Nature of hydroxy groups in MCM-41

The type and concentration of hydroxy groups in MCM-41 as a function of the Si:Al ratio have been investigated and compared with amorphous Si/Al oxide. Three hydroxy groups at 3745, 3715 and 3530 cm(-1) were observed in the IR spectra of siliceous MCM-41. For Al-containing samples the intensity of the bands at 3715 and 3530 cm(-1) increased with increasing Al concentration and additionally a broad band at ca. 3660 cm(-1) was observed. The bands were assigned to silanol groups terminating the particles (3745 cm(-1)), hydroxy groups in the interior (3715 cm(-1)) and to hydrogen-bonded internal SiOH groups (3530 cm(-1)). The band at 3660 cm(-1) was assigned to hydroxy groups on octahedrally coordinated aluminium oxide species. On siliceous samples only weak acid sites were detected, while for Al-containing samples Bronsted and strong Lewis-acid sites were also observed.