Surface area, pore volume distribution, and acidity in mesoporous expanded clay catalysts from hybrid density functional theory (DFT) and adsorption microcalorimetry methods

A hybrid density functional theory (DFT) method has been used to interpret the data for the adsorption of nitrogen at 77 K within the porous structure of a sample of synthetic saponite expanded with stable SiO2-TiO2 colloidal particles and dried by different methods. Air drying (AD) favor the formation of structures containing both micro- and mesopores. In contrast, drying with supercritical CO2(SCD) produces mesoporous solids containing only minor amounts of micropores. In SCD samples, the total PV is more than 5 times higher than in AD samples and their structure is hydrothermally stable after a 5-h exposure to 100% steam at 760 degreesC. Adsorption microcalorimetry experiments with ammonia, indicate that the inclusion of SiO2TiO2 clusters drastically increases the clay total (B + L) acidity; acid site strength and acid site density is greatest in AD samples. Both the BET and BJH methods underestimate the clay catalyst's surface area and only the DFT method yields reliable surface area and pore volume measurements over the entire micro-meso porosity range investigated.