On the location of different titanium sites in Ti–OMS-2 and their catalytic role in oxidation of styrene

Octahedral manganese oxide molecular sieves (OMS-2) modified by impregnation of TiO₂ exhibit a higher catalytic activity for oxidation of styrene with tert-butylhydroperoxide in comparison to titanium-incorporated OMS-2, where the styrene conversions were ca. 70% and 45–50%, respectively. The framework of titanium species has no effect on the enhancement of catalytic activity, while the non-framework of titanium species induces a synergetic effect that enhances the oxidation of styrene with tert-butylhydroperoxide.     

Catalytic Characterization of Mesoporous Ti–Silica Hollow Spheres

Titanium–silica hollow spheres (TSHS) with a mesoporous shell were synthesized using an inverse multiple oil–water–oil (O/W/O) emulsion. These novel materials show interesting potential for catalysis. Excellent catalytic performance was found in the epoxidation of cyclohexene with tert-butylhydroperoxide. The investigation of the titanium environment by UV–vis, Raman and FTIR spectroscopy showed that at low Ti loading only isolated species are present and catalytic measurements demonstrated that these species are the active sites for this reaction. At higher loading Ti–O–Ti microdomains are present and they do not exhibit any significant catalytic activity.     

Use of a New Polymer Anchored Cu(II) Azo Complex Catalyst for the Efficient Liquid Phase Oxidation Reactions

A new polymer anchored copper(II) azo complex has been synthesized and characterized by using scanning electron microscope (SEM), thermogravimetric analysis (TGA), elemental analysis, atomic absorption spectroscopy (AAS) and spectrometric methods like diffuse reflectance spectra of solid (DRS) and Fourier transform infrared spectroscopy (FTIR). The immobilized Cu(II) catalyst shows excellent catalytic activity in oxidation of cyclohexene, styrene, benzyl alcohol and ethylbenzene in presence of tert-butylhydroperoxide (TBHP) as an oxidant. The effects of different solvents, oxidants, temperature, substrate oxidant ratio and amount of catalyst were also studied. The catalytic results reveal that the polymer-anchored Cu(II) azo complex catalyst can be recycled more than five times without appreciable loss in the catalytic activity.     

Efficient Allylic Oxidation of Olefins Catalyzed by Polymer Supported Metal Schiff Base Complexes with Peroxides

Three homogeneous Cu(II), Co(II) and Ni(II) complexes of a Schiff base ligand and their heterogeneous complexes supported on poly(4-aminostyrene) were prepared and characterized by using elemental analysis, fourier transform infrared spectroscopy, UV–Vis diffuse reflectance spectroscopy, thermogravimetric analysis and scanning electron microscopy. The catalytic performance of both homogeneous and heterogeneous complexes was evaluated in the liquid phase oxidation of cyclohexene, styrene and trans-stilbene in acetonitrile with tert-butylhydroperoxide or hydrogen peroxide as the oxidant. All types of catalyst were active in oxidation; and, the complexes produce allylic oxidation products in all cases. Immobilized complexes are slightly more active than their homogeneous complexes. The polymer-supported Cu(II) complex shows a higher catalytic activity than the other metal species. The activities of the immobilized catalysts remained nearly the same after five cycles, suggesting the true heterogeneous nature of the catalyst.     

Oxidation of 2,6-di-tert-butylphenols to Diphenoquinones Catalysed by Schiff Base-Cu(II) Systems Immobilized on Polymer Support

Benzoquinone, diphenoquinones and its derivatives are important intermediates for industrial synthesis of a wide variety of special chemicals, such as pharmaceuticals, dyes and agricultural chemicals. The useful catalyst were obtained by aminolysis of vinylbenzyl chloride/divinylbenzene copolymer with ethylenediamine (1) or urotropine (2) and then modification by salicylaldehyde (1A, 2A) or picolinaldehyde (1B, 2B). The catalytic activity of Cu(II) complexes with Schiff base immobilized on the synthesized supports were tested in the oxidation reaction of 2,6-di-tert-butylphenol (DTBP) to diphenoquinone (PQ) with tert-butylhydroperoxide. The best oxidation degree of DTBP (60-70%) and the selectivity towards PQ (80%) is revealed by Cu(II) complexes with long Schiff base ligands derived from salicylaldimine (1A), which have CuL structure (EPR measurement).     

Oxovanadium(IV) Schiff base complexes derived from 2,2′-dimethylpropandiamine: A homogeneous catalyst for cyclooctene and styrene oxidation

Tetradentate Schiff base ligands, derived from aromatic aldehydes and aliphatic diamine (2,2′-dimethylpropandiamine), and their vanadyl complexes have been prepared and characterized. Catalytic potential of these complexes was tested for the oxidation of cyclooctene and styrene using tert-butylhydroperoxide (TBHP) as oxidant. The effects of molar ratio of oxidant to substrate, temperature and solvent have been studied. Excellent selectivity of epoxidation for cyclooctene and good selectivity for styrene were obtained. The mechanism of oxidation has also been discussed.     

Preparation of silica-immobilized titanium-containing silsesquioxane catalysts and activity for the epoxidation of alkenes

Silsesquioxanes containing tetrapodal titanium species and alkenylsilyl groups were immobilized onto dimethylsilyl-functionalized silica having mesopores by chemical tethering via hydrosilylative reaction in the presence of a Pt catalyst. The immobilized catalysts showed higher activity than their corresponding homogeneous catalyst towards the epoxidation of cyclooctene by tert-butylhydroperoxide, probably because of the improvement of the micro-environment around the titanium center. The reaction was found to be truly heterogeneous, since no further reaction proceeded after the separation of the catalyst by filtration. These catalysts also showed excellent catalytic activity for the epoxidation using hydrogen peroxide as an oxidant, while the parent homogeneous catalysts did not show any activity at all.     

Synthesis and characterization of elastoplastic poly(styrene‐co‐ethylene) using novel mono(η5‐cyclopentadienyl) titanium and modified methylaluminoxane catalysts

Elastoplastic poly(styrene‐co‐ethylene) with high molecular weight was synthesized using novel mono(η⁵‐pentamethylcyclopentadienyl)tribenzyloxy titanium [Cp*Ti(OBz)₃] complex activated with four types of modified methylaluminoxanes (mMAO) containing different amounts of residual trimethylaluminum (TMA). The ideal mMAO, used as a cocatalyst for the copolymerization of styrene with ethylene, contains TMA approaching to 17.8 wt %. The oxidation states of the titanium‐active species in different Cp*Ti(OBz)₃/mMAO catalytic systems were determined by the redox titration method. The results show that both active species may exist in the current system, where one [Ti(IV)] gives a copolymer of styrene and ethylene, and the second one [Ti(III)] only produces syndiotactic polystyrene (sPS). Catalytic activity, compositions of copolymerization products, styrene incorporation, and copolymer microstructure depend on copolymerization conditions, including polymerization temperature, Al/Ti, molar ratio, and comonomers feed ratio. The copolymerization products were fractionated by successive solvent extractions with boiling butanone and tetrahydrofuran (THF). The copolymer, chiefly existing in THF‐soluble fractions, was confirmed by ¹³C‐NMR, GPC, DSC, and WAXD to be an elastoplastic copolymer with a single glass transition temperature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1851–1857, 1999     

Styrene Epoxidation over Carbon Nanotube-Supported Gold Catalysts

Carbon nanotube supported gold catalysts prepared by deposition–precipitation with urea (DP urea) were characterized by various techniques. Its catalytic activity was examined for the oxidation of styrene using t-butylhydroperoxide as oxidant. This system showed good epoxide selectivity. The other factors, such as solvent, reaction time, concentrations of oxidant and catalyst, have also been investigated and reaction conditions are optimized. It is a novel highly active/selective and reusable heterogeneous catalyst for styrene epoxidation.     

OMS-2 Catalysts for Formaldehyde Oxidation: Effects of Ce and Pt on Structure and Performance of the Catalysts

OMS-2 (2 × 2 type octahedral molecular sieve) and cerium doped OMS-2 were synthesized by refluxing method. The platinum doped on both samples was prepared by impregnation method for catalytic oxidation of formaldehyde. These catalysts were characterized by BET, XRD, SEM, H₂-TPR and Laser Raman. Results show that OMS-2 formed needle-shape nanocrystal structure. The addition of Ce before the formation of the OMS-2 structure obstructs the process due to mismatch of the cation size. Rather than into the framework of the structure, the addition of Pt after the formation of the OMS-2 structure was loaded on the surface of catalysts, which strongly influenced the bonding of the Mn–O lattice of OMS-2. The properties of the catalyst structure determine the temperature of complete oxidation of HCHO while Pt promotes this reaction at a lower temperature range due to active surface oxygen species and lattice defects.     

Electron-rich salen-type Schiff base complexes of Cu(II) as catalysts for oxidation of cyclooctene and styrene with tert-butylhydroperoxide: A comparison with electron-deficient ones

Two square planar copper(II) complexes of tetradentate Schiff base ligands derived from aromatic aldehydes and 2,2′-dimethylpropandiamine (H₂{salnptn(3-OMe)₂}, H₂{hnaphnptn}) have been prepared and used as catalysts for oxidation of cyclooctene and styrene with tert-butylhydroperoxide (TBHP). Oxidation of cyclooctene with TBHP gave cyclooctene oxide as the sole product, but in the case of styrene a mixture of styrene oxide and benzaldehyde has been obtained in ca. 1:3 molar ratio. It has been shown that the rate and selectivity of reaction depend to the electron-donor ability of substituents at the phenyl groups of the ligand and can be improved by introduction of π-electron-donating groups at the aromatic rings of salen-type Schiff bases. The structure of Cu{salnptn(3-OMe)₂} has been determined by X-ray crystallography at 291 K with results generally in agreement with those previously reported. The results suggest that the symmetrical Schiff base ligands are bivalent anions with tetradentate N₂O₂ donors derived from the phenolic oxygen and azomethine nitrogen atoms.     

Complexes of poly(2-<Emphasis Type="Italic">N,N</Emphasis>-dimethylaminoethyl) methacrylate with heavy metals II. Oxidation of cyclohexene with <Emphasis Type="Italic">tert</Emphasis>-butylhydroperoxide

Metal complexes of poly(2-N,N-dimethylaminoethyl) methacrylate were prepared by complex-forming with aqueous solutions of salts of FeSO₄.2H₂O; CoCl₂.6H₂O; CuCl₂.2H₂O; VOSO₄.5H₂O; Na₂MoO₄.2H₂O and Na₂WO₄.2H₂O. The catalytic activity of the complexes was studied in the oxidation of cyclohexene as a model reaction. The activities of the complexes synthesized towards the reaction of cyclohexene epoxidation can be arranged by the following order: PDMAEM-MoO₂ ²⁺> PDMAEM-VO²⁺ > PDMAEM-WO₂ ²⁺> PDMAEM-Co²⁺ > PDMAEM-Fe³⁺> PDMAEM-Cu²⁺. The complexes catalyzing the homolytic decomposition of tert-butylhydroperoxide increased the maximum yield of 2-cyclohexene-1-ol and 2-cyclohexene-1-on. The yield of cyclohexene oxide and 2-cyclohexene-1-ol were 58% and 13%, respectively.     

Host (nanocavity of zeolite-Y)/guest (Mn(II), Co(II), Ni(II) and Cu(II) complexes of pentadendate Schiff-base ligand) nanocomposite materials (HGNM): application in the heterogeneous oxidation of cyclohexene

Transition metal (M = Mn(II), Co(II), Ni(II) and Cu(II)) complexes with pentadendate Schiff-base ligand; N,N′-bis(salicylidene)-2,6-pyridinediaminato, H₂ [sal-2,6-py]; was entrapped in the nanocavity of zeolite-Y by a two-step process in the liquid phase: (i) adsorption of bis(salicylaldiminato)metal(II); [M(sal)₂]-NaY; in the supercages of the zeolite, and (ii) in situ Schiff condensation of the metal(II) precursor complex with the corresponding 2,6-pyridinediamine; [M(sal-2,6-py)]-NaY. The new materials were characterised by several techniques: chemical analysis, spectroscopic methods (DRS, BET, FTIR and UV/Vis), conductometric and magnetic measurements. Analysis of the data indicates that the M(II) complexes are encapsulated in the nanodimensional pores of zeolite-Y and exhibit different from those of the free complexes, which can arise from distortions caused by steric effects due to the presence of sodium cations, or from interactions with the zeolite matrix. The Host–Guest Nanocomposite Materials (HGNM); [M(sal-2,6-py)]-NaY; catalyzes the oxidation of cyclohexene with tert-butylhydroperoxide (TBHP). Oxidation of cyclohexene with HGNM gave 2-cyclohexene-1-one, 2-cyclohexene-1-ol and 1-(tert-butylperoxy)-2-cyclohexene. [Mn(sal-2,6-py)]-NaY shows significantly higher catalytic activity than other catalysts.     

Sulfated TiO2 Decontaminate 2-CEES and DMMP in Vapor Phase

A hybrid complex Zr(PO₄)(AgHPO₄) was prepared by ion-exchange and its activity was examined on the allylic oxidation of cycloolefins using tert-butylhydroperoxide (TBHP) as oxidant under argon atmosphere. Under these reaction conditions, attack of the activated C–H bond was preferred instead of the epoxidation of C=C bond, yielding enols as the main products, this is a novel highly active/selective and reusable heterogeneous catalyst.     

Significant enhancement of catalytic activities of manganese oxide octahedral molecular sieve by marginal amount of doping vanadium

Vanadium doped hollandite-type manganese oxide octahedral molecular sieve (OMS-2) was synthesized by a simple hydrothermal route. The results of catalytic tests showed that marginal amount of doping vanadium had greatly enhanced catalytic activities of OMS-2 not only in low-temperature complete oxidation of formaldehyde at 140 °C but also in high-temperature complete oxidation of methane at 450 °C. The results of XPS, HRTEM and H₂-TPR measurements revealed that the enhancements of the catalytic activities primarily resulted from the increase of surface defect sites regardless of reaction temperatures and nature of reactants, rather than reducibility and average oxidation state of manganese.     

Metalloporphyrin/Iodine(III)‐Cocatalyzed Oxygenation of Aromatic Hydrocarbons

Hypervalent iodine species have a pronounced catalytic effect on the metalloporphyrin‐mediated oxygenations of aromatic hydrocarbons. In particular, the oxidation of anthracene to anthraquinone with Oxone readily occurs at room temperature in aqueous acetonitrile in the presence of 5–20 mol% of iodobenzene and 5 mol% of a water‐soluble iron(III)‐porphyrin complex. 2‐tert‐Butylanthracene and phenanthrene also can be oxygenated under similar conditions in the presence of 50 mol% of iodobenzene. The oxidation of styrene in the presence of 20 mol% of iodobenzene leads to a mixture of products of epoxidation and cleavage of the double bond. Partially hydrogenated aromatic hydrocarbons (e.g., 9,10‐dihydroanthracene, 1,2,3,4‐tetrahydronaphthalene, and 2,3‐dihydro‐1H‐indene) afford under these conditions products of oxidation at the benzylic position in moderate yields. The proposed mechanism for these catalytic oxidations includes two catalytic redox cycles: 1) initial oxidation of iodobenzene with Oxone producing the hydroxy(phenyl)iodonium ion and hydrated iodosylbenzene, and 2) the oxidation of iron(III)‐porphyrin to the oxoiron(IV)‐porphyrin cation‐radical complex by the intermediate iodine(III) species. The oxoiron(IV)‐porphyrin cation‐radical complex acts as the actual oxygenating agent toward aromatic hydrocarbons.     

Oxidation of alcohols with tert-butylhydroperoxide catalyzed by Mn (II) complexes immobilized in the pore channels of mesoporous hexagonal molecular sieves (HMS)

The oxidation of alcohols with tert-butylhydroperoxide (TBHP), in the presence of Mn²⁺ complexes immobilized in the pore channels of mesoporous hexagonal molecular sieves (HMS), were investigated. It was found that immobilized [Mn(bpy)₂]²⁺/HMS is an efficient catalyst for the oxidation of alcohols such as benzyl alcohol, n-hexanol and cyclohexanol. The effects of reaction time, amount of Mn²⁺ in the catalyst, type of substrates and oxidants in this catalysis system were investigated. At optimum conditions, TBHP is more efficient oxidant with respect to H₂O₂. Following order has been observed for the percentage of conversions of alcohols: benzylic >1° >2°.     

Activity and Mechanism of Action of Thio and Dithiobisphenols in the Stabilization of Polyolefins

Abstract

Diphenol substituted sulfides have been studied as stabilizers poypropylene against thermal oxidation. Both phenol groups of antioxidants are reacting with oxidation reaction centres, the sulfide group reacting with hydroperoxides.

The reaction of phenol-substituted disulfides with tert-butylhydroperoxide was also studied.

The role of unstable sulfur-containing intermediates is discussed.     

Catalytic oxidation of styrene by manganese(II) bipyridine complex cations immobilized in mesoporous Al-MCM-41

Manganese 2,2'-bipyridine (bpy) complex cations, [Mn(bpy)₂]²⁺, have been immobilized in mesoporous Al-MCM-41 (Si/Al=9) and used as a catalyst for the oxidation of styrene by iodosylbenzene, H₂O₂ and tert-butyl hydroperoxide (TBHP). The oxidation products included epoxide, diol and aldehyde. Al-MCM-41-immobilized [Mn(bpy)₂]²⁺ exhibited a higher catalytic activity for styrene oxidation than the corresponding homogeneous catalyst and showed no significant loss of catalytic activity when recycled. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Effect of Coating TiO<Subscript>2</Subscript> on the CO Oxidation of the Pt/γ-Alumina Catalysts

Abstract  

The effect of coating TiO₂ on the CO oxidation of the Pt/γ-alumina catalysts was observed through activity tests and surface characterization spectroscopy by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) experiments. XPS results evidenced the occurrence of different Pt²⁺ species and metallic Pt⁰ at the surface which suggest electron transfer of titanium (cation) to the platinum atom and the reduction of titanium (Ti⁴⁺ → Ti³⁺). FTIR analyses suggested oxygen spillover mechanism at the interface between titanium dioxide and platinum that may explain the catalytic activity of the platinum titania-supported catalysts. The apparent activation energy for the CO oxidation was 52.5 kJ/mol and similar for all catalysts. However, the frequency factor changed significantly, indicating interfacial phenomena caused by CO and oxygen adsorptions over TiO _x species and Al₂O₃ support with similar dispersions.