ZEOLITES VERSUS ALUMINOSILICATE CLUSTERS - THE VALIDITY OF A LOCAL DESCRIPTION

A comparison of force field calculations on extended systems and ab initio quantum chemical calculations on ring structures reveals that the relative energy content of neutral-framework silicas and aluminophosphates is determined by that of the smallest substructures. Energy-minimized structures containing only four, five, and six rings have the same energy content to within 10 kJ/mol. Hypothetical three-ring-containing structures have considerably higher energy content, which may well inhibit their synthesis in the pure SiO2 modification. The results accord with sparse experimental information, Using the same combination of techniques, we demonstrate that substitution of aluminum in silica causes an appreciable, albeit local, distortion of the lattice. The relaxation energy for such a substitution amounts to 100 200 kJ/mol, depending on the way charge compensation is accomplished.