Aromatics Production from Lignocellulosic Biomass: Shape Selective Dealkylation of Lignin-Derived Phenolics over Hierarchical ZSM-5

The selective conversion of lignin or lignin-derived products into bulk chemicals is the foremost challenge for near-future lignocellulosic biorefineries. This study investigates the production of phenol and propylene from lignin-derived 4-n-propylphenol (4-n-PP) via gas-phase dealkylation over hierarchical ZSM-5 zeolites in the presence of steam. A series of hierarchical ZSM-5 zeolites with different degrees of mesoporosity and acid properties were prepared by alkaline treatment and mild acid washing. The catalytic evaluation reveals a predominant contribution of the strong acid sites to the dealkylation catalysis. Hierarchization of ZSM-5 zeolites via desilication in alkaline conditions generates Lewis acid sites and reduces the amount of strong Brønsted acid sites. Despite their higher activation energy for dealkylation, reactions on the Lewis acid sites are faster at the thermodynamically required high temperature due to a larger entropic contribution to activation on these sites. In addition to the high catalytic activity, the hierarchical zeolites preserve the high phenol and propylene selectivity as bimolecular side reactions such as disproportionation and transalkylation and some C–C cleavage pathways were inhibited. This observation suggests that both the strong Lewis and the Brønsted acid sites are located in confined spaces in which selectivity is determined by transition state shape selectivity. The catalytic stability is improved upon hierarchization as the result of lowering of the total amount of strong acid sites and prevention of substantial product diffusion issues (caused by shortening of diffusion paths). Therefore, dealkylation over mesoporous zeolites can be processed under lower water partial pressure.