The dehydrogenation of isopropanol, 2-butanol and cyclohexanol to acetone, methylethylketone and cyclohexanone respectively may be effected industrially in the vapor phase at a relatively high temperature (300.degree.-550.degree. C.), in the presence of a dehydrogenaton catalyst. This catalyst may contain a heavy metal from one of groups I, II, VI, VII and VIII of the periodic classification of the elements and more particularly such a metal as copper, magnesium, nickel, zinc, etc. these metals having sometimes added thereto other metals or derivatives of metals such as tin and lead, for example. The operating pressure is generally low and often not substantially different from the atmospheric pressure.
The necessity of operating in the vapor phase at a relatively high temperature and low pressure is the main cause of a number of drawbacks:
1. substantial supply of heat at a high thermal level; PA1 2. deactivation of the catalyst by decrease of its active surface owing to the growth of the particles by re-agglomeration; PA1 3. deactivation of the catalyst by "coke" deposit making necessary a regeneration of the catalyst by combustion at about 500.degree. C., by means of an oxygen-nitrogen mixture containing, for example, 2% of oxygen, this regeneration being effected every 10 days in some cases; PA1 4. incomplete conversion of the alcohol which requires a fractionation of the reaction effluent followed with a recycling of the unconverted alcohol; PA1 5. parasitic reactions which decrease the total yield and the purity of the produced hydrogen. These reactions are, for example: PA1 6. the use of an apparatus which is rather impractical in view of the following facts: PA1 having a high boiling point, higher than the boiling point of the alcohol to be converted; PA1 being not subject to any conversion or degradation in the reaction medium; PA1 having a much lower adsorption coefficient on the catalyst than that of the alcohol to be converted.
(a) formation of degradation products such as methane and carbon monoxide; PA2 (b) dehydration of the alcohol to an olefin or to a cycloolefin which, by condensation with the formed ketone, may result in the formation of products of the diacetone alcohol type and, subsequently of mesityl oxide; PA2 arrangement of the catalyst in furnace tubes where the thermal regulation is not easy, PA2 periodical disassembling for cleaing or changing the catalyst.
The use of a process wherein the reaction is conducted in the liquid phase makes it possible to avoid most of the above-mentioned drawbacks.
In the prior art, it has been established, on the one hand, that the removal of hydrogen was necessary to displace the equilibrium towards the conversion of alcohol to ketone, thermodynamically unfavoured at low temperature and, on the other hand, that the removal of the ketone from the reaction medium, as it is formed, facilitates the obtention of a satisfactory reaction velocity. The second condition is satisfied by adding to the reaction medium certain solvents which do not react under the operating conditions and make it possible to conduct the reaction at a temperature higher than the boiling temperature of the ketone, under the prevailing pressure.
In the prior art, solvents have been selected particularly with the following characteristics of:
The following compounds could thus be used as solvents: heavy naphthenic hydrocarbons such as decahydronaphthalene, heavy paraffinic hydrocarbons, naphthenic hydrocarbons having at least one paraffinic chain such as hexadecyldecahydronaphthalene or mixtures of the preceding hydrocarbons as they can be found in heavy petroleum cuts.
However, such a process in the liquid phase is finally very disadvantageous as compared with the conventional gaseous phase process. As a matter of fact, in the liquid phase process, the reaction temperature scarcely exceeds 150.degree. C. and, consequently, the conversion is much lower than that obtained in the gaseous phase process at 400.degree. C.: the conversion is about 25% for the manufacture of acetone and about 10% for that of methylethylketone, thus requiring a very substantial recycling of the unconverted alcohol. Moreover, even at 150.degree. C., it is impossible to completely avoid a stripping of the solvent, so that an oversizing of the fractionation stage is required. The occurrence of parasitic hydrogenolysis reactions which produce light hydrocarbons such as methane, ethane, propane and butane do not allow the liquid phase process to attain a selectivity of 100%: the purity of the produced hydrogen is about 98% at a temperature close to 150.degree. C. The temperature increase which favours the hydrogenolysis reactions and the solvent evaporation does not seem to be a good solution.