Potassium incorporated alumina based CO2 capture sorbents: Comparison with supported amine sorbents under ultra-dilute capture conditions

Colloids and Surfaces A: Physicochem. Eng. Aspects 486 (2015) 78–85

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Potassium incorporated alumina based CO2 capture sorbents: Comparison with supported amine sorbents under ultra-dilute capture conditions Sumit Bali, Miles A. Sakwa-Novak, Christopher W. Jones ∗ School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332-0100, USA

g r a p h i c a l

a b s t r a c t

h i g h l i g h t s • • • • •

Carbon dioxide capture from ultra-dilute gas streams. ␥-alumina supported potassium based solid sorbents. Desorption under moderate thermal conditions for cyclic testing. Comparison of CO2 uptakes with amine based sorbents under similar conditions. Estimation of isosteric heats of adsorption.

a r t i c l e

i n f o

Article history: Received 4 June 2015 Received in revised form 26 August 2015 Accepted 4 September 2015 Available online 8 September 2015 Keywords: CO2 capture Dilute gas streams

a b s t r a c t Amines supported on silica or alumina supports are among the most well-studied sorbents for CO2 capture from ultra-dilute sources, such as ambient air. Another class of sorbents that may also bind CO2 strongly enough to function well under ultra-dilute conditions are supported alkali metal based compositions. In this work, the synthesis of alumina-supported potassium (AlK) via calcination of impregnated potassium acetate is described and the resulting materials are evaluated for CO2 capture under ultra-dilute conditions (1% CO2 ), including under simulated air capture conditions (400 ppm CO2 ). The sorbents are evaluated at different adsorption and regeneration temperatures alongside two benchmark alumina-supported amine sorbents. The potassium–alumina sorbents are found to adsorb more CO2

∗ Corresponding author. E-mail address: [email protected] (C.W. Jones). http://dx.doi.org/10.1016/j.colsurfa.2015.09.020 0927-7757/© 2015 Elsevier B.V. All rights reserved.

S. Bali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 486 (2015) 78–85 Alumina Potassium sorbents

79

than the amine benchmark sorbent at the tested 1% CO2 conditions, whereas the amine sorbent yielded a higher uptake under simulated air capture conditions of 400 ppm. Abbreviated adsorption isotherms are measured for the potassium-alumina sorbents at three temperatures, the isotherms are fit to a single site Toth adsorption model, and the assembled data are utilized to estimate the heat of adsorption using the Clausius–Clapeyron equation. The isosteric heat of adsorption for the potassium based sorbent is higher than those estimated and measured for amine based sorbents, which is consistent with the higher regeneration temperature required by the potassium based sorbents. Potassium–alumina sorbents may be useful adsorbents for ultra-dilute conditions due to their stability in oxygen relative to amine sorbents if plentiful heat is available for the significant temperature swings needed to effectively cycle the sorbents. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The uncontrolled emissions of the greenhouse gas CO2 are linked to climate change. To this end, much research has focused on technologies for decreasing CO2 emissions by capturing CO2 from gas streams being vented to the atmosphere, such as the exhaust from fossil fuel based power plants, which serve as the largest contributors of anthropogenic CO2 emissions [1,2]. In parallel, large amounts of CO2 are also being emitted by numerous small mobile sources such as cars, aircraft, and ships employed in the transportation sector. To address such emissions, research has also focused on capturing CO2 directly from the air, known as direct air capture [3–16]. Conventionally, amine scrubbing using aqueous solutions of amines is used to remove CO2 from gas streams [17–23]. However, the large amount of energy that is required to regenerate the solutions as well as the corrosion effects associated with amines have prompted the search for alternative sources for capture and removal of CO2 . Solid adsorbents are now widely studied as alternative CO2 capture media [24]. One such class of adsorbents is physical adsorbents such as zeolites [25,26] and activated carbons [27–31]. However due to the relatively shallow adsorption isotherms associated with the weak CO2 -solid binding in physisorbents, such sorbents are not suitable for capture of CO2 from streams with very large volumes of CO2 at very low partial pressures. Physisorbents generally have low CO2 adsorption capacities at low partial pressures as well as the high affinities for water, especially in the case of zeolites. Other classes of physisorbents such as metal organic frameworks (MOFs) [32–37] show promise for capturing CO2 from both flue gas and ultra-dilute gas streams, although both MOF stability under practical conditions as well as competition with water and other impurities in the gas streams can be problematic [37]. Another promising class of sorbents are amines incorporated in porous supports [24,38–43,44]. In these sorbents, both molecular amines and amine polymers have been used, with these species most often physically immobilized on the support (class 1) [38,39,45–47], or covalently attached to the support, often using aminosilanes (class 2) [40,41,43,48,49]. In a hybrid of these two approaches, amines have been incorporated by the in-situ polymerization of amine monomers onto the support (class 3) [50–56]. Although supported amines are being increasingly demonstrated to be promising materials for the CO2 capture applications under relatively concentrated (flue gas) as well as dilute conditions (air capture), their long-term stability under the conditions used for adsorption/desorption cycling is less well-understood. In particular, silica supports containing amines can be unstable in the presence of humidity or liquid water [57], and amines are known to oxidize in the presence of air at elevated temperatures [42,58–60]. Alternate sorbent compositions that capture significant amounts of CO2 at low temperatures and low partial pressures and that are oxidatively stable might offer opportuni-

ties for improved sorbent longevity, ultimately leading to reduced process costs. Another class of sorbents being explored for CO2 capture applications are the chemisorbents based on supported alkali oxides [61–68], and carbonates [69–90]. Of particular interest are CO2 sorbents based on carbonates on porous inorganic supports, which are most often prepared by potassium carbonate impregnation, followed by calcination to generate the adsorbing alkali species [64,78,79,81]. Reports describing the use of such materials for CO2 capture from dilute or ultra-dilute streams have generally been based on wet impregnation of K2 CO3 directly onto the support [78,79,82,84,88–90], and have been limited to adsorption uptake data of such systems. In this work, we report the synthesis of well dispersed potassium species on an alumina support via calcination of impregnated potassium acetate under an inert N2 atmosphere. The K-alumina sorbents thus formed are used to capture CO2 under dry conditions using both dilute (1%) and ultra-dilute 400 ppm (direct air capture) CO2 and the uptakes are compared with more commonly studied amine based sorbents under similar conditions. In spite of very low loadings of K (5% and 10%) as compared to previously reported Al–K systems, the sorbents exhibited outstanding uptakes under both dilute (1%) and ultra-dilute (air capture, 400 ppm) conditions. Furthermore, to better the understanding of such systems, experimental data from isotherm measurements of the synthesized K-alumina sorbents are used to calculate estimated isosteric heats of adsorption for these sorbents as well. 2. Experimental 2.1. Materials The following chemicals were used as received from the suppliers: pseudobomite (Catapal B, 74.3% Al2 O3, SASOL North America), Pluronic P-123® EO20-PO70-EO20 triblock copolymer (SigmaAldrich), potassium acetate (Sigma–Aldrich) poly(ethyleneimine) (PEI), branched Mw 800 (Sigma–Aldrich). 2.2. Synthesis of mesoporous -alumina ␥-Alumina was synthesized using P-123® as a surfactant for templating mesopores [43,91]. Typically, 13.75 g of pseudobomite from Sasol North America (Catapal B, 74.3% Al2 O3 ) was suspended in a mixture of 1.27 g nitric acid (Fischer Scientific, ∼70%) and 200 mL distilled water and sonicated for 90 min at room temperature. The sonicated suspension was subsequently stirred at 60 ◦ C for 17 h followed by cooling to room temperature. The obtained peptized alumina was then poured into a solution of 15.3 g Pluronic P123® in 200 mL ethanol (200 proof). The resulting solution was stirred at room temperature for 24 h, after which the solvent ethanol was evaporated at 60 ◦ C in an open beaker. The P-123® alumina composite thus obtained after removal of the solvent was then dried at 75 ◦ C for 24 h. The white sol-gel derived mesoporous

80

S. Bali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 486 (2015) 78–85

␥-alumina was obtained by calcination of this composite at 700 ◦ C for 4 h. The composite was heated at rate of 1 ◦ C/min to 150 ◦ C. The temperature was then held constant at 150 ◦ C for 1 h followed by increasing it to 700 ◦ C at 1 ◦ C/min, where the temperature was held at 700 ◦ C for 4 h to yield the desired mesoporous ␥-alumina. 2.3. Synthesis of potassium incorporated alumina sorbents The synthesized ␥-alumina was suspended in 100 mL ethanol (200 proof). To this suspension the desired amount of potassium acetate was added with constant stirring. The resulting suspension was sonicated for 0.5 h followed by removal of the ethanol solvent in an open beaker at 60 ◦ C. The resulting solid was then dried in an oven at 75 ◦ C for 12 h, followed by calcination under flowing N2 (50 mL/min) at 500 ◦ C with a heating ramp of 5 ◦ C/min and soak time of 5 h. After calcination the samples were stored in tightly closed vials in a desiccator. 2.4. Sorbent characterization Surface areas, pore volumes, and pore diameters were calculated from the collected cryogenic nitrogen isotherm data. Surface areas were calculated using the Brunauer Emmett Teller (BET) method [92,93], and pore diameters and pore volumes were calculated using the Broekhoff-de Boer-Frenkel Halsey Hill (BdB-FHH) method [94]. Powder X-ray diffraction (XRD) patterns were collected on a PANalytical X’pert diffractometer equipped with a Cu–K-alpha Xray source. The potassium content in the potassium-incorporated alumina (AlK) sorbents were determined at Galbraith laboratories using the inductively coupled plasma-optical emission spectrometry (ICP–OES) technique 2.5. CO2 sorption experiments A TA Instruments Q500 TGA was used to measure the sorption capacities of the materials under dry CO2 capture conditions. The fresh sorbent materials were loaded into the sample pan (platinum) and then helium was flowed through the sample chamber, with a simultaneous increase to a hold temperature of 110 ◦ C. The temperature was then held constant for 3 h followed by cooling to 25 ◦ C under helium flow. After stabilization under a helium flow at 25 ◦ C for 1 h, the gas flow was then switched to the required CO2 concentration (1% CO2 in helium or 400 ppm in helium). The subsequent weight gain because of sorption of CO2 was then measured over the desired time period (3 h and 12 h) for all the tested sorbents. For recycling studies on the synthesized sorbents, after the adsorption cycle, the thermal regeneration of the sorbents was carried out under a flow of inert helium with heating at a ramp of 5 ◦ C/min to the desired temperature of regeneration followed by a 3 h period holding the temperature constant. After this the sorbent was cooled to room temperature (RT, 25 ◦ C) under inert atmosphere for the next sorption cycle. 2.6. Isotherm model for calculation of heats of adsorption Adsorption isotherms were modeled using the single site Toth isotherm model, shown in Eq. (1). The Toth isotherm is a modification of the Langmuir isotherm model, and accounts for an energetically heterogeneous surface [95,96]. q=

ns bP t 1/t

(1)

[1 + (bp) ]

In the Toth model, ns is the surface coverage (mmol/g), b is a constant similar to the Langmuir equilibrium constant (1/Pa), P is pressure (Pa) and t is a second constant ranging from 0 to 1 describ-

Fig. 1. Wide angle XRD patterns for mesoporous alumina as well as AlK sorbents.

ing the degree of surface heterogeneity (dimensionless). Heats of adsorption were estimated using the Clausius–Clapeyron equation, shown in Eq. (2), assuming an ideal gas phase. dP HP = dT RT 2

(2)

In this equation, P is the pressure (Pa), T is the temperature (K), R is the gas constant (J/mol K), and H is the enthalpy relating equilibrium between the two phases, in this case the heat of adsorption (J/mol). From the Toth isotherm model, the slope of the line relating ln(P) vs 1/T at constant loading (n) was estimated using a linear regression at loadings ranging from 0.5 mmol/g to 1.2 mmol/g. These bounds in loading were chosen to minimize the extrapolation of the isotherm model beyond the range of the experimental data, especially in the low pressure region. 3. Results and discussion The wide angle XRD pattern of the synthesized alumina shown in Fig. 1 confirms the ␥-alumina structure of the synthesized support. The AlK sorbents prepared by potassium acetate impregnation on these supports are designated as AlK5 and ALK10, based on the approximate percentage weight of potassium in the synthesized sorbents as determined by ICP-OES. The XRD patterns of synthesized AlK5 and AlK10 sorbents are also shown in Fig. 1. The XRD patterns of AlK5 and AlK10 sorbents indicate the presence of potassium carbonate (K2 CO3 ), potassium bicarbonate (KHCO3 ) along with the KAl(CO3 )(OH)2 species being formed on the alumina support [69,84,88]. The mesoporous structures of the synthesized alumina as well as K-incorporated sorbents were confirmed by nitrogen physisorption measurements. Nitrogen adsorption- desorption isotherms for the alumina as well as AlK5 and AlK10 sorbents are shown in Fig. 2 and the corresponding pore size distributions calculated using the BdB-FHH method are shown in Fig. 3.The synthesized ␥-alumina exhibited a typical type IV isotherm with a type H1 hysteresis loop, having sharp uptakes at P/P0 = 0.8 − 0.9, indicating that the synthesized ␥-alumina contained uniform large mesopores. The BET surface area of the synthesized bare alumina was found to be 223 m2 /g while the pore volume (at P/P0 = 0.99) was calculated to be 1.17 cm3 /g. The average mesopore diameter of the bare alumina was 17.3 nm. The synthesized AlK sorbents (AlK5, AlK10) also yielded typical type IV isotherm shapes with a type H1 hystere-

S. Bali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 486 (2015) 78–85

81

Alumina AlK5 AlK10

700 600

CO2 uptake (mmol/g)

Quantity adsorbed (cm3/g) STP

800

500 400 300 200

1.0

0.5

100 0

0.0 0.0

0.2

0.4

0.6

0.8

1.0

h 3h 2h 2h ina _3 _1 5_ _1 10 um 10 lK5 Al A lK K l A AlK A

P/P0 Fig. 2. N2 physisorption isotherms for the ␥-alumina and AlK sorbents.

Fig. 4. CO2 uptakes of ␥-alumina and AlK sorbents under dry, 1% CO2 conditions.

Pore volume (cm3/g) STP

0.5 Alumina ALK5 ALK10

0.4 0.3 0.2 0.1 0.0

0

20

40

60

80

100

120

140

Pore diameter (nm) Fig. 3. Pore size distribution of the ␥-alumina and AlK sorbents. Table 1 N2 physisorption data for as synthesized ␥-alumina and AlK sorbents.

Alumina ALK5 ALK10 a

BET surface area (m2 /galumina)

Cumulative pore volume (cm3 /galumina)

Pore diameter (nm)

223 170 167

1.17 0.88 0.86

17.3 18.0 17.5

K content (%)a

6.6 10.5

Determined by ICP-OES.

sis loop, having sharp uptakes at P/P0 = 0.8-0.9, indicating that the synthesized AlK sorbents maintained the porosity of the parent alumina support. The BET surface area, pore size distribution and pore volume of the bare alumina as well as the synthesized AlK sorbents are summarized in Table 1. The surface area of the AlK sorbents gradually decreased upon addition of potassium to the support. The N2 physisorption isotherms and pore size distribution of the synthesized alumina as well as potassium-incorporated AlK sorbents are shown in Fig. 3. As can be seen, the AlK sorbents maintained a well-defined pore size distribution similar to the ␥-alumina support. The nitrogen physisorption isotherms for the synthesized AlK sorbents were qualitatively similar to the bare alumina support. There was no significant difference in the pore size distribution between the alumina and AlK sorbents. Some reduction in the pore volumes was observed for the synthesized AlK sorbents as compared to bare alumina support, consistent with the presence of potassium species inside the pores of the alumina in the case of

AlK sorbents. The potassium contents in AlK5 and AlK10 was found to be 6.6 and 10.5 wt% respectively. The AlK sorbents were subjected to dry CO2 capture under dilute conditions (1% CO2 , balance helium) as well as ultra-dilute conditions (400 ppm CO2 , balance helium), the latter mimicking arid air capture conditions. The sorbents were tested for different adsorption times of 3 h and 12 h of CO2 exposure, with the corresponding CO2 uptakes at 1% dry CO2 conditions at room temperature for the two AlK sorbents and bare alumina shown in Fig. 4. The CO2 capacity of both AlK5 and AlK10 were much higher than the bare alumina support, clearly demonstrating the effect of potassium incorporation. However no significant differences were observed between the AlK5 and AlK10 sorbents. Even at such low loadings of K, for the 12 h CO2 uptake experiment, the AlK5 sorbent exhibited a high CO2 uptake of ∼1.19 mmol/g using 1% CO2. The corresponding uptake for the AlK10 sorbent was only marginally higher at 1.28 mmol/g. On reducing the CO2 uptake time from 12 h to 3 h, the CO2 uptake of the AlK10 sorbent decreased slightly to 1.10 mmol/g while that of the AlK5 reduced to 1.08 mmol/g. The ALK sorbents were also compared with more commonly studied amine based sorbents under 1% CO2 capture conditions for benchmarking. The amine based sorbents that were used included a class 1 sample made by the physical impregnation of poly(ethyleneimine) (PEI) on a ␥-alumina as well as a class 2 material synthesized by covalent attachment of (3-aminopropyl) trimethoxysilane (APS) onto ␥-alumina The comparative uptakes for the amine based sorbents and AlK sorbents under 1% CO2 capture conditions for 3 h are shown in Fig. 5. From the data in Fig. 5, it was observed that the AlK sorbents had better uptakes than the amine based sorbents under the 1% CO2 capture conditions using an uptake time of 3 h. A maximum uptake of ∼1.1 mmol/g was achieved for the AlK10 sorbent. This was higher than any of the benchmark amine based sorbents under the tested conditions. The capacity of the AlK5 sorbent was also similar at 1.08 mmol/g. To further test the regeneration and recyclability of the synthesized AlK sorbents, the sorbents were subjected to cyclic CO2 uptake experiments with a thermal regeneration step using flowing helium for regeneration in between the consecutive adsorption steps. The regeneration of both the AlK5 and AlK10 sorbents was carried out in helium at three different temperatures of 110 ◦ C, 250 ◦ C and 350 ◦ C with the time for regeneration held constant at 3 h and using a temperature ramp of 5 ◦ C/min. The resulting regeneration profiles for the AlK5 and AlK10 sorbents are shown in Fig. 6. The data show that both the AlK5 and AlK10 sorbents maintained high CO2 uptakes over repeated cycles, but that the

82

S. Bali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 486 (2015) 78–85

1.0 0.8

CO2 uptake (mmol/g)

CO2 uptake (mmol/g)

1.0 0.8 0.6 0.4 0.2 0.0

0.6 0.4 0.2 0.0 Al_PEI35

5 0 h h I20 I3 _3 _3 S2 PE 10 K5 PE AP l _ K _ l _ l l A l A A A A

regeneration temperature was an important variable. At a regeneration temperature of 110 ◦ C, the CO2 uptake gradually decreased over multiple cycles, indicating that higher temperatures were required to fully regenerate the adsorbed CO2 . This decrease in CO2 uptake on regeneration at 110 ◦ C might be attributed to the formation of KAl(CO3 )(OH)2 , which was observed in XRD patterns and which requires temperatures upwards of 200 ◦ C for regeneration [85]. However when the regeneration was carried out at 250 ◦ C, there was no significant loss in CO2 uptake over five repeated cycles for both AlK5 and AlK10 sorbents. Similar results were also observed when the regeneration was carried out at 350 ◦ C, with slightly larger cyclic capacities. These results demonstrate that the synthesized AlK sorbents are stable under the conditions employed and can be regenerated under mild conditions of 250 ◦ C when 1% CO2 in helium was used as the adsorption gas. To further test the suitability of AlK systems for direct air capture, CO2 capture experiments were carried out using 400 ppm CO2 at room temperature, mimicking arid air capture conditions. As before, an amine based sorbent containing 35 wt% PEI supported on ␥-alumina was also tested and compared with the AlK sorbents (Fig. 7). The temperature of capture was kept at 25 ◦ C and the time for capture was 12 h for all the tested sorbents. The potassium incorporated alumina sorbents, AlK5 and AlK10, exhibited high CO2 uptakes of ∼0.86 and 0.78 mmol/g, respectively. However, this was slightly lower than the corresponding 35 wt% PEI on

0.5

350 oC

1.0

0.5

0.0 1

2

3

4

Regeneration cycle

5

110 oC 250 oC

AlK10 sorbents

1.5

CO2 utpake (mmol/g)

CO2 uptake (mmol/g)

1.0

0.0

␥-alumina sample which exhibited a CO2 uptake of 0.95 mmol/g. Thus, these first generation potassium incorporated alumina sorbents are slightly inferior to a benchmark amine sorbent under air capture conditions. To evaluate the regeneration of the AlK sorbents used for 400 ppm capture, the AlK5 and AlK10 sorbents were regenerated at 250 ◦ C under a flow of helium and cycled through five adsorption/desorption steps. The time for regeneration was kept constant for 3 h and the CO2 uptake time was also kept constant at 3 h for all the cycles. The uptakes obtained for the AlK5 and AlK10 sorbents are shown in Fig. 8. It can be observed that the CO2 uptakes of the AlK5 and AlK10 sorbents remained constant over the five cycles studied; suggesting the regeneration of the sorbent at that temperature was viable. Given the high desorption temperatures needed for regeneration of the AlK sorbents, it may be surmised that the heats of reaction with CO2 for these samples are considerably higher than those for the benchmark amine sorbents. Typical aminegrafted oxide sorbents have heats of adsorption in the range of 50–120 kJ/mol [97–100]. The experimental data from CO2 adsorption isotherms for the AlK10 sorbent measured under different partial pressures of CO2 (10%, 1% and 400 ppm) under different temperatures of 45, 65 and 85 ◦ C were used to estimate isosteric heats of adsorption of the AlK10 sorbent using the Clausius–Clapeyron equation and the single site Toth isotherm model, which has been previously used for amine based CO2 sorbents with reasonably good approximations [98]. Fig. 9 shows the equilibrium isotherm data obtained from TGA experiments on the AlK10 sorbent measured at three temperatures

250 oC 350 oC

1.5

AlK5

Fig. 7. Uptakes of AlK sorbents and amine based sorbent (35 wt% PEI on ␥-alumina) under dry air capture conditions (400 ppm).

Fig. 5. Comparative CO2 uptakes using 1% CO2 over the AlK20 sorbent and amine based benchmark sorbents. Al APS20: APS on alumina with 20% organic loading; Al PEI20: 20% PEI by weight on ␥-alumina; Al PEI35: 35% PEI by weight on ␥alumina.

AlK5 sorbents

AlK10

1

2

3

4

5

Regeneration cycle

Fig. 6. Regeneration of AlK5 and AlK10 sorbents at different temperatures under dry 1% CO2 capture conditions over five cycles.

S. Bali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 486 (2015) 78–85

AlK5 (regeneration at 250 oC) AlK10 (regeneration at 250 oC)

0.7

face coverage, a result of energetic heterogeneity on the sorbent surface. The heats estimated here are higher than those estimated for [98,99] and measured [97,101–104] for amine based sorbents being evaluated for CO2 capture from ambient air, and are similar to those estimated for inorganic sorbents being evaluated for the same application.

CO2 uptake (mmol/g)

0.6 0.5 0.4

4. Conclusions 0.3 0.2 0.1 0.0

1

2

3

4

5

Regeneration cycle Fig. 8. Regeneration of the AlK5 and AlK10 sorbents at 250 ◦ C used to capture CO2 under dry 400 ppm CO2 condtions.

Heats of Ads (kJ/mol)

(a) 136 134 132 130 128 126 124 122 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 Surf. coverage (mmol CO2/g sorbent) CO2 capacities (mmol / g)

83

1.4

(b)

1.2

45 oC

1.0 0.8

65 oC

0.6 0.4 0.2 0.0

The potassium incorporated alumina sorbents prepared by calcination of alumina impregnated with potassium acetate exhibited high CO2 uptakes under both dilute and ultra-dilute dry CO2 capture conditions at low (∼5% and ∼10% wt) potassium loadings. Under 1% CO2 conditions the uptakes of the potassium incorporated AlK sorbents were higher than that of two benchmark amine based sorbents under the conditions tested. Under simulated air capture conditions of 400 ppm dry CO2 , the AlK sorbents exhibited high uptakes as well, though lower than that observed for the amine based sorbent systems. Unlike the amine based sorbents, which could be effectively regenerated at a low temperature of 110 ◦ C, the AlK sorbents required regeneration temperatures of 250 ◦ C and 350 ◦ C under an inert gas flow. The AlK sorbents maintained the CO2 uptake over repeated cycles using the elevated regeneration temperatures when capturing CO2 from both 1% and 400 ppm gas streams. CO2 adsorption isotherms were measured for the AlK10 sorbent under different partial pressures of CO2 (10%, 1% and 400 ppm) at multiple temperatures (45, 65 and 85 ◦ C). The results were fit to a single site Toth isotherm model, and the results were used to calculate isosteric heats of adsorption of the AlK10 sorbent using the Clausius–Clapeyron equation. The isosteric heat of adsorption for the AlK10 sorbent was found to be higher than amine based sorbents but similar to other inorganic sorbents being tested under similar conditions. It is noteworthy that despite the higher estimated heat of adsorption of the AlK sorbent, the benchmark class 1 amine sorbent outperformed the AlK material under simulated air capture conditions. This suggests the two classes of materials have different isotherm shapes, and that the structure of the amine materials, perhaps the associated mobility of the CO2 binding sites (amines), makes capture under ultra-dilute conditions more favorable than in the case of the inorganic AlK sample. Further comparisons of inorganic and amine-based sorbents under ultra-dilute conditions are warranted.

85 oC 0

2 4 6 Pressure (kPa)

8

10

Fig. 9. (a) Isosteric heat of adsorption modeled using the Clausius–Clapeyron equation and a Toth isotherm fit to adsorption data for the AlK10 sorbent. (b) Adsorption isotherms of the AlK10 sorbent at varying temperatures. Squares represent measured data, and lines are fits of the Toth isotherm.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2015.09. 020. References

and three CO2 partial pressures. The capacities of the materials decreased with increasing temperature, as is expected for thermodynamically controlled adsorption processes with exothermic heats of adsorption. After fitting the isotherms to a Toth isotherm model, an estimate of the isosteric heat of adsorption was derived using the Clausius–Clapeyron equation. These data are also shown in Fig. 9, which depicts the isosteric adsorption heats from a starting surface coverage value of ∼0.5 mmol CO2 /g sorbent. This coverage was used because this is the lowest capacity measured for the 45 ◦ C sample, and its use as a lower bound eliminates the need for extrapolation of the isotherm model at exceedingly low gas partial pressures, where the accuracy of the model may be reduced relative to higher pressures. The isosteric heats derived here show the expected trend of decreasing magnitude with increasing sur-

[1] Y. Huang, M. Wang, P. Stephenson, S. Rezvani, D. McIlveen-Wright, A. Minchener, et al., Hybrid coal-fired power plants with CO2 capture: a technical and economic evaluation based on computational simulations, Fuel 101 (2012) 244–253. [2] N. MacDowell, N. Florin, A. Buchard, J. Hallett, A. Galindo, G. Jackson, et al., An overview of CO2 capture technologies, Energy Environ. Sci. 3 (2010) 1645–1669. [3] K.S. Lackner, P. Grimes, H.-J. Ziock, Capturing Carbon Dioxide from Air. LAUR-99-5113, Los Alamos Natl. Lab., Los Alamos, NM, 1999. [4] V. Nikulshina, M.E. Gálvez, A. Steinfeld, Kinetic analysis of the carbonation reactions for the capture of CO2 from air via the Ca(OH)2 –CaCO3 –CaO solar thermochemical cycle, Chem. Eng. J. 129 (2007) 75–83. [5] C.W. Jones, Capture from dilute gases as a component of modern global carbon management, Annu. Rev. Chem. Biomol. Eng. 2 (2011) 31–52. [6] A. Goeppert, M. Czaun, G.K. Surya Prakash, G.A. Olah, Air as the renewable carbon source of the future: an overview of CO2 capture from the atmosphere, Energy Environ. Sci. 5 (2012) 7833–7853.

84

S. Bali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 486 (2015) 78–85

[7] N.R. Stuckert, R.T. Yang, CO2 capture from the atmosphere and simultaneous concentration using zeolites and amine-grafted SBA-15, Environ. Sci. Technol. 45 (2011) 10257–10264. [8] A. Goeppert, H. Zhang, M. Czaun, R.B. May, G.K.S. Prakash, G.A. Olah, et al., Easily regenerable solid adsorbents based on polyamines for carbon dioxide capture from the air, ChemSusChem 7 (2014) 1386–1397. [9] S. Choi, J.H. Drese, P.M. Eisenberger, C.W. Jones, Application of amine-tethered solid sorbents for direct CO2 capture from the ambient air, Environ. Sci. Technol. 45 (2011) 2420–2427. [10] S. Choi, M.L. Gray, C.W. Jones, Amine-tethered solid adsorbents coupling high adsorption capacity and regenerability for CO2 capture from ambient air, ChemSusChem 4 (2011) 628–635. [11] Y. Belmabkhout, R. Serna-Guerrero, A. Sayari, Amine-bearing mesoporous silica for CO2 removal from dry and humid air, Chem. Eng. Sci. 65 (2010) 3695–3698. [12] T. Wang, K.S. Lackner, A.B. Wright, Moisture swing sorbent for carbon dioxide capture from ambient air, Environ. Sci. Technol. 45 (2011) 6670–6675. [13] T. Wang, K.S. Lackner, A.B. Wright, Moisture-swing sorption for carbon dioxide capture from ambient air: a thermodynamic analysis, Phys. Chem. Chem. Phys. 15 (2013) 504–514. [14] K.S. Lackner, The thermodynamics of direct air capture of carbon dioxide, Energy 50 (2013) 38–46. [15] J.A. Wurzbacher, C. Gebald, P. Tingaut, T. Zimmermann, A. Steinfeld, Amine-based nanofibrillated cellulose as adsorbent for CO2 capture from air, Environ. Sci. Technol. 45 (2011) 9101–9108. [16] R. Socolow, M. Desmond, R. Aines, J. Blackstock, O. Bolland, T. Kaarsberg, et al., Direct air capture of CO2 with chemicals A technology assessment for the APS panel on public affairs, Direct Air Capture CO2 with Chem. A Technol. Assess. APS Panel Public Aff. (2011). [17] N.A. Spector, B.F. Dodge, Removal of carbon dioxide from atmospheric air, Trans. Am. Inst. Chem. Eng. 42 (1946) 827–848. [18] N.A. Spector, B.F. Dodge, Colorimetric method for determination of traces of carbon dioxide in air, Anal. Chem. 19 (1947) 55–58. [19] S.S. Laddha, P.V. Danckwerts, Reaction of CO2 with ethanolamines – kinetics from gas-absorption, Chem. Eng. Sci. 36 (1981) 479–482. [20] D.A. Glasscock, J.E. Critchfield, G.T. Rochelle, CO2 absorption desorption in mixtures of methyldiethanolamine with monoethanolamine or diethanolamine, Chem. Eng. Sci. 46 (1991) 2829–2845. [21] C.-B. Yong, M.-H. Li, Kinetics of the absorption of carbon dioxide into mixed aqueous solutions of piperazine and triethanolamine, J. Chem. Eng. Jpn. 42 (2009) 29–38. [22] B. Mandald, S.S. Bandyopadhyay, Simultaneous absorption of CO2 and H2 S into aqueous blends of N-methyldiethanolamine and diethanolamine, Environ. Sci. Technol. 40 (2006) 6076–6084. [23] P.D. Vaidya, E.Y. Kenig, CO2 -alkanolamine reaction kinetics: a review of recent studies, Chem. Eng. Technol. 30 (2007) 1467–1474. [24] S. Choi, J.H. Drese, C.W. Jones, Adsorbent materials for carbon dioxide capture from large anthropogenic point sources, ChemSusChem 2 (2009) 796–854. [25] N. Konduru, P. Lindner, N.M. Assaf-Anid, Curbing the greenhouse effect by carbon dioxide adsorption with zeolite 13X, AIChE J. 53 (2007) 3137–3143. [26] A. Zukal, S.I. Zones, M. Kubu, T.M. Davis, J. Cejka, Adsorption of carbon dioxide on sodium and potassium forms of STI zeolite, ChemPlusChem 77 (2012) 675–681. [27] C. Pevida, M.G. Plaza, B. Arias, J. Fermoso, F. Rubiera, J.J. Pis, Surface modification of activated carbons for CO2 capture, Appl. Surf. Sci. 254 (2008) 7165–7172. [28] T.C. Drage, J.M. Blackman, C. Pevida, C.E. Snape, Evaluation of activated carbon adsorbents for CO2 capture in gasification, Energy Fuels 23 (2009) 2790–2796. [29] T.C. Drage, O. Kozynchenko, C. Pevida, M.G. Plaza, F. Rubiera, J.J. Pis, et al., Developing activated carbon adsorbents for pre-combustion CO2 capture, Energy Procedia 1 (2009) 599–605. [30] Y. He, W. Shen, S. Zhang, W. Fan, J. Li, Hierarchical porous polyacrylonitrile-based activated carbon fibers for CO2 capture, J. Mater. Chem. 21 (2011) 14036. [31] C. Zhang, W. Song, G. Sun, L. Xie, J. Wang, K. Li, et al., CO2 capture with activated carbon grafted by nitrogenous functional groups, Energy Fuels 27 (2013) 4818–4823. [32] A.R. Millward, O.M. Yaghi, Metal-organic frameworks with exceptionally high capacity for storage of carbon dioxide at room temperature, J. Am. Chem. Soc. 127 (2005) 17998–17999. [33] L. Bastin, P.S. Bárcia, E.J. Hurtado, J.A.C. Silva, A.E. Rodrigues, B. Chen, A microporous metal-organic framework for separation of CO2 /N2 and CO2 /CH4 by fixed-bed adsorption, J. Phys. Chem. C 112 (2008) 1575–1581. [34] J.-R. Li, Y. Ma, M.C. McCarthy, J. Sculley, J. Yu, H.-K. Jeong, et al., Carbon dioxide capture-related gas adsorption and separation in metal-organic frameworks, Coord. Chem. Rev. 255 (2011) 1791–1823. [35] S. Choi, T. Watanabe, T.-H. Bae, D.S. Sholl, C.W. Jones, Modification of the Mg/DOBDC MOF with amines to enhance CO2 adsorption from ultradilute gases, J. Phys. Chem. Lett. 3 (2012) 1136–1141. [36] Z. Zhang, Z.-Z. Yao, S. Xiang, B. Chen, Perspective of microporous metal–organic frameworks for CO2 capture and separation, Energy Environ. Sci. 7 (2014) 2868–2899.

[37] S. Keskin, T.M. van Heest, D.S. Sholl, Can metal–organic framework materials play a useful role in large-scale carbon dioxide separations? ChemSusChem. 3 (2010) 879–891. [38] X. Xu, C. Song, J.M. Andresen, B.G. Miller, A.W. Scaroni, Novel polyethylenimine-modified mesoporous molecular sieve of MCM-41 type as high-capacity adsorbent for CO2 Capture, Energy Fuels 16 (2002) 1463–1469. [39] X. Xu, C. Song, J.M. Andrésen, B.G. Miller, A.W. Scaroni, Preparation and characterization of novel CO2 molecular basket adsorbents based on polymer-modified mesoporous molecular sieve MCM-41, Microporous Mesoporous Mater. 62 (2003) 29–45. [40] R.A. Khatri, S.S.C. Chuang, Y. Soong, M. Gray, Thermal and chemical stability of regenerable solid amine sorbent for CO2 Capture, Energy Fuels 20 (2006) 1514–1520. [41] A. Sayari, R. Serna-Guerrero, Y. Belmabkhout, Amine-bearing mesoporous silica for CO2 removal from dry and humid air, Chem. Eng. Sci. 65 (2010) 3695–3698. [42] S. Bali, T.T. Chen, W. Chaikittisilp, C.W. Jones, Oxidative stability of aminopolymer–alumina hybrid adsorbents for carbon dioxide capture, Energy Fuels 27 (2013) 1547–1554. [43] S. Bali, J. Leisen, G.S. Foo, C. Sievers, C.W. Jones, Aminosilanes grafted to basic alumina as CO2 adsorbents—role of grafting conditions on CO2 adsorption properties, ChemSusChem (2014) 3145–3156. [44] P. Bollini, C.W. Jones, S.A. Didas, Amine-oxide hybrid materials for acid gas separations, J. Mater. Chem. 21 (2011) 15100. [45] T. Tsuda, T. Fujiwara, Polyethyleneimine and macrocyclic polyamine silica-gels acting as carbon-dioxide absorbents, J. Chem. Soc. Commun. 165 (1992) 1659–1661. [46] A.L. Chaffee, Z. Liang, P.A. Webley, D.J.N. Subagyono, G.P. Knowles, PEI modified mesocellular siliceous foam: a novel sorbent for CO2 , Energy Procedia 4 (2011) 839–843. [47] F.S. Li, W. Qiu, R.P. Lively, J.S. Lee, A. a Rownaghi, W.J. Koros, Polyethyleneimine-functionalized polyamide imide (Torlon) hollow-fiber sorbents for post-combustion CO2 capture, ChemSusChem 6 (2013) 1216–1223. [48] R. Serna-Guerrero, E. Da’na, A. Sayari, New Insights into the interactions of CO2 with amine-functionalized silica, Ind. Eng. Chem. Res. 47 (2008) 9406–9412. [49] P.J.E. Harlick, A. Sayari, Applications of pore-expanded mesoporous silica 5. Triamine grafted material with exceptional CO2 dynamic and equilibrium adsorption performance, Ind. Eng. Chem. Res. (2007) 446–458. [50] H.J. Kim, J.H. Moon, J.W. Park, A Hyperbranched poly(ethyleneimine) grown on surfaces, J. Colloid Interface Sci. 227 (2000) 247–249. [51] C.O. Kim, S.J. Cho, J.W. Park, Hyperbranching polymerization of aziridine on silica solid substrates leading to a surface of highly dense reactive amine groups, J. Colloid Interface Sci. 260 (2003) 374–378. [52] G.P. Knowles, S.W. Delaney, A.L. Chaffee, Diethylenetriamine[propyl(silyl)]-functionalized (DT) mesoporous silicas as CO2 adsorbents, Ind. Eng. Chem. Res. 45 (2006) 2626–2633. [53] J.C. Hicks, J.H. Drese, D.J. Fauth, M.L. Gray, G. Qi, C.W. Jones, Designing adsorbents for CO2 capture from flue gas-hyperbranched aminosilicas capable of capturing CO2 reversibly, J. Am. Chem. Soc. 130 (2008) 2902–2903. [54] W. Chaikittisilp, J.D. Lunn, D.F. Shantz, C.W. Jones, Poly(l-lysine) brush–mesoporous silica hybrid material as a biomolecule-based adsorbent for CO2 capture from simulated flue gas and air, Chem. Eur. J. 17 (2011) 10556–10561. [55] W. Chaikittisilp, S.A. Didas, H.-J. Kim, C.W. Jones, Vapor-phase transport as a novel route to hyperbranched polyamine–oxide hybrid materials, Chem. Mater. 25 (2013) 613–622. [56] J.H. Drese, S. Choi, R.P. Lively, W.J. Koros, D.J. Fauth, M.L. Gray, et al., Synthesis–structure–property relationships for hyperbranched aminosilica CO2 adsorbents, Adv. Funct. Mater. 19 (2009) 3821–3832. [57] W. Li, P. Bollini, S.A. Didas, S. Choi, J.H. Drese, C.W. Jones, Structural changes of silica mesocellular foam supported amine-functionalized CO2 adsorbents upon exposure to steam, ACS Appl. Mater. Interfaces 2 (2010) 3363–3372. [58] P. Bollini, S. Choi, J.H. Drese, C.W. Jones, Oxidative degradation of aminosilica adsorbents relevant to postcombustion CO2 capture, Energy Fuels 25 (2011) 2416–2425. [59] A. Heydari-Gorji, Y. Belmabkhout, A. Sayari, Degradation of amine-supported CO2 adsorbents in the presence of oxygen-containing gases, Microporous Mesoporous Mater. 145 (2011) 146–149. [60] G. Calleja, R. Sanz, E.S. Sanz-Pérez, A. Arencibia, Influence of drying conditions on amine-functionalized SBA-15 as adsorbent of CO2 , Top. Catal. 54 (2011) 135–145. [61] N.D. Hutson, S.A. Speakman, E.A. Payzant, Structural effects on the high temperature adsorption of CO2 on a synthetic hydrotalcite, Chem. Mater. 16 (2004) 4135–4143. [62] M.K. Ram Reddy, Z.P. Xu, G.Q. (Max) Lu, J.C. Diniz da Costa, Layered double hydroxides for CO2 capture: structure evolution and regeneration, Ind. Eng. Chem. Res. 45 (2006) 7504–7509. [63] R.V. Siriwardane, C. Robinson, M. Shen, T. Simonyi, Novel regenerable sodium-based sorbents for CO2 capture at warm gas temperatures, Energy Fuels 21 (2007) 2088–2097.

S. Bali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 486 (2015) 78–85 [64] R.V. Siriwardane, R.W. Stevens, Novel regenerable magnesium hydroxide sorbents for CO2 capture at warm gas temperatures, Ind. Eng. Chem. Res. 48 (2008) 2135–2141. [65] D.J. Couling, U. Das, W.H. Green, Analysis of hydroxide sorbents for CO2 capture from warm syngas, Ind. Eng. Chem. Res. 51 (2012) 13473–13481. [66] V. Manovic, E.J. Anthony, Steam reactivation of spent CaO-based sorbent for multiple CO2 capture cycles, Environ. Sci. Technol. 41 (2007) 1420–1425. [67] C. Vogt, G.P. Knowles, S.L.Y. Chang, A.L. Chaffee, Cadmium oxide/alkali metal halide mixtures – a potential high capacity sorbent for pre-combustion CO2 capture, J. Mater. Chem. A 1 (2013) 10962–10971. [68] E. Mostafavi, M.H. Sedghkerdar, N. Mahinpey, Thermodynamic and kinetic study of CO2 capture with calcium based sorbents: experiments and modeling, Ind. Eng. Chem. Res. 52 (2013) 4725–4733. [69] S.C. Lee, B.Y. Choi, T.J. Lee, C.K. Ryu, Y.S. Ahn, J.C. Kim, CO2 absorption and regeneration of alkali metal-based solid sorbents, Catal. Today. 111 (2006) 385–390. [70] H. Hayashi, J. Taniuchi, N. Furuyashiki, S. Sugiyama, S. Hirano, N. Shigemoto, et al., Efficient recovery of carbon dioxide from flue gases of coal-fired power plants by cyclic fixed-bed operations over K2 CO3 -on-carbon, Ind. Eng. Chem. Res. 37 (1998) 185–191. [71] A.G. Okunev, V.E. Sharonov, Y.I. Aristov, V.N. Parmon, Sorption of carbon dioxide from wet gases by K2 CO3 -in-porous matrix: influence of the matrix nature, React. Kinet. Catal. Lett. 71 (2000) 355–362. [72] C. Zhao, X. Chen, C. Zhao, CO2 absorption using dry potassium-based sorbents with different supports, Energy Fuels 23 (2009) 4683–4687. [73] C. Zhao, X. Chen, C. Zhao, Y. Wu, W. Dong, K2 CO3 /Al2 O3 for capturing CO2 in flue gas from power plants. Part 3: CO2 capture behaviors of K2 CO3 /Al2 O3 in a bubbling fluidized-bed reactor, Energy Fuels 26 (2012) 3062–3068. [74] C. Zhao, X. Chen, C. Zhao, Effect of crystal structure on CO2 capture characteristics of dry potassium-based sorbents, Chemosphere 75 (2009) 1401–1404. [75] R.R. Kondakindi, G. McCumber, S. Aleksic, W. Whittenberger, M.A. Abraham, Na2 CO3 -based sorbents coated on metal foil: CO2 capture performance, Int. J. Greenh. Gas Control 15 (2013) 65–69. [76] K. Kim, S. Yang, J.B. Lee, T.H. Eom, C.K. Ryu, S.-H. Jo, et al., Analysis of K2 CO3 /Al2 O3 CO2 sorbent tested with coal-fired power plant flue gas: effect of SOx, Int. J. Greenh. Gas Control 9 (2012) 347–354. [77] S.C. Lee, Y.M. Kwon, S.Y. Jung, J.B. Lee, C.K. Ryu, J.C. Kim, Excellent thermal stability of potassium-based sorbent using ZrO2 for post combustion CO2 capture, Fuel 115 (2014) 97–100. [78] S. Lee, Y. Kwon, Y. Park, W. Lee, J. Park, C. Ryu, et al., Structure effects of potassium-based TiO2 sorbents on the CO2 capture capacity, Top. Catal. 53 (2010) 641–647. [79] S.C. Lee, H.J. Chae, S.J. Lee, Y.H. Park, C.K. Ryu, C.K. Yi, et al., Novel regenerable potassium-based dry sorbents for CO2 capture at low temperatures, J. Mol. Catal. B Enzym. 56 (2009) 179–184. [80] S.C. Lee, M.S. Cho, Y.M. Kwon, H.J. Chae, S.Y. Jung, C.K. Ryu, et al., CO2 sorption properties of nano-sized zirconia-based sorbents promoted with alkali metal carbonates, J. Nanoelectron. Optoelectron. 7 (2012) 460–465. [81] S.C. Lee, H.J. Chae, Y.M. Kwon, W.S. Lee, H.S. Nam, S.Y. Jung, et al., Characteristics of new potassium-based sorbents prepared with nano-titanium oxide for carbon dioxide capture, J. Nanoelectron. Optoelectron. 5 (2010) 212–217. [82] S. Lee, B. Choi, C. Ryu, Y. Ahn, T. Lee, J. Kim, The effect of water on the activation and the CO2 capture capacities of alkali metal-based sorbents, Korean J. Chem. Eng. 23 (2006) 374–379. [83] S. Lee, H. Chae, B. Choi, S. Jung, C. Ryu, J. Park, et al., The effect of relative humidity on CO2 capture capacity of potassium-based sorbents, Korean J. Chem. Eng. 28 (2011) 480–486. [84] S.C. Lee, Y.M. Kwon, C.Y. Ryu, H.J. Chae, D. Ragupathy, S.Y. Jung, et al., Development of new alumina-modified sorbents for CO2 sorption and regeneration at temperatures below 200 ◦ C, Fuel 90 (2011) 1465–1470.

85

[85] S.C. Lee, Y.M. Kwon, H.J. Chae, S.Y. Jung, J.B. Lee, C.K. Ryu, et al., Improving regeneration properties of potassium-based alumina sorbents for carbon dioxide capture from flue gas, Fuel 104 (2013) 882–885. [86] S. Lee, J. Kim, Dry potassium-based sorbents for CO2 capture, Catal. Surv. Asia 11 (2007) 171–185. [87] C. Zhao, X. Chen, C. Zhao, Multiple-cycles behavior of K2 CO3 /Al2 O3 for CO2 capture in a fluidized-bed reactor, Energy Fuels 24 (2010) 1009–1012. [88] J.V. Veselovskaya, V.S. Derevschikov, T.Y. Kardash, O.A. Stonkus, T.A. Trubitsina, A.G. Okunev, Direct CO2 capture from ambient air using K2 CO3 /Al2 O3 composite sorbent, Int. J. Greenh. Gas Control 17 (2013) 332–340. [89] V.S. Derevschikov, J.V. Veselovskaya, T.Y. Kardash, D.A. Trubitsyn, A.G. Okunev, Direct CO2 capture from ambient air using K2 CO3 /Y2 O3 composite sorbent, Fuel 127 (2014) 212–218. [90] S.C. Lee, H.J. Chae, S.J. Lee, B.Y. Choi, C.K. Yi, J.B. Lee, et al., Development of regenerable MgO-based sorbent promoted with K2 CO3 for CO2 capture at low Temperatures, Environ. Sci. Technol. 42 (2008) 2736–2741. [91] W. Chaikittisilp, H.-J. Kim, C.W. Jones, Mesoporous alumina-supported amines as potential steam-stable adsorbents for capturing CO2 from simulated flue gas and ambient air, Energy Fuels 25 (2011) 5528–5537. [92] S. Brunauer, P.H. Emmett, E. Teller, Adsorption of gases in multimolecular layers, J. Am. Chem. Soc. 60 (1938) 309–319. [93] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, et al., Reporting physisorption data for gas solid systems with special reference to the determination of surface-area and porosity (Recommendations 1984), Pure Appl. Chem. 57 (1985) 603–619. [94] W.W. Lukens, P. Schmidt-Winkel, D. Zhao, J. Feng, G.D. Stucky, Evaluating pore sizes in mesoporous materials: a simplified standard adsorption method and a simplified Broekhoff–de Boer method, Langmuir 15 (1999) 5403–5409. [95] J. Tóth, Uniform interpretation of gas/solid adsorption, Adv. Colloid Interface Sci. 55 (1995) 1–239. [96] K.Y. Foo, B.H. Hameed, Insights into the modeling of adsorption isotherm systems, Chem. Eng. J. 156 (2010) 2–10. [97] M.A. Alkhabbaz, P. Bollini, G.S. Foo, C. Sievers, C.W. Jones, Important roles of enthalpic and entropic contributions to CO2 capture from simulated flue gas and Ambient air using mesoporous silica grafted amines, J. Am. Chem. Soc. 136 (2014) 13170–13173. [98] S.A. Didas, A.R. Kulkarni, D.S. Sholl, C.W. Jones, Role of amine structure on carbon dioxide adsorption from ultradilute gas streams such as ambient air, ChemSusChem 5 (2012) 2058–2064. [99] T. Watabe, K. Yogo, Isotherms and isosteric heats of adsorption for CO2 in amine-functionalized mesoporous silicas, Sep. Purif. Technol. 120 (2013) 20–23. [100] L. Wang, R.T. Yang, Increasing selective CO2 adsorption on amine-grafted SBA-15 by increasing silanol density, J. Phys. Chem. C 115 (2011) 21264–21272. ˇ [101] C. Knöfel, J. Descarpentries, A. Benzaouia, V. Zelenák, S. Mornet, P.L. Llewellyn, et al., Functionalised micro-/mesoporous silica for the adsorption of carbon dioxide, Microporous Mesoporous Mater. 99 (2007) 79–85. ¨ [102] C. Knofel, C. Martin, V. Hornebecq, P.L. Llewellyn, Study of carbon dioxide adsorption on mesoporous aminopropylsilane-functionalized silica and titania combining microcalorimetry and in situ infrared spectroscopy, J. Phys. Chem. C 113 (2009) 21726–21734. [103] M.R. Mello, D. Phanon, G.Q. Silveira, P.L. Llewellyn, C.M. Ronconi, Amine-modified MCM-41 mesoporous silica for carbon dioxide capture, Microporous Mesoporous Mater. 143 (2011) 174–179. [104] F.W.M. da Silva, D.A.S. Maia, R.S. Oliveira, J.C. Moreno-Piraján, K. Sapag, C.L. Cavalcante, et al., Adsorption microcalorimetry applied to the characterisation of adsorbents for CO2 capture, Can. J. Chem. Eng. 90 (2012) 1372–1380.