Tungsten-based catalysts for lignin depolymerization: the role of tungsten species in C–O bond cleavage

Abstract

Tungsten carbide has shown promising activity in lignin depolymerization, but the active site remains unclear yet due to its complicated surface tungsten species. Tungsten-based catalysts with designed species were therefore synthesized for hydrocracking β-O-4 model compounds and lignin. X-ray diffraction, Raman spectroscopy, physical adsorption, X-ray photoelectron spectroscopy, and transmission electron microscopy as well as the temperature-programmed desorption of ammonia and H₂ microcalorimetric adsorption characterizations were employed to identify and quantify the composition of the as-prepared catalysts. It was found that the catalyst prepared under a N₂ atmosphere at 500 °C (N₂-500) had a major tungsten trioxide (WO₃) phase and possessed dominantly acidic sites, while poor in terms of metallic sites. With the increasing treatment temperature, there was a clear evolution of the tungsten species from WO₃ to W, and then to W₂C and WC, along with an increase in metallic sites. As a result, the catalyst treated under 1000 °C (N₂-1000) exhibited a proper amount of both acidic sites and metallic sites. The structure–function relationship of the above catalysts was studied for transforming β-O-4 model compounds and real lignin. In the model compounds reactions, N₂-500 showed poor conversion due to the lack of tungsten carbide species, while N₂-1000 exhibited both dehydration and hydrogenolysis activity due to the existence of balanced tungsten trioxide and tungsten carbide species, and therefore provided much higher yields of β-O-4 cleavage products. The conversion results of beech dioxasolv lignin fitted well with the model compounds studied. The present work provides a deeper understanding of the role of different tungsten species in lignin depolymerization and makes an important contribution to the chemistry of tungsten-based catalysts in biomass conversion.