Revisiting Metal-Organic Frameworks Porosimetry by Positron Annihilation: Metal Ion States and Positronium Parameters.

Metal-organic frameworks (MOFs) stand as pivotal porous materials with exceptional surface areas, adaptability, and versatility. Positron Annihilation Lifetime Spectroscopy (PALS) is an indispensable tool for characterizing MOF porosity, especially micro- and mesopores in both open and closed phases. Notably, PALS offers porosity insights independent of probe molecules, which is vital for detailed characterization without structural transformations. This study explores how metal ion states in MOFs affect PALS results. We find significant differences in measured porosity due to paramagnetic or oxidized metal ions compared to simulated values. By analyzing CPO-27(M) (M = Mg, Co, Ni), with identical pore dimensions, we observe distinct PALS data alterations based on metal ions. Paramagnetic Co and Ni ions hinder and quench positronium (Ps) formation, resulting in smaller measured pore volumes and sizes. Mg only quenches Ps, leading to underestimated pore sizes without volume distortion. This underscores the metal ions' pivotal role in PALS outcomes, urging caution in interpreting MOF porosity.

Unravelling the Water Adsorption Mechanism in Hierarchical MOFs: Insights from In Situ Positron Annihilation Lifetime Studies.

Atmospheric water harvesting with metal-organic frameworks (MOFs) is a new technology providing a clean, long-term water supply in arid areas. In-situ positron annihilation lifetime spectroscopy (PALS) is proposed as a valid methodology for the mechanistic understanding of water sorption in MOFs and the selection of prospective candidates for desired applications. DUT-67-Zr and DUT-67-Hf frameworks are used as model systems for method validation because of their hierarchical pore structure, high adsorption capacity, and chemical stability. Both frameworks are characterized using complementary techniques, such as nitrogen (77 K) and water vapor (298 K) physisorption, SEM, and PXRD. DUT-67-Zr and DUT-67-Hf are investigated by PALS upon exposure to humidity for the first time, demonstrating the stepwise pore filling mechanism by water molecules for both MOFs. In addition to exploring the potential of PALS as a tool for probing MOFs during in situ water loading, this work offers perspectives on the design and use of MOFs for water harvesting.

Spatiotemporal Design of the Metal-Organic Framework DUT-8(M).

Switchable metal-organic frameworks (MOFs) change their structure in time and selectively open their pores adsorbing guest molecules, leading to highly selective separation, pressure amplification, sensing, and actuation applications. The 3D engineering of MOFs has reached a high level of maturity, but spatiotemporal evolution opens a new perspective toward engineering materials in the 4th dimension (time) by t-axis design, in essence exploiting the deliberate tuning of activation barriers. This work demonstrates the first example in which an explicit temporal engineering of a switchable MOF (DUT-8, [M1 M2 (2,6-ndc)2 dabco]n , 2,6-ndc = 2,6-naphthalene dicarboxylate, dabco = 1,4diazabicyclo[2.2.2]octane, M1  = Ni, M2  = Co) is presented. The temporal response is deliberately tuned by variations in cobalt content. A spectrum of advanced analytical methods is presented for analyzing the switching kinetics stimulated by vapor adsorption using in situ time-resolved techniques ranging from ensemble adsorption and advanced synchrotron X-ray diffraction experiments to individual crystal analysis. A novel analysis technique based on microscopic observation of individual crystals in a microfluidic channel reveals the lowest limit for adsorption switching reported so far. Differences in the spatiotemporal response of crystal ensembles originate from an induction time that varies statistically and widens characteristically with increasing cobalt content reflecting increasing activation barriers.

Comparative Study of Nitro- and Azide-Functionalized ZnII -Based Coordination Polymers (CPs) as Fluorescent Turn-On Probes for Rapid and Selective Detection of H2 S in Living Cells.

Selective detection of H2 S in the cellular systems using fluorescent CPs/MOFs is of great scientific interest due to their outstanding aqueous stability, biocompatibility and real-time detection ability. Fabrication of such materials using complete biologically essential elements and applying them as an efficient biosensor is still quite challenging. In this context, two newly synthesized CPs containing biologically essential metal ion (Zn) and nitro/azido functional groups into the framework to sense extracellular and intracellular H2 S by reducing into respective amines are presented. The CP-1 containing the azide group acted as an efficient fluorescent turn-on probe with the lowest detection limit (7.2 µM) and shortest response time (30 s) among the Zn-based probes reported till date. Moreover, CP-1 exhibited green luminescence in live cells after imaging a very low concentration of H2 S, whereas the nitro analogue CP-2 could not detect the target analyte due to its framework disruption.

Pt(II)-Decorated Covalent Organic Framework for Photocatalytic Difluoroalkylation and Oxidative Cyclization Reactions.

A new covalent organic framework (COF) based on imine bonds was assembled from 2-(4-formylphenyl)-5-formylpyridine and 1,3,6,8-tetrakis(4-aminophenyl)pyrene, which showed an interesting dual-pore structure with high crystallinity. Postmetallation of the COF with Pt occurred selectively at the N donor (imine and pyridyl) in the larger pores. The metallated COF served as an excellent recyclable heterogeneous photocatalyst for decarboxylative difluoroalkylation and oxidative cyclization reactions.

Porous Li-MOF as a solid-state electrolyte: exploration of lithium ion conductivity through bio-inspired ionic channels.

A rarely porous Li-MOF (Li-AOIA) with surface area of 605 m2 g-1 was employed for the formation of an emerging class of solid-state lithium ion electrolytes. Infiltration of LiBF4 into Li-AOIA afforded Li-AOIA@BF4 with ionic conductivity of 1.09 × 10-5 S cm-1 at room temperature and an activation energy of 0.18 eV.

Isoreticular Expansion of Metal-Organic Frameworks via Pillaring of Metal Templated Tunable Building Layers: Hydrogen Storage and Selective CO2 Capture.

The deliberate construction of isoreticular eea-metal-organic frameworks (MOFs) (Cu-eea-1, Cu-eea-2 and Cu-eea-3) and rtl-MOFs (Co-rtl-1 and Co-rtl-2) has been accomplished based on the ligand-to-axial pillaring of supermolecular building layers. The use of different metal ions resulted in two types of supermolecular building layers (SBLs): Kagome (kgm) and square lattices (sql) which further interconnect to form anticipated 3D-MOFs. The isoreticular expansion of (3,6)-connected Cu-MOFs has been achieved with desired eea-topology based on kgm building layers. In addition, two (3,6)-connected Co-rtl-MOFs were also successfully constructed based on sql building layers. The Cu-eea-MOFs were shown to act as hydrogen storage materials with appreciable amount of hydrogen uptake abilities. Moreover Cu-eea-MOFs have also exhibited remarkable CO2 capture ability at ambient condition compared to nitrogen and methane, due to the presence of amide functionalities.

Porous Metal-Organic Polyhedral Framework containing Cuboctahedron Cages as SBUs with High Affinity for H2 and CO2 Sorption: A Heterogeneous Catalyst for Chemical Fixation of CO2.

Development of active porous materials that can efficiently adsorb H2 and CO2 is needed, due to their practical utilities. Here we present the design and synthesis of an interpenetrated CuII metal-organic framework (MOF) that is thermally stable, highly porous and can act as a heterogeneous catalyst. The CuII -MOF contains a highly symmetric polyhedral metal cluster (Cu24 ) with cuboctahedron geometry as secondary building unit (SBU). The double interpenetration of such huge cluster-containing nets provides a high density of open metal sites, due to which it exhibits remarkable H2 storage capacity (313 cm3 g-1 at 1 bar and 77 K) as well as high CO2 capture ability (159 cm3 g-1 at 1 bar and 273 K). Further, its propensity towards CO2 sorption can be utilized for the heterogeneous catalysis of the chemical conversion of CO2 into the corresponding cyclic carbonates upon reaction with epoxides, with high turnover number and turnover frequency values.

Co(II)-Doped Cd-MOF as an Efficient Water Oxidation Catalyst: Doubly Interpenetrated Boron Nitride Network with the Encapsulation of Free Ligand Containing Pyridine Moieties.

Development of an efficient and inexpensive water oxidation electrocatalyst using the earth-abundant elements is still far to go. Herein, a novel strategy has been demonstrated for developing the OER electrocatalyst by doping Co(II) in to a three-dimensional Cd-based MOF that contains a naked pyridine moieties in the form of uncoordinated ligand. Electrochemically active CoCd-MOF was resulted through the doping of Co(II) into the inactive Cd-MOF. CoCd-MOF exhibited very high catalytic activity in water oxidation reaction. An overpotential of 353 mV is required to produce an anodic current density of 1 mA/cm2 under alkaline conditions. Further, the CoCd-MOF exhibits remarkable recyclability over 1000 cycles.