Aminopropyl-functionalised silicas as high capacity CO₂ adsorbents

The effects of mesopore structure, e.g., pore geometry, pore spacing, pore volume, surface area, and surface silanol concentration, on the extent of surface functionalization and consequent CO₂ adsorption capacities were studied. A new approach to characterizing the extent of surface modification (per unit of substrate surface area) was described and applied to the prepared materials. In each case, treatment with apts led to reduction in pore volume, pore diameter, and surface area and also led to an increase in both N and C content. Longer treatment times (H5c-p24) improved tether-loadings for the HMS substrates. However, extending the treatment beyond 24 hr(H5c-p96) or to higher temperature (H5c-p96r) did not lead to any further increase in tether-loading. The apparent equilibrium loadings for the HMS substrates were lower than for Silica 40 at the same apts concentration. All products exhibited capacities that corresponded to a CO₂/N molar ratio of ∼ 0.5:1, consistent with the postulated carbamate mechanism. Marginally greater CO₂/N ratios observed for products with lower tether-loadings were due to the occurrence of some additional CO₂ adsorption at surface silanol sites.