Kinetic Analysis for the Catalytic Pyrolysis of Polypropylene over Low Cost Mineral Catalysts

Abstract

A kinetic analysis of non-catalytic pyrolysis (NCP) and catalytic pyrolysis (CP) of polypropylene (PP) with different catalysts was performed using thermogravimetric analysis (TGA) and kinetic models. Three kinds of low-cost natural catalysts were used to maximize the cost-effectiveness of the process: natural zeolite (NZ), bentonite, olivine, and a mesoporous catalyst, Al-MCM-41. The decomposition temperature of PP and apparent activation energy (Ea) were obtained from the TGA results at multiple heating rates, and a model-free kinetic analysis was performed using the Flynn–Wall–Ozawa model. TGA indicated that the maximum decomposition temperature (Tmax) of the PP was shifted from 464 °C to 347 °C with Al-MCM-41 and 348 °C with bentonite, largely due to their strong acidity and large pore size. Although olivine had a large pore size, the Tmax of PP was only shifted to 456 °C, because of its low acidity. The differential TG (DTG) curve of PP over NZ revealed a two-step mechanism. The Tmax of the first peak on the DTG curve of PP with NZ was 376 °C due to the high acidity of NZ. On the other hand, that of the second peak was higher (474 °C) than the non-catalytic reaction. The Ea values at each conversion were also decreased when using the catalysts, except olivine. At <0.5 conversion, the Ea obtained from the CP of PP with NZ was lower than that with the other catalysts: Al-MCM-41, bentonite, and olivine, in that order. The Ea for the CP of PP with NZ increased more rapidly, to 193 kJ/mol at 0.9 conversion, than the other catalysts.

Critical issues for the deployment of plastic waste pyrolysis

“…19 The methodology for kinetic assessment of catalytic pyrolysis via lumped models follows the same approach described for thermal decomposition, hence kinetic parameters can be evaluated for different solid catalysts and polymers. 46,[163][164][165][166][167][168][169][170][171][172] As a general trend (see Table S3 in the ESI †), catalysts cause a reduction of the activation energy with respect to thermal pyrolysis and modify the values of the kinetic parameters fitted since the catalyst presence induces important changes in the reaction mechanisms. On the other hand, experimental validation is done via reactant conversion and (apparent) kinetic parameters are expected (both for thermal and catalytic pyrolysis).…”