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An effect of deblocking of small size (8R, D8R) pores in zeolites due to cation drift is analyzed by using ab initio molecular dynamics (AIMD) at the PBE-D2/PAW level. The effect of carbonate and hydrocarbonate species on the carbon dioxide uptake in NaKA zeolite is demonstrated. It is shown that a hydrocarbonate or carbonate anion can form strong complexes with K+ cation and withdraw it from the 8R window, so that the probability of CO2 diffusion through 8R increases. For the first time, correlations between cationic and HCO3–/CO32– positions are demonstrated in favor of their significant interaction leading to the cationic drift from 8R windows. This phenomenon explains a nonzero CO2 adsorption in narrow pore zeolites upon high Na/K exchange. In a gas mixture, such deblocking effect reduces the separation factor because of the possible passage of both components through the plane of partly open 8R windows.

Possible mechanism of oxidative carbonylation of methanol via >Cu(OCH3)2Cu< binuclear cationic oxo-clusters in the CuNaX zeolite with Cu2+ cations is analyzed theoretically within the scope of periodic boundary conditions with VASP. Such scheme was first derived by Saegusa et al. (J. Org. Chem.1970, 35, 2976) and was never tested by theoretical modeling to our best knowledge. For the CO attack we have computed the activation energy value that is close to experimental values obtained in Cu-zeolites. We suppose that this scheme can correctly describe the oxidative carbonylation at medium and high Cu loading when the copper oxo-clusters can be formed.

Our work is devoted to DFT calculations of the relative rotational and diffusional barriers for CO motions in zeolite NaY. The diffusion jump of CO adsorbed in NaY from NaII to Na'II has been confirmed as the favored way for CO re-coordination via either the C or the O atom to the Na cations instead of the CO rotation, hence explaining the mechanism which is responsible for the CO exchange between different positions and the changes in the intensities of the vibrational IR spectra. The fine structure of the vibrational C-O bands is explained by the different CO locations of adsorbed mono- and dicarbonyl species. The calculated activation energy of intra-cage CO diffusion from NaII-CO to Na'II-OC matches the respective experimental barrier observed in the NaX zeolite.

The energies of CO2 interaction with MeY zeolites containing different alkali cations (Me = Li, Na, K, Cs) at small coverage of MeY zeolites are studied using DFT modeling (PBE, PBE-D2, optPBE-vdW, optB88-vdW, and optB86b-vdW functionals). The role of CO2 interaction with HCO3− is analyzed to explain unusual decreasing order of the heats of CO2 adsorption in the sequence CsY > KY > NaY > LiY recently observed. The largest isosteric heat of adsorption in CsY is assigned to the interactions with HCO3− whose maximum concentration in CsY was confirmed via IR tools.

The activation barriers for Na jumps between the cationic sites NaI, NaII, and NaIII are calculated using VASP in the presence of anions (Cl−, Br−) or H2O as well as for the empty Na12A zeolite model. The inter-cage cationic NaIII→NaII’→NaIII′ path, where NaIII′ is located in the neighbor α–cage, was modeled via an intermediate state (NaII′) corresponding to two NaII cations in one 8R window. This NaII′ point can also serve as an intermediate for the second type of intra-cage Na drift, i.e., to a NaIII site in the same cage. Reasonable agreement with experimental activation energy was achieved for NaII’→NaIII′, NaII→NaIII, and NaIII→NaIII jumps. The lower activation energy is obtained for the NaII’→NaIII' jump than for NaII→NaIII, which indicates the possible role of the doubly occupied 8R window as an intermediate state in the Na transfer with high frequency and lower activation energy related to ionic current (inter-cage transfer) in Na12A.

Two possible schemes of carbonate formation (with and without water participation) in cationic form zeolites are considered. Activation energy for the formation of hydrogen carbonate in NaX zeolite from water and carbon dioxide is calculated at the DFT level with periodic boundary conditions, while the problems of modeling the formation of symmetric carbonate in the same zeolite are discussed. The formation of copper carbonate is studied using binuclear CuOCu clusters from CO2 where the influence of water on the barrier is discussed. The questions related to DFT application to binuclear copper clusters are also considered by comparison with the data obtained at the MP2 level. The reactivity of copper carbonate is tested in the reaction with methanol. © 2015 Wiley Periodicals, Inc.

The bulk and Young moduli and heats of hydration have been calculated at the DFT level for fully optimized models of all-siliceous and cationic zeolites with and without water, and then compared to the corresponding experimental data. Upon the addition of water, the monovalent alkali ion and divalent alkaline earth ion exchanged zeolites presented opposite trends in the elastic modulus. The main contribution to the decrease in the elastic modulus of the alkali ion exchanged zeolites appeared to be a shift of cations from the framework oxygen atoms upon water addition, with the coordination number often remaining the same. The contrasting increase in elastic modulus observed for the divalent (alkaline earth) ion exchanged zeolites was explained by cation stabilization resulting from increased coordination, which cannot be achieved within a rigid zeolite framework without water.

The alkali forms of MeRHO zeolite (Me = Li, Na, K, Cs) are studied theoretically at the DFT/PBE level using projector augmented wave method (PAW) and periodic boundary conditions (PBC). The equilibrium positions of the Me cations have been optimized at the D8R prisms where they control molecular transport in the pores of MeRHO. The effect of the displacement of alkali cations has been estimated due to coordination of both carbonate or hydrogen carbonate anions that can also induce RHO framework transformations. Admitting the CO 3 −2 and HCO 3 − formations via the reactions between H 2 O and CO 2 , the energy of respective reactions was evaluated at various Si/Al moduli and analyzed regarding possible influence on the gas adsorption.

The inter-relation between chemical and mechanical properties is demonstrated regarding Young’s modulus variation of the NaX and NaKX (83% exchange Na/K) zeolites upon carbonate formation or CO2 adsorption. The force field (FF) approach is based on the combination of known FFs for the zeolite atoms, alkali cations, CO2, and carbonate atoms. Contrary variations of the Young’s modulus are obtained in NaX upon CO2 adsorption (increase far from CO2 condensation conditions) or carbonate formation (decrease). Higher decreases of the Young’s and bulk moduli are predicted for K-exchanged form that was not yet tested experimentally.

The common main factor of the Na+ influence on the extra-framework aluminum (EFAL) transformations was observed while modeling partly hydrated zeolites at both periodic and cluster computational levels. It can be characterized as the attack of Lewis acid (Na+) toward electron pairs of different O atoms. The limiting step of EFAL formation is assigned to the transformation between the AlOH2+ and Al ( OH ) 2 + species. It allows the explanation of the poisoning effect of Na+ on the cracking reactions known experimentally. The role of electrostatic factor is also discussed.

Problem of Al–O–Al linkage in aluminosilicate materials or Al-avoidance is discussed for two zeolite structures (phillipsite and brewsterite) exchanged with Mg2+ cations. All models are fully optimized at periodic Hartree–Fock and hybrid density functional theory levels (the CRYSTAL code). Their properties are then calculated at the periodic level with the same basis sets. The reasons for the instability of the zeolite structures including the Al–O–Al moieties are interpreted on the basis of cell energy decomposition. This destabilization comes from an increase in kinetic energy if the Al–O(Mg)–Al moieties are present in zeolites. This effect is discussed in parallel with already known variational evidences in favor of dominating role of kinetic energy for stability of molecular systems.

Theoretical calculations are performed on a model zeolite A where sodium ions have successively been exchanged with potassium. Using both isolated cluster and periodic DFT calculations, we made an attempt to explain how the chemisorbed carbonate species in the material contribute to the exceptionally high CO 2 over N 2 selectivity of nearly 200 found in recent experiments [Liu et al. [1] ] with the zeolite NaKA as adsorbent to capture and separate carbon dioxide from a gas mixture containing nitrogen. We have shown that the high carbonate forming at the potassium positions in the 8R windows (KII) results in a larger 8R window diameter potentially enhancing the CO 2 uptake if adsorption is measured for individual gases.

In our paper, we show that carbonates can be formed with almost no energetic barrier from CO2 and metal-oxide binuclear MOXM species (M = Mg, Ca, Sr, Ba, with X = 1–4, depending on the cation) in alkaline earth zeolites, mordenite (MOR) and phillipsite (PHI), on the basis of quantum mechanical density functional theory (DFT) calculations at both isolated cluster and 3D periodic levels. The participation of MOXM species (X = 1 and 3) explains the source of the third O atom in CO3 species in dehydrated zeolites, on the basis of a good agreement between the calculated and experimental positions of the asymmetric and symmetric CO3 vibration bands, of the ratio of their intensities, and of the weak dependence versus the cation and framework type. The reaction of formation of dimethylcarbonate from CaCO3Ca in the 8-membered (8R) ring of MOR and methanol has also been considered, suggesting the carbonate activity as the source of CO2 at elevated temperatures.

A new CO oxidation mechanism occurring at (MgO 2Mg) 2+ oxide clusters in different zeolites is modeled at both the periodic DFT (PDFT) level for Mg-phillipsite and with the cluster approach for zeolite Y and mordenite. Using PDFT, we obtained a value of reaction activation energy (15.3 kcal/mol) that is lower than the one obtained using the isolated cluster approach (35 kcal/mol). The main reason of the lower barrier is the strong CO chemisorption, which was not obtained with the cluster approach. The lower oxidation activity of the Mg zeolite forms is due to the smaller number of active singlet MgO XMg species compared to those in the other alkaline earth zeolites. © 2011 Published by Elsevier B.V.

Water molecules were removed from the X-ray structure models of zeolites with comparatively small-sized unit cells (BIK, BRE, EDI, GIS, GOO, LAU, and PHI), and cations were replaced by cations of only one type (Mg or Zn). The dehydrated zeolite models obtained were optimized with the Bush and Catlow force fields. The dependences of atomic multipole moments up to the fourth order along the so-called cumulative coordinate were calculated for these models by the density functional method with periodic boundary conditions. These dependences were used to estimate the electrostatic potential and electric field in the cross section of a zeolite that was not used to calculate them. The data obtained with atomic charges borrowed directly from calculation results were quite satisfactory.

An important problem when studying the interaction between a CO probe molecule and a Na4Ca4A type zeolite is the estimation of the central repulsive coefficients versus the internuclear distance of CO. In particular, this dependence cannot be estimated in the case of the unstable linear “framework oxygen–CO molecule” pair due to the electrostatic repulsive interaction. Hence, we discuss the application of two approximate forms of this dependence either allowing or disregarding the repulsive contribution in the interval wherein the vibrational CO probability distribution cannot be neglected. The consequences of these approximations are compared through calculation of the interaction energy and band shift of CO adsorbed inside Na4Ca4A. The CO spatial parameters (semi-axes) are estimated by fitting both the band shift, corresponding to two different positions of CO relative to the zeolite, and the interaction energy values to the experimental data obtained at small coverage.

The decrease in elastic moduli (Young's, bulk, and shear modulus), the variations in their asymmetries, the Poisson's ratio and the linear compressibility due to carbonate formation in NaX, have been compared to those produced by dealumination of the zeolite HY framework, from the Al–Si–Al fragment positioned in joined 4R rings. All these systems have been considered at the density functional theory (DFT) level using periodic boundary conditions. The representativeness of the models has been checked by comparison of the calculated IR spectra of carbonate and hydrocarbonate species in NaX and of hydroxyl groups in HY with the experimental equivalents. The correlation between the destabilization energy of the systems and the displacement of Na or K cations coordinated to the carbonate or hydrocarbonate species, expressed in terms of Me–O bond elongation, has been confirmed for either one or two carbonate and hydrocarbonate species per unit cell (UC). Finally, a similar reduction in elasticity in FAU zeolites has been observed, due either to carbonate/bicarbonate formation in NaX or as a step in HY dealumination.

Distributed multipole analysis on the basis of periodic Hartree-Fock PHF calculations, using the CRYSTAL code, is applied to 12 all-siliceous zeolite models, plus one H-form type. A simple approximation of the dependence of the Mulliken-type charge of the framework oxygens, calculated with two Gaussian basis sets, is found with respect to the average Si-O distance and Si-O-Si angle. These results allow the estimation of the oxygen charges within zeolites with larger elementary unit cells which are still hardly tractable with the presently available computing facilities. The validity of such an estimation for the oxygens of silicalite is shown by comparison with results of direct PHF calculations. q 1998 Elsevier Science B.V. All rights reserved.

An iterative numerical procedure is proposed to evaluate the variation of the dependence versus the internuclear distance of several molecular properties (polarizabilities, multipole moments) of hydrogen adsorbed within zeolite A. Dealing with a method which includes only the vibrational perturbation, it is shown that the dependence on internuclear distance of the properties of H2 does not change upon adsorption in NaA as compared to the gas.

The influence of the choice of the intramolecular potential on the resulting frequency shift of the fundamental vibrational transition in the dihydrogen molecule adsorbed on zeolite NaA is estimated. It is shown that an improved Morse potential and the potential calculated by Koltsos and Wolniewicz lead to the same frequency shift value. Application of the Buckingham method for the frequency shift calculation to this case is discussed.

The vibrational frequency shift of physisorbed diatomic molecules is related to the interaction with the adsorbent expressed in terms of the internuclear distance ρ. It is calculated by the Schrodinger equation, the perturbation theory, and a simplified method. We show that it is sufficient to calculate the interaction potential for the values of ρ in the ground and first vibrational states in order to get a precision of 10% on the frequency shift. The comparison between the theoretical and experimental frequency shift of H2 adsorbed in NaA zeolite is used to adjust the interaction potential, especially in terms of the ionicity of the crystal.

A review of quantum chemical modeling of bivalent metal ion stabilization in zeolites is presented. Location of single metal ions in zeolite cationic positions and formation of polynuclear metal-oxo clusters are considered. Special attention is paid to the stabilization of single bivalent metal ions in the cationic positions with distant separation of the two lattice Al ions forming these exchange positions. It is shown that such a type of cation trapping generates increased number of the Lewis acid sites. Comparative stability and catalytic reactivity of different forms of cation species in zeolites are discussed on the example of Zn/HMFI. Dehydrogenation catalytic reaction of ethane molecule on the single Zn(II) and polynuclear zinc-oxo ions is considered. It is found that binuclear metal-oxo ions can be the effective traps of molecular oxygen and so they can direct the way of oxidation catalysis. This is demonstrated by the theoretical treatment of CO oxidation on alkaline earth zeolites. The CO2 molecule can be activated by the binuclear metal-oxo ions with possibility of further reaction functionalization. The last part of this review is devoted to discussion of the structure of single Fe(II) active sites in Fe/HZSM-5 zeolites. This system has attracted great attention as selective oxidation of hydrocarbons with N2O but up to now the understanding of the active structures remains challenging.

Ortho-para separation coefficients for H2 and D2 are estimated for small and medium coverage of 4A and 5A zeolites using the assumption that two types of adsorption sites occur. Charge distribution of zeolite ions is modelled by fitting theoretical to experimental band shifts of physisorbed diatomic molecules of hydrogen, deuterium and nitrogen. Computed and experimental values of ortho-para separation coefficients for H2 and small coverage of 4A zeolite have been demonstrated to agree reasonably well. The reasons for the discrepancy between the theoretical and experimental ortho-para separation coefficients for higher coverage are discussed.

A new series of calcium oxide clusters Ca2OX (X = 1–4) at cationic positions of mordenite (MOR) and faujasite (FAU) is studied via the isolated cluster approach. Active oxide framework fragments are represented via 8-membered window (8R) in MOR, and two 6R and 4R windows (6R+4R) possessing one common SiOSi moiety in FAU. Structural similarities between the Ca2OX(8R) and Ca2OX(6R+4R) moieties are considered up to X = 4. High oxidation possibilities of the Ca2O2(nR) and Ca2O3(nR) systems are demonstrated relative to CO, whose oxidation over the Ca-exchanged zeolite forms is well studied experimentally. Relevance of the oxide cluster models with respect to trapping and desorption of singlet dioxygen is discussed. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010

The vibrational frequency shift of physisorbed diatomic molecules is related to the interaction with the adsorbent expressed in terms of the internuclear distance ρ. The comparison between the theoretical and experimental frequency shift of H 2 , N 2 and O 2 adsorbed in NaA zeolite is used to discuss the interaction potential, especially in terms of the ionicity of the crystal.