Phase-Change Aminopyridines as Carbon Dioxide Capture Solvents

Publication Date (Web): June 9, 2017. Copyright ... A Novel Phase-Changing Nonaqueous Solution for CO2 Capture with High Capacity, Thermostability, an...
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Phase-Change Aminopyridines as Carbon Dioxide Capture Solvents Deepika Malhotra, Jordan P. Page, Mark E. Bowden, Abhijeet Karkamkar, David J. Heldebrant, Vassiliki-Alexandra Glezakou, Roger Rousseau, and Phillip K. Koech* Energy Processes and Materials Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: Carbon dioxide is the main atmospheric greenhouse gas released from industrial point sources. In order to mitigate adverse environmental effects of these emissions, carbon capture, storage, and utilization are required. To this end, several CO2 capture technologies are being developed for application in carbon capture, including aqueous amines and water-lean solvents. Herein we report new aminopyridine solvents with the potential for CO2 capture from coal-fired power plants. These four solvents, 2-picolylamine, 3-picolylamine, 4-picolylamine, and N′-(pyridin-4-ylmethyl)ethane-1,2-diamine, are liquids that rapidly bind CO2 to form crystalline solids at standard room temperature and pressure. These solvents have displayed high CO2 capture capacity (11−20 wt %) and can be regenerated at temperatures in the range of 120−150 °C. The advantage of these primary aminopyridine solvents is that a crystalline salt product can be separated, making it possible to regenerate only the CO2rich solid and ultimately reduce the energy penalty.



INTRODUCTION Carbon dioxide emitted from coal-fired power plants and other industrial processes is a major anthropogenic greenhouse gas released to the atmosphere. In order to mitigate the potential effects of CO2 emissions on the environment, global efforts have been dedicated to developing technologies for CO2 capture, utilization, and storage. To this end, aqueous alkanolamines such as monoethanolamine (MEA) have been widely used for natural gas purification1,2 and extensively studied for potential application in postcombustion CO2 capture from flue gas.3−5 Aqueous MEA-based solvents have high reactivity, low cost, and acceptable capture capacity; however, using them is energy intensive because water must be boiled and condensed during regeneration of the solvent. Other drawbacks of aqueous amines include high volatility, corrosivity to steel, and oxidative degradation.6,7 Implementing amine solvent systems for CO2 capture from flue gas would result in a 30% parasitic load to coal-fired power plants.8 The U.S. Department of Energy has set a goal to develop technologies capable of capturing 90% of CO2 from flue gas from coal-fired power plants with a combined cost (operational expenditures and capital expenditures) target of $20 per ton of CO2.9 Water-lean solvents are proposed to overcome the energy penalties inherent to regeneration of aqueous amines by reducing the amount of water in the solvent, which cuts parasitic loads to approximately half those of aqueous amines.10,11 Examples of these water-lean solvents are nonaqueous alkanolamines,12−16 task-specific ionic liquids,17,18 conventional ionic liquids,19 aminosilicones,20,21 siloxylated amines,22,23 nanomaterial organic hybrids (NOHMs),24 and CO2 binding organic liquids (CO2BOLs).25,26 Our group aims © XXXX American Chemical Society

to advance water-lean solvents by developing a deeper fundamental understanding of the molecular level interactions to enable design of cost-effective CO2 capture sorbents.27−29 CO2BOLs and other water-lean solvents show excellent CO2 uptake and CO2 mass-transfer capacities similar to those of aqueous amine solvents, in addition to favorable kinetic and thermodynamic properties; however, CO2-bound solution viscosities are an order of magnitude higher than those of aqueous amines.10,11 In the course of developing amine-based solvents that have low CO2-rich viscosities, we serendipitously discovered that liquid primary aminopyridines readily formed crystalline salts upon CO2 binding. This phase-change solvent comprises a primary amino group tethered by a short carbon chain to a pyridine ring that binds CO2 to form either a zwitterionic carbamate or a conventional carbamate anion/ammonium ion pair (Table 1 and Scheme 1). A majority of water-lean CO2 capture solvents described in the current literature do not undergo a liquid-to-solid phase change. In one of these rare examples, Perry and co-workers describe a liquid-to-solid phase-changing CO2 capture solvent, 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethylsiloxane (GAP-0).30 Other liquid-to-solid phase-change solvent systems include 1,2ethanediame in polyethylene glycol (PEG)31 and amino acid salt in water.32 Conversely, Brennecke et al. report phasechanging ionic liquids (PCILs), solids that convert to liquids upon CO2 uptake; these PCILs could exploit the heats of Received: Revised: Accepted: Published: A

March 1, 2017 May 6, 2017 June 9, 2017 June 9, 2017 DOI: 10.1021/acs.iecr.7b00874 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. CO2 Capture Capacity of Phase-Changing Aminopyridines

4-ylmethyl)ethane-1,2-diamine (Figure 1). These solvents, although highly volatile, possess high CO2 capture capacity. One common drawback of liquid-to-solid phase-changing solvents is sensitivity to moisture that makes solid carbamates sticky or semisolid. The aromatic nature of these new aminopyridine solvents allows the formation of crystalline solids that are water tolerant. Detailed experimental results of CO2 capture performance, regeneration studies, and mechanistic exploration are discussed herein.

Scheme 1. Potential Binding Modes of Aminopyridines with CO2



EXPERIMENTAL SECTION General Experimental Details. All CO2 uptake and regeneration experiments were performed under atmospheric conditions unless otherwise stated. Liquids were transferred via a plastic syringe or glass pipet. 2-picolylamine, 3-picolylamine, and 4-picolylamine were purchased from Sigma-Aldrich and either used without further purification or distilled under reduced atmosphere over MgSO4. Deuterated dimethyl sulfoxide (DMSO-d6) was purchased from Cambridge Isotope Laboratories and stored over 4 Å molecular sieves. CO2 (99.99%) was purchased from Oxarc. Proton nuclear magnetic resonance (1H NMR) spectra were recorded with 300 and 500 MHz spectrometers. Chemical shifts are reported in delta (δ) units, parts per million (ppm) downfield from tetramethylsilane. Carbon nuclear magnetic resonance (13C NMR) spectra were recorded with 300 (75.5 MHz) and 500 (125.7 MHz) spectrometers. Chemical shifts are reported in delta (δ), parts per million (ppm) relative to the center of the septet at 39.52 ppm for DMSO-d6. 13C NMR spectra were routinely run with broadband. Attenuated total reflectance infrared spectroscopy

melting and fusion to offset the enthalpy of CO2 binding and release and improve efficiency.33 The liquid-to-solid phasechanging property of these solvents is proposed to reduce the energy penalty of regeneration by concentrating or separating the CO2-rich solvent from the lean, which enables thermal stripping of CO2 from only the CO2-rich solid, thereby reducing the reboiler duty.30,33 Phase-changing solvents have a high enthalpy of reaction compared to aqueous amines; this enables them to be regenerated at higher temperatures, which provides a higher CO2 pressure in the stripper, thus potentially reducing CO2 compression costs,30 or allows use of the advanced stripper configurations proposed by Rochelle and coworkers.34 In this paper, we describe new phase-change solvents based on a class of pyridylamines such as 2picolylamine, 3-picolylamine, 4-picolylamine, and N′-(pyridin-

Figure 1. Phase-change aminopyridines as CO2 capture solvents. B

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urations, i.e., solvent misters, centrifuges, and augers that are completely different from absorbers and strippers of conventional solvent-based technologies. It is believed that effective phase-changing solvents for CO2 separations would require nonviscous liquids that rapidly and quantitatively bind CO2 to form powdery or crystalline solids under both wet and dry conditions to mitigate slurry formation, which would retard CO2 uptake by clogging or fouling infrastructure.30 Amines are known to rapidly bind CO2, forming carbamate salts; however, not all the products are easily processed, i.e., augured from absorber to stripper to be regenerated thermally. In this work, we postulated that amines with short aliphatic chains connected to an aromatic ring with a non-nucleophilic basic moiety would form crystalline or powdery solids. To test this hypothesis, we evaluated several aminopyridine solvents that have not been previously reported for CO2 capture (see Figure 1 and Table 1). All four aminopyridines reacted rapidly with CO2 (within 5−10 min) at ambient temperature and pressure (25 °C and 1 atm) to form solids. The CO2 capture capacity of these solvents was in the range of 11−21 wt %, which corresponded to 38−51 mol % (Table 1, columns 2 and 3). The CO2 uptake capacity was less than the theoretical maximum of 100 mol %, most likely because of mass-transfer limitations caused by the formation of a solid carbamate crust on the liquid interface that trapped unreacted liquid in the solid and prevented further reaction with CO2. This limitation might be mitigated by passing the solvent as an aerosol through a CO2-rich gas stream in order to maximize surface area and minimize mass-transfer resistance, leading to quantitative reaction and achieving the theoretical loading capacity of 50 wt % as demonstrated by Perry and co-workers.30 Heat of reaction for 3-picolylamine and CO2 obtained from DSC at 25 °C is 121 kJ/mol; this value includes contribution from the enthalpy of crystallization (see the Supporting Information). Aminopyridine molecules contain a nucleophilic amine moiety and a non-nucleophilic pyridine nitrogen atom that is weakly basic. This structural configuration potentially allows for two binding modes for CO2. Aminopyridines can capture CO2 via a 2:1 binding mode where the amine reacts with CO2 followed by proton transfer to a second aminopyridine molecule to form a carbamate ion pair (Scheme 1). Alternatively, a 1:1 binding mode is feasible where an amine group attacks the electrophilic CO2 carbon followed by intramolecular proton transfer to the pyridine nitrogen to form a carbamate zwitterion, which may be in equilibrium with the carbamic acid (Scheme 1). In our typical neat solvent experiments, we found the solvents did not achieve a CO2 loading above 50 mol %, which suggested that our solvent behaved as a 2:1 CO2 binding agent. However, if the solvent was dissolved in an equal volume of DMSO-d6 and sparged with CO2, the loading went well above 50 mol %. In our examples, the CO2 loading capacity for a solution of 2picolylamine in DMSO-d6 was 78.5 mol % and greater than 71 mol % for 3- and 4-picolylamine (Table 1, column 5), which indicated both 1:1 and 2:1 binding modes were present. To test whether the increase in CO2 uptake of aminopyridines in DMSO-d6 solutions is not due to physically dissolved CO2, neat DMSO-d6 was bubbled with CO2 for 45 min at 25 °C. There was no increase in mass, indicating no contribution of physically dissolved CO2. In addition, 13C NMR spectra for the neat DMSO-d6 and those of the aminopyridines in DMSOd6 had no signal at 125 ppm corresponding to physically dissolved CO2. These results indicate that the increase in CO2

(ATR-IR) measurements were performed on a diamond crystal (angle of incidence, 45.0°; number of bounces, 1.0; sample refractive index, 1.50) using a Nicolet Magna-750 spectrometer running OMNIC software. The spectra were recorded over 400−4000 cm−1 with a resolution of 4 cm−1. CO2 Uptake and Buret Release. Liquid aminopyridines were transferred via syringe to an oven-dried, cooled, and preweighed single-neck round-bottomed flask that contained a stir bar and was capped with a rubber septum punctured by two syringe needles. Each preweighed flask of aminopyridine liquid was charged with CO2 delivered via a regulator connected to a syringe needle and allowed to sparge for 45 min. Each sample was massed for CO2 uptake, and the amount of CO2 was calculated based on the difference in mass. In the subsequent CO2 stripping experiment, the solid carbamate sample was attached to a custom-built automated buret system,35 and a preheated oil bath (100−150 °C, solvent dependent) was raised until the flask was submerged to the ground glass joint. The amount and rate of CO2 released were quantified based on the volume of gas displaced in the buret as a function of time and compared to the postrelease sample mass. Synthesis of N′-(pyridin-4-ylmethyl)ethane-1,2-diamine. To a stirred solution of ethylene diamine (12.13 g, 13.50 mL, 202 mmol) in methanol at 0 °C was added 4pyridenecarboxaldehyde (9.66 g, 8.50 mL, 90.2 mmol) dropwise over 30 min. The mixture was allowed to stir a further 30 min at 0 °C, then was allowed to warm to room temperature, and stirred for 6 h. Next, sodium borohydride (5.69 g, 150 mmol) was added in small portions at room temperature, and the mixture was stirred for a further 12 h. The reaction was quenched with a minimum of water until no gas evolved, then the solvent was removed in vacuo. The deep yellow crude mixture was distilled at 110 μm (bp 100−105 °C) to produce 5.94 g of product as a clear light yellow oil (44% yield).36 Single-Crystal X-ray Diffraction (XRD). Single crystals for structure determination were held on nylon loop mounts with Paratone oil. Diffraction data were collected on a Bruker Apex II diffractometer using Mo radiation, integrated using the Bruker SAINT software, and corrected for absorption using the multiscan method. Structures were solved and refined using the SHELX37 and OLEX238 software suites. Powder XRD. Data were collected on a Panalytical Bragg− Brentano goniometer with Cu Kα radiation, a postdiffraction monochromator, and variable divergence slits. The powder was mixed with a weighed quantity of corundum powder (NIST SRM 674a), and the quantities of crystalline phases were determined by whole-pattern (Rietveld) refinement using TOPAS v5 (Bruker AXS). The corundum standard allowed phase fractions to be placed on an absolute scale; therefore, the amorphous content could be determined by difference. Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). The TGA-DSC data was collected on a Neztsch STA-449 simultaneous TG-DSC equipped with internal mass flow controllers. Approximately 25 mg of the sample was loaded in an Al2O3 crucible and held under a flow of N2 gas for 30 min. CO2 was introduced at a predetermined time, and the weight gain was measured along with the DSC signal.



RESULTS AND DISCUSSION Phase-changing CO2 capture processes are in the early stages of development because they require nonconventional configC

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which confirms the presence of an ion pair. The absorption at 3364 cm−1 in the CO2-free 2-picolyamine indicative of N−H stretching is absent in the CO2-bound spectrum, which further suggests a carbamate is present. The ATR-IR results for 3picolylamine and 4-picolylamine with and without CO2 have signals consistent with those of 2-picolylamine (see the Supporting Information). An ideal solvent system for CO2 capture has a high CO2 capture capacity and low regeneration energy. The advantage of aminopyridines is that they bind with CO2 to form crystalline salts, which can be separated from the lean liquid solvent for regeneration, and the high enthalpy of the reaction can be used to reduce the energy penalty during compression by enabling CO2 stripping at high pressure.30 To evaluate the regeneration rate and temperature of these solvents, CO2 release experiments were conducted on the CO2-bound solvents that used an automated gas buret system in order to quantify the volume of CO2 released.35 The amounts of CO2 released by 2-picolylamine, 3-picolylamine, and 4-picolylamine were measured at 100 and 120 °C, temperatures that are common for a stripper column for amines (Table 2). The CO2 release was rapid; most

loading capacity of aminopyridines in DMSO-d6 is due to the 1:1 binding mode, resulting from stabilization of the zwitterion and neutral species. Large single crystals of the reaction product of 3-picolylamine with CO 2 were obtained, which allowed for unambiguous identification of the product using single-crystal diffraction. Figure 2 shows the structure of the recovered crystal

Figure 2. X-ray crystallographic structure of 3-picolylamine.

with oxygen in red, carbon in gray, hydrogen in white, and nitrogen in blue. The crystal structure confirmed a 2:1 carbamate pair. To evaluate whether the 2:1 carbamate was representative of the entire sample, a powder diffraction pattern was obtained from a sample mixed with a known quantity of internal standard (α-Al2O3). Profile fitting of the pattern using the Rietveld method (see Figure S10) revealed that only the carbamate ion pair was present, as exhibited in Figure 2. Furthermore, the internal standard showed that this compound was present in 100% concentration, which ruled out the presence of significant quantities of any additional amorphous compounds. The CO2-rich aminopyridines were characterized using both 1 H and 13C NMR spectroscopy, and the spectra were compared to those of CO2-free solvents. Representative spectra for CO2free and -bound 2-picolylamine in DMSO-d6 are shown in Figure S1. The 1H NMR spectra showed the characteristic peaks for a benzylic proton shifted downfield to 4.41 ppm in the CO2-rich 2-picolylamine as compared to 3.78 ppm for its CO2-free form. This shift is attributed to the electronwithdrawing effect of the carboxylate group on the amine motif and confirms the presence of a carbamate (Scheme 1). A new broad acidic proton signal appears at 10.77 ppm, which can be assigned to the pyridinium proton that is in equilibrium with a carbamic acid proton. The 13C NMR spectrum shows two new peaks at 160.4 and 154.24 ppm that can be assigned to the formation of carbamate and the pyridinium carbons, respectively (Figure S2). An upfield chemical shift was observed for the benzylic carbon, which shifted from 47.40 to 46.63 ppm. Characteristic shifts in the 1H and 13C NMRs for both CO2-rich and -free 3-picolylamine, 4-picolylamine, and N′-(pyridin-4ylmethyl)ethane-1,2-diamine were consistent with those of 2picolylamine (see the Supporting Information). The CO2-free and CO2-bound aminopyridines were also characterized using ATR-IR spectroscopy. Figure S3 shows the spectra for 2picolylamine, and Figures S4 and S5 are for 3-picolylamine and 4-picolylamine, respectively. The appearance of a strong absorption signal at 1625 cm−1 for CO2-bound 2-picolylamine, which is attributed to carbonyl stretching for the carbamate, is consistent with literature assignments.39 Additionally, the broad absorption between 2600 and 3100 cm−1 is due to the O−H stretching for the carbamic acid, and the broad absorption signal at 2582 cm−1 is assigned to the ammonium cation NH3+,

Table 2. CO2 Release Experiments from Buret

of the CO2 was released within 5−10 min at 120 °C in all three cases (Figure 3). The volumetric buret release experiments were also compared with the gravimetric difference between rich and stripped solvents to validate the volumetric data and were found to be consistent (Table S1). In the case of N′(pyridin-4-ylmethyl)ethane-1,2-diamine, although the CO2 was released at a higher temperature (150 °C), the compound remained active over five cycles. The 1H and 13C NMR results of the recovered solvent after five cycles of CO2 uptake and release were consistent with that of the pristine N′-(pyridin-4ylmethyl)ethane-1,2-diamine, which indicated that no significant thermal decomposition occurred over the course of the experiment (see page S21 of the Supporting Information). To simulate the flue gas environment, we saturated the CO2 with water by passing it through a bubbler containing distilled water. In these studies, the capture solvent still formed solids, and the uptake capacities were similar to those in the DMSO experiments (Table S2). The solids were visibly wet, but no more “sticky” than the dry samples. To confirm the nature of the CO2-rich species for the hydrated aminopyridines, 13C NMR spectra for the solvents in DMSO-d6 were acquired and compared to those of anhydrous CO2-rich aminopyridines (Figures 4, S8, and S9). Figure 4 shows the 13C NMR spectra for CO2-rich 2-picolylamine containing water and that of the corresponding anhydrous solvent; it is clear from this spectrum that the hydrated sample contains bicarbonate, as evidenced by the presence of an extra carbonyl carbon signal at 159.4 ppm, D

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Figure 3. Rate of CO2 release from 2-picoylamine and 3-picolylamine at 100 and 120 °C.

Figure 4. 13C NMR of CO2-bound 2-picolylamine with and without water.

which is consistent with known similar chemical shifts.40 The presence of a bicarbonate explains the high CO2 uptake capacity (>70 mol %) of aminopyridines under wet CO2 conditions, due to the contribution from the 1:1 binding mode and entrained trace moisture (Table S2). This result shows that water is chemically reacting with CO2 and these aminopyridines to form a mixture of carbamate and bicarbonate, matching the speciation of aqueous amines.

aqueous amines, which enables regeneration at higher temperatures and leads to higher CO2 pressure in the stripper, potentially reducing CO2 compression cost. Further work will focus on functionalizing these solvents to address their high vapor pressure, and thermodynamics and kinetics property testing will be performed.

CONCLUSION We identified new phase-change aminopyridine-based solvents that readily form crystalline salts upon binding CO2 under both dry and wet conditions. This solvent system displayed high CO2 capture capacity (11−20 wt %) and facile thermal regeneration at temperatures between 120 and 150 °C. X-ray crystallographic data and powder diffraction patterns of CO2rich 3-picolylamine confirm the presence of a 2:1 carbamate ion pair binding mode, and the high CO2 uptake capacity of aminopyridine solution in DMSO indicates the presence of a zwitterionic 1:1 binding mode contribution. These phasechanging solvents have a high enthalpy of reaction compared to

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00874. Spectral data for CO2-free and CO2-bound compounds 2-picolylamine, 3-picolylamine, 4-picolylamine, and N′(pyridin-4-ylmethyl)ethane-1,2-diamine; powder XRD pattern of CO2-bound 3-picolylamine; uptake capacity in the presence of water; and gravimetric CO2 release (PDF)





ASSOCIATED CONTENT

S Supporting Information *



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 (509) 372-6891. E

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David J. Heldebrant: 0000-0002-5529-526X Vassiliki-Alexandra Glezakou: 0000-0001-6028-7021 Roger Rousseau: 0000-0003-1947-0478 Phillip K. Koech: 0000-0003-2996-0593 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors acknowledge the U.S. Department of Energy (DOE), Office of Fossil Energy, for funding this project through NETL FWP-65872 and the Pacific Northwest National Laboratory (PNNL) for facilities. PNNL is proudly operated by Battelle for the U.S. Department of Energy under Contract No. DE-AC05-76RL01830. This contribution was identified by Session Chair William J. Koros (Georgia Institute of Technology) as the Best Presentation in the session, “Novel Materials for Gas Separation, Storage & Utilization,” at the 252nd American Chemical Society National Meeting and Exposition in Philadelphia, PA in August 2016.



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DOI: 10.1021/acs.iecr.7b00874 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.7b00874 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX