Department of Chemistry

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IIT Hyderabad

Catalytic conversion of glycerol into value-added chemicals, particularly acrolein via acid-catalyzed dehydration route has received much attention due to the potential uses of acrolein. This work reports the synthesis of various alkaline metal substituted phosphotungstic acid (H3PW12O40, HPW) catalysts, namely M2.5H0.5PW12O40 (M = Cs, Rb and K) using a controlled precipitation method. A systematic structural, morphology, and chemical characterization were conducted using various analytical techniques. XRD studies revealed that the incorporation of alkaline metals in H3PW12O40 leads to decreased crystallite size and enhanced lattice strain. N2 adsorption–desorption studies show that the specific surface area of H3PW12O40 is significantly improved from 5 to 82 (K2.5H0.5PW12O40), 103 (Rb2.5H0.5PW12O40), and 94 m2/g (Cs2.5H0.5PW12O40). XRD, Raman, and FT-IR studies confirm the Keggin structure of all the alkaline metal substituted HPW catalysts. The acidity strengths estimated by NH3-TPD analysis were obtained in the following order: H3PW (2654.91 μmole/g) &gt; K2.5H0.5PW (1060.10 μmole/g) &gt; Rb2.5H0.5PW (762.08 μmole/g) &gt; Cs2.5H0.5.5PW (461.81 μmole/g). Although alkaline metal substituted H3PW12O40 catalysts exhibit higher specific surface area and smaller crystallite size compared to parent H3PW12O40 low glycerol conversions were found for substituted H3PW12O40 catalysts. As well, the parent H3PW12O40 catalyst shows an excellent acrolein selectivity (95%) which is much higher than that of Cs2.5H0.5.5PW (81.9%) and very close to the selectivities obtained over Rb2.5H0.5PW (95.1%) and K2.5H0.5.5PW (95.6%) catalysts. The catalytic performance of H3PW12O40 and M2.5H0.5PW12O40 materials is directly proportional to their acidic strengths, indicating that the catalyst acidity is a key factor for achieving better results in glycerol dehydration. © 2016

World University root node

Polyhedron

Catalytic dehydration of glycerol to acrolein over M2.5H0.5PW12O40 (M = Cs, Rb and K) phosphotungstic acids: Effect of substituted alkali metals

A series of novel yellowish-orange emitting phosphors [Ca3-xMxSi2O7 (M = Eu2+)] have been synthesized by high temperature solid state reaction. The powder X-ray diffraction patterns reveals a single phase formation with excellent crystalline nature of the phases. Photoluminescence excitation and emission studies show that the Eu2+ doped phosphors show broad absorption (300-525 nm) and emits yellowish-orange-red spectral region. Further efforts have been made to tune the Eu2+ emission by crystal chemical isovalent substitution of Mg2+ in Ca2+ site. Compositionally induced phase transformation has been observed at y ≥ 0.25 in Ca2.985-yMgyEu0.015Si2O7. The yellowish-orange-red emission (y = 0) tuned to green color (y = 1) and the corresponding Commission Internationale de I'Eclairage (CIE) color coordinates (x, y) are changed from (0.5647, 0.4202) to (0.3540, 0.5932). © 2016 The Royal Society of Chemistry.

RSC Advances

Eu2+ luminescence in Ca3Si2O7 and spectral widening and tuning of Eu2+ emission color (orangish-red to green) by crystal chemical substitution

The black-colored compound BaUSe3 has been synthesized at 1173 K by a stoichiometric reaction of the elements in a CsCl flux. BaUSe3 crystallizes in the GdFeO3 structure type. There is no change in structure between 100 and 298 K. The U atoms in this structure are octahedrally connected to six Se atoms. Each octahedral unit shares all six corners with neighboring octahedra, forming a three-dimensional network. BaUSe3 can be charge balanced as Ba2+U4+(Se2-)3. DFT electronic structure calculations found BaUSe3 to be antiferromagnetic in its ground state and to be a semiconductor with a band gap of 2.5 eV. The band gap is inconsistent with the black color of the material and with the small activation energy of 0.12(1) eV obtained from resistivity measurements. A UV-vis spectrum indicated that there was no band gap above 1 eV. It is possible that, for BaUSe3, intrinsic and extrinsic impurities from the flux create midgap states that lead to the experimentally measured narrow optical gap. More likely, BaUSe3 presents a challenge to DFT calculations as applied to 5f materials. © 2016 American Chemical Society.

Inorganic Chemistry

Synthesis, crystal structure, theoretical, and resistivity study of BaUSe3

Five new compounds belonging to the ABaMQ4 family were synthesized by solid-state chemistry at 1123 K. The compounds RbBaPS4, CsBaPS4, CsBaVS4, RbBaVSe4, and CsBaVSe4 are isostructural and have the TlEuPS4 structure type. They crystallize in space group D162h - Pnma of the orthorhombic system. Their structure consists isolated MQ4 tetrahedra separated by A and Ba atoms to form a salt-like structure. Density Functional Theory (DFT) calculations of the electronic structures with the use of the HSE functional suggest that the compounds are semiconductors with calculated band gaps of 3.3 eV (RbBaPS4), 3.4 eV (CsBaPS4), 2.3 eV (CsBaVS4), and 1.6 eV (RbBaVSe4). © 2015 Elsevier Inc. All rights reserved.

Journal of Solid State Chemistry

Syntheses, crystal structure, and electronic properties of the five ABaMQ4 compounds RbBaPS4, CsBaPS4, CsBaVS4, RbBaVSe4, and CsBaVSe4

Two new ternary lanthanide (Ln) uranium selenides, Er3USe8 and Yb3USe8, were synthesized at 1198 K using NaI as a flux. Single-crystal X-ray studies show these two compounds to be isostructural and to crystallize in space group D2h11-Pbcm of the orthorhombic crystal system. The Ln and U atoms are disordered on the same crystallographic site in these crystal structures. Each Ln/U atom is coordinated to eight Se atoms in a bicapped trigonal prism, and sharing of these (Ln/U)Se8 units creates a three-dimensional network. Se2 atoms are connected to each other to form infinite one-dimensional chains along the c axis. In these chains, the two Se atoms are separated by about 2.74 Å, a distance intermediate to those of a Se-Se single bond and a van der Waals interaction. Temperature-dependent resistivity measurements show that Er3USe8 and Yb3USe8 are semiconductors with activation energies of 0.08(1) and 0.17(1) eV, respectively. © 2015 Elsevier Inc. All rights reserved.

Fulltext

Syntheses, crystal structures, and resistivities of the two new ternary uranium selenides, Er3USe8 and Yb3USe8

Electrically conductive hydrogels are a fascinating class of materials that exhibit multifarious applications such as photoresponse, energy storage, etc., and the three-dimensional micro- and nanofibrillar structures of the gels are the key to those applications. Herein, we have synthesized a hybrid hydrogel based on folic acid (F) and polyaniline (PANI) in which F acts as a supramolecular cross-linker of PANI chains. The gels are mechanically robust and are characterized by field-emission scanning electron microscopy, transmission electron microscopy, and spectroscopic, rheological, and universal testing measurements. The hybrid xerogel exhibit a BET surface area 238 m2 g-1, conductivity of 0.04 S/cm, specific capacitance of 295 F/g at a current density of 1A/g, and photocurrent of ∼2 mA under white-light illumination. Silver nanoparticles (AgNPs) are in situ grown to elegantly improve the conductivity, energy storage, and photoresponse capability of the gels. The formation of AgNPs drastically improves the specific capacitances up to 646 F/g (at current density 1A/g), excellent rate capability (403 F/g at 20 A/g), and stable cycling performance with a retention ratio of 74% after 5000 cycles. The AgNPs embedded gel exhibits dramatic enhancement of photocurrent to 56 mA, and its time-dependent photoillumination corroborates faster rise and decay of current compared to those of folic acid-polyaniline hydrogel. © 2016 American Chemical Society.

ACS Applied Materials and Interfaces

Enhancement of Energy Storage and Photoresponse Properties of Folic Acid-Polyaniline Hybrid Hydrogel by in Situ Growth of Ag Nanoparticles

Dihybrid (GP) and trihybrid (GPPS) hydrogels constructed by 5,5′-(1,3,5,7-tetraoxopyrrolo[3,4-f]isoindole-2,6-diyl)diisophthalic acid (P), graphene oxide (GO) and P, GO, and PEDOT:PSS are studied. The gelation occurs through H-bonding and π-π stacking interactions. A magnificent improvement occurs in the mechanical properties of the hybrid gels compared to the P gel. The dc-conductivity of GPPS xerogels is 4-5 orders of magnitude higher compared to the GP and P xerogels. The current-voltage (I-V) characteristic curves of the GP and GPPS gels exhibit rectification properties with the highest rectification ratio of 61; however, the xerogels exhibit semiconducting nature. The GP and GPPS xerogels exhibit photoresponse behavior, and the on-off cycles display a stable photocurrent for the latter system. Dye-sensitized solar cells (DSSCs) are fabricated taking the GPPS gels as active materials and the power conversion efficiency (PCE) increases with the increase of PEDOT:PSS concentration showing a maximum PCE of 4.5%. The IPCE curve shows an absorption range of 360-700 nm with the maximum absorbance of ∼57%. Impedance spectroscopy indicates a considerable difference in the Nyquist plots between the xerogels and gels; the latter exhibit a semicircle with an additional spike corresponding to the Warburg impedance. The Cole-Cole plot of the DSSC consists of three semicircles and the equivalent resistance-capacitance circuits elucidate the PCE values. © The Royal Society of Chemistry 2016.

Journal of Materials Chemistry A

Robust hybrid hydrogels with good rectification properties and their application as active materials for dye-sensitized solar cells: Insights from AC impedance spectroscopy

A family of five isostructural butterfly complexes with a tetranuclear [Ln4] core of the general formula [Ln4(LH)2(μ2-η1η1Piv)(η2-Piv)(μ3-OH)2]⋅x H2O⋅y MeOH⋅z CHCl3(1: Ln=DyIII, x=2, y=2, z=0; 2: Ln=TbIII, x=0, y=0, z=6; 3: Ln=ErIII, x=2, y=2, z=0; 4: Ln=HoIII, x=2, y=2, z=0; 5: Ln=YbIII, x=2, y=2, z=0; LH4=6-{[bis(2-hydroxyethyl)amino]methyl}-N′-(2-hydroxy-3-methoxybenzylidene)picolinohydrazide; PivH=pivalic acid) was isolated and characterized both structurally and magnetically. Complexes 1–5 were probed by direct and alternating current (dc and ac) magnetic susceptibility measurements and, except for 1, they did not display single-molecule magnetism (SMM) behavior. The ac magnetic susceptibility measurements show frequency-dependent out-of-phase signals with one relaxation process for complex 1 and the estimated effective energy barrier for the relaxation process was found to be 49 K. We have carried out extensive ab initio (CASSCF+RASSI-SO+SINGLE_ANISO+POLY_ANISO) calculations on all the five complexes to gain deeper insights into the nature of magnetic anisotropy and the presence and absence of slow relaxation in these complexes. Our calculations yield three different exchange coupling for these Ln4complexes and all the extracted J values are found to be weakly ferro/antiferromagentic in nature (J1=+2.35, J2=−0.58, and J3=−0.29 cm−1for 1; J1=+0.45, J2=−0.68, and J3=−0.29 cm−1for 2; J1=+0.03, J2=−0.98, and J3=−0.19 cm−1for 3; J1=+4.15, J2=−0.23, and J3=−0.54 cm−1for 4 and J1=+0.15, J2=−0.28, and J3=−1.18 cm−1for 5). Our calculations reveal the presence of very large mixed toroidal moment in complex 1 and this is essentially due to the specific exchange topology present in this cluster. Our calculations also suggest presence of single-molecule toroics (SMTs) in complex 2. For complexes 3–5 on the other hand, the transverse anisotropy was computed to be large, leading to the absence of slow relaxation of magnetization. As the magnetic field produced by SMTs decays faster than the normal spin moments, the concept of SMTs can be exploited to build qubits in which less interference and dense packing are possible. Our systematic study on these series of Ln4complexes suggest how the ligand design can help to bring forth such SMT characteristics in lanthanide complexes. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chemistry - A European Journal

Observation of Slow Relaxation and Single-Molecule Toroidal Behavior in a Family of Butterfly-Shaped Ln4Complexes

The laboratory detection of previously unobserved resonance-stabilized C9H5 and C9H9 radicals in the supersonic expansion of a hydrocarbon discharge source is reported. The radicals are tentatively assigned as acetylenic-substituted cyclopentadienyl C9H5 and vinyl-substituted benzyl C9H9 species. They are found to feature visible absorption bands that coincide with a few very weak diffuse interstellar bands toward HD183143 and HD204827. &iuml;&iquest;&frac12; 2016. The American Astronomical Society. All rights reserved.

Astrophysical Journal

VISIBLE ABSORPTIONS of POTENTIAL DIFFUSE ISM HYDROCARBONS: C9H9 and C9H5 RADICALS

Spontaneous formation of metallogels through the self-assembly of unsubstituted nucleobases with Ag(i) ions is reported. Whereas adenine, cytosine, thymine and uracil form metallogels under deprotonated conditions with nanofibrillar morphology, gelation of guanine with Ag(i) occurs only under acidic conditions through the self-organization of nanoscale metal-organic particles. In situ reduction of Ag salts occurs in Ag-pyrimidine gels to yield Ag nanoparticles decorated on the gel nanofibers. The Ag-nucleobase hydrogels showed excellent antimicrobial properties against both Gram positive as well as Gram negative bacteria. © 2016 The Royal Society of Chemistry.