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Патент USA US2410118

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2,410,118
Patented Oct. 29, 1946
‘UNITED STATES PATENT‘ OFFICE
METHOD OF PRODUCING TRIALKYL
PHOSPHATE ESTERS
Willard H. Woodstock, Flossmoor, and Paul E.
Pelletier, Jr., Chicago Heights, 111., assignors to
Victor Chemical Works, a corporation of Illi
nois
No Drawing. Application April 5, 1944,
Serial N 0. 529,680
2 Claims. (Cl. 260-461)
1
g
trialkyl phosphate esters, and more particularly
to a method of producing these substances by
pyrolysis of acid esters.
Ordinarily trialkyl phosphate esters are pre
pared by reacting an alcohol with phosphorus
oxychloride at relatively low temperatures under
vacuum in accordance with the following theoret
ical equation:
2
mercial acid ster compositions produces consid
This invention relates to a method of producing
erable therm l decomposition when heated at py
rolysis temperatures. In the case of the mono
ester, no triester is formed on heating. . The or
dinary mixture of mono and diesters produces a
maximum of about 20% of triester in the case of
short chain alkyl groups, and no triester when the
alkyl group contains more than 4 carbon atoms.
The maximum yield on pyrolysis of commercial
10 dialkyl esters was about 60% for the methyl ester
The reaction is never quantitative and generally
the reaction product includes impurities such as
and progressively less with the higher alkyl esters.
We have found, however, that markedly im
proved yields of triesters may be produced by
carefully controlling the acidity of the composi
alkyl chlorides and alkyl acid esters resulting
from side reactions. Where long chain alkyl
groups are employed, yields are generally low.
The present invention makes it possible to ob
methyl phosphates obtained by reacting ‘three
tain good yields of neutral phosphate esters by
heated to a pyrolyzing temperature without and
with varying amounts of added caustic soda. The
heating partially neutralized acid esters.
Theoretical equations for the pyrolysis of acid ~
phosphate esters may be written as follows:
tion.
For example, a mixture of mono and di
moles of methanol and one mole of P205 was
yield of trimethyl phosphate wasonly about 18%
when no ' added "caustic soda was employed.
When su?icient causticsoda was added to give a
calculated distillation residue of hemisodium
phosphate (H3PO4.NaHzPO4) the yield was 53%.
We have found, however, that pyrolysis in ac
cordance with these theoretical equations is quite
impracticable.
Thermal decomposition takes
' When the caustic soda was su?icient to give a
calculated residue of monosodium phosphate
(NaH2PO4) the yield of trimethyl phosphate was
58%. With further added caustic soda to give a
place with the formation of large amounts of ole
residue of disodium phosphate the yield again
?n gases and low yields of the desired esters.
We have found, however, that by partially neu 30 dropped to about 18%. The'preferred degree of
neutralization is that required to give a calculated
tralizing the acid esters with a base, either prior
distillation residue substantially equivalent to
to or during the heating step, commercially prac
that of a monobasic phosphate salt. Thispre
ticable yields of the triesters can be obtained.
ferred condition for the pyrolysis reaction may
The best yields are obtained when the base is
‘added in su?lcient quantity to produce a distilla 35 be expressed by the following theoretical equa
tion residue corresponding to a monobasic in
tion:
organic phosphate or its dehydration derivative.
Dialkyl phosphate esters have been found to be
much more readily pyrolyzed than monoalkyl
esters. Mixtures of mono- and diesters, however, 40 In this equation the distillation residue is indi
cated as monosodium phosphate, but under prac
tical conditions the pyrolyzing temperature is
The dialkyl phosphates of commerce are not
provide suitable starting materials.
pure, butusually contain about 65% to 75% of
the dialkyl ester, with impurities ranging from
vabout 20% to 25% of the monoalkyl ester, 0-10%
ofthe trialkyl ester, and 0—-10% of free phos
phoric acid. Ordinary monoalkyl phosphates of
usually high enough to cause molecular dehydra
tion of the monosodium phosphate, thereby giv
ing a sodium pyro or metaphosphate residue.
Therefore, wherever we designate the calculated
distillation residues as inorganic acid phosphates,
we also intend to include the dehydration prod
commerce generally contain about 50-55% of
ucts of such inorganic phosphates.
the mono ester, 25 to 35% of the diester, and
Commercially acceptable yields may be ob
about 15 to 20% of free phosphoric acid. Mixed 50
tained within the range of calculated distilla
esters such as those made by reacting three moles
tionn residues from hemisodium phosphate to
of alcohol with one mole of P205 generally con
monosodium phosphate or even to a mixture of
tain about 55% diester, 35% mono ester, and 10%
mono and disodium phosphates. This substan
free acid.
Any one of the foregoing three types of com 55 tially corresponds to a partial neutralization of _
2,410,118
3
4
from 1/6 to 1/2 of the acidity of the starting alkyl
product to remove the soluble monobutyl phos
phate. The resulting water-insoluble ester con
taining 92.6% dib-utyl phosphate was treated with
the calculated amount of caustic soda to give a
calculated pyrolysis residue of monosodium phos
phate, and the mixture heated to a p-yrolyzing
temperature. A yield of 77% tri-n-butyl phos
acid phosphate esters. The acidity of the start
ing esters may be determined by titration with
standard caustic soda solution to a phenolphthal
ein endpoint. The titration is made in aqueous
solution for esters below amyl and in alcoholic
solution for amyl or higher esters.
Attempts to pyrolyze commercial monoalkyl
phate in substantially pure form was obtained.
In another example a commercial dioctyl phos
phosphates with and without added alkali were
not in general practicable. For example, no tri 10' phate was treated with caustic soda suf?cient to
give a monosodium phosphate residue and py
alkyl esters were formed without added alkali,
rolyzed at a temperature up to 270° C. A yield
but where suf?cient caustic soda was added to
give a theoretical monosodium phosphate distil
\of:8.3;7.% of trioctyl phosphate was obtained.
lation residue, the pyrolysis yield of trialkyl ester
The pyrolysisstep as herein described may be
was increased to 31.5% in the case of the methyl 15 used to supplement and improve the yields of
ester. Lower yields are produced with :higher
trialkyliphosphates prepared by the known oxy
alkyl groups. Using commercial dialkyl ‘phos
chloride method.
phate esters as starting materials, we were able
For example, an excess of normal propyl alco-,
to obtain practical pyrolysis yields of triesters
hol was reacted with one mole proportion of phos
where the alkyl groups ranged from methyl up 20 phorus oxychloride in known manner. The prin
to at vleast octyl. For example, ,irialkyl phos
cipal reaction ‘may ‘be expressed by the equa
phate yields of ‘70% and over may ‘be obtained
tion:
‘
when caustic soda "is added to ‘give a distillation
residue substantially equivalent to monosodium
phosphate.
Without acidity control the pyrolysis of the
'dialkyl phosphates gives yields ‘of the triesters
which are not practical except possibly in the
cases of the methyl and ethyl esters. For exam
‘ple, the yield of trimethyl phosphate is about
50-60%, the triethyl ester about 50%, the triamyl
ester about 10%, and no yield in the case of the
octyl ‘esters.
In all. cases better yields are ob
tained ‘by acidity control.
In the above discussion; we have shown that
‘in vthe ‘pyrolysis ‘of all acid alkyl phosphate esters
.the addition of .a partially neutralizing amount
of a base will greatly improve the yields of tri
alkyl esters, the yields "being higher as the pro
portion or dialkyl phosphate increases.
It is,
therefore, preferred that the so called dialkyl
phosphate ‘esters, namely, those containing more
than 50% dialkyl ‘phosphate, be employed as
starting materials for the production of trialkyl
esters by our pyrolysis method.
In a typical example of ourprocess, 150 g. of
After heating under vacuum to drive off the hy
drogen chloride, the temperature was further in
creased to distill off the tripropyl phosphate ester.
A 60% yield of the triester was obtained leav
ing a residual acidic liquid which upon vfurther
heating would-rapidly decompose-with the forma
tion of a noncondensible gas. This residual lig
uid contained mixed acid esters resulting from
side reactions. To 150 g. of this residual liquid
caustic soda was added equivalent in'amount to
one-third that required to neutralize the residue
to ‘a phenolphthalein ‘end point. The mix ‘was
then ‘heated in'vacuo and 95 g. of substanstially
pure tri-n-propyl phosphate was distilled o?,
representing a 14% increase in the overall yield
or a 74% yield by combining the two process
steps.
In another ‘modi?cation of ‘the prior art oxy
chloride method of producing trialkyl phosphates,
one mole of POCls was reacted with an excess of
octyl alcohol at 30 to 55° C. for three hours in
‘vacuo to remove most of the evolved HCl. The
commercial 'diethyl phosphate containing about
6.5% 'diethyl phosphate was partially neutralized
excess alcohol was then removed under vacuum
with 26. g. of 50% NaOH solution and the mix
titrated ('1 ‘cc. requiring 4.8 cc. N/ 10 NaOH) to a
.ture placed in a distilling ?ask and heated un- ‘
phenolphthalein end point in alcohol, and an
amount of NaOH "added equivalent to one-third
der vacuum '(17-25 mm.) until distillation was
substantially complete.
The distillate coming
over up to 150° 0.‘, consisting largely of water
and a. small amount of alcohol, was discarded.
Pyrolysis started at 177° C. with a gradual thick
ening of the charge. As the pyrolysis and dis
tillation proceeded the charge became ‘a slurry
and ?nally baked to a white solid and the tem
perature increased up to about 300° C. without
any evidence of destructive decomposition. The
upto ‘175" C.
A sample of the crude product was
of vthat indicated as required for neutralization.
The vcharge was then heated in vacuo to distill
off vtrioctyl phosphate ‘formed by direct reaction
asjwell as‘that formed by pyrolysis of the acid
octyl phosphate components of the POC13 reac
tion product. A 94% yield of crude trioctyl phos
phate was distilled over between 220° and 260° C.
The crude trioctyl ester analyzed 16.4% P205
compared to the ‘theoretical value of 16.3%. The
distilled product was a clear colorless liquid
ester had a boiling point of 195-196° at 5 mm.
weighing 89 g., and represented a yield of 75%
triethyl .phosphate having a boiling range of
Hg, pressure. The high yield is particularly re
markable since without‘the added alkali only a
small part of the charge is distillable before se
1004.050 C. at 16 mm. pressure.
In .another experiment a commercial dibutyl
phosphate containing 71.5% di-n-butyl phos
phate was ‘heated under vacuum with caustic
soda, equivalent to one-third of that required to
neutralize the product .to a phenolphthalein end
point, andthe distillate collected up to a tem
perature of ‘255° C. A yield of 55% of tributyl
phosphate was obtained.
In another case a dibutyl phosphate was pre
pared by‘reacting three moles of n-butyl alcohol
with one ‘mole of P205 and. washing the ester
vere thermal decomposition takes place. In car
.rying out ‘the above improved processon a larger
vscale, the amount of added caustic soda'was in
'creased‘to approximately half of that required'for
‘neutralization of the acidity. without .materially
reducing the yield of triester.
‘The present invention also provides a method
whereby economically practicable yields of‘trial
‘kyl phosphate esters may be obtained from ‘al
cohol and phosphorus pentoxide without the in
termediate use of phosphorus oxychloride. The
2,4 10,1 18
.
.6
5
V
separated into three fractions having the follow
new ‘process is carried out .in “three stages; It
requires the recycling of a definite‘ amount of‘the
ing boiling ranges and P205 contents:
trialkyl phosphate and may be represented by
the following equations showing the three steps:
1. ‘105-120° C., 39.4% P205
2. 120—135° -C., 35.8% P205
3. .135-150° :C., 31.6% P205
These analyses indicate the product to be .com
posed ofa mixture of monomethyl dibutyl :phos
10 .phate which has a ‘P205 content of 31.7% and
monobutyl dimethyl phosphate having :a P205
- content of 39.0%. Analysis indicates that no tri
One mole of R3PO4 from the third step is re
methyl nor tributyl phosphate was formed, and
turned to the ?rst step and the cycle repeated.
the product comprised neutral mixed esters.
While three distinct procedural steps are re
quired, the net material balance of the process
Upon redistillation in a suitable fractionating
column a better separation of these esters is pos
sible.
maybe represented by the equation:
In the pyrolysis of dialkyl esters illustrated
As an example of this cyclic combination proc
ess, 140 g. trimethyl phosphate is placed in a
above, caustic soda has been used to stabilize the
reaction during pyrolysis, but it should be un~
20
derstood that other bases capable of combining
with and holding the released phosphoric acid
three-necked ?ask equipped with thermometer,
in the still residue, are entirely suitable when
stirrer, and external bath, and 142 g. phosphoric
anhydride added slowly at ?rst so that the tem
perature does not exceed 50° C. Futher addi
tions can be made more rapidly and the charge
is maintained at 50° to 60° C. until the reaction
is substantially complete, generally 4 to 5 hours
regulated to yield or maintain suitable acidity’
for facilitating the pyrolysis. For example, com
mercial dimethyl phosphate treated with the
following bases in amounts to give calculated
monobasic acid phosphate residues, on pyrolysis
being required. The liquid methyl metaphos
phate from this step (282 g.) is then cooled and
following yields of the triesters: '
' 96 g. methyl alcohol added slowly with stirring
while maintaining the temperature below 50 to
55° C. After the alcohol addition is completed
the charge is heated to 60 to 65° C. for 2 hours
to complete the reaction. The resulting liquid
dimethyl phosphate ester (3'78 g.) from this step
in vacuo at temperatures up to 245° 0., gave the
Per cent
Ca(OH)2 gave yield of __________________ __
KOI-I gave yield of _____________________ __
NazCOa gave yield of ___________________ __
NaOH gave yield of ____________________ __
NI-LrOI-I gave yield of ___________________ __
63.0
68.5
51.0
68.5
57.0
is then treated with 80 g. 50% sodium hydroxide
In the above examples, the pyrolysis reaction
solution with cooling so that the temperature
does not exceed 50° C. The stirrer is then re
placed with a Liebig condenser, and the bath re
was followed by or simultaneously carried out
moved. Vacuum is applied, and the ?ask heated
with a flame. Water of neutralization and that
added with the alkali are distilled off With small
amounts of alcohol up to 120° C. and discarded.
When the temperature rises to 150 to 160° C.,
pyrolysis starts and the product distills over free
1y. The charge becomes milky, thickens, and
?nally changes to a solid white residue between
200 and 250° C. The distillate is a water-clear
with distillation of the resulting triester. ' Dis
tillation, while economical and desirable in many
cases, is not essential where the resulting tri
ester'is insoluble in water as is the case with tri
esters containing more than 3 carbon atoms in
the alkyl group.
In such cases the pyrolyzed ,
'mixture may be washed with water or suitable
solvent to dissolve out the residual inorganic
phosphate salt. The insoluble triester may then
be separated from the solvent medium by de
cantation or use of a separatory funnel.
In the
trimethyl phosphate ester which, upon redistilla 50 distillation procedures, it is dif?cult to completely
drive off the triester because of its being retained
by the solid inorganic residue. In such cases
where the alkyl group contains more than 3
pyrolysis step. Since 140 g. of the triester is re
carbon atoms it is generally possible to increase
turned to the ?rst step to be recycled in the
the trialkyl ester yield as much as 5 to 15% by
process, a net'yield of 112 g. trimethyl ester or
dissolving out the water soluble residue, and
80% is obtained based on the methyl alcohol in
separating the undistilled triester from its
put. The residue which may be recovered as so
dium metaphosphate or hydrated to monosodium
aqueous suspension.
phosphate is a valuable byproduct of the process.
We have shown that pyrolysis of mono and di
A modification of the process applicable to 60 esters or phosphoric acid maybe improved by
tion, yields 252 g. of a substantially pure trimeth
yl phosphate representing a 90% yield for the
methyl and ethyl esters involves the preparation
of neutral methyl or ethyl metaphosphates by
the reaction of P205 and methyl or ethyl ether,
then treating with alcohol to produce the dialkyl
esters which may be partially neutralizedwith a
base and pyrolyzed to the triesters.
Neutral mixed alkyl phosphate esters may also
be prepared by pyrolysis of mixed alkyl acid
phosphates. For example, 3 moles of methyl n
butyl acid orthophosphate was partially neutral
ized with 1 mole of caustic soda and heated in
vacuo up to 210° C. An 87% yield of neutral
esters was obtained with a boiling range of 105°
to 150° C. On fractionation the product was
partially neutralizing the esters before heating,
but from a practical standpoint the primary
feature of the invention is the pyrolysis of the di
alkyl esters under controlled conditions of acid
ity and the combining of this step with known
processes to obtain improved yields of trialkyl
phosphates. The acid ester starting materials
employed are limited to the primary alkyl esters
as most of the secondary and tertiary esters de
compose at the pyrolysis temperatures.
_
The foregoing detailed description has been
given’ for clearness of understanding only, and no
unnecessary limitations should be understood
therefrom.
2,410,118
7
.
What we claim as new, and desire to secure
by Letters Patent, is:
1. The method of producing trialkyl phosphate
which comprises heating a mixture of from 2
to 6 moles of an acid dialkyl phosphate with 1
mole of caustic soda at a temperature of 100°
8
portion of a dialkyl acid phosphate ester which
comprises adding a base to the dialkyl acid phos
phate ester composition in su?icient quantity to
neutralize from 1/6 to 1/2 of the acidity of the ester
composition, and. then heating the mixture at a.
temperature of from 150° to 300° C. to cause the
to 300°‘ C., and separating the trialkyl phosphate
formation of trialkyl phosphate ester and separ
produced.
ating the trialkyl phosphate ester. ,
v
2. The method of producing a primary trialkyl
phosphate ester from an acid phosphate ester 10
starting composition including a substantial pro
WILLARD H. WOODSTOCK.
PAUL E. PELLE'I'IER, JR.
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