US3437570A - Electrolytic polymerization of aromatic compounds - Google Patents

Description

United States Patent 3,437,570 ELECTROLYTIC POLYMERIZATION 0F AROMATIC COMPOUNDS Norvell E. Wisdom, Jr., Elizabeth, N.J., assignor to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Filed Dec. 11, 1967, Ser. No. 689,350 Int. Cl. B01k 3/00 US. Cl. 204-59 8 Claims ABSTRACT OF THE DISCLOSURE Polymeric aromatic compounds are prepared by electrolyzing a liquid solution of a C to C aromatic compound, such as biphenyl, and a ternary complex having the formula RzHXz2AlX where R is a C to C aromatic compound at least as basic as the aromatic in solution and X is chlorine or bromine in which the. potential of the working anode (vs. the saturated Cu:Cu Cl is 1.15 for the first two to ten minutes and then is 1.05 to 1.10 and in which the AlCl is preferably contained in a porous wall container.

Cross reference This case is related to Ser. No. 549,482, filed May 12, 1966 by David G. Walker and Norvell E. Wisdom, Jr.

Background This invention relates to a process of electrolytically polymerizing and oligomerizing aromatic compounds and more particularly relates to methods for improving current efficiency in such a process.

In the application Ser. No. 549,482, supra, there is described and claimed a process for the electrolytic polymerization of C to C aromatic compounds, such as benzene, toluene, xylene, biphenyl, mesitylene, and the like in which the electrolyte is a ternary complex having the formula RzHXz2AlX where R is a C to C aromatic compound at least as basic as the aromatic compound being polymerized and X is chlorine or bromine.

The efficiency of the current for this synthesis has been found to vary from 10 to 100% depending upon the conditions (calculated on the basis of a requirement of two electrons for each original aromatic molecule incorporated into the polymer in the center of a chain and one electron for each such molecule on the end of a chain. It has also been found that the se variations are due at least in part to a complex variation of the potential of the anode at which synthesis occurs i.e. the working electrode, with current density and time. However, no polymer can be formed at an anode whose potential is kept too low throughout the course of an experiment.

Summary In accordance with this invention it has been found that a current efficiency of 90 to 100% in the electrolytic polymerization of aromatic compounds using R:H:2AlX as the electrolyte may be reproducibly achieved by making the potential of the working anode 1.15 volts (vs. the saturated Cu:Cu Cl electrode.) for two to ten minutes at the beginning of the electrolysis, then reducing the electrode potential to 1.05 to 1.10 volts (vs. the saturated Cu:Cu Cl electrode) for the remainder of the electrolysis. The potential variation required can be advantageously and automatically accomplished by controlling the current density of the working electrode. The saturation of the electrolyte, specifically the anolyte, is maintained by pro viding a porous walled reservoir within the anode for holding solid aluminum chloride.

Preferred embodiments Thus, in accordance with the invention, described in Ser. No. 549,482, high yields of polymeric aromatic hydrocarbons are prepared by electrolyzing a solution comprising a C to C aromatic compound and a ternary complex having the formula R:HX:2AlX wherein R is a C to C aromatic compound at least as basic as the aromatic to be polymerized, and X is selected from the group consisting of chlorine and bromine. The overall electrolytic reaction may be illustrated, with respect to the preparation p-sexiphenyl, by the following expression:

3C H (biphenyl) C H (p-sexiphenyl) +2H (1) with p-sexiphenyl being produced at the anode and hydrogen gas at the cathode.

The ternary complex used in this invention has many interesting properties, among which two are particularly important to this process: (1) the ability to exist as an ionic phase according to the following equation:

which imparts a relatively high degree of electrical conductivity to the ternary complex, thereby allowing the ternary complex to function as the electrolyte in the electrolysis reaction; (2) the. ability to dissolve substantial amounts of aromatic compounds, over and above the amount required to form the ternary complex. The dissolved aromatic compounds may be referred to as excess aromatics and the solution produced thereby will be referred to as the complex phase.

The polymers which may be formed by utilizing the process of this invention are quite varied and dependent upon the material used as the excess aromatic in solution. Generally, the excess aromatic may be a C to C aromatic compound. Preferably, the aromatic may be selected from the group consisting of benzene, biphenyl, naphthalene, alkyl substituted benzenes, naphthalenes, naphthalenes, and biphenyls, and halo derivatives thereof,

the hydrocarbon compounds being preferred. While alkyl (llH 0 H I C H; C H;

from mesitylene, and various dimers, trimers, and higher oligomers from ferrocene o-xylene m-xylene p-xylene, 1,2, 4-trimethyl benzene, l,2,4,5-tetramethyl benzene, chlorobenzene and the like. The polymers produced herein are normally characterized by the loss of hydrogen at the coupling site.

The ternary complex which functions as the electrolyte in this process is represented by the formula:

wherein R is a C to C aromatic compound at least as basic, and preferably more basic than the aromatic to be polymerized. Preferred aromatic compounds are selected from the group consisting of benzene, biphenyl, naphthalene, alkyl benzenes, naphthalenes, and biphenyls, and halo derivatives thereof, preferably a hydrocarbon, more preferably C to C alkyl benzenes, and still more preferably C to C alkyl benzenes; and X is selected from the group consisting of chlorine and bromine. The basicity of a compound, as used herein, designates the tendency of that compound to accept a proton, i.e. the greater the basicity, the greater the tendency to accept a proton. Illustrative of the aromatic hydrocarbons which may be used as R in the ternary complex and listed in the order of increasing basicity are: benzene, biphenyl, toluene, xylene, pseudocumene, hemimellitene, durene, mesitylene, prehnitene, isodurene, pentamethylbenzene, hexamethylbenzene. Other compounds which also may be used are: isopropyl benzene, l,3,S-dimethylethylbenzene, the ethyl toluenes, methylnaphthalene, dimethylnaphthalene, ethylbenzene. A review of the relative basicities of methylbenzenes and the method used for determining basicity is presented in Ehrenson, J. Am. Chem. Soc. 84, 2681- 2687 (1962). For example, when biphenyl is the excess hydrocarbon, R may be biphenyl but preferably is more basic, e.g. toluene. However, more highly substituted alkyl benzenes, i.e. the C to C alkyl benzenes, are normally preferred in p-sexiphenyl production.

The ternary complex may be prepared in substantially pure form by mixing a suitable aromatic compound, as described above, with a stoichiometric excess of HCl or HBr and an aluminum halide, i.e. AlCl AlBr at a temperature between 50 C. and +30 C. A preferred method for preparing the ternary complex consists of mixing the aromatic compound with an anhydrous aluminum halide powder at room temperature. The mixture is stirred and anhydrous HCl or HBr is allowed to bubble through the mixture. It is necessary to provide a stoichiometric excess of both the hydrogen halide and aluminum halide to insure that all of the aromatic compound will be reacted. (The use of less than a stoichiometric amount of aluminum halide will tend to the formation of monomer complexes, wherein the aromatic2hydrogen halidezaluminum halide ternary compound will form in the mole ratio of 1:1:1. Impure compounds with an HXzAlX ratio of less than 1:2 may also form, but are not desirable in the process of this invention. The monomer complexes are not applicable to the process of this invention, unless restricted to the cathode chamber only of a cell with a porous diffusion barrier between anode and cathode. Electrolysis of an aromatic saturated monomer complex yields hydrogen evolution at the cathode and chlorine evolution at the anode along with the formation of chlorinated products at the anode.) Substantially pure ternary complexes prepared by either of the foregoing procedures will be saturated with respect to hydrogen halide and aluminum halide; however, the presence of these compounds at saturation will be small and will not be detrimental to the process of this invention.

The complex phase may be readily prepared by mixing excess aromatic with the ternary complex. Since the ternary complex is capable of dissolving excess aromatics, the complex phase will comprise a solution of excess aromatic and ternary complex. Normally, the ternary complex is capable of dissolving about five to seven moles of excess aromatic before saturation, depending upon the excess aromatic employed. However, in the case of hiphenyls or naphthalenes, the ternary complex will dissolve only about three moles of excess aromatic. In ordinary circumstances the complex phase should comprise at least 0.5 mole, and preferably 1.0 mole of excess aromatic per mole of ternary complex. Particularly preferred, however, is a ternary complex saturated with excess aromatic. Addition of excess aromatic above that required to form a saturated complex phase will not be deleterious, but will not enter into the electrolysis reaction since a separate non-conductive phase containing the excess aromatic will form. As noted, the ternary complex is capable of dissolving excess aromatic hydrocarbons. Therefore, the preparation of the complex should be carried out to keep amount of excess R, i.e. over and above that required to form the ternary complex, to a minimum, thereby avoiding undesirable side reactions during electrolysis.

The electrolysis may be carried out in any suitable type of cell, either with or without a diffusion hindering membrane. The aromatic polymer will for-m at the anode, or in the anode compartment when a membrane is employed. The anode is preferably selected from the platinum group metals, i.e. platinum, palladium, rhenium, ruthenium, osmium, iridium, or tantalum. Platinum, however, is particularly preferred. The cathode may be of any convenient material, e.g. aluminum, carbon; however, the platinum group metals are also preferred for the cathode. It is also possible, and in some instances economically desirable, to utilize base metals as the electrodes. When using base metals, they are preferably plated with one of the platinum group metals.

Diffusion hindering membranes may be utilized, if desirable. In general, such membranes may be of any material that is chemically inert to the ternary complex and will form a diffusion barrier while allowing ion transfer. Examples of such membranes are numerous, among which are: fritted glass, sintered glass, asbestos, porous ceramics, e.g. Alundum, zirconia, porous plastics, e.g. cellophane, paper products, e.g. parchment, perforated metals, and the like. When a diffusion hindering membrane is used, the ternary complex can be used in both compartments to function as the electrolyte. However, if desirable, the ternary complex need only be used in the anode compartment, while another electrolyte can be used in the cathode compartment. Generally, any electrolyte may be utilized in the cathode compartment which will not destroy the ternary complex at the interface. A preferred electrolyte is the monomer complex, which inhibits side reactions and produces only hydrogen at the cathode.

The operating conditions for the electrolysis reaction are not critical and may vary over a wide range. The reaction temperature need only be such that the reaction is effected in the liquid phase. Generally, however, temperatures will range from about -l0 C. to about +100 C., preferably about 0 C. to 50 C., and still more preferably, at room temperature, i.e. about 18 to 26 C. Pressure may also vary widely, i.e. from about 0.5 atm. to about 10 atm. and preferably at atmospheric pressure.

The efiiciency of the current for the synthesis described in Ser. No. 549,482 has been found to vary from 10 to 100% calculated on the basis of a requirement of two electrons for each original aromatic molecule incorporated into the polymer in the center of a chain and one electron for each such molecule on the end of a chain.

The present invention provides a means for overcoming these variations and maintaining a current efficiency of to by using a potential of 1.15 volts (vs. the saturated Cu:(h1 Cl electrode) for two to ten (preferably 2) minutes to activate the electrode and then reducing the voltage to 1.05 to 1.10 volts '(vs. the saturated CuzCu Cl electrode) for the remainder of the electrolysis in the synthesis of polyphenylene from benzene.

The potential variation required can be advantageously and automatically accomplished by controlling the current density of the Working electrode, While choosing the proper current density in accordance with the following table.

Table I.Variation of current efficiency for poly (paraphenylene) formation as a function of current density Current density Current Efficiency,

1 Ratio of theoretical to actual quantity used.

Table II.--Infiuence on electrical yield of added solid aluminum chloride in the electrosynthesis of polyphenyl Gms of solid AlCl added per Electrical yield,

hundred milliliters of anolyte percent 1 1 Electrolysis under optimum current density galvanostatlc conditions. Theoretical yield of polymer 0.20 gm.

As a further embodiment of this invention, the continuation of the product polymer with the A101 can be decreased and yet the required saturation can be obtained by confining the solid AlCl inside a porous walled reservoir added to the cell anode chamber. In this manner the dissolved AlCl is enabled to maintain its saturation level from the confined reservoir, yet the AlCl is easily kept separate from the polymer during separation of the latter by filtration from the cell electrolyte.

The polymeric material may be recovered by hydrolyzing the complex phase with ice and/or water. A twophase mixture will result: an inorganic phase comprising water, hydrogen halide, and aluminum halide; and an organic phase comprising any unreacted excess aromatic, the aromatic from the ternary complex, and the polymer, either in solution or suspended in the organic phase. The phases may then be separated by any convenient method, e.g. extraction, decanting, etc., and the polymer recovered from the organic phase by extraction, sublimation, distillation, etc. Alternatively, and usually preferably, the polymer may be filtered or centrifuged from the liquid electrolyte, and the electrolyte may then be returned to the cell for re-use. The solid polymeric material thus separated may be treated with ice and/or water as above.

The following examples will serve to further illustrate the process of this invention.

Example 1. Electrolysis of a complex phase of benzene and mesitylene:HCl:2AlCl In a glass U-tube, 50 ml. solution of the ternary complex mesitylene:HCl:2AlCl was saturated with 60 gm. of benzene and electrolyzed between two platinum electrodes. A current of 100 milliamps, corresponding to a current density of 8 milliamperes/square centimeter of electrode surface, was applied between the electrodes with cell voltage varying between 25 and 7 volts. The current was continued for about three hours and was continuously recorded. Using a reference electrode of Cu:'Cu Cl (saturated) in the electrolyte a potential of 1.15 volts was found sufficient to activate the electrode. After two minutes the electrode potential was reduced below 1.11 and kept between that potential and 1.07 for the remainder of the electrolysis. The contents of the cell were hydrolyzed with ice and water to destroy the complex phase. The organic products remaining after hydrolysis were extracted and the starting materials, benzene and mesitylene, were distilled away. From the remaining materials, 0.42 gm. of polyphenylene were recovered. This corresponds to 96% of theoretical yield, assuming two electrons per benzene ring incorporated into the polymer. (This assumption corresponds to that generalized above, except that the end groups are assigned two electrons as well as the center groups. This is unavoidable in this case because the ratio of end groups to center groups is not known exactly. However, it is likely that there are at least ten center rings for each end ring in the polymer, so that this assumption leads to relatively small error.)

Example 2 Example 1 was repeated except that the solid AlCl was confined inside a porous walled reservoir added to the cell anode chamber. The polymer obtained was filtered from the anolyte and was found to :be mixed with only 50% of its own weight of A1Cl as contrasted with more than 1000% of its own weight without the use of the confining chamber.

The nature of the present invention having thus been fully set forth and examples of the same given, what is claimed as new, useful and unobvious and desired to be secured by Letters Patent is:

1. In a process for preparing polymeric aromatic compounds which comprises electrolyzing a liquid solution comprised of a C to C aromatic compound and a ternary complex having the formula:

wherein R is a C to C aromatic compound at least as basic as the aromatic in solution and X is chlorine or bromine, the solution also being saturated with free AlCl the improvement which consists in making the potential of the working anode 1.15 (vs. the saturated Cu:Cu Cl electrode) for two to ten minutes at the beginning of the electrolysis then reducing the electrode potential to 1.05 to 1.10 (vs. the saturated Cu:Cu Cl electrode) for the remainder of the electrolysis.

2. The process of claim 1 in which the potential variation is achieved automatically by maintenance of a constant current density of 8 to 9 milliamperes/ square centimeter.

3. The process of claim 1 in which the saturation with A101 is achieved by maintaining solid AlCl in a confined porous zone.

4. The process of claim 3 wherein the temperature is about 10 C. to about +100 C.

5. The process of claim 3 wherein the aromatic in solution is selected from the group consisting of benzene, biphenyl, naphthalene, alkyl substituted benzenes, naphthalenes and biphenyls, and halo derivatives thereof.

6. The process of claim 5 wherein the aromatic in solution is benzene.

7. The process of claim 3 wherein R is selected from the group consisting of benzene, biphenyl, naphthalene, alkyl substituted benzenes, naphthalenes and biphenyls, and halo derivatives thereof.

8. The process of claim 7 wherein the ternary complex is mesitylene HCl 2AlOl RD S- WILLIAMS, P im ry Examiner.