Extended X-ray absorption fine structure study of manganese-ore species and related compounds on the surface of MCM-41 channels

It has been previously shown that passing Mn-2(CO)(10) vapour over MCM-41 mesoporous silica at 50 degrees C followed by heat treatment in air resulted in a material with high catalytic activity for propene oxidation. EXAFS studies of the Mn-2(CO)(10) precursor, MCM-41 treated with Mn-2(CO)(10) before and after the calcination in air have therefore been carried out and the results are presented in this paper. Upon immobilisation of the Mn-2(CO)(10), the EXAFS data indicate four Mn-(CO) groups with a Mn-Mn interaction at 3.112 Angstrom; new interactions such as Mn-O (at the much shorter distance of 1.705 Angstrom) and Mn-Si(via Mn-O-Si) at a distance of 3.529 Angstrom are evident. This clearly suggests that the Mn species anchor onto the surface of MCM-41 forming Mn-O-Si bonds, presumably through the reaction of surface silanol and Mn carbonyl species. After heat treatment in air, the EXAFS data indicate further loss of carbonyl groups with the formation of more Mn-O interactions(three Mn-O at 1.900 Angstrom and one Mn-O at 1.747 Angstrom). The best fitted model for the surface Mn-oxo species has a structure resembling Mn2O7. With reference to this structure, an active site in the Mn-modified MCM-41 for propene oxidation is therefore proposed.