Promoted ZSM-5 catalysts for the production of bio-aromatics, a review

This review covers the research and industrial activities on mainly Ga- and Zn-promoted ZSM-5 catalysts for the production of bio-aromatics in the thermochemical conversion of biomass and waste. The promoted ZSM-5 catalysts currently favoured in biomass aromatization have not initially been designed for biomass processing, but for the conversion of petrochemical feeds. In biomass pyrolysis, however, aromatization catalysts have to perform additional tasks to aromatization, such as cracking, reforming, decarboxylation, decarbonylation and the water-gas shift reaction. In addition, catalysts in biomass processing may have to operate with feeds containing acids, sulphur, nitrogen and minerals. Nevertheless, there is great similarity between the optimum catalysts for both petrochemical and biomass aromatization. The preferred aromatization catalysts for both gasification and pyrolysis are Ga- and Zn-promoted ZSM-5 catalysts that synergistically combine a dehydrogenation and an acidic function. Reduced extra-framework Ga cations residing at ZSM-5 exchange positions in close proximity to Brønsted acid sites are the likely active sites for aromatization. For Ga a reduction/oxidation activation procedure to form these sites is beneficial. For Zn/ZSM-5 catalysts ZnOH + species appear to be the active sites. For these catalysts no reduction/oxidation activation is required. The key process parameters for aromatization are a temperature of 400–550 °C and a low space velocity of 0.5–1.0 h −1 . In agreement with the bifunctional nature of Ga/ZSM-5 there exists an optimal Ga loading for each SAR. Best results were obtained with a molar ratio Ga/H + or Zn/H + close to 1.0. These catalysts also show the best stability. • The preferred aromatization catalysts are Ga- and Zn-promoted ZSM-5. • Extra-framework Ga cations or ZnOH + species are the likely active sites. • Proximity of metal sites to Brønsted acid sites is essential. • For Ga/ZSM-5 a reduction/oxidation activation procedure is beneficial. • A molar ratio Ga/H + or Zn/H + close to 1.0 is optimal for both catalyst activity and stability.