Gas phase hydrogenation of nitrocyclohexane over supported gold catalysts

We report the first continuous (gas phase) hydrogenation of nitrocyclohexane over oxide (Al2O3, TiO2, CeO2 and ZrO2) supported Au catalysts. Thermochemical analysis has established possible thermodynamic constraints and product distribution at equilibrium. The catalysts have been characterised by temperature-programmed reduction (TPR), H2/O2 chemisorption/temperature-programmed desorption (TPD), BET surface area/porosity, X-ray diffraction (XRD) and scanning/transmission electron microscopy (STEM/TEM) measurements. The effects of space velocity (2–6 × 104 h−1), temperature (353 and 473 K) and H2 partial pressure (8 × 10−4–0.93 atm) on catalyst performance have been examined. Selectivity to the target cyclohexanone oxime is sensitive to H2 pressure, where an increase in temperature favours cyclohexanone with amine/ketone condensation and subsequent reduction to dicyclohexylamine. An increase in turnover frequency was observed with decreasing (surface area weighted) mean Au size (from 7.0 to 4.3 nm) but a lower value was obtained for 3.0 nm Au (on CeO2) that is linked to suppressed H2 chemisorption (under reaction conditions) resulting from strong interaction with the partially reduced support. We establish a critical surface interplay between imine, H and –OH that governs selectivity. Au/Al2O3 exhibited the highest activity and oxime selectivity (maximum = 95%), Au/CeO2 promoted near exclusive production of cyclohexanone whereas Au/TiO2 and Au/ZrO2 generated a cyclohexylamine/cyclohexanone mixture.