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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.