Esther Asedegbega-Nieto

Selective 1,3-butadiene hydrogenation by gold nanoparticles deposited & precipitated onto nano-carbon materials

Graphene oxide and multiwall carbon nanotubes (CNTs) were chemically modified by treatment with urea and subsequent annealing at different temperatures. These materials were used as supports for gold nanoparticles and the resulting samples have been applied as catalysts in the 1,3-butadiene partial hydrogenation reaction. The supports and catalysts were exhaustively characterized. It was shown that urea treatments modified the graphene surfaces and the morphology of CNTs, in both cases with incorporation of significant amounts of different nitrogen surface groups. The presence of these groups on few layered graphene or on CNT surfaces modifies the gold precipitation–deposition process during catalyst preparation, giving place to different amounts of incorporated gold on the various supports. The obtained catalytic results suggested that the partial hydrogenation requires limited availability of hydrogen, and for this the migration through adsorbed species between the metal and support to initiate the hydrogenation, probably by a spillover mechanism, seems to be a required step. In general intramolecular selectivity is structure-sensitive meanwhile catalytic activity is not structure-sensitive, as evidenced when the gold nanoparticle sizes are decreased.

Role of Exposed Surfaces on Zinc Oxide Nanostructures in the Catalytic Ethanol Transformation.

For a series of nanometric ZnO materials, the relationship between their morphological and surface functionalities and their catalytic properties in the selective decomposition of ethanol to yield acetaldehyde was explored. Six ZnO solids were prepared by a microemulsion-precipitation method and the thermal decomposition of different precursors and compared with a commercial sample. All these materials were characterized intensively by XRD and SEM to obtain their morphological specificities. Additionally, surface area determinations and IR spectroscopy were used to detect differences in the surface properties. The density of acid surface sites was determined quantitatively using an isopropanol dehydration test. Based on these characterization studies and on the results of the catalytic tests, it has been established that ZnO basal surfaces seem to be responsible for the production of ethylene as a minor product as well as for secondary reactions that yield acetyl acetate. Furthermore, one specific type of exposed hydroxyl groups appears to govern the surface catalytic properties.

Difference in the deactivation of Au catalysts during ethanol transformation when supported on ZnO and on TiO2

Au nanoparticles of different sizes were supported by the deposition–precipitation method on two metal oxides: ZnO and TiO2. The resulting catalysts were tested in the ethanol catalytic transformation reaction. Both metal oxide support materials exerted a different influence on the achieved Au particle size as well as on the behavior of the subsequent catalyst, with regard to their initial conversion values, product distribution and stability. While TiO2 favors the formation of smaller nanoparticles, ZnO offers larger Au particle sizes when prepared under similar conditions. At the same time, TiO2 produced catalysts which displayed higher initial conversions in comparison with AuZnO catalysts, even when observing catalysts of each series with similar particle sizes. At the same time, catalysts supported on ZnO exhibited higher resistance to deactivation caused by coke formation. These results were evidenced employing different characterization techniques on both used and fresh catalyst samples. The decline in deactivation was generally accompanied by an increase in the carbon content on the catalysts surface.

Continuous Gas-Phase Condensation of Bioethanol to 1-Butanol over Bifunctional Pd/Mg and Pd/Mg-Carbon Catalysts

The condensation of ethanol to 1-butanol in the presence of different catalyst systems based on a Pd dehydrogenating/hydrogenating component and magnesium hydroxide-derived materials as basic ingredient was studied in a fixed-bed reactor. The metal was incorporated by wetness impregnation, and the resulting material was then reduced in situ with hydrogen at 573 K for 1 h before reaction. The bifunctional catalysts were tested in a fixed-bed reactor operated in the gas phase at 503 K and 50 bar with a stream of helium and ethanol. A bifunctional catalyst supported on a synthetic composite based on Mg and high surface area graphite (HSAG) was also studied. Improved catalytic performance in terms of selectivity towards 1-butanol and stability was shown by the Pd catalyst supported on the Mg-HSAG composite after thermal treatment in helium at 723 K, presumably due to the compromise between two parameters: adequate size of the Pd nanoparticles and the concentration of strongly basic sites. The results indicate that the optimal density of strongly basic sites is a key aspect in designing superior bifunctional heterogeneous catalyst systems for the condensation of ethanol to 1-butanol.

Physicochemical and catalytic properties of over- and low-exchanged Mo‒ZSM-5 ammoxidation catalysts

A series of Mo/ZSM-5 catalysts prepared by solid-state ion exchange at different Mo/Al molar ratios were characterized and tested in ethane and ethylene ammoxidation into acetonitrile. It has been concluded that the low-exchanged solid (Mo/Al = 0.2) stabilized MoO3, [Mo2O7]2− and [Mo7O24]6− species. However, besides these species, the solids prepared at Mo/Al = 0.5 and 1.5 stabilized [MoO4]2−. Nevertheless, only MoO3 and [Mo2O7]2− species were stabilized at Mo/Al = 1. The study performed by diffuse reflectance spectroscopy allowed the determination of the molar fraction relative to each Mo specie and, therefore, the calculation of the turnover frequency values. The catalytic activities of the various solids have been classified by taking into consideration the inefficiency of Al2(MoO3)4 phase, which inhibits the diffusion of reactants molecules towards the active sites, and amorphous MoO3 which catalyzes the undesired hydrocarbons’ combustion. However, [MoO4]2‒ species are efficient in the oxidative dehydrogenation of C2H6 into C2H4, while dimeric species catalyze the ammoxidation.