Reduction of lignin heterogeneity for improved catalytic performance of lignin nanosphere supported Pd nanoparticles

Lignin heterogeneity, including molecular weight polydispersity and structural complexity, causes undesirable properties of lignin in its valorization. In this regard, the current work focuses on the effect of lignin heterogeneity on the green synthesis of lignin nanosphere (LNS) supported Pd nanoparticles (Pd@LNS) and their catalytic performance. Three lignin fractions with reduced heterogeneity (F1, F2, and F3) were used for LNS fabrication and the as-prepared LNSs were further applied as carriers for Pd nanoparticles (Pd NPs) synthesized using lignin as an in-situ reducer. It was found that the three LNSs exhibited a gradually decreased particle size from F1 to F3, while all of them revealed improved particle size uniformity compared to that from the initial lignin. Owing to the superior reducing power of F1, the LNS prepared from F1 presented the highest loading amount of Pd NPs among the three LNSs. Moreover, these Pd NPs had the smallest particle size and they were dispersed in a much more uniform way than those on the LNSs from F2 and F3. Therefore, the Pd@LNS prepared from F1 exhibited significantly increased catalytic activity in the Cr(VI) reduction compared with those from F2 and F3, as well as the initial lignin. Consequently, the work implies that lignin heterogeneity has considerable influences on the green formation of Pd NPs on LNS, and the catalytic performance of Pd@LNS can be substantially improved using low molecular weight lignin fraction as the feedstock.