Characterization of copper-containing porphyrins in an insoluble fraction of a bio-crude obtained from hydrothermal liquefaction of wild microalgae

Among the various alternatives to fossil energies, microalga-based fuels are promising solutions. Thanks to their ability to grow under harsh conditions and trap industrial exhaust gases, these microalgae can help reduce global warming effects. One efficient way to process this feedstock is to use hydrothermal liquefaction (HTL) which leads to an intermediate bio-crude and a residual solid. This bio-crude can be further refined to produce usable biofuel by eliminating problematic heteroatoms such as nitrogen, oxygen, and metals. The residual solid, which can represent 50 % in mass is, however not useable nowadays. To valorize this insoluble fraction, the present work is focused on its molecular characterization and, in particular, on its metal content. Metals are particularly problematic for downstream processes when they are in an organic form such as porphyrins as they destroy catalysts. Identification and speciation of these metal organic species is thus needed. However, these analyses are challenging because of the high molecular diversity of these samples. Here, complementarity between electron paramagnetic resonance spectroscopy (EPR) and FTICR-MS, which is a state-of-the-art mass spectrometer, was used for the speciation of copper elements in the residual solid. A signal characteristic of a copper surrounded by four symmetric nitrogen atoms was observed in EPR, suggesting porphyrinic structures. The dynamic range of FTICR-MS broadband analyses was not sufficient to access the isotopic fine structure of the copper porphyrins. Continuous accumulation of selected ions analyses (CASI) was thus, performed to boost the dynamic range of the mass spectrometer in a narrower mass range allowed to confirm for the first time the presence of copper porphyrins in a residual solid of bio-crude. Different families of copper porphyrins were observed, and putative structures were proposed.