Specification
The invention relates to a process for the production of
five-membered or six-membered cyclic ethers, in particular
of anbydropolyols, by acid-catalysed cyclodehydration of
polyols that comprise at least two hydroxyl groups with a
spacing enabling ring formation - that is to say,
preferably with a spacing of 4 or 5 C atoms. The invention
is directed in particular towards the production of
anhydrotetritols, anhydropentitols and, particularly
preferred, anhydrohexitols, from tetritols, pentitols and
hexitols, respectively.
The acid-catalysed cyclodehydrogenation of polyhydric
alcohols accompanied by the formation of, in particular, 5-
membered cyclic ethers or hydroxyethers, designated
hereinafter as anhydropolyols, has been known for a long
time. The cyclodehydrogenation of a sugar alcohol from the
family of the hexitols results in a complex product mixture
of anhydrohexitols and dianhydrohexitols as well as
undesirable by-products arising by virtue of the production
process, among them polymers - see K Bock et al. in Acta
Chemica Scandinavica B 35 (1981) 441-449 and G Fleche et
al. in Starch/Starke 38 (1986) No. 1, 26-30. Increasing
interest is being shown in anhydropolyols that can be
produced from renewable raw materials such as sugars, for
instance 2,5-sorbitan and isosorbide from D-glucose or
sucrose via sorbitol, in connection with the production of
polyester resins, epoxy resins and surfactants.
In the article by G Fleche cited above, the influence of
the water content, the type of acid and che acid
concentration on the composition of the product and the
polymer content in connection with the acid-catalysed
cyclodehydration of sorbitol obtained by hydrolysis of
starch and hydrogenation of the D-glucose arising in the
process is dealt with. This article recommends that
dehydration be carried out, as far as possible, in the
absence of water. Mineral acids, organic cation-exchangers
or Lewis acids are employed as catalysts. The use of
mineral acids or Lewis acid as catalysts necessitates
neutralisation and elaborate separation of the salt from
the reaction mixture and disposal of the salt. A
disadvantage of all forms of implementation is the usually
high polymer content in the reaction mixture. The polymer
content is particularly disadvantageous when the reaction
mixture is to be supplied directly - that is to say,
without further elaborate purifying stages - to a stage for
further utilisation after removal by distillation of the
isosorbide which is formed.
According to DE-OS 31 11 092 cyclodehydrogenation can also
be carried out by means of gaseous hydrogen halide such as
HC1 as catalyst and, optionally in addition, of an organic
carboxylic acid as co-catalyst. Disadvantageous is the
very large quantity of catalyst, as well as a small
proportion of monoanhydro products. According to WO
89/00162 the cyclodehydrogenation of hexitols can also be
carried out at moderate temperature in liquid hydrogen
fluoride in the presence of a carboxylic acid, but this
process is very elaborate on account of the great dangers
of HF to both people and material.
Instead of using acidic catalysts, the cyclodehydration of
polyols such as glucitol can also be catalysed by means of
bimetcillic catalysts such as Cu-Pt, Cu-Au, Cu-Pd and Cu-Ru
in the presence of hydrogen - see C Montassier et al.,
Applied Catalysis A : General 121^Ug95j , 231^244.
However, in the course of this conversion, with increasing
conversion of glucitol {- sorbitol) into
monoanhydroglucitols and 1,4 -. 3,6-dianhydroglucitol a
decline in dehydration selectivity occurs. Although the
selectivity can be increased again by addition of NaCl, the
activity of the catalyst falls off considerably in the
process. According to EP-B 0 380 402 it is possible for a
maximum of 71 % anhydro compounds (49 % isosorbide and 22 %
isomeric monoanhydroglucitols) to be obtained from
glucitol. Disadvantageous are the high loss (29 %) of
parent compound, the long reaction-times and the high
catalyst demand or, as the case may be, the necessity for
catalyst regeneration. Unless long reacticn-times are
taken into consideration, the conversion of soarbitol is
incomplete, so that the cyclodehydrogenated reaction
mixture also contains, besides the monoanhydrohexitols and
isosorbide, considerable amounts of sorbitol. Although
isosorbide can easily be separated from the mixture by
distillation, besides the monoanhydrohexitols the
distillation sump remaining contains sorbitol, this latter
having a disadvantageous influence on consequent
conversions or necessitating elaborate purification.
The object of the invention is accordingly to make
available a process for the cyclodehydrogenation of
polyols, in particular of sugar alcohols, that results in
substantially quantitative conversion and that leads to
cyclodehydrogenation products containing less than 1 wt-%
polymers.
A process has been discovered for the production of five-
membered or six-membered cyclic ethers, in particular of
anhydropolyols, by cyclodehydration of polyols having at
least 4 C atoms and at least 2 hydroxyl groups, the polyol
being treated in the presence of water and an acidic
catalyst at a temperature of at least 100 °C, said process
being characterised in that the treatment is carried out in
the presence of an acid-stable hydrogenating catalyst in a
hydrogen atmosphere.
In the process according to the invention, polyols are
employed that contain at least two hydroxyl groups with a
spacing enabling cyclodehydrogenation. Ordinarily the
hydroxyl groups are separated from one another by four or
five, but preferably four, C atoms, so that in the course
of cyclodehydrogenation a 5-membered or 6-membered cyclic
ether is formed. It cannot be ruled out that 4-membered
cyclic ethers may also be formed in accordance with the
invention. The polyols employed preferably contain more
than two hydroxyl groups from which anhydropolyols can be
formed. The term 'anhydropolyols' in this connection is
taken to mean compounds that comprise one, two or more
cyclic ethers having five or six members, in particular
five members (= dihydrofuran derivatives and, in
particular, tetrahydrofuran derivatives), and in addition
one or more, in particular two to four, hydroxyl groups.
The polyols to be dehydrogenated contain at least 4 C
atoms, ordinarily 4 to 20 C atoms, but polyols having more
than 20 C atoms can also be employed, provided that they
are sufficiently water-soluble. Polyols having 4 to 6 c
atoms and 2 to 6 hydroxyl groups are particularly
preferred. The last group includes sugar alcohols from the
family of the tetritols such as erythritol, pentitols such
as ribitol, arabinitol and xylitol, and hexitols such as
glucitol, mannitol, galactitol and iditol. The tetritols,
pentitols and hexitols can be employed in the D form or L
form, in which they are ordinarily availe.ble in naturally
occurring raw materials or from such raw materials as can
be obtained, or in the form of the racemates or mixtures of
conformational isomers.
The cyclodehydrogenation according to the invention is
effected in the presence of water. The polyols are
ordinarily converted in the form of an aqueous solution,
preferably a solution having a polyol content in the range
from 10 to 80 wt-%, in particular 40 to 60 wt-%. Use may
be made, for example, of solutions such as can be obtained
in the course of the transformation of starch into alditols
(= hexitols) by hydrolysis with subsequent hydrogenation.
The conversion is effected at a temperature of at least
lOO0c, mostly in the range from 120 to 380 °C A
temperature in the range from 120 to 300 0C is preferred,
in particular 180 to 300 °C.
As in the state of the art, acidic catalysts are also
present in the process according to the invention. Use may
be made of mineral acids such as H2SO4, HCl and H3PO4,
organic carboxylic acids and sulfonic acids and also acidic
fixed catalysts, the H0 value of the Hammett acidity
function of which is less than +2, in particular less than
-3. Mineral acids are less preferred, because after the
conversion they have to be neutralised and the salts have
to be separated from the reaction mixture and disposed of.
In order to make processing of the cyclodehydrogenation
reaction mixture as simple as possible, according to a
preferred embodiment a carboxylic acid, the boiling-point
of which lies below that of the monoandropolyols or
dianhydropolyols to be distilled off, in particular a Cx to
C12 monocarboxylic acid, is employed as catalyst.
Particularly preferred is a carboxylic acid from the family
comprising formic acid, acetic acid and propionic acid.
Such carboxylic acids can be separated by distillation from

the reaction mixture and recycled.

The acidic fixed catalysts havingi H0 less than +2 are
substances from the following series: natural and synthetic
siliceous substances such as montmorillonite, mordenite and
acidic zeolites; acids permanently bonded to inorganic
carrier substances such as Si02, Al2Q3 or Ti02, such as, in
particular, phosphoric oxides/acids; oxides such as gamma-
A1203, Ti02, Zr02, Sn02, Bi205, Sb205, Mo03, W03; mixed
oxides such as Si02-Al203, Si02-Ti02, Al203-ZnO, Si02-Zr02,
Si02-Sn02, Si02-Mo03, Si02-W03; heteropoly acids, for
example polytungstosilicates and polytungstophosphates;
metallic salts such as A1P04, FeP04, Zn3(P04)2, Mg3(P04)2,
Ti3(P04)4, Zr3(P04)4; cat ion-exchangers such as exchangers
containing sulfonate groups and based on polystyrene,
polymeric perfluorinated resins or, preferably,
organopolysiloxanes (Deloxan®, available from Degussa AG).
Particularly preferred acidic fixed catalysts for the ester
formation and ester cleavage according to the invention are
zeolites of the H-Y and H-ZSM 5 types.
The required amount of acidic catalysts that are soluble in
the reaction mixture generally lies in the range from o.l
to 20 wt-%, in particular from 0.5 to 10 wt-%, in each case
relative to the polyol. The required amount of solid
acidic catalysts depends both on the activity thereof and
on the chosen reaction temperature but generally lies
within the range of the soluble catalysts. The optimal
amount required can be determined simply by means of
orienting experiments.
An essential feature of the invention is that, in addition
to the acidic catalyst, a conventional hydrogenating
catalyst: is present and the cyclodehydrogenation is carried
out in a hydrogen atmosphere. The partial pressure of
hydrogen is generally in the range from at least 0.1 to
20 MPa, preferably in the range from 1 to 15 MPa, and in
particular 3 to 10 MPa.
Although homogeneous and heterogeneous catalysts can be
> employed as hydrogenating catalysts, heterogeneous
catalysts are preferred, because a simple separation of the
catalyst from the reaction mixture, by filtration for
instance, is then possible. Conventional hydrogenating
catalysts contain by way of active component a noble metal
from the series Ru, Rh, Pd and Pt or a transition metal
from the series Cu, Cr, Co, Ni, Fe, including, in
particular, Raney catalysts and chromite catalysts; use can
also be made of bimetallic catalysts consisting of a
transition metal and noble metal. The use of a
hydrogenating catalyst containing one or more transition
metals is only expedient when the catalyst exhibits
sufficient acid stability under the reaction conditions.
Preferred hydrogenating catalysts for the process according
to the invention are noble-metal catalystis in metallic
form, such as so-called ethiops of Ru, Rh and, in
particular, Pd and Pt, or in a form bound to a carrier.
Suitable carrier materials for Ru, Rh, Pd and Pt are
activated carbon, aluminium oxide and other metallic oxides
and also silicates. The amount of noble metal in carrier-
bound noble-metal catalysts is usually in the range from
0.0001 to 10 wt-%, in the case of Pd and Ru preferably in
the range from 0.01 to 0.1 wt-%. The required amount of
noble-metal catalysts, which depends on the activity of the
i catalyst, the reaction temperature and the pressure of H2,
will be ascertained by a person skilled in the art by means
of orienting experiments. In general, the required amount
is in the range from 0.01 to 10 wt-%, in particular 0.5 to
5 wt-%, relative to the polyol. Noble-metal catalysts in
5 the form of an ethiops or in carrier-bound form can be
easily recycled, and they have a longer useful life than
bimetallic catalysts based on a noble metal and a
transition metal, such as were employed in the process
known previously for cyclodehydration in the absence of an
.0 acidic catalyst.
The process can be operated discontinuously or
continuously. In this connection the polyol and water can
be mixed upstream of the reactor or can be supplied to the
reactor in parallel. Where use is made of an acid that is
soluble in the reaction mixture by way of catalyst, this is
added to the reaction partner, to the water or to the
mixture of the two or is introduced into the reactor
separately. A solid hydrogenating catalyst can be employed
in the form of a suspended catalyst or in the form of a
fixed bed. Where use is made of a solid acidic catalyst
the latter may find application, in a manner analogous to
the hydrogenating catalyst, in the form of a suspension or
in the form of a fixed bed. It is also possible to employ
a catalyst containing both acidic and hydrogenation-active
functions, for example a zeolite that is partially charged
with a noble metal. The optimal reaction-time can easily
be ascertained by a person skilled in the art by means of
orienting experiments.
The cyclodehydrogenation reaction mixture can be processed
in simple manner. This processing may comprise the
filtration of a solid acidic catalyst and of a
heterogeneous hydrogenating catalyst. Where use is made of
a distillable acidic catalyst and of a heterogeneous
hydrogenating catalyst the processing comprises the
filtration of the hydrogenating catalysts and separation of
the acidic catalyst by distillation. The reaction mixture
remaininq after separation of the catalysts is processed by
distillation and/or? extraction, preferably by distillation.
'.......T"" .
Where necessary^ the processing may also comprise a
) crystallisation step. In the case of cyclodehydration of
hexitols, dianhydrohexitols that have been formed are
mostly separated by distillation, and a mixture of
monoanhydrohexitols remains in the distillation sump.
It is a surprising and therefore significant advantage of
the process according to the invention that polymers are
practically not formed - the rate of formation is below l
mole-%, relative to converted polyol. Hence the reaction
mixture which is free from catalysts and water can be
utilised further immediately or after separation of
distillable dianhydropolyols by distillation.
Monoanhydropolyols, in particular those consisting of
hexitols, are therefore valuable raw materials for various
fields of application where hitherto the content of
polymers was troublesome. Through the preferred use of a
low carboxylic acid and a heterogeneous hydrogenating
catalyst it is possible to recycle not only the
hydrogenating catalyst but also the carboxylic acid, after
separation thereof by distillation. Where use is made of
mineral acids, hitherto elaborate measures were necessary
to separate the salt arising as a result of neutralisation
of the acidic catalyst from the anhydropolyol reaction
mixture. Finally, by virtue of the combination, according
to the invention, of an acid catalyst and a hydrogenating
catalyst it is possible to employ noble-metal catalysts of
high stability, a factor which has an advantageous effect
on recyclability and therefore on the costs of the process.
In the course of the cyclodehydrogenation of hexitols,
after removal of dianhydrohexitols by distillation it is
possible for anhydrohexitol mixtures to be obtained having
typical compositions stated in the Table, the concrete
composition depending on the chosen reaction conditions.
*) DL form of the anhydro product
These mixtures of substances can be employed for the
production of surfactants and also as a component in
polycondensation resins and polyaddition resins.. In this
connection it is to be noted that an increased proportion
of 2,5-anhydrohexitols in relation to 1,4-anhydrohexitols
has clear advantages in subsequent processing: under
unfavourable processing conditions dianhydro compounds can
be formed from the 1,4-anhydrohexitols as a result of a
further condensation, whereas in the case of the 2,5-
anhydrohexitols this reaction is not possible, so that 4
hydroxy functions remain accessible for subsequent
processing.
Example 1
In a 2 0-1 autoclave there were submitted 8 kg D-sorbitol in
the form of a 50-wt-% solution in water, 5 wt-% propionic
acid, irelative to sorbitol, and 1 wt-% Pd/C catalyst having
a Pd content of 3 wt-%, relative to sorbitol. The reaction
mixture was heated to 2 70J|°C and stirred for 2 h at 60 bar
H2 pressure. After cooling, the catalyst was removed by
filtration and the water/propionic-acid mixture was removed
by distillation. According to analysis by gas
chromatography the yield relative to D-sorbitol amounted to
38 % isosorbide and 58 % anhydrohexitols {--- tetrols) and
less than 1 % polymers. The conversion of D-sorbitol was
practically quantitative.
Under high vacuum (< 10 Pa) at 130 °C 2.5 kg isosorbide ( =
1,4:3,6-dianhydro-D-sorbitol) were distilled off. The sump
(3.9 kg) left behind contained 20 % 2,5-anhydro-D-mannitol,
31 % 2, 5-anhydro-L-iditol, 34 % 1,4-anhydro-D-sorbitol and
5 % l,4-anhydro-D-mannitol, 5 % isosorbide and about 3 %
other monomeric polyols, but practically no polymers
(< 1 %) .
The analyses of all the Examples and Comparative Examples
were carried out by means of GC analytical methods of the
silylated polyols on a capillary column (DB-5) at 280MoC.
Detection is effected in a flame ionisation detector at
250 °c with helium as carrier gas. After retention-times
between 15 and 20 min the products were able to be eluted
and identified.
Example 2
Example 1 was repeated, with the difference that the
cyclodehydration was effected for 8 hours at 24o)j°C.
According to GC analysis the conversion was greater than
99 %; the yields (relative to sorbitol) amounted, to 20 %
isosorbide, 65 % monoanhydrohexitols (= tetrols) and less
than 1 % polymers.
The reaction mixture that was largely free from isosorbide
contained 14 % 2,5-anhydro-D-mannitol, 21 % 2,5-anhydro-L-
iditol, 44 % 1,4-anhydro-D-sorbitol, 4 % l,4-anhydro-D-
mannitol and about 16 % other polyols, but less than
1 % polymers.
Example 3
D-mannitol was employed instead of sorbitol in accordance
with Example 1, as a result of which a stereochemically
changed product composition was obtained. By way of
principal components there were formed (relative to D-
mannitol): 48 % 2,5-anhydro-D-sorbitol and 23 % isomannide
( = l,4:3,6-dianhydro-D-mannitol) , 10 % l,4-anhydro-D-
mannitol and 10 % 1,5-anhydro-D-mannitol. The mixture
contained less than 1 % D-mannitol and less than 1 %
j polymers.
Example 4
D-sorbitol was cyclodehydrogenated in a manner analogous to
Example 1, whereby, however, instead of propionic acid an
acidic zeolite (type Y zeolite) was employed in an quantity
of 1 wt-%, relative to D-sorbitol, the reaction temperature
amounted to 270J] °C and the react ion-time amounted to 4
hours. The conversion of D-sorbitol was practically
quantitative (greater than 99 %). The yields (relative to
sorbitol) amounted to 46 % isosorbide, 45 %
anhydrohexitols, about 8 % other low-molecular polyols and
less than 1 % polymers.
Example 5
D-sorbitol was dehydrogenated in a manner analogous to
Example 1, whereby, however, instead of the hydrogenating
catalyst Pd/C an Ru/C catalyst was employed having a
content of 5 % Ru in a quantity of 0.1 wt-%, relative to
sorbitol. The conversion amounted to 96 %; the yields,
relative to sorbitol, amounted to 25 % isosorbide, 55 %
anhydrohexitols, about 19 % other low-molecular polyols and
less than 1 % polymers.
Example 6
Cyclodehydrogenation was carried out in a manner analogous
to Example 5, but sucrose was employed as substrate instead
of D-sorbitol; furthermore, conversion was effected for 8 h
at 150!)°C and then for 4 h at 270 loC. In the course of
this conversion the hydrogenation of the sucrose to form D-
sorbitol and D-mannitol took place in situ with the
cyclodehydrogenation. The conversion of sucrose amounted
to 95 %. The reaction mixture consisted of 29 % isosorbide
> l,4:3,6-dianhydro-D-sorbitol), 7 % isomannide
(= 1, 4:3,6-dianhydro-D-mannitol), 12 % 2,5-anhydro-D-
sorbitol, 13 % 2,5-anhydro-D-mannitol, 14 % 2,5-anhydro-L-
iditol, 7 % 1,4-anhydro-D-sorbitol, 5 % 1,4-anhydro-D-
mannitol and 2 % 1,5-anhydro-D-mannitol and 11 % other low-
molecular polyols. Polymers were practically not formed.
Example 7
1,4-butanediol was employed as substrate.. Conversion was
effected otherwise in a manner analogous to Example 1. The
conversion of 1,4-butanediol amounted to 16 %, the yield of
tetrahydrofuran amounted to 52 %.
Comparative Example 1 (without acidic catalyst)
Example 2 was repeated, with the sole difference that no
acidic catalyst was employed. After 8 h at 240 °C the
conversion of D-sorbitol amounted to 92 %. The yields,
relative to D-sorbitol employed, amounted to 70 %
monoanhydrohexitols, 20 % isosorbide, about 9 % other low-
molecular polyols and less than 1 % polymers.
This example shows that in the absence of an acid only an
insufficient conversion of D-sorbitol is achieved.
Comparative Example 2 (without hydrogenating catalyst and
H2)
Example 2 was repeated, with the sole difference that no
hydrogenating catalyst was added. With almost quantitative
conversion of sorbitol, 8 % of the same was transformed
into white-yellow polymer. The yields, relative to D-
sorbitol employed, amounted furthermore to 29 % isosorbide,
54 % anhydrohexitols, 9 % other low-molecular polyols.
Comparative Examples 3 to 6
In the absence of an acidic catalyst, sorbitol in the form
of a 20-wt-% aqueous solution was treated for 8 h at 240 °c
and at a H2 pressure of 13 MPa with the hydrogenating
catalysts stated in the Table. The conversion of sorbitol
and also the yields relative to converted sorbitol of
isosorbide and monoanhydropolyols can be gathered from the
Table. Polymers arose in each case in a quantity amounting
to less than 1 %. By way of principal products, polyols
arose in the form of C2 to C4 fragments.
These tests show that the catalysts investigated are not
really suitable for the cyclodehydrogenation.
1. Process for the production of five-membered or six-membered cyclic
eihers, in particulars of anhydropofyofs, by cyclodehydration of pofyofs
having at least 4 C atoms and at least 2 hydroxyl groups, the polyol being
treated in the presence of water and an acidic catalyst such as herein
described employed in an amount from 0.1 to 20 wt.% relative to the
polyol at a temperature or at least 100 °C, characterised hi that the
treatment is carried out In the presence of an acid-stable hydrogenatlng
catalyst such as herein described employed in an amount from 0.001 to
10 wt.% relative to the polyol in a hydrogen atmosphere, the pressure
ranging from 0.1 to 20 Mpa.
2. Process as claimed in claim 1, wherein the cyciodehydration is carried out
at a hydrogen pressure In the range of 1 to 20 Mpa.
3. Process as claimed in claim 1, wherein the cyclodehydration is carried out
at a temperature In the range from 180 to 380 °C and at a H2 pressure In
the range from 3 to 10 Mpa.
4 Process as claimed in claims 1 and 2, wherein by way of hydrogenatlng
catalyst use is made of a catalyst containing one or more noble metals
from the series comprising ruthenium, rhodium, palladium and platinum in
elemental form or in the form of a noble metal compound.
5. Process as claimed in claims 1 to 3, wherein the hydrogenatlng catalyst is
employed in an amount from in particular 0.01 to 1 wt-%, relative to the
polyol.
6. Process as claimed in claims 1 to 4, wherein by way of acidic catalyst use
is made of an aliphatic carbaxylic acid having 1 to 10 C atoms, in particular
a monocarboxylfc acid from the family comprising formic acid, acetic acid
and propionic acid.
7. Process as claimed in claims 1 to 5, wherein the acidic catalyst Is
employed In an amount from 0.5 to 10 wt-%, relative to the polyoi.
8. Process as claimed in claim 1 to 6, wherein a polyoi from the family
comprising the tetrltols, perrtltols and hexttols Is cyclodehydrated.
9. Process as claimed in claims 1 to 8, wherein use is made of a catalyst
containing acidic and hydrogenation-active functions and based on a
zeolite charged with a noble metal From the series comprising Pd, Pt, Ru
and Rh and having an H0 value less than +2, in particular less than 43^

Five-membered or six-membered cyclic ethers, in particular
anhydropolyols, can be obtained by cyclodehydration of
polyols having at least 4 C atoms and at least 2 hydroxyl
groups in the presence of water and an acidic catalyst.
According to the invention the cyclodehydration is promoted
and the formation of polymers is avoided if the acid-
catalysed cyclodehydration is carried out in the presence
of an acid-stable hydrogenating catalyst in a hydrogen
atmosphere. Anhydrohexitols can be obtained both from
hexitols and from sucrose.