Tin(iv) phosphonates: porous nanoparticles and pillared materials

Abstract

Two types of tin(IV) phosphonates have been prepared; monophosphonates that form porous spherical aggregates and diphosphonates that form layered three-dimensional structures. The reaction of 4,4′-monophenyldiphosphonic acid with SnCl₄·5H₂O in a solvothermal reaction produces pillared layered porous materials with surface areas of 350–450 m² g⁻¹ and pore sizes in the 8–20 Å range. The isotherms are type I in character. With biphenyldiphosphonic acid, preparation in water–alcohol solutions also yields porous pillared materials but the isotherms are type IV. The pores are somewhat larger and the pore distribution range is also larger. However, carrying out the reactions in DMSO–water or DMSO–alcohol, yields porous materials with type I isotherms. The use of spacer groups such as methyl, phenyl and phosphite were found to change the pore structure and/or to increase the surface area. The structure of these porous materials cannot be determined directly as they yield minimal X-ray powder patterns. However, the structure of Sn(O₃PCH₃)₂ was determined from its powder pattern obtained from a sample treated hydrothermally at 220 °C for 30 days. The structure is indeed layered with pendant methyl groups forming a bilayer similar to the structure of zirconium phenylphosphonate. Using these structures to describe the porous pillared compounds led to a hypothesis of layer growth of these materials that explains their properties and their unique type of porosity. The usefulness of these porous materials resides in their ability to be functionalized to impart chemical reactivity.