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Computational investigation of isoeugenol transformations on a platinum cluster – I: Direct deoxygenation to propylcyclohexane
Full text linkThe growing demand for renewable and sustainable fuels, protagonists of an increasingly important research
area due to the exhaustion of fossil resources, has oriented our investigation towards the computational
mechanistic analysis of the catalytic hydrodeoxygenation (HDO) reaction of isoeugenol. Having the most com
mon functional groups, the isoeugenol molecule is actually considered as an experimental and computational
model for typical species of biomass origin. The reported computational investigation outlines the energy bar
riers and the intermediates along the path for the conversion of isoeugenol to propylcyclohexane through a direct
deoxygenation mechanims, catalyzed by a subnanometric metal cluster. For this purpose, the Pt10 platinum
cluster was chosen as the catalyst model, being this noble metal a reference for hydrogenation reaction. The
results obtained rule the formation of the 4-propylphenol intermediate as the rate determining step for the
considered branch of the mechanim, and as the pivotal point for further ramifications. The present is the first of a
series of studies aimed to a complete mapping of isoeugenol HDO on a Pt cluster, to be used as reference for
further, more focused, investigations, such as those regarding the effects of the support and of the metal particle
size, as well as the HDO reaction of biomass-derived compounds similar to isoeugenol
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
Computational investigation of isoeugenol transformations on a platinum cluster—II: Deoxygenation through hydrogenation to propylcyclohexane
Full text linkThe debate on climate change and the future of our Planet has brought to general attention the problem of fossil fuels (coal, oil and natural gas), among the main causes of pollution on Earth. At the same time, the necessity to encourage research and development of alternative and renewable energy resources has become increasingly relevant. In this context, biomass is an attractive option to produce biofuels and chemicals currently derived from petroleum. The hydrodeoxygenation process of bio-oils, produced by the rapid pyrolysis of biomass, is the most effective strategy for obtaining biofuels, hence the reason for the investigation of its mechanism on model biomass compounds. Having investigated the direct deoxygenation (DDO) mechanims in the first paper of this series, the present work aims to illustrate the deoxygenation-through-hydrogenation (HYD) mechanism, by which isoeugenol, a compound chosen as a model of bio-oils, is converted into propylcyclohexane on a ten-atom platinum cluster. DFT calculations highlight, from kinetic and thermodynamic perspectives, how the formation of propylcyclohexane takes place through 4-propyl-2-methoxycyclohexane-1-ol, the removal of –OCH3 as methanol and then of the –OH group as water. Microkinetic analysis, performed by joining findings on both DDO and HYD routes, reveals that the isoeugenol DDO mechanism is favored at any selected temperatures
DFT insights into competing mechanisms of guaiacol hydrodeoxygenation on a platinum cluster
Full text linkIn a scenario of declining fossil resources and increasing demand for renewable and sustainable alternatives, biomass is the only source able to offer an easy and gradual transition in the use of current energy technologies based on the exploitation of carbon derivatives. Its conversion to liquid fuels has oriented our study towards the computational mechanistic analysis of the guaiacol catalytic hydrodeoxygenation, which is currently considered one of the most challenging routes for upgrading biomass-derived bio-oils. For this purpose, a subnanometric Pt10 platinum cluster was chosen as the catalyst model, with Pt as a computational reference element for catalytic hydrogenation, and guaiacol as a model compound of bio-oils. DFT calculations revealed that the energy barriers related to the cleavage of C(sp2)-O bonds in the direct deoxygenation mechanism are significantly lower (by an average of 60 kJ mol−1) than those in the deoxygenation-through-hydrogenation mechanism in which C(sp3)-O bond breaking from a saturated ring occurs. Even if the ring hydrogenation is easier in the oxygenated compound, the analysis reveals that the direct deoxygenation mechanism is favoured at all temperatures. Furthermore, the results obtained highlight that, from a thermodynamic perspective, the removal of oxygen groups preferentially occurs by the elimination of the -OCH3 fragment as methanol and then of the -OH fragment as a water molecule
DFT Study of Pt Particle Growth inside β-Zeolite Cages
Full text linkThe preferred location and the corresponding energetics of zeolite-embedded single metal atoms and small metal particles are hot topics within active site optimization and catalyst tuning, even as part of bifunctional materials design. In this context, periodic density functional theory was used to provide insights on the interactions of a platinum atom with the microporous cages of a purely silicious β-zeolite (BEA) framework. Cluster growth was subsequently addressed, up to Pt3@BEA systems, following a one-by-one platinum atom addition; platinum migration between cages was taken into account as well. An unbiased approach was employed, which allowed a wide panorama of structures being considered in addition to a thorough analysis in terms of energetics, cluster geometries, and cavity distortions. Calculations revealed that the optimal interaction geometry for a single platinum atom is realized where two strong Pt-O bonds in almost linear arrangement can form, regardless of the cavity involved. This can cause distortions or even breaking of the zeolite structure, a factor which however is not decisive in determining the energetics of systems with two and three platinum atoms. Platinum migration is associated with energy barriers ranging from 100 to 200 kJ mol-1, depending on the cages. Up to the dimensions considered here, preference for clustering is observed, being the embedded Pt3 systems in almost all cases energetically favored with respect to isolated atoms within the BEA framework
Decomposition of guaiacol on a subnanometric platinum cluster: a DFT investigation followed by microkinetic analysis
No full textThe use of biomass as renewable feedstock for commodity chemicals may largely benefit from the successful development and application of heterogeneous catalysts for decomposition processes. The present investigation combines density functional theory and Christiansen-like microkinetic analysis to describe, at the atomistic level, the mechanisms related to the conversion of oxygenated biomass compounds to deoxygenated and semi-saturated hydrocarbons on a subnanometric Pt10 cluster. The DFT calculations and the kinetic analysis based on the evaluated free energy variations, associated with both elementary step barriers and rearrangement/desorption processes occurring on the cluster, suggest that benzene is the preferred product, together with a compound still bearing oxygen, cyclopentadienone, which would form as a minor product only at high temperature. Other than highlighting the role of the peculiar interaction between carbon and platinum, the reported investigation underlines the importance of cluster fluxionality and reorganization ability in promoting catalyzed reactions
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
DFT study on zeolites’ intrinsic Brønsted acidity: The case of BEA
Full text linkSince Brønsted acidity is a crucial aspect for the applications of zeolitic materials in heterogeneous catalysis, great effort was devolved to characterize the number, strength and location of the potentially active acidic sites. Quantum chemical calculations can turn out essential in estimating the intrinsic acidity by computing deprotonation energy (DPE) values, although each method comes with its own difficulties. In this context, three approaches within density functional theory were employed to study the intrinsic acidity of 30 topologically distinct Brønsted sites in the β-zeolite framework. Advantages and disadvantages of the three methods were outlined and the acidity order between the sites was assessed, being the DPE range 59 kJ mol−1 wide, with the proposed best approach. By dividing the range into three portions, the sites were classified as having high, medium and low acidity. Hydrogen bonds formation was found to be a contributing factor in determining a low Brønsted acidity
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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