Upgrading of Kraft Lignin-Derived Bio-Oil over Hierarchical and Nonhierarchical Ni and/or Zn/HZSM5 Catalysts

Abstract

Synthesized hierarchical and nonhierarchical Ni and/or Zn/HZSM5 catalysts were compared for upgrading Kraft lignin-derived liquefaction bio-oil in a batch reactor at 300 °C for 1 h under a H2 atmosphere in supercritical ethanol. The catalysts were characterized by N2 adsorption–desorption isotherms, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption of ammonia, inductively coupled plasma–optical emission spectroscopy, and X-ray diffraction techniques. Results revealed that incorporation of Ni and Zn did not significantly influence the HZSM5 crystalline structure. The hydrogenated bio-oils were analyzed by means of gas chromatography–mass spectrometry and elemental analysis. Upon bio-oil upgrading, the amounts of 4-ethylguaiacol and 4-propylguaiacol increased considerably on account of reduction in the amounts of unsubstituted guaiacol, 4-methylguaiacol, 4-propenylguaiacol, and homovanillic acid. The plausible bio-oil upgrading mechanism involved hydrogenation, alkylation, and deoxygenation. Hydrogenation and deoxygenation were prominent over hierarchical and nonhierarchical catalysts, respectively. Highest HHVBoie (∼29.93 MJ/kg) was obtained for both nonhierarchical 15Ni5Zn/HZSM5 and hierarchical 20Zn/HZSM5 catalysts, whereas the former was found to be more stable for bio-oil upgrading. Additionally, the upgraded bio-oils obtained over hierarchical and nonhierarchical catalysts were rich in hydrogen and carbon contents, respectively.