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

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Oxidation of Anthracene to Anthraquinone in Liquid- Phase with an Air/Oxygen/Nitric Acid System J. Chem. Tech. Bioiechnol. 1982,32,643-649 Oxidation of Anthracene to Anthraquinone in Liquid- Phase with an Air/Oxygen/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 dye-stuffs such as alizarin, 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 wood-pulp industry.3 The demand of anthraquinone (2500 t year-') may increase in the future. The conventional process for manufacturing anthraquinone depends on the petroleum-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- quinone 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 acetic acid, re- fluxed at 95°C and 20 cm3 of a solution containing conc. nitric acid (13~),acetic acid and water 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 phenanthrene and carbazole (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 nitrogen 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.
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