Secondary crystallization of SBA-15 pore walls into microporous material with MFI structure

Abstract

SBA-15 was prepared under reflux, resulting, unlike SBA-15 prepared under autogenous pressure, in a mesoporous material with thicker pore walls but with a slightly smaller unit cell parameter and pore diameter. Under the conditions employed, only a very minor portion of the aluminum present in the reaction mixture was found to have been incorporated in the solid, predominantly in the form of extraframework species. When SBA-15 was thermally treated at 180 °C in the presence of TPABr, fluoride anions and water, the mesopore structure collapsed and large crystals of microporous zeolite with MFI structure were obtained. On the other hand, when SBA-15 was transformed at 120 °C in the presence of hydroxyl anions and glycerol, the mesopore structure was partly maintained, although the formation of the microporous structure could not be observed. Due to the very low aluminum content, SBA-15 and the transformed samples were much less active in the isomerisation of m-xylene than a reference H-ZSM-5 catalyst (Si/Al = 11). However, irrespective of the modification method, the activity of the transformed SBA-15 samples increased with the time of thermal treatment. For the sample treated in the presence of fluoride anions, this behavior was attributed to the formation of microporous ZSM-5. For SBA-15 transformed in the presence of hydroxyl anions, the increased activity indicated that SBA-15 activity can be enhanced by this post-synthesis treatment without complete loss of mesoporosity.