Model compound study about catalytic Co-Pyrolysis of lignin and polyethylene by using online photoionization mass spectrometry and ReaxFF MD simulation

Catalytic co-pyrolysis of lignin and hydrogen rich plastics with zeolite can potentially mitigate the problems including low conversion and severe coking with lignin. However, controversial conclusions still exist regarding the synergistic effects and cross-reaction pathways between the two feedstocks, resulting from complex product composition and limited testing methods. To clarify the aforementioned problems, the current study investigated the catalytic co-pyrolysis of model compounds from lignin (guaiacol) and polyethylene with HZSM-5 by using in-situ sampling and online vacuum ultraviolet photoionization mass spectrometry (VUV-PIMS). Specifically, C6 hydrocarbons (n-hexane, hexene, and hexadiene) with different unsaturation degrees were chosen to understand the role of plastic-derived volatiles in co-pyrolysis. Pyrolysis of individual feedstock showed that the increase of double bonds in C6 hydrocarbons could enhance the production of aromatic hydrocarbons, while the conversion of guaiacol to aromatic hydrocarbons occurred following the pathways as: guaiacol→catechol→phenol&cresol→BTX (benzene, toluene, and xylene). Analyzing the evolution of major intermediates and products from co-pyrolysis showed that hydrogen transfer from C6 hydrocarbon to guaiacol significantly enhanced the conversion of catechol to phenol/cresol and subsequent hydrodeoxygenation to BTX. Hexadiene showed the strongest synergistic effects with guaiacol, as abundant hydrogen was generated from aromatization reaction and then transferred. Fitting the evolution curves with 1st order catalyst deactivation kinetics model indicated that the decay factors calculated based on the yield changes of BTX and typical phenolic products were significantly lower with co-pyrolysis. The synergistic effects to mitigate catalyst deactivation and coke formation were further confirmed by characterizations of spent catalysts, catalytic coke and coke precursors, strength of which followed the order as hexadiene (16.7%) > hexene (13.0%) > n-hexane (8.5%). These experimental observations were further corroborated by ReaxFF MD simulations, which provide complementary evidence for hydrogen-transfer interactions.