ZSM-5 zeolite nanosheets with improved catalytic activity synthesized using a new class of structure-directing agents

Abstract

A new series of multiquaternary ammonium structure-directing agents, based on 1,4-diazabicyclo[2.2.2]octane, was prepared. ZSM-5 zeolites with nanosheet morphology (10 nm crystal thickness) were synthesized under hydrothermal conditions using multiquaternary ammonium surfactants as the zeolite structure-generating agents. Both wide-angle and small-angle diffraction patterns were obtained using only a suitable structure-directing agent under a specific zeolite synthesis composition. A mechanism of zeolite formation is proposed based on the results obtained from various physicochemical characterizations. ZSM-5 materials were investigated in catalytic reactions requiring medium to strong acidity, which are important for the synthesis of a wide range of industrially important fine and specialty chemicals. The catalytic activity of ZSM-5 materials was compared with that of the conventional ZSM-5 and amorphous mesoporous aluminosilicate Al-MCM-41. The synthesis strategy of the present investigation using the new series of structure-directing agents could be extended for the synthesis of other related zeolites or other porous materials in the future. Zeolite with a structural feature as small as the size of a unit cell (5-10 nm) with hierarchically ordered porous structure would be very promising for catalysis. Zeolite nanosheets: 1,4-Diazabicyclo[2.2.2]octane (D)-based multiquaternary ammonium surfactants direct the formation of zeolite structures on both the mesoporous and microporous length scales, and yield ZSM-5 zeolite nanosheets of 10 nm crystal thickness (see figure). The sheetlike nanoarchitecture is stable at high temperatures. Acid sites on the external surface of the zeolites render them active in the catalytic conversion of large organic molecules.