1,720,974 research outputs found
Kinetics of Carboxylation of Styrene Using a Homogeneous Palladium Complex Catalyst
No full textKinetics of carboxylation of styrene to Me 2-phenylpropionate and Me 3-phenylpropionate has been investigated using a
homogeneous Pd(OAc)2/PPh3/p-toluenesulfonic acid catalyst system. The effect of concn. of styrene, catalyst, water,
and partial pressure of CO on the activity, selectivity, and rate of reaction has been studied in a temp. range of 338-358
K. The activity of the catalyst as well as the selectivity of the products was sensitive to all the reaction parameters
studied. The selectivity of 2-phenylpropionate was found to decrease substantially with an increase in temp. and was
enhanced by an increase in the partial pressure of CO and beyond a styrene concn. of 3.84 kmol/m3. The rate of
carboxylation varied linearly with the catalyst and was zero order with respect to styrene concn. in the range of 0.567-
3.84 kmol/m3. An unusual trend of an increase in activity was obsd. beyond a styrene concn. of 3.84 kmol/m3. The rate
also increased with an increase in the partial pressure of CO initially and was independent beyond 3.4 MPa. Water
showed a promoting effect up to a concn. of 9.244 × 10-2 kmol/m3. On the basis of the initial rate data obtained, a rate
equation was proposed and the kinetic parameters evaluated. The activation energy was found to be 65.49 kJ/mol.
Also, a plausible reaction mechanism for the carboxylation of styrene has been proposed
Hydroesterification of styrene using an in situ formed Pd(OTs)2(PPh3)2 complex catalyst
No full textJournal Molecular Catalysis: A Chemica
On the mechanism of hydroesterification of styrene using an in situ-formed cationic palladium complex
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Hydroesterification of 1-hexene
No full textIncludes bibliographical references.Hydroesterification is the carbonylation of an olefin and the successive addition of an alcohol or water yielding an ester or acid. A variety of soluble palladium complexes are widely used for the hydroesterification of olefins due to their high activity and selectivity. Branched and linear carboxylic acid esters are formed in these reactions and the regioselectivity is strongly dependent on the catalytic system and reaction conditions used. For potential industrial applications, there is a need to understand the effect of catalyst and reaction parameters on the initial rate to enhance the yield of the desired isomer and to develop a suitable rate equation. Standard carbonylation catalysts such as C02(CO)8, Fe(CO)s and Ni(CO)4 have been used to prepare fatty-acid esters. More recently, other catalysts based on Pd, Pt, Rh and Ru found widespread use because of their better performance under milder reaction conditions. Shell has intensively studied the use of palladium and ruthenium as halogen-free carbonylation catalysts. The palladium systems typically consist of palladium acetate, tertiary phosphines and strong acids such as mineral acids or acids with weak or non-coordinating anions such as sulfonic acids. Although a number of reaction conditions have been reported for homogeneously catalysed transformations of olefins to esters, it has not been clear what the key intermediate of the Pd(II)-catalysed hydroesterification is. The proposals for the mechanism originate from the accepted cobalt-catalysed hydroformylation mechanism with modifications from the Heck formulations. Two mechanistic pathways have been proposed viz. the hydride mechanism and the alkoxy mechanism where the catalytic cycle starts from the insertion of the olefin into a Pd-H or a Pd-COOMe species respectively. It is well known that the active species could either be a hydride Pd-complex or an alkoxy Pd-complex (in the presence of an alcohol). In this study, the kinetics of the hydroesterification of 1-hexene was investigated. The catalyst system used in this study was generated in situ from a mixture of Pd(OAch/PPh3/methanesulfonic acid. Reactions were carried out in a 100ml Hastelloy autoclave at 90°C and 70 bar carbon monoxide pressure. The performance of the in-situ generated, catalytic system was evaluated in terms of activity (turnover frequency, TOF) and the selectivity towards the various product compounds. For these reactions, the TOF was constant in the initial 15 minutes whereupon it decreased significantly. The following reaction and catalyst variables were studied: 1-hexene concentration, carbon monoxide pressure, methanol concentration, temperature; palladium, ligand and acid concentrations. The reaction rate was approximated to be first-order with respect to 1-hexene concentration. An increase in the carbon monoxide above 10 bar and palladium concentration strongly inhibited the hydro esterification of 1-hexene. On varying the methanol concentration, the TOF passed a pronounced maximum at ca. 7.7 mol/litre. The reaction became inhibited by methanol at high methanol concentrations. A similar trend was observed for the selectivity for the formation of esters, which passed a maximum at ca. 10 mol/litre. The molar ratio of triphenylphosphine to palladium showed a significant influence in the catalyst activity and product selectivity. The TOF passed a maximum at an excess of 30 indicating the need for a large excess of the ligand for the formation of the catalytically active complex. The selectivity for ester formation also passed a maximum with increasing ligand concentration. The presence of an acid was found to be necessary to form the catalytically active cationic species
Variations on the Author
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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