Conversion of polypropylene into valued-added gasoline range hydrocarbons by catalytic pyrolysis at atmospheric pressure: different composite catalysts of HZSM-5, HY and MCM-41

Background

The synergic effect of textural and acidity properties of zeolite composite catalysts needs to be investigated, to improve catalyst design for plastic pyrolysis. The proximity of the acid sites of the catalysts can increase accessibility and reduce the diffusion limitation of macromolecules, improving the performance of zeolite catalysts.

Methods

In this study, polypropylene (PP) pyrolysis was studied over different zeolites (ZSM-5, HY, and MCM-41) and their diverse composites in a dual-bed fixed bed reactor at atmospheric pressure. The parent catalysts were synthesized by hydrothermal method and two techniques (granular mixing and powder mixing) were applied to prepare the composite catalysts. The structure and acidity of the catalysts were characterized by FE-SEM, XRD, BET-BJH, NH₃-TPD, FT-IR, and TGA techniques.

Significant Findings

The catalysts included a uniform morphology, high crystallinity, high surface area (>360 m²g⁻¹), and mesopore structure. The MCM-41/HY composite catalyst had superior catalytic performance in both forms of granular mixing and powder mixing. The highest amount of gasoline hydrocarbons (76.6 %) was achieved by the powder mixing, including olefin (28.3 %), paraffin (1.7 %), isoparaffin (8 %), aromatic (5.6 %), and cyclo compounds (32.4 %). The developed catalyst showed high stability, reusability, and low coke tendency. Furthermore, high activity was achieved in the pyrolysis of real PP waste plastic. The composite catalysts offer improved shape selectivity and porosity, reducing the diffusion limitation of macromolecules and leading to high activity in multi-step/cascade reactions. The results confirmed the high potential of the zeolite composite catalysts in catalytic pyrolysis of PP.