NH3-assisted synthesis of microporous silicon oxycarbonitride ceramics from preceramic polymers: a combined N2 and CO2 adsorption and small angle X-ray scattering study

We have developed a simple and general synthesis strategy to tune the chemical composition and pore size as well as the surface area of microporous ceramics. This method is based on modifying the structure of preceramic polymers through chemical reactions with NH3 at 300-800 degrees C, followed by thermolysis under an Ar atmosphere at 750 degrees C. Under these synthesis conditions polysiloxane (SPR-212a, Starfire (R) Systems) and polysilazane (HTT-1800, KiON Specialty Polymers) transform to microporous ceramics, while materials derived from polycarbosilane (SMP-10, Starfire (R) Systems) remain non-porous, as revealed by N-2 and CO2 adsorption isotherms. Small angle X-ray scattering (SAXS) characterization indicates that samples prepared from polycarbosilane possess latent pores (pore size < 0.35 nm) which are not accessible in the gas adsorption experiments. The microporous silicon oxycarbonitride (SiCNO) ceramics synthesized from polysilazane and polysiloxane by the above-mentioned route possess a surface area and micropore volume of as high as 250-300 m(2) g(-1) and 0.16 cm(3) g(-1), respectively, as determined by the N-2 adsorption method. The analysis of CO2 adsorption isotherms by the Dubinin-Astakhov equation confirms a narrow pore size distribution in the ceramics derived from polysilazane. Our synthesis strategy provides tools to engineer the microstructure, that is the chemical structure and porosity, of microporous SiCNO ceramics for potential applications in the fields of catalysis, gas adsorption and gas separation.