Synthesises of Mesoporous Materials and Studies of Host/guest Interaction in Mesoporous Carbon by solid state NMR
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Beschreibung
A series of mesoporous silicas and carbon materials was successfully prepared via sol-gel techniques. In particular a new synthetic route for the synthesis of silica materials with and without functional groups was developed which is based on polyglycerol (PG) as templates. As an environmentally friendly and biodegradable chemical, PG can be completely removed from the as-synthesized silicas by three times of water extraction. Thus the pathway involved in silica mesoporous materials synthesis is free of organic solvent and therefore it is environmentally benign. The removals of PG template from pure and amino group modified silicas were mainly proved by 1H and 13C solid state NMR. The pure silicas templated by PG with various molecular weights were characterized by N2-isotherm at 77K. It showed large specific surface areas and narrow pore size distribution centered at 3.8 nm. Since PG in solution cannot form micelles with well-ordered structure, the pore structure of the PG templated silicas presented here is almost amorphous and not as ordered as in the case of MCM-41 and SBA-15. The role of PG in the self-organization is similar to the neutral template.
The mesoporous carbon materials were prepared directly by using resorcin and formaldehyde together with Pluronic F 68 as pore structure template. From the TEM pictures, after carbonization at 600°C and 800°C, the carbon materials (denoted as C600 and C800) showed ordered structures with a clear electron diffraction pattern which is typical for crystalline structure. The specific surface areas of the materials are all above 500 m2/g, with pore diameter centered at ca. 4 nm. Owing to the presence of remaining functional groups on the graphite-like surface, the pore diameter can be expanded up to 22 nm by oxidation under moderate conditions.
Another carbon material (denoted as C_SiO2), which is replicated from silica gel with very large surface area of 1500 m2/g was prepared. To probe the surface properties of carbon materials and their host/guest interactions with small molecules, water and three aromatic molecules (benzene, pyridine and pyrrole) were employed as surface sensors. The host/guest interactions were studied by a series of solid-state NMR experiments. The results of the system of water-carbon material reveal that the surface properties depend strongly on the temperature of carbonization and the method of synthesis. The NMR results showed a hydrophilicity of the carbon material surface and the existence of surface water. The latter could not be frozen at -50°C. At low levels of loading (16% w/w), only the surface species was found, which is characterized by high field shifted signals at -2.8 and –4.0 ppm in the proton spectra. The signal of bulk water dominated with a signal at ca. 4.7 ppm at higher loading above the amount of monolayered water.
In the next step three organic molecules, pyridine, benzene and pyrrole, were employed as surface probes. All proton signals of the surface species are shifted to high field, which we explain as a result of ring currents on the graphitic carbon material surface. Moreover, the protons and carbons of all surface species possess much shorter spin lattice relaxation times (T1) than those of the bulk materials. All these results indicate interactions between guest molecules and host material. 13C CP (Cross Polarization) signals of the three organic compounds were recorded, which demonstrate the existence of non-zero dipolar couplings between H and C on the surface, in contrast to a typical liquid state, where they average to zero. The measurements of the CP dynamics of the pyridine reveal that the TCH value (CP time constant) of the C2 carbon in pyridine is slight different to the TCP of the other carbons. From this we conclude that the pyridine molecule is mainly parallel to the surface and that there may be a very small tilt angle between the plane of pyridine molecule and carbon material surface, indicated by the small difference in TCP.
Compared to this, the TCH of the C2 position in pyrrole is more than 2 times larger than the TCP of the C3 carbon. This implies a longer distance to the surface and thus a larger tilt angle compared to the case of pyridine molecule.
Autor*in
Yeping Xu