Provas Pal

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.

Morphology-mediated tailoring of the performance of porous nanostructured Mn2O3 as an anode material

Tailoring of functional properties by varying the size and shape of porous nanostructured materials is an important frontier area of research. Herein, we report the successful synthesis of nanostructured Mn2O3 with desired 3D architectures such as porous hollow spheres, lotus shapes and tubular shapes, as well as aggregated nanoparticles, through the calcination of corresponding MnCO3 with the same architecture. Porous structures were formed upon evolution of CO2 during decomposition of the carbonate intermediate, and hollow structures were formed through a nonequilibrium interdiffusion process, i.e., the Kirkendall effect. The bare MnCO3 structures were synthesized using the chelating agents citric acid (CA), tartaric acid (TA), oxalic acid (OA), and ethylenediaminetetraacetic acid (EDTA), which mediated the growth of these MnCO3 structures by hydrothermal treatment of a precursor solution containing MnCl2, ammonium carbonate and chelating agent. A systematic evaluation of the effect of the morphology of the synthesized Mn2O3 on its performance as an anode material in Li-ion batteries reveals that the shape and the nature of pores of Mn2O3 strongly influence its Li-ion storage capacity. A superior specific capacity of 478 mAh g−1 is obtained for hollow spheres with 38% retention after 30 cycles compared to other shapes due its high accessible surface area and inner hollow architecture.

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.

Mesoporous Borated Zirconia: A Solid Acid–Base Bifunctional Catalyst

The development and use of reusable solid catalysts for the selective organic transformation in solvent‐free or environmentally benign solvent media is the key interest of modern frontier science. Herein a facile low temperature aqueous solution based chemical route for the synthesis of mesoporous borated zirconia, an acid–base bifunctional solid catalyst, using aqueous zirconium ammonium carbonate complex and borax in presence of cetyltrimethylammonium bromide is presented. The material has a very high surface area and acidity with weak basicity. The catalytic activity of the material was investigated for the solvent‐free Knoevenagel condensation reaction of benzaldehyde/substituted benzaldehyde and malononitrile/cyano ethylacetate to confirm the acid–base bifunctionality. High yield (>90 %) of the corresponding benzylidene was obtained within 15–30 min at room temperature. The evidence that the high catalytic activity is a result of acid–base bifunctionality of the synthesized borated zirconia material was further supported by performing Claisen–Schmidt condensation of benzaldehyde and acetophenone. The methodology was also extended for targeted synthesis of cinnamyl ethyl ester and coumarin or coumarin ester; and resulted in a good yield.

Heterogeneously Porous γ-MnO2-Catalyzed Direct Oxidative Amination of Benzoxazole through CH Activation in the Presence of O2

Oxidative amination of azoles through catalytic C-H bond activation is a very important reaction due to the presence of 2-aminoazoles in several biologically active compounds. However, most of the reported methods are performed under homogeneous reaction conditions using excess reagents and additives. Herein, we report the heterogeneous, porous γ-MnO2-catalyzed direct amination of benzoxazole with wide range of primary and secondary amines. The amination was carried under mild reaction conditions and using molecular oxygen as a green oxidant, without any additives. The catalyst can easily be separated by filtration and reused several times without a significant loss of its catalytic performance. Of note, the reaction tolerates a functional group such as alcohol, thus indicating the broad applicability of this reaction.

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.

Efficient oxidation of hydrocarbons over nanocrystalline Ce1−xSmxO2 (x = 0–0.1) synthesized using supercritical water

Selective oxidation of hydrocarbons to more functional oxygenated compounds is a challenging task for industrial research. Here we report the synthesis of highly crystalline Ce1−xSmxO2 (x = 0–0.1) using supercritical water and their excellent catalytic activity for selective oxidation of hydrocarbons (ethyl benzene, n-butylbenzene, biphenyl methane, 1,2,3,4-tetrahydro naphthalene, cyclohexene and cyclopentene) to corresponding ketone through the oxidation of activated proton. Materials characterization results revealed the formation of highly crystalline small cube shaped nanoparticles (∼8–10 nm) with highly exposed (100) facet and exhibiting a surface area of 83–96 m2 g−1. The catalytic study revealed that Ce0.95Sm0.05O2 is highly active towards selective oxidation of stable sp3 hybridized C–H bond of different hydrocarbons. The superior activity is most probably due to its high surface area, high degree of crystallinity with exposed high energy active (100) facet and presence of large amount Ce3+. In optimized condition as high as 90% conversion of ethyl benzene with 87% selectivity of acetophenone was observed. Among other different substrates n-butylbenzene and cyclopentene showed 100% selectivity towards corresponding ketone with the conversion of 60% and 73% respectively. The catalyst is re-usable for minimum 5 times without any deactivation of its activity.