Olga A. Simakova

“Double‐Peak” Catalytic Activity of Nanosized Gold Supported on Titania in Gas‐Phase Selective Oxidation of Ethanol

Recent years have seen a growing amount of fundamental research dealing with selective oxidation of alcohols and polyols using molecular oxygen (air) as a cheap and clean oxidant in the presence of solid catalysts. In this respect, supported gold nanoparticles have attracted great attention owing to their unique catalytic properties under mild conditions. Moreover, gold catalysts are becoming increasingly important for the conversion of biomass-derived alcohols, such as ethanol and glycerol, into other useful chemicals. Bioethanol, in particular, is an example of a promising renewable feedstock to obtain corresponding products of oxidation and concurrent reactions; acetaldehyde, 1,1-diethoxyethane, ethyl acetate and acetic acid, which are normally formed one by one with increase of temperature. As a result, more or less complex mixtures of these products or a predominant individual product can be obtained depending on the reaction conditions and the nature of the catalyst (gold particle size, support, preparation procedure). Notably, exactly the same spectrum of products, for example, aldehydes, acetals, esters and carboxylic acids, is typically produced in the presence of other supported metals (e.g. , Pt, Pd, Ru) or metal oxide catalysts (e.g. , V2O5, NbMoVOx), although generally under harsher conditions. Therefore, it seems quite possible that all reactions indicated above have similar mechanistic aspects. However, no generally accepted mechanism has, to date, been formulated for these reactions (see, however, Refs. [8–11]), as it has been for gold-catalyzed reactions of CO, O2, H2, and other small molecules. [12–13] During our monitoring of a gas-phase oxidation of ethanol, a large variety of solid catalysts, such as supported noble metals, metal oxides, and multicomponent systems have been tested. Among them, about thirty different supported gold catalysts were examined. Herein we report an unusual catalytic behavior of Au/TiO2, which was the only tested catalyst to give rise to a second low-temperature peak of activity. The most active gold catalysts are known to be those that contain small particles of gold (<10 nm in diameter), especially on reducible supports such as titania or ceria. 14] Taking this into account, several samples of 2 wt % Au/TiO2 were prepared according to a direct ion-exchange procedure 15] using ammonia as a washing agent to give an average gold particles size close to 2 nm (Figure 1; see also the Supporting Information). The catalytic performance of Au/TiO2 (d = 1.9 1 nm) sample

Generation of Reactive Oxygen Species on Au/TiO2 after Treatment with Hydrogen: Testing the Link to Ethanol Low‐Temperature Oxidation

The gold catalyzed aerobic oxidation of alcohols is currently of great interest for the following reasons: 1) biomass-derived alcohols are a promising, renewable organic feedstock; 2) they use cheap, green oxidants, such as molecular oxygen (air) ; and 3) gold catalysts show extraordinarily high activities under mild conditions. The mechanistic aspects of gold catalyzed oxidation reactions of CO, H2, and other small molecules have been extensively studied, but a generally accepted oxidation mechanism for alcohols under similar conditions has not yet been formulated (however, see references [7–10]). Notably, the efficient low-temperature (50–120 8C) gold catalyzed oxidation of alcohols is known to proceed only in the liquid phase during the course of a long-term batch reaction with high oxygen pressure or intensive O2 flow. [1–4] On the other hand, we recently found that metallic gold nanoparticles supported on TiO2 gave rise to “double peak” catalytic activity in the gasphase oxidation of ethanol to acetaldehyde. The temperature, at which the first peak of activity occurred, 120 8C, was unusually low for the gas-phase reaction of primary alcohols. In contrast, gold supported on Al2O3 and SiO2 showed more usual behaviors, which were analogous to the second peak activity of Au/TiO2 at temperatures above 200 8C. [11] To account for these results, we proposed that specific active oxygen species form on the Au/TiO2 surface under mild reaction conditions and suggested hydrogen as a probable cofactor in their generation. Indeed, H2 can be produced concurrently as a result of ethanol anaerobic dehydrogenation and can thus participate in catalytic activity. In the present work, our efforts were focused on the role of hydrogen in the catalytic activity of gold supported on TiO2, Al2O3, and SiO2 matrixes to provide insights into the different profiles of ethanol oxidation. For this purpose, the O2 isotope exchange technique was applied in order to estimate the relative activity of the surface oxygen species. Oxygen isotope exchange (OIE) is a very sensitive direct method for evaluating the reactivity of surface oxygen atoms, which is a subject of primary interest in heterogeneous oxidation catalysis. Usually, as in the case of metal oxides, OIE is observed at 220–700 8C, although some metal oxides induce OIE at low temperatures after calcination and cooling in a vacuum. A highly reactive oxygen species, so called aoxygen, is generated on the Fe-ZSM5 zeolite surface after treatment with N2O. These species readily oxidize organics (benzene, methane, etc.) as well as perform OIE at room temperature. A special but relevant case, although photoinduced, is the OIE over TiO2, which also occurs at room temperature due to the involvement of reactive oxygen species, probably O anion radicals. Notably, a correlation was found between the TiO2 activity for the photoinduced OIE and the oxidation of isobutane, methanol, and ethanol over TiO2 upon ultraviolet (UV) irradiation. Generally, in a heterophase system comprising molecular dioxygen (O2) in the gas phase and oxygen on the surface of a solid (O), two isotope exchange reactions take place: 1) the hetero-exchange reaction [Equation (1)] , which is conveniently monitored by a fraction of the O isotope in the gas phase (a), and 2) the homo-exchange reaction [Equation (2)] , which is monitored by the parameter y = *C34 C34 (the difference between the equilibrium (*C34) and the current (C34) values for the fraction of asymmetric isotopic OO molecules). Obviously, the low-temperature activity of a catalyst in an oxygen homoexchange can indicate the involvement of very active surface oxygen species by analogy with the cases of a-oxygen and illuminated TiO2. [17]

Selective Oxidation of D-Galactose over Gold Catalysts

The selective oxidation of D‐galactose to galactonic acid over Au/Al2O3 was studied isothermally in a semi‐batch shaker reactor under pH‐controlled conditions at atmospheric pressure. A series of Au/Al2O3 catalysts were prepared and calcined at different temperatures to achieve different gold‐particle sizes. The catalytic properties of the gold nanoparticles were affected by the cluster size. A detailed comparison of activity and selectivity of these catalysts was made, which demonstrated that Au/Al2O3 with a mean particle size of 2.6 nm exhibited the highest activity. The influence of the pH value of the reaction medium on the selective oxidation of D‐galactose was elucidated. Alkaline conditions were characterized by high catalyst activity and selectivity to aldonic acid. Inhibition of the catalytic activity was observed in the acidic medium. The intermediate species was present at low pH values, and this resulted in low conversion and selectivity to the main product, galactonic acid. The electrochemical potential of the catalyst was correlated to the catalytic activity.

Oxygen-Assisted Hydroxymatairesinol Dehydrogenation: A Selective Secondary-Alcohol Oxidation over a Gold Catalyst

Selective dehydrogenation of the biomass-derived lignan hydroxymatairesinol (HMR) to oxomatairesinol (oxoMAT) was studied over an Au/Al(2)O(3) catalyst. The reaction was carried out in a semi-batch glass reactor at 343 K under two different gas atmospheres, namely produced through synthetic air or nitrogen. The studied reaction is, in fact, an example of secondary-alcohol oxidation over an Au catalyst. Thus, the investigated reaction mechanism of HMR oxidative dehydrogenation is useful for the fundamental understanding of other secondary-alcohol dehydrogenation over Au surfaces. To investigate the elementary catalytic steps ruling both oxygen-free- and oxygen-assisted dehydrogenation of HMR to oxoMAT, the reactions were mimicked in a vacuum over an Au(28) cluster. Adsorption of the involved molecular species--O(2), three different HMR diastereomers (namely, one SRR and two RRR forms), and the oxoMAT derivative--were also studied at the DFT level. In particular, the energetic and structural differences between SRR-HMR and RRR-HMR diastereomers on the Au(28) cluster were analyzed, following different reaction pathways for the HMR dehydrogenation that occur in presence or absence of oxygen. The corresponding mechanisms explain the higher rates of the experimentally observed oxygen-assisted reaction, mostly depending on the involved HMR diastereomer surface conformations. The role of the support was also elucidated, considering a very simple Au(28) charged model that explains the experimentally observed high reactivity of the Au/Al(2)O(3) catalyst.

Aerobic Oxidative Esterification of Benzyl Alcohol and Acetaldehyde over Gold Supported on Nanostructured Ceria–Alumina Mixed Oxides

Au nanoparticles supported on nanostructured ceria‐alumina mixed oxides (10 and 30 wt % ceria) were prepared by a deposition–precipitation method. Their properties were studied by N2 adsorption, XRD, TEM, X‐ray photoelectron spectroscopy, and UV/Vis spectroscopy under temperature‐programmed reduction or oxidation. The materials catalyzed the liquid‐phase aerobic oxidative esterification of benzyl alcohol and benzaldehyde effectively and showed a much better performance than Au supported on the individual oxides. The reactions occurred with high turnover numbers (up to 19 000) in methanol solutions in the absence of any auxiliary base and gave mainly methyl benzoate. The strong synergetic effect of ceria and alumina can be explained by the enhanced oxygen storage capacity of the materials prepared from mixed oxides compared to that of pure alumina and ceria. The order of the catalytic activity (Au/Al2O3

CHAPTER 10:Selective Oxidation of Biomass-Derived Secondary Alcohols

Although various gold-catalysed oxidation reactions have been extensively studied and reported in the literature, transformations of biomass-derived compounds have been investigated to a lesser extent. This chapter describes the selective oxidation of the naturally occurring lignin, hydroxymatairesinol (HMR), to form another lignan oxomatairesinol (oxoMAT), which represents an example of the selective oxidation of biomass-derived secondary alcohols. The lignan oxoMAT has been shown to be beneficial for human health. But because it cannot be extracted directly from biomass in sufficient amounts, there is a need to synthesize it from the more abundant HMR. Gold catalysts demonstrate a complete selectivity in HMR oxidation to oxoMAT. The chapter describes the influence of the reaction conditions on product yield, catalyst deactivation, reaction structure sensitivity, reaction mechanism and reactions kinetics.

Gold Catalysts Stability

Long-term stability of catalysts is required for industrial applications. One of the most important advantages with respect to gold catalysts is their stability in oxidizing atmospheres. Catalyst deactivation can be caused, however, by other reasons, such as catalyst surface reconstruction, leaching of the active phase, strong adsorption of organic molecules and poisoning by inorganic compounds. A few studies addressed stability of gold catalysts in biomass conversion. In carbohydrates oxidation gold catalysts demonstrated long-term stability, while catalyst deactivation due to the coke formation was observed in α-pinene isomerization. Complete catalyst regeneration was achieved by catalyst calcination at high temperature, although the same approach was not efficient in case of the lignan HMR oxidation.

Selective Oxidation/Dehydrogenation Reactions

Gold catalysts were extensively studied in the selective oxidation of ethanol into acetic acid and ethyl acetate. Gold supported on titania was found to be the most promising catalysts, demonstrating the low temperature (120oC) catalytic activity even in the gas phase. Reaction kinetics and structure sensitivity were investigated. Gold catalysts were successfully applied in the selective oxidation of 5-hydroxymethyl-2-furfural (HMF) to 5-hydroxymethyl-2-furan-dicarboxylic acid (FDCA) via aldehyde group transformation. Mechanism of the reaction was proposed. Selective oxidation of sugars over gold was described in detail. Aqueous-phase reactions of different sugar units were investigated at low temperature. Biomass-derived lignan hydroxymatairesinol (HMR) was converted over gold catalysts with complete selectivity into another industrially valuable lignan via the reaction of secondary alcohol selective oxidation. Valorization of glycerol by selective oxidation over gold catalyst was described in numerous studies. Influence of the support, gold particle size, bimetallic Au catalysts, and base as well as reaction kinetics was investigated. Reaction mechanism was proposed, where the role of oxygen was addressed.

Selective Hydrogenation Reactions

Gold catalysts demonstrated high selectivity in hydrogenation of the multifunctional molecules. Selective hydrogenation of succinic anhydride (SAN) over gold catalysts was studied. Gold catalysts demonstrated higher activity and selectivity compared to other studied metals.

Double Bond Oxidation Reactions

Environmental friendly selective oxidation of terpene α-pinene to verbenol, verbenone, and α-pinene oxide was studied. Gold demonstrated outstanding activity and selectivity in the reaction. The reaction mechanism was proposed.