Tetracarboxylate Linker-Based Flexible CuII Frameworks: Efficient Separation of CO2 from CO2/N2 and C2H2 from C2H2/C2H4 Mixtures

Tailoring a robust Al-MOF for trapping C2H6 and C2H2 towards efficient C2H4 purification from quaternary mixtures

Light hydrocarbon separation is considered one of the most industrially challenging and desired chemical separation processes and is highly essential in polymer and chemical industries. Among them, separating ethylene (C₂H₄) from C2 hydrocarbon mixtures such as ethane (C₂H₆), acetylene (C₂H₂), and other natural gas elements (CO₂, CH₄) is of paramount importance and poses significant difficulty. We demonstrate such separations using an Al-MOF synthesised earlier as a non-porous material, but herein endowed with hierarchical porosity created under microwave conditions in an equimolar water/ethanol solution. The material possessing a large surface area (793 m² g⁻¹) exhibits an excellent uptake capacity for major industrial hydrocarbons in the order of C₂H₂ > C₂H₆ > CO₂ > C₂H₄ > CH₄ under ambient conditions. It shows an outstanding dynamic breakthrough separation of ethylene (C₂H₄) not only for a binary mixture (C₂H₆/C₂H₄) but also for a quaternary combination (C₂H₄/C₂H₆/C₂H₂/CO₂ and C₂H₄/C₂H₆/C₂H₂/CH₄) of varying concentrations. The detailed separation/purification mechanism was unveiled by gas adsorption isotherms, mixed-gas adsorption calculations, selectivity estimations, advanced computer simulations such as density functional theory (DFT), grand canonical Monte Carlo (GCMC) and ab initio molecular dynamics (AIMD), and stepwise multicomponent dynamic breakthrough experiments.

Industrially important C₂H₄ purification from multi-component hydrocarbon mixtures.

Two-dimensional MOFs: Design & Synthesis and Applications

For the past few years, two-dimensional materials have attracted widespread attention owing to their special properties and potential applications. It is well-known that graphene, transition metal disulfide compounds (TMDC), carbon nitride, transition metal carbonitrides (Mxenes), silene and hexagonal boron nitride are typical two-dimensional materials. Compared with these traditional two-dimensional materials, two-dimensional MOF is favored by numerous researchers because of its unique structure. Based on the unique metal ion and organic ligand coordination of MOF and two-dimensional layered structure, the applications of two-dimensional MOF were getting serious, including catalysis, supercapacitor, gas adsorption/separation, sensors and so on. This review presents a relatively comprehensive summary of the design & synthesis and applications of two-dimensional MOF over the past few years. Furthermore, the opportunities and challenges have been discussed to supply a promising prospect to this field.

Deciphering the Weak CO2···Framework Interactions in Microporous MOFs Functionalized with Strong Adsorption Sites-A Ubiquitous Observation

Carbon capture from industrial effluents such as flue gas or natural gas mixture (cf. landfill gas), the primary sources of CO₂ emission, greatly aids in balancing the environmental carbon cycle. In this context, the most energy-efficient physisorptive CO₂ separation process can benefit immensely from improved porous sorbents. Metal organic frameworks (MOFs), especially the ultramicroporous MOFs, built from readily available small and rigid ligands, are highly promising because of their high selectivity (CO₂/N₂) and easy scalability. Here, we report two new ultramicroporous Co-adeninato isophthalate MOFs. They concomitantly carry basic functional groups (-NH₂) and Lewis acidic sites (coordinatively unsaturated Co centers). They show good CO₂ capacity (3.3 mmol/g at 303 K and 1 bar) along with high CO₂/N₂ (∼600 at 313 K and 1 bar and ∼340 at 303 K and 1 bar) selectivity, working capacity, and smooth diffusion kinetics (D_c = 7.5 × 10^-9 m² s^-1). The MOFs exhibit good CO₂/N₂ kinetic separation under both dry and wet conditions with a smooth breakthrough profile. Despite their well-defined CO₂ adsorption sites, these MOFs exhibit only a moderately strong interaction with CO₂ as evidenced from their HOA values. This counterintuitive observation is ubiquitous among many MOFs adorned with strong CO₂ adsorption sites. To gain insights, we have identified the binding sites for CO₂ using simulation and MD studies. The radial distribution function analysis reveals that despite the amine and bare-metal sites, the pore size and the pore structure determine the positions for the CO₂ molecules. The most favorable sites become the confined spaces lined by aromatic rings. A plausible explanation for the lack of strong adsorption in these MOFs is premised from these collective studies, which could aid in the future design of superior CO₂ sorbents.

Modulation of CO2 adsorption in novel pillar-layered MOFs based on carboxylate-pyrazole flexible linker

Metal-organic frameworks (MOFs) have attracted significant attention as sorbents due to their high surface area, tunable pore volume and pore size, coordinatively unsaturated metal sites, and ability to install desired functional groups by post-synthetic modification. Herein, we report three new MOFs with pillar-paddlewheel structures that have been synthesized solvothermally from the mixture of the carboxylate-pyrazole flexible linker (H₂L), 4,4-bipyridine (BPY)/triethylenediamine (DABCO), and Zn(ii)/Cu(ii) ions. The MOFs obtained, namely [Zn^II(L)BPY], [Cu^II(L)BPY], and [Cu^II(L)DABCO], exhibit two-fold interpenetration and dinuclear paddle-wheel nodes. The Zn(ii)/Cu(ii) cations are coordinated by two equatorial L linkers that result in two-dimensional sheets which in turn are pillared by BPY or DABCO in the perpendicular direction to obtain a neutral three-dimensional framework that shows one-dimensional square channels. The three pillar-layered MOFs were characterized as microporous materials showing high crystalline stability after activation at 120 °C and CO₂ adsorption. All MOFs contain uncoordinated Lewis basic pyrazole nitrogen atoms in the framework which have an affinity toward CO₂ and hence could potentially serve as CO₂ adsorption material. The CO₂ uptake capacity was initially enhanced by replacing Zn with Cu and then replacing the pillar, going from BPY to DABCO. Overall, all the MOFs exhibit low isosteric heat (Q_st) of adsorption which signifies an advantage due to the energy required for the adsorption and regeneration processes.

Molecular motion in the nanospace of MOFs upon gas adsorption investigated by in situ Raman spectroscopy

Molecular motions taking place in the nanospace of metal-organic frameworks (MOFs) are an interesting research subject, although not yet fully investigated. In this work, we utilized in situ Raman spectroscopy in the ultralow-frequency region to investigate the libration motion (including the rotational motion of phenylene rings) of MOFs, in particular [Cu₂(bdc)₂(dabco)] (Cu-JAST-1), where bdc = 1,4-benzenedicarboxylate and dabco = 1,4-diazabicyclo[2.2.2]octane. The libration mode of Cu-JAST-1 was found to be significantly suppressed by the adsorption of various guest molecules, such as CO₂, Ar, and N₂. In addition, an appreciable correlation between the libration mode and adsorption equilibrium time was identified, which provides useful novel tools in the design of MOFs acting as molecular adsorption and separation materials.

Modulating Hierarchical Micro/Mesoporosity by a Mixed Solvent Approach in Al-MOF: Stabilization of MAPbBr3 Quantum Dots

Various hierarchical micro/mesoporous MOFs based on {[Al(μ-OH)(1,4-NDC)]⋅H₂ O} (MOF1) with tunable porosities (pore volume and surface area) have been synthesized by assembling Al^III and 1,4-NDC (1,4-naphthalenedicarboxylate) under microwave irradiation by varying water/ethanol solvent ratio. Water/ethanol mixture has played a crucial role in the mesopore generation in MOF1M₂₅ , MOF1M₅₀ , and MOF1M₇₅ , which is achieved by in situ formation of water/ethanol clusters. By adjusting the ratio of water/ethanol, the particle size, surface area and micro/mesopore volume fraction of the MOFs are controlled. Furthermore, reaction time plays a critical role in mesopore formation as realized by varying reaction time for the MOF with 50 % ethanol (MOF1M₅₀ ). Additionally, hierarchical MOF (MOF1M₅₀ ) has been used as a template for the stabilization of MAPbBr₃ (MA=methylammonium) perovskite quantum dots (PQDs). MAPbBr₃ PQDs are grown inside MOF1M₅₀ , where mesopores control the size of PQDs which leads to quantum confinement.

A highly robust heterometallic TbIII/NiII-organic framework for C2 hydrocarbon separation and capture

The exquisite combination of hetreometallic [TbIIINiII(COO)3(H2O)] clusters with a designed hexatopic ligand generates one highly robust three-dimensional heterometallic TbIII/NiII-organic framework material {[TbNi(HTDP)(H2O)]·3DMF·2H2O}n (NUC-2; H6TDP = 2,4,6-tri(2,4-dicarboxyphenyl)pyridine). Gas adsorption measurements reveal that the activated NUC-2 exhibits ultrahigh C2H2 adsorption capacity but negligible uptake of C2H4, which is ascribed to its high stability and ultrahigh functional internal surface. Furthermore, dynamic breakthrough experiments confirm that NUC-2 can effectively fulfill the separation of a C2H2/C2H4 (1 : 99) mixture. Such excellent performance makes NUC-2 a promising adsorbent for practical C2H2/C2H4 separation and provides new insights into heterometallic MOFs for gas separation.

A new honeycomb metal–carboxylate-tetrazolate framework with multiple functions for CO2 conversion and selective capture of C2H2, CO2 and benzene

A stable Cd-MOF was built by a carboxylate-tetrazolate ligand, which contains hexagonal channels and reveals multiple functions including separation of CO₂/CH₄, C₂H₂/CO₂, C₂H₂/CH₄ and benzene/cyclohexane, and catalytic conversion of CO₂ with epoxides.

Cobalt(ii) and zinc(ii)-coordination polymers constructed from ether-linked tetracarboxylic acid and isomeric bis(imidazole) linkers: luminescence-based Fe(iii) detection in aqueous media

Four Co(ii) and Zn(ii)-coordination polymers were synthesized and characterized. Compound 3 exhibited sensitive and selective detection towards Fe³⁺ ions in the presence of other competing agents with a detection limit of 3.23 ppm.

Highly selective C2H2 and CO2 capture and magnetic properties of a robust Co-chain based metal–organic framework

A robust Co-MOF based on uncommon 1D alternate Co₄ chain units was synthesized, which efficiently takes up C₂H₂ and CO₂ with significant selectivity for C₂H₂ and CO₂ over CH₄, as well as shows a slow freezing process of magnetic behavior.

Computational Exploration of IRMOFs for Xenon Separation from Air

Metal-organic frameworks (MOFs) found their well-deserved position in the field of gas adsorption and separation because of their unique properties. The separation of xenon from different gas mixtures containing this valuable and essential noble gas is also benefited from the exciting nature of MOFs. In this research, we chose a series of isoreticular MOFs as our study models to apply advanced molecular simulation techniques in the context of xenon separation from air. We investigated the separation performance of our model set through simulation of ternary gas adsorption isotherms and consequent calculation of separation performance descriptors, finding out that IRMOF-7 shows better recovering capabilities compared to the other studied MOFs. We benefited from visualization of xenon energy landscape within MOFs to obtain valuable information on possible reasoning behind our observations. We also examined temperature-based separation performance boosting strategy. Additionally, we noted that although promising candidates are present among the studied MOFs for xenon recovery from air, they are not suitable for xenon recovery from exhaled anesthetic gas mixture.