GB1591308A

GB1591308A - Regeneration of silver catalysts for the production of olefine oxides

Info

Publication number: GB1591308A
Application number: GB47212/76A
Authority: GB
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1977-10-25
Filing date: 1977-10-25
Publication date: 1981-06-17

Description

(54) REGENERATION OF SILVER CATALYSTS FOR THE PRODUCTION OF OLEFINE OXIDES (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 olefine oxides.

In the oxidation of olefines especially ethylene to the corresponding olefine oxides by reacting them with oxygen in the presence of silver containing catalysts it is sometimes found that the catalysts are unstable especially if they are highly selective in that they lose selectivity with time, i.e. a smaller proportion of the converted olefine is converted to the desired olefine oxide.

This invention provides a process of restoring selectivity to a silver containing catalyst which has lost selectivity in a process of oxidising an olefine to an olefin oxide which comprises wetting the catalyst with an organic solvent and evaporating the solvent, the solvent containing a dissolved potassium or preferably sodium compound.

It is preferred that the organic solvents should have boiling points under a pressure of one bar in the range 50 - 1500C. Suitable solvents include lower alcohols having for example 1-3 carbon atoms, and also dimethyl formamide and pyridine.

The catalyst may be an unstable catalyst, ie. a catalyst which after an initial conditioning period exhibits a substantial decline in selectivity, e.g. a decline of 1% or more e.g. 3, 4 or 5% or more per month in normal use or may be a catalyst which exhibits only a slow decline in selectivity over a long period.

After treatment according to this invention the catalyst may be subjected to a conditioning period prior to re-use.

The organic solvent may comprise a minor proportion of water sufficient to facilitate dissolution of the sodium or potassium compound present, for example 0.1 to 10%.

The sodium or potassium compound may suitably be a hydroxide, carbonate, bicarbonate or soluble salt for example an acetate, oxalate or citrate. It is preferred however that the sodium or potassium compound should not contain any component capable of poisoning the catalyst for example iodide or sulphide.

It is preferred that the amount of solution absorbed into the catalyst should at least for part of the time approach or be equal to the water porosity of the catalyst in terms of its volume. If desired the volume of solution to which the catalyst is exposed may exceed the water porosity but it is desirable in this case that excess solution should not be discarded but should be evaporated in contact with the catalyst in order to avoid loss of catalyst components.

Wetting with the solution may take place at any convenient temperature for example from 20 - 1500C. The solvent may be removed by evaporation at a temperature in the range 70 - 350 C.

It is prefereed that at least 50% of the silver particles be present as discrete particles adhering to a support and having equivalent diameters of less than 10,000A, preferably in the range 200 to 10,000 . By "equivalent diameter" is meant the diameter of a sphere of the same silver content as the particle. The dimensions of the silver particles may be determined by scanning electron microscopy.

It is preferred that the catalyst should comprise a porous heat resisting support. Such supports are suitably alumina, silicon carbide, silica, zirconia or silica/alumina supports, a-alumina or silicon carbide being preferred. The support is preferably a preformed porous heat resisting support into which the other catalyst components are introduced by impregnation. It preferably has a surface area in the range 0.04 to 10 m2/g, preferably 0.05 to 6mLlg and more preferably 0.2 to 5.0 m2/g as measured by the Brunauer Emmett and Teller method, an apparent porosity as measured by the mercury absorption method of at least 20%, preferably 30-65% and more preferably 40-60% for example 45-55% and median pore diameters of 0.3 to 15 microns, preferably 0.3 to 5 microns as measured by the mercury porosimetry method.

The catalyst preferably comprises 3 to 50% and more preferably 3 to 30% by weight of silver.

The preformed support may be impregnated with a solution comprising 3-50% of silver compound by weight and a nitrogen-containing ligand for example acrylonitrile, ammonia, and/or an amine for example pyridine, hydroxylamine, or a vicinal alkyl diamine having from 2-4 carbon atoms for example ethylene diamine optionally together with a vicinal alkanolamine. Any alkali metal compounds originally present in the catalyst may be introduced with the silver as a compound soluble in the silver solution or by a separate impregnation step before or after introduction of the silver. It may suitably be introduced as a halide for example a chloride, a sulphate, nitrate, nitrite or a carboxylate. It is preferred that it should be introduced as a hydroxide, carbonate, bicarbonate, nitrate, nitrite, formate, acetate, oxalate, citrate, or lactate.

It is desirable that silver should be deposited as discrete particles throughout substantially all of the available surface (inter alia within the pores) of the support as well as on the superficial surface. In order to ensure that as much of the silver is in the pores of the support as possible rather than being deposited on the external surface of the support it is preferred that the total amount of solution used should be such that it is substantially entirely absorbed into the support rather than an excess being left in contact with it. Any surplus solution may alternatively be drained off prior to drying the solid. The silver compound may be decomposed by heating the impregnated support to a temperature of 200 - 400"C, preferably 200 - 350"C.

It is preferred that the final sodium content after treatment according to this invention should be in the range 10-2 to 10% by weight of the catalyst. It is preferred that the sodium content should be in the range 3 X 10-5 to 5 X 10- grams of sodium per sq. metre of surface area of support. The preferred final content of potassium is in the range 2 X 10-2 to 3 X 10-5 grams per sq. metre of surface area of support.

The conversion of ethylene oxide using the catalysts of the invention may be carried out in a conventional manner. Pressures of from 1 to 35 bars absolute may be employed. The temperature is suitably in the range 190 to 2700C, and preferably 210 to 245"C. In general, a diluent, for example methane, is present in proportions of, for example 10 to 50% by weight. Generally 0.5 to 70% for example 2 to 20% of ethylene is converted and unconverted ethylene is recycled. Oxygen may be supplied, for example, in the form of air or commercial oxygen. Carbon dioxide is generally also present. A reaction modifier, for example vinyl chloride or ethylene dichloride, may be used.

The regenerated catalysts may also be used in propylene oxide production.

Example 1 30g. sodium acetate were dissolved into 15 mls of water and the solution was made up to 1 litre by the addition of methanol. 20 ml of this solution were used to impregnate 100g. of a commercial catalyst comprising 12% by weight of silver on an alumina support. This sample of catalyst had been used in the production of ethylene oxide for a period of seven years. The impregnated catalyst was then dried in a nitrogen purged oven at 1200C for one hour. The resulting catalyst (A) containing 0.1% sodium, was tested for activity towards ethylene oxidation in the following manner: 25g. of catalyst were loaded into a tubular glass reactor of internal diameter 8mm contained in a thermostatically controlled air oven. The catalyst was subjected to increasingly severe reactions over a period of two days. A gas mixture containing 30% ethylene, 8% oxygen, 62% nitrogen and 1.Sppm ethylene dichloride at a pressure of 20 p.s.i.a. was passed over the catalyst. The selectivity and activity of the catalyst was determined at a temperature of 260"C. The results of catalyst A are given in Table 1 below.

Example 2 Catalyst B was prepared and tested in an identical manner to catalyst A except that the final catalyst contained 0.05% potassium rather than 0.1% sodium. The selectivity and activity of catalyst B are given in the table below together with comparative data obtained on the fresh untreated silver catalyst (C) and the used untreated silver catalyst (D).

TABLE 1 Selectivity Activity (moles EO produced per 100 (moles ethylene Catalyst moles ethylene converted) converted per kg catalyst per hour) A 77 .18 B 77 .14 C 77 .15 D 74 .14 WHAT WE CLAIM IS: 1. A process of restoring selectivity to a silver containing catalyst which has lost selectivity in a process of oxidising an olefine to an olefine oxide which comprises wetting the catalyst with an organic solvent and evaporating the solvent, the solvent containing a dissolved sodium or potassium compound.

2. A process as claimed in Claim 1 in which the solvent contains sodium.

3. A process as claimed in claim 1 or 2 in which the solvent comprises an alcohol having 1 to 3 carbon atoms, dimethyl formamide or pyridine.

4. A process as claimed in Claim 1, 2 or 3 in which the organic solvent comprises a minor proportion of water sufficient to facilitate dissolution of the sodium or potassium compound.

5. A process as claimed in any preceding claim in which the sodium or potassium compound is a hydroxide, carbonate, bicarbonate, acetate, oxalate or citrate.

6. A process as claimed in any preceding claim in which the volume of solution absorbed into the catalyst is for at least part of the time substantially equal to the water porosity of the catalyst.

7. A process as claimed in any preceding claim in which wetting with the solution takes place at a temperature in the range 20-150"C.

8. A process as claimed in any preceding claim in which the solvent is evaporated at a temperature in the range 70 - 350"C.

9. A process as claimed in any preceding claim in which at least 50% of the silver particles are discrete particles adhering to a support and having equivalent diameters of less than 10,OO0it.

10. A process as claimed in any preceding claim in which the catalyst comprises a porous heat resisting support.

11. A process as claimed in Claim 10 in which the support is a-alumina or silicon carbide.

12. A process as claimed in Claim 10 or 11 in which the support has a surface area in the range 0.04 to 10 m2/g as measured by the Brunauer, Emmett and Teller method.

13. A process as claimed in Claim 10, 11 or 12 in which the support has an apparent porosity as measured by the mercury absorption method of 30-65%.

14. A process as claimed in any of Claims 10-13 in which the median pore diameter of the support is 0.3 to 15 microns.

15. A process as claimed in any preceding Claim in which the catalyst comprises 3 to 30% by weight of silver expressed as the element.

16. A process as claimed in any preceding claim in which the catalyst is produced by impregnating a pre-formed support with a solution comprising 3-50% of silver by weight which is present as a silver compound which is decomposable to silver by heating and a nitrogen containing ligand and decomposing the silver compound to silver.

17. A process as claimed in any preceding claim in which the final sodium content after the treatment is in the range 10 to 10% by weight of the catalyst.

18. A process as claimed in any preceding claim in which the final content of sodium in the catalyst is in the range 3 x 10-5 to 5 x 10~lg/m2 of surface area of support.

19. A process as claimed in any preceding claim in which the final content of potassium is in the range 2 x 10-2 to 3 x 10-5 g/m2 of surface area of support.

20. A process for the production of ethylene oxide by the oxidation of ethylene in the presence of a silver-containing catalyst which comprises restoring the selectivity of the catalyst by a process as claimed in any preceding claim.

21. A process of restoring the selectivity of a silver-containing catalyst as claimed in any of claims 1 to 19 whenever carried out substantially as described in any of the Examples.

22. A process for the production of ethylene oxide as claimed in Claim 20 whenever carried out substantially as described in any of the Examples.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. TABLE 1 Selectivity Activity (moles EO produced per 100 (moles ethylene Catalyst moles ethylene converted) converted per kg catalyst per hour) A 77 .18 B 77 .14 C 77 .15 D 74 .14 WHAT WE CLAIM IS:

1. A process of restoring selectivity to a silver containing catalyst which has lost selectivity in a process of oxidising an olefine to an olefine oxide which comprises wetting the catalyst with an organic solvent and evaporating the solvent, the solvent containing a dissolved sodium or potassium compound.

2. A process as claimed in Claim 1 in which the solvent contains sodium.

23. Silver containing catalysts whenever processed by a process as claimed in any of

claims 1 to 19 or 21.

24. Ethylene oxide whenever produced by a process as claimed in Claim 20 or 22.