Preparation Of Pd/CNTs Catalysts And Its Application In Hydrodehalogenation

Persistent organic compounds?POPs?are highly concerned due to difficult degradation,long-range migratory,bioaccumulation and high toxicity.In this paper,the liquid phase catalytic hydrodehalogenation of halogenated phenols and polychlorinated biphenyls?PCBs?was studied.The conclusions were summarized as follows:?1?Using activated carbon?AC?,carbon nanotube?CNTs?,aluminum oxide?Al2O3?and graphene?Gr?as supports,the Pd-supported catalysts were prepared and the catalytic hydrogenated performance was investigated in liquid phase using4-chlorophenol?4-CP?and nitrobenzene as reaction models.The results showed that Pd/CNTs exhibited the best catalytic hydrogenated activity.According to the catalyst characterization,it was found that?i?the mesopore structure of CNTs was advantageous to the diffusion and material mass transfer in the catalytic hydrogenation;?ii?Pd/CNTs had a small average Pd diameter and a narrow particle size distribution,which were possible main reasons to lead to highly catalytic hydrogenation performance in liquid phase compared with other prepared catalysts.?2?The catalytic hydrodechlorination?HDC?reactivity of 4-CP and the reaction kinetics were studied in the liquid-phase.The effects of reaction temperature,catalyst and NaOH were investigated.The optimized operating parameters included reaction temperature 40 ?,catalyst dosage 20 mg,molar ratio of chlorine atom and hydroxyl ion 1:1.1 were obtained.The kinetic equation of the HDC reaction of 4-CP was a zero-order kinetic model.The activation energy Ea was 40.12 kJ·mol-1andpre-exponential factor A was 1.66×107mol·L-1·min-1.?3?The hydrodehalogenation reactivity of 4-halogenated phenols was studied.The reaction rates showed the following order of 4-bromophenol?4-BP?> 4-CP >4-iodophenol?4-IP?> 4-fluorophenol?4-FP?.It was found that the strong adsorption of Iodide anion?I-?on the surface of the catalyst possibly led to the decline of catalytic activity of Pd/CNTs in the hydrodeiodination in aqueous solutions.?4?5% Pd/CNTs-PVP was prepared by Polyvinylpyrrolidone?PVP?as non-covalent modifier.The catalyst characterization showed that the particle sizes of PVP modified Pd/CNTs was decreased compared with non-PVP modified Pd/CNTs.The degree of dispersion in aqueous solutions was obviously strengthened.Compared with 5% Pd/CNTs and Pd/AC,Pd/CNTs-PVP showed the highest catalytic activity in the HDC of 4-CP.?5?The mechanism of the HDC of 2,4-dichlorophenol?2,4-DCP?over Pd/CNTs-PVP was investigated.The degradation pathway of 2,4-DCP was step-by-step degradation.The velocity-determined step was the adsorption and activation of hydrogen on the surface of palladium nanoparticles during the catalytic HDC of 2,4-DCP,which was a slow reaction step;the reaction between 2,4-DCP and activated hydrogen was a quick step.?6?Pd/CNTs-PVP was used in the catalytic HDC degradation of transformer oil-contained PCBs.The effects of four different types of proton solvents?methanol,ethanol,isopropanol,ethanol-water mixed solvent system?on the HDC of transformer oil-contained PCBs were investigated.The results showed that the HDC rate increased with the increase of the solvents polarity.In the ethanol-water system,PCBs could effectively be degraded by Pd/CNTs-PVP under normal conditions with H₂.The HDC rate of PCBs with NaOH and Et₃N as base was also studied.Both of them could effectively remove the toxic effects of HCl to the catalyst.The HDC rate of PCBs was faster in the presence of Et₃N.