Basicity and Porosity of a Calcined Hydrotalcite-Type Material from Nitrogen Porosimetry and Adsorption Microcalorimetry Methods

The hydrothermal transformation at 90−95 °C of a slurry of Al 2 O 3 /MgO/(CH 3 COOH)/H 2 O with molar oxide composition 1:4.4:2.2:1111 produced a hydrotalcite-like (HT) material free from phase impurities and having an approximate unit cell composition of Mg 0.66 Al 0.33 (OH) 2 (CHCOO) 0.33 . 13 C NMR and 27 Al NMR spectra have revealed that the air-dried solid contains acetate anions probably as charge-compensating anions in the HT interlamellar space and that all the aluminum is present in octahedral coordination. After calcination in air at 500 °C, the stacks of brucite layers that form the HT crystals exfoliate, forming a stable mesoporous “house-of-cards” structure with pore width in the range 2.5−20.0 nm, characterized by a DFT surface area of 254 m 2 /g and total pore volume of 0.33 cm 3 /g at 0.995 p /Po, that is available to sorption and catalysis. A 5-h exposure to 100% steam at 760 °C collapses the HT material forming a residue that contains a well-crystallized MgO and an Al spinel phase identified by XRD. At 30 °C, adsorption microcalorimetry with SO 2 revealed that the calcined HT material behaves like a solid base possessing a large (530 μmol SO 2 /g) population of basic sites with strength near 170 kJ/mol; the strength of the strongest basic site is 190 kJ/mol. The steam-aged HT retains the strength of its strongest basic sites but loses most of its sorption capacity for SO 2 . Thus, materials of this type should be efficient adsorbents for the removal of sulfur oxides and other acidic air pollutants in industrial applications that do not require the abovementioned hydrothermal stability.