Pyrolysis of Polystyrene over SAPO-34 Catalyst

 

Pyrolysis of Polystyrene over SAPO-34 Catalyst

Naime Aslı Sezgi, Tülay Bursalı, and Timur Doğu

Middle East Technical University Chemical Engineering Department, Ankara, Turkey

sezgi@metu.edu.tr

Thermoplastic materials have a wide variety of usage area like automotive, food and beverage industries due to their low costs, high capacity of production, and easy processing properties.  However, these materials cause a serious environmental pollution because of their long-term self-recycling. Landfilling, incineration are the main treatment methods for these materials. In the landfilling disposal method, plastic wastes are buried. They are generally nonbiodegradable, and landfilled plastic waste will be degraded after hundreds of years have been passed. Therefore, a great amount of space is required, and available free space is running out every day. In the incineration method, plastic waste is burned and highly toxic chemicals evolve in the effluent gas depending on the nature of plastic waste. Therefore, it is harmful for human health, and extra cleaning treatment units for the effluent gas are required. Because of toxic gases as a result of incineration and inadequate landfilling areas, new methods are now being searched. Considering energy need and clean environment, chemical recycling is a more proper method than the other ways. As compared with the previous techniques, it is an environment- friendly system and also more favored with its valuable end products. Moreover, the process time can be minimized and the end product efficiency can be maximized with using a proper catalyst like microporous and mesoporous materials [1-4].

The silica-alumina type catalyst, SAPO-34, was synthesized through hydrothermal synthesis route. In the synthesis of SAPO-34, tetraethylamonium hydroxide was used as a surfactant and aluminum isopropoxide and fumed silica were aluminum and silica sources, respectively. The aluminum source and the surfactant was mixed at 313 K. Then the silica source and phosphoric acid were added to the mixture. Final solution was transferred into a Teflon-lined stainless steel autoclave at 473 K for 48 h. The product was filtered and calcined in a dry air medium for 8 h at 823 K. For the first time, its performance was tested in the degradation reaction of polystyrene using a thermogravimetric analyzer. The analysis was performed under nitrogen atmosphere at a flow rate of 60 cc/min, in a temperature range of 30-550^oC with a heating rate of 5^oC/min. The ratio of catalyst to polymer was selected as 1/2.

XRD results showed that the synthesized material was SAPO-34. Its surface area and pore diameter were 308.5 m²/g and 0.28 nm, respectively. It exhibited Type I nitrogen isotherms, which indicated the microporosity of the material.  The structure of the synthesized material was cubic shape. Lewis and Bronsted acid sites were available in the structure of the SAPO-34 material. ²⁷Al MAS NMR spectrum of the material showed mainly tetrahedrally coordinated aluminum species in the structure. A standard power law model was used to describe the kinetics of polystyrene degradation reaction.  The overall order of the polystyrene degradation was found to be 1 and the activation energy was 0.73 times the activation energy of pure polystyrene. Gas product distribution indicated the presence of hydrocarbons, being lower than C5 and mainly methane, ethylene, and butane. In the presence of catalyst C₈-C₁₀ hydrocarbons in the liquid products were observed. As a conclusion, this catalyst, SAPO-34 was active for the conversion of polystyrene to lighter hydrocarbons. Value added chemicals could be recovered from the polystyrene waste under suitable conditions.

References

[1] Obalı Z., Sezgi N.A., Doğu T. Chem. Eng. Commun.196 (2009) 116–130. [2] Obalı Z., Sezgi N.A., Doğu T. Chem. Eng. J. 176-177 (2011) 202-210.

[3] Obalı Z., Sezgi N.A., Doğu T. Chem. Eng. J. 207-208 (2012) 421-425.

[4] Aydemir B., Sezgi N.A., Doğu T. AIChE J. 58 (2012) 2466-2472.