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 metalsupported 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.

Atom Doping Engineering of Metal/Carbon Catalysts for Biomass Hydrodeoxygenation

“…Metal/N-doped carbon materials are widely used in biomass HDO due to strong metal−support interactions, high stability, and excellent surface chemistry (wettability, basicity). 4,29,73,97 N, which is adjacent to C in the periodic table and has a similar atomic size to C, can effectively reduce the structural deformation and form stable N−C bonds of the carbon network, which has been the most introduced exotic element in metal/carbon catalysts due to its unique properties, easy synthesis, abundance, and excellent performance of biomass HDO. 102 Jiang et al 96 synthesized the cobalt loaded N-doped carbon catalyst (Co/NC) for the HDO of vanillin, and nearly quantitative MMP yields were obtained at 10 bar H 2 pressure and 150 °C.…”

“…Once the preferential adsorption of reactant molecules has occurred, the well-dispersed Pt nanoparticles undergo the next step of C−O bond cleavage. Mushtaq et al 97 investigated catalytic HDO of Kraft lignin (KL) over bimetallic Co-and ZnO-loaded N-doped CNTs (ZnO-Co/N-CNTs) in an aqueous medium, achieving almost 100% conversion of KL at 350 °C and 6 h reaction conditions (Figure 12). The hydrophilicity of N allows better dispersion of the catalyst in aqueous media and promotes catalyst −substrate contact.…”

“…HDO is one of the most promising technologies for biomass upgrading. − Many metals (e.g., Pd, Pt, Ru, Fe) combined with molecular sieves, CNTs, silica, metal and its oxides, and other catalysts have been widely used for HDO of biomass. ,− In the following, we highlight the latest progress of metal/heteroatom (N, B, S, P)-doped carbon catalysts for biomass HDO applications (Table ). Oxygen doping is excluded because many catalysts are oxidized by air during high temperature carbonization.…”

Selective Hydrogenolysis of Lignin Model Compounds to Aromatics over a Cobalt Nanoparticle Catalyst

“…25 Rautiainen et al reported that using formate (or formic acid) as a hydrogen donor and Co-phen/C as a catalyst, birch wood without treatment could be converted into monophenolic compounds in up to 34 wt % yield of total lignin. 26 of bio-oil (52 wt %) and lignin-derived monomers (12.1 wt %) over ZnO−Co/N−CNTs at 350 °C in 6 h. 27 In the previous work, acetophenone was always produced for the hydrogenolysis of β-O-4 ketone, or a mixture of ethylbenzene, acetophenone, and 1-phenylethanol was obtained due to the poor catalyst selectivity. It has been known that the hydrodeoxygenation of aromatic ketones/alcohols is a very important process in chemical synthesis and biomass upgrading.…”

“…Rautiainen et al reported that using formate (or formic acid) as a hydrogen donor and Co-phen/C as a catalyst, birch wood without treatment could be converted into monophenolic compounds in up to 34 wt % yield of total lignin . Very recently, Kim et al found that kraft lignin could be converted almost completely with a high yield of bio-oil (52 wt %) and lignin-derived monomers (12.1 wt %) over ZnO–Co/N–CNTs at 350 °C in 6 h …”