| dc.description.abstract | In the present work, catalytic hydrogenation of salicyloyl chloride to salicylaldehyde by Rosenmund reduction using Pd-C catalysts was extensively studied. A stirred slurry reactor was used for studying this reaction. The experiments were carried out under isothermal conditions at atmospheric pressure.
In the liquid phase hydrogenation, the overall rate of the reaction is influenced by mass transfer across the phase interfaces and in the pore system of the catalyst. Since it is difficult to entirely eliminate these transport hindrances, the best way to study the kinetics of this system is to determine the substrate concentration directly or to estimate the concentration decrease due to transport effects. In the present work, the system was investigated by measuring the concentration of the aldehyde in the reaction mixture at regular intervals of time as it was not possible to measure the concentration of dissolved hydrogen in the reaction mixture.
In the initial experiments carried out to study the influence of temperature, the reaction rate was found to decrease with increasing temperature. To clarify this erroneous observation, in the subsequent experiments, the reaction mixture was first saturated with hydrogen before adding the salicyloyl chloride and it was observed that this trend reversed and the rate as expected increased with increasing temperature. In the initial experiments where the reaction mixture was not saturated with hydrogen, apparently it was the adsorption of hydrogen on the catalyst surface that controlled the reaction rate and not the hydrogen solubility.
The effect of gas-liquid mass transfer resistance on the rate of the reaction was studied by carrying out experiments in which the quantity of the catalyst was varied. All the other variables like temperature, hydrogen flow rate, stirring, initial concentration of salicyloyl chloride were maintained constant. From these experiments it is observed that the resistance due to gas-liquid mass transfer was not negligible. The estimated values of the gas-liquid mass transfer resistance for 10% Pd-C and 5% Pd-C were in the range 4.1143 - 5.0436 min?¹ and 3.4468 - 5.7830 min?¹ respectively.
The liquid-solid mass transfer coefficient was estimated from the correlation developed by Acres and Cooper. The concentration drop across the liquid-solid interface was found to be negligible compared to that across the gas-liquid interface. From the estimated values of the liquid-solid mass transfer coefficient and the slopes of the lines obtained by plotting the reciprocal of the reaction rate against the reciprocal of catalyst quantity, the intrinsic reaction rate constant r?? was estimated and was found to be in the range 0.2041-0.5455 cm/min for 10% Pd-C catalyst and 0.0610-0.1542 cm/min for 5% Pd-C catalyst. The apparent activation energy was 14.558 KCal/mole for 10% Pd-C and 24.248 KCal/mole for 5% Pd-C catalysts.
The resistance to pore diffusion was negligible for 5% catalyst while with 10% Pd-C catalyst it was found to increase with increasing temperature.
In the conditions where the reaction rate was maximum (highest temperature and catalyst loading) salicylaldehyde was further hydrogenated to o-cresol. Salicyloyl alcohol was not detected in any of these experiments indicating that salicylaldehyde might undergo direct hydrogenation to o-cresol.
A model in which the gas-liquid mass transfer and the surface reaction resistances influence the overall reaction rate was found to adequately represent the experimental data, wherein the predicted values agreed within ±5% with the experimental values in the range of conditions employed in the present study.
The overall rate of the reaction was given by the equation:
-r? = k? a · K C? i
where
k? a, g, k?, V, K | |
