Weishen Yang

Molecular Sieve Membrane: Supported Metal–Organic Framework with High Hydrogen Selectivity

Microporous membranes with pore apertures below the nanolevel can exhibit size selectivity by serving as a molecular sieve, which is promising for overcoming Robeson s “upperbound” limits in membrane-based gas separation. Zeolites, polymers of intrinsic microporosity (PIMs), metal oxides, and active carbon are the typical materials used for this purpose. Metal–organic frameworks (MOFs) have attracted much research interest in recent years, and are emerging as a new family of molecular sieves. MOFs are novel porous crystalline materials consisting of metal ions or clusters interconnected by a variety of organic linkers. In addition to promising applications in adsorptive gas separation and storage or in catalysis, their unique properties, such as their highly diversified structures, large range in pore sizes, very high surface areas, and specific adsorption affinities, make MOFs excellent candidates for use in the construction of molecular sieve membranes with superior performance. The preparation of MOF membranes for gas separation is rapidly becoming a research focus. A number of attempts have been made to prepare supported-MOF membranes; however, progress is very limited and so far there are only very few reports of continuous MOF films on porous supports being used as separating membranes. Recently, Guo et al. reported a copper-net-supported HKUST-1 (Cu3(BTC)2; BTC= benzene-1,3,5-tricarboxylate) membrane exhibiting a H2/N2 selectivity of 7 [13] (separation factor of H2 over N2 is calculated as the permeate-to-retentate composition ratio of H2, divided by the same ratio for N2 as proposed by IUPAC) ; this is the first MOF membrane to show gasseparation performance beyond Knudsen diffusion behavior. Very recently, Ranjan and Tsapatsis prepared a microporous metal–organic framework [MMOF, Cu(hfipbb)(H2hfipbb)0.5; hfipbb= 4,4’-(hexafluoroisopropylidene)bis(benzoic acid)] membrane by seeded growth on an alumina support. The ideal selectivity for H2/N2, based on single permeation tests, was 23 at 190 8C. This higher selectivity, compared to the report from Guo et al., might be a result of the smaller effective pore size (ca. 0.32 nm of MMOF versus 0.9 nm of HKUTS-1), which results in a relatively low H2 permeance of this MMOF membrane (10 9 molm 2 s Pa 1 at 190 8C). The authors attributed this finding to the blockage of the onedimensional (1D) straight-pore channels in the membrane. Therefore, with regard to H2 separation, small-pore MOFs having three-dimensional (3D) channel structures are considered to be ideal membrane materials. Zeolitic imidazolate frameworks (ZIFs), a subfamily of MOFs, consist of transition metals (Zn, Co) and imidazolate linkers which form 3D tetrahedral frameworks and frequently resemble zeolite topologies. A number of ZIFs exhibit exceptional thermal and chemical stability. Another important feature of ZIFs is their hydrophobic surfaces, which give ZIF membranes certain advantages over zeolite membranes and sol–gel-derived silica membranes in the separation of H2 in the presence of steam. Very recently we reported the first result from permeation measurements on a ZIF-8 membrane. The ZIF-8 membrane showed a H2/CH4 separation factor greater than 10. Whereas the ZIF-8 pores (0.34 nm) are slightly larger than the kinetic diameter of CO2 (0.33 nm), and are very flexible, the H2/CO2 separation on this ZIF-8 membrane showed Knudsen selectivity. In the current work, we therefore chose ZIF-7 as a promising candidate for the development of a H2-selective membrane to satisfy the above requirements. ZIF-7 (Zn(bim)2) is formed by bridging benzimidazolate (bim) anions and zinc cations with soladite (SOD) topology. The pore size of ZIF-7 (the hexagonal window size in the SOD cage) estimated from crystallographic data is about 0.3 nm, which is just in between the size of H2 (0.29 nm) and CO2 (0.33 nm). We could therefore expect a ZIF-7 membrane to achieve a high selectivity of H2 over CO2 and other gases through a molecular sieving effect. In many cases, it was reported that the heterogeneous nucleation density of MOF crystals on ceramic supports is very low, 14] which makes it extremely difficult to prepare supported-MOF membranes by an in situ synthesis route. Chemical modifications of substrate surfaces have been proposed to direct the nucleation and orientation of the deposited MOF layers. Based on our knowledge in the development of zeolite membranes, we adopted a seeded secondary growth method for the ZIF-7 membrane prepara[*] Prof. Dr. Y.-S. Li, F.-Y. Liang, H. Bux, A. Feldhoff, Prof. Dr. J. Caro Institute of Physical Chemistry and Electrochemistry and the Laboratory for Nano and Quantum Engineering (LNQE) in cooperation with the Center for Solid State Research and New Materials, Leibniz Universit t Hannover Callinstrasse 3A, 30167 Hannover (Germany) Fax: (+49)511-762-19121 E-mail: yanshuo.li@pci.uni-hannover.de juergen.caro@pci.uni-hannover.de

Metal-organic framework nanosheets as building blocks for molecular sieving membranes

Layered metal-organic frameworks would be a diverse source of crystalline sheets with nanometer thickness for molecular sieving if they could be exfoliated, but there is a challenge in retaining the morphological and structural integrity. We report the preparation of 1-nanometer-thick sheets with large lateral area and high crystallinity from layered MOFs. They are used as building blocks for ultrathin molecular sieve membranes, which achieve hydrogen gas (H2) permeance of up to several thousand gas permeation units (GPUs) with H2/CO2 selectivity greater than 200. We found an unusual proportional relationship between H2 permeance and H2 selectivity for the membranes, and achieved a simultaneous increase in both permeance and selectivity by suppressing lamellar stacking of the nanosheets. Crystalline sheets exfoliated from layered metal-organic framework materials are formed into selective membranes. Metal-organic framework material membranes There continues to be a lot of interest in developing membranes for gas separations that go beyond the current polymer membranes used commercially for this purpose. Peng et al. took a porous metal-organic framework material with a layered structure and exfoliated it to give nanometer-thick molecular sieves. The membranes were exceptionally good at separating hydrogen gas from carbon dioxide both in terms of permeance and selectivity. Science, this issue p. 1356

Controllable Synthesis of Metal–Organic Frameworks: From MOF Nanorods to Oriented MOF Membranes

Tailoring of the crystal size and morphology of metal-organic framework (mof) materials and manipulation of mof films is possible by the solvothermal synthesis route introduced here. a c-out-of-plane zif-7 membrane (see figure) is obtained through evolutionary selection in a van der drift-type growth originating from randomly oriented seed layers. highly oriented mof thin films are important as molecular sieve membranes.

Ba effect in doped Sr(Co0.8Fe0.2)O3-δ on the phase structure and oxygen permeation properties of the dense ceramic membranes

BaxSr1-xCo0.8Fe0.2O3-δ (x=0∼1.0) composite oxides were prepared by a combined EDTA and citrate complexing method. The doping effect of Ba on the phase structure, phase stability and oxygen permeation properties of the mixed conductors was investigated with combined XRD, H2-TPR, O2-TPD techniques and oxygen permeation studies. It was found that the perovskite structure was sustained in the whole range of the oxygen activity investigated (0.21∼10−6 atm) when 0.3≤x≤0.5. But for the materials with a differed doping content of Ba (x=0∼0.1, 0.7∼1.0), a phase transition was accompanied which was deleterious to the integrality of the membrane during operation. The relationship of phase structure and stability with tolerance factor was proposed. The doping of Ba in Sr(Co0.8Fe0.2)O3−δ could not lead to the improvement of these materials in the resisting of reduction, but the synergetic effect between cobalt and iron in the material was improved. Considerable high oxygen permeation rates were found for the BSCFO membranes in the whole range of Ba doping content. The Ba0.3Sr0.7Co0.8Fe0.2O3−δ membrane had the highest oxygen permeation flux, (1.19 ml/cm2 min for 1.50 mm thickness membrane at 850°C), the lowest activation energy for oxygen transportation (37.8 KJ/mol at the temperature range of 731°C∼950°C), and the lowest critical temperature for the change in the activation energy (731°C).

An Organophilic Pervaporation Membrane Derived from Metal–Organic Framework Nanoparticles for Efficient Recovery of Bio-Alcohols†

adsorption of water before the onset of capillary condensation. [12] All the above-mentioned characteristics suggest that ZIF-8 nanoparticles could be used as fillers in mixed-matrix membranes (MMMs) for the recovery of organic compounds from aqueous solutions by adopting organophilic pervaporation (OPV) technology. Pervaporation is a membrane process based on a sorption–diffusion mechanism, and is considered the most promising technology for molecular-scale liquid/ liquid separations. [13] Herein we show that both pervaporation

Synthesis of a high-permeance NaA zeolite membrane by microwave heating

Zeolite membranes with high permeance and separation factors are highly desirable for practical applications. Although, in the past, very good separation factors have been obtained, it has proved difficult to achieve a high permeance. Ken a comparative study of microwave versus conventional heating in the hydrothermal synthesis of NaA zeolite membranes is made. It is demonstrated that membranes prepared by microwave heating have not only a higher permeance but also a considerably shorter synthesis time. These observations are rationalized by examining the mechanism of membrane formation.

Investigation of ideal zirconium-doped perovskite-type ceramic membrane materials for oxygen separation

Zirconium-doped perovskite-type membrane materials of BaCo0.4Fe0.6−x Zrx O3−δ (x = 0–0.4) with mixed oxygen ion and electron conductivity were synthesized through a method of combining citric and EDTA acid complexes. The results of X-ray diffraction (XRD), oxygen temperature-programmed desorption (O2-TPD) and hydrogen temperature-programmed reduction (H2-TPR) showed that the incorporation of proper amount of zirconium into BaCo0.4Fe0.6O3−δ could stabilize the ideal and cubic structure of perovskite. Studies on the oxygen permeability of the as-synthesized membrane disks under air/He gradient indicated that the content of zirconium in these materials had great effects on oxygen permeation flux, activation energy for oxygen permeation and operation stability. The high oxygen permeation flux of 0.90 ml cm −2 min −1 at 950 ◦ C, the single activation energy for oxygen permeation in the range of 600–950 ◦ C and the long-term operation stability at a relatively lower operational temperature of 800 ◦ C under air/He gradient were achieved for the BaCo0.4Fe0.4Zr0.2O3−δ material. Meanwhile, the effect of carbon dioxide on structural stability and oxygen permeability of this material was also studied in detail, which revealed that the reversible stability could be attained for it.

Oxygen permeation study in a tubular Ba0.5Sr0.5Co0.8Fe0.2O3-δ oxygen permeable membrane

Dense tubular Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCFO) membranes were successfully prepared by the plastic extrusion method. The oxygen permeation flux was determined at different oxygen partial pressures in the shell side and different temperatures between 700 and 900 degreesC. The oxygen vacancy diffusion coefficients (Dv) at different temperatures were calculated from the dependence of oxygen permeation flux on the oxygen partial pressure term based on the surface current exchange model. No unsteady-state of oxygen permeation flux was observed at the initial stage in our experiments. The reason is the equilibrium time is too short (less than 10 min) to observe the unsteady-state in time. The increase of the helium flow rate can increase the oxygen permeation flux, which is due to the decrease of the oxygen partial pressure in the tube side with increasing of the helium flow rate. The oxygen permeation flux can also be affected by the air flow rate in the shell side when the air flow rate is lower than 150 ml/min. But the oxygen permeation flux is insensitive to the air flow rate when the air flow is higher than 150 ml/min. The membrane tube was operated steadily for 150 It with oxygen permeation flux of 1.12 ml/(cm(2) min) at 875 degreesC. X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) analysis showed that both the surface exposed to air and the surface exposed to helium of the BSCFO membrane tube after permeation for 150 h are similar to the fresh membrane tube in composition and structure. These results indicated that the membrane tube exhibits high structure stability

Novel and ideal zirconium-based dense membrane reactors for partial oxidation of methane to syngas

A novel and ideal dense catalytic membrane reactor for the reaction of partial oxidation of methane to syngas (POM) was constructed from the stable mixed conducting perovskite material of BaCo0.4Fe0.4Zr0.2O3−δ and the catalyst of LiLaNiO/γ-Al2O3. The POM reaction was performed successfully. Not only was a short induction period of 2 h obtained, but also a high catalytic performance of 96–98% CH4 conversion, 98–99% CO selectivity and an oxygen permeation flux of 5.4–5.8 ml cm−2 min−1 (1.9–2.0 μmol m−2 S−1 Pa−1) at 850 °C were achieved. Moreover, the reaction has been steadily carried out for more than 2200 h, and no interaction between the membrane material and the catalyst took place.

Investigation on POM reaction in a new perovskite membrane reactor

A perovskite-type oxide of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCFO) shows mixed (electronic/oxygen ionic) conductivity at high temperatures. Membrane made of the oxide has high oxygen permeability under air/helium oxygen partial pressure gradient. At 850 ◦ C, oxygen permeation rate maintained about 1.15 ml/cm 2 min for more than 1000 h under ambient air/helium oxygen gradient. A membrane reactor constructed from the oxide membrane was applied for the partial oxidation of methane (POM) to syngas, LiLaNiOx/-Al2O3 with 10 wt.% Ni loading was used as the packed catalyst. At the initial stage, oxygen permeation rate, methane conversion and CO selectivity were closely related with the state of the catalyst. Less than 21 h was needed for the oxygen permeation rate to reach its steady state. A membrane reactor made of BSCFO was successfully operated for POM reaction at 875 ◦ C for about 500 h without failure, with methane conversion of >97%, CO selectivity of >95% and oxygen permeation rate of about 11.5 ml/cm 2 min. Under membrane reaction condition, the POM reaction mechanism was suggested to obey the CRR (complete combustion of CH4 to CO2 and H2O and a subsequent reforming reaction of the residual CH4 and with CO2 and H2 Ot o CO and H 2) mechanism.