Note: Descriptions are shown in the official language in which they were submitted.
HOE 81/F 121
The invention relates to a process for the pre-
paration of 3-alkylaminophenols by catalytic dehydrogena-
tion of 3-alkylamino-2-cyclohexenones in the gas phase.
3-Alkylaminophenols are important intermediate
products for drugs, dyestuffs (German Offenlegungsschrift
2,458,347), herbicides, optical brighteners and anti-
oxidants.
. ~ . ,~2~?,~?3
U.S. Patent ~e~2~e~ describes a process for the
preparation of 3-aminophenols, which comprises dehydrogenating
3-amino-2-cyclohexenones in the presence of a dehydrogena-
tion catalyst in the liquid phase at 150-300C. In
general, an inert solvent boiling between 150 and 300C
is required in this process. Hydrocarbons and poiyglycol
ethers are used in particular. In order to achievé high
yields,a quantity of solvent which is 5 to 10 times as
large as the quantity of 3-amino-2-cyclohexenone is
required, and the 3-aminophenols formed must be separated
from the solvent by distillation.
- The encumbrance caused by large quantities of
solvents, some of which are readily ignitable at the
reaction temperatures, is ~ disadvantage in this process.
It has now been ~ound that 3-alkylamino-2-cyclo-
hexenones, particularly 3-dimethylamino-2-cyclohexenone,
~ f~
-- 2 --
can be vaporized either on their o~"n or in conjunction
with an entraining agent and can be dehydrogenated in the
gas phase on dehydrogenation catalysts to give 3-alkyl-
aminophenols.
The invention therefore relates to a process for
the preparation of 3-alkylaminophenols of the general
formula I --
~ "_ R5 (I)
in which the radicals Rl to R5 independently of on.e
another can be hydrogen or alkyl groups having up to 12 C
atoms, and R6 denotes an alkyl group having up to 12 C
atoms or, conjointly with R5, denotes an alkylene ring
which has 4 to 6 C atoms and which can also contain an
oxygen atom or a substituted nitrogen atom, by catalytic
lS dehydrogenation of 3-amino-2-cyclohexenones of the general
formula II:
~ ~R5 (II)
~2 }I
in which the radicals Rl to R6 have the meaning indicated
above, which comprises carrying out the dehydrogenation
reaction in the gas phase at 200 to 500C and under a
pressure of 0.1 to 10 bar.
The alkyl groups which are suitable for the rad-
icals R1 to R6 ln this connection can be straight-chain,
~L2~
-- 3 --
b~anched or cyclic and, in the case of Rl to R5 , can
further be substi~uted by halogen atoms.
The 3-alkylamino-2-cyclohexenones required as the
starting compound for the process according to the inven-
tion are readily accessible by reacting the corresponding1,3-cyclohexanediones with primary or secondary amines
(U~S. Patent 4,212,823)~
The radicals Rl to R4 are preferably hydrogen,
methyl, ethyl, propyl or butyl, in particular hydrogen or
methyl. R5 and R6 are preferably methyl or ethyl.
The dehydrogenation catalysts used are ruthenium,
rhodium, palladium, osmium, iridium, platinum, copper,
silver, gold, iron, cobalt or nickel, preferably ruthenium,
rhodium, palladium9 osmium, iridium or platinum. The use
of palladium has proved to be particularly advantageous.
--, ...
In general, these elements, in the form of salts,
are applied to support materials, such as carbon, aluminum
oxide, silicon oxide, chromium oxide, aluminosilicates,
zeolites, magnesium oxide, calcium oxide or titanium
- 20 oxide. Carbon and sillcon oxide have proved particularly
suitable. The quantity of the metal applied is generally
between 0.1 and 20% by weight, preferably between 0.1 and
5% by weight. The particles of the support can have any
- desired external form; spheres or extruded pieces are preferably used.
Before the start of the reaction, it is generally
advantageous to reduce, by passing hydrogen over them at
250-500C or by means of other reducing agents, the salts
which have been applied to the support.
The 3-alkylamino~2-cyclohe~enones can be vaporized
..
8~
- 4 --
directly. In order to ensure the avoi-
dance of tarry by-products during the evaporation process,
ho~Jever, it is advantageous to use an inert solvent boiling
between 50 and 300C as an entraining agent for the
starting material~In general, the quantity of solvent is
between 10 and 200% by weight, relative to the 3-aminocyclo-
hexenone, preferably between 10 and 90% by weight and par-
ticularly 20 to 50% by weight. The following are
examples of suitable solvents: water, aliphatic or aro-
10 matic hydrocarbons, such as toluene and decalin, and ali-
phatic and aromatic ethers, such as diphenyl ether or
diethylene glycol dimethyl ether.
Water is particularly suitable: while the 3-
alkylamino-2-cyclohexenones used as the starting material
are readi'y soluble in water, the 3-alkylaminophenols
formed in the dehydrogenation reaction are largely insoluble
in water. This offers the advantage that the latter can
be isolated by filtration and are immediately available
for further use, after drying. The unreacted 3-alkylamino-
2 cyclohexenone remains dissolved in the water and can bere-used as feed material without further purification.
If water is used as the entraining agent, the
dehydrogenation reaction takes place with a higher degree
of selectivity in respect of the desired 3-alkylamino-
phenols than if organic solvents are used. A high space-
time yield of up to 300 g/l.hour is achieved. It is sur-
prising that, in the course of this reaction, the water
- does not effect hydrolysis of the 3-alkylamino-2-cyclo-
hexenones; in p~rticular, 1,3-cyclohexanedione and
8~
-- 5 --
resorcinol, the possible by-products, are not formed. The
use of water instead of the abovementioned organic sol-
vents is also to be preferred from the point of view
operatir.g safety; ~later can also bé employed as t~e
entraining agent together with organic solvents, pro-
vided that the latter are miscible with water, for ex~mple
- glycol ethers.
The vaporization of the starting compound is pre-
ferably effected by dropwise addition of a mixture ~f the
lattçr with an entraining agent on to a heated vaporiza-
tion zone located above the catalyst. This vaporization
zone is packed with an inert material, such as spheres of
glass or stainless steel. The high-boiling 3-alkyl&~no-
2-cyclohexenones pass over into the vapor phase at ~
vaporization temperature of 300-500C. It proves ~dv~.-
tageous to feed in an inert carrier gas, for example nitro-
gen, pre-heated to 300-500C, in order to achieve complete
vaporization ~lnd a high selectivlty. The
addition of hydrogen to the inert carrier gas counteracts
early deposition of carbon on the catalyst and can
therefore be recommended. It is preferable to feed in 1~0
to 500 i of nitrogen and 50 to 200 1 of hydrogen per mole
of 3-amino-2-cyclohexenone.
The vaporization of the starting compound can
also be effected by atomizing a mixture of the latter with
the entraining agent, with the aid of a pre-heated stre~n
of nitrogen or nitrogen/hydrogen, in a t~Jo-fluid nozzle
located above the catalyst.
a ~ The dehydrogenation reaction is carried out at
.
~V~;~B~
-- 6 --
200 to 500C, preferably 250 to 400C, and under a pressure
of 0.1 to 10 bar, preferably 0.5 to 2 bar and especially
under normal pressure.
Catalysts which are employed for a prolonged
period generally exhibit a slow decline in space-time
yield. One of the possible causes of this deactivation
is the deposition of carbon on the catalyst. These cata-
lysts can be regenerated by burning off the carbon in an
oxygen-nitrogen mixture at a temperature of 350 to 500C.
After the carbon has been burnt off, the metal catalysi
must be reduced again by passing hydrogen over it.
- The procedure which follows has proved particularly
advantageous for carrying out the process according to the
invention: ~he reaction is carried out in a vertical
reactor, which can be heated externally. The catalyst is
located in the middle section of ~his reactor. Above ~his
is the vaporization zone, on to which the mixture of
starting compound and entraining agent is added dropwise.
Pre-heated nitrogen and hydrogen are passedfrom above over
the catalyst. The reaction mixture condenses at the
bottom of the reactor. If water is used as the entraining
agent, the 3-alkylaminophenol is precipitated in a crystal-
line form and is filtered off. If organic entraining
agents are used, the products are isolated by distillation.
The following examples are intended to illustrate
the invention; the gas rates (l/hour) quoted are in all
cases measured under normal conditions~
Exarnple 1
Charcoal having a specific surface area of approx.
. .
. _ _ , . . , ..... . . . .. . .. , ~,, _,
8~
~ - 7 -
1,000 m2/g was coated with 0.6% by weight o~ Pt by being
impregnated with an aqueous solution of K2PtC16.
40 ml of this catalyst were charged to the middle
section of a vertical reactor which can be heated exter-
nally, and were reduced at 330C by passing 20 l/hour ofnitrogen and 10 l/hour of ~ydrogen over the catalyst. The
starting material, 3-dimethylamino-2-cyclohexenone, was
added dropwise, in the form of a 50% strength by weight
solution in toluene, on to the vaporization zone above the
catalyst and was passed from above over the catalyst in the
form of vapor at an LHSV* of 0.9 hours 1 at 330C, together
with 20 l/hour of nitrogen and 10 l/hour of hydrogen. The
mixture of products obtained at the bottom of the reactor
- was analyzed by means of gas chromatography. At a con-
15 version of 63%, the yield of 3-dimethylaminophenol was 7%.
~nenthe reaction temperature was increased to 370C, the
yield of 3-dimethylaminophenol was 31%, at a conversion
of 71%.
*LHSV ( = liquid hourly space velocity) =
liters of solution of starting compound
litërs of catalyst x hour
~xample 2
.
Silica having a specific surface area of 100 m2/g
was coated with 1% by weight of Pd in the form of palladium
25 acetate and 0.9% by weight of K in the form of potassium
hydroxide. 100 ml of this catalyst were charged to the
reactor described in Example 1 and were red~lced with
hydrogen. A 50% strength by weight solution of 3-dimethyl-
amino-2-cyclohexenone in toluene as entraining agent was
-- 8 --
vaporized, in the presence of 20 l/hour of hyd.ogen, at
an LHSV of 0. 25 hours 1 and at a reactor temperature of
- 330C, and was dehydrogenated in the gas phase.
- Analysis by gas chromatography indicated a con-
5 version of 97% and a yield of 3-dimethylaminophenol of
75%. 6% by weight of cyclohexanone, 4% by weight of
phenol and 3% by weight of N,N-dimethylaniline were also
~ormed.
After an operating period of about 50 hours, the
10 catalyst was regenerated at 400-450C by burning off,
using 20 l/hour of air. After the subsequent reduction
of the catalyst, the same yield of 3-dimethylaminopheno'
was obtained as before the regeneration. In a special
test run, a catalyst ~ife of 1,000 hours was achieved,
15 regeneration being carried out 11 times.
Example 3
The catalyst and the reactor described in Example
2 were used. A 50% strength by weight aqueous s~lution of
3-dimethylamino-2-cyclohexenone was vaporized and dehy-
20 drogenated at 330C in the gas phase. In the course ofthis, an additional 30 l/hour of nitrogen and 10 l/hour
of hydrogen were passed through the reactor. Complete
conversion was achieved at an LHSV of 0.5 hours 1. The
3-dimethylaminophenol was precipitated from the aqueous
25 solution of product in the form of white-brown crystals.
639 g of the feed material were dehydrogenated in
an operating time of 52 hours. After drying, 617 g of
3-dimethylaminophenol were obtained, corresponding to a
yield of 98%. The space-time yield was 250 g/l.hour. The
3~2~
g
product had a melting point of 70C.
The catalyst was th~n regenerated at 400-450C ~y
burning off, using 10 l/hour of air, and was then reduced
again.
5764 g of 3-dimethylaminophenol, corresponding to
a yield of 96%, were then obtained from 804 g of starting
material under the same conditions as before regeneration.
After a further regeneration of the catalyst, a
further 595 g of 3-dimethylaminophenol were obtain~d at a
yield of 78% in an operating time of 50 hours. The con-
version was 95%. The unreacted feed material remained
dissolved in the water, while the reaction product was
produced in crystalline form and was filtered off.
Example 4
15The reactor described in Example 1 was used.
50 ml of a catalyst containing 1% by weight of Pd and
0.9% by weight of ~ on silica having a specific surface
area of 100 m2/g were employed for the dehydrogenation of
3-diethylamino-2-cyclohexenone. The feed material was
vaporized in the form of an 80% strength solution in
water. In addition 20 l/hour of nitrogen and 10 l/hour
of hydrogen were fed in at a reactor ternperature of 350C
and an LHSV of 0.5 hours 1, The conversion was 100%. The
3-diethylaminophenol was precipitated in crystalline
form and was filtered off. The yield was 80%.
Example 5
Using the same catalyst and under the same reaction
conditions as in Example 4, 3-methylamino-2-cyclohexenone
r~ was vaporized in the ~orm of a 40% strength solution in
,
-- 10 --
water, and was dehydrogenated in the gas phase. The
reaction product formed an aqueous and an organic phase.
- The 3-methylamino-2-cyclohexenone which had uot reacted
remained in the aqueous phase; the conversion was 95%.
After vacuum distillation, the yield of 3-methylaminophenol
was 70%.
~xample 6
Under the same conditions as in Example 5, 57 g of
3-dimethylamino-5-methyl-2-cyclohexenone were vaporized
in the form of a 50% strength solution in water and were
dehydrogenated in the gas phase. At an LHSV of 0.3 hours 1,
the conversion was 100%. 41 g of 3-dimethylamino-5-
methylphenol, corresponding to a yield of 72%, were obtained
after distillation of the organic phase of the reaction
i5 product,
