Activity and stability studies of H-transfer reduction reactions of aldehydes and ketones over aluminium isopropoxide heterogenised catalysts

Aluminium isopropoxide Al(OⁱPr)₃ immobilised on various mesoporous supports (SiO₂, TiO₂ and γ-Al₂O₃) was tested for H-transfer reductions of various aldehydes and ketones in the presence of 2-propanol as a sacrificial agent. The heterogenised catalysts were characterised by N₂ physisorption, XRD, SEM-EDX, FTIR and ICP-OES. The characterisation results show a successful grafting of the homogeneous aluminium isopropoxide catalyst, covalently bound to the solid surface, with high dispersion over the mesoporous supports. The heterogenised catalysts show an excellent catalytic activity with high selectivity towards the desired alcohol product, with performances that are comparable with those of the homogeneous Al(OⁱPr)₃ catalyst. Al(OⁱPr)₃ grafted on SiO₂ shows higher activity compared to γ-Al₂O₃ and TiO₂ supported catalysts. The catalysts remain very active after 5 cycles of reuse and no leached Al(OⁱPr)₃ was found in the reaction mixture, hence showing an excellent stability. The work reported here shows that it is possible to effectively immobilise catalytic functions, usually working in the homogeneous phase, over solid supports, with the resulting heterogenised catalysts keeping the same catalytic activity of the homogeneous counterpart and excellent stability, and with the advantage of being able to recycle and reuse them, without loss of catalytic materials.