Tubular membrane reactors for immobilization of enzymes

Oxidation belongs among the most occurring reactions in industrial chemical processes, such as the synthesis of fine chemicals and the production of active pharmaceutical ingredients. Enzyme-catalysed oxidations offer an environmentally friendlier and sustainable alternative to conventional chemo-catalytic approaches; moreover, they open the potential for new reactions. This work aims to develop new biocatalytic reactors addressing instability and inefficient use of enzymes and gas limitations, which are some of the current challenges hindering oxygen- and other gas-dependent enzymes to be harnessed in sustainable chemical production. For this aim, polymer chemistry and chemical surface modifications were utilized to facilitate enzyme immobilization in gas/liquid continuous-flow reactors.

First, a versatile platform for studying the interaction between enzymes and surfaces of various chemical properties in a continuous-flow regime was developed. Surface-initiated atom transfer radical polymerization (SI-ATRP) was employed to manipulate, in a controlled manner, the surface properties of commercial hollowfibre polyethersulfone (PES) membranes. Membranes with a range of surface chemistries were prepared and characterized. The impact that surface properties have on enzyme immobilization and biocatalytic activity was demonstrated, particularly the beneficial effect of quaternary ammonium on the biocatalytic performance of coimmobilized glucose oxidase (GOx, EC 1.1.3.4) and horseradish peroxidase (HRP, EC 1.11.1.7).

Next, an approach to use one of the most gas-permeable materials – polydimethylsiloxane (PDMS) – for enzyme immobilization and application in gas/liquid continuous-flow bioreactors was presented. A simple, fast, and oxygen-tolerant chemical modification of PDMS via surface-initiated supplemental activation reducing agent atom transfer radical polymerization (SI-SARA-ATRP) was developed. Quaternary ammonium was introduced onto the PDMS surface using this method to exploit its beneficial effect. The surface-modified PDMS showed dramatically increased immobilization yield and biocatalytic activity of GOx and HRP, which could not be achieved on a pristine PDMS surface.

Finally, a highly flexible reactor design was introduced, and its ability to continuously operate over extended periods was demonstrated. The reactor comprises a thiol-functional porous monolith and a thin PDMS wall. The monolith, prepared via polymerized high-internal-phase emulsion, retains enzyme inside the reactor in a fashion similar to liquid chromatography, leading to efficient use. The wall acts as a membrane contactor providing transport of oxygen from atmospheric air to the immediate proximity of the enzyme inside the reactor. The reactor can be rejuvenated by periodic injection of a new enzyme. Moreover, the reactor can be optimized via multiple parameters to accommodate different gas-dependent biocatalytic transformations. Overall, this work provides new platforms enabling further studies of gas-dependent enzymes immobilized in continuous-flow reactors. The presented methods allow multiple modification routes to optimize enzyme activity, stability, and efficient use while providing sufficient gas delivery. Bringing gas-dependent biocatalysis closer to its industrial exploitation will ultimately lead to an environmentally friendly, sustainable future.