Characterisation of catalysts and their precursors prepared from supportedpalladium phosphine complexes

Hydrogenation catalysts prepared from the silica-supported binuclear pallad ium complexes [Pd2X4(PR3)(2)] (X = Cl, Br or I, and R = Me, Et and, for X = Br only, Pr-i and Bu) and mononuclear complexes [PdX2(PR3)(2)] (X = Cl, Br or I, and R = Me, Et) have been examined throughout their life cycle (indu ction period, selectively active catalyst and deactivation) by XPS, solid s tate P-31 NMR spectroscopy and transmission electron microscopy, and their performance and activities compared to a simple Pd/SiO2 catalyst. The binuc lear chloro complexes partially react with the silica surface producing som e Pd(0) species. The others retain their integrity until contacted by the s ubstrate and hydrogen. The selective solution hydrogenation of cinnamaldehy de to hydrocinnamaldehyde appeals to take place at oxidised palladium sites , most likely dipalladium(I) monophosphine species. The formation of phenyl propanol by some of the catalysts, on the other hand, occurs at Pd(0) sites . The induction period before the catalysts become active results from an a utocatalytic process during which the Pd(II) precursors are converted to th e active, Pd(I) or Pd(0) species. The catalysts are finally deactivated by a combination of continued conversion of the active species to a new, phosp horus-poisoned Pd(0) material and the restructuring of a hydrocarbonaceous overlayer. (C) 2001 Elsevier Science B.V. All rights reserved.