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Description  |
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The preparation of acetic anhydride on a technical scale is carried out by
oxidation of acetaldehyde or by reaction of acetic acid with ketone.
It is also known to prepare acetic anhydride by the reaction of acetic acid
methyl ester and carbon oxide while using nickel catalysts (German Pat.
No. 921,987), but this possibility for producing acetic anhydride is
plagued by fundamental disadvantages as compared to the above described
technical methods of synthesis, so that the preparation by means of nickel
catalyst never gained a wider significance. One of these disadvantages is,
for example, that pressures of preferably above 300 bar, especially from
500 to 700 bar, are applied and that even under these pressures the
reaction speed is slow; a further disadvantage is the fact that the
operation is generally carried out in the presence of a solvent alien to
the system, such as N-methylpyrrolidone and that, finally, larger
quantities of catalyst and solvent have to be circulated.
Surprisingly, a process for preparing acetic anhydride from acetic acid
methyl ester and carbon oxide has been found which comprises pressures of
from 1 to 500 bar and at temperatures of from 50.degree. to 250.degree. C
the reactants are conveyed over catalysts containing noble metals of the
8the subgroup of the periodic table or their compounds as well as iodine
and/or iodine compounds.
Under the reaction conditions acetic acid methyl ester and carbon oxide
react with each other in stoichiometric quantities. This does not mean,
however, that strictly stoichiometric quantities have to be used for the
reaction. Depending on the execution of the process there may be employed
methyl acetate as well as carbon oxide at excess quantities.
As suitable starting materials may be considered, in addition to the
commercially available methyl acetate, also mixtures of methyl acetate
with other chemical compounds such as mixtures of methyl acetate with
dimethyl ether. The latter is converted with carbonoxide under the
reaction conditions, either partially or completely, to yield methyl
acetate. In such a way it is possible, for example, to convert a mixture
of this kind entirely to acetic anhydride.
However, the methyl acetate which is employed as starting product, may also
contain methanol or minor quantities -- e.g. from 1 to 5% -- of water. In
that case the result is not only acetic anhydride, but additionally a
quantity of acetic acid being approximately equivalent to the quantity of
methanol or water. This acetic acid is generally a desirable product, too,
which can be separated easily -- e.g. by distillation -- from the main
product acetic anhydride. The use of methylacetate-methanol-mixtures
containing from 18 to 20% of methanol is very often particularly
economical, since mixtures of this kind are obtained as azeotropics during
other processes, e.g. upon esterification of acetic acid or
transesterification of acetic acid esters with methanol.
The carbon oxide which is needed for the reaction must not be pure either,
but may contain inert subcomponents such as nitrogen, carbon dioxide or
methane.
The presence of large quantities, e.g. of from 5 to 50% by volume, of
hydrogen in the reaction gas very often has a very favorable effect on the
execution of the reaction. The slight formation of soot and carbon dioxide
which can be observed ocasionally at reaction temperatures of above
150.degree. C; is suppressed in the presence of hydrogen. From this fact
results the advantage that in the presence of hydrogen the reaction can be
carried out at high temperatures at high reaction speeds and prolonged
life of the catalyst. On the other hand, when performing the reaction in
the presence of hydrogen, an increased formation of acetic acid is
generally noticed so that hydrogen is consumed.
The reaction is carried out at pressures of from 1 to 500 bar, preferably
from 10 to 300 bar, particularly preferred are pressures comprised between
50 and 200 bar.
The reaction is performed at a temperature range from 50.degree. to
250.degree. C, preference is given to the operation at from 120.degree. to
200.degree. C, because this temperature range most often allows for an
optimal relation of reaction speed to selectivity.
The reaction is executed in the presence of a catalyst which contains as
essential components noble metals and/or noble metal compounds of the 8th
subgroup of the periodic table, and of iodine or compounds thereof.
The noble metal component includes the following elements -- either single
or mixed--: ruthenium, rhodium, palladium, osmium, iridium and platinum.
Especially efficient are catalysts containing rhodium or
rhodium-compounds. Rhodium being a very expensive element, however, in
many cases use of slightly less efficient elements might technically make
sense, whereby especially palladium and platinum and their compounds may
be considered. The initial form of the metal applied may vary widely.
The metals may be used, for example, as such. Preference is given to the
use of finely distributed metals, such as they are formed e.g. by reducing
salts, oxides or hydroxides in solution or on carrier catalysts. As
examples for such metal catalysts may be cited raney-rhodium,
raney-palladium, raney-platinum, rhodium on active coal. Alloys of the
metals, besides the pure metals, e.g. the alloys with carbon and with the
carbonyl-forming non-metals, prove to be well efficient, too. Alloys of
metal with carbon are formed e.g. on those carrier catalysts which contain
finely dispersed metals, by conveying carbon oxide over these catalysts at
temperatures of above 100.degree. C. Alloys of metals with
carbonyl-forming non-noble metals may be prepared e.g. by reducing double
salts such as cobalt-rhodium-III-chloride or nickel-rhodium-III-cyanides
or of mixtures of the metal oxides, hydroxides or metal salts.
In many cases, however, the use of metal compounds instead of a metal or a
metal alloy is preferred. Good catalytic properties are ascribed e.g. to
metal halides such as RuCl.sub.3 . 3H.sub.2 O, RhCl.sub.3 . 3H.sub.2 O,
RhBr.sub.3 . 2H.sub.2 O, RhJ.sub.3, PdCl.sub.2, IrCl.sub.3, PtCl.sub.2 and
PtCl.sub.4, as well as oxides, hydroxides and metal salts of oxygen acids.
As examples of groups of these substances may be cited RuO.sub.2, Rh.sub.2
O.sub.3, Rh.sub.2 O.sub.3 . H.sub.2 O, Rh(OH).sub.3, sodium -- rhodite,
potassium platinite, PdO, OsO.sub.4. Salts of inorganic acids, such as
noble metal salts of nitric acid, phosphoric acid or sulfuric acid and
their hydrates are efficient catalysts for the reaction, too, for example
RH(NO.sub.3).sub.3 . 2H.sub.2 O, Rh.sub.2 (SO.sub.4).sub.3,
platinum-IV-phosphate and palladium nitrate. Suitable catalysts are also
the carboxylates of noble metals, such as the acetates, namely
rhodium-III-acetate, palladium-III-acetate or platinum-IV-acetate. Very
efficient catalysts are also complex compounds of noble metals, namely as
well neutral complexes as also complex acids and complex salts, too.
Ligands in such complexes are e.g. halogen, water, carbon oxide,
phosphines or compounds of the trivalent nitrogen. As such compounds are
to be considered e.g. K.sub.2 Rh Br.sub.5, Rh[P(C.sub.6 H.sub.5).sub.3
].sub.2 COCl, Rh(NH.sub.3).sub.5 Cl.sub.3, H.sub.2 Pt Cl.sub.6,
Pt(CH.sub.3)[P(CH.sub.2 H.sub.5).sub.3 ].sub.2 J.sub.2, [(n-C.sub.4
H.sub.9).sub.3 P].sub.2 Pd J.sub.2. Finally may be taken into
consideration metal carbonyles, metal carbonyl halides or metal carbonyl
halogen oxides as catalyst-components, e.g. RH.sub.2 Cl.sub.2 O . 3 CO,
Pt(CO).sub.n, Pt CO J.sub.2, Pd(CO).sub.n, Pd CO Cl.sub.2.
The iodine-containing component of the catalyst may also vary widely, it
may be added e.g. as elementary iodine or hydrogen iodide: Equally
suitable starting compositions are inorganic salts such as sodium iodide,
potassium iodide or cobalt iodide, as well as quaternary ammonium or
phosphonium compounds such as tetramethyl ammonium iodide or tetraethyl
phosphonium iodide which may be considered as catalyst components. In many
cases, however, use will be made of organo-iodine-compositions as catalyst
components, such as alkyl iodide, especially methyl iodide, or acyl
iodides, especially acetyl iodide.
Other halogens, especially bromine or bromine compounds have a certain
catalyzing effect, too.
It is recommendable to add to these essential catalyst components further
promotors with e.g. the following assignments: to reduce the quantity
needed of the aforementioned essential catalyst components, to improve the
durability of the catalysts, to increase their selectivity, to improve the
reaction speed. Such promotors are, for example complex-forming compounds
such as alkyl phosphines or aryl phosphines or even organic nitrogen
compounds such as pyridines, pyrrolidones, alkyl amines or
N-alkyl-derivatives of aniline. Numerous metals or metal compounds,
especially carbonyl-forming metals such as cobalt, nickel or iron show
also a good effect as promotors.
The catalyst is used as a suspension, a solid or a solution. Preference is
given to the use of catalyst solutions or catalyst suspensions. When using
catalyst suspensions, the liquids consist essentially in the starting
materials or the reaction products. Solid catalysts are preferably
prepared as carrier catalysts while utilizing the usual carrier materials
such as active coal, silica gel, silicates or aluminum oxides.
The concentration ratio of the aforementioned noble metals in the catalyst
varies generally from 0.0001 to 10 weight %, preferably keeping within the
range of from 0.001 to 1 weight %. As far as catalyst solutions and
suspensions are concerned, good technical results will be obtained with
low concentration rates, whilst carrier catalysts require higher
concentration rates for an economically most interesting embodiment of the
process. The concentration in iodine components of the catalyst varies
widely, too. Very often the best suitable concentration ratio is not only
determined by the noble metal components, but to a large extent also by
the rest of the reactants involved in each case. The concentrations keep
generally within from 0.01 to 20 weight %, but preferably from 0.1 to 10
weight %. Most often low reaction temperatures recommend high
concentration rates, though at higher reaction temperatures the most
economical embodiment of the process is brought about by lower
concentration rates.
In relation to carbonylation reactions there have been described quite
frequently in the past catalyst compositions being somewhat similar to
those specified herein. However, no description has been given so far of
an invention for the preparation of acetic anhydride under pressures below
500 bar from methyl acetate and carbon oxide at a high selectivity and
with good conversion rates. It is known that by means of such catalysts
carboxylic acids, especially acetic acid from methanol and carbon oxide,
may be prepared from alcohols and carbon oxide. As far as the
corresponding preparation of acetic acid from methanol and carbon oxide is
concerned, it is a known fact that the reaction mixture include high
concentrations in methyl acetate and that the reaction is leading to
acetic acid at a high selectivity.
It is, therefore, a surprise that under the conditions of the present
process acetic anhydride is obtained at a high selectivity.
It is further known that generally for carrying out the carbonylation
reactions with catalysts which are similar to those according to the
invention, the presence of large quantities of water are either required
or at least very favorable.
Surprisingly, the reaction is running speedily under the reaction
conditions as per the invention, i.e. also in absence of any noticeable
quantities of water.
The process may be carried out either continuously or discontinuously.
However, on an industrial scale preference is given to a continuous
reaction.
When carrying out the reaction continuously by using catalyst liquids or
suspensions, the reactor may consist, for example, of a tube mounted
vertically which is operated either as a bubble reactor or as
trickling-phase reactor. Carbon dioxide and methyl acetate are sent
through this reactor. The reaction products are worked up e.g. by
condensation and subsequent distillation. The non-reacted starting
materials and the discharged catalyst, which is generally being formed
during the work-up partially as column sump and partially as a low-boiling
substance, are recycled into the reactor.
The acetic anhydride which is prepared according to the invention suits all
known application fields, e.g. acetylation reactions or reacting with
alcohols to yield acetic acid esters.
The process according to the invention may be applied also to the
preparation of other anhydrides. As an example may be cited that the mixed
anhydride of acetic acid and propionic acid may be prepared from
ethylacetate and carbon oxide, or propionic anhydride may be obtained from
propionic acid ethyl ester and carbon oxide.
The following Examples illustrate the process of the invention, especially
concerning the preparation and efficiency of the catalysts.
EXAMPLE 1
The following substances are charged into a 200 ml agitator-autoclave:
50 g of methylacetate
5 g of methyl iodide
50 mg of Rh Cl.sub.3 . 3H.sub.2 O
200 mg of P(C.sub.6 H.sub.5).sub.3
After having heated to 140.degree. C, carbon oxide was introduced until the
pressure of 80 bars was reached. The temperature was then gradually
increased to 160.degree. C and for three hours carbon oxide was metered in
at such a rate that the pressure of 80 bar was maintained. The autoclave
was then cooled and the pressure released. About 60 g of a liquid mixture
containing about 27% of acetic anhydride were obtained. Further noticeable
products were mainly 2% (approx.) of acetic acid. The mixture was
submitted to a vacuum distillation at 20 mm Hg and at a heating
temperature of 80.degree. C so that a residue of about 7 g was obtained.
This liquid residue contained about 70 weight % of acetic anhydride and
was the catalyst of Example 2.
The vacuum distillation product was submitted to a fractional distillation
under a normal pressure. About 3 g of methyl iodide (boiling point at 760
mm Hg: 42.degree. C), about 35 g of methyl acetate (boiling point
57.degree. C at 760 mm Hg), about 1 g of acetic acid (boiling point
118.degree. C at 760 mm Hg) and about 11 g of acetic anhydride (boiling
point from 139.degree. to 140.degree. C at 760 mm Hg) were obtained.
EXAMPLE 2
A 200 ml agitator-autoclave was charged with the following substances:
50 g of methyl acetate
3 g of methyl iodide
7 g of the distillation residue of example 1
After having heated to 140.degree. C carbon oxide was introduced until a
pressure of 80 bar was reached. The temperature was then increased to
160.degree. C and carbon oxide was metered in for three hours at such a
quantity that the pressure of 80 bar was maintained. The autoclave was
then cooled and the pressure released by a condensation trap maintained at
-78.degree. C. More than 99% by volume of the waste gas consists in carbon
oxide.
About 64 g of a liquid mixture containing about 30% of acetic anhydride
were obtained. While working-up by distillation under a normal pressure,
about 2.5 g of methyl iodide and about 38 g of methyl acetate could be
recovered. The following products were eliminated by distillation: about 1
g of acetic acid and about 12 g of acetic anhydride leaving behind about
7.5 g of sump which consisted in about 70 weight % of acetic anhydride.
This sump product may be re-used as catalyst component.
EXAMPLES 3 - 12
Further Examples were carried out in analogy to Example 1, while varying
some catalyst components. The following details of Example 1 were
retained: the quantity of methyl acetate employed (50 g), the quantity of
methyl iodide used (5 g), the reaction temperature (160.degree. C), the
reaction pressure (80 bar) and the reaction time (3 hours).
The variations of the catalyst and the quantities of acetic anhydride
obtained are stated in the following table.
TABLE
__________________________________________________________________________
Catalyst
Example
Noble metal Promoter Acetic anhydride (g)
__________________________________________________________________________
3 80 mg RhCl.sub.3 . 3H.sub.2 O
-- 10
4 25 mg RhCl.sub.3 . 3H.sub.2 O
200 mg CO(OAc).sub.2
15
200 mg P (C.sub.3 H.sub.6).sub.2
5 200 mg Rh(NH.sub.3).sub.5 Cl.sub.3
-- 11
6 200 mg Rh[P(C.sub.6 H.sub.5).sub.3 ].sub.2 COCl
-- 14
7 200 mg Pd(OAc).sub.2
200 mg P (C.sub.6 H.sub.5).sub.3
6
8 200 mg raney-Pd
" 5
9 200 mg PtCl.sub.4
" 7
10 200 mg IrCl.sub.3
" 4
11 200 mg RuCl.sub.3 . 3H.sub.2 O
" 4
12 200 mg OsO.sub.4
" 3
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The quantity of acetic anhydride as stated in the table refers to the
quantity which was obtained by complete work-up of the reaction products,
i.e. no catalyst components were recycled to acetic anhydride solution
according to examples 1 and 2. All tests showed a selectivity of the
reaction of more than 90%.
The most important by-product was acetic acid.
EXAMPLE 13
This Example illustrates the effect of the presence of hydrogen.
The following substances were charged into a 200 ml agitator-autoclave:
50 g of methyl acetate
5 g of methyl iodide
80 mg of RhCl.sub.3 . 3H.sub.2 O
80 mg of P(C.sub.6 H.sub.5).sub.3
The test was then continued as follows:
a. After heating to 180.degree. C, carbon dioxide was added until the
pressure reached 80 bar. This pressure of 80 bar was maintained by
constantly metering in further carbon dioxide. The autoclave was cooled
after 3 hours and the pressure released. The release gas contained about
1% of carbon dioxide. 60 g of a dark liquid containing fine particles of
carbon black remained in the autoclave. About 25% of acetic anhydride and
about 1% of acetic acid were also contained in this liquid.
b. Prior to heating to 180.degree. C hydrogen was introduced at such a rate
that the pressure was adjusted to 10 bar. After having completed the
heating process to 180.degree. C, the pressure was increased to 80 bar by
means of carbon oxide and was maintained at this value by constantly
metering in further carbon oxide. The autoclave was cooled after three
hours and the pressure released. The release gas contained less than 0.1%
of carbon dioxide. About 61 g of liquid remained in the autoclave which
had a dark colour shade, but did not contain any particles of carbon
black. This liquid contained about 19% of acetic anhydride and about 12%
of acetic acid.
EXAMPLE 14
The following substances were charged into a 200 ml agitator-autoclave:
30 g of methyl acetate
20 g of methyl iodide/methanol at the weight ratio of 82:18 (azeotropic)
1 g of iodine
200 mg of RhCl.sub.3 . 3H.sub.2 O
200 mg of P(C.sub.6 H.sub.5).sub.3
After heating to 160.degree. C, the pressure was increased to 150 bar by
means of carbon oxide. This pressure of 150 bar was maintained for 5 hours
by constantly metering in further carbon oxide. The autoclave was then
cooled and the pressure released. We obtained about 60 g of a liquid
reaction mixture containing 12% of acetic acid and about 35% of acetic
anhydride. 30 g of methyl acetate were recovered by means of fractional
distillation. The products, namely 6.5 g of acetic acid (boiling point
118.degree. C under a pressure of 760 mm Hg) and 15 g of acetic anhydride
(boiling point 139.degree. - 140.degree. C under a pressure of 760 mm Hg),
were obtained by distillation. After 8 g of a liquid distillatin residue
remained, consisting of about 60 weight % of acetic anhydride. This
distillation residue was used as catalyst for carrying out the reaction of
Example 15.
EXAMPLE 15
The following substances were charged into a 200 ml agitator-autoclave:
30 g of methyl acetate
20 g of methyl acetate/methanol at the weight ratio of 82:18 (azeotropic)
8 g of the distillation residue of example 14
After heating to 160.degree. C, the pressure was increased to 150 bar by
means of carbon oxide and maintained at this level for 5 hours. The
autoclave was then cooled and the pressure released. We obtained about 67
g of a liquid reaction mixture. About 30 g of methyl acetate were
recovered by distillation, subsequently 7 g of acetic acid (boiling point
118.degree. C under a pressure of 760 mm Hg) and 20 g of acetic anhydride
(boiling point 139.degree. - 140.degree. C under a pressure of 760 mm Hg)
were distilled off the reaction mixture. The remaining distillation
residue of from 9 to 10 g was still containing about 60% of acetic
anhydride. It may be used as catalyst for another preparation of acetic
anhydride and acetic acid.
EXAMPLE 16
250 ml of active coal having a grain size of from 6 to 8 mm were soaked
with an aqueous solution containing 0.5 g of RhCl.sub.3 . 3H.sub.2 O and 1
g of KJ. The impregnated coal was then dried in a nitrogen current at
60.degree. C up to the constant weight.
The carrier catalyst being prepared in such a way was charged into a
reactor consisting in a steel tube of a 25 mm diameter and a 750 mm
length. A gas mixture of per hour 100 Nl (= Normal liter at 0.degree. C
and 760 mm Hg) of CO, 20 Nl of methyl acetate and 1 Nl of methyl iodide,
at a temperature of about 160.degree. C and under a pressure of 10 bar,
was conveyed over the catalyst. An exothermic reaction took place. The
reactor was therefore cooled, in order to maintain a reaction temperature
of 160.degree. C.
The gas mixture was cooled to about 25.degree. C under a pressure of about
10 bar while leaving the reactor. About 70 g per hour of a liquid were
produced, which contained -- besides about 3 g of methyl iodide and about
60 g of methyl acetate -- about 6 g of acetic anhydride.
More than 90% by volume of the gaseous phase, which could not be condensed
at 25.degree. C, consisted in carbon oxide. The only further substantial
components were methyl iodide and methyl acetate. The liquid was worked-up
by distillation under normal pressure.
The gaseous phase which could not be condensed and the recovered quantities
of methyl acetate and methyl iodide were fed back into the reactor.
The consumed quantity of methyl acetate and carbon oxide were replenished
by fresh substances, so that the aforementioned quantities of the starting
materials were maintained. As far as methyl iodide was concerned, there
were only very small quantities to be replaced, i.e. quantities inferior
to 0.1 Nl p/h.
The selectivity of the formation of acetic anhydride was 90% calculated on
methyl acetate and carbon oxide. The most important by-product was acetic
acid.
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