Process for separating mixtures of prefluorinated hydrocarbon compounds

A process for separation of mixtures of partially- or perfluorinated hydrocarbon compounds, which optionally contain additional chain members from the heteroatom group nitrogen, oxygen, boron, phosphorus and sulfur and/or can be substituted by further halogen atoms, characterized in that such partially- or perfluorinated compounds, which have an acyclic straight-chain structure and a critical molecule diameter in the range from 4 to 7 .ANG. or are substituted by residues with a straight-chain acyclic structure and with a critical molecule diameter in the range from 4 to 7 .ANG., of such partially- or perfluorinated compounds, which have no straight-chain acyclic residues, and whose critical molecule diameter amounts to at least 1.1-fold times the that of the aforementioned straight-chain acyclic residues, in liquid or gaseous phase are separated in that a mixture or a solution of the mixture in an inert organic solvent, the molecular diameter of which is larger that of the aforementioned straight-chain acyclic residue, is contacted with an amount of a molecular sieve sufficient for adsorption, the average pore diameter of which lies in the range of the critical molecule diameter of the aforesaid acyclic residue or up to 30% less, separating the molecular sieve laden predominantly with compounds containing acyclic residues, and desorbing the compounds adsorbed on the molecular sieve.

The present invention relates to a process for separating mixtures of 
perfluorinated or partially perfluorinated hydrocarbon compounds, 
especially perfluorocarbon mixtures as obtained in the electrochemical 
perfluorination of cyclic hydrocarbons. 
The so-called "Perfluorkohlenstoffe" (=perfluorocarbons) are perfluorinated 
organic compounds which consist of carbon and fluorine and may also 
contain hetero atoms such as nitrogen or oxygen, for example. 
Perfluorocarbons are chemically and biologically inert compounds insoluble 
in water, which can have a fluid to waxy consistency at room temperature 
under standard pressure. Such compounds are disclosed, for example, in 
European patent applications, publication numbers 77 114, 99 652 and 151 
679. The perfluorocarbon molecules are outstandingly masked by a uniform 
shell of fluorine atoms. Therefore perfluorocarbons are extraordinarily 
inert chemically and physiologically, i.e., nontoxic. On account of their 
extremely low intermolecular forces perfluorocarbons have a low boiling 
point in comparison to their molecular masses and an extremely low surface 
tension. The very weak intermolecular forces also result in the ability of 
the perfluorocarbons to dissolve large amounts of gases, such as oxygen or 
carbon dioxide, for example. On the basis of these properties, especially 
their ability to dissolve and transport oxygen physically, 
perfluorocarbons have found application in medicine for the preparation of 
aqueous oxygen-transporting perfluorocarbon emulsions which can be used, 
for example, as blood substitutes or perfusion media. Furthermore, 
perfluorocarbons are also appropriate for use in other technical fields in 
which nontoxic and chemically inert liquid or waxy substances are needed, 
or inert substances with the ability to dissolve gases are needed. Thus, 
perfluorocarbons and their mixtures are suitable as inert refrigerants, 
lubricants, sealing and hydraulic fluids, insulating media in electrical 
technology, and means for vapor-phase soldering or as additives in agents 
for the above-mentioned purposes. 
Perfluorinated or partially fluorinated hydrocarbon compounds are generally 
obtained in a manner known in itself by fluorination, e.g., 
electrochemical fluorination of corresponding nonfluorinated hydrocarbon 
compounds. In the fluorination of cyclic starting compounds, mixtures are 
often obtained which contain, in addition to the partially or 
perfluorinated cyclic products, likewise partially fluorinated byproducts 
or perfluorinated and hence chemically and physiologically inert 
byproducts, which have acyclic structural elements. In particular, 
fluorination products of cyclic starting compounds can contain such 
perfluorinated byproducts which have a molecular weight similar to that of 
the chief product, and thus boil in the same temperature range as the 
chief product, as for example isomers of the chief product containing 
acyclic structural elements. Such mixtures cannot be separated 
satisfactorily by fractional distillation. For many applications the 
mixtures can be used as-is because the presence of perfluorinated 
byproducts does not cause the application to be impaired. In other fields 
of application, however, in medicine for example, pure compounds are 
required. 
It is the object of the present invention to find a simple separating 
process which can be used for separating mixtures of perfluorinated or 
partially fluorinated hydrocarbon compounds which contain cyclic compounds 
and compounds having acyclic structural elements. 
The subject matter of the present invention is a process for the separation 
of mixtures of partially or perfluorinated hydrocarbon compounds which may 
still contain chain links from the group of the hetero atoms, nitrogen, 
oxygen, boron, phosphorus and sulfur, and/or can be substituted by 
additional halogen atoms, characterized in that such partially or 
perfluorinated compounds which have an acyclic straight-chain structure 
and a critical molecule diameter ranging from 4 to 7 .ANG., or are 
substituted by moieties having a straight-chain acyclic structure and a 
critical molecule diameter ranging from 4 to 7 .ANG., and which are 
hereinafter called A compounds, are separated in liquid or gaseous phase 
from those partially or perfluorinated compounds which have no 
straight-chain acyclic moieties and whose critical molecule diameter 
amounts at least to 1.1 times that of the aforesaid straight-chain acyclic 
moieties, and which are substituted only by those moieties whose critical 
molecule diameter amounts to at least 1.1 times that of the aforesaid 
straight-chain acyclic moieties, and which hereinafter are called B 
compounds, by: 
a) the mixture or a solution of the mixture in an inert organic solvent 
whose molecular diameter is greater than that of the aforesaid 
straight-chain acyclic moiety is placed in contact with a quantity of a 
molecular sieve sufficient for the adsorption of the A compounds, the 
average pore opening diameter of the molecular sieve being in the range of 
the critical molecule diameter of the acyclic straight-chain moiety of the 
A compounds or up to 30% below it, 
b) the molecular sieve laden chiefly with A compounds is separated from the 
liquid or gaseous phase depleted of A compounds and containing the B 
compounds, and 
c) the compounds adsorbed on the molecular sieve are desorbed. 
The process of the invention is especially suited for the separation of 
mixtures of perfluorocarbons containing in some cases hetero atoms from 
the group, nitrogen and oxygen, and containing, as B compounds, 
perfluorocarbons with a cyclic structure containing hetero atoms in some 
cases and, as A compounds, perfluorocarbons with an acyclic structure 
containing hetero atoms in some cases or contain moieties with an acyclic 
structure. In the scope of the present invention, saturated or aromatic, 
especially saturated, perfluorinated organic compounds which consist of 
carbon and fluorine and can in some cases also contain hetero atoms such 
as nitrogen, oxygen or sulfur, especially nitrogen or oxygen, in the 
carbon chain are called perfluorocarbons. The compounds can contain, for 
example, up to 20, especially 5-20 carbon atoms. Perfluorocarbons of the B 
compound type are, for example, mono- to tetracyclic compounds with 3-18, 
especially 6-12 ring members. They can be compounds with annulated ring 
systems, such as decalin [decahydronaphthalene] for example, or with 
diamantoid ring systems, such as adamantane for example, or compounds 
wherein two 5-member and/or 6-member rings are joined together, in some 
cases by a lower alkylene chain, such as cyclohexylmorpholine or 
1,3-dipiperidinopropane. The process is likewise also suited for the 
separation of mixtures of fluorinated A and B compounds which still 
contain a residue of hydrogen atoms, i.e., are only partially fluorinated, 
or which are substituted by additional halogen atoms. The process is 
suited especially for separating mixtures which contain a cyclic B 
compound and were obtained by fluorination, especially electrochemical 
fluorination of the nonfluorinated starting compound corresponding to the 
cyclic B compound. For example, the process is suitable for separating 
mixtures which contain cyclic B compounds, especially perfluorocarbons 
with a cyclic structure, containing hetero atoms in some cases, also 
isomeric A compounds which have an acyclic structure or contain moieties 
with an acyclic structure. The process is also suitable for separating 
mixtures which contain compounds of formula B, especially cyclic 
perfluorocarbons with a cyclic structure containing hetero atoms in some 
cases, along with homologous A compounds substituted by lower 
straight-chain perfluoroalkyl moieties, especially trifluoromethyl 
moieties. 
The critical molecule diameter of compounds can be calculated, for example 
by using the van der Waals atomic radii (cf. O. Grubner et al., 
"Molekularsiebe." VEB Deutscher Verlag der Wissenschaften, Berlin 1968, 
pages 63-64). The critical diameter of spherical molecules (e.g., noble 
gases) is equal to the diameter of a sphere describing this molecule. 
In the case of symmetrical molecules disposed tetrahedrically (e.g., 
CCl.sub.4), the critical diameter is equal to the diameter of the circle 
which describes the triangular wall of the tetrahedron. 
In the case of molecules arrayed octahedrically (e.g., SF.sub.6) the 
critical diameter is equal to the diameter of the circle that gives the 
square base of the octahedron. In the case of diatomic molecules their 
critical diameter is given by the diameter of the circle that describes 
these molecules in the plane perpendicular to their length. The critical 
diameters of n-paraffins whose molecules have a linear (zig-zag) 
arrangement are given by the diameter of the largest circle that describes 
the molecules lying in the plane perpendicular to the paraffin chain, and 
they are the same in all n-paraffins. Similar considerations apply also to 
acyclic perfluorocarbon compounds and moieties. 
Suitable as molecular sieve for the process of the invention are especially 
inorganic molecular sieves, such as aluminosilicates or silicalites. 
Natural and synthetic zeolites, especially type A and X zeolites, have 
proven effective. The pore size of the molecular sieves used can vary 
according to the critical molecule diameter of the straight-chain, acyclic 
moieties of the A compounds. In general, good separating effects are 
obtained with molecular sieves with mean pore opening diameters of 4.5 to 
6.5 .ANG.ngstroms. The molecular sieves can be used as particles of 
varying shape, e.g., in the form of granules or pearls. 
Mixtures of cyclic perfluorocarbons containing hetero atoms in some cases 
with acyclic perfluorinated byproducts containing straight-chain moieties, 
which can be separated by the process of the invention, can be obtained, 
for example, by fluorinating corresponding nonfluorinated cyclic compounds 
by methods known in themselves. The nonfluorinated starting compounds can 
be fluorinated by electrochemical fluorination by electrolyzing solutions 
of the compounds in liquid hydrofluoric acid. It is expedient to use for 
this purpose solutions of 4-30, preferably 5-10 wt.-% of an unfluorinated 
starting compound in liquid hydrofluoric acid. The electrolysis is 
advantageously performed at temperatures between -25.degree. and 
+10.degree. C., preferably -5.degree. and +5.degree. C., an anode current 
density of 2-30 mA/cm.sup.2, and a cell voltage of 3-10, especially 4-8 
volts. For further processing, the raw reaction product settling as a 
heavy phase on the bottom of the electrolysis cell is separated and, to 
decompose any partially fluorinated byproducts, it is subjected to a 
treatment with an alkali metal or alkaline earth metal base, especially an 
alkali metal or alkaline earth metal hydroxide, in the presence of water 
and, if desired, a lower aliphatic primary or secondary amine at a 
temperature sufficiently high to decompose any partially fluorinated 
byproducts. This step of the process can be performed by methods known in 
themselves. For example, the reaction mixture can be heated for a period 
of several hours to 8 days at ebullition with refluxing. The 
perfluorinated compounds can be isolated from the reaction mixture by 
fractional distillation. The products separated from the reaction mixture 
by fractional distillation are generally free of unperfluorinated 
products. In general, however, they are mixtures which contain, in 
addition to the perfluorinated cyclic chief product, perfluorinated 
byproducts which are isomeric to the chief product or have a molecular 
weight similar to that of the chief product and may contain acyclic 
moieties. For example, in compounds containing piperidine rings, a 
narrowing of the ring sometimes occurs under the conditions of the 
electrochemical perfluorination, so that to a lesser extent isomeric 
methylpyrrolidine compounds form in addition to the perfluorinated 
piperidine compounds. Such isomer mixtures can for example be separated by 
the process of the invention. 
According to the invention, the contacting of the mixture to be separated 
with the molecular sieve can be performed in a liquid phase or in a gas 
phase. The adsorption is the liquid phase can be performed at temperatures 
between room temperature and the boiling temperature of the mixture being 
separated. For example, mixtures of compounds liquid at room temperature 
or solutions of mixtures of compounds that are waxy to solid at room 
temperature can be introduced into a reactor containing the molecular 
sieve and kept therein in contact with the molecular sieve for a period of 
time sufficient for the adsorption of the A compounds containing 
straight-chain acyclic moieties. Suitable solvents are especially liquid 
perfluorocarbons, such as perfluorinated decalin. Provision is made for a 
thorough mixing of the mixture with the molecular sieve particles by 
shaking or stirring, for example. The adsorption of A compounds onto the 
molecular sieve can be performed at room temperature or at elevated 
temperatures below the boiling point of the reaction mixture. The amount 
of molecular sieve to be used can vary according to the type of molecular 
sieve and the type of adsorbing A compound. For example. 2-50 times the 
weight of the A compound to be adsorbed can be used. The contact of the 
molecular sieve with the mixture to be separated can also be performed in 
the gas phase by passing the mixture in gas form through a reactor filled 
with molecular sieve particles. For example, the mixture can be passed by 
means of an evaporator together with a carrier gas through the reactor 
containing the molecular sieve with such a velocity of flow that the time 
of contact with the molecular sieve suffices for the adsorption of the 
formula A compound. The adsorption from the gas phase can be performed at 
temperatures between 50.degree. and 300.degree. C. and at pressures from 
0.1 to 10 bar. 
After the adsorption of the A compounds is performed, the molecular sieve 
laden with compound A is separated from the unadsorbed compounds B. After 
a gas phase adsorption this can be performed by flushing out with carrier 
gas. If the adsorption was performed from the liquid phase, the molecular 
sieve particles laden with compounds A can be separated in a manner known 
in itself by filtering or decanting the unadsorbed liquid. The molecular 
sieve particles can also be laden with small amounts of molecules of 
compound B adhering externally, for example, depending on the amount and 
volume of the adsorbed molecules of compound A. 
The subsequent desorption of the formula A compounds from the molecular 
sieve particles can be performed in a manner known in itself, for example 
by displacement desorption or by driving them out with heat. A 
displacement desorption can be performed in a manner known in itself, for 
example, by treating the molecular sieve laden with compounds A with 
water. If the molecular sieve particles still contain small amounts of B 
compounds, a thermal desorption is recommended. In this case, B compounds 
adhering externally can first be evaporated and then the adsorbed A 
compounds can be desorbed by further heating. The thermal desorption can 
be performed by raising the temperature to a range between ebullition and 
the degradation temperature of the compounds to be desorbed. The 
desorption temperature can best be about 100.degree. to 150.degree. C. 
above the adsorption temperature in the gas phase. If the mixture being 
separated contains several adsorbable A compounds, a mixture of these A 
compounds can also be obtained by the desorption. Such mixtures of A 
compounds can be further separated, if desired, by chromatography in a 
manner known in itself, e.g., by thin-layer or gas chromatography. 
In accordance with the process of the invention, molecular sieves are used 
whose pore opening diameters are up to 30% below the critical molecule 
diameter of the A compounds to be adsorbed. Since the pore opening 
diameters of the molecular sieves used are thus smaller than the critical 
molecule diameter of all compounds contained in the mixture being 
separated, it is surprising that nevertheless a separation of the A 
compounds from the B compounds is possible by adsorption onto molecular 
sieves, such as zeolites, by the process of the invention. 
The following examples are intended to further explain the invention 
without, however, limiting its scope.

EXAMPLE 1 
Separation of a mixture of perfluorodecalin and perfluoromethyldecalin 
10 g of a zeolitic molecular sieve with a mean pore opening diameter of 0.5 
nm (Mfr. Merck) was activated by 15 hours of heat treatment at 500.degree. 
C. in the muffle kiln. 4 g of perfluorodecalin and 4 g of 
perfluoromethyldecalin were added to the molecular sieve particles thus 
treated, in a glass flask and shaken for two hours at room temperature. 
Then the unadsorbed liquid was separated from the molecular sieve. The 
unadsorbed phase contained about 4 g of pure perfluorodecalin. For the 
desorption of the adsorbed compound the molecular sieve particles in the 
glass flask were shaken with water for two hours at room temperature. The 
desorbed product was separated from the water by distillation into a 
chilled receiver and identified by gas chromatography and by .sup.19 F NMR 
spectroscopy as perfluoromethyldecalin. The yield of 
perfluoromethyldecalin was about 3.5 g. 
EXAMPLE 2 
Separation of a mixture of perfluorotrimethyl adamantane and 
perfluoroadamantane 
0.5 g of perfluorotrimethyl adamantane and 0.5 g of perfluoroadamantane 
were dissolved in 15 g of perfluorodecalin. The solution was shaken with 
20 g of the activated molecular sieve used in Example 1 (pore opening 
diameter 0.5 nm) for four hours at a temperature of 30.degree.-40.degree. 
C. Then the unadsorbed liquid was separated from the molecular sieve 
particles. The liquid contained the weighed-in amount of 
perfluoroadamantane and perfluorodecalin. The desorption of the adsorbed 
part of the starting mixture was performed by heating the molecular sieve 
at 180.degree. C. at reduced pressure (oil-pump vacuum) and collecting the 
volatilized product in a chilled receiver. Approximately 0.3 g of 
perfluorotrimethyl adamantane was obtained, which was identified by 
.sup.19 F-NMR spectroscopy and mass spectrometry. 
EXAMPLE 3 
Separation of a mixture of perfluorocyclohexyl morpholine and 
perfluoro-n-hexylmorpholine 
A) Preparation of the mixture: 
A 5 to 15 percent solution of morpholinocyclohexene-(1) in predried, 
chilled, liquid hydrofluoric acid was perfluorinated in an electrolysis 
cell at an anode current density of 3-10 mA/cm.sup.2, a cell voltage of 5 
to 6.5 V and a cell temperature of -8.degree. to +5.degree. C. From time 
to time additional morpholinocyclohexene-(1) was added and consumed 
hydrofluoric acid was replaced so as to permit continuous operation of the 
cell. The heavy phase gathering on the cell floor and containing the crude 
reaction product was let out from time to time. The crude product was 
treated with the same volumes each time of an aqueous 8N potassium 
hydroxide solution and dibutylamine. The mixture was refluxed for 8 days. 
Then the mixture was fractionally distilled. In the distillation a chief 
fraction is obtained boiling in the range of 145.degree.-148.degree. C. 
The gas chromatographic analysis showed that the distillate was a mixture 
of 65% perfluorocyclohexyl morpholine, 33% perfluoro-n-hexyl-morpholine 
and 2% other byproducts. 
B) Separation of the mixture obtained under A): 
The above-obtained mixture of perfluorocyclohexyl morpholine and 
perfluoro-n-hexyl morpholine at 120.degree. C. was fed through an 
evaporator into a glass column filled with a zeolitic molecular sieve with 
a pore size of 5 .ANG.ngstroms, through which a constant stream of helium 
was flowing at a rate of 60 ml/minute. While the perfluorocyclohexyl 
morpholine was passing through the column, the byproduct containing the 
C.sub.6 F.sub.13 moiety was held back. Thus, perfluorinated cyclohexyl 
morpholine was obtained with a boiling point of 147.5.degree. to 
149.5.degree. C. After all of the perfluorocyclohexyl morpholine and 
excess perfluoro-n-hexylmorpholine had left the column the temperature was 
raised to 300.degree. C. In a cold trap at the outlet of the column, pure 
perfluoro-n-hexyl morpholine condensed, which had a boiling point of 
149.degree. to 150.degree. C. 
EXAMPLE 4 
Separation of a mixture of perfluorinated 1,3-dipiperidinopropane and the 
perfluorinated 1-(3-methylpyrrolidino)-3-piperidinopropane isomeric 
therewith, and 1,3-di(3-methylpyrrolidino)-propane. 
A) Preparation of the mixture: 
A 10% solution of 1,3-dipiperidinopropane in predried, chilled liquid 
hydrofluoric acid was perfluorinated in an electrolysis cell at a 
temperature of +2.degree. C., an anode current density of 2.5 to 25 
m/cm.sup.2 and a cell voltage of 4.5 to 7.0 V. From time to time 
dipiperi-dinopropane dissolved in liquid hydrofluoric acid was added and 
consumed hydrofluoric acid was replaced so as to permit the cell to 
operate continuously. The raw reaction product collecting on the cell 
bottom was let out from time to time and processed as described in Example 
3A. In the distillation a chief fraction was obtained which boiled at 
195.degree. to 203.degree. C. A gas chromatographic separation of the 
mixture showed that it contained 58% of perfluorinated 
1,3-dipiperidinopropane, 36% of perfluorinated 
1-(3-methylpyrroli-dino)-3-piperidinopropane, and 6% of perfluorinated 
1,3-di(3-methylpyrrolidino)-propane. 
B) Separation of the mixture obtained above: 
The separation was performed by means of temperature-programmed gas 
chromatography on a preparative gas chromatograph. The mixture was passed 
through a gas chromatography separating column filled with a zeolite 
molecular sieve with a pore size of 5 .ANG. (glass column 1.7 mm.times.3 
mm inside diameter) at about 180.degree. C. While the perfluorinated 
1,3-dipiperidinopropane was passing through the column, the two byproducts 
containing a trifluoromethyl moiety were largely held back. Thus a pure 
perfluorinated 1,3-dipiperidinopropane was obtained with a boiling point 
of 191.degree. C. After all of the perfluorinated 1,3-dipiperidinopropane 
had left the column, thermal desorption at a temperature raised by 
100.degree. C. yielded a mixture of perfluorinated 
1-(3-methylpyrrolidino)-3-piperidinopropane and perfluorinated 
1,3-di-(3-methylpyrrolidino)-propane was obtained. Then the separation of 
the two compounds containing trifluoromethyl moieties was performed by 
preparative gas chromatography on a solid phase of methylsilicone oil on 
inorganic support material (=SE-30, Mfr. Chrompack). Perfluorinated 
1-(3-methylpyrrolidino)-3-piperidinopropane with a boiling point of 
189.degree. C. and 1,3-di(3-methylpyrrolidino)-propane with a boiling 
point of 186.degree. C. was obtained.