Topology-Dependent Coke Formation in the Catalytic Pyrolysis of Phenol Over HFAU and HZSM-5 Zeolites

Catalytic pyrolysis of lignin, the most abundant natural aromatic polymer, offers a route to obtain value-added products with a low carbon footprint. In such a process, the lignin structure undergoes decomposition through an intricate network of reaction routes. Despite the use of model compounds to gain insights into the decomposition pathways, the formation mechanism of coke and its role in critically affecting catalyst performance remain poorly understood. Herein, we use operando electron paramagnetic resonance (EPR) spectroscopy together with ex situ pulsed EPR experiments and density functional theory (DFT) calculations to understand coke formation in catalytic pyrolysis of phenol over HFAU and HZSM-5 zeolites. Our results pinpoint that coke formation is heavily influenced by zeolite topology. The large cages in HFAU facilitate the initial formation of linear configurations that grow to extended structures, whereas the narrower channels in HZSM-5 promote the formation of more linear structures. These results provide comprehensive mechanistic insights into coke formation and growth that are relevant for the development of lignin valorization strategies and for the general phenomenon of coke formation in zeolites and beyond.