The present invention relates to a process for the preparation of phenyl carbonates. More particularly, the present invention allows for the preparation of phenyl carbonates utilizing as reactants a phenol; acetic anhydride; carbon monoxide; and oxygen.

In U.S. Patent 4,533 _* 504 an integrated process for the preparation of aromatic carbonates was disclosed

10 wherein the methyl acetate by-product formed in the transesterification of a phenyl ester with a dialkyl carbonate was recycled by heating the alkyl ester to form a ketene and subsequently such ketene was j _|c reconverted by reaction with a phenol to form additional quantities of the phenyl ester.

Preparation of a ketene intermediate has proven costly and inefficient due to the high temperatures _Q involved. Recoveries on the order of only about 80 percent of theoretical or less are typical leading to the need to develop a more efficient means of generating phenyl carbonate precursors and disposing of alkyl ester 5 by-products.

According to the present invention, there is now provided a process for producing an aromatic carbonate comprising the steps of:

A. reacting acetic anhydride with a phenol to produce a phenyl acetate and acetic acid and separating the phenyl acetate;

B. reacting dimethyl carbonate with at least a portion of the phenyl acetate from step A to produce a phenyl carbonate and methyl acetate and separating the methyl acetate;

C. reacting the methyl acetate with carbon monoxide in the presence of a catalyst to prepare acetic anhydride; and

D. employing the acetic anhydride as at least a portion of the acetic anhydride reactant of step A.

The reaction of phenols with acetic anhydride to form a phenyl acetate and acetic acid, step A of the present invention, is a well known acetylation reaction, Suitable process conditions are disclosed, for example, in U.S. Patent 4,374,263. By the term "phenol" is included hydroxy benzene and Cι_ι_ _j alkyl or halo substituted phenols, however, the preferred reactant is hydroxybenzene. In schematic form, the reaction of a phenol and acetic anhydride may be depicted by the following illustration.

0 0 0

» ιι II

ArOH + (CH3O2O ArOCCH3 + HOCCH3

wherein Ar represents a ^arvl group or alkyl or halo substituted aryl group, particularly phenyl.

The reaction is conducted at moderate temperatures from 25°C to 180°C, more preferably 75°C to 150°C. The products are easily separated by fractional distillation or other suitable technique. While numerous catalysts are known for the esterification process, preferred catalysts are acids, especially heterogeneous macroporous ion exchange resins in the acid form.

The dimethyl carbonate reactant for step B may be prepared by the reaction between methanol, carbon monoxide and oxygen, according to any suitable procedure. Suitable are those processes disclosed in U.S. Patents 3,846,468, 3,980,690, 4,452,690, 4,533,504, and 4,360,477. The reaction is generally expedited by the use of a catalyst, particularly a copper or cobalt complex and the use of elevated temperatures and pressures. Preferred catalysts are cuprous chloride complexed with an organic base such as pyridine, dipyridyl, imidazole, alkyl or aryl phosphines, etc.

Preferred temperatures are from 20 to 75°C, more preferably 25 to 50°C. Preferred pressures of 0 ₂ and CO are from atmospheric to 20 atmospheres pressure.

Reaction of dimethyl carbonate and a phenyl acetate to give the desired phenyl carbonate and by¬ product alkyl acetate is a known chemical transformation. This step of the process may be illustrated schematically by the following diagram.

2 Ar0CCH3 + (CH ₃ )2Cθ3 > ArOC02Ar + 2 CH3OCCH3

wherein Ar is as previously defined.

I

The above reaction has been previously disclosed, for example, in U.S. Patent 4,182,726, and U.S. Patent 4,533,504. In a highly desirable embodiment of the present invention, this reaction is conducted in 0 the presence of a catalyst, particularly a Lewis acid, such as AlCl , or stannate or titanate catalysts. Temperatures of the reaction are from 100°C to 350°C,._ _ preferably from 150°C to 300°C. Pressures are generally from atmospheric to 25 atmospheres, more preferably 5 to 5 10 atmospheres.

The phenyl carbonate product is a highly desired article of commerce for conversion into polycarbonate resins. Processes for such conversion are 0 also well known having been disclosed in U.S. Patents

3,625,920, 3,888,826 and 4,330,664. Included in the synthesized product may be a mixture of both the mono- and ditransesterified products, that is both the diphenyl carbonate and phenyl methyl carbonate products 5 are formed.

The phenyl carbonate product and methyl acetate are readily separated by use of standard distillation techniques and the by-product, methyl acetate, is recycled by means of a reaction with carbon monoxide to produce acetic anhydride that is used in step A. Suitable processes for such reaction between methyl acetate and carbon monoxide are known and previously disclosed in the art, for example, in U.S. Patents 4,374,070, 4,559,183, and 4,046,807 and in S. W. Polichnowski, J. CHEM. ED., 6 , 206 (1986). A preferred process utilizes a noble metal catalyst and temperatures from 50°C to 250°C, preferably 100°C to 200°C, and pressures from atmospheric to 500 atmospheres, preferably 10 to 250 atmospheres.

It may be readily observed that the present process scheme allows for the overall conversion of a phenol to the corresponding phenyl carbonate utilizing as reactants acetic anhydride, carbon monoxide and oxygen. The recycle of methyl acetate intermediate allows for great economy and simplicity in the resulting process compared to the generation of a ketene intermediate. The present process utilizes much lower reaction temperatures and results in greatly improved efficiency of operation. Moreover, intermediate by¬ product acetic acid may also be recycled if desired by reaction with additional methanol to form methyl acetate and then carbonylated as above explained to form acetic anhydride. Alternatively, the acetic acid, which represents a significant increase in value over the starting material, methanol, may be sold.