Substituted AlPOs

Substituting divalent or trivalent elements for the A1 in the framework has been successfully carried out by several groups yielding novel heterogeneous catalysts (metal-substituted ALPOS, MALPOs Thomas et al 2001) for hydrocarbon oxidation and liquid phase oxidation. MALPO catalysts can be complementary to metal-doped silicalite catalysts. Particularly interesting compounds are MALPOs in which a divalent metal (Me) substitutes for the framework Al +, for example MALPO-36 (where M = Mg, Mn, Zn, Co) and MALPO-34 (M = Mg, Mn, Co etc). 

Methylation of aniline forms a range of products varying from N-alkylated to C-alkylated compounds. ALPOs and substituted ALPOs facilitate N-alkylation whereas ZSM-5 catalyse both C- and N-alkylations. ZSM-5 catalyst forms only mono methylated products. Both mono and di methylated products are formed with SAPOs and ALPOs. KY and CsY catalyse only mono methyl aniline formation. Conversions are high with ZSM-5, KY and CsY. Mechanism of reaction is proposed. 

It is interesting that the TPD-TGA curves for the substituted AlPO s appear to be essentially identical to those reported for H-ZSM-5, for which it has been argued that the decomposition feature is due to strong, Bronsted sites associated with framework Al atoms [2]. By analogy, the decomposition feature on the MeAPO s must be due to Brensted sites associated with framework Si, Mg, or Co atoms. The isopropylamine which desorbs unreacted below 550K appears to be unrelated to... 

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. 

The acidic properties of cobalt and silicon-substituted AlPO-5, -11, and -44 have been characterized by JSnchen et al. [111,276] by adsorption calorimetry of acetonitrile at 303 K, after activation at 720 K. Adsorption calorimetric measurements indicated that the adsorption potential of the samples for acetonitrile was enhanced upon cobalt incorporation. The heat curves exhibited at least two steps indicating the existence of acid sites of different strengths. The heats of adsorption indicated the formation of strong acid sites, due to the cobalt incorporation, as well as the presence of weaker acid sites, probably terminal P - OH groups. 

Akolekar [151] found a linear relation between the number of strong acid sites (determined by adsorption of pyridine at 400 °C) and the conversion of cumene over metal-substituted AlPO-ll with the exception of Mg-AlPO-11. A closer look at the acid site distribution, however, showed that this catalyst has the largest concentration of sites with lower add strength (pyridine desorption between 300 and 400 °C). As described above, the activity of these sites must be included. Tian et al. [152] studied SAPO-11 molecular sieves, which were dealuminated by EDTA to achieve different Al contents. After de-alumination the activity for cumene cracking improved, which was attributed to the formation of strong Bronsted add sites around Si domains and to a reduction of the Lewis acidity. 

Silicoaluminophosphates (SAPOs), along with their crystalline aluminum phosphate counterparts (ALPOs), first discovered by Union Carbide workers in the early 1970s [41, 42], derive their acidity through the substitution of framework phosphorous by silicon thereby creating the charge imbalance which, when compensated for by protons, creates acidic centers. SAPOs in general have seen limited use in bond-breaking applications primarily due to weaker acidity, framework stability, or technoeconomic reasons. Of the rich variety of structures available,... 

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... 

From a mechanistic viewpoint it is worth noting that the TS-1 catalyst contains the same chemical elements in roughly the same proportions as the Shell amorphous TiIV/Si02 catalyst referred to earlier. However, the former displays a much broader range of activities than the latter. A possible explanation may be that the TS-1 catalyst contains more (or more active) isolated titanyl centres than the amorphous Ti1v/Si02. Based on the quite remarkable results obtained with TS-1 we expect many more examples of redox zeolites, i.e. zeolites, alpos, etc. modified by isomorphous substitution with redox metal ions in the crystal lattice, as selective oxidation catalysts.66... 

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. 

Aluminum. Previous Al NMR studies have demonstrated four possible local environments for Al in SAPO materials (3,4). These environments are illustrated in Figure 3, and may be classified as either phosphorous rich (i.e., ALPO -like) with a chemical shift ranging from 30 to 40 ppm, or silicon rich (i.e., zeolite-like) with a chemical shift greater than 48 ppm. Both types of environments are characteristic of a substitution mechanism involving silicon substitution for phosphorus. A fifth possibility for an Al environment involves two Si and two P second nearest neighbors. However, no such environment has yet been identified by NMR, either because the Al chemical shift is similar to that for the silicon- or phosporous-rich environments, or because materials with an appropriate level of Si to give rise to... 

Mesoporous molecular sieves materials5-8 designated M41S (which include the MCM-41 class of materials) have made a further major impact on the area of synthesis of porous materials. A variety of open framework structures that are mesoporous have recently been reviewed by Thomas.9 Activated charcoal, MCM-41, mesoporous tungsten oxide, and substituted MCM-41 materials are mentioned. This article primarily emphasizes potential applications of such materials and possible mechanisms of reaction. The mesoporous sysems are compared briefly to microporous materials such as zeolites, ALPOs, MeALPOs and SAPOs. 

The recent EPR study by Kevan et al.[16] of cobalt substimted ALPO-5 illustrates the power of the EPR technique to characterise metal substituted zeolites. CoAPO-5 as synthesised is blue in colour the electronic spectrum is characteristic of tetrahedrally coordinated Co2+, suggesting that Co2+ has been incorporated into the AIPO4 lattice. The corresponding generation of Bronsted acid sites indicates that Co2 + substitutes for Al +.The material does however change colour to yellow-... 

We have found that the widely used TS-1 oxidation catalyst is distinctly inferior to Mn -framework-substituted microporous AlPOs (AlPO-5) which have been described [10, 11], along with similar M-AlPOs (M = Co ", Mn ", Fe "), in relation to its high performance in the one-step conversion of cyclohexane to adipic acid [9]. 

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