Membrane homogeneous catalysis

Several L-amino acids are produced on a large scale by enzymatic resolution of N-acetyl-D,L-amino adds (Figure A8.4). Acylase immobilised on DEAE-Sephadex is for example employed in a continuous process while Degussa uses the free acylase retained in a membrane reactor. In the latter process the advantage of reuse of the enzyme and homogeneous catalysis are combined. 

The separation of homogeneous catalysts by means of membrane filtration has been pioneered by Wandrey and Kragl. Based on the enzyme-membrane-reactor (EMR),[3,4] that Wandrey developed and Degussa nowadays applies for the production of amino acids, they started to use polymer-bound ligands for homogeneous catalysis in a chemical membrane reactor (CMR).[5] For large enzymes, concentration polarization is less of an issue, as the dimension of an enzyme is well above the pore-size of a nanofiltration membrane. 

The only ceramic membranes of which results are published, are tubular microporous silica membranes provided by ECN (Petten, The Netherlands).[10] The membrane consists of several support layers of a- and y-alumina, and the selective top layer at the outer wall of the tube is made of amorphous silica (Figure 4.10).[24] The pore size lies between 0.5 and 0.8 nm. The membranes were used in homogeneous catalysis in supercritical carbon dioxide (see paragraph 4.6.1). No details about solvent and temperature influences are given but it is expected that these are less important than in the case of polymeric membranes. 

Under carefully adjusted experimental conditions unmodified catalysts can be used in nanofiltration coupled homogeneous catalysis. Also non-dendritic but nanosized rigid catalytic systems can be retained by nanofiltration membranes. In this section, unmodified catalysts and rigid non-dendritic systems applied in continuous catalysis will be discussed. 

Sequential hydroformylation/reductive amination of dendritic perallylated polyglycerols with various amines in a one-pot procedure to give dendritic polyamines in high yields (73-99%). Furthermore, the use of protected amines provides reactive core-shell-type architectures after deprotection. These soluble but membrane filterable multifunctional dendritic polyamines are of high interest as reagents in synthesis or as supports in homogeneous catalysis as well as nonviral vectors for DNA-transfection (Scheme 18) [65]. 

An excellent production figure for (R)-mandelonitrile (2400 g/1 per day) was achieved by Kragl et al. [105] using a continuously stirred tank reactor in which an ultrafiltration membrane enables continuous homogenous catalysis to occur from an enzyme (PaHnl) which is retained within the reaction vessel. In order to quench the reaction the outlet of this vessel was fed into a vessel containing a mixture of chloroform and hydrochloric acid, which allowed for accurate product analysis. 

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). 

For the production of chemicals, food additives and pharmaceutical products, homogeneous catalysis offers some attractive features such as a high selectivity and activity, e.g. in asymmetric synthesis. However, since most homogeneous catalysts are relatively expensive, their current industrial application is limited [3]. On the other hand, heterogeneous catalysts can easily be separated from the products and can be recycled efficiently. Membrane separations with emphasis on nanofil-tration and ultrafiltration will allow for a similar recyclability of homogeneous catalysts, which is important both from an environmental as well as a commercial... 

Seen the list of demonstrated applications, numerous possibilities exist for the integration of homogeneous catalysis and a membrane separation. A complicating factor, however, is the relatively limited availability of solvent-resistant membranes. This will require a substantial development effort to obtain more solvent-stable membranes, including both polymeric and inorganic ones. 

By choosing the right combination of catalyst, polymer, and membrane reactor, tremendous progress has been made in continuous homogeneous catalysis. 

CMRs can offer viable solutions to the main drawback of homogeneous catalysis catalyst recycling. In addition, the membrane can actively take part in the reactive processes by controlling the concentration profiles thanks to the possibility to have membranes with well-defined properties by the modulation of the membrane material and structure. 

In general, two types of CFMRs are applied in homogeneous catalysis the dead-end-filtration reactor (Fig. 3B) and the loop reactor (Fig. 3C) [19]. In the dead-end-filtration reactor the nanosized catalyst is compartmentalized in the reactor and is retained by nanofiltration membranes. Reactants are continuously pumped into the reactor, whereas small molecules (products and substrates) cross the perpendicularly positioned membrane due to the pressure exerted. Unreacted materials can be processed by adding them back into the reactor in this set-up. Concentration polarization of the catalyst near to the membrane surface can occur using this technique. In contrast, when a loop reactor is used, such behavior is prevented, since the solution is continuously circulated through the reactor and no pressure is exerted in the direction of the parallel-positioned membrane, so small particles cross the membrane laterally. 

U. Kragl, C. Dreisbach, Membrane Reactors in Homogeneous Catalysis, in B. Cornils, W. A. Herrmann, Eds., Applied Homogeneous Catalysis with Organometallic Compounds, 2nd edn., Wiley-VCH Weinheim, 2002, p. 941. 

As mentioned in Chapter 8, many catalysts in homogeneous catalysis are liquid which are acids, bases or other ion-generating compounds. The corrosive nature of these catalysts and the reaction mixtures suggests the utility of inorganic membranes. Two major technical problems need to be solved. One is the immobilization or encapsulation... 

Continuous homogeneous catalysis is achieved by membrane filtration, which separates the polymeric catalyst from low molecular weight solvent and products. Hydrogenation of 1-pentene with the soluble pofymer-attached Wilkinson catalyst affords n-pentane in quantitative yield A variety of other catalysts have been attached to functionalized polystyrenes Besides linear polystyrenes, poly(ethylene glycol)s, polyvinylpyrrolidinones and poly(vinyl chloride)s have been used for the liquid-phase catalysis. Instead of membrane filtration for separating the polymer-bound catalyst, selective precipitation has been found to be very effective. In all... 

Goetheer ELV, Verkerk AW, van den Broeke UP, de Wolf E, Deelman B-J, van Koten G, and Keurentjes JTF. Membrane reactor for homogeneous catalysis in supercritical carbon dioxide. J. Catal. 2003 219 126-133. 

It is therefore not surprising that it was only when suitable methods for catalyst separation from the substrates and reaction products of homogeneous catalysis were developed that the importance of this type of process grew. The successful developments (thermal separation or chemical reaction (e. g., [26]), immobilization by means of supports and thus heterogenization (e. g., [44]), phase transfer catalysis [45], biphasic processes (e. g., [46, 47]) or separation with membrane modules [48, 49]) are described in the relevant sections of this book (cf. [50]). 

J. Kim and R. Datta, Supported liquid-phase catalytic membrane reactor-separator for homogeneous catalysis. AIChE ]., 3711 (1991) 165. 

Kragl, U. Dreisbach, C. Membrane reactors in homogeneous catalysis. In Applied Homogeneous Catalysis with Organometallic Compounds, 2nd Ed. Cornils, B., Hermann, W.A., Eds. Wiley-VCH Weinheim 2002 Vol. 2, 941-953. 

Although catalysis, in particular asymmetric catalysis, has emerged for the last century as the most irmovative and efficient tool for stereoselective transformations, its contribution to the expected discoveries of this century will be even more profound. The development of catalytic processes in ionic liquids is part of the today efforts to transform conventional homogeneous catalysis into green biphasic catalytic processes under the everyday increasing pressure of sustainability and economy issues. Efforts in these directions also include the great field of enzymes for example lipases, have been successfully used in bulk ILs and in IL-based membranes. 

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