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In this study, we report the direct synthesis of amides and ureas via the catalytic dehydrogenation of volatile alcohols and amines using the Milstein catalyst in a Pd-Ag/ceramic membrane reactor. A series of amides and ureas, which could not be synthesized in an open system by catalytic dehydrogenation coupling, were obtained in moderate to high yields via catalytic dehydrogenation of volatile alcohols and amines. This process could be monitored by the hydrogen produced. Compared to the traditional method of condensation, this catalytic system avoids the stoichiometric pre-activation or in situ activation of reagents, and is a much cleaner process with high atom economy. This methodology, only possible by employing the Pd-Ag/ceramic membrane reactor, not only provides a new environmentally benign synthetic approach of amides and ureas, but is also a potential method for hydrogen storage. [ABSTRACT FROM AUTHOR]

Intermetallic diffusion between the Pd layer and the porous metal support can limit the hydrogen permeability, and ultimately give rise to serious problems in the long-term stability of Pd alloy hydrogen separation membranes. In this study, we developed a multi-layered Pd–Ag membrane having a uniform-microstructure and thermally stable alloy composition by DC magnetron sputtering and Ag up-filling heat treatment. This multi-layered Pd–Ag alloy membrane was introduced in order to prevent Ag segregation, to block the intermetallic diffusion from the porous stainless steel (PSS), and to improve the stability of the membrane. Experimental results showed that the homogeneous Pd–Ag alloy membranes acted as an extremely effective diffusion barrier and provided significant improvement on the thermal stability over a period of 2000 h. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the results obtained with different composite membranes used on a membrane reactor for the production of high purity hydrogen. The dry reforming of methane was carried out over a Rh/La2O3 catalyst. Two types of composite membranes were synthesized, Pd and a Pd–Ag alloy. The metal films were deposited by electroless plating on modified porous stainless steel. The support surface was previously modified with a NaA zeolite by dip coating and hydrothermal synthesis. The composition and crystalline structures of the alloy films were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The membrane reactor was designed and built to use either the Pd or the Pd–Ag composite membrane whose crystalline structure and typical morphology were not affected after different reaction conditions. The Pd membrane showed the best behavior in the membrane reactor with the highest methane conversion, H2/CH4 selectivity and permeation flux. After operation of the membranes at 450–500°C on stream for up to 570h no modification of the permeation parameters was observed. [ABSTRACT FROM AUTHOR]

Abstract: A theoretical model is proposed to describe hydrogen permeation in palladium and silver–palladium membranes in presence of a non-inert gas as CO; it is known indeed that hydrogen flux through palladium-based membranes drastically decreases when H2 is fed in mixtures containing carbon monoxide due to the interaction of the latter gas with the membrane surface. To model this process, the adsorption step of the well-known approach suggested by Ward and Dao has been suitably modified, since it must be considered as a competitive adsorption of the different non-inert molecules on the metal interface. In particular, the competitive adsorption of CO and H2 has been examined accounting for the spectrum of information available for CO adsorption on palladium, as well as for hydrogen in palladium and palladium–silver alloys. A validation of the model proposed has been performed through a comparison between several literature data and model calculations, over a rather broad range of operating conditions. A quite good agreement was obtained in the different cases; the model, thus, can be profitably used for predictive purposes. [ABSTRACT FROM AUTHOR]

Abstract: In this work a novel modelling approach based on Computational Fluid Dynamics (CFD) for the prediction of the gas separation process in a Pd–Ag membrane module for H2 purification is presented. With this approach, the pressure and velocity flow fields of the gas mixture and the species concentration distribution in the selected three-dimensional domain are simultaneously and numerically computed by solving the continuity, momentum and species transport equations, including a gas-through-gas diffusion term derived from the Stefan–Maxwell formulation. As a result, the H2 permeation calculations depend on the local determination of the mass transfer resistances offered by the gas phase and by the membrane, which is modelled as a permeable surface of known characteristics. The applicability of the model to properly predict the separation process under a wide range of pressure, feed flow rate, temperature and gas mixtures composition is assessed through a strict comparison with experimental data. The influence of inhibitor species on the module performance, that is obtained by implementing in the CFD model a suitable literature correlation, is also discussed. [Copyright &y& Elsevier]

Abstract: For many years enhancement of the hydrogen permeation rate through Pd and Pd-alloy membranes has been accomplished by decreasing the membrane thickness. This approach is based on the idea that the permeation rate is limited by the diffusion process through the membrane bulk material. Theoretical modeling suggests that as membrane thickness is reduced into the micrometer range the rate-limiting permeation mechanism may shift to desorption from the permeate surface. In order to test the model predictions, free-standing Pd–Ag alloy membranes (23wt% Ag) with a thickness on the order of 1μm have been microfabricated and their hydrogen permeation behavior has been experimentally determined. At temperatures between 523 and 723K, hydrogen permeation is limited by the diffusion process and is characterized by a membrane permeability pre-exponential factor of 5.51E−8±1.34E−8mol/m/s/Pa0.5 and an activation energy of 10.8±1.25kJ/mol. At temperatures below 500K, the activation energy increases appreciably. This transition is consistent with diffusion-limited permeation in the presence of non-ideal absorption behavior of hydrogen in the Pd–Ag alloy. A transition into a desorption-limited permeation regime is not observed under the operating conditions in this study. Additionally, two distinct failure modes have been observed for these microfabricated devices. The operating conditions leading to failure and cause of each failure mode are discussed. [Copyright &y& Elsevier]

Abstract: Current theoretical models for the permeation behavior of hydrogen through palladium and palladium alloys predict that low temperature operation of thin membranes will result in permeation which is no longer controlled by the diffusion of hydrogen through the metal lattice. Specifically, the process of desorption from the downstream surface is predicted to become the dominant resistance to mass transfer. However, these models neglect the non-ideal absorption behavior of hydrogen in palladium which typically occurs at temperatures below 300°C. In this work a model is developed which accounts for the non-ideal behavior of hydrogen in palladium–silver alloys (25% silver by weight) which are typically used for hydrogen purification. This model predicts that the diffusion-limited regime should be characterized by an increase in the activation energy to ∼32kJ/mol at temperatures below 200°C. This prediction is supported by data available in the literature for thick (>125μm) palladium–silver alloy membranes. [Copyright &y& Elsevier]

Abstract: Pure and mixed gas permeation tests were performed on Pd-based hydrogen selective membranes at different experimental conditions. In particular the permeance of pure hydrogen as well as of binary and ternary mixtures containing hydrogen, nitrogen and methane was measured, at temperatures ranging from 673 to 773K and at pressure differences up to 6bar. The membranes, supplied by NGK Insulators Ltd., Japan, were formed by a selective Pd–Ag layer (20wt% Ag) deposited on a tubular ceramic support, and showed very high hydrogen permeance and a practically infinite selectivity toward hydrogen. Interestingly, the permeance values measured in pure gas experiments resulted always higher than those obtained in permeation tests with gas mixtures; in the latter case, moreover, the permeate flux significantly deviates from Sieverts’ law based on the hydrogen partial pressure in the bulk gas phase. Both facts suggest the existence of non-negligible resistances to hydrogen transport in the gas phase itself, in addition to that offered by the metallic membrane. Experiments performed with increasing feed flow rates, showed also an increase in hydrogen permeance thus revealing the importance of the concentration polarization effects inside the module. Gas phase mass transport coefficients were calculated and used to determine the role of such a resistance in the overall mass transport process. The Sherwood number was also evaluated and was found to follow a boundary layer type of correlation. A general sensitivity analysis was performed in order to compare the effects on the transmembrane hydrogen flux of the two resistances, with different physical dimensions, offered by the gas phase and the metallic membrane. The concentration polarization number thus introduced allows for an a priori identification of the leading resistance at any operating conditions and gives clear indications on the actions required to improve the module performance. [Copyright &y& Elsevier]

Abstract: The long-term stability of Pd–23%Ag/stainless steel composite membranes has been examined in H2/N2 mixtures as a function of both temperature and feed pressure. During continuous operation, the membrane shows a good stability at 400°C while the N2 leakage increases very slowly at a temperature of 450°C (P feed =10bar). After 100 days of operation (P feed =5–20bar, T =350–450°C), the N2 permeance equals 7.0×10−9 molm−2 s−1 Pa−1, which indicates that the H2/N2 permselectivity still lies around 500, based on a H2 permeance equal to 3.0×10−6 molm−2 s−1 Pa−1. Despite the generation of small pinholes, a membrane life-time of several (2–3) years (T ≤425°C) is estimated for the experimental conditions employed based on long-term stability tests over 100 days. Post-process characterisation shows a considerable grain growth and micro-strain relaxation in the Pd–23%Ag membrane after the prolonged permeation experiment. Changes in surface area are relatively small. In addition, segregation of Ag to the membrane surfaces is observed. The formation of pinholes is identified as the main source for the increased N2 leakage during testing at higher temperature. [Copyright &y& Elsevier]