Performance of MCM-22 zeolite for the catalytic fast-pyrolysis of acid-washed wheat straw

Abstract MCM-22 zeolite samples, having different Si/Al ratios, have been studied for the fast-pyrolysis of acid-washed wheat straw at two catalytic pyrolysis temperatures aimed to the production of partially upgraded bio-oil. The best combination of bio-oil deoxygenation activity and energy yield is obtained when the catalytic bed was operated at 450 °C using the MCM-22 sample with the lowest Al content (Si/Al = 40). Interestingly, the increase in the reaction temperature results in a lower amount of coke deposited over the zeolite. On the other hand, reducing the zeolite Si/Al ratio had a negative effect as a higher concentration of acid sites promotes non-desired reactions: severe cracking of the bio-oil vapours, leading to the enhanced production of gaseous hydrocarbons, and coke formation. Coke produced over MCM-22 zeolite exhibits high oxygen content, whereas the bio-oil fraction presents a high concentration of oxygenated aromatics. These results denote the limited aromatization activity of MCM-22 zeolite for producing aromatic hydrocarbons, in particular when compared with ZSM-5, being of interest for the selective production of phenolic compounds by biomass catalytic pyrolysis.