Lignin depolymerisation by nickel supported layered-double hydroxide catalysts
Lignin depolymerisation is traditionally facilitated with homogeneous acid or alkaline catalysts. Given the effectiveness of homogeneous basic catalysts for lignin depolymerisation, here, heterogeneous solid-base catalysts are screened for C–O bond cleavage using a model compound that exhibits a common aryl–ether linkage in lignin. Hydrotalcite (HTC), a layered double hydroxide (LDH), is used as a support material as it readily harbours hydroxide anions in the brucite-like layers, which are hypothesised to participate in catalysis. A 5 wt% Ni/HTC catalyst is particularly effective at C–O bond cleavage of a model dimer at 270 °C without nickel reduction, yielding products from C–O bond cleavage identical to those derived from a base-catalysed mechanism. The 5% Ni-HTC catalyst is shown to depolymerise two types of biomass-derived lignin, namely Organosolv and ball-milled lignin, which produces alkyl-aromatic products. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy show that the nickel is well dispersed and converts to a mixed valence nickel oxide upon loading onto the HTC support. The structure of the catalyst was characterised by scanning and transmission electron microscopy and X-ray diffraction, which demonstrates partial dehydration upon reaction, concomitant with a base-catalysed mechanism employing hydroxide for C–O bond cleavage. However, the reaction does not alter the overall catalyst microstructure, and nickel does not appreciably leach from the catalyst. This study demonstrates that nickel oxide on a solid-basic support can function as an effective lignin depolymerisation catalyst without the need for external hydrogen and reduced metal, and suggests that LDHs offer a novel, active support in multifunctional catalyst applications.