Investigation on the pyrolysis of waste poly(ethylene terephthalate) over NiMo bimetallic catalysts

With the increasing amount of plastic waste and the decreasing fossil energy, catalytic pyrolysis has become a promising technology for the production of fuels and chemicals from plastic waste. Polyethylene terephthalate (PET), a widely used plastic derived from non-renewable petroleum resources, is ubiquitous in daily life. However, the vast majority of PET is discarded within a year of use and is notoriously resistant to natural degradation, leading to significant environmental accumulation and pollution. This widespread waste and the inability to effectively recycle PET contribute to severe environmental pollution and resource depletion. In light of global trends toward sustainable development, circular economy principles, and pollution prevention, the development of efficient, sustainable, and environmentally friendly methods for processing waste PET has become an urgent priority. Among the potential solutions, catalytic pyrolysis has gained increasing attention as an effective strategy for recovering valuable chemical feedstocks from waste plastics. In this paper, we propose a novel strategy for catalytic pyrolysis of PET in a hydrogen (H2) atmosphere to produce high-value-added chemical products, such as terephthalic acid (TPA), benzoic acid, and olefins. To this end, a NiMo catalyst is designed and investigated for its performance in PET catalytic pyrolysis. The paper aims to explore the structure-activity relationship between the NiMo catalyst and PET, assess the influence of various reaction conditions on the catalytic pyrolysis process, and elucidate the reaction mechanism for the formation of TPA. In particular, the high value conversion of waste poly(ethylene terephthalate) (PET) to aromatic products rich in terephthalic acid (TPA) is successfully achieved by pyrolysis using NiMo bimetallic catalysts in H2. The catalytic performance of NiMo/Al2O3 and NiMo/ZSM-5 for pyrolysis of PET in a H2 atmosphere at different temperatures (350–550℃) was investigated. In addition, NiMo catalysts with different Ni contents (Mo: 25wt%, Ni: 1wt%–10wt%) were prepared for the pyrolysis of PET in a H2 atmosphere to evaluate the effect of Ni on the catalytic reaction. The experimental results show that the performance of NiMo catalyst is far superior to that of Ni or Mo catalyst. When NiMo/Al2O3 (Ni-5wt%) is used at 450℃, the highest aromatic compound yield of 62.6wt% is obtained and the TPA content reached 65.1%. Meanwhile, for the target product TPA, the catalytic performance of NiMo/ZSM-5 is slightly lower than NiMo/Al2O3. The excellent catalytic performance of the NiMo catalyst is due to a combination of effects. The characterization results show that active centers of the NiMo catalyst are dominated by MoO2 and Ni. The promotion effect of Ni on the catalytic reaction is not only due to its own excellent hydrogenation performance, but also limits the agglomeration growth of MoO2 particles, promotes the dispersion of MoO2, lowers the activation temperature of the catalyst to increase the active sites, improves the thermal stability of the catalyst, and enriches the acidic sites of the catalyst. In addition, the mechanism of PET catalytic pyrolysis in a H2 atmosphere was elucidated by in situ-GC. The main pathway for the conversion of PET to TPA is the transfer of Hβ and the cleavage of alkoxy groups to generate carboxylic acids and vinyl esters, followed by the hydrocracking of vinyl esters into carboxyl groups and alkenes. This work provides valuable insights into the potential for improving the sustainability and efficiency of waste PET recycling, addressing both environmental concerns and resource management challenges.