Microporous materials find their origin in the discovery by Crönsted during the thirteenth century of the zeolitic property of the mineral stilbite. The zeolite family is made up of the aluminosilicate minerals with formula {
M
n+
x/n[(AlO
2)
x(SiO
2)
y]
x·
wH
2O}, where
x indicates the number of
M
n+ cations necessary to compensate the negative charge of the whole framework. All these phases exhibit three-dimensional structures built up exclusively from corner-sharing
TO
4 (
T = Al, Si) tetrahedra, defining tunnels in which the
M
n+ cations and water molecules are located. Wilson
et al. (1982
![[triangle]](/corehtml/pmc/pmcents/rtrif.gif)
) discovered a new family of compounds, the microporous aluminophosphates. Since 1992, the research groups of Cavellec (Cavellec
et al., 1995
; Cavellec, Riou & Férey, 1997
; Cavellec, Férey & Grenèche, 1997
; Riou-Cavellec
et al., 1998
) have been interested in the synthesis of these microporous materials. This work was followed by studies of microporous oxides by several groups (Debord
et al., 1997
; Lii & Huang, 1997
a
,
b
,
c
; Huang
et al., 1998
; Zima
et al., 1998
; Zima & Lii, 1998
). Microporous materials derived from octahedral and tetrahedral frameworks currently boast an extensive chemistry and a number of them display useful properties as catalysts, sorbents and ionic exchangers (Davis & Lobo, 1992
; Breck, 1974
; Venuto, 1994
).
Two polymorphs of Al(H
2PO
4)
3 have been reported to date. The α-form is hexagonal with cell parameters
a = 7.849 (1) Å and
c = 24.87 (3) Å (Yoire, 1961
), and the hexagonal β-form has parameters
a = 13.69 (1) Å and
c = 9.135 (1) Å (Yoire, 1961
), also found by Brodalla
et al. (1981
). The α-form is isostructural with Fe(H
2PO
4)
3 (Baies
et al., 2006
) and consists of a three-dimensional framework of corner-sharing FeO
6 and PO
2(OH)
2 tetrahedra. The synthesis of a new monoclinic variety of iron aluminium phosphate, (Fe
0.81Al
0.19)(H
2PO
4)
3 (γ-form), is reported in this work.
(Fe0.81Al0.19)(H2PO4)3 is composed of a highly puckered sheet structure containing interconnected M
2P2 units (M = Fe, Al) connected laterally by Fe–O–P mixed bridges to form two-dimensional layers perpendicular to the b axis (Fig. 1). The oligomeric M
2P2 units are built up from alternating corner sharing of octahedral MO6 and tetrahedral PO4 units. The MO6 octahedra share six O atoms with adjacent P atoms, whereas the PO4 tetrahedra share only two O atoms. The projection of the sheet is shown in Fig. 2, viewed down the [010] axis. The M—O distances in (Fe0.81Al0.19)(H2PO4)3 have values intermediate between 1.944 (4) and 2.063 (4) Å (Table 1), consistent with the occupation of Fe and Al valencies in these sites. The interatomic angles reveal distortions of the octahedra, varying from O6—Fe1—O2(x, y, z − 1) = 86.66 (17)° to O1—Fe1—O6 = 178.84 (18)°. The dihydrogen phosphate ions, [H2PO4]−, can be described as slightly distorted tetrahedra, with a mean value for the P—OH bond distances of 1.578 Å and with P=O bond distances ranging from 1.504 (4) to 1.525 (4) Å. The O—P—O angles are in the range 101.6 (3)–118.1 (2)°.
| Table 1Selected geometric parameters (Å, °) |
The crystal structure of (Fe
0.81Al
0.19)(H
2PO
4)
3 is characterized by an extended hydrogen-bonding network. The layers are held together through strong hydrogen bonds between the terminal O atoms attached to the two-connected phosphate groups in adjacent layers. Analysis of the hydrogen bonds in (Fe
0.81Al
0.19)(H
2PO
4)
3 shows two different types of P—O—H
O—P bridges. Within the layer, adjacent [H
2PO
4]
− ions are connected into chains by short hydrogen bonds (Table 2) with an O
O distance of 2.614 (5) Å formed by one of the hydroxy groups, O3—H3
O6(
x −
, −
y +
,
z +
). Adjacent layers are linked by longer hydrogen bonds,
viz. O4—H4
O4(
x, −
y + 1,
z +
) [2.775 (6) Å], O12—H12
O12(
x, −
y + 2,
z −
) [3.055 (8) Å] and O11—H11
O12(
x, −
y + 2,
z +
) [3.220 (7) Å], which allow the layers to connect as observed in Fig. 1.
| Table 2Hydrogen-bond geometry (Å, °) |
A comparison between (Fe0.81Al0.19)(H2PO4)3 and the series of compounds (NH4, H3O, K)(Fe,Al)3(HPO4)2(H2PO4)6·4H2O and (C6H8N)[Al2P3O10(OH)2] is shown in Fig. 3. Detailed descriptions of their topology are also reported here. The aim of this comparison is to provide a review of possible approaches that can be used to establish the topology of microporous structures. For obvious reasons, we do not consider related octahedral–tetrahedral frameworks here. Most attention will be focused on network topology and the possibility of intercalating alkaline cations or organic molecules in the solid-state inorganic framework, which is important for both mineralogy and material sciences.
(Fe
0.81Al
0.19)(H
2PO
4) is considered a normal solid-state inorganic framework. A comparison between this compound and the series of compounds (NH
4, H
3O, K)(Fe,Al)
3(HPO
4)
2(H
2PO
4)
6·4H
2O is shown in Figs. 3(
a) and (
b). The common characteristic of these compounds is their bidimensionality. In (NH
4, H
3O, K)(Fe,Al)
3(HPO
4)
2(H
2PO
4)
6·4H
2O, the NH
4
+, H
3O
+ and K
+ cations are located inside 12-sided polyhedra, which are generated by the corner-sharing
MO
6 (
M = Fe, Al) and [H
2PO
4]
− units, while water molecules are located in the interlayer space (Mgaidi
et al., 1999
; Bosman
et al., 1986
; Anisimova
et al., 1997
). Figs. 3(
a) and (
c) show the comparison between (Fe
0.81Al
0.19)(H
2PO
4)
3 and the two-dimensional layered compound (C
6H
8N)[Al
2P
3O
10(OH)
2]. This structure contains macroanionic [Al
2P
3O
10(OH)
2]
− sheets that are charge-balanced by protonated 4-methylpyridine. The inorganic layers are constructed from alternating Al-centred units (AlO
4 and AlO
5) and P-centred units [PO
4, PO
3(OH) and PO
2(=O)(OH)] with triply and doubly bridging phosphate groups (Yu
et al., 2000
). This comparison provides an example of the concept of scale chemistry (Férey, 2000
). The cavities created by the framework, which are very small in typical solid-state inorganic frameworks and only able to accept alkaline cations or organic molecules, become larger and larger.
