Spectroscopic studies of synthesis, modification and characterization of novel mesoporous molecular sieves.

摘要

A novel family of mesoporous molecular sieves MCM-41 and MCM-48 are successfully synthesized. Some catalytically important metal ions such as Al, Mn, Cu, Pd and Fe are also incorporated into the structures of mesoporous materials by direct synthesis or ion-exchange. Different mesophases are characterized by powder x-ray diffraction (XRD), 29Si magic-angle-spinning (MAS) NMR spectroscopy and transmission electron microscopy (TEM) as well as by N₂ adsorption. MCM-41 materials are synthesized by different methods but MCM-48 materials are much more difficult to synthesize. So a reliable synthesis method is developed for MCM-48 materials. A series of mesophase transformations from disordered tubular mesophase (H₁) to a layered phase (L₁) to a cubic MCM-48 mesophase (V) to a second layered phase (L₂) is found during the formation of MCM-48. This sequence is found to be regulated by methanol redistribution, pH changes at the silica/surfactant interface and silica framework condensation. Therefore, a formation mechanism for MCM-48 is proposed by controlling the mesophase transformations.; The locations and adsorbate interactions of metal ions such as Mn, Cu, Pd and Fe in MCM-41 and MCM-48 are critical for acidic and redox catalysis properties. Electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies make it possible to characterize the incorporated paramagnetic transition metal ions in these materials. ESR results indicate that a framework transition metal ion such as Mn(II) in MnMCM-41 or MnMCM-48 shows different dehydration behavior from the extraframework Mn(II) in ion exchanged Mn-MCM-41 or Mn-MCM-48. Also, framework Mn(II) ions are found to be less accessible to adsorbates D₂O, CH₃OD and ND₃ than extraframework Mn(II). ESEM simulations further indicate that framework Mn(II) ions in MnMCM-41 and MnMCM-48 are interacting with fewer adsorbed D₂O and CH ₃OD than extraframework Mn(II) in Mn-MCM-41 and Mn-MCM-48. The same results are observed in Al-containing and Fe, Cu, Pd-containing mesoporous materials.