Catalyst, biomimetic

Both mechanisms can also rationalize an increase in due to the production of superoxide/HO2 (18.16), which appears to dominate the flux of partially reduced oxygen species generated by certain biomimetic catalysts [Boulatov et al., 2002 Boulatov, 2004]. It remains to be estabhshed if either of these two mechanisms does indeed operate in simple Fe porph5Tins, for example by carrying out single-turnover experiments similarly to the approach used to study ORR by C5hochrome c oxidase. 

So far, certain biomimetic catalysts (1 and 2b in Fig. 18.17) have been shown to reduce O2 to H2O under a slow electron flux at physiologically relevant conditions (pH 7,0.2-0.05 V potential vs. NHE) and retain their catalytic activity for >10" turnovers. Probably, only the increased stability of the turning-over catalyst is of relevance to the development of practical ORR catalysts for fuel cells. In addition, biomimetic catalysts of series 1,2,3, and 5, and catalyst 4b are the only metalloporphyrins studied in ORR catalysis with well-defined proximal and distal environments. For series 2, which is by far the most thoroughly studied series of biomimetic ORR catalysts, these well-defined environments result in an effective catalysis that seems to be the least sensitive among all metalloporphyrins to the electrode material (whether the catalyst is adsorbed or in the film) and to chemicals present in the electrolyte or in the O2 stream, including typical catalyst poisons (CO and CN ). 

In contrast to simple metalloporphyrins, or cofacial diporphyrins, the catalytic performance of these biomimetic catalysts improves at higher pH as a result, the smallest overpotential was observed at pH 8 (0.5 V) and at pH > 8 no partially reduced oxygen species could be detected at any potential. 

Figure 18.20 A plausible ORR catalytic cycle by biomimetic catalysts 2 (Fig. 18.17). Cu is ligated by three imidazoles (omitted for clarity) and potentially an exogenous ligand, whose nature is not known. All intermediates other than ferric-peroxo and ferric-hydroperoxo were prepared independently.

Figure 18.21 (a) FeAc is a simplified version of the biomimetic catalysts in series 1-4... 

The prevalence of the heme in O2 metabolism and the discovery in the 1960s that metallophthalocyanines adsorbed on graphite catalyze four-electron reduction of O2 have prompted intense interest in metaUoporphyrins as molecular electrocatalysts for the ORR. The technological motivation behind this work is the desire for a Pt-ffee cathodic catalyst for low temperature fuel cells. To date, three types of metaUoporphyrins have attracted most attention (i) simple porphyrins that are accessible within one or two steps and are typically available commercially (ii) cofacial porphyrins in which two porphyrin macrocycles are confined in an approximately stacked (face-to-face) geometry and (iii) biomimetic catalysts, which are highly elaborate porphyrins designed to reproduce the stereoelectronic properties of the 02-reducing site of cytochrome oxidase. 

Bilirubin oxidase, 603-606, 621-626 Biomimetic catalysts, 679-686 Bond-breaking electron transfer reactions, 43-44... 

Roth, K.M., Zhou, Y., Yang, W. and Morse, D.E. (2005) Bifunctional small molecules are biomimetic catalysts for silica synthesis at neutral pH. Journal of the American Chemical Society, 127, 325-330. 

Much work has been done to help understand how metal ions react or catalyze reactions in solution. Many enzymes also use bound metal ions to catalyze their reactions, and there is still need to understand how they work. When we do understand them in detail, we should be able to produce biomimetic catalysts for useful processes in manufacturing. 

Figure 13.3 Application of transition-metal-substituted polyxometalates as biomimetic catalysts.

Biorefineries New catalytic pretreatment of plant materials Valorization, pretreatment or disposal of co-products and wastes from biorefinery by catalytic treatments New and/or improved catalytic processes for chemicals production through the integration of the biorefinery concept and products into the existing chemical production chain New advanced catalytic solutions to reduce waste emissions (solid, air and, especially, water) New catalysts to selectively de-oxygenate products from biomass transformation Catalysts to selectively convert chemicals in complex multicomponent feedstocks New biomimetic catalysts able to operate under mild conditions Small catalytic pyrolysis process to produce stabilized oil for further processing in larger plants... 

Enzymes are known to show high enantio-selectivity, which is a parameter one wishes to install in the MIP as well. That this is possible was demonstrated in a recent paper on enantio-selective ester hydrolysis catalyzed by MIP. The MIP imprinted with the D-enantiomer preferentially hydrolyzed the D-ester with rate enhancements of up to three comp ared to the CP [117]. Although these findings may be far from outstanding, they represent remarkable results on the route towards the generation of competitive biomimetic catalysts. 

The evidence examined here pertains to hydrolysis of the peptide bond and of proteins in water under various conditions of pH and temperature. Selective hydrolysis by artificial or biomimetic catalysts is not discussed (e.g., [65] [66]). 

A sterically protected, water-soluble synthetic iron porphyrin could provide a readily available biomimetic catalyst for both basic research and potential industrial applications. Such a synthetic hemin might be superior to the enzyme, in that being a small molecule it could interact, with the polymeric lignin molecule more readily than can ligninase. 

A new trend in the field of oxidations catalyzed by metalloporphyrin complexes is the use of these biomimetic catalysts on various supports ion-exchange resins, silica, alumina, zeolites or clays. Efficient supported metalloporphyrin catalysts have been developed for the oxidation of peroxidase-substrates, the epoxidation of olefins or the hydroxylation of alkanes. 

While only tyrosinase catalyzes the ortho-hydroxylation of phenol moieties, both tyrosinase and catechol oxidase mediate the subsequent oxidation of the resulting catechols to the corresponding quinones. Various mono- and dinu-clear copper coordination compounds have been investigated as biomimetic catalysts for catechol oxidation [21,194], in most cases using 3,5-di-tert-butylcatechol (DTBC) as the substrate (Eq. 16). The low redox potential of DTBC makes it easy to oxidize, and its bulky tert-butyl groups prevent un-... 

The area between enzymatic and chemical catalyses, associated with simulation of biochemical processes by their basic parameters, is accepted as mimetic catalysis. The key aspect of the mimetic catalyst is diversity of enzyme and biomimetic function processes, which principally distinguishes the mimetic model from traditional full simulation. Based on the analysis of conformities and diversities of enzymatic and chemical catalysis, the general aspects of mimetic catalysis are discussed. An idealized model of the biomimetic catalyst and the exclusive role of the membrane in its structural organization are considered. The most important achievements in the branch of catalysis are shown, in particular, new approaches to synthesis and study of biomimetic catalase, peroxidase and monooxidases reactions. 

BIOMIMETIC CATALYSTS (BIOMIMICS) FOR CATALASE, PEROXIDASE AND MONOOXYGENASE REACTIONS... 

Similar to enzymes, the biomimetic catalysts mentioned operate in liquids. Their activity depends on the diluter origin, reaction mixture pH and cell effects. Gas-phase oxidation is free from these effects, which can be considered in the first approximation as oxidation under quasi-ideal conditions [53], The study of resonance Raman spectra [54] of PPFe3+ 0H/A1203 catalase mimic indicated its clear analogy with the fifth coordinate high-spin heme Fe3+ ion, bonded to tyrosine in catalase. 

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