Variation in Catalytic Efficacies of a 2D pH-Stable MOF by Altering Activation Methods

Although it is well-known that the Lewis acidity of Metal-Organic Frameworks (MOFs) can effectively enhance their catalytic activity in organic transformations, access to these Lewis-acidic sites remains a key hurdle to widespread applications of Lewis-acidic catalysis by MOFs. Easy accessibility of strong Lewis acidic sites onto 2D MOFs by using proper activation methods can be a cornerstone in attaining desired catalytic performance. Herein, we report a new 2D chemically stable MOF, IITKGP-60, which displayed excellent framework robustness over a wide pH range (2-12). Benefiting from the abundant open metal sites (OMSs) and framework robustness, the catalytic activity of the developed material was explored in one-pot three-component Strecker reaction and Knoevenagel condensation reaction. Moreover, the developed catalyst is superior in catalyzing the reactions involving sterically hindered substrate (1-naphthaldehyde) with high turnover number. A comparative catalytic study was conducted using different activation methods (chloroform and methanol exchanged activated samples), highlighting the significant effect of activation methods on its catalytic performances. The sustainable synthetic pathway under solvent-free conditions for a broad scope of substrates using low catalyst loading and excellent recyclability made the developed pH-stable framework a promising heterogeneous catalyst.

A Highly Chemically Robust 3D Interpenetrated MOF Heterogeneous Catalyst for the Synthesis of Hantzsch 1,4-Dihydropyridines and Drug Molecules

Metal-organic frameworks (MOFs) have attracted immense attention as efficient heterogeneous catalysts over other solid catalysts, however, their chemical environment instability often limits their catalytic potential. Herein, utilizing a flexible unexplored tetra-acid ligand and employing the mixed ligand approach, a 3D interpenetrated robust framework is strategically developed, IITKGP-51 (IITKGP stands for Indian Institute of Technology Kharagpur), which retained its crystallinity over a wide range of pH solution (4-12). Having ample open metal sites (OMSs), IITKGP-51 is explored as a heterogeneous catalyst in one-pot Hantzsch condensation reaction, with low catalyst loading for a broad range of substrates. The synthesis of drug molecules remains one of the most significant and emergent areas of organic and medicinal chemistry. Considering such practical utility, biologically important Nemadipine B and Nifedipine drug molecules (calcium channel protein inhibitor) are synthesized for the first time by using this catalyst and fully characterized via SC-XRD and other spectroscopic methods. This report inaugurates the usage of a MOF material as a catalyst for the synthesis of drug molecules.

Highly Robust Metal-Organic Framework for Efficiently Catalyzing Knoevenagel Condensation and the Strecker Reaction under Solvent-Free Conditions

Metal-organic frameworks (MOFs) have been recognized as one of the most promising porous materials and offer great opportunities for the rational design of new catalytic solids having great structural diversity and functional tunability. Despite numerous inherent merits, their chemical environment instability limits their practical usage and demands further exploration. Herein, by employing the mixed-ligand approach, we have designed and developed a robust 3D Co-MOF, [Co2(μ2-O)(TDC)2(L)(H2O)2]·2DMF (H2TDC = 2,5-thiophenedicarboxylic acid, L = 3,3'-azobispyridine), IITKGP-50 (IITKGP stands for the Indian Institute of Technology Kharagpur), which exhibited excellent framework robustness not only in water but also in a wide range of aqueous pH solutions (pH = 2-12). Taking advantage of superior framework robustness and the presence of high-density open metal sites, IITKGP-50 was further explored in catalyzing the two-component Knoevenagel condensation reaction and three-component Strecker reactions. Moreover, to verify the size selectivity of IITKGP-50, smaller to bulkier substrates in comparison with the MOF's pore cavity (8.1 × 5.6 Å2) were employed, in which relatively lesser conversions for the sterically bulkier aldehyde derivatives confirmed that the catalytic cycle occurs inside the pore cavity. The easy scalability, lower catalyst loading compared to that of benchmark MOFs, magnificent conversion rate over a wide range of substrates, and excellent recyclability without significant performance loss made IITKGP-50 a promising heterogeneous catalyst candidate.

Construction of multifunctional NH2-UiO-66 metal organic framework: sensing and photocatalytic degradation of ketorolac tromethamine and tetracycline in aqueous medium

Existence of pharmaceutical residues in water has endangered environmental pollution worldwide, which makes it ineludible to develop prospective bifunctional materials which not only possess excellent fluorescence behaviour to monitor pharmaceuticals but also exhibit simultaneous photocatalytic removal efficiency. Strengthened by functionalized metal organic framework (MOF) materials, we present here an amine functionalized zirconium-based MOF NH₂-UiO-66 which has been successfully synthesized using solvothermal approach. The as prepared MOF was subjected to numerous structural, morphological and compositional characterizations. Interestingly, featured by the excellent fluorescent intensity of MOF modulated by LMCT effect, NH₂-UiO-66 was screened to detect pharmaceutical compounds with KTC and TC in aqueous solution. The prepared functionalized MOF showcased excellent sensing platform with magnificent response range (0‒3 µM), lower limit of detection (160 nM; KTC and 140 nM; TC), excellent selectivity and influential anti-interference capability. More importantly, the practical utility of the proposed sensor was further explored for the determination of pharmaceutical drugs in real water samples with suitable recoveries. Simultaneously, the synthesized MOF also exhibited high photocatalytic efficiency towards the removal of KTC and TC under solar light irradiation. The degradation efficiency for KTC and TC was found to be 68.3% and 71.8% within 60 and 280 min of solar light, respectively. Moreover, excellent recyclability was demonstrated by the current synthesized system over five cycles. Overall, this study presents a feasible route for the utilization of functionalized MOFs as potential dual functional materials towards the simultaneous detection and degradation of specific pharmaceuticals from aqueous medium.

Successive constructions of regular tetra-, hexa- and octanuclear microporous polyoxovanadates(III) for gas adsorption

Pyrazole-assisted tetranuclear microporous polyoxovanadates(III) (POVs) (NH₄)₂K₂[V₄(μ₂-OH)₄(ox)₄(pz)₄]·9H₂O (1, ox = oxalate and pz = pyrazole) and (NH₄)₂Na₂[V₄(μ₂-OH)₄(ox)₄(4-mpz)₄]·7H₂O (2, 4-mpz = 4-methylpyrazole) have been constructed in reduced media, along with their triazole neutral hexa- and octanuclear products K₂[V₆(μ₂-OH)₆(ox)₆(Hdatrz)₆]Cl₂·29.5H₂O (3) and [V₈(μ₂-OH)₈(SO₃)₈(Hdatrz)₈]·38H₂O (4, Hdatrz = 1H-1,2,4-triazole-3,5-diamine) successively. Both polyanionic structures of 1 and 2 share similar inorganic building blocks that consist of regular {V₄(μ₂-OH)₄} skeletons with an inner diameter of 2.8 Å, while a paddle wheel-shaped cluster 3 contains a {V₆(μ₂-OH)₆} skeleton with two chlorides encapsulated around the center of the ring, occupying a hole of 3.7 Å. An interesting isolated intrinsic polyoxometalate-based metal-organic framework (POMOF) 4 exists as an octanuclear petaloid-like skeleton {V₈(μ₂-OH)₈(SO₃)₈} with an inner diameter of 5.2 Å. Bond valence sum calculations manifest that all V ions have severely reduced +3 oxidation states in 1-4, which are supported by charge balance, structural and magnetic data. Moreover, gas adsorptions indicate that 1, 2 and 4 can adsorb CO₂ and O₂ more favorably than N₂, CH₄ and H₂ gases. Compared with 1 and 2, due to the functionalization of microchannels with Lewis base amino and hydroxy groups and uncoordinated azolate N-donors inside POMOF 4, it should have notable affinities toward CO₂ adsorption.

Topologically Driven Pore/Surface Engineering in a Recyclable Microporous Metal-Organic Vessel Decorated with Hydrogen-Bond Acceptors for Solvent-Free Heterogeneous Catalysis

The use of metal-organic frameworks (MOFs) comprising custom-designed linkers/ligands as efficient and recyclable heterogeneous catalysts is on the rise. However, the topologically driven bifunctional porous MOFs for showcasing a synergistic effect of two distinct activation pathways of substrates (e.g., involving hydrogen bonding and a Lewis acid) in multicomponent organic transformations are very challenging. In particular, the novelty of such studies lies in the proper pore and/or surface engineering in MOFs for bringing the substrates in close proximity to understand the mechanistic aspects at the molecular level. This work represents the topological design, solid-state structural characterization, and catalytic behavior of an oxadiazole tetracarboxylate-based microporous three-dimensional (3D) metal-organic framework (MOF), {[Zn₂(oxdia)(4,4'-bpy)₂]·8.5H₂O}_n (1), where the tetrapodal (4-connected) 5,5'-(1,3,4-oxadiazole-2,5-diyl)diisophthalate (oxdia^4-), the tetrahedral metal vertex (Zn(II)), and a 2-connected pillar linker 4,4'-bipyridine (4,4'-bpy) are unique in their roles for the formation, stability, and function. As a proof of concept, the efficient utilization of both the oxadiazole moiety with an ability to provide H-bond acceptors and the coordinatively unsaturated Zn(II) centers in 1 is demonstrated for the catalytic process of the one-pot multicomponent Biginelli reaction under mild conditions and without a solvent. The key steps of substrate binding with the oxadiazole moiety are ascertained by a fluorescence experiment, demonstrating a decrease or increase in the emission intensity upon interaction with the substrates. Furthermore, the inherent polarizability of the oxadiazole moiety is exploited for CO₂ capture and its size-selective chemical fixation to cyclic carbonates at room temperature and under solvent-free conditions. For both catalytic processes, the chemical stability, structural integrity, heterogeneity, versatility in terms of substrate scope, and mechanistic insights are discussed. Interestingly, the first catalytic process occurs on the surface, while the second reaction occurs inside the pore. This study opens new ways to catalyze different organic transformation reactions by utilizing this docking strategy to bring the multiple components close together by a microporous MOF.

Effect of Unsaturated Metal Site Modulation in Highly Stable Microporous Materials on CO2 Capture and Fixation

We have designed and synthesized two unprecedented microporous three-dimensional metal-organic frameworks, {[Cd₆(TPOM)₃(L)₆]·12DMF·3H₂O}_n (1) and {[Zn₂(TPOM)(L)₂]·2DMF·H₂O}_n (2), based on a flexible quadritopic ligand, tetrakis(4-pyridyloxymethylene)methane (TPOM), and a bent dicarboxylic acid, 4,4'-(dimethylsilanediyl)bis-benzoic acid (H₂L). The networks of 1 and 2 share a 4-c uninodal net NbO topology but exhibit different metal environments due to coordination preferences of Cd(II) and Zn(II). The Cd(II) center in 1 is six-coordinated, whereas the Zn(II) center in 2 is only four-coordinated, making the latter an unsaturated metal center. Such modulation of coordination atmosphere of metal centers in MOFs with the same topology is possible due to diverse binding of the carboxylate groups of L^2-. Both 1 and 2 have relatively high thermal stability and exhibit permanent porosity after the removal of guest solvent molecules based on variable temperature powder X-ray diffraction and gas adsorption analysis. These materials exhibit similar gas adsorption properties, especially highly selective CO₂ uptake/capture over other gases (N₂ and CH₄). However, because of the presence of an unsaturated Lewis acidic metal site, 2 acts as a very efficient heterogeneous catalyst toward the chemical conversion of CO₂ to cyclic carbonates under mild conditions, whereas 1 shows very less activity. This work provides experimental evidence for the postulate that an unsaturated metal site in MOFs enhances adsoprtion of CO₂ and promotes its conversion via the Lewis-acid catalysis.

Current and future perspectives on catalytic-based integrated carbon capture and utilization

There exist several well-known methods with varying maturity for capturing carbon dioxide from emission sources of different concentrations, including absorption, adsorption, cryogenics and membrane separation, among others. The capture and separation steps can produce almost pure CO₂, but at substantial cost for being conditioned for transport and final utilization, with high economical risks to be considered. A possible way for the elimination of this conditioning and cost is direct CO₂ utilization, whether on-site in a further process but within the same plant, or in-situ, coupling both capture and conversion in the same unit. This approach is usually called integrated carbon capture and utilization (ICCU) or integrated carbon capture and conversion (ICCC), and has lately started receiving considerable attention in many circles. As CO₂ is already industrially employed in other sectors, such as food preservation, water treatment and conversion to high added-value chemicals and fuels such as methanol, methane, etc., among others, it is of great interest to explore the global ICCC approach. Catalytic-based processes play a key role in CO₂ conversion, and different technologies are gaining great attention from both academia and industry. However, the 'big picture of ICCU' and in which technology the efforts should focus on at large scale is still unclear. This review analyzes some promising concepts of ICCU specifically on CO₂ catalytic conversion, highlighting their current commercial relevance as well as challenges that have to be faced today and in the next future.

Rational Construction of a Responsive Azo-Functionalized Porous Organic Framework for CO2 Sorption

An azo-functionalized porous organic framework (denoted as JJU-1) was synthesized via FeCl3-promoted oxidative coupling polymerization. By virtue of a porous skeleton and a light/heat responsive azo functional group, this task-specific JJU-1 displays a reversible stimuli-responsive adsorption property triggered by UV irradiation and heat treatment. The initial Brunauer-Emmet-Teller (BET) surface area of this porous material is 467 m2 g-1. The CO2 sorption isotherms exhibit a slight decrease after UV irradiation because of the trans to cis conversion of the azo functional skeleton. It is worth mentioning that the responsive CO2 adsorption performance can be recycled for three cycles via alternating external stimuli, confirming the excellently reversible switchability of trans-to-cis isomerization and controllable CO2 adsorption.

Facile synthesis of porous porphyrin-based polymers by solvent-crosslinking method

Three solvents were employed as crosslinkers to synthesise porous porphyrin-based polymers via Friedel–Crafts alkylation reaction.

Organic-Inorganic Hybrid Nanomaterials for Electrocatalytic CO2 Reduction

Electrochemical CO₂ reduction (ECR) to value-added chemicals and fuels is regarded as an effective strategy to mitigate climate change caused by CO₂ from excess consumption of fossil fuels. To achieve CO₂ conversion with high faradaic efficiency, low overpotential, and excellent product selectivity, rational design and synthesis of efficient electrocatalysts is of significant importance, which dominates the development of ECR field. Individual organic molecules or inorganic catalysts have encountered a bottleneck in performance improvement owing to their intrinsic shortcomings. Very recently, organic-inorganic hybrid nanomaterials as electrocatalysts have exhibited high performance and interesting reaction processes for ECR due to the integration of the advantages of both heterogeneous and homogeneous catalytic processes, attracting widespread interest. In this work, the recent advances in designing various organic-inorganic hybrid nanomaterials at the atomic and molecular level for ECR are systematically summarized. Particularly, the reaction mechanism and structure-performance relationship of organic-inorganic hybrid nanomaterials toward ECR are discussed in detail. Finally, the challenges and opportunities toward controlled synthesis of advanced electrocatalysts are proposed for paving the development of the ECR field.

Design and synthesis of photoluminescent active interpenetrating metal-organic frameworks using N-2-aryl-1,2,3-triazole ligands

N-2-aryl-1,2,3-triazole derivatives were synthesized as new ligand systems for the construction of photoluminescent active metal-organic frameworks (MOFs). Crystal structures revealed that the five-membered triazoles show an unsymmetrical conformation with the two C4,C5-substituted benzenes adopting a "twisted-planar" geometry. As a result, a MOF constructed from this ligand exhibited cross-layer interactions with improved water stability (at 100 °C for 24 hours). Furthermore, enhanced photoluminescence emissions were observed upon the formation of MOF structures (Φ up to 30%), suggesting the potential applications of these photoactive porous materials through this new ligand design.

Metal–organic frameworks for the energy-related conversion of CO2 into cyclic carbonates

MOFs are promising heterogeneous catalysts for chemical fixation of CO₂ and epoxides into cyclic carbonates.