Electrolytic hydrodimerization of olefinic compounds

BACKGROUND OF THE INVENTION

It is known that unsaturated compounds such as olefinic nitriles, esters and carboxamides can be hydrodimerized in an electrolytic cell having a cathode compartment and anode compartment separated by a solid cation-permeable membrane. Ordinarily, an aqueous solution containing at least one of the nitriles, esters and/or carboxamides and an electrolytic salt (e.g., a quaternary ammonium or amine salt) is circulated through the cathode compartment of the cell while an aqueous solution of a strong acid (usually a mineral acid such as sulfuric acid) is circulated through the anode compartment. As electric current is passed through the solutions and the intermediate membrane, hydrogen ions from the anolyte permeate through the membrane into the cathode compartment and, in an electrolytic reaction utilizing such hydrogen ions, the nitriles, esters and/or carboxamides are dimerized at the cathode. General conditions under which the electrolysis can be suitably carried out are described in U.S. Pat. Nos. 3,193,476, 3,193,481 and (with particular reference to the electrolytic hydrodimerization of acrylonitrile to adiponitrile) 3,193,480. The disclosures of those patents are incorporated herein by reference.

One of the greatest problems encountered in commercial operation of the electrohydrodimerization process has been the development of a cell-dividing membrance that will satisfactorily perform the required ion-exchange function with a reasonably long and predictable service life. The problem has been complicated by the unusually severe stresses to which the membrance is subjected. For example, the process is generally carried out with current densities that are up to a hundred times as great as those used in more conventional electrolytic processes such as water desalinization and, for best results, with a substantial pressure differential (e.g., 10-15 p.s.i.) between the anode and cathode compartments. The cell is also normally operated with high anolyte and catholyte flow rates which may cause vibration of the membrane, at elevated temperatures (up to 60° C.) and in low pH ranges which may be very different on opposite sides of the cell.

Conventional ion-exchange membranes that are thick enough to withstand the stresses of the process generally have an impractically high resistance to the required flow of electric current. Thinner membranes, on the other hand, normally tend to deteriorate rapidly under such conditions with cracking, spalling and eventual penetration of the membrane after which the anolyte becomes contaminated by constituents of the catholyte and vice versa. The problems caused by such contamination (e.g., complication of hydrodimerization product purification, anode corrosion by the olefinic compound from the catholyte, difficulty in controlling catholyte concentrations and pH after leakage of aqueous acid from the anolyte, etc.) soon become so severe that they necessitate costly procedures for decontamination of the anolyte and catholyte as well as process interruptions for membrane replacement.

A variety of membrane-reinforcing materials have been tested in attempts to improve the length and consistency of membrane service life and thereby reduce contamination of the anolyte and catholyte without an intolerable sacrifice of process efficiency. For example, membranes containing a nonwoven fabric embedded within a polymeric matrix (as described in U.S. Pat. No. 3,356,607) have been tried without significant success. The use of a plurality of thin unreinforced membranes has also failed to adequately improve the length and reproducibility of membrane service life. In the absence of a satisfactory substitute for the conventional varieties of polymeric ion-exchange membrane material, a commercially acceptable type of reinforced membrane is very desirable, and it is an object of this invention to provide an electrohydrodimerization process utilizing such a reinforced membrane.

SUMMARY OF THE INVENTION

It has now been discovered that the aforedescribed electrohydrodimerization process can be carried out with a substantially longer and more consistent average membrane service life and with significantly lower levels of anolyte and catholyte contamination when the polymeric ion-exchange membrane is reinforced by a plurality of substantially parallel sheets of woven glass fabric. Thus, in generic scope, the present invention provides a process for producing hydrodimers of olefinic compounds which comprises passing electric current through an aqueous olefinic nitrile-, ester- or carboxamide-containing catholyte separated from an aqueous acidic anolyte by an ion-exchange membrance comprising a solid electrically conductive polymeric cation-permeable matrix reinforced by at least two substantially parallel sheets of woven glass fabric embedded within said matrix.

DESCRIPTION OF THE DRAWING AND PREFERRED EMBODIMENTS OF THE PROCESS

The invention may be more easily understood by reference to the following detailed description taken in conjunction with the accompanying drawing in which FIG. I is a perspective view of a rectangular piece of an ion-exchange membrane having a polymeric matrix partly broken away to show a plurality of layers of glass fabric embedded therein, FIG. II is a schematic vertical section view of an electrolytic cell assembly in which the process of this invention can be carried out with a membrane of the type shown in FIG. I, and FIG. Ill is a graph in which the durability of the membranes used in the process of this invention is compared with that of other membranes heretofore suggested for similar use by plotting the water leakage rate of each membrane as a function of the length of time it has been in use in the electrohydrodimerization process.

In FIG. I, membrane 3 which is composed of a solid polymeric ion-exchange material having a substantially uniform thickness is partially broken away to show an upper layer 4 and a lower layer 5 of woven glass fabric. The polymeric material can be of any composition that permits a suitable rate of permeation of cations (e.g., hydrogen ions) from the anolyte to the catholyte of the electrohydrodimerization cell described herein. A variety of polymeric materials having the desired general characteristics and methods for their preparation are described in U.S. Pat. No. 2,731,411. A composition preferred for use as the membrane matrix in the present invention may be prepared by polymerizing a mixture of compounds containing at least about 20 mol percent and preferably between about 30 and about 80 mol percent of one or more polyvinyl aromatic compounds e.g., divinyl benzenes, divinyl naphthalenes, divinyl diphenyls, alkyl-substituted derivatives thereof, etc.) and less than 80 mol percent of other monovinyl compounds which copolymerize with the polyvinyl aromatic compounds (e.g., styrene, vinyl naphthalenes, alkylsubstituted derivatives thereof, etc.) while maintaining the mixture in a suitable inert solvent (e.g., an aromatic hydrocarbon such as diethylbenzene) under conditions preferably preventative of evaporation of the solvent. Polymerization can be effected with any of the well-known expedients such as pressure, heat (e.g., 50°-100° C.) and/or a catalytic accelerator such as benzoyl peroxide, and is continued until an insoluble, infusible gel is formed substantially throughout the solution. The resulting gel structure is then sulfonated in a solvated condition and preferably to such an extent that there are not more than about four equivalents of sulfonic acid groups formed for each mol of polyvinyl aromatic compound in the polymer and not less than about one equivalent of sulfonic acid groups formed for each 10 mols of vinyl aromatic compound in the polymer.

In the production of a membrane of the type shown in FIG. 1, the polymeric matrix preparation just described is conveniently carried out after glass fabric sheets 4 and 5 have been arranged in desired position within the mixture of polymerizable compounds and solvent. Sheets 4 and 5 have

3,616,319 the visibly open structures (preferably but not necessarily 50-75 percent of total fabric area) that are characteristic of woven fabrics and are woven from glass fibers which are preferably staple fibers but which may be alternatively prepared from continuous filaments. Fabric weave and weight may be varied widely, although all X 14 fiber weave and a weight of 5-15 ounces per square yard are exemplary. The choice of a particular glass fabric depends on the desired properties of the membrane. Fiber orientation in the sheets can be parallel or biased and, if desired for greater rigidity, the filaments of adjacent sheets can be entangled by a needling or stitching operation before polymerization of the matrix.

In a particularly preferred method for preparation of the membrane, 9-ounce 21X14 thread count woven glass cloth is treated with a sizing compound (e.g., the mixture of methacrylic acid and chromium chloride known in the art as Volan) for greater polymer-glass adhesion and then cut into sheets having dimensions suitable for the intended use (e.g., 36 inches X40 inches for an electrohydrodimerization cell of preferred size). At least two of the sheets of sized glass cloth are horizontally stacked on a glass plate in a polymerization vessel having horizontal dimensions slightly larger than those of the sheets after which filaments of adjacent sheets may be entangled by needling or stitching. A second glass plate is then laid on the sheets of glass fabric and the stack of glass plates and sheets of glass fabric are covered with a mixture containing approximately equal amounts of polymerizable compounds and solvent plus a catalytically effective amount of a polymerization catalyst of the type described hereinbefore. The mixture is heated to a suitable polymerization temperature (preferably 80°-90° C.) and then maintained at such a temperature for several hours after which the resulting solid unfractured gel-like matrix with the glass fabric sheets embedded there in is removed from between the glass plates. Polymerization solvent can then be removed by washing although it is generally advantageous to replace it directly with a suitable sulfonation solvent (e.g., a hydrocarbon such as heptane) by leaching the polymerized gel in such a solvent.

Thereafter, the gel is rendered selectively cation-permeable by treatment with a suitable sulfonating agent such as sulfuric acid containing dissolved sulfur trioxide, e.g., at 50°-60° C., until sulfonation of the aromatic nuclei in the gel has taken place to the extent described hereinbefore. The sulfonating agent and solvent are then washed out of the gel, preferably by immersion in water which prevents drying and possible cracking of the gel before cell installation. Removal of solvent from the gel leaves a microporous structure in which the proportion of pore space determines the water transfer properties of the membrane during cell use. When the polymerization step is carried out with a mixture containing a proportion of solvent within the aforedescribed range, the resulting membrane normally has a degree of porosity which permits the iransfer of between about 20 and about 80 (even more generally 30 to 55) milliliters of water per Faraday of current passing through the membrane, as measured with a current density of 0.5 ampls/cm.² in a cell having aqueous 0.5M sulfuric acid in both anolyte and catholyte compartments. For use in the process of this invention, a membrane reinforced by two or three sheets of glass fabric is preferably at least about 0.1 centimeter thick for satisfactory rigidity but generally not more than about 0.25 centimeter thick to avoid the undesirable brittleness of a membrane containing too high a proportion of the polymer.

FIG. II schematically illustrates a system in which a membrane of the type shown in FIG. 1 can be employed in the electrohydrodimerization of one or more olefinic nitriles, esters and/or carboxamides. The FIG. II system includes an anode leaf 6 and a cathode leaf 7 which are assembled together and clamped under sufficient pressure to prevent fluid leakage. Anode leaf 6 has a plate-like anode 8 mounted on its right side and a plate-like cathode 9 is similarly mounted on the left side of cathode leaf 7. Anode 8 and cathode 9 can be made of any suitable electrode materials, e.g., metals such as lead or lead alloys. The ion-exchange membrane 10 is sealingly clamped between the peripheral portions of anode leaf 6 and cathode leaf 7, separating the space between leaves 6 and 7 into an anode compartment 11 and a cathode compartment 12.

In operation, an aqueous anolyte containing an acid such as sulfuric acid is pumped by anolyte pump 13 from surge tank 14 into anode compartment 11 through which it flows upwardly in contact with anode 8 and membrane 10 and thereafter back to tank 14. An aqueous catholyte containing the olefinic nitrile, ester and/or carboxamide and preferably an electrolytic salt (e.g., a quaternary ammonium or amine salt such as a tetraalkylammonium alkylsulfate or sulfonate) is similarly pumped by catholyte pump 15 from surge tank 16 into cathode compartment 12 through which it flows upwardly through the space defined by cathode 9 and membrane 10 and thereafter back to tank 16. As electric current is passed between anode 8 and cathode 9 via the anolyte, membrane 10 and the catholyte, hydrogen ions from the anolyte permeate through membrane 10 into the catholyte in which they participate in the electrolytic dimerization of the olefinic compound at the cathode 9.

In commercial operation, the cell is run continuously with the dimerization product continuously withdrawn from tank 16, oxygen and other gases allowed to escape from tank 14, and fresh acid and fresh olefinic feed continuously fed to tanks 14 and 16, respectively. It is in such operation that the tendency toward deterioration of the membrane 10 (primarily on the cathode side) is especially pronounced, the need for a consistently long membrane life is critically important, and the deterioration resistance of membranes reinforced by a plurality of sheets of woven glass fabric is unexpectedly great. For example, under normal operating conditions, the service life of such membranes has been generally two or more times the average service life of membranes having only one sheet of glass fabric similarly embedded in a matrix of the same polymeric material. Advantages of that kink and others will be apparent from the following examples which are included for purposes of illustration only and do not represent any limitations on the scope of the invention. All evaluations of water transfer and water leakage rates of the membranes in the examples are on the basis of 100 cm.² of membrane surface area except where noted otherwise.

EXAMPLE 1

Acrylonitrile was continuously electrohydrodimerized to adiponitrile as described hereinbefore in a series of plant-scale cells each utilizing an aqueous sulfuric acid anolyte, tetraethylammonium ethylsulfate as the electrolytic salt in the catholyte and an ion-exchange membrane made of a solid matrix of sulfonated divinylbenzene-styrene copolymer (0.106 cm. thick) reinforced by two substantially parallel sheets of woven 9-ounce 21x14 thread count glass fabric embedded within the polymeric matrix by the membrane preparation procedure described hereinbefore. The glass fabrics were made of staple fibers with parallel fiber orientation and the membranes had a porosity which permitted the passage of 32 milliliters of water from anolyte to catholyte per Faraday of electric current transmitted through the membrane, as measured with a current density of 0.5 amps/cm.² of membrane surface area in a cell having aqueous 0.5M sulfuric acid in both anolyte and catholyte compartments. The deterioration of the membranes was determined by measuring the rate of water leakage through each membrane with a hydrostatic differential pressure of 4 p.s.i. on the anolyte in the cell. After the membranes had been in use for 2,200 hours, their average water leakage rate was about 0.2 milliliters per hour per 100 cm.² of membrane surface area.

EXAMPLE 2

When the procedure of example 1 was repeated with the exception that the membranes were 0.14 cm. thick and had a porosity that permitted passage of 42 milliliters of water per

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Faraday of current, there was no measurable water leakage rate after the membranes had been in use for 1,500 hours.

EXAMPLE 3

When the procedure of example 1 was repeated with the exception that the membranes were 0.14 cm. thick and had a porosity that permitted passage of 56 milliliters of water per Faraday of current, the average water leakage rate was 0.2 milliliters per hour after the membranes had been in use for 1,400 hours.

EXAMPLE 4

When the procedure of example 1 was repeated with the exception that the membrane contained three sheets of the same type of woven glass fabric and had a thickness of 0.22 cm. and a porosity that permitted passage of 44 milliliters of water per Faraday of current, there was no measurable water leakage rate after the membranes had been in use for 1,500 hours.

COMPARATIVE EXAMPLE A

When the procedure of example 1 was repeated with the exception that two back-to-back membranes each containing one sheet of the woven glass fabric and having a porosity that permitted passage of 49 milliliters of water per Faraday of current were used in place of the membrane containing two glass fabric sheets, the average water leakage rate was about 0.2 milliliters per hour after 80 hours, 1.8 milliliters per hour after 575 hours, 2 milliliters per hour after 1,030 hours, and 4 milliliters per hour after the membranes had been in use for 1,600 hours.

COMPARATIVE EXAMPLE B

When the procedure of example 1 was repeated with the exception that the membranes each contained only one sheet of the woven glass fabric and had a thickness of 0.063 cm. and a porosity that permitted passage of 29 milliliters of water per Faraday of current, the average water leakage rate was about 2.7 milliliters per hour after 350 hours, 3 milliliters per hour after 540 hours, and 5.2 milliliters per hour after the membranes had been in use for 824 hours.

COMPARATIVE EXAMPLE C

When the procedure of example 1 was repeated with the exception that the membranes each contained only one sheet of the woven glass fabric and had a thickness of 0.06 cm. and a porosity that permitted passage of 50 milliliters of water per Faraday of current, the average water leakage rate was about 1.8 milliliters per hour after 820 hours and 7.25 milliliters per hour after the membranes had been in use for 960 hours.

COMPARATIVE EXAMPLE D

When the procedure of example 1 was repeated with the exception that the membranes each contained only one sheet of the woven glass fabric and had a porosity that permitted passage of 63 milliliters of water per Faraday of current, the average water leakage rate was about 2.5 milliliters per hour after 550 hours and 4.25 milliliters per hour after the membranes had been in use for 639 hours.

COMPARATIVE EXAMPLE E

When the procedure of example 1 was repeated with a membrane that contained only one sheet of 20-ounce woven glass fabric and had a thickness of 0.101 cm. and a porosity that permitted passage of 50 milliliters of water per Faraday of current, the membrane split after it had been in use for 168 hours.

COMPARATIVE EXAMPLE F

When the procedure of example 1 was repeated with the exception that the membranes each contained one sheet of woven Teflon cloth sandwiched between two sheets of nonwoven polypropylene fabric and had a porosity which permitted passage of 38 milliliters of water per Faraday of current, the average water leakage rate was about 1.9 milliliters per hour after 525 hours, 2.1 milliliters per hour after 700 hours, 2.8 milliliters per hour after 1,280 hours, and 4 milliliters per hour after the membranes had been in use for 1,780 hours.

COMPARATIVE EXAMPLE G

When the procedure of example F was repeated with the exception that the Teflon cloth was replaced with a sheet of woven glass fabric and the membranes had a porosity which permitted passage of 44 milliliters of water per Faraday of current, the average water leakage rate was about 0.4 milliliters per hour after 525 hours, 0.6 milliliters per hour after 700 hours, 1.2 milliliters per hour after 1,280 hours, 2.8 milliliters per hour after 2,000 hours, and 3.7 milliliters per hour after the membranes had been in service for 2,050 hours.

COMPARATIVE EXAMPLE H

When the procedure of example G was repeated with the exception that the membranes had a porosity which permitted passage of 49 milliliters of water per Faraday of current, the average water leakage rate was about 1.0 milliliters per hour after 700 hours, 1.6 milliliters per hour after 860 hours, 3.1 milliliters per hour after 1,020 hours, 7.3 milliliters per hour after the membranes had been in use for 1,240 hours.

The results of examples 1-4 and comparative examples A-H are graphically represented in FIG. Ill from which it is readily apparent that the durability of membranes containing at least two sheets of woven glass fabric is significantly greater in the process of this invention than that of ion-exchange membranes having other types of reinforcement in similar process use. In particular, FIG. Ill demonstrates that the leakage rate of the other membranes (e.g., those having only one sheet of glass or Teflon fabric, with or without additional reinforcement by nonwoven fabrics) increased during cell service at a rate of at last about seven times the leakage rate of the membranes used in the process of this invention.