Evaluation of CO2 storage properties and mobility through NMR technique in post-grafting synthetized organically functionalized porous silica

Functionalized porous silica materials were synthesized in alkaline conditions by post-grafting method using hexadecyltrimethylammonium bromide (CTAB) as templating agent, tetraethyl orthosilicate (TEOS) as silica precursors and trialkoxysilanes functionalised with different organic groups. Textural features, halfway between microporosity and mesoporosity, combined with the surface physico-chemical ones related to various organic groups, determined different CO2 storage properties highlighting a strong interaction with the precursor 3-Aminopropyl)triethoxysilane. NMR spectroscopy also showed how physisorption is the mainstream mechanism for CO2 adsorption in all the sorbents, except for the aforementioned organic group in which an important chemisorption contribution occurs. Diffusion and molecular mobility analysis revealed that at least two species coexist within the micro-/meso-pores of the sorbents, namely, CO2 molecules powerfully interacting with the pore surface (pore-surface CO2) and "bulk-like" CO2 filling the central region of the pores. Furthermore, the post functionalization suppresses the diffusion of CO2 molecules through the pore channels with all the functionalized materials exhibiting a single self-diffusion coefficient. On the other side, D// strictly depends on the average pore size of the sorbent.