Ethylene is an important building block in the chemical industry; state of the art ethylene production (steam cracking) has multiple drawbacks, including high energy consumption, coke formation, and ...significant CO2 and NO x emissions. We propose a chemical looping oxidative dehydrogenation (CL-ODH) process to convert ethane into ethylene in a two-step, cyclic redox scheme. In this process, lattice oxygen in a metal oxide based redox catalyst is used to combust the hydrogen formed in ethane dehydrogenation, thereby enhancing ethylene formation while retarding coke formation. The oxygen-deprived redox catalyst is subsequently regenerated with air, releasing heat to balance the overall heat requirement. CL-ODH can realize a reduction of over 80% in primary energy consumption and pollutant emissions. The key to this process is an efficient redox catalyst with high selectivity and facile oxygen transport. Previously we determined that oxides with an Mg6MnO8 structure allow high lattice oxygen mobility and satisfactory oxygen-carrying capacity for the proposed redox reactions. However, unpromoted Mg6MnO8 exhibits poor ethylene selectivity, producing primarily CO2. In the current study, we examine the effects of various sodium-containing promoters on Mg6MnO8 CL-ODH activity and mechanism. Sodium tungstate promoted Mg6MnO8 was the most effective redox catalyst, showing an ethylene selectivity of 89.2% and yield of 68.2%, a significant improvement of thermal cracking (38.9% yield). Temperature-programmed reaction (TPR) experiments indicate that the reaction proceeds via gas-phase ethane thermal cracking in parallel with selective hydrogen combustion on the redox catalyst surface. XPS analysis indicates that the decreased ethane/ethylene oxidation activity on the sodium tungstate promoted redox catalysts results from the suppression of near-surface Mn4+. This is due to a combination of decreased surface manganese content and reduction in average Mn oxidation state. The suppression of Mn4+ results in a decrease in electrophilic surface oxygen species, inhibition of ethylene combustion, and enhanced ethylene yield.
The sorption enhanced steam reforming (SESR) technology has the potential to produce high purity hydrogen by Le Chatelier's principle. However, its practical applicability is limited by sorbent ...sintering and deactivation at high reaction/decarbonation temperatures. Herein, we propose a novel strategy to enhance the stability of the SESR of glycerol (SESRG), in which misfit layered materials, i.e. calcium cobaltates (CCO), were used as a dual-functional material combining CO 2 absorption and catalytic reforming. Differing from the conventional approach of enhancing the robustness of catalysts/sorbents, we exploited the reversible phase change of CCO: Ca 3 Co 4 O 9 ↔ Co + CaO, during the decarbonation and reaction steps respectively. By doing so, the sintering of the CaO sorbent and the Co catalyst could be suppressed because they were homogenized into CCO on an atomic level in every decarbonation stage. The CCO catalyst displayed a very stable performance for producing high purity H 2 through SESRG for up to 120 reaction–decarbonation cycles, without noticeable changes in H 2 production and CO 2 absorption capacity. In situ XRD and microscopy studies demonstrated the reversible phase transition and the accompanied formation of hierarchical CCO micro-structures that facilitated the catalytic reforming and CO 2 absorption, benefited from the complex phase equilibria among different CCO compounds. The results in this study shed light on a new paradigm for the design of materials working at high temperatures thus suffering from serious sintering.
A microfluidic strategy is developed for continuous synthesis of monodisperse yolk–shell titania microspheres. The continuous flow synthesis of titania microparticles is achieved by decoupling the ...microdroplet formation and interfacial hydrolysis reaction steps by utilizing a polar aprotic solvent as the continuous phase in the microreactor. The decoupling of the precursor microdroplet formation and the hydrolysis reaction allows titania synthesis throughputs an order of magnitude higher than those previously reported in a single-channel flow reactor (∼0.1 g/h calcined microparticles), without affecting the microreactor lifetime due to clogging. Flow synthesis and dynamics across a broad range of precursor flow rates are examined, while effects of flow synthesis parameters, including the precursor to continuous phase flow rate ratio, precursor composition, and calcination temperature on the surface morphology, size, and composition of the resulting titania microparticles, are explored in detail. Titania microparticle size can be controlled by variation in the precursor to continuous phase flow rate ratio. The surface morphology and porosity of the in-flow synthesized titania microparticles can be varied by adjusting the precursor composition, while the crystalline phase can be tuned by varying the calcination temperature.
Oxygen evolution from water driven by visible light is one of the key reactions for solar fuel production. In this paper, we investigated the effect of the support on photocatalytic water oxidation ...under visible light using cobalt oxide as a multielectron catalyst. A range of supported Co3O4 nanoclusters have been successfully synthesized via wet impregnation and bisolvent methods. Compared with the wet impregnation approach, the bisolvent method allowed us to obtain a high quality catalyst with all the Co3O4 nanoclusters formed inside the mesoporous support using hexane/water as the combination. The resulting catalyst consists of Co3O4 nanoclusters with a very small particle size (∼25 nm) and narrow size distribution. Catalytic water oxidation experiments were performed in Ru(bpy)32+-persulfate (photochemical) and Ce4+/Ce3+ (chemical) systems, and it was found that smaller Co3O4 cluster sizes resulted in higher water oxidation activity. In addition, KIT-6 was found to be a better support than SBA-15, which is likely due to the fact that the 3D porous structure of KIT-6 offers more accessible pores than the 1D channels in SBA-15. To further elucidate the role of support in the photocatalytic oxygen evolution, bare Co3O4 nanoparticles together with two SiO2- and γ-Al2O3-supported ones were investigated. Photocatalytic studies show that both supported Co3O4 nanoparticles exhibited significant enhancement (50–80%) in oxygen evolution activity, compared with bare Co3O4 nanoparticles. However, switching from the SiO2 to γ-Al2O3 support does not significantly change the activities, indicating composition and surface properties of the support do not participate in rate-limiting steps in oxygen evolution. It can be concluded that the major role of catalyst supports in Co3O4-based water oxidation catalysts is to provide a medium to physically separate Co3O4 nanoclusters from aggregation, leading to superior photocatalytic activities.
The present study investigates the effect of sodium and tungsten promoters on Mg6MnO8-based redox catalysts in a chemical looping oxidative dehydrogenation (CL-ODH) scheme. CL-ODH has the potential ...to significantly lower energy consumption and CO2/NO x emissions for ethylene production compared with conventional steam cracking. Sodium tungstate (Na2WO4) was previously shown to be an effective promoter for Mg6MnO8-based redox catalysts. Overall, the CL-ODH reaction proceeds via parallel gas-phase cracking of ethane and selective combustion of H2 on the surface of the Na2WO4-promoted redox catalyst. Reaction testing indicates that both Na and W are necessary to form Na2WO4 and to achieve high ethylene selectivity. A Na:W ratio lower than 2:1 lead to significant formation of additional mixed tungsten oxide phases and decreases ethylene selectivity. Further characterizations based on low-energy ion scattering (LEIS) and differential scanning calorimetry (DSC) indicate that the NaW promoter forms a molten shell around the Mg6MnO8 redox catalyst. Methanol TPSR and in situ DRIFTS experiments indicate that the promoter significantly suppresses the number of basic sites on Mg6MnO8. 18O–16O exchange experiments reveal that the promoter decreases the rate of oxygen exchange. O2 cofeed studies indicate that below the melting temperature of Na2WO4, H2 and CO conversions are both inhibited, but above the melting temperature, H2 combustion significantly increased while CO combustion is still inhibited. On the basis of extensive characterizations, it was determined that H2 is primarily combusted at the gas–Na2WO4 molten shell interface via redox reactions of the tungsten salt, likely between the WO4 2– (tungstate) and WO3 – (tungsten bronze).
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•MoO3/Fe2O3 core-shell redox catalysts prepared.•Calcined catalysts have surface layers comprising Fe2(MoO4)3 and MoO3.•Core-shell catalysts exhibit enhanced ethane CL-ODH ...performance.•Mixed Mo-Fe oxide shell suppresses combustion and increases ODH activity.
Oxidative dehydrogenation (ODH) of ethane offers large reductions in energy consumption and associated greenhouse gas emissions when compared to conventional steam cracking for ethylene production; however, catalytic ODH of ethane using co-fed O2 requires expensive air separation. As an alternative, we are investigating novel core-shell catalysts that utilize lattice oxygen (O2−) as the sole oxidant and operate in a cyclic redox mode. In this work, redox catalysts having 1, 3 and 6 monolayer (ML) equivalents of MoO3 on α-Fe2O3 and a stoichiometric ferric molybdate, Fe2(MoO4)3, were prepared, characterized by powder x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), and temperature-programmed reduction (TPR) and evaluated for ethane ODH in a cyclic redox mode at 600 °C. The characterization data are consistent with a core-shell structure for the calcined MoO3/Fe2O3 catalysts with a mixed Mo-Fe oxide surface layer. H2 and ethane TPR evidence that the shell inhibits Fe2O3 reduction and decreases the ethane combustion activity of the fully oxidized catalyst. Covering the Fe2O3 core with MoO3 also increases ODH activity and ethylene selectivity. In cyclic redox mode at 600 °C, ethylene selectivity was 57–62% for catalysts with 3 and 6 ML equivalents of MoO3.
Oxygen evolution from water is one of the key reactions for solar fuel production. Here, two nanostructured K‐containing δ‐MnO2 are synthesized: K‐δ‐MnO2 nanosheets and K‐δ‐MnO2 nanoparticles, both ...of which exhibit high catalytic activity in visible‐light‐driven water oxidation. The role of alkaline cations in oxygen evolution is first explored by replacing the K+ ions in the δ‐MnO2 structure with H+ ions through proton ion exchange. H‐δ‐MnO2 catalysts with a similar morphology and crystal structure exhibit activities per surface site approximately one order of magnitude lower than that of K‐δ‐MnO2, although both nanostructured H‐δ‐MnO2 catalysts have much larger Brunauer–Emmett–Teller (BET) surface areas. Such a low turnover frequency (TOF) per surface Mn atom might be due to the fact that the Ru2+(bpy)3 sensitizer is too large to access the additional surface area created during proton exchange. Also, a prepared Na‐containing δ‐MnO2 material with an identical crystal structure exhibits a TOF similar to that of the K‐containing δ‐MnO2, suggesting that the alkaline cations are not directly involved in catalytic water oxidation, but instead stabilize the layered structure of the δ‐MnO2.
Alkaline‐cation‐containing δ‐MnO2 nanosheets and nanoparticles are fabricated, and both exhibit a high catalytic activity in visible‐light‐driven water oxidation. The alkaline‐cation‐containing δ‐MnO2 exhibit activities per surface site approximately one order magnitude higher than a H‐δ‐MnO2 catalyst with a similar morphology and crystal structure. The alkaline cations are not directly involved in the catalytic water oxidation, but stabilize the MnO2 layers.
The current study demonstrates a redox oxide @ molten salt core-shell architecture as a generalized redox catalyst design strategy for chemical looping – oxidative dehydrogenation of ethane. 17 ...combinations of redox active oxides and molten salts were prepared, evaluated, and characterized. X-ray diffraction indicates that the redox oxides and molten salts are fully compatible, forming separate and stable phases. X-ray photoelectron spectroscopy demonstrates that the molten salts aggregate at the redox oxide surface, forming a core-shell structure to block the non-selective sites responsible for COx formation. Up to ∼74 % single-pass olefin yields were achieved using the proposed redox catalyst design strategy. Statistical analyses of the performance data indicate the potential to achieve up to 86.7 % single-pass yield by simply optimizing the operating conditions using the redox catalysts reported in this study. Meanwhile, the generalizability of the catalyst design strategy offers exciting opportunities to further optimize the composition and performance of the redox catalysts for ethane ODH under a chemical looping scheme with significantly reduced energy consumption and CO2 emissions.
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