US3493477A

US3493477A - Electrochemical reduction of benzene

Info

Publication number: US3493477A
Application number: US689783A
Authority: US
Inventors: Eddie C French; Charles M Starks
Original assignee: Continental Oil Co
Current assignee: ConocoPhillips Co
Priority date: 1967-12-12
Filing date: 1967-12-12
Publication date: 1970-02-03
Anticipated expiration: 1987-02-03

Description

United States Patent US. Cl. 204-59 6 Claims ABSTRACT OF THE DISCLOSURE A process for electrochemically reducing benzene to 1,4 cyclohexadiene in the presence of liquid ammonia, an alkali metal chloride and an alcohol, all in the absence of iron and under anhydrous conditions.

DISCLOSURE This invention relates to a method of electrochemically reducing benzene to selectively produce 1,4 cyclohexadiene.

Benzene can be reduced chemically to a mixture of products, such as 1,3 and 1,4 cyclohexadienes, cyclohexene, and cyclohexane, in anhydrous low molecular weight amines with metallic lithium or sodium. These alkali metal reductions are of considerable synthetic use, however, the cost of lithium or sodium metal and the amines makes this process too expensive for large scale use. The products formed by this chemical reduction have similar properties and separation into pure components is nearly impossible. Aromatic compounds have been reduced electrolytically. Benzene has been reduced in methylamine while using lithium chloride as a current carrier.

It has now been unexpectedly discovered that benzene can be reduced to selectively produce 1,4 cyclohexadiene when said reduction is carried out electrolytically in the presence of liquid ammonia and an alcohol with a current carrier of sodium chloride. For highest yields it has been discovered that said reaction should be carried out in the absence of iron contamination and under anhydrous conditions.

An outstanding feature of this reaction is its selectivity of products. Benzene was converted to 1,4 cyclohexadiene with selectivity of greater than 98 percent. The remaining 2 percent was cyclohexene, probably formed by the reduction of the 1,4 cyclohexadiene. If the reaction was carried to near completion (reduction of all the benzene), the amount of cyclohexene would increase; however, the reaction can be carried to 55 percent conversion to 1,4 cyclohexadiene with only 2 percent cyclohexene byproduct and still maintain high current efficiencies. The remainder was unreacted benzene.

Current efficiencies vary over a wide range and are dependent on many factors. Efficiencies of 40 percent can be obtained by using the best known conditions and not carrying the reaction past 25 percent conversion. A current efficiency of 30 to 33 percent was obtained where the conversion was 55 percent.

The presence of 1 to 4 percent alcohol such as methanol, ethanol, or isopropanol is essential to obtain good current efiiciencies. In the absence of an alcohol, the current efficiencies are only about 1 to 2 percent.

Numerous materials can be used as cathodes. The desirable properties of the cathode are: (1) high hydrogen overvoltages, (2) resistance to attack by anhydrous ammonia, (3) good conductor, (4) readily available, and (5) low cost. Aluminum, zinc, and platinum have been used with aluminum being preferred because of its availability and low cost.

Numerous materials were tested as anodes. Included were platinum, gold, titanium, cast iron, stainless steel,

nickel, zirconium, molybdenum, tungsten, antimony, and tin. Good current efficiencies can be obtained with a platinum anode with 1 percent alcohol present.

The effect of curent density is closely related to the cathode potential as shown in the table.

TABLE Effect of cathode current density on current efficiency Current density, ma./in. Percent current efficiency Sodium formed at the cathode by the reduction of sodium ions present will react rapidly with any water present rather than the benzene. This results in a drop in current efficiency. Also, iron catalyzes the reaction of sodium with ammonia. If iron or other transition metals are present the sodium reacts with the ammonia and no reduction of benzene occurs.

The effect of benzene concentration was not as critical as other variables; however, highest current efficiencies were obtained when the benzene concentration was 15 to 20 percent.

Temperatures near -33 C. were necessary to obtain good current efficiencies. The yield was not affected by the temperature.

The over-all reaction for the reduction of benzene to 1,4 cyclohexadiene is:

This is a chemical reaction. The electrode reactions are:

Cathode- 6Na 6e- 6Na Anode 8NH 6e- N +6NH Total electrode reaction- 8NH 6Na+ 6Na +N 6NH A reasonable sequence of steps would be the reaction of sodium with benzene to form a sodium-benzene complex.

3 6Na 3N9. Na

The alcohol then acts as an acid in the ammonia, displaces the sodium, and forms the 1,4 cyclohexadiene.

The alcohol is regenerated by the reaction of the sodium alkoxide with ammonium ions which are formed at the anode.

Mass spectroscopy analysis showed the presence of nitrogen and hydrogen in the off-gases. Nitrogen was liberated at the anode from the oxidation of ammonia. Hydrogen was formed by the reaction of sodium with ammonia.

This reaction is catalyzed by the presence of iron and other transition metals. The competition between this reaction and the benzene-sodium reaction thus accounts for the observed 40% current efficiency.

Separation of the products formed can be accomplished by fractional crystallization or fractional distillation.

For a fuller understanding of the present invention, reference Will be had to the following example.

EXAMPLE Ten grams of benzene was dissolved in 200 grams of liquid ammonia along with 10 grams of sodium chloride. The solution was maintained at 20 C. in a stainless steel pressure .vessel lined with a polyethylene liner. The electrodes were strips of platinum with an area of 25 cm. each. A constant direct current of 20 ma. per cm. was passed through the solution for 20 hours. Benzene was reduced at the cathode to 1,4 cyclohexadiene. No other reduction products were formed during the electrolysis. The current eificiency was 25 percent. The remainder of the curent Went to form hydrogen gas from the ammonia.

Having thus described the invention by providing a specific example thereof, it is to be understood that no undue limitations or restrictions are to be drawn by reason thereof and that many variations and modifications are within the scope of the invention.

What is claimed is:

1. A process for electrochemically reducing benzene to 1,4 cyclohexadiene which comprises:

(a) dissolving benzene and an alkali metal chloride in liquid ammonia under substantially anhydrous conditions and in a substantially iron free vessel containing an anode and a cathode;

(b) passing a direct current of electricity through said solution for a period of time sufiicient to produce a mixture of essentially 1,4 cyciohexadiene and benzene; and

(c) recovering said 1,4 cyclohexadiene from said mixture.

2. The process of claim 1 comprising the additional steps of adding alcohol to step (a) of claim 1.

3. The process of claim 1 wherein said alkali metal chloride is sodium chloride.

4. The process of claim 1 wherein the recovery step of (c) is by fractional crystallization.

S. The process of claim 1 wherein the recovery of step of (c) is by fractional distillation.

6. The process of claim 2 wherein the temperature of the reaction is maintained at about 33 C.

References Cited UNITED STATES PATENTS 3,361,653 1/1968 Miller 204--59 HO\VARD S. WILLIAMS, Primary Examiner