Molecular Engineering of Methylated Sulfone-based Covalent Organic Frameworks for Back-reaction Inhibited Photocatalytic Overall Water Splitting.

Solar-to-hydrogen (H2) and oxygen (O2) conversion via photocatalytic overall water splitting (OWS) holds great promise for a sustainable fuel economy, but has been challenged by the backward O2 reduction reaction (ORR) due to its favored proton-coupled electron transfer (PCET) dynamics. Here, we report that molecular engineering by methylation inhibits the backward ORR of molecular photocatalysts and enables efficient OWS process. As demonstrated by a benchmark sulfone-based covalent organic framework (COF) photocatalyst, the precise methylation of its O2 adsorption sites effectively blocks electron transfer and increases the barrier for hydrogen intermediate desorption that cooperatively obstructs the PCET process of ORR. Methylation also repels electrons to the neighboring photocatalytic sulfone group that promotes the forward H2 evolution. The resultant DS-COF achieves an impressive inhibition of about 70% of the backward reaction and a three-fold enhancement of the OWS performance with a H2 evolution rate of 124.7 µmol h-1 g-1, ranking among the highest reported for organic photocatalysts. This work provides insights for engineering photocatalysts at the molecular level for efficient solar-to-fuel conversion.

Temporally Decoupled Ammonia Splitting by a Zn-NH3 Battery with an Ammonia Oxidation/Hydrogen Evolution Bifunctional Electrocatalyst as a Cathode.

Ammonia splitting to hydrogen is a decisive route for hydrogen economy but is seriously limited by the complex device and low efficiency. Here, we design and propose a new rechargeable Zn-NH3 battery based on temporally decoupled ammonia splitting to achieve efficient NH3-to-H2 conversion. In this system, ammonia is oxidized into nitrogen during cathodic charging (2NH3 + 6OH- → N2 + 6H2O + 6e-) with external electrical energy conversion and storage, while during cathodic discharging, water is reduced to hydrogen (2H2O + 2e- → H2 + 2OH-) with electrical energy generation. In this loop, continuous and efficient H2 production without separation and purification is achieved. With the help of the ammonia oxidation reaction (AOR) and hydrogen evolution reaction (HER) bifunctional catalyst of Mo2C/NiCu@C, a rechargeable Zn-NH3 battery is realized that exhibits a high NH3-to-H2 FE of 91.6% with outstanding durability for 900 cycles (300 h) at 20 mA/cm2, enabling efficient and continuous NH3-to-H2 conversion.

Coupling Electron Transfer and Redox Site in Boranil Covalent Organic Framework Toward Boosting Photocatalytic Water Oxidation.

The efficient polymeric semiconducting photocatalyst for solar-driven sluggish kinetics with multielectron transfer oxygen evolution has spurred scientific interest. However, existing photocatalysts limited by π-conjugations, visible-light harvest, and charge transfer often compromise the O2 production rate. Herein, we introduced an alternative strategy involving a boranil functionalized-based fully π-conjugated ordered donor and acceptor (D-A) covalent organic frameworks (Ni-TAPP-COF-BF2 ) photocatalyst. The co-catalyst-free Ni-TAPP-COF-BF2 exhibits an excellent ~11-fold photocatalytic water oxidation rate, reaching 1404 µmol g-1 h-1 under visible light irradiation compared to pristine Ni-TAPP-COF (123 µmol g-1 h-1 ) alone and surpasses to reported organic frameworks counterpart. Both experimental and theoretical results demonstrate that the push/pull mechanism (metalloporphyrin/BF2 ) is responsible for the appropriate light-harvesting properties and extending π-conjugation through chelating BF2 moieties. This strategy benefits in narrowing band structure, improving photo-induced charge separation, and prolonged charge recombination. Further, the lower spin magnetic moment of M-TAPP-COF-BF2 and the closer d-band center of metal sites toward the Fermi level lead to a lower energy barrier for *O intermediate. Reveal the potential of the functionalization strategy and opens up an alternative approach for engineering future photocatalysts in energy conversion applications.

Overcoming the Trade-Off between C2 H2 Sorption and Separation Performance by Regulating Metal-Alkyne Chemical Interaction in Metal-Organic Frameworks.

Designing porous materials for C2 H2 purification and safe storage is essential research for industrial utilization. We emphatically regulate the metal-alkyne interaction of PdII and PtII on C2 H2 sorption and C2 H2 /CO2 separation in two isostructural NbO metal-organic frameworks (MOFs), Pd/Cu-PDA and Pt/Cu-PDA. The experimental investigations and systematic theoretical calculations reveal that PdII in Pd/Cu-PDA undergoes spontaneous chemical reaction with C2 H2 , leading to irreversible structural collapse and loss of C2 H2 /CO2 sorption and separation. Contrarily, PtII in Pt/Cu-PDA shows strong di-σ bond interaction with C2 H2 to form specific π-complexation, contributing to high C2 H2 capture (28.7 cm3 g-1 at 0.01 bar and 153 cm3 g-1 at 1 bar). The reusable Pt/Cu-PDA efficiently separates C2 H2 from C2 H2 /CO2 mixtures with satisfying selectivity and C2 H2 capacity (37 min g-1 ). This research provides valuable insight into designing high-performance MOFs for gas sorption and separation.

The Combination of Charge and Energy Transfer Processes in MOFs for Efficient Photocatalytic Oxidative Coupling of Amines.

Combining electron and energy transfer processes is very significant for efficient photocatalytic oxidation of organic molecules. The first synthesized MOF, Co2(L)(2,6-NDC)2·xguest (FJI-Y10, L = bis(N-pyridyl) tetrachloroperylene peryleneimide, 2,6-NDC = 2,6-naphthalenedicarboxylic acid, FJI = Fujian Institute), shows a 2-fold interpenetrated pcu net, in which the 2,6-NDC ligand connects typical Co2(COO)4 paddle wheel clusters to form square lattices pillared by new PDI-type ligand L. FJI-Y10 as a heterogeneous and recyclable photocatalyst is applied for photo-oxidation of benzylamine and its derivatives with an excellent yield of 100%, which is much higher than that (59%) of the equivalent L ligand as a homogeneous photocatalyst under the same reaction conditions. Such a high-efficiency photocatalytic activity attributes to the combination of charge and energy transfer processes in catalyst FJI-Y10 during the catalytic process.

Optimizing H2, D2, and C2H2 Sorption Properties by Tuning the Pore Apertures in Metal-Organic Frameworks.

By adjustment of the arm lengths of two triphenylamine-based ligands, two nearly isostructural metal-organic frameworks (MOFs), namely, the reported nanoporous FIR-29 (FIR = Fujian Institute of Research) and the new microporous FJI-Y9 (FJI = Fujian Institute), are obtained, and all exhibit honeycomb lattices of hexagonal channels with Ca-COO chains connected by tris[(4-carboxyl)phenylduryl]amine (H3TCPA) ligands and 4,4',4''-nitrilotribenzoic acid (H3NTB) ligands, respectively. Although the Brunauer-Emmett-Teller (BET) surface area (1117 m2 g-1) and pore size (8.5 Å) of FJI-Y9 are much lower than those (BET surface area of 2061 m2 g-1 and pore size of 16 Å) of the reported FIR-29 because of the shorter arm lengths of H3NTB, the activated FJI-Y9-ht shows high H2 (202.3 cm3 g-1) and D2 (221.9 cm3 g-1) uptake under 77 K and 1 bar and C2H2 uptake of 168.9 cm3 g-1 under 273 K and 1 bar, which are all at least 48% enhancement over those of FIR-29-ht. The above results indicate that small pores in MOFs are beneficial to the uptake of some special gases including H2, D2, C2H2, etc.

Direct Solar-to-Electrochemical Energy Storage in a Functionalized Covalent Organic Framework.

A covalent organic framework integrating naphthalenediimide and triphenylamine units (NT-COF) is presented. Two-dimensional porous nanosheets are packed with a high specific surface area of 1276 m2 g-1 . Photo/electrochemical measurements reveal the ultrahigh efficient intramolecular charge transfer from the TPA to the NDI and the highly reversible electrochemical reaction in NT-COF. There is a synergetic effect in NT-COF between the reversible electrochemical reaction and intramolecular charge transfer with enhanced solar energy efficiency and an accelerated electrochemical reaction. This synergetic mechanism provides the key basis for direct solar-to-electrochemical energy conversion/storage. With the NT-COF as the cathode materials, a solar Li-ion battery is realized with decreased charge voltage (by 0.5 V), increased discharge voltage (by 0.5 V), and extra 38.7 % battery efficiency.

Design and synthesis of multifunctional metal-organic zeolites.

Metal-organic zeolites (MOZs) are an important branch of metal-organic frameworks (MOFs) and combine the advantages of zeolites and MOFs, such as high surface area and porosity as well as the exceptional stability of zeolites, which would have a significant impact on catalysis chemistry, inorganic chemistry, coordination chemistry, materials science and other areas. In this review, we focus on the recent advances in MOZs with a brief outline of the most prominent examples. In particular, we highlight the basic principles of the design and synthesis approaches toward the construction of MOZs. Obeying the principle of charge matching, tuning tetrahedral metal centers, using enlarged tetrahedral building units as clusters, introducing functional groups into ligands, and combining traditional inorganic TO4 sites in MOZs enable the final materials with diverse topological structures to exhibit superior performance for various applications, such as gas sorption/separation, catalysis, enantio-selectivity, luminescence, etc.

Rational design of a flu-type heterometallic cluster-based Zr-MOF.

Following the HSAB principle, the cooperative assembly of tetrahedral [Cu4I4(Ina)4]4- metalloligands and 8-connecting [Zr6(µ3-OH)8(OH)8]8+ building units leads to the first heterometallic cluster-based Zr-MOF (1). The results provide a successful strategy for rational design of heterometallic cluster-based Zr-MOFs.

Gas Sorption, Second-Order Nonlinear Optics, and Luminescence Properties of a Multifunctional srs-Type Metal-Organic Framework Built by Tris(4-carboxylphenylduryl)amine.

A chiral 8-fold interpenetrating srs-type metal-organic framework FIR-28 (FIR denotes Fujian Institute of Research) exhibits a surface area of 1029 m(2)/g and high C3H8/CH4 separation capacity in excess of 154 and displays strong powder second-harmonic-generation efficiency, with more than half over potassium dihydrogen phosphate powder. Moreover, the luminescence properties of FIR-28 are dependent on the solvent guests.

Integration of rigid and flexible organic parts for the construction of a homochiral metal-organic framework with high porosity.

Presented is a pair of homochiral metal-organic frameworks built from mixed ligands integrating rigid and flexible organic parts, and each compound shows high porosity and can be used for enantioselective separation of racemic 1-phenethylalcohol and methyl lactate.

Homochiral metal-organic frameworks with enantiopure proline units for the catalytic synthesis of ß-lactams.

Two enantiopure organic ligands integrating flexible proline units and rigid isophthalate units have been rationally designed and employed for the construction of four homochiral porous metal-organic frameworks (MOFs), respectively. One pair of these MOFs is used as heterogeneous catalysts to construct ß-lactam derivatives by oxidative coupling reactions.

Multifunctional anionic MOF material for dye enrichment and selective sorption of C2 hydrocarbons over methane via Ag(+)-exchange.

The anionic Zn-2,5-thiophenedicarboxylate framework material (1) built from the connection of Johnson cages can perform Ag(+)-exchange to upgrade the uptakes of C2 hydrocarbons (C2s) and separation properties of C2s over methane (C1). Moreover, its activated phase (1a) can enrich organic dyes from ethanol and make a significant red-shift in photoluminescent spectra of Rhodamine B (Rh B) via varying the aggregation states of dye molecules.

Highly selective and sensitive trimethylamine gas sensor based on cobalt imidazolate framework material.

A cobalt imidazolate (im) framework material [Co(im)2]n was employed to use as a trimethylamine (TMA) gas sensor and the [Co(im)2]n sensor can be easily fabricated by using Ag-Pd interdigitated electrodes. Gas sensing measurement indicated that the [Co(im)2]n sensor shows excellent selectivity, high gas response and a low detection limit level of 2 ppm to TMA at 75 °C. The good selectivity and high response to TMA of the sensor based on [Co(im)2]n may be attributed to the weak interaction between the TMA molecules and the [Co(im)2]n framework. That may provide an ideal candidate for detecting freshness of fish and seafood.

Cooperative ion-exchange and self-redox process to load catalytic metal nanoparticles into a MOF with Johnson-type cages.

Through a cooperative ion-exchange and self-redox process, a new MOF material with nanosized Johnson-type cages has successfully produced Pd- or Ag-NPs in its pores without any external reducing agents, leading to the formation of M-NP-embedded catalysts for the hydrogenation of styrene.

Nanoporous cobalt(II) MOF exhibiting four magnetic ground states and changes in gas sorption upon post-synthetic modification.

We present the syntheses, structural characterization, gas sorption, I2 uptake, and magnetic properties of a double-walled porous metal-organic framework, [Co(II)3(lac)2(pybz)2]·3DMF (1·3DMF, purple, where pybz = 4-pyridyl benzoate, lac = d- and l-lactate) and of its post-synthetic modified (PSM) congeners, [Co(II)3(lac)2(pybz)2]·xGuest (xGuest = 6MeOH, purple; 4.5EtOH, purple; 3PrOH, purple; 2C6H6, purple; 2.7I2, black), [Co(II)3(lac)2(pybz)2] (1, purple), [Co(II)3(pybz)2(lac)2(H2O)2]·7H2O (1a·7H2O, green), and [Co(III)Co(II)2(pybz)2(lac)2(H2O)2]I·2H2O·1.5DMSO (1b·I(-)·2H2O·1.5DMSO, yellow, DMSO = dimethyl sulfoxide). Crystallography shows that the framework is not altered by the replacement of DMF by different solvents or by the removal of the solvent molecules during the single-crystal to single-crystal (SC-SC) transformations, while upon exchange with H2O or partial oxidation by molecular iodine, the crystallinity is affected. 1 absorbs N2, H2, CH4, CH3OH, C2H5OH, PrOH, C6H6, and I2, but once it is in contact with H2O the absorption efficiency is drastically reduced. Upon PSM, the magnetism is transformed from a canted antiferromagnet (1·3DMF and 1·xGuest) to single-chain magnet (1), to a ferrimagnet (1a·7H2O), and to a ferromagnet (1b·I(-)·2H2O·1.5DMSO). Raman spectroscopy suggests the color change (purple to green 1a·7H2O or yellow 1b·I(-)·2H2O·1.5DMSO) is associated with a change of geometry from a strained octahedron due to the very acute chelating angle (∼60°) of the lactate of a cobalt center to a regular octahedron with a monodentate carboxylate and one H2O. The magnetic transformation is explained by the different interchain exchanges (J'), antiferromagnetic for 1·3DMF and 1·xSolvent (J' < 0), SCM for 1 (J' verge to 0), and ferromagnetic for 1a·7H2O (J' > 0), between homometal topological ferrimagnetic chains (two octahedral and one tetrahedral Co(II) ions) connected by the double walls of pybz at 13.3 Å (shortest Co···Co). For 1b·I(-)·2H2O·1.5DMSO the moment of the tetrahedral site is turned off, thus stabilizing a ferromagnetic state (J' > 0). The present stabilization of four magnetic ground states is unique in the field of metal-organic frameworks as well as the electrical conductivity of 1·2.7I2.

Structural diversity and photoluminescent properties of zinc benzotriazole-5-carboxylate coordination polymers.

A series of zinc(II) compounds, [Zn2(btca)2(im)2]·(DMA) (1; H2btca = benzotriazole-5-carboxylate acid, im = imidazole, and DMA = N,N-dimethylacetamide), [Zn(btca)(im)]·(DMF) (2; DMF = N,N-dimethylformamide), [Zn2(btca)2(tmdpy)]·2(DMF)·5(H2O) [3; tmdpy = 1,3-di(4-pyridyl)propane], and [H2N(CH3)2]2[Zn3(btca)4]·(DMF) (4), have been successfully synthesized via rational control of experimental conditions. Single-crystal X-ray diffraction analyses indicate that compounds 1 and 2 are isomers, and both of them exhibit two-dimensional structures with the same uninodal 3-connected fes topology. Additionally, the three-dimensional (3D) structure of 3 was obtained by using tmdpy instead of an im ligand under synthesis conditions similar to those of compound 2. Interestingly, compound 3 presents a pillared-layer structure with a (3,4)-connected (4.8(2).10(3))(4.8(2)) topology. When 4,4'-bipyridine was used to replace tmdpy, the assembly between Zn(2+) ions and H2btca ligands produced a chiral 3D framework of 4. Furthermore, compound 4 showed a new (3,4)-connected topology with a vertex symbol of (4.6.8(4))2(4.6.8)(4.8(2))(6.8(2))2(6.8(5)). A comparison of all compounds suggested that the structural diversity of the compounds could be tuned by altering the auxiliary ligand. In addition, the photoluminescent properties of compounds 1-4 were measured.

Tuning a layer to a pillared-layer metal-organic framework for adsorption and separation of light hydrocarbons.

Rational assembly from a layer structure to a pillared-layer metal-organic framework affords an ideal microporous material with large Langmuir surface area for highly selective separation of hydrocarbons.

Gas sorption, second-order nonlinear optics, and luminescence properties of a series of lanthanide-organic frameworks based on nanosized tris((4-carboxyl)phenylduryl)amine ligand.

By controlling the pH value of the reaction system, two sets of lanthanide (Ln)-tris((4-carboxyl)phenylduryl)amine (Ln = Ce, Pr, Nd, Sm) frameworks have been generated. Four isostructural noninterpenetrating frameworks (FIR-8 to FIR-11) are constructed from rod-shaped secondary building units and four other isostructural frameworks (FIR-12 to FIR-15) based on single Ln nodes are described as 8-fold interpenetrating dia-type nets. Gas sorption measurements for FIR-8 give a Langmuir surface area of 633.8 m(2)·g(-1) and a H2 uptake of 165.2 cm(3)·g(-1) at 77 K and 1 atm. However, FIR-12 with smaller pores can hardly adsorb any N2 and H2. Because both FIR-8 and FIR-12 crystallize in acentric space group, the second-harmonic generation (SHG) measurements indicate that both of them display strong powder SHG efficiencies, which are approximately 8 and 3 times as strong as that of a potassium dihydrogen phosphate powder. In addition, the fluorescent emissions of all compounds in the solid state are also investigated in detail.

A porous 4-fold-interpenetrated chiral framework exhibiting vapochromism, single-crystal-to-single-crystal solvent exchange, gas sorption, and a poisoning effect.

The synthesis and characterization of a 4-fold-interpenetrated pseudodiamond metal-organic framework (MOF), Co(II)(pybz)2·2DMF [pybz = 4-(4-pyridyl)benzoate], are reported. N,N-Dimethylformamide (DMF) of the channels can be removed to give the porous framework, and it can also be exchanged for methanol, ethanol, benzene, and cyclohexane. It is a rare example of a stable MOF based on a single octahedral building unit. The single-crystal structures of Co(II)(pybz)2·2DMF, Co(II)(pybz)2, Co(II)(pybz)2·4MeOH, and Co(II)(pybz)2·2.5EtOH have been successfully determined. In all of them, the framework is marginally modified and contains a highly distorted and strained octahedral node of cobalt with two pyridine nitrogen atoms and two chelate carboxylate groups. In air, the crystals of Co(II)(pybz)2·2DMF readily change color from claret red to light pink. Thermogravimetric analysis and Raman spectroscopy indicate a change in coordination, where the carboxylate becomes monodentate and an additional two water molecules are coordinated to each cobalt atom. In a dry solvent, this transformation does not take place. Tests show that Co(II)(pybz)2 may be a more efficient drying agent than silica gel and anhydrous CuSO4. The desolvated Co(II)(pybz)2 can absorb several gases such as CO2, N2, H2, and CH4 and also vapors of methanol, ethanol, benzene, and cyclohexane. If Co(II)(pybz)2 is exposed to air and followed by reactivation, its sorption capacity is considerably reduced, which we associate with a poisoning effect. Because of the long distance between the cobalt atoms in the structure, the magnetic properties are those of a paramagnet.