Process for the Preparation of Dialkyl Carbonates

1. Field of the Invention

The invention relates to a process for the preparation of dialkyl carbonates by reacting alkylene oxides with alcohols and carbon dioxide in the presence of catalysts.

2. Discussion of the Prior Art

It is surprising that dialkyl carbonates can be prepared at all by reacting alkylene oxides with alcohols and carbon dioxide.

Rather, it was to be expected that polymerisation reactions which lead, for example, to polycarbonates and polyglycols take place in the reaction of alkylene oxides with alcohols and carbon dioxide.

It is known, in fact, from U.S. Pat. Nos. 3,248,415 and 3,248,416 that alkylene oxides and carbon dioxide or alkylene oxides, carbon dioxide and glycol carbonates react in the presence of small amounts of diglycols or polyglycols, under the catalytic influence of bases, to give polycarbonates which, however, predominantly contain polyalkylene oxide units and therefore in the strict sense are to be considered polyether-polycarbonates.

Furthermore, it is known from Makromol. Chem. 130, 210 (1969) that in the reaction of alkylene oxides with carbon dioxide in the presence of zinc diethyl as the catalyst, large amounts of polyglycols are obtained in addition to small amounts of alternating copolymers of alkylene oxides and carbon dioxide.

In addition to the polymerisation reactions, the formation of alkyl glycol ethers was to be expected as a side reaction. Because of the large number of hydroxyl groups present, amounts of alcohols equivalent to the amounts of alkylene oxide are added. Thus, the alkylene oxide should react with the alcohol to a considerable degree to give alkyl glycol ethers.

It has now been found that dialkyl carbonates can be prepared in good yields if alkylene oxides are reacted with aliphatic and/or cycloaliphatic alcohols and carbon dioxide in the presence of catalysts at elevated temperature.

Possible aliphatic and/or cycloaliphatic alcohols, e.g. alkanols and cycloalkanols are those with 1 to 18, preferably with 1 to 12, carbon atoms. Examples which may be mentioned are: methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol, amyl alcohol, cyclohexanol, octanol, decanol and cyclododecanol.

Alkylene oxides which are employed in the process according to the invention are those with 2 to 8, preferably with 2 to 3, carbon atoms. Examples which may be mentioned are: vinyloxirane, epichlorohydrin, 1,2- and 2,3-butylene oxide, cyclohexene oxide, ethylene oxide and propylene oxide; ethylene oxide and propylene oxide are preferably employed.

Suitable catalysts for the process according to the invention include basic organic nitrogen compounds, in particular tertiary amines, e.g. tertiary alkyl, tertiary alkylene, tertiary hydroxy alkyl and tertiary hydroxy alkylene, tertiary aryl and tertiary amino having both aliphatic and aromatic groups attached to the nitrogen atom. These include amines with 3 to 18, preferably with 3 to 12, carbon atoms. Examples which may be mentioned are: triethylamine, tributylamine, triethanolamine, dimethylbenzylamine and dimethylstearylamine.

Furthermore, heterocyclic aromatic nitrogen compounds with 2 to 15, preferably with 2 to 12, carbon atoms are suitable. Examples which may be mentioned are pyridine, pyrimidine, imidazole, triazole, tetrazole, benzimidazole, benztriazole and benzthiazole.

Hydrazines such as hydrazine itself, dimethylhydrazine and dibenzylhydrazine as well as amidines, such as formamidine and acetamidine, cyclic amidines, such as 2-alkyl-imidazol-2-ines, guanidine and urea can also be used as catalysts. In addition, the salts of all the nitrogen compounds mentioned with weak and strong acids, such as carboxylic acids with 1 to 18 carbon atoms, carboxylic acid or hydrogen halide acids, as well as quaternization products of the nitrogen compounds mentioned, for example those which are obtained by reacting the nitrogen compounds with alkylating agents, for example alkyl halides, can be employed as catalysts. Examples of such compounds are tetramethylammonium bromide or iodide, tetrabutylammonium chloride, dimethylpiperidinium iodide, N-methyl-pyridinium bromide, N,N'-dimethylimidazolium chloride and N-methyl-benzthiazolium iodide.

Other salt-like compounds, such as the sulphonium and phosphonium salts of hydrogen halide acids, for example triethylsulphonium iodide, tetraethylphosphonium bromide, tetrabutylphosphonium iodide and triphenyl-benzyl-phosphonium chloride, are also suitable catalysts.

A further class of substances having a catalytic action comprises the oxides, hydroxides and salts of the alkali metals and alkaline earth metals, such as LiOH, LiCl, Li.sub.2 CO.sub.3, NaOH, NaOAc, K.sub.2 CO.sub.3, NaI, KBr, K stearate, Sr stearate, CaCl.sub.2 and CaI.sub.2, and furthermore the oxides, hydroxides, carbonates and hydrogen halide acid salts of heavy metals (In, Cd, Co, Fe, Ni, Mn, Pb, for example) such as ZnCl.sub.2, CdI.sub.2, CoBr.sub.2, FeCl.sub.3, NiCl.sub.2, MnI.sub.2, CoCo.sub.3 and PbCl.sub.2, and compounds of tin, above all also with organic radicals with up to 46 carbon atoms, such as SnCl.sub.2, (But).sub.2 SnO, (But).sub.2 Sn(laurate).sub.2, (But).sub.2 Sn(OCH.sub.3)Hd 2, Na.sub.2 SnO.sub.2 and Na.sub.2 SnO.sub.3, and finally the oxides, hydroxides and salts of thallium with inorganic and organic acids, such as Tl.sub.2 CO.sub.3, Tl.sub.2 O, TlOAc, TlNO.sub.3, Tl(OAc).sub.3, Tl.sub.2 O.sub.3 and TlT. In some cases it proves favorable to use a combination of the catalysts mentioned, such as NaI/Tl.sub.2 CO.sub.3, NaI/Li.sub.2 CO.sub.3, CoCo.sub.3 /TlI or N-methylpyridinium bromide/guanidine carbonate, or to first produce the catalyst in the reaction mixture, for example by employing triethylamine and ethyl iodide.

The catalysts can also be applied to solid supports, for example to MgO and Al.sub.2 O.sub.3, or can be used in the form of solid catalysts, for example as polystyrenes which are crosslinked with divinyl benzene and contain amino groups or quaternary ammonium groups.

The halides, alcoholates, oxides, hydroxides and carbonates of the alkali metals and of thallium and the halides of quaternary ammonium bases, in particular the bromides and iodides, are preferably used as catalysts. It is very advantageous to use mixtures of these compounds, in particular those which contain thallium compounds.

The molar ratio of the alkylene oxide to alcohol is not critical in the process according to the invention. It is usually in the range from about 1:1 to 1:20, preferably 1:2 to 1:15. However, the molar ratio can also be outside the range indicated. In general, an excess of alcohol is preferably employed in order to shift the reaction equilibrium in the direction of the desired dialkyl carbonates.

Carbon dioxide is usually employed in an excess of 0.1 to 100 mols/mol of ethylene oxides. The partial pressure of carbon dioxide should be in the range from about 1 to 1,000 bars here, preferably 3 to 500 bars and particularly preferably 5 to 200 bars.

The reaction temperatures of the process according to the invention are in general about 70.degree. to 300.degree. C., preferably 90.degree. to 280.degree. C. and particularly preferably 100.degree. to 250.degree. C.

The reaction can be carried out discontinuously or continuously, for example in a tube reactor. The reaction components are separated by customary methods, for example by fractional distillation. The catalyst remaining in the residue can be reused.

The dialkyl carbonates can be obtained in yields of over 97% of theory by the process according to the invention.

The products of the process according to the invention are suitable as solvents for cellulose derivatives and as starting materials for the preparation of diaryl carbonates, aliphatic and aromatic polycarbonates, medicaments and plant protection agents (DT-OS (German Published Specification) No. 2,528,412, DT-AS (German Published Specification) No. 1,031,512, U.S. Pat. No. 3,933,846, J. Amer. Chem. Soc. 52, 314 (1930) and Ullmann's Encyclop/a/ die der technischen Chemie, (Ullmann's Encyclopaedia of industrial Chemistry), 3rd edition, volume 9, page 776 et seq. (1957)).

The examples which follow are intended to illustrate the process according to the invention in more detail, but without limiting it to these examples.

A mixture of 640 g (20 mols) of methanol and 44 g (1 mol) of ethylene oxide is kept at 120.degree. C. in the presence of 1 g of sodium iodide and 0.2 g of thallium carbonate under a CO.sub.2 pressure of 70 bars for 2 hours. After cooling, the mixture is let down.

The gas chromatogram of the reaction mixture shows that about 7% by weight of dimethyl carbonate is present in the reaction mixture. The diglycol content is about 0.3% by weight, the triglycol content is 0.1% by weight and the methylglycol content is 0.5% by weight.

Working up the reaction mixture by distillation gives, in addition to the methanol fraction containing 51 g of dimethyl carbonate, 49 g of a fraction which boils at 60.degree.-80.degree. C./0.8 mm Hg and consists of about 30 g of glycol and 19 g of glycol carbonate. The residue is 1.8 g and virtually corresponds to the amounts of catalyst employed.

Example 1 is repeated. Following the reaction (2 hours at 150.degree. C. under a pressure of 88 bars of CO.sub.2), after cooling, the CO.sub.2 is allowed to escape and the temperature is kept at 150.degree. C. for a further 2 hours.

According to the gas chromatogram, the reaction mixture contains 0.1% of diglycol and triglycol and about 0.7% of methylglycol. After working up as in Example 1, a methanol fraction consisting of 85 g of dimethyl carbonate and a glycol fraction which contains 3-4 g of glycol carbonate are obtained. The distillation residue is 2.5 g.

The ethylene oxide employed is thus converted to the extent of over 94% into glycol, and an equivalent proportion of methanol and CO.sub.2 is converted into dimethyl carbonate. The glycol carbonate formed can be converted into dimethyl carbonate by trans-esterification, so that the total yield of dimethyl carbonate, relative to ethylene oxide employed, is then >97% of theory.

A mixture of 640 g (20 mols) of methanol, 53 g (1.2 mols) of ethylene oxide, 2 g of sodium iodide and 0.2 g of thallium hydroxide is kept at 160.degree. C. under 100 bars of CO.sub.2 for 2 hours, the CO.sub.2 is blown out and the mixture is kept at 150.degree. C. for a further 1/2 hour. After working up by distillation, a methanol fraction which contains 102 g of dimethyl carbonate and about 1 g of methylglycol is obtained. The distillation residue is 3 g and thus virtually corresponds to the amount of catalyst.

A mixture as in Example 1, which contains 1 g of sodium iodide as the catalyst and no thallium salt, is reacted at 150.degree. C. for 2 hours. The methanol fraction which has been separated off by distillation contains 57 g of dimethyl carbonate and 0.2 g of methylglycol. The sodium iodide employed remains as a distillation residue of 1 g. Ethylene oxide is not quantitatively converted.

A mixture as in Example 1 is reacted in the presence of 1 g of lithium hydroxide under a CO.sub.2 pressure of 50 bars at 150.degree. C. for 2 hours. With incomplete conversion of ethylene oxide after working up, 45 g of dimethyl carbonate and 0.3 g of methylglycol are obtained in the methanol fraction. The residue is 1.5 g.

A mixture as in Example 1 is allowed to react in the presence of 1 g of triethanolamine as the catalyst at 150.degree. C. and under a CO.sub.2 pressure of 30 bars for 2 hours. With incomplete conversion of the ethylene oxide, 33 g of dimethyl carbonate and 0.5 g of methylglycol are obtained in the methanol fraction. The residue is 2.5 g.

A mixture as in Example 1 is heated to 150.degree. C. in the presence of 0.2 g of sodium carbonate as the catalyst under a CO.sub.2 pressure of 80 bars for 2 hours. The methanol fraction which has been separated off contains 23 g of dimethyl carbonate and 0.5 g of methylglycol. 1.5 g remain as the residue.

A mixture as in Example 1 is heated to 150.degree. C. in the presence of 0.1 g of sodium iodide as the catalyst under a CO.sub.2 pressure of 80 bars for 2 hours. 19 g of dimethyl carbonate and 0.6 g of methylglycol are obtained in the methanol fraction.

A mixture as in Example 1 is heated to 150.degree. C. in the presence of 1 g of imidazole as the catalyst under a CO.sub.2 pressure of 20-30 bars for 2 hours. The yield of dimethyl carbonate is 18 g.

A mixture as in Example 1 is heated to 150.degree. C. in the presence of 1 g of 1,5-diazabicyclo-(0,4,5)-undec-5-ene as the catalyst under a CO.sub.2 pressure of 80 bars for 2 hours. The yield of dimethyl carbonate is 16 g.

Example 1 is repeated at 150.degree. with 0.5 g of tetraethylammonium bromide as the catalyst. The yield of dimethyl carbonate is 25 g.

If Example 10 is repeated with 0.5 g of tetrabutylammonium iodide as the catalyst, 28 g of dimethyl carbonate are obtained.

A mixture of 640 g (20 mols) of methanol and 132 g (3 mols) of ethylene oxide is kept at 150.degree. C. in the presence of 1 g of sodium iodide and 0.2 g of thallium hydroxide under a CO.sub.2 pressure of 110 bars for 2 hours. With incomplete conversion of the ethylene oxide, 123 g of dimethyl carbonate and 0.6 g of methylglycol are obtained in the methanol fraction. 2 g remain as the residue.

Example 1 is repeated at 150.degree. C. with 1 g of lithium methylate as the catalyst. The yield of dimethyl carbonate is 53 g.

Example 1 is repeated with 1 g of guanidine carbonate as the catalyst. The yield of dimethyl carbonate is 11 g.

Example 1 is repeated with 1 g of lithium iodide as the catalyst. The yield of dimethyl carbonate is 56 g.

Example 1 is repeated with 2 g NaJ and 0,2 g Tl.sub.2 CO.sub.3 under the conditions of example 1. The yield of dimethyl carbonate is 78 g.

A mixture of 796 g (17.3 mols) of ethanol and 36 g (0.82 mol) of ethylene oxide is reacted with one another under the conditions of Example 1.

The yield of diethyl carbonate is 22 g, corresponding to a conversion of the ethylene oxide employed of 23%. Ethylglycol is not found. The distillation residue is 2 g and thus corresponds to the amount of catalyst employed.

If Example 18 is repeated with 1 g of imidazole instead of the catalyst from Example 1 and the mixture is kept at 200.degree. C. under a CO.sub.2 pressure of 88 bars for 2 hours, an amount of diethyl carbonate which corresponds to a conversion of the ethylene oxide employed of 43% is obtained.

A mixture of 749 g (16.3 mols) of ethanol and 45 g (0.775 mol) of propylene oxide is kept under a CO.sub.2 pressure of 80 bars and in the presence of 0.18 g of thallium carbonate and 1.8 g of sodium iodide at 150.degree. C. for 2 hours. After blowing out the CO.sub.2, the mixture is kept at 200.degree. C. for a further 2 hours. The yield of diethyl carbonate is 76 g, corresponding to a conversion of propylene oxide of 83%. Ethylether of Propylenglycol has not formed. The amount of residue from the distillation corresponds to the amount of catalyst employed.

Example 1 is repeated with 88 g (2.0 mols) of CO.sub.2, corresponding to a molar ratio of CO.sub.2 to ethylene oxide of 2:1. After 2 hours at 150.degree. C. a yield of 65 g of dimethyl carbonate are obtained.

If EXAMPLE 21 is repeated with 52 g (1,2 mol) of CO.sub.2, corresponding to a molar ratio of CO.sub.2 to ethylene oxide of 1,2:1, 68 g of dimethyl carbonate are obtained. The distillation residue is 2 g.