Pore Structure Characterization of Porous Silica by ¹H NMR Using Water, Benzene, and Cyclohexane as Probe Molecules

Abstract

The pore size distribution of four commercial silica materials saturated with water have been derived from experimental 1H NMR intensity vs temperature curves (IT-curves) of the confined water. The observed melting points or transition temperatures of benzene and cyclohexane confined in the same materials are shown to be consistent with corresponding data obtained from the water IT-curves. In general, the freezing point depression (ΔT) of these fluids can be related to the pore radius R in accordance with a modified Gibbs−Thompson equation: ΔT = K/(R + d), where K and d are constants characteristic of the confined fluid. The melting point depressions of benzene and cyclohexane are shown to be more sensitive to pore radius than that of water; i.e., the two former have a larger K-value. Moreover, the average pore radius of these materials can be estimated from 1H NMR chemical shift measurements of the benzene-saturated samples. Simulation of 1H NMR spectra of benzene confined in mixtures of silica (mesopores) and zeolite (micropores) will be presented.