Gold Promoted Imine Production by Selective Gas Phase Reductive Coupling of Nitrobenzene and Benzaldehyde

The feasibility of gas phase continuous reductive coupling of nitrobenzene and benzaldehyde for imine (N-benzylideneaniline) synthesis over oxide (TiO2, Fe2O3, Al2O3, ZrO2 and MgO) supported Au and Pd (1.0–3.4% w/w) has been demonstrated. Reaction over Au/TiO2 (mean size = 3.2 nm) delivered 99% selectivity to the target imine. An equivalent level of selectivity was achieved over all the oxide supported Au catalysts with increasing turnover frequency (6 → 103 h−1) for smaller Au particles (7.7 → 3.2 nm), where nitrobenzene → aniline was rate determining. Lower TOF over Au/Fe2O3 is linked to the involvement of surface oxygen vacancies that serve to stabilise the nitro-reactant and lower reactivity. Under the same reaction conditions, Pd/TiO2 exhibited greater H2 chemisorption capacity and higher activity but lower imine selectivity (S = 83%) due to toluene formation (via benzaldehyde hydrogenolysis). Reaction at extended contact times (0.2 → 0.5 s) over Au/TiO2 served to enhance imine yield (to 70%).