GB1601635A

GB1601635A - Catalyst for the manufacture of ethylene oxide

Info

Publication number: GB1601635A
Application number: GB19168/78A
Authority: GB
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1977-05-12
Filing date: 1978-05-12
Publication date: 1981-11-04
Also published as: JPS5415491A; DE2819595A1; IT7823299A0; FR2390203A1; FR2390203B1; DE2819595C2; BE867045A; NL7805102A; IT1206597B; JPS6235813B2

Description

(54) CATALYST FOR THE MANUFACTURE OF ETHYLENE OXIDE (71) We, BASF AKTIENGESELLSCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, 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: The present invention relates to an improved catalyst for the manufacture of ethylene oxide, which catalyst comprises finely divided silver and, as promoters, lithium, with or without sodium, together with potassium, rubidium and/or cesium (hereafter referred to for brevity as heavy alkali metal.

It is known that ethylene oxide can be obtained by a direct addition reaction between elementary oxygen and ethylene in the gas phase in the presence of a silver catalyst. Based on considerations of the reaction mechanism, it is frequently assumed that the reaction can at most give a yield (selectivity) of about 85 mole %, based on ethylene converted, since a part of the ethylene is always combusted to carbon dioxide.

In practice, the best conventional catalysts give selectivities of from about 70 to 75%, but for the requirements of large industrial installations other considerations are also decisive, such as maximum conversion per unit space and unit time (space-time yield) and life of the catalyst. In this context, the fact that even higher yields can be achieved on an experimental scale and at low oxygen conversion is immaterial.

A plurality of processes for the manufacture of suitable silver catalysts have been disclosed. Normally, a coarse-pored, chemically inert catalyst carrier is used and silver is deposited thereon in a very fine form, for example by thermal decomposition of a silver salt of an organic carboxylic acid or of a silver complex (German Published ApplicationDAS 1,211,607 and German Laid-Open Application DOS 2,159,346). Furthermore, modifiers (promoters) are added. Proposed improvements to these catalysts therefore relate to the carrier used, to the addition of promoters and to the manufacturing process (R. Landau and R.E. Lidov in "Ethylene and its Industrial Derivatives", published by S.A. Miller, Ernest Benn, London (1969), page 513, and D.J. Hucknall, "Selective Oxidations of Hydrocarbons" Academic Press, London (1974), page 6).

Promoters proposed for the silver catalystsare above all compounds of the alkaline earth metals, earth metals and rare earth metals (U.S. Patent 2,238,474, German Published Application DAS 1,279,669 and German Laid-Open Application DOS 2,263,543). German Published Application DAS 2,300,512 alleges that compounds of the heavy alkali metals have an effect superior to that of any other promoters known previously. The promoter action of sodium is however stated to be substantially inferior to that of the heavy alkali metals. On the other hand, U.S. Patent 2,125,333 states that sodium has a beneficial effect on the selectivity of the catalysts and U.S. Patent 2,446,132 states that certain salts (namely the carboxylates) of sodium or lithium improve the catalyst properties.

We have now found that by using a certain combination of alkali metals as promoters of silver catalysts, it is possible to produce catalysts for the oxidation of ethylene which, in respect of their life, activity and selectivity are superior to the conventional catalysts containing, for example, promoters selected from the heavy alkali metals or the alkaline earth metals. Furthermore we have found, surprisingly, that using the catalysts of the invention the oxygen conversion and gas hourly space velocity can be increased without thereby accelerating a decrease in the activity of the catalyst.

The present invention provides a catalyst for the manufacture of ethylene oxide by reaction of ethylene with oxygen, which catalyst comprises silver and alkali metals on a carrier, and has been obtained by applying a thermally decomposable silver compound and alkali metal compounds, in an sequence to the carrier and then activating the catalyst, wherein the finished catalyst contains an effective amount, for example from 0.1 to 100 atom %, preferably from 0.1 to 10 and especially from 0.1 to 2 atom %, of lithium or lithium plus sodium, and from 0.05 to 0.35 atom % of potassium or from 0.003 to 0.25 atom % of rubidium or from 0.0005 to 0.2 atom % of cesium, or a corresponding amount (as hereinafter defined) of a mixture of two or more of these heavy alkali metals (i.e.

potassium, rubidium and cesium) the percentages being based on silver.

By the term "a corresponding amount" is meant an amount of each of the components of the mixture such that the mixture as a whole has a catalyst activity equivalent to that of an amount of potassium, rubidium or cesium alone which is within the ranges specified above.

It is Particularly advantageous to replace lithium partially, for example to the extent of up to 70% of its amount, by sodium (though the proportion of sodium should not exceed 2 atom % based on silver), so that a particularly advantageous catalyst comprises silver together with, for example, from 0.3 to 10 atom % of lithium and from 0.1 to 1 atom % of sodium (the percentages being based on silver) and, in addition, at least one of the heavy alkali metals potassium, rubidium or cesium, in the amount stated above.

The catalyst is obtained by applying a thermally decomposable silver compound and appropriate alkali metal compounds to the carrier in any sequence and activating the catalyst as by heating and/or by treatment with a reducing agent.

A particularly advantageous method to follow in manufacturing the catalyst is to treat the catalyst, which already contains silver in a reduced form, with or without sodium and/or lithium, with a solution, preferably an alcoholic solution, of a compound of potassium, rubidium and/or cesium which in addition contains a nitrile, an amine and/or ammonia or a comparable compound which forms silver complexes, and to use the- solution in such amount that the solution taken up by the catalyst corresponds (based on silver) to forum 0.05 to 0.35 atom % of potassium, from 0.003 to 0.25 atom % of rubidium or from 0.0005 to 0.2 atom % of cesium or to corresponding amounts of a mixture of these alkali metals, though these amounts should take into account any amounts of the alkali metals which may already be present on the catalyst.

The addition of the lithium, with or without sodium, can evidently in this case take place before applying the silver to the carrier or conjointly with the application of the silver or during subsequent treatment of a finished silver catalyst or even of a silver catalyst which has already been used.

The amounts of lithium or sodium to be used in the silver catalyst depend on the magnitude of the surface area of the carrier, on the amount of silver and, to a slight extent, also on the method of manufacture. The latter above all has an effect on the amounts of lithium and/or sodium with which an optimum catalyst is obtainable. The best amount can most easily be determined by a series of preliminary experiments, in which catalysts with varying amounts of the alkali metal are produced by a particular method of manufacture, and their performance is investigated.

In the case of the preferentially used coarse-pored carriers having a surface area of from 0.1 to 0.5 m2/g, the amount of lithium and sodium together is preferably from 0.4 to 3 atom % (based on silver), the amount of silver being from 5 to 10% by weight based on the weight of the catalyst, and the mean silver particle size being from 0.2 to 0.4 Fm.

The preferred amounts of promoters, selected from the heavy alkali metals, present in the catalysts are from 0.05 to 0.35 atom % of potassium, from 0.03 to 0.25 atom % of rubidium or from 0.005 to 0.2 , more preferably from 0.01 to 0.2, atom % of cesium or corresponding amounts of a mixture. The best amount of heavy alkali metal depends on the amount of lithium and/or sodium. If a high content of lithium and/or sodium is selected, the most suitable content of heavy alkali metal lies in the lower part of the stated concentration range. To achieve a long catalyst life it is advantageous to select a very high lithium and/or sodium content and a very low heavy alkali metal content.

We assume that in the finished catalyst the alkali metals are present as compounds. A suitable method of determination of the amounts of alkali metal has proved to be the determination of the solubility in dilute (10% strength) nitric acid.

In principle, all methods which are proposed in the literature for the manufacture of silver catalysts (R. Landau and R.E. Lidov, loc. cit. and D.J. Hucknall, loc. cit.) may be used to manufacture the catalysts according to the invention. It is advantageous to use methods in which the silver is applied to a carrier. For example, it is possible to tumble a carrier with a suspension of freshly precipitated, well-washed silver oxide (German Published Application DAS 1,279,669) or silver carbonate (U.S. Patent 3,043,854) and then to decompose the silver compound thermally to silver. A preferred method is to impregnate a coarse-pored carrier with a solution of a silver salt, eg. silver nitrate (U.S.

Patent 3,575,888) or silver lactate (German Published Application DAS 1,211,607) or of a silver complex (eg. a silver amine-carboxylate complex (German Laid-Open Application DOS 2,159,346)) and then to decompose the silver compound by treatment with a reducing agent or by heat treatment.

The lithium or sodium can be added in the form of a compound to the suspension or the impregnating solution. However, it is also possible to apply these alkali metals as compounds to the carrier in a prior step, as described in principle in German Laid-Open Application DOS 2,448,449, and to treat the dried carrier with the silver formulation in a subsequent step.

It is true that the heavy alkali metal can be added, simultaneously with lithium and/or sodium, in the form of a solution of a compound. However, it is also possible to apply the heavy alkali metal to the carrier in a separate step, ie. to impregnate the otherwise finished catalyst with a solution of a compound of the heavy alkali metal, preferably in an organic solvent.

Suitable silver complexes are silver salts which possess ligands bonded by coordination.

Particularly suitable complexing agents are amines, which form readily water-soluble silver salts which decompose easily, the undesired decomposition products being readily volatile.

Examples of such amines are primary amines which contain alkyl radicals of 1 to 8 carbon atoms, polyamines derived from alkanes up to hexane, mono-alicyclic amines, eg.

cyclohexylamine, and mono-heterocyclic amines, eg. pyrrolidine, piperidine and morpholine. It is true that ammonia is equally suitable but industrially its use is less advisable because explosive silver nitride is easily formed.

The choice of the anion of the silver complex is substantially optional; it is merely necessary that the anion or its decomposition products should be volatile on heating.- For example, virtually all carboxylates, as well as carbonate, isocyanate, cyanide, nitrite and nitrate are suitable anions.

Amine complexes of silver nitrate are preferred, because silver nitrate is the silver chemical which is cheapest and obtainable in the purest form. Advantageously, the amount of amine added to the impregnating solution, in order to form the complex, is that required for the stoichiometric formation of the complex of the formula Ag(amine)2 NO3 In this formula"amine" denotes one amino group or one equivalent ligand. According to the invention, preferred amines have the general formula

where R and R are aliphatic radicals.

The choice of the anion of the lithium or sodium compound and of the heavy alkali metal compound is also substantially optional. The use of compounds which are readily water-soluble and which contain anions which, when brought together with a solution containing silver ions, do not cause the latter to precipitate, is preferred. Compounds of which the anions contain catalyst poisons, eg. halogens and chalcogens, which can subsequently only be removed with difficulty, are less suitable.

In the preferred method of manufacture the carrier is impregnated with a solution or suspension of a silver compound in a first treatment step and next the silver compound is decomposed to silver. This can be done by subjecting the impregnated carrier, in a heated cabinet or oven, to a heat treatment in an atmosphere which may or may not contain steam up to saturation level. A treatment with a reducing gas (CO or H2) may be carried out in place of, or in addition to, the heat treatment.

The catalysts of the invention in general contain from 2 to 12%, especially from 5 to 10% by weight, of silver, based on the weight of the carrier. It is true that larger amounts of silver (for example up to 20% by weight) do not impair the catalyst activity, but from a technical point of view they are unnecessary.

The carrier preferably consists of a refractory, chemically inert material, for example a-aluminum oxide, silicon carbide, graphite or highly sintered aluminum silicate. The content of uncontrolled impurities which can be dissolved out by boiling with dilute nitric acid, preferably does not exceed 0.001who by weight of alkali metal ions and 0.03% by weight of alkaline earth metal ions and earth metal ions. Of course, in specific cases purification of the carrier is unnecessary if the known alkali metal content, if any, is taken into account in calculating the amounts of alkali metal to be added, and if other constituents, which would interfere, are absent.

Preferably, the porosity of the carrier will be very high (more than 40% by volume) and of such nature that the degree of utilization of the pores is virtually 1, ie. the carrier should preferably only have pores of diameter greater than 1,000 nm, which are readily accessible to the gas molecules by diffusion. Suitable carriers are available commercially.

The present silver catalysts may be used for all processes in which ethylene oxide is manufactured by direct oxidation of ethylene with molecular oxygen. A review is to be found in a series of articles in Hydrocarbon Processing (March 1976, 69-72, 73-77, 78-80).

For example, the catalysts may be used both in the "air oxidation process" and in the "oxygen oxidation process". Optimum selectivities are only achieved, as with other industrially used catalysts for the manufacture of ethylene oxide, if an inhibitor for controlling the catalytic action, for example a chlorinated polyphenyl compound, 1,2-dichloroethane or vinyl chloride, is present.

Catalysts within the invention are superior to previously described catalysts in respect of activity and selectivity in processes in which the synthesis gas contains approximately equal amounts of ethylene and oxygen, in addition to diluents, eg. nitrogen, carbon dioxide, methane or argon; see Use Example 1.

In processes where the synthesis gas contains a substantial excess of ethylene over oxygen, the superiority over the conventional catalysts is particularly significant in respect of activity and catalyst life.

EXAMPLE 1 13.9 g of silver nitrate are dissolved in 12 g of sec-butylamine. 0.3 ml of an 8% strength by weight CsNO3 solution and 0.25 ml of LiNO3 solution (22.75 g of LiNO3 made up to 100 ml with H2O) are added to the first solution. The mixture is made up with water to a volume corresponding to the anticipated liquid absorption of the carrier, as determined in a preliminary experiment. 100 g of carrier (based on a-aluminum oxide and obtainable, for example, as grade SA 5551 from Norton, USA) are impregnated under reduced pressure and left for 1 day at room temperature. The impregnated carrier is then transferred into an air circulation oven preheated to 240"C and charged with CO2 or N2. When the evolution of gas has subsided, the catalyst is removed from the oven. The catalyst is designated L 1.

EXAMPLE 2 The catalyst is produced as described in Example 1, but instead of the amounts of alkali metal compounds stated in Example 1, 0.1 ml of CsNO3 solution (8.0 g of CsNO3 made up to 100 ml with H2O), 0.25 ml of LiNO3 solution (22.75 g of LiNO3 made up to 100 ml with water) and 0.15 ml of NaNO3 solution (25.8 g of NaNO3 made up to 100 ml with water) are added. The catalyst is designated L 2.

USE EXAMPLE 1 The catalyst obtained as described in Example 1 or 2 is comminuted and 10 g of the 0.6 0.75 mm sieve fraction are packed into a glass reactor of 5 mm internal diameter. The reactor is introduced into a temperature-regulated metal bath. A gas comprising 7% of ethylene, 9.7% of oxygen, 0.3 ppm of inhibitor, remainder nitrogen, is passed over the catalyst under atmospheric pressure. The throughput is 2,000 1 (S.T.P.) of gas 1 of catalyst. h The temperature is regulated to give an oxygen conversion of 40%. After about 60 hours, the temperature has stabilized and after 90 hours samples are taken and the selectivity is determined. The results are summarized in Table 1.

USE EXAMPLE 2 The catalyst obtained as described in Example 1 or 2 is comminuted and 10 g of the 0.6 0.75 mm sieve fraction are packed into a stainless steel reactor of 5 mm internal diameter.

The reactor has a jacket through which a thermostatically controlled fluid is passed. A gas comprising 30% of ethylene, 8% of oxygen, 3 ppm of inhibitor, remainder nitrogen, is passed through the reactor. The pressure is 16 bars and the throughput is 3,300 1 (S.T.P.) of gas 1 of catalyst. h The temperature is regulated so that the oxygen conversion is 50%. After 2 days, samples are taken and the activity and selectivity are determined (Table 2).

EXAMPLE 3 11.25 g of silver nitrate are dissolved in 9.75 g of sec-butylamine. 2 ml of an aqueous lithium nitrate solution (22.75 g of LiNO3 per 100 ml of solution) are added to the preceding solution. The mixture is made up with water to a volume corresponding to the anticipated liquid absorption of the carrier, as determined in a preliminary experiment. 100 g of carrier (for example a carrier based on a-aluminum oxide, obtainable as grade SA 5551 from Norton, USA) are impregnated and stored for one day at room temperature. The impregnated carrier is then transferred into an air circulation oven which is preheated to 24Cand has been flushed with nitrogen. After the evolution of gas has subsided, the catalyst is taken out of the oven, cooled and treated with an amount, just sufficient for complete impregnation, of a methanol solution containing 1% by volume of sec-butylamine and 0.3 ml of a previously prepared cesium hydroxide solution (5.46 g of CsOH per 100 ml of methanol). The catalyst obtained is then dried in a drying oven flushed with nitrogen; the catalyst is designated L 3 and is examined as described in Use Example 2 (Tables 1 and 2).

TABLE 1 (USE EXAMPLE 1) Catalyst Silver Alkali metal content in the acid-soluble matter TemperatureSelectivity content (atom % based on silver) L 1 7.7 0.15 (Cs+) 0.09 (Na+) 0.03 (K+) 1.0 (Li+) 252 78.5 L 2 7.9 0.05 (Cs+) 0.55 (Na+) 0.02 (K+) 1.0 (Li+) 254 79.0 L 3 6.6 0.18 (Cs+) 0.07 (Na+) 0.03 (K+) 10 (Li+) TABLE 2 (USE EXAMPLE 2) Catalyst Temperature Selectivity L 1 214 80.5 L 2 216 81.0 L 3 212 81.0 WHAT WE CLAIM IS: 1. A catalyst comprising silver and alkali metals on a carrier and obtained by applying a thermally decomposable silver compound and alkali metal compounds, in any sequence, to the carrier and then activating the catalyst, wherein the finished catalyst contains, in addition to an effective amount of lithium or lithium plus sodium, from 0.05 to 0.35 atom per cent of potassium or from 0.003 to 0.25 atom per cent of rubidium or from 0.0005 to 0.2 atom per cent of cesium or a corresponding amount (as hereinbefore defined) of a mixture of two or more of potassium, rubidium and cesium, the percentages being based on silver.

2. A catalyst as claimed in claim 1, which contains from 0.1 to 100 atom per cent of lithium, based on silver.

3. A catalyst as claimed in claim 1, which contains from 0.1 to 2 atom per cent of lithium, based on silver.

4. A catalyst as claimed in claim 1, which contains a total of from 0.1 to 100 atom per cent of lithium plus sodium, based on silver, the sodium constituting not more than 70 atom per cent of the total of lithium plus sodium and not more than 2 atom per cent based on silver.

5. A catalyst as claimed in claim 4, which contains from 0.3 to 10 per cent of lithium and from 0.1 to 1 atom per cent of sodium, based on silver.

6. A catalyst as claimed in any of claims 1 to 5, wherein the carrier is a coarse-pored material having a surface area of from 0.1 to 0.5 m2/g, the amount of lithium with or without sodium is from 0.4 to 3 atom per cent based on silver, the amount of silver is from 5 to 10% by weight based on the catalyst and the mean silver particle-size is from 0.2 to 0.4 CL.

7. A catalyst as claimed in any of claims 1 to 6 which contains from 0.03 to 0.25 atom per cent of rubidium or from 0.01 to 0.2 atom percent of cesium.

8. A catalyst as claimed in any of claims 1 to 7 which has been obtained by a method including the step of contacting a carrier, which already contains silver in a reduced form, with a solution of at least one compound of potassium, of rubidium and/or of cesium containing in addition a nitrile, an amine and/or ammonia.

9. A catalyst as claimed in claim 1 and substantially as hereinbefore specifically described or exemplified.

10. A process for the manufacture of ethylene oxide by the catalytic oxidation of ethylene with molecular oxygen, wherein the oxidation is carried out in the presence of a catalyst as claimed in any of claims 1 to 9.

11. Ethylene oxide whenever produced by a process as claimed in claim 10.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. TABLE 2 (USE EXAMPLE 2) Catalyst Temperature Selectivity L 1 214 80.5 L 2 216 81.0 L 3 212 81.0 WHAT WE CLAIM IS:

1. A catalyst comprising silver and alkali metals on a carrier and obtained by applying a thermally decomposable silver compound and alkali metal compounds, in any sequence, to the carrier and then activating the catalyst, wherein the finished catalyst contains, in addition to an effective amount of lithium or lithium plus sodium, from 0.05 to 0.35 atom per cent of potassium or from 0.003 to 0.25 atom per cent of rubidium or from 0.0005 to 0.2 atom per cent of cesium or a corresponding amount (as hereinbefore defined) of a mixture of two or more of potassium, rubidium and cesium, the percentages being based on silver.

11. Ethylene oxide whenever produced by a process as claimed in claim 10.