Silicoaluminophosphate materials

The mesoporous silicoaluminophosphate materials were synthesized using cetyl tiimethyl ammonium bromide (CTAB) (99% S.D. Fine Chemicals) as structure directing agent. In a typical synthesis procedure, 8 gms. of aluminium isopropoxide (Loba Chemie) was mixed with dilute phosphoric acid (6.2 ml. H3PO4 in 60 ml H2O) and stirred vigorously for 1 hr at 333 K. This was followed by the addition of CTAB (8 gm in 20 ml H2O) with subsequent addition of appropriate amount of tetraethyl orthosilicate (TEOS) (Merck) in tetramethyl ammonium hydroxide. The pH of the resulting gel was around 2.5. The molar composition of the resulting gel thus obtained was ... 

Microwave heating has also been used to prepare other molecular sieves such as aluminophosphate aud silicoaluminophosphate materials. A comparison between conventional and nticrowave heating in the syntheses of SAPO-11 has been performed, with particular phasis on study effects on nncleation and crystal growth. Both were enhanced nsing microwave heating, narrower pore size distributions and more uniform crystal morphologies being observed. 

Keywords porous materials, silicoaluminophosphates, solid-state catalysts, structuredirecting agents... 

Up to now, a variety of non-zeolite/polymer mixed-matrix membranes have been developed comprising either nonporous or porous non-zeolitic materials as the dispersed phase in the continuous polymer phase. For example, non-porous and porous silica nanoparticles, alumina, activated carbon, poly(ethylene glycol) impregnated activated carbon, carbon molecular sieves, Ti02 nanoparticles, layered materials, metal-organic frameworks and mesoporous molecular sieves have been studied as the dispersed non-zeolitic materials in the mixed-matrix membranes in the literature [23-35]. This chapter does not focus on these non-zeoUte/polymer mixed-matrix membranes. Instead we describe recent progress in molecular sieve/ polymer mixed-matrix membranes, as much of the research conducted to date on mixed-matrix membranes has focused on the combination of a dispersed zeolite phase with an easily processed continuous polymer matrix. The molecular sieve/ polymer mixed-matrix membranes covered in this chapter include zeolite/polymer and non-zeolitic molecular sieve/polymer mixed-matrix membranes, such as alu-minophosphate molecular sieve (AlPO)/polymer and silicoaluminophosphate molecular sieve (SAPO)/polymer mixed-matrix membranes. 

Investigations performed by Minchev et al (215) indicated that the framework of crystalline silicoaluminophosphates can be damaged upon the rehydration of the template-free material. In the case of rehydrated template-free H-SAPO-5 and H-SAPO-34, for example, a strong loss of the crystallinity occurs in the presence of water. However, the crystallinity can be completely restored after an additional dehydration at 823 K. Hydration of H-SAPO-37 at room temperature causes irreversible structural changes and leads to a material that is totally amorphous to X-ray diffraction (216). At temperatures of more than 345 K, template-free H-SAPO-37 exhibits a high stability toward hydration (216). 

Hydration of the NH4-form of SAPO-34 and SAPO-37, that is, of materials that were ammoniated at the bridging OH groups, caused a coordination of water molecules exclusively to Al atoms in =P-O-A1= bridges. This process led to a hydrolysis of the framework (220). No hydrolysis of the silicoaluminophosphate framework occurred, provided that not only the bridging OH groups (SiOHAl), but also the aluminophosphate framework (=P-O-A1=) was covered by ammonia. The latter finding may explain the stabilizing effect of preloaded ammonia on silicoalumino-phosphates toward hydration and weak hydrothermal treatments as recently observed for H-SAPO-34 (227). 

Crystalline microporous silicoaluminophosphates have been patented as SAPO-n (1) or MCM-n (2) materials. The SAPO materials crystallize from an aqueous medium in the presence of organic templates, the MCM materials from a biphasic medium, using similar templates. Most of the actually known MCM s and SAPO s are crystallographically different apart from SAPO-34, SAPO-44, SAPO-47 and MCM-2 which have the chabasite topology (2,2) The structure of other MCM materials is presently unknown. 

Molecular sieves Aluminophosphates, silicoaluminophosphates and associated porous materials... 

Elements with different valences can be incorporated into AIPO4 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], metalaluminophos-phates (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 = Mg +, Mn +, Fe +, Co + and Zn +). The... 

The commercial importance and diverse uses of zeolites have long prompted attempts to synthesise related compounds. Many framework materials containing atoms other than silicon and aluminium are now known. These materials, known as zeotypes , are not zeolites in the strictest sense as they are not aluminosilicates. Examples include the aluminophosphates, commonly known as AlPOs (Wilson et al., 1982), silicoaluminophosphates - SAPOs (Lok et al., 1984) - and gallophosphates - GaPOs (Parise, 1985). 

However, the incorporation of metal cations whose valence is different from that of A1 or P leads to the formation of electronically unsaturated sites, as schematically shown in Figure 3. This addition of aliovalent metal cations into the lattice of AlPO-n generates solid acidity and ion-exchange sites. There are numerous reports on the incorporation of many different metal cations into the lattice of AlPO-n. Table 2 summarizes the reported isomorphous substituted AlPO-n. The family of AlPO-n substituted with metal cations is generally called metal aluminophosphates (MeAPO-n). The typical metal cations substituted into AlPO-n are Li, B, Be, Mg, Ti, Mn, Fe, Co, Zn, Ga, Ge, Si, and As. The Si-substituted AlPO-n is called a silicoaluminophosphate and denoted as SAPO-n, where n also means the framework structure, and it is distinct from the MeAPO-n materials.SAPO-n exhibits both structural diversity and compositional variation. In particular, the crystal structure of SAPO-n is of greatest interest, because the distribution of the Si atom in the framework is quite complicated. Some crystal structures, such as SAPO-40, are only found in SAPO-n and not in AlPO-n or zeolite. The mole... 

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