The tetrahedral atom arrangements of silicon, phosphorus, and aluminum in silicoaluminophosphates with sodalite structure have been elucidated. No evidence of Si-O-P linkages is provided. Silicon atoms are found to have silicon and/or aluminum neighbors while phosphorus is surrounded by aluminum only. Two distinct physicochemical environments of aluminum are observed: phosphorus rich and silicon rich.New crystalline, microporous molecular sieves have been reported recently.1 11These novel materials are aluminophosphate molecular sieves that have been incorporated with other elements. To date, 13 elements have been employed.1 The catalytic properties of several silicoaluminophosphates1-7 (SAPO-n)8 and other substituted aluminophosphates1,7,9 show that these materials exhibit unique conversion abilities for a broad spectrum of hydrocarbon reactions.These new solids consist of tetrahedral oxide frameworks. For example, the SAPO molecular sieves contain oxide tetrahedra of silicon, aluminum, and phosphorus arranged in a manner that can be considered as silicon substitution into a hypothetical AlP04-n framework. The silicon substitution can be for (1) aluminum, (2) phosphorus, or (3) an aluminum-phosphorus pair.4b Similar substitutions with other elements are postulated to occur in the formation of MeAPO, MeAPSO, E1APO, and ELAPSO1 materials. Lok et al.4b report that mechanisms 2 and 3 are more likely than mechanism 1 for the SAPO materials since they exhibit cation exchange and they show an excess of aluminum over phosphorus.We have been investigating the tetrahedral atom arrangements in substituted, aluminophosphate molecular sieves and have reported recently on SAPO-37.10 SAPO-37 is the silicoaluminophosphate with the structure of faujasite. This molecular sieve framework contains a homogeneous distribution of silicon, aluminum, and phosphorus and is negatively charged. We conclusively showed that silicon substitutes into the hypothetical aluminophosphate framework by mechanism 2 alone. Previous work on SAPO-n molecular sieves11,12 other than SAPO-37 provides speculative evidence for mechanisms 2 and/or 3. Thus, in order to generalize our conclusions from SAPO-37 to the complete category of SAPO-n molecular sieves, we expanded our investigation to include a series of SAPO's in which the framework structure is maintained constant while the composition of Si, Al, and P tetrahedral atoms is varied.The purpose of our work is to investigate the nature of silicon substitution into hypothetical aluminophosphate frameworks. We synthesized five molecular sieves with framework compositions spanning the range from aluminophosphate through the SAPO region to sodalite (aluminosilicate). All five materials possess the same framework structure, that being the sodalite structure.13 Complete study of these materials allows us to overcome the limitations in generalizing our previous conclusions for to the entire category of SAPO-n materials.Experimental Section a.Samples. The molecular sieves SAPO-20A, SAPO-20B, and SAPO-20C were s...
