US3546082A - Oxidation process - Google Patents

Description

United States Patent 3,546,082 OXIDATION PROCESS John Rickard Mansfield, Norton-on-Tees, England, assignor to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain No Drawing. Filed Apr. 29, 1968, Ser. No. 725,181 Claims priority, application Great Britain, May 5, 1967, 21,027/ 67 Int. Cl. C07f 15/00; B01k N00 US. Cl. 204--72 7 Claims ABSTRACT OF THE DISCLOSURE A Group VIII noble metal, e.g., palladium, is converted to a carboxylate by electrolysing a slurry of the metal in the anode compartment of a cell comprising anode and cathode compartments separated by an anion exchange membrane or salt bridge. The cathode compartment contains an electrically conducting carboxylic acid solution and the noble metal carboxylate is withdrawn from the anode compartment.

The present invention relates to an oxidation process, particularly to the oxidation of a metal from a lower to a higher valency state.

Processes are known in which the noble metals of Group VIII of the Periodic Table (platinum, palladium, rhodium, iridium, ruthenium and osmium) are used to oxidise olefinic compounds to aldehydes, ketones, unsaturated esters, unsaturated ethers or acetals. Such processes are described in British patent specification No. 964,001 and our co-pending U.S. applications Ser. Nos. 601,868 filed Dec. 15, 1966 and 646,490 filed June 16, 1967. In carrying out the oxidation the noble metal is reduced to the zero-valent form. The vero-valent form of the metal cannot be re-oxidised directly by molecular oxygen but only by the intervention of a redox system which is suitably a metal salt of variable valency, for example, a copper, iron or cobalt salt. Thus in oxidising the zero-valent noble metal the redox system is reduced to its lower valent form which may then be re-oxidised with molecular oxygen. In the absence of the redox system the noble metal on reduction is precipitated in metallic form.

In processes such as those mentioned above, the noble metal used may be palladium in the form of its carboxylate, for example palladous acetate. The redox systems employed may also be metal carboxylates, for example copper acetate, iron acetate or cobalt acetate.

In carrying out such processes however, despite the presence of a redox system and molecular oxygen, a precipitate of metallic palladium may build up after several hours of continuous operation. This precipitate may also contain cuprous salts which have precipitated without being re-oxidised to the cupric form.

It is known that while the dissolution of palladium in hydrochloric acid is readily effected its dissolution in carboxylic acids such as acetic acid is very difficult to achieve.

The present invention provides a process whereby a Group VIII metal such as palladium may be reconverted to a corresponding carboxylate. It also provides a process whereby the reduced form of a metal carboxylate redox system may be reconverted to the oxidised form. In one form the process of the present invention may be used in conjunction with the processes described in our specifications referred to above, to re-oxidise the palladium salt directly without the use of a redox system and molecular oxygen, or to re-oxidise the redox system without the use of molecular oxygen or to facilitate the re- 3,546,082 Patented Dec. 8, 1970 oxidation of the palladium salt by molecular oxygen with or without the use of a redox system.

According to the present invention, an oxidation process comprises introducing a slurry of a finely divided noble metal of Group VIII and/or a slurry or solution of the reduced form of a metal carboxylate redox system to the anode compartment of an electrolytic cell comprising anode and cathode compartments separated by an anion exchange membrane or a salt bridge, the cathode compartment containing a cathode and an electrically conducting carboxylic acid solution and the anode compartment an anode, applying a direct current between the anode and cathode, and withdrawing a solution of a noble metal carboxylate and/or the oxidised form of the metal carboxylate redox system from the anode compartment.

The noble metals of Group VIII are palladium, platinum, rhodium, ruthenium, osmium and iridium.

Hydrogen is liberated at the cathode in the process of the invention and if oxygen is supplied at the cathode the hydrogen may be converted to water with consequent liberation of energy which may usefully help in reducing the overall electrical energy required by the process.

The slurry or solution which is introduced to the anode compartment may be formed in a separate reaction zone. It may alternatively be produced in situ by oxidation of an olefinic compound as described in the above mentioned co-pending applications using the anode compartment as the oxidation reactor. The molecular oxygen which is a feature of the above described processes would however be used in the cathode compartment to convert the hydrogen formed to water.

The solution or slurry introduced to the anode compartment may comprise any suitably inert electrically conducting liquid. Suitably the liquid is a carboxylic acid for example acetic acid. As the slurry or solution may have been produced in the catalytic oxidation of an olefinic compound it may also contain other catalyst components such as alkali metal or alkaline earth metal salts, e.g. lithium acetate, nitrogen compounds such as oximes, Water and organic reactants and products. Other elemental metals may also be present in the slurry e.g. metallic copper and/or other metals such as metallic cobalt derived from metal carboxylate redox systems.

The dissolution of the noble metal is also improved by deliberately incorporating a complexing agent in the anode compartment solution. This agent, by complexing with the noble metal improves the solubility of the noble metal ion and facilitates electron transfer. Nitriles such as acetonitrile and benzonitrile, amides such as acetamide, metal carboxylates such as lithium acetate and, most preferably, oximes and nitrates, nitriles, nitrosyl and nitroso compounds such as are described in our copending US. application Ser. No. 601,868 filed Dec. 15, 1966 are suitable. Acetoxime, acetaldehyde oxime and formaldoxime are preferred complexing agents. The concentration of the complexing agent may suitably be in the range 0.01 to 1 molar.

The cathode compartment contains an electrically conducting carboxylic acid solution. Preferably the carboxylic acid contains up to four carbon atoms and more preferably is acetic acid. As will be discussed later the carbon number of the acid which is used dictates to a certain extent the design of the cell. To improve the conductivity of the carboxylic acid it is advantageous to include carboxylate ions other than those provided by self-ionisation of the carboxylic acid. The carboxylate ions are preferably provided in the form of an alkali metal or alkaline earth metal carboxylate, for example sodium or lithium acetate. The conductivity may be further improved by the presence of water. Suitable carboxylic acid solutions for use in the cathode compartment comprise, for example:

Percent by wt. Alkali metal or alkaline earth metal carboxylate 0.1-2.0 Acetic acid 65-96 Water 35-4 As carboxylate ions are continuously removed from the cathode compartment it is advisable when operating the process continuously to add carboxylic acid to the cathode compartment during the process to maintain the carboxylic acid concentration.

An electrochemical cell suitable for use in the above process comprises anode and cathode compartments separated by an anion exchange membrane or a salt bridge the cathode compartment containing a cathode and the anode compartment an anode, a direct current source being connected between the anode and cathode.

The cathode may be constructed of any suitable conductor of electricity which is chemically inert to the electrolyte. Thus cathodes made of carbon, copper, nickel or iron may be used. The cathode may be rod-like or plate-shaped or may be formed as a cylinder surrounding the anode.

The anode is preferably made of carbon and is preferably porous to give the maximum surface area. In an improved form of the cell the anode compartment contains a plurality of carbon granules preferably of 0.5 to mm. diameter. The anode compartment is preferably substantially filed with such granules.

The salt bridge may comprise any concentrated solution of carboxylate ions corresponding to the carboxylate it is desired to produce. The carboxylate ions may be provided by an alkali metal carboxylate particularly a lithium carboxylate.

The anion exchange membrane may comprise any suitable exchanger particularly one containing quaternary nitrogen atoms. The membrane may be heterogeneous e.g. finely ground anion exchange resin deposited on a thermoplastic matrix, or homogeneous, e.g. formed from the condensation product of a suitable nitrogen-containing compound and formaldehyde rolled out into a thin sheet before setting. It may be advantageous to provide a support for the membrane to prevent distortion. For example when the cathode is a plate and the cell is of a sandwich construction a perforated polytetrafluorethylene spacer may be placed between the cathode and the membrane. The carbon number of the carboxylic acid used dictates the choice of membrane as carboxylate ions have to pass through the membrane from the cathode to the anode compartment. As the molecular size of the carboxylate ion increases the passage becomes slower and more difiicult.

The voltage required to be supplied by the direct current source is a function of the cell construction and may be determined by simple experiment. Advantageously the voltage is maintained at the value required to oxidise the metals present and is not allowed to rise substantially in excess of this value as secondary oxidations of organic material present may take place.

A preferred form of the cell is one in which the anode compartment is elongated and filled with carbon granules to form a packed column. The granule size is chosen by reference to the nature of the slurry and 1 to 2 mm. diameter granules have proved suitable in many instances. The anode may be a rod running down the centre of the column and the cathode a cylinder surrounding the anode compartment being separated therefrom by a cylindrical anion exchange membrane. Such a design assists the recovery of the oxidised product from the anode compartment as it may be collected in the liquors percolating through the column.

The process of the present invention is particularly applicable to the following oxidations of olefinic compounds utilising noble metal of Group VIII catalysts particularly a palladium catalyst.

(1) The oxidation of olefines in the presence of carboxylate ions to unsaturated esters, e.g. ethylene to vinyl acetate and propylene to allyl acetate.

(2) The oxidation of allyl acetate to esters of glycerol as described in British patent specification No. 987,278.

(3) The oxidation of olefines in the presence of water to aldehydes or ketones e.g. ethylene to acetaldehyde and propylene to acetone.

(4) The oxidation of olefines in the presence of alcohols to ethers and acetals e.g. ethylene to methyl vinyl ether and dimethylacetal.

In the above processes palladium carboxylates, particularly palladous acetate, may be used as catalysts. The redox system may be a copper or iron carboxylate, e.g. copper acetate. In our co-pending US. application Ser. No. 622,430 filed Mar. 13, 1967, now abandoned, combinations of redox systems e.g. copper and cobalt carboxylates are disclosed for use in the above processes and such combined redox systems may be re-oxidised by the process of the present invention.

EXAMPLE 1 The cell comprised a carbon rod 4" long and diameter as the anode surrounded by a cylindrical anion exchange membrane forming an anode. compartment 1 /2" in diameter. The ends of the cylinder were sealed, glass tubes being provided to allow the introduction of liquid to the compartment and withdrawal of liquid from the compartment. The cathode comprised a cylinder of copper foil surrounding the membrane and supported in a glass tube, the ends of the tube being sealed to form the cathode compartment. The diameter of the cathode compartment was 3". A source of DC. current comprising a rectifier connected to the A.C. mains supply was connected between the anode and the cathode. The cathode compartment was filled with an electrolyte comprising:

Lithium acetate0.8 mole/litre Water20% by volume Acetic acid-% by volume The anode compartment was filled with a mixture of 50 mls. British Standard 10 to 16 mesh carbon granules and finely powdered metallic palladium.

milliamps of current at 4 volts was delivered to the cell while electrolyte of the above composition was introduced into the anode compartment and allowed to trickle through the carbon granules.

The solution of the electrolyte withdrawn from the anode compartment contained 4 grams/litre of palladous ions as palladous acetate. The rate of dissolution of palladium was 1 mole per cubic meter of carbon granules per hours.

EXAMPLE 2 The cell comprised a rectangular sandwich of crosssectional area 6" x 3 consisting of, in order, a A" polytetrafluoethylene back plate with two inlet ports cut in its face, a A" carbon cathode similarly equipped with two parts coincident with those in the back plate, a A" polytetrafluoethylene spacer with a cut away centre portion forming a cathode compartment, an anion exchange membrane, a /1." polytetrafluoethylene spacer with a cut away central portion or anode compartment filled with carbon granules and having inlet and exit ports on its shorter side edges, a carbon anode and a polytetrafluoethylene back plate. The whole was held together by longitudinal bolts. A source of DC current comprising a rectifier connected to the A.C. mains supply was connected between the anode and the cathode.

The cell-sandwich" was laid horizontally and the cathode compartment filled with electrolyte comprising:

Lithium acetate-0.1 mole% litre Acetic acid20% by volume Water-80% by volume A slurry of finely divided metallic palladium in a solution of the above composition was also fed into the anode compartment via the upper port. The solution emerging from the lower port was recycled to this compartment.

The temperature was ambient and 100 milliamps of current at 4 volts was delivered to the cell.

The rate of dissolution of the palladium was 3.0 moles per cubic meter of carbon granules per hour.

EXAMPLE 3 Example 2 was repeated incorporating 0.1 molar concentration of acetoxime in the electrolyte in the anode compartment. The rate of dissolution of the palladium increased to 7.6 moles per cubic meter of carbon granules per hour.

EXAMPLE 4 Example 1 was repeated employing a mixture of metals of composition metallic palladium 1.7%, metallic cobalt 44.4% and metallic copper 53.9%. The solution obtained contained 0.1% palladous ions, 0.06% cobaltous ions and 0.2% cupric ions.

I claim:

1. An oxidation process which comprises passing a direct electrolyzing current through at least one member of the group consisting of a slurry of a finely divided noble metal of Group VIII, a slurry of the reduced form of a metal carboxylate redox system from the group consisting of copper, cobalt and iron and mixtures thereof and a solution of a reduced form of said metal carboxylate redox system in an anodic zone, and an electrically conducting carboxylic acid solution in a cathodic zone, permitting carboxylate anions but not cations to pass from said cathodic zone to said anodic zone, the current being sufiicient to convert noble metal in said noble metal slurry to the carboxylate and the reduced form of the redox metal carboxylate to the oxidized form thereof and withdrawing at least one member of the group consisting of a solution of a noble metal carboxylate and a solution of the oxidized form of the metal carboxylate redox system from said anodic zone.

2. The process according to claim 1 in which the slurry or solution comprises a carboxylic acid and the noble metal is palladium.

3. The process according to claim 2 in which the carboxylic acid in the cathiodic zone contains up to four carbon atoms.

4. The process according to claim 3 in which the solution in the cathodic zone contains carboxylate ions other than those provided by self-ionisation of the carboxylic acid.

5. The process according to claim 4 in which the solution in the cathodic zone contains water.

6. The process according to claim 1 in which the solution in the anodic zone contains a complexing agent.

7. The process according to claim 6 in which the complexing agent is an oxime.

References Cited UNITED STATES PATENTS 3,119,874 1/1964 Paszthory et al. 260-597B 3,303,020 2/1967 Clement et al. 260597BX 3,365,498 1/1968 Bryant et al. 260597BX JOHN H. MACK, Primary Examiner I. C. EDMUNDSON, Assistant Examiner us. 01. XJR. 260-597