The effect of synthesis gel dilution on the physico-chemical properties of acid saponite clays

Acid saponite samples prepared by varying H ₂O/Si molar ratio of the synthesis gel are reported in this work aiming to tune their particles size and surface properties. In particular, H ₂O/Si molar ratio of 20, 50, 100 and 150 were used and the obtained samples were submitted to an ion exchange procedure in 0.01 M HCl solution in order to replace the interlayer Na ⁺ ions by protons. A combined experimental approach (XRD, HRTEM, N ₂ physisorption and solid-state MAS NMR) was used to finely characterize the synthesized materials. FTIR spectroscopy of adsorbed probe molecules with different basicity (e.g., CO and NH ₃) was used to monitor the surface acid properties (i.e. number and strength of acid sites) and to assess the acid site distribution from weak to strong acidity. Structural data evidenced that samples prepared by using H ₂O/Si ratio of 20, 50 and 110 are characterized by a layered structure typical of saponite. Moreover, the increase of synthesis gel dilution leads to a progressive reduction of the particles sizes along with cation exchange capacity (CEC), which depends on the gradual decrease of aluminum concentration in the tetrahedral sheets as found by ²⁷Al MAS NMR. In addition, the presence of an amorphous phase was observed for the saponite sample prepared with H ₂O/Si = 150. IR spectroscopy of adsorbed NH ₃ and CO probes pointed out that the H-exchanged samples prepared with higher synthesis gel dilution presented a reduced concentration of Bronsted acid sites, according to CEC data.