Hybrid catalysis

In the sixties of past century, a few patents issued to Bergbau Chemie [5,48,49] and to Mobil Oil [50-52], respectively described the use of CFPs as supports for catalytically active metal nanoclusters and as carriers for heterogenized metal complexes of catalytic relevance. For the latter catalysts the term hybrid phase catalysts later came into use [53,54], At that time coordination chemistry and organo-transition metal chemistry were in full development. Homogeneous transition metal catalysis was expected to grow in industrial relevance [54], but catalyst separation was generally a major problem for continuous processing. That is why the concept of hybrid catalysis became very popular in a short time [55]. 

Suppliers of POMS are Hybrid Plastics Inc., Hattiesburg, MS, USA, Hybrid Catalysis BV. 

Hybrid catalysis is a concept that combines biocatalysis with homogeneous chemical catalysis. Typically, a chemical catalyst (most often a transition-metal complex) is anchored inside a pocket of a biomolecule, and, according to the original concept, the metal complex should provide the reactivity while the chiral environment of the biopolymer determines and fine-tunes the selectivity. Meanwhile, several systems have been developed where such a clear-cut distribution of work is not possible anymore and the biomolecule contributes significantly to rate acceleration. 

The earliest example of hybrid catalysis originates from Wilson and Whitesides [1], who implanted a biotinylated rhodium(I) catalyst into avidin (Figure 18.1). The selectivity remained modest, and practical applications were lacking at that time. Twenty-five years later. Ward, Reetz, and others [2] developed approaches of directed evolution, combining either rational or combinatorial mutagenesis with screening in an iterative process. This work ultimately led to remarkable catalysts with high turnover frequencies and stereoselectivities for numerous chemical reactions. The field of protein-based hybrid catalysts has been extensively reviewed, and the reader is directed to these instructive reviews for further reference [3]. 

In addition to the major double-helical form, DNA also exists as triplexes and quadruplexes and forms complex three-way or four-way junctions. Single-stranded DNA has been shown in vitro to fold into complex, stable three-dimensional folds and to bind to smah molecules (such DNA molecules are named aptamers) or to catalyze reactions in a way similar to proteins (so-caUed deoxyribozymes or DNAzymes) [4]. This increased structural diversity makes it a more attractive host molecule for hybrid catalysis [5]. Furthermore, directed evolution approaches are easier to conduct with nucleic acids than with proteins, as nucleic acids can be directly rephcated and amphfied. 

The first published reports on DNA-based hybrid catalysis are from Roelfes, Feringa, and coworkers [6]. They could demonstrate that DNA isolated from salmon sperm (an inexpensive commodity chemical) could dramaticahy influence the stereochemistry of the Cu -catalyzed DAR in water between an aza-chalcone and cyclopentadiene when used in combination with different aminoacridine-aminomethylpyridine conjugates (Figure 18.2). In these experiments, Cu ions act as Lewis acids and the aminomethylpyridine is a bidentate Cu chelator, while the acridine moiety intercalates into DNA and thereby brings the copper catalyst into the chiral environment. In their first report, the authors could achieve moderate enantioselectivities (up to 49% ee for the endo... 

A detailed understanding of structure-function relations could not, however, be established so far in DNA-based hybrid catalysis. Likewise, applications in... 

Lin, H, Huang, Z., and Shangguan, W. (2008) Temperature-programmed oxidation of soot in a hybrid catalysis-plasma system. Chem. Eng. TechnoL, 31, 110-115. 

This process is highly suitable for rubbers with poor solubility. In this process, the rubber sheet is soaked in TEOS or quite often in TEOS-solvent mixture and the in situ sUica generation is conducted by either acid or base catalysis. The sol-gel reaction is normally carried out at room temperature. Kohjiya et al. [29-31] have reported various nonpolar mbber-silica hybrid nanocomposites based on this technique. The network density of the rubber influences the swelling behavior and hence controls the silica formation. It is very likely that there has been a graded silica concentration from surface to the bulk due to limited swelling of the rubber. This process has been predominantly used to prepare ionomer-inorganic hybrids by Siuzdak et al. [48-50]. 

Whereas the mono- and the S/S-dithioether moieties have been used to date, the 1,3-dithianyl motif was used for the first time in 2005 by Ricci et al. as a new hybrid ligand in asymmetric catalysis. Hence, a series of new chiral oxazoline-1,3-dithianes have been successfully applied to the copper-catalysed conjugate addition of ZnEt2 to enones (Scheme 2.16). The expected products were obtained in almost quantitative yields and enantioselectivities of up to 69% ee. 

Ujaque G, Maseras F (2004) Applications of Hybrid DFT/Molecular Mechanics to Homogeneous Catalysis 112 117-149 Umemoto K, see Saito S (2004) 109 41-57... 

Mok, Y.S. and Huh, Y.J. (2005) Simultaneous removal of nitrogen oxides and particulate matters from diesel engine exhaust using dielectric barrier discharge and catalysis hybrid system, Plasma Chem. Plasma Process. 25, 625-39. 

Extending these ideas to enzymatic catalysis, Jiang et al. reported the use of protamine-silica hybrid microcapsules in combination with a host gel-like bead structure to encapsulate several enzymes individually in the enzymatic conversion of C02 to methanol [20]. They used a layer-by-layer (LbL) method where alternately charged layers were deposited on an enzyme-containing CaC03 core. The layers, however, were not polyelectrolytes, but protamine and silica (Scheme 5.6). 

Most of the supports so far studied are conventional in the field of catalysis. Some new kinds of support have emerged including mesostructured materials,193 dendrimers, organic-inorganic hybrids, and natural polymers such as polysaccharides or polyaminoacids. Nevertheless, at the moment, supported catalysts still suffer from relatively poor stability when compared to classical heterogeneous catalysts, and from limited activity when compared to homogeneous catalysts. The driving force for this research is thus to make up some of these deficits. 

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