Article pubs.acs.org/IECR
Computational Insights into Adsorption of C4 Hydrocarbons in Cation-Exchanged ZSM-12 Zeolites Pavlo Kostetskyy and Giannis Mpourmpakis* Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *
ABSTRACT: Using ab initio electronic structure calculations we investigated the adsorption of C4 alkane and alkene isomers in pure-silica and cation-exchanged ZSM-12 zeolites. Alkali metal cations (Li+, Na+, and K+) in the ZSM-12 zeolite framework promoted the adsorption of alkenes relative to alkanes, due to electrostatic interactions between the cations and alkene π-electrons (cation-π interactions). Pure-silica zeolites exhibited the opposite trends, favoring adsorption of alkanes over alkenes. Linear alkenes were generally found to exhibit stronger binding on the cation sites versus branched species due to the bulkier molecular dimensions and steric constraints imposed by the zeolite pore. Calculations at different levels of (ONIOM-embedded method) theory were in perfect agreement with respect to the overall adsorption trends in the pores of the zeolites, and our results rationalize the impact of the zeolite pore to the adsorption of the species, especially for pore sizes close to the critical diameter of the adsorbates. Importantly, we demonstrate that the steric effects imposed by the zeolite pore, the alkali metal cation (size and electropositivity) exchange, and the degree of saturation of the adsorbates can all play a significant role in the adsorption behavior in zeolites. Our calculated adsorption energy differences between alkanes and alkenes in cation-exchanged zeolites can serve as a guide for experiments focusing on the separation of C4 hydrocarbons.
■
INTRODUCTION Separation of light hydrocarbons is an important component of unit operations in industrial processes that currently requires extensive energy expenditure and capital investments.1 Alternatives to existing separation technologies such as distillation1 could increase the separation selectivities and associated process efficiency.1 Adsorption-based separation processes have been evaluated by numerous authors1−6 as promising alternative separation methods. The adsorptionbased separation schemes leverage physical and chemical properties of sorbents and the corresponding adsorbates to maximize separation efficiency. Porous materials have shown promise as effective sorbents and include diverse classes of materials such as zeolites and metal−organic frameworks (MOFs).2 Zeolites are a subclass of aluminosilicate materials, characterized by regular pore structure and shown to be remarkably stable under a wide range of experimental conditions.3 The well-understood nature of these materials makes them attractive candidates for separations of light hydrocarbons via physical adsorption processes. There are over 200 known zeolite frameworks that have been characterized in terms of type of channel system, porosity, pore-size distribution, and many additional properties.7 Several zeolites have been studied extensively, based on their applicability in the field of petrochemical processing1,3,8−12 for both their adsorption and catalytic properties. For © XXXX American Chemical Society
adsorption applications, low Si/Al ratio acidic zeolites are typically not preferred, as the presence of Brønsted acid sites (from the presence of framework Al) may result in undesired reactions, like polymerization, dimerization, etc. Generally, high-silica or cation-exchanged zeolites are preferred in separations applications, because, in addition to reducing catalytic activity, the specific interactions between feed molecules and cations within the zeolite pores can result in preferential adsorption and thus separation selectivity.4−6,13,14 Conventional zeolite frameworks for which adsorption of hydrocarbons has been a major research focus include LTA, MFI, FAU, CHA, and others.3−6,11,15−17 A number of studies have investigated the adsorption of hydrocarbons (saturated and unsaturated) in zeolites containing acidic sites.18−20 Numerous reports have described the successful selective adsorption of small-chain hydrocarbons in cation-exchanged zeolites8,9,12,21,22 for the separation of linear and branched alkanes4,23,24 and alkane/alkene mixtures.4−6,9,16 However, many zeolites have not been examined for light hydrocarbon adsorption applications yet, exhibiting a large parameter space, Received: Revised: Accepted: Published: A
March 7, 2017 May 26, 2017 June 1, 2017 June 1, 2017 DOI: 10.1021/acs.iecr.7b00972 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
substitution and saturation of the adsorbed hydrocarbons, providing design guidelines for selective hydrocarbon separations.
including pore geometry, cation type, and cation loading that could potentially be optimized to maximize selective separation. The total cation concentration within the zeolite network can be tuned by controlling the total framework Si4+ to Al3+ ratio during synthesis. Higher loadings of Al will result in greater charge imbalance and higher concentration of cations (active sites) within the zeolite pore network. The positively charged ions are preferentially localized in the center of the sixmembered tetrahedral rings (6T) within the zeolite pore (Figure 1b).25 As stated previously, cation-exchanged zeolites
■
COMPUTATIONAL METHODS Density Functional Theory (DFT) has been a widely used computational approach to model zeolite-adsorbate interactions.25,28−30 However, the large size of the zeolite systems is computationally prohibitive to be treated using quantum mechanical (QM) calculations. Thus, we apply the ONIOM method (Our Own N-layered Integrated Molecular Orbital and Molecular Mechanics)29 to reduce computational costs associated with a large system size, while maintaining the accuracy of our calculations. ONIOM is a hierarchical method combining both the size of the system and the accuracy of theory that approximates an accurate (full QM) calculation for the full-size system. We implement the ONIOM method using the Gaussian 09 software and partition scheme that divides the energetic analysis of the system into three (ONIOM3) levels of theory.31 The ONIOM method as applied to the ZSM-12 zeolite is shown in Figure 1, with a three-layer partitioning scheme that consists of a 28-atom High Level system, a 34atom Intermediate system, and a 369-atom Low Level model. We applied two different levels of theory to the aforementioned partitioning scheme. In the first set of calculations the Molecular Mechanics (UFF) was used for Low-Level calculations, the Semi-Empirical (PM6) method was used for the Intermediate-Level system, and the hybrid M06-2X density functional of Truhlar and Zhao was used for the HighLevel system32−34 (M06-2X/6-31G(d,p): PM6: UFF). In the second set of calculations, the Intermediate-Level system was modeled at the HF/3-21G(d) QM level of theory, while maintaining the previous configuration for the High and Low level calculations (M06-2X/6-31G(d,p): HF/3-21G(d): UFF). The atoms in the Low and Intermediate layers were kept frozen in their crystallographic positions as reported by LaPierre et al.,35 while the High-Level atoms were allowed to relax during geometry optimization calculations. The lattice position of Al ions in the zeolite framework has been shown to influence adsorption of molecules in zeolites, as reported in recent publications.36 In addition, the degree of Al incorporation into zeolite frameworks has been shown to alter the binding of adsorbates on extra-framework cation sites, by presenting additional steric effects.37 The MTW framework has seven unique tetrahedral sites, and recent work by Fellah25 has shown the intersection of the 6T ring, sharing an edge with a neighboring 5T ring as the favorable Al position (shown in Figure 1(b)). The dangling bonds on the outer oxygen atoms of the Low layer were saturated with hydrogen atoms. The gasphase electronic binding energies (BE) of C4 hydrocarbons were calculated using eq 1
Figure 1. ONIOM3 partitioning scheme chosen for the study of C4 adsorption energies in the MTW framework. The layers are visualized and represented as ball-and-stick for the High Level, stick for the Intermediate Level, and line representation for the Low Level for (a) the Si-only ZSM-12 zeolite and (b) cation-exchanged ZSM-12.
are typically preferred for adsorption applications. However, an experimental study by Maes et al. showed that high-silica zeolites can be used to selectively separate C-5 alkene mixtures.5 The authors did not report reactions of alkenes on the acid sites, although possible reactions were not ruled out. In addition, Guo et al.8 were able to show a similar effect by modeling a cation-exchanged ZSM-5 zeolite, that significantly improved adsorption performance in the zeolite pores by decreasing the Si/Al ratio (increasing number of cationic sites). Peralta et al.26 observed qualitative trends in the adsorption of organic molecules in zeolite and metal−organic framework (MOF) materials with extraframework cations and demonstrated that the hydrocarbons bind in the following order: aromatics > alkenes > alkanes. Smit et al.3 summarized sorption properties of cation-exchanged zeolites as primarily being a function of framework Si/Al ratio and type of cation used. Finally, Göltl et al.27 have shown that saturated hydrocarbons interact with the inner walls of the zeolites via van der Waals interactions. The focus of this work is to screen cation-exchanged zeolite materials as sorbents for the selective separation of C4 hydrocarbons and understand the adsorption behavior as a function of the degree of saturation and branching. Using ab initio electronic structure calculations, we investigate a ZSM-12 zeolite (vide inf ra) system exchanged with three alkali cations Li+, Na+, and K+ as a porous sorbent, by quantifying the adsorption of seven C4 hydrocarbons of varying substitution, namely, n-butane, isobutane, 1-butylene, trans-2-butylene, cis-2butylene, isobutylene, and 1,3-butadiene. It should be noticed that all these hydrocarbons have very close boiling points, making a distillation-based separation very challenging. To the best of our knowledge, this is the first study to examine the adsorption of these C4 hydrocarbons in pure-silica and cationexchanged ZSM-12. Our work systematically addresses the role of the cations and pore effects of the zeolite, as well as the
BE = EONIOM3,ads − EONIOM3,zeolite − E high,C4
(1)
where EONIOM3, ads and EONIOM3, zeolite are ONIOM3 energies of the zeolite with the adsorbed hydrocarbon and the zeolite system, respectively, and Ehigh,C4 is the QM-level energy of hydrocarbons. This choice of methods allows us to accurately account for dispersion interactions between the adsorbates and the zeolite pore walls. Multiple adsorbate configurations were examined, to ensure stable minima for the adsorbates in question, and the corresponding energies were reported. In addition, binding energies (BE) of the C4 hydrocarbons to gas-phase Li+, Na+, and K+ cations were calculated using B
DOI: 10.1021/acs.iecr.7b00972 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research three DFT functionals: B3LYP,38,39 M06-2X,34 and ωB97XD,40 combined with the 6-311G* basis set (for purposes of comparison). BEs were calculated according to eq 2 BE = Eads − EC4 − Ecation
species with dc slightly larger than the pore diameter of the ZSM-12, given that the pores of the zeolites have flexibility). In addition to the size difference in dc of adsorbates (branched vs linear), we screened three alkali cations as candidates for zeolite cation-exchange. Figure 3(a) shows the calculated cation (Li+, Na+, K+) BEs on the hydrocarbons in question. We observe that the interactions are significantly stronger (ca. 30 kJ/mol) in alkenes compared to alkanes, for all three cations. In addition, the type of cation has a significant effect on the interaction energy and follows a periodic electropositivity trend (Li > Na > K) as shown in the case of alkali metal cations interacting with the walls of carbon nanotubes.41 The calculated BEs are in perfect agreement with those of Tielens et al.,12 who calculated BE values for butylene isomers (1-, cis-2-, trans-2-, and isobutylene) using B3LYP/6311+G** methodology. The more unsaturated species, 1,3butadiene (C4H6), was found to bind the cations the strongest, as the presence of two double bonds resulted in increased cation-π (electrostatic) interactions. Finally, a minor trend in BE is observed for the C4H8 species (one double bond) interacting with all cations. It appears that the location of the double bond on the hydrocarbon has an effect on the interaction energy for these species, with isobutylene binding the strongest due to the bulky nature of the molecule. Figure 3(b) examines the effect of different theoretical methods (functionals) in calculating the BE of the alkali cations on the C4 species. Very good agreement was observed for the calculated BEs between functionals that account for dispersion forces (M06-2X and ωB97XD) and the B3LYP functional. The BEs within the pure-Si zeolite pore as well as Li+-, Na+-, and K+-exchanged ZSM-12 were calculated for n-butane, isobutane, 1-butylene, trans-2-butylene, cis-2-butylene, isobutylene, and 1,3-butadiene. Ground-state adsorption configurations of n-butane and isobutylene in pure-silica and sodiumexchanged ZSM-12 are shown in Figure 4. These snapshots represent variability in the C4 molecular structure (linear vs branched) and the type of bonds present (alkane vs alkene). Calculated BEs of the seven C4 hydrocarbons in all cationexchanged zeolites are reported in Figure 5, as well as in Table 2 in an identical, sequential order. Snapshots of all ground-state geometries at this level of theory are reported in Figures S1−S4 in the SI. Several important observations can be made from the results presented in Figure 5 and Table 2. First, the binding behavior of all seven C4 species was significantly different in pure-silica ZSM-12 compared to the cation-exchanged zeolites. The BEs of alkenes increased significantly in the presence of cations, while those of alkanes decreased, when compared to pure-silica zeolite pores. Overall, the BEs of the C4 species in the cationexchanged zeolite correlate with cation electropositivity (Li > Na > K), in agreement with the molecular calculations. An exception is the case of (i) n-butane, which binds all the cationexchanged zeolites with approximately the same affinity, and (ii) butadiene, which binds the K-exchanged zeolite slightly stronger than the Na-exchanged one. The adsorption of hydrocarbons in pure-silica ZSM-12 (Figure S1) clearly follows a different trend than the cationexchanged zeolite. It can be generally concluded that the alkanes (n-butane, isobutane) bind stronger than the alkenes of the pure-silica zeolite pores, with mean BE values of −100 kJ/ mol and −56 kJ/mol, respectively. This is primarily due to hydrogen bonds between the hydrogens of the hydrocarbons and the framework oxygen atoms. The BEs of n-butane, 1-
(2)
where Eads, EC4, and Ecation correspond to the gas-phase electronic energies of the cation-adsorbate complex, hydrocarbon, and the cation in question, respectively.
■
RESULTS AND DISCUSSION A high-silica zeolite ZSM-12 was chosen as the model system. It has a one-dimensional pore structure and MTW framework and was first synthesized by Rosinski et al.7,10,17 It was chosen based on its internal pore accessibility to the adsorbates in question,10,25,35 simplicity of 1-d cylindrical pore architecture,35 and stability of the zeolite,17,35 with the accessible volume and specific surface area of 9.4% and 692.2 m2/g, respectively. Onedimensional projections of the zeolite framework unit cell and the model system chosen in this study are shown in Figures 2(a) and (b), respectively.
Figure 2. (a) Graphical representation (1-d projection) of the ZSM-12 unit cell and (b) the inner pore of ZSM-12 along the pore axis.7
We calculated the critical diameter (dc) of the seven C4 hydrocarbons considered in this work, which include linear and branched species, as shown in Table 1, using three different Table 1. C4 Hydrocarbons Investigated in This Study with Critical Diameter Values Calculated Using Three Density Functionals (B3LYP, M06-2X, and ωB97XD) dc (Å) species
B3LYP
M06-2X
ωB97XD
n-butane isobutane 1-butylene trans-2-butylene cis-2-butylene isobutylene 1,3-butadiene
5.46 6.74 5.47 5.45 5.51 6.54 5.51
5.45 6.73 5.47 5.45 5.51 6.52 5.51
5.45 6.73 5.47 5.45 5.51 6.53 5.51
functionals. The critical diameter, typically used in gas adsorption, is a quantitative measure of molecular size, defined as the minimum diameter of a cylinder that would circumscribe the molecule in the gas phase, including the van der Waals radii of hydrogen (1.2 Å). The values remain practically identical when calculated using the three different methods. The molecular dimensions of adsorbates were used in selection of the zeolite. The pore diameter of the ZSM-12 zeolite shown in Figure 2 is 6.3 Å, which is on the upper end of the dc of the adsorbates, potentially maximizing the interactions in the zeolite pores (we cannot exclude adsorption of the branched C
DOI: 10.1021/acs.iecr.7b00972 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 3. (a) DFT-calculated Li+, Na+, and K+ BEs on C4 hydrocarbons at the B3LYP level of theory. (b) Parity plot comparing BEs calculated with two functionals that include dispersion (M06-2X − red squares and ωB97XD − blue diamonds) to B3LYP-calculated values. The dotted line represents the parity y = x.
Figure 4. Ground-state adsorbed configurations of n-butane and isobutylene in pure-silica ZSM-12 (a, c) and Na-ZSM-12 (b, d), respectively.
and −128 kJ/mol, respectively. As reported previously for cation-only BEs, Li-exchanged zeolite bound adsorbates the strongest, followed by Na- and K-exchanged cases. The BE trends were nearly identical in the cases of Li+ and Na+ zeolites (Figure 5). Notably, the BE of 1,3-butadiene was highest among all C4 species in Li-exchanged ZSM-12 which can be attributed to the small radius of the Li cation, that localizes along the plane of the T6 ring of the zeolite and does not occupy available space within the ZSM-12 pore, thus resulting in an enhanced interaction of the diene with the zeolite.
butylene, trans-2-butylene, and cis-2-butylene in pure-silica zeolite were calculated to be −83 kJ/mol, −67 kJ/mol, −119 kJ/mol, and −37 kJ/mol, respectively. The remaining alkenes (isobutylene and 1,3-butadiene) bound considerably weaker, with BE values of −28 kJ/mol for both cases. In the case of cation-exchanged zeolite, the trends observed in pure-Si zeolite reverse, and the BE values are generally greater for alkenes when compared to alkanes as demonstrated by the values reported in Table 2. The BEs of alkenes in Li-, Na-, and K-exchanged zeolite were higher than in the case of pure-silica, with average values of −169 kJ/mol, −135 kJ/mol, D
DOI: 10.1021/acs.iecr.7b00972 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 5. ONIOM3-calculated adsorbate BEs for all C4 hydrocarbons for clean and Li+-, Na+-, and K+-ZSM-12 at the (M06-2X/631G(d,p):PM6:UFF) level of theory.
Figure 6. ONIOM3-calculated adsorbate BEs for all C4 hydrocarbons for Li+-, Na+-, and K+-doped ZSM-12 at the (M06-2X/6-31G(d,p):HF/3-21G(d):UFF) level of theory.
Table 2. C4 BEs for Pure-Silica (Si) and Cation-Exchanged (Li+, Na+, K+) ZSM-12 Zeolitesa n-butane isobutane 1-butylene trans-2-butylene cis-2-butylene isobutylene 1,3-butadiene
Si
Li+
Na+
K+
−83 −116 −67 −119 −37 −28 −28
−62 −100 −159 −168 −178 −141 −197
−64 −74 −147 −156 −156 −131 −85
−68 −69 −145 −140 −143 −116 −94
of the saturated hydrocarbons, in agreement with the trends observed in Figure 5 and Table 2 (medium level treated with PM6). Interesting enough, the repulsion interactions between the electron-rich double bonds of cis-2-butylene, isobutylene, 1,3-butadiene, and the framework oxygens result in endothermic adsorption energies. Additionally, in agreement with the trends observed in the previous partition scheme (medium level PM6), the pure-Si trends are nearly reversed in the case of cation-doped zeolites. The saturated hydrocarbons were found to bind cationic sites weaker relative to alkenes at the new level of theory for all three cations in question. The periodic electropositivity (Li > Na > K) BE trend observed at the M06-2X/6-31G(d,p):PM6:UFF level of theory is also observed, with the Li-exchanged zeolite exhibiting the strongest adsorption for all the unsaturated hydrocarbons (Figure 6). In addition, the calculated Li-ZSM-12 BE of 1,3-butadiene was found to be the highest among all C4 species (−138 kJ/mol) binding the Li-exchanged zeolite, in agreement with the results observed at the (M06-2X/631G(d,p):PM6:UFF) level of theory. The observed trends remain in agreement with the previous set of calculations, favoring the adsorption of unsaturated alkenes on the cation site of exchanged ZSM-12 zeolite relative to saturated alkanes. The average alkane BEs for Li-, Na-, and K-exchanged ZSM-12 were calculated to be −24 kJ/mol, −12 kJ/mol, and −14 kJ/ mol, while the average alkene BEs were −114 kJ/mol, −82 kJ/ mol, and −73 kJ/mol, respectively. From a computational perspective, one can observe that changing the level of theory in the medium layer of the zeolite from PM6 to HF resulted in less favorable (less exothermic) adsorption of all the hydrocarbon species. A detailed BE comparison (parity plot) between the two different levels of theory is presented in Figure S9. The BEs are on average 54− 61 kJ/mol more exothermic (more negative) when the medium layer is treated with PM6 than with HF. This rationalizes the importance of the pore structure in the overall adsorption, especially for zeolite pores close to the critical diameter of the adsorbates (Table 1). In addition, it underscores the sensitivity of the BEs to the method selected treating the pore atoms. Although the total energies are consistently shifted in a way that the BE trends hold between the two levels of theory, there are cases such as the cis-2-butylene, isobutylene, and 1,3-butadiene, which change from an exothermic (PM6-medium layer) to an endothermic (HF-medium layer) adsorption in the pure-silica
a
Energies are reported in kJ/mol and negative values indicate exothermicity (favorable adsorption).
To address the role of the surrounding environment (pore atoms) in the adsorption of the hydrocarbons, we modified the level of theory on the Medium layer from PM6 (semiempirical) to ab initio (HF/3-21G(d)), and we repeated all the calculations using the (M06-2X/6-31G(d,p):HF/3-21G(d):UFF) level of theory. The calculated BEs for the seven C4 hydrocarbons are reported in Table 3 and Figure 6, with ground-state snapshots reported in Figures S5−S8 of the SI. Table 3. Adsorbate Binding Energies at the (M06-2X/631G(d,p):HF/3-21G(d):UFF) Level of Theory for the Clean and Cation-Exchanged ZSM-12 Zeolite Pore Systema n-butane isobutane 1-butylene trans-2-butylene cis-2-butylene isobutylene 1,3-butadiene a
Si
Li+
Na+
K+
−26 −60 −6 −49 +22 +31 +34
−6 −42 −109 −110 −128 −85 −138
−10 −15 −98 −104 −99 −77 −32
−16 −12 −85 −89 −93 −59 −41
Values are reported in kJ/mol.
The majority of the C4 isomers were found to bind weakly the pure-silica ZSM-12, with isobutane BE of −60 kJ/mol being the highest for all branched species. It should be noticed that the isobutane exhibits the largest critical diameter (dc) among all the C4 species, maximizing the hydrogen bond interactions between the methyl-group hydrogens and the pore oxygens. The majority of the BEs calculated for the C4 alkene isomers and 1,3-butadiene were found to be generally lower than those E
DOI: 10.1021/acs.iecr.7b00972 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
enhance the adsorption of alkenes relative to alkanes for all cation species (Li, Na, K). Electrostatic interactions of metal cations with π-electron systems of alkenes resulted in high binding energies (BEs), while pure-Si systems exhibited the opposite trends, favoring the adsorption of alkanes rather than unsaturated alkenes. Local steric effects induced by the atomic radii of the cations play an important role in affecting hydrocarbon adsorption within the pores of the ZSM-12 zeolite, generally favoring smaller cation radii (Li+). Linear alkenes were found to exhibit stronger interactions with cation sites relative to branched hydrocarbons, which was attributed to steric constraints imposed by the inner pore structure. Two sets of theoretical methods applied to the partitioning scheme resulted in very good agreement in terms of the overall adsorbate BE trends, rationalizing, however, the importance of the zeolite pore in the overall BE of species exhibiting critical diameters close to the pore size of the zeolite. Leveraging the differences in adsorbate binding in cation-exchanged zeolites shown in this work can guide experimental work in the separations of light hydrocarbons.
zeolite. Although we observe a strong agreement in the reported trends between the two models used to compute the hydrocarbon BEs in the zeolites (uniform BE offset between the two methods), it is difficult to make a conclusive recommendation for applying a particular level of theory over another. However, it has been shown that applying QM methods to the layers surrounding the High-Level atoms produces more accurate results in molecular adsorption in acid zeolites.42 In addition, Harlfinger et al. have reported experimental adsorption enthalpies of butylene isomers in Na-X zeolite on the order of −60 kJ/mol,9 which is in greater agreement with our results, reported at the (M06-2X/631G(d,p):HF/3-21G(d):UFF) level of theory for Na-ZSM-12, as shown in Figure 6 and Table 3. As a final note, enthalpic and entropic contributions can affect adsorption thermodynamics (Gibbs binding free energy). We have calculated the entropic losses from the interaction of the hydrocarbons with the highlevel system, to be in the range of −159.8 to −171.5 J/(mol K) (see Table S1 and Figure S10). From a separations perspective, this work shows that cationexchanged ZSM-12 zeolites promote the adsorption of alkenes vs alkanes within the zeolite pore, due to the cation-π interactions. On the other hand, the adsorption of saturated hydrocarbons is favored in pure zeolite frameworks, due to enhanced hydrogen bonding between the alkanes and the framework oxygens. Experimental studies by a number of authors6,9,12,13,43 have examined adsorption of saturated and unsaturated hydrocarbons in (monovalent) cation-exchanged zeolites. The Henry coefficients and adsorption energies were shown to be greater for the olefin species relative to paraffins in all cases, as shown by Palmas,13 Peralta,43 and Harlfinger,9 in agreement with the computational adsorption trends reported here. In addition, experimental data for different siliceous sorbents and adsorbates, reported by Pham et al.14 and Sun et al.,11 show an adsorption preference toward saturated hydrocarbons vs olefins of identical chain length, also in strong agreement with the trends reported in this work. It is worth noting that the pore size of the ZSM-12 (close to the dc of the C4 species), in conjunction with the cation-π interactions (electropositivity of alkali metal) and the alkali metal size, results in a very similar adsorption behavior of the C4 species in Na- and K-ZSM-12 zeolites (slightly favoring adsorption in the Na-exchanged zeolite) and to an enhanced adsorption in the LiZSM-12 zeolite. Importantly, we note that all these effects can contribute to significantly enhancing the adsorption of a specific species: The 1,3-butadiene was unbound in the puresilica ZSM-12 zeolite, it intermediately bound the Na- and Kexchanged ZSM-12 zeolites (−32 and −41 kJ/mol, respectively), and it very strongly bound the Li-exchanged ZSM-12 zeolite (−138 kJ/mol) as shown in Figure 6. Finally, for the alkanes/alkenes separations, from an experimental point of view, sodium (Na+) could be more preferable as it has been shown that, based on its small ionic radii, Li-exchange can alter the zeolite framework44 lattice parameters and possibly distort the pore structure. Finally, the relative ease and availability of techniques for cation exchange and characterization45 make the selection of Na-exchanged zeolite a potential candidate for the separation of alkenes from alkanes.
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00972. Graphical snapshots of ground-state adsorption configurations of all adsorbate−zeolite combinations and relevant adsorption energy analysis at the different levels of ONIOM theory (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Giannis Mpourmpakis: 0000-0002-3063-0607 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Acknowledgment is made to Lubrizol Inc. and the Donors of the American Chemical Society Petroleum Research Fund (ACS-PRF 56989-DNI5) for support of this research, as well as to the Center for Simulation and Modeling (SAM) at the University of Pittsburgh for computational support.
■
REFERENCES
(1) Eldridge, R. B. Olefin paraffin separation technology - a review. Ind. Eng. Chem. Res. 1993, 32 (10), 2208−2212. (2) Hartmann, M.; Kunz, S.; Himsl, D.; Tangermann, O.; Ernst, S.; Wagener, A. Adsorptive separation of isobutene and isobutane on Cu(3) (BTC)(2). Langmuir 2008, 24 (16), 8634−8642. (3) Smit, B.; Maesen, T. L. M. Molecular simulations of zeolites: Adsorption, diffusion, and shape selectivity. Chem. Rev. 2008, 108 (10), 4125−4184. (4) Fuchs, A. H.; Cheetham, A. K. Adsorption of guest molecules in zeolitic materials: Computational aspects. J. Phys. Chem. B 2001, 105 (31), 7375−7383. (5) Maes, M.; Alaerts, L.; Vermoortele, F.; Ameloot, R.; Couck, S.; Finsy, V.; Denayer, J. F. M.; De Vos, D. E. Separation of C-5Hydrocarbons on Microporous Materials: Complementary Performance of MOFs and Zeolites. J. Am. Chem. Soc. 2010, 132 (7), 2284− 2292.
■
CONCLUSIONS Based on ab initio molecular and embedded cluster (ONIOM3) calculations performed in this work, we demonstrate that alkali cations, exchanged into the ZSM-12 zeolite framework, F
DOI: 10.1021/acs.iecr.7b00972 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research (6) Padin, J.; Yang, R. T.; Munson, C. L. New sorbents for olefin/ paraffin separations and olefin purification for C-4 hydrocarbons. Ind. Eng. Chem. Res. 1999, 38 (10), 3614−3621. (7) Baerlocher, C., McCusker, L. B. Database of Zeolite Structures. http://www.iza-structure.org/databases/ (accessed March 2015). (8) Guo, X. D.; Huang, S. P.; Teng, J. W.; Xie, Z. K. Adsorption of isobutene on Na(n)ZSM-5 type zeolite with various Si/Al ratios: Molecular simulation study. Chin. J. Chem. 2005, 23 (12), 1593−1599. (9) Harlfinger, R.; Hoppach, D.; Quaschik, U.; Quitzsch, K. Adsorption of C-4 hydrocarbons on X-zeolites containing Li+, Na+, K+, Rb+ and Cs+ cations. Zeolites 1983, 3 (2), 123−128. (10) Rosinski, E. J., Rubin, M. K. Crystalline zeolite zsm-12. 3832449, 1974. (11) Sun, M. S.; Shah, D. B.; Xu, H. H.; Talu, O. Adsorption equilibria of C(1) to C(4) alkanes, CO(2), and SF(6) on silicalite. J. Phys. Chem. B 1998, 102 (8), 1466−1473. (12) Tielens, F.; Denayer, J. F. M.; Daems, I.; Baron, G. V.; Mortier, W. J.; Geerlings, P. Adsorption of the butene isomers in faujasite: A combined ab-initio theoretical and experimental study. J. Phys. Chem. B 2003, 107 (40), 11065−11071. (13) Palmas, S.; Polcaro, A. M.; Carta, R.; Tola, G. Sorption and diffusion of light-hydrocarbons on Na-Y zeolites. J. Chem. Eng. Data 1991, 36 (1), 1−4. (14) Pham, T. D.; Lobo, R. F. Adsorption equilibria of CO2 and small hydrocarbons in AEI-, CHA-, SIT-, and RRO-type siliceous zeolites. Microporous Mesoporous Mater. 2016, 236, 100−108. (15) Gladden, L. F.; Hargreaves, M.; Alexander, P. Monte Carlo lattice dynamics studies of binary adsorption in silicalite. Chem. Eng. J. 1999, 74 (1−2), 57−66. (16) Freeman, C. M.; Catlow, C. R. A.; Thomas, J. M.; Brode, S. Computing the location and energetics of organic-molecules in microporous adsorbents and catalysts - a hybrid approach applied to isomeric butenes in a model zeolite. Chem. Phys. Lett. 1991, 186 (2− 3), 137−142. (17) Piccione, P. M.; Yang, S. Y.; Navrotsky, A.; Davis, M. E. Thermodynamics of pure-silica molecular sieve synthesis. J. Phys. Chem. B 2002, 106 (14), 3629−3638. (18) Jae, J.; Tompsett, G. A.; Foster, A. J.; Hammond, K. D.; Auerbach, S. M.; Lobo, R. F.; Huber, G. W. Investigation into the shape selectivity of zeolite catalysts for biomass conversion. J. Catal. 2011, 279 (2), 257−268. (19) Namuangruk, S.; Tantanak, D.; Limtrakul, J. Application of ONIOM calculations in the study of the effect of the zeolite framework on the adsorption of alkenes to ZSM-5. J. Mol. Catal. A: Chem. 2006, 256 (1−2), 113−121. (20) De Moor, B. A.; Reyniers, M. F.; Marin, G. B. Physisorption and chemisorption of alkanes and alkenes in H-FAU: a combined ab initiostatistical thermodynamics study. Phys. Chem. Chem. Phys. 2009, 11 (16), 2939−2958. (21) Fischer, M.; Bell, R. G. Cation-exchanged SAPO-34 for adsorption-based hydrocarbon separations: predictions from dispersion-corrected DFT calculations. Phys. Chem. Chem. Phys. 2014, 16 (39), 21062−21072. (22) Daems, I.; Leflaive, P.; Methivier, A.; Denayer, J. F. M.; Baron, G. V. A study of packing induced selectivity effects in the liquid phase adsorption of alkane/alkene mixtures on NaY. Microporous Mesoporous Mater. 2005, 82 (1−2), 191−199. (23) Vlugt, T. J. H.; Krishna, R.; Smit, B. Molecular simulations of adsorption isotherms for linear and branched alkanes and their mixtures in silicalite. J. Phys. Chem. B 1999, 103 (7), 1102−1118. (24) Vlugt, T. J. H.; Zhu, W.; Kapteijn, F.; Moulijn, J. A.; Smit, B.; Krishna, R. Adsorption of linear and branched alkanes in the silicalite1. J. Am. Chem. Soc. 1998, 120 (22), 5599−5600. (25) Fellah, M. F. Hydrogen adsorption on M-ZSM-12 zeolite clusters (M = K, Na and Li): a density functional theory study. J. Porous Mater. 2014, 21 (5), 883−888. (26) Peralta, D.; Chaplais, G.; Simon-Masseron, A.; Barthelet, K.; Chizallet, C.; Quoineaud, A.-A.; Pirngruber, G. D. Comparison of the
Behavior of Metal-Organic Frameworks and Zeolites for Hydrocarbon Separations. J. Am. Chem. Soc. 2012, 134 (19), 8115−8126. (27) Goltl, F.; Gruneis, A.; Bucko, T.; Hafner, J. Van der Waals interactions between hydrocarbon molecules and zeolites: Periodic calculations at different levels of theory, from density functional theory to the random phase approximation and Møller-Plesset perturbation theory. J. Chem. Phys. 2012, 137 (11), 114111. (28) Chung, L. W.; Sameera, W. M. C.; Ramozzi, R.; Page, A. J.; Hatanaka, M.; Petrova, G. P.; Harris, T. V.; Li, X.; Ke, Z.; Liu, F.; Li, H.-B.; Ding, L.; Morokuma, K. The ONIOM Method and Its Applications. Chem. Rev. 2015, 115, 5678. (29) Morokuma, K.; Froese, R. D.; Dapprich, S.; Komaromi, I.; Khoroshun, D.; Byun, S.; Musaev, D. G.; Emerson, C. L. The ONIOM (our own integrated N-layered molecular orbital and molecular mechanics) method, and its applications to calculations of large molecular systems. Abstr. Pap. Am. Chem. S. 1998, 215, U218−U218. (30) Courtney, T. D.; Nikolakis, V.; Mpourmpakis, G.; Chen, J. G.; Vlachos, D. G. Liquid-phase dehydration of propylene glycol using solid-acid catalysts. Appl. Catal., A 2012, 449, 59−68. (31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2013. (32) Stewart, J. J. P. Optimization of parameters for semiempirical methods V: Modification of NDDO approximations and application to 70 elements. J. Mol. Model. 2007, 13 (12), 1173−1213. (33) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. UFF, A full periodic-table force-field for molecular mechanics and molecular-dynamics simulations. J. Am. Chem. Soc. 1992, 114 (25), 10024−10035. (34) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120 (1−3), 215−241. (35) Lapierre, R. B.; Rohrman, A. C.; Schlenker, J. L.; Wood, J. D.; Rubin, M. K.; Rohrbaugh, W. J. The framework topology of ZSM-12 a high-silica zeolite. Zeolites 1985, 5 (6), 346−348. (36) Ghorbanpour, A.; Rimer, J. D.; Grabow, L. C. Periodic, vdWcorrected density functional theory investigation of the effect of Al siting in H-ZSM-5 on chemisorption properties and site-specific acidity. Catal. Commun. 2014, 52, 98−102. (37) Oleksiak, M. D.; Ghorbanpour, A.; Conato, M. T.; McGrail, B. P.; Grabow, L. C.; Motkuri, R. K.; Rimer, J. D. Synthesis Strategies for Ultrastable Zeolite GIS Polymorphs as Sorbents for Selective Separations. Chem. - Eur. J. 2016, 22 (45), 16078−16088. (38) Becke, A. D. Density-functional thermochemistry 0.3. The role of exact exchange. J. Chem. Phys. 1993, 98 (7), 5648−5652. (39) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the collesalvetti correlation-energy formula into a functional of the electrondensity. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37 (2), 785− 789. (40) Chai, J.-D.; Head-Gordon, M. Systematic optimization of longrange corrected hybrid density functionals. J. Chem. Phys. 2008, 128 (8), 084106. G
DOI: 10.1021/acs.iecr.7b00972 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research (41) Mpourmpakis, G.; Froudakis, G. Why alkali metals preferably bind on structural defects of carbon nanotubes: A theoretical study by first principles. J. Chem. Phys. 2006, 125 (20), 204707. (42) Patet, R. E.; Caratzoulas, S.; Vlachos, D. G. Adsorption in zeolites using mechanically embedded ONIOM clusters. Phys. Chem. Chem. Phys. 2016, 18 (37), 26094−26106. (43) Peralta, D.; Chaplais, G.; Simon-Masseron, A.; Barthelet, K.; Chizallet, C.; Quoineaud, A. A.; Pirngruber, G. D. Comparison of the Behavior of Metal-Organic Frameworks and Zeolites for Hydrocarbon Separations. J. Am. Chem. Soc. 2012, 134 (19), 8115−8126. (44) Kayiran, S. B.; Darkrim, F. L. Synthesis and ionic exchanges of zeolites for gas adsorption. Surf. Interface Anal. 2002, 34 (1), 100−104. (45) Inglezakis, V. J. The concept of ″capacity″ in zeolite ionexchange systems. J. Colloid Interface Sci. 2005, 281 (1), 68−79.
H
DOI: 10.1021/acs.iecr.7b00972 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
