1,721,112 research outputs found
Operando spectroscopies and heterogeneous olefin polymerization catalysis: A rare marriage with a lot of potential beyond usual understanding
Full text linkThis short review summarizes the main results achieved in the field of operando spectroscopies applied to heterogeneous catalysts for olefin polymerization. Following a chronological order, starting from the first spectroscopic measurements applied to the Phillips catalyst, the review retraces the main stages of this exciting story, reporting also some details on the experimental set-ups adopted from time to time, the difficulties encountered, the failures, and the solutions taken to address the problems. What emerges is a dynamic picture with still many unexplored potentialities, even though unfortunately limited to a few research groups systematically engaged in this field
Flexible ligands in heterogeneous catalysts for olefin polymerization: Insights from spectroscopy
Full text linkHeterogeneous catalysis for olefin polymerization is one of the most relevant industrial processes in terms of worldwide diffusion, material volume, and economic turnover. At the base of this process, two main families of multicomponent catalysts, the Ziegler-Natta and the Phillips ones, which have been developed independently since the 1950s and nowadays serve different sectors of the market. This review aims at unifying the picture of these catalysts, pointing out the often-neglected relevance of the ligands around the active sites, including not only the additives explicitly included in the general composition, but also some compounds that are generated in situ during the catalytic process by side reactions, and even the support material itself. All these components have a direct influence on the properties of the catalytic sites and, in turn, on the overall activity and on the properties of the produced polymers. A multi-technique spectroscopic investigation has the potential to shed light on the fleeting, but decisive interactions among all the components and their effect on the catalytic active sites, contributing to create a proper three-dimensional (nano-sized) environment and acting in a concerted way during the olefin polymerization. A few spectroscopic results obtained in our lab are reported, in order to show in parallel how the same principles of coordination chemistry can be effectively applied to both Ziegler-Natta and Phillips catalysts
I depositi quaternari di Borzano, Rio Groppo e Toscanella. Paleosuoli, sedimenti, Industrie
No full textVengono presentati i depositi quaternari di Borzano, Rio Groppo e Toscanella, presso il limite meridionale della Valle del Po, in Emilia-Romagna. Sono oggetto di analisi le caratteristiche geostratigrafiche, pedologiche, sedimentologiche e delle industrie litiche rinvenut
Delayed differential equation based study of sub-terahertz emission in multi-section quantum dot ring lasers
Full text linkWe present a study of the generation of subterahertz pulses in multi-section quantum dot (QD) ring lasers based on an improved version of the well-established delayed differential equation model, and taking into account an arbitrary number of gain and absorber sections. Results of the analysis of an 8-section ring laser emitting at 1.3 μm are presented. The proposed approach provides a significant insight for the understanding of the onset of the harmonic mode-locking in this family of devices and shows to be an effective tool for the optimization of the real devices in terms of pulses quality and generated RF intensity
The Active Sites in the Phillips Catalysts: Origins of a Lively Debate and a Vision for the Future
No full textIn this work, we summarize and critically compare some of the experimental results recently published on the Phillips catalyst, in the attempt to make the point on a few particularly debated questions that have recently animated the specialized literature; in particular, we discuss the structure of the active chromium sites and how ethylene polymerization initiates on them. The data collected in this article unequivocally demonstrate that the structural and electronic properties of the chromium sites strongly depend on the strain of the silica surface, which in turns is affected by both the activation treatment and the chromium loading. This explains, at least partially, the differences of results obtained in different research groups. Another fundamental message is the need of applying the largest possible set of characterization methods, including theoretical calculation on large and flexible models. Our final purpose (and hope) is to promote a positive and constructing discussion on this catalyst, as a premise to create a solid scientific base useful to both the young researchers approaching this field and the industrial researchers who daily work with it
Spectroscopic evidences for TiCl4/Donor complexes on the surface of MgCl2-supported Ziegler-Natta catalysts
No full textIn this work, we have addressed the question of the location of electron donors (we chose ethylbenzoate, EB, for practical reasons) on the surface of MgCl2-supported Ziegler–Natta catalysts by synergistically coupling (a) an in situ investigation of the EB adsorption process by Fourier transform infrared spectroscopy, (b) an evaluation of the MgCl2 surfaces available for the adsorption of CO as a molecular probe at each step of the precatalyst synthesis and (c) an accurate quantum mechanical density functional theory (DFT-D) study of a few TiCl4/EB complexes. Our experimental data indicate that homogeneous-like TiCl4/EB complexes are formed, loosely bonded to the MgCl2 surfaces, as a consequence of a rather high mobility of the adsorbed EB and TiCl4. Our DFT-D computational results demonstrate that monomeric TiCl4(EB) and TiCl4(EB)2 complexes might indeed exist on different catalytically relevant MgCl2 surfaces (although steric repulsion does not allow reaching a full surface coverage), whose computed infrared spectra are highly compatible with the experimental ones. The whole set of data converge to a final scenario whereby the internal donor has the function to induce a certain mobility for TiCl4 simultaneously acting as a “surfactant”, in certain cases providing a particularly exposed TiCl4 species readily available for further reaction with the aluminum alkyl activator. The relevant role of the MgCl2 support in assisting the self-organization of the catalyst components clearly emerges
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
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