Article pubs.acs.org/ac
Determination of Structural Building Blocks in Heavy Petroleum Systems by Collision-Induced Dissociation Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Kuangnan Qian,* Kathleen E. Edwards, Anthony S. Mennito, Howard Freund, Roland B. Saeger, Karl J. Hickey, Manny A. Francisco, Cathleen Yung, Birbal Chawla, Chunping Wu, J. Douglas Kushnerick, and William N. Olmstead ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801, United States S Supporting Information *
ABSTRACT: Collision-induced dissociation Fourier Transform ion cyclotron resonance mass spectrometry (CID-FTICR MS) was developed to determine structural building blocks in heavy petroleum systems. Model compounds with both single core and multicore configurations were synthesized to study the fragmentation pattern and response factors in the CID reactions. Dealkylation is found to be the most prevalent reaction pathway in the CID. Single core molecules exhibit primarily molecular weight reduction with no change in the total unsaturation of the molecule (or Z-number as in chemical formula CcH2c+ZNnSsOoVNi). On the other hand, molecules containing more than one aromatic core will decompose into the constituting single cores and consequently exhibit both molecular weight reduction and change in Z-numbers. Biaryl linkage, C1 linkage, and aromatic sulfide linkage cannot be broken down by CID with lab collision energy up to 50 eV while C2+ alkyl linkages can be easily broken. Naphthenic ring-openings were observed in CID, leading to formation of olefinic structures. Heavy petroleum systems, such as vacuum resid (VR) fractions, were characterized by the CID technology. Both single-core and multicore structures were found in VR. The latter is more prevalent in higher aromatic ring classes.
A
by Quann and Jaffe in 1992.21 The effort enables efficient crude valuation and refinery optimization on a molecular basis. Structure oriented lumping (SOL) was proposed for molecular description of mass spectrometric data of a petroleum system. SOL was later extended to vacuum resid with nominal boiling point greater than 538 °C (1000 °F) by invoking the concept of multicore structures.22 Relative to lower boiling petroleum cuts, 538+ °C petroleum cuts (vacuum resid, VRs) are much more challenging to characterize because of their low volatility, high asphaltene and aromatic carbon content, and high molecular weight distribution. The presence of multicore structures in VR makes analytical characterization even more difficult. Progress in resid characterization had been very slow until the introduction of high field Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) to heavy petroleum characterization23−27 in the early 2000s. The ultrahigh resolution capability of FTICR-MS provides unambiguous identification of molecular formulas for each mass peak detected. Record numbers (10−50 thousands) of molecular formulas have been identified for crude oils and asphaltenes. One of the major gaps in heavy petroleum characterization has been the fact that
nalytical techniques for determining the composition and chemical structure of petroleum fractions with boiling point below 538 °C (1000 °F) have been largely established. Molecules in naphtha range with boiling point up to 221 °C (430 °F) are routinely measured by high resolution GC PIONA or GC-MS PIONA (C4 to C12 paraffins, isoparaffins, olefins, naphtha, and aromatics).1,2 Middle distillates with nominal boiling point ranging from 221 to 343 °C (430 to 650 °F) are characterized by GC-field ionization mass spectrometry (GCFIMS) in low resolution3,4 or high resolution modes.5,6 These analyses are sometimes combined with GC-FID for normal paraffin measurements and/or supercritical fluid chromatography (SFC) for determination of hydrocarbon lumps, such as paraffins, naphthenes, 1−3 ring aromatics.7−9 Two-dimensional gas chromatography (2DGC) has also become increasingly popular for the middle distillate characterization.10−13 Vacuum gas oil with nominal boiling point ranges from 343 to 538 °C (650 to 1000 °F) requires multidimensional liquid chromatographic separations (such as silica gel and aromatic ring class) followed by low or high resolution mass spectrometry.14−17 A model of composition (MoC) can be developed by reconciling various bulk property and composition measurements.18,19 A summary of analytical technology for heavy petroleum can be found in a well-written book by Altgelt and Boduszynski.20 The primary driver of the high detail petroleum analysis is the molecule-based petroleum composition modeling introduced © 2012 American Chemical Society
Received: February 28, 2012 Accepted: April 23, 2012 Published: April 23, 2012 4544
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abundances of the structural building blocks in the petroleum system.
molecular structure assignments cannot be made only on the basis of molecular formula. Figure 1 illustrates the problem.
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EXPERIMENTAL SECTION CID-FTICR MS. All experiments were conducted on a 12 T Bruker Apex FTICR-MS equipped with electrospray ionization (ESI) and atmospheric pressure photoionization (APPI). APPI is the primary ionization method used to generate parent ions in this study. Detailed experimental parameters and conditions of APPI-FTICR MS can be found in the previous publications.29,30 Molecule ions formed by APPI were collected by 2-stage ion funnels and accumulated in an rf-only hexapole prior to injection into a quadrupole mass analyzer (Q1). Ions passing Q1 were sent into a collision cell with a pressure controlled at ∼10−2 mbar with argon as the collision gas. Kinetic energy of the ions was defined by the voltage difference between Q1 and the collision cell. Molecule ions are fragmented by CID with argon. Unreacted molecule ions and fragment ions are trapped and relaxed in the collision cell prior to injection into the FTICR cell for ultrahigh resolution mass analysis. In all model compound experiments, molecular ions of interests are isolated by Q1 with an isolation window set between 1 and 5 Da. For petroleum samples, Q1 is open and all ions were subjected to CID. The rf-only hexapole is operated at a voltage 200−400 Vpp at a frequency of 5 MHz. Ion accumulation time in the hexapole varies from 0.1 to 0.5 s. The collision cell is a linear quadrupole operated in rf-only mode with Vpp set at 690 V. Ion accumulation time in the collision cell varies from 0.5 to 2 s. An overview of the CID-FTICR MS process is shown in Supporting Information, Figure S1. The collision energy of a single collision event is controlled by the lab collision energy (Elab), mass of analyte ion, and mass of neutral molecule. Effective collision energy is determined by the center of mass (ECM) energy as given in eq 1.
Figure 1. Single core (a) and multicore (b) representations of molecular formulas, C58H68S2. The two presentations yield different products in refinery process.
ECM = MAr /(MAr + M ion) × E lab
Given a molecular formula of C58H68S2 (molecular weight 810 g/mol), one might assign two drastically different chemical structures. Figure 1a represents a single core molecule. Figure 1b represents a multicore molecule. The two molecules will produce very different products in refinery processing. In order to derive an accurate model of composition, the populations of multicore versus single-core structures and the distribution of building blocks of the multicore molecules need to be addressed. One way of determining resid core structures is to crack resid structure by thermal or other selective dealkylation chemistry.28 Coking reactions are a complication in the thermal cracking approach because of secondary reactions. Thermal cracking under hydrogen pressure may yield less coking but can still alter the building block structure by hydrodesulfurization. The quantitative measurement of building block distribution is very challenging. In this work, we discuss the development of collision-induced dissociation (CID) FTICR MS technology for the determination of aromatic building blocks and their distributions. Although CID has been widely studied and applied in mass spectrometric characterization of organic molecules and mixtures, its applications in petroleum structure characterization have not been fully explored. Here we demonstrate that CID can be effectively used to break multicore structures in a heavy petroleum system. Consequent ultrahigh resolution FTICR MS analysis of CID products yields identities and
(1)
Here, MAr is the mass of argon gas, and Mion is the mass of a parent ion. In model compounds studies, lab collision energy varies from 0 to 50 eV. In petroleum analysis, lab collision energy is fixed at 30 eV for vacuum resid (VR) and 20 eV for vacuum gas oils (VGO). In this paper, another energy unit (kcal/mol) is sometimes used to describe collision energy (1 eV equals to 23.06 kcal/mol). Equation 1 defines maximum available energy in a single collision event. Under our CID conditions, a large number of collisions are expected between ions and neutrals (multiple collisions CID). The collision energy of extra collisions gets smaller as ions slow down. However, accumulated energy deposition of multiple collisions is expected to be much higher than that defined by the ECM in the single collision event. Enhanced Fragmentation by Multipole Storage Assisted Dissociation (MSAD). CID fragmentation can be enhanced when ions accumulate to certain concentrations in the collision cell. This phenomenon was named as multipole storage assisted dissociation (MSAD).31,32 The effect was used in this work to enhance fragmentation of petroleum molecules and model compounds. The exact mechanism of MSAD is still a subject of research. However, it was speculated that once ion density reached the charge limit in the multipole, the Columbic force will push ion ensembles to spread out radially, enabling the ion to oscillate at higher magnitude. This would allow the coupling of the rf energy in the multipole rods to the ions, 4545
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Table 1. Model Compounds Synthesized for CID Studies
effectively accelerating them to higher kinetic energy. Extensive fragmentation can be achieved by collisions of excited ions with the gas molecules in the collision cell. Data Acquisition and Analysis. The fragment ions of model compounds and petroleum samples are consequently analyzed by FTICR-MS in ultrahigh resolution mode similar to that of parent ion analysis. The procedures of data calibration and analysis are similar to that of parent ion analysis.29,30 Before data analysis, the masses of fragment ions were converted into that of neutral products by adding the mass of a proton (1.007 285 Da) so that composition comparison (e.g., the change in Znumber) can be made on neutral molecule basis. Model Compounds and Petroleum Samples. Various single core and multicore model compounds were synthesized to support the CID study. Most of the model compounds were synthesized internally. Details on the organic synthesis can be found elsewhere.33 A complete list of the compounds and key structure features can be found in Table 1. There are multiple purposes of the synthesis. For example, di-C 16 alkyl naphthalene was used to study CID behavior of single core molecule. Phenanthrene-C14-dibenzothiophene (DBT) was used to study relative response factor of aromatic hydrocarbon core versus aromatic sulfur core in CID. Naphthalene-C14pyrene was used to study the impact of PNA core size on CID product distribution. C9 alkyl diaromatic sterane was purchased from Chiron AS, (Stiklestadveien 1, No-7041 Trondheim, Norway). This compound was used to understand ring-opening chemistry in CID. VR and VGO samples were generated by distillation of the whole crude oil via standard crude assay. Aromatic ring class (ARC) fractions were generated using methods similar to that described previously.14,15
definition contain one core. All others will be considered as multicore molecules. Description and Organization of Petroleum Molecules and CID Products. For the convenience of discussion of CID data, it is worthwhile to review the lumping and organization of petroleum molecules. A petroleum system can be described using a general chemical formula, CcH2c+ZNnSsOoVNi, where Z is typically referred as hydrogen deficiency. c, n, s, o are the numbers of carbon, nitrogen, sulfur, and oxygen atoms in a molecule, respectively. V and Ni are normally present in the forms of metalloporphyrins. Because of the existence of alkyl chains, mass spectrometric data of a petroleum system are commonly organized by homologous series. A homologous series has one unique Z number and heteroatom combination. For example, alkylated carbazoles have a carbazole core and follow the chemical formula CcH2c‑15N. The Z-number of the carbazoles is −15. The homologous series is often short-handed as “−15 N”. All benzothiophenes have a general chemical formula CcH2c‑10S. Its Z-number is −10. The benzothiophene homologous series can be shorthanded as “−10 S”. Hydrocarbons with no hetereoatoms are abbreviated as “HC”. For example, alkylated benzenes (CcH2c‑6) is “−6 HC”. The more negative the Znumber, the more unsaturated the core. The polyaromatic structures shown in Figure 1a,b before dissociation have the same elemental formulas and belong to “−48 S2” homologous series. Another term commonly used to describe a petroleum system is double bond equivalence (DBE), which describes total number of rings and double bonds in a molecule. Z and DBE have a simple relation:
RESULTS AND DISCUSSION Definition of Cores, Building Blocks, and Multicore versus Single Core Structures. In this paper, the terms “cores” and “building blocks” are defined as aromatic and naphthenic structures in a petroleum molecule that cannot be further broken up by the CID process. Cores and building blocks are used interchangeably. Aliphatic chains are not considered as building blocks. Single core molecules by
More examples of petroleum homologues can be found in Supporting Information (Table S1). The change in chemical structure can be tracked by monitoring the change in Z-numbers of the parent molecules and the fragments. If a molecule is a single core (such as structure a in Figure 1), only molecular weight reduction (dealkylation) is expected during CID. The degree of unsaturation (Z-number) of the CID product should be the same as that of the parent ion. In this case, Z-number remains
Z = −2 × (DBE − 1) + n
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Figure 2. (a) APPI-FTICR mass spectrum of di-C16 alkyl naphthalene. CID-FTICR mass spectra of the compound at low and high collision energies are shown in parts b and c, respectively.
Figure 3. Fragmentation efficiency curve of di-C16 alkyl naphthalene. The abundance of disubstituted products goes up first and then decreases as collision energy increases, reflecting further dissociation of fragmented ions. Product distribution flattened after 30 kcal/mol. The sum of total ion abundances is arbitrarily normalized to 1 000 000.
to be −48 after CID. If a molecule is a multicore (such as structure b in Figure 1), both molecular weight reduction and Z-number change (multicore dissociation) are expected. The absolute Z-numbers of the fragments will be lower than that of the parent multicore molecule as the latter is dissociated into single core structures. In this example, “−48 S2” is breaking apart into three single core products, acephenanthrenes (−20 HC), dibenzothiophenes (−16 S), and benzenes (−6 HC). Model Compounds Study. A series of model compounds were synthesized and studied to understand the CID chemistry of petroleum molecules and experimental factors affecting fragmentation patterns. A number of important questions about CID chemistry include the effect of the weak versus strong bonds in CID process, the impact of CID on naphthenic ring structures and product distribution, the CID charge efficiency of different core structures.
Dealkylation of Single Core Molecules. To study fragmentation behavior of model compounds as a function of collision energy, lab energy was varied from 0 to 50 eV. Figure 2 shows the CID mass spectra of di-C16 alkyl naphthalene. Both Elab and ECM are given in the figure. When CID is turned off, fragmentation does not occur as expected. With CID turned on, the degree of fragmentation increases with the increase of collision energy. At ECM = 15 kcal/mol, two distributions of fragmentation ions were observed. One distribution is between m/z of 360 and 550. The most abundant peaks have m/z values of 365, 379, and 393. A logical explanation is that dealkylation starts from one side chain via α, β, and γ cleavage, yielding C1C16, C2C16, and C3C16 substituted products, respectively. The second distribution is between m/z 140 and m/z 360 and is caused by the cleavage of the second chain. The most abundant peaks have m/z values of 141, 155, and 169, 4547
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Figure 4. (a) APPI mass spectrum of phenanthrene-C14-dibenzothiophene (DBT). (b and c) CID-FTICR mass spectra at two different collision energies. C1−C4 aromatic cores were observed, arising from direct cleavage of the alkyl linkage. Major products are C2-phenanthrene and C2-DBT.
of CID products thus provides “core” or “building block” information of the parent molecules. The abundances of A+ and B+ cores are governed by the CID chemistry and energetics. If the core components of a resid molecule have very different ionization potentials, it is expected that the CID products will favor the core that has the lowest ionization potential. To evaluate the impact of aromatic ring size and heteroatom contents on CID product distribution, three 2-core model compound systems were synthesized (see Table 1)33 and evaluated by CID-FTICR-MS. These are naphthalene-C14pyrene (−38 HC), phenanthrene-C14-dibenzothiophene (−36 S), and phenanthrene-C14-carbazole (−35 N). Figure 4 shows the CID mass spectrum of phenanthrene-C14-dibenzothiophene (DBT). C1−C4 substituted aromatic cores were observed, arising from cleavage of the alkyl linkage. The most abundant products are C 2 -phenanthrene and C 2 -DBT. Phenanthrene and DBT have similar molecular weights (178 and 184 g/mol) and ionization potentials (7.89 and 7.90 eV). As expected, their product abundances are similar. Figure 4 demonstrates that C14 alkyl linked multicore structures can easily cleave under CID conditions and result in smaller cores with the absolute Z-numbers (−16 and −18) lower than that of the original structures (−36). Single cores (phenanthrenes and DBTs) do not further dissociate, and the aromatic core structures are maintained. The other two model compound systems show a similar dissociation pattern and energy dependence. However, the constituting cores have very different ionization potentials and proton affinities. To evaluate relative response factors (RRF) of these aromatic cores in CID reaction, the product ions from the same cores were summed up and their relative abundances were compared at Elab = 25 eV (ECM = 39 kcal/mol) where the fragmentation pattern has been stabilized. RRF of all other cores are normalized to phenanthrene. The results are summarized in Table 2. Ionization potentials of the core
corresponding to C1, C2, and C3 naphthalenes, respectively. At E CM = 37.4 kcal/mol, almost all products are from fragmentation of both alkyl side chains. These products are substituted alkyl (C1 to C4) naphthalenes, with C2 substituted products being the most abundant. Most fragments are odd mass species, suggesting that they are even-electron (EE) ions formed via alkyl cleavage. Other single core molecules were also studied, including diC16 alkyl benzothiophenes and tetradecyl pyrenes. All compounds behave similarly, producing C1−C4 substituted cores. There is no change in Z-number between the parent and the product (on neutral molecule basis). Overall it is concluded that single core aromatics preserve the aromatic core structures in CID with lab energy up to 50 eV. Primary reaction chemistry is dealkylation to shorter chain products. In thermal chemistry, it is rare that alkyl chains are completely removed from aromatic cores. In CID, it was observed that multiple substituted aromatics were dealkylated down to C1 substituted structures because rearrangement reactions can happen during ion dissociation. The fragmentation efficiency curve of di-C16 alkyl naphthalene is shown in Figure 3. Fragmentation efficiency curves are constructed by normalizing summation of major product signals to 1 000 000. The abundances of products with one intact alkyl side chain goes up first and then decreases as collision energy increases, reflecting further dissociation of fragmented ions. The product distribution flattened when ECM is greater than 30 kcal/mol. Dissociation of Multicore Structures. Petroleum VRs have been speculated to contain multiple cores with different numbers of aromatic rings and cores with sulfur and nitrogen incorporations. If the speculation is true, then the dissociation of a multicore structure would produce a mixture of core fragments. CID of a two-core molecule (A−B) will generate “A+” ion and “B” neutral and “B+” ion and “A” neutral. Analysis 4548
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The fragmentation efficiency curves of three major ions can be found in Supporting Information (Figure S3). In summary, dealkylation, multicore dissociation, and naphthenic ring-opening are the major reactions observed in the CID of petroleum model compounds. Dealkylation of a single core structure conserves Z-number. Partial naphthenic ring destruction occurs, producing an olefin moiety. Low levels of aromatic ring closure were also observed in CID. For example, diphenyl sulfide can form DBT. Applications to Heavy Petroleum Systems. CID of petroleum samples is more complicated than that of model compounds because of the presence of a large number of parent molecule ions. CID product distribution is affected by many experimental parameters in addition to lab collision energy. Sample concentration and ion accumulation affect ion density in the collision cell. It has been observed that CID fragmentation can be significantly enhanced by high ion density in collision cell because of MSAD effects.32 Instrument parameters, such as beam steering, can affect ion transmission. ICR cell conditions (such as excitation) also affect mass distribution. To ensure consistent fragmentation of petroleum products, a quality control sample is always analyzed together with samples. Multicore Structures in Vacuum Resid. Our first set of CID experiments was performed on a vacuum resid aromatic ring class (ARC) fractions separated by HPLC. ARC1, ARC2, ARC3, and ARC4+ are petroleum fractions contain mostly 1, 2, 3, and 4 ring+ aromatic molecules, respectively. Figure 5 shows
Table 2. Ionization Potentials and CID Relative Response Factors (RRF) of Various Cores core
IP (eV)
RRF
naphthalene pyrene phenanthrene dibenzothiophene carbazole
8.14 7.43 7.89 7.90 7.57
0.85 1.15 1.00 0.99 5.33
molecules are also listed in the table. RRF increases with aromatic ring size, i.e., pyrene > phenanthrene > naphthalene. The trend follows that of ionization potentials. Phenanthrene and DBT have very close responses because they have very similar ionization potentials and molecular weights. Carbazole response is much higher than all other cores, because it forms a more stable protonated carbazole during CID. If a multicore is made of phenanthrene and carbazole, the abundance of carbazole core will be significantly overestimated if no response factors are applied for quantification. C1, C2, and Aromatic Sulfide Linkages. To evaluate the strength of various chemical linkages, a series of model compounds were synthesized and studied. The complete list of the model compounds can be found in Table 1. C22 alkylated p-ditolyl methane (C22-DTM) is a two core (benzene) system linked by CH2 (C1 linkage). C22 alkylated diphenyl sulfide (C22DPS) is a two core (benzene) system linked by sulfur. C22 alkylated dinaphthyl ethane (C22-DNE) is a two core (naphthalene) system linked by C2H4 (C2 linkage). At conditions specified in the Experimental Section, both C22DTM and C22-DPS behave like a single core compound, producing primarily C2 to C4 DTM and DPS, respectively. No alkylated benzenes were observed. On the other hand, C22DNE behaves like multicore compounds in CID, dissociating mostly into C2−C4 naphthalenes. Very little alkylated DNE fragments were observed. The results can be rationalized by the chemical bond energies of these linkages. Aromatic−aliphatic C−C linkage and aromatic C−S linkage have bond energies of 93 and 86 kcal/mol, respectively, which are much higher than that of aliphatic C−C bond (∼65 kcal/mol for C2 linkage).34 Aromatic−aromatic C−C bond (such as biphenyl) has even higher bond energy (∼99 kcal/mol), and we expect that the biaryl structure will also behave like a single core compound. It should be noted that CID normally involves multiple collisions. Higher energy deposition (than that defined by ECM) can be achieved in CID. A summary of the weak and strong bonds observed in the CID study of the model compounds can be found in the Supporting Information (Figure S2). At high collision energy, diphenyl sulfide also shows ring closure, forming dibenzothiophene. This reaction is similar to the thermal chemistry behavior of the compound. Naphthenic Ring-Opening in CID. One important question about CID is its impact on naphthenic ring structures. CID of C22 (C9 alkyl) diaromatic sterane containing both 5 and 6 member ring naphthenic structures was studied. The parent ion dominates below 10 kcal/mol (ECM). Between 10 and 30 kcal/ mol, the major product ion is C1 diaromatic sterane from dealkylation. In the high collision energy region (ECM > 50 kcal/mol), ring-opening and formation of cyclic olefin aromatic structures was observed. However, the Z number of this fragment is the same as the parent ion (Z = −16) because Z is determined by the total number of rings and double bonds.
Figure 5. Mass distribution of a VR ARC4+ fraction (mostly 4-ring plus aromatics) before and after CID. Significant molecular weight reduction indicates either dealkylation or multicore dissociation or both.
the changes in molecular weight distribution of VR ARC4+ fraction before and after CID. The mass distribution before CID ranges from 300 to 1500 Da and centers at around 700 Da. After CID, mass distribution is reduced to 80−500 Da. The drastic reduction in mass can be rationalized by dealkylation and/or multicore dissociation of VR molecules. If dealkylation is the only reaction in CID, it is expected that there is no change in aromatic cores (or Z-number distribution). Znumber distributions of the VR ARC4+ fraction before and after CID are shown in Figure 6. Before CID, the Z-number distribution ranges from −6 (benzenes) to −66 (10−12 aromatic ring systems) and peaks around −40 (6−7 aromatic ring systems). After CID, Z-number distribution becomes bimodal. The distribution between Z = −6 and −20 is small aromatic molecules with 1−3 aromatic rings. The distribution 4549
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Figure 6. Z-number distribution of the VR ARC4+ fraction before and after CID. Total concentrations in both cases are normalized to 100%. Y-axis shows the relative concentrations of Z-number series (homologous series). Bimodal distribution indicates the presence of small aromatic pendants and large aromatic cores in the resid structure.
Figure 7. Two-dimensional view (Z-number versus MW) of aromatic ring classes before and after CID. Each pixel represents one molecule. Relative abundances of the molecules are indicated by the color scheme. The stoichiometric limits of planar polynuclear aromatic hydrocarbons (PAHs) structures are indicated by the red lines. After CID, species are pushed toward left up corner and aligned on the red limit lines. Extensive dealkylations of the resid molecules yielded short alkyl substituted core molecules. Positive shift in Z-number distribution indicate dissociation of multicore structures.
after Z = −20 is more condensed aromatic structures (4−9 ring aromatics). These data confirmed the speculation of multicore structures in vacuum resid. These multicore compounds are likely made of both large condensed and small aromatic building blocks. The populations of multicore structures appear to be more abundant in large aromatic ring systems (or molecules with very negative Z-numbers). Since no compounds with Z-number between −50 to −66 were observed in CID products, the data suggested that all species with Z < −50 were multicore structures. For Z > −50, there is likely the presence of both single core and multicore structures. To understand the core structure distributions as a function of aromatic ring classes, ARC1, ARC2, and ARC3 of VR were also examined. Figure 7 is a two-dimensional view (Z-number versus MW) of these aromatic fractions together with ARC4+ before and after CID. Relative abundances of ions are normalized to each box and are indicated by the color scheme. Before CID reactions, the parent ion profiles are broad in both MW and Z-number. Average Z-number is moving toward more negative values as aromatic ring class increases. Average molecular weight decreases with the increasing of ring class mainly because of volatility limitations of APPI which has an upper boiling limit of ∼1300 °F (∼700 °C). For a given boiling point, a more condensed aromatic molecule has a lower mass than does a less condensed aromatic molecule. After CID, both MW and Z-number range have been reduced. Z-number is shifting toward positive direction as MW decreases. The extent of change in terms of Z-number is most dramatic for ARC4+ and least for ARC1, indicating that populations of multicore structures increase with aromatic ring class. Molecules were effectively reduced to their core structures by CID. The stoichiometric limits of planar polynuclear aromatic hydrocarbons (PAHs) structures are indicated by the red lines. The formation of nonplanar structures in CID was not observed. Other Observations. We also examined CID products of lower boiling petroleum fractions, such as VGO (see Supporting Information, Figure S4). Z-number distribution peaks at the same value, suggesting that VGO contains mostly single-core structures. This observation is consistent with
existing compositional and structural knowledge of VGO and other lower boiling petroleum streams. Finally we have noticed striking similarities in product distribution generated by CID versus that produced by thermal reaction (data not shown). It has been reported that internal energy distributions of parent ions in multiple collision CID look very much like Boltzmann distributions,35 implying that CID process is thermal in nature. There is one key difference between CID and thermal reaction under ambient and elevated pressures. There are no bimolecular reactions between analyte ions in CID due to charge repulsion between ions. Consequently, polyaromatic ring growth (coking) which commonly occurred in thermal processes is largely minimized in CID.
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CONCLUSIONS We report the development of CID-FTICR-MS technology to determine structural building blocks of heavy petroleum fractions. A wide range of model compounds were studied to understand CID chemistry and interpretation of petroleum CID data. Model compound experiments demonstrated dealkylation of single core structures and conservation of Znumber (or core structures) during the CID process. The 35− 40 kcal/mol of center of mass collision energy allows dealkylation of resid molecules to C1−C4 substituted cores. Heterocore types were studied to evaluate relative efficiency in core production. In general, the core that has lower ionization potential is more likely to carry charges. CID can effectively break aliphatic−aliphatic carbon bonds and aliphatic−heteroatom bonds in multicore structures. Aliphatic−aromatic carbon bond and aromatic sulfide bond were not broken under CID conditions described in this work. Substantial naphthenic ring-opening is observed at high collision energies (ECM > 50 kcal/mol). The presence of multicore structures in vacuum resid is confirmed. In reality, VR is likely a mixture of 4550
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Analytical Chemistry
Article
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single-core and multicore structures. Multicore features are more pronounced in higher boiling fractions and in higher aromatic ring classes.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was conducted over a two year period from 2007 to 2009 as a collective team effort on a heavy petroleum characterization project. In addition to the coauthors of this paper, many others have contributed to the development at different stages of the program including Drs. Larry A. Green, Charles Rebick, Richard J. Quann, Clifford C. Waters, David T. Ferrughelli, Kaiyuan He, Bryan E. Hagee, James M. Brown, Frank C. Wang, and Len Koenig. Management support from Steve W. Levine of Corporate Strategic Research and Manika Varma-Nair from Analytical Sciences Lab should also be acknowledged. We also wish to acknowledge discussions with Professor Hilkka Kenttamaa of Purdue University on CID chemistry.
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REFERENCES
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dx.doi.org/10.1021/ac300544s | Anal. Chem. 2012, 84, 4544−4551
