Recovery of Kraft lignin from industrial black liquor for a sustainable production of value-added light aromatics by the tandem catalytic pyrolysis

Lignin, isolated from black liquor, is an important feedstock to produce bio-aromatics through catalytic fast pyrolysis. First, the variation in chemical structure among the Kraft lignin precipitated by the organic acids (acetic acid and citric acid) and the inorganic acids (hydrochloric acid and sulfuric acid) was investigated. Then, a sustainable production approach of light aromatics from Kraft lignin was developed, namely the dual-catalyst catalytic fast pyrolysis (DC-CFP) by using the mesoporous Nb2O5 and microporous HZSM-5 as catalyst. Results showed that the molecular weight of lignin isolated by inorganic acids (1674–1841 g/mol) was much lower than that of lignin isolated by organic acids (2040–2441 g/mol), caused by the fact that inorganic acids promoted the breakage of ether bonds in lignin. In addition, compared with lignin precipitated by the organic acids, the CFP of lignin precipitated by the inorganic acids has a higher yield of light aromatics, especially the lignin precipitated by the hydrochloric acid, reaching 1.39 × 108 a. u./mg. Among the different layout modes of catalyst, Model II (Meso-Nb2O5 blended with lignin, HZSM-5 placed separately from lignin) presented highest yield of light aromatics during DC-CFP of lignin, reaching 1.78 × 108 a. u./mg.