Reduction of nitrates dissolved in water over palladium-copper catalysts supported on a strong cationic resin

Catalysts containing 4 and 1 wt.% of palladium and copper, respectively, supported on cationic resin (DOWEX I x 4, gel type poly(styrene-co-divinylbenzene) with -N(CH3)(3)Cl-+(-) groups, particle size 100-200 mesh) were prepared and tested in the liquid-phase hydrogenation of nitrates in water. The catalyst was prepared by reduction of anionic chloro complexes of palladium(II) and copper(II) immobilized in the resin. Different reduction agents were employed, namely sodium borohydride in ethanol, or in water, and dihydrogen in either water (0.1-0.5 MPa) or methanol (0.5 MPa) solution of Na2CO3 (2.5%, w/w). The characterisation by means of X-ray microprobe analysis (XRMA), X-ray powder diffraction (XRPD) and transmission electron microscopy (TEM) revealed the presence of palladium metal nanoclusters and copper compounds. A cationic resin as the support was expected to facilitate the transport of the nitrate anions into the catalyst in comparison to previously employed anionic resins. This was fully proved by the generally higher specific activity of Pd-Cu catalyst supported on cationic resin. The most active and relatively stable catalyst was obtained by reduction of the immobilized metal precursors with H-2 in aqueous Na2CO3 at 0.5 MPa. The catalysts exhibited different stability with respect to leaching of metals under duty. Owing to the mechanical stirring of the reaction mixture, the catalytic particles were gradually ground and handling the reaction mixture with the suspended catalyst became rather problematic.