Asamanjoy Bhunia

Highly stable nanoporous covalent triazine-based frameworks with an adamantane core for carbon dioxide sorption and separation

Adamantane substituted with two to four 4-cyanophenyl groups was used for preparation of a new series of robust Porous Covalent Triazine-based Framework (PCTF) materials. Novel adamantane PCTFs were synthesized in good yields (>80%) by the trimerization reaction of 1,3-bis-, 1,3,5-tris- and 1,3,5,7-tetrakis(4-cyanophenyl)adamantane, respectively, in the presence of ZnCl2 (Lewis acid condition) and CF3SO3H (strong Bronsted acid condition). From N2 adsorption isotherms, the Lewis acid condition gives higher surface areas than the strong Bronsted acid condition. The amorphous nano- to microporous frameworks (>50% micropore fraction) exhibit excellent thermal stabilities (>450 °C) with BET surface areas up to 1180 m2 g−1. A very similar ultramicropore size distribution between 4 and 10 A was derived from CO2 adsorption isotherms with a “CO2 on carbon based slit-pore model”. At 1 bar the gases H2 (at 77 K), CO2 (at 273 and 293 K) and CH4 (at 273 K) are adsorbed up to 1.24 wt%, 58 cm3 g−1 and 20 cm3 g−1, respectively. Gas uptake increases with BET surface area and micropore volume which in turn increase with the number of cyano groups in the monomer. From single component adsorption isotherms, IAST-derived ideal CO2:N2, CO2:CH4 and CH4:N2 selectivity values of up to 41 : 1, 7 : 1 and 6 : 1, respectively, are calculated for p → 0 at 273 K. The adamantane PCTFs have isosteric heats of adsorption for CO2 of 25–28 kJ mol−1 at zero loading and most of them also >25 kJ mol−1 over the entire adsorption range which is well above the heat of liquefaction of bulk CO2 or the isosteric enthalpy of adsorption for CO2 on activated carbons.

Covalent triazine-based frameworks (CTFs) from triptycene and fluorene motifs for CO2 adsorption

Two microporous covalent triazine-based frameworks (CTFs) with triptycene (TPC) and fluorene (FL) backbones have been synthesized through a mild AlCl3-catalyzed Friedel–Crafts reaction, with the highest surface area of up to 1668 m2 g−1 for non-ionothermal CTFs. CTF-TPC and CTF-FL show an excellent carbon dioxide uptake capacity of up to 4.24 mmol g−1 at 273 K and 1 bar.

Giant Zn14 molecular building block in hydrogen-bonded network with permanent porosity for gas uptake.

In situ imidazolate-4,5-diamide-2-olate linker generation leads to the formation of a [Zn14(L2)12(O)(OH)2(H2O)4] molecular building block (MBB) with a Zn6 octahedron inscribed in a Zn8 cube. The MBBs connect by amide-amide hydrogen bonds to a 3D robust supramolecular network which can be activated for N2, CO2, CH4, and H2 gas sorption.

A photoluminescent covalent triazine framework: CO2 adsorption, light-driven hydrogen evolution and sensing of nitroaromatics

A highly photoluminescent (PL) porous covalent triazine-based framework (PCTF-8) is synthesized from tetra(4-cyanophenyl)ethylene by using trifluoromethanesulfonic acid as the catalyst at room temperature. Due to triazine units in the framework, the PCTF-8 exhibits excellent thermal stability (>400 °C). The Brunauer–Emmett–Teller (BET) specific surface area of PCTF-8 is 625 m2 g−1 which is lower than the one obtained from the synthesis under Lewis acid conditions (ZnCl2). At 1 bar and 273 K, the PCTF-8 adsorbs a significant amount of CO2 (56 cm3 g−1) and CH4 (17 cm3 g−1) which is highly comparable to nanoporous 1,3,5-triazine frameworks (NOP-1-6, 29–56 cm3 g−1). This nitrogen rich framework exhibits good ideal selectivity (61 : 1 (85% N2 : 15% CO2) at 273 K, 1 bar). Thus, it can be used as a promising candidate for potential applications in post-combustion CO2 capture and sequestration technologies. In addition, photoluminescence properties as well as the sensing behaviour towards nitroaromatics have been demonstrated. The fluorescence emission intensity of PCTF-8 is quenched by ca. 71% in the presence of 2,4,6-trinitrophenol (TNP). From time-resolved studies, a static quenching behaviour was found. This high photoluminescence property is used for hydrogen evolving organic photocatalysis from water in the presence of a sacrificial electron donor and a cocatalyst.

Synthesis of a Co(II)–imidazolate framework from an anionic linker precursor: gas-sorption and magnetic properties

A Co(II)–imidazolate-4-amide-5-imidate based MOF, IFP-5, is synthesized by using an imidazolate anion-based novel ionic liquid as a linker precursor under solvothermal conditions. IFP-5 shows significant amounts of gas (N2, CO2, CH4 and H2) uptake capacities. IFP-5 exhibits an independent high spin Co(II) centre and antiferromagnetic coupling.

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.

Two linkers are better than one: enhancing CO2 capture and separation with porous covalent triazine-based frameworks from mixed nitrile linkers

Covalent triazine-based framework (CTF) materials were synthesized by combining two different nitrile building blocks: the tetranitrile tetrakis(4-cyanophenyl)ethylene (M) was reacted with either terephthalonitrile (M1), tetrafluoroterephthalonitrile (M2), 4,4′-biphenyldicarbonitrile (M3) or 1,3,5-benzenetricarbonitrile (M4) under ionothermal conditions (ZnCl2, 400 °C) to yield mixed-nitrile MM′-CTFs MM1 to MM4. Comparative 1H/13C and 19F/13C CP MAS analyses of MM2(300) (synthesized at 300 °C) suggest that the hydrogenated and fluorinated carbon atoms are in close vicinity (<5 A) to each other and support the formulation of the MM2(300) sample as a copolymeric CTF. Systematic N2, CO2 and CH4 gas sorption studies were performed up to 1 bar at 273 K and 293 K. The specific BET surface areas of MM1–MM4 were 1800, 1360, 1884 and 1407 m2 g−1, respectively. The CO2 uptake capacity of mixed-nitrile MM1, MM2 and MM4 was higher than the CO2 uptake of the respective individual single-nitrile M- or M′-CTF despite a higher surface area of the M-CTF PCTF-1 (2235 m2 g−1). The synergistic increase in the CO2 uptake of the mixed-nitrile MM′-CTFs is due to the higher CO2-accessible micropore volume Vmicro(CO2) and the higher micropore volume fraction V0.1/Vtot of the MM′-CTFs compared to the M- or M′-CTFs. The surface area of porous materials does not play the most important role in CO2 storage at low pressure but the CO2-accessible micropore volume is the more decisive factor. Further, MM2 shows the second highest (of known CTFs synthesized at 400 °C) CO2 uptake capacity of 4.70 mmol g−1 at 273 K and 1 bar because of its large micropore fraction (82%), which may be due to the release of fluorous decomposition products (‘defluorination carbonization’) during its synthesis. The CO2/N2 adsorption selectivities of mixed-nitrile MM1, MM2 and MM4 CTFs were also higher than those of the single-nitrile component M- or M′-CTFs.

Study of the Discrepancies between Crystallographic Porosity and Guest Access into Cadmium–Imidazolate Frameworks and Tunable Luminescence Properties by Incorporation of Lanthanides

An extended member of the isoreticular family of metal-imidazolate framework structures, IFP-6 (IFP=imidazolate framework Potsdam), based on cadmium metal and an in situ functionalized 2-methylimidazolate-4-amide-5-imidate linker is reported. A porous 3D framework with 1D hexagonal channels with accessible pore windows of 0.52 nm has been synthesized by using an ionic liquid (IL) linker precursor. IFP-6 shows significant gas uptake capacity only for CO2 and CH4 at elevated pressure, whereas it does not adsorb N2 , H2 , and CH4 under atmospheric conditions. IFP-6 is assumed to deteriorate at the outside of the material during the activation process. This closing of the metal-organic framework (MOF) pores is proven by positron annihilation lifetime spectroscopy (PALS), which revealed inherent crystal defects. PALS results support the conservation of the inner pores of IFP-6. IFP-6 has also been successfully loaded with luminescent trivalent lanthanide ions (Ln(III) =Tb, Eu, and Sm) in a bottom-up one-pot reaction through the in situ generation of the linker ligand and in situ incorporation of photoluminescent Ln ions into the constituting network. The results of photoluminescence investigations and powder XRD provide evidence that the Ln ions are not doped as connectivity centers into the frameworks, but are instead located within the pores of the MOFs. Under UV light irradiation, Tb@IFP-6 and Eu@IFP-6 (λexc =365 nm) exhibit observable emission changes to a greenish and reddish color, respectively, as a result of strong Ln 4 f emissions.

Development of a UiO-Type Thin Film Electrocatalysis Platform with Redox-Active Linkers

Metal-organic frameworks (MOFs) as electrocatalysis scaffolds are appealing due to the large concentration of catalytic units that can be assembled in three dimensions. To harness the full potential of these materials, charge transport to the redox catalysts within the MOF has to be ensured. Herein, we report the first electroactive MOF with the UiO/PIZOF topology (Zr(dcphOH-NDI)), i.e., one of the most widely used MOFs for catalyst incorporation, by using redox-active naphthalene diimide-based linkers (dcphOH-NDI). Hydroxyl groups were included on the dcphOH-NDI linker to facilitate proton transport through the material. Potentiometric titrations of Zr(dcphOH-NDI) show the proton-responsive behavior via the -OH groups on the linkers and the bridging Zr-μ3-OH of the secondary building units with pKa values of 6.10 and 3.45, respectively. When grown directly onto transparent conductive fluorine-doped tin oxide (FTO), 1 μm thin films of Zr(dcphOH-NDI)@FTO could be achieved. Zr(dcphOH-NDI)@FTO displays reversible electrochromic behavior as a result of the sequential one-electron reductions of the redox-active NDI linkers. Importantly, 97% of the NDI sites are electrochemically active at applied potentials. Charge propagation through the thin film proceeds through a linker-to-linker hopping mechanism that is charge-balanced by electrolyte transport, giving rise to cyclic voltammograms of the thin films that show characteristics of a diffusion-controlled process. The equivalent diffusion coefficient, De, that contains contributions from both phenomena was measured directly by UV/vis spectroelectrochemistry. Using KPF6 as electrolyte, De was determined to be De(KPF6) = (5.4 ± 1.1) × 10-11 cm2 s-1, while an increase in countercation size to n-Bu4N+ led to a significant decrease of De by about 1 order of magnitude (De(n-Bu4NPF6) = (4.0 ± 2.5) × 10-12 cm2 s-1).