In situ analysis of volatiles obtained by catalytic cracking of polyethylene with HZSM-5, HY, and HMCM-41

A variety of plastic waste recycling methods have been established and new recycling approaches are being developed to avoid placing polymers into landfills. One approach to waste plastic recycling, known as tertiary recycling, consists of decomposing plastics into useful chemicals or fuels. Repetitive injection thermal analysis gas chromatography mass spectrometry and thermal analysis mass spectrometry allow us to identify and quantify volatile products evolved from complex temperature-dependent systems. Volatile products from cracking/hydrocracking of low molecular weight polyethylene (LPE) were analyzed and activation energies of formation were determined when HZSM-5, HY, HMCM-41, and their platinum loaded analogs were employed as cracking catalysts.; When LPE is heated in helium with HZSM-5, paraffins are detected initially and olefins are produced at somewhat higher temperatures. Volatile paraffin formation by disproportionation reactions catalyzed by external HZSM-5 acid sites is favored due to the low activation energy values for this pathway at low temperatures. Small olefins (C₃–C₅) are the most abundant products when HZSM-5 and HMCM-41 catalysts are employed for cracking LPE. In contrast, cracking with HY produces primarily paraffin volatile products (C₄–C₈). HY pores are large enough and acid sites are strong enough to promote disproportionation reactions, which lead to formation of volatile paraffins.; When polymer/catalyst samples are heated in hydrogen, the extent of hydrogenation is reflected by reduced residue content and variations in E_a versus temperature curves. The addition of platinum increases volatile aromatic and olefin yields and/or residue content when polymer/catalyst samples are heated in helium. Bifunctional hydrogenation reactions dominate volatile product forming reactions, resulting in mainly paraffin products and small amounts of residue. Activation energy value differences between polymer/Ptcatalyst samples heated in hydrogen and the same samples heated in helium may be responsible for observed temperature shifts. The magnitudes of hydrogenation and/or platinum catalyzed effects appear to be related to catalyst pore size and acidity.; Volatile product slates derived from LPE cracking/hydrocracking differ significantly with temperature, reaction atmosphere, and catalyst physical characteristics (i.e. pore size, acidity, metal loading). When thermal analysis-gas chromatography mass spectrometry and thermal analysis-mass spectrometry results are considered, volatile product variations can be rationalized by effects of catalyst acidity and/or pore size on mono- and bifunctional cracking mechanisms.