Selectivity enhancement of microencapsulated enzymes with permselective shells
Special Symposium - EPIC-1: European Process Intensification Conference - 1
EPIC-1: New Concepts (NC)
Keywords: microencapsulation,microscale membrane reactor,selectivity inprovment
E. E. Barth (author) and D. W. Agar
University of Dortmund, Dep. of Biochemical and Chemical Engineering, 44221 Dortmund, Germany
Immobilised enzymes offer considerable advantages over dissolved enzymes, for example by virtue of their simple recovery from the reaction medium, greater stability and the lower enzyme losses in downstream processing. The retention of enzymes using membranes, which segregrate the large protein enzyme molecules from the remaining reaction medium, is a well-established technique for heterogenisation of the enzymatic catalysts, avoiding the deactivation or catalyst loss associated with alternative immobilisation procedures. The additional mass transfer resistance of the membrane material represents a major disadvantage of this immobilisation method, which can, however, be offset by employing the very high specific surface areas available in hollow fibres or microcapsules. By the use of permselective membranes one could even exploit the mass transfer resistance, in order to regulate the accessibility of the enzymatic catalyst for different substrates in a mixed system. In this manner one can achieve a more effective control of the enzymatic reaction through incorporation of an additional physical separation process, analogous to zeolithic catalysis [1], in which selectivity is enhanced by the integration of diffusive molecular sieving into the catalyst architecture. Standard techniques for immobilising enzymes on or in a membrane are encapsulation using hollow fibres, porous beads or in microcapsules [2], the last two methods being somewhat simpler than the first in terms of development, production and handling.
In the work presented, the non-selective hydrolytic enzyme α‑amylase and a permselective alginate membrane material were chosen as a suitable test system. Different types of hollow and solid spherical microcapsules containing α‑amylase were prepared by using a specially designed nozzle with a superimposed concentric air jet and varying the composition of the alginate with respect to the two component monomers ß‑D‑mannuronate and α‑L‑guluronate, which form the skeleton of the biopolymer, and the gelation solution, comprising different alkali earth metal ions at various concentrations [3]. The microbeads and hollow microspheres produced were characterised with respect to their structure, permeability and reactive behaviour. Variation of the experimental parameters enabled the systematic manipulation of both the size of the microcapsules and the thickness of the membrane (> 120 mm) for the hollow microspheres. Initial work demonstrated that the activity of the encapsulated biocatalyst in both types of microcapsules is inferior to that of free α‑amylase, but also that the hollow spheres (40% of free enzyme value) are significantly better than comparable microbeads (25% of free enyme value), as would be expected from the prevailing mass transfer resistances [4].
Subsequent studies revealed that the encapsulation of catalysts in microcapsules is an powerful method for improving their selectivity with help of a discriminating permeable membrane. Although the permselectivity of the alginate membranes is based the exclusion of larger molecules above a given molecular weight threshhold, the principle could be extended to alternative or multiple separation criteria to further enhance selectivity. The encapsulation of catalyst within a permselective shell can also be considered to be a form of process intensification yielding a microscale membrane reactor [5], with the high specific surface areas providing excellent harmonisation between reaction and diffusion rates.
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Presented Thursday 20, 15:00 to 15:20, in session EPIC-1: New Concepts (NC).
