US2537304A - Electrolytic process and apparatus - Google Patents

Description

Jan. 9, 1951 c. coN rr 2 ,537,304

ELECTROLYTIC PROCESS AND APPARATUS Filed Oct. 7, 1946 3 Sheets-Sheet 2 o '0 g g f WWW/ o ilj :g L L T] m r 2 lnveniof PA LC. COND/T Aiiorneys Jan. 9, 1951 P. c2. CONDIT 2,537,304

ELECTROLYTIC PROCESS AND APPARATUS Filed Oct. 7, 1946 5 Sheets-Sheet 5 g 8 Inventor Attorneys Patented Jan. 9, 1951 I ELECTROLYTIC PROCESS AND APPARATUS Paul C. Condit, Berkeley, Calif., assignor to California Research Corporation, San Francisco, Calif., a corporation of Delaware Application October 7, 1946, Serial No. 701,761

4 Claims.

This invention relates to a process and apparatus for electrolytic treatment of chemical compositions and, more particularly, to a process and apparatus for effecting electrolytic oxidation and reduction reactions.

An object of the invention is to provide an improved electrolytic process and apparatus for continuously treating chemical compositions to produce oxidation-reduction reactions with relatively high yields of reaction products while simultaneously maintaining both high current efficiency and superior cell capacity;

Additionally an object of this invention is the provision of an electrolytic cell for treating or producing sensitive chemicals while minimizing undesirable alteration thereof b reducing the contact time required for conversion in the cell.

Another object of the invention is to provide a non-clogging electrolytic cell capable of handling a continuously flowing, thin ribbon or film of an electrolyte carrying the chemical composition to be treated.

It is also an object of the invention to provide a process and apparatus capable of continuous and prolonged operation with substantially no cell poisoning. I

A further object of the invention comprises the provision of a continuous process for cathodically reducing a chemical composition in high yield and with high current efficiency.

Another object is to furnish an electrolytic reduction process and apparatus capable of effecting reduction in low treating time whereby sensitive chemicals may be treated or produced with maximum desired conversions and minimum deleterious alteration.

Additionally, an object is to furnish a cathodic reduction process wherein side reactions are minimized, currentv eificiency is increased, and the cell capacity is enhanced by passing an electric current transversely to the direction of fiow of a hydrodynamically balanced film or ribbon of an electrolyte carrying an electrolytically reducible chemical composition.

A further object is to provide a process of electrolytically treating a chemical composition in thin fluid films or ribbons while avoiding possibilitiesof clogging or impeding proper flow of said films by reason of inadvertent introduction of foreign particles or bodies.

Another object is to provide a cathodic reduction process in which cell poisoning substantially is eliminated.

Other objects and advantages of the invention will be apparent from the following description and drawings, in which Figure l is a semi-diagrammatic illustration of a hydraulic system in ,an electrolytic cell and of a process embodying the principles of this invention. Figure 2 is an enlarged and somewhat detailed cross-section of a presently preferred embodiment of the inven tion. Figure 3 is a diagrammatic fiowsheet of a plant or system for effecting electrolytic reactions in accordance with this invention.

Figure 4 is a transverse sectional view of a modified form of apparatus which may be employed in the practice of the invention- Figure 5 is a fragmentary longitudinal sectional view of a portion of the apparatus illustrated in Figure 4.

Figure 6 is a longitudinal sectional view of a further embodiment of the invention.

Figure 7 is a fragmentary sectional view of a portion of the apparatus shown in Figure 6.

Figure 8 is a longitudinal sectional view of a third form of apparatus which may be employed in the practice of the invention.

The process of this invention utilizes a flowing stream of an electrolyte in hydrodynamic balance between a semi-"permeable diaphragm and a cell electrode, said diaphragm and electrode being in hydraulic balance.

In accordance with a preferred embodimentof therinvention a chemical composition to be treated is dispersed in a liquid electrolyte to form tially to follow the contour of the cell diaphragm surface b selecting a diaphragm having a 'surface which is preferentiall wetby the liquid dis persion. Desirably, .the diaphragm surface should not be wet by the second immiscible liquid in order that the liquid dispersion will spread more readily into the desired film or ribbon, more faithfully follow the contour of the diaphragm surface, and give greater cell efficiency as shown hereinafter; V V

A, clearer understanding of theqforeg oing may be had by reference to Figure 1 of the drawing, wherein it represents a cell diaphragm of the semi-permeable type; i. e., V a diaphragm relatively highly permeable to ions of the electrolytic solution when under electrical potential gradient and relatively impermeable to cell liquids under simple hydraulic pressure. A thin stream of electrolyte I l flows along and follows the contour of the diaphragm surface as illustrated and is in hydrodynamic equilibrium with the hydrostatic pressure of the liquid mercury electrode l2. It should be observed that, as here shown, the electrolyte stream is in the form of a thin film squeezed between the semipermeable diaphragm I ll, on the one hand, and the liquid mercury electrode IE, on the other hand. The hydrostatic liquid pressure of the mercury electrode successively increases with the depth of the mercury and this pressure serves, first, to force the electrolyte stream into a thin film and, secondly, to provide a hydraulic slope for said electrolyte stream causing the same to flow upwardly along the surface of the porous diaphragm as indicated. Desirably, the mercury electrode does not wet the surface of the diaphragm, whereas, as here illus-- trated, the electrolyte perferentially wets said surface and thereby faithfully follows the contour of said surface rather than break away into the liquid mercury electrode. It should be apparent that the liquid electrolyte phase and the liquid electrode phase should be mutually immiscible to avoid dilution or dispersion each by the other. In this system the electrolyte stream flows at such a rate and in such a thickness as will place it in hydrodynamic equilibrium with the essentially static liquid pressures exerted by the liquid mercury electrode transversely to the direction of flow. Cell electric current likewise is passed through said electrolyte stream transversely to the direction of stream flow for effecting the desired chemical change in compositions dispersed or contained in said electrolyte liquid.

The foregoing system has various important advantages in both its electrolytic and mechanical aspects. Mechanically, the cell is nonclogging and automatic in operation features not heretofore obtained, so far as applicant is aware, in any truly continuous cell of the semi-permeable diaphragm type. When and if a foreign particle which normally would clog or plug a narrow passage comparable in thickness to that of the electrolyte stream is introduced into the present cell, the particle does not become mechanically wedged since the fluid pressure of the liquid mercury gives 'way to said particle and allows it to flow along with the electrolyte stream. Even if, by some peculiar circumstance, said particle should become fixed in the line of electrolyte flow, it is likewise apparent that the electrolyte may pass around said particle since the liquid mercury electrode is in hydrostatic balance with the flowing stream. Further, the hydrostatic pressure of the liquid mercury and the hydrodynamic forces of the flowing e ectrolyte stream automatically regulate the thickness of the film for any given flow rate since these forces are in equilibrium. Mechanical adjustments or controls for spacing cell electrodes to regulate film thickness are avoidedregulation of electrolyte flow rate being sufficient.

An additional advantage of this hydrodynamically balanced system is that current density appears to be automatically adjusted and correlated with concentration of reactant to give enhanced current efiiciency and high yields in the electrolytic reduction of organic compounds. The exact explanation for this automatic correlation and for the outstanding efiiciency of this system has not been established, but it is believed that since (1) rate of flow of the electrolyte stream through the cell is maintained substantially constant and (2) the total electric current flowing through the cell may likewise be maintained substantially constant, it is the current density variable which is automatically regulated by this invention to yield improved performance. (It is known that proper regulation of current density is a most important variable for controlling cell efliciency where other factors are held constant as above stated.)

Such automatic regulation of current density can be understood when it is considered that in the electrolytic reduction of organic compounds, the compound being reduced acts as a depolarizer. The electrodes used in cathodic reduction are subject to polarization and tend to develop a relatively high hydrogen over voltage. Polarization at the surface of the electrode increases the resistance to flow of electric current therethrough, and conversely the action of depolarizers serves to decrease the electrical resistance to the current and hence serves to increase current density through the cell at the point where depolarization occurs most effectively.

Keeping in mind the foregoing principles, it will be seen that the concentration of the organic compound being reduced (depolarizer) is greatest in the zone where the electrolyte enters the cell and reduction begins. This concentration is continuously diminished as the electrolyte stream progresses through the cell by reason of the electrolytic action of said cell in chemically converting the organic compound to its reduced form, in which form it no longer acts as a depolarizer. The resulting lower concentration of depolarizer in turn is thought to decrease depolarization and conversely to allow increased polarization at the surface of the mercury electrode as the electrolyte stream progresses through the cell. In accordance with the foregoing theory a highly depolarized, low resistance mercury electrode surface automatically is produced where the electrolyte first enters the cell, and a more polarized, higher resistance mercury electrode surface is obtained when and where electrolytic reduction is substantially completed. Thus, polarization and electrical resistance thus progressively increase from the point of entry along the electrolyte stream to the point of exit, and current density through the stream conversely is progressively decreased from the point of entry to the point of exit or complete reduction. This automatically maintains a relatively high current density at the point of entry and likewise automatically diminishes said current density progressively through the cell in direct correlation with the concentration of unreduced organic compound being treated. This correlation of current density with concentration of unreduced chemical in the flowing electrolyte stream is believed to be responsible for the remarkably high efficiency of the present process and apparatus in effecting cathodic reduction reactions.

It should be noted that one of the preeminent problems which has been encountered in electrolytically inducing reactions with organic compounds has been the accumulation or formation of cell poisons during cell operation. Little is known concerning the nature of these cell poisons nor is the mechanism of their poisoning action adequately understood, but their effect is unequivocally deleterious and the unsolved cell poisoning problem in many instances has been largely responsible for the limited acceptance of proposed electrolytic synthesis of organic com- In the above equations, F) represents a faraday of electricity and the remaining symbols have their usual chemical significance. Cell poisons increase the extent of reaction (2) above or other side reactions and hence decrease either the desired conversion by reaction (1) 01' the efiiciency of the cell; or both. The practical feasibility of the entire electrolytic process de-' pends upon the extent to which reaction (1) can be effected to the exclusion of reaction (2) above, and, in turn, therefore, on the extent to which cell poisoning can be ayoidedother factors being constant. The utilityof the present invention will be readily understood when it is appreciated that inaddition to the various advantages or simplicity and initial cellemciency the process and apparatus disclosed hereinafter are capable of substantially eliminating cell poisoning in a cathodic reduction.

The invention in its broader aspects, particularly the utility of the apparatus, is not limited to cathodic reduction but includes the electrolytic treatment of other compounds for other purposes, such as anodic oxidation. However, in order to simplify the description, the invention is here illustrated by an apparatus and process particularly adapted to cathodic reduction and more especially to the electrolytic selective reduction of only one carbon-to-carbon double bond in the benzene. ring of a phthalic acid to produce a cyclohexadiene dicarboxylic acid. The production of cyclohexadiene dicarboxylic acids specifically is exemplified by production of two alternative compounds in accordance with the following chemical reactions:

boxyhc acid-1,4

Additionally, other electrolytic reductions of organic compounds may be eifected with advantage according to the present invention. Such other reductions have been carried out by the process and in the apparatus of this invention and are exemplified by: reduction of the aldehyde group in glucose to give hexitols; reduction of the double bond in maleic acid to give succinic acid; reduction of the nucleus in pyridine to give piperidine and polypiperidyls; reduction of the cyclic amide group in cafieine to give desoxycaffeine; reduction of the nitro group. in p-nitroaniline to give p-phenylene diamine; reduction of the carbonyl group in acetone to give among other products mercury alkyls.

A preferred form of electrolytic cell embodying the principles of this invention and particularly adapted for effecting cathodic reductions of organic compounds is illustrated in Figure 2 of the drawing. Inspection of this figure will reveal that within a supporting frame having an upper cover plate I 3 and a lower supporting plate l4 secured to each other by through bolts IE is a cell container ll of glass or any other suitable material and desirably cylindrical in shape. Container I1 is clamped between the upper cover plate and the lower supportingframe l4, and in order to prevent relative motion the container is fitted into a gasket groove or slot E8 in the cover plate.

Container l1 forms the cathode compartment for the liquid cathode and is here shown filled with liquid mercury l9. Immersed in the liquid cathode I9 is a semi-permeable diaphragm 2| which in turn serves to form an inner anode com-' partment here illustrated as filled with any suitable anolyte 22. A liquid cooled anode is provided by means of a coiled tube 23, of lead or other suitable metal. This anode and anolyte are cooled by conducting water or similar cooling fluid through tube 23. The anode 23 is suitable connected to a source of direct electric current as illustrated. The electrical connection with the mercury cathode desirably is madethrough an upwardly projecting conduit 26 which is in fiuid communication with the mercury in the cathode chamber.

In the preferred operation of the cell the liquid mercury cathode is circulated through the oathode chamber by way of inlet line 26 and overflow line 21. This continuous replenishment with fresh mercury in the cathode followed by scrubbing of the discharged mercury as hereinafter described serves to eliminate any cell poisons which might accumulate in a particular operation after prolonged operation. However, in the electrolytic reduction of phthalic acid to A3,5-cyclohexadiene transdicarboxylic acid-1,2 poisoning has not been observed in the cell of Figure 2 and mercury circulation may be omitted in certain instances or merely utilized as a precaution against contamination from extraneously introduced contaminants.

Semi-permeable diaphragm 2| desirably is of a type which is not wet by the liquid cathode and preferably has a catholyte contact surface which is preferentially wet by the catholyte. Preferential wetting of the diaphragm surface by the catholyte and non-wetting by the liquid cathode yields better cell performance but is not presently regarded as absolutely essential to operativeness. It has been found that where the catholyte is aqueous and the liquid cathode desirably is mercury (as in the cathodic reduction hereinafter specifically described), a suitable semi-permeable diaphragm is of unglazed porous porcelain.

The diaphragm of Figure 2 is a porous porcelain cup having its upper rim ground plane to insure firm seating in slot or gasket 28. This cup when placed inside the cathode compartment is held firmly against cover plate IS in slot 28 by the buoyance of the liquid mercury cathode. It has been found that the hydrostatic pressure of the mercury tends to force a certain quantity of the electrolyte through the pores of the cup into the anode compartment. This tendency can be largely or entirely overcome by a suitable treatment of the porous cup prior to use.

It has been found for example that the porous porcelain diaphragm can be sealed by soaking first in hot dilute water glass and subsequently in hot dilute sulfuric acid to precipitate silica in the form of a gel or the like within the pores of the cup and form a diaphragm which is relatively highly permeable to ions and relatively impermeable to cell fluids. The porous cup is conditioned after the foregoing pretreatment by running in a conventional cell for about one-half hour with a current of amperes, for example. To further indicate the characteristics of such 2. treated porous diaphragm it is noted that the electrical resistance of the diaphragm after treatment in the foregoing manner rose only slightly from 0.075 to 0.980 ohm. Relative permeability is illustrated by the fact that when an untreated porous cup was placed in dilute sulfuric acid and a vacuum equivalent to inches of mercury drawn on it, the diaphragm passed 110 cc. through its pores in a half hour. The treated cup passed only 3 cc. in the same period and after two hours only 4 cc.

Catholyte is introduced into the cell of Figure 2 at a substantially constant rate through inlet tube 29 and is directed against a surface of porous diaphragm 2 I. As here shown the stream of catholyte 30 preferentially wets the diaphragm surface and flows therealong while being subjected to the hydrostatic pressure of the mercury cathode. This hydrostatic pressure furnishes th hydraulic slope necessary to cause flow of the catholyte along the diaphragm toward the surface of the cathode. The hydrostatic pressure of the mercury cathode simultaneously forces the catholyte into thin films or ribbons which follow substantially the contour of the diaphragm surface. Thus, for any given feed rate the velocity of the catholyte as well as the thickness of the catholyte films or ribbons is governed primarily by the hydrostatic mercury pressure and is such that the catholyte hydraulic system is in hydrodynamic balance with the hydrostatic pressure of the liquid mercury cathode.

In operation, direct electric current flowing through the cell is passed through the catholyte films or ribbons as the catholyte progresses along the diaphragm surface. The direction of this electric current is transverse to the flowing films or ribbons of catholyt and as here shown is substantially perpendicular to their general direction of flow.

It should be observed that as any given volume of catholyte is followed through the cell its electrolytic reduction occurs progressively and new cathode surface likewise is progressively encountered with a minimum of mixing and with only.

a relatively short contact time in the zone of electrolytic action. It is important that the total volume of catholyte subjected to treatment per unit of active cathode surface is extremely small and that contact time likewise is relatively short as compared with ordinary batch or semi-continuous operation. This is of particular advantage where sensitive compounds are treated or formed in the cell, since undesired alteration of such compounds may be largely avoided. For example in the reduction of orthophthalic acid to A 3,5-cyclohexadiene dicarboxylic acid-1,2, it has been found that the latter compound is sensitive at cell operating temperatures and tends to isomerize to the corresponding A 2,6 acid. This.

isomerization is minimized while simultaneously obtaining excellent conversions and high current efficiency by reason of the low contact times attainable with this invention. It should be recalled at this point that the electrolytic current density is automatically correlated with the concentration of unreduced chemical as the catholyte progresses along its path through the zone of electrolytic action. In accordance with theory and the previous explanations given herein, the highest current density occurs in the zone of entry of the catholyte and the lowest current density is found just below the surface of the mercury electrode in the zone of exit where electrolytic action ceases.

It has been found that a plain diaphragm surface tends to permit channeling of the catholyte by allowing flow of a catholyte stream up one side of the cup to the exclusion of other areas. Such channeling decreases the conversion capacity of the cell and suitable baffle means preferably should be provided on or in the diaphragm surfacev to prevent such channeling action. As shown in Figure 2, the outer surface of the porous diaphragm along which the catholyte flows is provided with a helical thread having a square profile to furnish such a baffle means. It will be apparent to those skilled in the art that various other baffle means may be substituted for the foregoing helical threading arrangement.

After reaching the surface of the mercury cathode, the catholyte forms a liquid layer 32 and is discharged outwardly by way of overflow conduit 2?. The treated catholyte may be collected and reduced product recovered therefrom by the method and in the system illustrated in Figure 3.

Performance data exemplifying the utility of the process and apparatus of this invention are given for electrolytic reduction of orthcphthalic acid to A3,5-cyclohexadiene transdicarboxylic acid, 1,2.

OPERATING PROCEDURE At the start of a run. the entire electrolytic cell assembly is placed in a temperature control bath to maintain the cell operating temperature at about 185 F. A sufiicient quantity of electrolyte for the run is prepared by dissolving phthalic anhydride in 5% sulfuric acid. Unless otherwise indicated the-concentration used is 40 grams of anhydride per liter of acid. The electrolyte is heated to about F. and is maintained at this temperature. The cathode compartment of the cell is filled with mercury to the overflow line and the anode compartment is filled with 5% sulfuric acid solution.

The cathode contact is inserted in the side line 26 of the cell, cooling water started through the anode and mercury circulation begun. The catholyte feed is then started, the electric circuit closed, and the amperage adjusted to the desired value. When uniform operation is established,

product is collected and recording of data commenced. After operation is completed lines may be flushed by pumping through distilled water.

CURRENT EFFICIENCY In the electrolytic reduction of phthalic acid the theoretical current requirement is 2 faradays per mol or 0.362 ampere hr. per gram of phthalic anhydride reduced. For a solution of 40 grams of phthalic anhydride per liter, this is equivalent to HA9 ampere hours per liter of electrolyte re- 9 duced. The current efficiency may therefore be calculated by the following formula:

N -current efiiciency in percentage S=feed rate in liters/hr. P=percentage of feed reduced I=applied current in amperes Current eificiency represents the per cent of the total applied current consumed in the desired reaction; and the difference between 100 and the per cent elficiency is the percentage going to side reactions.

In general it had been found that batch operations utilizing 20 amperes substantially reduced 750 cc. of electrolyte per hour. In the following series of runs data for which are given in Table I, the initial run was made under these conditions. However, it was discovered that the cell was much more efficient than batch cells previously run, and current was accordingly reduced in subsequent runs as indicated:

The per cent conversion given in the table is the average of several analyses determined on the product crystallized from the catholyte emerging from the cell. A very small amount of unreduced material could therefore remain in the mother liquor and not be shown in the analysis.

As may be seen, the current efficiency in a continuous cell of the type shown in Fig. 2 was much superior to that obtained in batch operation. In later runs at higher feed rates, better efiiciencies were obtained with complete conversion as will be apparent from the data exploring cell production capacity.

In batch operations utilizing an electrolytic cell substantially identical in size with the continuous cell illustrated in Fig. 2 and utilized in the runs herein described, reduction of about 750 cc. of catholyte per hour was the maximum capacity obtainable with reasonable efficiency. The foregoing series of runs showed that by reason of the enhanced efficiency of the continuous process and apparatus of this invention much greater current efficiency could be obtained by maintaining feed rate corresponding to the production capacity of a batch cell and lowering the current density. A second series of runs revealed that by maintaining the more efficient or a similar ratio of current density to feed but vastly increasing the catholyte feed rate, high efiiciency could be obtained while simultaneously increasing cell capacity many fold. In fact it was found that in general the catholyte would tend to break away from the cup or stream up its sides before the cell capacity was otherwise exceeded. In other words, cell capacity and efficiency are limited primarily by hydraulic capacity and not be electro-chemical phenomena as had been previously experienced. The following results were obtained with a cell utilizing a pulsating catholyte feed pump which tended to cause the catholyte to break away from the diaphragm surface at a flow rate of about 4 liters per hour.

Table II Feed Current R Applied Per Cent p No 37 23," Amps. Conversion EFSES The system for feeding catholyte to the cell was improved to provide smooth catholyte flow whereby the catholyte continued to follow the contour of the diaphragm surface at feed rates exceeding 4 liters per hour. sults were obtained after these changes:

Table III Feed Current Applied Per Cent Run Amps. Conversion 12 5 825 Some escapeof electrolyte from the diaphragm surface of the cell was noticed at 6 liters per hour and this increased slightly as the flow rate was raised. Variations in this escape rate undoubtedly are responsible for minor discrepancies in the foregoing resultsas is the case for data given in Table II. Aswill be observed from the data of Table I'll, even with the minor escape of catholyte from the cell diaphragm, cell efficiency and conversion surpassed that previously obtained.

CATHODE POISONING No evidence of cathode poisoning has been observed in the operation of the electrolytic cells hereindisclosed. In a test run catholyte was recycled and no poisoning was noticed. In previous operations with batch and semi-continuous type cells cathode poisoning became evident the first time electrolyte was returned to the cell.

CONTACT TIME Comparative contact times were determined for a diaphragm of the threaded cup type shown in Figure 2 and a porous cup of the same overall dimension but having a plain non-threaded surface. Average cont-act time was measured by filling the cathode chamber of the assembled cell to the point of overflow with mercury, starting the catholyte feed pump, and measuring the quantity of mercury displaced by the oatholyte in the cell. The measured quantities so obtained are given below for the two types of diaphragm along with contact times calculated with these values at the feed rates involved:

Catholyte, cc.

Contact Reference is now made to Figure 3 of the accompanying drawing which shows in diagrammatic form a system adapted for commercial op- The following reeration and utilizing the principles of this invention. The apparatus comprises an electrolytic cell 1?, similar to that of Figure 2, for inducing the desired organic reaction and an inert gas agitated mixing heater 33 for preparing an electrolyte solution of chemicals to be treated. A feed pump 3 3 conveys the solution to cell I"! by way of cell inlet pipe 35. As here shown, the electrolyte solution is a catholyte and after-reduction of the solute as it flows along the diaphragm in the cathode compartment of the electrolytic cell, the catholyte passes from the cell through discharge conduit 2'! and level controller 36 to any suitable means for effecting product separation. In the form here shown, a chiller 3'? is provided for crystallizing the conversion prodnot from the electrolyte solution and a filter 38 recovers the crystals from the electrolyte.

The electrolyte used in the cell will depend upon the particular electrolytic treatment to be effected and upon the chemical compound selected for the reaction. Many suitable electrolytes, usually aqueous, are known. For various electrolytic oxidation or reduction reactions any of the usual electrolytes may be utilized within the broader aspect of this invention. In the electrolyte reduction of phthalic acid to a cyclohexadiene dicarboxylic acid, the invention embraces an anolyte and a catholyte comprising a dilute aqueous acid solution, desirably an aqueous solution of a poly-basic mineral acid, and preferably sulfuric acid in water. A phthalic acid dispersion solution in the catholyte is formed by dissolving phthalic anhydride or phthalic acid in aqueous sulfuric acid. The reduction of phthalic acid to the desired product occurs by the reaction previously described herein. Since sulfuric acid concentration builds up in the anolyte and decreases in the anolyte, suitable adjustments are made either intermittently or continuously as desired.

In order to avoid contamination of the electrolytic cell with extraneously introduced impurities or with side reaction products which might poison the mercury electrode, it will be found ad vantageous to circulate the liquid mercury in the electrode body to and through a treater herein designated as a cell poison separator. The liquid mercury serves as a carrier which appears to extract poisons as formed in electrolytic reductions. Thus, the cathode mercury together with mercury in the conveying system of conduits, vessels and pumps furnishes a means for transporting contaminants or cell poisons from the liquid cathode body to the cell poison separator 39. More particularly the liquid mercury flows from the cathode body l9 through conduit 4| to a level controller 42 for maintaining the fluid level of the liquid electrode at a constant or predetermined height. The mercury carrier then passes from level controller 42 by way of conduit 93 to the mercury treater 39. This treater may take one of several forms, such as:

(l) A chemical treater for scrubbing impurities or poisons out of the carrier or decomposing the same by chemical action;

(2) A still or fractionatinlg column for separating the mercury from such contaminants by distillation; or

(3) A thermal treater for decomposing organic cell poisons by heat when of the thermally unstable-d type.

The presently preferred form of treater for the circulating mercury cathode comprises a caustic alkali scrubber for removing contaminants and/or 12 cell poisons in this treater. Aqueous caustic alkali solution may be fed to treater 39 by way of line 44 and removed through conduit 56. Preferably the chemical treating agent fills at least a substantial portion of a packed treater 39 to provide a relatively deep liquid body through which the mercury carrier is caused to fall in discrete droplets. As the mercury carrier droplets pass through the body of chemical treating agent contaminants are removed by chemical scrubbing or decomposition. The purified mercury then is collected in the bottom of treater 39 to form a mercury seal which prevents reverse flow and assures discharge of the chemical treating agent upwardly through the separator countercurrently to the flow of mercury. Mercury from the seal is then passed through outlet conduit 41 to level controller 48 and mercury storage 49 which furnishes a constant supply of mercury for recirculation to the cathode compartmtnt of the electrolytic cell. Suitable means such as a mercury recirculation pump 5! is provided for returning the mercury from storage by way of return line 59 to the cathode chamber. As the liquid cathode level is raised by the return of mercury thereto a portion of the mercury liquid is forced to overflow through outlet pipe 4| and again is circulated through caustic scrubber 39.

Despite the fact that recycling of electrolyte has been found to accelerate cell poisoning in processes such as those herein involved, the foregoing system for electrolytic reduction will continue to function with recycled electrolyte at an efficient level which may be maintained for prolonged periods with striking economy in both electrolyte and electrolytic current consumption as well as in product conversion and recovery. This recycle type of operation is illustrated in Figure 3 wherein the electrolyte may be returned from product filter 38 by way of conduit 52, valve controlled by-pass 53 and storage return conduit 54 to electrolyte storage tank 55. The stored electrolyte is returned to the electrolytic cell by way of valve controlled line 55 and mixing heater 33 in the same manner as previously described for fresh feed. Fresh electrolyte and phthalic are introduced into mixer 33 as needed.

In some instances it will be found advantageous to reduce the color body content of the recycled electrolyte and diminish other adsorba'cle contaminants by treatment with a solid absorbent, such as active carbon, acid treated decolorizing clay, or the like. Provision is made for this mode of operation in Figure 3 wherein active carbon is introduced at'58 into mixer 59 and thoroughly contacted with the recycle electrolyte to absorb impurities contained therein. The resulting electrolyte slurry then flows through conduit 5| to filter 62 for removal of the active carbon with its adsorbed impurities and the filtrate is passed to storage through recycle line 55. Alternatively this treatment with solid adsorbents may be effected before product separation as by inserting mixer 59 and filter 62 in level controller discharge line as where the electrolyte solution is above crystallization temperature.

In the operation of the apparatus and process of Figure 3 specific process conditions will vary among the reactants utilized, and even in the case of one type of reactant, preferred conditions may change with different cell structures, with ratio of volume of catholyte to surface area of cathode, as Well as with other variables such as temperature, electrolyte, and concentration in the catholyte of the chemical compound to be reduced. However, to illustrate suitable ranges of conditions for operating the apparatus of Figure 3 in the productionof A 3,5-cyclohexadiene transdicarboxylic acid-1,2 the following data are given.

Preferred operating temperatures for the catholyte are from about 80 C. to about 90 0., although temperatures as low as about 60 C. and as high as about 100 C. may be utilized. The concentration of sulfuric acid in the catholyte may vary from about 3% to about 20% by weight of concentrated sulfuric acid (specific gravity about 1.84). in water, and the anolytemay be approximately the same or higher concentration. From about 2% to about 10% preferably approximately 4% by weight of phthalic anhydride is dissolved or dispersedin the aque-- ous sulfuric acid catholyte solution.

In order to obtain maximum recovery of product it is desirable that 90% or more conversion be effected in the electrolytic cell, and that in product recovery the electrolyte should be chilled to just above its freezing point to make certain that most of the cyclohexadiene dicarboxylic acid is crystallized out and removed by filtration as indicated. Filtrate contains some uncrystallized cyclohexadiene dicarboxylic acid or phthalic acid, or both, and is recycled to avoid loss thereof. It has been found that even when the recycled electrolyte contains residual uncrystallized A 3,5-cyclohexadiene transdicarboxylic acid-1,2, this residual acid may be recycled through theelectrolytic cell without substantial further reduction or loss thereof despite its high degree of cycloolefinic unsaturation. However, some isomerization of the A 3,5 acid may occur to. form A 2,6-cyclohexadiene dicarboxylic acid-1,2.

An alternative form of apparatus for carrying out the process of this invention is illustrated in Figures 4 and 5 of the drawing. Inspection of these figures will reveal that the semi-permeable diaphragm of the electrolytic cell appears in the form of a horizontal cylinder 60 having a radially protruding lip El at each end thereof to prevent escape of the catholyte film. Within the cylindrical diaphragm there is provided an anode 62 immersed in anolyte t3. A cathode chamber is formed by a semi-circular tank 64 containing liquid mercury 66 as the cathode. Catholyte is introduced beneath the surf-ace of the liquid mercury cathode and projected against the cylindrical surface of porous diaphragm 6.0 by means of inlet conduit 67. The catholyte flows along the surface of said diaphragm as indicated and upwardly to the surface of the mercury cathode where it is trapped in catholyte compartment 68 formed by any suitable means such asaglassplate E9 extending below and above the mercury surface; Catholyte is removed from the cell through overflow conduit H and subjected to further processing as hereindisclosed. If desired, cylinder 5i} may be rotatably mounted to furnish fresh diaphragm surface in the zone ofu electrolytic reduction and permit washing or any other desired treatment of the diaphragm surface during the portion of its travel outside the mercury or cathode chamber. Likewise the surface of diaphragm 60 may be fluted or provided with any suitable form of baffles to insure unidissolved for retaining the liquid catholyte in contact with form how of the catholyte thereover and to prevent channelling of the liquid solution.

the porous diaphragm as the catholyte film flows therealong from inlet conduit 19 to catholyte outlet Bl. "As in previous descriptions the preferred cathode 82 is of liquid mercury. The bottom surface of inclined plate 16 desirably is provided with transverse slots 83 to prevent channelling of catholyte and secure substantially even distribution of the catholyte film over the inclined porous diaphragm surface.

In some instances depending upon the compound being treated and the reaction desired, it is possible to substitute other liquid electrodes for the liquid mercury electrode body. For example, other low melting metals or metal alloys may be adopted especially where high hydrogen over-voltage is not essential" to -satisfactory reaction. But where high hydrogen over-voltage is required as in the reduction of phthalic acid to cyclohexadiene"dicarboxylic acid, these metals have not been found satisfactory. However, a

."suitableamalgam such as sodium or potassium amalgam is not precluded where reduction is being effected in alkaline solution. The alkali metal in the amalgam will react with the electrolyte in this type operation, but additional amalgam is formed by electrolysis during the electrolytic reduction reaction so that it may be said that an amalgam cathode surface rather than pure mercury is the effective cathode. In such a system for reducing phthalic acid cyclohexene dicarboxylic acids rather than'cyclohexadiene dicarboxylic acids are produced.

In the mercury treating or cell poison separation step alkaline treating agents other than caustic alkali are operative. Potassium hydroxide is an alternative strong alkaline treating agent for removing impurities from the mercury carrier. Active oxidizing agents illustrated bypotassium permanganate may be utilized and nitric acid has been found operative although it attacks the mercury slightly with resulting increase in metals consumption.

In the modified form of Figure 8, the catholyte stream 05 enters the electrolytic cell by way of inlet conduit 36 and is hydrodynamically balanced between a semipermeable diaphragm 81 and a solid electrode 88 of suitable metal, preferably having a high hydrogen overvoltage, such as lead. The semipermeable diaphragm 8'! tends to float on the electrolyte film B5 flowing along its surface and is positioned over electrode 88 by loosely fitting pins 90. At least the diaphragm electrolyte and insure proper spreading of the film from center inlet conduit 86 outwardly along floating diaphragm 31 to a catholyte collecting "chamber from which the treated solution passes through conduit 92 to a suitable recovery system. If desired, baffle ribs 89 may be spiral instead of concentric to provide a spiral channel or conduit along and through which the electrolyte stream may flow while simultaneously contacting the Figures 6 and 7 show a further modifiedform- 7aelectrode 88 and semi-permeable diaphragm 87.

The.

- The versatility of the process and apparatus herein described is illustrated by application of the electrolytic cell and process of Figure 2 to various reactions as hereinafter described.

16 The catholyte eilluent from each of these runs was chilled to crystallize out the reduced prodnot and the crystals filtered off. In the first above run the crystals were contaminated with metallic REDUCTION OF ALDEHYDE GROUP IN 5 mercury accidentally spilled from the cathode GLUCOSE TO GIVE HEXITOLS This reduction was carried out according to the following reaction in the electrolytic cell of Figure 2:

Glucose to sorbz'tol and manm'tol The conditions chosen for the operation are set forth in Table. IV.

compartment and the product was therefore not worked up completely. In the last two runs above listed the mother liquor was evaporated down to about 100 cc., chilled, and a second crop of m crystals obtained. The amounts and inspections of these products are given in Table VI:

Table VI Crop No.

Equiv. F C

During the run the voltage required to main- The melting point of pure succinic acid is given tain a current of 54 amps. rose from 9.1 to 12.9 43 in the literature as 170 C. and its equivalent volts and hydrogen evolution which was at first very slight increased as the run progressed. The hydrogen evolution continued after the current was interrupted at the end of the run and weight is 59.0. The constants given in the foregoing table are on unpurified samples but are close enough to those of chemically pure succinic acid to be conclusive proof of identity of the while the cell was being washed with dilute acid. product when taken with the method of prepara There was, therefore, an appreciable quantity of sodium amalgam formed and dissolved in the cathode. Analysis of the reduced glucose solution revealed a 27% conversion to sorbitol and mannitol. :5 l

REDUCTION OF MALEIC ACID TO SUCCINIC ACID This reduction proceeds according to the re action: 55

OHCOOH 2H CHzCOOH CHGOOH CHzCOOH Maleic acid Succinic acid Three runs were made under the conditions (30 given in the following table:

tion.

REDUCTION OF PYRIDINE TO PIPERIDINE This reaction requires a high hydrogen overvoltage cathode and may be regarded as proceeding according to the following equation:

GE Cs: 0 CH C CH2 4 11 H A l H CH H: CH1

Pyridine Piperidine Conditions for the run are given in Table VII:

,17. a 18 "Table VII Catholyte Composition Current Feed Rate, Temp: Duration \I, .-1 G Liters/En '7 Percent 35 l ateria rams/llter Amps. Theory "1 V Pyridine 39.5 Q --r---- I r The catholyte product was rendered alkaline and distillediintil'about 300cc. of aqueous distillate'had collected. This distillate was diluted to 500 bciwitnwater and a cc. aliquot titrated with N/2 sodium hydroxide using a Becki na'n pH 5 meter. The titration curve revealed a first region of infiection' at pH 8and a second inflection-at about pH 4.5. These two inflections established that the weaker-base (pyridine) was converted in This pureproduct was identifiedby its melting point asdesoxyca'fieine monohydr'ate; The mono hydrate-was dried at 200 F. under vacuumand identified as deso'XycaiT-eine by a melting point determination. A further check on the identity of this product was obtaned by converting a samplej'o f the unrc-crystallized desoxyc'afieine to its pi'crate and a melting point determination part t'oi the stronger base" (piperidine). Distillaelipbn this derivative aJftrr' twice crystaltion bottoms contained di-"and' poly-pipe'ridyls which separateas a viscous brown oil.--

REDUCTION OF CAFFEINE TO DES'QXY CAFFEINE This reaction. was carried. out in accordance with the following equation:

,liz'ing from the water.

' REDUCTION OF NITRO GROUP To AMINO" v GROUP She reduct on of aromatic nitro compounds to amines was carried out by the following ex emplary'reaction:

QfialF-C Q, C a C 2 (sac tm' 4H 0: CN OHa NH? on on mo 6H +2H20 OH=N'C'N C'H3N- v Caffeine Desoxycafieine h 1' NEE: I L? Conditions for the run in Table VIII is l1ste I p-Nitroaniline D:Ph nY1 ne'diamine hereinbelow:

4 Table VIII 7 Catholyte Composition Current Feed Rate, Temp, g g Liters/Hr. F. Percent Hm Material Grams/liter Amps. Theory 0aiieine. 145. 5 1. 0 90 100 124 0.5 H SO 698 This reaction was effected under the following conditions:

Table IX 0 A Current D t Catholyte omposl' Feed Rate Temp. malon tlon, of Run, Mateml Grams/Liter lhi. Am S Per Cent Hrs.

. Theory p-nitro-aniline c7. 7 1. 0 163 100 127 1.0 H l 90 The reduced catholyte was diluted with water and neutralized with a slurry of calcium hydroxide. The very bulky precipitate of calcium sulfate was filtered off, washed with water and The reduced catholyte was chill-ed in an ice bath saturated with dry hydrogen chloride and filtered. The precipitate was dried under vacuum at 195 F. for twelve hours and identified as the combination of filtrate washings evaporated paraphenylene dianiine dihydrochloride.

on a steam bath under vacuum to about 500 cc. The concentrated solution was filtered, again extracted several times with chloroform and the chloroform solvent removed by heating in an at- The present invention is not limited to specific details set forth herein by Way of illustration, but in view of the numerous modifications which may be efiected therein without departmosphere of nitrogen. Th rty-three grams of a ing from the spirit and scope of the invention,

dark colored solid product was obtained. This crude product was purified by dissolving in 66 grams of 10% hydrochloric acid and again extracted with chloroform to remove organic imonly such limitations should be imposed as are recited in the appended claims.

I claim:

1. An electrolytic cell comprising an anode purities. The hydrochloric acid solution was then 70 compartment containing an anode, a cathode made alkaline with 109 grams of 10% sodium carbonate, treated with active carbon and extracted with chloroform. Removal of the solvent left 16 grams of product which was further puricompartment containing a liquid cathode, a semipermeable diaphragm disposed between said anode compartment and said cathode compartment, said diaphragm having a face subject to fled by recrystallization twice from ethyl acetate, the fluid pressure of said liquid cathode, and elec 19 trolyte inlet means disposed below said diaphragm for admitting a liquid electrolyte into said cathode compartment, said inlet means 156- ing surrounded by said liquid cathode.

2. An electrolytic cell comprising an anode compartment containing an anode, a cathode compartment containing a liquid mercury cathode, a semi-permeable diaphragm disposed between said anode compartment and said cathode compartment, said diaphragm having an irregularface subject to the fluid pressure of said liquid mercury cathode, an electrolyte inlet means disposed below said diaphragm for admitting a liquid electrolyte into said cathode compartment, said inlet means being surrounded by said liquid mercury cathode.

3. A process for subjecting a liquid electrolyte in the form of a continuously flowing thin film to the action of an electric currrent which comprises forming said film initially by introducing said electrolyte into a liquid mercury electrode at a point immediately below an interface of said electrode and a semi-permeable diaphragm, projecting the electrolyte so introduced against the interfacial surface of sa d diaphragm to squeeze said electrolyte into a flowing thin film between said liquid mercury electrode and said diaphragm,

continuing to introduce said electrolyte from the same point to maintain the flowing film so formed, said film being maintained in contact with the surface of said diaphragm throughout the process by the fluid pressure of the liquid mercury electrode, and passing an electric current through said continuously flowing thin fllm of electrolyte transversely to the direction of 5 flow thereof.

20 4. The process of claim 3 wherein the liquid electrolyte comprises a dispersion of a phthalic "acid in aqueous sulphuric acid solution, said solution containing from about 3% to about 20% by weight of sulphuric acid.

PAUL C. CONDI'I'.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 695,302 Gilmour Mar. 11, 1902 699,415 Reed May 6, 1902 735,464 Byrnes Aug. 4, 1903 1,209,835 Greenawalt Dec. 26, 1916 1,411,507 Paulus Apr. 4, 1922 1,538,389 Ewan May 19, 1925 1,927,289 Kyrides et a1 Sept. 19, 1933 1,981,498 Engelhardt et a1. Nov. 20, 1934 2,307,835 Gardiner Jan. 12, 1943 2,316,685 Gardiner Apr. 13, 1943 2,336,045 Taylor Dec. 7, 1943 2,477,579 Condit Aug. 2, 1949 FOREIGN PATENTS Number Country Date 471,912 Great Britain Sept. 13, 1937 OTHER REFERENCES Swann: Transactions of the Electrochemical Society, vol. 69, (1936), page 317.

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