Biomass Catalytic Pyrolysis over Zeolite Catalysts with an Emphasis on Porosity and Acidity: A State-of-the-Art Review

The massive consumption of petroleum and the increasingly deteriorating environment has triggered the attention and development of sustainable chemistry recently. Biomass source is the only renewable carbon source on Earth, which is regarded as the potential alternative to the current fossil fuels that cannot be regenerated in a short time. Past decades have witnessed great advances in the utilization of biomass with developed technologies. However, the quality of bio-oils obtained from pyrolysis at the various temperature is poor, which cannot be directly used as valuable chemicals and fuels. Molecular sieve catalyst (zeolite) have successfully applied in the catalytic conversion of different products after biomass pyrolysis, because of their uniform pore structure, strong acidity, and stable framework. This review emphasizes the traditional and the modification of the zeolite acidic active sites and pore structure and their roles in biomass catalytic pyrolysis. First, we compared the catalytic effects of biomass and its derivatives on traditional zeolites. The results show that ZSM-5 has the best catalytic effect (more high-value products), because of its shape selectivity. Different from the previous articles about the effect of modification on one side, this Review mainly introduces the combination of different methods to modify and optimize acidic sites and pores to improve the catalytic effect of rapid biomass pyrolysis. The zeolite was modified to improve the selectivity and yield of the target product, and a certain optimization effect was achieved. From different methods of mesopore formation to metal doping, the ways of adjusting zeolite catalysts and discussions of changes in acidity after metals being doped into zeolites are reviewed. Simultaneously, a hydrogen source was introduced to deal with the problem of insufficient hydrogen content, to improve the quality of bio-oils. Finally, the advantages and disadvantages of catalytic conversion over the modified zeolite (ZSM-5) are analyzed, and the prospects in this field are also discussed.