Substituted AlPOs silicon substitution

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... 

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... 

Similar reactions are used in the formation of the zeotypes They also have framework structures, but contain other elements rather than aluminium and silicon. For example, substitution of phosphorus for silicon produces the ALPO family of materials. These require no counterions in the mixture, but need mildly basic or slightly acidic conditions for crystallization. Often a small amount of hydrogen fluoride is added as a mineralizer (a material used to encourage crystallization). 

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. 

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. 

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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