PROCESS FOR THE ORTHO-ALKYLATION OF PHENOLS

Abstract: A phenol is ortho-alkylated by reaction with an alkanol in the presence of a catalyst comprising at least 87% w/w, calculated as Fe₂O₃, of an oxide of iron, and at least one other metal oxide, in which the other metal is selected from As, Cd, Co, Hg, In, Ni, Pb, lanthanide rare earths, Sc, Sn, Ti and Zr. By way of example, phenol can thus be converted to σ-cresol and/or 2,6-xylenol.

Description: This invention relates to a process for the ortho-alkylation of phenols. In particular, it relates to a process in which a phenol unsubstituted at at least one ortho position is reacted with an alkanol in the presence of a catalyst. It is well known to react a phenol which is unsubstituted at at least one ortho position with an alkanol in the presence of a catalyst. Such a reaction is used, for example, to prepare o -cresol (e.g., for herbicide manufacture), 2,6-xylenol (e.g., for the manufacture of polyphenylene ether) and 2,3,6-trimethylphenol (e.g., for vitamin E manufacture). Many catalysts for the ortho-alkylation of phenols are known. British Patent Specification No. 602,257 discloses alumina as catalyst, and a reaction temperature of 345°C, but appreciable quantities of phenol remain unreacted and meta- and para-alkylation products are formed in substantial amounts. U.S. Patent Specification No. 3,843,606 discloses magnesia as the catalyst and reports some what higher conversion of the phenol feedstock, and relatively high selectivities, but requires the temperature of operation to be in excess of 400°C, e.g. 460°C. It is known to use iron oxide as a component of a catalyst for use in the ortho-alkylation of phenols. U.S. Patent Specification No. 3,716,589 discloses such a catalyst, containing oxides of iron and another metal (M) in sufficient amounts to form a spinel of the formula MFe₂O₄. By conducting the alkylation reaction with this catalyst at about 350°C, high selectivity to orthoalkylation is achieved but, with quoted conversions not exceeding 71%, a major proportion of the phenol feedstock must be recycled. Other references which disclose ortho-alkylation catalysts include British Patent Specification No. 1,378,179 where the catalyst comprises oxides of iron, vanadium and a further metal. British Patent Specification No. 1,428,057 discloses a catalyst comprising oxides of iron and silicon. C.A. 77: 101173d discloses a catalyst comprising vanadium and iron oxides in comparable amounts and another metal oxide. C.A. 78: 3944g discloses a catalyst comprising a mixture of indium and iron oxides. C.A. 79: 78384d discloses a catalyst comprising a spinel of the formula MFe₂O₄ in which M is barium, calcium, magnesium or a transition metal. C.A. 81: 105003d discloses a catalyst comprising iron oxide and nickel oxide in which the Ni: Fe molar ratio is 1: 2. C.A. 82: 155769z discloses a catalyst comprising manganese and iron oxides, optionally together with cobalt oxide. C.A. 84: 58898z discloses a catalyst of the formula Zn₂Fe₂Mo 0.03 O₄. C.A. 90: 6086n discloses a catalyst of the formula MFe₂O₄ in which M is Mg, Ca, Ba, Cu, Ni, Cr, Co or Cu-Zn C.A. 93: 71286d discloses a catalyst comprising a mixture of the oxides of Fe, Cr, Sb, V and Si. C.A. 94: 103010y discloses a catalyst comprising oxides of iron and one of Mo, Sn, W and Zr. In the given example, the catalyst comprises Fe and Zr in an atomic ratio of 96: 4. European Patent Publication No. 19476, published 26th November 1980, discloses a catalyst comprising a major proportion of an iron oxide and a minor proportion of an oxide of a metal selected from Ga, Ge, Y, Nd, Hf, Bi and Ta. Examples 45, 46, 49 and 50 of this Publication disclose catalysts comprising three metal oxides, of iron, germanium and either Zr or Sn, in a metal atom ratio of 97: 1.8: 1.2. US-A-4,227,024 discloses a catalyst comprising a major proportion of iron oxide and minor proportions of oxides of Sn, Cr and K. It will thus be apparent that a wide variety of mixtures of metal compounds has been proposed for use as catalysts in the ortho-alkylation of phenols. It is difficult to make any clear inference as to the most suitable catalyst for use in this reaction. According to the present invention, a process for preparing a o -alkylphenol comprises reacting a monohydric phenol unsubstituted at at least one ortho position with a C₁₋₄ alkanol at 300 to 500°C, in the presence of a catalyst comprising at least 92% w/w, calculated as Fe₂O₃, of an oxide of iron, and 0.5 to 3% w/w of an oxide of another metal selected from Cd, Ce and Ti. The catalyst used in the present invention comprises a major proportion of an iron oxide. The catalyst will suitably be prepared from Fe₂O₃, although any suitable form of iron oxide is appropriate. In use, Fe₂O₃ may be converted to Fe₃O₄. The oxide of the metal in the catalyst other than iron may be of Group II of the Periodic Table (Cd), Group III (Ce), Group IV (Ti). If desired, the catalyst may be prepared from a salt or other compound of the desired metal which is converted to the oxide during preparation or in use of the catalyst. The amount of the metal other than iron in the catalyst, calculated as its oxide, is from 0.5 to 3% w/w. The preferred amount is about 1% w/w (here and throughout the specification, percentages of components in the catalyst are measured on the basis of the dry weight of the catalyst). The catalyst may comprise an oxide of a metal other than iron which is not detrimental to the reaction, usually in an amount of no more than 5% w/w. It is often preferred that the catalyst should comprise Cr₂O₃ in an amount of up to 5% w/w. It is also preferred that the catalyst should comprise an alkali metal oxide or salt, e.g. of Na or K. A suitable such compound is K₂CO₃. The amount of any such alkali metal compound may be up to 3% w/w. The catalyst may be prepared by conventional means. It may be produced in any form which is appropriate for a vapour-phase reaction. The phenol feedstock may be any phenol which is unsubstituted at at least one ortho position. The feedstock may be, for example, phenol or m -cresol. The alkanol is a C₁₋₄ alkanol and is most preferably methanol or ethanol. By the process of the invention, phenol may be reacted with methanol to give o -cresol and/or 2,6-xylenol, and m -cresol may be reacted with methanol to give 2, 3, 6-trimethylphenol. The alcohol is usually present in excess. Water is usually included in the alkanol/phenol feedstock, to extend the life of the catalyst. The temperature of the reaction is from 300 to 500°C, preferably from 320 to 420°C, and often about 330 to 360°C. The reaction may be carried out in fluidised or fixed bed equipment. The pressure is not especially critical but it will often be appropriate to conduct the reaction at atmospheric or slightly super-atmospheric pressure. The process may be conducted using liquid hourly space velocities of, for example, 0.3 to 3 litres of liquid feed per litre of catalyst per hour. This rate is often less than 1.0 when di-alkylation is required, and rather higher when appreciable amounts of mono-alkylation are desired. The following illustrates the preparation of catalysts for use in, and the process of, the invention. All parts and percentages are by weight, unless otherwise specified. Catalysts A to D, F and H are for the purposes of comparison. Catalyst compositions A to I 600 parts of ferric nitrate (Fe(NO₃)₃. 9H₂O) and 3.82 parts of chromic nitrate (Cr(NO₃)₃. 9H₂O) were dissolved in 6000 parts of water. To this solution were added 4.21 parts of zirconium nitrate (Zr(NO₃)₄. 5H₂O) in the form of a 10% solution in water. Dilute aqueous ammonia was then added with stirring until the solution was neutral. After a period of ageing, the resultant hydrogel was recovered by filtration, washed, and dried for 10 hours at 180°C. It was then soaked overnight in a solution containing 0.048 parts of potassium carbonate, redried and calcined for 4½ hours in air at 470°C, before being formed into 3.1 mm pellets (Catalyst A). In a similar way, using different amounts of zirconium nitrate, catalysts B, C and D were prepared. In a similar way, instead of using zirconium nitrate, solutions of titanium tetrachloride, lead nitrate, cadmium nitrate, stannic chloride and cerium nitrate were used to prepare catalysts E, F, G, H and I, respectively. Catalyst A to I have the following compositions, on a dry weight basis, the balance of each composition being Fe₂O₃: 60 ml of each of the above catalyst compositions were packed in a mild steel reaction tube and maintained at a background temperature of 345°C. A gaseous mixture of methanol, phenol and water at a molar ratio of 5: 1: 1 was passed through the catalyst bed at a liquid hourly space velocity (LHSV) of 0.4 or 0.6 kg of liquid feed/I catalyst/hr. The results which were obtained are tabulated below. Other catalysts for use in the invention may be prepared in a manner similar to that given for Catalysts A to I. For example, Ce, may be introduced in the form of its nitrate. In this way, varius catalysts may be prepared, with or without Cr₂O₃ and/or any alkali metal, the following being a further example: Fe₂O₃ + 1% CeO

Claims: 1. A process for preparing an o -alkylphenol, which comprises reacting a monohydric phenol unsubstituted at at least one ortho position with a C₁₋₄ alkanol at 300 to 500°C, in the presence of a catalyst comprising at least 92% w/w, calculated as Fe₂O₃, of an oxide of iron, characterised in that the catalyst also comprises 0.5 to 3% w/w of an oxide of another metal selected from Cd, Ce and Ti. 2. A process according to claim 1, in which the said other metal is Ti. 3. A process according to any preceding claim, in which the alkanol is methanol. 4. A process according to claim 5, in which the phenol is phenol itself and the product comprises o -cresol and/or 2, 6-xylenol. 5. A process for preparing o -cresol and/or 2, 6-xylenol, which comprises reacting phenol with methanol at 300 to 500°C in the presence of a catalyst comprising at least 92% w/w, calculated as Fe₂O₃, of an oxide of iron, characterised in that the catalyst also comprises 0.5 to 3% w/w of TiO₂. 6. A process according to any preceding claim, in which the catalyst additionally comprises Cr₂O₃, in an amount of up to 5% w/w. 7. A process according to any preceding claim, in which the catalyst additionally comprises an alkali metal, oxide or salt in an amount of up to 3% w/w. 8. A process according to any preceding claim, which is conducted at from 320 to 420°C. 9. A process according to any preceding claim, which is conducted in the presence of water.

Inventors: ADEY, KENNETH ARMSTRONG, YATES, FRANK SYLVESTER, YOUNG, JOHN HARRISON
Applicants: CRODA SYNTHETIC CHEMICALS LIMITED
Application year: 1981
Main classification: C07C37
IPC codes: C07C37, B01J23