Online Study on the Pyrolysis of Polypropylene over the HZSM-5 Zeolite with Photoionization Time-of-Flight Mass Spectrometry

The production of hydrocarbons through the pyrolysis of polyolefins is a promising way of fuel recycling. In this work, online single-photon ionization time-of-flight mass spectrometry (SPITOFMS) was used to study both thermal and catalytic decompositions of polypropylene (PP) in a tubular furnace. SPI produces few or no fragments of molecular ions, making the identification and interpretation of complex pyrolysis products in real time possible. The mass spectra at different reaction temperatures and time-evolved profiles of selected species during the pyrolysis processes were measured. The pyrolysis products can be classified into three groups: alkenes, dienes, and aromatics. As the coke precursors, aromatics mainly composed of benzene, toluene, and xylene (BTX) were observed at a very low temperature of 300 degrees C with the presence of HZSM-5, which cannot be detected for pure PP until 700 degrees C, indicating that HZSM-5 can accelerate the coke formation. Because of the secondary reactions, different tendencies of product intensities were exhibited as the reaction temperature increased. In addition, in comparison of the time-evolved profiles of the alkenes and BTX under high temperatures, a two-stage catalytic degradation process taking place on the external surface and the micropores of HZSM-5 was verified. A degradation mechanism was also proposed for the pyrolysis of PP with a low HZSM-5 content based on the time-evolved profiles performed at a low temperature. This work demonstrates the good performance of SPITOFMS for the online study of the polymer pyrolysis as well as the evaluation of the catalyst.