Nitrogen Fate in Polyurethane Decomposition: Pathways Uncovered by In Situ Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry

This study provides advanced molecular-level insights into the complex pyrolysis mechanisms of polyurethane (PU), particularly concerning the formation and fate of hazardous nitrogen-containing compounds. By uniquely combining advanced analytical techniques, including synchrotron vacuum ultraviolet photoionization mass spectrometry, gas chromatography–mass spectrometry, and thermogravimetric Fourier transform infrared spectroscopy-mass spectrometry, the research successfully identified approximately 45 pyrolysis species, including previously unreported intermediates like 4-isocyanato-1-methylbenzene and isocyanatobenzene, and dynamically tracked their evolution over time and temperature. Based on the experiments, two reaction mechanisms related to PU decomposition were proposed at the molecular level, including a detailed reaction pathway and the nitrogen migration and transformation mechanism in the three-phase products. The results indicated that the initial decomposition of PU involves two types of bonds breaking: C–O and C–N. The former formed 4,4’-methylenediphenyldiisocyanate then resulting in the formation of isocyanic acid, hydrogen cyanide, nitric oxide, nitrogen dioxide, and nitrous oxide. The latter generated 4,4’-methylenedianiline further decomposing to aniline, toluidine, and NH3. These findings are critically important for developing more accurate models for predicting toxic emissions during PU fires or waste thermal treatment and for designing safer PU materials or optimizing pyrolysis processes to minimize the release of harmful nitrogenous pollutants into the environment.