Investigation of the mechanism for the separation of nitrogen—oxygen mixtures on carbon molecular sieves

Sorption separation of nitrogen-oxygen mixtures by carbon molecular sieves, e.g. to produce nitrogen using the pressure swing adsorption principle, proceeds under the condition of non-equilibrium. Experimental data based upon measurements of sorption equilibria and kinetics for nitrogen and oxygen on several samples of carbon molecular sieve particles in the temperature range 0–50°C suggest the existence of a surface barrier in the pore mouths of slit-like micropores for these particular samples. The conclusion on this phenomenon is supported by the results of canonical and grand canonical Monte Carlo simulations. Both types of analysis give a deeper insight into the complex non-equilibrium separation process.     

EXAFS studies on MFI‐type gallosilicate molecular sieves

EXAFS spectroscopy shows that gallium atoms incorporated into the framework of MFI‐type ZSM‐5 gallosilicates have a typical Ga–O bond length of 1.82 ± 0.01 Å corresponding to a tetrahedral coordination. Accordingly, the coordination number, N, was found to take the value N = 4.2 ± 0.3. Template burning (at 773 K) causes little disturbance to the zeolite framework, and most of the gallium remains tetrahedrally coordinated. However, after a heat treatment at 923 K the apparent coordination number was found to decrease, as well as the corresponding Ga–O bond length. These results were interpreted in terms of formation of extra‐framework gallium oxide species which appear to be EXAFS silent. The thermal stability of the gallosilicate framework was found to increase with increasing Si/Ga ratio.     

Synthesis and characterization of boron‐containing molecular sieves

The synthesis, physico-chemical characterization and catalytic activity of boron-containing molecular sieves (boralites) are reviewed and discussed. Incorporation of boron in the silica framework of several phases has been assessed by structural and spectroscopic evidence. Compared with the aluminosilicate parent compounds, boralites display weaker acid strength sufficient, however, for catalyzing several reactions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Catalytic monoepoxidation of butadiene over titanium silicate molecular sieves TS‐1

Titanium silicate molecular sieves TS‐1 have been prepared under hydrothermal conditions with tetrapropylammonium hydroxide as structure‐directing agent. The structures of TS‐1 were characterized by means of XRD, TEM, FT‐IR, DR UV‐Vis, ICP‐AES, and N₂ adsorption/desorption techniques. Its catalytic performances in the epoxidation of butadiene with hydrogen peroxide were investigated; 3,4‐epoxy‐1‐butene was obtained with high selectivity. The influences of various parameters such as temperature, solvent, and titanium content on the activities of the catalysts were studied systematically. This revised version was published online in July 2006 with corrections to the Cover Date.     

Production of carbon molecular sieves by plasma treated activated carbon fibers☆

Carbon molecular sieves (CMS) are valuable materials for the separation and purification of gas mixtures. In this work, plasma deposition was used aiming to the formation of pore constrictions, by narrowing the surface pore system of commercial activated carbon fibers (ACF). For this reason propylene/nitrogen or ethylene/nitrogen discharges of 80 and 120 W were used. The molecular sieving properties of the plasma treated ACF were evaluated by measuring the adsorption of CO₂ and CH₄. The CO₂/CH₄ selectivity was significantly improved and depended on plasma treatment conditions (discharge gas and power). The optimum CO₂/CH₄ selectivity (26) was observed for C₂H₄/N₂ plasma treated ACF at 80 W. Sample scanning electron microscopy (SEM) analysis after plasma treatment revealed an external film formation and X-ray photoelectron spectroscopy (XPS) analysis showed the incorporation of nitrogen functional groups in the film, which probably interact with CO₂, thereby altering CO₂/CH₄ selectivity.     

Catalytic application of mesoporous molecular sieves to vapor‐phase Beckmann rearrangement of cyclohexanone oxime

The vapor‐phase Beckmann rearrangement of cyclohexanone oxime to ɛ‐caprolactam catalyzed by mesoporous molecular sieves was studied. The proton‐form mesoporous molecular sieves, H‐MCM‐41 and H‐FSM‐16, exhibited extremely high activity, selectivity and stability for ɛ‐caprolactam formation when 1‐hexanol was used as diluent. The weak acid sites and/or surface silanol groups of mesoporous molecular sieves play an important role in the selective ɛ‐caprolactam formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Breakthrough curves for single solutes in beds of activated carbon with a broad pore-size distribution—II. Measuring and calculating breakthrough curves of phenol and indole from aqueous solutions on activated carbons

The breakthrough curves of phenol and indole from aqueous solutions on four activated carbons have been measured under a range of experimental conditions. We obtained a break-time ranking of the carbons coinciding with the equilibrium capacities. The breakthrough curves are simulated using the two-zone surface diffusion model, and it is found that the measurements can be modelled applying the adsorption capacity values from equilibrium measurements if empty bed contact times exceed 4 min. The kinetic parameters estimated by curve-fitting are influenced by particle crushing, thus indicating the importance of macropore diffusion resistance. The two-zone model was necessary to describe diffusion in biporous active carbons, whereas in the case of a carbon with a narrow pore-size distribution a monopore model was sufficient for breakthrough curve simulation.     

Study on the reactivity of ultrafine cerium–molybdenum oxide particles in the absence of molecular oxygen

The cracking, hydrocracking and hydrodesulfurization activities of gallium–aluminum mixed oxide catalysts, with fluoride added as promoter, were investigated. The addition of fluoride to the Ni–Mo catalyst supported on the mixed oxides leads to a significant enhancement in the cracking (no metals) and the hydrocracking (Ni–Mo) reactions of the catalysts, respectively. This enhancement of activity is attributed to the promotional effect of fluoride and the presence of gallium in the support, which leads to catalysts that are more resistant to deactivation. The results of the hydrodesulfurization experiments are not so encouraging. The activity of the catalysts was depressed by the addition of fluoride and with increasing gallium oxide content of the support. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dissociation of molecular oxygen on unpromoted and cesium‐promoted Ag(111) surfaces

Density functional calculations for the dissociation of molecular oxygen on a Ag₁₀ cluster model (seven surface and three subsurface atoms) of the [111] surface of silver are presented. The calculations show that the reaction pathway for the dissociation of molecular oxygen depends on the amount of cesium present on the surface. For the unpromoted surface, dissociation should have a low rate as the process is activated. As a single Cs atom is added to the cluster, O₂ should readily dissociate since the process is kinetically and thermochemically favored. However, as a second Cs atom is added, the dissociation of O₂ is quenched as the process is highly activated. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of β-nucleating agent and zeolite molecular sieves on supercritical CO2-assisted cell and crystal nucleation during the long-chain-branched polypropylene foaming

It is of great interest and importance to design and fabricate the double crystal melting peak (DCMP) structure for the expanded polypropylene (EPP) bead foams during sintering. Two kinds of methods with different mechanisms were proposed to prepare long-chain-branched polypropylene (LCBPP) samples with a DCMP structure: One was that the DCMP structure was prepared by generating crystals of different perfection degrees and the other one was that the DCMP structure was prepared by adding β-NA to produce various crystal forms. In addition, zeolite molecular sieve (ZMS) as the synergist was introduced into LCBPP/β-NA system to increase the efficiency of β-NA. The crystallization behaviors of various PP and their foams were tested by differential scanning calorimetry and wide-angle X-ray diffraction, as well as the cellular morphologies of various LCBPP foams were observed by scanning electron microscope. The results showed that a DCMP structure was formed in pure LCBPP in the presence of CO₂ at 168–172 °C. The content of β-crystal in LCBPP/β-NA system was the highest when the content of β-NA optimized as 1.5 phr. The percentage of β-crystal in LCBPP/β-NA system was further increased when a small content (0.045 phr) of ZMS was added. Compared with that in unfoamed LCBPP, the percentage of β-crystal in LCBPP foams was obviously enhanced, due to the coupling effect of crystal nucleation and cell nucleation. In the LCBPP foams, the cell size became smaller and the cell density became larger, owing to the addition of ZMS. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48183.     

X-ray absorption spectroscopy of Ti-containing molecular sieves ETS-10, aluminum-free Ti-β, and TS-1

We report here the first structural characterization by Ti K-edge EXAFS and XANES of ETS-10 and aluminum-free Ti- molecular sieves. A TS-1 sample was also studied for comparison. The near-edge spectrum of TS-1 revealed an intense pre-edge feature characteristic of Ti atoms in tetrahedral coordination. However, the pre-edge peak for ETS-10 was quite small and shifted in energy, which is consistent with the known octahedral structure surrounding Ti atoms in the material. Analysis of the EXAFS for ETS-10 revealed an average Ti-O interatomic distance of 2.00 +- 0.01 Å which is longer than the Ti-O distance (1.95Å) in anatase TiO₂. For Ti-, an intense pre-edge peak is present in the near-edge spectrum and the intensity, energy and width of this peak are the same as for TS-1. In addition, the average Ti-O interatomic distance in Ti- and TS-1 was 1.80 +- 0.01 Å, which is significantly shorter than the distance measured in ETS-10. The EXAFS and XANES results indicate that the Ti sites in aluminum-free Ti- are structurally identical to the tetrahedral sites in TS-1. This conclusion is supported by the similarity of the UV absorption thresholds for the two samples. X-ray absorption spectra were also recorded with methanol or 2-propanol adsorbed on the Ti- and TS-1 molecular sieves. Alcohol adsorption decreased the intensity and broadened the Ti pre-edge peak for both samples, demonstrating a local chemical interaction with the Ti sites. Methanol adsorption lengthened the average Ti-O bond to 1.83 ± 0.01 Å. The similarity of the X-ray absorption spectra for aluminum-free Ti- and TS-1 in the presence of alcohols suggests that the Ti sites in the two frameworks are chemically indistinguishable for vapor-phase adsorption of alcohols at low concentrations.     

Modification of side chain terminals of PEGylated molecular bottle brushes—A toolbar of molecular nanoobjects

Water soluble molecular brush with poly(glycidyl methacrylate) (PGMA) as the main chain and mono-allyl terminated poly(ethylene glycol) (PEG) as the densely grafted side chains was efficiently synthesized via grafting-onto approach by copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Then highly efficient post-modification of the ends of the side chains was realized through UV light induced thiol–ene radical addition. Thus polymer brushes with various side chain terminal functional groups including hydroxyl group, carboxyl group, amino group, amino acid, sulfonic group and glucose unit were obtained.     

Effects of oxygen partial pressure on the thermodynamics of CaO–SiO2–VOx system at 1873 K

In this study, two aspects concerning with the thermodynamics of CaO–SiO₂–VO_x system were investigated. One aspect is about the effects of oxygen partial pressure on the phase relationships in CaO–SiO₂–VO_x system at 1873 K, and the other is the measurement of the standard Gibbs energy of formation of the vanadium compounds. In the first aspect, the phase relationships in CaO–SiO₂–VO_x system at 1873 K under oxygen partial pressure of 1.7 × 10⁻⁹ atm were determined, and the isothermal section diagram was constructed. Furthermore, by comparing this diagram with that under 6.9 × 10⁻¹¹ atm, the effects caused by oxygen partial pressure on this system were elucidated. With the increase in oxygen partial pressure, the compound CaV₂O₄ that exists stably under 6.9 × 10⁻¹¹ atm is oxidized to V₂O₃ and CaVO₃. The solubility limit of V₂O₃ in liquid phase also increases remarkably, which results in the enlargement of the single liquid area. In the second aspect, the standard Gibbs energy of formation of CaV₂O₄, CaVO₃, V₂O₃, and Ca₂Si_1−δV_δO₄ (0 < δ < 0.1) at 1873 K were measured. The determined values for CaV₂O₄ and CaVO₃ are −1 259 468.9 ± 5090.5 J/mol and −963 479.8 ± 3298.6 J/mol, respectively. Then, the limit of oxygen partial pressure for CaV₂O₄ existing stably at 1873 K is determined to be 4.4 × 10⁻¹⁰ atm.     

Experimental measurements of effective diffusion coefficient of oxygen–nitrogen mixture in PEM fuel cell diffusion media

PEM fuel cells are increasingly designed to operate at high current densities. At these densities, mass transport limitations become very significant, but they are not well understood, with many modeling studies but few experimental observations. The use of accurate transport coefficients to simulate the mass transport at high current densities is crucial. In this study, experimental measurements have been carried out to determine the effective diffusion coefficient in the carbon paper gas diffusion layer that is commonly used in PEM fuel cells. It was found that almost all the existing theoretical models significantly overpredict the effective diffusion coefficient by as much as 4–5 times; thus, underestimating the transport limitations considerably. Further, the effects of temperature, Teflon treatment for hydrophobicity and porosity on the effective diffusion coefficient were investigated. It was found that temperature does not affect the overall diffusibility of the gas. The diffusibility is decreased with the increase of Teflon treatment and decrease in porosity. Further work on better understanding the diffusion process in the gas diffusion layer is under way.     

Effect of molecular weight on molecular orientation and morphology of polypropylene sheets containing a β-nucleating agent

In this study, polypropylene (PP) films containing the β nucleating agent, N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide, were prepared using PP with three different molecular weights (low, medium, and high) by extrusion process with T-shaped die. The structure and morphology of the films were studied after stretching. It was found that a unique molecular orientation, in which both the c-axis and crystalline lamellae were oriented perpendicular to the flow direction, was formed in all undrawn film samples, irrespective of the molecular weights of the PP. In the PP sheets stretched in the machine direction, the low-molecular-weight sample containing the nucleating agent exhibited brittle properties owing to a lack of tie chains in the stretching direction. In contrast, cavitation was prominent in the medium (M-PP)- and high (H-PP)-molecular-weight samples. Notably, M-PP containing the nucleating agent, with a high degree of molecular orientation, promoted the formation of a large number of voids. In H-PP containing the nucleating agent, the presence of numerous tie chains inhibited cavitation, resulting in fewer voids. The experimental results demonstrated the influence of the molecular weight on the void structure, which will be useful in the field of microporous membranes.     

Unusual oxygen affinity of linear polyvinyl amine–cobalt complexes with hydroxyl counter ions: an efficient way of separation and chemical storage of molecular oxygen

Methylated poly(vinyl amine) and its copolymer with diallyldimethylammonium chloride (DADMAC) were prepared by polymerization of N-vinyl formamide (NVF) or copolymerization with DADMAC. Acid hydrolysis of the polymers and subsequent methylation yielded corresponding polyamines. Co(II) complexes of the polyamines were demonstrated to be reasonably fast in molecular oxygen binding, when counter anions of cobalt are exchanged with hydroxyl ions. The oxygenation tests performed with oxygen sensor, FT-IR, and gas-volumetric methods revealed that complexes either with 4/1 or with 2/1 [amine]/[Co] ratios give 1:1 molecular oxygen adducts. Fast oxygenation (c.a. 2–4 min), high storage capacity, and smooth oxygen releasing within 70–120 °C are typical characteristics of the oxygenation–deoxygenation. The system seems to be useful for harvesting and chemical storage of air oxygen.     

Epoxidation of alkenes with molecular oxygen catalyzed by a manganese porphyrin-based metal–organic framework

The flexible nature of reticular assemblies and high specific surfaces of metal–organic frameworks (MOFs) offers new opportunities for the design of heterogeneous catalysts capable of industrially relevant reactions. We demonstrate the first instance of alkene epoxidation at mild conditions using molecular oxygen by a manganese porphyrin containing MOF, MOF-525-Mn [Zr₆O₄(OH)₄(MgC₄₈H₂₄O₈N₄Cl)₃]. This zirconium MOF with a manganese porphyrin catalyst shows minimal deactivation over long periods and maintains its structure and high activity after multiple catalytic cycles. Kinetic studies of styrene epoxidation are in agreement with theoretical and experimental studies of homogeneous reactions with the same porphyrin unit, suggesting that the heterogeneous catalyst operates according to a similar mechanism as its homogeneous counterpart.     

Dielectric investigations on ‘MgAlON’ compounds: role of nitrogen content

The dielectric properties of dense polycrystalline magnesium aluminium oxynitride have been investigated up to 90 °C. The oxygen/nitrogen substitution on the anionic lattice of this solid solution enhances the charge trapping ability and the dielectric breakdown of ‘MgAlON’ compounds. Oxygen vacancies, which play the role of electron traps, can partially explain the good dielectric properties of this solid solution at low temperature; but they are ineffective for higher temperature where the charge trapping phenomenon remains active. This behaviour, specific to ‘MgAlON’ solid solution, is explained on the basis of a crystallographic model of atom repartition: this spinel structure offers rich-nitrogen zones randomly located as AlN₄ clusters which generate locally the heterogeneous zones of electric charge supposed to be responsible for the good dielectric performance of ‘MgAlON’ compounds. The local polarizability is modified which enhances the charge-trapping phenomenon.