1,721,006 research outputs found
Hydroesterification of cyclohexene using the complex Pd(PPh3)2(TsO)2 as catalyst precursor. Effect of a hydrogen source (TsOH, H2O) on the TOF and a kinetic study (TsOH: p-toluensulfonic acid)
No full textThe hydroesterification of cyclohexene is catalyzed by a preformed Pd(PPh3)2(TsO)2 complex I in methanol as solvent. The effect of PPh3, TsOH, and water on the TOF has been evaluated. The system I/PPh3/TsOH=1/6/8, in the presence of 800 ppm of H2O, at 373 K and under 2.0 MPa of CO leads to a TOF as high as 850 h−1. The increase of TOF observed adding a hydride source such as TsOH and H2O suggests that Pd-hydride species plays a key role in the first step of the catalytic cycle. The initial reaction rate increases linearly with the concentration of cyclohexene and of MeOH and passes through a maximum with increasing the pressure of CO. The rate equation r0=k1PCO (1+k2PCO+k3PCO2)−1 fits well the experimental data. The values of k1, k2, and k3 have been evaluated at different temperatures. From the plot ln k versus 1/T, E1=19.4 kcal/mol, E2=20.6 kcal/mol and E3=6.5 kcal/mol have been evaluated. On the basis of experimental evidences and of the kinetic study, a catalytic cycle mechanism has been proposed
Synthesis, characterization and catalytic activity in the carbonylation of ethene of cis-[Pd(H2O)2(PPh3)2]X2.nH2O (X=p-CH3C6H4SO3, n=2; X=CH3SO3, n=0). X-ray structure of cis-[Pd(H2O)2(PPh3)2](p-CH3C6H4SO3)2.2H2O and of cis-[Pd(H2O)2(PPh3)2](CH3SO3)2.2CH2Cl2
No full textThe complexes cis-[Pd(H2O)2(PPh3)2]X2 · nH2O (Ia: X=p-CH3C6H4SO3, n=2; IIa: X=CH3SO3, n=0) have been synthesized by reacting Pd(OAc)2, PPh3 and HX in acetone in the presence of H2O. They have been characterized by IR, 1H and 31P NMR spectroscopy and TA analysis. By re-crystallization of these complexes, crystals of Ia and IIa · 2CH2Cl2 suitable for the X-ray analysis have been obtained. The solid-state investigation of Ia reveals that the two p-toluenesulfonato units act as counter anions of a dicationic complex, in which the metal atom is surrounded in a square planar environment realized by two water molecules and two PPh3 moieties that are cis to each other. The X-ray investigation of IIa · 2CH2Cl2 shows that also in this case the two PPh3 are cis to each other and that in the acentric triclinic cell there are two independent [Pd(H2O)2(PPh3)2]2+ units, together with two methanesulfonato counter anions and two crystallization molecules of CH2Cl2. The cationic complexes Ia and IIa are easily inter-converted with the neutral species trans-[Pd(p-CH3C6H4SO3)2(PPh3)2] (Ib) and trans-[Pd(CH3SO3)2(PPh3)2] (IIb), respectively, depending on temperature and solvent. In chloroform at r.t., complex Ia catalyzes the carbonylation of ethene to a polyketone; at higher temperature in methanol it catalyzes the hydroesterification of ethene. In both cases catalysis is accompanied by CO2 evolution. These results suggest that catalysis occurs via initial formation of a Pd(II)–H species by interaction of H2O with CO on the metal center though a reaction closely related to that of the water gas-shift
Highly active [Pd(AcO)2(dppp)] catalysts for CO-C2H4 copolymerization in H2O-CH3COOH solvent [dppp= 1,3-bis(diphenylphosphino)propane]
No full textIn MeOH, [Pd(AcO)2(dppp)] becomes an efficient catalyst for the perfectly alternated CO-C2H4 copolymerization when used in combination of a relatively large amount of CH3COOH (CH3COOH/Pd, ca. 2.0×104). Under 4.56 MPa (CO/C2H4 = 1/1), at 90 °C, 7.5 kg polyketone/g Pd∗h are obtained. A significantly higher productivity is obtained in H2O-CH3COOH, in place of MeOH (27.5 kg polymer/g Pd∗h under 4.56 MPa (CO/C2H4 = 1/1), 90 °C, H2O=37% (mol/mol)). Under these conditions the catalyst undergoes only a minor deactivation with time as after 3 h the productivity decreases only by ca. 10%.
The NMR analysis shows that [Pd(AcO)2(dppp)] in CH3COOH-H2O yields a perfectly alternated polyketone of ca. 8000 g/mol bearing only ketonic end groups. During the course of the copolymerization reaction CO2 forms in significant amount. These findings suggest that the insertion of C2H4 into PdH bond starts the catalytic cycle, and that alternating insertions of the monomers followed by protonolysis of a PdC bond of a Pd-CH2-CH2-(CO-C2H4)n-H growing chain yields the polymer with only keto ending groups and a Pd-OH species; a reaction closely related to the WGS reproduces the starting Pd-hydride; deprotonation of this species to inactive Pd(0) is prevented by the acid
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
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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