CA1040581A - Electrochemical adiponitrile formation from acrylonitrile using ammonia - Google Patents

Abstract

ELECTROCHEMICAL ADIPONITRILE PROCESS
ABSTRACT

Electrochemical hydrodimerization of acrylonitrile to adiponitrile in an electrochemical, preferably undivided, cell containing acrylonitrile dissolved in an aqueous electrolyte, wherein ammonia is added to the cell and oxidized at or near the anode.

Description

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1~)4~5~1 -~ ~ ~ This inventio~ relates to the-elc~rochemic~l hydro, dimexization of acrylonitrile to adiponitrile, wherein ammonia~is oxidized at or near the anodè, More-particularly, . , i this invention relates to the electrochemical hydrodimerization ~ -of acrylonitrile to adiponitrile in an undivided cell, wherein .
- ammonia is oxidized at or near the anode.
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In one aspect of this invention there is provided the process for the electrochemical hydrodimerization of acrylonitrile to adiponitrile in an electrochemical cell con-taining acrylonitrile dissolved in an aqueous-electrolyte, ., .
wherein ammonia is added to the cell and oxidized at or near the anode.
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In another aspect of this invention there i8 provided the process for the electrochemical hydrodimerization of acrylonitrile ~o adiponitrile in an undivided electrochemical cell containing acrylonitrile dissolved in an aqueous-..
electrolyte, wherein ammonia is added to the cell and `~
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oxidized at or near the anode.
Adiponitrile is a very important intermediate in ;;-the production of nylon 6,6. Accordingly, there has been considerable research on new methods of producing this monomer. During the last 10 years, interest has focused on `!, various electrochemical methods of producing adiponitrile from `
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acrylonitrile. (See, for example, Monsanto U.S. Patents 3,193,476 to 483 und Asahi British 1,169,525.) The April 17, 1970 European Chcmical News rcports at pnge 47 that Monsanto has a 45 million pouncl per year plunt using a divided-cell hydrodimerization process for producing adiponitrile.
In these processes, the principal reaction at o:r near the cathode is

- 2 CH2 = CH-CN + 2H20 + 2e ~NC(CH2)4CN + 20H
and the principal reaction at or near the anode is H20 ~ 2H + 1/202 + 2e `:

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~s in all electrochemical reactions, the solutions must be conductive (contain an electrolyte) and the anode reaction must either be compatible with the cathode reaction or isolated from it. However, it is difficult to deslgn - electrochcmical systems where the cathode and anode reactions are compatible 5 since electrolytes stable at one electrode are usually unstable at the other and products of one electrode reaction are usually reactive at the opposite ;
electrode. The preferred electrolytes (tetraalkyl ammonium sulfonates) used in the Monsanto patents have the additional functions of promoting the solubility of acrylonitrile and preventing electroreduction of water at the 10 ~ cathode! These patents prefer the use of a divided cell (e.g., a cation ' exchange membrane), thereby preventing destruction of electrolyte, acrylonitrile and adiponitrile at the anode.
The use of a divided cell has several disadvantages . For examplc, lJ.S. Patcr1t 3,193,~81, points out at column 5, line 66, through column 6, 15 line 27, that alkalinity increases in the catholyte. The alkalinity must be controlled in order to avoid side reactions. In addition to its much higher cost, the divided cell has a relatively large electric-power loss and heat evolution. The total voltage drop of the divided cell is at least double the undivided and the resistivity drop is triple. Further, on a commercial basis, i . 1 20 ~ product buildup is limited to about 15% by weight in a divided cell due to higher electrical requirements as the solution viscosity increases. For example, as the adiponitrile concentr&tion increases from about 15% to 30% by weight in an undivided cell, voltage rises about 1. 5 volts . Other things being equal, voltage rises about 5 volts in the divided cell. Accordingly, the 25; electric-power cost is substantially greater in the divided cel l. Moreover, j separator life is reduced by heat buildup and chemical attack in the warm ; solution.
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~.~)4~581 The general object of this invention is to provide ~ new electro-chemical process for producing adiponitrile from acrylonitrile. The princi- `
pnl object of this invention is to provide n new electrochemical undivided ccll process for producing adiponitrile from acrylonitrile. Other object.s appear hereinafter.
The objects of this invention can be attained by the electrochemical ; hydrodimerization of acrylonitrile to adiponitrile in an electrochemical cell containing acrylonitrile dissolved in an aqueous electrolyte, wherein ammonia is added to the solution in an amount at least equal to that stoich- .' 10 iometrically required for service as a proton replenisher. The present invention, like U.S. Patent 3,699,020, makes use of the fact that ammonia can be oxidized electrochemically at a lower voltage than water. Since ammonia can be oxidized electrochemically without danger of oxidizing electrolyte, acrylonitrile or adiponitrile at the ; 15 anode, it is possible to carry out the process in an undivided cell. Accord~
ingly, when an undivided cell is employed in the process of this invention, :i , there i9 no substantial buildup of hydroxyl ions in the cathode chamber elimin~ting the alkalinity-control problems of the divided cell; there is less hent buildup, lower power costs, lowcr cquipment investment and the pos-sibility of rccovcring more concentratcd ndiponitrile compositions.
In somewhat greater detail, the electrochemical reaction in the present process can be represented as: ;
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(cnthode) 2CH2 = CH-CN + 2H20 + 2e ~ NC(CH2~4 CN -~ 20H

(anode) NH3 + 30H ~ 1/2N2 + 3H20 ~ 3e ~ -(overall) 2CH2 = CH-CN + 2/3NH3~ NC(CH2)4 CN + 1/3N2 , - : .

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~0~581 As indicated above (see the equations above), the present electrochemical process requires the presence of water, which is contin-uously regenerated, and also an electrolyte salt which must be soluble in - the combination of water and aprotic solvent. In general, the higher the concentration of water, the higher the current density. Accordingly, water ; should comprise at least 15% by weight of the electrolysis composition .
, Various aprotic solvents, such as acetonitrile, can be added to the system to increase the soiubility of acrylonitrile in the aqueous-electrolyte.

Suitable electrolyte salts include tetraalkyl arnmonium salts such as ' 10 tetraethylammonium bromide, tetramethyl ammonium chloride, tetrapropyl-- nmmonium bromidc, totrabutyIammonium bromide, tctracthylamrnonium l~aratoluoIlc sulfonato, etc. Of theso, best rosults have been obtllincd with the~ bromides, particularly totraethylnmmonium bromide.

Since acrylonitrile is consumed in this process, acrylonitrile can ~ 15 ; be added to the electrolysis cell through a separate dip tube. If the solution is continuously removed from the cell for recovery of adiponitrile and/or for cycling through a cooling system, acrylonitrile can be injected into the recycled solvent-electrolyte.

. ~mmonia is consumed in our procoss and accordingly should always bc prescnt in some OXC(,S8 concontration ovcr that stoichiomctrically rocluircd ~or the clectro-reduction. However, a high concentration of ammonia lowers selectivity. Accordingly, it has been found convenient to maintain the solvent-electrolyte system about 0.1 to 0.5 molar in ammonia, preferably 0. 2 to 0. 3 molar . This can be maintained approximately by continuously , bubbling ammonia into the solution contained in the electrolysis cell during ,. ' ,, .. ,j ~ . .. ~ , ., . . .... - . . ...
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~04~S8:a JCtiOIl. If the solution is continuously removed ~rom the cell for cycling through a cooling system, ammonia can be conveniently injected into the line, preferably downstream of the pumping means.

In the practice of our invention, carbon anodes, preferably . ~
graphite anodes, have been satisfactory. The carbon may be impregnated with a metal; e.g., platinum, although metals tend to oxidize and then plate out on the cathode. The cathode must be a material exhibiting a high hydrogen over-voltage such as lead, mercury, aluminum, tin, zinc and cadmium. Lead and mercury are especially suitable.
:~; 10 In our preferred system, ammonia is the most easily oxidized entity so that oxidation of halide ion is essentially eliminated. In general, an 'f, impressed current density in the range from about 30 to about 1, 000 amps/
Ft2, and preferably from 300 to 500 amps/Ft , may be employed. Cell voltage, will accordingly vary within the range from 2 to 10, and desirably 3 to 6, volts. Failure to supply ammonia to the system results in oxidation of the halide, formation of copious precipitates, decreased selectivity and low current efficiency.
Electrolysis temperature is not a critical variable and hydrodimer-ization may readily be conductcd within the range from 70 to 140 Ei . The f solvent-electrolyte solution must be fluid at the selected temperature and the soluhility of ammonia must be adequate to satisfy mass transfer (stoich-iometric) requirements. Operation at or near room temperature (80 to 120 F . ) is preferred . During electrolysis part of the solution may be c ontimlously removed from the cell, passed through a cooling coil and ~ returned to the cell.

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As our hydrodimeriæation progresses, the impressed voltage is~
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increased to !maintain the desircd current density whenever there is a slow I -, = . . :
corrosion of the graphite anode (for example, a loss amounting to about 0. 0005 inch per 24 hours) . Salt consumption is low, although it tends to .
increase at low water concentrations while hydrogen evolution occurs at high water concentrations. No separator is required in our system. Indeed, presence of a separator to afford compartments and minimize mixing between reagents could lead to an inoperable system by effecting precipitation of, for example, ammonium bromide on the separator frit, and thus gradually , , :.
shut off the electrochemical reaction. ~ :
When the reduction operation is complete, the cell contents can be transferred to a still and heutcd under mild vacuum to remove aprotic 1. :.
:~ solvent, wRter, ammonia, Llcrylonitrile and adiponitrile. Tetraalkyl ammonium salt is recovered from the residue. ~;
In continuous operation, our process comprises the use of banks of electrolytic cells, each bank including a plurality of cells arranged in a line and having abutting walls to conserve spuce. Each cell is relatively narrow and deep and fitted with thin, flat facing electrodes disposed vertically. One such cell-bank has a number of bielectrodes (anode-cathode p;lirs sealed back to back) of carbon and lead (or some other high hydrogen-overvoltage metal) punched in the center, faced at either end with a sep~aratc . anode and cathode, and spaced by plastic inserts. Solution is introduced at the center of the separate cathodes (or anode) and then flows in parallel, between the bielectrode and out to a reservoir. An exit line transports solution from the reservoir and after addition of ammonia and acrylonitrile, back to the cell-bank.
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-~ Within a cell--bank, only the two end clectrodes havl? electrical connections, i.e., the electrodes are arranged in series. Between cell-banks electrical connections may be either in series or in parallel arrangement.
;~ ï`he solution flow pattern may be arranged in parallel within a cell-bank and '~
in a series between a plurality of banks as required. Provision can be made for cooling the solution outside the cells as needed. Solution may also be recycled, in whole or in part, as required.
The following examples are merely illustrative.

EXA~PLE
.'~ 10 An electrolytic cell was fitted with disc electrodes 9 inches in diametor und ~ inch thick. A graphite anode was disposod hori~ontully near ~, the ~ottom of the ccll. A Icùd cnthodc was punched out at its centor to receive a hollow cylindrical support rod and was maintained parallel to and above the anode by insulating spacers placed therebetween. The separation l -15 .
' between the electrodes was 0 . 03 inch . A suction tube was extended from the cell to a circulating pump. The effluent line from the pump passed through a cooling bath and adapted to returning to the cell through the ; hollow support rod. The effluent line contained Ts for continuously adding ammonia and acrylonitrile downstream from the cooling coil. Sheathed wires connected the electrodcs to a D . C . powcr source . ~ voltmeter and an ammeter were provided in the circuit. The cell was filled with a solution containing 40 grams acrylonitrile, 90 grams water, 120 grams tetraethyl-ammonium bromide and 150 grams acetonitrile. Five thousand c.c. ammonia ~ (3 . 8 grams) was then added to provide a 0 . 3 molar solution of ammonia .

~The composition was electrolyzed at 35A (350 ma/cm ), 4.1 volts (average) and 29 C. for two hours while adding ammonia at 160 cc/min and acrylonitrile i 1 , .. . . . . ..

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i t 1.0 g/min. Aftcr two hours, by analysis, the electrolysis solution con- ;;
.. - tained 21'-~, by weight adiponi-trile, selectivity was 92%, current e~ficiency w~S 83',i and convcrsion w~si 77",.
EXAMPLE II
.. 5 Example I was repeated, except that the solution was electrolyzed ;- ,, at 40A (400 ma/cm ), 4.8 volts (average) and 26 C. for two hours while adding ammonia at 180 cc/min . and acrylonitrile at 1. 3 g/min . After two hours, by analysis, the electrolysis solution contained 24% by weight :~ adiponitrile, selectivity was 91%, current effïciency was 83% and conversion '' 10 was 77%.

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Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The process for the electrochemical hydrodimerization of acrylonitrile to adiponitrile in an electrochemical cell containing acrylonitrile dissolved in an aqueous-electrolyte, wherein ammonia is added to the cell and oxidized at or near the anode.

2. The process of Claim 1, wherein the electrolyte comprises acetonitrile.

3. The process of Claim 1, wherein the electrolyte comprises a tetraalkyl ammonium salt.

4. The process for the electrochemical hydrodimerization of acrylonitrile to adiponitrile in an undivided electro-chemical cell containing acrylonitrile dissolved in an aqueous-electrolyte, wherein ammonia is added to the cell and oxidized at or near the anode.

5. The process of Claim 4, wherein ammonia is added to the cell in an amount at least equal to that stoichiometrically required for service as a proton replenisher in the hydro-dimerization reaction.

6. The process of Claim 4, wherein acrylonitrile is continuously supplied to the cell.

7. The process of Claim 4, 5 or 6, wherein said electrolyte comprises an aprotic solvent.

8. The process of Claim 4, 5 or 6, wherein said aprotic solvent comprises acetonitrile.

9. The process of Claim 4, 5 or 6, wherein said electrolyte comprises a tetraethyl ammonium bromide.

10. The process of Claim 4, 5 or 6, wherein said electrolyte contains at least 15% by weight water.

11. The process of Claim 1, 2 or 3, wherein concentration of ammonia in the aqueous-electrolyte is maintained within a range from 0.1 to 0.5 molar percent of ammonia to the aqueous-electrolyte.

12. The process of Claim 1, 2 or 3, wherein concentration of ammonia in the aqueous-electrolyte is maintained within a range from 0.2 to 0.3 molar percent of ammonia to the aqueous-electrolyte.

13. The process of Claim 4, 5 or 6, wherein concentration of ammonia in the aqueous-electrolyte is maintained within a range from 0.1 to 0.5 molar percent of ammonia to the aqueous-electrolyte.

14. The process of Claim 4, 5 or 6, wherein concentration of ammonia in the aqueous-electrolyte is maintained within a range from 0.2 to 0.3 molar percent of ammonia to the aqueous-electrolyte.