Simpley, Try to graft organic groups such as Octyl or Phenyl on the surface...

Hi Atul,

SBA-15 is ordered-mesoporous silica, and also hydrophobic in nature.

J. Am. Chem. Soc. 1998, 120, 6024-6036

http://pubs.acs.org/doi/pdfplus/10.1021/ja974025i

I have to use this SiO2 supported catalysts with HNO3. So organic groups should have stability under such acidic conditions.

I am using 70% HNO3. Does it affect to SiO2?

Synthesis and Properties of Nanosilicon Stabilized by Butyl and Perfluorobutyl Ligands L. A. Aslanov, V. N. Zakharov, A. V. Pavlikov, S. V. Savilov, V. Yu. Timoshenko, A. V. Yatsenko

Russian Journal of Coordination Chemistry 01/2013; 39(6):427-431.

Nanosilicon covered with C4F9 simply to transform to SiO2 covered C4F9. 

It is relatively stable at mentioned acidic condition. you can analysis the catalysts before and after acid treatment. Let's pray !

Dear all, I have already used SiO2 based catalyst for nitration reactions using 70% HNO3. It is observed that catalyst  being active even after 1000 hrs under vapor phase nitration conditions 

http://pubs.rsc.org/en/content/articlelanding/2006/gc/b600094k#!divAbstract

So i guess stability of SiO2 under acidic condition (70% HNO3) may not be the issue.

Hi,

You may first prepare the sol of SiO2, then add hexamethyldisilazane (HMDS) to the sol and stir， you can get the hydrophobic SiO2.