Click Synthesis of Monolithic Silicon Carbide Aerogels from Polyacrylonitrile-Coated 3D Silica Networks

Abstract

SiC retains high mechanical strength and oxidation stability at temperatures above 1500 °C, representing a viable alternative to silica, alumina, and carbon, which have been in use as catalyst supports for more than 60 years. Preparation of monolithic porous SiC is usually elaborate and porosities around 30% v/v are typically considered high. This report describes the synthesis of monolithic highly porous (70% v/v) SiC by carbothermal reduction (1200−1600 °C) of 3D sol−gel silica nanostructures (aerogels) conformally coated and cross-linked with polyacrylonitrile (PAN). Synthesis of PAN-cross-linked silica aerogels is carried out in one pot by simple mixing of the monomers, whereas conversion to SiC is carried out in a tube reactor by programmed heating. Intermediates after aromatization (225 °C in air) and carbonization (800 °C under Ar) were isolated and characterized for their chemical composition and materials properties. Data are interpreted mechanistically and were used iteratively for process optimization. Solids 29 Si NMR validates use of skeletal densities (by He pycnometry) for the quantification of the conversion of silica to SiC. Consistent with the topology of the carbothermal process, data support complete conversion of SiO 2 to SiC requiring a C:SiO 2 ratio higher than the stoichiometric one (=3). The morphology of the SiC network is invariant of the processing temperature between 1300 and 1600 °C, and hence it is advantageous to carry out the carbothemal process at higher temperatures where reactions run faster. Those samples are macroporous and consist of pure polycrystalline β-SiC (skeletal density: 3.20 g cm −3 ) with surface areas in the range reported previously for biomorphic SiC (∼20 m 2 g −1 ). Although the micromorphology remains constant, the crystallite size of SiC increases with processing temperature (from 7.1 nm at 1300 °C to 16.5 nm at 1600 °C). Samples processed at 1200 °C are mesoporous and amorphous (by XRD), even though they consist of ∼75% mol/mol SiC. The change in the morphology of SiC in the 1200−1300 °C range has been explained by a melting mechanism. This comprises the first report of using a polymer cross-linked aerogel for the synthesis of another porous material.