Pore-size determination of mesoporous materials by 1H NMR spectroscopy

The pore-size distributions of a series of mesoporous silica materials were determined by measuring the H-1 nuclear magnetic resonance (NMR) signal from the nonfrozen fraction of organic probe molecules as a function of temperature. The melting point distribution curves of confined benzene reveal 2-3 transition points. The high-temperature transition point, corresponding to the temperature at the first maximum of the melting point distribution curve, is interpreted as the average depressed melting point of the confined substance. However, the intensity data reveal that a measurable portion of the confined benzene apparently remains nonfrozen even 120 K below the bulk melting point in the 4-10 nm pore systems. The component at lowest temperature is largely attributed to the liquidlike molecules at the pore wall, while the component at the intermediate temperature might result from pockets in the solid matrix or even a bimodal pore-size distribution. The average pore-size distributions obtained by NMR agree fairly well with those obtained by N-2 sorption. However, NMR gives a more detailed picture of the distribution, revealing two or three well-defined peaks. The peak at the smallest pore size, however, reflects the surface layer rather than a pore-size distribution.