Now showing 1 - 10 of 40
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    Selective oxidation of alkenes over uranyl-anchored mesoporous MCM-41 molecular sieves
    (10-02-2011) ;
    Ravat, Vilas M.
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    Krishna, Vidya
    Uranyl-anchored MCM-41 (UO22+/MCM-41) catalysts were prepared hydrothermally and systematically characterized employing various analytical and spectroscopic techniques: namely, X-ray diffraction (XRD), nitrogen sorption isotherms, transmission electron microscopy (TEM), electron diffraction (ED), and thermogravimetry-differential thermal analysis, inductively coupled plasma-atomic emission spectroscopy, diffuse reflectance ultraviolet-visible (DRUV-vis) spectroscopy and fluorescence spectroscopy, and Fourier transform-intrared (FT-IR) spectroscopy. XRD confirms the incorporation of uranyl ions into the silicate matrix and that TEM and ED investigations corroborate the highly ordered structure of uranyl-incorporated MCM-41. Further, these findings were well supported by DRUV-vis, fluorescence, and FT-IR spectra, indicating the nature of uranyl ion species as well as their interaction with the silicate framework. Well-characterized, high-quality UO22+/MCM-41 catalysts were employed for the liquid-phase allylic oxidation of α-pinene, β-pinene, and cyclohexene under moderate reaction conditions using various solvents and oxidants. Under the optimized experimental conditions, the catalysts showed high substrate conversion and excellent product selectivity. In addition, the influence of various other parameters (viz., temperature, time, recyclability, uranium content, etc.) were also performed. © 2011 American Chemical Society.
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    Crystal structure of bis(acetato-κO)diaqua(2,2′-bipyridine-κ2N,N′)manganese(II)
    (06-08-2014)
    Saravanan, Natarajan
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    In the title monomeric manganese(II) complex, [Mn(CH3COO)2(C10H8N2)(H2O)2], the metal ion is coordinated by a bidentate 2,2′-bipyridine (bpy) ligand, two water molecules and two axial acetate anions, resulting in a highly distorted octahedral environment. The aqua ligands are stabilized by the formation of strong intramolecular hydrogen bonds with the uncoordinated acetate O atoms, giving rise to pseudo-bridging arrangement of the terminal acetate groups. In the crystal, the molecules form [010] zigzag chains via O - H⋯O hydrogen bonds involving the aqua ligands and acetate O atoms. Further, the water and bpy ligands are trans to each other, and are arranged in an off-set fashion showing intermolecular π-π stacking between nearly parallel bipy rings, the centroid-centroid separations being 3.8147 (12) and 3.9305 (13) Å.
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    Acid-Mediated Synthesis of Ordered Mesoporous Aluminosilicates: The Challenge and the Promise
    (31-01-2017)
    Krishna, Nunna V.
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    A new intrinsic hydrolysis method was employed, for the first-time, to synthesize well-ordered H-AlSBA-15 with trivalent aluminium exclusively in the tetrahedral framework structure of SBA-15. Unlike other methods, which involve incorporation of aluminium ions in both the framework (Brønsted) and non-framework (Lewis) sites of the silicate matrix, the intrinsic hydrolysis method isomorphously substitutes aluminium ions in the tetrahedral network even at high aluminium content. This unique approach relies mainly on the hydrolysis rates of the inorganic (silicon and aluminium) precursors used for the preparation in such a way that the condensation occurs simultaneously so as to overcome the usually encountered difficulties in stabilizing aluminium ions in the silicate matrix. In this way, we could successfully synthesize high quality Brønsted acidic H-AlSBA-15, hitherto not reported. The synthesized materials were systematically characterized by various analytical, spectroscopic, and imaging techniques, including XRD, Brunauer–Emmett–Teller (BET) surface area measurements, TEM, SEM,29Si and27Al magic angle spinning NMR spectroscopy, X-ray fluorescence (XRF), and NH3temperature-programmed desorption (TPD). The characterization results reveal the presence of a highly porous structure (with narrow pores) and tetrahedrally coordinated trivalent aluminium in the silicate matrix with more medium to strong Brønsted acid sites. The resulting high quality catalysts exhibit excellent activity for tert-butylation of phenol with high selectivity towards para-tert-butyl phenol and 2,4-di-tert-butyl phenol.
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    A Remarkable Catalytic Activity of Hierarchical Zeolite (ZH-5) for Tertiary Butylation of Phenol with Enhanced 2,4-Di-t-Butylphenol Selectivity
    (20-09-2018)
    Parsapur, Rajesh K.
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    Hierarchical zeolites with the ordered arrangement at the nanoscopic level are of paramount importance in catalysis owing to their remarkable properties. Nevertheless, architecting such structures is extremely difficult owing to their challenging synthesis conditions. In this study, we report a designed synthesis of such a material with MFI topology viz., hierarchical nanoporous ZSM-5 (designated as ZH-5) with unique serrated morphologies. The synthetic approach was envisaged by considering the stable supramolecular aggregation, controlled zeolitization and strong organosilane interactions as the key factors for the hierarchical organization. Furthermore, for the first time, we emphasize the effect of dual-templating nature of organosilanes in stabilizing the crystalline mesophase for the development of a hierarchical pore-architecture. In addition, the catalysts have shown remarkable catalytic activities for the formation of 2,4-di-tert-butylphenol.
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    Crystal structure of (2-formylphenolato-κ2O,O′) oxido(2-{[(2-oxidoethyl) imino]-methyl}phenolato-κ3O,N,O′)-vanadium(V)
    (01-05-2015)
    Parimala, Sowmianarayanan
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    Baggio, R. F.
    In the unsymmetrical title vanadyl complex, [V(C9H9NO2)-(C7H5O2) O], one of the ligands (2-formylphenol) is disordered over two sets of sites, with an occupancy ratio of 0.55 (2):0.45 (2). The metal atom is hexacoordinated, with a distorted octahedral geometry. The vanadyl O atom (which subtends the shortest V - O bond) occupies one of the apical positions and the remaining axial bond (the longest in the polyhedron) is provided by the (disordered) formyl O atoms. The basal plane is defined by the two phenoxide O atoms, the iminoalcoholic O and the imino N atom. The planes of the two benzene rings are almost perpendicular to each other, subtending an interplanar angle of 84.1 (2)° between the major parts. The crystal structure features weak C - H⋯O and C - H⋯π interactions, forming a lateral arrangement of adjacent molecules.
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    Solar-light driven photocatalytic activity of mesoporous nanocrystalline TiO2, SnO2, and TiO2-SnO2 composites
    (01-09-2019)
    Alagarasi, A.
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    Rajalakshmi, P. U.
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    Shanthi, K.
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    Mesoporous nanocrystalline metal oxides such as TiO2, SnO2, and TiO2-SnO2 composites form a new class of mesoporous inorganic solids, which exhibit excellent structural, textural, optical, and photocatalytic properties. Furthermore, they also possess a large number of surface-exposed catalytic active sites in addition to their intrinsic flexible diffusion characteristics. In this investigation, we report the photocatalytic activity of high-quality mesoporous TiO2, SnO2, and TiO2-SnO2 composites. All the catalysts were tested for photocatalytic degradation of 4-chlorophenol under a solar simulator. The reaction results indicate that the anatase-phase mesoporous Ti0·9Sn0·1O2 exhibits enhanced activity as compared with the commercial TiO2 (P-25).
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    Sulfonic Acid Functionalized Ordered Mesoporous Silica and their Application as Highly Efficient and Selective Heterogeneous Catalysts in the Formation of 1,2-Monoacetone-D-glucose
    (21-12-2018)
    Krishna, Nunna V.
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    Anuradha, Sankaran
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    Ganesh, Reddi
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    Kumar, Velisoju V.
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    A series of sulfonic acid functionalized ordered mesoporous silica (OMS), designated as RSO3H-OMS (R=alkyl or aryl; OMS=MCM-41, IITM-56 or SBA-15), were prepared by post-synthesis grafting method. These catalysts, in general, exhibit strong acidic sites and, therefore, yield diacetone-D-glucose as main product in the D-glucose acetonation reaction. On the other hand, the functionalized catalyst can also be tuned in such a way to generate significant amount of weak-to-moderate acidic sites, which are in turn responsible for the formation of 1,2-monoacetone-D-glucose, hitherto not reported so far. These functionalized materials also show promise as they are water tolerant catalyst as well as exhibit varying acidic strengths, which allow greater flexibility for the desired product. In addition, the uniform mesopores with high surface area permit bulkier molecules to enter the active sites, thus the catalyst offers larger pliability in terms of yield and reusability. We report here, for the first time, RSO3H-SBA-15, with sizable amount of weak-to-moderate acidic sites, as a robust heterogeneous catalyst for the formation of the targeted molecule, 1,2-monoacetone-D-glucose.
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    Specific role of polymorphs of supporting titania in catalytic CO oxidation on gold
    (30-04-2011)
    Beck, A.
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    Magesh, G.
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    Kuppan, B.
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    Schay, Z.
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    Geszti, O.
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    Benkó, T.
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    Viswanath, R. P.
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    Viswanathan, B.
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    Guczi, L.
    Dependence of catalytic activity in CO oxidation on the structure of gold nanoparticles supported on various TiO2 polymorphs has been investigated. Pure brookite, brookite-anatase (45:55) and anatase-rutile (85:15) mixtures were compared as supports of Au/TiO2. Au nanoparticles were deposited on different TiO2 supports by adsorption of Au colloids of about 6 and 15 nm mean diameter and by deposition precipitation (DP) with urea providing Au particles smaller than 5 nm in size. Sol derived gold particles were more stable against sintering on brookite than on anatase containing supports. Taking into account the particle sizes of gold and titania for all types of gold deposition methods, the Au-anatase perimeter seems to be significantly more active in CO oxidation than the Au-brookite perimeter. The difference in activity should be originated from the different electronic, physical and surface properties of anatase and brookite. © 2010 Elsevier B.V. All rights reserved.
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    Novel ionic liquid-templated ordered mesoporous aluminosilicates: Synthesis, characterization and catalytic properties
    (01-02-2019)
    Kumar, Maddila A.
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    Krishna, Nunna V.
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    Highly crystalline ordered mesoporous aluminosilicates with hexagonal AlIITM-41 and cubic AlIITM-48 pore symmetries with varying silicon-to-aluminium ratios were hydrothermally synthesized, for the first time, using long-chain ionic-liquid (IL) such as 1-hexadecyl-3-methylimidazolium chloride (HDMIC), as structure directing agent. The prepared materials were systematically characterized by various analytical, spectroscopic and imaging techniques, viz., XRD, BET, TEM and XRF. In addition, the extent of aluminium incorporation and the nature of acid sites were deduced from 27Al MAS-NMR and NH3-TPD measurements. The structural and physico-chemical properties of both AlIITM-41 and AlIITM-48 were compared with analogous AlMCM-41 and AlMCM-48, respectively prepared using hexadecyltrimethylammonium bromide (CTAB). It was found that all the ionic liquid-templated materials, viz., AlIITM-41 and AlIITM-48, and exhibited large surface area, high crystallinity and thicker pore walls. The observed superior orderness of AlIITM-41 and AlIITM-48 could attribute to a well distributed positive charge and π-π stacking of aromatic imidazolium head group of ionic-liquid. In addition, it also assisted in the incorporation of all the aluminium species exclusively in the tetrahedral silicate framework even after the calcination as confirmed by 27Al MAS NMR. AlIITM-41 and AlIITM-48 possess more medium and strong Bronsted acid sites due to their thicker walls, high crystallinity and presence of tetrahedral aluminium in the framework, due to which they exhibited higher catalytic activity towards tertiary butylation of phenol with a good Para tertiary butyl phenol selectivity compared to commonly used aluminosilicates, H-AlMCM-41 and H-AlMCM-48.
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    Ordered mesoporous carbon-supported nano-platinum catalysts: Application in direct methanol fuel cells
    (01-01-2015) ;
    Kuppan, Balaiah
    During the past century, chemists have primarily focused on making and breaking strong covalent bonds.With this approach, it is possible to combine atoms into molecules and extended structures with nearly arbitrary atomic scale configurations (Service, 2005). Molecules of increased size and complexity require more demanding synthetic methods and for many years, meso-or macro-scale designed configurations, generated by molecular assembly, had limited accessibility. However, chemists have not viewed hydrogen bonds, Van der Wall’s forces, and medium to long range electrostatic forces, all of which are much weaker than covalent bonds, as chemical glue for assembling molecules into materials (Fan et al., 2008). This is in spite of the fact that nature is built on this approach; nearly all that surrounds us, from cells to trees, are knit together usingweak interactions betweenmolecules. The formation process formany of themeso-structuredmaterials, whereby a collection of small molecules, electrolytes, polymers, and co-solvents spontaneously combine into larger, well-defined supra-molecular assemblies or aggregates due to the weak forces, has historically been termed by chemists and materials scientists as “self-assembly”, “cooperative self-assembly”, or “molecular assembly”. Over the past two decades, researchers have made large advances in understanding the basic rules of molecular assembly, as well as in developingmethods to simultaneously control intermolecular interactions and reaction kinetics to create material systems with hierarchical ordering and complexity (Davis, 2002). Methodologies, which make use of molecular assembly, have been recognized as the most promising approach for the fabrication of a wide variety of meso-structured materials. At this juncture, it is important to note that platinum-supported carbon catalysts are generally used as electrode materials, e.g., methanol oxidation reaction at the anodes of direct methanol fuel cells (DMFCs) and oxygen reduction reaction at the cathodes of proton exchange membrane fuel cells (PEMFCs). These systems have received considerable attention as clean energy sources for various applications (Winter and Brodd, 2004). However, in order to achieve high dispersion, good stability, effective utilization and stupendous activity of platinummetal, porous carbonswith ordered pore structure, high surface area, nano-scale morphology, tunable pore characteristics with varied surface functionality, and good electric conductivity are highly desirable for electrocatalysts (Dicks, 2006; Gasteiger and Marković, 2009). In this regard, the most commonly used electro-catalyst, both for cathode and anode, is platinum supported on carbon blacks (Escudero et al., 2002; Lizcano-Valbuena et al., 2003; Kim et al., 2003; Tian et al., 2004). However, it is necessary to obtain a more effective catalyst, both in catalytic performance and electronic conductivity. To achieve a higher efficiency of the electro-catalysts, platinum has to be well dispersed on the support. For this reason, it is desirable that the supportmaterial provides a suitable specific area and surface chemistry as well as good electrical conductivity. In this chapter, we address such issues in a greater detail by taking into account of our recent work (Kuppan, 2014; Kuppan and Selvam, 2012; Selvam and Kuppan, 2012) on nano-platinum-supported mesoporous carbons such as NCCR-41, CMK-3, NCCR-11 and CMK-1 for DMFCs.