studies on oxidation of anthracene

Abstract

Anthraquinone is an important dye intermediate. The demand for anthraquinone in India is likely to go up in the next few years, making its manufacture promising. Among the various methods, vapour phase catalytic oxidation of anthracene appears to be the best for Indian conditions. This thesis embodies the work carried out on the vapour phase catalytic oxidation of anthracene to anthraquinone, with a view to elucidating the kinetics and mechanism of the reaction. A detailed thermodynamic analysis of the possible reactions in the vapour phase oxidation of anthracene was carried out. From the values of the equilibrium constants evaluated for the temperature range of 100°–400°C, it was found that all these reactions are feasible. The literature survey on the vapour phase oxidation of anthracene revealed that by a judicious combination of catalyst and operating conditions, it could be possible to obtain mainly anthraquinone with negligible by?products. From the catalysts recommended, vanadium pentoxide and cobalt molybdate were chosen for the present study. Experiments were conducted in a bench?scale isothermal flow reactor, and the effect of the following variables on rate and conversion was studied: time factor (W/F), temperature, mole ratio of oxygen to anthracene, partial pressure of anthracene and partial pressure of oxygen. It is seen that conversion increases with time factor, mole ratio, and temperature. Good yields of anthraquinone with negligible amount of by?products were obtained in the temperature range of 270° to 360°C with vanadium pentoxide catalyst and in the range of 280° to 340°C with cobalt molybdate catalyst. The rate of reaction increases with the increase in partial pressures of anthracene and oxygen. The results were analysed for elucidating the kinetics of the reaction according to Langmuir–Hinshelwood, Rideal and redox mechanisms and empirical equations. Fifteen different models were tested. The two?stage redox mechanism was found to explain the experimental data better than other models. The final model discrimination was carried out by the non?intrinsic parameter method. The redox mechanism was discussed in detail. The two?stage redox mechanism—the substance to be oxidized reduces the catalyst which, in turn, is oxidized by the oxygen from the feed—corresponds to the following rate equation: rate of reaction [mole of anthracene/(gm)(min)] where, Pg = partial pressure of anthracene [mm Hg] Po? = partial pressure of oxygen [mm Hg] k? = rate constant for catalyst reduction step [moles/(gm)(min)(mm Hg)] k? = rate constant for catalyst reoxidation step [moles/(gm)(min)(mm Hg)] In the range of variables studied, the order of the reaction was found to be one with respect to the partial pressure of anthracene and oxygen, for both the catalysts. After establishing the order and mechanism of the reaction, the rate constants k? and k? and the activation energies E? and E? for the reduction and reoxidation steps, respectively, were evaluated for both the catalysts by linear and non?linear least squares methods. The reparameterized temperature values were used in the evaluation of activation energies. In the case of both the catalysts studied, it was observed that the rate constant for the catalyst reoxidation step was several times smaller than the rate constant for the catalyst reduction step, at any temperature level. Thus, it appears that the catalyst reoxidation is the rate?controlling step in the vapour phase oxidation of anthracene to anthraquinone. This is in accordance with general observations made by many investigators. With vanadium pentoxide catalyst, in the temperature range of 270–330°C, the activation energy was nearly halved compared to that for the temperature range above 330°C. This behaviour was attributed to the change in the values of diffusion coefficients of the oxygen vacancies on the catalyst or to a possible disorder in the lattice structure. The development of, and the studies with, a reactor which combines the advantages of back?mix and plug?flow reactors were carried out. The ratio of the weights of the catalysts operating in these two regimes, under adiabatic conditions, was optimized to yield maximum conversion.