PORE-SIZE DETERMINATION OF MCM-41 MESOPOROUS MATERIALS BY MEANS OF H-1-NMR SPECTROSCOPY, N-2 ADSORPTION, AND HREM - A PRELIMINARY-STUDY

The aim of the present study is to derive a mathematical pore size dis tribution function containing a limited number of adjustable parameter s. These parameters can be determined exclusively from H-1 NMR measure ments. Regular mesoporous MCM-41 materials with different pore sizes, ranging from 20 to 30 Angstrom, were synthesized and characterized by HREM, N-2 adsorption, and H-1 NMR. The pore sizes determined by Nf ads orption and HREM were in good agreement. The H-1 NMR technique was use d to determine the freezing point of water enclosed in water-saturated samples. By combining N-2 adsorption and H-1 NMR measurements, a simp le relation was found between the freezing point depression (Delta T) and the pore radius (R(p)): Delta T = K/(R(p) - t(f)) with t(f) = 3.49 +/- 0 36 Angstrom. The observation that t(f) not equal 0 is tentative ly explained by the formation of a surface layer of nonfreezing water of thickness t(f), which effectively reduces the actual pore radius fr om R(p) to R(p) - t(f). A mathematical model is derived which enables the pore size distribution to be determined from H-1 NMR intensity vs temperature measurements of water-saturated materials. The pore size d istribution of amorphous' silica determined independently by H-1 NMR a nd N-2 adsorption agreed well. However, the pore size of a microporous VPI-5 material (R(p) = 6.05 Angstrom) could not be predicted by the p resent model.