Suresh Babu Kalidindi

GaN@ZIF-8: Selective Formation of Gallium Nitride Quantum Dots inside a Zinc Methylimidazolate Framework

The microporous zeolitic imidazolate framework [Zn(MeIM)(2); ZIF-8; MeIM = imidazolate-2-methyl] was quantitatively loaded with trimethylamine gallane [(CH(3))(3)NGaH(3)]. The obtained inclusion compound [(CH(3))(3)NGaH(3)]@ZIF-8 reveals three precursor molecules per host cavity. Treatment with ammonia selectively yields the caged cyclotrigallazane intermediate (H(2)GaNH(2))(3)@ZIF-8, and further annealing gives GaN@ZIF-8. This new composite material was characterized with FT-IR spectroscopy, solid-state NMR spectroscopy, powder X-ray diffraction, elemental analysis, (scanning) transmission electron microscopy combined with electron energy-loss spectroscopy, photoluminescence (PL) spectroscopy, and N(2) sorption measurements. The data give evidence for the presence of GaN nanoparticles (1-3 nm) embedded in the cavities of ZIF-8, including a blue-shift of the PL emission band caused by the quantum size effect.

Metal@COFs: Covalent Organic Frameworks as Templates for Pd Nanoparticles and Hydrogen Storage Properties of Pd@COF‐102 Hybrid Material

Three-dimensional covalent organic frameworks (COFs) have been demonstrated as a new class of templates for nanoparticles. Photodecomposition of the [Pd(η(3)-C(3) H(5))(η(5)-C(5)H(5))]@COF-102 inclusion compound (synthesized by a gas-phase infiltration method) led to the formation of the Pd@COF-102 hybrid material. Advanced electron microscopy techniques (including high-angle annular dark-field scanning transmission electron microscopy and electron tomography) along with other conventional characterization techniques unambiguously showed that highly monodisperse Pd nanoparticles ((2.4±0.5) nm) were evenly distributed inside the COF-102 framework. The Pd@COF-102 hybrid material is a rare example of a metal-nanoparticle-loaded porous crystalline material with a very narrow size distribution without any larger agglomerates even at high loadings (30 wt %). Two samples with moderate Pd content (3.5 and 9.5 wt %) were used to study the hydrogen storage properties of the metal-decorated COF surface. The uptakes at room temperature from these samples were higher than those of similar systems such as Pd@metal-organic frameworks (MOFs). The studies show that the H(2) capacities were enhanced by a factor of 2-3 through Pd impregnation on COF-102 at room temperature and 20 bar. This remarkable enhancement is not just due to Pd hydride formation and can be mainly ascribed to hydrogenation of residual organic compounds, such as bicyclopentadiene. The significantly higher reversible hydrogen storage capacity that comes from decomposed products of the employed organometallic Pd precursor suggests that this discovery may be relevant to the discussion of the spillover phenomenon in metal/MOFs and related systems.

A Cryogenically Flexible Covalent Organic Framework for Efficient Hydrogen Isotope Separation by Quantum Sieving

Conventional molecular sieves are often utilized in industryfor the purification of gas mixtures consisting of differentmolecular sizes. However, separation of hydrogen isotopesrequires special efforts because of their identical size, shape,and thermodynamic properties. Separation of isotope mix-tures is only possible with limited techniques, such ascryogenic distillation, thermal diffusion, and the Girdlersulfide process, but these methods have a low separationfactor and are excessively time- and energy-intensive.

Synergistic Hydrogenation over Palladium through the Assembly of MIL-101(Fe) MOF over Palladium Nanocubes

Enhancing catalytic performance of metal nanoparticles is highly sought for many industrial catalytic processes. In this regard, assembly of crystalline porous super-tunable metal-organic frameworks (MOFs) around preformed metal nanoparticles is an attractive prospect as this strongly influences the activity of the entire nanoparticle surface. Herein, we assembled a MlL-101(Fe) MOF onto the Pd nanocubes and evaluated the catalytic properties of the hybrid material for the hydrogenation of the α,β-unsaturated carbonyl compounds cinnamaldehyde, crotonaldehyde, and β-ionone. Owing to the synergestic effects originating from the Lewis acid sites present on MOF and Pd active sites, striking improvements in the activities and selectivities were observed for the Pd⊂MIL-101(Fe) hybrid material. The turnover frequency (TOF) values increased up to roughly 20 fold and in all three studied substrates, C=C was preferentially hydrogenated compared to C=O. Furthermore, the Pd⊂MIL-101(Fe) catalyst was readily reusable and highly stable.

Assembly of ZIF-67 Metal-Organic Framework over Tin Oxide Nanoparticles for Synergistic Chemiresistive CO2 Gas Sensing

Metal-organic frameworks (MOFs) are widely known for their record storage capacities of small gas molecules (H2 , CO2 , and CH4 ). Assembly of such porous materials onto well-known chemiresistive gas sensing elements such as SnO2 could be an attractive prospect to achieve novel sensing properties as this affects the surface chemistry of SnO2 . Cobalt-imidazole based ZIF-67 MOF was grown onto preformed SnO2 nanoparticles to realize core-shell like architecture and explored for greenhouse gas CO2 sensing. CO2 sensing over SnO2 is a challenge because its interaction with SnO2 surface is minimal. The ZIF-67 coating over SnO2 improved the response of SnO2 up to 12-fold (for 50 % CO2 ). The SnO2 @ZIF-67 also showed a response of 16.5±2.1 % for 5000 ppm CO2 (threshold limit value (TLV)) at 205 °C, one of the best values reported for a SnO2 -based sensor. The observed novel CO2 sensing characteristics are assigned to electronic structure changes at the interface of ZIF-67 and SnO2 .

Hybridization of Pd Nanoparticles with UiO-66(Hf) Metal-Organic Framework and the Effect of Nanostructure on the Catalytic Properties

Metal-organic frameworks (MOFs) have emerged as a new class of supports for metal nanoparticles(NPs) in heterogeneous catalysis because of possible synergetic effects between the two components. In addition, MOFs also can be coated over metal NPs to influence the entire nanoparticles surface. Herein, NPs were hybridized with UiO-66(Hf) MOF possessing Brønsted acidic sites (on secondary building units) and fabricated Pd@UiO-66 (Hf) core-shell and Pd/UiO-66(Hf) supported catalysts. These hybrid materials exhibited enhanced catalytic properties (TOF increased up to 2.5 times) compared to individual counterparts or their physical mixture for dehydrogenation of ammonia borane(AB) in non-aqueous medium(1,4-dioxane). Further, nanostructure of the hybrid material had pronounced influence on the catalytic properties. The core-shell catalyst exhibited highest activity towards H2 generation from AB owing to greater contact interface between Pd and MOF. Further, phenylacetylene semi-hydrogenation with AB over Pd@UiO-66 (Hf) furnished styrene selectivity as high as 93.2 % at ∼100 % conversion mostly due to the regulated phenylacetylene diffusion through UiO-66(Hf) shell.