Oxidative Dehydrogenation of Light Alkanes on Supported Molten Alkali Metal Chloride Catalysts

Abstract

Potential and limitations of molten alkali metal (Li, Na, and K) chlorides supported on Dy 2 O 3 /MgO were explored for the oxidative dehydrogenation of lower alkanes, such as ethane and propane. The catalysts have high activity and selectivity to olefins compared to conventional catalysts. Optimum performance is obtained with catalysts on which the alkali metal chloride phase is molten under reaction conditions. Lower chloride melting point correlates with higher selectivity. The high selectivity to ethene or propene is attributed to the high mobility of cations and anions, which facilitates desorption of alkene (limiting further oxidation) and the generation of spatially isolated hypochloride anions acting as the active sites for the primary C-H bond activation.

Oxidative Dehydrogenation of Ethane: Common Principles and Mechanistic Aspects

“…The mobility of the melt prevents the formation of defect sites (coordinatively unsaturated metal cations), which are speculated to initiate the reactions that lead to deep oxidation. In a melt, the surface re‐arranges, thereby quickly limiting the concentration of these exposed cations by covering the transiently present exposed cations 51. 67 The fact that predominantly anions that are not able to attack the double bond of the olefin are present on the outer surface further enhances the benefits of a molten overlayer.…”

“…Whereas supported alkali‐oxide catalysts show certain disadvantages, including inhibition by CO 2 and limited selectivity, materials with an overlayer of molten alkali chloride and no addition of alkali oxide have been reported to be catalytically active for the ODH of ethane 51. 67 Supported liquid‐phase catalysts (SLC) are a new generation of catalysts for the ODH of ethane, which requires comparatively high reaction temperatures (around 600 °C).…”

“…Activity and selectivity have been reported for catalysts with overlayers of pure alkali chlorides and several eutectica153 of alkali chlorides, such as LiCl, KCl, NaCl, LiKCl, LiNaCl, LiSrCl, and LiBaCl, that are supported on (Dy 2 O 3 ‐promoted) MgO 51. 67 Whereas LiCl leads to the most active catalysts, these catalysts are also the least selective.…”

“…Metal cations are presumably inactive towards CH activation, but they influence the catalytic behavior of the molten chloride overlayer. The oxygen solubility is the highest for LiCl,51 whereas larger cations (Na + , K + ) lead to fewer dissolved oxygen species 154. 155 Ethane solubilities in the highly polar melt are negligible 67…”

Chemical Technologies for Exploiting and Recycling Carbon Dioxideinto the Value Chain

“…[47,48] Thus, the couple CaO-CaCO 3 can be applied in the temperature range of 600-900 8C. [237][238][239][240][241] Moreover, Mg/Al, [242] Li/Dy/Mg, [243,244] and other mixed oxides are capable of reversibly adsorbing CO 2 . Other inorganic adsorbents for CO 2 are MnO 2 , Fe 2 O 3 , calcium hydroxyapatite, hydrotalcite, and zeolite (Na-ZSM-5).…”