Continuous method for producing substituted cyclohexylmethanols

1. A continuous process for preparing a 1-hydroxyalkylcyclohexane of the formula (I)

where

R 1 is isopropyl and

R 2 is hydrogen or a straight-chain or branched alkyl radical having from 1 to 3 carbon atoms, which comprises catalytically hydrogenating an aromatic carbonyl compound of the formula (II)

and/or by catalytically hydrogenating an aromatic alcohol of the formula (III)

in which the R 1 and R 2 radicals are each as defined in formula (I),

in the presence of hydrogen and in the presence of a catalyst which comprises, as an active metal, ruthenium applied to a support, the support comprising SiO 2 , and the catalyst being used in the form of a fixed bed catalyst in a hydrogenation reactor or in a plurality of hydrogenation reactors connected in series, and the fixed bed catalyst, based on the total weight of the finished catalyst, having a ruthenium content of from 0.1 to 0.5% by weight;

wherein the catalytic hydrogenation is performed in a cascade of n hydrogenation reactors connected in series comprising a first hydrogenation reactor and one or more further hydrogenation reactors, where n is an integer from 2 to 5, and where

a1) the reaction mixture which has been discharged continuously from an upstream hydrogenation reactor and not recycled, and hydrogen, are introduced continuously into the downstream hydrogenation reactor,

b1) the resulting reaction mixture comprising 1-hydroxyalkylcyclohexane of the formula (I) is discharged continuously from the particular hydrogenation reactor and optionally, partly recycled back into the particular hydrogenation reactor, and

c1) optionally, the 1-hydroxyalkylcyclohexane of the formula (I) is removed from the reaction mixture which has been discharged from the nth hydrogenation reactor in step b1) and not recycled.

2. The process according to claim 1 , wherein

a) the starting compounds of the formula (II) and/or (III) and hydrogen are introduced continuously into the first hydrogenation reactor,

b) the resulting reaction mixture comprising 1-hydroxyalkylcyclohexane of the formula (I) is discharged continuously from the first hydrogenation reactor and, if desired, partly recycled back into the first hydrogenation reactor and

c) optionally, the 1-hydroxyalkylcyclohexane of the formula (I) is removed from the reaction mixture which has been discharged from the first hydrogenation reactor in step b) and not recycled.

3. The process according to claim 1 , wherein an aromatic carbonyl compound of the formula (II) is used.

4. The process according to claim 1 , wherein a fixed bed catalyst which comprises only SiO 2 as a support is used.

5. The process according to claim 1 , wherein a fixed bed catalyst comprising, as an active metal, ruthenium alone or together with at least one further metal of transition groups IB, VIIB or VIII of the Periodic Table of the Elements, applied to a support comprising silicon dioxide as a support material, the amount of the active metal being <1% by weight, based on the total weight of the catalyst, and at least 60% by weight of the active metal being present in the coating of the catalyst up to a penetration depth of 200 μm, determined by means of SEM-EPMA, is used.

6. The process according to claim 1 , wherein the hydrogenation is performed without addition of solvents.

7. The process according to claim 1 , wherein the first hydrogenation reactor is operated in trickle mode.

8. The process according to claim 1 , wherein at least one of the further hydrogenation reactors is operated in liquid phase mode.

9. The process according to claim 1 , wherein the hydrogenation is performed at pressures in the range from 100 to 250 bar.

10. The process according to claim 1 , wherein the hydrogenation is performed at temperatures in the range from 80 to 160° C.

11. The process according to claim 1 , wherein an aromatic aldehyde of the formula (II) which has an acid value of from 0 to 5 mg KOH/g is used.