ADSORPTION BEHAVIOR ON DEFECT STRUCTURE OF MESOPOROUS MOLECULAR-SIEVEMCM-41

Adsorption isotherms of N-2, water, NH3, pyridine, n-hexane and cycloh exane were determined and used to study the adsorption behavior on hig h silica, mesoporous molecular sieve MCM-41 and trimethylsilylated. MC M-41. X-ray diffraction (XRD), and Si-29 and C-13 magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) were used to charac terize the surface composition and structure of the materials. These r esults and the comparison plot from the adsorption of nitrogen provide evidence that the wall surface of the mesopore is nonuniform. There a re micropores with similar to 0.50 nm openings on the wall of the pare nt sample. Trimethylsilylation replaces similar to 79% of silanol grou ps in the parent sample, changes the surface composition and structure , destroys the acidic center, and blocks the micropore. Therefore it i ncreases the hydrophobicity, contracts the pore size, greatly decrease s the adsorption at low partial pressure, and changes the adsorption i sotherms for various adsorbates. The process of capillary condensation in the mesoporous channels is discussed.