US2410118A - Method of producing trialkyl phosphate esters - Google Patents

Description

Patented Oct. 29, 1946 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 Illinois No Drawing. Application April 5, 1944, Serial N 0. 529,680

2 Claims.

1 g This invention relates to a method of producing trialkyl phosphate esters, and more particularly to a method of producing thes substances by pyrolysis of acid esters.

Ordinarily trialkyl phosphate esters are prepared by reacting an alcohol with phosphorus oxychloride at relatively low temperatures under vacuum in accordance with the following theoretical equation:

The reaction is never quantitative and generally the reaction product includes impurities such as 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 obtain good yields of neutral phosphate esters by heating partially neutralized acid esters.

Theoretical equations for the pyrolysis of acid phosphate esters may be written as follows:

We have found, however, that pyrolysis in accordance with these theoretical equations is quite impracticable. Thermal decomposition takes place with the formation of large amounts of olefin gases and low yields of the desired esters.

We have found, however, that by partially neutralizing the acid esters with a base, either prior to or during the heating step, commercially practicable yields of the triesters can be obtained. The best yields are obtained when the base is added in sufilcient quantity to produce a distillation residue corresponding to a monobasic inorganic phosphate or its dehydration derivative.

Dialkyl phosphate esters have been found to be much more readily pyrolyzed than monoalkyl esters. Mixtures of monoand diesters, however, provide suitable starting materials.

The dialkyl phosphates of commerce are not pure, butusually contain about 65% to 75% of the dialkyl ester, with impurities ranging from about 20% to 25% of the monoalkyl ester, -10% ofthe trialkyl ester, and 0-10% of free phosphoric acid. Ordinary monoalkyl phosphates of commerce generally contain about 50-55% of the mono ester, 25 to 35% of the diester, and about to of free phosphoric acid. Mixed esters such as those made by reacting three moles of alcohol with one mole of P205 generally contain about 55% diester, 35% mono ester, and 10% free acid.

Any one of the foregoing three types of commercial acid ster compositions produces considerable therm l decomposition when heated at pyrolysis temperatures. In the case of the mono ester, no triester is formed on heating. The ordinary 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 dialkyl esters was about for the methyl ester and progressively less with the higher alkyl esters.

We have found, however, that markedly improved yields of triesters may be produced by carefully controlling the acidity of the composition. For example, a mixture of mono and dimethyl phosphates obtained by reacting three moles of methanol and one mole of P205 was heated to a pyrolyzing temperatur without and with varying amounts of added caustic soda. The yield of trimethyl phosphate wasonly about 18% when no added "caustic soda was employed. When suflicient causticsoda was added to give a calculated distillation residue of hemisodium phosphate (H3PO4.NaHzPO4) the yield was 53%.

' When the caustic soda was sufiicient to give a In this equation the distillation residue is indicated as monosodium phosphate, but under practical conditions the pyrolyzing temperature is usually high enough to cause molecular dehydration of the monosodium phosphate, thereby giving 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 products of such inorganic phosphates.

Commercially acceptable yields may be obtained within the range of calculated distillationn residues from hemisodium phosphate to monosodium phosphate or even to a mixture of mono and disodium phosphates. This substantially corresponds to a partial neutralization of from /6 to /2 of the acidity of the starting alkyl acid phosphate esters. The acidity of the starting esters may be determined by titration with standard caustic soda solution to a phenolphthalein 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 phosphates with and without added alkali were not in general practicable. For example, no trialkyl esters were formed without added alkali, but where sufficient caustic soda was added to give a theoretical monosodium phosphate distillation residue, the pyrolysis yield of trialkyl ester was increased to 31.5% in the case of the methyl ester. Lower yields are produced with :higher alkyl groups. Using commercial dialkyl phosphate esters as starting materials, we were able to obtain practical pyrolysis yields of triesters where the alkyl groups ranged from methyl up to at least octyl. For example, ,irialkyl phosphate yields of 70% and over may be obtained 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 example, the yield of trimethyl phosphate is about 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 commercial 'diethyl phosphate containing about 6.5% 'diethyl phosphate was partially neutralized with 26. g. of 50% NaOH solution and the mix- .ture placed in a distilling flask and heated under vacuum '(17-25 mm.) until distillation was substantially complete. 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 thickenin of the charge. As the pyrolysis and distillation proceeded the charge became a slurry and finally baked to a white solid and the temperature increased up to about 300 C. without any evidence of destructive decomposition. The distilled product was a clear colorless liquid weighing 89 g., and represented a yield of 75% triethyl .phosphate having a boiling range of 1004.05 C. at 16 mm. pressure.

In .another experiment a commercial dibutyl phosphate containing 71.5% di-n-butyl phosphate 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 temperature of 255 C. A yield of 55% of tributyl phosphate was obtained.

In another case a dibutyl phosphate was prepared byreacting three moles of n-butyl alcohol with one mole of P205 and. washing the ester The distillate coming vacuo to remove most of the evolved HCl.

compared to the theoretical value of 16.3%.

product to remove the soluble monobutyl phosphate. The resulting water-insoluble ester containing 92.6% dib-utyl phosphate was treated with the calculated amount of caustic soda to give a calculated pyrolysis residue of monosodium phosphate, and the mixture heated to a p-yrolyzing temperature. A yield of 77% tri-n-butyl phosphate in substantially pure form was obtained.

In another example a commercial dioctyl phosphate was treated with caustic soda sufficient to give a monosodium phosphate residue and pyrolyzed at a temperature up to 270 C. A yield of:8.3;7.% of trioctyl phosphate was obtained.

The pyrolysisstep as herein described may be used to supplement and improve the yields of trialkyliphosphates prepared by the known oxychloride method.

For example, an excess of normal propyl alco-, hol was reacted with one mole proportion of phosphorus oxychloride in known manner. The principal reaction may be expressed by the equation:

After heating under vacuum to drive off the hydrogen chloride, the temperature was further increased to distill off the tripropyl phosphate ester. A ield of the triester was obtained leaving a residual acidic liquid which upon further heating would-rapidly decompose-with the formation of a noncondensible gas. This residual liguid contained mixed acid ester resulting from side reactions. To 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 ofi, representing a 14% increase in the overall yield or a 74% yield by combining the two process steps.

In another modification of the prior art oxychloride 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 The excess alcohol was then removed under vacuum upto C. A sample of the crude product was titrated ('1 cc. requiring 4.8 cc. N/ 10 NaOH) to a phenolphthalein end point in alcohol, and an amount of NaOH "added equivalent to one-third of that indicated as required for neutralization. The charge was then heated in vacuo to distill off vtrioctyl phosphate formed by direct reaction as well asthat formed by pyrolysis of the acid octyl phosphate components of the POC13 reaction product. A 94% yield of crude trioctyl phosphate was distilled over between 220 and 260 C.

The crude trioctyl ester analyzed 16.4% P205 The ester had a boiling point of -196 at 5 mm. Hg, pressure. The high yield is particularly remarkable since withoutthe added alkali only a small part of the charge is distillable before severe thermal decomposition takes place. In car- .rying out the above improved processon a larger scale, the amount of added caustic soda'was in- 'creasedto 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 oftrialkyl phosphate esters may be obtained from alcohol and phosphorus pentoxide without the intermediate use of phosphorus oxychloride. The

new process is carried out .in three stages; It requires the recycling of a definite amount ofthe trialkyl phosphate and may be represented by the following equations showing the three steps:

One mole of R3PO4 from the third step is returned to the first step and the cycle repeated. While three distinct procedural steps are required, the net material balance of the process maybe represented by the equation:

' 96 g. methyl alcohol added slowly with stirring while maintaining the temperature below to C. After the alcohol addition is completed the charge is heated to to C. for 2 hours to complete the reaction. The resulting liquid dimethyl phosphate ester (3'78 g.) from this step is then treated with g. 50% sodium hydroxide solution with cooling so that the temperature does not exceed 50 C. The stirrer is then replaced with a Liebig condenser, and the bath removed. Vacuum is applied, and the flask 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 finally changes to a solid white residue between 200 and 250 C. The distillate is a water-clear trimethyl phosphate ester which, upon redistillation, yields 252 g. of a substantially pure trimethyl phosphate representing a yield for the pyrolysis step. Since 140 g. of the triester is returned to the first step to be recycled in the process, a net'yield of 112 g. trimethyl ester or 80% is obtained based on the methyl alcohol input. The residue which may be recovered as sodium metaphosphate or hydrated to monosodium phosphate is a valuable byproduct of the process.

A modification of the process applicable to 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 nbutyl acid orthophosphate was partially neutralized 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 to 150 C. On fractionation the product was .6 V separated into three fractions having the following boiling ranges and P205 contents:

1. 105-120 C., 39.4% P205 2. 135 -C., 35.8% P205 3. .135- C., 31.6% P205 These analyses indicate the product to be .composed ofa mixture of monomethyl dibutyl :phosphate which has a P205 content of 31.7% and monobutyl dimethyl phosphate having :a P205 content of 39.0%. Analysis indicates that no trimethyl nor tributyl phosphate was formed, and the product comprised neutral mixed esters. Upon redistillation in a suitable fractionating column a better separation of these esters is possible.

In the pyrolysis of dialkyl esters illustrated above, caustic soda has been used to stabilize the reaction during pyrolysis, but it should be understood that other bases capable of combining with and holding the released phosphoric acid in the still residue, are entirely suitable when regulated to yield or maintain suitable acidity for facilitating the pyrolysis. For example, commercial dimethyl phosphate treated with the following bases in amounts to give calculated monobasic acid phosphate residues, on pyrolysis in vacuo at temperatures up to 245 0., gave the following yields of the triesters:

Per cent Ca(OH)2 gave yield of 63.0 KOI-I gave yield of 68.5 NazCOa gave yield of 51.0 NaOH gave yield of 68.5 NI-LrOI-I gave yield of 57.0

In the above examples, the pyrolysis reaction was followed by or simultaneously carried out with distillation of the resulting triester. Distillation, while economical and desirable in many cases, is not essential where the resulting triester'is insoluble in water as is the case with triesters 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 decantation or use of a separatory funnel. In the distillation procedures, it is difficult 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 carbon atoms it is generally possible to increase the trialkyl ester yield as much as 5 to 15% by dissolving out the water soluble residue, and separating the undistilled triester from its aqueous suspension.

We have shown that pyrolysis of mono and diesters or phosphoric acid maybe improved by partially neutralizing the esters before heating, but from a practical standpoint the primary feature of the invention is the pyrolysis of the dialkyl esters under controlled conditions of acidity 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 decompose at the pyrolysis temperatures.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom.

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 to 300 C., and separating the trialkyl phosphate produced. v

2. The method of producing a primary trialkyl phosphate ester from an acid phosphate ester starting composition including a substantial pro- 8 portion of a dialkyl acid phosphate ester which comprises adding a base to the dialkyl acid phosphate ester composition in sufiicient quantity to neutralize from /6 to /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 formation of trialkyl phosphate ester and separating the trialkyl phosphate ester.

WILLARD H. WOODSTOCK. PAUL E. PELLE'I'IER, JR.