Reducing the Usage of Silane Coupling Agent in the Synthesis of Amine-Grafted Mesoporous Silica and its Application for Co2 Adsorption

Abstract

Amine-grafted adsorbents are promising CO2 adsorbents; however, the excessive addition of an amino silane coupling agent during their synthesis increases the synthesis cost. We investigated the synthesis of 3-aminopropyltrimethoxysilane (APTMS)-grafted SBA-15 mesoporous silica using low amounts of silane and evaluated its CO2 adsorption performance. APTMS-grafted SBA-15 samples were prepared using impregnation and heating-filtration (grafting) methods. The obtained samples were characterized by X-ray diffraction spectroscopy, transmission electron microscopy, N2 adsorption/desorption, scanning electron microscopy, magic angle spinning (MAS) nuclear magnetic resonance (NMR), and elemental analysis. The characterization results revealed that the micropores of SBA-15 were preferentially blocked; silane occupied the mesopores with increasing amine loading. The CO2-adsorption performance of the adsorbents was measured by thermogravimetric analysis in dry conditions. For both synthesis methods, the amine immobilization efficiency, estimated from the amount of silane coupling agents used in the synthesis and that immobilized on the support, was high (78.3–92.2%). The adsorbents prepared by the two methods showed similar adsorption amounts, which were around 0.5 mmol g−1 in 400 ppm CO2 and around 1.0 mmol g−1 in 5 vol. % CO2. In contrast, the adsorption rate of the material was affected by the synthesis method even with similar amine loadings. The adsorption amounts obtained in this study are comparable to those of previously reported silane-coupling-agent-modified adsorbents. The results revealed that even with a low amount of the silane coupling agent, it was possible to synthesize high-performance amine-grafted CO2 adsorbents.

Keywords: mesoporous silica, Direct air capture, CO2 adsorption, SBA-15, 3-aminopropyltrimethoxysilane