Mechanistic insights into nitrogen transformation during zeolite-catalyzed fast pyrolysis of soybean protein

Catalytic fast pyrolysis of nitrogen-rich biomass offers a promising route for producing high-value N-containing chemicals while reducing NOₓ emissions. This study systematically examines the effects of pyrolysis temperature, residence time, zeolite Si/Al ratio, and external nitrogen sources on the conversion behavior of soybean protein. The results show that appropriately elevated catalytic temperatures and reduced space velocities significantly promote the formation of N-heterocycles such as indole and quinoline. Mildly acidic HZSM-5 with abundant micropores, as well as added inorganic nitrogen sources, further enhance product selectivity. Spectroscopic analysis reveals a sequential evolution of nitrogen species on the catalyst surface, in which protein-derived quaternary nitrogen gradually transforms into pyridinic N and subsequently into N-heterocyclic intermediates, accompanied by the release of CO₂ and NH₃. These findings elucidate the mechanistic pathway of nitrogen migration during catalytic pyrolysis and provide guidance for the targeted synthesis of high-value N-containing chemicals.