Waxphaltene Determinator Method for Automated ... - ACS Publications

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Waxphaltene Determinator Method for Automated Precipitation and Redissolution of Wax and Asphaltene Components John F. Schabron,*,† Joseph F. Rovani, Jr.,† Mark M. Sanderson,† Jenny L. Loveridge,† Leonard Nyadong,‡ Amy M. McKenna,‡ and Alan G. Marshall‡ †

Western Research Institute, 365 North 9th Street, Laramie, Wyoming 82072, United States National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States

‡

ABSTRACT: The new Waxphaltene Determinator method is based on the on-column precipitation and redissolution separation technique developed at Western Research Institute. Although high-performance liquid chromatography (HPLC) instrumentation and detectors are used, the separation does not involve chromatographic interaction between the material being separated and the stationary phase. It is based on freezing, melting, and solubility. The Waxphaltene Determinator uses methyl ethyl ketone at −24 °C to precipitate waxes and asphaltenes. The precipitated material is redissolved in four steps using a series of three solvents of increasing polarity, at different temperatures: heptane at −24 °C, heptane at 60 °C, toluene at 30 °C, and methylene chloride/methanol (98:2, v/v) at 30 °C. This approach allows for the detection of waxy, polar, and pericondensed aromatic components in minutes.

1. INTRODUCTION 1.1. Background. Petroleum residua consist of a continuum of molecules that includes aliphatic hydrocarbons, branched and n-paraffin waxes, aromatic and naphthene structures, and associated pericondensed asphaltene complexes dispersed in a low-polarity solvent phase by intermediate polarity resins. The on-column precipitation and redissolution technique developed at Western Research Institute (WRI) provides rapid separations by use of an automated continuous flow system to both precipitate and redissolve petroleum fractions based on solubility. The precipitated material is redissolved with one or more solvents. Although high-performance liquid chromatography (HPLC) equipment is used, the separation does not involve a chromatographic separation based on adsorption. The separation is performed by use of an inert stationary phase consisting of 0.42−0.25 mm (40−60 mesh) ground polytetrafluoroethylene (PTFE) and is strictly solubility-based. Initial on-column precipitation and redissolution method development and example separations with representative materials have been described recently.1−4 One of the new methods is called the Waxphaltene Determinator. It uses methyl ethyl ketone at −24 °C to precipitate and freeze and precipitate waxes and asphaltenes together. The precipitated material is melted and redissolved in four steps by use of solvents of increasing solubility at different temperatures: heptane at −24 °C, heptane at 60 °C, toluene at 30 °C, and methylene chloride/methanol (98:2, v/v) at 30 °C. This approach allows for the separation of waxy, polar, and pericondensed aromatic components in minutes. Another related method, the Asphaltene Determinator, involves precipitation of asphaltene components from a portion of oil by use of the heptane mobile phase with a polytetrafluoroethylene (PTFE)packed column.4 The precipitated material is redissolved at 30 °C in three steps by use of solvents of increasing solubility: cyclohexane, toluene, and methylene chloride/methanol (98:2, v/v). © 2012 American Chemical Society

These methods have been optimized with rigorous daily quality control (QC) checks and are now routinely used to characterize up- and downstream oils, process oils, residua, and asphalt binders. The current work presents a detailed optimization of the Waxphaltene Determinator method to provide an automated procedure for a routine standardized separation that is now used to gain insight into the contributions of waxy subfractions in various process applications. The new standardized conditions are provided in the Experimental Section. The Waxphaltene Determinator separation is an important tool because it makes distinguishing the waxy components from the pericondensed asphaltene components of oil possible. Figure 1 shows the size-exclusion chromatography (SEC) profiles for fractions of a preparative saturates, aromatics, resins, and asphaltenes (SARA) separation of a vacuum residuum that was obtained as part of a project in 1993, in which six residua were characterized.5 The separation was conducted by use of the toluene mobile phase and styrene−divinylbenzene stationary phase. Negative signals from the baselines for the refractive index detector profiles represent aliphatic hydrocarbon or waxy materials, which have a lower refractive index than toluene. A negative dip is apparent in the profile for the asphaltenes, which represents the entrained waxy components of the asphaltenes. 1.2. Petroleum Waxes. Waxes in petroleum consist of branched and linear aliphatic molecules. They are typically classified into two major classes: paraffinic and microcrystalline. Paraffin waxes are obtained from the light lube stock distillate of crude oil and typically have a melting range of 25−75 °C.6 Microcrystalline waxes are extracted from residual lube stock of refined petroleum and have a melting range between 55 and 100 °C. Received: January 30, 2012 Revised: February 23, 2012 Published: February 27, 2012 2256

dx.doi.org/10.1021/ef300184s | Energy Fuels 2012, 26, 2256−2268

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n-alkanes from this fraction by freezing and precipitation in MEK at −20 °C. They determined that n-alkanes with less than 20 carbon atoms do not precipitate in MEK at −20 °C, whereas larger n-paraffins freeze and precipitate. The upper limit of solubility in n-heptane at room temperature was determined to be about 40 carbon atoms (n-tetracontane). Lu and Redelius12 used two wax separation methods: preparative SEC and precipitation of the waxes from a distillate oil with diethyl ether/ethanol (1:1, v/v) at −20 °C for the determination of bitumen waxes.

2. EXPERIMENTAL SECTION 2.1. Materials. Residua samples were Strategic Highway Research Program (SHRP) reference asphalts.13 Crude oil samples were from prior work at WRI.1 Solvents and chemicals used in the study were reagent-grade. Paraffin and microcrystalline waxes were provided by International Group, Inc. (IGI), Farmers Valley, PA. GC profiles to quantify branched and n-paraffin distributions up to 70 carbons were provided by IGI. All of the wax materials contained both branched and n-alkanes. When the GC profiles are extrapolated, the range of branched and normal alkanes can be estimated. A summary of the n-paraffin and branched paraffin composition of these wax samples is provided in Table 1. These materials were evaluated for solubility in chlorobenzene at room temperature at 3% (w/v). Only the microcrystalline wax 5788A was fully soluble in chlorobenzene. 2.2. Waxphaltene Determinator Separation. The Waxphaltene Determinator precipitation and redissolution experiment was conducted by use of a Waters 717 autosampler, a Waters 60F pump with a model 600 controller, and a Waters 2424 evaporative light scattering detector (ELSD). Solutions of residua, waxes, and asphaltenes in chlorobenzene were injected onto a 7 mm inner diameter × 250 mm stainless-steel column packed with 0.25−0.42 mm ground PTFE (40− 60 mesh). An optical absorbance detector cannot be used routinely with the Waxphaltene Determinator separation for waxy materials because the waxy components tend to precipitate in the optical cell, resulting in flow path plugging, once separated from the peptizing matrix oils. Three column temperature baths were used. The −24 °C bath was a Julabo model F-24 refrigerated temperature bath, and the 60 and 30 °C baths were both Lindberg Blue M model WB11 20A-1 heated baths (Figure 2). The fluid used in all baths was propylene glycol/distilled water (70:30, v/v) obtained by evaporating water from red RV antifreeze to achieve the optimal glycol/water ratio. The column was moved between temperature baths at preset times during the separations by use of a Thermo Fisher F-3 industrial-grade robot (Figure 3). The solvent flow rate was 2 mL/min with step changes between solvents. The peak area integration was performed by use of Waters Empower software. The ELSD peak area was electronically corrected for small blank peaks because of the step gradient solvent changes prior to integration. 2.3. High-Temperature GC. High-temperature GC was conducted by use of a HP 5890 Series II GC with a flame ionization detector set at 395 °C. Injections were made with a flow inversion cup injector in the split mode at 375 °C. The column was a 15 m × 0.32 mm × 0.1 μm thickness DB-1 column operated initially at 200 °C with a temperature gradient ramp up to 385 °C with helium carrier gas. Samples were made by adding 1 mL of toluene to each fraction. The solutions were heated to 90 °C with a metal block heater and shaken immediately before injection of a 1 μL portion. To provide retention time points of reference, small amounts of n-hexacontane (n-C60),

Figure 1. Waxy components of heptane asphaltenes.5

Microcrystalline and paraffin waxes both consist of mixtures of linear and branched aliphatic hydrocarbons. Petrolatum is a related material that consists of a complex mixture of hydrocarbons and various waxy materials that have a gelled appearance. Historically, it was obtained from deposits that are known to foul equipment during oil production. Classic approaches for the gravimetric determination of waxes are typically accomplished in a separation process consisting of a minimum of two steps. The initial separation consists of procedures such as deasphaltening, distilling, and conducting a silica gel or alumina chromatographic separation, urea adduction, or separation by preparative size-exclusion chromatography (SEC), to isolate a fraction enriched in n-paraffins. The waxes separated in the first step can be analyzed directly by gas chromatography [GC, e.g., American Society for Testing and Materials (ASTM) D5442] or differential scanning calorimetry (DSC) or separated further by freezing and precipitating the waxes in a polar solvent at a sub-ambient temperature. Branthaver et al.7 precipitated waxes by mixing a toluene solution of heptane maltenes from tar sand oil with methyl ethyl ketone (MEK; 2-butanone) at −20 °C. McKay et al.8 isolated waxes from neutral fractions by ionexchange chromatography of asphalts with a 2-butanone/ toluene solvent mixture at −25 °C. Nwadinigwe and Eze9 and Netzel and Rovani10 used urea adduction followed by GC to characterize materials rich in n-paraffins. The European Standard Method 12606 for isolating waxes combines an initial distillation followed by precipitation of waxes from the distillate with diethyl ether/ethanol (1:1, v/v) at −20 °C. Lu et al.11 combined an initial separation by use of rod chromatography to isolate a fraction of saturates, followed by separation of

Table 1. Aliphatic Hydrocarbon Distribution for IGI Wax Samples normal

branched

sample

carbon number

maximum

carbon number

maximum

approximate branched/normal ratio

IGI 1245A paraffin IGI 5788A microcrystalline IGI 5910A microcrystalline

19−43 20−67 20−80

29 40 50

24−44 28−75 22−110

31 45 61

1:4 2:1 2:1

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Figure 2. Waxphaltene Determinator flow schematic diagram.

Figure 3. Thermo Fisher F3 robot for moving the PTFE-packed column between temperature baths. n-tetratetracontane (n-C44), and n-octacosane (n-C28) were spiked into the solutions. 2.4. Fourier Transform Infrared (FTIR) Spectroscopy. Attenuated total reflection (ATR) FTIR spectroscopy was performed with a Perkin-Elmer Universal ATR sampling accessory by smearing a

small amount of sample directly onto the ZnSe crystal and applying the pressure plate until the force gauge reaches a maximum of 150 (unitless). A total of 32 scans were performed on each sample. 2.5.1. Mass Spectrometry (MS). Sample Preparation. Waxphaltene fractions were provided by WRI. All solvents 2258


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were reagent grade (Sigma-Aldrich Chemical Co., St. Louis, MO). Prior to mass spectrometric analysis, ∼1 mg of each fraction was diluted with toluene to make a stock solution (1 mg/mL) that was further diluted to yield a final concentration of 250 μg/mL equal parts (v/v) methanol spiked with 1% (v/v) formic acid [positive electrospray ionization (ESI)] or 1% (v/v) tetramethylammonium hydroxide (25% by weight in methanol; negative ESI) prior to MS analysis. 2.5.2. ESI Time-of-Flight (TOF) MS. ESI TOF MS was performed with a Waters LCT Premier TOF mass spectrometer equipped with a Z-spray ESI source (Waters Corporation, Milford, MA). The LCT was operated in the V mode by use of MassLynx, version 4.1, software. The electrosprayed ions were transferred from the source to the orthogonal acceleration TOF mass analyzer by use of two ion guides and a hexapole. The instrument settings were as follows: capillary voltage, ±2.8 kV; cone voltage, 60 V; source temperature, 120 °C; desolvation temperature, 350 °C; and desolvation gas flow, 500 L/h. The sample was introduced by use of a built-in syringe pump at a flow rate of 25 μL/min. 2.5.3. ESI 9.4 T Fourier Transform Ion Cyclotron Resonance (FT-ICR) MS. Sample solutions were introduced into the mass spectrometer through a microelectrospray source14 (50 μm inner diameter fused silica emitter) at 400 nL/min by a syringe pump. Typical conditions for negative-ion formation were emitter voltage, −2.5 kV; tube lens, −350 V; and heated metal capillary current, 4 A. Positive-ion formation occurred at the same conditions as for negative ions but with positive voltages. The electrosprayed ions were analyzed with a custom-built FT-ICR mass spectrometer15 equipped with a 9.4 T horizontal 220 mm bore diameter superconducting solenoid magnet operated at room temperature (Oxford Instruments, Abingdon, Oxfordshire, U.K.). A modular ICR data station (Predator) facilitated instrument control, data acquisition, and data analysis. 16 Ions generated at atmospheric pressure were accumulated in an external linear octopole ion trap for 1−3 s and transferred by radio frequency (rf)-only octopoles (2.0 MHz and 240 Vp−p amplitude) to the ICR cell. ICR time-domain transients were collected from a seven segment open cylindrical cell with capacitively coupled excitation electrodes based on the Tolmachev configuration.15,17 A total of 150 individual transients of 5.6 s each were collected for each fraction and averaged, apodized with a full Hanning weight function, and zero-filed once prior to fast Fourier transformation. ICR frequencies were converted to ion masses based on the quadrupolar trapping potential approximation.18 Each m/z spectrum was internally calibrated with respect to an abundant homologous alkylation series differing in mass by integer multiples of 14.105 65 Da (mass of a CH2 unit) confirmed by an isotopic fine structure based on the “walking” calibration equation.19 Experimentally measured masses were converted from the International Union of Pure and Applied Chemistry (IUPAC) mass scale to the Kendrick mass scale to identify the homologous series for each heteroatom class (i.e., species with the same CcHhNnOoSs content, differing only by their degree of alkylation). Peak assignments were performed by Kendrick mass defect analysis as previously described.20 For each elemental composition, CcHhNnOoSs, the heteroatom class, type [double bond equivalents (DBE) = number of rings plus double bonds involving carbon], and carbon number, c, were tabulated for subsequent generation of heteroatom class relative abundance distributions and graphical DBE versus carbon number or H/C ratio versus carbon number images.

Figure 4. Response profile for injections of whole residua and asphaltenes.

Figure 5. Separation profiles for Lloydminster vacuum residuum QC sample and Lloydminster heptane asphaltenes.

3.2. Detector Considerations. ELSD response considerations have been discussed in detail previously.4 A plot of total ELSD peak area response versus total milligrams injected, which includes Lloydminster whole residua and Lloydminster n-heptane asphaltene injections, is provided in Figure 4. The plot shows that the Waters 2424 ELSD provides a linear response in our experimental region, up to 3 mg, and that the response factors for various portions of asphaltenes and whole residua are about the same. This observation shows that, for these separations, the ELSD relative peak areas represent the approximate weight percent distribution of the material in the fractions.

3. RESULTS AND DISCUSSION 3.1. Co-solvency and Melting. In the petroleum continuum, individual chemical components including waxes and pericondensed aromatic components that are not soluble in a particular solvent can dissolve readily if they are part of a mixture with other species that impart co-solvency or so-called peptization. In addition to co-solvency, the melting and freezing of waxy components can occur. These considerations have been discussed in detail previously.1 2259


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Figure 8. Waxphaltene Determinator separation profiles for Lloydminster and West Texas intermediate residua.

Figure 6. Waxphaltene Determinator separation profiles for waxy reference materials with the optimized system.

ethylene (PTFE) between the autosampler and the robotic arm. Stainless steel was used as tubing connection to the

3.3. Optimized Waxphaltene Determinator Conditions. To minimize the surface area and adsorption effects that can occur with stainless-steel tubing, most of the longer length portion of tubing was 0.76 mm inner diameter polytetrafluoro-

Figure 7. Waxphaltene Determinator separation profiles for n-C18 and n-C28.

Figure 9. Waxphaltene Determinator separation profiles for Boscan and Wyoming Sour residua. 2260


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The separation uses the lowest initial temperature that the cold bath can achieve, near −24 °C for the initial precipitation. Portions of 20 μL are injected for the analytical-scale Waxphaltene Determinator separation. The separation conditions for the optimized system are as follows: column, 7 mm inner diameter × 250 mm stainless-steel column; packing, 40− 60 mesh ground PTFE; detector, Waters 2424 ELSD (60 °C tube, 12 °C nebulizer, 35 psi nitrogen, and gain = 1); solutions, sample and QC solutions at 10 wt %/vol or less in chlorobenzene; injection amount, 20 μL; solvents used for step gradient changes, methyl ethyl ketone, n-heptane, toluene, and methylene chloride/methanol (98:2, v/v), all at 2 mL/min; step gradient times and column temperatures, methyl ethyl ketone at 0 min and −24 °C, n-heptane at 10 min and −24 °C, n-heptane at 20 min and 60 °C, n-heptane at 30 min and 30 °C, toluene at 35 min and 30 °C, methylene chloride/methanol (98:2) at 45 min and 30 °C, methyl ethyl ketone at 55 min and 30 °C, methyl ethyl ketone at 65 min and −24 °C, and next injection at 80 min. All separation profiles are electronically blank-subtracted prior to peak integration. A 20 μL injection of 10% Lloydminster vacuum residuum is made daily as a QC check sample to ensure that there is no adsorption occurring on the column. Separation profiles for this material and asphaltenes isolated from this material are provided in Figure 5. With the Waters 2424 ELSD, all five peaks including the MEK maltenes peak are on scale and can all be integrated from the same separation. Although some of the peaks appear to be very small, the high signal-to-noise ratio allows amplification for accurate integration of peak areas. Different levels of purity or different sources of the MEK precipitating solvent can result in slightly different separation profiles and relative peak area distributions. This potential shortcoming becomes an issue if the differences are significant over time. 3.4. Separation of Standard Materials. The separation profiles of standard materials used in the optimization experiments are shown in Figures 6 and 7. Results indicate that n-C28 mostly appears as a single large peak with n-heptane at 60 °C. The total peak area suggests that all of the n-C28 material is being detected. There is partial elution in n-heptane at −24 °C, which is indicative of either the presence of branched hydrocarbons or partial solubility in n-heptane at the lower temperature. n-Octadecane (n-C18) is not detected. To check relative volatility, 0.5 g each of n-C18 and n-C28 were placed in 4 oz glass bottles, which were placed in a vacuum oven at full vacuum and heated to 200 °C, and then allowed to cool back to ambient temperature. n-C18 was completely lost, and only 48% of n-C28 was lost. For the ELSD Waxphaltene Determinator profiles in Figure 6, n-C28 has a total area consistent with the area of microcrystalline wax, Vaseline, and Lloydminster asphaltenes. Thus, there is no significant loss because of volatilization in the detector. n-Tetracosane (n-C24) however has a significantly lower total area because of volatility in the detector, and most of it elutes in n-heptane at −24 °C (Figure 7). It appears on the borderline of detection and separation of n-paraffins in n-heptane at 60 °C. n-C18 is not detected at all because of its volatility in the detector. Thus, the cutoff for n-paraffin detection under the heptane peak at 60 °C appears to be for n-paraffins larger than C24. 3.5. Example Separations. Separation profiles with integrated peak areas for eight residua materials are provided in Figures 8−11. These materials are the SHRP reference asphalts.13 The profile for the West Texas residuum clearly

Figure 10. Waxphaltene Determinator separation profiles for Cosden, Oklahoma and California Valley residua.

Figure 11. Waxphaltene Determinator separation profiles for Redwater, B.C. and California Coastal residua.

column and was in direct contact with the various temperature fluids in the baths to better withstand the severe temperature changes. 2261


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Table 2. ELSD Area Percentages for Eight Residua ELSD area percentages residuum

SHRP sample

−24 °C MEK

Lloydminster Wyoming Sour Redwater, B.C. California Coastal West Texas Sour California Valley Boscan West Texas Intermediate

AAA-1 AAB-1 AAC-1 AAD-1 AAF-1 AAG-1 AAK-1 AAM-1

79.99 78.64 83.52 77.49 82.91 93.39 76.11 54.73

−24 °C heptane

60 °C heptane

30 °C toluene

30 °C CH2Cl2/MeOH (98:2)

8.00 10.22 9.90 7.68 8.68 4.16 9.25 34.09

0.71 1.79 2.24 0.83 1.93 0.27 1.18 6.90

9.61 7.58 3.27 10.44 5.45 1.04 10.81 3.29

1.71 1.78 1.09 3.56 1.03 1.15 2.64 1.00

Figure 13. Separation profile for devolatilized Tensleep crude oil.

and solubility considerations. Table 3 presents a summary of some likely chemical identities of the various peaks illustrated in Figure 14 for a waxy West Texas Intermediate residuum. The Waxphaltene Determinator separation is not readily amenable to being scaled up into a preparative mode because of the severe temperature changes involved. However, repeat separations can be made with the analytical system and fractions collected with an automated fraction collector (bypassing the ELSD) to provide small amounts of each fraction for further characterization by various methods, including high-temperature GC, to measure the wax component. To characterize the types of waxy composition eluting in the first three peaks of the Waxphaltene Determinator separation, a series of 22 repetitive 2 mg injections with automatic fraction collection were made of a waxy West Texas Intermediate residuum on the Waxphaltene Determinator system (Figure 14). Tentative peak assignments in the figure are based on injections of standard materials in this study and in earlier work. 1 Fractions from peaks 1, 2, and 3 were analyzed by high-temperature GC according to the procedure described by Netzel and Rovani.10 Peaks 4 and 5 represent the refractory pericondensed asphaltene material, which were not injected. The amount of sample in each fraction was ∼ 20 mg for peak 1 (methyl ethyl ketone at −24 °C), ∼20 mg for peak 2 (heptane at −24 °C), and ∼3 mg for peak 3 (heptane at 60 °C), after solvent evaporation. The high-temperature GC profiles for the three peak fractions are provided in Figure 15. There was no wax hydrocarbon

Figure 12. Separation profile for devolatilized Dakota crude oil and waxy deposit from Dakota crude oil.

shows the relatively high wax content (Figure 8). The SHRP sample designations and the relative ELSD area percentages are provided in Table 2. Another potential use for the new separation method is the predictability of wax fouling of pipelines at oil production sites.1 A separation profile for devolatilized Dakota crude is provided in Figure 12. This material shows the presence of a significant wax content eluting with n-heptane at 60 °C. This oil is known to develop chronic petrolatum-type deposits in production site pipelines. A separation profile for one of the deposits is also provided in Figure 12. Some other oils in the region that do not exhibit the heptane peak at 60 °C, such as Tensleep oil, do not exhibit this type of fouling (Figure 13). 3.6. Waxphaltene Determinator Peak Characterization by GC. An exploratory study of crude oils and pipeline fouling by an early version of the Waxphaltene Determinator with the 250 × 10 mm inner diameter column is described by Goual et al.1 This includes a discussion of possible wax types 2262


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Table 3. Likely Chemical Structures Represented by Waxphaltene Determinator Peaks eluting solvent

temperature (°C)

methyl ethyl ketone

−24

heptane heptane toluene methylene chloride/methanol (98:2, v/v)

−24 60 30 30

likely chemical structures relatively low-molecular-weight hydrocarbons, polar and aromatic functional groups, and some alkyl-substituted pericondensed aromatics n-paraffins C26−C40, isoparaffins C40−C50, and highly alkyl-substituted polar and aromatic functional groups n-paraffins C30−C60+, isoparaffins C50−C60+, and highly alkyl-substituted polar and aromatic functional groups alkyl-substituted pericondensed aromatics highly pericondensed precoke aromatics

Figure 14. Waxphaltene Determinator separation profile for 2 mg of waxy West Texas Intermediate residuum.

profile apparent for the material in fraction 1. The MEK maltenes likely contain various aromatic, polar, and other materials that are soluble in MEK and do not freeze at −24 °C. The high-temperature GC profile for the peak 2 fraction shows two prominent regions of waxy hydrocarbon elution. On the basis of retention times, these correspond predominately to n-C26−n-C40 and iso-C40−iso-C60+ hydrocarbons, with some smaller peaks throughout the elution time range, which represent both iso- and n-paraffins. The separation profile for the peak 3 fraction shows two prominent elution regions, which correspond predominantly to a complex mixture of n-C30− n-C60 and iso-C50−iso-C60+ hydrocarbons, with some smaller peaks throughout the elution time range indicating the presence of iso- and n-paraffins. All three fractions had some brown color, which is likely due to the presence of highly alkyl-substituted pericondensed aromatic material. In addition to the paraffins that were identified by high-temperature GC, there are other molecules present that are not pure wax hydrocarbons. However, these exhibit freezing and solubility behavior similar to the wax components, and they should not be ignored. Also, isoparaffins include molecules with complex degrees of branching at various locations in the molecules. Components eluting above n-C60 in the range larger than C60 were not well-resolved or detected. 3.7. Waxphaltene Determinator Fraction Characterization by Infrared Spectroscopy. A series of 49 automated injections of 2 mg each of a waxy West Texas Intermediate residuum was conducted in a separate experiment, and the fractions were collected with an automated fraction collector. The approximate amounts of sample material in the fractions were ∼30 mg for peak 1 (methyl ethyl ketone at −24 °C), ∼40 mg for peak 2 (heptane at −24 °C), ∼10 mg for peak 3 (heptane at 60 °C),

Figure 15. High-temperature GC separation profiles for peaks 1, 2, and 3 materials.

∼7 mg for peak 4 (toluene at 30 °C), and ∼3 mg for peak 5 [methylene chloride/methanol (98:2, v/v) at 30 °C]. The Waxphaltene Determinator separation profile for this material was similar to the profile shown in Figure 14. Portions of the fractions were analyzed by ATR FTIR spectroscopy and highresolution FT-ICR MS (described in the following section). The FTIR spectra for the whole material and fractions 2 and 3 are provided in Figure 16. The peak 1, 4, and 5 fraction materials showed significant carbonyl content near 1700 cm−1, some hydroxyl material near 3300 cm−1, and C−O absorbance near 1240 and 1160 cm−1, which is likely due to small amounts of aldol condensation products in the MEK solvent used in the separation or some oxidation during the repetitive fraction collection procedure. The peak 2 material also shows some carbonyl, which is also likely due to the MEK (Figure 16). The spectra all show typical aliphatic C−H profiles from 3000 to 2899 cm−1, −(CH2)n− chain 2263


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frequencies at 870, 810, and 743 cm−1, the least amount of aromatic and other functional group structures of all of the fractions is found in the peak 3 material, followed by a little more in the peak 2 fraction material. The FTIR spectrum of the peak 3 material confirms that this is primarily aliphatic hydrocarbon waxy material, with very little aromatic content. 3.8. Waxphaltene Determinator Fraction Characterization by ESI TOF and Ultrahigh-Resolution FT-ICR MS. ESI is a soft ionization, semi-quantitative technique that allows for the generation of unfragmented ions for mass spectrometric analysis. With this technique, every molecular formula in a complex mixture can be identified. Negative ESI with tetramethylammonium hydroxide (TMAH) was used to ionize and characterize acidic components because it minimizes drastic differences in ionization efficiency that are otherwise observed by ammonium hydroxide (NH4OH) deprotonation, which primarily ionizes only highly polar naphthenic acids.21 Positive ESI with formic acid was used to ionize and characterize basic components. Neither of these techniques can ionize the hydrocarbon wax molecules but can nevertheless provide insight into the carbon backbone structure of the polar species in the fractions. The two asphaltene fractions 4 and 5 failed to produce a sufficient signal because of poor solubility and the tendency of asphaltenes to self-associate into stable aggregates at concentrations required for ESI MS. However, the peak 1, 2, and 3 fractions were characterized on the basis of acidic species (negative ESI) and basic species (positive ESI) for compositional differences. Solvent modification with TMAH increases the compositional coverage of acidic species to include five-membered ring nitrogen species (pyrrolic) compounds that are otherwise masked by the much higher ionization efficiency of carboxylic acids (O2) with conventional solvent modification with ammonium hydroxide. Figure 17 shows the negative-ion ESI TOF mass spectra of peaks 1, 2, and 3 fractions. Mass analysis by TOF exhibits minimal mass discrimination because all ions of essentially all mass-to-charge ratio (m/z) reach the detector to provide an accurate representation of the molecular-weight distribution of the sample. A striking feature in the TOF MS spectra is the

Figure 16. ATR FTIR spectra of whole West Texas Intermediate residuum and fractions 2 and 3.

near 1465 cm−1, and CH3 near 1450−1375 cm−1. From the CC absorbance near 1600 cm−1 and the aromatic C−H

Figure 17. Negative-ion ESI TOF mass spectra of Waxphaltene Determinator peak fractions 1, 2, and 3. 2264


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Figure 18. Negative-ion ESI TOF mass spectrum (top) versus FT-ICR mass spectrum (bottom) of Waxphaltene Determinator peak fraction 1. The corresponding zoom insets show the necessity for ultrahigh resolution provided by FT-ICR MS to resolve and identify the thousands of peaks in the spectrum.

Figure 20. Distribution of acidic N and NO heteroatom classes for Waxphaltene Determinator peak fractions 1, 2, and 3.

identify the thousands of peaks in the spectrum as demonstrated by the corresponding mass scale expanded insets across a 0.5 Da m/z range. At a mass resolving power of ∼5000, TOF MS exhibits a single unresolved peak. FT-ICR MS, at an average mass resolving power of ∼500 000, enables resolution and assignment of 41 peaks of magnitude greater than 6 times the standard deviation of the baseline noise. Similar results were obtained for the peak 2 and 3 materials (data not shown). The relative abundances for several acidic species in the three fractions are provided in Figure 19. In petroleum, nitrogencontaining compounds detected by negative-ion ESI consist of pyrroles, carbazoles, and indoles.22 Compounds with oxygen can include carboxylic functional groups. In general, ionization efficiency in negative-ion ESI increases with the relative acidity of the analytes. Only a small portion of sulfur-containing compounds are ionized by negative-ion ESI,22 including

Figure 19. Negative-ion ESI heteroatom distribution of acidic molecules for Waxphaltene Determinator peak fractions 1, 2, and 3.

difference in the m/z distributions: m/z 250−1800 for peak 1, m/z 500−2700 for peak 2, and m/z 650−2750 for peak 3. Those broad m/z distributions could represent n-alkane and branched backbone structures that melt and elute with n-heptane at 60 °C. Thus, the high-temperature GC experiments discussed earlier accessed only relatively low-molecular-weight components. Figure 18 presents the TOF and FT-ICR m/z distributions for the peak 1 material. The distributions are similar, except for a somewhat lower signal magnitude at the high mass end in the FT-ICR spectrum, presumably because of TOF mass discrimination in the transfer octopole. However FT-ICR MS provides ultrahigh resolution and mass accuracy to resolve and 2265


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Figure 21. Distribution of acidic O and O2 heteroatom classes for Waxphaltene Determinator peak fractions 1, 2, and 3. Figure 23. Positive-ion ESI heteroatom distribution of basic molecules for Waxphaltene Determinator peak fractions 1, 2, and 3.

thiophenols, benzylic thiols, and other acidic compounds containing neutral sulfur. Electrospray-ionizable hydrocarbons include species with an acidic hydrogen moiety, such as the acidic aliphatic hydrogen on the five-membered ring of indene and fluorene. Figure 19 shows that the most prominent heteroatom compositions are N1 (one nitrogen), N1O1 (one nitrogen and one oxygen), O1 (one oxygen), O2 (two oxygens), and HC. Three-dimensional abundance-contoured plots of the H/C ratio versus carbon number for the N1 and N1O1 species are shown in Figure 20. The H/C ratio and the carbon number range for the peak 2 and 3 materials are greater than those for the peak 1 material, indicating greater relative aromatic content in the peak 1 material and more aliphatic chain components in the peak 2 and 3 materials. Similar plots (Figure 21) show that the O1 and O2 classes in the peak 2 and 3 materials are highly aliphatic. For the O2 class material, the H/C ratio is near 2,

which is diagnostic of oxygenated highly aliphatic waxy material. Positive-ion ESI FT-ICR mass spectra (Figure 22) represent the basic compounds in the three fractions. As for the acidic species, the peak 1 material has a lower m/z range than the peak 2 and 3 materials. Figure 23 shows the relative abundances for several basic species in the three fractions. In petroleum, the basic nitrogen-containing compounds are primarily pyridinic. Three-dimensional isoabundance contoured plots of the H/C ratio versus carbon number for N1 and N1O1 species are shown in Figure 24. As for the acidic species, the H/C ratio and carbon number range for the peak 2 and 3 materials are greater than those for the peak 1 material, which is diagnostic of greater

Figure 22. Positive-ion ESI FT-ICR mass spectra of Waxphaltene Determinator peak fractions 1, 2, and 3 (bases). 2266


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(NSF) Division of Materials Research through DMR-06-54118 and the State of Florida. Tensleep and Dakota crude oils and residue materials were provided by Lamia Goual and Brian Towler, University of Wyoming.1 The authors acknowledge Tony Munari for preparing the figures.

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Figure 24. Distribution of basic N and NO heteroatom classes for Waxphaltene Determinator peak fractions 1, 2, and 3.

relative aromatic content in the peak 1 material and much more aliphatic chain content in the peak 2 and 3 materials.

4. CONCLUSION The Waxphaltene Determinator separation was successfully optimized by use of initial precipitation of oil in MEK at −24 °C, followed by sequential redissolution of precipitated component materials with heptane at −24 °C, heptane at 60 °C, toluene at 30 °C, and methylene chloride/ethanol (98:2, v/v) at 30 °C. This separation has the potential of rapidly measuring branched waxes, n-paraffin waxes, asphaltenes, and highly pericondensed materials with only milligram quantities of oil.

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*Telephone: 307-721-2445. Fax: 307-721-2345. E-mail: [email protected]. Notes

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agencies thereof nor any of its employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed or represents that its use would not infringe on privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this study was provided by the U.S. Department of Energy (DOE) under Cooperative Agreements DE-FC2698FT40322 and DE-FC26-08NT43293 and BP Products North America, Chevron, ConocoPhillips, ExxonMobil Research and Engineering, Shell Global Solutions, and UOP. Funding for the work performed at the National High Magnetic Field Laboratory (NHMFL) was provided by the National Science Foundation 2267


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