Thermal behavior, reaction pathways and kinetic implications of using a Ni/SiO₂ catalyst for waste tire pyrolysis

Abstract

Catalytic pyrolysis has been used to upgrading the quality of pyrolytic liquids. Herein, we report a comprehensive study on the catalytic pyrolysis of waste tires using Ni/SiO₂ as catalysts. The analyses were carried out by combining thermogravimetry (TGA), TGA interfaced to a Fourier transform infrared spectrometer (TGA–FTIR), and pyrolysis coupled to gas chromatography/mass spectrometer (Py–GC/MS) techniques. During waste tire decomposition, the main functional groups detected in the FTIR were alkenes, aromatics, and heteroatoms-containing groups such as nitrogen, sulfur, and oxygen. Meanwhile, by Py–GC/MS were identified mainly D,L-limonene, isoprene, benzene, toluene, xylenes (BTX), and p-cymene. The Py–GC/MS experiments at three different temperatures (350, 400, and 450 °C) suggested an effect of the catalyst on product distribution. The Ni catalyst promoted cyclization reactions and subsequently aromatization, leading to an improved vapors composition. The use of iso-conversional kinetic models along with master plots allows proposing a multiple-step reaction mechanism, which was well described by the Avrami–Erofeev, Random Scission, and truncated Sestak–Berggren models. The values of activation energies show differences for the catalyzed and uncatalyzed pyrolysis (111.0 kJ mol⁻¹ and 168.4 kJ mol⁻¹ ), validating the effectivity of Ni/SiO₂. Finally, the thermal Biot (> 1) and PyI and PyII numbers (10–³ < PyI < 10⁻¹ and 10⁻² < PyII < 10⁻³) confirms that the process is being occurred between the kinetic and the convection-limited regimes.