Polymeric membranes for membrane reactors

Various methods have been used for the synthesis and smdies of the chemical and physical properties of the polymeric membrane reactor with embedded nanoparticles. These methods include sonochemical, ultraviolet (UV), and y irradiation, and chemical reduction by reducing agents such as borohydrides and hydrazine. Studies have revealed the successful synthesis of cellulose acetate thin film with embedded bimetallic nanopaiticles such as Ni/Fe, Pd/Co, and monometallic Pd for the catalytic dechlorination and hydrogenation processes (Liu et al., 1997 Liu et al., 2000 Meyer et al., 2004). Catalytically active... 

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. 

The use of a non-pervaporative extractor-type catalytic polymeric membrane reactor has been reported for light alcohol/acetic acid esterifications. A cross-linked poly(styrene sulfonic acid) (PSA)/PVA blend flat membrane was assembled in the reactor in a vertical configuration, separating two chambers. One of the chambers was loaded with an aqueous solution of ethanol and acetic acid, while the other chamber was filled with chlorobenzene. The esterification equilibrium is displaced to the product s side by the continuous extraction of the formed ester. In the esterifications of methanol, ethanol and n-propanol with acetic acid, the reactivity through the PSA/PVA membrane was higher than that with HCl as catalyst. In that of n-butanol with acetic acid, however, it was viceversa. 

Hie use of a forced-flow polymeric membrane reactor has also been described for room temperature polychlorinated biphenyl (PCB) dechlorination. Core/shell Fe/Pd nanoparticles were synthesized on the pore walls of PVDF microfiltration membranes functionalized with poly(acrylic acid) (PAA). PAA functionalization was achieved by in situ free radical polymerization of acrylic add in the microfiltration membrane pores. Ferrous ions were then introduced into the membranes by the ion-exchange process. Subsequent reduction resulted in the in situ formation of 20-40 nm Fe nanoparticles. Bimetallic nanoparticles could be formed by post-deposition of Pd. 

Forced-flow polymeric membrane reactors have also been successfully tested for the oxidation of benzene to phenol by Molinari and co-workers. Mixed-matrix membranes consisting of CuO powder or CuO nanoparti-cles dispersed in PVDF were prepared by the inversion phase method, by using dimethylacetamide (DMAc), dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP) as solvents and water as non-solvent. The membranes were assembled in a ultraliltration unit to which a solution of acetonitrile/benzene and hydrogen peroxide (HjOj) was fed. The best results were obtained with a PVDF membrane hlled with CuO nanoparticles, with a phenol yield of 2.3% at 35°C and a contact time of 19.4 s in a single pass, in the presence of ascorbic acid. 

Another catalytic polymeric membrane reactor operating in interfacial contact mode, but now for conducting the liquid/vapour phase oxyfunc-tionalization of n-hexane with H2O2 producing a mixture of hexanones and hexanols, was studied by Kaliaguine and co-workers. The catalytic... 

In spite of the growing research effort, with the exception of fuel cells, there are only a few examples of industrial applications of non-biocatalytic polymeric membrane reactors, such as the Remedia Catalytic Filter System for the destruction of dioxins and furans from industrial combustion sources or pervaporation-assisted esterification processes. More research is required in order to find long-lasting high-performance and cheap polymeric materials and catalysts that can effectively compete with the traditional processes. On pursuing this quest, mathematical modelling and simulation are fundamental tools for the better understanding of membranes behaviour and optimization. 

It turned out that for all the polymeric amphiphiles of the (EO) -(PO)m-(EO) type there was an increase in enantioselectivity compared with the reaction without amphiphile. Moreover, the ratio of the length of the (PO) block compared with the (EO) block seemed to determine enantioselectivity and activity and not the cmc (critical micelle concentration). A (PO) block length of 56 units works best with different length of the (EO)n block in this type of hydrogenation [30]. for the work-up of the experiments, G. Oehme et al. used the extraction method, but initial experiments failed and the catalyst could not be recycled that way. To solve this problem the authors applied a membrane reactor in combination with the amphiphile (EO)37-(PO)5g-(EO)37 (Tab. 6.1, entry 9) [31]. By doing so, the poly-mer/Rh-catalyst was retained and could be reused several times without loss of activity and enantioselectivity by more than 99%. 

Ultrafiltration has been used for the separation of dendritic polymeric supports in multi-step syntheses as well as for the separation of dendritic polymer-sup-ported reagents [4, 21]. However, this technique has most frequently been employed for the separation of polymer-supported catalysts (see Section 7.5) [18]. In the latter case, continuous flow UF-systems, so-called membrane reactors, were used for homogeneous catalysis, with catalysts complexed to dendritic ligands [23-27]. A critical issue for dendritic catalysts is the retention of the catalyst by the membrane (Fig. 7.2b, see also Section 7.5). 

Membrane reactors have been investigated since the 1970s 11). Although membranes can have several functions in a reactor, the most obvious is the separation of reaction components. Initially, the focus has been mainly on polymeric membranes applied in enzymatic reactions, and ultrafiltration of enzymes is commercially applied on a large scale for the synthesis of fine chemicals (e.g., L-methionine) 12). Membrane materials have been improved significantly over those applied initially, and nanofiltration membranes suitable to retain relatively small compounds are now available commercially (e.g., mass cut-off of 400—750 Da). 

All peptide-catalyzed enone epoxidations described so far were performed using insoluble, statistically polymerized materials (neat or on solid supports). One can, on the other hand, envisage (i) generation of solubilized poly-amino acids by attachment to polyethylene glycols (PEG) and (ii) selective construction of amino acid oligomers by standard peptide synthesis-linked to a solid support, to a soluble PEG, or neat as a well-defined oligopeptide. Both approaches have been used. The former affords synthetically useful and soluble catalysts with the interesting feature that the materials can be kept in membrane reactors for continuously oper-... 

Y. Zhu, R.G. Minet and T.T. Tsotsis, A Continuous Pervaporation Membrane Reactor for the Study of Esterification Reactions Using a Composite Polymeric/Ceramic Membrane, Chem. Eng. Sci. 51, 4103 (1996). 

Another type of reactor that may have considerable future potential for use in homogeneous catalytic reactions is called the membrane reactor. These reactors have been successfully used for the commercialization of manufacturing processes based on enzyme catalysis. In fact, 75% of the global production of l-methionine is performed in an enzyme reactor. A membrane is basically an insoluble organic polymeric film that can have variable thickness. The catalyst... 

The polymeric oxazaborolidine prepared from the linear copolymer of 29 and styrene was used in membrane reactor and resulted in high total turnover number with high enantioselectivity [44]. Another polystyrene-based soluble polymeric oxazaborolidine 38 was used in the same system. Polysiloxanes are also useful polymeric supports of catalyst 39 for the same purpose [45]. 

The main advantages of reactors with composite membrane catalysts arc the higher hydrogen permeability and smaller amount of precious metals in comparison with those presented in Section II. All constructions of the reactors with plane membrane catalyst may be used for composites of thin palladium alloy film and porous metal sheet The design of reactors with composite membranes on polymeric support may be the same as for diffusion apparatus with polymeric membranes (see, for example. Ref. 138). A very promising support for the composite membrane catalysts is hollow carbon fiber [139], once properly thermostable adhesives are found. 

Despite the unique properties of inorganic membranes vs. the rather well-established polymeric ones (see Table 1 for a comparison), issues such as membrane instability, insufficient permeability or permselectivity, or simply the unbearable costs implied still hamper the application of inorganic-membrane reactors in the process industry. 

Krmelj V, Habubn M, Vasic-Racki D, and Knez Z. Polymeric membranes for the use in high pressure membrane reactors. International Symposium on Supercritical Fluids, Atlanta, GA, April 8-12, 2000. 

Today, annual market membrane of ca. U.S. 4.5 x 10 for membranes and membrane modules (mostly polymeric ones) indicates that separation processes seem to be the largest application field, whereas membrane reactors are just on the verge of being considered as a competitive tool [274]. 

Zhu Y, Minet RG, and Tsotsis TT. A continuous pervaporation membrane reactor for the study of esterification reactions using a composite polymeric/ceramic membrane. Chem Eng Sci 1996 5(17) 4103-4113. 

Apart from radioactive tritium separation from reactor atmosphere or off-gas, polymeric membranes can be applied for separation of noble gases produced by nuclear power plants and fuel reprocessing plants as an alternative to commonly used adsorption or low-temperature distillation methods. 

On the other hand, the use of polymeric membranes in CMRs is increasing in interest [16]. The cost of polymeric membranes is generally lower in comparison with inorganic ones, and the preparation protocols allow a better reproducibility moreover, the relatively low operating temperatures are associated with a less stringent demand for materials used in the reactor construction [16]. 

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