Fast and Reversible Direct CO2 Capture from Air onto All-Polymer

Article pubs.acs.org/est

Fast and Reversible Direct CO2 Capture from Air onto All-Polymer Nanofibrillated CellulosePolyethylenimine Foams Houssine Sehaqui,† María Elena Gálvez,‡ Viola Becatinni,‡ Yi cheng Ng,‡ Aldo Steinfeld,*,‡,§ Tanja Zimmermann,*,† and Philippe Tingaut† Empa, Swiss Federal Laboratories for Materials Science and Technology, Applied Wood Materials Laboratory. Ü berlandstrasse 129, CH-8600, Dübendorf, Switzerland ‡ Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zurich, Switzerland § Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen, Switzerland †

S Supporting Information *

ABSTRACT: Fully polymeric and biobased CO2 sorbents composed of oxidized nanofibrillated cellulose (NFC) and a high molar mass polyethylenimine (PEI) have been prepared via a freeze-drying process. This resulted in NFC/PEI foams displaying a sheet structure with porosity above 97% and specific surface area in the range 2.7−8.3 m2.g−1. Systematic studies on the impact of both PEI content and relative humidity on the CO2 capture capacity of the amine functionalized sorbents have been conducted under atmospheric conditions (moist air with ∼400 ppm of CO2). At 80% RH and an optimum PEI content of 44 wt %, a CO2 capacity of 2.22 mmol·g−1, a stability over five cycles, and an exceptionally low adsorption half time of 10.6 min were achieved. In the 20−80% RH range studied, the increase in relative humidity increased CO2 capacity of NFC/PEI foams at the expense of a high H2O uptake in the range 3.8−28 mmol·g−1.

1. INTRODUCTION Direct air capture (DAC), i.e. carbon dioxide capture from atmospheric air, has been recently proposed, together with CO2 sequestration, as a perspective technology contributing to the reduction of CO2 concentration in the atmosphere.1 DAC can be furthermore used to purify air from carbon dioxide-containing streams in industrial processes (e.g., cryogenic separation of N2 and O2,2 alkaline fuel cells3). Moreover, the captured CO2 can be converted into a renewable fuel by means of solar thermochemical cycles, thus closing the carbon material cycle.4,5 Among the various strategies envisaged so far for DAC, those offering the possibility to capture CO2 directly from the atmosphere (in moist environment) are of outstanding importance for the future development of this technology, since any pretreatment would require the energy-intensive processing of 2600 mol of air per mole of CO2.6−11 The use of calcium oxide/hydroxide or basic ion exchange resins requires high energy or yields a relatively low-purity CO2 stream.12,13 Sodium hydroxide solutions do not require, in principle, any airconditioning, but need a regeneration at above 800 °C.14 The use of solid sorbents that can be regenerated at lower temperatures is therefore preferred.15−18 In this sense, amine-functionalized solid sorbents have been proposed for DAC. Amines react selectively with CO2 via a 2-step exothermic and reversible mechanism, during which zwitterion species are formed and subsequently converted to carbamate moieties via deprotonation by a free base.19,20 High purity CO2 can then be released in an endothermic step by supplying low grade heat (∼90 °C) by © XXXX American Chemical Society

means of temperature-vacuum-swing (TVS) or steam stripping processes.15,21 CO2 adsorbents include metal−organic frameworks22 and porous polymer network,23 while most of the aminefunctionalized sorbents described in the literature make use of inorganic supports such as silica or carbon-based substrates.21,24−33 Cellulose is the most abundant polymer on earth, is environmentally benign and comes from a renewable resource.34 Nanofibrillated cellulose (NFC) can be disintegrated from cellulose pulp through a water-based disintegration process,35−38 which yields fibrils with a diameter in the range 5−100 nm. Such fibrils provide excellent forming ability39−41 and large surface area for an effective functionalization.42−44 Furthermore, nanofibers with uniform diameters from 3 to 5 nm can be obtained through a 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) mediated oxidation pretreatment of the cellulose pulp before disintegration, a selective treatment which converts the primary alcohol of the glucose ring to negatively charged carboxylate entity.45 Since high porosity foams from NFC can be easily obtained through a freeze-drying process,46,47 porous materials elaborated from TEMPO-oxidized NFC represent promising substrates for the processing of all polymer-based adsorbents for DAC. Received: September 9, 2014 Revised: January 28, 2015 Accepted: January 28, 2015

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DOI: 10.1021/es504396v Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

frozen in liquid nitrogen, then freeze-dried under vacuum at ambient temperature in a Lyovac freeze-dryer (SRK system technik GMBH) yielding porous NFC/PEI foams. Physico-Chemical Characterization. Elemental analyses allowed the determination of the C, H, and N composition of the NFC/PEI sorbents (LECO CHN-900). Textural properties of the sorbents were evaluated by means of N2 adsorption at −196 °C (Coulter SA3100). Infrared spectra were recorded using a FTS 6.000 spectrometer. All spectra were recorded between 4.000 and 600 cm−1 with a resolution of 4 cm−1 and 32 scans and normalized with respect to the peak located at 897 cm−1 corresponding to the C−O−C stretching vibration of the cellulose β-(1−4)-glucosidic linkage.56 The microstructure of the PEI/NFC composites was studied by means of scanning electron microscopy (SEM) in a FEI Nova NanoSEM 230 microscope. Prior to analysis, samples were placed on a specimen holder, which was directly sputter-coated under Ar at 0.05 mbar, with a platinum layer of about 7.5 nm (BAL-TEC MED 020 Modular High Vacuum Coating Systems). Thermal degradation of the PEI/NFC sorbents was studied by means of thermogravimetric analysis (TGA). Experiments were performed in a Netzsch TG 209 F1 instrument under nitrogen flow (15 mL·min−1) from 30 to 600 °C at a rate of 20 K.min−1 and by maintaining in between the temperature at 100 °C for 60 min. From the TGA derivative curve, a second peak at higher temperature corresponds to the PEI degradation. The density of the PEI/NFC sorbents (ρ) was calculated as the ratio between the measured weight and volume of the freezedried materials. The porosity of these materials was calculated from eq 1, where W stands for the weight fraction of each component, considering 1500 and 1000 kg·m−3 as the densities for cellulose and PEI,57,58 respectively. ρ Porosity = 1 − (WNFC/ρNFC + WPEI/ρPEI )−1 (1)

In previous studies, adsorbents based on NFC functionalized with aminosilanes have been prepared and tested for DAC.48−50 The resulting materials displayed good CO2 capacity,48 regenerability48 and excellent stability over 100 adsorption/ desorption cycles.49 Polyethylenimine (PEI) seems to be a promising alternative to aminosilanes for the functionalization of different kind of substrates in the preparation of such aminebased sorbents for CO2 capture.21,24−27,31 An all-polymer sorbent based on the association of PEI and a NFC template seems to represent an interesting alternative to hybrid inorganics/PEI or carbon/PEI materials. Such combination of both materials has been previously successfully assayed via layerby-layer (LbL) deposition technique alternating monolayers of PEI and carboxymethylated NFC at basic pHs, in order to both, limit the compacting of PEI chains, and to promote the interactions between the free amine groups of PEI and the hydroxyls or carboxylate groups of NFC.51−54 Accordingly to these findings, we introduce in the present work a novel freezedried sorbent for CO2 DAC, based on the strong association of PEI with TEMPO-oxidized NFC. The influence of important parameters of sorbent preparation, such as PEI content, along with DAC operational parameters such as air moisture content, on both CO2 adsorption capacity and sorbent stability, are as well considered. Furthermore, the possibility to regenerate the sorbent is investigated. The performance registered for these novel sorbents over five consecutive adsorption/desorption cycles places them among the best materials reported so far in the literature for DAC application.

2. EXPERIMENTAL SECTION Beech pulp fibers (Arbocel) were kindly provided by J. Rettenmaier & Söhne, Germany. 2,2,6,6-Tetramethyl-1-piperidinyloxy free radical (TEMPO), Sodium hypochlorite (NaClO) solution (reagent grade, 10−15% chlorine) were purchased from VWR international. Branched polyethylenimine (PEI) in an aqueous solution at 50 wt % and having a molecular weight Mr = 600.000−1.000.000 g·mol−1 was purchased from Sigma-Aldrich. Deionized water was used in all experiments. Synthesis of the Oxidized Nanofibrillated Cellulose (NFC). Oxidized nanofibrillated cellulose (NFC) was prepared from beech pulp fibers according to a previously reported method by Saito et al.,38 where sodium bromide and TEMPO were added to mechanically beaten pulp (1 and 0.1 mmol per gram of pulp, respectively). Concentration of water was adjusted to 2 wt %. The oxidation reaction was subsequently started by the dropwise addition of sodium hypochlorite to the fiber suspension while maintaining the pH of the reaction at ca. 10 by NaOH addition. After all NaClO (10 mmol.g−1) was consumed, the oxidized pulp was washed, dispersed in water at a concentration of 0.65 wt % and disintegrated by one pass in a Microfluidizer M110Y (Microfluidics Ind.) equipped with 200 and 75 μm chambers to achieve an aqueous suspension of oxidized NFC. Carboxylate content of NFC is 1.95 mmol.g−1 as determined by electric conductivity titration method.55 Preparation of the NFC/PEI Sorbents. NFC/PEI sorbents of different compositions were prepared by mixing 0.65 g (dry weight) of NFC (pH 6.8) with PEI, at weight ratios NFC:PEI of 4:1, 2:1, 1:1, 1:2 or 1:4, maintaining the total volume at 250 mL by water addition. The pH of all suspensions was 11.2. The NFC:PEI mixture was stirred overnight and subsequently centrifuged at 4500 g during 15 min, in order to remove the supernatant comprising the nonadsorbed PEI. At this stage, composite hydrogels of PEI/NFC were formed. These were

In order to evaluate the moisture adsorption of the PEI/NFC sorbents, sorption measurements were performed in a VTI-SA dynamic vapor sorption analyzer (DVS, TA-Instruments). A microbalance and a moisture generation system were placed in an incubator which maintained a constant temperature. Ca. 15 mg of sample were placed in the sample cup and dried at 105 °C under inert atmosphere for 60 min. Relative humidity was set then to the desired value while the temperature was fixed at 25 °C. Weight was therefore continuously registered with a resolution of 0.1 μg and compared to a reference empty cup. CO2 Adsorption/Desorption: DAC Experiments. DAC adsorption/desorption experiments have been performed in the packed bed reactor setup. A scheme of the reactor and details of the DAC process are given in Supporting Information (SI) S1. Approximately 40−230 mg of sorbent were placed in a 27 mm inner diameter, 23 mm height stainless steel cylinder. The cylinder was placed in a closed water bath tank for temperature control. CO2 concentration at the reactor inlet and outlet was analyzed using an IR sensor, Siemens ULTRAMAT. Each DAC sorption cycle consisted of an isothermal adsorption step at 25 °C, followed by a desorption step in which temperature was raised up to 85 °C at 5 °C.min−1. During the adsorption step, ambient air was flown at 1L.min−1, until saturation of the sorbent, that is, reaching equilibrium adsorption capacity. Relative humidity was fixed at 20, 40, 60 or 80% during adsorption. Desorption of adsorbed CO2 was subsequently performed, flowing 0.5 L·min−1 of N2 through the sorbent bed. Although the CO2 adsorption and desorption values found in the B


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Table 1. Nitrogen and PEI Contents, BET, Density, Porosity, Water Uptake, Adsorption at Half Time and Amine Efficiency of Reference NFC (PEI-0) and NFC/PEI sorbents sample

N content (wt %)

PEI content (wt %)

BET (m2/g)

density (kg/m3)

porosity (%)

amine efficiency (E)

H2O uptake at 80% RH (gH2O/g)

adsorption at half time t1/2 (min)

PEI-0 PEI-19 PEI-31 PEI-44 PEI-52 PEI-62

6.3 10.1 14.4 17.1 20.2

0 19.3 31.0 44.2 52.5 62.0

18.7 7.9 5.9 8.3 7.1 2.7

7.0 9.5 12.9 12.3 21.0 30.8

99.5 99.3 99.0 99.0 98.2 97.3

0.05 0.17 0.22 0.18 0.12

0.28 0.32 0.39 0.54 0.60 0.71

5.0 10.5 10.6 33.2 47.3

symmetric bending at 1410 cm−1, C−O−C skeletal vibrations at 1055 cm−1. The additional band at 1600 cm−1 has been attributed to the carbonyl stretching vibration of the carboxylate groups,38 which were introduced into the material upon the selective oxidation of the primary hydroxyl groups in cellulose (C6 position on the glucose ring), as widely documented in the literature.60−64 The FTIR spectra of the PEI-containing sorbents evidence the characteristic vibrations of the nitrogen-based polymer at 3280 cm−1 (N−H stretching), 2825 cm−1 (C−H stretching), a shoulder at 1660 cm−1 (primary N−H bending), 1460 cm−1 (C−H bending), and 1305 cm−1 (C−N stretching). The intensity of these bands gradually increases from PEI-19 to PEI-62, as a consequence of the increasing PEI content in the sorbents. The presence of amide linkages (carbonyl stretching vibration around 1630−1690 cm−1) cannot be clearly observed in the FTIR spectra, pointing to a predominant noncovalent interaction between PEI and the NFC support in this series of sorbents. The BET specific surface area of the different sorbents is presented in Table 1. The PEI/NFC sorbents display a SSA in the range of 2.7−8.3 m2.g−1, which is consistent with other freeze-dried NFC-based materials reported in the literature43,48,65,66 and comparable to previous NFC-aminopropylmethyldimethoxysilane CO2 sorbent (AEAPDMS-NFC).48 The lowest surface area (2.7 m2.g−1) was measured for the sorbent containing the maximum amount of PEI (PEI-62), due to PEI agglomeration resulting in pore blockage at such high loading. SEM micrographs acquired for the different sorbents are depicted in Figure 2. Both, the raw NFC and the PEI/NFC sorbents are arranged in thin sheets, as a consequence of the freeze-drying process.43,48,65−67 The sheet structure was created during the freezing of the suspension when ice crystals formed and pushed the fibrils into the interstitial regions between the ice crystals. The sublimation step removed ice crystals template and afforded a porous structure composed of pores surrounded by thin sheets.46,67,68 Porosity could be observed in all the materials, with pore diameters in the micrometer range. The pore diameter seemed to be smaller for the highest PEI loadings (PEI-52 and PEI-62). Higher resolution SEM images of the PEI-44 and PEI62 samples are given in SI S4 and show the relatively dense aspect of the PEI-62 foam cell wall. Nevertheless, all the sorbents remained highly porous (≥97.3%) in spite of the PEI content, which is advantageous for a good permeability of air through the material. The TGA curves presented in Figure 3 evidence that the cellulosic backbone of NFC starts to be degraded at temperatures above 200 °C. As a result of increasing PEI content, the higher thermal stability of this polymer69 in comparison to NFC causes a progressive shift of the TGA degradation curves of the NFC/ PEI sorbents toward higher temperatures (Figure 3.a), and the main degradation temperature of the PEI in the sorbents

present study are fairly comparable (see SI S7), higher precision of CO2 desorption capacity is expected since integration of the signal of the IR sensor is performed over a shorter time period, as has been stated by Gebald.59 The CO2 capacity will therefore be subsequently taken as the desorbed amount of CO2.

3. RESULTS AND DISCUSSION Textural, Structural and Chemical Features of the NFC Support and the PEI/NFC Sorbents. SEM micrographs of the oxidized NFC used in this study are shown in SI S2. A typical NFC structure with interconnected nanofibers has been observed, as well as high aspect ratio nanofibers. The high external surface area of the nanofibrils network is of interest to maximize the area of interaction between the PEI and NFC that would favor an adequate distribution of PEI in the sorbents. In this study, we envisaged the adsorption of PEI on the nanofibers at a pH above 11 since basic conditions have been reported to limit the self-compacting of the PEI chains and expose free amine groups which are able to interact with NFC.51 Table 1 contains the N% wt. measured for the different PEI/NFC sorbents prepared in this work. It ranges from 6.3 to 20.2%, indicating that corresponding PEI weight contents from 19 to 62% wt. can be achieved by varying the initial concentration of PEI in the hydrogel before freeze-drying (details on PEI% wt. calculations are given in SI S3). Sorbents will be identified from now on in the text as a function of this PEI content, that is, PEI-x, where x denotes the weight percent of PEI. The FTIR spectrum of the raw NFC (PEI-0) is shown in Figure 1 and reveals the typical bands of cellulose, that is, O−H stretching at 3340 cm−1, C−H stretching at 2900 cm−1, CH2

Figure 1. FTIR spectra of reference NFC (PEI-0) and the NFC/PEI sorbents. C


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Figure 2. SEM micrographs of reference NFC (PEI-0) and the PEI/NFC sorbents with different PEI content. Scale bare is 100 μm.

Figure 3. a. TGA thermograms of reference NFC (PEI-0) and the PEI/NFC sorbents b. Evolution of the PEI degradation temperature as a function of the PEI content in the sorbents.

progressively increases with increasing PEI content from 379 °C for PEI-19 to 409 °C for PEI-62 (Figure 3.b). It is probable that the configuration of PEI in different foam samples was not identical which might have affected its degradation temperature. In the NFC/PEI foams, the extensive agglomeration of the PEI chains for the highest PEI loadings investigated might have increased the degradation temperature of the polymer. CO2 Capacity vs PEI Content. The CO2 equilibrium adsorption capacities of the PEI/NFC sorbents measured during the DAC cycles at 80% RH are presented in Figure 4, and their moisture uptake at the same RH is given in Table 1 and SI S5. The CO2 profiles during the adsorption and desorption are added in SI S6. As expected, the raw NFC sorbent does not adsorb any CO2, due to the absence of amine functionalities on its surface. Upon PEI addition to NFC, the CO2 capacity increases linearly for PEI contents between 19 and 44 wt % and reaches a maximum of 2.2 mmol·g−1 for the PEI-44 sample. At this PEI concentration range, the polymer is expected to be well dispersed and stabilized onto the surface of NFC, allowing an easier access to amine groups for the incoming air. Further increase in the PEI content leads to a decrease of the CO2 adsorption capacity, probably due to a reduced accessibility of the amine moieties to the CO 2 molecules upon extensive agglomeration of the components, as evidenced in the SEM

Figure 4. Evolution of the CO2 equilibrium adsorption capacity during DAC at 80% RH (expressed in mmol of CO2 per gram of dried NFC/ PEI sorbent) with the PEI content in the sorbents.

images shown in Figure 2. The existence of an optimum PEI content has been previously reported for Montmorillonite claysupported PEI sorbents.33 Gebald et al. furthermore reported that the accessibility of the amine functionalities in the material is even more important than the total nitrogen content.59 In general, all these works agree that an excess of amine-based polymer beyond the substrate capacity can be detrimental to the D


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desorption CO2 capacities for the five adsorption/desorption cycles are given in the SI S7. The adsorption capacities of the sorbents with a PEI content up to 44% are rather constant. PEI31 and PEI-44 show a good stability over the five cycles with a decrease in the performance of respectively 4% and 3% after five consecutive cycles. A significant reduction in the CO2 capture capacity can be observed for the sorbents with the highest PEI contents, PEI-52 and PEI-62, corresponding to a decrease in their adsorption capacity of respectively 16 and 27% over the five consecutive DAC cycles. Higher stability of the NFC/PEI sorbents is attained for PEI contents below 44%. Nevertheless, long-term DAC cycle studies are needed to assess the stability of our NFC/PEI sorbents in view of their real-scale application and will be pursued in future work. Kinetics of CO2 Uptake. Choi et al. used the adsorption half time (t1/2), that is, the time needed to achieve 50% of the total adsorption capacity, to evaluate kinetic performance of CO2 sorbents.28,72 The adsorption half time has been calculated for the different NFC/PEI sorbents and plotted in Figure 6.a as a function of the PEI content. Remarkably low t1/2 in the range 5.4−10.6 min have been obtained for the sorbents with PEI contents from 19 to 44% wt., which can be associated with the high porosity and large pores of these materials allowing good CO2 permeability and optimum interaction with the amine functions. At higher PEI contents, the kinetic of CO2 uptake dramatically slow down, as indicated by the longer t1/2 calculated for PEI-52 and for PEI-62 of 33 and 47 min, respectively. This fact can be attributed to the significant densification observed for these materials at high PEI loadings, which may in turn lead to limited CO2 permeability and reduced amine accessibility. Interestingly, the t1/2 values of our NFC/PEI are systematically lower than those reported up to now in the literature, for adsorption experiments involving an inlet CO2 concentration of 400 ppm. Figure 6.b shows t1/2 versus CO2 capacity of the present foams and different sorbents cited in the literature.28 PEI44 is located in the bottom right of the graph (see arrow in Figure 6.b) as it combines low t1/2 and high CO2 capacity. PEI-44 has a t1/2 of 10.6 min corresponding to an average CO2 adsorption rate of 105 μmol.g−1.min−1 during the adsorption half time, and this is more than an order of magnitude faster kinetic than that of sorbents cited in the literature. For instance, AEAPDMS-NFC displayed an adsorption half time of 92 min corresponding to an average CO2 adsorption rate of 7.6 μmol.g−1.min−1.48 The adsorption half time and corresponding average CO2 adsorption rate was 309 min and 3.8 μmol.g−1.min−1 for PEI/silica, 196 min

CO2 adsorption performance. In any case, among the different data presented in the existing literature, the CO2 adsorption capacity measured for PEI-44 at 80% RH can be considered as remarkably high (2.2 mmol·g−1), despite its relatively low surface area in comparison to other materials based on different supports and using other amine precursors.22,25,27,30,31,48 The amine efficiency (E), defined as the molar ratio of captured CO2 over the amines loading in the foam, has been calculated in our samples and compared with the E values from the literature. E varied between 0.05 and 0.22 in the present foams with a maximum noted for PEI-44 sample and compares well with sorbents used for atmospheric CO2 capture.25,27,31,70 Other sorbents have been found to display higher E, including AEAPDMS-NFC foam (0.26 in dry conditions, 0.34 at 21% RH and 0.51 at 91% RH),49 as well as the mesocellular silica foam (0.3 in the dry state).71 Taking into consideration that 64% of the amine functions of PEI can be used for CO2 capture (4 out of 11 amine moieties in PEI are tertiary amines which display negligible CO2 uptake under DAC conditions71), the efficiency of PEI-44 combining only primary and secondary amines would be 0.35. The efficiency of the PEI/NFC sorbent presented herein is then considerably high, since the theoretical maximum efficiency for CO2 chemisorption is 0.5. Sorbent Stability. DAC is conceived as a cyclic process in which the sorbent is continuously used and regenerated. The stability of the different PEI/NFC sorbents has been investigated over five consecutive DAC cycles (Figure 5). Adsorption and

Figure 5. Evolution of the CO2 adsorption capacity over five consecutive DAC cycles for the NFC/PEI sorbents. The cycle number is written on top of each bar.

Figure 6. a. Adsorption half time (t1/2) for NFC/PEI sorbents as a function of their PEI content. b. Comparison of adsorption capacity and adsorption half time of NFC/PEI sorbents (green circles) with NFC/AEAPDMS (red triangle),48 and different PEI/silica sorbents (black squares).28 E


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Environmental Science & Technology and 5.8 μmol.g−1.min−1 for silane stabilized PEI/silica, 210 min and 5.2 μmol/g/min for titanate stabilized PEI/silica, and 167 min and 5.2 μmol.g−1.min−1 for HAS6, respectively.72 The faster kinetic of CO2 adsorption of the present NFC/PEI foams as compared to kinetics of other sorbents cited is probably due to higher porosity and larger pores of the present sorbents. Indeed, PEI-44 has a porosity of 99%, which is higher than the porosity of AEAPDMS-NFC (∼96%) and much higher than porosity of silica-based sorbents such as SBA-15 and MCM-41. These latters have pores in the nanometer range. However, it should be noted that the particle size, the residence time, linear velocity of the incoming gas and length over diameter of the packed bed investigated here are different from those used by Choi et al.28 and Gebald et al., and this may in turn affect the kinetic of CO2 adsorption. The present samples with exceptionally high kinetic of CO2 uptake would be interesting in real applications as they would allow processing of larger amount of CO2 per time unit. Furthermore, the t1/2 was stable over 5 DAC cycles for NFC foams containing up to 44 wt % PEI loadings (see SI S8), which was in agreement with their stable cyclic CO2 adsorption capacity previously noted. PEI-52 and PEI-62 sorbents exhibited the lowest t1/2 in the first cycle (33 and 47 min, respectively), which increased in the fifth cycle to 47 and 66 min, respectively. CO2 Uptake vs Relative Humidity. The influence of the RH on the CO2 adsorption capacity has been evaluated for the NFC/PEI foams which displayed the lowest t1/2 and the most stable DAC cycle capacities, namely PEI-31 and PEI-44. Results are presented in Figure 7. The CO2 adsorption capacity of these

10.6−15.3 min and average CO2 adsorption rate of 55−105 μmol.g−1.min−1. The higher RH however is accompanied by a substantial H2O uptake of 7−21 mmolH2O.g−1 for PEI-31 and 9− 28 mmolH2O.g−1 for PEI-44 in the 40−80% RH range (SI S10). Water uptake by NFC-PEI multilayers was reported to be 0.4− 0.73 gH2O.g−1 in the wet state.51,52 For comparison, NFC-APDES sorbent displayed a far lower moisture uptake of 5.45 mmolH2O· g−1 at 71% RH probably due to its lower amine content and its hydrophobic character brought by numerous methyl groups. When the moisture uptake is reported to the amine content, PEI44 sample uptake is 2.7 gH2O/gN (gram of water per gram of nitrogen) at 70% RH vs 1.7 gH2O/gN for NFC-APDES at 71% RH. There is a need to use a low-cost waste heat for NFC/PEI sorbents regeneration for their good exploitation in real applications. Ideally, a good sorbent must be cheap, environmentally benign, regenerable over many cycles, must display fast kinetics in terms of CO2 adsorption and desorption, and it should operate under moist conditions and uptake a moderate amount of water. Many of these aspects are realized in the present NFC/ PEI sorbents, and optimum operating conditions should be determined by a multicriteria analysis taking into account all the mentioned aspects. In the end, an optimal balance between CO2 adsorption capacity and its ability to adsorb water needs to be found, since this fact may strongly influence the DAC operation, i.e. in terms of regeneration energy requirements.

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ASSOCIATED CONTENT

S Supporting Information *

Scheme of the packed bed reactor used for DAC, Scanning electron micrograph of the oxidized NFC, DAC profiles, CO2 adsorption and desorption values, moisture uptake of the sorbents for different PEI contents and different relative humidities, and adsorption half time for different regeneration cycles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +41 44 632 79 29; fax: +41 44 632 10 65; e-mail: [email protected] (A.S.). *Phone: +41 58 765 41 15; fax: +41 58 765 11 22; e-mail: [email protected] (T.Z.). Notes

Figure 7. CO2 capacity of PEI-31 and PEI-44 sorbents versus relative humidity.

The authors declare no competing financial interest.

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sorbents was relatively low at 20% RH and progressively increased with RH. The same trend has been reported for NFCaminopropylmethyldiethoxysilane (NFC-APDES) sorbent for which the capacity increased from 1.42 mmol/g in the dry state to 2.13 mmol.g−1 at 91% RH.50 Higher CO2 uptake under humid conditions is due to the fact that only one amino group is necessary to capture a molecule of CO2 while two amino groups are necessary under dry conditions (see details in SI S9). The presence of water is also advantageous as it stabilizes the PEIbased sorbent by inhibiting the formation of urea species.73 However, this trend is not always observed as some sorbents perform better under dry conditions, and this is unfavorable for DAC where RH between 50% and 90% is common. For instance, performance of fumed silica with 50% PEI loading is 62 mg.g−1 at 67% RH versus 75 mg.g−1 in the dry state.25 Over the prevailing atmospheric RH range of 40−80%, the PEI-44 performs well with a capacity of 1.5−2.2 mmol.g−1, an adsorption half time of

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