The immobilisation, or more generally heterogenisation, of homogeneous catalysts is a well-established technique for facilitating the separation or retention of an active catalytic component. For conventional supports the resulting mass transfer resistance often becomes a limiting factor, with only partial utilisation of the active internal surface area as a consequence. Such unfavourable concentration profiles within the support can reduce both conversions and selectivities. Such phenomena arise in most enzyme immobilisation techniques. One of these – microencapsulation – localises the resistance to mass transfer in the enclosing membrane, whilst free diffusion takes place within the liquid-filled capsule. This structured architecture can lead to higher effectiveness factors than for comparable entrapment immobilisation procedures. Furthermore, by bestowing permselective properties on the membrane shell, one can transform the curse of mass transfer resistance into the blessing of controlled catalyst access. By integration of the ‘unit operation' of permselective diffusion at the microlevel, one achieves both process intensification through a good harmonisation between the rates of reaction and separation [1] as well as a powerful tool for manipulating the behaviour of non-selective catalytic sites, in analogy to the size-exclusion behaviour in zeolitic catalysis.

In the work presented, the non-selective hydrolytic enzyme dextranase and a permselective alginate membrane material were chosen as a test system for this advanced microencapsulation concept. Various types of hollow and solid spherical microcapsules containing dextranase were prepared using a specially designed nozzle with a superimposed peripheral air jet. The composition of the alginate was varied with respect to the two constituent monomers: ß-D-mannuronate and α-L-guluronate, the composition of the gelation solution of alkali earth metal ions [2] and using additives such as silica. The microbeads and microcapsules generated were characterised with respect to their physical structure, their permeability behaviour, enzyme leakage and catalytic performance. Variation of the experimental parameters enabled the systematic manipulation of both microcapsule size (1.8 mm) and the membrane thickness (> 150 µm). Initial work [3] demonstrated that the activity of the encapsulated biocatalyst in both microbeads and microcapsules is inferior to that of free dextranase, but also, as expected, that hollow microcapsules are superior to solid microbeads [4]. Enzyme leaching could be cut to a tolerable rate using low levels of colloidal silica to fine tune the diffusive properties of the alginate membrane. Subsequent studies revealed that the encapsulation of catalysts in microcapsules can be exploited, as envisaged, to improve their selectivity. In a mixed substrate containing two distinct dextran fractions with different molecular weight ranges (1.1 vs. 15-20 kDa) only the shorter oligosaccharide chains were hydrolysed to glucose. Although the permselectivity of the alginate membranes is based the exclusion of larger molecules above a given molecular weight threshold, the principle could be extended to alternative or multiple separation criteria to further enhance substrate selectivity and thus has considerable potential for conducting multistep reactions in a single vessel.

[1] Chai Y. Gelation Conditions and Transport Properties of Hollow Calcium Alginate Capsules. Biotechnology and Bioengineering. 87 (2). 228 233. 2004

[2] Yamagiwa K. Effects of alginate compositions and gelling conditions on diffusional and mechanical properties of calcium alginate gel beads. J. Chem. Eng. Japan. 28 (4). 463 467.1995

[3] Barth E E, Ufer A, Agar D W. Selectivity enhancement of microencapsulated enzymes with permselective shells. European Congress of Chemical Engineering (ECCE-6). Copenhagen, Denmark. 2007

[4] Konsoula Z, Liakopoulou-Kyriakides M. Starch hydrolysis by the action of an entrapped in alginate capsules α-amylase from Bacillus subtilis. Process Biochemistry. 41. 343 349. 2006