Effect of calcium and potassium on V 2 O 5 /ZrO 2 catalyst for oxidative dehydrogenation of propane: a comparative study
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Catalysis Letters Vol. 102, Nos. 3–4, August 2005 ( 2005) DOI: 10.1007/s10562-005-5862-6
Effect of calcium and potassium on V2O5/ZrO2 catalyst for oxidative dehydrogenation of propane: a comparative study Mahuya De and Deepak Kunzru* Department of Chemical Engineering, Indian Institute of Technology, Kanpur, 208016, India
Received 11 January 2005; accepted 12 April 2005
The effect of calcium and potassium on the physiochemical properties and performance of V2O5/ZrO2 catalyst for oxidative dehydrogenation of propane was studied in the temperature range of 385–400 C. The vanadia loading was kept constant at 5 VOx/ nm2 and the atomic ratio A/V (A=Ca, K) was varied from 0.05 to 0.75. The vanadia surface structure was investigated using X-ray diffraction analysis (XRD), electron paramagnetic resonance (EPR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The redox property of the catalysts was studied by temperature programmed reduction (TPR) and temperature programmed oxidation (TPO) whereas surface acidity was measured by temperature programmed desorption (TPD) of ammonia. Calcium and potassium both interact with the surface V=O and stabilize the +5 oxidation state of vanadium. Interaction between calcium and vanadium was more intense though surface concentration of calcium was lower than that of potassium. For doped catalysts, the activity was lower due to an increase in reduction temperature as well as a lower extent of reduction and increased resistance to undergo redox cycles. On the other hand, removal of surface acidic sites by the dopants increased the propene selectivity. Potassium was more effective in decreasing the activity and increasing the propene selectivity. KEY WORDS: Vanadia–zirconia; calcium; potassium; oxidative dehydrogenation; propane; temperature-programmed reduction; temperature-programmed desorption.
1. Introduction Supported vanadia catalysts have been extensively used for study of oxidative dehydrogenation (ODH) of hydrocarbons. The most commonly used supports are silica, alumina and titania. Recently zirconia has been used, either as a single or mixed oxide support. The characteristics that favor zirconia over the conventional supports include, strong interaction with the active metal phase resulting in higher dispersion, higher thermal and chemical stability and unique combination of acid- base and reducing -oxidizing properties. The activity and selectivity of supported vanadia catalysts are strongly affected by the oxidation state and molecular structure of surface vanadia species, which in turn depend upon the nature of support, preparation methods and amount of vanadia content. The other important factor that strongly affects the physicochemical properties of the catalyst is the presence of additives. Limited literature is available on the effect of additives on the V2O5/ZrO2 system. Albrecht et al. [1] studied the alkali modified V2O5/ZrO2 catalyst for ODH of propane. They observed that all the alkali metals (Li, Na and K) lowered the activity of the catalyst. Sodium m
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