Betiana Campo

Acid-functionalized UiO-66(Zr) MOFs and their evolution after intra-framework cross-linking: structural features and sorption properties

The functionalization of metal–organic frameworks (MOFs) with free carboxylic groups is naturally difficult due to their potential coordination with metal ions. The impact of functionalizing the archetypical metal organic framework UiO-66(Zr) with free pending carboxylic groups was thus studied by a multi-technique approach. First, an environmentally friendly water synthesis route was developed to produce UiO-66(Zr)–(COOH)x (x = 1, 2) and the kinetics of crystallization was studied by in situ energy dispersive X-ray diffraction. In a second step, the structural features were studied by temperature-dependent X-ray diffractometry and further characterized by density functional theory calculations and solid-state nuclear magnetic resonance spectroscopy. The gas sorption properties, acidity and conductivity features were respectively assessed by gas isotherms and calorimetry, infrared spectroscopy and complex impedance spectroscopy. These data show the noticeable influence of the introduced acidic groups. Finally, it was shown that the thermal treatment of such solids leads to an intra-framework cross-linking associated with the formation of anhydride bridges, as evidenced by FTIR and solid-state NMR, and modelled by DFT simulations. These species have a strong impact on the acidity, but a limited effect on gas sorption properties at room temperature. The reversibility of the carboxylic acids to anhydride transformation was also assessed.

“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

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.

Kinetics of lactose and rhamnose oxidation over supported metal catalysts

Several mono- and bimetallic Pd, Pt, Rh and Ru supported on alumina and active carbon catalysts were characterized by CO chemisorption, nitrogen adsorption, XPS and XRD and acidity titrations were performed for active carbon supported catalysts. These catalysts were tested in oxidation of two sugars, namely lactose and rhamnose, at 60 °C and at 70 °C under slightly alkaline conditions (pH 8) with molecular oxygen. The results revealed that there is an optimum metal particle size in a range of 3-10 nm giving the highest initial TOFs for both oxidations. Furthermore, the catalytic activities and conversions were related to other catalyst properties, such as the type and amount of promoters and the presence of different phases. In situ catalyst potential measurements revealed that there is an inverse correlation between the increase of catalyst potential as a function of sugar conversion and the catalyst activity after prolonged reaction times. This method is a valuable tool for in situ characterization of catalysts correlating well with their activities.