A Theoretical Perspective to Decipher the Origin of High Hydrogen Storage Capacity in Mn(II) Metal-Organic Framework

Herein, we report a detailed periodic DFT investigation of Mn(II)-based [(Mn₄ Cl)₃ (BTT)₈ ]^3- (BTT^3- =1,3,5-benzenetristetrazolate) metal-organic framework (MOF) to explore various hydrogen binding pockets, nature of MOF…H₂ interactions, magnetic coupling and, H₂ uptake capacity. Earlier experiments found an uptake capacity of 6.9 wt % of H_2, with the heat of adsorption estimated to be ∼10 kJ/mol, which is one among the highest for any MOFs reported. Our calculations unveil different binding sites with computed binding energy varying from -6 to -15 kJ/mol. The binding of H₂ at the Mn²⁺ site is found to be the strongest (site I), with H₂ found to bind Mn²⁺ ion in a η² fashion with a distance of 2.27 Å and binding energy of -15.4 kJ/mol. The bonding analysis performed using NBO and AIM reveal a strong donation of σ (H₂ ) to the d_z ² orbital of the Mn²⁺ ion responsible for such large binding energy. The other binding pockets, such as -Cl (site II) and BTT ligands (site III and IV) were found to be weaker, with the binding energy decreasing in the order I>II>III>IV. The average binding energy computed for these four sites put together is 9.6 kJ/mol, which is in excellent agreement with the experimental value of ∼10 kJ/mol. We have expanded our calculations to compute binding energy for multiple sites simultaneously, and in this model, the binding energy per site was found to decrease as we increased the number of H₂ molecules suggesting electronic and steric factors controlling the overall uptake capacity. The calculated adsorption isotherm using the GCMC method reproduces the experimental observations. Further, the magnetic coupling computed for the unbound MOF reveals moderate ferromagnetic and strong antiferromagnetic coupling within the tetrameric {Mn₄ } unit leading to a three-up-one-down spin configuration as the ground state. These were then coupled ferromagnetically to other tetrameric units in the MOF network. The magnetic coupling was found to alter only marginally upon gas binding, suggesting that both exchange interaction and the spin-states are unlikely to play a role in the H₂ uptake. This is contrary to the O₂ uptake studied lately, where strong dependence on exchange-coupling/spin state was witnessed, suggesting exchange-coupling/magnetic field dependent binding as a viable route for gas separation.

Overview on recent advances of magnetic metal-organic framework (MMOF) composites in removal of heavy metals from aqueous system

Developing a novel, simple, and cost-effective analytical technique with high enrichment capacity and selectivity is crucial for environmental monitoring and remediation. Metal-organic frameworks (MOFs) are porous coordination polymers that are self-assembly synthesized from organic linkers and inorganic metal ions/metal clusters. Magnetic metal-organic framework (MMOF) composites are promising candidate among the new-generation sorbent materials available for magnetic solid-phase extraction (MSPE) of environmental contaminants due to their superparamagnetism properties, high crystallinity, permanent porosity, ultrahigh specific surface area, adaptable pore shape/sizes, tunable functionality, designable framework topology, rapid and ultrahigh adsorption capacity, and reusability. In this review, we focus on recent scientific progress in the removal of heavy metal ions present in contaminated aquatic system by using MMOF composites. Different types of MMOFs, their synthetic approaches, and various properties that are harnessed for removal of heavy metal ions from contaminated water are discussed briefly. Adsorption mechanisms involved, adsorption capacity, and regeneration of the MMOF sorbents as well as recovery of heavy metal ions adsorbed that are reported in the last ten years have been discussed in this review. Moreover, particular prospects, challenges, and opportunities in future development of MMOFs towards their greener synthetic approaches for their practical industrial applications have critically been considered in this review.

Construction of 3D lanthanide based MOFs with pores decorated with basic imidazole groups for selective capture and chemical fixation of CO2

Rational construction of three new 3D lanthanide-based MOFs exhibiting selective CO₂ capture and conversion to value-added cyclic carbonates under mild conditions is reported.

Construction of a bifunctional Zn(ii)–organic framework containing a basic amine functionality for selective capture and room temperature fixation of CO2

The construction of a novel 3D, porous, bifunctional Zn(ii)–organic framework featuring two-types of 1D channels for efficient fixation of CO₂ to cyclic carbonates under solvent-free mild conditions of RT is reported.

Construction of a 3D porous Co(ii) metal–organic framework (MOF) with Lewis acidic metal sites exhibiting selective CO2 capture and conversion under mild conditions

The application of a 3D Co(ii)-MOF as a recyclable heterogeneous catalyst for conversion of CO₂ to cyclic carbonates is reported.

Construction of bifunctional 2-fold interpenetrated Zn(ii) MOFs exhibiting selective CO2 adsorption and aqueous-phase sensing of 2,4,6-trinitrophenol

Design of Zn(ii) MOFs, [{Zn(BINDI)_0.5(bpa)_0.5(H₂O)}·4H₂O]_n (MOF1) and [{Zn(BINDI)_0.5(bpe)}·3H₂O]_n (MOF2) for selective CO₂ storage and aqueous-phase detection of TNP is demonstrated.

Cyclophosphazene-Based Hybrid Nanoporous Materials as Superior Metal-Free Adsorbents for Gas Sorption Applications

Cyclophosphazene-based inorganic-organic hybrid nanoporous materials (CHNMs) have been synthesized by a facile solvothermal method. The condensation of pyrrole with the reaction product of phosphonitrilic chloride trimer and 4-hydroxybenzaldehyde resulted in the formation of high-surface-area CHNMs. The maximum specific surface area (SA_BET) of 1328 m² g^-1 with hierarchical pore structures having micropores centered at 1.18 nm and mesopores in the range of 2.6-3.6 nm was estimated from the N₂ sorption analysis. Observation of high SA_BET could be attributed to the synergy effect exerted by the cyclophosphazene moiety owing to its three-dimensional paddle wheel structure. The metal-free adsorbent exhibited a high and reversible CO₂ uptake of 22.8 wt % at 273 K and 1 bar. The performance is on the higher side among the reported metal-free inorganic-organic hybrid nanoporous adsorbents. Moreover, the high H₂ uptake of 2.02 wt % at 77 K and 1 bar is an added advantage. The superior performance of the adsorbents for the gas sorption applications could be attributed to the combined effect of high SA_BET and hierarchical pore structure, which has made CHNMs good candidates for energy and environmental applications.

Construction of 3D homochiral metal–organic frameworks (MOFs) of Cd(ii): selective CO2adsorption and catalytic properties for the Knoevenagel and Henry reaction

Syntheses of two new homochiral, 3D metal–organic frameworks (MOFs) of Cd(ii) and the selective CO₂adsorption and catalytic properties of1has been demonstrated.