Framework Oxygen Atoms Research Articles

Chemical Bonding Effect on the Incorporation and Conduction of Interstitial Oxide Ions in Gallate Melilites

AbstractThe ability to incorporate high content of interstitial oxygen ions (Oi) in La1+xSr1‐xGa3O7+0.5x melilite owing to the good size match between La3+ and Sr2+ ions is well documented. Here, the complete substitution of Sr2+ by Pb2+ lone‐pair cations results in a significant loss of this ability, even though Sr2+ and Pb2+ have almost the same effective ionic radius. To explore the fundamental mechanism underlying this result, density functional theory (DFT) calculations are performed on both the LaSrGa3O7‐based and LaPbGa3O7‐based materials, revealing a new chemical bonding effect on the incorporation of mobile oxygen interstitial defects in melilites. For LaSrGa3O7‐based melilites, the interstitial oxygens have cooperatively weak antibonding interactions with the framework oxygen atoms (Of). This antibonding Oi‐Of interaction pushes Oi toward a 3‐linked Ga ion, enhancing the covalent bonding between this Ga ion and Oi. In addition, the antibonding Oi–Of interaction makes the oxygen interstitial defects and framework atoms highly active, benefiting the migration of defects. In contrast, for LaPbGa3O7‐based materials, the 6s2 electrons of Pb2+ point toward the c‐axis and form antibonding with framework O2−. This antibonding projects into the tunnel void, thereby directly hindering the entrance of interstitial oxygen atoms into the pentagonal rings.

Framework Modifications and Dehydration Path of a Ag+-Modified Zeolite with STI Framework Type

The effect of Ag+ incorporation into stellerite, a natural zeolite with STI framework type, was investigated by means of single-crystal X-ray diffraction (SC-XRD), molecular dynamics (MD) simulations, and X-ray absorption fine structure spectroscopy (XAFS). At room temperature (RT), the complete exchange of the original extraframework ions with Ag+ provoked a distortion of the framework accompanied by symmetry reduction from orthorhombic Fmmm to monoclinic F2/m space group. Ag+ ions were strongly disordered, with occupancies ranging from 0.02 to 0.24, at partially occupied sites within zeolitic cages. The combination of ab initio MD simulations and XAFS spectroscopy suggested that Ag+ is coordinated by three H2O oxygens at 2.37 A and two framework oxygen atoms between 2.55 and 3.00 A. The thermal stability was monitored in situ by SC-XRD (from 25 to 400 °C) and XAFS (from 25 to 650 °C). Upon heating, the structure transformed to three different topologies: B phase, D′ phase, observed here for the first ti...

Experimental and theoretical study of propene adsorption on alkali metal exchanged FER zeolites

Abstract Propene adsorption on Li and Na-FER zeolites was investigated combining IR spectroscopy and calorimetric measurements of adsorption heats with DFT calculations using a DFT/CC scheme based on the PBE density functional. Considering the good agreement between experimental and theoretical results, the following adsorption complexes of propene in the M-FER zeolites investigated in this study can be distinguished: (i) propene interacting with the zeolitic framework via dispersion interactions mainly populated in zeolites with a high Si/Al ratio and with a characteristic νC C vibrational band at 1646 cm−1 and adsorption heat of approximately 48 kJ/mol, (ii) propene interacting with cations coordinated in 6-rings characterized by IR bands at 1637 cm−1 (Li-FER) and 1636 cm−1 (Na-FER), (iii) propene adsorbed on remaining cationic positions excluding cationic positions in 6-rings characterized by IR bands at 1630 cm−1 (Li-FER) and 1633 cm−1 (Na-FER) and (iv) propene bridging two nearby sodium cations in dual-cation sites characterized by a vibrational band at 1626 cm−1 and with an adsorption heat of 85 kJ/mol, which is 6 kJ/mol higher than that of the strongest interaction with a single Na+ cation (79 kJ/mol). The population of bridged complexes in Na-FER was significantly lower than those in previously studied K-FER zeolites due to the preference of potassium cations for 8-rings, which is more suitable for the creation of dual-cation sites than 6-rings, wherein sodium cations are preferentially coordinated. No bridged complexes were found in the case of Li-FER because Li+ cations are closer to the framework oxygen atoms and thus relatively long distance from each other.

What Is Measured When Measuring Acidity in Zeolites with Probe Molecules?

Based on theoretical calculations of CO, NH3, and pyridine adsorption at different sites in MOR and MFI zeolites, we analyze how confinement effects influence the measurement of acidity based on the interaction of probe molecules with Brönsted acid sites. Weak bases, such as CO, form neutral ZH–CO adducts with a linear configuration that can be distorted by spatial restrictions associated with the dimensions of the pore, leading to weaker interaction, but can also be stabilized by dispersion forces if a tighter fitting with the channel void is allowed. Strong bases such as NH3 and pyridine are readily protonated on Brönsted acid sites, and the experimentally determined adsorption enthalpies include not only the thermochemistry associated with the proton transfer process itself, but also the stabilization of the Z––BH+ ion pair formed upon protonation by multiple interactions with the surrounding framework oxygen atoms, leading in some cases to a heterogeneity of acidities within the same zeolite structure.

Cation pair formation in copper and palladium exchanged MFI zeolite frameworks - a theoretical study.

The coordination of copper and palladium cations in the channels of MFI frameworks is examined by the QM/MM approach within the ONIOM method. Density functional theory is applied to the high layer of the ONIOM models, and different rings and ring associations are considered with the aim to find the optimal positions for cation pair formation. The Al atoms were located at tetrahedral sites which are known to have high occupancy factors from crystal structure studies. Divalent cations have a strong preference towards six-member ring coordination, and while the energy differences between five-member ring coordination and six-member ring coordination are not that large for the monovalent cations and hydroxyl-cations (M+ and M(OH)+, M = Cu, Pd) they still prefer six-member ring coordination in the straight channel. The monovalent cations form dimers [Pd-Pd]2+ and [Cu-Cu]2+ with bond lengths of 2.50-2.57 Å, but while palladium cations form dimers independent of the Si, Al ordering, the copper cation dimers are stable only with closely positioned negative framework charges. Separation of the two Cu+ cations at distances >4 Å is preferred, unless an oxygen bridge is formed. The cation dimers are exposed in the intersection volume of the straight and sinusoidal channels of MFI, which greatly favors the accessibility of reactants. The oxygen bridged Pd cations accept protons from Brønsted acid sites to form Pd-OH+-Pd and they are able to transfer this proton to reactants: upon co-adsorption of NO and NO2 on Pd-OH+-Pd, protonation of NO2 occurs. In contrast, Cu-OH+-Cu groups reach stabilization by forming hydrogen bonds to framework oxygen atoms. [Pd-Pd] cation dimers alone, or in oxygen-bridged form, provide an energetically favorable route for NO dissociation. Two different configurations with wide and narrow angle ∠MOM were examined for partial oxidation of methane to methanol. The narrow angle [Cu-O-Cu] was found to ease methanol desorption. The active site [Pd-O-Pd] is superior to [Cu-O-Cu], providing lower energy barriers for methane activation and methanol desorption.

Effect of the Method of Modification of Zeolite X on Selectivity of Catalytic Methylation of Toluene

The effect of the method of modification of catalysts based on zeolite X by cesium and magnesium cations (ion exchange and impregnation) on the selectivity of the alkylation of toluene with methanol was determined. It was shown by IR spectroscopy that the presence of strong Lewis acid sites (LAS) in the zeolites levels out the positive effect of the increased basicity of the framework oxygen atoms achieved by impregnation of (Mg-, Cs-)X samples with cesium nitrate. Ion-exchange modification of the catalysts, which eliminates the strong LAS, is an essential condition for increasing their selectivity for styrene and ethylbenzene.

Disproportionation of an Element in a Zeolite. III. Crystal Structure of a High-Temperature Sulfur Sorption Complex of Zeolite LTA Containing Two New Ions: Perthiosulfite, S42–, and the Trisulfur Cation, S32+

The structure of a high-temperature sulfur sorption complex of zeolite Na-A (LTA) has been determined by single-crystal crystallography. It was prepared by treating anhydrous Na-A with sulfur vapor at 623 K. Its crystal structure was refined in the space group Pm3m (a = 12.192(1) A) to the error index R1 = 0.075. Upon sorption, sulfur disproportionated fully to give one perthiosulfite anion, S42–, and one trisulfur cation, S32+, per unit cell. The perthiosulfite anion (S–S = 2.16(5) A and S–S–S = 105.4(22)°) is the perthio form of SO32–. It is in the large cavity where it bonds to one 6-ring and three 8-ring Na+ ions (S–Na = 2.83(6) A, 2.80(5) A, and 2.43(5) A). It appears to be the first example of a branched polysulfide ion. The electron-deficient trisulfur cation, S32+ (S–S = 2.11(8) A and S–S–S = 102.0(14)°), bonds to framework oxygen atoms in the sodalite cavity (S–O = 2.64(7) A). Twelve Na+ ions are found per unit cell at five crystallographic sites similar to but different from those in anhydrous ...

High-temperature behavior of natural ferrierite: In-situ synchrotron X-ray powder diffraction study.

In this paper, we report the results of the first study focused on the thermal stability and dehydration dynamics of the natural zeolite mineral ferrierite. A sample from Monastir, Sardinia [(Na0.56K1.19Mg2.02Ca0.52Sr0.14) (Al6.89Si29.04)O72·17.86H2O; a = 19.2241(3) Å; b = 14.1563(2) Å; c = 7.5106(1) Å, V = 2043.95(7) Å3] was investigated by thermogravimetric analysis and in-situ synchrotron X-ray powder diffraction. Thermogravimetric data show that H2O release begins already in the range 50-100 °C and is complete at ~600 °C. The results of the structure refinements performed in Immm space group by Rietveld analysis with data collected up to 670 °C show that ferrierite belongs to the group of zeolites that do not undergo phase transitions. Upon heating to 670 °C, ferrierite behaves as a non-collapsible structure displaying only a slight contraction of the unit-cell volume (ΔV = -3%). The unit-cell parameter reductions are anisotropic, more pronounced for a than for b and c (Δa = -1.6%; Δb = -0.76%; Δc = -0.70%). This anisotropic response to a temperature increase is interpreted as due to the presence in the ferrierite framework of five-membered ring chains of SiO4 tetrahedra, which impart a higher structural rigidity along b and c. Upon dehydration we observe: (1) the gradual H2O loss, beginning with the molecules hosted in the 10MR channel, is almost complete at 670 °C, in good agreement with the TG data; (2) as a consequence of the decreased H2O content, Mg and K migrate from their original positions, moving from the center of the 10MR channel toward the walls to coordinate the framework oxygen atoms. The observation of transmission electron microscopy selected-area electron diffraction patterns revealed defective crystals with an occasional and moderate structural disorder. Beyond providing information on the thermal stability and behavior of natural ferrierite, the results of this work have significant implications for possible technological applications. These data allow for comparison with the dehydration kinetics/mechanisms of the corresponding synthetic phases, clarifying the role played by framework and extra-framework species on the high-temperature behavior of porous materials with ferrierite topology. Moreover, the information on the thermal behavior of natural ferrierite can be used to predict the energetic performances of analogous synthetic Si-pure counterparts, namely "zeosil-electrolyte" systems, under non-ambient conditions. Specifically, the very high thermal stability of ferrierite determined in this study, coupled with the baric behavior determined in other investigations, suggests that the "Si-FER-electrolyte" system may be an excellent candidate for use as an energy reservoir. Indeed, ferrierite exhibits the so-called "spring behavior," i.e., upon compression in water or in an electrolyte solution, it converts the mechanical energy into interfacial energy, and-when pressure is released-it can completely restore the supplied mechanical energy accumulated during the compression step.

One year geopolymerisation of sodium silicate activated fly ash and metakaolin geopolymers

This paper presents the evolution of mechanical and chemical properties of sodium silicate activated fly ash and metakaolin geopolymers in one year time scale. The initial curing protocols are considered, which were chosen based on the isothermal heat release properties of both reaction systems. Increasing initial temperature contributes to the cumulative heat release, strength development and zeolite formation of both geopolymers, especially the fly ash based, but results in slightly worse pore characteristics. The formation of zeolite is rapid, indicating a simultaneous process with geopolymerisation. During sample handling and aging, natural carbonation took place. Two kinds of carbonation processes are detected: reaction of CO2 with mobile alkalis on the one hand and with framework oxygen atoms of the alunimosilicate gels on the other hand.

Monitoring the methanol conversion process in H-ZSM-5 using synchrotron X-ray powder diffraction-mass spectrometry

Using synchrotron X-ray powder diffraction-mass spectrometry in combination with theoretical calculations, we probed the structural changes of adsorbed methanol species in H-ZSM-5 for the first time during catalytic conversion of methanol. Preliminary experimental and computational findings suggested that associative mechanism was the dominant reaction pathway in the early stage of the reaction at 200 °C. Also, we observed a minor contribution from dissociative mechanism of the adsorbed methoxy intermediate species, CH3-Oz (where Oz is a framework oxygen atom), at Brønsted acid sites. It led to methoxy accumulation during the late stage of the reaction, which was postulated to be the initial step that produced ‘hydrocarbon pool’ and ‘coke’ through the formation of CC bonds.

Zeolite/dye hybrid composites: Organization of photoactive azobenzene molecules inside AlPO4-5

Organic/inorganic hybrid materials - like zeolites + dyes - are currently used in strategic areas, from sustainable energy technologies to biomedical sciences. In these systems, photoactive molecules are organized in one-dimensional nanostructures inside the zeolite channels. In this paper we present the results of a study aimed to the synthesis and the structural and spectroscopic characterization of azobenzene/AlPO4-5 hybrid composites, performed by a multi-technique approach based on material synthesis, thermal gravimetric analysis, synchrotron X-ray powder diffraction, UV-VIS and IR spectroscopies. The results indicate that azobenzene molecules are hosted in the 12-membered ring channel of AFI framework and the maximum loading is 0.9 molecules per unit cell. The combination of spectroscopic and diffractometric methods point out that, being AlPO4-5 starting material affected by Brönsted acidity, a consistent portion of the azobenzene molecules is protonated and interact with the framework oxygen atoms.

New Structural Model of Hydrous Sodium Aluminosilicate Gels and the Role of Charge-Balancing Extra-Framework Al

A new structural model of hydrous alkali aluminosilicate gel (N-A-S-H) frameworks is proposed, in which charge-balancing extra-framework Al species are observed in N-A-S-H gels for the first time. This model describes the key nanostructural features of these gels, which are identified through the application of 17O, 23Na, and 27Al triple quantum magic angle spinning solid-state nuclear magnetic resonance spectroscopy to synthetic 17O-enriched gels of differing Si/Al ratios. The alkali aluminosilicate gel predominantly comprises Q4(4Al), Q4(3Al), Q4(2Al), and Q4(1Al) Si units charge-balanced by Na+ ions that are coordinated by either 3 or 4 framework oxygen atoms. A significant proportion of Al3+ in tetrahedral coordination exist in sites of lower symmetry, where some of the charge-balancing capacity is provided by extra-framework Al species which have not previously been observed in these materials. The mean SiIV–O–AlIV bond angles for each type of AlIV environments are highly consistent, with compositional changes dictating the relative proportions of individual AlIV species but not altering the local structure of each individual AlIV site. This model provides a more advanced description of the chemistry and structure of alkali aluminosilicate gels and is crucial in understanding and controlling the molecular interactions governing gel formation, mechanical properties, and durability.

Structure of a cyclohexane sorption complex of partially dehydrated, fully Mn2+-exchanged zeolite Y (FAU, Si/Al = 1.56)

The structure of a cyclohexane sorption complex of partially dehydrated, fully Mn2+-exchanged zeolite Y has been determined at 100(1) K by single-crystal synchrotron X-ray diffraction techniques. It was refined using all intensities to the final error indices R1 = 0.052 and wR2 = 0.174. Cyclohexane molecules of symmetry 3¯2/m (D3d, chair form) occupy 5.7 of the 16 12-rings per unit cell. Each cyclohexane molecule is held in place by 18 weak hydrogen bonds between its six equatorial hydrogen atoms and all 12 oxygen atoms of its 12-ring: C⋯O ca. 3.85 Å. Mn2+ ions are found at four crystallographic sites, I, I′, II′, and II. Each of the 17.7(2) Mn2+ ions per unit cell at site II (opposite 6-rings in the supercage) coordinates tetrahedrally to three framework oxygen atoms and a water molecule. The cyclohexane molecules interact neither with the Mn2+ ions nor with the water molecules.

H2 Adsorption on Cu(I)–SSZ-13

We report H2 adsorption capacities reaching 0.05 wt % at 303 K and at 1 atm H2 pressure on solid-state CuCl-exchanged [Al]-SSZ-13 and [B]-SSZ-13 zeolites. Differential heat of H2 adsorption is found in the range between 16 and 48 kJ mol H2–1 on Cu(I)-SSZ-13 at 323 K and isosteric heat of adsorption is found between 20 and 55 kJ mol H2–1 on Cu(I)-[B]-SSZ-13 at temperatures between 293 and 323 K. Strong interactions between H2 and the copper cations in Cu(I)-SSZ-13 are evidenced using Rietveld refinements of neutron powder diffraction patterns revealing Cu–deuterium (D2) distances of 2.3(2) and 2.41(1) A. A temperature-dependent Cu(I) migration—away from the six-membered ring (6MR) and eight-membered ring (8MR)—is related to the high adsorption capacities of the samples at 303 K. At 10 K, access to Cu(I) at 6MR is sterically hindered by framework oxygen atoms (Cu–Oframework distance of 2.196(5) A), rationalizing the low H2 adsorption capacities of Cu(I)-SSZ-13 samples as compared to Na-SSZ-13 at 77 K.

Tandem Aromatization of Oxygenated Furans by Framework Zinc In Zeolites. A Computational Study

We have performed electronic structure calculations to characterize the active site of the zeolite CIT-6 (isomorphically substituted Zn-Beta) and to study the Diels–Alder dehydrative aromatization of methyl-5-(methoxymethyl)-furoate (MMFC) and of the dimethyl ester of 2,5-furan-dicarboxylic acid (DMFDC) with ethylene. Three types of active sites have been investigated: a site where the framework charge was balanced by two protons (Z0); a site with an H+ and a Li+ as counter-cations (Z1); and an active site with two Li+ counter-cations (Z2). Using NBO and Bader analysis, we conclude that Zn incorporation into the zeolite framework is not through covalent bonding to framework oxygen atoms but rather is through ionic bonding. Despite the ionic character of the active site and the generally strong Lewis acidic nature of Zn(II) cations, we find no catalysis of the Diels–Alder reaction for the two furans tested. On the other hand, the dehydration of the Diels–Alder cycloadduct can be either Bronsted or Lewis ac...

Progress toward Zeolite-Based Self-Luminous Sensors for Radioactive Isotopes such as 201Tl and 137Cs: Structures and Luminescence of Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A

Smart materials that both sorb and announce the presence of radioactive isotopes such as 201Tl and 137Cs are needed for human safety in environmental catastrophies. This study reports progress toward zeolite-based self-luminous sensors for radioactive isotopes. Hf4+ was introduced into zeolite A by treating Tl-A and Cs,Na-A with HfCl4(g) under anhydrous conditions. The crystal structures of the products, Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A, were determined by single-crystal crystallography using synchrotron X-radiation, with compositional confirmation by SEM-EDX analysis. Their luminescence properties upon X-ray, UV, and proton beam irradiation were studied. In Hf,Cl,Tl-A, some Hf4+ ions are 3-coordinate with three framework oxygen atoms of 6-rings; octahedral HfCl62– ions are at the very centers of 7% of the large cavities within Tl14HfCl612+ clusters. HfCl62– ions are also seen in Hf,Cl,Cs,Na-A; they similarly occupy 7% of the large cavities and are members of a Cs11HfCl69+ continuum. The X-ray-induced lumines...

Effect of cerium ions initial distribution on the crystalline structure and catalytic performance of CeY zeolite

CeY samples with different cerium (Ce) ions initial distribution were prepared by diverse preparation processes. The correlative influence of Ce ions distribution on the intracrystalline structure and catalytic mechanism of Y-type zeolite was studied by X-ray diffraction (XRD), Rietveld refinement, X-ray fluorescence spectrometry (XRF), energy-dispersive X-ray spectroscopy (EDS), infrared (IR) spectra, NH3 temperature-programmed desorption (NH3-TPD), N2 physical adsorption-desorption and micro-activity test (MAT). The results indicated that Ce ions were concentrated on the surface (supercage) of CeY zeolite would reduce the number of acid sites and the ratio of Brönsted acid and Lewis acid (B/L). Once Ce ions concentrated in the sodalite cage, it would participate in the coordination with framework oxygen atoms under the condition of steric hindrance. Meanwhile, Ce ions would preferentially promote the B/L value of weak acid by retaining more intracrystalline framework Al atoms in sodalite cage, which was beneficial to the conversion of heavy oil in FCC process. The optimal initial-distribution of Ce ions could not only save the consumption of rare earth resources but also give the best catalytic performance of CeY zeolite catalysts, and these advantages and attributes enabled it to have the potential to be applied in industrial applications.

Dehydration studies of natrolites: Role of monovalent extra-framework cations and degree of hydration

Rietveld refinements of natrolite analogs [M 16 Al 16 Si 24 O 80 ·nH 2 O, M-NAT, M = Li, Na, Ag, K, NH 4 , Rb, and Cs, 14.0(1) 2 O content and monovalent extra-framework cations (EFC) contained in the channels have on dehydration and thermal expansion. Dehydration temperatures are found to be inverse proportional to the size of the EFC. Isostructural K-, Rb-, and Cs-NAT with disordered EFC-H 2 O distribution exhibit negative thermal expansions before dehydration. The thermal expansion coefficients increase linearly from K-, Rb-, to Cs-NAT, the latter exhibits has the smallest thermal expansion coefficient of all NAT analogs [3.0(1) × 10 −6 K −1 ]. After dehydration, the EFC distribution of K-, Rb-, and Cs-NAT becomes ordered and their thermal expansion coefficients become positive. In the isostructural Li-, Na-, and Ag-NAT with ordered EFC-H 2 O distribution, the thermal expansion coefficients are positive for the Li- and Ag-NAT and negative for Na-NAT. After dehydration, this behavior is reversed, and Li- and Ag-NAT show negative thermal expansion coefficients, whereas Na-NAT exhibits a positive thermal expansion. Upon dehydration, the channels in Li- and Ag-NAT reorient: the rotation angles of the fibrous chain units, ψ, change from 26.4(2)° to −29.6(2)° in Li-NAT and from 22.3(2)° to −23.4(2)° in Ag-NAT. The structure models of the dehydrated Li- and Ag-NAT reveal that the change in the channel orientation is due to the migration of the Li + and Ag + cations from the middle of the channel to the walls where they are then coordinated by four framework oxygen atoms. Further heating of these dehydrated phases results in structural collapse and amorphization. X-ray O1s K -edge absorption spectroscopy reveals that the binding energy between the EFC and the oxygen of the framework (O f ) is larger in Li- and Ag-NAT than in Cs-NAT due to an increase of the basicity of the framework oxygen. The interaction between the H 2 O molecules and EFCs allow a clear separation in structures with disordered H 2 O molecules in the center of the channels (K-, NH 4 -, Rb-, and Cs-NAT) and those in close proximity to the aluminosilicate framework (Li-, Na-, and Ag-NAT), which leads to systematic dehydration and thermal expansion behaviors. Our structure work indicates that the effects of EFCs are more important to stabilize the NAT structure than the degree of hydration.

Exchanging noble and seminoble cations into zeolites by oxygen vacancy ion exchange (OVIE)

It is proposed that cation exchanged zeolites can be prepared by reacting their decationized forms, e.g., (SiO2)n(Al2O3)m, n ≥ 2m, with noble or seminoble metals and oxygen or ozone. The resulting oxide ions would very exoergically occupy vacancies in the zeolite framework and the cations would go to conventional extraframework positions. The noble metals Rh, Pd, Ir, Pt, and Au and the seminoble metals Ru, Ag, Re, Os, Hg, and Bi are suggested. A sample reaction using decationized zeolite LTA would be: 6Pd + 3O2 + Si12Al12O42 → Pd6Si12Al12O48. Decationized zeolites can be prepared by heating zeolites, especially high-Al zeolites, in their H+ or NH4+ forms, causing them to lose framework oxygen atoms as H2O. The reactions may occur by simply heating a mixture of starting materials in flowing oxygen or ozone, e.g., 6Pd + 3O2 + (NH4)12Si12Al12O48.nH2O → Pd6Si12Al12O48 + 12NH3 + (6 + n)H2O. This method may be extended to other inorganic ion exchangers.

The role of water in the elastic properties of aluminosilicate zeolites: DFT investigation

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.