High‐Efficiency Recovery of Au (III) From Acidic Metallurgical Wastewater Using Sulfonic Acid‐Functionalized UIO‐67‐NH 2 Metal–Organic Frameworks

Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2021Regulating Electronic Status of Platinum Nanoparticles by Metal–Organic Frameworks for Selective Catalysis Yu Shen†, Ting Pan†, Peng Wu, Jiawei Huang, Hongfeng Li, Islam E. Khalil, Sheng Li, Bing Zheng, Jiansheng Wu, Qiang Wang, Weina Zhang, Wei David Wei and Fengwei Huo Yu Shen† Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Ting Pan† Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Peng Wu Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Jiawei Huang Department of Chemistry, Center for Catalysis, University of Florida, Gainesville, FL 32611 , Hongfeng Li Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Islam E. Khalil Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Sheng Li Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Bing Zheng Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Jiansheng Wu Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Qiang Wang Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816 , Weina Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Wei David Wei *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Center for Catalysis, University of Florida, Gainesville, FL 32611 and Fengwei Huo *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 https://doi.org/10.31635/ccschem.020.202000278 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Selective hydrogenation of alkynes to alkenes remains challenging in the field of catalysis due to the ease of over-hydrogenated of alkynes to alkanes. Favorably, the incorporation of metal nanoparticles (MNPs) into metal–organic frameworks (MOFs) provides an opportunity to adjust the surface electronic properties of MNPs for selective hydrogenation of alkynes. Herein, we used different metal-O clusters of MOFs to regulate the electronic status of platinum nanoparticles (Pt NPs) toward overhydrogenation, semihydrogenation, and unhydrogenation of phenylacetylene. Specifically, Pt/Fe-O cluster-based MOFs are found to reduce the electronic density on Pt NPs and inhibit the overhydrogenation of styrene, leading to an 80% increase in selectivity toward a semihydrogenation product (styrene). Meanwhile, Cu-O cluster-based MOFs generate high oxidation states of Pt NPs and release Cu2+ ions, which worked together to deactivate Pt NPs in the hydrogenation reaction entirely. Thus, our studies illustrate the critical role of metal-O clusters in governing chemical environments within MOFs for the precise control of selective hydrogenation of alkynes, thereby, offering appealing opportunities for designing MNPs/MOFs catalysts to prompt a variety of reactions. Download figure Download PowerPoint Introduction Selective hydrogenation of alkynes to alkenes is a key transformation reaction in industrial manufacturing of fine chemicals, pharmaceuticals, polymers, and others.1–3 Achieving high selectivity of partial hydrogenation products in an economical, mild, and environmentally benign way is still a challenge because it is easy to overhydrogenate alkynes into alkanes.4,5 Heterogeneous metal catalysts are brought into the spotlight due to their high activity and stability.6 A traditional Lindlar catalyst, composed of palladium nanoparticles (Pd NPs) modified by lead ions and quinolone additives, has been used widely in the semihydrogenation industry for decades.7 However, toxic lead ions in Lindlar catalysts hamper their applications in the green chemical industry. Recently, metal oxides, organic molecule- or metal ion-modified metal nanoparticles (MNPs), multimetallic alloys, and single-atom catalysts have been developed for improving the selectivity of semihydrogenation.8–11 However, all those materials require complicated procedures to tune the electronic status of MNPs to achieve selective hydrogenation. Thus, developing new strategies to regulate the electronic status of MNPs remains a critical issue in the field of selective hydrogenation of alkynes. As an emerging porous material, metal–organic frameworks (MOFs) offer intriguing properties, including diverse organic–inorganic compositions, facile functionalization, and uniform yet tunable cavities, showing promising application prospects in gas separation, sensor platforms, heterogeneous catalysis, and so on.12–14 Besides, MOFs have been used as hosts for MNPs, and those hybrid catalysts combine both the molecular sieving effect of MOFs matrix and the high catalytic activity of MNPs.15–21 Recently, scientists found that MOFs could be used to modulate the electronic status of MNPs.22–24 For instance, Zhao et al.25 demonstrated that controlling the electron transfer in Pt/MOFs improved the catalytic selectivity for hydrogenation of α,β-unsaturated aldehydes. Also, Xiao et al.26 observed that tuning the electron transfer between platinum nanoparticles (Pt NPs) and porphyrinic MOFs allowed an increase in the surface electron densities of Pt NPs and enhanced the alcohol oxidation. Herein, we demonstrate the use of metal-O clusters within MOFs to regulate precisely the interfacial electronic status of Pt NPs for promoting the selective hydrogenation of phenylacetylene (Scheme 1). Specifically, Cr-O, Fe-O, and Cu-O cluster-based MOFs, which had similar ligands and coordination structures, were exploited to create different chemical environments for Pt NPs. We found that Pt/Fe-O cluster-based MOFs catalysts exhibited >99% conversion of phenylacetylene and ∼80% selectivity to styrene. Meanwhile, Pt/Cr-O cluster-based MOFs catalysts showed no influence on the selectivity, and thus, resulted in overhydrogenation, with the formation of ethylbenzene. Surprisingly, Cu-O cluster-based hybrid catalysts were found to lose catalytic activity totally in the hydrogenation reaction. Further, studies using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and density functional theory (DFT) confirmed the essential role of metal-O clusters within MOFs in modulating the electronic status of Pt NPs in promoting the activity and selectivity of alkyne hydrogenation. Scheme 1 | Precise control of selective hydrogenation of phenylacetylene using distinct metal-O cluster-based MOFs as modulators to regulate the electronic status of Pt NPs. MOFs, metal–organic frameworks; Pt NPs, platinum nanoparticles. Download figure Download PowerPoint Experimental Methods Preparation of Pt/MOFs catalysts About 30 mL as-synthesized Pt NPs solution (0.6 mM) and 50 mg MOFs powder were stirred under 500 rpm at room temperature for 6 h. Subsequently, Pt/MOFs were collected by centrifugation at 8000 rpm for 5 min and washed twice with ethanol or methanol. Finally, the obtained Pt/MOFs powder was dried at room temperature in a vacuum oven for 12 h. Catalytic hydrogenation of phenylacetylene In a typical procedure, 10 mg of the Pt NPs catalyst was dispersed in 5 mL of methanol solution, and then 100 μL phenylacetylene was added to the above solution. Subsequently, the solution was purged with a H2 balloon. During the catalytic process, the reaction solution was stirred magnetically at room temperature for the desired reaction time. After that, the catalysts were separated by centrifugation, and the solution was analyzed by gas chromatography (GC). Results and Discussion Catalysts preparation To actualize the concept while avoiding the interference of pore size within MOFs and MNPs morphology, the hybrid Pt/MOFs catalysts were rationally designed and synthesized, where presynthesized MNPs were deposited on the MOFs surface. Pt NPs with an average size of 2.8 nm were synthesized by established methods ( Supporting Information Figure S1).27 Several stable MOFs were selected as supports to explore the effect of distinct metal-O clusters on regulating the electronic status of Pt NPs, namely, Cr-based MIL-100(Cr) and MIL-101(Cr); Fe-based MIL-100(Fe), MIL-101(Fe), and MIL-88(Fe); and Cu-based MOF, HKUST-1, and MOF nanosheets, Cu-TCPP.28–30 Pt NPs were dispersed uniformly on the MOFs surface, as revealed by transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) elemental mapping data (Figures 1a–1d). As shown in the large-scale TEM images from Supporting Information Figures S2–S8, no free Pt NPs were observed in the catalysts. Additionally, powder X-ray diffraction (PXRD) patterns of Pt/MOFs composites were identical to those of the corresponding simulated MOFs, indicating that MOFs maintained their original crystal structures after the deposition of Pt NPs ( Supporting Information Figures S9–S15). An inductively coupled plasma mass spectrometry (ICP-MS) measurements confirmed the similarity of Pt concentration in the composites ( Supporting Information Table S1). Figure 1 | TEM, high-angle annular dark-field scanning TEM (HAADF-STEM), and the corresponding EDS elemental mapping images of Pt NPs anchored on different MOFs supports. (a) Pt/MIL-101(Cr), (b) Pt/MIL-101(Fe), (c) Pt/MIL-88(Fe), (d) Pt/HKUST-1. TEM, transmission electron microscopy; EDS, energy-dispersive X-ray spectroscopy, Pt NPs, platinum nanoparticles; MOFs, metal–organic frameworks. Download figure Download PowerPoint Catalytic performance Various Pt/MOFs catalysts were utilized in exploring the chemoselectivity of phenylacetylene hydrogenations (Figure 2a). In a typical catalytic reaction, Pt/MIL-100(Cr) and Pt/MIL-101(Cr) with Cr-O clusters generated mostly overhydrogenation products (ethylbenzene), which were similar to the Pt NPs catalysts (Figure 2b and Supporting Information Table S2). Surprisingly, Pt/MIL-100(Fe), Pt/MIL-101(Fe), and Pt/MIL-88(Fe) with Fe-O clusters showed a high conversion (>99%) and selectivity (∼80%) toward the semihydrogenation product (styrene; Figure 2b), while suppressing the overhydrogenation of styrene to ethylbenzene (Figure 2c). No significant decay in the selectivity was noticed even when the reaction time was prolonged to 24 h ( Supporting Information Figure S16). Pt/HKUST-1 and Pt/Cu-TCPP composite of Cu-O clusters in MOFs showed no activity of phenylacetylene and styrene hydrogenations (Figures 2b and 2c). For comparison, MOFs and the corresponding metal oxide supports were also tested for the phenylacetylene and styrene hydrogenation. As shown in Supporting Information Table S2, all MOFs support exhibited no hydrogenation activity, and Pt/metal oxide catalysts showed no hydrogenation selectivity. The evaluation of Pt/MOFs catalytic performance was based on the similar Pt NPs loading (2%) and full conversion of phenylacetylene ( Supporting Information Figure S17 and Table S1). In short, Pt/MOFs catalysts showed three distinct hydrogenation results: overhydrogenation, semihydrogenation, and unhydrogenation, indicating that metal-O clusters within MOFs functioned as modulators of Pt NPs and altered the catalytic performance of phenylacetylene hydrogenation reaction in Pt NPs. Figure 2 | Performance of various Pt/MOFs catalysts for the hydrogenation reaction yielding phenylacetylene and styrene. (a) Schematic of hydrogenation of phenylacetylene. (b) The yield of phenylacetylene hydrogenation on various catalysts. (c) The yield of styrene to ethylbenzene on various catalysts. MOFs, metal–organic frameworks. Download figure Download PowerPoint Mechanistic studies We sought to gain an understanding of how metal-O clusters within MOFs affected Pt NPs' activity and selectivity in the hydrogenation of phenylacetylene by exploring the mechanism of the electronic effects and the coordination environments of Pt NPs on MOFs. The electronic properties were experimentally confirmed by XPS and XAS spectra. The Pt 4f spectra of Pt/MIL-101(Cr) and Pt/MIL-101(Fe) showed two main peaks at 71.3 ± 0.1 and 74.6 ± 0.1 eV, corresponding to Pt 4f7/2 and Pt 4f5/2, respectively (Figure 3a).31 Interestingly, an 0.3 eV shift of Pt 4f toward the high-energy side was observed on Pt/HKUST-1, revealing the difference in the electronic status of the Pt NPs supported on HKUST-1, compared with that on MIL-101(Cr) and MIL-101(Fe). Furthermore, the Pt 4f7/2 was fitted with two components, including the predominant metallic Pt0 located in the binding energy of 71.3 eV in the spectra (denoted as red peaks) and Pt2+ at 72.3 eV (denoted as blue peaks).32 Pt/MIL-101(Cr) showed 28% amount of Pt2+ species, which was similar to that of the bare Pt NPs (27%), indicating a weak electronic interaction between Pt NPs and MIL-101(Cr).25 We speculated that this weak electronic interaction was mainly due to the less overlap of the d orbitals between Pt NPs and Cr-O clusters in MIL-101(Cr).23 Notably, the Pt2+ species ratio in Pt/MIL-101(Fe) and Pt/HKUST-1 catalysts increased to 32% and 36%, respectively, revealing that Pt NPs on these two MOFs became electron deficient, compared with the bare Pt NPs. In comparison with the Cr 2p, Fe 2p, and Cu 2p XPS spectra of MIL-101(Cr), MIL-101(Fe), HKUST-1, and Pt/MOFs composites (Figures 3b–3d), the Cr 2p states remained unchanged after the deposition of Pt NPs, whereas noticeable increases in Fe2+ species (from 61.3% to 67.7%) and Cu+ species (from 22% to 41%) were observed. These phenomena suggested that electrons from Pt NPs were transferred to Fe-O and Cu-O clusters-based MOFs across the heterogeneous interface. As shown in the normalized X-ray absorption near-edge structure (XANES) at the Pt L-edge ( Supporting Information Figure S18), Pt/HKUST-1 exhibited a higher white line intensity than Pt/MIL-101(Fe), MIL-101(Cr), and bore Pt NPs, further confirming the electron deficiency of Pt NPs in Pt/HKUST-1.31 The local coordination environment of Pt in Pt/MOFs catalysts was further characterized by extended X-ray absorption fine structure (EXAFS; Supporting Information Figure S19). The stronger peaks at 2.6 and 1.6 Å could be assigned to Pt–Pt and Pt–O coordination, respectively (Figure 3e).33 The intensity of Pt–O peaks exhibited a gradual increase upon the deposition of Pt NPs on MIL-101(Fe), and HKUST-1 supports. The quantitative EXAFS curve fitting analysis revealed that the coordination number of Pt–O bond in Pt/MIL-101(Cr) was 0.4, while in Pt/MIL-101(Fe) and Pt/HKUST-1, it increased to 0.8 and 1.2, respectively ( Supporting Information Table S3). The formation of abundant Pt–O bonds and charge-transfer interactions demonstrated that MOFs matrix shared the same function as inorganic and organic materials to regulate the surface electronic status of Pt NPs. DFT calculations further proved that the exposed metal-O clusters on MOFs could withdraw electrons from Pt atoms in the order of MIL-101(Cr) < MIL-101(Fe) < HKUST-1 and consequently increased the Bader charge of Pt atoms (Figure 3f and Supporting Information Figure S20 and Table S4). Figure 3 | Electronic status of Pt on various Pt/MOFs catalysts for phenylacetylene hydrogenation reaction. (a) XPS profiles of Pt 4f for Pt NPs supported on a series of MOFs. The metallic Pt0 located at the binding energy of 71.3 and 74.6 eV, and oxidation state Pt2+ located at 72.3 and 75.5 eV. The ratio of Pt2+/Pt0 in various catalysts was marked. (b–d) XPS profiles of Cr 2p, Fe 2p, and Cu 2p in pure MOFs and Pt/MOFs catalysts. The ratio of reduction state of Fe2+ and Cu+ within Fe-O and Cu-O cluster-based MOFs was marked. (e) EXAFS spectra of Pt foil, PtO2, and Pt/MOFs catalysts. (f) DFT calculations of Bader charge of Pt atoms on different MOFs. (g) DFT calculations of the binding energy of H atoms on the Pt surface. (h) Photographs of 5 mg Pt/MOFs catalysts mixed with 45 mg of WO3 before and after treatment with H2 gas at 25 °C for 5 min. MOFs, metal–organic frameworks; XPS, X-ray photoelectron spectroscopy; EXAFS, extended X-ray absorption fine structure; DFT, density functional theory; WO3, tungsten oxide. Download figure Download PowerPoint The surface electronic status of Pt NPs directly correlated to the interaction with active H atoms and unsaturated molecules, thus, delivering different catalytic activities and selectivities of alkyne hydrogenation.34,35 The binding energy of active H atoms on Pt(111) (2 × 2) surface with different charges was explored by DFT calculations (Figure 3g and Supporting Information Figure S21 and Table S5). The small binding energy (−0.42 eV) of H atoms on the neutrally charged Pt surface indicated that active H atoms could readily migrate to the adsorbed phenylacetylene and/or styrene, resulting in overhydrogenation on Pt NPs and Pt/MIL-101(Cr) catalysts. When the Pt(111) surface charge increased to a positive value through the interfacial electron transfer from Pt to MOFs, the higher Pt–H binding energy (−1.2 eV) prevented the migration of active H atoms toward the adsorbed phenylacetylene and/or styrene, suppressing the overhydrogenation on Pt/MIL-101(Fe) and Pt/HKUST-1 catalysts. These DFT results were evaluated further by the color change in tungsten oxide (WO3), since active H atoms could react readily with the yellow WO3 to form dark blue HxWO3.36 As depicted in Figure 3h, the Pt/HKUST-1 with WO3 powder mixture exhibited no change in color after the H2 treatment, whereas mixing Pt/MIL-101(Cr) and Pt/MIL-101(Fe) with WO3 powders showed a different extent of color changes, which revealed the distinct binding energies of active H atoms on the Pt surface. These differences in the color change of WO3 were consistent with DFT simulations and the performance of phenylacetylene hydrogenation on distinct Pt/MOFs catalysts. Furthermore, the partially oxidized Pt could adsorb phenylacetylene tightly on its surface, while weakening its interaction with styrene molecules,34 as verified by the lower catalytic activity (5%) of Pt/MOFs(Fe) catalysts for styrene (Figure 2c). The electronic environment of Pt NPs created by Fe-O clusters allowed for the semihydrogenation of phenylacetylene while styrene intermediates preferred desorption from the Pt surface. The results mentioned above demonstrated that the chemical environment of metal-O clusters within MOFs could regulate the catalytic performance of Pt NPs in the hydrogenation of phenylacetylene. It is known that the metal ions also affect the catalytic activity and selectivity of MNPs in the hydrogenation reaction.37 Therefore, we wondered whether the trace of metal ions released during the catalytic process would affect the catalytic performance. As shown in Supporting Information Figure S22, even when Fe3+ ions were increased to 6 ppm, no apparent decay of Pt NPs in the hydrogenation activity was observed. The high conversion (99%) of phenylacetylene to ethylbenzene indicated that the Fe3+ ions could hardly modify Pt NPs to realize alkyne semihydrogenation. In contrast, 6 ppm Cu2+ ions could hinder the hydrogenation activity completely, suggesting that the activity of Pt NPs was sensitive to Cu2+ ions. Considering the potential influence of metal ions on the activity of Pt NPs, we built a core–shell structure [email protected](Cr) as catalysts to better evaluate the influences of metal ions released from MIL-101(Fe) and HKUST-1 on phenylacetylene hydrogenation reaction ( Supporting Information Figure S23). [email protected](Cr) catalysts reached 99% conversion of phenylacetylene to ethylbenzene, and the reduced reaction rate was mainly a result of the diffusion of the reactant through the MOFs channel from the surface to the active sites ( Supporting Information Figure S24). When HKUST-1 powders were introduced to the reaction, the conversion of phenylacetylene only reached 35% within 1 h, after which no further increase occurred ( Supporting Information Figure S24, red dots). This result indicated that the synergistic effect of Cu-O metal clusters and released Cu2+ ions could poison Pt NPs. However, when the MIL-101(Fe) was added to the reaction, the conversion of phenylacetylene reached 100%, and the selectivity was similar to [email protected](Cr) ( Supporting Information Figure S24, blue dots). These results confirmed that the metal-O clusters within MOFs could regulate the catalytic chemoselectivity of Pt NPs in the hydrogenation of alkynes. Furthermore, the catalytic stability of Pt/MIL-101(Fe) catalyst for phenylacetylene hydrogenation was evaluated by PXRD, TEM, and XPS characterizations. There was a slight decrease in Pt NPs content of Pt/MIL-101(Fe) after three reaction cycles (from 2.2% to 2.0%; Supporting Information Figure S25) and negligible influence on catalytic performance ( Supporting Information Figure S26), indicating the catalytic stability of Pt/MIL-101(Fe) catalyst. The crystallinity of MIL-101(Fe) changed slightly ( Supporting Information Figure S27) as the sizes, and electronic status of Pt NPs were retained ( Supporting Information Figures S28 and S29), which confirmed the stability of the Pt/MIL-101(Fe) catalyst. Conclusion We successfully demonstrated that employing different metal-O clusters within MOFs enabled the regulation of interfacial electronic structures of Pt NPs for significant improvement of selective hydrogenation of phenylacetylene. We found that Pt/Fe-O cluster-based MOFs catalysts highly favored the semihydrogenation of phenylacetylene to form styrene while Pt/Cr-O cluster-based MOFs facilitated overhydrogenation to an alkane. More importantly, Pt NPs were deactivated completely when Cu-O metal cluster-based MOFs were used as supports. Our studies affirmed that the electronic status of MNPs modified by metal-O clusters and trace poisonous metal ions released from MOFs is crucial in regulating the hydrogenation of unsaturated molecules, thus, opening up new paths for designing a series of suitable MNP/MOFs catalysts to boost other selective hydrogenation reactions of alkynes. Supporting Information Supporting Information is available. Conflict of Interest The authors declare no conflict of interest. Funding Information This study was supported by the National Key R&D Program of China (no. 2017YFA0207201), the National Natural Science Foundation (nos. 21727808, 21574065, 21604038, 21971114, 21604040, and 51702155), the National Science Foundation for Distinguished Young Scholars (no. 21625401), and the Jiangsu Provincial Funds for Natural Science Foundation (nos. BK20160975, BK20160981, and BK20170975). Acknowledgments The authors are grateful to the Synchrotron Radiation Research Center (NSRRC) in Taiwan for their help on X-ray absorption spectroscopy measurements.

Benzene is an important air pollutant and a key chemical feedstock for the synthesis of cyclohexane. Because of the small difference of 0.6°C in their boiling points, the separation of benzene and cyclohexane is extremely challenging. Here, we report the high adsorption of benzene at low pressure and efficient separation of benzene/cyclohexane, achieved by the control of pore chemistry of two families of robust metal-organic frameworks, UiO-66 and MFM-300. At 298 K, UiO-66-CuII shows an exceptional adsorption of benzene of 3.92 mmol g−1 at 1.2 mbar and MFM-300(Sc) exhibits a high selectivity of 166 for the separation of benzene/cyclohexane (v/v = 1/1) mixture. In situ synchrotron X-ray diffraction and neutron powder diffraction, and multiple spectroscopic techniques reveal the binding mechanisms of benzene and cyclohexane in these materials. We also report the first example of direct visualization of reversible binding of benzene at an open Cu(II) site within metal-organic frameworks.

Enzyme immobilization in metal-organic frameworks (MOFs) offers retained enzyme integrity and activity, enhanced stability, and reduced leaching. Trapping enzymes on MOF surfaces would allow for catalysis involving large substrates. In both cases, the catalytic efficiency and selectivity depend not only on enzyme integrity/concentration but also orientation. However, it has been a challenge to determine the orientation of enzymes that are supported on solid matrices, which is even more challenging for enzymes immobilized/trapped in MOFs due to the interferences of the MOF background signals. To address such challenge, we demonstrate in this work the utilization of site-directed spin labeling in combination with Electron Paramagnetic Resonance spectroscopy, which allows for the first time the characterization of the orientation of enzymes trapped on MOF surfaces. The obtained insights are fundamentally important for MOF-based enzyme immobilization design and understanding enzyme orientation once trapped in solid matrices or even cellular confinement conditions.

Reticular chemistry for the rational design of mechanically robust mesoporous merged-net metal-organic frameworks

Metal-organic frameworks (MOFs) have shown promise in enhancing the stability of biomolecules. Herein, biliverdin (BVD), a photoacoustic (PA) and fluorescent agent, was immobilized within the pores of NH2-MIL-101 (Fe) (FeMOFs) and on the surface of CuBTC crystallites (CuMOFs). MOFs were found to enhance the fluorescence emission and quench the PA intensity of biliverdin. Fluorescence and PA studies, in tandem with DFT simulations, demonstrated that the spectral interactions between MOFs and BVD resulted from interactions between biliverdin and the MOF pores and surfaces in addition to alterations in the HOMO-LUMO energy gap. The MOF internal structure of the MOF played a role in BVD loading, with the FeMOFs enabling greater BVD encapsulation, while CuMOF interactions with BVD primarily took place on the MOF surface. The role of these surface vs pore interactions in the release of biliverdin was explored. This study demonstrates that the effects of the MOF internal structure, surface interactions, and energy interactions should be taken into consideration for biomolecule loading in MOFs.

Highly porous and robust metal-organic frameworks (MOFs) were constructed based on aromatics-rich octa-carboxylate ligands and copper paddle-wheel building units. Each octa-carboxylate ligand is linked to eight copper paddle wheels via the bridging carboxylate groups in a rectangular prismatic fashion to lead to very rare (4,8)-connected networks of the scu topology. The high-connectivity MOFs show remarkably high porosity and framework stability, as evidenced by a perfect agreement between experimental and theoretical surface areas and the maintenance of framework powder X-ray diffraction patterns after solvent removal. These aromatics-rich MOFs exhibit an exceptionally high hydrogen uptake of up to 2.5 wt% at 77 K and 1 atm. This work thus demonstrates the ability to construct highly porous and robust functional MOFs using multidentate bridging ligands of high connectivity. Such a rational synthetic strategy is complementary to the common reliance on high-nuclearity metal clusters for building stable and porous MOFs.

Precise Construction of Stable Bimetallic Metal–Organic Frameworks with Single-Site Ti(IV) Incorporation in Nodes for Efficient Photocatalytic Oxygen Evolution

Metal organic frameworks (MOFs) for the removal of dissolved silica before reverse osmosis desalination

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jul 2021Copper (I)–Organic Frameworks for Catalysis: Networking Metal Clusters with Dynamic Covalent Chemistry Rong-Jia Wei†, Hou-Gan Zhou†, Zhi-Yin Zhang, Guo-Hong Ning and Dan Li Rong-Jia Wei† College of Chemistry and Materials Science, Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications, Jinan University, Guangzhou 510632 , Hou-Gan Zhou† College of Chemistry and Materials Science, Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications, Jinan University, Guangzhou 510632 , Zhi-Yin Zhang College of Chemistry and Materials Science, Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications, Jinan University, Guangzhou 510632 , Guo-Hong Ning *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Chemistry and Materials Science, Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications, Jinan University, Guangzhou 510632 and Dan Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Chemistry and Materials Science, Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications, Jinan University, Guangzhou 510632 https://doi.org/10.31635/ccschem.020.202000401 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Metal clusters exhibit diverse structures, emerging functions, and applications; thus, incorporating clusters into metal–organic frameworks (MOFs) brings tremendous merits. Although the construction of cluster-based MOFs is sophisticated, the reticular materials constructed from a combination of the chemistry of metal clusters and covalent organic frameworks (COFs) remain unexplored. Herein, we prepared two Cu(I) cluster-based MOFs with cyclic trinuclear units (CTUs), termed JNM-1 and JNM-2, either by a stepwise synthetic approach or by a one-pot reaction, for networking clusters with dynamic covalent chemistry, rarely utilized in MOF synthesis. The generated JNMs exhibited excellent stability and could be used as recyclable catalysts for palladium-free Sonogashira coupling reactions with high efficiency and tolerance (>90% yield for nine examples), without loss of performance for at least five cycle runs. In addition, conjugated single molecular wires with lengths ranging from 1.6 to 2.7 nm were synthesized feasibly using the JNM-1 catalyst. Download figure Download PowerPoint Introduction Reticular chemistry1 endows chemists to link molecular building blocks into extended and crystalline framework structures such as metal–organic frameworks (MOFs)2–6 and covalent organic frameworks (COFs)7–11 via strong coordinate and covalent bonds, respectively. Owing to relatively weaker strengths of coordinate bonds, compared with covalent bonds, MOFs are often suffering from stability issues, especially in harsh chemical environments such as strong bases and acids, boiling water, and reactions involving highly reactive substrates.12 In contrast, with the development of dynamic covalent chemistry (DCC), COFs could achieve high stabilities toward harsh conditions.13 However, the lack of metals restricts their functionalities and further applications. Therefore, it is envisioned that "cream-skimming" of coordination chemistry and DCC would address these shortcomings and might bring unprecedented structural complexity, along with functional diversity. Recently, incorporation of a single metal ion or mononuclear metal complexes into COFs, namely metal–covalent organic frameworks (MCOFs),14 was proposed and even shown to facilitate crystal growth of COFs with unusual topology,15,16 leading to emerging applications in catalysis, molecular adsorption and separation, optics, and sensing.14 Compared with single metal ion units, metal clusters or polynuclear metal complexes are much more attractive because of their intriguing aesthetic and diverse structures, as well as fascinating functions such as magnetism, catalytic activities, and luminescence properties.17–19 Although the preparation of cluster-based MOFs has been well established,20,21 the construction of cluster-based, crystalline reticular materials via linkage of covalent bonds is highly challenging and remain scarcely explored.22–24 This is due to the incompatibility of the condition for cluster formation with those of DCC, and the stability along with solubility issues of clusters during their synthesis and crystallization. Cyclic trinuclear units (CTUs) with d10 metals are a class of metal clusters exhibiting unique properties such as unsaturated metal centers with a medium oxidation state, metallophilic attraction, π-acidity/basicity, and luminescence properties. Therefore, they are potentially useful for a wide range of applications, including chemical sensing, full-color display, gas absorption, and catalysis.25–28 In 2006, our group29 first introduced the solvothermal synthesis of a MOF with Cu3Pz3 CTUs (pyrazolate ligand [Pz]), with reaction conditions similar to those used in COF synthesis. Therefore, we reasoned that the Cu(I) cluster-based organic frameworks could be constructed in a hierarchical assembly fashion via a combination of metal clusters chemistry and COF, which is rarely adopted in MOFs' fabrication (Scheme 1).22–24 Unlike the one-pot synthesis, assembly of metal clusters-based COFs in a stepwise fashion could exclude the disturbance from other metal ions or ligands; thus, the extended structure could be predictable and designed precisely employing reticular chemistry. Scheme 1 | Stepwise and one-pot syntheses and structural illustration of the JNMs. Download figure Download PowerPoint In this study, we demonstrated the preparation of two-dimensional (2D) Cu(I) CTU-based organic frameworks, namely JNM-1 and JNM-2 (JNM represents Jinan material), from either imine condensation reaction between Cu3L3 [1H-pyrazole-4-carbaldehyde (HL)] as cluster units and organic linkers [i.e., 1,3,5-tris(4-aminophenyl)benzene ( 1) for JNM-1 or 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline ( 2) for JNM-2, respectively], or in a one-pot reaction (Scheme 1). Interestingly, these two MOFs featured much higher stability and porosity than those of Cu3L3 CTUs, making them promising platforms for catalytic study. Indeed JNM-1 exhibited excellent catalytic activities and broad substrate scope with various functional groups for the palladium (Pd)-free Sonogashira cross-coupling reaction. Besides, JNM-1 showed much better catalytic activities than those of Cu3L3, and it could be applicable in the synthesis of conjugated single molecular wires. Overall, the strategy of combining reticular chemistry of metal clusters and COF in a stepwise manner allowed us to merge their advantages for constructing new types of function-led reticular materials with rational design. Experimental Methods Synthesis of the complex Cu3L3 A mixture of the ligand 1H-pyrazole-4-carbaldehyde (HL) (24.0 mg, 0.25 mmol), Cu2O (14.3 mg, 0.1 mmol), 4 mL ethanol, and 0.1 mL pyridine was sealed in an 8 mL Pyrex tube, heated in an oven at 120 °C for 72 h, and then slowly cooled to room temperature at a rate of −5 °C·h−1. The light-yellow needle crystals of Cu3L3 formed were filtered and collected under a microscope manually. The yield of Cu3L3: 23.7 mg (75.8%, based on Cu2O). Chemical formula, C12H9Cu3N6O3: C, 30.29; H, 1.91; N, 17.66. Found: C, 30.45; H, 2.13; N, 17.42. IR (KBr, cm−1): 3481 w, 3109 w, 2782 w, 1667 s, 1537 s, 1416 m, 1337 w, 1203 s, 1044 m, 872 w, 767 m, 625 w. Solid-state 13C cross-polarization/magic-angle spinning nuclear magnetic resonance (CP/MAS NMR) (400 MHz), δ (ppm) 124, 142, 184. Stepwise synthesis of JNM-1 and JNM-2 A 10 mL Schlenk tube was charged with Cu3L3 (23.7 mg, 0.05 mmol), 1,3,5-tris(4-aminophenyl)benzene ( 1) (26.3 mg, 0.075 mmol) or 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline ( 2) (26.5 mg, 0.075 mmol), 0.5 mL of mesitylene, 0.5 mL of dioxane, and 0.1 mL of 6 M aqueous acetic acid. Each tube containing 1 or 2 was flash-frozen at 77 K in a liquid nitrogen bath and degassed with three freeze-pump-thaw cycles. Upon warming to room temperature, each tube was heated at 120 °C for 72 h. The pale green solid from each tube was isolated by filtration, washed, and solvent exchanged with tetrahydrofuran (THF) and fresh dimethylformamide (DMF). The resultant solids were dried under vacuum at 100 °C for 8 h to give JNM-1 and JNM-2 both as pale green powders. For JNM-1: Elemental analysis calcd (%) for C36H24Cu3N9: C, 55.92; H, 3.11; N, 16.31. Found: C, 53.58; H, 2.40; N, 15.71. Yield: 28.1 mg (73%, based on Cu3L3). IR (KBr pellets, cm−1): 3355 w, 3112 w, 2872 w, 1667 m, 1617 s, 1539 m, 1490 m, 1375 w, 1199 m, 1050 w, 863 m, 748 w, 641 m. For JNM-2: Elemental analysis calcd (%) for C33H21Cu3N12: C, 51.06; H, 2.70; N, 21.66. Found: C, 49.52; H, 2.01; N, 21.20. Yield: 29.5 mg (76%, based on Cu3L3). IR (KBr pellets, cm−1): 3369 w, 1667 m,1591 m, 1507 s, 1417 w, 1369 m, 1309 w, 1246 w, 1203 m, 1144 w, 1071 w, 1013 w, 877 w, 814 w. One-pot synthesis of JNM-1 and JNM-2 A 10 mL Schlenk tube was charged with Cu2O (10.7 mg, 0.075 mmol), HL (14.4 mg, 0.15 mmol), 1 (26.3 mg, 0.075 mmol) or 2 (26.5 mg, 0.075 mmol), 0.5 mL of mesitylene, 0.5 mL of dioxane, and 0.1 mL of 6 M aqueous acetic acid. The tube was flash-frozen at 77 K in a liquid nitrogen bath and degassed with three freeze-pump-thaw cycles. Upon warming to room temperature, the tube was heated at 120 °C for 72 h. The pale green solid was isolated by filtration, washed, and solvent exchanged with THF and DMF. The resultants were dried under vacuum at 100 °C for 8 h to give JNM-1 and JNM-2 as pale green powders for catalytic performance experiments. General procedure for the Sonogashira cross-coupling reaction Before the catalytic experiment, the catalysts were dried in a vacuum at 120 °C for 8 h. About 4 mol % of the dried catalysts and 5 mL of DMF were added into a 10 mL Pyrex tube. Then phenylacetylene (0.5 mmol, 51.5 mg), iodobenzene (0.6 mmol, 122.4 mg), and K2CO3 (1 mmol, 138.2 mg) were added into the tube, orderly. The mixture was stirred at 140 °C under N2 atmosphere for 8 h. After that, 50 µL of the reaction solution was taken and diluted with CH2Cl2 to 1 mL, followed by centrifugation at 10,000 rpm·min−1 for 5 min. Then the supernatant was analyzed by gas chromatography–mass spectrometry (GC–MS). The reaction conversion was calculated based on the phenylacetylene reference substrate. Also, after the 8 h completion of the reaction, the mixture was quenched with water. The aqueous layer was extracted with ethyl acetate (3 × 150 mL), and the combined organic layers were washed with water, dried with anhydrous MgSO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography to give a solid white product. Different catalysts, solvents, temperatures, catalyst loading, and others were investigated in a similar procedure. Results and Discussion The initial synthetic attempts were carried out in a stepwise manner, in which single crystals of discrete, planar Cu3L3 were obtained under solvothermal conditions. A single crystallographic analysis revealed that Cu3L3 complexes formed a column packing with intermolecular Cu⋯Cu distances of 3.74 Å, indicating weak metal–metal interactions (Figures 1a–c). The solvothermolysis of a suspension of triangular Cu3L3 and organic linkers 1 or 2 in a 5∶5∶1 (v/v) mixture of mesitylene, 1,4-dioxane, and 6 M aqueous acetic acid led to crystalline products with hexagonal symmetry of hxl lamellar structures of JNM-1 and JNM-2 (Scheme 1). Although the high crystalline JNMs could be prepared feasibly from a one-pot reaction of the Cu2O, HL, and 1 or 2, where Cu3L3 was formed in situ, it was hard to remove unreacted Cu2O and remained as impurities, as confirmed by powder X-ray diffraction (PXRD) patterns (see Supporting Information Figures S2 and S3). A similar observation of metal oxide impurities was also reported previously in the one-pot synthetic approaches.22 Figure 1 | X-ray structure of Cu3L3. (a) ORTEP diagram at 50% level; (b) top view and (c) side view showing the crystal packing of Cu3L3 with intermolecular Cu⋯Cu distance of 3.74 Å. (C, N, O, H, and Cu atoms are shown as gray, light blue, light red, white, and orange, respectively.) Structural modeling of JNM-1 exhibiting (d) AA and (e) AB packing modes shown as space-filling models. (f) PXRD structural analysis of JNM-1. Experimental (black) and refined (red) PXRD patterns of JNM-1 with difference curve (blue), and calculated profiles of JNM-1 displaying AA (purple) and AB (green) packing modes. N2 adsorption (filled) and desorption (open) isotherm profiles of (g) JNM-1 and (h) JNM-2 at 77 K. Inset, showing pore size distribution profiles of JNM-1 and JNM-2 calculated by nonlocal DFT modeling based on N2 adsorption data, showing a uniform pore size of 1.89 nm. ORTEP, Oak Ridge thermal ellipsoid plot; PXRD¸ powder X-ray diffraction; DFT, density functional theory. Download figure Download PowerPoint The Fourier-transform infrared (FT-IR) spectra of the JNMs confirm the formation of imine linkages, supported by the disappearance of the N−H stretching signals located at 3462–3208 cm−1 and exhibition of the C=N stretching bands located at 1623–1617 cm−1 ( Supporting Information Figures S4 and S5). Also, the solid-state 13C CP/MAS NMR spectra of the JNMs revealed the vanish of aldehyde carbon signals located at 184 ppm and the appearance of characteristic resonance peaks of imine carbons at 157 and 155 ppm for JNM-1 and JNM-2, respectively, which evidenced the existence of imine linkages ( Supporting Information Figures S7 and S8). Furthermore, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) displayed rod-shaped morphologies of the products and consisted of highly crystalline nanolayered structures ( Supporting Information Figures S9–S12). Energy-dispersive X-ray spectroscopy (EDS) elemental mapping of the JNM powder particles displayed a uniform distribution of C, N, and Cu in JNMs ( Supporting Information Figures S13 and S14). The PXRD experiments and theoretical simulations were performed to analyze the crystal structure of the microcrystals of JNMs obtained. The structural calculations were carried out using BIOVIA Materials Studio (Accelrys, San Diego, CA, USA; see Supporting Information Figures S15–S19) during which the eclipsed stacking (AA) and staggered stacking (AB) structures were simulated (Figures 1d and 1e). The PXRD patterns of JNM-1 show an intense peak at 4.27° accompanied by four small peaks at 7.42°, 8.55°, 11.43°, and 25.98°, which can be attributed to (100), (110), (200), (120), and (001) diffractions. The experimental PXRD patterns were in good agreement with the calculated PXRD profiles of the AA stacking model (Figure 1f), suggesting that JNM-1 featured a uniform pore distribution with an eclipsed structure. In particular, Pawley refinements gave a hexagonal space group P 6 ¯ with unit cell parameters of a = b = 24.4344 Å, c = 4.2298 Å, with refinement parameters of Rp = 3.84% and Rwp = 8.61%. The refined PXRD patterns match the experimental PXRD data, as confirmed by the negligible difference plot in Figure 1f. JNM-2 featured a similar AA stacking structure with JNM-1 (see Supporting Information for details). The nitrogen adsorption isotherms, measurements at 77 K of JNM-1 and JNM-2 both illustrate the Type IV adsorption curves featuring the mesoporous nature (Figures 1g and 1h). The Brunauer−Emmett−Teller (BET) surface areas of JNM-1 and JNM-2 were calculated to be 534.61 and 505.32 m2·g−1 and the total pore volumes were 0.28 and 0.39 cm3·g−1 (P/P0 = 0.99), respectively. The calculated eclipsed-stacked structures of JNM-1 and JNM-2 using the nonlocal density functional theory (DFT) suggested a narrow pore size distribution with an average pore width both of ∼1.89 nm (Figures 1g and 1h), identical to their theoretical values of 1.89 nm predicted from the eclipsed AA stacking structures, thereby further supporting the eclipsed structures of JNM-1 and JNM-2. Interestingly, the JNMs exhibited high stability toward heat, air, and water, in spite of the common observation of fast oxidation and decomposition of Cu(I) CTU-based compounds when exposed to air and water.30–32 Thermal gravimetric analyses (TGA) and various temperature PXRD spectra under N2 atmosphere proved that the JNMs had high thermal stability and remained the high crystallinity up to 320 °C ( Supporting Information Figures S21–S24). It is known that the Cu(I) ions in CTU-based MOFs underwent fast oxidation to give Cu(II) ions.31,32 In contrast, the JNMs exhibited superior stability even when exposed to air over 1 month. X-ray photoelectron spectroscopy (XPS) measurements evidenced only intense sharp and symmetrical Cu(I) 2p3/2 signals at 933.4 and 933.5 eV for JNM-1 and JNM-2, respectively, without satellite peaks, implying that the Cu(I) ions remained intact within the frameworks (Figure 2a and Supporting Information Figure S26). Furthermore, the crystallinities of the JNMs were sustained upon suspension in various organic solvents, water, and even NaOH solutions for 24 h, documented by PXRD analyses ( Supporting Information Figures S27 and S28). Figure 2 | XPS profiles of (a) JNM-1 exposed to air for over 1 month and before oxidation with H2O2; (b) JNM-1 after oxidation with H2O2 (the asterisk represents the Cu(II) satellite peaks); (c) oxidized JNM-1 was reduced in NMP at 160 °C. XPS, X-ray photoelectron spectroscopy; NMP, N-methyl-2-pyrrolidone. Download figure Download PowerPoint We tested the reversible redox reactivities of Cu(I) CTUs in JNMs by initially treating JNM-1 with a solution of H2O2 in THF at room temperature for 24 h (see Supporting Information Figure S29). PXRD analysis of the resultant dark green powder suggested a slight decrease in the crystallinity of JNM-1, which might be due to vigorous stirring. In addition, the XPS experiments revealed an intense asymmetrical Cu 2p3/2 peak along with satellite peaks that could be deconvoluted into two contributions located at 932.9 and 934.8 eV, corresponding to Cu(I) 2p3/2 and Cu(II) 2p3/2 with an integrated Cu(I):Cu(II) ratio of ∼4∶5, respectively (Figure 2b).32 These results confirmed that the Cu(I) ions in JNM-1 were able to oxidize to Cu(II) ions in the presence of an oxidant. When the samples of oxidized JNM-1 were heated in N-methyl-2-pyrrolidone (NMP) at 160 °C, the Cu(II) ions reduced entirely to Cu(I) ion, as shown by the XPS analysis (Figure 2c). The excellent stability and reversible redox reactivities of JNMs promoted us to investigate their catalytic performance further. Since the of the Sonogashira cross-coupling it has been used in synthetic chemistry as an for Recently, with a more and Cu catalyst has of the Sonogashira cross-coupling reaction by the JNMs was tested via an initial of a model reaction of phenylacetylene and We the reaction including reaction temperature, and catalyst shown in 1 and Supporting Information Figure at 140 °C, and in the presence of K2CO3 as the the mixture of and JNM-1 mol based on Cu formed the coupling in 8 h with the temperature to 120 °C or the reaction to 2 h the conversion to and respectively. Besides, the reaction in the of the catalyst. the a was indicating the high of the using the JNMs catalyst. It is that phenylacetylene underwent a coupling reaction in the presence of air and JNMs catalyst to give with a high yield of (see Supporting Information for details). 1 | JNM Sonogashira (%) 1 2 mol % JNM-1 DMF 140 2 mol % JNM-1 DMF 140 4 mol % JNM-1 DMF 140 4 5 mol % JNM-1 DMF 140 5 4 mol % JNM-1 THF 6 4 mol % JNM-1 4 mol % JNM-1 8 4 mol % JNM-1 DMF 100 4 mol % JNM-1 DMF 120 10 4 mol % JNM-2 DMF 140 6 mol % Cu2O DMF 140 4 mol % Cu3L3 DMF 140 phenylacetylene 0.5 mmol, iodobenzene mL), N2 and reaction is 8 h. The reported conversion is based on chromatography–mass spectrometry Although Cu3L3 exhibited similar catalytic activities with JNM-1 it after the reaction, confirmed by In addition, the green mixture further oxidation of Cu(I) to In contrast, the JNM-1 catalyst excellent stability and after five catalytic the crystallinity and structural of JNM-1 remained supported by the PXRD analysis ( Supporting Information Figure the catalyst could feasibly from the reaction mixture by and at least for five reaction without loss of catalytic performance ( Supporting Information Figure Furthermore, we investigated the of Cu ions in JNM-1 by XPS The XPS experiments of JNM-1 revealed an intense Cu(I) 2p3/2 at 933.4 eV without satellite peaks, that the Cu(I) ions are during the catalytic cycle and are and ( Supporting Information Figure the conditions in we further the scope of the JNM-1 coupling reaction with various We utilized with groups ( and and with ( and with both of which with excellent In addition, we investigated the tolerance of with reactive functional which could the imine or coordinate with metal the with and ( and also gave excellent ranging from to These results demonstrated that the and reactive functional groups were well in the Sonogashira cross-coupling reaction using the JNM-1 catalyst. 2 | of JNM-1 for Sonogashira 0.5 of 4 of the JNM-1 and 2 of DMF mL), 140 °C, N2 8 h. The reported are isolated The Sonogashira cross-coupling reaction is a approach for constructing a single molecular with a conjugated structure such as which are only for the electron also for We to such conjugated molecular by the JNM-1 catalyst JNM-1 showed superior catalytic compared with that of Cu3L3 The reaction of ( of and JNM-1 gave the ( with a good yield of only yield was obtained using Cu3L3 as a catalyst. yield of the might be to the stability of Cu3L3 in the presence of with which in Besides, with extended conjugated was synthesized with over yield using either JNM-1 or Cu3L3. JNM-1 was for with a conjugated up to 2.7 nm These results demonstrated that JNM-1 is a highly and promising catalyst for constructing molecular wires. | Synthesis of for 0.25 of for 0.5 of for 0.25 of and 4 of the JNM-1 or Cu3L3 2 of DMF mL), 140 °C, N2 h. The isolated yield using b JNM-1 or catalyst. Inset, the X-ray structure of and displaying the ORTEP diagram at a 50% and the calculated structure of showing the (C, O, and atoms are shown as red, and white, ORTEP, Oak Ridge thermal ellipsoid We a stepwise synthetic strategy for Cu(I) CTUs into extended frameworks by combining the chemistry of metal clusters and COFs, which is rarely demonstrated in These Cu(I) cluster-based JNM-1 and JNM-2, exhibited superior compared with that of their Cu(I) they reversible redox by their Cu(I) an of metal clusters and we illustrate that JNMs are promising platforms for the Sonogashira cross-coupling reactions with excellent performance and Besides, JNM-1 is a useful catalyst for constructing conjugated molecular wires and much higher catalytic activities than that of Cu3L3. the JNMs catalyst could be and for at least five reaction without loss of This a synthetic strategy for constructing function-led cluster-based reticular materials by networking metal clusters via linkage of dynamic covalent Supporting Information Supporting Information is of The of Information This was supported by the of and and the Guangdong of and is for the from Guangdong and Guangdong Scheme and the for the The and for the structural modeling and refinement and solid-state 13C measurements and their to Reticular Frameworks and Covalent Synthesis and the of Coordination and of of Frameworks to MOF and K. Chemistry and of Covalent Frameworks to of Covalent Frameworks on and Frameworks and for and of Li Covalent Li Frameworks A Frameworks and Covalent Zhang of into a Covalent Li Li K. of a Clusters of and Metal to the Metal and and Zhang Functional Coordination Li and K. as an of the Chemistry of and of and as a Li of from Different A for of of Supramolecular of and Li of Cyclic a to Cyclic d10 via and to the via and Ning Li Cyclic with to and to Li and Synthesis of Coordination and Zhang Coordination by Metal Zhang Metal and of Li Li in Frameworks from Sonogashira Synthesis of of with and and in Sonogashira A in and A with Chemical to Functional Information Chemical organic trinuclear and for the structural modeling and refinement and solid-state 13C measurements and their

Prior to envisage any implication of metal–organic framework (MOF) materials in industrial applications such as gas storage/separation, it is of primary importance to examine beyond their stability under humidity, the interactions between water and MOF surfaces. Regarding the MOF type MIL-53(Cr), the situation becomes more complex due to the breathing of its structure upon water adsorption that induces a structural transition between a narrow pore (NP) and a large pore (LP) forms. The resulting shrinkage/reopening of the framework leads to crucial modifications of the hydrophobicity/hydrophilicity character of the material. A combination of molecular simulations (Grand Canonical Monte Carlo and molecular dynamics) and experimental techniques (quasi-elastic neutron scattering and gravimetry/volumetry) allows a complete elucidation of the adsorption and diffusion mechanisms in play in this structure. It also provides a microscopic explanation of the hydrophilic and mildly hydrophobic behaviors of the NP and...

The copper-based metal-organic framework (MOF) HKUST-1 adsorbs organic molecules into its pores. When loaded with electron-rich oligothiophenes, the resulting system reacts under heat to initiate oxidative polymerization without the use of any other oxidant or catalyst. This reaction is not observed in the non-redox-active MOF MIL-100(Al). We have characterized the composites by optical and nanoscale microscopy, vibrational and UV-vis spectroscopy, X-ray photoelectron spectroscopy, N2 sorption analysis, and thermogravimetric analysis/residual gas analysis. Unsubstituted oligothiophenes polymerize within MOF pores, while 3,4-ethylenedioxythiophene forms a coating on the MOF surface. MOF composites with conjugated polymer dopants trapped inside their pores undergo profound shifts in the composite electronic structure. Reasoning from time-dependent density functional theory calculations of an HKUST-1 model system bound to monomers, we rationalize the observed reactivity and propose an initiation mechanism based on a ligand-to-metal charge-transfer state.

Creating more exposed active sites on the metal-organic framework (MOF) surface is crucial for enhancing the recognition ability of MOF artificial receptors. Here, a copper-based MOF Cu(im)2 (im = imidazole) was utilized to act as an artificial receptor, inhibiting the activity of α-chymotrypsin. The shortest diazole ligand reduced the distance between regenerative copper sites, creating as many active sites as possible on the MOF unit surface. The amount of copper(ii) centers on the Cu(im)2 surface was calculated to be 4.96 × 106μm-2. Thus, Cu(im)2 showed exceedingly higher inhibition performance than other copper-based MOFs. The ChT activity was almost inhibited (88.8%) after the incubation with only 20 μg mL-1 Cu(im)2 for 10 min. The binding between ChT and Cu(im)2 was very fast with high affinity. Further results proved that Cu(im)2 inhibited the activity of ChT through electrostatic interactions and coordination interactions via the mixed inhibition mode. This strategy to use short ligands to create more active sites on the MOF surface provides a new direction to enhance the inhibition efficiency.

A method for modifying the external surfaces of a series of nanoscale metal-organic frameworks (MOFs) with 1,2-dioleoyl-sn-glycero-3-phosphate (DOPA) is presented. A series of zirconium-based nanoMOFs of the same topology (UiO-66, UiO-67, and BUT-30) were synthesized, isolated as aggregates, and then conjugated with DOPA to create stably dispersed colloids. BET surface area analysis revealed that these structures maintain their porosity after surface functionalization, providing evidence that DOPA functionalization only occurs on the external surface. Additionally, dye-labeled ligand loading studies revealed that the density of DOPA on the surface of the nanoscale MOF correlates to the density of metal nodes on the surface of each MOF. Importantly, the surface modification strategy described will allow for the general and divergent synthesis and study of a wide variety of nanoscale MOFs as stable colloidal materials.

A method for modifying the external surfaces of a series of nanoscale metal–organic frameworks (MOFs) with 1,2‐dioleoyl‐sn‐glycero‐3‐phosphate (DOPA) is presented. A series of zirconium‐based nanoMOFs of the same topology (UiO‐66, UiO‐67, and BUT‐30) were synthesized, isolated as aggregates, and then conjugated with DOPA to create stably dispersed colloids. BET surface area analysis revealed that these structures maintain their porosity after surface functionalization, providing evidence that DOPA functionalization only occurs on the external surface. Additionally, dye‐labeled ligand loading studies revealed that the density of DOPA on the surface of the nanoscale MOF correlates to the density of metal nodes on the surface of each MOF. Importantly, the surface modification strategy described will allow for the general and divergent synthesis and study of a wide variety of nanoscale MOFs as stable colloidal materials.

Recovery of rare earths from secondary resources is of great strategic value and environmental significance. In this work, a phosphine-functionalized metal–organic framework (MOF) material (BPG@UiO-66-NH2) was prepared by modifying UiO-66-NH2 with the bis-phosphine-based ligand N,N-bis(phosphine hydroxymethyl)glycine (BPG) and applied to capture Nd(III) from wastewater. The results showed that BPG@UiO-66-NH2 could effectively capture Nd(III) at a pH range from 4 to 6, and the adsorption equilibrium was rapidly reached within 10 min with a maximum capture capacity of 190.51 mg/g. The kinetics and isotherms indicated that the adsorption of Nd(III) was more consistent with the Langmuir isotherm and pseudo-second-order model, and the adsorption belonged to monolayer adsorption on a homogeneous medium. The mass transfer results of the EDR-IDR and AOAS models illustrated that Nd(III) diffused rapidly into the adsorbent surface in solution and was dominated by the chemisorption reaction on the surface. Thermodynamic calculations indicated that the capture of Nd(III) was a spontaneous heat absorption process. Moreover, the KD value of BPG@UiO-66-NH2 for Nd(III) was up to 2278.53 mL/g, which was higher than those of the coexisting ions (Co(II), Zn(II), Ni(II), Mn(II)), demonstrating the superior adsorption selectivity for Nd(III). Meanwhile, BPG@UiO-66-NH2 exhibited excellent salt resistance and reusability with a high capture rate of 87.5%, even after five adsorption–desorption cycles. In addition, comprehensive characterization and density functional theory (DFT) calculations indicated that the adsorption mechanism was mainly attributed to the polydentate coordination binding of the phosphonic acid moiety in the BPG ligand to the Nd(III) “2:1” formula; electrostatic attraction and ion exchange also played an important role in the adsorption process. The superior performance of BPG@UiO-66-NH2 provides a practical solution for the efficient recovery of Nd(III).