Liquid phase microcalorimetry applied to evaluate surface polarity of MCM-41 type aluminosilicates

Abstract

Three calorimetric methods have been used to evaluate the surface polarity of three MCM-41 materials characterised by different Si:Al ratios (referred to as SiAl xC14, where x is the moles of Si:Al ratio, and 14 the chain length of the surfactant template). In the liquid phase, the heats of adsorption of 1-butanol from n-heptane and the heats of immersion in water, n-heptane and formamide were compared with the corresponding heat values obtained for two reference solids, viz. silica XOB015 and alumina. Surface (internal) energy of all MCM-41 samples appeared to be smaller than that of any reference solid, the differences being more marked in the case of alumina. In the gas phase, the combination of ammonia gas adsorption and mirocalorimetry measurements was used to evaluate the number and strength of acidic surface sites. With the exception of SiAl8C14, which contains 46% of strongly acidic sites, SiC14 and SiAl32C14 have low surface acidity. The broad distribution of adsorption energy values indicates the energetic heterogeneity of the acidic sites. Incorporating Al into the silicate matrix caused the surface polarity of the materials to increase.