Liquid adsorption of basic dye using silica aerogels with different textural properties

Monolithic silica aerogels with different pore structure were prepared by sol–gel process and CO2 supercritical drying by changing the cycle pressure of liquid CO2 which was used to replace ethanol in the autoclave before supercritical state of CO2 was obtained. N2 adsorption at −196 °C was used to characterize the texture properties of the silica aerogels as prepared. Equilibrium adsorption of methylene blue (MB) onto silica aerogels was investigated. It was found that silica aerogel prepared at 4.5 MPa possesses mainly macroporous structure, while samples prepared at 6.5 and 8.5 MPa are typical mesoporous material. These materials exhibit significant difference in dye adsorption behavior depending on the different porous structure. The MB adsorption capacity and adsorption rate for sample prepared at 4.5 MPa is higher than samples prepared at 6.5 and 8.5 MPa, and the silica aerogel prepared at 8.5 MPa possesses the lowest adsorption capacity and adsorption rate. All samples are best fitted by Langmuir equation, but the fitting of Freundlich equation shows that the adsorption is also favorable. The adsorption on macroporous silica aerogel prepared at 4.5 MPa is best described by the pseudo-first-order model, while mesoporous silica aerogels prepared at 6.5 and 8.5 MPa follow the pseudo-second-order kinetics model. Surface energy distribution obtained by using DFT method indicated that the surface energy distributions of all samples are very wide, and the surface heterogeneity not only affects the dye adsorption capacity but also the adsorption rate, i.e., both the adsorption capacity and adsorption rate are higher for the sample with more heterogeneous surface.