Production of high value-added phenolic compounds through lignin catalytic pyrolysis over ion-exchanged hierarchical ZSM-5 and Beta zeolites

Abstract

Synthesised H-ZSM-5 and H-Beta zeolites with hierarchical porosity (h-H-ZSM-5 and h-H-Beta) have been ion-exchanged with alkali (Na+ and K+) and alkaline-earth (Mg2+) metals and have been evaluated as catalysts to produce high value-added products through catalytic pyrolysis of lignin. In comparison with the thermal test, hierarchical zeolites in acid form are effective catalysts for lignin pyrolysis, favouring the production of valuable light compounds although reducing bio-oil* yield and increasing gas and coke formation. In this way, h-H-ZSM-5 zeolite promotes the formation of oxygenated aromatics, being guaiacols and syringols the major products. Alkali-exchanged variants of this zeolite enhance demethylation reactions improving the selectivity towards 2-methoxy-phenol and syringol, by modifying acid site properties. On the other hand, h-H-Beta zeolite, with larger pore size and stronger acidity, leads to higher concentration of oxygenated aromatics for both parent and ion-exchanged catalysts. Specifically, h-KH-Beta promotes the production of 2-methoxy-phenol and syringol. Besides, h-MgH-Beta stands out for its greater selectivity towards phenol and alkylphenols, such as dimethylphenol. Overall, the combination of accessibility, provided by the hierarchical porosity, with the different nature and strength of the acid sites, induced by the ion-exchange with alkali and alkaline-earth metals, allows tailoring the lignin catalytic pyrolysis process to selectively produce high value-added compounds.