Nature of Chemisorbed CO2 in Zeolite A | The Journal of Physical

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

The Nature of Chemisorbed CO in Zeolite A Przemyslaw Rzepka, Zoltan Bacsik, Andrew J. Pell, Niklas Hedin, and Aleksander Jaworski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04142 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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The Journal of Physical Chemistry

The Nature of Chemisorbed CO2 in Zeolite A Przemyslaw Rzepka† , Zoltán Bacsik, Andrew J. Pell, Niklas Hedin∗ , and Aleksander Jaworski∗

Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden ∗ Corresponding

authors: [email protected], [email protected]

† Current

address: Department of Chemistry and Applied Biosciences, ETH Z¨ urich, 8093 Z¨ urich, Switzerland

Abstract − Formation of CO2− 3 and HCO3 species without participation of the framework bridging oxygen

atoms (−O−) upon chemisorption of CO2 in zeolite |Na12 |-A is revealed. The transfer of O and H atoms is very likely to have proceeded via the involvement of residual H2 O or −OH groups. A combined study by solid-state 1 H and

13 C

MAS NMR, quantum chemical calculations, and

in situ IR spectroscopy showed that the chemisorption mainly occurred by the formation of HCO− 3. However, at a low surface coverage of physisorbed and acidic CO2 , a significant fraction of the HCO− 3 was deprotonated and transformed into CO2− 3 . We expect that similar chemisorption of CO2 would occur for low-silica zeolites and other basic silicates of interest for the capture of CO2 from gas mixtures. 1 ACS Paragon Plus Environment

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1

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Introduction

Adsorption-driven separation of CO2 from flue gas1–3 and raw biogas4, 5 are of considerable interest for anthropogenic reasons. In relation to these processes, adsorbents such as zeolites, activated carbons, porous polymers, and amine-modified silica materials are being extensively studied.1, 2, 6 It is generally found that CO2 typically adsorbs by weak intermolecular interactions (physisorption),7 but on sorbents of sufficiently high base strength chemisorption also occurs, especially when catalytic amounts of water are present.8–12 For example, on low-silica zeolites with monovalent cations, CO2 is both physisorbed and chemisorbed.13–17 In contrast to physisorption, chemisorption results in the molecular structure of CO2 being significantly altered. However, to date the mechanisms of chemisorption of CO2 on zeolites, and the nature of the species formed, are not well understood.

a

b

Figure 1: The unit cell of zeolite |Na12 |-A with single α-cavity marked (a), and (b) the model of chemisorption site used in quantum chemical calculations (terminating pseudohydrogen atoms omitted for clarity). Oxygen atoms are red, Na atoms blue, Si yellow, and Al tan.

The physisorption of CO2 on zeolites15, 18–20 is mainly due to the interaction between the relatively large molecular electric quadrupole moment of CO2 , and the significant electric field gradients near the zeolite framework. In parallel, low-silica zeolites such as zeolite X,19, 21–23 Y,14, 23, 24 and 2− A19, 22 also chemisorb CO2 resulting in the formation of HCO− 3 /CO3 species. Even though the

positioning of the chemisorbed species at the 8-membered ring of zeolite A α-cavity (Figure 1) was partially determined by Rzepka et al.,25 with in situ neutron diffraction (ND), the precise 2 ACS Paragon Plus Environment

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2− chemical nature of these HCO− 3 /CO3 species has hitherto remained unknown. It has been spec-

ulated that either CO2 reacts with the −O− bridges of the framework21–24 forming framework2− linked species,14, 19, 22, 24 or that residual H2 O or −OH groups are involved in forming HCO− 3 /CO3

species.13, 14, 24 This was the problem we undertook to address in this study, and we were able to identify the species formed on chemisorption of CO2 by using a combination of 1 H and

13 C

solid-

state nuclear magnetic resonance (NMR), density functional theory (DFT), and in situ infrared (IR) spectroscopy.

2

Experimental Methods

The two subsequent isotherms of CO2 adsorption on zeolite |Na12 |-A were recorded with a Micromeritics ASAP 2020 surface area and porosity analyzer in the regime of high vacuum−ambient pressure. The sample of zeolite |Na12 |-A was prepared in ASAP tube, evacuated for 10 h under high dynamic vacuum (0.001 µbar) at T = 623 K, backfilled to 1 bar of dry N2 at T = 323 K and weighted. Free space was evaluated with He gas, which is not adsorbed by zeolite A.26 The adsorption data were measured with precision of 99.9%) was supplied by the Linde Gas Company. Solid-state 1 H/13 C MAS NMR spectra were acquired at a magnetic field strength of 14.1 T (Larmor frequencies of 600.1 and 150.9 MHz for 1 H and III spectrometer. For the directly excited

13 C

13 C,

respectively) with a Bruker Avance

MAS NMR spectra zeolite samples were evacuated

accordingly to those for volumetric adsorption measurements and backfilled with 99%

13 C

labeled

CO2 gas supplied by Cambridge Isotope Laboratories Inc. Powders were packed into 7.0 mm zirconia rotors and spun at 5.00 kHz. The data were collected with 6.00 µs radiofrequency (rf) pulses with a spin nutation frequency of 42 kHz. On an average, 1600 signal transients per sample were accumulated with relaxation delay of 60 s, which was verified to be sufficient to provide quantitative spectra. For 1 H MAS and 1 H→13 C CPMAS NMR experiments zeolite samples were evacuated for 24 h under high dynamic vacuum (0.001 µbar) at T = 623 K and backfilled with argon. Rotors were packed in a glovebox under argon atmosphere. 1 H NMR acquisitions were performed with a 1.3 mm MAS probehead at MAS rate of 60.00 kHz and involved rotor-synchronized, double-adiabatic spinecho sequence with a 90 degree excitation pulse of 1.1 µs, followed by two 50.0 µs tanh/tan short high-power adiabatic pulses28, 29 with 5 MHz frequency sweep.30 All pulses operated at a nutation frequency of 220 kHz. 4096 signal transients with 5 s relaxation delay were accumulated for each zeolite sample, whereas 128 signal transients with 60 s relaxation delay were collected for solid NaHCO3 .

1 H→13 C

CPMAS NMR spectra were collected with 4.0 mm MAS probehead at MAS

rate of 14.00 kHz. Acquisitions involved a 2 ms contact interval and matched radio-frequency fields of νH = 19 kHz and νC = 5 kHz for 1 H and

13 C,

respectively, which obeyed the modified

4 ACS Paragon Plus Environment

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Hartmann-Hahn condition νH = νC + νr . 32768 signal transients with 4 s relaxation delay were accumulated for zeolite sample, and 128 scans with 60 s relaxation delay for solid NaHCO3 . Neat tetramethylsilane (TMS) was used for rf pulse calibrations and chemical shifts referencing for both 1H

and

3

13 C.

Computational Methods

Calculations exploited a cluster approach in order to apply accurate methods for predictions of 13 C

NMR chemical shifts of adsorbed CO2 species. A model of CO2 chemisorption site in zeolite

|Na12 |-A was created from in situ neutron diffraction data after Rzepka et al. The Na+ cations displacements upon CO2 adsorption were included in the model, although as revealed by in situ synchrotron X-ray diffraction experiments, were very minor.17 Oxygen atoms on the cluster perimeter were terminated with pseudohydrogens at the distance of 1.000 ˚ A in the direction of missing atoms. Charges of these terminating pseudohydrogens were evaluated to assure that they mimic electrostatics of the lattice beyond the broken bonds. Terminating pseudohydrogens were at least 4 bonds apart from the carbon atom of NMR interest, therefore its influence on the

13 C

NMR chem-

ical shift (which is a local property for closed-shell systems) was negligible. Atomic coordinates of the model are provided in the SI. All calculations were performed with the ORCA code31 using tight convergence tolerance of 10−8 Hartree. Atomic coordinates of the modeled chemisorbed CO2 species were optimized without constraints, whereas positions of zeolite framework atoms were not allowed to change. Geometry optimizations employed OLYP exchange-correlation DFT approximation32, 33 with D3(BJ) dispersion correction34, 35 and cc-pVTZ basis set on all atoms.36, 37 13 C

NMR shielding tensors were calculated with the GIAO approach38 at the level of the

perturbatively-corrected, double-hybrid DFT within DSD-PBEP86 approximation,39 and the



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second-order Møller-Plesset perturbation theory (MP2).40 Test results for different DFT approximations are provided in the SI. DFT calculations employed pcSseg-2 basis set on the carbon atom, pcseg-2 on the directly bonded oxygen atoms, and pcseg-1 on the rest of atoms.41 In analogy, for MP2 calculations the suite of aug-cc-pVTZ-J/cc-pVTZ/cc-pVDZ basis sets was employed.42 This approach for calculations of NMR shielding was proven robust,41, 43 and allowed to reduce the computational efforts associated with the second-order property calculations at the MP2 level. All electrons were included in the correlation (no frozen core). RIJCOSX approximation was used throughout with large auxiliary basis sets and accurate grid settings tested that errors introduced were found to be negligible. For the given level of theory the calculated

13 C

isotropic shieldings

(σiso ) were converted into isotropic NMR chemical shifts (δiso ) using the CO2 molecule in the gas phase as a secondary reference at δiso = 125.0 ppm.

4 4.1

Results and Discussion Volumetric Adsorption

The fraction of CO2 chemisorbed by the system in this study, zeolite |Na12 |-A, was assessed indirectly by volumetric adsorption measurements17, 44 of the first two adsorption-desorption cycles, as shown in Figure 2. A reduction in adsorption capacity during the second cycle relative to the first was observed, which was ascribed to the fraction of CO2 that had been irreversibly chemisorbed, since the large majority of chemisorbed species cannot be removed by evacuation under high dynamic vacuum. A long evacuation was used to remove fractionally entrapped CO2 before the second cycle,45–47 and it was found that approximately 6% of the adsorbed CO2 on zeolite |Na12 |-A was chemisorbed, corresponding to about 0.5 molecules per α-cage.


8 4 6 3 4

2

1st cycle 2nd cycle 2-site Langmuir model

1

2 0

0 0

200

400

600

800

1000

CO2 molecules per cage

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


CO2 quantity (mmol/g)

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Absolute pressure (mbar)

Figure 2: The quantity of adsorbed CO2 vs pressure on zeolite |Na12 |-A at 293 K for a pristine sample (1st cycle) and after 12 h of re-evacuation under high dynamic vacuum (2nd cycle). The data conform to the two-site Langmuir model.

4.2

Solid-State NMR

To identify the chemisorbed species, their quantities, and to gain insight into their dynamics, we employed

13 C

solid-state magic-angle spinning (MAS) NMR spectroscopy. This technique has

been recently and successfully used to investigate the details of chemisorbed CO2 in amine-modified clays9 and silicas,10 metal organic frameworks,11, 48 polymers,49 and layered hydroxides.50 However, 13 C

MAS NMR spectroscopy has to date not been used to study the chemisorption of CO2 on

zeolites. We employed a direct excitation (Bloch-decay) NMR protocol with 99%

13 C-enriched

CO2 to ensure a full visibility of the adsorbed species and quantitative character of the collected data. The

13 C

NMR spectrum at 5 kHz MAS of zeolite |Na12 |-A with 1 bar of CO2 is shown in Fig-

ure 3(a). Resonances with chemical shifts characteristic of both physisorbed and chemisorbed CO2 were detected. The signals with isotropic shifts at 125 and 162 ppm correspond to physisorbed CO2 ,51, 52 and HCO− 3 , respectively. The H-transfer was likely to have occurred with the involvement of H2 O or −OH groups.53 No CO2− 3 was observed under these conditions. The fraction of chemisorbed species as determined from integrating the

13 C

MAS NMR resonances was 0.06 at 1

bar of CO2 , which is in an excellent agreement with the fraction estimated from the volumetric adsorption data.



a

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b

Physisorption

iso = 125 ppm

DSD-PBEP86 pcSseg-2 MP2 aug-cc-pVTZ-J

Chemisorption 162

Backlled with 13CO2

165 172

*

c

Na Si

Al

2.96 Å

C

2.49

O

iso = 124.8 iso = 124.7

*

3.73

*

2.46

Spectra zoomed 20x

d Backlled with 13CO2

2.00

iso = 164.5 iso = 165.1

*

2.80 2.35 2.30

3.36 2.48

3.82

* *

*

*

*

*

Evacuated at 298K Backlled with N2

e

*

4.06 2.23

* 250

*

*

Evacuated at 353K Backlled with N2

*

200 150 100 13 C chemical shift (ppm)

iso = 173.2 iso = 174.2

3.81

2.32

2.41 2.52

50

Figure 3: (a) 13 C MAS NMR spectra of CO2 adsorbed in zeolite |Na12 |-A collected at 14.1 T and 5 kHz MAS rate. The spinning sidebands are marked with asterisks. (b) The α-cavity of zeolite 2− |Na12 |-A; fragments of the models of CO2 (c), HCO− 3 (d), and CO3 (e) species inside the α-cavity resulting from geometry optimization at the OLYP-D3(BJ)/cc-pVTZ level of theory. 13 C isotropic chemical shifts calculated at the DSD-PBEP86/pcSseg-2 (black) and MP2/aug-cc-pVTZ-J (grey) level of theory.


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The bottom two solid-state

13 C

MAS NMR spectra of Figure 3(a) were collected from zeolite

|Na12 |-A following saturation with 1 bar of CO2 , subjected to evacuation under high dynamic vacuum for 12 h at the respective temperatures of 298 and 353 K, and backfilled with 1 bar of N2 . In both these cases, the intensity of the signal with isotropic shift of 125 ppm was reduced substantially, indicating that very little physisorbed CO2 remained, and the resonances at 162−172 ppm originate exclusively from chemisorbed species. The signals with isotropic shifts at 165 and 162 ppm were hypothesized to correspond to two different types of HCO− 3 , whereas that at 172 ppm was attributed to CO2− 3 . Similar assignments were reported by Ishihara et al. for the adsorption of CO2 on hydrotalcite. The relative fraction of the CO2− 3 , to the full amount of chemisorbed CO2 , was 0.06 for the system evacuated at 298 K, and 0.11 for that evacuated at 353 K. The increased relative fraction of CO2− 3 was consistent with a heat- and vacuum-enhanced desorption of CO2 or H2 O, and in line with the in situ IR spectroscopy observations reported in the IR section. Concurrently with the elevated desorption temperature of CO2 , there were alterations towards higher populations of both carbonates (172 ppm), and bicarbonate species with a shift of 165 ppm, at the expense of the bicarbonate species with an isotropic shift of 162 ppm. To corroborate the NMR assignments and derive molecular representations for the adsorption of CO2 , we calculated the

13 C

NMR chemical shifts on DFT-derived models. The corresponding

molecular representations and calculated chemical shifts are presented in Figure 3(b−e). The 2− energy-optimized positions of the CO2 , HCO− 3 , and CO3 were in accordance to those refined from

ND data by Rzepka et al. At the dispersion-corrected OLYP-D3(BJ)/cc-pVTZ level of theory used in the optimization of the DFT models, the potential energy surface did not reveal an energy minimum that would correspond to CO2 binding with framework −O− bridge. Several starting positions were tested. The calculations indicate that the distance between the C atom of HCO− 3 and the framework O atom is 3.36 ˚ A (Figure 3(d)), which is in a good agreement with the value



Page 10 of 29

of 3.3 ˚ A from the previous ND refinement of the chemisorbed CO2 positions performed for the same system.25 At this distance, we could infer that there is no participation of the framework O-atom in covalent bonding with a HCO− 3 moiety. The same conclusion can be reached for the CO2− group (Figure 3(e)), where the C is 3.81 ˚ A from the framework O. Hence, in the light 3 2− of these results, we concluded that both the HCO− 3 and CO3 form as self-standing entities in

the zeolite α-cavity without participation of framework −O− bridge. For

13 C

chemical shifts

assignments the perturbatively-corrected DSD-PBEP86 DFT approximation, as well as the secondorder Møller-Plesset perturbation theory (MP2) were employed. These are currently the most accurate (affordable) methods for NMR shielding tensors calculations.54–56 We observed that the chemical shifts calculated on our models at both DSD-PBEP86 and MP2 level of theory were in excellent agreement with those measured experimentally. In particular, we note that the shifts in 2− Figure 3(d,e) of 164.5−165.1 and 173.2−174.2 ppm were due to HCO− 3 and CO3 ions, respectively,

and not to physisorbed CO2 , which gave a lower shift of 124.7−124.8 ppm (Figure 3(c)).

To provide experimental evidence for bicarbonate formation upon chemisorption of CO2 on zeolite |Na12 |-A, fast-MAS 1 H and 1 H→13 C cross-polarization (CP) MAS NMR experiments were performed. Here we note that traditional CPMAS approaches have very recently been shown to 12 be problematic in studies of HCO− 3 chemisorbed onto amine-modified mesoporous silica sorbents.

In Figure 4(a), high-resolution (60 kHz MAS) 1 H NMR spectra of activated zeolite |Na12 |-A (dried under high dynamic vacuum at 623 K for 24 h), as well as the zeolite containing chemisorbed CO2 are presented. The activated zeolite contained non-negligible amounts of H2 O and −OH groups, as was indicated by several overlapping 1 H resonances in the 0−8 ppm range. Upon chemisorption of CO2 signals at 12 and 16 ppm emerge at the expense of those from −OH groups and residual H2 O. These resonances were assigned to bicarbonate species, as their chemical shifts are in good agreement with the shift of 14 ppm from pure, solid NaHCO3 . Furthermore, the formation 10 ACS Paragon Plus Environment

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a Evacuated at 623K Equilibrated with 1 bar 13CO2 Evacuated at 293K Backlled with Ar

12

16 14 Solid NaHCO3 40

30

20 1

0

10

-10

-20

-30

H chemical shift (ppm) 162

b

Equilibrated with 1 bar 13CO2 Evacuated at 293K Backlled with Ar

165

166 Solid NaHCO3 180

170 13

160

150

140

C chemical shift (ppm)

Figure 4: (a) 1 H MAS NMR spectra of activated zeolite |Na12 |-A (black), zeolite |Na12 |-A equilibrated with 1 bar of 13 CO2 and backfilled with Ar (red), shown together with that of solid NaHCO3 (grey). The data collected at 14.1 T and 60 kHz MAS rate. (b) 1 H→13 C CPMAS spectrum of zeolite |Na12 |-A equilibrated with 1 bar of 13 CO2 and backfilled with Ar (black), and that of solid NaHCO3 (grey); both spectra acquired at 14.1 T and 14 kHz MAS rate.


c 3000

1600

2360

1250

1455

2344

3434

3500

b

3284

3635

3250

a

1385

3150

1408

1365

1726

CO2 (g)

Absorbance

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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1694 1624


2500

2000

1500

Wavenumber (cm-1)

Figure 5: In situ IR spectra of zeolite |Na12 |-A: under 1 bar of CO2 (a), 0.00003 bar of CO2 (b), and after loading with 1 bar of CO2 and evacuation under high dynamic vacuum for 4 h (c). Presented spectra were obtained using a background spectrum of zeolite activated at 523 K. of bicarbonate was unambiguously evidenced by 1 H→13 C CPMAS spectra in Figure 4(b).

13 C

chemical shifts of 162 and 165 ppm were close to that of 166 ppm from solid NaHCO3 , and same as directly-excited counterparts in Fig. 3(a), indicating two distinct bicarbonate environments, in line with 1 H spectrum in Figure 4(a). Note that in the directly-excited

13 C

MAS NMR spectra of the

evacuated samples (Figure 3(a); bottom traces) signal with isotropic shift of 172 ppm was observed. The absence of this resonance in 1 H→13 C CPMAS spectrum corroborates the assignment to CO2− 3 .

4.3

In situ IR Spectroscopy

We also studied the chemisorption of CO2 in zeolite |Na12 |-A with in situ IR spectroscopy. For zeolites with relatively high basicity, IR spectral features of chemisorbed CO2 are usually observed by C−O stretching bands in the range of 1800−1200 cm−1 , and typically assigned to carbonatelike species.13, 14, 24, 57, 58 However, the bands in this spectral region are quite complex and not always straightforward to interpret. The symmetrical (free) carbonate ion has a single band around


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1415 cm−1 , but upon lowering the symmetry, this double degenerated vibration splits into two distinct IR bands for the asymmetric stretching mode. The extent of this split has been used for identification of chemisorbed species,59 and for zeolite 4A these band pairs have been assigned to monodentate, bidentate, and chelating coordinated carbonate species.57 Although our IR spectra shown in Figure 5 are very similar to those reported by Montanari et al., the presence of bicarbonate cannot be excluded, because of the same nature of bands split for asymmetric C−O stretches of bicarbonate and these carbonate species. The band pair at 1726 and 1250 cm−1 originate from labile carbonate or bent CO2 species that are present only at higher CO2 pressures (Figure 5(a)). The overlapping band pairs around 1624 and 1365 cm−1 can either be assigned to bidentate carbonate species or to HCO− 3 . The bicarbonate formation is suggested by a broad H-bonded −OH group band around 3250 cm−1 at low partial pressures of CO2 (Figure 5(b)) that appear in parallel with the C−O bands at 1600 and 1385 cm−1 . Moreover, a negative intensities are observed in the −OH stretching region around 3635 cm−1 , which compares well with 3608 cm−1 reported for residual −OH groups in activated zeolite A by Montanari et al. This may indicate that −OH groups are “consumed” and the new band result from −OH vibrations in bicarbonate moiety. This effect is enhanced in the spectrum of evacuated zeolite, where negative intensities around 3635 cm−1 are accompanied by two distinct bands of H-bonded −OH groups at 3434 and 3284 cm−1 (Figure 5(c)). The appearance of these two bands is in agreement with 1 H MAS NMR spectra of Figure 4(a). At low CO2 pressure, and after evacuation (Figure 5(b,c)), there is a single band of the antisymmetric stretching mode at 1455 cm−1 observed, which we attribute to CO2− 3 . This band is a high frequency component of the band pair with its lower frequency counterpart appearing at 1392 cm−1 , beneath the band at 1385 cm−1 . The small split of 63 cm−1 (obtained from the spectrum of activated zeolite; see Figure S2(d)) suggests a relatively small break in the symmetry of these carbonate species. Note that the intensity ratios of these carbonates are not fully correct in the spectra of



Page 14 of 29

Figure 5(b,c), since these species are already present in activated zeolite (Fig. S2). The negative band at 1455 cm−1 in Figure 5(a) indicates that these symmetric carbonate species transform to other forms at high CO2 coverage. The transformations of different carbonate-like species upon CO2 pressure/coverage changes i.e. symmetric carbonates at low, and less symmetric species at high have also been observed by others on zeolite A57 and zeolite X.13

5

Conclusions

To conclude, it is established that the chemisorption of CO2 in low-silica zeolites involves basic sites (conjugate acid-base pairs).60–63 However, in this study we showed for the first time that carbonates did not integrate with the framework −O− bridges of zeolite |Na12 |-A. We also showed that two 2− different types of HCO− 3 formed and that CO3 occurred at very small pressures of CO2 . As the 12 it was consistent that CO2− was formation of CO2− 3 is favored only in highly basic environment, 3

stabilized at low concentrations of CO2 on the zeolite |Na12 |-A. We expect that similar equilibria among different coexisting chemisorbed species and their dependencies on the temperature and overall CO2 surface coverage to be observed for other zeolites and basic (alumino)silicates in general.

Acknowledgements The research was financially supported by the Swedish Research Council (VR), grant numbers: 2016-03441 (A.J.P. and A.J.) and 2016-03568 (N.H., P.R., and Z.B.). A.J. would like to thank Prof. Juha Vaara for fruitful discussions. Supporting Information Available: parameters of the two-site Langmuir model, 1 H MAS NMR spectrum of zeolite |Na12 |-A exposed to moisture, tests with different DFT approximations, atomic coordinates of the model used in quantum chemical calculations, in situ IR spectra with an empty cell as a background. This material is available free of charge via the internet at http://pubs.acs.org. 14 ACS Paragon Plus Environment

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Amine-Modified Nanoporous Materials Studied by Solid-State NMR and

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13 CO 2

adsorption.

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TOC Graphic



a

b

ACS Paragon Plus Environment

Page 24 of 29

8 4 6 3 4

2

1st cycle 2nd cycle 2-site Langmuir model

1

2 0

0 0

200

400

600

800

Absolute pressure (mbar)


1000

CO2 molecules per cage

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


CO2 quantity (mmol/g)

Page 25 of 29


a

Page 26 of 29

b

Physisorption δiso = 125 ppm

DSD-PBEP86 pcSseg-2 MP2 aug-cc-pVTZ-J

Chemisorption 162 Backfilled with 13CO2

165 172

c

*

Na Si

Al

2.96 Å

C

2.49

O

δiso = 124.8

*

δiso = 124.7

3.73

*

2.46

Spectra zoomed 20x

d Backfilled with 13CO2

2.80 2.00

δiso = 164.5

2.30

3.36

δiso = 165.1

*

2.35

2.48

3.82

* *

*

*

*

*

Evacuated at 298K Backfilled with N2

e

*

4.06 2.23

* 250

*

*

Evacuated at 353K Backfilled with N2

*

200 150 100 13 C chemical shift (ppm)

3.81

δiso = 173.2 δiso = 174.2

2.32

2.41 2.52

50


Page 27 of 29 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


a Evacuated at 623K Equilibrated with 1 bar 13CO2 Evacuated at 293K Backfilled with Ar

12

16 14 Solid NaHCO3 40

30

20 1

0

10

-10

-20

-30

H chemical shift (ppm) 162

b

Equilibrated with 1 bar 13CO2 Evacuated at 293K Backfilled with Ar

165

166 Solid NaHCO3 180

170 13

160

150

C chemical shift (ppm)


140

3434

c 3000

2500

2000

Wavenumber (cm-1)


1500

1250

1455 1385

1600

2360

b

2344

3250

3150

a

3284

3635

3500

1408

1365

1726

CO2 (g)

Absorbance

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

Page 28 of 29

1694 1624


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381x232mm (300 x 300 DPI)


Nature of Chemisorbed CO2 in Zeolite A | The Journal of Physical

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