US3692646A - Electrochemical chlorination of hydrocarbons in an hci-acetic acid solution - Google Patents

Abstract

A PROCESS OF ELECTROCHEMICALLY HALOGENATING HYDROCARBONS IN AN AQUEOUS HYDROHALIC ACID ELECTROLYTE COMPRISING PASSING CURRENT FROM A CATHODE TO AN ANODE IMMERSED IN AN ELECTROLYTE COMPRISING A MIXTURE OF AQUEOUS HYDROHALIC ACID AND 10-90 VOLUME PERCENT OF AN ALIPHATIC ACID OF 2-4 CARBON ATOMS. TYPICALLY, THE PROCESS INVOLVES CHLORINATING AN ALIPHATIC, CYCLOALIPHATIC OR AROMATIC HYDROCARBON IN AN ELECTROLYTIC CELL EMPLOYING GRAPHITE ELECTRODES USING A MIXTURE OF HYDROCHLORIC ACID AND ACETIC ACID. USING THE MIXED ACID ELECTROLYTE PREVENTS RAPID DETERIORATION OF THE ANODES AND INCREASES THE SOLUBILITY OF THE HYDROCARBON STARTING MATERIAL IN THE ELECTROLYTE.

Description

United States Patent Office Patented Sept. 19, 1972 3,692,646 ELECTROCHEMICAL CHLORINATION F HYDRO- CARBONS IN AN HCl-ACETIC ACID SOLUTION William B. Mather, Jr., Hopewell Junction, and Edwin R.

Kerr, Wappingers Falls, N.Y., assignors to Texaco Inc., New York, NY. N0 Drawing. Filed Sept. 2, 1971, Ser. No. 177,484 Int. Cl. C07b 9/00, 27/06 US. Cl. 20481 9 Claims ABSTRACT OF THE DISCLOSURE A process of electrochemically halogenating hydrocarbons in an aqueous hydrohalic acid electrolyte comprising passing current firom a cathode to an anode immersed in an electrolyte comprising a mixture of aqueous hydrohalic acid and 10-90 volume percent of an aliphatic acid of 2-4 carbon atoms. Typically, the process involves chlorinating an aliphatic, cycloaliphatic or aromatic hydrocarbon in an electrolytic cell employing graphite electrodes using a mixture of hydrochloric acid and acetic acid. Using the mixed acid electrolyte prevents rapid deterioration of the anodes and increases the solubility of the hydrocarbon starting material in the electrolyte.

BACKGROUND OF INVENTION This invention relates to a process for halogenating hydrocarbons. More particularly, it relates to an improved method for electrochemically chlorinating or brominating hydrocarbons using a modified solvent-electrolyte mixture which increases the solubility of the hydrocarbon reactant in the electrolyte and aids in reducing the rate of deterioration of the electrodes during electrolysis.

Though for the purpose of brevity, the description of the present invention will be, for the most part, specific to chlorination reactions, bromination reactions are substan' tially analogous and they are intended to be included herein.

Electrochemical halogenation, particularly chlorination of organic compounds is well known and widely practiced. The synthesis of chlorohydrin from ethylene and a neutral aqueous chloride solution is a commercial process. The electrochlorination of n-dodecane with aqueous HCl has also been disclosed (Ruehlen et al.--.l. Electrochem. Soc. 111, 1107 (1964)).

Electrolytic chlorinations are carried out in conventional electrolysis cells. The cell consists of a covered vessel containing electrolyte and equipped with an anode and a cathode extending down through the cover into the electrolyte. The cell is divided into interconnected anode and cathode compartments, each compartment being provided with a port to permit escape of gases produced during electrolysis. A source of DC power is connected across the electrodes.

Electrochemical chlorination involves the production of atomic chlorine or the chloronium ion (01+) by electrolytic dissociation of a chlorine-containing compound in the electrolyte. As noted above, the chlorine compound may be one which gives neutral or acidic aqueous solutions. To commence electrochlorination, the compound to be chlorinated is added to the anode compartment and a DC potential is applied across the electrodes. At the cathode, hydrogen gas is formed from the reduction of hydrogen ions, and at the anode, chlorine is generated by dissociation of the chlorine-containing electrolyte. The chlorine generated at the anode reacts with the hydrocarbon present in the anode compartment to give the desired chlorohydrocarbon. Excess chlorine or hydrogen produced during electrolysis is vented through the electrolysis cell ports provided for this purpose.

[In order for electrochemical chlorinations to be conducted in an economically feasible manner, the product yield and purity must be relatively high and the electrolytic cell, especially the electrodes, must be resistant to rapid deterioration during use.

From a standpoint of cell durability, it is preferred to conduct the chlorination in a neutral aqueous chloride solution since such electrolytes do not cause rapid deterioration of the cell and the electrodes. However, the neutral electrolyte process has several drawbacks which have an adverse impact upon its overall efiiciency. The main drawbacks are the lack of solubility of the hydrocarbon starting material in the electrolyte, the relatively poor conductivity of the electrolyte and the problem of by-product alkali which is expensive to separate from residual sodium chloride. Other difiiculties, such as the instability of the chloronium ion, necessary for the chlorination of olefin or aromatic hydrocarbons is much less stable in neutral than in acid solution.

The disadvantages attendant the use of a neutral electrolyte for chlorination of hydrocarbons are substantially avoided when the chlorination is carried out in a simple aqueous HCl electrolyte. Though it is advantageous from one standpoint to use an aqueous acidic electrolyte, aqueous acids cause rapid deterioration of the anode especially when a graphite anode is employed. Moreover, hydrocarbons used as starting materials are not generally soluble in aqueous HCl and since the solubility of the starting material is a determinant in product yields, yields of product based upon current input have not been as high as desired.

'In view of this state of the art, it is an object of the present invention to provide a modified electrochemical chlorination process for converting hydrocarbons to chlorinated derivatives thereof. It is another object of the present invention to provide a new method of electrochemically chlorinating hydrocarbons wherein the elec trolysis cell, especially the electrodes thereof, is not subject to rapid deterioration during electrolysis. Other objects will be apparent from the ensuing description of this invention.

SUMMARY AND DESCRIPTION OF INVENTION In accordance with one aspect of this invention, it has been found that electrochemical halogenation, e.g., chlorination and bromination, can be conducted efliciently in a manner which does not result in rapid deterioration of the electrodes. The method involves conducting the electrolysis in an electrolyte comprising a carboxylic acid of 2-4 carbons such as glacial acetic acid, propionic and/or butyric acids and aqueous concentrated hydrochloric or hydrobromic acid. It has been found that the carboxylic acid provides two beneficial effects. It increases the solubility of the hydrocarbons in the electrolyte and it reduces the deterioration of the anode. Dry hydrogen halides can be used, but higher conductivity and better current efficiencies are achieved with aqueous concentrated hydrohalic acids, e.g., 12 N HCl or HBr. A preferred electrolyte contains 50% by volume of glacial acetic acid and 50% by volume of 12 N HCl. However, beneficial effects are obtained when as little as 10% by volume of carboxylic acid or as much as is in the electrolyte. When the concentration of carboxylic acid is less than about 10%, solubility of the hydrocarbons is very low and deterioration of the graphite anode occurs. Too high a concentration of glacial acetic acid reduces the conductivity of the electrolyte thereby considerably increasing the lvoltage requirements for the reaction and raising the operating costs for the process.

The process of this invention is applicable to aliphatic, cycloaliphatic and aromatic hydrocarbons in general so long as they are soluble in the electrolyte. Examples of useful compounds are parafiins of 3 to 20 carbon atoms such as propane, nonane, cetane, etc.; cycloalkanes of 4 to 12 carbon atoms such as cyclobutane, cyclohexane, etc.; olefins of 2 to 20 carbons such as ethylene, dodecene- 1, etc.; cycloalkenes of 4 to 12 carbons such as cyclobutene, cyclohexene, etc.; and aromatics of 1-3 carbocyclic rings such as benzene, naphthalene, biphenyl anthracene and lower C alkyl or alkenyl substitution products thereof such as toluene, xylene, cumene and styrene.

The hydrocarbon reactant need not be completely soluble in the electrolyte for the chlorination to proceed. However, chlorination by the process-f this invention is more efiicient for olefinic and aromatic compounds, than for the less soluble paraffins. Gaseous, liquid and solid hydrocarbons can be chlorinated so long as the reactant hydrocarbon is brought into intimate contact with the electrolyte. In the case of gaseous hydrocarbon reactants, the hydrocarbon is bubbled into the electrolyte by positioning the reactant feed inlet below the level of the liquid electrolyte in the anode compartment. Solid reactants can be comminuted prior to being introduced 'or dispersed by high shear agitation in the electrolyte cell, and maintained in suspension by agitation during electrolysis. Liquid hydrocarbon reactants which do not dissolve in the electrolytic cell can be suspended in the electrolyte by agitation during the chlorination reaction.

The chlorination process of this invention advantageously can be conducted in conventional electrolysis cells of the aforedescribed type. The electrodes can be fabricated of graphite, preferably of the high density type. One advantage of using a carboxylic acetic acid solvent in the electrolyte is that it permits use of low cost graphite electrodes without the previously encountered serious problem of rapid electrode deterioration due to spalling away of carbon during electrolysis. Not only does this s-ur prising result beneficially influence the durability and efiiciency of the cell, but it also leads to the production of chlorinated hydrocarbon product not contaminated by particles of the graphite electrodes.

The yield of product is determined by amount of current used, time of electrolysis, time to quench, voltage, temperature, electrolyte compaction, and in certain cases, the presence or absence of short wave UV radiation.

The amount of current theoretically required is that which yields the number of electrons required to-eifect a particular chlorination reaction. One equivalent of electrons is required for monochlorination, two for dichlorination, etc. The actual current requirement is based upon the percent current efiiciency, i.e., the number of'electrons used to effect a particular reaction divided by the total electrons used and multiplied by 100. Current efficiency for any particular reaction increases until enough chlorine has been generated to react with the hydrocarbon starting material. The increase in current elliciency appears to be due to an initiation period in which current does not form products, because, it is speculated a certain concentration of chlorinating species (about 0.03 equiv.) must be generated before chlorination occurs. After the initiation period, there is a rapid increase in product formation until nearly all the hydrocarbon has reacted. After the equivalence point, current efficiency declines since there is no more hydrocarbon to react. Thus, the optimal use of current is fortunately achieved at highest starting material conversion levels. The electrolysis can be conducted at any temperature between ambient temperatures and the boiling point of the electrolyte.

In most electrochlorinations in accordance with this invention, current efiiciency and product selectivity are not greatly influenced by incident light and generally the reaction proceeds equally well in room light and with no light. However, the bromination of parafiins is significantly affected by radiation which is capable of dissociating halogen molecules into halogen atoms. Wavelengths shorter than 500 mp will dissociate halogen molecules into halogen atoms to some extent although maximum dissociation is around 340 III/1. or lower. In the case of parafiin halogenation the active species is atomic halogen and therefore the higher level of halogen dissociation aids in the attainment of greater conversion rates for the-paraffin strating material. In the case of olefins or aromatic hydrocarbons, the active species is either the chloronium or the bromonium ion. Since UV light does not facilitate the formation of the latter, and in fact, may inhibit their formation, only a minor or even a negative effect is noticed when aromatics or olefins are halogenated in the presence of light with wavelengths shorter than 500 my.

The major products of the electrohalogenation reaction of the present invention and mono and dihalogenated derivati'ves of the hydrocarbon starting material. In the case of olefin starting materials, the major products are dihalo derivatives of the starting materials, whereas in the cases of paraffin and aromatic compounds the major products are monohalo derivatives of the starting materials. The productsof halogenation can be separated from the electrolyte by conventional extraction or chromatographic separation techniques, the choice of which is dependent upon the nature of the product and the degree of product purity which is desired.

The following examples are presented to further ill-ustrate the present invention.

EXAMPLE 1 Chlorination of dodecene-l An electrolysis cell was made from a laboratory beaker which was fitted with a cover through which was passed a /3" graphite rod anode and a 1" (I.D.) bottomless Pyrex tube extending to A above the bottom of the beaker. Another graphite rod was used as the cathode within the Pyrex tube. The beaker was equipped with a magnetic stirrer and a Teflon-coated'stirring bar.

When the electrolysis was conducted in the absence of light, the beaker was wrapped with black plastic tape to exclude all outside light. A strip of the tape was removable to give an area 2 centimeters wide and 5 centimeters high to admit light into the cell. 8 watt long wave (about 360 m peak) and short wave (about 254 m peak) dis charge bulbs were placed next to the untaped area of the electrolysis beaker. The Pyrex glass in the beaker transmits ultraviolet light down to 280 mp The short wave length UV light had a peak intensity at 254 mp but a long radiation tail into the visible region. The long wave UV light had a peak intensity at 360 m and a short wave cutoff at 300 mp. Since chlorine absorbs radiation below 480 m to form chlorine atoms, both lights dissociated electrogenerated chlorine molecules.

The electrolyte, usually consisting of glacial acetic acid and concentrated aqueous HCl, was mixed with the hydrocarbon in the cell and the solution (or suspension) was stirred vigorously. A current was passed through the cell and voltage and current measurements were taken. After a predetermined time, the current was shut off and the reaction was allowed to stand for some hours in order to allow the chlorine to react further. The mixture was then diluted with an equal volume of water and the reaction quenched with excess sodium sulfite. Products were separated directly or extracted with chloroform and then weighed and analyzed.

Analyses were carried out on an Aerograph model 202 gas chromatograph (T.C. detector) using a 5 ft. by A" column of 20% SE-30 on 60/80 Chromsorb W with isothermal operation. Effiuent peaks were collected and identified by IR or NMR quantitative determinations.

Using the foregoing equipment and procedures, a series of experiments was conducted in which dodecene-l was electrochlorinated in 250 milliliters of an electrolyte consisting of 50% glacial acetic acid and 50% 12 N HCl. Current, concentration, time of electrolysis, time to quench, equivalents of current and light conditions were comparison, two runs were made in which an equivalent amount of sodium chloride was substituted for HCl as the source of chloride ions in the electrolyte. The results are presented in Table H.

TABLE IL-ELECTROCHLORINATION OF N-NONANE, N-DODECANE Gas chromatographic analysis (moles in product) Current Starting Current Products efi. for material used separated UV Chloro- Dichloro- Triehlorochlorination, n-Nonane (moles) (equiv.) (moles) light Nonane nonane nonane nonane percent 056 061 042 sw...- 038 003 9. 056 238 040 sw.--- .025 .012 7.6 140 476 124 SW. 072 044 12. 140 1. 245 144 sw.-.- 011 062 17. 3 140 539 127 sw--.. 120 006 1. 6 140 289 135 sw- 089 042 17. 4 140 2. 00 078 sw.... 018 057 3. 2 140 .887 138 No 125 .011 1. 6

Chloro- Dichloro- Dodecane dodecane dodecane 110 1. 71 088 No- 079 0. 66 110 713 125 sw..-. 073 10. 2 110 1. 033 097 sw---. .060 3. 6 110 1. 29 076 sw.-.- 044 2. 6 110 .478 112 sw-... 079 8. 6 110 597 123 sw.... .064 12. 6 110 1. 47 093 sw.-.. 023 7. 3 110 3.14 107 sw... 014 4. 4 .110 1.255 .082 No .081 .09

experiment were isolated,

varied. Products from EXAMPLE 3 Using the equipment and following the procedures described in Example 1, benzene and cumene were electro- Gas chromatographic analysis (moles in product) Current Starting Current Products 1,2-dieh1o 2-acetoxyefi. for material used separated U.V. Dodec- Chlororododel-chlorochlorination, (moles) (equiv.) (moles) light ene-l dodecane cane dodecane percent 225 1. 00 163 041 41 225 46 166 019 80 113 166 049 007 68 115 074 016 001 60 115 238 075 017 77 113 047 011 002 55 113 392 074 023 113 609 083 019 33 113 331 074 020 57 113 082 015 0003 46 113 117 024 004 48 113 037 0021 003 17 113 179 039 009 113 174 031 009 43 113 065 009 002 34 113 024 0027 2s 113 056 011 003 55 113 284 072 023 68 113 119 027 007 113 406 083 017 50 113 068 009 002 35 046 0306 0029 0004 24 047 0164 044 1w- 0422 0004 0013 0001 19 046 0080 043 1w- 0417 0003 0008 0002 29 .045 .0312 .044 sw..-- .0411 0008 0022 .0003 19 047 0134 046 sw- 0438 0009 0012 0001 26 045 0609 0445 sw. 0396 0014 0029 0005 13 N o attempt to exclude light.

EXAMPLE 2 55 Using the procedures and equipment described in Example 1, n-nonane and n-dodecane were electrochlorinated under varying conditions of current, light, time, etc. and the products were isolated and analyzed. For purposes of chlorinated in 250 milliliters of a 50% glacial acetic acid and 50% 12 N HCl electrolyte. The results are presented in Table III.

TABLE IIL-ELECTROCHLORINATION OF BENZENE AND CUMENE Gas chromatographic analysis Current (moles of product) efi. for Starting Current Products chlorinmaterial used separated Chloroo-Dichlop-Dichloation, Benzene (moles) (eqniv.) (moles) Benzene benzene robenzene robenzene percent Cumene o-Chlorop-Chlorocumene cumene 1 All runs except No. 7 were run in taped beaker (no exposure to light) ultraviolet light (360 mp).

. Run No. 7 was exposed to long wave From the foregoing examples, it can be seen that proding monochlorocyclohexane and dichlorocyclohexane was uct yields and product selectivity are quite high in the obtained. case of olefins and aromatics, and considerably lower in In the cases of Examples and 6, there were no signs of the case of paraffins. Thus, dodecene-l was chlorinated to deterioration of the graphite anode after electrolysis had a mixture containing 1,2-dichlorododecane (70% selec- 5 been completed. tivity) and Z-acetoxy-l-chloro-dodecane (20% selectivity) What is claimed is: representing a 50% yield of the dichloro compound. 1. In the process of electrolytically halogenating hydro- Paratfins were chlorinated in the same system with about carbons in an aqueous hydrochloric or hydrobromic acid 1% current efficiency in the absence of light, but up to electrolyte by passing current from a cathode to an anode 17% current efficiency in the presence of shortwave ultra- 1O immersed in said electrolyte whereby active halogen violet (254 mp. peak) light. Aromatics, like olefins, were species is formed and reacts with the hydrocarbon to prochlorinated with high current efliciency. The initial prodduce the desired chlorinated or brominated derivatives, uct of benzene chlorination was chlorobenzene formed in the improvement which comprises incorporating in the about 40% yield. Further chlorination yielded only o-dielectrolyte between and 90% by volume of an aliphatic chlorobenzene (33%) and p-dichlorobenzene (66%) with carboxylic acid of 2-4 carbon atoms. no more than 5% of the meta isomer. Overall current 2- Th pr ss of claim 1 wherein the anode and cathefliciency for benzene chlorination was therefore between Ode are fabricated 0f graphite and 50%. Cumene was chlorinated in a 50% yield to The process of claim 1 wherein the hydrocarbon i o-chlorocumene (49%) and p-chlorocumene (51%) with n l fi f 220 r n msno more than n-chlorocumene in th product, 20 4. The process of claim 1 wherein the hydrocarbon is Analogous bromination products are obtained when the an aromatic compound of cafbocyclic ringsforegoing procedures are modified by substituting HBr for The Process of Claim 1 wherein the hydrocarbon is HCl in the electrolytic composition. a parafiin of 3-20 carbon atoms.

EXAMPLE 4 v 6. The process of claim 5 wherein the electrolyte is ex- 5 posed to short wave UV radiation during the halogena- Using the equipment and the procedures of Example 1, tion. toluene was chlorinated in two separate runs with and 7. The process of claim 1 wherein the carboxylic acid without glacial acetic acid in the electrolyte. The reaction is glacial acetic acid.

conditions and results are presented in Table IV.

TABLE IV.ELECTROCHLORINATION OF TOLUENE Starting material Current used Run Solvent (mL) plus Voltage Time Product No. Gm. Moles Anode electrolyte (gm.) applied Current Amph Equiv. of elec. sep. (gm) Remarks 1 43.4 .470 graphite rod.-- HOAc (125) plus H01 (125)-..- 4. 3-4.7 .26.30 34.8 1.30 119 43. 1 Rirl lgwstlllqgstitution 2 43.4 .470 .do H20 (125) plus H01 (125) 2.6-5.5 .17-.30 14.9 .56 72 Anode badly attacked.

From the results of Example 4, it can be noted that the 8. The process of claim 1 wherein hydrochloric acid is presence of glacial acetic acid in the electrolyte was critiemployed. cal to the attainment of a substantial amount of the prod- 9. The composition of claim 1 wherein the electrolyte net and the preservation of the graphite anode. is a %-50% by volume mixture of aqueous concen- EXAMPLE 5 trated hydrohallc acid and glacial acetic acid.

Electrochlorination of methylcyclopentane 4,5 References Cited Following the procedure of Example 1 except for the UNITED STATES PATENTS substitution of methylcyclopentane for dodecene-l, a 3,632,489 197 Weinberg at a]. 4 product having the expected monochlorocyclopentane and dichlorocyclopentane was obtained. 0 JOHN H. MACK, Primary Examiner EXAMPLE 6 k R. L. ANDREWS, Assistant Examiner Electrochlorlnatlon of cyclohexane USI CL Following the procedure of Example 1 except for the 204-1571 R substitution of cyclohexane for dodecene-l, a product hav-