Insights on the use of different gases at 77 K to assess the specific surface area of silica nanoporous materials

DEICY BARRERA; JOSÉ JOAQUÍN ARROYO GÓMEZ; JHONNY VILLARROEL ROCHA; KARIM SAPAG

Fortaleza

Congreso; 13º ENCONTRO BRASILEIRO SOBRE ADSORÇÃO; 2020

Universidade Federal do Ceará

There is a well-known relationship between porous materials performance in several processes and their textural properties such as specific surface area, pore volume and pore size distribution. The most widely used experimental technique and adsorbate to determine the textural properties of porous materials are gas adsorption and N2 at 77 K, respectively. Due to its quadrupole moment, the N2 generates specific interactions with the surface groups as silanol ones present in ordered silica materials, causing a preferential orientation effect on the adsorbed N2 molecule. Herein, alternative adsorptives as Ar, O2 and CH4 were evaluated and compared with N2 to assess the specific surface area in ordered silica materials: MCM-41, MCM-48, SBA-15 and SBA-16. These adsorptives were chosen due to Ar and CH4 molecules do not present quadrupole moment; and that of O2 is four times less than N2. Additionally, at 77 K the Ar and CH4 are above their triple-point temperature, and N2 and O2 are in the same thermodynamic state (i.e., vapor). It was found that the variation of transversal area for the N2 molecule on silica materials was between 0.133 and 0.149 nm2, whereas in the case of O2 molecule at same temperature this value was almost constant, with an average of 0.123 nm2. These results showed that the quadrupole moment of the O2 at 77 K is low enough to not interact with surface silanol groups present in the samples, making oxygen a potential and reliable adsorptive to determine the specific surface area of silica materials.