Role of catalyst porosity and acidity in nitrogen transformation during catalytic fast pyrolysis of microalgae: Study on extracted protein and model amino acids

Valorizing defatted microalgae after lipid extraction maximizes the value derived from microalgae. Catalytic fast pyrolysis (CFP) of defatted microalgae effectively promotes denitrogenation, thereby advancing the sustainable production of aromatic hydrocarbons (AHs). This study explored how the intricate structures of various amino acids (lysine, proline, and tryptophan) and extracted microalgae protein influenced nitrogen transformation pathways by means of pyrolysis − gas chromatography/mass spectrometry (Py-GC/MS) at 500 °C. The roles of acidic sites and pore sizes of metal-doped (0.5Ni) and alkali-treated (0.05 M) HZSM-5 (Hydrogen Zeolite Socony Mobil-5) catalysts in denitrogenation and aromatization were focused upon. The doping of Ni led to a 2.5 % increase in medium acidity, whereas the alkaline pretreatment resulted in a 40.0 % increase in mesopore volume. The relative yields of AHs from extracted protein increased by 10.0, 10.3, and 10.5 times with the addition of HZSM-5, 0.05 M and 0.5Ni, respectively. The denitrogenation indices of the extracted protein were 0.22, 0.28 and 0.31 when HZSM-5, 0.05 M and 0.5Ni catalysts were applied, respectively. The results revealed that surface area enhanced the adsorption of intermediates from lysine, facilitating their entry into pore channels for subsequent reactions on acid sites. The formation of mesopores in the 0.05 M catalyst improved mass diffusion and accessibility of acids sites for the pyrolysis of proline and tryptophan which had a larger molecular size than lysine. A hydrogenation catalyst like Ni was crucial especially for the cleavage of N-heterocyclic amino acids with lower degree of saturation within N-containing bonds. This research provides a basic understanding of the roles that chemical structures of amino acids and catalysts synthesis play in the efficient denitrogenation and AHs production from microalgae pyrolysis.