PETpyrolysis and hydrolysis mechanism in the fixed pyrolyzer

Abstract

In the conventional polyethylene terephthalate (PET) pyrolysis process, the formation of char by excessive pyrolysis is mainly due to the dehydration mechanism, so water is considered an auxiliary agent that can effectively inhibit excessive pyrolysis. The preparation of terephthalic acid (TPA) by steam‐assisted pyrolysis of PET is an effective method to achieve closed‐loop recycling of waste PET. To ensure that the reaction is mild enough to reduce excessive cracking products such as char and benzoic acid and thus increase the yield of TPA, it is critical to reduce the reaction rate while maintaining a sufficient excess steam coefficient. Under the optimal operating conditions, when the temperature rise rate was 0.5 °C min−1 and the excess steam coefficient was 150, the yield of TPA was 72.5 wt.%, and the purity was 85.5%. Noticeably, the steam‐assisted pyrolysis system is a heterogeneous reaction system whose reaction mechanism is different from the conventional hydrolysis and pyrolysis reactions and has a unique reaction path. The mechanistic study indicates that, in addition to the thermal cracking of PET molecules occurring in conventional pyrolysis, hydroxyl attack and transfer, and supplementation of benzene ring hydrogen also occur between water and intermediate molecules. Meanwhile, it has also been proven that the intermolecular hydrogen transfer between intermediate molecules and water molecules is the key to reduce the intensity of the reaction and inhibit the formation of char. This discovery illustrates the mechanism of the reaction between water and PET in the steam‐assisted pyrolysis process in the fixed pyrolyzer and justifies the distinction between it and the pyrolysis and hydrolysis processes of PET. It provides a theoretical basis for optimizing the pyrolysis process of PET, which is essential for the industrialization of TPA preparation from PET steam‐assisted pyrolysis.

Catalytic Steam-Assisted Pyrolysis of PET for the Upgrading of TPA

“…In a previous study [ 22 ], it was noticed that water can contribute to enhancing the yield of TPA through both the hydroxyl attack pathway and hydrogen transfer pathway. To further clarify the effect of water on the different reaction pathways, isotope experiments using deuterated water were conducted as a tracer reagent.…”

“…When the water flow velocity was lower than 0.1 mL/min, the relative content of molecules with a mass-to-charge ratio (m/z) of 167 decreased rapidly from 8.72% to 3.37% as the flow rate decreased, but the change in the relative content of molecules with an m/z mass ratio of 166 was not significant. In previous studies [ 22 ], it has been clarified that a molecule with an m/z of 166 represents a reaction in which a hydroxyl group attacked the ester bond, and a molecule with an m/z of 167 represents a hydrogen transfer reaction on the benzene ring in addition to the hydroxyl group attack. As shown in Figure 1 e, when the water flow rate was 0.02 mL/min, the hydroxyl attack reaction had fully occurred, but the hydrogen transfer reaction on the benzene ring was incompletely carried out.…”

“…As shown in Figure 6 b, not only the migrated carbon, hydrogen, and oxygen atoms were dramatically reduced by the catalytic effect of the Pt@Hzsm-5, but also the O/C and H/C in the gas were significantly reduced. This was mainly due to the enhanced intermolecular hydrogen transfer efficiency due to the Pt@Hzsm-5, which allows for more hydrogen and oxygen to be immobilized on the TPA in the reaction [ 22 , 24 ].…”

“…At this point, it is noticed that since the benzene ring in the TPA comes entirely from PET, the mass of this fraction is not disturbed by water. Therefore, it was defined that the mass ratio between the product and the benzene ring in the raw material was used to calculate the yield to reflect the production of TPA as follows [ 22 ]. Among them, mTPAbr is the mass of the benzene ring in the produced TPA, and mPETbr is the mass of the benzene ring in the initially added PET.…”

“…In a previous study, it was found that the production of char could effectively be inhibited under a steam atmosphere, and the yield of TPA could be improved [ 22 ]. Through the study, it was found that as an exogenous hydrogen source, water effectively inhibited the excessive cracking of PET during the pyrolysis through an intermolecular hydrogen transfer reaction.…”