PORE STRUCTURE CHARACTERIZATION OF POROUS SILICA BY H-1-NMR USING WATER, BENZENE, AND CYCLOHEXANE AS PROBE MOLECULES

The pore size distribution of four commercial silica materials saturat ed with water have been derived from experimental H-1 NMR intensity vs temperature curves (IT-curves) of the confined water. The observed me lting points or transition temperatures of benzene and cyclohexane con fined in the same materials are shown to be consistent with correspond ing data obtained from the water IT-curves. In general, the freezing p oint depression (Delta T) of these fluids can be related to the pore r adius R in accordance with a modified Gibbs-Thompson equation: Delta T = K/(R + d), where K and d are constants characteristic of the confin ed fluid. The melting point depressions of benzene and cyclohexane are shown to be more sensitive to pore radius than that of water; i.e., t he two former have a larger K-value. Moreover, the average pore radius of these materials can be estimated from H-1 NMR chemical shift measu rements of the benzene-saturated samples. Simulation of H-1 NMR spectr a of benzene confined in mixtures of silica (mesopores) and zeolite (m icropores) will be presented.