Flexibility and distortion of the collapsible framework of NAT topology: the crystal structure of NH4-exchanged natrolite

Stuckenschmidt, E.; Kassner, Dethard; Joswig, W.; Baur, W. H.

doi:10.1127/ejm/4/6/1229

Cited by 30 publications

(12 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The as-prepared ammonium-form has been identified to be anhydrous and as a consequence exhibited a reduction in the unit-cell volume by ca. 4% compared to the original sodium natrolite (Stuckenschmidt et al 1992), whereas the lithium form maintained the same water content as the sodium natrolite but exhibited similar reduction in the unit-cell volume by ca. 5% due to the smaller cation size (Baur et al 1990).…”

mentioning

confidence: 66%

See 1 more Smart Citation

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

Lee

Seoung

Jang

et al. 2011

American Mineralogist

View full text Add to dashboard Cite

We report here for the first time that fully and partially NH 4 -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 NH 4 -exchanged natrolites at ambient conditions, (NH 4 ) 16(2) Al 16 Si 24 O 80 ·14.1(9)H 2 O and (NH 4 ) 5.1(1) K 10.9(1) Al 16 Si 24 O 80 ·15.7(3)H 2 O, are found to be larger than that the original sodium-natrolite by ca. 15.6% and 12.8%, respectively. Upon temperature increase, the fully NH 4 -exchanged natrolite undergoes dehydration at ca. 150 °C with ca. 16.4% contraction in the unit-cell volume. The dehydrated phase of the fully NH 4 -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 NH 4 -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 NH 4 -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 NH 4 -exchanged natrolite. In the hydrated model of the fully NH 4 -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 NH 4 -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 NH 4 -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.

show abstract

mentioning

confidence: 66%

“…It has been shown that the sodium cations in natrolite can be exchanged with smaller lithium cations or larger ammonium cations via high-temperature melt exchange (Baur et al 1990;Stuckenschmidt et al 1992). The as-prepared ammonium-form has been identified to be anhydrous and as a consequence exhibited a reduction in the unit-cell volume by ca.…”

mentioning

confidence: 98%

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

Lee

Seoung

Jang

et al. 2011

American Mineralogist

View full text Add to dashboard Cite

show abstract

“…A recent single crystal x-ray diffraction study of an anhydrous NH 4 ϩ -natrolite containing flexible eight-rings reports hydrogen bond lengths between 186 and 208 pm, and N-O distances between 282 and 287 pm. 42 A powder x-ray diffraction study of zeolite NH 4 -RHO which also possesses flexible eight-rings yielded N-O distances of 289 and 314 pm. 43 For the NH 3 molecule we find a similar adsorption struc-TABLE IV.…”

Section: Qm-pot Calculationsmentioning

confidence: 99%

Comparison of a combined quantum mechanics/interatomic potential function approach with its periodic quantum-mechanical limit: Proton siting and ammonia adsorption in zeolite chabazite

Brändle

Sauer

Dovesi

et al. 1998

The Journal of Chemical Physics

View full text Add to dashboard Cite

Comparison is made between a combined quantum mechanics/interatomic potential function approach ͑QM-Pot͒ and its fully quantum-mechanical limit, ab initio calculation applying periodic boundary conditions. The Hartree-Fock ͑HF͒ method is combined with ab initio-parametrized ion pair shell model potential functions. The CRYSTAL code is employed for the periodic Hartree-Fock calculations. The same double-/valence triple-zeta polarization basis sets are used in both the approaches. The proton siting and ammonia adsorption in a high-silica acidic zeolite catalyst, H-chabazite ͑Si/Alϭ11, space group P1, unit cell H-AlO 2 ͓SiO 2 ] 11 ) are examined. The combined QM-Pot relative stabilities and reaction energies deviate from the periodic full QM results by 4-9 kJ/mol only, which demonstrates the power of our combined approach. This conclusion is also supported by comparison of the electrostatic potential inside the zeolite pore, calculated from the periodic wave function and by the QM-Pot approach. Framework oxygen O1 is found to be the preferred proton site and on interaction with NH 3 the proton is predicted to move to NH 3 yielding NH 4ϩ . The NH 4 ϩ surface species is coordinated to two framework oxygen atoms. It is by 30-35 kJ/mol more stable than the neutral adsorption complex of NH 3 . Evidence is produced that the failure of previous periodic HF calculations to predict a stable NH 4 ϩ ion is due to the limitations of the minimum basis set used.

show abstract

“…However, cation exchange of natrolite was found to be more difficult than in other zeolites due to its small pore size and tightly bound cations [1]. For many years, only isovalent cation substitution was reported in Na-NAT by high-temperature melt exchange methods to produce partially Li-exchanged and anhydrous NH 4 -forms [7,8]. Recently, natrolites containing large isovalent (Rb + , Cs + , Ag + ) and divalent cations (Ca 2+ , Sr 2+ , Ba 2+ , Cd 2+ , Pb 2+ ) as well as hydrous NH 4 -NAT were synthesized through ion exchange using K-NAT as a starting material by Lee's group [9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Thermodynamic study of alkali and alkaline-earth cation-exchanged natrolites

Navrotsky

Lee

2013

Microporous and Mesoporous Materials

View full text Add to dashboard Cite

Flexibility and distortion of the collapsible framework of NAT topology: the crystal structure of NH4-exchanged natrolite

Cited by 30 publications

References 0 publications

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

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

Comparison of a combined quantum mechanics/interatomic potential function approach with its periodic quantum-mechanical limit: Proton siting and ammonia adsorption in zeolite chabazite

Thermodynamic study of alkali and alkaline-earth cation-exchanged natrolites

Contact Info

Product

Resources

About