SII Cations Research Articles

Cation Dynamics upon Adsorption of Methanol in Na−Y Faujasite Type Zeolites: A Dielectric Relaxation Spectroscopy Investigation

Dielectric relaxation experiments are carried out to address the question of cation location upon adsorption of methanol in a Na-Y faujasite system. They allow us to probe both the evolutions of the number of extraframework sodium ions in the different crystallographic sites and of their de-trapping energy as a function of the adsorbate loading. Redistributions of the cation positions are explained in light of previous molecular dynamic results. It is shown that SI cations remain mainly trapped in their initial sites whatever the methanol loading. At low and intermediate loadings, SII cations migrate within the supercage, due to strong interaction with the adsorbate molecules. They are followed by hopping of SI' cations from the sodalite cage toward the supercage to fill vacant SII sites. At higher loading, the cation motions are hindered due to steric effects induced by absorbates within the supercage. These cation motions are shown to be consistent with the evolution of the de-trapping energy of Na + .

Cation mobility upon adsorption of methanol in NaY faujasite type zeolite: A molecular dynamics study compared to dielectric relaxation experiments

Molecular Dynamics simulations have been carried out to address the question of cation migration upon adsorption of methanol in NaY Faujasite system as a function of the loading. It has been shown that at low and intermediate loadings, SII cations can migrate toward the center of the supercage due strong interaction with the adsorbates, followed by hopping of SI' cations from the sodalite cage into the supercage to fill vacant SII sites. SI cations mainly remain trapped in their initial sites whatever the loading. At higher loading, only limited motion is observed for SII cations due to steric effects induced by the adsorbates within the supercage. These simulated results are in good agreement with those extracted by Complex Impedance Spectroscopy measurements, which provided the evolution of the number of extraframework cations in the different crystallographic sites as a function of the treatment temperature.

Investigation of CO 2 adsorption in Faujasite systems: Grand Canonical Monte Carlo and molecular dynamics simulations based on a new derived Na +–CO 2 force field

Both Grand Canonical Monte Carlo and Molecular Dynamics simulations have been carried out to gain further insight into the CO 2 adsorption process at the microscopic scale in NaX and NaY Faujasites. A new Na +–CO 2 force field was derived using quantum calculations performed on realistic cluster directly excised from the periodic structure. Both the methodology and the force field were validated by a good agreement between both the simulated isotherms and the evolution of the differential enthalpies of adsorption as a function of adsorbate coverage and those previously obtained by Microcalorimetry. In addition, the microscopic mechanisms of CO 2 adsorption in both systems, consistent with the trends observed for the differential enthalpies of adsorption, were proposed. It was observed that two different types of adsorption behaviour exist for NaY and NaX, and can be defined by involving one (SII cations) or two (SIII′ and SII cations) preferential adsorption sites respectively.

Selectivity of Thiophene/Toluene Competitive Adsorptions onto Zeolites. Influence of the Alkali Metal Cation in FAU(Y)

In the framework of research considering adsorption for gasoline desulfurization purposes, dynamic adsorption experiments have been performed to study a comparison between thiophene and toluene affinities toward Y-faujasite adsorbents (Si/Al = 2.4) exchanged with various alkali metal cations (Li+, Na+, K+, Rb+, Cs+). These experiments have been carried out at two concentration ranges in liquid n-heptane solutions: around 25 mmol·L-1 for competitive adsorptions and around 1.25 mmol·L-1 for single-solute adsorptions. Complementary microcalorimetry studies have been carried out in order to investigate the interaction of nucleophilic compounds toward alkaline Y zeolites, by measuring the adsorption enthalpy of CCl4 onto NaY and CsY sorbents. The results clearly exhibit two opposite trends for the adsorption of thiophene and toluene molecules onto alkaline Y faujasites. The thiophene affinity increases with cation electropositivity, while the toluene affinity decreases as the cations are more electropositive. Thus, from a favorable adsorption for toluene onto the LiY zeolite (αthio/tol = 0.38 at ca. 26 mmol·L-1), the selectivities shift in favor of thiophene so that the αthio/tol value reaches 3.00 with the CsY zeolite. The calorimetry measurements of CCl4 adsorption have exhibited an increase of the adsorption enthalpy with more electropositive cations, which seems to indicate that thiophene molecules are adsorbed similarly to CCl4, i.e., by strong interaction between the nucleophilic S atom of thiophene and the cationic charge. However, toluene should be mainly stabilized on the adsorption sites through other interactions, as combined attractions between the H atoms of toluene and the O atoms of the zeolite framework surrounding the SII cation can be suggested.

Cation Migration upon Adsorption of Methanol in NaY and NaX Faujasite Systems: A Molecular Dynamics Approach

Molecular dynamics simulations have been carried out to address the question of cation migration upon adsorption of methanol in NaY and NaX faujasite systems as a function of the loading. For NaY, it has been shown that, at low and intermediate loadings, SII cations can migrate toward the center of the supercage due to strong interactions with the adsorbates, followed by a hopping of SI' from the sodalite cage into the supercage to fill the vacant SII site. A SI' cation can also migrate across the double six ring and takes a SI' vacant position. SI cations mainly remain trapped in their initial sites whatever the loading. At high loading, only limited motions are observed for SII cations due to steric effects induced by the presence of adsorbates within the supercage. For NaX, the SIII' cations which occupy the most accessible adsorption sites are significantly moving upon coordination to the methanol molecules; the extent of this mobility exhibits a maximum for 48 methanol molecules per unit cell before decreasing at higher loadings due to steric hindrance. In addition, the SI' and SII cations remain almost trapped in their initial sites whatever the loading. Indeed, the most probable migration mechanism involves SIII' cation displacements into nearby SIII' sites.

Influence of Si:Al-ratio of faujasites on the adsorption of alkanes, alkenes and aromatics

The influence of Si:Al-ratio on the adsorption of alkanes, alkenes and aromatics was determined in gas and liquid phase; i.e. at low and high degree of pore filling. In gas phase the pulse chromatographic method was used to determine the interaction of pure C 6–C 8 n-alkanes and benzene at 180–420 °C on NaX (Si:Al 1.23) and NaY (Si:Al 2.55). In agreement with the cation contents, higher Henry constants and enthalpies were measured on NaX compared to NaY for all components. To study the adsorption of alkane/alkene/aromatic mixtures at room temperature in liquid phase a batch technique was used. Binary adsorption isotherms of hexene, octene and dodecene in heptane and benzene in octane were determined on NaX (Si:Al 1.23) and NaY (Si:Al 2.79). The adsorption of benzene, toluene, m-xylene and mesitylene from their mixture with octene was studied on NaLSX (Si:Al 1.02), NaX (Si:Al 1.23) and NaY (Si:Al 2.79). All binary adsorption isotherms in liquid phase could be very well fitted with a multicomponent Langmuir–Freundlich adsorption isotherm model. In liquid phase conditions alkenes are preferentially adsorbed on NaX compared to NaY, while aromatics are unexpectedly more selectively adsorbed on NaY than on NaX. At high degree of pore filling arrangement of the molecules in the supercages largely affects the selectivity. SIII/SIII′ cations affect the position of aromatics adsorbed on SII cations or benzene adsorbed in the 12 Membered Ring-window, which causes a more restricted packing inside the supercages of NaX compared to NaY lacking these SIII/SIII′ cations.

Molecular Dynamics Simulation of the Cation Motion upon Adsorption of CO2 in Faujasite Zeolite Systems

Molecular Dynamics simulations have been carried out in NaX and NaY Faujasite systems to deepen understanding of the cation rearrangement during the CO2 adsorption process suggested by our recent diffusivity measurements. This study is a major contribution since the rearrangement of the cations in Faujasite, the most promising adsorbent for CO2 storage, can represent a significant breakthrough in understanding the adsorption and diffusion processes at the mircroscopic scale. For NaY, it has been shown that at low and intermediate loadings, SII cations can migrate toward the center of the supercage due to strong interactions with the adsorbates, followed by a hopping of SI'cation from the sodalite cage into the supercage to fill the vacant SII site. The SI cations are only displaced at a higher loading, leading to cation de-trapping out of the double six rings into the vacant SI' sites. For NaX, the SIII' cations which occupy the most accessible adsorption sites move significantly upon coordination to the carbon dioxide molecules. The SI' and SII cations remain consistently located in their initial sites whatever the loading. Indeed, the most probable migration mechanism involves SIII' cation displacements into nearby vacant SIII' sites.

CO 2 adsorption in alkali cation exchanged Y faujasites: A quantum chemical study compared to experiments

The adsorption of CO 2 in the alkali exchanged Y-faujasite type zeolite (Li +, Na +, K + and Cs +) has been investigated using density functional theory calculations at the PW91 level. A cluster centered around the SII cation was cut from the periodic faujasite structure including a whole zeolite supercage. The computed geometric parameters of the bare clusters are compared with those of the crystal structure obtained by X-ray or neutron diffraction. The cation-CO 2 geometry is then investigated as a function of the nature of the alkali cations. The calculated adsorption enthalpies show a decrease from Li + to Cs + and reproduce well experimental values obtained by microcalorimetry measurements.

Cation Mobility and the Sorption of Chloroform in Zeolite NaY: Molecular Dynamics Study

Molecular dynamics simulations at temperatures of 270, 330, and 390 K have been carried out to address the question of cation migration upon chloroform sorption in sodium zeolite Y. The results show that sodium cations located in different sites exhibit different types of mobility. These may be summarized as follows: (1) SII cations migrate toward the center of the supercage upon sorption, due to interactions with the polar sorbate molecules. (2) SI' cations hop from the sodalite cage into the supercage to fill vacant SII sites. (3) SI' cations migrate to other SI' sites within the same sodalite cage. (4) SI cations hop out of the double six-rings into SI' sites. In some instances, concerted motion of cations is observed. Furthermore, former SI' and SI cations, having crossed to SII sites, may then further migrate within the supercage, as in (1). The cation motion is dependent on the level of sorbate loading, with 10 molecules per unit cell not being enough to induce significant cation displacements, whereas the sorption of 40 molecules per unit cell results in a number of cations being displaced from their original positions. Further rearrangement of the cation positions is observed upon evacuation of the simulation cell, with some cations reverting back to sites normally occupied in bare NaY.

Double Resonance NMR and Molecular Simulations of Hydrofluorocarbon Binding on Faujasite Zeolites NaX and NaY: the Importance of Hydrogen Bonding in Controlling Adsorption Geometries

Double resonance NMR experiments have been used to study the binding of the asymmetric hydrofluorocarbons (HFCs) CF3CFH2 (HFC-134a), CF3CF2H (HFC-125) and CF2HCFH2 (HFC-143) on zeolites NaX and NaY. By exploiting the very large differences in 19F chemical shifts for the −CF3-xHx end groups of each molecule, individual 19F → 23Na cross-polarization (CP) build-up curves involving polarization transfer from different parts of the molecule have been obtained. CP efficiencies in the order CF3≪CF2H<CFH2 were found, indicating that the hydrogen-containing groups are bound more strongly to the zeolite framework. This effect is most pronounced for the lowest-sodium content zeolite studied (NaY: Si/Al = 7.6), and increases with HFC loading. Both the 1H NMR resonances and 1H → 27Al and 1H → 17O CP MAS NMR experiments are consistent with H-bonding interactions with the zeolite framework. Molecular dynamics and docking calculations for HFC-134 and 134a on model NaY and NaX zeolites revealed the importance of both H-bonding and Na−F interactions in determining the low-energy sorption sites. Stable binding sites for HFC-134a in NaY were found by docking to involve only CFH2−Na(SII) contacts, in qualitative agreement with the CP results. The simulations indicate that Na-binding to the groups with more H atoms is favored by the possibility of achieving shorter Na−F distances and maximizing the number of H-bonds with the framework. The higher sodium content zeolite containing both SII and SIII cations gives minimum energy binding sites involving multiple Na−F and H-bonding contacts. For HFC-134 (gauche conformer), the global energy minimum actually involves several H-bonds but no short Na−F contacts.

Sorption of Biphenyl in Nonacidic Faujasitic Y Zeolites: Modeling and Spectroscopic Studies

The predictions of the sorption energy, sorption site, conformation, mobility, and vibrational spectra of biphenyl (BP, C12H10) occluded in nonacidic faujasitic Y zeolites Mn(SiO2)192-n(AlO2)n (MnFAU, n = 0 or 56 and M = Na+, K+, or Cs+) were carried out using Monte Carlo simulations (MC), molecular mechanics (MM), and molecular dynamics (MD) calculations. The zeolite−biphenyl interactions have been tuned by varying the aluminum content n, the charge-balancing cation M+ of the zeolite, and the BP loading. No preferential sorption site was expected in purely siliceous FAU at 300 K, whereas well-defined location sites energetically favored were expected in M56FAU aluminated faujasite. BP lies in the cavity in a twisted conformation with one phenyl group facially coordinated to the SII cations and the other phenyl group engaged in the 12-ring windows. The accommodation of two BPs within the same supercage occurs at relatively low coverage (2 BP/UC) and is hindered by the M+ size. The diffuse reflectance UV−v...

Adsorption of ethane and ethene in Na–Y studied by inelastic neutron scattering and computation

Inelastic neutron scattering (INS) spectra were collected for ethane and ethene in Na zeolite–Y (Si/Al=2.0). The INS spectrum of ethane in Na–Y consists of a single broad adsorption band in which no individual peaks can be discerned. Monte-Carlo docking calculations predicted a large number of similar binding sites with energies ranging from 24 to 37 kJ mol −1 in the region of the four rings on the wall of the faujasite supercage. This is consistent with the broad feature observed in the INS spectrum. In contrast, the INS spectrum of ethene in Na–Y shows significant fine structure, and several peaks can be distinguished in the region<1000 cm −1. Monte-Carlo docking calculations with a molecular mechanics type force field were performed and predict two binding sites, one bound to the SII cation site (binding energy=36.2 kJ mol −1) and one lying in the window site (binding energy=19.4 kJ mol −1). Quantum mechanical calculations were also performed using a cluster model for the SII binding site but resulted in a significantly lower binding energy, even when correlation corrections were employed (18.2 kJ mol −1). Vibrational spectra were calculated using both quantum mechanical and molecular mechanics based techniques and the results compared with the INS spectra. A significantly better fit was obtained using the latter method and the peaks could be assigned based on the computed eigenvectors. Analogies are drawn between the assignments and other previous studies of Zeise's salt and related compounds.

The location of pyridine in sodium–silver–Y zeolite by powder synchrotron X-ray diffraction

The powder synchrotron X-ray diffraction pattern of a mixed sodium–silver–Y zeolite, Ag 56− x Na x Si 136Al 56O 384 x≈19, saturated with pyridine, has been analysed by the Rietveld method to reveal positions for the adsorbed molecules. Cations are distributed over three sites, SII, constrained to 100% occupancy, with 17.2(1) and 14.8(1) Ag(1) and Na(1) ions per unit cell, respectively, SI′ with 18.3(1) Ag(2) ions per cell, and SI with 1.8(1) Ag(3) ions per cell. The refinement suggests that approximately 7.5 pyridine molecules are adsorbed per supercage, located in two sites within the cavities of the zeolite. Pyridine(1) is in the 12-ring window connecting supercages. Three molecules project through each window and approach the SII cations in a supercage, with an average Ag(1)–N distance of 3.17(2) Å. An SII cation can be linked to three pyridine(1) molecules from three separate windows. These sites are full with 5.98(2) molecules per supercage. Pyridine(2) is found in the supercage, oriented with its π electron density towards the SII cations, with its centre at 2.87(2) Å from Ag(1). The average occupancy was fixed at 1.5 molecules per supercage, corresponding to 20% of the pyridine content. A local ordering scheme can be postulated, whereby alternate supercages are filled by pyridine(1) and pyridine(2) molecules, efficiently filling the channels of the zeolite.

Neutron Diffraction and Computational Study of Zeolite NaX: Influence of SIII‘ Cations on Its Complex with Benzene

The crystal structure of zeolite NaX (Si:Al = 1.2) and that of its complex with benzene have been determined at 5 K by Rietveld analysis of powder neutron diffraction data in space group Fd3. For NaX, a = 25.0328(5) Å, Rwp = 4.25%, and Rp = 3.27%. For NaX + benzene, a = 25.0496(7) Å, Rwp = 4.60%, and Rp = 3.52%. The sodium ions were located preferentially in sites SI‘ and SII, both of which are fully occupied. The remaining cations were found at the SIII‘ site in the 12-ring window. These cations are coordinated by two oxygen atoms, O(1) and O(4), in a corner of the 4-ring window where the aluminum is located. Our simulations on cation positioning in NaX (Si:Al = 1) predict the preferred occupancy of SIII‘ sites that are facing AlO4 tetrahedra and support our crystallographic results. Less than half the benzene molecules were found at the six-ring window in the supercage, where they interact with the SII cations. No benzene was found in the 12-ring window, presumably because of the presence of the SIII‘ sodium cations, but there remains a possibility that undetected benzene is adsorbed near these cations.

Changes with dealumination of the state of benzene adsorbed on faujasites

The adsorption of benzene on two faujasite-type zeolites with different cation contents on full exchange (Y and dealuminated Y) shows the existence of three possible states of adsorbed benzene: interaction with SII cations, interaction with oxygen atoms, and liquid benzene filling the cavity.

Far-infrared studies on zeolites

The far-infrared spectra of X-type zeolites were measured. In the far-infrared region vibration bands occur, which are sensitive to exchangeable cations. The behaviour of these cation vibration bands upon adsorption of H 2O, H 2S, C 6H 6 and other molecules is analysed. While the bands assigned to SIII cation vibrations vanish with increasing amount of adsorbed molecules, the bands assigned to SII cation vibrations shift to lower frequencies and low and high frequency shoulders appear if adsorption takes place.

Chromium ions within zeolites. Part 2.—A quantum-chemical study of the properties of chromium ions in faujasites

The CNDO (complete neglect of differential overlap) method is used for calculation of the physico-chemical characteristics of faujasites with CrII, CrIII, CrV and CrVI ions in the SII cation position, modelled by Si3Al3O6(OH)12Cr clusters. It is demonstrated that the Cr ions are bonded to the zeolite skeleton by electron donor–acceptor bonds whose formation leads to electron donation from the skeleton to the cation, thus stabilizing the valence states of the Cr ions. The most probable valence state of the Cr ions in the zeolites was found to be CrIII; however, it can readily be oxidized to CrV. In contrast, CrVI and CrV ions were found to exhibit high electron-acceptor ability and thus are readily reduced.