Second-Generation Aminosilicones as CO2 Capture Solvents

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Second Generation Aminosilicones as CO2 Capture Solvents Robert J. Perry Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01048 • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on June 4, 2017

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Second Generation Aminosilicones as CO2 Capture Solvents Robert J. Perry Abstract Silicones with a variety of linear, branched, star and cyclic architectures were synthesized that contained electron-donating ethylaminopropyl groups attached to the silicone core. These solvents were tested for CO2 uptake and their physical state after reaction. Compared to analogous materials that only possessed a primary amine, all the heteroatom substituted derivatives displayed excellent CO2 uptake and all maintained a liquid, flowable state after reaction. Optimal CO2 uptake was achieved at ambient temperatures rather than the typical 40 oC level. This was likely due to the lower heats of reaction for the secondary amine structure. β-Isomer present in the samples did not adversely affect the reaction with CO2 or the ability to remain in a liquid state upon complete reaction.

Introduction Aqueous solutions of organic amines are used as the workhorses in most carbon capture technologies that have progressed to pilot scale and beyond. These include the sterically hindered amines KS-1TM, KS2 TM and KS-3 TM developed by MHI [1], Cansolv’s proprietary amine system [2], Fluor’s Econamine FG and Econamine FG Plus solvents [3] and the Linde/BASF OASE® system. [4] However, recent developments in non-aqueous-based solvents for carbon capture have spurred increased interest in this area with imidazoles [5], guanidines and amidines [6], CO2 binding organic liquids (CO2BOLs) [7] and even traditional organic amines in glycol or alcoholic solvents [8] being examined. Aminosilicones have also been investigated as one class of such solvents that have been calculated to be more economical that conventional aqueous-based organic amines. [9] A mixture of an aminosilicone (GAP-1) and triethylene glycol (TEG) as a CO2 capture solvent (Figure 1) has been examined at the laboratory and bench scale [10] was recently tested in the pilot facility at the National Carbon Capture Center (NCCC). TEG was needed as a co-solvent as the reaction product of GAP-1 and CO2 formed a soft solid unsuitable for a liquid-based capture process. While the TEG did not absorb any appreciable amounts of CO2, it did permit the solvent to remain liquid throughout the process. The pilot testing showed excellent performance with regards to CO2 capture ability with 66% CO2 capture efficiency at 0.5 Mwe using a continuous stirred-tank reactor (CSTR) desorption unit. Use of a steam stripper desorption unit improved the CO2 capture efficiency to 95% at 0.5 Mwe. However, even this promising solvent composition had properties that might be improved. These included increased capture capacity, lower heat of reaction with CO2, decreased viscosity, and elimination of a co-solvent.

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Figure 1. GAP-1/TEG CO2 Capture Solvent. The use of secondary amine functionalized disiloxanes as CO2 capture solvents was reported earlier and these materials possessed several features that made them attractive candidates as improved solvents for CO2 capture [11]. Some of these qualities included lower heats of reaction, higher CO2 uptake capacities and, in several cases, the ability to maintain a flowable liquid state even when fully reacted with CO2 without the need for a co-solvent. These fully reacted materials only manifested the liquid state when a very specific arrangement of functional groups was present on the disiloxane backbone. The functional group possessed an electron-donating group on the ethylaminopropyl linking group as shown in Figure 2 where XR was OEt, OMe, or NMe2.

Figure 2. Preliminary 2nd Generation Aminosilicone CO2 Capture Solvents This unique property is thought to be derived from the capacity of the molecule to form intramolecular H-bonds as shown in Figure 3. This provided steric bulk around the ammonium nitrogen and diminished the propensity of the charged carbamate product to self-associate, as occurs with the unsubstituted primary amine. This contrasts with the preliminary results from molecular modeling which indicate that carbamate molecules cluster together rather than remain dispersed in the solvent matrix. [12] Similar arguments have been made for single component switchable ionic liquids. [13]

Figure 3. Intramolecular H-bonding in 2nd Generation Aminosilicone CO2 Capture Solvent This report examines the generality of imparting flowability to other core structures containing the alkoxyethylaminopropyl and similar groups and describes the synthesis and CO2 uptake experiments for an extended class of second generation materials as well as a qualitative assessment of their physical state upon reaction with CO2. Experimental All reactions were performed under a nitrogen atmosphere and all chemicals purchased from suppliers were used as received. Bis(3-hydroxylpropyl)siloxane 3. 1,1,3,3,5,5,7,7-octamethyltetrasiloxane (5.65g, 20 mmol) was dissolved in toluene (30 mL) and then 2 mL of allyl alcohol (2.8g, 48 mmol) in toluene (10 mL) was added at ambient temperature followed by 1 drop of Karstedt’s catalyst (4.3 wt % Pt in xylenes). The reaction mixture was heated to 40 oC and the remainder of the alcohol solution was added over 1 min. Heat was increased to reflux and the reaction allowed to continue for 18h. After this time, the solution was concentrated in vacuo, dissolved in MeOH, filtered through Celite® and concentrated to give 5.6 g (67%) 2 ACS Paragon Plus Environment

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of product as a viscous brown liquid. 1H NMR (CDCl3) δ: 3.83 (m, 1.2 H); 3.64 (m, 4H); 2.15 (br, 1.8H); 1.8 (m, 1.6 H); 1.62 (m, 4.1H); 0.7 (m, 1.5H); 0.55 (m, 4H); 0.07-0.14 (45H). lit bp 141-142/0.06 mm Hg). [14] Bis(3-chloropropyl)siloxane 4. Bis(3-hydroxypropyl)siloxane 3 (5.4g, 13.5 mmol)was cooled to 3 oC, then 1 drop of pyridine was added followed by thionyl chloride (16.1g, 135 mmol) dropwise with ice-bath cooling. After addition was complete, the bath was removed and the mixture stirred at ambient temperature for 4 h the reflux for an additional 7 h. Excess thionyl chloride was removed by distillation and the residue dissolved in CH2Cl2, washed with water, then brine then dried over MgSO4, concentrated and allowed to stand. After two days, the solid that had precipitated out was removed by filtration and the product as a clear orange liquid was obtained. (3.2 g, 54%). 1H NMR (CDCl3) δ: 3.53 (t, J = 7.1 Hz, 4H); 1.82 (m, 4H); 0.66 (m, 4H); 0.10 (s, 40 H). 13C {1H} NMR (CDCl3): 47.84, 27.06, 15.90, 1.15, 0.74, 0.27, 0.11 ppm. Bis(3-iodopropropyl)siloxane 5. Bis(3-chloropropyl)siloxane 4 (3.0g, 4.8 mmol) was dissolved in acetone (20 mL) then NaI (4.1g, 27.6 mmol) in acetone (40 mL) was added and the reaction mixture heated to reflux for 3 days. After this time, the reaction was concentrated, the residue dissolved in CHCl3, washed with water (2x), aq Na2SO3, brine, dried over MgSO4, and concentrated to give 3.26g (84%) product as an orange oil. 1H NMR (CDCl3) δ: 3.22 (m, 4H); 1.87 (m, 4H); 0.65 (m, 4H); 0.10 (s, 38 H). 13C {1H} NMR (CDCl3): 28.23, 20.22, 11.12, 1.09, 0.37, 0.19 ppm. Bis(3-(N-ethoxyethyl)aminopropyl)siloxane 7. Bis(3-iodopropyl)siloxane 5 (2.7g, 3.33 mmol) was added to ethoxyaminoethane 6 (6.7 g, 75 mmol) at ambient temperature over 15 min and stirred at ambient temperature for 22. Excess amine was removed in vacuo and the residue dissolved in heptane, treated with 10% NaOH (aq) and the layers separated. The organic layer was washed with water and brine and dried over Na2SO4, filtered and concentrated to give 1.9 g (79%) product as a light orange oil. 1H NMR (CDCl3) δ: 3.58 (m, 8H); 2.79 (t, J = 5.4 Hz, 4H); 2.62 (t, J = 7.4 Hz, 4H); 1.53(m, 4H); 1.51 (br, 2H); 1.22 (t, J = 7.1 Hz, 6H); 0.53 (m, 4H); 0.09-0.06 (ms, 38.8H). 13C {1H} NMR (CDCl3): 69.99, 66.41, 53.29, 49.53, 23.88, 23.83, 15.88, 15.73, 15.17, 1.17, 1.04, 0.29, 0.10 ppm. Tris(3-aminopropropyldimethylsiloxy)phenylsilane 11. A solution of allylamine (2.0 g, 35 mmol) in toluene (5 mL) was added dropwise to a solution of tris(dimethylsiloxy)phenylsilane 9 (3.0 g, 9.07 mmol) in toluene (5 mL). After 1 mL of the amine was added, 1 drop of Karstedt’s catalyst was added and then the mixture heated to 80 oC for a total of 8 h. An additional 1 mL of allylamine and 1 drop of Karstedt’s catalyst was added at this time and the reaction allowed to proceed for a total of 24 h. Solvent was removed in vacuo (3 h, 80 oC/1 mm Hg) to give 4.0 g (88%) product with 11% β-isomer. 1H NMR (CDCl3) δ: 7.54 (m, 2H); 7.33 (m, 3H); 2.88 (dd, J = 12.5, 4.7 Hz, 0.4H); 2.60 (t, J = 7.0 Hz, 3.6H); 2.22 (m, 0.2H); 1.40 (m, 3.6H); 1.07 (m, 4H); 0.96 (d, J = 7.4 Hz, 1.8H); 0.75 (m, 0.6H); 0.52 (m. 3.5H); 0.02-0.20 (m, 18H). 13 C {1H} NMR (CDCl3): 133.79, 133.73, 129.63(m), 45.41, 44.11, 27.56, 26.39, 15.12, 11.50, 0.11, -1.34 ppm. N-(2-ethoxyethyl)allylamine 13. Et3N (16 mL, 115 mmol) was added to a solution of allylamine (32.93 g, 577 mmol) in THF (40 mL), heated to 60 oC and then 2-bromoethyl ethyl ether 12 (17.65 g, 115 mmol) was added over 10 min. After 3 h excess solvent and allylamine were removed in vacuo, the residue 3 ACS Paragon Plus Environment

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dissolved in CHCl3, washed with water, dried over Na2SO4, filtered, concentrated and distilled to give 11.2 g (75%) product. 1H NMR (CDCl3) δ: 5.88 (m, 1H); 5.15 (dm, J = 17.2 Hz, 1H); 5.05 (dm, J = 10.4 Hz, 1H); 3.51 (m, 4H); 3.24 (m, 1H); 3.23 (m, 1H); 2.75 (t, J = 5.1 Hz, 2H); 1.57 (br, 1H); 1.18 (t, J = 7.1 Hz, 3H). 13 C {1H} NMR (CDCl3): 136.83, 115.78, 69.85, 66.38, 52.40, 48.81, 15.11 ppm. Tris(3-(N-ethoxyethyl)aminopropyldimethylsiloxy)phenylsilane 14. A solution of N-(2ethoxyethyl)allylamine 13 (2.6 g, 20 mmol) in toluene (5 mL) was added dropwise to a solution of tris(dimethylsiloxy)phenylsilane 9 (2.0 g, 6 mmol) in toluene (5 mL). after 1 mL of the olefin had been added to the hydride followed by 1 drop of Karstedt’s catalyst. The mixture was heated to 50 oC and then to 100 oC with an additional 1 drop of catalyst added. After 22h, the reaction mixture was concentrated, dissolved in CHCl3, washed with water, dried over Na2SO4, filtered, concentrated and volatiles removed at 100 oC/1 mm Hg for 1 h to yield 4.3 g (99%) product as a clear orange liquid with ~ 20% β-isomer. 1H NMR (CDCl3) δ: 7.54 (m, 2H); 7.32 (m, 3H); 3.51 (m, 10.1H), 2.75 (m, 5.2H); 2.59 (m, 4.3H); 1.48 (m, 6.1H); 1.21 (m, 8.4H); 1.17 (m, 1.14H); 0.57 (m, 3.7H); 0.12 (m, 19.4H). 13C {1H} NMR (CDCl3): 133.81, 133.76, 129.54, 127.53, 127.49, 69.97, 66.38, 53.22, 51.50, 49.36, 49.24, 23.73, 22.44, 15.66, 15.61, 15.17, 12.02, 1.07, 0.10 ppm. Tetrakis(3-aminopropropyldimethylsiloxy)silane 16. A solution of allylamine (4.4 g, 77 mmol) in toluene (5 mL) was added dropwise to a solution of tetrakis(dimethylsiloxy)silane 15 (5.0 g, 15.2 mmol) with 1 drop of Karstedt’s catalyst and heated to 85 oC. After 24 h additional allylamine (1g) and catalyst was added and the reaction allowed to continue for another 6 h. The solvent was removed in vacuo to give 7.0 g (82%) product with ~ 18% β-isomer. 1H NMR (CDCl3) δ: 4.85 (s, 2.6H); 2.90 (dd, J = 12.5, 4.7 Hz, 0.2H); 2.63 (t, J = 7.0 Hz, 1.8H); 2.56 (m, 0.2H); 1.54 (m, 2H); 1.06 (m, 0.6H); 0.86 (m, 0.2H); 0.61 (s, 2H); 0.15 (s, 10.6H). 13C {1H} NMR (CDCl3): 44.50 (m), 42.95, 26.34 (m), 25.49, 14.75, 10.44, -1.08, -2.54 ppm. Tetrakis(3-(N-ethoxyethyl)aminopropropyldimethylsiloxy)silane 17. A solution of N-(2-ethoxyethyl)allylamine 13 (6.0 g, 46.4 mmol) in toluene (5 mL) was added dropwise to a solution of tetrakis(dimethylsiloxy)silane 15 (3.0 g, 9.3 mmol) with 1 drop of Karstedt’s catalyst and heated to 85 oC. Two additional aliquots of olefin and catalyst added at 5 and 22h. A small amount of hydride remained after 24 hours so 1-hexene (0.5 mL) was added to quench any remaining S-H groups. The solvent was removed in vacuo to give 6.1 g (78%) product with ~ 25% β-isomer. 1H NMR (CDCl3) δ: 3.48 (m, 16H); 2.75 (m, 8H); 2.56 (m, 7H); 1.51 (br m, 6H); 1.17 (t, J = 6.8 Hz, 12H); 0.97 (m, 3H); 0.55 (m, 5.85H); 0.07 (s, 32H). 13C {1H} NMR (CDCl3): 69.91, 66.36, 55.23, 51.42, 23.67, 22.36, 15.53, 15.15, 11.91, 0.95, -0.12 ppm. Tetrakis-1,3,5,7-(3-(N-ethoxyethyl)aminopropropyl)-1,3,5,7-tetramethycyclotetrasiloxane 19. A solution of N-(2-ethoxyethyl)allylamine 1 (5.4g, 41.6 mmol) in toluene (2 mL) was added dropwise to a solution of 1,3,5,7-tetramethylcyclotetrasiloxane 15 (3.0 g, 9.3 mmol) in toluene (3mL) with 1 drop of Karstedt’s catalyst and heated to 90 oC. Two additional aliquots of olefin and catalyst added at 5 and 23h. A small amount of hydride remained after 24 hours so 1-hexene (0.5 mL) was added to quench any remaining S-H groups. The solvent was removed in vacuo to give 5.25 g (83%) product with ~ 50% βisomer. 1H NMR (CDCl3) δ: 3.50 (m, 7.9H); 2.75 (m, 4H); 2.70 (m, 1H); 2.59 (m, 2H); 2.47 (m, 1H); 1.67 (br,


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1.8H); 1.52 (m, 2H); 1.19 (m, 6H); 0.97 (m, 3.3H); 0.51 (m, 2H); 0.07 (7.8H). 13C {1H} NMR (CDCl3): 69.92, 66.36, 53.01, 51.40, 49.36, 23.45, 21.77, 15.15, 14.56, 11.95, 0.75, -2.14 ppm. Bis(3-aminopropyl)dimethylsiloxane 25. Allylamine (1.3 g, 22.8 mmol) was added to hydride capped siloxane 21 (5.02 g, 6.9 mmol) over 2 min with 1 drop of Karstedt’s catalyst and heated to 60 oC for 16h. Volatiles were removed In vacuo to give 5.39 g (93%) product with 25% β-isomer. 1H NMR (CDCl3) δ: 2.89 (dd, J = 12.6, 4.8 Hz, 0.72h); 2.65 (t, J = 6.8 Hz, 3.8H); 2.54 (dd, J = 12.6 Hz, 0.72H); 1.44 (m, 4H); 1.03 (br 4H); 0.97 (d, J = 7.3Hz, 2H); 0.74 (m, 0.7H); 0.52 (m, 4H); 0.06 (s, 78H). 13C {1H} NMR (CDCl3): 45.41, 44.27, 27.64, 26.37, 15.20, 11.51, 1.12, 0.99, 0.08 ppm. Bis(3-(N-ethoxyethyl)aminopropyl)siloxane 26. N-(2-ethoxyethyl)-allylamine 13 (2.0 g, 15.2 mmol) was added to hydride capped siloxane 21 (5.0 g, 6.9 mmol) over 2 min with 1 drop of Karstedt’s catalyst and heated to 80 oC for 18h. Volatiles were removed in vacuo to give 5.66 g (83%) product with 40% βisomer. 1H NMR (CDCl3) δ: 3.54 (m, 11.5H); 2.76 (m, 6.4H); 2.62 (m, 3.5H); 2.51 (0.9H); 1.59 (m, 3.7H); 1.22 (m, 8.6H); 1.03 (m, 3.7H); 0.59 (m, 4H); 0.09-0.14 (m, 94H). 13C {1H} NMR (CDCl3): 68.74, 66.04, 52.35, 51.03, 48.42, 22.84, 21.75, 15.22, 14.22, 11.07, 0.18, -0.93, -2.56 ppm. N-(2-methoxyethyl)allylamine 22. Allylamine (20.5g, 360 mmol) and NEt3 (13.9 mL, 100 mmol)were added together followed by slow addition (35 min) of 2-bromoethyl methylether (10.0g, 72 mmol) while heating to 50 oC. After 2h the reaction was two phases. The upper phase was isolated and concentrated to an orange oil that was diluted with heptane and washed with 10% NaOH, water and then dried over Na2SO4. The aqueous base contained most of the product and the base wash was extracted with CHCl3, concentrated and distilled (115-120 oC/300 mm Hg) to give 1.9g (23%) product. 1H NMR (CDCl3) δ: 5.85 (m, 1H); 5.12 (dd, J = 17.2, 1.5 Hz, 1H); 5.02 (dd, J = 10.4, 1.5 Hz, 1H); 3.44 (t, J = 5.2 Hz, 2H); 3.30 (s, 3H); 2.72 (t, J = 5.2 Hz, 2H); 3.21 (dt, J = 6.1, 1.4 Hz, 2H); 1.44 (br, 1H). 13C {1H} NMR (CDCl3): 136.76, 115.78, 71.99, 58.71, 52.35, 48.62 ppm. N-(2-ethylthioethyl)allylamine 23. 2-hydroxyethyl ethyl thioether (9.0 g, 85 mmol) was dissolved in CH2Cl2 (100 mL), cooled to 0 oC and phosphorous tribromide (8.0 mL, 85 mmol) was added dropwise over 30 min. The reaction mixture remained at 0 oC for 2 h then the reaction was allowed to come to ambient temperature and stirred for 18 h. The mixture was again cooled to 0 oC and water (10 mL) was added followed by sat. Na2CO3 (~60 mL) until pH=7. The layers were separated, dried over Na2SO4 and concentrated in vacuo to give 13.1 g (91%) 2-bromoethyl ethyl thioether as a colorless liquid. 1H NMR (CDCl3) δ: 3.49 (t, J = 7.7 Hz, 2H); 2.95 (t, J = 8.2 Hz, 2H); 2.59 (q, J = 7.3 Hz, 2H); 1.28 (t, J = 7.3 Hz, 3H). 13 C {1H} NMR (CDCl3): 33.71, 30.60, 26.16, 14.91 ppm. [15] Allylamine (15 g, 260 mmol), K2CO3 (43 g, 310 mmol) and THF (70 mL) were added together followed by dropwise addition (5 min) of 2-bromoethyl ethyl thioether (7.3 g, 43 mmol) at ambient temperature. After 24 h, an additional allylamine (9.8 g) and THF (40 mL) were added and the temperature increased to 40 oC. After 4 days, the mixture was filtered, concentrated in vacuo and distilled (160-163 oC/255 m Hg) to give 4.8 g (77%) product a a colorless liquid. 1H NMR (CDCl3) δ: 5.86 (m, 1H); 5.12 (dm, J = 17.2 Hz, 1H); 5.07 (dm, J = 10.1 Hz, 1H); 3.24 (m, 2H); 2.77 (m, 2H); 2.66 (m, 2H); 2.50 (m, 2H); 1.59 (br, 1H); 1.23 (m, 3H). 13C {1H} NMR (CDCl3): 136.66, 115.95, 52.00, 47.82, 31.84, 25.73, 14.83 ppm. [16]


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N-(2-dimethylaminoethyl)allylamine 24. Allylamine (3.9 g, 32 mmol) and NEt3 (4.5 mL, 32 mmol) were added together followed by dropwise addition (5 min) of allylbromide (3.9 g, 32 mmol) while heating to 60 oC. After 18h the reaction was diluted with CHCl3, washed with water, dried over Na2SO4 filtered and concentrated to give 1.0 g (24%) product which was flash distilled at 80 oC/100 mm Hg to remove color. 1 H NMR (CDCl3) δ: 5.91 (m, 1H); 5.17 (dd, J = 17.2, 1.7 Hz, 1H); 5.08 (dd, J = 10.1, 1.3 Hz, 1H); 3.27 (d, J = 6.1 Hz, 2H); 2.68 (t, J = 6.2 Hz, 2H); 2.41 (t, J = 6.1, 2H); 2.22 (s, 6H); 1.66 (br, 1H). 13C {1H} NMR (CDCl3): 136.93, 115.81, 59.17, 52.56, 46.68, 45.56 ppm. Bis(3-(N-2-methoxyethyl)aminopropyl)siloxane 27. N-(2-methoxyethyl)-allylamine 22 (1.5 g, 13 mmol) was added to hydride capped siloxane 21 (4.3 g, 6 mmol) over 1 min with 1 drop of Karstedt’s catalyst and heated to 50 oC for 1 h. Volatiles were removed in vacuo to give 5.1 g (89%) product with 55% βisomer. 1H NMR (CDCl3) δ: 3.48 (t, J = 5.0 Hz, 4H); 3.34 (s, 6H); 2.73 (m, 5.1 H); 2.59 (t J = 6.8 Hz, 1.8H); 2.47 (m, 1.2 H); 1.51 (m. 1.9H); 1.35 (br, 1.8H); 0.96 (m, 4.5 H); 0.54 (m, 1.8H); 0.06 (m, 60.7H). 13C {1H} NMR (CDCl3): 72.17, 72.14, 58.74, 58.71, 49.28, 49.20, 23.78, 22.38, 15.69, 12.05, 1.12, 0.05, -1.39 ppm. Bis(3-(N-2-ethylthioethyl)aminopropyl)siloxane 28. N-(2-ethylthioethyl)-allylamine 23 (1.9 g, 13 mmol) was added to hydride capped siloxane 21 (4.3 g, 6 mmol) with 1 drop of Karstedt’s catalyst. After 15 min, 4 more drops of catalyst were added and heated to 80 oC for 18 h. Volatiles were removed in vacuo to give 5.2 g (85%) product with 55% β-isomer. 1H NMR (CDCl3) δ: 2.76 (m, 2.4H); 2.69 (m, 2.2H); 2.60 (t, J = 7.4 Hz, 0.9H); 2.53 (m, 2.6H); 1.6 (br, 1H); 1.51 (m, 0.9H); 1.25 (t, J = 7.3 Hz, 3H); 0.97 (m, 2.1H); 0.52 (m, 0.9H), 0.02 (s, 30.8H). 13C {1H} NMR (CDCl3): 52.80, 51.39, 48.47, 48.44, 31.95, 31.93, 25.75, 23.77, 22.36, 15.67, 14.86, 12.08, 1.15, 1.01, 0.08, -1.34 ppm. Bis(3-(N-2-dimethylaminoethyl)aminopropyl)siloxane 29. N-(2-dimethylaminoethyl)-allylamine 23 (0.74 g, 5.8 mmol) was added to hydride capped siloxane 21 (1.96 g, 2.7 mmol) at 50 oC with 1 drop of Karstedt’s catalyst. After 6 h, 2 more drops of catalyst were added and heated to 75 oC for 24 h. Volatiles were removed in vacuo to give 1.95 g (74%) product with 40% β-isomer. 1H NMR (CDCl3) δ: 2.70 (m, 1.9H); 2.62 (m, 1.6H); 2.44 (m, 2.5H); 2.23 (s, 6 H); 2.02 (br, 1H); 1.54 (m, 1.2H); 0.99 (m, 1.6H); 0.55 (m, 1.2H), 0.09 (s, 73.7H). 13C {1H} NMR (CDCl3): 59.21, 59.12, 53.40, 51.84, 47.27, 47.19, 45.58, 23.72, 22.21, 15.72, 12.08, 1.17, 1.03, 0.16 ppm. 29Si NMR (CDCl3 + Cr(acac)3): 8.11, 7.55, -21.53, -21.95, -22.10 ppm. Bis(3-chloropropyl)siloxane 32. 1,3-bis(3-chloroprpyl)-1,1,3,3-teramethyldisiloxane (25 g, 87 mmol), D4 (64.5 g, 217 mmol) and Filtrol F-20 (8.3 g, acid-washed clay) were added together and heated at 80 oC for 1 day then 2 days at 60 oC. The reaction mixture was filtered, and stripped to give 60 g product. 1H NMR (CDCl3) δ: 3.53 (t, J = 7.0 Hz, 4H); 1.82 (m, 4H); 0.67 (m, 4H); 0.10 (s, 69H). 13C {1H} NMR (CDCl3): 47.69, 27.04, 15.82, 1.10, 0.07 ppm. Bis(3-iodopropyl)siloxane 32. Bis(3-chloropropyl)siloxane 33. (60 g, 60.6 mmol), NaI (40.8 g, 272 mmol) and acetone (250 mL) were refluxed together for 5 days. The reaction mixture was concentrated, filtered, dissolved in CHCl3, washed with water, Na2S2O3, dried over Na2SO4, filtered and stripped to give 62 g (87%) product. 1H NMR (CDCl3) δ: 3.22 (t, J = 7.1 Hz, 4H); 1.88 (m, 4H); 0.67 (m, 4H); 0.09 (s, 72H). 13 C {1H} NMR (CDCl3): 28.24, 20.20, 11.22, 1.18, 1.07, 0.18 ppm.


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Bis(3-(N-ethoxyethyl)aminopropyl)siloxane 37. 2-(ethoxyethyl) aminoethane (6.1 g, 68.5 mmol) was heated to 60 oC then bis(3-iodopropyl)siloxane 33 (7.5 g, 6.2 mmol) was added over 15 min. The reaction mixture was heated for 20h then excess aine was removed in vacuo and the residue diluted in CHCl3, washed with 10% NaOH, water, brine dried over Na2SO4, filtered and con entreated to give 4.3 g (61%) product as a light yellow liquid. 1H NMR (CDCl3) δ: 3.54 (m, 8H); 2.80 (m, 4H); 2.64 (m, 4H); 1.57 (m, 6H); 1.25 (m, 6H); 0.57 (m, 4H); 0.13 (m, 77H). 13C {1H} NMR (CDCl3): 69.96, 66.39, 53.21, 49.43, 23.85, 15.78, 15.09, 1.113, 0.99, 0.05 ppm. 29Si NMR (CDCl3 + Cr(acac)3): 87.54, -21.50, -21.98, -22.13 ppm. Bis(3-(N-2-methoxyethyl)aminopropyl)siloxane 38. 2-(methoxyethyl) aminoethane (7.5 g, 100 mmol) was heated to 60 oC then bis(3-iodopropyl)siloxane 33 (7.5 g, 6.2 mmol) was added over 15 min. The reaction mixture was heated for 16h then excess amine was removed in vacuo and the residue diluted in heptane, washed with 10% NaOH, water, brine dried over Na2SO4, filtered and con entreated to give 5.6 g (82%) product as a light yellow liquid. 1H NMR (CDCl3) δ: 3.50 (t, J = 5.2 Hz, 4H); 3.36 (s, 6H); 2.78 (t, J = 5.2 Hz, 4H); 2.62 (t, J = 7.2 Hz, 4H); 1.63 (br, 2H); 1.53 (m, 4H); 0.56 (m, 4H); 0.08 (m, 75H). 13C {1H} NMR (CDCl3): 72.14, 58.67, 53.15, 49.22, 23.77, 15.71, 1.07, 0.95, 0.04 ppm. Bis(3-(N-2-methylthioethyl)aminopropyl)siloxane 39. 2-(methylthioethyl) aminoethane (4.85 g, 53 mmol) was heated to 60 oC then bis(3-iodopropyl)siloxane 33 (6.08 g, 5.0 mmol) was added over 15 min. The reaction mixture was heated for 16h then excess amine was removed in vacuo and the residue diluted in heptane, washed with 10% NaOH, water, brine dried over Na2SO4, filtered and con entreated to give 4.3 g (69%) product as a light yellow liquid. 1H NMR (CDCl3) 2.81 (t, J = 6.3 Hz, 4H); 2.65 (t, J = 6.3 Hz, 4H); 2.61 (t, J = 7.4 Hz, 4H); 2.09 (s, 6H); 1.52 (m, 4H); 0.54 (m, 4H); 0.07 (m, 86H). 13C {1H} NMR (CDCl3): 52.73, 447.92, 34.57, 23.86, 15.69, 1.07, 0.93, 0.05 ppm. Bis(3-(N-2-dimethylaminoethyl)aminopropyl)siloxane 40. 2-(dimethylaminoethyl) aminoethane (6.1 g, 69.2 mmol) was heated to 60 oC then bis(3-iodopropyl)siloxane 33 (6.08 g, 5.0 mmol) was added over 15 min. The reaction mixture was heated for 16h then excess amine was removed in vacuo and the residue diluted in heptane, washed with 10% NaOH, water, brine dried over Na2SO4, filtered and con entreated to give 5.9 g (84%) product as a light yellow liquid. 1H NMR (CDCl3) 2.69 (m, 4H); 2.62 (m, 4H); 2.42 (m, 4H); 2.21 (s, 6H); 1.54 (m, 4H); 0.54 (m, 4H); 0.09 (m, 75H). 13C {1H} NMR (CDCl3): 59.37, 53.39, 47.36, 45.53, 23.79, 15.77, 1.11, 0.98, 0.08 ppm. N-(2-ethoxyethyl)aminopent-5-ene 42. 2-ethoxyethyl aminoethane 6 (38.2 g, 429 mmol), K2CO3 (10.1 g, 73.2 mmol) and THF (50 mL) were heated to 60 oC then 5-bromo-1-pentene (10.02 g, 67.2 mmol) was added over 25 min and the reaction allowed to continue for 22 h. The reaction mixture was filtered, concentrated and distilled (130 oC/130 mm Hg) to give 7.2 g (68%) product as a colorless liquid. 1H NMR (CDCl3) δ: 5.81 (m, 1H); 5.01 (d, J = 16.9 Hz, 1H); 4.93 (d, J = 10.1 Hz, 1H); 3.52 (t, J = 5.5 Hz, 2H); 3.49 (q, J = 7.1 Hz, 2H); 2.76 (t, J = 5.2 Hz, 2H); 2.62 (t, J = 7.3 Hz, 2H); 2.08 (q, J = 6.8 Hz, 2H); 1.60 (m, 2H); 1.46 (br, 1H); 1.19 (t, J = 7.1 Hz, 3H). 13C {1H} NMR (CDCl3): 138.49, 114.53, 69.88, 66.39, 49.48, 49.45, 31.54, 29.27, 15.14 ppm. Bis(3-(N-ethoxyethyl)aminopropyl)siloxane 43. A solution of N-(2-ethoxyethyl)aminopent-5-ene 42 (5.0 g, 31.8 mmol) in toluene (10 mL) was added dropwise to 1,1,3,3,5,5-hexamethyltrisiloxane 41 (3.32 g, 7 ACS Paragon Plus Environment

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15.9 mmol) heated at 55 oC with 1 drop of Karstedt’s catalyst. The temperature was raised to 65 oC after addition was complete and after 8h the reaction mixture was concentrated in vacuo to give 7.95 g (91%) product (95% pure with ~ 10 % β-isomer) with ~ 5% of unreacted olefinic amine isomers. 1H NMR (CDCl3) δ: 3.52 (t, J = 5.3 Hz, 4.5 H); 3.48 (q, J = 7.1 Hz, 4.5 H); 2.76 (m, 4.4 H); 2.59 (m, 4.4 H); 2.08 (q, J = 6.8 Hz, 2H); 1.58 (m, 0.3H); 1.48 (m, 4.4 H); 1.33 (m, 11.1 H); 1.18 (t, J = 7.0 Hz, 6.7H); 0.91 (m, 0.7H); 0.52 (m, 4H); 0.05-0.01 (m, 21.7H). 13C {1H} NMR (CDCl3): 69.94, 66.37, 50.05, 49.55, 31.12, 29.91, 23.17, 18.22, 15.13, 1.25, 1.16, 0.16 ppm. CO2 uptake experiments. A known amount of solvent (typically 2-4 g) was added to a pre-tared, 25 mL, 3-neck round-bottom flask equipped with a gas inlet tube, a mechanical stirrer with Teflon blade and a gas outlet adapter connected to a bubbler. The flask was either immersed in a constant temperature bath at 30 or 40 oC or left at ambient temperature. CO2 was introduced via the sublimation of dry ice that was passed through a CaCl2 drying tube prior to introduction into the reaction flask. The solvent was stirred at 200 rpm for 30 min – 2 hours depending on the experiment. The reaction was considered complete when either the preset time was reached or constant weight was achieved. The % of theory uptake calculation was based on the weight of CO2 absorbed divided by the theoretical amount which was predicted from the weight of the initial sample and the molecular weight of the candidate solvent.

Results & Discussion Synthesis It was clear from the preliminary studies reported above [12] that the tetramethyldisiloxane core did not inhibit the beneficial effects of the electron-donating ethylaminopropyl linking group. However, it was not clear if a longer siloxane chain or a more congested cyclic, branched or star core would show the same effects. To probe these various architectures, a number of derivatives were made. A short chain linear siloxane with ethoxyethylaminopropyl termini was made via the route shown in Scheme 1. Hydrosilylation of the starting hydride 1 with allyl alcohol 2 gave the chain lengthened 3hydroxypropyl intermediate 3 which was converted to the corresponding chloride 4 by refluxing in neat thionyl chloride. The chloride was converted to the more reactive iodide 5 via the Finklestein reaction. Reaction of the iodide with 2-ethoxyethylamine gave the final material 7 with an average chain length of 4.5 as determined by 1H NMR. The parent aminopropyl compound 8 was purchased and used as received and tested as a comparative example.


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Scheme 1. Preparation of linear siloxane with ethoxyethylaminopropyl end groups.

Branched compounds were made via hydrosilylation of the corresponding tris hydride 9 with allylamine or (2-ethoxyethyl)allylamine 13 as shown in Scheme 2. In both cases, some of the Markovnikov addition product giving the β-isomer was observed. For compound 11, 11% β-isomer was made and for 14, 20% was produced.

Scheme 2. Synthesis of Branched Aminosilicones.


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The candidates with a star core were produced in a similar manner as above with the tetrahydride 15 serving as the common starting material (Scheme 3). The aminopropyl derivative 16 contained 18% βisomer and the ethoxyethylaminopropyl material 17 had 25% β-isomer.

Scheme 3. Synthesis of Star Aminosilicones.


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The final core variation examined was derived from the cyclic siloxane, DH4 18. The 1,3,5,7-tetrakis(3aminopropyl)tetramethylcyclotetrasiloxane 20 was obtained from an independent source. Derivative 19 was synthesized from 18 via hydrosilylation as seen in Scheme 4. Compound 19 contained 25% of the βisomer.

Scheme 4. Synthesis of Cyclic Aminosilicone 19.


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In addition to expanding the silicone portion of the core, extending the functionality at the termini of the solvents was also explored. Previously, it was found that dimethylamino, methoxy and ethoxy endgroups all provided materials that readily absorbed CO2 and remained flowable liquids. [12] However, it was not clear if that was only a manifestation of having a small disiloxane core or if these groups could lower viscosity while maintaining high CO2 uptake with longer silicone chains. A series of linear silicones were prepared via the hydrosilylation of a hydride endcapped siloxane with either allyl amine or a variety of electron-donating allyl amine derivatives shown in Scheme 5. The addition reaction was not very selective and a large proportion of both β and γ isomers were produced as noted. Scheme 5. Synthesis of Linear Silicones.


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The olefins used to make linear solvents 25-29 were made as shown in Scheme 6. Three of the four olefins were obtained by simple displacement reaction of allyl amine with the corresponding bromoethane derivative. The dimethylamino compound 24 was also made via displacement but the nucleophile was N,N-dimethylethylenediamine.

Scheme 6. Preparation of Olefins.

As shown in Scheme 5, the solvent candidates possessed a significant amount of branching. To determine if this factor played a role in CO2 uptake or viscosity attributes, a similar series of linear materials were made that had only γ-isomer modes of attachment to the silicone core. The initial step was to make the backbone of required length. This was done as shown in Scheme 7 with the equilibration of D4 31 and 1,3-bis(3-chloropropyl)-1,1,3,3-tetramethyldisolaxane 30 followed by conversion of the chloride 32 to the more reactive iodide 33. Scheme 7. Synthesis of Linear Siloxane Precursor.


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The iodide was then allowed to react with the corresponding substituted ethylamine derivative as seen in Scheme 8. To avoid any significant amount of over alkylation, slow inverse addition of the iodide to a large excess of the amine was performed to give products 37-40 as shown in Scheme 8.

Scheme 8. Synthesis of all γ-Isomer Linear Siloxanes.

CO2 Uptake Testing Preliminary uptake measurements on the silicone solvents was performed by contacting dry CO2 gas, generated from dry ice and passed through a Drierite™ drying tube, with 2-4 g of solvent being mechanically stirred at 200 rpm for 30 min at 40 oC. The difference in weight gain was the amount of CO2 reacted and this was compared to the theoretical amount calculated assuming 1 mole of CO2 required 2 moles of primary or secondary amine for complete reaction. Additionally, the physical state of the reaction product after 30 min was observed and reported. The physical state was determined by manually turning the Teflon paddle on a stir shaft that was used during the CO2 absorption reaction. This qualitative assessment of the reaction product viscosity increased in the order: low viscosity liquid (10-2 Pa-s) < moderate viscosity liquid (10-1 Pa-s) < flowable liquid (100 Pa-s) < very viscous, flowable liquid(101 Pa-s) < very viscous, non-flowable liquid(102 Pa-s). The values in parentheses are order of magnitude estimates of the viscosity associated with these descriptors. Table 1 shows the comparison between the various core architectures with primary amine and substituted secondary amines.


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Table 1. Comparison Between Primary Amines and Substituted Secondary Amines at 40oC.

Entry

CO2 Uptake (% of theory)

Compound

8

90

7

87

25

91

26

78

11

55

73

14


Physical State Very viscous, flowable liquid Low viscosity liquid Very viscous, nonflowable liquid Low viscosity liquid

Solid

Moderate viscosity liquid

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64

16

74

17

Solid

Moderate viscosity liquid

NH2 H2N Me O Si Me Si O

18

O Si Me

26

solid

58

Flowable liquid

Si O Me

NH2

H2N

19

It is clear looking at the pairs of solvents that, upon reaction with CO2, the primary amine containing compounds form very viscous materials while the corresponding ethoxyethylamine derivatives remained as flowable liquids. Although the linear primary amines (8, 24) showed good CO2 reactivity with 90% or greater absorption, they would not be useful in a liquid-based process given their very high viscosities. In contrast, the ethoxyethylamine compounds gave very low viscosity liquids. It was odd that 26 only reached 78% of theoretical CO2 uptake but this will be discussed below.


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In a similar fashion, the branched (11, 15), star (16, 17) and cyclic (18, 19) series displayed the same trends. All the primary amines gave solids and very low conversions while the substituted analogs showed higher CO2 uptake and were all flowable liquids. Further comparisons were also made between the baseline primary amine 25 and a series of substituted silicones 26-29. These solvents all had a significant amount of branching, in the form of the β-isomer, and all showed modest CO2 uptake levels in the range of 48-78% at 40 oC. One explanation theorized to account for this observation was that the CO2 could not access the slightly more hindered substituted amine groups in solvents 26-29. This, along with the longer silicone chain in the core that could potentially shield the reacting CO2 might account for the modest theoretical uptake values. To probe this possibility, a series of similarly substituted materials were made via a route that did not permit any β-isomer to form. Compounds 37-40 were tested under similar conditions and modest yields of carbamates were obtained again. Obviously, the β-isomer had little impact on the CO2 uptake reaction. An alternate explanation, was that the reaction temperature was too high to permit effective CO2 absorption. Isotherms generated for the structure shown in Figure 1 where XR= OEt, showed that with 16.4% CO2 in N2, only 58% of the theoretical CO2 uptake was achieved at 40 oC. This was due to the much lower heat of reaction (~1860 kJ/kg CO2 vs 2560 kJ/kg CO2) compared to GAP-1. When the temperature was lowered to 23 oC, in all but one case, theoretical uptake was achieved. In several cases, slightly greater than theoretical absorption was observed. Some of this may be attributed to experimental error as most of the uptake experiments were run only once. However, replicates of some reactions provided reproducibility in the range of +/- 4%. There may have also been a small contribution of physical solubility of CO2 in the solvent.

Table 2. Comparison of Branched vs. Linear Functional Groups.

Compound

Entry

25

CO2 Uptake (% of theory @ 40 o C)

Physical State

CO2 Uptake (% of theory @ 23 o C)

Physical State

91

Very viscous, nonflowable liquid

-

-

78

Low viscosity liquid

96


25% β-isomer

26 40% β-isomer 17


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107


70


102


58


88


63


103


38

64


106


39

75


101


73


101


27

48


55% β-isomer

28 55% β-isomer

29 60% β-isomer

37

40

A final example of this new class of CO2 capture solvents that employ a longer alkyl chain spacer and a short siloxane core is shown in Scheme 9. A 5-carbon spacer with an ethoxyethylamino group was introduced via hydrosilylation of 42 on hexamethyltrisiloxane 41. Approximately 10% of the β-isomer was formed in 43 and CO2 uptake experiments showed ~106% theoretical uptake with the solvent maintaining a low viscosity profile upon full reaction with CO2.


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Energy & Fuels

Scheme 9. Long chain spacer.

If intra-molecular H-bonding is responsible for the decreased viscosity observed in these solvents on reaction with CO2, the same principle should apply to an inverted structure shown in Figure 4 wherein the reactive nitrogen is on the terminus and the H-bonding heteroatom is located internally.

Figure 4. Inverted Aminosilicone Structure. Several examples of these solvents are in preparation and the results of their testing will be reported later.

Conclusions Silicones with a variety of linear, branched, star and cyclic architectures were synthesized that contained electron-donating ethylaminopropyl groups attached to the silicone core. These solvents were tested for CO2 uptake and their physical state after reaction. Compared to analogous materials that only possessed a primary amine, all heteroatom substituted derivatives displayed excellent CO2 uptake and all maintained a liquid, flowable state after reaction. Optimal CO2 uptake was achieved at ambient temperatures rather than the typical 40 oC level. This was likely due to the lower heats of reaction for the secondary amine structure. β-Isomers present in the samples did not adversely affect the reaction with CO2 or the ability to remain in a liquid state upon complete reaction. Further examination of this class of materials is warranted to optimize performance under conditions of interest. Measurement of additional physical properties of the down-selected solvents is in progress.


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References 1. (a) Iijima, M.; Nagayasu, T.;Kamijyo, T.; Kishimoto, S.; Nakatani, S. Mitsubishi Heavy Industries Technical Review, 2012, 49, 37. (b) Rameshni, M. in http://stoppingclimatechange.com/Carbon%20Capture%20Overview%20(3).pdf 2. (a) Stéphenne, K. Energy Procedia, 2014, 63, 6106. (b) Shaw, D., Energy Procedia, 2009, 1, 237. 3. (a) Satish Reddy, S.; Scherffius, J.; Freguia, S. in https://netl.doe.gov/publications/proceedings/03/carbon-seq/PDFs/169.pdf (b) Scherfuss, J. R.; Reddy, S.; Klumpyan, J. P.; Armpriester, A., Energy Procedia 2013, 37, 6553. 4. (a) Moser, P.; Schmidt, S.; Stahl, K.; Vorberg, G.; Lozano, G. A.; Stoffregen, T.; Rösler, F., Energy Procedia 2014, 63, 902. (b) https://www.basf.com/us/en/company/news-and-media/newsreleases/2016/07/P-US-16-086.html

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(a) H Heldebrant, D.J.; Yonker, C. R.; Jessop, P. G.; Lam, P., Energy Environ. Sci., 2008, 1, 487. (b) Mathias, P. M.; Afshar,K.; Zheng, F.; Bearden, M. D.; Freeman,C. J.; Andrea,T.; Koech, P. K.; Kutnyakov,I.; Zwoster,A,; Smith,A,. R.; Jessop,P. G.; Nik, O. G.; Heldebrant, D. J., Energy Environ. Sci., 2013,6, 2233.

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Energy & Fuels

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Second-Generation Aminosilicones as CO2 Capture Solvents

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