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Energy & Fuels 1998, 12, 792-797
Copper Removal from Fuel by Solid-Supported Polyamine Chelating Agents Dhanajay B. Puranik,†,‡ Yan Guo,†,§ Alok Singh,† Robert E. Morris,| A. Huang,⊥ L. Salvucci,⊥ R. Kamin,⊥ V. David,‡ and E. L. Chang*,† Code 6930, Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, DC 20375-5348; Geo-Centers Inc., 10903 Indian Head Highway, Ft. Washington, Maryland 20744; Department of Biochemistry and Molecular Biology, University of Georgetown, 3900 Reservoir Road NW, Washington, DC 20007-2197; Code 6181, Navy Technology Center for Safety and Survivability, Naval Research Laboratory, Washington, DC 20375-5348; and Code PE33, Naval Air Warfare Center, 1440 Parkway Avenue, Trenton, New Jersey 08628 Received January 15, 1998. Revised Manuscript Received April 4, 1998
The presence of copper in hydrocarbon fuels impairs fuel stability and jet-engine performance. We report here the results of an investigation on the feasibility of removing copper from hydrocarbon liquid using chelators attached to various support materials. The chelators, 1,4,8,11-tetraazacyclotetradecane (cyclam) and N1-[3-(trimethoxysilyl)propyl]diethylenetriamine (DETA), were attached to polystyrene, agarose, or silica. Copper extraction from JP-5 and dodecane with the immobilized chelators, both cyclic (cyclam) and acyclic (DETA) ones, was performed and the amount of Cu(II) removed measured. All of the immobilized chelators showed an ability to remove copper ions from jet fuel, but DETA bound to silica exhibited the best results and was used for further Cu(II) binding studies under column flow. Results from JP-5, dodecane, and water tests are reported, and the potential of this approach for removing Cu(II) from jet fuel is discussed.
Introduction Jet fuel, in the presence of oxygen, undergoes autoxidative degradation at elevated temperatures. Sub parts per million levels of dissolved copper can promote this process, resulting in the formation of insoluble gums and/or sediments. Copper can be introduced into jet fuel from contact with many metals encountered during fuel transport, storage, and certain refining operations, such as copper sweetening. However, copper contamination is most often a serious concern for U. S. Navy JP-5 fuels, since copper-bearing alloys are used in many components of shipboard fuel handling systems. Copper concentrations as high as 1000 ppb have been reported in surveys of JP-5 fuel aboard U.S. Naval air-capable ships. In addition, significant quantities of JP-5 fuel are held in storage as unusable for aircraft use due to unacceptably high levels of copper. There is evidence that dissolved copper can promote certain prerequisite chemical reactions in jet fuel during storage,1 which enhance the autoxidation process and result in thermal degradation. The addition of soluble metal chelants has been the basis for present measures in counteracting * To whom correspondence should be addressed. E-mail: echang@ cbmse.nrl.navy.mil. † Center for Bio/Molecular Science and Engineering, Naval Research Laboratory. ‡ Geo-Centers Inc. § University of Georgetown. | Navy Technology Center for Safety and Survivability, Naval Research Laboratory. ⊥ Naval Air Warfare Center. (1) Pande, S. G.; Hardy, D. R. Energy Fuels 1997, 11, 1019.
the catalytic effects of dissolved copper in fuels. This approach is based upon the assumption that the copper is effectively tied up in a soluble catalytically inactive complex. Copper catalysis is thought to occur through an electron-transfer mechanism,2 although the influence of coordination on the redox potential of the copper atom is not explicitly known in organic media. It has been proposed3 that catalytic deactivation of the copper may be a consequence of the increased energy of activation for the change in valency. Therefore, the conversion of Cu(II) disalicylidene to Cu(I) disalicylidene would be unfavored, due to the difficulty of undergoing the change in geometry within the complex, thus rendering the copper catalytically inactive. However, findings from long-term high-temperature turbulent rig studies4,5 have indicated that these metal deactivating additives are not effective under the flow and temperature conditions that exist in certain regions of a typical military aircraft fuel system. Other reports have indicated that the effectiveness of a soluble copper chelant at the elevated temperatures reached in jet engines is limited by the thermal stability of the complex.6,7 Thus, there are many advantages in eliminating the reliance on soluble copper chelators to (2) Scott, G. Atmospheric Oxidation and Antioxidants; Elsevier Publishing Co.: Amsterdam, 1965. (3) Chalk, A. J.; Smith, J. F. Nature 1954, 174, 802. (4) Kendall, D. R.; Houlbrook, G.; Clark, R. H.; Bullock, S. P.; Lewis, C. Presented at the 1987 Tokyo International Gas Turbine Congress, III-41 to III-46, October 26-31, Tokyo, Japan, 1987. (5) Moses, C. A. Southwest Research Institute Report No. 281, contract DAAK70-92-C-0059, August, 1992. (6) Morris, R. E.; Turner, N. H. Fuel Sci. Technol. Int. 1990, 8, 32.
S0887-0624(98)00006-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/27/1998
Copper Removal from Fuel
provide acceptable levels of thermal stability of jet fuels in the presence of copper. The approach investigated in this study involves the removal of copper from jet fuel without otherwise changing the fuel constituency. We have previously described one method for the removal of copper from jet fuel based on using a cyclic chelator polymer.8 In this paper, we extend the approach to both cyclic and acyclic polyamine chelators and to different substrates. The chelators are actually composed of three sections, the chelating group itself, a linker arm that varies from three to sixteen carbons in length, and an attachment site for binding to a solid support. Each of these aspects can be optimized for a particular application or environment. Here we compare the relative performances of various combinations, with the most promising tested further in columns. Our goal is to be able to reduce the concentration of dissolved copper in jet fuel to levels that do not adversely affect the thermal stability. Polyamine chelators, especially the macrocyclic forms, are among the strongest metal chelators known.9,10 While the acyclic polyamines have an enhanced stability for complexing metal ions through the “chelator effect”, azamacrocyclic molecules have additional thermodynamic stabilization attributable to the “macrocyclic effect”, as first described by Cabbiness and Margerum.11 The high metal affinity and selectivity of the polyamines make them attractive as chelators of choice for metal sensors,12 redox systems,13 or for water remediation, i.e., ion-flotation collectors.14 Adsorption or chemical attachment of chelators to solid support have been reported previously for chromatography15-17 and for removal of metal ions from aqueous wastestreams.18-22 However, previous to our recent communication,8 no report has been found of applying immobilized chelators for removal of metal ions, specifically Cu(II), from jet fuel. The advantages of this approach are that the chelators have high specificity for transition metal ions, that (7) Morris, R. E.; Hasan, M. T.; Su, T. C. K.; Wechter, M. A.; Turner, N. H. Energy Fuels 1998, 12, 371. (8) Puranik, D. B.; David, V. A.; Morris, R. E.; Chang, E. L. Energy Fuels 1997, 11, 1311. (9) Kodama, M.; Kimura, E. J. Chem Soc., Dalton Trans. 1978, 1081. (10) Bianchi, A.; Micheloni, M.; Paoletti, P. Coord. Chem. Rev. 1991, 110, 17. (11) Cabbiness, D. K.; Margerum, D. W. J. Am. Chem. Soc. 1969, 91, 6540. (12) Singh, A.; Tsao, L.-I.; Markowitz, M.; Gaber, B. P. Langmuir 1992, 8, 1570. (13) De Santis, G.; Di Casa, M.; Mariani, M.; Seghi, B.; Fabbrizzi, L. J. Am. Chem. Soc. 1989, 111, 2422. (14) Yamada, K.; Koide, Y.; Yamanoduchi, H.; Ohmura, M.; Shosenji, H. Bull. Chem Soc. Jpn. 1989, 62, 2867. (15) Zhou, X. C.; Wu, C. Y.; Lu, X. R.; Chen, Y. Y. J. Chromatogr. 1994, 662, 203. (16) Lamb, J. D.; Smith, R. G. J. Chromatogr. 1991, 546, 73. (17) Blain, S.; Appriou, P.; Chaumeil, H.; Handel, H. Anal. Chim. Acta 1990, 232, 331. (18) Izatt, R. M.; Bradshaw, J. S.; Bruening, R. L.; Tarbet, B. J.; Bruening, M. L. Pure Appl. Chem. 1995, 67, 1069. (19) Tuncay, M.; Christian, S. D.; Tucker, E. E.; Taylor, R. W.; Scamehorn, J. F. Langmuir 1994, 10, 4688. (20) Izatt, R. M.; Bruening, R. L.; Tarbet, B. J.; Griffin, L. D.; Bruening, M. L.; Krakowiak, K. E.; Bradshaw, J. S. Pure Appl. Chem. 1990, 62, 1115. (21) Schlapfer, C. W. Process for removing metal ions from solution with a dipicolylamine chemically bound to the surface of a silicate. U.S.A. Pat. No. 5102640, 1992. (22) Macedo, P. B.; Barkatt, A.; Macedo, P. B., Litovitz, T. A. Fixation of dissolved metal species with a complexing agent. U.S.A. Pat. No. 4659512, 1987.
Energy & Fuels, Vol. 12, No. 4, 1998 793
copper ions are directly removed from the fuel, that no metal ions need to be exchanged for binding copper, that no chemicals need be added to the fuel, and that the copper may be recovered after treatment of the fuel. In designing this alternative cleanup method, we have tried to consider carefully both the chemistry of the process and the economics of implementing such a technology. The positive results obtained from the studies are encouraging and demonstrate the potential of this approach as a plausible alternative to present methods. Experimental Section Materials. Technical grade DETA -silane, N1-[3-(trimethoxysilyl)propyl]diethylenetriamine, was purchased from Aldrich (Milwaukee, WI). Amino(propyl)-modified silica was bought from Jones Chromatography. All other standard chemicals and materials were purchased from commercial chemical providers such as Aldrich, Fluka, Pierce, Sigma, Polychem, and Kodak and were used without further purification, unless otherwise stated. A copper-bearing JP-5 fuel used in the flow testing was prepared by introducing copper by long-term immersion with copper plates until a copper concentration of 368 ppb was reached. Equilibrium tests were conducted with n-dodecane and a different copper-free JP-5 to which 20 ppm copper from copper(II) ethylacetoacetate (Eastman) was added. Sample Preparation. DETA-Modified Silica. A suspension of 1:1 silica and the DETA-silane was refluxed in toluene for 20-24 h and filtered, and the silica was washed with methanol. The modified silica was then baked for 20-24 h at 70-90 °C. 1,4,8,11-Tetraazacyclotetradecane (Cyclam)-Modified Silica. Aminopropylsilane (0.3 g, 2.04 mmol) was stirred overnight with bromohexanoyl chloride (0.5 mL, 2.20 mmol) and pyridine (1.0 mL). The solution was then filtered, stirred for 72 h with cyclam (0.5 g) in chloroform, and filtered again, and the residual oil was refluxed with silica (2.77 g) in toluene. The toluene was next removed and the residual silica washed with toluene. The 1,4,8,11-tetraazacyclotetradecane immobilized silica was dried and used for copper removal. Agarose Coupled with Cyclam. Agarose (immunopure epoxyactivated agarose, Pierce, Rockford, IL), containing a 16-carbon chain that terminated with an epoxide (2 g, 3 mmol active epoxide/g), was added to cyclam (2.0 g, 10 mmol) in water. The suspension was refluxed for 45 h, washed with water, and dried under vacuum. Unreacted cyclam (1.4 g) was recovered, indicating the amount of immobilization was 30%. Polystyrene Coupled with Cyclam. Chloromethylpolystyrene (1.0 g, 1 mmol chloride/g) was refluxed with diaminooctane (1.0 g, 6.94 mmol) in DMF. The suspension was filtered and washed, sequentially, with DMF, water, chloroform, ethanol, and methanol. The aminated polystyrene was stirred with dibromooctane (2.0 g, 7.35 mmol) for 5 days in DMF. The suspension was filtered and washed with methanol. The brominated polystyrene was then stirred with cyclam (1.5 g) in DMF. The suspension was filtered, washed with chloroform, and dried. Tests. Slurry Tests. In a typical procedure, a sample, derivatized with cyclam, was combined with either dodecane or JP-5 doped up to 20 ppm copper from the addition of copper acetoacetate. After the mixture was stirred for 18 h, the supernatant was recovered by filtration through glass wool and then a 0.45 µm microporous nylon filter. The resulting filtrate was analyzed for copper content by graphite furnace atomic absorption. For DETA-modified silica, the slurry test was performed in tetradecane using the above procedure, varying the amount of time for slurry mixing and standing time
794 Energy & Fuels, Vol. 12, No. 4, 1998 Determination of Binding Capacities for DETA-Modified Silica in Aqueous Media. Samples of DETA-modified silica, typically 0.5 g, were mixed with 15 mL of 3000 ppm Cu(II) and let stand overnight. The supernatant was analyzed spectrophotometrically for copper content as described below. (a) Aqueous Media Analysis for Copper. Cu(I) binds with the disodium salt of bathocuporine disulfonic acid (Aldrich) to form a yellow colored, water-soluble complex used for onephase aqueous assays of copper. This method gives very sensitive and reliable results. Standard curves had regression factors (least-squares method) up to 0.999+ for the range of 40 ppb to 5 ppm Cu(I). The assays were verified by comparison with inductively coupled plasma (ICP) spectroscopy. Hydroxylamine hydrochloride was used as a reductant for Cu(II). Since the pH affects the stability of Cu(I)-bathocuproine complex, ammonium acetate was used as a buffer. Absorption measurements were obtained at 480 nm. A typical procedure for 10 mL samples was as follows. One milliliter of sample solution was transferred to a test tube, and the following aliquots were added sequentially to the tube, mixing well after each addition: 1 mL of 10% hydroxylamine hydrochloride solution, 1 mL of 0.1% bathocuproine solution, 1 mL of 10% ammonium acetate solution (for use as buffer around pH 7), and 6 mL of 18 MΩ water for a 1:10 dilution. The solution was shaken, and the absorption was measured at 480 nm to 483 nm. Reagent blanks were prepared by substituting 1 mL of water for the sample. The sample results were compared to a linear standard curve. From the difference between the initial Cu(II) concentration and the measured Cu(II) concentration, the amount of copper absorbed and the mass of copper absorbed per gram of modified silica gel were obtained. (b) Organic Media Analysis for Copper. A bathocuproine solution consisting of 225 ppm bathocuproine in ethanol and a 10% hydroxylamine solution were mixed in a 7:3 ratio, respectively. To a 16 mL portion of the copper-bearing hydrocarbon sample, 1 mL of the bathocuproine solution was added and mixed vigorously for 5-10 min. After standing for 24 h at room temperature, absorbance was measured at 475 nm. Copper content was calculated by comparison with a standard curve. Aqueous Flow Tests. Initial flow tests were conducted in the laboratory using 250 mm × 10 mm tempered borosilicate Omnifit columns (Supelco, Bellefont, PA) packed with approximately 8.5 g of DETA-modified or unmodified silica. A variable peristaltic pump was calibrated for each mesh silica and used to pass known concentrations of copper solutions through packed modified-silica gel. The contacting solution was made up to 500 ppm CuSO4(aq). The aqueous flow rates were kept between 4 and 5 mL/min. Initially, 30-60 mesh silica was used to maximize the flow rate and keep the backpressure as low as possible, but we found that modified 70-230 mesh silica was much more effective in removing copper. Therefore, most of the subsequent flow tests were done using the finer mesh silica. We have also examined other parameters, such as choice of solvent, silane-to-silica ratio, baking time, temperature, and other factors to optimize for the capacity of the modified materials. For the flow tests reported here, we used DETAmodified silica with a capacity of 50 mg Cu/g modified silica. Eluates were taken periodically and copper concentrations measured using the above-described method. Since the capacity of the columns could not be pushed to their limits for the laboratory fuel testing, these aqueous tests were performed, as an indirect test, to ensure that the binding capacity of the modified silica does not change greatly under flow. JP-5 Flow Tests. Approximately 30 g of modified silica was packed into drying tubes closed off at each end by glass wool. The JP-5 fuel that was doped with 368 ppb copper as described above was pumped at a rate of 10 mL/min for 8 h through the following mixtures of modified silica: 70-230 mesh DETA-
Puranik et al. modified silica, 70-230 mesh DETA-modified silica/30-60 mesh DETA-modified silica (1/1), aminopropylsilica, aminopropylsilica-70-230 mesh DETA-modified silica (1/1), aminopropylsilica-30-60 mesh DETA-modified silica (1/1), and 70230 mesh unmodified silica. Flow tests were also conducted at 20 mL/min using an Isco model 2350 HPLC pump. The copper-bearing JP-5 fuel flowed through polyethylene drying tubes packed with approximately 30 g of silica or modified silica at a backpressure of 20-30 psi. Samples were taken periodically and analyzed for copper content by graphite furnace atomic absorption (AA). Thermal Stability Tests. Copper-bearing fuel, which contained soluble copper for approximately nine months and at the time of the test contained 317 ppb copper, was passed through columns of either 70-230 mesh DETA-modified silica or unmodified silica gel. A total of 4 L of fuel was passed through each column, at a rate of 20 mL/min, and each liter of sample was collected and tested for thermal stability by the gravimetric JFTOT apparatus described elsewhere23,24 at a flow rate of 3 mL/min at 260 °C (500 °F) for 2.5 h. Aqueous Formation Constants. Formation constants in aqueous media were obtained 25 by titration and analysis of the titration data with HYPERQUAD, a suite of programs designed for analyzing chemical equilibria. Briefly, 1.0 mM of the fully protonated chelator was titrated with NaOH to obtain metal-free titration curves. These curves were analyzed by HYPERQUAD to obtain the protonation constants for the polyamines. Subsequent titration of the chelator-metal complex with NaOH and application of HYPERQUAD yielded the formation constants.
Results and Discussion Copper extraction from JP-5 and a model fuel with surfaces functionalized with either a macrocyclic chelator, cyclam, or an acyclic ligand, DETA, was examined. The structures of these compounds are shown in Figure 1. They were attached to substrates via a hydrocarbon bridging group, either propyl or octyl. The example in Figure 1 is for a DETA propyl silane. Cyclic Polyamines (Cyclam). The results of the slurry tests with cyclam-modified polystyrene, agarose, and silica are shown in Table 1. Neat dodecane contains negligible amounts of copper (within error of measurement) as measured by ICP-furnace AA, whereas the JP-5 was found to have around 20 ppb copper. Tests were carried out with n-dodecane and JP-5 doped with 20 000 ppb copper from copper ethylacetoacetate. All three materials modified with cyclam performed well. The agarose control appeared to be efficient in adsorbing copper from dodecane, but was not as effective in JP-5, where the copper was more tightly bound. Generally, the silanized silica yielded the lowest levels of copper, although the agarose-cyclam material was comparable on a per gram basis. The length of the linker arm was thought to be crucial because the chain has to be sufficiently long to provide adequate exposure of the chelator to the soluble copper in fuel but not so long as to impair the mechanical stability of the linkage. However, we did not observe a noticeable dependence (23) Beal, E. J.; Hardy, D. R.; Burnett, J. C. In Aviation Fuel: Thermal Stability Requirements; ASTM STP 1138; Kirklin, P. W., David, P., Eds.; American Society for Testing and Materials, Philadelphia, PA, 1992, p 138. (24) Beal, E. J.; Hardy, D. R.; Burnett, J. C. In Proceedings of the 4th International Conference on Stability and Handling of Liquid Fuels; Orlando, FL, Nov. 1991; p 245. (25) Puranik, D. B.; Singh, A. N.; Chang, E. L. J. Coord. Chem. 1996, 39, 321.
Copper Removal from Fuel
Energy & Fuels, Vol. 12, No. 4, 1998 795 Table 2. Comparison of Cyclic and Acyclic Formation Constants chelatora
log KCu(II)
log KMg(II)
log KCa(II)
27.0 20.0 16.0 20.0 16.0 14.0
3.0
10
cyclamb DETA HDCc HDTETA HDDETA
TETA ) triethylenetetramine, DETA ) diethylenetriamine, HDC ) hexadecylcyclam, HDTETA ) hexadecyltriethylenetetramine, HDDETA ) hexadecyldiethylenetriamine. b Cyclam value from Bianchi et al. (1991).10 c HDC value from Puranik et al. (1996).25 a
Table 3. Copper Content of 10 mL Solutions of Copper in Tetradecane after Static Exposure to 2 g of DETA-Modified Silicaa mixing time/standing time
initial [Cu] ) 5 ppm
initial [Cu] ) 20 ppm
30 s/2 min 5 min/10 min 5 h/overnight
