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Molecular Structure–Activity Relationships for the Oxidation of Organic Compounds Using Mesoporous Silica Catalysts Derivatised with Bis(halogeno)dioxomolybdenum( VI ) Complexes
Author(s) -
Nunes Carla D.,
Valente Anabela A.,
Pillinger Martyn,
Rocha João,
Gonçalves Isabel S.
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200304887
Subject(s) - catalysis , monomer , chemistry , extended x ray absorption fine structure , triethylamine , molybdenum , mesoporous material , alkene , mesoporous silica , x ray absorption fine structure , oxidation state , polymer chemistry , crystallography , nuclear chemistry , organic chemistry , spectroscopy , absorption spectroscopy , polymer , physics , quantum mechanics
Mo K ‐edge XAFS spectra have been measured for ordered mesoporous silica MCM‐41 grafted with the complexes [MoO 2 X 2 (thf) 2 ] (X=Cl, Br). For grafting reactions in the absence of triethylamine, materials with 1 wt. % Mo are obtained; the Mo K ‐edge EXAFS results indicate the co‐existence of isolated surface‐fixed monomeric species {MoO 2 [(O) 3 SiO] 2 (thf) n } and {MoO 2 [(O) 3 SiO]X(thf) n }. When Et 3 N is used in the grafting reactions, materials with 4 wt. % Mo are obtained. The EXAFS data for the material prepared using [MoO 2 Cl 2 (thf) 2 ] and Et 3 N indicate the presence of dinuclear species with two Mo VI centres, each with two MoO groups and each linked by one or two oxo bridges (Mo ⋅⋅⋅ Mo 3.27 Å). The molybdenum centres in the material prepared using the dibromo complex comprise mainly isolated four‐coordinate dioxomolybdenum( VI ) and trioxomolybdenum( VI ) monomeric species, with a small contribution from dimeric species. All materials were further characterised in the solid state by powder X‐ray diffraction, N 2 adsorption analysis, MAS NMR ( 13 C, 29 Si) and FTIR spectroscopy. The derivatised MCMs perform differently as catalysts in the liquid‐phase oxidation of various olefins and alcohols with tert ‐butyl hydroperoxide. The highest alkene epoxidation activity was recorded for the catalysts with low metal loading, whereas the material containing oxo‐bridged dimers had the highest activity for oxidation of alcohols. The recyclability of all the catalysts was tested: the catalytic activity of the derivatised materials tended to stabilize with ageing.