Note: Descriptions are shown in the official language in which they were submitted.
A ~ 9 5 j ~ 3 2 6 O
PROCES5 FOR P:RODUCING P(:)LYOLS
WITE1 AT LEAST ONE OXACXCLOPENTANE RING
.
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to anhydro sugar al-
cohols and particularly rela~es to a process for producing them.
The Prior Art
Anhydro sugar alcohols are well known. They can be
used for pharmaceutical purposes or as starting materials or inter-
mediates for chemical syntheses (of, for example, fine chemicals,
emulsifiers on the basis of esters, polyester and alkyd resins,
and polyurethanes by serving as starters or cross~linking agents)~
The anhydro alcohols are known to be produced by acid-
cataly~ed, intramolecular dehydration of sugar alcohols. This
reaction has been thoroughly studied and optimized particularly
with regard to the sugar alcohol most readily available, i.e.
sorbitol.
D-sorb.itol, when subjected to thermal treatment in
the presence of an acid, first forms a monoannydride~ l,A-anhydro-
D-sorbitol~ which immediately undergoes further reaction forming
1,4; 3,6-dianhydro-D-sorbitol~ By-products of this reaction
may be 2,5 monoanhydrosorbi~ol and, to a smaller extent, 3,6-
anhydrosorbitol.
This reaction today is commonly performed with so-called
homogeneous catalysts, i~e., catalyst.s which are molecularly di.s-
persed in the reaction system~ such as hydrocholoric acid or sul-
furic acidO The use of these acids, however, entails excessive
formation of by-products, particularly esters of the catalyzing
acid, by halogenations and requires neu~ralization of the reac-
tion product mixture with subsequent desalinationO Moreover,
the acids used in the reaction are highly corrosive, ~xtremely
volatile in part, and produce maximum yields of about 66~ which
are not fully satisfactory (Carbohydrate Chemistry, Vol. II,
191-lg8 (1963)).
It is also known to use organic solvents in performing
reactions of this type with azeotropic removal of the react.ion
water. This does not result in any major improvements~
~lthouyh some of ~he disadvantages of the processes
using acid catalysts dissolved in the reaction mixture, such as
~ corrosion problems and the formation of excessive amounts of
esterified by-products which are difficult to remove, may be
avoided by using heterogeneous catalysts, i.e., specific~ strong-
ly acidic, cation exchange resins, this is not satisfactory as it
has a number of other disadvantagesf particularly lower maximum
yields of about 35% and a need to include organic solvents as
'; '
reaction media and entrainers for removing water by azeotropic
distillation ~cf. Ropuszynski et al. in Przem Chem. 1969, 48
(11)/ 665-~ and Weisleder et al. in Carbohydr. Res. 79, 133~141
(1980~.) As far as the need to use carefully selected organic
solvents (solven~ mixtures) is concerned, ~eisleder et al. stress
that in a comparative assay in which 1,4; 3,6~dianhydro-D-
sorbitol is produced ~rom sorbitol by means of "dry distilla-
~ion wi~h acidic resin" a far lower yield was obtained, iOe.,
no more than S~.
An object of the invention therelore is~to provide a
process f6r producing anhydro sugar alcohols by ~hich the dis-
advantages of the prior ar~ are overcome. This includes avoid-
ing the problems arising in connection with homogeneous acid
catalysts in respect of by-product formation, corrosion~ and
lS low yields. Another object is to avoid the use o costly
organic solvents.
SUMMARY OF THE INVENTION
The objects have been accomplished by the invention
on ~he basis o the surprising ~inding tha~ in the dehydration
~0 o~ su~ar alcohols in the presence of heterogeneous catalysts,
tl~e absence of organic solvents results in an increase of the
yield to levels which are higher, sometimes even several times
higher, than the levels obtained in the most favorable known
processes.
The anhydrous sugar alcohols of the invention are
polyols with at least one oxacyclopentane ringO They have the
general formula:
~l OH
I ~ ~
R ~ /
where Rl is selected from the group consisting of hydroxyl ~nd
the ether oxygen atom of a second oxacyclopentane ring and R
is selected from the group consisting of hydrogen, hydroxymethyl,
and a bivalent radical having the formula -CH~-CHOH-, provided
that when ~2 is said bivalent radical, R is an ether oxyyen
atom and the polyol has the structural formula:
0~
., '' ' ,, /~ ~' .
II ~ ~
Y '.
OH
I)ETAILED DESCRIPTION OF THE INVENTION
The anhydro sugar alcohols of the invention are pre~
pared by dehydra~ing sugar alcohols having 4 to 6 carbon atoms
at ~levated temperatures in the presence of a strongly acidic
heterogeneous catalyst. Organic solvents are not used in the
process of the invention.
Sugar alcohols used as starting ma~erials for the pur-
poses of the invention are preferably hexitols and in particular
sorbitol, since these alcohols are readily available at low prices
and in practically unlimi~ed amoun~s. With these alcohols, the
process of the invention produces yields of at least 50% and mostly
over 60% 7 In the case of sorhitol, yields of 85 to 90~ and more
are generally achieved. It is noted that mixtures of sugar
alcohols like, for instance, a mixture of sorbitol and mannitol
which is obtained in the catalytic hydrogenation of mixtures of
glucose and fructose, are also suitable starting materials. A
startin~ material which is interesting in this regard consists
of C4 -C6 -polyol mixture~ which are obtainable by hydrogena-
tion of hemicellulose hydrolysatesO Such hydrolysates at the
present are prepared commercially in great amounts from side
products like for example corn shellst oat shells~ etcO These
hydrolysates are in particular used for preparing xylose and
xylitol, respectively. The tetroses, different pentoses and
~5 hexoses and their hydrogenation products, respectively inevitably
obtained thereby at the present are scarcely useably waste
products which according to the process of the present applica-
tion can be con~er~ed into valuable intermediates, especially
f or the plastics industry.
Heterogeneous catalysts that are useful in the present
invention include any strongly acidic acid catalysts which are
sufficiently heat resistant. Suitable catalysts i~clude "acidic"
molecular sieves such as zeolites, as well as the cracking or
hydrocracking catalysts employed chiefly by the mineral oil
industry~ The latter are of interest for purposes of the
invention mainly because they are widely used also or the
production of sugar alcohols by catalytic hydrogenation of the
corresponding sugars and thus open a way for producing polyols of
formula I and II from sugars by means o~ integrated uncomplicated
processes.
Best results can be achieved with highly acidic syn-
thetic cation exchange resins, in particular polystyrene sulfonic
acid cation exchange resins cross-linked with divinyl benzene
(DVB). Suita~le for the purpose are so-called "gel resins" which
should be partially cross-linked, i~e.~ with at most 4% divinyl
ben2ene, as well as macroporous resins which sho~ld preferably be
cross-linked to a higher degree, iOeO, with at least 10~ divinyl
ben~.ene.
In the process of the invention the catalyst is separated
from the reaction product mixture by simple solid/liquid separating
2S methods, e.gl, filtration, and may be re-used several times with-
out requiring any special ~reatment.
As the react~nts and/or reaction products and, in some
cases, even the catalysts are sensitive to oxidation, the process
of the invention is best carried out in an atmosphere of in~rt
gas, preferably nitrogen, i~ being advantageous as a rule to
bubble the inert gas through the reaction mixture to achieve a
good stirring effect and to improve removal of water from the
reaction mixture by distillation
According to a preferred embodiment of ~he inventionr
the reaction is carried out at reduce~ pressure, particularly a
pressure in the range of 0.001 to 0.5 bar. This i5 recommended
especially when using a relatively high melting starting material
whose melting point is depressed below the maximum ~emperature
of 160 C., preferably below 155 C. and more preferably below
145 C. by the addition of wa~er. It is also recommended if the
starting material is an aqueous solution of sugar alcohol which is
concentrated in situ. In the latter case, the vacuum is applied
and gradually increased, preferably after the water content has
dropped to a level at which the reaction mixture~ when exposed
to normal pressure, no longer boils in the ~emperature range of
100 C. to about 150 C. and in particular 110 C. to 140 C.
which is preferred according to the invention.
The process of the invention may be carried out not
only batchwise, but also continuously. In the continuous pro-
cess, agitated vessel-type or fluidiæed bed-type reac~ors are
2S particularly well suited and a two- or multistep operation is
preferable as a rule mainly because it facilitates adjustment
of the reaction condi~ions to the composition of the reaction
mixture.
When the reaction products o the present invention are
us~d for technical purposes, for instance a~ the polyol component
in synthetic resin syntheses, they may freyuently be used without
any further processing or purification. If a higher degree of
purity appears to be necessary or at least desirable with regard
to the intended use; the crude reaction product mixtures may be
further processed or purified~ This can be done, for instance,
by treating with activated carbon or ion exchange resins, or
applying optional commonly known methods such as fxactionated
crystallization, distillation or column chromatography. The
total yield may be increased in that fractions rich in unreacted
starting material or, in the case of hexitols~ monoanydropolyols,
which are obtained during the separating process, are recycled
into the reaction.
The invention is further illustrated by the following
examples.
~O ~ le 1
A commercial mixing vessel was charged with 25 kg of a
70~ sorbitol solution and 1 ]cg of a strongly acidic ion exchanger
in the H~ form (macroporous polystyrene sulfonic acid resin, cross
linked with 14~ DVB). The mixture was concentrated to dryness
-- 8 ~
and then stirred ~or two hours at a pressure of 0O03 bar and at
a temperature of 140 C,
The result.ing melt was removed from the ion exchanger
by filtering while hot~ The yield was 13,900 g of raw product
containing 91 w/w % of dianhydrosorbitol~
Example ~
250 g o sorbitol and 20 g of the ion exchanger de-
scribed in Example 1 were placed in a l-liter ~hree-necked flask
and treated as in Example lo Af~er a reaction time of 3 hours,
10 the melt was filtered off as describedO The yield was 190 ~ of
raw product containing B7 w/w % of dianhydrosorbitol~
Example 3
Example 2 was repeated except that macroporous poly-
styrene sulfonic acid resin cross-linked with 20% divinyl benzene
was used as catalyst and the reaction time was reduced to 2
hours. The yield was 191 g of raw product containing 89 w/w~
of dianhydrosorbitol.
Example. 4
Example 3 was repeated except that a strongly acidic
.0 polystyrene acid gel ion exchange resin (in the H form) cross-
linked with 2~ DVB was used. The yield was 191 g of raw product
containing ~7 w/w% o~ dianhydrosorbitol~
r_~ ` :
Example 4 was repeated except that a gel resin9 highly
cross-linked with 10~ DVB, was used as a catalyst and the reaction
time was extended to 5 hours. The yield was 190 g of raw product
containing 86 w/w% of dianhydrosorbitol,
Example 6
200 g of mannitol~ 50 ml of water and 30 g of the ion
exchanger described in Example 3 were placed in a l-liter three
necked flask and stirred for 2 hours at 1~0 C~ The pressure
was gradually reduced to 0.03 bar with the temperature being kept
constant causing water to be evaporated. The reaction mixture
was then stirred for another 5 hours under constant conditions,
after which the catalyst was remoYed by filtering wh}le hot. The
yield was 151 g of raw product containing 76 W/W~ of dianhydro-
mannitol.
Example 7
250 g of a mixture of equal parts by weight of sorbitol
and mannitol and 30 g of the catalyst described in Example 3 were
placed in a l-liter three-necked flask and stirred for 4 hours
7~ at 0.03 bar and 145 C. The catalyst was then removed by filter~
ing while hot. The yield was 192 g of raw product containing 46
w/w% of dianhydrosorbitol and 40 w/w% of dianhydromannitol.
-- 10 --
Example ~
Analogously to Example 3, 200 9 meso-erythrite was stirred
for 4 hours at 0.03 bar and 140 C. During the reaction process,
nitrogen was caused to bubble through the reaction mixture. The
yield was 175 g of raw product containing 86 w/w~ of anhydroeri=~
thrite.
Exam~le 9
Example 2 was repeated except that the reaction time
was extended to 5 hours. The reaction mixture was filtered as
usual. The yield was 189 9 of raw product which contained 94
w/w% of dianhydrosorbitol.
The results achieved according to the invention, as
evidenced in the examples, are unexpected, slnce one would
expect that, normally,
a) at high temperatures and more intimate contact and/or im-
proved transport of material to and from the hetero~eneous
catalyst brought about by the solvent should tend to at least
accelerate the approach of reaction equilibrium conditions
and
~0 b) the continuous removal of either or both of the two main
reaction proAucts from the system taking place in "dry dis-
tillation" would favor the formation of anhydropolyols.
Example 10
In a l-liter three-necked flask 250 y of a cata-
lytically hydrogenated anhydrous hemicellulose hydrolysat2
obtained from corn shells and 20 g of a strongly acidic ion
exchanger in the H~ -form (polystyrene sulfonic acid~gel resin,
2 % DVB cross-linked~ were placed and stirred 2 hours at 130 C.
at a pressure of 0,03 bar.
The resulting melt was removed from the ion exchanger
by filtering while hot, dissolved in water and treated with
active carbon. After filtration and concentration 180 g of
a product were obtained having a hydroxyl number of 985 in
comparison to a hydroxyl number of 1790 of the starting materialO
.
A HPLC-examination showed that the polyols present in
the raw material ~ca. 75 ~ pentitol, from which more than 55 %
consisted of xylitol, sorbitol and gallactitol) had been
converted into anhydroderivatives in an extent of more than
9~ %0
Having set ~orth the general nature and some examples
of the present invention, the scope is now particularly set
forth in the appended claims.
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