Catalytic Fast Pyrolysis of Biomass over Hierarchical Zeolites: Comparison of Mordenite, Beta, and ZSM-5

Abstract

Biomass catalytic fast pyrolysis is a promising process, but its development is still hampered by the formation of coke, which reduces the carbon selectivity of the process to the targeted aromatic hydrocarbons. It is important to better understand the effect of the zeolite structure on the mechanisms of coke deposit and how this coke deposit poisons the active sites [Brønsted acid sites (BAS)]. Here, we provide a systematic comparison of various zeolites: mordenite (MOR), beta (*BEA), and ZSM-5 (MFI) with different hierarchization methods (NaOH, HF, and NH 4 F). The effect of crystal size has been also studied for *BEA. These zeolites were tested with a constant biomass-to-catalyst ratio of 0.8 in a double fixed bed reactor which enables to decouple biomass pyrolysis and ex situ volatiles conversion. The formation of volatiles species was monitored online by photoionization mass spectrometry. Aromatic hydrocarbons were also quantified by GC/MS-FID. The spent catalysts were analyzed by a pyridine probe (for acidity), FTIR (for coke composition), and N 2 sorption (for porous structure). The structure of mordenite with a two-dimensional network of channels and weak connections between the channels leads to a fast deactivation by pore blocking even after different hierarchization methods. The hierarchical MFI (obtained by mild desilication with NaOH) was the best selective zeolite toward aromatic hydrocarbons. The selectivity toward aromatics and coke formation depends on the shape of canals but not on the H + cage environment. *BEA promotes the diffusion of species to BAS in larger pores than MFI but it leads to a lower shape-selectivity than MFI, especially after partial deactivation by coke.