Structures and H2 Adsorption Properties of Porous Scandium Metal-Organic Frameworks

A Multimetal Approach for the Reticulation of Iridium into Metal-Organic Framework Building Units

Noble metal elements are ubiquitous in our everyday life, from medical applications to electronic devices and synthetic chemistry. Iridium is one of the least abundant elements, and despite its scarcity, it remains essential for efficient and active catalytic processes. Consequently, the development of heterogeneous catalysts with the presence of active iridium sites is of enormous interest as it leads to the improvement of their recyclability and reusability. Here, we demonstrate a strategy to incorporate iridium atoms into metal-organic frameworks (MOFs), as part of their secondary building units (SBUs), resulting in robust and reusable materials with heterogeneous photocatalytic activity.

Expanding the Reticular Chemistry Building Block Library toward Highly Connected Nets: Ultraporous MOFs Based on 18-Connected Ternary, Trigonal Prismatic Superpolyhedra

The chemistry of metal-organic frameworks (MOFs) continues to expand rapidly, providing materials with diverse structures and properties. The reticular chemistry approach, where well-defined structural building blocks are combined together to form crystalline open framework solids, has greatly accelerated the discovery of new and important materials. However, its full potential toward the rational design of MOFs relies on the availability of highly connected building blocks because these greatly reduce the number of possible structures. Toward this, building blocks with connectivity greater than 12 are highly desirable but extremely rare. We report here the discovery of novel 18-connected, trigonal prismatic, ternary building blocks (tbb's) and their assembly into unique MOFs, denoted as Fe-tbb-MOF-x (x: 1, 2, 3), with hierarchical micro- and mesoporosity. The remarkable tbb is an 18-c supertrigonal prism, with three points of extension at each corner, consisting of triangular (3-c) and rectangular (4-c) carboxylate-based organic linkers and trigonal prismatic [Fe3(μ3-Ο)(-COO)6]+ clusters. The tbb's are linked together by an 18-c cluster made of 4-c ligands and a crystallographically distinct Fe3(μ3-Ο) trimer, forming overall a 3-D (3,4,4,6,6)-c five nodal net. The hierarchical, highly porous nature of Fe-tbb-MOF-x (x: 1, 2, 3) was confirmed by recording detailed sorption isotherms of Ar, CH4, and CO2 at 87, 112, and 195 K, respectively, revealing an ultrahigh BET area (4263-4847 m2 g-1) and pore volume (1.95-2.29 cm3 g-1). Because of the observed ultrahigh porosities, the H2 and CH4 storage properties of Fe-tbb-MOF-x were investigated, revealing well-balanced high gravimetric and volumetric deliverable capacities for cryoadsorptive H2 storage (11.6 wt %/41.4 g L-1, 77 K/100 bar-160 K/5 bar), as well as CH4 storage at near ambient temperatures (367 mg g-1/160 cm3 STP cm-3, 5-100 bar at 298 K), placing these materials among the top performing MOFs. The present work opens new directions to apply reticular chemistry for the construction of novel MOFs with tunable porosities based on contracted or expanded tbb analogues.

Sacrificial-Hydroxamate-Enabled Sizable Crystallization of Scandium Carboxylate Metal-Organic Frameworks

Starting from labile hydroxamic acid ligands that are strong chelators, here, we implemented a sacrificial modulating strategy to prepare a series of scandium carboxylate metal-organic frameworks. Overcoming conventional syntheses that use excessive carboxylate modulators, the present strategy greatly reduces the organics required and produces large single crystals of several Sc-MOFs for X-ray crystallography.

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.

Synthesis, supramolecular isomerism, and photoluminescence of scandium(III) complexes with tetrafluoroterephthalate ligand

Here we present a new family of seven Sc(III) complexes with tetrafluoroterephthalate ligand (tFBDC2−), having non-polymeric and polymeric 2D and 3D structures. These complexes are characterized by SC XRD, PXRD,...

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.

Isoreticular chemistry of scandium analogues of the multicomponent metal–organic framework MIL-142

MIL-142(Sc) is prepared and the limits of the isoreticular substitution of each linker type are explored and characterised by single-crystal XRD.

Two scandium-based coordination polymers: rapid ultrasound-assisted synthesis, crystal transformation, and catalytic properties

Assisted by ultrasound waves, a Sc-based coordination polymer CP 1 was synthesized successfully. With 1 as the precursor, another stable CP 2 can be obtained by single-crystal to single-crystal transformation and 2 exhibited good catalytic activities.

Effect of Larger Pore Size on the Sorption Properties of Isoreticular Metal-Organic Frameworks with High Number of Open Metal Sites

Four isostructural CPO-54-M metal-organic frameworks based on the larger organic linker 1,5-dihydroxynaphthalene-2,6-dicarboxylic acid and divalent cations (M=Mn, Mg, Ni, Co) are shown to be isoreticular to the CPO-27 (MOF-74) materials. Desolvated CPO-54-Mn contains a very high concentration of open metal sites, which has a pronounced effect on the gas adsorption of N2 , H2 , CO2 and CO. Initial isosteric heats of adsorption are significantly higher than for MOFs without open metal sites and are slightly higher than for CPO-27. The plateau of high heat of adsorption decreases earlier in CPO-54-Mn as a function of loading per mole than in CPO-27-Mn. Cluster and periodic density functional theory based calculations of the adsorbate structures and energetics show that the larger adsorption energy at low loadings, when only open metal sites are occupied, is mainly due to larger contribution of dispersive interactions for the materials with the larger, more electron rich bridging ligand.

The thermally induced decarboxylation mechanism of a mixed-oxidation state carboxylate-based iron metal-organic framework

Investigations into a thermally generated decarboxylation mechanism for metal site activation and the generation of mesopores in a carboxylate iron-based MOF, PCN-250, have been conducted. PCN-250 exhibits an interesting oxidation state change during thermal treatment under inert atmospheres or vacuum conditions, transitioning from an Fe(iii)₃ cluster to a Fe(ii)Fe(iii)₂ cluster. To probe this redox event and discern a mechanism of activation, a combination of thermogravimetric analysis, gas sorption, scanning electron microscopy, ⁵⁷Fe Mössbauer spectroscopy, gas chromatography-mass spectrometry, and X-ray diffraction studies were conducted. The results suggest that the iron-site activation occurs due to ligand decarboxylation above 200 °C. This is also consistent with the generation of a missing cluster mesoporous defect in the framework. The resulting mesoporous PCN-250 maintains high thermal stability, preserving crystallinity after multiple consecutive high-temperature regeneration cycles. Additionally, the thermally reduced PCN-250 shows improvements in the total uptake capacity of methane and CO₂.

Synthesis and Characterization of an SWCNT@HKUST-1 Composite: Enhancing the CO2 Adsorption Properties of HKUST-1

Synthesis of a new HKUST-1 composite based on single-walled carbon nanotubes (SWCNTs) was successfully achieved (SWCNT@HKUST-1). SWCNTs were used as templates to grow rod-like HKUST-1 crystals over the surface of the nanotubes. N₂ adsorption properties showed an increment on the surface area and pore volume for the SWCNT@HKUST-1 composite. Furthermore, the CO₂ capture increased, from 7.92 to 8.75 mmol g^-1 at 196 K up to 100 kPa, for the SWCNT@HKUST-1 composite. This enhancement was directly associated with the increase of the surface area of the composite. Additionally, an increase in the CO₂ heat of adsorption was estimated, from 30 to 39.1 kJ mol^-1 for the SWCNT@HKUST-1 composite. In situ Raman experiments corroborated the favored CO₂ adsorption for the composite and provided an insight into the augmented hydrophobicity of the SWCNT@HKUST-1. Ethanol adsorption isotherms corroborated an increase in the hydrophobicity of the material upon the incorporation of carbon nanotubes.

Two scandium coordination polymers: rapid synthesis and catalytic properties

Two new scandium coordination polymers were synthesized by MW and hydrothermal methods, and exhibited good catalytic performance.

Encapsulation of stilbazolium-type dyes into layered metal–organic frameworks: solvent-dependent luminescence chromisms and their mechanisms

Dye@Cd-LMOCs composites exhibit interesting solvent-dependent chromismic fluorescence, in which a two step solvent exchange process and a new electron transfer route are proposed.

Greener synthesis of Cu-MOF-74 and its catalytic use for the generation of vanillin

A greener synthesis of Cu-MOF-74 was obtained, for the first time, in methanol as the unique solvent and at room temperature. Full characterisation of the MOF material showed its purity and also its nanocrystalline nature. Complete activation (150 °C for 1 h and 10-3 bar) of Cu-MOF-74 afforded unsaturated Cu metal sites and this was corroborated by in situ DRIFT spectroscopy. The access to these Cu open metal sites was tested for the catalytic transformation of trans-ferulic acid to vanillin (yield of 71% and 97% selectivity) and a plausible catalytic reaction mechanism was postulated based on quantum chemical calculations.

CO2 capture in a carbazole-based supramolecular polyhedron structure: the significance of Cu(ii) open metal sites

Upon the removal of coordinated H₂O and DMF molecules to the paddlewheel of MOP 4 (followed by FTIR), it showed very interesting CO₂ capture properties at 196 K.

Confined methanol within InOF-1: CO2 capture enhancement

The CO₂ capture in InOF-1 was enhanced by confining small amounts of MeOH. DFT calculations coupled with forcefield based-MC simulations revealed that such an enhancement is due to an increase of the degree of confinement.

Synthesis of vanillin via a catalytically active Cu(ii)-metal organic polyhedron

Crystalline Cu (ii)-MOP 1 was employed for the first time in the catalytic conversion of trans-ferulic acid to vanillin.

HKUST-1 as a Heterogeneous Catalyst for the Synthesis of Vanillin

Vanillin (4-hydoxy-3-methoxybenzaldehyde) is the main component of the extract of vanilla bean. The natural vanilla scent is a mixture of approximately 200 different odorant compounds in addition to vanillin. The natural extraction of vanillin (from the orchid Vanilla planifolia, Vanilla tahitiensis and Vanilla pompon) represents only 1% of the worldwide production and since this process is expensive and very long, the rest of the production of vanillin is synthesized. Many biotechnological approaches can be used for the synthesis of vanillin from lignin, phenolic stilbenes, isoeugenol, eugenol, guaicol, etc., with the disadvantage of harming the environment since these processes use strong oxidizing agents and toxic solvents. Thus, eco-friendly alternatives on the production of vanillin are very desirable and thus, under current investigation. Porous coordination polymers (PCPs) are a new class of highly crystalline materials that recently have been used for catalysis. HKUST-1 (Cu3(BTC)2(H2O)3, BTC = 1,3,5-benzene-tricarboxylate) is a very well known PCP which has been extensively studied as a heterogeneous catalyst. Here, we report a synthetic strategy for the production of vanillin by the oxidation of trans-ferulic acid using HKUST-1 as a catalyst.

Enhancement of CO2 Adsorption and Catalytic Properties by Fe-Doping of [Ga2(OH)2(L)] (H4L = Biphenyl-3,3',5,5'-tetracarboxylic Acid), MFM-300(Ga2)

Metal-organic frameworks (MOFs) are usually synthesized using a single type of metal ion, and MOFs containing mixtures of different metal ions are of great interest and represent a methodology to enhance and tune materials properties. We report the synthesis of [Ga2(OH)2(L)] (H4L = biphenyl-3,3',5,5'-tetracarboxylic acid), designated as MFM-300(Ga2), (MFM = Manchester Framework Material replacing NOTT designation), by solvothermal reaction of Ga(NO3)3 and H4L in a mixture of DMF, THF, and water containing HCl for 3 days. MFM-300(Ga2) crystallizes in the tetragonal space group I4122, a = b = 15.0174(7) Å and c = 11.9111(11) Å and is isostructural with the Al(III) analogue MFM-300(Al2) with pores decorated with -OH groups bridging Ga(III) centers. The isostructural Fe-doped material [Ga(1.87)Fe(0.13)(OH)2(L)], MFM-300(Ga(1.87)Fe(0.13)), can be prepared under similar conditions to MFM-300(Ga2) via reaction of a homogeneous mixture of Fe(NO3)3 and Ga(NO3)3 with biphenyl-3,3',5,5'-tetracarboxylic acid. An Fe(III)-based material [Fe3O(1.5)(OH)(HL)(L)(0.5)(H2O)(3.5)], MFM-310(Fe), was synthesized with Fe(NO3)3 and the same ligand via hydrothermal methods. [MFM-310(Fe)] crystallizes in the orthorhombic space group Pmn21 with a = 10.560(4) Å, b = 19.451(8) Å, and c = 11.773(5) Å and incorporates μ3-oxo-centered trinuclear iron cluster nodes connected by ligands to give a 3D nonporous framework that has a different structure to the MFM-300 series. Thus, Fe-doping can be used to monitor the effects of the heteroatom center within a parent Ga(III) framework without the requirement of synthesizing the isostructural Fe(III) analogue [Fe2(OH)2(L)], MFM-300(Fe2), which we have thus far been unable to prepare. Fe-doping of MFM-300(Ga2) affords positive effects on gas adsorption capacities, particularly for CO2 adsorption, whereby MFM-300(Ga(1.87)Fe(0.13)) shows a 49% enhancement of CO2 adsorption capacity in comparison to the homometallic parent material. We thus report herein the highest CO2 uptake (2.86 mmol g(-1) at 273 K at 1 bar) for a Ga-based MOF. The single-crystal X-ray structures of MFM-300(Ga2)-solv, MFM-300(Ga2), MFM-300(Ga2)·2.35CO2, MFM-300(Ga(1.87)Fe(0.13))-solv, MFM-300(Ga(1.87)Fe(0.13)), and MFM-300(Ga(1.87)Fe(0.13))·2.0CO2 have been determined. Most notably, in situ single-crystal diffraction studies of gas-loaded materials have revealed that Fe-doping has a significant impact on the molecular details for CO2 binding in the pore, with the bridging M-OH hydroxyl groups being preferred binding sites for CO2 within these framework materials. In situ synchrotron IR spectroscopic measurements on CO2 binding with respect to the -OH groups in the pore are consistent with the above structural analyses. In addition, we found that, compared to MFM-300(Ga2), Fe-doped MFM-300(Ga(1.87)Fe(0.13)) shows improved catalytic properties for the ring-opening reaction of styrene oxide, but similar activity for the room-temperature acetylation of benzaldehyde by methanol. The role of Fe-doping in these systems is discussed as a mechanism for enhancing porosity and the structural integrity of the parent material.

Structural aspects of metal-organic framework-based energy materials research at Diamond

Large-scale central facilities such as Diamond Light Source fulfil an increasingly pivotal role in many large-scale scientific research programmes. We illustrate these developments by reference to energy-centred projects at the University of Nottingham, the progress of which depends crucially on access to these facilities. Continuing access to beamtime has now become a major priority for those who direct such programmes.

Sc2(pydc)2 unit based 1D, 2D and 3D metal–organic frameworks as heterogeneous Lewis acid catalysts for cyanosilylation

Three new Sc-MOFs have been synthesized using H₂pydc linker and Sc(iii) ions. 1 features a ladder-shaped chain, 2 displays a waved layer and 3 exhibits a (3,3)-connected nbo-a network. All three compounds exhibit good catalytic abilities for the cyanosilylation of aldehyde.

Catalytic activity of HKUST-1 in the oxidation of trans-ferulic acid to vanillin

HKUST-1 was used as a catalyst in the conversion, under mild oxidation conditions, of trans-ferulic acid to vanillin. When catalyst was activated under vacuum, the average reaction yield reached 95%.

High valence 3p and transition metal based MOFs

This article focuses on high valence 3p and transition metal based metal organic frameworks.

Structures, photoluminescence and photocatalytic properties of two novel metal–organic frameworks based on tetrazole derivatives

Two novel, efficient metal organic framework photocatalysts based on silver salts and tetrazolate derivatives have been successfully prepared and characterized.

H3O2 bridging ligand in a metal-organic framework. Insight into the aqua-hydroxo↔hydroxyl equilibrium: a combined experimental and theoretical study

A metal-organic framework (MOF) bearing the aqua-hydroxo species (O2H3)(-) in the framework, as well as the processes that govern the equilibrium aqua-hydroxo (O2H3)(-)↔hydroxyl (OH) in Sc-MOFs, are studied experimentally and theoretically. Computational studies were employed to determine the relative energies for the two compounds that coexist under certain hydrothermal conditions at pH < 2.8. The thermodynamically more stable [Sc3(3,5-DSB)2(μ-O2H3)(μ-OH)2(H2O)2] (from now on, (O2H3)Sc-MOF; 3,5-DSB = 3,5-disulfobenzoic acid) was obtained as a pure and stable phase. It was impossible to isolate [Sc3(3,5-DSB)2(μ-OH)3(H2O)4] as a pure phase, as it turned out to be the precursor of (O2H3)Sc-MOF. Additionally, a third compound that appears at pH between 3.5 and 4, [Sc3(3,5-DSB)(μ-OH)6(H2O)] and a fourth, [Sc(3,5-DSB)(Phen)(H2O)](H2O), in whose formula neither OH groups nor H3O2(-) anions appear, are reported for comparative purposes. A study of the (O2H3)Sc-MOF electronic structure, and some heterogeneous catalytic tests in cyanosilylation of aldehydes reactions, are also reported.