Effect of acidity of solid acid catalysts during non-oxidative thermal decomposition of LDPE

Thermal decomposition of low-density polyethylene (LDPE) was monitored by thermogravimetry under N₂ atmosphere in the presence of solid acid catalysts such as alumina (α-Al₂O₃, γ-Al₂O₃), crystalline silica-alumina (SA, molar ratio of Si/Al = 0.19) and amorphous silica-alumina catalysts (ASA, molar ratio of Si/Al = 4.9). Crystal structure and surface area of solid acid catalysts were measured by XRD and BET, respectively. The strength and distribution of acid sites of solid acid catalysts were estimated by NH₃-TPD. It was observed that total acidity strength is in the order of ASA (1.77 μmmol NH₃/g) > AS (1.42 μmol NH₃/g) > γ-Al₂O₃ (1.06 μmol NH₃/g) > α-Al₂O₃ (0.06 μmol NH₃/g). Thermal degradation behavior of LDPE with and without solid acid catalyst was monitored by TGA, where heating rates (β) of 5, 10, and 20 °C/min were employed under an inert atmosphere, and their activation energies (E_a), onset temperatures (T_initial), decomposition temperatures (T_decomp) were calculated and compared. The activation energy (E_a) was evaluated using the Coats-Redfern method. Solid acid catalysts with stronger acidity and higher surface area showed a decrease in activation energy and onset temperature. Activation energy of LDPE over ASA catalyst is decreased to 97.3 kJ/mol from thermal decomposition of LDPE without catalyst of 117.2 kJ/mol under heating rate of 10 °C/min. The isothermal decomposition of LDPE was monitored at 300 °C for 3 h with a heating rate of 10 °C/min, where 13.1% and 24.2% wt. loss were observed over SA and ASA, respectively, while only 0.7% wt. loss was observed for LDPE without a solid acid catalyst.

Graphical abstract

Single step decomposition of LDPE

Thermal degradation behavior of LDPE monitored by TGA, with different heating rates (β) of 5, 10, 20 °C/min.