Reoxidation Step Research Articles

A mechanistic study on the oxidation of 2,6-dimethyl-phenol by DMAP - and polystyrene-bound DMAP -based copper catalysts

A possible mechanism for the oxidation of 2,6-dimethylphenol by soluble- and polystyrene-bound Cu(II)-DMAP catalysts is described. From our earlier work it is known that, under our standard reaction conditions, the only Cu(II)-DMAP complexes that are initially present in significant amounts in the catalyst solution are mononuclear. From the difference in reaction order in copper for the phenol oxidation and the Cu(I) reoxidation, viz. 1 and 2 respectively, it had been concluded that dimerization of Cu(I) complexes is needed to allow the reoxidation step. For this dimerization, a small amount of copper-coordinating counter-ions proved to be required. The present study shows that under standard conditions for the low molar mass catalyst the dimerization reaction is rate-limiting. For the polymeric catalyst, however, the local copper concentration within the polymer coils is relatively high, the dimerization is accelerated and now the phenol oxidation becomes rate-limiting. The present work further shows that the phenol oxidation obeys Michaelis-Menten kinetics. For the Cu(I) reoxidation, an equilibrium is suggested in which molecular O 2 is reversibly bound to mononuclear Cu(I)-DMAP complexes, prior to Cu(I) dimerization and electron transfers. Combination of the present results with generally accepted steps in the oxidation of phenols allowed the construction of a possible reaction mechanism for our particular system.

Copper(II) complexes of “polystyrene-bound DMAP”: Synthesis, structure and catalytic activity in the oxidative coupling of 2,6-dimethylphenol

The oxidative coupling of 2,6-dimethylphenol by Cu(II) complexes of “polystyrene-bound DMAP” was studied. The polymer was prepared by radical copolymerization of styrene and 4-( N-methyl- N- p-vinylbenzylamino)pyridine ( 1). Monomer ( 1) was prepared as described by Tomoi et al. [7]. By purifying the monomer by column chromatography instead of distillation, however, we succeeded in raising its yield by some 20%. Catalytic experiments supported by UV and EPR experiments revealed that in the catalytically active solution an equilibrium exists between dinuclear and mononuclear Cu(II) complexes. The concentration of the catalytically most active, mononuclear species Cu(II)(ligand) 4(OH)Cl increases on enhancing the ligand/Cu ratio and decreases on addition of an excess of copper-coordinating hydroxide ions. From this structural point of view the polymeric catalyst proved to behave just like low molar mass Cu(II)-DMAP complexes, although the mononuclear polymeric catalyst is more stable because of a polydentate effect. From the difference in reaction order in copper for unbound and polystyrene-bound DMAP catalysts, it was concluded that for the reoxidation step dimerization of Cu(I) complexes is needed, whereas mononuclear Cu(II) complexes are the most active species for the oxidation of DMP. The mentioned dimerization is promoted by the polymer chain. The specificity of the polystyrene-bound DMAP catalyst for formation of polyphenyleneoxide exceeds 95%.

An e.p.r. study of the non-equivalence of the copper sites of caeruloplasmin.

The two Type 1 (blue) copper-binding sites of caeruloplasmin were spectroscopically differentiated by the kinetic analysis of the e.p.r. spectra during the redox cycle. One blue copper, with a hyperfine splitting constant (A parallel) of 6.8 mT, which was rapidly reduced, was not reoxidized by oxygen, whereas it was reoxidized by H2O2. The other blue copper (A parallel = 5.8 mT), which was reduced slowly, was rapidly reoxidized by either oxygen or H2O2. A conformational change of the Type 2 copper was concomitant with the fast reduction of Type 1 copper, whereas its reduction occurred during the slow phase. This sequence of events was reversed in the reoxidation step, that is, the Type 2 copper reappeared rapidly as the species with altered conformation and reverted to the symmetry typical of the native state in the slow phase. The specific reaction of a blue-copper site with the H2O2 can tentatively be related to the established ability of caeruloplasmin to prevent 'oxidative' attack of proteins and lipids.

Structure of copper 4-( n, n- dimethylamino)pyridine complexes and their catalytic activity in the oxidative coupling of 2,6-dimethylphenol

The oxidative coupling of 2,6-dimethylphenol (DMP) by copper complexes of 4-( N,N)-dimethylamino)pyridine (DMAP) has been studied. Catalytic experiments were carried out in which the DMAP-to-copper ratio and the amount and nature of the copper counter ions were varied. Supporting UV and EPR experiments were performed, and it was concluded that both dinuclear and mononuclear complexes are catalytically active, the mononuclear species being the more active. In solution both species are in equilibrium with one another. The mono/di ratio can be increased by addition of extra DMAP ligands. An excess of coordinating counter-ions increases the amount of dinuclear species. However, a few coordinating counterions are inevitable, and the catalytically most active species was found to be ‘Cu(DMAP) 4Cl(OH)’, the role of Cl − probably being that of a bridging counter-ion promoting the formation of dinuclear Cu(I) complexes for the reoxidation step. The DMAP ligands are coordinated to Cu(II) through the pyridine N-atoms, as was determined by X-ray analysis. The Cu(II)DMAP complexes are catalytically active even without initial hydroxide addition. It is believed that the strongly basic DMAP ligands produce some hydroxide from traces of water present in the reaction medium. The species ‘Cu(DMAP) 4Cl(OH)’ proved to be able to produce relatively high molecular weight polyphenylene oxide (PPO) in short time and with good specificity ( >95%).

Post Oxidation Anneal and Re-Oxidation Effects in 5 nm SiO2 Films

ABSTRACTUltrathin silicon dioxide films, 5 nm thick, were grown in a double-walled furnace at 850°C in dry O2. A consistent improvement in the electrical properties is observed following the oxidation either with a Post-Oxidation Anneal (POA) at 1000°C in N2 or with the same POA followed by a short re-oxidation (Re-Ox) step in which 1 nm of additional oxide was grown. We attribute these results to the redistribution of hydrogen and water related groups as well as to a change in the concentration of sub-oxide charge states at the Si-SiO2 interface. A further improvement observed after the short re-oxidation step had been attributed to the filling of the oxygen vacancies produced during the POA. High resolution Transmission Electron Microscopy cross-sectional observations of the Si-iSO2 interface have evidenced an increase in the interface roughness after the thermal treatment at high temperature. These results are in agreement with recent XPS data.

Catalytic Activity of MoO3 and V2O5 Highly Dispersed on TiO2 for Oxidation Reactions

Abstract The structure of Mo–Ti oxide catalysts has been investigated by using IR and XRD techniques. At a low Mo content, MoO3 is highly dispersed on TiO2, an amorphous phase being formed. Simultaneously, a shift of the Mo=O band from 995 cm−1 to 950 and 905 cm−1 takes place. The maximum rate of the oxidation of C2H5OH and C3H6 over Mo–Ti-6(atom% of Mo) is attributable to the weakening of the Mo=O bond. The 18O tracer and kinetic studies of the oxidation of C3H6 over Mo–Ti and V–Ti oxides suggest that a redox mechanism is applicable, the rate constants of reduction and reoxidation step being determined. The promoter action of TiO2 support is attributed mainly to the increase in the rate of reduction step.

Catalytic reduction of NO with ammonia over Cu(II) NaY

The mechanism of the catalytic reduction of NO with NH 3 over Cu(II) ion-exchanged Y-type zeolites [Cu(II) NaY] was examined by kinetic and isotopic tracer studies as well as relevant studies on the transient reaction of reduced catalysts. The rate of the NONH 3 reaction showed dependence on the partial pressures of both No and NH 3, in accordance with the Langmuir-Hinshelwood type of rate equation. The isotopic tracer studies using 15N-labeled ammonia and NO indicated that N 2 was mainly formed by bi-molecular reaction between NO and ammonia, while the nitrogen atom of nitrous oxide came soley from NO. These results support the reaction mechanism previously proposed. It has been shown that the “bell-shaped” catalytic activity-temperature relation of Cu(II) NaY is associated with the redox change of copper ions. The reoxidation step of Cu(I) ions to Cu(II) was studied in detail by allowing the reduced catalyst to react with gaseous mixtures of NO and NH 3 under various conditions. The results showed that Cu(I) ions were oxidized to Cu(II) by coupling with the stoichiometric disproportionation reaction of NO. The reaction was strongly promoted by the coexistence of NH 3. Based on these results, the mechanism and the rate-determining step of the catalytic NONH 3 reaction were discussed.

Covalent assembly of mouse immunoglobulin G subclasses in vitro: application of a theoretical model for interchain disulfide bond formation.

The pathways and kinetics of interchain disulfide bond formation have been determined in vitro for purified myeloma proteins representing the three major subclasses of mouse immunoglobulin G(IgG) using the reoxidation system described previously (Petersen, J.G.L. & Dorrington, K.J. (1974) J. Biol. Chem. 249, 5633-5641). Mixtures of oxidized and reduced glutathione were added to act as a disulfide interchange catalyst. The pathways of covalent assembly observed in vitro were qualitatively and quantitatively similar to those followed by the various subclasses in vivo. HH and HHL were the principle covalent intermediates seen with IgG1 (MOPC 31C) and IgG2a (MOPC 173 and clone 19). With IgG2b( MPC 11C), HL, HH and HHL were all prominant intermediates. The time courses of reoxidation were simulated using a theoretical model based on second-order reaction kinetics (Percy, J.R., Percy, M.E. & Dorrington, K.J. (1974) J. Biol. Chem. 250, 2398-2400). Two distinct phases were apparent in the reoxidation sequence. The first, which lasted for the initial 5-15 min, did not confirm to the theoretical model. The second phase could be accounted for by the model and represented the remainder of the covalent assembly process. The physico-chemical basis for this biphasic phenomenon was explored. Sedimentation velocity studies showed that noncovalent association was incomplete at the beginning of the reoxidation step for all proteins except IgG2b (MOPC 11C). No dissociation was apparent in the reduced and alkylated proteins at pH 5 in the absence of prior exposure to acid conditions. Thus, exposure to acid appears to affect the affinity between the subunits in the native proteins. Transfer of the proteins from pH 5 to pH 8.2 (the pH at which reoxidation proceeds) is accompanied by the generation of an absorption difference spectrum over an 8-10 min period. These data suggest that a pH-dependent conformational relaxation process may influence the early stages of reoxidation.

An investigation into the reaction mechanism of uranium(VI) reduction in acidic solutions by cyclic chronopotentiometry

The reduction-reoxidation mechanism for uranium(VI) in acidic perchlorate and nitrate solutions has been investigated by cyclic chronopotentiometry. By analysis of data for the second and third transition times it has been shown that possibly no single mechanism of either the reduction-disproportionation sequence or the e.c.e. mechanism is solely operative. The low order pH dependence, the negative dependence on ionic strength of the formal rate coefficient for the chemical reaction following electron transfer, and the consistently shorter third transition time with respect to the second, suggest interpretation based on the involvement of polynuclear complexes, on overlapping of reoxidation waves, and on the existence of an e.c.e. mechanism only in the reverse (reoxidation) step.

Chronopotentiometry with current reversal effect of unequal forward and reverse current densities

The theory of current reversal chronopotentiometry has been extended to take into account unequal forward and reverse current densities. Equations have been derived for the potential-time dependence on the current density ratio for reversible and irreversible re-oxidations. The dependence of the ratio of the reverse to forward transition time on the current density ratio was established. The time at which the potential is equal to the half-wave potential for the re-oxidation step was shown to depend on the current density ratio and verified experimentally. A study was made of the irreversible nickel-nickelous system in 1 M sodium per-chlorate using the equations derived. A value of k b,h° equal to 2.74·10 −4 cm sec −1 at 25° was calculated for the re-oxidation step and the value of −0.246 V vs. N.H.E. established as the formal reduction potential of the Ni 2+-Ni system.