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Calcined Layered Double Hydroxides: Catalysts for Xanthene, 1,4-Dihydropyridine, and Polyhydroquinoline Derivative Synthesis.

ABSTRACT: This work is generally focused on the synthesis of NiFeTi-layered double hydroxides (LDHs) using a hydrothermal route, which were calcined at various temperatures (varying from 200 to 600 °C). The synthesized materials were physicochemically characterized. X-ray diffraction results revealed the loss of the layered structure on calcination resulting in the formation of layered double oxides (LDOs) or mixed metal oxides, which was also supported by Fourier transform infrared studies. Scanning electron microscopy results also show loss of the layered structure and the creation of LDOs on increasing the temperature. These LDOs were tested as the catalysts for the synthesis of biologically significant xanthene, 1,4-dihydropyridine, and polyhydroquinoline derivatives. Among all, NiFeTi LDH calcined at 600 °C proved to be the best catalyst for the synthesis of these derivative compounds under optimized conditions. The advantages obtained were excellent yields in a lesser reaction time. Stability and reusability were also assessed; the catalyst was stable even after five cycles. Furthermore, the memory effect of the obtained NiFeTi CLDH calcined at 600 °C confirms that the material so formed is a calcined state of LDH itself. High catalytic efficiency, easy fabrication, and recycling ability of NiFeTi CLDH calcined at 600 °C make it a potential catalyst for the synthesis of xanthene, 1,4-dihydropyridine, and polyhydroquinoline derivatives.

SUBMITTER: Rathee G

PROVIDER: S-EPMC7331214 | biostudies-literature | 2020 Jun

REPOSITORIES: biostudies-literature

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Project description:Layered double hydroxides (LDHs) are an ideal platform to host catalytic metal centers for water oxidation (WO) owing to the high accessibility of water to the interlayer region, which makes all centers potentially reachable and activated. Herein, we report the syntheses of three iridium-doped zinc-aluminum LDHs (Ir-LDHs) nanomaterials (1-3, with about 80 nm of planar size and a thickness of 8 nm as derived by field emission scanning electron microscopy and powder X-ray diffraction studies, respectively), carried out in the confined aqueous environment of reverse micelles, through a very simple and versatile procedure. These materials exhibit excellent catalytic performances in WO driven by NaIO4 at neutral pH and 25 °C, with an iridium content as low as 0.5 mol % (∼0.8 wt %), leading to quantitative oxygen yields (based on utilized NaIO4, turnover number up to ∼10,000). Nanomaterials 1-3 display the highest ever reported turnover frequency values (up to 402 min-1) for any heterogeneous and heterogenized catalyst, comparable only to those of the most efficient molecular iridium catalysts, tested under similar reaction conditions. The boost in activity can be traced to the increased surface area and pore volume (>5 times and 1 order of magnitude, respectively, higher than those of micrometric materials of size 0.3-1 μm) estimated for the nanosized particles, which guarantee higher noble metal accessibility. X-ray absorption spectroscopy (XAS) studies suggest that 1-3 nanomaterials, as-prepared and after catalysis, contain a mixture of isolated, single octahedral Ir(III) sites, with no evidence of Ir-Ir scattering from second-nearest neighbors, excluding the presence of IrO2 nanoparticles. The combination of the results obtained from XAS, elemental analysis, and ionic chromatography strongly suggests that iridium is embedded in the brucite-like structure of LDHs, having four hydroxyls and two chlorides as first neighbors. These results demonstrate that nanometric LDHs can be successfully exploited to engineer efficient WOCs, minimizing the amount of iridium used, consistent with the principle of the noble-metal atom economy.

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Calcined Layered Double Hydroxides: Catalysts for Xanthene, 1,4-Dihydropyridine, and Polyhydroquinoline Derivative Synthesis.

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