A TOPOLOGICAL MODEL FOR THE SILICON INCORPORATION IN SAPO-37 MOLECULAR-SIEVES - CORRELATIONS WITH ACIDITY AND CATALYSIS

Changes in the unit cell parameter, composition (chemical and X-ray microanalysis), and Si-29 MAS n.m.r. spectra of a series of SAPO-37 with increasing Si contents suggest a topological model for the Si, Al, and P distribution in the SAPO-37 framework. Some Si atoms are not isolated but form islands bordered by Si-O-Al species typical of the SAPO-37 framework. The next-nearest neighbors of Si are different from those existing in Si-Al faujasites and consist of a significant number of P atoms (or of Al for Si (0 Al) species). The model agrees with a decrease in the number of acid sites as the Si content increases in the SAPO-37, leading to more atoms involved in Si islands. It is in line with a low catalytic activity in cracking reaction due to the P influence. No significant evidence is found for the presence of a pure Si-Al faujasite phase.