Production of light aromatics from the fast pyrolysis of lignin catalyzed by metal-modified H-ZSM-5 zeolites

A series of metal (Zn, Ga and Mg) modified H-ZSM-5 bifunctional catalysts were prepared by impregnation method. Three types of milled wood lignin (MWL), isolated from softwood (Chinese Fir, CF), hardwood (Poplar, P) and herbaceous plant (Corn Straw, CS), were served as starting material to produce light aromatics via the catalytic fast pyrolysis (CFP). The effect of metal modifying and lignin source on the component of bio-oil derived from the lignin CFP was investigated. The results indicate that: Among three types of MWL, CF-MWL has the highest carbon content (59.90%) and calorific value (23.05 MJ/kg), whereas CS-MWL has the highest hydrogen content (6.51%) and effective hydrocarbon ratio (0.43); Compared to H-ZSM-5, Ga/H-ZSM-5 and Zn/H-ZSM-5 can promote the production of light aromatics, whereas Mg/H-ZSM-5 inhibits the formation of light aromatics. Zn/H-ZSM-5 as a catalyst in the lignin CFP displays the highest yield of light aromatics, with 3.122 × 10⁹ a.u./mg for CF-MWL, 2.916 × 10⁹ a.u./mg for P-MWL, and 2.865 × 10⁹ a.u./mg for CS-MWL. Among three types of MWLs, the CFP of CF-MWL gives highest selectivity of 65.02% to BTX. The coke is mainly deposited on the outside surface of zeolite catalyst during the pyrolysis, leading to a great decrease in the number of strong acid sites.