FT-IR studies on the acidity of gallium-substituted mesoporous MCM-41 silica

Gallium-containing mesoporous MCM-41 silica was synthesized at a nominal Si:Ga ratio of 16:1. Synthesis was carried out from a parent gel containing no cations other than NH4+ and cetyltrimethylammonium (template), so that following thermolysis of the template agent and ammonium ions the protonic form, H-GaMCM-41, was obtained. Powder X-ray diffraction showed the characteristic pattern of MCM-41-type materials, and nitrogen adsorption at 77 K lead to a value of 535 m2 g−1 for the specific (BET) surface area. Infrared spectroscopy of carbon monoxide adsorbed at 77 K, and of pyridine and lutidine adsorbed at room temperature, showed the presence in H-GaMCM-41 of both Brønsted and Lewis acid sites. Brønsted acidity, assigned to structural Si(OH)Ga groups, was most distinctively proved by protonation of both pyridine and lutidine. Lewis acidity (coordinatively unsaturated Ga3+ ions) showed up by formation of the characteristic Lewis-type adducts with both CO and pyridine.