Continuous two-phase biocatalysis using water-in-oil Pickering emulsions in a membrane reactor: Evaluation of different nanoparticles

Pickering emulsions are currently receiving increased attention as a promising alternative to dispersions or surfactant stabilized emulsions in two-phase biocatalysis. In order to design a continuous membrane reactor using water-in-oil Pickering emulsions for biocatalysis, knowledge on the filterability of the Pickering emulsion is required. This can be influenced by a number of factors, e.g. type and size of stabilizing nanoparticles, applied solvent, and used membrane. In a previous application of water-in-oil Pickering emulsions for continuous biocatalysis in a membrane reactor, spherical silica particles were used to stabilize the emulsion. This resulted in densely packed filter cakes due to unbound residual particles and decreasing flux over time. Furthermore, rather large droplet sizes with small specific interfacial areas were achieved. In this work, the use of colloidal silica nanoparticle for the stabilization of bioactive Pickering emulsions was studied. The filterability of bioactive water-in-oil Pickering emulsions stabilized by spherical or colloidal silica nanoparticles was compared. Using colloidal silica nanoparticles was found to result in smaller emulsion drop sizes, higher filterability and better reproducibility of drop size distribution and flux. An increase in water volume fraction decreased the flux level, but filtration was still possible at industrially relevant fluxes. With the selected nanoparticles, continuous biocatalysis in a membrane reactor at constant flux, i.e. constant residence time, was performed twice for 30 h. Substrate and product concentrations were constant and reproducible and the enzyme was still active after 30 h. The productivity was higher than that obtained with spherical silica nanoparticles and the process duration is the longest so far reported for continuous biocatalysis in PE at industrially relevant residence times.