Mechanism of enhanced adsorption of volatile cyclic siloxane on cold-sintered mesoporous silica

Contamination of sensors, electrical contacts, and optics by volatile cyclic siloxanes leads to their dysfunction and shortens their service life. Application of mesoporous adsorbents can be effective to remove volatile siloxanes from the environment, potentially making them useful for protecting sensitive equipment. However, the practical application of mesoporous adsorbents in powder form may not be suitable due to handling and containment problems. In the present work, cold sintering of mesoporous silica powders with addition of sodium silicate water solution is applied to fabricate highly effective adsorbent for volatile siloxanes. The cold-sintered mesoporous silica pellets are characterized through nitrogen gas adsorption, scanning electron microscopy, low-angle X-ray diffraction, and Fourier-transform infrared spectroscopy (FTIR). Hexamethylcyclotrisiloxane (D3) serves as a model substance to assess the adsorption capability toward volatile cyclic siloxanes under static adsorption test conditions at room temperature. Furthermore, thermal desorption of D3 is investigated using thermogravimetry with in-situ gas analysis employing mass spectrometry. Results show that the cold sintering method preserves the mesoporous structure while transforming mesoporous silica powders into bulk pellets, resulting in an enhanced adsorption capacity for D3 compared to raw silica powder. Furthermore, the temperature required for siloxane desorption significantly increases indicating chemical adsorption mechanism and possible D3 ring-opening polymerization. FTIR spectroscopy reveals the significant involvement of hydroxyl groups in the adsorption and desorption processes of D3 on mesoporous silica.