Tailoring the Hydrophobic Character of Mesoporous Silica by Silylation for VOC Removal

Abstract

MCM-41 and non-ordered mesoporous silica were modified using hexamethyl-disilazane (HMDS). The hydrophobic and hydrophilic properties of grafted materials were studied and compared to a purely all silica BEA zeolite by using competitive and noncompetitive water toluene adsorption. A linear correlation between the silylation degree and the hydrophobicity measurements has been found for MCM-41 materials. Even if highly silylated MCM-41 material have more hydrophilic sites (silanol groups) than all silica zeolite, water molecule condensation is not observed because these sites are isolated. Thus, the highly silylated MCM-41 sample exhibits not only hydrophobicity 2.3 times higher than all silica BEA zeolite but also possesses a storage capacity for toluene and chlorobenzene 3 times higher than this zeolite. In competition with water, the organic molecule (toluene or chlorobenzene) adsorption is always favored even if water adsorption is enhanced by chlorobenzene polarity.

Keywords:

• adsorption
• hydrophobicity
• MCM-41
• mesoporous silica
• silylation

ACKNOWLEDGEMENTS

The authors would like to acknowledge E. Colnay for her helpful assistance for the TGA measurements.

Notes

^a From TGA analysis.

^a estimated from the t-plot plateau slope.

^b estimated at the relative pressure value of P/P₀ = 0.97.

^c estimated from t-plot plateau y-intercept.

^d estimated from t-plot by y-intercept of the linear regression calculated from t values between 0.44 and 0.74 nm.

^e estimated from the BJH desorption plots.