Selective oxidative dehydrogenation, propane

Selective Oxidative Dehydrogenation of Propane on Promoted Niobium Pentoxide... 

Smits, Seshan and Ross have studied the selective oxidative dehydrogenation of propane to propylene over Nb20s and Nb2C>5 supported on alumina.32 Vanadia supported on MgO has typically been used in these reactions, although reduced surface vanadyl ions can give rise to decreasing selectivity to propylene. Niobia on the other hand, is much more difficult to reduce than vanadyl but there have been few studies with this oxide in such reactions. 

With respect to the catalytic reactions, there are well-established industrial reactions (as occurs in the case of n-butane to maleic anhydride), reactions in the preindustrial stage (such as the transformation of propane to acrylonitrile), very promising reactions (such as ethane oxidative dehydrogenation to ethylene), and potential reactions whose economical viability will depend on the prices of crude and natural gas in the future (such as propane selective oxidation to acrylic acid or methane transformation). 

Concepcion, P, Lopez-Nieto, J.M., and Perez-Pariente, J. The selective oxidative dehydrogenation of propane on vanadium aluminophosphate catalysts. Catal. Lett. 1993,19, 333-337. 

Chaar, M.A., Patel, D., and Kung, H.H. Selective oxidative dehydrogenation of propane over V-Mg-O catalysts. J. Catal. 1988,109, 463. 

Chaar, M.A., Patel, D., Kung, M.C., and Kung, H.H. Selective oxidative dehydrogenation of butane over V-Mg-O catalysts. J. Catal 1987,105, 483. Kung, M.C. and Kung, H.H. The effect of potassium in the preparation of magnesium orthovanadate and pyrovanadate on the oxidative dehydrogenation of propane and butane. J. Catal 1992, 134, 668. 

The oxidative dehydrogenation of propane to give propene catalyzed by TS-1, Ti-beta, Ti-MCM-41, Ti02-silicalite-l, or others was investigated by Schuster et al (259). TS-1 was the best catalyst, with a selectivity of 82% for propene at a propane conversion of 11% (Fig. 42). Sulfation of TS-1 by H2S04 prior to the reaction increased the conversion to 17%, with a selectivity of about 74%. Although conversion of propane was higher on Ti-beta and Ti-MCM-41, selectivity for propene was much lower C02 was the main product. Lewis acid sites were considered to be the major active sites (259). 

Vanadia catalysts exhibit high activity and selectivity for numerous oxidation reactions. The reactions are partial oxidation of methane and methanol to formaldehyde, and oxidative dehydrogenation of propane to propene and ethane to ethcnc.62 62 The catalytic activity and selectivity of... 

Figure 1 is the catalytic behavior of VSU545 in propane oxidative dehydrogenation to propylene. Selectivities to propylene in the range of60-80% are obtained up to propane conversions of about 20-25% and reaction temperatures up to around 450- 500 C. For higher reaction temperatures and conversions the selectivity decreases due both to the formation of carbon oxides and of aromatics. As compared to pure silicalite, a significant increase in both the selectivity to propylene and the activity in propane conversion is observed. 

Figure 2. Comparison of the catalytic behavior of VSil samples in propane oxidative dehydrogenation to propylene. Conversion of propane and selectivity to propylene at 470 C. Exp. conditions as in Fig. 1.

Figure 4. Comparison of the behavior of VSil545 in propane oxidative dehydrogenation using N2O or O2 as oxidizing agents. Exp. conditions as in Fig. 1. The dotted lines represent the propane conversion and propylene selectivity observed in the absence of the catalyst (homogeneous gas phase). The activity of the catalyst in the absence of O2 or N2O is similar to that observed in the homogeneous gas phase, but the selectivity to propylene (around 50-60%) is lower.

The role of adsorbed oxygen species in the mechanism of alkane transformation, on the contrary, is more questionable. The effect induced by the substitution of O2 with N2O and IR indications are in agreement with this interpretation, but, on the other hand, activated electrophilic oxygen species form on reduced sites, preferably in tetrahedral coordination (79). The partial reduction of tetrahedral V =0 with formation of tetrahedral v after propane oxidative dehydrogenation can be observed using UV-Visible diffuse reflectance, ESR and V-NMR spectroscopies. It is thus not possible to assign unequivocally the active species in propane selective activation to a tetrahedral V =0 species or to or V -0-0 species formed in the... 

It has been shown that the activity of niobia for the oxidative dehydrogenation of propane can be increased by adding vanadium or chromium, while maintaining a high selectivity towards propylene. 

In this paper we present the results of experiments which show that the activity of niobia can be improved by adding other suitable elements, while maintaining high selectivity. These results bring the proces of oxidative dehydrogenation of propane closer to practical application. 

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