Liquid-Phase Oxidation of Anthracene in Acetic Acid with an Oxygen/Nitric Acid System

J. Chem. Tech. Biotechnol. 1990, 49, 5544

Liquid-phase Oxidation of Anthracene in Acetic Acid with an Oxygen/Nitric Acid System

C. K. Das & N. S. Das

Central Fuel Research Institute, PO FRI, District Dhanbad, Bihar PIN 828 108, India

(Received 18 September 1989; accepted 24 November 1989)

ABSTRACT

Oxidation of anthracene in acetic acid by the oxygenlnitric acid system has been studied and an attempt has been made to ascertain a practical limit of the amount of solvent keeping the commercial prospects in view. The effects of other reaction parameters such as flow rate, amount of nitric acid and water, residence time, etc., have been investigated. While opting for a solvent1 substrate ratio of 10, the optimum conversion of anthracene to anthraquinone fiee from nitro-compounds has been found to be 91 with purity of 98.7%, acceptable to dye-stuff industries.

Key words: anthracene, anthraquinone, oxidation in liquid phase, oxygen, nitric acid.

1 INTRODUCTION

Oxidation of anthracene in the liquid phase by nitric acid is a thermodynamically favourable reaction. Several attempts have been made earlier to develop the process, using this comparatively low-priced yet strongly oxidising agent to explore commercial viabilities.'" Oxidation with nitric acid alone is a stoichiometric reaction. Very dilute acid at lower temperature requires a long induction period, while with the use of strong nitric acid, the product purity may be affected, due to side reactions, such as nitration, which may also affect the overall economy of the process.'S2 However, nitric acid as co-oxidant with airloxygen has been found to be useful in oxidising anthra~ene.'.~.~The concentration of the acid requires proper adjustment to suppress the formation of undesired by-products and accordingly, an optimum limit has been suggested previously by the present authors.* In spite ofthe fact that the synthesis involves mild reaction conditions and simple techniques, thus

55 J. Chem. Tech. Biorechnol. 0268-2575/90/$03.50 0 1990 Society of Chemical Industry. Printed in Great Britain 56 C. K. Das, N. S. Das having commercial possibilities, the high solvent/substrate ratio has a limiting effect not only on the capacity of the reactor but also on the size of the solvent recovery unit. In an attempt to further reduce the amount of solvent needed, the present investigation has been undertaken to optimise the process, setting a practical limit to the minimum level of solvent required, without significantly affecting the yield and purity of anthraquinone.

2 EXPERIMENTAL 2.1 Materials Same as used earlier.' Nitric acid of sp. gr. 1.42 (Analar, IDPL, India) was used as a dilute solution in aqueous acetic acid. 2.2 Procedure Experiments were carried out following the same procedure as stated earlier.' A 10-20cm3 solution of nitric acid was used with pure oxygen gas (flow rate 034.0dm3h-') for the oxidation of log anthracene (of 92% purity) in 70-180 cm3 acetic acid at 95°C. Dropwise addition of HNO, solution (in the course of 20 min) was accompanied by brown fumes and anthraquinone began to separate out either from the clear solution or from the suspension after an interval of 40-60 min. In order to study the quality of the product commercially attainable, the reaction was discontinued after a definite period and the solid was collected as in Method I.' For the rest of the experiments, total anthraquinone was extracted out with water (Method II).' 2.3 Analysis The amount of anthraquinone present in the product mixture was analysed following the analytical method described earlier.' Products obtained by Method I were also analysed according to the I.S.I. specification No. IS: 6259, 1971.9 Lassaigne's sodium fusion test was carried out to detect the elemental nitrogen in the products. IR spectra (4Oo(MOO cm-') were recorded for the product samples responding to the nitrogen test, and the nitrocompounds were estimated quantitatively as relative peak area (%) at 1517 cm-' using 9-nitroanthracene (Purum, Fluka AG, Switzerland) as reference with the help of a supported software 'Quant' (Perkin Elmer 1720 FTIR).

3 RESULTS AND DISCUSSION

Conversion of anthracene to anthraquinone and purity of the products have been calculated as: 178 X Conversion ( %) = -. -. 100 208 Y X Purity (%)=loo- A Liquid-phase oxidation of anthracene 57

where A=the total amount of the product, X = amount of anthraquinone in A, Y=amount of anthracene in the feedstock.

Experimental data were subjected to the standard regressional analysis (linear, exponential, logarithmic, power and polynomials) applying the least squares method, and best possible curves were drawn to show the dependence of the oxidation reaction on the variables. The values of the coefficient of determination (R2)were supplied along with the individual curves (Figs 1-7). Complete dissolution of the reacting material/intermediate(s) and gradual separation of anthraquinone as fine crystals from the clear solution after a fixed interval, as observed with high solvent/substrate ratio, appear to have a direct implication for the quality of the product. High solvent ratios hinder the formation of nitrocompounds as well as facilitating the oxidation reaction to a greater degree in the liquid phase. A decrease in the amount of solvent would necessarily disturb the course of the reaction. With a minimum solvent/substrate ratio of 10, the dissolution phenomenon is still observed, but anthraquinone is precipitated in a bulk after a very short time (Table 1). This results in an increase in yield with less selectivity (cf. Ref. 8). With less solvent, the waterextracted products (by Method 11) have been found to be contaminated with nitrocompounds to an extent of 2-2.5% (Table 2). The dependence of the product quality on the amount of solvent is shown in Fig. 1. The maximum yield and conversion are found at solvent/substrate ratio around 10 and this limiting factor has been maintained while carrying out all other experiments. Figures 2 and 3 show the effects of the amount of nitric acid on the oxidation reaction at different flow rates of oxygen. At lower oxygen flow rate, both conversion and purity depend on the amount of nitric acid added. Considering the fact that the oxidation reaction does not proceed in the absence of nitric acid,'**the curves indicate that nitric acid probably competes with oxygen as oxidant. A higher rate of oxygen supply is accompanied by an enhancement of the reaction rate but selectivity decreases with increasing amount of HNO, beyond a limit of 0.5 mol mol- ' of anthracene. Here, nitric acid may act simply as an initiator or, alternatively, a probable loss of acid as oxides of nitrogen from the system may account for this. The dependence of the product quality on the oxygen flow rate at different concentrations of nitric acid is depicted in Figs 4 and 5. When the amount of nitric acid added is less, the oxidation reaction proceeds gradually towards completion with the increasing amount of oxygen. At higher nitric acid concentrations, the product quality is less dependent on the rate of oxygen supply, and the high conversion rate indicates the preferential oxidative attack on anthracene rather than the nitration side reaction. The absence of nitro-groups in these products as revealed from IR spectra (Table l),supports this observation. At the optimum flow rate of oxygen, 1.5 dm3 h-', the practical limit of HNO, can well be taken as 0.5 mol mol- of anthracene (Figs 3 and 5). A series of experiments at low and high oxygen flow rates was carried out varying TABLE 1 Effects of Various Reaction Parameterson the Oxidation of Anthracene to Anthraquinone in the Liquid-phase (Anthracene,10 g; Temperature, 95°C and Reaction Time 2 h)

Experiment Nitric acid Water Acetic Flow Dissolution Total Anthraquinone Amount of number (mol mol-’ (cm3) acid rate before product, in A nitro-compound anthracene) (cm’) (dm’ h-’) anthraquinone A (9) (%I separation (9)”

la 0-5 1 4-0 74.0 1.5 108 9-09 2-4 b 0.5 1 4-0 84-0 06 108 9.02 2.1 C 0-51 4-0 94.0 1.5 11.3 1047 < 05 d 0.5 1 4-0 94.0 1.5 109 1034 < 05 e 051 4.0 1040 06 1Q4 9.4 - f 051 4.0 124.0 1.5 11.0 1019 B 051 4-0 1640 1.5 9.75 9.33 h 051 4-0 184.0 1.5 101 9.62 2a 0.225 45 94.5 0.3 101 5.96 1-2 b 0225 4.5 94.5 0.6 103 6 80 - C 0 225 4.5 94.5 1.5 109 8.63 d 0.225 4.5 94.5 2.2 108 9.02 e 0.225 4.5 94.5 4-0 11.18 1006 3a 0.5 1 4.0 94.0 0.3 100 7.1 b 0.5 1 4.0 94.0 0.6 103 8.99 C 0.5 1 4-0 94.0 1.1 108 9.54 d 0.51 4-0 94.0 2.2 11.4 1056 e 0.5 1 4.0 94.0 3-0 11.3 1066 f 051 4.0 94.0 4-0 11.28 10.19 4a 0.5 1 98.0 4.0 10.0 6.4 2.0 b 051 - 98.0 0.6 102 6.73 2.7 C 051 9.0 89.0 4.0 11.4 9.87 051 d 9.0 89-0 0.6 104 8.4 e 0.51 18.0 80.0 4.0 1005 145 5a 0765 3.5 93.5 06 108 9.53 b 0.765 3.5 93.5 1.5 11.01 1012 C 0.765 3.5 93.5 2.2 11.00 1032 a Products extracted with 150 an3 of water (Method 11):

Wvl TABLE 2 Product Profile, as Dependent on Reaction Time, of Anthracene Oxidation (Conditions: Anthracene, log; HNO,, 05 mol mol-' of Anthracene, Water 4% (v/v) Acetic Acid, 94 cm3 and Oxygen Flow Rate, 1.5 dm3 h-')

Experiment Reaction Dissolution Method Total Anthraquinone Total Conversion Amount of number time before of product A anthraquinone (%) nitro- (min) anthraquinone extraction A" (9) none compound separation (9) (%) (%)

6a 40 I1 10.0 7.13 71.5 66-1 2.0 6b 60 I1 9-54 8.81 92.4 82-0 I1 1.8 lc 120 + 11.3 10.47 92.7 97.4 < 05 Id 120 + I1 10.9 10.34 94.5 96.2 < 0.5 6c 180 + I1 11.4 10.56 92.6 98.2 7a 60 I 8.1 8.04 99.3 74.8 7b 90 + I 8.9 8.8 98.9 81.9 7c 120 + I 9.25 9.2 99.5 85.6 7d 180 + I 9.9 9.77 98.7 909

Products recovered either as such from the suspension (Method I) or extracted with 150 cm3 of water (Method II).' Liquid-phase o.uidation q/ atithrucetre 61

Fig. I. Effect of the quantity of solvent on the oxidation of anthracene (10 9).Isolation by Method 11; temperature, 95°C; nitric acid (0.51 mol mol-' anthracene); 0, flow rate 1.5 dm3 h-' for 2 h: (O),"," purity; (A), :do conversion; (a),total product.

Nitric acid (rnol mol-' anthracene)

Fig. 2. Effect of the quantity of nitric acid on the oxidation of anthracene at O2 flow rates (--), 0.6dm3 hK' and (---), 2.2dm3h-'; (0).',; purity; (A),74 conversion; R2=l.00. the water content in the HNO, solution. Figure 6 indicates that a 5 % vol. ofwater is required to attain high yield and conversion and also to minimise the nitration side reaction (Table 1). The effect of time on the oxidation reaction is shown in Fig. 7. Although anthraquinone in such cases started to separate out at earlier stages (40-60 min), a reasonable conversion can be achieved only if the reaction is allowed to proceed for a longer period of about 2 h, beyond which the rate of oxidation remains unaffected. The possibility of nitro-compound formation at early stages of the reaction cannot be ruled out, as nitro-groups are found to be present in the initial products, obtained by Method IT (Table 2). The fact that a constant purity is maintained irrespective of increase in conversion and product yield at different intervals by Method I (Table 2 62 C.K. Das, N.S. Das

100 -

-60 2 c -50 .:

-0

0 0.2 0.4 0.6 0.8 Nltric acid (mol mol-’ anthracene)

Fig. 3. Effect of the quantity of nitric acid on the oxidation of anthracene under optimum reaction conditions: (O),% purity; (A),% conversion; (a),total product.

0 1.0 2.0 3.0 4.0 Flow rate (dm3 h-’1

Fig. 4. Effect of oxygen flow rate on the oxidation of anthracene in the presence of (-), 0255 mol and (---), @765mol nitric acid per mol of anthracene: (O), % purity; (A),% conversion; (O),total product.

and Fig. 7) shows that anthraquinone precipitates out in preference to the nitrocompounds, if formed. Absence of any nitroderivatives in these products (as inferred from IR spectra), indicates that if the process is to be applied commercially, any nitroderivatives formed would remain in the acetic acid solution and would not interfere in maintaining the product quality desired by industry. Liquid-phase oxidation of anthracene 63

- I

0 >

I I I I 190 0 1.0 2.0 3.0 4.0 Flow rate (drn3h-l)

Fig. 5. Effect of oxygen flow rate on the oxidation of anthracene in presence of the optimum amount of nitric acid: (O),% purity; (A),% conversion; (a),total product.

Fig. 6. Effect of additional water on the oxidation of anthracene with nitric acid (0.5 mol mol-' anthracene) at O2 flow rates -, 4.0dm3 h-' and ---, 06dm'hK': (01,% purity; (A), 'X, conversion; R2= 1.00.

4 CONCLUSION

A practical limit of solvent/substrate ratio of 10 has been set for the liquid-phase oxidation of anthracene with oxygen in the presence of a controlled amount of nitric acid, which results in the production of anthraquinone in fairly good yield. The product obtained, under the reaction conditions O2flow rate 1.5-2.0 dm3 h-' (15- 20 dm3 dm-3 solvent h-'), nitric acid 0-5 mol mol-' of anthracene at temperature 95°C and duration 3 h, is free from nitroderivatives and a 91 % conversion of anthracene into anthraquinone of 98.7 % purity has been achieved. 64 C. K. Das, N. S. Das

- m

- a0

30 60 90 120 150 iao Time (rnin)

Fig. 7. Effect of reaction time on the oxidation of anthracene: (0).7; purity; (A),76 conversion; (0). total product; (---), Method I (no nitrocompounds detected); (-), Method I1 (nitrocompounds detected in the products obtained after times up to 2 h).

ACKNOWLEDGEMENT

The authors thank G. S. Murthy, A. Banerjee and R. Sen for IR data, V. A. Krishnamurthy for encouragement and the Director of the Central Fuel Research Institute for permission to publish.

REFERENCES

1. Das, N. S. & Das, C. K., Oxidation of anthracene in liquid phase. J. Chem. Tech. Biotechnol., 39 (1987) 183-99. 2. Morley, J. O., Reactions of substituted anthracene with nitric acid. J. Appl. Chrm. Biotechnol., 27 (1977) 143-9, and references therein. 3. Alexander, W. N. & Verona, N. J., US Patent 2 821 534, 1958. 4. Koppers Co. Inc., German Patent 2 143 210, 1972. 5. Greenhalgh, C. W., Aspects of anthraquinone dyestuff chemistry. Endeavour, 34 (126) (1976) 1349. 6. Peters, G., US Patent 1 455 448, 1923. 7. Olivier, K. L. & Gothlich, L. V. E., In Oxidation of Organic Compounds Part 11. Adv. Chem. Ser. 76, American Chemical Society, Washington, DC, 1968, pp. 382-7. 8. Das, N. S. & Das, C. K., Oxidation of anthracene to anthraquinone in liquid phase with an air/oxygen/nitric acid system. J. Chem. Tech. Biotechnol., 32 (1982) 643-9. 9. Specification for Anthraquinone. Technical IS: 6259-1971, Indian Standards Institution, New Delhi, India, 1972.