Direct CO2 Capture from Air using Poly(ethylenimine)-Loaded

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Direct CO2 capture from air using poly(ethyleneimine)loaded polymer/silica fiber sorbents Achintya Sujan, Simon H Pang, Guanghui Zhu, Christopher W Jones, and Ryan P Lively ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06203 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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ACS Sustainable Chemistry & Engineering

Direct CO2 capture from air using poly(ethyleneimine)-loaded polymer/silica fiber sorbents Achintya R. Sujan, Simon H. Pang, Guanghui Zhu, Christopher W. Jones*, Ryan P. Lively* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, United States

Corresponding authors: *Email: [email protected] *Email: [email protected]

Keywords: Direct air capture, CO2 adsorption, fiber sorbents, poly(ethyleneimine), temperature-vacuum swing adsorption

0 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Direct CO2 capture from atmospheric air is gaining increased attention as one of the most scalable negative carbon approaches available to tackle climate change if coupled with the sequestration of CO2 geologically. Furthermore, it can also provide CO2 for further utilization from a globally uniform source, which is especially advantageous for economies without natural sources of carbon-based fuels. Solid-supported amine-based materials are effective for direct air capture (DAC) due to their high CO2 uptakes and acceptable sorption kinetics at ambient temperature. In this work, we describe the application of polymer/silica fiber sorbents functionalized with a primary amine rich polymer, poly(ethyleneimine) (PEI), for DAC. Monolithic fiber sorbents composed of cellulose acetate (CA) and SiO2 are synthesized via the dry-jet, wet quench spinning technique. These fibers are then functionalized with PEI (Mw 800 Da) in a simple and scalable post-spinning infusion step and tested for CO2 capture under pseudo-equilibrium conditions as well as under breakthrough conditions. An investigation to study the effect of feed flowrate, adsorption temperature, and presence of moisture in the feed on the CO2 breakthrough performance of a densely packed fiber sorbent module is conducted to highlight the potential application of this class of structured contactors in direct air capture. The pressure drop of these contactors at high gas velocities is also evaluated. Finally, a vacuum-assisted desorption step is demonstrated for production of high purity CO2 from both dry and humid ambient air mixtures.


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Introduction Direct air capture (DAC) with carbon sequestration was a concept first introduced for climate change mitigation in 1999.1 In this negative carbon approach, CO2 is extracted from ambient air for permanent storage.2 A significant advantage of DAC is that it does not require co-location with a concentrated point source of CO2. Moreover, technologies for direct air capture are not faced with the challenge of CO2 capture in the presence of high concentrations of other contaminants such as SOx, NOx, and mercury, which can be present at significant concentrations during flue gas capture. In a broader context, direct air capture addresses CO2 emissions released by transportation vehicles (planes, ships, buses, cars) and other mobile sources that are difficult for conventional technologies to handle.3 A recent assessment of negative CO2 emissions technologies suggests that scalable approaches tailored for DAC conditions offer the potential for cost-effective CO2 extraction from the air,4 with projected costs approaching an order of magnitude lower than the previous analyses5 that did not consider technologies tailored for ultra-dilute conditions. DAC via adsorption-based approaches has gained increased attention and involves the direct contact of a highly diluted CO2 stream with solid sorbents. These solid sorbents are typically comprised of relatively small sorbent particles in structured contactors that offer higher surface areas and reduced internal diffusion resistance compared to pellet-packed adsorption columns.6,7 However, the kinetics of CO2 adsorption can be reaction-limited (dependent on temperature, pressure, etc.) as well as diffusion-limited (dependent on pore size, pore shape). Sorbents composed of zeolites, activated carbons, or metal-organic frameworks (MOFs) are typically physical sorbents that bind CO2 more weakly, offering low uptakes and low selectivity at the low CO2 concentration (~400 ppm) present in ambient air.8 However, chemisorbents involving immobilized amines on potentially inexpensive supports such as cellulose,3,9 SiO2,10,11, and



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Al2O37,12 are more effective for DAC13,14 because of their selectivity for CO2 at low concentration,10 tolerance to moisture,15 and stability.16 The type of amine and the effect of amine structure for DAC applications has been investigated in detail.17 Branched poly(ethyleneimine) (PEI) with a low molecular weight (Mw 800) is the most commonly employed poly(amine) utilized to sorb CO2. The large density of amine groups and good stability under thermal/vacuum swing cycles make this polymeric amine a suitable choice for CO2 capture from the air, as compared to other amine rich organic molecules with lower molecular weight and higher volatility such as tris(2-aminoethyl)amine

(TREN),18

tetraethylenepentamine

(TEPA),19,20

and

pentaethylenehexamine (PEHA).21 Temperature swing adsorption (TSA) using relatively low volatility PEI is an efficient method to conduct DAC22 to minimize loss of CO2-sorbing material during the sorbent regeneration step. The DAC process is typically conducted under large operating air flow rates, over a range of temperatures (0-50 °C) depending on the geographical location, and often in the presence of moisture in the feed stream. An effective DAC process will involve structures that can easily move high volumes of gas with low-pressure drop and materials that can sustain their CO2 capture capacity in the presence or absence of moisture. There are relatively few scalable contactors utilizing solid sorbents that are applicable to DAC, with the most common contactor used in lab scale studies – fixed beds – being not deployable at scale due to prohibitively high-pressure drops. Most prior academic studies focus on maximizing equilibrium CO2 capacity and accelerating uptake kinetics, and as a result, sorbent regeneration or desorption is often performed using an inert purge stream such as N2, and Ar, or in rare cases more reactive gases/vapors such as CO2 or steam.23 The use of these inert gases during the desorption stage leads to desorbed CO2 in diluted form, unsuitable for sequestration. In contrast, for practical, scalable collection of concentrated


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CO2, a combined temperature-vacuum swing adsorption/desorption (TVSA) process using an amine-functionalized SiO2-based sorbent has been demonstrated in a packed bed configuration for extraction of high purity (>97%) CO2 from the air after rigorous optimization of operating parameters such as desorption pressure, desorption temperature, and relative humidity.24 Previously, the use of composite polymer fiber contactors containing silica particles functionalized with an amine-based polymer has been shown to be effective for CO2 removal from more concentrated feeds such as coal-derived flue gas.25 However, similar contactors for ultradilute feeds have not been reported. In this work, we demonstrate the application of polymer/silica fiber sorbents functionalized by branched PEI for CO2 capture from an ultra-dilute, simulated air stream (approximately 400 ppm). We describe the effect of temperature, operating flowrate, and the presence of humidity on the performance of these fiber sorbents via breakthrough, pressure drop, and desorption experiments. Finally, a temperature- and vacuum-assisted desorption approach was demonstrated to extract high purity CO2 from the dry simulated air mixture as well as CO2 from actual ambient lab air under humid conditions.

Experiment Materials Cellulose acetate (CA) (MW 50,000 Da) and mesoporous silica (C-803) were purchased from Sigma-Aldrich Inc. and Grace Davison Inc., respectively. N-methyl-2-pyrrolidone (NMP) (Reagent Plus, 99%) was purchased from Sigma-Aldrich and used as the solvent during the polymer-dope preparation. Poly(vinylpyrrolidone) (PVP) (MW 40,000 Da) was purchased from Sigma–Aldrich and used as the pore-forming agent responsible for the open pore network in the polymer/silica fiber sorbents. Methanol (ACS Reagent, 99.8%, and n-hexane (ACS Reagent, 3 ACS Paragon Plus Environment


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>98.5%) were used in the solvent exchange step to remove water and residual NMP in spun fibers and purchased from Sigma-Aldrich and Baker Inc., respectively. All solvents were used as-received from the manufacturer with no purification or modification. Branched poly(ethyleneimine) (PEI) (MW 800 Da) was purchased from Sigma-Aldrich and used as the amine source in post-spinning functionalization step. Potassium chloride (ACS grade, BDH) was used to create a saturated solution using DI H2O at 25°C for experiments under controlled relative humidity conditions. Single-component gases were purchased from Airgas as UHP grade. Gas mixtures were purchased from Matheson Trigas. A mixture composed of 395 ppm CO2/balance He was used for gravimetric adsorption experiments on the TGA. A mixture composed of 380 ppm CO2/397 ppm He/balance N2 was used for breakthrough adsorption experiments. Both mixtures used in the current study contained approximately 400 ppm CO2 that

simulated

the

CO2 concentration in dry air.

Preparation of fiber sorbents Polymer/silica monolithic fiber sorbents were spun via the dry-jet, wet quench spinning technique using a custom-built fiber spinning line (Figure S1). All polymers were dried under 25 in Hg vacuum at 110°C for 12 hours before polymer dope preparation. A detailed description of the preparation of polymer dope can be found elsewhere.25,26 Spinning parameters and dope compositions used in the current study are listed in Table S1. For the formation of PEI infused CA-SiO2 fiber sorbents, 2 g of dry CA-SiO2 monolithic fibers corresponding to approximately 1 g of SiO2 was placed in a vial containing 16 mL of methanol. The methanol-soaked fibers were then placed in another vial containing 16 mL of PEI in methanol solution at various PEI weight loadings for 4 h. The PEI/methanol soaked fibers were then placed


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in a vial containing 16 mL of n-hexane to solvent exchange the methanol while preserving the PEI content on the fibers. Finally, PEI-infused CA-SiO2 fibers were allowed to dry at ambient temperature overnight. For the majority of the experiments in this study, the infusion was performed using a 10 weight% PEI/methanol solution, which resulted in a PEI loading of 0.7 gPEI/gSiO2 in the final fibers. These fibers are referred to as PEI-CA-SiO2 throughout the text. Fiber modules were constructed from a 20 cm long stainless-steel tube with an inner diameter of 0.4 cm. An empty module was filled to a practical limit of 30 monolithic PEI-CA-SiO2 fibers, corresponding to a fiber packing fraction of 0.57 vol/vol. These modules were used in the pressure drop and column breakthrough experiments described below. For the formation of PEI impregnated SiO2 powdered sorbents, 0.7 g of PEI was dissolved in 20 mL of methanol and allowed to stir for 2 h. To this solution, 1 g of dried SiO2 was added and the mixture was allowed to stir for another 2 h. Then, the methanol was removed by rotary evaporation, and the remaining powder was dried under ambient temperature overnight. Finally, the powder was dried at 90°C under a high vacuum Schlenk line operating at 6×10-4 in Hg. The dried powder is referred to as PEI/SiO2 powdered sorbents throughout the text.

CO2 Adsorption Experiments Pseudo-equilibrium adsorption capacities for CO2 were measured gravimetrically for a span of 12 h at 35°C using a mixture of 395 ppm CO2/balance He on a TA Instruments Q500 TGA. Ambient air commonly exists over a wide temperature range spanning from well below freezing to in excess of 50°C. Typical indoor environments are controlled in the range of 20-25°C. In this study a sorption temperature of 35°C was chosen because it allows for controlled testing that is not subject to seasonal or daily room temperature variations and to represent the performance of



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the sorbents in regions such as India, China, Africa and so on where the atmospheric temperature is stable at 35°C for prolonged periods (for example, during summer months). Prior to CO2 adsorption, the fiber sorbents or powder samples were dried and pretreated by heating to 110 °C at a ramp rate of 10 °C min-1 under a flow of He and held for 2 h. The samples were then cooled to 35 °C and equilibrated at this temperature for 1 h. Subsequently, the gas flow was switched to a premixed gas containing 395 ppm CO2/He and held for 12 h, i.e., until the rate of mass change was less than 5×10-4 % min-1. Cyclic adsorption/desorption experiments were performed with a regeneration step between each adsorption cycle. The sample was initially heated at 110°C for 2 h under He and then equilibrated at 35°C for 1 h. Cycles of adsorption for 3 h at 35°C and desorption for 2 h at 110°C were then performed. He is used as a balance gas in the CO2 adsorption measurements by TGA because it is a non-adsorbing gas compared to CO2 when using amines for CO2 capture, and the use of a CO2/He mixture in the current study allows for comparison with similar amine loaded materials reported in the literature.7,27,28 Column breakthrough experiments were performed on a custom-built apparatus described elsewhere.29 In brief, for a typical experiment, a module containing PEI-CA-SiO2 monolithic fibers was wrapped in heat tape and dried at 90°C for 30 minutes under a 20 sccm flow of dry N2 and was then brought to the desired adsorption temperature. The inlet gas was then switched to a 380 ppm CO2/397 ppm He/balance N2 mixture at varying flow rates between 40 sccm and 200 sccm. The adsorption step was carried out for 4 to 5 h. After the sorption run was complete, the module was heated at 90°C under a 20 sccm flow of dry N2 for 25 min to regenerate the fiber sorbents. After the regeneration step was complete, the temperature of the module was then brought to the desired adsorption temperature and the protocol described above was repeated for multiple adsorption-desorption cycles. The effluent composition


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during column breakthrough experiments was determined with a Pfeiffer Vacuum QMS 200 Omnistar Mass Spectrometer. All experiments were conducted in triplicate to estimate the error. For experiments involving humidity, the fiber module was subjected to a pre-humidification step at the adsorption temperature, after regeneration but prior to starting adsorption. N2 was passed through a bubbler containing a saturated salt solution of KCl in DI water that was maintained at 25 °C, and then passed through the fiber module. The outlet concentration of water was monitored to ensure that the fiber module was completely saturated. Upon complete hydration of the module, the inlet gas was switched to a humidified flow of simulated air, similarly to the dry experiments described above. The tubing downstream of the module was held at 100 °C to prevent any condensation of H2O.

Characterization Scanning electron microscopy (SEM) was performed on a Hitachi SU8230 with a cold field emission gun at an accelerating voltage of 10 kV and an emission current of 15 μA. N2 physisorption at 77 K was conducted on a Microtrac Belsorp Max to determine the textural properties of the powders and fiber sorbents. Fibers were cut into lengths of 1.5 cm and inserted into the sample analysis tube. Prior to analysis, the samples were degassed below 10−2 kPa on a Belprep-Vac II. The degas protocol was as follows: (1) Heat from 25 to 110 °C at 5 °C/min and hold for 12 h, (2) Cool to 30 °C at 1 °C/min. The organic content of the samples was estimated from combustion TGA experiments on a Netzsch STA409PG TGA. Mass loss from 25 °C to 900 °C under a flow of N2 diluted air was recorded. The organic content was determined from mass loss between 120°C to 900°C. A Hiden IGASorp instrument was used to measure H2O vapor sorption isotherms. Samples were pre-treated at 110 °C for 3 h under N2 flow and cooled to 35°C.



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Samples were then exposed to various partial pressures of H2O under a flow of N2. Pressure drop measurements were conducted using a custom-built setup described in a previous study.30

Results and discussion Amine functionalized CA silica fiber sorbents SEM images of the monolithic fiber sorbents are shown in Figure 1. The average diameter of the monolithic fibers was found to be 530 (±5) microns, as depicted in Figure 1a. SiO2 particles with an average size of 3 to 5 microns appear to be held in place by the surrounding polymer matrix (Figure 1b) and uniformly embedded throughout the polymer matrix (Figure 1c).

Figure 1. SEM images of monolithic CA-SiO2 fiber sorbent (a) Face of fiber, (b) Edge of fibers, (c) Bulk of fiber. Combustion TGA measurements on bare CA-SiO2 monolithic fibers revealed a SiO2 loading of 48% by weight (Figure S2). This value is close to the original value in the dope (53%), as illustrated in Table S1. “Bare” CA-SiO2 fibers were then functionalized with solutions containing varying PEI concentrations in methanol to understand the effect of PEI loading on the structural integrity of the fibers.25 An image of dried monolithic fibers after infusion with various PEI concentrations is shown in Figure S3. Combustion TGA analysis was conducted to calculate the loading of PEI in the CA-SiO2 fibers, as shown in Figure S4. The PEI loading is calculated from the difference in the residual inorganic SiO2 content and total organic burn-off assuming the


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CA/SiO2 ratio remains unchanged during the PEI infusion process. The highest concentration of the PEI/methanol solution that produced fibers suitable for constructing a fiber module was 10 wt%, which resulted in fibers with a PEI content of 0.7 gPEI/gSiO2. Increasing the PEI content beyond this point resulted in fibers that spontaneously bent and appeared to become a gel-like material with a noticeable degree of swelling, thus making assembly into fiber modules impractical. We attribute this swelling behavior to be responsible for the collapse of the CA-SiO2 fiber structure at high concentrations of PEI in the impregnation solution. These observations are in accordance with a previous study.25 Approximately 90% of the pore volume of SiO2 in the fibers appeared to be filled with PEI as suggested by the low quantity of N2 adsorbed at 77K (Figure S5) and the pore volume after PEI infusion as shown in Table S2. The flexibility of a CA/silica/PEI fiber was evaluated by a bending test in which the minimum radius of curvature of the monolithic fiber is the outer radius of a coin at which the fiber breaks (21.2 mm in the case of PEI-CA-SiO2 with a 0.7 gPEI/gSiO2 loading, Figure S3 inset).31 The gravimetrically-measured kinetic uptake curves of the fiber sorbents and their powdered sorbent analog are shown in Figure S6. There is no significant difference in the rate of CO2 uptake at 380 ppm on the fibers as compared to the powdered specimen that contains the same concentration of PEI per gram of silica. Thus, the morphology of the fiber sorbent does not significantly affect the uptake kinetics at 380 ppm. Gravimetric CO2 uptake over multiple adsorption/desorption cycles is shown in Figure S7a. Rather than performing these experiments for 12 h to achieve the pseudo-equilibrium uptake, the working capacity of PEI-CA-SiO2 was determined for 3 h of adsorption time. Adsorption from 395 ppm CO2 performed for this amount of time was found to achieve approximately 95% of the pseudo-equilibrium capacity. An approximate 2% loss in the degassed (CO2-free) mass of the fiber



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sorbent was observed by the end of the 10th cycle. This mass loss appears as a decreasing baseline in the cyclic TGA profile shown in Figure S7a and can be attributed to the incremental loss of H2O from the fiber sorbent as previously observed for similar low molecular weight hydrophilic amine-based polymers under dry conditions.32,33 Nevertheless, it was found that branched PEI (Mw 800 Da) supported on the fiber sorbent structure is relatively stable, with no significant volatilization or deactivation since ~ 95% of the CO2 capacity was retained at the end of 10 cycles, as shown in Figure S7b.

Pressure drop in fiber sorbent modules The experimental pressure-drop as a function of gas superficial velocity in a densely packed fiber module containing PEI-CA-SiO2 fiber sorbents was measured under a flow N2 gas at 22°C. It can be seen that for a packed bed (void fraction = 0.42) composed of the powdered PEI-SiO2 samples (~ 500-micron particle size), an exponential rise in pressure drop at increasing superficial velocities occurs, as suggested by the Ergun equation.34 Indeed, the application of packed beds of such particle size is impractical, since the pressure drop will be prohibitively high under DAC operating flow rates. Even with larger particle sizes as shown in Figure 2, pressure drops will be too excessive at the air velocities needed for DAC applications.


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Figure 2. Column pressure drop as a function of superficial velocity calculated from the Ergun equation for particles of various diameters (lines). Experimentally-measured pressure drop for the packed fiber module at the same void fraction (circles). The measured pressure drop for the densely packed fiber module (void fraction = 0.43) as a function of superficial velocity is also plotted in Figure 2. The pressure drop values are corrected for the pressure drop that occurred in an empty module (i.e., no fibers) and is shown in Figure S8. The pressure drop in the empty tubes is due to friction drag, surface roughness, and stainless-steel fittings in the module assembly. The corresponding Reynold’s number for the 1 cm I.D. tube was calculated to be approximately 1700 at the highest operational superficial velocity of 2.6 m/s (assuming N2 gas has a dynamic viscosity at 25°C of 1.8×10-5 kg∙m/s; density of 1.17 kg/m3). This implies that the flowrates studied here are in the laminar flow regime, in which the head loss due to friction in the pipe as well as the head loss due to fittings and valves in the current module



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assembly can be neglected. A high pressure drop translates into higher parasitic power loss across the air compressor or fans used for gas delivery. In practice, the contacting system would be operated in extraction mode, rather than as a purification, with the CO2 uptake rate optimized with the pressure drop to give the lowest cost possible for capture.1,14

CO2 breakthrough experiments under dry conditions PEI infused CA-SiO2 monolithic fibers assembled in the modules were tested in a laboratory scale temperature swing adsorption (TSA) setup.25 Experiments were conducted to understand the effect of (i) adsorption temperature, and (ii) feed flowrate, as these two operating parameters significantly influence the performance of adsorbents. Figure 3 shows the CO2 breakthrough curves as a function of temperature between 35-55°C and in the flow rate range of 40-90 sccm.


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Figure 3. Breakthrough curves for CO2 as a function of temperature at various feed flowrates (a) 40 sccm; (b) 60 sccm; (c) 90 sccm. The mean residence time of the feed mixture through the bed is determined by observing the non-adsorbing He concentration profile at the outlet of the module as shown in Figure 3a-c. The range of operating conditions explored in the above experiments had a negligible impact on the shape and breakthrough time of the He tracer profile, which is expected when modules of similar dimensions and packing are utilized.35 Figure 3a shows the breakthrough curves for experiments conducted at a feed flow rate of 40 13 ACS Paragon Plus Environment


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sccm for 5 h. Initially, all of the CO2 in the feed is adsorbed by the fibers. However, after some time, CO2 begins to break through the column; the time at which the CO2 concentration is 5% of its inlet concentration is termed the breakthrough time. After breakthrough, the fiber sorbents reach equilibrium with the feed concentration of CO2 over a span of approximately 150 minutes. As the feed flow rate was increased to 60 sccm and 90 sccm, the difference in the CO2 breakthrough time as a function of temperature appeared to diminish. The breakthrough and pseudo-equilibrium capacities as a function of the flow rate at each sorption temperature are illustrated in Table 1. Table 1. Breakthrough and pseudo-equilibrium capacities of the PEI-CA-SiO2 fiber sorbent as a function of feed flowrate and sorption temperature. Breakthrough

capacity†

Pseudo-equilibrium capacity‡

(mmol/gfiber)

Sorption

(mmol/gfiber)

temperature

40 sccm

35°C

45°C

55°C

†

0.51 ± 0.01

0.41 ± 0.01

0.19 ± 0.01

60 sccm

0.48 ± 0.01

0.37 ± 0.01

0.13 ± 0.01

40

60

90

sccm

sccm

sccm

0.61 ±

0.62 ±

0.62 ±

0.01

0.01

0.01

0.55 ±

0.56 ±

0.56 ±

0.01

0.01

0.01

0.32 ±

0.33 ±

0.33 ±

0.01

0.01

0.01

90 sccm

0.44 ± 0.01

0.36 ± 0.01

0.08 ± 0.01

Capacities determined from the breakthrough curves and defined at 5% of C0 14 ACS Paragon Plus Environment

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Capacities determined from the breakthrough curves and defined at 95% of C0 The error mentioned above is taken as the standard deviation of experiments conducted in triplicate ‡

An increase in feed flow rate is expected to reduce the external mass transfer resistance that is partially responsible for retarding the approach to equilibrium. The pseudo-equilibrium capacity as a function of the feed flow rate is illustrated in Table 1. The subtle difference in the pseudoequilibrium capacity as a function of the flowrates under consideration, as reported in Table 1, is in accordance with the fact that the equilibrium capacity should not depend on the flow rate. On the other hand, a higher sorption temperature is responsible for the increased mobility of the aminopolymer chains, which in turn, can result in greater accessibility of the CO2 in the gas phase to the immobilized amines. Once the CO2 is fully accessible to the amines, the reaction is expected to proceed and follow thermodynamic trends. This phenomenon implies a reduction in the breakthrough and pseudo-equilibrium capacity of the fiber sorbents with an increase in the sorption temperature, as seen from values reported in Table 1 obtained upon analysis of the breakthrough curves shown in Figure 3. To investigate the stability and apparent kinetics of dry CO2 adsorption on a densely packed module containing PEI-CA-SiO2 fiber sorbents, the CO2 breakthrough curves were examined over 20 adsorption/desorption thermal swing cycles at a feed flow rate of 100 sccm and a sorption temperature of 35°C. This is shown in Figure S9a, which suggests that the sorption kinetics remained approximately constant. The pseudo-equilibrium capacity stabilized at 0.56 mmol/gfiber, as shown in Figure 4. Based on past studies, on poly(ethyleneimine) impregnated solid sorbents, one can expect a loss in CO2 capacity over several adsorption-desorption thermal cycles mainly due to volatilization of low molecular weight amino-polymers that ultimately leads to a lower amine density.36



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Figure 4. Pseudo-equilibrium and breakthrough CO2 capacity under dry adsorption conditions at 35°C. However, there was an apparent decrease in the CO2 breakthrough time between the fresh fiber sorbent and at the end of the 20th adsorption/desorption of about 20 min, corresponding to a 16% reduction in the breakthrough capacity (0.43 mmol/gfiber to 0.36 mmol/gfiber).

CO2 breakthrough experiments under humid conditions DAC usually involves co-adsorption of atmospheric moisture along with CO2. The water concentration in ambient air ranges between 2-3 mol% H2O, which is 50-75 times greater than the CO2 concentration in ambient air. As a result, the water sorption capacity per gram of sorbent material may become significantly higher than the CO2 capacity, which may impact the sorbent’s ability to adsorb CO2.39,40 An advantage of amine functionalized solid sorbents is their stability under moisture in the air,41,42 particularly compared to other adsorbents such as MOFs. The average 16 ACS Paragon Plus Environment

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relative humidity in the United States is estimated to be between 80-90% during the months of June-August.43 In the current study; we chose to work with a relative humidity of 85% for conducting humid breakthrough experiments on pre-hydrated fiber sorbents. The PEI infused CA-SiO2 fiber sorbents showed a sharp rise in water vapor uptake at this high relative humidity value compared to the individual components at 35°C, as shown in Figure S11, and so is expected to have a large effect on the CO2 capture behavior of these adsorbents. For breakthrough experiments under humid conditions, a pre-humidification step was used to ensure the module was saturated with water prior to CO2 adsorption, which prevents the water and CO2 breakthrough fronts from propagating through the bed at different velocities. High purity N2 gas was passed through a bubbler containing a saturated salt solution of KCl in DI water that was maintained at 25°C in an isothermal chamber. The water concentration was monitored downstream of the bubbler on a mass spectrometer, and the gas was bubbled for approximately 4 h or until a fixed water concentration of 2.6 ± 0.2 mol % was obtained. Then, this water-equilibrated N2 stream was admitted into the dry fiber module at 35°C at the rate of 200 sccm and the module was allowed to saturate with the wet N2 stream completely. In the meantime, the adsorption feed mixture comprised of 380 ppm CO2/397 ppm He/balance N2 was allowed to equilibrate in a separate bubbler containing KCl saturated solution until the water concentration at the outlet of the bubbler reached a similar value as noted above. Finally, upon complete pre-hydration of the fiber sorbent module with a wet N2 stream, the wet, CO2-laden feed stream was passed through the pre-hydrated fiber sorbents at 35°C at a rate of 200 sccm. Furthermore, the water concentration during the fresh and multi-cycle adsorption process under humid conditions was monitored and is shown in Figure S10b. For direct comparison, experiments performed in the absence of humidity were conducted on the same module immediately prior to and after humid experiments.



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The predominant effect of humidity is to increase the pseudo-equilibrium CO2 capacity of the fiber sorbents to 1.6 mmol/gfiber, which is approximately 2.5 times higher than the capacity under dry conditions (0.59 mmol/gfiber) (Figure 5). Additionally, it can be seen that the CO2 breakthrough time was longer in humidity, but that the breakthrough curve also broadened. This led to a breakthrough capacity of 0.64 mmol/gfiber in the presence of humidity, compared to approximately 0.36 mmol/gfiber for dry conditions. Multiple humid breakthrough cycles were conducted on fiber sorbents, wherein the fiber sorbent module was heated to 110°C under dry N2 gas at 200 sccm for a period of 2 h to regenerate the fiber sorbent module, desorbing CO2 while also removing the adsorbed water. At the end of the 2 h drying period, the water concentration in the module effluent reached less than 30 ppm. At this point, the process of pre-hydrating the fiber module was repeated, and the humid breakthrough experiment was conducted again. In Figure 5a, the comparison of ‘Humid CO2 (cycle 1)’ and ‘Humid CO2 (cycle 2)’ shows that there was a loss in equilibrium capacity after the first humid experiment. This loss was also accompanied by a slight sharpening of the CO2 breakthrough curve. After this first cycle, the pseudo-equilibrium and breakthrough capacities over five additional humid cycles was more or less constant according to the humid CO2 breakthrough behavior shown in Figures 5b and S10a, with minor fluctuations that could be due to slight changes in the water concentration during each cycle (Figure S10b). Such fluctuations are expected to be common under DAC conditions in both the lab and the field due to naturally occurring fluctuations in environmental conditions.


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Figure 5. (a) Dry and humid CO2 breakthrough curves at 35°C for a PEI-CA-SiO2 fiber sorbent module; (b) Pseudo-equilibrium and breakthrough CO2 capacity under dry and humid adsorption conditions at 35°C. Finally, a breakthrough experiment was conducted on a completely dry fiber module after six humid breakthrough cycles at 200 sccm with a resultant breakthrough capacity calculated to be 0.28 mmol/gfiber, corresponding to a 7% reduction in the breakthrough capacity obtained in the case of the dry breakthrough experiment (0.30 mmol/gfiber conducted at 200 sccm prior to the humid experiments). These results are shown by the dotted curve in Figure 5a, which indicates that the CO2 breakthrough and equilibrium capacity under dry conditions is mostly maintained after several humid breakthrough cycles. The increased CO2 capacity under humid conditions is in accordance with previous studies that suggest that the presence of water vapor caused a release of additional free amine groups to allow more carbamate ions to be produced.44 Some formation of bicarbonate species may also be possible.45,46

Vacuum-assisted desorption in TSA cycles In most academic laboratory work, temperature swing desorption occurs under a flow of the inert gas stream, which yields a dilute CO2 product stream. However, in practice, a stream of high



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purity CO2 can be obtained if desorption is carried out in a combined temperature-vacuum swing mode.24,47 Temperature-vacuum swing adsorption for CO2 capture from the air has been previously reported for an amine-based powder sorbent, but not for structured sorbent contactors where it is likely to be more applicable.3,48 In this study, a custom-built apparatus shown in schematic Figure S12 was used to recover instantaneous and highly concentrated CO2 from the desorption of PEI-CA-SiO2 fiber sorbents. As a proof of concept study, a densely packed fiber module consisting of 34 PEI infused hollow fibers with a fiber packing fraction of 0.55 was used. To examine the achievable desorbed CO2 purity, the fiber module containing 34 fibers was then dried under N2 and allowed to saturate under a dry stream of 380 ppm CO2/397 ppm He/balance N2 at 25°C with a flow rate of 500 sccm (Figure S13a). The CO2 concentration at the outlet reached a steady value at the end of 2.75 h of adsorption time, as shown in Figure 6. A vacuum was applied in the CO2 saturated module for 5 min, at which time the pressure in the module rapidly reduced from 1005 mbar (ambient) to 0.6 mbar over 5 s, owing to the low pressure drop in these fiber sorbent modules. This step removes the interstitial gas that is present in the headspace or void volume in a completely saturated fiber module; these gases would otherwise dilute the desorbing CO2 and reduce the purity of the recovered stream. Next, the inlet and outlet of the module were closed, and the module was heated under static vacuum from 22 °C to a final temperature 90°C, followed by holding at this temperature for 10 minutes. During the heating step, the pressure inside the module increased from 0.6 mbar to 6.8 mbar. The increase in pressure inside the module is due to previously-chemisorbed CO2 that is now present in the gas phase. A vacuum was also created in the mass spec capillary line and held for less than 10 minutes, during which stage there was a complete loss in ion current signal being recorded as shown in


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Figure S13b. This step eliminates any remaining gases in the capillary line from the previous step (sorption), which if present, could dilute the high purity desorbing CO2. The vacuum created in the mass spec capillary line also helped to achieve a pressure gradient and allowed the desorbed CO2 to self-sweep from the high-pressure side (i.e., inside the module) to low pressure side (i.e., inside the mass spectrometer). Finally, the valve at the module outlet was opened to the inlet of the mass spec and the concentration was monitored. A steep rise in CO2 concentration from the module outlet was observed in the mass spectrometer (Figure 6a), and the purity reached 88% within a few seconds under these ‘selfsweep’ conditions, in which the gas traveled from the module to the mass spectrometer under convective transport due to the aforementioned pressure difference between the module and the mass spectrometer. Subsequently, a pulse of N2 was applied to push the plug of high purity CO2 out of the module. This resulted in a 98% pure CO2 stream for a short span of 5 seconds before the CO2 concentration progressively dropped to less than 10 ppm in this unoptimized CO2 recovery operation (Figure 6b). At this stage, the temperature of the module was lowered to 22°C, and this marks the end of the desorption step.

Figure 6. (a) CO2 concentration profile during vacuum-assisted desorption of CO2 from a fiber 21 ACS Paragon Plus Environment


sorbent module saturated with CO2 under dry conditions; (b) Zoomed-in view of Figure 6a highlighting CO2 concentration during self-sweep and N2 sweep conditions. To truly perform DAC, the fiber module was exposed to the air in the lab for 48 hours and allowed to equilibrate with ambient air containing CO2 along with other gases and vapors. Similar desorption experiments as above were performed following this adsorption step. In this case, a vacuum of 1.1 millibar at 22°C was achieved within 5 s from a module saturated with ambient air at atmospheric conditions. The heating step under static vacuum was performed at 120°C, followed by holding at this temperature for 5 minutes. During this period, the pressure inside the module reached 10 millibar, due to desorption of previously adsorbed CO2 and H2O from the ambient lab air. A similar self-sweep step was performed by pulling vacuum in the mass spec capillary, as before. The concentration profile in this self-sweeping step was plotted and is shown in Figure 7.

c

100 90 80 70 60 50 40 30

N2 CO2

Vacuum in detector

Concentration (mol %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H2O O2

b

a

N2 sweep

Self-sweep

20 10 0 0

10

20

30

40

50

60

70

Time (min)

Figure 7. Vacuum-assisted thermal desorption of CO2 from a fiber sorbent module saturated with ambient air under humid conditions at 22°C. The outlet gas composition was also analyzed by periodically (i.e. at span a, b, and c) observing


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the ion currents as a function of atomic mass units using the secondary electron multiplier (SEM) detector, which is contained in the same mass spectrometer as shown in Figure S14. It can be seen that the various species labelled are mass fragments of compounds present in the atmosphere that are relatively more abundant as compared to trace species. The desorbed CO2 concentration reached about 65% at the end of the 20 minute self-sweep. This value is lower than that observed for the experiment performed with dry simulated air, due to the presence of water vapor. Finally, N2 is used to sweep the module and complete desorption. After an extended N2 sweep (span c), an SEM scan of the major species present indicate the abundance of mass fragments belonging to N2. It is envisioned that the achievable CO2 purity would be higher if all the water vapor is condensed before entering the MS since the balance of the gas in this was primarily water. Indeed, on a dry gas basis, the desorbed CO2 concentration reached a maximum of 94% in this experiment and an average value of 92% during the self-sweep step. The vacuum assisted desorption studies demonstrate how high concentration CO2 can be obtained from ambient air using these amine fiber sorbents, demonstrating similar performance to previous studies using amine-based powder sorbents. No attempt was made to optimize the desorption conditions and maximize purity and recovery; rather, this study serves as a proof-ofconcept for obtaining high purity CO2 from air using fiber sorbents. Additional study for the desorption process is recommended for future work.

Conclusion Poly(ethyleneimine) functionalized CA-SiO2 fiber sorbents represent a promising combination of state-of-the-art DAC sorbents with a scalable structured contactor not yet reported for DAC applications. Here, the amine-loaded monolithic fibers were successfully demonstrated in CO2



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capture under ultra-dilute conditions, under both dry and humid conditions. The performance of PEI infused CA-SiO2 fiber sorbents for CO2 capture was examined over a wide range of operating parameters such as adsorption temperature, feed flow rate, and varied humidity. Almost complete recovery of CO2 equilibrium and breakthrough capacity under dry conditions was obtained after the fiber sorbent was subjected to multiple humid breakthrough cycles. Extraction of CO2 from ambient air resulted in the ability to recover high purity CO2 in both the presence or absence of moisture by employing a vacuum- and temperature-assisted desorption step. In the case of a dry, simulated air mixture containing 380 ppm CO2, an instantaneous purity of 98% was obtained. CO2 can be extracted from the ambient air and recovered at high purity using the same desorption conditions but will also contain co-extracted H2O which in principle could be condensed out.

Acknowledgments This work was partially supported by the Center for Understanding and Control of Acid GasInduced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center, funded by U.S. Department of Energy (US DoE), Office of Science, Basic Energy Sciences (BES) under Award DE-SC0012577. Partial support also came from the U.S. Department of Energy through grant DE-FE0026433 for financial support. Any options, findings, conclusions or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the DOE. Partial support from the Love Family Professorship at Georgia Tech is also acknowledged. The authors would like to thank Ya Dong Chiang and Arantza Romero for conducting the H2O vapor sorption isotherm measurements and assistance with the vacuumassisted desorption experiments respectively.


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Supporting Information The layout of the fiber spinning apparatus. Combustion TGA analysis for determining the SiO2 loading in the monolithic fiber sorbents. CA-SiO2 fibers with various PEI loading (gPEI/gSiO2) obtained by soaking fibers in solutions of varying PEI concentration. Combustion TGA profile of CA-SiO2 fibers with various PEI contents and profile of a single PEI-SiO2 powdered specimen. N2 adsorption-desorption isotherms at 77K of PEI loaded fiber sorbents and PEI loaded powdered sorbent. Gravimetric CO2 uptake curves for PEI/SiO2 powdered sorbents and PEI-CA-SiO2 fiber sorbents. Cyclic TGA profiles and gravimetric uptake of CO2 on PEI-CA-SiO2 fiber sorbents. The difference in pressure drop between a densely packed fiber module and an empty module. Cyclic CO2 breakthrough curves of PEI-CA-SiO2 fiber sorbents. Dry CO2 breakthrough curves and humid CO2 breakthrough curve of PEI-CA-SiO2 fiber sorbents. Water vapor sorption isotherms for various materials at 35°C. Schematic of the apparatus vacuum assisted desorption of CO2. Mass spec ion current as a function of time during the desorption experiment under dry conditions. Spin dope compositions and spinning parameters used in the current study. Physical properties for sorbent materials.



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TOC

Synopsis Amine loaded fiber sorbents as low-cost, scalable, and stable materials combined with a process level advancement for efficient CO2 capture from ultra-dilute sources such as ambient air


Direct CO2 Capture from Air using Poly(ethylenimine)-Loaded

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