High-Pressure Molecular Sieving of High-Humidity C2H4/C2H6 Mixture by a Hydrophobic Flexible Metal-Organic Framework.

Molecular sieving is an ideal separation mechanism, but controlling pore size, restricting framework flexibility, and avoiding strong adsorption are all very challenging. Here, we report a flexible adsorbent showing molecular sieving at ambient temperature and high pressure, even under high humidity. While typical guest-induced transformations are observed, a high transition pressure of 16.6 atm is observed for C2H4 at 298 K because of very weak C2H4 adsorption (~16 kJ mol-1). Also, C2H6 is completely excluded below the pore-opening pressure of 7.7 atm, giving single-component selectivity of ca. 300. Quantitative high-pressure column breakthrough experiments using 1:1 C2H4/C2H6 mixture at 10 atm as input confirms molecular sieving with C2H4 adsorption of 0.73 mmol g-1 or 32 cm3(STP) cm-3 and negligible C2H6 adsorption of 0.001(2) mmol g-1, and the adsorbent can be completely regenerated by inert gas purging. Furthermore, it is highly hydrophobic with negligible water adsorption, and the C2H4/C2H6 separation performance is unaffected at high humidity.

Efficient Capture and Electroreduction of Dilute CO2 into Highly Pure and Concentrated Formic Acid Aqueous Solution.

High-purity CO2 rather than dilute CO2 (15 vol %, CO2/N2/O2 = 15:80:5, v/v/v) similar to the flue gas is currently used as the feedstock for the electroreduction of CO2, and the liquid products are usually mixed up with the cathode electrolyte, resulting in high product separation costs. In this work, we showed that a microporous conductive Bi-based metal-organic framework (Bi-HHTP, HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) can not only efficiently capture CO2 from the dilute CO2 under high humidity but also catalyze the electroreduction of the adsorbed CO2 into formic acid with a high current density of 80 mA cm-2 and a Faradaic efficiency of 90% at a very low cell voltage of 2.6 V. Importantly, the performance in a dilute CO2 atmosphere was close to that under a high-purity CO2 atmosphere. This is the first catalyst that can maintain exceptional eCO2RR performance in the presence of both O2 and N2. Moreover, by using dilute CO2 as the feedstock, a 1 cm-2 working electrode coating with Bi-HHTP can continuously produce a 200 mM formic acid aqueous solution with a relative purity of 100% for at least 30 h in a membrane electrode assembly (MEA) electrolyzer. The product does not contain electrolytes, and such a highly concentrated and pure formic acid aqueous solution can be directly used as an electrolyte for formic acid fuel cells. Comprehensive studies revealed that such a high performance might be ascribed to the CO2 capture ability of the micropores on Bi-HHTP and the lower Gibbs free energy of formation of the key intermediate *OCHO on the open Bi sites.

Utilisation of carbon dioxide and nitrate for urea electrosynthesis with a Cu-based metal-organic framework.

It is important and challenging to utilise CO2 and NO3- as a feedstock for electrosynthesis of urea. Herein, we reported a stable 2D metal-organic framework (MOF) Cu-HATNA, possessing planar CuO4 active sites, as an efficient electrocatalyst for coupling CO2 and NO3- into urea, achieving a high yield rate of 1.46 g h-1 gcat-1 with a current density of 44.2 mA cm-1 at -0.6 V vs. RHE. This performance surpasses most of the previously reported catalysts, revealing the great prospects of MOFs in sustainable urea synthesis.

Continuously Producing Highly Concentrated and Pure Acetic Acid Aqueous Solution via Direct Electroreduction of CO2.

It is crucial to achieve continuous production of highly concentrated and pure C2 chemicals through the electrochemical CO2 reduction reaction (eCO2RR) for artificial carbon cycling, yet it has remained unattainable until now. Despite one-pot tandem catalysis (dividing the eCO2RR to C2 into two catalytical reactions of CO2 to CO and CO to C2) offering the potential for significantly enhancing reaction efficiency, its mechanism remains unclear and its performance is unsatisfactory. Herein, we selected different CO2-to-CO catalysts and CO-to-acetate catalysts to construct several tandem catalytic systems for the eCO2RR to acetic acid. Among them, a tandem catalytic system comprising a covalent organic framework (PcNi-DMTP) and a metal-organic framework (MAF-2) as CO2-to-CO and CO-to-acetate catalysts, respectively, exhibited a faradaic efficiency of 51.2% with a current density of 410 mA cm-2 and an ultrahigh acetate yield rate of 2.72 mmol m-2 s-1 under neutral conditions. After electrolysis for 200 h, 1 cm-2 working electrode can continuously produce 20 mM acetic acid aqueous solution with a relative purity of 95+%. Comprehensive studies revealed that the performance of tandem catalysts is influenced not only by the CO supply-demand relationship and electron competition between the two catalytic processes in the one-pot tandem system but also by the performance of the CO-to-C2 catalyst under diluted CO conditions.

Highly Efficient Electroreduction of CO2 to Ethanol via Asymmetric C-C Coupling by a Metal-Organic Framework with Heterodimetal Dual Sites.

The electroreduction of CO2 into value-added liquid fuels holds great promise for addressing global environmental and energy challenges. However, achieving highly selective yielding of multi-carbon oxygenates through the electrochemical CO2 reduction reaction (eCO2RR) is a formidable task, primarily due to the sluggish asymmetric C-C coupling reaction. In this study, a novel metal-organic framework (CuSn-HAB) with unprecedented heterometallic Sn···Cu dual sites (namely, a pair of SnN2O2 and CuN4 sites bridged by µ-N atoms) was designed to overcome this limitation. CuSn-HAB demonstrated an impressive Faradic efficiency (FE) of 56(2)% for eCO2RR to alcohols, achieving a current density of 68 mA cm-2 at a low potential of -0.57 V (vs RHE). Notably, no significant degradation was observed over a continuous 35 h operation at the specified current density. Mechanistic investigations revealed that, in comparison to the copper site, the SnN2O2 site exhibits a higher affinity for oxygen atoms. This enhanced affinity plays a pivotal role in facilitating the generation of the key intermediate *OCH2. Consequently, compared to homometallic Cu···Cu dual sites (generally yielding ethylene product), the heterometallic dual sites were proved to be more thermodynamically favorable for the asymmetric C-C coupling between *CO and *OCH2, leading to the formation of the key intermediate *CO-*OCH2, which is favorable for yielding ethanol product.

Simultaneous Capture of CO2 Boosting Its Electroreduction in the Micropores of a Metal-organic Framework.

Integration of CO2 capture capability from simulated flue gas and electrochemical CO2 reduction reaction (eCO2 RR) active sites into a catalyst is a promising cost-effective strategy for carbon neutrality, but is of great difficulty. Herein, combining the mixed gas breakthrough experiments and eCO2 RR tests, we showed that an Ag12 cluster-based metal-organic framework (1-NH2 , aka Ag12 bpy-NH2 ), simultaneously possessing CO2 capture sites as "CO2 relays" and eCO2 RR active sites, can not only utilize its micropores to efficiently capture CO2 from simulated flue gas (CO2 : N2 =15 : 85, at 298 K), but also catalyze eCO2 RR of the adsorbed CO2 into CO with an ultra-high CO2 conversion of 60 %. More importantly, its eCO2 RR performance (a Faradaic efficiency (CO) of 96 % with a commercial current density of 120 mA cm-2 at a very low cell voltage of -2.3 V for 300 hours and the full-cell energy conversion efficiency of 56 %) under simulated flue gas atmosphere is close to that under 100 % CO2 atmosphere, and higher than those of all reported catalysts at higher potentials under 100 % CO2 atmosphere. This work bridges the gap between CO2 enrichment/capture and eCO2 RR.

Dicopper(I) Sites Confined in a Single Metal-Organic Layer Boosting the Electroreduction of CO2 to CH4 in a Neutral Electrolyte.

It is challenging and important to achieve high performance for an electrochemical CO2 reduction reaction (eCO2RR) to yield CH4 under neutral conditions. So far, most of the reported active sites for eCO2RR to yield CH4 are single metal sites; the performances are far below the commercial requirements. Herein, we reported a nanosheet metal-organic layer in single-layer, namely, [Cu2(obpy)2] (Cuobpy-SL, Hobpy = 1H-[2,2']bipyridinyl-6-one), possessing dicopper(I) sites for eCO2RR to yield CH4 in a neutral aqueous solution. Detailed examination of Cuobpy-SL revealed high performance for CH4 production with a faradic efficiency of 82(1)% and a current density of ∼90 mA cm-2 at -1.4 V vs. reversible hydrogen electrode (RHE). No obvious degradation was observed over 100 h of continuous operation, representing a remarkable performance to date. Mechanism studies showed that compared with the conventional single-copper sites and completely exposed dicopper(I) sites, the dicopper(I) sites in the confined space formed by the molecular stacking have a strong affinity to key C1 intermediates such as *CO, *CHO, and *CH2O to facilitate the CH4 production, yet inhibiting C-C coupling.

Self-Accelerating Effect in a Covalent-Organic Framework with Imidazole Groups Boosts Electroreduction of CO2 to CO.

Solvent effect plays an important role in catalytic reaction, but there is little research and attention on it in electrochemical CO2 reduction reaction (eCO2 RR). Herein, we report a stable covalent-organic framework (denoted as PcNi-im) with imidazole groups as a new electrocatalyst for eCO2 RR to CO. Interestingly, compared with neutral conditions, PcNi-im not only showed high Faraday efficiency of CO product (≈100 %) under acidic conditions (pH ≈ 1), but also the partial current density was increased from 258 to 320 mA cm-2 . No obvious degradation was observed over 10 hours of continuous operation at the current density of 250 mA cm-2 . The mechanism study shows that the imidazole group on the framework can be protonated to form an imidazole cation in acidic media, hence reducing the surface work function and charge density of the active metal center. As a result, CO poisoning effect is weakened and the key intermediate *COOH is also stabilized, thus accelerating the catalytic reaction rate.

"Ship-in-a-Bottle" Integration of Ditin(IV) Sites into a Metal-Organic Framework for Boosting Electroreduction of CO2 in Acidic Electrolyte.

The electrochemical CO2 reduction reaction (eCO2RR) under acidic conditions has become a promising way to achieve high CO2 utilization because of the inhibition of undesirable carbonate formation that typically occurs under neutral and alkaline conditions. Herein, unprecedented and highly active ditin(IV) sites were integrated into the nanopores of a metal-organic framework, namely NU-1000-Sn, by a "ship-in-a-bottle" strategy. NU-1000-Sn delivers nearly 100% formic acid Faradaic efficiency at an industry current density of 260 mA cm-2 with a high single-pass CO2 utilization of 95% in an acidic solution (pH = 1.67). No obvious degradation was observed over 15 hours of continuous operation at the current density of 260 mA cm-2, representing the remarkable eCO2RR performance in acidic electrolyte to date. The mechanism study shows that both oxygen atoms of the key intermediate *HCOO can coordinate to the two adjacent Sn atoms in a ditin(IV) site simultaneously. Such bridging coordination is conducive to the hydrogenation of CO2, thus leading to high performance.

Efficient electroreduction of CO to acetate using a metal-azolate framework with dicopper active sites.

Electrochemical reduction of CO to value-added products, especially C2 products, provides a potential approach to achieve carbon neutrality and overcome the energy crisis. Herein, we report a metal-azolate framework (CuBpz) with dicopper active sites as an electrocatalyst for the electrochemical CO reduction reaction (eCORR). As a result, CuBpz achieved an impressive faradaic efficiency (FE) of 47.8% for yielding acetate with a current density of -200 mA cm-2, while no obvious degradation was observed over 60 hours of continuous operation at a current density of -200 mA cm-2. Mechanism studies revealed that the dicopper site can promote C-C coupling between two C1 intermediates, thereby being conducive to the generation of the key *CH2COOH intermediate.

Cyclic Trinickel(II) Clusters in a Metal-Azolate Framework for Efficient Overall Water Splitting.

Herein, a stable metal-azolate framework with cyclic trinickel(II) clusters, namely [Ni3 (µ3 -O)(BTPP)(OH)(H2 O)2 ] (Ni-BTPP, H3 BTPP=1,3,5-tris((1H-pyrazol-4-yl)phenylene)benzene), achieved a current density of 50 mA cm-2 at a cell voltage of 1.8 V in 1.0 M KOH solution, while the current density of 20%Pt/C@NF||IrO2 @NF is just 35.8 mA cm-2 at 2.0 V under the same condition. Moreover, no obvious degradation was observed over 12 hours of continuous operation at a large current density of 50 mA cm-2 . Theoretical calculations revealed that the µ3 -O atom in the cyclic trinickel(II) cluster serves as hydrogen-bonding acceptor to facilitate the dissociation of a H2 O molecule adsorbed on the adjacent Ni(II) ion, giving a lower energy barrier of H2 O dissociation compared with Pt/C; meanwhile, the µ3 -O atom can also participate in the water oxidation reaction to couple with the adjacent *OH adsorbed on Ni(II) ion, providing a low-energy coupling pathway, thus Ni-BTPP achieves a high performance for overall water splitting.

Isolated Tin(IV) Active Sites for Highly Efficient Electroreduction of CO2 to CH4 in Neutral Aqueous Solution.

The development of efficient electrocatalysts with non-copper metal sites for electrochemical CO2 reduction reactions (eCO2 RR) to hydrocarbons and oxygenates is highly desirable, but still a great challenge. Herein, a stable metal-organic framework (DMA)4 [Sn2 (THO)2 ] (Sn-THO, THO6- = triphenylene-2,3,6,7,10,11-hexakis(olate), DMA = dimethylammonium) with isolated and distorted octahedral SnO6 2- active sites is reported as an electrocatalyst for eCO2 RR, showing an exceptional performance for eCO2 RR to the CH4 product rather than the common products formate and CO for reported Sn-based catalysts. The partial current density of CH4 reaches a high value of 34.5 mA cm-2 , surpassing most reported copper-based and all non-Cu metal-based catalysts. Our experimental and theoretical results revealed that the isolated SnO6 2- active site favors the formation of key *OCOH species to produce CH4 and can greatly inhibit the formation of *OCHO and *COOH species to produce *HCOOH and *CO, respectively.

Synergistic Effect in a Metal-Organic Framework Boosting the Electrochemical CO2 Overall Splitting.

It is a very important but still challenging task to develop bifunctional electrocatalysts for highly efficient CO2 overall splitting. Herein, we report a stable metal-organic framework (denoted as PcNi-Co-O), composed of (2,3,9,10,16,17,23,24-octahydroxyphthalocyaninato)nickel(II) (PcNi-(O-)8) ligands and the planar CoO4 nodes, for CO2 overall splitting. When working as both cathode and anode catalysts (i.e., PcNi-Co-O||PcNi-Co-O), PcNi-Co-O achieved a commercial-scale current density of 123 mA cm-2 (much higher than the reported values (0.2-12 mA cm-2)) with a Faradic efficiency (CO) of 98% at a low cell voltage of 4.4 V. Mechanism studies suggested the synergistic effects between two active sites, namely, (i) electron transfer from CoO4 to PcNi sites under electric fields, resulting in the raised oxidizability/reducibility of CoO4/PcNi sites, respectively; (ii) the energy-level matching of cathode and anode catalysts can reduce the energy barrier of electron transfer between them and improve the performance of CO2 overall splitting.

Rational Design of Metal-Organic Frameworks for Electroreduction of CO2 to Hydrocarbons and Carbon Oxygenates.

Since CO2 can be reutilized by using renewable electricity in form of product diversity, electrochemical CO2 reduction (ECR) is expected to be a burgeoning strategy to tackle environmental problems and the energy crisis. Nevertheless, owing to the limited selectivity and reaction efficiency for a single component product, ECR is still far from a large-scale application. Therefore, designing high performance electrocatalysts is the key objective in CO2 conversion and utilization. Unlike most other types of electrocatalysts, metal-organic frameworks (MOFs) have clear, designable, and tunable catalytic active sites and chemical microenvironments, which are highly conducive to establish a clear structure-performance relationship and guide the further design of high-performance electrocatalysts. This Outlook concisely and critically discusses the rational design strategies of MOF catalysts for ECR in terms of reaction selectivity, current density, and catalyst stability, and outlines the prospects for the development of MOF electrocatalysts and industrial applications. In the future, more efforts should be devoted to designing MOF structures with high stability and electronic conductivity besides high activity and selectivity, as well as to develop efficient electrolytic devices suitable for MOF catalysts.

Single-Product Faradaic Efficiency for Electrocatalytic of CO2 to CO at Current Density Larger than 1.2†A cm-2 in Neutral Aqueous Solution by a Single-Atom Nanozyme.

Electroreduction of CO2 to CO is a promising approach for the cycling use of CO2 , while it still suffers from impractical current density and durability. Here we report a single-atom nanozyme (Ni-N5 -C) that achieves industrial-scale performance for CO2 -to-CO conversion with a Faradaic efficiency (FE) exceeded 97 % over -0.8--2.4 V vs. RHE. The current density at -2.4 V vs. RHE reached a maximum of 1.23 A cm-2 (turnover frequency of 69.7 s-1 ) with an FE of 99.6 %. No obvious degradation was observed over 100 hours of continuous operation. Compared with the planar Ni-N4 site, the square-pyramidal Ni-N5 site has an increase and a decrease in the d z 2 ${{{\rm d}}_{{z}^{2}}}$ and dxz/yz orbital energy levels, respectively, as revealed by density functional theory calculations. Thus, the Ni-N5 catalytic site is more superior to activate CO2 molecule and reduce the energy barriers as well as promote the CO desorption, thus boosting the kinetic activation process and catalytic activity.

A Stable Metal-azolate Framework with Cyclic Tetracopper(I) Clusters for Highly Selective Electroreduction of CO2 to C2 Products.

It is of great significance for constructing electrocatalysts with accurate structures and compositions to pinpoint the active sites, thereby improving the C2 products (C2 H4 , C2 H5 OH and CH3 COOH) selectivity during electrocatalytic CO2 reduction raction. Here, we report a tetracopper(I) cluster-based metal-organic framework that exhibits long-term stability and remarkable performance for electroreduction CO2 towards C2 products in an H-type cell with a maximum Faradaic efficiency (FE) of 72%, and delivers a current density of 350 mA cm-2 with a FE(C2 ) up to 46% in a flow cell device, outperforming most of the Cu-based electrocatalysts such as Cu derivatives and Cu nanostructured materials. Importantly, no obvious degradation was observed at 350 mA cm-2 over 20 hours of continuous operation, strengthening the practicability. In-situ infrared spectroscopy analysis showed the cooperative effect of adjacent Cu(I) ions in tetracopper(I) cluster may promote the C-C coupling to generate C2 products.

A Porous π-π Stacking Framework with Dicopper(I) Sites and Adjacent Proton Relays for Electroreduction of CO2 to C2+ Products.

Crystalline porous materials sustained by supramolecular interactions (e.g., π-π stacking interactions) are a type of molecular crystals showing considerable stability, but their applications are rarely reported due to the high difficulty of their construction. Herein, a stable π-π stacking framework formed by a trinuclear copper(I) compound [Cu3(HBtz)3(Btz)Cl2] (CuBtz, HBtz = benzotriazole) with pyrazolate-bridged dicopper(I) sites is reported and employed for electrochemical CO2 reduction, showing an impressive performance of 73.7 ± 2.8% Faradaic efficiency for C2+ products [i.e., ethylene (44%), ethanol (21%), acetate (4.7%), and propanol (4%)] with a current density of 7.9 mA cm-2 at the potential of -1.3 V versus RHE in an H-type cell and a Faradic efficiency (61.6%) of C2+ products with a current density of ≈1 A cm-2 and a reaction rate of 5639 µmol m-2 s-1 at the potential of -1.6 V versus RHE in a flow cell device, representing an impressive performance reported to date. In-situ infrared spectroscopy, density functional theory calculations, and control experiments revealed that the uncoordinated nitrogen atoms of benzotriazolates in the immediate vicinity can act as proton relays and cooperate with the dicopper(I) site to promote the hydrogenation process of the *CO intermediate and the C-C coupling, resulting in the highly selective electroreduction of CO2 to C2+ products.

A Stable and Conductive Covalent Organic Framework with Isolated Active Sites for Highly Selective Electroreduction of Carbon Dioxide to Acetate.

Electroreduction of CO2 to acetate provides a promising strategy to reduce CO2 emissions and store renewable energy, but acetate is usually a by-product. Here, we show a stable and conductive two-dimensional phthalocyanine-based covalent-organic framework (COF) as an electrocatalyst for reduction of CO2 to acetate with a single-product Faradaic efficiency (FE) of 90.3(2)% at -0.8 V (vs. RHE) and a current density of 12.5 mA cm-2 in 0.1 M KHCO3 solution. No obvious degradation was observed over 80 hours of continuous operation. Combined with the comparison of the properties of other catalysts with isolated metal active sites, theoretical calculations and in situ infrared spectroscopy revealed that the isolated copper-phthalocyanine active site with high electron density is conducive to the key step of C-C coupling of *CH3 with CO2 to produce acetate, and can avoid the coupling of *CO with *CO or *CHO to produce ethylene and ethanol.

Coupling Ruthenium Bipyridyl and Cobalt Imidazolate Units in a Metal-Organic Framework for an Efficient Photosynthetic Overall Reaction in Diluted CO2.

Artificial photocatalytic CO2 reduction, using water as the reductant, is challenging mainly because it is difficult for multiple functional units to cooperate efficiently. Here, we show that the classic photosensitive and H2O-oxidizing ruthenium bipyridyl units and CO2-reducing cobalt imidazolate units can be incorporated into a metal-organic framework using a classic organic ligand, imidazo[4,5-f][1,10]phenanthroline. Under visible light without additional sacrificial agents and photosensitizers, the overall conversion of CO2 and H2O to CO and O2 was achieved by the multifunctional photocatalyst in the CH3CN/H2O mixed solvent with a high CO production rate of 11.2 µmol g-1 h-1 and CO selectivity of ca. 100%. Thanks to its ultramicroporous structure with moderately strong CO2 adsorption ability, the photocatalyst also exhibited high performances with CO/CH4 production rates of 5.15/0.62 and 4.26/0.20 µmol g-1 h-1 in the gas phase with pure and even diluted CO2, respectively. Photoluminescence emission spectroscopy and photoelectrochemical tests confirmed that the photosensitive and catalytic units cooperated well to give suitable photocatalytic redox potentials and fast electron-hole separation.

A Cu(111)@metal-organic framework as a tandem catalyst for highly selective CO2 electroreduction to C2H4.

Here, we report an improved tandem catalytic mechanism for electroreduction of CO2 to C2H4. Cu(111) nanoparticles with an average size of 5.5 ± 0.9 nm were anchored on a conductive Cu-based metal-organic framework (Cu-THQ) by in situ electrochemical synthesis. Compared to Cu(111) nanoparticles, the C2H4 faradaic efficiency of the tandem catalyst Cu(111)@Cu-THQ was increased doubly.