Reaction kinetics and product distributions in thermal and catalytic pyrolysis of agricultural mulch films over HZSM-5 zeolite

Mulch films support crop growth and food security by conserving resources, yet spent films contribute to microplastic pollution, posing disposal challenges. Pyrolysis offers a promising way to recycle these films, often mixed with soil and organic residues. This study explores thermal and catalytic pyrolysis of spent mulch films (over HZSM-5 zeolite), to assess reaction kinetics and product distribution. Characterization of fresh and spent films showed minor physical degradation and slight oxidation in the latter. Thermogravimetric analysis revealed similar thermal degradation patterns for both film types, though HZSM-5 reduced the peak temperature by 40–60 °C, with a more marked reduction in the mass loss rate for spent films. Kinetic analysis using a single-step model indicated that HZSM-5 lowered the activation energy from 144.5 kJ mol−1 to 89.8 kJ mol−1, underscoring its catalytic effect. Mulch film pyrolysis can be described by the Avrami model. Thermal pyrolysis generated ethene, propene, and liquid hydrocarbons with a wide carbon number range. In-situ catalytic pyrolysis enhanced gas yields rich in propane and propene, while ex-situ pyrolysis promoted H₂, C₁–C₃ hydrocarbons in the gas phase and naphthalenes in the condensed phase. The large number of hydrocarbons in the liquid phase reflected the roles of free radical and carbenium ion mechanisms (especially in in-situ pyrolysis), while polyaromatic hydrocarbons resulted from the catalyst's Brønsted acidic sites and high temperatures. Contaminants in spent films affected catalyst efficiency, suggesting a need for optimizing pyrolysis conditions. These findings provide insights into catalytic pyrolysis for sustainable mulch film recycling.