A review on the catalytic pyrolysis of biomass for the bio-oil production with ZSM-5: Focus on structure

A higher amount of oxygenated compounds in bio-oil obstructs its use in the existing petroleum-based infrastructures. In order to convert oxygenates into hydrocarbons and less reactive compounds, a number of catalysts have been studied in the catalytic fast pyrolysis. The ZSM-5 catalyst has drawn a considerable attention due to its potential to yield high quality bio-oil that is rich in aliphatic and aromatic hydrocarbons and deficient of oxygenating species. This review has focused on shape selectivity of ZSM-5 towards improving the quality of bio-oil from catalytic fast pyrolysis. The uniform pore size, unique channels equipped with elliptical and near-circular openings, mesoporosity, and tunable acidity are considered as the most selective for the production of aromatics and also are responsible for deoxygenation reaction. It has been revealed that pyrolysis intermediates diffuse into ZSM-5 pores and undergo catalytic cracking on the active sites while cleavage of C-O and C-C bonds releases oxygen and further transformation produces aromatic hydrocarbons and small molecular weight compounds. Furthermore, total acidity and the number of acid sites (Bronsted and Lewis acid) of ZSM-5 can be tuned that is promising rather than other acid catalysts for catalytic fast pyrolysis. At last, the catalytic effects of parent and modified ZSM-5 catalysts have been summarized. This review will carry the potential for the selection of catalyst with better understanding of pore size that will be a forwarding approach in the area of catalytic pyrolysis.