FRINK Linked Data Fragments server

Matches in SemOpenAlex for { <https://semopenalex.org/work/W1967218692> ?p ?o ?g. }

• W1967218692 endingPage "185" @default.
• W1967218692 startingPage "182" @default.
• W1967218692 abstract "Sift fluorene into a bowl: Manganese oxide octahedral molecular sieves (OMS-2), with the overall composition KMn8O16⋅n H2O, catalyze the mild, green, and efficient oxidation of 9H-fluorene to 9-fluorenone. The involvement of lattice oxygen species has been implicated in a free-radical chain mechanism. In terms of reaction kinetics, the breaking of the CH bond is rate controlling. 9H-Fluorene falls under the polycyclic alkylarene family of compounds, the oxidation of which has elicited much attention lately. For instance, polycene derivatives can be applied to organic semiconductors and transistors.1 However, many oxidation reactions have relied on stoichiometric amounts of CrVI, MnVII, and OsVIII that generate large quantities of toxic metal wastes2 and are performed under relatively forced conditions. Additionally, from the standpoint of environmental friendliness, economy, and atom efficiency, attention has shifted to the development of methods that employ clean oxidants such as molecular oxygen.3 9-Fluorenone is an important compound that is finding promising uses as a component in organic solar cells and display devices. It also represents an interesting class of compounds for biomedical applications.4 9-Fluorenone has been synthesized by the oxidation of 9H-fluorene in the presence of alkali-metal hydroxides,5 by using triton B as a catalyst in the presence of pyridine,6 and by using oxygen in the presence of cobalt bromide.7 Laboratory-scale preparation involves palladium-catalyzed cyclization of o-iodobenzophenones8 and carbonylation of o-halobiaryls.9 Here, we explore the detailed oxidation of 9H-fluorene to 9-fluorenone with manganese oxide octahedral molecular sieves (OMS) as a selective catalyst, using air as an environmentally friendly oxidant. The oxidation is carried out efficiently without use of any additives, promoters, or radical initiators, hence representing a benign process. Emphasis is laid on the kinetics of 9H-fluorene oxidation using the initial rates and kinetic isotope effect (KIE). We have also effectively identified OMS-2 catalysts that show excellent performance in this oxidation process and a solvent that is 100 % selective to the desired product. OMS-2 exhibits one-dimensional tunnels built by 2×2 edge- and corner-shared MnO6 octahedral chains, which form infinite 3D frameworks with molecule-sized tunnels similar to those found in naturally occurring zeolites.10, 11, 12 OMS is mixed valent with manganese found in oxidation states Mn2+, Mn3+, and Mn4+. OMS-2 materials have been synthesized by use of different synthetic routes and characterized.13 Some of the synthetic routes utilized are reflux methods14 (OMS-2R), solvent-free methods15 (OMS-2S), hydrothermal methods16 (OMS-2H), and constant-frequency microwave techniques17 (OMS-2MW). Reports suggest that only a small amount of Mn3+ exists and that the majority of manganese is in the Mn4+ state.18 The oxidation of 9H-fluorene goes to completion in 4 h (Scheme 1). 9H-fluorene is not oxidized in the absence of OMS-2, implying that OMS-2 catalyzes this reaction (Table 1, entry 1). The best solvent for the reaction was also studied. Having established that 9H-fluorene oxidation in toluene led to the formation of 9,9′-bifluorene, a side product, at selectivities of 4 and 5 % (Table 1, entries 3 and 4), other solvents were chosen to study the same reaction and determine their effect on selectivity. Non-polar solvents led to better conversions than polar solvents. These solvents are either aliphatic or aromatic hydrocarbons, and all of them led to formation of a side product except isooctane, a branched aliphatic that was 100 % selective to 9-fluorenone at a lower temperature. Isooctane suppressed the intermediates that lead to formation of 9,9′-bifluorene as a side product by not interacting adversely with the intermediate species, thus availing them to the active sites of the catalyst for completion of the oxidation cycle. However, other non-coordinating solvents such as octane led to formation of 9,9′-bifluorene. Thus, isooctane behaves quite uniquely in this reaction. Branched hydrocarbons are more stable than unbranched hydrocarbons as a result of the interplay between attractive and repulsive forces within the molecule (intramolecular forces). As a further test of this hypothesis, 2,4-dimethylpentane and 2,3,4-trimethylpentane were also used in fluorene oxidation. 2,3,4-trimethylpentane was 100 % selective, however, 2,4-dimethylpentane led to formation of the side product. Not only is branching important but so is the number of branches. Attractive forces within a branched hydrocarbon increase as the structure becomes more compact. The proton-exchanged catalyst, H-K-OMS-2, led to a lower conversion of 9H-fluorene (Table 1, entry 4), thus indicating that the reaction is inhibited by the presence of Brønsted acid sites. Proposed mechanism for the OMS-2-catalyzed oxidation of 9H-fluorene. Entry OMS-2 [mg] Solvent Conv. [%][b] Selectivity [%][c] 1 0[d] toluene 0 0 2 50 isooctane[e] 99 100 3 50 toluene 98 96 4 50[f] toluene 57 95 5 50 octane 74 95 6 50 CH3CN 12 98 7 50 THF 4 98 8 50 hexane 38 96 9 50 dioxane 0 0 The OMS-2 catalyst was washed thoroughly with methanol and dried following the oxidation reaction. The X-ray diffraction pattern of the regenerated catalyst revealed that the OMS-2 structure was maintained after reaction, indicating that it was not a stoichiometric oxidant. Used in a second reaction, the regenerated catalyst led to a decreased conversion of 30 % after 4 h. A turnover number (TON) of 16 after the first reaction was obtained, and a total TON of 20 was observed for both reactions. The lowering in activity may be attributed to the blocking of active sites by the organics after the first reaction. The kinetics of the oxidation of 9H-fluorene was followed at the very beginning when conversions were relatively low and hence could be relied on as being representative of the true kinetics of the reaction (Figure 1). 9H-Fluorene is oxidized selectively and at a faster rate in isooctane than in toluene. When the reaction is performed at 80 °C in toluene, the rate was lower than that for the reaction heated at reflux, indicative of the thermodynamic dependency of this reaction in toluene. After thermally treating the catalyst at 300 °C under nitrogen atmosphere and by using nitrogen instead of air in the oxidation reaction, no conversion of the starting material was observed implying that dissolved oxygen is involved in the oxidation cycle. To probe the outcome of the reaction when the catalyst was calcined at a lower temperature, OMS-2 was heated in air at 170 °C and the reaction was carried out under nitrogen atmosphere. Interestingly, there was activity as 9-fluorenone was obtained in 30 min but at a lower conversion of 10 %. The activity pointed to the involvement of lattice oxygen species in the oxidation cycle. Temperature-programmed desorption mass spectrometry and thermogravimetric analyses of this catalyst show that at 170 °C the only loss of weight from the catalyst is by evolution of water as no other species are detected with the mass spectrometer, but above 300 °C oxygen species are detected.19 This observation implies that surface oxygen species are still present at a lower temperature. There was a drastic drop in the rates of generation of 9-fluorenone when carbon tetrachloride (CCl4) was added into the reaction vessel. CCl4 lowered the rate of formation of the reactive intermediates by either quenching them, thus hindering the oxidation cycle, or by blocking the active sites thus making them unavailable for any further oxidation of 9H-fluorene. Kinetics of 9H-fluorene oxidation with octahedral molecular sieves under different conditions: in isooctane (□), in toluene at 110 °C (○), and in toluene at 80 °C (▵); oxidation of [D10]-fluorene in toluene at 110 °C (▿); oxidation of 9H-fluorene in toluene in the presence of 0.2 equiv CCl4 (×) and in toluene under nitrogen atmosphere (◊). Kinetically relevant elementary steps in the oxidation of 9H-fluorene, measured by replacing H with D in 9H-fluorene and its effect on the rates of reaction, gave a KIE value of 5.38 (in the absence of tunneling effects). From a pseudo-first-order plot of ln[9H-fluorene]o/[9H-fluorene]t versus time (ln Xo/X vs time; Figure 2), the slopes, with correlations of 0.9752 for the oxidation of 9H-fluorene and 0.9222 for oxidation of [D10]-fluorene, represent the rate constants kH and kD for 9H-fluorene and [D10]-fluorene, respectively. Early transition states for elementary CH bond-activation steps would give KIE values of approximately 1, and late transition states in which the relevant CH bond is significantly cleaved would give values as high as around 7.20, 21 Oxidation of 9H-fluorene on OMS-2 thus represents late transition states, in which the breaking of the CH bond is the rate-determining step. Kinetic isotope effect (KIE) in the oxidation of 9H-fluorene (○) and [D10]-fluorene (□) based on data for the oxidation of 9H-fluorene and [D10]-fluorene in toluene at 110 °C shown in Figure 1. Xo corresponds to the original concentration, and X represents the concentration at any particular time. On the basis of our findings, we propose a series of steps that plausibly constitute the reaction mechanism of OMS-2-catalyzed oxidation of 9H-fluorene. Mnn+ (n=2, 3, or 4), the lattice oxygen species (O2−, O2−, or O−), and dissolved oxygen (3O2 or 1O2) are involved in the oxidation cycle. The initiation step involves either dissolved oxygen in H abstraction to generate hydroperoxy radicals or CH bond cleavage, attributed to H abstraction by lattice metal-oxo or hydroxy species of OMS-2. The high KIE value is attributed to this step. Homolytic cleavage generates reactive species: free fluorene radicals. Once formed, the free radicals quickly react with dissolved molecular oxygen to form peroxy radicals. In an alternative route, they may also react with hydroperoxy radicals to generate hydroperoxides. The peroxy radicals are stabilized by reaction with neutral 9H-fluorene molecules to form hydroperoxides and generate further 9H-fluorene radicals. Manganese active sites are responsible for catalyzing the homolytic decomposition of the intermediate hydroperoxides to yield the ketone product with evolution of a molecule of water (Scheme 1). GC-MS and 1H NMR analyses did not reveal 9H-fluorenol as a product. As OMS-2 catalyzes the oxidation of alcohols22 and hydrocarbons, then the presence of an alcohol product is a priori not excluded, albeit, its existence may be curtailed by its short lifetime which results from its simultaneous oxidation on OMS-2. The side product 9,9′-bifluorene can be accounted for by the termination of the cycle involving two 9H-fluorene radicals. OMS-2 is regenerated at the end of the cycle. A systematic investigation of the oxidation of 9H-fluorene using different OMS-2 catalysts (Table 2) led to the isolation of the two most active catalysts for this reaction. OMS-2B and OMS-2S, prepared differently, were identified as the most active catalysts for 9H-fluorene oxidation. These were followed by OMS-2R, OMS-2MW, and OMS-2H, respectively. These catalysts have differing average oxidation states (AOS), crystallite sizes, surface areas, Lewis acidities, and mesopore volumes. Entry OMS AOS[b] Crystallite size [nm][c] Conv. [%][d] Selectivity [%][e] 1 OMS-2B 3.7 8 98 96 2 OMS-2R 3.9 18 51 98 3 OMS-2H 3.7 17 6.3 98 4 OMS-2S 3.7 10 95 97 5 OMS-2MW 3.9 18 37 50 The most active catalysts showed the lowest crystallite sizes as compared to the others (Table 2). They also exhibited low AOS values (3.7) and relatively higher mesopore volumes at 98 % and 92 % of total pore volumes, respectively. Small crystallites are the most apparent features of both and a reason for the high activity exhibited by both catalysts. At lower crystallite sizes, the material can fracture leading to disjointing of the tunnels, which creates more defects and exposes active sites. Other than crystallite sizes, a combination of these features may also be responsible for their high activity. A trend cannot effectively be tied to Lewis acidity. In summary, a mild, green, and efficient OMS-2 catalyzed oxidation of 9H-fluorene to 9-fluorenone was successfully performed. Our study effectively identified the most active mixed-valent OMS-2 catalysts. The use of isooctane as solvent makes this reaction very selective and sustainable. In terms of kinetics the breaking of the CH bond is rate-controlling, and mechanistically the involvement of lattice oxygen species has been implicated in a free-radical chain mechanism. This report with OMS-2 catalysts is seminal and markedly different to those for the selective oxidation of alcohols.17 There is a need to explore the nature of lattice oxygen species, the nature of dissolved oxygen, and the exact state of the manganese centers that act as active sites. We also hope to extend these findings to cover oxidation trends of other substrates in this family of hydrocarbons using solvents with a higher degree of branching. Synthesis of manganese oxide octahedral molecular sieves (OMS-2): OMS-2 was prepared by a modification to the reflux method.19 Thus, 1 % hydrogen peroxide (H2O2) solution (40 mL) was added to a buffer solution made by adding acetic acid (5 mL) to a solution of potassium acetate (5 g) in distilled deionized water (40 mL). A solution of potassium permanganate (6.5 g) in distilled deionized water (15 mL) was then added dropwise to this solution while stirring. The resulting mixture was heated at reflux for 24 h, and the product was filtered, washed thoroughly, dried overnight at 80 °C in air, and then further dried for 2 h in air at 120 °C to give OMS-2B. Other OMS-2 catalysts were prepared according to reported procedures (Supporting Information). Oxidation of 9H-fluorene: Prior to each reaction, OMS-2B (and other OMS-2 catalysts) was ground to a fine powder and activated by drying in an oven at 120 °C for 3 h. The following standard procedure was used for all oxidation runs: the activated catalyst (50 mg, 0.0625 mmol) was added to a mixture of 9H-fluorene (1 mmol) dissolved in the given solvent (15 mL) in a 25-mL round-bottomed flask. The flask was immersed in an isothermal (±0.5 °C) paraffin oil bath. Air at 1 atm was introduced to this set-up at an optimum flow rate. The reactions were carried out with vigorous stirring for up to 4 h. Kinetics of 9H-fluorene oxidation: Kinetics studies were performed using a 50-mL three-necked round-bottomed flask. OMS-2 (100 mg) was added to a mixture of 9H-fluorene (1 mmol) dissolved in toluene (15 mL). Air was introduced through one of the necks, and the flask was immersed in an isothermal paraffin oil bath. Periodic sampling was used to monitor the progress of the reaction. Sampling was performed using a GC injection needle fitted with an in-line filter to exclude any catalyst particles, and the product was analyzed and quantified. Time t=0 was defined as the time just after the three-necked flask was immersed in the isothermal oil bath. In other kinetics experiments, 0.2 equiv carbon tetrachloride (CCl4) was used or nitrogen was employed instead of air. Kinetic isotope effect (KIE): KIE was determined using [D10]-fluorene (1 mmol). Sampling was performed periodically for the first 35 min approximately. We thank the Department of Energy, Office of Basic Energy Sciences, and the Division of Chemical Sciences for supporting this research. We also appreciate input from and discussions with Edward K. Nyutu and Shanthakumar Shithambaram, and are grateful to Randy K. Jackson for running 1H NMR experiments. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2476/2008/z700094_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article." @default.
• W1967218692 created "2016-06-24" @default.
• W1967218692 creator A5030445732 @default.
• W1967218692 creator A5057443174 @default.
• W1967218692 creator A5062514204 @default.
• W1967218692 creator A5084155540 @default.
• W1967218692 date "2008-03-17" @default.
• W1967218692 modified "2023-10-12" @default.
• W1967218692 title "Kinetics and Mechanism of 9H-Fluorene Oxidation Catalyzed by Manganese Oxide Octahedral Molecular Sieves" @default.
• W1967218692 cites W1628347788 @default.
• W1967218692 cites W1992477591 @default.
• W1967218692 cites W1999665858 @default.
• W1967218692 cites W2000225983 @default.
• W1967218692 cites W2002557071 @default.
• W1967218692 cites W2012757735 @default.
• W1967218692 cites W2031954089 @default.
• W1967218692 cites W2042144643 @default.
• W1967218692 cites W2047074636 @default.
• W1967218692 cites W2052087768 @default.
• W1967218692 cites W2054495841 @default.
• W1967218692 cites W2057439332 @default.
• W1967218692 cites W2068047715 @default.
• W1967218692 cites W2073601355 @default.
• W1967218692 cites W2082536609 @default.
• W1967218692 cites W2083833784 @default.
• W1967218692 cites W2085836399 @default.
• W1967218692 cites W2093869507 @default.
• W1967218692 cites W2094007850 @default.
• W1967218692 cites W2125167161 @default.
• W1967218692 cites W2128615147 @default.
• W1967218692 cites W2332899488 @default.
• W1967218692 cites W2861074768 @default.
• W1967218692 cites W2950655361 @default.
• W1967218692 cites W2952227129 @default.
• W1967218692 cites W4230494025 @default.
• W1967218692 cites W4239615045 @default.
• W1967218692 doi "https://doi.org/10.1002/cssc.200700094" @default.
• W1967218692 hasPubMedId "https://pubmed.ncbi.nlm.nih.gov/18605202" @default.
• W1967218692 hasPublicationYear "2008" @default.
• W1967218692 type Work @default.
• W1967218692 sameAs 1967218692 @default.
• W1967218692 citedByCount "26" @default.
• W1967218692 countsByYear W19672186922012 @default.
• W1967218692 countsByYear W19672186922013 @default.
• W1967218692 countsByYear W19672186922014 @default.
• W1967218692 countsByYear W19672186922015 @default.
• W1967218692 countsByYear W19672186922016 @default.
• W1967218692 countsByYear W19672186922017 @default.
• W1967218692 countsByYear W19672186922018 @default.
• W1967218692 countsByYear W19672186922019 @default.
• W1967218692 countsByYear W19672186922020 @default.
• W1967218692 countsByYear W19672186922021 @default.
• W1967218692 countsByYear W19672186922022 @default.
• W1967218692 countsByYear W19672186922023 @default.
• W1967218692 crossrefType "journal-article" @default.
• W1967218692 hasAuthorship W1967218692A5030445732 @default.
• W1967218692 hasAuthorship W1967218692A5057443174 @default.
• W1967218692 hasAuthorship W1967218692A5062514204 @default.
• W1967218692 hasAuthorship W1967218692A5084155540 @default.
• W1967218692 hasConcept C105281773 @default.
• W1967218692 hasConcept C111472728 @default.
• W1967218692 hasConcept C115624301 @default.
• W1967218692 hasConcept C121332964 @default.
• W1967218692 hasConcept C138885662 @default.
• W1967218692 hasConcept C148898269 @default.
• W1967218692 hasConcept C161790260 @default.
• W1967218692 hasConcept C178790620 @default.
• W1967218692 hasConcept C179104552 @default.
• W1967218692 hasConcept C185592680 @default.
• W1967218692 hasConcept C198819137 @default.
• W1967218692 hasConcept C2776211377 @default.
• W1967218692 hasConcept C2993225395 @default.
• W1967218692 hasConcept C521977710 @default.
• W1967218692 hasConcept C528890316 @default.
• W1967218692 hasConcept C62520636 @default.
• W1967218692 hasConcept C65024703 @default.
• W1967218692 hasConcept C89611455 @default.
• W1967218692 hasConceptScore W1967218692C105281773 @default.
• W1967218692 hasConceptScore W1967218692C111472728 @default.
• W1967218692 hasConceptScore W1967218692C115624301 @default.
• W1967218692 hasConceptScore W1967218692C121332964 @default.
• W1967218692 hasConceptScore W1967218692C138885662 @default.
• W1967218692 hasConceptScore W1967218692C148898269 @default.
• W1967218692 hasConceptScore W1967218692C161790260 @default.
• W1967218692 hasConceptScore W1967218692C178790620 @default.
• W1967218692 hasConceptScore W1967218692C179104552 @default.
• W1967218692 hasConceptScore W1967218692C185592680 @default.
• W1967218692 hasConceptScore W1967218692C198819137 @default.
• W1967218692 hasConceptScore W1967218692C2776211377 @default.
• W1967218692 hasConceptScore W1967218692C2993225395 @default.
• W1967218692 hasConceptScore W1967218692C521977710 @default.
• W1967218692 hasConceptScore W1967218692C528890316 @default.
• W1967218692 hasConceptScore W1967218692C62520636 @default.
• W1967218692 hasConceptScore W1967218692C65024703 @default.
• W1967218692 hasConceptScore W1967218692C89611455 @default.
• W1967218692 hasIssue "3" @default.
• W1967218692 hasLocation W19672186921 @default.
• W1967218692 hasLocation W19672186922 @default.

• next