Approach for Selective Separation of Thiophenic and Sulfidic Sulfur Compounds from Petroleum by Methylation/Demethylation

Direct sulfur-containing compound speciation in crude oils and high-boiling fractions by APCI (+) FT-ICR mass spectrometry

In this study, we focus on advancing the methodology for detecting sulfur-containing compounds (SCCs) in crude oils and their derivatives. These compounds are critical for geochemical analysis, crude oil evaluation, and overcoming production and refining challenges. Although various analytical techniques exist, the precision and resolution power of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) stand out. However, the current methods for characterizing SCCs in petroleum products often lack standardization and tend to be complex and time-consuming. Our research introduces the use of Atmospheric Pressure Chemical Ionization (APCI) as an efficient alternative. We employed a mixture of toluene and methanol (1 : 1 ratio) for APCI, which demonstrated superior performance in sulfur speciation compared to mixtures of toluene and acetonitrile. Our specified method showed high repeatability, with coefficients of variation reported between 5% and 14%. This method effectively covers a wide range of double bond equivalents (DBEs) from 1 to 25 and various carbon numbers, demonstrating notable repeatability and reproducibility. Compared to results from ESI post-S-methylation and Atmospheric Pressure Photoionization (APPI), APCI offers a more comprehensive analysis of sulfur compounds, presenting a broad spectrum of molecular formulae and extending across a vast range of carbon numbers and DBEs. Here, we demonstrate that APCI is a robust and efficient method for direct and extensive sulfur speciation in crude oil and its high-boiling fractions, marking a significant advancement over existing techniques. This methodological improvement opens new pathways for more accurate and efficient sulfur compound analysis in petroleum products.

Convenient high resolution mass spectrometry characterization of aromatic sulfur-containing petroleum components following by preliminary S-alkylation with aliphatic alcohols

This paper describes rather suitable and variable preliminary derivatization strategy that may precede the molecular level characterization of sulfur-containing compounds of a particularly aromatic nature by high-resolution MALDI and ESI mass spectrometry. We demonstrated for the first time that free aliphatic alcohols (primary 1-alkanols C3-C20) in the presence of triflic acid provide easy S-alkylation of not only saturated sulfides but also most typical aromatic sulfur-containing compounds (benzothiophene, dibenzothiophene and their homologues) widely distributed and frequently analyzed in oil. The reaction proceeds quantitatively at rather mild conditions and gives rise to corresponding S-alkyl sulfonium salts the cation moieties of which can be detected using MALDI and ESI mass spectrometry with excellent signal/noise (S/N) ratios; the response ratios for target ions being quite close for both methods. Collision-induced dissociation (CID) of S-alkylsulfonium cations proceeds only by the elimination of entire S-alkyl group yielding protonated molecule of the analyte. This process can be useful for a reliable determination of target aromatic heterocyclic compounds in complex mixtures. The applicability of the method is illustrated by the analysis of deasphalted medium petroleum sample. The proposed derivatization principle is considered to be highly applicable as an alternative approach to routine characterization and sensitive determination of most typical sulfur-containing compounds and particularly of aromatic S-heterocycles in crude oils by soft-ionization mass spectrometry methods.

Origin and Geochemical Implications of Hopanoids in Saline Lacustrine Crude Oils from Huanghekou East Sag and Laizhouwan Northeastern Sag, Bohai Bay Basin

A suite of low-mature crude oils (five high-sulfur oils and six low-sulfur oils) from the Huanghekou and the Laizhouwan Sags, Bohai Bay Basin, are analyzed to investigate the fate of the hopanoids. Abundant hopanes, such as secohopanes, 25-norhopanes, benzohopanes, aromatized secohopanes, and sulfide hopanes, are identified, and their carbon isotope compositions are determined. Varying ¹³C isotope values of C₃₁ hopane (-38.7-34.0‰) and C_29-30 hopanes (-38.5-31.5‰) suggest different bacterial sources of these compounds. The presence of 25-norhopanes with enriched heavy carbon isotopes in severely biodegraded oils suggests that they are microbially mediated products. The detection of the isotopically depleted C₂₉ and C₃₀ D-ring-8,14-secohopanes (-45.6-41.2‰) indicates that secohopanes are from methane-oxidizing bacteria (methanotrophs). The presence of isorenieratane, lower aryl isoprenoid ratios, and a good correlation between the sulfur content and the gammacerane index indicate the presence of green sulfur bacteria (Chlorobiaceae) under photic zone euxinic conditions. Water column stratification results in good preservation of the organic matter, and it is in favor of diversity of aquatic microorganisms. The ratios of C₃₅/C₃₄ sulfide hopane, C₃₅ sulfide hopane-2/C₃₅ sulfide hopane-1, and C₃₅/C₃₄ benzohopane are influenced by the reducing environments in this region. In addition, the D-ring monoaromatized 8,14-secohopanoid/(D-ring monoaromatized 8,14-secohopanoid + benzohopanes) and C₃₁-C₃₅ secomoretanes/secohopanes are affected by the maturity. We hypothesize that the reducing environments and thermal effects are important markers for the hopanoid transformation, including the incorporation of inorganic sulfur in substituting functional groups, cyclizing, aromatizing, and opening ring C of the hopanoids.

Theoretical Study on the Unimolecular Pyrolysis of Thiophene and Modeling

Thiophenic sulfur is the most stable and abundant organic sulfur species in petroleum. Removal of thiophenes has profound significance in environmental protection. In this work, we investigate the unimolecular pyrolysis of thiophene from a kinetic perspective. High-level ab initio methods have been employed to deduce the potential energy surface. Rate coefficients of the elementary reactions are computed using variational transition-state theory at the CCSD(T)/CBS level to develop a kinetic model. By comparison with preceding experimental results, the kinetic model shows good performance in calculating the thiophene pyrolysis rate. The Arrhenius expression for thiophene unimolecular pyrolysis has been redetermined as k = 1.21 × 10¹³ × exp[(78.96 kcal/mol)/(RT)]. The unimolecular pyrolysis of thiophene is mainly initiated by the ring-H migrations, whereas the C-S bond rupture has limited contribution to the overall pyrolysis rate. Thioketene (SC₂H₂) and ethyne (C₂H₂) are the major pyrolysis products at all temperatures. Significant amounts of the thioformyl (HCS) radical and CS could also be yielded. By contrast, atomic sulfur and H₂S are difficult to be directly produced. Possible secondary reactions in the products have also been discussed.

Comprehensive Composition, Structure, and Size Characterization for Thiophene Compounds in Petroleum Using Ultrahigh-Resolution Mass Spectrometry and Trapped Ion Mobility Spectrometry

Thiophene compounds are the main concern of petroleum desulfurization, and their chemical composition and molecular configuration have critical impacts on thermodynamic and kinetic processes. In this work, atmospheric pressure chemical ionization (APCI) was employed for effective ionization of thiophene compounds in petroleum with complex matrix, in which carbon disulfide was used for generating predominant [M]^+• ions without the need of derivatization as for electrospray ionization. APCI coupled with ultrahigh-resolution mass spectrometry (UHRMS) was successfully applied to the composition characterization of thiophene compounds in both a low boiling petroleum fraction and a whole crude oil. APCI coupled with trapped ion mobility spectrometry (TIMS) was developed to determine the shape and size of thiophene compounds, providing configuration information that affects the steric hindrance and diffusion behavior of reactants in the desulfurization reaction, which has not been previously reported. Moreover, the comprehensive experimental structural data, expressed as the collision cross section (CCS) of the ions as surrogates of molecules, provided clues to the factors affecting the desulfurization reactivity of thiophene compounds. Further exploration showed that not only qualitative analysis of thiophene compounds can be achieved from the correlation between m/z and CCS, but also molecular size was found to be correlated with CCS that can be used as structural analysis. Overall, the molecular composition and dimension analysis together can provide substantial information for the desulfurization activity of thiophene compounds, facilitating the desulfurization process studies and catalyst design.

Molecular Characterization of Lignite Extracts of Methanol and Carbon Disulfide/N-Methyl-2-pyrrolidone by High-Resolution Mass Spectrometry

CS₂/N-methyl-2-pyrrolidone (NMP) could extract much more substance from coals than any other solvents. Investigation on the molecular composition of CS₂/NMP extracts from lignite is significant for the understanding of high extraction yield and the clean utilization of coal. The methanol-soluble portions from lignites and CS₂/NMP extracts of lignites were characterized by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The yield of CS₂/NMP extracts from lignite was quite higher than that of methanol extracts. Furthermore, the yield of methanol-soluble portions from CS₂/NMP extracts was far more than that of methanol extracts from lignite. At the level of molecular composition, the relative content of heteroatom compounds with more oxygen atoms, longer side chain, and higher condensation in the CS₂/NMP extract was also higher than that in the methanol extract. Despite great difference in the yield and the relative content of components, the distributions of species, molecular weight, carbon number, and double-bond equivalent were similar to those of most organic molecules for the methanol extract and methanol-soluble portions from the CS₂/NMP extract. These phenomena suggested that organic molecules with similar structure but different composition, nonuniformly distributed in the coal matrix, were released more in the CS₂/NMP extract compared to the single methanol extract.

Application of gas chromatography-high resolution quadrupole time-of-flight mass spectrometry in fingerprinting analysis of polycyclic aromatic sulfur heterocycles

Polycyclic aromatic sulfur heterocycles (PASHs), as a group of major sulfur-containing compounds, widely occur in crude oil and its refined products. Accurate analyses of these petrochemical components play an important role in monitoring oil quality, forensic source identification, and assessment of environmental impact of an oil spill. PASHs occur at relatively lower abundances in most crude oils and refined petroleum products than their corresponding aromatic hydrocarbons and are co-eluted together with some petroleum hydrocarbons in chromatographic analysis, resulting in high uncertainty for their quantitation. Capillary gas chromatography coupled with a quadrupole time-of-flight mass spectrometry (GC-QTOF-MS) provides high resolution and high mass accuracy, which facilitates discrimination of the delicate mass defects of isobaric compounds with the same nominal mass and external matrix material. In this work, GC-QTOF-MS was applied to analyze bicyclic to pentacyclic PASHs including benzothiophenes, dibenzothiophenes, benzonaphthothiophenes, dinaphthothiophenes and their C1- to C4- alkylated homologues in a number of crude oils, refined petroleum products, and environmental samples. GC-QTOF-MS analysis substantially improved the identification confidence and reduced quantitation uncertainty of PASHs and polycyclic aromatic hydrocarbons (PAHs) by eliminating the interferences presented in nominal mass chromatograms.

Molecular Characterization of Soluble Components in the Lignite by Sequential Solvent Extraction via Continuously Reducing Particle Size

A lignite was subjected to sequential solvent extraction via continuously reducing particle size from around 20 to more than 200 mesh. Five sets of n-hexane and methanol extracts from the particles were characterized by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) and gas chromatography-mass spectrometry. The total extract yield for lignite when using hexane and methanol as solvents could reach to 0.98 and 15.12%, respectively. The results showed that more molecules with a similar structure but different composition could be extracted by continuously reducing the particle size of the residues, indicating the nonuniform distribution of the low-solubility molecules trapped in the coal particles. The extracts were abundant in branched long-chain aliphatic moieties and oxygen-containing compounds. With the increasing of the extraction degree, the content of alkanes in the extracts decreased rapidly. On the contrary, the content of the compounds with higher condensation and more oxygen atoms increased. It should be noted that polycyclic aromatic hydrocarbons were almost steadily present in all the extraction steps. The molecular composition and distribution of organic molecules in the lignite provide clues to the understanding of coal structure, which is significant for the environmental emission and development of processing techniques for the clean and high value-added utilizations of such a low-rank and abundant coal resource.