Fabrice Gaslain

Factors affecting the reactivity of thiol-functionalized mesoporous silica adsorbents toward mercury(II).

Numerous mercaptopropyl-functionalized silica spheres have been prepared by either post-synthesis grafting of MCM-41 and MCM-48 or self-assembly co-condensation of mercaptopropyltrimethoxysilane (MPTMS) or mercaptopropyltriethoxysilane (MPTES) and tetraethoxysilane (TEOS) precursors in hydroalcoholic medium in the presence of a cationic surfactant as templating agent and ammonia as catalyst. These materials of approximately the same particle size and morphology featured different functionalization levels, various degrees of structural order, and variable distribution of thiol groups in the mesopores. Their reactivity in solution has been studied using Hg(II) as model analyte. Total accessibility (on a 1:1 S:Hg stoichiometry basis) was demonstrated and quantified for well-ordered materials whereas less open and less organized structures with high degrees of functionalization were subject to less-than-complete loadings. Capacities measured at pH 2 were lower than at pH 4 because of distinct mercury-binding mechanisms. Kinetics associated to the uptake process were studied by in situ electrochemical monitoring of Hg(II) consumption from aqueous suspensions containing the various adsorbents. They indicate only little difference between materials of the MCM-41 and MCM-48 series at similar functionalization levels, fast mass transport in well-ordered mesostructures in comparison to the poorly or non-ordered ones (except at pH 2 where charge formation induced some restriction in materials characterized by long-range structural order), and even faster processes in the wormlike frameworks (characterized by shorter range structural order). Hg(II) binding to thiol-functionalized materials obtained by post-synthesis grafting was found to occur more rapidly in the early beginning of the uptake process as a result of a higher concentration of binding sites at the pore entrance in comparison to the more homogeneous distribution of these groups in the mesochannels of materials obtained by co-condensation.

First zeolite carbon replica with a well resolved X-ray diffraction pattern

The present study demonstrates that for the nanocasting process with zeolites, a careful choice of the zeolite structure type (EMT) allows the formation of faithful carbon replica exhibiting up to three well resolved XRD peaks.

Structure and sorption properties of a zeolite-templated carbon with the EMT structure type.

An ordered microporous carbon material was prepared by the nanocasting process using the EMC-2 zeolite (EMT structure type) as a hard template. X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed long-range ordering in the material that resulted from the negative replication of the host template. The carbon porous network replicating the zeolite structure was modeled by overlapped spherical voids with diameters determined from the XRD pattern that displayed up to six distinct peaks. The surface delimiting the 3D interconnected porosity of the solid has a complex morphology. The pore size distribution calculated from the XRD-derived structural model is characterized by a maximum at 1.04 nm related to the long-range-ordered microporous network. Complementary studies by immersion calorimetry revealed that most of the porosity was characterized by a size above 1.5 nm. These porous features were compared to data resulting from classical analysis (DR, DFT, BET, etc.) of the N2 (77 K) and CO2 (low and high pressure, 273 K) physisorption isotherms. The limitations of these approaches are discussed in light of the pore size distribution consistently determined by XRD and immersion calorimetry measurements.

Synthesis of dithiocarbamate-functionalized mesoporous silica-based materials: interest of one-step grafting

The classical approach to prepare dithiocarbamate-functionalized silica involves a two-step procedure based on the reaction of carbon disulfide with a pre-formed amine modified silica sample. A novel approach is proposed here, involving the direct grafting of silica surfaces with the aid of a siloxydithiocarbamate precursor. Functionalization has been applied to three silica samples (a non-ordered silica gel and two ordered mesoporous solids with distinct pore size) to evidence possible effects of mesostructural order and/or pore size of the materials. The performance of the one-step method has been compared to the two-step one and qualitatively discussed in terms of derivatization efficiency regarding the integrity of dithiocarbamate organo-functional groups. Thanks to complementary physico-chemical techniques (N2adsorption/desorption, solid-state NMR, XPS, XRD), the interest of direct grafting has been clearly demonstrated as it allows to access in one step dithiocarbamate-functionalized silicas free of residual amine groups. Moreover, the use of a base in the two-step procedure is likely to induce some degradation of the silica materials. The advantageous features of the direct grafting process were also discussed with respect to Hg(II) adsorption to these materials.

Organically-modified ordered mesoporous siliceous solids

Publisher Summary This chapter gives an overview of the main advances in the development of ordered mesoporous siliceous solids functionalized by organic groups including periodic organo-silica mesoporous materials. The growing interest in such organic modifications can be attributed to the characteristics of ordered mesoporous silica (OMS) materials. Moreover, the amorphous character of their frameworks offers a large number of available silanol groups that can easily react with various functional groups and allows the strong and stable coupling of organic moieties to the silica network. An important attention has been also given to the shape modeling of these hybrid materials with a particular effort on the elaboration of thin films and monoliths. Three routes are commonly followed to organically modify OMS materials. Organic pendant groups can be anchored to the silica backbone by post-synthesis reactions with residual Si–OH groups (post-syntheis grafting) or by a direct synthesis process based on co-condensation of silica precursors with organoalkoxysilanes. Organically modified OMS prepared by post-synthesis or by direct synthesis have been compared in terms of structural and textural characteristics, organic group loadings, localization, and accessibility. Catalysis has been the most studied application for the organically modified mesoporous siliceous solids. Porous materials are commonly employed for the adsorption of molecules or elements and can be found in everyday applications. The large potential of these materials in terms of structural and textural characteristics, surface and bulk properties, processing, functionalities, and so on make them suitable materials for a large panel of applications.