Characterization of layered silicate HUS-5 and formation of novel nanoporous silica through transformation of HUS-5 ion-exchanged with alkylammonium cations

Abstract

Hiroshima University Silicate-1 (HUS-1) and its precursor (HUS-5), whose crystal structure was similar to that of β-HLS layered silicate, were prepared. The interlayer distance of HUS-5 was 0.4 nm; both Na⁺ cations and hydrated water were present in the interlayer. The interlayer distance of HUS-5 was expanded by ion-exchanging with several bulky organic cations such as dodecyltrimethylammonium, hexadecyltrimethylammonium, and benzyltrimethylammonium. The HUS-5 variants that were ion-exchanged with alkylammonium exhibited high catalytic activities base catalysts in a Knoevenagel condensation reaction. In particular, the HUS-5 that was ion-exchanged with hexadecyltrimethylammonium cations was converted to novel nanoporous silica by acetic acid treatment and subsequent calcination and was named HUS-6. The BET surface area and average pore diameter were 983 m² g⁻¹ and 1.6 nm, respectively. Formation of nanopores in HUS-6 was clearly confirmed by TEM. Nanopores (ca. 1.6 nm) of HUS-6 present a unique adsorption space; there was an intermediate state between micropores (ca. 0.6 nm) of silicalite-1 and mesopores (ca. 2.9 nm) of MCM-41.