Enzyme-Loaded Zeolite Membranes to Assist Bioconversions

Zeolite membranes separate in a continuous way mixtures of substances on the basis of different adsorption properties [1] and different molecular size and shape [2]. These membranes also present high chemical and thermal resistance and so they are interesting as membrane reactors [3]. However, their application at industrial level is hindered by reproducibility problem during the synthesis and for their high cost. Different methods are used for their preparation and the most popular are the one step [4] and the secondary growth [5]. The first one produces very thick membranes with a low concentration of defects and so they exhibit very low flux and high selectivity. The most promising method is the second one because decoupling zeolite nucleation from crystal growth permits to optimize the operating conditions of each step independently. In our institute an innovative route based on enzymatic production of l-DOPA from immobilized mushroom tyrosinase (EC 1.14.18.1) was developed [6]. The enzyme was chemically immobilised on the inner surface of zeolite membranes prepared by secondary growth method [7]. This approach combines the role of the zeolite membrane as support for the enzyme immobilization and as "scavenger" of the free radicals generated during the reaction. The experimental results showed as tyrosinase immobilised on the zeolite membrane exhibited good performance in terms of specific activity and productivity. In fact, comparing the performance of the immobilized and the free enzyme, it can be observed that the specific activity of the first one is 2.76 times higher than the second one (1.93 ?mol mg-1 min-1 and 0.70 ?mol mg-1 min-1, respectively). The possibility to regenerate the zeolite membranes by thermal treatment was also successfully demonstrated.