Accessible Metal Sites Research Articles

Engineering Two-Dimensional Metal-Organic Framework on Molecular Basis for Fast Li+ Conduction.

Metal-organic frameworks (MOFs) have been proposed as emerging fillers for composite polymer electrolytes (CPEs). However, MOF particles are usually served as passive fillers that yield limited ionic conductivity improvement. Building continuous MOF reinforcements and exploiting their active roles remain challenging. Here we demonstrate the feasibility of engineering fast Li+ conduction within MOF on molecule conception. Two-dimensional Cu(BDC) MOF is selected as an active filler due to its sufficient accessible open metal sites for perchlorate anion anchoring to release free Li+, verified by theoretical calculations and measurements. A novel Cu(BDC)-scaffold-reinforced CPE is developed via in situ growth of MOF, which provides fast Li+ channels inside MOF and continuous Li+ paths along the MOF/polymer interface for high Li+ conductivity (ambient 0.24 mS cm-1) and enables high mechanical strength. Stable cycling is achieved in solid-state Li-NCM811 full cell using the MOF-reinforced CPE. This molecule-basis Li+ conduction strategy brings new ideas for designing advanced CPEs.

Enhanced cracking conversion of supercritical fuel over stabilized Pt nanoparticles supported SiO2-Al2O3 catalyst coating

Catalytic cracking of fuel is an effective method to solve the thermal barrier of hypersonic aerocraft, but requires catalysts that are durable and highly active under high temperature. In this contribution, Pt nanoparticles loaded SiO2-Al2O3 catalyst (PSA-re) was synthesized through modified colloid deposition method (CDM), aiming to improve the stability and dispersion of Pt in the reaction. PSA-re exhibited high cracking activity that heat sink of above 3.9 MJ/kg at 750 °C and stand for 30 min without obvious activity decreasing. Characterizations with Py-FTIR, H2-TPR, CO chemisorption, XPS, XRD, STEM and CO-DRIFTS techniques on the catalysts revealed that the Pt nanoparticles were dispersed on the PSA-re uniformly and PSA-re have smaller particle size as well as higher Pt dispersion compared with catalyst (PSA-im) that prepared by incipient wetness impregnation method (IWIM). The high activity of PSA-re is strongly associated with abundant accessible Pt metal sites and appropriate acid sites distribution on the surface. Meantime, the outstanding stability is mainly due to anti-sintering ability of Pt particles and alleviated carbon deposition to reduce the loss of Pt metal sites and acid sites.

Two-dimensional bimetallic coordination polymers as bifunctional evolved electrocatalysts for enhanced oxygen evolution reaction and urea oxidation reaction

Two-dimensional coordination polymers (CPs) have aroused tremendous interest as electrocatalysts because the catalytic performance could be fine-tuned by their well-designed coordination layers with highly accessible and active metal sites. However, it remains great challenge for CP-based catalysts to be utilized for electrocatalytic oxidation reactions due to their inefficient activities and low catalytic stabilities. Herein, we applied a mixed-metal strategy to fabricate two-dimensional CoxNi1−x-CPs with dual active sites for electrocatalytic water and urea oxidation. By metal ratio regulation in the two-dimensional layer, an optimized Co2/3Ni1/3-CP exhibits a water oxidation performance with an overpotential of 325 mV at a current density of 10 mA cm−2 and a Tafel slope of 86 mV dec−1 in alkaline solution for oxygen evolution reaction. Importantly, a lower potential than that of commercial RuO2 is observed over 20 mA cm−2. Co2/3Ni1/3-CP also displays a potential of 1.381 V at 10 mA cm−2 for urea oxidation reaction and a Tafel slope of 124 mV dec−1. This mixed-metal strategy to maximize synergistic effect of different metal centers may ultimately lead to promising electrocatalysts for small molecule oxidation reaction.

Anion-Dependent Catalytic C-C Bond Cleavage of a Lignin Model within a Cationic Metal-Organic Framework.

The development of heterogeneous catalysts capable of selectively converting lignin model compounds into products of added value offers an exciting avenue to explore in the production of renewable chemical feedstocks. The use of metal-organic frameworks (MOFs) in such chemical transformations relies largely on the presence of accessible open metal sites found within highly porous networks that simultaneously allow for fast transport and strong interactions with desired substrates. Here, we present the first systematic study on the modulation of catalytic performance of a cationic framework, [Cu2(L)(H2O)2](NO3)2·5.5H2O (L = 1,1'-bis(3,5-dicarboxylatophenyl)-4,4'-bipyridinium), achieved through the exchange of anionic guests. Remarkably, the catalytic activity proves to be highly anion-dependent, with a nearly 10-fold increase toward the aerobic C-C bond cleavage of a lignin model compound when different anionic species are incorporated within the MOF. Moreover, we demonstrate that the cationic nature of the MOF, imparted by the incorporation of viologen moieties within the linker, tunes the electrophilicity of the active copper(II) sites, resulting in stronger interactions with the substrate. As such, the copper-based framework exhibits enhanced catalytic performance when compared to its neutral counterpart, emphasizing the appeal of charged frameworks for use as green heterogeneous catalysts.

Role of the metal cation in the dehydration of the microporous metal–organic frameworks CPO-27-M

The dehydration of the CPO-27-M (M-MOF-74, M = Zn, Co, Ni, Mg, Mn, Cu) metal-organic framework series has been investigated comprehensively using in situ variable temperature powder X-ray diffraction (VT-PXRD) and thermal analysis (TG) coupled with mass spectrometry (MS). Significant differences in the order of water desorption from different adsorption sites on heating are found with varying metal cation in the otherwise isostructural material. For all CPO-27-M (except M = Cu), water is bonded significantly more strongly to the accessible open metal sites, and these water molecules are only desorbed at higher temperatures than the other water molecules. CPO-27-Cu is an exception, where all water molecules desorb simultaneously and at much lower temperatures (below 340 K). MS and TG data show that all CPO-27-M start to release traces of CO2 already at 300–350 K, and thus long before bulk thermal decomposition is observed. Only for CPO-27-Co, the CO2 release is essentially constant on its baseline between 450 and 700 K, and it is the only CPO-27-M member that shows a stable plateau in the TG in this region. Additional rehydration studies on CPO-27-Co show that the MOF incorporates any water molecules present until the pores are fully loaded. CPO-27-Co consequently behaves as an efficient trap for any water present.

A library of carbon-supported ultrasmall bimetallic nanoparticles

Small-sized bimetallic nanoparticles that possess numerous accessible metal sites and optimal geometric/electronic structures show great promise for advanced synergetic catalysis but remain synthetic challenge so far. Here, an universial synthetic method is developed for building a library of bimetallic nanoparticles on mesoporous sulfur-doped carbon supports, consisting of 24 combinations of 3 noble metals (that is, Pt, Rh, Ir) and 7 other metals, with average particle sizes ranging from 0.7 to 1.4 nm. The synthetic strategy is based on the strong metal-support interaction arising from the metal-sulfur bonding, which suppresses the metal aggregation during the H2-reduction at 700 °C and ensure the formation of small-sized and alloyed bimetallic nanoparticles. The enhanced catalytic properties of the ultrasmall bimetallic nanoparticles are demonstrated in the dehydrogenation of propane at high temperature and oxidative dehydrogenations of N-heterocycles.

Comparative studies of methyl orange adsorption in various metal-organic frameworks by nitrogen adsorption and positron annihilation lifetime spectroscopy

Abstract In this work, typical metal-organic frameworks (MOFs) MIL-101-Cr, MIL-53-Al and ZIF-8 with high specific surface areas were synthesized and applied in the removal of anionic dye methyl orange (MO) from water. Nitrogen adsorption and positron annihilation lifetime spectroscopy (PALS) were applied to study the pore characteristics of MOFs and their adsorption of MO, respectively. The specific surface areas of the synthesized MOFs are in the order MIL-101-Cr > ZIF-8>MIL-53-Al. The pore sizes of MOFs obtained from PALS are essentially the same as those from their crystal structure, suggesting that PALS is a useful probe to characterize the pore properties of MOFs. After MO adsorption in MOFs, the nominal mean pore sizes for MIL-101-Cr-MO and MIL-53-Al-MO from nitrogen adsorption are mainly attributed to the interspaces between grains because their inherent pores were occupied by MO molecules as revealed by PALS, while that of ZIF-8-MO represents the inherent pores and interspaces between grains. Langmuir adsorption of MO was found to occur in the MOFs, whereas the MO adsorption behaviors are different depending on the unique pore structures of specific MOFs. MIL-101-Cr and MIL-53-Al show the MO adsorption capacities of 196.08 and 100.0 mg/g due to their large specific surface areas and accessible pore windows, which can provide more accessible unsaturated metal sites. However, despite that the specific surface area of ZIF-8 is larger than that of MIL-53-Al, MO adsorption capacity in ZIF-8 is rather low because the size of its pore windows is smaller than MO dimension, which results in MO molecules inaccessible to the pores. Results showed that methyl orange anions (MO−) were electrostatically attracted by unsaturated metal sites in the cages/channel pores of MIL-101-Cr and MIL-53-Al in aqueous solutions, whereas MO was only adsorbed in the interspaces between grains in ZIF-8.

Recent advances in CO2capture and simultaneous conversion into cyclic carbonates over porous organic polymers having accessible metal sites

This review describes the recent advances in the design and synthesis of metal-functionalized porous organic polymers featuring accessible active sites and discloses the structure–property relationship in the metal-catalyzed CO2cycloaddition reaction to epoxides.

Porous GdIII‐Organic Framework as a Dual‐Functional Material for Cyanosilylation of Aldehydes and Ablation of Human Liver Cancer Cells

Metal‐organic frameworks (MOFs) have gained great attention in recent years because they could behave as multifunctional materials which combine the advances of porous solids and coordination complexes. With the aim of constructing multifunctional MOFs, in this study, we choose a Y‐shaped tricarboxylic ligand biphenyl‐3,4′,5‐tricarboxylic acid (H3bpt) to react with GdIII ions to afford a new dual‐functional lanthanide‐organic framework with the chemical formula of [Gd2(bpt)2(H2O)2]·(DMF)2(H2O)6 (1) (DMF = N,N‐dimethylformamide) under solvothermal condition. The title complex was characterized by means of elemental analysis, FT‐IR spectroscopy, thermogravimetric and X‐ray diffraction analyses. Crystal structure analysis reveals that compound 1 is composed of 1D helical chain secondary building units that connect by the bpt3– ligands into a 3D framework with 1D nanosized channels running along the b axis. In view of its high porosity and accessible open metal sites, the activated 1 (1a) was studied for the cyanosilylation of aldehydes under solvent‐free conditions. The catalytic activity of 1a is much higher than that of compound 1, indicating that the exposed open metal sites of 1a is beneficial to the cyanosilylation reaction. In connection to these, the different cytotoxicities of 1 and 1a were also evaluated on four human liver cancer cells (SMMC‐7721, Bel‐7402, MHCC97 and Hep3B) by the MTT assay.

A Bifunctional Metal–Organic Framework Based on Linear Trinuclear Clusters for Solvent-Free Aldehyde Cyanosilylation and Growth Inhibition of Human Breast Cancer Cells

By using a rigid Y-shaped biphenyl-3,4′,5-tricarboxylic acid (H3bpt) organic ligand, a new cluster-based metal-organic framework (MOF) with the chemical formula of {[Cu3(bpt)2(H2O)4](DMA)(MeOH)2}n (1, DMA = N,N-dimethylacetamide) was synthesized under solvothermal condition. The structure of 1 was determined by single-crystal X-ray diffraction analyses (SCXRD) and further characterized by elemental analyses, IR spectroscopy, powder X-ray diffraction (PXRD), thermogravimetric (TG) analyses and gas sorption measurements. The SCXRD result reveals that complex 1 exhibits a 3D framework with 1D channels which contains opposite-handedness helical chains based on the trinuclear Cu(II) clusters. Considering its high pore volume and accessible open metal sites, complex 1 as well as its activated form (1a) was studied for cyanosilylation of aldehydes under solvent-free conditions, which reveals that 1a shows much better catalytic performance than that of 1, indicating the open metal sites play important roles in the catalytic process. Furthermore, the anticancer activates of complex 1 and 1a has been evaluated on three human breast cancer cells (MDA-MB-231, MDA-MB-468 and Hst578) via the MTT assay.

Hydroisomerization of n-dodecane over bi-porous Pt-containing bifunctional catalysts: Effects of alkene intermediates’ journey distances within the zeolite micropores

In this study, three micro/mesoporous composites with MCM-41 materials overgrowing on external surfaces of Y zeolite crystals were prepared. For the three composites, average crystal particle sizes of their zeolite components differed from each other, resulting from the different levels of alkali-treatments to the original Y zeolite before the composite synthesis. Hydroisomerization of n-dodecane (n-C12) was used to evaluate the catalytic performances of bi-porous Pt-loaded composite catalysts with similar metal-acid balances but different average zeolite crystal particle sizes, which leaded to the distinction of alkene intermediates’ journey distances inside the acid-rich micropores between two accessible Pt metal sites on three catalysts. The number of protonic sites encountered by alkene intermediates along their diffusion paths, determining the skeletal transformation frequencies, was considered to be proportional to the above-mentioned intermediates’ journey distances. Results showed that bifunctional catalysts with smaller average zeolite crystal particle sizes outperformed the catalyst containing the original Y zeolite in the regard of isomerization selectivity. This implied the significant effect of intermediates’ journey distances within the microporous arrays between two Pt sites on bifunctional behaviors of catalysts. It can be confirmed that shortening this distance within certain limits indeed contributed to inhibiting secondary reactions, including the skeletal multiple branching and cracking.

Luminescent Metal–Organic Framework Thin Films: From Preparation to Biomedical Sensing Applications

Metal-organic framework (MOF) thin films are receiving increasing attention in a number of different application fields, such as optoelectronics, gas separation, catalysis electronic devices, and biomedicine. In particular, their tunable composition and structure, accessible metal sites and potential for post-synthetic modification for molecular recognition make MOF thin films promising candidates for biosensing applications. Compared with solution-based powdery probes, film-based probes have distinct advantages of good stability and portability, tunable shape and size, real-time detection, non-invasion, extensive suitability in gas/vapor sensing, and recycling. In this review, we summarize the recent advances in luminescent MOF thin films, including the fabrication methods and origins of luminescence. Specifically, luminescent MOF thin films as biosensors for temperature, ions, gases and biomolecules are highlighted.

Catalyst stabilization by stoichiometrically limited layer-by-layer overcoating in liquid media

The use of metal oxide overcoats over supported nanoparticle catalysts has recently led to impressive improvements in catalyst stability and selectivity. The deposition of alumina is especially important for renewable catalysis due to its robustness in liquid-phase conditions. However, there are limited reports of work on alumina deposition and stabilization that goes beyond atomic layer deposition (ALD). Here, we present a layer-by-layer deposition technique for the controlled formation of conformal alumina overcoats in the liquid phase. This technique is easy to perform in common wet chemistry conditions. Alternated exposure of the substrate to stoichiometric amounts of aluminum alkoxide and water in liquid-phase conditions leads to the formation of a porous overcoat that was easily tunable by varying synthesis parameters. The deposition of 60 Al2O3 layers onto Al2O3-supported copper nanoparticles suppressed irreversible deactivation during the liquid-phase hydrogenation of furfural – a key biomass-derived platform molecule. The porous overcoat leads to highly accessible metal sites, which significantly reduces the partial site blocking observed in equivalent overcoats formed by ALD. We suggest that the ease of scalability and the high degree of control over the overcoat’s properties during liquid-phase synthesis could facilitate the development of new catalyst overcoating applications.

A New Class of Atomically Precise, Hydride-Rich Silver Nanoclusters Co-Protected by Phosphines.

Thiols and phosphines are the most widely used organic ligands to attain atomically precise metal nanoclusters (NCs). Here, we used simple hydrides (e.g., H-) as ligands along with phosphines, such as triphenylphosphine (TPP), 1,2-bis(diphenylphosphino)ethane [DPPE], and tris(4-fluorophenyl)phosphine [TFPP] to design and synthesize a new class of hydride-rich silver NCs. This class includes [Ag18H16(TPP)10]2+, [Ag25H22(DPPE)8]3+, and [Ag26H22(TFPP)13]2+. Our work reveals a new family of atomically precise NCs protected by H- ligands and labile phosphines, with potentially more accessible active metal sites for functionalization and provides a new set of stable NC sizes with simpler ligand-metal bonding for researchers to explore both experimentally and computationally.

Adsorbent–Adsorbate Interactions in the Oxidation of HMF Catalyzed by Ni-Based MOFs: A DRIFT and FT-IR Insight

The three Ni-based metal–organic frameworks (MOFs) Ni(BDP), Ni(BPEB), and Ni3(BTP)2 [H2BDP = 1,4-(4-bispyrazolyl)benzene; H2BPEB = 1,4-bis(1H-pyrazol-4-ylethynyl)benzene; H3BTP = 1,3,5-tris(1H-pyrazol-4-yl)benzene], possessing square planar, potentially accessible metal sites, were preliminarily tested as catalysts in the base-free selective oxidation of hydroxymethylfurfural to 2,5-diformylfuran (DFF). While Ni(BDP) undergoes degradation, Ni3(BTP)2 is the most active of the three MOFs, yielding 27% DFF after 24 h with a selectivity close to 100% under relatively mild reaction conditions (120 °C, 30 bar O2, water as solvent). Upon flowing a model probe, in situ DRIFT and FT-IR spectroscopy were employed to rationalize the different performances of Ni(BPEB) and Ni3(BTP)2 in terms of adsorbate–adsorbent interactions: Not only hydrogen bonds are at work between the hydroxyl functionality of the probe and the pore walls of the MOF, but also and more importantly, bands ascribed to Ni-OR stretching are detected...

Separation of CO2/CH4 and CH4/N2 mixtures using MOF-5 and Cu3(BTC)2

In this paper we used MOF-5 and Cu3(BTC)2 to separate CO2/CH4 and CH4/N2 mixtures under dynamic conditions. Both materials were synthesized and pelletized, thus allowing for a meaningful characterization in view of process scale-up. The materials were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). By performing breakthrough experiments, we found that Cu3(BTC)2 separated CO2/CH4 slightly better than MOF-5. Because the crystal structure of Cu3(BTC)2 includes unsaturated accessible metal sites formed via dehydration, it predominantly interacted with CO2 molecules and more easily captured them. Conversely, MOF-5 with a suitable pore size separated CH4/N2 more efficiently in our breakthrough test.

Hydrogen storage in a potassium-ion-bound metal-organic framework incorporating crown ether struts as specific cation binding sites.

To develop a metal-organic framework (MOF) for hydrogen storage, SNU-200 incorporating a 18-crown-6 ether moiety as a specific binding site for selected cations has been synthesized. SNU-200 binds K(+), NH4(+), and methyl viologen (MV(2+)) through single-crystal to single-crystal transformations. It exhibits characteristic gas-sorption properties depending on the bound cation. SNU-200 activated with supercritical CO2 shows a higher isosteric heat (Qst) of H2 adsorption (7.70 kJ mol(-1)) than other zinc-based MOFs. Among the cation inclusions, K(+) is the best for enhancing the isosteric heat of the H2 adsorption (9.92 kJ mol(-1)) as a result of the accessible open metal sites on the K(+) ion.

Hydrogen Storage in a Potassium‐Ion‐Bound Metal–Organic Framework Incorporating Crown Ether Struts as Specific Cation Binding Sites

AbstractTo develop a metal–organic framework (MOF) for hydrogen storage, SNU‐200 incorporating a 18‐crown‐6 ether moiety as a specific binding site for selected cations has been synthesized. SNU‐200 binds K+, NH4+, and methyl viologen(MV2+) through single‐crystal to single‐crystal transformations. It exhibits characteristic gas‐sorption properties depending on the bound cation. SNU‐200 activated with supercritical CO2 shows a higher isosteric heat (Qst) of H2 adsorption (7.70 kJ mol−1) than other zinc‐based MOFs. Among the cation inclusions, K+ is the best for enhancing the isosteric heat of the H2 adsorption (9.92 kJ mol−1) as a result of the accessible open metal sites on the K+ ion.

Copper-based MOF-74 material as effective acid catalyst in Friedel–Crafts acylation of anisole

Metal organic frameworks (MOFs) are emerging potential materials for catalytic applications. Among them, the honeycomb M2(dhtp) family, also known as MOF-74 or CPO-27 materials, are characterized by remarkable textural properties and accessible unsaturated metal sites (M). This work has been focused on the catalytic properties of novel Cu-MOF-74 material for the Friedel–Crafts acylation of anisole, as acid-catalyzed reaction test. The influence of reaction variables such as the type of acylating agent and solvent, and the reaction temperature have been studied. Additionally, the stability of Cu-MOF-74 was evaluated under different reaction conditions. Cu-MOF-74 achieved a remarkable catalytic performance in terms of anisole conversion and p-MAP yield. Additionally, the structural MOF-74 phase was preserved when using diluted concentrations of acetyl chloride as acylating agent. The relative acylating agent concentration was demonstrated to be crucial in the collapse of the crystalline MOF-74 phase. The catalytic activity of Cu-MOF-74, in terms of anisole conversion and selectivity to p-MAP product, was also superior to other Cu-containing MOF materials (HKUST-1) and conventional inorganic acid catalysts, such as aluminum-containing microporous zeolites (H-ZSM-5 and BETA) and mesoporous ordered materials (Al-MCM-41). Cu-MOF-74 also showed high stability and remarkable catalytic performance for seven consecutive reaction cycles when this MOF material was used in the best reaction conditions.

Integration of accessible secondary metal sites into MOFs for H2S removal

2,2′-Bipyridine-5,5′-dicarboxylic acid (H2bipy) was utilized for the synthesis of a zirconium based MOF isoreticular to UiO-67. The chelating bipyridine moiety was used for postsynthetic functionalization of the MOF with different metal salts giving materials with good performance in capture of toxic H2S.