Characteristics of carbon supported palladium catalysts for liquid phase hydrogenation of nitro aromatics

Abstract

Liquid phase hydrogenation of dinitrotoluene over carbon supported palladium catalyst was investigated. Effects of support properties and catalyst preparation conditions on metal dispersion, metal distribution, and catalytic activity were examined. The oxygen-hydrogen titration cycles(OHTC) and pulse-wise oxygen titration have been employed for the determination of Pd dispersion. $^{129}Xe$ NMR spectroscopy can be a sensitive probe for the characterization of amorphous carbon support. Oxidative treatment of carbon support appeared to increase the number of surface oxygen groups, which were characterized by acid-base titrations, TPD of the decomposition products, and infrared spectroscopy. The Pd dispersion increased with the increasing amount of surface oxygen groups, especially more acidic groups, but the catalytic activity did not improve in proportion to the dispersion. The promotional effect of surface oxygen groups on metal dispersion can be explained by the interaction between metal precursor and carbon support in catalyst preparation stages. Deposition of the metal precursor by adsorption and precipitation may be a dominant step in deciding the final state of metal dispersion on the catalyst prepared. The more acidic groups are expected to give a major influence on the metal dispersion by providing nucleation sites and enhancing the hydrophilicity of the carbon surface. Oxidative treatment with nitric acid has been used for the preparation of an ion-exchangeable carbon. Protons of carboxylic groups on the oxidized carbon support could be exchanged with palladiumammine complex captions. Extremely high metal dispersions have been achieved by this ion-exchange method. The precipitation followed by liquid-phase reduction may yield the most efficient catalyst for liquid-phase hydrogenation which is frequently affected by pore diffusional limitations. In this case the catalytic activity tends to be dependent on the type of metal distribution throughout the...