Oxidative Reaction Conditions Research Articles

Toluene Oxyfunctionalization with Air over Metalloporphyrins and Reaction Condition Optimization

Catalytic activities of a series of metalloporphyrin complexes in selective aerobic oxidation of toluene were investigated. The effects of different central metal ions in metalloporphyrins [T( p-Cl)PPMCl (M=Fe,Co,Mn,Cu)] on the reaction course had been examined and it was found that T( p-Cl)PPCu presented the highest catalytic activity in the reaction. The reaction conditions of toluene oxidation were optimized by using orthogonal experiment design. Five relevant factors were investigated: temperature, air pressure, catalyst loading, air flow rate and reaction time. The effects of the five factors on both toluene conversion and total yield of benzaldehyde and benzyl alcohol were discussed. The research results showed that the reaction temperature was the most significant factor influencing toluene oxidation. On the basis of the margin analysis, the optimum conditions for the toluene conversion and the total yield of benzaldehyde and benzyl alcohol respectively were achieved, under which the toluene conversion was up to 14.67% and the total yield of benzaldehyde and benzyl alcohol reached 5.89%.

Synthesis, characterization, and thermal stability of azomethine oligomer and its metal complexes

AbstractThe oxidative polycondensation reaction conditions of N,N′‐bis[(2‐hydroxy‐1‐naphthyl)methylene]urea (2‐HNMU) has been accomplished using NaOCl, H2O2, and air O2 oxidants in an aqueous alkaline medium. The structures of the obtained monomer and oligomer were confirmed by FTIR, UV–vis, 1H NMR, 13C NMR, and elemental analysis. The characterization was made by TG‐DTA, size exclusion chromatography (SEC), and solubility tests. At the optimum reaction conditions, the yield of oligo‐N,N′‐bis[(2‐hydroxy‐1‐naphthyl)methylene]urea (O‐2‐HNMU) was found to be 95% (for air O2 oxidant), 51% (for H2O2 oxidant), 96% (for NaOCl oxidant). According to the SEC analysis, the number‐average molecular weight (Mn), weight‐average molecular weight (Mw), and polydispersity index values of O‐2‐HNMU was found to be 1036, 1225 g/mol, and 1.182, respectively, using H2O2, and 765, 1080 g/mol, and 1.412, respectively, using air O2, and 857, 1105 g/mol, and 1.289, respectively, using NaOCl. TG‐DTA analyses showed that O‐2‐HNMU was more stable than 2‐HNMU. According to TG analyses, the carbonaceous residue of 2‐HNMU and O‐2‐HNMU was found to be 0.49% and 2.11% at 1000°C, respectively. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Polymer-anchored vanadium(IV), molybdenum(VI) and copper(II) complexes of bidentate ligand as catalyst for the liquid phase oxidation of organic substrates

Monobasic bidentate ligand 2-(2′-hydroxyphenyl)benzimidazole (Hhpbmz) has been covalently bonded to the chloromethylated polystyrene cross-linked with 5% divinylbenzene. Treatment of the resulted chelating resin, abbreviated as PS-Hhpbmz, with [VO(acac) 2], [MoO 2(acac) 2] (Hacac = acetylacetone) and Cu(CH 3COO) 2·H 2O, gave polymer-anchored complexes PS-[VO(hpbmz) 2], PS-[MoO 2(hpbmz) 2] and PS-[Cu(hpbmz) 2], respectively. The corresponding neat complexes with Hhpbmz have also been prepared similarly. Structures of these complexes have been established on the basis of elemental analyses, scanning electron micrographs and spectroscopic (infrared and electronic) as well as thermogravimetric studies. These complexes have been tested as catalyst for the oxidation of styrene, ethylbenzene and methyl phenyl sulfide. Reaction conditions for the maximum oxidation of these substrates have been optimised by considering the concentration of oxidant, amount of catalyst, volume of solvent and temperature of the reaction mixture. Under the optimised conditions styrene gave a maximum of 58.9% conversion with five reaction products namely styrene oxide, benzaldehyde, 1-phenylethane-1,2-diol, benzoic acid, and phenylacetaldehyde. Oxidation of ethylbenzene gave 47.7% conversion where benzaldehyde, phenyl acetic acid, styrene, 1-phenylethane-1,2-diol and benzoic acid have been obtained as major oxidation products. A maximum of 76.5% conversion of methyl phenyl sulfide, catalysed by PS-[VO(hpbmz) 2], has been achieved. Catalyst, PS-[MoO 2(hpbmz) 2] has shown comparable (75.1%) catalytic activity while only 44.8% conversion has been achieved with PS-[Cu(hpbmz) 2]. Irrespective of their catalytic performances, the selectivity for the formation of sulfoxide is almost similar (ca. 75%) for all these catalysts. These catalysts do not leach metal ions during catalytic activity and are recyclable.

Synthesis, characterization, thermal analysis, and band gap of oligo‐2‐methoxy‐6‐[(4‐methylphenyl)imino]methylphenol

AbstractThe oxidative polycondensation reaction conditions of 2‐methoxy‐6‐[(4‐methylphenyl)imino]methylphenol (2M‐4‐MPIMP) by using oxidants such as air O2, H2O2 and NaOCl were studied in an aqueous alkaline medium between 30 and 90°C. The structures of synthesized monomer and oligomer were confirmed by FTIR, UV–vis, 1H NMR, 13CNMR, and elemental analysis. The characterization was made by TG–DTA, size exclusion chromatography (SEC), and solubility tests. At the optimum reaction conditions, the yield of oligo (O)‐2M‐4‐MPIMP was 68% for air O2 oxidant, 68% for H2O2 oxidant, and 82% for NaOCl oxidant. According to the SEC analysis, the number– average molecular weight (Mn), weight–average molecular weight (Mw), and polydispersity index (PDI) values of O‐2M‐4‐MPIMP were found to be 695, 1000 g mol−1 and 1.439, using air O2, and 670, 795 g mol−1 and 1.187, using NaOCl, and 645, 790 g mol−1 and 1.225, using H2O2, respectively. TG–DTA analyses were shown to be stable of O‐2M‐4‐MPIMP against thermal decomposition. The weight losses of 2M‐4‐MPIMP and O‐2M‐4‐MPIMP were found to be 98.46% and 89.54% at 1000°C, respectively. Also, electrical conductivity of the O‐2M‐4‐MPIMP was measured, showing that the polymer is typical semiconductor. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of monomer and oligomer were determined from the onset potentials for n‐doping and p‐doping, respectively. Optical energy gaps (Eg) of 2M‐4‐MPIMP and O‐2M‐4‐MPIMP were calculated from UV–vis measurements. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

SYNTHESIS, CHARACTERIZATION AND ANTIMICROBIAL PROPERTIES OF 4-[(4-HYDROXYBENZYLIDENE) AMINO] PHENOL AND ITS POLYMER

The oxidative polycondensation reaction conditions of 4-[(4-hydroxybenzylidene) amino] phenol (4-HBAP) were studied with H2O2, air oxygen and NaOCl in an aqueous alkaline medium between 50 and 90°C. The structures of the obtained monomer and polymer were confirmed by FT-IR, UV-Vis, 1H- and 13C-NMR and elemental analysis. The characterization was made by TG-DTA, size exclusion chromatography (SEC) and solubility tests. At the optimum reaction conditions, the yield of poly[4-(4-hydroxybenzylidene amino) phenol] (P-4-HBAP) was found to be 48.3% (for H2O2 oxidant), 80.5% (for air O2 oxidant) and 86.4% (for NaOCl oxidant). According to the SEC analysis, the number-average molecular weight (Mn), weight-average molecular weight (Mw) and polydispersity index (PDI) values of P-4-HBAP was found to be 8950, 10970 g mol-1 and 1.225, respectively, using H2O2; and 11610, 15190 g mol-1 and 1.308 respectively, using air O2 and 7900, 9610 g mol-1 and 1.216, respectively, using NaOCl. According to TG-DTA analyses, P-4-HBAP was more stable than 4-HBAP against thermal decomposition. The weight loss of P-4-HBAP was found to be 49.27% at 1000°C. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) values calculated from electrochemical measurement. Electrochemical energy gaps (Eg′) of 4-HBAP and P-4-HBAP were found to be -5.46, -5.28; -2.26, -2.67; 3.20 and 2.61 eV, respectively. According to UV-Vis measurements, optical band gap (Eg) of 4-HBAP and P-4-HBAP were found to be 3.34 and 3.01 eV, respectively. Also, antimicrobial activities of 4-HBAP and P-4-HBAP were examined against selected some bacteria. The electrical conductivity of the polymer was measured after doping with iodine.

SOx Storage Materials under Lean−Rich Cycling ConditionsPart II: Influence of Pt, H2O, and Cycling Time

The role of Pt and the influence of the reaction conditions during lean-rich cycling experiments were studied on a second generation SOx trapping material. The combination of the Generalized 2-D Correlation Analysis, 2-D Sample-Sample Correlation Analysis, and Factor Analysis using the MCR-ALS technique was applied to identify the reactive species. Transient surface sulfate species were formed under oxidative reaction conditions (lean mode) and decomposed under reducing reaction conditions (rich operation mode). The reduction of this species was identified to be the main contribution to the SO2 release observed under dynamic flow conditions. Pt facilitates the formation of sulfates but also catalyzes the reduction of the transient surface sulfate species leading to a higher amount of SO2 released under rich conditions. In the presence of water, this effect was diminished, which was found to be mainly a result of the suppressed formation of surface sulfate species caused by the faster transport of SO2 into the bulk phase of the SOx trapping component (BaCO3). Increasing the time under reducing conditions in the cycles leads to an enhanced reduction of the surface during rich conditions. The presence of water did not influence the bulk type species. It is proposed that for effective SO2 storage materials, strong SOx adsorption sites on the surface, the presence of water, and a short time under reducing conditions are essential.

Synthesis, characterization, and thermal degradation of oligo‐2‐(morpholinoiminomethyl)phenol and its Pb(II) complex compound

AbstractThe oxidative polycondensation reaction conditions of 2‐(morpholinoiminomethyl)phenol were studied with H2O2, air O2, and sodium hypochloride (NaOCl) oxidants in an aqueous alkaline medium between 40 and 90°C. The structure of oligo‐2‐(morpholinoiminomethyl)phenol was characterized with 1H‐ and 13C‐NMR, Fourier transform infrared, ultraviolet–visible, size exclusion chromatography, and elemental analysis techniques. Under the optimum reaction conditions, the yield of oligo‐2‐(morpholinoiminomethyl)phenol was 28% for the H2O2 oxidant, 12% for the air O2 oxidant, and 58% for the NaOCl oxidant. According to the size exclusion chromatography analysis, the number‐average molecular weight, weight‐average molecular weight, and polydispersity index of oligo‐2‐(morpholinoiminomethyl)phenol were 2420 g/mol, 2740 g/mol, and 1.187 with H2O2, 1425 g/mol, 2060 g/mol, and 1.446 with air O2, and 1309 g/mol, 1401 g/mol, and 1.070 with NaOCl, respectively. Thermogravimetry/dynamic thermal analysis showed that the oligo‐2‐(morpholinoiminomethyl)phenol–lead complex compound was more stable than 2‐(morpholinoiminomethyl)phenol and oligo‐2‐(morpholinoiminomethyl)phenol against thermal degradation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:3795–3804, 2006

Synthesis, characterization, and antimicrobial properties of oligo‐4‐[(pyridine‐3‐yl‐methylene) amino] phenol

AbstractThe oxidative polycondensation reaction conditions of 4‐[(pyridine‐3‐yl‐methylene) amino]phenol (4‐PMAP) were studied using H2O2, atmospheric O2, and NaOCl oxidants in an aqueous alkaline medium between 30°C and 90°C. Synthesized oligo‐4‐[(pyridine‐3‐yl‐methylene) amino] phenol (O‐4‐PMAP) was characterized by 1H‐, 13C NMR, FTIR, UV–vis, size exclusion chromatography (SEC), and elemental analysis techniques. The yield of O‐4‐PMAP was found to be 32% (for H2O2 oxidant), 68% (for atmospheric O2 oxidant), and 82% (for NaOCl oxidant). According to the SEC analysis, the number–average molecular weight, weight–average molecular weight, and polydispersity index values of O‐4‐PMAP was found to be 5767, 6646 g mol−1, and 1.152, respectively, using H2O2, and 4540, 5139 g mol−1, and 1.132, respectively, using atmospheric O2, and 9037, 9235 g mol−1, and 1.022, using NaOCl, respectively. According to TG and DSC analyses, O‐4‐PMAP was more stable than 4‐PMAP against thermal decomposition. The weight loss of O‐4‐PMAP was found to be 94.80% at 1000°C. Also, antimicrobial activities of the oligomer were tested against B. cereus, L. monocytogenes, B. megaterium, B. subtilis, E. coli, Str. thermophilus, M. smegmatis, B. brevis, E. aeroginesa, P. vulgaris, M. luteus, S. aureus, and B. jeoreseens. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3327–3333, 2006

On the Morphology and Stability of Au Nanoparticles on TiO2(110) Prepared from Micelle-Stabilized Precursors

The morphology and stability of well-ordered, nanostructured Au/TiO2(110) surfaces, prepared by deposition of Au loaded micelles on TiO2(110) substrates and subsequent oxidative removal of the polymer shell in an oxygen plasma, was investigated by noncontact AFM, SEM and XPS. The resulting arrays of Au nanoparticles (particle sizes 1-5 nm) form a nearly hexagonal pattern with well-defined interparticle distances and a narrow particle size distribution. Particle size and particle separation can be controlled independently by varying the Au loading and the block-copolymers in the micelle shell. The oxygen plasma treatment does not affect the size and distance of the Au nanoparticles; the latter are fully metallic after subsequent UHV annealing (400 degrees C). The particles are stable under typical CO oxidation reaction conditions, up to at least 200 degrees C, making these surfaces ideally suited as defined model systems for catalytic studies. Significant changes in the height distributions of the Au nanoparticles are found upon 400 degrees C annealing in O2. For adlayers with small interparticle distances, this leads to a bimodal particle size distribution, which together with the preservation of the lateral order points to Ostwald ripening.

The Oxidative Polycondensation of 4-[(Phenylimino) Methyl] Phenol Using NaOCl, Air Oxygen, and H2O2 at Alkaline and Neutral Medium: Synthesis and Characterization

In this study, the oxidative polycondensation reaction conditions of 4-[(phenylimino) methyl] phenol (4-PIMP) with air oxygen, H2O2, and NaOCl oxidants in aqueous alkaline and neutral medium between 50° and 90°C were examined. 4-PIMP and its condensation product, oligo-4-[(phenylimino) methyl] phenol (O-4-PIMP), were characterized by 1H-13C-NMR, FT-IR, UV-vis techniques, and elemental analysis. At the optimum reaction conditions, the yield of oligo-4-[(phenylimino) methyl] phenol was found to be 73% for NaOCl, 65% for air O2, and 70% for H2O2. According to size exclusion chromatography (SEC) analyses, the number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) values of O-4-PIMP were 5268, 10320 g mol−1, and 1.959, respectively, using air O2; 8215, 17789 g mol−1, and 2.165, respectively, using NaOCl; and 4800, 5260 g mol−1, and 1.096, respectively, using H2O2. The carbon residues for 4-PIMP and O-4-PIMP were found to be 42.4% and 53.4%, respectively, at 1000°C by using TG-DTA analyses.

Synthesis, characterization, thermal stability, conductivity and band gap of oligo-4-[(2-hydroxybenzylidene)amino]benzoic acid

In this study, the oxidative polycondensation reaction conditions of 4-[(2-hydroxybenzylidene)amino]benzoic acid (4-HBAB) by using oxidants such as air O 2, H 2O 2 and NaOCl were studied in an aqueous alkaline medium between 40 and 90 °C. The structures of the synthesized monomer and oligomer were confirmed by FT-IR, UV–vis, NMR and elemental analysis. The characterization was made by TG-DTA, size exclusion chromatography (SEC) and solubility tests. At the optimum reaction conditions, the yield of oligo-4-[(2-hydroxybenzylidene)amino]benzoic acid (O-4-HBAB) was found to be 68% (for air O 2 oxidant), 70% (for air H 2O 2 oxidant) 53% (for NaOCl oxidant). According to the SEC analysis, the number-average molecular weight ( M n), weight-average molecular weight ( M w) and polydispersity index (PDI) values of O-4-HBAB were found to be 932, 1469 g mol −1 and 1.576, using H 2O 2, and 1895, 2560 g mol −1 and 1.354, using air O 2 and 2320, 3015 g mol −1 and 1.300, using NaOCl, respectively. According to TG analysis, the weight losses of 4-HBAB and O-4-HBAB were found to be 96.86% and 73.10% at 1000 °C, respectively. O-4-HBAB was shown higher stability against thermal decomposition. Also, electrical conductivity of the O-4-HBAB was measured, showing that the polymer is a typical semiconductor. Electrochemical HOMO, LUMO and energy gaps ( E g) of 4-HBAB and O-4-HBAB were found to be −6.28, −6.36; −2.39, −2.64; 3.89 and 3.72 eV, respectively. According to UV–vis measurements, optical band gap ( E g) of 4-HBAB and O-4-HBAB were found to be 3.23 and 3.09 eV, respectively.

A Bipyridyl Palladium Complex Covalently Anchored onto Silica as an Effective and Recoverable Interphase Catalyst for the Aerobic Oxidation of Alcohols.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis, Characterization, and Thermal Degradation of Oligo-2-[(4-Chlorophenyl) Imino Methylene] Phenol and Its Oligomer-Metal Complexes

The oxidative polycondensation reaction conditions of 2-[(4-chlorophenyl) imino methylene] phenol (CPIMP) were studied by air O2 and NaOCl oxidants at various temperatures and times. Optimum reaction conditions of air O2 and NaOCl were determined for CPIMP. Oligo-2-[(4-chlorophenyl) imino methylene] phenol (OCPIMP) was synthesized from the oxidative polycondensation of CPIMP with air O2 and NaOCl in alkaline medium between 50 and 90°C. The number-average molecular weight (Mn) weight-average molecular weight (Mw) and polydispersity index (PDI) values of OCPIMP were found to be 470 g mol−1, 895 g mol−1, and 1.90, using NaOCl, and 455 g mol−1, 765 g mol−1, and 1.68, using air O2, respectively. At the optimum reaction conditions, the yield of OCPIMP was found to be 62.80% (for air O2 oxidant) and 87.50% (for NaOCl oxidant). The OCPIMP was characterized by 1H-NMR, FT-IR, UV-Vis and elemental analysis. The thermogravimetric (TGA)-DTA analyses were shown to be stable of OCPIMP and its oligomer metal complexes (such as Co+2, Ni+2, and Cu+2) against thermo-oxidative decomposition. The weight loss of OCPIMP and its oligomer metal complexes (such as Co+2, Ni+2, and Cu+2) were found to be 98%, 85%, 80%, and 82%, respectively, at 1000°C.

A Bipyridyl Palladium Complex Covalently Anchored onto Silica as an Effective and Recoverable Interphase Catalyst for the Aerobic Oxidation of Alcohols

A new silica-based Pd(II) interphase catalyst 2 efficiently catalyzes the aerobic oxidation of various types of structurally diverse alcohols to the corresponding carbonyl compounds. The catalyst is also quite effective for the oxidation of primary benzylic alcohols under atmospheric pressure of air. The catalyst shows high thermal stability (up to 200 °C) and is also stable under the oxidation reaction conditions. To our knowledge this work is the first example of the application of a palladium complex covalently anchored to silica for the aerobic oxidation of alcohols.

6 + 3] Cycloaddition of Fischer aminocarbene complexes: An efficient annulation of fulvenes to indene derivatives

Alkenyl(amino)carbene chromium(0) complexes 1 undergo regioselective [6 + 3] cycloaddition to pentafulvenes 2, affording substituted dihydroindenes 5a– d and indene 6. In the case of 6,6-dimethylpentafulvene 2a dihydroindene chromium complexes 4a– c were also isolated. Isopropylfulvene 2b similarly produces 5d or, under oxidative reaction conditions, the indene 6. The reaction involves: (i) 1,2-nucleophilic addition of fulvene to the carbene carbon, (ii) [1,2]-Cr(CO) 5 migration, (iii) either metal–indene coordination or reductive metal elimination/aromatization.

Strained thin copper films as model catalysts in the materials gap

Thin copper films on silicon constitute model systems to investigate the influence of lattice strain on activity in heterogeneous catalysis. Thin copper films on silicon were investigated by ultraviolet photoelectron spectroscopy (UPS) to reveal the effect of strain in the copper films on the electronic structure of the surface. For cleaned and adsorbate-free surfaces, no effect of strain on the electronic structure was detected by UPS. Conversely, an oxygen-containing film exhibited a distinct effect of strain induced by cyclic heating and cooling on the electronic structure. Comparison with studies on a Cu single crystal under methanol oxidation reaction conditions revealed a characteristic hysteresis behavior in both the adsorbate structure and the catalytic properties of the metal surface. Hence, copper model systems that are suitable to unravel the correlation between strain and catalytic activity need to take the disordered microstructure of ‘‘real’’ copper catalysts into account. The present experiments reveal the correlation between surface restructuring and catalysis on the one side and the influence of lattice strain on either restructuring or the electronic structure of the surface on the other side.

Semi-quantitative method to evaluate the α-carbonyl content in lignin

A method to estimate the content of α-carbonyl structures in lignin was developed. This method consists of two successive treatments: NaBD4 treatment of pulp to reduce an α-carbonyl structure in lignin, and nitrobenzene oxidation. NaBD4 was used to convert an α-carbonyl structure to a deuterium-labeled hydroxymethine structure. The ratio of D-vanillin [(HO)(H3CO)C6H3CDO] to H-vanillin [(HO)(H3CO)C6H3CHO] or that of their syringyl analogues obtained by nitrobenzene oxidation was used as the measure of the content of α-carbonyl structure. Model experiments demonstrated that when sodium hydroxide was used as alkali for the nitrobenzene oxidation, the retention of deuterium at the side chain α-position was very low due to the displacement of deuterium with hydrogen by an unknown reaction mechanism. In order to depress this unexpected displacement, the reaction conditions of the nitrobenzene oxidation were modified. The modified nitrobenzene oxidation employs 0.5 mol/l of lithium hydroxide as a reaction medium instead of 2.0 mol/l sodium hydroxide. By this modification, this method could successfully trace the formation and the degradation of the α-carbonyl structure in milled wood lignins.

The Oxidative Polycondensation of Benzylidene-4′-hydroxyanilene using Air O2, NaOCl and H2O2: Synthesis and Characterization

In this work, the oxidative polycondensation reaction conditions of benzylidene-4′-hydroxyanilene (B-4′-HA) were studied using oxidants such as air O2, H2O2 and NaOCl in an aqueous alkaline medium between 40 and 95 ○C. Oligo-benzylidene-4′-hydroxyanilene was characterized by 1H-NMR, FT-IR, UV-Vis, size exclusion chromatography (SEC) and elemental analysis techniques. The solubility of oligomer using organic solvents such as DMF, THF, DMSO, methanol, ethanol, CHCl3, CCl4, toluene, acetonitrile, ethyl acetate was investigated. According to air O2 oxidant (flow rate 8.5 L/h), the conversion of B-4′-HA was 82.0% in optimum conditions such as [B-4′-HA]0=[KOH]0=0.1015 mol/L at 50 ○C for 25 h. According to the SEC analysis, the number-average molecular weight (Mn), weight-average molecular weight (Mw) and polydispersity index (PDI) values of O-B-4′-HA were found to be 1852 g mol−1, 3101 g mol−1 and 1.675; 2123 g mol−1, 4073 g mol−1 and 1.919; 2155 g mol−1, 4164 g mol−1 and 1.932, using air oxygen, NaOCl and H2O2 oxidants, respectively. Also, Thermo gravimetric analysis (TGA) showed oligo-benzylidene-4′-hydroxyanilene to be unstable against thermo-oxidative decomposition. The weight loss of O-B-4′-HA was found to be 95.87% at 1000 ○C.

In situ investigations of the bulk structural evolution of vanadium-containing heteropolyoxomolybdate catalysts during thermal activation

The bulk structural evolution of a vanadium-containing heteropolyoxomolybdate (HPOM), H 4[PVMo 11O 40] × 13H 2O, with vanadium substituting for Mo in the Keggin ion, was studied under reducing (propene) and partial oxidation reaction conditions (propene and oxygen) by in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) combined with mass spectrometry. During treatment in propene, the loss of crystal water in the temperature range from 373 to 573 K was followed by a partial decomposition, reduction of the average Mo valence, and formation of a characteristic cubic HPOM at 573 K. This behavior is similar to the structural evolution of H 3[PMo 12O 40] ×13H 2O during treatment in propene. The formation of cubic Mo x [PVMo 11− x O 40] with Mo centers on extra Keggin framework positions and V centers remaining in the lacunary Keggin ion coincides with the onset of catalytic activity at ∼ 573 K. Detailed investigations of the local structure around the vanadium centers in Mo x [PVMo 11− x O 40] have made it possible to propose a model for the geometric structure of the active site in Mo- and V-containing metal oxide catalysts. The cubic Mo x [PVMo 11− x O 40] phase prepared from H 4[PVMo 11O 40] × 13H 2O is stable in propene and oxygen up to ∼ 620 K and exhibits an onset of activity at ∼ 573 K. This onset of activity is correlated to characteristic changes in the average local Mo structure, indicating a reversible transition from the reduced state of the active site in Mo x [PVMo 11− x O 40] to an oxidized state under propene oxidation reaction conditions.

In situ bulk structural study on solid-state dynamics and catalytic activity correlations of a H4[PNbMo11O40] partial oxidation catalyst

Abstract We report on the preparation and characterization of a niobium containing Keggin type heteropolyoxomolybdate (HPOM), H4[PNbMo11O40] · 13 H2O, with Nb substituting Mo in the Keggin ion. H4[PNbMo11O40] · 13 H2O exhibits a long-range order structure similar to that of H3[PMo12O40] · 13 H2O. The structural evolution of H4[PNbMo11O40] · 13 2O was studied under reducing (propene) and partial oxidation reaction conditions (propene and oxygen) by in situ X-ray diffraction and X-ray absorption spectroscopy combined with mass spectrometry and corroborated by thermogravimetric analysis. Following the loss of crystal water in the temperature range from 373 K to 573 K, a partial decomposition and reduction of the molybdenum and the formation of a characteristic cubic HPOM is observed at 573 K, similar to the structural evolution of H3[PMo12O40] · 13 H2O and H4[PVMo11O40] · 13 H2O during treatment in propene. However, in contrast to H3[PMo12O40] · 13 H2O and H4[PVMo11O40] · 13 H2O, the cubic HPOM decomposes rapidly and the MoO3 formed is subsequently reduced to MoO2. Nb centers in the Keggin ion considerably destabilize the HPOM and accelerate the decomposition towards MoO3. A similar structural evolution is observed under propene and oxygen. The formation of the cubic HPOM with Mo centers on extra-Keggin framework positions coincides with the onset of catalytic activity. However, at about 673 K the active phase further decomposes to a mixture of MoO3 and a more stable cubic HPOM resulting in a pronounced decrease of the catalytic activity. Evidently, preparation and structure-activity relationships for Nb containing mixed metal oxide catalysts need to take into account the particular structure directing properties of Nb during thermal activation.