A comparative computational study of the reaction mechanisms for hydrocarbon oxidations catalysed by Mn- and Fe-doped nanoporous aluminophosphates shows distinctive features for each transition metal depending on its electronic configuration. Preactivation of Mn catalysts is easier due to the higher stability of MnII, but its oxidation during propagation requires activation barriers. In contrast, preactivation of Fe is more difficult and avoids a direct Fe reduction because of the low stability of FeII. FeII is only produced at the end of the propagation cycle, favoured by an energetic compensation caused by the simultaneous exothermic oxidation of an alcohol molecule. Fe-catalysed propagation is kinetically favoured since it requires lower activation barriers, and is further assisted by higher adsorption energies of the reactants and lower desorption energies of the products on the active site. The mechanistic information gained can be used for the rational design of improved oxidation catalysts.

Electronic structure calculations employing hybrid exchange DFT methods under periodic boundary conditions are applied to unravel the mechanism and energetics of the secondary oxidation reactions of hydrocarbons catalyzed by Mn-doped nanoporous aluminophosphates. Secondary oxidations are favored by the activated nature of the primary oxidative products, alcohols, which have weaker C–H bonds than the corresponding hydrocarbon molecules. Our model accounts for the secondary oxidations of terminal alcohols; a double H-abstraction from these compounds, first by MnIII complexes with radical-type oxo-based ligands, and then by framework O atoms nearest neighbor to Mn, yields aldehyde molecules or ketones if nonterminal alcohols are oxidized. These aldehydes are then transformed first into carbonyl-containing α-hydroperoxide derivatives through H-abstraction, addition of O2, and a subsequent H-transfer. These hydroperoxides can then only be decomposed by the action of MnII sites which, depending on the stereochemistry of the hydroperoxide adsorption onto the Mn sites, yield the carboxylic acid final products directly or after a new H-abstraction. The knowledge gained in our study allows us to propose a mechanism for the secondary oxidation of ketones to give carboxylic acids, a route that involves cleavage of C–C bonds adjacent to the carbonyl group and gives place to carboxylic acids of shorter chain-length than the initial hydrocarbon, products commonly found during the experimental reaction.

We apply electronic structure methods based on hybrid-exchange DFT functionals under periodic boundary conditions to study the catalytic aerobic oxidation of hydrocarbons in Mn-doped aluminophosphates. In particular, we focus on the mechanism of the propagation reactions. Hydrocarbon oxidation is achieved via a succession of H abstraction, O2 addition, and desorption reactions occurring on MnIII···OX complexes (X = H, CH2CH3, or OCH2CH3). The complexes MnIII···OH and MnIII···OCH2CH3 result from the decomposition of CH3CH2OOH by preactivated MnII sites, whereas MnIII···OOCH2CH3 is formed in preactivation or propagation routes. The radical nature of the oxo-type ligands (OX) allows for the homolytic H abstraction from new hydrocarbon molecules, leading to XO–H (HO–H, CH3CH2O–H and CH3CH2OO–H) and to CH3CH2· radicals that are stabilized by interaction with the H atoms transferred. Subsequent stereospecific O2 additions yield free peroxo radicals CH3CH2OO· that undergo a propagation subcycle to produce further CH3CH2OOH; these hydroperoxide molecules re-enter the oxidation cycle by reacting with MnII. The different H abstraction ability of the MnIII···OX complexes is related to the stability of the oxo radicals that act as ligands. Our results demonstrate that the role of the Mn sites in the propagation reactions is to stabilize the oxo radicals by forming complexes, but no redox process involving Mn takes place in this stage of the reaction; MnIII is the only active species throughout the propagation steps.Keywords: aerobic; heterogeneous catalysis; molecular modeling; nanoporous aluminophosphates; oxidation; reaction mechanism; zeolites;

Electronic structure methods based on periodic DFT with hybrid-exchange functionals are applied to study the reaction mechanism of the aerobic oxidation of hydrocarbons catalyzed by Mn-doped nanoporous aluminophosphates. Here, we focus on the regeneration of the active sites that closes the oxidation cycle. At this stage, the catalyst pores are accumulated with CH3CH2OOH (hydroperoxide) and MnIII···OOCH2CH3 complexes resulting from the propagation reactions. CH3CH2OOH intermediates can only be decomposed into the oxidation products by MnII sites; thus, a reaction pathway in which Mn sites in MnIII···OOCH2CH3 are reduced is essential for the oxidation cycle to proceed. We demonstrate that two different regeneration mechanisms take place at different times of the oxidation reaction: at the beginning, MnIII···OOCH2CH3 complexes are transformed into a molecule of aldehyde and MnIII···OH complexes by an intramolecular H transfer from the methylene C to the terminal O in the peroxo radical in a slow process that requires a high activation energy of 141 kJ/mol. Mn sites in MnIII···OH can then be regenerated by a H-transfer from a new hydrocarbon molecule to a framework O nearest neighbor to Mn, followed by coupling between the resulting alkyl radical and the OH ligand to give a molecule of ethanol and MnII sites. At later stages of the oxidation, when alcohol molecules are accumulated within the pores of the catalyst, Mn sites in MnIII···OOCH2CH3 can be regenerated by a more favorable mechanism with a double H-abstraction from the alcohol. First, a H atom in the methylene C of CH3CH2OH is transferred to the CH3CH2OO ligand to give CH3CH2OOH and CH3CH·OH radicals, followed by H transfer from this radical to a framework O to yield finally a molecule of aldehyde and MnII sites. The occurrence of alternative regeneration mechanisms along the oxidation reaction explains the variation of the alcohol-to-aldehyde ratio observed experimentally as a function of the reaction time.Keywords: aerobic; heterogeneous catalysis; nanoporous aluminophosphates; oxidation; reaction mechanism; regeneration; zeolites;

We apply state of the art electronic structure techniques, based on hybrid exchange-functionals in DFT and periodic boundary conditions, to unravel the reaction mechanism responsible for the initial stages of the aerobic oxidation of hydrocarbons catalyzed by Mn-doped aluminophosphates. In this preactivation step of the catalyst, which precedes the catalytic propagation cycle in which the final oxidation products (alcohol, aldehyde, and carboxylic acid) are formed, the MnIII ions initially present in the activated (calcined) catalyst are transformed by interaction with one alkane and one O2 molecule into new Mn-bearing species: a reduced MnII site and a MnIII···peroxo complex, which are active for the subsequent propagation cycle. The preactivation step has a high activation energy, calculated as 135 kJ/mol, explaining the long induction time observed experimentally. Our results further show that MnIII sites are able to produce the hydroperoxide intermediate from the reactants; however, this intermediate can be transformed into the oxidative products only through reduced MnII sites. The latter are formed from MnIII in the preactivation step, via a H-abstraction from the hydrocarbon, also yielding an alkyl radical (R•) that subsequently adds O2 in a stereospecific way to form a free peroxo radical, ROO•. Migration of ROO• in the AlPO nanopores frees MnII for the propagation cycle and forms MnIII···ROO• complexes also needed for propagation. We demonstrate the essential role of MnIII active sites at the initial stages of the reaction for activating the hydrocarbon molecules; such hydrocarbon activation catalyzed by Mn requires much lower activation energies than through noncatalytic pathways, where the hydrocarbon is activated by O2 alone.Keywords (keywords): heterogeneous catalysis; hydrocarbon; molecular modeling; nanoporous aluminophosphate; oxidation; preactivation; reaction mechanism; zeolite

Electronic structure methods based on hybrid-exchange functionals in Density Functional Theory (DFT) and periodic boundary conditions have been applied to study the reaction mechanism of the aerobic oxidation of hydrocarbons catalyzed by Mn-doped nanoporous aluminophosphates. In this paper we examine the decomposition of hydroperoxide intermediates (ROOH). The reaction takes place on MnII acid sites, charge-balanced by a proton on a nearest neighbor framework oxygen, resulting from the preactivation step. In this stage, the MnII sites catalyze the homolytic decomposition of the hydroperoxide molecules to produce MnIII and oxo-containing radical species, stabilized by complexation to MnIII, yielding in addition alcohol and water molecules. Two parallel reaction pathways have been identified for this process, through alkoxy (RO·) or hydroxy (HO·) radical-like intermediates. The occurrence of the two mechanisms depends on the stereochemistry of the initial adsorption of ROOH onto the active site, which takes place through H-bonding with the framework acid proton: adsorption via the hydroxylic O atom of ROOH leads to RO· intermediates, while adsorption via the nonterminal O atom in ROOH, which is less energetically stable, drives the decomposition toward HO· intermediates. In both cases, the hydroperoxide decomposition is assisted by the MnII sites, in a concerted mechanism that consists of a H-transfer from the framework to the O atom of the adsorbed ROOH involved in the H-bond, prompting the oxidation of MnII, and the O–O homolytic cleavage in ROOH, leading to the formation of RO· or HO· radicals that are stabilized by binding the oxidized MnIII site. The relative energetics of the two reaction pathways is explained in terms of the relative stability of the oxo-radicals produced: the higher stability of RO· radicals causes a more favorable decomposition of the hydroperoxide intermediate through this pathway. Our results demonstrate the crucial role of Mn in this stage of the aerobic oxidation of hydrocarbons, due not only to its redox activity but also, and fundamentally, to its coordinative unsaturation, that allows for the stabilization of the radicals produced by complexation.Keywords: heterogeneous catalysis; hydroperoxide; molecular modeling; nanoporous aluminophosphates; oxidation; reaction mechanism; zeolites

A comparative computational study of the reaction mechanisms for hydrocarbon oxidations catalysed by Mn- and Fe-doped nanoporous aluminophosphates shows distinctive features for each transition metal depending on its electronic configuration. Preactivation of Mn catalysts is easier due to the higher stability of MnII, but its oxidation during propagation requires activation barriers. In contrast, preactivation of Fe is more difficult and avoids a direct Fe reduction because of the low stability of FeII. FeII is only produced at the end of the propagation cycle, favoured by an energetic compensation caused by the simultaneous exothermic oxidation of an alcohol molecule. Fe-catalysed propagation is kinetically favoured since it requires lower activation barriers, and is further assisted by higher adsorption energies of the reactants and lower desorption energies of the products on the active site. The mechanistic information gained can be used for the rational design of improved oxidation catalysts.