GB1600747A - Production of alkylene oxides and catalysts therefor - Google Patents

Description

(54) PRODUCTION OF ALKYLENE OXIDES AND CATALYSTS THEREFOR (71) We, IMPERIAL CHEMICAL INDUSTRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the production of alkylene oxides.

In our co-pending British Patent 1,491,447, and cognate applications we have disclosed silver containing catalysts for alkylene oxide production which may comprise inter alia mixtures of potassium with certain other elements, especially barium, as promoters.

We have now found that when certain reaction modifiers are present in the reaction attractive performances are given by catalysts containing high amounts of silver together with certain concentrations of potassium, even if no other promoter is present in the catalyst.

The invention provides a process for the production of akylene oxides for example ethylene and propylene oxides by oxidation of the corresponding olefine with oxygen in the presence of a chlorine containing reaction modifier which does not readily dehydrochlorinate and of a catalyst which comprises more than 15% by weight of silver supported on a porous heat resisting support which has a specific surface area in the range of 0.05 to 10 m2/g and preferably 0.1 to 5 m2lg and more preferably 0.3 to 5 m2/g as measured by the Brunauer Emmett and Teller method, the catalyst also comprising more than 16y gram equivalents per square metre of surface area of the support of the potassium the said amount being also more than 1.5 x 10-5 gram equivalents per gram of catalyst, present in a form which is extractable by contact with water.

The support is suitably a preformed support.

By "promotion" is meant that the activity or preferably selectivity of the catalyst is enhanced at any time in the life of the catalyst; the initial selectivity may for example thus be higher and/or it may be maintained for longer.

In making the catalyst, silver may be introduced to a pre-formed porous heat resisting support as a suspension of silver or silver oxide in a liquid medium for example water or by impregnation of the support with a solution of a silver compound which can be reduced to silver metal if necessary by means of a reducing agent for example hydrogen. If necessary a heat treatment may be used to decompose the silver compound to silver. Suitably the impregnating solution contains a reducing agent which may be for example an anion, for example a formate, acetate, propionate, lactate, tartarate or preferably oxalate ion, of a silver compound in the solution. The reducing agent may be for example an aldehyde, for example formaldehyde or acetaldehyde or an alcohol preferably having 1 to 4 carbon atoms for example methanol or ethanol.

The solution may be a solution in water and/or an organic solvent, for example an aliphatic alcohol preferably having 1 to 4 carbon atoms, a polyhydric alcohol for example ethylene glycol or glycerol, a ketone for example acetone, an ether for example dioxan or tetrahydrofuran, a carboxylic acid for example acetic acid or molten lactic acid which is preferably used in the presence of water, or an ester for example ethyl acetate or a nitrogen containing base for example pyridine or formamide. An organic solvent may function as a reducing agent and/or complexing agent for the silver also.

If the silver is introdued by impregnating a support with a solution of a decomposable silver compound it is preferred that ammonia and/or a nitrogen containing base should be present. The nitrogen contaning base suitably acts as a ligand maintaining the silver in solution, for example it may be pyridine, acetonitrile, an amine, especially a primary or secondary amine havine 1-6 carbon atoms, or preferably ammonia. Other suitable nitrogen-containing bases include acrylonitrile, hydroxylamine and alkanolamines for example ethanolamine, alkylene diamines having from 2-4 carbon atoms or amides for example formamide or dimethyl formamide. The nitrogen containing base may be used alone or in admixture. Suitably the nitrogen containing base or bases are used together with water.

Alternatively the solution may be a neutral or acid solution for example it may be a solution of a silver carboxylate especially a formate, acetate, propionate, oxalate, citrate, tartarate or preferably lactate or for example a solution of silver nitrate.

The solutions preferably contain 3-50% of silver by weight.

Impregnation may be carried out in a single stage or if desired may be repeated one or more times. By this means higher silver contents of the catalyst may be achieved.

The silver compounds may generally be reduced to silver by heating in the range 100 to 350"C, for example for a period of 15 mins. to 4 hours.

The catalyst support preferably has an apparent porosity as measured by the mercury absorption method of at least 20%, for example 30-80% preferably 30-65% and more preferably 40-60% and mean pore diameters of 0.1 to 20 microns preferably 0.3 to 4 microns as measured by the mercury porosimetry method. The pore size distribution of the support may be bimodal, in which case the smaller pores preferably account for at least 70% of the total pore volume and have a mean pore diameter preferably in the range 0.1 and preferably 0.3 to 4 microns, and the larger pores preferably have a mean pore diameter in the range 25 to 500 microns.

Most of the silver content of the catalyst is preferably present in the form of discrete particles adhering to the support having equivalent diameters of less than 10,000 preferably in the range 20-10,000A and more preferably 40-8,000 . By equivalent diameter is meant the diameter of a sphere of the same silver content as the particle.

Preferably at least 80% of the silver is present as particles having equivalent diameters in the aforesaid range, the quantity of silver being judged in terms of the number of particles falling in that range. The silver may be present as silver and/or silver oxide. The dimensions of the silver particles may be determined by scanning electron microscopy.

The support may be an alumina, silicon carbide, silica, zirconia or silica/alumina support, but it is preferably composed of an aggregate of alpha-alumina particles which may be fused together or cemented together with, for example, silica or baryta.

The catalyst preferably comprises 15 to 50% and more preferably 16 to 35% for example 20 to 30% by weight of silver.

The preferred content of potassium in a form which is extractable by contact with water expressed as microgram equivalents per square metre of surface area of the support is in the range of above 16 up to 2000, more preferably 30 to 500. The form in which the potassium is present is determined by the conditions under which an oxidation of an olefin to an olefine oxide is carried out using the catalyst. It is preferred that catalyst poisons for example bromine, iodine and sulphur are substantially absent.

The potassium may be introduced to the support before during or after impregnation with a solution of the silver compound. It is suitably introduced as a solution of a potassium compound which solution may be in water and/or an organic solvent. They may comprise solvents, reducing agents and/or complexing agents as previously described. If it is desired to impregnate a catalyst which has already been used in the oxidation of an akylene oxide and has lost performance, this may be carried out also, whether or not the catalyst already contains one or more promoters. It may also be possible to regenerate a catalyst which has deteriorated in performance in use by contacting it with water and/or an organic solvent as previously described. The catalyst after treatment as aforesaid is heated to a temperature at which promotion becomes effective, normally in the range 100 to 350"C; such heating may serve to decompose the silver compound if present also.

Partial pressures of ethylene or propylene in the process may be in the range 0.1 - 30 and preferably 1 to 30 bars. The total pressure may be in the range of from 1 to 100 and preferably 3-100 bars absolute. The molar ratio of oxygen to ethylene or propylene may be in the range of 0.05 to 100. The partial pressure of oxygen may be in the range 0.01 and preferably 0.1 to 20 bars and preferably 1-10 bars. The oxygen may be supplied for example in the form of air or preferably as commercial oxygen. A diluent for example helium, nitrogen, argon, and/or carbon dioxide and/or preferably methane may be present in proportions of 10-80% and preferably 40-70% by volume in total. It is necessary to operate using gas compositions which are outside the explosive limits.

The temperature is suitably in the range 200-300"C and preferably in the range 220-290"C. Contact times should be sufficient to convert 0.5 - 70%, for example 2 to 20% and preferably 5-29% of the ethylene or propylene and unconverted ethylene or propylene is suitable recycled.

Suitable reaction modifiers which do not readily dehydrochlorinate under the reaction conditions contain a carbon-chlorine bond, for example methyl chloride, dichloromethane, 1 ,1-dichloroethylene and 1 ,2-dichloroethylene, or especially vinyl chloride. Chlorinated aromatic compounds for example chlorobenzene, dichlorobenzenes and chlorinated toluenes are also suitable. Ethylene dichloride readily dehydrochlorinates and is thus unsuitable for this invention. Preferably 0.1 to 200 and more preferably 5 to 80 parts of reaction modifier per million of the reaction medium by weight are present.

We have found that with appropriate concentrations of such reaction modifiers, especially vinyl chloride, attractive selectivity may be secured and that such selectivity is more stable than selectivity secured using for example dichloroethane which can undergo dehydrochlorination under reaction conditions.

Example 1 Catalysts for the oxidation of ethylene and propylene to ethylene oxide and propylene oxide were prepared as follows: 8.9g of silver oxalate were dissolved in 7ml of a solution of 50% 1,2-diamino ethane in water. The resulting solution was made up to 8 mls by the addition of ethanolamine.

Potassium acetate was added to this solution which was then used to impregnate 20 g. of the support material. The support used was a porous alpha-alumina in the form of cylindrical pellets 3mm diameter and 3mm long. The surface area of the support material was 2.2 m g , the mean pore diameter was 1 micron and the pore volume was 0.5 ml g-1.

The impregnated support was heated at 290"C for a period of 3 hours in a forced draught air oven. This procedure resulted in catalysts containing about 24% by weight of silver. The levels of potassium present in the final catalysts are given in Table 1.

Example 2 The catalysts prepared in Example 1 were tested for catalytic activity in the following way: 10g of catalyst were loaded into a stainless steel reactor (internal diameter 8mm). The catalyst was subjected to increasingly severe reaction conditions and, once the performance had stabilised, the catalyst selectivity and oxygen conversion were measured using a process gas stream containing 30% ethylene, 8% oxygen and 30 ppm vinyl chloride at a pressure of 15 psia. The temperature of the reactor was 240"C and the gas hourly space velocity was 2,000 hr The process gas pressure was then raised to 240 psia and the selectivity and oxygen conversion measured at 2400C at a GHSV of 15,000 hil after the catalyst performance had stabilized again. The results of the catalyst tests are shown in Table 1.

TABLE I Catalyst Potassium Selectivity Oxygen Select- Oxygen level @ 15 psia Conversion ivity @ Conversion (micro gram (%) @ 15 psia 240 psia @ 240 psia equivalents (%) (%) (%) per sq. metre) A 35 94 7 90 3 B 117 92 3 90 3 COMPARATIVE EXAMPLES C 2.3 85 10 80 10 D 11.7 87 10 81 6 GHSV = Gas hourly space velocity psia = pounds per square in absolute

Claims (17)

WHAT WE CLAIM IS:

1. A process of producing an alkylene oxide which comprises oxidising the corresponding olefin with oxygen in the presence of a chlorine containing reaction modifier which does not readily dehydrochlorinate and of a catalyst which comprises more than 15% by weight of silver supported on a porous heat resisting support which has a specific surface area in the range 0.05 to 10m2/g as measured by the Brunauer Emmett and Teller method, the catalyst also comprising more than 16 11 gram equivalents per square metre of surface area of the support of potassium the said amount being also more than 1.5 x 105 gram equivalents per gram of catalyst, present in a form which is extractable by contact with water.

2. A process as claimed in Claim 1 in which the porous heat resisting support has a specific surface area in the range 0.1 to 5 square metres per gram.

3. A process as claimed in Claim 1 or 2 in which the alkylene oxide is ethylene oxide.

4. A process as claimed in any preceding claim in which the catalyst support has an apparent porosity as measured by the mercury absorption method of 30 to 80% and mean pore diameters of 0.1 to 20 microns as measured by the mercury porosimetry method.

5. A process as claimed in any of claims 1 to 3 in which at least 80% of the silver is present as particles having equivalent diameters in the range 20 to 10,000 , the quantity of silver being judged in terms of the number of particles falling in that range.

6. A process as claimed in any preceding claim in which the support is an aggregate of alpha alumina particles fused or cemented together.

7. A process as claimed in any preceding claim in which the content of potassium in the catalyst in a form which is extractable by contact with water expressed as microgram equivalents per square metre of surface area of the support is 30 to 500.

8. A process as claimed in any preceding claim in which a partial pressure of ethylene or propylene in the process is in the range 1 to 30 bars, the total pressure is in the range of 1 to 100 bars and the molar ratio of oxygen to ethylene or propylene is in the range 0.05 to 100.

9. A process as claimed in Claim 12 in which the partial pressure of oxygen is in the range 0.1 to 20 bars.

10. A process as claimed in Claim 8 or 9 in which the temperature is in the range 200 to 300"C.

11. A process as claimed in any preceding claim in which the reaction modifier is vinyl chloride.

12. A process as claimed in any of Claims 1-11 in which the catalyst has been made by introducing silver to a pre-formed porous heat resisting support by impregnation of the support with a solution of a silver compound which can be reduced to silver metal.

13. A process as claimed in Claim 12 in which the impregnating solution contains a reducing agent.

14. A process as claimed in Claim 12 or 13 in which the impregnating solution comprises water.

15. A process as claimed in claim 12, 13 or 14 in which the impregnating solution comprises ammonia and/or nitrogen containing base.

16. A process as claimed in any preceding claim whenever carried out substantially as described in Example 2.

17. Alkylene oxides whenever produced by a process as claimed in any preceding claim.