Comprehensive chemical description of pyrolysis chars from low-density polyethylene (LDPE) by thermal analysis hyphenated to different mass spectrometric approaches

Abstract

In the last decades the production and demand of plastics has drastically increased, with severe environmental impact. Concerning climate change and the idea of a circular economy, waste incineration is not favored, and efficient recycling strategies are needed. As one of the most promising approaches, pyrolysis generates a certain amount of solid residue besides liquid and gaseous products.The chemical nature of this char is not fully understood, but it holds the potential to be used in material science or to generate further chemicals. To explore the value of this feedstock, thermal analysis with mass spectrometric detection of the evolved gas mixture is deployed. With the help of soft photoionization techniques, we were able to identify alkenes, dienes, and polycyclic aromatic hydrocarbons (PAHs), which were emitted at four distinct mass loss events. Here, resonance-enhanced multiphoton ionization allows selectively addressing the aromatic constituents, whereas single-photon ionization covered a comprehensive chemical range. Interestingly, we found an enrichment of UVstabilizers, such as benzophenone, within the macromolecular nature of the residue. The evolved gas mixture was found to be highly complex. Consequently, high-resolution mass spectrometry addressing the isobaric complexity and pyrolysis gas chromatography was used for structural elucidation. We hypothesize island-and archipelago-type structural motives comparable to asphaltenes but with almost no heteroatoms based on analyzing the complex mixture by carbon number versus double bond equivalent cartographies. A comprehensive set of thermal analysis mass spectrometric platforms enabled an in-depth chemical description of plastic pyrolysis char, valuable knowledge in reactor design and material science.