Structural and oxidative properties of Mn-MCM41 catalysts synthesised by MOCVD method

Molecular organic chemical vapour deposition (MOCVD) is employed to implant various gaseous manganese precursors onto mesoporous MCM-41 silica. After heat treatment in air these samples show significantly higher catalytic activity than Mn/MCM-41 samples prepared by a conventional impregnation method for trans-stilbene epoxidation to trans-stilbene oxide using tert-butylhydroperoxide (TBHP) as an oxidant. In contrast, the same MOCVD samples gave much poorer activity than the impregnation samples in diphenylmethane oxidation reaction using air as the oxidant. Catalyst characterisation (infrared, temperature programmed reduction, EXAFS, etc.) indicates that discrete entities of Mn species (Mn-oxo and/or bridging Mn-oxygen structures, etc.) are formed on the surface of the MCM-41 by the MOCVD technique. This is attributed to initial reactions of gaseous organo-manganese complexes with surface silanol groups, followed by their oxidation in air during the heat treatment. On the other hand, in the wet impregnation samples, bulk Mn oxide phases are deposited onto the MCM-41 structure. It is thus believed that the discrete supported manganese-oxygen species of high electrophilicity are the active sites for the oxygen transfer reaction in the trans-stilbene oxidation. However, the nucleophilic lattice oxygen of the bulk Mn oxide phases prepared by the wet impregnation is responsible for the effective hydrogen abstraction in the diphenylmethane oxidation. It is also evident that the catalytic activity of the MOCVD samples depends crucially on the type of precursors used. Thus, the result clearly suggests that active and ultraselective Mn-oxygen catalytic sites for a particular oxidation reaction may be tailored through the use of different chemical precursors using the MOCVD technique. (C) 2000 Elsevier Science B.V. All rights reserved.