16 research outputs found

    A heterogeneous mechanism for the catalytic decomposition of hydroperoxides and oxidation of alkanes over CeO2 nanoparticles: A combined theoretical and experimental study

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    [EN] A nanoparticulate CeO2 catalyst is presented that is able to oxidize cyclohexane to K/A-oil (a mixture of cyclohexanone and cyclohexanol) using hydroperoxides as oxidizing species. The improvement in selectivity with decreasing particle size suggests the existence of a structure-activity relationship in this process, and points to a preferential activation of cyclohexane at defective corner or edge sites. A detailed theoretical study of the reaction mechanism over three different CeO2 catalyst models shows that cyclohexane is preferentially activated by bicoordinated oxygen atoms present at the edges of small particles, following a Mars van Krevelen mechanism which has been confirmed by in situ IR spectroscopy and O-18/O-16 isotopic exchange experiments. The process is fully heterogeneous, and the catalyst can be reused without loss of activity up to four cycles.The authors thank Solvay, MINECO (Consolider Ingenio 2010-MULTICAT, CSD2009-00050 and Severo Ochoa program, SEV-2012-0267), Generalitat Valenciana (PROMETEOII/2013/011 Project) and the European Union (ERC-AdG-2014-671093 - SynCat-Match) for financial support. Red Espanola de Supercomputacion (RES) and Centre de Calcul de la Universitat de Valencia are gratefully acknowledged for computational facilities and technical assistance. The technical support of the Electronic Microscopy Service of UPV is kindly appreciated. T.L.-A. thanks ITQ for a contract. We are grateful to Special Chem and Fabien Ocampo for providing ceria materials, and to Konstantin Neyman for providing the Ce21O42 and Ce40O80 nanoparticle models.López Auséns, JT.; Boronat Zaragoza, M.; Concepción Heydorn, P.; Chouzier, S.; Mastroianni, S.; Corma Canós, A. (2016). A heterogeneous mechanism for the catalytic decomposition of hydroperoxides and oxidation of alkanes over CeO2 nanoparticles: A combined theoretical and experimental study. Journal of Catalysis. 344:334-345. https://doi.org/10.1016/j.jcat.2016.09.032S33434534

    Surface Organometallic Chemistry of Titanium: Synthesis, Characterization, and Reactivity of (Si-O)nTi(CH2C(CH3)3)4-n (n =1, 2) Grafted on Aerosil Silica and MCM-41

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    International audienceThe reaction of tetrakisneopentyl titanium, TiNp4 (1), with the surface of partially dehydroxylated Aerosil silica and MCM-41 and the reactivity of the resultant supported titanium alkyl product with water, alcohols, and oxygen are reported. Two methods of preparation have been investigated and compared for the grafting of TiNp4: (i) reaction of the support with the vapor of the sublimed complex and (ii) impregnation of the support with a solution of the complex. The second method appeared to be more reliable for “larger scale” preparations. The surface species thus obtained were characterized by infrared spectroscopy, solid state NMR, XAFS, elemental analysis, and various test reactions. Whereas on an Aerosil silica partially dehydroxylated at 500 °C, SiO2-(500), the surface complex is a monopodal titanium trisalkyl complex, SiO−Ti[CH2C(CH3)3]3, 2a, a bipodal titanium bisalkyl complex, (SiO)2Ti[CH2C(CH3)3]2, 2b, is obtained as the major species (ca. 65%) with 2a on MCM-41(500). The reason for this difference in behavior is discussed on the basis of the surface structure. The results obtained from hydrolysis confirmed the structure proposed for the supported alkyl complexes. For the reaction of the alkyl surface complexes with alcohols (MeOH, EtOH, tBuOH), the surface compounds obtained were characterized by the same techniques and by XPS and UV−vis. The results are consistent with the formation of monosiloxytrisalkoxy titanium complexes on SiO2-(500), SiO−Ti(OR)3, 3aOR, and of SiO−Ti(OtBu)3, 3aOtBu, and (SiO)2Ti(OtBu)2, 3bOtBu, on MCM-41(500), after reaction with tBuOH. The supported titanium alkyl, 2a, also reacts with oxygen, leading mainly to SiO−Ti[OCH2C(CH3)3]3, probably via an unstable surface compound such as SiO−Ti[OCH2C(CH3)3]2[OOCH2C(CH3)3], resulting from the incorporation of two molecules of oxygen in 2a

    Surface Organometallic Chemistry of Titanium: Synthesis, Characterization, and Reactivity of (Si-O)nTi(CH2C(CH3)3)4-n (n =1, 2) Grafted on Aerosil Silica and MCM-41

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    International audienceThe reaction of tetrakisneopentyl titanium, TiNp4 (1), with the surface of partially dehydroxylated Aerosil silica and MCM-41 and the reactivity of the resultant supported titanium alkyl product with water, alcohols, and oxygen are reported. Two methods of preparation have been investigated and compared for the grafting of TiNp4: (i) reaction of the support with the vapor of the sublimed complex and (ii) impregnation of the support with a solution of the complex. The second method appeared to be more reliable for “larger scale” preparations. The surface species thus obtained were characterized by infrared spectroscopy, solid state NMR, XAFS, elemental analysis, and various test reactions. Whereas on an Aerosil silica partially dehydroxylated at 500 °C, SiO2-(500), the surface complex is a monopodal titanium trisalkyl complex, SiO−Ti[CH2C(CH3)3]3, 2a, a bipodal titanium bisalkyl complex, (SiO)2Ti[CH2C(CH3)3]2, 2b, is obtained as the major species (ca. 65%) with 2a on MCM-41(500). The reason for this difference in behavior is discussed on the basis of the surface structure. The results obtained from hydrolysis confirmed the structure proposed for the supported alkyl complexes. For the reaction of the alkyl surface complexes with alcohols (MeOH, EtOH, tBuOH), the surface compounds obtained were characterized by the same techniques and by XPS and UV−vis. The results are consistent with the formation of monosiloxytrisalkoxy titanium complexes on SiO2-(500), SiO−Ti(OR)3, 3aOR, and of SiO−Ti(OtBu)3, 3aOtBu, and (SiO)2Ti(OtBu)2, 3bOtBu, on MCM-41(500), after reaction with tBuOH. The supported titanium alkyl, 2a, also reacts with oxygen, leading mainly to SiO−Ti[OCH2C(CH3)3]3, probably via an unstable surface compound such as SiO−Ti[OCH2C(CH3)3]2[OOCH2C(CH3)3], resulting from the incorporation of two molecules of oxygen in 2a
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