Zr K-edge XAS and ²⁹Si MAS NMR studies on hexagonal mesoporous zirconium silicate

Hexagonal mesoporous silicate (HMS) molecular sieve containing zirconium species (Zr-HMS) has been synthesized using a neutral template of hexadecylamine with ethanol and water. A disordered channel structure with a uniform diameter of 44.1 Å was confirmed by powder X-ray diffraction after calcination. From N₂ adsorption/desorption measurement, it is found that Zr-HMS has a very high surface area of about 910 m²/g (BET) and a mesopore with a size of 21 Å (BJH). According to the X-ray absorption spectroscopy at the Zr K-edge, it is confirmed that Zr atoms are coordinated by six oxygen neighbors with a distance of 2.13 Å, which is shorter than the (Zr-O) bond distance found in ZrO₂ or ZrOCl₂·8H₂O. The ²⁹Si MAS-NMR spectrum for Zr-HMS shows two additional peaks around -94.6 and -107.2 ppm due to the Q³Zr and Q⁴Zr components, respectively, besides three peaks at -91.1, -101.5, and -111.3 ppm due to the silicate network. The relation between the pore and crystal structures and the local environment around Zr is discussed in detail.