Evaluation of the hydrophobic properties of mesoporous FSM-16 by means of adsorption calorimetry

The hydrophobicity of the mesoporous FSM-16 surface was studied by measurements of adsorption isotherms and heats of adsorption of water at 298 K. The hydrophobic-hydrophilic change on the surface derived by water adsorption was also studied. The water adsorption isotherm of freshly prepared FSM-16 was of type V and the initial heat of adsorption of water was ca. 20 kJ/mol, which is indicative of weak adsorbent-adsorbate interactions of a hydrophobic character. However, once water was adsorbed the water adsorptivity increased at a lower relative pressure region and the initial heat of adsorption increased to 75 kJ/mol. The surface became hydrophilic to a small extent, and the original hydrophobic behavior was never restored even after evacuation at high temperatures, 573 and 823 K. An in situ IR analysis revealed that silanol groups were formed and some of them remained after reevacuation at high temperatures once water had been adsorbed on the freshly calcined FSM-16.