On the use of the Knudsen number and aerosol Knudsen number in the kinetics of Brownian coagulation of aerosols

We consider, on the basis of dimensional analysis, the use of the Knudsen number and aerosol Knudsen number in the kinetics of Brownian coagulation of aerosols. The aerosol Knudsen numberKn _p emerges naturally when one assumes that the aerosol particle motion can be described in terms of Brownian motion. Examples of dimensionless coagulation constants consistent with the use ofKn _p are also discussed.     

Gas permeation of porous organic/inorganic hybrid membranes

Selective gas permeation of porous organic/inorganic hybrid membranes via sol-gel route and its thermal stability are described. Separation performance of the hybrid membrane was improved compared with porous membranes governed by the Knudsen flow, and gas permeability was still much higher than that through nonporous membranes. Additionally, it was shown that these membranes were applicable at higher temperatures than organic membranes.SEM observation demonstrated that the thin membrane was crack-free. Nitrogen physisorption isotherms showed the pore size was in the range of nanometers. Gas permeability through this membrane including phenyl group was in the range of 10^–8 [cc(STP) cm/(cm² s cmHg)] at 25°C. The ratios of O₂/N₂ and CO₂/N₂ were 1.5 and 6.0, respectively, showing the permeation was not governed by the Knudsen flow. The permeability decreased as the temperature increased. Furthermore, the specific affinity between gas molecules and surface was observed not only in the permeation data of the hybrid membranes but in the physisorption data. These results suggested that the gas permeation through the hybrid membrane was governed by the surface flow mechanism.Thermal analysis indicated that these functional groups were still stable at higher temperatures. The phenyl group especially remained undamaged even at 400°C.     

NaA Zeolite Membrane with High Performance Synthesized by Vapor Phase Transformation Method

IntroductionZeolitemembranes ,asaremarkablebranchofinor ganicmembrane ,havepotentialadvantagesinmanyappli cationssuchascatalysisandseparation ,chemicalsensors ,ascousticwavedevices ,andmicroelectronicdevicesduetotheiruniformporesizeatthemolecularlevelandresis tancetohightemperature .1 5For 10years ,manyre searchershavepaidconsiderableattentiontosynthesisofzeolitemembraneswithhighperformance .Amongthere portedzeolitemembranes ,mostattentionwasfocusedonsynthesisofMFI typezeolitemembranebecausei…     

在多孔γ-Al2O3氧化铝载体上微波合成NaA分子筛膜及气体渗透分离性能

The paper presented novel synthesis of NaA zeolite membrane with good performance using microwave heating. The method involved two steps, prior seeding 120 nm of LTA crystals on substrate and then employing a secondary hydrothermal synthesis. Effects of seeding times, synthesis time and synthesis times were investigated in this work. The quality evaluation of membranes respectively used single component gases (HE and N2) and H2/N2 (equivalent volume) mixture. The ideal H2,/N2 selectivity increased from 1.90 of the substrate to 6.37 of the three-stage synthesized membrane, which was distinctly higher than the corresponding Knudsen diffusion selectivity of 3.74. However, the real H2/N2 selectivity of the three-stage synthesis was much lower than the corresponding ideal selectivity and close to the corresponding Knudsen diffusion selectivity of 3.74.     

Kinetics of the uptake of SO2 on mineral oxides: Improved initial uptake coefficients at 298 K from pulsed Knudsen cell experiments

The uptake of SO₂ on γ‐Fe₂O₃, γ‐Al₂O₃, and Saharan dust has been studied at T = 298 K using a Knudsen cell reactor operated in a steady‐state as well as in a pulsed mode. The initial uptake coefficients determined in the steady‐state mode have been found to be affected by surface saturation as well as bulk diffusion of SO₂ resulting in an apparent dependence of the initial uptake coefficients on the sample mass of the mineral oxides. However, by operating the Knudsen cell in the pulsed mode with shorter response time, these effects could be suppressed. Initial uptake coefficients of γ_ini (Fe₂O₃) = (8.8 ± 0.4) × 10^?2, γ_ini (Al₂O₃) = (7.4 ± 0.9) × 10^?3, and γ_ini (Saharan dust) = (7.6 ± 0.5) × 10^?2 were derived. This suggests that Fe₂O₃ is an important component in determining the reactivity of mineral dusts. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 242–249, 2006     

Pore Modification in Porous Ceramic Membranes With Sol-Gel Process and Determination of Gas Permeability and Selectivity

Summary: The aim of the study was to investigate the variation in total surface area, porosity, pore size, Knudsen and surface diffusion coefficients, gas permeability and selectivity before and after the application of sol-gel process to porous ceramic membrane in order to determine the effect of pore modification. In this study, three different sol-gel process were applied to the ceramic support separately; one was the silica sol-gel process which was applied to increase porosity, others were silica-sol dip coating and silica-sol processing methods which were applied to decrease pore size. As a result of this, total surface area, pore size and porosity of ceramic support and membranes were determined by using BET instrument. In addition to this, Knudsen and surface diffusion coefficients were also calculated. After then, ceramic support and membranes were exposed to gas permeation experiments by using the CO₂ gas with different flow rates. Gas permeability and selectivity of those membranes were measured according to the data obtained. Thus, pore surface area, porosity, pore size and Knudsen diffusion coefficient of membrane treated with silica sol-gel process increased while total surface area was decreasing. Therefore, permeability of ceramic support and membrane treated with silica sol-gel process increased, and selectivity decreased with increasing the gas flow rate. Also, surface area, porosity, pore size, permeability, selectivity, Knudsen and surface diffusion coefficients of membranes treated with silica-sol dip coating and silica-sol processing methods were determined. As a result of this, porosity, pore size, Knudsen and surface diffusion coefficients decreased, total surface area increased in both methods. However, viscous flow and Knudsen flow permeability were detected as a consequence of gas permeability test and Knudsen flow was found to be a dominant transport mechanism in addition to surface diffusive flow owing to the small pore diameter in both methods. It was observed that silica-sol processing method had lower pore diameter and higher surface diffusion coefficient than silica-sol dip coating method.     

铁基催化剂用于氨选择性催化还原氮氧化物研究进展

氨选择性催化还原NO_x技术可以有效控制氮氧化物的排放。V_2O_5-WO_3(MoO_3)/TiO_2脱硝催化剂虽然已经工业化应用,但其工作温度偏高,不能满足低温宽工作温度窗口等工况的需要。因此,开发具有宽工作温度窗口的低温脱硝催化剂成为研究热点。其中,铁基催化剂因其具有良好的氧化还原性,以及储量丰富、价格低廉、无毒无害等特点,使其在低温氨选择性催化还原(NH_3-SCR)反应中得到了广泛研究。基于Fe_2O_3在NH_3-SCR催化体系中所起的作用不同,从Fe_2O_3作为载体、助剂、活性组分以及新型结构的铁基催化剂等方面系统地介绍了近年来铁基催化剂在NH_3-SCR反应中的最新研究进展。此外,还总结了铁基催化剂的NH_3-SCR反应机理以及抗水抗硫性,并对该领域未来可能的发展方向进行了展望。     

Controllable Conversion of CO2 on Non-Metallic Gold Clusters

Gold nanoparticles in metallic or plasmonic state have been widely used to catalyze homogeneous and heterogeneous reactions. However, the catalytic behavior of gold catalysts in non-metallic or excitonic state remain elusive. Atomically precise Au_n clusters (n=number of gold atoms) bridge the gap between non-metallic and metallic catalysts and offer new opportunities for unveiling the hidden properties of gold catalysts in the metallic, transition regime, and non-metallic states. Here, we report the controllable conversion of CO₂ over three non-metallic Au_n clusters, including Au₉, Au₁₁, and Au₃₆, towards different target products: methane produced on Au₉, ethanol on Au₁₁, and formic acid on Au₃₆. Structural information encoded in the non-metallic clusters permits a precise correlation of atomic structure with catalytic properties and hence, provides molecular-level insight into distinct reaction channels of CO₂ hydrogenation over the three non-metallic Au catalysts.     

Temperature and pressure effects on CO2 and CH4 permeation through MFI zeolite membranes

Single gas and mixture permeances of CO₂ and CH₄ were measured as functions of pressure and temperature through three MFI zeolite membranes that have different fractions of their permeances through non-zeolite pores. The effect of pressure on CO₂ permeance, which was different for each membrane, was fit by a modified surface diffusion model. The differences in the pressure behavior of the membranes are attributed to pores with viscous and Knudsen flow. Membranes with the largest permeation through non-zeolite pores have the lowest CO₂/CH₄ mixture selectivity. The highest CO₂/CH₄ mixture selectivity is 5.5 at room temperature and decreases with temperature because of a decrease in competitive adsorption. Although increasing pressure at constant pressure drop increases the apparent CO₂/CH₄ selectivity, the ratio of the CO₂ and CH₄ fluxes decreases.     

Saturated Vapor Pressure of Iridium(III) Acetylacetonate

The temperature dependency of the saturated vapor pressure of Ir(acac)₃ has been measured by the method of calibrated volume (MCV), the Knudsen method, the flow transpiration method, and the membrane method. The thermodynamic parameters of phase transition of a crystal to gas were calculated using each of these methods, and the following values of ΔH _T ⁰ (kJ mol⁻¹) and ΔS _T ⁰ (J mol⁻¹K⁻¹), respectively, were obtained: MCV: 101.59, 156.70; Knudsen: 130.54, 224.40; Flow transpiration: 129.34, 212.23; Membrane: 95.45, 149.44 Coprocessing of obtaining data (MCV, flow transportation method and Knudsen method) at temperature ranges 110−200°C as also conducted:ΔH _T ⁰ =127.9±2.1 (kJ mol⁻¹ ); ΔS _T ⁰ =215.2±5.0 (J mol⁻¹ K⁻¹ ). This revised version was published online in August 2006 with corrections to the Cover Date.     

Controllable Conversion of CO2 on Non‐Metallic Gold Clusters

Gold nanoparticles in metallic or plasmonic state have been widely used to catalyze homogeneous and heterogeneous reactions. However, the catalytic behavior of gold catalysts in non‐metallic or excitonic state remain elusive. Atomically precise Au_n clusters (n=number of gold atoms) bridge the gap between non‐metallic and metallic catalysts and offer new opportunities for unveiling the hidden properties of gold catalysts in the metallic, transition regime, and non‐metallic states. Here, we report the controllable conversion of CO₂ over three non‐metallic Au_n clusters, including Au₉, Au₁₁, and Au₃₆, towards different target products: methane produced on Au₉, ethanol on Au₁₁, and formic acid on Au₃₆. Structural information encoded in the non‐metallic clusters permits a precise correlation of atomic structure with catalytic properties and hence, provides molecular‐level insight into distinct reaction channels of CO₂ hydrogenation over the three non‐metallic Au catalysts.     

“Living” radical polymerization of styrene catalyzed by cyclometalated ruthenium(II) complexes bearing nonlabile ligands

Cationic substitutionally inert cyclometalated ruthenium (II) and osmium (II) complexes, ([Mt(o‐C₆H₄‐2‐py)(LL)₂]PF₆), where LL‐1,10‐phenanthroline (phen) or 2,2′‐bipyridine (bipy), were used for radical polymerization of styrene. Gradual modification of the complexes within the series allowed comparison of the catalytic activity and the redox properties. There was no correlation between the reducing powers of the complexes and their catalytic activities. The osmium compound of the lowest reduction potential was not active. All the ruthenium complexes catalyzed the polymerization of styrene in a controlled manner; but the level of control and the catalytic activity were different under the same polymerization conditions. [Ru(o‐C₆H₄‐2‐py)(phen)₂]PF₆ demonstrated the best catalytic performance though its redox potential was the highest. It catalyzed the “living” polymerization with a reasonable rate at a catalyst‐to‐initiator ratio of 0.1. 1 equiv. of Al(OiPr)₃ accelerated the polymerization and improved the control, but higher amount of Al(OiPr)₃ did not speed up the polymerization and moved the process into the uncontrollable regime. Under the most optimal conditions, the controlled polymerization occurs fast without any additive and the catalyst degradation. Added free ligands inhibited the polymerization suggesting that the catalytically active ruthenium intermediates are generated via the reversible dechelation of bidentate phen or bipy ligands. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3814–3828, 2009     

Lipase bound cellulose nanofibrous membrane via Cibacron Blue F3GA affinity ligand

Cellulose (Cell) nanofibrous membranes were prepared by nucleophilic reaction of the cellulose hydroxyl with the triazinyl chloride of Cibacron Blue F3GA (CB) ligand and studied as affinity membranes for lipase enzyme. Cell nanofibrous membranes containing fibers with 200 nm average diameters were prepared by electrospinning of cellulose acetate (CA), followed by alkaline hydrolysis. The CB capacity of the Cell nanofibrous membranes was optimized by lengthening the nucleophilic reaction time and increasing CB concentration and ionic strength. The equilibrium adsorption isotherms of CB on the Cell nanofibrous membranes followed a typical Langmuir monolayer adsorption behavior. At 242 mg CB/g of Cell, the maximum lipase adsorption capacity (q_m) and the dissociation constant (K_d) values were 41.02 mg/g and 0.25 mg/mL, respectively. Optimal lipase adsorption capacity was obtained at pH 4.0, its isoelectric point, with added NaCl on Cell membranes 86 mg CB capacity per g of Cell. A facile lipase loading capacity of 16.21 mg/g of CB–Cell was achieved under moderated conditions and could be optimized to reach at least 150 mg/g. The CB–Cell bound lipase had similar catalytic rate and retained 86.2% activity as in its free form. These findings clearly show that the CB bound Cell nanofibrous membrane is a highly efficient ultra-high specific porous support for lipase enzyme and is potentially versatile for immobilizing other enzymes and as affinity membrane for proteins.     

Preparation and hydrogen permeation of SrCe0.95Y0.05O3−δ asymmetrical membranes

Asymmetrical thin membranes of SrCe_0.95Y_0.05O_3−δ (SCY) were prepared by a conventional and cost-effective dry pressing method. The substrate consisted of SCY, NiO and soluble starch (SS), and the top layer was the SCY. NiO was used as a pore former and soluble starch was used to control the shrinkage of the substrate to match that of the top layer. Crack-free asymmetrical thin membranes with thicknesses of about 50 μm and grain sizes of 5–10 μm were successfully pressed on to the substrates. Hydrogen permeation fluxes (J_H2) of these thin membranes were measured under different operating conditions. At 950 °C, J_H2 of the 50 μm SCY asymmetrical membrane towards a mixture of 80% H₂/He was as high as 7.6 × 10⁻⁸ mol/cm² s, which was about 7 times higher than that of the symmetrical membranes with a thickness of about 620 μm. The hydrogen permeation properties of SCY asymmetrical membranes were investigated and activation energies for hydrogen permeation fluxes were calculated. The slope of the relationship between the hydrogen permeation fluxes and the thickness of the membranes was −0.72, indicating that permeation in SCY asymmetric membranes was controlled by both bulk diffusion and surface reaction in the range investigated.     

Gas–liquid membrane contactors for CO2 removal

In the present work we use a membrane contactor for the separation of CO₂ from CH₄ and we systematically investigate the influence of both the type of membrane and the different process parameters on the overall process performance (permeability and selectivity). This work is important because it reports real process performance data (permeances and selectivities) for the total process consisting of absorption and desorption under practical conditions using feed mixtures. Commercially available porous PP hollow fiber membranes and asymmetric PPO hollow fiber membranes have been applied and MEA was used as absorption liquid in the membrane contactor. The proposed approach allows us to identify the operating window and potential of the process. Although the performance of the PP membranes outperforms the performance of the PPO membranes in terms of productivity and selectivity, the PP fibers are extremely sensitive to only small variations in the feed pressure, resulting in severe performance loss. In addition to that, extremely high liquid losses are observed for the PP fibers especially at elevated temperatures. Factors that are significantly reduced when asymmetric PPO membranes with a dense, ultrathin top layer are used, which thus improves the performance and significantly increases the operating window and potential of the membrane contactor process.     

Mass spectrometric determination of the electron affinity of C<Subscript>60</Subscript>(CF<Subscript>3</Subscript>)<Subscript>10</Subscript> and C<Subscript>60</Subscript>(CF<Subscript>3</Subscript>)<Subscript>12</Subscript> molecules

Knudsen cell mass spectrometry was used to study ion-molecular electron exchange reactions between some trifluoromethyl derivatives of C₆₀ fullerene. Electron affinity values were experimentally determined for C₆₀(CF₃)₁₀ and the S ₆ isomer of C₆₀(CF₃)₁₂ and compared with the results of calculations and the data in the literature.     

SiO2-TiO2 membranes by the sol-gel process

The use of membranes for gas separation represents an important alternative from the viewpoint of energy conservation in industrial separation processes. Polymeric Si-Ti sols prepared from titanium isopropoxide (Ti(OPrⁱ)₄) and tetraethoxysilane (TEOS) were used to deposit membranes on α-Al₂O₃ supports. Acetylacetone (2,4 pentanedione, acacH) and isoeugenol (2-methoxy-4-propenylphenol, isoH) were employed separately to chelate the Ti precursor in order to slow down the chemical reactivity, avoiding precipitation. The radial distribution functions (RDF) of the gels aging at room temperature were obtained. The xerogels were studied by Thermal Gravimetric (TGA) and Differential Thermal (DTA) Analysis in air. The Microporosity of the solids calcined at 773 K was determined by N₂-adsorption at 77 K. The membrane thickness was determined from SEM photographs. Preliminary permeance results of the supported membranes on commercial alumina support were obtained for He, N₂ and CO₂ in a single gas equipment. At 773 K the separation factors α(He/CO₂) and α(N₂/CO₂) for both membranes exceeds the theoretical Knudsen limit.     

The enhancement of oxygen flux on Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) hollow fibers using silver surface modification

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) hollow fibers were fabricated using a phase inversion/sintering method. As oxygen permeation of BSCF hollow fibers is controlled by the rate surface exchange kinetics, catalytic Ag particles were coated on both inner and outer surfaces using chemical deposition method, as verified by SEM and EDX. The Ag coated BSCF membranes showed up to 100% increase in oxygen permeation at 700 °C, and improvements lower than 10% were measured at 950 °C as compared with unmodified membranes. It was found that Ag catalyst surface loading was non-homogenous and concentrated on the perovskite grain boundaries. As a result, lighter Ag surface loading delivered improved oxygen flux while oxygen flux reached a maximum even though in the presence of excess catalyst loading. The catalytic activity of Ag was beneficial in enhancing surface reaction kinetics up to 850 °C attributed to the spillover effect. Above this temperature, the increase in oxygen permeation rate was marginally diminished due to the reduction of the spillover effect.     

Supramolecular systems based on alkylated <Emphasis Type="Italic">p</Emphasis>-sulfonatocalix[<Emphasis Type="Italic">n</Emphasis>]arenes: aggregation and catalytic and biological activity

Aggregation, catalytic activity, and the influence of alkylated p-sulfonatocalix[n]arenes (SCA; n = 4, 6, 8) on the energy exchange in cells of wheat roots were studied. In water SCA are surfactants, and their efficiency changes with an increase in the number of aromatic rings (n) and the length of hydrocarbon substituents (R) in the molecule. In the range of critical micelle concentration, SCA form micelles with an effective hydrodynamic diameter of ∼200 nm. The catalytic activity of micellar solutions of SCA (n = 4, 8; R = C₁₂H₂₅) in hydrolysis of ethyl 4-nitrophenyl chloromethylphosphonate (ENCP) decreases with an increase in n, whereas the binding constant of ENCP increases. The modifying effect of SCA on the membranes and the energy exchange in cells of wheat roots was revealed: the membranotropic activity of SCA increases with an increase in the hydrophobicity of substituents R and the macrocycle size.     

Polymerization of methyl methacrylate using the NiX<Subscript>2</Subscript>(PPh<Subscript>3</Subscript>)<Subscript>2</Subscript>/Zn catalytic system

The NiBr₂(PPh₃)₂/Zn catalytic system in combination with the PhI initiator ensures methyl methacrylate polymerization in a wide temperature range. The polymer yield is determined by the ligand environment of the nickel atom, the reaction temperature, and the ratio of the components of the catalytic system. An analysis of the macrokinetic relationships and of the molecular-weight characteristics of the products indicates that polymerization occurs in a controlled regime.