Structural Transformation of Porous Polyoxometalate Frameworks and Highly Efficient Biomimetic Aerobic Oxidation of Aliphatic Alcohols

Anderson-type polyoxometalate-based metal-organic framework as an efficient heterogeneous catalyst for selective oxidation of benzylic C-H bonds

Oxidative transformation of benzylic C-H bonds into functional carbonyl groups under mild conditions represents an efficient method for the synthesis of aromatic carboxylic acids and ketones. Here we report a high-efficiency catalyst system constructed from an Anderson-type polyoxometalate-based metal-Organic framework (POMOF-1) and N-hydroxyphthalimide (NHPI) for selective oxidation of methylarenes and alkylarenes under 1 atm O2 atmosphere. POMOF-1 exerted a synergistic effect originating from the well-aligned Anderson {CrMo6} clusters and Cu centers within the framework, and this entailed good cooperation with NHPI to catalyze the selective oxidation. Accordingly, the reactions exhibit good tolerance and chemical selectivity for a wide range of substrates bearing diverse substituent groups, and the corresponding carboxylic acids and ketones were harvested in good yields under mild conditions. Mechanism study reveals that POMOF-1 worked synergistically with NPHI to activate the benzylic C-H bonds of substrates, which are sequentially oxidized by oxygen and HOO˙ to give rise to the products. This work may pave a way to design high-efficiency catalysts by integration of polyoxometalate-based materials with NPHI for challenging C-H activation.

External Catalyst- and Additive-Free Photo-Oxidation of Aromatic Alcohols to Carboxylic Acids or Ketones Using Air/O2

We present an environment-friendly and highly efficient method for the oxidation of aromatic alcohols to carboxylic acids or ketones in air via light irradiation under external catalyst-, additive-, and base-free conditions. The photoreaction system exhibits a wide substrate scope and the potential for large-scale applications. Most of the desired products are easily obtained via recrystallization and separation from low-boiling reaction medium acetone in good yields, and the products can be subsequent directly transformed without further purification.

β-MnO2 nanoparticles as heterogenous catalysts for aerobic oxidative transformation of alcohols to carbonyl compounds, nitriles, and amides

β-MnO2 nanoparticles exhibit high catalytic performance for the aerobic oxidation of various aromatic, allylic, and heteroaromatic alcohols and one-pot tandem oxidation of alcohols to nitriles and amides in the presence of NH3.

A self-assembly solvothermal synthesis of SiMoVn@[Cu6O(TZI)3(H2O)6]4·nH2O for the efficient selective oxidation of various alkylbenzenes

A series of SiMoVn@rht-MOF-1 were isolated via a one-pot self-assembly solvothermal synthesis, exhibiting effective catalytic activity and excellent recyclability.

MOF-stabilized perfluorinated palladium cages catalyze the additive-free aerobic oxidation of aliphatic alcohols to acids

Extremely high electrophilic metal complexes, composed by a metal cation and very electron poor σ-donor ancillary ligands, are expected to be privileged catalysts for oxidation reactions in organic chemistry. However, their low lifetime prevents any use in catalysis. Here we show the synthesis of fluorinated pyridine-Pd 2+ coordinate cages within the channels of an anionic tridimensional metal organic framework (MOF), and their use as efficient metal catalysts for the aerobic oxidation of aliphatic alcohols to carboxylic acids without any additive. Mechanistic studies strongly support that the MOF-stabilized coordination cage with perfluorinated ligands unleashes the full electrophilic potential of Pd 2+ to dehydrogenate primary alcohols, without any base, and also to activate O 2 for the radical oxidation to the aldehyde intermediate. This study opens the door to design catalytic perfluorinated complexes for challenging organic transformations, where an extremely high electrophilic metal site is required.

Effective Heterogeneous Oxidative Desulfurization Catalyzed by H3 PMo9 W3 O40 @rht-MOF-1

With the increasingly strict standard for sulfur content in fuel, it is necessary to develop high-efficiency catalyst for extractive and catalytic oxidative desulfurization systems (ECODS). Herein, a series of three remarkable complexes H₃ PMo_(12-n) W_n O₄₀ @rht-MOF-1 (1 a, n=1; 2 a, n=2; 3 a, n=3) have been designed and prepared. Complexes 1 a, 2 a and 3 a were characterized by single-crystal X-ray diffraction and FT-IR, PXRD, SEM, N₂ adsorption-desorption isotherms, etc. Upon complex 3 a was applied as catalyst, it exhibited remarkably high catalytic activity in the ECODS reactions of aromatic sulfur compounds under optimal conditions. On the basis of its excellent heterogeneity, the catalyst could be recycled for nine consecutive cycles without significant losing of activity centers. Then, the reaction kinetics and mechanism were investigated and the activation energy have been calculated and discussed. Further, the complex 3 a is employed to catalyze the ODS of commercial diesel oil. As a result, the desulfurization efficiency reached 90%. These results provided important structure data for study the structure-property relationship and potential heterogeneous catalyst applied in ODS in industry.

{Ti6}/{Ti10} Wheel Cluster Substituted Silicotungstate Aggregates

Two novel Ti-oxo wheel cluster substituted silicotungstates (STs) [H₂N(CH₃)₂]₉H₉[Ti₆O₆(SiW₁₀O₃₇)₃]·11H₂O (1) and [H₂N(CH₃)₂]₁₆H₁₀[Ti₁₀O₁₁(SiW₁₀O₃₇)₂(SiW₉O₃₅)₂]·14H₂O (2) have been made by hydrothermal reactions. The polyoxoanion of 1 is a ring-shaped trimer where a Ti₆O₆ ({Ti₆}) wheel cluster is encapsulated by three divacant [SiW₁₀O₃₇]^10- (SiW₁₀O₃₇) fragments. However, 2 is built by two divacant SiW₁₀O₃₇ units and two rare trivacant [SiW₉O₃₅]^12- (SiW₉O₃₅) fragments and further installs an unprecedented Ti₁₀O₁₁ ({Ti₁₀}) double-wheel cluster. To the best of our knowledge, 2 is rare in POM chemistry. Studies on the catalytic oxidation properties reveal that 1 exhibits high catalytic activity toward the oxidation of various sulfides using H₂O₂ as an oxidant. Furthermore, 1 can be facilely recycled and reused for at least five cycles without obvious loss of catalytic activity.

A {Ti6W4}-Cluster-Substituted Polyoxotungstate: Synthesis, Structure, and Catalytic Oxidation Properties

A novel Ti-W-O-cluster-substituted tungstoantimonate (TA), [H₂N(CH₃)₂]₃Na₄H₉[{Ti₆W₄O₁₈(OH)(H₂O)₃}(B-α-SbW₉O₃₃)₃]·20H₂O (1), has been made by hydrothermal reactions of trivacant [B-α-SbW₉O₃₃]^9- units, Ti⁴⁺ cations, and WO₄^2- anions in the presence of [H₂N(CH₃)₂]·Cl and structurally characterized. Intriguingly, the polyoxoanion of 1 is constructed from three [B-α-SbW₉O₃₃]^9- units and a previously unobserved decanuclear heterometallic Ti-W-O cluster [Ti₆W₄O₁₈(OH)(H₂O)₃]¹¹⁺ ({Ti₆W₄}) that is comprised of an octahedral [Ti₆WO₆(H₂O)₃]¹⁸⁺ cluster and an edge-sharing [W₃O₁₂(OH)]^7- fragment via six W-O-Ti/W linkers. Furthermore, studies on the catalytic oxidation properties reveal that 1 possesses good catalytic activity toward the oxidation reactions of various sulfides and cyclooctene based on the environmentally friendly oxidant H₂O₂.

Merging N-Hydroxyphthalimide into Metal-Organic Frameworks for Highly Efficient and Environmentally Benign Aerobic Oxidation

Two highly efficient metal-organic framework catalysts TJU-68-NHPI and TJU-68-NDHPI have been successfully synthesized through solvothermal reactions of which the frameworks are merged with N-hydroxyphthalimide (NHPI) units, resulting in the decoration of pore surfaces with highly active nitroxyl catalytic sites. When t-butyl nitrite (TBN) is used as co-catalyst, the as-synthesized MOFs are demonstrated to be highly efficient and recyclable catalysts for a novel three-phase heterogeneous oxidation of activated C-H bond of primary and secondary alcohols, and benzyl compounds under mild conditions. Based on the high efficiency and selectivity, an environmentally benign system with good sustainability, mild conditions, simple work-up procedure has been established for practical oxidation of a wide range of substrates.

A Copper-Containing Polyoxometalate-Based Metal-Organic Framework as an Efficient Catalyst for Selective Catalytic Oxidation of Alkylbenzenes

A copper-containing polyoxometalate-based metal-organic framework (POMOF), Cu^I₁₂Cl₂(trz)₈[HPW₁₂O₄₀] (HENU-7, HENU = Henan University; trz = 1,2,4-triazole), has been successfully synthesized and well-characterized. In addition, the excellent catalytic ability of HENU-7 has been proved by the selective oxidation of diphenylmethane. Under the optimal conditions, the diphenylmethane conversion obtained over HENU-7 is 96%, while the selectivity to benzophenone is 99%, which outperforms most noble-metal-free POM-based catalysts. Moreover, HENU-7 is stable to reuse for five runs without an obvious loss in activity and also can catalyze the oxidation of different benzylic C-H with satisfactory conversions and selectivities, which implied the significant catalytic activity and recyclability.

Oxidation of 1-propanol to propionic acid with hydrogen peroxide catalysed by heteropolyoxometalates

BACKGROUND

Propionic acid as a very valuable chemical is in high demand, and it is industrially produced via the oxo-synthesis of ethylene or ethyl alcohol and via the oxidation of propionaldehyde with oxygen. It is urgent to discover a new preparation method for propionic acid via a green route. Recyclable amino-acid-based organic-inorganic heteropolyoxometalates were first used to high-efficiently catalyse the selective oxidation of 1-propanol to propionic acid with H2O2 as an oxidant.

RESULT

A series of amino-acid-based heteropoly catalysts using different types of amino acids and heteropoly acids were synthesized, and the experimental results showed proline-based heteropolyphosphatotungstate (ProH)3[PW12O40] exhibited excellent catalytic activity for the selective catalytic oxidation of 1-propanol to propionic acid owing to its high capacity as an oxygen transfer agent and suitable acidity. Under optimized reaction conditions, the conversion of 1-propanol and the selectivity of propionic acid reached 88% and 75%, respectively. Over four cycles, the conversion remained at >80%, and the selectivity was >60%. (ProH)3[PW12O40] was also used to catalyse the oxidations of 1-butanol, 1-pentanol, 1-hexanol, and benzyl alcohol. All the reactions had high conversions, with the corresponding acids being the primary oxidation product.

CONCLUSIONS

Proline-based heteropolyoxometalate (ProH)3[PW12O40] has been successfully used to catalyse the selective oxidation of primary alcohols to the corresponding carboxylic acids with H2O2 as the oxidant. The new developed catalytic oxidation system is mild, high-efficient, and reliable. This study provides a potential green route for the preparation propionic acid.

A Rh(iii)-catalyzed C–H activation/regiospecific annulation cascade of benzoic acids with propargyl acetates to unusual 3-alkylidene-isochromanones

We developed a new approach to synthesize isochromanones with benzoic acids and propargyl acetates, which introducing an unusual exocyclic C–C double bond at the 3-position with high regioselectivity and moderate to excellent yields.

Polyoxometalate-based metal–organic frameworks for heterogeneous catalysis

POM-based MOFs simultaneously possessing the virtues of POMs and MOFs exhibit excellent heterogeneous catalytic properties.

Sequential Connection of Mutually Exclusive Catalytic Reactions by a Method Controlling the Presence of an MOF Catalyst: One-Pot Oxidation of Alcohols to Carboxylic Acids

A functionalized metal-organic framework (MOF) catalyst applied to the sequential one-pot oxidation of alcohols to carboxylic acids controls the presence of a heterogeneous catalyst. The conversion of alcohols to aldehydes was acquired through aerobic oxidation using a well-known amino-oxy radical-functionalized MOF. In the same flask, a simple filtration of the radical MOF with mild heating of the solution completely altered the reaction media, providing radical scavenger-free conditions suitable for the autoxidation of the aldehydes formed in the first step to carboxylic acids. The mutually exclusive radical-catalyzed aerobic oxidation (the first step with MOF) and radical-inhibited autoxidation (the second step without MOF) are sequentially achieved in a one-pot manner. Overall, we demonstrate a powerful and efficient method for the sequential oxidation of alcohols to carboxylic acids by employing a readily functionalizable heterogeneous MOF. In addition, our MOF in-and-out method can be utilized in an environmentally friendly way for the oxidation of alcohols to carboxylic acids of industrial and economic value with broad functional group tolerance, including 2,5-furandicarboxylic acid and 1,4-benzenedicarboxylic acid, with good yield and reusability. Furthermore, MOF-TEMPO, as an antioxidative stabilizer, prevents the undesired oxidation of aldehydes, and the perfect "recoverability" of such a reactive MOF requires a re-evaluation of the advantages of MOFs from heterogeneity in catalytic and related applications.

Solvent and additive-free efficient aerobic oxidation of alcohols by a perovskite oxide-based heterogeneous catalyst

A perovskite oxide has been utilized for the solvent and additive-free heterogeneous oxidation of various alcohols.

Highly efficient oxidation of various thioethers catalyzed by organic ligand-modified polyoxomolybdates

Herein, four hybrid dimers based on the carboxylic acid ligand-modified polyoxomolybdates were prepared, which could rapidly and selectively oxidize various phenyl sulfides and the sulfur mustard simulant 2-chloroethyl ethyl sulfide (CEES).

A universal strategy for fabrication and morphology control of polyoxometalate-based metal–organic frameworks

Fabrication of POM@MOFs by utilizing the oxidization of POMs to metals and the effect of polyoxoanion charge on POM@MOF morphology.

Polyoxometalate-based room-temperature phosphorescent materials induced by anion–π interactions

A series of polyoxometalate-based host–guest materials emit strong red room-temperature phosphorescence attributed to intermolecular charge-transfer states which was caused by unorthodox anion–π interactions.

Multiunit Catalysts with Synergistic Reactivity: Three-Dimensional Polyoxometalate-Based Coordination Polymers for Highly Efficient Synthesis of Functionalized p-Benzoquinones

The rational design of highly efficient catalysts for the synthesis of functionalized p-benzoquinones (p-BQs) is of great significance for the manufacture of bioactive compounds. Herein, two 3D crystalline polyoxometalate-based coordination polymers (POMCPs) are used as heterogeneous catalysts for the synthesis of p-BQs, which are H[Cu^II(ttb)(H₂O)₃]₂[Cu^II(ttb)Cl]₂[PW₁₂O₄₀]·4H₂O (1) (Httb = 1-(tetrazol-5-yl)-4-(triazol-1-yl)benzene) and [ClCu₆^I(trz₎₄][ClCu₅^I(trz)₄]₂[Cu^II(H₂O)][PW₁₂O₄₀] (2) (trz = 1,2,4-triazole). Both compounds were characterized by elemental analysis, IR, XPS, solid diffuse reflective spectroscopy, TG analysis, and single-crystal X-ray diffraction. In 1, Keggin anions [PW₁₂O₄₀]^3- locate in 1D square channels constructed from wave-like Cu-ttb layers to form a 3D POMCP by coordinating to Cu ions, and in 2, [PW₁₂O₄₀]^3- anions situate in eight-membered Cu-trz channels via Cu···O interactions to yield a 3D POMCP structure. The catalytic activities of 1 and 2 have been evaluated in the selective oxidation of alkylphenols/alkoxybenzenes/methylnaphthalene, especially in the oxidation reaction of 2,3,6-trimethylphenol (TMP) to 2,3,5-trimethyl-p-benzoquinone (TMBQ, vitamin E key intermediate), with H₂O₂ as oxidant. By using catalysts 1 and 2 under optimal conditions, the yield of TMBQ can reach 99% and 96% within 10-20 min, respectively. Both catalysts demonstrated high turnover frequencies (300 h^-1 for 1 and 600 h^-1 for 2) and the truly heterogeneous nature. 1 and 2 catalyzed the synthesis of p-BQs on the basis of effective cooperative catalytic activities by POMs and metal nodes.

Directed Structural Transformations of Coordination Polymers Supported Single-Site Cu(II) Catalysts To Control the Site Selectivity of C-H Halogenation

A main difficulty in C-H bond functionalization is to undertake the catalyst control accurately where the reaction takes place. In this work, to achieve highly effective and regioselective single-site catalysts, a three-dimensional (3D) rhombus-like framework of {[Mn(Hidbt)DMF]·H₂O}_n (1) [H₃idbt = 5,5'-(1H-imidazole-4,5-diyl)-bis(2H-tetrazole)] containing coordinated DMF molecules was constructed. For the dissolution-recrystallization structural transformation process, attractive structural transformations proceeded from 1 to a new crystalline species formulated as {[Mn₃(idbt)₂(H₂O)₂]·3H₂O}_n (2) with a 3D windowlike architecture, and then the Mn ions in 2 could be exchanged with Cu ions through cation exchange in a single-crystal to single-crystal fashion to produce the Cu-exchanged product {[Mn₂Cu(idbt)₂(H₂O)₂]·3H₂O}_n (2a), which had a windowlike framework like that of 2. Furthermore, 2 and 2a were used as heterogeneous catalysts for the regioselective C-H halogenation of phenols with N-halosuccinimides (NCS and NBS) to produce the site selective single monohalogenated products. It was found that the catalytic activity and site selectivity of 2a were much higher than those of 2, because the unique structural features of 2a with the uniformly dispersed Cu^II active centers served as a single-site catalyst with a site-isolated and well-defined platform to promote the C-H halogenation reaction in regiocontrol and guide an orientation that favored the para selectivity during the reaction process.

Reversible Structural Transformations of Metal-Organic Frameworks as Artificial Switchable Catalysts for Dynamic Control of Selectively Cyanation Reaction

The synthesis of molecular-level artificial switchable catalysts, of which activity in different chemical processes can be switched by controlling different stimuli, has provided a new paradigm to perform mechanical tasks and measurable work. In this work, to obtain highly effective and regioselective artificial switchable catalysts, a hierarchical anion-pillared framework {(H₃ O)[Cu(CPCDC)(4,4'-bpy)]}_n (1; H₃ CPCDC=9-(4-carboxyphenyl)-9H-carbazole-3,6-dicarboxylic acid, 4,4'-bpy=4,4'-bipyridine), including free [H₃ O]⁺ ions as guest molecules, was constructed. Upon dissolve-exchange-crystallization behavior, fascinating reversible structural transformations proceeded between anion framework 1 and neutral 2D stair-stepping framework {[Cu(CPCDC)(4,4'-bpe)]}_n (2; 4,4'-bpe=4,4'-vinylenedipyridine). Moreover, frameworks 1 and 2 can act as heterogeneous artificial switchable catalysts to selectively promote the direct cyanation reaction of terminal alkynes and azobisisobutyronitrile. The results indicated that 1 and 2 exhibited excellent selectivity to generate vinyl isobutyronitrile skeletons or propiolonitrile frameworks, respectively, as unique products. Furthermore, indicating paper, GC-MS, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy analysis demonstrated that the reversible structural transformations endowed 1 and 2 with well-defined platforms to stabilize the isobutyronitrile and CN sources through the different catalytic pathways.

Hetero-metallic active sites coupled with strongly reductive polyoxometalate for selective photocatalytic CO2-to-CH4 conversion in water

The photocatalytic reduction of CO2 to value-added methane (CH4) has been a promising strategy for sustainable energy development, but it is challenging to trigger this reaction because of its necessary eight-electron transfer process. In this work, an efficient photocatalytic CO2-to-CH4 reduction reaction was achieved for the first time in aqueous solution by using two crystalline heterogeneous catalysts, H{[Na2K4Mn4(PO4) (H2O)4]⊂{[Mo6O12(OH)3(HPO4)3(PO4)]4[Mn6(H2O)4]}·16H2O (NENU-605) and H{[Na6CoMn3(PO4)(H2O)4]⊂{[Mo6O12(OH)3(HPO4)3(PO4)]4[Co1.5Mn4.5]}·21H2O (NENU-606). Both compounds have similar host inorganic polyoxometalate (POM) structures constructed with strong reductive {P4Mo6 V} units, homo/hetero transition metal ions (MnII/CoIIMnII) and alkali metal ions (K+ and/or Na+). It is noted that the {P4Mo6 V} cluster including the six MoV atoms served as a multi-electron donor in the case of a photocatalytic reaction, while the transition metal ions functioned as catalytically active sites for adsorbing and activating CO2 molecules. Additionally, the presence of alkali metal ions was believed to assist in the capture of more CO2 for the photocatalytic reaction. The synergistic combination of the above-mentioned components in NENU-605 and NENU-606 effectively facilitates the accomplishment of the required eight-electron transfer process for CH4 evolution. Furthermore, NENU-606 containing hetero-metallic active sites finally exhibited higher CH4 generation selectivity (85.5%) than NENU-605 (76.6%).

Versatile catalysts constructed from hybrid polyoxomolybdates for simultaneously detoxifying sulfur mustard and organophosphate simulants

Twelve new hybrid dimers based on carboxylic acid ligand modified polyoxomolybdates were prepared, which can rapidly oxidize the mustard gas simulant, CEES, and hydrolyze the nerve agent simulant, DECP, at room temperature.

Octahedron-shaped three-shell Ln14-substituted polyoxotungstogermanates encapsulating a W4O15 cluster: luminescence and frequency dependent magnetic properties

The octahedron-shaped three-shell structure of the [(Ln₁₄(H₂O)W₄(OH)O₁₄)(WO₄)₄(GeW₁₀O₃₈)₆]⁴³⁻ polyanion contains a W₄ tetrahedron, a Ln₁₄W₄ tetrahedron and a Ge₆W₆₀ octahedron.

Polyoxometalate-Based Metal-Organic Frameworks for Selective Oxidation of Aryl Alkenes to Aldehydes

Polyoxometalates (POMs) show considerable catalytic performance toward the selective oxidation of alkenes to aldehydes, which is commercially valuable for the production of pharmaceuticals, dyes, perfumes, and fine chemicals. However, the low specific surface area of POMs as heterogeneous catalysts and poor recyclability as homogeneous catalysts have hindered their wide application. Dispersing POMs into metal-organic frameworks (MOFs) for the construction of POM-based MOFs (POMOFs) suggests a promising strategy to realize the homogeneity of heterogeneous catalysis. Herein, we report two new POMOFs with chemical formulas of [Co(BBTZ)₂][H₃BW₁₂O₄₀]·10H₂O (1) and [Co₃(H₂O)₆(BBTZ)₄][BW₁₂O₄₀]·NO₃·4H₂O (2) (BBTZ = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene) for the selective oxidation of alkenes to aldehydes. Compound 1 possesses a non-interpenetrated three-dimensional (3D) cds-type open framework with a 3D channel system. Compound 2 displays a 3D polyrotaxane framework with one-dimensional channels along the [100] direction. In the selective oxidation of styrene into benzaldehyde, compound 1 can achieve a 100% conversion in 4 h with 96% selectivity toward benzaldehyde, which is superior to that of compound 2. A series of control experiments reveal that the co-role of [BW₁₂O₄₀]^5- and Co²⁺ active center as well as a more open framework feature co-promote the catalytic property of the POMOFs in this case. This work may suggest a new option for the development of POMOF catalysts in the selective oxidation of alkenes.

Polyoxometalate-based Gemini ionic catalysts for selective oxidation of benzyl alcohol with hydrogen peroxide in water

Polyoxometalate-based Gemini ionic hybrids with inherent phase transfer capability are highly efficient and recyclable catalysts in the selective oxidation of alcohols with H₂O₂ in water.