Study on the Pyrolysis of Solid Materials with Pyrolysis-Online Vacuum Ultraviolet Photoionization Mass Spectrometry

An online pyrolysis-vacuum ultraviolet photoionization mass spectrometer (PY-VUV PIMS) was built for the analysis of pyrolysis products of solid materials. Vacuum ultraviolet photoionization is a “soft” ionization method producing few or no fragments of molecular ions, making the identification and interpretation of complex pyrolysis products in real time possible. In this work, the gaseous pyrolysis of polypropylene at the temperatures range of 400 ℃ to 600 ℃ were firstly studied with this new-built setup. A series of mass spectra of gaseous products were obtained, and the time-evolved curves for the products like propylene (m/z 42) and pentadiene (m/z 68) were also recorded. It was found that the increase of temperature can dramatically shorten the reaction time. Due to the effect of secondary reactions, the formation time of pentadiene is a little later than that of propylene produced only from primary reaction. Reactions like chain fission, back-biting, and β-scission contribute to most of the primary reactions. The pyrolysis of tobacco was also performed with PY-VUV PIMS in the temperature range from 400 ℃ to 700 ℃, and the effects of reaction temperature on the pyrolysis products intensity were studied. For relative small molecular weight compounds like propylene (m/z 42), their relative intensities increase all the time as pyrolysis temperature rises. As for acetone/propanal (m/z 58) and some other compounds, their relative intensities first rise to the maximum and then decrease. In the case of relatively large molecular weight compounds like nicotine (m/z 162), they decrease with temperature increases, indicating that it have suffered secondary reactions under high temperature and decomposed into small molecules. Results show that vacuum ultraviolet photoionization mass spectrometry is a powerful method for pyrolysis study, which can offer important information for learning the pyrolysis mechanisms and dynamic processes.