Note: Descriptions are shown in the official language in which they were submitted.
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- HULS AKTIENGESELLSCHAFT - 1 - O.Z. 4863
- Patentabteilung -
Process for preparinq alkyl polyqlycosides
The invention relates to a novel process for preparingalkyl polyglycosides having C8 to C20 alkyl radicals from
alkyl glycosides having Cl to C6 alkyl radicals and alco-
hols having 8 to 20 C atoms by acid-catalysed transglyco-
sidation.
Alkyl polyglycosides having C8 to C20 alkyl radicals can
be prepared in whole or in part from renewable raw
materials. The alkyl polyglycosides, because of their
interesting surfactant properties with simultaneously
very good biodegradability are becoming increasingly
important. For applications in the home and the cosmetics
sector these products must satisfy high aesthetic
requirements. There is therefore interest in processes by
which alkyl polyglycosides can be prepared in transparent
aqueous solutions of unobjectionable colour.
To prepare alkyl polyglycosides having long-chain alkyl
groups, alkyl glycosides having Cl to C6 alkyl groups can
first be prepared by glycosidation of saccharides having
short-chain alcohols. These products are then converted
into the desired alkyl polglycosides by transglycosida-
tion using long-chain alcohols at elevated temperature.
However, the products thus prepared are dark in colour.
Reeping to defined mass ratios and with the use of
solubilizers, the polar saccharides can also be reacted
directly with the nonpolar long-chain alcohols to give
the alkyl polyglycosides. In this case also, without the
addition of colour improvers, if the reaction is carried
out in the stirred tank, dark-coloured products are
obtained.
In EP 0 077 167, a single-stage preparation process is
described in which an aldose or a ketose is reacted
directly with a long-chain alcohol in the molar ratio of
1:1.25 to 1:4. The reaction is carried out at low water
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contents in the presence of a reducing agent.
This single-stage process is improved in
DE-A-41 01 252 by a large excess of alcohol being used and
alkali metal hydroxide dissolved in alcohols being used for
the neutralization. The reaction is carried out here in a
stirred tank.
In addition, it is known that the colour quality
of the alkyl polyglycosides can be improved by measures
relating to equipment.
Thus, according to EP-A-0 482 325, in a two-stage
preparation process, the first stage, the glycosidation of
saccharides in aqueous solution using Cl to C6 alcohols, can
be carried out in a counter-current flow reaction column,
for example in a bubble-cap column.
Moreover, according to DE-A-41 16 665, the trans-
glycosidation, the second stage of the two-stage process,
can also be carried out in a reaction column and preferably
under counter-current flow conditions.
These processes can be still further improved and
simplified with respect to the reactors.
It is attempted according to the present invention
to simplify further the transglycosidation in a two-stage
process.
Thus, according to the invention, the transglycos-
idation is carried out using a tubular reactor under co-current
flow conditions.
Suitable tubular reactors generally have a diameter
23443-545
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of 0.5 to 50 cm and a length of 0.5 to 50 m, where the
length is to be at least four times the diameter. In the
simplest case this is a heatable tube. Preferably, however,
the tubular reactor also has a plurality of
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nozzles via which the readily volatile substances can be
taken off or expelled by means of a vacuum or using an
inert gas stream, respectively.
The tubular reactor can also contain chicanes and turbu-
lence-generating internals. The tube can be arranged
horizontally, at an incline or vertically. However,
columns are not considered as tubular reactors in the
context of this invention.
The saccharide moiety of the alkyl glycosides used can be
derived from aldoses or ketoses. Examples of these are
glucose, mannose, galactose and fructose, glucose being
preferably used. The alkyl radicals which are suitable
are for example methyl, ethyl, butyl or hexyl.
Alcohols which are suitable for the present process are,
for example, octanol, decanol, lauryl alcohol, myristyl
alcohol, palmityl alcohol and stearyl alcohol. Mixtures
of alcohols can also be used. Preferably, alcohols having
8 to 12 C atoms are used.
The alkyl glycoside/alcohol molar ratio is preferably in
the range from 1:2 to 1:10.
The catalysts which are suitable are mineral acids and
strong organic acids. Examples of these are sulphuric
acid, phosphoric acid and p-toluenesulphonic acid. The
catalyst is preferably used in concentrations of 0.2 to
5%, based on the saccharide.
The reaction is usually carried out at a temperature of
70 to 140C, temperatures of 100 to 140C being parti-
cularly preferred.
Simultaneously, a mean residence time of 5 to 90 minutes
is preferably set.
The process is preferably carried out continuously. The
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products generally have a mean degree of glycosidation of
1 to 10, mean degrees of glycosidation of 1.1 to 4 being
particularly preferably set. Very particular preference
is given to mean degrees of glycosidation of 1.1 to 1.5.
By means of the present invention, in particular in the
case of continuously operating synthesis plants, the
expenditure in terms of apparatus is greatly decreased.
The yield is increased and the space-time yield is
improved. Owing to the short residence times and the
narrow residence time distributions, because of the low
thermal stress, light products of high quality are
obtained. The products are of light colour if their 50%
strength aqueous solutions before H2O2 bleaching have
iodine colour values of ~60.
After the reaction the product is neutralized with a base
in a known manner, whereupon excess fatty alcohol is
separated off by distillation. The residual fatty alcohol
content is then generally below 1%. The product is then
generally mixed with water and bleached using H2O2.
When the invention is carried out in practice, stirred
tanks can also be provided upstream or downstream of the
tubular reactor. Thus the alkyl glycosides can react in
part with the long-chain alcohols for example in an
upstream tubular reactor. The reaction mixture is then
homogenized and passed into the tubular reactor. Down-
stream stirred reactors generally only serve to increase
the residence time and to complete the conversion.
Preferably, 5 to 80% of the reaction is carried out in a
tubular reactor.
Example
A stirred reactor is continuously charged with 40 kg/h of
fatty alcohol mixture comprising 68% of dodecanol, 26% of
tetradecanol and 6~ of hexadecanol, and with 20 kg/h of
butanolic butyl glucoside solution which contains 65% of
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butanol and 35% of butyl glucoside. A mean residence time
of 3.5 hours is set. By adding p-toluenesulphonic acid,
a catalyst concentration of 0.25 per cent by weight is
maintained in the reaction mixture. At 115C and 5 mbar,
approximately 13 kg/h of butanol are distilled off via a
column. 47 kg of reaction mixture which contains 77% of
fatty alcohol, 11.1% of Cl2 to Cl6 alkyl glucoside and
0.63% of butyl glucoside leave the stirred reactor per
hour.
This reaction mixture is passed at 112C through a tube
having an internal diameter of 25 cm and a length of
200 cm. When the theoretical residence time is approx-
imately 30 minutes, at the end of the tubular reactor
there is obtained a mixture which contains 76.5% of fatty
alcohol, 11.7% of Cl2 to Cl6 alkyl glucoside and 0.26% of
butyl glucoside. The mean degree of glycosidation of the
alkyl glucoside is 1.2.
