Process for the preparation of alpha-bêta unsaturated alcohols

Abstract: The invention relates to a process for the preparation of α - β unsaturated alcohols of the formula where R₁, R₂ and R₃, which may be equal or different, represent phe­ nyl, optionally substituted with one or more alkyl and/or alkoxy groups, the total number of C atoms of these substituents being at most 5, or hydrogen or an alkyl group with 1-10 C atoms, with at least one of the groups R₁, R₂ and R₃ representing the optionally substi­ tuted phenyl group, by hydrogenation in the liquid phase of the alde­ hyde corresponding with the desired alcohol, this being effected in the presence of a platinum-containing catalyst and of an alkali metal hydroxide and/or an alkali metal alkoxide as promotor, this process being characterized in that platinum-on-graphite is used as catalyst.

Description: The invention relates to a process for the preparation of α-β unsaturated alcohols of the formula where R₁, R₂ and R₃, which may be equal or different, represent phe­ nyl, optionally substituted with one or more alkyl and/or alkoxi groups, the total number of C atoms of these substituents being at most 5, or hydrogen or an alkyl group with 1-10 C atoms, with at least one of the groups R₁, R₂ and R₃ representing the optionally substi­ tuted phenyl group, by hydrogenation in the liquid phase of the alde­ hyde corresponding with the desired alcohol, this being effected in the presence of a platinum-containing catalyst and of an alkali metal hydroxide and/or an alkali metal alkoxide as promotor. Such alcohols in practice are of importance in the flavours and fragrances industry. A process as described above is known from EP-C-4122. This process is suitable for industrial-scale application. The highest efficiencies according to EP-C-4122 are achieved at low temperatures, for instance up to 20◊C. On a technical scale cooling to such a rela­ tively low temperature of such a strongly exothermic reaction necessi­ tates costly investments as ammonia or nitrite cooling, for instance, is to be applied. If the reaction is carried out at a higher tem­ perature, for instance at 40◊C, a simpler cooling system, such as cooling water will suffice, it is true, but then the selectivity of the hydrogenation is found to become substantially lower. The object of the invention is to provide a process as described in the preamble, is suitable for application on an industrial scale, in which hydrogenation can be effected with high selectivity at elevated temperature. The process according to the invention for the preparation of α-β unsaturated alcohols of the formula where R₁, R₂ and R₃, which may be equal or different, represent phe­ nyl, optionally substituted with one or more alkyl and/or alkoxy groups, the total number of C atoms of these substituents being at most 5, or hydrogen or an alkyl group with 1-10 C atoms, with at least one of the groups R₁, R₂ and R₃ representing the optionally substi­ tuted phenyl group, by hydrogenation in the liquid phase of the alde­ hyde corresponding with the desired alcohol, this being effected in the presence of a platinum-containing catalyst and of an alkali metal hydroxide and/or an alkali metal alkoxide as promotor, is charac­ terized in that platinum-on-graphite is used as catalyst. It is noted that in themselves various carriers were already known for platinum-containing catalysts. From US-A-3284517, for instance, kieselguhr, calcium sulphate, carbon and calcium carbonate are known. In EP-C-4122, palladium-on-activated-carbon is constantly used. In GB-A-1123837, too, preference is given to a platinum-on-activated-carbon catalyst. It was not to be anticipated that replacement of activated carbon by graphite would have such an advantageous effect that the drawback of the state of the art as referred to in the preamble can be overcome. Examples of aldehydes for which the process according to the invention is particularly suitable are: 3-phenyl-2-propenal (cinnam­ aldehyde), 2-n-pentyl-3-phenyl-2-propenal, 2-n-hexyl-3-phenyl-2-­ propenal, 2-methyl-3-phenyl-2-propenal, 2-phenyl-2-pentenal, 2-phenyl­ 2-propenal and 3-p-methoxyphenyl-2-propenal. The process according to the invention can be applied in one phase, for instance in isopropyl alcohol. By preference, however, the process according to the invention is applied in a two-phase system of water and a non-water-miscible organic solvent. In such an embodiment of the process according to the invention complete or almost complete conversion can be achieved at low catalyst consumption. In this case various non-water-miscible organic solvents can be used, such as, for instance, benzene, xylenes, cyclopentane, cyclohexane, n-pentane and n-hexane. By preference toluene is used. The ratio of the water to the organic solvent can be varied, for instance between 0.05 and 3 g water per g organic solvent. The amount of organic solvent per g of aldehyde to be converted can be chosen within a wide range, for instance bet­ ween 0.25 and 20 g, preferably between 0.5 and 5 g organic solvent per g of aldehyde to be converted. As catalyst, use can be made of the platinum-containing cata­ lysts that are known for hydrogenation, for instance in an amount corresponding with 0.03-5 mg, preferably 0.05-2.5 mg platinum per g of aldehyde to be hydrogenated. The platinum of the catalyst according to the invention is mounted on a graphite carrier. The weight percentage of platinum, calculated as metal, in itself is not of importance. This percentage may vary from, for instance, 0.1 to 10 wt.%. The amount of alkali metal hydroxide and/or alkali metal akoxide that is dissolved in the water is, for instance, 0.1-0.7 mol, by preference 0.2-0.5 mol per g water. In the hydrogenation according to the invention the tem­ perature chosen in principle is not too high, for instance between -5◊C and 60◊C. According to the invention it is preferred to use a temperature between 30◊ and 55◊C, since at that temperature no special cooling system is required, but the cooling water will suffice. In addition, the reaction proceeds faster than at, for instance, room temperature. If nevertheless, it is preferred to work at a lower tem­ perature, the selectivity that can be reached with the process according to the invention also is higher than that reached with prior art processes. The hydrogen pressure as such is not critical. A good result is mostly obtained when applying a partial hydrogen pressure of between 500 and 30,000 kPa. If the hydrogenation is effected in a two-phase system, upon completion the liquid reaction medium can be separated into an organic layer, which contains the desired reaction product, and an aqueous layer, in which the catalyst and the promotor applied are present. This aqueous layer can be used anew in the hydrogenation of the alde­ hyde. From the organic layer the unsaturated alcohol can be recovered, for instance by distillation, while the organic solvent can be re-used. If a one-phase system is applied, purification of the reac­ tion product can take place by first separating the catalyst by means of filtration and subsequently distilling. The process according to the invention will be elucidated in the following examples. Examples I-XI A 5-l autoclave provided with a stirrer and a cryostat cooling was charged with 700 g cinnamaldehyde, 467 g toluene, 210 g water, 8.8 g KOH (100%) and 16.8 g catalyst. The air present was expelled by means of nitrogen. After the desired temperature had been reached, hydrogen was introduced into the autoclave until the desired pressure was reached (Table 1). The set total pressure was maintained throughout the experiment by means of hydrogen. Upon completion of the experiment the catalyst with the aqueous phase was separated and extracted with toluene. The reaction mixture, which beside cinnamic alcohol also contained phenylpropanol, phenylpropanal as well as unconverted cinnamaldehyde, was analyzed for its composition using gas-liquid chromatography. On the basis of this analysis and the amount of material started from, the conversion and the selectivity could be calculated. The experiments were varied in weight percentage of platinum in the catalyst, reaction time, temperature and pressure under other­ wise identical conditions. These results are presented in Table 1.

Claims: 1. Process for the prep-ration of α-β unsaturated alcohols of the formula where R₁, R₂ and R₃, which may be equal or different, represent phenyl, optionally substituted with one or more alkyl and/or alkoxy groups, the total number of C atoms of these substituents being at most 5, or hydrogen or an alkyl group with 1-10 C atoms, with at least one of the groups R₁, R₂ and R₃ representing the optionally substituted phenyl group, by hydrogenation in the liquid phase of the aldehyde corresponding with the desired alco­ hol, this being effected in the presence of a platinum-containing catalyst and of an alkali metal hydroxide and/or an alkali metal alkoxide as promotor, this process being characterized in that platinum-on-graphite is used as catalyst. 2. Process according to claim 1, characterized in that the process is carried out in a two-phase system consisting of water and a non-­ water-miscible organic solvent. 3. Process according to claim 2, characterized in that toluene is used as organic solvent. 4. Process according to any of claims 1-3, characterized in that the catalyst contains 0.1-10 wt.% platinum, calculated as metal. 5. Process according to any of claims 1-4, characterized in that the reaction is carried out at a temperature between 30 and 55◊C. 6. Process according to any of claims 1-5, characterized in that as starting material use is made of 3-phenyl-3-propenal, 2-n-pentyl-3-phenyl-2-propenal, 2-n-hexyl-3-phenyl-2-propenal, 2-methyl-3-phenyl-2-propenal, 2-phenyl-2-pentenal, 2-phenyl-2-propenal and 3-p-methoxyphenyl-2-propenal. 7. Process as substantially described and/or elucidated in the examples. 8. α-β unsaturated alcohols obtained by application of any of the preceding claims.

Inventors: BOSMA, MARCEL ANTOINE ROBERT, VAN DE MOESDIJK, CORNELIS GERARDUS MARIA
Applicants: STAMICARBON B.V.
Application year: 1986
Main classification: C07C29
IPC codes: B01J23, C07B61, C07C29, B01J31, C07C43, C07C67, C07C41