Lignin Depolymerization by Catalytic Hydrodeoxygenation Performed with Smectitic Clay-Based Materials

The present investigation aims at exploring the application of low-cost catalysts based on smectitic clays to the hydrodeoxygenation (HDO) of Cynara cardunculus derived lignin. Two types of clay minerals, montmorillonite and saponite, are pillared and decorated with suitable metals by means of an optimized process and characterized by X-ray powder diffraction (XRPD), porosity, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR), UV-vis-NIR, Raman, and thermogravimetric analysis (TGA). Alkaline lignin is obtained from steam-treated cardoon. Lignin depolymerization is conducted in a bench-scale apparatus under subcritical conditions, at relatively low temperature (320 degrees C), for 3 h, under 5-6 MPa of total pressure. Four catalysts are tested, namely, Fe/Al-pillared montmorillonite (FAMO) and saponite (FASA), and Ni-exchanged Ni/Al-pillared montmorillonite (Ni-Ni/AMO) and saponite (Ni-Ni/ASA). Results indicate that mineral clay-based catalysts result effectively in the hydrogenolysis of lignin, producing several low molecular weight aromatics and other species, with overall conversions in the range 60-70%. Montmorillonite-based catalysts yield the highest substrate conversion, with a prevalence of phenols and substituted phenols, whereas only Ni-Ni/ASA generates a higher abundance of aliphatic hydrocarbons. Overall, data indicate that smectitic clays are promising catalyst supports for products-oriented HDO of lignin.