Template synthesis and characterisation of MCM-41 mesoporous molecular sieves containing various transition metal elements - TME (Cu, Fe, Nb, V, Mo)

A series of metallosilicates (transition metal elements-TME) MCM-41 (TME = Fe, Cu, Nb, V, Mo) mesoporous molecular sieves with variable Si/TME ratios have been synthesized and characterised by low temperature N-2 adsorption/desorption, XRD, XPS, H-2-TPR, FTIR combined with NO + NO2 adsorption, diffuse reflectance UV-Vis spectroscopy, and ESR study. All the materials exhibit hexagonal arrangement of uniform mesopores (with exception of CuMCM-41). Defect holes amid the nanochanels besides well-ordered mesopores characterise mainly Fe-containing materials, in which the highest TME loading was reached. Similar but smaller defects take place in NbMCM-41. The amount of TME included into MCM-41 structure under the preparation conditions used in this work changes in the order: Fe > Nb > Cu much greater than V much greater than Mo. This sequence is not related to the oxidation state of metals which was estimated in calcined materials as Fe3+, Nb5+, Cu2+, V5+, Mo6+. It does not also correlate with cation sizes in a simple way. The possibility of forming tetrahedral coordination seems to limit the TME incorporation into the MCM-41 skeleton if free metal cations are used in the gel (Cu2+, Fe3+, Nb5+). Al in the gel makes the isomorphously substitution of silicon by copper easier, but part of Cu occupies extra framework cationic positions in the final material. (C) 2003 Elsevier Ltd. All rights reserved.