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.     

Synthesis and permeation properties of ion-exchanged ETS-4 tubular membranes

Zeolite membranes, which were composed of ETS-4 with Na cations, were prepared on porous -alumina tubes by hydrothermal synthesis. The membranes, which were formed under optimized conditions, sharply rejected molecules with sizes larger than 0.4 nm. For mixtures of N₂–CO₂, N₂–O₂, N₂–Ar and N₂–CH₄ systems, N₂ permeated faster than the coexisting gas. The N₂/O₂ separation factor for an equimolar mixture was in the range of 2.3–3.5, and the N₂ permeance was in the range of (0.55–2.8)×10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ at permeation temperatures of 283–333 K. Moisture had some effect on the permeation properties for N₂–O₂ mixtures. The separation factor for the N₂/CH₄ system was larger than that of the N₂/O₂ system. When the membrane was ion exchanged with either Li⁺ or Sr²⁺, the separation factors for N₂/O₂ and N₂/CH₄ systems increased, while the permeances decreased.     

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.     

Influence of exchanged cations (Na, Cs, Sr and Ba) on xylene permeation through ZSM-5/SS tubular membranes

Na-ZSM-5 membranes were synthesized by secondary growth on the outer surface of stainless steel porous tubes. The membranes were ion-exchanged with Cs⁺, Ba²⁺ and Sr²⁺ to investigate their effect upon the separation of p-xylene from m-xylene and o-xylene. The permeation through the membranes was measured between 150 and 400 °C using each xylene isomer separately and a ternary mixture. All the membranes were selective to p-xylene in the temperature range studied. N₂ and xylene permeation measurements together with SEM observations were used to determine whether or not cracks and/or pinholes developed after exposure to the xylene isomers at high temperature (400 °C). Neither pore blockage nor extra-zeolitic pores developed after the ion exchange procedure and subsequent calcination. Furthermore, duplicate synthesized membranes of each cation form had similar separation factors and permeances. The duplicate values differ much less than the measurement error. The p-xylene permeation flux decreased in the order: Na-ZSM-5 > Ba-ZSM-5 > Sr-ZSM-5  Cs-ZSM-5 while the permeation flux of the m- and o-xylene decreased in the order Na-ZSM-5 > Sr-ZSM-5 > Ba-ZSM-5 > Cs-ZSM-5. The membrane that exhibited the best performance was Ba-ZSM-5, with a maximum p/o separation factor of 8.4 and a p-xylene permeance of 0.54 × 10⁻⁷ mol s⁻¹ m⁻² Pa⁻¹ at 400 °C.     

Catalyzing a cycloaddition with molecularly imprinted polymers obtained via immobilized templates

Polymeric catalysts to be applied in the Diels–Alder cycloaddition of hexachlorocyclopentadiene and maleic acid have been prepared via molecular imprinting with template molecules immobilized on silica particles. These enzyme mimicking polymers exhibit specific catalytic effects compared to non-imprinted control polymers or polymer-free solutions. It could be demonstrated that the activity of the molecularly imprinted material rises when increasing the temperature. By this means, the reduction of the activation energy (as expected for catalysts) from 63 to 55 kJ mol⁻¹ could be observed. Furthermore, the reaction was characterized based on the Michaelis–Menten model. For the diene compound a Michaelis constant of K_M=5.8 mmol l⁻¹ and an effective reaction rate of r_max,eff=0.4 μmol l⁻¹ s⁻¹, leading to a reaction rate constant k_eff=1.1×10⁻³ s⁻¹, were determined.     

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.     

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.     

Separation and enrichment of carbon dioxide by capillary membrane module with permeation of carrier solution

A novel facilitated transport membrane for gas separation using a capillary membrane module is proposed in which a carrier solution is forced to permeate the membrane. Both a feed gas and a carrier solution are supplied to the lumen side (high pressure side, feed side) of the capillary ultrafiltration membrane and flow upward. Most of the carrier solution which contains dissolved solute gas, CO₂ in the present case, permeates the membrane to the permeate side (low pressure side, shell side), where the solution liberates dissolved gas to form a lean solution. The lean solution is circulated to the lumen side. This type of capillary membrane module was applied to the separation of CO₂ from model flue gases consisting of CO₂ and N₂. Monoethanolamine (MEA), diethanolamine (DEA) and 2-amino-2-methyl-1-propanol (AMP) were used as carriers or absorbents of CO₂. The feed side pressure was atmospheric and the permeate side was evacuated at about 10 kPa. CO₂ in the feed gas was successfully concentrated from 5–15% to more than 98%. The CO₂ permeance was as high as 2.7×10⁻⁴ mol m⁻² s⁻¹ kPa⁻¹ (8.0×10⁻⁴ cm³ cm⁻² s⁻¹ cmHg⁻¹) when the CO₂ mole fraction in the feed was 0.1 and temperature was 333 K. The selectivity of CO₂ over N₂ was in the range from 430 to 1790. The membrane was very stable over a discontinuous one-month testing period.     

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.     

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.     

Pyrylium salt-photosensitised degradation of phenolic contaminants present in olive oil wastewaters with solar light: Part II. Benzoic acid derivatives

Solar light photodegradation, catalysed by a pyrylium salt, of seven benzoic acids present in olive oil mill, has been studied. Significant percentages of photodegradation (20–40%) have been achieved after 6 h of solar exposure for six of the acids, even though they were expected to be difficult to oxidise, due to the presence of an electron-withdrawing carboxylic acid group directly attached to the aromatic ring. Quenching constants for the electron-transfer process between the substrate and the excited catalyst were calculated by means of fluorescence measurements for syringic acid (66×10⁹ M⁻¹ s⁻¹), gallic acid (51×10⁹ M⁻¹ s⁻¹), veratric acid (51×10⁹ M⁻¹ s⁻¹), vanillic acid (48×10⁹ M⁻¹ s⁻¹), protocatechuic acid (37×10⁹ M⁻¹ s⁻¹) and p-hydroxybenzoic acid (15×10⁹ M⁻¹ s⁻¹); no quenching was found for benzoic acid. These photophysical measurements are in good correlation with the yields obtained in the pyrylium salt photocatalysed degradation of those phenolic acids.     

Probing cooperative jump-diffusion in zeolites: Neutron spin–echo measurements and molecular dynamics simulations of benzene in NaX

We report neutron spin–echo measurements of transport diffusivities for perdeuterated benzene in NaX zeolite. Corrected diffusivities from these neutron measurements were obtained for comparison with recently reported molecular dynamics simulations. Experimental diffusivities were measured with benzene loadings of 1, 2, 3 and 4.5 molecules per NaX cage, and at 300, 350 and 400 K. Both corrected and transport diffusivities increase from 1 to 3 molecules per cage, then decrease sharply to 4.5 molecules per cage. The comparison between experiment and simulation shows remarkably good agreement in both the loading dependence and in the overall magnitudes of the corrected diffusivities, with values mostly in the range 10⁻¹²–10⁻¹¹ m² s⁻¹.     

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.     

The condensation/polymerisation of dimethyl siloxane fluids in a three-phase trickle flow monolith reactor

For the production of siloxane fluids, the viability of using a multi-channel monolith as a catalyst support system in a three-phase reactor has been studied. The catalyst was tripotassium phosphate (K₃PO₄). Experiments were performed in a single-channel flow reactor (15 mm i.d. and 500 mm catalyst coated length). The rate of reaction was followed by monitoring the disappearance of the hydroxyl group (–OH). Reaction experiments were performed at a hydroxyl group concentration range from 150 to 170 mol m⁻³, T=373–413 K and P=7.9 kPa with a nitrogen purge. The maximum temperature of operation was restricted to 413 K to avoid the formation of undesirable by-products. In the regime controlled by chemical kinetics, reaction was of an apparent first order with respect to –OH concentration, and in the apparent rate constant, the pre-exponential factor was 4.19×10⁻⁴ ms⁻¹, and the apparent activation energy was 16.1 kJ mol⁻¹. These are only valid for the operating pressure and purge gas flowrate used, as both of these are shown to affect water removal from the liquid phase and, hence, reaction rates. Mass transfer coefficients from the liquid to the catalyst surface were estimated and these increased rapidly with flowrate and were higher than expected for a falling liquid film.     

Decomposition of NO over Cu-AITS-1 zeolites

H-AITS-1 zeolite with Si/Ti = 50 and Si/Al = 50 was employed in preparing catalyst samples by ion-exchange and impregnation with a copper nitrate solution to obtain 0.24–1.15 wt.% and 1.5, 2 and 2.5 wt.% Cu loading, respectively. The catalytic properties for the NO decomposition were compared with that of Cu-ZSM-5 (Si/Al = 25 with 2 wt.% Cu loading) and similarity was found between the AITS-1 based samples and Cu-ZSM-5. Due to the higher acidity, the activity at 500°C per total copper atoms (an apparent turnover frequency, TOF) was significantly higher over Cu based AITS-1 samples being 2–3 × 10⁻³ s⁻¹ as compared to 1 × 10⁻³ s⁻¹ measured on Cu-ZSM-5. For the ion-exchanged Cu-AITS-1 there was an increase in TOF with increasing copper content, whereas on the impregnated samples a decrease in TOF was found. On all catalysts there was a maximum in the NO conversion at 500–550°C. The amount of NO per copper atom measured by temperature programmed desorption (TPD) was about the same as that on Cu-ZSM-5 and the features of the TPD were also similar. At the first contact of the catalyst at 500°C with the 2 vol% NO/Ar gas a transient N₂O formation and a considerable delay in the O₂ formation was observed. This could, however, be reproduced only on fresh catalyst, while all further transients showed different but reproducible features using the same sample.     

Ion conduction in crosslinked β-cyclodextrin gels

Gels of crosslinked β-cyclodextrin have been prepared using dimethylacetamide containing lithium, sodium and potassium triflate salts.

Compositions were adjusted to produce materials with dry surfaces that showed no evidence of solvent leakage. Alternating current conductivity (σ) measurements of ion transport in these systems were made over the temperature range 290–360 K. Systems containing KCF₃SO₃ exhibited the best range of conductivity values from σ=10⁻⁴ S cm⁻¹ (293 K) to σ=1.8×10⁻³ S cm⁻¹ (360 K). These systems also show a linear dependence of log conductivity on 1/temperature, with activation energies for ion transport in the range 32–48 kJ mol⁻¹.     

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.     

Dilute solution properties of carboxymethylchitins in high ionic-strength solvent

The hydrodynamic characteristics in aqueous solution at ionic strength I=0.2  of carboxymethylchitins of different degrees of chemical substitution have been determined. Experimental values varied over the following ranges: the translational diffusion coefficient (at 25.0°C), 1.1<10⁷×D<2.9 cm² s⁻¹; the sedimentation coefficient, 2.4<s<5.0 S; the Gralen coefficient (sedimentation concentration-dependence parameter), 130<k_s<680 mL g⁻¹; the intrinsic viscosity, 130<[η]<550 mL g⁻¹. Combination of s with D using the Svedberg equation yielded ‘sedimentation–diffusion' molecular weights in the range 40 000<M<240 000 g mol⁻¹. The corresponding Mark–Houwink–Kuhn–Sakurada (MHKS) relationships between the molecular weight and s, D and [η] were: [η]=5.58×10⁻³ M^0.94; D=1.87×10⁻⁴ M^−0.60; s=4.10×10⁻¹⁵ M^0.39. The equilibrium rigidity and hydrodynamic diameter of the carboxymethylchitin polymer chain is also investigated on the basis of wormlike coil theory without excluded volume effects. The significance of the Gralen k_s values for these substances is discussed.