Hydrocarbon Group Type Separation of Gas Oil Resins by High

Aug 18, 2015 - An analytical method that provides chromatographic information regarding gas oil resins has been developed. This method uses a high per...
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Hydrocarbon Group Type Separation of Gas Oil Resins by High Performance Liquid Chromatography on Hyper-Cross-Linked Polystyrene Stationary Phase Patricia H. Arboleda,*,† Heather D. Dettman,† and Charles A. Lucy‡ †

Natural Resources Canada, CanmetENERGY, One Oil Patch Drive, Devon, Alberta Canada, T9G 1A8 Department of Chemistry, Gunning/Lemieux Chemistry Centre, University of Alberta, Edmonton, Alberta Canada, T6G 2G2

‡

ABSTRACT: An analytical method that provides chromatographic information regarding gas oil resins has been developed. This method uses a high performance liquid chromatographic system without backflushing and a single hyper-cross-linked polystyrene column. The use of the latter was warranted as it retains aromatic model compounds more effectively than a typically used silica-based amino-cyano column. Moreover, there was no need for extra solvent drying techniques because a retention time reproducibility of 2.9% RSD over 6 months was achieved. Using this method, gas oil resins were separated into seven chromatographic regions in 35 min on the hyper-cross-linked polystyrene column using a four-solvent-gradient procedure (hexane, chloroform, tetrahydrofuran, and methanol). On the basis of model compounds, separation was achieved between aromatics, sulfur-containing, and nitrogen-containing compounds. Determining composition has a large effect on the characteristics, conversion, and uses of the petroleum feedstock. As resins comprise a significant component of petroleum feedstockup to 54% of the source materialthe detailed characterization of resins using this method is expected to assist in improving the valorization of petroleum products.

1. INTRODUCTION Global crudes are complex mixtures of hydrocarbons containing varied contents of sizes of molecules and heteroatoms such as sulfur and nitrogen1−4 Such materials must be characterized in order to support growing demand for oil products and more efficient processing technologies, and to meet strict environmental regulations and specifications.5−9 Due to compositional complexity, preparative separations are almost always required. One of the most basic and routinely performed method of characterizing petroleum is distillation.1 The 204−524 °C boiling point distillate fraction is termed gas oil. Gas oil can subsequently be chromatographically separated into saturates, aromatics, and gas oil resins using preparative chromatography ASTM D2007 method.10 Saturates and aromatics can be further analyzed using gas chromatography−mass spectrometry (GC−MS), a technique which offers high resolving power of up to 100 000 theoretical plates. However, detailed characterization of gas oil resin is difficult due to their diversity, highly polar nature, and high sulfur and nitrogen content (>1%).1,11 In addition, the presence of large aromatic compounds (up to five rings) leads to poorly resolved chromatograms and assignments due to myriad possible isomeric forms.1,8 Petroleum feedstocks can be composed of up to 54 wt % resins, thus warranting the analysis and conversion of resins to more valuable petroleum products.7,12 In addition, it is important to characterize gas oil resins because they contain polycyclic aromatic hydrocarbons that can increase particulate emission in engine exhaust gases. Gas oil resins contain sulfur, in the form of polyaromatic thiophenes, sulfides, and thiols.1,13−18 Determining the distribution of sulfur compounds among these groups would aid in determining which sulfur species break down more easily during the desulfurization processes.1,19 In the petroleum feedstock, nitrogen compounds © 2015 American Chemical Society

include both basic and neutral species such as pyridines, amines, and pyrroles. Nitrogen species are undesirable compounds in refineries as they are involved in catalyst deactivation during refinery processes and are implicated in fouling and corrosion of refinery units.20,21 A detailed characterization of gas oil, including its resins, is important in the identification of heteroatom compounds in order to optimize hydrocracking and hydrotreating catalytic processes through more precise modeling.19 Overall, it is desirable to characterize gas oil resins to improve the refining processes and to optimize utilization of the petroleum resin feedstock.1,7,12,22−25· In the literature heavy fractions of crude oils are characterized using nuclear magnetic resonance (NMR) spectroscopy,11,26,27 comprehensive two-dimensional gas chromatography (GC × GC),27,28 supercritical fluid chromatography (SFC),11,29 mass spectroscopy (MS),11,27−29 thin-layer chromatography with flame-ionization detector (TLC−FID)30 and high performance liquid chromatography (HPLC).23,31,32 High performance liquid chromatography group-type separations of heavy distillates and coal derived products are often performed using multiple columns, gradients, backflushing, and valves.29 For example, Grizzle and Sablotny analyzed crude oil with two amino−bonded silica columns using n-hexane and dichloromethane solvents.33 Group type separation was achieved where saturated hydrocarbons and aromatic compounds were eluted, and polar (sulfur and nitrogen-containing aromatic compounds) were backflushed. Robbins analyzed a petroleum fraction with boiling points up to 700 °C with a Received: June 9, 2015 Revised: August 7, 2015 Published: August 18, 2015 6686

DOI: 10.1021/acs.energyfuels.5b01287 Energy Fuels 2015, 29, 6686−6694

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μm Rheodyne (Rohnert Park, California, U.S.A.) frit at each end.40 Separations were temperature controlled using an Eppendorf CH-30 column heater (Mississauga, Ontario, Canada). High-temperature simulated distillation (HTSD) was performed using an Agilent (Wilmington, Delware, U.S.A.) gas chromatograph and Analytical Control software (Bensalem, Pennsylvania, U.S.A.).42 The carbon, hydrogen, and nitrogen contents of each gas oil resins sample were determined using a LECO (Mississaugau, Ontario, Canada) CHN analyzer,43 and sulfur content was determined using a LECO sulfur analyzer.44 The nuclear magnetic resonance (NMR) spectra for the gas oil resins were run at room temperature (20 ± 1 °C) with a Varian XL300 NMR spectrometer, operating 75.429 MHz for carbon. The carbon spectra were acquired using an acquisition time of 0.9 s, a sweep of 16 500 Hz, a flip angle of 31.9° and a 4 s recycle delay.45 2.2. Standards and Samples. All solvents were HPLC grade, purchased from Fisher Scientific (Ottawa, Ontario, Canada). No additional drying of the solvents were performed. Helium (>99.99%) for sparging, was from BOC Gas (Mississauga, Ontario, Canada). Model compounds representative of those suspected to be in gas oil resins were used to qualify and optimize the separation. Nonpolar model compounds (>95% purity) were prepared at concentrations of 0.05 parts-per-thousand to 20 parts-per-thousand (weight/weight) in either hexane or 80:20 hexane:chloroform blend (volume %). Sulfur and nitrogen-containing model compounds were dissolved in chloroform at 0.03 parts-per-thousand to 20 parts-per-thousand (weight/weight), while gas oil resins were dissolved in chloroform 0.15−0.39 wt %. Samples were filtered with 0.2 μm polytetrafluoroethylene (PTFE)-fluorophore membranes from InnoSep (Samutsakorn, Thailand) before injection. Gas oil resins were isolated from extra-heavy and heavy oils from different geographical locations including: Alberta, Canada (AB1 and AB2), South America (SA), and the United States (US). Each bitumen or heavy oil sample was subjected to distillation (one theoretical plate) at reduced pressure between 0.13 and 6.7 kPa (1 and 50 mmHg) to collect the gas oil fraction (boiling point range 204−524 °C). The gas oil was divided into saturate, aromatic, and resin fractions using preparative chromatography.10 Each was dissolved in n-pentane and eluted through a column packed with clay in the upper section and blend of silica gel plus clay in the lower section. The saturated hydrocarbons fraction was eluted using n-pentane, then a fraction of aromatic compounds was eluted using 50:50 n-pentane:toluene blend (volume %), and finally the gas oil resins fraction was eluted using 50:50 toluene:acetone blend (volume %).10 Each fraction was dried using a rotary evaporator and then weighed to determine the mass composition of the gas oil distillation fraction. Gas oil resins required 100% chloroform to fully dissolve before injecting into the HPLC system. 2.3. Method. For a hexane or 80:20 hexane:chloroform blend (volume %) mobile phase, the flow rate was 1 mL/min, the oven temperature was 35 °C. All solvents were sparged with helium for 15 min at 100 mL/min before use, and then sparged at 50 mL/min for the duration of the experiment. The columns were equilibrated with the mobile phase at 1 mL/min for 60 min (15 column volumes) at the beginning of each day before making injections. For experiments using a hexane mobile phase, a refractive index detector was used. For the silica amino-cyano column, sample injections of 10 μL were made. For the hyper-cross-linked polystyrene column, the peaks were broader so the injection volume was 50 μL. For experiments using an 80:20 hexane:chloroform blend (volume %) mobile phase, a photodiode array (PDA) detector was used. An injection volume of 10 μL was used for both columns. Chromatograms were recorded over a wavelength range of 210−400 nm. Reference spectra were used to confirm compound identity. For the gradient elution and composition of gas oil resins the column temperature was 50 °C, the injection volume was 20 μL, and the flow rate was 1.5 mL/min. The linear four-solvent-gradient procedure was initially 80:20 hexane:chloroform blend (volume %), and changed linearly over 10 min to chloroform, followed by a linear gradient to tetrahydrofuran over 10 min, and linearly to methanol over

propylamino-cyano column and a dinitro-aminopropyl column using three solvents (n-hexane, dichloromethane, and 2propanol).31 Valves were used to direct mobile phase flow between different columns in order to elute saturates, 1−4 ring aromatics, and polars (sulfur- and nitrogen-containing aromatic compounds). Padlo et al. analyzed coal derived liquids with three columns (amino-cyano, dinitroaminopropyl-Si, and diolSi columns) using three solvents (n-pentane, dichloromethane, and 2-propanol).34 Valves were used to direct mobile phase flow between different columns to elute saturates, 1−4 ring aromatics and polars (sulfur and nitrogen-containing aromatic compounds). Gas oil resins have been analyzed using fluorescence spectroscopy,7 supercritical fluid extraction (SFE),9 comprehensive two-dimensional gas chromatography (GC × GC),8 and high performance liquid chromatography (HPLC).6,31 Using HPLC, Islas-Flores et al. analyzed petroleum gas oil resins by reversed and normal phase liquid chromatography followed by gel-permeation chromatography where each column was independently operated.6 Two fractions were obtained by reversed phase chromatography (C18 column) using acetone and dichloromethane solvents. Four fractions were obtained by normal phase with an amino modified silica column using three solvents (cyclohexane, dichloromethane, acetone). Gel-permeation chromatography was used to compare molecular size distribution of all fractions. However, the method was not validated using aromatic model compounds. Our goal is to develop an analytical method that provides chromatographic information regarding gas oil resins, using a simple chromatographic system consisting of a single column without valves; our methodology compared the performance of a standard silica-based amino-cyano column to that of a hypercross-linked polystyrene column. Hyper-cross-linked polystyrene network builds up due to introducing methylene bridges between phenyl groups of long polystyrene chains.35 Hypercross-linked polystyrene is usable in the pH 2−14 range, is compatible with both polar and nonpolar organic solvents, and its potential for hydrocarbon group separation has been earlier described.35−41 Previous reports have demonstrated hypercross-linked polystyrene use in reversed-phase, mixed, and quasi-normal-phase modes, and in gradients going from quasinormal-phase to mixed mode.35 For this study, model compounds characteristic of those found in gas oil resins are used to reference the chromatographic retention on the columns. Finally, gas oil resins from four different sources were analyzed and compared using the hyper-cross-linked polystyrene system.

2. EXPERIMENTAL SECTION 2.1. Apparatus. The HPLC system was composed of a Waters 600 controller and multisolvent delivery system (Waters, Milford, Massachusetts, U.S.A. and Waters Canada, Mississauga, Ontario, Canada), a Waters 717 autosampler, a Waters 996 multiwavelength photodiode array detector, and a Waters 2414 refractive index detector. Data was collected at an acquisition rate of 1 Hz and a response time of 1 s using Empower Pro 2 software (Waters). The columns studied were a silica amino-cyano Partisil 5PAC (5 μm, 4.6 mm ID × 250 mm, Whatman, Brentford, Middlesex, England) and a hyper-cross-linked polystyrene Chromalite 5HGN (Purolite International Limited, Wales, United Kingdom and Bala Cynwyd, Pennsylvania, U.S.A.). The 5 μm hyper-cross-linked polystyrene particles were slurry packed (50 mg/mL slurry in chloroform) inhouse into a 4.6 mm ID × 250 mm stainless steel column with a 0.5 6687

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Figure 1. Retention of model compounds on silica-based amino-cyano and hyper-cross-linked polystyrene columns. Conditions: hexane mobile phase; 1.0 mL/min; 35 °C; 250 mm × 4.6 mm ID; refractive index detection. Injection volume was 10 and 50 μL, respectively. Error bars represent one standard deviation. 10 min. After each run, the system was equilibrated with chloroform for 15 min, followed by 80:20 hexane:chloroform blend (volume %) for 15 min. Gas oil resins for 13C NMR spectroscopy were prepared by mixing 100 mg of materials in 600 μL of deuterated chloroform. 2.4. Calculations. Factors that affect resolution are retention factor, selectivity, and efficiency.46 Herein, resolution (Rs) was calculated based on width at half height.47,48 For a peak detected by the refractive index detector the retention factor (k′) was calculated using heptane as the unretained solute.46 The retention factor can be adjusted by modifying the mobile phase composition.48 Isocratic (one solvent mobile phase) separation has a preferred k′ of between 1 and 10; gradient elution (multiple solvent mobile phase) becomes desirable for samples that cover a wide range in k′-values (ex: 0.5 ≤ k′ ≤ 20).46 Selectivity (α) is the ability of the system to discriminate between two analytes, and is the ratio of retention factors.46 The selectivity is dependent on the solvent and column type.48 Efficiency (N), also known as the number of theoretical plates, is a measure of peakedness of the data curve. It accounts for the dispersion a peak undergoes while traveling through the system. The number of theoretical plates can be increased by increasing the length of the column or by decreasing the column plate height.48 Herein, efficiency (N) was calculated based on width at half height.47,48

linked polystyrene column according to the procedure described in Section 2.3.35 Figure 1 compares the retention characteristics of model aromatic compounds on the two columns using a hexane mobile phase. In both cases, the elution order is saturates, monoaromatics, and then polyaromatics. However, the aromatics are more strongly retained on the hyper-cross-linked polystyrene. The retention factors (k′) of the model compounds range between 0.3−2.9 on the aminocyano column and 0.2−8 on the hyper-cross-linked polystyrene column. Hyper-cross-linked polystyrene shows greater selectivity (α) for aromatics such as o-xylene (monoaromatic), naphthalene (diaromatic), and dibenzothiophene (sulfurcontaining diaromatic) yielding α = 3.3−6.6 compared to α = 1.8−2.2 on the silica-based amino-cyano column (Table 1). Gas Table 1. Elution of Model Compounds on a Silica-Based Amino-Cyanoa and a Hyper-Cross-Linked Polystyreneb Column Using a Hexane Mobile Phase retention time (min)

3. RESULTS AND DISCUSSIONS 3.1. Comparison of a Silica-Based Amino-Cyano and Hyper-Cross-Linked Polystyrene Columns. Often group type separation of crude oil, heavy fractions, and residues by HPLC uses bonded phase columns or a combination of silica and bonded phase columns, and an alkane as mobile phase.11,29,31,49,50 Typically two columns are used to separate saturates from aromatics, such as amino, cyano, or silica columns. Then the mobile phase flow is reversed so that the late eluting components elute out as one peak and are “backflushed” in order to speed up the analysis.11,51 Previous chromatographic separations of gas oil resins by normal and reversed phase liquid chromatography used a silica-based amino-cyano column.6 In this study, a comparison was made between a silica-based amino-cyano column with a hyper-cross-

relative standard deviation (%)

compound

a

b

a

b

cyclohexane o-xylene naphthalene 9,10-dihydroanthracene dibenzothiophene anthracene 9-methylanthracene phenanthrene

3.80 4.85 6.19 8.74 9.72 10.16 11.45 11.64

3.59 6.93 15.76 28.73 47.21 49.22 57.00 40.68

0.1 0.4 5.2 7.7 3.2 8.2 1.2 1.7

0.04 0.07 1.2 0.3 1.4 0.4 0.4 0.3

selecitivity (α) a

b

2.0c 1.8d

6.5c 3.3d

2.2e

3.6e

a 4.6 mm ID × 250 mm amino-cyano column, 1 mL/min, 35 °C, 10 μL injection, refractive index detector. bSame as (a) except with a hypercross-linked polystyrene column and 50 μL injection. cSelectivity between cyclohexane and o-xylene. dSelectivity between o-xylene and naphthalene. eSelectivity between naphthalene and dibenzothiophene.

6688

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Energy & Fuels oil resins contain polycyclic aromatic hydrocarbons with up to five rings to be separated.1,8 The polycylic aromatic hydrocarbons are more strongly retained on the hyper-cross-linked polystyrene column. The greater selectivity of the hyper-crosslinked polystyrene column for aromatics will be advantageous for resolving the different aromatic compounds in gas oil resins.1,8 Using a hexane mobile phase (Section 2.3), the relative standard deviation (RSD) of retention times varied up to 8.2% for four injections over 2 days on the amino-cyano column, while less than 1.4% on the hyper-cross-linked polystyrene column (Table 1). The larger retention time deviation observed using the silica-based amino-cyano column may be due to trace water in the mobile phase. Silica-based stationary phase is deactivated by small amounts of water, and so typically it is critical to control water content.52−56 The use of very dry solvents, such as dehydrated organic solvents kept dry with molecular sieves, is recommended especially for silica based columns.57 Conversely, the hyper-cross-linked polystyrene column, was not as sensitive to fluctuating atmospheric water content. Using minimal solvent preparation and equilibration time (Section 2.3), the hyper-cross-linked polystyrene column demonstrated excellent retention time reproducibility. Over a period of 6 months (15 injections), the observed RSD for this mobile phase and column was 2.9%. This is comparable to the 2−3% observed for a silica column and a bonded nitrile column over 10 months using dehydrated organic solvents kept dry with molecular sieves and with an equilibration of 20 times the column volume.57 Aromatic model compounds exhibited larger retention (k′) and gave improved reproducibility of retention times on the hyper-cross-linked polystyrene. Therefore, this column seems promising for separating the components of gas oil resins. However, the efficiency using hexane mobile phase was low (N = 410−910) and retention factor could be improved (0.5 ≤ k′ ≤ 20)  so other mobile phases were explored (Section 2.4). 3.2. Mobile Phase for Model Compounds on a HyperCross-Linked Polystyrene. The largest linear aromatic compound expected in gas oil resins is pentacene was anticipated to elute at 490 min using hexane as eluent, based on the retention times for naphthalene and anthracene (Table 1).1 Further, as noted above, the efficiencies were low using hexane alone. An 80:20 hexane:chloroform blend (volume %) had been previously used on the hyper-cross-linked polystyrene column in quasi-normal phase mode and so was investigated herein.35,40,41 Table 2 and Figure 2 present the elution order of mono- to penta-aromatic linear, biphenyl linked-, fused angled-, and dihydro-aromatics as well as sulfur- and nitrogencontaining model compounds. It was observed that the analysis time for pentacene decreased to 76.1 min (Table 2) using 80:20 hexane:chloroform blend (volume %) compared to the expected 490 min in hexane. In addition, the efficiencies of the aromatic model compounds increased from 410−910 to 1170−2340. The retention of aromatic compounds increased with the number of rings, which is consistent with the greater polarizability of the πelectrons in larger rings systems.35,40,41 The resolution was >5 between each of the linear aromatic compounds. For compounds with the same number of rings, biphenyl linkedor fused angled-aromatics eluted first, followed by lineararomatics and finally dihydro-aromatics. Feedstocks, depending on their source and processing history, can contain 0.05−14 wt % sulfur and ppm to 2 wt %

Table 2. Elution of Aromatic Model Compounds on a Hyper-Cross-Linked Polystyrene Column with 80:20 Hexane:Chloroform Mobile Phasea hydrocarbons

hydrocarbon group-type

retention time (min)

benzene tetralin biphenyl naphthalene 9,10dihydroanthracene p-terphenyl phenanthrene anthracene 5,12-dihydrotetracene pyrene p-quaterphenyl

linear monoaromatic dihydro monoaromatic biphenyl linked diaromatic linear diaromatic dihydro diaromatic

4.2 4.5 6.7 6.7 7.8

biphenyl linked triaromatic fused angled triaromatic linear triaromatic dihydro triaromatic fused angled tetra-aromatic biphenyl linked tetraaromatic fused angled tetra-aromatic linear tetra-aromatic fused angled pentaaromatic linear penta-aromatic

13.1 13.4 13.4 17.2 19.9 24.7

chrysene tetracene benzo(e)pyrene pentacene

Rs b

5.9

6.7

30.1 32.9 50.2

7.3

76.1

6.8

a

Conditions: as in footnote b of Table 1 except 80:20 hexane:chloroform (volume %) mobile phase, 10 μL injection, and photodiode array detector at 254 nm. bResolution is calculated using width at half height and is listed for each compound relative to the next lower molecular weight linear aromatic (5.9 between benzene and naphthalene).

Figure 2. Chromatogram of sulfur- and nitrogen-containing aromatic model compounds on hyper-cross-linked polystyrene. Conditions: as in footnote b of Table 1 except with 80:20 hexane:chloroform blend (volume %) mobile phase, 10 μL injection, and photodiode array detections at 254 nm.

nitrogen and are, therefore, of interest here.1,11 Sulfur and nitrogen introduces a permanent dipole moment, allowing interaction with the stationary phase through additional Debye induced-dipole forces. Figure 2 shows the separation of sulfurand nitrogen-containing model compounds. Phenylsulfide eluted first, followed by phenyl disulfide, dibenzothiophene, and then 2-naphthalenethiol. As the number of heteroatoms increased (from sulfide to disulfide), so did retention. Aromatic sulfur embedded within a ring (i.e., dibenzothiophene) exhibited greater retention than the two sulfides. The thiol (−SH) functionality in 2-naphthalenethiol results in hydrogen bonding with the stationary phase and thus strong retention. 6689

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Energy & Fuels The nitrogen-containing aromatic compounds are retained more strongly than the sulfur compounds. Elution order of the nitrogen-containing diaromatic compounds apparently based on decreasing acidity, with 2-aminonaphthalene (an amine, pKa = 4.15) and quinolone (a pyridine, pKa = 4.92), coeluting and then carbazole (a pyrrole, pKa = 19.9).58,59 Particularly interesting in Figure 2, is the latest eluting sulfur compound (2-naphthalenethiol) was resolved from the earliest eluting nitrogen compounds (quinolone and 2-aminonaphthalene). Similar results were observed in a comparison of hyper-crosslinked polystyrene and silica supported hyper-cross-linked polystyrene.41 However, using dichloromethane41 the pyridine is irreversibility retained; while here using chloroform, quinolone (pyridine) elutes before carbazole (pyrole).40 Further, hyper-cross-linked polystyrene in quasinormal phase mode (Table 2) eluted 5,12-dihydrotetracene (dihydro triaromatic) before pyrene (fused angled tetra-aromatic) which is advantageous over columns such as zirconia, titania, silica, and carbon columns for which no column completely separated all of the triaromatic compounds from the tetraaromatic compounds.50 Overall, herein sulfur-containing model compounds eluted first and were resolved from all the nitrogen-containing model compounds. Given the resolution and stability achieved with the aromatic model compounds, separations of gas oil resins were explored using an 80:20 hexane:chloroform blend (volume %) mobile phase (Section 3.4). 3.3. Characterization of Gas Oil. Bitumen and heavy oils are referenced by their place of origin in this paper. The four oil samples used here are from different geographical locations, including Alberta, Canada (AB1 and AB2), South America (SA), and the United States (US). The high temperature simulated distillation (HTSD) results for gas oil (Table 3a) indicate that the laboratory-scale distillation did not give sharp boiling point cuts for the 204−524 °C distillate fraction, which can be expected with the one theoretical plate distillation method (Section 2.2). The saturates, aromatics, and resins contents of the four gas oils are shown in Table 3b; of which 3.3−8.6 wt % are underutilized resins.7,12 The increase in initial boiling point (IBP) and weight % > 524 °C from gas oil to gas oil resins (Table 3a and c) indicate that the majority of the lighter eluting components have been distributed in the saturate and aromatic fractions. The resins containing up to 44 wt % boiling >524 °C would be difficult to analyze by GC−MS and up to 17 wt % boiling >750 °C would pose challenges to characterizing by chromatographic methods.8 Table 3d lists the elemental composition of the different gas oil resins, in which hydrogen to carbon ratio range from 1.0−1.2; US gas oil resin contains the most nitrogen whereas AB1 and AB2 gas oil resins contain the most sulfur. 3.4. Group Type Separation of Gas Oil Resins. Our objective is to develop an analytical method that provides chromatographic separation for gas oil resins samples in the shortest analysis time using a simple chromatographic setup (Section 3.1: one column and no backflushing). Resins contain components with a wide polarity range and based on the model compounds (Section 2.4 and Section 3.2: pentacene, 76.1 min and k′ > 10) the separation can be improved with a gradient elution.46 Section 3.2 demonstrated that an initial 80:20 hexane:chloroform blend (volume %) mobile phase gave excellent resolution (Rs > 5) for linear aromatic compounds ranging from benzene to pentacene. Considering gas oil resins were prepared from preparative chromatography using 50:50

Table 3. (a) Boiling Point and Weight Percent Composition of Gas Oil; Boiling Point Range and Elemental Composition of Gas Oil Resins (a) boiling point range of gas oil origin

IBP (°C)

United States South America Alberta 1 Alberta 2

saturates (weight %)

origin United States South America Alberta 1 Alberta 2 origin

United States South America Alberta 1 Alberta 2

weight % > 750 °C

aromatics (weight %)

44.7 56.0 41.3 43.0 (c) boiling point range of IBP (°C)

United States South America Alberta 1 Alberta 2

origin

weight % > 524 °C

187.6 3 205.6 7 200.4 4 187.6 3 (b) weight percent composition of gas oil

46.7 40.0 53.4 53.7 gas oil resins

weight % > 524 °C

< < <
750 °C

304.2 26 304.2 44 293.6 38 297.6 41 (d) elemental composition of gas oil resins

4 8 14 17

carbon (weight %)

hydrogen (weight %)

nitrogen (weight %)

sulfur (weight %)

87.10

9.88

1.99

1.03

88.36

8.56

1.03

2.05

86.00 85.30

8.66 9.70

0.91 1.14

4.44 3.87

toluene:acetone blend (volume %) (Section 2.2), the final solvent would probably be an alcohol.11 Hence, an initial 80:20 hexane:chloroform blend (volume %), transitioning to 100% chloroform was selected. To improve sample recovery, tetrahydrofuran and methanol were added into the gradient method.11,31,60 Further optimization of conditions (Section 2.3: 50 °C, 1.5 mL/min, 30 min), yielded seven regions in 35 min for all gas oil resins tested, which is comparable to the regions in other chromatographic runs and systems.31,60 Figure 3 shows the results of the four-solvent-gradient method (Section 2.3) for the four different gas oil resins. A mass balance was performed for each of the gas oil resins (total peak area compared for injections performed with and without the column). Recoveries were 95−105%, which is comparable to results presented in literature.29,31,33,34 The main advantage of the separation method presented herein is the use of only one column with no backflushing. Figure 4 compares the chromatograms of a 20 μL chloroform blank injection to the US gas oil resin sample (Section 2.3: 50 °C, 1.5 mL/min, 30 min, 20 μL injection). The blank injection had a nonflat baseline, which can occur in a gradient elution.46 To minimize the baseline drift effect, the elution was monitored at a longer wavelength of 300 nm.46 In addition, the column pressure and the wavelength absorbance (UV 210−410 nm, Section 2.3) were monitored for signs of impure mobile phase and contaminants being mistaken for sample peaks.61 The pressure and UV absorbance confirms an aromatic at each of the gas oil resins’ seven regions but no aromatic component from the blank injection. The features observed in the blank injection could be carryover from a previous injection or some 6690

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Table 4. Model Compounds Eluting in Each Gas Oil Resins Retention Time Windowa

Figure 3. Gas oil resins chromatogram from different geographic regions normalized to sample weight: (US = United States, SA = South America, AB1 and AB2= Alberta, Canada). Conditions: hypercross-linked polystyrene with linear gradients of: 80:20 hexane:chloroform blend (volume %) to 100% chloroform over 10 min, 100% chloroform to 100% tetrahydrofuran by 20 min, 100% tetrahydrofuran to 100% methanol by 30 min; 50 °C; 1.5 mL/min; 20 μL; and 300 nm. See Table 4 for related model compounds corresponding to regions A−G.

region

retention time window (min)

A B C

0−2.1 2.1−3.1 3.1−7

D

7−10

E

10−18.5

F G

18.5−28 28−40

model compound

retention time (min)

boiling point (°C)b

none found 1-phenyloctane naphthalene phenylsulfide phenyldisulfide dibenzothiophene anthracene 2-naphthalenethiol tetracene quinoline 2-aminonaphthalene carbazole pentacene acridine acridine orange

2.2 4.0 4.6 5.5 7.4 8.0 11.2 12.0 12.6 13.8 15.1 16.0 19.5 32.1

264 218 296 U 332 340 306 U 238 306 355 525 346 U

Conditions: 4.6 mm ID × 250 mm column 5 μm hyper-cross-linked polystyrene stationary phase, gradient 80:20 hexane:chloroform blend (volume %) linearly changing to 100% chloroform by 10 min, 100% chloroform to 100% tetrahydrofuran by 20 min, 100% tetrahydrofuran to 100% methanol by 30 min; 50 °C, 1.5 mL/min, 20 μL injection, absorbance at 300 nm. bU: unspecified. a

polystyrene stationary phase and the four-solvent-gradient method the linear aromatics are separated from each other (naphthalene, tetracene, and pentacene) whereas sulfur- and nitrogen-containing model compounds have longer retention times than their direct linear aromatic counterparts as exemplified by naphthalene that is separate from 2naphthalenethiol and 2-aminonaphthalene. Given the molecular complexity of petroleum products,1 each region in gas oil resins consists of multiple compounds (Table 4 and Figure 3). Given the physical size of the aromatic ring system and abundance of these large aromatic rings in resins the larger the size, the lower the transition energy300 nm was needed to be able to detect UV absorptivity.7 For such complex samples, group type separation is the objective rather than trying to separate each compound.29,31,33,34 Region A is essentially the dead time and so is comprised of unretained components. The region B retention time window encompasses the retention time of 1-phenyloctane. Region C covers naphthalene, phenyl sulfide, and phenyldisulfide. Thus, region C may be considered to represent diaromatics and sulfide compounds. However, the gas oil resins were derived after performing ASTM D2007. So regions A, B, and C will not be comprised of only the simple hydrocarbon types indicated by the model compounds but will include more complex species.30 The region D retention time window encompasses the retention time of dibenzothiophene and anthracene, suggesting that sulfur-containing diaromatic and triaromatics elute in this region. Region E covers 2-naphthalenethiol, tetracene, quinolone, 2-aminonaphthalene, carbazole, and pentacene. In terms of boiling point and size, pentacene is expected to be the largest linear aromatic compound in gas oil resins.1 Pentacene on a hyper-cross-linked polystyrene stationary phase eluted at 490 min in hexane mobile phase, 76.1 min in 80:20 hexane:chloroform blend (volume %), and 16.0 min in the four-solvent-gradient method. Overall the peak sharpens and elution time decreases with increasing mobile phase polarity

Figure 4. Raw chromatogram of blank (chloroform) and United States (US) gas oil resin. Conditions: hyper-cross-linked polystyrene with linear gradients of: 80:20 hexane:chloroform blend (volume %) to 100% chloroform over 10 min, 100% chloroform to 100% tetrahydrofuran by 20 min, 100% tetrahydrofuran to 100% methanol by 30 min; 50 °C; 1.5 mL/min; 20 μL; and 300 nm.

other contaminant. This can be a problem if it precludes accurate integration of the chromatogram as identified by Snyder et al.;46 therefore, the recoveries were calculated above (95−105%). If it is an impurity such as contaminated water, it would have minimal effect on hyper-cross-linked polystyrene as discussed in Section 3.1. Overall baseline drift is minor and does not hinder results. Model compounds were used to both qualify and optimize the separation achieved (Table 4). A spectrum purity check was performed to ensure the chromatographic peak is from the model compound (Section 2.3). Using hyper-cross-linked 6691

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validation procedures and comparisons made with model compounds demonstrate the method applicability for the purpose of chromatographic separation of gas oil resins.

and is expedited with a gradient method as can be expected in a normal phase chromatography.46 Region E encompasses fourand five-ring aromatics, thiols, and nitrogen diaromatics. Based on observed data, the general elution order in the foursolvent-gradient method is (Table 4): linear aromatics, sulfurcontaining aromatics, then nitrogen-containing aromatics. For example, the elution order of: naphthalene (region C 4.0 min), 2-naphthalenethiol (region E 11.2 min), and 2-aminonaphthalene (region E 13.8 min). Looking at Figure 3, it appears that there are later eluting components after pentacene. Upon looking for sulfur- or nitrogen-containing model compounds that elute in the vicinity of these last two regions, it was found that acridine (C13N tricycle)62 elutes in region F and acridine orange (C10N3 tricycle)62 elutes in region G. Overall, based on model compounds, separation was obtained between aromatics, and sulfur and nitrogen-containing compounds in gas oil resins. Due to the complexity of petroleum crude oil, preparative separations are almost always required.1,11,29 Compared with open column preparative techniques, HPLC is faster, more reproducible, and more readily automated.63 Tools such as HPLC are valuable as they can quickly confirm or rule out the presence of select compounds. Herein, separation on hypercross-linked polystyrene provides valuable group-wise separations of the gas oil resins. In comparison to the literature,29,31,33,34 which advocates using multiple columns, gradients, backflushing, and valves, the method presented herein is much simpler, requiring only one column and no backflushing and yet retains the precision and accuracy of the former. Figure 3 shows the chromatograms for all four gas oil resins samples normalized to their sample concentration. All four gas oil resins display the same characteristic regions, but the relative intensity of each region varies for the various gas oil resins. For example, for regions C and D, AB1 gives the most intense signal, followed by SA, AB2, and US (Figure 3). Interestingly, regions C and D encompass sulfide and sulfur-containing diaromatic compounds (Table 4) and AB1 has the most abundant elemental sulfur (Table 3d). But beyond this, it is impractical to make any additional generalizations. Elemental analysis (Table 3d) and 13C NMR spectroscopy data were based on the gas oil resins as a whole.43−45,64,65 The comparison between the gas oil resins’ UV absorptivity and their total sulfur, total nitrogen, and NMR spectroscopy aromatic carbon contents were attempted but no correlations were observed. The UV absorptivity of the gas oil resins are reported as atomic units per part-per-million (au ppm−1) because unlike model compounds, the molecular weights of components are unknown. Given the fact that UV absorbance depends upon the molar absorptivities and quantities of molecular species, and that molar absorptivity of species varies depending on aromatic carbon, nitrogen, and sulfur type, it would be fortuitous if molar absorptivity of the gas oil resins regions could be correlated with the total contents of aromatic, carbon, sulfur, and nitrogen. Instead, the potential for preparative HPLC fraction collection coupled with analysis by nuclear magnetic resonance spectroscopy, and comprehensive two-dimensional gas chromatography field ionization mass spectroscopy, would improve our knowledge about the seven chromatographic regions presented in this paper. In addition, retention behavior of compounds containing multiple heteroatoms (i.e., both sulfur and nitrogen) and molecular size variations have yet to be investigated. With the information presented in this paper, the

4. CONCLUSIONS Due to the compositional complexity of heavy oils, crude, and bitumen, preparative separations are almost always required. Gas oil resin characterization is challenging as it contains large aromatic (up to five rings), sulfur, and nitrogen, which leads to poorly resolved chromatograms and assignments due to myriad possible isomeric forms. Group type separation of gas oil resins into seven chromatographic regions in 35 min with quantitative recovery was achieved using a high performance liquid chromatography method comprising of a hyper-cross-linked polystyrene column and four-solvent-gradient method (hexane, chloroform, tetrahydrofuran, and methanol). The hyper-crosslinked polystyrene stationary phase yielded highly reproducible retention (RSD 2.9% over 6 months). On the basis of model compounds, separation was obtained between aromatics, sulfurcontaining, and nitrogen-containing components in gas oil resins. The advanced characterization of resins demonstrated herein is expected to assist in improving refining process by providing new information for the chemical composition of this complex fraction.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 780-987-8642. Fax: 780-987-5349. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Natural Resources Canada, the Natural Sciences and Engineering Research Council of Canada (NSERC), and the University of Alberta. Partial funding for this project was provided by government of Canada’s interdepartmental Program of Energy Research and Development (PERD) and the Alberta Innovates-Energy and Environment Solutions. The hyper-cross-linked polystyrene was provided by Purolite. P.H.A. would like to thank Dr. Richard E. Paproski for valuable conversations over the years and Dr. Rafal Gieleciak and Dr. Edward Little for comments and suggestions on revising the manuscript.

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REFERENCES

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