Tetrapropylammonium (TPA)‐containing precursors are the building blocks in the crystallization of silica. In the first steps slab‐shaped silicalite nanoparticles are formed by ordered combination of the precursors (see picture). These nanoslabs have MFI‐type zeolite framework topology and play a key role in TPA‐ion‐mediated zeolite crystallization from monomeric and polymeric silica sources.

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The crystallization of colloidal silicalite-1 from clear solution is one of the best understood zeolite formation processes. Colloidal silicalite-1 formation involves a self-assembly process in which nanoslabs and nanotablets with a silicalite-1 type connectivity are formed at intermediate stages. During the assembly process, with strongly anisometric particles present, regions appear with orientational correlations, as evidenced with measurements of dynamic light scattering, viscosity, and rotation of polarized light. The presence of such regions rationalizes the unexpected differences between the crystallization kinetics under microgravity and on earth. The discovery of the locally oriented regions sheds new light on currently poorly understood hydrodynamic effects on the zeolite formation processes, such as the influence of stirring on the phases obtained and the subsequent kinetics. Addition of surfactants or polymers modifies the ordering of the zeolitic building units in the correlated regions, and new types of hierarchical materials named zeogrids and zeotiles can be obtained.

Spherical, micrometer‐sized particles with a layered structure were obtained by precipitation of a Silicalite‐1 zeolite nanoslab suspension upon addition of cetyltrimethylammonium bromide (CTMABr) and subsequent calcination. The material had a specific micropore volume of 0.69 cm3 g–1, distributed over super‐ and ultra‐micropores. The formation process of this peculiar microporous solid was studied using X‐ray diffraction (XRD), 29Si MAS NMR spectroscopy, thermogravimetry (TG), and nitrogen adsorption. In the precipitate, the Silicalite‐1 nanoslabs were laterally fused into nanoplates and stapled into layers with intercalated surfactant molecules. Removal of the surfactant through calcination caused facial fusion, besides additional lateral fusion, of the nanoplates. Empty spaces left lying laterally between individual nanoplates were responsible for the super‐microporosity. The ultra‐micropores were zeolitic channels inside the fused nanoplates. The potential of these Silicalite‐1 zeogrids as molecular sieves was demonstrated with pulse gas‐chromatographic separation of alkane mixtures. The mass‐transfer resistance of a packed bed of zeogrid particles was considerably lower than of compacted zeolite powder.

Tetrapropylammonium(TPA)‐haltige Vorstufen sind die Bausteine bei der Silicalitkristallisation. In den ersten Schritten entstehen durch geordnete Zusammenlagerung der Vorstufen quaderförmige Silicalitnanopartikel (siehe Bild). Diese Nanoblöcke weisen die Topologie von Zeolith MFI auf und spielen eine Schlüsselrolle bei der TPA‐gesteuerten Kristallisation von Zeolithen ausgehend von mono‐ und polymeren Siliciumquellen.

Nanoslabs (2): Small square domains of lighter contrast observed with TEM on NaCl particles are attributed to tetrapropylammonium‐silicalite‐1 nanoslabs carefully extracted from a nanoslab suspension (see TEM image).

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n-Alkane hydroisomerisation and hydrocracking experiments reveal that ZSM-5 materials synthesized by self-assembly of nanoslabs show different molecular shape selectivity than ZSM-5 synthesized by hydrothermal methods.