SURFACE CHARACTERIZATION OF MESOPOROUS SILICOALUMINATES OF THE MCM-41TYPE - EVALUATION OF POLAR SURFACE SITES USING FLOW CALORIMETRY, ADSORPTION OF A CATIONIC SURFACTANT AS A FUNCTION OF PORE-SIZE AND ALUMINUM CONTENT

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 differe nt chain lengths as surfactant templates (x = 32 and n = 8, 12, 14, 16 , 18) and of aluminum isopropoxide as a source of aluminum with differ ent framework Si/Al ratios (n = 14 and x = 8, 32, and purely siliceous SiC14). The as-synthesized and calcined samples have been characteriz ed by powder X-ray diffraction (XRD), Al-27 magic angle spinning NMR, nitrogen gas adsorption, flow microcalorimetry, and adsorption of ioni c surfactants from aqueous solution. NMR spectra clearly shaw that all aluminum in the calcined solids is 4-coordinate (framework Al). The u nit cell parameter alpha(o), calculated from XRD patterns, increases w ith increase in the chain length n and decrease in the Si/Al ratio. Ad sorption isotherms of nitrogen at 77 K are of a reversible type IV in the IUPAC classification, with the sole exception of that obtained wit h a microporous SiAl32C8. The values of surface area and pore structur e 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 me sopore 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 introduce d into the silicate framework. The B.E.T. and pore surface areas are s hown 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 o f 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 po lar 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 octylbenzenesul fonate from aqueous solution at 298 K and free pH onto SiAl32C18 revea ls the existence of negatively charged sites in the surface under the experimental conditions employed. The acidic character of polar surfac e 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.