504
1 Introduction
Zeolites are crystalline, caged, tetrahedron framework molecular sieves comprising
(SiO 4 )
4−
and (AlO 4 )
5−
units, attached three dimensionally by Si-O-Al linkages.
Owing to their high acidic strength, thermal and mechanical stability, they have
been one of the most extensively studied and utilized solid acid catalysts till date.
Their unique pore apertures (<2 nm) set up a well-defined selectivity rules with
provisions of shape as well as size of the reactant and products at a molecular level.
Aluminophosphates, a novel class of crystalline microporous materials without silica, were firstly reported by Wilson et al. at Union Carbide [1, 2]. Often termed as
AlPO-n (n refers to a distinct structure type), their framework has alternating AlO 4
and PO 4 tetrahedra, cross-linked via O bridges. They have general formula [(AlO 2 )
x (PO 2 ) x ]·yH 2 O. Regardless of the structural analogy to zeolites, aluminophosphates
differ considerably from aluminosilicates. The first difference is the negatively
charged framework of aluminosilicates, compared to the electroneutrality of the
AlPO framework. Secondly, Al in aluminosilicate is always T d coordinated, whereas
in the AlPO framework, it may exhibit higher coordination (e.g., penta and hexacoordination), in addition to tetrahedral coordination. The framework of AlPO is
flexible compared to the rigid zeolite framework. Structural flexibility of AlPO
results in structural and compositional diversity, as aluminium and/or phosphorous
in the lattice framework can be isomorphously substituted by ions of similar charge
and size [2–4]. Incorporation of Si or any other metal (M) in the framework leads to
formation of silicoaluminophosphate molecular sieves, SAPO-n, and metal (M) aluminophosphates MAPO-n or MAPSO-n, respectively [2–7]. The introduction of
hetero-atoms in the aluminophosphates changes its acidic properties without altering the pore shape or size.
In the current book chapter, we summarize some important applications of these
silicoaluminophasphate (SAPO) materials for hydrocarbon conversions, and clean
fuel production from biomass/vegetable oil, and carbon dioxide.
2 Synthesis Methodology of SAPO-n
Conventional hydrothermal synthesis of SAPO involves the presence of inorganic
precursors particularly Si, Al, and P. These precursors in the presence of suitable
structure directing agents (SDAs) like alkali/ammonium cations and organic amines
at different crystallization hours lead to various-sized SAPO frameworks with multiple pore entrances.
Figure 1 illustrates how various factors like molar ratio of inorganic precursors,
nature of aluminium/silica source, choice of SDAs, crystallization temperature,
duration, etc., may lead to different topological frameworks. The framework topology has been broadly categorized as small (8-membered ring of T atoms (Si, Al, P
in case of SAPO), where T atoms are connected to each other via O), medium
R. Yadav and A. K. Singh
1 Introduction
Zeolites are crystalline, caged, tetrahedron framework molecular sieves comprising
(SiO 4 )
4−
and (AlO 4 )
5−
units, attached three dimensionally by Si-O-Al linkages.
Owing to their high acidic strength, thermal and mechanical stability, they have
been one of the most extensively studied and utilized solid acid catalysts till date.
Their unique pore apertures (<2 nm) set up a well-defined selectivity rules with
provisions of shape as well as size of the reactant and products at a molecular level.
Aluminophosphates, a novel class of crystalline microporous materials without silica, were firstly reported by Wilson et al. at Union Carbide [1, 2]. Often termed as
AlPO-n (n refers to a distinct structure type), their framework has alternating AlO 4
and PO 4 tetrahedra, cross-linked via O bridges. They have general formula [(AlO 2 )
x (PO 2 ) x ]·yH 2 O. Regardless of the structural analogy to zeolites, aluminophosphates
differ considerably from aluminosilicates. The first difference is the negatively
charged framework of aluminosilicates, compared to the electroneutrality of the
AlPO framework. Secondly, Al in aluminosilicate is always T d coordinated, whereas
in the AlPO framework, it may exhibit higher coordination (e.g., penta and hexacoordination), in addition to tetrahedral coordination. The framework of AlPO is
flexible compared to the rigid zeolite framework. Structural flexibility of AlPO
results in structural and compositional diversity, as aluminium and/or phosphorous
in the lattice framework can be isomorphously substituted by ions of similar charge
and size [2–4]. Incorporation of Si or any other metal (M) in the framework leads to
formation of silicoaluminophosphate molecular sieves, SAPO-n, and metal (M) aluminophosphates MAPO-n or MAPSO-n, respectively [2–7]. The introduction of
hetero-atoms in the aluminophosphates changes its acidic properties without altering the pore shape or size.
In the current book chapter, we summarize some important applications of these
silicoaluminophasphate (SAPO) materials for hydrocarbon conversions, and clean
fuel production from biomass/vegetable oil, and carbon dioxide.
2 Synthesis Methodology of SAPO-n
Conventional hydrothermal synthesis of SAPO involves the presence of inorganic
precursors particularly Si, Al, and P. These precursors in the presence of suitable
structure directing agents (SDAs) like alkali/ammonium cations and organic amines
at different crystallization hours lead to various-sized SAPO frameworks with multiple pore entrances.
Figure 1 illustrates how various factors like molar ratio of inorganic precursors,
nature of aluminium/silica source, choice of SDAs, crystallization temperature,
duration, etc., may lead to different topological frameworks. The framework topology has been broadly categorized as small (8-membered ring of T atoms (Si, Al, P
in case of SAPO), where T atoms are connected to each other via O), medium
R. Yadav and A. K. Singh