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Innovative catalytic processes for oxidation reactions
Full text linkThe study and the development of innovative metal-catalysed systems for the oxygenation of organic molecules with sustainable oxidants, and especially molecular oxygen, O2, or hydrogen peroxide, H2O2, is a very attractive perspective for the Chemical Industry.
In the Thesis project, different strategies have been addressed to implement benchmark oxidative transformations, including the autooxidation of benzylic derivatives, the hydroxylation of aromatic hydrocarbons and the epoxidation of olefins. In all cases, the research approach has been based on some key issues, namely the integrated use of: i) bulk oxidants with low environmental impact, such as O2 and H2O2; ii) multi-metallic catalysts with thermal, hydrolytic and oxidative resistance, tailored functionality and solubility; iii) heterogeneous catalysis techniques with membrane-based hybrid organic-inorganic functional materials and solvent-free protocols; iv) microwave irradiation and/or photoirradiation as non-conventional activation techniques, v) multiple catalysis techniques, with sequential and/or parallel processes (Concurrent Tandem Catalysis).
The choice of the catalyst package has been established within the class of molecular polyanionic metals oxides clusters, known as polyoxometalates (POMs), with general formula [XxMmOy]q-, where M is the main metallic component (Mo, V, W) and X is an eventual heteroatom such as P or Si.
These complexes offer a unique opportunity because of their prevalent inorganic, robust nature, and high versatility in terms of structure, chemical composition, electron density and polyanionic charge. Moreover, a rewarding approach has been recently devised for the catalyst upgrade, by decorating the POM scaffold with organic domains, yielding hybrid organic-inorganic catalysts with superior performances. Since fluorinated phases are of particular interest for performing oxidative transformations, the research activity has been focused on the synthesis, characterization and catalytic activity of novel fluorous-tagged polyoxometalates. Two diverse synthetic approaches have been adopted, based on counterion metathesis and on the covalent functionalization of the POM inorganic surface. With the first strategy, the decatungstate polyanion (W10O32)4- has been isolated in the presence of a fluorous-tagged tetraalkylammonium cation, yielding the fluorophilic salt ?[CF3(CF2)7(CH2)3]3CH3N}4W10O32 , (RfN4W10). Decatungstate is known to be an efficient initiator of autooxidation pathways under photoirradiation, in a oxygen atmosphere. Therefore, the photocatalyzed oxidation of benzylic hydrocarbons, including ethylbenzene and cumene, by RfN4W10 and O2, has been performed in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), both in homogeneous and heterogeneous conditions. The application of membrane technology for the heterogeneous catalysis, in particular, offers the combination of advanced molecular separation and selective transport properties, with the reactivity on a solid support. The photocatalyst heterogenisation has thus been obtained by incorporation of RfN4W10 in perfluorinated polymeric films of HYFLON AD 60X, thus providing novel hybrid materials to be employed and recycled in multi-turnover processes, in solvent-free conditions. Scanning electron microscopy (SEM) images of the film surface and cross-section highlight a highly dispersed, homogeneous distribution of the catalyst domains which appear as spherical particles with uniform size of approximately 2-3 ?m in diameter. Furthermore, in particular conditions, a porous membrane has been obtained, allowing the use of a continuous flow reactor. In the conditions explored, the photooxygenation by (RfN)4W10O32 yields the benzylic hydroperoxide and the corresponding alcohol and ketone. Noteworthy, tetraline and indane photooxygenation proceeds with TON>6000 and remarkable alcohol selectivity, thus providing a convenient alternative to other radical-centered oxygenation systems.
Fluorous-tagged polyoxometalates have also been synthesised through a covalent functionalization approach. This alternative strategy employs vacant polyoxotungstates and the fluorinated organosilyl chloride CF3(CF2)7CH2CH2SiCl3 (RfSiCl3) to afford hybrid derivatives through the covalent attachment of the organic groups on the POM surface. The resulting complexes, with general formula Q4[(RfSi)xOySiWwOz], isolated as tetrabutylammonium (Q+) salts, have been characterised and used as catalysts for the epoxidation of different olefins in the presence of H2O2. Kinetic and mechanistic studies have provided several insights on the synergistic effect between the catalysts and the fluorinated solvent used, the hexafluoro-isopropanol (HFIP). Noteworthy, under microwave irradiation, the epoxide is produced with quantitative yield and only after 20 minutes, even for terminal olefins. A further aspect concerns the self-assembly of the POM-based fluorous-tagged amphiphiles. Aggregation phenomena in HFIP solution have been studied by DLS and monitored in the solid state by electronic microscopy.
The upgrade to heterogeneous catalysis has also been achieved through the covalent functionalization approach. In this case, the vacant polyoxotungstate has been reacted with the organo silyl chloride CH2=CH(CH2)6SiCl3 (RSiCl3), bearing a terminal alkene residues. The morphology and structure of the resulting hybrid materials have been tuned upon variation of comonomers and porogenic solvents ratio. Interestingly, the hybrid polymer swells in fluorinated alcohols, where the epoxidation of cis-cyclooctene, occurs with quantitative yield after 15 min.
Finally, POM-based catalysis has also been applied to the synthesis of phenol which is one of the most valuable intermediate and commodity chemical on the market. To this aim the research activity has been focussed on the implementing both the autoxidation pathway, and the direct mono-hydroxylation of benzene. (i) The application of Tandem Catalysis techniques, so to exploit the membrane-based photocatalytic production of the cumene hydroperoxide and foster its decomposition to phenol through a second step by a acid POM catalyst; (ii) the screening of several molybdovanadate catalyst to be used with H2O2 for benzene hydroxylation. In the first case, the POM mediated tandem catalysis yields 63% phenol with respect to the initial amount of cumyl-hydroperoxide.; in the second case, with the vanadium mono-substituted undecamolybdate, H4Mo11VO40, an improvement of literature results has been obtained through reaction optimization, thus giving: 17 % conversion of benzene at 50°C in CH3CN, with selectivity = 90 % and TOF = 22.5 h-1.Lo studio e lo sviluppo di sistemi metallo-catalizzati innovativi per l’ossidazione di molecole organiche con ossidanti sostenibili, specialmente ossigeno molecolare, O2, o acqua ossigenata, H2O2, è una prospettiva di grande attrazione per l’Industria Chimica.
Nel progetto di Tesi di Dottorato, sono state sviluppate diverse strategie allo scopo di effettuare trasformazioni ossidative di riferimento quali l’autoossidazione di derivati benzilici, l’idrossilazione di idrocarburi aromatici e l’epossidazione di olefine. In tutti i casi, l’approccio alla ricerca si è basato su alcuni aspetti chiave che prevedono l’utilizzo integrato di: i) ossidanti a basso impatto ambientale quali O2 e H2O2; ii) catalizzatori multi-metallici ad elevata resistenza termica, idrolitica ed ossidativa; iii) tecniche di catalisi eterogenea per mezzo di materiali funzionali ibridi organici-inorganici costituiti da membrane catalitiche a matrice polimerica e protocolli che non prevedano l’impiego di solventi organici; iv) microonde o radiazioni fotochimiche quali tecniche di attivazione non convenzionali; v) tecniche di catalisi multipla con processi sequenziali e/o paralleli (Tandem Catalisi).
I catalizzatori impiegati appartengono alla classe degli ossidi polianionici metallici, detti poliossometallati (POMs), aventi formula generale [XxMmOy]q-, dove M è il componente metallico principale (Mo, V, W) e X è un eventuale eteroatomo (P o Si). Questi complessi sono molto vantaggiosi come catalizzatori perché di natura inorganica, resistenti e sono modulabili in termini di struttura, composizione chimica, densità elettronica e carica polianionica. Particolarmente interessante è la possibilità di funzionalizzare la porzione inorganica con domìni di natura organica, potendo così ottenere catalizzatori ibridi organici-inorganici dalle prestazioni catalitiche migliori. Poiché le fasi fluorurate sono di particolare interesse per ciò che concerne le trasformazioni ossidative, l’attività di ricerca si è focalizzata sulla sintesi, caratterizzazione e attività catalitica di nuovi poliossometallati fluorurati. Sono state seguite due strategie di sintesi differenti basate sulla metatesi di controcatione e sulla funzionalizzazione covalente di superfici di POM inorganici.
Con la prima strategia è stato isolato il polianione decatungstato (W10O32)4- con un controcatione tetraalchilammonio fluorurato ottenedo il sale fluorofilico ?[CF3(CF2)7(CH2)3]3CH3N}4W10O32 , (RfN4W10). Il decatungstato è noto per la sua capacità di essere iniziatore di processi radicalici se fotoirradiato in atmosfera di ossigeno. L’ossidazione fotocatalitica di etilbenzene ed altri idrocarburi benzilici tramite RfN4W10 e O2 è stata condotta in 1,1,1,3,3,3-hexafluoroisopropanolo (HFIP), sia in omogeneo che in fase eterogenea. L’applicazione della tecnologia delle membrane per la catalisi eterogenea offre, in particolare, numerosi vantaggi in termini di combinazioni di avanzati processi di separazione molecolare con proprietà di trasporto selettive, insieme alla reattività su supporti solidi. L’eterogeneizzazione del fotocatalizzatore è stata così ottenuta per incorporazione di RfN4W10 in film polimerici perfluorurati di HYFLON AD 60X, ottenendo così nuovi materiali ibridi da impiegare e reciclare in processi “multi-turnover” ed in assenza di solventi. Immagini di microscopia elettronica a scansione (SEM) della superficie e della sezione del film evidenziano una distribuzione omogenea ed altamente dispersa dei domini catalitici che appaiono come particelle sferiche a dimensioni uniformi e di diametro pari a circa 2-3 ?m. Inoltre, in condizioni controllate, è stato possibile ottenere una membrana porosa da poter utilizzare in un reattore a flusso continuo. Nelle condizioni testate, la fotoossigenazione con (RfN)4W10O32 dà benzilidroperossido ed il corrispondente alcool e chetone. E’importante evidenziare come la fotoossigenazione di tetralina ed indano proceda con TON>6000 e con una elevata selettività in alcool, fornendo così un’ importante alternativa ad altri sistemi di ossigenazione basati su meccanismi di tipo radicalico.
Poliossometallati fluorurati sono stati sintetizzati anche tramite l’approccio di funzionalizzazione covalente. Questa strategia prevede l’impiego di poliossotungstati lacunari e organosilil cloruri CF3(CF2)7CH2CH2SiCl3 (RfSiCl3) a dare derivati ibridi per mezzo dell’attacco covalente dei gruppi organici sulla superficie del POM. I complessi finali risultanti, con formula generale Q4[(RfSi)xOySiWwOz], isolati come sali di tetrabutilammonio (Q+) sono stati caratterizzati ed impiegati come catalizzatori per l’epossidazione di diverse olefine in presenza di H2O2. Studi cinetici e meccanicistici hanno fornito diverse indicazioni circa l’esistenza di un effetto sinergico fra i catalizzatori e il solvente per fluorurato utilizzato (HFIP). E’importante sottolineare come questa reazione attivata da microonde produca epossido in rese quantitative dopo soli 20 minuti anche per le olefine terminali. Ulteriori aspetti trattati riguardano le caratteristiche auto-assemblanti di questi complessi fluorurati anfifilici. Fenomeni di aggregazione in soluzione di HFIP sono stati studiati tramite DLS e tramite microscopia elettronica allo stato solido. Anche l’eterogeneizzazione di questo sistema è stata ottenuta tramite l’approccio di funzionalizzazione covalente. In questo caso, il poliossotungstato lacunare è stato fatto reagire con un silano che porta una catena alchilica insatura terminale: CH2=CH(CH2)6SiCl3 (RSiCl3). La morfologia e la struttura del materiale ibrido risultante sono state modulate attraverso la variazione del rapporto dei solventi porogenici e dei comonomeri impiegati nella miscela di polimerizzazione. Il polimero ibrido finale presenta l’interessante proprietà di rigonfiare in alcool fluorurati, dove l’epossidazione di cis-cicloottene avviene con rese quantitative in 15 minuti.
Infine, è stato studiato il processo POM-catalizzato per la sintesi di fenolo: uno degli intermedi e commodity a più alto valore di mercato. A questo scopo l’attività di ricerca è stata incentrata sullo studio sia del processo autoossidativo, sia della mono-idrossilazione diretta del benzene. (i) L’applicazione di tecniche di Tandem Catalisi, allo scopo di sfruttare la produzione con sistema a membrana foto catalitica del cumilidroperossido da cumene, e promuoverne la sua decomposizione a fenolo in un secondo stadio con un catalizzatore POM-acido, (ii) lo screening di diversi molibdovanadati come catalizzatori da usare con H2O2 per l’idrossilazione di benzene. Nel primo caso, la Tandem Catalisi permette di ottenere una resa in fenolo pari a 63% rispetto alle moli iniziali di cumilidroperossido, nel secondo caso, con H4Mo11VO40 si ha un miglioramento dei dati di letteratura per ottimizzazione della reazione con 17% di conversione, selettività = 90% e TOF = 22.5 h-1
Hydrogen peroxide activation by fluorophilic polyoxotungstates for fast and selective oxygen transfer catalysis
No full textFluorophilic polyoxotungstates perform the selective epoxidations of internal and terminal double bonds by hydrogen peroxide (H2O2) activation in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), under mild temperature conditions. A hybrid synergy of supramolecular interactions, involving the inorganic cluster and the fluorinated solvent, is envisaged to boost H2O2 activation and the oxygen transfer mechanism. 1,2-Epoxides have been obtained with >99% selectivity and 98% yield at T = 40-70 °C
Catalytic Membranes and Membrane Reactors: An Integrated Approach to Catalytic Process with a High Efficiency and a Low Environmental Impact
No full textThe design of new heterogeneous photooxygenation systems able to employ visible light, oxygen, mild temperatures, and solvent with a low environmental impact has been investigated. In particular, the heterogenization of decatungstate (W10O324−), a polyoxometalate with photocatalytic activity in oxidation reactions, has been carried out in polymeric membranes of polyvinylidenefluoride. The polymeric catalytic membranes prepared by phase inversion technique have been successfully applied in the aerobic mineralization of phenol in water, which was used as an example of organic pollutant. In order to evaluate the effect of the polymeric environment on the overall catalyst behavior, we have also heterogenized the decatungstate (opportunely functionalized) in perfluorinated membrane made of Hyflon. The photocatalytic composite membranes are characterized by different and tuneable properties depending on the nature of the polymeric micro-environment, in which the catalyst is confined. Moreover, the selective separation function of the membrane results in enhanced performance in comparison with homogeneous reactions
Hybrid photocatalytic membranes embedding decatungstate for heterogeneous photooxygenation
No full textThe incorporation of decatungstate in polymeric membranes provides new heterogeneous photocatalysts for the oxidation of
organic substrates under oxygen atmosphere at 25 C. Photocatalytic membranes have been prepared yielding polymeric films with
a high thermal, chemical and mechanical stability (PVDF, PDMS, Hyflon). Surface spectroscopy techniques including
transmittance and reflectance UV-Vis and FT-IR have been used to assess the photocatalyst integrity within the polymeric
support. Catalyst screening has been performed under both homogeneous and heterogeneous photooxygenation conditions. The
photocatalyst activity has been evaluated in terms of the substrate conversion, turnover numbers, and recycling experiments. A
membrane induced selectivity behavior has been evidenced by comparison with homogeneous oxidations
Solvent-free, heterogeneous photooxygenation of hydrocarbons by Hyflon® membranes embedding a fluorous-tagged decatungstate
No full textHybrid fluoropolymeric membranes with 25% loading of the fluorous-tagged (RfN)4W10O32 effect the solvent-free photooxygenation of benzylic C–H bonds with up to 6100 TONs in 4 hours
Hybrid polyoxotungstates as functional co-monomers in new cross-linked catalytic polymers for sustainable oxidations with hydrogen peroxide
No full textThe hybrid organic inorganic polyoxometalates [(CH2=CH(CH2)6Si)xOySiWwOz]4-, with x=2, w=11, y=1, z=39 (1); x=2, w=10, y=1, z=36 (2) and x=4, w=9, y=3, z=34 (3) have been synthesized and co-polymerized with methyl methacrylate, to obtain porous materials with homogeneous catalyst distribution. Their reactivity towards the activation of hydrogen peroxide in acetonitrile has been screened, using methyl p-tolyl sulfide as a model substrates. The hybrid material containing the bivacant polyanion (2) was also employed in octane, to evaluate the oxidation and removal of a well-known gasoline contaminant, dibenzothiophen
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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