Marco A. S. Garcia

Recent advances in the development of magnetically recoverable metal nanoparticle catalysts

O objetivo deste Account e fornecer uma visao geral de nossas atividades atuais de pesquisa, as quais envolvem a sintese e modificacao de nanomateriais superparamagneticos para aplicacao na area de separacao magnetica e catalise. Primeiramente, uma introducao ao magnetismo e a separacao magnetica e feita. Em seguida, estrategias sinteticas que tem sido desenvolvidas para gerar nanoparticulas superparamagneticas revestidas esfericamente por silica e outros oxidos, com destaque a sistemas bem caracterizados e preparados por metodologias que geram amostras de elevada qualidade e que, a principio, podem ser produzidas em maior escala, sao discutidas. Uma serie de catalisadores magneticamente recuperaveis preparados em nosso grupo de pesquisa pela combinacao unica de suportes superparamagneticos e nanoparticulas metalicas e destacada. Este Account e concluido com observacoes pessoais e perspectivas neste campo de pesquisa. The aim of this Account is to provide an overview of our current research activities on the design and modification of superparamagnetic nanomaterials for application in the field of magnetic separation and catalysis. First, an introduction of magnetism and magnetic separation is done. Then, the synthetic strategies that have been developed for generating superparamagnetic nanoparticles spherically coated by silica and other oxides, with a focus on well characterized systems prepared by methods that generate samples of high quality and easy to scale-up, are discussed. A set of magnetically recoverable catalysts prepared in our research group by the unique combination of superparamagnetic supports and metal nanoparticles is highlighted. This Account is concluded with personal remarks and perspectives on this research field.

Gold nanoparticles supported on magnesium ferrite and magnesium oxide for the selective oxidation of benzyl alcohol

Au nanoparticles (Au NPs) have gained significant attention as catalysts for the selective oxidation of alcohols; however, the catalytic activity is highly dependent on the presence of a base. Alternatives to the use of strong bases, such as NaOH, are still needed. Here, we explored the basicity of magnesium ferrite/oxide supports to study the catalytic behaviour of supported Au NPs for the oxidation of benzyl alcohol. The presence of Mg2+ ions in the ferrite structure improved the catalytic activity of supported Au NPs to ca. 35% conversion in the absence of an additional base. After modifying the support with MgO, the catalytic activity of supported Au NPs was further improved to ca. 50% conversion, but the catalyst deactivated in successive recycling tests. When the catalysts were tested in the presence of a sub-stoichiometric amount of K2CO3, they became more active and remained stable upon recycling with no loss of activity and selectivity for the preferential production of benzoic acid.

Oxidation of benzyl alcohol catalyzed by gold nanoparticles under alkaline conditions: weak vs. strong bases

Gold nanoparticles have shown excellent catalytic properties in selective oxidation of alcohols in the presence of base; however, the influence of the nature and concentration of the base on gold catalyst activity and selectivity is not completely understood. We here present a study of the effect of using weak and strong bases on the conversion and selectivity of PVA-stabilized gold nanoparticles supported on titania (AuPVA/TiO2) for benzyl alcohol oxidation. The increase in the concentration of base had little effect on conversion when a weak base was used (K2CO3, Na2B4O7 and Na(CH3COO)), due to the buffer effect, but strongly affected selectivity. The bases with higher pKa values provided higher conversions and increased production of benzoic acid. For the strong base NaOH, benzoic acid was always the major product, although conversion decreases in excess of base. The formation of benzoic acid is avoided by using K2CO3 in non-aqueous media; benzaldehyde is the main product in cyclohexane whereas benzyl benzoate is also formed in significant amounts in solvent-free conditions. The promotion effect observed in the presence of base was discussed in terms of reaction mechanism.

Rhodium Nanoparticles as Precursors for the Preparation of an Efficient and Recyclable Hydroformylation Catalyst

Despite all the advances in the application of nanoparticle (NP) catalysts, they have received little attention in relation to the hydroformylation reaction. Herein, we present the preparation of a hydroformylation catalyst through the immobilization of air‐stable rhodium NPs onto a magnetic support functionalized with chelating phosphine ligands, which serves as an alternative to air‐sensitive precursors. The catalyst was active in hydroformylation and could be used in successive reactions with negligible metal leaching. The interaction between the rhodium NPs and the diphenylphosphine ligand was evidenced by an enhancement in the Raman spectrum of the ligand. Changes occurred in the Raman spectrum of the catalyst recovered after the reaction, which suggests that the rhodium NPs are precursors of active molecular species that are formed in situ. The supported catalyst was active for successive reactions even after it was exposed to air during the recycling runs and was easily recovered through magnetic separation.

Polymer versus phosphine stabilized Rh nanoparticles as components of supported catalysts: implication in the hydrogenation of cyclohexene model molecule

The solution synthesis of rhodium nanoparticles (Rh NPs) was achieved from the organometallic complex [Rh(η3-C3H5)3] under mild reaction conditions in the presence of a polymer (PVP), a monophosphine (PPh3) and a diphosphine (dppb) as a stabilizer, leading to very small Rh NPs of 2.2, 1.3 and 1.7 nm mean size, with PVP, PPh3 and dppb, respectively. The surface properties of these nanoparticles were compared using a model catalysis reaction namely, hydrogenation of cyclohexene, first under colloidal conditions and then under supported conditions after their immobilization onto an amino functionalized silica-coated magnetite support. PVP-stabilized Rh NPs were the most active catalyst whatever the catalytic conditions as a result of a strong coordination of the phosphine ligands at the metal surface that blocks some surface atoms even after several recycles of the supported nanocatalysts and limit the reactivity of the metallic surface.

Support Functionalization with a Phosphine‐Containing Hyperbranched Polymer: A Strategy to Enhance Phosphine Grafting and Metal Loading in a Hydroformylation Catalyst

We present the design of a hydroformylation catalyst through the immobilization of air‐stable Rh nanoparticles (NPs) on a magnetic support functionalized with a hyperbranched polymer that bears terminal phosphine groups. The catalyst modification with the hyperbranched polymer improved the metal–support interaction, the metal loading, and the catalytic activity. The catalyst was active for the hydroformylation of natural products, such as estragole, and could be used in successive reactions with negligible metal leaching. The phosphine grafting played a key role in the recyclability of Rh NPs under hydroformylation conditions. The catalytic activity was maintained in successive reactions, even if the catalyst was exposed to air during each recovery procedure. The modification of the support with hyperbranched polyester allowed us either to increase the number of Rh active species or to obtain more active Rh species on the catalyst surface.

Magnesium surface enrichment of CoFe2O4 magnetic nanoparticles immobilized with gold: reusable catalysts for green oxidation of benzyl alcohol

Gold nanoparticles have shown excellent activity for selective oxidation of alcohols; such catalytic systems are highly dependent on the initial activation of the substrates, which must occur on the catalyst surface in heterogeneous catalysts. In many cases, an extra base addition is required, although the basicity of the support may also be of significant importance. Here, we explored the intrinsic basicity of magnesium-based enrichments on CoFe2O4 magnetic nanoparticles for the oxidation of benzyl alcohol using molecular oxygen as oxidant. The MgO and Mg(OH)2 enrichments enabled gold impregnation, which was not possible on the bare CoFe2O4 nanoparticles. The Au/MgO/CoFe2O4 and Au/Mg(OH)2/CoFe2O4 catalysts reached 42% and 18% conversion, respectively without base promotion, in 2.5 hour and 2 bar of O2. When the catalysts were tested with sub-stoichiometric amounts of base, they became more active (>70% of conversion) and stable in successive recycling experiments without metal leaching, under the same reaction conditions. We also showed the oxide phases of the enrichments performed using Rietveld refinements and how the Mg(OH)2 phase interferes with the activity of MgO-based materials.

Facile recycling approach for waste minimization of silica-coated magnetite nanoparticles synthesis

ABSTRACT The negative environmental impacts of industrial activity have been felt day-to-day, encouraging global actions to reduce the waste generation of harmful and persistent pollutants. Microemulsion synthesis of silica-coated iron oxide magnetic nanoparticles offers a high potential for several applications; however the process generates hazardous wastes that are difficult to treat and dispose of. In this study, cyclohexane and methanol were separated using phase decantation, distillation, and temperature reduction. The recovered solvents were reused in the same synthesis more than 20 times and the recovered surfactant IGEPAL® CO-520 was reused twice, suggesting the method applied has potential for continuous recycling.

Synthesis, Characterization and Catalytic Evaluation of Magnetically Recoverable SrO/CoFe2O4 Nanocatalyst for Biodiesel Production from Babassu Oil Transesterification

Biodiesel production has gained a lot of attention due to its environmental benefits and production from renewable sources. Here, it is reported a magnetically recoverable catalyst of SrO immobilized on a CoFe2O4 support (SrO/CoFe2O4), which was efficiently applied for babassu oil transesterification and used up to four cycles without a significant activity loss. The stability and performance of the catalyst were analyzed considering the solvent used for its synthesis. Characterizations such as vibrating sample magnetometer (VSM), thermogravimetry (TG), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) were performed. The present study demonstrated that the catalyst has a strong magnetic response, which reflects the nature of the nanoparticles. For the first run, the material presented a yield of 96% when synthesized in acetone in a molar proportion of SrO/CoFe2O4 of 5:1.

Determination of Metal Loading in Heterogeneous Catalyst by Slurry Sampling Flame Atomic Absorption Spectrometry

Quantification of metal loading in supported catalysts is imperative; however, the analytical methods are usually time-consuming, expensive, and require sophisticated equipment and sample extraction with hazardous and corrosive strong acids. Herein, we report a new method for quantification of metal in supported catalysts using slurry sampling flame atomic absorption spectrometry. The slurry sampling method was used for determination of gold and the results were similar to those obtained with the traditional extraction method using aqua regia; however, it overcomes the main drawbacks associated with sample digestion by avoiding the use of concentrated toxic reagents, minimizing sample manipulation, and consequently, reducing the risks of contamination and the sample preparation time. Ultrasound radiation optimization associated with the solvent, mass of sample, concentration, and matrix effect in the slurry analysis allowed direct analysis by using the calibration in aqueous medium. The slurry sampling procedure could be applied to other catalysts with different supports and in bimetallic systems.