Ultrahigh-Purity Vanadyl Petroporphyrins - Energy & Fuels (ACS

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Ultra-high Purity Vanadyl Petroporphyrins Bryson McKay Rytting, Indra Deo Singh, Michael R. Harper, Anthony S Mennito, Yunlong Zhang, and Peter K. Kilpatrick Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03358 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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Ultra-high Purity Vanadyl Petroporphyrins B. McKay Rytting,† ID Singh,‡ Michael R. Harper,¶ Anthony S. Mennito,¶ Yunlong Zhang,∗,¶ and Peter K. Kilpatrick∗,† Department of Chemical and Biomolecular Engineering, Notre Dame, IN 46556, Indian Institute for Petroleum, Dehradun, Uttarakhand, 248005, India, emeritus, and ExxonMobil Research and Engineering Company, Annandale, NJ 08801 E-mail: [email protected]; [email protected]

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 which poison catalysts and which may facilitate the aggregation, deposition, and emulsion formation exhibited by asphaltenes. Here they are extracted and enriched to ultra-high purities from several sources: an Athabasca Bitumen, a Canadian northern tier crude, and a North American heavy crude. 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 silica-packed columns) and chromatography (on alumina-packed columns). The process yields purified petroporphyrins in unprecedented quantities (>100mg). These purified petroporphyrins can be further refined to ultra-high purities (> 85% petroporphyrin by ∗ To

whom correspondence should be addressed of Notre Dame ‡ Indian Institute for Petroleum ¶ ExxonMobil Research and Engineering † University

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weight) by using temperature and centrifugation to fractionate them into more and less soluble fractions. Petroporphyrins are characterized by UV-Visible (UVVis) spectroscopy, X-ray fluorescence (XRF) spectroscopy, and mass spectrometry (Time of Flight (TOF) and Fourier Transform Ion Cyclotron Resonance (FT ICR)). The majority of the petroporphyrins are simple etio (407 nm Soret band) or DPEP (410 nm Soret band) types, but some are more functionalized compounds with highly broadened and shifted Soret bands.

Introduction Most crude oils contain trace amounts (10s to 1000s ppm) 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, as 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 the nickel ion and which have undergone alkylation and ring additions to the porphyrin macrocycle. 12–21 Because 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 to the asphaltene fraction. 11,22–24

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N

O

Et

Me

Et

Me

Me

Et

Me

Et

N

N

O

N

V V N

N

N

Et

Me

Me

Me

Me

N

Et

Et

(b) Deoxophylloerythroetio por-

(a) Etioporphyrin

phyrin (DPEP)

Figure 1: Common Vanadyl Porphyrin Types

Substitution with vanadyl ion or nickel 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 the 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-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 refining 30–32 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 formation of elastic films at organic-aqueous interfaces, 47–58 although the basic asphaltene structure and mechanism of aggregation continues 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

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self assembly, adsorption, and phase separation. 63 This could be by either bridging asphaltenes or groups of asphaltenes to promote aggregation or by 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 due to 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 UV-visible spectroscopy and mass spectrometry 2 along with metal detection. 25,27 Other characterization techniques (such as EPR, ENDOR, and NMR) complement these to provide a more complete understanding of petroporphyrin molecular structure. 66,70–74 Petroporphyrin purification requires multiple successive separation procedures, usually some combination of extraction, extrography, and chromatography. Extraction, typically utilizing a Soxhlet apparatus, is often the first step since it can accommodate a large quantity of material. Solvent choice requires prioritization of either yield or purity and solvent effectiveness varies depending on the parent crude, the fraction of the crude being extracted, and the desired product. Extrographic-chromatographic columns are used to purify the extracted fraction further, since 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

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asphaltenes with less polar or aromatic solvents, while those with more intermolecular interaction sites will be more intimately associated with 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 not only on the character of the porphyrin pendant groups, but also the metal coordinated to the center, so 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 vanadyl petroporphyrins. 76,80,81

Experimental Materials Crude Oils and Crude Residua Asphaltenes (both n-pentane insoluble and n-heptane insoluble) were separated from an Athabasca Bitumen (ETB) and a Canadian northern tier crude (EHO) using the method described by Spiecker et al. 48 Additionally, the vacuum residue of a metal-rich North American heavy crude (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 ETB Asphaltenes EHO Asphaltenes M Vacuum Resid

V (ppm) Ni (ppm) wt% nC7 Asphaltene (% of Whole Crude) 187.5 394 750

77.9 87 145

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14.5 14.2 9.24

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Solvents, Chemicals, and Other Materials HPLC-grade methanol, hexane, methylene chloride, and toluene were purchased from SigmaAldrich 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 A˚ pore size) and alumina column packing media (80-200 mesh) were purchased from VWR. Additional silica gel column packing media (150 and 300 A˚ pore size) was purchased from SiliCycle. Both silica gel and alumina packing media were activated at 100◦ C for 4 hours prior to column assembly unless otherwise specified. 180x60 mm cellulose Soxhlet extraction thimbles were purchased from VWR. 18x18 inch cellulose absorbent pads were purchased from Grainger and used for large-scale extractions. 0.2µm cellulose ester membrane filters were purchased from Advantec.

Petroporphyrin Characterization UV-Visible Spectrophotometry Petroporphyrins in varying states of purity were characterized by UV-Visible Spectrophotometry (UVVis). 27,29,92–95 A calibration curve for vanadyl petroporphyrins was developed using vanadyl octaethylporphyrin dissolved in toluene at room temperature. The UVVis 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 = 300-1000 nm. A baseline measurement with pure toluene was taken each day that new measurements were taken. 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 VOPP content in order 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 VOPP content rather than the extinction coefficient due to broadening

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and variability of extinction coefficients among petroporphyrins. 21,92 For presentation here, all UVVis spectra are normalized to A=1 at the peak of the Soret Band.

ICP-MS Metal Analysis Trace element analyses by inductively coupled plasma mass spectrometry (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) high resolution (HR)-ICP-MS. All measurements were conducted in medium mass resolution (M/M 3000), and at the start of each analytical session the instrument was tuned ng ). Standard and spike solutions were prepared and calibrated using a multi elemental solution (1 gm

from 1000 mg gm 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 ng (1 gm ) as the internal standard to monitor for instrumental drift and matrix effects. Concentrations

of V were calculated using an external calibration method.

XRF Spectrometry XRF spectrometry was used to measure metal content 25–27,96 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 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 five times for three minutes each. These five three-minute 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

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energy range to a metal count (nV ), and, therefore, a VOPP count (since molV = 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 LDI TOF MS or APPI FT-ICR MS, see next section): CVOPP = nV

1 1molVOPP (MWVOPP ) 1molV CSample mSolution

(1)

Mass Spectrometry Laser Desorption Ionization Time of Flight (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, then spotted onto a sample plate. No matrix was used to assist ionization, as 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 600-3000 amu. Select samples were additionally analyzed using Ultra High Resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS), 12 Tesla Bruker Apex Apollo II and 15T Bruker solariX XR apparatuses. Atmospheric Pressure Photoionization was employed to vaporize the porphyrin molecules into the gas phase using a 420◦ C-450◦ C heated nebulizer. Once in the gas phase a krypton lamp with 9.4eV was utilized to induce ionization. Samples were dissolved in toluene at a concentration of 10-100 ppm, infused into the instrument at 120ul/hr, and accumulated ions in the collision cell for 0.020-1.0 sec. 24-80 scans were collected from 150m/z to 2000m/z 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 by identifying a homolog series (core porphyrin with additions of -CH2 - alkyls 14.01565 dalton) 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

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-CH2 -, so 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 species 8–10,45,99 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 DPEP, etc.

Results and Discussion Petroporphyrin Extraction and Purification Vanadyl petroporphyrins (VOPP) 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 due to the 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 due to 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.

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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, then dried at 100◦ C under vacuum. The dried slurries were then extracted again. The extracts from both nC5 and nC7 asphaltenes were combined (nC5 ETB/EHO Extract with nC7 ETB/EHO Extract, nC5 ETB R/EHO R Extract with nC7 ETB R/EHO R Extract) and fractionated on several successive columns to produce purified VOPPs.

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ETB Asph al t en e Acet on e (n C7 Asph al t en es) or 1:7 M et h an ol :Acet on e (n C5 Asph al t en es) Ex t r act i on

Ex t r act

Resi d

Si l i ca Col u m n (SC-1)

Acet on e Ex t r act i on R Ex t r act

H

4:1 H:M C

1:1 H:M C

1:4 1:1 MC T:M eOH H:M C

Si l i ca Col u m n (RSC-1)

Si l i ca Col u m n (SC-2) H H

4:1 H:M C

1:1 H:M C

4:1 H:M C

1:1 H:M C

1:4 1:1 MC H:M C T:M eOH

1:4 1:1 MC H:M C T:M eOH Si l i ca Col u m n (RSC-2)

Al u m i n a Col u m n (AC-1) H H

1:1 H:M C

1:2 H:M C


4:1 H:M C

1:1 H:M C


Al u m i n a Col u m n (RAC-1) Pu r i f i ed VOPP H

1:1 H:M C

1:2 H:M C

MC 1:6 1:1 H:M C T:M eOH

Pu r i f i ed VOPP

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 an R prefix to specify that they are using the residuum from the original extraction

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EHO Asph al t en e Acet on e Ex t r act i on

H

4:1 H:T

Ex t r act

Resi d

Si l i ca Col u m n (SC-1)

Acet on e Ex t r act i on

1:1 H:T

1:3 3:2 H:T H:M C

2:3 H:M C

1:4 H:M C

R Ex t r act

1:1 T:M eOH


Si l i ca Col u m n (SC-2) H H

1:1 3:2 2:3 1:4 H:T H:M C H:M C H:M C

1:1 T:M eOH

1:1 3:2 2:3 1:4 H:T H:M C H:M C H:M C

1:1 H:T

1:3 3:2 H:T H:M C

2:3 H:M C

1:4 H:M C

1:1 T:M eOH


Al u m i n a Col u m n (AC-1)

H

4:1 H:T

1:1 3:2 H:T H:M C

H

2:3 H:M C

1:4 H:M C

1:1 T:M eOH

1:1 T:M eOH Al u m i n a Col u m n (RAC-1)

Pu r i f i ed VOPP H

1:1 3:2 2:3 1:4 H:T H:M C H:M C H:M C

1:1 T:M eOH

Pu r i f i ed VOPP

Figure 3: EHO Asphaltene VOPP Enrichment Procedure. See Figure 2 for a description of the structure of the schematic.

Purified VOPPs from ETB and EHO asphaltenes were characterized by measuring their metal content as well as performing UVVis and mass spectrometry (see Figure 4 for the UVVis 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 (Max. Peak), weight average MW, and relative percent (number of VOPPs of a Z-class compared to total number of VOPPs) are calculated (see Figures 5 and 6 and Table 2, as well as Tables 5, 6, 7, and 8 in supplementary material).

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1.2

EHO VOPP 1

Normalized Absorbance

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0.8 0.6 0.4 0.2 0 300

400

500

600

Wavelength (nm) Figure 4: UVVis spectrum of VOPPs purified from EHO asphaltenes (EHO VOPP)

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700

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Molecular Weight (gm/mol)

650

600

ETB VOPP ETB R VOPP EHO VOPP EHO R VOPP

550

500

450

-28 E

-30 D

-32 diD

-34 B

-36 D-B

-38 diD-B

-40 diB

Z-number Porphyrin Class

Figure 5: Average Molecular Weight 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.

35 ETB VOPP ETB R VOPP EHO VOPP EHO R VOPP

30 25

Relative %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 15 10 5 0

-28 E

-30 D

-32 diD

-34 B

-36 D-B

-38 diD-B

-40 diB

Z-number Porphyrin Class

Figure 6: Composition of purified VOPPs from EHO and ETB asphaltenes in percent of N4 O1V1 species reporting to each Z-number

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Table 2: ETB and EHO VOPP purity by metal content and UVVis (Extinction coefficient of OEP-VO at 573 nm) Sample ETB VOPP ETB R VOPP EHO VOPP EHO R VOPP

gm V (ppm) Purity Purity MW mol LDI-TOF-MS ICP-MS ICP-MS UVVis

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%

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 molecular weight estimated from LDI-TOF-MS. ETB VOPPs were not enriched to the same degree (22% pure), but were still significantly enriched compared to the starting crude at 187.5 ppm 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 V vs 394 ppm V in EHO and 750 ppm V in M vacuum resid). Both the height and integrated area of the Soret band vary depending on the porphyrins’ molecular structure, and since 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 due to asphaltene trapping and other interference. 104–107 Because of this, the Soret band is not necessarily an accurate measure of the total petroporphyrin content (whether measured by height or area). Because of these inherent uncertainties, purity estimation by metal analysis (in this case, ICP-MS) is preferable to UVVis. Since ICP-MS captures all of the metal content and the molecular weight 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 15


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trends (see Table 9). 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 molecular weights 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/o-CP) at 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 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, 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, in order 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. Figures 7 (for M-1 and M-2) and 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 six-liter Soxhlet extractor according to the scheme until the solvent became colorless. For M extractions, the mass 16


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of extract exceeded the solubility limit of the extraction solvent, so there were both solvated and precipitated portions which were characterized and further fractionated separately.

M-1 Petroporphyrin Enrichment and Purification 165.42 grams of M vacuum resid were deposited onto cellulose pads, which were then dried and extracted with 6 liters of methanol, yielding 15.41 grams of soluble extract and 16.11 grams of precipitated extract. The petroporphyrin content of the soluble extract was estimated to be 3% with UVVis spectroscopy. M Vac Resi d M et h an ol Ex t r act i on Sol u bl e

Pr eci p.

Resi d

Si l i ca Col u m n (SC-1) H

4:1 H:M C

1:1 H:M C

1:4 H:M C

MC

1:1 T:M eOH

MC

1:1 T:M eOH

Si l i ca Col u m n (SC-2) H

4:1 H:M C

1:1 H:M C

1:4 H:M C

Al u m i n a Col u m n (AC-2) H

1:2 H:T

T

1:1 T:M eOH

Pu r i f i ed VOPP

Figure 7: M Vacuum Resid Preparatory Scale VOPP Enrichment Procedure. See Figure 2 for a description of the structure of the schematic.

A 100 cm by 6 cm diameter column was packed with silica gel totaling 2.5 liters (SC-1), and 3 liters of each eluting solvent were used. A brightly-colored pink band eluted with 1:4 H:MC, yielding material which was around 30% petroporphyrin as estimated by UVVis. This fraction was loaded onto a 100 cm tall by 2 cm diameter column packed with 250 mL silica gel (SC-2) and

17


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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 four hours. After the aromatic maltenes eluted with hexane, seventeen aliquots were taken of 1:2 H:T followed by ten 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-Visible spectra of three of these fractions: 10, 20, and 24). The residual asphaltene concentration 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 FT-ICR MS analysis.

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1.2

M-1 AC-1 10 M-1 AC-1 20 M-1 AC-1 24


1 0.8 0.6 0.4 0.2 0

360

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Wavelength (nm) (a) Soret region, AC-1 10, 20, and 24

0.1


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M-1 AC-1 10 M-1 AC-1 20 M-1 AC-1 24

0.08

0.06

0.04

0.02

0

520

540

560

580

600

Wavelength (nm) (b) α/β region, AC-1 10, 20, and 24

Figure 8: UV-Visible spectra of AC-1 fractions 10, 20, and 24, corresponding to Soret band maxima of 407, 410, and 416 nm, respectively

19


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One significant and noteworthy feature of this set of UVVis 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 nm to 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 due to 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 sample is majority DPEP (Z=-30), which also is expected, since 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 Soret band at about 417 nm 27 ), 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% vs 5.26%) and reduced Z=-28 and =-32 (about two percentage points each, 8.01% vs 9.70% and 4.72% vs 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 Z=-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.

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70

407 nm SB 410 nm SB 416 nm SB

60

50

Relative %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

40

30

20

10

0

-28 E

-30 D

-32 diD

-34 B

-36 D-B

-38 diD-B

-40 diB

-42 -44 D-diB diD-diB

-46 B-N

-48 -50 D-B-N diD-B-N

Z-number Porphyrin Class Figure 9: Composition of purified VOPPs in three fractions of M-1 AC-1 having different Soret band (SB) maxima. Compositions are plotted as percent of N4 O1V1 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.

21


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100 407 nm SB 410 nm SB 416 nm SB

98 96 94

Relative %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 51

92 90 10 8 6 4 2 0

HC

1O

3O

3S1N

1S1O

3N1O

4N1O1V 1S4N1O1V

Hydrocarbon Class Figure 10: Composition of purified VOPPs in three fractions of M-1 AC-1 having different Soret band (SB) maxima. Compositions are plotted as percent of the total sample reporting to various hydrocarbon species

M-2 Petroporphyrin Enrichment and Purification 384.84 grams of vacuum resid were deposited onto cellulose pads, yielding 20.77 grams of soluble extract and 85.21 grams of precipitated extract. The mass of soluble extract is fairly similar to M-1, which is expected since 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 cm by 6 cm diameter column filled with 1.5 L silica gel (M-2 SC-1) and fractionated with 3L of each eluting solvent. The 1:4 hexane:methylene chloride fraction was the most enriched in vanadyl petroporphyrins (see Figure 11), and was further fractionated twice on 100 cm by 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 408-409-410 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.

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0.8


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H 4:1 H:MC 1:1 H:MC 1:4 H:MC MC 1:1 T:MeOH

0.6

0.4

0.2

0 380

390

400

410

420

430

440

Wavelength (nm) Figure 11: UVVis 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)

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, with “P” acronyms used later in the text referring to the separation process done 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 due to 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, so the solvent mixtures used were all richer in methylene chloride. Some UV-Visible spectra of the Soret band for M-2 AC-1 fractions are shown in Figure 12. Many of these fractions have very similar UV-Visible spectra compared to the most highly purified petroporphyrins from the M-1 experiment, but one significant difference is the lack of any frac23


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tions 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 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 resid (165 vs 385 grams) with the same amount of methanol (6 L), and ended up with similar amounts of soluble extract (15 vs 21 grams) but a sizable difference in the amount of precipitated extract (16 vs 85 grams). This suggests that the compounds in this vacuum resid which are extractable by methanol have a solubility limit around 3 g/L. Once the concentration in the stillpot exceeds that, precipitation begins to occur. Since more functionalized petroporphyrins are more likely to aggregate or deposit onto surfaces than their simpler counterparts, it would be unsurprising if they are more likely to precipitate out of the methanol.

1.2

M-2 AC-1 5 M-2 AC-1 10 M-2 AC-1 15 M-2 AC-1 20

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 51

0.8 0.6 0.4 0.2 0 380

390

400

410

420

430

440

Wavelength (nm) Figure 12: UVVis spectra of select eluted fractions from the M-2 AC-1 separation process

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1.2

M-2 PAC-1 3 M-2 PAC-1 7

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0.8 0.6 0.4 0.2 0 380

390

400

410

420

430

440

Wavelength (nm) Figure 13: UVVis spectra of select eluted fractions from the M-2 PAC-1 separation process

Figure 13 shows UVVis 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: 409.5-410.5 nm, mostly around 409.5 nm) than for M-2 AC-1 (range: 407-410 nm, mostly around 407 nm), there were none of the petroporphyrins with significantly red-shifted Soret bands that were found before. The UV-Visible 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 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 M vacuum residue was deposited onto cellulose pads and extracted with hexane to try to remove all of the maltenes, followed by extraction with a series of more polar solvents. UV-Visible spectra of the four extracts showed red-shifted Soret bands in the 1:1 toluene:methanol extract and the

25


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Page 26 of 51

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 (60A). 150 and 300A˚ pore size silica gels were also used to try to accommodate the larger molecules and aggregates in this most aromatic and polar fraction, but 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 UVVis or XRF spectroscopy.

M -3 Hex an e Ex t r act i on Si l i ca Col u m n (TM SC-1) Sol u bl e Pr eci p.

Resi d 1:1 H:M C

Acet on e:M eOH Ex t r act i on Sol u bl e Pr eci p.

Si l i ca Col u m n (TM SC-2)

Resi d

1:4 1:4 1:1 1:4 1:1 H:M C H:M C H:M C H:M C T:M eOH 2 3 1

Tol u en e:M eOH Ex t r act i on Sol u bl e Pr eci p.

1:1 1:4 H:M C T:M eOH

Resi d

Al u m i n a Col u m n (TM AC-1) M et hyl en e Ch l or i de Ex t r act i on

Pu r i f i ed VOPP

Sol u bl e Pr eci p. No Pu r i f i cat i on

Figure 14: M-3 Functionalized VOPP Enrichment Procedure. See Figure 2 for a description of the structure of the schematic.

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1.2

1:1 H:MC 1:4 H:MC 1 1:4 H:MC 2 1:4 H:MC 3

1


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Energy & Fuels

0.8 0.6 0.4 0.2 0

380

400

420

440

460

Wavelength (nm) Figure 15: UVVis 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.

Next, the 1:1 toluene:methanol extract was fractionated. UVVis 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 UVVis spectra were combined, resulting in three 1:4 H:MC fractions. The first of these had an unusual UV-Visible 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, so this sample was further fractionated on an alumina column. The UVVis spectra of these compounds, shown in Figure 16 with some M-1 AC-1 spectra for comparison in Figures 16a and 16b, 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 max vs ∼ 10 nm normally) than the Soret bands from previously-purified petroporphyrins. Later fractions had Soret bands that were shifted (up to 423 nm) and broadened (up to ∼ 40 nm at half max) dramatically, as well as 27


Energy & Fuels

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 413-427 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.

1.2

M-1 AC-1 10 M-3 TMAC-1 6


1 0.8 0.6 0.4 0.2 0 300

400

500

600

700

Wavelength (nm)

(a) Etio-dominated sample, M-3 TMAC-1 vs typical 1.2

M-1 AC-1 20 M-3 TMAC-1 8

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 51

0.8 0.6 0.4 0.2 0 300

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Wavelength (nm)

(b) DPEP-dominated sample, M-3 TMAC-1 vs typical

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1.2

M-3 TMAC-1 22 1


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0.8 0.6 0.4 0.2 0 300

400

500

600

700

Wavelength (nm)

(c) Highly functionalized and polydisperse M-3 TMAC-1 fraction

Figure 16: UV-Visible spectra of M-3 TMAC-1 fractions, with typical etio and DPEP petroporphyrin spectra for comparison. The additional Q band at 590 and Soret band shoulder at 450 are unique to the M-3 TMAC-1 VOPPs Additionally, these samples were visibly different from typical petroporphyrins, with the lessshifted 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 UVVis 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 detected 9 but never purified and characterized at the levels noted here.

Summary of Large-scale Purification of VOPPs From M Vacuum Residue In summary, several fractions containing different populations of VOPPs were extracted and purified to a high degree from M vacuum residue. The majority of the VOPPs purified were purple and had 407-410 nm Soret band maxima, corresponding to majority etio and DPEP types. These petroporphyrins had UVVis spectra that were almost identical to the UVVis spectrum of a synthetic standard, OEP-VO (see Figure 17). While it is not easy to purify these petroporphyrins from 29


Energy & Fuels

crude oil, the process is nonetheless straightforward and replicable. 1.2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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OEP-VO M-1 AC-1 VOPP M-2 AC-1 VOPP M-2 PAC-1 VOPP

1 0.8 0.6 0.4 0.2 0 300

400

500

600

700

Wavelength (nm)

Figure 17: UV-Visible spectra of highly-purified petroporphyrins vs synthetic standard (OEP-VO)

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 highlyfunctionalized, having Z-numbers concentrated below -40 and high proportions of petroporphyrins with thiophenic sulfur. 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 UVVis, XRF, and FT-ICR MS. Purities calculated from UVVis should be considered a lower bound, due to the issues of Soret band broadening, Soret band quenching, and extinction coefficient variability mentioned previously. As was mentioned previously, FT-ICR MS does not necessarily ionize all species, so the purity estimated from it should be considered an upper bound. XRF should be the most accurate, as it accounts for every vanadium atom in the sample and every vanadium is chelated to a VOPP compound. The molecular weight used to convert the vanadium measured with XRF to the mass of petroporphyrin is estimated with FT-ICR MS, which, though imperfect as mentioned above, gives us our best estimation of the average VOPP mass.

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Table 3: Select purified VOPPs from M vacuum residue. PP Designation

Calculated Petroporphyrin Content (%) UVVis XRF

M-1 AC-1 407 M-1 AC-1 410 M-1 AC-1 416 M-3 TMAC-1 407 M-3 TMAC-1 410 M-3 TMAC-1 420 M-3 TMAC-1 413-427

44 22 35 42 33 41 19

45 40 47 45 47 45 25

FT-ICR MS 4N1O1V 1S4N1O1V Total 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

We attempted several times to further purify these petroporphyrins on additional alumina columns but did not obtain any ultra-high purity VOPPs like the sample from EHO that was over 90% VOPP by weight. For M vacuum residue, there appear to be many compounds with very similar strengths of adsorption on alumina compared to VOPPs, so additional alumina columns do not further purity the porphyrinic compounds. An additional fractionation step is required that does not rely on 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, and then centrifuging at 15,000 RPM for 1 hour 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 nuclear magnetic resonance spectroscopy, fourier transform infrared spectroscopy, UVVis spectroscopy, and FT ICR-MS in conjunction with molecular models developed using density functional theory.

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Table 4: Ultra-high purity VOPPs (per metal analysis with XRF) precipitated from purified VOPP samples at low temperature

PP Description

Calculated VOPP Purity (%) 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

Conclusions Vanadyl petroporphyrins 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-Visible spectra of the simplest highly-purified petroporphyrins is almost identical to a vanadyl octaethyl porphyrin standard from Sigma. The simplest etio porphyrins 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, and 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 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 ETB petroporphyrins had significant amounts of porphyrins with di-DPEP and benzo groups. Vanadyl petroporphyrin 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 which contains meaningful amounts of vanadium. We achieve these high purities by using several sequen32


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tial separation processes (such as Soxhlet extraction and column extrography/chromatography) with various solvents and separation media (Celite, silica gel, alumina). The different interaction profiles of silica and alumina, in particular, allow us to reach high purities by using both materials in sequence on several columns. Purifying VOPPs to high and then ultra-high purity can be difficult for some crude 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 similar strength of adsorption, a final fractionation using some other method, such as temperature and centrifugation, is necessary for achieving ultra-high purities. More functionalized asphaltenes, such as those with 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, as the interaction strength between these more functionalized petroporphyrins and asphaltenes is much stronger than the simpler etio and DPEP petroporphyrins. Despite experiments with elevated temperature, many additional columns, silica gels with different pore sizes, and more aggressive solvent profiles, only small quantities (< 20mg) of these highly-functionalized petroporphyrins were separated and purified.

Acknowledgement The authors thank Theresa Sagartz and Christopher Coles for help with data collection and chromatographic column preparation and execution.

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Supporting Information Available Tabular Details of ETB, EHO, and M Purified VOPP Characterization Table 5: Detailed breakdown of petroporphyrins in ETB VOPP VOPP Z-class General Formula MW Range C-number Max. Peak Ave MW Relative % -28 -30 -32 -34 -36 -38 -40

375+14n 401+14n 455+14n 425+14n 451+14n 505+14n 531+14n

445-697 443-709 455-693 439-747 451-759 505-687 545-657

C25 -C43 C25 -C44 C26 -C43 C25 -C47 C26 -C48 C30 -C43 C33 -C41

501 527 497 495 507 505 573

520.9 525.8 518.4 517.8 535 546.6 566.9

32.8 32.7 16 8.9 4.2 2.9 2.5

Table 6: Detailed breakdown of petroporphyrins in ETB R VOPP VOPP Z-class General Formula MW Range C-number Max. Peak Ave MW Relative % -28 -30 -32 -34 -36 -38 -40

375+14n 401+14n 455+14n 425+14n 451+14n 505+14n 531+14n

445-739 457-751 455-763 453-747 451-773 505-771 545-755

C25 -C46 C26 -C47 C26 -C48 C26 -C47 C26 -C49 C30 -C49 C33 -C48

515 513 525 509 521 547 545

536.9 536.6 544.1 537.3 572.1 610.7 610.8

21.6 32.8 15.3 12.3 8.6 4.8 4.6

Table 7: Detailed breakdown of petroporphyrins in EHO VOPP VOPP Z-class General Formula MW Range C-number Max. Peak Ave MW Relative % -28 -30 -32 -34 -36 -38 -40

375+14n 401+14n 455+14n 425+14n 451+14n 505+14n 531+14n

445-571 443-569 455-567 453-579 465-577 505-561 545-573

34

C25 -C34 C25 -C34 C26 -C34 C26 -C35 C27 -C35 C30 -C34 C33 -C35


487 485 497 509 521 519 545

500.9 503.7 498.7 508.2 517.4 527.6 553.4

30.5 30.9 17.5 9.9 5.3 4 1.9

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Table 8: Detailed breakdown of petroporphyrins in EHO R VOPP VOPP Z-class General Formula MW Range C-number Max. Peak Ave MW Relative % -28 -30 -32 -34 -36 -38 -40

375+14n 401+14n 455+14n 425+14n 451+14n 505+14n 531+14n

445-683 443-695 455-707 453-691 451-703 505-687 545-685

C25 -C42 C25 -C43 C26 -C44 C26 -C43 C26 -C44 C30 -C43 C33 -C43

487 485 483 481 521 505 545

513.7 513.6 513 527 545.9 571.9 602.6

24.1 26.3 17.9 13.9 9.3 5 3.5

Table 9: Comparison of VOPPs from ETB and EHO. 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.

ETB VOPP Z-num -28 -30 -32 -34 -36 -38 -40

ETB R VOPP

EHO VOPP

EHO R VOPP

Class Ave MW Rel % Ave MW Rel % Ave MW Rel % Ave MW Rel % E D diD B B-D B-diD diB

520.9 525.8 518.4 517.8 535 546.6 566.9

32.8 32.7 16 8.9 4.2 2.9 2.5

536.9 536.6 544.1 537.3 572.1 610.7 610.8

21.6 32.8 15.3 12.3 8.6 4.8 4.6

500.9 503.7 498.7 508.2 517.4 527.6 553.4

30.5 30.9 17.5 9.9 5.3 4 1.9

513.7 513.6 513 527 545.9 571.9 602.6

Table 10: Detailed breakdown of petroporphyrins in M-1 AC-1 VOPP as measured by APPI FT-ICR MS VOPP Z-Class

Number Proportion In Sample 407 nm SB 410 nm SB 416 nm SB

-28 -30 -32 -34 -36 -38 -40 -42 -44 -46 -48 -50

35.9 27.5 12.7 8.55 5.26 3.11 3.64 2.17 0.614 0.553 0.0337 0.0694 35

9.70 64.4 6.55 6.46 5.26 3.28 1.95 1.51 0.421 0.330 0.125 0.0246


8.01 65.0 4.72 6.39 9.52 3.42 2.32 0.476 0.126 0.00 0.00 0.00

24.1 26.3 17.9 13.9 9.3 5 3.5

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Table 11: Detailed breakdown of hydrocarbons in M-1 AC-1 samples as measured by APPI FT-ICR MS Compound Class

Number Proportion In Sample 407 nm SB 410 nm SB 416 nm SB

HC 1O 3O 3S1N 1S1O 3N1O 4N1O1V 1S4N1O1V

0 0.453 0.518 0.00 0.00 0.856 92.7 3.65

0.0367 0.520 0.421 0.00 0.00 0.00 97.2 1.24

0.157 1.99 0.428 0.316 1.45 0.303 93.6 0.00

Additional XRF Calibration Development Figure 18a shows a set of typical XRF spectra, where the range used to calculate the amount of vanadium is highlighted, and Figure 18b zooms in closer to the vanadium range. In this way, the amount of vanadium or nickel (linked directly to petroporphyrin content in a 1:1 molar ratio) in a sample can be measured nondestructively in just a few minutes.

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(a) Full XRF spectra

(b) XRF spectra, vanadium range

Figure 18: XRF spectra of vanadyl petroporphyrins

An important consideration in using XRF is sample depth. XRF counts the total number of photons fluorescing and their energy values. If the sample is “infinitely thick” from the vantage point of the detector, then the variable of depth can safely be ignored and the total photon count is easily converted to a concentration using the calibration curve. The source emits x-rays at 40keV, which can penetrate several meters of hydrocarbons, but the lower-energy fluorescent photons from Ni and V can only penetrate several millimeters. Using the mass attenuation coefficients and

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approximate atomic concentrations of the constituents of typical samples, the maximum escape depth of fluorescent photons from vanadium (∼ 5 keV) and nickel (∼ 8 keV) was calculated to be about 4 mm and 14 mm respectively. Because the relationship between photon escape and sample depth is logarithmic and because there is a fair amount of noise in the data, empirical infinite thickness could be even shallower than those depths. Photon count was measured for many sample depths, and the minimum thickness required for both vanadium (3 mm, per Figure 19a) and nickel (6 mm, per Figure 19b) appears to be less than calculated.

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(a) Sample thickness vs vanadium signal

(b) Sample thickness vs nickel signal

Figure 19: Determination of infinite thickness for V, Ni

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(a) XRF vanadium calibration curve

(b) XRF nickel calibration curve

Figure 20: XRF elemental calibration curves

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Mass Spectrometry of Petroporphyrins With Matrix Figures 21a and 21b show mass spectra of vanadyl and nickel octaethyl porphyrin (OEP-VO and OEP-Ni) with and without dihydroxybenzoic acid (DHB) matrix. The OEP-VO spectra clearly show complexation between the matrix and porphyrins, including possibly inducing dimerization. This complexation effect is not observed with OEP-Ni, although the spectrum with matrix is slightly different.

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(a) LDI-TOF-MS of OEP-VO without (top) and with (bottom) matrix

(b) LDI-TOF-MS of OEP-Ni without (top) and with (bottom) matrix

Figure 21: LDI-TOF-MS of porphyrins

This material is available free of charge via the Internet at http://pubs.acs.org/.

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