Crystal-Defect-Induced Longer Lifetime of Excited States in a Metal-Organic Framework Photocatalyst to Enhance Visible-Light-Mediated CO2 Reduction.

We report the structural defects in Zr-metal-organic framework (MOFs) for achieving highly efficient CO2 reduction under visible light irradiation. A series of defective Zr-MOF-X (X = 160, 240, 320, or 400) are synthesized by acid-regulated defect engineering. Compared to pristine defect-free Zr-MOF (NNU-28), N2 uptake increases for Zr-MOF-X synthesized with the HAc modulator, producing a larger pore space and Brunauer-Emmett-Teller surface area. The pore size distribution demonstrates that defective Zr-MOF-X exhibits mesoporous structures. Electrochemistry tests show that defective Zr-MOF-X possesses a more negative reduction potential and a higher photocurrent responsive signal than that of pristine NNU-28. Consequently, the defective samples exhibit a significantly higher efficiency in the photoreduction of CO2 to formate. Transient absorption spectroscopies manifest that structural defects modulate the excited-state behivior of Zr-MOF-X and improve the photogenerated charge separation of Zr-MOF-X. Furthermore, electron paramagnetic resonance and in-suit X-ray photoelectron spectroscopy provide additional evidence of the high photocatalytic performance exhibited by defective Zr-MOF-X. Results demonstrate that structural defects in Zr-MOF-X also improve the charge transfer, producing abundant Zr(III) catalytically active sites, exhibiting a slower decay process than defect-free Zr-MOF. The long-lifetime Zr(III) species in defective Zr-MOF-X are fully exposed to a high-concentration CO2 atmosphere, thereby enhancing the photocatalytic efficiency of CO2 reduction.

Hydrophobic Porphyrin Titanium-Based MOFs for Visible-Light-Driven CO2 Reduction to Formate.

Three hydrophobic porphyrin titanium-based metal-organic frameworks (MOFs) (HPA/DGIST-1, DPA/DGIST-1, and OPA/DGIST-1) were synthesized through a postsynthetic coordination reaction by using alkylphosphonic acid of different lengths (HPA, hexylphosphonic acid; DPA, dodecylphosphonic acid; OPA, octadecylphosphonic acid). Compared with the hydrophilic DGIST-1, modified DGIST-1 exhibits excellent hydrophobicity and presents good stability in humid atmospheres. Due to the introduction of porphyrin ligands, HPA/DGIST-1, DPA/DGIST-1, and OPA/DGIST-1 showed good visible-light absorption (380-700 nm) and sensitive photogenerated charge responses. When acted as catalysts, these hydrophobic Ti-MOFs can selectively reduce CO2 to HCOO- under visible-light irradiation with average reaction rates of 150.9, 178.5, and 228.3 µmol·h-1·g-1, where these values are 1.3-2.0 times higher than the system mediated by the initial porphyrin Ti-MOF catalyst. 13C NMR spectroscopy demonstrates that the catalytic product HCOO- anion originates from the reactant CO2. The photocatalytic experiments, electron paramagnetic resonance, and photoluminescence spectra tests showed that porphyrin ligands and Ti-O units can act as catalytic activity centers to realize the conversion of CO2 to HCOO-. This work demonstrated that the combination of porphyrin titanium-based MOF and alkyl hydrophobic groups is an effective way to enhance the stability of titanium-based MOFs and maintain their high photocatalytic performance.

Confinement Synthesis of Atomic Copper-Anchored Polymeric Carbon Nitride in Crystalline UiO-66-NH2 for High-Performance CO2-to-CH3OH Photocatalysis.

Photocatalytic CO2 reduction to value-added fuels displays an attractive scenario to enhance energy supply and reduce global warming. We report herein the confinement synthesis of polymeric carbon nitride (PCN) incorporating with Cu single atoms (CuSAs) inside the crystalline UiO-66-NH2, which combines the merits of heterojunction photocatalysis and single-atom catalysis (SAC) to achieve high-performance CO2-to-CH3OH conversion. A series of spectral studies displays the formation of CuSAs@PCN inside the crystalline UiO-66-NH2. Remarkably, the ternary composite shows an excellent photocatalytic turnover frequency of 4.15 mmol ⋅ h-1 ⋅ g-1 for CO2-to-CH3OH conversion. Theoretical and experimental studies demonstrate the doping of CuSAs, as well as the formation of type-II heterojunction, are causal factors to achieve CH3OH generation. The study provides new insights designing high-performance photocatalyst for CO2 conversion to fuels at atomic scale.

Aluminum-Porphyrin Metal-Organic Frameworks for Visible-Light Photocatalytic and Sonophotocatalytic Cr(VI) Reduction.

In this study, four isostructural aluminum-based porphyrin metal-organic frameworks [Al-TCPP(M), M = H2 and Zn] with different morphologies and sizes were synthesized by the hydrothermal method. By adjusting the hydrothermal reaction time and the types of porphyrin ligands, Al-TCPP(M) MOFs exhibited diverse morphologies including tetragonal, rectangular, and carambola-like phase. In view of the introduction of porphyrin ligands and the strong coordination effect of Al-O units, Al-TCPP(M) MOFs exhibited good chemical stability, broad visible light harvesting capability, and fast photogenerated charge response. Four Al-TCPP(M) MOFs exhibited excellent photocatalytic activities for Cr(VI) in aqueous solution. Notably, the regulation to the nanoscale carambola-like morphology of Al-TCPP MOFs and metallization of the porphyrin ligand promoted the Cr(VI) photoreduction reaction where the catalytic activity of metallic carambola-like Al-TCPP increased 1.7 times compared to that of nonmetallic tetragonal MOFs. With the assistance of sonophotocatalysis, the Cr(VI) average reduction rates reached 0.658, 0.542, 0.785, and 0.629 mg·L-1·min-1 for Al-TCPP(H2)-24h, Al-TCPP(H2)-72h, Al-TCPP(Zn)-24h, and Al-TCPP(Zn)-72h, which are 1.2-1.4 times higher than that of photocatalysis. UV-vis absorption spectroscopy, electronic spin resonance, and fluorescence spectroscopy experiments demonstrated that the synergistic effect of photochemistry and sonochemistry promoted the transfer of photogenerated electrons from Al-TCPP(M) to Cr(VI), thus enhancing the catalytic activity. The combination of the sonophotocatalytic technology with aluminum-porphyrin MOFs may become an effective strategy to improve MOF-based photocatalytic systems.

Controlled Partial Linker Thermolysis in Metal-Organic Framework UiO-66-NH2 to Give a Single-Site Copper Photocatalyst for the Functionalization of Terminal Alkynes.

Metal-organic frameworks (MOFs) with expanding porosity and tailored pore environments are intriguing for catalytic applications. We report herein a straightforward method of controlled partial linker thermolysis to introduce desirable mesopores into mono-ligand MOFs, which is different from the classical thermolyzing method that starts from mixed-linker MOFs. UiO-66-NH2 , after partial ligand thermolysis, exhibits significant mesoporosity, retained crystal structure, improved charge photogeneration and abundant anchoring sites, which is ideal to explore single-site photocatalysis. Atomically dispersed Cu is then accommodated in the tailored pore. The resulting single-site Cu catalyst exhibits excellent performance for photocatalytic alkylation and oxidation coupling for the functionalization of terminal alkynes. The study highlights the advantage of controlled partial linker thermolysis to synthesize hierarchical MOFs to achieve the advanced single-site photocatalysis.

Titanium-Porphyrin Metal-Organic Frameworks as Visible-Light-Driven Catalysts for Highly Efficient Sonophotocatalytic Reduction of Cr(VI).

In this work, we synthesized and characterized four titanium-porphyrin metal-organic frameworks (MOFs) [DGIST-1(M), M = Co(II), Fe(III), Zn(II), and H2] and used them as visible-light-driven catalysts for sonophotocatalytic Cr(VI) reduction. DGIST-1(M) exhibited open-framework, broad light absorption stemmed from ligand and sensitive photocurrent responses owing to the integration of one-dimensional Ti-oxo chains and 4-connected conjugated TCPP ligand (TCPP = tetrakis(4-carboxyphenyl)-porphyrin). DGIST-1(M) presented efficient reduction of Cr(VI) to Cr(III) in aqueous solution when used as sonophotocatalytic catalysts. The average reduction rates upon Cr(VI) were 0.920, 0.476, 0.377, and 0.194 mg·L-1·min-1 for DGIST-1(H2), DGIST-1(Zn), DGIST-1(Co), and DGIST-1(Fe), which are 1.15-2.45 times higher than those in photocatalysis. Sonophotocatalytic experiments and electron paramagnetic resonance measurement proved that Ti-oxo chain units and porphyrin ligand in the structures of DGIST-1(M) existed as catalytic active centers for sonophotocatalytic reduction of Cr(VI). Photoluminescence and UV absorption spectra revealed that the unity of photocatalysis and sonochemistry strengthened the migration of photogenerated electrons from DGIST-1(M) to Cr(VI), which improved the activities of catalysts. This study suggested that the association of titanium-porphyrin MOFs and sonophotocatalytic technology is an impactful program for enhancing MOF-based photocatalytic systems.

One-Pot Dual Catalysis of a Photoactive Coordination Polymer and Palladium Acetate for the Highly Efficient Cross-Coupling Reaction via Interfacial Electron Transfer.

We report herein an exploration of the straightforward one-pot dual-catalysis strategy, i.e., direct combination of a photoactive coordination polymer (CP) with another metal catalyst, for carrying out the desirable photoinduced organic transformation. The strategy overcomes the necessity of the presynthesis of the metal/CP composite that has been demonstrated to be invalid in our case. A new two-dimensional CP showing the desirable properties of wide-range visible-light absorption and efficient photoinduced charge generation was synthesized via a solvothermal reaction. The synthesized CP was successfully applied to the photocatalytic C-C cross-coupling reaction via the one-pot dual-catalysis method, in combination with the simple and ligand-free palladium salt of Pd(OAc)2 as a metal catalyst. The reaction features a short reaction time, mild reaction conditions, good recyclability, and a high yield of Heck products from a broad variety of substrates. A comparative experiment showed the presynthesized Pd/CP composite was invalid for the reaction, demonstrating the significance of the one-pot dual-catalysis strategy. Mechanistic studies suggest the one-pot reaction depends on the synergy between the photocatalysis of a synthesized CP to generate reactive aryl radicals and Pd catalysis to generate target products, in which the interfacial electron transfer has been demonstrated to be vital for producing the transient and catalytically active Pd(0) species near the surface of the CP. The study shows the direct combination of a CP photocatalyst and a metal catalyst is a highly feasible method for the photochemical reaction and enhances the prospects of application of photoactive CPs.

Visible-Light-Driven Sonophotocatalysis for the Rapid Reduction of Aqueous Cr(VI) Based on Zirconium-Porphyrin Metal-Organic Frameworks with csq Topology.

Photochemical treatment of highly toxic Cr(VI) is a desirable and ecofriendly method to protect the environment and human beings. In this study, a MOF-based sonophotocatalytic system is established, in which visible-light-driven sonophotocatalytic reduction of toxic Cr(VI) to Cr(III) in water is investigated using zirconium-porphyrin metal-organic frameworks (MOFs) structured as PCN-222(M) [M = H2, Zn(II), Fe(III), Co(II)]. In the view of the synergistic effect of sonochemistry and photocatalysis, PCN-222(M) exhibited enhanced activities for Cr(VI) reduction compared with the photocatalytic process. Kinetic studies showed that apparent reaction rate constants in the sonophotocatalytic system of PCN-222(M) are 1.5-3.3 times higher than those in photocatalysis. Fluorescence and UV-vis absorption spectra measurements demonstrate that the sonophotocatalytic process promotes the transfer of photoinduced electrons from PCN-222(M) to Cr(VI), thus enhancing the catalytic performance. The innovative combination of porous MOFs and sonophotocatalytic technology might become a feasible strategy to improve the existing MOF-based photocatalytic systems.

Photoactive Metal-Organic Frameworks for the Selective Synthesis of Thioethers: Coupled with Phosphine to Modulate Thiyl Radical Generation.

Metal-organic framework (MOF) materials are intriguing photocatalysts to trigger radical-mediated chemical transformations. We report herein the synthesis and characterization of a series of isomorphic MOFs which show a novel structure, wide visible-light absorption, high chemical stability, and specific redox potential. The prepared MOFs were explored for the photoinduced single-electron oxidation of thiol compounds, generating reactive thiyl radicals to afford thioethers via a convenient thiol-olefin reaction. Importantly, we provide a widely applicable strategy by combing a photoactive MOF with phosphine to modulate the generation of thiyl radical in the reaction, thereby producing a single product of the thioether without the formation of a disulfide byproduct due to the dimerization of thiyl radicals. The photocatalytic reaction takes advantage of this strategy, showing great generality where tens of thiols and olefins have been examined as coupling partners. In addition, the strategy has also been demonstrated to be effective for the reactions catalyzed by other MOFs. Mechanism studies reveal that the selective synthesis of C-S products relies on a synergy between the photoinduced generation of a thiyl radical over the MOF and the in situ cleavage of S-S bond into a S-H bond by phosphine. It is notable that the synthesized MOFs show advanced performance in comparison with classical MOFs. The work not only provides a series of novel MOF photocatalysts that are capable of photoinduced thiol-olefin coupling but also indicates the great potential of MOFs for photochemical transformations mediated by reactive radicals.

RhB-Embedded Zirconium-Naphthalene-Based Metal-Organic Framework Composite as a Luminescent Self-Calibrating Platform for the Selective Detection of Inorganic Ions.

A series of dye@MOF composites were synthesized through in situ encapsulation of luminous rhodamine B (RhB) molecules into a blue-emissive zirconium-naphthalene-based metal-organic framework (Zr-MOF). The fabricated RhB@Zr-MOF composites exhibit tunable dual-emissive characteristics due to the process of resonant energy transfer from Zr-MOF to RhB. Notably, one of the RhB@Zr-MOF composites (R@D3) exhibited a weak emission at 420 nm and a strong emission at 607 nm, for which the two emissions possess large distinctions in location and intensity and can be referenced with each other in sensing analytes. By using relative fluorescence intensity instead of their absolute fluorescence intensity as the detection signals, R@D3 served as a built-in self-calibrated platform to selectively detect Fe3+ and Cr2 O7 2- ions in water. Compared with the pristine Zr-MOF, the R@D3 composite shows enhanced sensing selectivity to Fe3+ and higher sensibility to Cr2 O7 2- . This study displays the advantages of combining organic dyes with robust Zr-MOFs in tuning fluorescence and sensing performance.

Eosin Y-Embedded Zirconium-Based Metal-Organic Framework as a Dual-Emitting Built-In Self-Calibrating Platform for Pesticide Detection.

A series of eosin Y (EY)-embedded zirconium-based metal-organic frameworks (Zr-MOFs) were prepared by utilizing the synthetic encapsulating method. By virtue of effective resonant energy transfer between Zr-MOF and EY, not only does EY@Zr-MOF exhibit dual-emissive characteristics, but also the relative intensity of their double emission is greatly tuned with increasing EY loading quantity. As a consequence, the double emission of EY@Zr-MOF presented large distinctions in location and intensity. By using the relative fluorescence intensity instead of the absolute fluorescence intensity of emission peaks as detection signals, two EY@Zr-MOFs served as built-in self-calibrated fluorescence sensors to detect pesticides, where EY@Zr-MOF realized the selective detection of nitenpyram, a kind of nicotine pesticide. These results indicate that the integration of robust Zr-MOF and fluorescence molecules provides a new research platform for pesticide sensing and recognition.

Electrically Conductive Coordination Polymer for Highly Selective Chemiresistive Sensing of Volatile Amines.

A new electrically conductive coordination polymer is synthesized using anthracene-incorporated organic ligand. The compound exhibits a band-like structure and long-range π-π stacking of ligand. Interestingly, the fabricated chemiresisitor using this coordination polymer shows selective sensing behavior for volatile amine detection with characteristics of quick response, good reproducibility, and room-temperature operation. This study not only presents a rare example of electrically conductive coordination polymer but also illustrates the useful application of coordination polymer for chemiresistor.

Facile Fabrication of Well-Dispersed Pt Nanoparticles in Mesoporous Silica with Large Open Spaces and Their Catalytic Applications.

In this paper, a facile strategy is reported for the preparation of well-dispersed Pt nanoparticles in ordered mesoporous silica (Pt@OMS) by using a hybrid mesoporous phenolic resin-silica nanocomposite as the parent material. The phenolic resin polymer is proposed herein to be the key in preventing the aggregation of Pt nanoparticles during their formation process and making contributions both to enhance the surface area and enlarge the pore size of the support. The Pt@OMS proves to be a highly active and stable catalyst for both gas-phase oxidation of CO and liquid-phase hydrogenation of 4-nitrophenol. This work might open new avenues for the preparation of noble metal nanoparticles in mesoporous silica with unique structures for catalytic applications.

Ionothermal syntheses and characterizations of new open-framework metal borophosphates.

Three new open-framework metal borophosphates, [Na(6)Co(3)B(2)P(5)O(21)Cl]·H(2)O (JIS-4), K(5)Mn(2)B(2)P(5)O(19)(OH)(2) (JIS-5), (NH(4))(8)[Co(2)B(4)P(8)O(30)(OH)(4)] (JIS-6), have been prepared under ionothermal conditions using ionic liquid 1-ethyl-3-methylimidazolium ([Emim]Br) as the solvent. They are the first examples of metalloborophosphate prepared by the ionothermal method. Their structures are determined by single-crystal X-ray diffraction. The 3-D open framework of JIS-4 is made of CoO(5)Cl octahedra, CoO(5) square pyramids, and PO(4) and BO(4) tetrahedra forming 12-ring channels along the [010] direction. It is noted that JIS-4 is the first 3-D open-framework structure in the family of borophosphate with the B/P ratio of 2/5, which features a borophosphate cluster anionic partial structure. Such cluster anionic partial structures connect with MnO(6) octahedra and MnO(5) trigonal bipyramids resulting in the formation of the 2-D layer structure of JIS-5 with the same B/P ratio as JIS-4. The 2-D layer structure of JIS-6 belongs to the largest family of borophosphate with a B/P ratio of 1/2 which features a unique 1-D chain anionic partial structure. Crystal data for JIS-4, orthorhombic, Pnma, a = 14.0638(8) Å, b = 9.8813(7) Å, c = 14.0008(10) Å, V = 1945.7(2) Å(3), and Z = 2; for JIS-5, monoclinic, P2(1)/n, a = 14.4939(3) Å, b = 9.2539(3) Å, c = 14.8031(4) Å, ß = 101.4600(10)°, V = 1945.88(9) Å(3), and Z = 4. For JIS-6, triclinic, P1, a = 9.6928(3) Å, b = 9.8747(3) Å, c = 10.0125(2) Å, α = 62.057(2)°, ß = 82.456(2)°, γ = 76.095(2)°, V = 821.60(4) Å(3), and Z = 1.

CdS/Ag2S/g-C3N4 ternary composites with superior photocatalytic performance for hydrogen evolution under visible light irradiation.

CdS/Ag2S/g-C3N4 ternary composites as photocatalysts with different amounts of Ag2S were successfully synthesized through a simple chemical deposition method. These photocatalysts were characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) mapping and X-ray photoelectron spectroscopy (XPS) to obtain the information of the structure and composition. Compared with the pure samples and binary composites, CdS/Ag2S/g-C3N4 ternary composites showed enhanced hydrogen production activities, and the maximum hydrogen production rate of CdS/Ag2S(2%)/CN is about 1020.54 µmol g-1 h-1 in Na2S-Na2SO3 solution. Based on the photoluminescence and electrochemical results, the improved photocatalytic activity could be attributed not only to the synergic effect of ternary components in the composite, but also to the introduction of Ag2S that provided abundant active sites for H2 production. A possible mechanism was investigated in detail.

Pillared Metal-Organic Framework Initiating Intermolecular Atom-Transfer Radical Addition via Visible-Light-Induced Electron Transfer Activation of Haloalkanes.

Herein, a novel metal-organic framework (MOF) with a pillared-layer structure was rationally synthesized to initiate intermolecular atom-transfer radical addition (ATRA) via photoinduced electron transfer activation of haloalkanes. The MOF synthesized via the controllable pillared-layer method is of excellent visible-light absorption and high chemical stability. Photocatalytic experiments show the atom transfer of various alkyl halides (R-X, X = Cl/Br/I) onto diverse olefins was successfully achieved to produce functional ATRA products. The mechanism and experimental investigations reveal the prepared MOF serves as an efficient photocatalyst with strong reduction potential to activate haloalkane substrates via photoinduced electron transfer, generating a highly reactive alkyl radical to trigger the ATRA reaction. Key events in the ATRA reaction, including alkyl radical photogeneration as well as halide transfer, have been further regulated to achieve preferable photocatalytic performance with higher yields, shorter reaction time, and desirable cycling capability. It is notable that the work is the first report on photoinduced electron transfer activation of halides by a MOF photocatalyst for the ATRA reaction, providing a new blueprint for MOFs to develop photoinduced radical reactions.

Selective Photooxidation of Amines and Sulfides Triggered by a Superoxide Radical Using a Novel Visible-Light-Responsive Metal-Organic Framework.

Photocatalysis is an efficient and sustainable approach to convert solar energy into chemical energy, simultaneously supplying valuable chemicals. In this study, a novel metal-organic framework (MOF) compound is constructed from anthracene-based organic linkers, which shows visible-light absorption and efficient photoinduced charge generation property. It was applied for triggering photooxidation of benzylamines and sulfides in the presence of environmental benign oxidants of molecular oxygen or hydrogen peroxide. Results show that it is a highly selective photocatalyst for oxidation reactions to produce valuable imines or sulfoxides. We further investigate the underlying mechanism for these photocatalytic reactions by recognizing reactive oxygen species in the reactions. It has been demonstrated that the superoxide radical (O2•-), generated by electron transfer from a photoexcited MOF to oxidants, serves as the main active species for the oxidations. The work demonstrates the great potential of photoactive MOFs for the transformation of organic chemicals into valuable complexes.

Selective photooxidation of sulfides mediated by singlet oxygen using visible-light-responsive coordination polymers.

A novel coordination polymer has been synthesized using an anthracene-based linker, and the photocatalytic properties of this visible-light-responsive compound are evaluated where it shows highly selective photooxidation of sulfides to produce sulfoxides relying on reactive species of singlet oxygen.

Enhancement of visible-light-driven CO2 reduction performance using an amine-functionalized zirconium metal-organic framework.

By employing a conjugated amine-functionalized dicarboxylic ligand (H2L = 2,2'-diamino-4,4'-stilbenedicarboxylic acid, H2SDCA-NH2), we have successfully synthesized and characterized a porous and visible light responsive zirconium metal-organic framework ([Zr6O4(OH)4(L)6]·8DMF, denoted as Zr-SDCA-NH2). This Zr-MOF showed good chemical stability and broad visible light absorption with an absorption edge at about 600 nm. When used as a photocatalyst, Zr-SDCA-NH2 exhibits visible-light activity for CO2 reduction with a formate formation rate of 96.2 µmol h-1 mmolMOF-1, which is higher than the series of reported amine-functionalized Zr-MOFs. Mott-Schottky measurements, photoluminescence study and photocatalytic experiments demonstrated that the Zr6 oxo cluster through the LMCT process and the organic ligand both contributed to the CO2 photoreduction. This study indicates that the combination of amino groups and highly conjugated molecules is a feasible and simple strategy to extend light absorption of the organic ligand, which is beneficial for designing a visible light responsive MOF photocatalyst.

Electrical conductivity and electroluminescence of a new anthracene-based metal-organic framework with π-conjugated zigzag chains.

A new three-dimensional microporous MOF has been constructed using a highly conjugated anthracene-based ligand. The rarely occurring long-range π-stacking of the ligand in the form of a zigzag chain has been found in the MOF structure, which provides not only a new charge transport pathway with high electrical conductivity of 1.3 (±0.5) × 10(-3) S cm(-1) but also an electroluminescence property with an emission centred at 575 nm.