Rate of Access to the Binding Sites in Organically Modified Silicates. 3. Effect of Structure and Density of Functional Groups in Mesoporous Solids Obtained by the Co-Condensation Route

Abstract

Mercaptopropyl-functionalized silica spheres have been prepared by self-assembly co-condensation of mercaptopropyltrimethoxysilane (MPTMS) and tetraethoxysilane (TEOS) precursors in the presence of a cationic surfactant as templating agent and ammonia as catalyst. Several materials have been obtained by varying the MPTMS content from 5 to 100% in the synthesis medium, giving rise to a wide range of porous solids featuring different functionalization levels (denoted MPS- n %, with n ranging from 5 to 100) and distinct structural order/disorder over different length scales. They were characterized by N 2 sorption isotherms, XRD, scanning and transmission electron microscopy, and particle size analysis. Their reactivity in aqueous media was studied with respect to their binding properties toward Hg II species by complexation with thiol groups. Total accessibility (expressed on the basis of a 1:1 S/Hg stoechiometry) was demonstrated and quantified for well-ordered materials containing up to 30% MPTMS. Less open structures characterized by a high degree of functionalization were subject to less-than-complete sorption capacities, while, however, reaching maximum loading values as high as 750 mg g - 1 . The speed of the uptake process was studied from batch experiments by using a fast electrochemical technique to monitor the consumption of the reactant, as a function of time, as a consequence of Hg II sorption by the MPS materials. The associated apparent diffusion coefficients were calculated according to a spherical diffusion model, by appropriate fitting of the kinetic curves. They were strongly affected by both the structure and density of functional groups in the MPS sorbents. Whereas the long hexagonally packed one-dimensional channels of MPS-5% and MPS-10% may induce some diffusional restrictions for Hg II to reach the binding sites located deep in the mesopores, transport issues within MPS-15% to MPS-30% sorbents is facilitated by a shorter range structural order in the form of three-dimensional wormhole framework structures. Increasing further the content of organic groups in the materials led to poorly ordered (MPS-40% and MPS-50%) and even amorphous (MPS-70% and MPS-100%) solids, resulting in considerable lowering of mass transfer rates, to which the concomitant increase of hydrophobicity may also contribute to a significant extent. The differences in sorption rates exhibited by MPS materials appear therefore to result from differences in their relative long-range versus short-range structural order/disorder and their intrinsic hydrophobicity, which are induced by their functionalization levels.