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Separation of Petroporphyrins from Asphaltenes by Chemical Modification and Selective Affinity Chromatography Cindy-Xing Yin,† Jeffrey M. Stryker,*,‡ and Murray R. Gray*,† Department of Chemical and Materials Engineering and Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada ReceiVed December 4, 2008. ReVised Manuscript ReceiVed February 10, 2009
Part of the metalloporphyrin fraction was separated from asphaltenes by a combination of reactive modification and affinity chromatography. The targeted vanadyl porphyrin complexes were modified by reaction with oxalyl chloride followed by a long chain alkylamine or perfluoroalkylaniline to produce an imido vanadium(IV) derivative bearing an octadecyl or perfluoroctyl side chain. This derivatized asphaltene was then chromatographed on C18-silica gel or perfluorous (-C8F17) silica gel to remove the tagged metalloporphyrins from the remaining asphaltene. With the C18-tagging and reverse-phase chromatography method, from 15 to 40% of the total vanadium content together with comparable percentages of nickel was removed, with less than 5% mass loss from the asphaltene fraction. The fluorous tagging process gave better removal of metals on one sample but was limited by the lower recovery of asphaltenes on other samples. This approach requires further optimization for quantitative selective separation of metal complexes from heavy oils and to overcome the aggregation of the metals with other components of the asphaltenes.
Introduction Heavy metals in crude oil or bitumen, notably in the form of vanadium(IV) and nickel(II) petroporphyrin complexes, cause problems during processing and utilization, such as the poisoning of hydroprocessing and catalytic cracking catalysts. Essentially all of the vanadium and nickel species in crude oil are coordinated in tetrapyrrole environments similar to the porphyrins, as determined by XAFS spectroscopy,1 even after hydroprocessing. On this basis, we label all of the Ni and V as occurring in petroporphyrin complexes for simplicity. A fraction of these petroporphyrin complexes are characterized and quantified by their characteristic ultraviolet-visible spectra with an intense Soret band around 400 nm and two visible bands at lower energy.2 The remaining vanadium and nickel compounds, which we term the non-Soret petroporphyrins, do not exhibit distinct UV-visible spectroscopic bands and constitute the majority of the metal-containing components. Vanadium and nickel are currently removed from crude oil by solvent deasphalting or by catalytic hydrogenation. The former technology for removal of the metals is nonselective in its nature because of the large mass of asphaltenes precipitated together with the petroporphyrins; up to 30% mass loss is not uncommon with heavy crude oils.3 In the ideal case, selective * To whom correspondence should be addressed. E-mail: murray.gray@ ualberta.ca (M.R.G.),
[email protected] (J.M.S.); phone: 1-780-4927965 (M.R.G.), 1-780-492-3891 (J.M.S.); fax: 1-780-492-2881 (M.R.G.), 1-780-492-8231 (J.M.S.). † Department of Chemical and Materials Engineering. ‡ Department of Chemistry. (1) Miller, J. T.; Fisher, R. B.; Van der Eerden, A. M. J.; Koningsberger, D. C. Energy Fuels 1999, 13 (3), 719–727. (2) Smith, K. M. General features of the structure and chemistry of porphyrin compounds. In Porphyrins and Metalloporphyrins; Smith, K. M. Ed.; Elsevier Scientific Publishing Company: Amsterdam, 1975; pp 1-28. (3) Long, R. B.; Speight, J. G. The composition of petroleum. In Petroleum Chemistry and Refining; Speight, J. G. Ed.; Taylor & Francis: Washington, D.C., 1998; p 19.
removal of only the metal-bearing molecules would only cause 1-5% mass loss. Hydrogenation of the heavy fractions of crude oil removes the vanadium and nickel as metal sulfides. For feeds with total metal concentration below ca. 250 ppm, packed bed reactors are used. Higher metal contents require alternate reactor designs to cope with catalyst deactivation, such as replaceable guard beds to remove V and Ni before the feed enters the fixedbed, ebullated beds, or other provisions for online catalyst replacement.4-6 Selective removal of the molecules containing metals will give a low retention of other asphaltene components, hence a high recovery of the initial mass of asphaltene. The limited options for selective removal of metal-bearing compounds at the commercial scale are paralleled by a lack of methods for isolating and characterizing the petroporphyrins at the laboratory scale. Available preparative methods for extracting petroporphyrins from crude oils and bitumens use benzene-methanol, chloroform, or acetone/methanol extraction followed by column chromatography.7 Chromatographic separations of the heptaneor pentane-asphaltenes are avoided due to problems of irreversible adsorption and precipitation. The resulting extracts contain only the simple, low molecular weight metal-bearing components. Although this approach has enabled the determination of petroporphyrin structures by mass spectrometry8 or highpressure liquid chromatography,9 these methods fail to provide (4) Mitchell, P. C. H.; Scott, C. E. Catal. Today 1990, 7, 467–477. (5) Hubaut, R.; Aissi, C. F.; Dejonghe, S.; Grimblot, J. J. Chim. Phys. 1991, 88, 1741–1755. ¨ zu¨m, B. Petroleum Refining Processes; Marcel (6) Speight, J. G.; O Dekker, Inc.: New York, 2002; pp 454-455. (7) Quirke, J. M. E. Techniques for isolation and characterization of the geoporphyrins and chlorins. In Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F. Eds.; American Chemical Society: Washington, D.C, 1987: pp 308-331, and references therein. (8) Baker, E. W.; Yen, T. F.; Dickie, J. P.; Rhodes, R. E.; Clark, L. F. J. Am. Chem. Soc. 1967, 89 (14), 3631–3639. (9) Hajibrahim, S. K.; Tibbetts, P. J. C.; Watts, C. D.; Maxwell, J. R.; Eglinton, G.; Colin, H.; Guiochon, G. Anal. Chem. 1978, 50 (4), 549–553.
10.1021/ef801059y CCC: $40.75 2009 American Chemical Society Published on Web 04/01/2009
Separation of Petroporphyrins from Asphaltenes
a fraction that contains most of the metal compounds at a high molar concentration directly from the asphaltenes. Consequently, the molecular environment of the metals in the molecules that associate strongly with the asphaltenes remains entirely unknown.10,11 Recently, Qian et al.12 detected larger fused-ring and sulfur bearing vanadyl porphyrins in an asphaltene sample that may associate more strongly than the previously identified components. Selective demetalation of crude oil,13 vacuum residue, and asphaltenes has been attempted using various methods including treatment with strong oxidants (hydrofluoric acid,14 chlorine,15 sulfuryl chloride,15 sodium hypochlorite,16 peroxyacetic acid,16 organophosphorous reagents,17 etc.); biologicals (chloroperoxidase/H2O2,18 cytochrome c reductase/NADPH,19 microbial strains isolated from petroleum-contaminated soil20); electrolysis;21,22 ultrasonic irradiation combined with chromatography;23 photoirradiation followed by acid extraction;24 and absorption with Mo complexes.25 (See Table S1 in the Supporting Information for details.) The efficiency of the removal of vanadium and nickel from asphaltene samples ranged from 20-78%;16 however, the selectivity of the separation cannot be judged in most cases because the mass of recovered asphaltenes was not reported. The mechanisms of asphaltene demetalation are not clear and are seldom discussed in the literature. This gap is partly due to the difficulty of monitoring petroporphyrin content during demetalation treatment; for example, petroporphyrin concentration can only be semiquantitatively determined from the absorbance of the Soret band.26 Herein we report a novel approach for the selective removal of vanadium and nickel petroporphyrins by chemical modification at the metal center to append a long-chain tag, followed by affinity chromatography separation of the tagged metalbearing components from the remaining asphaltene fraction. The (10) Reynolds, J. G.; Biggs, W. R. Acc. Chem. Res. 1988, 21, 319– 326. (11) Bestougeff, M. A.; Byramjee, R. J. Chemical Constitution of Asphaltenes. In DeVelopments in Petroleum Science 40A (Asphaltenes and Asphalts 1); Yen, T. F., Chilingarian G. V. Eds.; Elsevier Science: Amsterdam, The Netherlands, 1994; pp 67-94. (12) Qian, K.; Mennito, A. S.; Edwards, K. E.; Ferrughelli, D. T. Rapid Commun. Mass Spectrom. 2008, 22, 2153–2160. (13) Ali, M. F.; Abbas, S. Fuel Process. Technol. 2006, 87, 573–584. (14) Kimberlin, C. N., Jr.; Ellert, H. G.; Adams, C. E.; Hamner, G. P. Demetallization with hydrofluoric acid. US patent No. 3203892, 1965. (15) Sugihara, J. M.; Branthaver, J. F.; Willcox, K. W. Oxidative demetallation of oxovanadium(IV) porphyrins. In The Role of Trace Metals in Petroleum; Yen, T. F. Ed.; Ann Arbor Science: Ann Arbor, 1975; pp 183-193. (16) Gould, K. A. Fuel 1980, 59 (10), 733–736. (17) Mann, D. P.; Kukes, S. G.; Coombs, D. M. Metals removal from oils with a light hydrocarbon and an organophosphorous compound. US patent No. 4518484, 1985. (18) Fedorak, P. M.; Semple, K. M.; Vazquez-Duhalt, R.; Westlake, D. W. S. Enzyme Microb. Technol 1993, 15 (5), 429–437. (19) Xu, G.-W.; Mitchell, K. W.; Monticello, D. J. Fuel product produced by demetalizing a fossil fuel with an enzyme. US patent No. 5726056, 1998. (20) Dedeles, G. R.; Abe, A.; Saito, K.; Asano, K.; Saito, K.; Yokota, A.; Tomita, F. J. Biosci. Bioeng 2000, 90 (5), 515–521. (21) Ovalles, C.; Rojas, I.; Acevedo, S.; Escobar, G.; Jorge, G.; Gutierrez, L. B.; Rincon, A.; Scharifker, B. Fuel Process. Technol. 1996, 48 (2), 159– 172. (22) Greaney, M. A.; Kerby, M. C., Jr.; Olmstead, W. N.; Wiehe, I. A. Method for demetallating refinery feedstreams. US patent No. 5529684, 1996. (23) Sakanishi, K.; Yamashita, N.; Whitehurst, D. D.; Mochida, I. Catal. Today 1998, 43 (3-4), 241–247. (24) Shiraishi, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 2000, 39, 1345–1355. (25) Sakanishi, K.; Saito, I.; Watanabe, I.; Mochida, I. Fuel 2004, 83 (14-15), 1889–1893. (26) Sugihara, J. M.; Bean, R. M. J. Chem. Eng. Data 1962, 7, 269– 271.
Energy & Fuels, Vol. 23, 2009 2601 Table 1. Composition Properties of the Asphaltenic Samples Used in This Study heptane-insoluble toluene-insoluble content (%) content (ppm) Cold Lake pentaneinsoluble (CL-C5) Cold Lake heptaneinsoluble (CL-C7)a Athabasca pentaneinsoluble asphaltene (AA-C5) Athabasca heptaneinsoluble (AA-C7)
90 100 50
Ni
850 320
0
790 280
97%, Fluorous Technologies Incorporated] was dissolved in 2 days, then eluted off the column, as compared to the normal procedure of loading the column and eluting immediately. (32) Tirant, M.; Smith, T. D. Inorg. Chim. Acta 1984, 90, 111–114.
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Figure 4. Metal and mass recovery from oxalyl chloride-treated asphaltenes in eluant from flow-through chromatography on aminederivatized stationary phases after elution with dichloromethane and tetrahydrofuran.
Figure 5. Remaining metal content percentages and mass recovery percentages of the oxalyl chloride/C18-aminated asphaltenes after elution from a C18-silica gel column with dichloromethane and tetrahydrofuran along with a control experiment on untreated AA-C5. Metal content was quantified using ICP-MS analysis.
The C18-alkylamine was then used to tag the asphaltenederived chlorinated vanadium petroporphyrin, using the long alkyl tail as an affinitive ligand to interact with the reversephase C18-silica gel. Three repeated chromatographic separations of the C18-derivatized Athabasca asphaltene (AA-C5) were performed using the same C18-column. The results showed the reusability of the column with an average removal of 37 ( 5% of vanadium and 36 ( 3% of nickel, with a loss of less than 5% of the mass of asphaltenes, see Figure 5. The data presented in Figure 5 show the results of the same chemical treatment as applied to different asphaltene materials, as listed in Table 1. The results (Figure 5) show that excellent mass recoveries of both Cold Lake asphaltene and Athabasca asphaltene were obtained from the combined dichloromethane and tetrahydrofuran eluents. The metal content of the asphaltene material obtained from the column by each eluent was reproducible. Overall, from 15 to 40% of the vanadium together with 17-39% of the nickel was removed using this tagging and chromatographic separation method, with substantially less than 5% weight loss. The vanadium removal efficiency is higher when the asphaltene has a lower heptanes-insoluble percentage (i.e., AA-C5 compared to CL-C5, see Table 1 and Figure 5). Additionally, the C18-alkylated silica gel showed minimal retention of the C18-derivatized asphaltene; after column separation, only a very thin dark layer was observed on the column head. For comparison, control experiments using untreated AAC5 showed an average of 17((4)% vanadium and 22((6)% nickel removal by flowing through the C18-alkylated silica gel column, also giving a thin dark layer at the top of the packing. Interestingly, nickel removal percentages were very similar to those of vanadium, although our chemical treatment was designed to target the vanadium only. When Ni(OEP) was reacted with excess (COCl)2 for 1 h, no changes were observed by 1H NMR of the mixture. In another control experiment, Ni(OEP) was reacted with oxalyl chloride and aniline and then separated on an SiO2-column to give a NiOEP fraction and a second fraction in a mass ratio of 55%:45%. The second fraction consisted of degraded products including demetallated free base H2OEP as detected by MALDI-MS. This result offers one
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Figure 6. Representative runs of remaining metal content percentages and mass recovery percentages of the oxalyl chloride/F-aminated asphaltenes after flowing through C8F17-silica gel column with dichloromethane and tetrahydrofuran along with a control experiment showing untreated AA-C5 eluted from a C8F17-silica gel column similarly. Metal content was quantified using ICP-MS analysis.
Figure 7. Runs of remaining metal content percentages and mass recovery percentages of the oxalyl chloride/F-aminated asphaltenes after flowing through C8F17-silica gel column with dichloromethane and tetrahydrofuran showing the degeneration of the reusability of C8F17silica gel column with less metal removal efficiency and higher asphaltene recovery (most notably in CL-C7 cases). Metal content was quantified using ICP-MS analysis.
possible explanation for the Ni removal under the current strategy: degradation apparently occurs when Ni petroporphyrins interact with unfunctionalized patches of on the perfluorous SiO2 or C18-SiO2 packings. Nickel petroporphyrin was previously reported to degrade on silica gel plates.33 A second chemical tagging/affinity chromatography strategy was evaluated using the combination of perfluorous aniline (4C8F17-C6H4NH2)/perfluorous C8F17-silica gel. The metal removal percentages of perfluorous separations were better than those of C18-separations, but the recovery of asphaltene was inconsistent between asphaltene samples (Figure 6). The reusability of perfluorous column was worse, due to the lesspassivated silica gel surfaces of perfluorous silica gel than C18silica gel (controls of asphaltene recoveries of perfluorous silica gel were 86%; C18-silica gel were 101%). The lower coverage of the silica gel by the end groups gave faster degeneration of metal removal ability upon reuse than the combination C18amine tagging/C18-silica gel, see Figures 6 and 7. Both Athabasca and Cold Lake gave less elution of C7 asphaltenes in comparison to the C5 fractions. Within the C7 samples, Cold Lake asphaltenes (CL-C7) showed lower recovery percentages on the perfluourous packing compared to Athabasca asphaltenes (AA-C7), possibly due to lower solubility after the reaction treatment. This difference could arise from the chemical structure and aggregation behavior of the two samples. The results of control experiments listed in Table 2 also show that several experimental variables in the procedure do not significantly change the metal removal efficiency, including (i) filtration through a 0.22 µm membrane, (ii) asphaltenes at lower concentration (0.11 mg of asphaltenes/1 mL of toluene compared to 5 mg of asphaltenes/1 mL of toluene under standard (33) Ali, M. F.; Perzanowski, H.; Bukhari, A.; Al-Haji, A. A. Nickel and vanadyl porphyrins in Saudi Arabian crude oils. Energy Fuels 1993, 7, 179–184.
Yin et al.
conditions), (iii) higher reaction temperature (60 or 100 °C34 compared to room temperature under standard conditions) when reacting with oxalyl chloride, and (iv) different ratios of asphaltene to oxalyl chloride. Separation using perfluorous silica gel that was further endcapped by treatment with chlorotrimethylsilane does not improve the asphaltene recovery percentage. 19F-NMR spectroscopy (19F-NMR) was used to determine the fate of the fluorous tags during the separation process. The perfluorous column retains most of the perfluorous tags: only very low intensity 19F-NMR resonances were observed in two different column-purified asphaltenic materials as compared to the original fluorous-tagged asphaltenic material (see Supporting InformationFigure S1). A continuous Soxhlet extraction with methanol from used perfluorous silica gel produced a metal-enriched asphaltene fraction that contained 1853 ppm of V and 475 ppm Ni, which was a 2-3 fold increase in concentration of vanadium from the original asphaltenic materials. The MALDI-TOF mass spectrum of this material showed a broad range of mass detection of mass/charge from 300-2000, similar to the untreated sample. This result confirmed the coabsorption of some untreated asphaltene on perfluorous silica. The highaffinity of the above methanol extract for adsorption on silica gel inhibited any further attempt to separate and identify the metal components in this fraction. The recovery of metal-rich material from spent C18-alkylated silica gel was less successful due to the lower retention of asphaltenic material on the C18silica gel. In previous studies of chromatographic enrichment of the metals from petroleum,7 asphaltenic fractions were segregated prior to any column chromatography, a consequence of problems associated with strong adsorption to packing materials. Apart from the loss of asphaltene to the column packing, the aggregation of the asphaltenes may also pose a limitation on selective removal of the tagged metalloporphyrins. Two factors related to aggregation could limit the effectiveness of the method: (1) aggregation of the asphaltenes in solution could limit the efficacy of the oxalyl chloride reaction and (2) the aggregation of tagged components could limit the interaction with the chromatographic packings. Further optimization of this method will involve the evaluation of different tagging reagents and development of analytical techniques to confirm the transformation of all the metals in the asphaltenes. The difficulty of confirming the extent of the chemical tagging transformations in the authentic asphaltene comes from the lack of spectroscopic tools to follow metal within the dark asphaltene matrix. The literature suggests that UV-visible spectra would not be representative of all the petroporphyrins, the EPR spectra of both the vanadyl porphyrin model compound and the imidovanadium porphyrin compound are very similar, and fluorescence signals would be obscured by the wide range of fluorescent emission from the asphaltene matrix. Such mechanistic issues and the further development of novel chemical tagging, in combination with selective separation methodologies, are currently under investigation. In contrast to the present work on selective separation, previous efforts have mainly focused on removal of the metal atom from the porphyrin ring. Roughly 50% efficiency for removal of metals has been observed (see Supporting Informa(34) The significant increase in the mass recovered (120-150%) from 100 °C treatment with oxalyl chloride then chromatography may be due to the formation of hydrochloric acid from hydrolysis induced by trace water in the eluting solvents, which further cleaves the siloxane bond, removing the fluorous functional group from the perfluorous silica gel and/or the C18alkyl chain from the C18-silica gel.
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Table 2. Summary of Control Experimentsa Showing Varying Reaction Conditions Did Not Change the Metal Removal Efficiency ppm (% over original metal level) Fluorous-AA-C5 obtained at 60 °C reaction temperature with (COCl)2 Fluorous-AA-C5 obtained at 100 °C reaction temperature with (COCl)2 Fluorous-AA-C5 obtained with smaller (COCl)2/asphaltene ratio Fluorous-AA-C5 separated with fluorous-SiO2 deactivated by treatment with chlorotrimethylsilane Fluorous-AA-C5 separated with C18-silica gel C18-AA-C5 obtained at 100 °C reaction temperature with (COCl)2 C18-AA-C5 obtained at lower asphaltene concentration C18-AA-C5 purified by filtration through 0.22 µm membrane to check for insoluble material a
V
Ni
asphaltene recovery (%)
350 (60%) 307 (53%) 249 (43%) 359 (62%) 317 (55%)
149 (65%) 120 (52%) 92 (40%) 136 (59%) 151 (66%)
97% 144% 161% 92% 88%
365 (63%) 428 (74%) 372 (64%) 344 (59%) 440 (76%)
181 (79%) 201 (87%) 157 (68%) 134 (58%) 179 (78%)
86% 104% 120% 94% 105%
Perfluorous aniline/perfluorous-SiO2 chromatography or C18-NH2/C18-SiO2 chromatography unless noted otherwise.
tion Table S1), although asphaltene mass balances were not uniformly determined. Prior reports include the demetalation of (1) Boscan asphaltene by oxidative demetalation with chlorine (V: from 3556 to 1783 ppm; 50%);15 (2) Cold Lake asphaltene by oxidative demetalation with NaOCl (78% V and 37% Ni removed, mass balance 112%);16 and (3) Hamaca asphaltene by electrolysis (V: from 2790 to 1394 ppm; 50%).21 One explanation for the limited effectiveness of the tagging method used in this study, as well as previous methods for metals removal, is that only the accessible petroporphyrins are amenable to derivatization and removal. The petroporphyrins covalently bound to groups that promote aggregation with other asphaltene components35 are either not subject to chemical treatment, or more likely are not subject to effective chromatographic separation due to aggregation in solution. The recent report by Qian, et al.,12 suggests that fused benzo- and sulfur-bearing rings could enhance such aggregation. This mechanism is also consistent with the small-angle neutron scattering experiments reported by Kilpatrick, et al., in which the vanadium and nickel content of an asphaltene was most strongly correlated to the asphaltene aggregate size and solubility parameter.36 Thus, the possibility of cleanly removing both Soret and non-Soret petroporphyrins from asphaltenes without significant loss of mass is intimately dependent on a successful strategy for disrupting self-aggregation in asphaltene structures. The high correlation of vanadium and nickel contents with aggregate size in asphaltenes, as determined by small-angle neutron scattering,36 supports this possibility. The challenge in separating the asphaltenes is to disrupt the aggregates without eliminating adsorption to a chromatrographic support. Increasing temperature, for example, will reduce both aggregate size and adsorp(35) Yin, C. X.; Tan, X.; Mu¨llen, K.; Stryker, J. M.; Gray, M. R. Energy Fuels 2008, 22, 2465–2469. (36) Gawrys, K. L.; Blankenship, G. A.; Kilpatrick, P. K. Energy Fuels 2006, 20, 705–714.
tion. Given the active development of mass spectrometric and fluorescence spectroscopic analysis of asphaltenes, chemical tagging could be valuable for characterization of metal petroporphyrins by enhancing detection within a complex mixture. Conclusions A new method of selective metal removal has been designed and examined for chemical modification and selective removal of part of the metal-bearing components directly from asphaltene samples. Metal-depleted asphaltenes were recovered by column chromatography, and metal-enriched components could be partially recovered by Soxhlet extraction of the chromatographic supports with methanol. The limitations of the current approach likely result from the aggregation of asphaltene structures with vanadium-containing components and deterioration of asphaltene solubility. Acknowledgment. We thank Dr. Michael J. Ferguson at the Department of Chemistry X-Ray Crystallography Laboratory, University of Alberta for the crystal structure determination. We thank James A. Dunn and Ron Myers from Imperial Oil and Greg P. Dechaine for useful discussions, and Brendon Boddez for his help with chromatography separations. The authors gratefully acknowledge financial support from the Imperial Oil-Alberta Ingenuity Center for Oil Sands Innovation and the Natural Sciences and Engineering Research Council. M.R.G. holds the NSERC/ Imperial Oil Chair in Oil Sands Upgrading and a Canada Research Chair. Supporting Information Available: Table of demetalation literature; 19F-NMR of different asphaltenic materials prior and after perfluorous column purification; crystal structure data of [(octaethylporphyrin)V(dNPh)]. This information is available free of charge via the Internet at http://pubs.acs.org. EF801059Y
