4.2.2. Monolithic Reactors

ω ń To address the disadvantages of packed-bed microreactors, monolithic microreactors have been prepared [93,121]. Monolithic reactors include a network of micro- or mesoporous materials that can be divided into two groups: inorganic monoliths (such as silica-based monoliths [122]) and polymeric monoliths (such as acrylamide copolymers [123] and glycidyl methacrylate copolymers [124]). The porosity of monolithic microreactors enables rapid rates of mass transfer with high flow rates possible at low pressures, resulting in improved performance when compared to conventional packed beds [93]. Biggelaar et al. immobilised ω-transaminases on silica monoliths for the enantioselective transamination reaction [125]. Szyma ´nska et al. developed a silica monolithic flow reactor for the esterification of a primary diol by immobilising acyltransferase on silica monoliths [126]. Sandig et al. used novel hybrid monolith materials (monolithic polyurethane matrix containing cellulose beads) for the immobilisation of lipase. The use of the continuous flow resulted in an efficient reactor with a high turnover number of 5.1 × 10<sup>6</sup> [127]. Logan et al. immobilized three enzymes (invertase, glucose oxidase, and horseradish peroxidase) on polymer monoliths in different regions of a microfluidic device [128]. Qiao et al. prepared a monolithic and a wall-coated reactor, using L-asparaginase immobilised on poly(GMA-*co*-EDMA). A lower value of K<sup>M</sup> was obtained with the monolith reactor

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(3.8 µM) in comparison to the wall-coated microreactor (7.7 µM) with a lower value for Vmax also reported (106.2 vs. 157 µMmin−<sup>1</sup> , respectively), indicating that at a lower substrate concentration, a higher reaction rate was achieved in a monolithic reactor. The monolithic reactor showed a better affinity between substrate and enzyme than wall-coated reactors [129].
