Volodymyr Bon

A pressure-amplifying framework material with negative gas adsorption transitions

Adsorption-based phenomena are important in gas separations, such as the treatment of greenhouse-gas and toxic-gas pollutants, and in water-adsorption-based heat pumps for solar cooling systems. The ability to tune the pore size, shape and functionality of crystalline porous coordination polymers—or metal–organic frameworks (MOFs)—has made them attractive materials for such adsorption-based applications. The flexibility and guest-molecule-dependent response of MOFs give rise to unexpected and often desirable adsorption phenomena. Common to all isothermal gas adsorption phenomena, however, is increased gas uptake with increased pressure. Here we report adsorption transitions in the isotherms of a MOF (DUT-49) that exhibits a negative gas adsorption; that is, spontaneous desorption of gas (methane and n-butane) occurs during pressure increase in a defined temperature and pressure range. A combination of in situ powder X-ray diffraction, gas adsorption experiments and simulations shows that this adsorption behaviour is controlled by a sudden hysteretic structural deformation and pore contraction of the MOF, which releases guest molecules. These findings may enable technologies using frameworks capable of negative gas adsorption for pressure amplification in micro- and macroscopic system engineering. Negative gas adsorption extends the series of counterintuitive phenomena such as negative thermal expansion and negative refractive indices and may be interpreted as an adsorptive analogue of force-amplifying negative compressibility transitions proposed for metamaterials.

Application of a chiral metal–organic framework in enantioselective separation

A modular approach for the synthesis of highly ordered porous and chiral auxiliary (Evans auxiliary) decorated metal-organic frameworks is developed. Our synthesis strategy, which uses known porous structures as model materials for incorporation of chirality via linker modification, can provide access to a wide range of porous materials suitable for enantioselective separation and catalysis. Chiral analogues of UMCM-1 have been synthesized and investigated for the enantioseparation of chiral compounds in the liquid phase and first promising results are reported.

Heterometallic CoIII4FeIII2 Schiff Base Complex: Structure, Electron Paramagnetic Resonance, and Alkane Oxidation Catalytic Activity

The heterometallic complex [Co(4)Fe(2)OSae(8)]·4DMF·H(2)O (1) was synthesized by one-pot reaction of cobalt powder with iron chloride in a dimethylformamide solution of salicylidene-2-ethanolamine (H(2)Sae) and characterized by single crystal X-ray diffraction analysis, magnetic measurements, high frequency electron paramagnetic resonance (HF-EPR), and Mössbauer spectroscopies. The exchange coupling in the Fe(III)-Fe(III) pair is of antiferromagnetic behavior with J/hc = -190 cm(-1). The HF-EPR spectra reveal an unusual pattern with a hardly detectable triplet signal of the Fe(III) dimer. The magnitude of D (ca. 13.9 cm(-1)) was found to be much larger than in related dimers. The catalytic investigations disclosed an outstanding activity of 1 toward oxidation of cycloalkanes with hydrogen peroxide, under mild conditions. The most efficient system showed a turnover number (TON) of 3.57 × 10(3) with the concomitant overall yield of 26% for cyclohexane, and 2.28 × 10(3)/46%, respectively, for cyclooctane. A remarkable turnover frequency (TOF) of 1.12 × 10(4) h(-1) (the highest initial rate W(0) = 3.5 × 10(-4) M s(-1)) was achieved in oxidation of cyclohexane. Kinetic experiments and selectivity parameters led to the conclusion that hydroxyl radicals are active (attacking C-H bonds) species. Kinetic and electrospray ionization mass spectrometry (ESI-MS) data allowed us to assume that the trinuclear heterometallic particle [Co(2)Fe(Sae)(4)](+), originated from 1 in solution, could be responsible for efficient generation of hydroxyl radicals from hydrogen peroxide.

Tailoring of network dimensionality and porosity adjustment in Zr- and Hf-based MOFs

Three Zr and Hf based metal–organic frameworks, namely DUT-52, DUT-53 and DUT-84 (DUT = Dresden University of Technology) were synthesized using linear 2,6-naphtalenedicarboxylate as a linker. By adjusting the modulator concentration only, the connectivity of SBU can be reduced from 12 to 8 and even to 6, which is reflected in different crystal structures possessing fcu (DUT-52), bcu (DUT-53) and (4,4)IIb (DUT-84) topologies, respectively. DUT-52 is isoreticular to UiO-66. DUT-53 is derived from DUT-52 by omitting four linker molecules from 12-connected SBU environment. In DUT-84 the dimensionality of the structure switches to 2D as a result of omitting further two linker molecules. The structure of DUT-84 is composed of double layers and involves 6-connected SBUs, which are observed for the first time in Zr-based metal–organic frameworks. All compounds are porous and thermally stable up to 450 °C. The BET area, amount to 1399 m2 g−1, 1097 m2 g−1, 782 m2 g−1, and 637 m2 g−1 for DUT-52(Zr), DUT-52(Hf), DUT-53(Hf) and DUT-84(Zr).

Dye Encapsulation Inside a New Mesoporous Metal–Organic Framework for Multifunctional Solvatochromic‐Response Function

Metal–organic frameworks (MOFs), hybrid materials built up from metal clusters and organic linkers, have shown a huge potential for a wide range of applications. In recent years, MOFs have set new records in terms of specific surface areas and pore volumes and therefore are highly suitable as storage materials for small and large molecules. The development of new materials is crucial for the improvement of storage devices, but MOFs are also ideal candidates for functionalization. One functionalization strategy is the integration of complex organic linker molecules containing secondary functional groups. However, this approach is synthetically demanding and not general, because functional donor atoms may affect the linker connectivity resulting in unexpected network topologies. A second powerful strategy is postsynthetic modification of the framework. In this case, the range of functions is restricted due to the limited stability of MOFs against aggressive chemicals. A modular and more versatile approach may be the encapsulation of functional guest molecules into the MOF material. However, for these systems leaching is critical. A crucial requirement in all cases is also the accessibility of MOF functionalities for guest molecules. In this context, a large pore size is highly beneficial. However, the development of such mesoporous frameworks is challenging, because expanded frameworks are often more fragile leading to a collapse of the framework during removal of included guests molecules. A very effective concept to achieve robust frameworks lies in the creation of hierarchical pore structures by combination of two different linker molecules. The most prominent examples of such copolymerization approach are UMCM-1/ 2/ 3, DUT-6 and DUT-23 or MOF210. In the case of DUT-23, auxiliary linker was used successfully to avoid interpenetration and to enhance the robustness, which resulted in highly porous structures. To date, this concept was restricted only to the combination of triand ditopic linkers. On the other hand, DUT-10(M) (M= Zn, Cu, Co) compounds based on the tetratopic N,N,N’,N’benzidinetetrabenzoate (benztb) ligand and the paddlewheel secondary building unit (SBU) undergo structural change upon solvent removal. By enhancing the connectivity of the framework by using a six-connecting [Zn4O] 6+

Understanding activity and selectivity of metal-nitrogen-doped carbon catalysts for electrochemical reduction of CO 2

Direct electrochemical reduction of CO2 to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO2-consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO2 to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M–Nx moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe–N–C and especially Ni–N–C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M–Nx moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M–Nx moieties and it provides predictive guidelines for the rational design of selective carbon-based CO2 reduction catalysts.Inexpensive and selective electrocatalysts for CO2 reduction hold promise for sustainable fuel production. Here, the authors report N-coordinated, non-noble metal-doped porous carbons as efficient and selective electrocatalysts for CO2 to CO conversion.

Integration of accessible secondary metal sites into MOFs for H2S removal

2,2′-Bipyridine-5,5′-dicarboxylic acid (H2bipy) was used for the synthesis of a zirconium based metal–organic framework with the aim to integrate an additional N-donor coordination functionality into the framework. The resulting MOF (UiO-67(bipy)) with a structure isoreticular to UiO-67 (UiO – University of Oslo) has a BET surface area up to 2500 m2 g−1. The chelating bipyridine moiety was used for postsynthetic functionalization of the MOF with different metal salts (MIIXy; M = Cu2+, Ni2+, Co2+; X = Cl−, NO3−, SO42−, acac2−) to give the corresponding metal_salt@UiO-67(bipy) materials. Due to the highly dispersed metal ions, metal_salt@UiO-67(bipy) shows good performance in toxic hydrogen sulfide capture. Especially, the copper loaded samples have high capacities of up to 7.8 wt%.

In Situ Observation of Gating Phenomena in the Flexible Porous Coordination Polymer Zn2(BPnDC)2(bpy) (SNU-9) in a Combined Diffraction and Gas Adsorption Experiment

The intrinsic structural dynamic during the adsorption of CO2 at 195 K and N2 at 77 K on flexible porous coordination polymer Zn2(BPnDC)2(bpy) (SNU-9) was studied in situ by powder XRD. The crystal structures of as made and solvent free (activated) phases were determined by single crystal X-ray diffraction. During the structural transformation caused by activation, the rearrangement of Zn-O bonds occurs that leads to changes in coordination environment of Zn atoms. Such changes lead to the contraction of the unit cell and to decreasing unit cell volume of nearly 28% in comparison to the pristine as made structure. The solvent accessible volume of the unit cell decreases from 40.8% to 12.8%. The adsorption of CO2 and N2 on SNU-9 proceeds in a different way: the formation of intermediate phase during the CO2 adsorption could be postulated, while the transformation from narrow pore form to the open structure occurs in quasi-one-step in the case of N2 adsorption (the intermediate phase is formed only in very narrow pressure region). The transformation of the structure is guest dependent and the differences in the structures of CO2@SNU-9 at 195 K and N2@SNU-9 at 77 K were proven by Pawley and Rietveld refinements of powder XRD patterns. The structure of N2@SNU-9 is identical to this of as synthesized phase, while the structure of CO2@SNU-9 differs slightly.

Postsynthetic Inner-Surface Functionalization of the Highly Stable Zirconium-Based Metal–Organic Framework DUT-67

A postsynthetic functionalization approach was used to tailor the hydrophobicity of DUT-67, a metal-organic framework (MOF) consisting of 8-connected Zr6O6(OH)2 clusters and 2,5-thiophenedicarboxylate as the ligand, using postsynthetic exchange of the modulator by fluorinated monocarboxylates. Water adsorption isotherms demonstrated that, by the incorporation of such hydrophobic molecules, the hydrophobicity of the inner surface of the network can be tuned. Furthermore, tolerance of the material toward the removal of adsorbed water can be significantly enhanced compared to the parent DUT-67 MOF.

Topological control of 3,4-connected frameworks based on the Cu2-paddle-wheel node: tbo or pto, and why?

Two trigonal tritopic ligands with different conformational degree of freedom: conformationally labile H3tcbpa (tris((4-carboxyl)phenylduryl)amine) and conformationally obstructed H3hmbqa (4,4′,4′′-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[9,1]quino-lizino[3,4,5,6,7-defg]acridine-2,6,10-triyl)tribenzoic acid) are assembled with square-planar paddle-wheel nodes with the aim of selective engineering of the frameworks with tbo and pto underlying net topologies. In the case of H3tcbpa, both topological types were obtained forming non-interpenetrated MOFs namely DUT-63 (tbo) and DUT-64 (pto). Whereas synthesis of DUT-63 proceeds under typical conditions, formation of DUT-64 requires an additional topology directing reagent (topological modifier). Solvothermal treatment of the conformationally hindered H3hmbqa ligand with the Cu-salt results exclusively in DUT-77 material, based on the single pto net. The possibility to insert the salen based metallated pillar ligand into networks with pto topology post-synthetically results in DUT-78 and DUT-79 materials (both ith-d) and opens new horizons for post-synthetic insertion of catalytically active metals within the above-mentioned topological type of frameworks.