Selective Hydroxylation Of Benzene Research Articles

Photocatalytic property of WO3 modified with noble metal co-catalysts towards selective hydroxylation of benzene to phenol under visible light irradiation

Direct synthesis of phenol from benzene in the presence of water and oxygen were conducted under visible-light irradiation. Effects of several noble metal co-catalyst on the selective hydroxylation of benzene over WO3 photocatalysts have been extensively studied. The photocatalytic activities of WO3 for phenol formation strongly depend on co-catalyst such as Pt, Au, Pd metal species, i.e., phenol formation was as follows: Pt-WO3 > Au-WO3 > Pd-WO3. Furthermore, the WO3 photocatalyst deposited bimetal co-catalysts such as Pd/Pt, Pd/Au and Pt/Au were also investigated. The Pd/Pt-WO3 photocatalyst was found to show the highest photocatalytic activity for phenol formation due to an effective oxygen reduction reaction (ORR) for bimetallic Pd/Pt cocatalyst. From the experimental and theoretical studies, phenol was confirmed to form by the insertion of OH derived from H2O into benzene, and simultaneous cleavage of benzylic carbon-hydrogen bond (Csp2-H) by an assist of co-catalyst surface through the push-pull process.

Selective hydroxylation of benzene to phenol via C[sbnd]H activation over mesoporous Fe2O3-TiO2 using H2O2

The omnipresence of CH bonds in organic molecules enamors chemists for the functionalization of CH bonds towards value-added molecular scaffolds. Mesoporous Fe2O3-TiO2 mix-oxide catalyst has been successfully synthesized via a triblock-copolymer (P123) mediated sol-gel method and catalytically tested in selective hydroxylation of benzene. The as-synthesized catalysts were comprehensively characterized by powder XRD, Raman spectroscopy, FT-IR, N2-physisorption, FESEM, HRTEM, XPS, and NH3-TPD techniques. The physicochemical characterizations evidence that Fe(III) ions incorporate into the pure anatase TiO2 lattice framework and the crystallite size decreases as iron concentration increases. The XPS analyses confirm the Fe3+ state (BE 710.3 eV) in Fe-O-Ti with a peak at 712.1 eV for Fe3+ of Fe2O3. The NH3-TPD results show that the 5 wt% Fe-TiO2 has a higher total acidity of 105 µmol/g. Interestingly, a further increase in iron amount decreases the total acidity. Herein, we observe that the surface acidity directs the selectivity towards phenol. The effect of different reaction parameters was carefully investigated. Under the optimized conditions, 5% Fe-TiO2 with notified strong acid sites gives a higher phenol yield of 31.2% with > 99% selectivity; moreover, the catalyst is reusable at least four times.

Highly Dispersed Vanadia Anchored on Protonated g-C3N4 as an Efficient and Selective Catalyst for the Hydroxylation of Benzene into Phenol

The direct hydroxylation of benzene is a green and economical-efficient alternative to the existing cumene process for phenol production. However, the undesired phenol selectivity at high benzene conversion hinders its wide application. Here, we develop a one-pot synthesis of protonated g-C3N4 supporting vanadia catalysts (V-pg-C3N4) for the efficient and selective hydroxylation of benzene. Characterizations suggest that protonating g-C3N4 in diluted HCl can boost the generation of amino groups (NH/NH2) without changing the bulk structure. The content of surface amino groups, which determines the dispersion of vanadia, can be easily regulated by the amount of HCl added in the preparation. Increasing the content of surface amino groups benefits the dispersion of vanadia, which eventually leads to improved H2O2 activation and benzene hydroxylation. The optimal catalyst, V-pg-C3N4-0.46, achieves 60% benzene conversion and 99.7% phenol selectivity at 60 oC with H2O2 as the oxidant.

La-Cu based heterogeneous perovskite catalyst for highly selective benzene hydroxylation under mild conditions.

Direct hydroxylation of benzene towards phenol with high conversion and selectivity remains a great challenge. We report herein an efficient La2 CuO4 perovskite catalyst for one-step oxidation of benzene using hydrogen peroxide under mild conditions. The catalyst was characterized using XRD, TEM, XPS, TG-DTA, and other advanced techniques. The one-pot hydroxylation reaction carried out at 60 °C under optimum reaction conditions in the presence of catalytic material shows benzene to phenol transformation with 51% conversion with >99% selectivity with 65 percent peroxide efficiency, respectively. The influence of reaction conditions such as temperature, amount of oxidant, reaction time and mode of addition of the oxidant was crucial in selectivity optimization.

Theoretical Investigation into Selective Benzene Hydroxylation by Ruthenium-Substituted Keggin-Type Polyoxometalates.

Benzene hydroxylation catalyzed by ruthenium-substituted Keggin-type polyoxometalates [RuV(O)XW11O39]n- (RuVOX; X = Al, Ga, Si, Ge, P, As, S; heteroatoms; 3 ≤ n ≤ 6) is investigated using the density functional theory approach. As a possible side reaction, the water oxidation reaction is also considered. We found that the rate-determining step for water oxidation by RuVOX requires a higher activation free energy than the benzene hydroxylation reaction, suggesting that all of the RuVOX catalysts show high chemoselectivity toward benzene hydroxylation. Additionally, the heteroatom effect in benzene hydroxylation by RuVOX is discussed. The replacement of Si by X induces changes in the bond length of μ4O-X, resulting in a change in the activation free energy for benzene hydroxylation by RuVOX. Consequentially, RuVOS is expected to be the most effective catalyst among the (RuVOX) catalysts for the benzene hydroxylation reaction.

Graphene vanadic acid (GVA) as a novel heterogeneous catalyst for highly selective benzene hydroxylation under mild conditions

In the present study, Graphene Vanadic Acid (GVA) is introduced as a novel catalyst for high-efficiency hydroxylation of benzene to phenol using hydrogen peroxide at ambient conditions. Graphene oxide as a famous carbon support has converted to a high-performance catalyst through the functionalization using vanadium oxytrichloride (VOCl3). This new carbon compound is regarded as the efficient catalyst due to the prominent advantages such as high-recativity and reusability, facile preparation, active Lewis and Bronsted acid sites, high potential for development of various oxidation reactions, and high thermal, chemical, and physical stability due to the regular carbon structure. GVA progressed the benzene oxidation toward the phenol formation as a useful raw material. Cause of the considerable reactivity of the synthesized catalyst, a minimum amount of hydrogen peroxide was required and the benzene oxidation carried out using an equimolar ratio of benzene/H2O2, and phenol obtained with a remarkable yield (28.2%) as the sole product.

Oxo-vanadium (IV) phthalocyanine implanted onto the modified SBA-15 as a catalyst for direct hydroxylation of benzene to phenol in acetonitrile-water medium: A kinetic study

Direct hydroxylation of benzene using molecular oxygen is an engaged approach to yield of phenol as a valuable compound with numerous applications. In the present research, highly selective benzene hydroxylation was successfully carried out in the presence of O2 using vanadyl phthalocyanine implanted onto the chloro-functionalized SBA-15 as a robust catalyst. Also, the effect of water as a co-solvent was investigated and it was concluded that the mixed solvent (CH3CN-H2O) can play a remarkable role in the development of the reaction progress due to its ability to act as an axial ligand. Then, the oxidation of benzene using CH3CN and CH3CN-H2O was kinetically investigated.

Selective Hydroxylation of Benzene to Phenol over Fe Nanoparticles Encapsulated within N-Doped Carbon Shells

Iron-containing heterogeneous catalysts hold great promising for the direct hydroxylation of benzene to phenol. However, their catalytic performance is greatly hampered by the inherent drawbacks of iron-containing solids, in particular their hydrophilic surface structure and inevitable leaching of iron species during the Fenton process, which result in poor selectivity and durability toward phenol production. Herein, we demonstrate that the encapsulation of iron nanoparticles with N-doped carbon layer as core–shell nanostructures (Fe@NC) is a promising synthetic strategy to advance iron-containing solids for benzene hydroxylation. The rigid carbon shells conformably coating on iron cores not only protect iron from leaching but also facilitate the selective adsorption of benzene molecules on Fe@NC because of their excellent stability against acid etching and hydrophobic surface with the unique π-conjugated electron system. As a result, Fe@NC exhibit a robust catalytic durability and good yield with high selectivity for the direct hydroxylation of benzene to phenol. Benefiting from their unique core–shell nanostructures and strong host–guest electron interaction between metals and carbons, Fe@NC are expected to be promising also for other liquid-phase organic synthesis.

Single Atomic Cu-N2 Catalytic Sites for Highly Active and Selective Hydroxylation of Benzene to Phenol.

SummarySearching for an efficient single-atom catalyst for benzene hydroxylation to phenol is of critical importance, but it still remains a challenge. Herein, a single-atom catalyst with unique Cu-N2 moieties (Cu1-N2/HCNS) was prepared and confirmed by HAADF-STEM and EXAFS. Turnover number (TON) over Cu1-N2/HCNS (6,935) is 3.4 times of Cu1-N3/HCNS (2,034) under the same reaction conditions, and both exhibit much higher phenol selectivity (close to 99%) and stability compared with Cu nanoparticles and nanoclusters. Experiments and DFT calculations reveal that atomically dispersed Cu species are active sites for benzene hydroxylation to phenol, and the Cu-N2 is more active than Cu-N3 owing to its much lower energy barrier concerning the activation of H2O2 led by its unique coordination state of local atomic structure. We envision that this work opens a new window for modulating coordination environments of single metallic atoms in catalysis design.

Stabilizing CuPd bimetallic alloy nanoparticles deposited on holey carbon nitride for selective hydroxylation of benzene to phenol

Phenol is one of the most common intermediates for the synthesis of industrial chemicals. In this paper, we report on the application of photocatalysis for phenol production by hydroxylation of benzene. A series of CuPd bimetallic alloy nanoparticles-coated holey carbon nitride materials have been synthesized. Compared with holey carbon nitride, a considerable increase in selectivity to phenol is achieved in the presence of a uniform distribution of CuPd bimetallic alloy nanoparticles coated on the surface. The catalyst with 0.5 wt% of CuPd bimetallic alloy particles shows an excellent photocatalytic performance where a selective transformation of benzene to phenol of ~89.6% is achieved within 90 min. We attribute this high performance in selective transformation to the uniform distribution of CuPd bimetallic alloy nanoparticles and the high photocatalytic activity of C3N4/CuPd composite photocatalysts.

NH3 -Driven Benzene C-H Activation with O2 that Opens a New Way for Selective Phenol Synthesis.

Catalytic benzene C-H activation toward selective phenol synthesis with O2 remains a stimulating challenge to be tackled. Phenol is currently produced industrially by the three-steps cumene process in liquid phase, which is energy-intensive and not environmentally friendly. Hence, there is a strong demand for an alternative gas-phase single-path reaction process. This account documents the pivotal confined single metal ion site platform with a sufficiently large coordination sphere in β zeolite pores, which promotes the unprecedented catalysis for the selective benzene hydroxylation with O2 under coexisting NH3 by the new inter-ligand concerted mechanism. Among alkali and alkaline-earth metal ions and transition and precious metal ions, single Cs+ and Rb+ sites with ion diameters >0.300 nm in the β pores exhibited good performances for the direct phenol synthesis in a gas-phase single-path reaction process. The single Cs+ and Rb+ sites that possess neither significant Lewis acidic-basic property nor redox property, cannot activate benzene, O2 , and NH3 , respectively, whereas when they coadsorbed together, the reaction of the inter-coadsorbates on the single alkali-metal ion site proceeds concertedly (the inter-ligand concerted mechanism), bringing about the benzene C-H activation toward phenol synthesis. The NH3 -driven benzene C-H activation with O2 was compared to the switchover of the reaction pathways from the deep oxidation to selective oxidation of benzene by coexisting NH3 on Pt6 metallic cluster/β and Ni4 O4 oxide cluster/β. The NH3 -driven selective oxidation mechanism observed with the Cs+ /β and Rb+ /β differs from the traditional redox catalysis (Mars-van Krevelen) mechanism, simple Langmuir-Hinshelwood mechanism, and acid-base catalysis mechanism involving clearly defined interaction modes. The present catalysis concept opens a new way for catalytic selective oxidation processes involving direct phenol synthesis.

Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites

A direct, catalytic conversion of benzene to phenol would have wide-reaching economic impacts. Fe zeolites exhibit a remarkable combination of high activity and selectivity in this conversion, leading to their past implementation at the pilot plant level. There were, however, issues related to catalyst deactivation for this process. Mechanistic insight could resolve these issues, and also provide a blueprint for achieving high performance in selective oxidation catalysis. Recently, we demonstrated that the active site of selective hydrocarbon oxidation in Fe zeolites, named α-O, is an unusually reactive Fe(IV)=O species. Here, we apply advanced spectroscopic techniques to determine that the reaction of this Fe(IV)=O intermediate with benzene in fact regenerates the reduced Fe(II) active site, enabling catalytic turnover. At the same time, a small fraction of Fe(III)-phenolate poisoned active sites form, defining a mechanism for catalyst deactivation. Density-functional theory calculations provide further insight into the experimentally defined mechanism. The extreme reactivity of α-O significantly tunes down (eliminates) the rate-limiting barrier for aromatic hydroxylation, leading to a diffusion-limited reaction coordinate. This favors hydroxylation of the rapidly diffusing benzene substrate over the slowly diffusing (but more reactive) oxygenated product, thereby enhancing selectivity. This defines a mechanism to simultaneously attain high activity (conversion) and selectivity, enabling the efficient oxidative upgrading of inert hydrocarbon substrates.

Iron Oxychloride as an Efficient Catalyst for Selective Hydroxylation of Benzene to Phenol

Selective hydroxylation of benzene is a felicitous strategy for the production of phenol that is deemed to be an alternative to conventional processes. Thus, the development of a durable and highly efficient catalyst for the selective hydroxylation of benzene should be a key topic. In this work, FeOCl was prepared by a chemical vapor transition method and characterized using various techniques including XRD, TEM, Raman spectroscopy, N2 adsorption–desorption, DLS, and TGA. The prepared FeOCl was applied as a heterogeneous catalyst in benzene hydroxylation, and the reaction conditions were optimized. The acquired data manifested that FeOCl has shown superiority over the other reported catalysts utilized in benzene hydroxylation. The superiority of FeOCl is attributed to the facile self-redox potential of FeOCl and its remarkable ability for the production of an overwhelming amount of hydroxyl radicals in a short period of time. The catalyst recovery and reuse test showed that FeOCl is able to endure the har...

Photooxygenation of Hydrocarbons with Molecular Oxygen By Electron Transfer

Phenol is an very important bulk organic compound, having a variety of applications in industry. Phenol is commercially produced through three-step cumene process via cumene hydroperoxide, which is the key intermediate. In addition acetone is produced as a co-product and a-methylstyrene is produced as a byproduct together with phenol. Thus, phenol synthesis from benzene in a one-step reaction with high benzene conversion and high phenol selectivity is highly desired from the viewpoints of an environmentally benign green process and economical efficiency. Extensive efforts have so far been devoted to develop the one-step direct hydroxylation of benzene using various oxidants. we would like to focus on the catalytic mechanisms of selective hydroxylation of benzene to phenol. First, catalytic mechanisms of liquid phase hydroxylation of benzene with H2O2 using metal complexes are discussed by identifying the reactive intermediates, which act as strong electrophiles for the benzene hydroxylation. Next, photochemical hydroxylation of benzene with an organic oxidant via benzene radical cation produced in photoinduced electron transfer from benzene to the excited state of the organic oxidant is discussed to clarify the reason of the selective hydroxylation to phenol. The reason why phenol is not further oxidized is discussed based on the Marcus theory of electron transfer. Finally, the photocatalytic hydroxylation of benzene with O2 to phenol is made possible by using an appropriate organic photocatalyst. The photochemical hydroxylation of benzene with an organic oxidant to phenol is also combined with the catalytic reoxidation of the reduced organic oxidant by O2 to make the selective hydroxylation of benzene with O2 to phenol catalytic. How overoxidation of phenol can be avoided is discussed in both the thermal and photochemical catalytic hydroxylation of benzene. Photochemical hydroxylation of benzene with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and water occurs under visible light irradiation to yield phenol and DDQH2. The yield of phenol was 99% with 99% conversion (>99% selectivity) [1]. The mechanism of the photochemical hydroxylation of benzene with DDQ was clarified by time-resolved transient absorption spectroscopy. The photochemical reaction was initiated by the photoinduced electron transfer from benzene to 3DDQ* to produce benzene p-dimer radical cation and DDQ•−. The free energy change for electron transfer from benzene to 3DDQ* is largely negative (DG et = −0.70 eV), as determined from the one-electron oxidation potential of benzene (E ox = 2.48 V vs. SCE) and the one-electron reduction potential of 3DDQ* (E red = 3.18 V vs. SCE). Then, benzene radical cation reacts with H2O to produce an OH-adduct radical of benzene. Finally, phenol was produced by hydrogen atom transfer from the OH-adduct radical to DDQ•–. Photoinduced oxygenation of neat cyclohexane in the presence of O2 also occurred under visible light irradiation of DDQ which acts as a super photooxidant. The products detected by GC and NMR analyses were cyclohexanol, cyclohexanone and cyclohexane hydroperoxide with 70, 40, 210% yields, respectively. When cyclohexane was replaced by n-butane, n-pentane and 3-methylpentane, the oxygenation reactions also took place to form the corresponding oxygenated products under the otherwise same photochemical reaction conditions [2-5]. [1] Ohkubo, K.; Fujimoto, A.; Fukuzumi, S. J. Am. Chem. Soc. 2013, 135, 5368. [2] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Chem.–Eur. J. 2015, 21, 2855. [3] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Photochem. Photobiol. Sci. 2016, 15, 731. [4] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Chem. Asia J. 2016, 11, 2218. [5] Fukuzumi, S.; Ohkubo, K. Asian J. Org. Chem. 2015, 4, 836 (Focus Review).

Catalytic Performance of a Dicopper-Oxo Complex for Methane Hydroxylation.

A dicopper(II) complex, [Cu2(μ-OH)(6-hpa)]3+, where 6-hpa is 1,2-bis[2-[bis(2-pyridylmethyl)aminomethyl]-6-pyridyl]ethane, generates an oxyl radical of CuIIO• and catalyzes the selective hydroxylation of benzene to phenol. From the structural similarity to methane activation catalysts (e.g., bare CuO+ ion, Cu-ZSM-5, and particulate methane monooxygenase), it is expected to catalyze methane hydroxylation. The catalytic performance for the hydroxylation of methane to methanol by this dicopper complex is investigated by using density functional theory (DFT) calculations. The whole reaction of the methane conversion involves two steps without radical species: (1) C-H bond dissociation of methane by the CuIIO• moiety and (2) C-O bond formation with methyl migration. In the first step, the activation barrier is calculated to be 10.2 kcal/mol, which is low enough for reactions taking place under normal conditions. The activation barrier by the other CuIIO2• moiety is higher than that by the CuIIO• moiety, which should work to turn the next catalytic cycle. DFT calculations show that the dicopper complex has a precondition to hydroxylate methane to methanol. Experimental verification is required to look in detail at the reactivity of this dicopper complex.

Direct Hydroxylation of Benzene to Phenol by Cytochrome P450BM3 Triggered by Amino Acid Derivatives

AbstractThe selective hydroxylation of benzene to phenol, without the formation of side products resulting from overoxidation, is catalyzed by cytochrome P450BM3 with the assistance of amino acid derivatives as decoy molecules. The catalytic turnover rate and the total turnover number reached 259 min−1 P450BM3−1 and 40 200 P450BM3−1 when N‐heptyl‐l‐proline modified with l‐phenylalanine (C7‐l‐Pro‐l‐Phe) was used as the decoy molecule. This work shows that amino acid derivatives with a totally different structure from fatty acids can be used as decoy molecules for aromatic hydroxylation by wild‐type P450BM3. This method for non‐native substrate hydroxylation by wild‐type P450BM3 has the potential to expand the utility of P450BM3 for biotransformations.

Specific Enhancement of Catalytic Activity by a Dicopper Core: Selective Hydroxylation of Benzene to Phenol with Hydrogen Peroxide.

A dicopper(II) complex, stabilized by the bis(tpa) ligand 1,2-bis[2-[bis(2-pyridylmethyl)aminomethyl]-6-pyridyl]ethane (6-hpa), [Cu2 (μ-OH)(6-hpa)]3+ , was synthesized and structurally characterized. This complex catalyzed selective hydroxylation of benzene to phenol using H2 O2 , thus attaining large turnover numbers (TONs) and high H2 O2 efficiency. The TON after 40 hours for the phenol production exceeded 12000 in MeCN at 50 °C under N2 , the highest value reported for benzene hydroxylation with H2 O2 catalyzed by homogeneous complexes. At 22 % benzene conversion, phenol (95.2 %) and p-benzoquinone (4.8 %) were produced. The mechanism of H2 O2 activation and benzene hydroxylation is proposed.

Specific Enhancement of Catalytic Activity by a Dicopper Core: Selective Hydroxylation of Benzene to Phenol with Hydrogen Peroxide

AbstractA dicopper(II) complex, stabilized by the bis(tpa) ligand 1,2‐bis[2‐[bis(2‐pyridylmethyl)aminomethyl]‐6‐pyridyl]ethane (6‐hpa), [Cu2(μ‐OH)(6‐hpa)]3+, was synthesized and structurally characterized. This complex catalyzed selective hydroxylation of benzene to phenol using H2O2, thus attaining large turnover numbers (TONs) and high H2O2 efficiency. The TON after 40 hours for the phenol production exceeded 12000 in MeCN at 50 °C under N2, the highest value reported for benzene hydroxylation with H2O2 catalyzed by homogeneous complexes. At 22 % benzene conversion, phenol (95.2 %) and p‐benzoquinone (4.8 %) were produced. The mechanism of H2O2 activation and benzene hydroxylation is proposed.

VOPO4·2H2O encapsulated in graphene oxide as a heterogeneous catalyst for selective hydroxylation of benzene to phenol

A heterogeneous catalyst was prepared through a strong interaction between graphene oxide and exfoliated VOPO4·2H2O, which can oxidize benzene to phenol with 100% selectivity and 33% conversion up to 5 catalytic cycles.

Highly Selective Hydroxylation of Benzene Catalyzed by Hybrids of Polyoxometalate/Ionic Liquid

Highly Selective Hydroxylation of Benzene Catalyzed by Hybrids of Polyoxometalate/Ionic Liquid