Shyam Biswas

New functionalized flexible Al-MIL-53-X (X = -Cl, -Br, -CH3, -NO2, -(OH)2) solids: syntheses, characterization, sorption, and breathing behavior.

Five new flexible functionalized aluminum hydroxo terephthalates [Al(OH)(BDC-X)]·n(guests) (BDC = 1,4-benzene-dicarboxylate; X = -Cl, 1-Cl; -Br, 2-Br; -CH(3), 3-CH(3); -NO(2), 4-NO(2); -(OH)(2), 5-OH(2)) were synthesized under solvothermal conditions. The as synthesized (Al-MIL-53-X-AS) as well as the activated compounds were characterized by X-ray powder diffraction (XRPD), IR spectroscopy, thermogravimetric (TG), and elemental analysis. Activation, that is, removal of unreacted H(2)BDC-X molecules and/or occluded solvent molecules, followed by hydration in air at room temperature, led to the narrow pore (NP) form of the title compounds [Al(OH)(BDC-X)]·n(H(2)O) (Al-MIL-53-X). Thermogravimetric analysis (TGA) and temperature-dependent XRPD (TDXRPD) experiments performed on the NP-form of the compounds indicate high thermal stability in the range 325-500 °C. As verified by N(2), CO(2), or H(2)O sorption measurements, most of the thermally activated compounds exhibit significant microporosity. Similar to pristine Al-MIL-53, the present compounds retain their structural flexibility depending on the nature of guest molecules and temperature, as verified by cell parameter determination from XRPD data. The breathing behavior of the functionalized frameworks upon dehydration-rehydration, investigated by temperature and time-dependent XRPD measurements, differs significantly compared to parent Al-MIL-53.

p-Xylene-Selective Metal-Organic Frameworks: A Case of Topology-Directed Selectivity

Para-disubstituted alkylaromatics such as p-xylene are preferentially adsorbed from an isomer mixture on three isostructural metal-organic frameworks: MIL-125(Ti) ([Ti(8)O(8)(OH)(4)(BDC)(6)]), MIL-125(Ti)-NH(2) ([Ti(8)O(8)(OH)(4)(BDC-NH(2))(6)]), and CAU-1(Al)-NH(2) ([Al(8)(OH)(4)(OCH(3))(8)(BDC-NH(2))(6)]) (BDC = 1,4-benzenedicarboxylate). Their unique structure contains octahedral cages, which can separate molecules on the basis of differences in packing and interaction with the pore walls, as well as smaller tetrahedral cages, which are capable of separating molecules by molecular sieving. These experimental data are in line with predictions by molecular simulations. Additional adsorption and microcalorimetric experiments provide insight in the complementary role of the two cage types in providing the para selectivity.

Enhanced selectivity of CO2 over CH4 in sulphonate-, carboxylate- and iodo-functionalized UiO-66 frameworks

Three new functionalized UiO-66-X (X = -SO(3)H, 1; -CO(2)H, 2; -I; 3) frameworks incorporating BDC-X (BDC: 1,4-benzenedicarboxylate) linkers have been synthesized by a solvothermal method using conventional electric heating. The as-synthesized (AS) as well as the thermally activated compounds were characterized by X-ray powder diffraction (XRPD), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, thermogravimetric (TG), and elemental analysis. The occluded H(2)BDC-X molecules can be removed by exchange with polar solvent molecules followed by thermal treatment under vacuum leading to the empty-pore forms of the title compounds. Thermogravimetric analysis (TGA) and temperature-dependent XRPD (TDXRPD) experiments indicate that 1, 2 and 3 are stable up to 260, 340 and 360 °C, respectively. The compounds maintain their structural integrity in water, acetic acid and 1 M HCl, as verified by XRPD analysis of the samples recovered after suspending them in the respective liquids. As confirmed by N(2), CO(2) and CH(4) sorption analyses, all of the thermally activated compounds exhibit significant microporosity (S(Langmuir): 769-842 m(2) g(-1)), which are comparable to that of the parent UiO-66 compound. Compared to the unfunctionalized UiO-66 compound, all the three functionalized solids possess higher ideal selectivity in adsorption of CO(2) over CH(4) at 33 °C.

A thiadiazole-functionalized Zr(IV)-based metal–organic framework as a highly fluorescent probe for the selective detection of picric acid

A new, strongly luminescent Zr(IV)-based metal–organic framework (MOF) material (1) having a UiO-68 (UiO = University of Oslo) framework topology and incorporating the π-conjugated, thiadiazole-functionalized H2BTDB {H2BTDB = 4,4′-(benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzoic acid} ligand was synthesized under solvothermal conditions (150 °C, 24 h) using ZrCl4 and the H2BTDB ligand in DMF (DMF = N,N-dimethylformamide). The phase-purity of as-synthesized 1 was ascertained by X-ray powder diffraction (XRPD) analysis, Fourier transform infrared (FT-IR) spectroscopy and elemental analysis. Based on the thermogravimetric analyses, 1 is thermally stable up to 400 °C. XRPD experiments verify that activated 1′ retains its crystallinity when exposed to water, acetic acid and 1 M HCl solutions. As revealed by the steady-state fluorescence titration experiments, the thermally activated form of the compound (1′) showed a selective sensing behaviour towards 2,4,6-trinitrophenol (TNP, commonly known as picric acid), even in the presence of other potentially competing nitroaromatic explosive compounds. The estimated detection limit of 1′ for sensing TNP in methanol suspension was found to be 1.63 × 10−6 M. The highest fluorescence quenching ability of TNP can be attributed to both energy and electron transfer processes as well as electrostatic interactions between the hydroxyl group of TNP and the Lewis basic N-donor sites of the BTDB ligand. Endowed with its excellent detection efficiency, 1′ is a promising luminescent sensor material for the long-term, practical sensing of TNP.

Partially fluorinated MIL-47 and Al-MIL-53 frameworks: influence of functionalization on sorption and breathing properties

Two perfluorinated metal hydroxo terephthalates [M(III)(OH)(BDC-F)]·n(guests) (M(III) = V, MIL-47-F-AS or 1-AS; Al, Al-MIL-53-F-AS or 2-AS) (BDC-F = 2-fluoro-1,4-benzenedicarboxylate; AS = as-synthesized) have been synthesized by a hydrothermal method using microwave irradiation (1-AS) or conventional electric heating (2-AS), respectively. The unreacted or occluded H(2)BDC-F molecules can be removed under vacuum by direct thermal activation or exchange of guest molecules followed by thermal treatment leading to the empty-pore forms of the title compounds [V(IV)(O)(BDC-F)] (MIL-47-F, 1) and [Al(III)(OH)(BDC-F)] (Al-MIL-53-F, 2). Thermogravimetric analysis (TGA) and temperature-dependent XRPD (TDXRPD) experiments indicate that the compounds are stable up to 385 and 480 °C, respectively. Both of the thermally activated compounds exhibit significant microporosity, as verified by N(2), CO(2), n-hexane, o- and p-xylene sorption analyses. The structural changes of 2 upon adsorption of CO(2), n-hexane, o- and p-xylene were highly influenced due to functionalization by -F groups, as compared to parent Al-MIL-53. The -F groups also introduce a certain degree of hydrophobicity into the frameworks, as demonstrated by the H(2)O sorption analyses.

Vanadium metal-organic frameworks: structures and applications

This perspective review paper describes the V-containing metal–organic frameworks that have been developed since the first systematic reports on MOFs almost 15 years ago. These hybrid crystalline materials, containing V(III) or V(IV) as metal nodes, show interesting behavior in oxidation catalysis and gas sorption. A significant amount of papers has appeared on the use of these structures in gas (hydrocarbon, CO2) separation. Promising future research and development of V-MOFs is suggested.

Cerium-based azide- and nitro-functionalized UiO-66 frameworks as turn-on fluorescent probes for the sensing of hydrogen sulphide

A new and an existing Ce-based metal–organic framework (MOF) having a UiO-66 framework topology and incorporating azide and nitro functional groups in their frameworks have been successfully used as turn-on fluorescent probes for the sensing of H2S under physiological conditions. The azide (1-N3) and nitro (2-NO2) functionalized Ce MOFs have been synthesized under similar solvothermal conditions (100 °C, 15 min) using ammonium cerium(IV) nitrate and H2BDC-X (BDC = 1,4-benzenedicarboxylate; X = –N3 for 1-N3 and –NO2 for 2-NO2) linkers in DMF/H2O (DMF = N,N-dimethylformamide) mixtures. The phase purity of both compounds has been confirmed by X-ray powder diffraction (XRPD) analyses, infrared spectroscopy and thermogravimetric (TG) analyses. The thermally activated forms of both compounds (1′-N3 and 2′-NO2) show fast response time, excellent selectivity and sensitivity for the detection of H2S under physiological conditions (HEPES buffer, pH 7.4) through the fluorescence ‘turn-on’ mechanism. The detection limits (12.2 μM for 1′-N3 and 34.8 μM for 2′-NO2) of both materials lie within the range of the H2S concentration observed in biological systems. The materials can selectively detect H2S even in the presence of other competing biomolecules. Apart from the sensing of H2S, both compounds exhibit high uptake of CO2 (2.6 mmol g−1 for 1′-N3 and 3.7 mmol g−1 for 2′-NO2) at 0 °C and 1 bar. Thus, the materials are promising candidates in the fields of H2S sensing and CO2 capture.

A highly stable dimethyl-functionalized Ce(IV)-based UiO-66 metal–organic framework material for gas sorption and redox catalysis

The new, dimethyl-functionalized Ce(IV)-based UiO-66 (UiO = University of Oslo) metal–organic framework (MOF) material Ce-UiO-66-(CH3)2 (1) was successfully synthesized under solvothermal conditions (100 °C, 15 min) by employing ammonium cerium(IV) nitrate and the 2,5-dimethyl-1,4-benzenedicarboxylate (H2BDC-(CH3)2) ligand in a DMF/H2O (DMF = N,N-dimethyl formamide) mixture. The phase purity of the as-synthesized and thermally activated form (1′) of the compound was confirmed by a combination of X-ray powder diffraction (XRPD) analysis, Fourier transform infrared (FT-IR) spectroscopy and thermogravimetric (TG) analysis. As verified by the thermogravimetric analysis, the compound is thermally stable up to 300 °C in air atmosphere. Based on the XRPD measurements, the material retains its crystallinity after treatment with water, methanol, acetic acid and 1 M HCl. As confirmed by the gas sorption experiments, the compound shows significant microporosity towards N2 (BET surface area = 845 m2 g−1) and CO2 (adsorption capacity = 34 cm3 g−1 at 0 °C and 1 bar). The catalytic activity of 1′ has been studied in the oxidation of styrene and cyclohexene using tert-butylhydroperoxide (TBHP) as the terminal oxidant. The catalyst is reusable for four cycles with no significant drop in its activity which is further confirmed by a hot filtration experiment.

Comparison of different solid adsorbents for the removal of mobile pesticides from aqueous solutions

Abstract The intensive use of mobile plant protection products, such as bentazon, clopyralid and isoproturon, in agriculture has led to an increasing contamination of groundwater and surface water. In this study, the sorption capacity of various activated carbon samples, metal–organic frameworks (MOFs) and resins were compared. After an initial screening the activated carbon samples and the activated carbon related resin Lewatit AF 5 were selected to characterize the total sorption dynamics. Based on the Freundlich isotherms, the zeolites and MOFs performed well because of their more rapid adsorption of a large amount of pesticides and the lower affinity for the pesticides in view of regeneration. On the other hand, the Temkin model showed physisorption for all adsorbents. Finally, the unstable structure of MOF-235, of which