On-stream modification of MFI zeolite membranes for enhancing hydrogen separation at high temperature

-Alumina-supported MFI zeolite membranes were modified by on-stream catalytic thermal cracking of methyldiethoxysilane (MDES) molecules inside the zeolitic channels during the separation of H₂/CO₂ gas mixture at 450 °C and atmospheric pressure. The MDES vapor was carried by the H₂/CO₂ feed gas and the effect of modification was monitored continuously through online analysis of the permeate stream. The modified membrane exhibited a significant increase in H₂ selectivity over CO₂ with a moderate decrease in H₂ permeance. At 450 °C, the modified MFI membrane obtained a H₂/CO₂ permselectivity of 17.5 with H₂ single gas permeance of 1.86 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ as compared to a permselectivity of 2.78 and permeance of 2.75 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ for the membrane before modification. The modified membrane also showed good performance and stability in separation of H₂/CO₂ gas mixture containing up to 28.4% water vapor at 450 °C and atmospheric pressure.     

Sorption kinetics of ammonia and ammonium ions on gel and macroporous sulphonic acid cation exchangers

Diffusion of ammonia and ammonium ions in sulphonic acid cation exchangers (gel Purolite SGC 100 × 10 MBH and macroporous Purolite C 160 MBH) from the solutions, representing the composition of “caustic condensate” (waste of nitrogen fertilizers production) is affected by pH of initial solution and structure of the matrix of cation exchanger. In gel matrix the effective intraparticle diffusivity (D_ef) depends greatly on the solution pH because of shrinkage in alkaline and swelling in acidic medium: on decreasing the initial concentration of ammonia from 0.214 to 0.003 and increasing that of ammonium nitrate from 0 to 0.214 mol l⁻¹ instead, the effect of ion exchange leads to a decrease in pH, resulting in swelling and increase in D_ef from 0.1 to 0.34 × 10⁻¹⁰ for gel Purolite SGC 100 × 10 MBH and variation of 0.18–0.11 × 10⁻¹⁰ m² s⁻¹ for macroporous Purolite C 160 MBH (resistant to shrinkage and swelling).

In Purolite C 160 MBH both macropore diffusivity (0.07–0.29 × 10⁻¹⁰ m² s⁻¹) and gel (solid phase) diffusivity (0.06–0.19 × 10⁻¹⁰ m² s⁻¹) are higher than micropore diffusivity (0.28–0.56 × 10⁻¹⁸ m² s⁻¹).

With respect to the effective intraparticle diffusivity, resistance to nitric acid, used for the regeneration, and high concentration of ammonium nitrate in eluate (up to 110 g l⁻¹), Purolite C 160 MBH has been installed for the conversion of ammonia and ammonium ions to ammonium nitrate reusable in the fertilizers production. This allows minimizing the economic loss and preventing the environmental contamination.     

Kinetics of hydrogen oxidation reaction on Nafion-coated Pt/C electrodes under high overpotentials

The influences of the Nafion film thickness and Pt loading on the kinetics of the hydrogen oxidation reaction on Nafion-coated 20 wt% Pt/C electrodes immersed in 0.5 M H₂SO₄ were investigated using a rotating disk electrode configuration. The coating of a Nafion film (8 μm) had a negligible effect on the electrochemical surface area of an electrode. The kinetic parameters were estimated at an overpotential of 0.4 V; the values obtained were shown to vary with the method of data treatment. The diffusional resistance for H₂ in the Nafion film was negligible when the film was thinner than 0.2 μm. The permeability of H₂ in the Nafion film ranged from 2.4 × 10⁻⁵ to 4.8 × 10⁻⁵ mM cm²/s. The error analysis demonstrated that the apparent kinetic current estimated was resulted from experimental errors, instead of resulting from a chemical process as proposed by some previous investigators.     

Poly-(3-methylthiophene)/carbon nanotubes hybrid composite-modified electrodes

The preparation of poly-(3-methylthiophene)—multi-walled carbon nanotubes hybrid composite electrodes is reported. The hybrid electrode shows a synergic effect of the electrocatalytic properties, and high active surface area of both the conducting polymer and carbon nanotubes, which gives rise to a remarkable improvement of oxidation of NADH with respect to polymer-modified electrodes, and CNTs-modified electrodes. SEM showed that carbon nanotubes served as nanosized backbone for P3MT electropolymerization. The amperometric NADH detection at +300 mV provided fast responses, a range of linearity between 5.0 × 10⁻⁷ and 2.0 × 10⁻⁵ mol l⁻¹, and a detection limit of 1.7 × 10⁻⁷ mol l⁻¹, which compares advantageously with those reported for other NADH CNT-based amperometric sensors. Furthermore, the direct electrochemistry of cytochrome c and FAD at the hybrid electrode is also checked.     

Reduction of nitrite by sulfamic acid and sodium azide from aqueous solutions treated by gliding arc discharge

Nitrous and nitric acids form in aqueous solutions exposed to a gliding arc discharge burning in humid air. The anions interfere when the concentration of particular solutes such as pollutants must be determined. In particular they falsify the COD measurements and spectral investigations and thus the efficiency of the plasma treatment in pollutant abatement. The nitrite anions must be thus removed, which require specific reagents. The influence of parameters such as solution pH and [reducers]/[NO₂⁻] ratio on the reduction reaction was evaluated. The reduction of nitrite into N₂ either by sulfamic acid or sodium azide is a first-order pH-dependant reaction with regard to nitrite and reducers (k₁ = 2.93 × 10⁻¹ m³ kmol⁻¹ s⁻¹; k₂ = 6.21 × 10⁻¹ m³ kmol⁻¹ s⁻¹, respectively). Sodium azide is thus more reactive than sulfamic acid.     

Nano-gold supported on TiO2 coated glass-fiber for removing toxic CO gas from air

A process for coating a layer of TiO₂ on the surface of glass fiber (Pyrex) was developed to support nano-gold on the fiber. The sol–gel method was utilized. The solution was composed of tetrapropyl titanate (TPT), isopropanol (i-PrOH), HCl and H₂O. The XRD pattern indicated that TiO₂ was in its anatase form after the coated fiber was calcined at 450 °C. In the preparation of nano-gold on the fibers (coated with TiO₂), deposition was performed in a pH-adjusted gold chloride solution. The catalytic activities of the resulting fibers were examined by the oxidation of CO in an air stream at room temperature. The gold containing fibers dried at room temperature contained less metallic gold and exhibited poorer CO oxidation activity than did those dried at 60 °C Moreover, the catalytic activities of the fibers depended on the gold concentration during deposition. Therefore, the gold fibers from the solution with gold concentrations of 2 × 10⁻⁴ M exhibited better CO oxidation activity than those from the solutions with concentrations of 1 × 10⁻³ and 0.7 × 10⁻⁴ M. TEM and A.A. analysis show that different concentrations of the gold solution were associated with different particle sizes and different gold loadings on the fibers, and therefore different catalytic activities of the fibers (per unit weight of fibers). 0.1 g of fibers prepared from the 2 × 10⁻⁴ M gold solution removed all CO from the air stream (containing 1% CO at a flow rate 110 cm³/min) at room temperature, approximately meeting the European Community EN403 (1993) standard for a qualified CO gas mask material.     

Electrodeposition of cesium at mercury electrodes in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide room-temperature ionic liquid

The electrochemistry of cesium was investigated at mercury electrodes in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide (Bu₃MeN⁺Tf₂N⁻) room-temperature ionic liquid (RTIL) by using cyclic staircase voltammetry, rotating disk electrode voltammetry, and chronoamperometry. The reduction of cesium ions at mercury exhibits quasireversible behavior with k⁰ = 9.8 × 10⁻⁵ cm s⁻¹ and = 0.36. The diffusion coefficient of Cs⁺ in this RTIL was 1.04 × 10⁻⁸ cm² s⁻¹ at 303 K. Bulk deposition/stripping experiments conducted at a rotating mercury film electrode gave an average recovery of 97% of the electrodeposited Cs. The density, absolute viscosity, and equivalent conductance of Bu₃MeN⁺Tf₂N⁻ were measured over the range of temperatures from 298 to 353 K. A polynomial equation describing the temperature dependence of the density is presented. Both the viscosity and conductance exhibited the non-Arrhenius temperature dependence typical of glass-forming liquids. The ideal glass transition temperature and the activation energies for viscosity and conductance were obtained by fitting the Vogel–Tammann–Fulcher (VTF) equation to the experimental data for these transport properties.     

Immobilized TiO2 photocatalyst during long-term use: decrease of its activity

When TiO₂ is immobilized on organic fibres, pumice stone or polymer film, the photocatalytic efficiency decreases slowly during long-term use. The efficiency of immobilized photocatalysts were tested on 500 ml of a 5×10⁻⁵ M solution of acid orange-7 (a classical azo dye) before and after treatment of 10⁻³ M solution of acid orange-7 during 4 weeks. It was observed that the efficiency was reduced approximately four, five and 10 times with polymer film containing TiO₂, TiO₂ on organic fibres and TiO₂ on pumice stone, respectively, after 4 weeks of use. Volumes treated were 40, 45 and 60 l, respectively. Nevertheless, the decomposition rate stays a little higher with TiO₂ on pumice stone than with the two other catalysts tested. The photocatalytic activity of immobilized TiO₂ was significantly reduced also during treated with wastewaters. For immobilized photocatalysts used, the decrease of activity is considered to be caused by the elimination of some particles from the catalyst surface during use and also by fouling of catalyst surface by the formation of by-products during the course of degradation process.     

Temperature dependence of electrical and electromechanical properties of Pb(Mg1/3Nb2/3)O3–Pb(Ni1/3Nb2/3)O3–Pb(Zr,Ti)O3 thin films

The electrical and electromechanical properties of Pb(Mg_1/3Nb_2/3)O₃–Pb(Ni_1/3Nb_2/3)O₃–Pb(Zr,Ti)O₃ (PMN–PNN–PZT, PMN/PNN/PZT = 20/10/70) on Pt/Ti/SiO₂/Si substrates by chemical solution deposition was investigated. The PMN–PNN–PZT films annealed at 650 °C exhibited slim polarization hysteresis curves and a high dielectric constant of 2100 at room temperature. A broad dielectric maximum at approximately 140–170 °C was observed. The field-induced displacement was measured by scanning probe microscopy, the bipolar displacement was not hysteretic, and the effective piezoelectric coefficient (d₃₃) was 66 × 10⁻¹² m/V. The effective d₃₃ decreased with temperature, but the value at 100 °C remained 45 × 10⁻¹² m/V.     

Kinetic determination of pyrogallol by a novel oscillating chemical reaction catalyzed by a tetraazamacrocyclic complex

A new analytical method for the determination of pyrogallol, by the perturbation caused by different amount of pyrogallol on a novel oscillation system is reported. This novel oscillation system, which is of Belousov–Zhabotinskii-type, involves a macrocyclic complex [CuL](ClO₄)₂ as catalyst and malic acid as the substrate. The ligand L in the complex is 5,7,7,12,14,14-hexemethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene. The experimental results show that the change in the amplitude of the potentiometric oscillation is linearly proportional to the logarithm of the concentration of pyrogallol in the range 1.5 × 10⁻⁶ to 1 × 10⁻⁴ M, with a correlation coefficient of 0.9959. The obtained relative standard deviation (R.S.D.) with eight samples is 1.6%. Some aspects of the potential mechanism of action of pyrogallol on the oscillating chemical system are also discussed.     

Preparation of methylene blue-doped silica nanoparticle and its application to electroanalysis heme proteins

In this paper, methylene blue (MB)-doped silica nanoparticles (NPs) were prepared in a reverse microemulsion and used as a novel matrix for biochemical application. By controlling the experiment conditions, MB was successfully captured in a three-dimensional cage of the silica network. The silica matrix, with hydrophilic groups, provides a biocompatible microenvironment for heme proteins retaining their native conformation and biological activity. The chemical and electrochemical stability of MB-doped silica NPs makes the organic–inorganic nanocomposite be a promising platform for bioelectrochemical analysis. Conjugated with chitosan, the MB-doped silica NPs were modified on the surface of glassy carbon (GC) electrode. The developed electrode exhibited the remarkable catalytic activity for the reduction of hemoglobin (Hb) and cytochrome c (Cyt c). The catalytic reduction peak currents for chitosan/MB-doped silica NPs were linearly proportional to the concentrations of Hb and Cyt c. The calculated detection limits (S/N = 3) were 4.5 × 10⁻⁹ mol/L for Hb and 5.0 × 10⁻⁸ mol/L for Cyt c, respectively.     

Effect of Pt on the water resistance of Co-zeolites upon the SCR of NOx with CH4

The objective of this work was to study the promotional effect of Pt on Co-zeolite (viz. mordenite, ferrierite, ZSM-5 and Y-zeolite) and Co/Al₂O₃ on the selective catalytic reduction (SCR) of NO_x with CH₄ under dry and wet reaction stream. After being reduced in H₂ at 350°C, the PtCo bimetallic zeolites showed higher NO to N₂ conversion and selectivity than the monometallic samples, as well as a combination of the latter samples such as mechanical mixtures or two-stage catalysts. After the same pretreatment, under wet reaction stream, the bimetallic samples were also more active. Among the other catalysts studied with 5% of water in the feed, (NO = CH₄ = 1000 ppm, O₂ = 2%), the NO conversion dropped to zero over Co_2.0Mor at 500°C and GHSV = 30,000 h⁻¹, whereas it is 20% in Pt_0.5Co_2.0Mor. In Pt/Co/Al₂O₃ the NO_x conversion dropped below 5% with only 2% of water under the same reaction conditions. The specific activity given as molecules of NO converted per total metal atom per second were 16.5 × 10⁻⁴ s⁻¹ for Pt_0.5Co_2.0Fer, 13 × 10⁻⁴ s⁻¹ for Pt_0.5Co_2.0Mor, 4.33 × 10⁻⁴ s⁻¹ for Pt_0.5Co_2.0ZSM-5 and 0.5 × 10⁻⁴ s⁻¹ for Pt/Co/Al₂O₃. The Y-zeolite-based samples were inactive in both mono and bimetallic samples. The species initially present in the solid were Pt° and Co°, together with Co²⁺ and Pt²⁺ at exchange positions. Co° seems not to participate as an active site in the SCR of NO_x. Those species remained after the reaction but some reorganization occurred. A synergetic effect among the different species that enhances both the NO to NO₂ reaction, the activation of CH₄ and also the ability of the catalyst to adsorb NO, could be responsible for the high activity and selectivity of the bimetallic zeolites.     

Adsorption and diffusion of light alkanes on nanoporous faujasite catalysts investigated by molecular dynamics simulations

Molecular dynamics simulations were performed for ethane, propane, and n-butane in siliceous faujasite for different numbers of molecules per unit cell (loadings) at 300 K. Both the adsorbed molecules and the zeolite framework were modeled as flexible entities. A new semiempirical analytical potential function for the systems was constructed. From the mean-square displacement of the molecules, self-diffusion coefficients of 18.7 × 10⁻⁵, 13.3 × 10⁻⁵, and 4.3 × 10⁻⁵ cm²/s were calculated for ethane, propane, and n-butane, respectively at a loading of 8 molecules/unit cell. They compare well with experimental values from pulsed-field gradient NMR measurements (10 × 10⁻⁵, 9 × 10⁻⁵, and 6 × 10⁻⁵ cm²/s, respectively). Besides depending on the size of the hydrocarbon, the heats of adsorption and self-diffusion coefficients also strongly depend on the loading of adsorbate molecules. The results suggest that the new intermolecular force field can reasonably describe the adsorption and diffusion behavior of ethane, propane, and n-butane in faujasite zeolite.     

Synthesis and proton conductivity of sulfonic acid functionalized zeolite BEA nanocrystals

The objective of this study is to prepare sulfonic acid functionalized zeolite BEA nanocrystals, and determine the bulk proton conductivity of this new material. Phenethyl functionalized zeolite BEA nanocrystals are synthesized using a mixture of fumed silica and phenethyltrimethoxysilane as silica sources. Contact of the phenethyl zeolite BEA nanocrystals with concentrated sulfuric acid removes the organic structure-directing agent (SDA) and simultaneously sulfonates the phenethyl moieties. Detailed characterizations of the zeolite BEA nanocrystals are performed using dynamic light scattering (DLS), Fourier transform Raman spectroscopy (FT-Raman), ²⁹Si cross-polarization magic angle spinning nuclear magnetic resonance spectroscopy (²⁹Si CPMAS NMR), ¹³C CPMAS NMR, ¹H MAS NMR, thermogravimetric (TGA) analyses, transmission electron microscopy (TEM) and powder X-ray diffraction (XRD). Sulfonic acid functionalized zeolite BEA nanocrystals have proton conductivities in the range of 1.2 × 10⁻³–1.2 × 10⁻² S/cm compared to 1.5 × 10⁻⁴ S/cm for unfunctionalizated zeolite BEA.     

New polyaniline(PAni)-polyelectrolyte (PDDMAC) composites: Synthesis and applications

Conducting and electroactive polymer blends of polyaniline (PAni) with polyelectrolyte, poly(diallydimethylammoniumchloride) (PDDMAC) have been synthesized by an in situ polymerization method and the resulting composites have been characterized by FT-IR, UV–Vis, XRD, AFM and electrochemical techniques. The blends are conducting and electroactive with even lower loadings of PAni and can be cast as films. The conductivity of the cast films containing 0.04–1.5 wt% PAni ranged from 4.5 × 10⁻⁶ to 42 × 10⁻⁶ S/cm. Some of the composites are tested for their corrosion inhibition property for pure iron in 1 M HCl solutions and were found to be active inhibitors.     

Selective permeation and separation of steam from water–methanol–hydrogen gas mixtures through mordenite membrane

Separation properties of a mordenite membrane for water–methanol–hydrogen mixtures were studied in the temperature range from 423 to 523 K under pressurized conditions. The mordenite membrane was prepared on the outer surface of a porous alumina tubular support by a secondary-growth method. It was found that water was selectively permeated through the membrane. The separation factor of water/hydrogen and water/methanol were 49–156 and 73–101, respectively. Even when only hydrogen was fed at 0.5 MPa, its permeance was as low as 10⁻⁹ mol m⁻² s⁻¹ Pa⁻¹ up to 493 K, possibly suggesting that water pre-adsorbed in the micropores of mordenite hindered the permeation of hydrogen. The hydrogen permeance dramatically increased to 6.5 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at 503 K and reached to 1.4 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ at 523 K because of the formation of cracks in the membrane. However, the membrane was thermally stabilized in the presence of steam and/or methanol.     

Phase transition phenomena of the adsorbed 1-dodecanol monolayer at the air–water interface

Phase transition phenomenon of the 1-dodecanol monolayer at the air/water interface was studied by the dynamic γ(t) curves and the adsorption isotherm obtained by ellipsometry at 20 °C. The surface-concentration adsorption isotherm clearly showed three abrupt increases at bulk concentration C of 1.3 × 10⁻⁹, 2 × 10⁻⁹ and 3.7 × 10⁻⁹ mol/mL, respectively. The 1st and the 3rd transitions observed herein, that were typical 2D first-order transitions, were consistent with the gas to liquid expanded (G–LE) and the liquid expanded to liquid condensed (LE–LC) phase transitions observed in a previous tensiometry study. The 2nd transition that occurred at C = 2 × 10⁻⁹ mol/mL was not identified from any previous dynamic surface-tension profiles. Judging from the substantial increase in the film thickness of the transition, it was believed that the orientation change of the adsorbed molecule was involved in the LE phase. A LE_h and a LE_v phase, that denoted the “lie-down” and “stand-up” types of adsorption, respectively, was used to describe this transition and a cusp, instead of a constant surface-tension region, was observed in the dynamic γ(t) curves for this transition. This suggested that, since the surface tension varied during the transition process, the newly identified LE_h and LE_v transition might not be the typical first-order type of phase transition.     

Liquid-phase methanol synthesis in apolar (squalane) and polar (tetraethylene glycol dimethylether) solvents

The kinetics of the three-phase methanol synthesis over a commercial Cu–Zn–Al₂O₃ catalyst were studied in an apolar solvent, squalane and a polar solvent, tetraethylene glycol dimethylether (TEGDME). Experimental conditions were varied as follows: P=3.0–5.3 MPa, T=488–533 K and Φ_vG/w=7.5×10⁻³–8×10⁻³ Nm³ s⁻¹kg⁻¹_cat. The nature of the slurry–liquid influences the activation energy and the kinetic rate constant by interaction between adsorbed species and solvent and by competitive adsorption of the solvent on the catalyst surface. The rate of reaction to methanol observed in TEGDME appeared to be about 10 times lower than in squalane. TEGDME reduces the reaction rate, which is a disadvantage for its use as a solvent.     

High flux mesoporous MCM-48 membranes: Effects of support and synthesis conditions on membrane permeance and quality

This communication reports experimental efforts to synthesize defect-free mesoporous MCM-48 membranes with improved gas flux. We demonstrate a facile and inexpensive method of synthesizing defect-free supported MCM-48 membranes with improved N₂ and CO₂ permeance (>2 × 10⁻⁷ mol/m² s Pa) employing asymmetric supports for the membrane synthesis which contain layers of macropores possessing different pore sizes. The membranes prepared on asymmetric -alumina supports displayed higher gas permeance than those fabricated on symmetric supports (N₂ permeance <10⁻⁷ mol/m² s Pa) as confirmed by unsteady-state gas permeation experiments. Further improvement in gas permeance was achieved by covering one face and the sides of the support with a ceramic tape during membrane synthesis.     

Electrocatalytic reduction of nitrate by foreign Pb adatoms

Au/Pb(ad) electrodes were prepared by the underpotential deposition of lead ions at various potentials in 1 M HClO₄ solution containing Pb(CH₃COO)₂ at concentrations of about 1 × 10⁻², 1 × 10⁻⁴ and 1 × 10⁻⁶ M, respectively. The best preparation condition was selected. These electrodes, modified by foreign metal atom were used to catalyze the reduction of nitrate in concentrated alkaline solution, and exhibited efficient electrocatalytic activity for this system.