Computer simulation of the Si,Al distribution in dealuminated zeolite Y

The distribution of Si and Al cations over the framework of dealuminated faujasites has been analysed by a Monte Carlo procedure, and the results have been compared to earlier 29Si NMR data. Modelled materials are dealuminated according to different criteria in order to determine the main characteristics of the actual dealumination processes. We find that the cation distribution after partial Al removal depends on the dealumination method. In particular, this distribution is more homogeneous for specimens dealuminated with SiCl4 than for those dealuminated by following a hydrothermal method. In the latter case, an appreciable gradient of atom concentrations is present, indicating that the dealuminating agent acts effectively only on a partial volume of the treated material. In all cases considered, dealumination yields cation distributions far from that of the state of lowest free energy.