The Thermodynamics of H2O and CO2 Absorption and Guest-Induced

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The Thermodynamics of HO and CO Absorption and Guest-Induced Phase Transitions in Zeolite RHO Xin Guo, David R. Corbin, and Alexandra Navrotsky J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06070 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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The Journal of Physical Chemistry

The Thermodynamics of H2O and CO2 Absorption and Guest-induced Phase Transitions in Zeolite RHO Xin Guo†, David R. Corbin‡ and Alexandra Navrotsky†* † Peter A. Rock Thermochemistry Laboratory, NEAT-ORU, University of California Davis, Davis, CA 95616 ‡ Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, KS 66047, USA.

*

Corresponding author email address:

[email protected] (Alexandra Navrotsky) Phone: (530) 752-3292 Fax: (530) 752-9307

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ABSTRACT Gas absorption calorimetry at low pressures has been employed to probe the interaction of water and carbon dioxide with several ion-exchanged zeolite RHO samples. It reveals guestinduced framework flexibility transitions from a less hydrated (initially anhydrous) acentric form to the more hydrated centric phase during water absorption. The differential enthalpy of absorption as a function of water loading directly identifies strengths of different interactions along with possible water binding mechanisms. Interactions with CO2 are weaker and no phase transition is seen except in Li,H-RHO. The most negative initial enthalpies have been obtained in both water and CO2 absorption for Cd and Cs-RHO. However, Li,H-RHO shows the strongest capture ability for CO2 despite a less exothermic initial enthalpy of absorption. The derived differential enthalpy, chemical potential and entropy elucidate the thermodynamic behavior of multiple interactions of small guest molecules (H2O and CO2) and ion-exchanged flexible zeolite frameworks.


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INTRODUCTION Zeolite RHO (Si/Al = 3) represents a family of highly porous aluminosilicates with an open framework structure of alpha cages uniformly linked by double eight membered rings (D8R).1-5 The body centered cubic structure of truncated cubo-octahedra secondary building units (SBU) give rise to two interpenetrating but not interconnected pore systems.6-9 There are three open windows of eight-membered rings (8R) connecting the alpha cages of six membered rings (6R) and four-membered rings (4R) which are too small for guest molecules to pass through their openings.9-13 The larger aperture of the 8R allows passage of small molecules.9 Pore accessibility varies with the type/distribution of extra-framework cations and framework flexibility.14-16 The charge-balancing extra-framework cations are preferentially distributed over three sites depending on their ionic radii and distinct coordination environment: the single 8-ring (8R), double 8-ring (D8R) and single 6-ring (6R), as shown in Figure 1. The RHO framework exhibits exceptional flexibility with particular sensitivity to cations, temperature, and hydration.17-20 Structural studies have shown that the RHO framework undergoes distortion and loss of symmetry upon dehydration.21-23 For instance, the centrosymmetric Im3തm structure in hydrated Na,Cs-RHO gives rise to the noncentrosymmetric I4ത3m upon dehydration.10, 24 Upon heating Cd-RHO, Cd2+ cations relocate to open the trapdoor accessible to the pores.7 The flexible RHO framework can accommodate a distortion of 8-rings from circular (3.6 Å) to elliptical (2.9 Å) with a contraction driven by relocation of cations and symmetry change.17, 25 This flexibility in provides the potential to introduce a high degree of molecular sieving ability with shape and size selectivity by controlled cation siting at reaction temperatures when zeolite RHO is used as a catalyst, support, or absorbent.



Zeolite RHO exhibits structural distortions and transformations induced by the absorption of guest molecules. RHO is as an excellent CO2 sorbent. At low pressures (< 100 kPa), the CO2 coordinates with cations in the 8R window regions and displaces them away from the pore aperture, opening access to the cavities.3 With increasing CO2 pressure, stronger modification of the structure occurs and a slow phase transition from the initial acentric to the centric polymorph is observed.12 Variable amounts of absorbed H2O can also modify the acidity and size of the pore system in a very responsive way.26-30 The hydrated framework exhibits smaller conformational distortion than the dehydrated structure.23 Due to the hydrophilicity of zeolite RHO, H2O content significantly affects stability.31-33 Despite these extensive structural studies, the energetics associated with CO2 and H2O absorption and the resulting “gate opening” (flexibility) phase transitions have not been studied quantitatively. The evolution of structure with varying degrees of hydration is also not well known. We stress that the hydration process which we call water absorption occurs throughout the structure in the pores rather than being adsorption on external surfaces. In this work, we employed gas absorption calorimetry to probe several cation exchanged zeolite RHO materials to determine the energetics of H2O and CO2 absorption as a function of loading from near zero coverage to saturation, to identify different strengths of interactions for guest molecules at various positions (cages and sites) along with binding mechanisms, and to study the energetics of possible structural phase transitions. An FTIR spectral analysis of hydrated zeolite RHO with different exchanged cations was also conducted.


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Figure 1. Schematic crystal structure of zeolite RHO showing framework T (Si, Al in blue) and O (red) atoms and illustrating cations in D8R (purple, Site I), 8R (green, Site II) and S6R (yellow, Site III) environments. D8R and 8R cations block the windows connecting cages.

EXPERIMENTAL METHODS Materials. These RHO samples are part of a suite of materials originating from the Dr. David R. Corbin Zeolite Library at the Center for Environmentally Beneficial Catalysis at the University of Kansas, Lawrence KS. Na,Cs-RHO, the parent material used for cation-exchange was prepared by a modification of the method described by Robson.34 Cd,Cs-RHO was prepared using standard ion-exchange in aqueous 10 % Cd(NO3)2 at 90 °C. The sample of Li,NH4-RHO was prepared by conventional NH4+ exchange followed by Li+ exchange of Na,Cs-RHO. A sample of Li,H-RHO was obtained by heating Li,NH4-RHO to 400 °C overnight under vacuum. Chemical analysis gave a unit cell composition of Cd3.6Cs4.8Na0.4Al12.0Si36.0O96 (Cd,Cs-RHO), Li4.98(NH4)5.65Cs0.08Na0.021Al11.9Si36.1O96 (Li,NH4-RHO), Na7.1Cs3.8Al11.5Si36.5O96 (Na,Cs-RHO), ,Li4.98H5.646Cs0.08Na0.021Al11.9Si36.1O96 (Li,H-RHO) respectively.



Characterization.

Powder X-ray diffraction(XRD) patterns were obtained using a

Bruker AXS D8 Advance diffractometer (Bruker, Madison, WI) operated with Cu Kα radiation (λ = 1.54 Å). The data were collected in the 2θ range of 5 to 70 °, with a step size of 0.02 ° and dwell time of 1 s per step. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) spectra were recorded using a Bruker VERTEX 70 IR spectrometer (with parameters: angle of incidence = 45 °, depth of penetration = 1.66 µm, 4 cm−1 resolution, spectral range of 100 − 4000 cm−1). An initial background scan was collected before sampling in order to subtract the contribution from the ambient environment. Calorimetry. Enthalpy measurements were performed by gas absorption calorimetry at 25 °C using a commercial gas dosing system (Micromeritics ASAP 2020) coupled to a Calvet microcalorimeter (Setaram Sensys) described previously.35 The sample (∼25 mg) was placed in one prong of a customized silica glass forked tube while the other prong remained empty as a reference. The prongs of this tube were jacketed in the twin chambers of the calorimeter with the head opening connected to the gas dosing manifold. The sample was degassed under vacuum (< 10−3 torr) overnight at 400 °C to remove any initial adsorbates. The gas dosing system was programmed in incremental dosing mode (10 µmol per dose). The absorption isotherm along with associated heat effects were recorded simultaneously. To provide and measure each water dose, the manifold was first dosed to a certain pressure with thermal equilibration and then the valve to the sample tube was opened to allow a dose of gas to the sample. When the new equilibrium pressure was attained, the difference in pressure between the previous and latter equilibrium was measured and used to calculate the amount of gas absorbed. This was controlled automatically by the Micromeritics software. The differential enthalpy of absorption, ∆Habs (kJ per mole of H2O), of each dose was calculated by dividing the observed heat effect by the 6 ACS Paragon Plus Environment

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amount of gas absorbed. All absorption calorimetry experiments were repeated at least once. A baseline was obtained with the empty tube under the same conditions.

RESULTS AND DISCUSSION 1. H2O Absorption

The chemical composition and structural parameters before and after gas absorption, obtained by Rietveld refinement, are listed in Table 1.Diffraction patterns are shown in Figure 2. The refinement was initiated using the starting structural parameters previously reported2 and accurate lattice parameters were achieved after completing the fitting refinement. The uncertainty can be evaluated by residual error of fit which is around 5% in our fitting. Both the initial hydrated form and the samples after dehydration followed by rehydration after gas absorption (H2O and CO2 sorption) adopt the centrosymmetric structure Im3m except for hydrated zeolite Cd,Cs-RHO although the loss of water leads to the noncentrosymmetric form. The framework structure is retained after several cycles of gas absorption. Several minor diffraction peaks appear in the Na,Cs-RHO after gas absorption, possibly owing to the remaining CO2 molecules coordinated to extra-framework cations. In Cd,Cs-RHO, there is no obvious difference in peak position but the peak becomes broader and less intense, which might be associated with larger residual lattice strain or distortion in the sample after cycles of degassing and gas absorption. Li,NH4-RHO shows the biggest difference in PXRD before degassing and



after gas absorption. It is possible that the splitting of the peaks or possible doubling of the unit cell found in the XRD pattern2 before degassing is due to partial redistribution of NH4 cations from 8R to D8R. Deammoniation is probably complete with only pure Li,H-RHO after the degassing and rehydration and no peak splitting is seen. The lattice parameter varies from hydrated Na,Cs-RHO to Li-RHO and there is almost no lattice parameter change after gas absorption for Na,Cs-RHO and Cd,Cs-RHO but the slight broadening of the peaks might come from the retained deformation strain of the zeolite framework during the absorption cycles. Li,NH4-RHO, however, has a large lattice constant change from 15.011(4) Å in the hydrated form to 14.676(3) Å in the rehydrated Li,H-RHO after deammoniation, leading to the shift of peak positions..The intensity ratio of peak (411) to (420) decreases from 100:45 in Na,Cs-RHO to 100 : 66 in Cd,Cs-RHO and 100:90 in Li,H-RHO.

Figure 2. Powder X-ray diffraction patterns of zeolite RHO before and after gas absorption at 25 oC.


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The IR spectra of all the compositions before and after gas absorption are shown in Figure 3. Extra-framework cations occupy three crystallographic sites,1 site I lying in the center of the D8R, site II close to the window plane of the 8R and site III sitting in the center of the 6R and recessed into the large alpha-cage cavity.19 The broad peaks near 3300 cm–1 can be assigned to stretching vibrations of O-H bonds .The O-H peaks for both Li,M-RHO (M = NH4+ or H+) and Cd,Cs-RHO occur at a lower wavenumber than those of Na,Cs-RHO.

This suggests that

chemisorbed water is dominant in Na,Cs-RHO but there are more weakly bonded water molecules in Li,M-RHO and Cd,Cs-RHO. Although the peaks for N-H stretching vibrations are possibly immersed in the O-H band and not discernable, there is an additional weak band around 1443 cm-1 that is only observed in Li,NH4-RHO which could be N-H bending. After degassing and gas absorption, this peak disappears, possibly due to deammonation. The peaks at 1018 cm–1 and 799 cm–1 in Li,NH4-RHO can be assigned to asymmetric stretching vibrations and symmetric stretching vibrations of bridge bonds in Si-O-Al linkages. The 718 cm–1 band may represent symmetric stretching vibrations of 8R while peaks at 586 and 413 cm–1 are bending vibrations of bridging O-Si-O bonds and bending vibrations of bridging O-Al-O bonds. It is possible that different distortions of the window 8R region due to interactions with cations sitting in various sites split the complex stretching vibrations of D8R. Similar situations are observed in Na,Cs-RHO and Cd,Cs-RHO except for a small shift to higher wavenumber as a result of smaller framework deformation by coordination to larger cations. In the far IR region, two bands appear at 296 and 214 cm–1 which may be associated with characteristic ring vibrations in the presence of M–O bonding.



Figure 3. FTIR spectra of zeolite RHO before and after gas absorption in the range of 4000-100 cm-1 at 25 oC. The water absorption isotherms of Na,Cs-RHO and the associated curves of differential enthalpy of absorption (DH), chemical potential (Dm) and differential entropy (DS) are presented in Fig. 4. The differential absorption enthalpy curve shows clear evidence of stepped interactions, consistent with a phase transition, as a function of water loading: The strongest affinity is at zero coverage with most negative differential enthalpy of adsorption, −108.27 ± 2.46 kJ/mol (see Table 2), followed by moderate absorption and a short plateau, and finally by gradually decreasing weak interaction. Water molecules prefer to occupy three sites aligned with the extra-framework cations, site I D8R and site II 8R within the window region and site III within the alpha cages around 6R.19 These sites offer water strong coordination and better confined environment. At low pressures < 0.05 P/Po, water molecules first adhere to the Na+ near the 6R walls of the alpha cage, and exhibit a local motion around ions with a negligible mobility.36 A few molecules also 10 ACS Paragon Plus Environment

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enter the window regions to interact with both the Cs+ ions at the center of D8R and Na+ in the center of 8R. The low diffusion rate of water at low pressure is confirmed by the observation that a much longer time is needed for the calorimetric curves to return to the initial baseline signal upon initial absorption. At higher pressures, water continues to fill the void space of the alpha cages and the central supercage, first the coordination sphere near the cations on both window and wall regions and then the center of the cages. Water molecules experience less interaction with cations solvated by hydration shells, corresponding to less negative enthalpies below 0.4 mol/mol TO2 than the initial absorption as shown in Figure 4B, and can move relatively freely in the large cavity.36 The increasing presence of mobile water promotes the mobility of cations.36 Before the water absorption experiment on the dehydrated sample, there is a large distortion of the 8MR when just one cation is present. This favors the stability of the acentric structure I43m .23 Upon increasing water molecules in the vicinity of cations, a gradual decrease of the distortion of 8MR induces a gradual opening of the window and the redistribution of Na ions from site II to III is facilitated by attractive forces due to the preferential adsorption of water in the vicinity of site II. The acentric structure is stable up to 0.4 mol/mol TO2 at 0.1 P/Po, see Figure 4. The TO2 here indicates the formula per mol of tetrahedra. Above that water content, there is a gradual phase transition with the addition of water, leading eventually to the experimentally observed centrosymmetric form Im3m . Further increase of water vapor pressure completes the phase transition, resulting in the pure Im3m zeolite RHO with a nearly circular pore and a cage-like structured water cluster forms within the alpha cages.37 When approaching the saturation pressure, the energetic plateau in Figure 4B implies that physisorption dominates by forming multilayered structure of absorbed water, and steric hindrance diminishes mobility of water molecules due to limited space available for motion.36



The chemical potential first increases and comes to an inflection at 0.18 mol H2O /mol TO2 in Figure 4C. The decreased chemical potential reflects the decreased P/Po at that point, which is due to the “gate opening” of the window region (cation relocations from D8R to 8R or from 8R to 6R6) in the framework requiring more gas molecules to establish the equilibrium and the same amount of dosing results in a lower equilibrated pressure with a smaller chemical potential. The unit cell expansion is associated with a depopulation of the 8R-site and occupancy of the 6R, suggesting a movement of the cations, which would open the pore space to absorption2.


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Figure 4. (A) H2O absorption isotherms, (B) corresponding differential enthalpies, (C) differential Gibbs free energies (chemical potential), and (D) differential entropies of H2O absorption curves at 25 oC in Na,Cs-zeolite RHO. The minor difference between the first run and second run is within experimental error. The differential entropy has the lowest value at near-zero coverage, indicating strong bonding and possible ordering of initially absorbed water molecules, probably coordinated with the extra-framework cations at 6R and 8R. The entropy increase with more water absorption is counteracted by the structural evolution during phase transition, accompanied by decreased slope and reaching a short plateau at 0.2 mol H2O /mol TO2 in Figure 4D, The relocation of cations 13 ACS Paragon Plus Environment


along with the gradually relieved framework distortion then occurs with increasing adjacent water molecules, leading to a balance between increasing disordering of water molecules and an ordered arrangement of the flexible framework at 0.4 mol H2O /mol TO2. At the final stage, entropy becomes much less negative with increasing absorption and reflects weak ordering for the bulk water molecules. The second and third cycles exhibit almost the same absorption process on the phase with the gate-opened structure after transition. The difference of the chemical potential from the first cycle in the initial absorption stage suggests that sample probably had severer structural deformation upon the same dehydration process due to lower initial hydration at ambient atmosphere. In the first-run isotherm of Cd,Cs- RHO shown in Figure 5A, there is a gentle linear increase before a sudden steep rise at 0.65 P/Po and corresponding 0.2 mol H2O/mol TO2 . The first run is obviously different from the second and third runs. After initial degassing, the sample lost water and showed stronger deformation of the acentric framework with high ellipticity and small apertures. It is possible that the water molecules first approach and coordinate to trapdoor cations in the window regions. At low pressure, they accumulate slowly around the cations by cooperative attractions in the window region and result in higher equilibrium pressure at the same dosing. The coordination of water molecules gradually relieves the strained framework of elliptical 8MR and mediates the cation relocation, resulting in gate opening of the window and making the pore space accessible to additional absorption at relatively high vapor pressure about 0.65 P/Po, see Figure 5. From the curve of differential enthalpy of absorption, there seem to be two plateaus, suggesting two processes of phase transition, including the relocation of Cd2+ from the 8R to the 6R site and possibly Cs+ from the D8R to the 8R, with a transformation of the acentric to the centric form.7 14 ACS Paragon Plus Environment

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Figure 5. (A) H2O absorption isotherms, (B) differential enthalpies, (C) differential Gibbs free energies (chemical potential), and (D) differential entropies of H2O absorption curves at 25 oC in Cd,Cs-zeolite RHO. The chemical potential in the first cycle reaches a plateau at around 0.2 mol/mol TO2, see Figure 5C, indicative of the beginning of phase transition. The most negative enthalpy (-163.74 ± 4.63 kJ/mol) occurs near zero coverage. The differential enthalpy near -70 kJ/mol suggests a similar interaction of water molecules before and after relocation during the phase transition. It then becomes much less negative at 0.6 mol/mol TO2, suggesting weak interaction, perhaps dominated by hydrogen bonding between water molecules. The entropy of the first cycle also is 15 ACS Paragon Plus Environment


consistent with phase transition with quickly increasing disorder from near-zero coverage to 0.2 mol/mol TO2, see Figure 5D. A flattened region represents a well- ordered arrangement of water molecules during the phase transition until there is increasing disorder in the physisorption region. The second and third cycles, however, represent water absorption on the phase with a gate-opened structure, i.e with high distortion of the acentric framework but with relocated cations. Near zero coverage, water molecules first absorb preferentially near cations at site III (6R) rather than at site II (8R), which is the location of the extra-framework Cd2+ ions in the anhydrous phase. The relatively constant enthalpy of water absorption is consistent with chemisorption onto the cations and the framework surface in the window region (8R). Near 0.6 mol/mol TO2, the additional filling of the large cage and cation hydration proceed with less negative enthalpy. The chemical potential of the second cycle shows no big difference from near-zero to 0.4 mol/mol TO2 while that for the third cycle indicates a more open framework with absorption of water to more available cation sites, producing more negative chemical potential at the same dosing. The two-step chemical potential curve reflects the two filling processes involving the alpha cage and window space. The less negative entropy of the second and third cycle from 0.1 to 0.4 mol/mol TO2 suggest increased disorder.


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Figure 6. (A) H2O absorption isotherms, (B) differential enthalpies, (C) differential Gibbs free energies (chemical potential) and (D) differential entropies of H2O absorption cat 25 oC on Li,Hzeolite RHO. The minor difference between the first run and second run is within experimental error. The starting sample is composed of Li,NH4-RHO but the degassing transforms the sample into Li,H-RHO. The sites occupied by NH4 group in the center of D8R and 8R become empty and accessible to guest molecules while only the 6R and 8R sites remain occupied by Li+ ions in dehydrated Li,H-RHO.21 The first run isotherm of Li,H-zeolite RHO in Figure 6A, shows a short tail before a steep rise at 0.03 P/Po and corresponding 0.16 mol/mol TO2. There is 17 ACS Paragon Plus Environment


only a gradual increase in the differential enthalpy and one small valley before further increase in Figure 6B. This complexity might be associated with three processes: the sequential filling of the alpha cage and window regions, and expansion of the 8R opening from an elliptical aperture in the acentric framework to the circular pore in the centric form, and relocation of remaining Li+ from the 8R to the 6R sites for better coordination driven by water molecules. The bridging interaction of water molecules with cations and framework oxygen reduces the distortion of framework and induces a more circular and planar 8R window. These changes are also reflected in the trend of chemical potential with a slight decrease to a plateau region near 0.3 mol/mol TO2 in Figure 6C, indicative of the gate opening process and phase transition. The short valley in the differential enthalpy suggests a stronger interaction of water molecules with the circular window after the phase transition. The quick entropy increase with increasing absorbed water molecules has been advanced by increased symmetry with more circular and planar 8R window and reduced framework distortion during the phase transition from 0.2 to 1.2 mol/mol TO2 and a valley-shape decrease represents a well-ordered arrangement of water molecules after the phase transition shown in Figure 6D. With additional water absorption, increasing disorder reaches the plateau in the physisorption region. After degassing again in the second and third cycles, the loss of water could give a repetitive scenario of change to more deformed structure with lower symmetry even without relocation of Li+ ions under vacuum.25


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2. CO2 absorption

Figure 7. (A) CO2 absorption isotherms, (B) differential enthalpies,(C) differential Gibbs free energies (chemical potential) and (D) differential entropies of CO2 absorption curves at 25 oC on the Na,Cs-zeolite RHO sample. The minor difference between the first run and second run is within experimental error. Almost linear CO2 uptake is observed in the absorption isotherms in Figure 7A. For all investigated RHO zeolites, the degassed form before CO2 sorption has the noncentrosymmetric I-43m structure with different 8R openings. In Na,Cs-zeolite RHO there are about half D8R sites occupied by Cs+ and Na+ cations occupy 8R in other windows and S6R of the large alpha-cages. 19 ACS Paragon Plus Environment


There is a prerequisite for CO2 absorption that Cs+ and/or Na+ must be able to dislodge to permit CO2 diffusion.19 No obvious phase transition was detected during the CO2 absorption under low pressure in the current experiments. Palomino et al.12 reported that a phase transition could take place above 1 bar at 25 oC and Cs+ is able to displace from D8R to 8R at 5 bar CO2 pressure.

The initial differential enthalpy of -40.68 ± 0.81 kJ/mol shown in Table 3 reflects relatively weak interaction of CO2 with Na,Cs-zeolite RHO This interaction appears insufficient to move Na+ cations away from the 8R sites. The CO2 molecules probably occupy two sites, one coordinated to Na+ cations in the 8R sites or Cs+ in the D8R (small fraction), and the other coordinated to 6R cations in the neighboring 6R within the alpha cages.38 At low pressures, CO2 molecules appear to first absorb preferentially to 6R Na+, showing the most negative enthalpies of absorption near zero coverage in Figure 7B. With increasing pressure, the momentary “trapdoor” motion of Na+ cations in the 8Rs occur to permit CO2 molecules to enter the alpha cage through window regions (8R or D8R), and the corresponding enthalpy reaches a plateau at 0.03 mol/mol TO2 (0.2 P/Po). With further loading of CO2, a weak cluster of CO2 might form inside the alpha cage due to quantum confinement of CO2 quadrupole by framework atoms and Na+ in the S6R and 8R windows.18 When approaching the saturation pressure, the interaction between CO2 molecules dominates. The chemical potential shows a monotonic increase with an inflection at 0.04 mol/mol TO2 in Figure 7C. This might indicate the quick absorption of CO2 onto the extra-framework cations with stronger binding and most negative potential, and increasing CO2 loading result in a higher chance of CO2-CO2 interaction and less chemical 20 ACS Paragon Plus Environment

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potential difference from the gaseous form. The initial entropy exhibits the lowest value and the ordering of absorbed CO2 molecules, aligned with the crystallographic sites of extra framework cations. At 0.04 mol/mol TO2, see Figure 7D, entropy reaches a plateau representing weak ordering for the physisorbed CO2 molecules.

Figure 8. (A) CO2 absorption isotherms, (B) differential enthalpies,(C) differential Gibbs free energies (chemical potential) and (D) differential entropies of CO2 absorption curves at 25 oC on the Cd,Cs-zeolite RHO sample. The minor difference between the first run and second run is within experimental error.



The absorption of CO2 on Cd, Cs-RHO adopts a different route with higher uptake as shown in Figure 8A. At low pressures, the steep rise of CO2 absorption may be a consequence of pore filling through incompletely blocked 8R in the dehydrated phase.7 The CO2 prefers to move closer to the Cd2+ sitting slightly outside the 8R aperture and the dynamic motion of cations Cd2+ allows an open access of partially blocked windows and CO2 absorption to occur.38 It shows the most negative enthalpies of absorption -46.45 ± 0.44 kJ/mol from near zero coverage to around 0.1 mol/mol TO2 and a gradual increase until 0.17 mol/mol TO2 indicating similar interactions with the cations throughout this stage in Figure 8B. After passing through the 8R windows, it is possible that more CO2 molecules enter the alpha cage and form closer coordination to Cd2+ in the S6R and 8R7. With increasing loading above 0.22 mol/mol TO2, additional CO2 is confined by framework atoms of the alpha cage without an interaction with cations but other CO2 molecules, and the corresponding enthalpy increases to less negative values. The chemical potential shows an almost linear increase with an inflection at 0.22 mol/mol TO2, see Figure 8C. This might indicate the termination of initial steep absorption which would reach saturation followed by a gradual increase due to increasing intermolecular CO2 interactions. Surprisingly the initial entropy is not the lowest value at near-zero coverage and probably the absorbed CO2 molecules constitute a more ordered arrangement at the 0.1 mol/mol TO2 level shown in Figure 8D. The further entropy increase is relatively flat through 0.22 mol/mol TO2, suggesting a well-coordinated ordering of absorbed CO2 molecules, distributed around the crystallographic sites of extra-framework cations.38 At the final stage, entropy increases with increasing CO2 loading representing weak ordering for the additional absorption of CO2 molecules.


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CO2 molecules prefer to occupy two sites in Li,H-RHO, one in the middle of each 8R within the window region and the other between two extra-framework cations in the neighboring 6R near the walls of the alpha cages.38,39 At low pressures, CO2 molecules appear to first absorb preferentially at 6R, bridging two extra-framework cations with different average M+···O=C=O distances,38 and partially at 8R where a small fraction of Li cations situate. Li-RHO shows the most negative enthalpies of absorption, -37.13 ± 0.65 kJ/mol, see Figure 9, near zero coverage. The CO2 molecules then accumulate around 8R without strong interaction with cations or with the remaining Li+ in the center of 8R. The enthalpy reaches a plateau with the least negative values in Figure 9B. The continuous filling of the void space in window regions gradually mitigates the distortion of the 8R, which originally favors the stability of the acentric structure I 43m , leading to a gradual phase transition to the centrosymmetric form I m3m upon increasing

CO2 molecules in the vicinity of window region labeled in Figure 9B, corresponding a relative pressure around 0.09 P/Po(0.1 bar). The CO2 molecules enter the alpha cage through gradually opening void window (8R or D8R), consistent with a small shoulder before the energetic valley. The increased coordination of Li+ cations at S6R by CO2 imposes a decreased distortion of the structure away from its high symmetry form. Further loading of CO2 molecules ends the phase transition and additional absorption fills the remaining void space of merely I m3m zeolite RHO through a nearly circular pores. When approaching the saturation pressure, the interaction between CO2 molecules takes the dominant place. The chemical potential first increases and then reaches a plateau at 0.15 mol/mol TO2, see Figure 9C. This might indicate the beginning point of the phase transition which would terminate, forming a single phase region, at the end of the plateau. The initial entropy has the lowest value at near-zero coverage and the phase transition counteracts the entropy increase of 23 ACS Paragon Plus Environment


CO2 molecules until a valley point at 0.3mol/mol TO2 in Figure 9D, indicating the increasing ordering of absorbed CO2 molecules, probably well-coordinated with the crystallographic sites of extra framework cations in the new phase with the gradually relieved framework distortion. At the final stage, entropy becomes much less exothermic with increasing amount of absorption and represent weak ordering for the bulk CO2 molecules.

Figure 9. (A) CO2 absorption isotherms, (B) corresponding differential enthalpies (C) differential Gibbs free energies (chemical potential) and (D) differential entropies of CO2 absorption at 25 oC on the Li,H-zeolite RHO sample. The minor difference between the first run and second run is within experimental error. 24 ACS Paragon Plus Environment

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CONCLUSIONS The structure of both hydrated zeolite RHO and its rehydrated form after gas absorption was investigated by XRD and FTIR analysis. Details of bonding with guest molecules, coordination environments for extra-framework cations distributed over specific sites and correlated structural evolution of the zeolite RHO framework have been surveyed. Among the materials assessed, Li,H-RHO exhibits exceptional performance characteristics for both water absorption and the capture of CO2. The gas absorption calorimetry effectively identifies the framework flexibility and “gate opening” phase transition from the dehydrated acentric form to the hydrated centric form during water absorption. The interactions with CO2 are weaker without an obvious phase transition except for Li,H-RHO in which CO2 sorption initiates a phase transition at low pressure around 0.1 bar, much lower than that other cation exchanged zeolite RHOs . Thus lithium appears to be the best performing ion to induce flexibility.19-21 This provides a promising avenue to activate the “gate opening” of the zeolite RHO framework using Li ion exchange and achieve better storage capacity due to its more dynamic response to guest molecules. Exceptional framework flexibility of zeolite RHO during water absorption leads to a phase transition with gate opening and enhanced absorption capability. It is likely that the total capacity can be more closely controlled by further modification of the hydrophilicity and gating cations. The potential flexibility of zeolite RHO can be tuned for selective absorption in processes operating in the presence of water or gas mixtures. Understanding this flexibility feature, which can be found in other zeolites and metal organic frameworks, will lead to applications in selective separations of small molecules, particularly in petrochemical applications.



ACKNOWLEDGMENTS The authors are grateful to Sergey Ushakov and Pinghui Zhang for helpful discussions on absorption measurement. DRC is grateful to Drs. Mark Shiflett, Bill Gilbert and David Minnick at the Center for Environmentally Beneficial Catalysis at the University of Kansas for their assistance in sample selection. This work was supported by the U.S. Department of Energy grant DE-SC0016573. REFERENCES 1

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The Thermodynamics of H2O and CO2 Absorption and Guest-Induced

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