Direct Hydroxylation of Benzene to Phenol with Molecular Oxygen over Phase Transfer Catalysts: Cyclodextrins Complexes with Vanadium-Substituted Heteropoly Acids

Cyclodextrins (CyDs) complexes with vanadium-substituted heteropoly acids (PMoV _n -β-CyDs, n = 1, 2) were prepared by simple mixing and their structures were characterized by FT-IR. Among various catalysts, PMoV₁-β-CyDs, an efficient phase transfer catalyst, exhibited the highest yield (13.1%) of phenol without observing the formation of catechol, hydroquinone and benzoquinone in direct hydroxylation of benzene to phenol in 80 vol% aqueous acetic acid with molecular oxygen and ascorbic acid used as the oxidant and the reducing reagent, respectively. The influences of the reaction temperature, the pressure of oxygen, the amount of ascorbic acid, the amount of catalyst, and the reaction time on the yield of phenol were investigated to obtain the optimal reaction conditions for phenol formation.     

Emission characteristics of PAHs, benzene and phenol group hydrocarbons in O2/RFG waste incineration processes

This study applies the oxygen/recycled flue gas (O₂/RFG) combustion technology for waste incineration in a laboratory-scale fluidized bed incinerator to investigate the effects of different RFG percentages and O₂ concentrations on the emission characteristics of organic pollutants (PAHs, phenol and benzene hydrocarbons). Experimental results show that most PAHs with high-ring structures were present in solid-phase and most low-ring PAHs were present in gas-phase. The major compounds of benzene and phenol hydrocarbons were benzene, toluene, trichlorobenzene and 2,4-dinitrophenol, phenol, dichlorophenol, respectively. As the O₂ concentration in feed gas was increased from 21% to 40%, the emissions of solid- and gas-phase PAHs and phenol compounds were decreased but not for benzene compounds. Increasing RFG percentages would decrease the emissions of gas-phase PAHs, benzene and phenol compounds, but increased those of solid-phase pollutants. The best operating conditions of such O₂/RFG combustion system to reduce the emissions of PAHs and phenol compounds were 40% O₂, 35% RFG, and that for benzene compounds was 21% O₂, 75% RFG. Comparing with conventional air combustion system, the best diminution efficiencies of PAHs, benzene and phenol compounds at such O₂/RFG conditions were 59.54%, 70.97% and 52.60%, respectively. With proper feed gas compositions and RFG percentages, the combustion efficiency and destruction efficiency of organic pollutants can be improved by this O₂/RFG combustion technology.     

Effects of vanadium supported on ZrO2 and sulfolane on the synthesis of phenol by hydroxylation of benzene with oxygen and acetic acid on palladium catalyst

The catalyst system consisting of Pd, transition metal-modified ZrO₂ and acetic acid was found to catalyze the hydroxylation of benzene with molecular oxygen without hydrogen and phenol was formed. Of transition metals employed, only vanadium additive was found to be effective for improving the rate of phenol formation as well as the selectivity, while any other transition metals such as iron, molybdenym, tungsten and yttrium were not promotive. Support effects on vanadium were in the order: V/ZrO₂> V/Al₂O₃> V/SiO₂. The highest rate of phenol formation was obtained at 0.5wt%V/ZrO₂ catalyst. Phenol selectivity was dramatically improved by the addition of sulfolane, while benzene conversion and STY of phenol formation decreased. It is assumed that Pd(II) and Pd(IV) intermediates derived from acetic acid, oxygen and palladium acetate could play an important role in hydroxylation of benzene.     

Study of Fe-silicalite catalyst for the N2O oxidation of benzene to phenol

A set of Fe-silicalite samples of MFI structure have been prepared by the hydrothermal technique, followed by steaming and by further chemical treating of the solid. After characterisation by nitrogen adsorption/desorption, X-ray diffraction (XRD), scanning electron microscopy-electron probe micro analysis (SEM-EPMA), the samples have been tested as catalysts for the oxidation of benzene to phenol by N₂O. The best performing catalyst has been studied also by temperature programmed desorption-mass spectrometry and temperature programmed reaction-mass spectrometry (TPD-TPR-MS), after pre-adsorption of both reactants and products. It was found that phenol forms when N₂O is adsorbed first, followed by benzene. Almost no phenol formation was observed when adsorbing benzene before N₂O. Furthermore, on this catalyst N₂O decomposed since 50°C or less, forming gaseous N₂ and adsorbed oxygen, which started to become available for the oxidation of benzene since 100–200°C. However, the so formed phenol remained adsorbed onto the catalyst. It desorbed within the 225–425°C temperature range, with a maximum around 300°C.     

Direct Hydroxylation of Benzene to Phenol with Molecular Oxygen over Pyridine-modified Vanadium-substituted Heteropoly Acids

Pyridine(Py)-modified Keggin-type mono-vanadium-substituted heteropoly acids (Py _n PMo₁₁V, n = 1–4) were prepared by a precipitation method as organic/inorganic hybrid catalysts for direct hydroxylation of benzene to phenol in a pressured batch reactor and their structures were characterized by FT-IR. Among various catalysts, Py₄PMo₁₁V exhibited the highest catalytic activity (yield of phenol 9.0%) with the high selectivity for phenol, without observing the formation of catechol, hydroquinone and benzoquinone in the reaction with 80 vol% aqueous acetic acid, molecular oxygen and ascorbic acid used as the solvent, oxidant and reducing reagent, respectively. The influences of the reaction temperature, the pressure of oxygen, the amount of ascorbic acid, the amount of catalyst, and the reaction time on the yield of phenol were investigated to obtain the optimal reaction conditions for phenol formation. Pyridine can greatly promote the catalytic activity of the Py-free catalyst (H₄PMo₁₁VO₄₀), mostly because the organic π electrons in the hydrid catalyst may extend their conjugation to the inorganic framework of heteropoly acid and thus dramatically modify the redox properties.     

New Way of Hydroquinone and Catechol Synthesis using Nitrous Oxide as Oxidant

The synthesis of dihydroxybenzenes (DHB) via the gas‐phase oxidation of phenol with nitrous oxide in the presence of benzene was studied. Addition of benzene to the feed mixture greatly improves the selectivity and catalytic stability of the Fe‐containing ZSM‐5 zeolite, that was previously considered to be a main obstacle to the development of a new process. Reaction conditions strongly affect the distribution of the DHB isomers: the ratio of hydroquinone to catechol may vary from 1.4 to 10, with the resorcinol fraction being nearly constant and comprising 3–5%. Some 40 h experiments on the oxidation of a phenol‐benzene mixture demonstrated the high efficiency of the formed FeZSM‐5 catalyst. With a good stability, the catalyst provides 97% phenol selectivity referred to DHB and 85–90% N₂O selectivity referred to the sum of DHBs and phenol. A new process for hydroquinone and catechol synthesis based on the neat oxidation of benzene with recycling of the phenol as an intermediate product is suggested.     

Comparative study of aromatization selectivity during N‐heptane reforming on sintered Pt/Al2O3 and Pt‐Re/Al2O3 catalysts

BACKGROUND: The metal dispersed over a support can be present as small crystallites with sizes less than 5 nm. The smaller crystallites favour aromatization while larger crystallites favour cracking/hydrogenolysis. Sintering results in the agglomerization of smaller metal crystallites. Correlation of size with aromatization selectivity was investigated. RESULTS: The primary products of n‐heptane reforming on fresh Pt were methane, toluene, and benzene, while on fresh Pt‐Re, the only product was methane. Both catalysts exhibited enhanced aromatization selectivity at different oxygen sintering temperatures. The reaction products ranged from only toluene at 500 °C sintering temperature to methane at a sintering temperature of 650 °C with no reaction at 800 °C for the Pt/Al₂O₃ catalyst. On Pt‐Re/Al₂O₃ catalyst, methane was the sole product at a sintering temperature of 500 °C while only toluene was produced at a sintering temperature of 800 °C. CONCLUSION: This is the first time that sintering has been used to facilitate aromatization of supported Pt and Pt‐Re catalysts. A superior selectivity behaviour associated with bi‐metallic Pt catalysts is established. It was found that no reaction occurred on Pt catalyst after sintering at 800 °C whereas sintering Pt‐Re at 800 °C promoted aromatization solely to toluene. Copyright © 2008 Society of Chemical Industry     

Benzene Selective Oxidation with N2O on Fe/MFI Catalysts: Role of Zeolite and Iron Sites on the Deactivation Mechanism

The catalytic performances of Fe-zeolites having MFI structures and in which the Fe introduced either by ion exchange or during the hydrothermal synthesis has undergone partial framework to extra-framework migration induced by controlled heat treatment are reported. In particular, the catalytic behavior as function of time-on-stream and the formation of carbonaceous species were studied. The results suggest that only a small fraction of the iron is active in the selective oxidation of benzene to phenol in the presence of N₂O. It is suggested that the active fraction is formed by isolated iron ions in a pseudo-octahedral configuration with the sites positioned in hydroxyl nests (defects) of the zeolite and is selective in phenol formation as a result of in situ reduction during the catalytic tests. Two possible pathways of carbonaceous species were identified, the first through the intermediate further hydroxylation of phenol and the second through the coupling of phenol with benzene or another phenol molecule. This second pathway is the dominant mechanism of formation of carbonaceous species, although the relative rate of the two pathways depends on the zeolite characteristics and iron loading. It is also suggested that the second pathway depends on the strong chemisorption of phenol, probably on Lewis acid sites, which hinders the fast back-desorption of phenol out from the zeolite channels and thus favors the formation of carbonaceous species. Catalysts prepared by hydrothermal treatment show a lower rate of deactivation than those prepared by ion exchange, although the latter show a comparable productivity to phenol for amounts of iron in extra-framework positions around 20 to 30 times lower. The results also indicate that the presence of Al in the zeolite framework is beneficial for reducing the rate of deactivation as compared to that of Fe-silicalite samples.     

Direct synthesis of phenol from benzene over hydroxyapatite catalysts

The direct synthesis of phenol from benzene in the gas phase was studied over hydroxyapatite catalysts. The reaction was carried out in a fixed-bed reactor at atmospheric pressure and reaction temperature of 450°C in the presence of ammonia. A high selectivity (about 97%) of phenol formation at about 3.5% conversion of benzene was achieved over catalysts containing Ca and Cu ions in the cation part of hydroxyapatite. Besides phenol as the main reaction product, aniline is also formed. The reaction mechanism involves formation of N₂O from NH₃ in the first step of reaction. Benzene is oxidized by active oxygen species which are formed on the catalyst by decomposition of N₂O.     

Comparing two new routes for benzene hydroxylation

The vapour phase hydroxylation of benzene to phenol by two different methods has been investigated. In the first, a mixture of oxygen and hydrogen using a Pd membrane tubular reactor with and without second catalyst was used. Hydrogen dissociated on the palladium layer and reacted with oxygen to give active oxygen species, which reacted with benzene to produce phenol. The slow step in the overall reaction is the formation of usable hydrogen peroxide. Using a second catalyst changed the productivity, and conversion of benzene was increased by changing the length and diameter of porous reactor tubes. Low phenol productivity and selectivity was observed and showed that hydroxylation of benzene using a Pd membrane reactor is a far from economic method. In the second, selective oxidation of benzene with N₂O on iron zeolite of different SiO₂/Al₂O₃ composition, with concentration of iron rating from 50 to 2000 ppm was investigated. The effects of temperature, reactant mole ratio, and contact time were investigated. Phenol was formed with near 97% selectivity and average productivity of 5 mmol g⁻¹ h⁻¹.     

Liquid-Phase Oxidation of Benzene to Phenol by Molecular Oxygen over La Catalysts Supported on HZSM-5

The liquid-phase oxidation of benzene to phenol over lanthanum oxide catalysts (LaOx/HZSM-5) supported on HZSM-5 was studied in the presences of oxygen as an oxidant and ascorbic acid as a reducing regent. LaO_x/HZSM-5 effectively catalyzes the formation of phenol at 353 K under the pressurization of oxygen. The LaO_x/HZSM-5 catalysts take place no leaching of lanthanum species from the catalysts to acidic solvent. Therefore, it is demonstrated that the supported lanthanum catalysts are stable and reusable for the benzene oxidation even in the acidic reaction solution.     

Photocatalytic conversion of benzene to phenol using modified TiO2 and polyoxometalates

The application of photocatalytic reactions to organic synthesis has attracted interests in view of the development of environmentally benign synthetic processes. This study investigated the effects of various parameters (electron acceptor, surface modification, and the combination of photocatalysts) on the direct synthesis of phenol from benzene using photocatalytic oxidation processes. The OH radicals generated on UV-illuminated TiO₂ photocatalyst directly hydroxylate benzene to produce phenol, hydroquinone, and catechol. The addition of Fe³⁺, H₂O₂, or Fe³⁺ + H₂O₂ highly enhanced the phenol production yield and selectivity in TiO₂ suspension. Surface modifications of TiO₂ had significant influence on the phenol synthetic reaction. Depositing Pt nanoparticles on TiO₂ (Pt/TiO₂) markedly enhanced the yield and selectivity. Surface fluorination of TiO₂ (F-TiO₂) increased the phenol yield two-fold because of the enhanced production of mobile (free) OH radicals on F-TiO₂. Polyoxometalate (POM) in phenol synthesis played the dual role both as a homogeneous photocatalyst and as a reversible electron acceptor in TiO₂ suspension. POM alone was as efficient as TiO₂ alone in the phenol production. In particular, the addition of POM to the TiO₂ suspension increased the phenol yield from 2.6% to 11% (the highest yield obtained in this study). Reaction mechanisms for each photocatalytic system were discussed in relation to the phenol synthesis.     

Influence of temperature on hydroxylation of benzene to phenol using molecular oxygen catalyzed by V/SiO2

Effect of temperature on hydroxylation of benzene to phenol with molecular oxygen as an oxidant was studied over V/SiO₂ using different reductants. The V/SiO₂ with highly dispersed vanadium species was prepared by a sol–gel process and characterized by diffuse reflectance UV–Vis and ESR. This work shows that the onset temperature range of benzene hydroxylation and the temperature reaching the maximum phenol yield differ corresponding to each reductant. The reducing capacity of reductant can be shown by changing temperature and affects, in turn, benzene conversion, product selectivity, and the amount of leaching of V species.     

Conversion of Ethane into Benzene on Re/ZSM-5

The reaction of ethane on pure and Re-containing ZSM-5 has been investigated at 773–973 K. Reaction products were analyzed using gas chromatography. ZSM-5 with Si/Al = 30 exhibited relatively high activity towards dehydrogenation, hydrogenolysis and aromatization of ethane above 773 K, whereas ZSM-5 with Si/Al = 280 showed very low activity. Deposition of 2% Re enhanced the conversion of ethane, the selectivity and the yield of benzene production even on the most effective ZSM-5. Further increase in the Re content did not lead to the improvement of the catalyst. The role of the Re is very likely the activation of ethane and the enhanced production of ethylene. ZSM-5 samples were found to be very active in the aromatization of ethylene, which was only slightly influenced by Re.     

Effects of Re addition on phase stability and mechanical properties of hexagonal OsB2

ReB₂‐type hexagonal Osmium diboride (OsB₂) has been predicted to exhibit higher hardness than its orthorhombic phase, but hexagonal‐orthorhombic phase transformation occurs at temperature higher than 600°C, resulting in the decrease in its hardness. Therefore, ReB₂‐type hexagonal OsB₂ samples with Re addition were produced by a combination of mechanochemical method and pressureless sintering technique, and the effects of Rhenium (Re) addition on phase composition, thermal stability and mechanical properties of OsB₂ were investigated in this study. X‐ray diffraction (XRD) analysis of the as‐synthesized powders by high‐energy ball milling indicates the formation of hexagonal Os_1‐xRe_xB₂ solid solution with Re concentration of 5 and 10 at.% without forming a second phase. After being sintered at 1700°C, part of the hexagonal phase in OsB₂ transformed to orthorhombic structure, while Os_0.95Re_0.05B₂ and Os_0.9Re_0.1B₂ maintained their hexagonal structure. This suggests that the thermal stability of the hexagonal OsB₂ was significantly improved with the addition of Re. Scanning electron microscopy (SEM) photographs show that all of the as‐sintered samples exhibit a homogeneous microstructure with some pores and cracks formed throughout the samples with the relative density >90%. The measurements of micro‐hardness, nano‐hardness, and Young's modulus of the OsB₂ increased with Re addition, and these properties of the sample with 5 at.% addition of Re is higher than that with 10 at.% Re.     

Cluster Model DFT Study of the Intermediates of Benzene to Phenol Oxidation by N2O on FeZSM-5 Zeolites

An Fe(II) ion at an -cation exchange position of ZSM-5 zeolite (Fe/Z) was taken as a model for the active site in the nitrous oxide decomposition and in the selective oxidation of phenol with nitrous oxide. The oxygen deposited by decomposition of N₂O is commonly referred to as -oxygen (OFe/Z). Cluster model DFT calculations show that the interaction of the OFe/Z center with benzene resulted easily in arene oxide formation. The results indicate a rather low activation energy for this step. Possible transformations of the adsorbed arene oxide are considered and the experimental evidence for the absence of the kinetic H/D isotope effect in phenol formation is discussed. It is concluded that the rate-limiting step for the in situ oxidation of benzene to phenol is the desorption of the product.     

Preparation of VO2(B) Nanoflake with Glycerol as Reductant Agent and its Catalytic Application in the Aerobic Oxidation of Benzene to Phenol

Nanostructured VO₂(B) was prepared by employing glycerol as reductant. Key parameters of the pH value of crystallization solution and the volume ratio of glycerol to water were optimized to prepare pure phase VO₂(B). The VO₂(B) could catalyze the aerobic oxidation of benzene to phenol with molecular oxygen in high selectivity.     

Polymeric Titanium Oxychloride Sorbent for 188W/188Re Nuclide Pair Separation

Abstract

The chemical synthesis conditions (TiCl₄: iPrOH reagent ratio and reaction temperature scheme) were optimized for the preparation of polymeric titanium oxychloride sorbent which met the requirements for clinically useful ¹⁸⁸W/¹⁸⁸Re generator production, such as high W-adsorption capacity, high ¹⁸⁸Re-elution yield, low ¹⁸⁸W-breakthrough, and good mechanical stability. This polymeric material was formed by polycondensation of titanium-oxychloride units, the chemical formula of which was supposed as [OTiO (Ti₄₀ Cl₈₀ (OH) ₈₀ (TiO₂)₉₅.60H₂O) OTiO]_n. The effect of the W-content of tungstate solution on the WO₄ ^2? ion adsorption (with minimizing the poly-tungstate ion adsorption) and its covalent bonding with the Ti metal atoms in the polymeric matrix were justified with respect to the optimal W-adsorption conditions for the preparation of a useful ¹⁸⁸W/¹⁸⁸Re generator column. The high W-adsorption capacity of about 515.6 mg W/g sorbent and ¹⁸⁸Re elution yield of higher than 85% wereachieved. The large difference in the distribution ratio values found for alumina and polymeric titanium oxychloride sorbent in 0.005% NaCl solution (D_{W, Re-188} = 50 and D_{W, Re-188} = 1.0, respectively) offered an advantage for the preparation of a consecutive-elution based ¹⁸⁸Re generator system which combined both ¹⁸⁸Re elution and ¹⁸⁸Re concentrating processes in one portable system. This generator system is of a tandem column type which consists of a polymeric titanium oxychloride sorbent coupled to an alumina column. This system gave a ¹⁸⁸Re concentration factor of approximately 10. The overall ¹⁸⁸Re yield achieved from this system was >80%. ¹⁸⁸W isotope and elemental tungsten breakthrough were not detected in its ¹⁸⁸Re eluate. This system thus offers a potential application for clinically useful ¹⁸⁸Re production using low specific radioactivity ¹⁸⁸W (around 500 mCi/g) producible in a medium neutron flux reactor.     

One-step hydroxylation of benzene to phenol. II. Gas phase N2O oxidation over Mo/Fe/borosilicate molecular sieve

The Fe/Mo/partially deboronated borosilicate molecular sieve catalyst prepared by the chemical vapor deposition (CVD) method was active for the selective formation of phenol by gas-phase N₂O oxidation of benzene. The impregnated counterpart exhibited lower activity than the CVD catalyst. The borosilicate molecular sieve itself also was active. Two mechanistic paths are postulated based on reactive oxygen species such as O^–, which can be generated via interaction of N₂O with the iron sites in the CVD-borosilicate molecular sieve catalyst, or OH⁺, which can be generated by Brønsted sites on the borosilicate molecular sieve itself.     

Oxidation of Benzene to Phenol with Hydrogen Peroxide Catalyzed by a Modified Titanium Silicalite (TS‐1B)

The hydroxylation of benzene to phenol with hydrogen peroxide was investigated using different solvents and a series of catalysts, obtained by modification of titanium silicalite (TS‐1). The best results were obtained after post‐synthesis treatment of TS‐1 with NH₄HF₂ and H₂O₂. The new catalyst (TS‐1B), used in the presence of a particular co‐solvent (sulfolane) is able to protect the produced phenol from over‐oxidation and dramatically enhanced the selectivity of the reaction.