Chemical removal of organic polymers from highly porous sol-gel-derived silica monoliths

Abstract

A new method for the removal of the organic polymer poly(ethylene glycol) (PEG) from silica monoliths exhibiting a hierarchical, bimodal porosity is described. The method is based on post-synthesis treatment with sulfuric acid and does not necessarily require calcination of the monoliths. This procedure preserves the mechanical integrity of the highly porous monoliths. Furthermore, it can be applied to remove PEG from one-pot-synthesized monoliths bearing organic functional groups that would not withstand high-temperature treatments but could survive in acidic conditions. The sulfuric acid treatment also results in less shrinkage than does calcination, which is related to an increase in the degree of silica condensation during the treatment. This could allow the removal of organic polymers to be carried out in the final monolith carrier, and hence reduce the number of steps needed for the fabrication of silica monoliths as HPLC columns, catalyst supports, etc. Furthermore, silica monoliths treated with sulfuric acid have larger amounts of surface silanol groups than calcined monoliths. They also show a higher degree of surface functionalization with functional silanes than calcined samples under the same reaction conditions.