Catalytic system, toward reduction

Since the first compound of this type, [Ru(NH2)5(N2)]Bt2 [15246-25-0] was synthesized (178), most transition metals have been found to form similar compounds (179,180). Many dinitrogen compounds are so stable that they ate unreactive toward reduction and so have Htde chance to form the basis of a catalytic system. 

Complex Base-Metal Oxides Complex oxide systems include the mixed oxides of some metals which have perovskite or spinel structure. Both the perovskites and the spinels exhibit catalytic activity toward cathodic oxygen reduction, but important differences exist in the behavior of these systems. 

The replacement of vanadia-based catalysts in the reduction of NOx with ammonia is of interest due to the toxicity of vanadium. Tentative investigations on the use of noble metals in the NO + NH3 reaction have been nicely reviewed by Bosch and Janssen [85], More recently, Seker et al. [86] did not completely succeed on Pt/Al203 with a significant formation of N20 according to the temperature and the water composition. Moreover, 25 ppm S02 has a detrimental effect on the selectivity with selectivity towards the oxidation of NH3 into NO enhanced above 300°C. Supported copper-based catalysts have shown to exhibit excellent activity for NOx abatement. Recently Suarez et al and Blanco et al. [87,88] reported high performances of Cu0/Ni0-Al203 monolithic catalysts with NO/NOz = 1 at low temperature. Different oxidic copper species have been previously identified in those catalytic systems with Cu2+, copper aluminate and CuO species [89], Subsequent additions of Ni2+ in octahedral sites of subsurface layers induce a redistribution of Cu2+ with a surface copper enrichment. Such redistribution... 

The catalytic activity toward hydrogenation reactions has been indeed observed in such bimetallic colloidal systems. The SERS spectrum of p-nitrobenzoate (PNBA) adsorbed on colloidal silver and the SERS spectra observed on Ag/Pd nanoparticles, immediately after the addition of PNBA and after 1 week, are shown in Fig. 20.7 A, B, and C, respectively. In the bimetallic colloid, instead of the SERS spectrum of PNBA (spectrum A), a different spectrum is obtained (spectrum B) that slowly evolves toward a different spectral feature, which becomes predominant after a week (spectrum C). This modification may be related to the initial formation of p-aminobenzoate as a result of the catalytic reduction of the nitro group, followed by slow oxidation to azodibenzoate by atmospheric oxygen (see Fig. 20.8). 

The role of metal-support interaction on the catalytic activity of carbon-supported Pt nanoparticles toward oxygen reduction and methanol oxidation was analyzed. It was observed that both dispersion and specific activity are influenced by the interaction of the active phase with the support, determining well-defined relationships that may be used for interpreting the electrochemical behavior of new, more advanced catalytic systems. 

An improvement of catalyst activity, especially for the oxidation of electron-poor, deactivated systems like p-toluic acid, can be reached by addition of other transition metal compounds to the Co/Mn/Br catalyst. The most prominent additive is zirconium(IV) acetate, which by itself is totally inactive. An addition of zirconi-um(IV) acetate (ca. 15 % of the amount of cobalt) can yield reaction rates which are higher than those observed using a tenfold amount of cobalt acetate. This amazing co-catalytic effect can be attributed to the common ability of zirconium to attain greater than sixfold coordination in solution, to the high stability of Zr toward reduction, and to the ability of zirconium or Hf to redistribute the dimer/ monomer equilibrium of dimerized cobalt acetates (Co 7Co, Co VCo " systems) by forming a weak complex with the catalytically more active monomeric Co species [17]. 

Aliphatic amines are amongst the most important bulk and fine chemicals in the chemical and pharmaceutical industry [13]. Hydroaminomethylation of alkenes to amines presents an atom-economic, efficient and elegant synthetic pathway towards this class of compounds. In hydroaminomethylation a reaction sequence of hydroformylation of an alkene to an aldehyde with subsequent reductive amina-tion proceeds in a domino reaction (see Eq. 4) [14]. Recently, the highly selective hydroamination of alkenes with ammonia to form linear primary and secondary aliphatic amines with a new Rh/Ir catalytic system (] Rh(cod)Cl 2], ] Ir(cod)Cl 2], aqueous TPPTS solution) has been described (see Scheme 2) [15]. The method is of particular importance for the production of industrially relevant, low molecular weight amines. 

CoPc modified carbon paste electrodes were reported by Chicharo et al. to show good catalytic activity towards the measurement of triazolic herbicides sueh as amitrole at low oxidation potential (+0.4 V, Table 7.1) in basic media, a detection limit of 0.04 jig mL was obtained using a injection system . A screen-printed carbon electrode which was impregnated with CoPc electrocatalyst, was employed in conjunction with acetylcholinesterase by Hartley and Hart for the reduction of organophosphate pesticides The detection limits were of the order of 10 and 10 ... 

If we first consider purely organic systems, one can mention the asymmetric reduction of ketmies with borane promoted by a chiral oxazaborolidine 1 developed by Corey, Bakshi, and Shibata (CBS reduction. Scheme 1) [13,14]. In this system, the nitrogen atom of the oxazaborolidine serves as Lewis base and coordinates BH3 thus improving its nucleoplulicity, while the endocyclic boron atom acts as the Lewis acid and activates the ketone toward the reduction. This seminal work constimtes an early example of metal-free catalysis and shows that cooperative effects can emerge from ambiphilic Lewis acid/base catalytic system. 

Some finer effects caused by additives were reported. Thus, the use of the pair rt-Bu4NOAc-KCl was used to control the selectivity of reductive elimination in the ary-lation of acrolein acetals (Scheme 2.7). Interestingly, a more usual reverse combination KOAc- -Bu4NC1 gave worse results with respect to selectivity [54, 55], which clearly shows that the Mizoroki-Heck reaction is often extremely sensitive towards subtle details of formulation of the catalytic system. As proposed by Cacchi and coworkers [54,68] for the -Bu4NOAc-KC1 cocktail [55-57], the coordination sphere of palladium is saturated to form an anionic complex 28 (28 29 + 30), in which palladium is not capable of coordinating to the aryl tt-system as in 31 (31 30, Scheme 2.7). 

In a given reacting system, usually not all palladium is engaged in the catalytic process. The amount engaged depends on kinetics and the concentrations of reactants. The excess of palladium exists either in palladium(II) (102) or as palladium(O) (36) both are unstable under the conditions of Mizoroki-Heck reactions and should be protected (Scheme 2.23) palladium(II) (102) is unstable towards reduction if not protected in the form of an SRPC (102 103), palladium(0) (36) is unstable towards clusterization and growth of metal particles (36 —101 —70) and is protected by additives or components of reaction media in the form of anionic complexes 100 [36,84]. If we consider the reversibihty of formation... 

One of the main efforts towards the development of sustainable chemistiy is the reduction in the generation of waste. The largest amount of waste produced by a reaction is typically associated with the solvent employed as it is the component added in the largest quantity and, usually not incorporated into the final product but rather removed and disposed of at the end of the process. For these reasons, many efforts are being devoted to the development of catalytic systems that can operate under solvent-free conditions. When the use of a solvent is mandatory, the efforts are directed towards the use of sustainable solvents. In this context, water, which is the only natural solvent, is the preferred choice. Other green alternatives are ionic liquids or supercritical carbon dioxide. 

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