Ravichandar Babarao

Chemically Stable Multilayered Covalent Organic Nanosheets from Covalent Organic Frameworks via Mechanical Delamination

A series of five thermally and chemically stable functionalized covalent organic frameworks (COFs), namely, TpPa-NO2, TpPa-F4, TpBD-(NO2)2, TpBD-Me2, and TpBD-(OMe)2 were synthesized by employing the solvothermal aldehyde-amine Schiff base condensation reaction. In order to complete the series, previously reported TpPa-1, TpPa-2, and TpBD have also been synthesized, and altogether, eight COFs were fully characterized through powder X-ray diffraction (PXRD), Fourier transform IR (FT-IR) spectroscopy, (13)C solid-state NMR spectroscopy, and thermogravimetric analysis. These COFs are crystalline, permanently porous, and stable in boiling water, acid (9 N HCl), and base (3 N NaOH). The synthesized COFs (all eight) were successfully delaminated using a simple, safe, and environmentally friendly mechanical grinding route to transform into covalent organic nanosheets (CONs) and were well characterized via transmission electron microscopy and atomic force microscopy. Further PXRD and FT-IR analyses confirm that these CONs retain their structural integrity throughout the delamination process and also remain stable in aqueous, acidic, and basic media like the parent COFs. These exfoliated CONs have graphene-like layered morphology (delaminated layers), unlike the COFs from which they were synthesized.

Molecular screening of metal-organic frameworks for CO2 storage.

We report a molecular simulation study for CO2 storage in metal-organic frameworks (MOFs). As compared to the aluminum-free and cation-exchanged ZSM-5 zeolites and carbon nanotube bundle, IRMOF1 exhibits remarkably higher capacity. Incorporation of Na(+) cations into zeolite increases the capacity only at low pressures. By variation of the metal oxide, organic linker, functional group, and framework topology, a series of isoreticular MOFs (IRMOF1, Mg-IRMOF1, Be-IRMOF1, IRMOF1-(NH2)4, IRMOF10, IRMOF13, and IRMOF14) are systematically examined, as well as UMCM-1, a fluorous MOF (F-MOF1), and a covalent-organic framework (COF102). The affinity with CO2 is enhanced by addition of a functional group, and the constricted pore is formed by interpenetration of the framework; both lead to a larger isosteric heat and Henrys constant and subsequently a stronger adsorption at low pressures. The organic linker plays a critical role in tuning the free volume and accessible surface area and largely determines CO2 adsorption at high pressures. As a combination of high capacity and low framework density, IRMOF10, IRMOF14, and UMCM-1 are identified from this study to be the best for CO2 storage, even surpass the experimentally reported highest capacity in MOF-177. COF102 is a promising candidate with high capacity at considerably low pressures. Both gravimetric and volumetric capacities at 30 bar correlate well with the framework density, free volume, porosity, and accessible surface area. These structure-function correlations are useful for a priori prediction of CO2 capacity and for the rational screening of MOFs toward high-efficacy CO2 storage.

Unprecedentedly High Selective Adsorption of Gas Mixtures in rho Zeolite-like Metal−Organic Framework: A Molecular Simulation Study

We report a molecular simulation study for the separation of industrially important gas mixtures (CO(2)/H(2), CO(2)/CH(4), and CO(2)/N(2)) in rho zeolite-like metal-organic framework (rho-ZMOF). Rho-ZMOF contains a wide-open anionic framework and charge-balancing extraframework Na(+) ions. Two types of binding sites for Na(+) ions are identified in the framework. Site I is in the single eight-membered ring, whereas site II is in the alpha-cage. Na(+) ions at site I have a stronger affinity for the framework and thus a smaller mobility. The binding sites in rho-ZMOF resemble those in its inorganic counterpart rho-zeolite. CO(2) is adsorbed predominantly over other gases because of its strong electrostatic interactions with the charged framework and the presence of Na(+) ions acting as additional adsorption sites. At ambient temperature and pressure, the CO(2) selectivities are 1800 for the CO(2)/H(2) mixture, 80 for the CO(2)/CH(4) mixture, and 500 for the CO(2)/N(2) mixture. Compared with other MOFs and nanoporous materials reported to date, rho-ZMOF exhibits unprecedentedly high selective adsorption for these gas mixtures. This work represents the first simulation study to characterize extraframework ions and examine gas separation in a charged ZMOF. The simulation results reveal that rho-ZMOF is a promising candidate for the separation of syngas, natural gas, and flue gas.

Post-synthetic structural processing in a metal-organic framework material as a mechanism for exceptional CO2/N2 selectivity

Here we report the synthesis and ceramic-like processing of a new metal-organic framework (MOF) material, [Cu(bcppm)H2O], that shows exceptionally selective separation for CO2 over N2 (ideal adsorbed solution theory, S(ads) = 590). [Cu(bcppm)H2O]·xS was synthesized in 82% yield by reaction of Cu(NO3)2·2.5H2O with the link bis(4-(4-carboxyphenyl)-1H-pyrazolyl)methane (H2bcppm) and shown to have a two-dimensional 4(4)-connected structure with an eclipsed arrangement of the layers. Activation of [Cu(bcppm)H2O] generates a pore-constricted version of the material through concomitant trellis-type pore narrowing (b-axis expansion and c-axis contraction) and a 2D-to-3D transformation (a-axis contraction) to give the adsorbing form, [Cu(bcppm)H2O]-ac. The pore contraction process and 2D-to-3D transformation were probed by single-crystal and powder X-ray diffraction experiments. The 3D network and shorter hydrogen-bonding contacts do not allow [Cu(bcppm)H2O]-ac to expand under gas loading across the pressure ranges examined or following re-solvation. This exceptional separation performance is associated with a moderate adsorption enthalpy and therefore an expected low energy cost for regeneration.

A route to drastic increase of CO2 uptake in Zr metal organic framework UiO-66

CO2 uptake in zirconium MOF UiO-66 almost doubles with post-synthetic exchange of Zr by Ti. This was due to smaller pore size and higher adsorption enthalpy, with good complementarity between experiment and simulation. Furthermore, the full effect is obtained with ~50% Ti loading, precluding the need to fully substitute frameworks for CO2 capture.

Diffusion and Separation of CO2 and CH4 in Silicalite, C168 Schwarzite, and IRMOF-1: A Comparative Study from Molecular Dynamics Simulation

Recently we have investigated the storage and adsorption selectivity of CO(2) and CH(4) in three different classes of nanoporous materialssilicalite, IRMOF-1, and C(168) schwarzite through Monte Carlo simulation (Babarao, R.; Hu, Z.; Jiang, J. Langmuir, 2007, 23, 659). In this work, the self-, corrected, and transport diffusivities of CO(2) and CH(4) in these materials are examined using molecular dynamics simulation. The activation energies at infinite dilution are evaluated from the Arrhenius fits to the diffusivities at various temperatures. As loading increases, the self-diffusivities in the three frameworks decrease as a result of the steric hindrance; the corrected diffusivities remain nearly constant or decrease approximately linearly depending on the adsorbate and framework; and the transport diffusivities generally increase except for CO(2) in IRMOF-1. The correlation effects are identified to reduce from MFI, C(168) to IRMOF-1, in accordance with the porosity increasing in the three frameworks. Predictions of self-, corrected, and transport diffusivities for pure CO(2) and CH(4) from the Maxwell-Stefan formulation match the simulation results well. In a CO(2)/CH(4) mixture, the self-diffusivities decreases with loading, and good agreement is found between simulated and predicted results. On the basis of the adsorption and self-diffusivity in the mixture, the permselectivity is found to be marginal in IRMOF-1, slightly enhanced in MFI, and greatest in C(168) schwarzite. Although IRMOF-1 has the largest storage capacity for CH(4) and CO(2), its selectivity is not satisfactory.

Functionalizing Porous Aromatic Frameworks with Polar Organic Groups for High-Capacity and Selective CO2 Separation: A Molecular Simulation Study

Porous aromatic frameworks (PAFs) were recently synthesized with the highest surface area to date; one such PAF (PAF-1) has diamond-like structure with biphenyl building blocks and exhibits exceptional thermal and hydrothermal stabilities. Herein, we computationally design new PAFs by introducing polar organic groups to the biphenyl unit and then investigate their separating power toward CO(2) by using grand-canonical Monte Carlo (GCMC) simulations. Among these functional PAFs, we found that tetrahydrofuran-like ether-functionalized PAF-1 shows higher adsorption capacity for CO(2) at 1 bar and 298 K (10 mol per kilogram of adsorbent) and also much higher selectivities for CO(2)/CH(4), CO(2)/N(2), and CO(2)/H(2) mixtures when compared with the amine functionality. The electrostatic interactions are found to play a dominant role in the high CO(2) selectivities of functional PAFs, as switching off atomic charges would decrease the selectivity by an order of magnitude. This work suggests that functionalizing porous frameworks with tetrahydrofuran-like ether groups is a promising way to increase CO(2) adsorption capacity and selectivity, especially at ambient pressures.

Defects in metal–organic frameworks: a compromise between adsorption and stability?

Defect engineering has arisen as a promising approach to tune and optimise the adsorptive performance of metal-organic frameworks. However, the balance between enhanced adsorption and structural stability remains an open question. Here both CO2 adsorption capacity and mechanical stability are calculated for the zirconium-based UiO-66, which is subject to systematic variations of defect scenarios. Modulator-dependence, defect concentration and heterogeneity are explored in isolation. Mechanical stability is shown to be compromised at high pressures where uptake is enhanced with an increase in defect concentration. Nonetheless this reduction in stability is minimised for reo type defects and defects with trifluoroacetate substitution. Finally, heterogeneity and auxeticity may also play a role in overcoming the compromise between adsorption and stability.

Hysteretic carbon dioxide sorption in a novel copper(II)-indazole-carboxylate porous coordination polymer

The synthesis, structural and gas sorption studies of a porous Cu(II) coordination polymer featuring 1H-indazole-5-carboxylic acid (H(2)L) are presented. [Cu(HL)(2)] is a thermally and hydrolytically robust 4-connected 3D coordination polymer of NbO topology and is replete with 1D channels that permit selective and hysteretic sorption of CO(2).

Exploiting stable radical states for multifunctional properties in triarylamine-based porous organic polymers

Redox-active porous organic polymers (POPs) have enormous potential in applications ranging from electrocatalysis to solar energy conversion. Exploiting the different electronic states offers exciting prospects for controlling host–guest chemistry, however, this aspect of multifunctionality has to date, remained largely unexplored. Here, we present a strategy for the development of multifunctional materials with industrially sought-after properties. A series of hydrophobic POPs containing redox-active triarylamines linked by ethynyl (POP-1), 1,4-diethynylphenyl (POP-2) and 4,4′-diethynylbiphenyl (POP-3) bridges have been synthesised and characterised by NMR and EPR spectroscopy, as well as spectroelectrochemistry and computational modelling. The facile electrochemical or chemical oxidation of the POPs generate mixed-valence radical cation states with markedly enhanced adsorption properties relative to their neutral analogues, including a 3-fold improvement in the H2 uptake at 77 K and 1 bar, and an increase in the isosteric heat of adsorption for CO2.