Effect of Mesoporosity, Acidity and Crystal Size of Zeolite ZSM-5 on Catalytic Performance during the Ex-situ Catalytic Fast Pyrolysis of Biomass

Abstract

The catalytic performance of a set of technical, zeolite ZSM-5 extruded materials have been studied in a bench scale unit for the ex-situ Catalytic Fast Pyrolysis (CFP) of lignocellulosic biomass. The set of catalysts include micro- and mesoporous materials, with and without ZrO2-promotion, with different Si/Al ratios and with micro- and nanosized zeolite crystals. Mesoporosity, acidity and crystal size play a key role on the overall catalytic performances in terms of activity and selectivity (i. e., in their ability to obtain the highest bio-oil fraction with the lowest oxygen content), and also importantly, stability. Detailed post-mortem bulk and micro-spectroscopic studies of the solid catalysts complement the catalytic testing. The obtained results point towards coke deposits as the main cause for catalyst deactivation. Details of the nature, formation and evolution of these coke deposits revealed essential insights, which serve to evaluate the design of catalysts for the ex-situ CFP of biomass. In particular the mesoporous catalysts are overall better preserved with increasing time-on-stream and deactivate later than their microporous counterparts, which suffer from pore blockage. Likewise, it was seen that ZrO2-promotion contributed positively in the prevention against deactivation by hard coke spreading, thanks to its enhanced Lewis acidity. The zeolite's crystal size is another important characteristic to combine the different components within catalyst bodies, ensuring the proper interaction between zeolite, promoter and binder. This is illustrated by the nanocrystalline ZrO2/n-ZSM-5-ATP material, which shows the best catalytic performance.