1,721,056 research outputs found
Thermal and Solvolytic Depolymerization Approaches for Lignin Depolymerization and Upgrading
No full textThe most direct approaches for lignin depolymerization typically utilize thermal environments (e.g., pyrolysis) or liquefaction (including in water or organic solvents). If tuned appropriately, these processes can yield fuel oils or upgradeable vapors and oils from lignin. This chapter reviews the extensive thermal and solvolysis literature and comments on the applicability of these approaches for particular lignin conversion strategies, primarily into fuels or fuel oils
Early-stage Conversion of Lignin over Hydrogenation Catalysts
Get PDFThe deconstruction of lignocellulosic materials producing high-quality pulps together with lignin streams of low molecular weight (Mw) with controlled molecular diversity constitutes the overarching objective of catalytic upstream biorefining (CUB) based on the early-stage catalytic conversion of lignin (ECCL). The CUB process based on ECCL or, simply, a “lignin first” approach builds on the lignin extraction with organic solvents in the presence of a hydrogenation catalyst under hydrogen pressure or employing an H-donor solvent (e.g. 2-propanol). Subjected to a hydrogenation catalyst, the lignin fragments undergo reductive processes, which decrease their reactivity and propensity to recondensation, directly upon their release from the lignocellulosic matrix. As a result, the isolated lignin fraction is a mixture of compounds of Mw 100–600 Da, with a high content of individual components. Herein, key concepts required for the rational deconstruction of lignocellulose based on ECCL are addressed. Notably, catalysis is now applied to the “pulping process” itself, opening up new horizons for the realistic valorization of lignin and full utilization of lignocellulose via downstream catalytic processing of better-defined lignin streams
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
Chain-End Controlled Depolymerization Selectivity in α,α-Disubstituted Propionate PHAs with Dual Closed-Loop Recycling and Record-High Melting Temperature
No full textWithin the large poly(3-hydroxyalkanoate) (PHA) family, C3 propionates are much less studied than C4 butyrates, with the exception of α,α-disubstituted propionate PHAs, particularly poly(3-hydroxy-2,2-dimethylpropionate), P3H(Me)2P, due to its high melting temperature (Tm ∼ 230 °C) and crystallinity (∼76%). However, inefficient synthetic routes to its monomer 2,2-dimethylpropiolactone [(Me)2PL] and extreme brittleness of P3H(Me)2P largely hinder its broad applications. Here, we introduce simple, efficient step-growth polycondensation (SGP) of a hydroxyacid or methyl ester to afford P3H(Me)2P with low to medium molar mass, which is then utilized to produce lactones through base-catalyzed depolymerization. The ring-opening polymerization (ROP) of the 4-membered lactone leads to high-molar-mass P3H(Me)2P, which can be depolymerized by hydrolysis to the hydroxyacid in 99% yield or methanolysis to the hydroxyester in 91% yield, achieving closed-loop recycling via both SGP and ROP routes. Intriguingly, the chain end of the SGP-P3H(Me)2P determines the depolymerization selectivity toward 4- or 12-membered lactone formation, while both can be repolymerized back to P3H(Me)2P. Through the formation of copolymers P3H(Me/R)2P (R = Et, nPr), PHAs with high tensile strength and ductility, coupled with high barriers to water vapor and oxygen, have been created. Notably, the PHA structure-property study led to P3H(nPr)2P with a record-high Tm of 266 °C within the PHA family
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