Mild reductive catalytic depolymerization of lignin in a continuous flow reactor using a Cu-enhanced Pd catalyst

Abstract

Mild reductive catalytic depolymerization (MRCD) of lignin offers a sustainable route to produce functionalized aromatic compounds. However, the economic viability is hindered by the need for expensive palladium (Pd) catalysts and the limited exploration of continuous flow reactors (CFRs), which are essential to achieve an adequate production scale. This study examines the impact of partial replacement of Pd with copper (Cu) on the performance, selectivity, active site characteristics, and deactivation of a γ-Al₂O₃ supported Pd catalyst in MRCD of lignin using a CFR. Despite containing 49 % less Pd, the PdCu catalyst achieves the same depolymerization degree as the Pd catalyst over 200 min of time on stream. During the reaction, metallic Pd is formed within the Pd catalyst and both a smaller (unordered) and larger (ordered FCC) metallic PdCu phase within the PdCu catalyst. The enhanced performance of the PdCu catalyst is attributed to synergistic effects between Pd and Cu and presence of differently sized metallic phases. A minimal impact of Cu on the selectivity, even in monomer yields, was observed. For both catalysts, the primary cause of deactivation is the hydration of the γ-Al₂O₃ support to boehmite, leading to loss of its acidity and morphological changes. Metal leaching and poisoning are insignificant, while nanoparticle growth likely arises from the reduction of the metallic phases during reaction. Only a very small amount of coke deposition is observed. Overall, the cost-effective partial replacement of Pd with of Cu forms metallic PdCu alloys during the reaction, enhancing activity without adversely affecting selectivity or deactivation.