Yanying Wei

A Two-Dimensional Lamellar Membrane: MXene Nanosheet Stacks

Two-dimensional (2D) materials are promising candidates for advanced water purification membranes. A new kind of lamellar membrane is based on a stack of 2D MXene nanosheets. Starting from compact Ti3 AlC2 , delaminated nanosheets of the composition Ti3 C2 Tx with the functional groups T (O, OH, and/or F) can be produced by etching and ultrasonication and stapled on a porous support by vacuum filtration. The MXene membrane supported on anodic aluminum oxide (AAO) substrate shows excellent water permeance (more than 1000 L m-2  h-1  bar-1 ) and favorable rejection rate (over 90 %) for molecules with sizes larger than 2.5 nm. The water permeance through the MXene membrane is much higher than that of the most membranes with similar rejections. Long-time operation also reveals the outstanding stability of the MXene membrane for water purification.

Water Transport with Ultralow Friction through Partially Exfoliated g-C3N4 Nanosheet Membranes with Self-Supporting Spacers

Two-dimensional (2D) graphitic carbon nitride (g-C3 N4 ) nanosheets show brilliant application potential in numerous fields. Herein, a membrane with artificial nanopores and self-supporting spacers was fabricated by assembly of 2D g-C3 N4 nanosheets in a stack with elaborate structures. In water purification the g-C3 N4 membrane shows a better separation performance than commercial membranes. The g-C3 N4 membrane has a water permeance of 29 L m-2  h-1  bar-1 and a rejection rate of 87 % for 3 nm molecules with a membrane thickness of 160 nm. The artificial nanopores in the g-C3 N4 nanosheets and the spacers between the partially exfoliated g-C3 N4 nanosheets provide nanochannels for water transport while bigger molecules are retained. The self-supported nanochannels in the g-C3 N4 membrane are very stable and rigid enough to resist environmental challenges, such as changes to pH and pressure conditions. Permeation experiments and molecular dynamics simulations indicate that a novel nanofluidics phenomenon takes place, whereby water transport through the g-C3 N4 nanosheet membrane occurs with ultralow friction. The findings provide new understanding of fluidics in nanochannels and illuminate a fabrication method by which rigid nanochannels may be obtained for applications in complex or harsh environments.

Oxidative Coupling of Methane with High C2 Yield by using Chlorinated Perovskite Ba0.5Sr0.5Fe0.2Co0.8O3−δ as Catalyst and N2O as Oxidant

Natural gas with methane as the main constituent is a natural resource that exceeds the reserves of oil in abundance. With inevitable depletion of crude oil, methane will become the major resource for chemicals and liquid fuels in future decades. Extensive efforts have been afforded to both direct and indirect conversion of methane. The direct-conversion approaches involve, for example, the selective oxidation of methane to methanol or formaldehyde, the oxidative coupling of methane (OCM) to C2 species (C2H6 and C2H4) and so on. Since the pioneering work of Keller and Bhasin, much research has been carried out into developing highly efficient catalysts for OCM. Unfortunately, the per pass C2 yields on nearly all reported catalysts are less than 30 %. OCM is believed to occur through a heterogeneous–homogeneous pathway. The mechanism proposes first the catalytic formation of methyl groups on catalyst surface, which desorb as free radicals (CH3·) into the gas phase and ultimately react by predominantly homogeneous pathways. 8] It is generally agreed that some oxygen species O* adsorbed on the catalyst surface are the active oxygen species for the activation of methane to form methyl radicals and play a key role in the selective generation of C2, whereas gaseous oxygen molecules mainly favor the formation of COx. [9–12] By assuming a reaction mechanism via methyl radicals formed by the reaction of methane with surface oxygen species, an upper limit of 28 % for the C2 yield is forecasted for a continuous, CH4/O2 co-fed, single pass process under conventional conditions, such as those used for laboratory screening. The inherent reason for such a limited C2 yield is that the gaseous oxygen—as required oxidant for OCM—can react with the more reactive C2 product to form COx and, thus, a higher C2 selectivity is always compromised with a lower CH4 conversion. [14] To attain a high C2 yield, it is necessary to reduce the nonselective gaseous oxygen concentration. Perovskite oxides with mixed conductivities have been attracting increasing attention due to their potential applications in air separation, as catalysts for selective oxidation, and as reactors for the partial oxidation of hydrocarbons or for other oxygen-involving reactions. Moreover, in the past decade, halide-modified oxides have been widely studied for oxidative dehydrogenation of ethane (ODE) to ethylene and were proven to offer a significant improvement in the performance of the selective oxidation. In this study, a novel chlorinedoped Ba0.5Sr0.5Fe0.2Co0.8O3 d (denoted as BSCFCls) was used as catalyst for the OCM in a fixed-bed reactor. Furthermore, N2O was chosen as the oxygen source because it is a relatively weak oxidant, which is suitable for selective oxidation. Herein, we want to show that (a) the introduction of Cl into Ba0.5Sr0.5Fe0.2Co0.8O3 d enhances the selectivity for the methyl radical formation and (b) N2O as oxidant can effectively reduce gaseous oxygen concentration, since it provides oxygen in the form of surface oxygen species rather than gas-phase oxygen molecules by decomposition on the catalyst surface. Figure 1 shows the concept of OCM on perovskite oxide catalysts using N2O as oxidant.

Introduction of metal precursors by electrodeposition for the in situ growth of metal–organic framework membranes on porous metal substrates

We report here a facile and efficient electrodeposition method to modify inexpensive porous stainless-steel nets for use as substrates in the in situ growth of metal–organic framework membranes, such as ZIF-8, ZIF-67 and HKUST-1. Using this method, different metal precursors can be electrodeposited depending on the central metals required in the target metal–organic frameworks. The inorganic modifiers prepared by this approach are sufficiently reactive for the one-step growth of continuous metal–organic framework membranes; their reactivity is comparable with that of organic functional groups. The procedure is also green and cost-effective, which is promising for use in large-scale production.

Tuning the separation performance of hydrogen permeable membranes using an anion doping strategy

Overcoming the dilemma between hydrogen permeability and stability is critical for realizing the widespread application of mixed protonic–electronic conducting (MPEC) membranes. Herein, fluoride-anion doping is for the first time reported for tuning the separation performance of MPEC membranes. Lanthanum tungstate oxyfluoride membranes, La5.5W0.6Mo0.4O11.25−δFx (x = 0, 0.025, 0.05, 0.10, 0.20, 0.50), exhibit improved hydrogen permeability and enhanced stability compared to their parent oxides, achieving a maximum value of 0.20 mL min−1 cm−2 at x = 0.05. Moreover, the declining hydrogen permeability performance of lanthanum tungstate MPEC membranes during high-temperature operation was systematically analyzed and relative solutions are put forward. The anion-doping and stability-improving strategies might accelerate the development and future practical applications of MPEC membranes.

CHLORINE-DOPED Ba0.5Sr0.5Co0.8Fe0.2O3-δ AS AN OXYGEN-PERMEABLE MEMBRANE AT INTERMEDIATE TEMPERATURE

A novel chlorine-doped oxygen permeable membrane based on Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is developed and shows an improved oxygen permeation flux at intermediate temperature compared to un-doped BSCF. X-ray photoelectron spectroscopy indicates the introduction of chlorine element weakened the metal-oxygen bond and increased the mobility of lattice oxygen in BSCF.

Selective gas diffusion in two-dimensional MXene lamellar membranes: insights from molecular dynamics simulations

Membrane gas separation has become increasingly important for modern industry, and the emerging two-dimensional (2D) lamellar membranes provide unprecedented possibilities to overcome the well-known permeability–selectivity trade-off of traditional membranes. However, the 2D materials currently available for lamellar membrane fabrication are very limited, and relevant experimental or simulation studies are scarce. Consequently, the understanding of gas diffusion in 2D nanochannels, though critical for developing more efficient lamellar membranes, is still quite poor. Very recently, we fabricated a 2D MXene lamellar membrane with exceptional gas separation performance (L. Ding, Y. Wei, L. Li, T. Zhang, H. Wang, J. Xue, L. X. Ding, S. Wang, J. Caro and Y. Gogotsi, Nat. Commun., 2018, 9, 155), and the gas transportation mechanism was studied thoroughly with Molecular Dynamics (MD) simulations in this work. The diffusion of different gases, such as H2, He, CH4, CO2 and N2, was simulated in 2D MXene nanogalleries with structural factors (e.g., interlayer distance and intercalating water) adjusted systematically. These gases were found to diffuse in the nanogalleries mainly via two mechanisms, activated diffusion and Knudsen diffusion. The main features of both diffusion mechanisms were discussed through studying the simulation trajectories carefully. The simulations also revealed that the MXene membrane structure significantly influenced the gas diffusion, such as the mechanism, diffusivity, and selectivity, and could thus be tuned to boost the gas separation performance as our recent experiment did. This simulation work provides a detailed microscopic understanding of gas diffusion in 2D nanochannels, and useful guidance for developing new 2D lamellar gas separation membranes with high performance.

Paralyzed membrane: Current-driven synthesis of a metal-organic framework with sharpened propene/propane separation

ZIF-8 membranes with inborn-suppressed linker mobility sharpen molecular sieving with C3H6/C3H8 separation factor above 300. Metal-organic framework (MOF) membranes show great promise for propene/propane separation, yet a sharp molecular sieving has not been achieved due to their inherent linker mobility. Here, zeolitic imidazolate framework ZIF-8–type membranes with suppressed linker mobility are prepared by a fast current–driven synthesis (FCDS) strategy within 20 min, showing sharpened molecular sieving for propene/propane separation with a separation factor above 300. During membrane synthesis, the direct current promotes the metal ions and ligands to assemble into inborn-distorted and stiffer frameworks with ZIF-8_Cm (a newly discovered polymorph of ZIF-8) accounting for 60 to 70% of the membrane composition. Molecular dynamics simulations further verify that ZIF-8_Cm is superior to ZIF-8_I 4¯3m (the common cubic phase) for propene/propane separation. FCDS holds great potential to produce high-quality, ultrathin MOF membranes on a large scale.