Structural elucidation of Kraft lignin isolated from black liquor by the gradient acid precipitation and highly selective production of light aromatics by tandem catalytic pyrolysis over dual-catalysts

Kraft lignin (KL), isolated from black liquor, serves as a significant feedstock for the production of bio-aromatics by catalytic fast pyrolysis (CFP). First, the influence of pH on the chemical structure of KL isolated by the gradient acid precipitation (GAP) was investigated. Then, the dual-catalyst CFP (DC-CFP) was employed to convert KL into light aromatics by using the hierarchical Nb2O5 (Hie-Nb2O5) and the microporous HZSM-5 as catalyst. Results showed that the decrease of pH during GAP promoted the breakage of the ether bond of KL, resulting in lower molecular weight and less content of hydroxyl group in KL. Among the four types of metal oxides (HieNb2O5, Al2O3, MgO, and CaO), the synergistic effect of dual-catalyst grouped by Hie-Nb2O5 and HZSM-5 was best for the production of light aromatics. In addition, the maximum production of light aromatics was reached at the pH of 2 and the mass ratio of Hie-Nb2O5-to-HZSM-5 at 2:2.