Acetic acid conversion reactions on basic and acidic catalysts under biomass fast pyrolysis conditions

The objective of this study was to investigate the reaction pathways of acetic acid, the most abundant organic acid in bio-oils, over acidic and basic catalysts under typical biomass fast pyrolysis conditions. The pyrolysis/cracking experiments were performed in a fluidized bed reactor at 500 degrees C over a ZSM-5 zeolite formulation, as the acidic catalyst, and MgO as the basic catalyst. Both exhibit high activity (>= 70 wt.% conversion) but distinctly different product selectivity. The primary reaction scheme on both acid and basic sites involves the ketonization of acetic acid to acetone, water and CO2, via different elementary routes. However, the consecutive reactions of acetone differ significantly depending on the nature of the active sites, leading to different product selectivity over the two types of catalysts. Aromatics, phenols, light olefins and CO were the main by-products over ZSM-5, while MgO favored the formation of higher ketones. TPD studies on acetic acid and acetone-saturated catalysts indicated that acetone undergoes self-condensation and oligomerization reactions at temperatures > 300 degrees C on MgO, yielding mainly CO2 and coke. On ZSM-5, both the direct dehydration of acetic acid and the secondary cracking of acetone condensation products play an equally important role, leading to the production of light olefins and small aromatic hydrocarbons. Phenols were also produced over ZSM-5 via acetone condensation and cyclization routes.