QUANTUM-MECHANICAL CALCULATIONS ON MOLECULAR-SIEVES .2. MODEL CLUSTER INVESTIGATION OF SILICOALUMINOPHOSPHATES

Ab initio molecular orbital calculations have been performed on model clusters simulating silicate, aluminosilicate, and silicoaluminophosphate dimeric and trimeric entities. This investigation was aimed at obtaining preliminary information on the relative stability of the Si-O-Al, Si-O-P, and Al-O-P bridges and on modifications generated by the presence of phosphorus in such and related systems. The results show that the Al-O-P bridge is more stable than Si-O-Al or Si-O-P linkages, which is in agreement with the high stability of framework aluminum phosphate structures. They also indicate a preferential tendency for P to replace Si rather than Al in aluminosilicate structures and agree with the observation that Si incorporation in aluminum phosphate molecular sieves occurs preferentially by replacement of P. Finally, the results also suggest that bridging hydroxyl groups in aluminum phosphates - the proton originating from nonstoichiometry, defects, or Si incorporation - should show acidic properties. © 1990 American Chemical Society.