PROCESS FOR PRODUCING ETHYL ACETATE BY HOMOLOGATION OF METHYL ACETATE

Abstract: A process for producing ethyl acetate by reacting methyl acetate with carbon monoxide and hydrogen at a tempera­ ture of between 150° and 250°C and at a pressure of between 50 and 200 atm., in the presence of a neutral or ionic catalyst of formula Ru(CO) x L y O z , in which L is a nitrogenated heterocyclic or alicyclic base or a phosphorated compound, x is a whole number between 1 and 3, y is 1 or 2, and z is 2 or 3.

Description: PROCESS FOR PRODUCING ETHYL ACETATE BY HOIlOLOGATIOI; OF 1tET}I!L ACETATE This invention relates to a new process for producing ethyl acetate by homologation of methyl acetate. Ethyl acetate is known as a product widely used in organic chemistry, and is by far the most industrially important acetic ester. A process is known (USA patent 4,189,441) for producing ethyl acetate from dimethyl ether or methyl acetate by reaction with carbon monoxide and hydrogen. The process is characterised by the use of a catalytic system constituted by a ruthenium carbonyl or an iodised or brominated compound. This process gives good results when dimethyl ether is used as the substrate, but is much less useful when applied to the conversion of methyl acetate. In this respect, in such a case in addition to unsatisfactory selectivity in terms of ethyl acetate and products for recycling, there is the simultaneous production of large hydrocarbon quantities, these being undesirable derivatives from the process aspect because they represent low value products. Moreover, at the end of the reaction there is a high concentration of water produced during the homologation, which forms an azeotrope with the ethyl acetate and complicates the normal distillation separation, and in addition gives rise to hydrolytic phenomena on the esters with the formation of methanol and ethanol which can interact with the Ru catalysts to give Ru alkyl intermediates which are easily hydrogenated to hydrocarbons. The production of high-boiling products (propionic acid and propionates) is also relatively high, with their consequent accumulation in the bottom fraction containing the catalyst, which is recycled. On the other hand, the considerable development of the BASF and Vionsanto processes for the direct carbonylation of methanol, by means of which a mixture of acetic acid and methyl acetate is produced in an extremely convenient manner, makes this latter particularly interesting as a raw material for the production of ethyl acetate. The object of the present invention is therefore to provide a new process for the homologation of methyl acetate ethyl acetate, which gives high yields of useful products and high ethyl acetate selectivity. A further object of the present invention is to provide an ethyl acetate preparation process which as its starting material uses a mixture of methyl acetate and acetic acid, in particular as originates from the direct carbonylation of methanol. The new ethyl acetate preparation process according to the present invention consists essentially of reacting methyl acetate in an acetic acid solution, with carbon monoxide and hydrogen at a temperature of between 150 and 250 C and at a pressure of between 50 and 200 atmospheres, in the presence of a catalyst which can be represented by the formulas: Ru(CO)xLyI2 (1) RuCCO)IJyI37 - (2) where L is a nitrogenated or phosphorated complexing agent, x is a whole number between 1 and 3, and y is 1 or 2. The catalyst of formula (1) or (2) which characterises the new process according to the present invention can be added to the reaction system in the form of a preformed complex, or alternatively can be formed directly in the reaction mixture by adding one or more compounds chosen from each of the following classes: a) carbonyl ruthenium compounds such as Ru3(CO)12, Ru(C0)4I2, NatRu(CO)3I3¯ 7, /Ru(CO),C12-7,, or ruthenium compounds which under the reaction conditions form ruthenium carbonyls in situ, such as finely divided metal ruthenium, ruthenium tris (acetylacetonate), ruthenium salts of carboxylic acids, sodium or potassium hexachlororuthenate, ruthenium halides and the like; b) iodised compounds chosen from the group consisting of iodine, hydriodic acid, alkyl, aryl or acyl iodides, inorganic iodides and tetralkylammonium iodides; c) nitrogenated or phosphorated ruthenium complexing agents chosen from the group consisting of nitrogenated heterocyclic or alicyclic bases and phosphines of formula R1R2RP in which R1, R2, R3, which may be the same or different, are alkyl groups of 1 to 6 carbon atoms, cycloalkyl groups of 3 to 6 carbon atoms, or simple or substituted aryl groups. Examples of nitrogenated bases which are particularly suitable for the purpose of the present invention are pyridine, quinoline, isoquinoline, imidazole, morpholine, piperidine, pyrimidine, pyridazine, thiazole, dipyridyl and benzimidazole, either unsubstituted or substituted. The compounds pertaining to said classes (a), (b), (c) must be added to the reaction system such that the ruthenium compound : iodised compound : complexing agent molar ratio lies between 1 : 2 : 1 and 1 : 50 : 50, and preferably between 1 : 5 : 5 and 1 : 10 : 10. As stated heretofore, preformed ruthenium iodocarbonyl complexing agents can be added directly to the reaction system, some examples of these being [RuI2(CO)2PPh3]2, Rul2 (CO)2(PPh3)2, [RuI2(CO)2PBu3]2, RuI2(CO)2Py2, RuI2(CO)3 (dipy), where Ph is phenyl, Bu is n.butyl, Py is pyridine, and dipy is s dipyridyl. In all cases, the catalytic complex must be present in a molar proportion of between 1 and 3% of the treated methyl acetate. The reaction according to the present invention must be carried out at a temperature of between 1500 and 250 C, The lower temperatures within the defined range can be used if the catalyst comprises a nitrogenated complexing agent, because this type of complexing agent accelerates the reaction. For example, if a catalyst comprising pyridine is used, the highest ethyl acetate selectivity is obtained at 180 -200 C. If however a catalyst is used comprising a phosphorated complexing agent, it is convenient to work at the higher temperatures, between 2000 and 2500C, because these complexing agents reduce the reaction rate. The CO/H mixture fed into the reaction space must ensure a 2 partial pressure of CO which is sufficient to prevent the ruthenium carbonyl derivative decomposing to metal ruthenium. In all cases, the total pressure must be at least 50 atm, and preferably 100-300 atm. The H/CO ratio in the gaseous reaction mixture can vary between 0.1 and 10. The preferred ratio lies between 0.5 and 2.5. A large hydrogen excess accelerates the reaction, but can limit selectivity by favouring the formation of methane and ethane. A large carbon monoxide excess reduces the reaction rate and favours carbonylation of the methyl acetate to acetic anhydride which, in the presence of the water formed in the homologation, is hydrolysed to acetic acid. The reaction is preferably carried out in the presence of acetic acid and possibly other recycle products and byproducts of the reaction, in which the catalyst is soluble. As stated heretofore, one of the advantages of the present invention is the fact that it is possible to use, as substrate, the reaction mixture originating directly from the carbonylation of methanol and consisting essentially of methyl acetate and acetic acid. A further advantage of the new catalytic system derives from its activity in the conversion of water gas: EMI5.1 which is displaced towards the production of hydrogen, used in the reaction, so consuming most of the water which forms in the reaction. The elimination of the reaction water is a particularly important aspect, because the ethyl acetate is recovered from the reacted mixture by distillation, and if water is present an azeotrope forms which requires a subsequent purification stage in order to remove the water. Moreover, as stated heretofore, water hydrolyses the methyl and ethyl esters with the production of alcohols which produce hydrocarbons by forming complexes with the Ru catalyst. In the described-process, a gaseous hydrocarbon fraction is generally produced which is eliminated together with small quantities of bottom products constituted by propionic acid and propionates. When the ethyl acetate has been separated, the intermediate fractions can be all recycled in that they consist of methyl acetate or products (acetic acid, methanol, ethanol, C1 and 2 ethers) which can be all reconverted into methyl acetate or ethyl acetate under the reaction conditions. In particular, the acetic acid fraction contains the catalyst in solution, and this is also recycled. borne significant examples are given hereinafter in order to illustrate the process according to the present invention. These examples are however in no way limiting with regard to the description given in the text. EXEXPLE 1 0.36 mmoles of Ru(C0)4I2 (prepared by the method described by F. Calderazzo and F. L'Eplattenier, Inorg. Chem., 6, 1220 (1967)), 3.6 mmoles of OH I and 3.6 mmoles of pyridine (Ru : OH3I : Py = 1 : 10 : 10) together with 0.180 moles of methyl acetate (15 ml) and 0.180 moles of acetic acid (10.5ml) are placed in a Hastelloy C autoclave of 150 ml capacity. A CO/H mixture of molar ratio 1 : 2 is compressed into the 2 autoclave to a pressure of 150 atm. The autoclave is then placed in a bath temperature-controlled at 200 C and kept stirring for 10 hours, while keeping the pressure constant at 240 + 5 atm by feeding a 1 : 1 mixture of H2 and CO. After the autoclave hÅas cooled, the liquid mixture (25.2 g) and gaseous mixture (22 N 1) representing the products is discharged and analysed by gas chromatography. The methyl acetate conversion is 41%or with the following product selectivities: ethyl acetate 76.7mop acetic acid 6.8to methanol 2.5% ethanol 2.Oc/o ethers Co1 and C2) 0.6% propionic acid 0.7% methane 10.7% In addition, the liquid product fraction contains 1.7% of H20 . A comparison test was carried out in the same autoclave on a reaction mixture concisting of 0.180 moles of methyl acetate and 0.180 moles of acetic acid. The operating conditions were identical to those described heretofore. However, in this case a catalytic system was used which did not comprise the nitrogenated complexing agent1 but consisted of 0.36 mmoles of Ru(CO)4I2 and 3.6 mmoles of CH3I. The following results were obtained: methyl acetate conversion 35.6e,0, product selectivities: ethyl acetate 65.5% acetic acid methanol ethanol 4.8% ethers'(C1 and C2) 2.65S propionic acid and propionates 1.1% methane 19.6% The liquid fraction contained 4.1% of H20. As can be seen from the given examples, when the complexing agent is absent in the catalyst, there is a substantially lower yield of useful products accompanied by greater production of hydrocarbons and high-boiling products. The quantity of H20 present in the final mixture is also greater. All these factors considerably complicate the separation and possible recycling of useful products. EXAs{PLES 2 - 11 Tables 1 and 2 relate to various homologation tests carried out in the same operating manner as described in example 1, but varying the catalytic system. TABLE 1 EXAi4PLE 2* 3 4 Methyl acetate (mmoles) 180 180 180 Acetic acid (mmoles) 180 180 180 Ru complex Ru(CO)4I2 Ru(CO)4I2 Ru(CO)4I2 (mg) 168 168 168 Iodised compound OH I OH I OH I 3 3 3 (mg) 511 511 511 Complexing agent pyridine pyridine pyridine (mg) 284 142 711 Ru/iodide/compl. agent 1/10/10 1/10/5 1/10/25 PCO when cold, atm. 50 50 50 PH2 when cold, atm. 100 100 100 CONVERSION % Methyl acetate 18 16 31 Acetic acid - 1.5 PRODUCTS SELECTIVITY , Ethyl acetate 59.1 61.5 57.7 Acetic acid 24.7 - 12.3 Methanol 5.7 8.8 0.5 Ethanol 1.4 1.2 0.4 Methyl ethyl ether - 0.4 Propionic acid and propionates - - 0.3 Methane 9.1 28.1 28.8 Water (mmoles) 10.2 16 3.2 Carbon dioxide (mmoles) - 15.7 18 * reaction temperature: 18000 TABLE 1 (continued) 5 6 7 180 180 180 180 180 180 RU(CO)4I2 Ru(CO)4I2 Ru(CO)4I2 168 168 168 NaI CH3I CH3I 540 511 511 pyridine piperidine morpholine 284 310 160 1/10/10 1/10/10 1/10/5 50 50 50 100 100 100 26.6 27 15.2 54 66.3 52.8 21.8 12.3 10.0 0.8 1.5 3.3 0.5 0.8 0.4 5.4 0.9 0.6 17.4 18.2 32.9 6.6 7.8 7.5 18 18 2 TABLE 2 EXAMPLE 8 9 Methyl acetate (mmoles) 180 180 Acetic acid (mmoles) 180 180 Ru complex [RuI2(CO)2Ph3]2 RuI2(CO)2(Ph3)2 (mg) 242 337 Iodised compound CH3I CH3I (mg) 511 511 Complexing agent (mg) Ru/iodide/compl. agent 1/10/1 1/10/2 PCo when cold, atm. 54 55 when cold, atm. 104 100 2 CONVERSION o Methyl acetate 24 8 Acetic acid 12 6 PRODUCTS SELECTIVITY % Ethyl acetate 67.5 58 Methanol 5.1 7 Ethanol 2.3 1.2 Methyl ethyl ether 3.2 2.5 Diethyl ether 0.5 0.4 Propionic acid and propionates 0.4 0.5 Methane 21 30.4 Water (mmoles) 24 12 TABLE 2 (continued) 10 11 12 180 180 180 180 180 180 [RUI2(CO)2PBu3]2 Ru(CO)4I2 Ru(CO)4I2 110 168 168 CH3I CH3I CH3I 256 511 511 PPh3 P(CH3)2Ph 71 37 1/10/1 1/10/1 1/10/1 50 55 50 100 105 100 4 16 21 0 12 12 50 70.5 69 19.3 6 6 2.5 2.7 3.3 2.5 0.5 0.5 0.6 0.7 30.7 16.6 18.6 10 24.5 27.2 EXAMPLE 12 1.68 g (3.6 mmoles) of Ru(O0)4I21 5.11 g (36 mmoles) of CH3I, 2.9 ml (36 mmoles) of pyridine, 133 g (1.8 moles) of methyl acetate and 108 g (1.8 moles) of acetic acid were placed in a 1 litre Hastelloy C reactor fitted with a rotating magnetic stirrer and devices for withdrawing liquid and gaseous samples. A 1:1 co/H2 mixture was then compressed into the autoclave to a pressure of 132 atm. The reactor was then heated to a temperature of 2000C while keeping the pressure constant at 180 + 5 atm. by feeding a 1:1 co/H2 mixture from a vessel under high pressure. The reaction was continued for 16 hours1 and the variation in the liquid product composition with time was followed by withdrawing and analysing samples at various times. Figure 1 shows the results of these analyses. After 16 hours, the reactor was sufficiently cooled, and the liquid products (240 g) and gaseous products (4.57 moles) were discharged and analysed by gas chromatography. The methyl acetate conversion was 62%, and the various product selectivities were as follows: ethyl acetate 45.1% acetic acid 27.3% methanol and ethanol 0.32ó propionic acid 0.3;ö propionic esters 0.7% methane and ethane 26.3% The gaseous products contained 0.15 moles of carbon dioxide, and the water concentration in the liquid products was less than 0.5% by weight. A test was carried out under the same operating conditions, but using a catalytic system which did not comprise the complexing agent, and consisted of 1.43 g (3.6 mmoles) of Ru(Ac ac)3 and 5.11 g (36 mmoles) of CH3I The reaction was continued for 14 hours, and the variation in the liquid product composition with time was followed by withdrawing and analysing samples, at various times. After 14 hours the reactor was cool, and the liquid products (274 g) and gaseous products (4.17 moles) were discharged and analysed by gas chromatography. The methyl acetate conversion w6a 76%. This high conversion is due to the, presence in the reaction mixture of a large water quantity (5.6% of the liquid fraction) which hydrolyses the acetate to ethanol. However, the presence of H20 considerably complicates ethyl acetate separation. The various product selectivitieS were ,as follows: ethyl acetate 46.3% acetic acid 11.1% methanol and ethanol 5.0% C1-C2 ethers 2.0 propionic acid Q.5% propionic esters and n-propyl derivatives 7.2%o methane and ethane 27.9 As can be seen, without the complexing agent in the catalyst, there is a total selectivity in terms of ethyl acetate and products for recycling of 64.4% against a selectivity of 72.7% in the corresponding example with pyridine. Moreover, in the absence of pyridine there are larger quantities of high-boiling products, hydrolysis products and hydrocarbons.

Claims: PATENT CLAIiG 1. A process for preparing ethyl acetate from methyl acetate using carbon monoxide and hydrogen, characterised in that the reaction is carried out in the presence of a catalyst of formula Ru(CO)xLyI2, in which L is a nitrogenated or phosphorated Ru complexing agent, x is a whole number between 1 and 3, and y is 1 or 2, at a temperature of between 1500 and 250 C and at a pressure of between 50 and 250 atm. 2. A process as claimed in claim 1, wherein the nitrogenated or phosphorated complexing agent is chosen from the group consisting of nitrogenated heterocyclic bases, nitrogenated alicyclic bases, or phosphorated compounds of formula R1R2R3P in which R1, R2, R3, which can be the same or different, are alkyl groups of 1 to 6 carbon atoms, cycloalkyl groups of 3 to 6 carbon atoms, or aryl groups, either simple or substituted. 3. A process as claimed in claim 1, wherein the catalyst is formed directly in the reaction mixture by adding a compound chosen from each of the following classes: a) carbonyl ruthenium compounds or compounds which form ruthenium carbonyls under the reaction conditions, b) iodised compounds, c) nitrogenated or phosphorated Ru complexing agents. 4. A process as claimed in claim 3, wherein the Ru compound of class (a) is chosen from the group consisting of Ru3(CO)12, - 3 12' Ru(00)4I21 Na/Ru(00)3I3J, /RuCOO)3 C12y2 finely divided ruthenium metal, Ru salts of carboxylic acids, sodium or potassium hexachlororuthenate, and ruthenium halides. 5. A process as claimed in claim 3, wherein the iodised compound of class (b) is chosen from the group consisting of iodine, hydriodic acid, alkyl iodides, aryl iodides, acyl iodides, inorganic iodides and tetraalkylammonium iodides. 6. A process as claimed in claim 3, wherein the complexing agent of class (c) is chosen from the group consisting of nitrogenated heterocyclic or slicyclic bases and phosphorated compounds of formula h R2R3P, in which R1, R2, 5 , which'can be the same or different, are alkyl groups of 1 to 6 carbon atoms, cycloalkyl groups of 3 to 6 carbon atoms1 or aryl groups, either simple or substituted. 7. A process as claimed in claim 6, wherein the nitrogenated heterocyclic base is chosen from the group consisting of pyridine, quinoline, isoquinoline, imidazole, morpholine, piperidine, pyrimidine, pyridazine, thiazole, dipyridyl, and benzimidazole. 8. A process 'as claimed in claim 3, wherein the compounds a:b:c are present in the catalytic system in proportions of between 1:2:1 and 1:50:50, and preferably between 1:5:5 and 1:10:10. 9. A process as claimed in claim 1, carried out in the presence of acetic acid.

Inventors: BRACA, GIUSEPPE, SBRANA, GLAUCO, VALENTINI, GIORGIO
Applicants: CONSIGLIO NAZIONALE DELLE RICERCHE
Application year: 1982
Main classification: C07C69
IPC codes: C07C69, B01J31, C07B61