Note: Descriptions are shown in the official language in which they were submitted.
0050/46633
CA 02247991 1998-08-28
Preparation of 1,6-hexanediol having a purity of over 99
The present invention relates to a process for preparing
1,6-hexanediol having a purity of at least 99 % which is, in
particular, essentially free of 1,4-cyclohexanediols, from a
carboxylic acid mixture which is obtained in the oxidation of
cyclohexane to cyclohexanone/cyclohexanol using oxygen or
l0 oxygen-containing gases by water extraction of the reaction
mixture, by esterification of the acids, fractionation of the
esterification mixture into an ester fraction free of
1,4-cyclohexanediols and a fraction comprising the
1,4-cyclohexanediols, hydrogenation of the ester fraction and
purification of the 1,6-hexanediol by distillation.
1,6-Hexanediol is a sought-after monomer building block which is
used predominantly in the polyester and polyurethane sector.
The aqueous solutions of carboxylic acids which are formed in the
oxidation of cyclohexane to cyclohexanol and cyclohexanone (cf.
Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition,
1 7, Vol. A8, p. 2/9) as by-products, hereinafter referred to as
dicarboxylic acid solution (DCS), generally comprise (calculated
in % by weight on an anhydrous basis) from 10 to 40 % of adipic
acid, from 10 to 40 % of 6-hydroxycaproic acid, from 1 to 10 % of
glutaric acid, from 1 to 10 % of 5-hydroxyvaleric acid, from 1 to
5 0 of 1,2-cyclohexanediols (cis and trans), from 1 to 5 0 of
1,4-cyclohexanediol (cis and trans), from 2 to 10 % by weight of
formic acid and also many further monocarboxylic and dicarboxylic
acids, oxo and oxa compounds whose individual amounts generally
do not exceed 5 0. Examples which may be mentioned are acetic
acid, propionic acid, butyric acid, valeric acid, caproic acid,
oxalic acid, malonic acid, succinic acid, 4-hydroxybutyric and
gamma-butyrolactone.
DE 2 321 101 and DE 1 235 879 disclose the hydrogenation of these
aqueous dicarboxylic acid solutions at from 120 to 300°C and
pressures of from 50 to 700 bar in the presence of catalysts
comprising predominantly cobalt to give 1,6-hexanediol as main
product. The hydrogenation products are preferably worked up by
distillation. Even with an extremely high distillation
efficiency, this work-up succeeds only incompletely, if at all,
in separating the 1,4-cyclohexanediols which are unchanged in the
hydrogenation from 1,6-hexanediol, so that the
1,4-cyclohexanediols which were initially present in the DCS are
CA 02247991 2004-09-16
2
still present in the 1,6-hexanediol in a concentration of
generally from 2 to 5 % by weight.
To counter this problem, some starting points for solutions are
known:
US 3 933 930 describes the conversion of 1,4-cyclohexanediol in
aqueous solutions of adipic acid and 6-hydroxycaproic acid into
cyclohexanol, cyclohexane and/or cyclohexene by catalytically
prehydrogenating the mixture. This process requires the use of
two different hydrogenation catalysts, one for the
prehydrogenation and one for the actual carboxylic acid
hydrogenation and is therefore complicated.
In DE A 2 060 548, very pure 1,6-hexanediol is obtained by
crystallization. This process too is very complicated and is also
associated with considerable yield losses.
A further possible way of obtaining highly pure 1,6-hexanediol is
to hydrogenate pure adipic acid or pure adipic esters in place of
DCS (K. Weissermel, H.J. Arpe, Industrielle Organische Chemie,
VCH-Verlagsgemeinschaft Weinheim, 4th Edition, 1994, page 263).
However, pure adipic acid, is very expensive in comparison with
DCS. In addition, the carboxylic acid mixture obtained in the
oxidation of cyclohexane is a waste product which should be
utilized in terms of the materials present, for environmental
reasons too.
It is an object of the present invention to develop a novel
process in which 1,6-hexanediol can be obtained in high purity.
in high yield and with justifiable outlay from DCS.
The invention, as claimed is concerned with a process for preparing 1,6-
hexanediol from an aqueous carboxylic acid mixture comprising adipic acid, 6-
hydroxy-caproic acid and small amounts of 1,4-cyclohexanediols which aqueous
carboxylic acid mixture is obtained as a by-product in the oxidation of
cyclohexane to cyclohexanone/cyclohexanol using oxygen or oxygen-containing
gases followed by water extraction, which process comprises esterifying the
aqueous carboxylic acid mixture followed by catalytic hydrogenation,
CA 02247991 2004-09-16
2a
a) reacting monocarboxylic and dicarboxylic acid present in the
aqueous carboxylic acid mixture with a low molecular weight alcohol to give an
esterification mixture comprising the corresponding carboxylic esters;
b) removing excess alcohol and low boilers from the esterification
mixture in a first distillation stage;
c) fractionating bottoms from the first distillation stage in a second
distillation stage to give an ester fraction essentially free of 1,4-
cyclohexanediols
and a fraction comprising at least a major amount of the 1,4-cyclohexanediols;
d) catalytically hydrogenating the ester fraction which is essentially
free of 1,4-cyclohexanediols to produce a hydrogenation product ; and
e) isolating 1,6-hexanediol from the hydrogenation product in a pure
distillation stage.
We have found that this object is achieved by a process for
preparing 1,6-hexanediol from a carboxylic acid mixture
comprising adipic acid, fi-hydroxycaproic acid and small amounts
of 1,4-cyclohexanediols which is obtained as a by-product in the
oxidation of cyclohexane to cyclohexanone/cyclohexanol using
oxygen or oxygen-containing gases by water extraction of the
reaction mixture, by esterification of the acids and
hydrogenation wherein
0050/46633
CA 02247991 1998-08-28
3
a) the monocarboxylic and dicarboxylic acids present in the
aqueous dicarboxylic acid mixture are reacted with a low
molecular weight alcohol to give the corresponding carboxylic
esters,
b) the resulting esterification mixture is freed of excess
alcohol and low boilers in a first distillation stage,
c) the bottoms are fractionated in a second distillation stage
to give an ester fraction essentially free of
1,4-cyclohexanediols and a fraction comprising at least the
major part of the 1,4-cyclohexanediols,
d) the ester fraction essentially free of 1,4-cyclohexanediols
is catalytically hydrogenated and
e) in a pure distillation stage, 1,6-hexanediol is isolated from
the hydrogenation product in a manner known per se.
It is surprising that in the separation of the ester mixtures
which are formed by esterification of the monocarboxylic and
dicarboxylic acids present in the DCS, the 1,4-cyclohexanediols,
which can of course likewise be present as esters of carboxylic
acids, can be separated off in such a way that after
hydrogenation and work-up the remaining very Iow
1,4-cyclohexanediol content of the 1,6-hexanediol is no longer of
any practical importance. Owing to the complicated mixtures to be
separated, it is surprising that it has been possible to remove
the 1,4-cyclohexanediols or their esters virtually completely
from the C6-esters used for the hydrogenation to 1,6-hexanediol
despite the unfavorable boiling point relationships and danger of
azeotrope formation.
The esterification can be carried out without addition of
catalysts, or preferably in the presence of catalysts. Suitable
low molecular weight alcohols are generally those having from 1
to 10 carbon atoms, in particular alkanols having from 1 to 8
carbon atoms. Diols such as butanediol or pentanediol are also
suitable in principle.
The industrially preferred alcohols used for the esterification
are n- or i-butanol and in particular methanol.
0050/46633
CA 02247991 1998-08-28
4
In the case of the esterification using methanol (variant A), the
procedure is to obtain, in the distillation stage (c), a methyl
carboxylate fraction essentially free of 1,4-cyclohexanediols at
the top of the column and a fraction comprising the high boilers
and the 1,4-cyclohexanediols as bottoms and to catalytically
hydrogenate the methyl carboxylate fraction in the hydrogenation
state (d).
If n- or i-butanol is used for the esterification (variant B),
the 1,4-cyclohexanediols together with the Iow boilers are
separated off at the top in the distillation stage (c) and the
butyl carboxylates are obtained as a side stream or as bottoms
comprising these and are subsequently introduced into the
hydrogenation stage (d). The process of the present invention and
its variants A (Fig. 1) and B (Fig. 2) are explained in general
as follows (where the term at the top means that the offtake is
above the feed point and as bottoms means that the offtake is
below the feed point):
Variant A
As shown in Fig. 1, the dicarboxylic acid solution (DCS), if
desired after dewatering, is fed together with a C1-C3-alcohol,
Preferably methanol, into the esterification reactor Rz in which
the carboxylic acids are esterified. The esterification mixture
obtained then goes to the column K1 in which the excess alcohol
(ROH), water and low boilers (LB) are distilled off at the top
and the ester mixture (EM) is taken off as bottoms and is fed
into the fractionation column K2. In this column, the mixture is
fractionated into an ester fraction (EF) essentially free of
1,4-cyclohexanediols and a bottoms fraction comprising high
boilers (HB) and 1,4-cyclohexanediols (1,4-CHDO). The ester
fraction (EF) is then catalytically hydrogenated in the
hydrogenation reactor R2 and the hydrogenation mixture is
fractionated in the distillation colomn K3 to give alcohol (ROH),
low boilers (LB) and pure 1,6-hexanediol.
Variant B
If alcohols having 4 or more carbon atoms, in particular n- or
i-butanol, are used for the esterification, the process as shown
in Fig. 2 differs only in that in the fractionation column K2 the
ester mixture (EM) is fractionated to give a top product of low
boilers (LB) in which the 1,4-cyclohexanediols (1,4-CHDO) are
present and an ester fraction (EF) essentially free of
1,4-cyclohexanediol which is obtained as a side fraction or as
0050/46633
CA 02247991 1998-08-28
bottoms comprising the ester fraction and is fed into the
hydrogenation stage (R2).
The process of the present invention is explained in detail
5 below. As shown in Fig. 3, the individual process steps are
classified into further stages, where the stages 2, 3, 4, 5, 6
and 7 are essential to the process and the stages 3 and 4 as well
as 6 and 7 may be combined. The stages 8, 9, 10 and 11 are not
strictly necessary, but may be useful for improving the economics
of the process.
The dicarboxylic acid solution (DCS) is generally an aqueous
solution having a water content of from 20 to 80 %. Since an
esterification reaction is an equilibrium reaction, it is usually
useful, particularly in an esterification using, for example,
methanol, to remove water present prior to the reaction,
especially when water cannot be removed~during the esterification
reaction, eg. cannot be removed as an azeotrope. The dewatering
ln~stage 1 can be carried out, for example, using a membrane
system or preferably by means of a distillation apparatus in
which water is removed at the top at from 10 to 250°C, preferably
from 20 to 200°C, particularly preferably from 30 to 200°C, and
a
pressure of from 1 to 1500 mbar, preferably from 5 to 1100 mbar,
particularly preferably from 20 to 1000 mbar, and higher
monocarboxylic acids, dicarboxylic acids and 1,4-cyclohexanediols
are taken off as bottoms. The bottom temperature is here
preferably selected such that the bottom product can be taken off
in liquid form. The water content at the bottom of the column can
be from 0.01 to 10 o by weight, preferably from 0.01 to 5 o by
weight, particularly preferably from 0.01 to 1 % by weight.
The removal of the water can be carried out such that the water
is obtained substantially free of acid or the lower
monocarboxylic acids present in the DCS, essentially formic acid,
can be mostly distilled off with the water so that they do not
bind any esterification alcohol in the esterification.
An alcohol having from 1 to 10 carbon atoms is mixed into the
carboxylic acid stream from stage 1: in the case of variant A an
alcohol having from 1 to 3 carbon atoms, viz. methanol, ethanol,
propanol or isopropanol, preferably methanol, in the case of
variant B an alcohol having from 4 to 10, in particular from 4 to
8, carbon atoms and particularly preferably n-butanol,
iso-butanol, n-pentanol and i-pentanol.
0050/46633
CA 02247991 1998-08-28
6
The mixing ratio of alcohol to carboxylic acid stream (mass
ratio) can be from 0.1 to 30, preferably from 0.2 to 20,
particularly preferably from 0.5 to 10.
5 This mixture goes as melt or solution into the reactor of stage 2
in which the carboxylic acids are esterified with the alcohol.
The esterification reaction can be carried out at from 50 to
400°C, preferably from 70 to 300°C, particularly preferably from
90 to 200°C. External pressure can be applied, but the
10 esterification is preferably carried out under the intrinsic
pressure of the reaction system. The esterification apparatus
used can be a stirred reactor or a flow tube or a plurality of
each these can be used. The residence time necessary for the
esterification is from 0.3 to 10 hours, preferably from 0.5 to
15 5 hours. The esterification reaction can proceed without addition
of a catalyst, but a catalyst is preferably added to increase the
reaction rate. This can be a homogeneously dissolved or a solid
catalyst. Examples of homogeneous catalysts are sulfuric acid,
phosphoric acid, hydrochloric acid, sulfonic acids such as
20 p_toluenesulfonic acid, heteropolyacids, such as
tungstophosphoric acid or Lewis acids such as aluminum, vanadium,
titanium and boron compounds. Preference is given to mineral
acids, in particular sulfuric acid. The weight ratio of
homogeneous catalyst to carboxylic acid melt is generally from
25 0,0001 to 0.5, preferably from 0.001 to 0.3.
Suitable solid catalysts are acid or superacid materials, eg.
acid and superacid metal oxides such as SiOz, A1z03, SnOz, Zr02 or
30 sheet silicates or zeolites, which can all be doped with mineral
acid radicals such as sulfate or phosphate to increase the
acidity, or organic ion exchangers containing sulfonic acid or
carboxylic acid groups. The solid catalysts can be used in a
fixed bed or as a suspension.
The water formed in the reaction is advantageously removed
continuously, eg. by means of a membrane or by distillation.
The completeness of the conversion of the free carboxyl groups
present in the carboxylic acid melt is determined by means of the
acid number (mg KOH/g) measured after the reaction. It is, with
subtraction of any acid added as catalyst, from 0.01 to 50,
preferably from 0.1 to 10. Not all of the carboxyl groups present
in the system are present as an ester of the alcohol used, but
some of them can be present in the form of dimeric or oligomeric
esters, eg. with the OH end of the hydroxycaproic acid.
UUSU/46633
CA 02247991 1998-08-28
7
The esterification mixture is fed to stage 3, a membrane system
or preferably a distillation column. If a dissolved acid has been
used as catalyst for the esterification reaction, the
esterification mixture is advantageously utilized with a base,
with from 1 to 1.5 equivalents of base being added per acid
equivalent of the catalyst. As bases, use is generally made of
alkali metal or alkaline earth metal oxides, carbonates,
hydroxides or alkoxides, or amines, as such or dissolved in the
esterification alcohol.
If a column is used in stage 3, the feed to the column is
preferably between the top and bottom streams. At the top, the
excess esterification alcohol ROH, water and, for example,
corresponding esters of formic acid, acetic acid and propionic
acid are taken off at from 0 to 150°C, preferably from 15 to
90°C
and in particular from 25 to 75°C, and pressures of from 1 to
1500 mbar, preferably from 20 to 1000 mbar, particularly
preferably from 40 to 800 mbar. This stream can either be
incinerated or preferably be worked up further in stage 11.
The bottom product obtained is an ester mixture consisting
predominantly of the esters of the alcohol ROH used with
dicarboxylic acids such as adipic acid and glutaric acid,
hydroxycarboxylic acids such as 6-hydroxycaproic acid and
5-hydroxyvaleric acid, and also of oligomers and free or
esterified 1,4-cyclohexanediols. It can be useful to allow an
amount of water and/or alcohol ROH of up to 10 % by weight of
each to remain in the ester mixture. The bottom temperatures are
from 70 to 250°C, preferably from 80 to 220°C, particularly
preferably from 100 to 190°C.
The stream from stage 3, which has been largely freed of water
and esterification alcohol ROH, is fed to stage 4. The latter is
a distillation column in which the feed is generally between the
low-boiling components and the high-boiling components. The
column is operated at from 10 to 300°C, preferably from 20 to
270°C, particularly preferably from 30 to 250°C, and pressures
of
from 1 to 1000 mbar, preferably from 5 to 500 mbar, particularly
Preferably from 10 to 200 mbar.
In variant A, ie. the esterification using C1-C3-alcohols, in
particular methanol, the stream from stage 3 is then separated
into a top fraction to be hydrogenated and a bottoms fraction
comprising the 1,4-cyclohexanediols.
UuSU/46633
CA 02247991 1998-08-28
8
The top fraction consists predominantly of remaining water and
remaining alcohol ROH, esters of the alcohol ROH with
monocarboxylic acids, predominantly C3-C6-monocarboxylic acids,
esters with hydroxycarboxylic acids such as 6-hydroxycaproic acid
and 5-hydroxyvaleric acid, and also, in particular, the diesters
with dicarboxylic acids such as adipic acid, glutaric acid and
succinic acid, also 1,2-cyclohexanediols, cagrolactone and
valerolactone.
The components mentioned can be taken off together at the top and
introduced into the hydrogenation (stage 5) or, in a further
preferred embodiment, fractionated in the column into a top
stream comprising predominantly remaining water and remaining
alcohol plus the abovementioned esters of the C3-C5-carboxylic
acids and a side stream comprising predominantly the
abovementioned esters of the C6-carboxylic acids and dicarboxylic
acids which then go to the hydrogenation.
The high-boiling components of the stream from stage 4,
consisting predominantly of 1,4-cyclohexanediols or their esters,
dimeric or oligomeric esters as well as sometimes polymeric
constituents of the DCS which are not defined in more detail, are
separated off in the stripping section of the column. These can
be obtained together or in such a way that the
1,4-cyclohexanediols are predominantly separated off via a side
stream of the column in the stripping section and the remainder
are separated off at the bottom. The 1,4-cyclohexanediols thus
obtained can be used, for example, as starting material for
active compounds. The high-boiling components, with or without
the 1,4-cyclohexanediols, can either be incinerated or, in a
preferred embodiment, go to the transesterification in stage 8.
In variant B, ie. the esterification using C4-Clo-alcohols, in
Particular n- or i-butanol, the stream from stage 3 can be
fractionated in stage 4 into a top fraction comprising the
1,4-cyclohexanediols, a side stream comprising predominantly the
C6-esters which goes to the hydrogenation and a bottom stream
comprising high boilers which can, if desired, go to stage 8.
The top fraction consists predominantly of remaining alcohol ROH,
C1-C3-monoesters of the alcohol ROH, valerolactone and 1,2- and
1,4-cyclohexanediols.
The side stream comprises predominantly diesters of succinic
acid, glutaric acid and adipic acid and also monoesters of
5-hydroxyvaleric acid and 6-hydroxycaproic acid. This side stream
UU50/46633
CA 02247991 1998-08-28
9
can be taken off either above or below the feed point of the
column and can be introduced into the hydrogenation (stage 5).
The bottom stream comprising oligomeric esters and other high
boilers can, in a similar way to variant A, either be incinerated
or advantageously go to stage 8.
According to a further embodiment, in stage 4 the C6-esters are
separated off together with the bottom stream and then, in a
further column, either separated as bottoms from the
above-described top fraction which consists predominantly of
remaining alcohol ROH, C1-C3-monoesters of the alcohol ROH,
valerolactone and 1,2- and 1,4-cyclohexanediols or separated as
top stream from the high boilers.
The fraction free or virtually free of .1, 4-cyclohexanediols from
stage 4, either the total stream or the side stream comprising
mainly esters of the C6-acids, is passed to the hydrogenation
stage 5.
The stages 3 and 4 can be combined, particularly when only
relatively small amounts are being processed. For this purpose,
for example, the C6 ester stream can be obtained in a batchwise
fractional distillation, again without 1,4-cyclohexanediols
getting into the stream fed to the hydrogenation.
The hydrogenation is carried out catalytically either in the gas
or liquid phase. Catalysts which can be used are in principle all
homogeneous and heterogeneous catalysts suitable for the
hydrogenation of carbonyl groups, for example metals, metal
oxides, metal compounds or mixtures thereof. Examples of
homogeneous catalysts are described, for example, in Houben-weyl,
Methoden der Organischen Chemie, Volume IV/1c, Georg Thieme
Verlag Stuttgart, 1980, pp. 45 - 67 and examples of heterogeneous
catalysts are described, for example, in Houben-weyl, Methoden
der Organischen Chemie, Volume IV/1c, pp. 16 to 26.
Preference is given to using catalysts comprising one or more of
the elements of transition groups I and VI to VIII of the
Periodic Table of the Elements, preferably copper, chromium,
molybdenum, manganese, rhenium, ruthenium, cobalt, nickel and
palladium, particularly preferably copper, cobalt or rhenium.
0050/46633
CA 02247991 1998-08-28
The catalysts can consist entirely of the active components or
the active components can be applied to supports. Suitable
support materials are, for example, Cr203, A1203, Si02, Zr02, Zn02,
Ba0 and Mg0 or mixtures thereof.
5
Particular preference is given to catalysts as are described in
EP 0 552 463. These are catalysts which, in the oxidic form, have
the composition
CuaAlbZr~MndOx,
where a > 0, b > 0, c ? 0, d > 0, a > b/2, b > a/4, a > c and a
> d and x is the number of oxygen ions required to maintain elec
trical neutrality of the formula unit. These catalysts can be
prepared, for example, as described in EP 552 463 by precipita-
tion of sparingly soluble compounds from solutions containing the
corresponding metal ions in the form of their salts. Suitable
salts are, for example, halides, sulfates and nitrates. Suitable
precipitants are all agents which lead to formation of insoluble
intermediates which can be converted into the oxides by thermal
treatment. Particularly suitable intermediates are the hydroxides
and carbonates or hydrogencarbonates, so that particularly pre-
ferred precipitants are alkali metal carbonates or ammonium car-
bonate. In the preparation of the catalysts, it is important that
the intermediates are thermally treated at from 500°C to 1000°C.
The BET surface area of the catalysts is from 10 to 150 m2/g.
Preference is given to using heterogeneous catalysts which are
either used as a fixed bed or as a suspension. If the
hydrogenation is carried out in the gas phase and over a
fixed-bed catalyst, temperatures of from 150 to 300°C and
pressures of from 1 to 100 bar, preferably from 15 to 70 bar, are
generally employed. In this hydrogenation, use is advantageously
made of hydrogen as hydrogenating agent and carrier gas in
amounts which are at least sufficient for the starting materials,
intermediates and products never to become liquid during the
reaction. The excess hydrogen is preferably circulated, with it
being possible to bleed off a small part as waste gas to remove
inerts such as methane. It is possible to use one reactor or a
plurality of reactors connected in series.
If the hydrogenation is carried out in the liquid phase using a
fixed-bed or suspended catalyst, it is generally carried out at
from 100 to 350°C, preferably from 120 to 300°C, and pressures
of
from 30 to 350 bar, preferably from 40 to 300 bar.
0050/46633
CA 02247991 1998-08-28
11
The hydrogenation can be carried out in one reactor or a
plurality of reactors connected in series. The hydrogenation in
the liquid phase over a fixed bed can be carried out either in
the downflow mode or in the upflow mode. According to a preferred
embodiment, use is made of a plurality of reactors, with the
predominant part of the esters being hydrogenated in the first
reactor and the first reactor preferably being operated with
circulation of liquid for heat removal and the downstream
reactors preferably being operated without circulation to
complete the conversion.
The hydrogenation can be carried out batchwise or preferably
continuously.
The hydrogenation product consists essentially of 1,6-hexanediol
and the alcohol ROH. Further constituents are principally, if the
total low-boiling stream from stage 4 of variant A has been used,
1,5-pentanediol, 1,4-butanediol, 1,2-cyclohexanediols and small
amounts of monoalcohols having from 1 to 6 carbon atoms and
water.
In stage 6, for example a membrane system or preferably a
distillation column, this hydrogenation product is fractionated
into the alcohol ROH which additionally contains the major part
of the further low-boiling components and a stream comprising
predominantly 1,6-hexanediol plus 1,5-pentanediol and the
1,2-cyclohexanediols. This is carried out at a pressure of from
10 to 1500 mbar, preferably from 30 to 1200 mbar, particularly
Preferably from 50 to 1000 mbar, top temperatures of from 0 to
120°C, preferably from 20 to 100°C, particularly preferably from
30 to 90°C, and bottom temperatures of from 100 to 270°C,
preferably from 140 to 260°C, particularly preferably from 160 to
250°C. The low-boiling stream can either be recirculated directly
to the esterification of stage 2 or go to stage 8 or stage 11.
The 1,6-hexanediol-containing stream is purified in a column in
stage 7. In this stage, 1,5-pentanediol, possibly the
1,2-cyclohexanediols and any further low boilers present are
separated off at the top. If the 1,2-cyclohexanediols and/or
1,5-pentanediol are to be isolated as additional desired
products, they can be separated in a further column. Any high
boilers present are removed at the bottom. 1,6-Hexanediol in a
purity of at least 99 ~ is taken from the column as a side
stream. This purification is carried out at pressures of from 1
to 1000 mbar, preferably from 5 to 800 mbar, particularly
preferably from 20 to 500 mbar, top temperatures of from 50 to
UU~U/4bb33
CA 02247991 1998-08-28
12
200°C, preferably from 60 to 150°C, and bottom temperatures of
from 130 to 270°C, preferably from 150 to 250°C.
If only relatively small amounts of 1,6-hexanediol are to be
prepared, the stages 6 and 7 can also be combined in a batchwise
fractional distillation.
To operate the process of the invention as economically as
possible, it is useful to recover the esterification alcohol ROH
and to reuse it for the esterification. For this purpose, the
stream comprising predominantly the alcohol ROH, for example
methanol, from stage 3 and/or 6 can be worked up in stage 11.
This is advantageously carried out using a column in which
components having boiling points lower than that of the alcohol
ROH are removed at the top, water and components having boiling
points higher than that of the alcohol ROH are removed at the
bottom and the alcohol ROH is isolated~as a side stream. The
column is advantageously operated at from 500 to 5000 mbar,
preferably from 800 to 3000 mbar.
In a further preferred embodiment of the process of the
invention, the high-boiling stream from stage 4 (in variant A) is
used to increase the total yield of 1,6-hexanediol based on
adipic acid and 6-hydroxycaproic acid introduced via the DCS
used. For this purpose, the dimeric and oligomeric esters of
adipic acid or hydroxycaproic acid present are reacted in stage 8
with further amounts of the alcohol ROH in the presence of a
catalyst. The weight ratio of alcohol ROH and the bottoms stream
from stage 4 is from 0.1 to 20, preferably from 0.5 to 10,
particularly preferably from 1 to 5. Suitable catalysts are in
principle those described above for the esterification in stage
2. However, preference is given to using Lewis acids. Examples of
these are compounds or complexes of aluminum, tin, antimony,
zirconium or titanium, for example zirconium acetylacetonate or
tetraalkyl titanates, eg. tetraisopropyl titanate, which are
employed in concentrations of from 1 to 10,000 ppm, preferably
from 50 to 6000 ppm, particularly preferably from 100 to 4000
ppm, based on the transesterification mixture. Particular
preference is given to titanium compounds.
The transesterification can be carried out batchwise or
continuously, in one reactor or a plurality of reactors, in
stirred vessels connected in series or tube reactors at from 100
to 300°C, preferably from 120 to 270°C, particularly preferably
from 140 to 240°C, and at the intrinsic pressures established. The
0050/46633
CA 02247991 1998-08-28
13
residence times required are from 0.5 to 10 hours, preferably
from 1 to 4 hours.
In the case of the esterification using methanol, this stream
from stage 8 can, for example, be returned to stage 3. To avoid
accumulations, especially of 1,4-cyclohexanediols, a substream of
the high boilers then has to be bled off at intervals or
continuously from stage 4. Another possibility is not to
recirculate the stream from stage 8 to stage 3 but, in a similar
way to stage 3, to fractionate it in a stage 9 into predominantly
alcohol ROH, which can then go to stage 2, 8 or 11, and a stream
comprising the esters.
This ester stream can in principle (with the proviso that
accumulation of the 1,4-cyclohexanediols is avoided) be
recirculated to stage 4 or is preferably fractionated in a
further stage 10 into the esters of the C6-acids and, relatively
unimportant in terms of amount, the esters of the C5-acids on the
one hand which are introduced either into stage 4 or directly
into stage 5 and, on the other hand, high boilers comprising the
1,4-cyclohexanediols, after which the high boilers are removed
from the system.
In this way, yields of 1,6-hexanediol of over 95 % can be
achieved at purities of over 99 %.
The novel process thus allows highly pure 1,6-hexanediol to be
obtained in high yield and in an economical manner from a waste
product.
The following example illustrates but in no way restricts the
process.
Example (variant A)
Stage 1 (dewatering):
0.1 kg/h of dicarboxylic acid solution (consisting essentially of
adipic acid, 6-hydroxycaproic acid, 1,4-cyclohexanediols,
glutaric acid, 5-hydroxyvaleric acid, formic acid, water) were
distilled continuously in a distillation apparatus (three-tray
bubble cap tray column having an external oil heating circuit,
oil temperature = 150°C, tray volume = about 25 ml each, feed
above the bubble cap trays) having a superposed packed column
(about 4 theoretical plates, no runback at the top). The top
UUSU/46633
CA 02247991 1998-08-28
14
product obtained comprised 0.045 kg having a formic acid content
in water of about 3 0. The water content of the bottoms stream
(5.5 kg) was about 0.4 0.
Stage 2 (esterification):
5.5 kg/h of the bottoms stream from stage 1 were reacted
continuously with 8.3 kg/h of methanol and 14 g/h of sulfuric
acid in a tube reactor (length 0.7 m, ~ 1.8 cm, residence time
2.7 h) The acid number of the product stream, excluding sulfuric
acid, was about 10 mg KOH/g.
Stage 3 (removal of excess alcohol and water):
The esterification stream from stage 2 was distilled in a 20 cm
packed column (1015 mbar, 65°C top temperature to 125°C bottom
temperature). 7.0 kg were taken off at the top. 6.8 kg were
obtained as bottoms.
Stage 4 (fractionation; 1,4-cyclohexanediol removal):
The bottoms stream from stage 3 was fractionally distilled in a
50 cm packed column (1 mbar, 70-90°C top temperature to 180°C
bottom temperature). The bottoms (1.9 kg) contained virtually all
the 1,4-cyclohexanediols.
0.6 kg of low boilers was distilled off (1,2:cyclohexanediols,
valerolactone, methyl 5-hydroxyvalerate, dimethyl glutarate,
dimethyl succinate, etc). 4.3 kg of the fraction comprising
predominantly dimethyl adipate and methyl 6-hydroxycaproate were
obtained.
The top stream, viz. the ester fraction, is passed to the
hydrogenation stage 5.
Stage 5 (hydrogenation):
4.3 kg of the C6-ester fraction from stage 4 were hydrogenated
continuously in a 25 ml reactor over a catalyst (catalyst: 70 0
by weight of CuO, 25 a by weight of ZnO, 5 % by weight of A1203)
which had previously been activated at 180°C in a stream of
hydrogen. The feed was 20 g/h, the pressure 220 bar and the
UUSU/46633
CA 02247991 1998-08-28
temperature 220°C. The ester conversion was 99.5 0, the
1,6-hexanediol selectivity was over 99 %. -
Alternatively, the ester fraction was hydrogenated continuously
5 in a two-stage reactor cascade (1st reactor:
2.5 1 of catalyst, downflow mode, 250 bar, product recirculation:
feed = 10 . 1, 220 - 230°C; 2nd reactor: 0.5 1 of catalyst,
downflow mode with straight passage, 260 bar, 220°C.) The catalyst
used was a catalyst comprising Cu0 (60 0), A1203 (30 0) and Mn203
10 (10 %) which had previously been activated at 180°C. The feed rate
was 1 kg/h. At 99.5 o conversion, the hexanediol selectivity was
over 99 %.
15 Stages 6 and 7:
4.0 kg of the hydrogenation product from stage 5 were
fractionally distilled (distillation flask with superposed 70 cm
packed column, reflux ratio: 2). 1 kg of methanol was distilled
off at 1013 mbar. After application of reduced pressure
(20 mbar), predominantly the 1,2-cyclohexanediols and
1,5-pentanediol distilled off. Subsequently (bp. 146°C),
1,6-hexanediol having a purity of 99.8 % distilled off.
(Remainder predominantly 1,5-pentanediol.)
Stage 8:
1.9 kg of the bottom product from stage 4 were admixed with 3.8
kg of methanol and 3.8 g of tetra-i-propyl titanate and reacted
continuously in a 1 m long, 440 ml capacity tube reactor which
was filled with 3 mm V2A rings. The mean residence time was about
2 hours.
Stage 9:
The product from stage 8 was fractionally distilled in an
apparatus similar to that described in stage 3. 3.5 kg
(predominantly methanol) were distilled off at a top temperature
of 65°C. 2.2 kg remained in the bottoms.
Stage 10:
The bottoms from stage 9 were fractionally distilled by a method
similar to stage 4 to a bottom temperature of 160°C. The
distillate obtained comprised 1.3 kg which can be directly
hydrogenated or returned to stage 4. Composition: 52 a of methyl
UUSU/46633
CA 02247991 1998-08-28
16
6-hydroxycaproate, 31 0 of dimethyl adipate, 5 % of dimethyl
glutarate, 4 0 of methyl 5-hydroxycaproate plus many further
components in unimportant amounts.
Stage 11:
7 kg of the top product from stage 3 were fractionally distilled
at 1015 mbar in a 20 cm packed column. 0.8 kg of a first fraction
was obtained at a top temperature of 59-65°C; this fraction
comprised predominantly methanol plus C1-C4-monomethyl esters. At
a top temperature of 65°C, 5.6 kg of methanol having a purity of
> 99 % were obtained. The bottoms (0.6 kg) consisted
predominantly of water.
20
30
40
