Christian Kjølseth

Direct conversion of methane to aromatics in a catalytic co-ionic membrane reactor

Membranes to make benzene from methane Methane gas is expensive to ship. It is usually converted into carbon monoxide and hydrogen and then liquefied. This is economically feasible only on very large scales. Hence, methane produced in small amounts at remote locations is either burned or not extracted. A promising alternative is conversion to benzene and hydrogen with molybdenumzeolite catalysts. Unfortunately, these catalysts deactivate because of carbon buildup; plus, hydrogen has to be removed to drive the reaction forward. Morejudo et al. address both of these problems with a solid-state BaZrO3 membrane reactor that electrochemically removes hydrogen and supplies oxygen to suppress carbon buildup. Science, this issue p. 563 Electrochemical processes extract hydrogen and maintain catalyst activity through oxygen injection. Nonoxidative methane dehydroaromatization (MDA: 6CH4 ↔ C6H6 + 9H2) using shape-selective Mo/zeolite catalysts is a key technology for exploitation of stranded natural gas reserves by direct conversion into transportable liquids. However, this reaction faces two major issues: The one-pass conversion is limited by thermodynamics, and the catalyst deactivates quickly through kinetically favored formation of coke. We show that integration of an electrochemical BaZrO3-based membrane exhibiting both proton and oxide ion conductivity into an MDA reactor gives rise to high aromatic yields and improved catalyst stability. These effects originate from the simultaneous extraction of hydrogen and distributed injection of oxide ions along the reactor length. Further, we demonstrate that the electrochemical co-ionic membrane reactor enables high carbon efficiencies (up to 80%) that improve the technoeconomic process viability.

Outstanding hydrogen permeation through CO2-stable dual-phase ceramic membranes

Mixed electronic- and protonic-conducting composites made up of physical mixtures of La5.5WO11.25−δ–La0.87Sr0.13CrO3−δ (LWO–LSC) have been evaluated as H2 separation membranes for operation at temperatures greater than 550 °C. The mixture of these two ion-conducting phases led to non-linear synergetic effects; i.e. unexpected enhancement of the total conductivity and well-balanced ambipolar conductivity, resulting in appealing H2 permeation fluxes through robust ceramic membranes. The preparation, primary characterization, H2 permeation and stability studies of various composites is presented. Mixing LWO and LSC phases makes it possible (1) to improve the LSC sintering behavior and to achieve very high membrane densities and (2) to obtain compounds with high total conductivity, higher than that shown for LWO and LSC, separately. The highest permeation rate is achieved for the 50 vol%-LWO–LSC membrane, though other composite compositions showed higher total conductivity. Moreover, the influence on the H2 permeation of the composite composition, the humidification of gas streams, temperature and the use of various catalytic coatings on the membrane surface is evaluated. The nature of the transport mechanism is investigated by the permeation studies using deuterium tracers. The H2 permeation rates reported in this investigation for a 370 μm thick 50 vol%-LWO–LSC membrane, e.g. 0.15 mL min−1 cm−2 at 700 °C, are the highest reported values, up to date, for any bulk mixed protonic-electronic membranes. The H2 permeation magnitude achieved at moderate temperatures along with the proven stability in CO2-rich atmospheres are firm steps towards the future application of this type of membrane for industrial processes.

Catalytic layer optimization for hydrogen permeation membranes based on La5.5WO11.25-δ/La0.87Sr0.13CrO3-δ composites

(LWO/LSC) composite is one of the most promising mixed ionic-electronic conducting materials for hydrogen separation at high temperature. However, these materials present limited catalytic surface activity toward H2 activation and water splitting, which determines the overall H2 separation rate. For the implementation of these materials as catalytic membrane reactors, effective catalytic layers have to be developed that are compatible and stable under the reaction conditions. This contribution presents the development of catalytic layers based on sputtered metals (Cu and Pd), electrochemical characterization by impendace spectroscopy, and the study of the H2 flow obtained by coating them on 60/40-LWO/LSC membranes. Stability of the catalytic layers is also evaluated under H2 permeation conditions and CH4-containing atmospheres.