Hydrogen source controlled hydrodeoxygenation of phenolic compounds to cycloalkanes/cycloalkanols over NiFeAlOx catalysts

Catalytic hydrodeoxygenation (HDO) of phenolic compounds is a promising strategy for bio-oil upgradation to produce high-value chemicals and fuels. Herein, a series of NiFeAlOx catalysts prepared by a co-precipitation method were applied in HDO of guaiacol as a typical model compound. The characterization results revealed that the catalyst with Ni/Fe/Al ratio of 3/0.5/2.5 possessed developed textural properties, small metal particles, abundant oxygen vacancies and acid sites. HDO of guaiacol occurred in two pathways with different hydrogen sources: the direct hydrogenation of benzene ring to 2-methoxycyclohexanone and the further hydrogenolysis and dehydration of intermediates to cyclohexane at 260 °C in n-hexane under H2 for 6 h, while demethoxylation affording phenol followed by hydrogenation of phenol to cyclohexanol at 220 °C in isopropanol under N2 for 2 h. The different product composition results from the type of reactive H species, where active hydrogen species are hydrogen radicals using H2 as the hydrogen source and hydrogen ions with opposite charges using isopropanol as the H-donor solvent, respectively. The Ni3Fe0.5Al2.5Ox catalyst maintained good performance after five recycle experiments with a slight decrease in the conversion of guaiacol. Moreover, the catalyst showed excellent universality for selective HDO of other phenolic compounds to cycloalkanes or cycloalkanols.