Liquid Phase Catalytic Hydrogenation Of Dichloroacetic Acid And Tetrabromobisphenol-A Over Pd Catalysts Supported On Modified Carbon Nanotubes

Dichloroacetic acid is a typical halogenated disinfection by-product. Owing to its hydrophilic and non-volatile properties, and high cancer risk, dichloroacetic acid has threatened the safety of drinking water. Tetrabromobisphenol-A is widely used as a brominated flame retardant, with the properties of bioconcentraiton and refractory against biodegradation. It had been easily found in environmental media. As an effective and clean water treatment technique, liquid phase catalytic hydrogenation has been widespread used for the removal of halogenated compounds and inorganic substances. Because of the large surface area, unique porous structure, stable mechanic properties, carbon nanotubes have widely used in catalytic field. They can be used as a catalyst support in the liquid phase catalytic hydrogenation system. Modification can optimize the performance of the carbon nanotubes and improve the activity of catalyst.Carbon nanotubes were modified by chitosan via a controlled surface-deposition and crosslinking method, and supported Pd catalysts with CS-CNTs and CNTs as the supports were prepared by the reduction method with NaBH4. The catalysts were characterized by Elemental analysis, ICP, TEM, XRD and XPS. The results indicated that carbon nanotubes were modified by chitosan successfully. After the modification, the PZCs increased and Pd particles were well-distributed due to the stronger metal-support interaction.The properties of support, Pd particle size, initial pH and concentration could influence the liquid phase catalytic hydrodechlorination of dichloroacetic acid. The catalyst activity was significantly improved after the support modification. The hydrodechlorination of dichloroacetic acid followed the Langumuir-Hinshelwood model, indicating that the hydrodechlorination was controlled by the adsorption of dichloroacetic acid on the catalyst surface. With increasing of solution pH, the existence form dichloroacetic acid and surface charge characteristics of catalyst changed. Hence, the reaction rate first increased, and then decreased with pH. In addition, Pd particle size was also an important factor influencing the catalytic activity. With the increasing of Pd loading amount, Pd particle size increased gradually, leading to the enhanced H2activation ability, but to declined C-Cl activation ability. Accordingly, the initial activity firstly increased and then decreased with Pd particle size.Pd(x)/CNTs exhibited a good catalytic activity for the catalytic hydrogenation debrominaion of tetrabromobisphenol-A. The catalytic hydrogenation of tetrabromobisphenol-A also followed the Langumuir-Hinshelwood model. The initial activity firstly increased and then decreased with the increasing of Pd loading amount.