Oxidation state active sites

Figure 11.8 Mechanism of redox reaction catalyzed by NAD dependent lactate dehydrogenase Lactate dehydrogenase (EC 1.1.1.27) is a tetrameric enzyme which catalyzes the reversible redox reaction between L-lactate and pyruvate via ordered kinetic sequence. The hydride ion is transferred to the proR side of the 4 position of NAD. His 195 acts as an acid-base catalyst removing the proton from lactate during oxidation. The active site loop (residues 98-110) carries Argl09 which helps stabilize the transition state during hydride transfer and contacts required for the substrate specificity.

The majority of model studies have focused on the synthesis of the oxidized enzyme active sites (or components thereof) but reduced active sites and chemically/catalytically functional systems boasting the inter-conversion of all three accessible enzyme oxidation states are also important targets for model studies. Models of reduced Mo hydroxylases are described in Sections 7.4.1.2 and 7.4.2.2. 

Only the surface layers of the catalyst soHd ate generaHy thought to participate in the reaction (125,133). This implies that while the bulk of the catalyst may have an oxidation state of 4+ under reactor conditions, the oxidation state of the surface vanadium may be very different. It has been postulated that both V" " and V " oxidation states exist on the surface of the catalyst, the latter arising from oxygen chemisorption (133). Phosphoms enrichment is also observed at the surface of the catalyst (125,126). The exact role of this excess surface phosphoms is not weH understood, but it may play a role in active site isolation and consequently, the oxidation state of the surface vanadium. 

Most of the Moco enzymes catalyze oxygen atom addition or removal from their substrates. Molybdenum usually alternates between oxidation states VI and IV. The Mo(V) state forms as an intermediate as the active site is reconstituted by coupled proton—electron transfer processes (62). The working of the Moco enzymes depends on the 0x0 chemistry of Mo (VI), Mo(V), and Mo (TV). 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Jorne et al. [36] investigated the reactivity of graphites in acidic solutions that are typically used for Zn/Cl2 cells. The degradation of porous graphite is attributed to oxidation to C02. The rate of C02 evolution gradually decreased with oxidation time until a steady state was reached. The decline in the C02 evolution rate is attributed to the formation of surface oxides on the active sites. 

The ultimate purpose of mechanistic considerations is the understanding of the detailed reaction pathway. In this connection it is important to know the structure of the active catalyst and, closely connected with this, the function of the cocatalyst. Two possibilities for the action of the cocatalyst will be taken into consideration, namely, the change in the oxidation state of the transition metal and the creation of vacant sites. In the following, a few catalyst systems will be considered in more detail. 

The function of the tetraethyltin is to create vacant sites so that coordination of alkene molecules becomes possible, and to change the oxidation state of the tungsten atom from +6 to +4. Similar behavior of the aluminum compound in the system WCL-CgHsAlCb is not probable, because it has been demonstrated that WCle-AlClg is also an active catalyst (22, 44), which suggests that C2H5AICI2 functions as a Lewis acid. Vacant sites can be created by a Lewis acid as follows ... 

The oxidation of phenol, ortho/meta cresols and tyrosine with Oj over copper acetate-based catalysts at 298 K is shown in Table 3 [7]. In all the cases, the main product was the ortho hydroxylated diphenol product (and the corresponding orthoquinones). Again, the catalytic efficiency (turnover numbers) of the copper atoms are higher in the encapsulated state compared to that in the "neat" copper acetate. From a linear correlation observed [7] between the concentration of the copper acetate dimers in the molecular sieves (from ESR spectroscopic data) and the conversion of various phenols (Fig. 5), we had postulated [8] that dimeric copper atoms are the active sites in the activation of dioxygen in zeolite catalysts containing encapsulated copper acetate complexes. The high substratespecificity (for mono-... 

Evaluating the results a clear kinetic picture of the catalysts has been obtained. In the steady state the active sites in Fe- and Cu-ZSM-5 are nearly fully oxidized, while for Co only -50% of the sites are oxidized. The former catalysts oporate in an oxidation reduction cycle, Fe /Fe and CuVCu. Coi in zeolites is hardly oxidized or reduced, but ESR studies on diluted solid solutions of Co in MgO indicate that Co -0 formation is possible, rapidly followed by a migration of the deposited oxygen to lattice oxygen and reduction back to Co [36]. For Fe-ZSM-5 such a migration has been observed, so a similar model can be proposed for the zeolitic systems. Furthermore, it is obvious that application of these catalysts strongly depends on the composition of the gas that has to be treated. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

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