Article pubs.acs.org/EF
CO2 Capture Using 2,2-Dialkylpropane-1,3-diamines Michael J. O’Brien,*,† Rachel L. Farnum,† and Robert J. Perry† †
GE Global Research, Chemisty and Chemical Engineering, 1 Research Circle, Niskayuna, New York 12309, United States ABSTRACT: A series of 2,2-dialkylpropane-1,3-diamines were synthesized and evaluated as potential phase-changing CO2 sorbents. In general, these compounds readily absorbed CO2 to form solid carbamate salts, which were relatively insensitive to the presence of moisture. This is one of the key performance attributes phase-changing sorbents must possess. However, these diamines were found to be less thermally stable in air than expected. The main reaction products obtained during heat aging at 150 °C appeared to be 1,4,5,6-tetrahydropyrimidine derivatives.
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INTRODUCTION Concern over the potential impact of anthropogenic carbon dioxide on global climate change has spawned numerous efforts aimed at mitigating CO2 emissions. The U.S. Department of Energy (DOE) has set a target for the capture and sequestration of 90% of the CO2 in flue gas generated from coal-fired power plants, with no more than a 35% increase in the cost of electricity.1 Alkanolamines have been the most widely studied materials for postcombustion CO2 capture from flue gas.2−7 Of these materials, aqueous monoethanolamine (MEA) has been the most commonly used solvent, having been used for over half a century for natural gas purification and food-grade CO2 production8−10 and more recently as a candidate for CO2 capture from flue gas.11−15 However, MEA-based systems have several negative attributes that have hindered their scale-up, including the huge parasitic energy demand required to heat and condense large quantities of water. This has resulted in an estimated increase in the cost of electricity (COE) of about 80% and a decrease in power plant efficiency of 30%.1 In addition, MEA is relatively volatile and corrosive16,17 and has poor thermooxidative stability.13,18,19 Our focus has been on the use of aminosilicones for the postcombustion capture of CO2. Initial work targeted using these materials in a process similar to that employed with the alkanolamines, in that the goal was to develop a sorbent solution that remained liquid throughout the absorption/desorption cycle. We reported that aminosilicones blended with glycol cosolvents are promising alternatives to the benchmark aqueous organic amine systems.20,21 More recently, we reported the use of amino disiloxanes as phase-changing sorbents in a new process as illustrated in Figure 1.22 In the first step, a liquid aminosilicone sorbent is sprayed into a stream of flue gas where it reacts with CO2 to form solid carbamate salts. These salts are then captured, allowing the scrubbed flue gas to be vented to the atmosphere. In the third process step, the solid is transported from a low to a high pressure regime, where it is then heated to decompose the salts and produce pure CO2, along with regenerated liquid aminosilicone, which can then be reused in Step 1. The optimum sorbent for use in this scheme would have many of the same attributes that are required in the classic aqueousbased amine process. For example, the material needs to have © 2012 American Chemical Society
Figure 1. General process for CO2 capture using phase-changing sorbent.
high CO2 capture efficiency, fast reaction with CO2, ready desorption on heating, low volatility, low specific heat, good thermal stability, low toxicity, low corrosivity, etc. However, in addition to these properties, the carbamate salt formed on reaction with CO2 must be a dry powder so that it can readily be collected and transported to the desorption unit. Furthermore, it must be fairly insensitive to moisture, which is present in the flue gas, so that the form of the solid does not change when it absorbs water. Particularly problematic would be a change from a dry powder under anhydrous conditions to a sticky semisolid in a moist environment. Of the amino functional disiloxanes evaluated as potential phase-changing sorbents, only two of them, namely bisReceived: October 10, 2012 Revised: November 30, 2012 Published: December 6, 2012 467
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water and once each with dilute sodium hydrosulfite, water, and saturated sodium chloride. After drying over anhydrous sodium sulfate, the ether was stripped under reduced pressure to give 8.9 g of crude product. This material was then recrystallized twice from a mixture of methanol and water (30 mL + 4 mL). The result was 6.9 g of a white solid (67% yield) with a melting point of 30−32 °C. The following spectral data were obtained: 1H NMR (CDCl3) δ 1.98 (q, J = 7.6 Hz, 2H), 1.89 (m, 2H), 1.65 (m, 2H), 1.2−1.45 (m, 17H), 0.87 (t, J = 7.0 Hz, 3H). 13C NMR 115.66, 38.77, 37.36, 31.86, 31.54, 29.48, 29.38, 29.26, 29.16, 28.84, 25.59, 22.66, 14.10, 9.87. Exact mass MS: Calc’d for C15H27N2 (M+H+); 235.2174. Found; 235.2166. 2,2-Dihexylmalononitrile (7).23 Malononitrile (6.90 g, 104 mmol) and tetrabutylammonium iodide (2.5 g, 6.8 mmol) were dissolved in THF (45 mL) under nitrogen. After cooling to 0 °C in an ice bath, potassium t-butoxide (11.7 g, 104 mmol) was added. The result was stirred for approximately 10 min, and then 1-iodohexane (22.1 g, 104 mmol) was added dropwise over several minutes. A few milliliters of fresh THF were used to rinse all of the alkyl halide into the reaction mixture. The ice bath was removed, and the reaction was allowed to warm to room temperature. A water bath was used to keep the reaction temperature from rising beyond this point. After 2 h, the mixture was cooled back down in an ice bath so that the second portions of reagents could be added. First, more KOtBu (14.1 g, 126 mmol) was added followed about 15 min later by the dropwise addition of another portion of 1-iodohexane (26.4 g, 124 mmol). At this point the ice bath was removed, and the reaction mixture was allowed to warm back up to room temperature where it was kept overnight. Next, the mixture was filtered to remove solids, and the THF was stripped on a rotary evaporator. Heptane was added, and the result was washed twice with 60 mL portions of a 2:1 mixture of methanol:water. The heptane phase was then washed with dilute sodium hydrosulfite and saturated sodium chloride solutions and dried over anhydrous sodium sulfate. A small amount of silica gel was added with the drying agent in order to take out most of the color on filtration. After the stripping the heptane on the rotary evaporator 21.9 g of crude product was obtained as a yellow oil. This was then crystallized from a mixture of 35 mL of methanol and 4.5 mL of water. After drying under vacuum at room temperature, 18.3 g (75% yield) of product was obtained as a light yellow solid. The melting point of this compound was found to be 26−28 °C. The following spectral data were obtained: 1H NMR (CDCl3) δ 1.90 (m, 4H), 1.66 (m, 4H), 1.26−1.44 (m, 12H), 0.90 (t, J = 7.2 Hz, 6H). 13 C NMR 115.85, 37.81, 31.30, 28.50, 25.51, 22.41, 13.95. 2-Decyl-2-ethylpropane-1,3-diamine (6). Lithium aluminum hydride (2.94 g, 310 mmol H) was mixed with ether (140 mL) under nitrogen and cooled in an ice bath. A solution of 2-decyl-2ethylmalononitrile (6.00 g, 51 mmol CN) in ether (10 mL) was added dropwise over 10−12 min. The ice bath was then removed, and the reaction mixture was allowed to warm to room temperature. After 3 h, the reaction was cooled back down using an ice bath, and then 16.5 mL water was added carefully to quench excess hydride. Once this was complete, 0.8 mL 50% sodium hydroxide was added, and the mixture was allowed to warm back up to room temperature. The solids were then filtered out, and the resulting solution was dried over anhydrous sodium sulfate and stripped on a rotary evaporator. The result was 6.02 g of product (97%) as a light yellow oil. 1H NMR (CDCl3) δ 2.39 (s, 4H), 0.95−1.2 (m, 20H), 0.93 (br.s., 4H), 0.75 (t, J = 7.0 Hz, 3H), 0.66 (t, J = 7.5 Hz, 3H). 13C NMR 45.55, 40.26, 31.87, 31.81, 30.58, 29.61, 29.54, 29.24, 24.44, 22.76, 22.58, 14.00, 7.28. Exact mass MS: Calc’d for C15H35N2 (M+H+); 243.2800. Found; 234.2792. 2,2-Dihexylpropane-1,3-diamine (8). Lithium aluminum hydride (4.64 g, 489 mmol H) was mixed with ether (150 mL) under nitrogen and cooled in an ice bath. A solution of 2,2-dihexylmalononitrile (9.4 g, 80 mmol CN) in ether (25 mL) was added dropwise over 10 min. The ice bath was then removed, and the reaction mixture was allowed to warm to room temperature. After 3 h, the reaction was cooled back down using an ice bath, and then 20 mL water was added carefully to quench excess hydride. Once this was complete, 0.8 mL of 50% sodium hydroxide was added, and the mixture was allowed to warm back up to room temperature. The solids were then filtered out, and the resulting solution was dried over
(aminopropyl) tetramethyldisiloxane (1) and the cyclic material (2), met this final criterion. All of the other materials explored, while readily forming solids on reaction with dry CO2, produced low quality semisolids in the presence of moisture. Given this, we hoped to be able to identify additional candidate materials. Based on the excellent results obtained with cyclocarbosiloxane (2) we decided to explore organic materials that closely emulated this structure. Thus, we wanted to prepare materials that combined a 1,3-propanediamine group capable of rapid reaction with CO2 with hydrophobic functionality that would render the resulting solid salts less susceptible to softening in the presence of moisture. Although it was anticipated that some thermal stability would be compromised in this way due to the lower oxidative stability of organics relative to siloxanes, it was felt that further candidates needed to be identified in order to fully vet the new phase-changing process. Included in this paper are the synthesis and characterization of 2,2-dialkylpropane-1,3-diamines of the general structure shown below:
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EXPERIMENTAL SECTION
General. 1H and 13C NMR spectra were obtained on a Bruker 400 MHz instrument. Mass spectra were acquired on a JEOL AccuTofF JMS T100 LC-MS instrument retrofitted with an Ionsense DART (Direct Analysis in Real Time) ion source in place of the normal Electrospray source used for LCMS. Helium (2.4 L/min) was used as the DART gas. The gas heater (post glow discharge) of the DART source was set to 240 °C. The analytes experience temperatures far below this during analysis. Melting points were measured on an Electrothermal Melting Point Apparatus and are uncorrected. Gas chromatographic analyses were performed on an HP6890 instrument with a TCD detector, using a 30 m, 0.32 mm inner diameter HP-5 column with 0.25 μm film thickness. The initial oven temperature was held at 40 °C for 2 min and then ramped at 10 °C/minute. The final oven temperature was 280 °C, which was held for 5 min. Synthesis of Diamines. 2-Decyl-2-ethylmalononitrile (5). To an ice cold solution of 2-ethylmalononitrile (4.7 g, 50 mmol) in THF (25 mL) under nitrogen was added potassium t-butoxide (KOtBu, 5.4 g, 48 mmol). The result was stirred for approximately 25 min at which point the reaction mixture was homogeneous. A solution of 1-iododecane (11.9g, 44 mmol) in THF (10 mL) was then added dropwise. After addition was complete, a few milliliters of fresh THF were used to rinse the addition funnel, and then the ice bath was removed so that the mixture could warm to room temperature. After 2 h, the reaction was checked by proton NMR- this showed it to be approximately 92% complete. Another sample was evaluated after 6 h; little additional reaction was observed. Thus another 0.4 g of 2-ethylmalononitrile and 0.5 g of potassium t-butoxide were added, and the reaction was allowed to continue overnight. At this point, the mixture was filtered to remove solids, and the resulting THF solution was stripped on a rotary evaporator. The residue was dissolved in ether, and the resulting solution was washed twice with 468
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anhydrous sodium sulfate and stripped on a rotary evaporator. The result was 9.41 g of product (97%) as a light yellow oil. GC analysis showed it to be 95.5% pure. Further purification could be accomplished via vacuum distillation (bp = 106−111 °C/0.78 mmHg). 1H NMR (CDCl3) δ 2.48 (s, 4H), 1.05−1.3 (m, 20H), 0.92 (br.s., 4H), 0.85 (t, J = 6.6 Hz, 6H). 13C NMR 46.11, 40.37, 32.50, 31.86, 30.29, 22.84, 22.68, 14.06. Exact mass MS: Calc’d for C15H35N2 (M+H+); 243.2800. Found; 234.2802. LAH Reduction of Dibenzylmalononitrile (9) in Ether/THF. To an ice cold slurry of lithium aluminum hydride (LAH, 1.94 g, 204 mmol H) in ether (60 mL) was added dropwise over 15 min a solution of dibenzylmalononitrile (4.20 g, 34 mmol CN) in a mixture of ether (10 mL) and THF (25 mL). Once addition was complete, a small amount of THF was used to rinse all of the nitrile into the reaction. At this point the ice bath was removed, and the mixture was allowed to warm to room temperature. After 3 h, the reaction mixture was once more placed in an ice bath, and 8.4 mL water was carefully added to quench the excess hydride. 0.4 mL of 50% NaOH was then added, and the mixture was allowed to warm to room temperature. The solids were filtered out, and the solution thus obtained was dried over anhydrous sodium sulfate and stripped on a rotary evaporator. This gave 3.43 g (79%) of crude product as a light yellow oil. Proton NMR indicated that most of the material was the desired product; however, there was a significant amount of another compound as well. A portion of this mixture was purified via column chromatography (silica gel with an approximate 85:14:1 mix (volume ratio) of chloroform:methanol:concentrated ammonium hydroxide) to provide analytical samples. The first peak to come off was the monoamine 2-benzyl-3-phenylpropan-1-amine (11). 1H NMR (CDCl3) δ 7.36 (t, J = 7.4 Hz, 4H), 7.25 (m, 6H), 2.69 (m, 6H), 2.09 (m, 1H), 1.10 (br s, 2H. 13C NMR 140.92, 129.22, 128.41, 125.98, 45.23, 44.30, 38.34. Exact mass MS: Calc’d for C16H20N2 (M+H+); 226.1596. Found; 226.1595. Then the diamine 2,2-dibenzylpropane-1,3-diamine (10) eluted. 1H NMR (CDCl3) δ 7.30 (t, J = 7.2 Hz, 4H), 7.24 (m, 6H), 2.69 (s, 4H), 2.55 (s, 4H), 1.15 (br s, 4H. 13C NMR 138.62, 130.51, 128.16, 126.18, 45.30, 43.80, 40.49. Exact mass MS: Calc’d for C17H23N2 (M+H+); 255.1861. Found; 255.1866. With the reference spectra in hand, it was determined that the initial molar ratio of di- to monoamine in the crude product was 54:46. Borane Reduction of Dibenzylmalononitrile (9). Sodium borohydride (3.9 g, 0.10 mols) was mixed with 80 mL of dimethoxyethane (DME) and cooled to −78 °C under nitrogen. A freshly prepared solution of bromine (2.4 mL, 0.047 mols) in DME was then added dropwise. Once addition was complete, a few milliliters of DME were used to rinse the addition funnel. After another 15 min, the dry ice/ acetone bath was replaced with a water ice bath. As the temperature approached 0 °C, a solution of dibenzyl malononitrile (5.0 g, 0.041 mols CN) in 35 mL of DME was added over a few minutes. At this point, the mixture was allowed to slowly warm to room temperature where it was kept overnight. The reaction was then quenched by carefully adding it to 125 mL of ice cold 10% HCl. Some foaming and solid formation were observed during this process. Therefore, small portions of isopropanol were added to knock the foam down. After the mixture warmed to room temperature, it was concentrated to about 40% of its original volume on a rotary evaporator. The result was cooled to 0 °C and made basic using 50% NaOH. The solution was allowed to warm to room temperature, and then it was extracted three times with chloroform. The combined organics were dried over anhydrous sodium sulfate, and the solvent was removed on a rotary evaporator. The result was 3.9 g of 2,2dibenzylpropane-1,3-diamine (10) (75% yield) as a viscous, slightly hazy, light yellow oil. Proton NMR indicated the material was 90−95% pure. 2,2-Bis(cyclohexylmethyl)malononitrile (13). (Bromomethyl)cyclohexane (13.9 mL, 99.6 mmol) was mixed with malononitrile (3.0 g, 45 mmol) and tetrabutylammonium iodide (1.68 g, 4.5 mmol) and stirred under nitrogen for about 30 min. The reaction mixture was then cooled in an ice bath, and potassium t-butoxide (5.56 g, 49.5 mmol) was added. The ice bath was removed, and the mixture was allowed to warm. As it neared room temperature an exotherm was noted. A room temperature water bath was then used to control this exotherm. After
stirring for an hour, the mixture was cooled once again in an ice bath, and another portion of potassium t-butoxide (5.56 g, 49.5 mmol) was added. The viscosity rapidly increased to the point where the magnetic stir-bar would no longer mix the blend. Thus, the ice bath was removed, and the reaction was allowed to warm to room temperature. Even at this point stirring could not be maintained. Thus the reaction mixture was heated to ca. 45 °C using an oil bath. It should be noted that efficient stirring resumed as the temperature reached about 35 °C. The reaction was then allowed to proceed overnight. Next, the mixture was cooled and diluted with 50 mL of methylene chloride, and the resulting slurry was filtered into a separatory funnel. The solution was washed twice with water and once with saturated sodium chloride. After drying over anhydrous sodium sulfate, the solvent was stripped on a rotary evaporator to yield 7.7 g (66%) crude product as an orange solid. The material was then recrystallized from methanol and dried to give 2.8 g of pure product. The melting point was found to be 95−97 °C. 1H NMR (CDCl3) δ 1.92 (br d, J = 12.4 Hz, 4H), 1.62−1.82 (m, 12H), 1.32 (m, 4H), 1.0−1.23 (m, 6H). 13C NMR 116.45, 46.03, 35.47, 34.09, 33.46, 25.88, 25.85. Exact mass MS: Calc’d for C17H27N2 (M+H+); 259.2174. Found; 259.2166. 2,2-Bis(cyclohexylmethyl)propane-1,3-diamine (14). To an ice cold slurry of lithium aluminum hydride (1.76 g, 186 mmol H) in ether (60 mL) under nitrogen was added a solution of 2,2bis(cyclohexylmethyl)malononitrile (4.0 g, 31 mmol CN) in ether (40 mL). At this point the ice bath was removed, and the reaction was allowed to warm to room temperature. After 3.5 h the mixture was cooled back down to ∼0 °C, and excess hydride was quenched via the careful addition of 6 mL of water. Next, 0.4 mL of 50% NaOH was added, and the mixture was allowed to warm to room temperature. At this point, the solids were removed via filtration. Several portions of fresh ether were used to wash these solids. The combined liquids were then dried over anhydrous sodium sulfate, and the ether was removed on a rotary evaporator. The result was 3.6 g (87% yield) of product as an offwhite solid. Proton NMR showed this material to be 90−95% pure. Recrystallization from heptane produced 2.4 g of product (57%) as a white solid with a melting point of 88−90 °C. 1H NMR (CDCl3) δ 2.50 (s, 4H), 1.52−1.68 (m, 10H), 1.08−1.32 (m, 8H), 1.07 (d, J = 5.0 Hz, 4H), 0.88−1.01 (m, 8H). 13C NMR 46.74, 42.29, 40.64, 36.01, 32,70, 26.66, 26.20. Exact mass MS: Calc’d for C17H35N2 (M+H+); 267.2800. Found; 267.2794. 1,1-Dicyano-4,4-dimethylcyclohexane (16). Malononitrile (4.97 g, 75.2 mmol) was dissolved in THF (35 mL) and cooled to 0 °C under nitrogen. Potassium t-butoxide (7.63 g, 68 mmol) was then added. After stirring 15 min, a solution of 3,3-dimethyl-1,5-diiodopentane (11.0 g, 62.5 mmol RI) in THF (10 mL) was added dropwise over approximately 10 min. At this point, the ice bath was removed, and the reaction was allowed to warm to room temperature. Proton NMR the next day showed the reaction to be about 27% complete. Thus the mixture was heated to 40 °C where it was kept for two more days. At this point the reaction was cooled to room temperature and filtered to remove solids. The THF was stripped under vacuum, and the resulting residue was partitioned between chloroform and water. The chloroform phase was washed twice more with water, once with dilute sodium hydrosulfite, and once with saturated sodium chloride. Anhydrous potassium carbonate and a small amount of silica gel were added, and then after several minutes, these solids were removed by filtration. Upon removal of the chloroform on a rotary evaporator, 3.9 g (78%) product was obtained as a light yellow solid. While the material could be used as in for the next reaction, further purification could be accomplished via recrystallization from heptane. The melting point was found to be 47− 49 °C. 1H NMR (CDCl3) δ 2.14 (t, J = 6.0 Hz, 4H), 1.53 (t, J = 6.2 Hz, 4H), 0.98 (s, 6H). 13C NMR 115.94, 34.26, 32.28, 30.82, 29.03, 27.82. Exact mass MS: Calc’d for C10H15N2 (M+H+); 163.1235. Found; 163.1229. 1,1-Bis(aminomethyl)-4,4-dimethylcyclohexane (17). To an ice cold slurry of lithium aluminum hydride (2.73 g, 288 mmol H) in ether (90 mL) under nitrogen was added dropwise a solution of 1,1-dicyano4,4-dimethylcyclohexane (3.93 g, 48 mmol CN) in ether (15 mL). After addition was complete, a few milliliters of fresh ether was used to rinse the addition funnel. The ice bath was removed, and the reaction mixture was allowed to warm to room temperature. After 3 h, the mixture was 469
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Figure 2. Synthesis of 2-decyl-2-ethylpropane-1,3-diamine (6).
Figure 3. Synthesis of 2,2-dihexylpropane-1,3-diamine (8). cooled back down to ∼0 °C, and excess hydride was quenched via the careful addition of 11.8 mL of water. Next, 0.5 mL of 50% NaOH was added, and the mixture was allowed to warm to room temperature. At this point, the solids were removed via filtration. Several portions of fresh ether were used to wash these solids. The combined liquids were then dried over anhydrous sodium sulfate, and the ether was removed on a rotary evaporator. The result was 3.08 g (75% yield) of product as an off-white solid with a melting point of 54−58 °C. Proton NMR showed this material to be 90−95% pure. After recrystallization from heptane, the resulting solid had a melting point of 60−62 °C. 1H NMR (CDCl3) δ 2.52 (s, 4H), 1.21(m, 4H), 1.51 (m, 4H), 1.07 (br s, 4H), 0.81 (s, 6H). 13 C NMR 47.03, 37.20, 34.31, 30.04, 28.51, 22.21. Exact mass MS: Calc’d for C10H23N2 (M+H+); 171.1861. Found; 171.1853. 5,5-Dihexyl-1,4,5,6-tetrahydropyrimidine (24). 2,2-Dihexylpropane-1,3-diamine (8) (1.0 g, 4 mmol) was mixed with formic acid (0.20 g, 4 mmol). An exothermic reaction ensued yielding a white solid formate salt (22). 1H NMR (CDCl3) δ 8.59 (s, 1H), 5.78 (br s, 1H), 2.82 (s, 4H), 1.1−1.3 (m, 20H), 0.86 (t, J = 7 Hz, 6H). 13C NMR 169.09, 48.07, 37.38, 32.53, 31.75, 29.95, 22.71, 22.62, 14.05. The reaction mix was further heated to 140−150 °C under vacuum for 2 h. This gave crude (24) as a viscous brown oil. 1H NMR (CDCl3) δ 7.05 (s, 1H), 5.08 (br s, 1H), 2.94 (s, 4H), 1.1−1.3 (m, 20H), 0.85 (t, J = 6.8 Hz, 6H). 13 C NMR 146.79, 50.32, 34.15, 31.81, 31.59, 30.13, 22.82, 22.65, 14.07. Exact mass MS: Calc’d for C16H33N2 (M+H+); 253.2644. Found; 253.2679. CO2 Uptake Procedure. A 25 mL round-bottom flask was equipped with a stir paddle/stir shaft and a gas outlet adapter into which was inserted a small amount of glass wool. This apparatus was then weighed on an analytical balance. The test amine was added, and the weight of sample was measured. The flask was then immersed in a 40 °C bath, attached to an overhead stirrer, and equipped with a glass pipet aimed slightly above the surface of the liquid through which the CO2 was introduced, and the outlet tube was connected to a bubbler filled with silicone oil. The gas stream was produced via sublimation of dry ice and was passed through a drying tube for the “dry CO2” experiments (filled with blue indicating Drierite) prior to entering the reaction flask. For the “wet CO2” experiments, the drying tube was replaced with a gas bubbler filled with water. In either case, the rate was adjusted so that a steady stream of bubbles was observed in the bubbler. Care was taken to keep the flow from being excessive. Once the test was complete, the CO2 flow was discontinued, as was stirring. The sample was then cooled to room temperature, and the outside of the flask was washed with isopropanol to remove any silicone oil remaining from the oil bath. After drying the outside of the flask, the sample weight was then remeasured. The percent weight gain was calculated by dividing the difference between the final and initial sample weights by the initial value and then multiplying the result by 100. The percentage of theoretical values were derived by comparing the experimentally determined % weight gains to those expected based on the molecular weight of the test amine and the assumption that two amines are required per molecule of CO2. Thermal Stability Testing Procedure. Samples (1 to 3 g) were placed in 48 mL glass pressure vessels and sealed with Teflon screw tops. The vessels were then heated to 150 °C using silicone oil baths. At periodic intervals, the vessels were removed from the baths and allowed
to cool to room temperature. Small samples were then removed for GC analysis. The vessels were then resealed and returned to the oil bath. The experiment completed under an argon atmosphere was set up in a glovebox and sealed prior to heating. The sample for GC analysis was collected at the end of the experiment in the glovebox to maintain the anaerobic environment.
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RESULTS AND DISCUSSION The 2,2-dialkylpropane-1,3-diamines used in this study were prepared via reduction of 2,2-dialkyl malononitrile derivatives as shown below.
When contemplating the choice of R1 and R2 groups, the main consideration was their impact on the molecular weight of the resulting diamine; a balance needed to be maintained between higher molecular weights, which would favor the low volatility required to minimize evaporative losses of sorbent during desorption, and lower molecular weights, which would lead to higher amine concentrations, and therefore higher theoretical CO2 uptake capability. Our target capacity was a minimum of 15% weight gain on reaction with carbon dioxide. Furthermore, the commercial availability of the requisite alkyl halide starting materials had to be taken into account. Based on these considerations, the overall number of carbons in the combination of R1 and R2 was targeted at 13−14. First, asymmetric 2-decyl-2-ethylmalononitrile (5) was formed from reaction of the potassium salt of 2-ethylmalononitrile with 1-iododecane. This compound was found to be a solid, which after recrystallization from methanol/water exhibited a melting point of 30−32 °C. This material was then reduced to diamine (6) using lithium aluminum hydride (LAH) in ether (see Figure 2). The symmetric 2,2-dihexylmalononitrile23 (7) (see Figure 3) was initially obtained via reaction of malononitrile with two equivalents of 1-iodohexane and potassium t-butoxide in THF. The melting point of this compound was found to be 26−28 °C after recrystallization. Although this “standard” procedure provided some material for testing, the reaction was slow, requiring several days to reach completion. During this time, other side reactions such as dimerization of the malononitrile would occur, and therefore the yield was lower than expected. Given this, an alternate procedure was explored where the materials were reacted neat using a phase transfer catalyst as described in the literature.24 In this procedure, malononitrile, 1bromohexane, and phase-transfer catalyst (tetrabutylammonium 470
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Figure 4. Proton NMR showing terminal olefin peaks.
Figure 5. Targeted reduction of dibenzylmalononitrile (9).
Figure 6. Reduction of dibenzylmalononitrile in ether/THF.
stirring was easily maintained, and a yield of 75% was obtained after recrystallization. 2,2-Dihexylmalononitrile (7) was also reduced to diamine (8) in high yield and purity using LAH in ether (see Figure 3). Two diamines with cycloalkyl substitution were targeted next. The first was envisioned to be available via the LAH reduction of dibenzylmalononitrile24 (9) as shown in Figure 5. Initially this reaction protocol seemed to work, although the poor miscibility of dibenzylmalononitrile in ether necessitated the use of THF cosolvent. However, upon analysis of the crude reaction product, it was clear that a mixture of compounds had been obtained. Column chromatography allowed the isolation of two compounds, which NMR and high-resolution mass spectrometry showed to be the desired 2,2-dibenzylpropane1,3-diamine (10) (54 mol %) along with a significant amount of monoamine 2-benzyl-3-phenylpropan-1-amine (11) (46 mol %) (see Figure 6). Interestingly, the only other time we have observed this reductive decyanation side reaction was in the LAH reduction of the cyclic dinitrile (12), the precursor to the previously described diamino functional cyclocarbosiloxane (2).21 Dibenzylmalononitrile (9) and dinitrile (12) are not particularly structurally similar, so it was not immediately clear why these materials produced comparable results. However,
iodide, Bu4NI) were combined, and then KOtBu base was added at 0 °C. However, on subsequent warming to room temperature, an exotherm was observed of sufficient intensity that a vapor cloud formed in the reaction headspace. NMR analysis (see Figure 4) of the crude reaction mixture indicated that terminal olefins were being produced, presumably via an E2 elimination reaction. Since alkyl halide was consumed in this side reaction, lower than desired yields of product were once again observed. Another issue with the neat reaction approach was high solution viscosity resulting from the fact that most of the inputs, the malononitrile, phase transfer catalyst, and the KOtBu, are solids. This made stirring difficult, particularly during the low temperature phase of the process. After exploring several variations of these two approaches, it was determined that the best results were obtained using a modified combination of both. Thus, a small amount of THF was used in order to facilitate stirring. Furthermore, the alkyl halide (1-iodohexane) and base (KOtBu) were added to the malononitrile and Bu4NI in two portions. The first half of each reagent was charged and allowed to react for about two hours to mainly form the monohexyl malononitrile. Then the rest of the reagents were added, and the reaction was allowed to continue overnight. In this way, a less significant exotherm was noted, 471
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dicyano-4,4-dimethylcyclohexane (16) in 78% yield as a light yellow solid. The purity of the material was high enough that it could be used directly in the next step. However, an analytical sample was prepared via recrystallization from heptane. Next, the dinitrile (16) was reduced with LAH under standard conditions. This resulted in a 75% yield of diamine as an off-white solid. Purification was accomplished via recrystallization from heptane. 1,1-Bis(aminomethyl)-4,4-dimethyl-cyclohexane (17) was thus obtained as a white solid with a melting point of 60−62 °C. Dialkylpropane-1,3-Diamine CO2 Uptake Experiments. The dialkylpropane-1,3-diamines were tested in both dry and wet CO2 to see how well they matched their theoretical weight gain and to determine if they produced high quality solid carbamate salts in both environments. 2-Decyl-2-ethylpropane-1,3-diamine (6) was found to gain 13.2% weight after 30 min at 40 °C in the presence of dry CO2. This is approximately 73% of its theoretical value. The reaction product was a hard solid on the outside with a softer, wax-like material on the inside. During the run, as CO2 was absorbed, the viscosity of the diamine increased rapidly, and the material ended up in clumps stuck to the stir paddle. Therefore, mixing was poor through most of the experiment, so that only the outside of the material was directly exposed to the gas over the whole 30 min. This type of mass transfer issue has been noted previously with neat amine functional materials.20−22 This material was next tested using wet CO2. The results were fairly comparable; a weight gain of 12.9% (71% of theoretical, 98% of the value observed with dry gas) was observed. Furthermore, the final form of the material was again a hard solid on the outside with a soft wax in the middle. The related compound 2,2-dihexylpropane-1,3-diamine (8) was found to fare somewhat better. It exhibited a weight gain of 15.3% (84% of theoretical) in dry CO2 and 14.8% (81% of theoretical, 97% of dry) using wet gas. However, like the previous material, it too ended up as a mixture of material forms. This included some free powder, along with bigger chunks of material that were hard on the outside and somewhat softer on the inside. In the case of both compounds (6) and (8), it seems likely that in a spray tower unit, such as that envisioned for the phase-changing absorbent process (Figure 1), that powdery carbamate salts would be obtained. This is due to the fact that the liquid droplets formed in this system would have a large surface area to volume ratio so that mass transfer issues should be minimized. As a point of reference, the carbamate salt generated from 2,2dihexylpropane-1,3-diamine (8) in dry CO2 was analyzed by NMR. This was facilitated by the fact that this material is much more soluble in CDCl3 than materials like bis(aminopropyl)tetramethyldisiloxane (GAP-0) (1) carbamate. Included in Figure 9 are proton spectra for the unreacted diamine, the carbamate salt solution, and the carbamate salt solution to which D2O had been added.
upon review of all the LAH reductions of malononitrile derivatives we had done over the last couple of years, it was determined that the one thing these two dinitriles do have in common is limited solubility in ether. In both cases, THF cosolvent had been used to help dissolve these compounds so they could be more easily added to the reaction mixture. Given this, modified LAH reductions of dibenzylmalononitrile were attempted. First, a reaction was run where the dibenzylmalononitrile was charged as a neat solid to a slurry of LAH in ether. This provided an improvement in yield of the desired product; the di- to monoamine molar ratio was 80:20. In a final attempt, the dibenzylmalononitrile was diluted with a large excess of ether (5 g of solid dibenzylmalononitrile in 225 mL of ether) and added to the LAH/ether slurry. The di- to monoamine ratio in this case was found to be approximately 84:16. It was subsequently determined that reduction of dibenzylmalononitrile without any measurable reductive decyanation could be accomplished using borane (prepared from reaction of sodium borohydride with bromine25) as the reducing agent instead of LAH. In this way, a sample of 2,2-dibenzylpropane-1,3diamine (10) suitable for testing was obtained. Next, 2,2-bis(cyclohexylmethyl)propane-1,3-diamine (14) was synthesized via the reaction sequence shown in Figure 7. First, malononitrile was reacted with two equivalents of (bromomethyl)cyclohexane in the presence of potassium tbutoxide and a catalytic amount of tetrabutylammonium iodide to give 2,2-bis(cyclohexylmethyl)malononitrile (13) as a crude orange solid in 66% yield. Recrystallization with methanol allowed isolation of pure product with a melting point of 95−97 °C. This dinitrile was then cleanly reduced with LAH in ether to give 2,2-bis(cyclohexylmethyl)propane-1,3-diamine (14) in 87% yield as an off-white solid. Proton NMR showed this material to be 90−95% pure. Recrystallization from heptane produced 57% yield of pure product as a white solid with a melting point of 88− 90 °C. The last dialkylpropane-1,3-diamine targeted was 1,1-bis(aminomethyl)-4,4-dimethyl-cyclohexane (17), a hydrocarbon equivalent of the carbosiloxane based cyclic diamine (2). The synthesis of this compound is shown in Figure 8. First, malononitrile was reacted with the known 1,5-diiodo-3,3dimethylpentane26 (15) to form 1,1-dicyano-4,4-dimethylcyclohexane (16). The reaction was considerably slower than its carbosiloxane counterpart had been, requiring two days at 40 °C to reach completion. Workup of the reaction provided 1,1-
Figure 7. Synthesis of 2,2-bis(cyclohexylmethyl)propane-1,3-diamine (14). 472
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Figure 8. Synthesis of 1,1-bis(aminomethyl)-4,4-dimethyl-cyclohexane (17).
Figure 9. Proton NMRs of 2,2-dihexylpropane-1,3-diamine (8) and carbamate salts.
piece of glass wool was placed in the reaction flask outlet to trap any powders before they could be blown out. The results for 2decyl-2-ethylpropane-1,3-diamine (6) were somewhat higher than those obtained with the original procedure at 14.0% weight gain (77% of theoretical, 0.7% higher than the previous result). The difference was more substantial for 2,2-dihexylpropane-1,3diamine (8) where the weight gain increased to 17.4% (96% of theoretical, 2.1% higher than the initial number). The larger change observed with the 2,2-dihexylpropane-1,3-diamine (8) can be attributed to the fact that it provided a harder solid CO2 reaction product, so that more fine powder was generated, a portion of which was lost in the absence of glass wool. Given these results, this became the standard procedure going forward. 2,2-Dihexylpropane-1,3-diamine (8) was next re-evaluated using the new procedure and wet CO2. In this case, the weight gain was 22.8% (125% of theoretical). Interestingly 13C NMR again only showed the presence of carbamate as evidenced by a single carbonyl peak at ca. 164 ppm. Thus, water clearly did not get involved chemically. It seems the extra weight gain in this case was due to water absorption by the carbamate salt. Significantly, this did not cause the salt to liquefy or even soften to a large extent; it was still largely powdery in nature. The next material to be evaluated was 2,2-dibenzylpropane1,3-diamine (10). It was determined that the weight gain in dry CO2 for this material was 10.4% (60% of theoretical). The relatively high initial viscosity of (10) appeared to cause mass transfer issues during the run thus preventing the weight gain to get closer to theoretical. Consequently, while some of the solid in this case ended up as powder, there was still a significant amount of material stuck on the stir paddle that was a soft wax.
The peak assignments for the carbamate salt are based on chemical shift values, relative integrals of peak height, and the changes in peaks observed on addition of the D2O. As can be seen, the CH2NH2 peak at approximately 2.5 ppm in the starting diamine becomes two peaks in the carbamate salt. The first is a broad peak at 2.95 ppm, which is most consistent with CH2NH3+. This peak sharpened considerably and shifted to slightly higher field (∼2.90 ppm) on addition of D2O. It should also be noted how much the NHx peaks changed on addition of the D2O; the larger NH3+ peak shrank considerably and shifted from 7.75 ppm to 5.82 ppm, while the carbamate NH peak also became considerably smaller and shifted from 4.26 ppm to 4.45 ppm. The carbamate methylene (CH2 -NH−CO 2−) was observed to be a sharp peak at 2.57 ppm. Finally, comparison of the integrals for the carbamate methylene peaks and the CH2NH2 peak from unreacted diamine indicated that about 90% of the amines had reacted. This was within the same range as the gravimetric weight gain data. 13 C NMR was also run, which showed only the presence of a carbamate salt carbonyl peak at 164 ppm as the CO2 reaction product - there was no bicarbonate/carbonate observed at ca. 160 ppm.27 During the course of additional uptake studies, it was determined that weight gain results using our standard test protocol can be artificially low in cases where powdery carbamate salts are generated. This is due to the fact that the CO2 gas flow can sweep some fine solids out of the reaction flask. In order to determine if this phenomenon had an impact on the results for these first two diamines (i.e., (6) and (8)), their CO2 uptake in dry gas was remeasured using a modified procedure, where a 473
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As discussed above, bis(cyclohexylmethyl)propane-1,3-diamine (14) was found to be a solid with a melting point of 88−90 °C. This temperature is too high for this material to be useful in a phase-changing sorbent based system where CO2 absorption needs to be done at the temperature of flue gas (40−50 °C). Nonetheless, the decision was made to test this material. Use of the modified procedure with glass wool trapping was particularly important in this case, as the diamine started as a fine powder. The weight gain in dry CO2 was found to be 16.5% (100% of theoretical), while that in wet CO2 was 16.3% (99% of theoretical). In both cases, the final form of the material was still a fine powder. Thus this compound would appear to be an excellent solid absorbent. The cyclic 1,1-bis(aminomethyl)-4,4-dimethyl-cyclohexane (17) was also found to have a melting point (60−62 °C) too high to be used in this type of process. However, the limited material that had been synthesized was tested in wet CO2. The weight gain was found to be 15.8% (61% of theoretical). The final form of the material was similar to that observed initially. Thus, this carbamate salt did not appear to be radically effected by moisture either. Thermal Stability. As mentioned in the Introduction, another attribute that phase changing CO2 sorbents need to possess is thermal stability. Desorption occurs at high temperatures, and it is imperative from both practical and economical perspectives to use a material that exhibits little change in composition even after multiple heat cycles. During the course of our work in the area of CO2 capture, we have explored the thermal stability of candidate sorbents by using an accelerated test, where materials are heated to 150 °C for up to three months, while the compositions are monitored using gas chromatography (GC). As the work described in this report was in progress, some thermal stability data were being generated on the previously described aminosiloxane derivatives.21,22 This showed that many of these compounds have good stability at this temperature. However, one of the more attractive phasechanging materials, namely the diamino functional cyclocarbosiloxane (2), did not.22 Initially it was not clear if the instability observed for (2) was due to its cyclic carbosiloxane core structure or the diamine portion of the molecule. Certainly cyclocarbosiloxanes are known to undergo ring-opening polymerization, although this is more common for 5-membered ring materials.28 However, since the amine structure of the 2,2dialkylpropane-1,3-diamines described herein is similar to that found in compound (2), and since we had already anticipated that some stability would be lost going from siloxanes to organics, their thermal stability was quickly tested. Like cyclocarbosiloxane (2), dialkylpropane-1,3-diamines were found to be less stable than standard aminodisiloxanes such as bis(aminopropyl)tetramethyldisiloxane (GAP-0) at 150 °C in air. However, they were more stable than (2). This is clearly illustrated for compound (8) in Figure 10. Interestingly, when the experiment was repeated in an argon atmosphere, no degradation of (8) was observed. Therefore, thermo-oxidative processes are clearly in operation.
Figure 10. Comparison of 150 °C stability of cyclic carbosiloxane (2) and dialkylpropane-1,3-diamine (8) to GAP-0 (1).
A few stabilizer screening experiments were also conducted. In these studies 0.1% concentrations of BHT (2,6-di-tert-butyl-4methylphenol), TEMPO (2,2,6,6,-tetramethyl-1-piperidineoxy free radical), and galvinoxyl (2,6-di-tert-butyl-α-(3,5-di-tertbutyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy free radical) were added to (8), and the thermal stability studies were repeated. Of these, the galvinoxyl and BHT did have some impact at longer heating times. However, much more work would need to be done in order to identify a stabilizer package capable of significantly impacting the results. Work was then initiated to determine the degradation products produced under these conditions, in order to provide further insight into the mechanism. A number of peaks in the GC-MS of heat aged samples were observed. However, the largest peaks found with both compounds (2) and (8) were at molecular weights that were ten mass units higher than the starting diamines (i.e., M + 10). The following structures met this requirement. In the case of the structures on the left (18 and 19a/b), one would have to add one carbon and remove two hydrogens from the diamine in order to get a mass of M+10. For the second set (20 and 21), one would have to add an oxygen and remove six hydrogens. Analysis of the 13C NMR spectra of heat aged samples of compound (8) indicated that the acyclic structures were unlikely to be present. First, there was only one significant CN carbon peak (∼147 ppm) observed, while in acylic structures (19a) and (19b), there are two distinct imines that should lead to two different CN peaks. Acylic compound (21) was excluded based on the fact that no nitrile carbon peaks were observed. Thus, we needed to decide between tetrahydropyrimidine (18) and the oxadiazine (20). Tetrahydropyrimidines are more commonly known than oxadiazines. In fact, the parent compound 1,4,5,6-tetrahydropyrimidine is commercially available. Proton NMR comparison between a reference spectrum for this compound and the heat aged dialkylpropane-1,3-diamine samples indicated that structure (18) was a reasonable candidate to be present, particularly given that distinct peaks were seen in both cases around 7 ppm for the proton attached to the CN carbon.
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dihexylpropane-1,3-diamine (8) could then presumably give compound (28).
In order to provide further evidence for the assignment of the M+10 peaks, an authentic sample of 5,5-dihexyl-1,4,5,6tetrahydropyrimidine (24) was prepared via reaction of the diamine (8) with formic acid as shown in Figure 11. Upon addition of formic acid to the diamine, an exothermic reaction occurred immediately which led to the solid formate salt (22). On heating to ca. 150 °C under vacuum, the salt presumably first decomposed to form formamide (23) and water. This material then underwent a condensation reaction to form the tetrahydropyrimidine (24) and an additional equivalent of water. Included in Figure 12 are comparisons between the tetrahydropyrimidine proton and CN carbon peaks for authentic 5,5-dihexyl-1,4,5,6-tetrahydropyrimidine (24) to those observed for heat aged material. As can be seen, the peaks correlated well. In addition, the mass spectral fragmentation patterns for the authentic 5,5-dihexyl-1,4,5,6-tetrahydropyrimidine and the main GC peak from the heat aged samples matched well; both had distinct molecular ions at 252, base peaks at 167 (M − C6H13), and peaks at M-15 (CH3) followed by a series of peaks that were each separated by 14 mass units (CH2 groups). Thus, we have good evidence for the formation of tetrahydropyrimidine (18) during heat aging of the 1,3-diamines. There were other peaks in the GC-MS of the heat aged samples of compound (8) that were also tentatively identified. The following structures were found to be consistent with molecular ion peaks observed:
Finally, it should be noted that tetrahydropyrimidines have also been reported to absorb CO2.29,30 Thus the uptake of authentic 5,5-dihexyl-1,4,5,6-tetrahydropyrimidine (24) was evaluated. Using dry CO2, a weight gain of 8.8% was observed after 30 min at 40 °C; the reaction product was an orange wax. If one assumes that a 1:1 molar ratio of tetrahydropyrimidine:CO2 is required, then this measured weight gain would be 50% of theoretical. On heating, the orange wax returned to a liquid with obvious evolution of gas. 13C NMRs of the starting tetrahydropyrimidine, the CO2 reaction product, and the material after desorption are shown in Figure 13. As can be seen, the tetrahydropyrimidine peaks at ca. 50 ppm (CH2 groups) and 147 ppm (C=N) in spectrum a changed significantly on reaction with CO2 (spectrum b); both peaks shrank and broadened considerable. In addition, a new peak appeared at approximately 157 ppm, which is presumably the carbonyl derived from the CO2. Interestingly this chemical shift is closer to that reported for bicarbonate/carbonate than carbamate.27 Finally, spectrum c shows that the material cleanly decarboxylated on heating, regenerating the starting tetrahydropyrimidine (24).
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SUMMARY A series of 2,2-dialkylpropane-1,3-diamines were synthesized and evaluated as potential phase-changing CO2 sorbents. In general compounds of this type were found to be capable of forming the type of moisture insensitive carbamates salts that are required for this process. Acylic compounds 2-decyl-2ethylpropane-1,3-diamine (6) and 2,2-dihexyl-propane-1,3-diamines (8) were the best candidates as they were low viscosity oils that gave solids on reaction with CO2. 2,2-Dibenzylpropane-1,3-diamine (10) was found to be a viscous liquid, which caused mass transfer issues during CO2 absorbance. Thus, the weight gain observed with this material was significantly lower than the theoretical value based on its amine content. 2,2-Bis(cyclohexylmethyl)propane-1,3-diamine (14) and 1,1bis(aminomethyl)-4,4-dimethyl-cyclohexane (17) were both found to be solids with melting points greater than the typical temperature of flue gas. Therefore, they would be unsuitable for use as phase-changing sorbents. However, they were both found to be efficient solid CO2 absorbents. Heat aging studies in air found that these dialkylpropane-1,3diamines are less thermally stable than aminosiloxanes like GAP-
Interestingly, several of these are also substituted 1,4,5,6tetrahydropyrimidines. These materials presumably could be formed via reaction of other oxidative cleavage products with (8). A potential intermediate in the formation of (28) was observed in the analysis of the nonamine based compounds isolated via partitioning the heat aged material between ether and 10% HCl. Analysis of the ether extract was consistent with the aldehyde (29). Proton NMR showed a diagnostic peak at approximately 9.5 ppm (doublet) and the 13C NMR contained a CO peak at 205.8 ppm. In addition, GC-MS showed the material to have a molecular weight of 212, matching the structure shown. Further oxidation of this aldehyde followed by condensation with
Figure 11. Synthesis of 5,5-dihexyl-1,4,5,6-tetrahydropyrimidine (24). 475
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Figure 12. NMR comparison between (a) heat aged 2,2-dihexylpropane-1,3-diamine (8) and (b) 5,5-dihexyl-1,4,5,6-tetrahydropyrimidine (24).
Figure 13. 13C NMRs of (a) 5,5-dihexyl-1,4,5,6-tetrahydropyrimidine (24), (b) CO2 reaction product, and (c) after desorption.
thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest.
0 (1) but more stable than diamine functional carbosiloxane (2). These materials were thermally stable when heated under argon. The main thermal degradation products found after 3 months at 150 °C in air were tetrahydropyrimidine derivatives such as 5,5dihexyl-1,4,5,6-tetrahydropyrimidine (24). Given these results, if these compounds are to be used in a practical phase-changing sorbent based process, an efficient thermal stabilizer package will need to be developed.
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ACKNOWLEDGMENTS We wish to thank Hans Grade for mass spectral analyses. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency − Energy (ARPA-E), U.S. Department of Energy, under Award Number DEAR0000084.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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Notes
Disclosure: The information, data, or work presented herein was funded in part by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency
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