| Abstract
| - Mass-transfer rates of various reactants (H+, HgII, CuII) in mesoporous silicas grafted with aminopropyl or mercaptopropyl groups have been investigated; they are strongly dependent on the materials structures, their pore size, the density of organic groups, and the reactant size and charge.
- Five different ordered mesoporous silica samples displaying various pore sizes andstructures (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 mercaptopropylgroups. 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 bothAPS and MPS. Special attention was given to characterize the accessibility to the bindingsites and to the speed at which the reactants are reaching these reactive centers inside themesoporous materials. Results have been obtained from batch experiments, by monitoringthe reactant depletion in suspensions containing APS or MPS particles, and discussed withrespect 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 mesoporoussolids than with amorphous materials of comparable porosity (average pore size ∼60−70Å). The ordered mesoporous structures of smaller pore size (∼35 Å) gave rise to the sameperformance as that of large-pore amorphous silica, only if pore blocking can be avoidedduring the grafting process; if not, the advantage of uniform pore structure over thecorresponding amorphous material did not exist anymore: a pore volume of at least 0.5 cm3g-1 remaining upon grafting was necessary to keep this advantage. Increasing the amountof grafted moieties in the mesopores also led to restricted mass-transfer rates because ofincreasing 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 probablybecause of strong repulsive electrostatic interactions in such a confined medium. Applyinga simplified diffusion model, the access rates of CuII in APS and HgII in MPS materialswere quantified via calculation of apparent diffusion coefficients, Dapp, by appropriate fittingof kinetic curves. Dapp values were found to decrease slowly upon gradual completion ofreaction because progressively less space was available for the ingress of reactants uponfilling the mesostructures. This work would help at selecting the most appropriate conditionsfor target applications of grafted mesoporous solids in terms of capacity, accessibility, andespecially, access rates to the active sites.
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