Process for Making a Silver-Gold Alloy Catalyst for Oxidizing Ethylene to Ethylene Oxide

This invention relates to a novel method of preparing a silver catalyst and its use in the process of making ethylene oxide by the partial oxidation of ethylene in the vapor phase. Silver-containing catalysts in which the catalytically active component is the metal itself are well known in the art. An important use for the catalyst is in the direct oxidation conversion of alkenes to the corresponding vicinal epoxides, particularly in preparing ethylene oxide from ethylene by reacting ethylene with oxygen in the vapor phase.

Methods known to the art for making such catalysts include soaking a carrier or support in aqueous solutions of silver salts to impregnate it. Thereafter the thus-impregnated salts are reduced to silver metal prior to utilization in the process for oxidizing ethylene. Reduction is normally accomplished by heating in the presence of a reducing agent or by thermal decomposition of the salt. This is done at temperatures within the range of 125.degree. C. to 400.degree. C. and preferably from 200.degree. C. to 300.degree. C. Alternatively, the silver salt may be deposited from a slurry. Either slurry or solution also may contain a reducing agent, or the reducing agent may be subsequently applied.

The commonly used reducing agents are organic compounds which include polyhydric alcohols, such as liquid glycols (e.g. ethylene, propylene, and butylene glycols), glycerol, aqueous sugar solutions, aqueous polyvinyl alcohol solutions, the polyglycols, (e.g. polyethylene and polypropylene glycols) preferably of relatively low molecular weight; also included are aqueous solutions of such polyglycols, the water soluble glycol alkyl ethers, and the like.

One of the criteria for commercially useful silver catalysts is that the silver be finely divided and relatively homogeneously dispersed on the catalyst support. To obtain such finely divided silver various methods have been suggested by the prior art. In U.S. Pat. No. 2,404,438 certain carboxylic acid salts of silver are taught as useful in preparing a finely divided particulate silver for a supported catalyst.

Representative silver salts which may be employed are silver salts of certain inorganic acids for example, silver nitrate, silver chlorate, and silver metaborate, or salts of carboxylic acids such as silver acetate, slver propionate and slver formate may be used. A preferred salt is silver nitrate because it is readily soluble in aqueous solution and easily reduced, either thermally or with an organic reducing agent or hydrogen.

Supports known to be useful for making silver catalysts are for example .alpha.-alumina, zirconia, corundum, mullite, silicon carbide and carbon. .alpha.-alumina is preferred and especially a porous alumina of low surface area, i.e. less than one square meter per gram.

While silver is the metal most useful from a commercial standpoint in providing the catalytic effect necessary to obtain ethylene oxide, most commercial catalysts additionally contain small amounts of a promoter. The amount employed is usually from a few parts per million up to one or two percent, based on the weight of the total catalyst. Representative promoters include the alkali and alkaline earth metals which are usually present as their oxides. Other metals known to be promoters for the silver catalyst for the oxidation of olefins to their oxides are platinum, palladium and gold. Still others taught by the art are copper, mercury, tin, nickel and iron. Among the patents which teach gold as a promoter are included U.S. Pat. Nos. 3,773,693 and 3,844,981.

A common way of applying the promoters is to add them as their salts to the solution of the silver salt which is applied to the suport which on subsequent heating are converted to their oxides.

Other ways known to the art of adding the promoter compound are to add it to the support prior to or subsequent to the application of the silver salt. In each case the particular salt applied is dried prior to applying the solution of the second salt. Generally the promoters, when applied first, are converted to their oxides and the silver salt when applied first, is reduced to silver.

The use of a promoter in the form of an alloy is taught in U.S. Pat. No. 2,143,371 wherein gold, copper and iron are alloyed with silver. In U.S. Pat. No. 3,962,285 rhenium, ruthenium and palladium are taught as useful combinations with silver in alloy form and are described as "comprising a bulk phase of substantially pure silver and a surface alloy phase of silver and said promoter metal."

The present invention differs from the known art in that a gold salt is applied to the catalyst first and subsequently reduced to the metal, after which the silver salt is applied and then reduced. This provides a purer alloy which will contain little or no chlorides or other anions which could act as catalyst poisons. The present catalyst is more selective for ethylene oxide and is more resistant to sintering, i.e. the small particles of the catalyst remain dispersed and do not readily migrate to form larger agglomerates.

The present invention is a method of preparing an alloy of gold and silver which provides an improved catalyst for making ethylene oxide via vapor phase oxidation of ethylene. A gold salt is applied first to a suitable support and subsequently reduced to the pure metal. A silver salt is then applied which is subsequently reduced to silver, after which diffusion of gold into the silver takes place to form the homogeneous alloy.

In the present method of producing a gold-silver alloy as a catalyst for ethylene oxide, gold is impregnated first, then reduced in hydrogen at 500.degree. C. These vigorous conditions lead to the complete removal of chlorine as HCl. After cooling and washing with water the catalyst is then impregnated with an aqueous solution or despersion of a silver salt. The silver salt may then be reduced by any common methods, after which the alloy forms. One theory is that the gold crystallites act as nucleation centers for the silver, and the alloy is then formed by diffusion of the gold along the grain boundaries of the silver crystallites. Formation of the alloy may occur during the catalyst preparation or during the first week of catalyst usage.

The catalyst supports useful in the practice of the invention include .alpha.-alumina, corundum, silica and zirconia with alumina being the preferred support. In depositing the gold on the support, gold salts such as the iodide, bromide and cyanide and others having volatile anions may be used. HAuCl.sub.4 and NaAuCl.sub.4 are equivalent and readily available. Gold salts also are generally applied by contacting the support with their aqueous solutions or dispersions.

The temperatures employed to reduce the gold salt to the metal are generally rather high in order to insure complete removal of the anion which can act as a catalyst poison if not substantially completely removed. Thus, temperatures of reduction are from about 350.degree. C. to about 700.degree. C. and preferable from about 450.degree.-550.degree. C. The reduction can be entirely thermal, conducted in an inert atmosphere, but preferably in the presence of hydrogen. Any remaining anion can be removed by washing with water.

The time for reduction can be accomplished in from about 3 to 18 hours, but preferably 2 to 5 hours for the gold salts. Any silver salts which are soluble can be employed, but silver carboxylates are preferred since a thermal reduction is more easily accomplished. Thus, silver oxalate, acetate and lactate can be used. Other silver salts such as silver nitrate can be employed but usually require a reducing agent.

The above silver salts require from about two to about 18 hours for reduction, but preferably from about 6 to about 12 hours at a temperature of from about 150.degree. to 200.degree. C.

The amount of silver in the alloy on the support can be from about 2% to about 20% by weight, while the gold comprises from about 0.1% to about 1.1% by weight, both based on the total weight of alloy and support.

A preferred catalyst is one containing from about 3% to about 8% silver and 0.1% to about 0.4% gold.

The following examples show the preparation of the catalyst and the utilization of it in the oxidation of ethylene with molecular oxygen in vapor phase.

An aqueous solution of NaAuCl.sub.4 was made which contained 281 mg Au in 50 ml of solution by diluting a stock solution. 50 g of highly crystalline .alpha.-alumina spherical support was dipped into, and vacuum impregnated with the solution. After draining off the excess solution, the support was heated to 500.degree. in flowing hydrogen for 5 hours. After cooling, the impregnated support was then extracted with water in a Soxhlet extractor overnight to remove excess NaCl.

A solution of silver oxalate was made by suspending 13.0 g of silver oxide in 50 ml of ethylene diamine and suspending 6.52 g of oxalic acid dihydrate in 50 ml of water. The silver oxide suspension was slowly added to the mixture of oxalic acid in water. After addition was complete, 6 ml of monoethanolamine was added.

The muddy, cloudy solution was divided into two parts. A quantity of 50 g of the gold-treated support was added to one portion and 50 g of support without gold was added to the other as a control. The excess liquor was drained off. The samples were dried at 200.degree. C. in flowing nitrogen for 12 hours. This treatment also caused decomposition of the silver oxalate to give the metal. The catalysts were then washed in a Soxhlet extractor with water for .about.12 hours each, and again dried at 150.degree.-200.degree. C. The catalysts each contained 4.22% silver, while the promoted catalyst contained 0.18% gold.

The catalysts were evaluated in a conventional laboratory reactor, consisting of a jacketed glass tube having a diameter of 1 inch and containing 50 g of catalyst and a reaction zone .about.2 inches long, by passing over the catalyst a gas feed containing .about.6% CO.sub.2, .about.6% C.sub.2 H.sub.4, 6.5% O.sub.2, all by volume, a trace (.about.5 ppm) of 1,2-dichloroethane, the balance being nitrogen. Temperature was about 250.degree. C. The silver-gold alloy showed an increase of about 6-10% in selectivity at the same conversion over the control and also approximately a 2.5% increase in selectivity at 30% conversion over a production catalyst on the same support containing 19% silver and .about.1000 ppm barium as a promoter.

An aqueous solution of NaAuCl.sub.4 containing 281 mg of gold in 50 ml of solution was prepared by dilution of a stock solution. A 50 g quantity of .alpha.-alumina (highly crystalline 1/4" porous spheres) was vacuum impregnated with the solution as in Example 1 . The gold was reduced in a stream of flowing hydrogen at 500.degree. C. for 6 hours. The catalyst was then washed with water in a Soxhlet extractor to remove NaCl.

A quantity of 50 g of the gold-impregnated beads and 50 g of unimpregnated beads were then vacuum impregnated with a 1-molar solution of AgNO.sub.3. The impregnated silver salt was then reduced to metallic silver using a method developed by a co-worker in the same laboratory. After excess AgNO.sub.3 solution was removed, the wet beads were placed into a flask containing ethyl acetate to which diethylamine and triethylamine were added in about equal amounts in sufficient quantity to provide a total of about 1 amine-equivalent per ion equivalent of silver. The temperature of reduction was about 68.degree.-70.degree. C. over a 48-minute period. The catalysts were then washed several times (twice with methanol and four times with water) and thereafter dried in a vacuum oven at 55.degree. C. The silver-gold catalyst contained 4.03% Ag and 0.12% AU, the silver control contained 3.89% Ag.

The catalysts were evaluated as described in Example 2. After 75 days, the silver-gold catalyst had a selectivity of 74.35% at 30% conversion , the silver control had a selectivity of 73.1% also at 30% conversion. A production catalyst containing 19% silver and .about.1000 ppm barium as a promoter had a selectivity of 73.45% at 30% conversion.