Molecular Catalysis

Published by Elsevier
Online ISSN: 1381-1169
Publications
Article
Computational investigation of the aldol reaction of benzaldehyde with acetone catalyzed by various proline derivatives and 2-azetidine carboxylic acid reveal the origins of stereoselectivities of these reactions. Structural differences between catalysts and transition states were analyzed with density functional theory geometries in order to establish the key factors that will help in the design of new catalysts.
 
Cross validation results of substrates from entries 11-20 (Table 2): Plot of predictions from leave-one-out models constructed from the 9 remaining substrates (y = 0.89x + 0.38, r 2 LOO = 0.61, CC LOO = 0.78)
Article
Computational models correlating substrate structure to enantioselection with asymmetric catalysts using the QMQSAR program are described. In addition to rapidly providing predictions that could be used to facilitate the screening of catalysts for novel substrates, the QMQSAR program identifies the portions of the substrate that most directly influence the enantioselectivity. The lack underlying relationship between all the substrates in one case, requires two quantitative structure selectivity relationships (QSSR) models to describe all of the experimental results.
 
Article
Ferric ion (Fe[III]) catalyzes the decomposition of hydrogen peroxide (H(2)O(2)) into strong oxidants such as hydroxyl radical ((•)OH) and ferryl ion (Fe[IV]) through the redox cycling of the iron couple (Fe[II]/Fe[III]). The use of these reactions for the catalytic oxidation of organic compounds is usually limited to the acidic pH region due to the low solubility of Fe(III) and the low efficiency of oxidant production at neutral pH values. The addition of phosphotungstate (PW(12)O(40) (3-)), a polyoxometalate extends the working pH range of the Fe(III)/H(2)O(2) system up to pH 8.5. PW(12)O(40) (3-) forms a soluble complex with iron that converts H(2)O(2) into oxidants. The coordination of Fe(II) by PW(12)O(40) (3-) also alters the mechanism of the reaction of Fe(II) with H(2)O(2) at neutral pH, resulting in formation of an oxidant capable of oxidizing aromatic compounds. The base-catalyzed hydrolysis of PW(12)O(40) (3-) gradually results in inactivation of the catalyst. In the absence of Fe(III), PW(12)O(40) (3-) was completely hydrolyzed after 1 day at pH 7.5, whereas the Fe(III)-PW(12)O(40) (3-) complex was active for at least four days under the same conditions.
 
Article
The transformation of acetone was carried out over a 0.4 wt% PtHMFI catalyst () under the following conditions: flow reactor, 160°C, pressures of acetone and hydrogen equal to 0.75 and 0.25 bar, respectively. Methylisobutylketone, propane and traces of mesityloxide are observed as primary products while the other main products: 2-methylpentane and diisobutylketone result from secondary transformation of methylisobutylketone. The reactivity of the reaction products and of probable intermediates: diacetone alcohol, isopropanol and propene was compared to that of acetone, which allows us to establish the complete scheme of acetone transformation. Acetone is competitively transformed through bifunctional catalysis into methylisobutylketone and into propane. The limiting step of methylisobutylketone formation is acetone aldolisation over the acid sites of the catalyst while that of propane formation is acetone hydrogenation over platinum sites. Methylisobutylketone undergoes the same competitive bifunctional transformations leading to diisobutylketone (limiting step: acid coaldolisation of acetone and of methylisobutylketone) and to 2-methylpentane (limiting step: hydrogenation of methylisobutylketone).
 
Article
Transformation of VOHPO4·0.5H2O precursor to well-crystallized (VO)2P2O7, for n-butane oxidation to maleic anhydride was studied by in situ Raman and XRD techniques. Atomic scale changes observed in the precursor structure at 583 K provided new insights into its transformation to (VO)2P2O7. In addition to (VO)2P2O7, nanocrystalline oxidized δ-VOPO4 invisible to XRD was detected during transformation in n-butane/air, possibly due to the specificity of the in situ conditions. Under catalytic reaction conditions the disordered nanocrystalline (VO)2P2O7 in the fresh catalysts (ca. 10–20 nm domains) gradually transformed into well-crystallized (VO)2P2O7 in the equilibrated VPO catalysts (>30 nm domains) with time on stream. Simultaneously, a disordered layer ca. 2 nm thick which was covering the surface (1 0 0) planes of (VO)2P2O7 disappeared yielding a solid with high steady-state catalytic performance. Only (VO)2P2O7 was observed both at room temperature and reaction temperature in the equilibrated VPO catalysts. Specific surface termination of the (1 0 0) planes of (VO)2P2O7 in the equilibrated VPO catalysts is believed to be responsible for high activity and selectivity of these catalysts for maleic anhydride formation.
 
Article
The bonding and the vibrations of isolated NH3 molecules adsorbed in the on-top and the three-fold hollow positions of a Ru(001) surface have been studied with the help of a density functional model cluster method. The calculated vibrational frequencies and adsorption energies have been compared to experimental data for the stronger bound α1 state and for the weaker bound α2 state of NH3 on Ru(001). The density functional results support the assignment of the α2 state to surface complexes with on-top located adsorbates. The calculated adsorption energy is smaller for NH3 in the three-fold hollow than in the on-top sites. This finding is at variance with a conceivable assignment of the α1 state to NH3 in hcp three-fold hollow sites of Ru(001).
 
Article
Platinum(II) complexes with the axially chiral phosphinyl phosphine (S)-BINAPO (1) have been prepared and their behaviour in solution has been studied by -NMR spectroscopy. Reaction of PtCl2(PhCN)2 with 1 in benzene leads to the isolation of a neutral complex, 4, which maintains the P,O-chelate coordination of the ligand even in solvents of low polarity. The hemilabile character of the ligand is apparent from the reactions with DMSO and with carbon monoxide which promote the cleavage of the chelate ring of 4 through displacement of the oxygenated arm. Insertion of tin(II) chloride into the Pt–Cl bond takes readily place at room temperature affording only one of the possible trichlorostannato derivatives (6) with complete selectivity. In the presence of SnCl2, the platinum complex 4 originates a catalyst of remarkable regioselectivity which, in the hydroformylation of styrene, is able to produce in up to 30% e.e. the branched aldehyde as the prevalent product.
 
Article
In situ-generated rhodium complexes of mono- and bisphosphorylated enantiopure BINAP ligands have been used for the asymmetric hydroformylation of styrene and vinylacetate. Corresponding Ru-complexes have been investigated in the homogeneous and biphasic asymmetric hydrogenation of dimethyl itaconate. An increase in the enantioselectivity of about 6–9% compared to BINAP was observed in the vinylacetate hydroformylation. For the aqueous biphasic hydroformylation of styrene the most enantioselective rhodium diphosphine catalyst (27% e.e.) up to now has been found.
 
Article
Aryl amines undergo smooth coupling with 2,2-dimethoxypropane (2,2-DMP) in the presence of 5mol% of bismuth triflate under solvent-free conditions to produce 1,2-dihydroquinolines in high yields under mild conditions. However, the condensation of o-phenylenediamines with 2,2-DMP gave the corresponding 1,5-benzodiazepines in excellent yields under identical conditions.
 
Article
The irreversible adsorbates of ethanol, 1,2-ethanediol and methyl-α-d-glucopyranoside (MGP) have been studied with FTIRS and cyclic voltammetry. Both ethanol and 1,2-ethanediol display full CC(OH) dissociative adsorption and dehydrogenation. In the case of ethanol adsorbed CO and C are formed of which the latter partially oxidizes further to adsorbed CO. In the case of 1,2-ethanediol CO is formed as the only adsorbate. The adsorption of MGP occurs similarly to the small alcohols; it decarbonylates to form adsorbed CO and a small fraction of C adatoms. It is shown that the catalytic alcohol oxidation can be regarded as an electrochemical process that consists of two independently acting half-reactions that determine the open circuit potential (o.c.p.). The roughness of the surface greatly affects the o.c.p. measured during catalytic alcohol oxidation; smooth platinum leads to high o.c.p. values and platinized platinum leads to low o.c.p. values. These low and high open circuit potentials correspond respectively to a diffusion limited regime where diffusion of oxygen is rate limiting and a kinetic regime. The reaction rate is considerably lower in the kinetic regime than in the diffusion limited regime. The surface is highly covered with adsorbed oxygen or hydroxyl during oxidation of ethanol and MGP in the kinetic regime, whereas the surface is devoid of adsorbed oxygen in the diffusion limited regime and is instead covered with a high steady state amount of CO and C species. The deactivation of the catalyst is found to occur both in the diffusion limited and in the kinetic regime of the MGP oxidation. Whereas in the diffusion limited regime, the deactivation is caused by a slow accumulation of carbonaceous residue, and in the kinetic regime, changes in the properties of adsorbed oxygen cause deactivation.
 
Article
Homogeneous catalysts prepared from rhodium trichloride in aqueous aromatic amines have been shown to reduce C–Cl bonds under mild water gas shift conditions (T=100°C, PCO=1.0 atm). In a 4-picoline/water solvent mixture, 1,2-dichloroethane is reduced to ethylene and ethane in yields compatible with the consumption of the reducing agent CO and with the formation of CO2. Variation of the catalyst solutions by using different substituted pyridines shows a pattern of catalytic activity parallel to that reported previously for H2 production from the shift reaction. There is a moderate dependence of activity on the basicity of the aromatic amine, but a methyl group at the α-position exercises a strong negative steric effect. Long term studies show decrease of the activity with the time perhaps due to the build up of chloride in the medium.
 
Article
Lamb pregastric lipase, extracted from the tongue and epiglottal region of lamb, has been partially purified and used to catalyse the hydrolysis of (9–10 3H) (1,2,3-tris-[(cis)-9-octadecenoyl]glycerol) over the pH range 5.50–7.50 and temperature range 20.0–40.0°C. Michaelis-Menten plots were constructed for each reaction condition, and allowed evaluation of Km and kcat. The values of kcat have been fitted to a three-dimensional activity profile. The optimum pH for reactivity was 6.3 ± 0.3 and the optimum temperature 32 ± 3°C. Under optimum conditions the values of kcat and Km were 0.073 μmol · min−1 · mg−1 and 9 mM, respectively. The reaction has also been studied in D2O as solvent at pD = 6.50 and T = 30.0°C. The kinetic isotope effects were 1.46 and 1.31 for the rate determining acylation and deacylation steps, respectively. The activity of the enzyme was inhibited by the presence of the bile salt, sodium taurocholate.
 
Article
Novel moisture and air stable, cationic palladium(II) amine complexes (1–4) of the general type [Pd(N∩N)(X)2](BF4)2, [N∩N=1,2-bis(N-indolinyl)ethane (BIE) 1, 3; 1,2-bis(N-1,2,3,4-tetrahydroquinolinyl)ethane (BTQE) 2, 4; X=NCCH3, H2O] were found to catalyze the polymerization reaction of bicyclo[2.2.1]hept-2-ene at room temperature. The amorphous polymer products consist of 2,3-linked norbornene units; no indications for ring opened species could be observed. The polymerization activity of the diaqua-complexes 3, 4 is superior compared to their acetonitrile analogues due to a facile activation by a Wacker-type reaction. The cationic Pd(II)-compounds are inactive towards homo- and copolymerization reactions of polar monomers, like acrylates or carbon monoxide. However, addition of methylacrylate resulted in polynorbornene products with increased molecular weight and narrow molecular weight distributions.
 
Article
The selective dimerization of styrene to 1,3-diphenyl-1-butene over Pd(β-diketonate)2/BF3OEt2 catalyst systems in both “phosphine-free” and “phosphine-modified” fashions has been investigated. For the Pd(acac)2 + 2PR3 + 7BF3OEt2 catalyst system the turnover of 75,000 mol styrene/mol palladium for 7 h was reached at 70 °C with selectivity to dimers of 93%. Styrene dimers up to 95% consists of trans-1,3-diphenyl-1-butene. Catalytic activity and selectivity can be controlled by varying the reaction parameters. The nature of substituents on the β-diketone did not affect the conversion of styrene. However, the selectivity to dimers increases almost linearly with decreasing acidity of β-diketonate ligand. The nature of the phosphine ligand has a strong effect on the conversion of styrene, but not on the selectivity. The order of the decrease in conversion parallels the increasing basicity (electronic effect) rather than steric properties of the phosphines. Reaction products were characterized with 1H and 13C NMR, IR, and GC–MS spectroscopies and GC analysis. Palladium hydride complexes are likely to be catalytically active species.Graphical abstractThe selective dimerization of styrene to 1,3-diphenyl-1-butene over Pd(β-diketonate)2/BF3OEt2 catalyst systems in both “phosphine-free” and “phosphine-modified” fashions has been investigated. Styrene dimers up to 95% consists of trans-1,3-diphenyl-1-butene. Reaction products were characterized with 1H, 13C NMR, IR, MS-GC spectroscopy and GC analysis. Palladium hydride complexes are likely to be catalytically active species.
 
Article
The hydrogenation and dehydrogenation reactions of cyclohexene on Pt(111) surface were investigated by surface vibrational spectroscopy via sum frequency generation (SFG) both under ultrahigh vacuum (UHV) and high pressure conditions with 10 Torr cyclohexene and various hydrogen pressures up to 590 Torr. Under UHV, cyclohexene on Pt(111) undergoes a change from π/σ-bonded, σ-bonded, and c-C6H9 surface species to adsorbed benzene when the surface was heated. A site-blocking effect was observed at saturation coverage of cyclohexene and caused the dehydrogenation to shift to higher surface temperature. At high pressures, however, none of the species observed in UHV condition were seen. 1,4-cyclohexadiene (CHD) was found to be the major species on the surface at 295 K even in the presence of nearly 600 Torr of hydrogen. Hydrogenation was the only detectable reaction at the temperature range between 300–400 K with 1,3-CHD on the surface as revealed by SFG. Further increasing surface temperature results in a decrease in hydrogenation reaction rate and an increase in dehydrogenation reaction rate with both 1,3-CHD and 1,4-CHD detectable on the surface simultaneously. Monitoring the reaction kinetics and the chemical nature of surface species together allows us to postulate a reaction mechanism: cyclohexene hydrogenates to cyclohexane via a 1,3-CHD intermediate, and dehydrogenates to benzene through both 1,4-CHD and 1,3-CHD intermediates. Both 1,3- and 1,4-CHD dehydrogenate to benzene at sufficiently high temperature on Pt(111).
 
Article
The homogeneous telomerization of 1,3-butadiene with alcohols for the selective synthesis of linear octadienyl ethers in the presence of catalysts prepared in situ from palladium(0) bis-dibenzylidene-acetone and different mono- and diphosphine ancillary ligands is described. With monophosphines, a correlation between basicity as well as steric hindrance of the ligand and activity and selectivity of the resulting catalyst was found. When diphosphines were used, the effect of the bite of the chelating ligand, as well as its basicity and steric hindrance on the activity and selectivity of the process was studied and discussed in terms of the relative stability of the metallacyclo moieties involved in the catalytic cycle. The above results have allowed to gain more light on the reaction mechanism.
 
Article
The transformation of the bis-1,3-trichloromethylbenzene depends on the experimental conditions (temperature, amount of HF and the presence of a Lewis acid). The formation of the 1-trichloromethyl-3-trifluoromethylbenzene is favored in the presence of HF in default. After the consumption of HF, the various fluorinated intermediates compounds were observed to lead to the more thermodynamically stable compounds, i.e., mainly the bis-1,3-trichloromethylbenzene and 1-trichloromethyl-3-trifluoromethylbenzene. These results were explained by theoretical calculations. The presence of a Lewis acid increases the formation of the more stable products at lower temperature. SbCl5 is the most efficient catalyst corresponding to a decrease of 150 °C to obtain similar results compared with HF alone.Graphical abstractThe transformation of the bis-1,3-trichloromethylbenzene depends on the experimental conditions (temperature, amount of HF and Lewis acid). The formation of the 1-trichloromethyl-3-trifluoromethylbenzene is favored in the presence of HF in default. The presence of a Lewis acid increases the formation of the more stable products at lower temperature.
 
Article
A new method for obtaining 2,6-dialkyl-1,4-benzoquinones by oxidation of 2,6-dimethyl and 2,6-ditertbutylphenols by molecular oxygen in a two-phase “water–organic” system and in the presence of P–Mo–V heteropoly acids has been proposed.
 
Article
Hydrogenation of 2-butyne-1,4-diol (B3D) using 1% Pt/CaCO3 catalyst was carried out to give 2-butene-1,4-diol (B2D) or butane-1,4-diol (B1D) selectively or a mixture of two diols eliminating the formation of acetal, aldehyde, and alcohols as side products. In presence of ammonia, nearly complete selectivity to B2D was obtained in a batch reactor while, in a fixed bed reactor total selectivity to B1D was obtained. Effect of concentration of ammonia, metal loading and catalyst pre-treatment on catalyst activity and selectivity has been investigated in a batch reactor. The formation of B1D and B2D is explained on the basis of associative or dissociative adsorption of B3D via carbene and carbyne type intermediates which react with adsorbed hydrogen to give the corresponding products. A kinetic model based on Langmuir–Hinshelwood (L–H) type mechanism has been proposed which shows a good agreement with experimental data.
 
Proposed structures of the catalysts. 
Proposed catalytic cycle for the oxidative aromatization of Hantzsch 1,4-dihydropyridines. 
Scheme 2. Oxidative aromatization of Hantzsch 1,4-dihydropyridines in the presence of Cat 1 using O2. 
Article
Cobalt complexes were covalently anchored onto the surface of silica by the complexation of organically modified silicas with Co(OAc)2 and CoCl2 and investigated for the oxidative aromatization of Hantzsch 1,4-dihydropyridines using molecular oxygen. The catalyst obtained by the complexation of Co(OAc)2 with organically modified 3-aminopropyl silica (Cat 1) was found to be most effective and stable under the reaction conditions. It can be easily recovered and recycled. The work-up procedure is simple and the corresponding pyridines were obtained in good to excellent yields.Graphical abstractThe oxidative aromatization of Hantzsch 1,4-dihydropyridines to the corresponding pyridines with molecular oxygen as oxidant is catalyzed by covalently anchored cobalt complexes onto the surface of silica gel. Cat 1 was found to be the most effective and stable under the reaction conditions. It can be easily recovered and recycled.
 
Article
Poly(1,4-phenylenemethylidynenitrilo-1,4-phenylenenitrilomethylidyne), (PIM), i.e. polyimine obtained via condensation of p-phenylenediamine and terephtalaldehyde, has been prepared and protonated with selected Keggin type heteropolyacids (H3PMo12O40 and H3PW12O40). Both, unprotonated and protonated PIM have been characterized by IR spectroscopy, X-ray diffraction measurements and thermal studies. Heteropolyacids inserted into PIM matrix preserve their structural identity. PIM-heteropolyacids systems exhibit high thermal stability. They are catalytically active in isopropanol conversion showing enhanced redox activity.
 
Article
The ligand 3,6,9,17,20,23,28,29-octaazatricyclo[23.3.1.111,15]tridecanedeca-1(28),2,9,11,13,15(30),16,23,25(29),26-ene (PD)2(DIEN)2, 1, was used together with Cu(CH3CN)4PF6 to prepare a dinuclear copper(I) complex (2) which, when oxygenated at 25.0°C produced a copper(II)-dioxygen adduct (3) as an initial species, which decomposed to a copper(II) complex (4). The complex cation of 4 was formed by the reaction of 1 with CuCl2. It is shown that both 3 and 4, in the presence of excess dioxygen, catalytically convert hydroquinone, t-butylhydroquinone, 2,6-di-t-butylphenol, 2,6-dimethoxyphenol and 3,5-di-t-butylcatechol to their respective oxidation products benzoquinone, t-butylbenzoquinone, 3,3′5,5′-tetra-t-butyldiphenoquinone, 3,3′,5,5′-tetramethoxydiphenoquinone and 3,5-di-t-butyl-1,2-benzoquinone. Under the same conditions 3 converts 2,4-di-t-butylphenol to 3,3′,5,5′-tetra-t-butyl-2-2′-dihydroxybiphenyl,4-t-butylcatechol to the γ-lactone of 3-hydroxy-4-t-butylmuconic acid ester, and 3,4-dimethylaniline to 3,3′4,4′-tetramethylazobenzene whereas 4 is inactive for these substrates. The complex 4 is found to oxidize 4-methylcatechol; however, for this substrate 3 is inactive. The oxidation of 3,5-di-t-butylcatechol is first order in 3 and zero order in substrate, having a pseudo first order rate constant of 1.3 × 10−3 s−1. Under pseudo first order conditions the reaction between 3,5-di-t-butylcatechol and 4 is first order in 4. The rates of stoichiometric reactions are determined to be greater for reactions with 3 than for those with 4 by factors which range between 5 and 100.
 
Article
To reveal the underlying problems and misleading results that can be obtained from uninitiated O2/aldehyde/olefin co-oxidative epoxidations that proceed by a radical-chain mechanism, a series of nearly identical experiments were set up at the University of Tokyo and Colorado State University. The main catalyst precursor studied is the oxidatively resistant, polyoxoanion-supported organometallic complex [(n-C4H9)4N]5Na3[(1,5-COD)Ir · P2W15Nb3O62], 1. Three other LnM · P2W15Nb3On−62 complexes were also examined (LnM = Re(CO)+3, Ru(C6H6)2+, and Rh(1,5-COD)+], as well as the framework-incorporated cobalt(II) complex, [(n-C4H9)4N]4H[PW11COIIO3−39]. Several types of important but often omitted control reactions were also performed, again to expose the pitfalls in studies of O2/aldehyde/olefin co-oxidative epoxidations, specifically: (i) the control of leaving out the catalyst completely, (ii) controls for O2-mass-transfer limitations, (iii) controls examining a range of different solvents (CH3CN, CH2Cl2, ClCH2CH2Cl, and Cl2CHCHCl2), and (iv) controls comparing uninitiated versus deliberately peroxide-initiated reactions. The resultant reproducibilities, product conversions, selectivities, and yields are presented and discussed, as are stereochemical results using cis- and trans-stilbene. Several important insights are generated for the area of co-oxidative epoxidations proceeding by a radical-chain mechanism, most notably that the catalyzed results are inferior to ROOH-initiated, uncatalyzed co-oxidative epoxidations examined under otherwise identical conditions.
 
Article
New bead-shaped insoluble polymer-supported multi-site (six-site) phase transfer catalyst (BSIMPTC) was synthesized from mesitylene as a starting material and its catalytic efficiency was studied with the kinetics of Darzen's condensation of 4-nonanolide with 1,6-dibromohexan-2one. The presence of six active-sites in the newly synthesized BSIMPTC, viz., 2,4,6-tris[(4-(2,2'-bis(N-triethylammoniummethylene chloride) eth-1-ene) phenoxy)benzene was characterized through FT-IR, solid H-1 NMR, C-13 NMR, SEM and [chloride ion] analyses and found that the BSIMPTC contains approximate to 6 active-site centers. The Darzen's condensation reaction of 4-nonanolide was performed at lower temperature (40 degrees C) under pseudo-first order rate conditions by taking lower concentration of aqueous NaOH (15 %, w/v, 3.75 M) and excess of 1,6-dibromohexan-2-one. The disappearance of 4-nonanolide was quantitatively monitored by a gas chromatograph for the calculation of the pseudo-first order rate constant. The presence of more number of active-sites in BSIMPTC was further confirmed from the comparative study of pseudo-first order rate constant with the rate constants of single-site polymer-supported phase transfer catalyst. The comparative rate constant results reveal that the BSIMPTC is highly active-than the corresponding soluble six-active-site and almost approximate to 4 times higher active-than with insoluble/soluble single-site PTCs. The effect of various experimental parameters such as [substrate], [catalyst], [NaOH], stirring speed, and temperature on the rate of the reaction have also been studied and found that each variables are influenced the rate of reaction. Based on the obtained kinetic results, a suitable mechanism is proposed.
 
Article
Nanoscale changes in the surface morphology of Pd particles that accompany the uptake of hydrogen have been studied in situ by Field Electron Microscopy (FEM). Exposure of a Pd tip to H2 at low temperatures led to the formation of extruding PdHx particles on top of the Pd tip. Growth of these particles proceeds in a “staccato” manner. When most of the hydrogen have been removed from the sample by heating in vacuum, Pd crystallites remain on the surface. They are quite stable up to about 700 K, where they melt back into the bulk of the tip. A sharp low temperature H2-peak (220 K) appears in the TD spectra as a result of decomposition of PdHx hydrides on Pd(110) single crystal surface. During the O2+Hads reaction, a hydrogen-modified Pt(100)-(hex) surface shows an increase in the population of atomic oxygen states: three Oads states are observed by high resolution electron energy loss spectroscopy (HREELS) as a result of the presence of defect sites. The reactivity of these new oxygen states to CO has been investigated with thermal desorption spectroscopy (TDS).
 
Article
The adsorption of ethylene on clean and oxygen-covered rhodium (111) was studied using extended Hückel calculations with the tight-binding approach. Ethylene binds preferentially in a two-fold bridging site with the CC and RhRh bonds parallel on clean Rh(111). The CC bond is significantly weakened, both by donation of electrons from the π orbital to the surface and back-donation from the surface to the π∗ orbital on the clean surface. The addition of a quarter of a monolayer of oxygen modifies the surface electronically, so that the three-fold site is preferred by ethylene and the on-top site becomes more stable than the two-fold. Steric effects are important for high oxygen coverage (θ ≈ 0.5) where close contacts between O and either C or H atoms of the alkene lead to highly repulsive interactions. Steric effects dominate on the Rh(111)-(2 × 1)-O surface. The CC bond coordinates across a RhO bond to form an oxametallacycle structure which is the lowest energy geometry found for this oxygen rich surface. This species is proposed as a plausible intermediate along the path to ketone production and rationalizes the experimental finding that oxygen-rich conditions favor partial oxidation.
 
Article
The forms of the atomic oxygen adsorbed on the Ag(111) face were investigated using the semi-empirical NDDO/MC method within the modified cluster approximation. Two oxygen positions — surface (over the octahedral hollow) and the subsurface (in the hollow) one — between the first two layers of (111) face were studied. A cross-section of the potential energy for the oxygen penetration into subsurface positions has been computed. A new approach was applied to take into account the lattice relaxation caused by the interaction with adsorbate. The doubly-excited-CI-like correlation corrections to the diffusion and desorption barriers were estimated. The barrier of the oxygen atom diffusion through the surface was shown to be substantially lower than that of the desorption.
 
Article
13C MAS NMR has been performed in situ to investigate the mechanism of benzene alkylation with propane over catalyst. Propane 2-13C was used as a labelled reactant. In order to clarify the main reaction pathways, conversions of the individual starting materials and some of the reaction intermediates such as cumene and n-propylbenzene were also studied under similar conditions.Benzene alkylation with propane begins at 573 K, toluene and ethylbenzene being the major primary products. The reaction is preceded by the induction period caused by stronger adsorption of benzene that prevents propane activation. The main reaction pathway includes bifunctional propane activation on Brønsted and Ga sites which leads to protonated pseudocyclopropane (PPCP) intermediate, which in turn decomposes preferentially to CH4 and C2H+5, or C2H6 and CH+3. The reaction of C2H+5 or CH+3 carbenium ions with benzene leads to toluene or ethylbenzene, respectively. The less favourable reaction routes give propenium and cycloproponium ions upon PPCP decomposition, and result in observation of the traces of cumene and n-propylbenzene. These routes are reversible, the equilibrium being shifted towards reactants at 573 K.
 
Article
Oxidative conversion of methane, ethane, propane, benzene, hydrogen and their binary mixtures R1–R2–N2O–He (where R1, R2 are substances under study) were studied on HZSM-5 at 350–450°C. Relative reactivities were estimated, rate of conversion of hydrocarbons correlating with the strength of CH bond. 13C label distribution in the product of 13CH4–C6H6–N2O feed was studied by GC-MS, 1H and 13C NMR. It was shown that methane was capable to alkylate the aromatic ring under reaction condition, giving toluene and xylenes from benzene.
 
Article
Direct esterification of butanol to butylbutanoate in the absence of butanoic acid has been investigated by a series of green solid acid catalysts, including H14[NaP5W30O110], H14-P5, H14[NaP5W29MoO110], H14-P5Mo and silica supported H14[NaP5W30O110], H14-P5/SiO2, with H2O2 as oxidizing agent at reflux and at room temperatures. The performance of these eco-friendly catalysts were compared with H2SO4. Maximum butylbutanoate yield and product selectivity (100%) was observed by using H14-P5Mo as the catalyst. The effects of various parameters such as catalyst type, reaction times, reaction temperatures, and molar ratio of n-butanol to hydrogen peroxide have been studied. The green catalysts can be easily recovered and recycled with retention of their initial structure and activity.Graphical abstractCatalytic activity of recyclable Preyssler acid, H14-P5 and its different forms such as H14-P5Mo and H14-P5/SiO2 have been investigated, in liquid phase in the presence of hydrogen peroxide. A systematic study has been made to find the effect of catalyst type, the molar ratio of alcohol to hydrogen peroxide, temperature and the duration of reaction on the synthesis of the ester. All the catalysts have been found to be active and exhibit 100% selectivity and recyclable.
 
Article
Catalytic properties of holmiumdecatungstate modified with cetylpyridinium cations [Cetyl-Ho(III)W10] for H2O2-oxidations of alcohols and olefins were investigated under various organic solvents-aqueous H2O2 biphasic conditions. Generally, secondary alcohols and benzyl alcohols and internal olefins such as cyclooctene were efficiently catalyzed by Cetyl-Ho(III)W10, while primary alcohols and terminal olefins were almost not. The activity for 2-octanol grew much by using a smaller amount of nonpoler solvents such as CHCl3. Especially, in an organic-solvent-free condition as a limited case, Cetyl-Ho(III)W10 was elucidated to be workable as a highly active but quite mild catalyst in comparison with the other heteropolyoxometalates well known. A schematical model on the working states was successfully presented. The present H2O2-Cetyl-Ho(III)W10 catalyst system will be insisted, finally, to be responsible for industrial oxidation processes such as being performed in a non-organic solvent system as environmental concerns.
 
Article
Vanadium oxide supported on mesoporous SBA-16 (VOx/SBA-16) catalysts have been prepared by the impregnation method and characterized by small-angle XRD, wide-angle XRD, TEM, N2-physisorption, DRUV–vis, Raman spectrum and H2-TPR. The VOx/SBA-16 catalysts retained the cubic cage-like pore structure of SBA-16. The dispersion and the nature of the vanadium species depend strongly on the V amount. At V loading of less than 3.6wt%, isolated tetrahedral VO4 is the main existence species that is highly dispersed in the pores of the support SBA-16. With the increase in V loading, the aggregation of isolated tetrahedral VO4 species occurred to form polymerized VO4 units. When the V loading was above 5.5wt%, nanostructured V2O5 crystallites were formed besides aggregation of polymerized VO4 units. VOx/SBA-16 with 7.3wt% V showed excellent activity for the hydroxylation of benzene. The effect of reaction time, temperature and the amount of catalyst was investigated over VOx/SBA-16 (7.3wt%) catalyst. The highest phenol yield and turnover number were 13.8% and 32.4, respectively, which were attributed to the formation of highly dispersed VO4 species and polymerized VO4 units. The TON and yield of phenol also increase in the presence of the V2O5 crystallites but its selectivity decreases.
 
Article
Using 17O and 1H NMR, peroxotitanium species formed in the reaction of TiO(acac)2 and Ti(OEt)4 with H2O2 in CHCl3 were characterized. Dinuclear μ-oxo, μ-peroxo complex [Ti(acac)2]2O(O2) was prepared via reaction of TiO(acac)2 with H2O2 in CHCl3. This complex is inert towards alkenes oxidation. When Ti(OEt)4 reacts with an equimolar amount of 95% H2O2 in CHCl3 at 293 K, ca. 75% of the initial titanium complex converts to the oligomeric peroxotitanium species, containing no alkoxo ligands, ca. 20% of titanium exists in solution in the form of dinuclear μ-oxo, μ-peroxo complexes of the type [Ti(OEt)2L2]2O(O2), where L is solvent or EtOH, and 3–5% in the form of mononuclear peroxo complexes of the type Ti(O2)(OEt)2L2. Oligomeric and dinuclear peroxotitanium species were inert towards alkenes and phenol; in contrast mononuclear ones oxidize cyclohexene at 293 K predominantly into cyclohexene oxide and phenol into a 1:3 catechol/hydroquinone mixture. Peroxo complex Ti(O2)(OEt)2L2 is the first peroxotitanium complex active towards oxidation of organic substrates.
 
Article
The oxidation of benzyl alcohol to benzaldehyde with potassium permanganate (KMnO4) was studied in a batch reactor using 18-crown-6 (crown ether) as phase transfer catalyst in a solid–liquid system. Benzene was used as the solvent and KMnO4 was taken as the solid reactant. KMnO4 is not soluble in benzene in normal condition but the addition of 18-crown-6 forms a complex with KMnO4 and makes it soluble in benzene. The oxidation reaction was studied at different degrees of agitation, temperature, catalyst concentration and mole ratio of benzyl alcohol to KMnO4. The reaction usually occurs in two steps where the first step is the oxidation of benzyl alcohol to benzaldehyde and the second step is the formation of benzoic acid from benzaldehyde. In the chosen reaction condition benzaldehyde was obtained as the only product for all the cases. However, at higher temperature and substantial amount of catalyst concentration benzoic acid was obtained along with benzaldehyde. The initial rate was found to increase with increase in concentration of catalyst (18-crown-6) and benzyl alcohol. A semi-empirical model for the reaction was depicted to illustrate the proposed mechanism. The activation energy and frequency factor of the reaction were found to be 9.149kJ/mol and 1.85×106 respectively.
 
Article
A new method for Biginelli reaction via heterogenous catalyst has been developed. This method uses natural Heulandite type zeolite (HTMA) as a catalyst for the one pot condensation of an aldehyde, urea and a 1,3-dicarbonyl compound under mild conditions in excellent yields.Graphical abstractA new method for Biginelli reaction via a heterogenous catalyst has been developed. This method uses natural Heulandite type zeolite (HTMA) as a catalyst for the one pot condensation of an aldehyde, urea and a 1,3-dicarbonyl compound under mild conditions in excellent yields.
 
Article
Method and conditions of preparation strongly affect the hydroformylation activity of the ruthenium cluster derived Ru3(CO)12/2,2′-bipyridine/SiO2 catalyst. The highest activities have been achieved with a dichloromethane impregnated catalyst. A major problem with this catalyst is poor reproducibility of the hydroformylation activity, probably due to uncontrolled formation of several supported surface species. Reproducibility can be improved by using a non-chlorinated impregnation solvent such as tetrahydrofurane. In hydroformylation, Ru3(CO)12/2,2′-bipyridine/SiO2 has a strong tendency to convert alkenes directly to alcohols. The limiting step in this process is the initial hydrocarbonylation of alkenes to aldehydes. Ru3(CO)12/2,2′-bipyridine/SiO2 catalyzes the second step, hydrogenation of aldehydes to alcohols, in good yield. Use of an effective, aldehyde-producing cocatalyst such as Rh together with Ru3(CO)12/2,2′-bipyridine/SiO2 allows very high alcohol yields to be achieved. The detailed surface structure of Ru3(CO)12/2,2′-bipyridine/SiO2 is not known, but one probable active species is oligomeric or polymeric [{Ru(bpy) (CO)2}n]. Monomeric and dimeric ruthenium monobipyridines showed at most only moderate activity in hydroformylation. By contrast, [Ru(bpy) (CO)2Cl(C(O)OCH3)] and [Ru(bpy) (CO)2CIH] are highly active in hydrogenation of C7 aldehydes to C7 alcohols.
 
Article
Ring-opening polymerization of 2,2-dimethyltrimethylene carbonate (DTC) has been carried out by using single component rare earth tris(4-tert-butylphenolate)s (Ln(OTBP)3) initiators for the first time. The influences of rare earth element, solvent, temperature, monomer and initiator concentration as well as reaction time on the polymerization were investigated. The kinetics indicates that the polymerization rate is first order with respect to monomer concentration and initiator concentration, respectively. The overall activation energy of the ring-opening polymerization amounts to 78.7kJ/mol. Some living character of the reaction was confirmed by adding another portion of DTC monomer into an almost completely polymerized system. PDTC was characterized by 1H NMR, GPC, IR and DSC. PDTC obtained has no ether unit which resulted from CO2 elimination. DSC data identified two crystalline modifications and showed some influences of molecular weight on the melting point and melting enthalpy. Mechanism studies showed that monomer inserted into the growing chains with the acyl-oxygen bond scission rather than the break of alkyl-oxygen bond.
 
Article
The ligands bipyridine (bpy), 4,4′-dimethoxy-2,2′-bipyridyl (dMeObpy), 4,4′-dimethyl-2,2′-bipyridyl (dMebpy), 4,4′-dichloro-2,2′-bipyridyl (dClbpy) and 4,4′-dinitro-2,2′-bipyridyl (dNO2bpy), when combined with copper(II) nitrate render active catalysts. The highest activity was found for bpy as the ligand. The ligands dMeObpy and dMebpy, with their electron-donating substituents, render the copper ions less electrophilic than unsubstituted bpy and so decrease the oxidation rate of 2,6-dimethylphenol (DMP). One would have expected an increase in the oxidation rate of DMP with the ligands dClbpy and dNO2bpy, having electron-withdrawing substituents, which render the copper ions more electrophilic. However, probably due to a stabilisation of the copper(I) species, which retards the reoxidation to Cu(II), the overall reaction rate decreases with dClbpy and dNO2bpy.
 
Article
A novel and stereocontrolled glycosidation of 3,4,6-tri-O-acetyl-d-glucal with various alcohols to give the corresponding 2,3-unsaturated glycopyranosides using [bmim]Cl–1.5FeCl3 based ionic liquid is presented. This ionic liquid has proved to be an efficient reaction medium, playing a dual role of a catalyst as well as of a solvent. Salient features of this simple methodology include non-hazardous reaction conditions, good yields in short reaction times and high anomeric selectivity.Graphical abstract
 
Article
Formation of catalytically active species in the system AlEt3-Co(acac)2 known as a catalyst for hydrogenation and polymerization is discussed based on the results of chemical, kinetic and physico-chemical (ESR, IR, UV, TEM, XRD) analysis. The formation of nanoscale cobalt metal particles is demonstrated. The particles are assumed to comprise the metal core and a stabilizing coating consisting of AlEt3, Et2Al(acac) and the products of their transformations. The dependence of hydrogenation activity of the catalytic system on the Al/Co ratio passes through maximum, that is explained by changes of the composition of the nanoparticle stabilizing coating.
 
Article
Polynuclear manganese complexes of 2,5-dihydroxyterephthalaldehyde (dhterH2) and its Schiff base polymer ligand (PdhterenH2) [MnII(dhter)]n, [MnnII(Pdhteren)] and [MnnIII(Pdhteren)(OAc)n] were synthesized. The manganese complexes were characterized by elemental analysis, thermal analysis, IR and EPR spectroscopic techniques. Catalytic epoxidation of olefins with hydrogen peroxide was studied using the above manganese complexes under heterogenized homogeneous reaction conditions in the presence of a base. The effect of imidazole and olefin concentrations on effective oxygen transfer were studied. [MnnII(Pdhteren)] and [MnnIII(Pdhteren)(OAc)n] are more efficient than the oxygen-bonded manganese complex [MnII(dhter)]n for the activation of hydrogen peroxide.
 
Article
Oxidation of various olefins into methylketones by a catalytic system constituted of palladium, copper, phosphomolybdovanadic acid and per(2,6-di-O-methyl)-β-cyclodextrin is investigated. The role of each redox catalytic system component is discussed from experiments under argon atmosphere and from vanadium NMR spectroscopic studies. The influence of various parameters, such as the per(2,6-di-O-methyl)-β-cyclodextrin concentration and the nature of the phosphomolybdovanadic acid is also reported. Finally, a catalytic cycle is also proposed.
 
Article
The influence of varying concentrations and ratios of phenol, base and copper on the copper/N-methylimidazole catalysed oxidative coupling of 2,6-dimethylphenol (DMP) has been studied. The reaction obeys simple Michaelis–Menten kinetics with respect to the phenol. The amount of DPQ formed during the reaction increases linearly with the increasing initial amount of DMP. At higher base-to-copper ratios an oxidative coupling experiment takes longer to complete, despite higher initial rates, which is probably due to the formation of inactive copper hydroxide species in the later stages of the reaction. The phenol oxidation step is most likely the rate-determining step, and the fractional reaction orders in copper are determined by the position of the equilibrium between mono- and dinuclear copper species, the latter being the active one.
 
Article
The complex of copper(II) nitrate with N-methylimidazole (Nmiz) ligand has been studied as a catalyst for the oxidative coupling of 2,6-dimethylphenol by means of kinetic and spectroscopic measurements. The order of the reaction in copper is fractional and depends on the ratio and the base/Cu ratio, indicating that there are at least two possible rate-determining steps, i.e. the formation of a dinuclear copper species and the phenol oxidation. EPR spectroscopy performed on frozen solutions with varying ligand to copper ratios shows that all Cu(II) is converted into the precursor complex at a ratio of 4 to 1, whereas in kinetic experiments, maximum activity and selectivity are reached only at a ratio of at least 30 to 1. Base is needed as a co-catalyst, and the maximum reaction rate is reached at a base to copper ratio of 1.8 to 1. The solid-state X-ray structure of the catalyst precursor complex has been determined to be [Cu(Nmiz)4(NO3)2], monoclinic, space group , a = 8.452(1) Å, , , β = 94.88(2) °, Z = 1, R = 0.049 for 3525 reflections. This structure consists of an axially elongated octahedral CuN4O2 chromophore, which is in agreement with frozen-solution EPR spectra. Investigations under conditions where water and dioxygen were carefully excluded, have shown that for the phenol oxidation step the presence of dioxygen is not required. However, the reaction does require a trace of water (or hydroxide) to form the reactive intermediate. A modified reaction mechanism for the oxidative coupling is presented with special attention to the first steps of the reaction and the equilibrium species present in solution. The role of dioxygen appears to be only to reoxidize the formed Cu(I) species and to regenerate base.
 
Article
An optimization study of the reaction conditions of Fe(TDCPP)Cl when it is used as catalyst in the hydroxylation of cyclohexane by iodosylbenzene (PhIO) has been carried out. It was found that Fe(TDCPP)Cl follows the classical PhIO mechanism described for Fe(TPP)Cl, which involves the monomeric active species FeIV(O)P+ (I). In the optimized condition ([Fe(TDCPP) = 3.0 × 10−4 mol l−1 in 1,2-dichloroethane (DCE); ultrasound stirring at 0°C; molar ratio = 100), this FeP led to a yield of cyclohexanol (C-ol) of 96% and a turnover number of 96. Therefore, Fe(TDCPP)Cl may be considered a good biomimetic model and a very stable, resistant and selective catalyst, which yields C-ol as the sole product. DCE showed to be a better solvent than dichloromethane (DCM), 1 DCE:1 MeOH mixture or acetonitrile (ACN). Since the FeIV(O)P+ is capable of abstracting hydrogen atom from DCM, MeOH or ACN, the solvent competes with the substrate. Presence of O2 lowers the yield of C-ol, as it can further oxidize this alcohol to carboxylic acid in the presence of radicals. Presence of H2O also causes a decrease in the yield, since it converts the active species I into FeIV(OH)P, which cannot oxidize cyclohexane. Addition of excess imidazole or OH− to the system results in a decrease in the yield of C-ol, due to the formation of the hexaccordinated complexes Fe(TDCPP)Im+2 (low-spin, β2 = 2.5 × 108 mol−2 l2) and Fe(TDCPP)(OH)−2 (high-spin, β2 = 6.3 × 107 mol−2 l2). The formation of both Fe(TDCPP)Im+2 and Fe(TDCPP)(OH)−2 complexes were confirmed by EPR studies. The catalytic activities of Fe(TDCPP)Cl and Fe(TFPP)Cl were compared. The unusually high yields of C-ol with Fe(TFPP)Cl obtained when ultrasound, DCM and O2 atmosphere were used, suggest that a parallel mechanism involving the μ-oxo dimer form, O2 and radicals may also be occurring with this FeP, besides the PhIO mechanism.
 
Article
In this work we have made use of the study of the interaction between Fe(TDCPP)+ and the axial ligands OH− and imidazole in order to help characterize the heterogenized catalysts Fe(TDCPP)SG and Fe(TDCPP)IPG through UV–VIS and EPR spectroscopies and thus, better understand their different catalytic activity in the oxidation of cyclohexane by PhIO. We have found out that in Fe(TDCPP)SG (containing 1.2×10−6 mol Fe(TDCPP)+/g of support), the FeP bis-coordinates to silica gel through Fe–O coordination and it is high-spin FeIIIP species. In Fe(TDCPP)IPG 1 (containing 1.1×10−6 mol Fe(TDCPP)+ and 2.2×10−4 mol imidazole/g of support), the FeP is bis-ligated to imidazole propyl gel through Fe-imidazole coordination and using NO as a paramagnetic probe, we present evidence that Fe(TDCPP)+ is present as a mixture of low-spin FeIIIP and FeIIP species. This catalyst led to a relative low yield of cyclohexanol (25%) because the bis-coordination of the FeIIIP to the support partially blocks the reaction between Fe(TDCPP)+ and PhIO, thus leading to the formation of only a small amount of the active species FeIV(O)P·+, while the FeIIP species do not react with the oxygen donor. Increasing the amount of Fe(TDCPP)+ and decreasing the amount of imidazole in the support led to the obtention of high-spin FeIIIP EPR signals in the spectra of Fe(TDCPP)IPG 5 (containing 4.4×10−6 mol Fe(TDCPP)+ and 2.2×10−5 mol imidazole/g of IPG), together with low-spin FeIIIP species. This latter catalyst led to better cyclohexanol yields (67%) than Fe(TDCPP)IPG 1. Fe(TDCPP)IPG 5 was further used in a study of the optimization of its catalytic activity and in recycling experiments in the optimized conditions. Recycling oxidation reactions of Fe(TDCPP)IPG 5 led to a total turnover number of 201 and total cyclohexanol yield of 201%, which could not be attained with Fe(TDCPP)Cl in homogeneous solution (turnover=96) due to the difficulty in recovering and reusing it.
 
Article
Coupling polymerization of 2,6-dimethylphenol to poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was carried out in an 1:1 aqueous–chloroform biphasic mixture using Cu–surface active ligand complex catalysts. Bidentate ligands were superior to monodentate ligands in promoting the oxidative coupling possibly due to their higher surface-activity and ability to form a binuclear complex with two metal ions. In particular, N,N-dibutylethylenediamine complex of copper gave the highest reaction rates and produced a polymer with an average molecular weight of 48,000 in a 95% yield. Spontaneous phase separation induced by the surface-active ligand made it possible to recover the catalyst complex segregated at the middle emulsion layer in a compact form. Biphasic reaction with a suitable surface-active ligand was found to be more effective than homogeneous counterpart to promote the oxidative coupling reaction and was more favorable for forming higher molecular weight polymer products.
 
Article
The preparation of VOPO4·2H2O is described and discussed. Three samples of the dihydrate are prepared with different ageing times following the initial reflux of V2O5 with H3PO4 in water for 24h. The materials were characterised using a combination of powder XRD, BET surface area measurement, laser Raman spectroscopy and scanning electron microscopy. A sample of VOPO4·2H2O was isolated by immediate filtration of the reaction mixture and comprised flat oval crystallites with a broad size distribution between ca. 2 and 20μm in diameter. Materials isolated following ageing of the initial reaction mixture (20°C, 24h) comprise square platelets again with a very broad size distribution. Using pyrophosphoric acid as the phosphorus source in place of phosphoric acid also affected the morphology of the VOPO4·2H2O. The dihydrates were reacted with isobutanol to form VOHPO4·0.5H2O and these were transformed to (VO)2P2O7 by reaction with 1.7% n-butane in air at 400°C. The most active catalyst was derived from VOPO4·2H2O prepared from ageing a reaction mixture following the removal of the first crop of crystals. The study shows that the method of preparation of VOPO4·2H2O and, in particular, its morphology is of importance in the preparation of vanadium phosphate catalysts using the two stage method based on the reaction of the dihydrate with an alcohol to form the hemihydrate precursor.
 
Article
This work presents the results of theoretical and experimental characterization of supported (nBuCp)2ZrCl2 on organosilanes pre-treated SiO2. Several possibilities are analyzed and considered as an attempt to explain the observed zirconocene loadings, catalyst performance in ethylene polymerization and resulting polymer properties of these supported zirconocenes. The effect of silica pre-treatment with Ph3SiCl, Me3SiCl and Me2HSiCl on the immobilization of (nBuCp)2ZrCl2 was modeled. The main considered aspects were the role of the surface concentration of the organosilanes, the presence or absence of residual chlorine and the possibility of interactions between the organosilanes and the zirconocene. All these facts might afford different surface species at the impregnation step prior to the MAO activation during the polymerization reaction.
 
Article
In the presence of the cobaloxime(II) catalyst [Co(Hdmg)2(Ph3P)2], at room temperature and 1 atm O2 or air, the dihydroxystilbene derivative referred to in the title (H2StQ) undergoes oxidative dehydrogenation to the corresponding stilbenequinone (StQ), and parallel oxidative cleavage at the CC double bond to afford 2,6-di-tert-butyl-4-hydroxybenzaldehyde (Ald). The two products are formed via a common anion radical intermediate stable enough for detection by ESR spectroscopy. A detailed kinetic analysis has been carried out by volumetric, spectrophotometric and HPLC techniques. The observed kinetic behavior is consistent with the formation of a superoxocobaloxime(III) intermediate, which generates the free radical intermediate from H2StQ. In the steady state, the catalyst is present in a hydroxo-cobaloxime(III) precursor state, from which it is released via reduction to cobaloxime(II) by the anion radical.
 
Top-cited authors
Majid M. Heravi
  • Alzahra University
Mannepalli lakshmi Kantam
  • Tezpur University
Kulamani Parida
  • ITER, Siksha 'O' Anusandhan University
Yong-Wang Li
  • Chinese Academy of Sciences
Alfons Baiker
  • ETH Zurich