(183aa) Dual Role of Surfactants As Structure-Directing Agents and Mesoporogens in the Preparation of Zeolites

Zeolites are crystalline microporous aluminosilicates with unique properties, such as tunable acidity and exceptional (hydro)thermal stability, which are utilized in commercial processes ranging from ion-exchange and adsorption to catalysis and separations. Surfactant templating has emerged as one of the most effective and versatile strategies for the construction of well-defined porous architectures in zeolites.¹ Despite tremendous efforts to elucidate the mechanisms of surfactant templating in zeolite synthesis, these pathways are not well understood. Here, we will discuss the dual roles of the cationic surfactant cetyltrimethylammonium (CTA) as an organic structure-directing agent (OSDA) and as a mesostructuring agent in the rational design of zeolites.

One of the most common zeolites is ZSM-5 (MFI type), which is used as a catalyst and sorbent in a wide range of industrial applications.² In ZSM-5 synthesis it is challenging to identify new and inexpensive OSDAs that can tailor the physicochemical properties of the final product. The most frequently used OSDA in ZSM-5 synthesis is tetrapropylammonium (TPA); however, recent studies have shown that CTA can be used as an alternative.^3,4 Here, we will present the effects of using CTA and TPA as OSDAs in combination with a variety of alkali metals as inorganic structure-directing agents. Our findings reveal that the selection of inorganic/organic combinations has a significant impact on the kinetics of ZSM-5 crystallization, as well as the properties of the resulting crystals. Notably, we show that TPA/Na and CTA/K are optimal combinations of structure-directing agents that can markedly alter the size, morphology, and aluminum distribution in ZSM-5. Using a combination of experiments and molecular modeling, we explore the use of CTA as an alternative OSDA for zeolite MFI and show that we can achieve smaller crystals (ca. 600 nm) in similar time (< 24 h) as syntheses employing TPA.

In addition to OSDA design, the development of mesoporosity in zeolites has been a long-standing goal in catalysis to alleviate the diffusion limitations imposed by micropores.^5,6 One area of research that has garnered considerable interest, yet is not fully understood, is the rearrangement of zeolite crystals post-synthesis to accommodate mesoporosity. Here, we will present in situ observations of intracrystalline mesoporosity in USY zeolite (FAU type) assisted by CTA using atomic force microscopy. Our findings capture in real time the structural, morphological, and textural evolution of initially rough crystals to smooth crystals with a uniform distribution of mesopores. Collectively, these findings provide new and unequivocal insights that confirm surfactant templating is a facile and efficient method to induce reconstruction of the crystal and the healing of defects towards the generation of optimal zeolites for catalytic applications.

(1) Choi, M et al. Nature 461 (2009) 246–249

(2) Chawla, A et al. Mol. Syst. Des. Eng. 3 (2018) 159-170

(3) Meng, L.; Mezari, B.; Goesten, M. G.; Hensen, E. J. M.; Chem. Mater. 29 (2017) 4091–4096

(4) Moteki, T.; Keoh, S. H.; Okubo, T.; Chem. Commun. 50 (2014) 1330–1333

(5) Sun, J.; C. Bonneau, et al. Nature 458 (2009) 1154

(6) Chawla, A et al. Chem. Mater. (2019) In Press DOI: 10.1021/acs.chemmater.9b00435