(621et) 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.