The effect of catalyst shaping on the process stability of continuous reductive catalytic depolymerization of lignin

Lignin is a renewable resource co-generated at large scale in biorefineries and has the potential to replace a significant fraction of fossil-derived aromatic polymer building blocks. However, technical lignins typically exhibit high polydispersity, low reactivity, and poor solubility, limiting direct valorisation. Continuous reductive catalytic depolymerization (RCD) of lignin enables the production of large quantities of well-defined lignin-derived building blocks suitable for material applications. To enable industrial implementation and to facilitate the transition from batch processing to continuous flow operation, maximizing catalyst lifetime and performance through optimized catalyst bed design is essential. This study investigates the impact of catalyst geometry on process stability and depolymerization efficiency during continuous RCD of hydrolysis lignin. Palladium on γ-Al2O3 catalyst was shaped as spheres, wash-coated honeycombs, and 3D-printed monoliths. The catalyst geometry strongly affected results of screening tests conducted at 215°C. Spherical catalysts enabled effective depolymerization but failed after 25 h of continuous operation due to rapid pressure buildup associated with coke formation. Honeycomb structures enabled stable operation for 49 h but showed limited depolymerization efficiency. Optimized 3D-printed monoliths with a high open frontal area provided the best compromise, combining improved process stability with efficient depolymerization. In a full-reactor configuration, 3D-printed catalysts permitted stable RCD for 84 h, yielding sufficient depolymerized product for the synthesis of lignin-based acrylic resins exhibiting competitive mechanical and thermal properties. The results of this study clearly demonstrate the potential of 3D-printing for shaping RCD catalysts and highlight the relevance of this lignin valorisation pathway for biobased polymer applications.