Hexagonal Mesoporous Silica for carbon capture: Unrevealing CO2 microscopic dynamics by Nuclear Magnetic Resonance

Abstract

Global warming and environmental concerns have triggered global efforts to reduce anthropogenic carbon emission. Against this background, mesoporous silica materials have experienced a great expansion in the latest years due to their interesting carbon capture properties. Despite the very vast literature currently available in synthetic strategies and adsorption behaviour of MSs, the internal arrangement and diffusion properties of carbon dioxide (CO2) inside silica mesopores, to date, have been barely clarified. Here, mesoporous silica material with hexagonal framework (MCM-41) has been synthesized by a modified Stöber process, and its CO2 capture properties were assessed yielding a maximum amount of CO2 adsorbed of 18 wt% at 15 bar and RT. Fort the first time, microscopic distribution of CO2 and its mobility in the MCM-41 mesopores have been investigated by 13C NMR techniques. Peak fitting analysis of the 13C NMR spectra and T1-relaxometry data revealed a multiple component configuration for CO2 adsorbed in the silica mesopores with at least two physisorbed species coexisting. 13C PFG NMR self-diffusion study showed anisotropic long-range mobility for carbon dioxide adsorbed in MCM-41. The fastest self-diffusion coefficient, ascribed CO2 molecules diffusing parallel to the axis of the MCM-41 channels, was in the order of 10−5 cm2 s-1, i.e., two order of magnitude higher that of gas diffusing perpendicular to the mesopore. This study is crucial to properly address the future design and preparation of mesoporous silica materials with improved CO2 capture performances.