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www.nature.com/scientificreports/ During the past few years, heterogeneous catalyst systems offer a benign alternative to accomplish the organic transformations. Further advancement in the field of green chemistry and nanotechnology has introduced mag- netically retrievable nanocatalysts which provide immense surface area, excellent activity, selectivity, recyclability and long lifetime44–56. Among various solid nanomaterials57, silica-coated magnetite nanosupports have garnered much attention, mainly because of their unique characteristics, such as chemical stability, non-toxicity, economic viability and simple preparation methods which can be practiced profitably by the industries. In addition to that, the fact that they are magnetically separable provides an alternative to cumbersome filtration and centrifugation techniques, saving time, energy as well as the catalyst58,59. As a part of our ongoing research work on advanced nanomaterials for the development of sustainability and nanocatalysis58–65, herein, we describe the synthesis and characterisation of an efficient, easily generated, copper-based magnetic nanocatalyst as Cu is abundant, inexpen- sive, less toxic, readily available and excellent catalyst in comparison to the earlier reported metals66. Notably, this catalytic system fixes the carbon dioxide under atmospheric pressure, solventless and organic halide free reaction conditions, rendering the present protocol sustainable, straightforward, superior and cost effective. To the best of our knowledge, this is the first report, wherein a copper-based magnetic nanocatalyst has been utilised for the direct conversion of CO2 and epoxides into cyclic carbonates under mild reaction conditions. Methods Materials and reagents. Tetraethyl orthosilicate (TEOS), 3-aminopropyltriethoxysilane (APTES) and 2-acetylbenzofuran (ABF) were purchased from Fluka, Alfa Aesar and Sigma Aldrich respectively. Ferrous sul- phate heptahydrate and ferric sulphate hydrate were commercially obtained from Sisco Research Laboratory (SRL). All epoxides and other reagents were bought from Alfa Aesar and Spectrochem Pvt. Ltd. and used without further purification. Double distilled water was used for the synthesis and washing purposes. Characterisations. XRD peaks were recorded using a Bruker diffractometer (D8 discover) with 2θ range of 10–80° (scanning rate = 4°/min, λ = 0.15406 nm, 40 kV, 40 mA). TEM experiments were carried out on a FEITECHNAI (model number G2 T20) transmission electron microscope (operated at 200 kV). The elemen- tal mappings were obtained by STEM-EDS with an acquisition time of 20 min. Sample preparation was per- formed by dispersion of powder samples in ethanol followed by ultrasonication for 5 min. One drop of this solution was placed on a copper grid with holey carbon film. The sample was dried at room temperature. Field emission-scanning electron microscopic analysis (FE-SEM) was carried out by a Tescan MIRA3 FE-SEM micro- scope. Powdered sample was immediately placed on metal stub covered with carbon tape. Sample was further sputter-coated with a JEOL JEC-3000 FC auto fine coater gold sputtering machine. The magnetisation of the samples was measured by VSM (Model number EV-9, micro sense, ADE). The FT-IR spectra were recorded using a PerkinElmer Spectrum 2000 and employing KBr disks. The amount of copper in the catalyst and filtrate was estimated by an inductively coupled plasma-optical emission spectrometer (ICP-OES) on Varian (Australia) Vista MPX equipped with an argon saturation assembly, CCD detector, and software 4.1.0 complying with 21 CFR 11. The products were analysed and verified by Agilent gas chromatography (6850 GC) with a HP-5MS 5% phenyl methyl siloxane capillary column (30.0 m × 0.25 mm × 0.25 μm) and quadrupole mass filter equipped 5975 mass selective detector (MSD) using helium as carrier gas. The oxidation state of the copper in the catalyst was analysed using Omicron Make XPS system with mon- ochromatised AlKα X-Ray radiation (1486.7 eV) with hemispherical energy analyser and resolution of 0.6 eV. Preparation of catalyst. The first step towards the synthesis of copper-based magnetic nanocatalysts involved the preparation of Fe3O4 magnetic nanoparticles (MNPs). In order to avoid agglomeration of MNPs, they were coated with silica, using TEOS to form silica-coated magnetic nanoparticles (SMNPs). For attaining the amine functionalised surface, we employed APTES to form amino-functionalised silica-coated magnetic nano- particles ASMNPs. Further, the ligand 2-acetylbenzofuran (ABF) was immobilized on the surface of ASMNPs via Schiff base reaction to obtain ABF-grafted-ASMNPs (ABF@ASMNPs). Finally, the resultant nanoparticles were metalated with copper (II) acetate to obtain the final copper based magnetic nanocatalyst (Cu-ABF@ASMNPs) (Fig. 1). Synthesis of nanosupport composites. Magnetic (Fe3O4) nanoparticles were prepared by co-precipitation method67. Ferric sulfate hydrate (6.0 g) and ferrous sulfate heptahydrate (4.2 g) were dissolved in 250 mL water and stirred at 60 °C till yellowish-orange solution was obtained. Further, 25% NH4OH (15 mL) was added and the solution was stirred vigorously for 30 min. After some time, the colour of the bulk solution changed to black. The precipitated Fe3O4 nanoparticles were separated by an external magnet and washed several times with ethanol and water. Silica coating of these nanoparticles was carried out via a sol-gel technique68. A solution of 0.5 g Fe3O4 and 0.1 M HCl (2.2 mL) was prepared in the mixture of 200 mL ethanol and 50 mL water under sonication. Then 25% NH4OH (5 mL) was added to this solution followed by addition of 1 mL TEOS at room temperature. The mixture was subsequently stirred for 6 h at 60 °C. The obtained SMNPs were separated mag- netically and washed with ethanol and water. Finally, 0.5 mL APTES was added to the solution of 0.1 g SMNPs in 100 mL ethanol and the resultant solution was stirred for 6 h at 50 °C. The derived ASMNPs were again separated magnetically and washed with water and ethanol and dried in vacuum oven. Preparation of Cu-ABF@ASMNPs catalyst. For the preparation of the catalyst, 4.0 mmol ABF and 2 g ASMNPs were added in 250 mL ethanol and refluxed for 3 h. The resultant 1 g ABF@ASMNPs was stirred with a solution of 2 mmol of copper acetate in 100 mL acetone for 4 h. Finally, the copper-based magnetic nanocatalyst was separated by an external magnet and dried in a vacuum oven. SCientifiC RepoRts | (2018) 8:1901 | DOI:10.1038/s41598-018-19551-3 2PDF Image | copper-based magnetic nanocatalyst for the fixation of carbon dioxide
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