PROCESS FOR THE PREPARATION OF ALPHA, BETA UNSATURATED ACID ESTERS

Abstract: A process for forming an alpha, beta unsaturated ester from the corresponding carboxylic acid ester is disclosed. In the process the carboxylic acid ester, in the vapor phase, is reacted with gaseous oxygen. In the presence of gaseous me­ thanol, the methanol present in a molar concentration at least equal to that of the carboxylic acid ester, to form the cor­ responding alpha, beta unsaturated ester. In a preferred embo­ diment the carboxylic acid ester is methylisobutyrate and cor­ responding alpha, beta unsaturated ester is methylmeth­ acrylate.

Description: PROCESS FOR THE PREPARATION OF ALPHA, BETA UNSATURATED ACID ESTERS BACKGROUND OF THE DISCLOSURE Field of the Invention The instant invention is directed to a method for converting a carboxylic acid ester to its corresponding alpha, beta unsaturated ester by means of an oxidative dehydrogenation procedure which is characterized by the absence of significant amounts of the corresponding saturated and alpha, beta unsaturated acids in the product stream. More specifically, the instant invention is directed to a process for converting the carboxylic acid ester, methylisobutyrate, into its corresponding alpha, beta unsaturated ester, methylmethacrylate. Background of the Prior Art There are many processes described in the prior art directed to methods of converting a carboxylic acid ester into its corresponding alpha, beta unsaturated ester by means of an oxidation process. These methods are characterized by their unique catalyst systems. Of these processes for converting carboxylic acid esters into the corresponding alpha, beta unsaturated ester, the most important specific reaction is the conversion of methylisobutyrate (MIB) into methylmethacrylate (MMA). These reactions, unfortunately, are characterized by the formation of the undesirable by-products, methylacrylic acid (MAA) and isobutyric acid (IBA). The reactions that occur in a typical prior art process involving the oxidation of MIB into MMA in the vapor phase are as follows: (1) H3 C-CH(COOCH3)CH3+¸1O2- > H2C=CH(COOCH3)CH3+H2O (MIB) " (MMA) (2) H3C -CH(COOCH3 )CH3+H2O + H3C-CH(COOH)CH3+CH30H (IBA) (3) H2C=CH ( COOCH3 ) CH3+H2O 4 H3C-CH(COOH)CH3+CH30H (MAA) The latter two reactions, reactions 2 and 3 are undesirable hydrolysis reactions. The formation of these unwanted acids, IBA and MAA, can be separated from the desired product, methylmethacrylate. However, this operation is very expensive owing to the high capital cost of building the units necessary to effectuate this separation. Moreover, there are significant energy costs associated with separating the acids from the ester. In addition, the acids must be converted back to the corresponding saturated esters and thereafter recycled to the oxidative dehydrogenation reactor. SUMMARY OF INVENTION A major problem associated with the formation of alpha, beta unsaturated esters by conversion of the corresponding carboxylic acid ester through an oxidative dehydrogenation procedure has now been overcome. It has now been found that the formation of the undesirable by-products, the corresponding saturated acid and the corresponding alpha, beta unsaturated acid, can be eliminated. This is effectuated by adding methanol to the reactants introduced into the oxidative reactor. The addition of the methanol effectively eliminates the formation of the undesirable corresponding saturated and alpha, beta unsaturated acids. This process has particular application to the specific carboxylic acid ester, methylisobutyrate, which is converted to the corresponding alpha, beta unsaturated ester, methylmethacrylate. The addition of methanol effectively eliminates the formation of the undesirable saturated acid, isobutyric acid, and the corresponding alpha, beta unsaturated acid, methacrylic acid. In accordance with the instant invention a process is provided cc--a.irising the reaction of a carboxylic acid ester, in the vapor phase, with oxygen gas in the presence of gaseous methanol, said methanol present in a molar excess, compared to said carboxylic acid ester, to form the corresponding alpha, beta unsaturated ester. In particular, the process comprises the reaction of gaseous methylisobutyrat. with oxygen gas in the presence of gaseous methanol, sali methanol present in a molar excess compared to the :-Lhylisobuty- e, to form methylmethdcrylate . DETAILED DESCRIPTION The current invention is directed to a process for forming an alpha, beta unsaturated ester by the conversion of the corresponding carboxylic acid ester in the presence of methanol. More specifically, the instant invention is directed to a process in which methylisobutyrate (MIB) is converted into methylmethacrylate (MMA) by reacting the MIB in the vapor phase with oxygen. In this process gaseous methanol is introduced into the reactor along with the MIB. The effect of this reaction is to substantially eliminate the formation of undesirable by-products, the corresponding saturated acid and the corresponding alpha, beta unsaturated acid, which in the preferred species, results in the elimination of the formation of isobutyric acid (IBA) and methacrylic acid (MAA) respectively. The methanol is preferably added to the reactor downstream of the MIB inlet. It is emphasized, however, that the process of this invention contemplates also, as an alternate preferred embodiment, the introduction of the methanol directly into the MIB feed. Introduction of the methanol downstream of the point of introduction of the MIB is preferred since the methanol is thus introduced at a point subsequent to the initiation of reaction between the oxygen and the methylisobutyrate. This minimizes methanol losses due to minimization of the reaction between methanol and oxygen. Another advantage of introducing the methanol downstream of the point of introduction of the methylisobutyrate is the availability of employing different catalysts for the oxidative dehydrogenation reaction and esterification reaction, which suppresses acid formation. The catalyst employed in this reaction can be contained in series in the same reactor vessel or in separate vessels connected in series. Suitable catalysts for the esterification reaction include iron phosphorous oxides, other supported and unsupported heteropoly acid oxides, activated aluminas, silica alumina, transition metal oxides, such as tungsten oxide, chromia, molecular sieves and the like. The operating condition under which the reaction occurs is a function of the type of catalyst employed. In general, the tempera ture of reaction may range between about 2000C and 5000C. More preferably, the temperature is in the range of between about 300"C and 4000C. The pressure under which the reaction occurs is in the range of between about 0.1 and 10 atmospheres. The time of reaction, that is, residence time, is in the range of between about 0.1 and 10 seconds. The molar ratio of methanol to methylisobutyrate is in the range of between about 1 and 20 moles of methanol per mole of methylisobutyrate. More preferably, the methanol to methylisobutyrate ratio is in the range of between about 1 and 10 moles of methanol per mole of methylisobutyrate. The process of this invention, the gaseous oxydehydrogenation reaction of methylisobutyrate to methylmethacrylate can be carried out in a fixed bed, fluidized bed or transport reactor system. The following examples are given to illustrate the scope and spirit of the instant invention. Example 1 The following feed composition: Methylisobutyrate 2.2 mole percent Methanol 15.4 mole percent Oxygen 1.8 mole percent Nitrogen 80.6 mole percent was introduced into a tubular stainless steel reactor containing 10.5 grams of an iron-phosphorous oxide catalyst. The temperature of the reactor was controlled by a liquid heat transfer medium into which the reactor was immersed. The space velocity of the gaseous reactants was 1,700 hour , measured at standard temperature and p:-esure. A temDerature of 360"C was maintained in the reactor. The products of this reaction were measured at various times during the continuous operation of this reaction. Table 1 below summarizes the analysis of the reactants at three specific time periods during the operation of this reaction. TABLE 1 Selectivity*, Mole% MIB Hours on Stream Conv.,% MMA MAA IBA CO 1 65 51 0 0 11 2 61 52 0 0 11 3.5 55 52 0 0 11 * Unaccounted for materials are unidentified light end byproducts. It is additionally noted that surprisingly no significant quantity of methanol oxidation products were found in the analysis of the reaction products of this reaction. Comparative Example 1 Example 1 was repeated with the single change that the feed composition during the running of this continuous operation did not include any methanol, which was present in the feed composition of Example 1 at a molar ratio of 7 moles of methanol per mole of methylisobutyrate. Thus, the feed composition in this comparative example was as follows: Methylisobutyrate 2.2 mole percent Oxygen 1.8 mole percent Nitrogen 96.0 mole percent based on same ratio of the three constituents absent methanol. The product stream of this reaction was sampled three times during the continuous operation of this reactor. The results of this analysis appear below in Table 2. TABLE 2 Selectivity*, Mole% MIB Hours on Stream Conv.,% MMA MAA IBA COx 1 72 54 8 7 12 2.5 65 51 9 12 13 3.5 60 51 10 14 14 * Unaccounted for materials are unidentified light end byproducts. As can be seen by a comparison of Tables 1 and 2, representing the process of this invention and the process of the prior art respectively, no undesirable saturated acid, isobutyric acid (IBA), was formed in Example 1, representative of the process of this invention. IBA, however, was formed in Comparative Example 1, an example of the process of the prior art. Similarly, the process of the instant invention, illustrated in Example 1, did not result in the formation of any alpha, beta unsaturated acid, methacrylic acid (MAA). However methacrylic acid was formed in the process of the prior art, that is, in Comparative Example 1. Example 2 Example 1 was repeated except that the feed ratio of methanol to methylisobutyrate was reduced from a molar ratio of 7 to a molar ratio of 2. This resulted in a feed composition as follows: Methylisobutyrate 2.5 mole percent Methanol 5.0 mole percent Oxygen 1.8 mole percent Nitrogen 90.7 mole percent During the continuous operation of the reactor, product samples were obtained resulting in the analysis which is summarized in Table 3 below. TABLE 3 Selectivity*, Mole% MIB Hours on Stream Conv.,% MMA MAA IBA CO 1 65 51 0 0 11 2 61 52 0 0 11 3.5 55 52 0 0 11 * Unaccounted for materials are unidentified light end byproducts. These results again indicate the surprisingly significant improvement in the suppression of the acid products, methacrylic acid (MAA) and isobutyric acid (IBA), with the addition of even a lesser excess of methanol to methylisobutyrate. As in Example 1, no significant quantity of methanol oxidation products were found in the analysis of the reaction products of this reaction. The above preferred embodiment and examples are given to illustrate the scope and spirit of this invention. They should not, however, be construed as limiting the invention to those embodiments and examples given hereinbefore. These embodiments and examples will suggest other embodiments and examples to those skilled in the art. These embodiments and examples are also within the scope and spirit of the instant invention. Thus, the instant invention should be limited only by the appended claims.

Claims: CLAIMS: 1. A process for converting a carboxylic acid ester into a corresponding alpha, beta unsaturated ester, the process characterised in that the carboxylic acid ester is reacted, in the vapor phase, with oxygen gas, in the presence of gaseous methanol, said methanol present in a molar concentration at least equal to that of said carboxylic acid ester, to form the corresponding alpha, beta unsaturated ester. 2. A process according to claim 1 characterised in that said carboxylic acid ester is methylisobutyrate and wherein said alpha, beta unsaturated ester is methylmethacrylate. 3. A process according to claim 2 characterised in that said molar ratio of methanol to methylisobutyrate is in the range of between about 1 and 30 4. A process according to claim 3 characterised in that said molar ratio of said methanol to methylisobutyrateis in the range of between about 1 and 10. 5. A process according to any of claims 2 to 4 characterised in that said methanol is added to the reaction downstream of the point at which the methylisobutyrate is fed into the reactor. 6. A process according to any of the preceding claims characterised in that said reaction is catalyzed bya catalyst selected from the group consisting of iron-phosphorous oxides, other supported and unsupported heteropoly acid oxides, activated aluminas, slicia alumina, transition metal oxides, choromia and molecular sieves. 7. A process according to claim 6 characterised in that said catalyst is an iron-phosphorous oxide.

Inventors: GELBEIN, ABRAHAM P.
Applicants: CHEM SYSTEMS, INC.
Application year: 1983
Main classification: C07C69
IPC codes: C07C69, C07C67, C07B61, B01J27