Support effects in a Rh diamine complex heterogenized on carbon materials

The Rh diamine complex [Rh(COD)NH2(CH2)2NH(CH2)3Si(OCH3)3] BF4 was heterogenized by covalent bonding on two carbon xerogels and on carbon nanofibers, with the objective of preparing hydrogenation hybrid catalysts. Gas adsorption, SEM, TEM, DTP, ICP-OES and XPS were used for characterization. The results indicate that the active molecule is mainly located in supermicropores and produces microporosity blockage. The hybrid catalysts are more active than the homogeneous complex, but the Rh complex is partially reduced upon reaction. This modification is related to the nature of the support, which also shows effects in the stabilization against sintering of the Rh particles formed. The support porosity is a key factor in the selectivity differences between the catalysts.

CuH-ZSM-5 as Hydrocarbon Trap under Cold Start Conditions

Cold start tests are carried out to evaluate the performance of copper-exchanged zeolites as hydrocarbon traps under simulated gasoline car exhaust gases, paying special attention to the role of copper in the performance of these zeolites. It is concluded that the partial substitution of the protons in the parent H-ZSM-5 zeolite is highly beneficial for hydrocarbon trapping due to the formation of selective adsorption sites with specific affinity for the different exhaust components. However, it is also observed that uncontrolled exchanging process conditions could lead to the presence of CuO nanoparticles in the zeolite surface, which seem to block the pore structure of the zeolite, decreasing the hydrocarbon trap efficiency. Among all the zeolites studied, the results point out that a CuH-ZSM-5 with a partial substitution of extra-framework protons by copper cations and without any detectable surface CuO nanoparticles is the zeolite that showed the best performance under simulated cold start conditions due to both the high stability and the hydrocarbon retaining capacity of this sample during the consecutive cycles.

Heterofunctional Supports in Enzyme Immobilization: From Traditional Immobilization Protocols to Opportunities in Tuning Enzyme Properties

A heterofunctional support for enzyme immobilization may be defined as that which possesses several distinct functionalities on its surface able to interact with a protein. We will focus on those supports in which a final covalent attachment between the enzyme and the support is achieved. Heterofunctionality sometimes has been featured in very old immobilization techniques, even though in many instances it has been overlooked, giving rise to some misunderstandings. In this respect, glutaraldehyde-activated supports are the oldest multifunctional supports. Their matrix has primary amino groups, the hydrophobic glutaraldehyde chain, and can covalently react with the primary amino groups of the enzyme. Thus, immobilization may start (first event of the immobilization) via different causes and may involve different positions of the enzyme surface depending on the activation degree and immobilization conditions. Other “classical” heterofunctional supports are epoxy commercial supports consisting of reactive covalent epoxy groups on a hydrophobic matrix. Immobilization is performed at high ionic strength to permit protein adsorption, so that covalent attachment may take place at a later stage. Starting from these old immobilization techniques, tailor-made heterofunctional supports have been designed to permit a stricter control of the enzyme immobilization process. The requirement is to find conditions where the main covalent reactive moieties may have very low reactivity toward the enzyme. In this Review we will discuss the suitable properties of the groups able to give the covalent attachment (intending a multipoint covalent attachment), and the groups able to produce the first enzyme adsorption on the support. Prospects, limitations, and likely pathways for the evolution (e.g., coupling of site-directed mutagenesis and thiol heterofunctional supports of enzyme immobilization on heterofunctional supports) will be discussed in this Review.

Nanoarchitectures Based on Layered Titanosilicates Supported on Glass Fibers: Application to Hydrogen Storage

This work reports on the synthesis of nanosheets of layered titanosilicate JDF-L1 supported on commercial E-type glass fibers with the aim of developing novel nanoarchitectures useful as robust and easy to handle hydrogen adsorbents. The preparation of those materials is carried out by hydrothermal reaction from the corresponding gel precursor in the presence of the glass support. Because of the basic character of the synthesis media, silica from the silicate-based glass fibers can be involved in the reaction, cementing its associated titanosilicate and giving rise to strong linkages on the support with the result of very stable heterostructures. The nanoarchitectures built up by this approach promote the growth and disposition of the titanosilicate nanosheets as a house-of-cards radially distributed around the fiber axis. Such an open arrangement represents suitable geometry for potential uses in adsorption and catalytic applications where the active surface has to be available. The content of the titanosilicate crystalline phase in the system represents about 12 wt %, and this percentage of the adsorbent fraction can achieve, at 298 K and 20 MPa, 0.14 wt % hydrogen adsorption with respect to the total mass of the system. Following postsynthesis treatments, small amounts of Pd (<0.1 wt %) have been incorporated into the resulting nanoarchitectures in order to improve their hydrogen adsorption capacity. In this way, Pd-layered titanosilicate supported on glass fibers has been tested as a hydrogen adsorbent at diverse pressures and temperatures, giving rise to values around 0.46 wt % at 298 K and 20 MPa. A mechanism of hydrogen spillover involving the titanosilicate framework and the Pd nanoparticules has been proposed to explain the high increase in the hydrogen uptake capacity after the incorporation of Pd into the nanoarchitecture.

Hydrogen Peroxide in Biocatalysis. A Dangerous Liaison

Hydrogen peroxide is a substrate or side-product in many enzyme-catalyzed reactions. For example, it is a side-product of oxidases, resulting from the re-oxidation of FAD with molecular oxygen, and it is a substrate for peroxidases and other enzymes. However, hydrogen peroxide is able to chemically modify the peptide core of the enzymes it interacts with, and also to produce the oxidation of some cofactors and prostetic groups (e.g., the hemo group). Thus, the development of strategies that may permit to increase the stability of enzymes in the presence of this deleterious reagent is an interesting target. This enhancement in enzyme stability has been attempted following almost all available strategies: site-directed mutagenesis (eliminating the most reactive moieties), medium engineering (using stabilizers), immobilization and chemical modification (trying to generate hydrophobic environments surrounding the enzyme, to confer higher rigidity to the protein or to generate oxidation-resistant groups), or the use of systems capable of decomposing hydrogen peroxide under very mild conditions. If hydrogen peroxide is just a side-product, its immediate removal has been reported to be the best solution. In some cases, when hydrogen peroxide is the substrate and its decomposition is not a sensible solution, researchers coupled one enzyme generating hydrogen peroxide “in situ” to the target enzyme resulting in a continuous supply of this reagent at low concentrations thus preventing enzyme inactivation. This review will focus on the general role of hydrogen peroxide in biocatalysis, the main mechanisms of enzyme inactivation produced by this reactive and the different strategies used to prevent enzyme inactivation caused by this “dangerous liaison”.

Química Verde: trabajo de laboratorio en la Microescala

Actualmente, los estudiantes de cualquier ámbito, y en especial en la Educación Superior, deben tender a formarse en las diferentes disciplinas dentro del marco de un aprendizaje integral que contemple amplios criterios de responsabilidad ética, cívica y medioambiental. Desde esta perspectiva, el profesorado debe comprometerse a ofrecer los recursos necesarios y la información adecuada para una plena concienciación del estudiante en temas de materia medioambiental para un desarrollo sostenible. Ambas facetas se contemplan en los estudios conducentes a la obtención del Grado en Química de la Universidad de Alicante mediante la impartición de diferentes asignaturas tanto obligatorias como optativas. El objetivo del presente trabajo es aumentar las posibilidades de concienciación de los estudiantes en estas materias por medio de la realización de prácticas instrumentales en el Grado en Química bajo los criterios de “Química Verde” y “Trabajo en la Microescala”. Como ejemplo se muestra la adaptación de una práctica convencional, elegida entre las que cumplen los requisitos de la Química Verde, para su realización a microescala. Esta adaptación, además de cumplir con los fines formativos ya mencionados, disminuye los costes económicos asociados tanto al consumo de reactivos como a la generación de residuos.

Influence of peroxometallic intermediaries present on polyoxometalates nanoparticles surface on the adipic acid synthesis

The cyclohexene oxidation by hydrogen peroxide catalysed by polyoxometalates (POM) has been shown as an adequate green route for the adipic acid synthesis. In this study, it has been demonstrated that POM's salts are effective catalysts for this reaction and how peroxopolyoxometalates intermediaries are the truly responsible species of the POM's salts catalytic activity and solubility. However, the latter can be reduced by calcining the catalyst previously. Polyoxomolybdates salts generally present a higher activity than polyoxotungstenates salts. Finally, it must be remarked the positive effect exerted by the acetic acid stabilising the peroxide of hydrogen against its decomposition.

Growth of Carbon Nanotubes on Surfaces: The Effects of Catalyst and Substrate

We report a study of synthesising air-stable, nearly monodispersed bimetallic colloids of Co/Pd and Fe/Mo of varying compositions as active catalysts for the growth of carbon nanotubes. Using these catalysts we have investigated the effects of catalyst and substrate on the carbon nanostructures formed in a plasma-enhanced chemical vapour deposition (PECVD) process. We will show how it is possible to assess the influence of both the catalyst and the support on the controlled growth of carbon nanotube and nanofiber arrays. The importance of the composition of the catalytic nuclei will be put into perspective with other results from the literature. Furthermore, the influence of other synthetic parameters such as the nature of the nanoparticle catalysts will also be analysed and discussed in detail.

Congreso de Estudiantes de Química de la Comunidad Valenciana. Otra forma de aprender es posible

En abril de 2013 se celebró en la Universidad de Alicante el I Congreso de Estudiantes de Química de la Comunidad Valenciana. Estas jornadas presentan como principal novedad que la totalidad del comité organizador está compuesto por estudiantes de Química (cinco de la Universidad de Alicante y dos de la Universidad de Valencia) apoyados por un comité científico formado por profesores de ambas universidades. Estos estudiantes habían participado en años anteriores en redes de innovación docente o proyectos similares, mostrando interés por proponer alternativas a las clases magistrales para la adquisición de competencias, tanto de tipo transversal como específicas de la titulación. El congreso se enfocó teniendo en cuenta tres puntos básicos que son de especial interés para estudiantes de Química: (i) Salidas profesionales; (ii) Búsqueda activa de empleo y como enfrentarse a una entrevista de trabajo; (iii) Estudios de postgrado. Además, los más de 100 estudiantes que participaron en las jornadas ya sea mediante una comunicación oral, un póster o una actividad de divulgación científica se mostraron muy satisfechos con la experiencia. Actividades de este tipo y su gran acogida son capaces de mostrarnos, tanto a estudiantes como profesores, que existen formas de aprendizaje fuera de las aulas que permiten la adquisición de competencias transversales y específicas.

Seguimiento del Grado en Química

Con la implantación del 4º curso del grado en Química, que se ha realizado en el presente curso académico, se han completado los cuatro cursos del grado. En la Facultad de Ciencias se han constituido ocho comisiones de semestre, en las que participan profesores de todos los departamentos que imparten docencia en la titulación y la Comisión de Grado, formada por los coordinadores de las comisiones de semestre que, a su vez, forman parte de una red docente para el seguimiento del grado. Desde estas comisiones se está realizando un intenso trabajo cooperativo cuyo objetivo es alcanzar una coherencia tanto en la distribución de contenidos, como en las metodologías docentes y de evaluación de las materias que componen el plan de estudios del Grado en Química de la Universidad de Alicante. La coordinación horizontal entre semestres de un mismo curso y la coordinación vertical entre cursos forman parte de las tareas que se desarrollan. Los resultados de este trabajo permiten identificar las deficiencias en el proceso de implantación y plantear posibles propuestas de mejora en la organización docente de la titulación.

Chiral rhodium complexes covalently anchored on carbon nanotubes for enantioselective hydrogenation

Chiral rhodium hybrid nanocatalysts have been prepared by covalent anchorage of pyrrolidine-based diphosphine ligands onto functionalized CNTs. This work constitutes the first attempt at covalent anchoring of homogeneous chiral catalysts on CNTs. The catalysts, prepared with two different chiral phosphines, were characterized by ICP, XPS, N2 adsorption and TEM, and have been tested in the asymmetric hydrogenation of two different substrates: methyl 2-acetamidoacrylate and α-acetamidocinnamic acid. The hybrid nanocatalysts have shown to be active and enantioselective in the hydrogenation of α-acetamidocinnamic acid. A good recyclability of the catalysts with low leaching and without loss of activity and enantioselectivity was observed.

Investigation of Pd nanoparticles supported on zeolites for hydrogen production from formic acid dehydrogenation

Catalysts based on palladium nanoparticles supported on different zeolites (BETA, ZSM-5 and Y) were prepared and their catalytic performance in formic acid dehydrogenation was studied. The effects of the zeolite structure and porous texture on the catalytic activity were investigated by comparing the behavior of these samples. The results revealed that the samples based on BETA zeolite are promising catalysts for this application.

Hydrothermal Carbons from Hemicellulose-Derived Aqueous Hydrolysis Products as Electrode Materials for Supercapacitors

Acid pretreatment of lignocellulosic biomass, required for bioethanol production, generates large amounts of by-products, such as lignin and hydrolyzed hemicellulose fractions, which have found so far very limited applications. In this work, we demonstrate how the recovered hemicellulose hydrolysis products can be effectively utilized as a precursor for the synthesis of functional carbon materials through hydrothermal carbonization (HTC). The morphology and chemical structure of the synthesized HTC carbons are thoroughly characterized to highlight their similarities with glucose-derived HTC carbons. Furthermore, two routes for introducing porosity within the HTC carbon structure are presented: i) silica nanoparticle hard-templating, which is shown to be a viable method for the synthesis of carbonaceous hollow spheres; and ii) KOH chemical activation. The synthesized activated carbons (ACs) show an extremely high porosity (pore volume≈1.0 cm3 g−1) mostly composed of micropores (90 % of total pore volume). Because of their favorable textural properties, the ACs are further tested as electrodes for supercapacitors, yielding very promising results (300 F g−1 at 250 mA g−1) and confirming the high suitability of KOH-activated HTC carbons derived from spruce and corncob hydrolysis products as materials for electric double layer supercapacitors.

BETA Zeolite Thin Films Supported on Honeycomb Monoliths with Tunable Properties as Hydrocarbon Traps under Cold-Start Conditions

A high percentage of hydrocarbon (HC) emissions from gasoline vehicles occur during the cold-start period. Among the alternatives proposed to reduce these HC emissions, the use of zeolites before the three-way catalyst (TWC) is thought to be very effective. Zeolites are the preferred adsorbents for this application; however, to avoid high pressure drops, supported zeolites are needed. In this work, two coating methods (dip-coating and in situ crystallization) are optimized to prepare BETA zeolite thin films supported on honeycomb monoliths with tunable properties. The important effect of the density of the thin film in the final performance as a HC trap is demonstrated. A highly effective HC trap is prepared showing 100 % toluene retention, accomplishing the desired performance as a HC trap, desorbing propene at temperatures close to 300 °C, and remaining stable after cycling. The use of this material before the TWC is very promising, and works towards achieving the sustainability and environmental protection goals.

Trabajo de coordinación para la implementación del cuarto curso del Grado en Química

En el curso 2013-14 se implantará el cuarto y último curso del Grado en Química. La experiencia adquirida durante la implementación de los tres primeros cursos ha puesto de manifiesto la conveniencia de realizar un proceso de coordinación entre los profesores de las diferentes asignaturas que garantice la consecución de las competencias previstas en la memoria del título. Por ello, se ha creado en la Facultad de Ciencias de la Universidad de Alicante una red de investigación en docencia universitaria que ha estado trabajando desde el inicio del presente curso académico en este tema. Dicha red está constituida por el Vicedecano de Ordenación Académica de la Facultad de Ciencias, la Coordinadora Académica de Química y los profesores coordinadores de todas las asignaturas del 4º curso del grado (excepto Prácticas Externas y Trabajo Fin de Grado). En esta comunicación se presentarán los resultados del trabajo de investigación realizado por estos profesores que ha permitido elaborar las guías docentes de las asignaturas, planificar y coordinar las actividades a realizar para que los alumnos adquieran las competencias transversales, realizar un cronograma de actividades de evaluación y otro de prácticas de laboratorio que asegure la distribución homogénea del trabajo del alumno durante el curso académico.