Comparative performance of Fe-MOx/SiO2 (M = V, Mn, Co, Ni, Cu, Mo) and FeOx/SiO2 catalysts in reductive liquefaction of switchgrass in ethanol

Abstract

The catalytic hydrothermal liquefaction of biomass under a hydrogen atmosphere is a promising technology to produce stable biocrude oil as a sustainable alternative to petroleum crude. A series of iron-based non-noble mix metal-oxide-on-silica catalysts were evaluated to mimic the natural transformation that may have led to the conversion of terrestrial biomass to fossilized fuels. Switchgrass powder was liquefied to a stable bio-oil with a 71.2% yield by using FeO_x/SiO₂ catalyst in ethanol under a 5.5 MPa hydrogen atmosphere at 210 °C. The use of Fe-MO_x/SiO₂ (M = V, Mn, Co, Ni, Cu and Mo) type bimetallic oxide catalysts instead of FeO_x/SiO₂ can produce improvements in liquefaction yields by using Mn, Co, Ni, and Cu as the second metal. The highest liquefaction yield of 78.8% was observed with the Fe-CuO_x/SiO₂ catalyst. Liquefaction oils were formed that were composed of complex mixtures of C6-C12 alcohols, esters, aldehydes, and phenols. The lignin products:holocellulose products ratio changed in the range 0.35 to 0.15 and the composition of oils changed significantly with the use of mixed metal oxides in place of single metal FeO_x/SiO₂ The most effective catalyst, Fe-CuO_x/SiO₂ could be reused in five cycles with a small loss in liquefaction yield from 78.8% to 70.0% after four reuse cycles and after regeneration of the catalyst at 500 °C for 3 h in air.