Highly-efficient and magnetically-separable ZnO/Co@N-CNTs catalyst for hydrodeoxygenation of lignin and its derived species under mild conditions

Abstract

A catalyst comprising highly-efficient and magnetically-separable bimetallic ZnO and Co nanoparticles (NPs) deposited on N-doped carbon nanotubes (ZnO/Co@N-CNTs) was synthesized by the direct calcination of the bimetallic Zn/Co zeolitic imidazolate framework (Zn/Co-ZIF) for the effective hydrogenation (HD) and hydrodeoxygenation (HD) of lignin and its derived species. During the calcination of Zn/Co-ZIF, Zn was dislocated from the framework to the particle surface to form amorphous ZnO NPs and metallic Co NPs, which activated the growth of the N-CNTs. Because of the highly Lewis acidic amorphous ZnO, high HD/HDO ability of metallic Co NPs, and high wettability of the N-CNT, an almost complete conversion of vanillin into its corresponding deoxygenated species, creosol, was achieved in an aqueous medium without the production of byproducts under mild reaction conditions (150 degrees C, 0.7 MPa H-2, a reaction time of 2 h). When kraft lignin and bio-oil derived from concentrated strong acid hydrolysis lignin were converted over ZnO/Co@N-CNTs, high degrees of deoxygenation of 74.2% and 34.4%, respectively, could be achieved at 350 degrees C, 5.0 MPa H-2, and a reaction time of 6 h in water. A detailed chemical composition analysis of the deoxygenated bio-oil revealed that cyclohexanone and its alkyl group-substituted derivatives were the major species. To gain insight into the HD/HDO mechanisms, various types of ligninderived monomers (syringaldehyde, acetovanillone, acetosyringone, 2-phenoxy-1-phenylethanol, cinnamaldehyde, isoeugenol) and holocellulose-derived monomers (furfural and 5-hydroxymethyl furfural), different types of catalysts, and various reaction parameters were tested. The mild reaction conditions, use of a non-noble metal catalyst, and use of water as the solvent make it possible to develop a costeffective, easy to scale up, and environmental-benign process for biofuel and biochemical production.