Surface acidity and hydrophilicity of coprecipitated Al₂O₃-SiO₂ xerogels prepared from aluminium nitrate nonahydrate and tetraethylorthosilicate

Amorphous aluminosilicate xerogels with various chemical compositions were prepared by coprecipitation, and their surface acidity and hydrophilicity were investigated by NH₃ gas temperature programmed desorption (TPD), water vapor adsorption-desorption isotherms, and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR). The xerogels were synthesized by adding conc. NH₄OH to an ethanol solution of calculated amounts of aluminium nitrate nonahydrate and tetraethylorthosilicate, and calcined at 300°C for 4 h. All the NH₃ TPD spectra of the xerogels showed similar asymmetric peak profiles at around 200°C tailing to the higher temperature side. The amount of acidity evaluated from the peak area of the TPD spectra showed a maximum at around 10 mol% Al₂O₃ composition. The change as a function of composition showed a good correlation with the total amount of four and five coordinated Al atoms in the xerogels deduced from the ²⁷Al MAS NMR spectra. The water vapor adsorption isotherms of the xerogels were all of type IV irrespective of the composition. The maximum amounts of water vapor adsorbed by these xerogels were about 600-700 ml(STP)/g and were relatively high compared with those for various other adsorbents reported so far. Since the thickness of the adsorbed water vapor layer of the xerogels in the low relative pressure region increased with increasing Al₂O₃ content, the surface of the xerogels is considered to become more hydrophilic with increasing Al₂O₃ content of the xerogels.