Ana C. Estrada

Homogeneous catalytic oxidation of styrene and styrene derivatives with hydrogen peroxide in the presence of transition metal-substituted polyoxotungstates

The tetrabutylammonium (TBA) salts of the Keggin-type polyoxotungstates with general formula [XW11M(H2O)O39](n−m)−, where X = P, B or Si and M = Mn, Fe or Co, were evaluated as catalysts in the oxidation of styrene, α-methylstyrene, p-methylstyrene, α,p-dimethylstyrene, p-chlorostyrene, p-nitrostyrene, and p-methoxystyrene under mild conditions, using aqueous H2O2 as an eco-sustainable oxidant. In this study, the influence of the catalysts and of the different styrene substituents on the oxidation reaction profile was evaluated in terms of conversion and selectivity. For all the performed catalytic studies, the main product results from the oxidative cleavage of the vinyl double bond, except in the case of the oxidation of p-methoxystyrene catalysed by BW11Mn, for which p-methoxyphenol is the main product. The catalysts BW11Mn and SiW11Co give rise to 100% conversion for almost all of the substrates, excluding p-methoxystyrene and p-nitrostyrene for both catalysts and α,p-dimethylstyrene only in the case of BW11Mn. The selectivity for CC cleavage products resulting from the oxidative cleavage of the vinyl double bond can be as high as 98%, reaching 98% conversion for p-nitrostyrene when SiW11Co was used as a catalyst. Possible pathways are discussed and the oxidation of a few presumed intermediates was carried out. The systematic study of several substituted styrene derivatives suggests a possible reactivity order for these compounds in the catalytic system considered.

Photothermally enhanced drug release by κ-carrageenan hydrogels reinforced with multi-walled carbon nanotubes

Temperature and near infrared (NIR) light responsive multi-walled carbon nanotube (MWCNT)–κ-carrageenan hydrogel composites have been prepared. The effects of the MWCNTs on the microstructure, strength, swelling and release properties of the resultant materials were investigated. MWCNTs acted as reinforcing fillers and enhanced the mechanical properties of the hydrogels, the effect being mostly nanotube concentration dependent. Surface functionalization of nanotubes had a major influence on the swelling of the composites. The increased release of a model drug (methylene blue) in in vitro conditions, from κ-carrageenan hydrogel composites due to the NIR photothermal effect of MWCNTs was demonstrated at the physiological temperature. Thus, these composites are promising materials for the development of carriers for remotely activated drug delivery.

Oxidation of Polycyclic Aromatic Hydrocarbons with Hydrogen Peroxide in the Presence of Transition Metal Mono-Substituted Keggin-Type Polyoxometalates

The oxidation of anthracene, naphthalene, 1‐ethylnaphthalene, and 2‐ethylnaphthalene with environmentally benign hydrogen peroxide has been studied in the presence of the tetrabutylammonium salts of iron and manganese mono‐substituted Keggin‐type polyoxometalates [XW11M(H2O)O39]n− (X=P, Si, or B, M=MnIII or FeIII). The reactions were performed under homogeneous conditions at 80 °C, using acetonitrile as the solvent. For all the catalysts studied, anthracene was selectively oxidized to 9,10‐anthraquinone at 100 % conversion in the presence of [BW11Mn(H2O)O39]6−. The oxidation of 1‐ethylnaphthalene and 2‐ethylnaphthalene occurs mainly at the alkyl substituent, along with the formation of phthalic anhydrides, which resulted from aromatic ring oxidation, with iron substituted anions. Naphthalene could not be oxidized with the conditions used. To understand the reaction pathways, the oxidation of several products was performed in the presence of [PW11Fe(H2O)O39]4−. Under the conditions used, the oxidation of vinylnaphthalene derivatives gave rise to mainly carbon–carbon double‐bond cleavage, affording the corresponding naphthaldehydes in high yields (>67 %). Possible reaction pathways are presented and the role of different catalysts is discussed.

Mono-substituted silicotungstates as active catalysts for sustainable oxidations: homo- and heterogeneous performance

A series of tetrabutylammonium (TBA) salts of the transition metal mono-substituted silicotungstates [SiW11M(H2O)O39]n−, M = CoII, FeIII, MnIII, (SiW11M) were explored as homogeneous catalysts for the oxidation of geraniol and styrene with H2O2. The most active homogeneous catalysts (SiW11Co and SiW11Fe) were immobilized onto an amine-functionalized SBA-15 (aptesSBA-15) and the resulting composites were characterized using several techniques (FT-IR, FT-Raman, UV-Vis/DRS, elemental analysis, powder XRD, SEM and N2 adsorption–desorption isotherms). The catalytic performance of the new composites SiW11Co@aptesSBA-15 and SiW11Fe@aptesSBA-15 was investigated under similar experimental conditions to those used for homogeneous counterparts. 2,3-Epoxygeraniol and benzaldehyde were the main products obtained from geraniol and styrene oxidation, respectively, for all the catalysts. SiW11Co and SiW11Co@aptesSBA-15 showed to be the most active catalysts for the oxidation of geraniol and styrene. The recyclability of the composite SiW11Co@aptesSBA-15 was investigated for three reaction cycles. The stability of the composites was confirmed using several techniques after catalytic cycles.

A General Route for Growing Metal Sulfides onto Graphene Oxide and Exfoliated Graphite Oxide

Graphene-based materials are elective materials for a number of technologies due to their unique properties. Also, semiconductor nanocrystals have been extensively explored due to their size-dependent properties that make them useful for several applications. By coupling both types of materials, new applications are envisaged that explore the synergistic properties in such hybrid nanostructures. This research reports a general wet chemistry method to prepare graphene oxide (GO) sheets decorated with nanophases of semiconductor metal sulfides. This method allows the in situ growth of metal sulfides onto GO by using metal dialkyldithiocarbamate complexes as single-molecule precursors. In particular, the role of GO as heterogeneous substrate for the growth of semiconductor nanocrystals was investigated by using Raman spectroscopic and imaging methods. The method was further extended to other graphene-based materials, which are easily prepared in a larger scale, such as exfoliated graphite oxide (EGO).

An ionic liquid route to prepare copper sulphide nanocrystals aiming at photocatalytic applications

Copper sulphide crystallizes in a variety of phases that exhibit distinct optical properties including different optical band gaps in the visible region. The crystalline phases are associated with unique properties shown by semiconductor nanocrystals that could be exploited to produce new photocatalysts with tuned optical behavior. This research reports on the use of ionic liquids as a reaction medium to produce copper sulphide nanocrystals whose crystalline phase depends on the synthesis conditions. Noteworthy, these solvents allow the production of water dispersible copper sulphides following the thermal treatment of Cu(II) alkyldithiocarbamates complexes, used here as single-molecule precursors, thus improving their potential for heterogeneous photocatalysis in the aqueous phase. Finally, the ensuing copper sulphides have been preliminarily assessed for their photocatalytic activity in the degradation of rhodamine B (RhB). The results indicate that the ionic liquid route described here has an impact on the copper sulphides produced and thereby on their performance as photocatalysts.