Marco A. Fraga

Hydrogenation of citral over ruthenium-tin catalysts

Citral was chosen to probe the kinetic behavior of the hydrogenation of ,-unsaturated aldehydes over HTR and LTR titania-supported Ru-Sn catalysts. The catalysts were prepared by coimpregnation with ethanolic solutions of RuCl 3 and SnCl2 and characterized using temperature-programmed reduction (TPR). The reaction was studied in a liquid phase semi-batch reactor at 399 K and a hydrogen pressure of 5 MPa. The global activity for the citral hydrogenation of HTR Ru-Sn/TiO2 catalysts as well as the selectivity to the unsaturated alcohols of both LTR and HTR Ru-Sn/TiO2 increased with the Sn/Ru atomic ratio, reaching a maximum at a value of Sn/Ru ∼ 0.2. This suggested a promotion of the CO bond hydrogenation by particular bimetallic Sn-Ru ensembles. Besides, while for Sn/Ru ≤ 0.1 the support effect of titania dominated, for higher values of the atomic ratio the performance of the catalysts was controlled by the promoter effect of tin. A combined effect of the support titania with the promoter tin was clearly detected.

Hemicellulose-derived chemicals: one-step production of furfuryl alcohol from xylose

One-pot production of furfuryl alcohol via xylose dehydration followed by furfural hydrogenation was investigated over a dual catalyst system composed of Pt/SiO2 and sulfated ZrO2 as metal and acid catalysts, respectively. All samples were characterized by XRD, XRF, N2 physisorption, TG-MS and FTIR regarding their most fundamental properties for the studied process. A systematic study is reported on the effects of the reaction temperature, the composition of the binary solvent and the molar ratio between acid and metal sites in the catalyst system. The results revealed the feasibility of the one-step process for furfuryl alcohol synthesis and showed that the occurrence of both acid and metal sites is compulsory in order to promote the dehydration of xylose to furfural and its further hydrogenation to furfuryl alcohol. Selectivity towards furfuryl alcohol was found to be strongly dependent on the solvent, which can inhibit its polymerization to some extent.

Tuning Surface Basic Properties of Nanocrystalline MgO by Controlling the Preparation Conditions

Different nanocrystalline magnesias were synthesized by precipitation and hydrothermal treatments of aqueous salt solutions in an attempt to tune their surface basicity. CO(2) was chosen as an acidic molecule to probe the basic sites by both temperature-programmed desorption and infrared spectroscopy. All samples were shown to be crystalline, and except that obtained by nitrate decomposition, they all possessed high surface areas. The oxides presented different basic site distributions, evidencing the significant role of the preparation conditions on tuning the surface basicity: while medium-strength basic centers are dominant in the samples prepared by precipitation aging or hydrothermal treatment, the one obtained by precipitation features a roughly equal concentration of medium-strength and strong centers. Infrared spectra revealed that hydrogen carbonate and monodentate and bidentate carbonates were formed in distinct proportion on all oxides. However, the bidentate complexes were shown to have different thermal stabilities; the more stable species are thought to be formed on acid-base pair centers associated with an anionic vacancy. Distinct morphological and structural characteristics were also observed by high-resolution transmission electron microscopy. It was consistently found that the high-surface area samples are formed by aggregates of nanoparticles (2-5 nm) randomly oriented and with a high concentration of structural defects. These findings allowed us to conclude that the surface heterogeneity promoted during synthesis increases the concentration of basic sites and plays an important role in tuning the basicity of the solids.

Examination of the surface chemistry of activated carbon on enantioselective hydrogenation of methyl pyruvate over Pt/C catalysts

Pt/C catalysts were prepared on oxidized activated carbons. Different oxidation treatments were used both in liquid and gas phase using HNO3 and 5% O2/N2 as the respective oxidizing agents. Thus, different surface chemical properties were achieved. The enantioselective hydrogenation of methyl pyruvate with (−)-cinchonidine as modifier revealed that the catalytic activity is dependent on the concentration of the surface oxygen remaining after activation as measured by XPS. It is suggested that it is a result of the increase in the carbon hydrophilicity that would enable the reactants to go through the catalyst. It is also suggested that the active sites created by the decomposition of the functional groups would allow hydrogen chemisorption and, therefore, the C=O polarization via hydrogen bond interaction. The chemical nature of the surface functional groups did not seem to influence either the catalytic activity or enantioselectivity.

Lactic acid production from hydroxyacetone on dual metal/base heterogeneous catalytic systems

Oxidative aqueous-phase catalytic conversion of hydroxyacetone to lactic acid was investigated under mild and green reaction conditions. Dual metal/base catalyst systems were applied to replace the alkali hydroxide and address the separation requirements and corrosiveness issues usually involved in similar processes. Pt/Al2O3 was taken as the supported metal catalyst while Mg–Al layered double hydroxides (LDH) and their derived 3D mixed oxides were exploited as basic solid catalysts. The anion embedded within the LDH interlamellar domain (nitrate or hydroxyl) and the LDH topology (lamellar or exfoliated 2D nanosheets) were also examined. Activity and chemoselectivity of the dual system were shown to be tailored by the morphology, topology and basicity of the base catalyst. Catalytic activity was up to five times higher by adding a heterogeneous base solid to the metal catalyst. Surface site distribution was also shown to be quite decisive to the catalyst performance; the proximity of metal and basic centres on the same surface rendered highly selective bifunctional catalysts. A remarkable 100% selectivity to lactic acid was reached and the undesired cascade oxidation of lactic to pyruvic acid was hindered.

Relationship between Acid–Base Properties and the Activity of ZrO2‐Based Catalysts for the Cannizzaro Reaction of Pyruvaldehyde to Lactic Acid

The Cannizzaro reaction of pyruvaldehyde to lactic acid is investigated in a flow reactor with ZrO2 catalysts with different structures and acid–base properties. The results show that a difference in the crystalline structures of two ZrO2 polymorphs strongly affects the conversion of pyruvaldehyde. The monoclinic phase of zirconia is the most active for this reaction. A good correlation is observed between the reaction rate and the concentration of Lewis acid sites of sufficient strength, which shows that these sites play a major role in the reaction. A reaction mechanism is proposed involving coordinatively unsaturated Zr4+ cations as sites for activating pyruvaldehyde molecules, whereas Zr4+−O2− pairs generate terminal OH groups through water dissociation.

Alcohol synthesis over bulk aluminium-nickel-copper based catalysts from syngas

The catalytic performance of bulk aluminium-nickel-copper based catalyst was investigated with regard to alcohol synthesis. It was shown that, in spite of forming methanol as the main product, these catalysts are promising for the production of higher alcohols. Higher activation temperature increased the production of higher alcohols. The change of the salt concentration decreased the selectivity, while the pH had basically no effect on the selectivity.

Perovskite as nickel catalyst precursor – impact on catalyst stability on xylose aqueous-phase hydrogenation

Precursors materials with formula, La1−xCexAl0.18Ni0.82O3 (x = 0.0, 0.1, 0.5, 0.7), were successfully used as precursors to prepare Ni nanoclusters to be used as catalysts in the hydrogenation of xylose to xylitol. For the Ce free and lower Ce content (x = 0.0; 0.1), the perovskite structure was obtained, whereas for a higher Ce content (x = 0.5; 0.7), an ordinary CeO2–La2O3 solid dissolution with no perovskite structure were obtained. Under the reduction conditions, the perovskite structure leads to ∼30% of metallic Ni without any loss of the perovskite structure (x = 0.0; 0.1) and the CeO2–La2O3 solid dissolution allows a Ni reduction of ∼40% of Ni (x = 0.5; 0.7). As expected, the similar reduced Ni content does not show large differences in the aqueous-phase xylose hydrogenation product distribution, or large difference in the precursors. The perovskite structure (x = 0.0; 0.1) and solid dissolution (x = 0.5; 0.7) highlight the importance of the perovskite structure in the remarkable leaching resistance showed by the Ni perovskite-precursor (x = 0.0; 0.1) catalysts, showing no Ni leaching over 6 h of reaction in an aqueous medium.

Acrylic resin water sorption under different pressure, temperature and time conditions

Received: April 24, 2000; Revised: November 22, 2000The purpose of this work was to analyze water sorption by polymerized acrylic resins underdifferent pressure, temperature and time treatments. A thermo-cured acrylic resin was used as thedenture base (Classico Ltda.) and ethylene glycol di-methacrylate as a cross-linking agent, withprocessing carried out in a water bath at 73 °C for nine hours. Forty-five samples were preparedfollowing the criteria and dimensions of specification # 12 of the American Dental Association(ADA), using a matrix in the shape of a stainless steel disc with 50 ±1 mm diameter and0.5 ±0.05 mmthickness. Thecontrolgroupsampleswerestoredin distilledwaterfor30 days,whilegroups GII to GIX were placed in a polymerization device with adjustable pressure, time andtemperature. An analysis of the variance of the results revealed the influence of different factors onwater sorption only, with significant factors being temperature, time, pressure and the interactionbetween timeandtemperature.Otherinteractionsexerted no significantinfluenceon watersorption.Neither additional treatments nor the control group (GI) showed any significant difference incomparison to the averages of other treatments.

Tandem dehydration–transfer hydrogenation reactions of xylose to furfuryl alcohol over zeolite catalysts

Dehydration and transfer hydrogenation tandem reactions of xylose to furfuryl alcohol in water/isopropanol were performed on MFI, FAU and BEA zeolites. An unprecedented selectivity of 75% was accomplished on zeolite beta, evidencing the crucial role of the configuration of tetrahedral framework Al-Lewis centres.