Preparation, characterization, and surface modification of periodic mesoporous silicon-aluminum-carbon-nitrogen frameworks

Periodic mesoporous silicon-aluminum-carbon-nitrogen (Si/Al/C/N) frameworks with P6mm hexagonal symmetry were synthesized by a solvent nanocasting route using mesoporous carbon (CMK-3) as hard template and preceramic polymers containing both -[R₁R₂Si-N(R₃)]_n- and -[R₄Al-N(R₅)]_n- backbones (with R ₁ = R₂ = R₃ = R₄ = H and R ₅ = CH₂CH₃) as ceramic precursors. The preceramic polymers are prepared through a simple and cost-effective procedure by blending poly(perhydropolysilazane) and poly(ethyliminoalane) as precursors of silicon nitride/silicon (Si₃N₄/Si) and carbon-containing aluminum nitride (Al/C/N), respectively. The blended polymers with various and controlled Al:Si ratios were infiltrated into the porous structure of CMK-3, followed by a pyrolysis-template removal cycle performed under nitrogen at 1000 C (2 h, ceramic conversion), then in an ammonia atmosphere at 1000 C (5 h, template removal). This procedure resulted in the formation of periodic mesoporous Si/Al/C/N frameworks with surface areas of 182-326 m² g^-1, a pore size distribution of 4.1-5.9 nm, and pore volumes in the range of 0.51-0.65 cm³ g^-1. The uniformity of the mesopores and periodicity of the obtained amorphous micrometer-size powders, studied by transmission electron microscopy (TEM), small-angle X-ray diffraction (SA-XRD), and N₂ sorption, are affected by the Al:Si ratio. Amorphous materials did not exhibit weight change up to 1400-1470 C in flowing nitrogen, and their behavior in air, up to 1000 C (with dwelling time of 5 h), is dependent on the proportion of AlN and Si ₃N₄ phases. The as-obtained powders then were decorated with Pt (nano)particles by impregnation to form supported catalysts. The as-formed catalysts showed attractive reactivity and robustness in our probe reaction, namely, the hydrolysis of an alkaline solution of sodium borohydride at 80 C. Our main results are reported therein.