Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
1999-11-15
2002-08-06
Shippen, Michael L. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
Reexamination Certificate
active
06429344
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for the preparation of d,l-menthol by catalytic hydrogenation of compounds which have the carbon skeleton of menthane containing at least one C═C double bond and are 3-substituted by oxygen and/or of menthone or isomenthone or mixtures of such compounds using hydrogen at temperatures of 100 to 200° C. and at hydrogen partial pressures between 2 and 50 bar and/or by rearrangement of menthol stereoisomers in the presence of hydrogen at temperatures of 0 to 140° C. and at hydrogen partial pressures between 0.1 and 20 bar in the presence of noble-metal-containing catalysts.
Surprisingly, it has been found that high hydrogen partial pressures markedly decrease the rate of formation of d,l-menthol, so that high temperatures are necessary to achieve high d,l-menthol contents. At low pressures, particularly high d,l-menthol contents can be achieved at low temperatures.
BACKGROUND OF THE INVENTION
Among the naturally occurring cyclic terpene alcohols, l-menthol occupies a special position owing to its cooling and refreshing action. l-Menthol is the main constituent of peppermint oil and is used in the perfume, flavoring and pharmaceutical industry.
Preparation of menthol by catalytic hydrogenation of compounds which have the resolved into carbon skeleton of menthane containing at least one C═C double bond and are their organoleptic properties. l-Menthol has a characteristic peppermint odor and the refreshing action already mentioned; it is therefore the most valuable of the menthol stereoisomers. Attempts are therefore made to carry out hydrogenation in such a way that as much d,l-menthol, from which l-menthol can be obtained by resolution of the racemate, is formed, or to rearrange as effectively as possible menthol stereoisomers, as arise, for example, in the hydrogenation of thymol.
DE 2,314,813 and EP-A-0,563,611 disclose that aromatic or partially hydrogenated cyclic compounds which have the carbon skeleton of menthane containing at least one C═C double bond and are 3-substituted by oxygen can be hydrogenated with hydrogen and/or stereoisomers of menthol can be rearranged in the presence of hydrogen, a cobalt/manganese catalyst or a fixed-bed catalyst which comprises palladium, ruthenium, rhodium or a mixture of these elements as active constituents and alkali metal hydroxides and alkali metal sulphates as promoters on a support doped with a rare earth (RE) metal and with manganese being used.
U.S. Pat. No. 2,843,636 describes the isomerization of stereoisomers of menthol to give d,l-menthol using hydrogen in the presence of a copper chromite catalyst. In this case, about 5% of hydrocarbons which are no longer reusable are produced.
All of these previously described processes for the preparation of menthol by hydrogenation and isomerization starting from compounds having the carbon skeleton of menthane are carried out at high pressures from 200 to 350 bar and high temperatures from 160 to over 220° C. In none of the processes described is more than 59.8% of d,l-menthol formed starting from thymol.
Since industrial apparatuses for carrying out high-pressure hydrogenations are complex and expensive, and a large amount of energy is needed for the compression of hydrogen to the required pressures, on the processing side also, there is considerable potential for improving the production of d,l-menthol by hydrogenation/isomerization starting especially from thymol.
SUMMARY OF THE INVENTION
The object of the invention was therefore to find a selective and industrially simple process for the preparation of d,l-menthol which makes possible d,l-menthol contents of over 60% in the product mixture and avoids, as much as possible, the formation of unwanted by-products.
It has been found that this purpose can be achieved starting from compounds which have the carbon skeleton of methane containing at least one C═C double bond and are 3-substituted by oxygen and/or from menthone or isomenthone or mixtures of such compounds, for example thymol, if hydrogenation is carried out with hydrogen at temperatures of 100 to 200° C. and at hydrogen partial pressures of 2 to 50 bar and then rearrangement is carried out at 0 to 140° C. and a hydrogen partial pressure between 0.1 and 20 bar and for this, noble-metal catalysts containing elements of subgroup VIII are used.
DETAILED DESCRIPTION OF THE INVENTION
The invention is surprising with respect to the fact that just avoiding high pressures causes a marked acceleration of the formation of d,l-menthol; very particularly, at low temperatures. The invention is further surprising in that particularly high d,l-menthol contents can be apparently achieved at the low temperatures. The invention, therefore, relates to a process for the preparation of d,l-menthol by catalytic hydrogenation of compounds which have the carbon skeleton of menthane containing at least one C═C double bond and are 3-substituted by oxygen and/or of menthone or isomenthone or mixtures of such compounds using hydrogen and/or by catalytic rearrangement of menthol stereoisomers in the presence of hydrogen, characterized in that the hydrogenation is carried out with hydrogen at hydrogen partial pressures between 2 and 50 bar, preferably between 4 and 10 bar and at temperatures of 100 to 200° C., preferably 120 to 180° C., and the catalytic rearrangement is carried out at a hydrogen partial pressure between 0.1 and 20 bar, preferably between 1 and 10 bar, and at temperatures of 0 to 140° C., preferably at 40 to 100° C., particularly preferably at 70 to 90° C. and the catalysts used are noble-metal catalysts containing metals of subgroup VIII.
The process of the present invention can be carried out, for example, batchwise or continuously in the bottom phase with suspended catalysts or in reactors for operating stationary catalyst beds. Suitable reactors for the process of the present invention using suspended catalysts are, for example, stirred tanks, bubble columns and loop reactors, as are known from the prior art.
Suitable reactors for the process of the invention using stationary catalyst beds are, for example, thermostated shell and tube reactors in which the catalyst is situated in the tubes or around the tubes for heat dissipation, or adiabatic tube reactors, preferably having zones for cooling the product stream. The reactor for the hydrogenation is preferably operated cooled and for the isomerization is preferably operated adiabatically.
Preferably, the process of the present invention is carried out in reactors having stationary catalyst beds.
The reactors for stationary catalyst beds can be individual low-pressure steel or steel alloy tubes, which are completely or partially packed with shaped catalyst bodies, whereas in the case of higher tube cross-sectional areas, the use of shaped bodies on trays (for example, wire baskets or similar items) can also be useful; however, tube bundles can also be employed within a collective shell, with the individual tubes, in turn, being completely or partially packed with the shaped bodies.
The above-mentioned reactor types are to be understood as examples and are not to be interpreted restrictively.
The process of the present invention can be carried out continuously in two successive reactors, the first, principally serving for the hydrogenation and the second, principally serving for the catalytic rearrangement.
This reactor cascade can be charged with starting material in such a manner that the mixture of the compounds to be hydrogenated is first hydrogenated in the first reactor to give a mixture of the isomeric menthols to which are admixed menthols to be recycled before being fed into the rearrangement reactor, wherein the menthols are produced, for example, in the isolation of l-menthol, but the menthols to be recycled can alternatively be completely or partially run into the hydrogenation reactor together with the mixture to be hydrogenated.
The process of the present invention can likewise be carried out continuously in a reactor in which the conditions ar
Langer Reinhard
Petruck Gerd-Michael
Bayer Aktiengesellschaft
Cheung Noland J.
Gil Joseph C.
Shippen Michael L.
LandOfFree
Process for the preparation of d,I-menthol does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for the preparation of d,I-menthol, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for the preparation of d,I-menthol will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2938613