1,721,037 research outputs found
Propene adsorption and reaction on zeolites and pillared clays
No full textComparative IR and UV-Vis spectroscopic studies of propene adsorption and reaction on H-mordenite, dealuminated H-mordenite, dealuminated mazzite, montmorillonite and Al~3-pillared montmorillonite have been carried out. On all systems propene is first transformed into polymeric species (C.H2.+~+). On HMOR (both as such and dealuminated), allylic carbocations are successively produced by loss of H 2, the monoenic species (C.H2~., +) being formed at room temperature and the dienic (C,H2..3 § and trienic (C.Hz,_s +) species at higher temperatures. These species are not observed on the other systems, although they are presumably formed as unstable intermediates. In fact, on all the zeolites studied here two cyclic penta-atomic and hexa-atomic allylic carbocations have been observed for the first time. On all systems, the final products of reaction are polyaromatic species which, on the basis of their reaction with NH 3 still exhibit unsaturated carbocation behaviour. The activity of the various samples depends on their pore dimensions and on the nature of acidic sites involved: the larger the available pore space, the more branched is the polymer and the more difficult it is to observe allylic carbocations. Evidence is provided for a Bronsted-induced mechanism
Cation migrations and Bronsted sites formation in NH4-form of zeolite omega: a combined in situ time-resolved Synchrotron and Neutron powder diffraction study
No full textAcidic zeolites are usually exploited by petroleum industries as high-performing catalysts. Thus far, they have been successfully employed with different purposes, such as isomerization of xylenes, paraffin hydro- isomerization, catalytic cracking, and heavy oil hydrocracking [1]. Their catalytic properties arises from the presence of protons bonded to framework oxygen atoms, which act as Brønsted acid sites. To predict the zeolite performances as acid catalysts, a detailed structural study is needed in order to determine location, nature, strength and density of acid sites. Based on this, the aim of this work is to give an exhaustive picture of thermal behaviour of zeolite omega (a synthetic counterpart of the natural mazzite zeolite) [2] in its NH4-form, from RT to 900°C in order to follow the structural modifications induced by calcinations of ammonium ions. The formation of Bronsted sites was also monitored by neutron diffraction in order to detect their location A sample of zeolite omega (Na6.6TMA1.8(H2O)22.2[Al8.4Si27.6O72]-MAZ) was synthetized at the Laboratoire de Matèriaux Catalytiques et Catalyse en Chimie Organique, (CNRS, Montpellier, France and then NH4-ion exchanged three times at room temperature and other three times at 90°C (Na2.4TMA0.9(H2O)4.2(NH4)20.0[Al8.4Si27.6O72]). The “in-situ” time resolved diffraction measurements were performed at ESFR, Grenoble (heating rate was 5°C/min). The loss of extraframework species (residual TMA and NH4) was accompanied by strong deformation to the zeolite framework (i.e., variation on O-O distances and T-O-T angles for 6MR, 8MR and gmelinite cage) and by Na cations migration towards the 6MR. The decrease in T-O2-T angle causes a shift of O2 framework oxygen towards the centre of gmelinite cage, as proved by the decrease of O1-O2 distance addressed by the formation of a O2-H Brønsted acid site, whose location was highlighted by neutron powder data collected at the diffraction beamline D2B (ILL Grenoble). The neutron structure refinement showed that the mean T-O distances are consistent with Si, Al disorder, showing that the small difference (0.015Å) indicate that more Al is in T2 (or A) than in T1 (or B) in agreement with the results obtained from single crystal X-ray refinement of natural mazzite [3]. Above 770°C, the strongly inhomogeneous lengthening of the T-O5 distance indicates that a dealumination process occurs. All these informations provide a basis for the interpretation of the omega shape selectivity in reactions of growing importance in the oil and petrochemical industries
1. Corma, A. (1995) Chem. Rev. 95, 559.
2. Martucci, A., Alberti, A., de Lourdes Guzman-Castillo, M., Di Renzo, F., & Fajula, F. (2003). Microporous and mesoporous materials, 63(1-3), 33-42.
3. Galli, E., Passaglia, E., Pongiluppi, D., & Rinaldi, R. (1974) Contributions to Mineralogy and Petrology, 45(2), 99-105
Ethene adsorption and reaction on some zeolites and pillared clays
No full textIn the acidic solids used in this study, ethene undergoes a cationic polymerisation started by proton transfer. The larger the dimensions of the micropores, the higher is the degree of branching of the polymeric cationic species obtained. Such saturated polymeric cationic species evolve upon thermal treatment by loss of hydrogen to allylic carbocations (linear mono-, di- and trienic species as well as penta-cyclo and hexacyclo monoenic species), which are precursors to aromatics. All species have been characterised both in the infrared (IR) and ultraviolet (UV) regions. The overall chemistry is very close to that of propene on the same systems, though ethene appears less reactive
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
Microporosity and connections between pores in SBA-15 mesostructured silicas as a function of the temperature of synthesis
No full textImaging of the platinum replica of the porous structure and low-pressure argon adsorption allowed us to elucidate the complicated porous structure of SBA-15. These techniques enabled us to draw a coherent picture of the evolution of the SBA-15 precursor mesophase as a function of the synthesis temperature. TEM of the platinum replicas has been unable to show bridges between the structural mesopores of SBA-15 synthesized at low temperature, whereas mesoporous bridges are clearly observed for samples formed at higher temperature. Argon adsorption has evidenced the ultramicroporosity of the materials formed at low temperature, as well as its evolution to secondary porosity with diameters greater than 1.5 nm under more severe hydrothermal treatment
Crystal structure of zeolite omega, the synthetic counterpart of the natural zeolite mazzite
No full textRietveld refinement of the structure of synthetic zeolite omega \Na(6.6)TMA(1.8)(H2O)(22.2)\[Al8.4Si27.6O72]-MAZ, using high-resolution X-ray powder diffraction data, by Rietveld method, provided the missing evidence that this material has the same framework type (MAZ) as the natural mazzite. Its real symmetry is P6(3)/mmc, the same as the topological one, where a = 18.215 Angstrom, c = 7.634 Angstrom. TMA molecules are located inside the gmelinite cage, with two possible orientations, which are symmetric with respect to the plane passing through the center and orthogonal to the threefold axis of the cage. Sodium cations are distributed over two sites: one, at the center of the 8-ring channel, hosts most of Na and is 8-fold coordinated to six framework oxygens and two water molecules, while the other, located along the axis of the 12-ring channel, coordinates nine H2
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