Selective oxidation of indole by chloroperoxidase immobilized on the mesoporous molecular sieve SBA-15

The selective oxidation of hydrocarbons is an important value-enhancing chemical transformation in particular with respect to fine chemicals and pharmaceuticals production. Enzymatic oxidations operate under mild reaction conditions and produce little if any waste. However, its industrial use is still limited mainly due to their high cost and the low space time yields. In the present work, chloroperoxiase from Calariomyces fumago immobilized on the mesoporous molecular sieve SBA-15 was applied for the oxidation of indole to 2-oxoindole using hydrogen peroxide or tert.-butyl hydroperoxide as oxidants. The performance of the immobilized enzyme was found to be superior to native chloroperoxidase with respect to maximum conversion and pH range applicable. However, immobilized CPO is still sensitive to high concentrations of hydrogen peroxide. The use of tert.-buty hydroperoxide is found to avoid this problem, but the reaction rate is significantly reduced.     

Asymmetric Epoxidation of α,β-Unsaturated Ketones over Heterogenized Chiral Proline Diamide Complex Catalyst in the Solvent-Free Condition

Chiral copper proline diamide complex has been immobilized on surface of mesoporous silica. These heterogenized complex catalysts were examined as asymmetric catalysts for the epoxidation of α,β-unsaturated carbonyl compounds with hydrogen peroxide, tert-butyl hydroperoxide and urea hydroperoxide as the oxidants under solvent-free condition. Enantiomeric excesses of up to 84% can be achieved conveniently with a good conversion using these promising catalysts.     

Optimizing reaction parameters for the enzymatic synthesis of epoxidized oleic acid with oat seed peroxygenase

Peroxygenase is a plant enzyme that catalyzes the oxidation of a double bond to an epoxide in a stereospecific and enantiofacially selective manner. A microsomal fraction containing peroxygenase was prepared from oat (Avena sativa) seeds and the enzyme immobilized onto a hydrophobic membrane. The enzymatic activity of the immobilized preparation was assayed in 1 h by measuring epoxidation of sodium oleate (5 mg) in buffer-surfactant mixtures. The pH optimum of the reaction was 7.5 when t-butyl hydroperoxide was the oxidant and 5.5 when hydrogen peroxide was the oxidant. With t-butyl hydroperoxide as oxidant the immobilized enzyme showed increasing activity to 65°C. The temperature profile with hydrogen peroxide was flatter, although activity was also retained to 65°C. In 1 h reactions at 25°C at their respective optimal pH values, t-butyl hydroperoxide and hydrogen peroxide promoted epoxide formation at the same rate. Larger-scale reactions were conducted using a 20-fold increase in sodium oleate (to 100 mg). Reaction time was lengthened to 24 h. At optimized levels of t-butyl hydroperoxide 80% conversion to epoxide was achieved. With hydrogen peroxide only a 33% yield of epoxide was obtained, which indicates that hydrogen peroxide may deactivate peroxygenase.     

An Efficient Approach for Immobilizing the Oxo‐Vanadium Schiff Base onto Polymer Supports using Staudinger Ligation

The present paper describes the use of Staudinger ligation as an efficient and high‐yielding approach for the immobilization of oxo‐vanadium Schiff base on polymeric supports via covalent attachment under mild conditions. The described strategy is simple in use, versatile, highly efficient with respect to better catalyst loading and proceeds under mild, metal‐free conditions. The catalytic potential of the prepared polymer immobilized oxo‐vanadium Schiff bases was tested for the oxidation of sulfides using aqueous tert‐butyl hydroperoxide (TBHP) as oxidant. The polymer‐supported catalysts could easily be recovered from the reaction mixture and reused for four runs without loss in activity and no metal leaching was observed during this course.     

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.     

Chemical degradation of aldicarb in water using ozone

The use of ozone to degrade aldicarb in water was investigated under different conditions. The oxidation develops through the direct attack of ozone since the presence of hydroxyl radical inhibitors, such as tert-butanol, does not affect the degradation rate of aldicarb. The combination of ozone with hydrogen peroxide does not improve the oxidation rate which also confirms the absence of radical reactions to eliminate aldicarb. However, TOC removal increases 51% in the presence of hydrogen peroxide after 65 min of oxidation. The oxidation rate is strongly affected by the type of device for feeding ozone, which indicates that a fast gas-liquid reaction is taking place. Therefore, mass transfer and chemical reaction steps are important factors in the establishment of the global rate of oxidation. Application of kinetic equations derived from gas absorption theories allows the determination of the rate constant of the direct ozone–aldicarb reaction, which was found to be: k = 3·18 × 10¹¹ exp(–6000/T) m³ mol^?1s^?1.     

Effect of Hydrogen Peroxide on the Photocatalytic Degradation of Methyl tert-butyl Ether

The photocatalytic degradation of methyl tert-butyl ether (MTBE) was investigated in the aqueous slurry of titanium dioxide (TiO₂) particles irradiated with xenon lamp in a batch reactor, in the presence and absence of hydrogen peroxide. The reaction was found to follow a pseudo-first-order kinetics and the initial reaction rate constant increased by raising the TiO₂ loading and reached an apparent optimum value at 0.5 g TiO₂/L. The addition of small amounts of hydrogen peroxide to the TiO₂ slurry, which is generally known to enhance the oxidation process in treating organic pollutants, decreased the initial MTBE degradation rate by as much as nearly 50%. However, this trend was reversed and reaction rate approached a plateau at higher concentration levels of hydrogen peroxide.     

Cyclohexane oxidation catalyzed by 2,2′-bipyridil Cu(II) complexes

In this work we present the use of three mononuclear Cu(II) complexes containing the ligand 2,2′-bipyridil as catalyst in the cyclohexane oxidation, using hydrogen peroxide and tert-butyl hydroperoxide as oxidant. The reactions were carried out in acetonitrile–H₂O at room temperature and at 50 °C. The complexes [Cu(bipy)₃]Cl₂, [Cu(bipy)₂Cl]Cl, [Cu(bipy)Cl₂] were able to oxidize cyclohexane into cyclohexanol, cyclohexanone and cyclohexyl hydroperoxide. Yields up to 43.7% were obtained for the system [Cu(bipy)Cl₂]/H₂O₂.     

Formation of triacylglycerol core aldehydes during rapid oxidation of corn and sunflower oils with tert-butyl hydroperoxide/Fe2+

The lipid ester core aldehydes formed during a rapid oxidation (7.8 M tert-butyl hydroperoxide, 90 min at 37°C) of the triacylglycerols of purified corn and sunflower oils were isolated as dinitrophenylhydrazones by preparative thin-layer chromatography and identified by reversed-phase high-performance liquid chromatography with on-line electrospray ionization mass spectrometry and by reference to standards. A total of 113 species of triacylglycerol core aldehydes were specifically identified, accounting for 32–53% of the 2,4-dinitrophenylhydrazine (DNPH)-reactive material of high molecular weight representing 25–33% of the total oxidation products. The major core aldehyde species (50–60% of total triacylglycerol core aldehydes) were the mono(9-oxo)nonanoyl- and mono(12-oxo)-9,10-epoxy dodecenoyl- or (12-oxo)-9-hydroxy-10,11-dodecenoyl-diacylglycerols. A significant proportion of the total (9-oxo)nonanoyl and epoxidized (12-oxo)-9,10-dodecenoyl core aldehydes was found in complex combinations with hydroperoxy or hydroxy fatty acyl groups (6–10% of total triacylglycerol core aldehydes). Identified were also di(9-oxo)nonanoylmonoacylglycerols (0.5% of total) and tri(9-oxo)nonanoylglycerols (trace). The identification of the oxoacylglycerols was consistent with the products anticipated from tert-butyl hydroperoxide oxidation of the major species of corn and sunflower oil triacylglycerols (mainly linoleoyl esters). However, the anticipated (13-oxo)-9,11-tridecadienoyl aldehyde-containing acylglycerols were absent because of further oxidation of the dienoic core aldehyde. A significant proportion of the unsaturated triacylglycerol core aldehydes contained tert-butyl groups linked to the unsaturated fatty chains via peroxide bridges (2–9%). The study demonstrates that rapid peroxidation with tert-butyl hydroperoxide consitutes an effective method for enriching natural oils and fats in triacylglycerol core aldehydes for biochemical and metabolic testing.     

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.     

Pro- and Antioxidative Effect of α-Tocopherol on Edible Oils, Triglycerides and Fatty Acids

Using advanced electron paramagnetic resonance techniques (EPR), oxidation of crude vegetable oils and their components (fatty acids and triglycerides) by radicals generated from hydrogen peroxide was investigated. The correlation rotational times were determined allowing us to characterize radicals formed during edible oils oxidation. Additionally ¹H- and ¹⁴N-hyperfine coupling constants differentiate the fatty acids dependently on their unsaturation. The acids with a higher number of unsaturated bonds exhibit higher A_N values of PBN/·lipid adduct. The waste oil with high free fatty acids content underwent the oxidation reaction more efficiently, however due to saturation and the high content of the fatty acids the carbon-centered radicals formed (upon hydrogen peroxide radicals) and their PBN (N-tert-butyl-α-phenylnitrone) adducts were less stable. The antioxidant effect was dependent on the amount of α-tocopherol added. In small amounts of up to 0.35 mg/1 g of fatty acid or triglyceride, it inhibited the creation of PBN/·lipid adducts while with higher amounts it intensified adduct formation. The α-tocopherol (AT) addition influence was also studied as spin scavenging dependence and indicated that any addition of the antioxidant in the investigated samples led to free radical scavenging and the effect increased with the increase in AT content.     

Synthesis and cross-linking of polyisoprene latexes

Seeded and ab initio emulsion polymerizations of isoprene using redox initiation systems were investigated and suitable reaction conditions determined to prepare polyisoprene latexes with minimal cross-linking. Polymerizations initiated with the potassium persulfate/sodium bisulfite (KPS/SBS) redox couple had a significant inhibition period and low yield. Polymerizations initiated with the tert-butyl hydroperoxide/tetraethylene-pentamine redox couple showed reasonable yields and no apparent inhibition. It is postulated that the lipophilic nature of the t-butyl group plays a favorable role in the entry of hydroperoxide-initiated oligomeric radicals, while persulfate-initiated radicals are more likely to undergo aqueous phase termination before entry. The cross-linking reaction by benzoyl peroxide (BPO) at 70 °C was investigated using this lightly cross-linked polyisoprene latex. ¹H NMR and gel permeation chromatography results were consistent with a reaction mechanism in which the radicals formed by the decomposition of BPO react exclusively with polyisoprene to abstract a hydrogen atom, and the resulting radicals react by termination to form cross-links. No loss of double bonds was found, suggesting that radical formation is overwhelmingly achieved by hydrogen abstraction and cross-linking occurs by termination between two radicals. Cross-linking was accompanied by chain scission, which was observed only at the beginning of the reaction. At low weight-fractions of polymer, the rate of cross-linking was dependent on the concentrations of BPO and abstractable hydrogens in a manner consistent with the postulated mechanism.     

Mild oxidation of hydrocarbons catalyzed by iron corrole with tert-butylhydroperoxide

Catalytic alkane and alkene oxidation by iron complex of 5,10,15 tris-(pentafluorophenyl)corrole, using 70% tert-butyl hydroperoxide as oxidant at room temperature is reported for the very first time. Involvement of freely diffusing radicals in the oxidizing system is observed. A reaction mechanism is proposed based on the experimental results.     

Epoxidation of Unsaturated Rubbers with Alkyl Hydroperoxide using Molybdenum Compounds as Catalyst in the Reactive Processing Equipment

A novel approach to the epoxidation of unsaturated rubbers in a reactive processing equipment is presented for the first time in this paper. The effects of the structures of unsaturated rubbers and catalysts on the epoxidation reaction were examined. The experimental results show that the reactivity of three typical unsaturated rubbers follows the decreasing order: ethylene‐propylene‐diene rubber (EPDM) > butadiene rubber (BR) > styrene‐butadiene rubber (SBR). No obvious further oxidation side reaction occurs during the epoxidation of EPDM and BR, while SBR has a significant degradation during reaction. Moreover, investigation of the catalytic system composed of tert‐butyl hydroperoxide (TBHP) and three different molybdenum compounds reveals that a blue molybdenum compound (Mo‐b), which is the product of the reaction of hydrogen peroxide (30% aqueous solution) with an excess of molybdenum powder, gives the highest catalytic activity and the highest efficiency of TBHP.     

Effects of hydrogen peroxide concentration and ultraviolet light intensity on methyl tert-butyl ether degradation kinetics

This paper aims to study methyl tert-butyl ether (MTBE) degradation in the presence of ultraviolet (UV) light and hydrogen peroxide, and to determine the effects of operating conditions on the reaction kinetics. Two key parameters investigated in this study include the concentration of hydrogen peroxide and the average UV light intensity in the solution. A 1000 W xenon arc lamp was used as the light source. The degradation of MTBE was found to follow a pseudo-first-order kinetics in terms of MTBE concentration. Although reaction rate increased with the addition of hydrogen peroxide, higher levels of hydrogen peroxide had a diminishing return since the excessive hydrogen peroxide could act as a scavenger for the hydroxyl radicals. On the other hand, the average UV light intensity in the solution was found to proportionally increase the reaction rate constant. A regression model was developed for the pseudo-first-order rate constant (k) as a function of the hydrogen peroxide concentration and the average light intensity in the solution:

     

Electrode reaction mechanisms for the reduction of tert-butyl peracetate, lauryl peroxide and dibenzoyl peroxide

The reduction of tert-butyl peracetate, lauryl peroxide and dibenzoyl peroxide has been investigated at platinum and glassy carbon electrodes in DMF. It is known from the studies of Maran et al. that peroxides reduction mechanisms generally involve at first a slow electron transfer concerted with the scission of the peroxide bond. Then the radical produced at the electrode is immediately reduced, which leads to a two electrons consumption. Similar results were obtained in this study for tert-butyl peracetate, lauryl peroxide and dibenzoyl peroxide. Besides, the determination of the number of electrons involved in the reduction at different time scales gave a number of two for lauryl peroxide, dibenzoyl peroxide, tert-butyl peroxybenzoate and di-tert-butyl peroxide and showed that this transfer is followed, in the case of tert-butyl peracetate, by a subsequent chemical reaction consuming a part of the substrate.     

Selective oxidation of benzaldehyde by molecular oxygen over molybdovanadophosphoric acid supported MCM-41

A series of molybdovanadophosphoric acid (MVPA) supported on mesoporous silica was synthesized by an incipient wetness impregnation method. The catalysts were characterized by nitrogen adsorption?Cdesorption, X-ray powder diffraction, Fourier-Transform Infra red spectroscopy (FT-IR), UV?CVis Diffused reflectance spectroscopy (UV?CVis DRS), Temperature programmed reduction (TPR) and ³¹P MAS Nuclear magnetic resonance(NMR) study. The characterization data reveals the incorporation of vanadium in phosphomolybdic acid and retention of intact Keggin ion on the support. The catalytic activities were evaluated for oxidation of benzaldehyde using molecular oxygen as oxidant as the new green reaction system. Among all the promoted catalysts, 50wt% molybdovanadophosphoric acid supported on MCM-41 exhibits highest catalytic activity in oxidation of benzaldehyde, giving 95% conversion. Other oxidants like H₂O₂ and tert-butyl hydrogen peroxide (TBHP) were also tested for benzaldehyde oxidation reaction.     

Oxidation of pinane over phthalocyanine complexes supported on activated carbon: Effect of the support surface treatment

The oxidation of cis-pinane with tert-butyl hydroperoxide, at room temperature and atmospheric pressure, was carried out in the presence of iron-phthalocyanines supported on activated carbons. The carbon supports were prepared from a NORIT activated carbon, which was modified by different chemical and thermal treatments (including oxidation in the gas and liquid phases). The carbon samples were characterized by nitrogen adsorption, mass titration and temperature programmed desorption (TPD). The TPD profiles were analysed by a simple deconvolution method, allowing for the determination of the amount of oxygen containing functional groups on the carbon surface. The main reaction product is 2-pinane hydroperoxide (77% selectivity at 91% conversion). Formation of 2-pinanol, pinocampheol and verbanol and the respective ketones was also observed. The influence of the surface chemistry of the carbon supports on catalytic activity and product selectivity is studied. The catalysts prepared from supports with very high or very low oxygen content exhibit low activity, whereas for supports with intermediate oxygen contents a good correlation between the amount of phenols and lactones and catalytic activity is obtained.     

Engineering TM1459 for Stabilisation against Inactivation by Amino Acid Oxidation

Oxidative alkene cleavage is a highly interesting reaction to obtain aldehydes and ketones. The Mn-dependent protein TM1459 from Thermotoga maritima can catalyse alkene cleavage of styrene derivatives in the presence of tert-butyl hydroperoxide. Despite the high thermal stability of the enzyme, it gets inactivated during the reaction. The data reported here indicate that auto-oxidation is responsible for the low stability of TM1459 in the oxidative environment required for the alkene cleavage reaction. By targeting the exchange of residues prone to oxidation, this phenomenon was successfully prevented. Importantly, the stability to oxidation conveyed by the amino acid exchanges led to increased enzyme activity. However, the exchanges resulted in slightly modified positions of two of the four metal-binding amino acids, thereby strongly impacting metal binding.     

Accumulation of 2-tert-Butyl-1,4-Benzoquinone in Frying Oil and Fried Food during Repeated Deep Fat Frying Processes

2-tert-Butyl-1,4-benzoquinone (TBBQ), the main oxidation product of tert-butyl-hydroquinone (TBHQ) during frying, is cytotoxic and its residual levels in frying oils and foods are unknown. In this study, TBBQ residues have been evaluated during the preparation of french fries. Results showed that frying at 140 °C resulted in the highest TBBQ peak concentration (48.42 mg kg⁻¹) compared with frying at 190 or 170 °C. This unexpected finding can be attributed to more extensive hydrolytic reaction when frying at the lower temperature, generating more peroxyl radicals. TBBQ concentrations proved to be independent of the oil type among various unsaturated oils. However, higher TBBQ levels were observed in saturated palm oil and crude soybean oil than in unsaturated oil or refined oil. Continuous frying leads to the accumulation of a large amount of TBBQ in fried food. After frying 1–5 batches, TBBQ levels in both the frying oil and fries were above 10 mg kg⁻¹, exceeding its critical cytotoxic concentration (IC₅₀ value of 10.71 mg kg⁻¹ for RAW 246.7 cells in our previous study), warranting concern with respect to the safety of fried food. FTIR has been utilized as an effective tool for visually monitoring the degree of oxidation in the frying medium with respect to its hydrogen peroxide level, which contributes to the increased level of TBBQ derived from TBHQ therein.