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

Abstract

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+NO 2 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≫V≫Mo. This sequence is not related to the oxidation state of metals which was estimated in calcined materials as Fe 3+, Nb 5+, Cu 2+, V 5+, Mo 6+. 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 (Cu 2+, Fe 3+, Nb 5+). 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.