Two-stage crystallization of meso-and macroporous MFI and MEL zeolites using tributylamine-derived diquaternary ammonium cations as organic structure-directing agents

Nanoporous materials have been utilized in a wide range of applications, e.g., adsorption, catalysis, energy storage, and drug delivery. Meso- and macroporous MFI and MEL aluminosilicate zeolites were synthesized via a two-stage crystallization method using diquaternary ammonium cations (N,N,N,N',N',N"-hexabutylheptanediammonium, Bu₆-diquat-7) as an organic structure-directing agent. In this two-stage process, both low- and high-temperature stages were found to be crucial for the formation of zeolites. Bu₆-diquat-7 was more selective to the formation of MFI and MEL zeolites than its monoquaternary counterpart under the present synthesis conditions. Interestingly, the synthesis time of the first (low temperature) stage highly affected the particle sizes of the resulting zeolites, wherein zeolites with smaller sizes were obtained under prolonged synthesis time. Gas physisorption analyses and scanning transmission electron microscopy observation indicated that the obtained zeolites possessed ink-bottle-like pores consisting of meso- and macroporous internal cavities with very narrow pore necks.