Chemical membrane reactors

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. 

Simulations based on kinetic modelling of the reduction of acetophenone with propan-2-ol, using polymer-enlarged and the unmodified catalysts, revealed that comparable performance cannot be obtained by batch operation. Polymer enlargement allowed a continuous operation of transfer hydrogenation in a chemical membrane reactor.353... 

For example, POPAM dendrimers of 1,3-diaminopropane type have been used in membrane reactors as supports for palladium-phosphine complexes serving as catalysts for allylic substitution in a continuously operated chemical membrane reactor. Good recovery of the dendritic catalyst support is of advantage in the case of expensive catalyst components [9]. It is accomplished here by ultra-or nanofiltration (Fig. 8.2). 

Fundamental aspects of chemical membrane reactors (MRs) were introduced and discussed focusing on the peculiarity of MRs. Removal by membrane permeation is the novel term in the mass balance of these reactors. The permeation through the membrane is responsible for the improved performance of an MR in fact, higher (net) reaction rates, residence times, and hence improved conversions and selectivity versus the desired product are realized in these advanced systems. The permeation depends on the membranes and the related separation mechanism thus, some transport mechanisms were recalled in their principal aspects and no deep analysis of these mechanisms was proposed. 

The most interesting candidate reactions for chemical membrane reactors will be those currently compromised... 

Y. Ye, L. Rikho-Struckmann, B. Munder, et al., Feasibility of an electro-chemical membrane reactor for partial oxidation of n-butane to maleic anhydride. 

Kragl et al.100 described the retention of diaminopropyl-type metallodendrimers bearing palladium phosphine complexes on ultra- or nanofiltration membranes and their use as catalysts for allylic substitution in a continuously operating chemical membrane reactor. Their results demonstrated a viable procedure for catalyst recovery, because these metallodendrimers acting as catalyst supports offered an advantage in that the intrinsic viscosity of the solution is smaller, facilitating filtration. 

A concise overview of recycling modes for chiral catalysts, including enzymes, is given by Kragl et al. [15]. The technical aspects of the application of continuously operated chemical membrane reactors was covered by Woltinger et al. [16, 17]. 

The oxidative reagent was formed by ffJ-CPBA and NMO. Upon recycling it was observed an enhancement of the stability of the catalyst, the total turnover number inereasing from 23.5 for a single batch to 80 in fom repetitive batches. Moreover, some metal leaehing occurred because the maximum conversion decreased from 98 to 75%, but enantioselectivities were less affected they decreased from 95 to 88% ee from the first to the fourth batch. Nevertheless, in a continuously operated chemical membrane reactor, the TTN reached up to 240 after 20 residence times with conversions up to 70% and ee up to 92%. 

Barbieri G and Scura F (2009), Fundamental of chemical membrane reactors , in Drioli E and Giorno L, Membrane Operations, Innovative Separations and Transformations, Weinheim, Wiley-VCH, 287-308. 

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