Acid sites in mesoporous materials: A DRIFTS study

A range of MCM-41 materials have been prepared and characterised employing diffuse reflectance FTIR spectroscopy (DRIFTS) to investigate fundamental and combination frequencies of the OH groups in amorphous and crystalline aluminosilicates. Comparative characterisation of mesoporous materials shows that SiOH groups in Al-MCM-41 are acidic, unlike those in Si-MCM-41. At the same time, no indication of the presence of bridging hydroxyls in any of the studied MCM-41 samples has been found. It appears that the 'mild acidity' of A1-MCM-41 materials is not due to the bridging A1(OH)Si groups, which are commonly found in zeolites, but it is associated with SiOH groups similar to those in amorphous aluminosilicates. Acidic properties of these OH groups are probably modified by the inductive effect of the Al present in the structure, which however does not affect their stretching frequency. A significant concentration of Bronsted acid sites is observed in the Al grafted sample obtained via a post-synthesis modification of Si-MCM-41 indicating that this may be a suitable way to generate acid sites compared with the aluminium introduction during the synthesis. © 2001 Elsevier Science B.V. All rights reserved.