PROCESS FOR THE PREPARATION OF UNSYMMETRICAL ALIPHATIC KETONES

Abstract: Unsymmetrical aliphatic ketones, e.g. methylisopropyl ketone and pinacolone, are prepared by passing two diffe­ rent carboxylic acids over a catalyst comprising one or more rare earth metals on a gamma-alumina-based carrier. The catalyst and the products of the process are also claimed.

Description: PROCESS FOR THE PREPARATION OF OF ALIPHATIC KETONES This invention relates to a process for the preparation of unsymmetrical aliphatic ketones by heating two different carboxylic acids having at least two aliphatic carbon atome in the presence of a catalyst. Ketones may be represented by the general formula RÚ-CO-Rê, wherein R1 and R2 are hydrocarbyl groups. In a symmetrical ketone and are identical, whereas in an unsymetrical ketone they are not. If R1 and R2 together forum one biradical group, the ketone is cyclic. Ketones may be prepared by the so-called decarboxylative condensation of two carboxylic acids (excluding formic acid) according to the following general equation: EMI1.1 <tb> R1COCBI <SEP> + <SEP> R200CH <SEP> heat <SEP> 1 <SEP> 2 <SEP> + <SEP> CO <SEP> + <SEP> H <SEP> O <tb> <SEP> -R- <SEP> R <SEP> C10, <SEP> R <tb> <SEP> catalyst <SEP> 2 <tb> Obviously, if R1,R2, only the symmetrical ketone RÚ-CO-RÚ will be formed, and quite high yields have been obtained using various catalysts, e.g. thoria. However, if R1 and R2 represent different hydrocarbyl groups, generally a mixture of three ketones, two symmetrical and one unsymmetrical, is formed. The above reaction therefore is not considered to be generally suited for the preparation of unsymmetrical ketones, due to the presence of two, sometimes difficultly removable, symmetrical ketones, and the low selectivity associated therewith. Thus it has been described once to use certain rare earth metals deposited on quartz or asbestos, in order to catalyze the above reaction. Apparently these catalysts are quite active in the preparation of symmetrical ketones, but the yield of unsymmetrical aliphatic ketones - Wher. using two different carboxylic acids - is very low. One would be inclined to conclude that supported rare earth metals are not very effective in the preparation of unsymmetrical aliphatic ketones. Surprisingly it has now been found that in the decarboxylative condensation of two different aliphatic carboxylic acids the formation of symmetrical ketones can be suppressed and, simultaneously, the formation of unsymmetrical ketones can be promoted, by employing a catalyst based on gamma-alumina, and also containing rare earth mentals. Accordingly the invention relates to a process for the preparation of unsymmetrical aliphatic ketones by heating two different carboxylic acids having at least two aliphatic carbon atoms in the presence of a rare earth metal containing catalyst, characterised in that the catalyst comprises one or more rare earth metals on a gama-alunina-based carrier. Suitably the catalyst contains fran 5 to 30% of rare earth metal based on the weight of the carrier, preferably from 10 to 28% and in particular from 17 to 26%. It has also been found that increasing the amount of rare earth metal promotes the yield of unsymmetrical ketone, at the expense of the formation of symmetrical ketones. The yield of the unsyirrtetrical ketone, however, becomes constant at higher rare earth metal percentages, and for this reason the range of fran 17 to 26% by weight is particularly preferred as already stated. These amounts refer to the rare earth elements calculated as mental, although mostly these metals are present as compounds, e.g. as oxides, sulphides and the like. The carrier is based on gamma-alumina and may contain minor amounts of other knawn carrier materials. The expression "based on" as used herein means that the carrier should consist of &gamma;Al2O3 for at least 70% by weight. It has been observed, however, that the addition of Ca0 or Li20, in general the addition of alkali metal or alkaline earth metal oxides, decreases the selectivity towards the unsymmetrical ketone desired. The same adverse effect has also been observed with Si02-based carriers such as quartz or silica, and with Si02-Al203 mixed carriers, including asbestos. These oxides preferably are virtually absent from the present catalyst, therefore. Preferably a pure go'trra-alurnina is used. In order to be able to increase the content of rare earth metal it is advantageous to use a carrier material with a large specific area. Excellent results have been obtained with the gamma-alumina having a specifica area of 50 to 260mê/g. Such aluminas are oommercially available, both for fluid and for fixed bed application. The rare earth metals comprise the elements scandium, yttrium, lanthanum and the lanthanides, i.e. the elements having atomic numbers 58 to 71 inclusive. The rare earth elements often are found together and are rather difficult to separate. Conveniently, therefore, sometimes mixtures of rare earth elements in naturally occurring ratios are used, without determining with element actually is the most active catalyst. Very satisfactory results have been obtained when as earth metal lanthanum, cerium, praeseodymium and/or neodymium is used. A mixture of rare earth elements excluding cerium is commercially obtainable - at a reasonable price - under the name of didymium, and excellent results have been obtained herewith. Preferably therefore a mixture of (at least) lanthanum, praeseodymium and neodymium, in a naturally or commercially occurring ratio, is used as rare earth metal [please refer to Kirk-Othmer, Encyclopedia of Chemical Technology (1968), Vol. 17, p. 147, for ore information on rare earth mixtures]. The rare earth petal or metals may be introduced onto the carrier by any convenient method, for example by impregenation of aqueous solutions of soluble salts of the rare earth metal(s). Preferably the rare earth metal(s) is (are) present as oxide(s). Suitably the carrier is dried and calcined after a solution of rare earth metal nitrate or acetate has been absorbed. The calcination is effected preferably at a temperature in the range from 350 to 6000C. The two different carboxylic acids should have at least two carbon atans, which means that e acetic acid and higher homologs can be used in the preparation of ketones. Formic acid, however, can not be used since it gives rise to aldehydes, which usually decompose. The smallest unsymetrical ketone is methyl ethyl ketone which, according to the present inveniton, may be prepared from acetic acid and propionic acid. Suitably two different carboxylic acids are used having not use than 30 carbon atoms each. The carboxylic acids may be aliphatic including arylaliphatic; e.g. phenylacetic acid is included. Excellent results are obtained when acetic acid is used as carboxylic acid. The invention thus provides in particular a process for the preparation of methyl ketones, CH3(CO)R, in which R represents a hydrocarbyl group, preferably an alkyl group, by reacting acetic acid with a carboxylic acid RDOOR. In a preferred embodiment an aliphatic acid having no or only one alpha-hydrogen atam is used as carboxylic acid. It appears that such acids give rise to relatively little hxnrxlandensation, so that still higher selectivities towards the unsymmetrical ketone are obtained. Acids having no alpha-hydrogen atom include pivalic or trimethylacetic acid, trichloroacetic acid and &alpha;,&alpha;-dimethyl- butyric acid. Pivalic acid is preferably used as the acid having no alpha-hydrogen atom, since after reaction with acetic acid it yields the important chemical product pinacolone, or tertiary butyl methylketone. Examples of acids having only one razz alph-hydrogen atan are isobutyric acid, &alpha;-methylbutyric acid, &alpha;-bromopropionic acid and &alpha;-phenylpropionic acid. Preferably iscbutyric acid is used as the acid having only one alpha-hydrogen atom, since after reaction with a acetic acid it yields the important solvent methyl isoprcpyl ketone. It is also possible to use the anhydride form of carboxylic acids. For instance, passing acetic anhydride over the Catalyst according to the invention results in the formation of dimethyl ketone (acetone), and passing acetic anhydride mixed with another acid would result in an unsymmetrical ketone. According to the invention the selectivity towards unsymmetrical ketones is increased and consequently the yield of symwetrical ketones is relatively small. It has been found, bower, that the catalyst according to the invention also catalyzes the reaction between a symmetrical ketone and a carboxylic acid t yield an other ketone. This now makes it possible to increase thf yield of unsymmetrical ketone, by recycling any undesired symmetrical ketones and converting these to the desired unsymmetrical ketone. Advantageously thus any symmetrical ketone formed is separated fran the unsymmetrical ketone and subsequently recycled to the catalyst. The molar ratio of the two different carboxylic acids may be roughly stoechiometric, i.e. about 1, but rather large excesses of one of the two may have some advantages, sometimes. If one of tte two carboxylic acids happens to have a relatively low homo- condensation tendency, as is the case with acids having no -H atoms, it may be present in excess to the other acid in order to suppress any homocondensation of that acid. The already high yield of unsymmetrical ketone may thus be further increased. an the hand it is desirable to use an excess of the cheaper acid in order to convert all of the more expensive acid into unsymmetrical ketone. Usually, acids having no or one ac-H atan are relatively expensive The molar ratio therefore, of the two carboxylic acids preferably lies between 10 and 0.1, in particular between 5 and 0.2, depending on the above considerations. These ratio limits may be extended even further if one of thf two acids, preferably the one with the higher hanoondensation tendency, is slowly and gradually added to a large volume of the other acid. At the beginning of the reaction the molar ratio thtr is almost zero (or infinity), at the end it may be one. The most preferred way of reacting the two acids, however, is to feed them - premixed - over a heated catalyst. The reactants may have been preheated to effect their vaporization, e.g. to a temperature of 100 to 4000C. The preparation, i.e. the actual decarbcylative (hetero) condensation reaction, is preferably carried out at a temperature in the range of 380 to 5800C, in particular of 425 to 5250C. Advantageously the preparation is carried out at a weight hourly space velocity of 0.1 to 5kg carboxylic acids/kg catalyst/hour. The two carboxylic acids may be used per se, or diluted with an inert carrier gas such as nitrogen or argon. Air, however, is not inert and causes a decrease in selectivity towards the desired unsymmetrical ketone. Addition of steam has no detrimental effect on the production rate of unsymmetrical ketone, and has the extra advantage of keeping the carbon deposits on the catalyst at a low level, thus reducing the frequency of reeneration. Preferably, therefore, the preparation is carried out in the presence of steam The molar ratio of the carboxylic acids (together) to the steam preferably lies between 0.1 and 100, in particular between 0.5 and 5.0. The pressure used may be atmospheric pressure or slightly above. EXAMPLES The following Examples illustrate the invention. The identity of the products was confirmed by infra-red and nuclear magnetic resonance analysis and by gas-liquid chromatography as necessary. All catalysts were prepared by adding a solution of rare earth metal nitrate in water to the carrier in an amount equal to its pore-volume, and removing the water in the pores by stirring the mixture at about 100-150 C. Then the catalysts were calcined in air at 5000C for 2 hours. All catalyst compositions are given in pbw of metal per 100 parts of carrier. 6g of calcined catalyst were transferred to a micro-fluid-bed reactor. A mixture of carboxylic acids, steam and nitrogen, at atmospheric pressure, was fed through the catalyst at various temperatures (T, C), various molar ratios and various weight hourly space velocities (WHSV, kg/kg/hour). This corresponded generally to a residence time of 1-5 seconds. The conversion of acid and the yield of ketones were measured by gas liquid chromatography and nmr analysis. Example 1 The reaction of pivalic acid and acetic acid to yield pinacolone was tested using four catalysts based on garrrna-alumina, E-type, i.e. an alumina suitable for use in fluid beds, having a specific surface area or 125mê/g and manufactured by Ketjen. All catalysts contained 17.2 parts by weight of metal, based on 100 parts of alumina carrier. Actually the petals were present as oxides, i.e. (presumably) as Di2O3, CeO2, Nd2O3, or La2O3, Di2O3 stands for didymium oxide, a rare earth netal mixture having the following composition 45.8% La203, 9.5% Pr6011, 32.5% Nd203, 5.5% Sm203, 3.5% Gd2O3, 0.5% Y203, 2.7% other rare earth oxides (all percentages are by weight). The WHSV and the conversion percentage of pivalic acid (PAC) and the yield molar percentage of pinacolone (PON) are tabulated in Table I, together with the calculated selectivity of pivalic acid towards pinacolone. The molar ratio of pivalic acid, acetic acid, steam and nitrogen amounted 1/1.4/1.4/1.1. TABLIF: I Experiment Catalyst WHSV T Conversion Yield Selectivity kg/kg/h C PAC, % PCN, %m PON, % PAC 1 17.2 Di 0.96 431 53.0 48.9 92.3 2 17.2 Di 0.96 415 33.5 32.7 97.6 3 17.2 Di 0.96 450 82.0 74.3 90.6 4 17.2 Ce 0.97 430 36.4 33.8 92.9 5 17.2 Nd 0.97 435 59.6 57.8 97.0 6 17.2 La 0.99 430 50.7 47.8 94.3 7 17.2 La 0.49 430 88.4 76.0 86.0 It can be seen that of the metals tested, cerium is less effective than didymium, lanthanum is about equally active and neodymium is rrore effective than didymium in the synthesis of pinacolone. Example 2 me reaction between pivalic acid and acetic acid according to Example 1 was carried out using a series of didymium catalysts, based on different carriers, including a silica and a silica-alumina carrier, as comparative examples. The results are shown in Table II, using the same units as in Table I, but adding the yield figures of dimethyl ketone (DMK). The molar yield to DMK refers to the conversion of acetic acid to dimethyl ketone, whereas the yield to PON refers to the conversion of pivalic acid (and acetic acid) to pinacolone. It should be noted that 1 mole of acetic acid yields 1/2 mole of DMK and that 1 mole of pivalic acid yields 1 stole of E(N, assuming 100% conversion and the selectivity. In all experiments a mixture of pivalic acid/acetic acid/steam/ nitrogen was fed to the reactor in a molar ratio of 1/1.4/1.4/1.1, except in experiment 14, where this ratio was 1/2/2/2.5. II Experiment Catalyst T WHSV Conv. Yield Sel. kg/kg/h % %m % C PAC PAC ON DMK PON 8* 19.6 Di/Si02 ) 468 2.10 46.3 19.3 73.8 41.7 9* 19.6 Di/SiO2-Al2O3ê) 465 2.48 100 0 64.8 0 10 17.2 Di/Al2O3 ) 480 0.96 99.5 92.4 33.6 92.9 11 17.2 Di/Al2O2 ) 450 0.96 82.0 74.3 46.9 90.6 12 26.2 Di/Al2O34) 398 1.01 19.4 19.3 77.8 99.5 13 8.6 Di/Al2O3 ) 462 0.97 54.2 50.8 57.3 93.7 5 14* 8.6 Di/A1203 ) 530 0.83 38.5 38.5 70.6 99.9 * comparative 1) silica, type F-22 ex. Ketjen, specific surface area 400 mê/g. 2) silica-alumina, type LA-HPV ex Ketjen, S.A. 600m2/g, containing 13%w Al2O3. 3) r alumina, type E ex Ketjen, S.A. 125ê/g. 4) &gamma; alumina, type D ex Ketjen, S.A. 250m2/g. 5) &alpha;alumina, type SAEHS-5, ex Carborundum, S.A. 1.4 mê/g. Evidently the catalysts based on pure alumina are superior to those based on silica or silica-alumina: the pivalic acid conversion and/or the selectivity towards pinacolone is better, whereas the yield of dimethylketone is lower (note, however, that experiment 12 represents a non-optimized case at a too low temperature). The gamma-alumina having a specific surface area of 125m2/g gives the best results. Example 3 Example 2 was repeated using catalysts based on alumina but containing different amounts of didymium. The results are shown in Table III, using the same abbreviations, notes and units as in Table II. In experiments 15 the molar ratio pivalic acid/acetic acid/ steam/nitrogen was 1/4/4/4, in experiment 16 it was 1/2/2/2.5, and in experiments 17-19 it was 1/1.4/1.4/1.1. TABLE III Experiment Catalyst T WHSV Conv. Yield kg/kg/h % C PAC PAC PON 15* 5.7 Di/A12035) 530 0.21 99.5 87.6 77.2 88.0 16* 8.6 Di/Al2O35) 530 0.33 60.1 55.5 65.0 92.3 17 8.6 Di/Al2033) 438 0.97 30.4 29.6 69.5 97.4 18 17.2 Di/A1203 ) 431 0.96 53.0 48.9 57.1 92.3 3 19 26.2 Di/A1203 ) 432 0.83 62.6 61.3 53.2 97.9 It appears that increasing the didymium concentration and/or the surface of the support resulted in a more effective composition of acetic acid towards pinacolone {relatively less homocondensation). This is reflected in a decrease of the acetic acid/pivalic acid molar ratio. Example 4 Experiments 17, 18 and 19 of the preceding Example were repeated at other temperatures and space velocities. It appeared that both the activity and the selectivity of the catalyst increased with increasing didymium content. After plotting the activity versus the Di-cantent, it was concluded that the most efficient catalyst should contain 20 parts Di per 100 parts &gamma;Al2O3. Example 5 The catalyst containing 26.2% w Di on 100% A1203(E) was chosen for further experiments, including recycle of symmetrical product to increase the yield of unsymmetrical ketone. It was found that feeding a mixture of pivalic acid, acetic acid, steam and nitrogen, having a molar ratio of 1/1.15/2.7/1, at a WHSV of 0.9kg.kg-1 .h-1 pivalic acid and a temperature of 5200C, resulted in a product stream containing 0.2 enrol acetone, 0.9 mol pinacolone and 0.1 mol pivalic acid per mol of pivalic acid fed to the reactor. Recycle of the acetone formed was simulated by feeding to the reactor the above mentioned mixture, plus 0.2 mol acetone per mol of pivalic acid. This resulted, under the same conditions, in 100% conversion of pivalic acid to pinacolone, i.e. with a selectivity of 100%. This catalyst was further tested under the above conditions for an extended period of 60 hours. No deactivation was observed. Example 6 The catalyst of Example 5 was tested using a mixture of pivalic acid, propionic acid and steam (molar ratio: 1/1.6/3.2) at a WHSV of 0.7kg pivalic acid/kg catalyst/hour, at atmospheric pressure and at temperatures of 505 and 5300C, respectively. The products were absorbed in CH3CI and analyzed by gas liquid chromatography. The resulting weight percentages are shown in Table IV. IV Product 5050C 5300c propionic acid trace pivalic acid 27.4 3.0 diethyl ketone 37.0 42.7 t.butylethyl ketone 31.2 36.0 unknown 3.3 11.5 Notwithstanding the fact that propionic acid has two alpha hydrogen atoms, and a rather high homocondensation tendency therefore, a sizeable amount of the hetero-condensate is formed, which amount could even be increased by recycling the diethyl ketone formed. Example 7 Example 6 was repeated using a mixture of isobutyric acid, acetic acid steam (molar ratio:1/1.5/2.5) at a WHSV of 0.77kg isobutyric acid/kg catalyst/hour and temperatures of 390, 425, 450 and 4850C, respectively. The resulting weight percentages are given in Table V. TABLE V Product 3900C 425 OC 4500C 485 C acetic - - - isobutyric acid 30.4 dimethyl ketone 22.7 13.0 11.8 11.1 methylisopropyl ketone 46.9 83.9 83.3 80.6 diiscprcoyl ketone - 1 4.9 6.5 A sizeable amount of the imoprtant solvent methylisopropyl ketone is formed. Recycle of symmetrical products would even increase the yield of unsymmetrical product.

Claims: CALIMS 1. Process for the preparation of unsymetrical aliphatic ketones by heating two different carboxylic acids having at least two aliphatic carbon atoms in the presence of a rare earth metal containing catalyst, characterized in that the catalyst comprises one or morse rare earth metals on a gamma-alumina- based carrier. 2. A process as claimed in claim 1, characterized in that the catalyst contains from 5 to 30% of rare earth mental based on the weight of the carrier. 3. A process as claimed in cleim 1 or 2, characterized in that the gamma-alumina has a specific area of 50 to 260mê/g. 4. A process as claimed in any of claims 1 to 3, characterized in that as rare earth mental lanthanum, cerium, praeseodymium and/or neodynium is used. 5. A process as claimed in any one of claims 1 to 4, characterized in that the rare earth metal(s) is (are) present as oxide(s). 6. A process as claimed in any one of claims 1 to 5, characterized in that as carboxylic acid acetic acid is used. 7. A process as claimed in any one of claims 1 to ó, characterized in that as carboxylic acid an aliphatic acid having no or only one alpha-hydrogen atom is used. 8. A process as claimed in claim 7, characterized in that pivalic acid issued as the acid having no alpha-hydrogen atom. 9. A process as claimed in claim 7, characterized in that isobutyric acid is used as the acid having only one alpha-hydrogen atom. 10. A process as claimed in any one of claims 1 to 9, characterized in that any symmetrical ketone formed is separated fran the unsymetrical ketone and subsequently recycled to the catalyst. 11. A process as claimed in any one of claims 1 to 10, characterized in that the preparation is carried out at a weight hourly space velocity of 0.1 to 5kg carboxylic acids/kg catalyst/hour, a temperature of 380 to 5800C and a molar ratio of the two different carboxylic acids between 10 and 0.1. 12. A process as claimed in claim 1 substantially as described hereinbefore and with particular reference to the Examples. 13. An unsymetrical aliphatic ketone when prepared by a process as claimed in any one of claims 1 to 12. 14. Catalytic composition for the preparation of unsymetrical aliphatic ketones by heating two different carboxylic acids having at least two aliphatic carbon atoms, comprising a gamma-alimina-based carrier, containing from 5 to 30%, based on the weight of the carrier, of one or more rare earth mentals.

Inventors: WATTIMENA, FREDDY
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1983
Main classification: C07C45
IPC codes: C07B61, C07C45, C07C49, B01J21, B01J23, C07C67