In Situ Resonant UV-Raman Spectroscopy of Polycyclic Aromatic Hydrocarbons

PAHs are hazardous and persistent pollutants, also found as byproducts of some petrolchemical reaction (e.g., MTH) in relation to the catalyst deactivation, i.e., to the formation of coke species. The analysis of such deactivation products is typically performed by means of chromatographic techniques, with some drawbacks: the extraction and separation of the molecules from the matrix (the catalyst) is always required, and the solubility of the larger ones is often very low also in nonpolar solvents, so that their analysis is not possible with a standard approach. Spectroscopies can represent an interesting alternative for the qualitative analysis of PAHs: in particular Raman spectroscopy has been demonstrated to be a powerful tool in the characterization of carbonaceous materials and PAHs, and the possibility to exploit the resonance effect (allowing the selective enhancement of vibrational features of the resonant species) can be a considerable advantage in the analysis of very diluted species. In the present work, PAHs have been characterized by means of UV Raman spectroscopy: the 244 nm excitation wavelength allowed to exploit the resonance effect and in the meantime to avoid interference due to the visible fluorescence typical of these molecules. A pool of representative PAHs have been analyzed in their pure form, in diluted solution, and dispersed on high surface area microporous supports: the collected data constitute a reference database to be compared with the deactivation products observed during petrolchemical reactions, offering the possibility to univocally identify them.