Benchtop NMR Spectroscopy of Pyrolysis Oils From Mixed Plastic Waste: Parameter Optimization for Oil Characterization and Detection of Heteroatom‐Containing Compounds

Using pyrolysis oils from plastic waste as an alternative to fossil feedstocks presents a sustainable opportunity for the petrochemical industry. However, depending on feedstock and pyrolysis conditions, the resulting pyrolysis oils differ in chemical composition and may contain heteroatoms exceeding the concentrations limits of petrochemical processes. This publication evaluates benchtop NMR spectroscopy for fast analysis of pyrolysis oils with respect to potential online measurements. The signal‐to‐noise ratio (SNR) of 1 H‐NMR experiments was enhanced by optimizing the measurement and postacquisition parameter on two state‐of‐the‐art spectrometers. To simulate the detection of heteroatoms in an industrial process, two polyolefin‐based pyrolysis oils were mixed with ε‐Caprolactam, acrylonitrile, benzoic acid, acetophenone, and phenylacetaldehyde in varying concentrations from 10 to 0.1 wt% and analyzed using 1 H‐NMR and 2D‐correlation spectroscopy (COSY) experiments. The limit of detection (LOD, SNR > 3) within 35 min measurement time in 1 H‐NMR for a concentration series of phenylacetaldehyde in a hydrocarbon model mixture was determined as 0.001 wt% (equivalent to 0.64 proton‐ppm). However, the detection of the investigated heteroatom‐containing substances in real pyrolysis oils remains challenging and the detection limit of 1–0.1 wt% was much higher due to overlapping peaks in 1 H experiments and comparably low sensitivity of COSY measurements.