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Home , Anthracene, Anthraquinone

J. Chem. Tech. Bioiechnol. 1982,32,643-649

Oxidation of to in Liquid- Phase with an Air//Nitric Acid System

Chandra Kumar Das and Nikhilendu Sekhar Das

Central Fuel Research Institute PO FRI Dist. Dhanbad, Bihar, PIN, 828108, India

(Paper received 17 November 1981 and accepted 26 February 1982)

Oxidation of anthracene to anthraquinone by air/oxygen in the presence of small amounts of nitric acid was studied in the liquid phase. The effects of various process variables (e.g. temperature, flow rate, amount of nitric acid and solvent substrate ratio) were investigated. An attempt was made to minimise the amount of nitric acid. The optimum conversions of anthracene into anthraquinone free from nitro comp- ounds, using air and oxygen was found to be 81.7 and 96.2% respectively with corresponding purities of 98.5 % and 99.6 %.

1. Introduction Anthraquinone is a very important intermediate for the manufacture of various -stuffs such as , indanthrene, etc, having all round fastness, brightness and a wide range of colour shades.1.2 Quite recently anthraquinone has gained an entry into the - industry.3 The demand of anthraquinone (2500 t year-') may increase in the future. The conventional process for manufacturing anthraquinone depends on the -derived feedstock. In the context of increasing prices and uncertain availability of petroleum crude coal-tar processing industries have been receiving more attention. A vapour-phase catalytic oxidation taking anthracene of 92% purity (obtained from anthracene cake by a two-stage purification)6 as feed material has been developed in CFRI. Since commercial vapour-phase oxidation technology calls for a high degree of expertise in chemical engineering, an effort has been made by us to investigate liquid-phase oxidation involving mild reaction conditions for which published information is scanty.7-l2 Often,7*9-*1however, an excess of nitric acid has been used and this appears to be unsatisfactory because of the concomitant formation of nitro c0mpounds.1~The present work, in contrast, considered the oxidation of anthracene by air/oxygen in the presence of a little nitric acid, avoiding the formation of nitro compounds and minimising corrosion. A similar study was reported in 19238 but with few details. Further some improvements, particularly regarding the purity of the product, an essential requirement for its commercial application, (where a purity of anthra- 398.5 % is an essential requirement for its use in the dye industry) is also reported here.

2. Experimental 2.1. Materials used Anthracene (92 % purity), procured from CFRI pilot-plant runs for the two-stage solvent extraction of crude anthracene,6 was used throughout. All other chemicals used were of BDH AnalaR quality.

2.2. Procedure 2.2.1. Method I In a typical experiment 10 g anthracene was taken in a flask containing 180 cm3 of , re- fluxed at 95°C and 20 cm3 of a solution containing conc. nitric acid (13~),acetic acid and 0142-0356/82/0600-0643 $02.00 0 1982 Society of Chemical Industry 643 644 C. K. Das and N. S. DPS

(2:9:9, by vol.) was added dropwise to the well-stirred mixture, while air was bubbled through at the rate of 3.0 dm3 h-1. The addition took about 20 min, deep-brown fumes evolved and the mixture went into solution within 40 min from the start. The turbidity appeared within 1 hand yellow crystals of anthraquinone were soon precipitated. After the reaction was completed (normally within 4 h) the vessel was cooled and the product allowed to settle for 1 h. It was then filtered. The residue, washed three times with warm water, was dried and weighed. The filtrate contains and (originally present in anthracene) as such or in the form of reaction by-products in the filtrate. Anthracene and its by- products also remain in the filtrate.

2.2.2. Method II Since a part of the anthraquinone remains in solution, the above method does not give any quantita- tive measure of the conversion. However, if the process is to be applied to commercial production, then the recovery of a small amount of anthraquinone from solution will not be justified as the solvent must be recycled/recovered. Thus the above method gives a precise idea of the conversion commercially attainable. However, for the study of the effects of reaction variables with a view to determining optimum reaction conditions, it is necessary to measure quantitatively the total amount of anthraquinone in the reaction products. For this, the experimental method must be slightly modified. Again, if the reactions are allowed to proceed for a longer period, the extent of conversion reaches such a high level that the effect of individual reaction variables becomes masked. Hence to obviate these difficulties the time of the reaction was confined to 2 h only, so as to arrest the degree of conversion. Further, with a view to measurement of the total quantity of anthraquinone present in the reaction products, the vessel was cooled and 150 cm3 of water was added. It was then left for 1 h to allow complete precipitation of anthraquinone. The rest of the procedure was as described in section 2.2.1. for method I.

2.3. Analysis The analytical method described by Nair and Ghosh4 was adopted here to estimate anthraquinone quantitatively in the product mixture. As deduced from i.r. data, the impurities associated with anthraquinone consist of anthracene mostly and nitro compounds (only in cases with an excess of nitric acid) and other unidentified compounds.

3. Results and discussion The oxidation of anthracene by air or oxygen alone, (i.e. in the absence of nitric acid/catalyst) does not take place at all at normal pressure and at 80-1 10°C; nitric acid is essential to initiate the oxida- tion. For air oxidation of anthracene, the presence of about 0.5 mol of nitric acid mol-1 of anthracene was required to achieve a reasonable rate and yield. In contrast, in a atmosphere instead of air or oxygen (other reaction conditions remaining the same) the conversion was found to be much lower (figure 1, Table 1). The conversion, however, is dependent on the amount of nitric acid as it is the only oxidising agent. The stoichiometric amount of nitric acid required was about 2 mol mol-l of anthracene giving a conversion of only 81.3% even after 6 h of reaction. Besides, the product was invariably contaminated with nitro compounds (as detected from i.r. spectra), when the molar proportion of nitric acid and anthracene was about 1.5. Conversely, oxidation was fast and the conversion and product purity recorded marked improvement with only half the amount (i.e. 0.75 mol of nitric acid) when nitrogen was replaced by air. Further, when oxygen was used, much smaller amounts of nitric acid (about 0.5 mol mol-l of anthracene) were found to be sufficient for the attainment of fast complete conversion, with a high degree of product purity (Figure 4, Table 2). Since the rate of the reaction tends to be very fast when the proportion of nitric acid in the system exceeds 0.5 mol (mol-1 of anthracene present), this limiting factor was maintained in all experi- T

00

30

30 70 - -2? at” aa 50

40 Figure 1. Effect of the quantity of nitric acid on the oxidation of anthracene to anthraquinone. 0, In nitrogen; A, in air; 30 0,in oxygen: -, % conversion: - - -, 7i purity. !O

Nitric acid (mol m0L-I of anthracene)

Table 1. Effects of various reaction variables on the oxidation of anthraccne to anthraquinone

Anthracene Anthra- converted Nitric acid quinone Anthra- into (mol in quinone anthra- Expt mol-1 of Water Solvent Temp. Flow rate Products product in (A) quinone no. anthracene) (cm3) (cm*) (“C) (dm3 h-l) (g) (A) (A) (%) (l3) ( %)

la 0.5 9 189 95 3.0 9.0 26.7 2.4 b 1 .o 8 188 95 3.0 8.3 80.4 6.7 C 1.5 1 187 95 3.0 9.0 79.7 7.2

2a 0.2(5) 9(.5) 189(.5) 95 3.0 9.0 53 .O 4.7 b 0.5 9 189 95 3.0 9.0 78.0 7.0 C 0.7(5) 8 (.5) 188(.5) 95 3.0 9.5 81.o 8.3 d 1 .o 8 I88 95 3.0 9.9 85.7 8.5 e I .5 1 187 95 3.0 10.9 76.3 8.3 77.1 3a 0.2(5) 9(.5) 189(.5) 95 1.5 8.8 89.3 7.9 b 0.5 9 I89 95 1.5 10.3 94.2 9.7 C 1 .o 8 188 95 I .5 9.7 97.3 9.4 d 1.5 7 I87 95 1 .5 10.4 93 .O 9.1 4a 0.5 9 I89 95 0.7 9.0 93.2 8.4 b 0.5 9 I89 95 3.0 10.4 94.4 9.8 5a 0.5 9 I89 95 1 .5 8.2 77 .O 6.3 b 0.5 9 189 95 4.2 9.0 78.0 7.0 C 0.5 9 I89 95 5.6 9.7 74.1 7.2

6a 0.5 9 I89 85 3.0 8.5 75.0 6.4 b 0.5 9 I89 I05 3.0 9.0 14.0 6.6

7a 0.5 - 198 95 3.0 8.3 70.3 5.8 b 0.5 18 I88 95 3.0 9.3 71 .O 6.6 61.3 8a 0.5 9 139 95 3.0 9.4 64.0 6.0 55.8 b 0.5 9 89 95 3 .O 9.7 60.0 5.8

Amount of anthracene (92% pure)= 10 g. duration=2 h. Method I1 was used for analysis. Table 2. Synthesis of anthraquinone through oxidation of anthracene in the liquid phase

Conversion of anthracene Anthracem HNOs (mol Purity of into Expt (92 % pure) mol-' of Acetic acid Water Flow rate Duration Products anthraquinone Anthraquinone anthraquinone no. (9) anthracene) (cm9 (cms) (dm3 h-1) (h) (g) ( %) (9) ( %)

1 10 1.5 187 7 3 6 9.8 89.1 8.7 81.2 (Nitrogen)

2 10 0.7 (5) 188 (.5) 8 (.5) 3 4 8.9 98.5 8.8 81.7 (Air)

3 10 0.5 189 9 3 4 8.7 88.0 7.7 71.5 (Air)

4 100 0.5 I890 90 30 3 106.0 99.8 105.7 98.2 (Oxygen)

5a 100 0.5 I890 90 15 3 104.0 99.6 103.6 96.2 (Oxygen) b 100 0.5 1910 I5 3 105.3 98.5 103.7 96.3 (Used solvent (Oxygen) +60 cmS of acetic acid fresh) C 100 0.5 1910 30 I5 3 95.0 91 .O 86.4 80.3 (As above) (Oxygen)

Method I was used for analysis. Reaction temperature =95"C. Oxidation of rnthrncene to anthnquinone 647

Figure 2. Effect of temperature on the oxidation of anthracene to anthraquinone in air. -, % conversion; - - -, % purity.

Temperature ("C 1

ments, particularly involving air oxidation. By taking recourse to this precaution, it was possible to determine the effect of individual reaction variables on the oxidation. The reaction was studied at 85,95 and 105°C; the optimum reaction temperature was found to be 95°C (Figure 2, Table 1). The effect of the amount of the nitric acid has already been discussed. Further, to decide on the optimum amount of nitric acid required, several factors including purity were also considered. The amount was kept as low as possible thereby eliminating the formation of nitro compounds and minimising corrosion. In fact, with 1.5 mol of nitric acid (mol-1 of anthracene), the nitro compounds were detected from i.r. spectra, even when air was used as a co-oxidising agent. Therefore, to eliminate the possibility of side reactions, the optimum amount of nitric acid was taken as 0.75 mol (mol-1 of anthracene) instead of 1.O mol, whereas for oxygen the optimum amount was found to be 0.5 mol (mol-l of anthracene) as shown in Figure 1. The flow rate of air was varied from 1.5 to 5.6 dm3 h-1. A flow rate of 3.0 dm3 h-1 was found to be optimum whereas 1.5 dm3 h-1 was found to be the optimum oxygen flow rate. The rate of oxidation was low at lower flow rates as a result of an inadequate supply of oxygen (Figure 3). However, a rapid increase in conversion was recorded when the flow rate doubled and it remained almost steady thereafter.

- loo -100 +--+------90

L 80 -60 2 0 -A------d------a- - - -1 f .a 5 Figure 3. Effect of flow 70 rate of air (A) and oxygen (0) on the oxidation of 6o anthraceneto anthraquinone. -, % conversion; - - -, purity. % 50 I I 1 1 I 50 0 I 2 3 4 5 6 Flow rote ldm3 h-'1

In a gasliquid reaction various factors (e.g. of the gas in the liquid phase, contact time, diffusion and bubble size) usually play an important role. An attempt was made to conduct the reaction in a vertical reactor, where the gas-liquid contact was better maintained. The absence of any improvement in the reaction rates was taken to indicate that mass transfer was not a limiting factor. The effects of impurities on the oxidation reaction was investigated. Addition of either small amount of azobis-isobutyronitrile or benzoyl peroxide did not improve the conversion significantly. Further, addition of had no great influence on the reaction. Nothing can in fact be con- 648 C. K. Das and N. S. Dm cluded as the conventional initiators and inhibitors behave in an anomalous manner in the presence of nitric acid.'* The 9,lO positions in anthracene, having the lowest atom localisation energy, are exceedingly reactive towards addition reactions and it may be possible that oxidation in these positions proceed via the formation of addition compounds such as 9-nitro- or 9-acetoxy-anthracene derivatives, etc. In fact such compounds have been isolated previ~usly.~~The oxidising entity in nitric acid in a solution is HzNOa+, NOz+ or Ntd08 and not Nos-. This explains why with soluble nitrates (NOS-), comparatively low conversions were obtained (Table 3). The ease with which these ions

Table 3. Effect of additives on the air oxidation of anthracene in the liquid phase at 95°C

Additive Purity of Conversion of (mol anthra- Anthra- anthracene Any other Expt mol-I of Products quinone quinone into anthra- conditions no. Metal salt/additive anthracene) (g) ( %) (9) quinone (%) specified

~ I Potassium nitrate ~0.5mol 9.8 52.3 5. I 47.4 No nitric acid of HNOs added 2a Benzoyl peroxide 0.01 8.3 75.2 6.2 57.6 b Azobis-isobutyroni- 0.01 8.9 71.3 6.3 58.5 trile (AIBN) c Phenol 0.02 9.0 75.6 6.8 63.3 3a Acetic anhydride 10 (ems) 8.3 73.8 6.1 56.7 Acetic acid ( 188 cmS), no water taken b Propionic acid - 7.6 74.3 5.7 52.5 In place of acetic acid

Amount of anthracene (92% pure)= 10 g. nitric acid=0.5 mol, flow rate of air =3.0 dms h-1, acetic acid = 189 cm3, water=9 cm3, duration=2 h. Method I1 was used. are formed in presence of a proton donorlBmay be one of the reasons why acetic acid is a better solvent media than other organic solvents such as propionic acid, chlorobenzene, dichlorobenzene and , etc. A solvent/substrate ratio of about 20:l was found to be optimum. This high solvent ratio, which may be due to the solubility limit of anthracene, limits the capacity of the reactor and calls for re- covery or reuse of the solvent. In oxidations with oxygen, it is possible to use the solvent at least two times, but subsequent reuse affects both conversion and quality. Therefore, the solvent needs to be recovered by distillation. Under optimum reaction conditions (i.e. with 0.75 mol of nitric acid mol-1 of anthracene, air flow rate 3.0 dm3 h-1, anthracene acetic acid water 1 :20:l (by vol.) at 95°C and 4 h) the conversion of anthracene into anthraquinone of 98.5% purity was 81.7%. When air is replaced by oxygen, the amount of nitric acid could be further reduced (up to 0.5 mol) and the reaction time can be reduced to 3 h without sacrificing either the yield or purity. In fact the yield increases (96.2 %) and the product purity is high (99.6%).

4. Conclusion Liquid-phase oxidation of anthracene with air and in the presence of controlled amounts of nitric acid under mild reaction conditions results in fairly good yields of anthraquinone, free from nitro compounds. However, with oxygen instead of air, the amount of nitric acid, reaction time, etc., can be further reduced while retaining high conversions (96.2 %) and purity (99.6 %). Oxidation of anthracene to anthraqulnone 649

Acknowledgments We thank G. S. Murty for i.r. data, G. Bhattacharjee for analytical data, P. N. Mukherjee and S. K. Ray for encouragement and the Director, CFRI, for permission to publish.

References 1. Venkatraman, K. (Ed.) The Chemistry of Synthetic Vol. 2, Academic Press, New York, 1952, pp. 803-881/ Vol. 3, Academic Press, New York, 1970, p. 391. _. Cireenhalgh, C. W. Endeavour 1976, 126, 134. 3. Trevor, I. Tenn. Chem. Eng. 1979, 26, 64. 4. Nair, C. S. B.; Ghosh, A. K. Proc. Symp. Chemicals and Oil from Coal CFRI, 1972, p. 339. 5. Internal Technical Report on Vapour-Phase Oxidation of Anthracene to Anthraquinone, CFRI. 1980. 6. Internal Technical Report on Pilot Plant Studies on Purification of Crude Anthracene, CFRI, 1980. 7. Alexander, W. N.; Verona, N. J. US Patent 2 821 534 (1958). 8. Peters, G. US Patent 1 455 448 (1923). 9. German Patent 2 832 13 (1913). 10. German Patent 2 341 468 (1974). II. I. G. Farbenindustrie FIAT 1313, Vol. 2, p. 19. 12. Olivier. K. L.; Gothlich, L. V. E. Adv. Chem. Ser. 76, Am. Chem. SOC.,Washington, DC, 1968, p. 382. 13. Morley, J. 0. J. appl. Chem. Biorechnol. 1977, 27, 143. 14. Trahanovsky, W. S. (Ed.) oxidation in Organic Chemistry Part C, Academic Press, New York, 1978. p. 295. 15. Barnett, De Barry Anthracene and Anthraquinone Bailliere, Tindell and Cox, London, 1921, p. 50. 16. Levitt, L. S. J. Org. Chem. 1955, 20, 1297.

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