Adsorption and Thermogravimetric Characterization of Mesoporous Materials with Uniform Organic−Inorganic Frameworks

Abstract

Two-dimensional hexagonal HMM-1 and three-dimensional hexagonal HMM-2 ordered mesoporous materials with hybrid ethanesilica frameworks were characterized using nitrogen adsorption, argon adsorption, and high-resolution thermogravimetry (TGA) under nitrogen and air atmosphere. Nitrogen and argon adsorption isotherms for these materials featured rather narrow capillary condensation steps. Adsorption−desorption hysteresis in primary mesopores was observed in the case of argon adsorption at 77 K. The pore diameter of HMM-1 was determined on the basis of the geometrical relation among the pore size, pore volume, and unit-cell size and was found to be in excellent agreement with the pore diameter calculated according to the method calibrated using MCM-41 silicas. This provided a confirmation of the applicability of the geometrical method for two-dimensional hexagonal materials with arbitrary framework composition. Low-pressure adsorption data provided evidence that the surfaces of HMM-1 and HMM-2 interact somewhat more weakly with nitrogen adsorbate than the silica surface, which can be explained as a consequence of the substitution of weakly interacting siloxane bridges by larger and presumably even more weakly interacting ethane bridges present in the frameworks of the hybrid materials. At low pressures, the statistical thickness of nitrogen film in the HMM-1 pores was found to be somewhat lower than that in the MCM-41 pores. Some differences in surface interactions with nitrogen and argon were also observed between HMM-1 and HMM-2, and these differences may be attributable to a different degree of exposure of organic groups on the pore surface of these two materials. The surfactant content in as-synthesized materials was found to correlate with the pore volume after surfactant removal. The latter was essentially complete after the solvent extraction, as judged from TGA. Thermogravimetric data also suggest that ethane groups of HMM-1 and HMM-2 were stable up to at least 553 K in air and up to at least 813 K under nitrogen atmosphere.