Chemistry Scholarships

Molecular dynamics of tethered organics in mesoporous solids

This project will explore how high temperature free-radical organic reactions are influenced by confinement in a restricted volume. This will be achieved by covalently attaching molecules to the surface of hexagonal mesoporous silicas, such as SBA-15 and MCM-41. This project will involve a mixture of computational and experimental studies, with the principal focus being on molecular dynamics measurements.

Background: Molecular dynamics modeling has shown that there is a tendency for tethered phenethyl phenyl ether (PPE) molecules to ‘hug’ the internal surface of MCM-41, even at quite high temperatures2. However, as the density of the tethers is increased, within the confined volume of the mesoprous support, steric crowding causes them to extend increasingly into the central pore volume. The data are consistent with pore size measurements on experimentally prepared hybrid materials and are important to provide a thorough understanding of reaction dynamics in such systems.
Now there is a need to develop more detailed models that will facilitate investigations into the effects of pore size (of the substrate) as well as the size, structure and shape of a variety of tethered organic moieties. Also, it is planned to use molecular dynamics calculations to measure the diffusion of free radical groups (no longer tethered) that are released into the pores as a result of pyrolytic reactions (thermal decomposition). This will help reveal the key bimolecular reaction steps that determine reaction rates and product selectivities.  Following this new models are  required to extend the study to other confined systems such as, for example silica-titania surfaces that are important in photochemical reactions.

Model for mesoporous silica containing tethered PPE molecules (density 1.33 nm-2).
Yellow - Si; red - O; grey – C, white – H.

This is a collaborative project with the Oak Ridge National Laboratory, Kentucky, USA and will ideally involve the candidate spending some time at this site.

Funding: This project is supported through a research agreement with Oak Ridge National Laboratory, USA.

References:
1 AL Chaffee, Molecular modeling of HMS-hybrid materials for CO2 adsorption. Fuel Processing Technology, 86 (14-15), 1471-1484 (2005).
2 MK Kidder, PF Britt, AL Chaffee and AC Buchanan, III, Confinement effects on product selectivity in the pyrolysis of phenethyl phenyl ether in mesoporous silica. Chem. Comm., 2007, 52-54.
3 MK Kidder, PF Britt, Z Zhang, S Dai, EW Hagaman, AL Chaffee and AC Buchanan, III - Pyrolysis of Mesoporous Silica-Immobilized 1,3-DPP. Impact of Pore Confinement and Size. J Am Chem Soc; 127, 6353-6360 (2005),