Selective Oxidation Of Alkanes Research Articles

Selective Catalytic Oxidation of Alcohols, Aldehydes, Alkanes and Alkenes Employing Manganese Catalysts and Hydrogen Peroxide

AbstractThe manganese‐containing catalytic system [MnIV,IV2O3(tmtacn)2]2+ (1)/carboxylic acid (where tmtacn=N,N′,N′′‐trimethyl‐1,4,7‐triazacyclononane), initially identified for the cis‐dihydroxylation and epoxidation of alkenes, is applied for a wide range of oxidative transformations, including oxidation of alkanes, alcohols and aldehydes employing H2O2 as oxidant. The substrate classes examined include primary and secondary aliphatic and aromatic alcohols, aldehydes, and alkenes. The emphasis is not primarily on identifying optimum conditions for each individual substrate, but understanding the various factors that affect the reactivity of the Mn‐tmtacn catalytic system and to explore which functional groups are oxidised preferentially. This catalytic system, of which the reactivity can be tuned by variation of the carboxylato ligands of the in situ formed [MnIII,III2(O)(RCO2)2(tmtacn)2]2+ dimers, employs H2O2 in a highly atom efficient manner. In addition, several substrates containing more than one oxidation sensitive group could be oxidised selectively, in certain cases even in the absence of protecting groups.magnified image

Redox State Dynamics at the Surface of MoVTe(Sb)NbO M1 Phase in Selective Oxidation of Light Alkanes

The influence of Te or Sb on the catalytic properties of the M1 phase of MoVTe(Sb)NbO catalysts in the oxidation of propane or oxidative dehydrogenation of ethane has been investigated. The results show that Te and Sb affect the intrinsic activity of the catalyst for ethane and propane oxidation in the same way and that the Te-containing M1 phase was significantly more active than the Sb-containing phase, even when they contained the same amount of vanadium, which is known to be the active species for the reactions. This feature has been explained by a strong modification of the relative number of available V5+ sites, which is related to the fact that Sb tend to stabilize vanadium in active site, in a reduced state. Sb and Te were also shown to affect the selectivity in the case of propane oxidation but not in the case of ethane oxidative dehydrogenation. This demonstrated that they should be involved directly in the further transformation of the alkene molecules that are formed as an intermediate and presumably play a role in the α-hydrogen abstraction from these molecules, as proposed earlier. Blocking problems related to the substitution of Te by Sb in view of industrialization are also discussed.

Fe(PyTACN)‐Catalyzed cis‐Dihydroxylation of Olefins with Hydrogen Peroxide

AbstractA family of iron complexes with general formula [Fe(II)(R,Y,XPyTACN)(CF3SO3)2], where R,Y,XPyTACN=1‐[2′‐(4‐Y‐6‐X‐pyridyl)methyl]‐4,7‐dialkyl‐1,4,7‐triazacyclononane, X and Y refer to the groups at positions 4 and 6 of the pyridine, respectively, and R refers to the alkyl substitution at N‐4 and N‐7 of the triazacyclononane ring, are shown to be catalysts for efficient and selective alkene oxidation (epoxidation and cis‐dihydroxylation) employing hydrogen peroxide as oxidant. Complex [Fe(II)(Me,Me,HPyTACN)(CF3SO3)2] (7), was identified as the most efficient and selective cis‐dihydroxylation catalyst among the family. The high activity of 7 allows the oxidation of alkenes to proceed rapidly (30 min) at room temperature and under conditions where the olefin is not used in large amounts but instead is the limiting reagent. In the presence of 3 mol% of 7, 2 equiv. of H2O2 as oxidant and 15 equiv. of water, in acetonitrile solution, alkenes are cis‐dihydroxylated reaching yields that might be interesting for synthetic purposes. Competition experiments show that 7 exhibits preferential selectivity towards the oxidation of cis olefins over the trans analogues, and also affords better yields and high [syn‐diol]/[epoxide] ratios when cis olefins are oxidized. For aliphatic substrates, reaction yields attained with the present system compare favourably with state of the art Fe‐catalyzed cis‐dihydroxylation systems, and it can be regarded as an attractive complement to the iron and manganese systems described recently and which show optimum activity against electron‐deficient and aromatic olefins.

A sandwich N-doped graphene/Co3O4 hybrid: an efficient catalyst for selective oxidation of olefins and alcohols

A sandwich-like N-doped graphene/Co3O4 hybrid was prepared via a simple one-pot hydrothermal reaction in a solution of NH3. Characterizations disclosed that highly dispersed Co3O4 nanoparticles with dominant exposed {112} and {110} planes were fabricated on both sides of well-exfoliated N-doped graphene; N-dopants in the graphene matrix can prevent re-graphitization of graphene, strengthen the interaction between Co3O4 and the graphene matrix, and improve the dispersion of Co3O4. This hybrid (Co3O4/RGO-N) exhibited superior activity and stability for the epoxidation of styrene compared to bulk Co3O4 and N-free graphene supported Co3O4. At the same time, the resulting catalyst also showed high compatibility with various olefins and alcohols with good conversion and high selectivity. This synergistic strategy can provide simple, efficient and versatile blue-prints for low-cost fabrication of graphene-based nanocomposites for extending applications where graphene has rarely been exploited and beyond.

Structural and electronic promotion with alkali cations of silica-supported Fe(III) sites for alkane oxidation

Promoters and precursors can control oxide phase, dispersion, and per-site reactivity of supported oxide catalysts. Previously, dispersed FeOx–SiO2 resulted from Fe3+ ethylenediaminetetraacetate (FeEDTA−) precursors, with NaFeEDTA giving enhanced dispersion and oxidation rates vs. NH4FeEDTA. Here, catalysts were synthesized by sequential alkali deposition and Fe3+ impregnation. At up to 0.9Fenm−2 from NH4FeEDTA and equimolar alkali, UV–visible and H2 TPR were consistent with isolated Fe3+ and small FeOx clusters. Omitting alkali, using Fe(NO3)3, or using Fe/alkali >1 gave evidence of larger agglomerates. For Fe/alkali ≤1 on non-porous SiO2, initial turnover frequencies in adamantane oxidation using H2O2 were independent of surface density. TOF increased as 6.3, 8.8, 15.4, and 20.9 (±0.3)ks−1 for Li+, Na+, K+, and Cs+, respectively, increasingly linearly with decreasing electronegativity. These results give a synthesis–structure–function taxonomy with alkali as an electronic and structural promoter of dispersed FeOx species for alkane selective oxidation.

ChemInform Abstract: Rhenium Nanochemistry for Catalyst Preparation

AbstractReview: 57 refs.

Wacker oxidation of ethylene over vanadia nanotube supported Pd catalysts

The selective oxidation of olefins to aldehydes and ketones by O2 using PdCl2/CuCl2 catalyst system in hydrochloric acid solution is known as Wacker process. Because of the advantages of heterogeneous catalytic systems there is great industrial interest in the development of solid Wacker catalyst. The present paper relates to the preparation and properties of novel Pd/vanadia nanotube catalysts and their activity in the Wacker oxidation of ethylene. The VOx nanotubes, having coil structure, were shown to be stabilized by interlayer cations. The as-synthesized VOx nanotubes had diameter of about 70–120nm and contained exchangable interlayer alkylammonium (AA) cations. By ion exchange VO2+ and Cu2+ cationic forms were obtained. All three forms were tested as catalyst support. Palladium nanoparticles of 8–12nm sizes were generated on the surface of the VOx tubes. The best acetaldehyde selectivity was achieved with the Pd/AA–VOx catalyst.

Rhenium Nanochemistry for Catalyst Preparation

The review presents synthetic approaches to modern rhenium-based catalysts. Creation of an active center is considered as a process of obtaining a nanoparticle or a molecule, immobilized within a matrix of the substrate. Selective chemical routes to preparation of particles of rhenium alloys, rhenium oxides and the molecules of alkyltrioxorhenium, and their insertion into porous structure of zeolites, ordered mesoporous MCM matrices, anodic mesoporous alumina, and porous transition metal oxides are considered. Structure-property relationships are traced for these catalysts in relation to such processes as alkylation and isomerization, olefin metathesis, selective oxidation of olefins, methanol to formaldehyde conversion, etc.

Unprecedented Selective Oxidation of Styrene Derivatives using a Supported Iron Oxide Nanocatalyst in Aqueous Medium

AbstractIron oxide nanoparticles supported on mesoporous silica‐type materials have been successfully utilized in the aqueous selective oxidation of alkenes under mild conditions using hydrogen peroxide as green oxidant. The supported catalyst could be easily recovered after completion of the reaction and reused several times without any loss in activity (no metal leaching observed during the reaction), constituting a facile and straightforward example of aqueous oxidation chemistry promoted by iron‐based heterogeneous systems.

Total Synthesis of the Potent cAMP Signaling Agonist (−)-Alotaketal A

We have developed a convergent synthetic route to the potent cAMP signaling agonist (-)-alotaketal A that employs two stages of SmI(2)-mediated reductive allylation reactions for assembling the polycycle and fragment coupling. Also notable are a Hg(OAc)(2)-mediated selective alkene oxidation and the subtlety of the formation of the unprecedented spiroketal ring system. The probes AKAR4 and ICUE3 were used to evaluate the cAMP singaling agonistic activity of (-)-alotaketal A and elucidate its structure-activity relationship.

Properties of metal oxide clusters in non-traditional oxidation states

Oxygen vacancies on metal oxide surfaces create active sites for catalytic processes, ranging from selective oxidation of alkenes to photocatalytic decomposition of water. Experimentally, there are numerous challenges associated with fully characterizing the role of oxygen vacancies in bulk systems. We present a bottom-up approach, probing the structures and electronic properties of small metal oxide clusters, and relating these properties to chemical reactivity. The work presented focuses on unary and binary Group 6 metal oxides (Mo and W) and their reactivity toward H2O and CO2. We additionally highlight more recent studies on binary Group 5/Group 6 metal oxides.

Nanoaggregates of Mn(III)tetraperfluorophenylporphyrin: a greener approach for allylic oxidation of olefins

Organic nanoparticles of metalloporphyrins can be a versatile catalyst for the selective oxidation of alkenes and other hydrocarbons. The catalytic activity of the metalloporphyrin depends on the nature of the central metal atom, peripheral groups, and the architecture of the porphyrin macrocycle. Herein, we report the catalytic activity of organic nanoparticles of 5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)porphyrinato manganese(III), Mn (III)TPPF20, for the oxidation of cyclohexene using molecular oxygen as an oxidant in aqueous solvent under ambient conditions. While the solvated metalloporphyrins catalytically oxidize alkenes to the corresponding epoxide with a modest turn-over numbers, ca. 30 nm organic nanoparticles of Mn (III)TPPF20 have enhanced catalytic activity with up to a two-fold greater turn-over number and yields only allylic oxidation products. The activity of organic nanoparticles is slow compared to the solvated metalloporphyrins. These organic nanoparticles catalytic systems facilitate a greener reaction since ca. 89% of the reaction medium is water, molecular oxygen is used in place of man-made oxidants, and the ambient reaction conditions require less energy. This organic nanoparticle catalytic system also avoids using halogenated solvents commonly used in solution phase reactions. The enhanced catalytic activity of these organic nanoparticles is unexpected because the metalloporphyrins in the nanoaggregates are in the close proximity and the turn-over number should diminish by self-oxidative degradation.

Manganese Triazacyclononane Oxidation Catalysts Grafted under Reaction Conditions on Solid Cocatalytic Supports

Manganese complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) are highly active and selective alkene oxidation catalysts with aqueous H(2)O(2). Here, carboxylic acid-functionalized SiO(2) simultaneously immobilizes and activates these complexes under oxidation reaction conditions. H(2)O(2) and the functionalized support are both necessary to transform the inactive [(tmtacn)Mn(IV)(μ-O)(3)Mn(IV)(tmtacn)](2+) into the active, dicarboxylate-bridged [(tmtacn)Mn(III)(μ-O)(μ-RCOO)(2)Mn(III)(tmtacn)](2+). This transformation is assigned on the basis of comparison of diffuse reflectance UV-visible spectra to known soluble models, assignment of oxidation state by Mn K-edge X-ray absorption near-edge spectroscopy, the dependence of rates on the acid/Mn ratios, and comparison of the surface structures derived from density functional theory with extended X-ray absorption fine structure. Productivity in cis-cyclooctene oxidation to epoxide and cis-diol with 2-10 equiv of solid cocatalytic supports is superior to that obtained with analogous soluble valeric acid cocatalysts, which require 1000-fold excess to reach similar levels at comparable times. Cyclooctene oxidation rates are near first order in H(2)O(2) and near zero order in all other species, including H(2)O. These observations are consistent with a mechanism of substrate oxidation following rate-limiting H(2)O(2) activation on the hydrated, supported complex. This general mechanism and the observed alkene oxidation activation energy of 38 ± 6 kJ/mol are comparable to H(2)O(2) activation by related soluble catalysts. Undesired decomposition of H(2)O(2) is not a limiting factor for these solid catalysts, and as such, productivity remains high up to 25 °C and initial H(2)O(2) concentration of 0.5 M, increasing reactor throughput. These results show that immobilized carboxylic acids can be utilized and understood like traditional carboxylic acids to activate non-heme oxidation catalysts while enabling higher throughput and providing the separation and handling benefits of a solid catalyst.

ChemInform Abstract: The Role of Higher Oxidation State Species in Platinum‐Mediated C—H Bond Activation and Functionalization

AbstractReview: 103 refs.

Low-temperature aerobic oxidation of decane using an oxygen-free radical initiator

The direct selective oxidation of long chain alkanes by O 2 is a highly demanding reaction. We have shown that it is possible to oxidise n-decane in the presence of the oxygen-free radical initiator azobisisobutyronitrile. Formation of a range of oxygenated products has been observed under relatively mild conditions (70 °C in air). Although the presence of a catalyst is not essential when the initiator is used, ceria-based catalysts have been found to increase the selectivity to alcohols by modifying the oxyfunctionalisation of decane.

Preparation of Pt@Fe2O3 Nanowires and their Catalysis of Selective Oxidation of Olefins and Alcohols

Iron oxide coated platinum nanowires (Pt@Fe(2)O(3)NWs) with a diameter of 2.8 nm have been prepared by the oxygen oxidation of FePt NWs in oleylamine. These "cable"-like NWs were characterised by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and X-ray absorption fine structure analysis. These Pt@Fe(2)O(3) NWs were used as "non-support" heterogeneous catalysts in oxidation of olefins and alcohols. The results revealed that it is an active and highly selective catalyst. Styrene derivatives were tested with molecular oxygen as the sole oxidant, with benzaldehyde successfully obtained from styrene in an absolute yield of 31%, whereas the use of tert-butyl hydroperoxide as the sole oxidant in the oxidation of alcohols led to yields of more than 80% of the corresponding ketone or aldehyde. This unsupported catalyst was found to be more active (TOF=96.5 h(-1)) than other reported Fe(2)O(3) nanoparticle catalysts and could be recycled multiple times without any notable decrease in activity. Our findings will extend the use of such nanomaterial catalysts to new catalytic systems.

Atmospheric‐Pressure, Liquid‐Phase, Selective Aerobic Oxidation of Alkanes Catalysed by Metal–Organic Frameworks

Aerobic oxidation of cyclooctane to its corresponding ol/one mixture at atmospheric pressure and in the liquid phase is efficiently promoted by an Fe(BTC) (BTC=1,3,5-benzenetricarboxylate) metal-organic framework, incorporating N-hydroxyphthalimide and in several cases reaches a selectivity over 90% at 28% conversion. This catalytic system is further extended to other hydrocarbons, such as ethylbenzene and 1,2,3,4-tetrahydronaphthalene (tetralin), with high selectivity (>85%). This high selectivity in the product distribution arises from a radical reaction mechanism that occurs inside a hydrophobic cavity that preferentially adsorbs hydrocarbons over their corresponding alcohols. The system can be reused although there is a gradual decrease in turnover frequency, caused by minor changes in the crystal structure due to the formation of iron oxide nanoparticles. Given the sustainable nature of the oxidant and the mild conditions used, this discovery could serve to develop a new catalyst generation for the oxyfunctionalisation of hydrocarbon feedstocks with the real possibility of finding industrial application.

Surface speciation and alkane oxidation with highly dispersed Fe(III) sites on silica

When highly dispersed, supported Fe oxides are selective alkane oxidation catalysts, but new syntheses are required to reliably produce such materials. Here, highly dispersed, supported Fe3+ catalysts are prepared via incipient wetness impregnation of SiO2 with aqueous Fe complexes of ethylenediaminetetraacetic acid (FeEDTA), followed by calcination. With Na+ countercations, UV–visible diffuse reflectance spectra are entirely below 300nm and H2 temperature-programmed reduction only shows reduction at ∼630°C for all loadings up to 2.15wt%, the maximum loading for a single impregnation cycle. These characteristics indicate isolated sites not seen for Fe(NO3)3 precursors even at 0.3wt%. NH4+ countercations lead to amorphous oxide oligomers and a minority species with unusual reducibility at 310°C. Na+ countercations produce ‘single-site’ behavior in adamantane oxidation using H2O2 with a specific turnover frequency of 9.2±0.8ks−1, constant for all Fe loadings and approximately 10 times higher than that of other well-dispersed Fe/SiO2 materials. Similar turnover frequencies are obtained when counting only the highly reducible species on the NH4+-derived catalyst, allowing these sites to be tentatively assigned as small, undercoordinated clusters that are both easily reduced and participate in alkane oxidation, reminiscent of Fe-exchanged MFI zeolites.

Selective oxidation of olefins and aromatic alcohols with tert‐butylhydroperoxide catalyzed by polymer‐anchored transition‐metal complexes

AbstractThree polymer‐anchored metal complexes (Co, Cu, and Pd) were synthesized and characterized. The catalytic performance of these complexes was tested for the oxidation of olefins and aromatic alcohols. These complexes showed excellent catalytic activity and high selectivity. These complexes selectively gave epoxides and aldehydes from olefins and alcohols, respectively. Individually, the effect of various solvents, oxidants, substrate oxidant molar ratios, temperatures, and catalyst amounts for the oxidation of cyclohexene and benzyl alcohol were studied. Under optimized reaction conditions, 96, 81, and 71% conversions of cyclohexene and 86, 79, and 73% conversions of benzyl alcohol were obtained with Co(II), Cu(II), and Pd(II) catalysts, respectively. The catalytic results reveal that these complexes could be recycled more than five times without much loss in activity. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Selective oxidation of alkenes using graphite-supported gold-palladium catalysts

Oxidation is an important route for the activation of chemical feedstocks for the synthesis of chemical intermediates. Alkene epoxidation by the electrophilic addition of oxygen to a carbon–carbon double bond is a major challenge in oxidation catalysis. In particular it is important to use molecular oxygen as the oxidant to avoid the formation of reagent by-products. We report the oxidation with air using graphite-supported gold-palladium catalysts of two alkenes, cis-cyclooctene, which gives mainly the epoxide, and crotyl alcohol (trans-but-2-en-1-ol). With cyclooctene, the reaction requires catalytic amounts of t-butyl hydroperoxide. The Au–Pd ratio has a major effect on the conversion with very low activities being associated with Au:Pd ratios of ca. 4:1 and 1:4 by weight. The selectivity to the epoxide is not affected by the Au:Pd ratio. With crotyl alcohol, t-butyl hydroperoxide was not required for activity. In the absence of Pd, crotonaldehyde was formed, but the introduction of Pd leads to an isomerisation pathway to 3-buten-1-ol being favoured over epoxidation and crotonaldehyde was a minor product.