Two-Stage Crystallization of Meso- and Macroporous MFI and MEL Zeolites Using Tributylamine-Derived Diquaternary Ammonium Cations as Organic Structure-Directing Agents

Abstract

Abstract Meso- and/or macroporous zeolites have received particular attention because the presence of secondary meso- and/or macropores, in addition to the intrinsic micropores of zeolites, can provide additional cavities at a nanometer-length scale and shorten molecular diffusion path lengths. Here, synthesis of meso- and macroporous MFI and MEL aluminosilicate zeolites 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 (OSDA) is described. In this two-stage process, both low- and high-temperature stages were found to be crucial for the formation of zeolites. By carefully varying the amount of sodium hydroxide in the reactants, both MFI and MEL zeolites can be obtained with the same OSDA. It was revealed that Bu6-diquat-7 was more selective to the formation of MFI and MEL zeolites than its monoquaternary counterpart (i.e., tetrabutylammonium) under the present synthesis conditions. Interestingly, the synthesis time of the first (low temperature) stage highly influenced the particle sizes of the resulting zeolites; zeolites with smaller sizes were obtained when the synthesis time was prolonged. Gas physisorption analyses and scanning transmission electron microscopy observation suggested that the obtained zeolites possessed ink-bottle-like pores consisting of meso- and macroporous internal cavities with very narrow pore necks.