Effect of the anchorage of Rh(III) ions by imidazoline type ligands on the location of Rh0 particles within the calcined form of the SBA-15 support

Two series of mesoporous SBA-15 silica materials with N-propyl-4,5-dihydroimidazole groups have been prepared. It is shown that up to 15% of the silicon atoms can be provided by the organosilane precursor, N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole (Si-Imi), either by one-pot (SBA-OP x%) or post-synthesis (SBA-GR x%) procedures. The hybrid solids were characterised by elemental analysis, XRD, TEM, nitrogen sorption isotherms, 29Si or 13C NMR, FTIR and UV–Vis spectroscopy. Higher apparent yields of Si-Imi incorporation were obtained in the grafting mode. However, a large number of ethoxysilane groups remain unaltered in comparison to those of SBA-OP x% solids. Furthermore, post-synthesis conditions favour the deposition of large fractions of the organic function on the external surface. Well structured SBA-15 type solids with a large proportion of imidazoline groups inside the pores were obtained by the co-condensation method provided that TEOS was pre-hydrolysed. Complexes of rhodium(III) with imidazoline ligands supported by SBA-GR or OP 5% were successfully tested as precursors of small rhodium(0) nanoparticles. Their location in the two solids (Rh0-SBA-Gr 5% and Rh0-SBA-OP 5%), obtained by a two-step calcination/reduction process, appears to reflect that of the ligands. Particles generated in the reference solids prepared by the impregnation of SBA-15 with rhodium(III) in the presence of NH3 are not included in the pores. The different materials were tested in the room-temperature hydrogenation of styrene and anisole in 1 atm H2. Their catalytic activities vary in the following order: Rh0-SBA-NH3 ≪ Rh0-SBA-GR 5% < Rh0-SBA-OP 5%.