Porosity characterization of SBA-15 silicas with thermoporosimetry of water and n-alkanes – The effect of the probe liquid nature

• Differences in the behavior of odd and even-numbered alkanes in TPM experiments.
• The ability of heptane to differentiate the u-shaped and straight pores in SBA-15.
• Lower degree of pore filling by n-hexane in sample with the largest pore size.
• Different packing of C6 and C7 alkane molecules in the solidified state.

Thermoporosimetry, based on DSC measurements of melting point depression of water, n-hexane and n-heptane confined in mesopores has been used for determination of the pore size distribution, average pore diameter and pore volume of series of SBA-15 mesoporous silicas. Porosity of these materials was additionally characterized by low-temperature nitrogen adsorption and quasi-equilibrated thermodesorption of n-nonane. In general, the pore sizes and pore volumes determined using water and alkanes TMP reveal the same tendency as those calculated with other methods, although the absolute values show some differences. In particular, pore diameters found from water and n-hexane TPM experiments are in most cases smaller in comparison with the results obtained from other methods. On the other hand, in the case of solids with largest pores, the pore diameters found on the basis of n-heptane TPM are significantly overestimated. The values of the pore volume corresponding to the integral intensity of the TPM PSD peaks are smaller than those calculated with other methods. The possible reasons for the observed deviations are discussed. For the first time important differences in the behavior of n-hexane and n-heptane in TPM experiments are reported and attributed to different packing of C6 and C7 alkane molecules in the solidified state due the alternations in the properties of odd and even numbered hydrocarbon chains.

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