Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a process for the
production of dipropylene glycol dibenzoate.
Dimethyl terephthalate can be produced commercially by
catalytically oxidizing _-xylene and methyl-_-tolate with air, by
esterifying the products of oxidation with methanol, and by
separating the methyl esters obtained. The esters (mainly methyl-
_-tolate, dimethyl terephthalate and methyl esters of other mono-
functional and polyfunctional acids) are separated from one another
by distillation or recrystallisation or by a combination of these
two processes. Dimethyl terephthalate is recovered, valuable
intermediate products are recycled, and secondary products are
- removed. Some of the secondary products consist of impure methyl-
benzoate containing a number of compounds with substantially the
~same boilingpoint as methylbenzoate. Unless these compounds are
removed beforehand, for example by pretreatment under oxidising
conditions in accordance with Applicant's German Auslegeschrift
No. 1,927,554 of May 17, 1973, pure methylbenzoate cannot be
obtained, even with considerable technical outlay on distillation.
The impure methylbenzoate obtained in industrial plants for the
; 20 manufacture of dimethyl terephthalate generally contains more than
80% of pure methylbenzoate.
Attempts to transesterify such impure methylbenzoate
with dipropylene glycol (also known as ~,~'-di-hydroxy-di-_-propyl
ether) in the presence of conventional transesterification cata-
lysts have shown that some of these catalysts, for example anti-
mony -trioxide and boron trioxide, are unexpectedly totally inactive
whilst others, for example sodium methylate, catalyse the reaction
to a very limited extent only. This is indicative of the formation
of inhibiting substances from the impurities in the methylbenzoate.
Although a number of other transesterification catalysts, such
as tetrabutyl titanate, zinc compounds and magnesium compounds,
are sufficiently active, the esters obtained are brown to
brown-black in colour with iodine colour numbers of up
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to t50. In many cases, the es-ters are so dark brown to black in
colour that their iodine colour numbers cannot be mea~ured. A few
examples are set out in Table 1, which shows the iodine colour
numbers o~ the transesterifiaation products of commercial grade
methylbenzoate and dipropylene glycol, measured in identical
standard batches and under comparable conditions using different
catalysts. The examples are selected in such a way that products
with substantially the same degree of transesterification, charac-
terised by hydroxyl numbers in the range from 10 to 30, were ob-
tianed.
Table 1
. . . . . . .
Catalyst Quantity of Transesteri- OH- iodine
catalyst fication ti- number colour ~`
me (hours) number
.
Sodium methylate 0.2 11 24100
Magnesium acetate 0.2 5 3080
Magnesium methylate 0.2 3 19130
Zinc stearate 0.2 7 13100
Zinc dust 0.2 3 1830
Dibutyl tin ~ilaurate0.2 4 1770
Tin (IV) butylate 0.2 4 black-brown
Titanium silicon 0.2 4 black_brown
alkoxide
lycerol titanate 0.2 8 1780
etrabutyl titanate 0.2 2760
etrabutyl titanate 0.1 4 3060
Tetrabutyl titanate 0.5 20 70
The deep colour of the esters cannot be lightened to any
appreciable extent by bleaches such a~ active carbon or fuller's
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earths. Similarly, oxidising agents, such as hydrogen peroxide or
ozone, do not have any significant effect. ~istillation in vacuo
produces deep-yellow distillates. r~he eolour quality is inadequate
for commercial application. ln addidition, the ~ster undergoes
thermolysis at the high temperatures used accompanied by the libe-
ration of benzoic acid, leaving the distillates with acid contents,
reducing yield and giving rise to considerable teclmical difficul-
ties through the accumulation of sublima-tes in the apparatus used.
Accordingly, it has not higherto been possible to use
the methylbenzoate obtained in considerable quantities as a commer-
cial se~ondary product for the production of light-coloured trans-
esterification products.
Accordingtothe present invention, there is provided a
process for the preparation of dipropylene glycol dibenzoate, which
eomprises transesterifying methylbenzoate with dipropylene glycol
in the presence o~ an aluminium alkoxide and/or an aluminium-sili-
con alkoxide as a catalyst at a temperature above 150C.
Surprisingly, it has been found that the process of the
invention can be used for the preparation of light-coloured dipro-
pylene glycol dibenzoate from impure methylbenzoate containing atleast 80~ by weight of methylbenzoate, for example that obtained
as a secondary produet in the manufacture of dimethyl terephthala-
te from P-xylene and/or methyl-~-tolate as deseribed above, for
instance, in aecordance with German ~atent No. 1,041,945 of May 27,
1959 to Chemisehe Werke Witten. The compounds bolling at lower
temperatures than dipropylene glyeol dibenzoate are distilled, off,
preferably in Yacuo, on eompletion of the liberation of methanol,
and the product of transesterification~ cooled to below 150C, is,
if neeessary, ozonised to improved its colour and optionally
refined in a known manner,
The alkoxy groups of the aluminium alkoxides and the alu-
minium-silieon alkoxides (the latter of which eontain the group
,.~ '
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AlOSi in the nucleus) are identical or different and preferably
contain from 1 to 4 carbon atoms. ~xample,~, of such catalysts aie
Al-ethylate, Al-n-propylate, Al-sec-butylate and (C2H50-)3SiOAl
(-0-sec C4Hg)2.
Transesterification is preferably effected under an
inert atmosphere. The inert atmosphere is established by displa-
cing the air with gases contai.ni.ng little or no oxygen, such as
nitrogen, helium or, optio.nally, hydrogen.
'rhe methyl benzoate is preferably used in a quantity of
at least 2 mol~ based on its methylbenzoate conte.nt in the case of ~
impure methylbenzoate, per mol of dipropylene glycol. A slight ,.
excess of up to 25 mol % of methyl benzoate can be useful i.n rapid- ~:
. ly completing the reaotio.n.
If impure methyl benzoate containing less than 80~ by
weight of methylbenæoate is to be used, it may be e~riched to at
least 80% by weight by simplé distillation.
The concentration o~ catalyst should not exceed 0.8%.
Preferably, amounts of from 0.02 to 0.5%, more preferably from 0.05
to 0.2%, based on the sum of the weights of methylbenzoate and di-
propylene glycol, are used. `~
Whe.n the transesterification mixtures are heated, sm~
quantities of low-boiling compounds and water i.nitially distil off.
~he liberation of methanol begins at a vessel temperature of around
150C. During transesterificatio.n, the reactio.n temperature rises
steadily up to æound 270C. ~ra~sesterification is best c æ ried
out in an inert gas, for example nitrogen. -
When the catalysts used in accordance with the inventio~
are employed and impure methyl.benzoate is used, the reactive impu-
rities i.n the impure methylbenzoate (which impurities otherwise
readily change into deep-coloured products duri.ng transesterifica-
tio.n as can be seen from '~able 1) surprisingly remain substantiallY
unchanged, which accounts for the light colour of the transesteri-
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ficatio.n products. ~y virtue of the considerable differe.nce in
the boilin~ points of the impurities and the dipropylene ~lycol
dibenzoate, the impurities can readily be removed by distilla-
tion, preferably in vacuo, o.n completion of transesterificatio.n.
If an excess of methyl be.nzoate has been used, it is also distil-
led off. ~he methylbenzoate-co.ntaining distillate can be used in
accordance with the inventio.n in a subsequent batch without in any
way adversely affecting the colour quality of the end product. I
Removal of the low-boiling co.nstituents by di~tillation leaves
light-coloured dipropyleLe glycol dibenzoate If desired, the co-
lour can be further lightened by treatment with an ozone-contai.n-
ing gas stream, in complete contrast-to the aforeme.ntioned disco- ~
very that the deep colour of the esters produced with conventio.nal L
catalysts is.not significantly impro~ed by treatment with oxidisi.ng
agents. . I
If the methylbenzoate used contains small quantities of 1.
free acids, the end product has corresponding acid .numbers. In
such a case, it is possible to carry out a simple de-acidifying
refiDing operatio.n of the kind.normally applied in the production
of plasticisers, for example by treatme.nt with aqueo.us alkal~e
- so.lutions. ~inally, small quantities of readily volatile substan-
ce~ can be removed by passing air or steam through the product.
. Dipropylene glycol dibe.nzoate is a high-grade, quick-
gelling plasticiser for polymers, for example for polyvinyl chlori-
de.
~he process according to the invention will now be bet-
ter illustrated with refere.nce to the following non-restrictive
~xamples.
EXAMPIE 1
2144 parts by weight of dipropylene glycol (t6 mols)
and 5227 parts by weight of commercial-grade methylbenzoate with a `,
content of 93.6% by weight (36 mols of pure ester) were heated in
-- 5 -- .
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the presence of 7.4 parts by weight oP aluminium-sec.-butylate as
Catalyst (0.1% by weight, based on the total sum of the startlng
materials) i.n a 10-litre flask equipped with an electrical heater,
a stirrer, a fractionating column co.ntaining glass Raschig ri.ngs
and a nitrogen inlet. Readily volatile compounds and water ini- ..
tially distilled off from the mixture at temperatures in the reac-
tion flask of up to 160C. ~he liberation of methanol began at
160C. The temperature of the flask rose steadily to 240C over a
period of 5 hours, after which the liberation of methanol was over.
The methanol-containing distillate had a methanol content of 95%, .
as determined by gas chromatography. The balance consisted of im-
purities in the commercial-~rade methylbe.nzoate used as starti.ng
material. ~ow-boiling constituents and the excess of methylbenzoa-
te were subsequently distilled off o.n a ~acuum of 20 Torr after
cooling to temperatures of 200C. ~he dipropylene glycol dibenzoa-
te, obtained in a yield of 98~o of the theoretical yield based o.n
the dipropylene glycol used, had an acid number of 1.6, a hydroly-
sis .number of 326.5 (calculated 328) and an iodine colour number
of 10. After cooling to 150C, the product was ozonised for 30
miDutes using a Type S2 ozoniser supplied by Argentox, Hamburg,
Germany, having an output of 1.5 g of 03 per hour at 12 KV/90 W.
After ozonisation, the iodine colour number wa~ 3, and, after
aqueous-alkaline refining, was 2. The end product had an acid ~um-
ber of 0.3 and a hydrolysis.number of ~29.5.
EXA~P~E 2
~ he same procedure as in Example 1, was carried out, ex-
cept that 5818 parts by weight (40 mols of pure ester) of commer-
cial-grade methylbenzoate (methylbe.nzoate conte.nt 93.6% by weight) f
were used instead of 522~ part~ by weight. Under otherwise the sa-
me co.nditions, transesterification was complete after 4 hours.
EXAMPIE 3
The procedure of Example 1, was repeated using the cata-
.
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~45636
lysts given in Table 2. The reaction times and iodine colour
numbers of the products are given in the Table,
'~able 2
. .____
Catalyst Quantity Reaction Iodi~e colour
- (~ by weight) time number of the end
_ _ product
Al-ethylate O.05 5,5 hours 1 - 2
Al-ethylate 0.1 . 4 " 1 - 2
Al-ethylate O.2 3.5 " 1 - 2
Al-n-propylate 0.1 4.5 " 1 - 2
Al-i-propylate 0.1 4 " 1 - 2 .
1ate ¦ 0 1 ¦ 5
.
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