Mild hydrodeoxygenation of phenolic lignin model compounds over a FeReO_x/ZrO₂ catalyst: Zirconia and rhenium oxide as efficient dehydration promoters

Strong adsorption of phenolics on zeolite acid sites causes high trapping inside zeolite channels and low catalytic activity of zeolite-supported catalysts in atmospheric pressure hydrodeoxygenation (HDO) of lignin-derived phenolics. This adsorption is more severe at low temperatures, and restricts the atmospheric HDO of phenolics to high reaction temperatures. The purpose of this research was to develop a catalyst with low phenolic trapping potential and high HDO efficiency under mild reaction conditions. Among the tested catalysts (Fe/HBeta, FeReO_x/HBeta, Fe/MCM-41, ReO_x/MCM-41, FeReO_x/MCM-41, Fe/ZrO₂ and FeReO_x/ZrO₂), the novel catalyst of FeReO_x/ZrO₂ exhibited the highest catalytic efficiency for mild-condition (pressure: 1 atm and temperature <350 °C) HDO of phenolics (guaiacol, m-cresol and anisole), and led to the selective production of BTX aromatics. Compared to Fe/HBeta(38) as a zeolite-supported catalyst, FeReO_x/ZrO₂ displayed remarkably enhanced performance, and its catalytic activity for the HDO of m-cresol at 350 °C was almost twice higher than that of Fe/HBeta(38) at 500 °C. Importantly, FeReO_x/ZrO₂ revealed a high HDO efficiency (BTX yield of 50.5 wt% with phenolic trapping below 5 wt%) at a low temperature of 250 °C, while Fe/HBeta(38) almost lost its entire catalytic activity at this temperature, and gave a low BTX yield of 3.0 wt% with a high trapped phenolic yield of 83.1 wt%. The remarkable catalytic activity of FeReO_x/ZrO₂ in the HDO of phenolics at atmospheric pressure and temperatures as low as 250 °C is a result of its mesoporosity and oxophilicity as well as its well-balanced acidity induced by both rhenium oxide and zirconia support causing a high dehydration efficiency.