An examination of catalyst deactivation in p-chloronitrobenzene hydrogenation over supported gold

The stability of Au/Al₂O₃ in the continuous gas phase (423K) hydrogenation of p-chloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN) has been investigated over an inlet H₂/p-CNB=4-390, i.e. from close to stoichiometry to H₂ far in excess. The catalyst (activated unused and spent) has been characterised with respect to specific surface area (SSA)/porosity, temperature programmed reduction (TPR), powder XRD, H₂ chemisorption, STEM, XPS, elemental analysis and TGA-DSC measurements. Activation of Au/Al₂O₃ by TPR in hydrogen generated a narrow Au size distribution (1-8nm, mean=3.6nm) with evidence (from XPS) of (support→metal) charge transfer to generate surface Au^δ-. Exclusive p-CAN production was achieved under conditions of kinetic control, which were established by parameter estimation and experimental variation of contact time, catalyst particle size and p-CNB/catalyst ratio. A temporal decline in activity was observed that was more pronounced at H₂/p-CNB ≤39. The spent catalyst exhibited equivalent SSA/porosity, Au particle size (from STEM) and electronic character (from XPS) relative to activated unused Au/Al₂O₃. A significant carbon content (6.3% w/w) was determined from elemental analysis and confirmed by XPS and TGA-DSC. This carbon deposit hindered H₂ chemisorption under reaction conditions, leading to suppressed hydrogenation activity. Catalyst regeneration by oxidative/reductive treatment resulted in a restoration of the initial hydrogenation activity, retaining exclusive selectivity to p-CAN.