A Computational Exploration of the CO Adsorption in Cation

Article pubs.acs.org/JPCC

A Computational Exploration of the CO Adsorption in CationExchanged Faujasites Z. Nour,† D. Berthomieu,*,† Q. Yang,‡ and G. Maurin*,† †

Institut Charles Gerhardt Montpellier, UMR 5253 CNRS, UM2, UM1, ENSCM, Université Montpellier 2, Place Eugène Bataillon, 34095 Montpellier cedex 05, France ‡ State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, P.O. Box 100 Beijing 100029, China ABSTRACT: Molecular simulations have been employed to explore at the microscopic scale the adsorption of CO in two families of sodium exchanged faujasite, NaX and NaY. As a preliminary step, density functional theory calculations have been conducted to derive new sets of potential parameters for accurately describing the interactions between CO and the extra-framework cations present within the supercage that act as preferential adsorption sites for the guest molecules. Two different parametrizations have been considered to discriminate the Na+ sites, Na+SII and Na+SIII′, involved in the interactions with CO. On the basis of these forcefields, Grand Canonical Monte Carlo simulations were further realized to first predict the adsorption properties (isotherms and enthalpies) of these two types of faujasites up to high pressure. This was followed by a careful analysis of the microscopic mechanism in play along the whole adsorption process with a special emphasis on understanding the arrangements of CO in the vicinity of the Na+ whether they occupy SII or SIII′ sites. These findings were discussed in light of the enthalpy profile obtained as a function of the loading and a few experimental data available in the literature. Finally, complementary simulations were realized with mobile extra-framework cations upon CO adsorption. It was evidenced that Na+SII can migrate toward SIII′ sites in NaY, while in NaX, there is no cation redistribution within the supercage due to a steric hindrance. Such cation dynamics were shown to not drastically affect the adsorption properties of both Faujasites; however, this is a critical prerequisite to allow CO to form a double type interaction with both Na+SIII′ and Na+SII via its C- and O-ends respectively, as predicted in cation-exchanged zeolites using quantum chemical calculations.

1. INTRODUCTION Carbon oxides are involved in many chemical and industrial processes and their removal to reduce detrimental environmental and climate impact is of great importance.1,2 The selective adsorption of these carbon oxides for the purification of hydrogen produced from reformed natural gas usually called syngas is also an important economical issue.3 Zeolites are generally classified among the most competitive porous solids for physisorption based applications industrially employed for the selective capture of such gases.4−7 Within this class of materials, the sodium-exchanged faujasites are known to be one of the most efficient adsorbents for carbon capture and the selective adsorption of carbon oxides over other gases as involved for instance in the purification of natural gas and syngas.4,6,7 These outstanding performances result from the interactions between the guest molecules and the extraframework cations present within their porosities, than can be tuned by changing the nature, the position, and the density of these cations.8−12 While the thermodynamic properties for the CO2/Faujasites system have been massively explored from both experimental and theoretical standpoints,8,9,13−18 in comparison, the studies related to the CO adsorption up to high pressure is scarcer. Indeed, the high toxicity of carbon monoxide19 renders rather complex the use of conventional adsorption experimental tools such as gravimetry/manometry © 2012 American Chemical Society

and microcalorimetry techniques, emphasizing the need to deploy a computational strategy to predict the thermodynamic properties of such systems. In this paper, molecular simulations combining quantum and classical approaches have been conducted to probe the CO adsorption properties of two sodium containing faujasites, labeled as NaX and NaY, that differ by their Si/Al ratio and thus by the concentration of cations present within their supercages. As a preliminary step, density functional theory (DFT) calculations were realized to derive a set of potential parameters able to accurately capture the interactions between CO and each type of Na+ sites present in both Faujasites. The forcefields available in the literature for describing the Na+/guest interactions generally consider the same potential parameters whatever the location of the extraframework cations within the cavities.16,20−23 This holds true for the existing forcefield that has been employed for treating the interactions between CO and the alkali cation exchanged X faujasite.22 This approach can be relatively crude for zeolite systems such as NaX that contains cations distributed over distinct crystallographic sites. For example, Na at SIII′ sites are less coordinated to the framework than Na at SII sites and thus Received: May 27, 2012 Revised: October 18, 2012 Published: October 26, 2012 24512

dx.doi.org/10.1021/jp305145s | J. Phys. Chem. C 2012, 116, 24512−24521

The Journal of Physical Chemistry C

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atom carrying a negative charge counterbalanced by a positive charge centered on the carbon atom seems to be inappropriate to address both types of interactions. In contrast, a CO model with a negative charge centered on both C and O atoms is expected to be more suitable to make C-end and O-end interactions with Na+ possible. Such a CO model previously reported by Straub et al.25 was selected in the present study. The CO molecule is described by a three-point charged model where two negative charges are centered on both the carbon and the oxygen atoms, and a positive point charge is fixed at the molecular center of mass (c.o.m.) to make the model neutral. The oxygen and carbon atoms are also considered as LenardJones (LJ) sites with associated LJ parameters summarized in Table 1. These set of parameters were refined in order to

more accessible to guest molecules. Such geometric features are expected to significantly affect the strength of the Na+/guest interactions. Indeed, a parametrization of such interactions for each type of interacting cation sites is of relevance for further understanding, precisely the adsorption mechanism in play. These newly derived forcefields were then combined with three- and four-point charge models for representing CO issued from the literature24,25 that was selected for its a priori ability to envisage the interactions of CO via both its C- and O-end with the sodium cations. This whole set of potential parameters was implemented in a Grand Canonical Monte Carlo scheme to predict the adsorption isotherms and the evolution of the adsorption enthalpies for the two faujasites in a wide range of pressure up to 30 bar. The determination of both the adsorbed amount at a given pressure and the energetic of the adsorption are of crucial interest for the applications since the most appropriate porous solids for the CO capture must combine large adsorption uptake and moderate interaction energy for being regenerable under mild conditions. Besides, CO is widely employed as a probe molecule to determine the cation site positions and the coordination types, using IR spectroscopy (for a review, see ref 26 and therein). In this domain, there are still open questions concerning the adsorption of CO in sodium exchanged faujasites. For instance, a new coordination type of CO in cations-exchanged faujasites was predicted using DFT calculations in which CO is coordinated both by its C- and Oends respectively to one cation at site II and one at site III′.27 In the present study, GCMC simulations in a wide range of pressures were conducted to access the adsorption mechanisms occurring in both NaY and NaX. Finally, as previous studies on zeolites have shown that the inclusion of guest molecules may induce a redistribution of the cation within the porosity,17,20,21,27−31 GCMC simulations allowing cations to move were investigated in order to quantify the impact of the cation mobility upon CO adsorption on the thermodynamics properties.

Table 1. LJ Parameters and Partial Charges Centred on the Different Sites of the CO Model Developed by Straub and Karplus25 and Piper et al.24 site

ε (K)

σ (Å)

charges

3.83 3.12

−0.75 −0.85 +1.6

Straub et al. C O c.o.m.

13.185 80.067 Piper et al.

X1 C X2 O

39.89

3.385

61.57

2.885

−0.636 +0.831 −0.195 0.000

reproduce the lattice constant, the vibrational frequencies, and the sublimation enthalpy of the α-CO.25 Another physical model was also considered which represents CO as a four point X1-C-X2-O charged model reported by Piper et al.,24 where only the oxygen and carbon atoms are considered as LJ sites (see Table 1). 2.3. Interatomic Potentials. To ensure a successful simulation of the adsorption process of CO in both NaX and NaY, accurate interatomic potentials are required. Each interatomic potential must account for the four nonbonded interactions: (i) zeolite framework/Na+ extra-framework cations, (ii) CO/CO, (iii) CO/zeolite framework and (iv) CO/Na+ cations. All these interactions are represented by the sum of a Coulombic term (for the electrostatic part) and a repulsion-dispersion contribution. The interactions between the zeolite framework and the Na+ extra-framework cations were considered when the Na+ cations were treated as mobile in our simulations. To that purpose, we used the interatomic potential developed by Ramsahye et al.,21 the repulsion-dispersion term being represented by a Buckingham potential whose parameters are shown in Table 2. While the adsorbate/adsorbate interactions were treated using the model described above (Table 1), the LJ potential parameters for describing the repulsion/dispersion interactions between both the carbon and oxygen atoms of CO and the zeolite framework were taken from our previous investigation which successfully reproduced the CO2 adsorption properties in NaX and NaY.13 The potential parameters for the interactions between CO and both types of interacting Na+ (SII and SIII′ sites) were further derived by our own DFT calculations following the strategy detailed below that was successfully employed in the past to derive the interatomic potential between the extra-

2. COMPUTATIONAL METHODS 2.1. Microscopic Model for the Faujasites. The crystal structures of the two NaX and NaY faujasites were taken from the literature.28,32 The corresponding chemical formula Si192‑xAlxNaxO384 with x = 92 and 56 were considered in order to mimic the experimental Si/Al ratios of 1.1 and 2.4 for NaX and NaY, respectively.13 We further considered for NaY, the distribution of the cations elucidated by Fitch et al.,28 that is 6 Na+ cations in SI sites, 18 in SI′ sites, and 32 in SII sites, while for NaX, we used the distribution proposed by Zhu et al.,32 that is, 32 Na+ cations in SI′ sites, 32 in SII sites, and 28 in SIII′ sites. The faujasite systems were assumed to be semi-ionic with atoms carrying the partial charges expressed in electron units, previously employed: Si (+2.4), Al (+1.7), Oz (−1.2), and Na (+0.7).13,33 In our simulations, the zeolite structure was maintained rigid. This assumption is justified as it has been unambiguously shown that such faujasites undergo a volume change lower than 1% upon adsorption of wide variety of guest molecules.28 2.2. Microscopic Model for CO. A few infrared spectroscopic studies and some DFT calculations have suggested that the interaction between CO and Na+ cations mostly occurs via the C-end atom of the CO molecule but the O-end interaction can also be envisaged.26,29,34−38 It is a crucial prerequisite to select a realistic CO microscopic model that can capture this scenario. In this context, a model with the oxygen 24513



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very well the interaction between CO and a series of alkali cations either bare or incorporated in small AlH(OH)3‑ and AlH(OSiH3)3‑ clusters calculated at the QCISD(T) level of theory.34−36 It has been also recently shown by Berthomieu et al. that the B3LYP functional coupled with the basis set mentioned above correctly reproduces CO adsorption energy values in sodium- and copper-exchanged faujasite zeolite.27,29 Because van der Waals contributions are expected to be not negligible in the Na+−CO interactions and since Hartree−Fock method reports part of the dispersion contribution, it is not surprising that that the admixture of Hartree−Fock exchange in B3LYP provides interaction energy values in good agreement with the experimental ones.29 The corresponding structural and energetic (ΔE) parameters of the so-obtained equilibrium geometries for CO in the two clusters, considering the two possible interactions type of CO via its carbon and oxygen atom, are summarized in Table 3. One observed that the most stable conformation for CO in both selected clusters corresponds to the Na+ - CCO adduct. Such a more favorable arrangement leads to an interaction energy which is significantly higher than those obtained for the Na+ - OCO geometry, from 4 to 5 kJ/mol for Na+ in sites II and III′ respectively, in good agreement with previous theoretical studies.27,29,34,35,37,38,51 On another hand, one can see from Table 3 that the angles for both (OCO...Na+) and (CCO... Na+) angles range from 168.3 to 179.2°, emphasizing that a quasi-linear adduct is obtained when CO interacts either by its C-end, or by its O-end atom with the Na+ cation located at SII site (cluster A) and at SIII′ site (cluster B). This result is in very good agreement with previous conclusions to a linear arrangement for the monocarbonyl complex formed during the adsorption of CO at low pressure in sodium-exchanged zeolite, from experimental data.52−54 Furthermore, the Na+−C(CO) and Na+−O(CO) distances calculated either for cluster A or B are comprised in the domain [2.460−2.499 Å] and [2.644−2.698 Å] respectively. These values are within the range of values previously obtained for sodium-exchanged faujasites,27,29 or other zeolite

Table 2. Interatomic Potential Parameters for the Adsorbate/Adsorbent Interactions Buckingham potentials atom pair

A (eV)

Na −Oz NaSII−C(CO) NaSII−O(CO) NaSIII′−C(CO) NaSIII′−O(CO)

8200 5600 5000 5300 4750

σ (Å)

C (eV·Å6)

0.218 11.8 4.19 31.8 3.75 31.8 4.22 31.8 3.78 31.8 Lennard-Jones potential

ref 21 this this this this

paper paper paper paper

atom pair

ε (eV)

σ (Å)

ref

C(CO)−Oz O(CO)−Oz

0.00363 0.00600

3.90 3.48

13 13

framework Na+ embedded in zeolites and various adsorbates such as CO2, CH4, etc.16,39 Two 20 T clusters (“T” refers to SiO4 or AlO4 tetrahedra) containing one Na+ at SII and at SIII′ sites were cut from the NaY and NaX periodic crystal structure, respectively, according to the procedure previously described by Berthomieu et al.29,40,41 Only one Al atom was included in the present models either in a six-membered or in a fourmembered ring close to the Na+ at the SII site (Figure 1a) and the Na+ at site III′ (Figure 1b) respectively. The hydrogen atoms, added to saturate the dangling bonds, were maintained fixed during the geometry optimization. On the basis of the use of the Gaussian G03 program,42 a preliminary DFT calculation was carried out to optimize the geometries of both Na-FAUCO clusters using the B3LYP hybrid functional43,44 and a basis set composed of 6-311++G(d,p) for the sodium cation and the CO molecule, and 6-31G(d) for the atoms of the zeolite framework (i.e., Si, Al, Oz).45−50 The use of such a large basis set allows us to avoid the BSSE corrections. During the geometry optimization, the CO molecule and all the atoms of the cluster were allowed to relax. One can justify the use of B3LYP functional based on previous elementary studies of Ferrari et al. which have shown that this functional reproduces

Figure 1. Representation of the two 20T clusters used in DFT calculations for the derivation of Na+(NaSII and NaSIII′)/CO potentials. (a) Cluster A and (b) Cluster B correspond to the cluster centered on a Na+ at the sites SII and SIII′, respectively. The silicon, oxygen, aluminum, and hydrogen atoms and the Na+ cation are represented in yellow, red, pink, and white and purple, respectively. The most stable arrangements of CO in both clusters are also provided, the characteristic Na+−CO distances and Na+−CO angles being reported in Å and degree, respectively. 24514



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Table 3. Structural (Distances in Å and Angles in deg) and Energetic (kJ/mol) Data from the DFT Optimized Geometries for the Clusters A and B in Interaction with the CO Molecule, Considering the Two Possible Interactions Type of CO: via Its Carbon and Oxygen Atomsa Na+−CO interaction Cluster A + CO Cluster B + CO a

(Na+SII−CO) (Na+SIII′−CO)

Na+−OC interaction

Na −C(CO)

Na −C−O

ΔE(CO)

Na −O(CO)

Na+−O−C

ΔE(CO)

2.698 2.644

174.3 179.2

−26 −34

2.499 2.460

168.3 175.5

−22 −29

+

+

+

Note that the binding energy ΔE has been calculated as follows: ΔE(CO) = E(cluster/CO) − (E(bare cluster) + E(CO)).

Figure 2. Typical arrangements of the CO molecules in NaY for low (a) 6 CO/u.c., intermediate (b) 33 CO/u.c., and high (c) 49 CO/u.c. CO loading issued from the GCMC simulations realized at 300 K for the NaY−CO system. The aluminum, silicon, oxygen atoms and Na+SII cations are represented in pink, yellow, red, and magenta, respectively. The characteristic distances and angles are reported in Å and deg, respectively.

calculate the interaction energies for each Na+SII/SIII′ − C(CO) and Na+SII/SIII′ − O(CO) distance in order to produce the energy profile. The resulting potential energy curves were fitted using a combination of a Coulombic contribution and a Buckingham term with Na+ carrying a charge of +0.7 (electron unit) as mentioned above.16 The corresponding Buckingham potential parameters for the Na+SII/SIII′ − C(CO) and Na+SII/SIII′ − O(CO) pairs are reported in Table 2. 2.4. Monte Carlo Simulations. Based on the interatomic potentials defined above, Grand canonical Monte Carlo (GCMC) simulations were further conducted to explore the adsorption behavior of CO at 300 K in the NaY and NaX using the simulation code CADSS (Complex Adsorption and Diffusion Simulation Suite).57,58 For such simulations, molecules involve four types of trials: attempts (i) to displace a molecule (translation or rotation), (ii) to regrow a molecule at a random position, (iii) to create a new molecule, and (iv) to delete an existing molecule. More details can be found elsewhere.58 The simulation box consisted of 1 unit cell for both NaY and NaX. A cutoff radius of 12.0 Å was applied to the LJ interactions, while the long-range electrostatic interactions were handled by the Ewald summation technique. Periodic boundary conditions were considered in all three dimensions. Peng−Robinson equation of state was used to convert the pressure to the corresponding fugacity used in the GCMC simulations.59 For each state point, GCMC simulations

types such as Na-LTA,55 Na-ZSM-5,38 and Na-FER.37 One can also observe that the Na+−C(CO) and Na+−O(CO) distances are significantly shorter when the Na+ are located at SIII′ site versus at the SII site. This result is in line with a less bounded environment of the Na+ cation to the zeolite framework when located at the SIII′ site, this more “open” position inducing a stronger interaction with any type of adsorbates13,27,56 as confirmed here by a higher Na+SIII′/C(CO) simulated binding energy compared to Na+SII/C(CO). DFT optimized geometries provided suitable starting structures for generating potential energy curves corresponding to the interactions of both Na+SII and Na+SIII′ with CO. We used the same strategy as the one previously reported by Plant et al. to derive the potential energy curve corresponding to the interaction between CO2 and sodium cations in faujasite systems.16 Each potential energy curve was obtained by varying the Na+−C(CO) and Na+−O(CO) distances from 1.5 Å to 7.5 Å for Na+SII (cluster A) and for Na+SIII′ (cluster B) while maintaining all the rest of the atoms fixed. The increment step was 0.1 Å, the geometry being constrained in order to maintain the previously defined equilibrium (CO and OC...Na+) angles as defined in Table 3. At each increment a single point energy calculation was performed using the functional and basis set mentioned above. An additional single-point energy calculation was performed along the vector at a distance of 70 Å. The resultant energy value was then used as a “zero” point to 24515



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Figure 3. Radial distribution functions (RDF) (a) Na+SII−C(CO) and (b) Na+SII−O(CO) for 6 (black line), 19 (blue line), 33 (green line), and 41 (red line) carbon monoxide molecules per unit cell, calculated at 300 K for the NaY−CO system.

Figure 4. Typical arrangements of the CO molecules in NaX for low (a) 5 CO/u.c., intermediate (b) 36 CO/u.c., and high (c) 60 CO/u.c. loading issued from the GCMC simulations realized at 300 K for the NaX−CO system. Illustration of the CO double interaction where CO is connected by its C-end to Na+SIII′ and by its O-end to Na+SII is shown in structure d. The aluminum, silicon, and oxygen atoms and Na+SII and Na+SIII′ cations are represented in pink, yellow, red, magenta, and green, respectively. The characteristic distances and angles are reported in Å and deg, respectively.

consisted of 2 × 107 steps to ensure the equilibration, followed by 2 × 107 steps to sample the desired thermodynamic properties. The GCMC calculations were first run considering the sodium cations fixed in their crystallographic positions assuming in a first step that the CO adsorption only slightly affects the partition of the extra-framework cations. This was followed by complementary simulations where the cations were allowed to be mobile upon adsorption. In this later case, at each value of the CO pressure, canonical Monte Carlo moves were performed for the extra-framework cations, allowing them to be redistributed over the space available within the sodalite and the supercage. Such a strategy has been widely employed for the adsorption of water in different cation containing zeolites.23,60 Indeed, for calculating the Na+/CO interactions with the most appropriate forcefield parameters, at each Monte Carlo step, a given Na+ cation was considered as Na+SII or

Na+SIII′ if its location was closer to the position of the crystallographic SII or SIII′ sites. Furthermore, dummy atoms with appropriate van der Waals radii were placed in the sodalite cages to prevent the introduction of the adsorbate molecule in these cages and to allow the gas accessibility only in the supercages. Indeed, as the van-der-Waals radius of the CO molecule, about 0.14 nm, is larger than the dimensions of the 6ring-opening windows of the sodalite cages (0.11 nm), carbon monoxide can neither access the sodalite cages nor the hexagonal prism.61 Further, for the calculation of the adsorption enthalpies (ΔH) as a function of the loading, Monte Carlo simulations in the canonical (NVT) ensemble were performed using the revised Widom’s test particle method.62 Such calculations were also run to calculate the radial distribution functions (RDFs) between the guest molecule and the Na+ cations for various loadings. From the integration of these 24516



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Figure 5. Radial distribution functions (RDF) (a) Na+SIII′−C(CO) and (b) Na+SII−C(CO) for 5 (black line), 17 (blue line), 36 (green line), and 60 (red line) carbon monoxide molecules per unit cell, calculated at 300 K for the NaX−CO system.

Figure 6. Absolute adsorption isotherms for carbon monoxide (a) and evolution of the adsorption enthalpies as a function of the coverage (b) obtained from GCMC simulations realized at 300 K up to 30 bar: NaY (square symbols) and NaX (circle symbols). The results with fixed and mobile cations are reported in solid lines−full symbols and dash lines−empty symbols, respectively.

RDFs, the mean number of CO molecules around the Na+SII (Na+SIII′) along the adsorption process was calculated.

in very good agreement with the previous experimental and theoretical values that span in the range [−21.0, −25.0 kJ/ mol].29,54,63−66 In contrast, when one used the microscopic model reported by Piper et al.,24 the CO is preferentially connected by its O-end to Na+ at the SII site with a mean characteristic distance of 2.90 Å resulting in an adsorption enthalpy of −19.0 kJ/mol much lower than the values reported above. Such a statement holds true in the whole range of loading. Indeed, such a model failed to capture both the type and the strength of interactions, which led us to keep only the model described by Straub et al.25 for the rest of the study. Regarding now NaX, the CO molecules interact preferentially with the cation located at the SIII′ site again via its C-end and adopting almost a linear arrangement as illustrated in Figure 4a, such geometry being in agreement with what has been experimentally and theoretically reported elsewhere.22,27,29,37,38,54,55,63−66 Further, the mean characteristic Na+SIII′−CCO distance of 2.67 Å derived from the corresponding RDF (Figure 5a) is slightly shorter than that observed for the Na+SII−CCO adduct in NaY. This is consistent with a calculated adsorption enthalpy which becomes higher in NaX than in NaY (−28.5 vs −23.0 kJ/mol), the so-obtained enthalpy value being in good agreement with recent experimental and theoretical isosteric heats of adsorption of −26.1 and −28.9 kJ/mol, respectively.22 Such a higher affinity

3. RESULTS AND DISCUSSION As a preliminary step, the transferability of the derived forcefield was tested by conducting GCMC simulations at very low pressure first for NaY using the two selected microscopic models for representing CO. Using the model reported by Straub et al.,25 one observed that the CO is preferentially connected by its C-end to Na+ at the SII site forming a quasi-linear Na+SII−C(CO) complex with geometric characteristics in terms of Na + SII−C (CO) distance and (OCO....Na+) angle, which matches well with the conclusion issued from the DFT calculations performed on cluster A. A typical illustration of such an arrangement of CO within the supercage is provided in Figure 2a. Indeed, the radial distribution function (RDF) for the Na+SII−CCO pair (Figure 3a) calculated from this GCMC run shows an average distance centered around 2.70 Å while the mean angle (OCO....Na+) is ∼164.5°, again consistent with previous findings reported in the literature for the same system or other Na+ cation exchange zeolites.27,29,37,38,52−55 From an energetic standpoint, the calculated adsorption enthalpy of −23.0 kJ/mol, consistent with the binding energy of −26.0 kJ/mol extracted from the DFT optimized Na+SII−C(CO) geometry (cluster A, Table 2), is 24517



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can coordinate with up to ∼0.9 CO molecule in its first coordination sphere for a loading of 49 CO molecules/u.c. corresponding roughly to the saturation capacity. Such a result also clearly shows that there is a significant fraction of CO molecules that do not interact directly with the Na+SII. This trend is more pronounced when the loading increases as illustrated in Figure 2b,c which shows a number of CO molecules free from interactions with the Na+SII. Such noninteracting CO molecules give rise to a signature in the RDF with a characteristic band centered around 7 Å whose intensity grows with the CO concentration. This microscopic adsorption behavior should be associated with a decrease of the host/guest interaction. Indeed, this is confirmed by the decreasing profile of the adsorption enthalpy when the CO concentration increases (see Figure 6b), which results from a decrease of the NaY/CO interaction (Figure 7) and an increase

for NaX zeolite has been previously reported for a series of guest molecules including CO2 and CFCl313,16,67 and is ascribed to the interactions between the CO and the more accessible cations located at site III′ in NaX as mentioned above. Figure 6a, reports the simulated absolute adsorption isotherms for CO in NaY and NaX up to 30 bar. One observes that both faujasites show a type I isotherm as usually expected for the adsorption type in microporous materials. The slope of the isotherms in the low domain of pressure is more pronounced for NaX than for NaY, supporting a higher CO affinity for this former faujasite, consistent with the trend found above for ΔHCO by GCMC simulations at very low coverage and by DFT calculations. Further, the simulated adsorbed amount of CO is significantly larger for NaX compared to NaY in the whole range of pressure up to 30 bar, the saturation capacity at the highest investigated pressure being 50 and 62 molecules/u.c. for NaY and NaX respectively. This result differs with the case of CO2 where the cation density within the supercage of the faujasite was shown to not affect the adsorbed amount at high pressure, despite a larger affinity for CO2 was also evidenced for NaX.13,16 This observation suggests a different microscopic packing mechanism of CO along the adsorption process compared to CO2. Indeed, in NaY, the linear Na+SII−CCO geometry observed at low coverage holds true for the whole range of pressures with a mean characteristic Na+SII−CCO distance that remains centered around 2.70 Å whatever the considered pressure, as depicted in the corresponding RDFs plotted in Figure 3a. One also observes that the main peak of the RDF between Na+SII and OCO (Figure 3b) is located at 3.80 Å, a distance which is much longer than the equilibrium one mentioned above for the DFT optimized Na+SII−OCO geometry (∼2.50 Å). This result suggests the absence of this latter geometry along the adsorption process as confirmed by the illustrative snapshots provided in Figures 2. Further, no multiple CO adsorption on Na+SII is detected at intermediate and even high CO loading (Figure 2b,c). Such an adsorption behavior is consistent with previous experimental findings reported by Hadjjivanov et al. on the CaNaY Faujasite,53 while it contrasts with those previously obtained for CO2 in the same NaY system, where the formation of the Na+SII−(CO2)2 species has been evidenced at high loading.53 Such a different adsorption behavior between CO and CO2 could be explained by a larger kinetic diameter of CO (3.75 Å vs 3.30 Å68) that prevents a higher coordination of the extra-framework cations present in the relatively confined SII sites. This conclusion drawn from the visual inspection of the Monte Carlo configurations is confirmed by the evolution of the summed n(r) value reported in Table 4 corresponding to the integration of the RDF Na+SII−CCO peak between 0 and 3.85 Å. Indeed, while there is a gradual increase of the n(r) values with the CO concentration, it is observed that the Na+SII

Figure 7. Evolution of the adsorbate/adsorbate (square symbols) and the adsorbate/adsorbent (circle symbols) energetic contributions as a function of the loading for NaY (solid lines) and NaX (dash lines) calculated from the GCMC simulations realized at 300 K for each system.

of the energetic contribution of the CO/CO interaction (Figure 7) consecutive of a shortening of the CO−CO distance when the pressure increases, this increase being not large enough to reverse the decreasing trend. Regarding NaX, at the initial stage of loading, while the CO molecules sit preferentially around the Na+SIII′ via a linear interaction with its C-end (Figure 4a) with a mean characteristic distance of 2.67 Å (Figure 5a), there is also a very small fraction of adsorbates that interact with the Na+SII in a similar way as in NaY. This adsorption of CO at site II is evidenced in the corresponding RDF (Figure 5b) and characterized by a band of low intensity centered ∼2.70 Å. Further, the analysis of the configurations generated by Monte Carlo simulations leads to the conclusion that the Na+SIII′ can coordinate 2 CO molecules in the intermediate and high pressure domains (Figures 4b,c). This is confirmed by the n(r) value reported in Table 5 corresponding to the integration of the RDF Na+SIII′− CCO peak between 0 and 3.60 Å. Indeed, it is observed that the first coordination sphere of the Na+SIII′ continuously increases with the loading, this cation type containing in average ∼1.7 CO molecule in its nearest environment sphere when the saturation capacity is reached. The predicted formation of such geometries, usually referred to as a geminal species, has been already identified by experimental measurements realized in the

Table 4. Summed n(r) Values Calculated from the RDFs Reported in Figure 3a for NaY/CO (the Integrations Are Calculated from 0 to 3.85 Å) loading (no. (CO)/u.c.)

n(r) Na+SII−CCO

6 19 33 49

0.634 0.743 0.832 0.899 24518



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Table 5. Summed n(r) Values Calculated from the RDFs Reported in Figure 5a,b for NaX/CO (the Integrations Are Calculated Both from 0 to 3.60 Å) loading (no. (CO)/u.c.)

n(r) Na+SIII′−CCO

n(r) Na+SII−CCO

5 17 36 60

0.832 1.174 1.368 1.698

0.064 0.113 0.212 0.297

the supercages). The relative increase of the adsorption enthalpy in presence of mobile cations is interpretated by tiny fluctuations of the positions for the cations Na+SIII′ around their mean crystallographic sites that lead to a slight increase of the Na+SIII′/CO interactions. Regarding NaY, the scenario is different compared to NaX. One first observes in the adsorption isotherm (Figure 6a) that (i) the saturation capacity obtained at 30 bar is slightly increased (53 CO/u.c. vs 49 CO/u.c.) and (ii) the enhancement of the adsorbed amount at the initial stage of adsorption is more pronounced in line with an increase of the adsorption enthalpy at low coverage reported in Figure 6b that varies from −23 to −25 kJ/mol. To explain these changes, again a deep analysis of the configurations generated during the GCMC simulations has been realized. It has been evidenced that some cation redistribution occurs within the supercage once the first adsorbate molecules are absorbed, leading to migrations of a fraction of Na+SII to vacant SIII′ sites. Such a rearrangement of Na+ cations have been previously evidenced in this same faujasite upon adsorption of a variety of guest molecules.17,20,21 The CO molecules thus preferentially interact with these newly generated Na+SIII′ following a similar adduct mechanism than was formerly described in the case of the NaX with fixed cations. The resulting stronger Na+SIII′/CO interactions than those with Na+SII explain the increase of the simulated adsorption enthalpy compared to the “rigid” case as reported in Figure 6b. As the loading increases, ∼10 Na+ move from SII to SIII′, leading to the formation of geminal species Na+SIII′− (CO)2 as was depicted above in NaX. This cation redistribution also induces the occurrence of the double interaction where the CO molecule interacts with both Na+SIII′ via its C-end and a neighbor Na+SII by means of its O-end (Figure 4d). This observation, consistent with some previous quantum chemical calculations that predict the presence of such geometries in this same NaY system,27 can be only obtained if the cation migration from SII to SIII′ sites occurs. It is the first time that a forcefield based simulation is able to support the existence of these arrangements in the NaY system. This concordance clearly states that even if considering the cation dynamics does not drastically affect the macroscopic adsorption properties of the faujasites, it is a crucial prerequisite to get a reliable microscopic picture of the adsorption mechanism.

low temperature range [80−100] K in this NaX system.52,69−72 It is noticeable that the more “open” position occupied by the cations in SIII′ allows a higher degree of coordination compared to that of the Na+SII in NaY. For intermediate and high loadings, the adsorbates continue to occupy sites of lower energy by interacting with Na+SII sites as evidenced by a relative increase of the corresponding n(r) values (Table 5). However, the Na+SIII′ remains the preferential adsorption site whatever the loading range when one compares its extent of coordination to those of the Na+SII (n(r) ≈ 0.3). Further, one can also notice the formation of a few species having a peculiar arrangement of CO, where the adsorbate molecules interact with both Na+SIII′ via its C atom and a neighbor Na+SII by means of its O atom (Figure 4d). This type of “double interaction” for CO was recently evidenced by DFT calculations on different zeolites including NaY27 and CuNaY29 and other types of alkali- and alkali earth-zeolite such as Na- and K-ZSM5,38 Na-,37 K-73 and Mg-74FER, and Na-LTA.55 Such a finding shows that the microscopic model we have selected to describe the carbon monoxide (the three site model), and the forcefield we have developed to describe the Na+(SII and SIII′)/CO interactions efficiently capture both types of geometries, Na+−C(CO) and Na+−O(CO), as was experimentally detected. The so-elucidated microscopic adsorption mechanism along with the whole range of pressure can explain the decreasing profile of the adsorption enthalpy reported in Figure 6b. Indeed, the CO/NaX interaction energy decrease observed in Figure 7 is due to the progressive coordination process of the Na+SIII′ and the occupancy of lower energetic site around Na+SII. This contribution, combined with an increase of the CO/CO interaction energy in a lesser extent, leads to a decrease of the adsorption enthalpy as the coverage increases. As a further step, the thermodynamic properties of the two faujasites with respect to CO were reconsidered by running GCMC simulations allowing the cations to move upon the whole adsorption process. The corresponding adsorption isotherms and enthalpies for both NaY and NaX are compared in Figure 6 to those obtained by maintaining the fixed cations in their initial positions. One first observes that the adsorbed amount and the enthalpy are very similar in both situations for the NaX, suggesting that the distribution of the cations within the supercage is not affected by the presence of CO. This was confirmed by a careful analysis of the configurations generated by Monte Carlo for different loadings of CO which clearly states that the majority of the cations Na+SII and Na+SIII′ remain in their crystallographic positions consistent with what has been previously observed for a series of polar and quadrupolar guest molecules including carbon dioxide,16,17 water,23 and methanol30 that are known to give rise to even stronger energetic interactions with the extra-framework cations. This behavior is not surprising when one considers the strong repulsion present in the supercage of NaX induced by the very high density of cations (60 over the total 92 cations per unit cell are present in

4. CONCLUSIONS A new set of potential parameters for representing the interaction between CO and the extra-framework sodium cations embedded in faujasite-type zeolites was derived using a cluster based density functional approach. The originality of this work was to parametrize distinct forcefields for the CO/ Na+SIII′ and CO/Na+SII pairs as the strength of the CO/Na+ interactions is expected to depend on the local environment of the cation site and its accessibility for the guest molecules. These forcefields were validated by a very good agreement between (i) the structural arrangements of CO in both NaY and NaX at low loading issued from DFT calculations and infrared spectroscopy measurements and those predicted by GCMC simulations conducted under similar conditions and (ii) the simulated adsorption enthalpies at zero coverage and those estimated by microcalorimetry measurements. GCMC calculations were further conducted to not only simulate the adsorption isotherms and enthalpies for both NaY and NaX in a wide range of pressure up to 30 bar, but also to carefully explore the microscopic adsorption mechanism along the whole 24519



Article

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adsorption process. It was pointed out that while in NaY the Na+SII are the preferential adsorption sites for CO with the formation of single Na+SII−CCO linear adduct, the NaX is characterized by a different packing mechanism that proceeds via the creation of geminal species around the Na+SIII′ corresponding to the coordination of these cations with 2 CO molecules (Na+SIII′−(CCO)2). A redistribution of the cations upon CO adsorption has been further pointed out in NaY, corresponding to a migration of Na+ from SII to SIII′ sites. This was followed by a peculiar arrangement of the CO molecules that tends to give rise to a double interaction with both Na+SIII′ and Na+SII via its C- and O-ends, respectively, previously suspected experimentally and first confirmed by a force field based computational approach. Such a result provides an additional validation of the derived forcefields. A future work would consist of using these transferable forcefields for screening a wide variety of sodium containing zeolites for predicting the most appropriate solids for CO capture without the need to perform risky experiments. This set of potential parameters is compatible with those we previously derived for treating the CO2/ Na+ interactions, suggesting that the CO2/ CO selectivity, which is of great interest for the purification of hydrogen and natural gas, will be simulated for such sodium containing zeolites.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; dorothee. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The density functional theory calculations were carried out on the IBM SP4 computers of the CINES (Centre Informatique National de l’Enseignement Supérieur) in Montpellier, France (Grants No. 2010-x2010081071 and 2011-x2011081071). The authors acknowledge the CNRS and the Region LanguedocRoussillon for the financial support of the PhD fellowship of Z.N.

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A Computational Exploration of the CO Adsorption in Cation

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