Rate of Access to the Binding Sites in Organically Modified Silicates. 1. Amorphous Silica Gels Grafted with Amine or Thiol Groups

The speed at which selected reactants are reaching active sites grafted on porous silica gels of various nature and porosity was evaluated using some model systems: protonation of aminopropyl-grafted silica (APS), mercury(II) binding on mercaptopropyl-grafted silica (MPS), and accumulation of copper(II) on APS. Data were obtained from batch experiments, by monitoring the consumption of reactant in solutions containing the solid phases as dispersed particles (average size: 60−150 μm). Various grafted solids were studied, with pore diameter ranging between 4 and 15 nm and organic group contents of typically 1.4−1.9 mmol g - 1 . Diffusion processes in the porous organically modified silicas were found to be dramatically restricted as compared to those observed in homogeneous solution (≈10 3 −10 4 times slower). They were dependent on several factors such as the pore size of the material and the size of the reactant, the density of grafted organic sites, and the nature of the starting silica gel. Evaluation of apparent diffusion coefficients was achieved by applying a simplified model based on spherical diffusion. This has allowed us to point out a significant decrease in the access rates upon gradual completion of reactions: as the reactant concentration in the vicinity of increasing amounts of grafted groups is raised progressively, there is less room available in the porous structure to enable the probe to reach rapidly the remaining active sites. The apparent diffusion coefficient was found to drop dramatically after typically 30−50% reaction completion, depending on the nature of the probe. This study allows highlighting the optimal conditions that should be required to ensure efficient application of grafted silica gels, that is, in the fields of catalysis or heavy metal extraction.