three-fold coordination, connected to three phosphorus atoms via bridging oxygens
located at the vertices of a trigonal pyramid; the lone pair occupies the fourth vertex of a hypothetical tetrahedron. For three-dimensional systems based on the alternation of PO 4 tetrahedra and SnO 3 trigonal pyramids, this yields a framework of
general formula [Sn 4 P 3 O 12
À ] with pores in which the organic amine species are
occluded [61]. The channels found to date include both 8- and 12-rings, but the
effective size for accessibility is smaller than anticipated due to the hindrance of
the lone pair of electrons pointing towards the centers of the channels. In the oxophosphate phase, SnOPO-1 [62], the Sn:P ratio is 2:1 and tin atoms are connected
together via a common oxygen atom which coordinates three tin atoms. In a system with a Sn:P ratio of 3:2, the presence of both trigonal pyramidal SnO 3 and
square pyramidal SnO 4 units also results in SnaOaSn linkages [63]. As in the
AlPO systems, monomeric, chain, layered and 3-D structures have also been observed, and there is evidence that a simple tetramer, Sn 2 P 2 O 8 , may be a fundamental building unit in many of these structures [64].
The discovery of the tin(II) phosphate family led to the search for other systems
containing lone pair cations, such as antimony(III). This indeed has turned out to
be feasible, and an extensive range of antimony(III) phosphates and fluorophosphates has been discovered [65]. As with the tin compounds, the structures appear
to be open but the presence of lone pairs protruding into the cavities reduces their
porosity.
18.4.4
Transition Metal Phosphates
Following the successful introduction of transition metals into the zeolitic AlPOs
and GaPOs and the parallel discovery of open framework solids with polyhedral
coordinations larger than four, it was natural that workers in this field should attempt to synthesize open-framework transition metal phosphates. The presentation will follow a chronological sequence.
18.4.4.1 Molybdenum and Vanadium Phosphates
By using hydrothermal techniques in combination with large removable cations,
Haushalter et al. [66] successfully prepared some MoPOs with open frameworks.
Typical reactions involved a molybdenum source (MoO 3 þ Mo or MoCl 5 ), phosphoric acid and an organic cationic template heated in an autoclave between 200
and 400
C. Several open framework solids were synthesized in this way by
changing the nature of the template; they contain a variety of cluster units, such as
tetramers and octamers, but have relatively poor thermal stability [67, 68].
Vanadium phosphates were made in an analogous manner by Haushalter and
Fe ´rey. The association of different vanadium valence states (V, IV, III) with their
various polyhedra (tetrahedra, square pyramids, distorted and regular octahedra)
leads to a very large diversity for the interesting structures. Some eye-catching results originate from this research, including the first evidence of an inorganic
double helix in [(CH 3 ) 2 NH 2 ]K 4 [V 10 O 10 (H 2 O) 2 (OH) 4 (PO 4 ) 7 ] Â 4H 2 O [69] and the
18 Nanoporous Materials
600
located at the vertices of a trigonal pyramid; the lone pair occupies the fourth vertex of a hypothetical tetrahedron. For three-dimensional systems based on the alternation of PO 4 tetrahedra and SnO 3 trigonal pyramids, this yields a framework of
general formula [Sn 4 P 3 O 12
À ] with pores in which the organic amine species are
occluded [61]. The channels found to date include both 8- and 12-rings, but the
effective size for accessibility is smaller than anticipated due to the hindrance of
the lone pair of electrons pointing towards the centers of the channels. In the oxophosphate phase, SnOPO-1 [62], the Sn:P ratio is 2:1 and tin atoms are connected
together via a common oxygen atom which coordinates three tin atoms. In a system with a Sn:P ratio of 3:2, the presence of both trigonal pyramidal SnO 3 and
square pyramidal SnO 4 units also results in SnaOaSn linkages [63]. As in the
AlPO systems, monomeric, chain, layered and 3-D structures have also been observed, and there is evidence that a simple tetramer, Sn 2 P 2 O 8 , may be a fundamental building unit in many of these structures [64].
The discovery of the tin(II) phosphate family led to the search for other systems
containing lone pair cations, such as antimony(III). This indeed has turned out to
be feasible, and an extensive range of antimony(III) phosphates and fluorophosphates has been discovered [65]. As with the tin compounds, the structures appear
to be open but the presence of lone pairs protruding into the cavities reduces their
porosity.
18.4.4
Transition Metal Phosphates
Following the successful introduction of transition metals into the zeolitic AlPOs
and GaPOs and the parallel discovery of open framework solids with polyhedral
coordinations larger than four, it was natural that workers in this field should attempt to synthesize open-framework transition metal phosphates. The presentation will follow a chronological sequence.
18.4.4.1 Molybdenum and Vanadium Phosphates
By using hydrothermal techniques in combination with large removable cations,
Haushalter et al. [66] successfully prepared some MoPOs with open frameworks.
Typical reactions involved a molybdenum source (MoO 3 þ Mo or MoCl 5 ), phosphoric acid and an organic cationic template heated in an autoclave between 200
and 400
C. Several open framework solids were synthesized in this way by
changing the nature of the template; they contain a variety of cluster units, such as
tetramers and octamers, but have relatively poor thermal stability [67, 68].
Vanadium phosphates were made in an analogous manner by Haushalter and
Fe ´rey. The association of different vanadium valence states (V, IV, III) with their
various polyhedra (tetrahedra, square pyramids, distorted and regular octahedra)
leads to a very large diversity for the interesting structures. Some eye-catching results originate from this research, including the first evidence of an inorganic
double helix in [(CH 3 ) 2 NH 2 ]K 4 [V 10 O 10 (H 2 O) 2 (OH) 4 (PO 4 ) 7 ] Â 4H 2 O [69] and the
18 Nanoporous Materials
600