Note: Descriptions are shown in the official language in which they were submitted.
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1
Method for producing formic acid by reacting carbon dioxide with hydrogen
Description
The invention relates to a process for preparing formic acid by reacting
carbon
dioxide with hydrogen in a hydrogenation reactor in the presence of a catalyst
comprising an element of group 8, 9 or 10 of the Periodic Table, a tertiary
amine
and a polar solvent to form formic acid/amine adducts as intermediates which
are
subsequently thermally dissociated.
Adducts of formic acid and tertiary amines can be thermally dissociated into
free
formic acid and tertiary amine and therefore serve as intermediates in the
preparation of formic acid.
Formic acid is an important and versatile product. It is used, for example,
for
acidification in the production of animal feeds, as preservative, as
disinfectant, as
assistant in the textile and leather industry, as mixture with its salts for
deicing
aircraft and runways and also as synthetic building block in the chemical
industry.
The abovementioned adducts of formic acid and tertiary amines can be prepared
in
various ways, for example (i) by direct reaction of the tertiary amine with
formic
acid, (ii) by hydrolysis of methyl formate to formic acid in the presence of
the tertiary
amine or (iii) by catalytic hydrogenation of carbon monoxide or hydrogenation
of
carbon dioxide to formic acid in the presence of the tertiary amine. The
latter
process of catalytic hydrogenation of carbon dioxide has the particular
advantage
that carbon dioxide is available in large quantities and is flexible in terms
of its
source.
The catalytic hydrogenation of carbon dioxide in the presence of amines (W.
Leitner, Angewandte Chemie 1995, 107, pages 2391 to 2405; P. G. Jessop, T.
Ikariya, R. Noyori, Chemical Reviews 1995, 95, pages 259 to 272) appears to be
especially promising from an industrial point of view. The adducts of formic
acid and
amines formed here can be thermally dissociated into formic acid and the amine
used, which can be recirculated to the hydrogenation.
The catalyst required for the reaction comprises one or more elements from
group
8, 9 or 10 of the Periodic Table, i.e. Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and/or
Pt. The
catalyst preferably comprises Ru, Rh, Pd, Os, Ir and/or Pt, particularly
preferably
Ru, Rh and/or Pd and very particularly preferably Ru.
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To make an economical process possible, the catalyst used has to be separated
off
ideally completely from the product stream and recirculated to the reaction,
for two
reasons:
(1) large losses of the expensive catalyst would incur considerable additional
costs and would be prohibitive for economical operation of the process.
(2) very little catalyst should be present in the thermal dissociation of
the formic
acid/amine adducts since it would, in the absence of a CO2 and/or H2
pressure, also catalyze the backreaction and thus lead to losses of the formic
acid formed.
cat. A vacuum
CO2 + H2 ________________________ , NR3 _____ X HCOOH*NR3 ______________ 1-
1COOH + NR3
recirculation
Formation of formic acid by hydrogenation of CO2 (x = 0.4 ¨ 3)
cat. A, vacuum
x HCOOWN ________________________________ CO2 H2 NR3
Decomposition of formic acid/amine adducts in the presence of a catalyst (x =
0.4 ¨
3)
The transition metal-catalyzed decomposition of formic acid has been described
in
detail, especially recently: C. Fellay, N. Yan, P.J. Dyson, G. Laurenczy Chem.
Eur.
J. 2009, 15, 3752-3760; C. Fellay, P.J. Dyson, G. Laurenczy Angew. Chem. 2008,
120, 4030-4032; B. Loges, A. Boddien, H. Junge, M. Beller Angew. Chem. 2008,
120, 4026-4029; F. Joe) ChemSusChem 2008, 1, 805-808; S. Enthaler
ChemSusChem 2008, 1, 801-804; S. Fukuzumi, T. Kobayashi, T. Suenobu
ChemSusChem 2008, 1, 827-834;A. Boddien, B. Loges, H. Junge, M. Beller
ChemSusChem 2008, 1, 751-758.
The catalysts used here are in principle also suitable for the hydrogenation
of CO2
to formic acid (P.G. Jessop, T. lkariya, R. Noyori Chem. Rev. 1995, 95, 259-
272;
P.G. Jessop, F. Joo, C.C. Tai Coord. Chem. Rev. 2004, 248, 2425-2442; P.G.
Jessop, Homogeneous Hydrogenation of Carbon Dioxide, in: The Handbook of
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Homogeneous Hydrogenation, editor: J.G. de Vries, C.J. Elsevier, Volume 1,
2007,
Wiley-VCH, pp. 489-511). Thus, the hydrogenation catalysts have to be
separated
off before the thermal dissociation in order to prevent the undesirable
decomposition of formic acid.
WO 2008/116799 discloses a process for hydrogenating carbon dioxide in the
presence of a catalyst which comprises a transition metal of transition group
VIII
(groups 8, 9, 10) and is suspended or homogeneously dissolved in a solution, a
tertiary amine having at least one hydroxyl group and a polar solvent to give
an
adduct of formic acid and the tertiary amine. As a result of the hydroxyl
group(s) in
the tertiary amine, an increased, compared to the triethylamine which is
otherwise
usually used, carbon dioxide solubility is achieved. RuH2L4 having monodentate
phosphorus-based ligands L and RuH2(LL)2 having bidentate phosphorus-based
ligands LL and, as particularly preferred, RuH2[P(C6F15)3]4 are mentioned as
preferred homogeneous catalysts. As polar solvents, mention is made of
alcohols,
ethers, sulfolanes, dimethyl sulfoxide and amides whose boiling point at
atmospheric pressure is at least 5 C above that of formic acid. The tertiary
amines
which are preferably used also have a boiling point above that of formic acid.
Since
no phase separation takes place, the total reaction output is worked up by
distillation, optionally after prior removal of the catalyst, with the
resulting adduct of
formic acid and the tertiary amine being thermally dissociated and the formic
acid
liberated being obtained as overhead product. The bottom product comprising
tertiary amine, polar solvent and possibly catalyst is recirculated to the
hydrogenation stage.
A disadvantage of this process is transfer of the entire liquid reaction
output into the
apparatus for thermal dissociation and distillation, optionally after prior
specific
removal of the homogeneous catalyst by means of a separate process step such
as
an extraction, adsorption or ultrafiltration stage. A consequence of this is
that the
apparatus for the thermal dissociation and distillation has to be made larger
and
more complex because of the higher liquid loading and also because of the more
specific separation properties, which, inter alia, is reflected in the capital
costs (for
example via engineering input, material, space requirement). In addition, the
higher
liquid loading also results in a higher energy consumption.
However, the fundamental studies on the catalytic hydrogenation of carbon
dioxide
to formic acid were carried out as early as the 1970s and 1980s. The processes
of
BP Chemicals Ltd patented in EP 0 095 321 A, EP 0 151 510 A and
EP 0 181 078 A may well have arisen therefrom. All three documents describe
the
hydrogenation of carbon dioxide in the presence of a homogeneous catalyst
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comprising a transition metal of transition group VIII (groups 8, 9, 10), a
tertiary
amine and a polar solvent to form an adduct of formic acid and the tertiary
amine.
As preferred homogeneous catalysts, EP 0 095 321 A and EP 0 181 078 A each
mention ruthenium-based carbonyl-, halide- and/or triphenylphosphine-
comprising
complex catalysts and EP 0 151 510 A mentions rhodium-phosphine complexes.
Preferred tertiary amines are C1-C10-trialkylamines, in particular the short-
chain C1-
C4-trialkylamines, and cyclic and/or bridged amines such as 1,8-
diazabicyclo[5.4.0]undec-7-ene, 1,4-diazabicyclo[2.2.2]octane, pyridine or
picolines.
The hydrogenation is carried out at a carbon dioxide partial pressure of up to
6 MPa
(60 bar), a hydrogen partial pressure of up to 25 MPa (250 bar) and a
temperature
of from about room temperature to 200 C.
EP 0 095 321 A and EP 0 151 510 A teach the use of an alcohol as polar
solvent.
However, since primary alcohols tend to form formic esters (organic formates),
secondary alcohols, in particular isopropanol, are preferred. In addition, the
presence of water is said to be advantageous. According to the examples of
EP 0 095 321 A, the reaction output is worked up by a directly subsequent two-
stage distillation in which the low boilers, alcohol, water, tertiary amine
are
separated off overhead in the first stage and the adduct of formic acid and
the
tertiary amine are separated off overhead under vacuum conditions in the
second
stage. EP 0 151 510 A likewise teaches a work-up by distillation, but with
reference
to EP 0 126 524 A with subsequent replacement of the tertiary amine in the
adduct
which has been separated off by distillation by a weaker, less volatile
nitrogen base
before thermal dissociation of the adduct in order to aid, or even make
possible, the
subsequent thermal dissociation to prepare the free formic acid.
EP 0 181 078 A teaches the targeted selection of the polar solvent on the
basis of
three main criteria which have to be fulfilled simultaneously:
(i) the homogeneous catalyst has to be soluble in the polar solvent;
(ii) the polar solvent must not have an adverse effect on the hydrogenation;
and
(iii) the resulting adduct of formic acid and the tertiary amine should be
able to be
separated off easily from the polar solvent.
As particularly suitable polar solvents, mention is made of various glycols
and
phenylpropanols.
According to the teaching of EP 0 181 078 A, the reaction output is worked up
by
firstly separating off the gaseous components (first and foremost unreacted
starting
materials hydrogen and carbon dioxide) at the top of an evaporator and
separating
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off the homogeneous catalyst dissolved in the polar solvent at the bottom and
recirculating them to the hydrogenation stage. The adduct of formic acid and
the
tertiary amine is then separated off from the remaining liquid phase
comprising the
adduct of formic acid and tertiary amine, free tertiary amine and possibly
water and
5 the remaining part of the liquid phase comprising the free tertiary amine
and
possibly water is recirculated to the hydrogenation stage. The separation can
be
effected by distillation or by phase separation of the two-phase system
(decantation).
A further important teaching from EP 0 181 078A is the subsequent, compulsory
replacement of the tertiary amine in the isolated adduct by a weaker, less
volatile
nitrogen base before thermal dissociation of the adduct in order to aid, or
even
make possible, the subsequent thermal dissociation to prepare the free formic
acid.
Imidazole derivatives such as 1-n-butylimidazole are mentioned as particularly
suitable weaker nitrogen bases.
A disadvantage of the process of EP 0 181 078 A is the very complicated four-
stage
work-up of the reaction output by
(i) separating off the gaseous components and the homogeneous catalyst and
the polar solvent in an evaporator and recirculating them to the hydrogenation
stage;
(ii) separating off the adduct of formic acid and the tertiary amine in a
distillation
column or a phase separator and recirculating the remaining liquid stream to
the hydrogenation stage;
(iii) replacing the tertiary amine in the adduct of formic acid and the
tertiary amine
by a weaker, less volatile nitrogen base in a reaction vessel with superposed
distillation column and recirculating the liberated tertiary amine to the
hydrogenation stage; and
(iv) thermally dissociating the adduct of formic acid and the weaker nitrogen
base
and recirculating the liberated weaker nitrogen base to the base replacement
stage.
A further, important disadvantage of the process of EP 0 181 078 A, and also
of the
processes of EP 0 095 321 A and EP 0 151 510 A, is the fact that the adduct of
formic acid and the tertiary amine partly redissociates into carbon dioxide
and
hydrogen in the presence of the homogeneous catalyst in the work-up in the
evaporator. In EP 0 329 337 A, the addition of a decomposition inhibitor which
reversibly inhibits the homogeneous catalyst is therefore proposed as a
solution to
the problem. As preferred decomposition inhibitors, mention is made of carbon
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monoxide and oxidants. However, disadvantages of this are introduction of
further
substances into the total process and the necessity of reactivating the
inhibited
homogeneous catalyst before it is used again.
EP 0 357 243 A also addresses the disadvantage of partial redissociation of
the
adduct of formic acid and the tertiary amine in the process of EP 0 181 078 A
by
joint work-up of the reaction output in the evaporator. The process proposed
in
EP 0 357 243 A teaches the use of a homogeneous catalyst comprising a
transition
metal of transition group VIII (groups 8, 9, 10), a tertiary amine and two
different
solvents, namely a nonpolar solvent and a polar solvent, in each case inert,
which
form two immiscible liquid phases in the catalytic hydrogenation of carbon
dioxide
to form an adduct of formic acid and a tertiary amine. As nonpolar solvents,
mention
is made of aliphatic and aromatic hydrocarbons, but also phosphines having
aliphatic and/or aromatic hydrocarbon radicals. Polar solvents mentioned are
water,
glycerol, alcohols, polyols, sulfolanes or mixtures thereof, with water being
preferred. The homogeneous catalyst dissolves in the nonpolar solvent, and the
adduct of formic acid and the tertiary amine dissolves in the polar solvent.
After the
reaction is complete, the two liquid phases are separated, for example by
decantation, and the nonpolar phase comprising the homogeneous catalyst and
the
nonpolar solvent is recirculated to the hydrogenation stage. The polar phase
comprising the adduct of formic acid and the tertiary amine and the polar
solvent is
then subjected to a compulsory replacement of the tertiary amine in the adduct
by a
weaker, less volatile nitrogen base before the adduct is thermally dissociated
in
order to aid, or even make possible, the subsequent thermal dissociation to
prepare
free formic acid. Here too, imidazole derivatives such as 1-n-butylimidazole
are, in a
manner analogous to EP 0 181 078 A, mentioned as particularly suitable weaker
nitrogen bases.
A disadvantage of the process of EP 0 357 243 A is the very complicated, three-
stage work-up of the reaction mixture by
(i) separating the two liquid phases and recirculating the phase comprising
the
homogeneous catalyst and the nonpolar solvent to the hydrogenation stage;
(ii) replacing the tertiary amine in the adduct of formic acid and the
tertiary amine
of the other phase by a weaker, less volatile nitrogen base in a reaction
vessel
with superposed distillation column and recirculating the liberated tertiary
amine to the hydrogenation stage; and
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(iii) thermally dissociating the adduct of formic acid and the weaker nitrogen
base
and recirculating the liberated weaker nitrogen base to the base replacement
stage.
A further disadvantage of the process of EP 0 357 243 A is the use of two
solvents
and thus introduction of a further substance into the total process.
As an alternative, EP 0 357 243 A also discloses the possibility of using only
one
solvent. In this case, the addition of the polar solvent in which the adduct
of formic
acid and the tertiary amine would otherwise dissolve is omitted. The only
solvent
used here is the nonpolar solvent which dissolves the homogeneous catalyst.
However, this alternative also has the disadvantage of the very complicated,
three-
stage work-up as described above.
DE 44 31 233 A likewise describes the hydrogenation of carbon dioxide in the
presence of a catalyst comprising a transition metal of transition group VIII
(groups
8, 9, 10), a tertiary amine and a polar solvent and water to form an adduct of
formic
acid and the tertiary amine, in which, however, the catalyst is present in
heterogeneous form and the active component is applied to an inert support.
Preferred tertiary amines are C1-C8-trialkylamines, polyamines having from 2
to 5
amino groups, aromatic nitrogen heterocycles such as pyridine or N-
methylimidazole and also cyclic and/or bridged amines such as N-
methylpiperidine,
1,8-diazabicyclo[5.4.0]undec-7-ene or 1,4-diazabicyclo[2.2.2]octane. As
suitable
polar solvents, mention is made of the low-boiling C1-C4-monoalcohols, with
secondary alcohols being preferred, in a manner analogous to EP 0 095 321 A.
The
hydrogenation is carried out at a total pressure of from 4 to 20 MPa (from 40
to
200 bar) and a temperature of from 50 to 200 C. To work up the resulting
adduct of
formic acid and tertiary amine, DE 44 31 233 A teaches the use of known
methods
with explicit reference to the work-up with replacement of the tertiary amine
in the
adduct of formic acid and the tertiary amine by a weaker, less volatile
nitrogen
base, as disclosed in EP 0 357 243 A. In the process according to DE 44 31 233
A,
too, the very complicated, three-stage work-up of the reaction output is also
disadvantageous, in a manner analogous to the process of EP 0 357 243 A.
It was an object of the present invention to provide a process for preparing
formic
acid by hydrogenating carbon dioxide, which does not have the abovementioned
disadvantages of the prior art or has them only to a significantly reduced
extent and
leads to concentrated formic acid in high yield and high purity. Furthermore,
the
process should allow simple or at least simpler operation than that described
in the
prior art, in particular a simpler process concept for working up the output
from the
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hydrogenation reactor, simpler process stages, a smaller number of process
stages
or simpler apparatuses. In addition, the process should also be able to be
carried
out with a very low energy consumption. In particular, an efficient solution
to the
previously only unsatisfactorily solved problem of recirculation of the
catalyst while
simultaneously ensuring a high activity of the hydrogenation catalyst should
be
offered.
The work-up of the output from the hydrogenation reactor should, in
particular, be
carried out using exclusively the materials already present in the process,
without
additional auxiliaries, and these should be recycled completely or largely
completely
in the process.
The object is achieved by a process for preparing formic acid by reacting
carbon
dioxide (1) with hydrogen (2) in a hydrogenation reactor (I) in the presence
of
- a catalyst comprising an element of group 8, 9 or 10 of the Periodic
Table,
- a tertiary amine comprising at least 12 carbon atoms per molecule and
a polar solvent comprising one or more monoalcohols selected from among
methanol, ethanol, propanols and butanols,
to form formic acid/amine adducts as intermediates which are subsequently
thermally dissociated,
where a tertiary amine having a boiling point which is at least 5 C higher
than that
of formic acid is used and
a reaction mixture comprising the polar solvent, the formic acid/amine
adducts, the
tertiary amine and the catalyst is formed in the reaction in the hydrogenation
reactor
(I) and is discharged from the reactor as output (3),
wherein
the output (3) from the hydrogenation reactor (I) is, optionally after
addition of water,
fed directly to an extraction unit (II) and the work-up of the output (3)
comprises the
following process steps:
1) extraction of the catalyst from the output (3) from the hydrogenation
reactor (I)
in the extraction unit (II), where the same tertiary amine which was used in
the
hydrogenation is used as extractant, to give an extract (4) which comprises
the major part of the tertiary amine and the catalyst and is recycled to the
hydrogenation reactor (I) and also a raffinate (5) which comprises the major
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part of the polar solvent and the formic acid/amine adducts and is passed on
to a distillation unit (III) in which
2) a distillation is carried out to give an overhead stream (6) which
comprises
predominantly the polar solvent and is recycled to the hydrogenation reactor
(I) and also a bottom stream (7) which comprises predominantly the formic
acid/amine adducts and the tertiary amine and is according to process step
3) fed to a phase separation vessel (IV) in which
a) a phase separation is carried out to give an upper phase which comprises
predominantly the tertiary amine and is recycled as stream (11) to the
extraction unit (II) as extractant for the catalyst and a lower phase which
comprises predominantly the formic acid/amine adducts and is fed as stream
(8) to a thermal dissociation unit (V) in which
b) a thermal dissociation is carried out to give a stream (9) which
comprises the
tertiary amine and is recirculated to the phase separation vessel (IV) and a
stream (10) comprising the formic acid
or according to process step
4) is fed to a thermal dissociation unit (V) in which
a) a thermal dissociation is carried out to give a stream (10) comprising
the
formic acid and a stream (9) which comprises the tertiary amine and the
formic acid/amine adducts and is fed to a phase separation vessel (IV) in
which
b) a phase separation is carried out to give an upper phase which comprises
predominantly the tertiary amine and is recycled as stream (11) to the
extraction unit (II) as extractant for the catalyst and a lower phase which
comprises predominantly the formic acid/amine adducts and is fed as stream
(8) to a thermal dissociation unit (V).
In an embodiment of the present invention, only a substream of the tertiary
amine-
comprising stream (11) from the phase separation vessel (IV) is introduced
into the
extraction unit (II) as selective extractant for the catalyst and the
remaining part of
the tertiary amine-comprising stream is introduced directly into the
hydrogenation
reactor (I).
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For the purposes of the present invention, "fed directly to an extraction unit
(II)"
means that the output (3) from the hydrogenation reactor (I) is, optionally
after
addition of water, fed without further work-up steps to an extraction unit
(II).
5
In one embodiment, the process of the invention therefore comprises the
process
steps 1), 2) and 3). In a further embodiment, the process of the invention
comprises
the steps 1), 2) and 4).
10 The catalyst used in the hydrogenation of carbon dioxide in the process
of the
invention is preferably a homogeneous catalyst. It comprises an element of
group 8,
9 or 10 of the Periodic Table (in the IUPAC version), i.e. Fe, Co, Ni, Ru, Rh,
Pd, Os,
Ir and/or Pt. The catalyst preferably comprises Ru, Rh, Pd, Os, Ir and/or Pt,
particularly preferably Ru, Rh and/or Pd, and very particularly preferably Ru.
The elements mentioned are present in the form of complexes homogeneously
dissolved in the reaction mixture. The homogeneous catalyst should be selected
so
that it accumulates together with the tertiary amine in the same liquid phase
(B).
Here, "accumulated" means a partition coefficient of the homogeneous catalyst
P = [concentration of homogeneous catalyst in liquid phase (B)] /
[concentration of homogeneous catalyst in liquid phase (A)]
of > 1. The homogeneous catalyst is generally selected by means of a simple
experiment in which the partition coefficient of the desired homogeneous
catalyst is
experimentally determined under the planned process conditions.
Liquid phase (A) here is the raffinate in process stage 2).
Owing to their good solubility in tertiary amines, complexes comprising an
element
of group 8, 9 or 10 of the Periodic Table and at least one phosphine group
having at
least one unbranched or branched, acyclic or cyclic aliphatic radical having
from 1
to 12 carbon atoms, where individual carbon atoms can also be substituted by
>P-,
are preferably used as homogeneous catalysts in the process of the invention.
Branched cyclic aliphatic radicals therefore also include radicals such as -
CH2-
C6H1 1. As suitable radicals, mention may be made by way of example of methyl,
ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl, 1-(2-methyl)propyl, 2-(2-
methyl)propyl, 1-
pentyl, 1-hexyl, 1-heptyl, 1-octyl, 1-nonyl, 1-decyl, 1-undecyl, 1-dodecyl,
cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl, methylcyclopentyl,
methylcyclohexyl, 1-(2-methyl)pentyl, 1-(2-ethyl)hexyl, 1-(2-propyl)heptyl and
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norbornyl. The unbranched or branched, acyclic or cyclic aliphatic radical
preferably
comprises at least one carbon atom and preferably not more than 10 carbon
atoms.
In the case of an exclusively cyclic radical in the above sense, the number of
carbon atoms is from 3 to 12 and preferably at least 4 and also preferably not
more
than 8. Preferred radicals are ethyl, 1-butyl, sec-butyl, 1-octyl and
cyclohexyl.
The phosphine group can comprise one, two or three of the abovementioned
unbranched or branched, acyclic or cyclic aliphatic radicals. These can be
identical
or different. The phosphine group preferably comprises three of the
abovementioned unbranched or branched, acyclic or cyclic aliphatic radicals,
with
particular preference being given to all three radicals being identical.
Preferred
phosphines are P(n-CH2n.1)3 where n is from 1 to 10, particularly preferably
tri-n-
butylphosphine, tri-n-octylphosphine and 1,2-bis(dicyclohexylphosphino)ethane.
As mentioned above, individual carbon atoms can also be substituted by >P- in
the
abovementioned unbranched or branched, acyclic or cyclic aliphatic radicals.
Thus,
polydentate, for example bidentate or tridentate, phosphine ligands are thus
also
>P-CH2CH2-P<
encompassed. These preferably comprise the group or
>P-CH2CH2-P-CH2CH2-P<
If the phosphine group comprises further radicals other than the
abovementioned
unbranched or branched, acyclic or cyclic aliphatic radicals, these generally
correspond to those which are otherwise customarily used in phosphine ligands
for
complex catalysts. Examples which may be mentioned are phenyl, tolyl and
xylyl.
The complex can comprise one or more, for example, two, three or four, of the
abovementioned phosphine groups having at least one unbranched or branched,
acyclic or cyclic aliphatic radical. The remaining ligands of the complex can
have
various natures. Illustrative examples which may be mentioned are hydride,
fluoride, chloride, bromide, iodide, formate, acetate, propionate,
carboxylate,
acetylacetonate, carbonyl, DMSO, hydroxide, trialkylamine, alkoxide.
The homogeneous catalysts can be produced either directly in their active form
or
only under reaction conditions from conventional standard complexes such as
[M(p-
cymene)C12]2, [M(benzene)C12], [M(COD)(allyI)], [MCI3 x
H20],
[M(acetylacetonate)3], [M(DMS0)4C12] where M is an element of group 8, 9 or 10
of
the Periodic Table with addition of the appropriate phosphine ligand(s).
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Homogeneous catalysts which are preferred in the process of the invention are
[Ru(PnI3u3)4(H)2], [Ru(Pnocty13)4(H)2], [Ru(PnE3u3)2(1,2-
bis(dicyclohexylphosphino)-
ethane)(H)2]. [Ru(Pnocty13)2(1,2-
bis(dicyclohexylphosphino)ethane)(H)2],
[Ru(PEt3)4(H)2]. These enable TOF (turnover frequency) values of greater than
1000 h-1 to be achieved in the hydrogenation of carbon dioxide.
When homogeneous catalysts are used, the amount of the specified metal
component in the metal-organic complex used is generally from 0.1 to 5000 ppm
by
weight, preferably from 1 to 800 ppm by weight and particularly preferably
from 5 to
800 ppm by weight, in each case based on the total liquid reaction mixture in
the
hydrogenation reactor.
The partition coefficient of the homogeneous catalyst based on the amount of
ruthenium in the amine phase and the product phase comprising the formic
acid/amine adduct is in the range of P greater than 0.5, preferably greater
than 1.0
and particularly preferably greater than 4, after the hydrogenation.
The tertiary amine to be used in the hydrogenation of carbon dioxide in the
process
of the invention has a boiling point which is at least 5 C higher than that of
formic
acid. Here, as is customary, when the relative position of boiling points of
compounds is indicated, the boiling points each have to be based on the same
pressure. The tertiary amine is selected in this way and matched to the polar
solvent in which the tertiary amine accumulates in the upper phase in the
hydrogenation reactor. Here, "accumulates/accumulated" means a proportion by
weight of > 50% of the free, i.e. not bound in the form of the formic
acid/amine
adduct, tertiary amine in the upper phase based on the total amount of free,
tertiary
amine in the two liquid phases. The proportion by weight is preferably > 90%.
The
tertiary amine is generally selected by means of a simple experiment in which
the
solubility of the desired tertiary amine in the two liquid phases is
experimentally
determined under the planned process conditions. The upper phase can
additionally comprise amounts of the polar solvent and of a nonpolar inert
solvent.
The tertiary amine to be used preferably has a boiling point which is at least
10 C
higher, particularly preferably at least 50 C higher and very particularly
preferably at
least 100 C higher, than that of formic acid. A restriction in terms of an
upper limit
for the boiling point is not necessary since a very low vapor pressure of the
tertiary
amine is in principle advantageous for the process of the invention. In
general, the
boiling point of the tertiary amine at a pressure of 1013 hPa abs, if
necessary
extrapolated from vacuum by known methods, is less than 500 C.
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The tertiary amine which is preferably to be used in the process of the
invention is
an amine comprising at least 12 carbon atoms per molecule of the general
formula
(la)
NR1R2R3 (la),
where the radicals R1 to R3 are identical or different and are each,
independently of
one another, an unbranched or branched, acyclic or cyclic, aliphatic,
araliphatic or
aromatic radical having in each case from 1 to 16 carbon atoms, preferably
from 1
to 12 carbon atoms, where individual carbon atoms can also, independently of
one
another, be substituted by a heterogroup selected from the group consisting of
¨0-
and >N- and two or all three of the radicals can also be joined to one another
to
form a chain comprising at least four atoms.
As suitable amines, mention may be made by way of example of:
= tri-n-butylamine, tri-n-pentylamine, tri-n-hexylamine, tri-n-heptylamine,
tri-n-
octylamine, tri-n-nonylamine, tri-n-decylamine, tri-n-undecylamine, tri-n-
dodecylamine, tri-n-tridecylamine, tri-n-tetradecylamine, tri-n-
pentadecylamine,
tri-n-hexadecylamine, tri-(2-ethylhexyl)amine.
= dimethyldecylamine, dimethyldodecylamine, dimethyltetradecylamine,
ethyldi(2-
propyl)amine (bPi 1313 hPa = 127 C), dioctylmethylamine, dihexylnnethylamine.
= tricyclopentylamine, tricyclohexylamine, tricycloheptylamine,
tricyclooctylamine
and the derivatives thereof substituted by one or more methyl, ethyl, 1-
propyl, 2-
propyl, 1-butyl, 2-butyl or 2-methyl-2-propyl groups.
= dimethylcyclohexylamine, methyldicyclohexylamine, diethylcyclohexylamine,
ethyldicyclohexylamine, dimethylcyclopentylamine, methyldicyclopentylamine.
= triphenylamine, methyldiphenylamine, ethyldiphenylamine,
propyldiphenylamine,
butyldiphenylamine, 2-ethylhexyldiphenylamine,
dimethylphenylamine,
diethylphenylamine, dipropylphenylamine, dibutylphenylamine,
bis(2-
ethylhexyl)phenylamine, tribenzylamine,
methyldibenzylamine,
ethyldibenzylamine and the derivatives thereof substituted by one or more
methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl or 2-methyl-2-propyl
groups.
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= N-C1-C12-alkylpiperidines, N,N-di-C1-C12-alkylpiperazines, N-C1-
C12-
alkylpyrrolidines, N-C1-C12-alkylimidazoles and derivatives thereof
substituted by
one or more methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl or 2-methy1-2-
propyl groups.
= 1,8-diazabicyclo[5.4.0]undec-7-ene ("DBU"), 1,4-diazabicyclo[2.2.2]octane
("DABCO"), N-methyl-8-azabicyclo[3.2.1]octane ("tropane"), N-methy1-9-
azabicyclo[3.3.1 ]nonane ("granatane"), 1 -
azabicyclo[2.2.2]octane
("quinuclidine").
It is naturally also possible to use mixtures of any number of different
tertiary
amines in the process of the invention.
Particular preference is given to using an amine of the general formula (la)
in which
the radicals R1 to R3 are selected independently from the group consisting of
C1-
C12-alkyl, C5-C8-cycloalkyl, benzyl and phenyl as tertiary amine in the
process of the
invention.
Particular preference is given to using a saturated amine, i.e. an amine
comprising
only single bonds, of the general formula (la) as tertiary amine in the
process of the
invention.
Very particular preference is given to using an amine of the general formula
(la) in
which the radicals R1 to R3 are selected independently from the group
consisting of
C5-C8-alkyl, in particular tri-n-pentylamine, tri-n-hexylamine, tri-n-
heptylamine, tri-n-
octylamine, dimethylcyclohexylamine, methyldicyclohexylamine,
dioctylmethylamine
and dimethyldecylamine, as tertiary amine in the process of the invention.
In particular, an amine of the general formula (la) in which the radicals R1
to R3 are
selected independently from among C5- and C8-alkyl is used as tertiary amine.
In the process of the invention, the tertiary amine is preferably present in
liquid form
in all process stages. However, this is not an absolute requirement. It would
also be
sufficient for the tertiary amine to be dissolved in suitable solvents.
Suitable
solvents are in principle those which are chemically inert in respect of the
hydrogenation of carbon dioxide and the thermal dissociation of the adduct and
those in which the tertiary amine and, if a homogeneous catalyst is used, also
the
latter dissolve(s) readily while polar solvent and the formic acid/amine
adducts are
sparingly soluble. Possibilities are therefore in principle chemically inert,
nonpolar
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solvents such as aliphatic, aromatic or araliphatic hydrocarbons, for example
octane and higher alkanes, toluene, xylenes.
The polar solvent to be used in the hydrogenation of carbon dioxide in the
process
5 of the invention has a boiling point which is at least 5 C lower than the
temperature
required for dissociation of the formic acid/amine adducts at the same
pressure.
The polar solvent should be selected or matched to the tertiary amine so that
the
lower phase is enriched in the polar solvent. Here, "enriched" means= a
proportion
by weight of > 50% of the polar solvent in the lower phase based on the total
10 amount of polar solvent in the two liquid phases. The proportion by
weight is
preferably > 70%. The polar solvent is generally selected by means of a simple
experiment in which the solubility of the desired polar solvent in the two
liquid
phases is experimentally determined under the planned process conditions.
15 The polar solvent can be a pure polar solvent or a mixture of various
polar solvents
as long as the abovementioned conditions in respect of boiling point and phase
behavior which the solvent has to meet are complied with.
The polar solvent to be used preferably has a boiling point which is at least
10 C
lower, particularly preferably less than 50 C lower, than the temperature
required
for dissociation of the formic acid/amine adducts at the same pressure. In the
case
of solvent mixtures, the boiling points of the solvent mixture used or of an
azeotrope
or heteroazeotrope are at least 10 C lower, particularly preferably at least
50 C
lower, than the temperature required for dissociation of the formic acid/amine
adducts at the same pressure.
Classes of substances which are suitable as polar solvents are preferably
alcohols
and the formic esters thereof and water. The alcohols have a boiling point
which is
at least 10 C lower, particularly preferably at least 50 C lower, than the
temperature
required for dissociation of the formic acid/amine adducts at the same
pressure so
as to keep esterification of the alcohols by formic acid as low as possible.
Suitable alcohols are alcohols in the case of which the trialkylammonium
formates
preferably dissolve in a mixture of the alcohol with water and this product
phase has
a miscibility gap with the free trialkylamine. As suitable alcohols, mention
may be
made by way of example of methanol, ethanol, 2-methoxyethanol, 1-propanol, 2-
propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol. It is also possible to
use
mixtures of one or more alcohols and water as polar solvent. In one embodiment
of
the invention, a mixture of one or more monoalcohols selected from among
methanol, ethanol, propanols and butanols with water is used as polar solvent.
In a
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further embodiment, a mixture of methanol and/or ethanol with water is used as
polar solvent. The ratio of alcohol to water should be selected so that the
mixture
together with the trialkylammonium formate and the trialkylamine forms a two-
phase
mixture in which the major part of the trialkylammonium formate, the water and
the
polar solvent is present in the lower phase, which is generally determined by
means
of a simple experiment in which the solubility of the desired polar solvent
mixture in
the two liquid phases is experimentally determined under the planned process
conditions.
The molar ratio of the polar solvent or solvent mixture to be used in the
process of
the invention to the tertiary amine used is generally from 0.5 to 30 and
preferably
from 1 to 20.
In an embodiment of the invention, water is added to the output (3) from the
hydrogenation reactor (I) before the output (3) is fed to the extraction unit
(II).
The invention therefore also provides a process for preparing formic acid by
reacting carbon dioxide (1) with hydrogen (2) in a hydrogenation reactor (I)
in the
presence of
- a catalyst comprising an element of group 8, 9 or 10 of the Periodic
Table,
- a tertiary amine comprising at least 12 carbon atoms per molecule and
- a polar solvent comprising one or more monoalcohols selected from among
methanol, ethanol, propanols and butanols,
to form formic acid/amine adducts as intermediates which are subsequently
thermally dissociated,
where a tertiary amine having a boiling point which is at least 5 C higher
than that
of formic acid is used and
a reaction mixture comprising the polar solvent, the formic acid/amine
adducts, the
tertiary amine and the catalyst is formed in the reaction in the hydrogenation
reactor
(I) and is discharged from the reactor as output (3),
wherein
the output (3) from the hydrogenation reactor (I) is, after addition of water,
fed
directly to an extraction unit (II) and the work-up of the output (3)
comprises the
following process steps:
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1) extraction of the catalyst from the output (3) from the
hydrogenation reactor (I)
in the extraction unit (II), where the same tertiary amine which was used in
the
hydrogenation is used as extractant, to give an extract (4) which comprises
the major part of the tertiary amine and the catalyst and is recycled to the
hydrogenation reactor (I) and also a raffinate (5) which comprises the major
part of the polar solvent and the formic acid/amine adducts and is passed on
to a distillation unit (III) in which
2) a distillation is carried out to give an overhead stream (6) which
comprises
predominantly the polar solvent and is recycled to the hydrogenation reactor
(I) and also a bottom stream (7) which comprises predominantly the formic
acid/amine adducts and the tertiary amine and is according to process step
3) fed to a phase separation vessel (IV) in which
a) a phase separation is carried out to give an upper phase which comprises
predominantly the tertiary amine and is recycled as stream (11) to the
extraction unit (II) as extractant for the catalyst and a lower phase which
comprises predominantly the formic acid/amine adducts and is fed as stream
(8) to a thermal dissociation unit (V) in which
b) a thermal dissociation is carried out to give a stream (9) which
comprises the
tertiary amine and is recirculated to the phase separation vessel (IV) and a
stream (10) comprising the formic acid
or according to process step
4) is fed to a thermal dissociation unit (V) in which
a) a thermal dissociation is carried out to give a stream (10) comprising
the
formic acid and a stream (9) which comprises the tertiary amine and the
formic acid/amine adducts and is fed to a phase separation vessel (IV) in
which
b) a phase separation is carried out to give an upper phase which comprises
predominantly the tertiary amine and is recycled as stream (11) to the
extraction unit (II) as extractant for the catalyst and a lower phase which
comprises predominantly the formic acid/amine adducts and is fed as stream
(8) to a thermal dissociation unit (V).
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The addition of water after hydrogenation has been carried out has the effect
that
the partition coefficients of the catalyst are improved by the addition of
water in
favor of accumulation of the catalyst in the amine phase and that in this way
efficient recirculation of the catalyst is made possible without the
hydrogenation
activity thereof being reduced. The hydrogenation output can, depending on the
polar solvent used and the concentration of formic acid-amine adducts, consist
of
one or two phases, with the formic acid-amine adducts then accumulating in the
product phase (raffinate) as result of the addition of water. Water is
preferably
added in such an amount that a water content in the raffinate (5) of from 0.1
to 50%
by weight, based on the total weight of the raffinate (5) from the extraction
unit (II),
is obtained, particularly preferably from 2 to 30% by weight, based on the
total
weight of the raffinate (5) from the extraction unit (II). The water to be
introduced
can originate from the distillation unit (III) in which the major part of the
polar solvent
and also the water are separated off from the raffinate (5) and/or can also be
water
which is freshly introduced into the process. The water can, when the output
(3) is
depressurized, be added either after depressurization of the output from the
hydrogenation reactor to atmospheric pressure or else before depressurization
of
the output.
In a further embodiment of the invention, no water is added to the output (3)
from
the hydrogenation reactor (I). The invention therefore also provides a process
for
preparing formic acid by reacting carbon dioxide (1) with hydrogen (2) in a
hydrogenation reactor (I) in the presence of
- a catalyst comprising an element of group 8, 9 or 10 of the Periodic
Table,
- a tertiary amine comprising at least 12 carbon atoms per
molecule and
a polar solvent comprising one or more monoalcohols selected from among
methanol, ethanol, propanols, butanols and water,
to form formic acid/amine adducts as intermediates which are subsequently
thermally dissociated,
where a tertiary amine having a boiling point which is at least 5 C higher
than that
of formic acid is used and
a reaction mixture comprising the polar solvent, the formic acid/amine
adducts, the
tertiary amine and the catalyst is formed in the reaction in the hydrogenation
reactor
(I) and is discharged from the reactor as output (3),
wherein
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the output (3) from the hydrogenation reactor (I) is fed directly to an
extraction unit
(II) and the work-up of the output (3) comprises the following process steps:
1) extraction of the catalyst from the output (3) from the hydrogenation
reactor (I)
in the extraction unit (II), where the same tertiary amine which was used in
the
hydrogenation is used as extractant, to give an extract (4) which comprises
the major part of the tertiary amine and the catalyst and is recycled to the
hydrogenation reactor (I) and also a raffinate (5) which comprises the major
part of the polar solvent and the formic acid/amine adducts and is passed on
to a distillation unit (III) in which
2) a distillation is carried out to give an overhead stream (6) which
comprises
predominantly the polar solvent and is recycled to the hydrogenation reactor
(I) and also a bottom stream (7) which comprises predominantly the formic
acid/amine adducts and the tertiary amine and is according to process step
3) fed to a phase separation vessel (IV) in which
a) a phase separation is carried out to give an upper phase which
comprises
predominantly the tertiary amine and is recycled as stream (11) to the
extraction unit (II) as extractant for the catalyst and a lower phase which
comprises predominantly the formic acid/amine adducts and is fed as stream
(8) to a thermal dissociation unit (V) in which
b) a thermal dissociation is carried out to give a stream (9) which
comprises the
tertiary amine and is recirculated to the phase separation vessel (IV) and a
stream (10) comprising the formic acid
or according to process step
4) is fed to a thermal dissociation unit (V) in which
a) a thermal dissociation is carried out to give a stream (10) comprising
the
formic acid and a stream (9) which comprises the tertiary amine and the
formic acid/amine adducts and is fed to a phase separation vessel (IV) in
which
b) a phase separation is carried out to give an upper phase which comprises
predominantly the tertiary amine and is recycled as stream (11) to the
extraction unit (II) as extractant for the catalyst and a lower phase which
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comprises predominantly the formic acid/amine adducts and is fed as stream
(8) to a thermal dissociation unit (V).
Here, the ratios of monoalcohol(s), tertiary amine and water in the
hydrogenation
5 reactor (I) are preferably selected so that a water content in the
raffinate (5) of from
0.1 to 50% by weight, based on the total weight of the raffinate (5) from the
extraction unit (II), is obtained, particularly preferably from 2 to 30% by
weight,
based on the total weight of the raffinate (5) from the extraction unit II.
10 The extract (4) obtained in the extraction unit (II) comprises the major
part of the
tertiary amine and the catalyst. The raffinate (5) obtained in the extraction
unit (11)
comprises the major part of the polar solvent and the formic acid/amine
adducts.
"Major part" in relation to the extract (4) means that the extract (4) has a
higher
15 concentration of tertiary amine and catalyst than the raffinate (5).
Here, tertiary
amine is the amine which is present in free form and is not bound to formic
acid in
the form of the formic acid/amine adduct.
"Major part" in relation to the raffinate (5) means that the raffinate (5) has
a higher
20 concentration of polar solvent and the formic acid/amine adducts than
the extract
(4).
In one embodiment of the process of the invention, a substream of the bottom
stream (7) obtained in the distillation unit (111) is fed to the phase
separation vessel
(IV) and the remainder of the bottom stream (7) is fed to the thermal
dissociation
unit (V). A substream of the bottom stream (7) is worked up according to
process
step 3) and the remainder of the bottom stream is worked up according to
process
step 4).
The carbon dioxide to be used in the hydrogenation of carbon dioxide can be
used
in solid, liquid or gaseous form. It is also possible to use industrially
available gas
mixtures comprising carbon dioxide as long as these are largely free of carbon
monoxide, i.e. have a proportion by volume of < 1% of CO. The hydrogen to be
used in the hydrogenation of carbon dioxide is generally gaseous. Carbon
dioxide
and hydrogen can also comprise inert gases such as nitrogen or noble gases.
However, the content of these is advantageously below 10 mol% based on the
total
amount of carbon dioxide and hydrogen in the hydrogenation reactor. Although
larger amounts may likewise still be tolerable, they generally require use of
a higher
pressure in the reactor, as a result of which further compression energy is
required
and the outlay in terms of apparatus increases.
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The hydrogenation of carbon dioxide is carried out in the liquid phase,
preferably at
a temperature of from 20 to 200 C and a total pressure of from 0.2 to 30 MPa
abs.
The temperature is preferably at least 30 C and particularly preferably at
least 40 C
and preferably not more than 150 C, particularly preferably not more than 120
C
and very particularly preferably not more than 80 C. The total pressure is
preferably
at least 1 MPa abs and particularly preferably at least 3 MPa abs and
preferably, not
more than 15 MPa abs.
The partial pressure of the carbon dioxide is generally at least 0.5 MPa and
preferably at least 1 MPa and generally not more than 8 MPa. The partial
pressure
of the hydrogen is generally at least 0.5 MPa and preferably at least 1 MPa
and
generally not more than 25 MPa and preferably not more than 10 MPa.
The molar ratio of hydrogen to carbon dioxide in the feed to the hydrogenation
reactor is preferably from 0.1 to 10 and particularly preferably from 0.2 to
5, in
particular from 0.5 to 3.
The molar ratio of carbon dioxide to tertiary amine in the feed to the
hydrogenation
reactor is generally from 0.1 to 10 and preferably from 0.2 to 5, in
particular from
0.5 to 3.
As hydrogenation reactors, it is in principle possible to use all reactors
which are
basically suitable for gas/liquid reactions at the given temperature and the
given
pressure. Suitable standard reactors for gas-liquid and for liquid-liquid
reaction
systems are indicated, for example, in K.D. Henkel, "Reactor Types and Their
Industrial Applications", in Ullmann's Encyclopedia of Industrial Chemistry,
2005,
Wiley-VCH Verlag GmbH & Co. KGaA, DOI: 10.1002/14356007.b04_087,
chapter 3.3 "Reactors for gas-liquid reactions". Examples which may be
mentioned
are stirred tank reactors, tube reactors and bubble column reactors.
The hydrogenation of carbon dioxide can be carried out batchwise or
continuously
in the process of the invention. In batch operation, the reactor is charged
with the
desired liquid and optionally solid starting materials and auxiliaries and the
reactor
is subsequently pressurized with carbon dioxide and hydrogen to the desired
pressure at the desired temperature. After the reaction is complete, the
reactor is
generally depressurized and the two liquid phases formed are separated from
one
another. In continuous operation, the starting materials and auxiliaries,
including the
carbon dioxide and hydrogen, are introduced continuously. Correspondingly, the
liquid phase is continuously discharged from the reactor so that the liquid
level in
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22
the reactor stays the same on average. Preference is given to the continuous
hydrogenation of carbon dioxide.
The average residence time in the hydrogenation reactor is generally from
5 minutes to 5 hours.
The formic acid/amine adducts formed in the hydrogenation of carbon dioxide in
the
presence of the catalyst to be used, the polar solvent and the tertiary amine
generally have the general formula (11a)
NR1R2R3 = x, HCOOH (11a),
where the radicals R1 to R3 correspond to the radicals described for the
tertiary
amine (la) and xl is from 0.4 to 5, preferably from 0.7 to 1.6. The respective
average composition of the amine-formic acid ratio in the product phases in
the
respective process steps, i.e. the factor xi, can be determined, for example,
by
determining the formic acid content by titration with an alcoholic KOH
solution
against phenolphthalein and the amine content by gas chromatography. The
composition of the formic acid/amine adducts, i.e. the factor xõ can change
during
the various process steps. Thus, for example, adducts having a relatively high
formic acid content with x2 > x1 and x2 from 1 to 4 are generally formed after
removal of the polar solvent, with the excess, free amine being able to form a
second phase.
The output (3) from the hydrogenation reactor (I), which comprises the polar
solvent, the formic acid/amine adducts, the tertiary amine and the catalyst
and to
which water has optionally been added, is extracted with streams of free
tertiary
amine originating from the respective phase separation vessels and
recirculated to
the hydrogenation reactor. This is done to separate off the catalyst. Without
this
extraction, hydrogenation catalyst could get into the apparatus for thermal
dissociation of the adduct of tertiary amine and formic acid and there
catalyze the
decomposition of formic acid and thus reduce the yield of formic acid.
Residual
amounts of hydrogen and carbon dioxide are disposed of as offgas.
The extraction of the catalyst from the output from the hydrogenation reactor
can,
for example, be carried out after depressurization, for example to about or
close to
atmospheric pressure, and cooling of the output (3), for example to about or
close
to ambient temperature.
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Here, the extract (4) obtained in the extraction, which comprises the tertiary
amine
and the catalyst, should be brought separately to the reaction pressure before
recirculation to the hydrogenation reactor (I). One or more suitable
compressors
designed for the pressure difference to be overcome or a pump are provided for
the
gas and liquid phases to be recirculated.
In the context of the present invention, it has surprisingly been found that a
raffinate
(5) enriched in the formic acid/amine adducts and the polar solvent and an
extract
(4) enriched in the tertiary amine and, if a homogeneous catalyst is used,
also in
this can be obtained in the extraction, even at a significantly elevated
pressure. For
this reason, the solvents can be selected in the process of the invention so
that
separation of the phase enriched in the formic acid/amine adducts and the
polar
solvent from the other phase enriched in the tertiary amine and also
recirculation of
the phase enriched in tertiary amine to the hydrogenation reactor at a
pressure of
from 0.1 to 30 MPa abs can take place. In accordance with the total pressure
in the
hydrogenation reactor, the pressure is preferably not more than 15 MPa abs. It
is
therefore even possible to separate the two liquid phases from one another
without
prior depressurization and to recirculate the extract (4) from the extraction
to the
hydrogenation reactor without an appreciable increase in pressure. In this
case,
and also in the case of only slight depressurization, it is possible to
dispense with
recirculation of any gas phase entirely.
The process of the invention can, in one embodiment, therefore be carried out
with
the pressure and the temperature in the hydrogenation reactor and in the
extraction
unit (II) being the same or approximately the same, where, for the present
purposes, "approximately the same" means a pressure difference of up to +/- 5
bar
or a temperature difference of up to +/- 5 C.
It has also surprisingly been found that in the present system the two liquid
phases
can be separated very well from one another even at an elevated temperature
corresponding to the reaction temperature. It is therefore also not necessary
to
carry out any cooling for the phase separation and subsequent heating of the
upper
phase to be recirculated, which likewise saves energy.
The extraction is preferably carried out at temperatures in the range from 20
to
100 C, particularly preferably from 40 to 80 C.
The extraction of the hydrogenation catalyst can be carried out in any
suitable
apparatus known to those skilled in the art, preferably in countercurrent
extraction
columns, cascades of mixer-settlers or combinations of mixer-settlers with
columns.
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24
The raffinate (5) from the extraction unit (II), which comprises the major
part of the
polar solvent and the formic acid/amine adducts, is fed to the distillation
unit (III) in
order to separate the polar solvent or solvent mixture from the formic
acid/amine
adduct. The phase comprising tertiary amine and hydrogenation catalyst from
the
extraction unit (II) is recirculated to the hydrogenation reactor.
Amounts of individual components of the polar solvent in the liquid phase to
be
extracted are sometimes dissolved in addition to the catalyst in the
extractant, viz.
the amine stream. This is not a disadvantage for the process since the amount
of
solvent already extracted does not have to be fed to the solvent removal and
may
thus save vaporization energy and apparatus costs.
It can be advantageous to integrate an apparatus for adsorbing traces of
hydrogenation catalyst into the plant for carrying out the process between the
extraction unit (II) and the distillation unit (III). Numerous adsorbents are
suitable for
this absorption. Examples are polyacrylic acid and salts thereof, sulfonated
polystyrenes and salts thereof, activated carbons, montmorillonites,
bentonites,
silica gels and zeolites.
In the extraction of the catalyst from the output (3) in the extraction unit
(II), an
extract (4) comprising the tertiary amine and the catalyst and a raffinate (5)
comprising the polar solvent and the formic acid/amine adducts are obtained.
The
raffinate (5) is enriched in the formic acid/amine adducts and the polar
solvent. With
regard to the formic acid/amine adducts, "enriched" means a partition
coefficient of
the formic acid/amine adducts
P = [concentration of formic acid/amine adduct (II) in liquid phase (A)] /
[concentration of formic acid/amine adduct (II) in liquid phase (B)]
of > 1. The partition coefficient is preferably 2 and particularly preferably
5. The
extract (4) is enriched in the tertiary amine. When a homogeneous catalyst is
used,
this likewise accumulates in the raffinate. The extract (4) is recycled to the
hydrogenation reactor.
The liquid phases (A) and (B) have the meaning defined above.
Recirculation of a gas phase comprising unreacted carbon dioxide and/or
unreacted
hydrogen to the hydrogenation reactor may also be advantageous. This may be
desirable, for example, for discharging undesirable by-products or impurities,
part of
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the upper phase and/or part of the liquid or gaseous phases comprising carbon
dioxide or carbon dioxide and hydrogen from the process.
The major part of the polar solvent of the raffinate (5) is separated off
thermally
5 from the formic acid/amine adducts in a distillation unit (III), with the
polar solvent
which has been removed by distillation being recirculated to the hydrogenation
reactor (I). The pure formic acid/amine adducts and also free tertiary amine
are
obtained at the bottom of the distillation unit (III) since formic acid/amine
adducts
having a relatively low amine content are formed on removal of the polar
solvent, as
10 a result of which a two-phase bottoms mixture comprising an amine phase
and a
formic acid/amine adduct phase is formed.
The thermal separation of the polar solvent or solvent mixture, see above, is
preferably carried out at a temperature at the bottom at which, at the given
15 pressure, no free formic acid is formed from the formic acid/amine
adduct having
the higher (x1) or lower (x2) amine content. In general, the temperature at
the
bottom of the thermal separation unit is at least 20 C, preferably at least 50
C and
particularly preferably at least 70 C and generally not more than 210 C,
preferably
not more than 190 C. The pressure is generally at least 1 hPa abs, preferably
at
20 least 50 hPa abs and particularly preferably at least 100 hPa abs and
generally not
more than 1 MPa abs and preferably 0.1 MPa abs.
The thermal removal of the polar solvent or solvent mixture is carried out
either in
an evaporator or in a distillation unit comprising an evaporator and column,
filled
25 with ordered packing, random packing elements and/or trays. The solvent
can be
condensed after the thermal separation, and the enthalpy of condensation
liberated
can in turn be utilized, for example, to preheat the solvent with amine/formic
acid
adduct mixture coming from the extraction to evaporation temperature.
As an alternative, only parts of the solvent mixture can be separated off.
This
applies particularly to solvent components which can be separated off via a
side
stream in the later formic acid distillation.
The formic acid/amine adducts which are obtained after the thermal removal of
the
polar solvent or solvent mixture or parts of the solvent in the distillation
unit (111) are
then, as bottom stream (7), thermally dissociated into free formic acid and
free
tertiary amine in a thermal dissociation unit (V), for example a distillation
unit, with
the free formic acid formed being distillatively removed and the free tertiary
amine
comprised in the bottoms from the distillation unit being recirculated to the
hydrogenation reactor (I). Here, the free amine obtained as second phase in
the
:A 02821642 2013-06-13
26
thermal removal of the polar solvent in the distillation unit (III) can be
separated off
beforehand in a phase separation vessel, together with the bottom product from
the
thermal dissociation unit (V) for isolation of formic acid in a common phase
separation vessel or fed directly as two-phase mixture to the dissociation
unit (V)
(see general embodiments). The liberated formic acid can be taken off, for
example, (i) at the top, (ii) at the top and as side offtake stream or (iii)
only as side
offtake stream. If formic acid is taken off at the top, a formic acid purity
of up to
99.99% by weight is possible. When formic acid is taken off as side offtake
stream,
aqueous formic acid is obtained, with the mixture comprising about 85% by
weight
of formic acid being of particular importance in industrial practice.
Depending on the
water content of the feed to the distillation unit, the formic acid is taken
off to an
increased extent as overhead product or as side product. If necessary, it is
even
possible to take off formic acid only as side product, in which case the
required
amount of water may even not be able to be explicitly added. The thermal
dissociation of the formic acid/amine adduct is generally carried out at the
process
parameters in respect of pressure, temperature and configuration of the
apparatus
which are known from the prior art. Reference may thus be made, for example,
to
the descriptions in EP 0 181 078 A or WO 2006/021411. The distillation unit to
be
used generally comprises a distillation column which generally comprises
random
packing elements, ordered packings and/or trays.
In general, the temperature at the bottom of the distillation column
(dissociation unit
(V)) is generally at least 130 C, preferably at least 140 C and particularly
preferably
at least 150 C, and generally not more than 210 C, preferably not more than
190 C
and particularly preferably not more than 185 C. The pressure is generally at
least
1 hPa abs, preferably at least 50 hPa abs and particularly preferably at least
100 hPa abs, and generally not more than 500 hPa abs, preferably not more than
300 hPa abs and particularly preferably not more than 250 hPa abs.
A water-comprising stream of formic acid may optionally also be taken off as
side
product.
The process of the invention has a number of advantages over the integrated
processes which have previously been described in EP 181 078 B1 and
EP 357 243 B1: the same tertiary amine is used for binding the formic acid in
the
hydrogenation and the thermal dissociation of the formic acid/amine adducts.
This
amine, which is obtained in free form in the thermal dissociation, is then
used for
extraction of the catalyst from the product phase in order to recirculate the
catalyst
together with the amine to the reaction vessel. It has a higher stability than
the N-
alkylimidazoles previously described. Losses of noble metals virtually do not
occur.
:A 02821642 2013-06-13
27
The catalyst is prevented from getting into the thermal dissociation unit and
there
catalyzing the decomposition of formic acid. It is a great advantage that it
can be
separated off in its active form and recirculated. High formic acid yields and
a high
product purity are achieved. The extraction replaces two distillation systems.
As a
result, energy costs and capital costs are reduced.
The process of the invention makes it possible to isolate concentrated formic
acid in
high yield and high purity by hydrogenation of carbon dioxide. It has, in
particular, a
particularly simple mode of operation which involves, compared to the prior
art, a
simpler process concept, simpler process stages, a smaller number of process
stages and also simpler apparatuses. Thus, for example, an appropriate choice
of
the tertiary amine and of the polar solvent enables, in the case of the use of
a
homogeneous catalyst, the latter to be separated off by extraction of formic
acid/amine adducts and recirculated without further work-up steps to the
hydrogenation reactor. The extraction can also be carried out under
superatmospheric pressure. Owing to the prompt separation of the catalyst from
the
formic acid/amine adducts formed, a backreaction with decomposition into
carbon
dioxide and hydrogen is suppressed.
Furthermore, no complicated, separate base replacement is necessary in the
process of the invention, so that the formic acid/amine adducts formed in the
hydrogenation reactor can be used directly for thermal dissociation. The use
of a
low-boiling polar solvent makes it possible for this solvent to be separated
off
thermally under mild conditions in a stage preceding the thermal dissociation
of
formic acid, as a result of which esterification of alcohols used and
decomposition
of the formic acid are minimized, a lower energy consumption is necessary and
a
higher purity of the formic acid can be achieved. As a result of the simpler
process
concept, the production plant required for carrying out the process of the
invention
is more compact in terms of a lower space requirement and the use of fewer
apparatuses compared to the prior art. It has a lower capital outlay and a
lower
energy consumption.
The invention is illustrated below with the aid of examples and a drawing.
Example A1-4 hydrogenation, catalyst extraction and reuse of the catalyst:
A 100 ml or 250 ml autoclave made of Hastelloy C and equipped with a blade
stirrer
or magnetic stirrer was charged under inert conditions with the trialkylamine,
polar
solvent and the catalyst. The autoclave was subsequently closed and CO2 was
injected at room temperature. H2 was subsequently injected and the reactor was
:A 02821642 2013-06-13
28
heated while stirring (700-1000 rpm). After the reaction time, the autoclave
was
cooled and the reaction mixture was depressurized. After the reaction,
trialkylamine
and optionally water were added to the reaction output to effect extraction
and the
mixture was stirred at room temperature for 10 minutes. A two-phase product
product phase by extraction and reused for the hydrogenation. Relatively high
degrees of enrichment for the ruthenium can be achieved by simple connection
of a
plurality of extraction steps in series, e.g. in a cascade of mixer-settlers
or in a
countercurrent extraction, using the same amount of amine.
29
'
Table 1.1
Example A-la (first hydrogenation Example A-lb (reuse of the
Example A-2a (first Example A-2b (reuse of the
and extraction) catalyst and extraction)
hydrogenation) catalyst and extraction)
Autoclave 250m1 250m1
100 ml 100 ml
Tertiary amine 65.0 g of trihexylamine Upper phase from A-la
20.0 g of trihexylamine 15.0 g of upper phase from
A-2a
Polar solvent 25.0 g of methanol 25.0 g of methanol
10.0 g of methanol 10.0 g of methanol
(introduced) 2.0 g of water 2.0 g of water
1.0 g of water 1.0 g of water
Catalyst 0.32 g of [Ru(PnOcty13)4(1-1)2],
Upper phase from A-la 0.16 g of [Ru(PnOcty13)4(H)2], 15.0 g of upper
phase from
0.08 g of 1,2- 0.04 g of 1,2- A-2
bis(dicyclohexylphosphino)ethane
bis(dicyclohexylphosphino)ethane
Injection of CO2 To 3.1 MPa abs
To 3.0 MPa abs To 2.2 MPa abs To 2.4 MPa abs.
2.
Injection of H2 To 12.0 MPa abs To 12.0 MPa
To 8.0 MPa abs To 8.0 MPa abs 2
Heating 70 C 70 C
70 C 70 C E
.
EH
Reaction time 1 hour 1 hour
1 hour 1 hour
8
Amine addition to the 65.0 g of trihexylamine 65.0 g of
trihexylamine 15.0 g of trihexylamine 15.1 g of trihexylamine
extraction after the
reaction
Water addition after the - 4.0 g
- -
reaction
Upper phase 77.3 g 118.0 g
19.6 g 25.8 g
Lower phase 80.9 g 50.5 g
26.0 g 17.6 g
8.6% of formic acid 6.4% of formic acid
7.73% of formic acid 5.3% of formic acid
cRu in trialkylamine phase - 210 ppm
290 ppm 170 ppm
after extraction
cRu in lower phase after - 3 ppm
75 ppm 18 ppm
extraction
Table 1.2
30
..
Example A-3a (first hydrogenation Example A-3b (reuse of
Example A-4a (first Example A-4h (reuse of the
and extraction) the catalyst and extraction)
hydrogenation) catalyst and extraction)
Autoclave 250 ml 250 ml
250 ml 250 ml
Tertiary amine 65.0 g of trihexylamine Upper phase from A-3a
65.0 g of trihexylamine Upper phase from A-4a
Polar solvent 25.0 g of methanol 25.0 g of methanol
25.0 g of methanol 25.0 g of methanol
(introduced)
3.0 g of water .
Catalyst 0.16 g of [Ru(PnOcty13)4(H)2], Upper
phase from A-3a 0.16 g of [Ru(PnOcty13)4(H)2], Upper phase from A-4a
0.04 g of 1,2-
0.04 g of 1,2-
bis(dicyclohexylphosphino)ethane
bis(dicyclohexylphosphino)ethane
Injection of CO2 To 2.4 MPa abs To 2.3 MPa abs
To 2.7 MPa abs To 2.2 MPa abs
Injection of H2 To 12.0 MPa abs To 12.0 MPa To
12.0 MPa abs To 12.0 MPa
Heating 70 C 70 C
70 C 70 C
Reaction time 1 hour 1 hour
1 hour 1 hour
Amine addition to the 66.0 g of trihexylamine-
66.6 g of trihexylamine - 5
extraction after the
reaction
Water addition after the 4.1 g 4.0 g
- -
reaction
Upper phase 97.7 g 67.3 g
91.5 g 67.0 g
Lower phase 61.7 g 54.6 g
69.1 g 50.0 g
8.3% of formic acid 6.0% of formic acid
8.6% of formic acid 5.0% of formic acid
cRu in trialkylamine phase . 75 ppm
95 ppm 80 ppm 100 ppm
after extraction .
cR, in lower phase after 27 ppm 4 ppm
25 ppm 4 ppm
extraction
:A 02821642 2013-06-13
31
Examples B1 and B2 (thermal removal of the polar solvent from the
trialkylamine/solvent/formic acid mixtures and dissociation of the formic acid-
amine
adducts; process steps 3 and 4):
Alcohol and water are distilled off from the product phase (comprises the
formic
acid/amine adduct) under reduced pressure by means of a rotary evaporator. A
two-phase mixture (trialkylamine phase and formic acid/amine adduct phase) is
formed as bottoms and the two phases are separated. The composition of the
distillate (comprising the major part of the methanol and of the water), the
upper
phase (comprising the free trialkylamine) and the lower phase (comprising the
formic acid-amine adduct) was determined by gas chromatography and by
potentiometric titration of the formic acid against 0.1 N KOH in Me0H using a
"Mettler Toledo DL50" titrator. The formic acid is then thermally split off
from the
trialkylamine in the lower phase from the first step in a vacuum distillation
apparatus
using a 10 cm Vigreux column. Complete removal of the formic acid leaves
single-
phase bottoms comprising the pure trialkylamine which can be used for
extraction
of the catalyst and recirculation to the hydrogenation. The formic acid and
residual
water are present in the distillate. The composition of the bottoms and of the
distillate was determined by gas chromatography and by potentiometric
titration of
the formic acid against 0.1 N KOH in Me0H using a "Mettler Toledo DL50"
titrator.
The parameters and results of the individual experiments are shown in Table
1.15.
Examples B1 and B2 show that various polar solvents can be separated off under
mild conditions from the product phase in the process of the invention,
forming a
lower phase which is relatively rich in formic acid and an upper phase
comprising
predominantly tertiary amine. The formic acid can then be split off from the
trialkylamine in this lower phase which is relatively rich in formic acid at
elevated
temperatures, giving the free trialkylamine. The formic acid obtained in this
way still
comprises some water but this can be separated off from the formic acid by
means
of a column having a relatively high separation power. The trialkylamine
obtained
both in the removal of the solvent and also in the thermal dissociation can be
used
for removing the catalyst from the product stream in step 2.
32
.
Table 1.3
Example B-la Example B-lb Example B-2a
Example D-2b
(removal of the polar (dissociation of the formic
(removal of the polar (dissociation of the formic acid-
solvent) acid-amine adduct) solvent)
amine adduct)
Feed mixture 199.8 g Lower phase from D1-a 199.8 g
Lower phase from D2-a
( /0 by weight) 8.9% of formic acid 7.8% of
formic acid
28.4% of methanol 33.0% of
methanol
5.6% of water 15.1% of
water
57.1% of trihexylamine 44.0% of
trihexylamine
Formic acid : amine in 1 : 1.1 1 : 0.64 1 : 1
1 : 0.89
feed mixture
N;
Pressure 200 mbar 90 mbar 200 mbar
90 mbar E.
Temperature 120 C 153 C 120 C
153 C g
8
E.'
Lower phase in the 79.8 g 63.6 g 69.4 g
55.5 g
bottoms after distillation 22.1% of formic acid 100% of
trihexylamine 14.9% of formic acid 99.7% of trihexylamine
(% by weight) 1.5% of water 6.9% of water
0.3% of water
76.4% of trihexylamine 78.2% of
trihexylamine
Upper phase in the 50.5 g Single-phase 32.7 g
Single-phase
bottoms after distillation 100% of
trihexylamine 99.7% of trihexylamine
0.3% of water
,
Distillate 66.6 g 14.9 g 93.1 g
12.9 g
0.3% of formic acid 92.1% of formic acid 70.1% of
methanol 85.0% of formic acid
81.2% of methanol 7.9% of water 29.9% of
water 15% of water
18.5% of water
:A 02821642 2013-06-13
33
In the drawings:
Figure 1 shows a block diagram of a preferred embodiment of the process
of the
invention,
Figure 2 shows a block diagram of a preferred embodiment of the process of the
invention,
Figure 3 shows a block diagram of a further preferred embodiment of the
process of the invention,
Figure 4 shows a block diagram of a preferred embodiment of the process of the
invention,
Figures 5A, 5B and 5C each show different, preferred variants for the
fractional
removal of polar solvent and water for the preferred embodiment in
figure 3.
In the embodiment shown in figure 1, carbon dioxide, stream (1) and hydrogen,
stream (2), are fed into the hydrogenation reactor I. In this reactor, they
are reacted
in the presence of a catalyst comprising an element of group 8, 9 or 10 of the
Periodic Table, a tertiary amine and a polar solvent to form formic acid-amine
adducts. The output from the hydrogenation reactor (I) (stream (3)), which can
consist of one or two phases, is fed to the extraction unit II in which the
catalyst is
extracted by means of the tertiary amine from the phase separation vessel (IV)
(stream (11)). The tertiary amine with the catalyst (stream (6)) from the
extraction
unit (II) is recirculated to the hydrogenation reactor (I). The product phase
(5) from
the extraction unit (II) is fed to the thermal separation unit (III) in order
to separate
off the polar solvent and the water thermally from the formic acid-amine
adducts.
The stream (5) can additionally be passed over an adsorber bed to remove last
traces of catalyst from this stream before the thermal separation. The polar
solvent
which is thermally separated off in the distillation unit 111 is recirculated
as stream (6)
to the hydrogenation reactor (I) and the two-phase bottoms mixture (stream 7)
from
the distillation unit (III), which comprises the formic acid-amine adducts and
the
tertiary amine, is fed to the phase separation vessel (IV). The formic acid-
amine
adducts are separated off in the phase separation vessel (IV) and fed as
stream (8)
to the distillation unit (V) in which they are thermally dissociated into free
formic acid
and tertiary amine. The free formic acid is taken off as overhead product,
stream
(10), and the two-phase bottoms from the distillation unit (V), which comprise
tertiary amine and undissociated formic acid/amine adducts, stream (9) are fed
:A 02821642 2013-06-13
34
back into the phase separation vessel (IV). The tertiary amine which is
separated
off in the phase separation vessel (IV) is fed as stream (1 1) to the
extraction unit (II)
in order to extract the catalyst, or parts of stream (1 1) can be recirculated
directly to
the hydrogenation reactor (I) if not all the trialkylamine is required for the
extraction.
In the embodiment shown in figure 2, carbon dioxide, stream (1), and hydrogen,
stream (2), are introduced into the hydrogenation reactor (I). In this
reactor, they are
reacted in the presence of a catalyst comprising an element of group 8, 9 or
10 of
the Periodic Table, a tertiary amine and a polar solvent to form formic acid-
amine
adducts. The output from the hydrogenation reactor (I) (stream (3)), which can
consist of one or two phases, is fed to the extraction unit II in which the
catalyst is
extracted by means of the tertiary amine from the phase separation vessel (IV)
(stream (11)). The tertiary amine with the catalyst (stream (6)) from the
extraction
unit (II) is recirculated to the hydrogenation reactor (I). The product phase
(5) from
the extraction unit (II) is fed to the thermal separation unit (III) in order
to separate
off the polar solvent and the water thermally from the formic acid-amine
adducts.
The stream (5) can be passed over an adsorber bed to remove last traces of
catalyst from this stream before the thermal separation. The polar solvent
which is
separated off thermally in the distillation unit III is recirculated as stream
(6) to the
hydrogenation reactor (I) and the two-phase bottoms mixture from the
distillation
unit (III), which comprises the formic acid-amine adducts and the tertiary
amine
(stream 7), is fed to the distillation unit (V). In this distillation unit,
the formic
acid/amine adducts are thermally dissociated into free formic acid and
tertiary
amine. The free formic acid is, for example, removed as overhead product,
stream
(10). The two-phase bottoms from the distillation unit (V), stream (9), which
comprise tertiary amine and undissociated formic acid/amine adducts, are fed
to the
phase separation vessel (IV). The tertiary amine which is separated off in the
phase
separation vessel (IV) is fed as stream (11) to the extraction unit (II). Part
of stream
(11) can also be recirculated directly to the hydrogenation reactor (I) if not
all the
amine is required for the extraction.
In the embodiment shown in figure 3, carbon dioxide, stream (1), and hydrogen,
stream (2), are introduced into the hydrogenation reactor (I). In this
reactor, they are
reacted in the presence of a catalyst comprising an element of group 8, 9 or
10 of
the Periodic Table, a tertiary amine and a polar solvent to form formic acid-
amine
adducts. The output from the hydrogenation reactor (I) (stream (3)), which can
consist of one or two phases, is admixed with the predominantly water-
comprising
stream (12) from the distillation vessel (III) in order to obtain better phase
separation and catalyst distribution in the extraction together with high
:A 02821642 2013-06-13
hydrogenation activity. This stream is fed to the extraction unit (11) in
which the
catalyst is extracted by means of the tertiary amine from the phase separation
vessel (IV) (stream (11)). The tertiary amine with the catalyst (stream (6))
from the
extraction unit (11) is recirculated to the hydrogenation reactor (I). The
product phase
5 (5) from the extraction unit (11) is fed to the thermal separation unit
(111) in order to
separate off the polar solvent and the water thermally from the formic acid-
amine
adducts. The stream (5) can be passed over an adsorber bed to remove last
traces
of catalyst from this stream before the thermal separation. The polar solvent
which
is separated off thermally in the distillation unit 111 is recirculated as
stream (6) to the
10 hydrogenation reactor 1, the water is recirculated as stream (12) to the
output from
the hydrogenation reactor (stream (3)) and the two-phase bottoms mixture from
the
distillation unit 111, which comprises the formic acid-amine adducts and the
tertiary
amine (stream (7)), is fed to the phase separation vessel (IV). The formic
acid-
amine adducts are separated off in the phase separation vessel (IV) and fed as
15 stream (8) to the distillation unit (V) in which they are thermally
dissociated into free
formic acid and tertiary amine. The free formic acid is taken off as overhead
product, stream (10), and the two-phase bottoms from the distillation unit
(IV),
which comprise tertiary amine and undissociated formic acid/amine adducts,
stream
(9), are fed back into the phase separation vessel (IV). The tertiary amine
which is
20 separated off in the phase separation vessel (IV) is fed as stream (11)
to the
extraction unit (11) in order to extract the catalyst or part of stream (11)
can be
recirculated directly to the hydrogenation reactor (1) if not all the
trialkylamine is
required for the extraction.
25 The embodiment shown in figure 4 differs from the embodiments in figures
1 and 2
in that part of the two-phase bottoms mixture (stream (7)) is fed to the phase
separation vessel IV and part of the two-phase bottoms mixture (stream (7)) is
fed
to the distillation unit (V).
30 Figures 5A, 5B and 5C each schematically show different variants for the
thermal
removal of water and polar solvent for the embodiment shown in figure 3.
Here, figure 5A shows the removal of two streams, i.e. the stream (6)
comprising
predominantly polar solvent and the stream (12) comprising predominantly
water, in
35 a single distillation column which can be a column having a side offtake
or a
dividing wall column.
Figure 5B schematically shows an embodiment with a 2-column variant (columns
(111a) and (111b)), where the lower-boiling component, generally the polar
solvent, is
:4 02821642 2013-06-13
36
separated off in the first column, IIla, and the intermediate-boiling
component,
generally the water, is separated off as stream (12) in the second column,
(111b), to
which a feed stream (5a) depleted in polar solvent is fed.
Figure 5C shows a further embodiment for the removal of water and polar
solvent,
in which the phase (7) comprising the formic acid/amine adducts and the
tertiary
amine is firstly separated off in a first distillation column (111c) and a
stream (5b) is
subsequently fractionated in a second distillation column (111d) to give a
stream (6)
comprising predominantly polar solvent and a stream (12) comprising
predominantly water.
