Framework of SAPO

Solid-state NMR spectroscopy has been demonstrated as a well established technique for characterization of zeolites and other porous materials with respect to structure elucidation, pore architecture, catalytic behaviour and mobility properties. The latest progress in the development of NMR techniques, both with respect to software and hardware improvements, has contributed to the present state of the art for NMR within the field of characterization of zeolitic materials. Furthermore, the introduction of NMR imaging (110), two-dimensional quintuple-quantum NMR spectroscopy (111) and transfer of populations in double resonance (TRAPDOR) NMR (112,113) will extent the horizons of zeolite characterization science. As a final example, the Al => Si TEDOR experiment directly proves, for the first time, that silicon substitutes for phosphorous atoms in the framework of SAPO-37 (114). The Al... 

X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data of the CoAPO-5 indicates that Co is surrounded by four oxygen atoms at a distance of 0.195 nm, and that Co is in a tetrahedral environment. On the other hand, the methanol-treated sample shows an increase in the Co-0 distances while they retain the distorted environment. Chen et al. concluded that the increase in the Co-0 distance by methanol treatment is caused by the reduction of Co to Co ". On the other hand, it is suggested that the reducibility of Ni in the framework is quite different from that of Ni ion exchanged into SAPO-11. It is suggested that catalysis of Ni in the framework of SAPO-11 is different from that of ion-exchanged Ni. ... 

In summary, if the framework structure is analogous, the ion-exchange sites in SAPO-n are generally similar to those of aluminosilicate zeolites, i.e., the sites are octahedrally coordinated to three zeolitic oxygens in the framework of SAPO. However, the number of studies on the ion-exchanged sites in SAPO-n or MeAPO-n is limited but... 

NiAPSO-34 is particularly selective for the formation of ethylene from methanol, with a value as high as 90 piercent being reported. Although Ni seems to be substituted into the framework of SAPO-34, the mechanism leading to such high selectivity to ethylene from methanol is not clear. However, the narrow pore size and the mild acidity of NiAPSO-34 seem to selectively convert methanol to ethylene. 

Kang, M. (2000) Effect of cobalt incorporated into the framework of SAPO-34 (CoAPSO-34s) on NO removal. /. Mol Catal A Chem., 161, 115-123. 

In the metal aluminophosphate (MeAPO) family the framework composition contains metal, aluminum and phosphorus [27]. The metal (Me) species include the divalent forms of Co, Fe, Mg, Mn and Zn and trivalent Fe. As in the case of SAPO, the MeAPOs exhibit both structural diversity and even more extensive composihonal variation. Seventeen microporous structures have been reported, 11 of these never before observed in zeoUtes. Structure types crystallized in the MeAPO family include framework topologies related to the zeolites, for example, -34 (CHA) and -35 (LEV), and to the AIPO4S, e.g., -5 and -11, as well as novel structures, e.g., -36 (O.Snm pore) and -39 (0.4nm pore). The MeAPOs represent the first demonstrated incorporation of divalent elements into microporous frameworks. 

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). 

Hydrothermal treatment of the initial gel for an extended period leads to the transformation of the prophase to a mixture of triclinic and trigonal phases of SAPO-34. Although the later forms in the presence of the fluoride anion, no F was found in the trigonal phase. For Si incorporation, Si(OAl)4 coordination exists in the prephase, but the amount of Si is very small. It appears that there are only two sites, P(l) and P(2), for Si incorporation, which may have implication in the catalytic sites in the framework. The mixture of triclinic and trigonal phases exists at much higher Si content, but most of the Si may locate only in the trigonal phase. 

For the normal SDE, the presence of SDA is necessary but not sufficient for the formation of a specific structure. Under different gel compositions and crystallization conditions, the same SDA can direct to many different structures. For example, the framework of CoAPO (AFY) and SAPO-40 (AFR) could only be synthesized in the presence of "Pr2NH and Pr4NOH, respectively. However, Pr4NOH can direct to AlP04-5, MAPO-36, and ZSM-5 as well. AlPO4-20 (SOD) could be synthesized under the influence of TMA+, while TMA+ is a common SDA in the synthesis of other microporous compounds. 

In the case of SAPO-11 materials, silicon was observed to incorporate the AEL framework with some difficulty, in order to induce medium strength acidity by substitution for P. It could also correspond to specific A1 environments, i.e., not all the A1 atoms are tetracoordinated, but some have six neighbors or even five. Such a concept may broaden the field of AlPO-type materials which exhibit novel topologies. 

Aluminophosphate based molecular sieves are known to exist in a wide range of structural and compositional diversity . Substitution of silicon in the framework of aluminophosphate molecular sieves (SAPO) imparts acidity to the material and thus makes it active for acid catalyzed reactions. Through controlled substitution of the amount of Si in aluminophosphate, the catalytic activities due to its acidic properties can be altered. The extent of Si substitution in the aluminophosphates is however limited and is determined by the topology of the structure. 

In some of the AlP04-n molecular sieves discovered in 1982 [1], it is possible to substitute part of the P and Al framework elements with Si [2]. In the resulting SAPO-n materials, isolated Si atoms occupy P sites, while patches of Si atoms replace locally P as well as Al atoms [3,4]. The degree of Si substitution and the substitution mechanism depend on the topology of the framework and on the synthesis method [4,5]. While the synthesis method does not seem to be critical for the incorporation of traces of Si in SAPO-5 and SAPO-11, extensive Si incorporation in these structures can be achieved only by using very specific synthesis recipes [4,6]. Silicon-rich crystals of SAPO-5 and SAPO-11, e.g., can be prepared by using aluminium isopropoxide as a source of aluminium and specific templates, viz. dipropylamine for SAPO-11 and cyclohexylamine for SAPO-5 [4,6]. 

A milestone was achieved with the discovery of aluminum phosphate molecular sieves [1]. These compounds have neutral frameworks of very low acidity. Thus, their use as catalysts and to some extent as sorbants is limited. However, it was subsequently shown that silicon could be incorporated into the aluminum phosphate framework [3-4] (SAPO s). This discovery was followed in short order by introduction of many metals (Mg2+, Mn +, Fe -, Cq2+, Zn2+) [5-6] into the framework by isomorphous... 

Davis and coworkers [104] studied " Xe NMR of xenon adsorbed in several SAPOs, ALPOs, and Y zeolites. From a comparison of the xenon chemical shift extrapolated to zero pressure, these authors concluded that Xe atoms feel significantly smaller electrostatic fields and field gradients in the aluminophosphates compared to aluminosilicates. The extrapolated chemical shift decreased from 97 ppm in erionite to 60 ppm in Y zeolite and to 27 ppm in AIPO4-5, with the values for SAPOs being intermediate to Y zeolites and AlPOs as would be expected from the acidity trends. They concluded as well that SAPO-37 does not contain separate aluminophosphate and aluminosilicate islands. Dumont et al. [105] also carried out xenon NMR experiments in SAPO-37. From xenon sorption capacity and the decrease in the chemical shift, their conclusion was that the framework of calcined SAPO-37 is unstable when exposed to moist air. 

The acid properties of SAPO-5 samples can be modified either by the con x>sition of framework, or by ion-exchange or impregnation. These changes in the acid properties can have certain implications on the catalytic performances. This assertion is proved by the results shown in Figure 3, where it can be observed that the decrease of the phosphorus cont t and partially of the aluninium determines an increase of the activity, but the selectivity is kept constant. The difference between the framework compositions deter-... 

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... 

Solid Acidity of SAPO-n and MeAPO-n. - The largest difference between AlPO-n and AlPO-n substituted with metal cations (SAPO-n or MeAPO-n) is its solid acidity and, consequently, its ion-exchange properties. Solid acidity is caused by substitution of a part of A1 or P in framework with metal cations. However, the number of acid sites caused by metal substitution does not increase linearly as the amount of substituted metal cations increases. This is very much in contrast with aluminosilicate zeolites. It is well known that, in the case of zeolites, the number of acid sites increases linearly as the number of A1 increases in the framework. Consequently, the acidity is often expressed simply by the Si/Al ratio in the zeolite. However, the number of acid sites as well as their strength depends on the amount of substituted metal cations in a complicated way. This is because two sites for substitution, A1 and P, exist in the framework of AlPO-n, and the substituted cations are not always substituted at the same site. The acidity of SAPO-n or MeAPO-n has been studied in connection with the state of substituted metal. In this section, acidity of SAPO-n and MeAPO-n will be briefly reviewed. 

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