Two series of MCM-41 silicoaluminates (referred to as SiAlxCn, where x is the mole ratio Si/Al and n the surfactant chain length) have been synthesized making use of alkyltrimethylammonium bromides with different chain lengths as surfactant templates (x = 32 and n = 8, 12, 14, 16, 18) and of aluminum isopropoxide as a source of aluminum with different framework Si/Al ratios (n = 14 and x = 8, 32, and purely siliceous SiC14). The as-synthesized and calcined samples have been characterized by powder X-ray diffraction (XRD), Al-27 magic angle spinning NMR, nitrogen gas adsorption, flow microcalorimetry, and adsorption of ionic surfactants from aqueous solution. NMR spectra clearly shaw that all aluminum in the calcined solids is 4-coordinate (framework Al). The unit cell parameter alpha(o), calculated from XRD patterns, increases with increase in the chain length n and decrease in the Si/Al ratio. Adsorption isotherms of nitrogen at 77 K are of a reversible type IV in the IUPAC classification, with the sole exception of that obtained with a microporous SiAl32C8. The values of surface area and pore structure parameters were determined on the basis of the alpha(s)-method. The calcined MCM-41 materials have large B.E.T. (600-1000 m(2) g(-1)) and pore surface (500-900 m(2) g(-1)) areas, as well as relatively high mesopore volumes (0.5-0.8 cm(3) g(-1)) with pore diameters of the order of a few nanometers. The effective pore size increases with increasing chain length n but remains essentially unchanged when Al is introduced into the silicate framework. The B.E.T. and pore surface areas are shown to increase with increase in the template chain length from n = 8 to n = 14 and to decrease from n = 14 to n = 18. A decrease in the Si/Al ratio causes both surface parameters to diminish. The enthalpies of adsorption of 1-butanol from n-heptane at 293 K were used to detect Lewis acidic-basic sites in MCM-41 surfaces. The percentage of such polar sites ranges between 43 and 70%. Surface polarity is increased by incorporating Al into the framework and by lengthening the surfactant template chain. Comparison between adsorption of a cationic tetradecyl-trimethylammonum bromide (TTAB) and an anionic sodium octylbenzenesulfonate from aqueous solution at 298 K and free pH onto SiAl32C18 reveals the existence of negatively charged sites in the surface under the experimental conditions employed. The acidic character of polar surface sites increases with increase in the degree of Al incorporation into the framework. The effectiveness of TTAB adsorption is significantly reduced as the pore size diminishes.
