Product characterization and synergistic studies of in situ co-pyrolysis and catalytic pyrolysis of xylan and polyvinyl chloride

Abstract

Through the co-pyrolysis of PVC and xylan, the HCl released during pyrolysis effectively catalyzes the deoxygenation of oxygenated intermediates (such as aldehydes, ketones, and acids) derived from xylan pyrolysis, thereby suppressing the formation of these coke precursors and reducing char residue. To further enhance the co-pyrolysis efficiency of PVC and xylan, this study employed two types of medium- to high-temperature heterogeneous catalysts for the catalytic pyrolysis of PVC and xylan mixtures. An innovative approach combining self-developed in situ pyrolysis photoionization time-of-flight mass spectrometry with in thermogravimetric infrared spectroscopy was utilized for in-depth investigation. Specifically, a detailed analysis of the catalytic effects and mechanisms during the co-pyrolysis of PVC and xylan was conducted using representative aluminosilicate catalysts (HZSM-5, ZSM-5, USY) and a metal oxide catalyst (ZnO). The results indicate that, compared to the pyrolysis of pure xylan, the relative intensity of polycyclic aromatic hydrocarbons (PAHs) generated from the co-pyrolysis of PVC and xylan decreased (for instance, by 25.4% and 20.6% at m/z = 130 and 180, respectively). Furthermore, both types of heterogeneous catalysts promote PVC pyrolysis to release HCl and effectively lower the formation temperature of co-pyrolysis products from PVC and xylan, with zinc oxide demonstrating the most pronounced catalytic effect. The acid sites of aluminosilicate catalysts facilitate the deoxygenation of xylan, while ZnO promotes the formation of polyene chains from PVC and the dehydrogenation of xylan-derived molecules. Overall, catalytic pyrolysis is a highly promising method for chemical recycling, enabling the transformation of waste plastics and xylan into high-value chemicals or fuels.