Discovery of novel cage compounds of diamondoids using multi-dimensional mass spectrometry
Graphical abstract
For the first time, two categories of diamondoid compounds, thiaethanodiamondoids (1-7 cages) and higher ethanodiamondoids (4-7 cages), were discovered and characterized. The conformers of the compounds were also identified. The discovery relied on the innovative multi-dimensional MS strategy, which could also be used for the MS characterization of unreported natural products present in complex matrices.
Introduction
Diamondoids that resemble a diamond lattice are a series of three-dimensional (3D) caged molecules entirely composed of cyclohexane in chair conformation (Hoft, 2000). Face-fused cages could be viewed as repeating subunits of diamondoids. Each additional cage meant that a subunit was formed by constructing cyclohexanes in chair conformation in the original framework. Due to the unique strain-free interlocking polycyclic systems, diamondoids and their analogs (diamondoid compounds), such as ethanodiamondoids and thiadiamondoids, are characterized by conformational rigidity. They exhibit ultra-high heat stability and anti-destructive ability, (Waltman and Ling, 1980) and are regarded as important measuring sticks for oil preservation and the earth’s crust (Dahl et al., 1999, Hanin et al., 2002, Wei et al., 2007, Wei et al., 2012, Grice et al., 2000). The compact 3D networks of covalent bonds give diamondoid compounds unique optical and electronic properties, which make them spotlighted in the area of nanomaterials (Zones et al., 1998, Meador, 1998, Çagin et al., 1999, Lifshitz et al., 2002, Yang et al., 2007, Vörös and Gali, 2009, Cheng et al., 2017, Bradac et al., 2017).
Crude oil is the only known natural source of all known diamondoids and their analogs, including ethanodiamondoids and thiadiamondoids. The compounds are present in significantly low concentrations (≤1–100 ppm) (Dahl et al., 1999). It was observed that the diamondoid compounds could resist pyrolysis and biodegradation during geochemical evolution (Dahl et al., 1999, Grice et al., 2000). This indicated their high stability. Thus, the “concentration effects” could be used to assess the cracking degree, (Dahl et al., 1999) thermal maturity, (Wei et al., 2007, Chen et al., 1996) and extent of biodegradation of crude oils (Grice et al., 2000). For example, the adamantane to diamantane ratio was used to study the crude oils characterized by high maturity and high degrees of biodegradation, which could not be completed by conventional markers such as steranes and hopanes (Chen et al., 1996). Thiadiamondoids are excellent indicators of thermochemical sulfate reduction (TSR) alteration in a petroleum reservoir (Hanin et al., 2002, Wei et al., 2012, Wei et al., 2007, Zhu et al., 2016). In the TSR process, the diamondoid cage undergoes C–C bond cleavage, and then the replacement of the secondary carbon atom with a sulfhydryl group, resulting in the production of the “pseudo” diamondoidthiol unit. Finally, thiadiamondoid is produced following C–S bond formation and cyclization. Results from isotope analysis revealed that the sulfur atom in thiadiamondoid originated from the sulfates in the reservoir (Hanin et al., 2002, Zhu et al., 2016, Gvirtzman et al., 2015). Ethanodiamondoids are considered as a most stable substance that can be used as an additive to improve material stability under high-temperature conditions (Meador, 1998, Zhu et al., 2018a). Unlike lower diamondoid compounds with 1–3 cages, higher ones with more than three cages exhibited nanoscale molecular dimensions. The complexity of stereo-caged isomerism in these compounds increases with an increase in the cage number (Dahl et al., 2003a). Higher diamondoid compounds with tailing capability and tunable structures were more promising as molecular building blocks than the lower ones in the field of nanomaterial science (Merkle, 2000, Tkachenko et al., 2006, Garcia et al., 2009, Sivaraman et al., 2016). Higher diamondoid compounds characterized by excellent stability could play a better role than lower ones to indicate high evolution or high maturity in geochemistry studies. For a long time, lower diamondoids and ethanodiamondoids were synthesized following the process of thermodynamically controlled carbocation rearrangement of polycyclic hydrocarbons catalyzed by Lewis acids (Von and Schleyer, 1957, Williams et al., 1966). However, the synthesis of higher diamondoids has seldom been realized following this process. The only successful synthesis of the 5-cage diamondoid, pentamantane, was achieved via a free radical mechanism. The synthetic condition resembled the conditions required for the formation and accumulation of crude oil where high pressure and temperature led to the natural occurrence of higher diamondoid compounds (Dahl et al., 2010). Increasing attention is being paid to the investigation and identification of higher diamondoids, which are being widely conducted as the results can potentially help advance in the field of geochemistry and diamondoid synthesis.
To date, the highest diamondoids, ethanodiamondoids and thiadiamondoids have been discovered in crude oils with cage numbers reported as 11, (Dahl et al., 2003b) 3, (Zhu et al., 2018a) and 6 (Wei et al., 2011) respectively. The structural identification of the diamondoid compounds relies on the use of single-crystal X-ray diffraction (XRD), (Dahl et al., 2003a) nuclear magnetic resonance (NMR) spectroscopy, (Dahl et al., 2003b, Hanin et al., 2002) and gas chromatography coupled with mass spectrometry (GC–MS) techniques (Wei et al., 2007, Zhu et al., 2018a, Wei et al., 2011, Zhu et al., 2018b). The single-crystal XRD and NMR spectroscopy techniques asked for the separation and purification of the targets, which was a complex and time-consuming procedure. For example, the purification of higher diamondoids characterized by 4–11 cages proceeded over several steps: the step involving high-temperature pyrolysis, preconcentration step involving column separation, isolation step involving the two-dimensional high-performance liquid chromatography (2D-HPLC) technique, and recrystallization steps that proceeded over 1–2 weeks (Dahl et al., 2003a). For ethanodiamondoids and thiadiamondoids existing at a much lower concentration than their corresponding diamondoids, purified compounds, except for the dimethyl substituted single-cage thiadiamondoid, were not obtained (Hanin et al., 2002). As the cage number increases, the boiling point increases, and the naturally existing concentration of the diamondoid compounds decreases. Under these conditions, complex stereo-caged isomerism was observed, increasing the complexity of the purification or concentration process of the compounds. The GC–MS technique can be used for online separation and trace analysis. Comprehensive GC × GC techniques coupled with the high-resolution MS represented the most powerful ability of GC technology in separation, column capacity and qualitative analysis. The use of the technique effectively promoted the discovery of ethanodiamondoids characterized by two and three cages (Zhu et al., 2018a). However, the GC-based methods were still powerless in the discovery and identification of higher diamondoid compounds whose boiling points are beyond the boiling point-working limit of the GC system. To date, there have been no reports on the identification of higher diamondoid compounds with more than six cages using the GC-based methods. Ultra-high-resolution MS (full width at half-maximum (FWHM) ≥300,000) such as Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) (Marshall and Rodgers, 2004, Liu et al., 2020, Smith et al., 2018, Krajewski et al., 2017) and Orbitrap MS, (Cho et al., 2017, Schmidt et al., 2018, Schneider et al., 2020, Kostyukevich et al., 2018) has achieved great success in petroleomics and other complex organic systems. The compounds could be identified by analyzing the molecular weight with ultra-high-resolution rather than the chromatographic separation. Based on accurate molecule assignment, compound structures could be determined using the tandem MS technique (Tachon et al., 2011, Qian et al., 2012, Podgorski et al., 2013).
A multi-dimensional MS methodology was proposed and developed to efficiently identify higher diamondoid compounds. The developed method can provide the much-needed breakthrough in the field of higher diamondoid exploration and identification. Ultra-high-resolution MS, tandem MS, and trapped ion mobility spectrometry MS (TIMS-MS) were combined to investigate and identify the naturally existing diamondoid compounds. Thiaethanodiamondoids (1–7 cages) and higher ethanodiamondoids (4–7 cages) were discovered and reported for the first time. The presence of higher diamondoids and thiadiamondoids (the cage numbers were higher than the those of previously reported compounds) was confirmed in the ZS1C crudes. It was also observed that various conformational isomers of regular linear catamantanes (Balaban and Ragé Schleyer, 1978) and regular branched catamantanes could be recognized and differentiated using this method.
Section snippets
Chemicals and materials
HPLC grade n-hexane, toluene, carbon disulfide (CS2) and dichloromethane (DCM) were obtained from Thermo Fisher Scientific (Waltham, MA). Both the alumina and silica gel were of 100–200 mesh and purchased from Sinopharm Group Co., Ltd. Before the separation process, alumina was activated at 500 °C for 4 h, while silica gel was activated at 140 °C for 8 h.
The ZS1C crude oil was highly mature and obtained from the 6861 to 6944 m interval, which penetrated the lower Cambrian strata. All fractions
Multi-dimensional MS strategy
Extensive and comprehensive information on compounds is required for the efficient characterization and exploration of unreported diamondoids, ethanodiamondoids, thiadiamondoids, and thiaethanodiamondoids present in trace amounts in a complex matrix of crude oil. Therefore, a multi-dimensional MS strategy was proposed. The first-dimensional qualitative analysis involves the acquisition of accurate molecular weight and molecular composition (step 1; Fig. 1) using the ultra-high-resolution MS
Conclusion
A multi-dimensional MS method was established to explore the possible existence of trace amounts of diamondoids, ethanodiamondoids, thiadiamondoids, and thiaethanodiamondoids in crude oils with a complex matrix. To the best of our knowledge, this is the first paper that reports on the natural existence of thiaethanodiamondoids (1–7 cages) and higher ethanodiamondoids (4–7 cages). Meanwhile, the previously reported species of higher diamondoids and higher thiadiamondoids present in ZS1C crude
CRediT authorship contribution statement
Yinghao Wang: Methodology, Investigation, Visualization, Writing – original draft. Guangyou Zhu: Methodology, Conceptualization, Writing – review & editing, Resources. Meng Wang: Methodology, Investigation, Resources. Jianxun Wu: Methodology, Investigation. Dali Fu: Methodology, Investigation, Visualization. Qingqing Xie: Methodology, Investigation. Quan Shi: Writing – review & editing, Resources. Chun-Ming Xu: Resources.Yehua Han: Methodology, Conceptualization, Writing – review & editing,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was financially supported by the National Natural Science Foundation of China (No. 22021004).
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Y. Wang and G. Zhu contributed equally to this paper.