For the unsupported silica or modified silica membrane, the microporous structure can be estimated by means of gas sorption as well as the pore size distribution (PSD). PSD is an interesting tool to estimate the pore width of these materials from the adsorption data. However, the microporosity results reported in the literature highly depends on the probe molecules used for the sorption anal- ysis, either N2, CO2 and/or Ar. In addition to that, the PSD results could differ somehow depending mainly on the model used for the PSD determination [14-16,20,26,27]. The earlier mentioned strategies (preparation of hybrid silica and metal-doped silica) en- abled researchers to produce sol–gel derived microporous silica or modified silica materials. However, the actual values of the pores width for the supported silica or modified silica materials may dif- fer somehow compared with the PSD results for the unsupported one. This is because there are several factors that should be consid- ered such as the support resistance and membrane thickness.