Role of acidic pretreatment of layered silicate RUB-15 in its topotactic conversion into pure silica sodalite

Topotactic conversion of crystalline layered silicates into zeolite provides an opportunity to create new chemical compositions, framework types, and macroscopic morphologies that are difficult to achieve by conventional hydrothermal synthesis. We have recently reported the successful synthesis of pure silica sodalite with a unique sheet-like morphology from layered silicate RUB-15 occluding interlayer TMA⁺ cations. Pretreatment of RUB-15 with acetic acid was found to be crucial for topotactic dehydration-condensation between the silicate layers upon heating. In this study, a homologous series of carboxylic acids of varying concentrations is examined for their capability to generate an ordered intermediate state, and important factors for topotactic conversion are determined. Both length of the alkyl chains and concentration of the carboxylic acids strongly affected the crystallinity of the products, and well-crystallized sodalite was obtained using either acetic or propionic acid. Transmission electron microscopy showed that the sodalite with sheet-like morphology has the thickness of several hundred nanometers in which the (110) plane is oriented parallel to the surface. Two key factors elucidated for successful conversion are (i) proton-exchange of interlayer TMA⁺ cations to shorten the interlayer distance and to form Si-OH groups and (ii) intercalation of carboxylic acid molecules between the layers to maintain the well-ordered layered structure prior to calcination.