1,720,957 research outputs found
An FT-IR study of the reactivity of hydrocarbons on the acid sites of HZSM5 zeolite
No full textThe interaction of HZSM5 zeolite with n-butane, iso-butane, n-heptane, 1-butene, cis-2-butene, trans-2-butene, iso-butene, benzene and toluene in the temperature range 100-773 K has been investigated using FT-IR spectroscopy Low temperature experiments allowed to characterise hydrogen-bonded species of all adsorbates both with the internal bridging OHs of HZSM5 and with the external terminal OHs. H-bonds apparently involve C-C sigma-type orbitals of alkanes and C=C pi-type orbitals of alkenes and aromatics. Evidence is provided for a partial steric hindrance in the interaction of benzene and toluene with the internal OHs. Hydrogen bondings with alkanes are precursors for dehydrogenation with the formation of carbenium ions that go fast towards the tert-butyl-sec-butyl equilibrium, giving rise to skeletal isomerization and cracking. Poly-isobutene like species are formed at the surface by high-temperature interaction with both alkanes and alkenes. Isobutene is a likely precursor for such species, that are intermediates of coking and can contribute also to cracking to light olefins. H-bonded species are also thought to be intermediates for the formation of the so-called 'sigma-bonded' intermediate of electrophilic aromatic substitution, giving rise to toluene dealkylation and/or dismutation, obtained in the IR cell. (C) 2000 Elsevier Science B.V. All rights reserved
FTIR study of the interaction of some branched aliphatic molecules with the external and internal sites of H-ZSM5 zeolite
No full textThe interaction of the branched molecules pivalonitrile (2,2-dimethylpropionitrile, PN), methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA), and 2,2-dimethylbutane (DMB), as well as of methylcyclohexane, benzonitrile and benzene over different H-ZSM5 zeolite samples was investigated by FTIR spectroscopy. PN does not enter the cavities of MFI structures at room temperature and 0-25 Torr pressure, at least when Al content is quite high. Also, TBA and MTBE do not enter the cavities under these conditions although isobutene produced by their decomposition, easily enters and polymerizes in the internal acidic sites. Conversely, DMB slowly enters the MFI zeolite cavities. On the other hand, the co-presence of benzene allows PN to enter the cavities and to interact with the internal sites. This shows that the access of single molecules can be influenced by other molecules in a mixture. The experiments also confirmed that two kinds of terminal silanol groups, indistinguishable from the point of view of the OH stretching band but differing in their Bronsted acidity, are located at the external surface of the ZSM5 zeolite crystals. Additionally, it was established that the external ZSM5 zeolite surface, even for low Al content samples, contains two types of Lewis acid sites. Conversely, no evidence was found for the presence of bridging Si-(OH)-Al sites at the external surface of the ZSM5 zeolite samples. This further corroborates the proposal that the bridging "zeolitic'' Bronsted acid sites are actually formed as a consequence of the porous structure of the zeolites and do not simply arise from the substitution of Al for silicon in a silica framework
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
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
Conversion and hydroconversion of hydrocarbons on zeolite-based catalysts: an FT-IR study
No full textThe interaction of HZSM5 and Mo-ZSM5 with benzene, naphthalene, toluene, ortho-xylene, pam-xylene, n-butane, isobutane, n-heptane, and methylcyclohexane, in the range 100-773 K has been investigated using FT-IR spectroscopy. Hydrogen bonded species with the internal bridging and the external terminal OHs has been detected. The reactivity at high temperature has also been studied. The access to the internal cavities and to the strongly acidic OHs is at least partly hindered in the case of Mo-ZSM5. The catalytic activity of ZSM5 was moderated by the addition of molybdenum, with lower cracking and higher liquid yields. (C) 2001 Elsevier Science B.V. All rights reserved
An FT-IR study of the adsorption of aromatic hydrocarbons and of 2,6-lutidine on H-FER and H-ZSM-5 zeolites
No full textThe interaction of aromatic hydrocarbons benzene, toluene, o-, m- and p-xylene and of methyl pyridines (in particular 2,6-lutidine) with H-ZSM-5 and H-FER zeolites has been studied. Two different H-ZSM-5 samples with strongly different intensity ratios between the two main OH stretching bands have been used. Benzene, toluene and p-xylene enter easily the cavities and give rise to three different H-bonded complexes. Two of them are strongly bonded while the third is likely a very hindered and distorted one. o-Xylene enters slowly the cavities and m-xylene even more slowly. Faster diffusion occurs at higher temperatures. On the contrary, xylenes do not enter the FER cavities. In spite of its steric hindrance, supposed to be the same of nt-xylene, 2,6-lutidine enters fast the ZSM channels and is protonated by the internal sites. On the contrary, it does not enter the FER cavities, but it is protonated too on the external silanols sites. Evidence is provided for some kind of heterogeneity of the internal sites of ZSM-5 zeolite. Additionally, it is concluded that other effects besides the molecular sieving effect may play a role in the access and diffusion of molecules into the zeolite channels. (C) 2001 Elsevier Science BN. All rights reserved
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