1,721,003 research outputs found
A spectroscopic investigation of the structure and interactions of hierarchical zeotype catalysts for the Beckmann Rearrangement
No full textThe acid-catalysed Beckmann rearrangement (BR) of cyclohexanone oxime is a key step in the production of ε-caprolactam, a high-value bulk chemical and the precursor to nylon-6. Conventionally, the BR is effected by a mineral acid catalyst, but the hazards and inefficiencies of the homogeneous process have motivated research into heterogeneous alternatives. Whilst zeotype materials have proven effective as solid-acid catalysts for the BR, their performance is subject to the steric constraints of the micropores. In this regard, the development of hierarchically-porous (HP) zeotypes has provided a means to combine the catalytic properties of a microporous (MP) framework with the mass transport properties of a secondary, mesoporous network.Recently, organosilane-templated HP silicoaluminophosphates (SAPOs) were investigated as catalysts for the vapour-phase BR, with the result of enhanced activity and lifetime relative to their MP analogues. In this case, the improved performance of the HP SAPOs was not solely attributed to mass transport effects, since the organosilane mesoporogen also created weakly acidic silanol sites in the mesopores. The silanol sites were implicated in the catalytic activity of the HP SAPOs, yet the mode and degree to which they might have facilitated the BR was unknown. To this end, an integrated empirical and theoretical study has been undertaken to determine whether the siliceous mesopores play a catalytic role in the BR, or simply facilitate mass transport to the Brønsted acid sites in the micropores. In this undertaking, neutron spectroscopies have provided a unique insight into the substrate-framework interactions in HP SAPOs, allowing reactive species to be studied near independent of the catalyst matrix. Inelastic neutron spectroscopy (with complementary FTIR, NMR, and computational studies) has evidenced substrate-activating interactions in the mesopores, which facilitate the protonation of cyclohexanone oxime in a physical mixture at ambient temperature. Moreover, quasi-elastic neutron spectroscopy has revealed that these substrate-silanol interactions lead to an interesting interplay with the molecular dynamics in HP SAPO. Significantly, these studies suggest that the HP structure allows the siliceous mesopores to act in concert with the Brønsted sites in the micropores to catalyse the BR.These investigations also provided a rationale for the application of the organosilane mesoporogen in the synthesis of a new HP SAPO-37 material. By this synthetic route, HP SAPO-37 was prepared the crystalline FAU-type structure, notwithstanding the mesoporositydetected by low-angle XRD, gas adsorption, positron annihilation lifetime spectroscopy, and TEM imaging. Probe-based characterisation (including TPD, FTIR, and NMR) revealed that the acidity of SAPO-37 was moderated by soft-templating, yet this proved effective for the vapour-phase BR (300 °C), in which HP SAPO-37 maintained > 99 % conversion of cyclohexanone oxime (whilst the activity of MP SAPO-37 declined to < 35 %) over 8 hours on-stream. The sustained performance of HP SAPO-37 also elaborated the role of the siliceous mesopores, which were identified to increase resistance to deactivation through coking.These investigations demonstrate how a multifaceted spectroscopic and computational analysis can support the design of targeted, solid acid-catalysts
The molecular design of active sites in nanoporous materials for sustainable catalysis
No full textAt the forefront of global development, the chemical industry is being confronted by a growing demand for products and services, but also the need to provide these in a manner that is sustainable in the long-term. In facing this challenge, the industry is being revolutionised by advances in catalysis that allow chemical transformations to be performed in a more efficient and economical manner. To this end, molecular design, facilitated by detailed theoretical and empirical studies, has played a pivotal role in creating highly-active and selective heterogeneous catalysts. In this review, the industrially-relevant Beckmann rearrangement is presented as an exemplar of how judicious characterisation and ab initio experiments can be used to understand and optimise nanoporous materials for sustainable catalysis
Expanding beyond the micropore: catalysis with hierarchical architectures
No full textRecent advances in design strategies for tailoring active sites within hierarchical architectures are highlighted. We illustrate the potential of hierarchically-porous catalysts to host a range of active sites, expanding the scope of applications of porous materials in catalysis. These hierarchically-porous materials, with their dual porosity, represent a new class of catalysts that have the potential to build upon the advantages of their microporous counterparts, for a range of industrially-significant catalytic transformations
Comprehensive vibrational spectroscopic characterization of nylon-6 precursors for precise tracking of the Beckmann rearrangement
No full textAs a key step in nylon‐6 synthesis, the Beckmann rearrangement is an ongoing target of catalytic studies that seek to improve the sustainability of polymer manufacture. Whilst solid‐acid catalysts (predominantly zeotypes) have proven effective for this transformation, the development of more active and selective systems demands an understanding of fundamental catalytic mechanisms. In this undertaking, in situ and operando characterization techniques can be informative, provided rigorous spectroscopic groundwork is in place. Thus, to facilitate mechanistic studies we present a detailed investigation of the vibrational spectra of cyclohexanone oxime, ε‐caprolactam and their D10‐isotopomers, in the solid state. Variable‐temperature infrared (150 ‐ 300 K) and Raman (10 ‐ 300 K) spectra are reported alongside inelastic neutron scattering data. Moreover, where key vibrational modes have been assigned with the aid of periodic density functional theory calculations, it has been possible to include hydrogen‐bonding interactions explicitly
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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