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 Brønsted and strong Lewis-acid sites were also observed.