Gas permeation and isobutane dehydrogenation over very thinPd/ceramic membranes

Abstract

An attempt was made in this study to prepare very thin palladium membranes. Pd was electrolessly deposited on the inner side surface of a mesoporous ceramic tube using a peristaltic pump to circulate the electroless solution through the tube. To obtain defect-free Pd membranes, a two-step deposition procedure from a dilute Pd2' bath and a concentrated Pd2' bath was practised with great care to avoid the difficulties associated with the mismatch of thermal expansion coefficients between Pd and the ceramic substrate. The various membranes produced were characterized through a study of their permeation behaviors. A dense Pd membrane about 2 pm thick was permselective to hydrogen. It was found that in this very thin Pd membrane case, the hydrogen permeation flux was better correlated to hydrogen pressure using a power higher than 0.5, showing deviation from Sieverts' law. To evaluate the efficiency of the prepared thin Pdlceramic membranes, iso-butane dehydrogenation was used as a model reaction. Enhanced dehydrogenation yields were obtained in the membrane reactor due to the separation of hydrogen from the reaction medium.Le present article porte sur la preparation de membranes de palladium ultraminces. Le depdt de palladium a ete fait par placage sans electrode sur la paroi interieure d'un tube mesoporeux de ceramique avec I'aide d'un pompe peristaltique qui fait circuler la solution de deposition a travers le tube. Pour obtenir des membranes composites sans defaut, le depdt a bte effectue en deux etapes. Dans la premiere etape, un bain de Pd2' dilue a ete utilise pour deposer des particules isolees de Pd sur la surface du substrat. Dans la deuxieme etape, un film de Pd continu est forme dans un bain de Pd2' concentre. I1 faut mettre beaucoup de soin durant le depbt pour surmonter les difficultes associees a la grosse difference des coefficients de dilatation thermique entre le Pd et le substrat ceramique. Diverses membranes preparees ont ete caracterisees par I'etude de leur comportement a la permeation des gaz. Une membrane non-poreuse d'une epaisseur de 2 pm n'etait permeable qu'a I'hydrogene. Dans ce cas-la, le flux de permeation d'hydrogene est relie a la pression d'hydrogene a un exposant superieur a 0.5. Le deshydrogenation de I'isobutane a ete choisie comme reaction modble pour evaluer les membranes ainsi preparees. Les rendements de la reaction de deshydrogenation ont ete plus eleves dans le reacteur a membrane du fait de la separation de I'hydrogene.

A New Preparation Technique for Pd/Alumina Membranes with Enhanced High-Temperature Stability

“…(b) Since Sn has a low melting point (505 K), it could be enhancing metallic diffusion along the grain boundaries, widening channels for the diffusion of gases. (c) The mismatch in the thermal expansion coefficient between Pd and alumina could result in film cracking or delamination. , (d) Residual porosity in the Pd film produced during electroless plating could easily transform into pores. We believe that all four of these processes play a role in the observed pore formation and selectivity decline.…”

“…Restoration of the permselectivity of membrane 11 upon lowering the temperature as discussed above may have been from the opening of diffusion paths because of stress induced by a mismatch in the thermal expansion coefficient, which then closed when the stress was relieved by returning to a lower temperature. Shu et al have attempted to alleviate this problem by forming the Pd film using two different plating baths which create different structures . The first bath deposits a more porous film in order to help absorb stress from a mismatch in the thermal expansion coefficient while the second bath deposits a denser, selective layer.…”

Experimental study and numerical simulation of hydrogen/isobutane permeation and separation using MFI-zeolite membrane reactor

“…A temperature of 723 K was chosen because the experimental studies of isobutane dehydrogenation in a catalytic membrane reactor are normally performed at this temperature [3,[5][6][7]. Previous calculations of viscous flow over diffusion ratio [13] have shown that convective contribution to transport is negligible (less than 3% of the global flux).…”

“…Indeed continuous and selective hydrogen removal can either improve olefins yields, or allows lower operating temperatures for the same conversions. Experimental studies illustrating such an improvement on isobutane dehydrogenation have been reported [3][4][5].…”

High-temperature membrane reactors: potential and problems

“…In this context, studies on dehydrogenations of alkanes and aromatic compounds have still received most attention (see e.g. those performed on Pd membranes: Gobina et al, 1995;Gobina and Hughes, 1996b;Sheintuch et al, 1997;Shu et al, 1997). Attention has also been paid to the steam-and CO -reforming of methane to syngas, led to high per-pass conversions by selective permeation of hydrogen through Pd}Ag membranes (Shu et al, 1995).…”