CTS-0086777 Edward J. Maginn U. of Notre Dame
This project concerns the synthesis, characterization and evaluation of functionalized mesoporous silica-based materials for adsorption-based separations critical to environmental applications. Specifically, we seek to develop high performance adsorbents for two applications: the remediation of aqueous streams contaminated with heavy metals, and the separation of paraffin/olefin gas mixtures.
Heavy metal contamination is a major source of environmental concern, and existing technologies are not always adequate for meeting stringent regulatory limits. There has been renewed interest recently in developing highly selective adsorbents for the removal of heavy metals such as mercury from aqueous streams, but most of the attention has focused on optimizing the selectivity of materials. Many practical engineering concerns such as stability, regeneration capability and mass transfer issues have been largely ignored.
Olefin/paraffin separation represents another opportunity where advanced adsorbents can play a large role in environmental protection. Due to the high capital costs associated with distillation of these mixtures, many chemical production facilities flare recycle purge streams containing valuable olefin species. This results in a significant waste of feedstock as well as increased emissions. A low capital adsorption-based process that could separate these mixtures is highly desirable, but requires the development of new types of adsorbent materials.
Using molecular modeling as a guide, we will follow a strategy in which mesoporous silicas from the M41S family of materials are tailored for either heavy metal remediation or olefin/paraffin separation by functionalizing the pore walls with ligands. The ligands will be chosen so as to tune the pore diameter and interaction strength between the target species and the ligand to achieve desirable separation performance. This means that the interaction strength must be great enough to achieve high selectivity and capacity, but weak enough so that the material can be easily regenerated. We will also engineer the material to achieve optimum mass transfer characteristics. Our synthetic strategy will build on some of our recent work in which we have attached organosilane ligands onto the pore walls of mesoporous silica. These materials can be made in powdered form using conventional techniques. We have also been able to synthesis self-supporting functionalized "macrostructures" using an emulsion process.
The project involves three main components. Detailed molecular modeling studies using Monte Carlo and molecular dynamics techniques will be conducted to probe fundamental issues pertaining to the way in which ligands interact with guest species to change the adsorption thermodynamics and diffusion properties of the system. Using these results as a guide, we will then incorporate different ligands into mesoporous silica and characterize the materials using a range of techniques, including X-ray diffraction, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption. For the heavy metal adsorbents, we will initially investigate the use of amine-terminated alkoxysilane ligands and sulfonated ligands with exchangeable cations. For the olefin/paraffin adsorbents, we will test whether metal cations such as silver can be used with the sulfonated ligands to selectively adsorb olefins. The performance of the materials will then be evaluated by measuring mixture isotherms to obtain selectivities and adsorption capacity. Importantly, we will also perform breakthrough curve analyses and regeneration tests using a packed bed. These tests will help determine the feasibility of using these materials for industrial and consumer applications.
