PROCESS FOR THE PREPARATION OF EPOXIDES FROM ALKYLENE CARBONATES

Abstract: A homogeneous catalytic process for converting an alkylene carbonate to the corresponding epoxide wherein a liquid phase comprising said alkylene carbo­ nate and a catalyst is heated at a temperature of from about 100°C to about 250°C to form a product mixture comprising said epoxide and carbon dioxide. The im­ provement comprises conducting the reaction of alkylene carbonate to epoxide at a pressure below one atmos­ phere thereby minimizing the formation of undesired aldehyde and/or ketone by-products.

Description: This invention relates to a process for converting alkylene carbonates to their corresponding epoxides. More particularly, this invention is concerned with a process for generating an alkylene carbonate such as ethylene oxide from the corresponding alkylene carbonate with a minimum formation of undesired aldehyde and/or ketone byproducts. Processes for the efficient conversion of alkylene carbonates, particularly ethylene and propylene carbonates, to the corresponding epoxides (namely, ethylene oxide and propylene oxide) have heretofore been the focus of relativeLy limited study in the prior art. Processes to synthesize the alkylene carbonate starting material from an alkylene oxide and carbon dioxide are more common in the art and are amgly described in the patent literature. U. S. Patent Nos. 2,773,070; 2,873,282; 2,907,771; and 2,994,704 are descriptive of such carbonateforming process. Belgian patent No. 872,960 corresponding to U. S. Patent application Serial No. 863,354, filed December 22, 1977, assigned to Uhiorx Carbide Corporation is directed to a particularly efficient process for alkylene carbonate formation which is characterized by alkylene carbonate efficiencies above 99% and an oxide conversion of greater than 99.57.. In view of. the relatively advanced state of the art of alkylene carbonate formation as reflected by the patents disclosed above, a process which efficiently converts an alkylene carbonate, such as ethylene carbonate, to ethylene oxide could be advantageously combined with an efficient alkylene carbonate forming process so as to allow ethylene carbonate to be used as an economical, safe and non-explosive medium of transportation for ethylene oxide. The ethylene oxide could thus be efficiently generated from ethylene carbonate upon demand and thereby avoid the inherent hazards of shipping and storing the flammable oxide prior to use. Moreover, in instances where the reactant alkylene carbonate is produced from non-epoxide starting materials, such carbonate would represent a particularly desirable source for generating alkylene oxide. A major source of inefficiency in the. conversion of an alkylene carbonate to an alkylene oxide and carbon dioxide is the formation of aldehydes and/or ketones as byproducts of the reaction. For processes where the alkylene carbonate reactant is, for example, ethylene carbonate, acetaldehyde is generally formed as an undesired byproduct. Similarly, the conversion of propylene carbonate usually results in the undesired formation of propionaldehyde and/or acetone; the particular aldehyde and/or ketones thus produced being dependent upon the choice of alkylene carbonate reactant. The presence of such aldehyde and ketone byproducts is deemed unacceptable in commercial grade alkylene oxides because they adversely affect the products ultimately formed by the catalyzed reactions of such alkylene oxides. Moreover, the separation of such aldehydes and ketones from alkylene oxide is troublesome and costly. For example, the separation of acetaldehyde from ethylene oxide is a relatively difficult operation inasmuch as both materials are liquids having relatively similar volatilities. As a consequence, separation is usually effected in. commercial operation by a relatively elaborate fractional distillation. SUMMARY OF THE INVENTION The invention describes a homogeneous process for converting an alkylene carbonate to the corresponding epoxide wherein a 'liquid phase containing the alkylene carbonate and a catalyst is reacted to form a product mixture comprising predominantly alkylene oxide and carbon dioxide. The process is characterized by the improvement which comprises conducting the reaction of alkylene carbonate to epoxide at a pressure less than one atmosphere. The present invention is predicated on the discovery that the formation of undesired aldehyde and/or ketone byproducts of the reaction can be substantially reduced by carrying out the reaction under sub-atmospheric pressure. Although the mechanism by which such undesired byproducts are-minimized is not fully understood, it is believed that the effect of operating under vacuum con editions is to minimize the residence time of alkylene oxide in the liquid phase. The availability of such alkylene oxide to interact with alkylene carbonate and the homogeneous catalyst is therefore correspondingly reduced thereby preventing the undesired side reactions which result in the formation of aldehyde and/or ketone byproducts. DETAILED DESCRIPTION OF THE INVENTION The preferred alkylene carbonates contemplated by the invention are ethylene carbonate and propylene carbonate, such carbonates being reacted by the process of the invention to produce ethylene oxide and propylene oxide, respectively. Other alkylene carbonates may, however be. also advantageously used in accordance with the invention to produce the corresponding epoxide. The invention contemplates the utilization of known catalysts for the conversion of alkylene carbonate to alkylene oxide. Thus, the catalyst may be any of those which are specifically defined by the prior art. For example, U. S. Patent No. 4,069,234 describes the use of phosphonium halide catalysts for the formation of vicinal epoxides from carbonates. Alkali metal halides, such as described in Shapiro et al., J. Org. Chem. U.S.S.R., 5(2), p. 222 (1969), are generally preferred for the reaction. The catalysts may be employed in amounts ranging from as little as 0.001 moleslliter in the liquid phase, based on the alkylene carbonate reactant, to about 0.1 mole/liter. Generally a concentration of from about 0.01 to 0.05 mole/liter is desirable. The primary materials contained in the homogeneous liquid phase in the conversion of alkylene carbonate to alkylene oxide is the alkylene carbonate reactant and a catalyst. A solvent is generally not necessary but if desired, an inert solvent such as tetraglyme or tetralin may be added to the liquid phase. This is particularly desirable in a continuous reaction system as a means of maintaining the catalyst in solution during catalyst recycle. The process of the invention is carried out under a sub-atmospheric pressure,which may include pressures 1,3mbar to about 798 mbar. from about EMI4.1 However, the broad 1,33 mbar range of' operable pressures are generally from about EMI4.2 - - 532 mbar, 39,9 mbar EMI4.3 to about EMI4.4 with a range of from about EMI4.5 266 mbar to about EMI4.6 being particularly preferred. The process temperature of the reaction is generally maintained between 100 C and 250 C. The minimum temperature of the reaction is that temperature at which the catalytic decomposition of alkylene carbonate to alkylene oxide and carbon dioxide will occur. At temperatures below 100 C the rate of reaction of such carbonate decomposition is generally too low for practical consideration. Temperatures above 2500C are generally avoided so as to minimize the rate of reaction to undesired aldehyde and/or ketone byproducts. For most effective operation of the process, a tempera- ture range of from about 1700C to about 2200C is preferred. DESCRIPTION OF TEST PRO z PROCEDURE The test reactor used in the following examples was a 75 ml carius tube sealed at the top with a metal cap having ports for adding catalyst and alkylene carbonate to the reactor and sampling the effluent gas produced by the reaction. The test reactor was immersed in an oil bath to maintain a constant reaction temperature. The reactor was used in a continuous mode in which liquid alkylene carbonate was fed at- a rate sufficient to maintain a constant liquid level. The effluent vapors leaving the reactor were passed into a 316 stainless steel manifold prior to being sampled and analyzed for ethylene oxide and acetaldehyde in a Perkin-Elmer model 990 gas chromatograph with a thermal conductivity detector. The manifold was connected to a vacuum pump so as to provide a sub-atmospheric pressure in the reactor. The effluent or off-gas of the reactor was condensed and collected using an acetone/dry ice condenser. Doubly-distilled ethylene carbonate was distilled from calcium hydride prior to its use in the examples below. A fraction having a boiling point of from 128-1290C/124mm was collected. The ethylene carbonate thus recovered was weighed and introduced into the reactor which was maintained at the indicated reaction temperature by the oil bath. Referring to Table I below, Example 1 was a control example in which a large amount of acetaldehyde (8400 ppm) byproduct was formed at ambient pressure. In Example 2, the reaction was carried out under the nearly identical reaction conditions of Example I except that the 133 mbar reaction pressure was reduce & to EMI6.1 resu-lting in a greater than 80% reduction in acetaldehyde formation. The effect of minimized aldehyde formation was also observed in Examples 3 and 4 which used a different reaction temperature and a different catalyst, respectively, relative to that used in Examples 1 and 2. TABLE I CONVERSION OF ETHYLENE CARBONATE (EC) TO ETHYLENE OXIDE AND CO2 CATALYZED BY ALKALI METAL SALTS EMI7.1 Catalyst <SEP> Reaction <SEP> Reaction <SEP> Rate <SEP> of <SEP> EC <SEP> (1) <tb> Concentration <SEP> Temp. <SEP> Reaction <SEP> Time <SEP> Conversion <SEP> Acetaldehyde <tb> Example <SEP> Catalyst <SEP> (Moles/Liter) <SEP> ( C) <SEP> Pressure <SEP> (hr) <SEP> (Mole/Liter/Hr) <SEP> (ppm) <tb> 1 <SEP> NaBr <SEP> 0.066 <SEP> 180 <SEP> 1 <SEP> bar <SEP> 6.0 <SEP> 1.4 <SEP> 8400 <tb> 2 <SEP> NaBr <SEP> 0.072 <SEP> 180 <SEP> 133 <SEP> mbar <SEP> 8.0 <SEP> 2.6 <SEP> 1400 <tb> 3 <SEP> NaBr <SEP> 0.122 <SEP> 160 <SEP> 133 <SEP> mbar <SEP> 8.0 <SEP> 0.7 <SEP> 1400 <tb> 4 <SEP> LiBr <SEP> 0.018 <SEP> 180 <SEP> 133 <SEP> mbar <SEP> 1.5 <SEP> 3.1 <SEP> 2200 <tb> Footnote: (1) The ppm of acetaldehyde are relative to ethylene oxide.

Claims: P a t e n t C l a i m s 1. In a homogeneous catalytic process for converting an alkylene carbonate to' the corresponding carobna wherein a liquid phase comprising said alkylene carbonate and a catalyst is heated at a temperature of from about ture to about 250 C to form a gaseous product mixture comprising said imizing and carbon dioxide, the improvement for minimizing the formation of undesired aldehyde and/or ketone byproduct which comprises conducting the reaction of alkylene carbonate to epoxide at a pressure below one atmosphere. 2. The process of claim I wherein said 39s9 mbar to about 266 mbar. pressure is from about EMI8.1 3. The process of claim 1 wherein said reaction temperature is from about 170 C to about 2200C.

Inventors: MCMULLEN, CHARLES HENRY
Applicants: UNION CARBIDE CORPORATION
Application year: 1981
Main classification: C07D301
IPC codes: C07D301, C07B61, C07D303, B01J23