Chemoselective crotonaldehyde hydrogen transfer reduction over pure and supported metal nitrates

A series of catalysts based on calcined aluminium, gallium, indium and zirconyl nitrates was synthesised. Gallium and zirconyl nitrate were also supported (impregnation method) over ZrO2 and TiO2. The whole set of catalysts was characterised and tested for the gas-phase chemoselective reduction of crotonaldehyde by hydrogen transfer from 2-propanol (Meerwein–Ponndorf–Verley process, MPV). Catalyst characterisation revealed that calcined nitrates consisted of metal oxides with residual metal nitrates as evidenced by FT-Raman and XPS measurements. Those systems showed high-strength acid sites (as determined by pyridine TPD). A relationship between high strength Brønsted plus Lewis acidity and the yield to 2-butenol was found for all the systems, calcined zirconyl nitrate being the solid leading to better catalytic results. Pyridine poisoning experiments confirmed the role of the strong acid sites in the MPV reduction of crotonaldehyde. However, all catalysts presented a severe deactivation process that seemed to consist of both irreversible and reversible processes since reactivation cycles only led to a partially recovered catalytic activity. The strong adsorption of polymeric species formed over the strong acid sites accounts for the irreversible deactivation process.