Search

A selectivity of 71 is achieved in the separation of a gas phase water–propanol mixture, using a mordenite/ZSM-5/chabazite membrane that is hydrothermally synthesized onto the inner surface of a porous α-alumina tubular support.

A new microporous titanosilicate umbite membrane has been synthesized on commercial porous TiO2 tubular supports. This membrane is able to separate H2/N2 mixtures (with selectivities as high as 48) even in the presence of water.

A new method for the selective excitation of diatomic molecules in single vibrational states on excited electronic potentials by two-photon absorption is proposed. The method implies the use of two chirped strong pulse lasers detuned from the optical transition to an intermediate electronic state. We show under what scenarios the method is successful on the time–energy scale in which the pulses operate. They involved a long-time (nanosecond) weak-field regime and a short-time (picosecond) strong-field regime. The adiabatic representation in terms of energy levels or in terms of light-induced potentials is used to interpret the physical mechanism of the excitation. The efficiency and robustness of the scheme are demonstrated by the excitation of the ground vibrational state of the 1Σg(4s) electronic potential of the Na2 molecule.

Preliminary results of a classical trajectory study of the dynamics of the four-center elimination of hydrogen fluoride from 1,1.1-trifluoroethane over an empirical potential surface with an exit barrier are presented. The characteristic features of the surface beyond the saddle point determine the partitioning of energy between the fragments, leading to a moderate enhancement of the HF vibrational energy and to high excitation of the olefinic CC bond, with a small repulsion between the fragments at the very end of the process appearing as relative translational energy. A comparison of our results with experiment and other theoretical treatments is given.

Miscibility of several binary fluid systems, placed in an external gravitational field, has been studied by Molecular Dynamics calculation. The results show inmiscibility and phase separation to appear as a consequence of differences in the Lennard-Jones energy parameter of molecular interaction. Two situations are primarily considered: The case of a big difference in the energy parameters of pure components and the case of a weak interaction between unlike molecules.

Using a standardized minicomputer program, we have investigated the interpretation of the observed of F + HCl→HF+Cl in terms of both LEPS and HMF potential energy hypersurfaces. A reasonable best fit has been selected. Predicted excitation functions, impact parameter dependences and isotope effects are presented and discussed.

Monte Carlo simulations of liquid carbon tetrachloride have been carried out for a system of 125 molecules described by five interaction centres of the (12-6) Lennard-Jones type. Radial distribution functions and structure factors for several pairs of interaction parameters are compared with experimental values and with the molecular dynamics results of other authors.

In four-center elimination reactions such as hydrogen halide elimination from halogenated hydrocarbons the energy barrier is higher than the difference in enthalpy of formation between the parent molecule and its fragments (HX and olefin). This determines that the energy available to products has two origins: the reverse reaction barrier (fixed energy), and the excess energy (energy above the barrier). Both types of energy are partitioned among products following different laws: more or less statistical for excess energy and non-statistical for fixed energy. In a study of CF 3 -CH 3 decomposition, we describe a practical method, based on the variation of product energy partitioning with excess energy, to determine the partitioning of the fixed energy among different types of product energy, thus defining the exact nature of the reverse reaction energy barrier. We applied this model to other types of reactions, such as three-center molecular eliminations.

A Monte Carlo study for liquid benzene, where the pair potential is described as a sum of Lennard-Jones (12, 6) interactions acting between six equivalent centres of each molecule, has been carried out. The potential parameters have been chosen in such a way that the internal energy and virial pressure were in agreement with the experimental values. The liquid structure has been examined in detail by computing several distribution functions: the radial, the angular correlations and the radial-angular distribution between molecular planes.

We present the first successful synthesis of monodisperse carbon nanodots (CNDs) with tunable photoluminescence (PL) carried out by laser pyrolysis of two common volatile organic precursors such as toluene and pyridine. Remarkably, the initial chemical composition of the precursor determines the formation of undoped or N-doped CNDs and their corresponding absorption response in the visible range (expanded for the latter). We demonstrate the control and versatility of this synthesis method to tune the final outcome and its potential to explore a great number of potential solvent candidates. Furthermore, we have successfully exploited these CNDs (both undoped and N-doped) as effective sensitizers of TiO2 nanoparticles in the visible-light driven photo-degradation of a cationic dye selected as model organic pollutant., Financial support from the European Research Council (ERC Advanced Grant CADENCE number 742684) is gratefully acknowledged. We also acknowledge the support from NANOLIGHT (REA grant number 294094), CIBER-BBN and MINECO (Spain) with project CTQ2016-79419-R.

Cu-based Metal Organic Frameworks (MOF) microdevices are applied insampling and preconcentration of nerve agents (NAs) diluted in gaseous streams. An in-situelectrochemical assisted synthesis of Cu-BTC thick film is carried out to functionalize a Cumodified glass substrate. This simple, rapid, reproducible and easy to integrate MOF synthesisapproach, enables the microfabrication of functional microprenconcentrators with large BETsurface area (above 2000 cm2) and active pore volume (above 90 nanoliters) for the efficientadsorption of nerve agent molecules along the microfluidic channel 2.5 cm in length. Equilibriumadsorption capacity of the bulk material has been characterized through thermogravimetricanalysis after exposure to controlled atmospheres of a sarin gas surrogate, dimethylmethylphosphonate (DMMP), in both dry and humid conditions (30% RH at 293 K).Breakthrough tests at ppm level (162 mg/m3) reveal equilibrium adsorption capacities up to 691mg/g. The preconcentration performance of such μ-devices when dealing with highly dilutedsurrogate atmosphere, i.e. 520 ppbV (2.6 mg/m3) at 298 K, leads to preconcentration coefficientsup to 171 for sample volume up to 600 STP cm3. We demonstrate the potentialities of Cu-BTCmicropreconcentrators as smart first responder tools for "on field" detection of nerve agents ingas phase at relevant conditions.

We present a study of R.F. sputtered CuInSe2 films from a 5% Se excess compound target in Ar and Ar/H2 atmospheres. The introduction of H2 in the sputtering chamber allows a fine control of film stoichiometry. A relation between Se contents of films and the formation of the chalcopyrite phase is established. Chalcopyrite CuInSe2 films with nearly stoichiometric composition, resistivity up to 104 Ωcm, and 1 µm average grain size have been obtained.

We present a gas-liquid microfluidic system as a reactor to obtain magnetite nanoparticles with an excellent degree of control regarding their crystalline phase, shape and size. Several types of microflow approaches were selected to prevent nanomaterial aggregation and to promote homogenous size distribution. The selected reactor consists of a mixer stage aided by ultrasound waves and a reaction stage using a N2-liquid segmented flow to prevent magnetite oxidation to non-magnetic phases. A milli-fluidic reactor was developed to increase the production rate where a magnetite throughput close to 450 mg/h in a continuous fashion was obtained.

This overview describes some of the main features of the use of zeolite membranes for separation applications. Four different types of separations are considered: separation of non-adsorbing compounds, of organic molecules, of permanent gases from vapors, and of water (or polar molecules) from organic (or non-polar) species. Several factors, such as the limiting pore size and pore size distribution, surface diffusion, capillary condensation, shape selectivity and molecular sieving, contribute to the separations observed However, in most of the high selectivity separations reported in the literature, preferential adsorption is the dominant characteristic. In this case, the adsorption of one component in the mixture is stronger and this blocks or hinders the passage of the other components through the membrane pores.

Diverse carbon materials have been used for tissue engineering and clinical implant applications with varying success. In this study, commercially available reticulated vitreous carbon (RVC) foams were tested in vitro and in vivo for compatibility with primary cell adhesion and tissue repair. Pores sizes were determined as 279 ± 98 µm. No hydroxyapatite deposition was detected after immersion of the foams in simulated body fluid. Nonetheless, RVC provided an excellent support for adhesion of mesenchymal stem cells (MSCs) as well as primary chondrocytes without any surface pre-treatment. Live cell quantification revealed neutral behaviour of the material with plastic adhered chondrocytes but moderate cytotoxicity with MSCs. Yet, rabbit implanted foams exhibited good integration in subcutaneous pockets and most importantly, total defect repair in bone. Probably due to the stiffness of the material, incompatibility with cartilage regeneration was found. Interestingly and in contrast to several other carbon materials, we observed a total lack of foreign body reactions. Our results and its outstanding porous interconnectivity and availability within a wide range of pore sizes convert RVC into an attractive candidate for tissue engineering applications in a variety of bone models and for ex vivo cell expansion for regenerative medical applications.

Ultrathin superlattices of 123 superconducting cuprates have been fabricated by high oxygen pressure sputtering technique. The superlattices show high quality, with negligible step disorder, roughness and interdiffusion. The insulating PrBa2Cu3O7 (PBCO) layer thickness has been kept constant at 5 unit cells (u.c.) and the superconducting YBa2Cu3O7 (YBCO) layer thickness has been changed from 8 u.c. to 1 u.c. In these uncoupled multilayers the critical temperature (Tc0) changes from 44 (1 u.c. YBCO) to 80 K (8 u.c. YBCO). In the vortex liquid regime, at constant temperature, the angular dependence of the magnetoresistance scales down with the applied magnetic field values in all the temperature range. This behavior allows us to study the anisotropy and dimensionality behavior of the mixed state in ultrathin superlattices. A dimensional crossover 3D-2D is found in the anisotropy with the 1 u.c. YBCO superlattices following a pure 2D behavior.

We present Admittance Spectroscopy measurements on two different ionically conducting materials, Gd 2 Ti 2-y Zr y O 7 and Li 0.5-x Na x -La 0.5 TiO 3 . Electrical relaxation data have been found to be well described by stretched exponential functions of the form Φ(t) = exp(-(t/ τ) 1-n ). In Gd 2 Ti 2-y Zr y O 7 the concentration of mobile ions in the whole series is very low, from 1% to 0.02%, and we find that by strongly decreasing the density of mobile ions the value of the exponent n shows a significant decrease from 0.44 ± 0.01 to 0.16 ± 0.01. In Li 0.5-x -Na x La 0.5 TiO 3 we have investigated changes in ion dynamics when approaching the percolation threshold for lithium diffusion by increasing the number of immoble sodium ions, and found that the exponent n increases towards a value of 0.81 ± 0.01 as the mobility of lithium ions becomes more and more constrained close to the percolation threshold. Both results are discussed in terms of the importance of ion-ion correlations in the dispersive behavior of the electrical conductivity.

Ni/NiAl2O4catalyst has been used for the hydrogenation of acetylene to ethylene and ethane at temperatures between 423 and 493 K. The results presented include data on the physicochemical characterization of the catalyst, the evolution of its activity and selectivity and its coking behavior. These studies have been carried out on fresh catalyst and on catalyst samples subjected to an accelerated thermal aging process. The results show that the catalytic activity increases with time on stream (activation period), until a point is reached in which catalyst deactivation predominates (deactivation period). Catalyst aging in this system was found to reduce coke formation and to increase the hydrogenation activity observed.

The effect of aging in nitrogen upon the regeneration characteristics of the coke deposits on chromia-alumina catalysts has been investigated. To this end, the coked catalysts have been subjected to various treatments in nitrogen, and the chemical composition and reactivity of the deposits have been investigated. The results show that the process of aging in nitrogen gives rise to significant changes in both the composition and reactivity of the coke deposits, due to the stripping of the coke fractions with a higher volatility. This obviously has important consequences upon the subsequent regeneration, which are also discussed and tested in regeneration experiments using coked catalyst of different ages.

Zeolite (silicalite-1) micromembranes having only approximately 0.005 mm2 of permeation area have been prepared on laser-perforated, 75-μm-thick stainless steel sheets. Arrays of these micromembranes performed well in the gas-phase separation of a propane/N2 mixture, showing a hydrocarbon permeance at least an order of magnitude larger than those of the conventional zeolite membranes.

A fluidized-bed reactor with separate butane and oxygen feeds has been studied in the oxidative dehydrogenation of butane. Under suitable conditions, feed segregation allows the creation of separated oxidation and reduction zones in the same reaction vessel, between which a V/MgO catalyst is recirculated. The behavior of the reactor has been characterized, with the separation of oxidation and reduction zones as the focus of the study. The performance of the two-zone reactor has been compared to that of a conventional fluidized-bed reactor.

Methane oxidative coupling has been carried out over Li/MgO, Li/Sn/MgO, Li/Na/MgO, Li/La/MgO, Na/Sm2O3, NaCl/CsCl/MgO and Na/W/Mn/SiO2 catalysts, using a fixed bed reactor and an inert ceramic membrane reactor in which the membrane acted as an oxygen distributor to the catalytic bed. In most cases, the ceramic membrane reactor provided a significantly higher selectivity than the fixed bed reactor, at the same conversion level.

A simulation study has been carried out of methane oxidative coupling in a fixed bed catalytic reactor with a distributed oxygen feed. To this end, a lumped kinetic model which takes into account 4 main species, namely CH4, O2, COx ( CO and CO2) and C2 (ethane and ethylene) has been used coupled with a pseudohomogeneous plug flow reactor model. The model contemplates several oxygen inputs along the reactor length, and also the possibility of heat exchange between the reactor and a cooling medium. The simulation yields the variation of temperature along the bed, together with the concentration profiles for the species considered. The results indicate that a distributed oxygen feed may significantly improve the yield to hydrocarbons in the oxidative coupling process.

Silicalite, ZSM5, and mordenite membranes were prepared on alumina and stainless steel porous tubes by several procedures. The N 2 permeation flux was measured before and after exposure to water, n-octane, or n-butane under different conditions (capillary condensation and adsorption). The reduction of permeation flux that takes place in the presence of adsorbable compounds can be related to the quality and separation properties of a given membrane. Also, a good correlation was observed between the separation selectivity and the time required to reach steady state after exposure of the membrane to quasi-saturation conditions. Both measurements can be used as fast and reliable techniques for membrane characterization.

This overview discusses some of the developments and outstanding opportunities in the field of catalytic reactors based on porous ceramic membranes, both inert and catalytic. This is an emerging area, where inputs from heterogeneous catalysis, material science and reactor engineering are playing the key roles. Rather than attempting a thorough review of the relevant literature, this work deals with some general concepts and then concentrates on a few selected examples that illustrate the application of membrane reactors.

The regeneration of fixed bed catalytic reactors has been simulated a pseudo-homogenous model. The influence of a number of variables on the temperature rise has been studied. In particular the effect of coke content and composition have been determined. It was found that fixed beds with series and parallel types of coke distribution behave differently when these variables are changed. Regeneration policies in which a stepped oxygen feed is employed, are discussed.

Several catalytic membranes have been studied, with different loadings and radial distribution of the catalytic material. Two types of catalytic membranes have been prepared on top of alumina supports: (a) V/MgO membranes, where MgO was deposited in the support pores before impregnation with V, and (b) V/Al2O3 membranes, where the alumina supports were impregnated with a boehmite sol before V deposition. In both cases the introduction of V was carried only from the internal side of the porous tube, and using short impregnation times, in order to obtain a radial distribution of active material. The membranes prepared have been tested under reaction conditions, giving yields to propene of up to 15%. It was found that the best V/MgO membranes are more selective but less active than the best V/Al2O3 membranes.

A zeolite membrane was employed to selectively remove water from the reaction atmosphere during the gas-phase synthesis of methyl-tert-butyl ether (MTBE) from tert-butanol and methanol. This reaction was carried out over a bed of Amberlyst™ 15 catalyst packed on the inside of a zeolite tubular membrane. The results obtained with different hydrophilic membranes (mordenite or NaA zeolite) are presented. Prior to reaction, the zeolite membranes were characterized by measuring their performance in the separation of the equilibrium mixture containing water, methanol, tert-butanol, MTBE and isobutene. The results obtained with zeolite membrane reactors (ZMR) were compared with those of a fixed bed reactor (FBR) under the same operating conditions. MTBE yields obtained with the ZMR at 334 K reached 67.6%, under conditions where the equilibrium value without product removal (FBR) would be 60.9%.

A flow-through catalytic membrane has been used to carry out consecutively the oxidative and non-oxidative dehydrogenation of butane, with the aim of achieving thermal and chemical coupling of both processes. To this end, two distinct zones were created across the membrane radius: the first, containing V supported on MgO, was prepared by sequential impregnation, and the second, containing Pt-Sn on γ-Al 2 O 3 , was prepared by sol-gel procedures. The investigation highlights some of the problems encountered during membrane preparation, which are discussed and related to the reaction results observed. When the preparation procedures are tailored to achieve segregation of the catalytic materials in both layers, and when the operating conditions are such that complete oxygen consumption within the oxidative dehydrogenation layer is obtained, the use of the flow-through, two-layer membrane allows stable operation and, at the same time, gives rise to increased conversion and selectivity.

A study of the deep catalytic oxidation of volatile organic compounds (VOCs) contained in air streams on catalytic membrane tubes is presented. The complete combustion of toluene and methyl ethyl ketone as single components and in binary mixtures, with concentrations between 100 and 7000 ppm, was achieved by permeating the VOC-containing stream through a Pt/γ-Al2O3 catalytic membrane operating under the Knudsen diffusion regime. The membrane combustor performed efficiently, allowing total VOC destruction at low temperatures. In addition, substantial changes in the gas feedrate were accommodated, with only a small variation in the combustion temperatures required. A discussion is presented regarding the ability of the flow-through membrane combustor to attain a lower mass-transfer resistance than a conventional catalytic reactor.

A mordenite layer with a high accessibility has been synthesised on cordierite monolith supports; substantial loadings of mordenite were achieved (above 50 wt%) under the synthesis conditions used.

A novel reactor concept is proposed for partial oxidation reactions that combines membrane and fluid bed reactor technology in a single vessel. Air fluidizes the shell side in which both membrane tubes — charged with catalyst — and cooling coils are immersed. Oxygen transport through the membrane wall is controlled by pressure drop. Model simulations, based on the kinetics for the oxidative dehydrogenation of propane to propene, show improved performance compared to conventional fixed bed technology. The controlled oxygen addition along the axis improves propene selectivity and broadens the operating range with respect to hydrocarbon and oxygen feed rates. On propose un nouveau concept de reacteurs pour les reactions d'oxydation partielle combinant la technologie des reacteurs a membranes et des reacteurs a lit fluide dans une cuve unique. L'air fluidise le cote de l'enveloppe ou sont immerges les tubes a membranes — charges de catalyseur — et les serpentins de refroidissement. Le transport de l'oxygene a travers la paroi de la membrane est regule par la perte de charge. Des simulations modeles, basees sur la cinetique de deshydrogenation oxydante du propane en propene, montrent une performance amelioree par rapport a la technologie des lits fixes conventionnelle. Un ajout s'oxygene regule le long de l'axe amelioree la selectivite en propene et elargit la gamme de fonctionnement par rapport aux taux s'alimentation en hydrocarbures et en oxygene.

Catalytic membranes operating in a mixed permeation regime (i.e., with significant Knudsen and laminar contributions) have been developed. The membranes prepared had wide pores and presented a low pressure drop. After the addition of γ-Al2O3 and Pt, the resulting catalytic membranes were active for the combustion of VOCs. Their performance was compared with that of similar catalytic membranes operating under the Knudsen diffusion regime.

An inert membrane has been used to distribute oxygen to a fixed bed of VPO catalyst in the partial oxidation of butane to maleic anhydride. Two different reactor configurations have been used, namely, a standard inert membrane reactor (IMR), in which oxygen permeates inward from the shell side and the catalyst is packed on the tube side, and an outward flow inert membrane reactor (OFIMR), in which the catalyst bed is packed on the shell side and oxygen flows outwardly from the tube side. The radial variation of both the oxygen and hydrocarbon concentrations and of the temperature is different in the two types of reactors investigated, and as a consequence a considerably better performance is obtained for the OFIMR.

We report large magnetoresistance (in excess of 1000%) in ferromagnet / superconductor / ferromagnet structures made of La0.7Ca0.3MnO3 and YBa2Cu3O7 in the current in plane (CIP) geometry. This magnetoresistance has many of the ingredients of the giant magnetoresistance of metallic superlattices: it is independent on the angle between current and magnetic field, depends on the relative orientation of the magnetization in the ferromagnetic layers, and takes very large values. The origin is enhanced scattering at the F/S interface in the anti parallel configuration of the magnetizations. Furthermore, we examine the dependence of the magnetoresistance effect on the thickness of the superconducting layer, and show that the magnetoresistance dies out for thickness in excess of 30 nm, setting a length scale for the diffusion of spin polarized quasiparticles.

A silicalite membrane has been used to separate a homogeneous catalyst from its mixture with solvent and product of the Heck reaction.

A catalytically active zeolite membrane has been used to displace equilibrium by selective water permeation during ethanol esteri#cation. Unlike previous works in which water separation was carried out by zeolitic membranes that did not take part in the reaction, the H-ZSM-5 membrane used in this work had su5cient catalytic activity to carry out the esteri#cation ofethanol with acetic acid, and at the same time was selective for water permeation. As a consequence, the reaction and separation functions could be coupled very e5ciently, and the conversion obtained at the same feed rate and catalyst loading was greater than in conventional #xed bed reactors, or in reactors where the zeolite membrane was kept separated from the catalyst. ? 2002 Elsevier Science Ltd. All rights reserved.

A fluidized bed reactor has been used to study the influence of the operation parameters on the reactor behavior during oxidative coupling of methane. A vibration device was employed to avoid the agglomeration of the catalyst particles, thus allowing operation at low gas velocities, near the minimum fluidization velocity. An improved selectivity for hydrocarbons was obtained in this way, compared to the results at high gas velocities.

LaCoO 3 , LaMnO 3 , La 0.8 Sr 0.2 CoO 3 , and La 0.8 Sr 0.2 MnO 3 perovskites have been used for the combustion of VOCs (toluene and methyl-ethyl-ketone) in air, at temperatures between 200 and 300°C. Reaction tests have been carried out using the single compounds, as well as binary mixtures in different proportions. Several characterization techniques have been employed in order to gain insight into the changes taking place on the catalyst surface, especially during the combustion of binary mixtures. The discussion attempts to relate these changes to the catalytic performance observed.

In spite of the presence of inter-crystalline defects, high selectivities can be obtained in the separation of gas phase mixtures with zeolite membranes.

A systematic study of the chemical and physical changes induced by exposure to UV (248 nm) excimer laser light of unvarnished tempera paint samples has been undertaken as a part of the research activities included in the European project "Advanced workstation for controlled laser cleaning of artworks". The direct exposure of the paint to the UV laser configures the worst case scenario of laser cleaning, as a thin protective layer of varnish is normally left to minimize the dose of UV radiation that reaches the paint surface. However, in the practice of laser cleaning, there is a need to characterize and quantify the possible effects of direct UV laser irradiation of unvarnished paints. To this purpose, a broad range of techniques have been used including profilometry, colorimetry, optical and vibrational spectroscopic techniques, such as laser-induced fluorescence (LIF), laser-induced breakdown spectroscopy (LIBS), Fourier transform Raman (FTR) and infrared (FTIR), and analytical mass spectrometric techniques, like direct-temperature-resolved mass spectrometry (DTMS) and laser desorption and ionization time of flight mass spectrometry (LDI-TOF). Integration of the results obtained by these techniques allowed the investigation of the nature and degree of change of the irradiated paint systems. These were observed to strongly depend on the type of paint system. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.