PROCESS FOR THE PREPARATION OF ALDEHYDES

Abstract: An aldehyde is prepared by passing an aromatic car­ boxylic compound or an aliphatic carboxylic compound having at most one α-hydrogen atom in the acyl group, in the presence of hydrogen at elevated temperature over a catalyst comprising at least 25%w of iron oxide expressed as Fe₂0₃, calculated on the total catalyst.

Description: The invention relates to a process for the preparation of aldehydes from carboxylic compounds, such as carboxylic acids, and esters, anhydrides and salts thereof. A useful method of preparing an aldehyde starting from a carboxylic acid is the so-called Rosenmund reduction in which the acid is converted into an acyl chloride which is reduced with hydrogen to the aldehyde. In EP-A-101,111 a process for the preparation of aldehydes is described in which certain carboxylic compounds are directly hydrogenated to the corresponding aldehydes over a catalyst comprising at least one rare earth metal and at least one additional metal, e.g. iron, on a carrier. The quantity of the additional metal(s) amounts to 0.1 to 20% w and preferably 0.5 to 10% w, calculated as the element(s) and based on the weight of the carrier. The catalyst shows a rather good reactivity and a fair selectivity to the aldehydes. However, it appeared that during the hydrogenation of a carboxylic acid over the above catalyst a considerable loss of the rare earth metal(s) took place. This loss is probably due to the formation of rare earth metal salts of the carboxylic acid to be hydrogenated and the subsequent sublimation of the salts thus obtained. It has now been found that iron oxide can be used as a catalyst in the preparation of aldehydes, provided that the catalyst comprises considerable quantity of iron oxide. The presence of a rare earth metal is not required. Such a catalyst can very suitably be used in the hydrogenation of aromatic carboxylic compounds and aliphatic carboxylic compounds having at most one α-hydrogen atom in the acyl group. The present invention therefore relates to a process for the preparation of an aldehyde form an aromatic carboxylic compound or an aliphatic carboxylic compound having at most one α-hydrogen atom in the acyl group, in which process the carboxylic compound is contacted at elevated temperature with hydrogen in the presence of a catalyst comprising from 25%w to 97%w of iron oxide expressed as Fe₂O₃, calculated on the total catalyst. By an aromatic carboxylic compound is understood a compound in which the carbon atom of the carboxylic group is directly connected to a carbon atom of the aromatic moiety. In general the aromatic moiety comprises a phenyl group, but a polynuclear aromatic group or a heterocyclic aromatic group is possible, too. The iron oxide catalyst as defined above appears to show an even greater reactivity and selectivity than those described in EP-A-101,111. Moreover, the catalyst is stable in that no catalytically active material has been found to disappear from the catalyst. The catalyst is advantageously applied in the form of particles, the maximum, dimension of which may, for example, lie in the range of from 0.5 to 15 mm. The shape of the particles is not critical; for instance spherical, cylindrical or annular particles may be used. The catalyst used in the process according to the invention may, if desired, contain up to 97 % w of iron oxide. A catalyst consisting exclusively of iron oxide shows an excellent initial reactivity and selectivity. However, in course of time a deactivation occurs due to the pulverization of the catalyst particles. To avoid pulverization the catalyst preferably contains a binding material. The binding material serves to retain the physical structure of the catalyst particles. The catalyst preferably comprises 50 to 95% w of iron oxide expressed as Fe₂O₃, calculated on the total catalyst. Catalysts which contain, as binding material, at least one oxide of silicon, of aluminium or of one or more transition elements tend to exhibit a very high physical stability. It is therefore preferred to employ such binding materials in catalysts used in the process according to the invention. Chromium is an especially preferred such transition element. The quantity of the binding material may vary. So it is possible to use up to about 75% w of binding material, calculated on the total catalyst. In the case of aluminium or silicon oxide as binding material the quantity of aluminium or silicon oxide may conveniently range from 25 to 75% w of the total catalyst. It will be appreciated by those skilled in the art that a catalyst comprising 25% w iron oxide, expressed as Fe₂O₃, and 75% w aluminium or silicon oxide (carrier) represents the catalyst having the lowest iron:carrier ratio, and that in such a catalyst, the quantity of iron calculated as the element and based on the weight of the carrier is greater than 23% w. In catalysts which comprise at least one oxide of the transition elements the quantity thereof preferably does not exceed 50% w of the total catalyst. A most preferred catalyst contains 5 to 15% w of chromium oxide expressed as Cr₂O₃ calculated on the total catalyst. The catalysts may be prepared in any conventional method. When the catalyst to be prepared should contain relatively little iron oxide, e.g. 25 to 45% w, it is conveniently prepared by impregnation of the binding material with a solution of an iron salt, which impregnation is optionally followed by drying and calcination. When the catalyst should contain over 45% w of iron oxide a very suitable way of preparing the catalyst is to precipitate the hydroxides or carbonates of iron and of aluminium, silicon and/or transition elements together and to dry and calcine the composition thus obtained. Another suitable method to prepare the catalyst is to form intimate mixtures of hydroxides, oxides or salts of the above elements, and optionally to dry and calcine the resulting mixture. During compacting or tabletting of the catalyst it is sometimes advantageous to use a small amount of a lubricating agent which is added to the mixture of metal oxides. The quantity of the lubricating agent depends to a large extent on the properties and effectiveness of the particular agent. A quantity of 0.5 to 7.5% w of lubricating agent, calculated on the total mixture of metal oxides, is generally appropriate. Suitable lubricating agents include for example graphite and high molecular weight fatty acids (e.g. stearic, palmitic or oleic acid). According to the present invention carboxylic compounds which are either aromatic or aliphatic provided they have at most one α-hydrogen atom in the acyl group, can be converted into the corresponding aldehydes. The carboxylic compounds can be selected from acids, esters, anhydrides or even salts. An example of a suitable salt is the ammonium salt. Aliphatic carboxylic compounds having two or more α-hydrogen atoms in the acyl group appear not to produce aldehydes in good yield when being contacted with hydrogen over an iron catalyst, but yield ketones and/or hydrocarbons instead. Ketones are formed due to a decarboxylative condensation of two molecules of the carboxylic compound to be treated. The acyl group preferably contains 4 to 20, in particular 5 to 12 carbon atoms. The acyl group may contain one or more substituents such as a halogen atom or, a hydroxy, nitro, nitroso, amino, alkylthio, cyano or alkoxy group. Since a carboxylic acid is generally more readily available than the derivatives thereof, the carboxylic compounds is preferably a carboxylic acid. Preferred esters which can be converted according to the invention, are C₁-C₆ alkyl esters of the above carboxylic acids. Preferred acids are benzoic acid, optionally substituted by e.g. an alkyl, e.g. C₁-C₄ alkyl, alkoxy, e.g. C₁-C₄ alkoxy, hydroxy, phenoxy or halogen moiety and aliphatic acids having no α-hydrogen atoms in the acyl group, trialkyl-substituted acetic acids, e.g. tri-(C₁-C₆ alkyl)-substituted acetic acids, especially such acids containing up to 10 carbon atoms such as pivalic acid, 2-ethyl-2-methyl butanoic acid and 2,2-dimethyl pentanoic acid, being in particular preferred, since the hydrogenation of these carboxylic acids to the corresponding aldehydes according to the invention is extremely selective. A very suitable trialkyl-substituted acetic acid is pivalic acid (in which each alkyl group is a methyl group). The actual hydrogenation reaction is preferably carried out by passing the reactants over the catalyst, either in a fixed, in a moving or in a fluidized bed. Advantageously the process is carried out in a fixed bed. The catalyst and/or the reactants may have been preheated to a suitable reaction temperature, advantageously to at least the vaporization temperature of the carboxylic acid or the derivatives thereof, e.g. to a temperature of 100 to 400°C, since the carboxylic compound is preferably in the gaseous phase when being contacted with hydrogen in the presence of the catalyst. It is, however, also possible to carry out the preparation starting from liquid carboxylic compounds. The hydrogenation can be carried out using hydrogen generated in situ, e.g. by the dehydrogenation of methanol or other alcohols, but preferably hydrogen gas or a mixture of gases containing hydrogen gas is used. The carboxylic compound is preferably hydrogenated using hydrogen gas in a molar ratio of at least 0.1:1 relative to the carboxylic compound. Advantageously the molar ratio of hydrogen to the carboxylic compound is in the range from 12:1 to 0.1:1. Preferably, the molar ratio of hydrogen to the carboxylic compound is in the range from 4:1 to 1:1, especially for catalysts comprising iron oxide and chromium oxide. In EP-A-101,111 only experiments are described in which the molar ratio between hydrogen and the carboxylic acid is substantially greater than 4. Since lower molar ratios can very well be applied in the process according to the present invention, a considerable advantage is obtainable over the process according to EP-A-101,111 as less (expensive) hydrogen is required while a similar or higher aldehyde yield is attained. The preparation, i.e. the actual hydrogenation, may conveniently be effected at a temperature in the range from 250 to 550°C, more particularly from 400 to 500°C. Advantageously the preparation is carried out at a weight hourly space velocity of 10 to 10,000, preferably 100 to 1,000, kg.m⁻³.h⁻¹, i.e. kg of carboxylic compound per m³ of the catalyst bed per hour. The pressure used may be atmospheric pressure or a greater or lower pressure, e.g. from 0.1 to 10 bar. The reactants may be used per se, or diluted with an inert gas such as nitrogen or argon. Steam or water may be added to the feed to effect hydrolysis of anhydrides into the corresponding carboxylic acids, which are subsequently reduced to the aldehydes, and to minimize the formation of carbonaceous material on the catalyst. An inert solvent may be used to facilitate dosage of any solid starting materials. Toluene or benzene is very suitable. The aldehydes prepared can find industrial application as aroma chemicals or an intermediates in the production of a broad range of chemicals. The process will now be elucidated by means of the following Examples. Examples In the following experiments four catalysts were used. Comparison catalyst 1 contained 17.7% w Fe₂O₃ and 82.3% w Al₂O₃. The catalyst was prepared by impregnating γ-alumina with a soluton of ferric nitrate and by drying and calcining the impregnated composition obtained. (Catalyst density: 0.930 g/ml; particle size: 14-30 mesh = 0.6-1.4 mm). Catalyst 2 contained 26.3% w of Fe₂O₃ and 73.7% w of Al₂O₃ and was prepared in a similar way as catalyst 1 (catalyst density: 0.827 g/ml; particle size: 1/16″ extrudate (= 1.59 mm), 5-10 mm long). Comparison catalyst 3 was prepared by compressing Fe₂O₃ powder, followed by crushing and sieving. Particles of 10-20 mesh (about 0.8-1.6 mm) were used. The catalyst consisted for 100% w of Fe₂O₃ (Catalyst density: 0.995 g/ml). Catalyst 4 was a commercially available Fe₂O₃-Cr₂O₃ composition, comprising about 86.0% w of Fe₂O₃ 9,7% w of Cr₂O₃ and 4.3% w of graphite. (Catalyst density: 1.00 g/ml; particle size: 510 mesh = 1.7-4.0 mm). In all experiments the catalyst was charged to a fixed bed reactor and a mixture of a carboxylic compound hydrogen was fed through the catalyst bed at atmospheric pressure. In experiments 1, 2 and 5-10 pivalic acid was fed to the reactor in a toluene solution in which the pivalic acid/toluene molar ratio was 1.0:0.3. Further reaction conditions are indicated in more detail hereinafter. Example I The reduction of pivalic acid to pivalaldehyde was performed using catalysts having different iron oxide content. Conditions and results are indicated in Table I. The results of the above experiments clearly show that the catalysts 2 and 4 are more active and/or selective than catalysts 1 and 3, showing a higher conversion at higher space velocities and lower H₂/acid molar ratios. Catalyst 4 (Fe₂O₃-Cr₂O₃ composition) apparently is the best catalyst showing an excellent conversion and selectivity at the highest space velocity and lowest H₂/acid ratio. Example II The experiments were run with catalyst 2 and 4 at various H₂/acid ratios and space velocities, as indicated in Table II. From the results it is apparent that for both catalysts an increased aldehyde yield can be obtained by increasing the H₂/acid ratio or decreasing the space velocity. It is also evident that for optimal performance a lower H₂/acid molar ratio may be applied with catalyst 4 than with catalyst 2. Additionally, higher space velocities can be dealt with, using catalyst 4. Example III The reduction of several carboxylic acids and two esters was carried out using catalyst 2 or 4, as illustrated in Table III. The results of experiments 17 to 28 show that the process according to the present invention can very successfully be applied to a variety of carboxylic compounds. Comparison of the results of experiments 18 and 19 shows that catalyst 4 is excellent even when the space velocity is increased.

Claims: Process for the preparation of an aldehyde from an aromatic carboxylic compound or an aliphatic carboxylic compound having at most one alpha-hydrogen atom in the acyl group, in which process the carboxylic compound is contacted at elevated temperature with hydrogen in the presence of a catalyst comprising from 25% w to 97% w of iron oxide expressed as Fe₂O₃, calculated on the total catalyst. Process according to claim 1, in which the catalyst comprises 50 to 95% w of iron oxide, expressed as Fe₂O₃ calculated on the total catalyst. Process according to claim 1 or 2, in which the catalyst contains at least one oxide of silicon, of aluminium or of one or more transition elements. Process according to claim 3, in which the catalyst contains 5 to 15% w of chromium oxide, expressed as Cr₂O₃ calculated on the total catalyst. Process according to,any one of claims 1 to 4, in which the carboxylic compound is benzoic acid, a substituted benzoic acid or an aliphatic carboxylic acid having no alpha-hydrogen atoms in the acyl group. Process according to claim 5, in which the carboxylic compound is trialkyl substituted acetic acid. Process according to claim 6, in which the carboxylic compound is pivalic acid. Process according to any one of claims 1 to 7 in which the carboxylic compound is in the gaseous phase when being contacted with hydrogen in the presence of the catalyst. Process according to any one of claims 1 to 8, in which the molar ratio of hydrogen to the carboxylic compound is in the range 4:1 to 1:1. Process according to any one of claims 1 to 9, in which the temperature is in the range of from 250 to 550°C, the pressure is in the range of from 0.1 to 10 bar and the weight hourly space velocity of the carboxylic compound is in the range of from 10 to 10.000 kg.m⁻³.h⁻³.

Inventors: JOHN, CHRISTOPHER STEPHEN
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1985
Main classification: C07C45
IPC codes: C07C45, C07B61, B01J23, C07C47, C07C67