1,721,144 research outputs found
Modified Nanocarbons for Catalysis
No full textNanocarbons represent useful scaffolds in the preparation of last generation nanostructured catalysts, and their chemical functionalization through covalent or non-covalent modification is becoming an important tool for introducing well-distributed anchoring points and, in the meantime, could be the first step toward the assembling of hybrid nanostructured materials with a hierarchical order. In this Review are reported synthesis and catalytic applications of chemically modified nanocarbons such as fullerene, carbon nanotubes, graphene, nanohorns and nanodiamonds in organocatalytic and metal-based (metal nanoparticles, organometallic complexes) reactions, covering major chemical reactions encompassing, the oxidation of alcohols, aldehydes, olefins, and silanes, hydrogenation reactions of aldehydes, ketones, and alkenes, dehydrogenative coupling of silanes, C−C coupling reactions, epoxidation of alkenes, CO2 fixation into cyclic carbonates, and asymmetric reactions, among others
Reconsidering TOF calculation in the transformation of epoxides and CO2 into cyclic carbonates
Full text linkThe combination of Lewis acids and Lewis bases, currently defined as catalysts and co-catalysts (or promoter) respectively, in the reaction between epoxides and CO2 to give cyclic carbonates, is discussed, starting from examples in which the Lewis base was used in larger amount with respect to the Lewis acid. In these cases, turnover frequency (TOF) values have been usually calculated taking into account solely the amount of the Lewis acid employed. The occurrence of two distinct reaction pathways, one catalysed by the sole Lewis base and the other one catalysed by the Lewis acid/Lewis base couple, in which the Lewis acid alone does not play a catalytic role, should bring researchers to reconsider the TOF calculation based solely on the amount of Lewis acid, especially when the amount of the Lewis base used is much larger than the Lewis acid. In order to draw a more precise picture of how the use of a Lewis base/Lewis acid co-catalytic system affects the reaction course, we analytically examined a general mechanistic scheme able to model the overall process. For these cases, we propose a simple equation for a better estimation of TOF values and a normalization approach for a more consistent comparison between various catalytic systems
A competitive reactivity study on the oxidative cyclization of thiosemicarbazones into 1,3,4-thiadiazolidines
Full text linkAbstract
In order to obtain useful insights on the mechanism of formation of 2(3H)-imino-1,3,4-thiadiazoles by
oxidative cyclization of aldehyde thiosemicarbazones with Cu(II) or Fe(III) salts, a competitive reactivity study
was performed on a suitable set of diversely substituted substrates, by means of HPLC techniques. This
approach enabled to exploit Hammett’s equation without performing otherwise difficult-to-run kinetic
experiments. The results presented herein support the hypothesis that the formation of the thiadiazole ring is
induced by the attack of the oxidizing Lewis acid metal cation onto the imine-like nitrogen atom of the
thiosemicarbazone substrate. Beyond mechanistic interpretation, the paper particularly focuses onto the
methodological issues implied
Antifouling Systems Based on a Polyhedral Oligomeric Silsesquioxane-Based Hexyl Imidazolium Salt Adsorbed on Copper Nanoparticles Supported on Titania
Full text linkThe reaction of octakis(3-chloropropyl)octasilsesquioxane with four equivalents of 1-hexylimidazole or 1-decylimidazole gave two products labelled as HQ-POSS (hexyl-imidazolium quaternized POSS) and DQ-POSS (decyl-imidazolium quaternized POSS) as regioisomer mixtures. An investigation of the biological activity of these two compounds revealed the higher antimicrobial performances of HQ-POSS against Gram-positive and Gram-negative microorganisms, proving its broad-spectrum activity. Due to its very viscous nature, HQ-POSS was adsorbed in variable amounts on the surface of biologically active oxides to gain advantages regarding the expendability of such formulations from an applicative perspective. Titania and 5 wt% Cu on titania were used as supports. The materials 10HQ-POSS/Ti and 15HQ-POSS/5CuTi strongly inhibited the ability of Pseudomonas PS27 cells—a bacterial strain described for its ability to handle very toxic organic solvents and perfluorinated compounds—to grow as planktonic cells. Moreover, the best formulations (i.e., 10HQ-POSS/Ti and 15HQ-POSS/5CuTi) could prevent Pseudomonas PS27 biofilm formation at a certain concentration (250 μg mL−1) which greatly impaired bacterial planktonic growth. Specifically, 15HQ-POSS/5CuTi completely impaired cell adhesion, thus successfully prejudicing biofilm formation and proving its suitability as a potential antifouling agent. Considering that most studies deal with quaternary ammonium salts (QASs) with long alkyl chains (>10 carbon atoms), the results reported here on hexylimidazolium-based POSS further deepen the knowledge of QAS formulations which can be used as antifouling compounds
Supported Polyhedral Oligomeric Silsesquioxane-Based (POSS) Materials as Highly Active Organocatalysts for the Conversion of CO2
No full textVery high turnover numbers (TON) and productivity values up to 7875 and 740 respectively have been obtained for the conversion of CO2 into cyclic carbonates by using hybrid materials based on imidazolium modified polyhedral oligomeric silsesquioxanes (POSS-Imi) grafted on amorphous silica (SiO2) and mesostructured SBA-15. The heterogeneous organocatalysts were easily prepared via a straightforward synthetic procedure allowing to generate high local concentration spots of imidazolium active sites surrounding the POSS core. This synthetic procedure is also a promising approach for the design of a wide library of hybrid functional materials. The materials do not possess other co-catalytic species with Lewis or Brønsted acid functionalities which still represents a challenging aspect for the outcome of the process. The recyclability of the catalysts was successfully verified for four consecutive runs. The catalytic versatility was proved with a wide range of epoxides and with the most challenging oxetane on large scale (105–210 mmol) showing higher performances in comparison with other unmodified imidazolium-based catalytic systems. The new hybrids based on supported POSS nanostructures allowed the sustainable conversion of carbon dioxide under solvents- and metal-free reaction conditions with a full selectivity toward cyclic carbonates
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Organocatalysts and metal-based catalysts: a journey toward the development of new catalytic materials
Full text linkOrganocatalysis and metal-based catalysis represent two of the main pillars of catalytic reactions and have witnessed a huge interest in the last decade. Immobilization, recovery and reuse of these catalysts is of primary importance because of the large amount used especially in the case of organocatalysts. On the other hand, metal-based catalysts must be recovered even if used in low amount, in order to avoid contamination of the product. In this context, we started several years ago investigations on the use of supported ionic liquid phases for the asymmetric organocatalysis mediated by proline.1 This approach is an example of a “release and catch” catalytic system.2 Starting from this example, we describe the development of new catalytic materials based on the use of i) supported ionic liquid phase (Scheme 1a) ii) covalently linked catalysts (scheme 1b) iii) ionic liquid-modified fullerenes (Scheme 1c), with the goal to obtain recyclable catalytic materials for C-C coupling reactions and alcohols oxidation.3 These studies have paved the way for the development of new hybrid materials such as silica-fullerenes or CNT-IL or POSS-IL
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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