Catalyst environmental 426 - redox

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

An environmentally friendly synthesis of 1,2-methylenedioxybenzene (MDB) can be efficiently carried out in the gas phase, by feeding pyrocatechol (PYC) and formaldehyde acetals and using a catalyst containing weak acid sites and redox sites. The Ti-silicalite (TS-1) was identified as the most active and selective catalyst, indicating the role of well-dispersed octahedrally-coordinated Ti" ions in comparison with some model catalysts. 

The multi-component systems developed quite recently have allowed the efficient metal-catalyzed stereoselective reactions with synthetic potential [75-77]. Multi-components including a catalyst, a co-reductant, and additives cooperate with each other to construct the catalytic systems for efficient reduction. It is essential that the active catalyst is effectively regenerated by redox interaction with the co-reductant. The selection of the co-reductant is important. The oxidized form of the co-reductant should not interfere with, but assist the reduction reaction or at least, be tolerant under the conditions. Additives, which are considered to contribute to the redox cycle directly, possibly facilitate the electron transfer and liberate the catalyst from the reaction adduct. Co-reductants like Al, Zn, and Mg are used in the catalytic reactions, but from the viewpoint of green chemistry, an electron source should be environmentally harmonious, such as H2. 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

Since S03/H2S04 is clearly not the most desirable system for industrial applications, a formidable challenge is to find an oxidant that oxidizes Pt(II) much faster than S03 does, operates in an environmentally friendly solvent, and can be (like SVI/SIV) reoxidized by oxygen from air. Ideally, the reduced oxidant would get reoxidized in a continuous process, such that the oxidant acts as a redox mediator. In addition, the redox behavior has to be tuned such that the platinum(II) alkyl intermediate would be oxidized but the platinum(II) catalyst would not be completely oxidized. Such a system that efficiently transfers oxidation equivalents from oxygen to Pt(II) would be highly desirable. A redox mediator system based on heteropolyacids has been reported for the Pt-catalyzed oxidation of C-H bonds by 02, using Na8HPMo6V6O40... 

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

Metal oxides are an important elass of heterogeneous catalysts. They find direct application in a variety of reactions, from acid-base to redox reactions, in photocatalytic processes, and as catalysts for environmental protection. In addition, they are widely used as supports for other active components (metal particles or other metal oxides), although often they act not only as a support, but actively participate in the reaction mechanism." ... 

We report here three studies that address three separate but significant issues in the emerging area of selective catalytic oxidation by TMSP-type complexes. The first study establishes for the first time that some TMSP complexes are compatible with basic oxidants and basic conditions. The second study reports the first oxidation, in this case selective alkene epoxidation, by the economically and environmentally desirable oxidant, aqueous hydrogen peroxide, catalyzed by TMSP complexes. The third study demonstrates that redox active polyoxometalates can be derivatized with alcohols in a manner that should prove useful for fabricating future generations of more sophisticated and selective TMSP catalysts. 

To understand these complex catalysts we describe some basic HTSC (high Tc) oxides and the important role played by EM in elucidating the nanostructure and chemistry of these complex oxides. The redox behaviour of Cu in various oxidation states of HTSC may be important in environmental control catalysis. 

Catalysts and their effects on chemical reactions aid in efficiency, effectiveness and selectivity. A recent example of current research is redox and ligand exchange reactions of the oxygenation catalyst (N,N -bis(salicylidene)ethylenediaminato)co-balt(II), Co(SALEN)2 (below), and its one-electron oxidation product, Co(salen) 2-These were investigated in DMF, pyridine, and mixtures of these solvents. Solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition are important considerations (Eichhorn, 1997). The results in these solvents should be compared with other work with catalysts using more environmentally benign media (Collins et al., 1998). 

The reduced iron atoms of complex C, being inert to dioxygen, are readily oxidized by nitrous oxide into complex D to give adsorbed species of a-oxygen, Oa. As Figure 7.3 shows, the reversible redox transition Fc" <-> Fe provides the catalytic activity of FeZSM-5 both the oxidation cycle due to the oxygen transfer from N20 to a substrate and the decomposition cycle of N20 into N2 and 02 due to recombination of a-oxygen into the gas phase. The decomposition is an environmentally important process, and FeZSM-5 zeolites are considered to be the best catalysts for this reaction (see review [117] and references therein). 

A semiconductor suitable for an efficient photo-oxidation reactions of environmental relevance should fulfil several requirements. Its bandgap should allow the use of solar radiation, ie the catalyst has to absorb in the visible or near-UV light region. The redox potentials of 0H7H20 and 02/02 couples should lie within this bandgap ( 0H./H20 = 2.8V [62], E°02/0.- = -0.16V [63]) in order to facilitate... 

In both preparative methods the integrity of the catalyst is compromised. Exposure to ambient conditions, exterior chemical environments, and exposure to high vacuum in the microscope may alter the catalyst properties. Employment of special chambers for redox pretreatments and/or environmental microscopes provides some assistance in overcoming these problems (9). It appears that regardless of the approach taken, compromises in specimen preparation or instrumental performance must be made. These factors must be kept in mind when interpreting results obtained from electron microscopic measurements. 

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