Heterogeneous biocatalysis

Special attention is given to the integration of biocatalysis with chemocatalysis, i.e., the combined use of enzymatic with homogeneous and/or heterogeneous catalysis in cascade conversions. The complementary strength of these forms of catalysis offers novel opportunities for multi-step conversions in concert for the production of speciality chemicals and food ingredients. In particular, multi-catalytic process options for the conversion of renewable feedstock into chemicals will be discussed on the basis of several carbohydrate cascade processes that are beneficial for the environment. 

The objective of this NoE is to strengthen research in catalysis by the creation of a coherent framework of research, know-how and training between the various disciplinary catalysis communities (heterogeneous, homogeneous, and biocatalysis) with the objective of achieving a lasting integration between the main European Institutions in this area. IDECAT will create the virtual European Research Institute on Catalysis (ERIC) that is intended to be the main reference point for catalysis in Europe. 

Because enzymes are insoluble in organic solvent, mass-transfer limitations apply as with any heterogeneous catalyst. Water-soluble enzymes (which represent the majority of enzymes currently used in biocatalysis) have hydrophilic surfaces and so tend to form aggregates or stick to reaction vessel walls rather than form the fine dispersions that are required for optimum efficiency. This can be overcome by enzyme immobilization, as discussed in Section 1.5. 

HNLs comprise a heterogenous enzyme family, since hydroxynitrile lyase activity has evolved in different structural frames by convergent evolution [17, 18]. Thus, (S) -specific HNLs based on an a/P-hydrolase fold framework from Manihot esculmta (cassava) [19-21], Hevea hrasilensis (rubber tree) [22-26], and Sorghum hicolor (millet) [27-33] have been described. (R)-specific HNLs based on the structural framework of oxidoreductases were isolated from Linum usitatissimum (flax) [30, 34-37] and Rosaceae (e.g., bitter almonds) [31, 38]. Despite their potential in biocatalysis only few HNLs (from cassava and rubber tree) are available by recombinant gene expression, which is a prerequisite for their technical application [20, 24]. Thus, cloning, recombinant expression, and... 

There are many factors that influence the outcome of enzymatic reactions in carbon dioxide. These include enzyme activity, enzyme stability, temperature, pH, pressure, diffusional limitations of a two-phase heterogeneous mixture, solubility of enzyme and/or substrates, water content of the reaction system, and flow rate of carbon dioxide (continuous and semibatch reactions). It is important to understand the aspects that control and limit biocatalysis in carbon dioxide if one wants to improve upon the process. This chapter serves as a brief introduction to enzyme chemistry in carbon dioxide. The advantages and disadvantages of running reactions in this medium, as well as the factors that influence reactions, are all presented. Many of the reactions studied in this area are summarized in a manner that is easy to read and referenced in Table 6.1. 

This chapter outlines the principles of green chemistry, and explains the connection between catalysis and sustainable development. It covers the concepts of environmental impact, atom economy, and life-cycle analysis, with hands-on examples. Then it introduces the reader to heterogeneous catalysis, homogeneous catalysis, and biocatalysis, explaining what catalysis is and why it is important. The last two sections give an overview of the tools used in catalysis research, and a list of recommended books on specialized subjects in catalysis. 

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... 

Before any catalysis can occur, at least one of the substrates must coordinate to the catalyst. This means that the catalyst must have a vacant active site. In homogeneous metal complex catalysis and biocatalysis, this will be a vacant coordination site at the metal atom. In heterogeneous catalysis, the vacant site could be a metal crystallite or an ion on the surface. For the latter, we speak of desorption and adsorption instead of dissociation and coordination. Remember that our reactions are not in vacuum, so there is no vacant site . Thus, before any chemical species can coordinate to the metal complex (or to the active site in heterogeneous catalysis or biocatalysis) the species already occupying this space must first vacate it. This happens constantly, as the system is dynamic (Figure 3.3) [15]. At any given moment... 

An interesting alternative that combines the advantages of both classical and quantum mechanics is to use hybrid QM/MM models, first introduced by Arieh Warshel for modeling enzymatic reactions [7]. Here, the chemical species at the active site are treated using high-level (and therefore expensive) QM models, which are coupled to a force field that describes the reaction environment. Hybrid models can thus take into account solvent effects in homogeneous catalysis, support structure and interface effects in heterogeneous catalysis, and enzyme structure effects in biocatalysis. 

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