Fractal dimension of zeolite catalysts

Abstract

Atom—atom analyses of the geometric descriptors, topological indices and fractal dimension D are applied to active-site models of Brønsted acid zeolites. The results are compared with those from the literature for rings and cavities. A method is tested similar to that used in previous works for crystal fragments. The results obtained are encouraging and the good quality of the analysis is clear from a comparison with cavity results. The active sites are modelled by sets of Al—OH—Si units. These bridges form 2–12 membered rings. Indices for the models are calculated. An analysis shows that the maximal Dsi contribution corresponds to the 6-ring, matching the maximal Dcavity. It is suggested that Si plays a main role in the catalytic activity. Most cavities show no fractal character, while for the 6–8 cavities D is the greatest and is a maximum for the 6-cavity, which is expected to be the most reactive. This is in agreement with the greatest flexibility and with the adsorption of ions in the 6-ring reported in the literature. Work is in progress to test the effect of local deformations on the opening and closing of ring apertures and to check the role of Si.