Mesostructured aluminosilicates have been synthesized using gels prepared by reacting colloidal silica (Ludox AS ) with Al(OH ) 3
, aluminum isopropoxide (Al(iPrO) 3 ) or NaAlO 2 in the presence of a surfactant. The hydrothermal transformation at 110Β°C of these gels produced solids with the hexagonal structure typical of MCM-41-type materials. These crystals have been characterized by X-ray diffraction, thermal analysis (TG/DTA), N 2 sorption and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The type, strength and density of acid sites have been studied using microcalorimetry and infrared (IR) spectroscopy.
As a general trend, pore volume and average pore size decreased as the gel SiO 2 /Al 2 O 3 ratio decreased from 32-8, while the pore wall thickness remained in the 1.0-1.5 nm range. Except for one sample, Al was incorporated in tetrahedral coordination inside the pristine crystals. However, as expected, dealumination occurred upon calcination at 600Β°C/12 h, yielding materials having both tetrahedral and octahedral Al-species. Fourier transform infrared (FTIR) experiments with pyridine have indicated that these mesostructured aluminosilicates contain both Bro Β¨nsted and Lewis acid sites and that acidity is strongest in samples prepared with NaAlO 2 . Microcalorimetry experiments with ammonia as the probe molecule have shown that Al insertion into the mesoporous silicate framework affects acid site strength and distribution in a manner controlled by synthesis condition. Samples prepared with Al(OH ) 3 contain a wide distribution of acid site strengths, indicating the absence of preferred locations of Si-O-Al groups within the pore walls. In contrast, distinct populations of acid sites with strengths in the 130-140 kJ mol-1 range or near 150 kJ mol-1, appear in materials prepared with Al(iPrO) 3 or with NaAlO 2 , respectively. Al sites may be located at the surface of the pore wall, where they interact directly with the basic probe molecule. They may also be sandwiched between silica layers within the pore wall (~3-4 Si layers thick), giving acid sites with a strength comparable with that of smectites. Finally, they may also be present in the pore (or within the pore walls) as extraframework Al( VI )-species. The nature, size, concentration, ease of hydrolysis and condensation of the aluminum precursors during synthesis control aluminum incorporation, distribution and location within the structure and with it the acidity of the resulting mesoporous aluminosilicate.