As described in this research paper:
Immobilization of Jacobsen type catalysts on modified silica, Jairo Cubillos , Edwing Grajales, Santiago Vásquez, Consuelo Montes de Correa, Rev. Fac. Ing. Univ. Antioquia N.° 57 pp. 38-48. Enero, 2011 (link)
The Jacobsen type catalysts are attached unto the silica through a reaction between Silanol that is already part of the silica and 3-APTES. However, there is no mention on how these Silanol molecules were already present in the silica. Any idea?