Yongmoon Lee

Natrolite may not be a “soda-stone” anymore: Structural study of fully K-, Rb-, and Cs-exchanged natrolite

Abstract Since its first discovery in nature, natrolite has been largely known as a sodium aluminosilicate zeolite, showing very limited preference toward cation exchange. Here we show that fully K-exchanged natrolite can be prepared from natural Na-natrolite under mild aqueous conditions and used to subsequently produce Rb- and Cs-exchanged natrolites. These cation-exchanged natrolites exhibit successive volume expansions by ca. 10, 15.7, and 18.5% for K-, Rb-, and Cs-forms, respectively, compared to the original Na-natrolite. This constitutes the largest, ever-reported volume expansion via cation substitution observed in zeolites and occurs by converting the elliptical channels into progressively circular ones. The observed cation-dependent changes in the channel volume and shape thus show the flexibility limits of the natrolite framework and suggest the possible existence of compositionally altered analogues in suitable environments as well as a novel means to tailor the cation selectivity of this class of small pore zeolites toward various industrial and environmental applications.

Super-Hydrated Zeolites: Pressure-Induced Hydration in Natrolites

High-pressure synchrotron X-ray powder diffraction studies of a series of alkali-metal-exchanged natrolites, A16Al16Si24O80·nH2O (A=Li, K, Na, Rb, and Cs and n=14, 16, 22, 24, 32), in the presence of water, reveal structural changes that far exceed what can be achieved by varying temperature and chemical composition. The degree of volume expansion caused by pressure-induced hydration (PIH) is inversely proportional to the non-framework cation radius. The expansion of the unit-cell volume through PIH is as large as 20.6% in Li-natrolite at 1.0 GPa and decreases to 6.7, 3.8, and 0.3% in Na-, K-, and Rb-natrolites, respectively. On the other hand, the onset pressure of PIH appears to increase with non-framework cation radius up to 2.0 GPa in Rb-natrolite. In Cs-natrolite, no PIH is observed but a new phase forms at 0.3 GPa with a 4.8% contracted unit cell and different cation-water configuration in the pores. In K-natrolite, the elliptical channel undergoes a unique overturn upon the formation of super-hydrated natrolite K16Al16Si24O80·32H2O at 1.0 GPa, a species that reverts back above 2.5 GPa as the potassium ions interchange their locations with those of water and migrate from the hinge to the center of the pores. Super-hydrated zeolites are new materials that offer numerous opportunities to expand and modify known chemical and physical properties by reversibly changing the composition and structure using pressure in the presence of water.

Natrolite is not a “soda-stone” anymore: Structural study of alkali (Li+), alkaline-earth (Ca2+, Sr2+, Ba2+) and heavy metal (Cd2+, Pb2+, Ag+) cation-exchanged natrolites

Abstract We report here the preparation and structural models of alkaline-earth (Ca2+, Sr2+, Ba2+) and heavy metal (Cd2+, Pb2+, Ag+) cation-exchanged natrolites at ambient conditions and compare them to the alkali (Li+, Na+, K+, Rb+, Cs+) cation forms. The latter two groups all crystallize in the orthorhombic Fdd2 symmetry as the natural sodium natrolite, whereas the alkaline earth analogues are all found in the monoclinic Cc symmetry as scolecite, the natural calcium counterpart. We find the existence of a universal linear relationship between the unit-cell volume and the non-framework cation radius in natrolite. The rotation angles of the fibrous chain units are distributed between 25.5° (Li-form) and 2.9° (Cs-form) to show its inverse proportionality to the non-framework cation radius and the channel opening area. We also propose a possible threshold in the cation radius that dictates the distribution pattern of the non-framework cations and water molecules in the ordered and disordered fashions in natrolite.

Irreversible xenon insertion into a small-pore zeolite at moderate pressures and temperatures

Pressure drastically alters the chemical and physical properties of materials and allows structural phase transitions and chemical reactions to occur that defy much of our understanding gained under ambient conditions. Particularly exciting is the high-pressure chemistry of xenon, which is known to react with hydrogen and ice at high pressures and form stable compounds. Here, we show that Ag16Al16Si24O8·16H2O (Ag-natrolite) irreversibly inserts xenon into its micropores at 1.7 GPa and 250 °C, while Ag(+) is reduced to metallic Ag and possibly oxidized to Ag(2+). In contrast to krypton, xenon is retained within the pores of this zeolite after pressure release and requires heat to desorb. This irreversible insertion and trapping of xenon in Ag-natrolite under moderate conditions sheds new light on chemical reactions that could account for the xenon deficiency relative to argon observed in terrestrial and Martian atmospheres.

In-situ dehydration studies of fully K-, Rb-, and Cs-exchanged natrolites

Abstract In-situ synchrotron X-ray powder diffraction studies of K-, Rb-, and Cs-exchanged natrolites between room temperature and 425 °C revealed that the dehydrated phases with collapsed frameworks start to form at 175, 150, and 100°C, respectively. The degree of the framework collapse indicated by the unit-cell volume contraction depends on the size of the non-framework cation: K-exchanged natrolite undergoes an 18.8% unit-cell volume contraction when dehydrated at 175 °C, whereas Rband Cs-exchanged natrolites show unit-cell volume contractions of 18.5 and 15.2% at 150 and 100°C, respectively. In the hydrated phases, the dehydration-induced unit-cell volume reduction diminishes as the cation size increases and reveals increasingly a negative slope as smaller cations are substituted into the pores of the natrolite structure. The thermal expansion of the unit-cell volumes of the dehydrated K-, Rb-, and Cs-phases have positive thermal expansion coefficients of 8.80 × 10−5 K−1, 1.03 × 10−4 K−1, and 5.06 × 10−5 K−1, respectively. Rietveld structure refinements of the dehydrated phases at 400 °C reveal that the framework collapses are due to an increase of the chain rotation angles, ψ, which narrow the channels to a more elliptical shape. Compared to their respective hydrated structures at ambient conditions, the dehydrated K-exchanged natrolite at 400°C shows a 2.2-fold increase in ψ, whereas the dehydrated Rb- and Cs-natrolites at 400°C reveal increases of ψ by ca. 3.7 and 7.3 times, respectively. The elliptical channel openings of the dehydrated K-, Rb-, to Cs-phases become larger as the cation size increases. The disordered non-framework cations in the hydrated K-, Rb-, and Csnatrolite order during dehydration and the subsequent framework collapse. The dehydrated phases of Rb- and Cs-natrolite can be stabilized at ambient conditions

Sonochemical Synthesis and Properties of Nanoparticles of FeSbO4

A sonochemical method was employed to prepare reactive nanoparticles of FeSbO(4) at 300 °C, which is the lowest calcination temperature reported so far for preparing FeSbO(4). A systematic evolution of the FeSbO(4) phase formation as a function of temperature was monitored by in situ synchrotron X-ray measurements. The 300 and 450 °C calcined powders exhibited specific surface areas of 116 and 75 m(2)/g, respectively. The X-ray photoelectron spectra analysis confirmed the presence of mainly Fe(3+) and Sb(5+) in the calcined powder. The response of the fabricated sensors (using both 300 and 450 °C calcined powders) toward 1000 ppm and 1, 2, 4, and 8% hydrogen, respectively, has been monitored at various operating temperatures. The sensors fabricated using 300 °C calcined powder exhibited a response of 76% toward 4% H(2) gas at an operating temperature of 300 °C, while those fabricated using 450 °C calcined powder exhibited a higher response of 91% with a quick recovery toward 4% H(2) gas at 300 °C. The results confirmed that a higher calcination temperature was preferred to achieve better sensitivity and selectivity toward hydrogen in comparison to other reducing gases such as butane and methane. The experimental results confirmed that the sonochemical process can be easily used to prepare FeSbO(4) nanoparticles for various catalytic applications as demonstrated. Here, we project FeSbO(4) as a new class of material exhibiting high sensitivity toward a wide range of hydrogen gas. Such sensors that could detect high concentrations of hydrogen may find application in nuclear reactors where there will be a leakage of hydrogen.

Immobilization of Large, Aliovalent Cations in the Small‐Pore Zeolite K‐Natrolite by Means of Pressure

High-pressure ion exchange of small-pore zeolite K-natrolite allows immobilization of nominally non-exchangeable aliovalent cations such as trivalent europium. A sample exchanged at 3.0(1) GPa and 250 °C contains about 4.7 Eu(III) ions per unit cell, which is equivalent to over 90 % of the K(+) cations being exchanged.

Pressure‐Induced Hydration and Insertion of CO2 into Ag‐Natrolite

CO2 insertion under pressure: In silver-exchanged natrolite, a low and technically achievable onset pressure-induced hydration has been established at 0.4 GPa accompanied by an approximately 5 % expansion in the unit-cell volume (see figure). This unique property has been utilized to trap CO2 under pressure in nominally non-penetrating natrolite pores.

Spectroscopic characterization of alkali-metal exchanged natrolites

Abstract Synchrotron infrared (IR) and micro-Raman spectroscopic studies have been performed on zeolite natrolites as a function of the non-framework composition at ambient conditions. This establishes the spectroscopic characterization of the ion-exchanged natrolites in the alkali-metal series both in the as-prepared hydrated (M-NAT-hyd, M = Li, Na, K, Rb, and Cs) and some stable dehydrated forms (MNAT- deh, M = Rb and Cs). The former series exhibits non-framework cation-size dependent opening of the helical channels to span ca. 21° range in terms of the chain rotation angle, ψ (or ca. 45° range in terms of the chain bridging angle, T-O2-T). For these hydrated phases, both IR and Raman spectra reveal that the degree of the red-shifts in the frequencies of the helical 8-ring channel as well as the 4-ring unit is proportional to the ionic radius of the non-framework cations. Linear fits to the data show negative slopes of -55.7 from Raman and -18.3 from IR in the 8-ring frequencies and ionic radius relationship. The spectroscopic data are also used to identify the modes of the dehydration-induced “collapse” of the helical 8-ring channels as observed in the stable anhydrous Rb-NAT-deh and Cs- NAT-deh. In addition, we demonstrate that the spectroscopic data in the hydrated series can be used to distinguish different water arrangements along the helical channels based on the frequency shifts in the H-O-H bending band and the changes in the O-H stretching vibration modes.

Structural studies of NH4-exchanged natrolites at ambient conditions and high temperature

Abstract We report here for the first time that fully and partially NH4-exchanged natrolites can be prepared in hydrated states using the solution exchange method with potassium-natrolite. The structural models of the as-prepared hydrated phases and their dehydrated forms at elevated temperature were refined in space group Fdd2 using in situ synchrotron X‑ray powder diffraction data and Rietveld methods. The unit-cell volumes of the hydrated NH4-exchanged natrolites at ambient conditions, (NH4)16(2) Al16Si24O80·14.1(9)H2O and (NH4)5.1(1)K10.9(1)Al16Si24O80·15.7(3)H2O, are found to be larger than that the original sodium-natrolite by ca. 15.6% and 12.8%, respectively. Upon temperature increase, the fully NH4-exchanged natrolite undergoes dehydration at ca. 150 °C with ca. 16.4% contraction in the unit-cell volume. The dehydrated phase of the fully NH4-exchanged natrolite exhibits marginal volume expansion up to 425 °C and then becomes amorphized during temperature decrease and exposure to atmospheric condition. In the case of the partially NH4-exchanged natrolite, the dehydration starts from ca. 175 °C with ~15.1% volume contraction and leads to a partial phase separation to show a phase related to the dehydrated K-natrolite. The degree of the phase separation decreases with temperature increase up to 475 °C, concomitant to the gradual volume contraction occurring in the partially NH4-exchanged natrolite in the dehydrared state. Upon temperature decrease and exposure to atmospheric condition, only the dehydrated K-natrolite is recovered as a crystalline phase from the partially NH4-exchanged natrolite. In the hydrated model of the fully NH4-exchanged natrolite, the ammonium cations and water molecules are statistically distributed along the elliptical channels, similar to the disordered pattern observed in natrolites exchanged with larger alkali metal cations such as the K-, Rb-, and Cs-forms. The dehydrated model of the fully NH4-exchanged natrolite at 400 °C is essentially same as the one reported previously from the sample prepared by direct melt exchange method using sodium-natrolite. Both the hydrated and dehydrated structures of the partially NH4- exchanged natrolite at RT and at 400 °C, respectively, are characterized by having two separate sites for the ammonium and potassium cations. Comparing the structural models of the monovalent cation forms studied so far, we find that the rotation angle of the natrolite chain is inversely proportional to the cation radius both in the hydrated and dehydrated phases. The distribution pattern of the nonframework species along the natrolite channel also seems to be related to the non-framework cation radius and hence to the chain rotation angle.