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Publication Detail
Aerobic Oxidation of Hydrocarbons Catalyzed by Mn-Doped Nanoporous Aluminophosphates(I): Preactivation of the Mn Sites
  • Publication Type:
    Journal article
  • Publication Sub Type:
    Article
  • Authors:
    Gomez-Hortiguela L, Cora F, Catlow CRA
  • Publisher:
    AMER CHEMICAL SOC
  • Publication date:
    01/2011
  • Pagination:
    18, 28
  • Journal:
    ACS CATAL
  • Volume:
    1
  • Issue:
    1
  • Print ISSN:
    2155-5435
  • Language:
    EN
  • Keywords:
    oxidation, reaction mechanism, preactivation, nanoporous aluminophosphate, hydrocarbon, zeolite, molecular modeling, heterogeneous catalysis, MOLECULAR-SIEVE CATALYSTS, SELECTIVE OXIDATION, SUBSTITUTED ALUMINOPHOSPHATES, CYCLOHEXANE OXIDATION, AERIAL OXIDATION, LINEAR ALKANES, ADIPIC ACID, AUTOXIDATION, OXYGEN, CO
  • Addresses:
    Gomez-Hortiguela, L
    UCL
    Dept Chem
    London
    WC1H 0AJ
    England
Abstract
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 Mn-III ions initially present in the activated (calcined) catalyst are transformed by interaction with one alkane and one O-2 molecule into new Mn-bearing species: a reduced Mn-II site and a Mn-III center dot center dot center dot 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 Mn-III sites are able to produce the hydroperoxide intermediate from the reactants; however, this intermediate can be transformed into the oxidative products only through reduced Mn-II sites. The latter are formed from Mn-III in the preactivation step, via a H-abstraction from the hydrocarbon, also Yielding an alkyl radical (R center dot) that subsequently adds O-2 in a stereospecific way to form a free peroxo radical, ROO center dot. Migration of ROO center dot in the AlPO nanopores frees Mn-II for the propagation cycle and forms Mn-III center dot center dot center dot ROO center dot complexes also needed for propagation. We demonstrate the essential role of Mn-III 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 O-2 alone.
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