CO2 adsorption on APTES functionalized mesocellular foams obtained from mesoporous silicas

• Four different APTES-grafted silicas have been successfully prepared with pending amino groups for CO2 capture.
• TEOS was used as silicon source and trimethyl-benzene and heptanes as swelling agents and NH4F as solubility enhancer.
• CO2 uptakes of 2.4 mmol g−1 or 0.64 mol CO2 per mol N were achieved at 1 bar and 25 °C and anhydrous conditions.
• One adsorbent could be successfully regenerated at 100 °C, maintaining a constant capacity for 3 adsorption-desorption cycles.

The CO2 adsorption capacity of different APTES-grafted mesoporous silicas of SBA-15 type has been investigated and the influence of support textural properties and the role of the presence of silanol groups on the adsorption capacity are analyzed. Four adsorbents based on SBA-15 were prepared using tetraethyl orthosilicate (TEOS) as silicon source, with and without the addition of trimethyl-benzene (TMB) and n-heptane as swelling agents, and adding in some cases ammonium fluoride as a solubility enhancer. 3-(triethoxysilyl)propylamine (APTES) was then used as grafting agent by reaction with free silanol groups on the silica surface so as to provide pending amino groups for CO2 capture. The adsorption behavior for all supports was adequately described by a Freundlich model, whereas for the APTES-grafted silica, a dual-site Langmuir model was applied, which allowed us to quantify and qualify two different adsorption sites. The addition of n-heptane as swelling agent led to pore sizes beyond 10 nm and improved significantly the grafting efficiency, leading to higher CO2 uptakes as compared to the starting supports. At 1 bar and 25 °C and anhydrous conditions, CO2 uptakes of 2.4 mmol g−1 or 0.64 mol CO2 per mol N were achieved (which reveals a significant contribution of physisorption). This sample could be successfully regenerated at 100 °C, maintaining a constant capacity for 3 adsorption–desorption cycles. At 0.15 bar and 60 °C, anhydrous conditions, CO2 uptake reaches 1.5 mmol g−1. This value may be theoretically doubled in the presence of humidity, and there is room for further improvement if supports with the same pore size (13 nm) and higher surface areas (e.g. 1000 m2/g) are successfully synthesized.

CO2 adsorption/desorption isotherms on HTT34 for the first (squares), second (triangles) and third (circles) adsorption/desorption cycles at 25 °C. Desorption branches are highlighted by empty symbols.Figure optionsDownload as PowerPoint slide