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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?

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    $\begingroup$ As mentioned in the answer, silanol groups are are normal terminal groups on silica, but you mention "modified" silica... So are you asking how was it modified? It's quite possible it was processed, for example to get good surface density of the groups. $\endgroup$
    – Mithoron
    Commented Aug 30, 2021 at 15:33

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The bulk of silica is indeed silicon dioxide, but at surface there is a discontinuity. The dangling O, exist as OHs, as silanols. Therefore silica surface itself is full of silanols, which are used for functionalizing silica. Native silica looks like the following with various types of silanols. APTES is a very common chemistry in the silica modification chemistry.

Note silicon dioxide, like quartz may not be easily to functionalized because the surface area is very low and therefore the number of silanols is also low.

enter image description here

Ref:High-Performance Precoated Stationary Phases, Colin F. Poole, in Instrumental Thin-Layer Chromatography, 2015

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