PA-41
Preparation, characterization and catalytic properties of vanadium species highly dispersed on the SBA-16 catalyst
Yongli Dong, Fulong Yuan, Yujun Zhu*, Lina Zhao, Honggang Fu
Education Office of Heilongjiang Province Key Laboratory of Functional Materials, School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080,
E-mail: [email protected]
The SBA-16 silica was synthesized as described in Ref [1]. Vanadium species highly dispersed on the SBA-16 catalysts (VOx/SBA-16) were prepared by impregnation. The XRD pattern, the N2-physisorption isotherms and TEM image of the VOx/SBA-16 (7.2 wt %) catalyst indicate that the pore structure is well retained and the introduced vanadium species would be
highly dispersed on the SBA-16 mesoporous silica.
Figure 1 depicts the Raman spectra and wide-angle XRD patterns (inset) of VOx/SBA-16 catalysts. At low loadings (3.7 wt %), the spectra is dominated by a strong band at around 1035 cm-1 with a shoulder at 1068 cm-1, which is characteristic for the V＝O stretch vibration of tetrahedral VO4 species to the support surface [2].
When the content of V is 7.2 wt %, the intensity of the 1035 cm-1 band decreases, and an additional band appears at 998 cm-1, which is attributed to crystalline V2O5 [3]. However, XRD patterns (inset) show no evidence for the formation of a crystalline fraction of V2O5. This
result reveals that an aggregation of the vanadium centers results in the formation of some micro-crystallites of V2O5 besides the small VOx clusters at this loading. Its formation also demonstrates that with increasing V loading the molecular structure of the surface vanadium species changed
significantly.
The HRTEM images of VOx/SBA-16 are shown in Figure 2. There are only highly dispersed vanadium species on the catalyst (Fig.2 a) when the content of vanadium is 3.7 wt %. However, some composites of VOx
species (Fig.2 b) with a diameter size around 10 nm and some bigger micro-crystallites of V2O5
(Fig.2 c) outside of the pores are observed on the VOx/SBA-16 (7.2 wt %). This result is in agreement with the Raman spectra discussed above.
The VOx/SBA-16 (7.2 wt %) shows an excellent catalytic activity that the selectivity and the yield of phenol is 97.5 and 13.80 %, respectively. The catalytic turnover number is 32.8.
In conclusion, the VOx/SBA-16 (7.2 wt %) catalysts with a 3-D cubic (Im3m)
800 1000 1200 1400
10 20 30 40 50
Intensity (a.u.)
2 Theta (degree)
7.2 wt %
3.7 wt % 7.2 wt %
3.7 wt %
Raman Intensity (a.u.)
Raman Shift (cm-1)
Fig. 1 Raman spectra of VOx/SBA-16 catalysts and corresponding wide-angle XRD patterns (inset)
c
b
Fig. 2 HRTEM images of VOx/SBA-16 (a) 3.7 wt %, (b) 7.2 wt % and (c) V2O5 outside of pores of VOx/SBA-16 (7.2 wt %)
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pore structure are prepared by impregnation method. It shows an excellent catalytic activity to the hydroxylation of benzene due to the vanadium species highly dispersed mainly inside the pores of SBA-16 support.
References
1. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky. J. Am. Chem. Soc. 120 (1998) 6024-6036
2. C. Hess, Journal of Catalysis 248 (2007) 120-123
3. C. Hess, J.D. Hoefelmeyer, T.D. Tilley. J. Phys. Chem. B, 108 (2004) 9703-9709
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