The Role of Mg(OH)2 in the So-Called "Base-Free" Oxidation of Glycerol with AuPd Catalysts.

Mg(OH)2 - and Mg(OH)2 -containing materials can provide excellent performance as supports for AuPd nanoparticles for the oxidation of glycerol in the absence of base, which is considered to be a result of additional basic sites on the surface of the support. However, its influence on the reaction solution is not generally discussed. In this paper, we examine the relationship between the basic Mg(OH)2 support and AuPd nanoparticles in detail using four types of catalyst. For these reactions, the physical interaction between Mg(OH)2 and AuPd was adjusted. It was found that the activity of the AuPd nanoparticles increased with the amount of Mg(OH)2 added under base-free conditions, regardless of its interaction with the noble metals. In order to investigate how Mg(OH)2 affected the glycerol oxidation, detailed information about the performance of AuPd/Mg(OH)2 , physically mixed (AuPd/C+Mg(OH)2 ) and (AuPd/C+NaHCO3 ) was obtained and compared. Furthermore, NaOH and Mg(OH)2 were added during the reaction using AuPd/C. All these results indicate that the distinctive and outstanding performance of Mg(OH)2 supported catalysts in base-free condition is in fact directly related to its ability to affect the pH during the reaction and as such, assists with the initial activation of the primary alcohol, which is considered to be the rate determining step in the reaction.

The preparation of large surface area lanthanum based perovskite supports for AuPt nanoparticles: tuning the glycerol oxidation reaction pathway by switching the perovskite B site.

Gold and gold alloys, in the form of supported nanoparticles, have been shown over the last three decades to be highly effective oxidation catalysts. Mixed metal oxide perovskites, with their high structural tolerance, are ideal for investigating how changes in the chemical composition of supports affect the catalysts' properties, while retaining similar surface areas, morphologies and metal co-ordinations. However, a significant disadvantage of using perovskites as supports is their high crystallinity and small surface area. We report the use of a supercritical carbon dioxide anti-solvent precipitation methodology to prepare large surface area lanthanum based perovskites, making the deposition of 1 wt% AuPt nanoparticles feasible. These catalysts were used for the selective oxidation of glycerol. By changing the elemental composition of the perovskite B site, we dramatically altered the reaction pathway between a sequential oxidation route to glyceric or tartronic acid and a dehydration reaction pathway to lactic acid. Selectivity profiles were correlated to reported oxygen adsorption capacities of the perovskite supports and also to changes in the AuPt nanoparticle morphologies. Extended time on line analysis using the best oxidation catalyst (AuPt/LaMnO3) produced an exceptionally high tartronic acid yield. LaMnO3 produced from alternative preparation methods was found to have lower activities, but gave comparable selectivity profiles to that produced using the supercritical carbon dioxide anti-solvent precipitation methodology.

Oxidation of Aliphatic Alcohols by Using Precious Metals Supported on Hydrotalcite under Solvent- and Base-Free Conditions.

Precious metal nanoparticles supported on magnesium-aluminum hydrotalcite (HT), TiO2 , and MgO were prepared by sol immobilization and assessed for the catalytic oxidation of octanol, which is a relatively unreactive aliphatic alcohol, with molecular oxygen as the oxidant under solvent- and base-free conditions. Compared with the TiO2 - and MgO-supported catalysts, platinum HT gave the highest activity and selectivity towards the aldehyde. The turnover number achieved for the platinum HT catalyst was >3700 after 180 min under mild reaction conditions. Moreover, the results for the oxidation of different substrates indicate that a specific interaction of octanal with the platinum HT catalyst could lead to deactivation of the catalyst.

Base-free oxidation of glycerol using titania-supported trimetallic Au­Pd­Pt nanoparticles.

Base-free selective oxidation of glycerol has been investigated using trimetallic Au­Pd­Pt nanoparticles supported on titania and their corresponding bimetallic catalysts. Catalysts were prepared by the sol-immobilization method and characterized by means of TEM, UV/Vis spectroscopy, diffuse reflectance infrared fourier transform spectroscopy, X-ray photoelectron spectroscopy, and microwave plasma­atomic emission spectroscopy. It was found that of the bimetallic catalysts, Pd­Pt/TiO2 was the most active with high selectivity to C3 products. The addition of Au to this catalyst to form the trimetallic Au­Pd­Pt/TiO2, resulted in an increase in activity relative to Pd­Pt/TiO2. The turnover frequency increased from 210 h(−1) with the Pd­Pt/TiO2 catalyst to378 h(−1) for the trimetallic Au­Pd­Pt/TiO2 catalyst with retention of selectivity towards C3 products.

Gold-nanoparticle-based catalysts for the oxidative esterification of 1,4-butanediol into dimethyl succinate.

The oxidation of 1,4-butanediol and butyrolactone have been investigated by using supported gold, palladium and gold-palladium nanoparticles. The products of such reactions are valuable chemical intermediates and, for example, can present a viable pathway for the sustainable production of polymers. If both gold and palladium were present, a significant synergistic effect on the selective formation of dimethyl succinate was observed. The support played a significant role in the reaction, with magnesium hydroxide leading to the highest yield of dimethyl succinate. Based on structural characterisation of the fresh and used catalysts, it was determined that small gold-palladium nanoalloys supported on a basic Mg(OH)2 support provided the best catalysts for this reaction.

Au-Pd nanoalloys supported on Mg-Al mixed metal oxides as a multifunctional catalyst for solvent-free oxidation of benzyl alcohol.

Au-Pd nanoalloys supported on Mg-Al mixed metal oxides prepared using sol-immobilisation are found to be highly efficient and reusable catalysts for the solvent-free oxidation of benzyl alcohol using molecular oxygen under low pressure. When using this support alloying Pd with Au resulted in an increase in both activity and selectivity to benzaldehyde and moreover an improved resistance to catalyst deactivation compared with the monometallic Pd and Au catalysts. The turnover number for the Au/Pd 1:1 molar ratio catalyst achieved 13,000 after 240 min and the selectivity to benzaldehyde was maintained at 93%; this high catalytic activity can be retained in full after three successive uses. The ensemble and electronic effect of Au-Pd nanoalloys were studied by IR spectroscopy using CO chemisorption, XPS and HRTEM. Moreover, the bifunctional nature of the acid-base MgAl-MMO support was found to be important as the acid sites are considered to be responsible for the improvement of catalytic activity; while, the basic sites gave rise to high selectivity. A possible mechanism with Au-Pd nanoparticles as the active sites has been proposed, illustrating that the oxidation of benzyl alcohol can proceed through the cooperation between the Au-Pd nanoalloys and the base/acid sites on the surface of the support.

Understanding the solvent effect on the catalytic oxidation of 1,4-butanediol in methanol over Au/TiO2 catalyst: NMR diffusion and relaxation studies.

The effect of water on the catalytic oxidation of 1,4-butanediol in methanol over Au/TiO(2) has been investigated by catalytic reaction studies and NMR diffusion and relaxation studies. The addition of water to the dry catalytic system led to a decrease of both conversion and selectivity towards dimethyl succinate. Pulsed-field gradient (PFG)-NMR spectroscopy was used to assess the effect of water addition on the effective self-diffusivity of the reactant within the catalyst. NMR relaxation studies were also carried out to probe the strength of surface interaction of the reactant in the absence and presence of water. PFG-NMR studies revealed that the addition of water to the initial system, although increasing the dilution of the system, leads to a significant decrease of effective diffusion rate of the reactant within the catalyst. From T(1) and T(2) relaxation measurements it was possible to infer the strength of surface interaction of the reactant with the catalyst surface. The addition of water was found to inhibit the adsorption of the reactant over the catalyst surface, with the T(1)/T(2) ratio of 1,4-butanediol decreasing significantly when water was added. The results overall suggest that both the decrease of diffusion rate and adsorption strength of the reactant within the catalyst, due to water addition, limits the access of reactant molecules to the catalytic sites, which results in a decrease of reaction rate and conversion.

Facile removal of stabilizer-ligands from supported gold nanoparticles.

Metal nanoparticles that comprise a few hundred to several thousand atoms have many applications in areas such as photonics, sensing, medicine and catalysis. Colloidal methods have proven particularly suitable for producing small nanoparticles with controlled morphologies and excellent catalytic properties. Ligands are necessary to stabilize nanoparticles during synthesis, but once the particles have been deposited on a substrate the presence of the ligands is detrimental for catalytic activity. Previous methods for ligand removal have typically involved thermal and oxidative treatments, which can affect the size or morphology of the particles, in turn altering their catalytic activity. Here, we report a procedure to effectively remove the ligands without affecting particle morphology, which enhances the surface exposure of the nanoparticles and their catalytic activity over a range of reactions. This may lead to developments of nanoparticles prepared by colloidal methods for applications in fields such as environmental protection and energy production.