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A FTIR evidence of two distinct sites for cationic exchange in BEA zeolite: consequences on acid and basic properties in presence of Cs
No full textThe oxidative dehydrogenation of propane over V-containing mesoporous silicas: the effect of vanadium dispersion, surface acidity and support properties on the catalytic activity
No full textAmong the many materials considered for propane ODH, V-containing mesoporous silicas characterized by high surface area, large pores and well-dispersed VOx species prove to be most effective [1,2]. V-based catalysts are usually obtained by impregnation: it has been observed, though, that V-containing mesoporous systems (i.e. V-SBA-15 and V-MCF) prepared by direct synthesis exhibit better V-dispersion and superior catalytic performances in both selective and total oxidation reactions, reactions both based on V reduction and subsequent reoxidation (MvK mechanism) [3,4]. As it is known, isolated V-species are beneficial in terms of propene selectivity, whereas higher activity is usually associated to polyvanadates (up to the formation of VOx monolayer) [1-5].In this work, a V-SBA-15 and a V-MCF sample [3,4] (V cont. 2.5 wt.%) prepared by direct synthesis were tested in propane ODH. Their physico-chemical and catalytic properties were compared with those of materials with the same V-content but prepared by different ways: i) two impregnated samples (V-SBA-15-i and V-MCF-i) and ii) a non-porous sample obtained by flame pyrolysis (V-SiO2), a high temperature synthesis technique allowing high V-dispersion to be achieved [2]. As a whole, both V-SBA-15 and V-MCF exhibited better textural properties (i.e. higher SSA) and higher V dispersion (isolated V-species), as compared to impregnated ones, in which polymeric VOx and micro-crystalline V2O5 were detected by TEM, H
2-TPR micro-Raman and DR UV-Vis [4,5]. On the other hand, both impregnated samples showed higher Brønsted acidity (related to V-OH groups), as shown by IR-spectroscopy of adsorbed CO and NH3
Better catalytic performances in terms of selectivity to propene were achieved with both samples directly synthesized, as consequence of higher V dispersion and lower surface acidity. Brønsted acidity seems indeed to be negatively correlated with the selectivity to propene, since stronger Brønsted acidic sites favor the formation of CO.. In contrast, samples directly synthesized show supported V species that can be converted into Lewis acidic sites by dehydroxylation much more easily than impregnated samples (Figure 1).
x and other by-products. A similar correlation was previously observed with a set of non-porous VOx-SiO2 samples prepared by flame pyrolysis (results to be published). Moreover, higher selectivity to propene was obtained over V-MCF as compared to V-SBA-15 (a similar trend was observed with impregnated ones). This behavior was related to the different porous network of the two catalysts: the structure of MCF, featuring a 3-D network with ultra-large cells (20-40 nm), favors molecular diffusion along each direction, unlike cylindrical channels of SBA-15 (3-6 nm) occurring in one dimension mainly (axial diffusion). Molecules can be much more retained inside channels of SBA-15 (deeper oxidation) as compared to MCF. On the other hand, the non-porous sample, with V-species well dispersed and incorporated into the silica framework, exhibited worse catalytic results than mesoporous samples, confirming the important role of mesoporosity in oxidation processes.
References
[1] F. Cavani, N. Ballarini, E. Cericola, Catal. Today 127 (2007) 113.
[2] I. Rossetti, L. Fabbrini, N. Ballarini, F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E.A. Blekkan, L. Forni, J. Catal. 256 (2008) 45.
[3] M. Piumetti, B. Bonelli, P. Massiani, S. Dzwigaj, I. Rossetti, S. Casale, L. Gaberova, M. Armandi, E. Garrone, Catal. Today, 176 (2011) 458.
[4] M. Piumetti, B. Bonelli, P. Massiani, S. Dzwigaj, I. Rossetti, S. Casale, M. Armandi, C. Thomas, E. Garrone, Catal. Today doi:10.1016/j.cattod.2011.06.028
[5] H. Dai, A.T. Bell, E. Iglesia, J. Catal 221 (2004) 49
Effect of vanadium dispersion and of support properties on the catalytic activity of V-containing mesoporous silicas
No full textIntroduction
SBA-15 and MCF mesoporous silicas that are characterized by large and uniform pores size, thick walls and high surface area, represent very interesting supports for catalytically active species (e.g. vanadium) allowing a large concentration of accessible, isolated and well defined active sites to be obtained [1]. On the other hand, vanadium-based systems are effective in many oxidation reactions, which can be kinetically modeled through a MvK mechanism.
In this work, two series of highly dispersed vanadium-containing systems (namely V-SBA-15 and V-MCF) were prepared by direct synthesis and tested in the catalytic decomposition of dichloromethane, the most stable chlorinated-alkane.
Physico-chemical and catalytic properties of both V-SBA-15 and V-MCF systems were compared with those of V-containing mesoporous samples prepared by wet impregnation and of a non-porous V-containing silica sample.
Experimental part
V-SBA-15 materials with different V-loadings (0.7-4.8 wt.%) were prepared by direct synthesis as reported in the literature [2]. Likewise, two V-MCF samples (2.0-4.5 wt.% vanadium) were prepared by a new direct synthesis (currently under publication). For comparison, two catalysts were synthesized by impregnation of SBA-15 and MCF supports (referred to as V/SBA(i) and V/MCF(i), respectively). A non-porous sample with 3 wt.% vanadium (denoted as V-Si) was obtained by flame pyrolysis method, according to Ref. [3].
Samples were characterized by means of powders XRD, FESEM and TEM microscopies, N2 sorption isotherms at -196 °C, H2-TPR, EPR, 51 V MAS NMR, DR UV-vis, micro-Raman and FT-IR spectroscopies.
The decomposition of dichloromethane was used as probe reaction for catalytic oxidation of Cl-VOCs. Catalytic measurements were carried out in a fixed-bed reactor under both aerobic and anaerobic conditions at different reaction temperatures (200-500 °C).
Results and discussion
The samples prepared by direct synthesis showed higher specific surface areas (up to 820 m2g-1 for V-SBA-15 and 925 m2g-1 for V-MCF) as compared to impregnated ones (530 m2g-1 for V/SBA-15(i) and 645 m2g-1 for V/MCF(i)), in which a decrease occurred of both surface area and porous volume. Micro-Raman spectroscopy of dehydrated V-SBA-15 and V-MCF samples showed V=O stretching modes of isolated V species in tetrahedral coordination (1035 cm-1), whereas typical bands of polymeric VOx species and micro-crystalline V2O5 were observed with V-impregnated samples. Likewise, TEM images revealed the presence of VOx aggregates only at the surface of impregnated samples. DR UV-vis spectra of dehydrated V-SBA-15 systems showed the occurrence of V5+ species in tetrahedral coordination (absorption band at about 260 nm), whereas upon rehydration such species were converted to octahedral ones by coordination of water molecules (absorption band at about 380 nm). By contrast, a higher amount of octahedral coordinated V species was observed with impregnated samples under dehydrated conditions. Indeed, IR spectroscopy showed that more abundant and acidic sites were obtained by impregnation. H2-TPR analysis showed a much easier V reducibility in both V-SBA-15 and V-MCF samples, due to better V dispersion reached by direct synthesis. Higher catalytic activity for the dichloromethane decomposition was achieved over V-SBA-15 and V-MCF samples directly synthesized with respect to impregnated samples, as consequence of their much higher V dispersion (Fig. 1). Furthermore, better dichloromethane conversions were obtained over V-SBA-15 as compared to V-MCF systems (a similar trend was observed with impregnated samples).
These findings seem to be related to the different mesoporous systems under study: molecules should be more retained within pores of SBA-15 framework as compared to ultra-large pores of three-dimensional MCF systems. On the other hand, a non-porous sample (V-Si), with V species well dispersed and incorporated into the silica framework, exhibited lower activity than mesoporous samples, confirming the important role of mesoporosity in total oxidation reactions.
[1] J.M. Thomas, J.C. Hernandez-Garrido, R. Raja, R.G. Bell, Phys. Chem. Phys. 11 (2009) 2799.
[2] M. Piumetti, B. Bonelli, M. Armandi, L. Gaberova, S. Casale, P. Massiani, E. Garrone, Micropor. Mesopor. Mater. (2010) doi:10.1016/j.micromeso.2010.04.011
[3] I. Rossetti, L. Fabbrini, N. Ballarini, F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E. A. Blekkan,L. Forni, J. Catal. 256 (2008) 4
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
Characterization and catalytic properties of mesoporous silica catalysts containing highly dispersed V species
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