Influence of the Nature of the Promoter in NiO Catalysts on the Selectivity to Olefin During the Oxidative Dehydrogenation of Propane and Ethane

Abstract

A comparative study of the catalytic properties for the oxidation of C2-C3 alkanes and olefins has been carried out over unpromoted and M-promoted NiO catalysts (Me= K, La, Ce, al, Zr, Sn, Nb). The catalysts have been characterized by several physicochemical techniques (UV Raman, Visible Raman, FTIR of adsorbed CO and XPS). The characteristics of promoter elements are of paramount importance, since they are able to modify both the nature of the active nickel and the concentration of electrophilic O2 -/Ooxygen species. Thus, a relatively high acidity and valence of the promoter oxide (with oxidation state higher than +3) are necessary to achieve high selectivity to olefins during the oxidative dehydrogenation (ODH) of C2-C3 alkanes. In addition, an inverse correlation between the selectivity to the corresponding olefin and the concentration of electrophilic oxygen species has been observed, although the selectivity to propene during propane ODH is lower than the selectivity to ethylene achieved during ethane ODH. On the other hand, a very low influence of alkane conversion on the selectivity to the corresponding olefins is observed. This behaviour can be explained by considering that the reaction rate for olefin combustion are lower to the reaction rate for alkane oxidation.However, the comparative study of the oxidation of alkanes and olefins suggest that the differences observed between the ODH of propane and ethane are not related to the reactivity of olefins, but to the different number and reactivity of C-H bonds in both alkanes. A discussion on the importance of the concentration of active sites and the characteristics of the alkanes fed on the selectivity to olefin during the alkane ODH is also presented.

Evolution of the optimal catalytic systems for the oxidative dehydrogenation of ethane: The role of adsorption in the catalytic performance

“…4). Furthermore, in previous comparative studies of undoped and Zr-or Nb-doped NiO catalysts for ethane ODH at 350 °C, it was observed that the r C2H4 /r C2H6 ratio shows small differences between 0.41 and 0.53 [62], similar to that reported here. However, supported vanadium oxide catalysts present r C2H4 /r C2H6 ratios between 2 and 10, depending on the hydrocarbon feed and the support [16].…”

Unraveling the Active Vanadium Sites and Adsorbate Dynamics in VOx/CeO2 Oxidation Catalysts Using Transient IR Spectroscopy

“…As suitable catalysts for ODH of short alkanes, especially propane, mainly vanadia but also NiO- and boron-oxide-based catalysts have been discussed. ,, In particular, catalysts with amorphous surface vanadia species (VO x ) as the active phase have shown promising performances during propane ODH, but further catalyst optimization is still required. ,− Besides the active phase, the choice of the support material is of great importance and has been proposed to increase the catalytic performance in the case of active support materials such as CeO 2 and TiO 2 due to their direct participation in the reaction. ,, Recently, ceria-supported vanadia (VO x /CeO 2 ) catalysts have attracted increased attention, both theoretically − and experimentally. ,,,,− Nevertheless, direct experimental insight into the reaction intermediates and the surface dynamics have not been reported yet.…”

Correlation between the Properties of Surface Lattice Oxygen on NiO and Its Reactivity and Selectivity towards the Oxidative Dehydrogenation of Propane

“…[24] Doping NiO with heteroatom, such as, Nb, Al, Sn, Ti, and W, has been reported to improve the selectivity of alkenes. [25,26] It has been suggested that electronic structure change due to solid solution formation resulted in the reduced concentration of the electrophilic O species [27] and suppressed further oxidation of alkene. Although these studies demonstrated the improvement of selectivity and yield of alkene by modifying NiO, how the modification affects the ODH mechanism remains unclear.…”