High-Yield Production of Deoxygenated Monomers from Kraft Lignin over ZnO-Co/N-CNTs in Water

Abstract

Catalytic conversion of technical lignin to value-added chemicals and fuels is important for realizing economically viable lignocellulosic biomass refineries. The choice of catalysts and solvents is critical for the effective conversion of the technical lignin to chemicals and fuels by the cleavage of the C–C bonds. In this study, catalytic depolymerization and hydrodeoxygenation of Kraft lignin (KL) were investigated over bimetallic ZnO and Co deposited on N-doped carbon nanotubes (ZnO-Co/N-CNTs) in an aqueous medium. The catalytic activity of ZnO-Co/N-CNTs was compared with those of various noble and non-noble metal-based catalysts. Almost complete KL conversion with a very low solid residue yield (5 wt %), a high bio-oil yield (52 wt %), a high degree of deoxygenation (DOD, 59.0%), and a high monomeric yield (12.1 wt %) was achieved over ZnO-Co/N-CNTs at 350 °C and 6 h reaction time. The monomers mainly consisted of cyclohexanone and its alkyl-substituted derivatives and alkylated phenols. At 400 °C, the monomeric yield and DOD increased to 24.4 wt %and 61.0%, respectively. In addition, the produced bio-oil exhibited high-calorific values of 34.3–37.0 MJ kg–¹ because of the high activity of ZnO-Co/N-CNTs for hydrodeoxygenation. ZnO-Co/N-CNTs outperformed most of the metal-supported catalysts including 5 wt % Pd, 5 wt % Ru, 5 wt % Pt, 66 wt % Ni, and CoMo on various supports of activated carbon and alumina. The use of water as the solvent resulted in much higher bio-oil and monomeric yields than those using methanol, isopropyl alcohol, and n-hexane (12–37 and 3.2–4.1 wt %, respectively). The high bio-oil and monomeric yields with a high DOD in water make ZnO-Co/N-CNTs highly attractive in the development of an environmentally friendly technical lignin conversion process.