MCM-69, a novel layered analogue of EU-19

Abstract

A new form of crystalline silica has been prepared and is designated MCM-69. The material has a crystalline layered structure which can be delaminated and which contains a regular array of reactive, surface silanols. Specifically, one out of every three Si atoms in the MCM-69 structure contains a silanol group. To maintain layer separation, considerable care must be taken in the preparation of MCM-69 to avoid dehydration of these silanols and inadvertent crosslinking of the silica layers, which would prevent subsequent delamination. Stoichiometrically, MCM-69 has the idealized formula, H 2O·6SiO 2, and is derived from MCM-69(P), a piperazine (pipz) silicate precursor. MCM-69(P) appears to have the same topology as EU-19. MCM-69(P) differs from EU-19 in its behavior, most notably in the removal of its pipz to yield MCM-69, and its subsequent dispersability. MCM-69 can be swollen and dispersed in aqueous solution by reacting the silanol groups with amines, such as pyrrolidine, n-propylamine, or n-octylamine. The resultant products all show crystallinity as characterized by a strong X-ray diffraction (XRD) peak below 10.5° 2 θ. The exact position of the low-angle peak indicates the size of the amine controls the interlayer separation. The larger the amine, the lower the angle of the XRD peak, and the larger the separation between the silica hydrate layers. The silanol groups react with hydroxide ion and with surface functionalizing agents such as hexamethyldisilazane.