Article Cite This: Energy Fuels 2018, 32, 5711−5724
pubs.acs.org/EF
Ultrahigh-Purity Vanadyl Petroporphyrins B. McKay Rytting,† I. D. Singh,‡ and Peter K. Kilpatrick*,† †
Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States Indian Institute for Petroleum, Dehradun, Uttarakhand 248005, India
‡
Michael R. Harper,§ Anthony S. Mennito,§ and Yunlong Zhang*,§ §
ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801, United States S Supporting Information *
ABSTRACT: Petroporphyrins contribute to many of the challenges encountered when producing, transporting, and refining heavy crude oil and bitumen. They are the source of heavy metals that poison catalysts and may facilitate the aggregation, deposition, and emulsion formation exhibited by asphaltenes. Here, they are extracted and enriched to ultrahigh purities from several sources: an Athabasca bitumen, a Canadian northern tier crude oil, and a North American heavy crude oil. Our motivation is to produce usable quantities that can be characterized and used in model studies to understand the molecular structure of asphaltenes and to probe asphaltene−petroporphyrin intermolecular interactions, in the bulk and at interfaces. Extraction is performed in a Soxhlet apparatus. The porphyrin-rich extract is then further purified using extrography (on silicapacked columns) and chromatography (on alumina-packed columns). The process yields purified petroporphyrins in unprecedented quantities (>100 mg). These purified petroporphyrins can be further refined to ultrahigh purities (>85% petroporphyrin by weight) using temperature and centrifugation to fractionate them into more and less soluble fractions. Petroporphyrins are characterized by ultraviolet−visible spectroscopy, X-ray fluorescence spectroscopy, and mass spectrometry (time of flight and Fourier transform ion cyclotron resonance). The majority of the petroporphyrins are simple etioporphyrin (407 nm Soret band) or deoxophylloerythroetioporphyrin (410 nm Soret band) types, but some are more functionalized compounds with highly broadened and shifted Soret bands.
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like compound.25−27 The nickel atom is small enough to fit into the center of the macrocycle,28,29 while vanadyl ion complexes axially, resulting in both the vanadium and oxygen atoms being out of plane with the macrocycle.28 Interest in petroporphyrins is, to a large degree, motivated by their harmful effects during refining30−32 and their interactions with asphaltenes (although porphyrins themselves are also interesting in many novel self-assembly and photophysical applications).33−46 It is common knowledge that intermolecular interactions, such as hydrogen bonding, dipole−dipole interactions, and π−π stacking, between asphaltenes lead to aggregation into nanoaggregates in the bulk oil phase and the formation of elastic films at organic−aqueous interfaces,47−58 although the basic asphaltene structure and mechanism of aggregation continue to be debated. A rigorous study of the interactions of petroporphyrins with asphaltenes may shed light on this debate. Petroporphyrins are often intimately associated with asphaltenes and many report to the asphaltene fraction.11,59−62 Dickie and Yen were the first to suggest that, having many of the same interactive groups as asphaltenes, petroporphyrins could also play a role in asphaltene self-assembly, adsorption, and phase separation.63 This could be by either bridging asphaltenes or groups of
INTRODUCTION Most crude oils contain trace amounts (from tens to thousands of parts per million) of nickel and vanadium (in the form of vanadyl ion) complexed in a tetrapyrrolic compound known as a petroporphyrin (petroleum-based porphyrin).1−3 Porphyrinic compounds (see Figure 1) contain four covalently bonded pyrrolic rings that are conjugated as a macrocycle, and the periphery of the macrocycle is decorated with pendant groups, which can be aliphatic, alicyclic, aromatic, heteroatomic, or some combination of those.4−11 The origins of these macrocycles are likely chlorins and bacteriochlorins, because petroporphyrins are presumably remnants of naturally occurring porphyrin and chlorin structures in plants and bacteria that have metamorphosed during diagenesis to contain the vanadyl or nickel ion and which have undergone alkylation and ring additions to the porphyrin macrocycle.12−21 As a result of the variability of those pendant groups and the resulting influence on polarity and interactions with asphaltenes, petroporphyrins are highly polydisperse, with some reporting to the maltene or resin fractions and others reporting to the asphaltene fraction.11,22−24 Substitution with vanadyl or nickel ion is thought to be due to the much higher binding affinity of these metallic forms for the porphyrinic exocycle and the availability of these metallic forms in the rock deposits in which kerogen undergoes diagenesis. It is widely believed that all or nearly all nickel and vanadium in crude oil is contained in a porphyrin or porphyrin© 2018 American Chemical Society
Received: October 31, 2017 Revised: April 10, 2018 Published: April 10, 2018 5711
DOI: 10.1021/acs.energyfuels.7b03358 Energy Fuels 2018, 32, 5711−5724
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Figure 1. Common vanadyl porphyrin types.
asphaltenes and, therefore, require more polar or aromatic solvents to separate them, if separation is even possible. The polydispersity of both petroporphyrins and the crude oils from which they derive means solvents that work very well for one porphyrin type in one crude oil may not work well in other situations. The ability of solvents to extract petroporphyrins preferentially is dependent upon not only the character of the porphyrin pendant groups but also the metal coordinated to the center; therefore, while methanol and acetonitrile are both good extraction solvents for petroporphyrins, nickel petroporphyrins are often preferentially extracted by acetonitrile, and the opposite is true for VOPPs.76,80,81
asphaltenes to promote aggregation or breaking up asphaltenes and preventing aggregate and/or film formation by occupying individual interaction sites but not connecting to anything else.1,22,64−69 Thus, petroporphyrins could play similar roles as asphaltenes. The relationship between petroporphyrins and asphaltenes, however, is largely unexplored, possibly in part as a result of the difficulty of obtaining purified petroporphyrins. Our goal is to purify significant quantities of vanadyl petroporphyrins (VOPPs), of high enough purity, from a crude oil source to enable future research to investigate the physical relationships between porphyrins and asphaltenes. These petroporphyrins will be characterized by ultraviolet− visible (UV−vis) spectroscopy and mass spectrometry,2 along with metal detection.25,27 Other characterization techniques [such as electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and nuclear magnetic resonance (NMR)] complement these to provide a more complete understanding of the petroporphyrin molecular structure.66,70−74 Petroporphyrin purification requires multiple successive separation procedures, usually some combination of extraction, extrography, and chromatography. Extraction, typically using a Soxhlet apparatus, is often the first step because it can accommodate a large quantity of material. Solvent choice requires prioritization of either yield or purity, and solvent effectiveness varies depending upon the parent crude oil, the fraction of the crude oil being extracted, and the desired product. Extrographic−chromatographic columns are used to purify the extracted fraction further, because they cannot accommodate as much material as the Soxhlet apparatus but can provide more distinct solubility fractions in a shorter time and in a single procedure. Several columns using polarity gradient solvent series can result in high-purity petroporphyrins. Many successive separation procedures can purify petroporphyrins significantly. These separation processes have been used successfully by many, but despite fairly abundant literature on the topic, petroporphyrin extraction and enrichment still present many challenges, including similarity of petroporphyrin solubility to other hydrocarbons, such as asphaltenes or resins, difficulty of detection and quantification of petroporphyrins, and scarcity of petroporphyrin compounds in the starting material.5,27,75−91 Porphyrins with fewer polar or aromatic sites should be more susceptible to separation from asphaltenes with less polar or aromatic solvents, while those with more intermolecular interaction sites will be more intimately associated with
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EXPERIMENTAL SECTION
Materials. Crude Oils and Crude Residua. Asphaltenes (both npentane insoluble and n-heptane insoluble) were separated from an Athabasca bitumen (ETB) and a Canadian northern tier crude oil (EHO) using the method described by Spiecker et al.48 Additionally, the vacuum residue of a metal-rich North American heavy crude oil (M) was used as a starting material. Table 1 summarizes these key values of the starting materials.
Table 1. Key Characteristics of Starting Materials starting material
V (ppm)
Ni (ppm)
weight percent of n-C7 asphaltene (% of whole crude oil)
ETB asphaltene EHO asphaltene M vacuum residue
187.5 394 750
77.9 87 145
14.5 14.2 9.24
Solvents, Chemicals, and Other Materials. High-performance liquid chromatography (HPLC)-grade methanol, hexane, methylene chloride, and toluene were purchased from Sigma-Aldrich and used as received. Vanadyl octaethylporphyrin (OEP-VO, 95% purity) purchased from Sigma-Aldrich Chemicals was used as a vanadyl porphyrin standard. Silica gel column packing media (100−200 mesh, 60 Å pore size) and alumina column packing media (80−200 mesh) were purchased from VWR. Additional silica gel column packing media (150 and 300 Å pore size) were purchased from SiliCycle. Both silica gel and alumina packing media were activated at 100 °C for 4 h prior to column assembly, unless otherwise specified. The 180 × 60 mm cellulose Soxhlet extraction thimbles were purchased from VWR. The 18 × 18 in. cellulose absorbent pads were purchased from Grainger and used for large-scale extractions. The 0.2 μm cellulose ester membrane filters were purchased from Advantec. Petroporphyrin Characterization. UV−Vis Spectrophotometry. Petroporphyrins in varying states of purity were characterized by UV− 5712
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Figure 2. ETB asphaltene VOPP enrichment procedure. Here, ETB is subjected to an extraction, which produces an extract and a residuum. The extract is further fractionated on a silica column (SC-1) using a series of solvents shown below the silica column, including hexane (H), hexane:methylene chloride mixtures (H:MC), methylene chloride (MC), and toluene:methanol mixtures (T:MeOH). The 1:4 H:MC and MC eluents are combined and further fractionated on an additional silica column (SC-2). Again, the 1:4 H:MC and MC eluents from SC-2 are combined and further fractionated on an alumina column (AC-1), where the 1:6 H:MC eluent contained purified VOPPs. The residuum from the original extraction is also further fractionated on several columns, which are named with a R prefix to specify that they are using the residuum from the original extraction. vis spectrophotometry.27,29,92−95 A calibration curve for VOPPs was developed using OEP-VO dissolved in toluene at room temperature. The UV−vis spectrophotometer used to measure absorbance was a Shimadzu UV-1800, equipped with both tungsten (λ = 320−1100 nm) and deuterium (λ = 190−370 nm) lamps and with a 1.0 nm spectral bandwidth. All samples were dried of all solvents and then redissolved in toluene, transferred to a 1.0 cm quartz cuvette, and scanned from λ = 300 to 1000 nm. A baseline measurement with pure toluene was taken each day that new measurements were taken. The Soret peak area was calculated by simple peak skimming. For experiments using ETB and EHO asphaltenes, the extinction coefficient of OEP-VO at 573 nm was used to quantify the VOPP content to minimize the background arising from hydrocarbon impurities. For experiments using M vacuum residue, the integrated molar absorptivity of the Soret band was used to quantify the VOPP content rather than the extinction coefficient as a result of broadening and variability of extinction coefficients among petroporphyrins.21,92 For presentation here, all UV−vis spectra are normalized to A = 1 at the peak of the Soret band. Inductively Coupled Plasma Mass Spectrometry (ICP−MS) Metal Analysis. Trace element analyses by ICP−MS were performed within the MITERAC ICP−MS Facility at the University of Notre Dame. Solution mode (SM)-ICP−MS analysis of all samples, standards, and blanks was carried out using the Nu Attom (Nu Instruments) highresolution (HR)-ICP−MS. All measurements were conducted in medium mass resolution (M/ΔM ≈ 3000), and at the start of each analytical session, the instrument was tuned and calibrated using a multi-elemental solution (1 ng/g). Standard and spike solutions were prepared from 1000 mg/g elemental standard solutions (BDH ARISTAR certified reference standards). The aliquots obtained during the optimization and calibration phase of the chemical separation protocol were prepared for SM-ICP−MS analysis using an external calibration method that involved In (1 ng/g) as the internal standard
to monitor for instrumental drift and matrix effects. Concentrations of V were calculated using an external calibration method. X-ray Fluorescence (XRF) Spectrometry. XRF spectrometry was used to measure the metal content25−27,96 with a Bruker Tracer III-SD. This spectrometer is equipped with a rhodium target, which can be operated at voltages up to 40 kV and a 10 mm2 XFlash SDD detector with 145 eV resolution at 100 000 counts per second (cps). Filters were used in all measurements to optimize the energy of the incident X-rays to create the highest signal-to-noise ratio for the metal of interest. A 1 mm Teflon filter was selected for vanadium detection. The spectrometer was set to 40 kV and 11 μA for all measurements. Each sample was measured 5 times for 3 min each. These five 3 min measurements were averaged together to calculate a total number of counts. Samples being characterized for VOPP purity were dried and redissolved in toluene for XRF measurement. The equation from the calibration curve converted the total counts of a specific energy range to a metal count (nV) and, therefore, a VOPP count (because 1 molV = 1 molVOPP) for the dissolved sample, which was then converted to a petroporphyrin concentration (CVOPP) of the dry sample using the concentration and mass of the measured solution (Csample and msolution) and an estimated molecular weight (MWVOPP) from laser desorption/ ionization time of flight mass spectrometry (LDI−TOF MS) or atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry (APPI FT-ICR MS) (see the next section).
⎛ ⎞ ⎛ 1 molVOPP ⎞ 1 ⎟⎟ C VOPP = nV ⎜ ⎟(MWVOPP)⎜⎜ ⎝ 1 molV ⎠ ⎝ Csamplemsolution ⎠
(1)
Mass Spectrometry. LDI−TOF mass spectra were collected using a Bruker UltrafleXtreme and analyzed using flexAnalysis 3.4. Samples were fully dried and redissolved in methylene chloride and then 5713
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Figure 3. EHO asphaltene VOPP enrichment procedure. See Figure 2 for a description of the structure of the schematic. spotted onto a sample plate. No matrix was used to assist ionization, because all tested matrix materials complexed with both synthetic and petroleum-based vanadyl porphyrins. The instrument was set to detect positive ions. Calibration was performed at the start of every data acquisition session using a combination of proteins ranging from 600 to 3000 amu. Select samples were additionally analyzed using ultrahigh-resolution FT-ICR MS, with 12 T Bruker Apex Apollo II and 15T Bruker solariX XR apparatuses. APPI was employed to vaporize the porphyrin molecules into the gas phase using a 420−450 °C heated nebulizer. Once in the gas phase, a krypton lamp with 9.4 eV was used to induce ionization. Samples were dissolved in toluene at a concentration of 10−100 ppm, infused into the instrument at 120 μL/h, and accumulated ions in the collision cell for 0.020−1.0 s. A total of 24−80 scans were collected from m/z 150 to 2000 and co-added. Data calibration was performed using Bruker Data Analysis with Agilent HP Mix for external calibration. Internal calibration was performed by HP Mix spiked in the sample blend or identifying a homologue series (core porphyrin with additions of −CH2− alkyls at 14.01565 Da) and constructing a calibration list for the mass profile. Further data interpretation was accomplished using in-house software. VOPPs have a small number of base structures, and their pendant groups most often vary by −CH2−; therefore, they can easily be distinguished from the asphaltene background by their large peaks separated by 14 amu.97,98 With FT-ICR MS, we can unambiguously assign molecular formulas to these porphyrinic species8−10,45,99 and describe them by Z class (or Z number), also known as hydrogen deficiency, which was developed to describe the number of double bonds and rings in a hydrocarbon.100 Z = −28 corresponds to the simplest alkyl-substituted etioporphyrin; Z = −30 corresponds to alkyl-substituted deoxophylloerythroetioporphyrin (DPEP); etc.
polydispersity and intermolecular interactions of asphaltenes (and petroporphyrins).48 Asphaltenes and petroporphyrins as a whole are toluene-soluble, but each individual asphaltene and petroporphyrin molecule has its own level of solubility or insolubility in toluene. Many of the larger and more functionalized asphaltenes and petroporphyrins are solvated in toluene by smaller molecules, including smaller asphaltenes, porphyrins, maltenes, and resins. These compounds interact with each other to enable the most insoluble asphaltenes to dissolve in toluene. However, when asphaltenes are fractionated into more and less soluble portions using alkanes, the less soluble portion is also less soluble in toluene and some asphaltene fractions become completely insoluble as a result of the lack of more soluble compounds that aid in their dissolution.101−103 When a narrow solubility range is isolated, such as in the case of individual petroporphyrin fractions from extrography, the solubility of the various components of that fraction change and subsequent columns, even with similar or identical solvent sequences, are effective at continuing to purify petroporphyrins. Petroporphyrin Purification from ETB and EHO Asphaltenes. The separation and purification procedures to purify petroporphyrins from ETB and EHO asphaltenes were identical with the exception of the solvents used (see Figures 2 and 3). Asphaltenes were ground in a mortar and pestle and deposited into a cellulose thimble, where they were extracted until the solvent became colorless. The extracts (ETB/EHO extract) were transferred to weighed vials using methylene chloride and dried in a vacuum oven at 100 °C. The residual (ETB/EHO R) asphaltenes were dissolved in ortho-dichlorobenzene (o-DCB)/ortho-chlorophenol (o-CP) (52:48, v/v) at 80 °C with sonication and slurried with Celite and then dried at 100 °C under vacuum. The dried slurries were then extracted again. The extracts from both n-C5 and n-C7 asphaltenes were combined (n-C5 ETB/EHO extract with n-C7 ETB/EHO
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RESULTS AND DISCUSSION Petroporphyrin Extraction and Purification. VOPPs from ETB, EHO, and M were purified according to the schemes depicted in Figures 2, 3, 7, and 14. Very similar or identical solvent series for extrography or chromatography are effective on consecutive columns as a result of the 5714
DOI: 10.1021/acs.energyfuels.7b03358 Energy Fuels 2018, 32, 5711−5724
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Energy & Fuels extract and n-C5 ETB R/EHO R extract with n-C7 ETB R/ EHO R extract) and fractionated on several successive columns to produce purified VOPPs. Purified VOPPs from ETB and EHO asphaltenes were characterized by measuring their metal content as well as performing UV−vis and mass spectrometry (see Figure 4 for
but were still significantly enriched in comparison to the starting crude oil at 187.5 ppm of V, which would correspond to about 0.2% VOPP. The lower final purity could be partially due to the lower initial concentration of VOPPs in ETB (187.5 ppm of V versus 394 ppm of V in EHO and 750 ppm of V in M vacuum residue). Both the height and integrated area of the Soret band vary depending upon the molecular structure of the porphyrins, and because petroporphyrins are highly polydisperse, it is nearly impossible to determine definitively the appropriate extinction coefficient or integrated molar absorptivity for any subset.21,27 This is complicated further by the potential for highly shifted Soret bands and Soret quenching as a result of asphaltene trapping and other interference.104−107 As a result of this, the Soret band is not necessarily an accurate measure of the total petroporphyrin content (whether measured by height or area). As a result of these inherent uncertainties, purity estimation by metal analysis (in this case, ICP−MS) is preferable to UV−vis. Because ICP−MS captures all of the metal content and the MW can be estimated from LDI−TOF MS, the purity according to metal analysis is highly accurate. The makeup of porphyrin species in the purified fractions from EHO and ETB is remarkably consistent. A comparison of the amounts of etio (Z = −28), DPEP (Z = −30), etc. between EHO VOPPs and ETB VOPPs as well as between EHO R VOPPs and ETB R VOPPs shows similar trends (see Table 9 of the Supporting Information). At least in the case of EHO and ETB VOPPs, it seems that these separation processes access the same petroporphyrin populations and molecular distributions. Also notable in the breakdowns of petroporphyrin data is the difference between the VOPPs and the R VOPPs. The R VOPP distributions in both cases are shifted toward more functionalized petroporphyrins, and the average MWs of the R VOPPs are higher as well. This larger, more functionalized VOPP population is more tightly bound to asphaltenes and is not extracted initially. It is only when the residuum from the first extraction is dissolved in a very strong solvent (52:48 o-DCB/oCP) at a high temperature (100 °C) that those associations are broken down and those petroporphyrins are released. Slurrying with Celite at this point provides the asphaltenes and
Figure 4. UV−vis spectrum of VOPPs purified from EHO asphaltenes (EHO VOPP).
the UV−vis spectrum of purified VOPPs from EHO). From their mass spectra, porphyrin type (etio, DPEP, benzo, or combinations of those), molecular weight (MW) range, most prevalent mass (maximum peak), weight-average MW, and relative percent (number of VOPPs of a Z class compared to the total number of VOPPs) are calculated (see Figures 5 and 6 and Table 2 as well as Tables 5−8 of the Supporting Information). These results show that significant purification of VOPPs is possible in several steps, including Soxhlet extraction and multiple extrographic columns. Following this procedure, EHO VOPPs were enriched to over 90% purity, as calculated using metal analysis and the MW estimated from LDI−TOF MS. ETB VOPPs were not enriched to the same degree (22% pure)
Figure 5. Average MW of purified VOPPs from EHO and ETB asphaltenes, sorted by Z number. The class listed (where E = etio, D = DPEP, and B = benzo) is a potential class based on the Z number but is not necessarily the only potential VOPP class that corresponds to that Z number. For instance, a tri-DPEP VOPP and a benzo VOPP have identical Z numbers. 5715
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Figure 6. Composition of purified VOPPs from EHO and ETB asphaltenes in percentage of N4O1V1 species reporting to each Z number.
Table 2. ETB and EHO VOPP Purity by Metal Content and UV−Vis (Extinction Coefficient of OEP-VO at 573 nm) MW (g/mol)
V (ppm)
purity (%)
purity (%)
sample
LDI−TOF MS
ICP−MS
ICP−MS
UV−vis
ETB VOPP ETB R VOPP EHO VOPP EHO R VOPP
524.3 547.9 505 524.4
21175 11872 94256 37799
21.8 12.8 93.3 38.9
11 8 43.8 26.7
petroporphyrins something to adhere to, preventing reaggregation and facilitating extraction of previously inaccessible VOPPs, which can then be purified. Petroporphyrin Purification from Vacuum Residuum of Crude Oil M. We performed several preparatory-scale extraction and enrichment experiments (M-1, M-2, and M-3), intended to purify sufficient amounts of petroporphyrins for other experiments, including interfacial adsorption, molecular self-assembly, chemical characterization, and interactions with asphaltenes. The objective of M-1 and M-2 is purification of less-functionalized etio and DPEP VOPPs at two different larger scales, to assess the scalability of the separation and purification processes. The objective of M-3 is purification of more-functionalized VOPPs of higher Z number. Our efforts to purify diverse populations of petroporphyrins from these large quantities of starting materials presented many challenges, which are discussed in the following sections along with our attempts to overcome them. Figure 7 (for M-1 and M-2) and Figure 14 (for M-3) detail the schemes of solvents and packing media used. Additional details are included in the sections below. The vacuum residue of M was dissolved in methylene chloride and deposited onto a cellulose pad. The solvent was completely evaporated, and the residue was extracted in a 6 L Soxhlet extractor according to the scheme until the solvent became colorless. For M extractions, the mass of extract exceeded the solubility limit of the extraction solvent; thus, there were both solvated and precipitated portions, which were characterized and further fractionated separately. M-1 Petroporphyrin Enrichment and Purification. A total of 165.42 g of M vacuum residue was deposited onto cellulose pads, which were then dried and extracted with 6 L of
Figure 7. M vacuum residue preparatory-scale VOPP enrichment procedure. See Figure 2 for a description of the structure of the schematic.
methanol, yielding 15.41 g of soluble extract and 16.11 g of precipitated extract. The petroporphyrin content of the soluble extract was estimated to be 3% with UV−vis spectroscopy. A 100 × 6 cm diameter column was packed with silica gel totaling 2.5 L (SC-1), and 3 L of each eluting solvent were used. A brightly colored pink band eluted with 1:4 H:MC, yielding material that was around 30% petroporphyrin, as estimated by UV−vis. This fraction was loaded onto a 100 cm tall × 2 cm diameter column packed with 250 mL silica gel (SC-2) and eluted with 500 mL of each solvent mixture. The 1:4 H:MC fraction eluted with about 50% pure petroporphyrins and was subsequently further fractionated on an alumina column. Packing media for the alumina column (AC-1) was prepared by activating alumina at 100 °C for 4 h. After the aromatic maltenes eluted with hexane, 17 aliquots were taken of 1:2 H:T, followed by 10 aliquots of pure toluene. This large-scale separation procedure resulted in several fractions of purified petroporphyrins, labeled AC-1 1, AC-1 2, ..., AC-1 29 (see Figure 8 for UV−vis spectra of three of these fractions: 10, 20, and 24). The residual asphaltene concen5716
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Soret band at about 417 nm27), which would shift the Soret band to longer wavelengths overall, but the differences in proportions of those species seem too little to shift it 6 nm. The additional Z = −36 (about four percentage points, 9.52 versus 5.26%) and reduced Z = −28 and = −32 (about two percentage points each, 8.01 versus 9.70% and 4.72 versus 6.55%, respectively) would not account for such a dramatic shift on their own. Additionally, the 410 nm sample has more VOPP at each Z number from Z = −42 to −50, and those more functionalized species should have longer wavelength Soret bands than DPEP. If the observed Soret band is determined by the individual species Soret bands added together proportionally, there should be very little or no difference in the Soret band maximum between the 410 and 416 nm samples. One possible source of this 6 nm shift is the non-porphyrinic compounds in these samples, which can be seen in Figure 10. The 416 nm sample has a smaller proportion of VOPPs and much more 1O, 3S1N, and 1S1O compounds than either the 407 or 410 nm samples, as reported by APPI FT-ICR MS. It is possible that these non-porphyrinic compounds are interacting with the VOPPs to red shift the Soret band. M-2 Petroporphyrin Enrichment and Purification. A total of 384.84 g of vacuum residue was deposited onto cellulose pads, yielding 20.77 g of soluble extract and 85.21 g of precipitated extract. The mass of soluble extract is fairly similar to M-1, which is expected because the same amount of methanol was used in both extractions and presumably the solubilities of the M-1 and M-2 extracts are similar. The soluble portion of the extract was loaded onto a 100 × 6 cm diameter column filled with 1.5 L silica gel (M-2 SC-1) and fractionated with 3 L of each eluting solvent. The 1:4 hexane:methylene chloride fraction was the most enriched in VOPPs (see Figure 11) and was further fractionated twice on 100 × 2 cm diameter columns having 250 mL of packing media to produce purified VOPPs. Figure 11 also shows the Soret band shifting to longer wavelengths as the polarity of the fraction increases, going from 408 to 409 to 410 for 1:4 H:MC, pure MC, and 1:1 toluene:methanol. This Soret band shift indicates that functionalization of VOPPs is related to their affinity for silica. We also purified petroporphyrins from the precipitated portion of the M-2 extract, following the scheme in Figure 7 but using the precipitated portion rather than the soluble portion, with “P” acronyms used later in the text referring to the separation process performed with the precipitated portion (e.g., PAC-1 of the precipitated extract is analogous to AC-1 of the soluble extract). Fractionating the precipitated portion of the extract proved to be problematic as a result of the difficulty of handling this very thick, sticky material. It was more difficult to dissolve and deposit onto packing media, and several silica gel columns were required to increase the purity enough to produce purified VOPPs on alumina gel. These columns were assembled as before, but the solvent series was modified from the soluble extract to account for the likely increased polarity of these compounds; therefore, the solvent mixtures used were all richer in methylene chloride. Some UV−vis spectra of the Soret band for M-2 AC-1 fractions are shown in Figure 12. Many of these fractions have very similar UV−vis spectra compared to the most highly purified petroporphyrins from the M-1 experiment, but one significant difference is the lack of any fractions having Soret peak maxima at wavelengths higher than 410 nm. The lack of red-shifted Soret bands in these purified VOPPs motivated us to investigate the precipitated portion of this extract. Highly
Figure 8. UV−vis spectra of AC-1 fractions 10, 20, and 24, corresponding to Soret band maxima of 407, 410, and 416 nm, respectively.
tration was calculated using the Beer−Lambert law, and the intensity of the Rayleigh scattering suggests over 90% VOPP content, which is similar to the VOPP purity estimated by FTICR MS analysis. One significant and noteworthy feature of this set of UV−vis spectra is the shifting of the Soret band seen in Figure 8a as well as the shifting of the α and β bands in Figure 8b. The Soret band for simple etio VOPPs peaks at 407 nm, but several of these fractions had spectra shifted by as much as 9−416 nm. The largest red shift is with the final fractions eluting from the column. It is conceivable that there are some petroporphyrins that are bound more strongly to the alumina and are therefore more difficult to elute, likely as a result of the presence of additional aromatic and/or polar functional groups in the VOPP, which adhere more strongly to alumina. Figure 9 displays the distribution of porphyrin species in three of these purified fractions as measured by FT-ICR MS, with Soret band maxima of 407, 410, and 416 nm. The 407 nm VOPP, as expected, has a significant amount of etioporphyrin, which has mostly disappeared in the 410 and 416 nm samples. The 410 nm sample is majority DPEP (Z = −30), which also is expected, because the Soret band maximum for DPEP in toluene has been reported to be about 410 nm. However, the 416 nm sample also has mostly DPEP. It has less etio and more Z = −36 (likely benzo-DPEP, which we might expect to have a 5717
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Figure 9. Composition of purified VOPPs in three fractions of M-1 AC-1 having different Soret band (SB) maxima. Compositions are plotted as a percentage of N4O1V1 species reporting to each Z number. The porphyrin type listed (where E = etio, D = DPEP, and B = benzo) is a potential class based on the Z number but is not necessarily the only potential VOPP class that corresponds to that Z number. For instance, a tri-DPEP VOPP and a benzo VOPP have identical Z numbers.
Figure 10. Composition of purified VOPPs in three fractions of M-1 AC-1 having different Soret band (SB) maxima. Compositions are plotted as a percentage of the total sample reporting to various hydrocarbon species.
aggregate or deposit onto surfaces than their simpler counterparts, it would be unsurprising if they are more likely to precipitate out of methanol. Figure 13 shows UV−vis spectra for the two M-2 PAC-1 fractions, 3 and 7, which are the first and last highly purified petroporphyrin fractions from M-2 PAC-1. While the average wavelength of the Soret band maxima was at a longer wavelength for M-2 PAC-1 (range of 409.5−410.5 nm, mostly around 409.5 nm) than for M-2 AC-1 (range of 407−410 nm, mostly around 407 nm), there were no petroporphyrins with significantly red-shifted Soret bands that were found before. The UV−vis spectra of the M-2 PAC-1 fractions suggest that they are slightly less pure than the M-2 AC-1 or M-1 AC-1
functionalized petroporphyrins likely have a higher affinity for asphaltenes and other functionalized petroporphyrins, causing them to precipitate. Red-shifted Soret band petroporphyrins, such as those observed in M-1, may report to the precipitated portion of the extract. M-2 initially used much more residue (165 versus 385 g) with the same amount of methanol (6 L) and ended up with similar amounts of soluble extract (15 versus 21 g) but a sizable difference in the amount of precipitated extract (16 versus 85 g). This suggests that the compounds in this vacuum residue that are extractable by methanol have a solubility limit of around 3 g/L. Once the concentration in the stillpot exceeds that, precipitation begins to occur. Because more functionalized petroporphyrins are more likely to 5718
DOI: 10.1021/acs.energyfuels.7b03358 Energy Fuels 2018, 32, 5711−5724
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extracted with hexane to try to remove all of the maltenes, followed by extraction with a series of more polar solvents. UV−vis spectra of the four extracts showed red-shifted Soret bands in the 1:1 toluene:methanol extract and the methylene chloride extract. The methylene chloride extract appeared to be a good candidate for extracting and purifying petroporphyrins of high functionality, but it was also highly viscous and difficult to solvate, even in methylene chloride, and the solutes in this extract occluded the pores of the silica gel matrix (60 Å). The 150 and 300 Å pore size silica gels were also used to try to accommodate the larger molecules and aggregates in this most aromatic and polar fraction; however, again, the pores were occluded, and no fractionation was achieved. Next, a jacketed column was used at several temperatures from 30 to 90 °C, with the expectation that the higher temperatures would deaggregate the asphaltenes and allow them to move through the column. The higher temperature (>50 °C) experiments were successful in facilitating asphaltene movement down the column, but no significant enrichment of VOPPs was detected in either UV−vis or XRF spectroscopy. Next, the 1:1 toluene:methanol extract was fractionated. UV−vis spectra of 500 mL aliquots of the 1:4 H:MC eluent from the second silica column (M-3 TMSC-2) revealed that the Soret band changed significantly from one aliquot to the next. All aliquots with identical UV−vis spectra were combined, resulting in three 1:4 H:MC fractions. The first of these had an unusual UV−vis spectrum, with a Soret band that had a strong shoulder at about 420−425 nm (see the M-3 TMSC-2 1:4 H:MC 1 curve in Figure 15). VOPPs with Soret bands shifted past 420 nm had not previously been observed or purified; therefore, this sample was further fractionated on an alumina column.
Figure 11. UV−vis spectra from the six M-2 SC-1 eluted fractions. Vertical lines are drawn to indicate the Soret band maxima of the porphyrin-containing fractions (4:1 H:MC-408, MC-409, and 1:1 T:MeOH-410).
Figure 12. UV−vis spectra of select eluted fractions from the M-2 AC1 separation process.
Figure 14. M-3 functionalized VOPP enrichment procedure. See Figure 2 for a description of the structure of the schematic. Figure 13. UV−vis spectra of select eluted fractions from the M-2 PAC-1 separation process.
The UV−vis spectra of these compounds, shown in Figure 16, with some M-1 AC-1 spectra for comparison in panels a and b of Figure 16, were striking, as most of the first several fractions had Soret bands with fairly common maxima (407− 410 nm), indicating significant amounts of etio- and DPEP-type petroporphyrins, but they were much wider (∼20 nm at half maximum versus ∼10 nm normally) than the Soret bands from previously purified petroporphyrins. Later fractions had Soret
fractions. That may be the case, or there may be a small proportion of functionalized petroporphyrins that do not report to the Soret band. M-3 Petroporphyrin Enrichment and Purification. The M vacuum residue was deposited onto cellulose pads and 5719
DOI: 10.1021/acs.energyfuels.7b03358 Energy Fuels 2018, 32, 5711−5724
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Figure 15. UV−vis spectra of fractions from M-3 TMSC-2. The shape of the Soret band of the 1:4 H:MC 1 fraction is unusual for its higher wavelength shoulder.
bands that were shifted (up to 423 nm) and broadened (up to ∼40 nm at half maximum) dramatically as well as completely different Q band characteristics (only one observed band at ∼590 nm rather than α and β bands at ∼575 and ∼535 nm, respectively). One sample, whose spectrum is shown in Figure 16c, even had an unexpected Soret “plateau” with a flat region from 413 to 427 nm rather than a well-defined peak. This feature was observed at several different concentrations and is not due to the absorbance exceeding the maximum measurable absorbance of the spectrophotometer. Additionally, these samples were visibly different from typical petroporphyrins, with the less shifted samples having a red− green color and the more highly shifted samples being completely green. These wider and more-shifted Soret bands suggest that these samples are both highly polydisperse and functionalized. The unique UV−vis spectral and visible characteristics are likely caused by the presence of sulfur (detected by FT-ICR MS) in the form of thiophenic pendant groups. Thiophenic petroporphyrins have been previously detected9 but never purified and characterized at the levels noted here. An alternative explanation for the green color of these petroporphyrins is that are chlorin or bacteriochlorin molecules. Summary of Large-Scale Purification of VOPPs from the M Vacuum Residue. In summary, several fractions containing different populations of VOPPs were extracted and purified to a high degree from the M vacuum residue. The majority of the VOPPs purified were purple and had 407−410 nm Soret band maxima, corresponding to a majority of etio and DPEP types. These petroporphyrins had UV−vis spectra that were almost identical to the UV−vis spectrum of a synthetic standard, OEPVO (see Figure 17). While it is not easy to purify these petroporphyrins from crude oil, the process is nonetheless straightforward and replicable. Small amounts of highly purified VOPPs having uncommon characteristics were also obtained. Some purple fractions had Soret band maxima at around 416 nm, while others were reddish green or completely green and had Soret peak maxima shifted to wavelengths as short as 405 nm and as long as 423 nm. The fractions with Soret bands at the longest wavelengths tended to be highly functionalized, having Z numbers concentrated below −40 and high proportions of petroporphyrins with thiophenic sulfur.
Figure 16. UV−vis spectra of M-3 TMAC-1 fractions, with typical etio and DPEP petroporphyrin spectra for comparison. The additional Q band at 590 nm and Soret band shoulder at 450 nm are unique to the M-3 TMAC-1 VOPPs.
Table 3 contains purity estimates of several purified VOPP samples, labeled by the final separation process (M-1 AC-1 or M-3 TMAC-1) and the wavelength of their Soret band maximum (407, 410, etc.), as measured by UV−vis, XRF, and FT-ICR MS. Purities calculated from UV−vis should be considered a lower bound as a result of the issues of Soret band 5720
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Figure 17. UV−vis spectra of highly purified petroporphyrins versus synthetic standard (OEP-VO).
Table 3. Select Purified VOPPs from the M Vacuum Residue calculated petroporphyrin content (%) FT-ICR MS PP designation M-1 M-1 M-1 M-3 M-3 M-3 M-3
AC-1 407 AC-1 410 AC-1 416 TMAC-1 407 TMAC-1 410 TMAC-1 420 TMAC-1 413−427
UV−vis
XRF
4N1O1V
1S4N1O1V
total
44 22 35 42 33 41 19
45 40 47 45 47 45 25
92.7 97.2 93.6 84.1 80.8 65.4 53.3
3.7 1.2 0.0 15.3 16.9 32.9 44.4
96.4 98.4 93.6 99.3 97.7 98.3 97.7
broadening, Soret band quenching, and extinction coefficient variability mentioned previously. As was mentioned previously, FT-ICR MS does not necessarily ionize all species; therefore, the purity estimated from it should be considered an upper bound. XRF should be the most accurate because it accounts for every vanadium atom in the sample, and every vanadium is chelated to a VOPP compound. The MW used to convert the vanadium measured with XRF to the mass of petroporphyrin is estimated with FT-ICR MS, which, although imperfect as mentioned above, gives us our best estimation of the average VOPP mass. We attempted several times to further purify these petroporphyrins on additional alumina columns but did not obtain any ultrahigh-purity VOPPs like the sample from EHO that was over 90% VOPP by weight. For the M vacuum residue, there appears to be many compounds with very similar strengths of adsorption on alumina compared to VOPPs; therefore, additional alumina columns do not further purify the porphyrinic compounds. An additional fractionation step is required that does not rely on the strength of adsorption on silica or alumina. We fractionated three porphyrin fractions by dissolving them in toluene near their solubility limit, lowering their temperature to induce precipitation, then centrifuging at 15 000 rpm for 1 h, and pipetting the supernatant from the precipitant. The precipitated VOPPs were purified significantly (see Table 4), with metal analysis showing >85% purity for all three fractionated VOPPs. The VOPPs in the supernatant were all lower in purity than the original purified fraction. We are investigating the supernatant and precipitant fractions to determine the nature of the impurity as well as potential differences in petroporphyrin population between the two fractions and will share those results in an upcoming paper focused on detailed VOPP characterization using NMR spectroscopy, Fourier transform infrared spectroscopy, UV−
Table 4. Ultrahigh-Purity VOPPs (Per Metal Analysis with XRF) Precipitated from Purified VOPP Samples at a Low Temperature calculated VOPP purity (%) PP description
whole
supernatant
precipitant
407 nm Soret band purple VOPP 410 nm Soret band purple VOPP 410 nm Soret band green VOPP
47 52 55
33 19 27
98 86 89
vis spectroscopy, and FT ICR-MS in conjunction with molecular models developed using density functional theory.
■
CONCLUSION VOPPs have been extracted from crude oil and then sequentially purified using solvent polarity gradient column extrography and chromatography. Final purities of petroporphyrins of >85% have been achieved as reported by FT-ICR MS or ICP−MS/XRF metal analysis. The UV−vis spectra of the simplest highly purified petroporphyrins are almost identical to a OEP-VO standard from Sigma. The simplest etioporphyrins have Soret bands at 407 nm in toluene and are purple in toluene solution. They are less tightly bound to asphaltenes and adsorb more weakly than other petroporphyrins to silica and alumina. DPEP-type porphyrins have Soret bands at 410 nm in toluene, are slightly more tightly bound to asphaltenes, and adsorb more strongly onto silica and alumina. Etio and DPEP petroporphyrins are comparatively easy to separate and purify. No extreme measures are required to separate and purify hundreds of milligrams of these types of petroporphyrins, which occur most commonly in the crude oil sources reported here. The petroporphyrins purified from EHO and ETB had more variety than M petroporphyrins, which tended to have much higher proportions of only etio and DPEP, while EHO and 5721
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Energy & Fuels ETB petroporphyrins had significant amounts of porphyrins with di-DPEP and benzo groups. VOPP extraction and purification processes scale fairly well with size. Our success at purifying both M and EHO VOPPs to over 85% purity and enriching ETB VOPPs to over 20% suggests that petroporphyrins can likely be enriched from any crude oil that contains meaningful amounts of vanadium. We achieve these high purities using several sequential separation processes (such as Soxhlet extraction and column extrography/ chromatography) with various solvents and separation media (Celite, silica gel, and alumina). The different interaction profiles of silica and alumina, in particular, allow us to reach high purities using both materials in sequence on several columns. Purifying VOPPs to high and then ultrahigh purity can be difficult for some crude oil sources. The principal factor in achieving high purity is the strength of adsorption of the asphaltenes onto the silica and/or alumina packing material. Because there are some hydrocarbons with a similar strength of adsorption, a final fractionation using some other method, such as temperature and centrifugation, is necessary for achieving ultrahigh purities. More functionalized asphaltenes, such as those with a high heteroatom content, could adsorb more strongly onto those packing materials, which may facilitate separation from less strongly adsorbed VOPPs. There are more functionalized petroporphyrins with one or more saturated or aromatic rings that are very tightly bound to asphaltenes and have Soret bands at wavelengths longer than 415 nm. This final group is very difficult to separate from the asphaltenes, because the interaction strength between these more functionalized petroporphyrins and asphaltenes is much stronger than the simpler etio and DPEP petroporphyrins. Despite experiments with elevated temperatures, many additional columns, silica gels with different pore sizes, and more aggressive solvent profiles, only small quantities (
