Energy & Fuels 2001, 15, 949-954
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Characterization of Large Nonvolatile Polyaromatic Molecules by a Combination of In-Source Pyrolysis and Field Desorption Mass Spectrometry Kuangnan Qian,* Kathleen E. Edwards, and Mike Siskin ExxonMobil Research and Engineering Co., 1545 Route 22 East, Clinton Twp., Annandale, New Jersey 08801-3059 Received January 31, 2001. Revised Manuscript Received March 30, 2001
Nonvolatile hydrocarbon systems, such as those formed via molecular weight growth reactions in refinery processes, normally have a low H/C atomic ratios (600 °C). These materials pose a great challenge to mass spectrometric analysis due to difficulties in vaporization and ionization. In this work, the combination of In-Source Pyrolysis Mass Spectrometry (ISPyMS) and Field Desorption Mass Spectrometry (FDMS) were utilized to obtain compositional information for a nonvolatile organic deposit (foulant) generated by a hydrocracking process. A mixture of polystyrene standards was also analyzed for reference purposes. FDMS yields intact molecular ions for the large polyaromatic systems (∼15 aromatic rings on average). ISPyMS, on the other hand, converts the large multi-core aromatic systems into relatively small single aromatic cores that could be analyzed in detail. In this case, 28 aromatic cores containing short alkyl chains were identified by ISPyMS. Average foulant composition was projected on the basis of IsPyMS and FDMS data. The projected H/C atomic ratio and aliphatic carbon/aromatic ratio match well with those measured by elemental analysis and solid-state 13C NMR.
Introduction Unsaturated hydrocarbons can undergo molecular weight growth reactions to form nonvolatile coke-like solids under thermal and/or catalytic conditions 1-3 posing significant threat to the operations of various refinery processes, such as hydrocracking, catalytic cracking, etc. To learn to effectively manage and prevent the formation of such deposits (foulants), detailed compositional and structural information on the deposit is often needed. Although mass spectrometry has been extensively used for characterizing complex liquid hydrocarbon mixtures, such as gasoline,4,5 diesel fuel,6,7 gas oils,8,9 and even the heavy boiling fraction of the petroleum,10-13 its application to nonvolatile hydrocar* Author to whom correspondence should be addressed. (1) Guisnet, M.; Magnoux, P. Appl. Catal. 1989, 54, 1-27. (2) Qian, K.; Tomczak, D. C.; Rakiewicz, E. F.; Harding, R. H.; Yaluris, G.; Cheng, W. C.; Zhao, X.; Peters, A. W. Energy Fuels 1997, 11, 596-601. (3) Sharma, Y. K.; Singh, I. D.; Bhatt, K. P.; Agrawal, K. M. Proc. Int. Conf. Stab. Handl. Liq. Fuels 1998, 2, 699-715. (4) Mathiesen, M. D.; Lubeck, A. J. J. Chromatogr. Sci. 1998, 36 (9), 449-456. (5) Malhotra, R.; Coggiola, M. J.; Young, S. E.; Spindt, C. A. Proc. Int. Conf. Stab. Handl. Liq. Fuels 1998, 2, 527-541. (6) Bansal, V.; Vatsala, S.; Kapur, G. S.; Sarpal, A. S.; Basu, B. Int. Symp. Fuels Lubr., Symp. Pap., 2nd 2000, 2, 659-664. (7) Hsu, Chang S.; Genowitz, M. W.; Dechert, G. J.; Abbott, Doug J.; Barbour, R. Molecular characterization of diesel fuels by modern analytical techniques. Book of Abstracts, 216th ACS National Meeting, Boston, August 23-27. 1998, PETR-001. (8) Qian, K.; Peru, D. A.; Petti, T. F.; Zhao, X.; Yaluris, G.; Harding, R. H.; Cheng, W.-C.; Rajagopalan, K. Prepr. Pap.- Am. Chem. Soc., Div. Pet. Chem. 1998, 43, 169-171. (9) Guan, S.; Marshall, A. G.; Scheppele, S. E. Anal. Chem. 1996, 68, 46-71. (10) Qian, K.; Hsu, C. S. Anal. Chem. 1992, 64, 2327-2333
bon systems, such as coke and other condensed deposits from various refinery processes, has been very limited because of difficulties in vaporization and ionization of the analyte molecules. Thermal degradation (pyrolysis) at high temperatures (normally >800 °C) can effectively convert large nonvolatile materials (e.g., polymer) into smaller volatile compounds that can be better characterized by Mass Spectrometry (MS).14-18 Pyrolysis Mass Spectrometry (PyMS) is often used in combination with other spectroscopic techniques,19-21 such as Nuclear Magnetic Resonance (NMR) spectroscopy, X-ray Photoelectron Spectroscopy (XPS), and Infrared (IR) spectroscopy, to provide information on composition and structure of (11) Hsu, C. S.; Qian, K. Energy Fuels 1993, 7, 268-272. (12) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Energy Fuels 2001, 15, 492-498. (13) Li, C. Z.; Herod, A. A.; John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Humphrey, P.; Chapman, J. R.; Rahman, M. Rapid Commun. Mass Spectrom. 1994, 8, 823-828. (14) Qian, K.; Killinger, W. Z.; Casey, M.; Nicol, G. K. Anal. Chem. 1996, 68, 1019-1027. (15) June, C. R.; Cramer, C. A. Analytical Pyrolysis; Elsevier: New York, 1977. (16) Zoller, D. L.; Johnston, M. V.; Qian, K.; Lohse, D. J. Macromolecules 2000, 33, 5388-5394. (17) Johnston, M.; Zoller, D.; Cox, F.; Qian, K. Book of Abstracts, 219th ACS National Meeting, San Francisco, CA, March 26-30, 2000, POLY-275. (18) Zoller, D. L.; Sum, S. T.; Johnston, M. V.; Hatfield, G. R.; Qian, K. Anal. Chem. 1999, 71, 866-872. (19) Pasterova, Z.; Botto, R. Z.; Artsz, P. W.; Boom, J. Carbohydr. Res. 1994, 262, 27-47. (20) Cho, W. J.; Choi, C. H.; Ha, C. S. J. Polym. Sci.-Part A: Polym. Chem. 1994, 32, 2301-2309. (21) Jamenez-Meteos, J. M.; Fierro, J. L. G. Interface Anal. 1996, 24, 223-236.
10.1021/ef010023e CCC: $20.00 © 2001 American Chemical Society Published on Web 05/15/2001
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organic solids. Pyrolysis can be conducted inside the ion source of a mass spectrometer using a heated insertion filament similar to that described for Direct Chemical Ionization (DCI) experiments.22 In-source pyrolysis MS (ISPyMS) minimizes secondary reaction and condensation of pyrolysates because of the high vacuum environment and the very short time gap between the pyrolysis and the ionization events. When combined with soft ionization techniques, such as low voltage electron ionization, ISPyMS provides carbon number distributions of aromatic hydrocarbon families and yields important information on the building blocks of the polyaromatic material. Since its introduction in late 1960s, Field Desorption Mass Spectrometry (FDMS)23 has evolved into a major analytical tool for characterizing heavy hydrocarbon systems and nonpolar polymers. Although there are significant advances in mass spectrometry ionization methods in the past three decades, such as the development of Fast Atom Bombardment (FAB)24 and MatrixAssisted Laser Desorption Ionization (MALDI),25 FDMS remained to be the choice for hydrocarbon characterization. The technique does not use any matrix (such as 2,5-dihydroxy benzoic acid in MALDI or glycerol in FAB) to promote ionization. This avoids the complication of mass spectra introduced by the matrix background. In general, FDMS is more sensitive to hydrocarbon molecules. It generates “true” molecular ion [M+] by electron removal rather than by forming pseudo molecular ions, such as protonated ions or cation adducts, and thereby simplifies mass spectral interpretation. In this work, we explored the combination of ISPyMS and FDMS to characterize a large condensed polyaromatic system generated from a hydrocracking process. ISPyMS was used to obtain detailed composition of the foulant building blocks. Field desorption mass spectrometry yields information on the size of the foulant molecules. Experimental Section The polystyrene standards used in this study were obtained from Aldrich Chemical Co. The nonvolatile deposit sample was generated from a commercial hydrocracking process. The sample was isolated from the top of the catalyst bed by sieving and density separation in 1,1,2-trichlorotrifluorethane (F ) 1.57 g/cm3). Because of its significant lower density, the organic foulant constitutes the “float” fraction in the density separation, whereas the catalyst is in the “sink” fraction. The organic solid has a “coke-like” appearance and is insoluble in toluene. The elemental analysis and solid state 13C NMR analysis of the deposit is given in Table 1. Field Desorption Mass Spectrometry (FDMS) was conducted on a VG-ZAB mass spectrometer. The polymer standards were dissolved in toluene. The foulant sample was dissolved with CS2 and placed into an ultrasonic bath for 10 min. The foulant material formed an emulsion-like solution. Approximately 1 µL of the liquid slurry was directly placed onto a FD emitter using a syringe. Molecules were ionized by an intense electric (22) Cotter, R. J. Anal. Chem. 1980, 52, 1589A-1604A. (23) Beckey, H. D. Int. J. Mass Spectrom. Ion Phys. 1969, 2, 500507. (24) Ballistrei, A.; Garozzo, D.; Giuffrida, M.; Montando, G. Anal. Chem. 1987, 59, 2024-2027. (25) Karas, M.; Buchman, D.; Hillenkamp, F. H. Anal. Chem. 1985, 57, 2935-2941
Qian et al. Table 1. Elemental (wt %) and Solid State Analysis of the Foulant Sample C H N S ash H/C (atomic ratio) CP% (paraffinic carbon %) CAr% (aromatic carbon %)
13C
NMR
94.41 3.97 0.09 0.54 0.33 0.50 7.9 92.1
field (107 V/cm) applied between carbon dendrimers on a thin filament and the source electrode. Electrons were removed from the analyte molecule via a process known as the quantum tunneling effect. The emitter was heated with a ramping current from 0 to 65 mA to assist the desorption of the foulant molecules. The extraction electrodes were heated with 1.2 A current (corresponding to about 225 °C) to avoid analyte condensation. This process generates intact foulant molecular ions with minimal fragmentation. In-Source Pyrolysis Mass Spectrometry (ISPyMS) was conducted on a VG 70 mass spectrometer. About 1 µL of the liquid slurry sample was deposited directly onto the filament of the DCI probe, and was placed into the ion source of the mass spectrometer. An electric current was gradually applied to the filament from 0 to 1200 mA at a rate of 100 mA/min. Most of the foulant pyrolyzes between 600 and 1000 mA. It is estimated that 1 mA roughly corresponds to 1 °C. The pyrolysates were immediately ionized by electron impact. The electron energy was set at 15 eV to minimize the fragmentation of the molecules.
Results and Discussion Molecular Ion Profiles by FDMS. FDMS has been widely used to characterize low molecular weight polymers and some heavy hydrocarbon systems, such as vacuum resid. FD is a soft ionization technique that generates molecular-ion-only mass spectra. The technique can provide molecular weight information for a polymer with molecular weight up to 5000 Da. To illustrate that FDMS can ionize high boiling/high molecular weight polyaromatic molecules, a polystyrene mixture (1:1 PS580 and PS2550 by weight) was analyzed. The FDMS spectrum in Figure 1a demonstrates that only molecular ions were produced, with each mass peak corresponding to one polystyrene oligomer. The composition of the polymer (repeating unit and end group) can be directly determined by FDMS. The mass difference between the two major mass peaks (104 Da) corresponds to the mass of a styrene monomer. The spectrum showed clearly two mass distributions, corresponding to PS580 and PS2550, respectively. The FDMS spectrum of the hydrocracker foulant is given in Figure 1b. The molecular weight of the foulant molecules ranges from 200 to 2000 Da and peaked around 500 Da. The complexcity of the foulant is more evident in the expanded segment of the spectrum (inset, m/z 350 to 750) which shows mass peaks at every nominal mass. The average molecular weight (Mn and Mw) of the foulant were calculated to be 673.4 and 794.7, respectively, using polymer statistics as shown in Eqs 1 and 2:26 (26) Zhu, H.; Yalcin, T.; Li, L J. Am. Soc. Mass Spectrom. 1988, 9, 275-281.
Large Nonvolatile Polyaromatic Molecules
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Figure 1. FDMS spectra of (a) a mixture of polystyrene standards (PS 580 and PS 2550), (b) a hydrocracker foulant. Inset is an expanded segment (m/z 350-750) of (b), showing the complexity of the foulant molecules.
∑MiNi/∑Ni Mw ) ∑MiNi2/∑MiNi Mn )
(1) (2)
where Ni and Mi are the intensity and mass of molecule i, respectively. Because of the low H/C atomic ratio (0.5) of the foulant, a molecular weight of 673.4 would give an atmospheric equivalent boiling point (AEBP) of 827 °C (or 1521°F).27 Many hydrocarbons would pyrolyze at this temperature. The mass resolving power (M/∆M10%) of our instrument was set to ∼1000 to maximize the sensitivity for FDMS analysis. The identities of the mass peaks cannot be assigned because only nominal masses were resolved at this mass resolving power. Core Structures by ISPyMS. Pyrolysis converts large nonvolatile material into small volatile molecules that can be better characterized (e.g., thermal degradation of polymers). Pyrolysis GC/MS has been widely used by industrial labs to identify nonvolatile compositions. The technique, however, has limitations in analyzing high-boiling and polar pyrolysates. Unlike Py-GC/MS, ISPyMS pyrolyzes and ionizes sample molecules inside the ion source of a mass spectrometer. Therefore, it can detect pyrolysates with much higher boiling points. Another advantage of ISPyMS is the short time gap between the pyrolysis and ionization, which minimizes the possibility of secondary reactions, such as polymerization or condensation of the pyrolysates. The pyrolysis products therefore better reflect the backbone structures of the analyte. The polystyrene mixture was first analyzed by ISPyMS to illurstrate the effect of InSource pyrolysis on the polyaromatic system. Figure 2a shows only one intense peak at m/z 104 (styrene monomer) from the “unzipping” of polystyrene. The process effectively reduced molecules with different (27) Boduszynski, M. M.; Altght, K. H. Energy Fuels 1992, 6, 7276.
molecular weights into one common core structure. When the hydrocracker foulant was analyzed by the same technique under identical conditions, a much more complicated mass spectrum was generated as shown in Figure 2b, suggesting the presence of a large number of core structures. Compared with the FDMS spectrum in Figure 1b, the molecular weight distribution of the pyrolysates is clearly shifted to the low mass region (
