hide
Free keywords:
-
Abstract:
Hydrocarbons are one of the most important compound classes on earth. They consist of only two elements (carbon and hydrogen). Nevertheless, due to the various bonding possibilities of carbon, they are very diverse in their molecular structures. Such a wide structural diversity leads to a broad range of physical and chemical properties among the compounds of this class. With depletion of conventional crude oil resources, the widest margin of hydrocarbon structural diversity is detected in unconventional crude oil resources. (e.g. bitumen and heavy oil). Hydrocarbons derived from these resources are of extreme importance due to their application as source of: fuel, heat, electricity and industrial chemicals which improve quality of our modern life (e.g. polymers, pharmaceuticals, etc). Therefore, it is very important to have molecular level, structural information about the diverse range of petroleum hydrocarbons. Such information helps us to prevent most of the industrial challenges and to comply with the sustainability and environmental protection goals. Due to the high complexity of crude oil samples, a pre-simplification step is the prerequisite of a detailed analysis. In this step, hydrocarbon constituents are separated into different fractions. One of the conventional fractionation techniques is the so-called SARA (Saturate, Aromatic, Resin, Asphaltene) fractionation. Structural complexity, ratio of heteroatom content and aromaticity of the compounds in these fractions increase accordingly from the saturate towards the asphaltene fraction. Thence, the saturate fraction is considered to contain the simplest and the most volatile compounds of crude oil. Contrarily, the asphaltene fraction contains more complex compounds with condensed aromatic structures, a higher ratio of heteroatom containing organic compounds and inorganic compounds.
To date, there are only few reports available about the structural diversity of the saturate fraction, which in all of them gas chromatography (GC) or GC intermediated mass spectrometry technique is applied for analysis. Ultrahigh resolution mass spectrometry (UHRMS) is the method of choice for analysis of complex petroleum samples; however, it is never applied for analysis of saturate fraction. In this method using the ultra-high resolving power and based on accurate mass measurements, unique elemental compositions are assigned for the detected compounds (molecular level details). In this work, for the first time, saturate fractions from different crude oil samples are analyzed through direct infusion, ultra-high resolution mass spectrometry (UHRMS), using different ionization techniques. Result of this work shows that direct infusion UHRMS analysis provides a comprehensive image of saturate fraction. Such information is not achievable through application of GC-intermediated analytical techniques.
In the heaviest fraction of crude oil, interaction of inorganic compounds (clay minerals) and hydrocarbons of asphaltene was investigated. Organic Reach Solids (ORSs) from an unconventional crude oil were analyzed using EM. Characterization of ORSs using EM allows studying different treatment methods for elimination of inorganic compounds from ORSs and crude oil samples. In this work, two different treatment methods are tested for removing inorganic content of asphaltene fraction. Effectiveness of those methods is confirmed by EM and energy dispersive x-ray (EDX) spectrometer analysis. These results shows that application of chemical treatment methods leads to considerable decrease in the amount of inorganic compounds in crude oil, which causes most of industrial challenges.
In the asphaltene related reports, hydrocarbon constituents of asphaltene fraction are presented to be planar, condensed polyaromatic structures (i.e. island and archipelago models). Results of this work, with support of different analytical methods, breaks more than a century dogma for the planar structure of petroleum hydrocarbons. The carbon only series of (Cn) compounds, up to C148, are detected using UHRMS analysis. Meanwhile, their 3D-structures (fullerene or buckyball) are confirmed using tandem mass spectrometry. Here, buckybowls as the building blocks of the fullerenes and carbon nanotubes (CNTs) are also detected. Moreover, azafullerenes (azaballs) and azabowls are also detected as heteroatom-containing 3D-structures in the same sample. In all above-mentioned cases, the structural property and stability of these compounds are confirmed using high level of Density Functional Theory (DFT) calculations combined with coupled cluster calculations.
Investigation of the sample for detection of giant fullerenes and CNTs was beyond the capabilities of mass spectrometry. Consequently, application of further analytical methods was mandatory. Therefore, further analysis was performed using electron microscopy (EM), which shows presence of different types of (single-walled, double-walled and multi-walled) CNTs in asphaltene samples. This is the first evident report of CNTs with natural origin.
