Continuous preparation of aldehydes and ketones

Aldehydes and ketones of the general formula I ##STR1## where R.sup.1 is hydrogen or an organic radical of 1 to n carbon atoms and R.sup.2 is a non-aromatic organic radical of 1 to m carbon atoms, (m+n) ranging from 2 to 24 and R.sup.1 and R.sup.2 being combinable to form a 4- to 12-membered ring, are prepared in a continuous manner by oxidizing an alcohol of the general formula II ##STR2## with oxygen or an oxygen-containing gas at elevated temperatures in the gas phase in the presence of a catalyst, by effecting the oxidation by means of a supported catalyst composed of an inert carrier having a smooth surface and from 0.1 to 20% by weight, based on the amount of carrier, of an active layer of copper, silver and/or gold in a tubular reactor or tube bundle reactor where the internal diameter D of the tube or tubes ranges from 10 to 50 mm and the largest diameter d of the coated supported catalysts is subject to the relationship d=from 0.1 to 0.2 D.

The present invention relates to a novel process for the continuous 
preparation of an aldehyde or ketone of the general formula I 
##STR3## 
where R.sup.1 is hydrogen or an organic radical of 1 to n carbon atoms and 
R.sup.2 is a non-aromatic organic radical of 1 to m carbon atoms, (m+n) 
ranging from 2 to 24 and R.sup.1 and R.sup.2 being combinable to form a 4- 
to 12-membered ring, by oxidizing an alcohol of the general formula II 
##STR4## 
with oxygen or an oxygen-containing gas at elevated temperatures in the 
gas phase in the presence of a catalyst. 
It is known from U.S. Pat. No. 2,042,220 that 3-methyl-3-buten-1-ol can be 
oxidized with an excess of oxygen at from 360.degree. to 550.degree. C. in 
the presence of metal catalysts, for example copper and silver catalysts, 
to 3-methyl-3-buten-1-al. The catalysts can be alloys, metal compounds or 
elemental metal. Preference is given to activated catalysts; the 
activating operations specified comprise surface amalgamation of the metal 
and subsequent heating of the metal surface. The preparation of copper and 
silver catalysts in the examples consists in reducing copper oxide 
particles at 300.degree. C. under hydrogen, or in amalgamating and heating 
silver wire networks. It is also known from DE-B No. 2,041,976 that the 
known process gives rise to appreciable amounts of undesirable byproducts. 
DE-A No. 2,517,859 describes the dehydrogenation of unsaturated alcohols 
over a copper catalyst which has a specific surface area of from 0.01 to 
1.5 m.sup.2 /g, substantially in the absence of oxygen at from 150.degree. 
to 300.degree. C. If the starting materials are .beta.,.gamma.-unsaturated 
alcohols, the by-products formed are .alpha.,.beta.-unsaturated aldehydes 
and saturated aldehydes; the selectivity for .alpha.,.gamma.-unsaturated 
aldehydes is low (page 2, last paragraph). Mixtures of this type need to 
be separated into their components in expensive operations. 
DE-B Nos. 2,020,865 and 2,041,976 describe the dehydrogenation of 
.beta.,.gamma.-unsaturated alcohols or .alpha.,.beta.-unsaturated alcohols 
to .alpha.,.beta.-unsaturated aldehydes. The dehydrogenation catalysts 
mentioned also include mixed catalysts, for example those made of copper 
and silver. It is a disadvantage, however, that appreciable amounts of 
nucleophilic substances need to be added. In the case of conversion of 
3-methyl-3-buten-1-ol, satisfactory results are only obtained with 
incomplete conversions, which, according to DE-B No. 2,243,810, leads to 
difficulties in separating off the unconverted starting material. 
Dehydrogenation of 3-methyl-3-buten-1-ol by the method of DE-B No. 
2,517,859 over metallic copper in the absence of oxygen produces 
appreciable amounts of isovaleraldehyde, and the activity of the catalysts 
decreases rapidly within a few days, so that frequent regeneration is 
necessary. 
FR-A No. 2,231,650 describes the preparation of aldehydes and ketones from 
the corresponding alcohols by air oxidation at 250.degree.-600.degree. C. 
in the presence of a gold catalyst. The advantage of a gold catalyst is 
the higher selectivity compared with copper and silver catalysts, so that 
byproduct formation is reduced. The disadvantage with this process is the 
expensive catalyst, since a solid gold catalyst is used. 
DE-B No. 2,715,209 and EP No. B-55,354 describe the oxidative 
dehydrogenation of 3-alkyl-buten-1-ols in the presence of molecular oxygen 
over catalysts which consist of layers of silver and/or copper crystals. 
The amount of oxygen ranges from 0.3 to 0.7 mole per mole of starting 
material. The disadvantage with this process is that catalyst expenses are 
high, because solid silver is used, and high selectivities can only be 
obtained if defined catalyst particle sizes or particle size distributions 
are used in a layer structure, even in some instances requiring specific 
mixtures of layers of copper and silver crystals. As a consequence, not 
only is the reactor expensive to charge but the catalyst is expensive to 
recover. In addition, the high reaction temperatures employed in this 
process cause sintering of the metal crystals, which leads to a pressure 
increase and short times-on-stream. 
JP-A No. 60/246,340 describes the gas phase oxidation of 
3-methyl-2-buten-1-ol to 3-methyl-2-buten-1-al at 300.degree.-600.degree. 
C. in the presence of oxygen over a supported catalyst. The supported 
catalyst has to be prepared in a complicated manner by impregnating the 
carrier with aqueous solutions of AgNO.sub.2, Cu(NO.sub.3).sub.2 
.times.3H.sub.2 O and Mg(NO.sub.3).sub.2 .times.6 H.sub.2 O, drying, 
calcining within a certain temperature range and activating under 
hydrogen. It is true that the catalyst gives a high selectivity of 96.6%, 
but only at the price of a low conversion, so that this catalyst is not a 
realistic option for use in industry. 
JP-A No. 58/059,933 describes the preparation of aldehydes and ketones by 
oxidative dehydrogenation of alcohols in the presence of a silver catalyst 
which additionally contains phosphorus. To maintain the selectivity of the 
reaction, a phosphorus compound is additionally introduced into the 
alcohol stream, so that product contamination is inevitable. Given that 
the aldehydes are used for scents and vitamins, the addition of an 
organophosphorus compound is evidently undesirable. 
Since all these known processes for preparing aldehydes and ketones are 
unsatisfactory in respect of simplicity and economy of operation, catalyst 
life and purity of reactor output, it is an object of the present 
invention to eliminate these disadvantages. 
We have found that this object is achieved with a process for the 
continuous preparation of an aldehyde or ketone of the general formula I 
##STR5## 
where R.sup.1 is hydrogen or an organic radical of 1 to n carbon atoms and 
R.sup.2 is a non-aromatic organic radical of 1 to m carbon atoms, (m+n) 
ranging from 2 to 24 and R.sup.1 and R.sup.2 being combinable to form a 4- 
to 12-membered ring, by oxidizing an alcohol of the general formula II 
##STR6## 
with oxygen or an oxygen-containing gas at elevated temperatures in the 
gas phase in the presence of a catalyst, which comprises effecting the 
oxidation by means of a supported catalyst composed of an inert carrier 
having a smooth surface and from 0.1 to 20% by weight, based on the amount 
of carrier, of an active layer of copper, silver and/or gold in a tubular 
reactor or tube bundle reactor where the internal diameter D of the tube 
or tubes ranges from 10 to 50 mm and the largest diameter d of the coated 
supported catalysts is subject to the relationship d=from 0.1 to 0.2 D. 
If desired, the catalyst can also be diluted with an inert material without 
an active coating. Suitable inert materials, which are also suitable for 
use as carrier materials, are ceramics, such as aluminum oxide, silicon 
dioxide, magnesium oxide, silicon carbide and in particular steatite. 
However, a catalyst layer should contain not less than 10% of particles of 
active material. 
Suitable inert shapes for the catalyst are primarily spheres and other 
structures such as ellipsoids, cylinders or rings. The diameter d of the 
spheres, or the largest diameter of the other shapes, can range from 1 to 
10 mm the diameters being subject, as defined, to the relationship with 
the internal diameter of the reactor tubes. 
The active catalyst metal is preferably applied to the inert material by 
flame spraying, but it is also possible to use other methods, for example 
impregnating or plasma spraying, provided the coating produced is 
abrasion-resistant; it should also be very smooth. 
Compared with the prior art, the process according to the invention 
surprisingly produces, in a simpler and more economical manner, a better 
overall result in terms of yield, space-time yield and purity of end 
product. In particular, the amount of product relative to the catalyst 
used is very much larger, so that using the selective noble metal 
catalysts silver or even gold can result in an economical process. The 
catalyst is simple to prepare and, especially if of spherical shape, 
simple to introduce into the reactor. A further advantage of the regular 
shape of the catalyst is that, without further measures, an ordered, 
relatively close packing is obtained in the reactor and, in the case of 
tube bundle reactors, the uniform packing produces a similar pressure loss 
in every individual tube. Since the pressure loss is the same in the many 
tubes of a tube bundle reactor, the flow through the individual tubes is 
the same, which evidently leads to a substantial improvement in the 
selectivity of the reaction. In the course of the reaction, no one tube is 
put under more stress than any other, so that the catalyst life is very 
long, amounting in practice to several months. 
The process is carried out by converting alcohol II into the gas phase and 
passing this gas together with oxygen over the catalyst at from 
300.degree. to 600.degree. C., preferably at from 350.degree. to 
450.degree. C. 
The oxidizing agent used can be not only pure oxygen but also a gas 
containing free oxygen, in particular air. Oxygen and alcohol II are 
expediently used in a molar ratio of from 0.2 to 0.6, in particular from 
0.3 to 0.5, mole of oxygen per mole of alcohol. If desired, it is possible 
to employ a solvent which is inert under the reaction conditions, for 
example water or ether. It is an advantage of the process according to the 
invention that, in addition to the nitrogen contained in the air, there is 
no need to use any further inert gas. Compared with the prior art 
processes, this results in a reduction in the inert gas content and hence 
in a simplification of the workup, since the useful product can be 
condensed from a smaller total gas stream. 
Preference is given to using a tube bundle reactor of from 100 to 10,000 
tubes of from 10 to 30 cm in length. For experimental purposes, it is 
sufficient to use just one tube. 
Expediently, the catalyst is operated at a space velocity of from 0.5 to 4 
tonnes, in particular from 0.7 to 2 tonnes of starting material II per m2 
of catalyst bed cross-section per hour. 
The reaction mixture is worked up in a conventional manner. For example, 
the hot reaction gas is absorbed with a solvent such as water or 
dimethylformamide or preferably in the condensed product mixture. 
The starting compound can in principle be any desired primary alcohol II. 
Or it can be any desired secondary alcohol, unless both R.sup.1 and 
R.sup.2 are aromatic. R.sup.1 and R.sup.2 can in turn have inert 
substituents such as fluorine, chlorine, bromine, cyano or tertiary amino 
and be interrupted by oxygen or oxygen-containing groups such as --CO-- 
and --O--CO--.

Examples are: 
saturated aliphatic primary alcohols of 1 to 24, preferably 6 to 18, carbon 
atoms, suitable being in the main branched alcohols and ether alcohols 
such as 2-ethoxy-ethanol; 
in particular preferably unsaturated aliohatic primary alcohols of 3 to 24, 
preferably 4 to 12, carbon atoms where the double bond or bonds are not in 
the 1,2-position, e.g. 
but-2-enol 
but-3-enol 
pent-3-enol 
pent-4-enol 
hex-3-enol 
hex-4-enol 
hex-5-enol 
hept-3-enol 
hept-4-enol 
hept-5-enol 
oct-4-enol 
oct-5-enol 
and in particular the C.sub.1 -C.sub.4 -alkyl homologs, in particular the 
methyl homologs, of these alcohols, of which in particular the butenols 
IIa and IIb 
##STR7## 
where R' is C.sub.1 -C.sub.4 -alkyl, in particular methyl. In general, the 
process is suitable in particular for alk-2-enols, since a group with 
--CH.dbd.CH--(C.dbd.O)-- conjugation forms, which energywise favors the 
dehydrogenation; 
cycloaliphatic/aliphatic primary alcohols such as hydroxymethylcyclohexane, 
hydroxymethylcyclohex-1-ene and the hydroxymethyltetrahydrofurans; 
araliphatic primary alcohols such as 2-phenylethanol; 
secondary aliphatic alcohols of 3 to 24, preferably 4 to 12, carbon atoms, 
for example butan-2-ol 
methylbutan-2-ol 
but-3-en-2-ol 
methylbut-3-en-2-ol 
pentan-2-ol 
pent-3-en-2-ol 
pent-4-en-2-ol 
pentan-3-ol 
pent-1-en-3-ol 
seconary araliphatic alcohols such as 1-phenylethanol, 
1-phenylpropanol and 1-phenylprop-2-enol; 
cyclic secondary alcohols, in particular those of 5 or 6 ring members, such 
as cyclohexanol, 3-hydroxytetrahydrofuran, 3-hydroxytetrahydropyran and 
4-hydroxytetra-hydropyran. 
The alcohols II are either known or obtainable by known methods. 
The carbonyl compounds I preparable by the process of the invention are 
useful starting materials for the preparation of dyes, pesticides, 
pharmaceuticals, plastics, scents, such as citral, vitamins, such as 
vitamins A and E, and chrysanthemumcarboxylic acid. 
The novel process is highly economical. In general it permits space 
velocities of from 50 to 500 moles per hour per liter of catalyst volume. 
Preparation of catalyst 
0.5 l of steatite spheres ranging from 1.6 to 2.0 mm in diameter was 
introduced into an open rotary drum and coated with silver powder 16 
micrometers in particle size by the method of flame spraying. This 
comprised introducing the silver powder into an oxygen/acetylene flame and 
transporting it with the flame gases in partially molten form onto the 
carrier. The powder quantity was dimensioned in such a way that the 
completed catalyst contained about 4% by weight of silver, based on the 
total weight. 
During the coating step, the carrier was tumbled in the rotary drum at a 
rate of 30 revolutions per minute. 
The temperature in the material to be coated was about 600.degree. C., and 
the coating took about 30 minutes. 
The flame spraying process produced catalyst spheres having a continuous 
smooth and non-porous coating of metallic silver. 
EXAMPLE 1 
Preparation of 3-methyl-3-but-3-enal using a silver catalyst 
11 ml of the catalyst were introduced into a reactor tube of 10 cm in 
length and 13 mm in internal diameter. A mixture of 30.1 l[S.T.P.]/h of 
technicalgrade 3-methyl-3-buten-1-ol (water content around 12-15% by 
weight) and 45.9 l[S.T.P.]/h of air was then passed through the tube at 
390.degree. C. and under 1.1 bar. The amount of oxygen in the mixture was 
0.32 mole per mole of 3-methyl-3-butenol. The reaction gases were 
collected and analysed. The alcohol conversion was 73% and the selectivity 
89%; this result did not change in the course of the test period of 10 
days. 
EXAMPLE 2 
Preparation of 3-methylbut-3-enal using a copper catalyst 
50 ml of a catalyst (steatite spheres 3.5-4.5 mm in diameter, layer of 
copper with a weight proportion of 9.57%, based on the inert material) 
which had been prepared in the same way as the silver catalyst were 
charged in a reactor tube 22 mm in internal diameter and 20 cm in length 
with a mixture of 27.7 l[S.T.P.]/h of 3-methyl-3-buten-1-ol and 75 
l[S.T.P.]/h of air at a reaction temperature of 390.degree. C. and under a 
pressure of 1.1 bar. The reaction products were trapped out of a bleed 
stream of the reaction gas, and analysed. A conversion of 78.6% and an 
aldehyde selectivity of 78.1% were found. 
Formaldehyde and acids were only produced in trace amounts. These results 
were obtained after the catalyst had been operated for a total of 9 days. 
No loss in activity or selectivity was found even after 4 weeks on stream. 
EXAMPLE 3 
Preparation of oct-1-en-3-one using a silver catalyst 
Example 1 was repeated, reacting 22.5 l[S.T.P.]/h of oct-1-en-3-ol and 45.2 
l[S.T.P.]/h of air at 380.degree. C. The alcohol conversion was 90% and 
the selectivity for the ketone 82%.