MASON EARING, DONALD R. JACKSON, ARTHUR ASH', and KENNETH AOKl Wyandotte Chemicals Corp., Wyandotte, Mich.
Nitric Acid Oxidation of Polyethylene Glycols and Their Monomethyl Ethers O S I D A T I O ? \ T ofthe terminal hydroxyl groups of polyethylene glycols and their monomethyl ethers would provide a possible synthesis of the corresponding carboxylic acids. Since the glycols and ethers are commercially available in varied average molecular weights. such a process would yield a series of monoand dibasic acids analogous to the straight-chain fatty acids but differing from those in being water-soluble even a t relatively high molecular weights. Reaction of these acids with other materials should provide a series of hitherto unobtainable products. Monoesterification of the dibasic acids with long-chain alcohols, for example, would produce surface-active agents containing both oxyethylene groups and a carboxyl group in the hydrophilic portion. The use of nitric acid as the oxidant is attractive because of its economic feasibility for commercial scale operation. The nitric acid oxidation of this type of compound was first applied by IVurtz in 1863 to the oxidation of triethylene glycol to ethylene bis (glycolic acid) ( 3 ) . Examples of the method are also disclosed in the patent literature as applied to the oxidation of the dodecyl and octadecyl monoethers of triethylene glycol (2). The nitric acid oxidation of long-chain glycols and their ethers has not been reported. The present investigation was, therefore, undertaken to study the effect of reaction variables and to extend the procedure to the synthesis of relatively high molecular weight acids. The basic reactions for the oxidation are shown in Equations 1 and 2. The glycols are oxidized to dibasic acids and the methyl ethers to monobasic acids:
the reaction conditions. If the oxides are not reduced past the level of nitric oxide, recovery and recycle of the nitric acid is possible through air oxidation. Thus, under suitable conditions the reaction can be considered essentially a n air oxidation in which nitric acid and oxides of nitrogen are used as capriers of the oxygen. Experimental
The polyethylene glycols and their monoalkyl ethers used in this work were commercial grade materials. The average molecular weights were determined by acetylation of the terminal hydroxyl groups. T h e nitric acid !vas the 70Yc C.P. grade. Oxidation. The apparatus consisted of a three-necked flask fitted \vith an electrically driven stirrer! pressure-equalking dropping funnel, thermometer, and condenser. A vent line led from the top of the condenser through a bubble counter to a water scrubber for disposal of the oxides of nitrogen. The reaction flask )vas charged with the desired quantity of nitric acid. and nitrogen dioxide was added with stirring from a cylinder until a definite orange color was observed. The charge was then heated with stirring to 45' C., and about 20 ml. of the glycol was added dropwise over a period of 5 minutes. \\'hen the reaction had definitely started, as shown by the darkening of the charge and the evolution of oxides of nitrogen, the remaining glycol was added continuously a t a rate to maintain a vigorous reaction as judged by the rate of gas evolution. The temperature of the reaction mixture was maintained between 45' and 50' C. by raising and lowering an ice bath. To4 (0) ward the end of the reaction period exHOCHrCHjO( CH?CHyO 1,CH:CH,OH+ ternal heating was generally necessary to maintain the reaction temperature. The 0 oxidation was considered to be complete HOeCH.O(CHiCHIO),CHr~OH (1) when oxides of nitrogen were no longer evolved. 2 (0) CH30( CH,CH~O),CH~CH?OH---+ Removal of Excess Nitric Acid. Nitric acid remaining after completion of 0 'I the oxidation was removed in a falling C H ~ O ( C H , C H , O ) , C H ~ O H( 2 ) film evaporator. This evaporator consisted essentially of a 1-inch stainless steel The nitric acid is, of course. reduced to pipe, 45 inches long, surrounded by a various oxides of nitrogen depending on Dowtherm jacket. The inner wall of American Machine and Foundry Comthe pipe was lined with stainless steel pany, Turboengineering Department, screening to minimize channeling of the 12270 Montague Blvd., Pacoima, California. liquid. The experimental conditions
9
\vere adjusted so that the products emerged a t the bottom of the pipe when the evaporation of water and nitric acid was just completed. Typical experimental conditions employed in the stripping of the oxidation product of polyethylene glycol 200 were: 175' to 185' C:. jacket temperature: 15 mm. of Hg. and a feed rate of about 15 ml. per minute. With polyethylene glycols of other molecular weights, slight alterations in the temperature and pressure were necessary for optimum results. The air that was bled into the system a t the bottom of the evaporator to adjust the pressure was also effective in forcing the vapors rapidly to the condenser and in increasinq the raw of evaporation The excess nitric acid could also be removed by ordinary distillation at prrssures sufficiently low to maintain the boiling point below about 60' C. After the bulk of the nitric acid had been rcmoved in this manner, the pressure \vas further reduced to 3 to 10 mm. and water was introduced into the kettlr in 5-10-ml. portions until about 500 ml. had been added or until the nitric acid concentration had been reduced to less than 2%,. This procedure. howevcr. was time-consuming and resulted i n overoxidation of the product. if not carefully controlled. A small amount of material reacting as nitric acid by analysis remained in the effluent from the evaporator. Completr elimination of this material could br achieved by saturating the product with sulfur dioxide followed by heating under reduced pressure in a stream of inert gas ( 7 ) . After this treatment no trace of nitrates or nitrites could be detrcted even by a diphenylamine spot test. The acids obtained by the oxidation of the monomethyl ethers of diethylene and triethylene glycol could be purified further by fractional distillation after th:. removal of the free nitric acid. Discussion The chief variables studied in the oxidation were temperature. time of reaction. mole ratio of nitric acid to glycol, the concentration of nitric acid, and the possible use of metal salts as catalysts. The acid and saponification equivalents of the purified oxidation products were expressed as assays for the acid VOL. 48, NO. 9
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corresponding to the starting glycol. Thus. the assay for acid obtained in the form of ester measured the extent to which the resulting acid had combined with the starting glycol or the extent to to which carbonyl groups had been produced within the oxyethylene chain. The total of the assays for free acid and acid as ester should not be greatrr than 10070 if the polyglycol chain \vas not affected in the oxidation. T h e extent to which the total exceeded 100yc was taken as a measure of the degradation of the starting glycol. Although iron and vanadium are mentioned in the literature as catalysts for nitric acid oxidations, no advantage \vas found from their use i n this work. However, a small amount of nitrogen dioxide in the nitric acid appears to be necessarv to initiate the oxidation O n e of the most important considerations was that the reaction temperature should be maintained as low as possible, consistent with a reasonable reaction rate, regardless of the concentration of nitric acid. Reaction temperatures greater than about 70' C. caused extensive overoxidation with degradation of the polyglycol chain. I n the experiments shown the temperature was maintained between 45' and 50' C. since below 45' the oxidation failed to be thermally self-sustaining, The effects of varied mole ratios of nitric acid to polyethylene glycol 200 are as follows: Moles of H.VOa/ M o l e of Glycol
Reaction
1 2
2 3
Time. Hr. 3 4
3 4 5 6 7
4
4
Run
8
5 6 7 8 10
5 5 5 3.5 6.5
Assay, As Acid 29 45 90 87 97 98 98
104
5;
As Ester
52 22
8 9
6 5 2 2
.qlthough titration of the oxidized material indicated that approximately 2.0 to 7.5 moles of nitric acid are consumed per hydroxyl group oxidized. a considerable excess of nitric acid over this amount is required to compete successfully with the side reaction of ester formation. I n general, about 3 moles of nitric acid per hydroxyl group represents a minimum practical working ratio. Larser excess of nitric acid or increasrd residence time do not cause appreciable degradation of the product (runs 7 and 8). The concentration of nitric acid, of course! is not a precisely defined variable, referring only to the initial concentration, which becomes progressively less as the reaction proceeds. The final concentration depends mainly on the mole ratio of nitric acid to glycol. if the reaction conditions are not so strenuous that overoxidation results. When an adequate mole ratio of nitric acid to glycol was employed. the initial concentration of nitric acid did not appear to be a n important factor as long as it was greater than about 40'5. The gas evolved in the oxidation \vas found by analysis to consist mainly of nitrogen dioxide and nitrogen tetroxide which are recoverable as nitric acid by air oxidation and solution in water. The gases Lvhich are not recoverable as nitric acid correspond to less than 10yGof the nitric acid charged. The evolved gases are. of course. relatively richer in nitrogen dioxide a t higher mole ratios of nitric acid to glycol. The use of the falling film cvaporator is preferable to simplr vacuum distillation in removing the excess nitric acid. I n the latter proccdure thc product is continuously exposed to the hot nitric acid which concentrates due to maximumboiling azeotrope formation and accentuatrs the overoxidation hazard. Experi-
mental conditions for operation of the evaporator were determined so that the products emerged a t the bottom when the removal of nitric acid was just coniplete. T h e conditions shown in the experimental section for the oxidation product of polyethylene glycol 201, are typical. The stripped product containcd a srnal! amount of material, probably nitrate or nitrite esters. which reacted as nitric acid in the analysis but resisted elimination by physical methods. .Uthough the amount was relatively small, its removal was considered to be desirable because of possible oxidizing and corrosive action in technical applications. The elimination in the final treatment with sulfur dioxide probably results from a reduction of the nitrate or nitrite esters to volatile products, which may be rtxnovrd together with the excess sulfur dioxide by heating under diminished pressure. The products are typically strawcolored and clear in appearance. Common physical properties together with assays for the product acids are shown in Table I. .4s would be expected. the totals of the assays for product as free acid and as ester reveal a n increasing tendency toward cleavage of the chain and ester formation with increasing molecular Iveight of the starting glycol or ether. Formation of esters probably results from the oxidation of carbon atoms i n the chain to carbonyl groups. The conversion of the terminal hydroxy! groupof the glycols and their ethers to the carboxyl produces a marked increase in viscosity. .411 the acids as well as their calcium salts are soluble i n water in all proportions. The acids also appear to be more highly ionized than the corresponding aliphatic acids. Thneutralizing capacity toward base, of course, is quite small in the higher molecular weight acids as the carboxyl content is very low per unit weight of acid. The esters with aliphatic alcohols of high molecular weight are readily prepared and show the expected surfaceactive properties. Thus, monorsterification of the oxidation product of polyethylene glycol 200 with technical lauryl alcohol yielded a material which produced a marked lowering of thr surface tension of water and gavc very stable kerosine-\vatcr emulsions. Literature Cited ( 1 i .Ish. A , ! Earing,
I.I. (to Wyandotte Chemicals Corp.), U. S. Patent 2,659,754 (1953). ( 2 ) Haussman ( t o I. G. Farbenindustrie) U. S. Patent 2,183,853 (1939). (3) Wurtz, .4.. -Inn. Chirn. 69. 351 (1863). RECEIVED for review September 14, 1955 ACCEPTED March 26. 1956 Division of Industrial and Engineering Chemistry, 128th Meeting, Minneapolis, hiinn.. September 1955.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
After 1 minute in solveni
Figure 1.
After 3 0 minuies i n solvent
Unsulfonated “standard” copolymer spheres under crossed nicols in swelling with methylene chloride
I
IRVING M. ABRAMS Chemical Process Co., Redwood City, Calif.
High Porosity Polystyrene Cation Exchange Resins I N T H E development of ion exchange resins the cross-linked polystyrene matrix has emerged as a highly satisfactory structure on which to attach cation-active and anion-active groups. The most commonly used cation exchangers are made by sulfonating a copolymer of styrene. ethylvinylbenzene, and divinylbenzenr ( 6 ) . These products are known commercially by such trade names as Chempro C-20, =\mberlite IR-120. Dowex 50 (Salcite H C R ) , and Permutit Q and are referred to herein as “standard” copolymer exchangers. During the sulfonation of these copolymers and in the subsequent displacement of the sulfonating agent with water the resin matrix is subjected to considerable strain because of the swelling forces involved. Much effort has been expended to produce the resin in bead form without breakage or cracks. Wheaton and Harrington (70) have described a method of obtaining such a product involving the use of a swelling solvent prior to sulfonation and dilution of the acid-impregnated resin with high concentrations of aqueous strong electrolytes. These processes are described in patents issued to Boyer (5) and Bauman ( I ) . Bodamer ( 3 ) has described a method of making a highly porous polystyrene type cation exchanger by sulfonating a “popcorn” or “proliferous” polymer. This procedure produces a very low density resin having a low volume capacity. Kone of these methods overcomes the objection commonly encountered with polystyrene in general-namely, brittleness. .A unique method of polymerization has been developed in this laboratory which results in a copolymer which resists breakage on sulfonation and possesses
other properties dissimilar to those of the standard copolymer used to make ion exchange resin beads. The sulfonated copolymer is being manufactured under the trade name Duolite C-25. Some of the properties of the unsulfonated copolymer and the final product \chich will be described here point to a degree of porosity greater than is obtained with analogous standard copolymers. Physical Properties of Copolymers Polystyrene normally is a highly brittle resin having a high degree of rigidity. Numerous attempts have been made to find a suitable plasticizer for this resin without apparent success. The use of low concentrations of divinylbenzene (DVB) to cross link polystyrene results in a toughened product; higher concentrations (above 5%), used in making cation exchangers? result in copolymers having lower tensile and impact strengths ( d ) . The method used to make the copolymer described here seems to impart a plasticizing effect, especially with regard to eliminating breakage during sulfonation and subsequent hydration. M’hereas the standard copolymer for making cation resin consists of hard, crystal-clear spheres, the Duolite (2-25 itraw” beads are opaque and milky white in appearance. Of greater significance for ion exchange purposes is the fact that the new type copolymer is much more rapidly and easily penetrated by various liquids. even a t higher degrees of cross linking. Information concerning bead structure can also be obtained by observing the optical properties of the resins. As shown by Wheaton and Harrington (70): observance of the effect of a swelling solvent on the unsulfonated resin beads
under crossed nicols (nicolprisms) is a useful technique. Figures 1 and 2 are crossed-nicol photomicrographs of unsulfonated copolymer beads of Chempro C-20 and Duolite C-25 (both 8Tc DVB). Transmitted light \vas used in taking these and all other photomicrographs for this report. Those in Figures 1‘1 and 2.1 were taken after 1 minute of swelling in methylene chloride! \\,hewas those in Figures 1B and 2B \vere taken after hour swelling in the same solvrnt. The sharply defined pattern shown in Figure l d and the briqht interference colors (not sho\vn but very evident on visual examination) indicate a high degree of anisotropy duriny the initial part of the swelling process. .\ difference in refractive index betwren the outer swollen shell and the inner core can also be seen in white light in the Chempro C-20 copolymer beads. This anisotropy seems to be due to the strain resulting from the compression and to the density difference between the swollen and unswollen portions. The diffuse cross pattern seen in Figure 1B generally does not persist beyond 40 minutes. During the initial swelling (first 10 minutes) of the Duolite (2-25 copolymer, the pattern is considerably more diffuse (Figure 2‘4) than with the Cheinpro C-20 beads and the interferencr colors are virtually absent. This could result from the presence of true pores which cushion the stresses so evident in the standard copolymer. The phase difference between shell and core is barely detectable in ichite light. This is probably due to the extremely rapid diffusion of the solvent into the copolymer spheres. The porosity of a given material is generally related inversely to the density. Specific gravity was determined on both VOL. 48, NO. 9
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