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

Abstract

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.

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

“…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].…”

“…In-situ XRD revealed that, except for NH 4 -NAT, natrolites are thermally persistent, with the onset temperature of dehydration decreasing with increasing cation size [9][10][11][12]. Once dehydrated, the elliptical channels collapse and unit cell volume contracts [9,10].…”

Solid-state NMR study of various mono- and divalent cation forms of the natural zeolite natrolite

“…for the reported crystallographic data on the hydrated form of various monovalent cation-exchanged natrolites (Table 2). 7,20,21 In fact, the 9-ring channels including Si 2 -O-T angles along the c-axis in the NAT structure become more circular with increasing the size of extraframework cations. Also, there is a linear relationship between the unit cell volume and the cation size.…”

“…The values in parentheses are those obtained using the d cal values calculated using the equation derived here. e Average Si-O-T angles in degrees from the crystallographic data for metanatrolite 8 and for the other cation forms of natrolite 21,22 which were collected under atmospheric pressure at 275 1C and under vacuum at 400 1C, respectively.…”

Structural characterization of various alkali cation forms of synthetic aluminosilicate natrolites

“…This zeolite is known to have a well-ordered distribution of framework Si and Al atoms and of extraframework Na þ cations and water molecules with an ideal composition Na 16 . While all of them are characterized by Si/Al ratios close to 1.5, their extraframework compositions, crystal symmetries, and/or framework tetrahedral atom (T-atom) distributions are rather different from one another.…”