Engineering of Phenylpyridine- and Bipyridine-Based Covalent Organic Frameworks for Photocatalytic Tandem Aerobic Oxidation/Povarov Cyclization.

Covalent organic frameworks (COFs) are emerging as a new class of photoactive organic semiconductors, which possess crystalline ordered structures and high surface areas. COFs can be tailor-made toward specific (photocatalytic) applications, and the size and position of their band gaps can be tuned by the choice of building blocks and linkages. However, many types of building blocks are still unexplored as photocatalytic moieties and the scope of reactions photocatalyzed by COFs remains quite limited. In this work, we report the synthesis and application of two bipyridine- or phenylpyridine-based COFs: TpBpyCOF and TpPpyCOF. Due to their good photocatalytic properties, both materials were applied as metal-free photocatalysts for the tandem aerobic oxidation/Povarov cyclization and α-oxidation of N-aryl glycine derivatives, with the bipyridine-based TpBpyCOF exhibiting the highest activity. By expanding the range of reactions that can be photocatalyzed by COFs, this work paves the way toward the more widespread application of COFs as metal-free heterogeneous photocatalysts as a convenient alternative for commonly used homogeneous (metal-based) photocatalysts.

Pyrene-Based Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Production.

Four highly porous covalent organic frameworks (COFs) containing pyrene units were prepared and explored for photocatalytic H2 O2 production. The experimental studies are complemented by density functional theory calculations, proving that the pyrene unit is more active for H2 O2 production than the bipyridine and (diarylamino)benzene units reported previously. H2 O2 decomposition experiments verified that the distribution of pyrene units over a large surface area of COFs plays an important role in catalytic performance. The Py-Py-COF though contains more pyrene units than other COFs which induces a high H2 O2 decomposition due to a dense concentration of pyrene in close proximity over a limited surface area. Therefore, a two-phase reaction system (water-benzyl alcohol) was employed to inhibit H2 O2 decomposition. This is the first report on applying pyrene-based COFs in a two-phase system for photocatalytic H2 O2 generation.

Linker Engineering of 2D Imine Covalent Organic Frameworks for the Heterogeneous Palladium-Catalyzed Suzuki Coupling Reaction.

Covalent organic frameworks (COFs) are an emerging class of porous organic polymers that have been utilized as scaffolds for anchoring metal active species to act as heterogeneous catalysts. Though several examples of such COFs exist, a thorough experimental and computational analysis on such catalysts is limited. In this work, a series of two-dimensional (2D) imine COFs (TTA-DFB COF (N), TTA-TBD COF (N∧O), and TTA-DFP COF(N∧N)) were synthesized by using suitable building units to obtain three different coordination sites (N, N∧O, and N∧N). These were post-modified with Pd(II) to catalyze the Suzuki-Miyaura coupling reaction. Pd@TTA-DFB COF, where Pd(II) was coordinated to N sites, showed the fastest reactivity and lower stability. Pd@TTA-DFP COF showed highest stability but slowest reactivity. Pd@TTA-TBD COF was the best among the three with both high stability and fast reactivity. By combining both experimental and computational results, we conclude that the Pd(II) to Pd(0) reduction is a key step in the difference between the catalytic reactivities of the three COFs. This study demonstrates the importance of the building block approach to design COFs for efficient heterogeneous catalysis and to understand the fate of the reaction profile.

Engineering Covalent Organic Frameworks as Heterogeneous Photocatalysts for Organic Transformations.

Covalent organic frameworks (COFs) are an emerging category of organic polymers with highly porous crystalline structures. In the last decade, reports on the use of COFs as heterogeneous photocatalysts for organic transformations have shown significant progress. Still, comprehensive reviews on the mechanisms of the photocatalytic organic transformations using COFs are lacking. This Review provides a comprehensive and systematic overview of COF-based photocatalysts for organic transformations. Firstly, we discuss the photophysical properties and the characterization methods of COF-based photocatalysts. Then, the general photocatalytic mechanism, the advantages, and the strategies to improve the photocatalytic efficiency of COF-based photocatalysts are summarized. After that, advanced examples of COF-based photocatalysts for organic transformations are analyzed with regard to the underlying mechanisms. The Review ends with a critical perspective on the challenges and prospects.

A Green Alternative for the Direct Aerobic Iodination of Arenes Using Molecular Iodine and a POM@MOF Catalyst.

Iodoarenes are important precursors for fine chemicals and pharmaceuticals. The direct iodination of arenes using molecular iodine (I2) has emerged as an attractive green synthesis method. Most of the direct iodination protocols are still homogeneous systems that require harsh conditions and use or produce toxic products. We report a new heterogeneous catalytic route for the direct aerobic iodination of arenes under mild conditions using a PMoV2 polyoxometalate (POM) embedded in the metal-organic framework (MOF) MIL-101 (PMoV2@MIL-101). The catalyst shows full yield for the conversion of mesitylene to 2-iodomesitylene at a rate that is similar to the homogeneous POM system. Moreover, the catalyst is applicable for a wide range of substrates in an oxygen atmosphere without using any co-catalysts or sacrificial agents. To the best of our knowledge, this is the first report on designing a sustainable and green MOF-based heterogeneous catalytic system for the direct iodination reaction using molecular oxygen and iodine.

Creation of Exclusive Artificial Cluster Defects by Selective Metal Removal in the (Zn, Zr) Mixed-Metal UiO-66.

The differentiation between missing linker defects and missing cluster defects in MOFs is difficult, thereby limiting the ability to correlate materials properties to a specific type of defects. Herein, we present a novel and easy synthesis strategy for the creation of solely "missing cluster defects" by preparing mixed-metal (Zn, Zr)-UiO-66 followed by a gentle acid wash to remove the Zn nodes. The resulting material has the reo UiO-66 structure, typical for well-defined missing cluster defects. The missing clusters are thoroughly characterized, including low-pressure Ar-sorption, iDPC-STEM at a low dose (1.5 pA), and XANES/EXAFS analysis. We show that the missing cluster UiO-66 has a negligible number of missing linkers. We show the performance of the missing cluster UiO-66 in CO2 sorption and heterogeneous catalysis.

Generating Catalytic Sites in UiO-66 through Defect Engineering.

UiO-66 is regarded as an epitome of metal-organic frameworks (MOFs) because of its stability. Defect engineering has been used as a toolbox to alter the performance of MOFs. UiO-66 is among the most widely explored MOFs because of its capability to bear a high number of defects without undergoing structural collapse. Several representative works in the field of MOF-based defect engineering are available based on UiO-66. In this review, more emphasis is given toward the construction of catalytic sites by engineering defects in UiO-66 as a representative including all the detailed synthesis procedures for inducing defects, and the characterization techniques used to analyze these defects in UiO-66 are discussed. Furthermore, a comprehensive review for the defects themselves and the support using defects in catalysis is provided to accentuate the importance of defect engineering.

Hydrogenative Ring-Rearrangement of Furfural to Cyclopentanone over Pd/UiO-66-NO2 with Tunable Missing-Linker Defects.

Upgrading furfural (FAL) to cyclopentanone (CPO) is of great importance for the synthesis of high-value chemicals and biomass utilization. The hydrogenative ring-rearrangement of FAL is catalyzed by metal-acid bifunctional catalysts. The Lewis acidity is a key factor in promoting the rearrangement of furan rings and achieving a high selectivity to CPO. In this work, highly dispersed Pd nanoparticles were successfully encapsulated into the cavities of a Zr based MOF, UiO-66-NO2, by impregnation using a double-solvent method (DSM) followed by H2 reduction. The obtained Pd/UiO-66-NO2 catalyst showed a significantly better catalytic performance in the aforementioned reaction than the Pd/UiO-66 catalyst due to the higher Lewis acidity of the support. Moreover, by using a thermal treatment. The Lewis acidity can be further increased through the creating of missing-linker defects. The resulting defective Pd/UiO-66-NO2 exhibited the highest CPO selectivity and FAL conversion of 96.6% and 98.9%, respectively. In addition, the catalyst was able to maintain a high activity and stability after four consecutive runs. The current study not only provides an efficient catalytic reaction system for the hydrogenative ring-rearrangement of furfural to cyclopentanone but also emphasizes the importance of defect sites.

Ru Catalyst Encapsulated into the Pores of MIL-101 MOF: Direct Visualization by TEM.

Ru catalyst nanoparticles were encapsulated into the pores of a Cr-based metal-organic framework (MOF)-MIL-101. The obtained material, as well as the non-loaded MIL-101, were investigated down to the atomic scale by annular dark-field scanning transmission electron microscopy using low dose conditions and fast image acquisition. The results directly show that the used wet chemistry loading approach is well-fitted for the accurate embedding of the individual catalyst nanoparticles into the cages of the MIL-101. The MIL-101 host material remains crystalline after the loading procedure, and the encapsulated Ru nanoparticles have a metallic nature. Annular dark field scanning transmission electron microscopy, combined with EDX mapping, is a perfect tool to directly characterize both the embedded nanoparticles and the loaded nanoscale MOFs. The resulting nanostructure of the material is promising because the Ru nanoparticles hosted in the MIL-101 pores are prevented from agglomeration-the stability and lifetime of the catalyst could be improved.

Rigid Nanoporous Urea-Based Covalent Triazine Frameworks for C2/C1 and CO2/CH4 Gas Separation.

C2/C1 hydrocarbon separation is an important industrial process that relies on energy-intensive cryogenic distillation methods. The use of porous adsorbents to selectively separate these gases is a viable alternative. Highly stable covalent triazine frameworks (urea-CTFs) have been synthesized using 1,3-bis(4-cyanophenyl)urea. Urea-CTFs exhibited gas uptakes of C2H2 (3.86 mmol/g) and C2H4 (2.92 mmol/g) at 273 K and 1 bar and is selective over CH4. Breakthrough simulations show the potential of urea-CTFs for C2/C1 separation.

Salen-decorated Periodic Mesoporous Organosilica: From Metal-assisted Epoxidation to Metal-free CO2 Insertion.

We clicked a salen ligand onto a thiol-ethane bridged periodic mesoporous organosilica (Salen-PMO) using a photo-initiated thiol-ene click reaction. This process resulted in a covalently bonded salen ligand on the PMO material. The final BET surface area amounts 511 m2 /g and the pore size diameter is approximately 7 nm. The functionalized PMO material showed an excellent carbon dioxide uptake capacity of 1.29 mmol/g at 273 K and 1 bar. More importantly, by coordinating a MoO2 2+ complex onto the Salen-PMO material, we obtained a heterogeneous catalyst with a good catalytic performance for the epoxidation of cyclohexene. The catalyst was highly reusable, as no decrease in its activity was observed for at least four runs (99% conversion). Finally, the metal-free Salen-PMO showed an exceptional catalytic performance in the cycloaddition of CO2 to epoxides. The obtained results clearly demonstrate the versatility of the Salen-PMO material not only as metal-free catalyst but also as a support material to anchor metal complexes for specific catalytic applications. With the same catalytic platform, we were able to firstly create epoxides out of alkenes, and subsequently turn these epoxides into cyclic carbonates, consuming CO2 .

Bifunctional Noble-Metal-Free Catalyst for the Selective Aerobic Oxidation-Knoevenagel One-Pot Reaction: Encapsulation of Polyoxometalates into an Alkylamine-Modified MIL-101 Framework.

One-pot reactions offer economic and environmental advantages. Therefore, the design and synthesis of multifunctional catalysts capable of catalyzing multistep organic transformations are highly important. Herein, an effective bifunctional heterogeneous catalyst is presented. For the first time, the encapsulation of H5PMo10V2O40 (PMoV2) polyoxometalate into the cages of an alkylamine-modified MIL-101 using an optimized double-solvent method is reported. The obtained PMoV2@DETA-MIL-101 material displays a great catalytic performance (99% conversion of alcohols) for the selective aerobic oxidation-Knoevenagel one-pot reaction. To the best of our knowledge, this is one of the first reports on the usage of noble-metal-free catalysts for the aerobic oxidation-Knoevenagel one-pot reaction without the addition of additives. The catalyst is very stable and can be used for at least five cycles with no leaching of the active sites.

Identification of vanadium dopant sites in the metal-organic framework DUT-5(Al).

Studying the structural environment of the VIV ions doped in the metal-organic framework (MOF) DUT-5(Al) ((AlIIIOH)BPDC) with electron paramagnetic resonance (EPR) reveals four different vanadium-related spectral components. The spin-Hamiltonian parameters are derived by analysis of X-, Q- and W-band powder EPR spectra. Complementary Q-band Electron Nuclear DOuble Resonance (ENDOR) experiments, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX), X-Ray Diffraction (XRD) and Fourier Transform InfraRed (FTIR) measurements are performed to investigate the origin of these spectral components. Two spectral components with well resolved 51V hyperfine structure are visible, one corresponding to VIV[double bond, length as m-dash]O substitution in a large (or open) pore and one to a narrow (or closed) pore variant of this MOF. Furthermore, a broad structureless Lorentzian line assigned to interacting vanadyl centers in each other's close neighborhood grows with increasing V-concentration. The last spectral component is best visible at low V-concentrations. We tentatively attribute it to (VIV[double bond, length as m-dash]O)2+ linked with DMF or dimethylamine in the pores of the MOF. Simulations using these four spectral components convincingly reproduce the experimental spectra and allow to estimate the contribution of each vanadyl species as a function of V-concentration.

Oxygen-rich poly-bisvanillonitrile embedded amorphous zirconium oxide nanoparticles as reusable and porous adsorbent for removal of arsenic species from water.

A new oxygen-rich porous polymer based on bisvanillonitrile was synthesized and characterized. This polymer was employed as support for the anchoring of 14.5 w% amorphous zirconium oxide nanoparticles. The formation of homogeneously dispersed nanoparticles in the poly-bisvanillonitrile (PBVN) host material was confirmed using N2-sorption, XRPD, XPS and electron microscopy. The combination of zirconium oxide nanoparticles having active adsorption sites with the porous supporting material showed excellent adsorption of arsenic species. The resulting adsorption capacities of the hybrid material extend to 245 mg g-1 for arsenite (AsIII) and 115 mg g-1 for arsenate (AsV). Moreover, adsorption kinetics showed a fast removal of both arsenic species with initial adsorption rate h of 0.0646 mg g-1 min-1 for arsenite and 0.0746 mg g-1 min-1 for arsenate. The immobilization was not interfered by the presence of other compounds in solution, indicating the applicability in real working environments. The material could be regenerated in a continuous mode using a 0.1 mol L-1 sodium hydroxide solution at 70 °C to desorb arsenic.

A Visible-Light-Harvesting Covalent Organic Framework Bearing Single Nickel Sites as a Highly Efficient Sulfur-Carbon Cross-Coupling Dual Catalyst.

Covalent Organic Frameworks (COFs) have recently emerged as light-harvesting devices, as well as elegant heterogeneous catalysts. The combination of these two properties into a dual catalyst has not yet been explored. We report a new photosensitive triazine-based COF, decorated with single Ni sites to form a dual catalyst. This crystalline and highly porous catalyst shows excellent catalytic performance in the visible-light-driven catalytic sulfur-carbon cross-coupling reaction. Incorporation of single transition metal sites in a photosensitive COF scaffold with two-component synergistic catalyst in organic transformation is demonstrated for the first time.

A Ru-Complex Tethered to a N-Rich Covalent Triazine Framework for Tandem Aerobic Oxidation-Knoevenagel Condensation Reactions.

Herein, a highly N-rich covalent triazine framework (CTF) is applied as support for a RuIII complex. The bipyridine sites within the CTF provide excellent anchoring points for the [Ru(acac)2(CH3CN)2]PF6 complex. The obtained robust RuIII@bipy-CTF material was applied for the selective tandem aerobic oxidation-Knoevenagel condensation reaction. The presented system shows a high catalytic performance (>80% conversion of alcohols to α, ß-unsaturated nitriles) without the use of expensive noble metals. The bipy-CTF not only acts as the catalyst support but also provides the active sites for both aerobic oxidation and Knoevenagel condensation reactions. This work highlights a new perspective for the development of highly efficient and robust heterogeneous catalysts applying CTFs for cascade catalysis.

Regeneration of Hopcalite used for the adsorption plasma catalytic removal of toluene by non-thermal plasma.

A dielectric barrier discharge reactor packed with both Hopcalite & glass beads has been investigated for the total oxidation of toluene adsorbed on Hopcalite. The catalytic activity and selectivity through the possible formation of by-products during the NTP discharge for the abatement of irreversibly adsorbed toluene have been investigated by FT-IR and mass spectrometer. The regeneration of the used Hopcalite by NTP discharge has been established by (i) determining the amount of toluene adsorbed on NTP regenerated Hopcalite, (ii) investigating the catalytic activity of NTP regenerated Hopcalite and (iii) comparing the bulk and surface properties of the fresh calcined and NTP regenerated Hopcalite. The ratio of amount of irreversibly adsorbed toluene to that of the total amount of adsorbed toluene adsorbed is similar for the fresh calcined and NTP (I) regenerated Hopcalite. The catalytic activity of the NTP (I) regenerated Hopcalite is slightly enhanced when compared to that of the fresh calcined Hopcalite. Although the first NTP treatment induces partial transformation of Hopcalite into Mn3O4 with no detected related CuOx and reduces specific surface area by a factor of 2, the toluene adsorption capacity remains less affected. A plausible reaction scheme for toluene decomposition in Hopcalite PBDBD reactor is proposed.

Strongly Reducing (Diarylamino)benzene-Based Covalent Organic Framework for Metal-Free Visible Light Photocatalytic H2O2 Generation.

Photocatalytic reduction of molecular oxygen is a promising route toward sustainable production of hydrogen peroxide (H2O2). This challenging process requires photoactive semiconductors enabling solar energy driven generation and separation of electrons and holes with high charge transfer kinetics. Covalent organic frameworks (COFs) are an emerging class of photoactive semiconductors, tunable at a molecular level for high charge carrier generation and transfer. Herein, we report two newly designed two-dimensional COFs based on a (diarylamino)benzene linker that form a Kagome (kgm) lattice and show strong visible light absorption. Their high crystallinity and large surface areas (up to 1165 m2·g-1) allow efficient charge transfer and diffusion. The diarylamine (donor) unit promotes strong reduction properties, enabling these COFs to efficiently reduce oxygen to form H2O2. Overall, the use of a metal-free, recyclable photocatalytic system allows efficient photocatalytic solar transformations.

Metal-free activation of molecular oxygen by covalent triazine frameworks for selective aerobic oxidation.

Oxygen activation is a critical step in ubiquitous heterogeneous oxidative processes, most prominently in catalysis, electrolysis, and pharmaceutical applications. We present here our findings on metal-free O2 activation on covalent triazine frameworks (CTFs) as an important class of N-rich materials. The O2 activation process was studied for the formation of aldehydes, ketones and imines. A detailed mechanistic study of O2 activation and the role of nitrogen heteroatoms were comprehensively investigated. The electron paramagnetic resonance (EPR) and control experiments provide strong evidence for the reaction mechanism proving the applicability of the CTFs to activate oxygen into superoxide species. This report highlights the importance of a self-templating procedure to introduce N functionalities for the development of metal-free catalytic materials. The presented findings reveal an important step toward the use of CTFs as inexpensive and high-performance alternatives to metal-based materials not only for catalysis but also for biorelated applications dealing with O2 activation.

Engineering a Highly Defective Stable UiO-66 with Tunable Lewis- Brønsted Acidity: The Role of the Hemilabile Linker.

The stability of metal-organic frameworks (MOFs) typically decreases with an increasing number of defects, limiting the number of defects that can be created and limiting catalytic and other applications. Herein, we use a hemilabile (Hl) linker to create up to a maximum of six defects per cluster in UiO-66. We synthesized hemilabile UiO-66 (Hl-UiO-66) using benzene dicarboxylate (BDC) as linker and 4-sulfonatobenzoate (PSBA) as the hemilabile linker. The PSBA acts not only as a modulator to create defects but also as a coligand that enhances the stability of the resulting defective framework. Furthermore, upon a postsynthetic treatment in H2SO4, the average number of defects increases to the optimum of six missing BDC linkers per cluster (three per formula unit), leaving the Zr-nodes on average sixfold coordinated. Remarkably, the thermal stability of the materials further increases upon this treatment. Periodic density functional theory calculations confirm that the hemilabile ligands strengthen this highly defective structure by several stabilizing interactions. Finally, the catalytic activity of the obtained materials is evaluated in the acid-catalyzed isomerization of α-pinene oxide. This reaction is particularly sensitive to the Brønsted or Lewis acid sites in the catalyst. In comparison to the pristine UiO-66, which mainly possesses Brønsted acid sites, the Hl-UiO-66 and the postsynthetically treated Hl-UiO-66 structures exhibited a higher Lewis acidity and an enhanced activity and selectivity. This is further explored by CD3CN spectroscopic sorption experiments. We have shown that by tuning the number of defects in UiO-66 using PSBA as the hemilabile linker, one can achieve highly defective and stable MOFs and easily control the Brønsted to Lewis acid ratio in the materials and thus their catalytic activity and selectivity.