Conversion of short-chain alkanes to olefins is a very important process for the chemical industry.  The large availability of lower alkanes from refineries as well as their abundance in natural gas reserves and oil shale makes these reactions economically very relevant. A major challenge in the dehydrogenation of alkanes is that these reactions are highly endothermic.  At lower temperatures, thermodynamic equilibrium limits the conversion, making use of high temperatures a necessity.  When they are exposed to high temperatures in an oxygen-free environment, however, cracking/decomposition of alkanes becomes dominant, leading to coking, and hence deactivation of the catalyst, as well as feedstock loss through coke formation. Oxidative dehydrogenation (ODH) has been studied as an alternative to direct dehydrogenation.  In this reaction, hydrogen abstraction is achieved oxidatively, hence removing the thermodynamic limitation.   Also, ODH reactions are exothermic and do not require additional energy input to the reaction.  In ODH, the major challenge is controlling the selectivity at high conversion levels. The presence of gas phase oxygen at high temperatures leads to complete oxidation products, mostly through further oxidation of the olefin product, which is often more reactive than the alkane.

Electrocatalytic reactors using an oxide ion-conducting membrane offer a potential solution to control the selectivity to the desired olefins by regulating the availability of oxygen to the alkane. In this scheme, the hydrocarbons and gas-phase oxygen never come in contact, and the selectivity can be controlled by tuning the adsorption/desorption characteristics of the anode electro-catalyst and by controlling the oxide ion transfer rate.  We have been working with Ti-based perovskites as anode electro-catalysts for this application. Lanthanum strontium titanates, La_xSr_1-xTiO_3-δ or LSTs, as well as chlorine-doped perovskites of the same form (LST-Cls) have been examined for potential anode catalysts in electrocatalytic ODH reactions.

Materials and Methods

          LST catalysts, with varied La doping from 10 to 50 mol% at the A-site, were prepared via a modified Pechini route. Cl-doped LST catalysts were also synthesized using a similar method. These catalysts were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier Transform spectroscopy, electrical conductivity measurements, and other techniques such as surface area analysis and scanning electron microscopy.

Electrocatalytically-asssisted ODH tests were performed in an electrochemical reactor consisting of an yttria-stabilized zirconia (8 mole% doped YSZ) membrane, an LSM (Lanthanum strontium manganite)-YSZ composite cathode and titanate-perovskite type catalyst on the anode. The cell was sealed using SCHOTT GM31107 glass sealant. A Keithley 6220 current source was used to apply current and the reactor effluent was monitored using a Shimadzu GC 2014.  Oxygen flux tests were conducted in the 550-700°C temperature range.

Results and Discussion

          XRD confirmed cubic perovskite structure of the LST and LST-Cl catalysts after calcination. At lanthanum-doping levels of 40% or higher at the A-site of the perovskite, other phases begin to precipitate out of the structure. Because of this, lanthanum-dopant of 0, 10, 20, and 30% were of most interest. Although these materials are similar to each other in behavior and surface features, characterization confirmed 20% doping as an optimal value in this particular application.

Oxygen flux tests displayed a good match between the actual and predicted oxygen flux values (by Faraday’s Law), for all temperatures. This confirms the reactor’s ability to regulate oxygen amount as a function of applied current. Ethane ODH tests using the same reactor showed the most promising results for LST28-Cl (La_0.2Sr_0.8TiO_3-δCl_σ). Although activity improvement is needed, these studies showed that the anode electrocatalyst characteristics can be fine tuned by dopant levels, and oxygen flux can be controlled by the current applied.

Acknowledgement

The financial support provided by the National Science Foundation through the Grant CHE-1213443 and the Ohio Development Services Agency is gratefully acknowledged.

References:

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