Effectiveness and stability of commercial Pd/C catalysts in the hydrodechlorination of meta-substituted chlorobenzenes

Abstract

The catalytic properties of commercial Pd/C catalysts containing 3%, 5% and 10% palladium were studied in the hydrodechlorination of five meta-substituted chlorobenzenes. The morphology and structure of the catalysts were investigated by TEM, selected area electron diffraction (SAED), X-ray powder diffraction (XRD) and hydrogen chemisorption. TEM data showed the Pd particles to be distributed uniformly on the carbon surface, their mean size increasing from 2.8nm to 3.3nm and 5.3nm with the Pd loading rising from 3 to 5 and 10%. The mean particle sizes estimated from H2 chemisorption were greater than those from TEM and it were equal to 4.3, 5.5 and 12.1nm, respectively. Suppressed H2 chemisorption was ascribed to carbon contamination of the Pd phase. The hydrogenolysis kinetics of 3-chloroaniline, 1-chloro-3-fluorobenzene, 3-chloroanisole, 1,3-dichlorobenzene and 3-chlorotoluene was investigated in the presence of these catalysts in n-heptane–alkaline aqueous solution at 60°C and atmospheric pressure of hydrogen. The main reaction products were dechlorinated ones. With all catalysts the initial rates of hydrogenolysis increased in the following sequence: 3-chlorotoluene, 1,3-dichlorobenzene, 3-chloroanisole, 1-chloro-3-fluorobenzene and 3-chloroaniline, and for the most active 10% Pd/C catalyst, corresponding TOFs values were equal to 1.04, 1.12, 1.23, 1.32 and 1.61 (s−1), respectively. Additionally, the stability and effectiveness of the Pd/C catalysts were tested in the presence of aq NaOH or without base, using 3-chlorotoluene as the model compound. Under basic conditions, the 3% Pd/C catalyst was found to be the most stable, but without base the 10% Pd/C catalyst retained its activity for longer than the 3% and 5% Pd/C catalysts. The formation of HCl, as well as leaching of Pd and formation of biphenyls—by-products of the hydrogenolysis reaction, were probably the main reasons for catalyst deactivation.