Coke formation and mineral accumulation on HZSM-5/Al2O3 catalysts during in-situ catalytic fast pyrolysis of microalgae over multiple regeneration cycles

Abstract

Catalyst deactivation caused by coke formation and mineral accumulation occurs simultaneously and intensively during the catalytic pyrolysis of biomass. A comprehensive understanding of their distinction and interactions during the deactivation and regeneration process of shaped catalysts are essential for catalyst advancement and longevity in industrial application. In this study, we report the reversible and non-reversible catalyst deactivation caused by coke formation, mineral accumulation and the interactions thereof by a multi-technique characterization of a HZSM-5/Al2O3 catalyst (crushed extrudates; particle size 1–3 mm) in fast pyrolysis. A total of 5 cycles of catalyst reaction/regeneration were performed in a bench scale reactor (60 g/h feedstock feeding rate; pyrolysis temperature of 500°C) using both raw microalgae (Scenedesmus almeriensis, or SA) and citric acid treated microalgae (CA-SA). The measured properties of the fresh and used catalysts (SA-R5, 5th recycled and reacted catalyst mixed with SA) disclosed the anticorrelation of the metal contents (increased by a factor of 2.5 times) with surface area (-4.4 %) and Brønsted acid (-70.4 %). The presence of minerals in the feedstock might catalytically alter the pyrolysis chemistry, interfere with the shape selectivity and acidity of the catalysts, thus reduced the formation of coke. Additionally, metal species from the microalgae ash that ended up on the catalyst could promote coke combustion during the regeneration process. This work revealed the demineralization of the microalgae prior to fast pyrolysis minimized the rate of irreversible catalyst deactivation. Whereas, the trade-off between promoted coke formation after the ash removal of microalgae and accelerated minerals accumulation without microalgae demineralization should be evaluated.