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Production of valuable chemicals from plastic wastes containing polyethylene and polypropylene

Habib, Abdul Rehman Rajabali
Global plastic consumption has significantly increased recently creating a serious environmental threat due to the non-biodegradability of plastics. Disposal methods like landfilling and incineration result in soil pollution and emission of toxic gases, respectively. Thus the catalytic thermal degradation of plastic offers a safer and economical alternative. In this study, aluminium and/or tungstophosphoric acid (TPA) loaded silica aerogel was used in the pyrolysis of polyethylene and polypropylene. The silica aerogel was synthesized using sol-gel technique and wet impregnation was used to incorporate the metals into the silica aerogel framework using different Al and W/Si molar ratios. Aluminum isopropoxide and tungstophosphoric acid hydrate were used as metal sources. The pyrolysis reactions were performed in the temperature range of 400- 450°C with a heating rate of 5°C/min, nitrogen atmosphere at a flow rate of 60 cc/min and a catalyst to polymer weight ratio of 1/2. All synthesized materials exhibited Type IV isotherms with pore sizes in the mesoporous material range. DRIFTS analysis revealed the existence of Lewis and Brønsted acid sites in the synthesized materials. SEM images revealed the mesoporous structure of silica aerogel. Both aluminum and TPA impregnated silica aerogel catalysts reduced the degradation reactions’ activation energy and the degradation temperature. Gas products of catalytic pyrolysis revealed that aluminium loaded silica aerogel had high methane and acetylene selectivity while TPA loaded silica aerogel had high propylene and isobutane selectivity for both polyethylene and polypropylene degradation reactions. Liquid products of catalytic pyrolysis revealed an increase in the amounts of gasoline C5-C12 hydrocarbon range. The SA-7Al-3W and SA-3Al-7W catalysts resulted in the best results for the catalytic pyrolysis of PE and PP, respectively