Pore size engineering of mesoporous silicon nitride materials

Abstract

Nitrogen physisorption measurements of silicon nitride materials obtained from silicon diimide show the valuable porosity of these compounds in the mesopore regime and very high specific surface areas up to 1000 m² g⁻¹, suitable for catalytic applications. The pore size of the solids is effectively tailored by the variation of parameters in the manufacture of the diimide source. Si(NH)₂ is obtained by reacting silicon halides with gaseous ammonia in an organic solvent followed by sublimation of the by-products in an ammonia flow at elevated temperatures (773–1273 K). The nature of the solvent, halide source and precipitation temperature strongly affect the average pore size and specific surface area of the product. The pore size can be adjusted in a range from 5.6 to 9.1 nm, as derived from nitrogen physisorption isotherms. The elemental composition of the porous nitrides varies from Si₂N₂(NH) to Si₃N₄ and depends on the temperature of the heat treatment. IR measurements indicate a high number of amino groups present on the inner surface of the solids, suitable for further functionalization. The materials are amorphous according to XRD powder patterns and show broad ²⁹Si MAS NMR lines. The low sintering tendencies in vacuum, ammonia and under ammonothermal conditions are essential for development of the compounds as high temperature catalysts.