Catalysts reoxidation

Repladng air by either N2 or 02 did not affect the conversion obtained in the experiment just described. Consequently, the higher conversion does not result from catalyst reoxidation. It must be due solely to stripping S03 from the catalyst. 

The rate constants of elementary reactions (see Scheme 3) were estimated for the PVP-Cu,Mn catalyst. For example, the rate constant of electron-transfer (ke) and of catalyst reoxidation (ko) were determined by measuring the decrease and the increase in the d-d absorption of Cu(n). The ke value for Cu(n) ->-Cu(I) 14 min-1 was much larger than that for Mn(ni) - -Mn(n). ko were PVP-Mn (0.042 min-1 ) PVP-Cu,Mn (0.040)>PVP-Cu(0.013), respectively. Furthermore the following rapid redox reaction was regoc-nized. 

The reaction rate varies with the change in the solvent composition. The catalysis of pyridine-Cu in DMSO-benzene mixed solvent is summarized in Fig. 4 (a). The rate constant of the catalyst reoxidation (k0) and the overall rate increase although the rate constant of electron-transfer (ke) decreases with the benzene content. Instead of the benzene solvent, the copolymer of vinylpyridine with styrene (PSP) was used as a polymer ligand, as shown in Fig. 4 (b). The overall rate and k0 increase with the styrene content in the PSP ligand, just as the solvent effect of benzene. Only several times amount of styrene unit to Cu ion (as polymer concentration ca. 0.1 wt% of the solvent) affects... 

Figure 4. Catalytic activity of the pyridine-Cu catalyst in DMSO-benzene solvent (a) and activity of the PSP-Cu catalyst in DMSO (b) (O) oxidative polymerization rate of XOH (A) rate constant of electron transfer step (ke) (0) rate constant of catalyst reoxidation step...

Recovery of the immobilized catalyst 48 was accomplished by simple filtration, and catalyst reoxidation and repeated recycling (seven times) was possible with no loss of reactivity or enantioselectivity. 

Abstract Palladium-catalyzed oxidation reactions are among the most diverse methods available for the selective oxidation of organic molecules, and benzoquinone is one of the most widely used terminal oxidants for these reactions. Over the past decade, however, numerous reactions have been reported that utilize molecular oxygen as the sole oxidant. This chapter outlines the fundamental reactivity of benzoquinone and molecular oxygen with palladium(O) and their catalyst reoxidation mechanisms. The chemical similarities... 

In a subsequent study of oxygen heterocychzation, Andersson et al. investigated various catalyst reoxidation conditions with the Pd(OAc)2/DMSO catalyst system (Eq. 27, Table 3) [ 150]. Several conditions result in high substrate conversion to the product, including the use of BQ, BQ with methanesulfonic acid, and molecular oxygen, with and without copper(II) salts as a cooxidant. Only the aerobic methods enable formation of the product 37 with high regio-selectivity. The presence of a copper cocatalyst enhances the rate but is not necessary for catalysis. 

In the catalyst reoxidation step, contrary to the electron-transfer step, the polymer ligand should shrink because of the formation of the Cu(II) complex. Therefore, the polymer chain may partially repeat are expansion and contraction occurring during the catalytic cycle. When one has a view of the polymer-Cu catalyst as a whole, each part of the polymer catalyst domain, which is drifted in solution, is seen to be fluctuating during the catalytic process [Fig. 32(b)]. The fluctuating shape of biopolymers in enzymic reactions has been pointed out, and the dynamically conformational change of a flexible polymer chain is considered to be one of the effects of the polymer catalyst. 

Palladium(II) and nitrate ion with oxygen as final oxidant give an excellent yield of cyclohexenyl acetate from cyclohexene (92% at 50°C).703 The catalyst reoxidation sequence includes a palladium nitro-nitrosyl redox couple. 

The reaction between propene and the catalyst is, in general, rate-determining, as catalyst reoxidation is a relatively fast reaction. This implies that the degree of catalyst reduction under steady state reaction conditions is fairly low (i.e. less than 10% with respect to the total amount of oxygen that can be removed with propene). Thus the observed kinetics... 

C2H4 -f 2CuClo -h H2O CH3CHO + 2CuCl + HCl Carbonyl Reaction 2CuCl + 2HC1 + 1 /2O2 2CuCl2 + H2O Catalyst Reoxidation... 

Figure 2 X-ray diffiaction pattern (a) fresh catalyst reduced in N2-H2 at 503 K, (b) fresh catalyst reduced in N2-H2 -H2O at 503 K,(c) aged catalyst, (d) catalyst reoxided at 623 K and reduced at 773 K. References 1= ZnO, 2= Cu, 3= CuO, 4= Cu AI2O4, 5= Cu-Zn 6= Cu AI2O2...

These equations are applicable for a reaction proceeding under pseudo-first-order conditions, i.e. when the concentration of the solute species is constant right up to the gas/liquid interface. It is thus possible to examine the possibility that reaction may occur in a film for the catalyst reoxidation and reduction reactions separately, if the two-stage redox mechanism is appropriate. The penetration theory leads to a series of coupled nonlinear partial differential equations which have to be solved numerically with appropriate boundary conditions. For example, if y is the distance from the melt surface, the equation governing the concentration of species B in time and space is given in (15). 

All selective oxidation and ammoxidation catalysts possess redox properties. They must be capable not only of reduction during the formation of acrolein or acrylonitrile, but also subsequent catalyst reoxidation in which gaseous oxygen becomes incorporated into the lattice as to replenish catalyst vacancies (Scheme 2). As mentioned earlier, the incorporation of such redox properties into solid state metal oxides was one of the salient working hypotheses on which the development of the Sohio ammoxidation process was based (2). Later, Keulks (70) confirmed the involvement of lattice oxygen in propylene oxidation by using as a vapor phase oxidant. The results showed that the incorporation of O into the acrolein (and CO2) increases with time (Fig. 11), which is consistent with the above redox mechanism. 

Figure 4. Kinetic cim es of the change of V " ions number during catalyst reoxidation fOal = 2.7a101 molec./cm3...

It turned out that the rate of consumption of a, P-unsaturated aldehyde is not affected by the concentration of a,p-unsaturated acid and vice versa. With increasing a,P-unsaturated aldehyde concentration the rate of aldehyde and oxygen consumption passes a maximum. The decreasing reaction rate at elevated aldehyde concentration is interpreted by adsorptive blocking of reduced sites of the catalyst by aldehyde so that the rate of reoxidation is reduced. The increase of the rate of aldehyde oxidation by co-feeding of water vapor could be interpreted by an enhanced rate of catalyst reoxidation which has been found also in the absence of aldehyde and the related acid. The selectivity towards a,P-unsaturated acid is mainly determined by the consecutive oxidation of a, P-unsaturated acid while the parallel reaction of a, P-unsaturated aldehyde to by-products is almost negligible. 

Both the rate of acrolein oxidation and the rate of catalyst reoxidation depend on the concentration of acrolein so that also steps towards the formation of acrylic anion must be taken into account as rate determining. In the whole the acrolein oxidation appears as the result of a network of coordinated reaction steps. 

The influence of water on the rate of acrolein respectively oxygen consumption is still included in the reaction rate constant of acrolein oxidation k, and in the constant of catalyst reoxidation klOj),. These constants are listed in the table for different water vapor contents of the reaction gas. 

Oxidative amination carried out under improved catalyst reoxidation conditions permits the use of alkenes as limiting reagents/ ... 

In fact, when the net rate of step 12 is high enough to compete with R the catalyst reoxidation could not proceed via the step (6) and N2O may behave only as a reductant, making N2O oscillations to disappear. Of course, the possibility the rate of step (12) increases is related with a decrease of the reaction temperature and/or an increase of O2 concentration. 

To do this, the differential equations (10 - 13) have been modified by including the terms corresponding to the catalyst reoxidation, equation (9), and transformed into a system of algebraic equations by taking the coverages as constant (ddjdi = 0). 

HREM also provided interesting information about nanostructural effects induced by reoxidation. Reoxidation temperatures ranging from 373 to 1173 K have been investigated, as well as samples pre-reduced at temperatures up to 1173 K. For catalysts reduced at < 773 K, reoxidation up to 773 K led only to minor nanostructural changes in the catalysts. Reoxidation at 773 K does not allow any recovery from decoration or alloying phenomena for NM/Ce02 catalysts. Much higher reoxidation temperatures are needed to achieve this objective [117]. 

A key in the use of dioxygen as a terminal oxidant in catalyzed oxidations lies in equations 8-10, namely the separation of catalyst (polyoxometalate) reduction - substrate oxidation, equation 8, from the reduced catalyst reoxidation by O2, equation 9. In part, as the reduced forms of the polyoxometalates are usually low in reactivity and very stable under turnover conditions, equations 8 and 9 can be separated from one another in time and/or in space. As radicals and other reactive species that can initiate radical chain oxidation by O2 (autoxidation), the dominant mode of organic oxidation by this oxidant, are generated in equation 8, autoxidation can be avoided by separating equations 8 and 9. This fact has been appreciated by other groups working in this area. We turn now to another aspect of the chemistry in equations 8-10 that is subtle but has considerable potential consequences for the metal-catalyzed or facilitated C>2-based oxidations and that is the nature of the O2 reoxidation step, equation 9. 

For the reduction of alkenes or alkynes to alkanes in laboratory we use metal catalysts such as Pt or Pd and often high pressures. The heating of alkane precursors with these metal catalysts reoxidizes alkanes to alkenes. In biosynthesis these reactions proceed with special reagents like flavine adenine dinucleotide FAD or its reduced form FADH2. 

Important technical parameters in choosing a suitable catalytic system are the easiness of low valence metal reoxidation by inexpensive oxidants (such as molecular oxygen) and the selectivity toward competitive reactions, mainly alcohol oxidation and formation of CO2. The latter is co-produced, under catalytic conditions, in the presence of water, generated when oxygen is used for the simultaneous catalyst reoxidation see equation 2. 

Golbig, K. G., Werther, J., 1997, Selective Synthesis of Maleic Anhydride by Spatial Separation of n-Butane Oxidation and Catalyst Reoxidation, Chem. Engng Sci. 52,583. 

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