Controlling carbonization of tannin for the effective Pd loading to hydrogenate lignin-derived aldehydes

Finding new catalytic applications for tannin, a biomass resource, exploiting its structural features and renewable nature, helps contribute toward carbon neutrality in catalysis. In this study, novel Pd-supported, carbon-neutral catalysts have been synthesized using tannic acid. First, tannic acid was cross-linked and coated on the surface of nano-SiO2. This material was then subjected to controllable carbonization, with the temperature being optimized, to obtain the catalyst support SiO2@TA-T (T: carbonization temperature). Experimental results demonstrated that carbonizing at 400 °C remained part of the phenolic hydroxyls of tannin with satisfactory reducing ability. Meanwhile, the condensation and the deoxygenation of tannin increased the thermal stability of this support and introduced porous structures and abundant defect sites. This material was then used to adsorb and in situ reduce Pd2+ to develop Pd-based catalysts. To this end, this synthesized Pd/SiO2@TA-400 catalyst was then used to hydrogenate vanillin, with a conversion of 99% to vanillyl alcohol at a pressure of 0.5 MPa initial H2 and a temperature of 20°C for 60 min. Such remarkable results were corroborated in the hydrogenation of several lignin-derived aromatic aldehydes, with our catalyst achieving high conversion and outstanding recyclability (5 recycles).