields a sur­ face aluminium vanadate, in contrast with state-of-the-art α-alumina carriers. US-A-3,240,805 discloses that, according to the state of the art, it should be attempted to deposit the vanadium, as di-vanadium pentoxide, on the α-alumina in the form of crystalline particles having a size < 1 µm, which of course is something quite different from a surface vanadate. From the publication by N.K. Nag et al. (Appl. Catalysis, 9 , 225-33 (1984) it is known to prepare a molecularly disperse aluminium vanadate by impregnation of γ-Al₂O₃ (180 m²/g) with an acidified stoichiometric solution of NH₄VO₃, the amount of vanadium that can be deposited being up to 12 wt.%. The BET surface area, calculated per carrier weight unit, then remains constant, a further indication of the formation of a monolayer, with at most one vanadium group being bound to three hydroxyl groups of the carrier according to the schedu­ le: Such a surface vanadate is distinguished from V₂O₅ also in spectroscopic respect (N.K. Nag, loc. cit.). The vanadium being present on the carrier surface in a mono­ molecular dispersion, for the Wacker synthesis now also use can be made of carrier materials having a large specific surface. By pre­ ference carrier materials with surface areas of 100-400 m²/g are applied. Spectrometric examination (XPS/AES) of both fresh and spent catalyst, prepared using H₂PdCl₄, has shown the atomic ratio between palladium and chlorine to be 4, so that it must be concluded that the palladium must be present in the catalyst as H₂PdCl₄, and not in the form of a palladium that is chemically bound to the vanadium, as is suggested in the publication by Evnin et al. (Journal of Catalysis, 30 , 109-117 (1973), loc. cit.). The amount of vanadium that can be applied depends on the type of carrier material (number of OH groups per m²) and on the spe­ cific surface of the carrier (J.B. Peri (J. Phys. Chem. 69 211 (1965)) reports that after drying at 150 ◊C γ-alumina only has chemically bound OH groups on its surface, viz. approx. 12.5 × 10¹⁸ OH groups per m²). By preference a vanadium content of 1-30 wt.%, relative to the carrier material, is applied. The amount of palladium preferably is 0.01-3 wt.%, relative to the carrier material. Preparation of the catalyst according to the invention pre­ ferably takes place in two steps: (a) preparation of the monolayer dispersion (the surface vanadate); this can be done by any of the techniques suitable for this, for instance the liquid method (F. Roosenboom et al., loc. cit.), in which, for instance, a solution of ammonium metavanadate (NH₄VO₃) is brought at a pH of 4 with H₂SO₄, after which this solution is passed through a column filled with carrier material until the metavanadate is no longer being bound and the solution leaves the column unchanged. Starting from gaseous V₂O₃(OH)₄, also a vanadium monolayer dispersion can be prepared (F. Roosenboom et al., loc. cit.). Other methods to obtain a surface vanadate can also be used. (b) Preparation of the Pd-V carrier-catalyst: a carrier thus loaded is washed, air-dried and calcined. By dry impregnation (such that the volume of the impregnation liquid is at most equal to the pore volume), a palladium-containing solution, e.g. an H₂PdCl₄ solu­ tion, is deposited until the caking point (the point at which the volume of liquid added equals the pore volume of the carrier) is reached, after which the catalyst is dried. The catalyst prepared in this way is eminently suitable for heterogeneous gas-phase oxidation by the Wacker process of alkenes or cycloalkenes, whether substituted or not. Alkenes which can be oxidised according to the invention in particular are C₂-C₁₂ alkenes, whether or not they are substituted with substituents that do not affect the reaction, as well as C₆-C₁₂ cycloalkenes likewise whether they are substituted or note. As alkene or cycloalkene substituents, in particular groups having an electron-­ attracting effect may be present, such as - C N group, a - halogen group, or an - NO₂ group. Dienes of C₃-C₁₂, such as, for instance, butadiene-1,3 and pentadiene-1,4, can also be oxidized by the subject process to the corresponding α-aldehyde, whereby a double band migration in the molecule takes place. It is preferred to use cyclohexene as cyclic alkene, but cyclopentene and cycloheptene can also be oxi­ dized by the process according to the invention. The temperature and pressure at which the reactions are carried out must always be chosen such that the reactants to be supplied are gaseous under those chosen conditions. The temperature, therefore, is in the 20-300 ◊C range, while the pressure is between 0.1 and 10 MPa. Though the invention is not tied up with any theoretical explanation, we assume that under said process conditions the pores of the catalyst carrier are wholly or partly filled via capillary conden­ sation with, inter alia, the water present in the reaction gas, the degree of pore filling being determined by, inter alia, the process conditions and the pore size distribution. This makes it likely that the palladium is present as a homogeneous catalyst in the capillary-­ condensed liquid during alkene or cycloalkene oxidation, while the fixed vanadium is responsible for the reoxidation of the palladium reduced during the oxidation. Besides the intrinsic activity for a certain alkene or cycloalkene, the catalyst activity therefore depends on the solubility of the alkene or cycloalkene in the capillary-­ condensed liquid (mainly water). In a special embodiment of the invention, solubility promoters are applied to increase the said solubility of the alkene or cycloalkene to be oxidized. Examples of suitable promoters are C₁-C₈ alcohols, in particular methanol or ethanol, and further dioxane, dimethyl sulphoxide, sulpholane. Such a substance may, for instance, be dosed to the reactor in the gaseous form. It has, surprisingly, been found that in an oxidation, according to the invention, of alkenes or cycloalkenes it is desirable for the carrier surface to be covered with vanadium to the fullest possible extent, so as to achieve maximum selectivity to the desired product. It is, therefore, preferred to aim at a degree of coverage of more than 0.25 vanadium atoms per OH surface group. The invention will be elucidated on the basis of the following, non-restrictive examples. In the figures relating to the examples, the abscissa givens the duration of the reaction (t) in hours, and the ordinate the ethylene conversion to acetaldehyde (x) in mole %. Figure I 6 g γ-alumina of Akzo (type 000-1.5 E, S (BET) = 180 m²/g) was dried at 150 ◊C to constant weight in a 24-hour period. The material was introduced into a column and irrigated with distilled water up to a point just above the catalyst bed. After this, a 1 wt.% NH₄VO₃ solution (150 ml/h) of pH 4 (sulphuric acid) was passed over during 24 hours at a preferred temperature of 80 ◊C until the ammonium metavanadate broke through. After removal and washing of the loaded carrier, a degree of loading of approx. 8.5 wt.% surface vanadate, calculated as V₂O₅, per unit carrier weight, was found to have been attained. The loaded γ-Al₂O₃ was washed with distilled water and air-­ dried at 80 ◊C. The material was then calcined at 400 ◊C during 1 hour and stored in the air at 150 ◊C. H₂PdCl₄ was subsequently deposited by dry impregnation with an H₂PdCl₄ solution containing 18 mg Pd per cm³ until the caking point was reached. The caking point was reached at 0.45 cm³/g, corresponding with 0.8 wt.% Pd per g catalyst. This was followed by drying at 120 ◊C. The Wacker oxidation of ethylene was carried out, using the catalyst obtained, in a setup with a continuous flow, the product stream being subjected to continuous gas chromatographic analysis. A microreactor with a length of 10 cm and a diameter of 0.9 cm was charged with 3 g catalyst, after which the catalyst was pre-treated in the reactor at 125 ◊C with 116 cm³ (NTP) gas per minute, the molar ratio of this gas stream being 6.95 % parts water, 70.48 % parts nitrogen and 22.57 % parts oxygen, the total pressure being 1.2 bar and the duration 2 hours. After this, the composition of the gas feed-stream was changed to 0.01 part C₂H₄, 0.23 part H₂O, 0.18 part O₂ and 0.58 part N₂. The reaction temperature was 140 ◊C. The gas flow rate was set at 137 cm³ (NTP) per minute, and the total pressure was 1.2 bar. Figure 1 presents the acetaldehyde production versus time. Acetaldehyde was produced at a constant and stable rate during 18 hours, with a C₂H₄ conversion to CH₃CHO at 19 %. The material balance proved that no by-products were formed (certainly less than 5 %). Example II The catalyst (3 g) was prepared in the same way as in Example I, but the degree of loading with surface vanadate, calculated as V₂O₅, was 9.5 wt.%. The pre-treatment was as in Example I. Reaction temperature: 125 ◊C; pressure 1.1 bar. Reactor feed: 130 cm³ (NTP) gas per minute, 0.01 aprt C₂H₄, 0.18 part H₂O, 0.19 part O₂, 0.62 part N₂. Figure 2 shows how the reaction proceeded in time. There is a constant C₂H₄ conversion to CH₃CHO of 33 % during 25 hours. The material balance proved that the amount of by-products formed, if any, was less than 5%. Example III To demonstrate that a higher ethylene concentration in the feed is also allowable, the experiment of Example II was repeated with 0.032 part C₂H₄, 0.14 part H₂O, 0.20 part O₂ and 0.628 part N₂. The C₂H₄ conversion to CH₃CHO now contantly was 48 % during 25 hours. Comparative experiment A The catalyst consisted of V₂O₅ particles deposited on porous α-Al₂O₃. The catalyst preparation is described in the article by Evnin (J. Catal., 30 , 109-17 (1973)); the V₂O₅ content was 5.5 wt.%. The reaction conditions were as follows: reaction temperature 140 ◊C; 3 g catalyst; feed flow rate 138 cm³ (NTP) min⁻¹; total pressure 1.1 bar; feed composition: 0.01 part C₂H₄, 0.23 part H₂O, 0.18 part O₂ and 0.58 part N₂. Fig. 3 shows that the conversion started at 17 % C₂H₄ con­ version to acetaldehyde, but this conversion was not stable; after 15 hours it had dropped to 2 %. Comparative experiment B The same as Comparative experiment A, but with 6 wt.% V₂O₅. Conversion started at 20 %, but had dropped to 6 % after 4 hours (see Fig. 4). Example IV The heterogeneous Wacker oxidation of cyclohexene was studied using a catalyst of the following composition: carrier material: γ-alumina of Akzo, type 000-1.5 E, with S(BET) = 253.2 m²/g. The carrier was loaded with a surface vanadate corresponding with 9.5 wt.% V₂O₅ in the way described in Example I. By dry impregnation, subsequently an amount of H₂PdCl₄ corresponding with 0.8 wt.% Pd was deposited. The catalyst was air-dried at 80 ◊C during one hour. An amount of 3 g catalyst was pre-treated in the reactor at 125 ◊C in a gas stream, fed at 116 cm³ (NTP) per minute, composed of 6.95 % water vapour, 70.48 % nitrogen and 22.75 % oxygen. The total pressure was 1.1 bar. The pre-treatment lasted two hours. Afterwards, the following feed was passed over: 0.8 % cyclohexene, 19.2 % oxygen, 10.4 % water vapour and 70 % nitrogen. The glas flow rate was 127 cm³ (NTP) per minute. After two hours the initial reactor temperature of 110 ◊C was lowered to 75 ◊C. The cyclohexene conversion after this amounted to 3.6 %, and the reaction product consisted of 70 % cyclohexanone, 24 % cyclohexe­ none and 5 % phenol. After 40 hours' experimenting, no appreciable change in con­ version or in selectivities was observed.

Claims: 1. Catalyst, suitable for the catalytic heterogeneous gas phase oxidation of an alkene or cycloalkene to the corresponding ketone or aldehyde in the presence of H₂O and O₂, containing a vanadium and a palladium component, mounted on a carrier, characterized in that the surface of the carrier is covered with hydroxyl groups and the vanadium is present in the form of a surface vanadate. 2. Catalyst according to claim 1, characterized in that the carrier consists of γ-alumina. 3. Catalyst according to either of claims 1-2, characterized in that the carrier has a surface area of 100-400 m²/g. 4. Catalyst according to claim 1, characterized in that the vanadium content of the catalyst is 2-30 wt.%., relative to the carrier material. 5. Catalyst according to claim 4, characterized in that the degree of coverage with vanadium of the catalyst is higher than 0.25 vanadium atom per surface hydroxyl group. 6. Process for the preparation of a catalyst according to claim 1, characterized in that a carrier with surface hydroxyl groups is started from, a monolayer dispersion of vanadium is deposited, which is followed by dry impregnation with a palladium-containing solution until the caking point is reached. 7. Process for the heterogeneous Wacker oxidation of an alkene or a cycloalkene, characterized in that use is made of a catalyst according to claim 1. 8. Process according to claim 7, characterized in that the oxidation is carried out at a temperature of 20-200 ◊C. 9. Process according to claim 7, characterized in that a pressure of 0.01-10 MPa is applied. 10. Process according to claim 7, characterized in that use is also made of a solubility promoter for the alkene or cycloalkene in the capillary condensing liquid. 11. Process according to claim 10, characterized in that methanol or ethanol is used as solubility promoter.

Inventors: SCHOLTEN, JOSEPH JOHANNES FRANCISCUS, VAN DER STEEN, PETRUS JOHANNES
Applicants: STAMICARBON B.V.
Application year: 1986
Main classification: B01J23
IPC codes: C07C47, C07C49, B01J23, C07C45, B01J27, C07C67, C07B61