Studies on the Interaction of Vanadia with Modified Silica Supports and Catalytic Activity for Oxidative Dehydrogenation of Propane: Effect of Support Modification by Al3+, Zr4+ or Y3+
✍ Scribed by Rajamma Sasikala; Vasanth Sudarsan; Shailendra K. Kulshreshtha
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- English
- Weight
- 136 KB
- Volume
- 2006
- Category
- Article
- ISSN
- 1434-1948
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✦ Synopsis
Abstract
Detailed structural studies of vanadia supported on SiO~2~ and modified SiO~2~ (Si~0.8~M~0.2~O~2±~~δ~ where M = Al^3+^, Zr^4+^ and Y^3+^) were carried out using different techniques such as powder XRD, IR spectroscopy and ^29^Si, ^27^Al and ^51^V MAS NMR spectroscopy. All samples except vanadia impregnated on Zr^4+^ or Y^3+^ modified SiO~2~ existed as amorphous phases. A crystalline phase of ZrO~2~ was seen in Zr^4+^ modified SiO~2~ and YVO~4~ was seen in Y^3+^ modified SiO~2~ after vanadia impregnation. Based on ^29^Si MAS NMR and IR spectroscopic studies it was concluded that substitution of Si^4+^ by M results in the breakage of the Si–O–Si network and formation of silicon structural units with Si–O–M type linkages. Unlike all other modified SiO~2~, the silicon structural units remained unaffected in Al^3+^ modified SiO~2~ before and after vanadia impregnation. A profound change in the interaction of vanadia with the support was observed when the SiO~2~ support was modified with different cations and in all cases the V^5+^ preferentially interacted with the substituted cations. Two types of vanadia species, viz. the octahedral and tetrahedral V^5+^, were present in vanadia impregnated silica and Al^3+^ modified silica but the amount of tetrahedral V^5+^ was more in the latter. In Zr^4+^ modified silica, V^5+^ was present in a crystalline environment which is the V^5+^ dissolved in the ZrO~2~ phase. Vanadia impregnated on Y^3+^ modified silica exhibited very little chemical shift anisotropy indicating a symmetric environment around V^5+^ in YVO~4~. The catalytic activity for the selective oxidation of propane to propene was tested on these supported catalysts. Vanadia supported on Zr^4+^ modified silica showed maximum selectivity whereas vanadia on silica and Al^3+^ modified silica showed comparable selectivity for propene formation. Vanadia on Y^3+^ modified silica did not show the formation of propene at all. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)