Catalytic Pyrolysis of Lipid-Extracted Microalgae over NaOH-Treated HZSM-5: Porosity–Acidity Synergy in Aromatic Production and Denitrogenation

Catalytic pyrolysis of byproducts after lipid extraction of microalgae with HZSM-5 is inefficient in converting heavy intermediates, primarily due to the catalyst’s pore-size constraints and the feedstock’s high nitrogen content. To promote the diffusion and cracking of these heavy and nitrogen-rich compounds, NaOH solution was used to pretreat HZSM-5 for tailoring its pore structure and acidity distribution. The effect of NaOH concentration in modifying HZSM-5 and its influence on the aromatic hydrocarbons (AHs) production and denitrogenation (DeN) were systematically investigated via pyrolysis–gas chromatography and mass spectrometry (Py–GC/MS) at a pyrolysis temperature of 500 °C. The quantitative evaluation of deoxygenation and denitrogenation performance as well as the correlation of zeolite properties and AH production/DeN was studied. Results demonstrated that NaOH-treated HZSM-5 retained the MFI topology while developing substantial mesoporosity. The 0.05MZ (0.05 M NaOH-treated) catalyst exhibited an 8.0% increase in strong acid sites, leading to about 73% and 86% reductions in amides and N-heterocyclic compounds, respectively, while doubling the nitrile yield. The corresponding deoxygenation and denitrogenation efficiencies reached 0.91 and 0.49, respectively. Optimal aromatic hydrocarbon production was achieved with the 0.1MZ (0.1 M NaOH-treated) catalyst, showing an 11% increase in relative yield compared to that of HZSM-5. A negative correlation was observed between the mesopore volume and the formation of AHs, while strong acidity was positively correlated with both AHs production and denitrogenation. These findings may guide the optimization of HZSM-5 catalysts to enhance the molecular diffusion and shape selectivity for heavy nitrogen-containing compounds from microalgae pyrolysis.