The amalgamation of different disciplines is at the heart of reticular chemistry and has broadened the boundaries of chemistry by opening up an infinite space of chemical composition, structure, and material properties. Reticular design has enabled the precise prediction of crystalline framework structures, tunability of chemical composition, incorporation of various functionalities onto the framework backbone, and as a consequence, fine-tuning of metal-organic framework (MOF) and covalent organic framework (COF) properties beyond that of any other material class. Leveraging the unique properties of reticular materials has resulted in significant advances from both a fundamental and an applied perspective. Here, we wish to review the milestones in MOF and COF research and give a critical view on progress in their real-world applications. Finally, we briefly discuss the major challenges in the field that need to be addressed to pave the way for industrial applications.

The Current Status of MOF and COF Applications

The variety of functionalities and porous structures inherent to metal-organic frameworks (MOFs) together with the facile tunability of their properties makes these materials suitable for a wide range of existing and emerging applications. Many of these applications are based on processes involving interaction of MOFs with guest molecules. To optimize a certain process or successfully design a new one, a thorough knowledge is required about the physicochemical characteristics of materials and the mechanisms of their interaction with guest molecules. To obtain such important information, various complementary analytical techniques are applied, among which vibrational spectroscopy (IR and Raman) plays an important role and is indispensable in many cases. In this review, we critically examine the reported applications of IR and Raman spectroscopies as powerful tools for initial characterization of MOF materials and for studying processes of their interaction with various gases. Both the advantages and the limitations of the technique are considered, and the cases where IR or Raman spectroscopy is preferable are highlighted. Peculiarities of MOFs interaction with specific gases and some inconsistent band assignments are also emphasized. Summarizing the broad analytical possibilities of the IR and Raman spectroscopies, we conclude that it can be applied in combinations with other techniques to explicitly establish the structure, properties, and reactivity of MOFs.

Power of Infrared and Raman Spectroscopies to Characterize Metal-Organic Frameworks and Investigate Their Interaction with Guest Molecules

Microporous framework membranes such as metal-organic framework (MOF) membranes and covalent organic framework (COF) membranes are constructed by the controlled growth of small building blocks with large porosity and permanent well-defined micropore structures, which can overcome the ubiquitous tradeoff between membrane permeability and selectivity; they hold great promise for the enormous challenging separations in energy and environment fields. Therefore, microporous framework membranes are endowed with great expectations as next-generation membranes, and have evolved into a booming research field. Numerous novel membrane materials, versatile manipulation strategies of membrane structures, and fascinating applications have erupted in the last five years. First, this review summarizes and categorizes the microporous framework membranes with pore sizes lower than 2 nm based on their chemistry: inorganic microporous framework membranes, organic-inorganic microporous framework membranes, and organic microporous framework membranes, where the chemistry, fabrications, and differences among these membranes have been highlighted. Special attention is paid to the membrane structures and their corresponding modifications, including pore architecture, intercrystalline grain boundary, as well as their diverse control strategies. Then, the separation mechanisms of membranes are covered, such as diffusion-selectivity separation, adsorption-selectivity separation, and synergetic adsorption-diffusion-selectivity separation. Meanwhile, intricate membrane design to realize synergistic separation and some emerging mechanisms are highlighted. Finally, the applications of microporous framework membranes for precise gas separation, liquid molecule separation, and ion sieving are summarized. The remaining challenges and future perspectives in this field are discussed. This timely review may provide genuine guidance on the manipulation of membrane structures and inspire creative designs of novel membranes, promoting the sustainable development and steadily increasing prosperity of this field.

Microporous framework membranes for precise molecule/ion separations.

The luminescence response application in chemical sensing by metal–organic frameworks (MOFs) and their hybrid materials has been reported extensively in recent years. However, the reviews on this topic are mainly focused on the sensing materials (MOFs and their functionalized hybrid materials) and analytes (all types of species). In this comprehensive review, the luminescence response mode and chemical sensing mechanism for lanthanide-functionalized MOF hybrids (abbreviated as LnFMOFH) are systematically summarized. Specifically, ten sections are subdivided, and importantly involve three main topics. Firstly, LnFMOFH and the luminescence responsive chemical sensing for these hybrids are introduced. Secondly, three single luminescence response modes, including “Turn-Off”, “Turn-On” “Turn–Off–On” and dual mode, as ratiometric luminescence (RL) for chemical sensing in LnFMOFH materials are summarized. Thirdly, some types of chemical sensing mechanisms observed in LnFMOFH materials are outlined, which involve the ligand–metal energy transfer (LMET) for the luminescence response in chemical sensing of enhanced or weakened analytes; the fluorescence (Forster) resonance energy transfer (FRET) and photo-induced electronic transfer (PET) mechanism depending on the energy match between the probe and analyte; and some special chemical interactions for their luminescence response upon chemical sensing, such as hydrogen bonding interaction, coordination interaction and other reactions. Finally, the conclusion and prospects are given for this topic. It needs to be noted that pure lanthanide MOFs (lanthanide ions as the framework metal, including mixed lanthanide MOFs) are not the topic of this review and are only mentioned for background knowledge. Herein, LnFMOFH materials in which the lanthanide ions behave only as photoactive species to functionalize MOF units via post-synthetic modification (PSM) by researchers are the emphasis of this review.

Luminescence response mode and chemical sensing mechanism for lanthanide-functionalized metal–organic framework hybrids

Porous materials such as MOFs are interesting candidates for gas separation and storage. An important parameter to gain deeper insights to the adsorption process of an adsorptive on an adsorbent is the isosteric enthalpy of adsorption, ΔHads which is defined as the heat to be released/required when an adsorptive binds to/detaches from the solid surface of an adsorbent. Two or three adsorption isotherms at different but close temperatures with ΔT ≤ 20 K for two and ΔT ≈ 10 K for three isotherms are the basis to derive the isosteric enthalpy of adsorption through the Clausius-Clapeyron approach or the virial analysis. This Perspective presents the procedure of the common (dual-site) Freundlich-Langmuir fit/Clausius-Clapeyron approach and the virial fit of the isotherms with usable Excel sheets and Origin files for the subsequent derivation of ΔHads. Exemplary adsorption isotherms of CO2, SO2 and H2 at two temperatures on MOFs are analyzed. The detailed computational description and comparison of the Clausius-Clapeyron approach and the virial analysis to determine ΔHads outlines the limitations of the two methods with respect to the available experimental data, especially at low pressure/low uptake values. It is emphasized that no extrapolation beyond the experimental data range should be done. The quality of the important and underlying isotherm fits must be checked and ensured with logarithmic-scale n/p isotherm plots for the (dual-site) Freundlich-Langmuir fit in the low-pressure region and through low standard deviations for the coefficients in the virial analysis.

A practical guide to calculate the isosteric heat/enthalpy of adsorption via adsorption isotherms in metal-organic frameworks, MOFs.

Metal-organic frameworks (MOFs) can contain open metal sites (OMS) or coordinatively unsaturated sites (CUS) or open coordination sites (OCS) when vacant Lewis acid sites on the metal ions or cluster nodes have been generated. This review combines for the first time all aspects of OMS in MOFs, starting from different preparation strategies over theoretical studies on the effects of OMS with host-guest interactions up to distinct OMS-MOF applications. In the experimental part the focus of this review is on MOFs with proven OMS formation which are not only invoked but are clearly verified by analytical methods.

Coordinatively unsaturated metal sites (open metal sites) in metal–organic frameworks: design and applications

New acetylenedicarboxylate (ADC) and chlorofumarate (Fum-Cl) based hafnium-metal–organic frameworks have been synthesized by alternatively reacting acetylenedicarboxylic acid in DMF or water with appropriate hafnium salt, in the presence of acetic acid modulator. The two materials of respective ideal formulas [Hf6O4(OH)4(ADC)6] (Hf-HHU-1) and [Hf6O4(OH)4(Fum-Cl)6] (Hf-HHU-2) have been structurally characterized by powder X-ray diffraction to be UiO-66 isostructural, consisting of octahedral [Hf6O4(OH)4]12+ secondary building units each connected to other units by 12 ADC or Fum-Cl linkers into a microporous network with fcu topology. This structure was confirmed by Rietveld refinement. Hf-HHU-2 is formed by in situ hydrochlorination of acetylenedicarboxylic acid to chlorofumarate. Its presence has been determined by combined Raman spectroscopy, solid-state NMR, scanning electron microscopy, energy dispersive X-ray and X-ray photoelectron spectroscopies. Hf-HHU-1 and Hf-HHU-2 exhibit very high hydrophilicit...

Acetylenedicarboxylate and In Situ Generated Chlorofumarate-Based Hafnium(IV)–Metal–Organic Frameworks: Synthesis, Structure, and Sorption Properties

The report is the first broader evaluation of the gas sorption properties of CAU-23 for the adsorptives CO2, H2, CH4, and SO2. CAU-23 is of intermediate porosity among Al-MOFs with specific BET surface areas of the order of MIL-100>MIL-53>CAU-23>MIL-160>MIL-53-TDC>Alfum>CAU-10-H and total pore volumes of the order of MIL-100>MIL-53>CAU-23>Alfum=MIL-160>MIL-53-TDC>CAU-10-H. CO2 uptake (3.97 mmol g−1, 293 K) and H2 uptake (10.25 mmol g−1, 77 K) of CAU-23 are second in the series and only slightly smaller than for MIL-160. The CH4 uptake of CAU-23 (0.89 mmol g−1, 293 K) is unremarkable in comparison with the other Al-MOFs. The SO2 uptake (8.4 mmol g−1, 293 K) follows the porosity and higher SO2 uptakes were only observed for MIL-53 and MIL-100. CAU-23 is one of the best Al-MOFs for high-pressure sorption of CO2, with an uptake of 33 wt.-% at 20 bar, 293 K. Gas sorption measurements at two different temperatures gave near zero-coverage enthalpy of adsorptions, ▵Hads0 for CO2 of −22 kJ mol−1 and of SO2 for −38 kJ mol−1 which is at the low end of the other Al-MOFs (−22 to −39 kJ mol−1 for CO2; −41 to −51 kJ mol−1 for SO2), yet ▵Hads increases for CAU-23 with CO2 and SO2 to −25 and −57 kJ mol−1, respectively. For CO2/CH4 and SO2/CO2 separation, ideal adsorbed solution theory (IAST) predicted gas selectivities of 5 and 27–50 (depending on molar ratio and model), respectively, in line with 4.5–6.3 and 17–50, respectively, with most of the other Al-MOFs, where only MIL-53-TDC with 83 and MIL-160 with 126 gave a higher SO2/CO2 selectivity at a molar ratio of 0.5. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/zaac.202200170

Unravelling gas sorption in the aluminum metal‐organic framework CAU‐23: CO2, H2, CH4, SO2 sorption isotherms, enthalpy of adsorption and mixed‐adsorptive calculations

Two‐dimensional Cobalt‐Carboxylate Framework with Hourglass Trinuclear Co3(COO)6(DMA)3 Secondary Building Unit

Two novel isostructural phosphonate-monoethylcarboxylate MOFs with the structural formula of [M2(EtBCP)2(DABCO)0.5]·2DMA (M = Zn (2), Co (3); H2EtBCP = O-ethyl-P-(4-carboxyphenyl)phosphonic acid, DABCO = 1,4-diazabicyclo[2.2.2]octane, DMA = N,N-dimethylacetamide) were synthesized and characterized. The frameworks of 2 and 3 are sustained by {Zn2(PO2(OEt))2}n metal-phosphonate- and DABCO-extended {Zn2(COO)4(DABCO)}n paddle-wheel carboxylate chain-SBUs. The chains providing connectivity in three, mutually orthogonal directions are running parallel and are combined in a framework, which could be interpreted as having a pcu topology. The simple structure-organization principle, which suggests the possibility of the elongation of the bifunctional ligand with scaling in two directions, allows to view the structures of 2 and 3 as prototypes for an isoreticular series. The porosity of both compounds, based on a relatively short ligand, is low: no adsorption of N2 was registered, however, CO2 is adsorbed readily allowing to estimate the surface area at ∼330 m2 g−1 (∼900–1060 m2 g−1 geometric estimate). The compounds demonstrate a two-step CO2 adsorption isotherm both at 195 K (0–1 bar) and 298 K (0–20 bar). The adsorption isotherms are characterized by a gradual (type “F-I”), albeit still relatively steep onset of the second step, associated with structural flexibility/bistability. The estimated pore volumes at the start of the transformation (195 K) for 2 and 3 are ∼0.11 (0.08P/P0) and 0.12 cm3 g−1 (0.12P/P0) respectively, which corresponds considerably well to the geometrically calculated accessible volume of ∼0.07 cm3 g−1 for the experimental structure (3.3 A probe diameter). The structural prerequisites of the observed flexibility of the framework, which might be associated with the non-planarity of the metal-phosphonate moieties, acting as ‘levers’ for propagating mechanical stress, are discussed.

Flexible bifunctional monoethylphosphonate/carboxylates of Zn(II) and Co(II) reinforced with DABCO co-ligand: paradigmatic structural organization with pcu topology

Anna Goldman

Papers

Coordinatively unsaturated metal sites (open metal sites) in metal–organic frameworks: design and applications

Acetylenedicarboxylate and In Situ Generated Chlorofumarate-Based Hafnium(IV)–Metal–Organic Frameworks: Synthesis, Structure, and Sorption Properties

Unravelling gas sorption in the aluminum metal‐organic framework CAU‐23: CO2, H2, CH4, SO2 sorption isotherms, enthalpy of adsorption and mixed‐adsorptive calculations

Two‐dimensional Cobalt‐Carboxylate Framework with Hourglass Trinuclear Co3(COO)6(DMA)3 Secondary Building Unit

Flexible bifunctional monoethylphosphonate/carboxylates of Zn(II) and Co(II) reinforced with DABCO co-ligand: paradigmatic structural organization with pcu topology