Study on Catalyst Deactivation During the Hydrodeoxygenation of Model Compounds

Abstract

To address the pressures associated with an increasing energy demand and rising cost of liquid fuel production, alternative feedstocks are being developed. One such potential replacement is bio-oils, which are derived from biomass, however the oil produced is often chemically unstable, corrosive and has low heating values due to the high oxygen content. Thus, an upgrading process is required to lower the oxygen content, with catalytic hydrodeoxygenation being one such technology currently being developed, with mechanistic studies commonly completed utilising model compounds. The current study examines factors that influence catalyst deactivation during hydrodeoxygenation of common bio-oil model compounds over zeolite-supported nickel catalysts, indicating the contribution of functional groups on the formation of condensed-ring products and catalyst deactivation. It was determined that phenolic-hydroxyl groups present in model compounds can facilitate catalyst deactivation through the formation of condensed-ring compounds, identified through decreased cycloalkane yields, causing blockage of catalyst pores. Conversely, when toluene, cyclohexanol and anisole were used as model compound feeds, there were no notable changes in cycloalkane yields, indicating aromatic, alkyl–OH and aromatic–OCH3 have negligible effect on catalyst deactivation. The effect of catalyst metal loading was also examined, and while increased metal content enhanced cyclohexane yields, so to was the formation of condensed-ring products, suggested to be the result of an increased concentration of surface cyclohexane carbocations. Modifications to reaction conditions was studied, with catalyst activity and stability found to improve with increasing reaction temperatures up to 230 °C. Based on the product distributions observed, a coupling reaction pathway (leading to the formation of condensed-ring products and cycloalkanes) is proposed as the hydrodeoxygenation mechanism.