Process for the preparation of diaryl carbonates

A process and a catalyst for the homogeneous liquid phase reaction of aromatic haloformates with aromatic hydroxy compounds for the production of diaryl carbonates with the elimination of anhydrous hydrogen halide. The catalysts of the present invention comprise at least one aromatic heterocyclic nitrogen compound. These catalysts permit the production of the products in very high yield, and the reaction proceeds at high rates.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process and a catalyst for the 
production of diaryl carbonates, and more particularly to a process and a 
homogeneous catalyst for the homogeneous liquid phase reaction of aromatic 
haloformates with aromatic hydroxy compounds for the production of diaryl 
carbonates with the elimination of anhydrous hydrogen halide. 
2. Description of the Related Art 
Prior art methods for the production of diaryl carbonates have used the 
interfacial route involving a two phase reaction system, and various 
homogeneous catalytic systems. The interfacial route involves the 
neutralization of the aromatic hydroxy compound with caustic and the 
subsequent reaction of an aqueous solution of the phenate type salt of the 
aromatic hydroxy compound with a carbonyl halide, usually phosgene. In the 
case where the desired product is diphenyl carbonate, excess caustic to 
insure the complete neutralization of phenol results in a loss of about 20 
percent of the phosgene. Salt which represents the loss of two 
chlor/alkali equivalents is produced. As a consequence, the aqueous stream 
coming from this reaction process requires treatment prior to disposal. 
Caustic equivalents include the Group 1, 2, 11 and 12 hydroxides, oxides, 
carbonates and phosphates. 
The prior art alternatives to the above described interfacial route to 
diaryl carbonates are various homogeneous catalytic processes. U.S. Pat. 
No. 2,362,865 discloses the use of metal phenates as catalysts in the 
reaction of phenol and phosgene to form diphenyl carbonate in a process in 
which the phenol is in the liquid phase. U.S. Pat. Nos. 3,234,261 and 
3,234,263 relate to the formation of diaryl carbonates from various 
chloroformates by reaction with metal oxides, with the process of the '263 
patent employing a tertiary amine base as a catalyst. Related processes 
are disclosed in French Patent No. 1,361,228 and U.S. Pat. No. 3,251,873. 
U.S. Pat. No. 4,366,102 discloses a process which employs various organic 
phosphorous compounds as catalysts for the reaction of a phenol and 
phosgene to form an aromatic chloroformic ester. 
A process for the reaction of aromatic hydroxy compounds with carbonyl 
halides to produce diaryl carbonates which employs a heterogeneous 
catalyst system is described in U.S. patent application Ser. No. 429,954 
filed on Oct. 26, 1989, by Harley et al. 
The use of organophosphines as catalysts for the reaction of an aromatic 
haloformate with an aromatic hydroxy compound which is carried out in an 
inert reaction medium is describe in U.S. patent application Ser. No. 
451,894, filed of even date herewith, by Rand. 
U.S. Pat. No. 3,170,946 discloses a process for the preparation of 
arylchloroformates using aromatic amine catalysts, and U.S. Pat. Nos. 
3,211,774, 3,211,776 and 3,275,674 disclose processes for the preparation 
of aromatic esters of chloroformic acid using aromatic amine catalysts. 
U.S. Pat. No. 4,012,406 discloses a process for the preparation of diaryl 
carbonates by the reaction of aromatic monohydroxy compounds with phosgene 
with the aid of an aromatic heterocyclic basic nitrogen compound as a 
catalyst. Many such catalysts are effective for the conversion of 
haloformates and aromatic hydroxy compounds into diaryl carbonates, as 
would be expected, since a haloformate is an intermediate in the reaction 
of an aromatic hydroxy compound and phosgene to form the same product. The 
'406 patent teaches that the catalyst may be any basic nitrogen compound 
in which the nitrogen is contained in an aromatic 5- or 6-membered ring 
and which does not have any other functional groups (e.g. --NH.sub.2 or 
--OH groups) which are liable to form firm bonds with phosgene or 
carbonates under reaction conditions. 
SUMMARY OF THE INVENTION 
The diaryl carbonates produced by the present invention may be converted 
into polycarbonate resins for use as molding resins by application of heat 
or some other suitable technique. 
The general objective of the present invention is to avoid the 
disadvantages of the prior art methods of production of diaryl carbonates. 
These include the water and salt disposal problem associated with the 
interfacial method, and catalyst degradation and regeneration problems 
associated with various homogeneous catalytic systems. Another objective 
of the present invention is to employ homogeneous organic catalyst systems 
with their numerous technical advantages. Surprisingly, contrary to the 
teachings of the prior art, it has been found that aromatic heterocyclic 
nitrogen compounds which are activated by reactive substituents promote 
the reaction between aromatic hydroxy compounds and aromatic haloformates 
to form diaryl carbonates in very high yields. The rates observed in the 
process of the present invention are much faster than those observed for 
prior art processes employing as catalysts heterocyclic nitrogen bases 
which are unsubstituted or substituted with non-reactive groups. 
The process of the present invention for the production of aromatic 
carbonates comprises contacting an aromatic haloformate with an aromatic 
hydroxy compound in the presence of a catalytic amount of a catalyst which 
comprises at least one aromatic heterocyclic nitrogen compound which has 
been activated by at least one reactive substituent. The process is 
carried out in an inert reaction medium comprising an inert atmosphere, 
and, optionally, a noninteracting solvent. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Desirable aromatic hydroxy starting materials are represented by the 
general formula 
##STR1## 
where Ar is an aryl or substituted aryl group with one or more fused 
rings, R independently selected each occurrence is alkyl, aryl, alkenyl, 
aryloxy, or alkoxy of 1-12 carbon atoms, and n is an integer. A preferred 
aromatic hydroxy starting material is represented by the formula 
##STR2## 
where R independently selected each occurrence is alkyl, aryl, alkenyl, 
aryloxy, or alkoxy of 1-12 carbon atoms, and n is an integer of 0-5. More 
highly preferred are phenols wherein R independently selected each 
occurrence is alkyl, aryl, alkenyl, aryloxy, or alkoxy of 1-6 carbon atoms 
and n is an integer of 0-3. Other desirable aromatic hydroxy starting 
materials are bisphenols and naphthols. Highly preferred aromatic hydroxy 
starting materials are phenol and Bisphenols A and F. 
Suitable aryl haloformates of the formula R.sub.n --[Ar]--O--C(O)--X 
include those in which R.sub.n --[Ar]--O-- is selected from the same group 
as R.sub.n --[Ar]--O-- of the aromatic monohydroxy compound, as discussed 
above. The R.sub.n --[Ar]--O-- group of the haloformate may be the same or 
different from that of the monohydroxy compound. X is a halogen, and a 
preferred halogen is chlorine. 
In a preferred embodiment the aromatic monohydroxy compound is phenol, the 
aromatic haloformate is phenyl chloroformate and the products of the 
reaction are diphenyl carbonate (DPC) and anhydrous hydrogen chloride. 
Catalysts for the process of the present invention comprise at least one 
aromatic heterocyclic nitrogen compound which has been activated by at 
least one reactive substituent. A preferred reactive substituent is --OH. 
Any non-reactive substituent can be placed on the ring in any position and 
in any combination so long as there is at least one reactive substituent 
in one activating position. Examples include 2-hydroxypyridine, 
3-hydroxypyridine, 4-hydroxypyridine, 2-hydroxy-4-methylpyridine, 
2-hydroxy-4-methoxypyridine and 4-chloro-2-hydroxy-pyridine. In addition 
to nitrogen the ring may also contain other heteroatoms such as oxygen, 
sulfur or additional nitrogen atoms. Examples include 
2-hydroxy-4-methylpyrimidine and 2-hydroxypyrimidine. Additional aromatic 
rings may be fused to the basic structure of the catalyst as well, with 
examples including 8-hydroxyquinazoline and 2-hydroxyquinoline. 
A catalytic amount of the catalyst may be dissolved, dispersed or supported 
in the reaction medium. In one embodiment of the present invention the 
catalyst is simply dispersed in the reaction medium. If the reaction 
medium includes a noninteracting solvent it is desirable that the catalyst 
dissolve in the solvent. 
The concentration of catalyst which provides a catalytic amount of the 
catalyst in the reaction system of the process of the present invention 
can range from about 0.1 percent to about 10 percent on a mole percent 
basis based on the reactants. A preferred range for the concentration of 
the catalyst is from about 0.5 mole percent to about 5 mole percent, with 
the most preferred range being from about 2 mole percent to about 4 mole 
percent. 
Under the reaction conditions used in the process of the present invention 
the catalyst of the present invention to some extent is converted from the 
free base form into the hydrohalide. Since the position of the equilibrium 
depends upon the equilibrium constant for the dissociation equilibrium and 
other factors, such as temperature and solvent, various relative amounts 
of free base and salt forms of the catalyst may be present. If the base is 
introduced to the system as the hydrohalide, it will dissociate to yield 
the same balance as would prevail after some reaction has taken place when 
introduced as the free base. 
The process of the present invention desirably is carried out in an inert 
reaction medium which comprises an inert atmosphere, preferably nitrogen. 
The reaction may be run with or without a noninteracting solvent. In one 
embodiment solvents are used which dissolve the catalyst. Suitable 
solvents include aromatic hydrocarbons, which may be halogenated, of from 
6 to 16 carbon atoms. Examples of desirable solvents include xylene, 
toluene, ethyleenzene, cumene, diisopropylbenze, chlorobenzene and 
dichlorobenzene. Other desirable solvents include aliphatic halogenated 
hydrocarbons such as trichloroethylene, methylene chloride and 
tetrachloroethylene. A preferred solvent is 1,2-dichlorobenzene (ODCB). A 
mixture of two or more solvents may be used. 
In another embodiment the aromatic haloformate serves as the reaction 
medium as well as being a reactant. 
The process of the present invention may be carried out at temperatures up 
to the temperature at which the catalyst becomes unstable and decomposes. 
The desired temperature range is from about 80.degree. C. to about 
250.degree. C., with the preferred temperature range being from about 
150.degree. C. to about200.degree. C. 
The mole ratio of the reactants can vary. However, a preferred ratio of 
aromatic haloformate to aromatic hydroxy compound is from about 0.9:1 to 
about 1:1.5. 
The hydrogen chloride produced in the reaction can be removed continuously 
or intermittently, as desired, and as necessary to relieve the pressure 
build-up due to the production of this gaseous product. 
The following examples and comparative examples are provided to illustrate 
the process of the present invention, and are not intended to limit the 
scope of the present invention in any way.

EXAMPLE 1 
A series of experiments were run under a standard set of conditions which 
utilizes the following ratio of solvent, reactants and catalyst: 
1,2-dichlorobenzene (15 mL), phenol (22 mmol), phenyl chloroformate (11 
mmol) and three mol percent of catalyst based on the total number of 
phenyl groups (33 mmol). The phenol and phenyl chloroformate are weighed 
into a vial, diluted with 15 mL of 1,2-dichlorobenzene (ODCB) and added 
through a septum to a five necked 25 mL round bottomed flask which has 
been purged with nitrogen. When the reaction temperature is constant at 
150.degree.-152.degree. C., the catalyst, dissolved in 3 mL of ODCB, is 
added to the hot solution. The extent of reaction is measured by titration 
of the evolved HCl, since, from the stoichiometry of the reaction, the 
amount of HCl evolved over time is equal to the amount of diphenyl 
carbonate (DPC) which is formed. Corrections for salt formation between 
the catalyst and an equivalent of HCl are included when appropriate. The 
final yields of DPC are verified by liquid chromatography (LC) analysis. 
In the above manner phenyl chloroformate(1.877 g) and phenol (2.1053 g) 
were allowed to react in the presence of 2-hydroxypyridine (0.0954 g). 
Rapid evolution of HCl was immediately evident. After about 10 minutes the 
yield was approximately 70%. Within experimental limitations, at 40 
minutes the reaction essentially went to completion as analyzed by both 
titration (95% yield of DPC) and LC analysis versus an internal standard 
(99% yield of DPC). LC analysis also clearly demonstrated that the 
catalyst was unchanged and present in the amount originally added. 
In comparison, under comparable experimental conditions, when catalyzed 
with pyridine the reaction rate was much slower and a lower yield was 
obtained. Experiments utilizing 2-chloropyridine, 2-mercaptopyridine and 
3,5,6-trilchloro-2-pyridinol as the catalyst showed that the catalytic 
activity of these compounds is inferior even to pyridine. 
4-N,N-dimethylaminopyridine, which is a known catalyst for the reaction of 
acid chlorides and chloroformates with nucleophiles such as phenol, also 
showed inferior catalytic activity.