Selective Palladium-Catalysed Aerobic Oxidation of Alcohols

Palladium has a significant track record as a catalyst for a range of oxidation reactions and it has been explored for the selective oxidation of alcohols for many years. This chapter focuses on the two main types of aerobic Pd catalysts: heterogeneous and ligand-modulated systems. In the case of heterogeneous systems, the mechanistic understanding of these systems and the use of in situ and operando techniques to obtain this knowledge are discussed. The current state-of-the-art is also summarized in terms of catalytic performance and substrate scope for heterogeneous Pd-based catalysts. In terms of ligand-modulated systems, leading examples of molecular Pd(ii) catalysts which undergo direct O2 coupled turnover are highlighted. The catalyst performance for such catalysts is exemplified and mechanistic understanding for these molecular systems is discussed.

A Highly Efficient Palladium(II)/Polyoxometalate Catalyst System for Aerobic Oxidation of Alcohols

A simple catalyst system composed of Pd(OAc)2, phosphomolybdic acid and tetrabutylammonium acetate oxidises a range of alcohols efficiently, with turnover numbers (TONs) of up to 10 000.

Quantum yield measurements for the photocatalytic oxidation of Acid Orange 7 (AO7) and reduction of 2,6-dichloroindophenol (DCIP) on transparent TiO2 films of various thickness

This work comprises the photoactivity assessment of transparent sol–gel TiO2 coatings of various thickness using two test systems. The initial rates of both photocatalytic reactions, namely the oxidative bleaching of Acid Orange 7 (AO7) and the reductive bleaching of 2,6-dichlorindophenol (DCIP) increase linearly with increasing titania film thickness as well as with increasing absorbed light flux. The latter work revealed quantum yields (QY) of 0.19% and 92% for the AO7 and DCIP test system, respectively. The low QY for the AO7 oxidation is due to the combination of a slow irreversible reduction of oxygen and also for the oxidation of AO7, thus favouring the high efficiency for electron–hole recombination that is typical for aqueous organic pollutants. In contrast, the very high QY for the photocatalysed reduction of DCIP is due to the presence of a vast excess of glycerol which traps the photogenerated holes efficiently and so allow time for the slower reduction of dye to take place. Furthermore, the oxidation of glycerol results in the generation of highly reducing R-hydroxyalkyl radicals that are able to also reduce DCIP. As a consequence of this ‘current doubling’ effect, the observed QY (92%) is much higher than the apparent theoretical value of 50%.

Arene cis-Diol Dehydrogenase-Catalysed Regio- and Stereoselective Oxidation of Arene-, Cycloalkane- and Cycloalkene-cis-diols to Yield Catechols and Chiral α-Ketols

Benzene cis-diol dehydrogenase and naphthalene cis-diol dehydrogenase enzymes, expressed in Pseudomonas putida wild-type and Escherichia coli recombinant strains, were used to investigate regioselectivity and stereoselectivity during dehydrogenations of arene, cyclic alkane and cyclic alkene vicinal cis-diols. The dehydrogenase-catalysed production of enantiopure cis-diols, α-ketols and catechols, using benzene cis-diol dehydrogenase and naphthalene cis-diol dehydrogenase, involved both kinetic resolution and asymmetric synthesis methods. The chemoenzymatic production and applications of catechol bioproducts in synthesis were investigated.

Hydrogen peroxide enhanced photocatalytic oxidation of microcystin-lR using titanium dioxide

Cyanobacterial toxins present in drinking water sources pose a considerable threat to human health. Conventional water treatment systems have proven unreliable for the removal of these toxins and hence new techniques have been investigated. Previous work has shown that TiO₂ photocatalysis effectively destroys microcystin-LR in aqueous solutions, however non-toxic by-products were detected. It has been shown that photocatalytic reactions are enhanced by utilisation of alternative electron acceptors. We report here enhanced photocatalytic degradation of microcystin-LR following the addition of hydrogen peroxide to the system. It was also found that hydrogen peroxide with UV illumination alone was capable of decomposing microcystin-LR although at a much slower rate than found for TiO₂. No HPLC detectable by-products were found when the TiO₂/UV/H₂O₂ system was used indicating that this method is more effective than TiO₂/UV alone. Results however indicated that only 18% mineralisation occurred with the TiO₂/UV/H₂O₂ system and hence undetectable by-products must still be present. At higher concentrations hydrogen peroxide was found to compete with microcystin-LR for surface sites on the catalyst but at lower peroxide concentrations this competitive adsorption was not observed. Toxicity studies showed that both in the presence and absence of H₂O₂ the microcystin solutions were detoxified. These findings suggest that hydrogen peroxide greatly enhances the photocatalytic oxidation of microcystin-LR.

Evidence To Challenge the Universality of the Horiuti-Polanyi Mechanism for Hydrogenation in Heterogeneous Catalysis:Origin and Trend of the Preference of a Non-Horiuti-Polanyi Mechanism

The Horiuti-Polanyi mechanism has been considered to be universal for explaining the mechanisms of hydrogenation reactions in heterogeneous catalysis for several decades. In this work, we examine this mechanism for the hydrogenation of acrolein, the simplest alpha,beta-unsaturated aldehyde, in gold-based systems as well as some other metals using extensive first-principles calculations. It is found that a non-Horiuti-Polanyi mechanism is favored in some cases. Furthermore, the physical origin and trend of this mechanism are revealed and discussed regarding the geometrical and electronic effects, which will have a significant influence on current understandings on heterogeneous catalytic hydrogenation reactions and the future catalyst design for these reactions.

Origin of extraordinarily high catalytic activity of Co3O4 and its morphological chemistry for CO oxidation at low temperature

Understanding and then designing efficient catalysts for CO oxidation at low temperature is one of the hottest topics in heterogeneous catalysis. Among the existing catalysts. Co3O4 is one of the most interesting systems: Morphology-controlled Co3O4 exhibits exceedingly high activity. In this study, by virtue of extensive density functional theory (OFT) calculations, the favored reaction mechanism in the system is identified. Through careful analyses on the energetics of elementary reactions on Co3O4(1 1 0)-A, Co3O4(1 1 0)-B, Co3O4(1 1 1) and Co3O4(1 0 0), which are the commonly exposed surfaces of Co3O4, we find the following regarding the relation between the activity and structure: (i) Co3+ is the active site rather than Co2+: and (ii) the three-coordinated surface oxygen bonded with three Co3+ may be slightly more reactive than the other two kinds of lattice oxygen, that is, the two-coordinated 0 bonded with one Co2+ and one Co3+ and the three-coordinated 0 bonded with one Co2+ and two Co3+. Following the results from Co3O4, we also extend the investigation to MnO2(1 1 0), Fe3O4(1 1 0), CuO(1 1 0) and CuO(1 1 1), which are the common metal oxide surfaces, aiming to understand the oxides in general. Three properties, such as the CO adsorption strength, the barrier of CO reacting with lattice 0 and the redox capacity, are identified to be the determining factors that can significantly affect the activity of oxides. Among these oxides, Co3O4 is found to be the most active one, stratifying all the three requirements. A new scheme to decompose barriers is introduced to understand the activity difference between lattice O-3c and O-2c on (1 1 0)-B surface. By utilizing the scheme, we demonstrate that the origin of activity variance lies in the geometric structures. (C) 2012 Elsevier Inc. All rights reserved.

An understanding and implications of the coverage of surface free sites in heterogeneous catalysis

The crucial roles of the coverage of surface free sites in determining catalytic activity trend are quantitatively addressed with the help of density functional theory and microkinetics. First, by analyzing activity trends of NO oxidation catalyzed by Ru, Rh, Pd, Os, Ir, and Pt surfaces with full kinetic considerations, we identify that the activity trend is in general determined by the competition between the reaction barrier and the coverage of surface free sites. Second, since the dissociation of many important molecules, such as the dissociation of N(2), O(2), and CO, follows the same Bronsted-Evans-Polanyi relationship, the coverage of surface free sites is usually a decisive term that affects the overall activity. Third, an equation is derived for the coverage of surface free sites and it is found that the coverage of surface free sites contains not only all the key thermodynamic parameters but also all the kinetic properties in the catalytic system. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3140202]

Direct oxidation of amines to nitriles in the presence of ruthenium-terpyridyl complex immobilized on ILs/SILP

The immobilization of a ruthenium complex (Ru2Cl4(az-tpy)2) within a range of supported ionic liquids ([C4C1im]Cl, [C4C1im][NTf2], [C6C1im]Cl, [C4C1pyrr]Br, [C4C1im]Br, [C4C1pyrr]Cl) dispersed silica (SILP) operates as an efficient heterogeneous catalyst in oxidation of long chain linear primary amines to corresponding nitriles. This reaction follows a “green” route using a cheap and easy to handles oxidant (oxygen or air). The conversion was found to be strongly influenced by the alkyl chain length of the amine substrate and the choice of oxidant. No condensation reaction was observed between the starting amines and the selectivity to nitrile is 100%. Moving from a composition of 20 atm N2/5 atm O2 to 5 atm N2/20 atm O2 led to enhancements in the conversion (n-alkylamines) and selectivity (benzonitrile) which have been correlated with an increase of the solubilized oxygen. This was further supported by using different inert gas (nitrogen, helium, argon)/oxygen mixtures indicating that the O2 solubility in the SILP system, has an important effect on conversions and TON in this reaction using SILP catalysts. Experiments performed in the presence of CO2 led to a different behaviour due to the formation of amine-CO2 adducts. The application of the Weisz–Prater criterion confirmed the absence of any diffusional constraints.

Comparative studies between classic and wireless electrochemical promotion of a Pt catalyst for ethylene oxidation

A comparative study between a classic and a wireless electrochemical promotion experiment was undertaken as a tool towards the better understanding of both systems. The catalytic modification of a platinum catalyst for ethylene oxidation was studied. The catalyst was supported on yttria-stabilised-zirconia (YSZ), a known pure oxide ion conductor, for the classic experiment and La _0.6Sr_0.4Co_0.2Fe_0.8O _3-δ-a mixed oxide ion electronic conductor-was used for the wireless experiment. The two systems showed certain similarities in terms of the reaction classification (in both cases electrophobic behaviour was observed) and the promotion mechanism. Significant difference was observed in the time scales and the reversibility of the induced rate modification. © 2008 Springer Science+Business Media B.V.

Electrochemical promotion of catalysis controlled by chemical potential difference across a mixed ionic-electronic conducting ceramic membrane - An example of wireless NEMCA

A La_0.6Sr_0.4Co_0.2F_0.8O₃ mixed ionic electronic conducting (MIEC) membrane was used in a dual chamber reactor for the promotion of the catalytic activity of a platinum catalyst for ethylene oxidation. By controlling the oxygen chemical potential difference across the membrane, a driving force for oxygen ions to migrate across the membrane and backspillover onto the catalyst surface is established. The reaction is then promoted by the formation of a double layer of oxide anions on the catalyst surface. Thelectronic conductivity of the membrane material eliminates the need for an external circuit to pump the promoting oxide ion species through the membrane and onto the catalyst surface. This renders this "wireless" system simpler and more amenable for large-scale practical application. Preliminary experiments show that the reaction rate of ethylene oxidation can indeed be promoted by almost one order of magnitude upon exposure to an oxygen atmosphere on the sweep side of the membrane reactor, and thus inducing an oxygen chemical potential difference across the membrane, as compared to the rate under an inert sweep gas. Moreover, the rate does not return to its initial unpromoted value upon cessation of the oxygen flow on the sweep side, but remains permanently promoted. A number of comparisons are drawn between the classical electrochemical promotion that utilises an external circuit and the "wireless" system that utilises chemical potential differences. In addition a 'surface oxygen capture' model is proposed to explain the permanent promotion of the catalyst activity. © 2007 Springer Science+Business Media, LLC.

Electrochemical promotion of catalysis using solid-state proton-conducting membranes

A solid-state electrochemical reactor with ceramic proton-conducting membrane has been used to study the effect of electrochemically induced hydrogen spillover on the catalytic activity of platinum during ethylene oxidation. Suitable proton-conducting electrolyte membranes (Gd-doped BaPrO ₃ (BPG) and Y-doped BaZrO₃ (BZY)) were fabricated. These materials were chosen because of their protonic conductivity in the operational temperature region of the reaction (400-700 °C). The BZY-based electrochemical cell was used to investigate the open-circuit voltage (OCV) dependence on H₂ partial pressure with comparison being made to the theoretical OCV as predicted by the Nernst equation. Furthermore, the BZY pellets were used to study the effect of proton transfer of the catalytic activity of platinum during ethylene oxidation. The reaction was found to exhibit electrochemical promotion at 400 °C and to be electrophilic in nature, i.e. proton addition to the platinum surface resulted in an increase in reaction rate. At higher temperatures, the rate was not affected, within experimental error, by proton addition or removal. Under similar conditions, AC impedance showed that there was a large overall cell resistance at 400 °C with significantly decreased resistance at higher temperatures. It is possible that there could be a relationship between large cell resistances and the onset of electrochemical promotion in this system but there is, as yet, no conclusive evidence for this. © 2003 Elsevier B.V. All rights reserved.