Thermodynamic permselectivity

I. Exposure to aqueous sodium tosylate solutions under thermodynamically permselective conditions. Electrochim Acta 45 3801. 

Membranes can be applied to catalysis in different ways. In most of the literature reports, the membrane is used on the reactor level (centimeter to meter scale) enclosing the reaction mixture (Figure 10.3). In most cases, the membrane is used as an inert permselective barrier in an equilibrium-limited reaction where at least one of the desired products is removed in situ to shift the extent of the reaction past the thermodynamic equilibrium. 

One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [1-3]... 

Novel unit operations currently being developed are membrane reactors where both reaction and separation occur simultaneously. Through selective product removal a shift of the conversion beyond thermodynamic equilibrium is possible. The membrane itself can serve in different capacities including (i) a permselective diffusion barrier, (ii) a non-reactive reactant distributor and (iii) as both a catalyst and permselective membrane [44]. 

The interpretation of the behaviour of PBT is more subtle. Overall mass changes upon total PBT oxidation / reduction are similar to the counter ion ("dopant") molar mass, for example FAM/Q = 93 g mol"1 in 0.01-0.1 mol dm 3 Et4N+BF47CH3CN compared to mgp — = 87 g mol"1. These results apparently imply permselectivity with little or fto solvent transfer at low electrolyte concentration, and permselectivity failure at high electrolyte concentration. As we show in the next section, this apparent permselectivity is entirely fortuitous, and results from a compensating combination of mobile species transfers. The message here is that a combination of thermodynamic and kinetic data is required to unequivocally attribute the mass change to the relevant species transfers. 

Apparent permselectivity and compensatoiy motion. Although normalised mass change data for the "doping/undoping" of PBT films were very similar to those predicted by permselectivity in the absence of solvent transfer (cf. electroneutrality), the differences were real. Furthermore, systematic variation of the anion or cation or deuteration of the solvent produced consistent trends in the departure from "simple" behaviour. Insight into the overall processes involved (the thermodynamics) is gained by considering the kinetics of mobile species transfer. 

Kinetic permselectivity. When permselectivity is not achieved in a thermodynamic sense (for example at high electrolyte concentration), we propose that it may be possible to achieve it kinetically. We aim to exploit the differing rates of mobile species transfer. In a transient experiment the response on a short time scale will be dominated by the fastest moving species. The converse will be true on a long time scale. 

Secondly, selectivity is not always achievable. For example, permselectivity of ion-exchanging polymer films fails at high electrolyte concentration. We have shown that even if permselectivity is not thermodynamically found, measurements on appropriate time scales in transient experiments can lead to kinetic permselectivity. To rationalise this behaviour we recall that the thermodynamic restraint, electrochemical potential, can be split into two components the electrical and chemical terms. These conditions may be satisfied on different time scales. Dependent on the relative transfer rates of ions and net neutral species, transient responses may be under electroneutrality or activity control. 

Preliminary results obtained in an effort to model the dehydrogenation of ethylbenzene to styrene in a "membrane reactor" are described below. The unique feature of this reactor is that the walls of the reactor are conprised of permselective membranes through which the various reactant and product species diffuse at different rates. This reaction is endothermic and the ultimate extent of conversion is limited by thermodynamic equilibrium constraints. In industrial practice steam is used not only to shift the ec[uilibrium extent of reaction towards the products but also to reduce the magnitude of the ten erature decrease which accon anies the reaction when it is carried our adiabatically. 

The implications of being able to increase the conversion of an equilibrium reaction by using a permselective membrane are several. First, a given reaction conversion may be attained at a lower operating temperature or with a lower mean residence time in a membrane reactor. This could also prolong the service life of the reactor system materials or catalysts. Second, a thermodynamically unfavorable reaction could be driven closer to completion. Thus, the consumption of the feedstock can be reduced. A further potential advantage is that, by being able to conduct the reaction at a lower temperature due to the use of a membrane reactor system, some temperature-sensitive catalysts may find new applications [Matsuda et al., 1993]. 

Very little is known about the influence of grain growth, or crystallization if the membrane is composed of an amorphous alloy, on membrane durability. The as-fabricated permselective metal membrane will be polycrystalline or amorphous, depending on the alloy composition and fabrication method. Amorphous, or metallic glass, structures are far less common than are polycrystalline structures. Both amorphous and polycrystalline structures are quasi-stable, meaning that structures are kinetically stabilized and slow to rearrange to the thermodynamically favored structure. In both cases, this would be a single crystal of the metal. 

Several approaches to the understanding of membrane permselectivity can be taken these include the use of the Nernst-Planck transport equations, irreversible thermodynamics, and... 

A pseudo-quantitative application of the theoretical formalism has been made for Nafion. The values for the requisite molecular parameters were estimated from a combination of experimental bulk thermodynamic data and molecular structure calculations using both molecular and quantum mechanics (23,24). A constraint was imposed in the development of the structural formalism. The model was constructed so that the predicted structural information could be used in a computer simulation of ion transport through an ionomer, that is, modeling the ionomer as a permselective membrane. 

Dehydrogenation of paraffins extracts a mole of hydrogen from each molecule and introduces an olefinic double bond in the conversion to olefins. Since this reaction is reversible, conversion is limited by thermodynamics [2]. Carrying out the process in a membrane reactor having hydrogen-permselective walls would allow... 

At low electrolyte concentration (typically c < 0.1 mol dm ), polypyrrole films in both redox states are permselective at equilibrium, that is, exclude salt , but the undoped film accumulates salt under kinetically controlled conditions [149]. Thus, the first redox cycle from an equilibrated reduced film must result in anion entry, but the accumulation of some cation (salt) during the redox cycle allows a mixed anion/cation mechanism to occur in a subsequent redox cycle. This is a consequence of the typical experimental timescale (10-100 s) being much shorter than the equilibration time for salt and/or solvent transfer (up to 1000 s) extended holding of the film at a given potential eventually restores equilibrium. When one moves to thermodynamically nonpermselective conditions (c < 0.1 mol dm ), the situation becomes much more complicated [150] since the presence of both anions and cations in hoth redox states opens up aU the mechanistic possibilities. 

The ideal separation foctor, equal to the ratio of the permeabilities of the two components, is also inteipretable as a product of two factors a "solubUiQr selectivity and a "mobiUty selectivity. These two selectivity contributions, consisting of the ratios of the respective conqionent solubilities and diffiisivities, indicate the relative importance of thermodynamic and kinetic fecKirs in the permselection process. Unfortunately, optimization of product permeability and membrane selectivity is oftmi difficult, and trade-offs in the two parameters may be necessary on economic grounds. A brief discussion of characterization methods and typical forms of sorption isotherms and local difiiiaon coeffidents for gases and vapors in polymers is presented below. This discussion serves as a background for rationalizing pressure dependencies of permeabilities and selectivities. 

The Gibbs-Donnan potential occurs when a nonpermeant ion (for biological systems, usually a polyion such as a protein) is unequally distributed between the two electrolyte solutions separated by a permselective membrane, which allows certain electrolyte ions to move freely between the two solutions. The second law of thermodynamics and the principle of electroneutrality restrict this movement. The first restriction requires that each permeant ion species moves only down its electrochemical potential gradient, the latter requires the sum of all positive charges (cations) to be equal to that of all negative charges (anions) in each solution. When the system reaches its equilibrium, there is no flux of any permeant species i, = 0. 

The thermodynamic limitations are partially reduced with the help of hydrogen permselective membranes they can remove hydrogen from the reaction mixture, and this produces better performances. Many configurations have been tested in most of the cases, membranes are only a tool to remove hydrogen, and they do not possess any catalytic power, with just a few examples about membranes that act as catalysts to be found in the literature, to our knowledge. 

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