Catalytic pyrolysis of lignin over hierarchical HZSM-5 zeolites prepared by post-treatment with alkaline solutions

The application of hierarchical HZSM-5 zeolite catalysts in the catalytic fast pyrolysis (CFP) of lignin was investigated using a pyrolyzer coupled with gas chromatography-mass spectrometry (Py-GC-MS). A series of modified HZSM-5 zeolites were prepared by the post-treatment desilication of commercial HZSM-5 zeolite with varying concentrations of sodium hydroxide (NaOH) solutions (0.1 - 0.5 mol/L) and the following solutions: 0.3 mol/L of sodium aluminate (NaAlO2), 0.3 mol/L of sodium carbonate (Na2CO3), and 0.3 mol/L of tetrapropylammonium hydroxide (TPAOH). The catalyst samples were characterized by XRD, XRF, TEM, N-2 adsorption-desorption, and NH3-TPD techniques. The crystallinity, the SiO2/Al2O3, the morphology, the pore properties, and the density of acid sites of the HZSM-5 zeolites were changed after alkali treatment. The alkali treatment improved the catalytic performance of HZSM-5 zeolite for cracking bulky oxygenates released from lignin (such as guaiacol, syringol, and their derivatives from the pyrolysis of lignin) to produce aromatic hydrocarbons. The HZSM-5 zeolite treated with 0.3 mol/L NaOH was the optimal choice for the catalytic fast pyrolysis of lignin for producing aromatic hydrocarbons.