Aluminophosphates AlPO substituted AlPOs

Besides the conventional zeolites, several novel zeolite analogues such as the ALPOs (aluminophosphates), MeALPOs (divalent-metal (Me) substituted aluminophos-phates), SAPOs (silicon substituted aluminophosphates) and so on have been synthesized (Davis Lobo, 1992). Wilson et al. (1982) first reported the synthesis of microporous ALPOs. ALPO synthesis differs from zeolite synthesis in that it involves acidic or mildly basic conditions and no alkali metal ions. Some members in the ALPO... 

Much effort has been devoted to aluminophosphates with isomorphous Co2+ substitution (Co-AlPOs) (162). These structures are of interest for two reasons. First, Co is immobilized at lattice sites, and formation of clusters of Co is therefore impossible. Such clusters, which are catalytically less active, can in contrast be formed in materials with mobile Co, for example, Co-exchanged zeolite Y. Second, calcination of Co-AlPOs brings at least... 

The first breakthrough was provided by Flanigen et al. [7] who, playing on the similarity 2 Si4+ -o-A13+ -f P5+, synthesized microporous aluminophosphates (hereafter noted AlPOs) with structures related to those of zeolites. The structural studies [8] showed however a striking difference between the two families. As already mentioned, the framework of zeolites is built up exclusively from connected tetrahedra which can accept small amounts (<10% of substitution) of other metals, whereas in aluminophosphates and homologous gallophosphates, Al and Ga polyhedra can adopt five and sixfold coordinations, which change [9] the connectivity of the framework, and therefore the shape of the windows. 

The two SAPO-20 materials have very different bulk concentrations. SAPO-20A has a composition consistent with a "silicon-substituted aluminophosphate", while the high bulk concentration of Si in SAPO-20B is more consistent with a "phosporous-substituted aluminosilicate". The other SAPO samples all have sufficiently low concentrations of Si to be considered as silicon-substitued ALPO s. 

Silico)aluminophosphate-molecular sieves are interesting materials as catalysts for the condensation of phenoi with carbonyl compounds, presumably because of the low acidity and the spacious constraints, decreasing catalyst deactivation. Especially ALPO-5, having a very low acid strength, gives good yields in the reaction of phenol with formaldehyde towards dihydroxydiphenylmethanes. In case of the phenol/isobutanai reaction, mainly monosubstituted products are obtained shape selectivity seems to retard a second condensation step. Ortho-substitution prevails, possibly caused by a contribution of Lewis-acid catalysis. 

Aluminophosphates denoted as AlPO-n (n means the framework structure) are a new family of molecular sieves with unique zeolitic pore structure. Since the first report on the synthesis of AlPO-n by Wilson et al., synthesis of various types of AlPO-n as well as the isomorphous substitution of metal cation into the AlPO-n structure have been reported. At present, more than 50 different crystal structures of AlPO-n have been reported, which is comparable to that of aluminosilicate zeolites, making the aluminophosphates an important class of microporous crystals." ... 

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... 

Several aluminophosphate molecular sieves with AEL topology structure were synthesized and modified by Pd for direct transformation reaction of -butane to isobutene. The effect of pore geometry of the molecular sieves was studied. Pd modified 10-member ring SAPO-11 and metal-substituted AlPO-l 1 and SAPO-11 showed high selectivity towards isobutene. The incorporation of metal into the molecular sieves had effect on the product distribution. Catalytic properties and the result of chemical adsorption of monoxide predicted the interaction between the transition metal for substitution and the supported palladium. 

Studies of this type have shown that incorporation of magnesium into the CHA aluminophosphate structure can be random, within the Af site. In the aluminophosphate form of sodalite, (AlPO-20) magnesium substitutes in an ordered way for aluminium, and Barrie and Klinowski interpret the observed P NMR intensities in terms of an ordered distribution where there are never two magnesium cations in the same 4MR. ... 

Later, the synthesis of the family of crystalline silicoaluminophosphate (SAPO- ) was reported. Some of them have structures topologically related to zeolites or AlPOs-n, some having novel structures. SAPO-n composition can be considered in terms of silicon substitution into a hypothetical aluminophosphate framework, the predominant substitution mechanism being silicon substitution for phosphorus. 

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