EPR spin probe investigation into the synthesis of mesoporous silica from the water/acetonitrile/n-dodecylamine system

Mesoporous silica prepared in the dodecylamine/acetonitrile (ACN)/water/tetraethoxysilane (TEOS) system, has led, by functionalization, to a series of catalytically active materials with well-defined narrow pore size distributions. In this contribution, we follow the mechanism of templated synthesis of mesoporous silica in the nonfunctionalized parent system, for different ACN/water proportions. Electron paramagnetic resonance (EPR) of spin probes was employed to follow structural changes in the template structure during precipitation, while X-ray diffraction (XRD) and porosimetry determinations have in parallel reported on the time evolution of the mesoporous structure. Three solvent mixtures were studied, with largely different solvent compositions, and EPR data have shown that a 0.3 M dodecylamine (DDA) solution in (i) ACN/water = 1/1 v/v was a true micellar solution, in (ii) ACN/water = 2/8 v/v was a biphasic system with a lamellar liquid crystalline phase, and in (iii) ACN/water = 8/2 was a molecular solution. In spite of these extreme differences, after 20 min reaction with TEOS, all compositions presented a well-developed mesoporous structure with similar pore dimensions. In case i, the mechanism of precipitation on preformed micelles is straightforward. In case ii, EPR of spin probes has provided evidence that tetraethoxysilane (TEOS) addition leads to an immediate phase transformation of the liquid crystalline phase to a micellar solution, on which silica precipitation occurs. Compared with i and ii, no DDA aggregates were found in solution before or during precipitation for the third solvent mixture iii, pointing to a true cooperative mechanism. XRD and nitrogen adsorption/desorption measurements entirely support these data. Interestingly, the precipitate formed in the first 5 min was nonstructured for all systems, even for i where preformed micelles were present.