Sandip Kumar Pahari

A rapid and green synthetic approach for hierarchically assembled porous ZnO nanoflakes with enhanced catalytic activity

Three dimensionally (3D) assembled hierarchical porous ZnO structures are of key importance for their applications in sensors, lithium-ion batteries, solar cells and in catalysis. Here, the controlled synthesis of 3D hierarchically porous ZnO architectures constructed of two dimensional (2D) nano-sheets through the calcination of a hydrozincite [Zn5(CO3)2(OH)6] intermediate is presented. The intermediate 3D hierarchical hydrozincite has been synthesized by a novel organic surfactant and solvent free aqueous protocol at room temperature using an aqueous solution of ammonium carbonate and laboratory grade bulk ZnO in a short time (20–30 min). The amount of carbonate and the reaction temperature play a crucial role in the formation of the 3D hierarchical morphology and on the basis of the experimental results a probable reaction mechanism is proposed. On calcination, the synthesized 3D hierarchical hydrozincite resulted in ZnO with an almost identical morphology to the parental hydrozincite. On decomposition a porous structure having a surface area of 44 m2 g−1 is obtained. The synthesized hierarchical ZnO morphology exhibits an improved catalytic activity for the synthesis of 5-substituted-1H-tetrazoles with different nitriles and sodium azide than that of nanocrystalline ZnO and bulk ZnO, as well as other developed solid catalysts. The catalyst is easily recyclable without a significant loss in catalytic activity.

Hierarchically order porous lotus shaped nano-structured MnO2 through MnCO3: chelate mediated growth and shape dependent improved catalytic activity

Design of hierarchical nanostructures towards a specific morphology is an important research area due to their shape dependent properties. Here, 3D hierarchically assembled lotus shaped porous MnO2 is synthesized using a simple aqueous solution based chelating agent (citric acid) mediated growth of MnCO3 followed by calcination at 350 °C. MnCO3 in other shapes, such as rods, spheres and nano-aggregates, is also synthesized just by varying the chelating agents. It is observed that the geometry and strength of the chelating ligands has a crucial role in the controlled shape selective synthesis and based on this a probable chelating agent driven formation mechanism is discussed. The synthesized porous MnO2 shapes exhibit excellent shape dependent catalytic oxidation of α-pinene to verbenone using molecular oxygen as the oxidant. The lotus shaped porous MnO2 shows superior activity, with 94% conversion of α-pinene and 87% selectivity of verbenone, to that of other MnO2 shapes. The activity is reasonably high compared to heterogeneous as well as homogeneous catalysts reported in the literature and bulk MnO2 with respect to both their conversion and selectivity. The synthesized lotus shaped MnO2 also showed good catalytic activity towards oxidation of allylic compounds to corresponding ene–ones using molecular oxygen as oxidant and is reusable.

Room temperature synthesis of protonated layered titanate sheets using peroxo titanium carbonate complex solution.

We report the synthesis of peroxo titanium carbonate complex solution as a novel water-soluble precursor for the direct synthesis of layered protonated titanate at room temperature. The synthesized titanates showed excellent removal capacity for Pb(2+) and methylene blue. Based on experimental observations, a probable mechanism for the formation of protonated layered dititanate sheets is also discussed.

Organic free low temperature direct synthesis of hierarchical protonated layered titanates/anatase TiO2 hollow spheres and their task-specific applications

Layered protonated titanates and corresponding anatase TiO2 shapes, specifically the hollow spheres, are the most important functional materials and have attracted much attention because of their superior performance. Here, a facile organic substrate (both as solvent or surfactant) free, low temperature aqueous solution based chemical protocol for the direct synthesis of 3D arranged hierarchical hollow spheres of protonated layered dititanate (H2Ti2O5·H2O), is proposed. The spheres had a high surface area (as high as 334 m2 g−1), and were prepared through hydrothermal treatment of freshly prepared peroxo titanium carbonate complex in the presence of ammonium hydroxide. Ammonium hydroxide is crucial in the formation of spherical arrangement of titanate sheets and the size of the spheres is tunable by changing the amount of ammonium hydroxide. The titanate spheres can easily be converted to pure anatase TiO2 with identical morphology on subsequent calcination. The synthesized titanate spheres showed very high removal capacity for toxic heavy metals like Pb2+ and methylene blue from aqueous solution. Corresponding anatase TiO2 spheres manifested as a brilliant anode material for lithium ion batteries with excellent cyclability. TiO2 spheres also showed good photocatalytic activity.

CeO2 nanowires with high aspect ratio and excellent catalytic activity for selective oxidation of styrene by molecular oxygen

CeO2 is a most promising oxidation catalyst and its superior oxidation performance is highly dependent on the extent of its Ce4+/Ce3+ redox cycle, shape, surface area and surface structure. Herein, a simple, efficient and aqueous solution based hydrothermal synthetic route for uniform CeO2 nanowires (NWs), with high aspect ratio and surface area, using an aqueous solution of cerium ammonium carbonate complex as precursor and poly(ethylene glycol) (PEG) as structure directing agent, is described. Cobalt incorporated CeO2 NW (Co–CeO2) were also synthesized by impregnation followed by calcination. Structural and morphological characterization by XRD, SEM and TEM showed that synthesized CeO2 NWs are of cubic fluorite crystal structure, with approximately 7 ± 2 nm width and several micrometers in length, bundled, grown through the (110) surface keeping the active (100) surface exposed. XPS and TPD analysis revealed the presence of both Ce3+ and Ce4+ with higher amount of Ce3+ as well as Co2+ and Co3+ species. The amount of PEG is crucial for the synthesis of uniform CeO2 wires and other varying shapes. A probable formation mechanism of wires through the (110) surface is proposed. Synthesized CeO2 shapes were employed as catalyst for selective oxidation of styrene to styrene oxide using molecular oxygen as oxidant. Shape selective catalytic studies revealed that the synthesized Co–CeO2 NWs showed excellent catalytic activity. Kinetic study revealed that the oxidation reaction followed the Langmuir–Hinshelwood model. The synthesized CeO2 NW catalysts are recyclable with no significant loss in catalytic activity in subsequent cycles.

Synthesis of monodispersed nanocrystalline materials in supercritical ethanol: a generalized approach

We report a generalized approach for synthesis of a variety of nearly monodisperse inorganic nanostructured materials with different sizes and shapes using supercritical ethanol (SCE). In the present synthetic strategy the inorganic precursor and stabilizer (long chain amines) were co-solubilised in ethanol. Decomposition of the precursor occurred under supercritical conditions and facilitated rapid homogeneous nucleation, growth, and crystallization of the desired nanoparticles in near monodisperse state. The size and shape of the synthesized nanoparticles were tuned by varying the synthetic parameters. The particles exhibited size- and shape-dependent properties, and could be readily dispersed in hexane and toluene. Controlled evaporation of solvent from the dispersions yielded fairly ordered 2D structures.

Room temperature Baeyer–Villiger oxidation using molecular oxygen over mesoporous zirconium phosphate

Lactones have found wide applications as key molecules for the synthesis of important bioactive compounds, natural products and polymers; and represent a valuable family of synthons for various organic transformations. A series of lactones or esters are synthesized from their corresponding ketones employing Baeyer–Villiger (BV) oxidation at room temperature using mesoporous zirconium phosphate (m-ZrP) as a solid acid catalyst, molecular oxygen (O2)/benzaldehyde as an oxidizing agent, in a solvent free reaction medium. The oxidation reaction is studied in detail by varying the reaction parameters like molar ratio of the reactants, reaction temperature, time and catalyst loading. The m-ZrP showed high catalytic activity for the BV oxidation of cyclohexanone as well as other ketones in the presence of reduced amount of benzaldehyde with 100% selectivity for the corresponding lactones/esters. The protocol is suitable even for bulkier cyclic ketones like adamantanone. The m-ZrP catalyst showed excellent reusability.

Synthesis of nearly monodispersed metal oxide nanoparticles in water

We report a generalized aqueous route for the synthesis of a variety of nearly monodisperse nanostructured metal oxides with controllable sizes and shapes using aqueous metal ammonium carbonate solution as a precursor and decanoic acid under reflux as well as hydrothermal conditions. The procedure is based on direct formation of metal oxide through hydrolysis followed by dehydration of a metal ammonium carbonate complex in basic medium, in situ dissolution and surface modification of an oxide nanoparticle by decanoic acid through the formation of ammonium salt of decanoic acid. All the synthesized metal oxide nanostructures were characterized by transmission electron microscopy, powder X-ray diffraction and IR spectroscopy. The characterization result shows that all the particles are monodispersed in size and shape, and highly crystalline in nature. A probable formation mechanism is proposed to explain the formation of the size and shape selective CeO2.

Allylic and Benzylic Oxidation over CrIII-Incorporated Mesoporous Zirconium Phosphate with 100 % Selectivity

The oxidation of allylic compounds to their corresponding aldehydes and ketones (carbonyls, enones) is a challenging and important reaction, as these compounds are relevant for the synthesis of pharmaceutical, agricultural, and natural products, as well as for the synthesis of resins, steroids, and fine chemicals. Traditionally, various chromium-based catalysts are utilized in stoichiometric amounts for these reactions. However, use of large amounts of noxious and harmful chromium-based catalysts and the sometimes uncontrollable oxidation reaction, makes the procedures cumbersome. Cobalt-, selenium-, and manganese-based catalysts were also reported, but these have a poor selectivity for allylic aldehydes or ketones. 8–12] Chromium-based solid catalysts, such as Cr-MCM, Cr-silicates, and Cr-zeolite, have an excellent catalytic activity and selectivity (>90 %) for allylic carbonyls. The zirconium phosphate was reported to have a good catalytic activity for different organic transformations. 19] Xiao et al. reported the oxidation of allylic compounds to the corresponding enones with poor selectivity, except for acetophenone, over Cr-pillared layered zirconium phosphate. Recently, we reported on the synthesis of mesoporous zirconium phosphate (mZrP) with a high specific surface area, narrow pore size distribution, and an excellent catalytic activity towards different organic reactions. Herein, we report the synthesis of chromium-incorporated mZrP (Cr-mZrP) and its catalytic activity towards the oxidation of allylic and benzylic compounds to their corresponding carbonyl compounds with 100 % selectivity. After a simple regeneration/reactivation step, the catalyst was used several times. The synthesis of mZrP was achieved by following the reported procedure using zirconium carbonate as the precursor in a basic medium. Chromium was then incorporated in the calcined mZrP (for a detailed experimental procedure, see the Experimental Section).The small-angle XRD pattern of Cr-mZrP depicts a broad peak at 2 q= 2.2–2.38, corresponding to d-values of approximately 4 to 3.8 nm, which is almost identical to the values of pristine mZrP (Figure 1 a). The presence of a broad diffraction peak at small angles also signifies the for-

Porous cesium impregnated MgO (Cs–MgO) nanoflakes with excellent catalytic activity for highly selective rapid synthesis of flavanone

Magnesium oxide (MgO) is an excellent base catalyst and its performance is well controlled by its morphology, surface area and surface structures. Here, a simple methodology for the synthesis of porous cesium impregnated MgO (Cs–MgO) nano flakes, with enhanced surface area (156 m2 g−1), basic properties and improved catalytic activity for flavanone synthesis, is presented. The synthesis of Cs–MgO nano flakes is performed through impregnation of CsNO3 on a nesquehonite [Mg(HCO3)OH·2H2O] rod, followed by calcination. During impregnation the metastable nesquehonite rod rehabilitated to hydromagnesite [4 MgCO3. Mg(OH)2·4H2O] flakes. The flakes were porous, constructed by building blocks of small nanoparticles (10–25 nm) with a large number of edges and corners, step edges and step corners and numerous base sites of various strength (surface hydroxyl groups, low coordinate O2− sites). It is observed that the amount of cesium in the MgO surface has a strong effect on its properties as well as its activity. The synthesized Cs–MgO nanoflakes showed significant improvement in the yield of flavanone through the Claisen–Schmidt condensation. A substantial increase in the reaction rate was also observed when DMF was used as a solvent without catalyst deactivation. As much as ∼90% conversion of 2′-hydroxyacetophenone with ∼81% selectivity of flavanone was observed in just 15–20 min using the synthesized 0.5% Cs loaded MgO nanoflakes as a catalyst and DMF as a solvent. The improved catalytic activity of Cs–MgO as a catalyst and the promotion effect of DMF is discussed by studying the interaction of the substrate and the solvent on the catalyst surface and identification of intermediates formed on the catalyst surface under the reaction conditions using FT-IR.