ABSTRACT Cobalt and niobium oxide solids modified with alkaline earth metals (Mg, Ca, and Sr) were synthesized and characterized to compare the second element effect on the catalyst structural and chemical properties. The materials were prepared by co-precipitation of cobalt nitrate and niobium oxalate solutions using ammonium hydroxide as precipitating agent. The samples were characterized by XRD, SEM, Ra-man, BET, TPR, ammonia TPD and cyclohexane dehydrogenation reaction (CHD). Results showed that Mg, Ca or Sr addition changed cobalt niobate crystalline structure, leading to alkaline earth metal nio-bates with lower crystallinity. It was observed that Mg, Ca or Sr increased the solids acidity and changed cobalt niobate reduction behavior so that the lowest amount of metallic cobalt was obtained when Mg was incorporated. Modifiers also decreased cobalt niobate dehydrogenation activity. This effect was higher with Mg or Sr addition.

The effect of noble metal catalysts supported on an ionic exchange resin for the enantioselective hydrogenation of ethyl pyruvate to ethyl lactate was studied. High activity and moderate enantioselectivity levels were obtained. Pt was the best enantioselective metallic phase, followed by Pd and Au; Rh was the less enantioselective metallic phase. The effect of the active site was evaluated for the most enantioselective catalyst: 2Pt-WA30 (prefix indicates the metal loading). A novel method for sintering the Pt metal particles by nitriles reduction was used, and a controlled migration of the metal particles to the surface layers was observed. There existed an optimal particle size with maximization of the enantiomeric excess, corresponding to 24 h sintering. The hydrogenation of ethyl pyruvate to ethyl lactate was found to be structure-sensitive and the importance of the metal particle size for the adsorption of the chiral modifier was proved. Two different chirality induction methods were tried for the 2Pt-WA30 catalyst: (i) the addition of the chiral modifier (CM) to the reaction media, or (ii) the modification of the catalyst in a swelling medium prior to the reaction in order to encapsulate CM. Lower activity but higher enantioselectivity levels were obtained with method (ii). Finally, the reuse of the catalysts was also assessed by means of consecutive reaction tests.

In this paper, magnesium aluminate spinels were prepared by microwave-assisted combustion, using urea as fuel. In order to evaluate the effect of the urea excess used in the synthesis, its mass was added in a range of 1.0 to 2.5 times the stoichiometric amount required. The materials produced were characterized by TGA, FTIR, XRF, XRD, BET, SEM and TEM. The results demonstrated that magnesium aluminate spinel was formed in all solids, but the crystallinity degree and the specific surface area increased according to the amount of urea that was used in the synthesis. The use of excess fuel in the preparation was therefore advantageous, since it eliminated the need of the calcination step, at high temperatures, in order to stabilize the spinel phase and improve the crystallinity of the solid. The heat generated in the combustion process was sufficient to develop the formation of a nanocrystalline structure and produce high purity materials, with a save of 99.4% in energy.

Rhodium nanoparticles were successfully stabilized by the (S,S)-1,2-ethanediylbis[(2-ethoxyphenyl)phenylphosphine] ((S,S)-DIPAMP) deposited on SiO2 prepared by a facile reduction and impregnation method. The chiral catalysts obtained were efficient for the enantioselective hydrogenation reactions of prochiral compounds. The catalysts synthesized from [Rh(µ-OMe)(COD)]2 in the presence of different amounts of a ligand used as chiral stabilizer, showed good metal dispersion. As shown by TEM, the metal particle size ranged between 1 and 2 nm using stabilizer and 9.1 nm was achieved without chiral ligand. Other techniques were used to characterize the chiral catalysts such as X-ray diffraction (XRD), electron diffraction, thermogravimetric analysis (TGA) and N2 adsorption-desorption isotherms. (S,S)-DIPAMP was used as the stabilizer of metal particles to prevent growing and agglomeration, and it also acts as chiral modifier inducing enantioselectivity in the asymmetric hydrogenation of 3,4-hexanodione (HD), ethyl pyruvate (EP), ketopantolactone (KP), and acetophenone (AP). Under specific conditions such as 25 °C, 40 bar of H2 and substrate/Rh=100, 1%Rh-(S,S)-DIPAMP/SiO2 chiral catalysts showed excellent catalytic performance with conversion and enantiomeric excess (ee) levels up to 99% and 54% respectively.

Supported nanoparticles (NPs) in presence of chiral ligand (L) were synthesized for their use in enantioselective hydrogenation reactions. Catalysts were obtained by chemical reduction from rhodium chloride hydrate, RhCl3x3H2O, in presence of (-)-DIOP ligand ((4R,5R)-4,5-Bis(diphenylphosphino-methyl)-2,2-dimethyl-1,3-dioxolane) that allows to control NPs growing and to obtain solids having chiral surfaces. Chirally stabilized rhodium NPs on SiO2 were characterized using techniques such as: TEM, electron diffraction, EDS, nitrogen adsorption-desorption isotherms and XPS. This work includes the study of some variables such as metal loading and ligand concentration and their effect in metal core sizes, catalytic activity and enantioselectivity. Catalysts properties have also been evaluated in the hydrogenation of substrates: acetophenone (AP), 1-phenyl-1,2-propanedione (PPD), 3,4-hexanedione (HD), 2,3-butanedione (BD) and ethyl pyruvate (EP) as reaction test. Ligand plays a fundamental role in the synthesis of NPs and enantioselectivity in hydrogenations reactions. That is, due to it generates metal particle size < 5.8 nm compared with unstabilized systems that generate average diameter around 14 nm. Results indicate increased activity in catalytic systems obtained from the stabilization of NPs. Enantioselectivity levels reach values up to 53% due to the chiral ligand is on the catalysts surface.

A hidrogenação enantiosseletiva de piruvato de etila e de 1-fenil-1,2-propanodiona foi estudada a 298 K e sob 40 bar de H2 sobre um catalisador de platina coloidal estabilizado com cinchonidina (CD) e suportado em sílica. O catalisador foi preparado por impregnação da platina coloidal em sílica, atingindo 1% em massa do metal no suporte. O colóide foi preparado a partir de uma solução aquosa de H2PtCl6 estabilizada com diferentes quantidades de CD. O catalisador foi caracterizado por isotermas de adsorção-desorção de N2 a 77 K e por XPS, XRD e TEM. As reações foram efetuadas em batelada em um reator de aço inoxidável, usando cicloexano como solvente e cinchonidina como modificador quiral. A adição de CD ao catalisador permite a estabilização e o controle do tamanho de partícula da platina, e também afeta o excesso enantiomérico (ee) do sistema, levando a um maior rendimento do produto com configuração R. A relação entre a concentração de CD adicionada in situ e a enantiosseletividade apresenta uma curva do tipo sino nas duas reações estudadas; este comportamento é um indicativo da importância da adsorção competitiva do modificador e do substrato na superfície do catalisador.

The enantioselective hydrogenation of ethyl pyruvate and 1-phenyl-1,2-propanedione was studied at 298 K and 40 bar of H2 over colloidal Pt stabilized with cinchonidine (CD) and supported on silica. The catalysts were prepared by impregnation of colloidal Pt on SiO2, leading to a metal load close to 1 wt.%. The colloid was prepared from an aqueous solution of H2PtCl6 and stabilized with different quantities of CD. The catalysts were characterized by nitrogen adsorption-desorption isotherms at 77 K, XPS, XRD and TEM techniques. The reactions were carried out in a stainless steel batch reactor using cyclohexane as solvent and cinchonidine as chiral modifier. The addition of CD to the catalysts allows the stabilization and control of the platinum particle size and also affects the enantiomeric excess (ee) of the system, affording a higher proportion of the (R)-product. The relationship between the enantioselectivity and the CD concentration (added in situ) exhibits a bell type curve for both reactions, this being indicative of the importance of a competitive adsorption of the modifier and substrate on the catalyst surface.

The continuous enantioselective hydrogenation of ethyl pyruvate (EP) in a continuous flow reactor at 25°C on l%Pt-CD/Si0(2) catalysts has been studied. The catalytic study included the analysis of variables such as: particle size, feed flow, H2 pressure and the addition of chiral modifier to the reactor feed. The catalysts were prepared using impregnation of Pt colloids which are stabilized with different quantities of cinchonidine (CD) on Si0(2). This process gives chiral properties to the catalyst and allows changing the particle size for the reaction of interest. The catalysts were characterized by adsorption/desorption isotherms of N2 a 77 K, TEM, XPS and the analysis of producis by GC-MS. The monitoring of the reaction indicates a significant increases in the conversion when deereasing the contact time and a maximum in the enantiomeric excess (ee) of 63 % to the (R)-ethyl lactate which is obtained after using 200mg of l%Pt/Si0(2) catalyst (1.6nm) at 10 bar of H2 and a flow of 0.3 mL/min of a mixture EP (0.01M) and CD (0.034M) in cyclohexane.