Experimental and Theoretical Study of Propene Adsorption on K-FER

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Experimental and Theoretical Study of Propene Adsorption on K-FER Zeolites: New Evidence of Bridged-Complex Formation Miroslav Rubeš, Eva Koudelková, Francisca Solanea de Oliveira Ramos, Michal Trachta, Ota Bludsky, and Roman Bulánek J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12706 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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The Journal of Physical Chemistry

Experimental and Theoretical Study of Propene Adsorption on K-FER Zeolites: New Evidence of Bridged-Complex Formation

Miroslav Rubeš1, Eva Koudelková2, Francisca Solanea de Oliveira Ramos2, Michal Trachta1, Ota Bludský1, Roman Bulánek*2

1

Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech

Republic, Flemingovo nám. 2, 162 10 Prague, Czech Republic 2

Department of Physical Chemistry, Faculty of Chemical Technology, University of

Pardubice,

Studentská

573,

532

10

Pardubice,

[email protected]

ACS Paragon Plus Environment

Czech

Republic

E-mail:

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

The interaction of propene with K-FER zeolites was investigated by a combination of IR spectroscopy, adsorption calorimetry and theoretical study. Periodic DFT calculations were performed using the DFT/CC scheme based on the PBE density functional for the description of the interaction between propene and K-FER zeolite. On the basis of good agreement between experimental and theoretical results, three types of adsorption complexes were identified: (i) propene adsorbed on single K+ cation sites characterized by a νC=C vibrational band at 1639 cm-1, (ii) propene bridging two nearby K+ cations in dual-cation sites represented in the IR spectra by a νC=C vibrational band at 1633 cm-1, and (iii) propene molecules interacting with the zeolite framework mainly by dispersion interactions characterized by the νC=C vibrational band at 1645 cm-1. The DFT calculations show that propene binds to the potassium cation via a cation-π interaction. The propene molecule adsorbed in the dual-cation site also exhibits the cation-π binding mode and it is stabilized by 14 kJ/mol with respect to the adsorption complex at the isolated K+ cation site. The population of such bridged complexes increases with a decreasing Si/Al ratio. The knowledge of various types of adsorption complexes, their properties and parameters influencing their population is crucial for understanding the adsorption properties of zeolites as well as their ability to separate, purify and store various gases, especially hydrocarbons.


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Introduction

Zeolites play an important role in the industry especially as adsorbents, catalysts and ion exchangers. Alkali metal-exchanged zeolites, either as membrane components or powder packed beds, can be applied as adsorbents into purification procedures and separations of different gas mixtures, e.g. of alkene/alkane,1-4 benzene/cyclohexane,5 oxygen from air,6-7 CO2 from flue gases8-16 and n-hexane from branched, cyclic and aromatic hydrocarbons.17 Extra-framework cations play the role of specific adsorption sites in these materials, whose properties are determined primarily by the nature of these cations, their localization within the zeolite channel or cavity, and their coordination to the oxygen atoms of the zeolite framework. In the middle of the last decade, experimental spectroscopic studies combined with periodic DFT calculations showed that adsorbed molecules as small as CO18-24 could interact with two (or more) extra-framework cations, giving rise to dual-cation sites (or multiple-cation sites). Such bridge carbonyl complexes were characterized by a lower stretching frequency of C-O vibration and a stronger interaction of the CO molecule with zeolite than CO on a single-cation site.25 The concept of dual-cation sites in zeolites could explain some of the observed peculiar adsorption and catalytic behavior of the zeolites, and the new knowledge of the factors influencing the heat of adsorption could be exploited in industrial gas separation and storage technologies. This stimulated further systematic research. In the paradigmatic case of CO, the study of carbon monoxide molecule interactions with alkali-metal, Mg2+, Ca2+ and Cu+ cations coordinated in FER,26-30 MFI,19,

27

BEA,18

LTA,23 FAU31 and LTL32 zeolites showed that the existence and population of complexes formed in zeolites depend on (i) the zeolite topology, (ii) the concentration of the cations (determined by the Si/Al ratio) and (iii) the cation size. Studies extended to other gas



molecules (such as CO2, NO, O2, N2 and acetylene11, 14-16, 33-41) have shown that the ability to form bridge complexes also depends on the size, geometry and electronic structure of the molecules. The main factors influencing the formation of the bridged complexes are still under debate. The size of molecule, which influence the optimal distance of the cations forming dual cationic site, and steric factors (bulkiness of the molecules and distance of the molecule adsorbed in bridged complex from channel wall) belong to the most important ones. In addition, dynamic effect will play important role especially in the case of light and weakly interacting molecules. Studies have also provided the knowledge that the existence of bridged complexes may not always be accompanied by the stabilization effect, resulting in the higher heat of adsorption. Among examples of the systems where the interaction with two cations can lead to the weakening of the interaction, one can mention the heteroionic CO/Cu+,M+FER29-30 (where M denotes the alkali-metal cation) and CO/ Cu+,Na+-FAU42 zeolite systems along with the acetonitrile interaction with Na+ cations in LTA and FER zeolites.43 In addition, it was inferred from the known data that a pair of nearby extra-framework cations in the zeolite can constitute a dual-cation site for the adsorption of a specific molecule and not for other molecules.25 In summary, it is not easy to predict whether the bridged complexes will be formed in the specific molecule/zeolite system and what role these complexes will play in the adsorption and separation. Therefore, it is necessary to investigate the adsorption of other industrially or scientifically important gases in zeolites. The relevant literature data on the nature of propeneadsorption complexes in alkali-metal zeolites characterized by IR spectroscopy are rather scarce due to the fact that the majority of the reported studies of propene interaction with zeolites concern propene adsorption onto Brønsted acid sites in H-zeolites. Generally, the shift of νC=C vibration is sensitive to changes in electron density on the olefin double bond upon interaction with the adsorbent. To our knowledge, no IR spectra of propene adsorbed on


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potassium-modified zeolites have been previously reported in the literature. Busca et al.44 have shown that the frequency shifts of the C=C vibration (free propene molecule exhibits wavenumber 1652 cm-1) are more pronounced for propene adsorbed over Na-Y zeolite (1634 cm-1) than for amorphous SiO2 (1640 cm-1). Similarly, the band at 1635 cm-1 in the IR spectra of propene on Na-Y has been reported by Gautam et al.45. Two bands at 1635 and 1645 cm-1 have been observed in the IR spectra of propene adsorbed on Na-ZSM-5.45 The existence of bridged complexes of olefins in zeolites is also scarcely reported. One of the systems where bridged complexes of unsaturated hydrocarbons (ethene and ethyne) have been predicted by theoretical calculations and identified experimentally is Cu(I)-ZSM-5 zeolite.

38-39, 41, 46

However, no information on propene adsorption in dual-cation sites of alkali metal-exchanged zeolites is available in literature. In this paper, we study the adsorption of propene (as a representative of short unsaturated hydrocarbons, whose adsorption behavior is determined by the existence of a C=C double bond) in K-FER zeolites, in which dual-cation sites for CO were identified for the first time.21 The interaction of propene molecules with K-FER zeolites of two different Si/Al ratios was investigated experimentally by FTIR spectroscopy and adsorption microcalorimetry and theoretically by DFT calculations employing a fully periodic model and the DFT/CC approach for dispersion interaction taking into account. All the bands in our reported IR spectra are red-shifted as compared to the free propene molecule, suggesting that the propene C=C vibration is strongly perturbed by the interaction with K+ cations and/or zeolite framework. The band frequency depends on the type of K+ site interacting with the propene molecule. As a result of the study, we present the evidence of the interaction of a propene molecule with two nearby potassium cations characterized by the νC=C vibration band at 1633 cm-1 and leading to significant stabilization of such complexes by 10–14 kJ/mol. Quantum chemical modeling has revealed that (i) the propene molecule interacts with extra-



framework cations via a C=C bond, (ii) bridged complexes are formed in the main channel between cations localized at intersection sites and (iii) propene adsorbed on dual-cation site exhibits a characteristic vibrational band in the region of C=C double-bond vibration, which has also been identified in the experimental IR spectra.

Experimental Materials and Experimental Procedures The K-FER zeolites used in this study were investigated previously in the adsorption of CO21,

27, 47

and CO216. Briefly, the parent NH4+-FER zeolites (supplied by the Research

Institute of Inorganic Chemistry in Ústí nad Labem (with the Si/Al ratio of 8.6) and by Zeolyst International (Si/Al = 27.5)) were converted into a potassium form by an ionexchange procedure repeated five times using 0.5 M aqueous solution of KNO3 at 313 K for one day. Complete ion-exchange was checked by the disappearance of the IR bands corresponding to either ammonium ion or Brønsted acid Si(OH)Al sites. In the whole manuscript, the zeolite samples will be labelled as K-FER-X, where X stands for the Si/Al ratio. Chemical analysis pefrormed by X-Ray Fluorescence spectroscopy (ElvaX spectrometer, Elvatech, Ukraine) estimated the amount of potassium cations to 1.12 mmol/g for K-FER-8.6 and 0.51 mmol/g for the K-FER-27.5 sample. The synthesis of the pure silica FER was performed as described in Ref.48 A solution composed of 1.5 SiO2 : 4 propylamine : 8 H2O : 2 HF : 16 pyridine was obtained by mixing pyridine and propylamine in a polypropylene beaker under magnetic stirring. After this homogenization step, concentrated hydrofluoric acid was carefully added dropwise to this initial mixture. It was vigorously stirred for 1 h at room temperature. Subsequently, fumed silica (Cab-O-Sil M5, Havel Composites CZ) was slowly added in portions, obtaining a gel. Finally, distilled water was added to obtain a clear solution, which was kept under magnetic stirring at room temperature


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for another hour. Finally, the solution was transferred to a Teflon-lined stainless autoclave and submitted to static hydrothermal treatment at 453 K for 12 days.49 The product was filtered, washed with methanol and distilled water and dried at 373 K overnight. The solid was then calcined at 823 K to remove the organic template completely. The details of the structure and the basic characteristics of the samples are reported in the Supporting Information (Figs. S1– S3 and Table S1). For FTIR study, the self-supporting wafers of K-FER samples with a density of ca 9 mg cm-2 were activated in dynamic vacuum (residual pressure < 10−5 mbar) at 723 K (the temperature was increased at the rate of 5 K/min) for at least 1 h in a home-made IR cell connected to a vacuum system. After that, wafers were calcined at a pressure of 100 mbar in O2 atmosphere at 723 K overnight to remove any carbonaceous contaminants, which was followed by final evacuation at the same temperature for 1 h. An equilibrium-pressure study with activated K-FER wafers was performed at room temperature by the step-by-step dosing of the propene into the IR cell to apply various propene pressures (ranging from vacuum to 100 mbar). Each dose was equilibrated for 20 min, after which the IR spectra were recorded. The spectrum of an empty IR cell (without a sample in the beam) with each desired propene pressure was taken as a background for each collected spectrum of the propene/K-FER system. For the assessment of adsorption-complex stabilities, series of time-resolved FTIR spectra were recorded upon the outgassing of K-FER samples equilibrated with 60 mbar of propene at room temperature for 20 minutes. All FTIR spectra (the accumulation of 32 scans with a 1cm-1 resolution) were recorded using a Nicolet 6700 spectrometer equipped with a MCT/A cryodetector. The FTIR spectra were normalized to the same sample amount using the intensity of the overtones and combination modes of the zeolite framework (2066–1721 cm-1) as an internal standard. For the analysis of the propene spectra, the spectrum of the



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dehydrated blank sample was subtracted from each spectrum of zeolite interacting with propene molecules. Adsorption

heat

was

measured

using

an

isothermal

Tian-Calvet

type

of

microcalorimeter (BT 2.15, SETARAM) combined with a home-made static volumetric adsorption device. Details on the apparatus can be found elsewhere.15,

29

Before the

calorimetric measurement, the sample (400 mg) was degassed under the same conditions as for the FTIR measurement by means of an external treatment vacuum line and transferred under vacuum to the calorimeter. The adsorption experiment was performed at 303 K by the step-by-step dosing of a known amount of propene to the cell up to an equilibrium pressure of 600 mbar. The system was equilibrated for 90 min after each dose of propene and the time dependence of the heat flow and pressure was recorded. The integral adsorption heat, the amount of propene adsorbed and the equilibrium pressure were determined for each step (dose) of the experiment. All experiments consisted of at least 45 steps.

Models and Computational Methods The periodic DFT calculations for the propene/K-FER system were performed with a double unit cell (a=19.018 Å, b=14.303Å, c=15.082 Å, and α=β=γ=900) consisting of 72 silicon and 144 oxygen atoms.50 Models for the single and dual adsorption sites in K-FER were obtained by replacing one or two silicon atoms in the double unit cell with aluminum atoms (see Figs. S4–6 in the Supporting Information). Throughout this paper, the IZA notation for the four topologically distinct T-atoms (T1 to T4) has been used.50 Note that the IZA numbering differs from the notation introduced in the previous adsorption studies on KFER with T1 and T4 interchanged.51 Only the most stable K+ sites in the vicinity of each Tatom were considered. For Al in T1 and T2 positions, the most stable site is close to the center


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of the eight-member entrance window to the perpendicular channel (P-cage). For Al in T3 and T4 positions, the K+ cation is located in the main channel and the P-cage, respectively.21 The DFT/CC correction scheme52 based on the PBE density functional53 has been used to describe the interaction between propene and the K-FER zeolite. Geometry optimizations at the PBE/CC level were converged to 10-2 eV/Å using an energy cutoff of 400 eV for the plane wave basis and Γ-point sampling of the Brillouin zone. The details of the implementation of DFT/CC and optimized geometries for all investigated adsorption complexes are provided in the Supplementary Information. Harmonic frequencies were calculated by the finitedifference method with four displacements (0.005 Å) along each Cartesian coordinate. Periodic calculations were performed using the VASP code with PAW pseudopotentials.54-55 MP2 and CCSD(T) cluster calculations have been used in the parameterization of the DFT/CC corrections and C=C stretching frequency evaluations. The calculations were performed with aug-cc-pwCVXZ basis sets for potassium and Dunning’s aug-cc-pVXZ basis sets for all the other atoms.56-58 The employed basis set is denoted as AVXZ (X=D,T). The MP2 and CCSD(T) interaction energies were calculated with the inner-shell correlation (3s, 3p) for potassium atoms. The effect of inner-shell correlation on interaction energies is shown in Table S2 (SI). The complete basis set (CBS) limit was estimated using the procedure proposed by Halkier et al.59-60 The counterpoise-corrected CCSD(T)/CBS interaction energies were evaluated as follows:

,

(1)

where MP2/CBS was obtained from the AVDZ/AVTZ extrapolation. The cluster calculations were performed using the MOLPRO quantum chemistry package.61 The corresponding DFT



calculations were carried out for cluster models inside the periodic box (20×20×20 Å) using VASP with the same settings as described in the previous paragraph.53

Results FTIR spectroscopy The C=C vibration region of the FTIR spectra of propene adsorbed on FER samples at room temperature under various equilibrium pressures up to 100 mbar of propene is shown in Figure 1. The IR absorptions that fall in the δC-H region of =CH2 and CH3 groups (1480–1360 cm-1) and the νC-H region of CH3 groups (3150–2800 cm-1) are shown in Figures S7 and S8 in the Supporting Information and will not be discussed here. Three main spectral components are visible at 1645, 1639 and 1633 cm-1 in the IR spectra of the νC=C vibration mode region. The intensity of the IR bands strongly depends on the Si/Al ratio and potassium cation content. The band at 1633 cm-1 dominates the IR spectra of the K-FER-8.6 (Fig. 1 A). In addition, a shoulder at 1639 cm-1 is clearly evident in the spectra, especially at the relatively high propene dose (high propene pressures). For the K-FER-27.5 sample (see Fig. 1B), the situation is the opposite. The relative intensity of the band at 1633 cm-1 is significantly lower than in the case of K-FER-8.6. Both bands appear simultaneously in the spectra, which are dominated by the band at 1639–40 cm-1. A new band begins to appear at 1645 cm-1 from the pressures higher than 0.5 mbar and becomes dominant at the highest pressures. These results indicate that different adsorption sites (K+ cations sites) are present in the K-FER samples. The spectra recorded on the siliceous FER zeolite in contact with propene at room temperature (Fig. 1C) exhibit only a single vibrational band at 1646 cm-1. This band increases in intensity with increasing propene pressure, but it does not change position and no other bands in the νC=C vibration region are observed. Thus, the band at 1646–45 cm-1 could be assigned to the νC=C vibration of propene molecules interacting with the zeolite wall only by


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dispersive interactions, whereas the bands at 1639 and 1633 cm-1 are related to the interaction of propene with K+ cations in the cationic sites of zeolite. For the assessment of the stability of the adsorption complexes, the rate of propene desorption from individual K-FER samples was studied by means of time-resolved IR spectra recorded during the dynamic evacuation of the samples at room temperature. In order to ensure that all K+ sites could interact with propene, the samples were equilibrated with 60 mbar of propene at room temperature for 1h, after which the equilibrium pressure was reduced to zero by switching the system to dynamic vacuum and IR spectra were measured as a function of evacuation time (see Fig. 2). The vibrational band at 1645 cm-1 progressively decreased its intensity with evacuation time and disappeared very quickly – within two minutes, as observed in Figure 2A spectrum i. At the same time, the bands at 1639 and 1633 cm-1 decreased in intensity significantly slower than the band at 1645 cm-1. Time changes in the intensities of the bands at 1639 and 1633 cm-1 are more easily observed on the K-FER-8.6 sample (Fig. 2B). It is evident from Fig. 2B that the band at 1633 cm-1 is much more stable than the band at 1639 cm-1. It is worth noting that evacuation for 1.5 h was sufficient to almost complete the desorption of propene from the K-FER-27.5 sample (Fig. 2A spectrum u), while the band at 1633 cm-1 was still visible in the IR spectra of the K-FER-8.6 sample even after 4.5 hours of evacuation (Fig. 2B, spectrum t). This significant difference in the rate of propene desorption is evidently related to the high population of the adsorption complexes characterized by the band at 1633 cm-1 in the K-FER-8.6 sample.

Adsorption microcalorimetry To obtain more detailed information on the strength of the interaction of propene with the zeolites, propene adsorption heats at 303K were measured by means of isothermal microcalorimetry. Figure 3 shows the adsorption isotherms of propene (Fig. 3A) and the



dependence of the propene heat of adsorption on the amount adsorbed (Fig. 3B) for both KFER samples and pure silica zeolite. It is evident that the adsorption isotherms differ in shape especially in the low pressure region, thus pointing to the differences in interaction energies, which are obvious in calorimetric curves. The adsorption isotherm is the steepest in the lowpressure region for K-FER-8.6 while the growth of the adsorbed amount is the least steep for pure-silica zeolite. This is in line with the adsorption heats recorded by microcalorimeter. The K-FER-8.6 sample exhibits the highest adsorption heats, starting at 77 kJ/mol for the lowest coverage, which monotonically and slowly decreases to 55 kJ/mol at the amount adsorbed equal to 1.2 mmol/g, close to the content of potassium cations in the sample. The propene adsorption heat measured on the K-FER-27.5 sample exhibits a similar trend, but it is lowered by 7–10 kJ/mol. The zero-coverage limit of the adsorption heat is about 70 kJ/mol and it systematically decreases with the increasing amount adsorbed. At high adsorbed amount, both calorimetric curves of propene adsorption on K-FER samples approach the calorimetric curve recorded on the pure silica FER zeolite. The propene heat of adsorption on the pure silica zeolite starts at 53 kJ/mol and slowly decreases to 46 kJ/mol. Calorimetric results show that (i) pure dispersive interactions of propene with the silica wall of the zeolite channels lead to heat of adsorption equal to 53–46 kJ/mol and (ii) the interaction of propene with K+ cations in the cationic sites of FER zeolite stabilizes the complexes and increases adsorption heat by up to 20–30 kJ/mol depending on the Si/Al ratio and thus on the cation content and the location of individual cations.

Calculations The structure of K+…propene complexes in the K-FER zeolite is determined by the directional (monopole-quadrupole) cation-π interaction. For the K+ sites in the center of the entrance window to the perpendicular channel cavity (Al in T1 and T2 positions), there are two


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possible ways to bind the propene molecule: (i) at the intersection of the main and perpendicular channels and (ii) inside the P-cage. The other two K+ sites form adsorption complexes with propene either in the main channel (Al in T3) or in the P-cage (Al in T4). The most stable single-site complexes of propene in K-FER are illustrated in Figs. 4 and S5. The calculated interaction energies and adsorption enthalpies for single-site complexes are given in Table 1. Due to strong dispersion interactions, the complexes with propene inside the Pcage (T1/P, T2/P, T4/P) have the largest adsorption heats of 72–73 kJ/mol. The other complexes are slightly weaker; heats of about 65 kJ/mol were calculated for complexes at the intersection (T1/I, T2/I) and 61 kJ/mol for the complex in the main channel (T3/M). The same behavior was observed for siliceous ferrierite, where the dispersion interaction should be the dominant contributing term. The PBE/CC adsorption enthalpies are -56 kJ/mol and 48 kJ/mol for propene in the P-cage and the main channel, respectively. Dual-K+ sites were investigated for all possible pairs of K+ ions at a distance of 7–8 Å. Interestingly, the only stable dual site complex with propene was found for the K+ ions in the center of the P-cage entrance windows (T1 and T2). The corresponding bridged complexes (T1-T1/I, T2-T2/I, and T1-T2/I) are shown in Figs. 4 and S6. The adsorption heats of the bridged complexes are substantially higher (78–79 kJ/mol) than those for the single-site complexes. The hypothetical Tx-Ty/P bridged complexes with even higher adsorption heats are not considered as the P-cage would be inaccessible for the propene molecule with both entrance windows blocked by K+ ions. The C=C frequency shifts, ∆ω, calculated at the PBE level for the single-site complexes, range from -12.5 to -10.4 cm-1. The only exception is the least stable T3/M complex with a harmonic shift of -4.9 cm-1 (see Table 1), which is only marginally larger than the redshift in siliceous ferrierite (P-cage) of about -2 cm-1. The shifts for bridged complexes range from -24.4 to -22.7 cm-1. The PBE calculations thus seem to suggest that single-site and



bridged complexes give rise to two distinct IR bands. This finding should be taken with caution, however, as the PBE description of K+-propene interaction is not particularly reliable. The error on the PBE harmonic shifts can be estimated from more accurate MP2 frequencies. A detailed analysis of a simple C=C(ethylene)…dipole model (Table S3) shows that the PBE error, ∆∆ω = ∆ω(PBE) - ∆ω(MP2), depends on the magnitude of the dipole moment and to a lesser extent on its orientation. The ∆∆ω values estimated from the C=C(ethylene)…dipole model (0.8e) are on average -3 cm-1 and -7 cm-1 for the single-site and bridged complexes, respectively. The inclusion of anharmonic corrections does not introduce significant changes in the C=C frequency shifts (for more details, see Table S3 in the Supporting Information).

Discussion The adsorption of propene on K-FER zeolites at room temperature gives rise to three well-distinguished vibrational bands in the region of νC=C vibration at 1633, 1639 and 1645 cm-1 (see Figs 1 and 2). Their intensities (populations) depend on the equilibrium pressure and the framework Si/Al ratio of FER zeolite. Experimental observation is in good qualitative agreement with periodic DFT/CC calculations, showing also three groups of the C=C bond vibration frequencies for propene adsorbed in K-FER zeolite (see Table 1). The DFT/CC calculations indicate that the shifts in the frequency of the νC=C vibration are caused by the formation of cation-π complexes with potassium atoms. The noncovalent cation-π complexes between the neutral C=C bonds (Lewis base) and the alkali-metal cations (positively-charged Lewis acid sites) have been observed experimentally in supramolecular chemistry62 as well as in Na-zeolites44-45. The magnitude of calculated shifts depends on the strength of the cation-π bond. A complex at the T3/M site, for example, cannot achieve its optimum geometry due to steric effects (Fig. S5 and Table S4), and consequently both the adsorption heat (61.3 kJ/mol)


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and the frequency shift (-4.9 cm-1) are significantly lower than those for other single-site adsorption complexes reported in Table 1. On the other hand, the difference in adsorption heats at the T1/P (P-cage) and T1/I (intersection) sites is governed by dispersion interactions, and therefore it is not accompanied by the corresponding change in frequency shifts. The lowest PBE shifts relative to the C=C stretching frequency of the free propene molecule with ∆ω(C=C) ranging from -1.3 cm-1 to -2.4 cm-1 have been found for the dispersion-bound adsorption complexes, where propene interacts only with the siliceous zeolite framework. The results obtained for the 6T model (Fig. S10) of the silica surface have shown that the PBE redshifts are most likely underestimated. The PBE and MP2 values calculated for the 6T model are -2 cm-1 and -6 cm-1, respectively. The medium frequency shifts, ∆ω(C=C) from -10.4 cm-1 to -12.5 cm-1, have been found for propene molecules binding into sites with isolated potassium cations (see Fig. S5). The largest frequency shifts from -22.7 cm-1 to -24.5 cm-1 have been found for adsorption complexes formed by a pair of adjacent (dual-site) K+ cations located close to T1 and/or T2 crystallographic positions. The bridging propene molecule is coordinated by the C=C bond (see Fig. 4). The average PBE frequency shifts relative to the free propene molecule for the three identified adsorption complexes (-2 cm-1, -12 cm-1, -23 cm-1) compare nicely with experiment (-5 cm-1, -11 cm-1, -17 cm-1) considering that the first calculated value is slightly underestimated and the third overestimated at the PBE level of theory (cf. Table S3). The assignment of the individual IR bands to individual types of adsorption complexes can be confirmed/refined through a comparison of the stability of the adsorption complexes represented by individual IR absorption bands and heats measured by calorimetry with calculated interaction energies and enthalpies of propene at various adsorption sites of K-FER zeolite (Fig. 3). Experimentally determined heats of propene adsorption in the K-FER zeolites show a rather broad range of values (from 46 kJ/mol to 77 kJ/mol) mostly depending on the



content of potassium cations in the zeolite (Si/Al ratio) and the amount of adsorbed propene. As a consequence of these large differences in adsorption heats, different adsorption complexes are formed in the C3H6/K-FER system under various equilibrium pressures of propene as evidenced by changes in IR spectra (see Fig. 1), and these complexes are removed by the evacuation of samples at different rates (see Fig. 2). This assumption makes it possible to interpret the changes in the IR spectra recorded at various propene pressures (Fig. 1) and evacuation times (Fig. 2), and the course of calorimetric curves by a comparison of experimental data with theoretical calculations (Table 1). The only IR band in the region of the νC=C stretching vibration observed for propene adsorbed in the siliceous FER zeolite is centered at 1645 cm-1. This band increases its intensity with increasing propene pressure without any shift in the position or development of a shoulder or satellite band. The band at the same frequency appeared in the spectra of the KFER-27.5 sample at propene pressures higher than 1 mbar. Based on volumetric adsorption isotherms measured at the same temperature as IR spectra (Fig. 3A), the coverage (defined as a number of adsorbed propene molecules per potassium cation) reaches a ratio of approximately 1:1 at this pressure (1 mbar), so it can be expected that some propene molecules have limited possibility to interact with potassium cations at higher pressures. Note that adsorption complexes with more than one propene molecule occupying the same K+ site can be formed at T1 and T2 positions (e.g. T1/P and T1/I). Other possibilities can be excluded by steric considerations (see Fig. S5). The band at 1645 cm-1 is not observed in the IR spectra of the C3H6/K-FER-8.6 system even at the highest propene pressure (100 mbar) due to the fact that the coverage does not exceed the 1:1 ratio at the measured pressures for this sample (see the volumetric adsorption isotherm in Fig. 3A). The calorimetric heat of adsorption of propene in siliceous FER zeolite is not constant but slowly decreases from 53 to 46 kJ/mol (Fig. 3B) as the P-cage and main channel are filled by propene. The calorimetric curves for K-


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FER zeolites decline to the curve for siliceous FER irrespective of the initial adsorption heats (at the zero coverage) as all K+ cations become occupied and propene interacts only with zeolite framework. For propene molecules interacting by dispersion interaction with siliceous inner surfaces, the DFT/CC calculations predict adsorption enthalpies ranging from -56 kJ/mol (P-cage) to -48 kJ/mol (the main channel) in good agreement with observed experimental values. Based on these facts, the band at 1645 cm-1 can be undoubtedly assigned to the propene molecules trapped in the channels of FER zeolite interacting only with the zeolite walls. The band at 1639 cm-1 is present in the IR spectra of both K-FER zeolites. This band is significantly more populated in the K-FER-27.5 zeolite, but even in this sample it is accompanied by a band at 1633 cm-1, which dominates the IR spectra of the C3H6/K-FER-8.6 system. A desorption experiment has clearly shown that the intensity of the band at 1639 cm-1 decreases faster than the intensity of the band at 1633 cm-1 in both samples, thus the band at 1639 cm-1 is less stable and propene interacts with the potassium cation with lower interaction energy than in complexes characterized by the band at 1633 cm-1. The higher population of the low-frequency (more stable) band in K-FER-8.6 is also reflected in 7–10 kJ/mol higher adsorption heat than in K-FER-27.5. The DFT/CC calculations predict the heat of adsorption to be 61–65 kJ/mol for the propene interacting with a single K+ cation in the main channel and 72–73 kJ/mol for the propene localized in the P-cage. When a propene molecule interacts with two K+ cations in dual-cation sites, adsorption complexes are stabilized by 14 kJ/mol, resulting in the adsorption heat of about 79 kJ/mol. These predicted values fit very well the experimentally observed adsorption heats on the K-FER sample. The initial heat of adsorption for the K-FER-8.6 sample, 77 kJ/mol, is in good agreement with the heat expected for the propene adsorbed in dual-cation sites (79 kJ/mol). These complexes are represented in the IR spectra by the band at 1633 cm-1, which dominates the IR spectra of K-FER-8.6 at low



coverage in good agreement with the theoretically predicted largest frequency shift for such bridged complexes (see the discussion above). The initial adsorption heat recorded on the KFER-27.5 sample is 70 kJ/mol. It can be interpreted in light of DFT/CC calculations as a result of the simultaneous formation of adsorption complexes on single K+ sites with a small contribution from the adsorption of propene on dual K+ sites as evidenced by IR spectroscopy (the band at 1633 cm-1 is present with low intensity in the spectra of the C3H6/K-FER-27.5 system). For both K-FER samples, the adsorption heat slowly decreases with increasing coverage to 55–57 kJ/mol at the coverage close to 1:1 (ca 0.5 mmol/g for K-FER-27.5 and 1.2 mmol/g for K-FER-8.6) as a consequence of the occupation of less energetic sites, where propene interacts with isolated K+ cations in the main channel and at the intersections. The decrease of heats is reflected in the IR spectra of both K-FER samples by the growing intensity of the band at 1639 cm-1. At even higher coverage, the calorimetric curves converge to the curve recorded for the siliceous FER zeolite as reflected by the presence of the band at 1645 cm-1 in the IR spectra, especially in the case of K-FER-27.5 (cf. Figs. 1 and 2).

Conclusion The interaction of a propene molecule with K-FER zeolites with various Si/Al ratios (including the siliceous FER sample) has been investigated both experimentally and theoretically. A comparison of theoretical results with detailed spectral features in the C=C bond characteristic vibration region of the IR spectra of adsorbed propene molecules together with the measured heats of adsorption and the rates of propene desorption has led to identification of three main types of adsorption complexes formed upon propene adsorption on K-FER zeolites: (i) propene adsorbed on an isolated single K+ cation, (ii) propene bridging two nearby K+ cations in so-called dual-cation sites, and (iii) the propene molecule interacting


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only with channel walls. To our knowledge, detailed analysis of bridged propene complexes with alkali-metal cations is reported here for first time. Based on the very good agreement between predictions based on DFT calculations and experimentally observed IR spectra and calorimetric curves, it can be concluded that bridged propene complexes with K+ cations in dual-cation sites are the most stable adsorption complexes in the C3H6/K-FER system with the adsorption heat of 77 kJ/mol, which are characterized by an IR band at 1633 cm-1. The propene molecule adsorbed on a single isolated K+ cation exhibits vibration of the C=C bond at 1639 cm-1 and adsorption heat between 55 and 70 kJ/mol. Theoretical results have shown that the interaction energy of propene with an isolated K+ cation depends on the localization of the potassium cation and propene adsorbed in the channel system. The complexes with propene in the P-cage have the largest interaction energy due to a stronger dispersion interaction, followed by complexes at the intersection of the channels, and the lowest adsorption heat is characteristic of propene complexes at the main channel. The propene molecule adsorbed in siliceous FER zeolite or in K-FER zeolite, where no free K+ cations are available for adsorption, exhibits the stretching frequency of the C=C bond at 1645 cm-1 and the heat of adsorption around 50 kJ/mol. This study has shown that a simultaneous interaction of a propene molecule with two nearby cations results in larger adsorption heat and the characteristic vibrational band in the IR spectrum of the C=C bond vibration. The probability of bridged-complex formation increases with the Si/Al ratio as evidenced by differences in the IR spectra of both K-FER zeolites. It can be expected that such complexes formed for other unsaturated hydrocarbons in various zeolites might have a great influence on the competitive adsorption of saturated and unsaturated hydrocarbons and their separation from gaseous mixtures.

Acknowledgement



The authors acknowledge the financial support of the Czech Science Foundation under project No. 17-07642S. The authors thank prof. Kirk Peterson for providing the basis set for potassium atom.

Supporting Information Description of the experimental procedures and results of the textural, crystalline and morphology characterization of the samples investigated (Figs. S1-3 and Table S1), IR spectra of propene adsorbed on K-FER in the region of the νC-H of CH3 groups and δC-H of =CH2 and CH3 groups recorded on K-FER-27.5 (Fig S7) and K-FER-8.6 (Fig. S8), theoretical details on most stable K+ sites in FER (Fig. S4), K+ single-site complexes (Fig. S5) and K+ dual-site complexes (Fig. S6), interaction energy of propene in the 1T-K…propene DFT/CC model as a function of the distance (Fig. S9), the 6T model of silica surface interacting with propene molecule (Fig. S10), effect of MP2 and CCSD(T) interaction energies values for the 1TK…propene complex on basis set (Table S2) and harmonic and anharmonic shifts of C=C vibration as a function of the dipole moment and orientation (Table S3) are included in the Supporting information.

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Table 1 Propene interaction energies and adsorption enthalpies (in kJ/mol), and the C=C harmonic frequencies and shifts (in cm-1) calculated at the PBE/CC level for single-site and bridged complexes in K-FER. Experimental isosteric heats (in kJ/mol) and frequencies of measured IR bands (in cm-1) are given for comparison. K+ site / propene locationa

Eint

∆H0 (0 K)

ω (C=C)

∆ω (C=C)b

T1/P

-74.7

-72.2

1647

-12.5

T1/I

-68.4

-65.3

1648

-11.7

T2/P

-74.2

-71.5

1648

-11.2

T2/I

-67.2

-64.5

1647

-12.4

T3/M

-65.0

-61.3

1655

-4.9

T4/P

-76.8

-72.7

1649

-10.4

T1-T1/I

-82.7

-79.2

1637

-22.9

T1-T2/I

-81.2

-78.2

1637

-22.7

T2-T2/I

-80.8

-78.0

1635

-24.4

/P

-57.2

-56.1

1658

-2.4

/M

-49.4

-47.5

1659

-1.3

Experimentc

qst

ν (C=C)

∆ν (C=C)

K-FER-27.5

-70

1639

-11

K-FER-8.6

-77

1633

-17

Siliceous FER

-53

1645

-5

Single-site complexes

Bridged complexes

Siliceous FER

a

P – P-cage, I – intersection, M – main channel. Relative to the C=C stretching frequency of 1659.6 cm-1 calculated at the PBE level for non-interacting propene. c Isosteric heats measured at 303 K, frequencies of dominant IR bands for each material, and frequency shifts, ∆ν, relative to C=C stretching frequency of 1650 cm-1. b



Figure caption Figure 1 IR spectra of propene adsorbed on K-FER-8.6 (A), K-FER-27.5 (B) and Si-FER (C) zeolites at room temperature under various equilibrium pressures of propene: a – 0.07 mbar, b – 0.121 mbar, c – 0.191 mbar, d – 0.259 mbar, e – 0.347 mbar, f – 0.416 mbar, g – 0.471 mbar, h – 0.512 mbar, i – 1.087 mbar, j – 4.999 mbar, k – 10.074 mbar, l – 20.001 mbar, m – 40.034 mbar, n – 59.9 81 mbar, o – 88.981 mbar, p – 99.981 mbar.

Figure 2 IR spectra of propene adsorbed on K-FER-27.5 (A) and K-FER-8.6 (B) at room temperature and equilibrium pressure 60 mbar. After 20 min of equilibration, desorption at room temperature starts by dynamic continuous evacuation of cell. The IR spectra were recorded at various times of evacuation. (A) K-FER-27.5: evacuation time of 0 s (a), 15 s (b), 30 s (c), 45 s (d), 60 s (e), 75 s (f), 90 s (g), 105 s (h), 120 s (i), 4 min (j), 5 min (k), 6 min (l), 8 min (m), 10 min (n), 13 min (o), 17 min (p), 20 min (q), 24 min (r), 29 min (s), 40 min (t), 90 min (u). (B) K-FER-8.6: evacuation time of 0 s (a), 30s (b), 45s (c), 1min (d), 3min (e), 5 min (f), 7 min (g), 8 min (h), 9 min (i), 10 min (j), 13 min (k), 16 min (l), 19 min (m), 22 min (n), 37 min (o), 47 min (p), 64 min (q), 129 min (r), 200 min (s), 266 min (t).

Figure 3 The adsorption isotherms (A) and heats of adsorption (B) of propene adsorbed at 303 K on K-FER-8.6 (circles), K-FER-27.5 (squares) and Si-FER (up triangles).

Figure 4 The most stable single-site and bridged complexes of propene in K-FER: a – T1/P, b – T2/P, c – T3/M, d – T4/P, and e – T1-T1/I. The main channel view along c-axis (left) and Pcage view along b-axis (right). The K, Al, and C atoms are depicted as violet, blue, and brown balls, respectively.


Page 26 of 31

Page 27 of 31

(A)

1633

Absorbance (a.u.)

1639 0.1

p

a 1680

1660

1640

1620

1600

1620

1600

1620

1600

-1

Wavenumber (cm ) (B)

1645

Absorbance (a.u.)

1640

0.1 1633

p

a 1680

1660

1640 -1

Wavenumber (cm ) 1646

(C)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


h

1680

a 1660

1640 -1

Wavenumber (cm )

Fig. 1



A

1633

B

1633

0.1

1640 Absorbance (a.u.)

1645

0.1

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

1639

a t

a u

1680

1660 1640 -1 Wavenumber (cm )

1620

1600

1680

1660

1640

1620 -1

Wavenumber (cm )

Fig. 2


1600

Page 29 of 31

80

1.8 1.6

K-FER-8.6 K-FER-27.5 Si-FER

A

1.4

B 70

1.2

Qads (kJ/mol)

na (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.8

60

50

0.6 0.4

K-FER-8.6 K-FER-27.5 Si-FER

40

0.2 0.0 0.01

30 0.1

1

10

100

1000

0.0

0.2

0.4

0.6

0.8

na (mmol/g)

p (mbar)

Fig. 3


1.0

1.2

1.4


a

b

c

d

e

Fig. 4


Page 30 of 31

Page 31 of 31

IR spectra C3H6/K-FER@303K

Calorimetry C3H6/K-FER@303K

1633 1639

80

Dual cation site

K-FER-8.6

70

1645

Single cation site

60

Qads (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


K-FER-27.5

50

siliceous FER 1660

1650

1640

1630

1620 -1

Wavenumbers (cm )

1610

1600

40 0.0

0.2

0.4

0.6

0.8

1.0

1.2

na (mmol/g)

TOC Graphic


Experimental and Theoretical Study of Propene Adsorption on K-FER

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