Process for the enrichment or separation of organic substance mixtures

Abstract

Salts of organic carboxylic acids can be separated from non-salt organic compounds using a semipermeable membrane consisting of a perfluorosulfonic acid polymer or a salt of such a polymer, by bringing a solution of the salts and the organic compounds in a C.sub.1 -C.sub.4 alkanol into contact with one side of the membrane and having the pure solvent on the opposite side of the membrane. The process can be used, for example, in purification processes or for recovering reactants from reaction residues.

Description

The present invention relates to a process for the enrichment or separation of salts of organic carboxylic acids from non-salt organic compounds, in which a solution of those salts and compounds in a low molecular weight alkanol is brought into contact with one side of a semipermeable membrane consisting of a perfluorosulfonic acid polymer, the pure low molecular weight alkanol being present on the opposite side of the membrane.

U.S. Pat. No. 4,846,977 describes a separating process for mixtures of polar and non-polar liquids using semipermeable membranes consisting of perfluorosulfonic acid polymers. Suitable polar liquids are, for example, lower alkanols and especially water, and suitable non-polar liquids are, for example, hydrocarbons, ethers, ketones, esters or organic acids.

It has surprisingly been found that using the same membranes it is also possible to enrich or separate mixtures of salts of organic carboxylic acids and non-salt organic compounds in the form of solutions in low molecular weight alkanols.

The invention relates to a process for the enrichment or separation of salts of organic carboxylic acids from non-salt organic compounds using a semipermeable membrane consisting of a perfluorosulfonic acid polymer or a salt of such a polymer, wherein a solution of the salts and organic compounds in an unsubstituted or C.sub.1 -C.sub.3 alkoxy-substituted C.sub.1 -C.sub.4 alkanol, mixtures of said alkanols or mixtures of said alkanols with ethers is brought into contact with one side of the membrane and the pure solvent is present on the opposite side of the membrane.

The concentration of the salts and organic compounds is preferably from 0.0001 to 10%, especially from 0.001 to 5% and more especially from 0.001 to 3% by weight, based on the solution.

The thickness of the membrane can be, for example, from 5 to 300 .mu.m, preferably from 20 to 200 .mu.m.

Perfluorosulfonic acid polymers and salts of those polymers are known and are described, for example, in U.S. Pat. No. 4,846,977. Some of those polymers are commercially available under the brand name NAFION.RTM. (DuPont).

In a preferred embodiment, the membrane consists of a perfluorosulfonic acid polymer having recurring structural elements of formula I ##STR1## wherein R.sub.1 and R.sub.2 are each independently of the other F or C.sub.1 -C.sub.10 perfluoroalkyl, w is a number from 5 to 15, x is a number from 0 to 6, y is a number from 1 to 16, z is a number from 0 to 16, and M is H.sup..sym., an ammonium cation or a metal cation, R.sub.1 and R.sub.2 are preferably fluorine or C.sub.1 -C.sub.3 perfluoroalkyl, especially fluorine or trifluoromethyl and more especially fluorine. In formula I, preferably w is a number from 5 to 10, x is a number from 0 to 2, y is a number from 1 to 6 and z is a number from 0 to 6, especially from 0 to 2.

M as an ammonium cation may be NH.sub.4.sup..sym. or an ammonium cation of a primary, secondary or tertiary open-chain amine having preferably from 1 to 20, especially from 1 to 12, carbon atoms, or an ammonium cation of a monocyclic or bicyclic secondary or tertiary amine or of a tricyclic tertiary amine having preferably from 4 to 12 carbon atoms.

M as a metal cation may be a mono- to tri-valent cation of a metal of the main and subsidiary groups, the transition metals and the noble metals. Mono- or di-valent metal cations are preferred. Examples of metals are Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, In, Sn, Pb, Cu, Ag, Au, Zn, Cd, Hg, Cr, Mo, Mn, Fe, Co, Ni, Rn, Rh, Pd, Ir, Pt, Sb, Bi, and also the group of the rare earth metals. Preferred metals are the alkali metals and alkaline earth metals, Cu, Ag, Au, Fe, Co, Ni, Zn, Cd and Mn.

In a preferred embodiment, M is NH.sub.4.sup..sym., an ammonium cation having a total of from 1 to 18 carbon atoms, or a mono- to tri-valent metal cation. M is more especially an alkali metal cation or Ag.sup..sym..

The permeation of a salt of an organic carboxylic acid or of a non-salt organic compound can be influenced by the cation M chose. When membranes having relatively small monovalent cations are used, it is generally the non-salt compound that permeates preferentially. When relatively large mono- or poly-valent cations are used, it is generally the salt that permeates preferentially.

It has been found that in especially favourable cases the salt is retained virtually completely when M in formula I is Ag.sup..sym.. Conversely, the relatively non-polar organic compound is retained virtually completely when M in formula I is Cs.sup..sym.. Preference is therefore given to a process wherein M in formula I is Ag.sup..sym. or Cs.sup..sym..

The alkanol used as solvent contains from 1 to 4 carbon atoms and preferably from 1 to 3 carbon atoms. It may be substituted, for example by methoxy or ethoxy. Examples thereof are methanol, ethanol, n- and iso-propanol, n-, iso- and tert-butanol, methoxyethanol, ethoxyethanol, propoxyethanol, 1-methoxypropan-3-ol and 2-methoxypropan-1-ol. Preferred solvents are methanol, ethanol, 1- or 2-propanol and 2-methoxyethanol. It is also possible to use mixtures of alkanols with one another or with ethers, for example diethyl ether or ethylene glycol dimethyl ether.

The salt of the organic carboxylic acid may be an ammonium or metal salt, for example NH.sub.4.sup..sym., an ammonium cation of a primary, secondary or tertiary amine having a total of from 1 to 20 carbon atoms, an alkali metal salt or alkaline earth metal salt. Alkali metal salts and ammonium salts are preferred. NH.sub.4.sup..sym. and Li.sup..sym. salts are especially preferred.

The organic carboxylic acid may be, for example, a mono-, di-, tri- or tetra-carboxylic acid. Aliphatic, cycloaliphatic, aromatic and heterocyclic or heteroaromatic monocarboxylic acids containing from 1 to 18 carbon atoms, preferably from 1 to 12 carbon atoms, are preferred.

In a preferred embodiment, the organic acid corresponds to formula (II)

R.sub.3 --X--COOH (II) wherein X is a direct bond, C.sub.1 -C.sub.4 alkylene, C.sub.2 -C.sub.4 alkylidene or C.sub.2 -C.sub.4 alkenylene, R.sub.3 is H or C.sub.1 -C.sub.12 alkyl, C.sub.2 -C.sub.12 alkenyl, C.sub.2 -C.sub.12 alkynyl, C.sub.3 -C.sub.12 cycloalkyl, C.sub.3 -C.sub.12 cycloalkenyl, C.sub.6 -C.sub.16 aryl, C.sub.3 -C.sub.12 heterocycloalkyl, C.sub.3 -C.sub.12 heterocycloalkenyl, C.sub.6 -C.sub.16 heteroaryl, each of which is unsubstituted or substituted by --OH, --SH, --CN, --NO.sub.2, halogen, C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 alkoxy, C.sub.1 -C.sub.6 alkylthio or by C.sub.1 -C.sub.6 alkyl-Y-- in which Y is --CO--, --SO--, --SO.sub.2 --, --CO--O--, --O--CO--, --CO--NR.sub.4 R.sub.5 -- or --NR.sub.4 R.sub.5 --CO--, and R.sub.4 and R.sub.5 are each independently of the other H, C.sub.1 -C.sub.6 alkyl, C.sub.2 -C.sub.4 hydroxyalkyl or R.sub.4 and R.sub.5 together are tetramethylene, pentamethylene or 3-oxapentyl-1,4-ene.

Examples of R.sub.3 as alkyl, which may be linear or branched, are methyl, ethyl, n- and iso-propyl, n- and iso-butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl and dodecyl.

Examples of R.sub.3 as alkenyl, which may be linear or branched, are vinyl, crotonyl, allyl, but-1-en-1-yl, but-1-en-2-yl, but-1-en-3-yl, but-1-en-4-yl, but-2-en-1-yl, but-2-en-2-yl, but-2-en-4-yl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl and dodecenyl.

Examples of R.sub.3 as alkynyl, which may be linear or branched, are ethynyl, prop-2-yn-1-yl, prop-2-yn-3-yl, butynyl, pentynyl, hexynyl, heptynyl, octynyl, decynyl and dodecynyl.

R.sub.3 as cycloalkyl and cycloalkenyl preferably contains from 4 to 8 carbon atoms, especially 5 or 6 carbon atoms. Examples thereof are cyclopropyl and cyclopropenyl, cyclobutyl and cyclobutenyl, cyclopentyl and cyclopentenyl, cyclohexyl and cyclohexenyl, cycloheptyl and cycloheptenyl, cyclooctyl and cyclooctenyl.

R.sub.3 as aryl preferably contains from 6 to 12 carbon atoms. Some examples thereof are phenyl, biphenyl and naphthyl.

R.sub.3 as heterocycloalkyl and heterocycloalkenyl preferably contains from 4 to 8, especially from 4 to 6, ring carbon atoms. Preferred hetero atoms are those from the group O, S and NR.sub.6 wherein R.sub.6 is H, C.sub.1 -C.sub.6 alkyl or C.sub.1 -C.sub.7 acyl. R.sub.3 as heteroaryl preferably contains from 4 to 11 ring carbon atoms and preferably hetero atoms from the group O, S and --N.dbd.. Some examples of heterocycles are pyrrolidine, tetrahydrofuran, tetrahydrothiophene, pyrroline, dihydrofuran, dihydrothiophene, indane, dihydrocoumarone, dihydrobenzothiophene, carbazole, dibenzofuran, dibenzothiophene, pyrazolidine, imidazolidine, pyrazoline, imidazoline, benzimidazolidine, oxazolidine, oxazoline, thiazolidine, thiazoline, isooxazolidine, isooxazoline, isothiazolidine, isothiazoline, benzoxazolidine, benzisooxazolidine, benzthiazolidine, 1,2,3- or 1,2,4-triazolidine, 1,2,3- or 1,2,4-triazoline, 1,2,3- or 1,2,4-oxazolidine or -oxazoline, piperidine, di- and tetra-hydropyridine, dihydro- and tetrahydro-pyran, di- and tetra-hydrothiopyran, piperazine, dehydropiperazine, morpholine, thiomorpholine, 1,3- and 1,4-dioxane, 1,4-dithiane, azepan, 1,3-dioxolane, 1,3-dithiolane, pyrrole, indole, imidazole, benzimidazole, furan, thiophene, benzofuran, benzothiophene, carbazole, dibenzofuran, dibenzothiophene, oxazole, isooxazole, thiazole, isothiazole, benzoxazole, benzothiazole, pyridine, pyrimidine, pyrazine, pyridazine, quinoline, isoquinoline, acridine, chromene, chromane, pyran, thiapyran, phenazine, phenoxazine, phenolthiazine and purine.

Examples of X in formula II are methylene, ethylene, 1,2- or 1,3-propylene, 1,2-, 1,3- or 1,4-butylene, ethylidene, 1,1- or 2,2-propylidene, 1,1- or 2,2-butylidene, ethenylene, prop-1-en-1,3- or -1,2- or -2,3-ylene.

In a preferred embodiment, the salt is a Li.sup..sym. or NH.sub.4.sup..sym. salt of a carboxylic acid from the group furan-2-carboxylic acid, benzoic acid, methylbenzoic acid, phenylacetic acid, cinnamic acid, sorbic acid or a C.sub.2 -C.sub.8 alkanecarboxylic acid.

The non-salt organic compound preferably contains from 2 to 20, especially from 2 to 16 and more especially from 2 to 12, carbon atoms. The organic compound is especially an ester of an organic monocarboxylic acid having a total of from 2 to 16 carbon atoms, an ether having from 2 to 12 carbon atoms, a ketone having from 3 to 16 carbon atoms or an alcohol having from 5 to 16 carbon atoms.

In a preferred embodiment, the organic compound is a C.sub.1 -C.sub.6 alkyl ester of furan-2-carboxylic acid, benzoic acid, methylbenzoic acid, phenylacetic acid, cinnamic acid, sorbic acid or an C.sub.2 -C.sub.8 alkanecarboxylic acid; a C.sub.5 -C.sub.12 alkanol or benzyl alcohol; a dialiphatic ketone having from 3 to 10 carbon atoms, a C.sub.1 -C.sub.6 alkyl phenyl ketone, or diphenyl ketone; or a dialiphatic ether having from 2 to 8 carbon atoms, C.sub.1 -C.sub.6 alkyl phenyl ether, or diphenyl ether.

The membrane can be constructed in various forms for carrying out the process according to the invention and can be incorporated into separating modules of customary design. For example, flat membranes or asymmetric membranes can be combined to form two-chamber or multi-chamber systems. It is also possible to use tubular membranes or hollow fibres which are generally used in the form of bundles. In order to increase mechanical stability the membranes can be mounted on a supporting framework.

The process according to the invention is generally carried out at room temperature. In order to obtain a sufficient rate of flow it is advantageous to establish increased pressure on the solution side. The pressure is preferably from 1 to 10 MPa, more especially from 1 to 6 MPa. In a special embodiment, the process is carried out in accordance with the counter-current principle.

The process according to the invention can be used, for example, as an enrichment or purification process or for recovering or separating reactants or secondary products from reaction residues or reaction mixtures or for the isolation or purification of intermediates, especially when thermally unstable substances are involved.

The following Examples illustrate the invention in more detail.

EXAMPLES 1-37

a) Manufacture of the membranes

The starting material used is a commercially available polymeric perfluorosulfonic acid membrane (H form) (Nafion.RTM.-117, DuPont) that is approximately 200 .mu.m thick. For the purpose of conversion into the salt form, the membrane is placed for 1 to 30 days in a 1:1 mixture consisting of methanol and a 1M aqueous solution of the hydroxide or chloride of the desired cation. The membrane is then washed with distilled water and methanol. Before use, the membrane is placed in the respective solvent until equilibrium swelling has taken place (1 to 5 days). For the manufacture of membranes having silver cations, sodium salts of the perfluorosulfonic acid membrane are used as starting material and are reacted with silver nitrate.

b) Permeation tests

In the centre of a pressure cell, the membrane (diameter 4.7 cm) is mounted in the holder. Each chamber is connected to a reservoir for the solution or the pure solvent to which a pump is connected. The reservoirs are mounted on scales. Between the reservoirs and the pumps there are arranged conductivity cells and between the outlet and the reservoirs there are arranged UV detectors. A pressure of 2.5 MPa is established on the solvent side and the solution and the solvent are circulated in the same direction. .sup.14 C measurements are taken using a liquid scintillation detector. The measurement data are evaluated using a computer program. The detectors are each calibrated with the organic compound and the salt of the carboxylic acid and the calibration curves are converted into algorithms from which the desired data are calculated.

The selectivity S is defined as follows, G.sup.P being the proportion by weight in the permeate and G.sup.L being the proportion by weight in the solution: ##EQU1##

Further data can be found in Tables 1 and 2 below.

TABLE 1__________________________________________________________________________ Metal cationExample in the Carboxylic Relatively non-polarNo. membrane Solvent acid salt organic compound__________________________________________________________________________ 1 Li.sup..sym. methanol Li cinnamate methyl cinnamate 2 Li.sup..sym. methanol Li cinnamate methyl cinnamate 3 Li.sup..sym. methanol Li cinnamate methyl cinnamate 4 Li.sup..sym. methanol Li cinnamate methyl cinnamate 5 Li.sup..sym. methanol Li cinnamate methyl cinnamate 6 Li.sup..sym. methanol Li cinnamate methyl cinnamate 7 Li.sup..sym. methanol Li cinnamate methyl cinnamate 8 Li.sup..sym. methanol Li cinnamate methyl cinnamate 9 Li.sup..sym. methanol Li cinnamate methyl cinnamate10 Li.sup..sym. methanol Li cinnamate furan-2-carboxylic acid ethyl ester11 Li.sup..sym. methanol Li cinnamate benzyl alcohol12 Li.sup..sym. methanol p-toluic acid Li salt p-toluic acid ethyl ester13 Li.sup..sym. methanol phenylacetic acid phenylacetic acid methyl Li salt ester14 Li.sup..sym. methanol benzoic acid Li salt benzoic acid methyl ester15 Li.sup..sym. methanol sorbic acid Li salt sorbic acid ethyl ester16 Li.sup..sym. methanol Li cinnamate acetophenone17 Li.sup..sym. methanol Li cinnamate methyl phenyl ether18 Li.sup..sym. methanol Li acetate methyl acetate19 Li.sup..sym. 2-methoxyethanol Li cinnamate methyl cinnamate20 Na.sup..sym. methanol Li cinnamate methyl cinnamate21 Na.sup..sym. ethanol Li cinnamate methyl cinnamate22 K.sup..sym. methanol Li cinnamate methyl cinnamate23 K.sup..sym. methanol Li cinnamate methyl acetate24 Ag.sup..sym. methanol Li cinnamate methyl cinnamate25 Ag.sup..sym. methanol Li cinnamate methyl cinnamate26 Cs.sup..sym. methanol Li cinnamate methyl cinnamate27 Cs.sup..sym. methanol Li cinnamate methyl cinnamate28 Ni.sup.2.sym. methanol Li cinnamate methyl cinnamate29 Co.sup.2.sym. methanol Li cinnamate methyl cinnamate30 Cu.sup.2.sym. methanol Li cinnamate methyl cinnamate31 NH.sub.4.sup..sym. methanol Li cinnamate methyl cinnamate32 N(C.sub.4 H.sub.9).sub.4.sup..sym. methanol Li cinnamate methyl cinnamate33 quinuclidinium methanol Li cinnamate methyl cinnamate (1-azabicyclo [2.2.2]octane)34 Li.sup..sym. methanol/di- ammonium benzoate methyl acetate ethylene glycol dimethyl ether 1:135 Li.sup..sym. methanol Li benzoate acetophenone36 Li.sup..sym. methanol ammonium benzoate methyl benzoate37 Li.sup..sym. methanol Li cinnamate methyl cinnamate38 Li.sup..sym. methanol octylammonium methyl cinnamate cinnamate39 octylammonium.sup..sym. methanol Li cinnamate methyl cinnamate40 Li.sup..sym. methanol di-Li phthalate.sup.1 methyl benzoate41 Li.sup..sym. methanol di-Li phthalate.sup.1 tris(2-ethylhexyl)trimellitate42 Li.sup..sym. methanol tri-Li trimellitate.sup.2 dimethyl phthalate43 Li.sup..sym. methanol tetra-Li methyl benzoate pyromellitate.sup.344 Li.sup..sym. methanol di-Li 2-carboxy- methyl cinnamate cinnamate.sup.4__________________________________________________________________________ .sup.1 98% dilithium salt + 2% monolithium salt .sup.2 35% trilithium salt + 65% dilithium salt .sup.3 45% tetralithium salt + 55% trilithium salt .sup.4 65% dilithium salt + 35% monolithium salt TABLE 2__________________________________________________________________________ Test end:Permea- Relatively non- Amounts oftion rate Carboxylic acid salt polar organic comp. components in(mg .multidot. Detection (amount start of test) (amount start of test) Test permeate (mg)Example min.sup.-1 .multidot. [UV (nm)/ Amount Conc. Amount Conc. time Organic Selec-No. cm.sup.-2) C 14] (mg) (wt. %) (mg) (wt. %) (min.) Salt compound tivity__________________________________________________________________________ 1 1.675 305 48.75 0.00975 1.25 0.00025 1080 0.714 0.340 18.7 2 1.583 305 45.00 0.00900 5.00 0.00100 1080 0.280 0.342 11.0 3 1.737 305 35.00 0.00700 15.00 0.00300 1050 0.425 1.005 5.5 4 1.721 305 25.00 0.00500 25.00 0.00500 1080 0.399 1.321 3.3 5 1.725 305 15.00 0.00300 35.00 0.00700 1080 0.300 1.864 2.7 6 1.741 305 5.00 0.00100 45.00 0.00900 1080 0.159 2.484 1.7 7 1.749 305 1.25 0.00025 48.75 0.00975 1050 0.044 2.828 1.6 8 1.767 305 5.00 0.00100 5.00 0.00100 1080 0.085 0.276 3.3 9 1.732 305 250.00 0.05000 250.00 0.05000 1080 4.176 12.974 3.110 1.232 260 250.00 0.05000 250.00 0.05000 1080 2.379 14.467 6.111 1.207 260 250.00 0.05000 250.00 0.05000 1080 2.173 34.200 15.712 1.685 255 25.00 0.00500 25.00 0.00500 1080 0.495 1.473 3.013 1.653 250 250.00 0.05000 250.00 0.05000 1080 4.617 18.687 4.114 1.690 255 250.00 0.05000 250.00 0.05000 1080 4.465 19.138 4.315 1.680 275 75.00 0.00500 75.00 0.00500 1050 0.787 1.506 1.916 0.717 280 250.00 0.05000 250.00 0.05000 1080 2.165 17.186 7.917 1.083 280 250.00 0.00500 250.00 0.05000 1040 2.187 14.878 6.818 1.284 C 14 7.50 0.00500 7.50 0.00500 1320 0.260 1.980 10.019 0.350 310 125.00 0.02500 125.00 0.02500 270 0.194 0.381 2.020 0.563 305 25.30 0.00500 25.30 0.00500 1080 0.170 0.569 3.421 0.067 300 25.00 0.00500 25.00 0.00500 1080 0.010 0.171 18.022 0.024 305 25.30 0.00500 25.30 0.00500 3240 0.295 0.443 1.523 0.010 C 14 10.00 0.00500 10.00 0.00500 5730 0.049 0.302 6.524 0.358 305 25.30 0.00500 25.30 0.00500 6500 0.000 7.400 .infin.25 0.189 305 1500.00 1.00000 3000.00 2.00000 1080 0.146 37.044 253.726 0.020 305 25.30 0.00500 25.30 0.00500 3600 0.390 0.000 0.027 0.037 305 1500.00 1.00000 1500.00 1.00000 260 0.234 0.000 0.0 340 0.260 0.077 0.328 0.031 305 25.30 0.00500 25.30 0.00500 2400 0.327 0.071 0.229 0.033 305 25.30 0.00500 25.30 0.00500 3510 0.772 0.466 0.630 0.064 305 25.30 0.00500 25.30 0.00500 2700 0.042 1.842 0.331 0.604 305 25.30 0.00500 25.00 0.00500 1080 0.331 0.719 2.232 0.314 305 25.30 0.00500 25.00 0.00500 1080 0.032 0.512 16.133 0.090 305 25.30 0.00500 25.00 0.00500 1080 0.040 0.116 2.934 0.352 C 14 10.00 0.00500 10.00 0.00500 4350 1.880 3.780 2.635 0.945 255 250.00 0.05000 250.00 0.05000 1350 1.586 15.854 10.036 1.740 255 250.00 0.05000 250.00 0.05000 1680 8.839 29.195 3.337 1.422 305 5000.00 1.0000 5000.00 1.00000 60 4.800 7.672 1.638 1.483 305 25.00 0.00500 25.00 0.00500 2070 1.024 3.093 3.339 1.099 305 25.00 0.00500 25.00 0.00500 2040 0.441 2.602 6.540 1.400 270 75.00 0.01500 75.00 0.01500 2040 1.216 14.340 14.341 1.392 270 75.00 0.01500 75.00 0.01500 2040 1.547 10.779 8.042 1.464 270 75.00 0.01500 75.00 0.01500 2040 1.601 13.096 9.743 1.587 270 2.50 0.00050 2.50 0.00050 2520 0.001 0.105 109.644 1.563 305 75.00 0.01500 75.00 0.01500 2040 1.434 9.361 7.3__________________________________________________________________________

Claims

1. A process for the enrichment of salts of organic carboxylic acids from non-salt organic compounds using a semipermeable membrane consisting of a perfluorosulfonic acid polymer or a salt of such a polymer, wherein the process comprises:

contacting one side of the membrane with a solution of said salts and non-salt organic compounds in a solvent selected from an unsubstituted or C.sub.1 -C.sub.3 alkoxy-substituted C.sub.1 -C.sub.4 alkanol, a mixture of said alkanols, or a mixture of said alkanols with ethers;

contacting the opposite side of the membrane with a solvent which is the same solvent selected to be present in the in said solution of salts and non-salt organic compounds;

and collecting the enriched or separated salts at one side of the membrane by preferentially permeating either the salts of organic carboxylic acids or the non-salt organic compound through the membrane.

2. A process according to claim 1, wherein the concentration of the salts and organic compounds is from 0.0001 to 10% by weight, based on the solution.

3. A process according to claim 1, wherein the membrane has a thickness of from 5 to 300 .mu.m.

4. A process according to claim 1, wherein the membrane consists of a perfluorosulfonic acid polymer having recurring structural elements of formula I ##STR2## wherein R.sub.1 and R.sub.2 are each independently of the other F or C.sub.1 -C.sub.10 perfluoroalkyl, w is a number from 5 to 15, x is a number from 0 to 6, y is a number from 1 to 16, z is a number from 0 to 16, and M is H.sup..sym., an ammonium cation or a metal cation.

5. A process according to claim 4, wherein M is NH.sub.4.sup..sym., an ammonium cation having a total of from 1 to 18 carbon atoms, or a mono- to tri-valent metal cation.

6. A process according to claim 4, wherein M is an alkali metal cation or Ag.sup..sym..

7. A process according to claim 1, wherein the solvent is methanol, ethanol, 1- or 2-propanol or 2-methoxyethanol.

8. A process according to claim 1, wherein the organic carboxylic acid salt is an ammonium or metal salt.

9. A process according to claim 8, wherein the salt is an alkali metal salt or an ammonium salt.

10. A process according to claim 8, wherein the salt is a NH.sub.4.sup..sym. salt or Li.sup..sym. salt.

11. A process according to claim 1, wherein the organic carboxylic acid forming the salt is an aliphatic, cycloaliphatic, aromatic, heterocyclic or heteroaromatic monocarboxylic acid having from 1 to 18 carbon atoms.

12. A process according to claim 11, wherein the organic acid corresponds to formula (II)

R.sub.3 --X--COOH (II)

wherein X is a direct bond, C.sub.1 -C.sub.4 alkylene, C.sub.2 -C.sub.4 alkylidene or C.sub.2 -C.sub.4 alkenylene, R.sub.3 is H or C.sub.1 -C.sub.12 alkyl, C.sub.2 -C.sub.12 alkenyl, C.sub.2 -C.sub.12 alkynyl, C.sub.3 -C.sub.12 cycloalkyl, C.sub.3 -C.sub.12 cycloalkenyl, C.sub.6 -C.sub.16 aryl, C.sub.3 -C.sub.12 heterocycloalkyl, C.sub.3 -C.sub.12 heterocycloalkenyl, C.sub.6 -C.sub.16 heteroaryl, each of which is unsubstituted or substituted by --OH, --SH, --CN, --NO.sub.2, halogen, C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 alkoxy, C.sub.1 -C.sub.6 alkylthio or by C.sub.1 -C.sub.6 alkyl--Y-- in which Y is --CO--, --SO--, --SO.sub.2 --, --CO--O--, --O--CO--, --CO--NR.sub.4 R.sub.5 -- or --NR.sub.4 R.sub.5 --CO--, and R.sub.4 and R.sub.5 are each independently of the other H, C.sub.1 -C.sub.6 alkyl, C.sub.2 -C.sub.4 hydroxyalkyl or R.sub.4 and R.sub.5 together are tetramethylene, pentamethylene or 3-oxapentyl-1,4-ene.

13. A process according to claim 1, wherein the salt is a Li.sup..sym. or NH.sub.4.sup..sym. salt of a carboxylic acid selected from the group consisting of furan-2-carboxylic acid, benzoic acid, methylbenzoic acid, phenylacetic acid, cinnamic acid, sorbic acid and a C.sub.2 -C.sub.8 alkanecarboxylic acid.

14. A process according to claim 1, wherein the non-salt organic compound is an ester of an organic monocarboxylic acid having a total of from 2 to 16 carbon atoms, an ether having from 2 to 12 carbon atoms, a ketone having from 3 to 16 carbon atoms or an alcohol having from 5 to 16 carbon atoms.

15. A process according to claim 14, wherein the non-salt organic compound is a C.sub.1 -C.sub.6 alkyl ester of furan-2-carboxylic acid, benzoic acid, methylbenzoic acid, phenylacetic acid, sorbic acid or a C.sub.2 -C.sub.8 alkanecarboxylic acid; a C.sub.5 -C.sub.12 alkanol or benzyl alcohol; a dialiphatic ketone having from 3 to 10 carbon atoms, a C.sub.1 -C.sub.6 alkyl phenyl ketone, or diphenyl ketone; or a dialiphatic ether having from 2 to 8 carbon atoms, C.sub.1 -C.sub.6 alkyl phenyl ether, or diphenyl ether.

16. A process according to claim 1 wherein said process is carried out at room temperature.

17. A process according to claim 1 wherein a pressure of from 1 MPa to 10 MPa is established on the solution side.

18. A process according to claim 1 wherein said process is carried out in counter-current mode.