Heterogeneous H2O2-based selective oxidations over zirconium tungstate α-ZrW2O8.

Catalytic properties of a crystalline zirconium tungstate, ZrW2O8, the material known mainly for its isotropic negative coefficient of thermal expansion, have been assessed for the liquid-phase selective oxidation of a range of organic substrates comprising CC, OH, S and other functional groups using aqueous hydrogen peroxide as the green oxidant. Samples of ZrW2O8 were prepared by hydrothermal synthesis and characterised by N2 adsorption, PXRD, SEM, EDX, FTIR and Raman spectroscopic techniques. Studies by IR spectroscopy of adsorbed probe molecules (CO and CDCl3) revealed the presence of Brønsted acidic and basic sites on the surface of ZrW2O8. It was demonstrated that ZrW2O8 is able to activate H2O2 under mild conditions and accomplish the epoxidation of CC bonds in alkenes and unsaturated ketones, oxidation of thioethers to sulfoxides and sulfones, along with the oxidation of alcoholic functions to produce ketones and aldehydes. The oxidation of tetramethylethylene and α-terpinene over ZrW2O8 revealed the formation of peroxidation products, 2,3-dimethyl-3-butene-2-hydroperoxide and endoperoxide ascaridole, respectively, indicating the involvement of singlet oxygen in the oxidation process. The ZrW2O8 catalyst preserves its structure and morphology under the turnover conditions and does not suffer from metal leaching. It can be easily recovered, regenerated by calcination, and reused without the loss of activity and selectivity.

Resolving the Mechanism for H2O2 Decomposition over Zr(IV)-Substituted Lindqvist Tungstate: Evidence of Singlet Oxygen Intermediacy.

The decomposition of hydrogen peroxide (H2O2) is the main undesired side reaction in catalytic oxidation processes of industrial interest that make use of H2O2 as a terminal oxidant, such as the epoxidation of alkenes. However, the mechanism responsible for this reaction is still poorly understood, thus hindering the development of design rules to maximize the efficiency of catalytic oxidations in terms of product selectivity and oxidant utilization efficiency. Here, we thoroughly investigated the H2O2 decomposition mechanism using a Zr-monosubstituted dimeric Lindqvist tungstate, (Bu4N)6[{W5O18Zr(µ-OH)}2] ({ZrW5}2), which revealed high activity for this reaction in acetonitrile. The mechanism of the {ZrW5}2-catalyzed H2O2 degradation in the absence of an organic substrate was investigated using kinetic, spectroscopic, and computational tools. The reaction is first order in the Zr catalyst and shows saturation behavior with increasing H2O2 concentration. The apparent activation energy is 11.5 kcal·mol-1, which is significantly lower than the values previously found for Ti- and Nb-substituted Lindqvist tungstates (14.6 and 16.7 kcal·mol-1, respectively). EPR spectroscopic studies indicated the formation of superoxide radicals, while EPR with a specific singlet oxygen trap, 2,2,6,6-tetramethylpiperidone (4-oxo-TEMP), revealed the generation of 1O2. The interaction of test substrates, α-terpinene and tetramethylethylene, with H2O2 in the presence of {ZrW5}2 corroborated the formation of products typical of the oxidation processes that engage 1O2 (endoperoxide ascaridole and 2,3-dimethyl-3-butene-2-hydroperoxide, respectively). While radical scavengers tBuOH and p-benzoquinone produced no effect on the peroxide product yield, the addition of 4-oxo-TEMP significantly reduced it. After optimization of the reaction conditions, a 90% yield of ascaridole was attained. DFT calculations provided an atomistic description of the H2O2 decomposition mechanism by Zr-substituted Lindqvist tungstate catalysts. Calculations showed that the reaction proceeds through a Zr-trioxidane [Zr-η2-OO(OH)] key intermediate, whose formation is the rate-determining step. The Zr-substituted POM activates heterolytically a first H2O2 molecule to generate a Zr-peroxo species, which attacks nucleophilically to a second H2O2, causing its heterolytic O-O cleavage to yield the Zr-trioxidane complex. In agreement with spectroscopic and kinetic studies, the lowest-energy pathway involves dimeric Zr species and an inner-sphere mechanism. Still, we also found monomeric inner- and outer-sphere pathways that are close in energy and could coexist with the dimeric one. The highly reactive Zr-trioxidane intermediate can evolve heterolytically to release singlet oxygen and also decompose homolytically, producing superoxide as the predominant radical species. For H2O2 decomposition by Ti- and Nb-substituted POMs, we also propose the formation of the TM-trioxidane key intermediate, finding good agreement with the observed trends in apparent activation energies.

Catalytic Performance of Zr-Based Metal-Organic Frameworks Zr-abtc and MIP-200 in Selective Oxidations with H2 O2.

The catalytic performance of Zr-abtc and MIP-200 metal-organic frameworks consisting of 8-connected Zr6 clusters and tetratopic linkers was investigated in H2 O2 -based selective oxidations and compared with that of 12-coordinated UiO-66 and UiO-67. Zr-abtc demonstrated advantages in both substrate conversion and product selectivity for epoxidation of electron-deficient C=C bonds in α,ß-unsaturated ketones. The significant predominance of 1,2-epoxide in carvone epoxidation, coupled with high sulfone selectivity in thioether oxidation, points to a nucleophilic oxidation mechanism over Zr-abtc. The superior catalytic performance in the epoxidation of unsaturated ketones correlates with a larger amount of weak basic sites in Zr-abtc. Electrophilic activation of H2 O2 can also be realized, as evidenced by the high activity of Zr-abtc in epoxidation of the electron-rich C=C bond in caryophyllene. XRD and FTIR studies confirmed the retention of the Zr-abtc structure after the catalysis. The low activity of MIP-200 in H2 O2 -based oxidations is most likely related to its specific hydrophilicity, which disfavors adsorption of organic substrates and H2 O2 .

Heterogeneous epoxidation of menadione with hydrogen peroxide over the zeolite imidazolate framework ZIF-8.

The zeolite imidazolate framework ZIF-8 exhibits superior catalytic performance in the epoxidation of the electron-deficient C[double bond, length as m-dash]C bond in menadione using aqueous hydrogen peroxide as the oxidant. The catalysis has a truly heterogeneous nature and the framework structure remains intact. This is the first example of oxidation catalysis with ZIF-8.

Nucleophilic versus Electrophilic Activation of Hydrogen Peroxide over Zr-Based Metal-Organic Frameworks.

Zr-based metal-organic frameworks (Zr-MOF) UiO-66 and UiO-67 catalyze thioether oxidation in nonprotic solvents with unprecedentedly high selectivity toward corresponding sulfones (96-99% at ca. 50% sulfide conversion with only 1 equiv of H2O2). The reaction mechanism has been investigated using test substrates, kinetic, adsorption, isotopic (18O) labeling, and spectroscopic tools. The following facts point out a nucleophilic character of the peroxo species responsible for the superior formation of sulfones: (1) nucleophilic parameter XNu = 0.92 in the oxidation of thianthrene 5-oxide and its decrease upon addition of acid; (2) sulfone to sulfoxide ratio of 24 in the competitive oxidation of methyl phenyl sulfoxide and p-Br-methyl phenyl sulfide; (3) significantly lower initial rates of methyl phenyl sulfide oxidation relative to methyl phenyl sulfoxide (kS/kSO = 0.05); and (4) positive slope ρ = +0.42 of the Hammett plot for competitive oxidation of p-substituted aryl methyl sulfoxides. Nucleophilic activation of H2O2 on Zr-MOF is also manifested by their capability of catalyzing epoxidation of electron-deficient C═C bonds in α,ß-unsaturated ketones accompanied by oxidation of acetonitrile solvent. Kinetic modeling on methyl phenyl sulfoxide oxidation coupled with adsorption studies supports a mechanism that involves the interaction of H2O2 with Zr sites with the formation of a nucleophilic oxidizing species and release of water followed by oxygen atom transfer from the nucleophilic oxidant to sulfoxide that competes with water for Zr sites. The nucleophilic peroxo species coexists with an electrophilic one, ZrOOH, capable of oxygen atom transfer to nucleophilic sulfides. The predominance of nucleophilic activation of H2O2 over electrophilic one is, most likely, ensured by the presence of weak basic sites in Zr-MOFs identified by FTIR spectroscopy of adsorbed CDCl3 and quantified by adsorption of isobutyric acid.

Carbon Nanotubes Modified by Venturello Complex as Highly Efficient Catalysts for Alkene and Thioethers Oxidation With Hydrogen Peroxide.

In this work, we elaborated heterogeneous catalysts on the basis of the Venturello complex [PO4{WO(O2)2}4]3- (PW4) and nitrogen-free or nitrogen-doped carbon nanotubes (CNTs or N-CNTs) for epoxidation of alkenes and sulfoxidation of thioethers with aqueous hydrogen peroxide. Catalysts PW4/CNTs and PW4/N-CNTs (1.8 at. % N) containing 5-15 wt. % of PW4 and differing in acidity have been prepared and characterized by elemental analysis, N2 adsorption, IR spectroscopy, HR-TEM, and HAADF-STEM. Studies by STEM in HAADF mode revealed a quasi-molecular dispersion of PW4 on the surface of CNTs. The addition of acid during the immobilization is not obligatory to ensure site isolation and strong binding of PW4 on the surface of CNTs, but it allows one to increase the PW4 loading and affects both catalytic activity and product selectivity. Catalytic performance of the supported PW4 catalysts was evaluated in H2O2-based oxidation of two model substrates, cyclooctene and methyl phenyl sulfide, under mild conditions (25-50°C). The best results in terms of activity and selectivity were obtained using PW4 immobilized on N-free CNTs in acetonitrile or dimethyl carbonate as solvents. Catalysts PW4/CNTs can be applied for selective oxidation of a wide range of alkenes and thioethers provided a balance between activity and selectivity of the catalyst is tuned by a careful control of the amount of acid added during the immobilization of PW4. Selectivity, conversion, and turnover frequencies achieved in epoxidations over PW4/CNTs catalysts are close to those reported in the literature for homogeneous systems based on PW4. IR spectroscopy confirmed the retention of the Venturello structure after use in the catalytic reactions. The elaborated catalysts are stable to metal leaching, show a truly heterogeneous nature of the catalysis, can be easily recovered by filtration, regenerated by washing and evacuation, and then reused several times without loss of the catalytic performance.

Synthesis of coenzyme Q0 through divanadium-catalyzed oxidation of 3,4,5-trimethoxytoluene with hydrogen peroxide.

The selective oxidation of methoxy/methyl-substituted arenes to the corresponding benzoquinones has been first realized using aqueous hydrogen peroxide as a green oxidant, acid tetrabutylammonium salts of the γ-Keggin divanadium-substituted phosphotungstate [γ-PW10O38V2(µ-O)2]5- (I) as a catalyst, and MeCN as a solvent. The presence of the dioxovanadium core in the catalyst is crucial for the catalytic performance. The reaction requires an acid co-catalyst or, alternatively, a highly protonated form of I can be prepared and employed. The industrially relevant oxidation of 3,4,5-trimethoxytoluene gives 2,3-dimethoxy-5-methyl-1,4-benzoquinone (ubiquinone 0 or coenzyme Q0, the key intermediate for coenzyme Q10 and other essential biologically active compounds) with 73% selectivity at 76% arene conversion. The catalyst retains its structure under turnover conditions and can be easily recycled and reused without significant loss of activity and selectivity.

Mechanism of thioether oxidation over di- and tetrameric ti centres: kinetic and DFT studies based on model Ti-containing polyoxometalates.

The oxidation of thioethers by the green oxidant aqueous H2 O2 catalysed by the tetratitanium-substituted Polyoxometalate (POM) (Bu4 N)8 [{γ-SiTi2 W10 O36 (OH)2 }2 (µ-O)2 ], as a model catalyst comprising tetrameric titanium centres, was investigated by kinetic modelling and DFT calculations. Several mechanisms of sulfoxidation were evaluated by using methyl phenyl sulfide (PhSMe) as a model substrate in the experiments and dimethyl sulfide in the calculations. The first mechanism assumes that the active hydroperoxo species forms directly through interaction of the Ti2 (µ-OH)2 group in [{γ-SiTi2 W10 O36 (OH)2 }2 (µ-O)2 ](8-) (1 D) with H2 O2 . The second mechanism includes hydrolysis of Ti-O-Ti bonds linking two γ-Keggin units in structure 1 D to produce the monomer [(γ-SiW10 Ti2 O38 H2 )(OH)2 ](4-) (1 M), followed by the formation of an active hydroperoxo species upon interaction of the Ti hydroxo group with H2 O2 . Both kinetic modelling and DFT calculations support the mechanism through the monomeric species that involves the hydrolysis step. According to the DFT studies the activation of H2 O2 by compound 1 M is preferred by 6.5 kcal mol(-1) with respect to anion 1 D due to the more flexible Ti environment of the terminal Ti hydroxo group in 1 M. The calculations also indicate that for the "monomeric" mechanism two pathways are operative: the mono- and the multinuclear pathway. In the mononuclear mechanism, the active group is the terminal TiOH group, whereas in the multinuclear path the active group is the bridging Ti2 (µ-OH) moiety. Moreover, unlike previous studies, the sulfoxidation is preferred through a ß-oxygen atom transfer from the Ti hydroperoxo group because the α-oxygen atom transfer leads to an unfavourable seven-fold coordinated Ti environment in the transition state. Finally, we have generalised these results to other Ti-containing POMs: the Ti-monosubstituted α-Keggin ion [α-PTi(OH)W11 O39 ](4-) and the dititanium-substituted sandwich-type ion [Ti2 (OH)2 As2 W19 O67 ](8-) .

Efficient epoxidation of olefins by H2O2 catalyzed by iron "helmet" phthalocyanines.

High yields of epoxides were obtained in the oxidation of a large range of olefins using 1.2-2 equiv. of H2O2 in the presence of iron helmet phthalocyanines. The involvement of high-valent iron oxo species was evidenced using cryospray mass spectrometry.

Mechanistic insights into oxidation of 2-methyl-1-naphthol with dioxygen: autoxidation or a spin-forbidden reaction?

Oxidation of 2-methyl-1-naphthol (MNL) with molecular oxygen proceeds efficiently under mild reaction conditions (3 atm O(2), 60-80 °C) in the absence of any catalyst or sensitizer and produces 2-methyl-1,4-naphthoquinone (MNQ, menadione, or vitamin K(3)) with selectivity up to 80% in nonpolar solvents. (1)H NMR and (1)H,(1)H-COSY studies revealed the formation of 2-methyl-4-hydroperoxynaphthalene-1(4H)-one (HP) during the reaction course. Several mechanistic hypotheses, including conventional radical autoxidation, electron transfer mechanisms, photooxygenation, and thermal intersystem crossing (ISC), have been evaluated using spectroscopic, mass-spectrometric, spin-trapping, (18)O(2) labeling, kinetic, and computational techniques. Several facts collectively implicate that ISC contributes significantly into MNL oxidation with O(2) at elevated pressure: (i) the reaction rate is unaffected by light; (ii) C-C-coupling dimers are practically absent; (iii) the reaction is first order in both MNL and O(2); (iv) the observed activation parameters (ΔH(‡) = 8.1 kcal mol(-1) and ΔS(‡) = -50 eu) are similar to those found for the spin-forbidden oxidation of helianthrene with (3)O(2) (Seip, M.; Brauer, H.-D. J. Am. Chem. Soc.1992, 114, 4486); and (v) the external heavy atom effect (2-fold increase of the reaction rate in iodobenzene) points to spin inversion in the rate-limiting step.

Iron tetrasulfophthalocyanine immobilized on metal organic framework MIL-101: synthesis, characterization and catalytic properties.

Iron tetrasulfophthalocyanine (FePcS) has been irreversibly inserted into nanocages of the metal organic framework MIL-101 to give a hybrid material FePcS/MIL-101 which demonstrated a superior catalytic performance in the selective oxidation of aromatic substrates with (t)BuOOH than homogeneous FePcS.