Oxana A. Kholdeeva

Hybrid Polyoxotungstate/MIL-101 Materials: Synthesis, Characterization, and Catalysis of H2O2-Based Alkene Epoxidation

Polyoxotungstates [PW(4)O(24)](3-) (PW(4)) and [PW(12)O(40)](3-) (PW(12)) have been inserted into nanocages of the metal organic framework MIL-101. The hybrid materials PW(x)/MIL-101 (x = 4 or 12) containing 5-14 wt % of polyoxotungstate have been obtained and characterized by elemental analysis, N(2) adsorption, FT-IR, Raman, and (31)P NMR MAS spectroscopic techniques. Their catalytic performance was assessed in the selective oxidation of alkenes with aqueous hydrogen peroxide under mild reaction conditions ([H(2)O(2)] = 0.1-0.2 M, 50 degrees C, MeCN). PW(x)/MIL-101 enclosing 5 wt % of polyoxotungstate demonstrated fairly good catalytic activities in the epoxidation of various alkenes (3-carene, limonene, alpha-pinene, cyclohexene, cyclooctene, 1-octene), the turnover frequencies (TOF) and alkene conversions were close to the corresponding parameters achieved with homogeneous PW(x). For the oxidation of substrates with aromatic groups (styrene, cis- and trans-stilbenes), a higher level of olefin conversion was attained using PW(12)/MIL-101. Moreover, confinement of PW(12) within MIL-101 nanocages allowed us to reach higher epoxide selectivities at higher alkene conversions. The hybrid PW(x)/MIL-101 materials were stable to leaching, behaved as true heterogeneous catalysts, were easily recovered by filtration, and reused several times with the maintenance of the catalytic performance.

Cyclohexane selective oxidation over metal–organic frameworks of MIL-101 family: superior catalytic activity and selectivity

Mesoporous metal-organic frameworks Cr- and Fe-MIL-101 are highly efficient, true heterogeneous and recyclable catalysts for solvent-free selective oxidation of cyclohexane with molecular oxygen and/or tert-butyl hydroperoxide under mild conditions.

Mechanistic Insights into Alkene Epoxidation with H2O2 by Ti- and other TM-Containing Polyoxometalates: Role of the Metal Nature and Coordination Environment

The oxidation of alkenes by H(2)O(2) catalyzed by Ti(IV)-containing polyoxometalates (POMs) as models of Ti single-site catalysts has been investigated at DFT level and has been compared with other early transition-metal-substituted polyoxometalates. We have studied in detail the reaction mechanism of the C(2)H(4) epoxidation with H(2)O(2) mediated by two different POMs, the Ti-monosubstituted Keggin-type POM [PTi(OH)W(11)O(39)](4-) and the Ti-disubstituted sandwich-type POM [Ti(2)(OH)(2)As(2)W(19)O(67)(H(2)O)](8-). These species exhibit well-defined 6- and 5-coordinated titanium environments. For both species, the reaction proceeds through a two-step mechanism: (i) the Ti-OH groups activate H(2)O(2) with a moderate energy barrier yielding either Ti-hydroperoxo (Ti(IV)-OOH) or Ti-peroxo (Ti(IV)-OO) intermediate, and (ii) the less stable but more reactive Ti-hydroperoxo species transfers oxygen to alkene to form the epoxide, this latter step being the rate-determining step. The higher activity of the sandwich anion was attributed to the absence of dimer formation, and its higher selectivity to the larger energy cost of homolytic O-O bond breaking in the hydroperoxo intermediate. We also propose several requisites to improve the efficiency of Ti-containing catalysts, including flexible and 5-fold (or lower) coordinated Ti environments, as well as reagent-accessible Ti sites. Calculations on other TM-containing Keggin-type POMs [PTM(OH)W(11)O(39)](4-) (TM = Zr(IV), V(V), Nb(V), Mo(VI), W(VI), and Re(VII)) showed that when we move from the left to the right in the periodic table the formation of the epoxide via peroxo intermediate becomes competitive because of the higher mixing between the orbitals of the TM and the O-O unit.

Titanium-monosubstituted polyoxometalates: relation between homogeneous and heterogeneous Ti-single-site-based catalysis

The similarity in the catalytic behaviour of titanium-monosubstituted Keggin type polyoxometalates [PTi(L)W11O39]n- (Ti-POMs) and heterogeneous titanium single-site catalysts in selective oxidations with H2O2 is demonstrated. Recent achievements in the application of Ti-POMs as soluble molecular models for studying mechanisms of oxidation catalysis are reviewed.

A new environmentally friendly method for the production of 2,3,5-trimethyl-p-benzoquinone

A new environmentally friendly method for the production of 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, Vitamin E precursor) based on the oxidation of 2,3,6-trimethylphenol (TMP) with aqueous H 2 O 2 over various Ti-containing mesoporous silicate materials is reported. Both well-organized Ti-containing mesoporous mesophase silicate (Ti-MMM), having hexagonal arrangement of uniform mesopores, and amorphous TiO 2 -SiO 2 mixed oxides (aerogels and xerogels) produced TMBQ with good to high yield. All the materials studied have been proved to operate as truly heterogeneous catalysts. No titanium leaching occurred from the solid matrixes during the oxidation process. Titanium dispersion and its accessibility were found to be crucial factors determining the catalytic properties. For samples with similar titanium loading, both the catalytic activity and TMBQ yield appeared to fall in the order TiO 2 -SiO 2 aerogel > Ti-MMM > TiO 2 -SiO 2 xerogel and correlate with average mesopore diameter and mesopore volume. The best results (96-98% selectivity to TMBQ at 99-100% TMP conversion) were obtained with TiO 2 -SiO 2 aerogels, containing 1.7-6.5 wt.% Ti.

Metal–organic frameworks of the MIL-101 family as heterogeneous single-site catalysts

In this short review paper, we survey our recent findings in the catalytic applications of mesoporous metal–organic frameworks of the MIL-101 family (Fe- and Cr-MIL-101) and demonstrate their potential in two types of liquid-phase processes: (i) selective oxidation of hydrocarbons with green oxidants—O2 and tert-butyl hydroperoxide—and (ii) coupling reaction of organic oxides with CO2. A comparison with conventional single-site catalysts is made with special attention to issues of the catalysts resistance to metal leaching and the nature of catalysis.

Recent developments in liquid-phase selective oxidation using environmentally benign oxidants and mesoporous metal silicates

This Perspective article surveys recent advances in the synthesis of mesoporous transition-metal-containing silicate materials and their use for the liquid-phase selective oxidation of organic compounds (alkanes, alkenes, aromatics, S-compounds, etc.) with environmentally friendly oxidants – molecular oxygen, hydrogen peroxide and organic hydroperoxides. A selection of the most relevant results reported thus far in the literature is provided, with particular attention paid to the issues of the nature of catalysis, catalyst stability and reusability. Approaches elaborated in recent years to create hydrothermally stable mesoporous metal silicates are considered, and the scope and limitations of such catalysts in liquid-phase oxidations are discussed.

Hexazirconium‐ and Hexahafnium‐Containing Tungstoarsenates(III) and Their Oxidation Catalysis Properties

Exploring the interaction of lacunary polyoxometalates (POMs) with Group 4 (Ti, Zr, and Hf) transition-metal ions has been mainly driven by the fact that potential products may serve as oxidation catalysts or even as soluble molecular analogues of known Tiand Zr-containing heterogeneous catalysts. Bearing in mind that zirconium(IV) and hafniACHTUNGTRENNUNGum(IV) can have larger coordination numbers than titaniACHTUNGTRENNUNGum(IV), the chemistry of the former pair compared to the lighter congener is expected to be different, leading to different structural assemblies. There are significantly fewer reports on Zr/Hf-POMs than Ti-POMs in the literature, and the former can be classified according to the composing lacunary POM fragments as follows: Keggin type ([PW11O39] 7 , [SiW11O39] 8 , [GeW11O39] 8 , [b-SiW10O37] 10 , [g-SiW10O36] 8 , [PW9O34] 9 , and [SiW9O34] 10 ), Wells– Dawson type ([a2-P2W17O61] 10 , [a-P2W16O59] 12 , and [aP2W15O56] 12 ), and Lindqvist type ([W5O18] 6 ). Our group has reported the first two examples of peroxo-Zr/Hf polyACHTUNGTRENNUNGanions.[2h,k] In addition, the unique, asymmetric structure [Zr2ACHTUNGTRENNUNG(m-OH)ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(AsOH)2ACHTUNGTRENNUNG(AsW7O28) ACHTUNGTRENNUNG(AsW10O36)]9 has been obtained from the reaction of [NaAs4W40O140] 27 with Zr ions, and does not belong to any of the above-mentioned classes. Whereas the reactivity of the classical trilacunary Keggin ions [PW9O34] 9 and [SiW9O34] 10 with Ti, Zr, and Hf ions is fairly well understood (vide supra), this is not the case for the lone pair containing Keggin family [XW9O33] 9 (X=As, Sb, Bi), which displays different chemical reactivity and consequently results in structurally distinct products. For all the above-mentioned reasons we decided to study in detail the reactivity of [AsW9O33] 9 with zirconium(IV) and hafnium(IV). Here we report on the two Zr6and Hf6-containing tungstoarsenates ACHTUNGTRENNUNG(III) [M6O4(OH)4 ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(CH3COO)5ACHTUNGTRENNUNG(AsW9O33)2]11 (M=Zr, 1; Hf, 2, see Figure 1), which have been isolated as the hydrated mixed cesium–sodium salts Cs6Na5ACHTUNGTRENNUNG[Zr6O4(OH)4 ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(CH3COO)5ACHTUNGTRENNUNG(AsW9O33)2]·80 H2O (CsNa-1) and Cs6Na5ACHTUNGTRENNUNG[Hf6O4(OH)4ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(CH3COO)5ACHTUNGTRENNUNG(AsW9O33)2]·80H2O (CsNa-2). These compounds were characterized in the solid state by single-crystal X-ray dif-

Alkene and thioether oxidations with H2O2 over Ti- and V-containing mesoporous mesophase catalysts

Abstract Well-organized Ti- and V-containing silicate mesoporous mesophase materials (MMM) were synthesized using C 16 H 33 N(CH 3 ) 3 Br and characterized by elemental analysis, IR, DRS-UV, XRD, and N 2 adsorption. Catalytic properties of these materials in alkene and thioether oxidations with aqueous H 2 O 2 were examined. The structure–activity study was performed for the C16-(Ti,Si)-MMM catalysts with the Si/Ti atomic ratio ranged from 19 to 166. The samples with the Si/Ti atomic ratio in the range of 49–124 showed the highest catalytic activities. Both structure perfection of the silicate matrix and degree of isolation of titanium ions in it appeared to be crucial factors determining catalytic activity of C16-(Ti,Si)-MMM in alkene oxidation. For efficient oxidation of more nucleophilic substrates (thioethers) the structural perfection of the C16-(Ti,Si)-MMM catalysts was less important. No titanium leaching occurred even in 1.1 M H 2 O 2 solution at 353 K, whereas vanadium leaching was already pronounced at [H 2 O 2 ]=0.05 M and room temperature. The oxidation processes proved to be true heterogeneous for C16-(Ti,Si)-MMM and mostly homogeneous for C16-(V,Si)-MMM.

Magnetically separable titanium-silicate mesoporous materials with core–shell morphology: synthesis, characterization and catalytic properties

The preparation and characterization (XRD, N2 adsorption, TEM, EDX, DRS-UV) of a novel catalytic material Ti-SMCMS (solid magnetic core–mesoporous shell) is reported. The material has quasi spherical particles with an ordered mesoporous silicate shell containing isolated Ti atoms and a silica core which, in turn, comprises superparamagnetic iron oxide nanoparticles. The magnetic kernels are embedded in the nonporous silica core and, thus, are completely protected from ambient medium. The material was found to combine the advantages of high activity and selectivity in H2O2-based selective oxidations with the merits of an easy magnetic separation from the reaction mixture.