water groups complete the coordination sphere of the aluminum, leading to connection modes already encountered in mineral aluminophosphates in which AlO n
polyhedra may share corners or edges through AlaOaAl linkages. It can also result
in the formation of odd-numbered rings. The channel size appears to be limited as
a consequence of the increase of the aluminum coordination, which thus seems to
be a drawback for producing large pores.
Several studies have explored the use of non-aqueous solvents, mainly alcohols
(butanol, triethylene glycol, etc.), for the synthesis of open framework aluminum
phosphates [41]. Better crystallization is generally observed, which is helpful for
X-ray structural characterization. In many cases, low-dimensional AlPOs [42, 43]
have been obtained, corresponding to several series of layered and chain solids,
with P:Al ratios ranging from 4:3 to 2:1, in which the interlamellar or interchain
space is occupied by organic cations. In a recent examination of the formation of
aluminophosphates under non-aqueous conditions, Ozin and co-workers [44] have
suggested that intermediates based upon one-dimensional chains could be involved in the crystallization of two-dimensional porous layers and finally the threedimensional open framework materials. We shall show in the following sections
that the versatility of aluminum phosphates in terms of dimensionality is a common feature of systems that are prone to form open-framework architectures. The
other feature is the P:Al ratio, which often differs from unity in phases obtained
from non-aqueous solvents. All of the AlO 4 tetrahedra share their vertices, but this
is not the case for the phosphate groups, some of which exhibit terminal PaOH
bonds giving rise to an interrupted framework.
The use of fluoride in phosphate synthesis was first used by Kessler and coworkers [45, 46]. When fluorine is present in the final structure, it can adopt specific configurations in which certain AlPO 4 structural types are essentially preserved. For example, in AlPO 4 -5 (AFI) [47] the fluorine is located in the 4-ring
columns between two 4-rings. It is linked to two aluminum atoms with rather long
distances (AlaF 0.219 nm). Similarly, in the aluminophosphate analogues of zeolites chabazite (CHA) and gismondine (GIS), two fluorines bridge two aluminum
atoms across the 4-ring units, rendering them octahedrally coordinated rather than
tetrahedrally coordinated as in the aluminosilicate forms. The most exciting feature of the fluorine method is its use in the synthesis of new aluminophosphate
architectures incorporating fluorine atoms into the open framework. Two situations are found for the fluorine. First, it seems to stabilize the double 4-ring cage
(D4R), i.e. a cube in which the corners are occupied by TO 4 tetrahedra. The occurrence of this octameric unit is found in aluminophosphates such as AlPO 4 -16
(AST) [48] synthesized in the presence of hydrofluoric acid. This building unit
has also been prepared as an isolated cluster in Mu-1 [49] and as linear chains in
Mu-3 [50].
Alternatively, fluorine can participate more directly in the coordination sphere of
the metal and increase the coordination number from V to VI. Studies performed
by Fe ´rey and co-workers [51, 52] in Le Mans gave rise to a new series of oxyfluorinated phosphates called ULM-n. In this family, the basic building unit can be
either tetramers, as is usually found in the crystal chemistry of phosphates [53],
18.4 Open-Framework Metal Phosphates 597
polyhedra may share corners or edges through AlaOaAl linkages. It can also result
in the formation of odd-numbered rings. The channel size appears to be limited as
a consequence of the increase of the aluminum coordination, which thus seems to
be a drawback for producing large pores.
Several studies have explored the use of non-aqueous solvents, mainly alcohols
(butanol, triethylene glycol, etc.), for the synthesis of open framework aluminum
phosphates [41]. Better crystallization is generally observed, which is helpful for
X-ray structural characterization. In many cases, low-dimensional AlPOs [42, 43]
have been obtained, corresponding to several series of layered and chain solids,
with P:Al ratios ranging from 4:3 to 2:1, in which the interlamellar or interchain
space is occupied by organic cations. In a recent examination of the formation of
aluminophosphates under non-aqueous conditions, Ozin and co-workers [44] have
suggested that intermediates based upon one-dimensional chains could be involved in the crystallization of two-dimensional porous layers and finally the threedimensional open framework materials. We shall show in the following sections
that the versatility of aluminum phosphates in terms of dimensionality is a common feature of systems that are prone to form open-framework architectures. The
other feature is the P:Al ratio, which often differs from unity in phases obtained
from non-aqueous solvents. All of the AlO 4 tetrahedra share their vertices, but this
is not the case for the phosphate groups, some of which exhibit terminal PaOH
bonds giving rise to an interrupted framework.
The use of fluoride in phosphate synthesis was first used by Kessler and coworkers [45, 46]. When fluorine is present in the final structure, it can adopt specific configurations in which certain AlPO 4 structural types are essentially preserved. For example, in AlPO 4 -5 (AFI) [47] the fluorine is located in the 4-ring
columns between two 4-rings. It is linked to two aluminum atoms with rather long
distances (AlaF 0.219 nm). Similarly, in the aluminophosphate analogues of zeolites chabazite (CHA) and gismondine (GIS), two fluorines bridge two aluminum
atoms across the 4-ring units, rendering them octahedrally coordinated rather than
tetrahedrally coordinated as in the aluminosilicate forms. The most exciting feature of the fluorine method is its use in the synthesis of new aluminophosphate
architectures incorporating fluorine atoms into the open framework. Two situations are found for the fluorine. First, it seems to stabilize the double 4-ring cage
(D4R), i.e. a cube in which the corners are occupied by TO 4 tetrahedra. The occurrence of this octameric unit is found in aluminophosphates such as AlPO 4 -16
(AST) [48] synthesized in the presence of hydrofluoric acid. This building unit
has also been prepared as an isolated cluster in Mu-1 [49] and as linear chains in
Mu-3 [50].
Alternatively, fluorine can participate more directly in the coordination sphere of
the metal and increase the coordination number from V to VI. Studies performed
by Fe ´rey and co-workers [51, 52] in Le Mans gave rise to a new series of oxyfluorinated phosphates called ULM-n. In this family, the basic building unit can be
either tetramers, as is usually found in the crystal chemistry of phosphates [53],
18.4 Open-Framework Metal Phosphates 597