Dense polymeric catalytic membrane

In a dense polymeric catalytic membrane the catalyst can be a thin layer on the membrane surface or distributed in the thickness of the polymeric matrix. An exhaustive review of methods for the preparation of catalytic polymeric membranes has been reported by Ozdemir et al. (2006). Vankelecom (2002) thoroughly reviewed the application of polymeric membranes in catalytic reactors. 

Abstract The objective of this chapter is to give an overview of the use of polymeric membranes in membrane reactors. Since the stndy of polymeric membrane reactors is a multidisciplinary activity, the chapter begins with some basic concepts of polymer science and polymer membranes. In the following, the different types of polymeric membrane reactors, classified into two main groups - polymeric inert membrane reactors (PIMRs) and polymeric catalytic membrane reactors (PCMRs), are presented and discussed. For each of these group , examples of the main reactor types are given extractors, forced-flow or contactors. Finally, there is a discussion of the modelhng aspects of membrane reactors with dense polymeric catalytic membranes reported in the literature. 

Sousa J M, Cruz P and Mendes A M (2001), A study on the performance of a dense polymeric catalytic membrane reactor , Catal Today, 67,281-291. 

Sousa J M and Mendes A M (2003), Simulation study of a dense polymeric catalytic membrane reactor with plug-flow pattern , Chem EngJ, 95,67-81. 

Yawalkar et al. (2001) has developed a model for a three-phase reactor based on the use of a dense polymeric composite membrane containing discrete cubic zeolite particles (Fig. 4.5) for the epoxidation reaction of alkene. Catalytic particles of the same size are assumed vdth a cubic shape and uniformly dispersed across the polymer membrane cross-section. Effects of various parameters, such as peroxide and alkene concentration in liquid phase, sorption coefficient of the membrane for peroxide and alkene, membrane-catalyst distribution coefficient for peroxide and alkene and catalyst loading, have been studied. The results have been discussed in terms of a peroxide effidency defined as the ratio of flux of peroxide through the membrane utilized for alkene oxidation to the total flux of organic peroxide through the membrane. The paper aimed to show that, by using an organophilic dense membrane and the catalysts confined in the polymeric matrix, the oxidant concentration (in that reaction peroxides) can be controlled on the active site with an improvement of the peroxide efficiency and selectivity to desired products. 

A membrane-induced structure—reactivity trend, which may be exploited to achieve selective processes, was observed in polymeric catalytic membranes prepared by embedding decatungstate within porous membranes made of PVDF or dense polydimethyl-siloxane (PDMS) membranes. These photocatalytic systems are characterized by different and tunable properties depending on the nature of the polymeric microenvironment (Bonchio et al., 2003). The polymeric catalytic membranes prepared were used for the batch-selective photooxidation of water-soluble alcohols. Membrane-induced discrimination of the substrate results from the oxidations of a series of alcohols with different polarity, through comparison with the homogeneous reactions (Fig. 27.7). 

Polymeric catalytic membranes, when compared with inert membranes, offer the possibility of tuning the sorption of reactants and products in the close vicinity of the catalytic active sites, by a careful selection of the polymeric environment. As was mentioned above, in porous catalytic membranes the choice of the polymer is of less importance, since permeation does not take place through the polymer matrix. However, in the case of dense membranes, sorption and transport properties are crucial for the catalytic performance and are strongly affected by the polymeric matrix. 

More recently, composite polymeric catalytic membranes consisting of a dense layer of a mixed-matrix of tiny particles of Amberlyst-35 dispersed in PVA cross-linked with maleic add cast over a commercial PVA membrane (PERVAP 1000), were effidently used in the pervaporation-assisted esterification of acetic acid and ethanol. After 8 h of reaction, a 60% increase in conversion was observed for the catalytic membrane configuration, compared to an inert membrane/fiuidized-bed configuration. 

Because of their intrinsic characteristics, mostly dense rubbery polymers have been considered for the preparation of polymeric catalytic membranes. The mass-transport mechanism considered has been the well-known sorption-diffusion model.ii Modelling the kinetics of the reaction(s) occurring at the occluded catalyst level is a much more complex task. The reaction may be carried out under special operating conditions, for example in a batch reactor where the catalyst is dispersed in a support - or directly inside the catalytic membrane, -" a reaction-rate model is assumed and the related parameters are determined by fitting the global model to the experimental data. In other cases, the kinetic models determined by other authors are used. - In some theoretical studies, a hypothetical reaction-rate model and the respective model parameters are assumed. 

The same reaction has been studied, in recent years, by Frisch and co-workers [2.43] using dense polymeric membranes. The dense polymeric membranes were prepared by blending polyethylacrylate with a 13X zeolite, which contained a dehydrogenation catalyst (Ti or Ni). They also prepared catalytic polymeric membranes by free radical polymerization of the monomer in the presence of the zeolite. These membranes were shown to be active for the cyclohexane dehydrogenation reaction at low temperatures. The recent development of thermally resistant polymeric membranes (Koros and Woods [2.44] and Rezac and Schoberl [2.45]) provides promise for the more widespread use of such membranes in CMR applications. 

Sousa et al [5.76, 5.77] modeled a CMR utilizing a dense catalytic polymeric membrane for an equilibrium limited elementary gas phase reaction of the type ttaA +abB acC +adD. The model considers well-stirred retentate and permeate sides, isothermal operation, Fickian transport across the membrane with constant diffusivities, and a linear sorption equilibrium between the bulk and membrane phases. The conversion enhancement over the thermodynamic equilibrium value corresponding to equimolar feed conditions is studied for three different cases An > 0, An = 0, and An < 0, where An = (ac + ad) -(aa + ab). Souza et al [5.76, 5.77] conclude that the conversion can be significantly enhanced, when the diffusion coefficients of the products are higher than those of the reactants and/or the sorption coefficients are lower, the degree of enhancement affected strongly by An and the Thiele modulus. They report that performance of a dense polymeric membrane CMR depends on both the sorption and diffusion coefficients but in a different way, so the study of such a reactor should not be based on overall component permeabilities. 

Figure 4.3e considers a dense catalytic layer which is permeable to both the reactants present in the segregated phases. This is the case of several catalytic polymeric membranes, either unsupported or supported on porous substrates. The reaction rate is governed by the relative rate of diffusion and reaction in the thickness of the catalytic membrane. In this case the solubility of the components in the membrane layer should also be taken into account. 

A particular case of the contact mode sketched in Fig. 4.3f is represented by the use of catalytic dense polymeric membranes working in cross-flow mode on the liquid feed side and in pervaporation mode through the membrane (Bengston et al, 2002). This particular class will be not discussed further, since Chapter 1 of Handbook of membrane reactors Volume 1 Fundamental materials science, design and optimisation is dedicated to polymeric membrane reactors.. 

The following discussion will focus exclusively on membrane reactors with polymeric catalytic dense membranes. Cases of membrane reactors with a polymeric non-catalytic membrane are not presented and discussed, as the membrane only performs some separation task and the models to be considered are independent of the type of membrane, considering that an adequate transport equation is considered. The cases of porous polymeric membranes with catalyst supported in the porous walls, typical in the biomembrane reactors, photocatalysis, amongst others, are also not considered here. 

Porous catalytic polymeric membranes dense catalytic pervaporative membranes... 

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