Esterification of acetic acid with methanol, which was used as a model reaction in the stabilization of bio-oil, was tested over propylsulfonic acid-functionalized SBA-15 materials. Due to the significant amount of water content in bio-oil, the water tolerance properties of the materials were also investigated in detail. The results showed that the functionalized mesoporous materials demonstrated a comparable site activity to H2SO4 in acetic acid esterification proving its potential for application. The effect of propylsulfonic acid loading showed that the intrinsic per site activity increased with the number of acid sites, which suggested the presence of acidic site cooperativity. Water tolerance experiments showed that SO3H-SBA-15 exhibited less reaction inhibition due to water than did H2SO4, which was at least partially ascribed to the hydrophobic propyl groups bonded to sulfonic acid groups. Further functionalization of SO3H-SBA-15 with hydrophobic propyl groups further increased the water tolerance property of the catalyst in the esterification reaction. The reusability of the catalyst was also investigated and showed multiple cycle stability for acetic acid esterification without significant loss of activity. (C) 2009 Elsevier B.V. All rights reserved