development is warranted on the basis of economic considerations. Acknowledgment
?‘he author thanks the research and development staff of
M & ’I’ Chemicals, Inc., for conducting the tin analyses and for
(2) Summerson, T., Page; H., Zedler, R., Miller, S.; .Wafer. Protection 3, 62-71 (1964). (3) \Voods Hole Oceanographic Institution, “Marine Fouling and Its Prevention,” up. .. 250-2. 256. U. S. S a v a l Inst.. Annapolis. Md., 1952. (4) Zedler, R. J., Am. Paint J . 45, 78 (1961). (5) Zedler, R. J., “Proc. du CongrPs International de la Corrosion Marine et des Salissures,” Cannes, France: in press.
its helpful suggestions.
RECEIVED for review January 31: 1964 ACCEPTED July 8, 1964
Literature Cited
( I ) Mar inr
Laboratory: Uniwrsity of hliami, “Antifouling Potentials of Pesticidal hiatrrials.” rrport to Bureau of Naval LVeapons, March 1961 (unpublished).
Division of Organic Coatings and Plastic Chemistry, 147th Meeting, XCS, Philadelphia, Pa., :\pril 1964. Portions of the work supported by the Bureau of Naval \Yeapoiis, Bureau of Ships. and M RC ?‘ Chemicals, Inc.
PREPARATION OF DIMETHYL-2,6NAPHTHA LEN ED ICARBOXY LATE H E N R Y J. P E T E R S O N , A R C H I B A L D P. S T U A R T , A N D W I L L I A M D . V A N D E R W E R F F Rpsearch and Dmelopment Diiiszon, Sun Ozl Co., ..Marcus Hook, Pa.
With the advent of petroleum-derived naphthalene, large quantities of dimethylnaphthalene are available. An improved method of oxidation of 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid i s presented. The method consists of oxidizing the hydrocarbon, b y means of nitrogen dioxide and selenium, to an approximately equimolar mixture of 2,6-naphthalenedicarboxylic acid and 6-formyl-2-naphthoic acid; separation of the two components during esterification; and oxidation of the 6-formyl-2-naphthoic acid. The effect of the reaction variables is described. The over-all yield of polymer grade dimethyl-2,6-naphthalenedicarboxylate i s 85 to 90 mole
70.
TETE
of aromatic carboxylic acids- and especially aromatic dicarboxylic acids by the oxidation of alkylaromatic hydrocarbons- has bcen pursued vigorously in recent y-ears, the products finding extensive application in synthetic resins, fibers: and films. T h e ease of oxidation as described in elementary texts is an oversimplification as is vividly illustrated by the effort expended in developing commercial procewes for the oxidation of p-xylene to terephthalic acid. This laboratory has been concerned primarily with the oxidation of 2.6-dimethylnaphthalene to 2.6-naphthalenedicarboxylic acid, a procesq further complicated by the increased reactivity of the naphthalene nucleus ovcr the benzene ring. 2.6-9aphthalenedicarboxylic arid has been prepared by fusing dipotarcium-2,6-naphthalenedisulfonate \vith potassium cyanide to give the corregponding dinitrile, Lvhich is hydrolyzed ( 3 ); by a combination of diazotization and potassium c>-anide fusion on 2-naphthylamine-6-sulfonate to >-ield 2.6-dicyanonaphthalene. \vhich is h>-drolyzed ( I ) ; by oxidation of 2methyl-6-acerylnaphthalene \vith dilute nitric acid a t 200’ C . ( 9 ): by thermal disproportionation of potassium naphthoates a t 430” C . ( 6 i ; by thermal tearrangement of dipotassium 1,8naphthalenedicarboxylate a t 425’ C . (7. 8) ; and by oxidation of 2.6-dimeth~-lnaphthalrnrby nitrogen dioxide in trichlorobenzene solution using a selenium catalyst (9). The sclcniuni-nitrogeri dioxide process. ivhich has been licensed by Carbogen Corp. and TVilmot and Cassidy. Inc., to Sun Oil Co.. proved to be rhe most convenient and improved procrdure and greatly simplified and shortened the purification 5tei)s. PRoDuc’rIos
230
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
Experimental
Materials. All of the reagents and chemicals used in these studies were available commercially. and were used without purification. T h e nitrogen dioxide \vas obtained from hlatheson Coleman & Bell; the 2.6-dimethylnaphthalene from Rutgerswerke A.G. ; the trichlorobenzene, from the Hooker Chemical C o . ; and the selenium, from Baker and ;2damron. Procedure. Figure 1 diagrams the apparatus found to be most effective for conducting the oxidation. T h e reactor is a 3-liter resin flask. and efficient stirring \vas accoinplished by using a high-speed stirrer equipped with triple turbine blades capable of 4500 to 5500 r.p.m. under reaction conditions. A further aid to stirring \vas the incorporation of stainless steel baffles. Two general modes of the reaction were used: a ”batch” process and an “incremental” process: the names referring to the manner of addition of the substrate. In both processes, the solvent (approximately 2000 ml.) and selenium were heated to the reaction temperature (usually 195’ C . ) ! and then the selenium \vas oxidized to selenium dioxide with nitrogen dioxide. In the batch process, all of the 2,6-dimethylnaphthalene was added (in hot solvent) after the oxidation of the sclenium. tlien nitrogen dioxide \vas passed through the mixture until the exit gases, originally colorless (nitric oxide), sho\ved a tinge of bronm coloration olving to unreacted nitrogen dioxide; the reaction \vas then stopped by replacing the nitrogen dioxide flow Iiith nitrogen. and cooling to room temperature. I n the incremental process, the 2,6-dimethyl-
naphthalene (and sometimes the selenium) is added in small increments in hot trichlorobenzene solution, each increment being added after the exit gases turn brown folloiving addition of the preceding increment. I n both instances: the nitrogen dioxide flow rate was approximately 2 grams per minute. T h e reaction mixture, after cooling to room temperature under a nitrogen flow. \cas suction-filtered through a medium fritted glass funnel ; washed \cith solvent, benzene, and finally pentane; and dried by stirring under an infrared lamp. I n order to determine the completeness of solvent removal. a 10-gram sample of the product was heated for several hours under vacuum a t 100' C. in an Abderhalden drying apparatus; if any Lcright loss occurred, the crude tveight kcas corrected accordingly. and the dried sample \vas used for analyses. T h e analysis of the acidic oxidation products \cas done by an acid number technique, and a quantitative esterification technique (5). 'l'he acid number technique had to be modified by a prior warm water \\rash to remove any traces of selenium dioxide (acid number 1010). T h e quantitative esterification technique was used in conjunction with vapor phase chromatography: and in this way traces (2Tc or less) of the oxidation intermediates \cere detected.
N2
Figure 1.
Diagram of oxidation apparatus
Results Table I.
I n the remainder of this study, standard conditions of oxidation will he employed, unless otherwise stated. 'The conditions are temperature, 195' C. ; hydrocarbon, 200 grams; solvent, 2000 rnl.; selenium, 2.0 grams. I n the incremental process, all of the selenium is added initially, and 500 ml. of the total solvent are used to dissolve the hydrocarbon for the increments. Effect of iMode of Reaction. Four modes of reaction were tried (Table I ) . 'The reaction was conducted as a batch process in which all of the 2,G-dimethylnaphthalene was added initially, or as a n incremental process in Lchich the 2,G-dimethylnaphthalene was added in small increments. Selenium gives a n improvement in either case. Effect of Selenium on Batch Process. T h e data (Table 11) show that in the batch process the concentration of selenium above a certain minimum (less than 0.5 gram per liter) has little effect on the yield; the lolver yield of 2.6-naphthalenedicarboxi\-lic acid in the first example is the result of increased 2,6-dimethylnaphthalene concentration (see Effect of 2,6Dimethylnaphthalene Concentration on the Batch Process. below). not of lo\ver selenium concentration. I n the absence of selenium, the hatch process-as \cell as the incremental process---yields a highly colored. multicomponent product (owing mostly to nitration). Effect of Selenium on the Incremental Process. T h e solubility of selenium in trichlorobenzene a t 195' C. is 2 grams per liter and that of selenium dioxide is someivhat lo\cer; hence. ivhen \ r e speak of the increased yield obtained by using 3 grams per liter of selenium. Lve are not referring to the actual concentration, but rather to the amount of selenium used (Table 111). T h e reason for the noticeable influence of catalyst quantities in excess of the solubility is to be found in the ease of sublimation of selenium dioxide. During the reaction, srlrnium dioxide is continually sublimed aivay from the sphere of the oxidation. T h e effect of decreased selenium concentration is not only decreased yield of 2,G-naphthalenedicarbox)-lic acid. but also more highly colored product, less efficient utilization of nitrogen dioxide, and increased need for purification of solvent. Effect of 2,6-Dimethylnaphthalene Concentration on the Batch Process. T h e combined 2,G-naphthalenedicarboxylic
Effect of Mode of Reaction
Dcscript ion
Batch process without selenium Incremental process ~ i t h o u selenium t Batch process Incremental process 0
2,6-.~aphtha:enrdicarboxylic acid.
Table II.
0.5 1 .0 3.0
b
43 59 47 69
58 68 96 84
15 9 49 15
6-Formyl-2-naphthoic acid.
Effect of Selenium on the Batch Process
63c 49 49
35c 4: 51
98c 96 100
a 2:6-.\hphtho~~nr.dic~rbo.~ylicacid. 6-Formyl-2-naphthoic acid. A 7 t o 5 ( g r a m / m i . ) w i g h t to i,olunie ratio b z t w m 2.6-dim~thylnaphthalrnv and soivmt i t a s U S P ~ .
Table 111.
Effect of Selenium on the Incremental Process
Selenium used, g./l: 0 0.2 1.0 3.0 3.0 Increments of selenium 0 1 1 1 4 Weight of crude product, g. 187 211 230 244 260 2,6-NI>C.\12> mole (j; 88 80 83 86 89 Yield of 2 ~ 6 - N 1 X A a5;~ 59 61 69 76 84 0
Z.t;-,\hphthalrnedicnrbox).iic
aczd.
acid plus S-formyl-3-naphthoic acid yield of 96 to 1007, in the hatch process remains independent of the initial 2,G-dimethylnaphthalene ~oiicrntration (Lcithin the range studied) ; however, the ratio of 6-formyl-2-naphthoic acid to 2.6-naphthalenedicarboxylic acid is increasecl with increasing initial 2,Gdiniethylnaphihalene concentration ('Tahle 11.1 Effect of Increments on the Incremental Process. As the number of increments is increased from one to 30. the )-ield of 2.6-naphthalenedicarbosylicacid increases. hut so does byprodiict formation and degradation. If the number of increments is made infinite icontinnous addition). then excessive VOL. 3
NO. 3
SEPTEMBER
1964
231
-
1.0
I
/
Table IV.
,'
'8
0
50
100
150
200
250
TIME (MINUTES) Figure 2. Plot of composition v s . time for a seleniumcatalyzed batch oxidation of 2,6-dimethylna phthalene 2,6-Dimethylnaphthalene
--- 6-Methyl-2-naphthaldehyde 2,6-Naphtholenedialdehyde
. - . . _Acids:
6-methyl-2-naphthoic 6-formyl-2-naphthoic 2,6-naphtholenedica rboxylic
degradation occurs and both total Lreight and yield are decreased (Tahle \-). Effect of Temperature on the Incremental Process. .A temperature of 195' to 200' C:. has been sholrn to be the optimum for the standard incremental process (Table VI). T h e effrct of temperature on the batch process \vas not investigated. but i t seems reasonable that it \\.oilid be similar. The factors influencing the optimum temperatiire are : the boiling point of trichlorobenzene (21 5 2 1 7 ' C.) ; nitration of 2.6-dimeth~-lnaphtlialenrand intrrinediate oxidation products, favored by lo\\, trmperaturc; and the oxidation of selenium to selenium dioxide (the catalyst regeneration cycle): which is quite sloiv at temperatures below 150' C .
Effect of 2,6-DMN Concentration on the Batch Process
25 6a 100 47 200 35 2.h- Vaphtlial~nedica~boxylic acid.
29 49
96
63
98
9'
[)-F4rmyl-2-nophthozcacid
dioxide will now be available for oxidation of the aldehydes. As these aldrh>.dcs are consumed, nitration of the products would become considerable were it not for the low solubility of the 6-formyl-2-naphthoic acid and especially the 2.6riaphthalenedicarbo~~lic acid (at 195' C.. the solubility in trichlorobenzene is 29 grams per liter for the 6-formyI-2naphtholic acid and 1 gram per liter for the 2.6-naphthalenedicarboxylic acid) which effectively removes them from the sphere of the reaction (and this is kvhy the product al~vays contains some 6-formyl-2-naphthoic acid). T h i s series of events is illustrated in Figure 2 . In the batch process. the high initial 2.6-dirnethylnaphthalenc concentration ensures rapid reduction of selcniuni dioxide to selenium. which ensures complete utilization of nitrogen dioxide in oxidizing selenium; thus little nitration occurs. As 2,G-dimethylriaphthalene is oxidized to aldehydes by selenium dioxide. reduction of selenium dioxide becomes slower arid nitrogen dioxide bccornes increasingly available for oxidation of aldehydes to acids. I his process is complicated by the bifunctionality of the 2,6-dimethylnaphthalenc. 'There \rill then be some oxidation of the 6-methyl-2-naphthaldehyde by nitrogen dioxide while oxidation of the rest of the 2,6iiaphthalenedialdehyde is occurring. A s the reaction continucq;.selrnium dioxidc oxidi7es 6-methyl-2-naphthoic acid to G-formyl-2-naphtholic acid and nitrogen dioxide oxidizes 6-formyl-2-naphthoic acid to 2.6-naphthalenedicarboxylic acid.
CH3
mCH3
Discussion of Results
L-nder the proper conditions of relenium concentration, 2.6-dimrthylnaphthalene concentration, and nitrogen dioxide flow rate. the nitrogen dioxide that enters the reactor is completely consumed by the rapid reaction \rith selenium. The resultant selenium dioxide is then reduced back to selenium by the 2.6-dimethylnaphthalene a t an even more rapid rate. Thus. if a sufficient 2.6-dimethylnaphthalene concentration is maintained. all of the nitrogen dioxide \?ill be consumed in the oxidation of selenium. and the 2.6-diniethylnaphthalene oxidation products \Till he aldehydcs. uncontaminatrd with by-products resulting from nitrogen dioxide attack on the 2,6-dimrthylnaphthalene. If the 2.6-dimethylnaphthalene concentration is too loiv. then the amount of ?eleniuin dioxide rrdueed by the 2.G-dimethylnaphthalene \vi11 diminish to the point \\.here i t cannot keep pace tvith the reoxidation of selenium by nitrogen dioxide ; therefore. excess nitrogen dioxide \rill be prewnt in the reaction niixture. Thic nitt-oqrn dioxide \vi11 thrn oxidize aldehydes to acids. but i t can also nitrate any of these substancrs. A s oxidation progrrsses, eventually all the 2.6-dimcthylnaphthalrnr \\,ill be consumed, the reduction of selenium dioxide \vi11 cease. and the nitrogen 232
l&EC
PRODUCT RESEARCH A N D DEVELOPMENT
CHO
\
CHO
COOH
CH 3/D
C
0
O
H
mcooH mcoou
I he high concmtration ol subctrate usually cmplo)-ed in the batch process rewlts in prccipitation of much of the 6-formyI-2naphthoic acid. thereb>- limiting i t s further oxidation and givins a product rich in 6-formyl-2-naphthoic acid. T h e incremental procccs is effectively a series of dilute batch proccssrs. Herr. the 2.6-dimethylnaphthalene concentration is
Application Table V.
Effect of Increments on the Incremental Process
Lt't. uf .\a of Iiic renientJ
2.6-.\ DCA'
Lt7t. o j Product, G.
in
Pruduct,
'/I
2,6-.VDC.4' Yield, 5 0 40
65 69 51 $1
L , t i - . ~ a ~ h t h u l e n e d i i a r b o u ~ lacid. ic
Table VI.
Effect of Temperature on the Incremental Process"
Reaction temperature, ' C . Weight of crude product, g. 2,6-N11CAb content o f crude, Yield of 2.6-NDCA, $b
55
165 188 80
54
180 222 81 65
200 260 89
215
84
80
254 87
Srlrriiun! u r d m i s -1.0grams added in 5 increments during oxidation. Z,O-~\'(iptit/~(ilunr dicarboxylic nczd. '1
b
always much lower than in the batch process; in order, therefore, to make the rate of reaction of 2,6-dimethylnaphthalene with selenium dioxide fast enough to keep u p with oxidation of selenium by nitrogen dioxide, a higher concentration of selenium is required (also helpful is a lower nitrogen dioxide flow rate). 'l'his is still not completely effective and more nitration occurs in the incremental process. At the concentrations present throughout most of the incremental process, the 6-formyl-2-naphthoic acid formed is completely soluble so that oxidation of 6-formyl-2-naphthoic acid to 2,6-naphthalenedicarboxylic acid is complete except in the later stages of the reaction. I n the idealized reaction scheme, 2,6-dimethylnaphthalene is oxidized first by selenium dioxide to 6-methyl-2-naphthaldehyde and then to 2:6-naphthalenedialdehyde ; then, there being no longer anything capable of reducing selenium dioxide to selenium, nitrogen dioxide \vi11 be available to oxidize the 2,6-naphthalene dialdehyde to 6-formyl-2-naphthoic acid and this to 2,6-naphthalenedicarboxylic acid. I n practice, as shown in Figure 2, acids are present in the reaction mixture long before any 2,6-naphthalenedialdehydeis produced. T h e explanation is t h a t : T h e reaction of nitrogen dioxide with aldehydes is not markedly slower than its reaction with selenium (but is faster than its reaction with 2,6-dimethylnaphthalene). T h e concentratio? of the various organic species is almost always much greater than that of selenium so that even though the reaction of nitrogen dioxide with selenium is faster, in the presence of much higher concentrations of a less reactive substrate there will be a noticeable reaction with this species. Thus, instead of the idealized stepwise oxidation, we actually have extensive overlapping of these steps so that all these oxidation products are present in the reactor throughout most of the reaction.
T h e batch ~ I O C C S Sis capable of the oxidation of 2,6-dimethylnaphthalene in higher yield with less solvent, less selenium, and less nitration of holvent and product (hence less nitrogen dioxide loss) than the process defined as incremental. T h e method for the utilization of the more efficient batch process involves five essential steps: 1. Batch pi.ocess oxidation of 400 grams of 2,6-dimethylnaphthalene in 2 liters of trichlorobenzene containing 2 grams of selenium to give a 95 to 100 mole yGyield of a one-to-one weight ratio of 6-formyl-2-naphthoic acid and 2,6-naphthalenedicarboxylic at id. 2 . Esterification ( 2 ) of 100 grams of the product from step 1 by heating in a glass-lined pressure reactor with 1 liter of methanol containing 5 ml. of 85yGsulfuric acid fur 2 hours a t 150" C. 3. Separation uf the esters is accomplished by filtration of the cold reaction mixture from step 2 . 'I'he solids are 95yC, dimethyl-2,6-naplithalenedicarbox)late~ and the filtrate contains 6-carbornethoxy-2-naphthaldehyde in 95YG purity. Further purification of the diinethy1-2,6-naphthalenedicarboxylate is accomplibhed by crystallization (conveniently from trichlorobenzene; the liquors can then be used as the solvent in step 4 ) . 4. Oxidation kvith nitrogen dioxide (no selenium required) of 400 grams of the crude 6-carbomethoxy-2-naphthaldehyde from step 3 in 2 liters of trichlorobenzene is conducted a t 100' to give 390 grams of 6-carbomethoxy-2-naphthoic acid. 5. Eaterification (2) of the 6-carbomethoxy-2-naphthoic acid, as in step 2, to give the dimethyl-2,6-naphthalenedicarboxylate in an over-all yield of 85 to 90 mole yG. Acknowledgment
T h e authors thank the Sun Oil Co. for permission to release this paper. \$'e albo express our gratitude for the able assistance of M:. R. Cherry and many technicians, especially R \V. Shinn and D. L. Kerr. References
(1) Bradbrook, E. F., J . Chem. Soc. 1936, p. 1739. (2) Convery, I
