Modulating the cavity micro-environments of Fe-MOFs for high-performance gas chromatographic separations

Precise modulation of cavity micro-environment in metal-organic frameworks (MOFs) is important for achieving significant separation performance. Herein, the Fe-MOF (MIL-142-BTB-BDC) with different tridentate and bidentate ligands to form cavities, was chosen as the platform to precisely modulate the cavity micro-environment and investigate the influence of cavity windows and cavity walls on gas chromatographic separations. Changing tridentate ligands on the cavity walls led to MIL-142-TATB-BDC while changing bidentate ligands on the cavity windows produced MIL-142-BTB-BDC-NH2. Mechanism investigation revealed that for MIL-142-BTB-BDC and MIL-142-TATB-BDC, altering the ligands of cavity walls had little influence on the thermodynamic interactions between MOFs and analytes while slightly reducing the kinetic diffusion rate of analytes. On the contrary, introducing amino groups on cavity windows in MIL-142-BTB-BDC-NH2 not only increased the thermodynamic interactions with analytes but also slowed down the kinetic diffusion of analytes, which resulted in poor separation performance of the MIL-142-BTB-BDC-NH2 coated column. This work provides a guide to precisely modulating the cavity micro-environment and analyzing the relationship between the cavity micro-environment and application properties of MOFs.

Merging and Clipping Nets for the Synthesis of Three- and Two-Merged Net Metal-Organic Frameworks

Herein, we report how merging and clipping nets in metal-organic frameworks (MOFs) can be controlled in a single-crystal-to-single-crystal fashion using three different approaches─the merged net, clip-off chemistry, and linker reinstallation─to design and synthesize three- and two-merged net MOFs. Initially, we show the formation of three isoreticular three-merged net MOFs by linking a trimeric Sc3+ cluster, Sc3(μ3-Ο)(-COO)6, with ditopic zigzag and tritopic linkers. The resulting MOFs exhibit three-merged edge-transitive nets─kgd + hxl + pcu─for the first time. Then, using these three-merged net MOFs as precursors, we selectively remove one of these subnets, the hxl net, via clip-off chemistry to form two-merged net MOFs. This process involves the cleavage of olefinic groups via ozonolysis, providing the resulting two-merged net MOFs with free carboxylic acid groups that can be used to tune their sorption properties such as the removal of cationic organic pollutants. Finally, we use the linker reinstallation approach to convert the two-merged net MOFs back to the three-merged net MOFs. This approach allows for the postsynthetic addition of the previously removed hxl merged net, enabling recovery of the initial three-merged net MOFs or synthesis of new ones using novel ditopic zigzag linkers.

Rational Design of Improved Ru Containing Fe-Based Metal-Organic Framework (MOF) Photoanode for Artificial Photosynthesis

Metal-Organic Frameworks (MOFs) recently emerged as a new platform for the realization of integrated devices for artificial photosynthesis. However, there remain few demonstrations of rational tuning of such devices for improved performance. Here, a fast molecular water oxidation catalyst working via water nucleophilic attack is integrated into the MOF MIL-142, wherein Fe3O nodes absorb visible light, leading to charge separation. Materials are characterized by a range of structural and spectroscopic techniques. New, [Ru(tpy)(Qc)(H2O)]+ (tpy = 2,2':6',2″-terpyridine and Qc = 8-quinolinecarboxylate)-doped Fe MIL-142 achieved a high photocurrent (1.6 × 10-3 A·cm-2) in photo-electrocatalytic water splitting at pH = 1. Unassisted photocatalytic H2 evolution is also reported with Pt as the co-catalyst (4.8 µmol g-1 min-1). The high activity of this new system enables hydrogen gas capture from an easy-to-manufacture, scaled-up prototype utilizing MOF deposited on FTO glass as a photoanode. These findings provide insights for the development of MOF-based light-driven water-splitting assemblies utilizing a minimal amount of precious metals and Fe-based photosensitizers.

Multicomponent Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are constructed from metal ions or clusters and organic linkers. Typical MOFs are rather simple, comprising just one type of joint and linker. An additional degree of structural complexity can be introduced by using multiple different components that are assembled into the same framework In the early days of MOF chemistry, conventional wisdom held that attempting to prepare frameworks starting from such a broad set of components would lead to multiple different phases. However, this review highlights how this view was mistaken and frameworks comprising multiple different components can be deliberately designed and synthesized. When coupled to structural order and periodicity, the presence of multiple components leads to exceptional functional properties that can be understood at the atomic level.

Isoreticular Contraction of Metal-Organic Frameworks Induced by Cleavage of Covalent Bonds

Isoreticular chemistry, in which the organic or inorganic moieties of reticular materials can be replaced without destroying their underlying nets, is a key concept for synthesizing new porous molecular materials and for tuning or functionalization of their pores. Here, we report that the rational cleavage of covalent bonds in a metal-organic framework (MOF) can trigger their isoreticular contraction, without the need for any additional organic linkers. We began by synthesizing two novel MOFs based on the MIL-142 family, (In)BCN-20B and (Sc)BCN-20C, which include cleavable as well as noncleavable organic linkers. Next, we selectively and quantitatively broke their cleavable linkers, demonstrating that various dynamic chemical and structural processes occur within these structures to drive the formation of isoreticular contracted MOFs. Thus, the contraction involves breaking of a covalent bond, subsequent breaking of a coordination bond, and finally, formation of a new coordination bond supported by structural behavior. Remarkably, given that the single-crystal character of the parent MOF is retained throughout the entire transformation, we were able to monitor the contraction by single-crystal X-ray diffraction.

Controlling the Flexibility of MIL-88A(Sc) Through Synthetic Optimisation and Postsynthetic Halogenation

Breathing behaviour in metal-organic frameworks (MOFs), the distinctive transformation between a porous phase and a less (or non) porous phase, often controls the uptake of guest molecules, endowing flexible MOFs with highly selective gas adsorptive properties. In highly flexible topologies, breathing can be tuned by linker modification, which is typically achieved pre-synthetically using functionalised linkers. Herein, it was shown that MIL-88A(Sc) exhibits the characteristic flexibility of its topology, which can be tuned by 1) modifying synthetic conditions to yield a formate-buttressed analogue that is rigid and porous; and 2) postsynthetic bromination across the alkene functionality of the fumarate ligand, generating a product that is rigid but non-porous. In addition to providing different methodologies for tuning the flexibility and breathing behaviour of this archetypal MOF, it was shown that bromination of the formate-bridged analogue results in an identical material, representing a rare example of two different MOFs being postsynthetically converted to the same end product.

Metal-organic framework structures of fused hexagonal motifs with cuprophilic interactions of a triangular Cu(I)3(pyrazolate-benzoate) metallo-linker

The reaction of the N,O-heteroditopic bifunctional ligand 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid (H2mpba) with Cu(NO3)2·2.5H2O and Zn(NO3)2·4H2O or Zn(CH3COO)2·2H2O in N,N-dimethylformamide (DMF) results in concomitant formation of three bimetallic metal-organic frameworks (MOFs) with...

Sc(iii)-Based metal–organic frameworks

In the universe of MOFs, their construction with Sc(iii) is rather limited. This highlight shows the exciting chronological development of Sc(iii)-MOFs which have afforded promising applications due to their exceptional chemical stability.

Design of Metal-Organic Polymers MIL-53(M3+): Preparation and Characterization of MIL-53(Fe) and Graphene Oxide Composite

This article presents a crystal chemical analysis, generalization, and systematization of structural characteristics of metal-organic polymers MIL-53(M3+) with M = Al, Cr, Ga, and Fe. The division of the MIL-53(M3+) structures into a morphotropic series was performed, which made it possible to predict the formation of new compounds or solid solutions with the corresponding composition and structure. The change in the symmetry of MIL-53(M3+) and the causes of polymorphs formation are explained on the basis of crystal chemical rules. The efficiency of the revealed regularities in the structural characteristics of the MIL-53(M3+) phases were experimentally confirmed for MIL-53(Fe) and composite MIL-53(Fe)/GO (GO-graphene oxide) by several methods (powder X-ray, X-ray absorption, and photoelectron spectroscopy). For the first time, different coordination numbers (CN) (CNFe = 4.9 for MIL-53(Fe)—two types of coordination polyhedra with CNFe = 6 and CNFe = 4; CNFe = 4 for MIL-53 (Fe3+)/GO) and the formal charges (FC) of iron ions (variable FC of Fe (2+δ)+ in MIL-53(Fe3+) and Fe2+ in MIL-53(Fe3+)/GO) were found. These experimental data explain the higher photocatalytic activity of MIL-53(Fe3+)/GO in photo-Fenton reactions—RR195 decomposition.