CO2 adsorption on amine modified mesoporous silicas: Effect of the progressive disorder of the honeycomb arrangement

In this work, we have investigated how the typical honeycomb arrangement of SBA-15 and the mechanisms of CO 2 uptake are affected by the addition of chemicals upon synthesis (to modify pore width and length) and post-synthesis amine functionalization. SBA-15 was synthesized hydrothermally by a conventional route (S00) and such synthesis was then subject to modifications: by adding heptane as a swelling agent (S10) and increasing amounts of ammonium fluoride (S11, S12, S13 and S14) to shorten channels length. All of the synthesized silicas were then subject to APTES grafting and PEI impregnation. CO 2 isotherms were obtained for the pure and amine-loaded samples. They were fitted to the Dual-Site Langmuir model for all functionalized adsorbents, so that it was possible to discriminate the relative importance of chemisorption and physisorption sites. The increasingly disordered frameworks brought about by fluoride addition led to the formation of mesocellular silica foams (MCFs), which were not advantageous to increase CO 2 uptakes in the case of amine-grafting because they have a lower silanol density and hence bind less nitrogen as compared to the conventional SBA-15. In the case of PEI-impregnation, it was found that an addition of a low amount of fluoride is important in the regulation of the chemisorption/physisorption ratio. Promising CO 2 uptakes (2.3 mmol CO 2 /g or 4.6 mmol/g PEI at 25 °C and 1 bar) were found for a load of 50% (wt) PEI in MCF synthesized with the highest NH 4 F/SiO 2 molar ratio (equal to 0.065), which was attributed to a better accessibility of amino groups to CO 2 , as compared to the conventional SBA-15 support subject to the same PEI load.