Insights of adsorption isotherms with different gases at 77 K and their use to assess the BET area of nanoporous silica materials

Abstract

There is a well-known relationship between porous materials performance in a given process and their textural properties. These properties include specific surface area, among others, where the most widely used experimental technique to determine them is gas adsorption. Although the most used adsorptive gas is N2 at 77 K up to atmospheric pressure, its quadrupole moment generates specific interactions with surface groups, as silanols in silica materials, causing a preferential orientation effect on the adsorbed N2 molecule affecting the specific surface area value. In this sense, we analyzed the adsorption–desorption isotherms at 77 K of nanoporous silica materials using different adsorptives. From these data, we obtained the specific surface area (SBET) values of the samples by applying the BET method with the IUPAC recommendations for each gas. The selected materials were MCM-41, MCM-48, SBA-15, and SBA-16, and the adsorptives used were Ar, O2, and CH4, along with N2. Among the chosen adsorptives, Ar and CH4 do not have a quadrupole moment, whereas this value is present for N2 and O2, being the latter four times smaller than nitrogen. In addition, at 77 K, both Ar and CH4 are below their triple-point temperature, while N2 and O2, which are above their triple-point temperature, are in the same thermodynamic state. Taking the SBET obtained by Ar at 77 K as the referential value of each sample, the corresponding molecular transversal areas of the other adsorptives were estimated. It was found that the variation of transversal area for the N2 molecule at 77 K on silica materials was between 0.133 and 0.149 nm2 (below its common value of 0.162 nm2). In contrast, in the case of the O2 molecule at 77 K, this value was almost constant, with an average of 0.123 nm2. These results showed that the quadrupole moment of the O2 does not play an important role in the interaction with surface silanol groups present in the samples, making oxygen at 77 K a potential and reliable adsorptive to determine the specific surface area of silica materials.