An overloaded pure silica zeolite ISV synthesized using a phosphonium cation

Abstract

A pure silica zeolite with ISV structure was synthesized using the tricyclopentylmethylphosphonium (Cp₃MP⁺) cation under fluoride media. Analysis of the organic content in the zeolite suggested the presence of more than four cations per unit cell, while fluoride anions were exclusively located in the four double four-membered rings of the structure. ²⁹Si solid-state NMR demonstrated a significant concentration of Q³ Si species, i.e., (OSi)₃(OH), in the structure, which afforded charge balance but contrasted with the most common observation of defect-free pure silica zeolites prepared in fluoride media at near-neutral pH. Calculations showed that improved stability was obtained in an overloaded (i.e., containing more Cp₃MP⁺ than F⁻ and three-channel crossings) defective zeolite. The calculated stabilization of ISV was larger with Cp₃MP⁺ than with the cyclohexyl analog used to produce ZEO-1, a zeolite with an extra-large 3D pore system. Substitution of small amounts of Cp₃MP⁺ by tetraethylammonium stopped crystallization instead of producing ISV/BEC intergrowths, despite calculations on the bare zeolites suggesting improved stability of the intergrowths. This rendered the observed difficulty in obtaining such intergrowths puzzling and likely dependent on a disruption of the host–guest assembly. Compared with prior ISV materials, this zeolite displayed a minimum amount of disorder, which, however, was sufficient to impede proper Rietveld refinement.