Carbon Dioxide Sorbents with Propylamine Groups−Silica Functionalized with a Fractional Factorial Design Approach

Abstract

Mesoporous silica particles (Davisil) were functionalized with aminopropyltriethoxysilane (APTES) in a fractional factorial design with 19 different synthesis and uptake experiments. The number of amino groups and the uptake of CO₂ were optimized in a 2_V⁵⁻¹ design. Most important to functionalization was the amount of water present during synthesis, the reaction time, and pretreating the silica with a mineral acid; certain two-way interactions were shown to be statistically significant as well. Modifications performed at 110 or 80 °C showed no significant differences concerning amine content or uptake of CO₂. Properly choosing center points for the discrete variables is problematic and is somewhat related to the lack of fit with respect to CO₂ uptake; the regression was good. Solid-state ²⁹Si NMR showed that the APTES was mainly fully condensed. Specific surface areas did not correlate with the number of n-propylamine groups on the silica, which is indicative of differential levels of heterogeneity in the coverage of propylamines. The uptake of CO₂ and N₂ was measured from −20 to 70 °C and from 0 to 1 bar and parametrized by the Freundlich isotherm. Amine-modified silica adsorbed significant amounts of CO₂, especially at the low partial pressure, which is important for CO₂ capture from flue gas. At such pressures, samples with a high density of amine (4 amines/nm²) showed a much higher uptake of CO₂ than did those with densities of ∼2−3 amines/nm², reflecting differential tendencies to form propylammonium−propylcarbamate ion pairs; these require close proximity among amine groups to form. Water affected the uptake of carbon dioxide in different ways. Certain samples took up more moist CO₂ gas than dry CO₂, and others took up less moist CO₂ than dry CO₂, which is indicative of differential tendencies toward water adsorption. We conclude that experimental design is a time-efficient approach to the functionalization of silica with propylamine groups.