Liquid phase catalytic hydrodechlorination of 2,4-dichlorophenol over Pd/Al2O3: Batch vs. continuous operation

The three-phase (atmospheric pressure, T = 303 K, bulk solution pH 12) catalytic hydrodechlorination (HDC) of 2,4-dichlorophenol (2,4-DCP) over Pd/Al2O3 has been studied in batch and continuous flow reactors. In both cases, 2,4-DCP yielded 2-chlorophenol and phenol as products of partial and complete HDC, respectively. Cyclohexanone, resulting from further ring hydrogenation, was also isolated but was formed with low selectivity (≤4%). Conditions have been established in both reactors where HDC proceeds under catalytic control. A move from discontinuous to continuous operation resulted in a more efficient (five-fold) H2 transfer, which can be attributed to an extended gas/liquid interface available for mass transport. Higher HDC rate and increased selectivity to phenol were obtained in the continuous flow reactor, which we attribute to a lower pH at the liquid/solid interface relative to batch operation. Furthermore, the productive catalyst lifetime was extended in the flow reactor due to the more effective removal of HCl from the catalyst surface. The results presented establish the feasibility of continuous, liquid phase HDC under mild conditions as a means of treating environmentally toxic 2,4-DCP.