Correlation between reduction potential of vanadium-containing heteropolyacid (HPA) catalysts and their oxidation activity for cyclohexanol dehydrogenation

Vanadium-containing H_6+xP₂Mo_18−xV_xO₆₂ (x = 0, 1, 2 and 3) Wells-Dawson heteropolyacid (HPA) and H_3+xPMo_12−xV_xO₄₀ (x = 0, 1, 2 and 3) Keggin HPA catalysts were applied to the vapor-phase dehydrogenation of cyclohexanol. The catalytic oxidation activity showed a volcano-shaped curve with respect to vanadium substitution for both families of HPA catalysts. The Wells-Dawson HPA showed a better catalytic oxidation performance than the Keggin HPA at the same level of vanadium substitution.     

Molybdovanadophosphoric Acid Catalyzed Oxidation of Hydrocarbons by H2O2 to Oxygenates

Heteropoly acids of the general formula H_3+x[PMo_12-xV_xO₄₀] (where x = 1,2,3) catalyzed the oxidation of aromatic hydrocarbons at 65°C with H₂O₂ to give oxygenated products. Among the catalysts, H₄[PMo₁₁VO₄₀] was found to be a more active catalyst and its activities have been reported in the oxidation of cyclohexane, methyl cyclohexane, naphthalene, 1-methyl naphthalene and biphenyl.     

Reductive amination of cyclohexanol/cyclohexanone to cyclohexylamine using SBA-15 supported copper catalysts

Amination of cyclohexanol was investigated in vapour phase over copper catalysts supported on mesoporous SBA-15. The different products identified during reductive amination of cyclohexanol reaction were cyclohexanone, cyclohexylamine, along with small amounts of N-Cyclohexylidinecyclohexylamine and dicyclohexylamine. Among several catalysts tested for the reductive amination, 5% Cu supported on SBA-15 exhibited better catalytic performance than other catalysts with 36% selectivity towards cylclohexylamine at 80% cyclohexanol conversion. The optimum reaction conditions employed to achieve the best catalyst performance were at 250 °C, 0.1 MPa of H₂/NH₃, TOS-10h. The active Cu sites, acidity of the catalyst, and effect of reaction parameters play a pivotal role in the reductive amination reaction. The prepared catalysts were characterized by XRD, BET, SEM, H₂-TPR and NH₃-TPD. The dispersion of Cu, particle size, and metal surface area (m²/g) calculated from pulse N₂O decomposition method. TPR findings reveal the presence of substantially dispersed copper oxide species at lower loadings which is easily reducible than the bulk copper oxide species found at higher Cu loadings. The acidity measurements by NH₃-TPD analysis suggest that the maximum acidic strength was obtained at 5 wt% copper on porous SBA-15, and decreased with Cu loadings. The catalytic properties are well in agreement with the findings of catalysts characterization.     

Polyferroorganosiloxanes as catalysts of oxidation processes

Polyferroorganosiloxanes were studied as catalysts for homogeneous oxidation of alkanes by hydrogen peroxide tinder mild conditions. In the oxidation of cyclohexane the catalysts are characterized by high efficiency (conversion of hydrogen peroxide is 25%) and stability Kip to 80 cycles per gat NJ The min product of the oxidation W do presence a 2,4,6-tri-tert-btitylphenol is cyclohexanol (tip to 35% per H₂O₂).     

Oxidations by the system ‘hydrogen peroxide-[Mn2L2O3][PF6]2 (L=1,4,7-trimethyl-1,4,7-triazacyclononane)-carboxylic acid’. Part 10: Co-catalytic effect of different carboxylic acids in the oxidation of cyclohexane, cyclohexanol, and acetone

Hydrogen peroxide oxidation of cyclohexane in acetonitrile solution catalyzed by the dinuclear manganese(IV) complex [LMn(O)₃MnL](PF₆)₂ (L=1,4,7-trimethyl-1,4,7-triazacyclononane, TMTACN) at 25 °C in the presence of a carboxylic acid affords cyclohexyl hydroperoxide as well as cyclohexanone and cyclohexanol. A kinetic study of the reactions with participation of three acids (acetic acid, oxalic acid, and pyrazine-2,3-dicarboxylic acid, 2,3-PDCA) led to the following general scheme. In the first stage, the catalyst precursor forms an adduct. The equilibrium constants K₁ calculated for acetic acid, oxalic acid, and 2,3-PDCA were 127±8, (7±2)×10⁴, and 1250±50 M⁻¹, respectively. The same kinetic scheme was applied for the cyclohexanol oxidation catalyzed by the complex in the presence of oxalic acid. The oxidation of cyclohexane in water solution using oxalic acid as a co-catalyst gave cyclohexanol and cyclohexanone, which were rapidly transformed into a mixture of over-oxidation products. In the oxidation of cyclohexanol to cyclohexanone, varying the concentrations of the reactants and the reaction time we were able to find optimal conditions and to obtain the cyclohexanone in 94% yield based on the starting cyclohexanol. Oxidation of acetone to acetic acid by the system containing oxalic acid was also studied.     

Synthesis of polyoxyethylene grafting nylon 6 and the selective separation of cyclohexane/cyclohexanone/cyclohexanol mixture through its membranes

A polymer membrane having polyoxyethylene grafting nylon 6 was prepared by reacting of nylon 6 and ethylene oxide. The chemical compositions of the polyoxyethylene grafting nylon 6 were determined by ¹H NMR. Degree of substitution for amide group, x, and degree of polymerization for polyoxyethylene, n, in bulk polymerization at 80°C for 4–9.5 h were evaluated: x = 0.32 ± 0.01–0.56 ± 0.02 and n = 2.8 ± 0.1–6.0 ± 0.3. The polyoxyethylene grafting nylon 6 membrane showed a selective separation of cyclohexanol from a cyclohexane/cyclohexanone/cyclohexanol mixture by a pervaporation technique. The FTIR and flux analyses verified that the selectivity for cyclohexanol was attributed to the hydrogen-bonding interaction between hydroxyl group in cyclohexanol and the hydroxyl group in polyoxyethylene grafting chain. The pervaporation and an adsorption experiment of cyclohexanol through the present membrane showed that hydroxyl group in graft chain acted as a carrier for cyclohexanol.     

Effect of substitution of Fe3+ in CuCr2O4 matrix for the hydrogenation of nitrobenzene

The hydrogenation of nitrobenzene to aniline over reduced Cu(Fe_xCr_2–x)O₄ series of catalysts (where x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) has been studied at 250°C in a fixed bed flow type reactor. All the catalysts were characterized by XRD, IR and DRS analysis and by measurements of electrical conductivity and surface area. The conversion of nitrobenzene to aniline is maximum over the catalysts with composition x=0.4. By comparing the results of reversible and irreversible adsorption of carbon monoxide with hydrogenation activity, it can be concluded that univalent copper at octahedral sites is more active for hydrogenation than metallic copper. The second cations [Cr(III) or Fe(III)] develop their catalytic activity by sharing anionic vacancies.     

Chemoselective hydrogenation of phenol to cyclohexanol using heterogenized cobalt oxide catalysts

Cobalt oxide nanoparticles (NPs) supported on porous carbon (CoO_x@CN) were fabricated by one-pot method and the hybrids could efficiently and selectively hydrogenate phenol to cyclohexanol with a high yield of 98%.     

Selective oxidation of methane to formaldehyde by oxygen over silica-supported iron catalysts (Special column of methane transformation, Article)

FeO_x-SiO₂ catalysts prepared by a sol-gel method were studied for the selective oxidation of methane by oxygen. A single-pass formaldehyde yield of 2.0% was obtained over the FeO_x-SiO₂ with an iron content of 0.5 wt% at 898 K. This 0.5 wt% FeO_x-SiO₂ catalyst demonstrated significantly higher catalytic performances than the 0.5 wt% FeO_x/SiO₂ prepared by an impregnation method. The correlation between the catalytic performances and the characterizations with UV-Vis and H₂-TPR suggested that the higher dispersion of iron species in the catalyst prepared by the sol-gel method was responsible for its higher catalytic activity for formaldehyde formation. The modification of the FeO_x-SiO₂ by phosphorus enhanced the formaldehyde selectivity, and a single-pass formaldehyde yield of 2.4% could be attained over a P-FeO_x-SiO₂ catalyst (P/Fe = 0.5) at 898 K. Raman spectroscopic measurements indicated the formation of FePO₄ nanoclusters in this catalyst, which were more selective toward formaldehyde formation.     

Co-doped MnCeOx/ZrO2 catalysts for low temperature selective catalytic reduction of NO

Co-doped MnCeO_x/ZrO₂ catalysts were synthesized by impregnation method and their low temperature deNOx performance were evaluated. The physicochemical properties of the catalysts were studied. The results showed that the doped Co could promote the deNOx performance of MnCeO_x/ZrO₂ significantly, and the doped catalyst with the Co/Mn molar ratio of 1:2 possessed the best catalytic performance. Compared with pure MnCeO_x/ZrO₂ catalyst, the deNOx efficiency of the optimal 1Co₂MnCeO_x/ZrO₂ was higher to 93% at 100 °C, improved nearly by 17%. The complete removal of NO was achieved at the temperature range of 120–250 °C. The promoted catalytic performance of Co-doped MnCeO_x/ZrO₂ catalyst was mainly attributed to the improvement of the catalyst support structure and surface acidity by Co. The catalytic reaction of NO with NH₃ over 1Co2MnCeO_x/ZrO₂ catalyst follows both Eley–Rideal mechanism and Langmiur–Hinshelwood mechanism.

     

Structure, reduction and catalytic properties of perovskite-like LaMn1?xNixO3 oxides

A series of LaMn_1–xNi_xO₃ (x=0–1.0) catalysts with perovskite structure has been prepared. The relationship among composition, crystal structure, reducilibity and the catalytic activities for CO oxidation has been investigated. XRD showed that samples for x=0.0–0.4 and x=0.8–1.0 had rhombohedral symmetry, and then pseudo-cubic symmetry for x=0.5–0.8. Research by IR and TPR indicated there were interactions between Mn and Ni coexisting in the B site of LaMn_1–xNi_xO₃. The oxidation activities of the catalysts were also measured.     

Cyclohexanol dehydrogenation over Cu-loaded TiO2 photocatalyst under solar illumination

Abstract  

The liquid-phase selective dehydrogenation of cyclohexanol has been investigated using two classes of catalyst containing either Cu₂O or CuO on TiO₂ under solar light in deaerated conditions at room temperature using acetonitrile medium in a batch reactor. The effect on dehydrogenation of three conditions, cyclohexanol concentration, copper loading on TiO₂, and amount of catalyst, were investigated. The maximum yield of cyclohexanone obtained was 40%. The catalysts were characterized by XRD, UV–DRS, TEM, SEM–EDAX, and XPS. It was found that 1% (w/w) Cu₂O/TiO₂ was 100% selective for photocatalytic dehydrogenation of cyclohexanol.     

The metal complexes of new Schiff bases containing phosphonate groups and catalytic properties for alkane oxidation

In this study, two new salicylidene phosphonate ligands (HL¹ and HL²) and their metal complexes (Cu²⁺, VO²⁺ and La³⁺) were synthesized and characterized by spectroscopic and analytical methods. The molecular structure of the ligand HL¹ was determined by single‐crystal X‐ray diffraction study. In the structure of the ligand, there is an intramolecular phenol‐imine hydrogen bond. The synthesized compounds exhibit only one emission maximum upon excitation at 270–295  nm range. Complexation of the Schiff base ligands with metal ions did not cause a considerable quenching effect. Finally, the complexes prepared were used as catalysts in cyclohexane oxidation under microwave irradiation. The complexes showed high conversion rates (> 90%) for cyclohexane oxidation; however, poor selectivity was observed for all complexes. The La³⁺ complexes showed better selectivity for cyclohexane → cyclohexanol transformation with about 45% selectivity.     

Apparent Dipole Moments of 1-Butanol, 1-Propanol, and Chlorobenzene in Cyclohexane or Benzene, and Excess Molar Volumes of (1-Propanol or Chlorobenzene + Cyclohexane or Benzene) at T = 298.15K

Apparent dipole moments and relative permittivities of {x1-butanol + (1 – x) cyclohexane}, {x1-propanol + (1 – x)cyclohexane or (1 – x)benzene} and {xchloro- benzene + (1 – x)cyclohexane or (1 – x)benzene} were determined for the mole fraction range of 0.0003 < x < 0.1 at a temperature of T = 298.15 K and at a frequency of f = 100 kHz. The apparent dipole moments were calculated using Frohlich equation. The molar excess volumes for {x1-propanol + (1 – x)cyclohexane or (1 – x) benzene} and {xchlorobenzene + (1 – x)cyclohexane} were determined by a vibrating-tube densimeter at T = 298.15 K.     

Alkane Oxidation by an Immobilized Nickel Complex Catalyst: Structural and Reactivity Differences Induced by Surface‐Ligand Density on Mesoporous Silica

Immobilized nickel catalysts SBA*‐ L ‐x/Ni ( L =bis(2‐pyridylmethyl)(1H‐1,2,3‐triazol‐4‐ylmethyl)amine) with various ligand densities ( L content (x)=0.5, 1, 2, 4 mol % Si) have been prepared from azidopropyl‐functionalized mesoporous silicas SBA‐N₃‐x. Related homogeneous ligand LtBu and its Ni^II complexes, [Ni( LtBu )(OAc)₂(H₂O)] ( LtBu /Ni) and [Ni( LtBu )₂]BF₄ (2 LtBu /Ni), have been synthesized. The L /Ni ratio (0.9–1.7:1) in SBA*‐ L ‐x/Ni suggests the formation of an inert [Ni L ₂] site on the surface at higher ligand loadings. SBA*‐ L ‐x/Ni has been applied to the catalytic oxidation of cyclohexane with m‐chloroperbenzoic acid (mCPBA). The catalyst with the lowest loading shows high activity in its initial use as the homogeneous LtBu /Ni catalyst, with some metal leaching. As the ligand loading increases, the activity and Ni leaching are suppressed. The importance of site‐density control for the development of immobilized catalysts has been demonstrated.     

Pt and PtRh nanowire electrocatalysts for cyclohexane-fueled polymer electrolyte membrane fuel cell

Electrochemical dehydrogenative oxidation of cyclohexane to benzene is studied over Pt and Pt₁Rh₁ nanowire electrocatalysts fabricated by electrospinning method, which shows the higher catalytic activities in a polymer electrolyte membrane fuel cell anode than the conventional Pt nanoparticle catalysts such as carbon-supported Pt or Pt black. The improved performances over the Pt₁Rh₁ nanowire electrocatalyst can be rationalized by enhanced electrical property and pertinent interface formation with nanowire catalysts in the high Pt-loaded cyclohexane fuel cell system.     

Transformations of cyclohexane and benzene on the bimetallic Ru-Pt oxide catalysts

The bimetallic Ru-Pt/Al₂O₃ catalysts with an overall metal content of 1 wt. % and Pt: Ru weight ratios from 1: 3 to 3: 1 were studied. The catalytic activity for cyclohexane and benzene transformations, including hydrogenation, hydrogenolysis, and skeletal isomerization of the initial substrates and products of intermediate transformations, was studied at temperatures 180–350 °C and H₂ pressures from 1.0 to 5.0 MPa. The maximum yield of n-hexane from cyclohexane and benzene was obtained on the catalysts with a ruthenium content of 0.75–1.0%, being ∼29–30 wt.% at 40% selectivity. The selectivity to form n-hexane decreases with an increase in the cyclohexane conversion and is almost independent of the composition of the Ru-Pt system. On the catalysts under study, benzene is converted to the same products but at temperatures by 60 °C lower as compared to cyclohexane conversion. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 633–637, April, 2006.     

Thermodynamic aspects of the Ru(III)—EDTA—ascorbate—molecular oxygen system for the oxidation of saturated and unsaturated organic compounds

The activation and thermodynamic parameters corresponding to rate and equilibrium constants, respectively, for the homogeneous oxidation of the saturated substrates, cyclohexane to cyclohexanol, cyclohexanol to cis-1,3-cyclohexane diol and olefin, cyclohexene to epoxide by Ru(III)—EDTA—ascorbateO₂ system were determined by measuring the various rates and equilibrium constants at four different temperatures in the range 288–313 K and μ = 0.1 M KNO₃ in a 50% (V/V) mixture of 1,4-dioxane and water in acidic medium. The kinetics of the oxidation of these substrates at each particular temperature was studied as a function of the concentration, the substrates, hydrogen ion, catalyst, ascorbic acid and molecular oxygen. The orders of the reaction in cyclohexanol and cyclohexene concentrations are one, and those in cyclohexane and hydrogen ion concentration are fractional and inverse first-order, respectively. For all substrates the reaction is first order with respect to the concentrations of molecular oxygen, ascorbic acid and catalyst. The source of the oxygen atom transferred to the substrates was confirmed by ¹⁸O₂ isotope studies in which the ¹⁸O was incorporated in the oxidized products. The kinetics and solvent isotope effect were studied for the oxidation of C₆H₁₂, C₆D₁₂, C₆H₁₁OH and C₆D₁₁OD. The order of the reactivity observed in the oxidation of the substrates studied is cyclohexene > cyclohexanol > cyclohexane. A comparison of the rates of oxidation of the substrates and the corresponding activation parameters with the catalytic systems Ru(III)—EDTAO₂ and Ru(III)—EDTA—ascorbateH₂O₂ indicated that activation parameters become more favourable in the presence of ascorbic acid, where the system acts as a mono-oxygenase and the activation energies are drastically reduced. Highly negative entropies are associated with all oxygen atom transfer reactions, indicating that the oxidation process is associative in nature.     

Vapor phase reduction of cyclohexanone to cyclohexanol on CexZr1-xO2 solid solutions

Reduction of cyclohexanone to cyclohexanol using propane-2-ol as hydrogen donor has been carried out in vapor phase on Ce_xZr_1-xO₂ solid solutions synthesized by ombustion synthesized at 302°C. The solid solutions around 0.4 mol% cerium content show better catalytic activity compared to pure ZrO₂ and the selectivity to cyclohexanol is 98%. A moderate acid-base and good redox properties of Ce_xZr_1-xO₂ solid solutions are seen to be responsible for the catalytic activity. A possible mechanism of hydride transfer has been proposed with cerium ions as promoters. This revised version was published online in June 2006 with corrections to the Cover Date.     

Microwave effects on the oxidative coupling of methane over Bi2O3-WO3 oxygen ion conductive oxides

The oxidative coupling of methane over (Bi₂O₃)_1-x(WO₃)_x (x=0.2, 0.3, 0.4) oxygen ion conductive oxide catalysts irradiated by microwave has been studied. Compared with a conventional heating mode, the temperature of the catalytic bed is much lower with microwave irradiation and there is a change in selectivity favoring the production of C₂ products.