Energy & Fuels 1995,9, 90-96
90
High Performance Liquid Chromatography (HPLC) of Coal Liquefaction Process Streams Using Normal-Phase Separation with Diode Array Detection Daniel E. McKinney,?David J. Clifford,? Lei Hou,? Michael R. Bogdan,* and Patrick G. Hatcher”?? Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, and Sci Tech, Inc., 200 Innovation Blvd., State College, Pennsylvania 16803 Received June 30, 1994@
A method is described for a normal-phase separation and quantification of polynuclear aromatic hydrocarbons (PAHs) using high performance liquid chromatography (HPLC) coupled with Wdiode array detection. The method employed takes advantage of the inherent capabilities of a “charge-transfer” normal-phase HPLC column (Holstein, W. Chromatographia 1981, 14, 4681, the Hypersil PAH-2 column (Keystone Scientific, Inc., Bellefonte, PA), and the ability of the diode array detector to provide UV spectra for each eluting compound having chromophores in the W range which allows for separation, identification, and quantification in the normal phase of individual PAHs. Elaborate sample preparations, which in the past have accompanied determination and separation of individual PAHs, can be avoided for the analysis of PAHs in either extracts of natural samples or complex fuel mixtures. The potential for this method was demonstrated by the separation and quantification of five well-studied coal liquefaction process stream samples. PAHs having from two rings up to nine rings, along with their isomers and alkylated derivatives, were separated and quantified using this method. Clear differences were observed among liquefaction process streams due, in large part, to differences in feed coals and process conditions.
Introduction Since its introduction as an analytical method in the late 1960s and early 19709,high performance liquid chromatography (HPLC) has become widely used in environmental studies, especially for the analysis of polycyclic aromatic hydrocarbons (PAHs). PAHs are widespread environmental contaminants and much effort has been devoted to the development of liquid chromatographic techniques which are sensitive to PAH separation. Schmit et a1.2first described the separation of PAHs by liquid chromatography using a reversephase column consisting of a chemically bonded CIS stationary phase. Since then, reverse-phase HPLC on chemically bonded CIShas become the method of choice and a review by Wise et for the separation of PAHs,~-~ aL7 has described general protocols for the separation of PAHs using reverse-phase HPLC. These general procedures have been applied to studies of PAHs in a variety of fuel-related materials including coal tar t The Pennsylvania State University.
t. Sci Tech, Inc.
Abstract published in Advance ACS Abstracts, November 1,1994. (1)Holstein, W. Chromatographia 1981, 14, 468. (2)Schmit, J. A.; Henry, R. A.; Williams, R. C.; Dieckman, J. F. J. Chromatogr. 1971, 9, 645. (3)Wise, S. A. Handbook of Polycyclic Aromatic Hydrocarbons; Marcel Dekker: New York, 1983;Vol. I, Chapter 5,p 183. (4) Wise, S. A. Handbook of Polycyclic Aromatic Hydrocarbons-Emission Sources a n d Recent Progress in Analytical Chemistry; Marcel Dekker: New York, 1985;Vol. 11, Chapter 5,p 113. (5)Fetzer, J. C. Chemical Analysis of Polycyclic Aromatic Compounds; Wiley: New York, 1989;Chapter 5, p 59. ( 6 ) Bartle, K. D.; Lee, M. L.; Wise, S. A. Chem. SOC. Rev. 1981, IO, 113. (7)Wise, S. A.; Sander, L. C.; May, W. E. J.Chromatogr. 1993,642, 329.
mixtures,8 petroleum crude and shale oil.l0 However, in many of these studies, direct analysis of PAHs in fuels using reverse-phase HPLC is complicated as sample preparations have become elaborate, due in large part to the fact that most complex fuel-related materials contain compounds that are not usually miscible in acetonitrile, the solvent of choice for reversephase HPLC separations of PAHs. With this problem in mind, a variety of techniques have been introduced for the separation of PAHs in fossil fuel related materials using HPLC. One method that has become popular in the separation of PAHs is the use of multidimensional liquid chromatography. In general, the material to be analyzed is first subjected to normal-phase HPLC, followed by fraction collection, and further chromatography on a reverse-phase column or on-line coupling of the normal-phase HPLC system to a reverse-phase HPLC system where the normal-phase HPLC provides only marginal separation of the The intricacies of this method can become laborious and great care must be taken with the PAH fraction in order t o remain quantitative with the PAH concentrations. Another method involves fractionation of the starting (8) Wise, S. A.; Benner, B. A.; Byrd, G. D.; Cheder, S. N.; Rebbert, R. E.; Schantz, M. M. Anal. Chem. 1988, 60, 887. (9)Kline, W. F.; Wise, S. A,; May, W. E. J. Chromatogr. 1985, 8,
223. (lO)Hertz, H. S.; Brown, J. M.; Cheder, S. N.; Guenther, F. R.; Hilpert, L. R.; May, W. E.; Parris, R. M.; Wise, S. A. Anal. Chem. 1980, 52, 1650.
(11)Sonnefeld, W. J.;Zoller, W. H.; May, W. E.; Wise, S. A. Anal. Chem. 1982, 54, 723. (12)Palmentier, J.-P. F.; Britten, A. J.; Charbonneau, G. M.; Karasek, F. W. J. Chromatogr. 1989,469, 241. (13)Packham, A. J.;Fielden, P. R. J. Chromatogr. 1991,552, 575.
0887-0624/95/2509-0090$09.00/00 1995 American Chemical Society
HPLC of Coal Liquefaction Process Streams material into general compound class categories14-16 (i.e., aliphatics, polar compounds, aromatics, resins, and asphaltenes) before separation of PAHs can be performed. In this technique, normal-phase HPLC is usually relegated t o general cleanup and isolation of the total PAH fraction making analysis time of the PAH fraction extensive. Separation of PAH’s by use of normal-phase chromatography is also possible. Under such conditions, solvent solubility would not be as problematical as it is with reverse-phase separations. Holstein1 demonstrated that the TCPP-modified silica separated aromatic compounds with respect to the number of fused aromatic rings, but separations were independent of the degree of alkylation. TCPP-modified silica belongs to the family of n-acidic phases which separate n-electronrich PAHs and have been used for some time.lJ7-19Even recently, Herren et a1.20 have reported the use of TCPPmodified silica in the separation of higher fullerenes. This stationary phase reacts strongly with electron-rich species by a mechanism which involves the delocalization of electrons between the support material and the analyte. The analyte and support material form a reversible complex by electron transfer, which result from a n-n interaction mechanism inherent in polynuclear aromatic species. The extent t o which this donor (analyte) and acceptor (bonded phase) form a complex is dictated by solvent polarity. Holstein1 first reported the use of this type of column for the separation of PAHs in complex mixtures such as coal liquids and extracts. However, few studies have taken advantage of this technique for the analysis of PAHs in coal liquids since Holstein’s article. Another problem associated with HPLC analysis of complex fuel mixtures and other extracts of natural samples is that, in the past, normal-phase and reversephase HPLC have depended upon either fluorescence detection or monochromatic W absorbance detection. Fetzer and Biggs21have pointed out that these detection methods are too selective and many compounds go unobserved due to varying optimal wavelengths for different compounds. Developments in the past 15 years of a full spectrum W absorbance detector enable full spectrum detection of HPLC eluates. A distinct advantage of the diode array HPLC technique, which provides W spectra of separated fractions as a function of time, is its ability to identify, by spectral comparisons, the molecular components of the eluates, including the isomers of PAHs. Sullivan et ~ 1 and. B o~ d u~~ z y n s k i ~ ~ utilized the diode array detector to investigate hydrotreating and hydrocracking reactions and heavy petroleums, respectively, but their analyses of the PAH fraction were only devoted to the mapping of the number of fused aromatic rings present in the samples and not individual PAHs. (14) Satou, M.; Nemoto, H.; Yokoyama, S.; Sanada, Y. Energy Fuels 1991,5,632. (15) Haas, J. W.; Uden, P. C. J. Chromatogr. 1991,550, 609. (16)Akhlaq, M. S. J. Chromatogr. 1993,644,253. (17) Nondek, L.; Malek, J. J . Chromatogr. 1978,155, 187. (18) Lochmuller, C. H. in Silylated Surfaces; Leyden, D. E., Collins, W., Eds.; Gordon and Breach; New York, 1980; p 231. (19) Grizzle, P. L.; Thomson, J. S. Anal. Lett. 1987,20,1171. (20) Herren, D.; Thilgen, C.; Calzaferri, G.; Diederich, F. J . Chromatogr. 1993,644,188. (21) Fetzer, J. C.; Biggs, W. R. J. Chromatogr. 1993,642,319. (22) Sullivan, R. F.; Boduszynski, M. M.; Fetzer, J. C. Energy Fuels 1989,3, 603. (23) Boduszynski, M. M. Energy Fuels 1988,2,597.
Energy & Fuels, Vol. 9, No. I , 1995 91
In this paper, we provide a solution for the two major problems associated with PAH detection by demonstrating a first step method for the efficient separation, identification, and quantification of multi-ring PAHs, their isomers, and alkylated derivatives without elaborate sample preparations or on-line coupling of a reverse-phase HPLC system. This procedure is accomplished by combining two well-documented techniques for PAH separation and detection. The first technique involves the use of a “charge-transfer” stationary phase, which allows for separation of n-electronrich PAHs and their isomers1 to be possible in the normal phase using solvents which are generally miscible with complex organic systems. The column used in this study was a tetrachlorophthalimidopropyl(TCPP)modified silica normal-phase column which was first reported by Holstein1 to have separated 60 different PAHs, including heterocyclic PAHs, and PAHs in a variety of coal liquids in the normal-phase. The other technique utilized is the UV-diode array detector method described by Fetzer and Biggsa21 This method, as mentioned above, provides W spectral data as a function of time of the eluting compounds allowing for better characterization of organic materials subjected to HPLC analysis.
Experimental Section Instrumentation. The dilute process stream samples were analyzed and separated into multiple fractions of selected peaks using a Waters 600E HPLC and Waters 991 photodiode array detector operated at a spectral range of 200-400 nm. The column used for HPLC separations was a Hypersil Green PAH-2 column purchased from Keystone Scientific, Inc. (Bellefonte, PA). Mass spectral data were collected for each fraction using the solids injection probe of a Kratos MS-80 double-focusing high-resolution mass spectrometer. The ionization mode on the mass spectrometer was electron impact (EI, 70 eV). Instrument control and data collection were accomplished by using a computer-aided Data General DS90 software system. Sample Description. A standard consisting of a mix of 16 PAHs was obtained from Supelco, Inc., Bellefonte, PA. The coal-derived liquids used for two-dimensional, normal-phase HPLC separation, were supplied by CONSOL, Inc. and consisted of five liquefaction process streams representing different liquefaction systems, different feed coals, and different degrees of catalytic activity (Table 1). Sample 1was obtained from Hydrocarbon Research Incorporated‘s (HRI) catalytic, two-stage, ebullated bed, bench scale liquefaction unit (CTSL) using a feed coal, subbituminous in rank, obtained from the Wyodak and Anderson seam, Black Thunder Mine (runno. CC-15, period 11)where a predispersed (impregnated) hydrated iron oxide catalyst was employed. The actual sample was a pressure-filtered liquid (PFL) obtained from filtering the atmospheric-still bottoms through a pressure filter and was the major second-stage product and the major component of the slurry oil. Sample 2 was obtained from HRI’s CTSL using Burning Star 2 coal (Illinois No. 6 seam) as feed (run no. CC-16). The run was 13 days in length and this sample was obtained on day 4 and is a pressure-filtered liquid obtained in similar fashion t o sample 1. This liquefaction run was designed to test Amoco EXP-AO-60 catalyst and validate it as a commercially available catalyst for use in CTSL direct liquefaction. Sample 3 was obtained from the same process stream as sample 2 but was obtained from day 13 and is also a pressurefiltered liquid. Sample 4 was obtained from the Wilsonville facility (run 257)performed with Illinois No. 6 coal in the catalytidcatalytic
McKinney et al.
92 Energy &Fuels, Vol. 9, No. 1, 1995
Table 1. General Information Including Percent Solubilities for Samples 1-6 sample no. sample designationa sourceb run no. and period feed coal comments percent solubility (%) HRI CC-15 Wyodak and Anderson filtered process stream 91.7 1 PFL 11
2
PFL
HRI
CC-16
Illinois No. 6
filtered process stream
92.9
Illinois No. 6
filtered process stream
92.6
Illinois No. 6
heavy distillate
96.8
Wyodak and Anderson heavy distillate
97.6
4
3
PFL
HRI
4
V-1067 Dist.
W
5
V-1067 Dist.
W
CC-16 13 257 composite 262 composite
a PFL = pressure-filtered liquid; V-1067 Dist. = 850 O F - distillate Inc.; w = Wilsonville. close-coupled integrated two-stage liquefaction (CC-ITSL) mode using 1/12-in. Amocat 1C catalyst. The actual sample was an 850 O F - distillate of the second-stage flashed bottoms. Sample 5 was obtained from the Wilsonville facility (run 262) operated with a feed coal, subbituminous in rank, obtained from the Wyodak and Anderson seam, Black Thunder Mine. The run used a slurry and supported molybdenum catalyst in the first and second reactor stages, respectively, in the CC-ITSL mode with ash recycle. The actual sample was an 850 O F - distillate of the second-stage flashed bottoms. Procedure. The coal liquefaction process stream samples are weighed and a 5060 hexane/dichloromethane (DCM) solution is used to solubilize them. The mixtures are then filtered through preweighed 0.2 pm filters (Supelco brand ISODISC N-32 3 mm diameter, nylon membrane, 0.2 pm pore size filters), and the filters are dried under a N2 stream at room temperature. The filters are reweighed to determine the weight of insolubles, allowing calculation of percent sample solubility (Table 1). The standard PAH mix is subjected to HPLC analysis t o derive response factors for internal standard quantitative calculations. These response factors were derived from a maxiplot of the standard. The internal standards were benzo[blfluoranthene and benzo[g,h,i]perylene, which were added t o each diluted sample at an appropriate concentration level. The former was used as the internal standard for samples 1, 2, and 3 because the compound elutes in a region containing few intense peaks in the sample eluates and appeared to be absent in the samples. For samples 4 and 5 , we used the latter internal standard because some significant peaks were observed in the elution range of benzo[b]fluoranthene and these would co-elute with the standard. Also, it appeared that benzo[g,hilperylene was absent from samples 4 and 5. The column, equilibrated with 100% hexane, is operated in the gradient elution mode. Samples are injected onto the PAH-2 column, and following an initial 10 min isocratic period, a linear gradient from 100%hexane to 100%dichloromethane is used up to 80 min followed by a final hold for 5 min. Compounds eluting from the column and detected by the diode array detector are identified in one of three ways: direct UV spectral matches t o the standard, comparison of obtained UV spectra t o the Stadtler UV spectral library, or a fraction cut of the selected peak is obtained. The fractions of selected peaks collected from the HPLC eluate are dried with N2 at room temperature and reconstituted in 25 pL of a 50150 hexane/DCM solvent mix. Samples for mass spectral analysis were prepared by placing 2 pL of the reconstituted fraction in a 2 cm long capillary tube. The capillary tube was in turn placed at the tip of the heated probe and inserted into the MS source. The temperature of the heated probe was increased from 25 to 325 "C over a period of approximately 20 min while the mass range of mlz 50-600 was scanned under electron impact conditions of 70 eV at a rate of 0.6 sldecade. Products were identified by comparison of the measured mass spectra to mass spectral libraries and other published spectra. Quantitative analyses of the HPLC data involve an internal standard method where response factors for PAHs in a
of second-stage flashed bottoms. b HRI = Hydrocarbon Research
2.0
3 1.6 a
11
r
9
1.2
8
0.0
H 1 1:
5 rings" re1 wt % of resolved PAH's
