Microporosity and CO2 Capture Properties of Amorphous Silicon Oxynitride Derived from Novel Polyalkoxysilsesquiazanes

Polyalkoxysilsesquiazanes ([ROSi(NH)1.5]n, ROSZ, R = Et, nPr, iPr, nBu, sBu, nHex, sHex, cHex, decahydronaphthyl (DHNp)) were synthesized by ammonolysis at -78 °C of alkoxytrichlorosilane (ROSiCl₃), which was isolated by distillation as a reaction product of SiCl₄ and ROH. The simultaneous thermogravimetric and mass spectrometry analyses of the ROSZs under helium revealed a common decomposition reaction, the cleavage of the oxygen-carbon bond of the RO group to evolve alkene as a main gaseous species formed in-situ, leading to the formation of microporous amorphous Si-O-N at 550 °C to 800 °C. The microporosity in terms of the peak of the pore size distribution curve located within the micropore size range (<2 nm) and the total micropore volume, as well as the specific surface area (SSA) of the Si-O-N, increased consistently with the molecular size estimated for the alkene formed in-situ during the pyrolysis. The CO₂ capture capacity at 0 °C of the Si-O-N material increased consistently with its SSA, and an excellent CO₂ capture capacity of 3.9 mmol·g-1 at 0 °C and CO₂ 1 atm was achieved for the Si-O-N derived from DHNpOSZ having an SSA of 750 m²·g-1. The CO₂ capture properties were further discussed based on their temperature dependency, and a surface functional group of the Si-O-N formed in-situ during the polymer/ceramics thermal conversion.