Coordination state of gallium in MFI structures prepared by direct synthesis and by postsynthetic modification of boralites

Abstract

High-field ⁷¹Ga MAS n.m.r. spectroscopy shows that gallium occupies preferentially the framework positions in Ga-MFI structures prepared by direct synthesis from a gel. However, modifications of boralite with Ga³⁺ in autoclave experiments provide structures with extraframework gallium species and only partially occupied boron vacancies in framework positions. Evidence of gallium oxohydrates in the modified samples was given by X-ray analysis. Extraframework gallium species with a quadrupole coupling constant of 8.8 MHz could be detected, but part of the extraframework gallium remains n.m.r. invisible.