DETERMINATION OF THE NITROGEN CHEMICAL STRUCTURES IN PETROLEUM ASPHALTENES USING XANES SPECTROSCOPY

Extensive nitrogen K-edge X-ray absorption studies have been performed for the first time on seven petroleum asphaltenes and nitrogen standard compounds for the purpose of determining chemical forms of nitrogen present in the asphaltenes; such an objective is difficult to achieve by any other method. Sulfur XANES studies on fossil fuel samples have provided a rich source of information for thc last ten years; similar XANES studies of nitrogen in fossil fuels have only recently been successfully performed using new, advanced fluorescence detection. Generally, the spectra of different chemical forms of nitrogen produce readily distinguishable features, thereby facilitating asphaltene analysis. Approximate contributions from different nitrogen structures present in the asphaltenes are calculated by comparing normalized areas under corresponding resonances in the spectra of the asphaltenes and the model compounds. The standard model compounds of nitrogen studied are pyrroles, pyridines, saturated amines, and metalloporphyrins. Most of the nitrogen in asphaltenes is found to be present in aromatic forms, with a very small amount as saturated amine. The pyrrole form of nitrogen is more abundant than the pyridine form in asphaltenes, and the pyridine fraction in different asphaltenes is somewhat variable. Pyridine, which is more basic than pyrrole, shows pi* resonances shifted to significantly lower energies than those observed for pyrroles. The relative positions of nitrogen pi* resonances are determined according to whether the nitrogen lone pair of electrons is shared in the pi aromatic system. These spectral shifts are largely produced by resonance effects rather than inductive effects, which are observed in the sulfur case. Spectra of more complicated molecules such as porphyrins and imidazoles are explained along similar lines. The saturated amine shows only a sigma* resonance. Sensitivity of the spectra to surface effects is explored by comparing electron yield and fluoresence data.