Lipase, carbon monoliths

Figure 32 includes results illustrating the performance of lipase/car-bon monolith systems in an acylation reaction. For comparison, the free lipase and a commercial immobilized lipase (Novozyme) were also tested. As expected, in all cases the specific activity of immobilized lipase was foimd to be lower than that of the free enzyme. Such a difference is usually ascribed to conformational changes of the enzyme, steric effects, or denaturation. For the monolithic biocatalysts, the activity of the immobilized catalyst relative to that of the pure enzyme was found to be 30-35%, and for the Novozyme catalyst about 80% in the first rim. However, the Novozyme catalyst underwent significant deactivation, in contrast to the carbon monolith-supported catalysts. The deactivation of the Novozyme catalyst in consecutive runs is probably a consequence of the instability of the support matrix under reaction conditions (101,102). 

FIGURE 32 Initial rates of reaction in catalysis by free lipase and immobilized lipase (Novozyme, and catalyst supported on 200 cpsi carbon monolith) in the acylation of butanol with vinyl acetate in an organic medium at 300 K. 

TABLE 4 Overall Performance of Carbon-Coated, Lipase-Loaded Monolithic Catalysts in the Acylation of Butanol. 

Carbon monolith Carbon loading (wt%) Lipase adsorption (mg/gcarbon) Enzyme activity (mmol/s/genzyrne) Overall activity of monolith (pmol/gmonolith )... 

In this example, lipase is immobilized on different carbon monoliths and applied in a transesterification reaction in toluene. The biocatalysts are compared in terms of carrier preparation, enzyme immobilization, and performance. A commercially available immobilized lipase is used as a comparison. A convenient tool to compare monolithic biocatalysts is the monolithic stirrer reactor (MSR), consisting of two monoliths that have the catalyst immobilized on the wall of their channels. These monoliths work as stirrer blades that can easily be removed from the reaction medium, thereby eliminating the need for a filtration step after reaction [37]. 

Different carbon monoliths were prepared from a sucrose coating [8,43], a polyfurfuryl coating [9], a furan coating [44] via the dipcoating method, and from methane [14,35] via the CVD method over deposited Ni. Ruthenium was loaded on furan-based monoliths by impregnation from [RuCls H20] in diluted hydrochloric acid, and lipase was loaded on the other monoliths by physical adsorption from a phosphate buffer (pH 7). 

Catalytic tests with the lipase-monolithic catalysts were performed in a monolithic stirrer reactor consisting of a glass vessel equipped with a stirrer motor (V = 2.5 dm ). 1-Butanol and vinyl acetate concentrations were 0.6 M and 1 M, respectively. Activity tests with immobilized lipase Candida antarctica) were performed at varying stirrer rates and temperatures. Carbon monoliths (Westvaco integral carbon monoliths, with a loading of 30 wt% of microporous activated carbon, wall thickness 0.3 mm) were used as a reference material. 

Several monolithic enzyme biocatalysts were prepared and characterized with carbon coatings consisting of carbonized sucrose, carbonized polyfurfuryl alcohol, and carbon nanofibers. The coated carbon monoliths were also compared with an integral (composite) carbon monolith. A lipase from Candida antarctica was adsorbed on the monolithic supports. Adsorption on carbon coatings can be very effective, depending on the carbon microstructure. For a high lipase loading. 

Using the reaction rate observed (in moFs m ataiyst) in experiments performed at 150 rpm, the Wheeler-Weisz modulus was calculated for aU monoUths by estimating the layer thickness (L) from the carbon yield. inside the carbon layers was estimated to be 1 to 3 x 10 ° m /s inside the varions carbons. The valnes for are presented in Table 11.5. For 1-Al, consisting mainly of macroporons carbon, it is likely that internal diffusion limitations are present inside the carbon walls (where the lipase is located). This was also indicated by the decreased activity per gram of enzyme for these monoliths (Table 11.5). For ceramic monoliths, all adsorbed enzyme is used effectively (a constant tnmover freqnency for all carbon types), as confirmed by the low valnes for... 

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