Silicalite-1 Zeogrid: A New Silica Molecular Sieve with Super- and Ultra-Micropores

Abstract

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.