Number and Strength of Surface Acidic Sites on Porous Aluminosilicates of the MCM-41 Type Inferred from a Combined Microcalorimetric and Adsorption Study

Abstract

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:Al ratio and n the chain length of the surfactant template). 29Si 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 27Al 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 μmol 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 Brönsted-type sites and extraframework aluminum. The density of acidic sites increases to 0.11 μmol 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 μmol 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.