Article pubs.acs.org/ac
Analysis of Saturated Hydrocarbons by Redox Reaction with Negative-Ion Electrospray Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Xibin Zhou,† Quan Shi,† Yahe Zhang,† Suoqi Zhao,*,† Rui Zhang,† Keng H. Chung,‡ and Chunming Xu*,† †
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249 China Well Resources Inc., 3919-149A Street, Edmonton, Alberta, Canada T6R 1J8
‡
S Supporting Information *
ABSTRACT: A novel technique was developed for characterization of saturated hydrocarbons. Linear alkanes were selectively oxidized to ketones by ruthenium ion catalyzed oxidation (RICO). Branched and cyclic alkanes were oxidized to alcohols and ketones. The ketones were then reduced to alcohols by lithium aluminum hydride (LiAlH4). The monohydric alcohols (O1) in the products obtained from the RICO and RICO-LiAlH4 reduction reactions were characterized using negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) for identification of iso-paraffins, acyclic paraffins and cyclic paraffins. Various model saturated compounds were used to determine the RICO reaction and ionization selectivity. The results from the FTICR MS analysis on the petroleum distillates derived saturated fraction were in agreement with those from field ionization gas chromatography time-of-flight mass spectrometry (FI GC-TOF MS) analysis. The technique was also used to characterize a petroleum vacuum residue (VR) derived saturates. The results showed that the saturated molecules in the VR contained up to 11 cyclic rings, and the maximum carbon number was up to 92.
P
etroleum derived saturated hydrocarbons consist of nparaffins, iso-paraffins, and cyclic paraffins with a wide range of carbon numbers. Gas chromatography (GC) and mass spectrometry (MS) have been successfully used to characterize this complex mixture in the distillate fraction of petroleum. However, these techniques are not effective for characterizing high boiling point saturated hydrocarbons, to identify cyclic paraffins with more than 6 cyclic rings, and to determine the distribution of the branched alkanes. MS is a common method used in hydrocarbon characterization. The standard high-voltage electron impact (EI) method can significantly fragment the molecular ions of saturates. On the basis of the intensities of the various fragmented ions, the distributions of alkanes and 1- to 6-ring cyclic alkanes can be determined but not information on molecular weight.1−3 Conventional low voltage EI is highly selective for analyzing aromatic hydrocarbons.4,5 A slightly modified EI ion source, a supersonic molecular beam EI, allows for the ionization and molecular ion identification for n-alkanes, iso-paraffin, and cycloparaffins by high/low voltage EI.6 Chemical ionization (CI) by proton transfer and charge exchange has been used for ionization of saturated hydrocarbons. Discharge nitric oxide chemical ionization (TDNOCI)7 and carbon disulfide charge exchange (CS2/CE)8,9 have been used to ionize saturated hydrocarbons, but there were noticeable amounts of fragment © 2012 American Chemical Society
ions in addition to the molecular ions. Field ionization (FI)/ field desorption (FD) are the most common ionization methods used for generating molecular ions of saturates.10−13 Online liquid chromatography field ionization mass spectrometry (LC FI MS), 14 off-line LC FI MS, 11 and gas chromatography field ionization time-of-flight mass spectrometry (GC FI-TOF MS)15 have been applied to characterize petroleum derived saturates. Branched hydrocarbons undergo fragmentation under FI/FD operating conditions which allow for their selective identification.16,17 The desorption electrospray ionization (DESI) technique was also used for analyzing saturated hydrocarbons.18 During the DESI discharge, hydrocarbon fragmentation was not observed, even for highly branched squalane. However, DESI has low responses for large alkanes. Several gas-phase transition-metal ions or organometallic ions have been chosen as the CI reagent ions for ionization of saturated hydrocarbons19−21 and polyethylene (PE).22−25 Silver salts are good cationization reagents for the direct speciation of alkanes and nonpolar synthetic polymers without fractionation.20,26 Recently, CI by cyclopentadienyl cobalt radical cation Received: November 29, 2011 Accepted: February 16, 2012 Published: February 16, 2012 3192
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(CpCo·+)27−29 or ClMn(H2O)+ ions30,31 combined with laserinduced acoustic desorption (LIAD)32,33 was demonstrated to ionize saturated hydrocarbons with equal desorption/ionization efficiency and negligible fragmentation. However, the LIAD/ CpCo·+ ions yielded different type of adduct ions (e.g., [CpCo(R-H2)]+ and/or [CpCo(R-2H2)]+) for alkanes. Consequently, it may be difficult to determine the cyclic paraffins.30 The LIAD/ClMn(H2O)+ method produces [ClMn + R]+ for each type of alkane. Therefore it is easy to determine the type and molecular weight of alkanes. Electrospray ionization (ESI) is a common ionization technique used in modern MS, which evaporates and ionizes polar compounds and can be easily coupled with Fourier transform ion cyclotron resonance (FTICR) MS.34−42 Since saturates lack functional groups, ESI is not suitable for saturated hydrocarbon analysis. To accomplish the detection of nonpolar compounds by ESI mass analysis, chemical pretreatment is conducted to convert nonpolar compounds into polar compounds. Methylation is a sample pretreatment method for petroleum sulfur compounds prior to ESI FTICR MS analysis.43,44 Chemical derivation has been reported to facilitate the ionization of saturated polyethylene (PE) samples,45,46 in which the vinyl-terminated PE molecules were converted to organic salts or added ionized functional groups. However, the vinyl-terminated series may not be representative of the main PE series. Saturated hydrocarbons are relatively refractory and difficult to transform chemically. Transformation of saturated hydrocarbons can be carried out by introducing metal complexes.47−52 Ruthenium ion catalyzed oxidation (RICO) is an effective reaction for transforming saturated hydrocarbons into alcohols and ketones.53−55 The RICO reaction can also convert aromatic hydrocarbons to aliphatic and naphthenic acids, which was used for determining the structure of asphaltenes,56−58 coal,59,60 and kerogen.61 The traditional high-resolution MS cannot identify cyclic paraffins with more than six cyclic rings. This is because the mass doublets between alkanes of CaHb with n cyclic-rings and alkanes of Ca+1Hb−12 with n + 7 (n ≥ 0) cyclic rings is only 9.39 mDa,62 for example, C75H150 (MW = 1051.173 76) and C76H138, (MW = 1051.079 85). This can be overcome by using FTICR MS. The ultrahigh resolution and mass accuracy of FTICR MS are capable of assigning a unique elemental composition to each peak in the mass spectrum of complex hydrocarbons such as petroleum. Combined with various soft ionization techniques, FTICR MS is capable of determining the composition of a wide range of complex species.44,63−66 In this work, a chemical pretreatment technique was developed, which was used in conjunction with ESI FTICR MS for characterizing acyclic, isomeric, and cyclic paraffins. Various model saturated hydrocarbons were used to determine the oxidation and ionization efficiencies. This method was further applied to characterize saturates in petroleum derived distillates and vacuum residues.
saturates in the distillates and vacuum residue were 85.2 and 24.7 wt %, respectively. Model saturate compounds such as noctane, n-hexacosane, n-triacontane, n-dotriacontane, squalane, cycloheptane, methylcyclohexane, n-dodecylcyclohexane, nnonadecylcyclohexane, bicyclohexyl, decahydronaphthalene (mixture of cis- and trans-), adamantane, and cholestane were purchased from J&K Chemical Ltd. A mixture of normal C30− C120 alkanes (catalog no. 59.50.100B) was purchased from Analytical Controls Ltd. Analytical grade toluene, methanol, carbon tetrachloride (CCl4), acetonitrile (CH3CN), chloroform (CHCl3), and tetrahydrofuran (THF) were obtained from Beijing Chemical Reagents Company and were distilled twice before use. Ruthenium trichloride (RuCl3) was obtained from J&K Chemical Ltd. Sodium periodate (NaIO4) and potassium hydroxide (KOH) were purchased from Beijing Chemical Reagents Company. Reaction of Model Saturate Compounds. Model alkane or a mixture of model alkanes (0.073 mmol) was mixed with 2 mL of CCl4, 2 mL of CH3CN, 3 mL of water, 0.25 g of NaIO4, (1.17 mmol) and 5 mg of RuCl3 in 12 mL sample vials with Teflon caps. The mixture was continuously stirred with a magnetic bar for 24 h at 40 °C. The organic and aqueous phases of the reaction products were collected. The aqueous phase was extracted with 1 mL of CHCl3 three times. The organic extracts in CHCl3 were combined and dried with sodium sulfate (Na2SO4). The light saturated hydrocarbon products (carbon number 10) was divided into two portions. Portion I was analyzed by GC and GC/MS and then subjected to silica chromatography to obtain the monohydric alcohols for negative-ion ESI FTICR MS analysis. Portion II was dried in a rotary evaporator and then reduced by 0.2 g of lithium aluminum hydride (LiAlH4) (5.27 mmol) in dry tetrahydrofuran (THF). The RICO-LiAlH4 reduction products were subjected to silica chromatography with CHCl3 eluent to yield the monohydric alcohols for negative-ion ESI FTICR MS analysis (see Figure S-1 in the Supporting Information for the scheme). Saturates Fraction of Distillates and Vacuum Residue. Figure 1 shows the reactions and analysis scheme of distillates and vacuum residue derived saturated fractions. In total, 30 mg of each sample was used for the RICO reaction. The treatment of reaction products of distillates and vacuum residue derived saturated fractions was similar to that of reaction products of model saturated compounds, as described above. It is known that the saturated fraction obtained from SARA separation contains alkylated aromatics67 which can be converted to acids by the RICO reaction. These acids can be removed by potassium hydroxide (KOH) modified silica gel chromatography.68 Neutral silica gel was packed into the bottom of a glass chromatography column (25 cm × 1 cm i.d.). The length of the silica gel packing in the column was 20 cm. The KOH modified silica gel was packed into the column above the neutral silica gel. The length of the KOH modified silica in the column was 3 cm. Each RICO product sample was divided into two portions. Portion I was subjected to solid phase extraction (SPE) by the KOH modified silica to remove acids, followed by conventional silica chromatography with a CHCl 3 eluent to yield monohydric alcohols. The monohydric alcohols were analyzed by negative-ion ESI FTICR MS for iso- and cyclic paraffins. Portion II was also subjected to KOH modified silica chromatography using a THF/CHCl3 (1:1) mixture as the
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EXPERIMENTAL SECTION Materials. The petroleum distillates (395−425 °C) and vacuum residue (>560 °C) were obtained from PetroChina Dagang refinery. The vacuum residue derived saturated fraction was obtained by saturates/aromatics/resins/asphaltenes (SARA) fractionation using the standard method (Chinese Standard Analytical Method for Petrochemical Industry: SH/T 0509-92, equivalent to ASTM D2007-93). The amounts of 3193
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Supporting Information, Table S-1). Model alkanes without tertiary C−H bonds were oxidized to ketones at extremely high product selectivity (>99%). By contrast, model alkanes with tertiary C−H bonds were oxidized to alcohols and ketones with moderate product selectivity for alcohols. This is because the reactivity of the C−H bonds decreases in the following order: tertiary > secondary ≫ primary.52 For alkanes without tertiary C−H bonds, the secondary C−H bonds undergo oxidation and convert to secondary alcohols which have a higher reactivity than the corresponding alkanes. These secondary alcohols can be easily oxidized to the corresponding ketones in the presence of excess oxidizers. The alkanes with secondary and tertiary C− H bonds can be easily oxidized to tertiary alcohols which cannot be further oxidized. The alkanes that oxidized to secondary alcohols can be further oxidized to ketones. Table S1 in the Supporting Information shows that the reaction selectivity varies over a wide range (60−80%) for alkanes with tertiary C−H bonds, depending on the reactivity, number of the tertiary C−H bonds, the ratio of tertiary to secondary C−H bonds, and the steric hindrance to tertiary C−H bonds. For different type alkanes with similar carbon number, (e.g., noctane and methylcyclohexane, n-hexacosane and n-nonadecylcyclohexane, n-triacontane and squalane), conversion of alkanes with tertiary C−H bonds is higher than that of alkanes without tertiary C−H bonds. This is because the reactivity of tertiary C−H bonds is higher than that of secondary C−H bonds. For the same type of saturated hydrocarbons (e.g., nalkanes series and n-alkyl cyclohexane series), the conversion rate of saturated hydrocarbons increases with carbon number. This is because the number of secondary C−H bonds and/or tertiary C−H bonds increases with carbon number. Monohydric alcohols, mono ketones, polyhydric alcohols, poly ketones, and alcohol-ketones were found in the RICO product, as detected by GC/MS. Negative-ion ESI has different ionization selectivities for alcohols and ketones. Alcohols can be ionized and detected by negative-ion ESI, whereas ketones cannot be ionized. Hence, it is possible to determine different types of saturated hydrocarbons by taking advantage of RICO reaction selectivity for saturated hydrocarbons without tertiary C−H bonds and ionization selectivity of negative-ion ESI for ketones (for the reaction-analytical test scheme, see the Supporting Information Figure S-2). The branched alkanes and most of cycloalkanes with tertiary C−H bonds were oxidized to tertiary alcohols which can be analyzed by negative-ion ESI FTICR MS. It
Figure 1. Reaction-analytical test scheme for the petroleum derived saturated fraction. Note: similar polarity aromatic and saturated hydrocarbons are in the same phase.
eluent to remove acids. The acid removed sample was reduced by LiAlH4 (0.2 g, 5.27 mmol) in dry THF. The LiAlH4 reduction product was analyzed by silica chromatography with CHCl3 eluent to yield monohydric alcohols for negativeion ESI FTICR MS analysis. No sulfide peaks were observed in the ESI FTICR mass spectrum. This is consistent with the results from a previous study indicating that alkyl sulfides, thiolanes, and thianes were completely oxidized to sulfones by RICO.56 Moreover, the Dagang crude has a low amount of sulfur.
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RESULTS AND DISCUSSION
Selective Oxidation and Characterization of Model Saturated Hydrocarbons. Various model alkanes were used to determine RICO reactivity and selectivity (see the
Figure 2. (a) Broadband negative-ion ESI FTICR mass spectrum of the product from RICO-LiAlH4 reduction of cholestane. The insert shows the expanded mass scale spectrum at m/z 380−440 and (b) broadband negative-ion ESI FTICR mass spectrum of the monohydric of cholestanols separated by silica chromatography. DBE values were calculated for the neutral states of the compounds. 3194
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opening molecules contain at least two oxygen atoms; the molecules with an oxygen atom are not the ring-opening products. Hence, the monohydric alcohols in the oxidation and oxidation−reduction product saturates have the same elemental composition of carbon and hydrogen as saturates in the feed sample. Figure 2b shows the broadband negative-ion ESI FTICR mass spectrum of the O1 class species obtained from column chromatography of the RICO-LiAlH4 reduction of cholestane. The analysis indicated that only O1 class species with 4 DBE (C27H47O) were present. Although GC analysis showed that monohydric alcohols and monoketones were dominant in the RICO products of cholestane (85.2 wt % of the total products by GC), the MS peak intensities of the O2 and multiple O-class species in Figure 2a were much higher than that of the O1 class species. This is likely due to high ionization efficiencies of dihydric and multihydric alcohols compared to that of monohydric alcohols. For the reaction products of cholestane, the ratio of ionization efficiency for O1 to O2 is 1:400 as determined by GC and MS data (Figure S-3 in the Supporting Information). Therefore, to properly characterize O1 class species, they must be isolated from the RICO product. Parts a-1 and a-2 of Figure 3 show the FTICR mass spectra of the O1 class species in RICO products of a mixture of ndotriacontane and squalane (1:1 molar ratio) before and after LiAlH4 reduction, respectively. Since only squalanols were observed in Figure 3a-1, it could be deduced that the oxidation reactivity of n-alkane to alcohols is much lower than that of branched alkanes. The high reactivity of squalane (an isoprenoid species) in RICO reactions was due to its 6 tertiary C−H bonds. After the RICO product was subjected to reduction reactions with LiAlH 4, both squalanols and dotriacontanols were observed, as shown in Figure 3a-2, although the MS peak intensity of squalanols was much higher than that of dotriacontanols. Parts b-1 and b-2 of Figure 3 show the FTICR mass spectra of the O1 class species in RICO products of a mixture of nhexacosane and n-nonadecylcyclohexane (1:1 molar ratio) before and after LiAlH4 reduction, respectively. Only nonadecylcyclohexanols were observed in Figure 3b-1. Figure 3b-2 showed both nonadecylcyclohexanols and hexacosanols were detected after LiAlH4 reduction. In summary, the results in Figure 3 show that the different types of alkanes can be characterized by using negative-ion ESI FTICR MS analysis in conjunction with RICO-LiAlH 4 reduction reactions. The results also indicate that the structural fingerprint of feed saturated hydrocarbons can be deduced from the monohydric alcohols in the product alkanes of RICO and RICO-LiAlH4 reduction. Saturates in Petroleum Distillates. Highly alkylated aromatic hydrocarbons in petroleum, such as long chained alkyl benzene, monoaromatic steroid hydrocarbons, and benzohopanes are difficult to separate from paraffins completely. In a previous study, the saturates fraction of the petroleum distillate was subjected to solid-phase extraction (SPE), with silver nitrate impregnated silica to separate aliphatic and aromatic hydrocarbons.67 In this work, the aromatic hydrocarbons were completely converted to carboxylic acids by the RICO reaction71 and these acids were removed by KOH modified silica chromatography.68 The petroleum distillates derived saturated fraction was subjected to RICO and RICO-LiAlH4 reduction reactions. The ESI FTICR MS and FI GC-TOF MS were used to analyze the
should be noted that all cycloalkanes in the high boiling point fractions of petroleum derived distillates and vacuum residue contain tertiary C−H bonds. The linear alkanes and a small fraction of cycloalkanes without tertiary C−H bonds (e.g., cyclohexane) were converted to ketones, which cannot be ionized by negative-ion ESI. When the RICO products were subjected to reduction reactions with LiAlH4, the ketones were converted to alcohols which can be analyzed by negative-ion ESI FTICR MS. It is possible to identify open chain alkanes, branched alkanes, and cycloalkanes in the saturated hydrocarbons using this reactionanalytical test scheme. Figure 2a shows the broadband negative-ion ESI FTICR MS of the product obtained from RICO-LiAlH4 reduction of cholestane. The inserts in Figure 2a show the expanded mass scale spectrum at 380−445 m/z. The molecular formula of cholestane is C27H48 and its double-bond equivalents (DBE) value is 4. When cholestane was subjected to RICO, the oxidized products were cholestanols and cholestanones. When the RICO product was subjected to reduction reactions with LiAlH4, only the carbonyls were converted to hydroxyls. Hence, cholestanol was the overall final reaction product, which has 4 DBE (the same as cholestane) with the molecular formula C27H47On (n ≥ 1). However, Figure 2a shows that the O2, O3, and O4 class species were present with 3 and 4 DBE (C27H45On and C27H47On, respectively, with n = 2,3,4). This indicated that not all the alcohols derived from RICO of cholestane had the same structural fingerprint as cholestane. Previous studies showed that cycloalkanes can be overly oxidized to form ring-opening products.53,55,69 On the basis of the reaction mechanism of RICO of saturated hydrocarbons,70 a possible ring-opening reaction mechanism of cholestane is shown in Scheme 1. First, a hydrogen atom at a specific site in Scheme 1. Possible Ring-Opening Reaction Mechanism of Cholestane Base on the Mechanism of RuO4 Oxidations70
the molecular structure is substituted with a hydroxyl. Under severe oxidation conditions, an adjacent carbon−carbon bond of the hydroxyl breaks and forms a dicarbonyl compound which can be further oxidized to multioxygen compounds. The presence of O2, O3, and O4 class species with 3 DBE in Figure 2a indicates that the ring-opening reaction occurs during the RICO of cholestane. Scheme 1 also shows that the ring3195
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Figure 3. Broadband negative-ion ESI FTICR mass spectrum of the O1 species from the RICO (a-1) and RICO-LiAlH4 reduction (a-2) product of n-dotriacontane and squalane (1:1 molar ratio) mixture and the RICO (b-1) and RICO-LiAlH4 reduction (b-2) product of the n-hexacosane and nnonadecylcyclohexane (1:1 molar ratio) mixture. Note: the peaks marked with an asterisk are likely the contaminants. DBE values were calculated for the neutral states of the compounds.
Figure 4. The iso-abundance plots of DBE as a function of carbon number for the O1 class species in the product of the (a) RICO and (b) RICOLiAlH4 reduction of the saturate fraction of Dagang petroleum distillates (395−425 °C), respectively, and (c) the dot plots of DBE as a function of carbon number for the saturate fraction of Dagang petroleum distillates from FI GC-TOF MS analysis. The radius (not area) of the plots corresponds to the relative abundance of hydrocarbons. DBE values were calculated for the neutral states of the compounds.
reaction products. Parts a and b of Figure 4 shows the isoabundance plots of DBE as a function of carbon number determined by ESI FTICR MS for the O1 class species in the products of RICO and RICO-LiAlH4 reduction reactions, respectively. The DBE values were determined for the neutral states of the compounds. Data treatment used for GC FI-TOF MS analysis which yields the dot-plot in Figure 4c was similar to that of ESI FTICR MS analysis. The O1 class species with 0 DBE in Figure 4a,b were the branched alkanes and total open chain alkanes, respectively. The O1 class species with 1−7 DBE in Figure 4a,b were 1−7 cyclic-ring cycloalkanes. A comparison of the data in Figure 4a,b shows that the carbon number of O1 class species with 0 DBE in Figure 4a is higher than in Figure 4b, suggesting that the branched alkanes had higher carbon numbers compared to the total open chain alkanes. This finding is consistent with the boiling points of alkanes. At a given boiling point temperature, the carbon number of the branched alkane will be higher than that of the linear alkane. The relative abundance of O1 class species with 0 DBE in Figure 4b is much higher than that shown in Figure 4a, indicating that the content of linear alkanes was much higher than branched alkanes in this saturate fraction. A comparison of the data in Figure 4b,c showed that the abundance distribution of saturated hydrocarbons determined by ESI FTICR MS and FI GC-TOF MS was similar. These two analytical techniques show that the dominant type of saturates in the petroleum distillates were open chain alkanes, in which C25H52 was the most abundant. The species with 4−5 DBE and
27−30 carbon numbers were more abundant than the peripheral species. Steranes (4 DBE, 27−30 carbon numbers) and hopanes (5 DBE, 27−30 carbon numbers) were present in petroleum distillates, which were confirmed by GC/MS analysis. The ratios (peak heights) of the biomarkers obtained from the −ESI were similar to those obtained by FI. The abundance of low carbon number species shown in Figure 4b was slightly lower than that in Figure 4c. This is likely due to the ion-transfer system of FTICR MS used in this work. After the ions were accelerated, the time that the ions reached the analyzer varied with m/z. The gate trapping interval of the analyzer was 1 ms at which most of the ions were trapped. During this time interval, a small portion of relatively low m/z ions may be lost. Saturates in the Vacuum Residue. Figure 5 shows the high-temperature gas chromatography (HT-GC) spectrum and broadband ESI FTICR mass spectrum of the O1 class species in the synthetic wax derived products from RICO and RICOLiAlH4 reduction reactions. The carbon numbers of the corresponding n-alkanes are marked on the HT-GC chromatogram and the mass spectrum. Discrimination of relatively low and high carbon number species was observed in the FTICR MS spectrum. The relative lower m/z ions may be lost during the gate trapping of the analyzer. Also, RICO is not as effective in oxidizing relatively low carbon number alkanes as large carbon number alkanes, as shown in Table S-1 in the Supporting Information. For relatively high carbon number 3196
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special class of saturated hydrocarbons: isoprenes. Isoprene alkanes with low carbon numbers (
