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High-Field Orbitrap Mass Spectrometry and Tandem Mass Spectrometry for Molecular Characterization of Asphaltenes. Leonard Nyadong, Jinfeng Lai, Carol Thompsen, Chris J. LaFrancois, Xin-heng Cai, Chunxia Song, Jieming Wang, and Wei Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03177 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017
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High-Field Orbitrap Mass Spectrometry and Tandem Mass Spectrometry for Molecular Characterization of Asphaltenes Leonard
Nyadong,*,†
LaFrancois,†
Jinfeng
Lai,†
Carol
Thompsen,†
Xinheng Cai,‡ Chunxia Song,‡ Jieming Wang,
Chris ‡
J.
and Wei
Wang.‡ †Phillips
66 Technology, Highway 60 and 123, Bartlesville, Oklahoma 74003, USA.
‡SINOPEC
Research Institute for Petroleum Processing, 18 Xueyuan Road, Beijing
100083, People’s Republic of China.
*Corresponding author. Ph: +1 918 977 7356 (LN) E-mail:
[email protected], (LN)
Resubmitted to Energy & Fuels (Manuscript ID: ef-2017-03177x): December 5, 2017
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(ABSTRACT) This work introduces a novel approach by use of high-energy collision-induced dissociation for fragmenting asphaltenes into their constituent stable aromatic cores as a means for determining the relative proportions of island-to-archipelago structures. This approach is particularly useful for comparing asphaltenes from various crude oils. Ion generation from asphaltenes was performed by use of atmospheric pressure photoionization, which has been demonstrated to provides hydrogen-to-carbon ratios consistent with bulk measurements by combustion analysis with less than 10 % relative error. The fragmentation behavior of asphaltenes was first evaluated with model compounds consisting of island and archipelago structures by use of low- and highenergy collision-induced dissociation (CID and HCD). Unlike CID, HCD enables dissociation of model compounds to their stable aromatic cores. This allows facile classification as either island or archipelago based on the discrepancy in the doublebond equivalents between the precursors and stable aromatic cores. Model compound studies also showed that when HCD is utilized for the simultaneous dissociation of multiple precursor ions, efficient fragmentation of all precursors only occurs when ions within a narrow mass window are presented for analysis. The HCD approach was then applied to characterize narrow mass segments of crude oil asphaltenes, including those derived from hydrotreated resids. Observed island-to-archipelago proportions were consistent with the chemical transformations that occur during the hydrotreating process. Importantly, the method also demonstrates that the proportion of island-toarchipelago structures in asphaltenes decreases with increase molecular weight.
KEYWORDS: APPI, Island, Archipelago, HCD, CID, Collisional Dissociation, Field Ionization, Asphaltene Model Compounds, Petroleum.
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INTRODUCTION Asphaltenes represent the most complex petroleum-derived materials and are operationally defined as those species that precipitate from crude oil upon addition of an excess amount of paraffinic solvent (pentane, hexane, and heptane) but remain soluble in aromatic solvents (toluene and benzene).1-3 Asphaltenes constitute a significant proportion of heavy oil deposits and because of self-aggregation, which originates from pressure, temperature, and fluid compositional changes,3-5 frequently cause flow assurance problems in pipelines as well as refinery processing issues. Consensus exists in
the
scientific
community
regarding
asphaltenes’
critical
nanoaggregation
concentration (CNAC),6-8 cluster size,9 bulk elemental composition,10 molecular weight distribution of monomeric species,6,8,11-14 and number of fused rings per polyaromatic hydrocarbon (PAH).9 However, the specific structural elements of asphaltenes have been a subject of debate for decades.9,15,16 From a refining standpoint, knowledge about the structure of asphaltenes should enable development of optimized approaches to circumventing asphaltene-related processing issues and enable prediction of products and yields during upgrading. From a molecular-structure standpoint, asphaltenes are proposed to consist of complex PAH structural motifs described as island9 and archipelago.16 The island model stipulates that asphaltenes consist predominantly of alkylated condensed polycyclic aromatic cores, whereas the archipelago model indicates they consist of polycyclic aromatic cores with cycloalkane moieties linked via alkyl bridges. Plenty of evidence exists for the presence of island structures in asphaltenes, especially in studies involving
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their analysis from the liquid phase and by laser-based techniques, in which the asphaltenes samples are prepared by spotting from solution. Solvent evaporation during analysis in spray-based techniques or sample drying prior to analysis in laser-based techniques can result in asphaltenes aggregation even if the sample is prepared at a concentration below its CNAC. Such analyses are therefore biased toward the determination nanoaggregate.
of
the
The
asphaltene unaggregated
molecular species
species observed
most
loosely
during
such
held
in
the
analyses
are
predominantly alkylated polyaromatic species, which supports the island model. On the other hand, techniques that are not limited by the need for disaggregation, together with asphaltene residue processing observations by techniques such as thermal cracking and pyrolysis, tend to support the archipelago model.16 For example, mild thermal cracking of asphaltenes generates products with a wide range of boiling points and, under severe cracking conditions, yields residue that correlates well with aromatic content, which is consistent with archipelago structures.16 Evidence from pyrolysis gas chromatography MS analysis and ruthenium-ion-catalyzed oxidation also indicates that asphaltenes consist of polydispersed molecules composed of interconnected large polycyclic aromatic and hydroaromatic units.15 A recent study that combined the use of atomic force microscopy and scanning tunneling microscopy enabled access to individual asphaltene molecules, revealing PAH moieties and primary sites of intermolecular interactions. That approach provided near-conclusive evidence for the presence of archipelago-type structures in asphaltenes and the dominance of island-type molecules.17 Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and tandem mass spectrometry (MS/MS)-
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based techniques by use of infrared laser multiphoton dissociation (IRMPD)18 and beamtype collision-induced dissociation (CID)19 have also been used to show that asphaltenes consist of a mixture of island and archipelago structures. In beam-type CID, a precursor ion beam is directed into a collision region in which collision excitation takes place on the time frame of the passage of the ions through the collision cell.20 The ions often undergo multiple collisions that could result in extensive fragmentation. When coupled to FT-ICR, the fragment ions are first trapped and relaxed in the collision cell prior to injection into the mass analyzer for ultrahigh resolution mass analysis.19 On the other hand, IRMPD, which is mostly applicable for trapped ions, occurs directly in the FT-ICR mass analyzer. That fragmentation technique does not require single-frequency excitation because ions of all mass-to-charge (m/z) values or a mass-isolated segment can be dissociated simultaneously. The ions are irradiated with multiple photons for a user-defined period and their extent of fragmentation depends on the irradiation period. IRMPD was recently used for exhaustive fragmentation of pentane-insoluble asphaltenes from a de-asphalted oil (DAO) to reveal the stable aromatic core building blocks.18 An alternative dissociation approach that provides beam-type CID fragmentation is the high-energy collision induced dissociation (HCD) available on the high-field Orbitrap Elite.21 The high-field orbitrap mass analyzer employs high frequencies of ion oscillations resulting in sufficiently high resolving power in excess of 350,000 at m/z 524 to enable reliable petroleomic analysis.22,23 The Orbitrap Elite platform is a hybrid mass analyser that consists of a linear ion trap analyser coupled to a high-field orbitrap analyser. That instrument platform allows complementary dissociation approaches by use of ion trap CID and HCD. In HCD, ions are fragmented in a collision cell rather than
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an ion trap and then transferred back through the C-trap for analysis at high resolution in the orbitrap. Compared with ion trap-based CID, HCD has no low-mass cutoff, thereby allowing detection of all product ions generated by that process. HCD also employs higher energy dissociations than those used in ion-trap CID, thus enabling a wider range of fragmentation pathways. In this work, HCD dissociation on a high-field orbitrap MS platform is employed as a means of fragmenting asphaltenes into constituent stable aromatic cores for determination of the relative proportions of island and archipelago structures. The approach is being evaluated as a means of comparing asphaltenes from different crude oils and was tested on a total of six asphaltene samples, including those isolated from a hydrotreated resid, and their solid phase extraction (SPE) solubility-based fractions.
EXPERIMENTAL Samples and reagents All reagents were used without additional purification. All solvents used for isolation, fractionation, and dissolution of asphaltenes (pentane, hexane, heptane, methylene chloride, methanol, water, and toluene) were high performance liquid chromatography (HPLC) grade and purchased from Sigma Aldrich (≥99%, Sigma Aldrich, St. Louis, MO, USA). Silica used for fractionation of asphaltenes and porphyrins was purchased from Supelco (LC-Si, Supleco, Bellefonte, PA). Asphaltenes were isolated from various crude oils and resid including: Arabian Medium (AM) crude oil, West Canadian Select (WCS) crude oil, West Sak (WS) crude oil, Shengli vacuum resid (SVR), and Tahe
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vacuum resid (TVR) and its hydrotreated vacuum resid (THVR). Asphaltene model compounds were obtained from the University of Alberta, Canada.
Isolation of Asphaltene Asphaltene samples were prepared by use of pentane insolubles for Arabian medium, Jones Creek and West Sak crude oils while heptane insolubles followed by soxhlet extraction was used for for Shengli and Tahe vacuum resids. Precipitation with pentanes was used to maximize the amount of collected asphaltenes due to the limited amounts of Arabian medium, Jones Creek, and West Sak crude oils. Pentane-insoluble asphaltenes were prepared by mixing pentane with crude in a 25/1 wt/wt ratio. A total of 4 L of pentane was added to 100 g of crude oil by adding the solvent 1 L at a time and stirring for 1 h in between additions until the total volume of pentane was added to the crude oil. The sample was then held overnight for flocculation. The flocculated mixture was vacuum filtered with Whatman #40 filter paper. The material that collected on the filter paper was washed with pentane until the filtrate became colorless. The solid material on the filter was then dried in an oven at 110 °C for 2 h and weighed. Asphaltenes from the Tahe vacuum resids were prepared by mixing 1.5 g of vacuum resid with 30 mL of heptane and refluxing the mixture for 1 h. The mixture was then left to equilibrate overnight at ambient temperature. The heptane-insoluble material was isolated by filtration with Whatman #40 filter paper. The filter paper with asphaltenes was soxhlet extracted with n-heptane until the solvent drops were colorless. Benzene was used to dissolve the asphaltenes from the filter paper and the benzene was
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subsequently evaporated to generate dry asphaltenes, which were then weighed. Each asphaltene sample was evaluated by combustion analysis per ASTM D529124 for bulk carbon, hydrogen, nitrogen, and sulfur (CHNS) content and by inductively coupled plasma atomic emission spectroscopy for iron, nickel, and vanadium content (Table SI). Fractionation of Asphaltenes Asphaltenes were fractionated by solid-phase extraction (SPE) on a silica stationary phase packed in a glass tube with a teflon frit (LC-Si, Supleco, Bellefonte, PA) as follows: The asphaltene sample (100 mg) was dissolved in 5 mL of toluene and 1.25 g of dried silica was added to the solution to make a slurry. The solvent was evaporated and the silica-bound asphaltenes were placed on top of the LC-Si SPE cartridge that had previously been conditioned with hexane. Hexane was passed through the cartridge and the eluent collected until the liquid emanating from the cartridge became colorless (~15 mL). Various other solvent mixtures, including 15:85 methylene chloride/hexane, 30:70 methylene chloride/hexane, 50:50 methylene chloride/hexane, and 80:20 methylene chloride/methanol, in that order, were passed through the cartridge and the corresponding eluent fractions were collected (Figure S1). The solvent from each of those fractions was evaporated under a flow of nitrogen and each fraction, except for the hexane fraction, was reconstituted in toluene for analysis.
The hexane fraction was
evaporated to about 10 µL and analyzed by field desorption mass spectrometry (vide infra)
Atmospheric Pressure Photoionization (APPI) MS and Tandem MS (MS/MS)
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Asphaltene model compounds were dissolved in toluene to a final concentration of 5 µmol and the asphaltene sample/fraction or IMs were diluted to a final concentration of 100 µg/mL prior to analysis. Sample introduction to the mass spectrometer was performed by direct flow injection at a flow rate of 100 µL/mL. The flow was directed to a heated nebulizer probe in which the sample was vaporized to generate gas-phase molecules that were subsequently ionized by APPI. The probe temperature was set at 400 °C. Nitrogen was used as a sheath gas and auxiliary gas at a flow rate of 60 and 5 au (arbitrary units), respectively. A vacuum ultraviolet (VUV) krypton discharge lamp emitting 10 eV photons (Syagen Technology Inc. Tustin, CA) and mounted on an Ion Max source (Thermo Fisher Scientific) was used to generate analyte ions, which were vacuumed into the inlet of the mass spectrometer. The mass spectrometer capillary inlet temperature and probe temperature were set at 350 °C. The S lens RF level was set at 60%. MS/MS analyses were performed by CID and HCD with an instrument-defined ion trap and orbitrap normalized collision energy (CE, %) of 35 and 200, respectively, and an isolation window of 1 Da unless stated otherwise. Mass analysis was performed with a hybrid linear ion trap orbitrap mass spectrometer (Orbitrap Elite, Thermo Scientific, Sans Jose, CA). The ion trap mass analyzer was operated with automatic gain control (AGC) set at 30,000 ions and the AGC target for the orbitrap mass analyzer was set at 1,000,000 with a maximum injection time of 100 ms. Mass analysis was performed in the orbitrap mass analyzer at a resolving power of 480,000 at m/z 400, unless stated otherwise. Full-scan and MS/MS mass spectra data were acquired with Xcalibur version 2.2.0 software. Xcalibur raw data files were converted to Notepad files (.txt) prior to data
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processing with commercial software (PetroOrg, Florida State University, Tallahassee, FL). Mass spectra recalibration and peaks assignment by the software was performed on the basis of Kendrick mass defect analysis. The data was grouped by heteroatom “class” composition and plotted as color-contour plots of double-bond equivalents (DBE) versus carbon number.
Field Desorption (FD) MS The hexane-soluble fraction of Arabian medium asphaltene was analyzed by FDMS with a dedicated field ionization/field desorption (FI/FD) ion source (Jeol Inc., Peabody, MA). The sample was deposited on the emitter of a direct-insertion probe and introduced into the FI source through a vacuum lock. Sample deposition onto the emitter was achieved by exposing a droplet of sample (~1 mg/mL in hexane) held at the tip of a syringe needle to the emitter and moving the emitter from side to side to expose the entire emitter surface to the sample. The emitter (CarboTec, Gesellschaft für instrumentelle Analytik mbH, Germany) consists of a 10 µm diameter tungsten wire onto which carbon microneedles had been grown. The analysis was performed at an emitter flashing current of 40 mA, a duration of 8 msec, and a wait time of 15 msec with the ion source heater turned off. Mass analysis was performed with a time-of-flight (TOF) mass spectrometer (AccuTOFTM GCx, Jeol Inc., Peabody, MA). Data acquisition was performed by using MsAxel software with a sampling frequency of 40 GHz and acquisition period of 40 msec. Mass spectral data preprocessing, including signal threshold determination, peaks centrioding, and drift
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compensation, was performed in MsAxel. Drift compensation was accomplished by use of the most abundant peak in the one-ring naphthene homologous series. The centroided data was exported to .txt files for final processing in PetroOrg (vide supra).
Results and Discussion MS and MS/MS Applied to Asphaltene Model Compounds Preliminary investigation into the molecular structure of asphaltenes was evaluated by use of model compounds representing island and archipelago structures (Table I). The fragmentation behavior of those model compounds was evaluated by MS/MS with CID. A fundamental characteristic of MS/MS is the ability to distinguish between different classes of compounds based on measurement of their collision energies,25,26 which is the amount of energy required to dissociate 50% of a precursor ion. Figure 1 shows the ion-trap CID fragmentation efficiency curves for each of the model compounds evaluated. The collision energy increases in the order 2-5 < 2-4 < 1, 3, 6, 8-THP
