Hydrodeoxygenation of guaiacol as a model compound of lignin-derived pyrolysis bio-oil over zirconia-supported Rh catalyst: Process optimization and reaction kinetics.

Graphical abstract Highlights • Deoxygenation of guaiacol with the synthesized Rh/ZrO 2 catalyst was achieved. • High yield of oxygen-free product (87.7mol%, cyclohexane) was obtained. • The suitable reaction conditions were 5 wt.% guaiacol, 300 °C, 7 MPa and 3 h. • Kinetics at four temperatures fit well to a pseudo-first-order kinetic model. Abstract To explore the mechanism for hydrodeoxygenation (HDO) of bio-oil, which contains large compounds that share similar molecular structures of phenyl ring with oxygen-containing functional groups, guaiacol as a typical model compound of lignin-derived bio-oil was hydrotreated at 150–350 °C under 3–7 MPa (H 2) using the synthesized zirconia-supported Rh catalyst in a batch reactor. With Rh/ZrO 2 catalyst, guaiacol can be completely converted where reaction temperature plays a decisive role and significantly affects the degree of hydrogenation. HDO Process was optimized with insight into effects of different reaction parameters, including H 2 pressure, reactant concentration, reaction time and temperature, on the whole process of HDO and formation of undesired oxygen-contained products. The optimum reaction conditions were 5 wt. % guaiacol, 300 °C and 7 MPa in which guaiacol could be completely deoxygenated with 87. 7 mol% oxygen-free product of cyclohexane in 3 h and the desired O/C and H/C ratios of products were obtained. A specific reaction network including three main steps: "guaiacol → 1-methyl-1,2-cyclohexanediol → cyclohexanone and cyclohexanol → completely deoxygenated compounds of cycloalkanes", was deduced by a comprehensive study on different reaction parameters and a typical key reaction network for HDO of lignin-derived bio-oil is proposed based on model compounds studies. Kinetic model for HDO of guiaacol with the Rh/ZrO 2 catalyst was proposed based on the credible pathway and it fits well to a pseudo-first-order kinetic model that the R 2 values obtained for the fittings were all above 0.98 at four temperatures. At low temperature of 150 °C, the kinetically relevant step is hydrogenation of the aromatic ring, yielding 90 mol% 1-methyl-1,2-cyclohexanediol, while at high temperature (≥300 °C), the kinetically relevant step is the complete deoxygenation of oxygen-containing functional groups which mainly yields cyclohexane. These results would be helpful to further understand the HDO mechanism of lignin-derived bio-oil, and the excellent performance of Rh/ZrO 2 demonstrated its potential application in bio-oil hydrodeoxygenative upgrading process. [ABSTRACT FROM AUTHOR]