Hydroxylation Of Phenol Research Articles

Mechanism of Iodine(III)-Promoted Oxidative Dearomatizing Hydroxylation of Phenols: Evidence for a Radical-Chain Pathway.

The oxidative dearomatization of phenols with the addition of nucleophiles to the aromatic ring induced by hypervalent iodine(III) reagents and catalysts has emerged as a highly useful synthetic approach. However, experimental mechanistic studies of this important process have been extremely scarce. In this report, we describe systematic investigations of the dearomatizing hydroxylation of phenols using an array of experimental techniques. Kinetics, EPR spectroscopy, and reactions with radical probes demonstrate that the transformation proceeds by a radical-chain mechanism, with a phenoxyl radical being the key chain-carrying intermediate. Moreover, UV and NMR spectroscopy, high-resolution mass spectrometry, and cyclic voltammetry show that before reacting with the phenoxyl radical, the water molecule becomes activated by the interaction with the iodine(III) center, causing the Umpolung of this formally nucleophilic substrate. The radical-chain mechanism allows the rationalization of all existing observations regarding the iodine(III)-promoted oxidative dearomatization of phenols.

Microwave-assisted method to degrade phenol using persulfate or hydrogen peroxide catalyzed by Cu-bearing silicon carbide.

The radical generation properties of hydrogen peroxide and persulfate for phenol degradation were investigated under microwave irradiation using copper-doped silicon carbide (Cu/SiC) composites as catalyst. The results showed that 90% and 70% of phenol and total organic carbon (TOC), respectively, were removed within 7 min. Microwave activation of hydrogen peroxide and sodium persulfate in terms of thermal effects and accelerated electron transfer was analyzed by degradation kinetics and X-ray photoelectron spectroscopy (XPS). The microwave activation of Na2S2O8 demonstrated that the hot spots promote decomposition of persulfate more rapidly and the rate of persulfate decomposition was more than three times the activation rate of a normal heating method. There is a synergistic effect between Cu and microwave radiation, which is highlighted by the H2O2 activation; ·OH was generated due to the redox cycle between Cu(I)/Cu(II) and was responsible for phenol degradation using H2O2. High performance liquid chromatography (HPLC) analysis indicated that hydroxylation and sulfate radicals addition of phenol were the initial oxidation reaction steps of hydrogen peroxide and persulfate, respectively, followed by further oxidation to form short-chain carboxylic acids.

Combining two active states of FeOx in-situ in molecular sieve to deliver enhanced catalytic activity via creating special configuration and synergy

Combining different active states of metal species together in special configuration might trigger interesting synergetic effect for catalytic process. As a proof-of-concept, the present study has developed an ingenious approach to reach functional mesoporous molecular sieve which in-situ encapsulates two different states of highly-active FeOx inside centered core and nano-tunnel of mesoporous silica, respectively. The α-Fe2O3in-situ immobilized nanoneedles coated by the porous silica afford ∼5 nm diameter, plus highly-dispersed Fe2O3 cluster-like species attachment in confined mesopore, which synergistically contributes to high activity and stabilization of catalyst. The combined iron oxides in special configuration deliver an appreciable catalytic activity in phenol hydroxylation (46.4% conversion, 87.9% benzenediol selectivity) during multiple catalytic cycles, compared to some reported results, which benefited from accelerating production of oxidative free radicals from H2O2 and the tailored micrpopore (∼1.4 nm) which could enhance the catalytic selectivity of targeted products. Furthermore, the photocatalytic antimicrobial effect of Fe catalyst against Escherichia coli was also demonstrated via the constructed synergy of two FeOx nanospecies with assistance of light.

Synthesis and characterization of new binuclear Co(II) and Ni(II) complexes derived from N, N′-bis(4-dimethyl-aminobenzylidene)-benzene-1,3-diamine as active catalysts for hydroxylation of phenol and their antibacterial properties

In the current study, both of Ni(II) and Co(II) complexes were derived from N,N′-bis(4-dimethyl-aminobenzylidene)-benzene-1,3-diamine as a novel ligand. The ligand and complexes were characterized by elemental analysis (CHNS), Fourier transform infrared spectroscopy (FT-IR), Ultraviolet–visible spectroscopy (UV–Vis), ESI mass spectroscopy, conductance, and magnetic moment measurements. Regarding magnetic moment measurements and spectral studies, an octahedral structure was suggested. The free ligand and metal complexes were evaluated in vitro against three bacterial pathogens, including; Staphylococcus aureus, Escherichia coli, and Bacillus subtilis. The growth inhibition zone of Ni(II) and Co(II) complexes was far more extended than the norm. Furthermore, we studied the catalytic activity of these complexes through the hydroxylation of phenol. Comparisons revealed that the catalytic performance of the Ni(II) complex was considerable than that of the Co(II) complex. Four parameters (i.e., duration of reaction, temperature, catalyst amount, and oxidant amount) were regarded for Ni(II) complex (as a catalyst) to achieve an optimum condition for hydroxylation of phenol. Ultimately, adequate time for the reaction was recognized in the 2 h duration and at 60 °C.

Alginate-based aerogels with double catalytic activity sites and high mechanical strength

Aerogel nanocomposites from biomass are one of the attractive emerging materials in the field of material chemistry and catalyst industry, but their lower catalytic efficiency and mechanical performance have limited their wide range applications. In this work, alginate-based aerogels with perfect catalytic performance and great mechanical strength were prepared through incorporating graphene oxide (GO) and SiO2 via a simple one-pot reaction. GO played as the second catalytic activity sites and endowed the aerogels the perfect catalytic performance (conversion of phenol reached 90.1 % in phenol hydroxylation reaction). Meanwhile, SiO2 and GO could improve the particle compressive strength of the aerogel to 36.0 kg/cm2. This alginate-based aerogel provides a novel and environmentally friendly catalyst for next generation catalytic industry.

Postsynthesis of Ti-UZM-35 titanosilicate as efficient catalyst for phenol hydroxylation reaction

Abstract The MSE-type aluminosilicate UZM-35 was efficiently prepared via adding a rational amount of MCM-68 as seed and using commercially available dimethyldipropylammonium hydroxide (DMDPAOH) as the structure-directing agent. The operation window of Si/Al ratio was widened significantly and the synthetic system was less sensitive to the Na+/K+ ratio, compared to the synthetic systems of MCM-68 and traditional UZM-35 without seed. The crystallization time was also shortened to 4 days, compared to MCM-68 (14 days) and traditional UZM-35 (7 days). Moreover, the framework of UZM-35 kept intact after the dealumination process including high temperature calcination and acid treatment. Tetrahedrally coordinated Ti4+ ions were introduced into the framework of highly dealuminated UZM-35 via solid-gas reaction with TiCl4 vapor and the obtained Ti-UZM-35 exhibited comparable catalytic activity to Ti-MCM-68 in the phenol hydroxylation reaction. The H2O2 feeding style was adjusted in present work to improve the H2O2 utilization efficiency up to 70%. In addition, the Ti-UZM-35 catalyst was stable in the regeneration-reuse cycles.

Iron-Containing TS-1 Zeolites with Controllable Mesopores by Desilication and Their Application in Phenol Hydroxylation

Hierarchical iron-containing TS-1 zeolites (FTS) were successfully synthesized via post-treatment desilication over microporous Fe-TS-1, which were prepared through a one-step hydrothermal synthesi...

Crystallization of as-Mesocellular Silica Foam into Hierarchical TS-1 Zeolites by Conventional Hydrothermal and Dense-Gel Routes for Oxidation Reactions.

Crystallization of as-mesocellular silica foam (as-MCF) into TS-1 zeolites by conventional hydrothermal and dense-gel routes is reported. The TS-1 synthesized through the dense-gel route (TS-1-d) showed higher meso-macroporosity (0.311 vs 0.233 cm3/g), smaller particle size (130 vs 305 nm), and enhanced external surface area (86 vs 31 m2/g) than that synthesized under conventional hydrothermal conditions (TS-1-h). These might suggest that an organic template from as-MCF acted as mesoporogen in the dense-gel synthesis; however, segregation of the organic template and the synthesis mixture occurred in the conventional hydrothermal synthesis. The obtained TS-1 from the crystallization of as-MCF showed an enhanced framework Ti. The turnover frequencies (h–1) of TS-1-d toward hydroxylation of phenol and oxidation of dibenzothiophene (DBT) were 2 and 5 times, respectively, higher than those by TS-1-h. The DBT conversion by TS-1-d reached >99.0%, while it was only 63% by TS-1-h, suggesting the potential of TS-1-d for deep desulfurization of the fuels. Consequently, the crystallization of as-mesoporous materials through the dense-gel route is a promising approach to prepare TS-1 zeolites with smaller particle sizes and enhanced mesoporosity.

High selective hydroxylation of phenol catalyzed by PNP ligand‐containing [FeFe]‐hydrogenase model complexes

AbstractBACKGROUNDDihydroxybenzene (DHB) which is an important raw material and intermediate of the organic chemical industry can be prepared by hydroxylation of phenol, so it promotes the study of highly selective catalyst for hydroxylation of phenol.RESULTSTwo novel PNP ((Ph2P)2NPh) ligand‐substituted di‐iron complexes were synthesized under controllable reaction conditions via carbon monoxide (CO)/ligand substitution of the parent complex (μ‐budt)Fe2(CO)6 [budt = SCH(CH3)CH2CH2)S]. The substituted complexes 1 and 2 were fully characterized by solution infrared spectroscopy, nuclear magnetoresonance spectroscopy, single crystal X‐ray diffraction, cyclic voltammetry and density functional theory calculations. Due to its favorable redox properties, Complex 1 was adopted first as an efficient catalyst for the hydroxylation of phenol to DHB (dihydroxybenzene) with hydrogen peroxide (H2O2) as oxidant and H2O as solvent. Under the optimum reaction conditions, the conversion of phenol achieved 42.0%, the selectivity of DHB was ≤88.6% and the catechol:hydroquinone ratio (mol mol−1) was 2.2. Furthermore, only trace amounts of benzoquinone, as a deeply oxidized product, was detected (yield c. 0.1%).CONCLUSIONComplex 1 could be a new type of catalyst for the hydroxylation of phenol to hydroquinone owing to the mild reaction conditions and excellent catalytic performance. In addition, a possible catalytic reaction mechanism was related to the special structure of Complex 1 which features an internal base of amine in PNP ligand.

Study on the Structure of Cu/SiO2 and Their Catalytic Performance in the Reaction of Phenol Hydroxylation

Three kinds of Cu/SiO2 were prepared by the impregnation method using the synthesized and purchased SiO2 with different structures as the support.The samples were characterized by X-ray diffraction(XRD),N2 isothermal absorption-desorption,ultraviolet-visible diffuse reflectance spectra,X-ray photoelectron spectroscopy,inductively coupled plasma atomic emission spectrometry and H2 temperature programmed reduction.The catalystic activity of the samples in the phenol hydroxylation to dihydroxybenzenes using H2O2 was investigated.The results showed that good stability of MS-1 possessing MFI topology and mesoporous structure were advantageous to diffuse copper species from surface to pore and further formed more highly dispersed copper species.The Cu/MS-1 catalysts gave a desired dihydroxybenzenes yield(38.91%)with excellent selectivity(81.5%).The dihydroxybenzenes yield reached up to 37.64%and copper loading of the Cu/MS-1 dropped slightly from 2.5%to 2.1%after use ten runs.The results demonstrated good reusability of Cu/MS-1,and the highly dispersed CuO in samples was main active species of the selective oxidation of phenol with H2O2 produces catechol,hydroqinone and benzoquinone.We believed that MS-1 prepared in this study was a promising metial-supported and was widely used to catalytic field,particularly for those employing in macromolecular liquid reaction.

Facile Synthesis of CuMgFe Layered Double Hydroxides for Efficient Catalytic Phenol Hydroxylation under Mild Conditions

AbstractA series of CuMgFe layered double hydroxides (denoted as X/CuMgFe–LDHs) with different Cu contents were synthesized by co‐precipitation as catalysts for phenol hydroxylation under mild conditions. The as‐prepared samples were analyzed by X‐ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma emission spectroscopy (ICP‐ES), N2 adsorption–desorption, Fourier transform infrared (FT‐IR), thermogravimetric differential scanning calorimetry (TG‐DSC), X‐ray photoelectron spectroscopy (XPS) and hydrogen temperature‐programmed reduction (H2‐TPR) to investigate the morphologies and physicochemical properties. As presented by the characterization results, introducing a certain amount of copper not only increased the amount of active metal centers but also formed more oxygen vacancies and lattice oxygen to promote electron transport between the double layers in LDHs. However, the excess copper would significantly disrupt the X/CuMgFe–LDH framework, declining the activity. Therefore, the 20/CuMgFe–LDH sample presented the best catalytic performance among all the catalysts in terms of activity, which was correlated with the layered structure, low‐temperature reducibility, oxygen species and low Cu+/Cu2+ and Fe2+/Fe3+ ratios. The catalyst could maintain good stability even after recycling for five times. Finally, a possible mechanism for phenol hydroxylation via Cu+–OV–Fe2+ and Cu2+–O–Fe3+ redox cycles on CuMgFe–LDHs was proposed.

Fe-Doped Mesoporous Alumina: Facile One-Pot Synthesis, Modified Surface-Acidity and Its Enhanced Catalytic Performance in Phenol Hydroxylation

Fe-doped mesoporous alumina (MA) was successfully synthesized via a facile one-pot synthesis method. The resulting Fe-doped MA samples possess typical mesoporous structure, relatively high BET surface area, and narrowed pore size diameter. Besides, the iron species are well-dispersed in the alumina matrix, and more importantly, some small oligonuclear iron oxide clusters are linked on the surface of alumina, leading to the elimination of strong acid sites. The reaction of phenol hydroxylation was preceded at room temperature for 2 h. The introduction of mesoporous structure in alumina would be favorable for the adsorption and diffusion process of reactant and product molecules, and also the well-dispersed iron species in alumina matrix and large amount of acid sites stimulate more active hydroxyl radicals which are greatly beneficial for the catalytic process, especially the elimination of strong acid sites would inhibit the over-oxidation reaction. In this case, the sample of 5Fe–MA displays the best catalytic performance especially with the extremely high dihydroxybenzene selectivity of 93.2%, and the good catalytic stability is also evidenced by the five times recycling tests. In this work, the Fe-doped mesoporous alumina (MA) was successfully prepared. The iron species are well-dispersed in the alumina matrix, and some small oligonuclear iron oxide clusters are linked on the surface of alumina, leading to the elimination of strong acid sites. The advanced properties of mesoporous structure, well-dispersed metal active centers and lack of strong acid sites in Fe-doped MA are greatly beneficial for the catalytic process, especially for the dihydroxybenzene selectivity with extremely high value of 93.2%.

Direct Hydroxylation of Phenol to Dihydroxybenzenes by H2O2 and Fe-based Metal-Organic Framework Catalyst at Room Temperature

A semi-crystalline iron-based metal-organic framework (MOF), in particular Fe-BTC, that contained 20 wt.% Fe, was sustainably synthesized at room temperature and extensively characterized. Fe-BTC nanopowders could be used as an efficient heterogeneous catalyst for the synthesis of dihydroxybenzenes (DHBZ), from phenol with hydrogen peroxide (H2O2), as oxidant under organic solvent-free conditions. The influence of the reaction temperature, H2O2 concentration and catalyst dose were studied in the hydroxylation performance of phenol and MOF stability. Fe-BTC was active and stable (with negligible Fe leaching) at room conditions. By using intermittent dosing of H2O2, the catalytic performance resulted in a high DHBZ selectivity (65%) and yield (35%), higher than those obtained for other Fe-based MOFs that typically require reaction temperatures above 70 °C. The long-term experiments in a fixed-bed flow reactor demonstrated good Fe-BTC durability at the above conditions.

Computational Biotransformation Profile of Emerging Phenolic Pollutants by Cytochromes P450: Phenol-Coupling Mechanism.

Phenols are ubiquitous environmental pollutants, whose biotransformation involving phenol coupling catalyzed by cytochromes P450 may produce more lipophilic and toxic metabolites. Density functional theory (DFT) computations were performed to explore the debated phenol-coupling mechanisms, taking triclosan as a model substrate. We find that a diradical pathway facilitated by compound I and protonated compound II of P450 is favored vs alternative radical addition or electron-transfer mechanisms. The identified diradical coupling resembles a "two-state reactivity" from compound I characterized by significantly high rebound barriers of the phenoxy radicals, which can be formulated into three equations for calculating the ratio [coupling]/[hydroxylation]. A higher barrier for rebound than for H-abstraction in high-spin triclosan can facilitate the phenoxy radical dissociation and thus enable phenol coupling, while H-abstraction/radical rebound causing phenol hydroxylation via minor rebound barriers mostly occurs via the low-spin state. Therefore, oxidation of triclosan by P450 fits the first equation with a ratio [coupling]/[hydroxylation] of 1:4, consistent with experimental data indicating different extents of triclosan coupling (6-40%). The high rebound barrier of phenoxy radicals, as a key for the mechanistic identification of phenol coupling vs hydroxylation, originates from their weak electron donor ability due to spin aromatic delocalization. We envision that the revealed mechanism can be extended to the cross-coupling reactions between different phenolic pollutants, and the coupling reactions of several other aromatic pollutants, to infer unknown metabolites.

Influences of synthesis conditions on preparation and characterization of Ti-MWW zeolite membrane by secondary hydrothermal synthesis

Influences of synthesis conditions on the growth and catalytic performance of the Ti-MWW zeolite membrane are investigated in this work. Medium crystallization time, crystallization temperature, molar ratios of the precursor synthesis gels (TiO 2 /SiO 2 , H 2 O/SiO 2 and B 2 O 3 /SiO 2 ) are crucial to prepare the compact and good catalytic performance Ti-MWW zeolite membranes. High crystallization temperature and long crystallization time cause a hetero zeolite phase membrane on the mullite support. Excessive titanium in the precursor synthesis gel could easily arise TiO 2 particles and non-framework TiO 6 species on the membrane surface, which are greatly decreased the catalytic performance of the Ti-MWW zeolite membrane. Dilute precursor synthesis gel is not favor for preparing dense Ti-MWW zeolite layer. The as-synthesized Ti-MWW zeolite membrane is applied as catalytic membrane reactor for phenol hydroxylation by pervaporation in this study, the phenol conversion and benzenediol selectivity are 41.42% and 98.33%. • Excessive titanium of precursor synthesis gel cause TiO 2 particles and non-framework TiO 6 species. • High crystallization temperature and long crystallization time could lead to form a hetero phase Ti-MWW zeolite membrane. • Dilute precursor synthesis gel was not favor for preparing dense Ti-MWW zeolite layer. • Phenol conversion and benzenediol selectivity of Ti-MWW zeolite membrane are 41.42% and 98.33%.

Binuclear furanyl-azine metal complexes encapsulated in NaY zeolite as efficiently heterogeneous catalysts for phenol hydroxylation

Two different methods A and B were used for preparing binuclear furanyl-azine metal complexes encapsulated in NaY zeolite. These new heterogeneous catalysts based on Fe(II) or Cu(II) complexes with a metal/ligand molar ratio of 2:1, were characterized by different spectroscopic techniques and chemical analysis which confirm the presence of the metal complexes inside the supercages of the zeolite. Mössbauer spectroscopy technique analysis confirms the presence of the Fe3+- complexes in octahedral coordination. The new heterogeneous catalysts were catalytic evaluated by phenol hydroxylation and compared with the encapsulated metal furanyl-azine complexes in NaY zeolite. The zeolite themselves do not present any activity and the presence of the metal complexes improve their activity. All heterogeneous catalysts enhance higher conversion of phenol to catechol.

Hydroxylation of Phenol Catalyzed by Iron Metal-Organic Framework (Fe-BTC) with Hydrogen Peroxide

Liquid phase catalytic hydroxylation of phenol by Fe-containing metal-organic framework, Fe-BTC (BTC = 1,3,5-benzenetricarboxylate) using 30% H2O2 as an oxidant and H2O as solvent showed good activity and stability under mild reaction conditions. Phenol reacts with hydrogen peroxide over Fe-BTC to produce two main products, viz., catechol and hydroquinone. The effect of temperature, time, substrate/hydrogen peroxide mole ratio and amount of catalyst on catalytic performance were studied. The catalyst could be reused four times without losing significant loss of catalytic performance. The crystallinity and structure of catalyst were unchanged during the catalysis reaction, as confirmed by comparison of XRD and SEM of the fresh and reused catalyst. A reaction mechanism is proposed based on the experimental results.

Zeolite encapsulated Ni(ii) Schiff-base complexes: improved catalysis and site isolation

Site isolation and distorted geometry of the zeolite encapsulated complex govern the improved catalysis for phenol hydroxylation reaction.

The diquaternary ammonium surfactant-directed synthesis of single-unit-cell nanowires of ZSM-5 zeolite.

Hierarchical ultrathin ZSM-5 zeolite samples with controllable crystal growth directions were realized by designing bifunctional soft templates with different hydrophobic alkyl chain groups. MFI-type crystal growth along the a-c plane and stacking along the b-axis could be controlled to form nanowires of ZSM-5, which exhibited better catalytic activity for phenol hydroxylation than nanosheets of ZSM-5.

Two Synthesis Methods for Fe(III)@MOF‐5‐derived Porous Carbon Composites for Enhanced Phenol Hydroxylation

AbstractMOF‐5‐derived porous carbon (MDPC) materials, MDPC‐600 and MDPC‐1000, were prepared by pyrolysis at 600 °C in nitrogen atmosphere prior to acid treatment and at 1000 °C in nitrogen atmosphere, respectively. The samples were characterized by X‐ray diffraction (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), X–MaxN energy spectrum, and transmission electron microscopy (TEM). The test results of MDPC‐1000 and MDPC‐600 show that both are microporous amorphous carbon with the 2.7 nm diameter pore size, indicating that the micropore of MOF‐5 is well preserved during pyrolysis. MDPC‐1000 and MDPC‐600 have specific surface areas of 1570.9 m2/g and 1029.5 m2/g, respectively. MDPC‐1000 and MDPC‐600 were loaded with Fe ions to prepare Fe(III)/MDPC composite materials, which were used as catalysts for phenol hydroxylation. The results show that Fe(III)/MDPC‐1000 has higher catalytic effect than Fe(III)/MDPC‐600. During one hour phenol hydroxylation at 80 °C with 3 wt.% Fe concentration and a mass ratio of catalyst to phenol of 0.053, Fe(III)/MDPC‐1000 provides maximum phenol conversion, dihydroxybenzene yield, and dihydroxybenzene selectivity of 61.4%, 54.3%, and 88.4%, respectively.