Preparation of aldehydes

A process for preparing aldehydes of the general formula I ##STR1## where R.sup.1, R.sup.2 and R.sup.3 are each hydrogen, C.sub.1 -C.sub.6 -alkyl, C.sub.3 -C.sub.8 -cycloalkyl, aryl, C.sub.7 -C.sub.12 -alkylphenyl, C.sub.7 -C.sub.12 -phenylalkyl and R.sup.1 and R.sup.2 are joined together to form a 3-, 4-, 5-, 6- or 7-membered cycloaliphatic ring, PA1 R.sup.1 and R.sup.3 are each C.sub.1 -C.sub.4 -alkoxy, phenoxy, methylamino, dimethylamino or halogen, and PA1 R.sup.1 is additionally hydroxyl or amino comprises reacting a carboxylic acid or ester of the general formula II ##STR2## where R.sup.1, R.sup.2 and R.sup.3 are each as defined above, and PA1 R.sup.4 is hydrogen, C.sub.1 -C.sub.6 -alkyl, C.sub.3 -C.sub.8 -cycloalkyl, aryl, C.sub.7 -C.sub.12 -alkylphenyl or C.sub.7 -C.sub.12 -phenylalkyl, with hydrogen in the gas phase at temperatures from 200.degree. to 450.degree. C. and pressures from 0.1 to 20 bar in the presence of a catalyst whose catalytically active mass comprises from 60 to 99.9% by weight of zirconium oxide and from 0.1 to 40% by weight of one or more elements of the lanthanides.

The present invention relates to a process for preparing aldehydes by 
reacting the corresponding carboxylic acids or esters with hydrogen in the 
gas phase in the presence of catalysts comprising zirconium oxide and 
elements of the lanthanides. 
It is known to convert carboxylic acids such as benzoic acid or 
cyclohexanecarboxylic acid or their esters into the corresponding 
aldehydes by hydrogenation in the gas phase. 
U.S. Pat. No. 3,935,265 discloses that it is possible to hydrogenate alkyl 
esters of aromatic carboxylic acids with hydrogen over Al.sub.2 O.sub.3 at 
from 400.degree. to 600.degree. C. For example, methyl benzoate is 
converted into benzaldehyde with a selectivity of 37% (conversion: 39%). 
Other catalysts used for the hydrogenation of aromatic and aliphatic 
carboxylic acids include, for example, Ru/Sn (EP-A-539 274), manganese 
oxide (EP-A-290 096, U.S Pat. No. 4,585,899), iron oxide (EP-A-304 853), 
vanadium oxide and/or titanium dioxide (U.S. Pat. No. 4,950,799, 
EP-A-414065), Cu/Y.sub.2 O.sub.3 (U.S. Pat. No. 4,585,900), Cr.sub.2 
O.sub.3 /ZrO.sub.2 (EP-A-150961, EP 439115), Cr.sub.2 O.sub.3 (U.S. Pat. 
No. 5,306,845) or lanthanide oxides/Al.sub.2 O.sub.3 (U.S. Pat. No. 
4,328,373, EP-A-101 111). 
The prior art hydrogenation processes produce only unsatisfactory yields 
and selectivities in most cases, in part because of very high 
hydrogenation temperatures. 
It is an object of the present invention to remedy the aforementioned 
disadvantages. 
We have found that this object is achieved by a novel and improved process 
for preparing aldehydes of the general formula I 
##STR3## 
where R.sup.1, R.sup.2 and R.sup.3 are each hydrogen, C.sub.1 -C.sub.6 
-alkyl, C.sub.3 -C.sub.8 -cycloalkyl, aryl, C.sub.7 -C.sub.12 
-alkylphenyl, C.sub.7 -C.sub.12 -phenylalkyl and R.sup.1 and R.sup.2 are 
joined together to form a 3-, 4-, 5-, 6- or 7-membered cycloaliphatic 
ring, 
R.sup.1 and R.sup.3 are each C.sub.1 -C.sub.4 -alkoxy, phenoxy, 
methylamino, dimethylamino or halogen, and 
R.sup.1 is additionally hydroxyl or amino, 
which comprises reacting a carboxylic acid or ester of the general formula 
II 
##STR4## 
where R.sup.1, R.sup.2 and R.sup.3 are each as defined above, and 
R.sup.4 is hydrogen, C.sub.1 -C.sub.6 -alkyl, C.sub.3 -C.sub.8 -cycloalkyl, 
aryl, C.sub.7 -C.sub.12 -alkylphenyl or C.sub.7 -C.sub.12 -phenylalkyl, 
with hydrogen in the gas phase at temperatures from 200.degree. to 
450.degree. C. and pressures from 0.1 to 20 bar in the presence of a 
catalyst whose catalytically active mass comprises from 60 to 99.9% by 
weight of zirconium oxide and from 0.1 to 40% by weight of one or more 
elements of the lanthanides. 
The process of the present invention can be carried out as follows: 
The novel hydrogenation of the carboxylic acid or ester II with hydrogen in 
the presence of a catalyst whose catalytically active mass comprises from 
60 to 99.9, in particular from 80 to 99.9%, by weight of zirconium oxide 
and from 0.1 to 40, in particular from 0.1 to 20%, by weight of one or 
more elements of the lanthanides is generally carried out at temperatures 
from 200.degree. to 450.degree. C., preferably from 250.degree. to 
400.degree. C., particularly preferably from 300.degree. to 380.degree. 
C., and pressures from 0.1 to 20 bar, preferably from 0.7 to 5 bar, 
particularly preferably at atmospheric pressure. The temperature and 
pressure required are dependent on the catalyst activity and the thermal 
stability of precursor and product. 
Suitable catalysts include supported catalysts, preferably solid catalysts 
of zirconium oxide in cubic, tetragonal or monoclinic phase, preferably in 
monoclinic phase, which have been doped with one or more elements of the 
lanthanide series. The catalytically active mass comprises in general from 
80 to 99.9% by weight, preferably from 90 to 99.9% by weight, particularly 
preferably from 92 to 99% by weight of zirconium oxide and from 0.1 to 20% 
by weight of one or more elements of the lanthanides, preferably from 0.1 
to 10% by weight of lanthanum, cerium, praseodymium, neodymium, samarium, 
europium or mixtures thereof, particularly preferably from 1 to 8% by 
weight of lanthanum(III) oxide. The doping is generally effected by 
saturating the zirconium oxide with salt solutions (aqueous or alcoholic) 
of the lanthanides. 
The catalyst may additionally include further dopants (e.g. chromium, iron, 
yttrium, manganese) in amounts from 0.001 to 10% by weight. Preference is 
given to catalysts without such additions. 
The BET surface area of the zirconium oxide can vary within wide limits and 
is generally from 5 to 150 m.sup.2 /g, preferably from 20 to 150 m.sup.2 
/g, particularly preferably from 40 to 120 m.sup.2 /g. 
Catalysts of this type are produced in a conventional manner, for example 
by saturating preformed carrier elements such as pellets, balls or 
extrudates, drying and calcining. 
The preferred supported catalysts are very active over a prolonged period. 
Deactivated catalysts can be regenerated by treatment with gases 
containing molecular oxygen, e.g. air, at temperatures from 350.degree. to 
500.degree. C. 
The weight hourly space velocity over the catalyst is held in general 
within the range from 0.01 to 10, preferably within the range from 0.01 to 
3, kg of carboxylic acid or ester per kg of catalyst per hour. 
The hydrogen concentration in the feed gas depends on the carboxylic acid 
or ester concentration. The molar ratio of hydrogen to carboxylic acid or 
ester is in general within the range from 2:1 to 100:1, preferably within 
the range from 10:1 to 70:1. The hydrogen can also come from formic acid 
used as source. 
It can also be advantageous to add an inert diluent. Typically, nitrogen, 
water or gaseous reaction-inert compounds such as hydrocarbons, aromatics 
or ethers are employed. 
The reaction can be carried out in the gas phase, continuously as a fixed 
bed reaction with a fixed bed catalyst, for example in an upflow or 
downflow (trickle) process, or as a fluidized bed reaction with the 
catalyst in the fluidized state. Preference is given to the use of a fixed 
bed. 
By-products of the hydrogenation, e.g. alcohols, can be recycled into the 
synthesis to increase the selectivity. 
The substituents R.sup.1, R.sup.2, R.sup.3 and R.sup.4 in compounds I and 
II each have independently of the others the following meanings: 
R.sup.1, R.sup.2, R.sup.3 and R.sup.4 : 
hydrogen, 
C.sub.1 -C.sub.6 -alkyl, preferably C.sub.1 -C.sub.4 -alkyl such as methyl, 
ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl and tert-butyl, 
particularly preferably methyl and ethyl, 
C.sub.3 -C.sub.8 -cycloalkyl such as cyclopropyl, cyclobutyl, cyclopentyl, 
cyclohexyl, cycloheptyl and cyclooctyl, preferably cyclopentyl, cyclohexyl 
and cyclooctyl, particularly preferably cyclopentyl and cyclohexyl, 
aryl such as phenyl, 1-naphthyl, 2-naphthyl, 1-anthryl, 2-anthryl and 
9-anthryl, preferably phenyl, 1-naphthyl and 2-naphthyl, particularly 
preferably phenyl, 
C.sub.7 -C.sub.12 -alkylphenyl such as 2-methylphenyl, 3-methylphenyl, 
4-methylphenyl, 2,4-dimethylphenyl, 2,5-dimethylphenyl, 
2,6-dimethylphenyl, 3,4-dimethylphenyl, 3,5-dimethylphenyl, 
2,3,4-trimethylphenyl, 2,3,5-trimethylphenyl, 2,3,6-trimethylphenyl, 
2,4,6-trimethylphenyl, 2-ethylphenyl, 3-ethylphenyl, 4-ethylphenyl, 
2-n-propylphenyl, 3-n-propylphenyl and 4-n-propylphenyl, preferably 
2-methylphenyl, 3-methylphenyl, 4-methylphenyl, 2,4-dimethylphenyl, 
2,5-dimethylphenyl, 2,6-dimethylphenyl, 3,4-dimethylphenyl and 
3,5-dimethylphenyl, particularly preferably 2-methylphenyl, 
3-methylphenyl, 4-methylphenyl, 
C.sub.7 -C.sub.12 -phenylalkyl such as benzyl, 1-phenethyl, 2-phenethyl, 
1-phenyl-propyl, 2-phenyl-propyl, 3-phenyl-propyl, 1-phenyl-butyl, 
2-phenyl-butyl, 3-phenyl-butyl and 4-phenyl-butyl, preferably benzyl, 
1-phenethyl and 2-phenethyl, particularly preferably benzyl, 
R.sup.1 and R.sup.3 : 
C.sub.1 -C.sub.4 -alkoxy such as methoxy, ethoxy, n-propoxy, isopropoxy, 
n-butoxy, isobutoxy, sec-butoxy and tert-butoxy, preferably methoxy, 
ethoxy, n-propoxy and isopropoxy, particularly preferably methoxy and 
ethoxy, 
phenoxy, 
methylamino, 
dimethylamino, 
halogen such as fluorine, chlorine, bromine and iodine, preferably 
fluorine, chlorine and bromine, particularly preferably chlorine and 
bromine, and 
R.sup.1 : 
hydroxyl, 
amino, 
R.sup.1 and R.sup.2 together a 3-, 4-, 5-, 6-, or 7-membered cycloaliphatic 
ring such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and 
cycloheptyl, preferably cyclopentyl, cyclohexyl and cycloheptyl, 
particularly preferably cyclopentyl and cyclohexyl, in which case R.sup.3 
is preferably hydrogen or C.sub.1 -C.sub.6 -alkyl. 
The starting materials used are carboxylic acids or esters of the formula 
II, e.g. phenylacetic acid, diphenylacetic acid, triphenylacetic acid, 
methyl phenylacetate, pivalic acid, isobutyric acid, phenylmethylacetic 
acid, cyclohexanecarboxylic acid or ester, cyclopentanecarboxylic acid or 
ester, dimethylhydroxyacetic acid, diphenylchloroacetic acid, 
dimethylmethoxyacetic acid. 
The process of the present invention provides a simple way of selectively 
preparing previously difficult-to-obtain aldehydes. 
Aldehydes I are useful as aromas and flavorings and as intermediates, for 
example for drugs and crop protection agents (Ullmann's Encyclopedia of 
Industrial Chemistry, Vol. A3, pages 469 to 474).

EXAMPLES 
Catalyst Preparation 
Example 1 
Monoclinic zirconium dioxide (BET surface area: 53 m.sup.2 /g) in the form 
of tablets (catalyst A) was saturated with an aqueous solution of 
lanthanum nitrate by thorough mixing and the mixture was held at room 
temperature for 2 h. The catalyst was then dried at 120.degree. C. for 15 
hours and then heat-treated at from 400.degree. to 500.degree. C. for from 
2 to 4 hours. 
The catalyst thus prepared had a lanthanum content of 3% by weight. 
Example 2 
Hydrogen at 100 1/h was used to vaporize 11 g/h of pivalic acid (as melt) 
and pass it at 330.degree. C. in the downflow direction through 100 ml 
(128 g) of catalyst A. The gaseous reaction effluent was condensed in cold 
traps and analyzed by gas chromatography. The pivaldehyde yield was found 
to be 98% (conversion 100%). The corresponding alcohol was obtained as a 
recyclable by-product with a yield of 1%. 
Example 3 
Example 2 was repeated to react the carboxylic acids and the carboxylic 
ester of the table below, which also shows the results of the reaction. 
TABLE 
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Aldehyde Conver- 
Selectivitity 
Carboxylic acid or 
Temperature 
yield 
Alcohol yield 
sion 
(incl. alcohol recycle) 
ester .degree.C.! 
%! %! %! %! 
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Cyclohexanecarboxylic 
330 90 1 97 94 
acid 
2-Methylpropionic 
310 68 -- 83 82 
acid 
Methyl 310 43 15 85 61 
3,5-dimethyloctanoate 
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