Synthesis and Characterization of Tungsten Containing Ultra Large Pore Mesoporous Silicate KIT-6

Synthesis and characterization of tungsten containing ultra large pore mesoporous silicate KIT-6

Anand Ramanthan^a, Bala Subramaniam^a,  Rajamanickam Maheswari^b and Ulf Hanefeld^c

^aCenter for Environmentally Beneficial Catalysis, The University of Kansas, Lawrence, KS 66047, USA

^bDepartment of Chemistry, Anna University, Chennai – 600025, India

^cGebouw voor Scheikunde, Technische Universiteit Delft, Julianalaan 136, 2628 BL Delft, The Netherlands

Tungsten was incorporated into ultra large pore mesoporous silicate, KIT-6[1], via hydrothermal synthesis method using a Pluronic P123 triblock copolymer as the structure directing agent and n-butanol as additive. Four W-KIT-6 samples with Si/W ratio of 200, 100, 40 and 20 were prepared. Calcined W-KIT-6 samples were characterized by XRD, elemental analysis, N₂ sorption, HR-TEM, FTIR, DR-UV-Vis and NH₃-TPD. Cubic Ia3d structure was confirmed by presence of 211 reflections around 0.9° (2θ) in Low angle XRD. The incorporation of tungsten is inferred by a slight decrease in unit cell parameter with tungsten content [2,3]. The presence of crystalline WO₃ was observed in high angle XRD for samples with Si/W ratio £ 40. Elemental analysis measured by instrumental neutron activation analysis (INAA) revealed that most of the tungsten in the synthesis gel was retained in to the KIT-6 silica matrix. W-KIT-6 samples showed a very high specific surface area of 780 – 930 m²/g with pore volume of 1.19 – 1.45 cm³/g and possessed a narrow pore size distribution of 6.4 – 6.7 nm. The BET surface area and pore volume decreased with increase in tungsten content. The presence of well-ordered cubic 3D mesopores of W-KIT-6 samples was seen by High resolution TEM studies. The average thickness of the wall is found to be 4–5 nm and the pore diameter is around 6-7 nm. This is in agreement with the results of the N₂ adsorption study. The incorporation of tungsten was further evident by FTIR spectra which showed a band at 960 cm^-1 [4] representative for Si-O-M linkages (M = heteroatom). An intense band at 216 nm observed in Diffuse reflectance UV-Vis spectra confirm this findings, which is due to ligand to-metal charge transfer in isolated [WO₄] tetrahedral species [5] and is a direct proof for the framework incorporation of tungsten species in silica framework. Existence of partially polymerized W species in octahedral coordination with low nuclearity is also observed in these spectra. NH₃-TPD study revealed presence of low-medium acid strength sites in these samples.

References

[1]   T.W. Kim, F. Kleitz, B. Paul and R. Ryoo, J. Am. Chem. Soc. 127 (2005) 7601.

[2]   O. Klepel, W. Bohlman, E.B. Ivanov, V. Riede and H. Papp, Micropor. Mesopor. Mater.  76 (2004) 105.

[3]   P. Trens, V. Stathopoulos, M. J. Hudson and P. Pomonis, Appl. Catal. A 263 (2004) 103.

[4]   D. Zhao, A. Rodriguez, N.M. Dimitrijevic, T. Rajh and R.T. Koodali, J. Phys. Chem. C 114 (2010) 15728.

[5]   M. S. Morey, G. D. Stucky, S. Schwarz and M. Froba, J. Phys. Chem. B 103 (1999) 2037 –2041.