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Benzene rich oil by the decarboxylation of PET
Guido Grause1), Tadaaki Mizoguchi2), Toshiaki Yoshioka1), Akitsugu Okuwaki1),
Tomohiko Handa2)

1)

Graduate School for Environmental Studies, Tohoku University, Aramaki Aza Aoba 6-6-07
Aoba-ku, Sendai 980-8579
2)
Research Institute for Environmental Conservation, Tohoku University, Aramaki Aza Aoba
6-6-11, Aoba-ku, Sendai 980-8579,
1. Abstract
Benzene can be obtained by the hydrolysis and subsequent decarboxylation of PET. Since
benzene is a valuable product (the prize increased by 300% in the last four years), it isdesirable to obtain a pure product. Since it is possible to obtain pure PET for processing,benzene can be recovered in yields between 70 % and 75 % with a purity of 97 % employing
a heating rate of 2 K/min between 300 °C and 500 °C in the presence of CaO in steam
atmosphere. CaO accelerates the hydrolysis of PET and decarboxylates the resulting
terephthalic acid. CO2 is mainly absorbed by CaO.
2. Introduction
PET can be obtained as a pure material after use, especially from bottles. Due to this fact, it ispossible to focus on the recovery of a single main product depending on the reaction
conditions. Terephthalic acid [1] and benzene [2] were obtained in the past in different ways.
Especially benzene can be achieved by the decarboxylation of PET in the presence ofCa(OH)2 in high yields (88 %) and purity (79 wt%). But for the production of chemicals, ahigh purity is as important as a high yield, therefore, it reduces the number of purification
steps. In this paper we describe a way of further purification during the pyrolysis of PET in
the presence of CaO.
3. Experimental
In opposite to the one-step reactor used withCa(OH)2 [2], the reaction with CaO was
carried out in a fixed bed reactor with two
separate controlled heating zones (Figure 1).
The sample consisting either of PET (0.40 g)
and CaO (1.17 g at each position) or just
PET was dropped into the upper heatingzone in a semi batch process. The second
heating zone was filled with CaO before theexperiment was started. The hydrolysis took
place in a steam atmosphere over a time of30 min resulting in terephthalic acid. The
hydrolysis products passed the CaO layer Figure 1 Experimental apparatus
with the gas stream and the terephthalic acidwas decarboxylized producing benzene. The products were condensed and analysed.
In the packed column reactor, the reactor was filled with 50 g CaO and the sample of 0.50 g
PET was added at once when the reactor reached a temperature of 300 °C. Then thetemperature increased with heating rates from 2 to 10 K/min to a maximum temperature of550 °C.

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4. Results and Discussion
There were big differences in the product distribution depending on the position of the CaO
bed (Figure 2). When CaO was placed in the upper position (450 °C), little benzene wasproduced. It was possible to double the benzene yield by the deposition of CaO in the lower
position at 700 °C. The lack of benzene resulted in the formation of calcium terephthalate in
the upper position. It can be seen that terephthalic acid was released slowly, while at thistemperature terephthalic acid was little decarboxylated. The deposition of CaO in the lower
position led to an effective reduction of terephthalic acid and benzoic acid. The benzene yield
increased drastically, when PET was hydrolysed in the absence of CaO in the upper position.
The terephthalic acid formed during the hydrolysis was transported unhindered to the lower
CaO layer, where it was decarboxylised. The residue consisted mainly of CO2 and unreacted
PET in the upper position and char and CO2 in the lower position.
Two Step process
One Step process

1001008080

100Benzene
Residue

40
Sublimation
products

060

Other oils
BenzeneCO2 as CaCO3
100 % Benzene yield
Otheｒ oil40

Benzene200
1st&2n2ndMixed with PET60Gas
4020Gas

1stYield ［C%］Yield［C%］

100 % Benzene yield

Benzene purity [%]Benzene yield ［%］602080Benzene recovery

Residue02510Hold at 400℃

Heating rate［K/min］

CaO
PET : CaO = 1: 5（molar ratio）、700℃、steam ratio = 89%

Figure 2 PET-degradation in one andtwo steps

PET = 0.50[g]、CaO = 50[g]、steam ratio = 89%

Figure 3 Product distribution at different
heating rates

More effective as the two step reactor was the packed column reactor. The highest benzene
yield was obtained at a heating rate of 5 K/min (75 %) with a purity of 92 wt%, but thehighest purity (97 wt%) was reached at a heating rate of 2 K/min with a benzene yield of
74 %. The results for 10 K/min were similar to an experiment where the temperature was kept
at 400 °C over the whole reaction time. The CO2 was mainly absorbed in the CaO column.
The residue might consist of unreacted PET and undecarboxylised terephthalic acid.
5. Conclusion
These experiments show that it is possible to obtain benzene from PET waste in high yields
and purity. The best results were achieved with the packed column reactor at a heating rate of2 K/min. A benzene yield of 74 % with a purity of 97 wt% was obtained. Instead of that, thetwo step reactor was little effective. Dependent on the CaO position, little benzene with poor
purity was obtained. The high temperature of 700 °C in the lower CaO position promoted theformation of biphenyl and especially of char. The formation of char was avoided by themoderate temperature (<550 °C) used in the packed column reactor.
6. References[1] G. Grause, W. Kaminsky, G. Fahrbach: Polym. Degrad. Stab. 85 (2004) 571-575[2] T. Yoshioka, E. Kitagawa, T. Mizoguchi, A. Okuwaki: Chem. Lett. 33 (2004) 282-283

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