Multinuclear Magnetic Resonance Spectroscopy Studies of the Structuration of the Tunnels of Sepiolite in the Presence of Intracrystalline Pyridine Molecules

摘要

Sepiolite,a naturally occurring magnesiosilicate nanoporous clay mineral with a tunnel structure,contains two types of water molecules in the structure: zeolitic water trapped inside the tunnels and structural water molecules which interact strongly with magnesium cations.The zeolitic water was removed by heating the sepiolite to 120 deg C.The partially dehydrated sepiolite absorbed pyridine vapor to produce an intercalated material where the pyridine takes the place of the zeolitic water.~1H solid-state MAS NMR spectroscopy showed that there is isotopic H/D exchange between pyridine-d_5 and the remaining structural water molecules of the sepiolite framework.The exchange takes place at room temperature over several days.Wide line solid state ~2H NMR of the sequestered pyridine-d_5 showed that two populations of pyridine molecules coexist in the material: one very mobile physisorbed population,which can be removed by heating at 90 deg C,and a population due to pyridine trapped in the tunnels.Except for small in-plane librations,the trapped pyridine was shown to be held rigidly by the sepiolite.When the pyridine intercalated material is heated at 400 deg C the structural water and some of the pyridine is lost.The remaining pyridine takes the place of the structural water to produce a new inorganic-organic nanohybrid material with the pyridine bound to the terminal Mg-(II) in the structure.The pyridine in this material as well as the intercalated material was characterized by slow-spinning ~(15)N and ~(13)C CP/MAS NMR spectroscopy.The ~(15)N NMR was shown to be a very sensitive probe to characterize the various types of pyridine.The data indicate that pyridine molecules in the inorganic-organic nanohybrid material are directly bound to magnesium cations exposed in the tunnels of sepiolite.