Number and strength of surface acidic site on porous aluminosilicates of the MCM-41 type inferred from a combined microcalorimetric and adsorption study

A combined microcalorimetry and adsorption study has been used to characterize the surface acidity of two series of MCM-41 aluminosilicates (referred to as SiAlxCn, where x is the mole Si:AI ratio and n the chain length of the surfactant template). (29)Si magic angle spinning NMR spectra of a selected sample (SiAl32C16) indicates the presence of siloxane groups, Si(OSi)(4), and three types of silanol groups, that is, single (SiO)(3)-Si-OH, hydrogen-bonded (SiO)(3)-SiOH...HO-Si-(SiO)(3), and geminal (SiO)(2)-Si(OH)(2). It is also possible to detect the contributions from Si(3Si,1Al) and Si(2Si,2Al) sites. Generally, the width of NMR spectra is characteristic of amorphous materials and suggests a large variety of Si-O-Si and Si-O-Al bond angles and lengths. The (27)Al MAS NMR study made on the hydrogen-exchanged SiAl32C14 sample (H(+)-SiAl32C14) and the one rich in aluminum (SiAl8C14) clearly points to the presence of extraframework six-coordinate Al in the calcined materials. The volumetric and calorimetric measurements of gas ammonia adsorption at 353 K were used to determine the number and strength of surface acidic sites. With the exception of H(+)-SiAl32C14 and SiAl8C14, all samples have low surface acidity. The density of acidic sites ranges between 0.04 and 0.09 mu mol m(-2), increasing a little with decreasing pore size. These sites are relatively homogeneous and weak. For H(+)-SiAl32C14, surface acidity is enhanced as a result of the formation of strongly acidic Bronsted-type sites and extraframework aluminum. The density of acidic sites increases to 0.11 mu mol m(-2) and the fraction of strongly acidic sites is about 60%. In the case of the SiAl8C14 sample, the density of acidic sites reaches a value of 0.3 mu mol m(-2). These sites are very heterogeneous: the corresponding differential enthalpy of ammonia adsorption decreases from -160 to -80 kJ mol(-1). Following the pyridine-TPD study on this sample, Lewis acid sites producing surface pyridine complexes constitute the strongest acidic sites.