Hydrogenation of Guaiacol and Pyrolysis of Biomass Using Nickel and Niobium-Based Catalysts

The hydrogenation reaction of guaiacol in the gas phase at 300 °C and atmospheric pressure of hydrogen was investigated over Ni-based catalysts supported on niobium oxide (Nb 2 O 5 ) and exfoliated potassium hexaniobate (Nb-exf). The catalysts were prepared by two different methods: photodeposition (pd) and incipient wetness impregnation (im) and were characterized by N 2 adsorption–desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of ammonia (NH 3 -TPD) analyses. XRD analysis showed that all catalysts exhibited approximately the same crystallite size of the metallic nickel (15 nm). Regarding the catalytic tests, guaiacol was mainly converted to catechol (59.5–80.6%) and phenol (4.4–14.2%) regardless of the catalyst. Small amounts of benzene were only observed over Ni(im)/Nb 2 O 5 . The catalytic experiments showed that the main reaction pathway is the conversion of guaiacol through demethylation reaction producing catechol followed by phenol formation via dehydroxylation reaction. Ni(im)/Nb 2 O 5 was tested for catalytic upgrading of pyrolysis vapor of different biomasses (lignin, microalgae, and malt residue), and it was able to reduce the yield of oxygenated products. The catalytic upgrading of pyrolysis vapor of microalgae using a catalyst-to-biomass weight ratio (C:B) of 3:1 showed the highest selectivity of hydrocarbons (57.44%). The catalyst concentration and the biomass used in the test showed a high influence on the product distribution. Graphical Abstract