Recent Advances in Pd-Based Membranes for Membrane Reactors

Alba Arratibel Plazaola, David Pacheco Tanaka, Martin Van Sint Annaland, Fausto Gallucci
2017 Molecules  
A.Arratibel.Plazaola@tue.nl (A.A.P.); M.v.SintAnnaland@tue.nl (M.V.S.A.) Abstract: Palladium-based membranes for hydrogen separation have been studied by several research groups during the last 40 years. Much effort has been dedicated to improving the hydrogen flux of these membranes employing different alloys, supports, deposition/production techniques, etc. High flux and cheap membranes, yet stable at different operating conditions are required for their exploitation at industrial scale. The
more » ... ntegration of membranes in multifunctional reactors (membrane reactors) poses additional demands on the membranes as interactions at different levels between the catalyst and the membrane surface can occur. Particularly, when employing the membranes in fluidized bed reactors, the selective layer should be resistant to or protected against erosion. In this review we will also describe a novel kind of membranes, the pore-filled type membranes prepared by Pacheco Tanaka and coworkers that represent a possible solution to integrate thin selective membranes into membrane reactors while protecting the selective layer. This work is focused on recent advances on metallic supports, materials used as an intermetallic diffusion layer when metallic supports are used and the most recent advances on Pd-based composite membranes. Particular attention is paid to improvements on sulfur resistance of Pd based membranes, resistance to hydrogen embrittlement and stability at high temperature. Molecules 2017, 22, 51 2 of 53 and Oyama [7] . Membrane preparation methods were also reported by Paglieri and Way in 2002, who described their application in a membrane reactor, fuel cell, hydrogen isotopes separation and novel applications such as quantification of produced hydrogen by microbes, among others [8] . Before separation, hydrogen is produced mainly by steam reforming (SR) or autothermal (ATR) reactions of different feedstocks, such as methane, higher hydrocarbons, methanol and ethanol; the produced hydrogen can be separated downstream from the exhaust gas stream using hydrogen perm-selective membranes. More interestingly, the separation can be integrated in the reactor (in a membrane reactor-MR) such that the in-situ hydrogen separation will also positively affect the conversion and selectivity of the often equilibrium limited reactions (shift effect according to Le Châtelier's principle). When using a membrane reactor, the practical configuration can be either in a packed (fixed) bed or as a fluidized bed, where the catalyst is free to move around the membranes. Micro-structured membrane reactors have also been investigated in view of their good heat and mass transfer characteristics [9] [10] [11] [12] [13] , however, the exploitation of this kind of configuration is hampered by the very active catalytic activity required [10], the extremely high surface area to catalyst volume ratio (in the order 10 5 m 2 /m 3 [13] ) and by the challenging manifolding and difficulty of accessing damaged channels in a system where hundreds of channels would be required even for smallscale applications. The advantages of packed-beds lie in its simplicity in construction and well established and validated models for its design and scale-up. Additionally, the catalyst is kept in a fixed position, thus any damage of the membranes due to erosion is circumvented while scratches on the thin membrane surface can only happen when loading and unloading the catalyst from the reactor. On the other hand, the main disadvantages of this kind of reactors are the unavoidable temperature gradients that the reactor (and thus the membrane) is experiencing in both endothermic (steam reforming like) or exothermic (autothermal reforming) reaction systems [14] . More importantly, as thinner membranes are produced with much higher fluxes, the concentration polarization (or better the bed-to-wall mass transfer limitation) prevailing in packed beds will be extremely detrimental for the hydrogen recovery and thus for the overall performance of the membrane reactor [14, 15] . These limitations affect fluidized beds to a lesser extent, since they can be operated at virtually uniform temperature and reduced mass transfer limitations. However, much more attention should be paid to possible damages of membranes by erosion. This was already reported in a previous review by Gallucci et al. [16], where the authors showed and discussed in detail different reactor configurations, such as packed-bed (Figure 1 left) and fluidized-bed (Figure 1 right) membrane reactors as well as other concepts.
doi:10.3390/molecules22010051 pmid:28045434 fatcat:a6gssxjyvfftbitqfxkbyiqrpe