Effect of SiO2/Al2O3 ratios of HZSM-5 zeolites on the formation of light aromatics during lignite pyrolysis

In this study, the effects of HZSM-5 zeolites with different SiO2/Al2O3 ratios on the formation and distribution of light aromatics (mainly BTEXN) were systematically investigated by in situ pyrolysis gas chromatography/mass spectrometry (Py-GC/MS), and model compounds were used to analyze the catalytic routes of HZSM-5 zeolites for the formation of light aromatics during the pyrolysis of coal. Results revealed that the role played by HZSM-5 zeolites for the catalytic upgrading of uncondensed coal tar from the pyrolysis of coal is strong related to the Bronsted and Lewis acid sites and pore structure. HZ-25 zeolite (HZSM-5 zeolite with a SiO2/Al2O3 mole ratio of 25) with a high content of Bronsted and Lewis acid sites exhibited the best promotion for the catalytic upgrading of coal tar during the pyrolysis of Coal A. Model compounds of anthracene, benzyl phenyl ether, and petroleum ether represented the condensed aromatic ring; C-O bridge bond, and aliphatic structure in coal, respectively, and BTEXN was generated over the HZSM-5 zeolite catalyst. HZ-25 zeolite with a higher content of acid sites and high specific surface area is prone to facilitating the formation of BTEXN during the pyrolysis of petroleum ether, anthracene and benzyl phenyl ether. Notably, the obtained results indicated that HZSM-5 zeolites exhibit a dual promoting role for the aromatization of aliphatic hydrocarbons and catalytic cracking of macromolecular fragments during the pyrolysis of coal.