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The design of water-tolerant catalysts is crucial to the upgrading of lignin streams to produce high-energy density transportation fuels. In the hydrotreating of lignin into hydrocarbons, niobium oxide has been receiving increasing attention as a potential water-tolerant acidic support for hydrogenation catalysts. Herein, we address the design of Ni-supported on niobia nanorods as a catalyst for the hydrotreating of lignin streams produced by the lignin-first biorefining. By hydrothermal synthesis, Nb2O5 nanorods with pseudohexagonal crystalline structure, exhibiting both Bronsted and Lewis acid sites on the surface, were obtained. The incorporation of 5-25 wt% Ni on the Nb2O5 nanorods led to a decrease in the population of Bronsted acid sites, whereas Lewis acidity was mostly preserved. These materials were evaluated in the hydrotreating of diphenyl ether, a model compound representative of the lignin’s 4-O-5 ether bond motifs, at 160 and 200 oC under 4 MPa H2. The optimized balance between hydrodeoxygenation (HDO) extension and selectivity to fully deoxygenated products was achieved for the 15%Ni/Nb2O5 catalyst. This catalyst showed excellent recyclability, as indicated by the sustained HDO performance as well as robust textural, structural and surface acidity properties throughout five consecutive reaction runs carried out at 200oC. The potential of 15%Ni/Nb2O5 catalyst was further examined in the hydrotreating of a lignin oil obtained from the lignin-first biorefining of poplar wood. At 300 oC under 7 MPa H2, extensive HDO with the primary formation of cycloalkanes in the liquid product was achieved. However, under these conditions, lignin also underwent condensation forming a carbonaceous layer on the acidic Nb2O5 support. As a result, Lewis acidity of the catalyst was poisoned. Thereby, significant changes in the product selectivity in the subsequent recycling tests were observed. Overall, this paper highlights crucial aspects to be considered in the development of water-tolerant catalysts for the hydrotreating of lignin and lignin-derived molecules. |
