Framework Metal Sites Research Articles

Facile synthesis of bimetallic MIL-100(Fe, Al) for enhancing CO2 Adsorption performance

Bimetallic metal-organic frameworks, having two different metal species in the framework metal sites, demonstrate higher effectiveness in CO2 capture and separation performance than their monometallic counterpart. For this reason, a series of bimetallic MIL-100(Fe, Al) was synthesized using a hydrothermal reaction system in the presence of Fe and Al metal precursors with various molar ratios. The physicochemical characterizations of the products were influenced by adding Al species in the starting materials, inducing the successful incorporation of a certain amount of Al species inside the framework. The effect of Al incorporation was analyzed by inductively coupled plasma, field-emission scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, NH3-temperature program desorption, and a N2 porosimetry. Among the investigated samples, MIL-100(Fe, Al)#2 showed the highest CO2 adsorption capacity of 3.27 mmol g−1 with an IAST-CO2/N2 selectivity of 76.5 at 25 °C and 1 bar. Compared to the MIL-100(Fe) parent sample, the results improved by approximately 26% for CO2 uptake amount and by 1.5-fold for CO2/N2 selectivity due to the increment of micropore volume and presence of the new adsorptive sites of unsaturated metal sites (Al3+) and H+ from the bridges between tiny particles of AlOOH and –COOH groups. This facile synthetic route for bimetallic MOFs has the potential to enhance CO2 adsorption performance.

Heteroatom zeolites with a high content of framework metal sites as Lewis-acid catalysts for the conversion of ethanol–acetaldehyde to 1,3-butadiene

Heteroatom zeolites with a high content of framework metal sites as Lewis-acid catalysts via improved dissolution–reconstruction strategy.

On the nature of post synthetic isolated zirconium site development in *BEA framework for ethanol-to-butadiene conversion : A mechanism study

Metallosilicates with isolated framework metal sites as well as Lewis acidity, e.g., Zr-β zeolite, is proven to be one of the most active for the conversion of ethanol to 1,3-butadiene (ETB), which is a potential process for sustainable butadiene production. However the development process of isolated Zr sites in *BEA framework and their structure–activity relationship in ETB process are still under debate. In this work, three post-synthetic procedures, Liquid Phase Incorporation (LPI), Solid State Ion Exchange (SSIE), and Direct Loading were applied for isolated framework Zr sites development, and the procedure as well as catalytic performance and mechanism were systematically investigated. The isolated framework sites could be developed by Liquid Phase Incorporation (LPI) process, where calcination was the key step for Zr-O-Si bond formation, thus representing Zr incorporation into *BEA framework, accompanying the absence of Zr ion exchange. Isolated Zr framework site was the active site in ETB process, for the strong Lewis acidity upon Zr incorporation. DRIFT and TPSR results revealed that the MPV reduction of crotonaldehyde on Zr site was rate-determining in ETB process.

Observation of an Intermediate to H2 Binding in a Metal-Organic Framework.

Coordinatively unsaturated metal sites within certain zeolites and metal-organic frameworks can strongly adsorb a wide array of substrates. While many classical examples involve electron-poor metal cations that interact with adsorbates largely through physical interactions, unsaturated electron-rich metal centers housed within porous frameworks can often chemisorb guests amenable to redox activity or covalent bond formation. Despite the promise that materials bearing such sites hold in addressing myriad challenges in gas separations and storage, very few studies have directly interrogated mechanisms of chemisorption at open metal sites within porous frameworks. Here, we show that nondissociative chemisorption of H2 at the trigonal pyramidal Cu+ sites in the metal-organic framework CuI-MFU-4l occurs via the intermediacy of a metastable physisorbed precursor species. In situ powder neutron diffraction experiments enable crystallographic characterization of this intermediate, the first time that this has been accomplished for any material. Evidence for a precursor intermediate is also afforded from temperature-programmed desorption and density functional theory calculations. The activation barrier separating the precursor species from the chemisorbed state is shown to correlate with a change in the Cu+ coordination environment that enhances π-backbonding with H2. Ultimately, these findings demonstrate that adsorption at framework metal sites does not always follow a concerted pathway and underscore the importance of probing kinetics in the design of next-generation adsorbents.

Spectroscopic characterization of metal ligation in trinuclear iron-μ3-oxo-based complexes and metal-organic frameworks.

The adsorption-based functionalities of porous metal-organic framework (MOF) materials that lead to applications such as catalysis and gas separation rely on specific host-guest interactions often involving the framework metal sites. These interactions are difficult to probe on the molecular level and consequently poorly understood. Conventional X-ray absorption spectroscopy (XAS) methods can provide molecular-level insights but, as the sole method of characterization, often lack the ligand sensitivity required to probe the relevant local metal coordination changes associated with MOF adsorption processes. Here, we investigate a series of trinuclear iron μ3-oxo-based MOFs under different metal-coordinating guest environments (water, pyridine, propylene, and guest-free) using a multipronged spectroscopy approach, including valence-to-core X-ray emission spectroscopy (vtc-XES) along with conventional XAS and vibrational spectroscopy, in an effort to characterize their local metal site coordination environments, including ligand identity. Closely related iron μ3-oxo reference complexes with known coordination are characterized as well for comparison to evaluate the ligand diagnostic nature of the combined spectroscopy approach. Density functional theory calculations aid the vtc-XES band assignments and provide insights into the molecular orbital parentage of the vtc transitions. This series of MOFs and complexes illustrates the advantages and limitations of using this combination of complementary techniques for distinguishing subtle differences in framework metal node coordination environments.

Unraveling the Arrangement of Al and Fe within the Framework Explains the Magnetism of Mixed-Metal MIL-100(Al,Fe).

Properties of mixed-metal MOFs depend on the distribution of different metals within their frameworks. Determination of this distribution is often very challenging. Using an example of aluminum- and iron-containing MIL-100, we demonstrate that 27Al NMR spectroscopy, when combined with first-principles calculations and magnetic, X-band electron paramagnetic resonance, Fe K-edge extended X-ray absorption fine structure, and Mössbauer measurements, enables one to accurately determine the arrangement of Al and Fe within the metal trimers, which are the basic building units of MIL-100. In this particular material, the incorporation of Fe and Al on the framework metal sites is random. Crucial for deciphering the arrangement is detecting NMR signals, shifted because of the strong hyperfine interaction between the 27Al nuclei and the unpaired electronic spins of Fe3+ ions, assigning the shifted signals aided by first-principles calculations of hyperfine couplings, and quantitatively evaluating the NMR intensities and the measured effective magnetic moment.

Interrogating the Lewis Acidity of Metal Sites in Beta Zeolites with 15N Pyridine Adsorption Coupled with MAS NMR Spectroscopy.

The Lewis acidity of isolated framework metal sites in Beta zeolites was characterized with 15N isotopically labeled pyridine adsorption coupled with magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The 15N chemical shift of adsorbed pyridine was found to scale with the acid character of both Lewis (Ti, Hf, Zr, Nb, Ta, and Sn) and Brønsted (B, Ga, and Al) acidic heteroatoms. The 15N chemical shift showed a linear correlation with Mulliken electronegativity of the metal center in the order Ti < Hf < Zr < Nb < Ta < Sn < H+. Theoretical calculations using density functional theory (DFT) showed a strong correlation between experimental 15N chemical shift and the calculated metal-nitrogen bond dissociation energy, and revealed the importance of active site reorganization when determining adsorption strength. The relationships found between 15N pyridine chemical shift and intrinsic chemical descriptors of metal framework sites complement adsorption equilibrium data and provide a robust method to characterize, and ultimately optimize, metal-reactant binding and activation for Lewis acid zeolites. Direct 15N MAS NMR detection protocols applied to the Lewis acid-base adducts allowed the differentiation and quantification of framework metal sites in the presence of extraframework oxides, including highly quadrupolar nuclei that are not amenable for quantification with conventional NMR methods.

Metalloenzyme-Like Zeolites as Lewis Acid Catalysts for C-C Bond Formation.

The use of metalloenzyme-like zeolites as Lewis acid catalysts for C-C bond formation reactions has received increasing attention over the past few years. In particular, the observation of direct aldol condensation reactions enabled by hydrophobic zeolites with isolated framework metal sites has encouraged the development of catalytic approaches for producing chemicals from biomass-derived compounds. The discovery of new Diels-Alder cycloaddition/dehydration routes and experimental and computational studies of Lewis acid catalyzed carbonyl-ene reactions have given a further boost to this rapidly evolving field.

Anion stripping as a general method to create cationic porous framework with mobile anions.

Metal-organic frameworks (MOFs) with cationic frameworks and mobile anions have many applications from sensing, anion exchange and separation, to fast ion conductivity. Despite recent progress, the vast majority of MOFs have neutral frameworks. A common mechanism for the formation of neutral frameworks is the attachment of anionic species such as F(-) or OH(-) to the framework metal sites, neutralizing an otherwise cationic scaffolding. Here, we report a general method capable of converting such neutral frameworks directly into cationic ones with concurrent generation of mobile anions. Our method is based on the differential affinity between distinct metal ions with framework anionic species. Specifically, Al(3+) is used to strip F(-) anions away from framework Cr(3+) sites, leading to cationic frameworks with mobile Cl(-) anions. The subsequent anion exchange with OH(-) further leads to a porous network with mobile OH(-) anions. New materials prepared by anion stripping can undergo ion exchange with anionic organic dyes and also exhibit much improved ionic conductivity compared to the original unmodified MOFs.

Urothermal Synthesis of Crystalline Porous Materials

Pores from urea: Urea derivatives are shown to be a highly versatile solvent system for the synthesis of crystalline solids. In particular, reversible binding of urea derivatives to framework metal sites has been utilized to create a variety of materials that integrate both porosity and open metal sites (see picture).

Oxidation of ethyl benzene and diphenyl methane over ordered mesoporous M-MCM-41 (M = Ti, V, Cr): Synthesis, characterization and structure–activity correlations

Transition metals (M = Ti, V, Cr) were incorporated into the MCM-41 framework by surfactant-assisted direct hydrothermal (DHT) methods and were characterized in detail by XRD, N 2 adsorption–desorption, TG–DTG/DTA, SEM/TEM, DR UV–Vis, XPS and EPR. The structural regularity as well as the textural properties of the M-MCM-41 materials was confirmed by XRD, TEM and adsorption–desorption techniques. Nature of metal species and the percentage of framework as well as extra framework metal sites in the MCM-41 matrix are discussed in detail by various spectroscopic techniques. The catalytic activity as well as the selectivity of the M-MCM-41 catalysts were explored in the one-step liquid-phase oxidation reaction of ethyl benzene and diphenyl methane. Reaction data showed that the oxidation activity is more when H 2O 2 was used an oxidant, acetonitrile as solvent and V-MCM-41 as catalyst. However, the selectivity towards the desired keto derivatives (ethyl benzene to acetophenone and diphenyl methane to benzophenone) follows the order, Ti-MCM-41 > V-MCM-41 > Cr-MCM-41.

Phase relations in Na xCr xTi 8−xO 16 at 1350 °C and crystal structure of hollandite-like Na 2Cr 2Ti 6O 16

Phase relations of rutile, freudenbergite, and hollandite structures were examined in the pseudobinary system NaCrO 2–TiO 2 (i.e., Na x Cr x Ti 8− x O 16) at 1350 °C. The hollandite structure was obtained in the composition range 1.7⩽ x⩽2.0. The symmetry of the samples at room temperature was tetragonal for x = 1.7 and 1.75, and monoclinic for x = 1.8 and above. Single crystals of monoclinic hollandite Na 2Cr 2Ti 6O 16 were grown and the structure refinement has been carried out using an X-ray diffraction technique. The space group was I2/ m and cell parameters were a = 10.2385 ( 11 ) , b = 2.9559 ( 9 ) , c = 9.9097 ( 11 ) Å , and β = 90.545 ( 9 ) ° with Z = 1 . The Na ion distribution in the tunnel was markedly deformed from that in the tetragonal form. It was suggested that Cr/Ti ratios were different between the two framework metal sites.

Vanadium-containing ordered mesoporous silicates: Does the silica source really affect the catalytic activity, structural stability, and nature of vanadium sites in V-MCM-41?

A series of vanadium-substituted mesoporous materials were synthesized hydrothermally with the use of two commonly used silica sources, fumed silica and tetra ethyl orthosilicate. The extent of mesopore structural ordering was confirmed from X-ray diffraction, N 2 physisorption, SEM, and TEM, and the presence and nature of vanadium species inside the framework of the MCM-41 matrix was confirmed in detail with the use of various characterization techniques like FT-IR, 29Si MAS NMR, DRUV–vis, EPR, 51V MAS NMR, and Raman analysis. It is deduced from the above characterization techniques that, regardless of the silica source, vanadium is incorporated into the silica framework and thereby increases the structural ordering and wall thickness of the mesoporous material. Thermal and hydrothermal studies performed over the V-MCM-41 catalysts show that the Si O Si inorganic backbone from a fumed silica source is more resistant to severe thermal treatments and hydrolysis than the tetraethyl orthosilicate-synthesized catalysts. Spectroscopic characterization reveals the existence of easily accessible isolated tetrahedral vanadium sites on V-MCM-41 catalyst prepared from fumed silica catalyst, whereas the sample obtained from a tetraethyl orthosilicate silica source shows vanadium in more disordered sites. Catalytic results show that both catalysts display excellent activity toward the epoxidation reaction of bulkier olefins, and the exceptional activity of the fumed silica catalyst may arise from the more isolated tetrahedral sites and the complementary textural characteristics, which may facilitate the easy access of substrate to the isolated framework metal sites.

Multiwavelength X-ray diffraction studies of ZnGaPOs; distinguishing isoelectronic framework cations

The crystal structures of three zinc–gallium phosphates have been determined using single-crystal X-ray diffraction at a synchrotron source: (C 7H 14N)[ZnGa 3(PO 4) 4] ( I) (monoclinic, P2 1/ c, a=14.3987(10), b=16.3241(11), c=8.7421(6) Å, β=90.261(2)°), (C 5H 6N)[ZnGa 2(PO 4) 3] ( II) (monoclinic, C2/ c, a=15.044(1), b=13.188(1), c=15.243(1) Å, β=112.248(5)°), and (C 6H 14N 2) 2[Zn 4Ga 5(PO 4) 9] ( III) (monoclinic, I2/ a, a=14.953(1), b=17.645(1), c=15.725(1) Å, β=97.117(5)°), have frameworks constructed from alternating PO 4 and MO 4 (M=Zn and Ga) tetrahedra with CGS, LAU and CGF topologies respectively. The use of anomalous X-ray scattering has enabled the distribution of the formally isoelectronic Zn and Ga cations over the framework metal sites to be determined and represents the first example of this type of experiment.