A review on asphaltenes characterization by X-ray diffraction: Fundamentals, challenges, and tips

Asphaltenes with complicated molecular structures consist of many aromatic portions, alkyl chains, and different functional groups. Various methods can determine their average structure. X-ray diffraction (XRD) is one of the most powerful techniques for investigating structural characteristics, especially in asphaltenes. The main goals of this paper are a comprehensive assessment of obtained crystallite parameters of asphaltene with various sources such as crude oil, vacuum residue, atmospheric residue, oil shale, coals, and asphalts, as well as investigation of the impact level of different kinds of processes such as hydro-treating, thermal cracking, thermal maturation, thermal decomposition, mild thermal processing reaction, aging, washing, catalytic aquathermolysis, and solvent addition. Results indicate that the coal-derived asphaltenes possess the highest aromaticity values and the extracted asphaltene of crude oil has the largest number of aromatic sheets in a cluster. Values belong to distances between aromatic sheets and saturated portions are almost equal in all categories of asphaltenes. Calculated XRD parameters are in good agreement with other techniques like Nuclear Magnetic Resonance (NMR), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). Thermal processes are generally accompanied by aromaticity increment, while aging causes aromaticity diminution. Notwithstanding some deficiencies about selected assumptions and approaches by the operator for data treating, XRD is considered a precise technique for determining structural parameters.