PROCESS FOR COSYNTHESIS OF ETHYLENE GLYCOL AND DIMETHYL CARBONATE

Abstract: A process is disclosed for the cosynthesis of ethylene glycol and dimethyl carbonate by reacting methanol and ethylene carbonate in the presence of a catalyst selected from the group consisting of zirconium, titanium and tin.

Description: This invention concerns a process for co-synthesis of ethylene glycol and dimethyl carbonate by the transesterification reaction of ethylene carbonate and methanol in the presence of a homogeneous or heterogeneous catalyst selected from zirconium and titanium oxides, salts and complexes. In addition to the fact that substantially fewer moles of methanol are needed in the methanol-ethylene carbonate feedstock per mole of dimethyl carbonate produced, this invention is advantageous in that the catalysts are in many cases found to perform better than sodium carbonate, which has been used in the art. Generally the prior art reports that the transesterification of aliphatic hydroxy compounds with carbonic acid, aliphatic diesters and aromatic diesters occurs readily in the presence of a basic catalyst and is a convenient method of synthesis of higher carbonates. Several references deal with the transesterification of glycol carbonates using an aliphatic alcohol. Most demonstrate the use of methanol and ethylene carbonate. US-A-4 307 032 discloses a process for the preparation of a dialkyl carbonate by contacting a glycol carbonate of a 1,2-diol containing 2 to 4 carbon atoms with a selected alcohol to form the corresponding carbonate of said alcohol at a temperature of between 50 and 250°C, in the presence of an improved catalyst which is a thallium compound, allowing the reaction to take place under milder conditions. Thallium is however expensive and very toxic. US-A-4 181 676 discloses a method for preparation of dialkyl carbonate by contacting a glycol carbonate of a 1,2-diol having 2 to 4 carbon atoms with a selected group of alcohols at an elevated temperature in the presence of an alkali metal or alkali metal compound wherein the improvement comprises employing less than 0.01 percent by weight of alkali metal or alkali metal compound based on the weight of the reaction mixture. It is known that alkyl carbonates of the type ROCOOR can be obtained from alcohols and cyclic carbonates corresponding to the above formula through a transesterification reaction in the presence of alkali alcoholates or hydrates. Only moderate amounts of inorganic compounds are produced by these reactions, however, and must be removed by methods which may unfavourably affect the general economy of the process. US-A-4 062 884 reports a finding that dialkyl carbonates can be prepared by reacting alcohols with cyclic carbonates in the presence of organic bases, which makes it unnecessary to remove inorganic compounds and allows the catalyst to be totally recovered by means of simple distillation. The preferred organic base is a tertiary aliphatic amine. US-A-4 349 486 teaches a monocarbonate transesterification process comprising contacting a betafluoroaliphatic carbonate, a compound selected from the class of monohydroxy aliphatic alcohols, monohydroxy phenols and ortho-positioned dihydroxy aromatic compounds in the presence of a base. The invention claims to greatly reduce undesirable side reactions and only small amounts of carbonic acid-aliphatic-aromatic mixed diester are associated with the isolated aromatic monocarbonate reaction. US-A-3 803 201 discusses problems associated with the separation of the methanol, dimethyl carbonate azeotrope and teaches one solution, wherein dimethyl carbonate is isolated from the azeotrope by a combination of low temperature crystallization and fractional distillation. In another article, in J. Org. Chem. 49(b) 1122-1125 (1984), Cella and Bacon discuss the results of their work. Among other things, they found that the alkylation of alkali metal bicarbonate and carbonate salts with alkyl halides in dipolar aprotic solvents and phase-transfer catalysts produces alkyl carbonates in good yields. The major limitation of this method is the failure of activated aryl halides or electronegatively substituted alkyl halides to produce carbonates due to the facility with which the intermediate alkoxy carbonate salts decompose. Disadvantages of the methods discussed above include in many cases the fact that it is necessary to use a large amount of methanol feedstock relative to the amount of dimethyl carbonate produced. Also, in many cases, alkali metal halides are co-produced and these halides present disposal problems. JP-A-54-63023 discloses a process for the preparation of a dialkyl carbonate by contacting an alcohol such as methanol with the carbonate of a 1,2-diol in the presence of a catalyst comprising a tin alkoxide. It would be a substantial advance in the art to devise an efficient process for co-producing dimethyl carbonate and ethylene glycol, which is homogeneous and and does not necessitate difficult product-catalyst separations. The dimethyl carbonate produced by such a process would be useful as a gasoline extender. According to the present invention there is provided a process for co-synthesis of ethylene glycol and dimethyl carbonate which comprises reacting methanol with ethylene carbonate in the presence of a homogeneous or heterogeneous catalyst which is selected from zirconium and titanium oxides, salts or complexes thereof at a temperature of 20° to 200°C until the desired products are formed. The operative gauge pressure will normally be from zero to 3.5 × 10⁷ Pa (5000 psig), until the desired products are formed. A particular advantage of these systems over the prior art is the high selectivities to dimethyl carbonate (DMC) and ethylene glycol (EG), relative to the charged amounts of ethylene carbonate (EC) and methanol (MeOH). These selectivities are illustrated in the accompanying Example I for the zirconium acetylacetonate catalyst and Example VII for the ziroconium diperchlorate oxide catalyst precursor. In the narrower and more preferred practice of this invention, dimethyl carbonate and ethylene glycol are prepared simultaneously by a transesterification process which comprises reacting ethylene carbonate and methanol in the presence of a homogeneous zirconium, or titanium catalyst, at a temperature of between 50°C and 150°C, and a gauge pressure of at least 3.5 × 10⁵ Pa (50 psig), until the desired products are formed. Recovery of the desired ethylene glycol and dimethyl carbonate can generally be carried out by distillation and crystallization. More specifically, methanol and ethylene carbonate are pumped into a tubular reactor upflow at a flow rate of 0.1 to 100 liquid hourly space velocity (LHSV). The reactor temperature is held at between 20 and 200°C and a back pressure of zero to 3.5 × 10⁷ Pa (5000 psig) is maintained throughout the experiment. The homogeneous catalyst systems suitable for the practice of this invention generally comprise a zirconium or titanium compound. The compound can be in the form of a salt or complex. The zirconium-containing catalyst compound comprises a salt of zirconium or a complex. Suitable examples include zirconium salts of strong (mineral) acids, such as zirconium tetrachloride, ZrCl₄, zirconium bromide, ZrBr₄, zirconium fluoride, zirconium nitrate, zirconium sulfate, Zr(SO₄)₂4H₂O, zirconium mixed halides and zirconium tetraiodide, zirconium ethoxide and zirconium isopropoxide, zirconium salts of weak acid such as zirconium acetate and zirconium acetylacetonate, Zr(O₂C₅H₇)₄, as well as zirconium compounds containing the zirconyl moiety, as for example, zirconium diperchlorate oxide, ZrO(CLO₄)₂ 8H₂O and zirconium oxide nitrate, ZrO(NO₃)₂ X H₂O. The preferred zirconium catalyst precursors are zirconium acetylacetonate and zirconium diperchlorate oxide. The titanium-containing catalyst compound may likewise comprise a salt of titanium or a complex. Suitable examples include titanium methoxide; other titanium alkoxides such as titanium isopropoxide, titanium acetate and titanium acetylacetonate also work. The preferred titanium compound is titanium isopropoxide. Also in some cases, the analogous zirconium, or titanium heterogeneous catalyst precursors may also be effective. Examples of suitable heterogeneous catalysts for the desired ethylene carbonate-methanol transesterification include zirconium oxide, ZrO₂, and titanium oxide. Said heterogeneous zirconium or titanium catalysts may be in the form of pellets, extrudates, granules or powders. Also effective may be zirconium carbide, zirconium nitride and zirconium silicate. A particularly effective catalyst for the co-synthesis of dimethyl carbonate and ethylene glycol is a solution of zirconium diperchlorate oxide dissolved in the ethylene carbonate-methanol feed mix. This reaction solution is illustrated in accompanying Example VII. During the co-synthesis of ethylene glycol and dimethyl carbonate by the reaction of ethylene carbonate with methanol, a large excess of methanol is normally employed in the prior art. Usually the initial molar ratio of methanol to ethylene carbonate is in the range of 5 or greater, and preferably at least 10. This preferred ratio range is illustrated by US-A-3 803 201. In the practice of this invention, by contrast, the initial weight ratio of ethylene carbonate to methanol is preferably 2 to 5. Such a range of weight ratios is illustrated by the accompanying examples. Potential advantages to operating at this ethylene carbonate-to-methanol weight ratio include: a) More efficient transesterification. b) Lower levels of methanol required to be re-cycled after the transesterification step. Ethylene glycol-dimethyl carbonate synthesis using the homogeneous catalyst described above can be conducted at reaction temperatures in the range from 20° to 200°C. The preferred operating temperature range is 50°-150°C. The reaction can be conducted under atmospheric pressure. A pressure reactor is nevertheless required in the case of low-boiling point components if the reaction is to be carried out in the upper temperature range and in the liquid phase. The pressure is not critical. In general the reaction is allowed to proceed under the autogenous pressure of the reactants. However, the reaction can also be carried out under elevated pressure, for example, under an inert atmosphere. A gauge pressure of zero to 3.5 × 10⁷ Pa (5000 psig) is appropriate here. An operating gauge pressure of greater than 3.5 × 10⁵ Pa (50 psig) is suitable and the preferred pressure was in the range of 3.5 to 10.5 × 10⁵ Pa (50 to 150 psig). The residence time for the ethylene carbonate and methanol reactants in the tubular reactor may vary over a wide range according to the temperature of reaction, the molar ratios of carbonate/alcohol feedstocks, etc. Using the homogeneous catalysts of this invention, the necessary residence time in the reactor may range from 0.01 hours to 10 hours,although it may be extended beyond 10 hours without danger of additional by-products being formed. The preferred residence time is in the range of 0.1 to 5 hours. The desired products of this process according to the invention are ethylene glycol and dimethyl carbonate. By-products include diethylene glycol, 1,1-dimethoxyethane, 1,2-dimethoxyethane, methyl 1,3-dioxolane, glycol monomethyl ether and dimethyl ether. Products have been identified in this work by gas chromatography (gc), NMR, IR and gc-IR or a combination of these techniques. Zirconium and titanium analyses were by atomic absorption (AA). All liquid product analyses have, for the most part, been by gc; all temperatures are in degrees centigrade and all gauge pressures in Pascal and pounds per square inch gauge. The following Examples illustrate the process of this invention. EXAMPLE I This example illustrates the co-synthesis of dimethyl carbonate and ethylene glycol from ethylene carbonate plus methanol, in good selectivity, using a homogeneous zirconium catalyst derived from zirconium acetylacetonate dissolved in the EC/MeOH feed mix. The weight ratio of ethylene carbonate to methanol is 2:3. To a 1 kg mixture of ethylene carbonate (EC) and methanol (typical composition: 59.0% MeOH, 41.0% (EC) was added 50g of zirconium acetylacetonate. The mixture was stirred to dissolve the zirconium salt, cooled in wet ice and the clear solution pumped through a 50 cc capacity, stainless steel, tubular reactor upflow at a rate of 25 cc/hr. The reactor temperature was held at 130°C and a back-pressure of 7.0 × 10 Pa (100 psig) was maintained throughout the experiment. After feeding the ethylene carbonate-methanol mix for several (3-8) hours, the liquid effluent was sampled at regular time intervals and analysed by gas-liquid chromatography. Typically, this liquid effluent has the following composition: 10.8 wt% dimethyl carbonate (DMC) 6.9 wt% ethylene glycol (EG) 30.4 wt% ethylene carbonate (EC) 50.1 wt% methanol (MeOH) Estimated molar selectivity to DMC, basic EC converted = Estimated molar selectively to DMC, basic MeOH converted = EG selectivity basis EC converted: EG selectivity basis MeOH converted: (in the above molar selectivity estimates: DMC, FW = 90.0; EC, FW = 88.0; EG, FW = 62.0; MeOH, FW = 32.0. "FW" is an abbreviation of "formula weight".) EXAMPLES II to VI Table 1 shows the co-synthesis of dimethyl carbonate and ethylene glycol from ethylene carbonate plus methanol using a variety of homogeneous zirconium and titanium catalyst system. Here the most effective catalyst precursors are: zirconium acetylacetonate titanium isopropoxide EXAMPLE VII This example illustrates the co-synthesis of dimethyl carbonate and ethylene glycol from ethylene carbonate plus methanol, in good selectivity, using a homogeneous zirconium diperchlorate oxide catalyst precursor. To a 1 kg mixture of ethylene carbonate and methanol (66.6% MeOH, 33.3% 1/EC) was added 50g of zirconium diperchorate oxide, ZrO(ClO₄)₂ 8H₂O. The mixture was stirred to dissolve the zirconyly salt (1.3% Zr), cooled in wet ice, and fed to the 50 cc tubular reactor at a rate of 25 cc/hr. using the procedures of Example I. The reactor temperature was held at 100°C, and a back pressure of 7.0 × 10⁵ Pa (100 psig) was maintained throughout the experiment. Typical liquid effluent showed the following composition: 10.4 wt% dimethyl carbonate 9.0 wt% ethylene glycol 24.2 wt% ethylene carbonate 54.2 wt% methanol The reactor temperature was then raised to 130°C. Typical liquid product now showed the following composition: 14.8 wt% dimethyl carbonate 9.0 wt% ethylene glycol 14.9 wt% ethylene carbonate 53.1 wt% methanol In the latter experiment: Estimated molar selectivity to DMC, basis EC converted = 89%. Estimated molar selectivity to DMC, basis MeOH converted = 76%. Estimated molar selectivity to EG, basis EC converted = >98%. Estimated molar selectivity to EG, basis MeOH converted = 95%. EXAMPLE VIII This example also illustrates dimethyl carbonate/ethylene glycol co-synthesis, but uses a homogeneous zirconyl nitrate catalyst precursor. To a 1 kg mixture of ethylene carbonate and methanol (57.0% MeOH, 38.5% EC) was added 50g of zirconium dinitrate oxide, ZrO(NO₃)₂ X H₂O. The mixture was stirred to dissolve the zirconyl salt (1.5% Zr), cooled in wet ice, and fed to the 50 cc reactor at a rate of 25 cc/hr., as in Example I. The reactor temperature was held at 130°C, and a back pressure of 7.0 × 10⁵ Pa (100 psig) was maintained through the experiment. Typical liquid effluent showed the following composition: 4.8 wt% dimethyl carbonate 8.3 wt% ethylene glycol 27.3 wt% ethylene carbonate 56.0 wt% methanol The reactor temperature was then raised to 150°C. Under these conditions the liquid product showed the following composition: 6.8 wt% carbonate 14.0 wt% ethylene glycol 24.1 wt% ethylene carbonate 52.5 wt% methanol EXAMPLE IX This example illustrates the co-synthesis of dimethyl carbonate and ethylene glycol from ethylene carbonate plus methanol, in good selectivity, using a heterogeneous zirconium oxide catalyst. To the 50 cc tubular reactor of Example I, packed with 3.2 mm pellets of zirconium oxide (98% ZrO₂), is pumped a solution of ethylene carbonate plus methanol (67.6% MeOH, 31.9% EC) at a rate of 50 cc/hr. Reactor temperature was held at 130°C, the back pressure was 7.0 × 10⁵ Pa (100psig). Typical liquid effluent showed the following composition: 3.8 wt% dimethyl carbonate 2.7 wt% ethylene glycol 29.9 wt% ethylene carbonate 63.1 wt% methanol The reactor temperature was then raised to 160°C. Typical liquid product under equilibrium conditions, using this higher reactor temperature were as follows: 7.9 wt% dimethyl carbonate 5.2 wt% ethylene glycol 25.2 wt% ethylene carbonate 60.8 wt% methanol No zirconium could be detected in the product liquid, basic atomic absorption analyses (AA).

Claims: A process for cosynthesis of ethylene glycol and dimethyl carbonate which comprises reacting methanol with ethylene carbonate at a temperature of 20° to 200°C in the presence of a homogeneous or heterogeneous catalyst, characterized in that the catalyst is selected from zirconium and titanium oxides, salts or complexes thereof. A process as claimed in claim 1, characterized in that the homogeneous catalyst is a soluble salt or complex of zirconium or titanium. A process as claimed in claim 2, characterized in that the homogeneous catalyst is a salt or complex of zirconium which is a zirconium salt of strong (mineral) acid, a zirconium alkoxide, a zirconium salt of a weak acid or a zirconyl compound. A process as claimed in claim 2, characterized in that the homogeneous catalyst is a salt or complex of titanium which is titanium acetylacetonate, titanium isopropoxide or titanium methoxide. A process as claimed in claim 1, characterized in that the heterogeneous catalyst is an oxide of zirconium or titanium. A process as claimed in any one of claims 1 to 5, characterized in that the operating temperature is between 50° and 150°C, A process as claimed in any one of claims 1 to 6, characterized in that the operating pressure is between zero and 3.5 x 10⁷ Pa (5000 psig). A process as claimed in any one of claims 1 to 7, characterized in that the weight ratio of methanol to ethylene carbonate is in the range of 2:1 to 5:1.

Inventors: KNIFTON, JOHN FREDERICK
Applicants: TEXACO DEVELOPMENT CORPORATION
Application year: 1987
Main classification: C07C68
IPC codes: C07C29, B01J27, B01J31, C07C27, C07B61, C07C68, C07C69, C07C31, C07C67