Rate of Access to the Binding Sites in Organically Modified Silicates. 2. Ordered Mesoporous Silicas Grafted with Amine or Thiol Groups

Five different ordered mesoporous silica samples displaying various pore sizes and structures (two small-pore MCM-41, two large-pore MCM-41, and one small-pore MCM-48) and one amorphous silica gel have been grafted with either aminopropyl or mercaptopropyl groups. The resulting aminopropyl-grafted silicas (APS) and mercaptopropyl-grafted silicas (MPS) have been studied in solution via protonation of APS and metal ion binding on both APS and MPS. Special attention was given to characterize the accessibility to the binding sites and to the speed at which the reactants are reaching these reactive centers inside the mesoporous materials. Results have been obtained from batch experiments, by monitoring the reactant depletion in suspensions containing APS or MPS particles, and discussed with respect to the structure and porosity of the organic−inorganic hybrids. As a general trend, both accessibility and rate of access to the reactive sites were higher with ordered mesoporous solids than with amorphous materials of comparable porosity (average pore size ∼60−70 Å). The ordered mesoporous structures of smaller pore size (∼35 Å) gave rise to the same performance as that of large-pore amorphous silica, only if pore blocking can be avoided during the grafting process; if not, the advantage of uniform pore structure over the corresponding amorphous material did not exist anymore: a pore volume of at least 0.5 cm3 g-1 remaining upon grafting was necessary to keep this advantage. Increasing the amount of grafted moieties in the mesopores also led to restricted mass-transfer rates because of increasing steric hindrance. Moreover, protonation of mesoporous APS displaying uniform-sized channels was found to be dramatically slow at protonation levels higher than 50%, leading even to less-than-complete occupancy levels after 24 h of equilibration, most probably because of strong repulsive electrostatic interactions in such a confined medium. Applying a simplified diffusion model, the access rates of CuII in APS and HgII in MPS materials were quantified via calculation of apparent diffusion coefficients, Dapp, by appropriate fitting of kinetic curves. Dapp values were found to decrease slowly upon gradual completion of reaction because progressively less space was available for the ingress of reactants upon filling the mesostructures. This work would help at selecting the most appropriate conditions for target applications of grafted mesoporous solids in terms of capacity, accessibility, and especially, access rates to the active sites.