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

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 beta-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 H-2 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 N-2 atmosphere at 500 degrees C (N-2-500) 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 degrees C (N-2-1000) exhibited a proper amount of both acidic sites and metallic sites. The structure-function relationship of the above catalysts was studied for transforming beta-O-4 model compounds and real lignin. In the model compounds reactions, N-2-500 showed poor conversion due to the lack of tungsten carbide species, while N-2-1000 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 beta-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.