and have up to nearly 50% pore volume. The pure silica zeolite polymorphs have
remarkable stability and in some cases will survive calcination at 1000
C.
18.4
Open-Framework Metal Phosphates
18.4.1
Aluminum Phosphates
The extensive development of nanoporous aluminosilicates has led to the search
for new families of three-dimensional open frameworks. The aluminophosphates
(AlPO 4 ) were considered first since they are isoelectronic with SiO 2 and can be
regarded as the III–V analogues. Several allotropic forms of AlPO 4 corresponding
to condensed silica polymorphs are known, e.g. berlinite, which is analogous to
a-quartz. In such materials, aluminum and phosphorus are observed in tetrahedral
coordination, as in the silicas. From these considerations, Flanigen and co-workers
at Union Carbide Corporation discovered a new generation of nanoporous aluminophosphate materials, AlPO 4 -n (n refers to a structural type), in the early 1980s
[2]. These materials, and related ones made subsequently, are the subject of this
section.
The synthetic procedure is similar to that used for zeolites: the aluminophosphates are prepared hydrothermally under autogeneous pressure in the temperature range 100–250
C. They are typically obtained from a mixture of aluminum
oxyhydroxide or alkoxide and phosphoric acid in water, leading to the formation of
an aluminophosphate gel. An amine or quatenary ammonium salt, which plays a
templating or structure-directing role, is then added to the mixture. The crystallisation time depends on multiple variables such as temperature, reactant composition, and structural type, and ranges from several hours up to several weeks. The
pH of the reaction, which is less than that observed in zeolite synthesis, varies
within the range from 3 to 10 and goes towards neutral values at the end of the
reaction. The organic molecule, which is trapped within the pores, plays a critical
role in producing the open framework, and in the absence of such a molecule, only
dense AlPO 4 or AlPO 4 ÁnH 2 O hydrates are formed. The thermal stability of the
open-framework solids is similar to that obtained for zeolites and the template can
often be removed by calcination in air at 500–600
C. In the AlPO 4 -n series, the
Al/P ratio is 1 and the frameworks are electrostatically neutral. A typical example,
AlPO-5 [37], is shown in Figure 18.2.
Elements with different valences can be incorporated into AlPO 4 frameworks in
order to modify the chemical properties for catalysis, ion-exchange and so on. In
this respect, Union Carbide researchers synthesized several new series of materials, in particular the silicoaluminophosphates (SAPO-n) [38], metalaluminophosphates (MePO-n) and metalsilicoaluminophosphates (MeAPSO-n) [39]. The strategy was first to explore the divalent cations of the periodic table which can adopt
tetrahedral coordination (e.g. Me ¼ Mg
2þ , Mn
2þ , Fe
2þ , Co
2þ and Zn
2þ ). The
18.4 Open-Framework Metal Phosphates 595
remarkable stability and in some cases will survive calcination at 1000
C.
18.4
Open-Framework Metal Phosphates
18.4.1
Aluminum Phosphates
The extensive development of nanoporous aluminosilicates has led to the search
for new families of three-dimensional open frameworks. The aluminophosphates
(AlPO 4 ) were considered first since they are isoelectronic with SiO 2 and can be
regarded as the III–V analogues. Several allotropic forms of AlPO 4 corresponding
to condensed silica polymorphs are known, e.g. berlinite, which is analogous to
a-quartz. In such materials, aluminum and phosphorus are observed in tetrahedral
coordination, as in the silicas. From these considerations, Flanigen and co-workers
at Union Carbide Corporation discovered a new generation of nanoporous aluminophosphate materials, AlPO 4 -n (n refers to a structural type), in the early 1980s
[2]. These materials, and related ones made subsequently, are the subject of this
section.
The synthetic procedure is similar to that used for zeolites: the aluminophosphates are prepared hydrothermally under autogeneous pressure in the temperature range 100–250
C. They are typically obtained from a mixture of aluminum
oxyhydroxide or alkoxide and phosphoric acid in water, leading to the formation of
an aluminophosphate gel. An amine or quatenary ammonium salt, which plays a
templating or structure-directing role, is then added to the mixture. The crystallisation time depends on multiple variables such as temperature, reactant composition, and structural type, and ranges from several hours up to several weeks. The
pH of the reaction, which is less than that observed in zeolite synthesis, varies
within the range from 3 to 10 and goes towards neutral values at the end of the
reaction. The organic molecule, which is trapped within the pores, plays a critical
role in producing the open framework, and in the absence of such a molecule, only
dense AlPO 4 or AlPO 4 ÁnH 2 O hydrates are formed. The thermal stability of the
open-framework solids is similar to that obtained for zeolites and the template can
often be removed by calcination in air at 500–600
C. In the AlPO 4 -n series, the
Al/P ratio is 1 and the frameworks are electrostatically neutral. A typical example,
AlPO-5 [37], is shown in Figure 18.2.
Elements with different valences can be incorporated into AlPO 4 frameworks in
order to modify the chemical properties for catalysis, ion-exchange and so on. In
this respect, Union Carbide researchers synthesized several new series of materials, in particular the silicoaluminophosphates (SAPO-n) [38], metalaluminophosphates (MePO-n) and metalsilicoaluminophosphates (MeAPSO-n) [39]. The strategy was first to explore the divalent cations of the periodic table which can adopt
tetrahedral coordination (e.g. Me ¼ Mg
2þ , Mn
2þ , Fe
2þ , Co
2þ and Zn
2þ ). The
18.4 Open-Framework Metal Phosphates 595