Synthesis of spherical mesoporous silica beads with tunable size, stiffness and porosity

Abstract

We present an innovative template-free water-based sol–gel method to produce uniform mesoporous silica beads of millimeter size, which have tunable size, stiffness and porosity, and could be used for adsorption applications. Our protocol exploits an in-situ enzymatic reaction to produce spherical beads of hydrogel from a charge-stabilized suspension of silica nanoparticles confined in a millimetric drop suspended in a non-miscible oil. Once the gelation step is complete, the spherical bead of gel is cleaned from oil and deposited onto a hydrophobic surface and let dry. Separating the gelation to the drying steps ensures a spatially uniform gel and allows us to perform a solvent exchange before drying. For all beads, we observe a crack-free drying process leading to the formation of stiff quasi-spherical beads with diameter in the range 1 to 5 mm and Young modulus in the range $(0.1-2)\mathrm{GPa}$ and narrow pore size distribution, centered around 10 to $25\mathrm{nm}$ depending on the experimental conditions. Finally, to demonstrate the potentiality of these materials, we graft on the bead surface aminosilane molecules, and quantify their CO${}_2$ adsorption efficiency. Overall, the production method we have developed is simple, readily adaptable, and offers promising materials for adsorption, storage, catalysis and chromatography.