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APPLIED CHEMISTRY Polymers with Multiple Ligand Sites for Metal Extractions in Dense-Phase Carbon Dioxide† Kimberly R. Powell,‡,§ T. Mark McCleskey,*,‡ William Tumas,‡ and Joseph M. DeSimone*,§ Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290
We have synthesized a series of CO2-soluble polymeric extractants with multiple ligand sites for CO2-based metal extractions. The CO2-soluble polymers were prepared via free-radical copolymerization of a fluorinated acrylate with a series of acrylate- or styrene-based monomers functionalized with ligand sites or ligand precursors. These polymers have high solubility in CO2; up to 30 wt % of a polymer with 15 mol % of a ligand-based monomer can be solubilized in liquid CO2 at 25 °C at 140 bar. Copper and europium extractions have been performed with β-diketone- and phosphonate-functionalized polymers, respectively. Preliminary extractions with copper nitrate were carried out at ligand-to-metal ratios of 1:1 and 2.7:1, resulting in 25-37% and 59% efficiency, respectively, suggesting a ligand binding stoichiometry of 2 for efficient extraction. Europium luminescence studies demonstrate that europium is bound to the polymer along with four water molecules in the inner coordination sphere. Introduction The use of liquid and supercritical carbon dioxide (scCO2) is gaining increasing interest as an environmentally benign alternative to organic solvents for a wide range of applications including metal extractions and chemical processing.1-6 We report a new class of highly CO2-soluble fluorinated acrylate polymers which incorporate multiple ligands for efficient metal extractions. These polymers can be modified with a wide range of specific metal-binding ligands, such as β-diketones and phosphonates, and extract metals into liquid CO2 with high ligand-to-metal efficiency (ligand efficiency). The polymer-ligand-metal complexes can be readily precipitated from solution by altering the CO2 pressure. We have used in situ absorption and emission spectroscopy to quantify the metal extractions and to characterize the polymer-bound metal species. Luminescence studies on europium-bound systems are used to probe the environment surrounding the metal as extracted in carbon dioxide. Wai pioneered the use of scCO2 for extracting metal ions from a wide range of matrixes almost a decade ago. The impetus for metal extractions using CO2 has been to capitalize on enhanced diffusivity (mass transfer) and pressure tunability, which allows for facile separations. Most of these efforts have employed common ligands as extractants, such as β-diketones, dithiocarbamates, and †This paper is dedicated to Dr. Joseph Breen (deceased July 19, 1999). * To whom correspondence should be addressed. Phone: 505667-5636. Fax: 505-667-9905. E-mail:
[email protected]. ‡ Los Alamos National Laboratory. § University of North Carolina.
organophosphorus reagents.1-4 The solubilities of the metal complexes are, however, often very low, limiting practical applications.5a The solubility of the metal complexes can be enhanced by incorporating fluorinecontaining moieties into the chelating ligands; however, the ligand efficiency is not greatly improved.1f,2a For example, at a 10-fold excess of chelating agent, acetylacetone had an extraction efficiency of 19% versus 37% for hexafluoroacetylacetone in the extraction of Zn2+ in liquid carbon dioxide and 16% versus 20%, respectively, in scCO2.2a Even in the case of the highly soluble uranyl complex, UO2(NO3)2‚2TBP (0.49 M, 225 atm, 40 °C; TBP ) [CH3(CH2)3O]3PO),1d a large excess of extractant is commonly used to effect efficient extraction.1e,h Beckman has shown that attaching “CO2-philic” moieties consisting of either highly fluorinated or polysiloxane groups, including oligomer or polymer chains, to ligands dramatically increases the solubility of the resulting metal complex and the ligand efficiency.5a-d For example, highly CO2-soluble perfluoropolyether-based extractants with dithiocarbamate, dithiol, and picolylamine headgroups require only 1.5 equiv of chelate for efficient extractions of mercury (57-87%), lead (20-75%), and arsenic (31-58%) in liquid CO2 (140 bar at room temperature). Our approach differs significantly from strategies to date in that we incorporate multiple ligand sites onto the backbone of a single highly soluble polymer chain, allowing for very high concentrations (30 wt % polymer in CO2) of ligand-containing polymer. This strategy also greatly increases the ability to incorporate a variety of traditional chelating groups for extraction by simple copolymerization and may allow for ligand recycle.
10.1021/ie000811b CCC: $20.00 © 2001 American Chemical Society Published on Web 02/07/2001
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Experimental Section Materials and Methods. The fluorinated monomer CH2CHCO2(CH2)2(CF2)nF (n ) 6-10, n ∼ 8; ZONYL TA-N) was obtained from DuPont. 2-(Methacryloyloxy)ethyl acetoacetate, 2-(dimethylamino)ethyl methacrylate (DMAEMA), 2-hydroxyethyl methacrylate (HEMA), and glycidyl methacrylate (GMA) were purchased from Aldrich. 2-Bromoethyl acrylate (BEA) was obtained from Polysciences, Inc. 4-Vinylbenzyl acetylacetone (VBA) was synthesized according to known methods.7 All monomers were purified by passing through a column of alumina under nitrogen immediately prior to use. A warm water jacket around the column was used to maintain the ZONYL TA-N monomer as a liquid during purification. R,R,R-Trifluorotoluene (TFT; 99+ % pure, Aldrich) was sparged with nitrogen immediately prior to use. 2,2′-Azobis(isobutyronitrile) (AIBN; Kodak) was recrystallized from methanol. Triethyl phosphite and 1,1,2-trichlorotrifluoroethane were used as received from Aldrich. SFC/SFE-grade carbon dioxide was purchased from Air Products and Chemicals, Inc., and used without further purification. Extractions were carried out in custom-built8 stainless steel reactors modified with three sapphire windows to allow for luminescence spectroscopy studies. ICP-AES analyses were carried out using a Varian Liberty ICP-AES. 31P NMR spectra were obtained on a Bruker 500 MHz spectrometer. Copolymer Syntheses. A representative synthesis is given for the 10 mol % bromofunctionalized polymer. Copolymers of 5, 15, and 25 mol % were synthesized analogously to copolymers with 2-(methacryloyloxy)ethyl acetoacetate, DMAEMA, HEMA, GMA, and VBA. p(TA-N-co-BEA). 10 mol % BEA Copolymer. A flame-dried, 200-mL round-bottomed flask was charged with 10 g (19 mmol) of ZONYL TA-N monomer and 100 mL of TFT. The solution was sparged with nitrogen. BEA (0.26 mL, 2.1 mmol) was added via syringe to the stirred solution. AIBN initiator of 1 mol % (0.035 g, 0.21 mmol) was added as a solid. The solution was heated under a nitrogen atmosphere at 65 °C for approximately 20 h. The polymer was precipitated by pouring the TFT solution into MeOH. After filtration, the white powder was dried in vacuo (7.63 g, 74% yield). Phosphonate Polymer. A 50-mL round-bottomed flask was charged with 10 mol % BEA copolymer (3.00 g) and excess triethyl phosphite (0.106 mL, 0.103 g, 0.6 mmol). The flask was fitted with a reflux condenser and the neat mixture was heated at reflux for 2 h. Volatiles were distilled in vacuo at 90 °C. The resulting phosphonate polymer was characterized by 31P NMR using 1,1,2-trichlorotrifluoroethane (Freon-113) as the solvent with an acetone-d6 insert. Multiple broad phosphorus resonances are indicative of phosphonate [-P(O)(OR)2] substitution. Solubility Studies. Cloud-point data were measured in liquid and scCO2 by adjusting the pressure in a highpressure view cell equipped with sapphire windows2 for visual observation of the solution turbidity. Loaded polymers of 5, 10, 15, and 25 mol % exhibited the following solubility characteristics at 4 w/v % in CO2. Results are summarized in Table 1. A variable-volume cell was used to measure the cloud point of the 15% acetoacetate-functionalized polymer as a function of concentration. The cloud point was designated as the pressure measured at the first visible sign of solution turbidity. Representative cloud-point data are presented in Figure 1. Up to 30 w/w % polymer
Table 1. Cloud Points in Carbon Dioxide (4 w/v %)a CP in bar % acetoacetate
0 °C
25 °C
40 °C
60 °C
5 10 15 25
72
99 103 112 190
161 162 166 249
217 229 236 317
CP in bar % VBA
0 °C
25 °C
40 °C
60 °C
5 10 15
-
100 114 131
163 167 183
218 230 256
a
-, soluble at vapor pressure.
Figure 1. Cloud-point curve for a 15 mol % acetoacetate polymer at 25 °C.
solutions were tested and found to be soluble in CO2, although at these extremely high concentrations the polymer/CO2 solution was slightly hazy, prohibiting a determination of the exact cloud point by the above criteria. Extractions. Extractions were carried out with both solid copper nitrate and copper nitrate spiked on filter paper. For solid copper nitrate extractions, Cu(NO3)2‚ 2H2O was weighed and placed in the extraction vessel. The large crystals were then pulverized using a spatula. Filter paper samples were prepared by spiking a 1 × 1 cm square of Whatman 42 filter paper with an aqueous stock solution of copper nitrate applied using a microsyringe. The filter paper was allowed to air-dry overnight. The stock solution was diluted and analyzed by ICPAES. A representative filter paper sample prepared in the same manner as the sample used in the extraction was treated with a 1:1 nitric acid solution overnight. The acid solution was diluted and analyzed by ICP-AES to determine the initial amount of metal deposited on the matrix. A high-pressure vessel (20 or 6.4 mL) with three sapphire windows was charged with a copper nitrate sample (crushed solid or filter paper) and polymer (acetoacetate or acetylacetone) along with a Tefloncoated magnetic stir bar. The cell was pressurized to 140 bar using an ISCO syringe pump. In all extractions the polymer dissolves completely in CO2, and no precipitation of the polymer is observed as long as the pressure is maintained at 140 bar. Stirring of the mixture of polymer solution and copper nitrate sample was interrupted periodically for UV-vis spectroscopy studies. When the amount of copper in solution had reached a maximum as judged by UV-vis, separation of the polymer with bound metal from the residual copper salts was carried out. Typically, the extraction was stirred for 3 days. A representative series of UVvis experiments is shown in Figure 2. Two techniques were used for the separation of the polymer with chelated metal from the residual copper
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Figure 2. UV-vis spectra for extraction of copper nitrate with a 10 mol % acetoacetate polymer.
salts. One technique involved flushing the polymer out of the reactor by dynamically flowing CO2 through the reactor, with the CO2 and polymer exiting the cell through a 1/16-in. stainless steel tube inserted into a flask or vial of 1,1,2-trichlorotrifluoroethane. Purging with CO2 continued until all of the polymer was removed as judged by inspection of the sapphire window of the depressurized reactor for residual polymeric material. Typically the volume of CO2 purged through the reactor at 140 bar was 3 times the reactor volume. This dynamic extraction technique frequently suffered from restrictor clogging. A second separation technique involving filtration alleviated restrictor clogging. A 0.5-µm in-line filter was used to connect the extraction vessel (20 mL) with an empty reactor (6.4 mL). A valve between the two cells was opened, and the pressure was allowed to equilibrate between the two. The valve was closed again, and CO2 was added to the 20-mL vessel to pressurize back up to 140 bar. The pressure between the two cells was equilibrated again, and the procedure was repeated until the pressure in both cells reached 140 bar. The receiving vessel was then depressurized through a 1/16in. stainless steel tube inserted into a vial of 1,1,2trichlorotrifluoroethane to capture any polymer flushed out of the cell. This procedure of emptying the extraction vessel into the receiving vessel was repeated until the receiving cell had been emptied seven times (over two 20-mL cell volumes). Both cells were then depressurized. The polymer was collected from the receiving cell by rinsing with 1,1,2-trichlorotrifluoroethane, and the filter paper was collected from the extraction vessel for analysis. Luminescence Measurements. Steady-state luminescent measurements were done on a PTI C-60 fluorimeter with a 75-W xenon lamp and a R955 PMT detector. Lifetime studies were done with a nitrogen dye laser using 395-nm excitation and a gated PMT detector. Lifetimes were determined by monitoring the europium luminescence intensity at 618 nm. Samples of EuCl3 hexahydrate were used as received from Strem, and the hydrated D2O complex was prepared by dissolving anhydrous EuCl3 in D2O and then drying the material in a vacuum oven overnight. The concentrations of H2O and D2O were well below the solubility of water in CO2 under the experimental conditions. It should be noted that copper can potentially interfere with europium luminescence in the mixed-metal experiments. Copper quenches europium because the absorption spectrum overlaps with the emission spectrum of europium. This type of Forster energy transfer is highly distancedependent (1/r6) and is expected to be negligible for metals bound to the polymer. Analyses. In extractions carried out with pulverized solid copper nitrate, residual copper nitrate was re-
moved from the cell by rinsing with deionized water (several cell volumes). The aqueous rinses were combined and diluted to a known volume for ICP-AES analysis. Filter paper samples with residual copper nitrate were allowed to stand overnight in 50% nitric acid, followed by dilution, and ICP-AES analysis. Competition Studies. A 20-mL high-pressure cell was charged with 0.0319 g (0.087 mmol) of EuCl3‚6H2O (pulverized), 0.0268 g (0.12 mmol) of Cu(NO3)2‚2(H2O) (pulverized), and 1.17 g (0.239 mmol of acetoacetate functionality) of acetoacetate copolymer. The cell was pressurized to 140 bar, and the extraction was monitored by both UV-vis and luminescence spectroscopy. Copper extraction was evident by the characteristic UV-vis spectrum. No emission was observed for europium in the CO2 phase. Results and Discussion Our polymer-based extraction system capitalizes on the fluorinated acrylate polymers that DeSimone et al. have demonstrated to be highly soluble in liquid and scCO2.9 We have developed a general approach for incorporation of ligands into these polymers in two synthetic steps or less through copolymerization of functionalized monomers bearing either the ligand or a precursor. Free-radical copolymerization of a fluorinated acrylate (CH2CHCO2(CH2)2(CF2)6-10F, ZONYL TA-N) with 5-25 mol % of an acrylate, methacrylate, or styrenic monomer containing a ligand or point of attachment for a ligand (eq 1) leads to a random copolymer that is soluble in CO2 and fluorocarbons. Ligands can be incorporated directly with comonomers such as 4-vinylbenzyl acetylacetone (2a) and 2-(methacryloyloxy)ethyl acetoacetate (2b).
Alternatively, polymer modification can be carried out postpolymerization to attach ligands. For example, phosphonate ligands were attached via Arbuzov reaction of triethyl phosphite with the bromofunctionalized copolymer (3e; eq 2). These two synthetic approaches make it possible to introduce a wide variety of potential ligand sites with ease.
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Solubility studies indicate that these polymer-ligand systems are extremely soluble in both liquid and scCO2 (greater than 20 wt %). Cloud points for these materials decrease with decreasing temperature, indicating an increase in solubility with increasing CO2 density. Typically, for up to 15 mol % incorporation of ligandcontaining comonomer, the copolymers 3a-f were found to be readily soluble in liquid CO2 at 25 °C at 140 bar. Remarkably, they are even soluble at 0 °C at the CO2 vapor pressure (40 bar). Increasing the concentration of polymer in liquid CO2 at 25 °C has little effect on the cloud point. In fact, concentrations as high as 30 w/w % of 15 mol % 3b have been achieved in liquid carbon dioxide. The observation that the acetoacetate monomer weight percent does not significantly alter the cloud point from the polymer, whereas a consistent increase in the cloud-point pressure is readily apparent for the VBA monomer because the weight percent increases from 5% to 15% can be attributed to either a simple difference in the CO2 solubility of the monomers or a difference in the polymer configuration in CO2. It is possible that the acetoacetate polymer is able to fold in a manner to generate ligand domains that are insulated from the CO2 solvent as observed for block copolymers. We have demonstrated the utility of these highly CO2soluble ligand-polymer extractants for two simple metal systems, copper and europium. Copper was successfully extracted from pulverized crystals and from spiked filter paper samples utilizing β-diketone-functionalized polymers 3a and 3b at 10% and 15 ligand mol %, respectively. Homogeneously clear, lightly colored solutions were formed in liquid CO2, by stirring pulverized copper nitrate in CO2 at 140 bar at room temperature. The in situ UV-vis spectroscopy shows an absorbance in the visible range at 650-750 nm, corresponding to the d-d transition for copper (in agreement with values previously reported for copper β-diketones in CO2).4f The peak maximum is observed to blue shift slightly over time presumably because of polymer rearrangement after the initial metal binding. The shift to higher energy is consistent with displacement of water and binding of a second ligand over time, which would lead to a stronger ligand field around the copper center. The more intense, higher energy band at
