Adsorption Microcalorimetry of CO2 in Confined Aminopolymers

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Adsorption Microcalorimetry of CO2 in Confined Aminopolymers Matthew E. Potter, Simon H. Pang, and Christopher W Jones Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03793 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016

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Adsorption Microcalorimetry of CO2 in Confined Aminopolymers Matthew E. Potter,a Simon H. Pang,a Christopher W. Jones*a a) School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Dr., Atlanta, GA, 30332, USA

ABSTRACT

Aminopolymers confined within mesoporous supports have shown promise as materials for direct capture of CO2 from ambient air. In spite of this, relatively little is known about the energetics of CO2 binding in these materials, and the limited calorimetric studies published to date have focused on materials made using molecular aminosilanes, rather than amine polymers. In this work poly(ethyleneimine) (PEI) is impregnated within mesoporous SBA-15, and the heats of CO2 adsorption at 30 oC are investigated using a Tian-Calvet calorimeter with emphasis on the role of PEI loading, and CO2 pressure in the compositional region relevant to direct capture of CO2 from ambient air. In parallel, CO2 uptakes of these materials are measured using multiple complimentary approaches, including both volumetric and gravimetric methods, and distinct changes in uptake as a function of CO2 pressure and amine loading are observed. The CO2 sorption behavior is directly linked to textural data describing the porosity and PEI distribution in the materials.

ACS Paragon Plus Environment

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INTRODUCTION Currently the combustion of fossil fuels is the leading contributor to the global energy supply, resulting in increased levels of CO2 in the atmosphere. After rising from 320 ppm to 405 ppm over the course of the last 60 years, the atmospheric concentration of CO2 continues to increase more than 2 ppm a year.1,2 The continued increasing rate of combustion of fossil fuels cannot be considered sustainable, given the correlation between the atmospheric CO2 level and undesirable climate change.1,2 As the use of alternative fuel sources (biomass, renewable hydrogen etc.) is still in the earliest stages of development, the most effective method to lower the amounts of CO2 released to the atmosphere is to capture and sequester it. The benchmark industrial process utilizes liquid amines (e.g. monoethanolamine) to capture CO2 from large flue gas sources, which typically have concentrations ranging from 5-15 % CO2. The aqueous solution used, as well as the volatile and corrosive nature of the amines, makes this process costly.3 A range of alternative technologies for CO2 capture have recently garnered attention, including adsorption and membrane separations processes utilizing zeolites,4,5 MOFs,6,7 activated carbons,8 composite fibers,9 and supported amine materials.10-14 The latter group of materials is particularly promising for post-combustion capture and direct air capture (DAC),15-17 combining strong, yet reversible, amine-CO2 interactions with a robust solid support, making the materials more practical for a range of applications. Supported amine materials for CO2 separations have been organized into three classes (however very recently a 4th class has been proposed).18,19 Class 1 materials are based on amine containing small molecules, or more commonly aminopolymer species, that are confined, but not chemically bound, within a porous support.20,21 In contrast, class 2 sorbents typically utilize organosilyl species (such as 3-aminopropyltrimethoxy silane) covalently grafted onto the support.22,23 Class 3 materials have attributes of both class 1 and class


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2, containing aminopolymer species, prepared via in situ polymerization, covalently bound to the surface. The most common class 3 materials have been referred to as hyperbranced aminosilicas.24 Class 1 materials have generated significant interest in the research community due to their ease of preparation and high equilibrium sorption capacities, particularly those based on poly(ethyleneimine) (PEI) (Figure S1). Such systems are very well suited for carbon capture, as their polymeric nature offers high amine loadings, translating into multiple sites for CO2 adsorption.25-27 Further, the high density of amines enhances the interactions with CO2, resulting in strong binding energies, which are highly desirable for low concentration carbon capture technologies such as DAC.28,29 Commercially available low-molecular weight PEI has a carbon:nitrogen ratio of 2:1 and contains a distribution of amine sites - typically 44% primary, 33% secondary and 22% tertiary amines (Figure S1). The range of amine sites increases the number of possible intramolecular interactions that are typically used to capture CO2 under dry conditions (see below). As a consequence, fundamental characterization of such systems is challenging, with the complexity of the system being further increased due to varied potential support-aminopolymer interactions.30-34 These interactions can have a profound impact on the efficacy of the sorbents. Bulk, unsupported PEI is known to form a ‘plug-like’ morphology; with no support on which to disperse the polymer, polymer-polymer interactions are dominant.32,33 This limits the efficiency of the system, as amines in the core of the plug can be difficult to access, and may therefore be unable to interact with CO2.32,33 Confining the PEI within a porous support increases the efficiency of the system. Recent molecular dynamics and small-angle neutron scattering studies show that PEI initially forms a monolayer coating within the pores of a mesoporous silica SBA15 support, due to the preferred interactions between the primary amines and surface silanol


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species.33 As the loading of PEI increases further, fewer amine-silanol interactions are possible, forcing the remaining PEI to occupy a bulk, plug-like state within the mesopores. These findings correlate well with previous literature findings on the effect of amine loading on amine efficiency for PEI/SBA-15 systems. Initially the amine efficiency increases with amine content, as more free amines are added to the system; however the amine efficiency will reach a maximum, typically in the region of 30-40 wt% PEI, as the pores eventually become occluded and CO2 diffusion is hindered.25-27,35 A delicate balance exists in PEI-based systems between maximizing the amount of amine sites and maintaining pore accessibility. At high amine loadings, a larger number of sorption sites are present; however, the CO2 uptake may become limited by diffusion due to pore occlusion. Thus, identifying the ideal amine loading is a crucial step for a given support, and the correlation between this ideal loading and the structure of the support must be understood for the effective design of CO2 sorbents. Many spectroscopic techniques have been used to study CO2-amine interactions, with in FT-IR and NMR being the most prominent.13,36-41 These techniques are well suited to class 2 materials, as the typically (more) uniform nature of the amines makes it possible to discern and assign the different vibrational bands and chemical shifts to specific species. However, the more complex polymeric systems make this a more challenging prospect, as many distinct but chemically similar species may exist. For example, under dry conditions, alkyl ammonium carbamates (which require two amines to bind CO2) and carbamic acid species are most commonly formed on supported amine materials.36-41 In class 2 materials containing a single type of amine, the nature of the adsorbed species can be assigned from vibrational and NMR spectra with some degree of certainty. In contrast, for class 1 materials containing PEI, an array of amine-CO2-amine pairs can be created, making definitive and precise characterization of the


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nature of the adsorbed CO2 species more challenging. Many researchers have attempted to overcome this difficulty by observing the influence of CO2 binding onto small oligomeric systems, such as tetraethylenepentamine (TEPA). Such systems have given significant insight into the behavior of class 1 materials, particularly from FT-IR, exploring the influence of deactivation and amine density.42-44 While these spectroscopic experiments are able to identify the nature of the active site they are not well-suited to measuring the energetics of adsorption, as such, additional, complimentary techniques need to be explored. Calorimetric measurements are a well-established method to experimentally measure isosteric heats of adsorption.45,46 However, in the CO2 adsorption literature, most authors have computed isosteric heats of adsorption from measured adsorption isotherms at multiple temperatures by fitting CO2 adsorption isotherms using Toth, Langmuir or related models, and then applying the Clausius-Clapeyron equation.10,47,48 However, this approach involves many assumptions and/or the use of empirical parameters, and recent work has suggested that heats of adsorption estimated in this manner for aminosilica materials may not be accurate.49,50 Previously, we demonstrated that measurement of adsorption isotherms via the pressure decay method in a customized TianCalvet calorimeter can be used to great effect to measure the heats evolved from CO2 adsorption on class 2 amine species,49,50 where it was found that the nature of the amine and the amine density have a significant effect on the heat of adsorption.50 Furthermore, it was shown that the measured heats of adsorption deviated substantially from what was estimated from fitting adsorption isotherms and applying the Clausius-Clapeyron equation.10,50 Studies on bare silica supports have placed the CO2 heat of adsorption in the range of 20 – 40 kJ/mol.49-51 Introducing primary amines grafted on the surface (3-aminopropyl silane species) greatly increases the adsorption energy to between 130 to 65 kJ/mol, depending on the density of amine sites


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available.50-54 Using similar techniques it has been shown that when the amine density is sufficiently high (> 1.2 mmol N/g) amine clustering occurs, allowing multiple amines to interact with one CO2 molecule under dry conditions, forming a strong alkylammonium carbamate species (~ 90 kJ/mol).50,55 Below this threshold, the amines become more isolated, and are only able to achieve adsorption energies of 60 kJ/mol. Secondary amines species (associated with nmethylaminopropyl silane, MAPS) showed similar adsorption energies at similar amine densities, yet are known to be less effective for CO2 capture.10 Such studies have not proven effective for tertiary amines, as these are not able to chemisorb any significant quantity of CO2 under dry conditions.50 In this work, we investigate the influence of the aminopolymer loading on the CO2 adsorption energy for a prototypical class 1 system based on PEI-impregnated SBA-15 silica. We sought to correlate our findings with recent microcalorimetry50,51 and theoretical findings on the roles of the amine sites and the polymer morphology.32,33 This is one of the first times that silica impregnated with an aminopolymer has been explored experimentally using microcalorimetry and the findings help to deepen the structure-property relationships that exist for this class of CO2 sorbents. EXPERIMENTAL METHODS Materials All chemicals were obtained from Sigma Aldrich and used as received without further purification. SBA-15 synthesis


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First, 24.01 g of Pluronic P123 was initially dissolved in 630 mL of H2O and 120 mL of concentrated hydrochloric acid (37 wt%) while rapidly stirring at 40 oC to give a clear solution with white foam. Next, 52.68 g of tetraethyl orthosilicate was added dropwise, after which the system was left to stir for 20 hours at 40 oC. The system was then aged under static conditions at 100 oC for 24 hours. The mixture was then filtered and washed with 6 L of deionized water. The white powder was subsequently dried at 75 oC overnight prior to calcination. The powder was calcined by heating to 200 oC at a rate of 1.2 oC/min, holding for 1 hour, and then increasing the temperature to 550 oC at a rate of 1.2 oC/min and holding for 4 hours to yield a white powder. PEI Impregnation Initially, 1.0 g of the above SBA-15 was stirred with 15 mL of methanol at room temperature for 1 hour. A 20 mL solution of methanol containing either 450, 30 or 9 mg (as appropriate) of 800 MW branched poly(ethyleneimine) (PEI) was added to the slurry and stirred for a further 24 hours at room temperature. The solvent was then removed under reduced vacuum at 50 oC to yield a white powder. The sample was then dried at 110 oC for 12 hours at 10 mbar. Nitrogen Physisorption Nitrogen physisorption was performed on a Micrometrics Tristar 3020 instrument at 77 K. Samples were degassed for 12 hours at 110 oC prior to analysis. Surface area was calculated using the BET model. Pore volumes and pore-size distributions were calculated using the BdBFHH model.53 Gravimetric CO2 adsorption


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Dry CO2 sorption capacities were measured using a TA instruments Q500 TGA using 10% CO2 in helium. Samples were pretreated under a 100 mL/min flow of helium at 110 oC for 3 hours. The CO2 uptake was then measured from the gain in mass after subsequent exposure to 10% CO2 in helium, at 30 oC, flowed at 90 mL/min for 6 hours, with a 10 mL/min balance helium flow. Organic content of sorbent The PEI content of the samples was measured using a Netzsch STA409PG TGA. The weight loss between 150 to 750 oC, under a combined flow of 90 mL/min of air and a 30 mL/min of nitrogen, was used to estimate the organic content in the sorbents. The data were collected between 25 and 900 oC at a heating at a rate of 10 oC/min. Simultaneous Volumetric CO2 adsorption and microcalorimetry CO2 adsorption capacities and heats of adsorption were measured using a combined calorimetricvolumetric adsorption apparatus, consisting of a Tian-Calvet calorimeter. In a typical experiment, 50 mg of the pelletized (1000 psi, 150-250 µm) sorbent was inserted into one side of the two-pronged sample cell, the other being left empty as a reference. The cell is encased in an aluminum block with highly sensitive thermopiles. The sample was pretreated at 120 oC for 3 hours under 15 Pa of vacuum, before cooling to 30 oC. Two pressure transducers connected to the cell and reservoir areas, which were maintained at 30 oC with heating tape, were used to measure the CO2 uptake. The amount of adsorbed CO2 was calculated using a mole balance with initial and final pressure values from the cell and reservoir. Heats of CO2 adsorption were simultaneously recorded while the dosed CO2 was being adsorbed. When the cell pressure change was less than 10-2 Pa/min, the system was assumed to have reached a pseudo equilibrium, signifying the end of the collection for that data point. The CO2 pressure was incrementally


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increased to simultaneously generate an adsorption isotherm and calculate the heat of adsorption. Thus, the reported heats of adsorption are for the incremental amount of CO2 adsorbed, and not an average over all CO2 adsorbed. Volumetric CO2 adsorption CO2 adsorption isotherms were also measured on a Micromeritics ASAP 2020 at 30 °C using approximately 100 mg of sample. Prior to analysis, the samples were degassed under vacuum on the instrument (< 5 mTorr) for 12 h at 110 °C. An equilibration interval of 30 s was used for pressures less than 5500 Pa and 10 s for all other pressures. RESULTS AND DISCUSSION The synthesis of the SBA-15 support was confirmed using nitrogen adsorption-desorption profiles and the pore size distribution calculated from the isotherm, with a sharp pore-size distribution of 7 – 8 nm and pore volumes (Figure 1a, Figure S2) and surface areas of 0.93 cm3/g and 789 m2/g, respectively, showing the N2 physisorption and powder XRD pattern (Figure S3) are in good agreement with literature data.25,29,33 To explore the influence of PEI loading on CO2 adsorption, the support was impregnated with varying quantities of PEI in methanol to generate a series of PEI-X/SBA-15 samples, where X corresponds to the amine loading per g of sample. Thermogravimetic analysis (Figure 1b) showed that the three PEI containing samples had organic contents 8.0, 15.9 and 33.3 wt%, corresponding to amine loadings of 1.5, 3.3 and 7.5 mmol/g, respectively (Table 1). These values were calculated by the decrease in mass between 150 and 750 oC to eliminate the mass of physisorbed species (solvent, water, CO2 sorbed from ambient conditions) from the calculation (Figure 1b). The associated dynamic scanning calorimetry curves show that the initial degradation of PEI occurred in the region of 220-235 oC.


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Beyond this region, the organic species continued to break down into high boiling point ‘cokelike’ molecules, before eventually degrading and/or evaporating. The minimum of this feature shifts to a lower temperature with increased PEI loading, suggesting that the less PEI, the more strongly it interacts with the silica support (Figure S4).32,33 The specific PEI loadings chosen represent the evolution from a monolayer of PEI oligomers on the silica pore walls to a sorbent with more bulk-like PEI properties.32,33 Higher loading samples were considered, however these were discarded because the diffusion limitations of CO2 moving through densely-packed liquid PEI made calorimetric measurements poorly reproducible, as the sorbent is not solid. On adding increasing amounts of PEI into the system, the surface areas and pore volumes were both found to significantly decrease, with the highest loading sample (PEI-7.5/SBA-15) possessing a pore volume and surface area of 0.51 cm3/g and 236 m2/g respectively (Table 1). However, the average pore diameter does not decrease monotonically with increasing PEI loading (Figure S2); initially, the average pore diameter decreases due to conformal monolayer coating of the SBA-15 pore surface. As the loading is increased, additional PEI aggregates into plugs that grow along the pore axis, rather than continued conformal coating and growth of multilayers. Thus, the pore volume continually decreases as PEI loading increases, but the average pore diameter does not change after the initial PEI monolayer. This phenomenon has been studied previously via neutron diffraction studies. 33


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1000

a

Nitrogen Adsorbed/(cm3/g)

900 800

SBA-15 PEI-1.5/SBA-15 PEI-3.3/SBA-15 PEI-7.5/SBA-15

700 600 500 400 300 200 100 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

105

b

100 95 90

Mass/wt%

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85 80 75


70 65 60 200

400

600 o

Temperature/ C

Figure 1. a) N2 adsorption-desorption isotherms of bare SBA-15 and PEI-X/SBA-15 species (isotherms are offset by 100 cm3/g for clarity), b) thermogravimetric analysis of bare SBA-15 and PEI-X/SBA-15 species to determine PEI loadings.


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Table 1. Physical characteristics of PEI-impregnated SBA-15 sorbents. Sample name

Surface area / (m2/g) Pore volume / (cm3/g) Organic content / wt%

SBA-15

789

0.93

1.8

PEI-1.5/SBA-

512

0.77

8.0

15PEI-3.3/SBA-

361

0.65

15.9

15PEI-7.5/SBA-

236

0.51

33.3

15 The CO2 sorption properties of the samples were initially evaluated using TGA measurements with 10% CO2, a concentration relevant to flue gas streams, under dry conditions (Figure 2).50 Bare SBA-15 showed a low CO2 uptake of 0.08 mmol/g, consistent with previous work, due to the lack of basic sites in the support, and the poor sorption capability of the surface silanol groups.33,50 Only a slight increase in CO2 uptake was observed upon introducing small quantities of PEI into the system (PEI-1.5/SBA-15; 0.12 mmol/g of CO2). It should be noted that this amine loading is within the range that has been studied extensively for class 2 aminosilane species (typically 1 – 2 mmol N/g).33,50 Direct comparison to previous studies shows that the amine efficiency of the PEI-1.5/SBA-15 sample is inferior to class 2 analogues with grafted APS or MAPS species.50 Under dry conditions, the theoretical maximum amine efficiency is 0.39 for this PEI system (due to the amine distribution of 44:33:22, with only primary and secondary amines adsorbing CO2), 0.5 for aminopropyl silane (APS) or methylaminopropyl silane (MAPS) and 0 for dimethylaminopropyl silane (DMAPS). The PEI-1.5/SBA-15 sample has an amine efficiency of just 0.04; even after accounting for the 22% of inactive tertiary amines in the PEI, this is notably worse than previous studies on APS/SBA-15 and MAPS-SBA-15. With similar amine


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loadings (1.4 and 1.8 mmol/g, respectively), both class 2 samples showed amine efficiencies of roughly 0.1. This reduced efficacy of the class 1 PEI system over analogous class 2 aminosilica species is attributed to the support-polymer interactions of PEI. Namely the primary amines are suggested to interact strongly with pendant silanol species to stabilize the PEI within the mesopores.32,33 At low loadings of PEI, these interactions force the primary amines to largely reside adjacent to the pore walls, leaving the less effective secondary and tertiary amines free in the center of the pore.33 Therefore, for a CO2 molecule to interact with a primary amine site, it must diffuse through the ‘outer’ PEI surface of tertiary and secondary amines and then compete with the silanol species for access to the amine. Furthermore, as noted above, under dry conditions, the primary mode of CO2 adsorption requires two amines to form an alkyl ammonium carbamate, capturing one CO2 molecule.38 This is thus significantly impacted by the density of available amines. As the majority of primary amines are involved in silanol interactions for the low PEI loading sample, PEI-1.5/SBA-15, it is less favorable for them to form the carbamate species required, lowering the CO2 uptake.


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1.6

0.20

1.4 0.15

1.2 1.0

0.10

0.8 0.6

0.05

0.4

Amine efficiency

CO2 Adsorbed/(mmol/g)

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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0.2 0.00 0.0 -1

0

1

2

3

4

5

6

7

8

Amine loading/(mmol/g)

Figure 2. CO2 uptake (solid line) and amine efficiency (dashed line) at 30 oC, using 10 mL/min of dry 10% CO2/He, as a function of amine loading. Increasing the PEI loading initially shows a subtle increase in the amine efficiency, as shown in Figure 2 (PEI-3.3/SBA-15; 0.05). However, increasing the organic loading further shows a significant improvement in the amine efficiency (PEI-7.5/SBA-15; 0.18). This is attributed to the differing morphology of the PEI species within the pore, as reported by Holewinski et al.33 As there is little difference between PEI-1.5/SBA-15 and PEI-3.3/SBA-15 this suggests they have similar (monolayer or near monolayer) PEI morphology, and that only at amine loadings greater than 3.3 mmol/g are the pore walls sufficiently saturated with PEI to achieve improved uptakes using this specific support. As the surface coverage of the PEI on the SBA-15 reaches a certain threshold, the primary-amine – silanol interactions become saturated, allowing the primary amines to reside closer to the center of the pore, away from the pore walls.32 This then makes


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them more available for CO2 capture, increasing the chance for two available amines to be in close proximity to form the necessary carbamate species.

1.6 1.4


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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1.2 1.0 0.8 0.6 0.4


0.2 0.0 0

20000

40000

60000

80000

100000

Pressure/Pa

Figure 3. CO2 adsorption isotherms obtained at 30 °C onbare SBA-15 and PEI impregnated SBA-15 with different PEI loadings. To gain a greater understanding of the CO2 adsorption behavior, CO2 adsorption isotherms were collected in a Micromeritics ASAP 2020 at 30 oC (to be consistent with previous calorimetry experiments,50,51 Figure 3 and S5). In agreement with previous literature by Sayari et al., the three PEI-impregnated samples show typical two-site behavior, corresponding to chemisorption and physisorption processes, as seen by the sharp rise in CO2 adsorption up to 7000 Pa, followed by a gradual linear increase thereafter.47 This two-site behavior was fit to a simple dual-mode sorption model, which was a combination of Langmuir and Henry adsorption for chemisorption and physisorption, respectively (Table S1). On bare SBA-15, two-site behavior was observed with limited Langmuir adsorption, suggesting that bare SBA-15 does not adsorb CO2 strongly, as


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expected. Similarly, on the SBA-15 with a low loading of PEI, the maximum amount of strongly adsorbed CO2 was limited due to strong interactions between the silanols and amine sites. The extent of Langmuir-like adsorption increased as the loading of PEI increased, both in terms of Langmuir adsorption constant and also in the maximum amount of chemisorbed CO2 observed. In particular, the number of sites available for chemisorption increased from 0.17 to 1.1 mmol/g as the PEI loading is increased from PEI-1.5/SBA-15 to PEI-7.5/SBA-15 (Table S1). We calculate a sharp increase in both the Langmuir adsorption constant and maximum amount of chemisorbed CO2 when moving from the PEI-3.3/SBA-15 to PEI-7.5/SBA-15 sample, consistent with an increase in amine efficiency at higher PEI loading and more extensive intermolecular amine-CO2-amine interactions capturing CO2. As discussed below, this leads to increased heats of adsorption. Conversely, the Henry’s law constant decreased consistently as the PEI loading was increased, suggesting that physisorption was correlated with available surface area and pore volume. Indeed, when normalized to either of these parameters, the observed Henry’s law constant was similar in all cases (Table S2). At higher pressures where physisorption is the dominant process, bare SBA-15 was able to adsorb more CO2 than SBA-15 with low loadings of PEI. The CO2 uptake behavior was further studied using microcalorimetry to experimentally measure the isosteric heats of adsorption as a function of PEI loading and CO2 adsorbed (Figures 4a, 4b and S6).65 The pressure range in this study is smaller than other recent studies,48-55 to focus more heavily on the appropriate range for DAC applications (400 ppm of CO2). All samples show a decrease in adsorption energy with increasing CO2 adsorbed, which is typical behavior for adsorbents, as the sites with the strongest CO2 interactions will bind first.50 The findings discussed above are further reinforced, as increasing the PEI loading leads to an increased


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amount of adsorbed CO2. The bare SBA-15 sample shows very similar behavior to previous studies,49-51 initially binding to CO2 with an energy of 40 kJ/mol, with this value decreasing to 36 kJ/mol after adsorbing just 0.02 mmol/g of CO2. This suggests that SBA-15 is able to chemisorb a very limited quantity of CO2, before physisorption is the dominant mechanism. This is in good agreement with the CO2 adsorption isotherm findings above.

100 90

Heat of Adsorption/(kJ/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

Langmuir

80 70 60 50 40 30


20 10 0 0.001

0.01

0.1

1


Figure 4. Calorimetrically measured isosteric heats of adsorption of CO2 at incrementally increased CO2 pressures for bare SBA-15 and PEI-loaded SBA-15 samples for low pressure CO2 uptakes. The PEI-1.5/SBA-15 sample shows adsorption energies of 10 kJ/mol higher than the SBA-15 sample, initially starting at 50 kJ/mol and dropping to 40 kJ/mol after 0.05 mmol/g of CO2 is adsorbed. It is again of interest to contrast this system with the findings of the class 2 grafted SBA-15 with an APS loading of 1.4 mmol/g. The adsorption heats of the class 2 sorbents are


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much higher, ranging from 90 – 70 kJ/mol for the same quantity of CO2 adsorbed. This is again attributed to a lower density of ‘free’ primary-amines in the PEI system, limiting the formation of the alkyl ammonium carbamate species.57 When this species is not able to efficiently form, the CO2 adsorption energy has been previously observed to be below 60 kJ/mol.50 This behavior has also been observed via in situ FT-IR in class 2 materials.58 Contrasting high- and low-loading APS modified silica systems shows that a high-loading of amines leads predominantly to the formation of ammonium carbamate pairs (due to the availability of nearby amines to form these pairs) typically showing a pair of IR bands around 1554 and 1495 cm-1, representing the carbamate and ammonium ions respectively. Lowering the amine density then hinders the formation of these pairs, forcing the system to form carbamic acid and bound carbamates with the silanol species.58 Therefore, the monolayer PEI species in PEI-1.5/SBA-15 is able to chemisorb some CO2, but the quantity is limited due to the more isolated nature of the free primary amines (with most interacting with the surface). It is also noted that the adsorption energy begins to approach that of the bare SBA-15 at higher pressures, as expected, again showing the similarities between these two materials.50,51 In stark contrast, the highest loading system (PEI-7.5/SBA-15) shows much higher adsorption energies during CO2 sorption, dropping slightly from 93 to 87 kJ/mol while sorbing up to 0.2 mmol/g of CO2 (Figure 4b). These values are in excellent agreement with those achieved for class 2 APS- and MAPS-functionalized materials with amine loadings above 1.5 mmol/g, showing that at the higher PEI loadings, the favored alkyl ammonium carbamates can be readily formed, owing to the high density of available primary and secondary amines, as seen in previous FT-IR work.29,34 However, unlike the class 2 materials, the PEI-7.5/SBA-15 species is able to maintain a high adsorption energy until 0.5 mmol/g of CO2 has been adsorbed.50,51 This is


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attributed to the significantly higher amine loadings present in the aminopolymer system, simply allowing more CO2 to be chemisorbed than the class 2 materials. Past 0.5 mmol/g of CO2 adsorbed the heat of adsorption begins to sharply decrease, down to a value of 38 kJ/mol after adsorbing 1.2 mmol/g of CO2, in agreement with the heat of adsorption with SBA-15.50,51 This is concurrent with previous spectroscopic observations. As alkyl ammonium carbamate pairs form the likelihood of isolated unbound amines increases. This forces the isolated amines to bind to CO2 via alternative pathways such as the formation of carbamic acid species,58 which are consequently facilitated by the presence of silanol groups or other amines on the surface.51,54 Such species are far less effective at binding to CO2 due to their relative instability compared to the alkyl ammonium carbamate species,14,54,59 similar to those for silica-CO2 physisorption. Such a significant decrease in energy is not seen in class 2 materials, though Yoo et al showed that by capping silanol species the CO2 adsorption energies can be lowered by roughly 10 kJ/mol.51 It is reasonable to assume that at these high PEI loadings (33.3 wt%) the overwhelming majority of the silica surface is covered by PEI, capping the silanol species and stopping them from interacting with CO2. This means that once the amine-CO2-amine interactions are sated then physisorption, and not amine-silanol CO2 interactions, are likely to primarily occur, as all other adsorption sites are blocked. As the latter are much lower in energy (40-30 kJ/mol), a decline in the rate of CO2 uptake is observed, giving similar heat of adsorption values as the bare SBA-15. The PEI-3.3/SBA-15 system shows intermediary behavior between the higher loading PEI7.5/SBA-15 and the lower loading PEI-1.5/SBA-15, which is consistent with the monolayer of PEI beginning to form multilayers.50,51 In the class 2 materials, previous work has assigned approximate heats of adsorption values for different forms of adsorbed CO2 - ~90 kJ/mol for amine-amine interactions leading to alkyl


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ammonium carbamate formation, ~60 kJ/mol for isolated species from amine-silanol interactions and ~40 kJ/mol for pure silanol interactions.50,51,57 The class 1 materials studied here offer the potential to form similar species, but also many other forms of adsorbed CO2 are also possible, due to the mixture of three different types of amines a range of sorption sites, and the steric constraints within the mesopore introduced by the amine polymer. More work is needed to elucidate the types of adsorbed species that exist on the low and medium loading samples, in this regard. As an example, the PEI-3.3/SBA-15 system has an initial heat of adsorption of 65 kJ/mol. This heat falls in a range where an array of different forms of adsorbed CO2 could form, potentially diluting the higher energy interactions. As the PEI loading is varied, the amount of free silanol species, the number of available primary amines, and the morphology of the polymer all change. To this end, additional work using in situ spectroscopy to examine the nature of the adsorbed species as a function of amine coverage is warranted to fully understand the nature of class 1 materials. While some information can be derived from oligomeric species,42-44 more emphasis on polymeric class 1 systems is required for a full understanding. The ability to tailor the nature of the available adsorption sites by changing the amine loading and support surface properties would add a further level of control over the design of CO2 sorbents. While this paper is confined to studying the adsorption of CO2, one must also consider the implications of these findings on desorption, as this is commonly the practical limitation of such processes. Modeling of the thermodynamic 2nd law efficiency for temperature swing CO2 adsorption/desorption processes has shown that the optimal heat of adsorption increases as the driving force for adsorption decreases.60 This is the case for direct air capture, where the concentration of CO2, and thus driving force for adsorption, is low (400 ppm). The PEI/SBA-15 materials presented here have a high heat of adsorption for CO2, making them suitable for direct


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air capture; however, these materials may not be ideal for capture from flue gas where the concentration of CO2 is much higher, and so high adsorption/desorption efficiency may be hampered by such a strongly binding adsorbent. A fundamental understanding of the heat of adsorption of CO2 in various sorbent materials can allow for design of materials to suit the particular adsorption process.

CONCLUSIONS To further understand the CO2 adsorption behavior of class 1 aminopolymer CO2 sorbents, the isosteric heat of CO2 adsorption of a series of PEI-impregnated SBA-15 has been directly measured via calorimetry as a function of PEI loading. The behavior of the aminosilica system as a function of PEI loading was in good agreement with our previous findings describing the morphology of PEI within the mesoporous silica, allowing for development of more complete structure-property relationships for this class of materials. While increasing the PEI loading, the obtained uptake data are in agreement with other literature reports that suggest that the quantity of available primary amine sorbent sites increased,32 as the binding between surface silanols and primary amines was sated as the monolayer of PEI was completed. This lead to a greater density of free primary amines, eventually leading to a maximum observed CO2 binding energy of 90 kJ/mol, as seen in class 2 APS materials, attributed to the formation of alkyl ammonium carbamate species. Given the complexity of the class 1 sorbents, one may surmise that a much broader array of types of adsorbed CO2 may be possible over such materials, and the data here support this hypothesis, suggesting that a continuum of CO2 binding energies exist in these


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materials, and that multiple factors such as silanol coverage and free amine density may be tuned. ASSOCIATED CONTENT Supporting Information. Supporting information includes pore-size distribution, differential scanning calorimetry data and further CO2 adsorption isotherms and microcalorimetry figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported as part of UNCAGE-ME, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0012577. REFERENCES

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Adsorption Microcalorimetry of CO2 in Confined Aminopolymers

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