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The mechanism of the acid-catalyzed condensation between levulinic acid and phenol to form diphenolic acid (DPA) was investigated using a combination of sulfonated hyperbranched poly(arylene oxindole)s and thiols. Taft-type steric and electronic parameters were applied to study the correlation between the nature of the thiols and the condensation rate. The kinetic effect of thiols to tune the regiochemical control was shown to be dependent on the substituent size of the thiols. Although thiols cause an increase in the regioselectivity to the desired p,p′-DPA, the isomer distribution is found to converge to a thermodynamic equilibrium at high conversions of levulinic acid. The hitherto unconsidered acid-catalyzed isomerization of p,p′-DPA to o,p′-DPA was demonstrated by condensation reactions with both m-cresol and 13C-labeled phenol supported by density functional theory (DFT) calculations.

Sn einfach eingebaut: Ein einfaches und schnelles Verfahren fur die Synthese von Lewis-sauren Sn-β-Zeolithen wurde entwickelt (siehe Schema). Die Materialien sind katalytisch hochaktiv und ergeben in der Baeyer-Villiger-Oxidation von Cyclohexanon und der Synthese von Ethyllactat aus Dihydroxyaceton Raum-Zeit-Ausbeuten, die mehr als eine Grosenordnung uber den Werten konventioneller Katalysatoren liegen.

Tin goes in: The solid Lewis acid Sn-zeolite β is obtained in a simple and scalable procedure (see scheme). A high metal content can be obtained, without undesirable side-effects. The space-time-yields of the resulting catalysis are over one order-of-magnitude larger than those of the state-of-the-art materials for the Baeyer–Villiger oxidation of cyclohexanone and the synthesis of ethyl lactate from the triose dihydroxyacetone.

The application of high-amplitude ultrasound to liquids triggers cavitation. By the collapse of the thereby appearing vacuum cavities, high temperatures can be reached in a transient manner. The high temperatures in these hot-spots can lead to homolytic scission of chemical bonds. The thereby generated radicals are usually utilized in aqueous systems for the degeneration of organic pollutants. In this contribution, we demonstrate that the radicals can also be used for synthetic purposes: under an oxygen atmosphere, they trigger the oxidation of an aldehyde substrate.

The economically viable oxidative upgrading of methane presents one of the most difficult but rewarding challenges within catalysis research. Its potential to revolutionalise the chemical value chain, coupled with the associated supremely challenging scientific aspects, has ensured this topic's high popularity over the preceeding decades. Herein, we report a non-exhaustive account of the current developments within the field of oxidative methane upgrading and summarise the pertaining challenges that have yet to be solved.

The oxidative upgrading of amines offers great opportunities for the sustainable production of key N-containing building-blocks for the modern chemical industry. Compared to other oxyfunctionalizations, and despite their potential, amine oxidation reactions are barely explored in the literature. This review aims at drawing attention to this important area and highlights both the major achievements and the challenges that still remain.

Molybdenum(VI)peroxide species, formed in the reaction of MoVI complexes with peroxides, are able to epoxidize >CC< double bonds heterolytically. In this study, theoretical and experimental evidence is provided for a kinetically competing reaction reaction of such molybdenum(VI)peroxide species with additional peroxide reagent, leading to molybdenum(VI)trioxide species, which easily decompose into radicals. Under epoxidation conditions, those radicals will reduce the selectivity, due to the formation of allylic byproducts. The involved reaction pathways are characterized by DFT calculations, providing kinetic parameters that are in good agreement with the experimental observations.

The aerobic autoxidation of hydrocarbons proceeds through a complicated reaction mechanism, mediated by free radicals. Most reported autoxidation catalysts enhance the radical formation rate via homolytic activation of hydroperoxide products. Whereas our knowledge of the product formation mechanisms has significantly improved over the last couple of years, the chain-initiation is still poorly understood. In this contribution, the thermal and catalytic initiation rate for the oxidation of the renewable olefin α-pinene is quantified, thereby providing evidence for a substrate-assisted thermal initiation. The kinetics of the catalytic initiation is in good agreement with previous studies under model conditions.

Humulene is a sesquiterpene with an important biochemical lead structure, consisting of an 11-membered ring, containing three nonconjugated C═C double bonds, two of them being triply substituted and one being doubly substituted. As observed by many groups, one of the two triply substituted C═C double bonds is significantly more reactive. In order to rationalize this peculiar regioselectivity, the conformational space of humulene has been explored computationally using various DFT functionals. Four different conformations were identified. Each conformation is chiral and has two enantiomeric forms, yielding a total of eight conformers. The potential energy surface for the interconversion of these conformers was characterized via intrinsic reaction coordinate analyses. Furthermore, an evaluation of the microcanonical partition functions allowed for a quantification of the entropy contributions and the calculation of the temperature dependent equilibrium composition. The results strongly suggest that the high regioselectivity is related to a strong, hyper-conjugative σ(Cα-Cβ)-π(C═C) orbital overlap in the predominant conformations that discriminates one triply substituted double bond from the other. Furthermore, the order of magnitude of the calculated activation energies for the interconversions of the conformers is supported by NMR measurements at different temperatures.

Rhodium catalysts have been developed that can be tuned to produce two different products from methane. The findings could aid the development of industrial processes that exploit this abundant resource. Rhodium catalysts have been developed that can be tuned to produce two different products from methane. The findings could aid the development of industrial processes that exploit this abundant resource.

The conformational space of cyclooctene has been explored computationally in order to rationalize its high epoxidation selectivity. Four different conformations were identified. Each conformation is chiral and has two enantiomeric forms. The degeneracy is further increased by a ring-inversion process, yielding a total of 16 conformers. The potential energy surface for the interconversion of these conformers was characterized via intrinsic reaction coordinate analyses. Furthermore, an evaluation of the microcanonical partition functions allowed for a quantification of the entropy contributions and hence the calculation of the equilibrium composition at different temperatures. The results strongly suggest that the high epoxidation selectivity, typically observed for cyclooctene, is related to a poor σ(C-αH)-π(C═C) orbital overlap in the predominant conformation, disfavoring αH-abstraction by radical species and thus allylic byproduct formation via undesired homolytic side-reactions.

Topics in Catalysis, 54 (10-12), ISSN:1022-5528, ISSN:1572-9028

During the autoxidation of cyclohexane, abstraction of the αH-atom of the hydroperoxide product by chain-carrying peroxyl radicals produces both the desired alcohol and ketone products, as well as the majority of by-products. Rationalizing the impact of this reaction, one should aim for a (catalytic) destruction of this hydroperoxide without the intervention of peroxyl chain-carriers. Starting from these new insights in the molecular mechanism, attempts for rational catalyst design are initiated.

Burned greasy deposits were found inside shells of the large Nile bivalve Chambardia rubens, excavated in an eight- to tenth- century AD church of the Coptic monastery of Bawit, Egypt, and supposedly used as oil lamps. The residues were subjected to a combination of chromatographic residue analysis techniques. The rather high concentrations of unsaturated fatty acids, as analysed by gas chromatography (GC) in the methylated extract, suggest the presence of a vegetal oil. Analysis of the stable carbon isotopes (delta C-13 values) of the methyl esters also favoured plants over animals as the lipid source. In the search for biomarkers by GC coupled to mass spectrometry on a silylated extract, a range of diacids together with high concentrations of 13,14-dihydroxydocosanoate and 11,12-dihydroxyeicosanoate were found. These compounds are oxidation products of erucic acid and gondoic acid, which are abundantly present in seeds of Brassicaceae plants. Liquid chromatography coupled to mass spectrometry analysis showed low concentrations of unaltered triglycerides, but revealed sizeable amounts of triglycerides with at least one dihydroxylated acyl chain. The unusual preservation of dihydroxylated triglycerides and alpha,omega-dicarboxylic acids can be related to the dry preservation conditions. Analysis of the stereoisomers of the dihydroxylated fatty acids allows one to determine whether oxidation took place during burning of the fuel or afterwards. The results prove that the oil of rapeseed (Brassica napus L.) or radish (Raphanus sativus L.) was used as illuminant in early Islamic Egypt, and that not only ceramic lamps but also mollusk shells were used as fuel containers. ispartof: Analytical and Bioanalytical Chemistry vol:390 issue:2 pages:783-793 ispartof: location:Germany status: published

Herein we report a new, one-step procedure for the metal-free ketonization of unsaturated fatty acid methyl esters and triacylglycerol mixtures with nitrous oxide (N 2 O). The conversion of various substrates can be tuned by parameters such as temperature, reaction time and N 2 O partial pressure. This ketonization chemistry offers various advantages over the classic Wacker catalytic process and a state-of-the-art two-step procedure via intermediate epoxides.

In this work, a complementary experimental and theoretical approach is used to unravel the formation of byproducts in the autoxidation of cyclohexane. The widely accepted vision that cyclohexanone would be the most important precursor of undesired products was found inconsistent with several experimental observations. However, the propagation reaction of cyclohexyl hydroperoxide, which we recently put forward as the missing source of cyclohexanol and cyclohexanone, is now unambiguously identified also as the dominant path leading to byproducts. Indeed, this overlooked reaction produces large amounts of cyclohexoxy radicals, able to ring-open via a beta-C--C cleavage to omega-formyl radicals. The pathway by which these radicals are converted into the observed and quantified byproducts is derived in this work. In this liquid-phase reaction, solvent cages were found very important, steering the fate of nascent species.

The catalytic performance of supported metal oxides is often controlled by their two- or three-dimensional dispersion. Silica, one of the popular inert supports, triggers the undesired formation of three-dimensional nanoparticles at significantly lower loadings than other conventional supports like Al2O3, TiO2, Nb2O5, or ZrO2. This observation has been ascribed to the lower reactivity of surface SiOH groups toward the precursor, compared to other metal hydroxyl groups on different supports. In this contribution, we show that by promoting amorphous silica with low amounts of sodium, the surface density of two-dimensional metal oxide species can be significantly enhanced to the same level as all other oxide supports previously reported in the literature. This effect is demonstrated for the case of supported vanadia using a variety of spectroscopic techniques (i.e., Raman, diffuse reflectance UV–vis, and 51V-MAS NMR), as well as a catalytic activity study for the oxidative dehydrogenation of propane (ODHP), a structure-sensitive probe reaction. The propane consumption rate was found to increase linearly with the vanadium surface density while the propylene selectivity was not affected until a monolayer coverage of ca. 9 vanadia per nm2 was surpassed. The method is also applicable to other group V metals (i.e., Nb- and Ta-oxide), opening new perspectives for supported metal oxides., ACS Catalysis, 5 (10), ISSN:2155-5435

Copper-exchanged SSZ-13 is a very efficient material in the selective catalytic reduction of NO(x) using ammonia (deNO(x)-SCR) and characterizing the underlying distribution of copper sites in the material is of prime importance to understand its activity. The IR spectrum of NO adsorbed to divalent copper sites are modeled using ab initio molecular dynamics simulations. For most sites, complex multi-peak spectra induced by the thermal motion of the cation as well as the adsorbate are found. A finite temperature spectrum for a specific catalyst was constructed, which shows excellent agreement with previously reported data. Additionally these findings allow active and inactive species in deNO(x)-SCR to be identified. To the best of our knowledge, this is the first time such complex spectra for single molecules adsorbed to single active centers have been reported in heterogeneous catalysis, and we expect similar effects to be important in a large number of systems with mobile active centers.

Herein, we demonstrate that the chain-initiating dissociation of cyclohexyl hydroperoxide, CyOOH, is substantially accelerated by H-bond acceptors (e.g. Teflon), which assist O-O bond breaking by stabilising the leaving *OH radical. This is a completely new approach to boost the chain-propagating radical concentration. Indeed, up to now, literature has remained focussed on transition metal catalysis. In addition to this initiation effect, we demonstrate how inert perfluorinated compounds are also able to steer the selectivity at the molecular level, by promoting the conversion of the intermediate cyclohexyl hydroperoxide to the most desired end-product, cyclohexanone. This effect is explained by an enhanced, H-bond-assisted, hydroperoxide propagation. This hitherto overlooked hydroperoxide propagation was recently presented by us as the dominant cyclohexanone and cyclohexanol source. We herein thus confirm our previously reported autoxidation scheme, and illustrate its usefulness as a solid basis for designing new catalytic systems.

In this article, we present a thorough study on the autoxidation of cyclohexane, a model substrate for other (saturated) hydrocarbons. Despite the industrial impact of autoxidation reactions, a detailed mechanism is still missing. We present a combined experimental and computational study on the formation of both the major products (cyclohexylhydroperoxide, cyclohexanol and cyclohexanone), and the formation of ring-opened side-products. Up to now, these by-products, mainly adipic acid, were thought to originate from cyclohexanone. However, we found strong evidence that the subsequent propagation of ketone is much slower than assumed, and can only account for some 25% of ring-opened products. On the other hand, the hitherto completely overlooked propagation of the hydroperoxide, via fast αH-abstraction by chain-carrying peroxyl radicals, is identified as the major source of not only alcohol and ketone, but also by-products. In the case of N-hydroxyphthalimide (NHPI) catalysed oxidations, where mostly phthalimide N-oxyl (PINO ) radicals are propagating the chain, the situation is slightly different, as PINO reacts more selectively with the alkane substrate than peroxyl radicals. This results in an increase in hydroperoxide selectivity. Lowering of the ROOH concentration by its, e.g. cobalt-catalyzed decomposition, leads to an enhanced catalytic efficiency, as a result of the shift in the ROO + NHPI ⇌ ROOH + PINO equilibrium to the more efficient PINO chain carrier.

In spite of its industrial importance, the detailed reaction mechanism of cyclohexane autoxidation by O2 is still insufficiently known. Based on quantum chemical potential energy surfaces, rate coefficients of the primary and secondary chain propagation steps involving the cyclohexylperoxyl (CyOO) radical were evaluated using multiconformer transition-state theory. Including tunneling and hindered-internal-rotation effects, the rate coefficient for hydrogen-atom abstraction from cyclohexane (CyH) by CyOO was calculated to be k(T)= 1.46 x 10(-11) x exp(-17.8 kcal mol(-1)/ RT) cm3s(-1) (300-600K), close to the experimental data. A "Franck-Rabinowitch cage" reaction between the nascent cyclohexylhydroperoxide (CyOOH) and cyclohexyl radical, products from CyOO + CyH, is put forward as an initially important cyclohexanol (CyOH) formation channel. alphaH abstraction by CyOO. from cyclohexanone was calculated to be only about five times faster than that from CyH, too slow to explain all the observed side products. The a-hydrogen (alphaH) abstractions from CyOH and CyOOH by CyOO. are predicted to be about 10 and 40 times faster, respectively, than the CyOO. +CyH reaction. The very fast CyOO.+CyOOH reaction proceeds through the unstable Cy-alphaH .OOH radical that decomposes spontaneously into the ketone (Q=O) plus the OH radical; the "hot" .OH is found to produce the bulk of the alcohol via a second, "activated cage" reaction analogous to that above. It is thus shown how the very reactive CyOOH intermediate is the predominant source of ketone and alcohol, while it also leads to some side products. The alpha-hydroxycyclohexylperoxyl radical formed during the moderately fast oxidation of CyOH is shown to decompose fast into HO2 + cyclohexanone in a rapidly equilibrated reaction, which constitutes a smaller, second ketone source. These two fast cyclohexanone forming routes avoid the need for unfavorable molecular routes hitherto invoked as ketone sources. The theoretical predictions are supported and complemented by experimental findings. The newly proposed scheme is also largely applicable to the oxidation of other hydrocarbons, such as toluene, xylene, and ethylbenzene.

A comparative theoretical study is presented on the formation and decomposition of alpha-hydroxy-alkylperoxyl radicals, Q(OH)OO* (Q = RR'C:), important intermediates in the oxidation of several classes of oxygenated organic compounds in atmospheric chemistry, combustion, and liquid-phase autoxidation of hydrocarbons. Detailed potential energy surfaces (PESs) were computed for the HOCH2O2*==HO2* + CH2O reaction and its analogues for the alkyl-substituted RCH(OH)OO* and R2C(OH)OO* and the cyclic cyclo-C6H10(OH)OO*. The state-of-the-art ab initio methods G3 and CBS-QB3 and a nearly converged G2M//B3LYP-DFT variant were found to give quasi-identical results. On the basis of the G2M//B3LYP-DFT PES, the kinetics of the approximately equal to 15 kcal/mol endothermal alpha-hydroxy-alkylperoxyl decompositions and of the reverse HO2*+ ketone/aldehyde reactions were evaluated using multiconformer transition state theory. The excellent agreement with the available experimental (kinetic) data validates our methodologies. Contrary to current views, HO2* is found to react as fast with ketones as with aldehydes. The high forward and reverse rates are shown to lead to a fast Q(OH)OO*==HO2* + carbonyl quasi-equilibrium. The sizable [Q(OH)OO*]/[carbonyl] ratios predicted for formaldehyde, acetone, and cyclo-hexanone at the low temperatures (below 220 K) of the earth's tropopause are shown to result in efficient removal of these carbonyls through fast subsequent Q(OH)OO* reactions with NO and HO2*. IMAGES model calculations indicate that at the tropical tropopause the HO2*-initiated oxidation of formaldehyde and acetone may account for 30% of the total removal of these major atmospheric carbonyls, thereby also substantially affecting the hydroxyl and hydroperoxyl radical budgets and contributing to the production of formic and acetic acids in the upper troposphere and lower stratosphere. On the other hand, an RRKM-master equation analysis shows that hot alpha-hydroxy-alkylperoxyls formed by the addition of O(2) to C(1)-, C(2)-, and C(3)-alpha-hydroxy-alkyl radicals will quasi-uniquely fragment to HO2* plus the carbonyl under all atmospheric conditions.

Degradation mechanisms of peroxide-containing compounds, either stable or transients in the degradation of volatile organic compounds, are still poorly understood, despite their importance in atmospheric chemistry. A theoretical DFT and Coupled Cluster theory analysis of the stability of alkyl radicals with hydroperoxyl- (HOO–) or alkylperoxyl (ROO–) substituents on the radical carbon atom revealed that such radicals are unstable, dissociating by O–O scission to a carbonyl compound and a hydroxy or alkoxy radical even for multiple substituted compounds. This dissociation occurs sequentially after completion of the reaction forming the α-peroxyl-substituted radicals, such that the latter is independent of the O–O scission.

Developing inorganic materials that can mimic nature's ability to selectively oxidise inert C-H bonds remains a topic of intense scientific research. In recent years, zeolitic materials containing Fe and/or Cu have been shown to be highly active, heterogeneous catalysts for the selective oxidation of alkanes (including methane), amongst a range of other related oxidation challenges. By using resonance-enhanced Raman spectroscopy, we demonstrate that, following high-temperature pre-treatment (activation), Fe-containing ZSM-5 possesses an active binuclear core, and forms a key Fe-OOH intermediate upon activation with H2O2. Both factors are reminiscent of biological oxidation catalysts, and may account for the unique ability of this material to selectively oxidise methane to methanol at low temperature. A pre-ferryl cat: Through insitu resonance-enhanced Raman spectroscopy, we identify an active, binuclear Fe-O(H)-Fe core and an FeIII-OOH intermediate in Fe-containing ZSM-5 following activation with H2O2. The pre-ferryl nature of this biomimetic intermediate may account for the unique ability of this solid catalyst to selectively oxidise methane to methanol under mild conditions.

Boron nitride was recently identified as a highly selective catalyst for the oxidative dehydrogenation of propane (ODHP). In this communication we report that other boron-containing materials such as boron carbide, titanium boride, nickel boride, cobalt boride, hafnium boride and tungsten boride, as well as elemental boron itself show the same exceptional behavior as boron nitride. X-ray photoelectron and infrared spectroscopy suggest oxyfunctionalization of the surface. This observation disproves previous mechanistic hypotheses that edge sites on the boron nitride would be the active sites and rather suggests the formation of an analogous surface-stabilized BOx active site for all tested boride catalysts.

International audience; Dynamic nuclear polarization surface enhanced NMR (DNP-SENS), Mossbauer spectroscopy, and computational chemistry were combined to obtain structural information on the active-site speciation in Sn-beta zeolite. This approach unambiguously shows the presence of framework Sn-IV-active sites in an octahedral environment, which probably correspond to so-called open and closed sites, respectively (namely, tin bound to three or four siloxy groups of the zeolite framework).

In our attempt to synthesize β-caryophyllene oxide in food-compatible conditions, we observed the uncatalyzed and highly selective epoxidation of β-caryophyllene, a strained bicyclic sesquiterpene, in ethanol with aqueous H2 O2 under radical-suppressing conditions without the addition of a catalyst. The unusual reactivity of β-caryophyllene allowed us to use it as a probe for the mechanism of the solvent-assisted epoxidation in a wide range of organic solvents. A kinetic study was performed to investigate the epoxidation mechanism; an excellent correlation was found between the observed epoxidation rates in different solvents and the Abraham's hydrogen bond formation parameters of these solvents. By means of computational analysis, it was found that the main role of the solvent consists of the stabilization of the elongated OO bond of H2 O2 in the transition state through hydrogen-bond donation to the leaving OH moiety of H2 O2 . α-Humulene was found to possess similar reactivity as β-caryophyllene whereas isocaryophyllene-the unstrained isomer of β-caryophyllene-was unreactive.

The sequential transfer hydrogenation/hydrogenolysis of furfural and 5-hydroxymethylfurfural to 2-methylfuran and 2,5-dimethylfuran was studied over in situ reduced, Fe2 O3 -supported Cu, Ni, and Pd catalysts, with 2-propanol as hydrogen donor. The remarkable activity of Pd/Fe2 O3 in both transfer hydrogenation/hydrogenolysis is attributed to a strong metal-support interaction. Selectivity towards hydrogenation, hydrogenolysis, decarbonylation, and ring-hydrogenation products is shown to strongly depend on the Pd loading. A significant enhancement in yield to 62%, of 2-methylfuran and 2-methyltetrahydrofuran was observed under continuous flow conditions.

The aerobic oxidation of amines offers a promising route towards many versatile chemical compounds. Within this contribution, we extend our previous investigations of iridium oxide-catalyzed alcohol oxidation to amine substrates. In addition to demonstrating the versatility of this catalyst, particular attention is focused on the mechanisms of the reaction. Herein, we demonstrate that although amines are oxidized slower than the corresponding alcohols, the catalyst has a preference for amine substrates, and oxidizes various amines at turnover frequencies greater than other systems found in the open literature. Furthermore, the competition between double amine dehydrogenation, to yield the corresponding nitrile, and amine-imine coupling, to yield the corresponding coupled imine, has been found to arise from a competitive reaction pathway, and stems from an effect of substrate-to-metal ratio. Finally, the mechanism responsible for the formation of N-benzylidene-1-phenylmethanamine was examined, and attributed to the coupling of free benzyl amine substrate and benzaldehyde, formed in situ through hydrolysis of the primary reaction product, benzyl imine.

Correction for ‘Supported two- and three-dimensional vanadium oxide species on the surface of β-SiC’ by Carlos A. Carrero et al., Catal. Sci. Technol., 2017, DOI: 10.1039/c7cy01036b.

The oxidative upgrading of amines offers great opportunities for the sustainable production of key N-containing building-blocks for the modern chemical industry. Compared to other oxyfunctionalizations, and despite their potential, amine oxidation reactions are barely explored in the literature. This review aims at drawing attention to this important area and highlights both the major achievements and the challenges that still remain.

The selective oxidation of alcohols is a pivotal transformation within industrial and synthetic chemistry. In this contribution we use O₂ as terminal oxidant and (H)NO(x) species as oxygen shuttle in order to mediate the oxidation of primary and secondary alcohols under the influence of a solid acid catalyst. Process optimization and intensification through the use of a continuous three-phase flow reactor is demonstrated. Space-time yields were found to increase by two orders of magnitude with respect to batch experiments, along with additional gains in selectivity and a decrease of N₂O formation.

The dehydration of D-fructose to 5-hydroxymethylfurfural was studied under single-phase conditions in the low boiling solvent 1,4-dioxane at moderate temperatures in the presence of the solid acid-catalyst Amberlyst-15. The reaction was first examined and optimized under batch conditions, where it was found that the yield could be increased up to 75 % by adding small amounts of DMSO. Subsequently, the reaction was performed under continuous flow in a fixed bed reactor. Internal and external mass transfer limitations could be eliminated by changing the particle size and by adjusting the flow rate. Under continuous conditions, the HMF yield could be further increased to 92 %; the space–time yield was found to be 75 times higher compared to the batch case. A long-term stability test (96 h), including solvent regeneration, demonstrated that the catalyst is stable over time. Additionally, it was shown that even small amounts of water have a negative effect on the HMF yield. Overall, the present system shows a good alternative to other systems presented in literature because high space–time yields and selectivities were obtained under relatively mild and continuous conditions.

The thermal oxidation of the renewable olefin β-pinene with molecular oxygen was experimentally and computationally investigated. Peroxyl radicals abstract weakly bonded allylic hydrogen atoms from the substrate, yielding allylic hydroperoxides (i.e., myrtenyl and pinocarvyl hydroperoxide). In addition, peroxyl radicals add to the C=C bond of the substrate to form an epoxide. It was found that a relatively high peroxyl radical concentration, together with the high rate of peroxyl cross-reactions, make radical-radical reactions surprisingly important for this particular substrate. Approximately 60 % of these peroxyl cross-reactions lead to termination (radical destruction), keeping a radical chain length of approximately 4 at 10 % conversion. Numerical simulation of the reaction-based on the proposed reaction mechanism and known or predicted rate constants-demonstrate the importance of peroxyl cross-reactions for the formation of alkoxyl radicals, which are the precursor of alcohol and ketone products.

The Co(II)/Co(III)-induced decomposition of hydroperoxides is an important reaction in many industrial processes and is referred to as deperoxidation. In the first step of the so-called Haber-Weiss cycle, alkoxyl radicals and Co(III)-OH species are generated upon the reaction of the Co(II) ion with ROOH. The catalytic cycle is closed upon the regeneration of the Co(II) ion through the reaction of the Co(III)-OH species with a second ROOH molecule, thus producing one equivalent of the peroxyl radicals. Herein, the deperoxidation of tert-butylhydroperoxide by dissolved cobalt(II) acetylacetonate is studied by using UV/Vis spectroscopy in situ with a noninteracting solvent, namely, cyclohexane. Kinetic information extracted from experiments, together with quantum-chemical calculations, led to new mechanistic hypotheses. Even under anaerobic conditions, the Haber-Weiss cycle initiates a radical-chain destruction of ROOH propagated by both alkoxyl and peroxyl radicals. This chain mechanism rationalizes the high deperoxidation rates, which are directly proportional to the cobalt concentration up to approximately 75 μM at 333 K. However, at higher cobalt concentrations, a remarkable decrease of the rate is observed. The hypothesis put forward herein is that this remarkable autoinhibition effect could be explained by the hitherto overlooked chain termination of two Co(III)--OH species. The direct competition between the first-order Haber-Weiss initiation and the second-order termination can indeed explain this peculiar kinetic behavior of this homogeneous deperoxidation system.

Functionalizing organic molecules is an important value-creating step throughout the entire chemical value-chain. Oxyfunctionalization of e.g. C-H or C=C bonds is one of the most important functionalization technologies used industrially. The major challenge in this field is the prevention of side reactions and/or the consecutive over-oxidation of the desired products. Despite its importance, a fundamental understanding of the intrinsic chemistry, and the subsequent design of a tailored engineering environment, is often missing. Industrial oxidation processes are indeed to a large extent based on empirical know-how. In this mini-review, we summarize some of our previous work to help to bridge this knowledge gap and elaborate on our ongoing research.

Functionalizing organic molecules is an important value-creating step throughout the entire chemical value-chain. Oxyfunctionalization of value-chain. Oxyfunctionalization of e.g. C–H or C=C bonds is one of the most important functionalization technologies used industrially. The major challenge in this field is the prevention of side reactions and/or the consecutive over-oxidation of the desired products. Despite its importance, a fundamental understanding of the intrinsic chemistry, and the subsequent design of a tailored engineering environment, is often missing. Industrial oxidation processes are indeed to a large extent based on empirical know-how. In this mini-review, we summarize some of our previous work to help to bridge this knowledge gap and elaborate on our ongoing research.

A combined experimental and theoretical approach is used to study the thermal autoxidation of alpha-pinene. Four different types of peroxyl radicals are generated; the verbenyl peroxyl radical being the most abundant one. The peroxyl radicals propagate a long radical chain, implying that chain termination does not play an important role in the production of the products. Two distinct types of propagation steps are active in parallel: the abstraction of allylic H atoms and the addition to the unsaturated C=C bond. The efficiency for both pathways appears to depend on the structure of the peroxyl radical. The latter step yields the corresponding epoxide product, as well as alkoxyl radicals. Under the investigated reaction conditions the alkoxyl radicals seem to produce both the alcohol and ketone products, the ketone presumably being formed upon the abstraction of the weakly bonded alphaH atom by O2. This mechanism explains the predominantly primary nature of all quantified products. At higher conversion, co-oxidation of the hydroperoxide products constitutes an additional, albeit small, source of alcohol and ketone products.

Topics in Catalysis, 52 (9), ISSN:1022-5528, ISSN:1572-9028

During the autoxidation of cyclohexane, abstraction of the αH-atom of the hydroperoxide product by chain-carrying peroxyl radicals produces both the desired alcohol and ketone products, as well as the majority of by-products. Rationalizing the impact of this reaction, one should aim for a (catalytic) destruction of this hydroperoxide without the intervention of peroxyl chain-carriers. Starting from these new insights in the molecular mechanism, attempts for rational catalyst design are initiated.

ion of hydrogen atoms by pthalimide-N-oxyl radicals is an important step in the N-hydroxyphthalimide catalyzed autoxidation of hydrocarbons. In this contribution, the temperature dependency of this reaction is evaluated by a detailed transition state theory based kinetic analysis for the case of toluene. Tunneling was found to play a very important role, enhancing the rate constant by a factor of 20 at room temperature. As a result, tunneling, in combination with the existence of two distinct rotamers of the transition state, causes a pronounced temperature dependency of the pre-exponential frequency factor, and, as a consequence, marked curvature of the Arrhenius plot. This explains why earlier experimental studies over a limited temperature range around 300 K found formal Arrhenius activation energies and pre-factors that are 4 kcal mol(-1) and three orders of magnitude smaller than the actual energy barrier and the corresponding frequency factor, respectively. Also as a consequence of tunneling, substitution of a deuterium atom for a hydrogen atom causes a large decrease in the rate constant, in agreement with the measured kinetic isotope effects. The present theoretical analysis, complementary to the experimental rate coefficient data, allows for a reliable prediction of the rate coefficient at higher temperatures, relevant for actual autoxidation processes.

Aerobic oxidation of toluene (PhCH3) is investigated by complementary experimental and theoretical methodologies. Whereas the reaction of the chain-carrying benzylperoxyl radicals with the substrate produces predominantly benzyl hydroperoxide, benzyl alcohol and benzaldehyde originate mainly from subsequent propagation of the hydroperoxide product. Nevertheless, a significant fraction of benzaldehyde is also produced in primary PhCH3 propagation, presumably via proton rather than hydrogen transfer. An equimolar amount of benzyl alcohol, together with benzoic acid, is additionally produced in the tertiary propagation of PhCHO with benzylperoxyl radicals. The "hot" oxy radicals generated in this step can also abstract aromatic hydrogen atoms from PhCH3, and this results in production of cresols, known inhibitors of radical-chain reactions. The very fast benzyl peroxyl-initiated co-oxidation of benzyl alcohol generates HO2* radicals, along with benzaldehyde. This reaction also causes a decrease in the overall oxidation rate, due to the fast chain-terminating reaction of HO2*with the benzylperoxyl radicals, which causes a loss of chain carriers. Moreover, due to the fast equilibrium PhCH2OOH+HO2* right harpoon over left harpoonPhCH2OO* + H2O2, and the much lower reactivity of H2O2 compared to PhCH2OOH, the fast co-oxidation of the alcohol means that HO2* gradually takes over the role of benzylperoxyl as principal chain carrier. This drastically changes the autoxidation mechanism and, among other things, causes a sharp decrease in the hydroperoxide yield.