Molecular Simulation of Propane Adsorption in ... - ACS Publications

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Molecular Simulation of Propane Adsorption in FAU Zeolites Junfang Zhang,*,† Nick Burke,‡ and Yunxia Yang‡ †

CSIRO CESRE, 26 Dick Perry Ave, WA 6151, Australia CSIRO CESRE, Ian Wark Laboratory, Bayview Ave, Clayton, Vic 3168, Australia

‡

ABSTRACT: The purpose of this study is to investigate propane adsorption capacity in sodium and calcium form of faujasite (FAU)-type zeolites X and Y by performing Monte Carlo simulations in the grand-canonical ensemble (GCMC). Our results have shown that the nonframework cation number density and the interaction potentials between propane molecules and the nonframework cations are very important factors for influencing propane adsorption in FAU zeolites. For a given ratio of Si/Al, the adsorption of propane in both sodium and calcium FAU increases with the increasing nonframework cation number density, suggesting that an increase in the number of cations produces extra adsorption sites and leads to a higher amount of adsorption. For a given nonframework cation number density, the higher ratio of Si/Al allows the higher adsorption capacity due to the reduced charge density, i.e., the fewer calcium cations, but more sodium cations making the interaction energy more negative and consequently higher affinity between propane and the nonframework cations.

1. INTRODUCTION Natural gas demand is expected to continue to grow in the midterm, mainly in industrial, services and residential sectors. It is expected that the consumption of natural gas on the electricity generation sector will consistently rise over the next decade due to its economical and environmental benefit of producing far less emission gases and particulates than petrol and coal. Raw natural gas produced from the wellhead typically consists of methane and paraffinic hydrocarbons such as ethene, propane, and other heavier hydrocarbons, and it must be treated to meet consumer specifications before pipeline distribution. Among all the technologies available for natural gas processing, separation by adsorption is a very promising chemical engineering method, widely considered because of its reliability, flexibility, and low energy consumption. Generally, the performance of any adsorptive separation or purification process is directly determined by the quality of the adsorbent.1 Ideally, the adsorbent should be tailored with specific attributes to meet the needs of each specific application. Development of better adsorbents can not only improve the performance of the current commercial processes but reduce the operating cost as well. A lot of successful applications have been made in the gas separation field such as CO2 and natural gas adsorption.2−7 The past two decades have seen an explosion in the development of new nanoporous materials. Tremendous advances have been made in the capabilities to tailor the porosity and surface chemistry of oxide molecular sieves among which zeolites are widely used in adsorption and separation processes.8−13 Propane-propylene binary adsorption in sodium forms of zeolite was studied via molecular simulations.14 Sodium and calcium forms of zeolite X were investigated via the gas chromatographic method to determine the potential of separation of ethylene from ethene and methane.15 The isotherms of propane and water vapor adsorption on X and Published 2012 by the American Chemical Society

Y zeolites with different exchange cations were measured in a calorimeter.16 Molecular dynamics and Monte Carlo simulations have become increasingly important and popular in gas adsorption in zeolites.17−27 Calero et al.17 developed a united atom force field able to accurately describe the adsorption properties of linear alkanes in the sodium form of FAU-type zeolites. The force field was validated by comparing the simulation results with those obtained experimentally.28−31 ́ Garcia-Pé rez et al.18 extended upon the force field of Calero et al. by including calcium-type ions and the force field was calibrated by using a set of experimental data.32−35 It is known that the adsorption interaction on zeolites is determined by the nature of the cations, the degree of exchange, population of single-cation positions, and by other factors.16,20,36 However, systematic studies on how zeolite Si/Al ratio, cation charge, and number density affect the adsorption are still limited. Fundamental knowledge is still needed to fully understand the adsorption mechanism. In this article, we present the results of a molecular study of propane adsorption in FAU-type zeolites X and Y with varying content of exchange cations for a pressure range of 102−106 Pa and a temperature of 299 K. We focus on the effect of the number density of the nonframework cations, the ratio of Si/Al and Na+/Ca2+ on propane adsorption. We choose to model FAU-type zeolites as we are currently undertaking experimental work with this adsorbent. It is anticipated that the molecular modeling and experimental work will be complementary and that the molecular simulations will shed new light on experimental findings. Received: February 22, 2012 Revised: March 28, 2012 Published: March 29, 2012 9666

dx.doi.org/10.1021/jp301780z | J. Phys. Chem. C 2012, 116, 9666−9674

The Journal of Physical Chemistry C

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2. SIMULATION DETAILS 2.1. Zeolite Structure and Compositions. FAU unit cell composition with Si/Al = 1.18 and 2.56 corresponding to 88 Table 1. Composition of Zeolites and Their Exchange Cation Contents composition

Ca2+/ uc

Na+/ uc

Na+,Ca2+/ uc

Ca44Na0Al88Si104O384 Ca40Na8Al88Si104O384 Ca30Na28Al88Si104O384 Ca20Na48Al88Si104O384 Ca0Na88Al88Si104O384 Ca27Na0Al54Si138O384 Ca25Na4Al54Si138O384 Ca21Na12Al54Si138O384 Ca14Na26Al54Si138O384 Ca0Na54Al54Si138O384

44 40 30 20 0 27 25 21 14 0

0 8 28 48 88 0 4 12 26 54

44 48 58 68 88 27 29 33 40 54

zeolites zeolite X (Si/Al = 1.18)

zeolite Y (Si/Al = 2.56)

Table 2. Intermolecular Potential Parameters and Charges for Faujasitea

a

interaction

σ (Å)

ε/kB (K)

OSi/OAl−CH3 OSi/OAl−CH2 OSi/OAl−Na+ OSi/OAl−Ca2+ CH3−Na+ CH3−Ca2+ CH2−Na+ CH2−Ca2+ charge q (e)

3.48 3.58 3.40 3.45 2.65 2.6 2.95 2.80

93.00 60.50 23.00 18.0 443.73 440.00 310.00 440.73 Si +2.05

OAl −1.20

OSi −1.025

Na +1.00

Ca +2

Al +1.75

Values are taken from ref 18.

Table 3. Energy Terms and Parameters for United Atom Force Field of Propanea component harmonic bond harmonic bend a

parameter value k1/kB = 96 500 (K/Å2), r0 = 1.54 Å k2/kB = 62 500 (K), θ0 = 114°

energy Ubond = (1/2)k1(r − r0)2

Figure 1. (a). Simulation unit cell projected on the x−y plane. Chained spheres represent propane molecules, and the rest is zeolite structure. (b). Snapshot of the simulation box, which consists of Na+ (gray spheres), Ca2+ (blue spheres), propane (blue chains), and zeolite frame (red and green lines).

Ubend = (1/2)k2(cos θ − cos θ0)2

Values are taken from ref 18.

and 54 aluminum atoms per unit cell is used. The simulation boxes are obtained by random replacement of silicon by Al atoms, automatically following the Lowenstein rule. The negative charges introduced by replacing Si by Al are compensated by Na+ and Ca2+. Atomic charges are chosen as qNa = +1, qCa = +2, qSi = +2.05, and qAl = +1.75.20 The crystal structure used in this work has a dehydrated composition CaxNa88−2xAl88Si104O384 for FAU zeolite X, which is constructed by replacing 2x Na+ cations with x Ca2+ cations in unit cell of NaX zeolite (x = 0, Na88Al88Si104O384) with lattice parameter of 25.099 Å.37 Similarly, the crystal structure for FAU zeolite Y is generated with a composition of CaxNa54−2xAl54Si138O384. Cations present in the zeolite framework are Ca2+, Na+, or their mixture. The zeolite is assumed to be rigid, as Vlugt and Schenk have shown that framework flexibility only has a minor influence on adsorption properties.38 Simulation box with 0, 8, 28, 48, and 88 Na+ cations (44, 40, 30, 20, and 0 Ca2+ cations) for zeolite X and 0, 4, 12, 26, and 54

Na+ cations (27, 25, 21, 14, and 0 Ca2+ cations) for zeolite Y are used in this work. We have varying content of the nonframework cations intending to study the influence of their type, density, and ratio on propane adsorption. The unit cell composition of zeolites and their exchange cation contents are given in Table 1. 2.2. Interaction Potential Model. Interactions of nonframework cations with zeolite atoms and propane molecules are considered. The nonframework cations are allowed to move freely in the system. The propane is described with a united atom model, in which each CH3 and CH2 group is treated as a single interaction center with their own effective potentials, and the united atoms are connected by harmonic bond length and angle potentials.18,21 The propane−propane, propane−zeolite host framework, and propane−cation interactions are modeled by Lennard-Jones potentials. The interactions of propane 9667



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Figure 1b is a snapshot from the simulation. We have varying content of the nonframework cations intending to study the influence of their type, density, and ratio on propane adsorption. The interaction potentials and the adsorption isotherms can be obtained directly from the simulation by computing the ensemble average of the potential energy and the number of propane molecules in the unit cell. The evolution of the total energy over the Monte Carlo steps is monitored to check for the equilibration conditions.

molecules with the zeolite host framework are dominated by the forces between the pseudoatoms and the oxygen atoms of zeolite. The contribution of silicon and aluminum are taken into account through an effective potential with only the oxygens as the polarizability of silicon and aluminum are much lower than those of oxygen atoms. The interactions between cations and the zeolite host framework are modeled by Lennard-Jones and Coulomb potentials. The Coulomb interactions are calculated using the Ewald summation.39 The LJ potential is expressed as follows: ⎧ ⎡⎛ ⎞12 ⎛ ⎞6 ⎤ σij ⎥ ⎪ ⎢⎜ σij ⎟ ⎪ 4εij⎢⎜ ⎟ − ⎜⎜ ⎟⎟ ⎥ for (rij ≤ rcut) u(rij) = ⎨ ⎣⎝ rij ⎠ ⎝ rij ⎠ ⎦ ⎪ ⎪0 for (rij > rcut) ⎩

3. RESULTS AND DISCUSSION The gas adsorption among the zeolites with varying content of cations is analyzed in terms of the adsorption energy, C3H8

(1)

Cross-interactions, except for the interactions with the nonframework cations, are calculated by the Lorentz−Berthelot mixing rules as given below: σij =

εij =

1 (σii + σjj) 2

εiiεjj

(2) (3)

The LJ parameters for the oxygen of the zeolite host framework, cations, propane, and partial charges are taken ́ rez et al.18 and summarized in Table 2. We use a from Garcia-Pé truncated and shifted potential with cutoff radius of 12 Å. Energy terms and parameters for united atom force field of propane are listed in Table 3. The partial charges on Si (+2.05e), Al (+1.75e), OAl (−1.20e), and OSi (−1.025e) of the zeolite host framework system are fixed. It should be noted that OAl are oxygens bridging one Si and one Al, and OSi are oxygens bridging two Si atoms. The partial charges on OAl and OSi are adjusted to take into account the polarization effects for the cation−zeolite interactions, and effective Lennard-Jones interactions between the cations and propane are used to include the polarization effects for the cation-propane interactions. Interaction potentials, adsorption capacity, and structural properties are obtained for propane in FAU-type zeolites, NaX, NaY, CaX, CaY, NaCaX, and NaCaY by GCMC simulations for a pressure range of 102−106 Pa and a temperature of 299 K. In a GCMC simulation, instead of setting the chemical potential, it is more intuitive to set the reservoir pressure, which is related to the chemical potential by μ = μ0 + RT ln(ϕP/p0), where μ is the chemical potential, and p0 and μ0 are the standard pressure and chemical potential, respectively. P is the reservoir pressure, and ϕ is the fugacity coefficient. The volume V, temperature T, and the chemical potential of the adsorbed phase μ, which is assumed to be in equilibrium with a gas reservoir, are fixed. The GCMC algorithm allows the system density to fluctuate with insertion and deletion of propane molecules. Equilibrium is attained when the number of successful insertion and deletion attempts balances each other. Details of the GCMC method can be found in ref 39. The simulations are carried out using the open source package RASPA 1.0 developed by Dubbeldam et al.40 The simulations are performed using one orthorhombic unit cell of dimensions 25.099 × 25.099 × 25.099 Å3 with a typical number of Monte Carlo steps of 5 millions. The simulation unit cell, shown in Figure 1a, is a projection onto the x−y plane, and

Figure 2. Comparison of the interaction potentials between cations (Na+ and Ca2+) and the united atoms (CH2 and CH3) in propane.

Figure 3. Interaction energy (kJ/mol of propane) between propane and the FAU zeolite host framework at 299 K. (a) NaX, CaX, and NaCaX; (b) NaY, CaY, and NaCaY. 9668



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Figure 6. Effect of the number of the nonframework cations on the adsorption isotherms of propane at 299 K in FAU-type zeolites, NaX, CaX, and NaCaX.

Figure 4. Interaction energy (kJ/mol of propane) between propane and the nonframework cations in FAU zeolites at 299 K. (a) NaX, CaX, and NaCaX; (b) NaY, CaY, and NaCaY.

Figure 7. Effect of the number of the nonframework cations on the adsorption isotherms of propane at 299 K in FAU-type zeolites, NaY, CaY, and NaCaY.

and cation, the CH3−Ca2+ and CH3−Na+ potentials are more attractive having a van der Waals radius ∼0.2 and 0.3 Å smaller

Figure 5. Propane−nonframework cation interaction energy (kJ/mol of propane) as a function of the number of cations for both X and Y zeolites at 299 K.

Table 4. Specific Retention Volumes of Hydrocarbon Gases C1−C4 and CO, Expressed at Flowmeter Temperature, 20 °Ca

loading, and structural properties. The effect of the cation number density, Si/Al ratio, and interaction potential on the adsorption capacity is discussed. 3.1. Interaction Potentials. A comparison of the interaction potentials between cations (Na+ and Ca2+) and the united atoms (CH2 and CH3) in propane is shown in Figure 2. For the interaction between united atom CH2 and cation, it is clear that van der Waals radius for CH2−Ca2+ is smaller than that for CH2−Na+; Meanwhile, the well depth of CH2−Ca2+ is more negative than that of the CH2−Na+. Compared with the interaction between the united atom CH2

a

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components

NaX (84 Na+)

NaY (64 Na+)

methane ethene propane butane carbon monoxide ethylene propylene

25.9 56.0 67.0 194.0 63.8 62.9 71.1

14.3 25.4 20.8 118.9 25.7 26.4 54.3

Values are taken from ref 36. dx.doi.org/10.1021/jp301780z | J. Phys. Chem. C 2012, 116, 9666−9674


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negative to a small negative value in the order of −13.13 < −10.69 < −8.54 < −6.30 < −5.68 (kJ/mol of propane), which corresponds to the number of cations in the order of 88 > 68 > 58 > 48 > 44 per unit cell. This move from large negative to small negative value signifies a decrease in affinity between propane and the cations. Affinity increases with the increasing number of the cations in the system studied. Similarly, we observe the same trend for FAU-type zeolite Y in Figure 4b. When the number of the cations decreases in the order of 54 < 40 < 33 < 29 < 27 per unit cell, the interaction energy becomes less negative in the order of −9.52 < −7.14 < −5.70 < −5.14< −4.50 (kJ/mol of propane). In order to fully investigate how the total number of the nonframework cations affects the interaction potential, their correlation is plotted in Figure 5, where the interaction energy as a function of the number of the nonframework cations for both X and Y zeolites is depicted. It is clear that the interaction energy between propane and the nonframework cations strongly depends on the total number of the nonframework cations. It becomes more negative when the total number of the nonframework cations is increased, indicating that a higher affinity interaction is associated with more total number of the nonframework cations. We also observe that the curve for X zeolite is above the other for Y zeolite in the overlap region, where the total number of cations is from 44 (minimum number of cations for X with 88 Al) to 54 (maximum number of cations for Y with 54 Al). This implies that if both X and Y are allowed to have the same amount of cations, X zeolite will have less negative interaction potential than Y zeolite, suggesting that not only the total number of the nonframework cations but also the Si/Al ratio and the type of the cations influences the interaction potential. For example, at a total number of 54 nonframework cations for both X and Y zeolites, X zeolite has 20 Na+ and 34 Ca2+, while Y zeolite has 54 Na+ (X

Figure 8. Adsorption capacity of methane at one atmosphere and at the temperature of 0 °C. Adapted with permission from ref 41. Copyright 1968 Elsevier.

than CH 2 −Ca 2+ and CH 2 −Na + , respectively. CH 3 −Na + potential with the more negative well depth than CH3−Ca2+ potential suggests a more attractive interaction. Figure 3 shows the interaction energy in the unit of kJ/mol of propane between propane and the zeolite host framework. As it is shown, an average propane and host interaction potential of around −17 kJ/mol of propane is obtained for both X (Figure 3a) and Y (Figure 3b) zeolites, suggesting the effect of the type and the number of the nonframework cations (Na+ and Ca2+) on the interaction energy between propane and the host framework of zeolite X and Y is negligible. The interaction energy between propane and the nonframework cations in zeolites X and Y with varying content of exchange cations is presented in Figure 4. For FAU-type zeolite X, shown in Figure 4a, the interaction energy is systematically more negative if the number of cations keeps increasing. The interaction energy varies from a large

Figure 9. Loading of propane per unit cell vs the total number of cations at different pressures: (a) 103 Pa; (a) 104 Pa; (a) 105 Pa; and (a) 106 Pa. 9670



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Figure 10. Radial distribution functions for propane with nonframework cations (Na+ and Ca2+) and different atoms in FAU zeolite X (OAl, OSi, Si, and Al) at the pressure of 105 Pa and temperature of 299 K.

Figure 11. Radial distribution functions for CH3 and CH2 groups in propane with nonframework cation Na+ in FAU zeolite Y at the pressure of 105 Pa and temperature of 299 K: (a) 26 Na+ and 14 Ca2+; (b) 12 Na+ and 21 Ca2+; and (c) 4 Na+ and 25 Ca2+.

zeolite with 88 Al atoms has 34 more Ca2+ but 34 fewer Na+ than Y zeolite with 54 Al atoms). From Figure 5, we know the interaction potential for Y zeolite is more negative than that for X zeolite at a given total number of cations in the overlap region. The reason for this can be explained by Figure 2, where it is shown that CH3−Na+ potential has more negative well depth than CH3−Ca2+ potential. Therefore, X zeolite having 34 fewer Na+ cations presents less negative energy than Y zeolite. We infer that the interactions between CH3 pseudoatoms and the cations dominate the energetics between propane and cations after assessing the importance of the energetics (potential well depth) and atomic volume (van der Waals radius) by relating the interaction energy between cations (Na+ and Ca2+) and propane, shown in Figure 4 to the interaction potentials between cations and the united atoms (CH2 and CH3) in propane, given in Figure 2. 3.2. Adsorption of Propane. Adsorption isotherms are simulated for propane at 299 K in FAU zeolite X with different cation densities. Our results presented in Figure 6 show the effect of the number of the nonframework cations on the adsorption capacity of propane in FAU structures. It is demonstrated that the adsorption of propane increases with the increasing number of the nonframework cations, for a given Si/Al ratio of 1.18. The total number of the cations inside CaX

framework (44 Ca2+) is half of the number of cations in NaX (88 Na+) since Ca2+ can replace two Na+, leading to a significant drop in the number of adsorbed propane molecules from 20 to 1.54 at 103 Pa, 40.55 to 22.65 at 104 Pa, and 47.11 to 42.84 at 105 Pa . The same behavior is true for the structures of FAU zeolite Y with Si/Al ratio of 2.56 as observed in Figure 7. In order to compare our simulation results with the experimental studies, we extract the relevant chromatographic data reported by Tsitsish et al.36 and listed it in Table 4. It indicated that the values of the specific retention volumes of all studied compounds (hydrocarbon gases C1−C4 and CO) were decreased with a decrease of the total number of cations per unit cell. For propane, the specific retention volume is 20.8 for NaY with 62 Na+ per unit cell but increases to 67.0 for NaX with 84 Na+ per unit cell. The observed trend of the increased adsorption with increasing cation numbers per unit cell in our simulation is in agreement with that obtained from the chromatographic experiment. Calorimeter data obtained by Dzhigit et al.16 also showed that a decrease in the concentration of monovalent exchange cations in hydrated synthetic zeolites of the FAU-type results in a reduction in the amount of propane adsorbed (see Figure 1b, the isotherms of propane adsorption at 24 °C on NaX (76 Na+/uc) and NaY (54 Na+/ uc), in ref 16). In Figure 8, one atmosphere methane adsorption capacity at 0 °C, extracted from chromatographic 9671



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Figure 12. Radial distribution functions for CH3 and CH2 groups in propane with nonframework cation Ca2+ in FAU zeolite Y at the pressure of 105 Pa and temperature of 299 K: (a) 26 Na+ and 14 Ca2+; (b) 12 Na+ and 21 Ca2+; and (c) 4 Na+ and 25 Ca2+. Figure 13. Radial distribution functions between nonframework cations in FAU zeolite Y at the pressure of 105 Pa and temperature of 299 K: (a) 26 Na+ and 14 Ca2+; (b) 12 Na+ and 21 Ca2+; and (c) 4 Na+ and 25 Ca2+.

data shown in Figure 3 of ref 41 is plotted. A strong dependence of the adsorption of methane on the Na+ numbers is observed. For sodium cation, the adsorption of methane in FAU increases with increasing the cation numbers in the unit cell. The experimentally observed trend for FAU zeolites agrees with our simulation results. In contrast, an increase in the amount of adsorption is observed with removal of cations from the MFI structure and also with introduction of divalent cations (divalent cations can replace two Na+ ions) in the molecular simulation work on the adsorption of alkanes in MFI-type zeolites.20 Because of the medium sized pore structure of MFI zeolite, there exists a space restriction for the adsorbed alkane and nonframework cations to fit next to each other in a pore. However, FAU structure with large pore size allows the propane molecules to have favorable van der Waals interactions with the nonframework cations, which create extra adsorption sites in the FAU-type zeolites. In Figure 9, we plot the loading of propane per unit cell vs the total number of cations at different pressures. For all the pressures specified, the number of propane molecules adsorbed increases with the increasing cation number. The propane loading curve for FAU zeolite Y is above the one for FAU zeolite X in the overlap region of the number of cations from 44 to 54. An examination of loading amount in Figure 9

displays a correlation with the interaction energy shown in Figure 5. As we have analyzed in 3.1, in the overlap region of the number of cations, if both X and Y have the same total number of cations, Y zeolite with 54 Al atoms has more negative interaction potential than X zeolite with 88 Al atoms resulting in a greater affinity between the nonframework cations and propane molecules and accordingly more loadings of propane. 3.3. Radial Distribution Functions (RDFs). The structures of the system were examined by using the RDFs. It is defined as the probability that two centers, A and B, are separated by a distance r. It was calculated by binning site pair distances periodically throughout the simulations. Figure 10 shows the RDFs for propane with nonframework cations (Na+ and Ca2+) and different atoms in FAU zeolite X (OAl, OSi, Si, and Al) at the pressure of 105 Pa. We observe a distinct peak in the RDFs of propane and cations at 2.98 Å. This indicates that propane molecules have favorable van der Waals interactions with the nonframework cations, which create extra adsorption sites in the zeolite structure. This observation agrees with the 9672



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result of the interaction energy between propane and cations discussed in section 3.1 and the adsorption isotherm results analyzed in section 3.2. One can see that the position of the first maximum of RDFs between propane and cations does not change with the varying content of cations, but the relative peak height for CH3−Na+ and CH3−Ca2+ depends on the cation contents. For all three FAU zeolites X with different cation contents, the RDFs of CH3−Si and CH3−Al are very similar. They have a first peak at 4.5 Å, and the second one occurring at around 7.4 Å and the third one at 9.6 Å relating to the structure of its nanoporous host. In Figure 11, which shows the comparison of the RDFs between CH3−Na+ and CH2−Na+ for FAU zeolite Y with Si/Al ratio of 2.56 and varying cation numbers, we observe that it is the CH3 group that interacts more strongly with Na+. CH3−Na+ shows a strong first peak, while CH2−Na+ has a broad peak. The positions of these first peaks for CH3−Na+ are left shifted from those of CH2−Na+. This result is not surprising as Figure 2 has shown that CH3− Na+ potential is more attractive than CH2−Na+ and has a van der Waals radius 0.3 Å smaller than CH2−Na+. Figure 12 depicts the RDFs for CH3−Ca2+ and CH2−Ca2+. CH3 groups have greater access to the Ca2+. Once again, we observe that the interactions between CH3 pseudoatoms and the cations dominate the energetics between propane and cations. The RDF results agree with the adsorption energy results shown in Figure 5 and the adsorption capacity presented in Figure 9. In Figure 13, the RDFs between the nonframework cations were presented. The peaks and locations vary with the varying content of exchange cations and no regular patterns are obtained.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Compute Infrastructure (NCI) national facility for a generous allocation of computing time and technical support during the course of this work.

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4. CONCLUSIONS We have performed molecular simulations of the FAU zeolites X and Y with varying content of exchange cations at a pressure range from 102 to 106 Pa at a temperature of 299 K. We calculated the interaction potentials between propane and the nonframework cations, propane and host framework of FAU zeolites X and Y, adsorption isotherms, and RDFs to investigate propane adsorption capacities on NaX, NaY, CaX, CaY, NaCaX, and NaCaY zeolites and their dependence on the number of the nonframework cations, the ratio of Si/Al and Na+/Ca2+. The results indicate that, for the structure of FAU zeolites X and Y with large pores, propane molecules have favorable van der Waals interactions with nonframework cations. For the same ratio of Si/Al, the higher cation number leads to the higher adsorption capacity. We infer that an increase in the number of cations produces extra adsorption sites in the FAU-type zeolites. We observed that the effect of the type and the number of the cations on the interaction energy between propane and the host framework of zeolites X and Y is negligible. However, an increase in the number of the nonframework cations makes the interaction energy between propane and the cations more negative. This move from less negative to more negative value signifies an increase in affinity between propane and the cations, and consequently, the adsorption capacity is facilitated. For a given cation density, the higher ratio of Si/Al allows the higher adsorption capacity due to the more monovalent sodium cations making the interaction energy more negative and consequently higher affinity between propane and the nonframework cations. Our results of the potential energy, adsorption isotherm, and radial distribution functions are self-consistent. Our results also agree with the experimental findings on FAU-type zeolites.16,36,41 9673



Article

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