Comparison of kinetics, activity and stability of Ni/HZSM-5 and Ni/Al ₂O₃-HZSM-5 for phenol hydrodeoxygenation

We have investigated the detailed kinetics of phenol hydrodeoxygenation in liquid aqueous medium over Ni supported on HZSM-5 or HZSM-5 with 19.3 wt.% γ-Al₂O₃ binder. The individual reaction steps on two Ni catalysts followed the rate order r₁ (phenol hydrogenation) < r₂ (cyclohexanone hydrogenation) < r₃ (cyclohexanol dehydration) ≪ r₄ (cyclohexene hydrogenation), so that phenol hydrogenation was the rate determining step. As Ni/Al₂O ₃-HZSM-5 showed up to five times higher catalytic activity for phenol hydrogenation than Ni/HZSM-5, it also delivered higher rates for overall phenol hydrodeoxygenation, verified by both the kinetic study monitored by GC and in situ IR spectroscopy to trace the product concentrations. Under the present conditions, Ni leaching was almost negligible from Ni/Al₂O ₃-HZSM-5 after 90 h. The HZSM-5 support was stable, but the Al ₂O₃-HZSM-5 support lost 7 wt.%. The catalytic activity gradually decreased when catalyst was recovered and reused, mainly due to Ni particle sintering tracked by transmission electron microscopy. The change of acid sites on the fresh and used catalysts monitored by IR of adsorbed pyridine demonstrated that Bronsted acid sites of HZSM-5 could be irreversibly transformed to Lewis acid sites during calcination.