1,721,055 research outputs found
Alumina as Acid Catalyst in the MonoMethyl Esterification of Adipic Acid: a Study of Surface Interactions
No full textThe selective protection of one of two identical functional groups in symmetrical positions in a molecule, is an important goal in organic synthesis. In this area the preparation of monoesters of symmetrical diacids represents an important challenge, mainly when the cyclic anhydrides are not readily available.
Dicarboxylic acids can be monoprotected by reaction with diazomethane or dimethyl sulfate in the presence of alumina or silica gel [1]; the transesterification approach, utilizing butyl formate in octane, is also used [2].
Here, preliminary studies concerning the monoesterification of adipic acid with methanol are reported.
The reaction was tested with some commercial Al2O3 powders and an ordered mesoporous alumina prepared by a surfactant assisted synthesis. The Al2O3 catalysts were characterized by different techniques including nitrogen physisorption at 77 K, temperature-programmed desorption of ammonia (NH3-TPD) and DRIFT-IR spectroscopy. The reaction, carried out at RT for 22 hrs, affords the adipic acid monomethyl ester in 57% yield and 95% selectivity. The effect of the catalyst amount and the role of the water will be discussed
The metathesis of alpha-olefins over supported Re-catalysts in supercritical CO2
No full textThe self-metathesis of -olefins has been accomplished according to a procedure never previously reported, based on the combined use of heterogeneous catalysts (supported Re-oxides) and of supercritical carbon dioxide as the solvent. The reaction outcome is affected by the nature of the catalytic support (-Al2O3 and silica), and by the properties of compressed CO2 which offers advantages in terms of replacement of conventional toxic solvents (e.g. n-heptane and toluene), of favourable reaction rates, and of easier recovery of final products.
Keywords: metathesis, Re-supported catalysts, supercritical CO2.
1. Introduction
The metathesis of olefins is one of the best and more general carbon-carbon bond forming methodologies.
The reaction has also been acknowledged as the archetype Green Chemistry process, for clean productions of pharmaceuticals and polymer with reduced environmental emissions of hazardous wastes. In this field however, notwithstanding the enormous efforts for the development of new catalysts able to extend the scope of the metathetic process, the replacement of conventional solvents, typically hexane, toluene, and dichloromethane, 3, with alternative safer and greener media, have been poorly, if any, investigated. In particular, to the best of our knowledge, the combined use of heterogeneous catalysts and CO2 solvent has never been claimed for the metathesis of olefins. We report here that in presence of Re2O7 supported on -Al2O3, not only the self-metathesis of -olefins occurs efficiently in supercritical carbon dioxide (scCO2), but interestingly, the reaction is more rapid than in n-heptane and toluene.
2. Experimental
The catalysts were prepared through conventional impregnation methods of aqueous solutions of NH4ReO4 over both -Al2O3 and silica, followed by calcination. The metathesis of 1-alkenes was performed in dense CO2 at pressure of 80-150 bars. Since previous methodologies were not available, a new Schlenk apparatus was arranged ad hoc, to allow multiple operations under an inert (N2) atmosphere: i) the activation of the catalyst at a high-temperature (550 °C); ii) the charge of the catalyst and of the reactant olefins (RCH=CH2, R = C4-C6) in a stainless-steel autoclave; iii) the reaction step under CO2 pressure. The method was also compared to conventional procedures carried out at atmospheric pressure, under liquid-phase conditions, using both n-heptane and toluene as solvents.
3. Results and discussion
Table 1 reports the results for the metathesis of 1-octene carried out at 35 °C, in presence of Re2O7 supported on both -Al2O3 and silica, and with different solvents. In all cases, regardless of the conditions used, the formation of the product of self-metathesis (7-tetradecene, 1a) is accompanied by different co-products identified as isomers of 1-octene (iso: 2-, 3-, and 4-octene) and linear olefins C9-C13. Data indicate that:
i) Only the catalyst prepared on γ-Al2O3 is active for the transformation (entries 1-2, and 4). The different surface acidity between alumina and silica mostly account for this result.
ii) In scCO2, the average reaction conversion is over 30% higher than in n-heptane and toluene. Plausible reasons are based on the gas-like diffusivity and viscosity and the liquid-like density of scCO2, which may improve the mass transfer (with respect to conventional liquids).
iii) In all cases, the self-metathesis selectivity was of 95-98%.
Conclusions
Overall, the combined use of heterogeneous and scCO2 appears a viable alternative for the replacement of conventional liquid-phase methods ordinarily used in the metathesis of olefins
Olefin epoxidation over Molybdenum(VI) catalysts derived from dithiocarbamate complexes grafted onto silica
No full text
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
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
MCM-41-TBD as a new, efficient, supported heterogeneous catalyst for the synthesis of thioureas
No full textThe preparation of thioureas, by reaction of carbon disulfide with primary amines, can be efficiently catalyzed, under heterogeneous conditions, by MCM-TBD as a new and reusable catalyst
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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