Porphyrins in Heavy Petroleums: A Review

Abstract

Vanadium and nickel are the most abundant and troublesome metal compounds present in the organic portions of fossil fuel deposits. These metal compounds may cause significant detrimental impact during refining processes, leading to the deactivation of catalysts used for sulfur and nitrogen removal. Therefore, it is highly desirable to remove vanadium and nickel from petroleum fractions before catalytic hydrogenation and cracking. Vanadium and nickel complexes generally have been classified into porphyrins and non-porphyrins. Studies of the porphyrins have focused extensively on their isolation and identification since their discovery in crude oils and shales. Although it was proposed that non-porphyrins will contain atypical porphyrin or pseudo aromatic tetradentate systems, no non-porphyrin molecules have been identified in crude oil. Ultraviolet–visible (UV–vis) spectroscopy and mass spectrometry are the common analytical techniques used to identify and quantify porphyrins. Due to the high intensity and sensitivity of electronic absorption of UV–vis radiation by porphyrins, approximately half of the vanadium and nickel porphyrins can be identified and quantified by their characteristic UV–vis spectra. The remaining vanadium and nickel compounds, the non-porphyrins, are defined by an absence of distinct UV–vis spectroscopic bands. However, the results of X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy indicated that these non-porphyrins are indeed still bound in a porphyrinic structure, although these metal compounds do not exhibit the characteristic UV–visible absorption. In addition, the recent results of mass spectrometry showed that majority of vanadium and nickel compounds existed in the form of porphyrins, including alkyl porphyrins, sulfur-containing porphyrins, nitrogen-containing porphyrins, and oxygen-containing porphyrins. The porphyrins with O, S, and N atoms should associate more strongly with the asphaltenes than the less polar components. Formation of complexes with other components would shift and attenuate the Soret absorption in UV–visible spectroscopy. The porphyrins are generally believed to chelate or non-covalently associate with aromatic asphaltene components by π–π interactions. Therefore, a portion of the vanadium and nickel compounds give strong optical absorption in the Soret band at ca. 400 nm, and the remainder do not, likely due to formation of complexes or due to chemical modification of the porphyrin ring. The implications of their chemical environments for alternative separation methods are important. In this review article, the current understanding of the forms of vanadium and nickel compounds in heavy petroleum is critically discussed and the methods of separation and demetallization evaluated. Effective separation and ultrahigh mass resolution are needed to resolve these vanadium and nickel compounds. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), which has the highest available broadband mass resolution, mass resolving power, and mass accuracy, is shown to be a significant method for the qualitative analysis of vanadium and nickel compounds in oil fractions.