Enhancing the performance of heterogeneous palladium based catalysts in the mild reductive depolymerization of soda lignin through addition of a non-noble metal and tuning of the preparation strategy

Abstract

Research towards mild reductive depolymerization of lignin is gaining momentum because of its potential for producing sustainable functionalized aromatics, but achieving high yields still relies on expensive noble metal catalysts. This study aims to improve the catalysts' cost effectiveness through addition of a non-noble metal to a Pd nanoparticle catalyst, supported on γ-Al₂O₃. Six Pd based catalysts (Pd, PdCu, PdNi, PdFe, PdCo, and PdMo) were synthesized and prepared through either calcination or thermal reduction, and their activity and selectivity in lignin depolymerization were evaluated as a function of batch time. Principal component analysis (PCA) of the entire datapool revealed that, albeit to varying degrees, the addition of a secondary metal shifts the behavior of a Pd catalyst more towards that of pure solvolysis and that the preparation strategy has no effect on Pd and PdMo. Regarding activity, it was found that the addition of Cu, Ni, Fe, Co and Mo significantly enhances the catalyst's activity and that the preparation strategy is also important, with calcination being preferred for PdCu and PdFe and thermal reduction for PdNi and PdCo. Using a plethora of analysis techniques to assess the selectivity at increasing depths, it was revealed that the shift in selectivity, as identified in the PCA results, is caused by variations in dehydration of aliphatic OH groups and hydrogenation of aliphatic double bonds. Moreover, due to a size exclusion effect during the reaction, differences in selectivity between the catalysts are most pronounced at lower molecular weights.