Elucidating the Geometric Substitution of Petroporphyrins by

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Article Cite This: Energy Fuels 2019, 33, 6088−6097

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Elucidating the Geometric Substitution of Petroporphyrins by Spectroscopic Analysis and Atomic Force Microscopy Molecular Imaging Yunlong Zhang,*,† Fabian Schulz,‡ B. McKay Rytting,§ Clifford C. Walters,† Katharina Kaiser,‡ Jordan N. Metz,† Michael R. Harper,† Shamel S. Merchant,† Anthony S. Mennito,† Kuangnan Qian,† J. Douglas Kushnerick,† Peter K. Kilpatrick,*,§ and Leo Gross*,‡ Downloaded via 31.40.211.191 on July 30, 2019 at 01:53:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801, United States IBM Research−Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland § Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡

S Supporting Information *

ABSTRACT: Determination of the molecular structures of petroporphyrins has been crucial to understand the diagenetic pathways and maturation of petroleum. However, these studies have been hampered by their structural complexity and the challenges associated with their isolation. In comparison to the skeletal macrocyclic structures, much less is known about the substitutions, which are more sensitive to the maturation and diagenesis pathways. While these isolated vanadyl petroporphyrins largely consist of etioporphyrin and deoxophylloerythroetioporphyrin as expected, surprisingly, we find evidence that one or a few β hydrogens are present in petroporphyrins of low carbon numbers using a combination of ultraviolet−visible spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and non-contact atomic force microscopy. Petroporphyrins with β hydrogens were not anticipated on the basis of their biological precursors. The data support dealkylation under catagenesis but not transalkylation or random alkylation of the β and meso positions, despite the fact that more complex porphyrin structures are formed.

1. INTRODUCTION Porphyrins have been extensively studied in biology, chemistry, physics, medicine, and materials science for their key roles in biochemical processes and their interesting properties and numerous applications.1,2 Since their discovery in shales by Alfred Treibs in mid-1930s, which provided the first molecular evidence for the biogenic origins of petroleum,3−6 porphyrins have played a significant role in petroleum science. On the basis of the identification of etioporphyrin III (etio; Figure 1) and deoxophylloerythroetioporphyrin (DPEP), Treibs proposed a degradative scheme for the conversion of heme to etio and chlorophyll a to DPEP involving reactions of metal displacement, decarboxylation, reduction, and aromatization.7−9 The Treibs hypothesis has been continuously verified and expanded with the advancements in characterization and separation techniques.10 These compounds, along with many more molecules with carbon skeletons that have clear biological origins (biomarkers), provided important insights into the geological processes and the evolving conditions that determine why petroleum molecules have the structures that they have.11−14 Despite their precursors being easily associated with specific biological tetrapyrroles, porphyrins from petroleum are a complex mixture of structures that are not identical to known biological porphyrins as a result of diagenetic alterations; hence, they were appropriately designated as petroporphyrins by Corwin in the late 1950s.15−17 The chelate metals in petroporphyrins are primarily vanadyl (VO) and nickel,18−20 © 2019 American Chemical Society

while biological porphyrins are mostly chelate iron (heme) or magnesium (chlorophyll). Other known alterations include cleavage and cyclization or aromatization of side chains and the addition of aromatic and thiophenic rings, oxygen, sulfur, and possibly pendant moieties.21−23 A tremendous amount of work on characterization of porphyrins has been performed using various analytical techniques, including ultraviolet−visible (UV−vis),6 nuclear magnetic resonance (NMR),24 X-ray scattering,25,26 and mass spectrometry (MS).27−29 In most of these studies, petroporphyrins are assigned to etio, DPEP, di-DPEP, and rhodo (or benzo) porphyrins according to degrees of unsaturation or double bond equivalent (DBE), with etio and DPEP detected as the most abundant species.27,28,30 Because the ratio of chlorophyll- to heme-type tetrapyrroles preserved in the geosphere in living organisms is estimated at ∼105:1,31 etioporphyrins are now believed to be mostly produced from chlorophylls via the degradation of DPEP, instead of directly derived from hemes, with a DPEP/etio ratio dependent upon thermal maturity.32 Complete determination of the molecular structure of a petroporphyrin requires its isolation as a single component in sufficient quantities to permit detailed characterization. Chromatographic methods are only partially successful in Received: March 17, 2019 Revised: May 11, 2019 Published: June 12, 2019 6088

DOI: 10.1021/acs.energyfuels.9b00816 Energy Fuels 2019, 33, 6088−6097

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Figure 1. General chemical structures and nomenclature of porphyrins and related compounds. The basic structures of porphyrin, chlorin, and bacteriochlorin, with key differences highlighted in green. The numbering of porphyrin and related macrocycles is shown with α, β, and meso positions indicated in black, blue, and red, respectively. Common biological precursors, heme b and chlorophyll a and b, as well as the structures of the commonly found petroporphyrins, etio and DPEP, are shown as examples.

separation and typically provide limited sample amounts.30,33 Hence, most studies have been conducted within the complex matrix of petroleum molecules, and methods are applied to study petroporphyrins without isolation.34−36 Thus far, only a few petroporphyrins have been isolated sufficiently for their structure to be unambiguously characterized.9,25,26 However, unambiguous assignment of the structures is essential to understand the details of the transformation of porphyrins as well as to address numerous remaining questions, including the pattern of aromatic condensation or naphthenic (cycloalkane ring) fusion onto the macrocyclic tetrapyrrole, the attachment of side chains or linkers and their locations, and heteroatom incorporation. We recently established a method to isolate ultrahigh-purity petroporphyrins from the high boiling fractions of petroleum resids and characterized their purity with various measures,37 with evidence of multiple S- and Ocontaining petroporphyrins and their unique distribution.22 In this study, we report a detailed characterization on the molecular structure of isolated petroporphyrins using a combination of spectroscopic studies [UV−vis, NMR, Fourier transform infrared (FTIR), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS)] and high-resolution scanning tunneling microscopy (STM)/atomic force microscopy (AFM) on metalloporphyrins and their demetalated counterparts. In recent years, high-resolution scanning probe microscopy (SPM, e.g., STM and AFM) has emerged as an alternative method for molecular structure elucidation. In contrast to ensemble-averaging techniques, SPM combines single-molecule sensitivity with the ability to obtain structural information at the atomic level. In particular, it is possible to image individual organic molecules with atomic resolution using AFM with carbon monoxide (CO)-functionalized tips (CO-AFM).38 The CO functionalization results in a chemically passivated tip apex, which can be approached close enough to a molecule to probe the repulsive forces arising as a result of Pauli’s exclusion principle.39 Because these forces depend upon the total charge density, they are strongest at the positions of atoms and covalent bonds, thereby resolving the

chemical structure of the imaged molecule. Since its initial demonstration, CO-AFM has been employed to measure bond order, 40 image and identify the structure of natural compounds,41,42 and follow the intermediate steps of onsurface reactions.43−45 In addition to imaging well-defined model systems and pure compounds, CO-AFM was recently extended to study complex molecular mixtures of natural origin and identify their individual constituents, e.g., asphaltenes,46 heavy oil fractions,47 fuel pyrolysis products,48 marine dissolved organic carbon,49 and incipient soot molecules.50 Thus, CO-AFM is uniquely capable of studying the molecular structure of petroporphyrins in real space.

2. MATERIALS AND METHODS 2.1. Materials. Petroporphyrins were isolated from the vacuum resid of a North American crude containing 140 ppm of nickel and 750 ppm of vanadium. Using a combination of Soxhlet extraction, extrography, and chromatography, a series of fractions highly enriched with petroporphyrins were obtained by monitoring with UV−vis spectroscopy (Figure 2) and was labeled according to their maximal absorption at the Soret bands (407, 410, and 416 nm).37 These fractions were further refined using centrifugation to yield highly purified isolates containing >85% petroporphyrin by weight based on metal analysis. The isolates contain ∼90 000 ppm of V by X-ray fluorescence (XRF), indicating an enrichment of 2 orders of magnitude for vanadium over its concentration in the oil. 2.2. Demetalation of Petroporphyrins. Petroporphyrins were analyzed in both metalated and demetalated states. The demetalation procedure was first tested using commercial standards. Nickel octaethylporphyrins (Ni-OEP, ∼10 mg) were stirred in a solution of sulfuric acid (98%) diluted in trifluoroacetic acid (1:3, v/v) for about 1 h. Stronger reaction conditions are required to obtain high yields of demetalated vanadyl porphyrins. Vanadyl octaethylporphyrin (VO-OEP, 10 mg) was dissolved into a sulfuric acid/trifluoroacetic acid solution (1:3, v/v) and brought to 100 °C for a few hours under nitrogen. In all cases, the reaction flask was wrapped in foil to protect it from light degradation. The demetalated porphyrins were isolated from the acidic solution using a chloroform−water liquid−liquid extraction and purified via flash chromatography. Demetalation of isolated vanadyl petroporphyrin (VO-PP) followed the procedure 6089


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Figure 2. UV−vis absorption of isolated petroporphyrin. (a) Three fractions featuring a continuous shift of the Soret band maximum at 407 nm (black), 410 nm (blue), and 416 nm (red), with insets showing details of the Soret band at 400−430 nm and Q band at 500−650 nm. (b) Overlay of absorption spectra of petroporphyrin (407 nm fraction), demetalated, and octaethylporphyrin (OEP). required for treating VO-OEP standards. A 50 mL round-bottomed flask equipped with a stirring bar, reflux condenser, and nitrogen flow was charged with 7.2 mg of purified petroporphyrin. Approximately 5 mL of a 20% solution (v/v) of sulfuric acid diluted in trifluoroacetic acid was slowly added, and the contents were brought to 100 °C for approximately 4 h. Reaction progress was monitored via UV−vis. The demetalated porphyrin was isolated from the acidic solution using a chloroform−water liquid−liquid extraction until the aqueous layer was neutralized by added triethylamine. The chloroform layers were collected and dried down as the final product (4.7 mg, 65% yield). 2.3. FT-ICR MS. FT-ICR MS was conducted on 15 T Bruker solariX XR FT-ICR-MS (Bruker Daltonics, Inc., Billerica, MA, U.S.A.). A petroporphyrin solution in toluene at a concentration of 25−100 ppm was prepared and injected into an atmospheric pressure photoionization (APPI) source (Syagen Technology, Inc., Tustin, CA, U.S.A.). Nitrogen was used as both the nebulizing and drying gas. Data acquisition was set between the m/z 300 and 3000 range with an accumulation time between 40 and 60 ms. Data analysis was performed with Bruker Data Analysis (DA) software and calibrated with an internal homologous mass series (Figure 3). 2.4. Non-contact Atomic Force Microscopy (nc-AFM). SPM characterization was carried out in a home-built combined STM/ AFM setup operated at a temperature of 5 K and under ultrahighvacuum (UHV) conditions (base pressure p ≈ 5 × 10−11 mbar). An Ag(111) single crystal partially covered by bilayer, (100)-oriented NaCl islands was used as a substrate [denoted as NaCl(2 ML)/ Ag(111)]. The Ag(111) surface was cleaned by repeated cycles of Ne+ sputtering and annealing to ∼550 °C. Bilayer NaCl was grown by evaporating NaCl onto the Ag(111) crystal held at a temperature of ∼150 °C, which also resulted in a small fraction of trilayer NaCl islands. Ultrapure petroporphyrins were sublimed onto the NaCl(2 ML)/Ag(111) substrate by first applying a small amount of petroporphyrins ex situ onto a piece of silicon wafer, which was subsequently transferred into the UHV system. The wafer was positioned in front of the cold sample (Tsample ≈ 10 K) and quickly heated to ∼600 °C by passing current through it, resulting in a submonolayer coverage of petroporphyrins on the NaCl(2 ML)/Ag(111) surface. This sublimation procedure is known to work reliably for molecular masses up to ∼1000 Da (for detailed discussions on the limitation of this procedure as well as on the statistical significance of STM/AFM characterization for molecular mixtures, see refs 46−51). A sub-monolayer amount of CO for SPM tip functionalization38,52 was dosed onto the NaCl(2 ML)/Ag(111) surface by deliberately introducing CO into the UHV system with the sample held at ∼10 K. SPM measurements were performed using a qPlus quartz sensor53 operated in the frequency modulation mode54 (resonance frequency f 0 ≈ 28.8 kHz, spring constant k ≈ 1800 kN/m, quality factor Q ≈ 100 000, and oscillation amplitude A = 50 pm), allowing for simultaneous STM/AFM operation. STM images were acquired in the constant-current mode, with the bias voltage applied to the sample. All AFM images were acquired with CO-functionalized tips,

in the constant-height mode, and with a bias voltage of 0 V (Figure 4). The AFM images show a grayscale map of the frequency shift of the quartz sensors arising as a result of the forces acting between the tip and sample, where brighter values correspond to more positive frequency shifts.

3. RESULTS Petroporphyrins were initially enriched with Soxhlet extraction from a vacuum residuum and were then isolated using silica gel and alumina chromatography as previously reported.37 The resulting petroporphyrins of ultrahigh purity were enriched with vanadyl porphyrins (VOPP), because the isolation procedure was not selective for Ni porphyrins (NiPP), which were likely removed during isolation and purifications.37 The purity of isolated VOPP is estimated to be ∼90% based on the metal content by XRF (90 000 ppm or 9 wt % V) and given that vanadium is ∼10 wt % of petroporphyrin molecules (molecular weight of 470−580). This indicates that roughly 9 of 10 molecules are vanadyl porphyrins. UV−vis absorption of isolated ultrapure petroporphyrins not only provided a convenient means to monitor and verify the isolation process25,55 but also provided important insights into their electronic structure (Figure 2). Detailed characterization of isolated petroporphyrin fractions was carried out with UV−vis, infrared (IR), NMR (1H and 13C), and FT-ICR MS (Figures 2 and 3).22 To understand the substituents of the macrocycle and overcome the peak broadening in NMR by the paramagnetic effect of transition metals (V), demetalation of isolated native petroporphyrins was carried out under acidic conditions (Figures S1−S5 of the Supporting Information). Ultimately, the structures of selected petroporphyrins were directly imaged with nc-AFM to confirm these results (Figures 4 and 5). 3.1. Spectroscopic Evidence on the Substitution of Petroporphyrins. Three fractions of isolated ultrapure vanadyl petroporphyrin were initially characterized with UV−vis spectroscopy (Figure 2a). The major absorption around 400−430 nm is well-known as the Soret band as a result of the transition to the S2 states of the 18π system of the macrocycle, and the absorptions at 500−700 nm are known as the Q bands as a result of the S1 transition. Changes in Soret and Q bands can be induced by perturbation of the electronic structures, such as protonation, metal chelation, and substituents (Figures S1−S4 of the Supporting Information).56,57 The shift in the Soret absorption from 407 to 420 nm can be associated with etio, DPEP, and rhodo types according to their 6090


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Figure 3. FT-ICR MS spectra of petroporphyrin (a) before and (b) after vanadyl is removed from petroporphyrins, showing the loss of VO. The 65 mass units indicate the difference between the vanadyl petroporphyrins and the free base porphyrin formed.

of the Supporting Information). We conducted demetalation of isolated petroporphyrin fractions to characterize their substitution patterns by the Q bands of their free base forms. After heating in acid mixtures for several (2−4) hours, the loss of VO is evident from a blue shift of ∼8 nm in the Soret band from 407 to 399 nm as well as the splitting of the two Q bands at 537 and 575 nm into four Q bands at 619, 564, 534, and 501 nm in an increasing manner (Figure 2b, and Figures S3 and S4 of the Supporting Information). The relative peak intensity suggests six or more alkyl substituents at the β positions of porphyrin.58 The removal of VO is also confirmed by the significant peak sharpening in 1H NMR, the manifestation of the internal NH around −2 ppm, and the loss of VO stretch at 991 cm−1 by FTIR (Figure S5 of the Supporting Information). In summary, these results are consistent with the expectation of meso hydrogens in etioporphyrin fractions and 6−8 alkyl groups at β positions in this etioporphyrin fraction (407 nm), but the exact substituents and their locations cannot be determined. 3.2. Minimal Carbon Number in Petroporphyrins by FT-ICR MS. Further evidence on the substitution patterns on the porphyrin rings can be obtained from FT-ICR MS (APPI). Demetalation of porphyrins is evident by the shift to lower molecular weight as a result of the loss of VO2+ and addition of two hydrogens (65) from the isolated metalloporphyrins to produce the free base porphyrins. Most of the molecular ion ([M +1 ]) peaks (identified as 1−8 in Figure 3) of petroporphyrins have corresponding peaks in the demetalated masses. Because of the ultrahigh resolution, DBE can be unambiguously determined from molecular formulas and two types of porphyrins can be assigned, i.e., etio (310 + 14n, with

DBEs of 17, 18, and 20, respectively. Although DBE of 17 can be associated with either porphyrin base structures or a chlorophyll with an extra double bond (cf. Figure 1), it is generally associated with etioporphyrins in accordance with previous literature.30 This is also supported by the fact that the UV−vis absorption of the 407 nm fraction (etioporphyrins) overlaps very well with the UV−vis spectra of vanadyl octaethylporphyrin (Figure S1 of the Supporting Information), indicating the structural similarity to this compound. However, Soret absorptions cannot reliably characterize etio and DPEP mixtures. For example, both etio and DPEP porphyrins are observed in the 407 nm fraction based on DBE by mass spectrometry (Figure 3), although the substitution pattern still cannot be unambiguously identified. Characterizing substitutional structural patterns of etioporphyrins is important to understand other more complicated structure types (e.g., rhodo types) that are supposedly produced from them. Although the mechanism of conversion from DPEP to etio has been reported, 32 questions remain on the exact substitution patterns. For example, the β positions in both etio and DPEP are expected to be fully occupied by substituents (mostly alkyl), and meso positions are expected to be hydrogens on the basis of their biological precursors. This feature is in striking contrast to many synthetic porphyrins (e.g., tetraphenylporphyrin) that often have meso substitutions from synthesis routes. It has been reported that the number of Q bands and their relative intensity are known to provide information on the substituents.58 For example, two Q bands in metalloporphyrins or protonated porphyrins will increase to four Q bands in free base porphyrins as a result of reduced symmetries (Figure S2 6091


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Figure 4. (a) Constant-height AFM images with CO-functionalized tips of petroporphyrins with a strongly repulsive center. (b) Constant-height AFM images with CO-functionalized tips and corresponding Laplace-filtered images of petroporphyrins where atomic resolution of the porphyrin core was achieved. Note that the image of compound M8 is composed of two AFM scans taken at different tip−sample distances, where the scan shown in the lower half was taken at a 1.4 Å larger tip height than the upper half. (c) Schematic representation and corresponding AFM images of the two different adsorption positions observed for vanadyl phthalocyanine.

C25−C57 for DBE of 17 and C27−C60 for DBE of 18 for vanadyl etioporphyrins using positive-ion FT-ICR MS.21,59 Recently, Ramirez-Pradilla et al. also reported C27 or less of N4VO species by direct electron-transfer matrix-assisted laser desorption/ionization (MALDI) analysis.60 However, to the best of our knowledge, the significance of these findings has not been discussed, probably as a result of the low abundance and uncertainty in MS detection.16,23 Biktagirov et al. investigated vanadyl porphyrins using electron−nuclear double resonance (ENDOR), and the high hyperfine coupling constants indicated the likely presence of β hydrogen.61 While the great potential of ENDOR spectroscopy in studying VOPP was demonstrated,62,63 it is unclear if the in situ measurement can be directly applied to heterogeneous petroporphyrins in a complex hydrocarbon mixture. More importantly, a recent study by Mannikko and Stoll reported the use of sub-megahertz hyperfine couplings to speciate the substitution pattern in a series of vanadyl porphyrin model compounds.64 Encouraged by these findings, we applied complementary techniques on isolated petroporphyrins to provide direct evidence for the presence of β hydrogen in low-carbon-number petroporphyrin. The presence of β hydrogens is considered unusual because dealkylation of natural porphyrins is expected to be more difficult based on the stronger bonds associated

n = 7−10) and DPEP (308 + 14n, with n = 11−15). DPEP was found to have higher molecular weight than that of etio, with three of the eight identified peaks (1−3) with lower molecular weights in the series associated with a homologous series of etioporphyrins, with 8−10 carbons beyond the tetrapyrrole core structure or 0−2 carbons beyond the basic octamethyl etio types. The remaining five major ions (4−8) can be identified as DPEP porphyrins starting with mass 527, with 2− 6 carbons more than the minimal porphyrin structure with an exocyclic moiety. Their assigned structures are shown in Figure 3, although the nature of each substituent and their chain lengths cannot be determined unambiguously by MS (e.g., these carbons can be one or more aliphatic substituent). As mentioned above, if all petroporphyrins were from biological precursors (e.g., chlorophyll, and bacteriochlorin) and all β substitutions were preserved, the expected minimal molecular weight change can be determined. This minimal structure, vanadyl octamethylporphyrin (VO-OMP), has a mass of 487 (C28) with all β positions occupied with methyl groups (shown in Figure 3). Only one mass of m/z 473 (C27) is observed below m/z 487 that would indicate the presence of petroporphyrins with one free β hydrogen, given that methyl groups occupy the remaining seven β positions. Porphyrins with a low molecular weight have been noted in earlier studies with MS.16 For example, the study by Zhao et al. has detected 6092


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Figure 5. Proposed structures for compounds M5, M6, M7, and M8 from Figure 4b for the O-down vanadyl petroporphyrins.

with α cleavage than β cleavage. This feature was not detected with 1H NMR after demetalation. However, this is easily attributed to the analytical limits of NMR as a result of the low concentration of these species and the low number of β hydrogens in each structure within the mixture.65 Examination of FT-ICR MS spectra of a few isolated petroporphyrin samples, as exemplified in Figure 3, confirmed the presence of smaller masses that correspond to C27 or C26 vanadyl porphyrins with one or more β hydrogens. Vanadyl and demetalated equivalents differ by 65 Da (1H2-51V16O), and the detection of the corresponding masses in both spectra provide strong evidence that the formulas are correct. The m/z 408 ion in the demetalated sample is at substantially lower abundance than expected from the corresponding C27 etio vanadyl porphyrin with a β hydrogen. This is likely due to its instability under the harsh conditions needed for demetalation, similar to previous reports.24 3.3. Imaging of Petroporphyrins with AFM. Molecular imaging with AFM was conducted to confirm the molecular structure of petrophyrins and their substitution positions. We deposited ultrapure petroporphyrins from the 407 nm fraction (see section 2.1) onto the NaCl(2 ML)/Ag(111) surface. Because the molecules are deposited onto the cold sample (T = 10 K) and all subsequent measurements are carried out at low temperatures (T = 5 K) as well, the molecules are essentially monodispersed on the surface.62 In addition, because we use clean NaCl(2 ML)/Ag(111) as a substrate, we can exclude the alteration of the petroporphyrin properties by proton-containing groups from the surface, as was found in previous work using amorphous SiO2 surfaces.63 We found that many of the molecules were inadvertently manipulated by the SPM tip during scanning, indicating the frequent presence of bulky and nonplanar side groups attached to the porphyrin core. The contrast in CO-AFM depends upon the repulsive forces as a result of Pauli’s exclusion principle, which exhibits a steep dependence upon the tip−sample distance. Because the AFM images are acquired in the constant-height mode, such bulky side groups prevented us in many cases from obtaining AFM images with optimal resolution. Only for a limited number of petroporphyrins were we able to obtain CO-AFM images of sufficient resolution to assign a molecular structure. In general, two different kinds of contrast were observed in the CO-AFM images of petroporphyrins, as shown in Figure 4. The molecules in Figure 4a (M1, M2, M3, and M4) all exhibit a very repulsive feature in the center, which prevents the tip from approaching the molecule closely enough to resolve the porphyrin core. In addition, several side groups can be observed, with a varying degree of bulkiness. In the case of compound M4, two petroporphyrin cores were in close proximity to each other. However, we were not able to unambiguously determine if these two molecules are actually

connected to each other by a chemical bond or if they are just coincidentally abutting each other. If covalently bonded, this would provide direct evidence for the formation of a multicore structure from porphyrins. It should be noted that formulas with multiple vanadyls were not detected in the FT-ICR mass spectra; however, linked petroporphyrins may not be easily volatilized or ionized or at such a low concentration that they escape detection. The second kind of contrast is demonstrated in Figure 4b. For molecules M5, M6, M7, and M8, we were able to resolve most of the atomic structure of the porphyrin core. This is highlighted by the Laplace-filtered images shown along the AFM images, which reveal that pyrrolic nitrogens appear particularly prominent, while the central cavity seems to be empty. Similar to the molecules in Figure 4a, different side groups attached to the porphyrin core can be seen as well. Note that the image of compound M8 is composed of two AFM scans taken at different tip−sample distances to show the porphyrin core and the bulky side group together. To gain further insight into the two kinds of contrast, we studied vanadyl phthalocyanine (VOPC). VOPC contains a central VO group similar to petroporphyrins but no bulky groups at either β or meso positions, which makes it an excellent model compound for high-resolution AFM imaging. In close analogy to the petroporphyrins, we find VOPC molecules with two different AFM contrasts after deposition onto the NaCl(2 ML)/Ag(111) substrate. As shown in Figure 4c, we either find molecules with a strongly repulsive center with only faint atomic resolution on the macrocycle or are able to image the entire macrocycle with atomic resolution and with an apparently empty central cavity. Both kinds of contrast are highly reproducible. We assign the former to VOPC adsorbed with the vanadyl group pointing away from the surface (“Oup”) and the latter to VOPC adsorbed with the vanadyl group pointing toward the surface (“O-down”). This interpretation is supported by previous studies on VOPC on these two adsorption positions.66−68 We find both species in an approximately 1:1 ratio. The image of VOPC O-down also shows increased repulsive contrast on pyrrolic nitrogens, similar to the petroporphyrins in Figure 4b. We attribute this contrast to a slight downward bending of the macrocycle toward the surface as a result of attractive van der Waals interactions, which, in turn, results in a slight lifting of central nitrogens above the plane of the macrocycle. Thus, the VOPC model compound indicates that the apparent cavity in the center of the macrocycle observed in some of the petroporphyrins is not due to the lack of a metal center. Instead, the two different AFM contrasts observed for petroporphyrins can be attributed to different adsorption conformations, resulting from two different orientations of the central vanadyl group. 6093


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4. DISCUSSION It is generally well-understood that DPEP is converted to etio and that the DPEP/etio ratio decreases with increasing thermal maturity. However, the exact geometric positions of substituents remain elusive. Such knowledge is important to fully understand the transformation of chlorophylls and other biochemical precursors to porphyrins. In this study, advanced and complementary analytical tools were employed to study the diverse and complicated structures of petroporphyrins. A detailed structural understanding of these molecules will not only help to elaborate the diagenesis pathway in geochemistry but also shed light on the compositional space of heavy oils and asphaltenes in which porphyrins can be seen as internal tracer molecules and carriers of metals that have poisoning effects on hydroprocessing catalysts.56 Overall, the Gaussian-like distribution of molecular weight distribution for petroporphyrin as well as that of petroleum has been described as the compositional and structural continuum of petroleum.73,74 The origin for this has not been wellestablished, although it has been attributed to extensive transalkylation via an acid-catalyzed mechanism.16 Petroporphyrins with high carbon numbers would necessitate transalkylation, and a low carbon number would require dealkylation. Both reactions are reasonable considering geologic time and conditions. Thus, the presence of β hydrogen and the lack of substitution at meso positions imply a limit imposed on the extension of the transalkylation process and shed light on the complementary dealkylation mechanisms on the aromatic systems. It should be noted that dealkylation of side groups requires breaking an unusually strong bond connected to aromatic carbon, which is at least 30 kcal/mol stronger than cleaving the Cα−Cβ bond producing methyl groups, suggesting that an ionic mechanism rather than a radical pathway is likely involved in diagenesis. Furthermore, a possible hypothesis is that porphyrins with β hydrogen at C-7 are specifically formed from chlorophyll b (Figure 1) rather than a, because a carboxaldehyde group at C7 of chlorophyll b will be more prone to be subject to dealkylation but a methyl group at C-7 of chlorophyll would be equivalent to other methyl groups at C-2, C-12, and C-18 or even alkyl groups on other positions. Therefore, β hydrogen at location C-7 is more likely derived from chlorophyll b.

On the basis of these observations, we assign compounds M1, M2, M3, and M4 to petroporphyrins adsorbed with the vanadyl group pointing away from the surface (O-up) and compounds M5, M6, M7, and M8 to petroporphyrins adsorbed with the vanadyl group pointing toward the surface (O-down). As a result of the lack of resolution on the porphyrin core, we will refrain from assigning molecular structures to the former O-up conformation. For the O-down conformation, Figure 5 shows the proposed molecular structures. The assignment of the side groups is based on comparison to previous AFM studies on model compounds with various aliphatic46,69 and oxygen-containing moieties42,70 as well as the insights gained from the NMR and MS experiments of petroporphyrins discussed above. We also take into account the influence of the CO flexibility at the tip apex on the imaging contrast.39,71,72 Nevertheless, we were not able to completely identify all of the side groups, and unknown substitutions are denoted by “R”. We mostly find methyl substitutions as well as some ethyl groups (M5 and M7) and one longer aliphatic chain, most likely a n-pentyl- [(CH2)4− CH3] group (M8). All four proposed structures have the etioporphyrin core; however, it should be noted that, as a result of the very bulky side group R1 in compound M2 and the aliphatic chain in compound M10, there is some uncertainty in the assignment of the porphyrin core for these two molecules, such that we cannot definitely rule out the presence of a cyclopentyl moiety that is hidden by those side groups. AFM imaging of only etio structures does not contradict the distributions from previous MS (Figure 3) and NMR data. While it is possible to obtain statistically significant results from AFM on some molecular mixtures,47 the number of observations obtained in this study are inadequate to draw conclusions concerning relative abundance and the influence of molecular structure inducing imaging bias has not be fully explored. Unsubstituted β hydrogens in petroporphyrins were directly observed by AFM (M5 and M6). Such hydrogens were already proposed on the basis of ENDOR measurements,61 and their presence is also indicated but not proven in our MS data discussed above. Although prior studies show that the sublimation process does not alter the structure of alkylated hydrocarbons, we cannot definitively rule out that (some of) the β hydrogen positions were initially substituted by larger side groups. Once again, it should be noted that we cannot make a definitive statement concerning the relative abundance of porphyrins with unsubstituted β hydrogens based on the AFM images as a result of the small number of assigned structures. In fact, because molecules with unsubstituted β hydrogens are less likely to contain bulky side groups that could prevent us from obtaining high-resolution AFM images, such molecules might be over-represented in our data set compared to their actual abundance in the sample. Finally, it is important to note the low symmetry of our imaged petroporphyrins, for both the molecules with oxygen pointing up (Figure 4a) and pointing down (Figure 4b). However, even a molecule that is symmetric in the gas phase can have reduced apparent symmetry once it adsorbs on a surface. This is true in particular for molecules with bulky side groups that have many degrees of freedom and, thus, can adsorb in different positions on the surface.

5. CONCLUSION In this study, we focus on petroporphyrins with low minimal carbon numbers and characterized their substitution patterns. UV−vis spectroscopy of the demetalated vanadyl petroporphyrins indicates the presence of six or more substituents on the β positions. Ultrahigh-resolution FT-ICR MS indicates the series of etioporphyrins and the presence of C27 etioporphyrins, which is lower than the minimal structure for eight substituents expected from the biological precursor. The remaining ambiguity of the distribution of these carbons at different substitution positions (one or more) was confirmed with AFM by obtaining real space images of individual petroporphyrin molecules. These data not only support the Treibs hypothesis that petroporphyrins are derived from defunctionalization of chlorophylls but also showed that geometric substitution patterns are preserved. The data support dealkylation under severe maturation but not transalkylation or random alkylation of the β and meso positions, despite the fact that more complicated structures are formed. 6094


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(9) Gueneli, N.; Brocks, J. J.; McKenna, A. M.; Ohkouchi, N.; Boreham, C. J.; Beghin, J.; Javaux, E. J. 1.1-billion-year-old porphyrins establish a marine ecosystem dominated by bacterial primary producers. Proc. Natl. Acad. Sci. U.S.A. 2018, 115 (30), E6978− E6986. (10) Keely, B. J.; Prowse, W. G.; Maxwell, J. R. The Treibs hypothesis: An evaluation based on structural studies. Energy Fuels 1990, 4 (6), 628−634. (11) Baker, E. W. Porphyrins. In Organic Geochemistry: Methods and Results; Eglinton, G., Murphy, M. T. J., Eds.; Springer: Berlin, Germany, 1969; pp 464−497, DOI: 10.1007/978-3-642-87734-6_24. (12) Biesaga, M.; Pyrzynska, K.; Trojanowicz, M. Porphyrins in analytical chemistry. A review. Talanta 2000, 51 (2), 209−224. (13) Milgrom, L. R. The Colours of Life: An Introduction to the Chemistry of Porphyrins and Related Compounds; Oxford University Press: Oxford, U.K., 1999. (14) Kräutler, B. Chlorophyll Breakdown. In Tetrapyrroles: Birth, Life and Death; Warren, M. J., Smith, A. G., Eds.; Springer: New York, 2009; pp 274−285, DOI: 10.1007/978-0-387-78518-9_17. (15) Baker, E. W.; Corwin, A. H. Structure studies on petroporphyrins. Prepr.Am. Chem. Soc., Div. Pet. Chem. 1964, 9 (1), 19−23. (16) Baker, E. W. Mass Spectrometric Characterization of Petroporphyrins1. J. Am. Chem. Soc. 1966, 88 (10), 2311−2315. (17) Baker, E. W.; Corwin, A. H.; Klesper, E.; Wei, P. E. Porphyrin studies. XXXVI. Deoxophylloerythroetioporphyrin. J. Org. Chem. 1968, 33 (8), 3144−8. (18) Furimsky, E. On Exclusivity of Vanadium and Nickel Porphyrins in Crude Oil. Energy Fuels 2016, 30 (11), 9978−9980. (19) Woltering, M.; Tulipani, S.; Boreham, C. J.; Walshe, J.; Schwark, L.; Grice, K. Simultaneous quantitative analysis of Ni, VO, Cu, Zn and Mn geoporphyrins by liquid chromatography-high resolution multistage mass spectrometry: Method development and validation. Chem. Geol. 2016, 441, 81−91. (20) Zheng, F.; Hsu, C. S.; Zhang, Y.; Sun, Y.; Wu, Y.; Lu, H.; Sun, X.; Shi, Q. Simultaneous Detection of Vanadyl, Nickel, Iron, and Gallium Porphyrins in Marine Shales from the Eagle Ford Formation, South Texas. Energy Fuels 2018, 32 (10), 10382−10390. (21) Liu, T.; Lu, J.; Zhao, X.; Zhou, Y.; Wei, Q.; Xu, C.; Zhang, Y.; Ding, S.; Zhang, T.; Tao, X.; Ju, L.; Shi, Q. Distribution of Vanadium Compounds in Petroleum Vacuum Residuum and Their Transformations in Hydrodemetallization. Energy Fuels 2015, 29 (4), 2089−2096. (22) Qian, K.; Fredriksen, T. R.; Mennito, A. S.; Zhang, Y.; Harper, M. R.; Merchant, S.; Kushnerick, J. D.; Rytting, B. M.; Kilpatrick, P. K. Evidence of naturally-occurring vanadyl porphyrins containing multiple S and O atoms. Fuel 2019, 239, 1258−1264. (23) Yakubov, M. R.; Milordov, D. V.; Yakubova, S. G.; Borisov, D. N.; Gryaznov, P. I.; Mironov, N. A.; Abilova, G. R.; Borisova, Y. Y.; Tazeeva, E. G. Features of the composition of vanadyl porphyrins in the crude extract of asphaltenes of heavy oil with high vanadium content. Pet. Sci. Technol. 2016, 34 (2), 177−183. (24) Quirke, J. M. E.; Eglinton, G.; Maxwell, J. R. Petroporphyrins. 1. Preliminary characterization of the porphyrins of gilsonite. J. Am. Chem. Soc. 1979, 101 (26), 7693−7. (25) Hummer, D. R.; Noll, B. C.; Hazen, R. M.; Downs, R. T. Crystal structure of abelsonite, the only known crystalline geoporphyrin. Am. Mineral. 2017, 102 (5), 1129−1132. (26) Ekstrom, A.; Fookes, C. J. R.; Hambley, T.; Loeh, H. J.; Miller, S. A.; Taylor, J. C. Determination of the crystal structure of a petroporphyrin isolated from oil shale. Nature 1983, 306, 173−174. (27) Lash, T. D. Geochemical origins of sedimentary benzoporphyrins and tetrahydrobenzoporphyrins. Energy Fuels 1993, 7 (2), 166− 71. (28) Qian, K.; Mennito, A. S.; Edwards, K. E.; Ferrughelli, D. T. Observation of vanadyl porphyrins and sulfur-containing vanadyl porphyrins in a petroleum asphaltene by atmospheric pressure photonionization Fourier transform ion cyclotron resonance mass spectrometry. Rapid Commun. Mass Spectrom. 2008, 22, 2153−2160.

The same might be true for the formation of other petroleum species derived from porphyrins or similar structures. A more complete picture on how petroporphyrins are formed during diagenesis and catagenesis should emerge as the structures are more accurately and fully defined. Future research will focus on the structures of other petroporphyrins, such as those with nickel chelated, high carbon numbers, high DBE, condensed with aromatic rings, or incorporated with heteroatoms (sulfur or oxygen).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.9b00816.

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Characterization data of petroporphyrins before and after demetalation using UV−vis, IR, FT-ICR MS, and NMR (Figures S1−S7) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yunlong Zhang: 0000-0002-3071-8625 Fabian Schulz: 0000-0002-1359-4675 Katharina Kaiser: 0000-0001-7519-8005 Peter K. Kilpatrick: 0000-0001-7897-4081 Leo Gross: 0000-0002-5337-4159 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Rolf Allenspach and Michael Siskin for helpful comments and Thomas Fredriksen for assisting FTICR MS analyses. This work received partial funding from the European Research Council (ERC) Consolidator Grant “AMSEL” (Agreement 682144).

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REFERENCES

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