Natália J. S. Costa

Magnetic nanomaterials in catalysis: advanced catalysts for magnetic separation and beyond

While magnetic separation techniques have long been in use, intensive research into superparamagnetic nanomaterials has accelerated the development of magnetically recoverable catalysts. Preparation techniques are currently undergoing rapid development and magnetic separation has been studied to facilitate the handling and recovery of enzyme, organo-, metal complex-, and nanoparticle-catalysts. In this article, we emphasize the preparation of support materials, because the choice of the correct support and the immobilization strategy are of primary importance in the development of high-quality magnetically recoverable catalysts. We summarize the representative methods for the synthesis of well-defined uncoated and coated magnetic nanomaterials. Recent scientific progress on the preparation of surface-modified magnetic nanomaterials and the most common synthetic approaches to attach or immobilize non-magnetic catalytic active phases onto magnetic nanomaterials were discussed. Moreover, better control and understanding of the magnetic properties is now an essential tool not only in selecting the best preparation route for recoverable catalysts, but also for designing reactors (e.g., magnetic fluidized-bed reactors) and for developing magnetic field-driven technologies (e.g., changes in the catalytic output operating under an applied magnetic field).

Ligand-Assisted Preparation of Palladium Supported Nanoparticles: a Step toward Size Control

Supported nanoparticles (SNPs) with narrow size distribution were prepared by H(2) reduction of Pd(2+) previously bound to ligand-modified silica surfaces. Interestingly, the size of the Pd(0) SNPs was tuned by the ligand grafted on the support surface. Amino- and ethylenediamino-functionalized supports formed Pd(0) SNPs of ca. 6 and 1 nm, respectively. The catalytic properties of both Pd(0) SNPs were investigated.

Synthesis of supported metal nanoparticle catalysts using ligand assisted methods

The synthesis and characterization methods of metal nanoparticles (NPs) have advanced greatly in the last few decades, allowing an increasing understanding of structure-property-performance relationships. However, the role played by the ligands used as stabilizers for metal NPs synthesis or for NPs immobilization on solid supports has been underestimated. Here, we highlight some recent progress in the preparation of supported metal NPs with the assistance of ligands in solution or grafted on solid supports, a modified deposition-reduction method, with special attention to the effects on NPs size, metal-support interactions and, more importantly, catalytic activities. After presenting the general strategies in metal NP synthesis assisted by ligands grafted on solid supports, we highlight some recent progress in the deposition of pre-formed colloidal NPs on functionalized solids. Another important aspect that will be reviewed is related to the separation and recovery of NPs. Finally, we will outline our personal understanding and perspectives on the use of supported metal NPs prepared through ligand-assisted methods.

Taking advantage of a terpyridine ligand for the deposition of Pd nanoparticles onto a magnetic material for selective hydrogenation reactions

A hybrid terpyridine ligand was designed to functionalize a magnetic support constituted of magnetite cores surrounded by a silica shell with the aim of improving the stabilization of supported-palladium nanoparticles for the later application of the obtained composite nanomaterial in hydrogenation catalysis. The preparation of the nanomaterial was performed by direct decomposition of the organometallic complex [Pd2(dba)3] on the terpyridine-modified magnetic support providing well-dispersed Pd NPs of 2.5 ± 0.6 nm mean size. This new nanomaterial is a highly active catalyst for the hydrogenation of cyclohexene under mild conditions reaching turnover frequencies up to ca. 58 000 h−1 or 129 000 h−1 when corrected for surface Pd atoms. Furthermore, in the hydrogenation of β-myrcene, this nanocatalyst is highly selective for the formation of monohydrogenated compounds. When compared to a similar nanocatalyst consisting of palladium nanoparticles supported on an amino-modified magnetic support or on Pd/C, the activity and selectivity of the nanocatalyst are largely increased. These results show how the design of an appropriate hybrid ligand used to functionalize the support can strongly influence the catalytic properties of supported metal nanoparticles.

Magnetic nanocatalysts: supported metal nanoparticles for catalytic applications

Abstract This review is focused on metal nanoparticles (NPs) supported on magnetic responsive solids and their recent applications as magnetically recoverable nanocatalysts. Magnetic separation is a powerful tool for the fast separation of catalysts from reaction medium and an alternative to time-, solvent-, and energy-consuming separation procedures. Metal NPs attached to a magnetic solid can be easily carried and recovered by magnetic separation. Some examples of magnetically recoverable metal NPs used in hydrogenation, oxidation, C-C coupling reactions, photocatalysis, and other organic reactions will be given.

A recoverable Pd nanocatalyst for selective semi-hydrogenation of alkynes: hydrogenation of benzyl-propargylamines as a challenging model

We describe a recyclable heterogeneous palladium nanocatalyst for the selective hydrogenation of alkynes to alkenes. The catalyst was prepared through the decomposition of the organometallic precursor Pd2(dba)3 over a magnetic support, obtaining well-dispersed Pd nanoparticles that formed exclusively on the support surface, with average diameter of 3.5 ± 0.8 nm. The catalytic activity was investigated in the hydrogenation reactions of alkenes and alkynes, and the chemo- and stereoselectivity were evaluated in the hydrogenation of benzyl-propargylamines. The catalyst is highly selective in performing semi-hydrogenation reactions under mild conditions and short reaction times, with good overall yields. Furthermore, it can be easily recovered and recycled, with no leaching of palladium detected, and activities and selectivity retained over multiple reaction cycles.

Recreational use of marijuana during pregnancy and negative gestational and fetal outcomes: An experimental study in mice.

The prevalence of marijuana use among pregnant women is high. However, the effects on gestation and fetal development are not well known. Epidemiological and experimental studies present conflicting results because of the route of administration, dose, time of exposure, species used, and how Cannabis toxicity is tested (prepared extracts, specific components, or by pyrolysis). In this study, we experimentally investigated the effects of maternal inhalation of Cannabis sativa smoke representing as nearly as possible real world conditions of human marijuana use. Pregnant mice (n=20) were exposed (nose-only) daily for 5min to marijuana smoke (0.2g of Cannabis) from gestational day (GD) 5.5 to GD17.5 or filtered air. Food intake and maternal weight gain were recorded. Ultrasound biomicroscopy was performed on 10.5 and 16.5dpc.On GD18.5, half of the dams were euthanized for the evaluation of term fetus, placenta, and resorptions. Gestation length, parturition, and neonatal outcomes were evaluated in the other half. Five minutes of daily (low dose) exposure during pregnancy resulted in reduced birthweight, and litter size was not altered; however, the number of male pups per litter was higher. Besides, placental wet weight was increased and fetal to placental weight ratio was decreased in male fetuses, showing a sex-specific effect. At the end of gestation, females from the Cannabis group presented reduced maternal net body weight gain, despite a slight increase in their daily food intake compared to the control group. In conclusion, our results indicate that smoking marijuana during pregnancy even at low doses can be embryotoxic and fetotoxic.

A green route for the synthesis of a bitter-taste dipeptide combining biocatalysis, heterogeneous metal catalysis and magnetic nanoparticles

There is increasing demand for green technologies to produce high-solubility and low-toxicity compounds with potential application in the food industry. This study aimed to establish a clean, synthetic route for preparing the bitter-taste dipeptide Ala-Phe, a potential substitute for caffeine as a food additive. Synthesis of Z-Ala-Phe-OMe starting from Z-Ala-OH and HCl·Phe-OMe was catalysed by thermolysin at 50 °C in buffer (step 1). Z-Ala-Phe-OMe ester hydrolysis to give Z-Ala-Phe-OH at 37 °C in 30% acetonitrile/buffer was catalysed by α-bovine chymotrypsin (αCT), protease with esterase activity (step 2). Hydrogenation of Z-Ala-Phe to give the desired Ala-Phe was catalysed by C/Pd in methanol (step 3). Steps 2 and 3 were optimized by using the magnetically recoverable recycling enzyme Fe3O4@silica–αCT and the magnetically recoverable metal nanocatalyst Fe3O4@silica–Pd, respectively. This inspiring combination of technologies and the original results demonstrate the suitability of using enzymes, metal catalyst and magnetic nanoparticles for easy, economical, stereoselective, clean production of an important target compound. Besides, they add to the development of peptide chemistry and catalysis.

Synthesis, properties, and application in peptide chemistry of a magnetically separable and reusable biocatalyst

Enzyme-catalyzed chemical processes are selective, very productive, and generate little waste. Nevertheless, they may be optimized using enzymes bound to solid supports, which are particularly important for protease-mediated reactions since proteases undergo fast autolysis in solution. Magnetic nanoparticles are suitable supports for this purpose owing to their high specific surface area and to be easily separated from reaction media. Here we describe the immobilization of bovine α-chymotrypsin (αCT) on silica-coated superparamagnetic nanoparticles (Fe3O4@silica) and the characterization of the enzyme-nanoparticle hybrid (Fe3O4@silica-αCT) in terms of protein content, properties, recovery from reaction media, application, and reuse in enzyme-catalyzed peptide synthesis. The results revealed that (i) full acid hydrolysis of the immobilized protease followed by amino acid analysis of the hydrolyzate is a reliable method to determine immobilization yield; (ii) despite showing lower amidase activity and a lower Kcat/Km value for a specific substrate than free αCT, the immobilized enzyme is chemically and thermally more stable, magnetically recoverable from reaction media, and can be consecutively reused for ten cycles to catalyze the amide bond hydrolysis and ester hydrolysis of the protected dipeptide Z-Ala-Phe-OMe. Altogether, these properties indicate the potential of Fe3O4@silica-αCT to act as an efficient, suitably stable, and reusable catalyst in amino acid, peptide, and protein chemistry as well as in proteomic studies.

Antimicrobial Activity of Ethylenediaminedisuccinate Metal Complexes. Short Communication

It is shown that metal complexes of the biodegradable ligand ethylenediaminedisuccinic acid (edds) present antimicrobial activity towards fungi and bacteria. [Cd(edds)], in particular, is more toxic than free Cd2+ to Aspergillus niger, behaving as a ‘Trojan Horse’ in the facilitated delivery of the toxic metal into the fungus.