Hydrodeoxygenation of guaiacol as a model compound of pyrolysis lignin-oil over NiCo bimetallic catalyst: Reactivity and kinetic study

Abstract

Catalytic hydrodeoxygenation (HDO) is a popular route to upgrade the pyrolysis lignin-oil that is abundant of phenolic compounds, and the synergistic effect of bimetal enables to improve the catalytic activity significantly. In this work, a series of non-noble metal catalysts are synthesized and their HDO performances of guaiacol are compared. Optimized NiCo/SiO2-ZrO2 catalyst expresses a 100% conversion and a 99.9% selectivity of cyclohexane product, which is higher than those over Ni/SiO2-ZrO2 catalyst. Catalyst characterization results demonstrate that the interaction between Ni and Co promotes the reduction of metal site and the adsorption of hydrogen, which is the crucial factor during HDO process. Besides, the electron transfer from Co to Ni species weakens the C-O bond of substrate intermediate and results in its cleavage to transform cyclohexanol to cyclohexane. This is also the rate determining step in the HDO of guaiacol. Specific kinetic study is conducted to verify the reaction pathway and the structure–activity relationship. The HDO reaction of guaiacol on NiCo/SiO2-ZrO2 fits the first order kinetic model well. The apparent Ea of 55.9 kJ·mol−1 is lower than those of the noble metal catalysts reported in previous studies. In addition, the upgrading of raw pyrolysis lignin-oil on this non-noble bimetallic catalyst also achieves a high HDO efficiency, with the hydrocarbon contents increased from 4.2% to 60.6%. The good HDO performance of this cheap and easy-prepared NiCo catalyst proves its promising potential in the upgrading of lignin-oil for the production of hydrocarbon fuels.