Characterization of Heavy Petroleum Saturates by Laser Desorption

Nov 12, 2013 - Silver Cationization and Fourier Transform Ion Cyclotron Resonance ... A comparison of Z number distributions between VGO and VR satura...
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Characterization of Heavy Petroleum Saturates by Laser Desorption Silver Cationization and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Anthony S. Mennito and Kuangnan Qian* ExxonMobil Research Engineering Company, 1545 Route 22 East, Annandale, New Jersey 08801, United States ABSTRACT: A method was developed for the analysis of high-boiling petroleum saturates by laser desorption and silver cationization (LDI-Ag) coupled to Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Commercial polywax standards, petroleum vacuum gas oil (VGO), and vacuum resid (VR) were successfully analyzed with a carbon number span from 30 to 100. Representative model compounds were studied to evaluate relative sensitivity of the ionization method. “Soft” ionizations have been observed for all hydrocarbon types, generating primarily M + Ag+ • ions. Because of the existence of silver isotopes, an ultrahigh mass resolution [e.g., resolving power (RP) > 65 000 at mass ∼ 1000 Da] is found necessary to resolve the key mass overlap (107AgH2/109Ag doublet; ΔM ∼ 16 mDa). Petroleum saturate composition was arranged by homologous series with a general chemical formula (CcH2c + Z), where c stands for the carbon number and Z is generally referred as the hydrogen deficiency index. A comparison of Z number distributions between VGO and VR saturates revealed significant differences in the compositions and structures. The Z number of VGO ranges from 2 to −12, corresponding to 0−7 naphthenic rings, respectively. This result is consistent with the prior knowledge on VGO composition from conventional analytical methods. For VR saturates, Z numbers ranging from 2 to −24 (0−13 naphthenic rings) have been observed. Because olefin and aromatic contents in saturate samples are very low, the large Z number (Z less than −12) are attributed to the presence of highorder naphthenic ring structures (>7 ring naphthenes).

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INTRODUCTION Significant progresses have been made in detailed characterization of vacuum resid (VR) or 538 °C+ (1000 °F+) bottoms over the past decade, largely thanks to the rapid advances in ultrahigh-resolution mass analyzers, in particular, the high-field (>9.4 T) Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS), and availability of various soft ionization methods for petroleum characterization. Similar to the analysis of lower boiling petroleums (538 °C−), VR characterization also involves pre-separation of hydrocarbons into chemical categories with similar core structures, such as saturates, 1−4+ ring aromatics, sulfides, polars, and asphaltenes.1,2 Ionization is the first step in mass spectrometry (MS) analysis of petroleum. Soft ionization is the key to the heavy petroleum analysis because of the complex nature of the petroleum samples. Soft ionization of saturated hydrocarbons is challenging. Ionization potentials of these molecules are substantially higher than that of aromatics. The traditional high energy ionization method (such as 70 eV electron ionization) causes extensive fragmentation of aliphatic chains. Reduction of the ionization voltage greatly reduces the ionization efficiency of the saturates. In FTICR-MS analysis of VR, atmospheric pressure photoionization (APPI) and electrospray ionization (ESI) are the two mostly used ionization methods.3−6 APPI is able to ionize aromatic molecules (60−90% of the total constituents). The technique is based on thermal vaporization assisted with high flow nebulizing gas, followed by charge transfer with solvent ions (typically toluene ions for hydrocarbon analysis). ESI relies on protonation or deprotonation as in the case of positive- and negative-ion ESI, respectively. ESI only detects base, acid, and © 2013 American Chemical Society

other surface-active molecules in petroleum. Because of the lack of charge sites and high ionization potential of the saturate compound class, such as paraffins and cyclic paraffins (or naphthenes), ionization efficiency of these saturate molecules by APPI or ESI is extremely low. Field ionization/field desorption (FI/FD) is by far the most universal ionization technique for petroleum, capable of ionizing both saturate and aromatic molecules, generating primarily molecular ion (M+ •) via electron removal. However, FIMS is limited to analysis of saturates with a boiling point lower than 538 °C. FDMS can be used to generate molecular weight distribution of VR.7 However, the low ion yields of FDMS and the need to ramp the FD emitter temperature during data collection made it difficult to couple FDMS with high-resolution FTICR-MS operation, where co-addition of spectra over an extended period of time (e.g., 10−30 min) is needed. There have been continued efforts to develop new ionization methods for saturate hydrocarbons. Continuous field desorption FTICR-MS enabled co-addition of FTICR-MS spectra. However, fragmentation of saturates, especially paraffins, has been observed8 because of the shorter lifespan of parent ions relative to the long ion transfer time from the ion source to the ICR cell. Laser-induced acoustic desorption (LIAD) was successfully combined with MnCl(H2O)+ chemical ionization inside a FTICR cell for base oil analysis.9 Atmospheric pressure LIAD chemical ionization was developed and applied for ultrahigh-resolution FTICR-MS analysis of petroleum saturates Received: September 17, 2013 Revised: November 8, 2013 Published: November 12, 2013 7348

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Figure 1. LDI-Ag FTICR-MS spectra of polywax 655 and 1000.

and model compounds.10 Both M+ • and (M − H)+ ions were observed. Saturate hydrocarbons can also be oxidized into acids via RICO chemistry and analyzed by ESI FTICR-MS.11 Most recently, paper spray was used to ionize paraffins (alkanes).12 Nitrogen insertion into the C−C bond to form iminium cations was believed to be the ionization mechanism. In this work, laser desorption combined with Ag cationization (LDI-Ag) was investigated as a tool to ionize the elusive saturate molecules. Fine cobalt powder was used as a substrate to assist vaporization. The use of silver salts and cobalt powder for hydrocarbon polymers was first proposed by Li’s group.13 They demonstrated effective ionization of polyethylene up to 3000 Da via a low-resolution time-of-flight (TOF) mass spectrometer. There were several follow-up research works on the use of the methods for the ionization of synthetic polymers and hydrocarbon mixtures.14−18 Here, we extended the LDI-Ag approach to the analysis of high boiling petroleum VR using an ultrahigh-resolution FTICR-MS.

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Fine cobalt powder (30 μm in diameter from Aldrich) is dissolved in isopropanol to form a slurry of 150 mg/mL. Saturated silver nitrate solution in ethanol is prepared by dissolving >31 g of silver nitrate (from Sigma) in 1000 g of 190 proof ethanol (from Aaper). A ∼25 mg/mL petroleum sample solution is prepared in toluene. Sets of three ∼1 μL aliquots of the cobalt slurry were deposited on the MALDI target plate. A 1:1 ratio of silver nitrate and sample solutions is then mixed and deposited on the dried cobalt bed. The organic matrix, such as 2,5-dihydroxybenzoic acids, can also be added to assist ionization. However, no significant advantage was observed with the use of the matrix. Consequently, all data discussed in this work did not use the organic matrix. Once the deposited sample mixture is dried, the target plate is inserted into the mass spectrometer source area and desorbed using the UV laser. The role of cobalt powder is primarily a heat vehicle that can transfer heat effectively to the analyte and the organic matrix. Data Collection and Analysis. FTICR-MS data were collected in a broadband mode (100−3000 Da). For ultrahigh-resolution experiments, data were collected in a 4 Mega word size. Typically, 100−400 FTICR spectra were co-added to achieve the desired signal-to-noise ratio. Ion accumulation times are 1−4 s. Organization of Saturate Molecules. FTICR-MS provides three layers of chemical information for a petroleum system. The first level is heteroatomic classes (or compound classes), such as hydrocarbons (HC), 1 sulfur molecule (1S), 1 nitrogen molecule (1N), 2 oxygen molecules (2O), 1 nitrogen and 1 oxygen molecules (1N1O), etc. The second level is Z number distribution (or homologous series distribution). Z is defined as hydrogen deficiency as in the general chemical formula, CcH2c + ZNnSsOo. The more negative the Z number, the more unsaturated the molecule. Another commonly used term is called double bond equivalent (DBE). For a typical petroleum system, DBE = 1 − (Z − n)/2, where n is the number of nitrogen atoms. The third level of information is the total carbon number distribution or molecular weight distribution of each homologue. If a compound core structure is known, total alkyl side-chain information can be derived by

EXPERIMENTAL SECTION

MS measurements were carried out using a Bruker Apex-Qe FTICRMS with a 12 T actively shielded superconducting magnet. The Bruker FTICR-MS is equipped with an Apollo II dual electrospray ionization/ matrix-assisted laser desorption ionization (ESI/MALDI). In the LDI mode, ions are generated by the focused laser beam on the LDI plate positioned in front of the ion funnel. The laser device employed in MALDI experiments is an ultraviolet (UV) Nd:YAG laser (yttrium aluminum garnet crystals doped with neodymium) at pulse durations of about 5−15 ns and at wavelengths of 355 nm (tripled frequency) or 266 nm (4-fold frequency), respectively. The beam is focused by suitable optics onto the sample inside the ion source to a diameter of ∼200 μm. The laser power can be adjusted with an attenuator. Laser power is set at 70−95% of full power, and pulse energy is set at 460 mJ. Each data point was generated with 500 shots. 7349

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Figure 2. Zoom-in mass spectrum of polywax 1000 at m/z of 1023−1045.

1033. Silver has two abundant stable isotopes, 107Ag and 109Ag, with exact atomic weights of 106.9051 and 108.9048, respectively. The exact masses of [C66H132 + 109Ag]+ • and [C66H134 + 107Ag]+ • are 1033.9371 and 1033.9531, respectively. Differentiation of the 107AgH2/109Ag doublet (16.0 mDa) at mass 1033 requires a mass resolving power (RP = M/ ΔMfwhm) of ∼65 000. Ultrahigh-resolution FTICR-MS confidently resolved and identified Ag adducts of the hydrocarbon species. In addition to the silver adducts, other adducts were also detected, such as M + Ag3+ •, M + Ag2NO3+ •, M + Ag3(NO3)2+ •, etc. The presence of these multiple adducts for a single hydrocarbon component introduced complexity in data analysis. In addition, lower levels of (M − 2H) + Ag+ • adduct ions were also observed. These ions were generated during the ionization process. They could be confused with olefins or cyclohexanes. The level of (M − 2H) + Ag+ • to M + Ag+ • is about 1:10. It has been noted that the straight-chain alkanes produce less (M − 2H) + Ag+ • ions. The use of Kendrick mass (KM) and Kendrick mass defect (KMD) in data processing can simplify the analysis of the LDIAg data. Both KM and KMD have been defined in the previous publications.19−21 In International Union of Pure and Applied Chemistry (IUPAC) mass scale, the mass of 12C was defined to be exactly 12. While in KM scale, the mass of 12C1H2 was defined to be exactly 14. The benefit of KM is that members of the homologous series have the same KMD values. Mathematically, KM and KMD can be calculated using eqs 1 and 2.

subtracting the carbon number of the core from the total carbon number. Petroleum saturates contain a minimum amount of heteroatoms, such as S, N, and O. In general, for 538 °C− molecules, Z span ranges from 2 to −12 with an increment interval of 2. The corresponding structures are non-cyclic alkanes, 1−6 ring naphthenes. For 538 °C+ molecules, Z span can range from 2 to −24, corresponding to alkanes and 1−13 ring naphthenes. Samples. Polywax 650 and 1000 were purchased from Petrolite (Tulsa, OK). Other model compounds were purchased from Aldrich. VGO and VR saturate and aromatic ring class (ARC) fractions were generated using high-performance liquid chromatography (HPLC) at the ExxonMobil Laboratory using methods described previously.1,2 The purity of saturate fractions were examined by both proton (1H) and 13C nuclear magnetic resonance (NMR) to ensure the lack of aromatic and olefinic moieties. The typical detection limits are about ∼0.1 mol % for aromatic and olefinic protons and 1 mol % for aromatic/olefinic carbons.

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RESULTS AND DISCUSSION Analyses of Polywax 655 and 1000. Polywax 655 and 1000 standards were examined in the initial experiments to investigate the method. These samples are saturated homopolymers of ethylene materials with nominal molecular weights of 655 and 1000 g/mol, respectively, based on gel permeation chromatography (GPC). Because there are no olefins or aromatics in these samples, we can conduct the experiment without the interference of other classes of molecules. The FTICR-MS ion transmission and detection were tuned to each molecular mass range separately. Results of the two polywax samples are shown in Figure 1. The centers of molecular weight distribution of polywax 655 and 1000 are around m/z 750 and 1050, respectively. After subtraction of the silver atomic weight (∼108 Da), the centers of the distributions are around m/z 650 and 950, respectively, which are consistent with product specifications. Figure 2 shows a zoom-in mass spectrum at m/z

KM = 14 × IUPAC mass/14.0565

(1)

KMD = nominal mass − KM

(2)

Figure 3 shows the KMD spectrum of polywax 1000, which plots KMD against nominal masses. There are three major groups of ions. The first group is between KMD of 0.2 and 0.3. This group arises from M + Ag+ •. The second group is around 7350

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electron bond between the metal ions and C−H or C−C bonds.16,22,23 In the case of aromatic compounds, Ag+ • will interact with the π electrons of aromatic molecules and form more stable complexes. Thus, it is expected that aromatics will have higher sensitivity during the LDI-Ag ionization. Comparison of LDI-Ag FTICR-MS and FD TOF MS. Saturates isolated from a VR were analyzed by LDI-Ag FTICRMS and FD TOF MS. The latter has a mass resolving power of ∼2000. FDMS produces primarily molecular ions (M+ •) for petroleum and has been routinely used in this lab to determine molecular weight distributions of heavy petroleum systems.7 Figure 5 compares the mass spectra obtained by LDI-Ag Figure 3. KMD plot of polywax 1000.

KMD of 0.48−0.55. This group arises from M + (AgNO3) + Ag+ •. The third band is around KMD of 0.62−0.7, arising from M + Ag3+ •. In this work, only M + Ag+ • ions with KMD between 0.2 and 0.4 were used. Other adduct ions and noises were removed before analysis. Analysis of Model Compound Mixtures. LDI-Ag ionization has different ionization efficiencies for different types of compounds. Figure 4 shows a LDI-Ag FTICR-MS spectrum for three types of compounds with similar molecular weights. The spectrum shows signal peaks corresponding to a C24 n-paraffin (saturated alkane), nonadecylcyclohexane (a C25 naphthene), and nonadecyl benzene (a C25 monoaromatic compound). Comparable amounts of each of the compounds were used in sample preparation. As shown in Figure 4, the peak intensity (signal magnitude) for the aromatic compound is much greater than the peak intensity for the two saturate molecules. The result suggests that aromatics would be preferentially detected if the method is directly applied to the analysis of a whole crude oil, resulting in higher apparent aromatic contents and lower apparent saturate contents. The ionization selectivity was confirmed by experiments. In this work, aromatics were removed from the saturates to overcome the limitation. To a lesser extent, the naphthene (cyclic alkane) in Figure 4 also shows higher peak intensity than that of the paraffin molecule. To account for the differences in ionization efficiency, a response factor or weighting factor will be needed for quantitation purposes. The current method cannot differentiate normal versus branched paraffins. The detailed ion complex chemistry is beyond the scope of this work. It has been postulated that Ag+ • can form intermediates with small alkanes involving a three-center two-

Figure 5. LDI-Ag FTICR-MS and FD TOF MS of VR.

FTICR-MS and FD TOF MS. Molecules with a molecular weight of 500−1300 Da were observed in the LDI-Ag spectrum (top mass spectrum of Figure 5), corresponding to a neutral molecular weight of 400−1200 Da (after subtracting silver atomic weight). This molecular weight range is similar to that observed by FD low-resolution MS, as shown in the bottom mass spectrum of Figure 5. The center of LDI-Ag FTICR-MS is around 850, while that of FD TOF MS is around 750 Da. The difference is about the mass of a Ag ion. The ultrahighresolution FTICR-MS spectra (top mass spectrum of Figure 5) were further processed to generate detailed compositional information (see discussions below).

Figure 4. LDI-Ag FTICR-MS spectrum of three model compounds of the same concentration. 7351

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Table 1. Zoom-in Mass Spectrum (m/z 805.5−805.7) and Formula Assignments peak number

experimental mass (Da)

1 2 3 4 5 6 7 8

805.6713 805.6609 805.6553 805.6455 805.6353 805.619 805.5774 805.5617

formula

theoretical mass (Da)

mass error (mDa)

intensity

mass resolution

S/N

Z

heteroatom type

C50H98107Ag C4813C2H96107Ag 12 C50H96109Ag 12 C48C13H94Ag 12 C49H94O107Ag 12 C49H92O109Ag C51H86107Ag 12 C51H84107Ag

805.6714 805.6625 805.6554 805.6465 805.6350 805.6190 805.5775 805.5615

−0.1 −1.6 −0.1 −1.0 0.3 0.0 −0.1 0.2

12670122 2230440 10145959 2308518 1639588 1580705 2532507 1198872

295091 296764 284304 212801 299566 334521 357567 332293

166.7 27.9 133.1 29 20.1 19.3 32 14.2

−2 −4 −4 −6 −4 −6 −16 −18

HC HC HC HC O O HC HC

12 12

Figure 6. Z number distribution of VGO and VR saturates.

Transition between Saturate and Aromatic Compositions. As briefly mentioned in the Introduction, separation of petroleum into molecules with similar core structures (e.g., saturates, 1−4 ring aromatics, etc.) is a critical step to generate reliable petroleum composition. Separation overcomes limitations of MS in total petroleum composition measurements, including narrow dynamic range and large variations in ionization efficiencies. In this work, VR was separated into eight chemical categories based on the core structure, such as saturates, 1−4+ ring aromatics, sulfides, polars, and asphaltenes. APPI has been used as the primary method for the ionization of aromatic-containing hydrocarbons. Figure 7 shows the contour plot of saturates obtained by LDI-Ag and other heavy

Comparison of VR and VGO Saturate Composition. The detailed examination of the top mass spectrum in Figure 5 between m/z 805.5 and 805.7 revealed eight components with a signal-to-noise ratio greater than 10 (Table 1). A unique elemental formula can be assigned to all peaks with mass errors