Heterogeneous catalysis biocatalysts

Catalysis is known as the science of accelerating chemical transformations. In general, various starting materials are converted to more complex molecules with versatile applications. Traditionally, catalysts are divided into homogeneous and heterogeneous catalysts, biocatalysts (enzymes), photocatalysts, and electrocatalysts, which are mainly used... 

This example treats a diffusion-reaction process in a spherical biocatalyst bead. The original problem stems from a model of oxygen diffusion and reaction in clumps of animal cells by Keller (1991), but the modelling method also applies to bioflocs and biofilms, which are subject to potential oxygen limitation. Of course, the modelling procedure can also be applied generally to problems in heterogeneous catalysis. 

In the case of gel entrapped biocatalysts, or where the biocatalyst has been immobilised in the pores of the carrier, then the reaction is unlikely to occur solely at the surface. Similarly, the consumption of substrate by a microbial film or floe would be expected to occur at some depth into the microbial mass. The situation is more complex than in the case of surface immobilisation since, in this case, transport and reaction occur in parallel. By analogy with the case of heterogeneous catalysis, which is discussed in Chapter 3, the flux of substrate is related to the rate of reaction by the use of an effectiveness factor rj. The rate of reaction is itself expressed in terms of the surface substrate concentration which in many instances will be very close to the bulk substrate concentration. In general, the flux of substrate will be given by ... 

Biocatalysis is a rather special case, somewhere between homogeneous and heterogeneous catalysis. In most cases, the biocatalyst is an enzyme - a complex protein that catalyzes the reactions in living cells. Enzymes are extremely effident catalysts. An enzyme typically completes 1000 catalytic cycles in one second. Compared to this, conventional homogeneous and heterogeneous catalysts are slow and inefficient (100-10000 cydes per hour). Speed, however, is not the only advantage enzymes specialize in converting one specific reactant into another... 

Case studies, biocatalysis vs. heterogeneous catalysis, systematic comparison of overall processes and of catalytic steps, ecological parameters Combination of chemical and biocatalytic steps, toolbox of biocatalysts, back integration Recombinant enzyme, process development, process comparison concerning waste Overview of various production methods, types of biocatalysts... 

For most applications, enzymes are purified after isolation from various types of organisms and microorganisms. Unfortunately, for process application, they are then usually quite unstable and highly sensitive to reaction conditions, which results in their short operational hfetimes. Moreover, while used in chemical transformations performed in water, most enzymes operate under homogeneous catalysis conditions and, as a rule, cannot be recovered in the active form from reaction mixtures for reuse. A common approach to overcome these limitations is based on immobilization of enzymes on solid supports. As a result of such an operation, heterogeneous biocatalysts, both for the aqueous and nonaqueous procedures, are obtained. 

Catalytic transformations can be divided on the basis of the catalyst-type - homogeneous, heterogeneous or enzymatic - or the type of conversion. We have opted for a compromise a division based partly on type of conversion (reduction, oxidation and C-C bond formation, and partly on catalyst type (solid acids and bases, and biocatalysts). Finally, enantioselective catalysis is a recurring theme in fine chemicals manufacture, e.g. in the production of pharmaceutical intermediates, and a separate section is devoted to this topic. 

In this communication a study of the catalytic behavior of the immobilized Rhizomucor miehei lipase in the transesterification reaction to biodiesel production has been reported. The main drawbacks associated to the current biodiesel production by basic homogeneous catalysis could be overcome by using immobilized lipases. Immobilization by adsorption and entrapment have been used as methods to prepare the heterogeneous biocatalyst. Zeolites and related materials have been used as inorganic lipase supports. To promote the enzyme adsorption, the surface of the supports have been functionalized by synthesis procedures or by post-treatments. While, the enzyme entrapping procedure has been carried out by sol-gel method in order to obtain the biocatalyst protected by a mesoporous matrix and to reduce its leaching after several catalytic uses. 

Adsorption on a solid catalyst surface, complex formation in homogeneous catalysis with metallo-organic complexes and in biocatalysis with enzymes share the same principle, i.e. the total number of sites is constant. Therefore, the rate expressions for reactions on heterogeneous, homogeneous and biocatalysts have a similar form. The constant number of active sites results in rate expressions that differ from homogeneous gas phase kinetics. Partial pressures are usually used in rate expressions for gas-phase reactions, while concentrations are used when the reactions take place in the liquid phase. It appears that definitions and nomenclature of particular kinetics constants in the different sub-communities differ sometimes. In the following sections the expressions used by the different subdisciplines will be compared and their conceptual basis outlined. 

Solid-catalyzed reactions are generally examined in continuous-flow reactors, while homogeneously catalyzed reactions (including reactions catalyzed by non-immobilized biocatalysts) are usually studied in batch reactors. An important difference between chemical catalysis and biocatalysis is that it is relatively easy to study the biocatalyst either as a homogeneous or a heterogeneous catalyst, as desired. 

---