Hydrogen-free hydrodeoxygenation of guaiacol using phase controlled NiAl layered double hydroxides-derived catalysts

Abstract

The effective and sustainable conversion of lignin-derived aromatics into valuable chemicals is pivotal in reducing dependence on fossil-based resources. This study investigates the selective hydrodeoxygenation (HDO) of guaiacol via aqueous-phase reforming of methanol in the absence of external hydrogen, targeting the selective cleavage of the methoxy group in guaiacol and the hydrogenation of the aromatic ring. Highly dispersed Ni catalysts were prepared through nanoscale phase transformation of NiAl-layered double hydroxide (NiAl LDH) precursors. We demonstrate that increasing calcination temperatures enhances the surface concentration of Ni-0 nanophases while decreasing Ni2+ content. While metallic Ni serves as a key active site, the controlled NiAlOx phases, functioning as acid sites, are essential for stabilizing NiO and boosting guaiacol conversion. The NiAl-500 catalyst exhibited higher performance in hydrogen-free conversion of guaiacol to cyclohexanol, outperforming many non-noble catalysts. Furthermore, the developed catalyst also converted other lignin-derived phenolics to alkyl cyclohexanols via selective demethoxylation and hydrogenation. In situ characterizations elucidated the guaiacol HDO mechanism, highlighting the synergistic roles of Ni2+ and Ni-0, alongside a distinctive Ni-O-Al bond in the LDH-derived catalyst. This work paves the way for the development of highly efficient, hydrogen-free HDO catalysts with high Ni dispersibility and content (>70 wt%) through precise tuning of the physical and chemical properties of LDH-based systems.