Palladium supported on chitosan hollow fiber for nitrotoluene hydrogenation

Palladium catalyst supported on chitosan hollow fibers was used for the continuous catalytic hydrogenation of nitrotoluene into o-toluidine. The reaction took place at room temperature using hydrogen gas under atmospheric pressure (i.e., moderate operating conditions). Chitosan hollow fibers were crosslinked with glutaraldehyde prior to palladium sorption at pH 2. The reduction of palladium immobilized on the fiber was performed using hydrogen generated in situ. The influence of palladium content, nitrotoluene concentration, contact time (or flow rate), hydrogen pressure (around atmospheric pressure 0.25-1.25 bar) and solvent has been tested using the conversion yield and the turn-over frequency as the process criteria. Hydrogen pressure (in the carried out range) had a weak impact on catalytic activity, contrary to palladium content and temperature. The palladium content had a strong impact on sorption performance due to catalytic metal availability and its impact on the size of Pd nanocrystals and on diffusion properties. A number of complementary techniques (TEM, SEM, ESEM, SEM-EDAX, X-ray diffraction, XPS, BET surface analysis, diffusion....) were used for the characterization of Pd-catalysts supported on chitosan hollow fibers. These techniques were used at the different stages of the preparation of the fibers (cross-linking treatment, metal sorption, Pd reduction,...). These treatments may control the diffusion of reagents, hydrogen donor as well as Pd catalyst availability and reactivity, which in turn may affect the catalytic activity. A significant mesoporosity was observed at around 10 nm; after fiber modification (cross-linking, metal sorption or Pd reduction) a supplementary porosity (around 25 nm) was observed due to the deformation of porous structure. High-specific surface were detected around 150-200 m(2) g(-1). Increasing Pd content revealed defavorable for the diffusion of a calibrated molecule (vitamin B12), probably due to a limitation of pore size and to a weak transient sorption of the target molecule. Increasing metal content also increased the size of Pd crystals as evidenced by TEM and X-ray analysis. A compromise should be found for maximum reactivity and optimal use of the catalytic metal in relation with the size of Pd crystals, and diffusion properties. These characteristics may explain the differences observed in the catalytic properties of fibers prepared with different metal contents. (c) 2008 Elsevier B.V. All rights reserved.